[Federal Register Volume 73, Number 60 (Thursday, March 27, 2008)]
[Rules and Regulations]
[Pages 16436-16514]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: E8-5645]
[[Page 16435]]
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Part II
Environmental Protection Agency
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40 CFR Parts 50 and 58
National Ambient Air Quality Standards for Ozone; Final Rule
Federal Register / Vol. 73, No. 60 / Thursday, March 27, 2008 / Rules
and Regulations
[[Page 16436]]
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ENVIRONMENTAL PROTECTION AGENCY
40 CFR Parts 50 and 58
[EPA-HQ-OAR-2005-0172; FRL-8544-3]
RIN 2060-AN24
National Ambient Air Quality Standards for Ozone
AGENCY: Environmental Protection Agency (EPA).
ACTION: Final rule.
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SUMMARY: Based on its review of the air quality criteria for ozone
(O3) and related photochemical oxidants and national ambient
air quality standards (NAAQS) for O3, EPA is making
revisions to the primary and secondary NAAQS for O3 to
provide requisite protection of public health and welfare,
respectively. With regard to the primary standard for O3,
EPA is revising the level of the 8-hour standard to 0.075 parts per
million (ppm), expressed to three decimal places. With regard to the
secondary standard for O3, EPA is revising the current 8-
hour standard by making it identical to the revised primary standard.
EPA is also making conforming changes to the Air Quality Index (AQI)
for O3, setting an AQI value of 100 equal to 0.075 ppm, 8-
hour average, and making proportional changes to the AQI values of 50,
150 and 200.
DATES: This final rule is effective on May 27, 2008.
ADDRESSES: EPA has established a docket for this action under Docket ID
No. EPA-HQ-OAR-2005-0172. All documents in the docket are listed on the
www.regulations.gov Web site. Although listed in the index, some
information is not publicly available, e.g., confidential business
information or other information whose disclosure is restricted by
statute. Certain other material, such as copyrighted material, is not
placed on the Internet and will be publicly available only in hard copy
form. Publicly available docket materials are available either
electronically through www.regulations.gov or in hard copy at the Air
and Radiation Docket and Information Center, EPA/DC, EPA West, Room
3334, 1301 Constitution Ave., NW., Washington, DC. This Docket Facility
is open from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding
legal holidays. The Docket telephone number is 202-566-1742. The
telephone number for the Public Reading Room is 202-566-1744.
FOR FURTHER INFORMATION CONTACT: Dr. David J. McKee, Health and
Environmental Impacts Division, Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency, Mail Code C504-06,
Research Triangle Park, NC 27711; telephone: 919-541-5288; fax: 919-
541-0237; e-mail: [email protected].
SUPPLEMENTARY INFORMATION:
Table of Contents
The following topics are discussed in this preamble:
I. Background
A. Summary of Revisions to the O3 NAAQS
B. Legislative Requirements
C. Review of Air Quality Criteria and Standards for
O3
D. Summary of Proposed Revisions to the O3 NAAQS
E. Organization and Approach to Final Decision on O3
NAAQS
II. Rationale for Final Decision on the Primary O3
Standard
A. Introduction
1. Overview
2. Overview of Health Effects
3. Overview of Human Exposure and Health Risk Assessments
B. Need for Revision of the Current Primary O3
Standard
1. Introduction
2. Comments on the Need for Revision
3. Conclusions Regarding the Need for Revision
C. Conclusions on the Elements of the Primary O3
Standard
1. Indicator
2. Averaging Time
3. Form
4. Level
D. Final Decision on the Primary O3 Standard
III. Communication of Public Health Information
IV. Rationale for Final Decision on the Secondary O3
Standard
A. Introduction
1. Overview
2. Overview of Vegetation Effects Evidence
3. Overview of Biologically Relevant Exposure Indices
4. Overview of Vegetation Exposure and Risk Assessments
B. Need for Revision of the Current Secondary O3
Standard
1. Introduction
2. Comments on the Need for Revision
3. Conclusions Regarding the Need for Revision
C. Conclusions on the Secondary O3 Standard
1. Staff Paper Evaluation
2. CASAC Views
3. Administrator's Proposed Conclusions
4. Comments on the Secondary Standard Options
5. Administrator's Final Conclusions
D. Final Decision on the Secondary O3 Standard
V. Creation of Appendix P--Interpretation of the NAAQS for
O3
A. General
B. Data Completeness
C. Data Reporting and Handling and Rounding Conventions
VI. Ambient Monitoring Related to Revised O3 Standards
VII. Implementation and Related Control Requirements
A. Future Implementation Steps
1. Designations
2. State Implementation Plans
3. Trans-boundary Emissions
4. Monitoring Requirements
B. Related Control Requirements
VIII. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review
B. Paperwork Reduction Act
C. Regulatory Flexibility Act
D. Unfunded Mandates Reform Act
E. Executive Order 13132: Federalism
F. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
G. Executive Order 13045: Protection of Children From
Environmental Health & Safety Risks
H. Executive Order 13211: Actions That Significantly Affect
Energy Supply, Distribution or Use
I. National Technology Transfer and Advancement Act
J. Executive Order 12898: Federal Actions to Address
Environmental Justice in Minority Populations and Low-Income
Populations
K. Congressional Review Act
References
I. Background
A. Summary of Revisions to the O3 NAAQS
Based on its review of the air quality criteria for O3
and related photochemical oxidants and national ambient air quality
standards (NAAQS) for O3, EPA is making revisions to the
primary and secondary NAAQS for O3 to provide protection of
public health and welfare, respectively, that is appropriate under
section 109, and is making corresponding revisions in data handling
conventions for O3.
With regard to the primary standard for O3, EPA is
revising the level of the 8-hour standard to a level of 0.075 parts per
million (ppm), to provide increased protection for children and other
``at risk'' populations against an array of O3-related
adverse health effects that range from decreased lung function and
increased respiratory symptoms to serious indicators of respiratory
morbidity including emergency department visits and hospital admissions
for respiratory causes, and possibly cardiovascular-related morbidity
as well as total nonaccidental and cardiorespiratory mortality. EPA is
specifying the level of the primary standard to the nearest thousandth
ppm.
With regard to the secondary standard for O3, EPA is
revising the standard by making it identical to the revised primary
standard.
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B. Legislative Requirements
Two sections of the Clean Air Act (CAA) govern the establishment
and revision of the NAAQS. Section 108 (42 U.S.C. 7408) directs the
Administrator to identify and list ``air pollutants'' emissions of
which ``in his judgment, cause or contribute to air pollution which may
reasonably be anticipated to endanger public health or welfare,'' whose
``presence * * * in the ambient air results from numerous or diverse
mobile or stationary sources,'' and for which the Administrator plans
to issue air quality criteria, and to issue air quality criteria for
those that are listed. Air quality criteria are to ``accurately reflect
the latest scientific knowledge useful in indicating the kind and
extent of identifiable effects on public health or welfare which may be
expected from the presence of [a] pollutant in ambient air, in varying
quantities * * *.'' Section 109 (42 U.S.C. 7409) directs the
Administrator to propose and promulgate ``primary'' and ``secondary''
NAAQS for pollutants listed under section 108. Section 109(b)(1)
defines a primary standard as one ``the attainment and maintenance of
which in the judgment of the Administrator, based on such criteria and
allowing an adequate margin of safety, are requisite to protect the
public health.'' \1\ A secondary standard, as defined in section
109(b)(2), must ``specify a level of air quality the attainment and
maintenance of which in the judgment of the Administrator, based on
such criteria, is requisite to protect the public welfare from any
known or anticipated adverse effects associated with the presence of
[the] pollutant in the ambient air.'' \2\
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\1\ The legislative history of section 109 indicates that a
primary standard is to be set at ``the maximum permissible ambient
air level * * * which will protect the health of any [sensitive]
group of the population,'' and that for this purpose ``reference
should be made to a representative sample of persons comprising the
sensitive group rather than to a single person in such a group'' [S.
Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970)].
\2\ Welfare effects as defined in section 302(h) (42 U.S.C.
7602(h)) include, but are not limited to, ``effects on soils, water,
crops, vegetation, manmade materials, animals, wildlife, weather,
visibility and climate, damage to and deterioration of property, and
hazards to transportation, as well as effects on economic values and
on personal comfort and well-being.''
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The requirement that primary standards provide an adequate margin
of safety was intended to address uncertainties associated with
inconclusive scientific and technical information available at the time
of standard setting. It was also intended to provide a reasonable
degree of protection against hazards that research has not yet
identified. Lead Industries Association v. EPA, 647 F.2d 1130, 1154 (DC
Cir 1980), cert. denied, 449 U.S. 1042 (1980); American Petroleum
Institute v. Costle, 665 F.2d 1176, 1186 (DC Cir. 1981), cert. denied,
455 U.S. 1034 (1982). Both kinds of uncertainties are components of the
risk associated with pollution at levels below those at which human
health effects can be said to occur with reasonable scientific
certainty. Thus, in selecting primary standards that provide an
adequate margin of safety, the Administrator is seeking not only to
prevent pollution levels that have been demonstrated to be harmful but
also to prevent lower pollutant levels that may pose an unacceptable
risk of harm, even if the risk is not precisely identified as to nature
or degree. The CAA does not require the Administrator to establish a
primary NAAQS at a zero-risk level or at background concentration
levels, see Lead Industries Association v. EPA, 647 F.2d at 1156 n. 51,
but rather at a level that reduces risk sufficiently so as to protect
public health with an adequate margin of safety.
The selection of any particular approach to providing an adequate
margin of safety is a policy choice left specifically to the
Administrator's judgment. Lead Industries Association v. EPA, 647 F.2d
at 1161-62. In addressing the requirement for an adequate margin of
safety, EPA considers such factors as the nature and severity of the
health effects involved, the size of the population(s) at risk, and the
kind and degree of the uncertainties that must be addressed.
In setting standards that are ``requisite'' to protect public
health and welfare, as provided in section 109(b), EPA's task is to
establish standards that are neither more nor less stringent than
necessary for these purposes. Whitman v. America Trucking Associations,
531 U.S. 457, 473. Further the Supreme Court ruled that ``[t]he text of
Sec. 109(b), interpreted in its statutory and historical context and
with appreciation for its importance to the CAA as a whole,
unambiguously bars cost considerations from the NAAQS-setting process *
* *'' Id. at 472.\3\
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\3\ In considering whether the CAA allowed for economic
considerations to play a role in the promulgation of the NAAQS, the
Supreme Court rejected arguments that because many more factors than
air pollution might affect public health, EPA should consider
compliance costs that produce health losses in setting the NAAQS.
531 U.S. at 466. Thus, EPA may not take into account possible public
health impacts from the economic cost of implementation. Id.
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Section 109(d)(1) of the CAA requires that ``not later than
December 31, 1980, and at 5-year intervals thereafter, the
Administrator shall complete a thorough review of the criteria
published under section 108 and the national ambient air quality
standards * * * and shall make such revisions in such criteria and
standards and promulgate such new standards as may be appropriate in
accordance with section 108 and [109(b)].'' Section 109(d)(2) requires
that an independent scientific review committee ``shall complete a
review of the criteria * * * and the national primary and secondary
ambient air quality standards * * * and shall recommend to the
Administrator any new * * * standards and revisions of existing
criteria and standards as may be appropriate under section 108 and
[section 109(b)].'' This independent review function is performed by
the Clean Air Scientific Advisory Committee (CASAC) of EPA's Science
Advisory Board.
C. Review of Air Quality Criteria and Standards for O3
Ground-level O3 is formed from biogenic and
anthropogenic precursor emissions. Naturally occurring O3 in
the troposphere can result from biogenic organic precursors reacting
with naturally occurring nitrogen oxides (NOX) and by
stratospheric O3 intrusion into the troposphere.
Anthropogenic precursors of O3, specifically NOX
and volatile organic compounds (VOC), originate from a wide variety of
stationary and mobile sources. Ambient O3 concentrations
produced by these emissions are directly affected by temperature, solar
radiation, wind speed and other meteorological factors.
The last review of the O3 NAAQS was completed on July
18, 1997, based on the 1996 O3 Air Quality Criteria Document
(EPA, 1996a) and 1996 O3 Staff Paper (EPA, 1996b). EPA
revised the primary and secondary O3 standards on the basis
of the then latest scientific evidence linking exposures to ambient
O3 to adverse health and welfare effects at levels allowed
by the 1-hour average standards (62 FR 38856). The O3
standards were revised by replacing the existing primary 1-hour average
standard with an 8-hour average O3 standard set at a level
of 0.08 ppm, which is equivalent to 0.084 ppm using the standard
rounding conventions. The form of the primary standard was changed to
the annual fourth-highest daily maximum 8-hour average concentration,
averaged over 3 years. The secondary O3 standard was changed
by making it identical in all respects to the revised primary standard.
EPA initiated this current review in September 2000 with a call for
information (65 FR 57810) for the development of a revised Air Quality
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Criteria Document for O3 and Other Photochemical Oxidants
(henceforth the ``Criteria Document''). A project work plan (EPA, 2002)
for the preparation of the Criteria Document was released in November
2002 for CASAC O3 Panel \4\ (henceforth, ``CASAC Panel'')
and public review. EPA held a series of workshops in mid-2003 on
several draft chapters of the Criteria Document to obtain broad input
from the relevant scientific communities. These workshops helped to
inform the preparation of the first draft Criteria Document (EPA,
2005a), which was released for CASAC Panel and public review on January
31, 2005; a CASAC Panel meeting was held on May 4-5, 2005 to review the
first draft Criteria Document. A second draft Criteria Document (EPA,
2005b) was released for CASAC Panel and public review on August 31,
2005, and was discussed along with a first draft Staff Paper (EPA,
2005c) at a CASAC Panel meeting held on December 6-8, 2005. In a
February 16, 2006 letter to the Administrator, the CASAC Panel offered
final comments on all chapters of the Criteria Document (Henderson,
2006a), and the final Criteria Document (EPA, 2006a) was released on
March 21, 2006. In a June 8, 2006 letter (Henderson, 2006b) to the
Administrator, the CASAC Panel offered additional advice to the Agency
concerning chapter 8 of the final Criteria Document (Integrative
Synthesis) to help inform the second draft Staff Paper.
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\4\ The CASAC O3 Review Panel includes the seven
members of the chartered CASAC, supplemented by fifteen subject-
matter experts appointed by the Administrator to provide additional
scientific expertise relevant to this review of the O3
NAAQS.
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A second draft Staff Paper (EPA, 2006b) was released on July 17,
2006 and reviewed by the CASAC Panel on August 24 and 25, 2006. In an
October 24, 2006 letter to the Administrator, CASAC Panel provided
advice and recommendations to the Agency concerning the second draft
Staff Paper (Henderson, 2006c). A final Staff Paper (EPA, 2007a) was
released on January 31, 2007. Around the time of the release of the
final Staff Paper in January 2007, EPA discovered a small error in the
exposure model that when corrected resulted in slight increases in the
human exposure estimates. Since the exposure estimates are an input to
the lung function portion of the health risk assessment, this
correction also resulted in slight increases in the lung function risk
estimates as well. The exposure and risk estimates discussed in this
final rule reflect the corrected estimates, and thus are slightly
different than the exposure and risk estimates cited in the January 31,
2007 Staff Paper.\5\ In a March 26, 2007 letter (Henderson, 2007), the
CASAC Panel offered additional advice to the Administrator with regard
to recommendations and revisions to the primary and secondary
O3 NAAQS.
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\5\ EPA made available corrected versions of the final Staff
Paper (EPA, 2007b, henceforth, ``Staff Paper'') and the human
exposure and health risk assessment technical support documents on
July 31, 2007 on the EPA Web site http://www.epa.gov/ttn/naaqs.
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The schedule for completion of this review has been governed by a
consent decree resolving a lawsuit filed in March 2003 by a group of
plaintiffs representing national environmental and public health
organizations, alleging that EPA had failed to complete the current
review within the period provided by statute.\6\ The modified consent
decree that currently governs this review provides that EPA sign for
publication notices of proposed and final rulemaking concerning its
review of the O3 NAAQS no later than June 20, 2007 and March
12, 2008, respectively. The proposed decision (henceforth ``proposal'')
was signed on June 20, 2007 and published in the Federal Register on
July 11, 2007.
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\6\ American Lung Association v. Whitman (No. 1:03CV00778,
D.D.C. 2003).
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A large number of comments were received from various commenters on
the proposed revisions to the O3 NAAQS. Significant issues
raised in the public comments are discussed throughout the preamble of
this final action. A comprehensive summary of all significant comments,
along with EPA's responses (henceforth ``Response to Comments''), can
be found in the docket for this rulemaking.
Various commenters have referred to and discussed a number of new
scientific studies on the health effects of O3 that had been
published recently and therefore were not included in the Criteria
Document (EPA, 2006a, henceforth ``Criteria Document).\7\ EPA has
provisionally considered any significant ``new'' studies, including
those submitted during the public comment period. The purpose of this
effort was to ensure that the Administrator was fully aware of the
``new'' science before making a final decision on whether to revise the
current O3 NAAQS. EPA provisionally considered these studies
to place their results in the context of the findings of the Criteria
Document.
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\7\ For ease of reference, these studies will be referred to as
``new'' studies or ``new'' science, using quotation marks around the
word new. Referring to studies that were published too recently to
have been included in the 2004 Criteria Document as ``new'' studies
is intended to clearly differentiate such studies from those that
have been published since the last review and are included in the
2004 Criteria Document (these studies are sometimes referred to as
new (without quotation marks) or more recent studies, to indicate
that they were not included in the 1996 Criteria Document and thus
are newly available in this review.
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As in prior NAAQS reviews, EPA is basing its decision in this
review on studies and related information included in the Criteria
Document and Staff Paper, which have undergone CASAC and public review.
The studies assessed in the Criteria Document, and the integration of
the scientific evidence presented in that document, have undergone
extensive critical review by EPA, CASAC, and the public during the
development of the Criteria Document. The rigor of that review makes
these studies, and their integrative assessment, the most reliable
source of scientific information on which to base decisions on the
NAAQS, decisions that all parties recognize as of great import. NAAQS
decisions can have profound impacts on public health and welfare, and
NAAQS decisions should be based on studies that have been rigorously
assessed in an integrative manner not only by EPA but also by the
statutorily mandated independent advisory committee, as well as the
public review that accompanies this process. As described above, EPA's
provisional consideration of these studies did not and could not
provide that kind of in-depth critical review.
This decision is consistent with EPA's practice in prior NAAQS
reviews. Since the 1970 amendments, the EPA has taken the view that
NAAQS decisions are to be based on scientific studies and related
information that have been assessed as a part of the pertinent air
quality criteria, and has consistently followed this approach. See 71
FR 61144, 61148 (October 17, 2006) (final decision on review of PM
NAAQS) for a detailed discussion of this issue and EPA's past practice.
As discussed in EPA's 1993 decision not to revise the NAAQS for
O3 ``new'' studies may sometimes be of such significance
that it is appropriate to delay a decision on revision of a NAAQS and
to supplement the pertinent air quality criteria so the studies can be
taken into account (58 FR at 13013-13014, March 9, 1993). In the
present case, EPA's provisional consideration of ``new'' studies
concludes that, taken in context, the ``new'' information and findings
do not materially change any of the broad scientific conclusions
regarding the health effects of O3 exposure made in the
Criteria Document. For this reason, reopening the air quality criteria
review would not be warranted even if there were time to do so under
the court order
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governing the schedule for this rulemaking. Accordingly, EPA is basing
the final decisions in this review on the studies and related
information included in the O3 air quality criteria that
have undergone CASAC and public review. EPA will consider the newly
published studies for purposes of decision making in the next periodic
review of the O3 NAAQS, which will provide the opportunity
to fully assess them through a more rigorous review process involving
EPA, CASAC, and the public. Further discussion of these ``new'' studies
can be found in the Response to Comments document.
This action presents the Administrator's final decisions on the
review of the current primary and secondary O3 standards.
Throughout this preamble a number of conclusions, findings, and
determinations made by the Administrator are noted. They identify the
reasoning that supports this final decision and are intended to be
final and conclusive.
D. Summary of Proposed Revisions to the O3 NAAQS
For reasons discussed in the proposal, the Administrator proposed
to revise the current primary and secondary O3 standards.
With regard to the primary O3 standard, the Administrator
proposed to revise the level of the 8-hour O3 standard to a
level within the range of 0.070 ppm to 0.075 ppm, based on a 3-year
average of the fourth-highest maximum 8-hour average concentration.
Related revisions for O3 data handling conventions and for
the reference method for monitoring O3 were also proposed.
These revisions were proposed to provide increased protection for
children and other ``at risk'' populations against an array of
O3-related adverse health effects that range from decreased
lung function and increased respiratory symptoms to serious indicators
of respiratory morbidity, including emergency department visits and
hospital admissions for respiratory causes, and possibly
cardiovascular-related morbidity, as well as total nonaccidental and
cardiorespiratory mortality. EPA also proposed to specify the level of
the primary standard to the nearest thousandth ppm. EPA solicited
comment on alternative levels down to 0.060 ppm and up to and including
retaining the current 8-hour standard of 0.08 ppm (effectively 0.084
ppm using current data rounding conventions).
With regard to the secondary standard for O3, EPA
proposed to revise the current 8-hour standard with one of two options
to provide increased protection against O3-related adverse
impacts on vegetation and forested ecosystems. One option was to
replace the current standard with a cumulative, seasonal standard
expressed as an index of the annual sum of weighted hourly
concentrations, cumulated over 12 hours per day (8 am to 8 pm) during
the consecutive 3-month period within the O3 season with the
maximum index value, set at a level within the range of 7 to 21 ppm-
hours. The other option was to make the secondary standard identical to
the proposed primary 8-hour standard. EPA solicited comment on
specifying a cumulative, seasonal standard in terms of a 3-year average
of the annual sums of weighted hourly concentrations; on the range of
alternative 8-hour standard levels for which comment was being
solicited for the primary standard, including retaining the current
secondary standard, which is identical to the current primary standard;
and on an alternative approach to setting a cumulative, seasonal
secondary standard.
E. Organization and Approach to Final O3 NAAQS Decisions
This action presents the Administrator's final decisions regarding
the need to revise the current primary and secondary O3
standards. Revisions to the primary standard for O3 are
addressed below in section II, and a discussion on communication of
public health information regarding revisions to the primary
O3 standard is presented in section III. The secondary
O3 standard is addressed below in section IV. Related data
completeness and data handling and rounding conventions are addressed
in section V, and federal reference methods for monitoring
O3 are addressed below in section VI. Future implementation
steps and related control requirements are discussed in section VII. A
discussion of statutory and executive order reviews is provided in
section VIII.
Today's final decisions are based on a thorough review in the
Criteria Document of scientific information on known and potential
human health and welfare effects associated with exposure to
O3 at levels typically found in the ambient air. These final
decisions also take into account: (1) Staff assessments in the Staff
Paper of the most policy-relevant information in the Criteria Document
as well as quantitative exposure and risk assessments based on that
information; (2) CASAC Panel advice and recommendations, as reflected
in its letters to the Administrator, its discussions of drafts of the
Criteria Document and Staff Paper at public meetings, and separate
written comments prepared by individual members of the CASAC Panel; (3)
public comments received during the development of these documents,
either in connection with CASAC Panel meetings or separately; and (4)
extensive public comments received on the proposed rulemaking.
II. Rationale for Final Decisions on the Primary O3 Standard
A. Introduction
1. Overview
This section presents the Administrator's final decisions regarding
the need to revise the current primary O3 NAAQS, and the
appropriate revision to the level of the 8-hour standard. As discussed
more fully below, the rationale for the final decision on appropriate
revisions to the primary O3 NAAQS includes consideration of:
(1) Evidence of health effects related to short-term exposures to
O3; (2) insights gained from quantitative exposure and
health risk assessments; (3) public and CASAC Panel comments received
during the development and review of the Criteria Document, Staff
Paper, exposure and risk assessments and on the proposal notice.
In developing this rationale, EPA has drawn upon an integrative
synthesis of the entire body of evidence \8\ relevant to examining
associations between exposure to ambient O3 and a broad
range of health endpoints (EPA, 2006a, Chapter 8), focusing on those
health endpoints for which the Criteria Document concluded that the
associations are causal or likely to be causal. This body of evidence
includes hundreds of studies conducted in many countries around the
world. In its assessment of the evidence judged to be most relevant to
decisions on elements of the primary O3 standards, EPA has
placed greater weight on U.S. and Canadian studies, since studies
conducted in other countries may well reflect different demographic and
air pollution characteristics.
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\8\ The word ``evidence'' is used in this notice to refer to
studies that provide information relevant to an area of inquiry,
which can include studies that report positive or negative results
or that provide interpretative information.
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As discussed below, a significant amount of new research has been
conducted since the last review, with important new information coming
from epidemiological, toxicological, controlled human exposure, and
dosimetric studies. Moreover, the newly available research studies
evaluated in the Criteria Document have undergone intensive scrutiny
through multiple layers of peer review, with extended
[[Page 16440]]
opportunities for review and comment by CASAC Panel and the public. As
with virtually any policy-relevant scientific research, there is
uncertainty in the characterization of health effects attributable to
exposure to ambient O3, most generally with regard to
whether observed health effects and associations are causal or likely
causal in nature and, if so, the certainty of causal associations at
various exposure levels. While important uncertainties remain, the
review of the health effects information has been extensive and
deliberate. In the judgment of the Administrator, this intensive
evaluation of the scientific evidence provides an adequate basis for
regulatory decision making at this time. This review also provides
important input to EPA's research plan for improving our future
understanding of the relationships between exposures to ambient
O3 and health effects.
The health effects information and quantitative exposure and health
risk assessment were summarized in sections II.A and II.B of the
proposal (72 FR at 37824-37862) and are only briefly outlined below in
sections II.A.2 and II.A.3. Subsequent sections of this preamble
provide a more complete discussion of the Administrator's rationale, in
light of key issues raised in public comments, for concluding that the
current standard is not requisite to protect public health with an
adequate margin of safety, and it is appropriate to revise the current
primary O3 standards to provide additional public health
protection (section II.B), as well as a more complete discussion of the
Administrator's rationale for retaining or revising the specific
elements of the primary O3 standards (section II.C), namely
the indicator (section II.C.1); averaging time (section II.C.2); form
(section II.C.3); and level (section II.C.4). A summary of the final
decisions on revisions to the primary O3 standards is
presented in section II.D.
2. Overview of Health Effects
This section outlines the information presented in Section II.A of
the proposal on known or potential effects on public health which may
be expected from the presence of O3 in ambient air. The
decision in the last review focused primarily on evidence from short-
term (e.g., 1 to 3 hours) and prolonged ( 6 to 8 hours) controlled-
exposure studies reporting lung function decrements, respiratory
symptoms, and respiratory inflammation in humans, as well as
epidemiology studies reporting excess hospital admissions and emergency
department visits for respiratory causes. The Criteria Document
prepared for this review emphasizes a large number of epidemiological
studies published since the last review with these and additional
health endpoints, including the effects of acute (short-term and
prolonged) and chronic exposures to O3 on lung function
decrements and enhanced respiratory symptoms in asthmatic individuals,
school absences, and premature mortality. It also emphasizes important
new information from toxicology, dosimetry, and controlled human
exposure studies. Highlights of the evidence include:
(1) Two new controlled human-exposure studies are now available
that examine respiratory effects associated with prolonged
O3 exposures at levels at and below 0.080 ppm, which was the
lowest exposure level that had been examined in the last review.
(2) Numerous recent controlled human-exposure studies have examined
indicators of O3-induced inflammatory response in both the
upper respiratory tract (URT) and lower respiratory tract (LRT), while
other studies have examined changes in host defense capability
following O3 exposure of healthy young adults and increased
airway responsiveness to allergens in subjects with allergic asthma and
allergic rhinitis exposed to O3.
(3) New evidence from controlled human exposure studies showing
that asthmatics have greater respiratory-related physiological
responses than healthy subjects and new evidence from epidemiological
studies showing associations between O3 exposure and lung
function and respiratory symptom responses; these findings differ from
the presumption in the last review that people with asthma had
generally the same magnitude of respiratory responses to O3
as those experienced by healthy individuals.
(4) Animal toxicology studies provide new information regarding
potential mechanisms of action, increased susceptibility to respiratory
infection, and biological plausibility of acute effects as well as
chronic, irreversible respiratory damage observed in animals.
(5) Numerous epidemiological studies published during the past
decade offer added evidence of associations between acute ambient
O3 exposures and lung function decrements and respiratory
symptoms in physically active healthy subjects and asthmatic subjects,
as well as new evidence regarding additional health endpoints,
including relationships between ambient O3 concentrations
and school absenteeism and between ambient O3 and cardiac-
related physiological endpoints.
(6) Several additional studies have been published over the last
decade examining the temporal associations between acute O3
exposures and both emergency department visits for respiratory diseases
and respiratory-related hospital admissions.
(7) A large number of newly available epidemiological studies have
examined the effects of acute exposure to PM and O3 on
premature mortality, notably including large multi-city studies that
provide much more robust information than was available in the last
review, as well as recent meta-analyses that have evaluated potential
sources of heterogeneity in O3-mortality associations.
Section II.A of the proposal provides a detailed summary of key
information contained in the Criteria Document (chapters 4-8) and in
the Staff Paper (chapter 3), on the known and potential effects of
O3 exposure and information on the effects of O3
exposure in combination with other pollutants that are routinely
present in the ambient air (72 FR 37824-37851). The information there
summarizes:
(1) New information available on potential mechanisms for morbidity
and mortality effects associated with exposure to O3,
including potential mechanisms or pathways related to direct effects on
the respiratory system, systemic effects that are secondary to effects
in the respiratory system (e.g., cardiovascular effects);
(2) The nature of effects that have been associated directly with
exposure to O3 or indirectly with the presence of
O3 in ambient air, including premature mortality,
aggravation of respiratory and cardiovascular disease (as indicated by
increased hospital admissions and emergency department visits), changes
in lung function and increased respiratory symptoms, as well as new
evidence for more subtle indicators of cardiovascular health;
(3) An integrative interpretation of the health effects evidence,
focusing on the biological plausibility and coherence of the evidence
and key issues raised in interpreting epidemiological studies, along
with supporting evidence from experimental (e.g., dosimetric and
toxicological) studies as well as the limitations of the evidence; and
(4) Considerations in characterizing the public health impact of
O3, including the identification of sensitive and vulnerable
subpopulations that are potentially at risk to such effects, including
active people, people with pre-existing lung and heart diseases,
children and older adults, and people with increased responsiveness to
O3.
[[Page 16441]]
3. Overview of Human Exposure and Health Risk Assessments
To put judgments about health effects that are adverse for
individuals into a broader public health context, EPA developed and
applied models to estimate human exposures and health risks. This
broader public health context included consideration of the size of
particular population groups at risk for various effects, the
likelihood that exposures of concern would occur for individuals in
such groups under varying air quality scenarios, estimates of the
number of people likely to experience O3-related effects,
the variability in estimated exposures and risks, and the kind and
degree of uncertainties inherent in assessing the exposures and risks
involved.
As discussed in more detail in section II.B of the proposal, there
are a number of important uncertainties that affect the exposure and
health risk estimates. It is also important to note that there have
been significant improvements since the last review in both the
exposure and health risk models. The CASAC Panel expressed the view
that the exposure analysis represents a state-of-the-art modeling
approach and that the health risk assessment was ``well done, balanced
and reasonably communicated'' (Henderson, 2006c).
In modeling exposures and health risks associated with just meeting
the current and alternative O3 standards, EPA simulated air
quality just meeting these standards based on O3 air quality
patterns in several recent years and on how the shape of the
O3 air quality distributions has changed over time based on
historical trends in monitored O3 air quality data. As
discussed in the proposal notice and in the Staff Paper (section
4.5.8), recent O3 air quality distributions were
statistically adjusted to simulate just meeting the current and
selected alternative standards. Specifically, the exposure and risk
assessment included estimates for a recent year of air quality and for
air quality adjusted to simulate just meeting the current and
alternative standards based on O3 season data from a recent
three-year period (2002-2004). The O3 season in each area
included the period of the year for which routine hourly O3
monitoring data are available. Typically this period spans from March
or April through September or October, although in some areas it
includes the entire year. Three years were modeled to reflect the
substantial year-to-year variability that occurs in O3
levels and related meteorological conditions, and because the standard
is specified in terms of a three-year period. The year-to-year
variability observed in O3 levels is due to a combination of
different weather patterns and the variation in emissions of
O3 precursors. Nationally, 2002 was a relatively high year
with respect to the 4th highest daily maximum 8-hour O3
levels observed in urban areas across the U.S. (see Staff Paper, Figure
2-16), with the mean of the distribution of annual 4th highest daily
maximum 8-hour O3 levels for urban monitors nationwide being
in the upper third among the years 1990 through 2004. In contrast, on a
national basis, 2004 was the lowest year on record with respect to the
mean of the distribution of annual 4th highest daily maximum 8-hour
O3 levels for this same 15 year period. The 4th highest
daily maximum 8-hour levels observed in most, but not all of the 12
urban areas included in the exposure and risk assessment, were
relatively low in 2004 compared to other recent years. The 4th highest
daily maximum 8-hour O3 levels observed in 2003 in the 12
urban areas and nationally generally were between those observed in
2002 and 2004. As a result of the variability in air quality, the
exposure and risk estimates associated with just meeting the current or
any alternative standard also will vary depending on the year chosen
for the analysis. Thus, exposure and risk estimates based on 2002 air
quality generally show relatively higher numbers of children affected
and the estimates based on 2004 air quality generally show relatively
fewer numbers of children affected.
These simulations do not reflect any consideration of specific
control programs or strategies designed to achieve the reductions in
emissions required to meet the specified standards. Further, these
simulations do not represent predictions of when, whether, or how areas
might meet the specified standards.\9\ Instead these simulations
represent a projection of the kind of air quality levels that would be
likely to occur in areas just attaining various alternative standards,
when historical patterns of air quality, reflecting averages over many
areas, are applied in the urban areas examined.
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\9\ For informational purposes only, modeling that projects how
areas might attain alternative standards in a future year as a
result of Federal, State, local, and Tribal efforts is presented in
the final Regulatory Impact Analysis being prepared in connection
with this decision.
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a. Exposure Analyses
As discussed in section II.B.1 of the proposal, EPA conducted human
exposure analyses using a simulation model to estimate O3
exposures for the general population, school age children (ages 5-18),
and school age children with asthma living in 12 U.S. metropolitan
areas representing different regions of the country where the current
8-hour O3 standard is not met. The emphasis on children
reflected the finding of the last review that children are an important
at-risk group. Exposure estimates were developed using a probabilistic
exposure model that is designed to explicitly model the numerous
sources of variability that affect people's exposures. This exposure
assessment is more fully described and presented in the Staff Paper and
in a technical support document, Ozone Population Exposure Analysis for
Selected Urban Areas (EPA, 2007c; henceforth ``Exposure Analysis
TSD''). As noted in the proposal, the scope and methodology for this
exposure assessment were developed over the last few years with
considerable input from the CASAC Panel and the public.
As discussed in the proposal notice and in greater detail in the
Staff Paper (chapter 4) and Exposure Analysis TSD, EPA recognized that
there are many sources of variability and uncertainty inherent in the
input to this assessment and that there was uncertainty in the
resulting O3 exposure estimates. In EPA's judgment, the most
important uncertainties affecting the exposure estimates are related to
the modeling of human activity patterns over an O3 season,
the modeling of variations in ambient concentrations near roadways, and
the modeling of air exchange rates that affect the amount of
O3 that penetrates indoors. Another important uncertainty
that affects the estimation of how many exposures are associated with
moderate or greater exertion is the characterization of energy
expenditure for children engaged in various activities. As discussed in
more detail in the Staff Paper (section 4.3.4.7), the uncertainty in
energy expenditure values carries over to the uncertainty of the
modeled breathing rates, which are important since they are used to
classify exposures occurring at moderate or greater exertion. These are
the relevant exposures since O3-related effects observed in
clinical studies only are observed when individuals are engaged in some
form of exercise. The uncertainties in the exposure model inputs and
the estimated exposures have been assessed using quantitative
uncertainty and sensitivity analyses. Details are discussed in the
Staff Paper (section 4.6) and in a technical memorandum describing the
exposure modeling uncertainty analysis (Langstaff, 2007).
The exposure assessment, which provided estimates of the number of
people exposed to different levels of
[[Page 16442]]
ambient O3 while at elevated exertion \10\, served two
purposes. First, the entire range of modeled personal exposures to
ambient O3 was an essential input to the portion of the
health risk assessment based on exposure-response functions from
controlled human exposure studies, discussed in the next section.
Second, estimates of personal exposures to ambient O3
concentrations at and above specified benchmark levels while at
elevated exertion provided some perspective on the public health
impacts of health effects that we cannot currently evaluate in
quantitative risk assessments but that may occur at current air quality
levels, and the extent to which such impacts might be reduced by
meeting the current and alternative standards. In the proposal, we
referred to exposures at and above these benchmark levels while at
elevated exertion as ``exposures of concern.''
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\10\ As discussed in section II.A of the proposal, O3
health responses observed in controlled human exposure studies are
associated with exposures while subjects are engaged in moderate or
greater exertion on average over the exposure period (hereafter
referred to as ``elevated exertion'') and, therefore, these are the
exposures of interest.
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Based on the observation from the exposure analyses conducted in
the prior review that children represented the population subgroup with
the greatest exposure to ambient O3, EPA chose to model 8-
hour exposures at elevated exertion for all school age children, and
separately for asthmatic school age children, as well as for the
general population in the current exposure assessment. While outdoor
workers and other adults who engage in moderate or greater exertion for
prolonged periods while outdoors during the day in areas experiencing
elevated O3 concentrations also are at risk for
O3-related health effects, EPA did not focus on developing
quantitative exposure estimates for these population subgroups due to
the lack of information about the number of individuals who regularly
work or exercise outdoors. Thus, as presented in the proposal and in
the Staff Paper the exposure estimates are most useful for making
relative comparisons of estimated exposures in school age children
across alternative air quality scenarios. This assessment does not
provide information on exposures for adult subgroups within the general
population associated with the air quality scenarios.
EPA noted in the proposal key observations that were important to
consider in comparing exposure estimates associated with just meeting
the current NAAQS and alternative standards considered. These included:
(1) As shown in Table 6-1 of the Staff Paper, the patterns of
exposures in terms of percentages of the population exceeding given
exposure levels were very similar for the general population and for
asthmatic and all school age (5-18) children, although children were
about twice as likely as the general population to be exposed at any
given level.
(2) As shown in Table 1 in the proposal (72 FR 37855), the number
and percentage of asthmatic and all school age children aggregated
across the 12 urban areas estimated to experience 1 or more exposures
of concern declined from simulations of just meeting the current
standard to simulations of alternative 8-hour standards by varying
amounts, depending on the benchmark level, the population subgroup
considered, and the air quality year chosen.\11\
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\11\ While the proposal notice stated in the text that
``approximately 2 to 4 percent of all and asthmatic children'' were
estimated to experience exposures of concern at and above the 0.070
ppm benchmark level for standards in the range of 0.070 to 0.075 ppm
(72 FR 37879), the correct range is about 1 to 5 perecent consistent
with the estimates provided in Table 1 of the proposal (72 FR
37855).
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(3) Substantial year-to-year variability in exposure estimates was
observed over the three-year modeling period.
(4) There was substantial variability observed across the 12 urban
areas in the percent of the population subgroups estimated to
experience exposures at and above specified benchmark levels while at
elevated exertion.
(5) Of particular note, there is high inter-individual variability
in responsiveness such that only a subset of individuals who were
exposed at and above a given benchmark level while at elevated exertion
would actually be expected to experience any such potential adverse
health effects.
(6) In considering these observations, it was important to take
into account the variability, uncertainties, and limitations associated
with this assessment, including the degree of uncertainty associated
with a number of model inputs and uncertainty in the model itself.
b. Quantitative Health Risk Assessment
As discussed in section II.B.2 of the proposal, the approach used
to develop quantitative risk estimates associated with exposures to
O3 builds upon the risk assessment conducted during the last
review.\12\ The expanded and updated assessment conducted in this
review includes estimates of (1) risks of lung function decrements in
all and asthmatic school age children, respiratory symptoms in
asthmatic children, respiratory-related hospital admissions, and non-
accidental and cardiorespiratory-related mortality associated with
recent short-term ambient O3 levels; (2) risk reductions and
remaining risks associated with just meeting the current 8-hour
O3 NAAQS; and (3) risk reductions and remaining risks
associated with just meeting various alternative 8-hour O3
NAAQS in a number of example urban areas. The health risk assessment
was discussed in the Staff Paper (chapter 5) and presented more fully
in a technical support document, Ozone Health Risk Assessment for
Selected Urban Areas (Abt Associates, 2007a). As noted in the proposal,
the scope and methodology for this risk assessment was developed over
several years with considerable input from the CASAC Panel and the
public.
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\12\ The methodology, scope, and results from the risk
assessment conducted in the last review are described in Chapter 6
of the 1996 Staff Paper (EPA, 1996) and in several technical reports
(Whitfield et al., 1996; Whitfield, 1997) and publication (Whitfield
et al., 1998).
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EPA recognized that there were many sources of uncertainty and
variability inherent in the inputs to these assessments and that there
was a high degree of uncertainty in the resulting O3 risk
estimates. Such uncertainties generally relate to a lack of clear
understanding of a number of important factors, including, for example,
the shape of exposure-response and concentration-response functions,
particularly when, as here, effect thresholds can neither be discerned
nor determined not to exist; issues related to selection of appropriate
statistical models for the analysis of the epidemiologic data; the role
of potentially confounding and modifying factors in the concentration-
response relationships; and issues related to simulating how
O3 air quality distributions will likely change in any given
area upon attaining a particular standard, since strategies to reduce
emissions are not yet fully defined. While some of these uncertainties
were addressed quantitatively in the form of estimated confidence
ranges around central risk estimates, other uncertainties and the
variability in key inputs were not reflected in these confidence
ranges, but rather were partially characterized through separate
sensitivity analyses or discussed qualitatively.
Key observations and insights from the O3 risk
assessment, together with important caveats and limitations, were
discussed in section II.B of the proposal. In general, estimated risk
reductions associated with going from current O3 levels to
just meeting the current and
[[Page 16443]]
alternative 8-hour standards show patterns of increasing estimated risk
reductions associated with just meeting the lower alternative 8-hour
standards considered. Furthermore, the estimated percentage reductions
in risk were strongly influenced by the baseline air quality year used
in the analysis (see Staff Paper, Figures 6-1 through 6-6)
Key observations important in comparing estimated health risks
associated with attainment of the current NAAQS and alternative
standards included:
(1) As discussed in the Staff paper (section 5.4.5), EPA has
greater confidence in relative comparisons in risk estimates between
alternative standards than in the absolute magnitude of risk estimates
associated with any particular standard.
(2) Significant year-to-year variability in O3
concentrations combined with the use of a 3-year design value to
determine the amount of air quality adjustment to be applied to each
year analyzed, results in significant year-to-year variability in the
annual health risk estimates upon just meeting the current and
potential alternative standards.
(3) There is noticeable city-to-city variability in estimated
O3-related incidence of morbidity and mortality across the
12 urban areas analyzed for both recent years of air quality and for
air quality adjusted to simulate just meeting the current and selected
potential alternative standards. This variability is likely due to
differences in air quality distributions, differences in estimated
exposure related to many factors including varying activity patterns
and air exchange rates, differences in baseline incidence rates, and
differences in susceptible populations and age distributions across the
12 urban areas.
(4) With respect to the uncertainties about estimated policy-
relevant background (PRB) concentrations,\13\ as discussed in the Staff
Paper (section 5.4.3), alternative assumptions about background levels
had a variable impact depending on the health effect considered and the
location and standard analyzed in terms of the absolute magnitude and
relative changes in the risk estimates. There was relatively little
impact on either absolute magnitude or relative changes in lung
function risk estimates due to alternative assumptions about background
levels.\14\ With respect to O3-related non-accidental
mortality, while notable differences (i.e., greater than 50 percent)
were observed in some areas, particularly for more stringent standards,
the overall pattern of estimated reductions, expressed in terms of
percentage reduction relative to the current standard, was
significantly less impacted.
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\13\ PRB O3 concentrations used in the O3
risk assessment were defined in chapter 2 of the Staff Paper (EPA,
2007, pp. 2-48, 2-54) as the O3 concentrations that would
be observed in the U.S. in the absence of anthropogenic emissions of
precursors (e.g., VOC, NOX, and CO) in the U.S., Canada,
and Mexico. Based on runs of the GEOS-CHEM model (a global
tropospheric O3 model) applied for the 2001 warm season
(i.e., April to September), monthly background daily diurnal
profiles for each of the 12 urban areas for each month of the
O3 season were simulated using meteorology for the year
2001. Based on these model runs, the Criteria Document states that
current estimates of PRB O3 concentrations are generally
in the range of 0.015 to 0.035 ppm in the afternoon, and they are
generally lower under conditions conducive to high O3
episodes. They are highest during spring due to contributions from
hemispheric pollution and stratospheric intrusions. The Criteria
Document states that the GEOS-CHEM model applied for the 2001 warm
season reports PRB O3 concentrations for afternoon
surface air over the United States that are likely 10 ppbv too high
in the southeast in summer, and accurate within 5 ppbv in other
regions and seasons.
\14\ Sensitivity analyses examining the impact of alternative
assumptions about PRB were only conducted for lung function
decrements and non-accidental mortality.
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(5) Concerning the part of the risk assessment based on effects
reported in epidemiological studies, important uncertainties include
uncertainties (1) surrounding estimates of the O3
coefficients for concentration-response relationships used in the
assessment, (2) involving the shape of the concentration-response
relationship and whether or not a population threshold or non-linear
relationship exists within the range of concentrations examined in the
studies, (3) related to the extent to which concentration-response
relationships derived from studies in a given location and time when
O3 levels were higher or behavior and /or housing conditions
were different provide accurate representations of the relationships
for the same locations with lower air quality distributions and/or
different behavior and/or housing conditions, and (4) concerning the
possible role of co-pollutants which also may have varied between the
time of the studies and the current assessment period. An important
additional uncertainty for the mortality risk estimates is the extent
to which the associations reported between O3 and non-
accidental and cardiorespiratory mortality actually reflect causal
relationships.
As discussed in the proposal, some of these uncertainties have been
addressed quantitatively in the form of estimated confidence ranges
around central risk estimates; others are addressed through separate
sensitivity analyses (e.g., the influence of alternative estimates for
policy-relevant background levels) or are characterized qualitatively.
For both parts of the health risk assessment, statistical uncertainty
due to sampling error has been characterized and is expressed in terms
of 95 percent credible intervals. EPA recognizes that these credible
intervals do not reflect all of the uncertainties noted above.
B. Need for Revision of the Current Primary O3 Standard
1. Introduction
The initial issue to be addressed in this review of the primary
O3 standard is whether, in view of the advances in
scientific knowledge reflected in the Criteria Document and Staff
Paper, the current standard should be revised. As discussed in section
II.C of the proposal, in evaluating whether it was appropriate to
propose to retain or revise the current standard, the Administrator
built upon the last review and reflected the broader body of evidence
and information now available. In the proposal, EPA presented
information, judgments, and conclusions from the last review, which
revised the level, averaging time, and form of the standard, from the
Staff Paper's evaluation of the adequacy of the current primary
standard, including both evidence- and exposure/risk-based
considerations, as well as from the CASAC Panel's advice and
recommendations. The Staff Paper evaluation, CASAC Panel's views, and
the Administrator's proposed conclusions on the adequacy of the current
primary standard are presented below.
a. Staff Paper Evaluation
The Staff Paper considered the evidence presented in the Criteria
Document as a basis for evaluating the adequacy of the current
O3 standard, recognizing that important uncertainties
remain. The extensive body of human clinical, toxicological, and
epidemiological evidence, highlighted above in section II.A.2 and
discussed in section II.A of the proposal, serves as the basis for
judgments about O3-related health effects, including
judgments about causal relationships with a range of respiratory
morbidity effects, including lung function decrements, increased
respiratory symptoms, airway inflammation, increased airway
responsiveness, and respiratory-related hospitalizations and emergency
department visits in the warm season, and about the evidence being
highly suggestive that O3 directly or indirectly contributes
to non-accidental and cardiorespiratory-related mortality.
[[Page 16444]]
These judgments take into account important uncertainties that
remain in interpreting this evidence. For example, with regard to the
utility of time-series epidemiological studies to inform judgments
about a NAAQS for an individual pollutant, such as O3,
within a mix of highly correlated pollutants, such as the mix of
oxidants produced in photochemical reactions in the atmosphere, the
Staff Paper noted that there are limitations especially at ambient
O3 concentrations below levels at which O3-
related effects have been observed in controlled human exposure
studies. The Staff Paper also recognized that the available
epidemiological evidence neither supports nor refutes the existence of
thresholds at the population level for effects such as increased
hospital admissions and premature mortality. There are limitations in
epidemiological studies that make discerning thresholds in populations
difficult, including low data density in the lower concentration
ranges, the possible influence of exposure measurement error, and
variability in susceptibility to O3-related effects in
populations.
While noting these limitations in the interpretation of the
findings from the epidemiological studies, the Staff Paper concluded
that if a population threshold level does exist, it would likely be
well below the level of the current O3 standard and possibly
within the range of background levels. This conclusion is supported by
several epidemiological studies that have explored the question of
potential thresholds either by using a statistical curve-fitting
approach to evaluate whether linear or non-linear models fit the data
better using, or by analyzing, sub-sets of the data where days over or
under a specific cutpoint (e.g., 0.080 ppm or even lower O3
levels) were excluded and then evaluating the association for
statistical significance. In addition to consideration of the
epidemiological studies, findings from controlled human exposure
studies indicate that prolonged exposures produced statistically
significant group mean FEV1 decrements and symptoms in
healthy adult subjects at levels down to at least 0.060 ppm, with a
small percentage of subjects experiencing notable effects (e.g., >10
percent FEV1 decrement, pain on deep inspiration).
Controlled human exposure studies evaluated in the last review also
found significant responses in indicators of lung inflammation and cell
injury at 0.080 ppm in healthy adult subjects. The effects in these
controlled human exposure studies were observed in healthy young adult
subjects, and it is likely that more serious responses, and responses
at lower levels, would occur in people with asthma and other
respiratory diseases. These physiological effects can lead to
aggravation of asthma and increased susceptibility to respiratory
infection. The observations provide support for the conclusion in the
Staff Paper that the associations observed in the epidemiological
studies, particularly for respiratory-related effects such as increased
medication use, increased school and work absences, increased visits to
doctors' offices and emergency departments, and increased hospital
admissions, extend down to O3 levels well below the current
standard (i.e., 0.084 ppm) (p. 6-7).
The newly available information reinforces the judgments in the
Staff Paper from the last review about the likelihood of causal
relationships between O3 exposures and respiratory effects
and broadens the evidence of O3-related associations to
include additional respiratory-related endpoints, newly identified
cardiovascular-related health endpoints, and mortality. Newly available
evidence also led the Staff Paper to conclude that people with asthma
are likely to experience more serious effects than people who do not
have asthma. The Staff Paper also concluded that substantial progress
has been made since the last review in advancing the understanding of
potential mechanisms by which ambient O3, alone and in
combination with other pollutants, is causally linked to a range of
respiratory-related health endpoints, and may be causally linked to a
range of cardiovascular-related health endpoints. Thus, the Staff Paper
found strong support in the evidence available since the last review,
for consideration of an O3 standard that is at least as
protective as the current standard and finds no support for
consideration of an O3 standard that is less protective than
the current standard. This conclusion is consistent with the advice and
recommendations of the CASAC Panel and with the views expressed by all
interested parties who provided comments on drafts of the Staff Paper.
While the CASAC Panel and some commenters on drafts of the Staff Paper
supported revising the current standard to provide increased public
health protection and other such commenters supported retaining the
current standard, no one who provided comments on drafts of the Staff
Paper supported a standard that would be less protective than the
current standard.
i. Evidence-Based Considerations
In looking more specifically at the controlled human exposure and
epidemiological evidence, the Staff Paper first noted that controlled
human exposure studies provide the clearest and most compelling
evidence for an array of human health effects that are directly
attributable to acute exposures to O3 per se. Evidence from
such human studies, together with animal toxicological studies, help to
provide biological plausibility for health effects observed in
epidemiological studies. In considering the available evidence, the
Staff Paper focused on studies that examined health effects that have
been demonstrated to be caused by exposure to O3, or for
which the Criteria Document judges associations with O3 to
be causal or likely causal, or for which the evidence is highly
suggestive that O3 contributes to the reported effects.
In considering the epidemiological evidence as a basis for reaching
conclusions about the adequacy of the current standard, the Staff Paper
focused on studies reporting effects in the warm season, for which the
effect estimates are more consistently positive and statistically
significant than those from all-year studies. The Staff Paper
considered the extent to which such studies provide evidence of
associations that extend down to ambient O3 concentrations
below the level of the current standard, which would thereby call into
question the adequacy of the current standard. In so doing, the Staff
Paper noted that if a population threshold level does exist for an
effect observed in such studies, it would likely be at a level well
below the level of the current standard. The Staff Paper also attempted
to characterize whether the area in which a study was conducted likely
would or would not have met the current standard during the time of the
study, although it recognizes that the confidence that would
appropriately be placed on the associations observed in any given
study, or on the extent to which the association would likely extend
down to relatively low O3 concentrations, is not dependent
on this distinction. Further, the Staff Paper considered studies that
examined subsets of data that include only days with ambient
O3 concentrations below the level of the current
O3 standard, or below even lower O3
concentrations, and continue to report statistically significant
associations. The Staff Paper judged that such studies are directly
relevant to considering the adequacy of the current standard,
particularly in light of reported responses to O3 at
[[Page 16445]]
levels below the current standard found in controlled human exposure
studies.
The Staff Paper evaluation of such studies is discussed below and
in section II.C.2.a of the proposal, focusing in turn on studies of (1)
lung function, respiratory symptoms and other respiratory-related
physiological effects, (2) respiratory hospital admissions and
emergency department visits, and (3) mortality.
(1) Lung function, respiratory symptoms and other respiratory-
related physiological effects. Health effects for which the Criteria
Document continued to find clear evidence of causal associations with
short-term O3 exposures include lung function decrements,
respiratory symptoms, pulmonary inflammation, and increased airway
responsiveness. In the last review, these O3-induced effects
were demonstrated with statistical significance down to the lowest
level tested in controlled human exposure studies at that time (i.e.,
0.080 ppm). Two new studies are notable in that they are the only
controlled human exposure studies that examined respiratory effects,
including lung function decrements and respiratory symptoms, in healthy
adults at lower exposure levels than had previously been examined.
EPA's reanalysis of the data from the most recent study shows small
group mean decrements in lung function responses to be statistically
significant at the 0.060 ppm exposure level, while the author's
analysis did not yield statistically significant lung function
responses but did yield some statistically significant respiratory
symptom responses toward the end of the exposure period. These studies
report a small percentage of subjects experiencing lung function
decrements (>= 10 percent) at the 0.060 ppm exposure level. These
studies provide very limited evidence of O3-related lung
function decrements and respiratory symptoms at this lower exposure
level.
The Staff Paper noted that evidence from controlled human exposures
studies indicates that people with moderate-to-severe asthma have
somewhat larger decreases in lung function in response to O3
relative to healthy individuals. In addition, lung function responses
in people with asthma appear to be affected by baseline lung function
(i.e., magnitude of responses increases with increasing disease
severity). This newer information expands our understanding of the
physiological basis for increased sensitivity in people with asthma and
other airway diseases, recognizing that people with asthma present a
different response profile for cellular, molecular, and biochemical
responses than people who do not have asthma. New evidence indicates
that some people with asthma have increased occurrence and duration of
nonspecific airway responsiveness, which is an increased
bronchoconstrictive response to airway irritants. Controlled human
exposure studies also indicate that some people with allergic asthma
and rhinitis have increased airway responsiveness to allergens
following O3 exposure. Exposures to O3
exacerbated lung function decrements in people with pre-existing
allergic airway disease, with and without asthma. Ozone-induced
exacerbation of airway responsiveness persists longer and attenuates
more slowly than O3-induced lung function decrements and
respiratory symptom responses and can have important clinical
implications for asthmatics.
The Staff Paper also concluded that newly available human exposure
studies suggest that some people with asthma also have increased
inflammatory responses, relative to non-asthmatic subjects, and that
this inflammation may take longer to resolve. The new data on airway
responsiveness, inflammation, and various molecular markers of
inflammation and bronchoconstriction indicate that people with asthma
and allergic rhinitis (with or without asthma) comprise susceptible
groups for O3-induced adverse effects. This body of evidence
qualitatively informs the Staff Paper's evaluation of the adequacy of
the current O3 standard in that it indicates that controlled
human exposure and epidemiological panel studies of lung function
decrements and respiratory symptoms that evaluate only healthy, non-
asthmatic subjects likely underestimate the effects of O3
exposure on asthmatics and other susceptible populations.
The Staff Paper noted that in addition to the experimental evidence
of lung function decrements, respiratory symptoms, and other
respiratory effects in healthy and asthmatic populations discussed
above, epidemiological studies have reported associations of lung
function decrements and respiratory symptoms in several locations. Two
large U.S. panel studies which together followed over 1,000 asthmatic
children on a daily basis (Mortimer et al., 2002, the National
Cooperative Inner-City Asthma Study, or NCICAS; and Gent et al., 2003),
as well as several smaller U.S. and international studies, have
reported robust associations between ambient O3
concentrations and measures of lung function, daily respiratory
symptoms (e.g., chest tightness, wheeze, shortness of breath), and
increased asthma medication use in children with moderate to severe
asthma. Mortimer et al. (2002) found that of the pollutants measured
(including O3, NO2, SO2 and
PM10), O3 was the only one that had a
statistically significant effect on lung function. (Mortimer et al.
2002) also found associations between NO2, SO2
and PM10 and respiratory symptoms that were stronger than
those between O3 and respiratory symptoms. Gent et al.
(2003) found that in co-pollutant models, O3 but not PM2.5
significantly predicted increased risk of respiratory symptoms and
rescue medication use among children using asthma maintenance
medication. Overall, the multi-city NCICAS (Mortimer et al., 2002),
(Gent et al. 2003), and several other single-city studies indicate a
robust positive association between ambient O3
concentrations and increased respiratory symptoms and increased
medication use in asthmatic children.
In considering the large number of single-city epidemiological
studies reporting lung function or respiratory symptoms effects in
healthy or asthmatic populations, the Staff Paper noted that most such
studies that reported positive and often statistically significant
associations in the warm season were conducted in areas that likely
would not have met the current standard. In considering the large
multi-city NCICAS (Mortimer et al., 2002), the Staff Paper noted that
the 98th percentile 8-hour daily maximum O3 concentrations
at the monitor reporting the highest O3 concentrations in
each of the study areas ranged from 0.084 ppm to > 0.10 ppm. However,
the authors indicate that less than 5 percent of the days in the eight
urban areas had 8-hour daily O3 concentrations exceeding
0.080 ppm. Moreover, the authors observed that when days with 8-hour
average O3 levels greater than 0.080 ppm were excluded,
similar effect estimates were seen compared to estimates that included
all of the days. There are also a few other studies in which the
relevant air quality statistics provide some indication that lung
function and respiratory symptom effects may be occurring in areas that
likely would have met the current standard (EPA, 2007b, p. 6-12).
(2) Respiratory hospital admissions and emergency department
visits. At the time of the last review, many time-series studies
indicated positive associations between ambient O3 and
increased respiratory hospital admissions and emergency room visits,
providing strong evidence for a relationship between O3
exposure and increased exacerbations of
[[Page 16446]]
preexisting lung disease extending below the level of the then current
1-hour O3 standard (EPA 2007b, section 3.3.1.1.6). Analyses
of data from studies conducted in the northeastern U.S. indicated that
O3 air pollution was consistently and strongly associated
with summertime respiratory hospital admissions.
Since the last review, new epidemiological studies have evaluated
the association between short-term exposures to O3 and
unscheduled hospital admissions for respiratory causes. Large multi-
city studies, as well as many studies from individual cities, have
reported positive and often statistically significant O3
associations with total respiratory hospitalizations as well as asthma-
and chronic obstructive pulmonary disease (COPD)-related
hospitalizations, especially in studies analyzing the O3
effect during the summer or warm season. Analyses using multipollutant
regression models generally indicate that copollutants do not confound
the association between O3 and respiratory hospitalizations
and that the O3 effect estimates were robust to PM
adjustment in all-year and warm-season only data. The Criteria Document
concluded that the evidence supports a causal relationship between
acute O3 exposures and increased respiratory-related
hospitalizations during the warm season.
In looking specifically at U.S. and Canadian respiratory
hospitalization studies that reported positive and often statistically
significant associations (and that either did not use GAM or were
reanalyzed to address GAM-related problems), the Staff Paper noted that
many such studies were conducted in areas that likely would not have
met the current O3 standard, with many providing only all-
year effect estimates, and with some reporting a statistically
significant association in the warm season. Of the studies that provide
some indication that O3-related respiratory hospitalizations
may be occurring in areas that likely would have met the current
standard, the Staff Paper noted that some are all-year studies, whereas
others reported statistically significant warm-season associations.
Emergency department visits for respiratory causes have been the
focus of a number of new studies that have examined visits related to
asthma, COPD, bronchitis, pneumonia, and other upper and lower
respiratory infections, such as influenza, with asthma visits typically
dominating the daily incidence counts. Among studies with adequate
controls for seasonal patterns, many reported at least one significant
positive association involving O3. However, inconsistencies
were observed which were at least partially attributable to differences
in model specifications and analysis approach among various studies. In
general, O3 effect estimates from summer-only analyses
tended to be positive and larger compared to results from cool season
or all-year analyses. Almost all of the studies that reported
statistically significant effect estimates were conducted in areas that
likely would not have met the current standard. The Criteria Document
concluded that analyses stratified by season generally supported a
positive association between O3 concentrations and emergency
department visits for asthma in the warm season. These studies provide
evidence of effects in areas that likely would not have met the current
standard and evidence of associations that likely extend down to
relatively low ambient O3 concentrations.
(3) Mortality. The 1996 Criteria Document concluded that an
association between daily mortality and O3 concentrations
for areas with high O3 levels (e.g., Los Angeles) was
suggested. However, due to inconsistencies in the results from the very
limited number of studies available at that time, there was
insufficient evidence to determine whether the observed association was
likely causal, and thus the possibility that O3 exposure may
be associated with mortality was not relied upon in the 1997 decision
on the O3 primary standard.
Since the last review, the body of evidence with regard to
O3-related health effects has been expanded by animal,
controlled human exposure, and epidemiological studies and now
identifies biologically plausible mechanisms by which O3 may
affect the cardiovascular system. In addition, there is stronger
information linking O3 to serious morbidity outcomes, such
as hospitalization, that are associated with increased mortality. Thus,
there is now a coherent body of evidence that describes a range of
health outcomes from lung function decrements to hospitalization and
premature mortality.
Newly available large multi-city studies and related analyses (Bell
et al., 2004; Huang et al., 2005; and Schwartz, 2005) designed
specifically to examine the effect of O3 and other
pollutants on mortality have provided much more robust and credible
information. Together these studies have reported significant
associations between O3 and mortality that were robust to
adjustment for PM and different adjustment methods for temperature and
suggest that the effect of O3 on mortality may be immediate
but may also persist for several days. Further analysis of one of these
multi-city studies (Bell et al., 2006) examined the shape of the
concentration-response function for the O3-mortality
relationship in 98 U.S. urban communities for the period 1987 to 2000
specifically to evaluate whether a threshold level exists. Results from
various analytic methods all indicated that any threshold, if it
exists, would likely occur at very low concentrations, far below the
level of the current O3 NAAQS and nearing background levels.
New data are also available from several single-city studies
conducted worldwide, as well as from several meta-analyses that have
combined information from multiple studies. Three recent meta-analyses
evaluated potential sources of heterogeneity in O3-mortality
associations. All three analyses reported common findings, including
effect estimates that were statistically significant and larger in warm
season analyses. Reanalysis of results using default GAM criteria did
not change the effect estimates, and there was no strong evidence of
confounding by PM.
Overall, the Criteria Document (p. 8-78) found that the results
from U.S. multi-city time-series studies, along with the meta-analyses,
provide relatively strong evidence for associations between short-term
O3 exposure and all-cause mortality even after adjustment
for the influence of season and PM. The results of these analyses of
studies considered in this review indicate that copollutants generally
do not appear to substantially confound the association between
O3 and mortality. In addition, several single-city studies
observed positive associations of ambient O3 concentrations
with total nonaccidental and cardiorespiratory mortality.
Finally, from those studies that included assessment of
associations with specific causes of death, it appears that effect
estimates for associations with cardiovascular mortality are larger
than those for total mortality; effect estimates for respiratory
mortality are less consistent in size, possibly due to reduced
statistical power in this subcategory of mortality. For cardiovascular
mortality, the Criteria Document (p. 7-106) suggested that effect
estimates are consistently positive and more likely to be larger and
statistically significant in warm season analyses. The Criteria
Document (p. 8-78) concluded that these findings are highly suggestive
that short-term O3 exposure directly or indirectly
contributes to nonaccidental and cardiorespiratory-related mortality,
but
[[Page 16447]]
additional research is needed to more fully establish underlying
mechanisms by which such effects occur.\15\
---------------------------------------------------------------------------
\15\ In commenting on the Criteria Document, the CASAC Ozone
Panel raised questions about the implications of these time-series
results in a policy context, emphasizing that ``* * * while the
time-series study design is a powerful tool to detect very small
effects that could not be detected using other designs, it is also a
blunt tool'' (Henderson, 2006b). They note that ``* * * not only is
the interpretation of these associations complicated by the fact
that the day-to-day variation in concentrations of these pollutants
is, to a varying degree, determined by meteorology, the pollutants
are often part of a large and highly correlated mix of pollutants,
only a very few of which are measured'' (Henderson, 2006b). Even
with these uncertainties, the CASAC Ozone Panel, in its review of
the Staff Paper, found ``* * * premature total non-accidental and
cardiorespiratory mortality for inclusion in the quantitative risk
assessment to be appropriate.'' (Henderson, 2006b)
---------------------------------------------------------------------------
ii. Exposure- and Risk-Based Considerations
In evaluating the adequacy of the current standard, the Staff Paper
also considered estimated quantitative exposures and health risks, and
important uncertainties and limitations in those estimates, which are
highlighted above in section II.A.3 and discussed in section II.B of
the proposal. These estimates are derived from an EPA assessment of
exposures and health risks associated with recent air quality levels
and with air quality simulated to just meet the current standard to
help inform judgments about whether or not the current standard
provides adequate protection of public health.
The Staff Paper (and the CASAC Panel) recognized that the exposure
and risk analyses could not provide a full picture of the O3
exposures and O3-related health risks posed nationally. The
Staff Paper did not have sufficient information to evaluate all
relevant at-risk groups (e.g., outdoor workers, children under age 5)
or all O3-related health outcomes (e.g., increased
medication use, school absences, and emergency department visits that
are part of a broader pyramid of effects), and the scope of the Staff
Paper analyses was generally limited to estimating exposures and risks
in 12 urban areas across the U.S., and to only five or just one area
for some health effects included in the risk assessment. Thus, due to
the limited geographic scope of the exposure and risk assessments, EPA
recognizes that national-scale public health impacts of ambient
O3 exposures would be much larger than the quantitative
exposure and risk estimates associated with recent air quality or air
quality that just meets the current or alternative standards in the 12
urban areas analyzed. On the other hand, inter-individual variability
in responsiveness means that only a subset of individuals in each group
estimated to experience exposures at and above a given benchmark level
while at elevated exertion would actually be expected to experience
such adverse health effects.
The Staff Paper estimated exposures and risks for the three most
recent years (2002-2004) for which data were available at the time of
the analyses. As discussed above in section II.A.3.a, within this 3-
year period, 2002 was a year with relatively higher O3
levels in most, but not all, areas and simulation of just meeting the
current standard based on 2002 air quality data provides a generally
higher-end estimate of exposures and risks, while 2004 was a year with
relatively lower O3 levels in most, but not all, areas and
simulation of just meeting the current standard using 2004 air quality
data provides a generally lower-end estimate of exposures and risks.
The Staff Paper consideration of such exposure and risk analyses is
discussed below and in section II.C.2.b of the proposal, focusing on
both the exposure analyses and the human health risk assessment.
(1) Exposure analyses. EPA's exposure analysis estimated personal
exposures to ambient O3 levels at and above specific
benchmark levels while at elevated exertion to provide some perspective
on the potential public health impacts of respiratory symptoms and
respiratory-related physiological effects that cannot currently be
evaluated in quantitative risk assessments but that may occur at
current air quality levels, and the extent to which such impacts might
be reduced by meeting the current and alternative standards. As noted
above in section II.A.3, the Staff Paper referred to exposures at and
above these benchmark levels as ``exposures of concern.'' The Staff
Paper noted that potential public health impacts likely occur across a
range of O3 exposure levels, such that there is no one
exposure level that addresses all relevant public health impacts.
Therefore, with the concurrence of the CASAC Panel, the Staff Paper
estimated exposures of concern not only at 0.080 ppm O3, a
level at which there are demonstrated effects, but also at 0.070 and
0.060 ppm O3. The Staff Paper recognized that there will be
varying degrees of concern about exposures at each of these levels,
based in part on the population subgroups experiencing them. Given that
there is clear evidence of inflammation, increased airway
responsiveness, and changes in host defenses in healthy people exposed
to 0.080 ppm O3 and reason to infer that such effects will
continue at lower exposure levels, but with increasing uncertainty
about the extent to which such effects occur at lower O3
concentrations, the Staff Paper focused on exposures at or above
benchmark levels of 0.070 and 0.060 ppm O3 while at elevated
exertion for purposes of evaluating the adequacy of the current
standard.
Exposure estimates were presented in the Staff Paper and in section
II.B (Table 1) of the proposal for the number and percent of all school
age children and asthmatic school age children exposed, and the number
of person-days (occurrences) of exposures, with daily 8-hour maximum
exposures at or above several benchmark levels while at intermittent
moderate or greater exertion. The percent of population exposed at any
given level is very similar for all and asthmatic school age children.
Substantial year-to-year variability in exposure estimates is observed,
ranging to over an order of magnitude at the current standard level, in
estimates of the number of children and the number of occurrences of
exposures at both of these benchmark levels while at elevated exertion.
The Staff Paper stated that it is appropriate to consider not just the
average estimates across all years, but also to consider public health
impacts in years with relatively higher O3 levels. The Staff
Paper also noted that there is substantial city-to-city variability in
these estimates, and notes that it is appropriate to consider not just
the aggregate estimates across all cities, but also to consider the
public health impacts in cities where these estimates are higher than
the average upon meeting the current standard.
About 50 percent of asthmatic of all school age children,
representing nearly 1.3 million asthmatic children and about 8.5
million school age children in the 12 urban areas examined, are
estimated to experience exposures at or above the 0.070 ppm benchmark
level while at elevated exertion (i.e., these individuals are estimated
to experience 8-hour O3 exposures at or above 0.070 ppm
while engaged in moderate or greater exertion 1 or more times during
the O3 season) associated with 2002 O3 air
quality levels. In contrast, about 17 percent of asthmatic and all
school age children are estimated to experience exposures at or above
the 0.070 ppm benchmark level while at elevated exertion associated
with 2004 O3 air quality levels. Just meeting the current
standard results in an aggregate estimate of about 20 percent of
asthmatic or 18 percent of all school age children likely to experience
exposures at or above the
[[Page 16448]]
0.070 ppm benchmark level while at elevated exertion using the 2002
simulation. The exposure estimates for this benchmark level range up to
about 40 percent of asthmatic or all school age children in the single
city with the highest estimate among the cities analyzed. Just meeting
the current standard based on the 2004 simulation, results in an
aggregate estimate of about 1 percent of asthmatic or all school age
children experiencing exposures exceeding the 0.070 ppm benchmark level
while at elevated exertion.
At the benchmark level of 0.060 ppm, about 70 percent of all or
asthmatic school age children are estimated to experience exposures at
or above this benchmark level while at elevated exertion for the
aggregate of the 12 urban areas associated with 2002 O3
levels. Just meeting the current standard would result in an aggregate
estimate of about 45 percent of asthmatic or all school age children
likely to experience exposures at or above the 0.060 ppm benchmark
level while at elevated exertion using the 2002 simulation. The
exposure estimates for this benchmark level range up to nearly 70
percent of all or asthmatic school age children in the single city with
the highest estimate among the cities analyzed associated with just
meeting the current standard using the 2002 simulation. The Staff Paper
indicated an aggregate estimate of about 10 percent of asthmatic or all
school age children would experience exposures at or above the 0.060
ppm benchmark level while at elevated exertion associated with just
meeting the current standard using the 2004 simulation.
(2) Risk assessment. The health risk assessment estimated risks for
several important health endpoints, including: (1) Lung function
decrements (i.e., >= 15 percent and >= 20 percent reductions in
FEV1) in all school age children for 12 urban areas; (2)
lung function decrements (i.e., >= 10 percent and >= 20 percent
reductions in FEV1) in asthmatic school age children for 5
urban areas (a subset of the 12 urban areas); (3) respiratory symptoms
(i.e., chest tightness, shortness of breath, wheeze) in moderate to
severe asthmatic children for the Boston area; (4) respiratory-related
hospital admissions for 3 urban areas; and (5) nonaccidental and
cardiorespiratory mortality for 12 urban areas for three recent years
(2002 to 2004) and for just meeting the current standard using a 2002
simulation and a 2004 simulation.
With regard to estimates of moderate lung function decrements,
meeting the current standard substantially reduces the estimated number
of school age children experiencing one or more occurrences of
FEV1 decrements >= 15 percent for the 12 urban areas, going
from about 1.3 million children (7 percent of children) under 2002 air
quality to about 610,000 (3 percent of children) based on the 2002
simulation, and from about 620,000 children (3 percent of children) to
about 230,000 (1 percent of children) using the 2004 simulation. In
asthmatic children, the estimated number of children experiencing one
or more occurrences of FEV1 decrements >= 10 percent for the
5 urban areas goes from about 250,000 children (16 percent of asthmatic
children) under 2002 air quality to about 130,000 (8 percent of
asthmatic children) using the 2002 simulation, and from about 160,000
(10 percent of asthmatic children) to about 70,000 (4 percent of
asthmatic children) using the 2004 simulation. Thus, even when the
current standard is met, about 4 to 8 percent of asthmatic school age
children are estimated to experience one or more occurrences of
moderate lung function decrements, resulting in about 1 million
occurrences (using the 2002 simulation) and nearly 700,000 occurrences
(using the 2004 simulation) in just 5 urban areas. Moreover, the
estimated number of occurrences of moderate or greater lung function
decrements per child is on average approximately 6 to 7 in all children
and 8 to 10 in asthmatic children in an O3 season, even when
the current standard is met, depending on the year used to simulate
meeting the current standard. In the 1997 review of the O3
standard a general consensus view of the adversity of such moderate
responses emerged as the frequency of occurrences increases, with the
judgment that repeated occurrences of moderate responses, even in
otherwise healthy individuals, may be considered adverse since they may
well set the stage for more serious illness.
With regard to estimates of large lung function decrements, the
Staff Paper noted that FEV1 decrements > 20 percent would
likely interfere with normal activities in many healthy individuals,
therefore single occurrences would be considered to be adverse. In
people with asthma, large lung function responses would likely
interfere with normal activities for most individuals and would also
increase the likelihood that these individuals would use additional
medication or seek medical treatment. Single occurrences would be
considered to be adverse to asthmatic individuals under the ATS
definition. They also would be cause for medical concern in some
individuals. While the current standard reduces the occurrences of
large lung function decrements in all children and asthmatic children
from about 60 to 70%, in a year with relatively higher O3
levels (2002), there are estimated to be about 500,000 occurrences in
all school children across the entire 12 urban areas, and about 40,000
occurrences in asthmatic children across just 5 urban areas. As noted
above, it is clear that even when the current standard is met over a
three-year period, O3 levels in each year can vary
considerably, as evidenced by relatively large differences between risk
estimates based on 2002 to 2004 air quality. The Staff Paper expressed
the view that it was appropriate to consider this yearly variation in
O3 levels allowed by the current standard in judging the
extent to which impacts on members of at-risk groups in a year with
relatively higher O3 levels remain of concern from a public
health perspective.
With regard to other O3-related health effects, the
estimated risks of respiratory symptom days in moderate to severe
asthmatic children, respiratory-related hospital admissions, and non-
accidental and cardiorespiratory mortality, respectively, are not
reduced to as great an extent by meeting the current standard as are
lung function decrements. For example, just meeting the current
standard reduces the estimated average incidence of chest tightness in
moderate to severe asthmatic children living in the Boston urban area
by 11 to 15%, based on 2002 and 2004 simulations, respectively,
resulting in an estimated incidence of about 23,000 to 31,000 per
100,000 children attributable to O3 exposure (Table 6-4).
Just meeting the current standard is estimated to reduce the incidence
of respiratory-related hospital admissions in the New York City urban
area by about 16 to 18%, based on 2002 and 2004 simulations,
respectively, resulting in an estimated incidence per 100,000
population of 4.6 to 6.4, respectively. Across the 12 urban areas, the
estimates of non-accidental mortality incidence per 100,000 relevant
population range from 0.4 to 2.6 (for 2002) and 0.5 to 1.5 (for 2004).
Meeting the current standard results in a reduction of the estimated
incidence per 100,000 population to a range of 0.3 to 2.4 based on the
2002 simulation and a range of 0.3 to 1.2 based on the 2004 simulation.
Estimates for cardiorespiratory mortality show similar patterns.
In considering the estimates of the proportion of population
affected and the number of occurrences of the health effects that are
included in the risk assessment, the Staff Paper noted that
[[Page 16449]]
these limited estimates are indicative of a much broader array of
potential O3-related health endpoints that we consider part
of a ``pyramid of effects'' that include various indicators of
morbidity that could not be included in the risk assessment (e.g.,
school absences, increased medication use, emergency department visits)
and which primarily affect members of at-risk groups. While the Staff
Paper had sufficient information to estimate and consider the number of
symptom days in children with moderate to severe asthma, it recognized
that there are many other effects that may be associated with symptom
days, such as increased medication use, school and work absences, or
visits to doctors' offices, for which there was not sufficient
information to estimate risks but which are important to consider in
assessing the adequacy of the current standard. The same is true for
more serious, but less frequent effects. The Staff Paper estimated
hospital admissions, but there was not sufficient information to
estimate emergency department visits in a quantitative risk assessment.
Consideration of such unquantified risks in the Staff Paper reinforced
the Staff Paper conclusion that consideration should be given to
revising the standard so as to provide increased public health
protection, especially for at-risk groups such as people with asthma or
other lung diseases, as well as children and older adults, particularly
those active outdoors, and outdoor workers.
iii. Summary of Staff Paper Considerations
The Staff Paper concluded that the overall body of evidence clearly
calls into question the adequacy of the current standard in protecting
at-risk groups against an array of adverse health effects that range
from decreased lung function and respiratory symptoms to serious
indicators of respiratory morbidity including emergency department
visits and hospital admissions for respiratory causes, nonaccidental
mortality, and possibly cardiovascular effects. These at-risk groups
notably include asthmatic children and other people with lung disease,
as well as all children and older adults, especially those active
outdoors, and outdoor workers.\16\ The available information provides
strong support for consideration of an O3 standard that
would provide increased health protection for these at-risk groups. The
Staff Paper also concluded that risks projected to remain upon meeting
the current standard are indicative of risks to at-risk groups that can
be judged to be important from a public health perspective. This
information reinforced the Staff Paper conclusion that consideration
should be given to revising the level of the standard so as to provide
increased public health protection.
---------------------------------------------------------------------------
\16\ In defining at-risk groups this way we are including both
groups with greater inherent sensitivity and those more likely to be
exposed.
---------------------------------------------------------------------------
b. CASAC Views
The CASAC Panel unanimously concluded in a letter to the
Administrator that there is ``no scientific justification for
retaining'' the current primary O3 standard, and the current
standard ``needs to be substantially reduced to protect human health,
particularly in sensitive subpopulations'' (Henderson, 2006c, pp. 1-2).
In its rationale for this conclusion, the CASAC Panel concluded that
``new evidence supports and builds-upon key, health-related conclusions
drawn in the 1997 O3 NAAQS review'' (id., p. 3). The Panel
noted that several new single-city studies and large multi-city studies
have provided more evidence for adverse health effects at
concentrations lower than the current standard, and that these
epidemiological studies are backed-up by evidence from controlled human
exposure studies. The Panel specifically noted evidence from the recent
Adams (2006) study that reported statistically significant decrements
in the lung function of healthy, moderately exercising adults at a
0.080 ppm exposure level, and importantly, also reported adverse lung
function effects in some healthy individuals at 0.060 ppm. The CASAC
Panel concluded that these results indicate that the current standard
``is not sufficiently health-protective with an adequate margin of
safety,'' noting that while similar studies in sensitive groups such as
asthmatics have yet to be conducted, ``people with asthma, and
particularly children, have been found to be more sensitive and to
experience larger decrements in lung function in response to
O3 exposures than would healthy volunteers (Mortimer et al.,
2002)'' (Henderson, 2006c, p. 4).
The CASAC Panel also highlighted a number of O3-related
adverse health effects that are associated with exposure to ambient
O3, below the level of the current standard based on a broad
range of epidemiological studies (Henderson, 2006c). These adverse
health effects include increases in school absenteeism, respiratory
hospital emergency department visits among asthmatics and patients with
other respiratory diseases, hospitalizations for respiratory illnesses,
symptoms associated with adverse health effects (including chest
tightness and medication usage), and premature mortality
(nonaccidental, cardiorespiratory deaths) reported at exposure levels
well below the current standard. ``The CASAC considers each of these
findings to be an important indicator of adverse health effects''
(Henderson, 2006c).
The CASAC Panel expressed the view that more emphasis should be
placed on the subjects in controlled human exposure studies with FEV1
decrements greater than 10 percent, which can be clinically
significant, rather than on the relatively small average decrements.
The Panel also emphasized significant O3-related
inflammatory responses and markers of injury to the epithelial lining
of the lung that are independent of spirometric responses. Further, the
Panel expressed the view that the Staff Paper did not place enough
emphasis on serious morbidity (e.g., hospital admissions) and mortality
observed in epidemiological studies. On the basis of the large amount
of recent data evaluating adverse health effects at levels at and below
the current O3 standard, it was the unanimous opinion of the
CASAC Panel that the current primary O3 standard is not
adequate to protect human health, that the relevant scientific data do
not support consideration of retaining the current standard, and that
the current standard needs to be substantially reduced to be protective
of human health, particularly in sensitive subpopulations (Henderson,
2006c, pp. 4-5).
Further, the CASAC letter noted that ``there is no longer
significant scientific uncertainty regarding the CASAC's conclusion
that the current 8-hour primary NAAQS must be lowered'' (Henderson,
2006c, p. 5). The Panel noted that a ``large body of data clearly
demonstrates adverse human health effects at the current level'' of the
standard, such that ``[R]etaining this standard would continue to put
large numbers of individuals at risk for respiratory effects and/or
significant impact on quality of life including asthma exacerbations,
emergency room visits, hospital admissions and mortality'' (Henderson,
2006c).
c. Administrator's Proposed Conclusions
At the time of proposal, in considering whether the current primary
standard should be revised, the Administrator carefully considered the
conclusions contained in the Criteria Document, the rationale and
recommendations contained in the Staff Paper, the advice and
recommendations
[[Page 16450]]
from CASAC, and public comments to date on this issue. In so doing, the
Administrator noted the following: (1) That evidence of a range of
respiratory-related morbidity effects seen in the last review has been
considerably strengthened, both through toxicological and controlled
human exposure studies as well as through many new panel and
epidemiological studies; (2) that new evidence from controlled human
exposure and epidemiological studies identifies people with asthma
(including children with asthma) as an important susceptible population
for which estimates of respiratory effects in the general population
likely underestimate the magnitude or importance of these effects; (3)
that new evidence about mechanisms of toxicity further contributes to
the biological plausibility of O3-induced respiratory
effects and is beginning to suggest mechanisms that may link
O3 exposure to cardiovascular effects; (4) that there is now
relatively strong evidence for associations between O3 and
total nonaccidental and cardiopulmonary mortality, even after
adjustment for the influence of season and PM; and (5) the limits of
the available evidence. Relative to the information that was available
to inform the Agency's 1997 decision to set the current standard, the
newly available evidence increased the Administrator's confidence that
respiratory morbidity effects such as lung function decrements and
respiratory symptoms are causally related to O3 exposures,
that indicators of respiratory morbidity such as emergency department
visits and hospital admissions are causally related to O3
exposures, and that the evidence is highly suggestive that
O3 exposures during the O3 season contribute to
premature mortality.
The Administrator judged that there is important new evidence
demonstrating that exposures to O3 at levels below the level
of the current standard are associated with a broad array of adverse
health effects, especially in at-risk populations that include people
with asthma or other lung diseases who are likely to experience more
serious effects from exposure to O3, children and older
adults with increased susceptibility, as well as those who are likely
to be vulnerable as a result of spending a lot of time outdoors engaged
in physical activity, especially active children and outdoor workers.
Examples of this important new evidence include demonstration of
O3-induced lung function effects and respiratory symptoms in
some healthy individuals down to the previously observed exposure level
of 0.080 ppm, as well as very limited new evidence at exposure levels
well below the level of the current standard. In addition, there is now
epidemiological evidence of statistically significant O3-
related associations with lung function and respiratory symptom
effects, respiratory-related emergency department visits and hospital
admissions, and increased mortality, in areas that likely would have
met the current standard. There are also many epidemiological studies
done in areas that likely would not have met the current standard but
which nonetheless report statistically significant associations that
generally extend down to ambient O3 concentrations that are
below the level of the current standard. Further, there are a few
studies that have examined subsets of data that include only days with
ambient O3 concentrations below the level of the current
standard, or below even much lower O3 concentrations, and
continue to report statistically significant associations with
respiratory morbidity outcomes and mortality. The Administrator
recognized that the evidence from controlled human exposure studies,
together with animal toxicological studies, provides considerable
support for the biological plausibility of the respiratory morbidity
associations observed in the epidemiological studies and for concluding
that the associations extend below the level of the current standard.
However, the Administrator recognized that in the body of
epidemiological evidence, many studies reported positive and
statistically significant associations, while others reported positive
results that were not statistically significant, and a few did not
report any positive O3-related associations. In addition,
the Administrator judged that evidence of a causal relationship between
adverse health outcomes and O3 exposures became increasingly
uncertain at lower levels of exposure.
Based on the strength of the currently available evidence of
adverse health effects, and on the extent to which the evidence
indicates that such effects likely result from exposures to ambient
O3 concentrations below the level of the current standard,
the Administrator judged that the current standard does not protect
public health with an adequate margin of safety and that the standard
should be revised to provide such protection, especially for at-risk
groups, against a broad array of adverse health effects.
In reaching this judgment, the Administrator had also considered
the results of both the exposure and risk assessments conducted for
this review, to provide some perspective on the extent to which at-risk
groups would likely experience ``exposures of concern'' \17\ and on the
potential magnitude of the risk of experiencing various adverse health
effects when recent air quality data (from 2002 to 2004) are used to
simulate meeting the current standard and alternative standards in a
number of urban areas in the U.S.\18\ In considering the results of the
health risk assessment, as discussed in the proposal notice (section
II.C.2), the Administrator noted that there were important
uncertainties and assumptions inherent in the risk assessment and that
this assessment was most appropriately used to simulate trends and
patterns that could be expected, as well as providing informed, but
still imprecise, estimates of the potential magnitude of risks.
---------------------------------------------------------------------------
\17\ As discussed in section II.A.3 above, ``exposures of
concern'' are estimates of personal exposures while at moderate or
greater exertion to 8-hour average ambient O3 levels at
and above specific benchmark levels which represent exposure levels
at which O3-related health effects are known or can with
varying degrees of certainty be inferred to occur in some
individuals. Estimates of exposures of concern provide some
perspective on the public health impacts of health effects that may
occur in some individuals at recent air quality levels but cannot be
evaluated in quantitative risk assessments, and the extent to which
such impacts might be reduced by meeting the current and alternative
standards.
\18\ As noted above in section II.A.3, recent O3 air
quality distributions have been statistically adjusted to simulate
just meeting the current and selected alternative standards. These
simulations do not represent predictions of when, whether, or how
areas might meet the specified standards.
---------------------------------------------------------------------------
In considering the exposure assessment results at the time of
proposal, the Administrator considered analyses that define ``exposures
of concern'' by three benchmark exposure levels: 0.080, 0.070, and
0.060 ppm. Estimates of exposures in at-risk groups at and above these
benchmark levels while at elevated exertion, using O3 air
quality data in 2002 and 2004, provide some indication of the potential
magnitude of the incidence of health outcomes that cannot currently be
evaluated in a quantitative risk assessment, such as increased airway
responsiveness, increased pulmonary inflammation, increased cellular
permeability, and decreased pulmonary defense mechanisms. These
respiratory-related physiological effects have been demonstrated to
occur in healthy people at O3 exposures as low as 0.080 ppm,
the lowest level tested for these effects. These physiological effects
provide plausible mechanisms underlying observed associations with
aggravation of asthma, increased medication use, increased school and
work absences,
[[Page 16451]]
increased susceptibility to respiratory infection, increased visits to
doctors' offices and emergency departments, and increased admissions to
hospitals. In addition, these physiological effects, if repeated over
time, have the potential to lead to chronic effects such as chronic
bronchitis or long-term damage to the lungs that can lead to reduced
quality of life.
In considering these various benchmark levels for exposures of
concern at the time of proposal, the Administrator focused primarily on
estimated exposures at and above the 0.070 ppm benchmark level while at
elevated exertion as an important surrogate measure for potentially
more serious health effects in at-risk groups such as people with
asthma. This judgment was based on the strong evidence of effects in
healthy people at the 0.080 ppm exposure level and the new evidence
that people with asthma are likely to experience larger and more
serious effects than healthy people at the same level of exposure. In
the Administrator's view at the time of proposal, this evidence did not
support a focus on exposures at and above the benchmark level of 0.080
ppm O3, as it would not adequately account for the increased
risk of harm from exposure for members of at-risk groups, especially
people with asthma. The Administrator also judged that the evidence of
demonstrated effects is too limited to support a primary focus on
exposures down to the lowest benchmark level considered of 0.060 ppm.
The Administrator particularly noted that although the analysis of
``exposures of concern'' was conducted to estimate exposures at and
above three discrete benchmark levels (0.080, 0.070, and 0.060 ppm)
while at elevated exertion, the concept is appropriately viewed as a
continuum. In so doing, the Administrator sought to balance concern
about the potential for health effects and their severity with the
increasing uncertainty associated with our understanding of the
likelihood of such effects at lower O3 exposure levels.
The Administrator observed that based on the aggregate exposure
estimates for the 2002 simulation (summarized in section II.B.1, Table
1, of the proposal) for the 12 U.S. urban areas included in the
exposure analysis, upon just meeting the current standard up to about
20 percent of asthmatic or all school age children are likely to
experience one or more exposures at and above the 0.070 ppm benchmark
level while at elevated exertion; the 2004 simulation yielded an
estimate of about 1 percent of such children. The Administrator noted
from this comparison that there is substantial year-to-year
variability, ranging up to an order of magnitude or more in estimates
of the number of people and the number of occurrences of exposures at
and above this benchmark level while at elevated exertion. Moreover,
within any given year, the exposure assessment indicates that there is
substantial city-to-city variability in the estimates of the children
exposed or the number of occurrences of exposure at and above this
benchmark level while at elevated exertion. For example, city-specific
estimates of the percent of asthmatic or all school age children likely
to experience exposures at and above the benchmark level of 0.070 ppm
while at elevated exertion ranges from about 1 percent up to about 40
percent across the 12 urban areas upon just meeting the current
standard based on the 2002 simulation; the 2004 simulation yielded
estimates that range from about 0 up to about 7 percent. The
Administrator judged that it was important to recognize the substantial
year-to-year and city-to-city variability in considering these
estimates.
With regard to the results of the risk assessment, the
Administrator focused on the risks estimated to remain upon just
meeting the current standard. Based on the aggregate risk estimates
(summarized in section II.B.2, Table 2, of the proposal), the
Administrator observed that upon just meeting the current standard
based on the 2002 simulation, approximately 8 percent of asthmatic
school age children across 5 urban areas (ranging up to about 11
percent in the city with the highest estimate among the cities
analyzed) would still be estimated to experience moderate or greater
lung function decrements one or more times within an O3
season. These estimated percentages would be approximately 3 percent of
all school age children across 12 urban areas (ranging up to over 5
percent in the city with the highest estimate among the cities
analyzed). The Administrator recognized that, as with the estimates of
exposures of concern, there is substantial year-to-year and city-to-
city variability in these risk estimates.
In addition to the percentage of asthmatic or all children
estimated to experience one or more occurrences of an effect, the
Administrator recognized that some individuals are estimated to have
multiple occurrences. For example, across all the cities in the
assessment, approximately 6 to 7 occurrences of moderate or greater
lung function decrements per child are estimated to occur in all
children and approximately 8 to 10 occurrences are estimated to occur
in asthmatic children in an O3 season, even upon just
meeting the current standard. In the last review, a general consensus
view of the adversity of such responses emerged as the frequency of
occurrences increases, with the judgment that repeated occurrences of
moderate responses, even in otherwise healthy individuals, may be
considered adverse since they may well set the stage for more serious
illness. The Administrator continued to support this view.
Large lung function decrements (i.e., >= 20 percent FEV1
decrement) would likely interfere with normal activities in many
healthy individuals, therefore single occurrences would be considered
to be adverse. In people with asthma, large lung function responses
(i.e., >= 20 percent FEV1 decrement), would likely interfere
with normal activities for most individuals and would also increase the
likelihood that these individuals would use additional medication or
seek medical treatment. Not only would single occurrences be considered
to be adverse to asthmatic individuals under the ATS definition, but
they also would be cause for medical concern for some individuals. Upon
just meeting the current standard based on the 2002 simulation, close
to 1 percent of asthmatic and all school age children are estimated to
experience one or more occurrences of large lung function decrements in
the aggregate across 5 and 12 urban areas, respectively, with close to
2 percent of both asthmatic and all school age children estimated to
experience such effects in the city that receives relatively less
protection from this standard. These estimates translate into
approximately 500,000 occurrences of large lung function decrements in
all children across 12 urban areas, and about 40,000 occurrences in
asthmatic children across 5 urban areas upon just meeting the current
standard based on the 2002 simulation; the 2004 simulation yielded
estimates that translate into approximately 160,000 and 10,000 such
occurrences in all children and asthmatic children, respectively.
Upon just meeting the current standard based on the 2002
simulation, the estimate of the O3-related risk of
respiratory symptom days in moderate to severe asthmatic children in
the Boston area is about 8,000 symptom days; the 2004 simulation
yielded an estimate of about 6,000 such symptoms days. These estimates
translate into as many as one symptom day in six, and one symptom day
in eight, respectively, that are attributable to O3 exposure
during the O3 season of the total number of symptom days
associated with all
[[Page 16452]]
causes of respiratory symptoms in asthmatic children during those
years.
The estimated O3-related risk of respiratory-related
hospital admissions upon just meeting the current standard based on the
2002 simulation is greater than 500 hospital admissions in the New York
City area alone, or about 1.5 percent of the total incidence of
respiratory-related admissions associated with all causes; the 2004
simulation yielded an estimate of approximately 400 such hospital
admissions. For nonaccidental mortality, just meeting the current
standard based on the 2002 simulation results in an estimated incidence
of from 0.3 to 2.4 per 100,000 population; the 2004 simulation resulted
in an estimated incidence of from 0.3 to 1.2 per 100,000 population.
Estimates for cardiorespiratory mortality show similar patterns (Abt
Associates, 2007a, Table 4-26).
The Administrator recognized that in considering the estimates of
the proportion of population affected and the number of occurrences of
those specific health effects that are included in the risk assessment,
these limited estimates based on 2002 and 2004 simulations are
indicative of a much broader array of O3-related health
endpoints that are part of a ``pyramid of effects'' (discussed in
section II.A.4.d of the proposal) that include various indicators of
morbidity that could not be included in the risk assessment (e.g.,
school absences, increased medication use, emergency department visits)
and which primarily affect members of at-risk groups. Moreover, the
Administrator noted that the CASAC Panel supported a qualitative
consideration of the much broader array of O3-related health
endpoints, and specifically referred to respiratory emergency
department visits in asthmatics and people with other lung diseases,
increased medication use, and increased respiratory symptoms reported
at exposure levels well below the current standard.
The Administrator expressed the view in the proposal that the
exposure and risk estimates discussed in the Staff Paper and summarized
above are important from a public health perspective and indicative of
potential exposures and risks to at-risk groups. In reaching this
proposed judgment, the Administrator considered the following factors:
(1) The estimates of numbers of persons exposed at and above the 0.070
ppm benchmark level; (2) the risk estimates of the proportion of the
population and number of occurrences of various health effects in areas
upon just meeting the current standard; (3) the year-to-year and city-
to-city variability in both the exposure and risk estimates; (4) the
uncertainties in these estimates; and (5) recognition that there is a
broader array of O3-related adverse health outcomes for
which risk estimates could not be quantified (that are part of a
broader ``pyramid of effects'') and that the scope of the assessment
was limited to just a sample of urban areas and to some but not all at-
risk populations, leading to an incomplete estimation of public health
impacts associated with O3 exposures across the country. The
Administrator also noted that it was the unanimous conclusion of the
CASAC Panel that there is no scientific justification for retaining the
current primary O3 standard, that the current standard is
not sufficiently health-protective with an adequate margin of safety,
and that the standard needs to be substantially reduced to protect
human health, particularly in at-risk subpopulations.
Based on all of these considerations, the Administrator proposed
that the current O3 standard is not requisite to protect
public health with an adequate margin of safety because it does not
provide sufficient protection and that revision would result in
increased public health protection, especially for members of at-risk
groups.
2. Comments on the Need for Revision
The above section outlines the health effects evidence and
assessments used by the Administrator to inform his proposed judgments
about the adequacy of the current O3 primary standard.
General comments received on the proposal that either supported or
opposed the proposed decision to revise the current O3
primary standard are addressed in this section. Comments on the health
effects evidence, which includes evidence from controlled human
exposure and epidemiological studies, are considered in section
II.B.2.a below. Comments on human exposure and health risk assessments
are considered in section II.B.2.b, and comments on other policy-
related issues are considered in section II.B.2.c, below. Comments on
specific issues, health effects evidence, or the human exposure and
health risk assessments that relate to consideration of the appropriate
averaging time, form, or level of the O3 standard are
addressed below in sections II.C.3 and II.C.4. General comments based
on implementation-related factors that are not a permissible basis for
considering the need to revise the current standard are noted in the
Response to Comments document.
a. Consideration of Health Effects Evidence
With regard to the need to revise the current primary O3
standard, sharply divergent comments were received from two general
sets of commenters. Many public comments received on the proposal
asserted that the current O3 standard is insufficient to
protect public health, especially the health of sensitive groups, with
an adequate margin of safety and revisions to the standard are
appropriate. Among those calling for revisions to the current primary
standard were medical groups, including for example, the American
Medical Association (AMA), the American Thoracic Society (ATS), the
American Academy of Pediatrics (AAP), and the American College of Chest
Physicians (ACCP), as well as medical doctors and academic researchers.
For example, the ATS stated:
We believe that the Administrator has correctly stated that,
beyond any degree of scientific uncertainty, convincing and
compelling evidence has demonstrated that exposure to ozone at
levels below the current standard is responsible for measurable and
significant adverse health effects, both in terms of morbidity and
mortality. * * * The known respiratory, cardiac and perinatal
effects of ozone pollution are each in their own right major public
health issues. In combination they provide immediate, actionable
information and require a meaningful public health policy response
from the EPA. [ATS et al. pp. 1, 11]
Similar conclusions were also reached in comments by many national,
State, and local public health organizations, including, for example,
the American Lung Association (ALA) in a joint set of comments with
several environmental groups, the American Heart Association (AHA), the
American Nurses Association (ANA), the American Public Health
Association (APHA), and the National Association of County and City
Health Officials (NACCHO), as well as in letters to the Administrator
from EPA's advisory panel on children's environmental health
(Children's Health Protection Advisory Committee; Marty et al., 2007a,
2007b). Environmental groups also commented in support of revising the
standard, including the Sierra Club, Environmental Defense, the Natural
Resources Defense Council (NRDC), Earthjustice, and the U.S. Public
Interest Research Group (US PIRG). All of these medical, environmental
and public health commenters stated that the current O3
standard needs to be revised and that an even more protective standard
than proposed by EPA is needed to protect the health of sensitive
population
[[Page 16453]]
groups. Many individual commenters also expressed such views.
The majority of State and local air pollution control authorities
who commented on the O3 standard supported revision of the
current O3 standard, as did the National Tribal Air
Association (NTAA). Environmental agencies that supported revising the
standard include agencies from: Arkansas; California; Delaware; Iowa;
Illinois; Michigan; North Carolina; New Mexico; New York; Oklahoma;
Oregon; Pennsylvania; Utah; Wisconsin; and Washington, DC. State
organizations, including the National Association of Clean Air Agencies
(NACAA), Northeast States for Coordinated Air Use Management (NESCAUM),
and the Ozone Transport Commission (OTC) urged that EPA revise the
O3 standard. All of these commenters supported revisions to
the current standard, with most supporting a standard consistent with
CASAC's recommendations.
In general, the commenters noted above primarily based their views
on the body of evidence assessed in the Criteria Document, finding it
to be stronger and more compelling than in the last review. Some
specifically agreed with the weight of evidence approach taken by the
Criteria Document. These commenters generally placed much weight on
CASAC's interpretation of the body of available evidence and the
results of EPA's exposure and risk assessments, both of which formed
the basis for CASAC's recommendation to revise the O3
standard to provide increased public health protection.
In recent years, a broad scientific consensus has emerged that
EPA's current air quality standards for ozone are not sufficient to
protect public health, and that the levels and form must be greatly
tightened. This consensus is evidenced by the by the strong
unanimous comments of the CASAC, which was backed by the endorsement
of over 100 leading independent air quality scientists, EPA's
Children's Health Protection Advisory Committee, and many others. In
the face of this strong consensus, it is untenable to cite
``uncertainty'' as a rationale for failing to propose tighter
standards. [ALA et al., p. 15]
Medical and public health commenters also expressed the view that EPA
must not use uncertainty in the scientific evidence as justification
for retaining the current O3 standard.
EPA generally agrees with these commenters' conclusion regarding
the need to revise the current primary O3 standard. The
scientific evidence-related health effects to O3 exposure
noted by these commenters was generally the same as that assessed in
the Criteria Document and the proposal. EPA agrees that this
information provides a basis for concluding that the current
O3 standard is not adequately protective of public health.
For reasons discussed below in sections II.C.3 and II.C.4, however, EPA
disagrees with aspects of these commenters' views on the level of
protection that is appropriate and supported by the available
scientific information.
Another group of commenters representing industry associations and
businesses opposed revising the current primary O3 standard.
These views were extensively presented in comments from the Utility Air
Regulatory Group (UARG), representing a group of electric generating
companies and organizations and several national trade associations,
and in comments from other industry and business associations
including, for example: Exxon Mobil Corporation; the Alliance of
Automobile Manufacturers (AAM); the National Association of
Manufacturers (NAM), the American Petroleum Institute (API). The API
sponsored a workshop at the University of Rochester in June 2007 to
review the scientific information and health risk assessment considered
by EPA during the review of the O3 NAAQS. Although the
report (hereafter, ``Rochester Report'') from this workshop does not
offer judgments on the specific elements of the current or proposed
standard, it has been cited in a number of public comments that opposed
revision of the current 8-hour standard. The Annapolis Center for
Science-Based Public Policy issued a report (hereafter, ``Annapolis
Center'') on the science and health effects of O3, which
explicitly opposed revising the current O3 primary standard.
Several State environmental agencies also opposed revising the current
O3 primary standard, including agencies from: Georgia;
Indiana; Kentucky; Louisiana; Nevada; and Texas.
As discussed more fully below in sections dealing with specific
comments, these and other commenters in this group generally mentioned
many of the same studies from the body of evidence in the Criteria
Document that were cited by the commenters who supported revising the
standards, but highlighted different aspects of these studies in
reaching substantially different conclusions about their strength and
the extent to which progress has been made in reducing uncertainties in
the evidence since the last review. They then considered whether the
evidence that has become available since the last review has
established a more certain risk or a risk of effects that is
significantly different in character from those that provided a basis
for the current standards, or whether the evidence demonstrates that
the risk to public health upon attainment of the current standards
would be greater than was understood when EPA established the current
O3 standard in 1997. These commenters generally expressed
the view that the current standard provides the requisite degree of
public health protection.
In supporting their view that the present primary O3
standard continues to provide the requisite public health protection
and should not be revised, UARG and others generally stated: That the
effects of concern have not changed significantly since 1997; that the
uncertainties in the underlying health science are as great or greater
than in 1997; that the estimated number of exposures of concern and
health risks upon attainment of the current O3 standard has
not changed or decreased since 1997; and that ``new'' studies not
included in the Criteria Document continue to demonstrate uncertainties
about possible health risks associated with exposure to O3
at levels below the current standard. As noted above, EPA disagrees
with this general assessment, and agrees with the general position that
the available information provides a basis for concluding that the
current O3 standard is not adequately protective of public
health. The rationale for this position is discussed more fully in the
responses to specific comments that are presented below.
More specific comments on the evidence and EPA's responses are
discussed below. Section II.B.2.a.i contains comments on evidence from
controlled human exposure studies; section II.B.2.a.ii contains
comments on evidence from epidemiological studies, including
interpretation of the evidence and specific methodological issues.
Comments on evidence pertaining to at-risk subgroups for O3-
related effects can be found in section II.B.2.a.iii below. EPA notes
here that most of the issues and concerns raised by commenters
concerning the health effects evidence, including both the
interpretation of the evidence and specific technical or methodological
issues, were essentially restatements of issues raised during the
review of the Criteria Document and the Staff Paper. Most of these
issues were highlighted and thoroughly discussed during the review of
these documents by the CASAC. More detailed responses related to the
interpretation of the health effects evidence and its role in the
decision on the O3 NAAQS are contained in the Response to
Comments document.
[[Page 16454]]
i. Evidence from Controlled Human Exposure Studies
As noted in the overview of health effects evidence, section II.A.2
above, two new controlled human-exposure studies (Adams 2002, 2006) are
now available that examine respiratory effects associated with
prolonged O3 exposures at levels at and below 0.080 ppm,
which was the lowest exposure level that had been examined in the last
review. One group of commenters that included national medical (e.g.,
ATS, AMA, ACCP) and national environmental and public health
organizations (e.g., ALA in a joint set of comments with Environmental
Defense, Sierra Club), agreed with EPA's reanalysis of the Adams' data
while disagreeing with EPA's characterization of the evidence from the
Adams studies as ``very limited'' (72 FR 37870). These commenters
expressed the view that the Adams studies provide evidence of effects
at lower concentrations than had previously been reported. They noted
that Adams, while finding small group mean changes at 0.060 ppm,
reported total subjective symptom scores reached statistical
significance (relative to pre-exposure) at 5.6 and 6.6 hours, with the
triangular exposure scenario, and that pain on deep inspiration values
followed a similar pattern to total subjective symptoms scores. In
addition, Adams (2002) reports that ``some sensitive subjects
experience notable effects at 0.060 ppm,'' based on a greater than 10%
reduction in FEV1. These commenters made the point that the
responses of individuals are more important than group mean responses
and that when the Adams (2002, 2006) study data are corrected for the
effects of exercise in clean air, 7 percent of subjects experience
FEV1 decrements greater than 10% at the 0.040 and 0.060 ppm
exposure levels. They expressed the view that while 2 of 30 tested
subjects responding at the 0.060 ppm level may seem like a small
number, a 7 percent response rate is far from trivial. Seven percent of
the U.S. population is 21.2 million people (ALA et al., p. 51). Noting
that the subjects in the Adams' studies were all healthy adults, these
groups expressed concern that ``in some vulnerable populations the
magnitude of the response would be greater and the exposure level at
which responses are observed to occur would be lower'' (ATS, p. 4).
These commenters generally supported EPA's reanalysis of the Adams'
data, stating that EPA has undertaken a careful reanalysis of the
underlying data in the Adams studies to assess the change in
FEV1 following exposure to 0.060 ppm O3 and
filtered air, and concluding that ``the reanalysis employs the standard
approach used by other researchers, and supported by CASAC'' (ALA et
al., p. 49), and ``we believe that the Adams study shows significant
health effects at 0.06 ppm exposure levels'' (ATS, p. 5). The American
Thoracic Society, AMA and other medical organizations conclude:
The Adams study confirms our understanding that in healthy
populations, an important fraction of the population will experience
larger-than-average decrements in FEV1 when exposed to
low levels of ozone. It is reasonable to assume that these effects
would be even greater when extrapolated to other populations known
to have sensitivities to ozone (children, asthmatics, COPD
patients). We feel the correct conclusion to draw from the Adams
study is that there is a significant fraction of the population that
will express significant responses to low levels of ozone. [ATS, p.
5]
EPA generally agrees with most of the comments summarized above,
while placing more emphasis on the limited nature of the evidence
addressing O3-related lung function and respiratory symptom
responses at the 0.060 and 0.040 ppm exposure levels. As characterized
in the proposal notice, EPA's reanalysis of the data from the most
recent Adams study shows small group mean decrements in lung function
responses to be statistically significant at the 0.060 ppm exposure
level, while acknowledging that the author's analysis did not yield
statistically significant lung function responses. The Adams studies
report a small percentage of subjects experiencing lung function
decrements ([gteqt]10 percent) at the 0.060 ppm exposure level. EPA
disagrees with these commenters that the percent of subjects that
experienced FEV1 decrements greater than 10% in this study
of 30 subjects can appropriately be generalized to the U.S. population.
The Administrator concludes that these studies provide very limited
evidence of O3-related lung function decrements and
respiratory symptoms at this lower exposure level.
The second group of commenters, who opposed revision of the
standard, raised many concerns about the role of the Adams studies and
EPA's reanalysis of the Adams data in the decision. With regard to the
results reported by Adams, these commenters expressed the view that the
group mean FEV1 decrement measured at 0.060 ppm was small,
less than 3%, which is within the 3 to 5% range of normal measurement
variability for an individual (UARG, p. 12). Moreover even the reported
group mean FEV1 decrements in Adams subjects when exposed to
an O3 concentration of 0.080 ppm were described as quite
minimal, likely non-detectable by the subjects and within the range
that the EPA would consider to be normal or mild (UARG, p. 13); With
respect to the larger decrements in FEV1 ([gteqt] 10%)
experienced by some subjects in the Adams studies, these commenters
stated the view that such decrements would not be considered adverse in
healthy individuals, and that ``reliance on the individual responses of
such a miniscule number of subjects (2 of 30) is woefully inadequate as
any basis for a nationwide O3 standard'' (UARG, p.14). Some
of these commenters put the results of the Adams studies (2002, 2006)
in the context of the 1997 decision on the O3 standard to
reach the conclusion that there is no basis for revising that standard.
They stated that the data from Adams (2002, 2006) on O3
levels below 0.080 ppm was too limited to support a revised standard,
and noted that responses reported in the Adams studies at 0.080 ppm
were similar to responses reported previously (Horstmann et al., 1990
and McDonnell et al., 1991), and therefore, provided no new information
on O3 that was not known at the time of EPA's last review
(Exxon Mobil, pp. 5-6).
These commenters raised one or more of the following concerns about
EPA's reanalysis of the Adams data: (1) EPA's re-analysis was not
published or peer-reviewed, and therefore neither the scientific
community nor the public was afforded opportunity to appropriately
review the analysis (Exxon Mobil, p. 6); (2) EPA has misinterpreted the
studies of Dr. Adams, and over his objections used a different
analytical methodology to reach a different conclusion; (3) EPA's
reanalysis did not employ an appropriate statistical test; the ANOVA
statistical test employed by Adams was preferred over the statistical
test used in EPA's reanalysis (paired t-test); and (4) the reanalysis
of the Adams data is evidence that EPA interpreted and presented
scientific information in a systematically biased manner, reflecting
purposeful bias because the reanalysis supported staff policy
recommendations and Adams' own analysis did not, and the 10% decrement
in FEV1 was a post-hoc threshold chosen for compatibility
with EPA staff policy recommendations (NAM, p. 19).
First, EPA agrees that the group mean lung function decrement
observed in the Adams study at the 0.060 ppm exposure level is
relatively small. However, EPA and the CASAC Panel observed that the
study showed some individuals experienced lung function decrements >=
10 percent, which is the most
[[Page 16455]]
important finding from this study in terms of public health
implications. The magnitude of changes in the group mean do not address
whether a subset of the population is at risk of health effects. The
clinical evidence to date makes it clear that there is significant
variability in responses across individuals, so it is important to look
beyond group mean to the response of subsets of the group to evaluate
the potential impact for sensitive or susceptible parts of the
population. The Administrator also agrees with both EPA staff and
CASAC's views that this level of response may not represent an adverse
health effect in healthy individuals but does represent a level that
should be considered adverse for asthmatic individuals.
Second, EPA notes that its reanalysis of the Adams (2006) study was
prepared in response to the issues and analysis raised by a public
commenter who made a presentation to the CASAC Panel at its March 5,
2007 teleconference. EPA replicated the analysis and addressed issues
raised in these public comments concerning the statistical significance
of 0.060 ppm O3 exposure on lung function response in the Adams (2006)
publication. EPA documented its response in a technical memorandum
(Brown, 2007), which was placed in the rulemaking docket prior to
publication of the proposal. EPA has clearly stated that the additional
statistical analyses conducted by both the public commenter and by EPA
staff do not contradict or undercut the statistical analysis presented
by Dr. Adams in his published study, as EPA and the author were
addressing different questions. While the author of the original study
was focused on determining whether the changes observed on an hour-by-
hour basis were statistically significant for different exposure
protocols, EPA's reanalysis was focused on the different question of
whether there was a statistically significant difference in lung
function decrement before and after the entire 6.6 hour exposure period
between the 0.060 ppm exposure protocol and filtered air.
Third, with respect to the concerns raised by Dr. Adams and other
commenters that EPA had used an inappropriate statistical approach to
address the question regarding statistical significance of the average
lung function response at 0.060 ppm, members of the CASAC Panel noted
on the March 5, 2007 teleconference the very conservative nature of the
approach used by Adams to evaluate the research questions posed by the
author. These same CASAC Panel members also supported the use of the
statistical approach (i.e., paired-t test) used in the analysis
prepared by the public commenter, which was the same approach later
used in EPA's reanalysis, as the preferred method for analyzing the
pre-minus post-exposure lung function responses reported in this study.
EPA agrees with the characterization of the Adams (2006) study in the
Rochester Report, which stated, ``Although these findings have not been
confirmed or replicated, the responses to 0.06 ppm ozone in this
[Adams] study are consistent with the presence of an exposure-response
curve with responses that do not end abruptly below 0.08 ppm.'' This
same report also concluded,
The statistical test used in Adams (2006) did not identify the
response of the 0.06 ppm exposure as statistically different from
that of the filtered air exposure. However, alternative statistical
tests suggest that the observed small group mean response in
FEV1 induced by exposure to 0.06 ppm compared to filtered
air is not the result of chance alone. [Rochester Report, p. 56].
Fourth, EPA rejects the contention that the conduct and
presentation of its reanalysis of the Adams (2006) study to address
issues raised by public commenters represents purposeful bias and was
developed only to support a pre-determined policy position. As
discussed above, EPA's reanalysis addressed a different question than
the author's analysis contained in the publication. Other controlled
human exposure studies had routinely examined the same question EPA's
reanalysis addressed, whether or not there was a statistically
significant group mean response for the entire exposure period compared
to filtered air.
ii Evidence from Epidemiological Studies
This section contains major comments on EPA's assessment of
epidemiological studies in the proposal and the Agency's general
responses to those comments. Many of the issues discussed below are
addressed in more detail in the Response to Comments document. Comments
on EPA's interpretation and assessment of the body of epidemiological
evidence are discussed first and then comments on methodological issues
and particular study designs are discussed. EPA notes here that most of
the issues and concerns raised by commenters on the interpretation of
the epidemiological evidence and methodological issues are essentially
restatements of issues raised during the review of the Criteria
Document and Staff Paper. EPA presented and the CASAC Panel reviewed
the interpretation of the epidemiological evidence in the Criteria
Document and the integration of the evidence with policy considerations
in the development of the policy options presented in the Staff Paper
for consideration by the Administrator. CASAC reviewed both the O3
Criteria Document and O3 Staff Paper and approved of the scientific
content and accuracy of both documents. The CASAC chairman sent to the
Administrator one letter (Henderson, 2006a) for the O3 Criteria
Document and another letter for the O3 Staff Paper (Henderson, 2006c)
indicating that these documents provided an appropriate basis for use
in regulatory decision making regarding the O3 NAAQS.
As with evidence from controlled human exposure studies, sharply
divergent comments were received on the evidence from epidemiological
studies, including EPA's interpretation of the evidence. One group of
commenters from medical, public health and environmental organizations,
in general, supported EPA's interpretation of the epidemiological
evidence (72 FR 37838, section II.a.3.a-c) with regard to whether the
evidence for associations is consistent and coherent and whether there
is biological plausibility for judging whether exposure to O3 is
causally related to respiratory and cardiovascular morbidity and
mortality effects. Comments of public health and environmental groups,
including a joint set of comments from ALA and several environmental
groups, note that more than 250 new epidemiological studies, published
from 1996 to 2005, were included in the Criteria Document and point to
a figure from the Staff Paper and proposal (72 FR 37842, Figure 1) of
short-term O3 exposures and respiratory health outcome showing
consistency in an array of positive effects estimates and health
endpoints observed in multiple locations in Canada and the U.S. Medical
commenters, including ATS and AMA, stated that these ``real world''
studies support the findings of chamber studies to show adverse
respiratory health effects at levels below the current 8-hour O3
standard. These commenters generally expressed agreement with the
weight of evidence approach taken by the Criteria Document and the
conclusions reached, which were reviewed by CASAC, that the effects of
O3 on respiratory symptoms, lung function changes, emergency department
visits for respiratory and cardiovascular effects, and hospital
admissions can be considered causal.
EPA generally agrees with this interpretation of the
epidemiological evidence. The Criteria Document concludes that positive
and robust
[[Page 16456]]
associations were found between ambient O3 concentrations and various
respiratory disease hospitalization outcomes and emergency department
visits for asthma, when focusing particularly on results of warm-season
analyses. These positive and robust associations are supported by the
human clinical, animal toxicological, and epidemiological evidence for
lung function decrements, increased respiratory symptoms, airway
inflammation, and increased airway responsiveness. Taken together, the
overall evidence supports a causal relationship between acute ambient
O3 exposures and increased respiratory morbidity outcomes resulting in
increased emergency department visits and hospitalizations during the
warm season (EPA, 2006a, p. 8-77).
However, in contrast with EPA, these commenters from ALA and other
environmental, medical and public health groups asserted that the
causal associations extend down to the lowest ambient O3 concentrations
reported in these studies. These commenters also expressed the view
that the respiratory and cardiovascular system effects are well-
supported by the Hill criteria\19\ of judging causality: strength of
association, consistency between studies, coherence among studies, and
biological plausibility (ALA et al., pp. 51-52). They also noted that
recent studies provide compelling evidence that exposure to O3 results
in adverse cardiovascular health effects (ATS, p. 6-7).
---------------------------------------------------------------------------
\19\ The Hill criteria, published by Sir Bradford Hill (1965),
are commonly used criteria for reaching judgments about causality
from observed associations, and these criteria were the basis for
the critical assessment of the epidemiological evidence presented in
the Criteria Document (pp. 7-3-7-4).
---------------------------------------------------------------------------
EPA disagrees with the assertion of these commenters that the
causal associations extend down to the lowest ambient O3 concentrations
reported in these studies. The biological plausibility of the
epidemiological associations is generally supported by controlled human
exposure and toxicological evidence of respiratory morbidity effects
for levels at and below 0.080 ppm, but that biological plausibility
becomes increasingly uncertain at much lower levels. Further, at much
lower levels, it becomes increasingly uncertain as to whether the
reported associations are related to O3 alone rather than to the
broader mix of air pollutants present in the ambient air. With regard
to cardiovascular health outcomes, the Criteria Document concludes that
the generally limited body of evidence from animal toxicology, human
controlled exposure, and epidemiologic studies is suggestive that O3
can directly and/or indirectly contribute to cardiovascular-related
morbidity, and that for cardiovascular mortality the Criteria Document
suggests that effects estimates are more consistently positive and
statistically significant in warm season analyses but that additional
research is needed to more fully establish the underlying mechanisms by
which such mortality effects occur (EPA, 2006a, pp. 8-77-78).
The second group of commenters, mostly representing industry
associations and some businesses opposed to revising the primary O3
standard, disagreed with EPA's interpretation of the epidemiological
evidence. These commenters expressed the view that while many new
epidemiological studies have been published since the current primary
O3 standard was promulgated, the inconsistencies and uncertainties
inherent in these studies as a whole should preclude any reliance on
them as justification for a more stringent primary O3 NAAQS. They
contend that the purported consistency is the result of inappropriate
selectivity in focusing on specific studies and specific results within
those studies (UARG, p. 15). With regard to daily mortality, the
proposal emphasizes the multi-city studies, suggesting that they have
the statistical power to allow the authors to reliably distinguish even
weak relationships from the null hypothesis with statistical
confidence. However, these commenters note that these studies are not
consistent, with regard to the findings concerning individual cities
analyzed in the multi-city analyses. One commenter asserted that each
of the multi-city studies and meta-analyses cited by EPA involves
cities for which the city-specific estimates of O3 effects have been
observed to vary over a wide range that includes negative [i.e.,
beneficial] effects (API, p. 15). To illustrate this point, many
commenters point to EPA's use of the study by Bell et al., 2004. They
note that in focusing on the national estimate from Bell of the
association between 24-hour average O3 levels and daily mortality, the
Administrator overlooks the very significant and heterogeneous
information of the individual analyses of the 95 cities used to produce
the national estimate and, based on this inconsistency, question
whether what is being seen is actually an O3 mortality association at
all (UARG, p. 16).
EPA has accurately characterized the inconsistencies and
uncertainties in the epidemiological evidence and strongly denies that
it has inappropriately focused on specific positive studies or specific
positive results within those studies. EPA's assessment of the health
effects evidence in the Criteria Document has been reviewed by the
CASAC Panel. EPA has appropriately characterized the heterogeneity in
O3 health effects in assessing the results of the single-city and
multi-city studies and the meta-analyses, as discussed in section 7.6.6
of the Criteria Document. In general, in the proposal, the
Administrator recognized that in the body of epidemiological evidence,
many studies reported positive and statistically significant
associations, while others reported positive results that were not
statistically significant, and a few did not report any positive O3-
related associations. In addition, the Administrator judged that
evidence of a causal relationship between adverse health outcomes and
O3 exposures became increasingly uncertain at lower levels of exposure.
More specifically, the Bell et al. (2004) study observed a
statistically significant, positive association between short-term O3
concentrations (24-hour average) and all-cause mortality using data
from 95 U.S. National Morbidity, Mortality, and Air Pollution Study
(NMMAPS) communities. The objective of the NMMAPS was to develop an
overall national effect estimate using multi-city time-series analyses,
by drawing on information from all of the individual cities. The
strength of this approach is the use of a uniform analytic methodology,
avoidance of selection bias, and larger statistical power. Significant
intercity heterogeneity was noted in the Bell et al. and other multi-
city studies, probably due to many factors, including city-specific
differences in pollution characteristics, the use of air conditioning,
time spent indoors versus outdoors, and socioeconomic factors. Levy et
al. (2005) found suggestive evidence that air conditioning prevalence
was a predictor of heterogeneity in O3 risk estimates in their meta-
analysis.
Several commenters argued that EPA overstates the probability of
causal links between health effects and exposure to O3, especially at
the lower concentrations examined, and that the statistical
associations found in the cited epidemiological studies do not
automatically imply that a causal relationship exists. These commenters
expressed the view that the correlation between health effects and O3
exposure must be rigorously evaluated according to a standard set of
criteria before concluding that there is a causal link and that EPA
fails to articulate and
[[Page 16457]]
follow the weight of the evidence or established causality criteria for
evaluating epidemiological studies in drawing conclusion regarding
causality (Exxon Mobil, pp. 10-11).
In the proposal, EPA explicitly stated that epidemiological studies
are not themselves direct evidence of a causal link between exposure to
O3 and the occurrence of effects (72 FR 37879). Throughout the O3
review, a standard set of criteria have been used to evaluate evidence
of a causal link. The critical assessment of epidemiological evidence
presented in the Criteria Document was conceptually based upon
consideration of salient aspects of the evidence of associations so as
to reach fundamental judgments as to the likely causal significance of
the observed associations in accordance with the Hill criteria
(Criteria Document, pp. 7-3--7-4). Moreover, consistent with the
proposal the Administrator has specifically considered evidence from
epidemiological studies in the context of all the other available
evidence in evaluating the degree of certainty that O3-related adverse
health effects occur at various levels at and below 0.080 ppm,
including the strong evidence from controlled human exposure studies
and the toxicological studies that demonstrate biological plausibility
and mechanisms for effects. More detailed discussion of the criteria
used to evaluate evidence with regard to judgments about causality can
be found in the Response to Comments document.
Several commenters made the point that the results of the new
epidemiological studies included in this review are not coherent. They
state that although EPA notes that estimates of risk from
cardiovascular mortality are higher than those for total mortality and
indicates that these findings are highly suggestive that short-term O3
exposure directly or indirectly contributes to cardiovascular
mortality, the Agency fails to contrast the mortality studies to
studies of hospital admissions for cardiovascular causes. Most studies
of cardiovascular causes have not found statistically significant
associations with O3 exposures (UARG, pp. 16-17).
EPA strongly disagrees that it has failed to appropriately
characterize the association between O3 exposure and potential
cardiovascular morbidity and mortality effects. As noted above, the
Criteria Document characterizes the overall body of evidence as
limited, but highly suggestive, and concludes that much needs to be
done to more fully integrate links between ambient O3 exposures and
adverse cardiovascular outcomes (EPA, 2006a, p. 8-77). Some field/panel
studies that examined associations between O3 and various cardiac
physiologic endpoints have yielded limited epidemiological evidence
suggestive of a potential association between acute O3 exposure and
altered HRV, ventricular arrhythmias, and incidence of myocardial
infarction (Criteria Document, section 7.2.7). In addition, there were
approximately 20 single-city studies of emergency department visits and
hospital admissions for all cardiovascular diseases or specific
diseases (i.e., myocardial infarction, congestive heart failure,
ischemic heart disease, dysrhythmias). In the studies using all year
data, many showed positive results but few were statistically
significant. Given the strong seasonal variations in O3 concentrations
and the changing relationship between O3 and other copollutants by
season, inadequate adjustment for seasonal effects might have masked or
underestimated the associations. In the limited number of studies that
analyzed data by season (6 studies), statistically significant
associations were observed in all but one study (Criteria Document,
section 7.3.4). Newly available animal toxicology data provide some
plausibility for the observed associations between O3 and
cardiovascular outcomes. EPA believes that its characterization of the
evidence for O3-related cardiovascular system effects is appropriate.
It is clear that coherence is stronger in the much larger body of
evidence of O3-related respiratory morbidity and mortality effects.
Many commenters who did not support revising the current O3 primary
standard also submitted comments on specific methodological issues
related to the epidemiological evidence, including: The adequacy of
exposure data; confounding by copollutants; model selection; evidence
of mortality; and, new studies not included in the Criteria Document.
Some of the major comments on methodological issues raised by these
commenters are discussed below. The Response to Comments document
contains more detailed responses to many of these comments, as well as
responses to other comments not considered here.
(1) Adequacy of exposure data. Many commenters expressed concern
about the adequacy of exposure data both for time-series and panel
studies. These commenters argued that almost all of the epidemiological
studies on which EPA relies in recommending a more stringent O3
standard are based on data from ambient monitors for which there is a
poor correlation with the actual personal exposure subjects receive
during their daily activities. They questioned the Administrator's
conclusion that in the absence of available data on personal O3
exposure, the use of routinely monitored ambient O3 concentrations as a
surrogate for personal exposures is not generally expected to change
the principal conclusions from epidemiological studies. These
commenters also note that, in its June 2006 letter, the CASAC Panel
raised the issue of exposure error, concluding that it called into
question whether observed associations could be attributed to O3 alone
(API, p. 17). One of these commenters cited studies (e.g., Sarnat et
al., 2001; Sarnat et al., 2005) that show a lack of correlation between
personal exposures and ambient concentrations (NAM, p. 22). Another
cited studies (Sarnat et al., 2001, 2005, and 2006; and Koutrakis et
al., 2005) that have found that the ability of ambient gas monitors to
represent personal exposure to such gases is similarly quite limited,
including: (1) Most personal exposures are so low as to be not
detectable at a level of 5 parts per billion (ppb), resulting in very
low correlation between concentrations reported from central ambient
monitors and personal monitors; (2) O3 measurements from ambient
monitors are a better surrogate for personal exposure to
PM2.5 than to O3; and (3) populations expected to be
potentially susceptible to O3, including children, the elderly, and
those with COPD, are at the low end of the population exposure
distribution (Exxon Mobil, pp. 15-16). These commenters contended that
without such a correlation there is no legitimate way for EPA to
conclude that O3 exposure has caused the reported health effects, or to
conclude that use of routinely monitored ambient O3 concentrations as a
surrogate for personal exposures is adequate. Some of these commenters
also contended that EPA incorrectly concludes that the exposure error
in epidemiological studies results in an underestimate of risk (Exxon
Mobil, p. 20).
With regard to the views on exposure measurement error expressed by
CASAC, while the commenter is correct that the CASAC Panel raised the
question of exposure error and whether observed associations could be
attributed to O3 alone, the commenter failed to note that CASAC's
comment was focused on the association between O3 and mortality, at
very low O3 concentrations and in the group of people most susceptible
to premature mortality. The CASAC Panel in its June 2006 letter stated:
[[Page 16458]]
The population that would be expected to be potentially
susceptible to dying from exposure to ozone is likely to have ozone
exposures that are at the lower end of the ozone population
distribution, in which case the population would be exposed to very
low ozone concentrations, and especially so in winter. Therefore it
seems unlikely that the observed associations between short-term
ozone concentrations and daily mortality are due solely to ozone
itself. [Henderson 2006b, pp. 3-4]
This section of the quote, which was not addressed in the comment
submitted by API, together with the conclusions in the final CASAC
letter (Henderson, 2007), leads EPA to conclude that contrary to the
commenters' assertion, the CASAC Panel was not calling into question
the association between O3 exposure and the full range of
morbidity effects found in panel or time-series studies that rely on
ambient monitoring data as a surrogate for personal exposure data. It
is important to note that EPA agrees that the evidence is only highly
suggestive that O3 directly or indirectly contributes to
mortality, as compared to the stronger evidence of causality for
respiratory morbidity effects.
EPA agrees that exposure measurement error may result from the use
of stationary ambient monitors as an indicator of personal exposure in
population studies. There is a full discussion of measurement error and
its effect on the estimates of relative risk in section 7.1.3.1 of the
Criteria Document. However, the possibility of measurement error does
not preclude the use of ambient monitoring data as a surrogate for
personal exposure data in time-series or panel studies. It simply means
that in some situations where the likelihood of measurement error is
greatest, effects estimates must be evaluated carefully and that
caution must be used in interpreting the results from these studies.
Throughout this review, EPA has recognized this concern. The Criteria
Document states that there is supportive evidence that ambient
O3 concentrations from central monitors may serve as valid
surrogate measures for mean personal O3 exposures
experienced by the population, which is of most relevance to time-
series studies, in which individual variations in factors affecting
exposure tend to average out across the study population. This is
especially true for respiratory hospital admission studies for which
much of the response is attributable to O3 effects on
asthmatics. In children, for whom asthma is more prevalent than for
adults, ambient monitors are more likely to correlate reasonably well
with personal exposure to O3 of ambient origin because
children tend to spend more time outdoors than adults in the warm
season. EPA does not agree that the correlation between personal
exposure and ambient monitoring data is necessarily poor, especially in
children. Moreover, the CASAC Panel supported this view as they noted
that ``[p]ersonal exposures most likely correlate better with central
site values for those subpopulations that spend a good deal of time
outdoors, which coincides, for example, with children actively engaged
in outdoor activities, and which happens to be a group that the ozone
risk assessment focuses upon.'' (Henderson, 2006c. p. 10). However, the
Criteria Document notes that there is some concern in considering
certain mortality and hospitalization time-series studies regarding the
extent to which ambient O3 concentrations are representative
of personal O3 exposures in another particularly susceptible
group of individuals, the debilitated elderly, as the correlation
between the two measurements has not been examined in this population.
A better understanding of the relationship between ambient
concentrations and personal exposures, as well as of the factors that
affect the relationship, will improve the interpretation of observed
associations between ambient concentration and population health
response.
With regard to the specific comments that reference the findings of
studies by Sarnat et al. (2001, 2005, 2006) and Koutrakis et al.
(2005), the fact that personal exposure monitors cannot detect
O3 levels of 5 ppb and below may in part explain why there
was a poor correlation between personal exposure measurements and
ambient monitoring data in the winter relative to the correlation in
the warm season, along with differences in activity patterns and
building ventilation. In one study conducted in Baltimore, Sarnat et
al. (2001) observed that ambient O3 concentrations showed
stronger associations with personal exposure to PM2.5 than
to O3; however, in a later study conducted in Boston (Sarnat
et al., 2005), ambient O3 concentrations and personal
O3 exposures were found to be significantly associated in
the summer. Another study cited by the commenter, but not included in
the Criteria Document, conducted in Steubenville (Sarnat et al., 2006),
also observed significant associations between ambient O3
concentrations and personal O3. The authors noted that the
city-specific discrepancy in the results may be attributable to
differences in ventilation. Though the studies by Sarnat et al. (2001,
2005, and 2006) included senior citizens, the study selection criteria
required them to be nonsmoking and physically healthy. EPA is not
relying on studies that are not in the Criteria Document, such as
Sarnat et al. (2006), to refute the commenters. However, EPA notes that
Sarnat et al. (2006) does not support the conclusion drawn by the
commenters that this study shows very limited associations between
ambient O3 concentrations and personal exposures.
Existing epidemiologic models may not fully take into consideration
all the biologically relevant exposure history or reflect the
complexities of all the underlying biological processes. Using ambient
concentrations to determine exposure generally overestimates true
personal O3 exposures (by approximately 2- to 4-fold in the
various studies described in the Criteria Document, section 3.9), which
assuming the relationship is causal, would result in biased
descriptions of underlying concentration-response relationships (i.e.,
in attenuated effect estimates). From this perspective, the implication
is that the effects being estimated in relationship to ambient levels
occur at fairly low personal exposures and the potency of O3
is greater than these effect estimates indicate. On the other hand, as
very few studies evaluating O3 health effects with personal
O3 exposure measurements exist in the literature, effect
estimates determined from ambient O3 concentrations must be
evaluated and used with caution to assess the health risks of
O3 (Criteria Document, pp. 7-8 to 7-10). Nonetheless, as
noted in section II.C.3 of the proposal, the use of routinely monitored
ambient O3 concentrations as a surrogate for personal
exposures is not generally expected to change the principal conclusions
from O3 epidemiologic studies. Therefore, population risk
estimates derived using ambient O3 concentrations from
currently available observational studies, with appropriate caveats
about personal exposure considerations, remain useful (72 FR 37839).
(2) Confounding by copollutants. Many commenters argued that known
confounders are inadequately controlled in the epidemiological studies
of O3 and various health outcomes and that the health
effects of O3 are often not statistically significant when
epidemiological studies consider the effects of confounding air
pollutants (e.g., PM2.5, CO, nitrogen dioxide
(NO2) in multi-pollutant models. For example, Mortimer et
al. (2002), a large multi-city asthma panel study, found that when
[[Page 16459]]
other pollutants, i.e., sulfur dioxide (SO2),
NO2, and particles with an aerodynamic diameter less than or
equal to a nominal 10 micrometers (PM10), were placed in a
multi-pollutant model with O3, the O3-related
associations with respiratory symptoms and lung function became non-
significant.
The National Cooperative Inner-City Asthma Study (Mortimer et al.,
2002) evaluated air pollution health effects in 846 asthmatic children
in 8 urban areas. The pollutants evaluated included O3,
PM10, SO2, and NO2. Three effects were
evaluated: (1) Daily percent change in lung function, measured as peak
expiratory flow rate (PEFR); (2) incidence of (>= 10% reduction in lung
function (PEFR); and, (3) incidence of symptoms (i.e., cough, chest
tightness, and wheeze). EPA notes that in this study, O3 was
the only pollutant associated with reduction in lung function. Nitrogen
dioxide had the strongest effect on morning symptoms, and the authors
concluded it ``* * * may be a better marker for the summer-pollutant
mix in these cities'' but had no association with morning lung
function. In a two-pollutant model with NO2, the
O3 effect on morning symptoms remained relatively unchanged.
Sulfur dioxide had statistically significant effects on morning
symptoms but no association with morning lung function. Particulate
matter (PM10), which was measured daily in 3 cities, had no
statistically significant effect on morning lung function. In a two-
pollutant model with O3, the PM10 estimate for
morning symptoms was slightly reduced and there was a larger reduction
in the O3 estimate, which remained positive but not
statistically significant. A more general discussion and response to
this issue concerning confounding by copollutants is presented in the
Response to Comments document.
(3) Model selection. Commenters who did not support revision of the
primary O3 standard raised issues regarding the adequacy of
model specification including control of temporal and weather variables
in the time-series epidemiological studies that EPA has claimed support
the finding of O3-related morbidity and mortality health
outcomes. Specifically, concerns were expressed regarding the following
issues: (i) Commenters noted that recent meta-analyses have confirmed
the important effects of model selection in the results of the time-
series studies, including the choice of models to address weather and
the degree of smoothing, in direct contradiction of the Staff Paper's
conclusion on the robustness of the models used in the O3
time-series studies (Exxon Mobil, p. 41); (ii) commenters contended
that there were no criteria for how confounders such as temperature or
other factors were to be addressed, resulting in arbitrary model
selection potentially impacting the resulting effect estimates; and
(iii) commenters expressed the view that to appropriately address
concerns about model selection in the O3 time-series
studies, EPA should rely on an alternative statistical approach,
Bayesian model averaging, that incorporates a range of models
addressing confounding variables, pollutants, and lags rather than a
single model.
In response to the first issue, EPA agrees that the results of the
meta-analyses do support the conclusion that there are important
effects of model selection and that, for example, alternative models to
address weather might make a difference of a factor of two in the
effect estimates. However, as noted in the Criteria Document, one of
the meta-analyses (Ito et al., 2005) suggested that the stringent
weather model used in the Bell et al. (2004) NMMAPS study may tend to
yield smaller effect estimates than those used in other studies
(Criteria Document, p. 7-96), and, thus concerns about appropriate
choice of models could result in either higher or lower effect
estimates than reported. In addressing this issue, the Criteria
Document concluded,
Considering the wide variability in possible study designs and
statistical model specification choices, the reported O3
risk estimates for the various health outcomes are in reasonably
good agreement. In the case of O3-mortality time-series
studies, combinations of choices in model specifications * * * alone
may explain the extent of difference in O3 risk estimates
across studies. (Criteria Document, p. 7-174)
Second, the issues surrounding sensitivity to model specifications
were thoroughly discussed in the Criteria Document (see section
7.1.3.6) and evaluated in some of the meta-analyses reviewed in the
Criteria Document and Staff Paper. As stated in the Criteria Document,
O3 effect estimates ``were generally more sensitive to
alternative weather models than to varying degrees of freedom for
temporal trend adjustment'' (Criteria Document, p. 7-176). The Criteria
Document also concluded that ``although there is some concern regarding
the use of multipollutant models * * * results generally suggest that
the inclusion of copollutants into the models do not substantially
affect O3 risk estimates'' and the results of the time-
series studies are ``robust and independent of the effects of other
copollutants'' (Criteria Document, p. 7-177). Overall, EPA continues to
believe that based on its integrated assessment, the time-series
studies provide strong support for concluding there are O3-
related morbidity effects, including respiratory-related hospital
admissions and emergency department visits during the warm season, and
that the time-series studies provide findings that are highly
suggestive that short-term O3 exposure directly or
indirectly contributes to non-accidental and cardiorespiratory-related
mortality.
The Administrator acknowledges that uncertainties concerning
appropriate model selection are an important source of uncertainty
affecting the specific risk estimates included in EPA's risk assessment
and that these quantitative risk estimates must be used with
appropriate caution, keeping in mind these important uncertainties, as
discussed above in section II.A.3. As discussed later in this notice,
the Administrator is not relying on any specific quantitative effect
estimates from the time-series studies or any risk estimates based on
the time-series studies in reaching his judgment about the need to
revise the current 8-hour O3 standard.
Third, in response to commenters who suggested that EPA adopt an
alternative statistical approach, i.e., Bayesian model averaging, to
address concerns about potential arbitrary selection of models, the
Criteria Document evaluated the strengths and weaknesses of such
methods in the context of air pollution epidemiology. The Criteria
Document noted several limitations, especially where there are many
interaction terms and meteorological variables and where variables are
highly correlated, as is the case for air pollution studies, which
makes it very difficult to interpret the results using this alternative
approach. EPA believes further research is needed to address concerns
about model selection and to develop appropriate methods addressing
these concerns.
(4) Evidence of mortality. Many commenters, including those that
argued for revising the current O3 standard as well as those
that argued against revisions, focused on the new evidence from multi-
city time-series analyses and meta-analyses linking O3
exposure with mortality. Again, the comments were highly polarized. One
set of commenters, including medical, public health, and environmental
organizations argued that recent published research has provided more
robust, consistent evidence linking O3 to cardiovascular and
respiratory
[[Page 16460]]
mortality. The ATS, AMA, and others stated that data from single-city
studies, multiple-city studies, and meta-analyses show a consistent
relationship between O3 exposure and mortality from
respiratory and cardiovascular causes. These commenters noted that this
effect was observed after controlling for co-pollutants and seasonal
impacts. These commenters stated that research has demonstrated that
exposure to O3 pollution is causing premature deaths, and
has also provided clues on the possible mechanisms that lead to
premature mortality (ATS, p. 4). These commenters noted that people may
die from O3 exposure even when the concentrations are well
below the current standard. They pointed to a study (Bell et al., 2006)
in which the authors followed up on their 2004 multi-city study to
estimate the exposure-response curve for O3 and the risk of
mortality and to evaluate whether a threshold exists below which there
is no effect. The authors applied several statistical models to data on
air pollution, weather, and mortality for 98 U.S. urban communities for
the period 1987 to 2000. The study reported that O3 and
mortality results did not appear to be confounded by temperature or PM
and showed that any threshold, if it existed, would have to be at very
low concentrations, far below the current standard (ALA et al., p. 74).
Another approach also indicated that the mortality effect is unlikely
to be confounded by temperature. A case-crossover study (Schwartz 2005)
of over one million deaths in 14 U.S. cities, designed to control for
the effect of temperature on daily deaths attributable to
O3, found that the association between O3 and
mortality risk reported in the multi-city studies is unlikely to be due
to confounding by temperature (ALA et al., p. 76). These commenters
argue that meta-analyses also provide compelling evidence that the
O3-mortality findings are consistent. They point to three
independent analyses conducted by separate research groups at Johns
Hopkins University, Harvard University and New York University, using
their own methods and study criteria, which reported a remarkably
consistent link between daily O3 levels and total mortality.
In response, EPA notes that the Criteria Document states that the
results from the U.S. multi-city time-series studies provide the
strongest evidence to date for O3 effects on acute
mortality. Recent meta-analyses also indicate positive risk estimates
that are unlikely to be confounded by PM; however, future work is
needed to better understand the influence of model specifications on
the risk coefficient (EPA, 2006a, p. 7-175). The Criteria Document
concludes that these findings are highly suggestive that short-term
O3 exposure directly or indirectly contributes to non-
accidental and cardiorespiratory-related mortality but that additional
research is needed to more fully establish the underlying mechanisms by
which such effects occur (72 FR 37836). Thus while EPA generally agrees
with the direction of the comment, EPA believes the evidence supports a
view as noted above. In addition, it must be noted that the
Administrator did not focus on mortality as a basis for proposing that
the current O3 standard was not adequate. In the proposal,
the Administrator focused on the very strong evidence of respiratory
morbidity effects in healthy people at the 0.080 ppm exposure level and
new evidence that people with asthma are likely to experience larger
and more serious effects than healthy people at the same level of
exposure (72 FR 37870). With regard to the ambient concentrations at
which O3-related mortality effects may be occurring, EPA
recognized in the proposal that evidence of a causal relationship
between adverse health effects and O3 exposures becomes
increasingly uncertain at lower levels of exposure (72 FR 37880). This
is discussed more fully in section (b) below.
Several industry organizations argued against placing any reliance
on the time-series epidemiological studies, especially those studies
related to mortality effects. The Annapolis Center (p. 46) makes the
point that although there may be somewhat more positive associations
than negative associations, there is so much noise or variability in
the data that identifying which positive associations may be real
health effects and which are not is beyond the capability of current
methods. They cite the view that the CASAC Panel expressed in a June
2006 letter (Henderson, 2006b), noting that ``Because results of time-
series studies implicate all of the criteria pollutants, findings of
mortality time-series studies do not seem to allow us to confidently
attribute observed effects specifically to individual pollutants.''
Because of the importance of the O3 mortality multi-city
studies in EPA's analysis of this issue, several of these commenters
focused on them in particular, arguing that, although these studies
have the statistical power to distinguish weak relationships between
daily O3 and mortality, they do not provide reliable or
consistent evidence implicating O3 exposures as a cause of
mortality. Several reasons were given, including: (a) The multi-city
studies cited by EPA involve a wide range of city-specific effects
estimates, including some large cities that have very slight or
negligible effects (e.g., Los Angeles) (Bell et al., 2004), thus
causing several commenters to question the relevance of a ``national''
effect of O3 on mortality and argue that a single national
O3 concentration-mortality coefficient should be used and
interpreted with caution (Rochester Report p. 4); (b) the multi-city
mortality studies did not sufficiently account for other pollutants,
for example, Bell et al. (2004) adjusted for PM10 but did
not have the necessary air quality data to adequately adjust for
PM2.5, which EPA has concluded also causes mortality and is
correlated with O3, especially in the summer months
(Annapolis Center, p. 42); and (c) these studies contain several
findings that are inconsistent or implausible, such as premature
mortality reported at such low levels as to imply that O3-
related mortality is occurring at levels well within natural
background, which is not biologically plausible (Annapolis Center, p.
42).
Evidence supporting an association between short-term O3
exposure and premature mortality is not limited to multi-city time-
series studies. Most single-city studies show elevated risk of total,
non-accidental mortality, cardiorespiratory, and respiratory mortality
(> 20 studies), including one study in an area that would have met
current standard (Vedal et al., 2003). Three large meta-analyses, which
pool data from many single-city studies to increase statistical power,
reported statistically significant associations and examined sources of
heterogeneity in those associations (Bell et al., 2005; Ito et al.,
2005; Levy et al. 2005). These studies found: (1) Larger and more
significant effects in the warm season than in the cool season or all
year; (2) no strong evidence of confounding by PM; and (3) suggestive
evidence of publication bias, but significant associations remain even
after adjustment for the publication bias.
Moreover, EPA asserts that the biological plausibility of the
epidemiological mortality associations is generally supported by
controlled human exposure and toxicological evidence of respiratory
morbidity effects for levels at and below 0.080 ppm, but that
biological plausibility becomes increasingly uncertain especially below
0.060 ppm, the lowest level at which effects were observed in
controlled human exposure studies. Further, at lower levels, it becomes
increasingly
[[Page 16461]]
uncertain as to whether the reported associations are related to
O3 alone rather than to the broader mix of air pollutants
present in the ambient air. EPA agrees that the multi-city times series
studies evaluated in this review do not completely resolve this issue.
It also becomes increasingly uncertain as to whether effect thresholds
exist but cannot be clearly discerned by statistical analyses. Thus,
when considering the epidemiological evidence in light of the other
available information, it is reasonable to judge that at some point the
epidemiological associations cannot be interpreted with confidence as
providing evidence that the observed health effects can be attributed
to O3 alone.
In the letter cited, the CASAC Panel did raise the issue of the
utility of time-series studies in the standard setting process with
regard to time-series mortality studies. Nevertheless, in a subsequent
letter to the Administrator, CASAC noted these mortality studies as
evidence to support a recommendation to revise the current primary
O3 standard. ``Several new single-city studies and large
multi-city studies designed specifically to examine the effects of
ozone and other pollutants on both morbidity and mortality have
provided more evidence for adverse health effects at concentrations
lower than the current standard (Henderson, 2006c, p. 3).''
With regard to the specific issues raised in the comments as to why
the times-series mortality studies do not provide reliable or
consistent evidence implicating O3 exposure as a cause of
mortality, EPA has the following responses:
(a) The purpose of the NMMAPS approach is not to single out
individual city results but rather to estimate the overall effect from
the 95 communities. It was designed to provide a general, nationwide
estimate. With regard to the very slight or negligible effects
estimates for some large cities (e.g., Los Angeles), an important
factor to consider is that the Bell et al. (2004) study used all
available data in their analyses. Bell et al., reported that the effect
estimate for all available (including 55 cities with all year data) and
warm season (April-October) analyses for the 95 U.S. cities were
similar in magnitude; however, in most other studies, larger excess
mortality risks were reported in the summer season (generally June-
August when O3 concentrations are the highest) compared to
all year or the cold season. Though the effect estimate for Los Angeles
is small compared to some of the other communities, it should be noted
that all year data (combined warm and cool seasons) was used in the
analyses for this city, which likely resulted in a smaller effect
estimate. Because all year data was used for Los Angeles, the median
O3 concentration for Los Angeles is fairly low compared to
the other communities, ranked 23rd out of 95 communities. The median
24-hour average O3 concentration for Los Angeles in this
dataset was 22 ppb, with a 10th percentile of 8 ppb to a 90th
percentile of 38 ppb. The importance of seasonal differences in
O3-related health outcomes has been well documented.
(b) In section 7.4.6, O3 mortality risk estimates
adjusting for PM exposure, the Criteria Document states that the main
confounders of interest for O3, especially for the northeast
U.S., are ``summer haze-type'' pollutants such as acid aerosols and
sulfates. Since very few studies included these chemical measurements,
PM (especially PM2.5) data, may serve as surrogates.
However, due to the expected high correlation among the constituents of
the ``summer haze mix,'' multipollutant models including these
pollutants may result in unstable coefficients; and, therefore,
interpretation of such results requires some caution.
In this section, Figure 7-22 shows the O3 risk estimates
with and without adjustment for PM indices using all-year data in
studies that conducted two-pollutant analyses. Approximately half of
the O3 risk estimates increased slightly, whereas the other
half decreased slightly with the inclusion of PM in the models. In
general, the O3 mortality risk estimates were robust to
adjustment for PM in the models.
The U.S. 95 communities study by Bell et al. (2004) examined the
sensitivity of acute O3-mortality effects to potential
confounding by PM10. Restricting analysis to days when both
O3 and PM10 data were available, the community-
specific O3-mortality effect estimates as well as the
national average results indicated that O3 was robust to
adjustment for PM10 (Bell et al., 2004). As commenters
noted, there were insufficient data available to examine potential
confounding by PM2.5. One study (Lipfert et al., 2000)
reported O3 risk estimates with and without adjustment for
sulfate, a component of PM2.5. Lipfert et al. (2000)
calculated O3 risk estimates based on mean (45 ppb) less
background (not stated) levels of 1-hour max O3 in seven
counties in Pennsylvania and New Jersey. The O3 risk
estimate was not substantially affected by the addition of sulfate in
the model (3.2% versus 3.0% with sulfate) and remained statistically
significant.
Several O3 mortality studies examined the effect of
confounding by PM indices in different seasons (Figure 7-23, section
7.4.6, Criteria Document). In analyses using all-year data and warm-
season only data, O3 risk estimates were once again fairly
robust to adjustment for PM indices, with values showing both slight
increases and decreases with the inclusion of PM in the model. In the
analyses using cool season data only, the O3 risk estimates
all increased slightly with the adjustment of PM indices, although none
reached statistical significance.
The three recent meta-analyses (Bell et al., 2005; Ito et al.,
2005; Levy et al., 2005) all examined the influence of PM on
O3 risk estimates. No substantial influence was observed in
any of these studies. In the analysis by Bell et al. (2005), the
combined estimate without PM adjustment was 1.75% (95% PI: 1.10, 2.37)
from 41 estimates, and the combined estimate with PM adjustment was
1.95% (95% PI: -0.06, 4.00) from 11 estimates per 20 ppb increase in
24-hour average O3. In the meta-analysis of 15 cities by Ito
et al. (2005), the combined estimate was 1.6% (95% CI: 1.1, 2.2) and
1.5% (95% CI: 0.8, 2.2) per 20 ppb in 24-hour average O3
without and with PM adjustment, respectively. The additional time-
series analysis of six cities by Ito et al. found that the influence of
PM by season varied across alternative weather models but was never
substantial. Levy et al. (2005) examined the regression relationships
between O3 and PM indices (PM10 and
2.5) with O3-mortality effect estimates for all
year and by season. Positive slopes, which might indicate potential
confounding, were observed for PM2.5 on O3 risk
estimates in the summer and all-year periods, but the relationships
were weak. The effect of one causal variable (i.e., O3) is
expected to be overestimated when a second causal variable (e.g., PM)
is excluded from the analysis, if the two variables are positively
correlated and act in the same direction. However, EPA notes that the
results from these meta-analyses, as well as several single- and
multiple-city studies, indicate that copollutants, including PM,
generally do not appear to substantially confound the association
between O3 and mortality.
(c) With regard to the biological plausibility of O3-
related mortality occurring at levels well within natural background,
EPA concluded in the proposal that additional research is needed to
more fully establish underlying mechanisms by which mortality effects
occur (72 FR 37836). Such research would likely also help determine
whether it is plausible that mortality would occur at such low levels.
As noted above, the multi-city
[[Page 16462]]
times series studies evaluated in this review can not resolve the issue
of whether the reported associations at such low levels are related to
O3 alone rather than to the broader mix of air pollutants
present in the ambient air.
(5) ``New'' studies not included in the Criteria Document. Many
commenters identified ``new'' studies that were not included in the
Criteria Document that they stated support arguments both for and
against the revision of the current O3 standard. Commenters
who supported revising the current O3 standard identified
new studies that generally supported EPA's conclusions about the
associations between O3 exposure and a range of respiratory
and cardiovascular health outcomes. These commenters also identified
new studies that provide evidence for associations with health outcomes
that EPA has not linked to O3 exposure, such as cancer, and
populations that EPA has not identified as being susceptible or
vulnerable to O3 exposure, including African-American men
and women. Commenters who did not support revision of the current
O3 standard often submitted the same ``new'' studies, but
focused on different aspects of the findings. Commenters who did not
support revision of the current O3 standard stated that
these ``new'' studies provide inconsistent and sometimes conflicting
findings that do little to resolve uncertainties regarding whether
O3 has a causal role in the reported associations with
adverse health outcomes, including premature mortality and various
morbidity outcomes. More detail about the topic areas covered in the
``new'' studies can be found in the Response to Comments document.
To the extent that these commenters included ``new'' scientific
studies, studies that were published too late to be considered in the
Criteria Document, in support of their arguments for revising or not
revising the standards, EPA notes, as discussed in section I above,
that as in past NAAQS reviews, it is basing the final decisions in this
review on the studies and related information included in the
O3 air quality criteria that have undergone CASAC and public
review and will consider newly published studies for purposes of
decision making in the next O3 NAAQS review. In
provisionally evaluating commenters' arguments, as discussed in the
Response to Comments document, EPA notes that its provisional
consideration of ``new'' science found that such studies did not
materially change the conclusions in the Criteria Document.
iii. Evidence Pertaining to At-Risk Subgroups for O3-Related
Effects
This section contains major comments on EPA's assessment of the
body of evidence, including controlled human exposure and
epidemiological studies, related to the effects of O3
exposure on sensitive subpopulations. Since new information about the
increased responsiveness of people with lung disease, especially
children and adults with asthma, was an important consideration in the
Administrator's proposed decision that the current O3
standard is not adequate, many of the comments focused on this
information and the conclusions drawn from it. There were also comments
on other sensitive groups identified by EPA, as well as comments
suggesting that additional groups should be considered at increased
risk from O3 exposure. Many of the issues discussed below,
as well as other related issues, are addressed in more detail in the
Response to Comments document.
As with the comments on controlled human exposure and
epidemiological studies, upon which judgments about sensitive
subpopulations were based, the comments about EPA's delineation of
these groups were highly polarized. In general, one group of commenters
who supported revising the current O3 primary standard,
including medical associations, public health and environmental groups,
agreed in part with EPA's assessment of the subpopulations that are at
increased risk from O3 exposure, but commented that there
are additional groups that need to be considered. A comment from ATS,
AMA and other medical associations noted:
Within this population exists a number of individuals uniquely
at much higher risk for adverse health effects from ozone exposures,
including children, people with respiratory illness, the elderly,
outdoor workers and healthy children and adults who exercise
outdoors. [ATS, p. 2]
These commenters agreed with EPA that, based on evidence from
controlled human exposure and epidemiology studies, people with asthma,
especially children, are likely to have greater lung function
decrements and respiratory symptoms in response to O3
exposure than people who do not have asthma, and are likely to respond
at lower levels. Because of this, these commenters make the point that
controlled human exposure studies that employ healthy subjects will
underestimate the effects of O3 exposures in people with
asthma.
These commenters agreed with EPA's assessment that epidemiological
studies provide evidence of increased morbidity effects, including lung
function decrements, respiratory symptoms, emergency department visits
and hospital admissions, in people with asthma and that controlled
human exposure studies provide biological plausibility for these
morbidity outcomes. Further, the Rochester Report, funded by API,
evaluated some of the same the studies that EPA did and found similar
results with regard to the increased inflammatory responses and
increased airway responsiveness of people with asthma when exposed to
O3. The Rochester Report reached the same conclusion that
EPA did, that this increased responsiveness provides biological
plausibility for the respiratory morbidity effects found in
epidemiological studies.
Several new studies have demonstrated that exposure of
individuals with atopic asthma to sufficient levels of ozone
produces an increase in specific airway responsiveness to inhaled
allergens* * * These findings, in combination with previously
observed effects of ozone on nonspecific airway responsiveness and
airway inflammation, supports the idea that ambient ozone exposure
could result in exacerbation of asthma several days following
exposure, and provides biological plausibility for the epidemiologic
studies in which ambient ozone concentration has been associated
with increased asthma symptoms, medication use, emergency room
visits, and hospitalizations for asthma. [Rochester Report, pp. 57-
58]
Commenters also often mentioned the increased susceptibility of
people with COPD, and in this case cited new studies not considered in
the Criteria Document.
They identify one potentially susceptible subpopulation that EPA
did not focus on in the proposal is infants. Commenters from medical
associations, and environmental and public health groups expressed the
view that O3 exposure can have important effects on infants,
including reduced birth weight, pre-term birth, and increased
respiratory morbidity effects in infants. Exposure to O3
during pregnancy, especially during the second and third trimesters,
was associated with reduced birth weight in full-term infants. Although
this effect was noted at relatively low O3 exposure levels,
the ATS notes that, ``* * * the reduced birth weight in infants in the
highest ozone exposures communities equaled the reduced birth weight
observed in pregnant women who smoke'' (ATS, p. 7).
In general, EPA agrees with comments that there is very strong
evidence from controlled human exposure and epidemiological studies
that people with lung disease, especially children and adults with
asthma, are susceptible to O3 exposure and are likely to
[[Page 16463]]
experience more serious effects than those people who do not have lung
disease. This means that controlled human exposure studies that employ
subjects who do not have lung disease will likely underestimate effects
in those people that do have asthma or other lung diseases.
In summarizing the epidemiological evidence related to birth-
related health outcomes, the Criteria Document (p. 7-133) concludes
that O3 was not an important predictor of several birth-
related outcomes including premature births and low birth weight.
Birth-related outcomes generally appeared to be associated with air
pollutants that tend to peak in the winter and are possibly traffic-
related. However, given that most of these studies did not analyze the
data by season, seasonal confounding may have therefore influenced the
reported associations. One study reported some results suggestive of
associations between exposures to O3 in the second month of
pregnancy and birth defects, but further evaluation of such potential
associations is needed. With regard to comments about effect in
infants, EPA notes that some of the studies cited by commenters were
not considered in the Criteria Document. More detailed responses to
studies submitted by commenters but not considered in the Criteria
Document can be found in the Response to Comments document.
The second group of commenters, mostly representing industry
associations and some businesses opposed to revising the primary
O3 standard, asserted that EPA is wrong to claim that new
evidence indicates that the current standard does not provide adequate
health public health protection for people with asthma. In support of
this position, these commenters made the following major comments: (1)
Lung function decrements and respiratory symptoms observed in
controlled human exposure studies of asthmatics are not clinically
important; (2) EPA postulates that asthmatics would likely experience
more serious responses and responses at lower levels than the subjects
of controlled human exposure experiments, but that hypothesis is not
supported by scientific evidence; and, (3) EPA recognized asthmatics as
a sensitive subpopulation in 1997, and new information does not suggest
greater susceptibility than was previously believed.
With regard to the first point, these commenters expressed the view
that asthmatics are not likely to experience medically significant lung
function changes or respiratory symptoms at ambient O3
concentrations at or even above the level of the current standard. Many
of these commenters cited the opinion of one physician who was asked on
behalf of a group of trade associations and companies to provide his
views on the health significance for asthmatics of the types of
responses that have been reported in controlled human exposure studies
of O3. This commenter (McFadden) reviewed earlier controlled
human exposure studies of asthmatics (from the last review) as well as
the recent controlled human exposure studies of healthy individuals
(Adams 2002, 2003a,b, and 2006) at 0.12, 0.08, 0.06, and 0.04 ppm and
expressed the view that ``* * * these studies on asthmatics indicate
that ozone exposures at ~ 0.12 ppm do not produce medically significant
functional changes and are right around the inflection point where one
begins to see an increase in symptoms; however, that increase is
small'' (McFadden, p. 3). This commenter went on to express the view
that responses to O3 exposure at levels < 0 .08 ppm would be
even less and that the available data are not sufficiently robust to
indicate that such exposures would present a significant health concern
even to sensitive people like asthmatics.
EPA notes that this commenter based his comment on the group mean
functional and respiratory symptom changes in the studies he reviewed.
EPA agrees that group mean changes at these levels are relatively small
and has described them as such in both the previous review and this one
(72 FR 37828). The importance of group mean changes is to evaluate the
statistical significance of the association between the exposures and
the observed effects, to try to determine if the observed effects are
likely due to O3 exposure rather than chance. In the
previous review as well as in this one, EPA has also focused on the
fact that some individuals experience more severe effects that may be
clinically significant. With regard to the significance of individual
responses, this commenter (McFadden, p. 2) states ``* * * transient
decreases in FEV1 of 10-20% are not by themselves
significant or meaningful to asthmatics* * *. It has been my experience
from examining and studying thousands of patients for both clinical and
research purposes that asthmatics typically will not begin to sense
bronchoconstriction until their FEV1 falls about 50% from
normal.'' EPA strongly disagrees with this assessment. As stated in the
Criteria Document (Table 8-3, p. 8-68) for people with lung disease,
even moderate functional responses (e.g., FEV1 decrements >=
10% but < 20%) would likely interfere with normal activities for many
individuals, and would likely result in more frequent medication use.
EPA notes that in the context of standard setting, CASAC indicated
(Henderson, 2006c) that a focus on the lower end of the range of
moderate functional responses (e.g., FEV1 decrements >= 10%)
is most appropriate for estimating potentially adverse lung function
decrements in people with lung disease.
With regard to the second point, whether asthmatics would likely
experience more serious responses and responses at lower levels than
the subjects of controlled human exposure experiments and EPA's
discussion of the relationship of increased airway responsiveness and
inflammation experienced by asthmatics to exacerbation of asthma, this
commenter stated that ``there simply are no data to support the
sequence described'' and that ``the assumption that these responses
would lead to clinical manifestations in terms of exacerbations of
asthma or other adverse health effects remains unproven theory''
(McFadden, p. 3).
In these sections of the proposal (72 FR 37826 and 37846-37847),
EPA describes the evidence indicating that people with asthma are as
sensitive as, if not more sensitive than, normal subjects in
manifesting O3-induced pulmonary function decrements.
Controlled human exposure studies show that asthmatics present a
differential response profile for cellular, molecular, and biochemical
parameters that are altered in response to acute O3
exposure. Asthmatics have greater O3-induced inflammatory
responses and increased O3-induced airway responsiveness
(both incidence and duration) that could have important clinical
implications.
There are two ways to interpret these comments. One way to
interpret them is that because these controlled human exposure studies
have not produced exacerbations of asthma in study subjects resulting
in the need for medical attention, there are no data to support the
clinical significance of the results. EPA rejects this interpretation
because it would be unethical to knowingly conduct a controlled human
exposure study that would lead to exacerbation of asthma. Controlled
human exposure studies are specifically designed to avoid these types
of responses. The other interpretation is that the commenter does not
agree that the differences in lung function, inflammation and increased
airway responsiveness found in these
[[Page 16464]]
controlled human exposure studies support the inference that asthmatics
are likely to have more serious responses than healthy subjects, and
that these responses could have important clinical implications. EPA
rejects this interpretation as well. EPA did not base its increased
concern for asthmatics solely on the results of the controlled human
exposure studies, but has appropriately used a weight of evidence
approach, integrating evidence from animal toxicological, controlled
human exposure and epidemiological studies as a basis for this concern.
The Criteria Document concludes that the positive and robust
epidemiological associations between O3 exposure and
emergency department visits and hospitalizations in the warm season are
supported by the human clinical, animal toxicological and
epidemiological evidence for lung function decrements, increased
respiratory symptoms, airway inflammation, and increased airway
responsiveness (72 FR 37832). The CASAC Panel itself expressed the view
that people with asthma, especially children, have been found to be
more sensitive to O3 exposure, and indicated that EPA should
place more weight on inflammatory responses and serious morbidity
effects, such as increased respiratory-related emergency department
visits and hospitalizations (Henderson, p. 4). Moreover, the Rochester
Report, cited above, reaches essentially the same conclusions as EPA
did, that the evidence from controlled human exposure studies provides
biological plausibility for the epidemiological studies in which
ambient O3 concentrations have been associated with
increased asthma symptoms, medication use, emergency room visits, and
hospitalizations for asthma. Therefore, EPA continues to assert that
there is strong evidence that asthmatics likely have more serious
responses to O3 exposure than people without asthma, and
that these responses have the potential to lead to exacerbation of
asthma as indicated by the serious morbidity effects, such as increased
respiratory-related emergency department visits and hospitalizations
found in epidemiological studies.
With regard to the third point, commenters expressed the view that
there is no significant new evidence establishing greater risk to
asthmatics than was accepted in 1997, when EPA concluded that the
existing NAAQS was sufficiently stringent to protect public health--
including asthmatics--with an adequate margin of safety (UARG, pp. 22-
23). To support this view, these commenters noted the points made above
and expressed the view that epidemiological studies of asthmatics that
provide new evidence of respiratory symptoms and medication use in
asthmatic children are subject to the limitations of epidemiological
studies discussed above (e.g., confounding by co-pollutants,
heterogeneity of results). In addition, these commenters identified a
new, large multi-city panel study, not included in the Criteria
Document, by Schildcrout et al. (2006), which the commenters
characterize as reporting no association between O3
concentrations and exacerbation of asthma.
At the time of the last review, EPA concluded that people with
asthma were at greater risk because the impact of O3-induced
responses on already-compromised respiratory systems would noticeably
impair an individual's ability to engage in normal activity or would be
more likely to result in increased self-medication or medical
treatment. At that time there was little evidence that people with pre-
existing disease were more responsive than healthy individuals in terms
of the magnitude of pulmonary function decrements or symptomatic
responses. The new results from controlled exposure and epidemiologic
studies indicate that individuals with preexisting lung disease,
especially people with asthma, are likely to have more serious
responses than people who do not have lung disease and therefore are at
greater risk for O3 health effects than previously judged in
the 1997 review. EPA notes that comments on the limitations of
epidemiological studies and evidence from ``new'' studies (not in the
Criteria Document) have been addressed above. As with other ``new''
studies, this study by Schildcrout et al. (2006) is specifically
discussed in the Response to Comments document.
b. Consideration of Human Exposure and Health Risk Assessments
Section II.A.3 above provides a summary overview of the exposure
and risk assessment information used by the Administrator to inform
judgments about exposure and health risk estimates associated with
attainment of the current and alternative standards. EPA notes here
that most of the issues and concerns raised by commenters concerning
the methods used in the exposure and risk assessments are essentially
restatements of concerns raised during the review of the Criteria
Document and the development and review of these quantitative
assessments as part of the preparation and review of the Staff Paper
and the associated analyses. EPA presented and the CASAC Panel reviewed
in detail the approaches used to assess exposure and health risk, the
studies and health effect categories selected for which exposure-
response and concentration-response relationships were estimated, and
the presentation of the exposure and risk results summarized in the
Staff Paper. As stated in the proposal notice, EPA believes and CASAC
Panel concurred, that the model selected to estimate exposure represent
the state of the art and that the risk assessment was ``well done,
balanced and reasonably communicated'' and that the selection of health
endpoints for inclusion in the quantitative risk assessment was
appropriate (Henderson, 2006c). EPA does not believe that the exposure
or risk assessments are fundamentally biased in one direction or the
other as claimed in some of the comments.
Comments received after proposal related to the development of
exposure and health risk assessments, interpretation of exposure and
risk results, and the role of the quantitative human exposure and
health risk assessments in considering the need to revise the current
8-hour O3 standard generally fell into two groups. One group
of commenters that included national environmental and public health
organizations (e.g., joint set of comments by ALA and several
environmental groups including Environmental Defense and Sierra Club),
NESCAUM, and some State and local health and air pollution agencies
argued that the exposure and health risk assessments underestimated
exposure and risks for several reasons including: (1) The geographic
scope was limited to at most only 12 urban areas and thus
underestimates national public health impacts due to exposures to
O3; (2) the assessments did not include all relevant at risk
population groups and excluded populations such as pre-school children,
outdoor workers, adults who exercise outdoors; and (3) the risk
assessment did not include all of the health effect endpoints for which
there is evidence that there are O3-related health effects
(e.g., increased medicine use by asthmatics, lung function decrements
and respiratory symptoms in adults, increased doctors' visits,
emergency department visits, school absences, inflammation, and
decreased resistance to infection among children and adults); and (4)
EPA's exposure assessment underestimates exposures since it considers
average children, not active children who spend more time outdoors and
repeated exposures are also underestimated. The joint set of
[[Page 16465]]
comments from ALA and several environmental groups contended that the
``exposures of concern'' metric presented in the Staff Paper and
proposal is ``an inappropriate basis for decisionmaking'' and urged EPA
to set the standard based on the concentrations shown by health studies
to cause adverse effects, not on how much O3 Americans
inhale. This same set of commenters stated that if exposures of concern
were to be considered then the benchmark level of 0.060 ppm should be
the focus, and not higher benchmark levels. These same commenters also
stated that EPA should have estimated and considered total risk without
excluding risks associated with PRB levels because there is no rational
basis for excluding natural and anthropogenic sources from outside
North America and that the NAAQS must protect against total exposure.
While disagreeing with EPA's approach of estimating risks only above
PRB, these same commenters supported the use of the GEOS-CHEM model as
the ``best tool available to derive background concentrations'' should
EPA continue to pursue this approach. These comments are discussed in
turn below.
EPA agrees that the exposure and health risk assessments are
limited to certain urban areas and do not capture all of the
populations at risk for O3-related effects, and that the
risk assessment does not include all potential O3-related
health effects. The criteria and rationale for selecting the
populations and health outcomes included in the quantitative
assessments were presented in the draft Health Assessment Plan, Staff
Paper, and technical support documents for the exposure and health risk
assessments that were reviewed by the CASAC Panel and the public. The
CASAC Panel indicated in its letter that the health outcomes included
in the quantitative risk assessment were appropriate, while recognizing
that other health outcomes such as emergency department visits and
increased doctors' visits should be addressed qualitatively (Henderson,
2006c). The Staff Paper (and the CASAC Panel) clearly recognized that
the exposure and risk analyses could not provide a full picture of the
O3 exposures and O3-related health risks posed
nationally. The proposal notice made note of this important point and
stated that ``national-scale public health impacts of ambient
O3 exposures are clearly much larger than the quantitative
estimates of O3-related incidences of adverse health effects
and the numbers of children likely to experience exposures of concern
associated with recent air quality or air quality that just meets the
current or alternative standards'' (72 FR 37866).
However, as stated in the proposal notice, EPA also recognizes that
inter-individual variability in responsiveness to O3 shown
in controlled human exposure studies for a variety of effects means
that only a subset of individuals in any population group estimated to
experience exposures exceeding a given benchmark exposure of concern
level would actually be expected to experience such adverse health
effects. The Administrator continues to recognize that there is a
broader array of O3-related adverse health outcomes for
which risk estimates could not be quantified (that are part of a
broader ``pyramid of effects'') and that the scope of the assessment
was limited to just a sample of urban areas and to some but not all at-
risk populations, leading to an incomplete estimation of public health
impacts associated with O3 exposures across the country. The
Administrator is fully mindful of these limitations, along with the
uncertainties in these estimates, in reaching his conclusion that
observations from the exposure and health risk assessments provide
additional support for his judgment that the current 8-hour standard
does not protect public health with an adequate margin of safety and
must be revised. For reasons discussed below in section II.C.4,
however, the Administrator disagrees with aspects of these commenters'
views on the level of the standard that is appropriate and supported by
the available health effects evidence and quantitative assessments
associated with just meeting alternative standards.
EPA does not agree that consideration of exposure estimates is not
permitted or is somehow inappropriate in decisions concerning the
primary standard. EPA has considered population exposure estimates as a
consideration in prior NAAQS review decisions, including the 1997
revision of the O3 primary standard and the 1994 decision on
the carbon monoxide (CO) standard. As indicated in the proposal,
estimating exposures of concern is important because it provides some
indication of potential public health impacts of a range of
O3-related health outcomes, such as lung inflammation,
increased airway responsiveness, and changes in host defenses. These
particular health effects have been demonstrated to occur in some
individuals in controlled human exposure studies at levels as low as
0.080 ppm O3 but have not been evaluated at lower levels.
While there is very limited evidence addressing lung function and
respiratory symptom responses at 0.060 ppm, this evidence does not
address these other health effects.
As noted in the proposal, EPA emphasized that although the analysis
of ``exposures of concern'' was conducted using three discrete
benchmark levels (0.080, 0.070, 0.060 ppm), the concept was more
appropriately viewed as a continuum, with greater confidence and less
uncertainty about the existence of health effects at the upper end and
less confidence and greater uncertainty as one considers increasingly
lower O3 exposure levels. EPA recognized that there was no
sharp breakpoint within the continuum ranging from at and above 0.080
ppm down to 0.060 ppm. In considering the concept of exposures of
concern, the proposal noted that it was important to balance concerns
about the potential for health effects and their severity with the
increasing uncertainty associated with our understanding of the
likelihood of such effects at lower levels.
As noted above, environmental and public health group comments
expressed the view that if exposures of concern were considered, then
the Administrator should focus only on the 0.060 ppm benchmark based on
the contention that adverse health effects had been demonstrated down
to this level. In contrast, other commenters, primarily industry and
business groups focused on comparisons of the exposures of concern at
the 0.080 ppm benchmark level based on their view that there was no
convincing evidence demonstrating adverse health effects at levels
below this benchmark. In view of the comments received related to the
definition and use of the term ``exposure of concern'' at the time of
proposal, the Administrator recognizes that that there is a risk for
confusion, as it could be read to imply a determination that a certain
benchmark level of exposure has been shown to be causally associated
with adverse health effects. As a consequence, the Administrator
believes that it is more appropriate to consider such exposure
estimates in the context of a continuum rather than focusing on any one
discrete benchmark level, as was done at the time of proposal, since
the Administrator does not believe that the underlying scientific
evidence is certain enough to support a focus on any single bright-line
benchmark level. Thus, the Administrator believes it is appropriate to
consider a range of benchmark levels from 0.080 down to 0.060 ppm,
recognizing that exposures of concern must be considered in the
[[Page 16466]]
context of a continuum of the potential for health effects of concern,
and their severity, with increasing uncertainty associated with the
likelihood of such effects at lower O3 exposure levels.
EPA recognizes that the 0.080 ppm benchmark level represents a
level at which several health outcomes including lung inflammation,
increased airway responsiveness, and decreased resistance to infection
have been shown to occur in healthy adults. The Administrator places
relatively great weight on the public health significance of exposures
at and above this benchmark level given these physiological effects
measured in healthy adults at O3 exposures of 0.080 ppm and
the evidence from controlled human exposure studies showing that people
with asthma have more serious responses than people without asthma.
However, the Administrator does not agree with those commenters who
would only consider this single benchmark level. While the
Administrator places less weight on exposures at and above the 0.070 pm
benchmark level, given the increased uncertainty about the fraction of
the population and severity of the health responses that might occur
associated with exposures at and above this level, he believes that it
is appropriate to consider exposures at and above this benchmark as
well in judging the adequacy of the current standard to protect public
health. Considering exposures at and above the 0.070 ppm benchmark
level provides some consideration for the fact that the effects
observed at 0.080 ppm were in healthy adult subjects but sensitive
population groups such as asthmatics are likely to respond at lower
O3 levels than healthy individuals. The Administrator
considered but placed very little weight on exposures at and above the
0.060 ppm benchmark given the very limited scientific evidence
supporting a conclusion that O3 is causally related to
various health outcomes at this exposure level.
EPA does not agree that it is inappropriate or impermissible to
assess risks that are in excess of PRB or that EPA must focus on total
risks when using a risk assessment to inform decisions on the primary
standard. Consistent with the approach used in the risk assessment for
the prior O3 standard review and consistent with the
approach used in risk assessments for other prior NAAQS reviews,
estimating risks in excess of PRB is judged to be more relevant to
policy decisions regarding the ambient air quality standard than risk
estimates that include effects potentially attributable to
uncontrollable background O3 concentrations. EPA also notes
that with respect to the adequacy of the current standard taking total
risks into account would not impact the Administrator's decision, since
he judges that the current standard is not adequate even when risks in
excess of current PRB estimates are considered. In addition, EPA notes
that consideration of the evidence itself, as well as exposures at and
above benchmark levels in the range of 0.060 to 0.080 ppm, are not
impacted at all by consideration of current PRB estimates.
EPA does agree with the ALA and environmental groups comment that
the GEOS-CHEM model represents the best tool currently available to
estimate PRB as recognized in the Criteria Document evaluation of this
issue and the CASAC Panel support expressed during the review of the
Criteria Document.
The second group of commenters mostly representing industry
associations, businesses, and some State and local officials opposed to
revising the 8-hour standard, and most extensively presented in
comments from UARG, API, Exxon-Mobil, AAM, and NAM, raised one or more
of the following concerns: (1) That exposures of concern and health
risk estimates have not changed significantly since the prior review in
1997; (2) that uncertainties and limitations underlying the exposure
and risk assessments make them too speculative to be used in supporting
a decision to revise the standard; (3) that EPA should have defined PRB
differently and that EPA underestimated PRB levels which results in
health risk reductions associated with more stringent standards being
overestimated; (4) that exposures are overestimated based on specific
methodological choices made by EPA including, for example,
O3 measurements at fixed-site monitors can be higher than
other locations where individuals are exposed, the exposure estimates
do not account for O3 avoidance behaviors, and the exposure
model overestimates elevated breathing rates; and (5) that health risks
are overestimated based on specific methodological choices made by EPA
including, for example, selection of inappropriate effect estimates
from health effect studies and EPA's approach to addressing the shape
of exposure-response relationships and whether or not to incorporate
thresholds into its models for the various health effects analyzed.
These comments are discussed in turn below. Additional detailed
comments related to the development, presentation, and interpretation
of EPA's exposure and health risk assessments, along with EPA's
responses to the specific issues raised by these commenters can be
found in the Response to Comments document.
(1) In asserting that the estimated exposures and risks associated
with air quality just meeting the current standard have not appreciably
changed since the prior review, comments from Exxon-Mobil, the
Annapolis Center and others have compared results of EPA's lung
function risk assessment done in the last review with those from the
Agency's risk assessment done as part of this review and have concluded
that lung function risks upon attainment of the current O3
standard are below those that were predicted in 1997 and that
uncertainties about other health effects based on epidemiological
studies remain the same. These commenters used this conclusion as the
basis for a claim that there is no reason to depart from the
Administrator's 1997 decision that the current 8-hour standard is
requisite to protect public health.
EPA believes that this claim is fundamentally flawed for three
reasons, as discussed in turn below: (i) It is factually inappropriate
to compare the quantitative risks estimated in 1997 with those
estimated in the current rulemaking; (ii) it fails to take into account
that with similar risks, increased certainty in the risks presented by
O3 implies greater concern than in the last review, and
(iii) it fails to recognize that the Administrator has used these
estimates in a supportive role, in light of significant uncertainties
in the exposure and risk estimates, to inform the conclusions drawn
primarily from integrative assessment of the controlled human exposure
and epidemiological evidence on whether ambient O3 levels
allowed under the current standard present a serious public health
problem warranting revision of the O3 standard.
With respect to the first point, the 1997 risk estimates, or any
comparison of the 1997 risk estimates to the current estimates, are
irrelevant for the purpose of judging the adequacy of the current 8-
hour standard, as the 1997 estimates reflect outdated analyses that
have been updated in this review to reflect the current science. Just
comparing the results for lung function decrements ignores these
differences. In particular, as discussed in section 4.6.1 of the Staff
Paper, there have been significant improvements to the exposure model
and the model inputs since the last review that make comparisons
inappropriate between the prior and current review. For example, the
geographic areas modeled are larger
[[Page 16467]]
than in the previous review and when modeling a larger area, extending
well beyond the urban core, there will be more people exposed, but a
smaller percentage of the modeled population will be exposed at high
levels, if O3 concentrations are lower in the extended
areas. In the prior review, only typical years, in terms of
O3 air quality were modeled, while the current review used
the most recent three-year period (i.e., 2002-2004). Also, the prior
review estimated exposures for children who spent more time outdoors,
while the assessment for the current review included all school age and
all asthmatic school age children. Therefore, the population groups
examined in the exposure assessment are different between those
considered in the 1997 and current review, making comparison of the
resulting estimates inappropriate. Another important difference making
comparison between the 1997 health risk assessment and the current
assessment inappropriate is that a number of additional health effects
were included in the current review (e.g., respiratory symptoms in
moderate/severe asthmatic children, non-accidental and
cardiorespiratory mortality) based on health effects observed in
epidemiological studies that were not included in the risk assessment
for the prior review. These commenters only compare the risk estimates
with respect to lung function decrement, and fail to account for
differences in additional and more severe health endpoints not covered
in the 1997 assessment, as well as the fact that there are somewhat
different and more urban areas included in the current assessment.
Second, it is important to take into account EPA's increased level
of confidence in the associations between short-term O3
exposures and morbidity and mortality effects. In comparing the
scientific understanding of the risk presented by exposure to
O3 between the last and current reviews, one must examine
not only the quantitative estimate of risk from those exposures (e.g.
the numbers of increased hospital admissions at various levels) but
also the degree of confidence that the Agency has that the observed
health effects are causally linked to O3 exposure at those
levels. As documented in the Criteria Document and the recommendations
and conclusions of CASAC, EPA recognizes significant advances in our
understanding of the health effects of O3 based on new
epidemiological studies, new human and animal studies documenting
effects, new laboratory studies identifying and investigating
biological mechanisms of O3 toxicity, and new studies
addressing the utility of using ambient monitors to assess population
exposures to ambient O3. As a result of these advances, EPA
is now more certain that ambient O3 presents a significant
risk to public health at levels at or above the range of levels that
the Agency had considered for these standards in 1997. From this more
comprehensive perspective, since the risks presented by O3
are more certain and the current quantitative risk estimates include
additional important health effects, O3-related risks for a
wider range of health effects are now of greater concern at the current
level of the standard than in the last review.
Third, quantitative risk estimates were not the only basis for
EPA's decision in setting a level for the O3 standard in
1997, and they do not set any quantified ``benchmark'' for the Agency's
decision to revise the O3 standard at this time. While EPA
believes that confidence in the causal relationships between short-term
exposures to O3 and various health effects reported in
epidemiological studies has increased markedly since 1997, the
Administrator also recognizes that the risk estimates for these effects
must be considered in the light of uncertainties about whether or not
these O3-related effects occur at very low O3
concentrations. The Administrator continues to believe that the
exposure and risk estimates associated with just meeting the current
standard discussed in the Staff Paper and summarized in the proposal
notice are important from a public health perspective and are
indicative of potential exposures and risks to at-risk groups. In
considering the exposure and risk estimates, the Administrator has
considered the year-to-year and city-to-city variability in both the
exposure and risk estimates, the uncertainties in these estimates, and
recognition that there is a broader array of O3-related
adverse health outcomes for which risk estimates could not be
quantified (that are part of a broader ``pyramid of effects'') and that
the scope of the assessment was limited to just a sample of urban areas
and to some, but not all, at-risk populations, leading to an incomplete
estimation of public health impacts associated with O3
exposures across the country.
(2) In asserting that uncertainties and limitations associated with
the exposure and health risk assessments make them too speculative to
be used in supporting a decision to revise the standard, comments from
industry associations and others cited a number of issues including:
(i) Uncertainties about the air quality adjustment approach used to
simulate just meeting the current and alternative standards; (ii)
uncertainties and limitations associated with the definition and
estimation of PRB concentrations; (iii) uncertainties about whether the
respiratory symptoms, hospital admissions, and non-accidental and
cardiorespiratory mortality effects included in the health risk
assessment are actually causally related to ambient O3
concentrations, particularly at levels well below the current standard;
and (iv) uncertainties about the shape of the exposure-response
relationships for lung function responses and concentration-response
relationships for the health effects based on findings from
epidemiological studies and the assumption of a linear non-threshold
relationship for these responses. In summary, these commenters contend
that the substantial uncertainties present in the exposure and risk
assessments preclude the Administrator from using any of the results to
support a conclusion that the current 8-hour standard does not
adequately protect public health.
Several of the issues raised, including whether EPA's judgments
about causality for the effects included in the risk assessment are
appropriate, the shape of concentration-response relationships, and use
of a linear non-threshold relationship for the health outcomes based on
the epidemiological evidence, have been discussed in the previous
section on health effects evidence. Concerns expressed about the
definition and estimation of PRB levels for O3 and the role
of PRB in the risk assessment are addressed as a separate item below.
These issues also are addressed in more detail in the Response to
Comments document.
With respect to the air quality adjustment approach used in the
current review to simulate air quality just meeting the current and
alternative O3 standards, as discussed in the Staff Paper
(section 4.5.6) and in more detail in a staff memorandum (Rizzo, 2006),
EPA concluded that the quadratic air quality adjustment approach
generally best represented the pattern of reductions across the
O3 air quality distribution observed over the last decade in
areas implementing control programs designed to attain the
O3 NAAQS. While EPA recognizes that future changes in air
quality distributions are area-specific, and will be affected by
whatever specific control strategies are implemented in the future to
attain a revised NAAQS, there is no empirical evidence to suggest that
future reductions in ambient O3 will be significantly
different from past
[[Page 16468]]
reductions with respect to impacting the overall shape of the
O3 distribution.
As discussed in the proposal notice, EPA recognizes that the
exposure and health risk assessments necessarily contain many sources
of uncertainty including those noted by these commenters, and EPA has
accounted for such uncertainties to the extent possible. EPA developed
and presented an uncertainty analysis addressing the most significant
uncertainties affecting the exposure estimates. With respect to the
health risk assessment, EPA conducted and presented sensitivity
analyses addressing the impact on risk estimates of different
assumptions about the shape of the exposure-response relationship for
lung function decrements and alternative assumptions about PRB levels.
EPA notes that most of the comments summarized above concerning
limitations and uncertainties in these assessments are essentially
restatements of concerns raised during the development and review of
these quantitative assessments as part of the preparation and review of
the Staff Paper and assessments. The CASAC Panel reviewed in detail the
approaches used to assess exposure and health risks and the
presentation of the results in the Staff Paper. EPA believes, and the
CASAC Panel concurred, that the model used to estimate exposures
represents a state-of-the-art approach and that ``there is an explicit
discussion of the limitations of the APEX model in terms of variability
and quality of the input data, which is appropriate and fine''
(Henderson, 2006c, p. 11). The CASAC Panel also found the risk chapter
in the Staff Paper and the risk assessment ``to be well done, balanced,
and reasonably communicated'' (Henderson, 2006c, p. 12). Although EPA
agrees that important limitations and uncertainties remain, and that
future research directed toward addressing these uncertainties is
warranted, EPA believes that overall uncertainties about population
exposure and possible health risks associated with short-term
O3 exposure have diminished since the last review. The
Administrator has carefully considered the limitations and
uncertainties associated with these quantitative assessments but
continues to believe that they provide general support for concluding
that exposures and health risks associated with meeting the current 8-
hour standard are important from a public health perspective and that
the 8-hour standard needs to be revised to provide additional
protection in order to protect public health with an adequate margin of
safety.
(3) Comments from several industry organizations, businesses, and
others related to PRB included: (i) That EPA should have defined PRB
differently so as to include anthropogenic emissions from Canada and
Mexico; (ii) that EPA underestimated PRB levels by relying on estimates
from the GEOS-CHEM model using 2001 meteorology and EPA should instead
rely on O3 levels observed at remote monitoring locations or
sites that represent PRB conditions; and (iii) that the use of
underestimated PRB levels in the risk assessment results in
overestimated health risks associated with air quality just meeting the
current standard. Finally, some commenters cited concerns expressed by
the CASAC Panel that ``the current approach to determining PRB is the
best method to make this estimation'' (Henderson, 2007, p. 2). Each of
these concerns is addressed below and in more detail in the Response to
Comments document.
First, the U.S. government has influence over emissions at our
borders that affect ambient O3 concentrations entering the
U.S. from Canada and Mexico through either regulations or international
agreements, and therefore EPA does not agree that these emissions are
uncontrollable. PRB is designed to identify O3 levels that
result from emissions that are considered uncontrollable because the
U.S. has little if any influence on their control, and in that context
anthropogenic emissions from Mexico or Canada should be excluded from
PRB. EPA has consistently defined PRB as excluding anthropogenic
emissions from Canada and Mexico in NAAQS reviews over more than two
decades and sees no basis in the comments to alter this definition.
Second, the criticisms raised concerning the use of a modeling
approach (GEOS-CHEM using 2001 meteorology) and the alternative
approach of using remote monitoring data to estimate PRB were
considered by EPA's scientific staff and the CASAC Panel during the
course of reviewing the Criteria Document. Both EPA's experts and CASAC
endorsed the use of the peer-reviewed, thoroughly evaluated modeling
approach (GEOS-CHEM) described in the Criteria Document as the best
current approach for estimating PRB levels. The Criteria Document
reviewed detailed evaluations of GEOS-CHEM with O3
observations at U.S. surface sites (Fiore et al., 2002, 2003) and
comparisons of GEOS-CHEM predictions with observations at Trinidad
Head, CA (Goldstein et al., 2004) and found no significant differences
between the model predictions and observations for all conditions,
including those reflecting those given in the current PRB definition.
The Criteria Document states that the current model estimates indicate
that PRB in the U.S. is generally 0.015 to 0.035 ppm that declines from
spring to summer and is generally < 0.025 ppm under conditions
conducive to high O3 episodes. The Criteria Document
acknowledges that PRB can be higher, especially at elevated sites in
the spring due to stratospheric exchange. However, unusually high
springtime O3 episodes tied to stratospheric intrusion are
rare and generally occur at elevated locations and these can be readily
identified and excluded under EPA's exceptional events rule (72 FR
13560) to avoid any impact on attainment/non-attainment status of an
area.
Third, many of the commenters who raised the concern that EPA's
estimates of PRB were too low and had the impact of exaggerating the
risks associated with the current standard ignored the fact that the
risk assessment included a sensitivity analysis which showed the
potential impact of both lower and higher estimates of PRB or only
focused on the impact of higher estimates of PRB. The choices of lower
and higher estimates of PRB included in the risk assessment sensitivity
analyses were based on the peer-reviewed evaluation of the accuracy of
GEOS-CHEM model. The Criteria Document states ``in conclusion, we
estimate that the PRB O3 values reported by Fiore et al.
(2003) for afternoon surface air over the United States are likely 10
parts per billion by volume (ppbv) too high in the southeast in summer,
and accurate within 5 ppbv in other regions and seasons.'' These error
estimates are based on comparison of model output with observations for
conditions which most nearly reflect those given in the PRB definition,
i.e., at the lower end of the probability distribution. As discussed in
the Criteria Document and Staff Paper, it can be seen that GEOS-CHEM
overestimates O3 for the southeast and underestimates it by
a small amount for the northeast. These commenters generally ignored
the scientific conclusion presented in the Criteria Document that for
some regions of the country the evidence suggests that the model
actually overestimates PRB. Thus, the influence of alternative
estimates of PRB on risks in excess of PRB associated with meeting the
current standard can be to lower or increase the risk estimates. While
the choice of estimates for PRB contributes to the uncertainty in the
risk estimates, EPA does not agree that the approach used is biased
since peer-reviewed evaluations of the model have shown relatively good
[[Page 16469]]
agreement (i.e., generally within 5 ppb for most regions of the
country).
Finally, EPA believes that some commenters have misread the CASAC
Panel concern ``that the current approach to determining PRB is the
best method to make this estimation'' (Henderson, 2007, p. 2) as a
criticism of the use of the GEOS-CHEM modeling approach and/or support
for primary reliance on estimates based on remote monitoring sites.
However, the CASAC Panel went on to state that one reason for its
concern was that the contribution to PRB from beyond North America was
uncontrollable by EPA and that ``a better scientific understanding of
intercontinental transport of air pollutants could serve as the basis
for a more concerted effort to control its growth . . .'' (Henderson,
2007, p. 3). Hence, CASAC's concern appeared to be more with defining
what emissions to include in defining PRB, and the role that PRB should
play, as compared to the technical question of the best way to estimate
PRB levels. In reviewing the Staff Paper, the atmospheric modeling
expert on the CASAC Panel in his comments on how PRB had been estimated
using the GEOS-CHEM model concluded that the ``current approach has
been peer-reviewed, and is appropriate'' (Henderson, 2006b, p. D-48).
(4) Some commenters raised concerns about aspects of the exposure
modeling that they felt resulted in overestimates of modeled exposures,
including: (i) O3 measurements at downwind monitors are
usually higher than the overall area and may not reflect the overall
outdoor exposures in the area; (ii) O3 exposures near
roadways will be below that measured at the monitor due to titration of
O3 from automobile emissions of NO; (iii) O3
concentrations are lower at a person's breathing height compared to
measurement height, (iv) exposure estimates do not account for
O3 avoidance behaviors; and (v) the APEX model over predicts
elevated ventilation rate occurrences, which results in an
overestimation of the number of exposures of concern and risk estimates
for lung function decrements.
The concern raised in the first point is unfounded since all
O3 monitors in each area are used to take into account the
spatial variations of O3 concentrations. The geographic
variation of O3 concentration is accounted for by using
measurements from the closest O3 monitor to represent
concentrations in a neighborhood and the measurements at downwind
monitors are applied only to the downwind areas.
Second, the reduction in O3 concentrations near roadways
due to titration of O3 from automobile emissions of NO is
accounted for and explicitly modeled in APEX and thus does not bias
estimates of exposures. This phenomenon was modeled through the use of
``proximity factors,'' which adjust the monitored concentrations to
account for the titration of O3 by NO emissions (the
monitored concentrations are multiplied by the proximity factors).
Three proximity factor distributions were developed, one for local
roads, one for urban roads, and one for interstates, with mean factors
of 0.75, 0.75, and 0.36 respectively (section 3.10.2, Exposure Analysis
TSD). Furthermore, the uncertainty of these proximity factor
distributions was included in the exposure uncertainty analysis.
Third, as discussed in the exposure uncertainty analysis, data were
not available to quantify the potential biases of differences between
O3 concentrations at a person's breathing height compared to
the heights of nearby monitors. EPA believes that these biases, to the
extent that they exist, are relatively small during warm summer
afternoons when O3 concentrations tend to be higher.
Fourth, behavior changes in response to O3 pollution or
in response to AQI notification alerts (``avoidance behavior'') is not
explicitly taken into account in the exposure modeling. There is not
much information about the extent to which people currently modify
their activities in response to O3 alerts. However, under
the scenarios modeled for just meeting alternative standards,
O3 alerts would be infrequent relative to the number of
alerts that currently occur in the nonattainment areas modeled.
Consequently, EPA does not feel that this is an influential factor in
the estimation of exposure for the scenarios simulating just meeting
the current or proposed standards.
Fifth, a comparison of ventilation rates predicted by APEX to
measurements showed APEX overpredicting ventilation rates for ages 5 to
10, underpredicting ventilation rates for ages 11 to 29 and greater
than 39, and in close agreement for ages 30 to 39. The overall
agreement was judged favorable, and the errors of the predicted
ventilation rates were partially incorporated into the overall
uncertainty analysis with the uncertainties of the metabolic
equivalents (METs), which are the primary drivers of ventilation rates.
(5) Comments from a number of industry organizations, businesses,
and others contended that EPA's health risk assessment was biased and
that the resulting risk assessment is ``much higher than would have
been obtained using objective methods'' (NAM), and commenters raised
one or more of the following points in support of this view: (i) EPA
inappropriately based its risk assessment for respiratory symptoms,
hospital admissions, and non-accidental and cardiorespiratory mortality
on positive studies with high risk coefficients while ignoring negative
studies and studies with lower coefficients; (ii) EPA focused on
combined ``national'' effect estimates from multi-city studies when it
should have relied on individual city effect estimates from these
studies in its risk assessment; (iii) the risk assessment presented
single-pollutant model results that overstate the likely impact of
O3 when co-pollutant model results were available which
should have been used; (iv) the risk assessment used linear
concentration-response relationships for the health endpoints based on
epidemiological studies when non-linear or threshold models should have
been used; and (v) the lung function portion of the risk assessment
should not rely on what they characterized as ``outlier'' information
to define exposure-response relationships, with reference to the data
from the Adams (2006) study, but rather should focus on group central
tendency response levels. Each of these issues is discussed below and
in more detail in the Response to Comments document.
First, several commenters asserted that the results of time-series
studies should not be used at all in quantitative risk assessments,
that risk estimates from single-city time-series studies should not be
used since they are highly heterogeneous and influenced by publication
bias, and that the panel study which served as the basis for the
concentration-response relationships for respiratory symptoms in
asthmatic children suffered from various weaknesses and was
contradicted by a more recent study. EPA notes that the selection of
specific studies and effect estimates was based on a careful evaluation
of the evidence evaluated in the Criteria Document and that the
criteria and rationale for selection of studies and effect estimates
were presented and extensively reviewed and discussed by the CASAC
Panel and in public comments presented to the CASAC Panel. EPA notes
that the CASAC Panel judged the selection of the endpoints based on the
epidemiological studies for inclusion in the quantitative risk
assessment to be ``appropriate'' and that the risk
[[Page 16470]]
assessment chapter of the Staff Paper and its accompanying risk
assessment were ``well done, balanced and reasonably communicated''
(Henderson, 2006c, p. 12).
While EPA notes that two of the meta-analyses, Bell et al. (2005)
and Ito et al. (2005), provided suggestive evidence of publication
bias, O3-mortality associations remained after accounting
for that potential bias. The Criteria Document (p. 7-97) concludes that
the ``positive O3 effects estimates, along with the
sensitivity analyses in these three meta-analyses, provide evidence of
a robust association between ambient O3 and mortality.''
Concerns about the heterogeneity of responses observed across different
urban areas, particularly for O3-related mortality are
addressed in the section above on health effect considerations.
Second, as discussed in more detail in the Staff Paper (section
5.3.2.3), there are different advantages associated with use of single-
city and multi-city effect estimates as the basis for estimating health
risks in specific urban areas. Therefore, the risk assessment included
estimates based on both types of effect estimates where such
information was available.
Third, the risk assessment included risk estimates based on both
single pollutant and multi-pollutant concentration-response
relationships where such information was available for the health
outcomes included in the assessment. Issues related to the
consideration of single versus multi-pollutant models have been
addressed in the section above on health effects evidence.
Fourth, EPA's approach of using linear concentration-response
relationships for the health outcomes based on epidemiological studies
and whether or not to include any non-linear models or assumed
threshold were reviewed and discussed by the CASAC Panel during the
development of the Staff Paper and risk assessment, and the Panel
concurred with the approach used. As discussed in the proposal notice,
Staff Paper (section 3.4.5), and above in the prior section on health
effects evidence, EPA recognizes that the available epidemiological
evidence neither supports or refutes the existence of thresholds at the
population level for effects such as increased hospital admissions and
premature mortality. Noting the limitations of epidemiological evidence
to address such questions, EPA concluded that if a population threshold
does exist, it would likely be well below the level of the current
O3 standard. The Administrator is very mindful of the
uncertainties related to whether the observed associations between
O3 concentrations at levels well below 0.080 ppm and the
health outcomes reported in the epidemiological studies reflect actual
causal relationships, and has taken this into account in considering
the risk assessment estimates in his decision.
Fifth, consistent with the prior review, the lung function
component of the risk assessment has focused on the number and
percentage of children that are estimated to experience a degree of
lung function decrement that represents an adverse health effect. EPA
does not agree that the focus of the quantitative risk assessment
should be on the average lung function response in the population,
since such an assessment would not address the public health policy
question concerning to extent to which a portion of the population
would likely experience health effects of concern. Looking at just the
average for the population would ignore the evidence of health effects
for sensitive subpopulations, an important aspect of public health
impact in this and other O3 reviews. EPA believes that it is
appropriate to include all of the individual data from the series of
controlled human exposure studies that address lung function responses
associated with 6.6-hour exposures to O3 and which were
reviewed and included in the final Criteria Document, and this includes
the Adams (2006) study. EPA notes that the CASAC Panel clearly did not
judge the responses observed in this study to be an ``outlier.''
Rather, CASAC stated in its comments on the Staff Paper's discussion of
this study, ``there were clearly a few individuals who experienced
declines in lung function at these lower concentrations. These were
healthy subjects so the percentage of asthmatic subjects, if they had
been studied, would most likely be considerably greater'' (Henderson,
2006c, p. 10).
Having considered comments on the quantitative exposure and health
risk assessments from both groups of commenters, the Administrator
finds no basis to change his position on these quantitative assessments
that was taken at the time of proposal. That is, as discussed above,
while the Administrator recognizes that the assessments rest on a more
extensive body of data and is more comprehensive in scope than the
assessment conducted in the last review, he is mindful that significant
uncertainties continue to underlie the resulting quantitative exposure
and risk estimates. Nevertheless, the Administrator concludes that the
exposure and risk estimates are sufficiently reliable to inform his
judgment about the significance of the exposures and risk of health
effects in susceptible and vulnerable populations at O3
levels associated with just meeting the current 8-hour standard.
However, the Administrator disagrees with aspects of these commenters'
views on the level of the standard that is appropriate and supported by
the available health effects evidence and quantitative assessments
associated with just meeting alternative standards.
3. Conclusions Regarding the Need for Revision
Having carefully considered the public comments, as discussed
above, the Administrator believes the fundamental scientific
conclusions on the effects of O3 reached in the Criteria
Document and Staff Paper, briefly summarized above in section II.A.2
and discussed more fully in section II.A of the proposal, remain valid.
In considering whether the primary O3 standard should be
revised, the Administrator places primary consideration on the body of
scientific evidence available in this review on the health effects
associated with O3 exposure, as summarized above in section
II.B.1. The Administrator notes that there is much new evidence that
has become available since the last review, including an especially
large number of new epidemiological studies. The Administrator believes
that this body of scientific evidence is very robust, recognizing that
it includes large numbers of various types of studies, including
toxicological studies, controlled human exposure studies, field panel
studies, and community epidemiological studies, that provide consistent
and coherent evidence of an array of O3-related respiratory
morbidity effects and possibly cardiovascular-related morbidity as well
as total nonaccidental and cardiorespiratory mortality. The
Administrator observes that (1) the evidence of a range of respiratory-
related morbidity effects seen in the last review has been considerably
strengthened, both through toxicological and controlled human exposure
studies as well as through many new panel and epidemiological studies;
(2) newly available evidence from controlled human exposure and
epidemiological studies identifies people with asthma as an important
susceptible population for which estimates of respiratory effects in
the general population likely underestimate the magnitude or importance
of these effects; (3) newly available evidence
[[Page 16471]]
about mechanisms of toxicity more completely explains the biological
plausibility of O3-induced respiratory effects and is
beginning to suggest mechanisms that may link O3 exposure to
cardiovascular effects; and (4) there is now relatively strong evidence
for associations between O3 and total nonaccidental and
cardiopulmonary mortality, even after adjustment for the influence of
season and PM. The Administrator believes that this very robust body of
evidence, taken together, enhances our understanding of O3-
related effects relative to what was known at the time of the last
review. Further, he believes that the available evidence provides
increased confidence that respiratory morbidity effects such as lung
function decrements and respiratory symptoms are causally related to
O3 exposures, that indicators of respiratory morbidity such
as emergency department visits and hospital admissions are causally
related to O3 exposures, and that the evidence is highly
suggestive that O3 exposures during the warm O3
season contribute to premature mortality.
Further, the Administrator judges that there is important new
evidence demonstrating that exposures to O3 at levels below
the level of the current standard are associated with a broad array of
adverse health effects. This is especially true in at-risk populations
that include people with asthma or other lung diseases, who are likely
to experience more serious effects from exposure to O3,
children, and older adults with increased susceptibility, as well as
those who are likely to be vulnerable as a result of spending a lot of
time outdoors engaged in physical activity, especially active children
and outdoor workers. The Administrator notes that this important new
evidence demonstrates O3-induced lung function effects and
respiratory symptoms in some healthy individuals down to the previously
observed exposure level of 0.080 ppm, as well as very limited new
evidence at exposure levels well below the level of the current
standard. In addition, the Administrator notes that (1) there is now
epidemiological evidence of statistically significant O3-
related associations with lung function and respiratory symptom
effects, respiratory-related emergency department visits and hospital
admissions, and increased mortality, in areas that likely would have
met the current standard; (2) there are also many epidemiological
studies done in areas that likely would not have met the current
standard but which nonetheless report statistically significant
associations that generally extend down to ambient O3
concentrations that are below the level of the current standard; (3)
there are a few studies that have examined subsets of data that include
only days with ambient O3 concentrations below the level of
the current standard, or below even much lower O3
concentrations, and continue to report statistically significant
associations with respiratory morbidity outcomes and mortality; and (4)
the evidence from controlled human exposure studies, together with
animal toxicological studies, provides considerable support for the
biological plausibility of the respiratory morbidity associations
observed in the epidemiological studies and for concluding that the
associations extend below the level of the current standard.
Based on the available evidence, the Administrator agrees with the
CASAC Panel and the majority of public commenters that the current
standard is not requisite to protect public health with an adequate
margin of safety because it is does not provide sufficient protection
and that revision of the current O3 standard is needed to
provide increased public health protection. The Administrator notes
that extensive critical review of this body of evidence and related
uncertainties during the criteria and standard review process,
including review by the CASAC Panel and the public of the basis for
EPA's proposed decision to revise the primary O3 standard,
has identified a number of issues about which different reviewers
disagree and for which additional research is warranted. Nonetheless,
on balance, the Administrator believes that the remaining uncertainties
in the available evidence do not diminish confidence in the causal
relationships between O3 exposures and indicators of serious
respiratory morbidity effects, or the highly suggestive evidence of
associations between O3 exposures and premature mortality,
nor do they diminish confidence in the conclusion that the associations
extend below the level of the current standard.
Beyond a primary consideration of the available evidence, the
Administrator has also taken into consideration the Agency's exposure
and risk assessments to help inform his evaluation of the adequacy of
the current standard. As at the time of proposal, the Administrator
believes the results of those assessments inform his judgment on the
adequacy of the current standard to protect against health effects of
concern. In considering the exposure analysis results at this time, the
Administrator recognizes that that there is a risk for confusion in the
term ``exposure of concern'' that was used at the time of proposal, as
it could be read to imply a determination that a certain benchmark
level of exposure has been shown to be causally associated with adverse
health effects. As a consequence, the Administrator believes that it is
more appropriate to consider such exposure estimates in the context of
a continuum rather than focusing on any one discrete benchmark level,
as was done at the time of proposal, since the Administrator does not
believe that the underlying scientific evidence is certain enough to
support a focus on any bright-line benchmark level. In so doing, the
Administrator recognizes that associations between O3
exposures and health effects of concern become increasingly uncertain
at lower O3 exposure levels. Thus, the Administrator has
taken into consideration the pattern of such exposure estimates across
the range of discrete benchmark levels considered in EPA's exposure
assessment to provide some indication of the potential magnitude of the
incidence of health outcomes that could not be evaluated in the
Agency's quantitative risk assessment but which have been demonstrated
to occur in healthy people at O3 exposures as low as 0.080
ppm, the lowest level at which such health outcomes have been
tested.\20\
---------------------------------------------------------------------------
\20\ As noted above, such health outcomes include increased
airway responsiveness, increased pulmonary inflammation, increased
cellular permeability, and decreased pulmonary defense mechanisms.
These physiological effects provide plausible mechanisms underlying
observed associations with aggravation of asthma, increased
medication use, increased school and work absences, increased
susceptibility to respiratory infection, increased visits to
doctors' offices and emergency departments, and increased admissions
to hospitals. In addition, these physiological effects, if repeated
over time, have the potential to lead to chronic effects such as
chronic bronchitis or long-term damage to the lungs that can lead to
reduced quality of life.
---------------------------------------------------------------------------
More specifically, the Administrator has considered the pattern of
reductions in such exposures across the benchmark levels of 0.080,
0.070, and 0.060 ppm, which span the level at which there is strong
evidence of effects in healthy people down to a level at which the
Administrator judges the evidence of effects to be very limited. The
Administrator observes that based on the aggregated exposure estimates
for the 2002 simulation for the 12 urban areas included in the exposure
analysis, upon just meeting the current standard, the percentages of
asthmatic or all school age children likely to experience one of more
exposures at and above these benchmark levels of 0.080, 0.070, and
0.060 ppm (while at moderate or greater exertion) are approximately 4%,
[[Page 16472]]
20%, and 45%, respectively. As noted at the time of proposal, the
Administrator recognizes that there is substantial year-to-year and
city-to-city variability in these estimates and that it is important to
recognize this variability in considering these estimates. For example,
for the 0.080, 0.070, and 0.060 ppm benchmark levels, these percentages
are estimated to range from approximately 1 to 10%, 1 to 40%, and 7 to
65%, respectively, across each of the 12 urban areas based on the 2002
simulation, and from approximately 0 to 1%, 0 to 7%, and 1 to 25%,
respectively, based on the 2004 simulation.
With regard to the results of the risk assessment, the
Administrator again considered the risks estimated to remain upon just
meeting the current standard. The Administrator takes note of the
estimated magnitudes of such risks, which are presented above in
section II.B.1.c for a range of health effects including moderate and
large lung function decrements (including percentages of children and
number of occurrences), respiratory symptom days, respiratory-related
hospital admissions, and nonaccidental and cardiorespiratory mortality,
as well as year-to-year and city-to-city variability, and the
uncertainties in these estimates. Further, the Administrator recognizes
that these estimated risks for the specific health effects that could
be analyzed in the Agency's risk assessment are indicative of a much
broader array of O3-related health endpoints that are part
of a ``pyramid of effects'' that include various indicators of
morbidity that could not be included in the risk assessment (e.g.,
school absences, increased medication use, emergency department visits)
and which primarily affect members of at-risk groups.
In considering these quantitative exposure and risk estimates, as
well as the broader array of O3-related health endpoints
that could not be quantified, the Administrator believes that they are
important from a public health perspective and indicative of potential
exposures and risks to at-risk groups. The Administrator thus finds
that the exposure and risk estimates provide additional support to the
evidence-based conclusion, reached above, that the current standard
needs to be revised. Based on these considerations, and consistent with
CASAC Panel's unanimous conclusion that there is no scientific
justification for retaining the current standard, the Administrator
concludes that the current primary O3 standard is not
sufficient and thus not requisite to protect public health with an
adequate margin of safety, and that revision is needed to provide
increased public health protection. It is important to note that this
conclusion, and the reasoning on which it is based, does not address
the question of what specific revisions are appropriate. That requires
looking specifically at the current indicator, averaging time, form,
and level of the O3 standard, and evaluating the evidence
relevant to determining whether and to what extent any of these
elements should be revised, as is discussed in the following section.
C. Conclusions on the Elements of the Primary O3 Standard
1. Indicator
In the last review of the air quality criteria for O3
and other photochemical oxidants and the O3 standard, as in
other prior reviews, EPA focused on a standard for O3 as the
most appropriate surrogate for ambient photochemical oxidants. In this
review, while the complex atmospheric chemistry in which O3
plays a key role has been highlighted, no alternatives to O3
have been advanced as being a more appropriate surrogate for ambient
photochemical oxidants.
The Staff Paper (section 2.2.2) noted that it is generally
recognized that control of ambient O3 levels provides the
best means of controlling photochemical oxidants. Among the
photochemical oxidants, the acute exposure chamber, panel, and field
epidemiological human health database provides specific evidence for
O3 at levels commonly reported in the ambient air, in part
because few other photochemical oxidants are routinely measured.
However, recent investigations on copollutant interactions have used
simulated urban photochemical oxidant mixes. These investigations
suggest the need for similar studies to help in understanding the
biological basis for effects observed in epidemiological studies that
are associated with air pollutant mixtures, where O3 is used
as the surrogate for the mix of photochemical oxidants. Meeting the
O3 standard can be expected to provide some degree of
protection against potential health effects that may be independently
associated with other photochemical oxidants but which are not
discernable from currently available studies indexed by O3
alone. Since the precursor emissions that lead to the formation of
O3 generally also lead to the formation of other
photochemical oxidants, measures leading to reductions in population
exposures to O3 can generally be expected to lead to
reductions in population exposures to other photochemical oxidants.
The Staff Paper noted that while the new body of time-series
epidemiological evidence cannot resolve questions about the relative
contribution of other photochemical oxidant species to the range of
morbidity and mortality effects associated with O3 in these
types of studies, control of ambient O3 levels is generally
understood to provide the best means of controlling photochemical
oxidants in general, and thus of protecting against effects that may be
associated with individual species and/or the broader mix of
photochemical oxidants, independent of effects specifically related to
O3. No public comments specifically suggested changing the
indicator for the O3 NAAQS.
In its letter to the Administrator, the CASAC Panel noted that
O3 is ``the key indicator of the extent of oxidative
chemistry and serves to integrate multiple pollutants.'' The CASAC also
stated that ``although O3 itself has direct effects on human
health and ecosystems, it can also be considered as indicator of the
mixture of photochemical oxidants and of the oxidizing potency of the
atmosphere'' (Henderson, 2006c, p. 9).
Based on the available information, and consistent with the views
of EPA staff and the CASAC, the Administrator concludes that it is
appropriate to continue to use O3 as the indicator for a
standard that is intended to address effects associated with exposure
to O3, alone or in combination with related photochemical
oxidants. In so doing, the Administrator recognizes that measures
leading to reductions in population exposures to O3 will
also reduce exposures to other photochemical oxidants.
2. Averaging Time
a. Short-Term and Prolonged (1 to 8 Hours)
The current 8-hour averaging time for the primary O3
NAAQS was set in 1997. At that time, the decision to revise the
averaging time of the primary standard from 1 hour to 8 hours was
supported by the following key observations and conclusions:
(1) The 1-hour averaging time of the previous NAAQS was originally
selected primarily on the basis of health effects associated with
short-term (i.e., 1- to 3-hour) exposures.
(2) Substantial health effects information was available for the
1997 review that demonstrated associations between a wide range of
health effects (e.g., moderate to large lung function
[[Page 16473]]
decrements, moderate to severe respiratory symptoms and pulmonary
inflammation) and prolonged (i.e., 6- to 8-hour) exposures below the
level of the then current 1-hour NAAQS.
(3) Results of the quantitative risk analyses showed that
reductions in risks from both short-term and prolonged exposures could
be achieved through a primary standard with an averaging period of
either 1 hour or 8 hours. Thus establishing both a 1-hour and an 8-hour
standard would not be necessary to reduce risks associated with the
full range of observed health effects.
(4) The 8-hour averaging time was more directly associated with
health effects of concern at lower O3 concentrations than
the 1-hour averaging time. It was thus the consensus of the CASAC
``that an 8-hour standard was more appropriate for a human health-based
standard than a 1-hour standard.'' (Wolff, 1995)
(5) An 8-hour averaging resulted in a significantly more uniformly
protective national standard than the then current 1-hour standard.
(6) An 8-hour averaging time effectively limits both 1- and 8-hour
exposures of concern.
In looking at the new information that is discussed in section
7.6.2 of the current Criteria Document, the Staff Paper noted that
epidemiological studies have used various averaging periods for
O3 concentrations, most commonly 1-hour, 8-hour and 24-hour
averages. As described more specifically in sections 3.3 and 3.4 of the
Staff Paper, in general the results presented from U.S. and Canadian
studies showed no consistent difference for various averaging times in
different studies. Because the 8-hour averaging time continues to be
more directly associated with health effects of concern from controlled
human exposure studies at lower concentrations than do shorter
averaging periods, the Staff Paper did not evaluate alternative
averaging times in this review and did not conduct exposure or risk
assessments for standards with averaging times other than 8 hours.
The Staff Paper discussed an analysis of a recent three-year period
of air quality data (2002 to 2004) which was conducted to determine
whether the comparative 1- and 8-hour air quality patterns that were
observed in the last review continue to be observed based on more
recent air quality data. This updated air quality analysis (McCluney,
2007) was very consistent with the analysis done in the last review in
that it indicated that only two urban areas of the U.S. have such
``peaky'' air quality patterns such that the ratio of 1-hour to 8-hour
design values is greater than 1.5. This suggested that based on recent
air quality data, it was again reasonable to conclude that an 8-hour
average standard at or below the current level would generally be
expected to provide protection equal to or greater than the previous 1-
hour standard of 0.12 ppm in almost all urban areas. Thus, the Staff
Paper again concluded that setting a standard with an 8-hour averaging
time can effectively limit both 1- and 8-hour exposures of concern and
is appropriate to provide adequate and more uniform protection of
public health from both short-term and prolonged exposures to
O3 in the ambient air. In its letter to the Administrator,
the CASAC Panel unanimously supported the continued use of an 8-hour
averaging time for the primary O3 standard (Henderson 2007,
p. 2).
With respect to comments received on the proposal, most public
commenters did not address the issue of whether EPA should consider
additional or alternative averaging time standards. A few commenters,
most notably the CA EPA and joint comments by ALA and several
environmental groups, expressed the view that consideration should be
given to setting or reinstating a 1-hour standard, in addition to
maintaining the use of an 8-hour averaging time, to protect people in
those parts of the country with relatively more ``peaky'' exposure
profiles (e.g., Los Angeles). These commenters pointed out that when
controlled exposure studies using triangular exposure patterns (with
relatively higher 1-hour peaks) have been compared to constant exposure
patterns with the same aggregate O3 dose (in terms of
concentration multiplied by time), ``peaky'' exposure patterns are seen
to lead to higher risks. The CA EPA made particular note of this point,
expressing the view that a 1-hour standard would more closely represent
actual exposures, in that many people spend only 1 to 2 hours a day
outdoors, and that it would be better matched to O3
concentration profiles along the coasts where O3 levels are
typically high for shorter averaging periods than 8 hours.
For the reasons discussed in the Staff Paper and summarized above
and considering the unanimous views of the CASAC Panel supporting the
continued use of an 8-hour averaging time for the primary O3
standard, the Administrator finds that, in combination with the
decisions on form and level described below, the 8-hour standard
provides adequate protection from both short-term (1 to 3 hours) and
prolonged (6 to 8 hours) exposures to O3 in the ambient air
and that it is appropriate to continue use of the 8-hour averaging time
for the O3 NAAQS.
b. Long-term
During the last review, there was a large animal toxicological
database for consideration that provided clear evidence of associations
between long-term (e.g., from several months to years) exposures and
lung tissue damage, with additional evidence of reduced lung elasticity
and accelerated loss of lung function. However, there was no
corresponding evidence for humans, and the state of the science had not
progressed sufficiently to allow quantitative extrapolation of the
animal study findings to humans. For these reasons, consideration of a
separate long-term primary O3 standard was not judged to be
appropriate at that time, recognizing that the 8-hour standard would
act to limit long-term exposures as well as short-term and prolonged
exposures.
Taking into consideration the currently available evidence on long-
term O3 exposures, discussed above in section II.A.2.a.ii,
the Staff Paper concluded that a health-based standard with a longer-
term averaging time than 8 hours is not warranted at this time. The
Staff Paper noted that while potentially more serious health effects
have been identified as being associated with longer-term exposure
studies of laboratory animals and in epidemiology studies, there
remains substantial uncertainty regarding how these data could be used
quantitatively to develop a basis for setting a long-term health
standard. Because long-term air quality patterns would be improved in
areas coming into attainment with an 8-hour standard, the potential
risk of health effects associated with long-term exposures would be
reduced in any area meeting an 8-hour standard. Thus, the Staff Paper
did not recommend consideration of a long-term, health-based standard
at this time.
In its final letter to the Administrator, the CASAC Panel offered
no views on the long-term exposure evidence, nor did it suggest that
consideration of a primary O3 standard with a long-term
averaging time was appropriate, and instead the CASAC Panel agreed with
the choice of an 8-hour averaging time for the primary O3
NAAQS suggested by Agency staff (Henderson, 2007). Similarly, no public
commenters expressed support for considering such a long-term standard.
Taking into account the evidence, the CASAC Panel's views, and the
public comments, the Administrator finds that there is not a sufficient
basis for setting
[[Page 16474]]
a long-term primary O3 NAAQS at this time.
c. Administrator's Conclusions on Averaging Time
In considering the information discussed above, the CASAC Panel's
views and public comments, the Administrator concludes that a standard
with an 8-hour averaging time can effectively limit both 1- and 8-hour
exposures of concern and that an 8-hour averaging time is appropriate
to provide adequate and more uniform protection of public health from
both short-term (1-to 3-hour) and prolonged (6- to 8-hour) exposures to
O3 in the ambient air. This conclusion is based on the observations
summarized above, particularly: (1) The fact that the 8-hour averaging
time is more directly associated with health effects of concern at
lower O3 concentrations than are averaging times of shorter duration
and (2) results from quantitative risk analyses showing that attaining
an 8-hour standard reduces the risk of experiencing health effects
associated with both 8-hour and shorter duration exposures.
Furthermore, the Administrator observes that the CASAC Panel agreed
with the choice of averaging time (Henderson, 2007). Therefore, the
Administrator finds it appropriate to retain the 8-hour averaging time
and to not set a separate 1-hour standard. The Administrator also
concludes that a standard with a long-term averaging time is not
warranted at this time.
3. Form
In 1997, the primary O3 NAAQS was changed from a ``1-expected-
exceedance'' form per year over three years \21\ to a concentration-
based statistic, specifically the 3-year average of the annual fourth-
highest daily maximum 8-hour concentrations. The principal advantage of
the concentration-based form is that it is more directly related to the
ambient O3 concentrations that are associated with health effects of
concern. With a concentration-based form, days on which higher O3
concentrations occur would weigh proportionally more than days with
lower concentrations, since the actual concentrations are used in
determining whether the standard is attained. That is, given that there
is a continuum of effects associated with exposures to varying levels
of O3, the extent to which public health is affected by exposure to
ambient O3 is related to the actual magnitude of the O3 concentration,
not just whether the concentration is above a specified level.
---------------------------------------------------------------------------
\21\ The 1-expected-exceedance form essentially requires that
the fourth-highest air quality value in 3 years, based on
adjustments for missing data, be less than or equal to the level of
the standard for the standard to be met at an air quality monitoring
site.
---------------------------------------------------------------------------
During the 1997 review, consideration was given to a range of
alternative forms, including the second-, third-, fourth- and fifth-
highest daily maximum 8-hour concentrations in an O3 season,
recognizing that the public health risks associated with exposure to a
pollutant without a clear, discernable threshold can be appropriately
addressed through a standard that allows for multiple exceedances to
provide increased stability, but that also significantly limits the
number of days on which the level may be exceeded and the magnitude of
such exceedances. Consideration was given to setting a standard with a
form that would provide a margin of safety against possible, but
uncertain, chronic effects and would also provide greater stability to
ongoing control programs. The fourth-highest daily maximum was selected
because it was decided that the differences in the degree of protection
against potential chronic effects afforded by the alternatives within
the range were not well enough understood to use any such differences
as a basis for choosing the most restrictive forms. On the other hand,
the relatively large percentage of sites that would experience O3 peaks
well above 0.08 ppm and the number of days on which the level of the
standard may be exceeded even when attaining a fifth-highest 0.08 ppm
concentration-based standard, argued against choosing that form.
As an initial matter, the Staff Paper considered whether it is
appropriate to continue to specify the level of the O3 standard to the
nearest hundredth (two decimal places) ppm, or whether the precision
with which ambient O3 concentrations are measured supports specifying
the standard level to the thousandth (three decimal places) ppm (i.e.,
to the part per billion (ppb)). The Staff Paper discussed an analysis
conducted by EPA staff to determine the impact of ambient O3
measurement error on calculated 8-hour average O3 design value
concentrations, which are compared to the level of the standard to
determine whether the standard is attained (Cox and Camalier, 2006).
The results of this analysis suggested that instrument measurement
error, or possible instrument bias, contribute very little to the
uncertainty in design values. More specifically, measurement
imprecision was determined to contribute less than 1 ppb to design
value uncertainty, and a simulation study indicated that randomly
occurring instrument bias could contribute approximately 1 ppb. EPA
staff interpreted this analysis as being supportive of specifying the
level of the standard to the thousandth ppm. If the current standard
were to be specified to this degree of precision, the current standard
would effectively be at a level of 0.084 ppm, reflecting the data
rounding conventions that are part of the definition of the current
0.08 ppm 8-hour standard. This information was provided to the CASAC
Panel and made available to the public.
In evaluating alternative forms for the primary standard in
conjunction with specific standard levels, the Staff Paper considered
the adequacy of the public health protection provided by the
combination of the level and form to be the foremost consideration. In
addition, the Staff Paper recognized that it is important to have a
form of the standard that is stable and insulated from the impacts of
extreme meteorological events that are conducive to O3 formation. Such
instability can have the effect of reducing public health protection,
because frequent shifting in and out of attainment due of
meteorological conditions can disrupt an area's ongoing implementation
plans and associated control programs. Providing more stability is one
of the reasons that EPA moved to a concentration-based form in 1997.
The Staff Paper considered two concentration-based forms of the
standard: the nth-highest maximum concentration and a percentile-based
form. A percentile-based statistic is useful for comparing datasets of
varying length because it samples approximately the same place in the
distribution of air quality values, whether the dataset is several
months or several years long. However, a percentile-based form would
allow more days with higher air quality values in locations with longer
O3 seasons relative to places with shorter O3 seasons. An nth-highest
maximum concentration form would more effectively ensure that people
who live in areas with different length O3 seasons receive the same
degree of public health protection. For this reason, the exposure and
risk analyses were based on a form specified in terms of an nth-highest
concentration, with n ranging from 3 to 5.
The results of some of these analyses are shown in the Staff Paper
(Figures 6-1 through 6-4) and specifically discussed in chapter 6.
These figures illustrate the estimated percent change in risk estimates
for the incidence of moderate or greater decrements in lung function
([gteqt] 15 percent FEV1) in all school age children and
moderate or
[[Page 16475]]
greater lung function decrements ([gteqt] 10 percent FEV1)
in asthmatic school age children, associated with going from meeting
the current standard to meeting alternative standards with alternative
forms based on the 2002 and 2004 simulations. Figures 6-5 and 6-6
illustrate the estimated percent change in the estimated incidence of
non-accidental mortality, associated with going from meeting the
current standard to meeting alternative standards, based on the 2002
and 2004 simulations. These results are generally representative of the
patterns found in all of the analyses. The estimated reductions in risk
associated with different forms of the standard, ranging from third- to
fourth-highest daily maximum concentrations at 0.084 ppm, and from
third- to fifth-highest daily maximum concentrations at 0.074 ppm, are
generally less than the estimated reductions associated with the
different levels that were analyzed. As seen in these figures, there is
much city-to-city variability, particularly in the percent changes
associated with going from a fourth-highest to third-highest form at
the current level of 0.084 ppm, and with estimated reductions
associated with the fifth-highest form at a 0.074 ppm level. In most
cities, there are generally only small differences in the estimated
reductions in risks associated with the third- to fifth-highest forms
at a level of 0.074 ppm simulated using 2002 and 2004 O3 monitoring
data.
The Staff Paper noted that there is not a clear health-based
rationale for selecting a particular nth-highest daily maximum form of
the standard from among the ones analyzed. It also noted that the
changes in the form considered in the analyses result in only small
differences in the estimated reductions in risks in most cities,
although in some cities larger differences are estimated. The Staff
Paper concluded that a range of concentration-based forms from the
third-to the fifth-highest daily maximum 8-hour average concentration
is appropriate for consideration in setting the standard. Given that
there is a continuum of effects associated with exposures to varying
levels of O3, the extent to which public health is affected by exposure
to ambient O3 is related to the actual magnitude of the O3
concentration, not just whether the concentration is above a specified
level. The principal advantage of a concentration-based form is that it
is more directly related to the ambient O3 concentrations that are
associated with health effects. Robust, concentration-based forms, in
the range of the third-to fifth-highest daily maximum 8-hour average
concentration, including the current 4th-highest daily maximum form,
minimize the inherent lack of year-to-year stability of exceedance-
based forms and provide insulation from the impacts of extreme
meteorological events. Such instability can have the effect of reducing
public health protection by disrupting ongoing implementation plans and
associated control programs.
With regard to the precision of the standard, in its letter to the
Administrator, the CASAC concluded that current monitoring technology
``allows accurate measurement of O3 concentrations with a precision of
parts per billion'' (Henderson, 2006c). The CASAC recommended that the
specification of the level of the O3 standard should reflect this
degree of precision (Henderson, 2006c). While the CASAC Panel
unanimously supported specifying the level of the standard to this
degree of precision, public comments were mixed. Environmental
organizations (e.g., ALA et al.) and some State/regional agencies
(e.g., NESCAUM, PA Department of Environmental Protection) supported
the proposed increased precision and but did not support truncating to
the third decimal. However, several industry associations (e.g., API,
EMA, AAAM) suggested that there is not sufficient evidence to modify
the 1997 decision to round to two decimal places. These comments are
addressed in the Response to Comments document.
The Administrator concludes that the level of the standard should
be specified to the thousandth ppm (three decimal places), based on the
staff's analysis and conclusions discussed in the Staff Paper that
current monitoring technology allows accurate measurement of O3 to
support specifying the 8-hour standard to this degree of precision, and
on the CASAC Panel's reasoning and recommendation with respect to this
aspect of the standard.
With regard to the form of the standard, in its letter to the
Administrator prior to proposal, the CASAC recommended that ``a range
of concentration-based forms from the third-to the fifth-highest daily
maximum 8-hour average concentration'' be considered (Henderson, 2006c,
p. 5). Several commenters supported maintaining the current form of the
standard because it strikes an appropriate balance between stability
and protection, as well as because EPA used this form in their analyses
(e.g., EMA, NESCAUM, and Pennsylvania Department of Environmental
Protection). Some public commenters that expressed the view that the
current primary O3 standard is not adequate also submitted comments
that supported a more health-protective form of the standard than the
current form (e.g., a second-or third-highest daily maximum form)
(e.g., ALA et al.). Most commenters who expressed the view that the
current standard should not be revised did not provide any views on
alternative forms that would be appropriate for consideration should
the Administrator consider revisions to the standard. A few industry
association and business commenters supported changing to a 5th highest
form (e.g., Dow Chemical, AAM). One commenter (Oklahoma Department of
Transportation) suggested the use of a 6th or 7th highest daily maximum
form.
The Administrator recognizes that there is not a clear health-based
threshold for selecting a particular nth-highest daily maximum form of
the standard from among the ones analyzed in the Staff Paper and that
the current form of the standard provides a stable target for
implementing programs to improve air quality. The Administrator also
agrees that the adequacy of the public health protection provided by
the combination of the level and form is a foremost consideration.
Based on this, the Administrator finds that the form of the current
standard, 4th-highest daily maximum 8-hour average concentration,
should be retained, recognizing that the public health protection that
would be provided by this standard is based on combining this form with
the increased health protection provided by the lower level of the
standard discussed in the section below.
4. Level
a. Proposed Range
For the reasons discussed below, and taking into account
information and assessments presented in the Criteria Document and
Staff Paper, the advice and recommendations of the CASAC, and the
public comments received prior to proposal, the Administrator proposed
to revise the existing 8-hour primary O3 standard. Specifically, the
Administrator proposed to revise the level of the primary O3 standard
to within a range from 0.070 to 0.075 ppm.
The Administrator's consideration of alternative levels of the
primary O3 standard builds on his proposal, discussed above, that the
overall body of evidence indicates that the current 8-hour O3 standard
is not requisite to protect public health with an adequate margin of
safety because it does not provide sufficient protection, and that
revision would result in increased public health protection, especially
for
[[Page 16476]]
members of at-risk groups, notably including asthmatic children and
other people with lung disease, as well as all children and older
adults, especially those active outdoors, and outdoor workers, against
an array of adverse health effects. These effects range from health
outcomes that could be quantified in the risk assessment, including
decreased lung function, respiratory symptoms, serious indicators of
respiratory morbidity such as hospital admissions for respiratory
causes, and nonaccidental mortality, to health outcomes that could not
be directly estimated, including pulmonary inflammation, increased
medication use, emergency department visits, and possibly
cardiovascular-related morbidity effects. In reaching a proposed
decision about the level of the O3 primary standard, the Administrator
considered: the evidence-based considerations from the Criteria
Document and the Staff Paper; the results of the exposure and risk
assessments discussed above and in the Staff Paper, giving weight to
the exposure and risk assessments as judged appropriate; CASAC advice
and recommendations, as reflected in discussions of drafts of the
Criteria Document and Staff Paper at public meetings, in separate
written comments, and in CASAC's letters to the Administrator; EPA
staff recommendations; and public comments received during the
development of these documents, either in connection with CASAC
meetings or separately. In considering what 8-hour standard is
requisite to protect public health with an adequate margin of safety,
the Administrator noted at the time of proposal that he was mindful
that this choice requires judgment based on an interpretation of the
evidence and other information that neither overstates nor understates
the strength and limitations of the evidence and information nor the
appropriate inferences to be drawn.
The Administrator noted that the most certain evidence of adverse
health effects from exposure to O3 comes from the clinical studies and
that the large bulk of this evidence derives from studies of exposures
at levels of 0.080 and above. At those levels, there is consistent
evidence of lung function decrements and respiratory symptoms in
healthy young adults, as well as evidence of inflammation and other
medically significant airway responses. Moreover, there is no evidence
that the 0.080 ppm level is a threshold for these effects. Although the
Administrator took note of the very limited new evidence of lung
function decrements and respiratory symptoms in some healthy
individuals at the 0.060 ppm exposure level, he judged this evidence
too limited to support a primary focus at this level. The Administrator
also noted that clinical studies, supported by epidemiological studies,
provide important new evidence that people with asthma were likely to
experience larger and more serious effects than healthy people from
exposure to O3. There were also epidemiological studies that provide
evidence of statistically significant associations between short-term
O3 exposures and more serious health effects, such as emergency
department visits, hospital admissions, and premature mortality, in
areas that likely would have met the current standard. The
Administrator also took note of the many epidemiological studies done
in areas that likely would not have met the current standard but which
nonetheless report statistically significant associations that
generally extend down to ambient O3 concentrations that were below the
level of the current standard. Further, there were a few studies that
have examined subsets of data that include only days with ambient O3
concentrations below the level of the current standard, or below even
much lower O3 concentrations, and continued to report statistically
significant associations with respiratory morbidity outcomes and
mortality. In considering this evidence, the Administrator noted that
the extent to which these studies provide evidence of causal
relationships with exposures to O3 alone, down to the lowest levels
observed, remains uncertain. EPA sought comment on the degree to which
associations observed in epidemiological studies reflect causal
relationships between important health endpoints and exposure to O3
alone at ambient O3 levels below the current standard.
Therefore, the Administrator judged at the time of proposal, and
continues to judge as discussed in section II.B.3, that revising the
current standard to protect public health with an adequate margin of
safety is warranted and would reduce risk to public health, based on:
(1) The strong body of clinical evidence in healthy people at exposure
levels of 0.080 and above of lung function decrements, respiratory
symptoms, pulmonary inflammation, and other medically significant
airway responses, as well as some indication of lung function
decrements and respiratory symptoms at lower levels; (2) the
substantial body of clinical and epidemiological evidence indicating
that people with asthma are likely to experience larger and more
serious effects than healthy people; and (3) the body of
epidemiological evidence indicating associations are observed for a
wide range of serious health effects, including respiratory emergency
department visits, hospital admissions, and premature mortality, at and
below 0.080 ppm. The Administrator also judged at the time of proposal
and continues to conclude that the estimates of exposures of concern
and risks remaining upon just meeting the current standard or a
standard at the 0.080 ppm level provide additional support for this
view. For the same reasons stated in the proposal notice and discussed
above in section II.B on the adequacy of the current standard, the
Administrator judges that the standard should be set below 0.080 ppm, a
level at which the evidence provides a high degree of certainty about
the adverse effects of O3 exposure even in healthy people.
The Administrator next considered what standard level below 0.080
ppm would be requisite to protect public health with an adequate margin
of safety that is sufficient, but not more than necessary, to achieve
that result, recognizing that such a standard would result in increased
public health protection. The assessment of a standard level calls for
consideration of both the degree of additional protection that
alternative levels of the standard might be expected to provide as well
as the certainty that any specific level will in fact provide such
protection. In the circumstances present in this review, there is no
evidence-based bright line that indicates a single appropriate level.
Instead there is a combination of scientific evidence and other
information that needs to be considered holistically in making this
public health policy judgment and selecting a standard level from a
range of reasonable values.
The Administrator noted that at exposure levels below 0.080 ppm
there is only a very limited amount of evidence from clinical studies,
indicating effects in some healthy individuals at levels as low as
0.060 ppm. The great majority of the evidence concerning effects below
0.080 ppm is from epidemiological studies. The epidemiological studies
do not identify any bright-line threshold level for effects. At the
same time, the epidemiological studies are not in and of themselves
direct evidence of a causal link between exposure to O3 and the
occurrence of the effects. The Administrator considers these studies in
the context of all the other available evidence in evaluating the
degree of
[[Page 16477]]
certainty that O3-related adverse health effects would occur at various
ambient levels below 0.080 ppm, including the strong human clinical
studies and the toxicological studies that demonstrate the biological
plausibility and mechanisms for the effects of O3 on airway
inflammation and increased airway responsiveness at exposure levels of
0.080 ppm and above.
Based on consideration of the entire body of evidence and
information available at this time, as well as the recommendations of
the CASAC, the Administrator proposed that a standard within the range
of 0.070 to 0.075 ppm would be requisite to protect public health with
an adequate margin of safety. As noted at the time of proposal, a
standard level within this range is estimated to reduce the risk of a
variety of health effects associated with exposure to O3,
including the respiratory symptoms and lung function effects
demonstrated in clinical studies, and in emergency department visits,
hospital admissions, and mortality effects indicated in the
epidemiological studies. All of these effects are indicative of a much
broader array of O3-related health endpoints, as represented
by the pyramid of effects, such as school absences and increased
medication use that are plausibly linked to these observed effects.
The Administrator also considered the degree of improvements in
public health that potentially could be achieved by a standard of 0.070
to 0.075 ppm, giving weight to the exposure and risk assessments as he
judged appropriate. As discussed in the proposal notice (section
II.D.4) in considering the results of the exposure assessment, the
Administrator primarily focused on exposures at and above the 0.070 ppm
benchmark level as an important surrogate measure for potentially more
serious health effects for at-risk groups, including people with
asthma. In so doing, the Administrator noted that although the analysis
of ``exposures of concern'' was conducted to estimate exposures at and
above three discrete benchmark levels, the concept is appropriately
viewed as a continuum. As discussed above, the Administrator strives to
balance concern about the potential for health effects and their
severity with the increasing uncertainty associated with our
understanding of the likelihood of such effects at lower O3
exposure levels. In focusing on this benchmark, the Administrator noted
that upon just meeting a standard within the range of 0.070 to 0.075
ppm based on the 2002 simulation, the number of school age children
likely to experience exposures at and above this benchmark level in
aggregate (for the 12 cities in the assessment) was estimated to be
approximately 2 to 4 percent of all and asthmatic children and
generally less than 10 percent of children even in cities that receive
the least degree of protection from such a standard in a recent year
with relatively high O3 levels. A standard within the 0.070
to 0.075 ppm range would thus substantially reduce exposures of concern
by about 90 to 80 percent, respectively, from those estimated to occur
upon just meeting the current standard. While placing less weight on
the results of the risk assessment, in light of the important
uncertainties inherent in the assessment, the Administrator noted that
the results indicated that a standard set within this range would
likely reduce risks to at-risk groups from the O3-related
health effects considered in the risk assessment, and by inference
across the much broader array of O3-related health effects
that could only be considered qualitatively, relative to the level of
protection afforded by the current standard. This lent support to the
proposed range.
The Administrator judged that a standard set within the range of
0.070 to 0.075 ppm would provide a degree of reduction in risk that is
important from a public health perspective and that a standard within
this range would be requisite to protect public health, including the
health of at-risk groups, with an adequate margin of safety. EPA's
evaluation of the body of scientific evidence and quantitative
estimates of exposures and risks indicated that substantial reductions
in public health risks would occur throughout this range. As noted in
the proposal notice, because there is no bright line clearly directing
the choice of level within this reasonable range, the choice of what is
appropriate, considering the strengths and limitations of the evidence,
and the appropriate inference to be drawn from the evidence and the
exposure and risk assessments is a public health policy judgment. To
further inform this judgment, EPA sought public comment on the extent
to which the epidemiological and clinical evidence provide guidance as
to the level of a standard that would be requisite to protect public
health with an adequate margin of safety, especially for at-risk
groups.
In considering the available information, the Administrator also
judged that a standard level below 0.070 ppm would not be appropriate.
In reaching this judgment, the Administrator noted that there was only
quite limited evidence from clinical studies at exposure levels below
0.080 ppm O3. Moreover, the Administrator recognized that in
the body of epidemiological evidence, many studies reported positive
and statistically significant associations, while others reported
positive results that were not statistically significant, and a few did
not report any positive O3-related associations. In
addition, the Administrator judged that evidence of a causal
relationship between adverse health outcomes and O3
exposures became increasingly uncertain at lower levels of exposure.
The Administrator also considered the results of the exposure
assessments in reaching his judgment that a standard level below 0.070
ppm would not be appropriate. The Administrator noted that in
considering the results from the exposure assessment, a standard set at
the 0.070 ppm level, with the same form as the current standard, was
estimated to provide substantial reductions in exposures of concern
(i.e., approximately 90 to 92 percent reductions in the numbers of
school age children and 94 percent reduction in the total number of
occurrences) for both all and asthmatic school age children relative to
just meeting the current standard based on a simulation of a recent
year with relatively high O3 levels (2002). Thus, a 0.070
ppm standard would be expected to provide protection from the exposures
of concern that the Administrator had primarily focused on for over 98
percent of all and asthmatic school age children even in a year with
relatively high O3 levels, increasing to over 99.9 percent
of children in a year with relatively low O3 levels (2004).
In considering the results of the health risk assessment, as
discussed in the proposal notice (section II.C.2), the Administrator
noted that there were important uncertainties and assumptions inherent
in the risk assessment and that this assessment was most appropriately
used to simulate trends and patterns that could be expected, as well as
providing informed, but still imprecise, estimates of the potential
magnitude of risks. The Administrator particularly noted that as lower
standard levels were modeled, including a standard set at a level below
0.070 ppm, the risk assessment continued to assume a causal link
between O3 exposures and the occurrence of the health
effects examined, such that the assessment continued to indicate
reductions in O3-related risks upon meeting a lower standard
level. As discussed above, however, the Administrator recognized
[[Page 16478]]
that evidence of a causal relationship between adverse health effects
and O3 exposures becomes increasingly uncertain at lower
levels of exposure. Given all of the information available to him at
the time of the proposal, the Administrator judged that the increasing
uncertainty of the existence and magnitude of additional public health
protection that standards below 0.070 ppm might provide suggested that
such lower standard levels would likely be below what is necessary to
protect public health with an adequate margin of safety.
In addition, the Administrator judged that a standard level higher
than 0.075 ppm would also not be appropriate. This judgment took into
consideration the information discussed in the proposal notice
(sections II.A and B) and was based on the strong body of clinical
evidence in healthy people at exposure levels of 0.080 ppm and above,
the substantial body of clinical and epidemiological evidence
indicating that people with asthma are likely to experience larger and
more serious effects than healthy people, the body of epidemiological
evidence indicating that associations are observed for a wide range of
more serious health effects at levels below 0.080 ppm, and the
estimates of exposure and risk remaining upon just meeting a standard
set at 0.080 ppm. The much greater certainty of the existence and
magnitude of additional public health protection that such levels would
forego provides the basis for judging that levels above 0.075 ppm would
be higher than what is requisite to protect public health, including
the health of at-risk groups, with an adequate margin of safety.
For the reasons discussed in more detail in the proposal notice and
summarized above, the Administrator proposed to revise the level of the
primary O3 standard to within the range of 0.070 to 0.075
ppm.
At the time of proposal, the Administrator recognized that sharply
divergent views on the appropriate level of this standard had been
presented to EPA as part of the NAAQS review process, and he solicited
comment on a wide range of standard levels and alternative approaches
to characterizing and addressing scientific uncertainties. One such
alternative view focused very strongly on the uncertainties inherent in
the controlled human exposure and epidemiological studies and
quantitative exposure and health risk assessments as the basis for
concluding that no change to the current 8-hour O3 standard
of 0.084 ppm was warranted. In sharp contrast, others viewed the
controlled human exposure and epidemiological studies as strong and
robust, and generally placed more weight on the results of the
quantitative exposure and risk assessments and the unanimous CASAC
recommendations as a basis for concluding that an 8-hour standard at or
below 0.070 ppm was warranted. As discussed below, the same sharply
divergent views were generally repeated in comments on the proposal by
the two distinct groups of commenters identified in II.B.2 above.
b. Comments on Level
i. Health Evidence Considerations
With regard to the evaluation and consideration of the health
effects evidence and how such information should be considered in the
decision on the standard level, EPA notes that the commenters fell into
the same two groups discussed above in section II.B.2. The two groups
often cited the same studies and evidence, but they reached sharply
divergent conclusions as to what standard level is supported by the
health effects evidence. The general views of both groups on the
interpretation and use of the health effects evidence are presented
above in section II.B.2.a, with most comments from one group arguing
that this evidence supports a decision to revise the 8-hour standard to
0.060 ppm or below, and the other group arguing that it supports a
decision not to revise the current 8-hour standard.
With regard to the evidence from controlled human exposure studies,
commenters that included public health and environmental groups who
supported revising the current standard expressed the view that the
large body of evidence available at the time of the last review,
demonstrating an array of adverse health effects (i.e., reduced lung
function, respiratory symptoms, increased airway responsiveness,
inflammation, and increased susceptibility to respiratory infection),
at concentrations of 0.080 ppm O3, indicated that the
standard should have been set at a lower level. These commenters noted
that standards must be set below the level shown to cause effects in
healthy subjects in order to protect sensitive populations with an
adequate margin of safety. As discussed in section II.B.2.a above,
these commenters focused on the results of the Adams studies (2002,
2006) as evidence that exposure to 0.060 ppm O3 will result
in a significant proportion (i.e., 7%) of the adult population who do
not have asthma or other lung diseases experiencing notable lung
function decrements (FEV1 decrement ([gteqt]10%), and
furthermore that larger decrements in FEV1 would be expected
in more susceptible populations. This evidence caused these commenters
to reject EPA's proposed range:
Clearly, EPA's proposed standard of 0.070 to 0.075 ppm cannot be
considered protective of public health in light of experimental
evidence demonstrating adverse respiratory effects in healthy
individuals exposed to 0.060 ppm, and the legal requirements to
protect sensitive populations with an adequate margin of safety.
[ALA et al., p. 51]
The second group of commenters, who opposed revision of the
standard, expressed the view that the group mean changes reported in
the Adams studies (2002, 2006) were small, that such decrements should
not be considered to be adverse, and that the individuals who
experienced larger responses were too few to serve as a basis for a
revised O3 standard. This group included virtually all
commenters representing industry associations and businesses. These
general comments are addressed above in section II.B.2.a and in more
detail in the Response to Comments document.
In considering comments received on controlled human exposure
studies, and how these studies support a focus on particular standard
levels, the Administrator observes that in general the comments support
his original view that these studies provide the most certain evidence
of adverse health effects, and that the large bulk of evidence derives
from studies of exposures at levels of 0.080 ppm and above. The
Administrator notes that since the last review important new evidence
includes demonstration of O3-induced lung function effects
and respiratory symptoms in some healthy adults down to the previously
observed exposure level of 0.080 ppm, as well as very limited new
evidence of the same effects at exposure levels well below the level of
the current standard (Adams, 2002, 2006). EPA disagrees with these
commenters that the percent of subjects that experienced
FEV1 decrements greater than 10% in this study of 30
subjects can appropriately be generalized to the U.S. population. Based
on careful consideration of the comments, the Administrator again
concludes that while the Adams studies provide evidence that some
healthy individuals will experience lung function decrements and
respiratory symptoms at the 0.060 ppm exposure level, this evidence is
too limited to support a primary focus at this level. Moreover, the
Administrator notes that while the CASAC Panel supported a level of
0.060 ppm, they also supported a level above 0.060, indicating that
they disagree with the commenters' view that
[[Page 16479]]
the results of Adams studies mean that the level of the standard has to
be set at 0.060 ppm.
With regard to the information from epidemiological studies,
commenters representing public health, environmental, and medical
organizations generally asserted that the large body of new
epidemiological studies provides evidence of causal associations
between O3 exposures and a wide array of respiratory and
cardiovascular morbidity effects, including emergency department visits
and hospital admissions. They expressed the view that a significant
body of strong, consistent evidence links short-term exposures to
premature mortality and noted that this evidence is supported by new
research that provides biological plausibility for such effects. These
commenters noted that various approaches, including air quality
assessments which show that statistically significant associations
occurred in areas that likely would have met the current standard, or
statistical approaches that examined subsets of the data which indicate
that statistically significant associations remain down to very low
ambient O3 levels, show effects well below the level of the
current standard. Moreover they identified particular studies,
including some ``new'' studies not considered in the Criteria Document,
that indicated there are additional sub-populations that are likely to
be sensitive to O3, including infants, women, and African-
Americans, that should be considered in deciding the requisite level of
protection. They asserted that this information supports a standard set
at a level no higher than 0.060 ppm O3.
With regard to the information from epidemiological studies, the
second group of commenters focused strongly on EPA's interpretation of
the epidemiological evidence and the uncertainties they saw in this
evidence as a basis for concluding that no change to the current level
of the 8-hour O3 standard is warranted. In commenting on the
proposed range of levels, these commenters generally relied on the same
arguments presented above in section II.B.2.a as to why they believed
it would be inappropriate for EPA to make any revisions to the primary
O3 standard. That is, they asserted that the health effects
of concern associated with short-term or prolonged exposures to
O3 have not changed significantly since 1997; that the
inconsistencies and uncertainties inherent in these studies as a whole
should preclude any reliance on them as justification for a more
stringent standard; and that ``new'' science not included in the
Criteria Document continues to increase uncertainty about possible
health risks associated with exposure to O3. Specific
methodological issues cited as additional support for their conclusions
included: adequacy of exposure data; potential confounding by
copollutants; model selection; inconsistent evidence relating
O3 exposure to mortality, and ``new'' studies that provide
additional evidence of inconsistencies. These general comments are
addressed above in section II.B.2.a, and in greater detail in the
Response to Comments document.
In considering these comments on the epidemiological evidence with
regard to the interpretation of the epidemiological evidence and
methodological issues, the Administrator notes that in general, most of
the issues and concerns raised by those who do not support any
revisions to the primary O3 standard with regard to the
interpretation of the epidemiological evidence and methodological
issues, are essentially restatements if issues raised during the review
of the Criteria Document and Staff Paper. The same is true of the views
of commenters who supported a level of the standard no higher than
0.060 ppm O3. EPA presented and the CASAC Panel reviewed the
interpretation of the epidemiological evidence in the Criteria Document
and the integration of the evidence with policy considerations in the
development of the policy options presented in the Staff Paper for
consideration by the Administrator. CASAC reviewed the scientific
content of both the Criteria Document and Staff Paper and advised the
Administrator that these documents provided an appropriate basis for
use in regulatory decision making. Therefore, these comments do not
provide a basis for the Administrator to reach fundamentally different
conclusions than he reached at the time of proposal.
Moreover, the Administrator notes that epidemiological evidence is
most appropriately evaluated in the context of all available evidence,
including evidence from controlled human exposure and toxicological
studies. In general, the Administrator agrees with the weight of
evidence approach used in the Criteria Document and believes that this
body of scientific evidence across all types of studies is very robust,
recognizing that it includes a large number of various types of studies
that provide consistent and coherent evidence of an array of
O3-related respiratory morbidity effects and possibly
cardiovascular-related morbidity as well as total nonaccidental and
cardiorespiratory mortality. More specifically, the Administrator
judges that the body of epidemiological evidence indicating
associations with a wide range of serious health effects, including
respiratory emergency department visits and hospital admissions and
premature mortality, at and below 0.080 ppm supports revising the
current standard to protect public health. While the great majority of
evidence concerning effects below 0.080 ppm was from epidemiological
studies, the epidemiological studies do not identify any bright-line
threshold level for effects. At the same time, the epidemiological
studies are not themselves direct evidence of a causal link between
exposure to O3 and the occurrence of the effects. Therefore,
Administrator has considered these studies in the context of all the
other available evidence in evaluating the degree of certainty that
O3-related adverse health effects would occur at various
ambient levels below 0.080 ppm. In that context, there is only quite
limited evidence from controlled human exposure studies at exposure
levels below 0.080 ppm O3. The Administrator recognizes that
in the body of epidemiological evidence, many studies reported positive
and statistically significant associations, while others reported
positive results that were not statistically significant, and a few did
not report any positive O3-related associations. In
addition, the Administrator judged that evidence of a causal
relationship between adverse health outcomes and O3
exposures became increasingly uncertain at lower levels of exposure.
Based on this the Administrator continues to believe that the body of
epidemiological evidence does not support setting a standard as low as
0.060 as suggested by some commenters.
The Administrator also notes the many epidemiological studies done
in areas that likely would not have met the current standard but which
nonetheless report statistically significant associations that
generally extend down to ambient O3 concentrations that were
below the level of the current standard. Further, there were a few
studies that have examined subsets of data that include only days with
ambient O3 concentrations below the level of the current
standard, or below even much lower O3 concentrations, and
continued to report statistically significant associations with
respiratory morbidity outcomes and mortality. In the context of the
strong clinical evidence of adverse effect in healthy adults at 0.080,
the Administrator finds that the body of epidemiological evidence does
not
[[Page 16480]]
support retaining a standard of 0.080, as suggested by commenters.
Both groups of commenters also considered evidence from controlled
human exposure and epidemiological studies of increased susceptibility
in people with lung disease, especially people with asthma, but they
reached sharply divergent conclusions about what standard level is
supported by this evidence. As discussed above in section II.B.2.a,
medical organizations and public health and environmental groups agreed
with EPA that, based on evidence from controlled human exposure and
epidemiological studies, people with asthma, especially children, are
likely to have greater lung function decrements and respiratory
symptoms in response to O3 exposure than people who do not
have asthma, and are likely to respond at lower levels. Furthermore,
these commenters noted that epidemiological studies have identified
other potentially sensitive subpopulations, including for example,
infants, women and African-Americans, and that effects in these groups
should be part of the consideration in providing an adequate margin of
safety. These commenters concluded that the appropriate level for the
primary O3 standard is 0.060 ppm, to provide protection for
members of sensitive groups, especially people with asthma, who are
likely to have more serious responses and to respond at lower levels
that healthy people. They also contended that a standard set at this
level also would provide protection against anticipated, but as yet
unproven effects in the additional groups cited. The Administrator
agrees with these commenters that important new evidence shows that
asthmatics have more serious responses, and are more likely to respond
at lower O3 levels, than healthy individuals. Moreover, he
agrees that this evidence supports a standard set at a level below
0.080 ppm O3, based on the strong evidence from human
clinical studies in healthy adults at this level. However, for the
reasons described above, he does not agree that the controlled human
exposure and epidemiological evidence provide support for a standard
set at 0.060 ppm, for the reasons discussed above.
In contrast, industry association and business commenters asserted
that EPA is wrong to claim that new evidence indicates that the current
standard does not provide adequate health public health protection for
people with asthma. In support of this position, these commenters made
the following major comments: (1) The lung function decrements and
respiratory symptoms observed in clinical studies of asthmatics are not
clinically important; (2) EPA postulates that asthmatics would likely
experience more serious responses and responses at lower levels than
the subjects of controlled human exposure experiments, but that
hypothesis is not supported by scientific evidence; and, (3) EPA
recognized asthmatics as a sensitive subpopulation in 1997, and new
information does not suggest greater susceptibility than was previously
believed. EPA has generally responded to these comments and those
summarized in the paragraph above in section II.B.2.a above, and in
greater detail in the Response to Comments document.
After careful consideration of these comments, the Administrator
continues to judge that there is important new evidence demonstrating
that exposures to O3 at levels below the level of the
current standard are associated with a broad array of adverse health
effects, especially in at-risk populations that include people with
asthma or other lung diseases who are likely to experience more serious
effects from exposure to O3, as well as children and older
adults with increased susceptibility, and those who are likely to be
vulnerable as a result of spending a lot of time outdoors engaged in
physical activity, especially active children and outdoor workers. The
Administrator notes that this important new evidence demonstrates
O3-induced lung function effects and respiratory symptoms in
some healthy individuals down to the previously observed exposure level
of 0.080 ppm, as well as very limited new evidence at exposure levels
well below the level of the current standard. In addition, there are
many epidemiological studies done in areas that likely would not have
met the current standard but which nonetheless report statistically
significant associations that generally extend down to ambient
O3 concentrations that were below the level of the current
standard. Further, there were a few studies that have examined subsets
of data that include only days with ambient O3
concentrations below the level of the current standard, or below even
much lower O3 concentrations, and continued to report
statistically significant associations with respiratory morbidity
outcomes and mortality. The Administrator recognizes that in the body
of epidemiological evidence, many studies reported positive and
statistically significant associations, while others reported positive
results that were not statistically significant, and a few did not
report any positive O3-related associations. In addition,
the Administrator judged that evidence of a causal relationship between
adverse health outcomes and O3 exposures became increasingly
uncertain at lower levels of exposure. This body of evidence provides a
strong basis for the Administrator's judgment that the standard needs
to be revised to provide more protection, and that a revised standard
must be set at a level appreciably below 0.080 ppm, the level at which
there is considerable evidence of effects in healthy people. At the
same time, for the reasons discussed above the Administrator judges
that this body of evidence does not support setting a standard as low
as 0.060, as suggested by other commenters.
ii. Exposure and Risk Considerations
With regard to considering how the quantitative exposure and health
risk assessments should factor into a decision on the standard level,
EPA notes that both groups of commenters generally consider these
assessments in their comments on the standard level, but they reach
sharply divergent conclusions as to what standard level is supported by
these assessments. The general views of both groups on the implications
of the exposure and risk assessment are presented above in section
II.B.2.b, with one group arguing that it supports a decision to revise
the 8-hour standard to 0.060 ppm or below, and the other group arguing
that it supports a decision not to revise the current 8-hour standard.
A joint set of comments from ALA and several environmental groups
expressed the view that EPA cannot use exposures of concern to justify
a standard in the range of 0.070 to 0.075 ppm. These commenters
contended that standards in the proposed range would continue to expose
too many asthmatic children, as well as other at risk groups such as
outdoor workers and preschool children, to ``demonstrably unhealthy
levels of ozone pollution'' in only 12 cities which does not represent
a national estimate (ALA et al., p. 106). These same commenters
asserted that if EPA were to consider exposures of concern, then the
benchmark level must be defined as 0.060 ppm based on the considerable
evidence of adverse health effects occurring at this level. As
discussed in section II.B.2.b above, they also cited various reasons
why the exposure estimates were underestimated, including: only 12
cities were included in the assessment, various at risk groups
including outdoor workers and preschool children were not included in
the assessment, and EPA's exposure assessment underestimated exposures
since it
[[Page 16481]]
considers average children, not active children who spend more time
outdoors and repeated exposures also were underestimated.
In contrast, industry association and business group commenters
expressed the view that the concept of exposures of concern should not
be considered as a basis for revising the level of the standard because
it provided no indication of the probability that individuals would
actually experience an adverse health effect. These same commenters
also provided various reasons why the exposure estimates were
overestimated based on specific methodological choices made by EPA
including, for example, O3 measurements at fixed-site
monitors can be higher than other locations where individuals are
exposed, the exposure estimates do not account for O3
avoidance behaviors, and the exposure model overestimates elevated
breathing rates. Finally, these commenters also contended that the
estimates of exposures of concern associated with just meeting the
current standard, using the 0.080 ppm benchmark levels, have not
appreciably changed since the prior review and, thus provide no support
for revising the current standard.
EPA has responded to the criticisms from both groups of commenters
related to concerns that the exposure estimates are either
underestimated or overestimated in section II.B.2.b above and in more
detail in the Response to Comments document. EPA also has addressed the
issues raised by both groups of commenters concerning the
appropriateness of considering exposures at and above various benchmark
levels as an element in the decision on the adequacy of the current
standard in section II.B.2.b.
As discussed in section II.B.2b, the Administrator believes that it
is appropriate to consider such exposure estimates in the context of a
continuum rather than focusing on any one discrete benchmark level, as
was done at the time of proposal, since the Administrator does not
believe that the underlying evidence is certain enough to support a
focus on any single bright-line benchmark level. Thus, the
Administrator believes it is appropriate to consider a range of
benchmark levels from 0.080 down to 0.060 ppm, recognizing that
exposures at and above these benchmark levels must be considered in the
context of a continuum of the potential for health effects of concern,
and their severity, with increasing uncertainty associated with the
likelihood of such effects at lower O3 exposure levels.
The Administrator recognizes that the 0.080 ppm benchmark level
represents a level at which several health outcomes, including lung
inflammation, increased airway responsiveness, and decreased resistance
to infection have been shown to occur in healthy adults. The
Administrator places great weight on the public health significance of
exposures at and above this benchmark level given the greater certainty
that these adverse health responses are likely to be observed in a
significant fraction of the at-risk population. With respect to his
decision on the level of the 8-hour standard, the Administrator notes
that upon just meeting a standard within the range of 0.070 to 0.075
ppm based on the 2002 simulation, the number of school age asthmatic
children likely to experience exposures at and above the 0.080 ppm
benchmark level in aggregate (for the 12 cities in the assessment) is
estimated to range from 0.1 to 0.4 percent of asthmatic school age
children. Based on the 2004 simulation, the estimates are even lower,
with no asthmatic children estimated to experience exposures at and
above the 0.080 ppm benchmark level. Similar patterns are observed for
all school age children. Recognizing the uncertainties inherent in the
exposure assessment, the Administrator concludes that the exposure
assessment suggests that exposures at and above the 0.080 ppm level,
where several health effects have been shown to occur in healthy
individuals, are eliminated or nearly eliminated depending on the
modeling year upon just meeting a standard within the range of 0.070 to
0.075 ppm.
The Administrator does not agree with those commenters who would
only consider the single benchmark level of 0.080 ppm. While the
Administrator places less weight on exposures at and above the 0.070 pm
benchmark level, given the increased uncertainty about the fraction of
the population and severity of the health responses that might occur
associated with exposures above this level, he believes that it is
appropriate to consider exposures at this benchmark as well in judging
the adequacy of the current standard to protect public health.
Consideration of the 0.070 ppm benchmark level recognizes that the
effects observed at 0.080 ppm were in healthy adult subjects and
sensitive population groups, such as asthmatics, are expected to
respond at lower O3 levels than healthy individuals. The
Administrator notes that upon just meeting a standard within the range
of 0.070 to 0.075 ppm based on the 2002 simulation, the number of
asthmatic school age children likely to experience exposures at and
above the 0.070 ppm benchmark level in aggregate (for the 12 cities in
the assessment) is estimated to range from about 2 to 5 percent of
asthmatic school age children. Based on the 2004 simulation, the
estimates are substantially lower, with 0 to 0.6 percent of asthmatic
children estimated to experience exposures at and above the 0.070 ppm
benchmark level upon just meeting a standard within the range of 0.070
to 0.075 ppm.
Finally, the Administrator has considered but places very little
weight on the benchmark level of 0.060 ppm given the very limited
scientific evidence supporting a conclusion that O3 is
causally related to various health outcomes at this exposure level.
Nevertheless, the Administrator observes that there is a similar
pattern of reductions in exposures of concern for all and asthmatic
school age children at this benchmark level as well when comparing the
0.070 ppm and 0.075 ppm 8-hour standards.
Given the degree of uncertainty associated with the exposure
assessment discussed in the Staff Paper and uncertainty assessment
(Langstaff, 2007), the Administrator judges that for each specific
benchmark level examined there is not an appreciable difference, from a
public health perspective, in the estimates of exposures associated
with air quality just meeting an 8-hour standard at 0.075 ppm versus an
8-hour standard set at 0.070 ppm. For example, given the uncertainty in
the exposure estimates, the difference between an estimate of 2 percent
and 5 percent of asthmatic children for the exposure benchmark of 0.070
is not an appreciable difference from a public health perspective.
While directionally there are likely to be fewer exposures at and above
this benchmark for a standard of 0.070 than a standard of 0.075 ppm,
given the uncertainty in the exposure assessment it is not at all clear
that the actual difference is large enough to present a public health
concern.
With regard to considering how the quantitative risk assessment
should factor into a decision on the standard level, as noted above
both groups of commenters generally considered the risk assessment in
their comments on the standard level, but they reached sharply
divergent conclusions as to what standard level is supported by the
risk assessment. More specifically, the environmental, public health,
and most medical organizations, and some State and regional air
pollution agencies (e.g., California, NESCAUM) contended that EPA's
proposed range of 0.070 to 0.075 ppm would result in significant
residual
[[Page 16482]]
public health risks. As articulated most fully in the joint set of
comments from ALA and several environmental organizations, these
commenters expressed the view that EPA's risk assessment clearly
demonstrates that a more stringent 8-hour O3 standard of
0.065 ppm, the most stringent standard analyzed by EPA, would
significantly decrease O3-related lung function decrements,
respiratory symptoms, hospital admissions, and mortality and that ``EPA
must adopt a more stringent ozone standard of 0.060 ppm or below--a
level that incorporates a more adequate margin of safety'' (ALA et al.,
p. 108). These same commenters also cited various reasons for asserting
that the risk assessment likely underestimates health risks to a
substantial degree, including the limited nature of the assessment with
respect to number of cities, populations covered, and health endpoints
analyzed. EPA has responded to the comments concerning the scope of the
risk assessment and assertion that health risks are likely
underestimated both in section II.B.2.b above and in more detail in the
Response to Comments document. The Administrator's reasoning and
conclusions regarding the weight he places on the health risk
assessment in reaching a judgment about the appropriate level for the
primary standard are discussed below in section II.C.4.c.
In contrast, industry association and business group commenters who
supported not revising the level of the current 8-hour standard
generally asserted the following points: (1) That risk estimates have
not changed significantly since the prior review in 1997; (2) that
uncertainties and limitations underlying the risk assessment make it
too speculative to be used in supporting a decision to revise the
standard; (3) that EPA should have defined PRB differently and that EPA
underestimated PRB levels, which results in health risk reductions
associated with more stringent standards being overestimated; and (4)
that health risks are overestimated based on specific methodological
choices made by EPA including, for example, selection of inappropriate
effect estimates from health effect studies, EPA's approach to
addressing the shape of exposure-response relationships, and whether or
not to incorporate thresholds into its models for the various health
effects analyzed. EPA has responded to these comments both in section
II.B.2.b above and in more detail in the Response to Comments document.
In summary, the Administrator concludes that the exposure
assessment suggests that exposures at and above the 0.080 ppm benchmark
level, where several health effects have been shown to occur in healthy
individuals, are essentially eliminated for standards in the range of
0.070 to 0.075 ppm. He also concludes that at the 0.070 ppm benchmark
level, the exposures are substantially reduced and eliminated for the
vast majority of people in at-risk groups, and that the very low
estimates of such exposures are not appreciably different, from a
public health perspective, between those exposures associated with just
meeting a standard set at 0.070 ppm or 0.075 ppm. Further, the
Administrator places relatively little weight on the exposures using
the 0.060 ppm benchmark level given the very limited scientific
evidence supporting a conclusion that O3 is causally related
to health outcomes at this exposure level. Considering the
uncertainties associated with the exposure assessment, the
Administrator concludes that the exposure estimates associated with
each of the benchmark levels are not appreciably different, between a
0.070 or 0.075 ppm standard, and therefore, the exposure assessment
does not provide a basis for choosing a level within the proposed
range.
While the Administrator places less weight on the results of the
risk assessment, he notes that the results indicate that a standard set
within the proposed range would likely reduce risks to at-risk groups
from the O3-related health effects considered in the
assessment, and by inference across the much broader array of
O3-related health effects that can only be considered
qualitatively, relative to the level of protection afforded by the
current standard. Moreover, he notes that the results of the assessment
suggest a gradual reduction in risks with no clear breakpoint as
increasingly lower standard levels are considered. In light of this
continuum and the important uncertainties inherent in the assessment
discussed above and in the proposal, the Administrator concludes that
the risk assessment does not provide a basis for choosing a level
within the proposed range.
c. Conclusions on Level
Having carefully considered the public comments on the appropriate
level of the O3 standard, as discussed above, the
Administrator believes the fundamental scientific conclusions on the
effects of O3 reached in the Criteria Document and Staff
Paper, briefly summarized above in section II.A.2 and discussed more
fully in section II.A of the proposal, remain valid. In considering the
level at which the primary O3 standard should be set, the
Administrator continues to place primary consideration on the body of
scientific evidence available in this review on the health effects
associated with O3 exposure, as summarized above in section
II.C.4.a, while viewing the results of exposure and risk assessment,
discussed above in section II.C.4.b, as providing information in
support of his decision. In considering the available scientific
evidence he judges that, as at the proposal, a focus on the proposed
range of 0.070 to 0.075 ppm is appropriate in light of the large body
of controlled human exposure and epidemiological and other scientific
evidence. As discussed above, this body of evidence does not support
retaining the current standard, as suggested by some commenters. Nor
does it support setting a level just below 0.080 ppm because, based on
the entire body of evidence, such a level would not provide a
significant increase in protection compared to the current standard.
Further, such a level would not be appreciably below the level in
controlled human exposure studies at which adverse effects have been
demonstrated (i.e., 0.080 ppm). This body of evidence also does not
support setting a level of 0.060 ppm or below, as suggested by other
commenters. The Administrator has also evaluated the information from
the exposure assessment and the risk assessment, and judges that this
evidence does not provide a clear enough basis for choosing a specific
level within the range of 0.075 to 0.070 ppm. In making a final
judgment about the level of the O3 standard, the
Administrator notes that the level of 0.075 ppm is above the range
recommended by the CASAC (i.e., 0.070 to 0.060 ppm). Placing great
weight on the views of CASAC, the Administrator has carefully
considered its stated views and the scientific basis and policy views
for the range it recommended. In so doing, the Administrator notes that
he fully agrees that the scientific evidence supports the conclusion
that the current standard is not adequate and must be revised.
With respect to CASAC's recommended range of standard levels, the
Administrator observes that the basis for its recommendation appears to
be a mixture of scientific and policy considerations. The Administrator
notes that he is in general agreement with CASAC's views concerning the
interpretation of the scientific evidence. The Administrator also notes
that there is no bright line clearly directing the choice of level, and
the choice of what is appropriate is clearly a public health
[[Page 16483]]
policy judgment entrusted to the Administrator. This judgment must
include consideration of the strengths and limitations of the evidence
and the appropriate inferences to be drawn from the evidence and the
exposure and risk assessments. In reviewing the basis for the CASAC
Panel's recommendations for the range of the O3 standard,
the Administrator observes that he reaches a different policy judgment
than the CASAC Panel based on apparently placing different weight in
two areas: the role of the evidence from the Adams studies and the
relative weight placed on the results from the exposure and risk
assessments. While he found the evidence reporting effects at the 0.060
ppm level from the Adams studies to be too limited to support a primary
focus at this level, the Administrator observes that the CASAC Panel
appears to place greater weight on this evidence, as indicated by its
recommendation of a range down to 0.060 ppm. The Administrator also
observes that while the CASAC Panel supported a level of 0.060 ppm,
they also supported a level above 0.060, indicating that they do not
believe that the results of Adams studies mean that the level of the
standard has to be set at 0.060 ppm. The Administrator also observes
that the CASAC Panel appeared to place greater weight on the results of
the risk assessment as a basis for its recommended range. In referring
to the results of the risk assessment results for lung function,
respiratory symptoms, hospital admissions and mortality, the CASAC
Panel concluded that: ``beneficial effects in terms of reduction of
adverse health effects were calculated to occur at the lowest
concentration considered (i.e., 0.064 ppm)'' (Henderson, 2006c, p. 4).
However, the Administrator more heavily weighs the implications of the
uncertainties associated with the Agency's quantitative human exposure
and health risk assessments, as discussed above in section II.A.3.
Given these uncertainties, the Administrator does not agree that these
assessment results appropriately serve as a primary basis for
concluding that levels at or below 0.070 ppm are required for the 8-
hour O3 standard.
After carefully taking the above comments and considerations into
account, and fully considering the scientific and policy views of the
CASAC, the Administrator has decided to revise the level of the primary
8-hour O3 standard to 0.075 ppm. In the Administrator's
judgment, based on the currently available evidence, a standard set at
this level would be requisite to protect public health with an adequate
margin of safety, including the health of sensitive subpopulations,
from serious health effects including respiratory morbidity, that is
judged to be causally associated with short-term and prolonged
exposures to O3, and premature mortality. A standard set at
this level provides a significant increase in protection compared to
the current standard, and is appreciably below 0.080 ppm, the level in
controlled human exposure studies at which adverse effects have been
demonstrated. At a level of 0.075, exposures at and above the benchmark
of 0.080 ppm are essentially eliminated, and exposures at and above the
benchmark of 0.070 are substantially reduced or eliminated for the vast
majority of people in at-risk groups. A standard set at a level lower
than 0.075 would only result in significant further public health
protection if, in fact, there is a continuum of health risks in areas
with 8-hour average O3 concentrations that are well below
the concentrations observed in the key controlled human exposure
studies and if the reported associations observed in epidemiological
studies are, in fact, causally related to O3 at those lower
levels. Based on the available evidence, the Administrator is not
prepared to make these assumptions. Taking into account the
uncertainties that remain in interpreting the evidence from available
controlled human exposure and epidemiological studies at very low
levels, the Adminisitrator notes that the likelihood of obtaining
benefits to public health with a standard set below 0.075 ppm
O3 decreases, while the likelihood of requiring reductions
in ambient concentrations that go beyond those that are needed to
protect public health increases. The Administrator judges that the
appropriate balance to be drawn, based on the entire body of evidence
and information available in this review, is a standard set at 0.075.
The Administrator believes that a standard set at 0.075 ppm would be
sufficient to protect public health with an adequate margin of safety,
and does not believe that a lower standard is needed to provide this
degree of protection. This judgment by the Administrator appropriately
considers the requirement for a standard that is neither more nor less
stringent than necessary for this purpose and recognizes that the CAA
does not require that primary standards be set at a zero-risk level,
but rather at a level that reduces risk sufficiently so as to protect
public health with an adequate margin of safety.
D. Final Decision on the Primary O3 Standard
For the reasons discussed above, and taking into account
information and assessments presented in the Criteria Document and
Staff Paper, the advice and recommendations of the CASAC Panel, and the
public comments to date, the Administrator has decided to revise the
existing 8-hour primary O3 standard. Specifically, the
Administrator is revising (1) the level of the primary O3
standard to 0.075 ppm and (2) the degree of precision to which the
level of the standard is specified to the thousandth ppm. The revised
8-hour primary standard, with a level of 0.075 ppm, would be met at an
ambient air monitoring site when the 3-year average of the annual
fourth-highest daily maximum 8-hour average O3 concentration
is less than or equal to 0.075 ppm. Data handling conventions are
specified in the new Appendix P that is adopted, as discussed in
section V below.
At this time, EPA is also promulgating revisions to the Air Quality
Index for O3 to be consistent with the revisions to the
primary O3 standard. These revisions are discussed below in
section III. Issues related to the monitoring requirements for the
revised O3 primary standard are discussed below in section
VI.
III. Communication of Public Health Information
Information on the public health implications of ambient
concentrations of criteria pollutants is currently made available
primarily through EPA's Air Quality Index (AQI) program (40 CFR 58.50).
The current Air Quality Index has been in use since its inception in
1999 (64 FR 42530). It provides accurate, timely, and easily
understandable information about daily levels of pollution. The AQI
establishes a nationally uniform system of indexing pollution levels
for O3, CO, NO2, PM and SO2. The AQI
converts pollutant concentrations in a community's air to a number on a
scale from 0 to 500. Reported AQI values enable the public to know
whether air pollution levels in a particular location are characterized
as good (0-50), moderate (51-100), unhealthy for sensitive groups (101-
150), unhealthy (151-200), very unhealthy (201-300), or hazardous (301-
500). The AQI index value of 100 typically corresponds to the level of
the short-term NAAQS for each pollutant. For the 1997 O3
NAAQS, an 8-hour average concentration of 0.084 ppm corresponds to an
AQI value of 100. An AQI value greater than 100 means that a pollutant
is in one of the unhealthy
[[Page 16484]]
categories (i.e., unhealthy for sensitive groups, unhealthy, very
unhealthy, or hazardous) on a given day; an AQI value at or below 100
means that a pollutant concentration is in one of the satisfactory
categories (i.e., good or moderate). Decisions about the pollutant
concentrations at which to set the various AQI breakpoints, that
delineate the various AQI categories, draw directly from the underlying
health information that supports the NAAQS review.
The Agency recognized the importance of revising the AQI in a
timely manner to be consistent with any revisions to the NAAQS.
Therefore, EPA proposed to finalize conforming changes to the AQI, in
connection with the Agency's final decision on the O3 NAAQS
if revisions to the primary standard were promulgated. These conforming
changes would include setting the 100 level of the AQI at the same
level as the revised primary O3 NAAQS, and also making
proportional adjustments to AQI breakpoints at the lower end of the
range (i.e., AQI values of 50, 150 and 200). EPA did not propose to
change breakpoints at the higher end of the range (from 301 to 500),
which would apply to State contingency plans or the Significant Harm
Level (40 CFR 51.16), because the information from this review does not
inform decisions about breakpoints at those higher levels.
EPA received relatively few comments on the proposed changes to the
AQI. Three major issues came up in the comments, including: (1) Whether
the AQI should be revised at all, even if the primary standard is
revised; (2) whether the AQI should be revised in conjunction with this
rulemaking, or in a separate rulemaking; and, (3) whether an AQI value
of 100 should be set equal to or lower than the level of the short-term
primary O3 standard, and the other breakpoints adjusted
accordingly. UARG asserted that EPA should not revise the AQI at all,
even if EPA does revise the primary O3 standard. In support
of this view, UARG noted that there is no requirement for EPA to set an
AQI value of 100 equal to the level of the short-term standard, and
cited the 1999 decision to set an AQI value of 100 for PM2.5
equal to 40 [mu]g/m\3\, when the level of the short-term standard was
then 65 [mu]g/m\3\. UARG also expressed the view that lowering the
ambient concentrations associated with different AQI values would
confuse and mislead the public about actual trends in air quality,
which UARG asserted are improving. ALA and other environmental groups
in a joint set of comments did not support revising the AQI in
conjunction with this rulemaking. ALA et al. expressed the view that
since EPA did not propose specific breakpoints in its proposed
revisions to the AQI, EPA should conduct a separate rulemaking,
specifying the proposed breakpoints to allow the public an opportunity
to comment on them. Several State agencies, including agencies from
Pennsylvania, Wisconsin and Oklahoma, and State organizations,
including NACAA and NESCAUM, supported revising the AQI at the same
time that the standard is revised. NACAA expressed the view that: ``The
effectiveness of the AQI as a public health tool will be undermined if
EPA undertakes regulatory changes to the ozone NAAQS without
simultaneously revising the AQI.'' (NACAA, p. 5) The Wisconsin
Department of Natural Resources (WI DNR) further noted that:
``* * * when the 24-hour PM2.5 standard was revised,
EPA missed an opportunity to adopt conforming changes to the AQI.
The Administrator signed the Federal Register notice promulgating a
revised fine-particle standard in September 2006, but EPA still has
not changed the AQI to reflect the revised standard. We recommend
that the AQI be amended to be consistent with the revised ozone and
PM2.5 standards.'' [WI DNR, p. 3]
Finally, ALA et al. and NESCAUM expressed the view that an AQI
value of 100 should be set at an ambient concentration below the range
for the proposed primary standard. These commenters cited the health
evidence showing adverse health effects below the proposed range of the
standard, the recommended range of CASAC, and also cited the 1999
decision to set an AQI value of 100 for PM2.5 equal to 40
[mu]g/m\3\ when the level of the short-term standard was 65 [mu]g/m\3\,
as support for this view. Most other State commenters supported setting
an AQI value of 100 equal to the level of the primary O3
standard.
Recognizing the importance of the AQI as a communication tool that
allows the public to take exposure reduction measures when air quality
may pose health risks, EPA agrees with State agencies and organizations
that favored revising the AQI at the same time as the primary standard.
EPA agrees with State agency commenters that its historical approach of
setting an AQI value of 100 equal to the level of the revised primary
standard is appropriate, both from a public health and a communication
perspective.
Both UARG and ALA et al. cite the 1999 AQI rulemaking, which set an
AQI value of 100 for PM2.5 equal to 40 [mu]g/m\3\, a lower
level than the level of the short-term PM2.5 standard, as
support for their view that an AQI value of 100 does not need to be set
at the level of the revised O3 standard. However, the sub-
index for PM2.5 was developed using an approach that was
conceptually consistent with past practice for selecting the air
quality concentrations associated with the AQI breakpoints. The
Agency's historical approach to selecting index breakpoints had been to
simply set the AQI value of 100 at the level of the short-term standard
(e.g., 24 hours) for a pollutant. This method of structuring the index
is appropriate in the case where a short-term standard is set to
protect against the health effects associated with short-term exposures
and/or an annual standard is set to protect against health effects
associated with long-term exposures. In such cases, the short-term
standard in effect defines a level of health protection provided
against short-term risks and thus can be a useful benchmark against
which to compare daily air quality concentrations.
In the case of the 1997 PM2.5 standards, EPA took a
different approach to protecting against the health risks associated
with short-term exposures. The intended level of protection against
short-term risk was not defined by the 24-hour standard (set at a level
of 65 [mu]g/m\3\) but by the combination of the 24-hour and the annual
standards working in concert. In fact, the annual standard (set at a
level of 15 [mu]g/m\3\) was intended to serve as the principal vehicle
for protecting against both long-term and short-term PM2.5
exposures by lowering the entire day-by-day distribution of
PM2.5 concentrations in an area throughout the year. See
generally 62 FR at 38668-70 (July 18, 1997). Because the 24-hour
standard served to provide additional protection against very high
short-term concentrations, localized ``hotspots,'' or risks arising
from seasonal emissions that would not be well-controlled by a national
annual standard, EPA consequently concluded that it would be
appropriate to caution members of sensitive groups exposed to
concentrations below the level of the 24-hour standard. EPA also
concluded that it would be inappropriate to compare daily air quality
concentrations directly with the level of the annual standard by
setting an AQI value of 100 at that level. EPA wanted to set the AQI
value of 100 to reflect the general level of health protection against
short-term risks offered by the annual and 24-hour standards combined,
consistent with the underlying logic of the historical approach to
establishing AQI 100 levels. Therefore EPA set the AQI value of 100
[[Page 16485]]
at the midpoint of the range between the annual and the 24-hour
PM2.5 standards (i.e., 40 [mu]g/m\3\) in order to reflect
the combined role of the 24-hour and the annual PM2.5
standards in protecting against short-term risks. Therefore, this
approach for defining an AQI value of 100 is conceptually consistent
with the proposed decision to set an AQI value of 100 equal to the
level of the primary O3 standard.
Therefore, EPA is revising the AQI for O3 by setting an
AQI value of 100 equal to 0.075 ppm, 8-hour average, the level of the
revised primary O3 standard. EPA is also revising the
following breakpoints: An AQI value of 50 is set at 0.059 ppm, an AQI
value of 150 is set at 0.095 ppm, and an AQI value of 200 is set at
0.115 ppm. All these levels are averaged over 8 hours. As indicated in
the proposal, these levels were developed by making proportional
adjustments to the other AQI breakpoints (i.e., AQI values of 50, 150
and 200). The proportional adjustments were modified slightly to allow
for each category to span at least a 0.015 ppm range to allow for more
accurate forecasting. So, for example, simply making a proportional
adjustment to the level of an AQI value of 150 (0.104 ppm) would result
in a level of about 0.092 ppm. Since most of these ranges are rounded
to the nearest 5 thousandths of a ppm, that rounding would have
resulted in a 0.014 ppm range (i.e., 0.076 to 0.090 ppm). So, the
number was rounded upward to the nearest 5 thousandths of a ppm, to
allow for at least a 0.015 ppm range for forecasting. The same
principle applies to the calculation of an AQI value for 200 (0.115
ppm). EPA believes that the finalized breakpoints provide a balance
between proportional adjustments to reflect the revised O3
standard and providing category ranges that are large enough to be
forecasted accurately, so that the new AQI for O3 can be
implemented more easily in the public forum for which the AQI
ultimately exists.
IV. Rationale for Final Decision on Secondary O3 Standard
A. Introduction
1. Overview
This section presents the rationale for the Administrator's final
decisions regarding the need to revise the current secondary
O3 NAAQS, and the appropriate revisions to the standard. As
discussed more fully below, the rationale for the final decisions on
appropriate revisions to the secondary O3 NAAQS is based on
a thorough review of the latest scientific information on vegetation
effects associated with exposure to ambient levels of O3, as
assessed in the Criteria Document. This rationale also takes into
account: (1) Staff assessments of the most policy-relevant information
in the Criteria Document regarding the evidence of adverse effects of
O3 to vegetation and ecosystems, information on
biologically-relevant exposure metrics, and staff analyses of air
quality, vegetation exposure and risks, presented in the Staff Paper
and described in greater detail in the associated Technical Report on
Ozone Exposure, Risk, and Impact Assessments for Vegetation (Abt,
2007), upon which staff recommendations for revisions to the secondary
O3 standard were based; (2) CASAC Panel advice and
recommendations as reflected in discussion of drafts of the Criteria
Document and Staff Paper at public meetings, in separate written
comments, and in CASAC's letters to the Administrator (Henderson,
2006a, b, c; 2007); (3) public comments received during development of
these documents either in conjunction with CASAC meetings or separately
and on the proposal notice; (4) consideration of the degree of
protection to vegetation potentially afforded by the revised 8-hour
primary standard; and (5) the limits of the available evidence.
In developing this rationale, EPA has again focused on direct
O3 effects on vegetation, specifically drawing upon an
integrative synthesis of the entire body of evidence, published through
early 2006, on the broad array of vegetation effects associated with
exposure to ambient levels of O3 (EPA, 2006a, chapter 9). In
addition, because O3 can also indirectly affect other
ecosystem components such as soils, water, and wildlife, and their
associated ecosystem goods and services, through its effects on
vegetation, a qualitative discussion of these other indirect impacts is
also included, though these effects are not quantifiable at this time.
As was concluded in the 1997 review, and based on the body of
scientific literature assessed in the current Criteria Document, the
Administrator believes that it is reasonable to conclude that a
secondary standard protecting the public welfare from known or
anticipated adverse effects to trees, native vegetation and crops would
also afford increased protection from adverse effects to other
environmental components relevant to the public welfare, including
ecosystem services and function. The peer-reviewed literature includes
studies conducted in the U.S., Canada, Europe, and many other countries
around the world. In its assessment of the evidence judged to be most
relevant to making decisions on the level of the O3
secondary standard, however, EPA has placed greater weight on U.S.
studies, due to the often species-, site- and climate-specific nature
of O3-related vegetation response.
As with virtually any policy-relevant vegetation effects research,
there is uncertainty in the characterization of vegetation effects
attributable to exposure to ambient O3. As discussed below,
however, research conducted since the last review provides important
information coming from field-based exposure studies, including free
air, gradient and biomonitoring surveys, in addition to the more
traditional controlled open top chamber (OTC) studies. Moreover, the
newly available studies evaluated in the Criteria Document have
undergone intensive scrutiny through multiple layers of peer review and
many opportunities for public review and comment. While important
uncertainties remain, the review of the vegetation effects information
has been extensive and deliberate. In the judgment of the
Administrator, the intensive evaluation of the scientific evidence that
has occurred in this review has provided an adequate basis for
regulatory decision-making at this time. This review also provides
important input to EPA's research plan for improving our future
understanding of the effects of ambient O3 at lower levels.
Information related to vegetation and ecosystem effects,
biologically relevant exposure indices, and quantitative vegetation
exposure and risk assessments were summarized in sections IV.A through
IV.C of the proposal (72 FR at 37883-37895), respectively, and are only
briefly outlined below in sections IV.A.2 through IV.A.4. Subsequent
sections of this preamble provide a more complete discussion of the
Administrator's rationale, in light of key issues raised in public
comments, for concluding that the current standard is not requisite to
protect public welfare from known or anticipated adverse effects, and
it is appropriate to revise the current secondary O3
standard to provide additional public welfare protection (section IV.B)
by making the secondary standard identical to the revised primary
standard (section IV.C). A summary of the final decisions on revisions
to the secondary O3 standard is presented in section IV.D.
[[Page 16486]]
2. Overview of Vegetation Effects Evidence
This section outlines the information presented in section IV.A of
the proposal on known or potential effects on public welfare which may
be expected from the presence of O3 in ambient air.
Exposures to O3 have been associated quantitatively and
qualitatively with a wide range of vegetation effects. The decision in
the last review to set a more protective secondary standard primarily
reflected consideration of the quantitative information on vegetation
effects available at that time, particularly growth impairment (e.g.,
biomass loss) in sensitive forest tree species during the seedling
growth stage and yield loss in important commercial crops. This
information, derived mainly using the OTC exposure method, found
cumulative, seasonal O3 exposures were most strongly
associated with observed vegetation response. The Criteria Document
prepared for this review discussed a number of additional studies that
support and strengthen key conclusions regarding O3 effects
on vegetation and ecosystems found in the previous Criteria Document
(EPA, 1996a, 2006a), including further clarification of the underlying
mechanistic and physiological processes at the subcellular, cellular,
and whole system levels within the plant. More importantly, however, in
the context of this review, new quantitative information is now
available across a broader array of vegetation effects (e.g., growth
impairment during seedlings, saplings and mature tree growth stages,
visible foliar injury, and yield loss in annual crops) and across a
more diverse set of exposure methods, including chamber, free air,
gradient, model, and field-based observation. These non-chambered,
field-based study results begin to address one of the key data gaps
cited by the Administrator in the last review.
Section IV.A of the proposal provides a detailed summary of key
information contained in the Criteria Document (EPA, 2006, chapter 9)
and in the Staff Paper (EPA, 2007, chapter 7) on known or potential
effects on public welfare which may be expected from the presence of
O3 in ambient air (72 FR 37883-37890). The information in
that section summarized:
(1) New information available on potential mechanisms for
vegetation effects associated with exposure to O3, including
information on plant uptake of O3, cellular to systemic
responses, compensation and detoxification responses, changes to plant
metabolism, and plant responses to chronic O3 exposures;
(2) The nature of effects on vegetation that have been associated
with exposure to O3 including effects related to
carbohydrate production and allocation, growth effects on trees and
yield reductions in crops, visible foliar injury, and reduced plant
vigor, as well as consequent potential impacts on ecosystems including
potential alteration of ecosystem structure and function and effects on
ecosystem services and carbon sequestration; and
(3) Considerations in characterizing what constitutes an adverse
welfare impact of O3, including an approach that expands the
consideration of adversity beyond the species level by making explicit
the linkages between stress-related effects such as O3
exposure at the species level and at higher levels within an ecosystem
hierarchy.
3. Overview of Biologically Relevant Exposure Indices
This section outlines the information presented in section IV.B of
the proposal on biologically relevant exposure indices that relate
known or potential effects on vegetation to exposure to O3
in ambient air. The Criteria Document concluded that O3
exposure indices that cumulate differentially weighted hourly
concentrations are the best candidates for relating exposure to plant
growth responses (EPA, 2006a). This conclusion followed from the
extensive evaluation of the relevant studies in the 1996 Criteria
Document (EPA, 1996a) and the recent evaluation of studies that have
been published since that time (EPA, 2006a). The depth and strength of
these conclusions are illustrated by the following observations that
are drawn from the 1996 Criteria Document (EPA, 1996a, section 5.5):
(1) Specifically, with respect to the importance of taking into
account exposure duration, ``when O3 effects are the primary
cause of variation in plant response, plants from replicate studies of
varying duration showed greater reductions in yield or growth when
exposed for the longer duration'' and ``the mean exposure index of
unspecified duration could not account for the year-to-year variation
in response'' (EPA, 1996a, pg. 5-96).
(2) ``[B]ecause the mean exposure index treats all concentrations
equally and does not specifically include an exposure duration
component, the use of a mean exposure index for characterizing plant
exposures appears inappropriate for relating exposure with vegetation
effects'' (EPA, 1996a, pg. 5-88).
(3) Regarding the relative importance of higher concentrations than
lower in determining plant response, ``the ultimate impact of long-term
exposures to O3 on crops and seedling biomass response
depends on the integration of repeated peak concentrations during the
growth of the plant'' (EPA, 1996a, pg. 5-104).
(4) ``[A]t this time, exposure indices that weight the hourly
O3 concentrations differentially appear to be the best
candidates for relating exposure with predicted plant response'' (EPA,
1996a, pgs. 5-136).
At the conclusion of the last review, the biological basis for a
cumulative, seasonal form was not in dispute. There was general
agreement between the EPA staff, CASAC, and the Administrator, based on
their review of the air quality criteria, that a cumulative, seasonal
form was more biologically relevant than the previous 1-hour and new 8-
hour average forms (61 FR 65716).
The Staff Paper prepared for this review evaluated the most
appropriate choice of a cumulative, seasonal form for a secondary
standard to protect the public welfare from known and anticipated
adverse vegetation effects in light of the new information available in
this review. Specifically, the Staff Paper considered: (1) The
continued lack of evidence within the vegetation effects literature of
a biological threshold for vegetation exposures of concern and (2) new
estimates of PRB that are lower than in the last review. The form
commonly called W126 was evaluated in the last review and was compared
with the form called SUM06, which incorporates a threshold level above
which exposures are summed, that was proposed in the last review. The
concentration-weighted form commonly called W126 is defined as the sum
of sigmoidally weighted hourly O3 concentrations over a
specified period, where the daily sigmoidal weighting function is
defined in the Staff Paper (EPA, 2007a, p. 7-16.) as:
[[Page 16487]]
[GRAPHIC] [TIFF OMITTED] TR27MR08.000
Regarding the first consideration, the Staff Paper noted that the W126
form, by its incorporation of a continuous sigmoidal weighting scheme,
does not create an artificially imposed concentration threshold, yet
also gives proportionally more weight to the higher and typically more
biologically potent concentrations, as supported by the scientific
evidence. Second, the index value is not significantly influenced by
O3 concentrations within the range of estimated PRB, as the
weights assigned to concentrations in this range are very small. Thus,
the Staff Paper concluded that it would provide a more appropriate
target for air quality management programs designed to reduce emissions
from anthropogenic sources contributing to O3 formation. On
the basis of these considerations, the Staff Paper and the CASAC Panel
concluded that the W126 form is the most biologically-relevant
cumulative, seasonal form appropriate to consider in the context of the
secondary standard review.
4. Overview of Vegetation Exposure and Risk Assessments
This section outlines the information presented in section IV.C of
the proposal on the vegetation exposure and risk assessments conducted
for this review, which improved and built upon similar analyses
performed in the last review. The vegetation exposure assessment was
performed using interpolation and included information from ambient
monitoring networks and results from air quality modeling. The
vegetation risk assessment included both tree and crop analyses. The
tree risk analysis included three distinct lines of evidence: (1)
Observations of visible foliar injury in the field linked to recent
monitored O3 air quality for the years 2001-2004; (2)
estimates of seedling growth loss under current and alternative
O3 exposure conditions; and (3) simulated mature tree growth
reductions using the TREGRO model to simulate the effect of meeting
alternative air quality standards on the predicted annual growth of a
single western species (ponderosa pine) and two eastern species (red
maple and tulip poplar). The crop analysis includes estimates of the
risks to crop yields from current and alternative O3
exposure conditions and the associated change in economic benefits
expected to accrue in the agriculture sector upon meeting the levels of
various alternative standards. Each element of the assessment is
outlined below, together with key observations from this assessment.
a. Exposure Characterization
The exposure analyses examined O3 air quality patterns
in the U.S. relative to the location of O3 sensitive species
that have a known concentration-response in order to predict whether
adverse effects are occurring at current levels of air quality, and
whether they are likely to occur under alternative standard forms and
levels. The most important information about exposure to vegetation
comes from the O3 monitoring data that are available from
two national networks: (1) Air Quality System (AQS; http://www.epa.gov/ttn/airs/airsaqs) and (2) Clean Air Status and Trends Network (CASTNET;
http://www.epa.gov/castnet/). In order to characterize exposures to
vegetation at the national scale, however, the Staff Paper concluded
that it could not rely solely on limited site-specific monitoring data,
and that it was necessary to use an interpolation method to
characterize O3 air quality over broad geographic areas. The
analyses used the O3 outputs from the EPA/NOAA Community
Multi-scale Air Quality (CMAQ) \22\ model system (http://www.epa.gov/asmdnerl/CMAQ, Byun and Ching, 1999; Arnold et al. 2003, Eder and Yu,
2005) to improve spatial interpolations based solely on existing
monitoring networks.
---------------------------------------------------------------------------
\22\ The CMAQ model is a multi-pollutant, multiscale air quality
model that contains state-of-the-science techniques for simulating
all atmospheric and land processes that affect the transport,
transformation, and deposition of atmospheric pollutants and/or
their precursors on both regional and urban scales. It is designed
as a science-based modeling tool for handling many major pollutants
(including photochemical oxidants/O3, particulate matter,
and nutrient deposition) holistically. The CMAQ model can generate
estimates of hourly O3 concentrations for the contiguous
U.S., making it possible to express model outputs in terms of a
variety of exposure indices (e.g., W126, 8-hour average).
---------------------------------------------------------------------------
Based on the significant difference in monitor network density
between the eastern and western U.S., the Staff Paper concluded that it
was appropriate to use separate interpolation techniques in these two
regions: AQS and CASTNET monitoring data were solely used for the
eastern interpolation, and in the western U.S., where rural monitoring
is more sparse, O3 values generated by the CMAQ model were
used to develop scaling factors to augment the interpolation. In order
to characterize uncertainty in the interpolation method, monitored
O3 concentrations were systematically compared to
interpolated O3 concentrations in areas where monitors were
located. In general, the interpolation method used in the current
review performed well in many areas in the U.S., although it under-
predicted higher 12-hour W126 exposures in rural areas. Due to the
important influence of higher exposures in determining risks to plants,
this feature of the interpolated surface could result in an under-
estimation of risks to vegetation in some areas. Taking these
uncertainties into account, and given the absence of more complete
rural monitoring data, this approach was used in developing national
vegetation exposure and risk assessments that estimate relative changes
in risk for the various alternative standards analyzed.
To evaluate changing vegetation exposures and risks under selected
air quality scenarios, the Staff Paper utilized adjusted 2001 base year
O3 air quality distributions with a rollback method (Horst
and Duff, 1995; Rizzo, 2005, 2006) to reflect meeting the current and
alternative secondary standard options. The following key observations
were drawn from comparing predicted changes in interpolated air quality
under each alternative standard form and level scenario analyzed:
(1) The results of the exposure assessment indicate that current
air quality levels could result in significant impacts to vegetation in
some areas. For example, for the base year (2001), a large portion of
California had 12-hr W126 O3 levels above 31 ppm-hour, which
has been associated with approximately up to 14 percent biomass loss in
50 percent of tree seedling cases studies. Broader multi-state regions
in the east (NC, TN, KY, IN, OH, PA, NJ, NY, DE, MD, VA) and west (CA,
NV, AZ, OK, TX) are predicted to have levels of air quality above the
W126 level of 21 ppm-hour, which is approximately equal to the
secondary standard proposed in 1996 and is associated with
approximately up to 10 percent biomass loss in 50 percent of tree
seedling cases studied. Much of the east and Arizona and California
have 12-hour W126 O3 levels above 13 ppm-hour which has been
associated with approximately up to 10 percent biomass loss in 75
percent of tree seedling cases studied.
(2) When 2001 air quality is rolled back to meet the current 8-hour
[[Page 16488]]
secondary standard, the overall 3-month 12-hour W126 O3
levels were somewhat improved, but not substantially. Under this
scenario, there were still many areas in California with 12-hour W126
O3 levels above 31 ppm-hour. A broad multi-state region in
the east (NC, TN, KY, IN, OH, PA, MD) and west (CA, NV, AZ, OK, TX)
were still predicted to have O3 levels above the W126 level
of 21 ppm-hour.
(3) Exposures generated for just meeting a 0.070 ppm, 4th-highest
maximum 8-hour average alternative standard (the lower end of the
proposed range for the primary O3 standard) showed
substantially improved O3 air quality when compared to just
meeting the current 0.08 ppm, 8-hour standard. Most areas were
predicted to have O3 levels below the W126 level of 21 ppm-
hr, although some areas in the east (KY, TN, MI, AR, MO, IL) and west
(CA, NV, AZ, UT, NM, CO, OK, TX) were still predicted to have
O3 levels above the W126 level of 13 ppm-hour.
(4) While these results suggest that meeting a proposed 0.070 ppm,
8-hour secondary standard would provide substantially improved
protection in some areas, the Staff Paper recognized that other areas
could continue to have elevated seasonal exposures, including forested
park lands and other natural areas, and Class I areas which are
federally mandated to preserve certain air quality related values. The
proposal notes that this is especially important in the high elevation
forests in the Western U.S. where there are few O3 monitors
and where air quality patterns can result in relatively low 8-hour
averages while still experiencing relatively high cumulative exposures
(72 FR 37892).
To further characterize O3 air quality in terms of
current and alternative secondary standard forms, an analysis was
performed in the Staff Paper to evaluate the extent to which county-
level O3 air quality measured in terms of various levels of
the current 8-hour average form overlapped with that measured in terms
of various levels of the 12-hour W126 cumulative, seasonal form.\23\
This analysis was limited by the lack of monitoring in rural areas
where important vegetation and ecosystems are located, especially at
higher elevation sites. This is because O3 air quality
distributions at high elevation sites often do not reflect the typical
urban and near-urban pattern of low morning and evening O3
concentrations with a high mid-day peak, but instead maintain
relatively flat patterns with many concentrations in the mid-range
(e.g., 0.05-0.09 ppm) for extended periods. These conditions can lead
to relatively low daily maximum 8-hour averages concurrently with high
cumulative values so that there is potentially less overlap between an
8-hour average and a cumulative, seasonal form at these sites. The
Staff Paper concluded that it is reasonable to anticipate that
additional unmonitored rural high elevation areas important for
vegetation may not be adequately protected even with a lower level of
the 8-hour form.
---------------------------------------------------------------------------
\23\ The Staff Paper presented this analysis using recent (2002-
2004) county-level O3 air quality data (using 3-year
average data as well as data from each individual year) from AQS
sites and the subset of CASTNET sites having the highest
O3 levels for the counties in which they are located.
---------------------------------------------------------------------------
The Staff Paper indicated that it further remains uncertain as to
the extent to which air quality improvements designed to reduce 8-hour
O3 average concentrations would reduce O3
exposures measured by a seasonal, cumulative W126 index. The Staff
Paper indicated this to be an important consideration because: (1) The
biological database stresses the importance of cumulative, seasonal
exposures in determining plant response; (2) plants have not been
specifically tested for the importance of daily maximum 8-hour
O3 concentrations in relation to plant response; and (3) the
effects of attainment of a 8-hour standard in upwind urban areas on
rural air quality distributions cannot be characterized with confidence
due to the lack of monitoring data in rural and remote areas. These
factors are important considerations in determining whether the current
8-hour form can appropriately provide requisite protection for
vegetation.
b. Assessment of Risk to Vegetation
The Staff Paper presented results from quantitative and qualitative
risk assessments of O3 risks to vegetation. In the last
review, crop yield and seedling biomass loss OTC data provided the
basis for staff analyses, conclusions, and recommendations (EPA,
1996b). Since then, several additional lines of evidence have
progressed sufficiently to provide a basis for a more complete and
coherent picture of the scope of O3-related vegetation
risks, especially those currently faced by seedling, sapling and mature
tree species growing in field settings, and indirectly, forested
ecosystems. Specifically, new research reflects an increased emphasis
on field-based exposure methods (e.g., free air exposure and ambient
gradient), improved field survey biomonitoring techniques, and
mechanistic tree process models. Key observations and insights from the
vegetation risk assessment, together with important caveats and
limitations, were discussed in section IV.C of the proposal. Highlights
from the analyses that addressed visible foliar injury, seedling and
mature tree biomass loss, and effects on crops are summarized below:
(1) Visible foliar injury. Recent systematic injury surveys
continue to document visible foliar injury symptoms diagnostic of
phytotoxic O3 exposures on sensitive bioindicator plants.
These surveys produced more expansive evidence than that available at
the time of the last review that visible foliar injury is occurring in
many areas of the U.S. under current ambient conditions. The Staff
Paper presented an assessment combining recent U.S. Forest Service
Forest Inventory and Analysis (FIA) biomonitoring site data with the
county level air quality data for those counties containing the FIA
biomonitoring sites. This assessment showed that incidence of visible
foliar injury ranged from 21 to 39 percent of the counties during the
four-year period (2001-2004) across all counties with air quality
levels at or below that of the current 0.08 ppm 8-hour standard. Of the
counties that met an 8-hour level of 0.07 ppm in those years, 11 to 30
percent of the counties still had incidence of visible foliar injury.
The magnitude of these percentages suggests that phytotoxic exposures
sufficient to induce visible foliar injury would still occur in many
areas after meeting the level of the current secondary standard or
alternative 0.07 ppm 8-hour standard. While the data show that visible
foliar injury occurrence is geographically widespread and is occurring
on a variety of plant species in forested and other natural systems,
linking visible foliar injury to other plant effects is still
problematic. However, its presence indicates that other O3-
related vegetation effects might also be present.
(2) Seedling and mature tree biomass loss. In the last review,
analyses of the effects of O3 on trees were limited to 11
tree species for which C-R functions for the seedling growth stage had
been developed from OTC studies. Important tree species such as quaking
aspen, ponderosa pine, black cherry, and tulip poplar were found to be
sensitive to cumulative seasonal O3 exposures. Work done
since the last review at the AspenFACE site in Wisconsin on quaking
aspen (Karnosky et al., 2005) and a gradient study performed in the New
York City area (Gregg et al., 2003) have confirmed the detrimental
effects of O3 exposure on tree growth in field studies
without chambers and beyond the seedling stage (King et al., 2005). To
update the seedling biomass loss analysis, C-R functions for biomass
loss
[[Page 16489]]
for available seedling tree species taken from the Criteria Document
and information on tree growing regions derived from the U.S.
Department of Agriculture's Atlas of United States Trees were combined
with projections of air quality based on 2001 interpolated exposures,
to produce estimated biomass loss for each of the seedling tree species
individually.\24\ In summary, these analyses showed that biomass loss
still occurred in many tree species when O3 air quality was
adjusted to meet the current 8-hour standard. For instance, black
cherry, ponderosa pine, eastern white pine, and aspen had estimated
median seedling biomass losses over portions of their growing range as
high as 24, 11, 6, and 6 percent, respectively, when O3 air
quality was rolled back to just meet the current 8-hour standard. The
Staff Paper noted that these results are for tree seedlings and that
mature trees of the same species may have more or less of a response to
O3 exposure. Due to the potential for compounding effects
over multiple years, a consensus workshop on O3 effects
reported that a biomass loss greater than 2 percent annually can be
significant (Heck and Cowling, 1997). Decreased seedling root growth
and survivability could affect overall stand health and composition in
the long term.
---------------------------------------------------------------------------
\24\ Maps of these biomass loss projections were presented in
the Staff Paper (chapter 7).
---------------------------------------------------------------------------
Recent work has also enhanced our understanding of risks beyond the
seedling stage. In order to better characterize the potential
O3 effects on mature tree growth, a tree growth model
(TREGRO) was used to evaluate the effect of changing O3 air
quality scenarios from just meeting alternative O3 standards
on the growth of mature trees.\25\ The model integrates interactions
between O3 exposure, precipitation and temperature as they
affect vegetation, thus providing an internal consistency for comparing
effects in trees under different exposure scenarios and climatic
conditions. The TREGRO model was used to assess O3-related
impacts on the growth of Ponderosa pine in the San Bernardino Mountains
of California (Crestline) and the growth of yellow poplar and red maple
in the Appalachian mountains of Virginia and North Carolina, Shenandoah
National Park (Big Meadows) and Linville Gorge Wilderness Area
(Cranberry), respectively. Ponderosa pine is one of the most widely
distributed pines in western North America, a major source of timber,
important as wildlife habitat, and valued for aesthetics (Burns and
Honkala, 1990). Red maple is one of the most abundant species in the
eastern U.S. and is important for its brilliant fall foliage and highly
desirable wildlife browse food (Burns and Honkala, 1990). Yellow poplar
is an abundant species in the southern Appalachian forest. It is 10
percent of the cove hardwood stands in southern Appalachians which are
widely viewed as some of the country's most treasured forests because
the protected, rich, moist set of conditions permit trees to grow the
largest in the eastern U.S. The wood has high commercial value because
of its versatility and as a substitute for increasingly scarce
softwoods in furniture and framing construction. Yellow poplar is also
valued as a honey tree, a source of wildlife food, and a shade tree for
large areas (Burns and Honkala, 1990).
---------------------------------------------------------------------------
\25\ TREGRO is a process-based, individual tree growth
simulation model (Weinstein et al. 1991) and has been used to
evaluate the effects of a variety of O3 scenarios and
linked with concurrent climate data to account for O3 and
climate/meteorology interactions on several species of trees in
different regions of the U.S. (Tingey et al., 2001; Weinstein et
al., 1991; Retzlaff et al., 2000; Laurence et al., 1993; Laurence et
al., 2001; Weinstein et al., 2005).
---------------------------------------------------------------------------
The Staff Paper analyses found that just meeting the current
standard would likely continue to allow O3-related
reductions in annual net biomass gain in these species. This is based
on model outputs that estimate that as O3 levels are reduced
below those of the current standard, significant improvements in growth
would occur. Though there is uncertainty associated with the above
analyses, it is important to note that new evidence from experimental
studies that go beyond the seedling growth stage continues to show
decreased growth under elevated O3 (King et al., 2005); some
mature trees such as red oak have shown an even greater sensitivity of
photosynthesis to O3 than seedlings of the same species
(Hanson et al., 1994); and the potential for cumulative ``carry over''
effects as well as compounding must be considered since the
accumulation of such ``carry-over'' effects over time may affect long-
term survival and reproduction of individuals and ultimately the
abundance of sensitive tree species in forest stands.
(3) Crops. Similar to the tree seedling analysis, an analysis that
combined C-R information on crops, crop growing regions, and
interpolated exposures during each crop growing season was conducted
for commodity crops, fruits and vegetables. NCLAN crop functions
developed in the 1980s were used for commodity crops, including 9
commodity crop species (i.e., cotton, field corn, grain sorghum,
peanut, soybean, winter wheat, lettuce, kidney bean, potato) that
accounted for 69 percent of 2004 principal crop acreage planted in the
U.S. in 2004. The C-R functions for six fruit and vegetable species
(tomatoes-processing, grapes, onions, rice, cantaloupes, Valencia
oranges) were identified from the California fruit and vegetable
analysis from the last review (Abt, 1995). The risk assessment
estimated that just meeting the current 8-hour standard would still
allow O3-related yield loss to occur in some commodity crop
species and fruit and vegetable species currently grown in the U.S. For
example, based on median C-R function response, in counties with the
highest O3 levels, potatoes and cotton had estimated yield
losses of 9-15 percent and 5-10 percent, respectively, when
O3 air quality just met the level of the current standard.
Estimated yield improved in these counties when the alternative W126
standard levels were met. The very important soybean crop had generally
small yield losses throughout the country under just meeting the
current standard (0-4 percent).
The Staff Paper also presented estimates of monetized benefits for
crops associated with the current and alternative standards. The
Agriculture Simulation Model (AGSIM) (Taylor, 1994; Taylor, 1993) was
used to calculate annual average changes in total undiscounted economic
surplus for commodity crops and fruits and vegetables when current and
alternative standard levels were met. Meeting the various alternative
standards did show some significant benefits beyond the current 8-hour
standard. However, the Staff Paper recognized that the modeled economic
benefits from AGSIM had many associated uncertainties which limited the
usefulness of these estimates.
B. Need for Revision of the Current Secondary O3 Standard
1. Introduction
The initial issue to be addressed in this review of the
O3 standard is whether, in view of the advances in
scientific knowledge reflected in the Criteria Document and Staff
Paper, the current standard should be revised. As discussed in section
IV.D of the proposal, in evaluating whether it was appropriate to
propose to retain or revise the current standard, the Administrator
built upon the last review and reflected the broader body of evidence
and information now available. In the proposal, EPA presented
information, judgments, and conclusions from the last review, which
revised the secondary O3 standard by
[[Page 16490]]
setting it identical to the revised primary O3 standard, and
from the current review's evaluation of the adequacy of the current
secondary standard, including both evidence- and exposure/risk-based
considerations in the Staff Paper, as well as from the CASAC Panel's
advice and recommendations. The Staff Paper evaluation, the CASAC
Panel's views, and the Administrator's proposed conclusions on the
adequacy of the current secondary standard are presented below.
a. Staff Paper Evaluation
The Staff Paper considered the evidence presented in the Criteria
Document as a basis for evaluating the adequacy of the current
O3 standard, recognizing that important uncertainties
remain. The Staff Paper concluded that the new evidence available in
this review as described in the Criteria Document continues to support
and strengthen key policy-relevant conclusions drawn in the previous
review. Based on this new evidence, the current Criteria Document once
more concluded that: (1) A plant's response to O3 depends
upon the cumulative nature of ambient exposure as well as the temporal
dynamics of those concentrations; (2) current ambient concentrations in
many areas of the country are sufficient to impair growth of numerous
common and economically valuable plant and tree species; (3) the
entrance of O3 into the leaf through the stomata is the
critical step in O3 effects; (4) effects can occur with only
a few hourly concentrations above 0.08 ppm; (5) other environmental
biotic and abiotic factors are also influential to the overall impact
of O3 on plants and trees; and (6) a high degree of
uncertainty remains in our ability to assess the impact of
O3 on ecosystem services.
In light of the new evidence, as described in the Criteria
Document, the Staff Paper evaluated the adequacy of the current
standard based on assessments of both the most policy-relevant
vegetation effects evidence and exposure and risk-based information,
highlighted above in section IV.A and discussed in sections IV.A-C of
the proposal. In evaluating the strength of this information, the Staff
Paper took into account the uncertainties and limitations in the
scientific evidence and analyses as well as the views of CASAC. The
Staff Paper concluded that progress has been made since the last review
and generally found support in the available effects- and exposure/
risk-based information for consideration of an O3 standard
that is more protective than the current standard. The Staff Paper
further concluded that there is no support for consideration of an
O3 standard that is less protective than the current
standard. This general conclusion is consistent with the advice and
recommendations of CASAC.
i. Evidence-Based Considerations
In the last review, crop yield and tree seedling biomass loss data
obtained in OTC studies provided the basis for the Administrator's
judgment that the then current 1-hour, 0.12 ppm secondary standard was
inadequate (EPA, 1996b). Since then, several additional lines of
evidence have progressed sufficiently to provide a more complete and
coherent picture of the scope of O3-related vegetation
risks, especially those currently faced by sensitive seedling, sapling
and mature growth stage tree species growing in field settings, and
their associated forested ecosystems. Specifically, new research
reflects an increased emphasis on field-based exposure methods (e.g.,
free air, ambient gradient, and biomonitoring surveys). In reaching
conclusions regarding the adequacy of the current standard, the Staff
Paper considered the combined information from all these areas
together, along with associated uncertainties, in an integrated,
weight-of-evidence approach.
Regarding the O3-induced effect of visible foliar
injury, observations for the years 2001 to 2004 at USDA FIA
biomonitoring sites showed widespread O3-induced leaf injury
occurring in the field, including in forested ecosystems, under current
ambient O3 conditions. For a few studied species, it has
been shown that the presence of visible foliar injury is further linked
to the presence of other vegetation effects (e.g., reduced plant growth
and impaired below ground root development) (EPA, 2006), though for
most species, this linkage has not been specifically studied or where
studied, has not been found. Nevertheless, when visible foliar injury
is present, the possibility that other O3-induced vegetation
effects could also be present for some species should be considered.
Likewise, the absence of visible foliar injury should not be construed
to demonstrate the absence of other O3-induced vegetation
effects. The Staff Paper concluded that it is not possible at this time
to quantitatively assess the degree of visible foliar injury that
should be judged adverse in all settings and across all species, and
that other environmental factors can mitigate or exacerbate the degree
of O3-induced visible foliar injury expressed at any given
concentration of O3. However, the Staff Paper also concluded
that the presence of visible foliar injury alone can be adverse to the
public welfare, especially when it occurs in protected areas such as
national parks and wilderness areas. Thus, on the basis of the
available information on the widespread distribution of O3-
sensitive species within the U.S. including in areas, such as national
parks, which are afforded a higher degree of protection, the Staff
Paper concluded that the current standard continues to allow levels of
visible foliar injury in some locations that could reasonably be
considered to be adverse from a public welfare perspective. Additional
monitoring of both O3 air quality and foliar injury levels
are needed in these areas of national significance to more fully
characterize the spatial extent of this public welfare impact.
With respect to O3-induced biomass loss in trees, the
Staff Paper concluded that the new body of field-based research on
trees strengthens the conclusions drawn on tree seedling biomass loss
from earlier OTC work by documenting similar seedling responses in the
field. For example, recent empirical studies conducted on quaking aspen
at the AspenFACE site in Wisconsin have confirmed the detrimental
effects of O3 exposure on tree growth in a field setting
without chambers (Isebrands et al., 2000, 2001). In addition, results
from an ambient gradient study (Gregg et al., 2003), which evaluated
biomass loss in cottonwood along an urban-to-rural gradient at several
locations, found that conditions in the field were sufficient to
produce substantial biomass loss in cottonwood, with larger impacts
observed in downwind rural areas due to the presence of higher
O3 concentrations. These gradients from low urban to higher
rural O3 concentrations occur when O3 precursors
generated in urban areas are transported to downwind sites and are
transformed into O3. In addition, O3
concentrations typically fall to near 0 ppm at night in urban areas due
to scavenging of O3 by NOX and other compounds.
In contrast, rural areas, due to a lack of nighttime scavenging, tend
to maintain elevated O3 concentrations for longer periods.
On the basis of such key studies, the Staff Paper concluded that the
expanded body of field-based evidence, in combination with the
substantial corroborating evidence from OTC data, provides stronger
evidence than that available in the last review that ambient levels of
O3 are sufficient to produce visible foliar injury symptoms
and biomass loss in sensitive vegetative species growing in natural
environments. Further, the Staff Paper
[[Page 16491]]
judged that the consistency in response in studied species/genotypes to
O3 under a variety of exposure conditions and methodologies
demonstrates that these sensitive genotypes and populations of plants
are susceptible to adverse impacts from O3 exposures at
levels known to occur in the ambient air. Due to the potential for
compounded risks from repeated insults over multiple years in perennial
species, the Staff Paper concluded that these sensitive subpopulations
are not afforded adequate protection under the current secondary
O3 standard. Despite the fact that only a relatively small
portion of U.S. plant species have been studied with respect to
O3 sensitivity, those species/genotypes shown to have
O3 sensitivity span a broad range of vegetation types and
public use categories, including direct-use categories like food
production for human and domestic animal consumption; fiber, materials,
and medicinal production; urban/private landscaping. Many of these
species also contribute to the structure and functioning of natural
ecosystems (e.g., the EEAs) and thus, to the goods and services those
ecosystems provide (Young and Sanzone, 2002), including non-use
categories such as relevance to public welfare based on their
aesthetic, existence or wildlife habitat value.
The Staff Paper therefore concluded that the current secondary
standard is inadequate to protect the public welfare against the
occurrence of adverse levels of visible foliar injury and tree seedling
biomass loss occurring in tree species (e.g., ponderosa pine, aspen,
black cherry, cottonwood) that are sensitive and clearly important to
the public welfare.
ii. Exposure- and Risk-Based Considerations
In evaluating the adequacy of the current standard, the Staff Paper
also presented the results of exposure and risk assessments, which are
highlighted above in section IV.A.3 and discussed in section IV.C of
the proposal. Due to multiple sources of uncertainty, both known and
unknown, that continue to be associated with these analyses, the Staff
Paper put less weight on this information in drawing conclusions on the
adequacy of the current standard. However, the Staff Paper also
recognized that some progress has been made since the last review in
better characterizing some of these associated uncertainties and,
therefore concluded that the results of the exposure and risk
assessments continue to provide information useful to informing
judgments as to the relative changes in risks predicted to occur under
exposure scenarios associated with the different standard alternatives
considered. Importantly, with respect to two key uncertainties, the
uncertainty associated with continued reliance on C-R functions
developed from OTC exposure systems to predict plant response in the
field and the potential for changes in tree seedling and crop
sensitivities in the intervening period since the C-R functions were
developed, the Staff Paper concluded that recent research has provided
information useful in judging how much weight to put on these concerns.
Specifically, new field-based studies, conducted on a limited number of
tree seedling and crop species to date, demonstrate plant growth and
visible foliar injury responses in the field that are similar in nature
and magnitude to those observed previously under OTC exposure
conditions, lending qualitative support to the conclusion that OTC
conditions do not fundamentally alter the nature of the O3-
plant response. Second, nothing in the recent literature suggests that
the O3 sensitivity of crop or tree species studied in the
last review and for which C-R functions were developed has changed
significantly in the intervening period. Indeed, in the few recent
studies where this is examined, O3 sensitivities were found
to be as great as or greater than those observed in the last review.
The Staff Paper consideration of such exposure and risk analyses is
discussed below and in section IV.D.2.b of the proposal, focusing on
seedling and mature tree biomass loss, qualitative ecosystem risks, and
crop yield loss.
(1) Seedling and mature tree biomass loss. Biomass loss in
sensitive tree seedlings is predicted to occur under O3
exposures that meet the level of the current secondary standard. For
instance, black cherry, ponderosa pine, eastern white pine, and aspen
had estimated median seedling biomass losses as high as 24, 11, 6, and
6 percent, respectively, over some portions of their growing ranges
when air quality was rolled back to meet the current 8-hr standard with
the 10 percent downward adjustment for the potential O3
gradient between monitor height and short plant canopies applied. The
Staff Paper noted that these results are for tree seedlings and that
mature trees of the same species may have more or less of a response to
O3 exposure. Decreased root growth associated with biomass
loss has the potential to indirectly affect the vigor and survivability
of tree seedlings. If such effects occur on a sufficient number of
seedlings within a stand, overall stand health and composition can be
affected in the long term. Thus, the Staff Paper concluded that these
levels of estimated tree seedling growth reduction should be considered
significant and potentially adverse, given that they are well above the
2 percent level of concern identified by the 1997 consensus workshop
(Heck and Cowling, 1997).
Though there is significant uncertainty associated with this
analysis, the Staff Paper recommended that this information should be
given careful consideration in light of several other pieces of
evidence. Specifically, limited evidence from experimental studies that
go beyond the seedling growth stage continues to show decreased growth
under elevated O3 levels (King et al., 2005). Some mature
trees such as red oak have shown an even greater sensitivity of
photosynthesis to O3 than seedlings of the same species
(Hanson et al., 1994). The potential for effects to ``carry over'' to
the following year or cumulate over multiple years, including the
potential for compounding, must be considered (see 72 FR 37885;
Andersen et al., 1997; Hogsett et al., 1989; Sasek et al., 1991; Temple
et al., 1993; EPA, 1996). The accumulation of such ``carry-over''
effects over time may affect long-term survival and reproduction of
individual trees and ultimately the abundance of sensitive tree species
in forest stands.
(2) Qualitative Ecosystem Risks. In addition to the quantifiable
risk categories discussed above, the Staff Paper presented qualitative
discussions on a number of other public welfare effects categories. In
so doing, the Staff Paper concluded that the quantified risks to
vegetation estimated to be occurring under current air quality or upon
meeting the current secondary standard likely represent only a portion
of actual risks that may be occurring for a number of reasons.
First, as mentioned above, out of the over 43,000 plant species
catalogued as growing within the U.S. (USDA PLANTS database, USDA,
NRCS, 2006), only a small percentage have been studied with respect to
O3 sensitivity. Most of the studied species were selected
because of their commercial importance or observed O3-
induced visible foliar injury in the field. Given that O3
impacts to vegetation also include less obvious but often more
significant impacts, such as reduced annual growth rates and below
ground root loss, the paucity of information on other species means the
number of O3-sensitive species that exists within the U.S.
is likely greater than what is now known. Since no state in the lower
48 states has less than seven known O3-
[[Page 16492]]
sensitive plant species, with the majority of states having between 11
and 30 (see Appendix 7J-2 in Staff Paper), protecting O3-
sensitive vegetation is clearly important to the public welfare at the
national scale.
Second, the Staff Paper also took into consideration the
possibility that more subtle and hidden risks to ecosystems are
potentially occurring in areas where vegetation is being significantly
impacted. Given the importance of these qualitative and anticipated
risks to important public welfare effects categories such as ecosystem
impacts leading to potential losses or shifts in ecosystem goods and
services (e.g., carbon sequestration, hydrology, and fire disturbance
regimes), the Staff Paper concluded that any secondary standard set to
protect against the known and quantifiable adverse effects to
vegetation should also consider the anticipated, but currently
unquantifiable, potential effects on natural ecosystems.
(3) Crop Yield Loss. Exposure and risk assessments in the Staff
Paper estimated that meeting the current 8-hour standard would still
allow O3-related yield loss to occur in several fruit and
vegetable and commodity crop species currently grown in the U.S. These
estimates of crop yield loss are substantially lower than those
estimated in the last review as a result of several factors, including
adjusted exposure levels to reflect the presence of a variable
O3 gradient between monitor height and crop canopies, and
use of a different econometric agricultural benefits model updated to
reflect more recent agricultural policies (EPA, 2006b). Though these
sources of uncertainty associated with the crop risk and benefits
assessments were better documented in this review, the Staff Paper
concluded that the presence of these uncertainties make the risk
estimates suitable only as a basis for understanding potential trends
in relative yield loss and economic benefits. The Staff Paper further
recognized that actual conditions in the field and management practices
vary from farm to farm, that agricultural systems are heavily managed,
and that adverse impacts from a variety of other factors (e.g.,
weather, insects, disease) can be orders of magnitude greater than that
of yield impacts predicted for a given O3 exposure. Thus,
the relevance of such estimated impacts on crop yields to the public
welfare are considered highly uncertain and less useful as a basis for
assessing the adequacy of the current standard. The Staff Paper noted,
however, that in some experimental cases, exposure to O3 has
made plants more sensitive or vulnerable to some of these other
important stressors, including disease, insect pests, and harsh weather
(EPA, 2006a). The Staff Paper therefore concluded that this remains an
important area of uncertainty and that additional research to better
characterize the nature and significance of these interactions between
O3 and other plant stressors would be useful.
iii. Summary of Staff Paper Considerations
In summary, the Staff Paper concluded that the current secondary
O3 standard is inadequate. This conclusion was based on the
extensive vegetation effects evidence, in particular the recent
empirical field-based evidence on biomass loss in seedlings, saplings
and mature trees, and foliar injury incidence that has become available
in this review, which demonstrates the occurrence of adverse vegetation
effects at ambient levels of recent O3 air quality, as well
as evidence and exposure- and risk-based analyses indicating that
adverse effects would be predicted to occur under air quality scenarios
that meet the current standard.
b. CASAC Views
In a letter to the Administrator (Henderson, 2006c), the CASAC
O3 Panel, with full endorsement of the chartered CASAC,
unanimously concluded that ``despite limited recent research, it has
become clear since the last review that adverse effects on a wide range
of vegetation including visible foliar injury are to be expected and
have been observed in areas that are below the level of the current 8-
hour primary and secondary ozone standards.'' Therefore, ``based on the
Ozone Panel's review of Chapters 7 and 8 [of the Staff Paper], the
CASAC unanimously agrees that it is not appropriate to try to protect
vegetation from the substantial, known or anticipated, direct and/or
indirect, adverse effects of ambient O3 by continuing to
promulgate identical primary and secondary standards for O3.
Moreover, the members of the Committee and a substantial majority of
the Ozone Panel agree with EPA staff conclusions and encourage the
Administrator to establish an alternative cumulative secondary standard
for O3 and related photochemical oxidants that is distinctly
different in averaging time, form and level from the currently existing
or potentially revised 8-hour primary standard'' (Henderson,
2006c).\26\
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\26\ One CASAC Panel member reached different conclusions from
those of the broader Panel regarding certain aspects of the
vegetation effects information and the appropriate degree of
emphasis that should be placed on the associated uncertainties.
These concerns related to how the results of O3/
vegetation exposure experiments carried out in OTC can be
extrapolated to the ambient environment and how C-R functions
developed in the 1980s can be used today given that he did not
expect that current crop species/cultivars in use in 2002 would have
the same O3 sensitivity as those studied in NCLAN
(Henderson, 2007, pg. C-18).
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c. Administrator's Proposed Conclusions
At the time of proposal, in considering whether the current
secondary standard should be revised, the Administrator carefully
considered the conclusions contained in the Criteria Document, the
rationale and recommendations contained in the Staff Paper, the advice
and recommendations from CASAC, and public comments to date on this
issue. In so doing, the Administrator recognized that the secondary
standard is to protect against ``adverse'' O3 effects, as
discussed in section IV.A.3 of the proposal. In considering what
constitutes a vegetation effect that is also adverse to the public
welfare, the Administrator took into account the Staff Paper
conclusions regarding the nature and strength of the vegetation effects
evidence, the exposure and risk assessment results, the degree to which
the associated uncertainties should be considered in interpreting the
results, and the views of CASAC and members of the public. On these
bases, the Administrator proposed that the current secondary standard
is inadequate to protect the public welfare from known and anticipated
adverse O3-related effects on vegetation and ecosystems.
Ozone levels that would be expected to remain after meeting the current
secondary standard were judged to be sufficient to cause visible foliar
injury, seedling and mature tree biomass loss, and crop yield
reductions to degrees that could be considered adverse depending on the
intended use of the plant and its significance to the public welfare,
and the current secondary standard does not provide adequate protection
from such effects. Other O3-induced effects described in the
literature, including an impaired ability of many sensitive species and
genotypes within species to adapt to or withstand other environmental
stresses, such as freezing temperatures, pest infestations and/or
disease, and to compete for available resources, would also be
anticipated to occur. In the long run, the result of these impairments
(e.g., loss in vigor) could lead to premature plant death in
O3 sensitive species. Though effects on other ecosystem
components
[[Page 16493]]
have only been examined in isolated cases, effects such as those
described above could have significant implications for plant community
and associated species biodiversity and the structure and function of
whole ecosystems. These considerations also support the proposed
conclusion that the current secondary standard is not adequate and that
revision is needed to provide additional public welfare protection.
2. Comments on the Need for Revision
The above section outlines the vegetation and ecosystem effects
evidence and assessments used by the Administrator to inform his
proposed judgments about the adequacy of the current O3
secondary standard. General comments received on the proposal that
either supported or opposed the proposed decision to revise the current
O3 secondary standard are addressed in this section.
Comments related to the vegetation and ecosystem effects evidence and
information related to exposure indices are considered in section
IV.B.2.a below, and comments on vegetation exposure and risk
assessments are considered in section IV.B.2.b. Comments on specific
issues, vegetation and ecosystem effects evidence, information on
exposure indices, or the vegetation exposure and risk assessments that
relate to consideration of the appropriate form, averaging time, or
level of the O3 standard are addressed below in section
IV.C. General comments based on implementation-related factors that are
not a permissible basis for considering the need to revise the current
standard are noted in the Response to Comments document.
a. Evidence of Effects and Exposure Indices
Sections IV.A.2 and IV.A.3 above provide a summary overview of the
information on vegetation and ecosystem effects and exposure indices
used by the Administrator to inform his proposed judgments about the
adequacy of the current O3 secondary standard. As discussed
more fully below, comments received on the proposal regarding the
nature and strength of the vegetation and ecosystem effects
information, information on exposure indices, and the conclusions that
could appropriately be drawn from such information fell generally into
two groups.
One group of commenters that included national and local
environmental organizations (e.g., Environmental Defense, Appalachian
Mountain Club, Rocky Mountain Clean Air Action), NESCAUM, NACAA,
individual States, Tribal Associations, and the National Park Service
(NPS) argued that the available science clearly showed that
O3-induced vegetation and ecosystem effects are occurring at
and below levels that meet the current 8-hour standard, and therefore
provides a strong basis and support for the conclusion that the current
secondary standard is inadequate. In support of their view, these
commenters relied on the entire body of evidence available for
consideration in this review, including evidence assessed previously in
the last review. These commenters pointed to the information and
analyses in the Staff Paper and the conclusions and recommendations of
CASAC as providing a clear basis for concluding that the current
standard does not adequately protect vegetation from an array of
O3-related effects. For example, the NPS noted that
``[w]idespread foliar injury has been documented in areas meeting the
current standard; field and chamber studies indicate that
O3-induced significant growth reductions are also occurring
at levels below the current standard'' (NPS, p. 3).
In addition to the body of information already considered by EPA in
this review, these same commenters also presented new information for
the Administrator's consideration, including a number of ``new''
studies published after completion of the Criteria Document, as well as
additional information on air quality and vegetation exposures and
effects pertaining to local conditions within their State, Tribal or
federal lands, as additional support for their views that the current
standard is inadequate. For example, NESCAUM, NY, PA, and NPS all
provided air quality information describing typical O3
concentrations in areas that rarely, if ever, exceeded the level of the
current 8-hour standard in areas that still showed O3-
related vegetation effects, particularly visible foliar injury.
Building on EPA's qualitative discussions of the potential linkage
between O3 vegetation effects and effects on ecosystems, a
number of these commenters expressed concern that the possible impact
of O3-related reductions in plant productivity could result
in a reduced capacity of vegetation to serve as a carbon sink to
mitigate the impacts of rising CO2 in a changing climate,
citing to a ``new'' study on that topic (Sitch et al., 2007). Many of
these same commenters also cited to ``new'' field-based studies in the
Great Smoky Mountain National Park that find a relationship between
O3 exposure, tree stem growth loss, tree water use and
stream flow as evidence that current ambient O3 levels can
impact ecosystems and that ecosystems should be afforded protection
from such potential effects. For example, some of these commenters note
that ``new'' studies in the Great Smoky Mountain National Park
(McLaughlin, et al., 2007a, b) have found that (1) ambient
O3 caused substantial growth reductions in mature trees in a
mixed deciduous forest, which was due in part to increased
O3-induced water loss and led to seasonal losses in stem
growth of 30-50 percent for most species in a high-ozone year; (2)
increasing ambient O3 levels also resulted in depletion of
soil moisture in the rooting zone and reduced late-season streamflow in
the watershed; and (3) O3 may amplify the adverse effects of
increasing temperature on forest growth and forest hydrology and may
exacerbate the effects of drought on forest growth and stream health.
Other ``new'' research noted by these commenters as supporting EPA's
findings that current O3 exposures cause significant biomass
losses in sensitive seedlings of various tree species include a study
that predicted up to 31 percent growth loss in aspen in certain areas
of its North American range in 2001-2003 (Percy, et al., 2007). These
commenters encouraged the Administrator to consider these ``new''
studies in making his final decision.
This group of commenters strongly supported revising the current
standard, not only because in their view the available evidence
conclusively demonstrates that the current standard is inadequate to
protect sensitive vegetation, but also because the Staff Paper provides
abundant evidence that it is appropriate to establish an alternative
cumulative, seasonal secondary standard that is distinctly different in
form from the current or revised primary standard. For example, NESCAUM
states that ``[i]n light of the EPA Staff and CASAC recommendations,
and the extensive body of historical and recent monitoring and research
data upon which these recommendations were based, the option of
equating the ozone secondary NAAQS with the 8-hour primary is
inappropriate and clearly not supported by the weight of scientific
evidence.''
EPA agrees with these commenters that when evaluated as a whole,
the entire body of vegetation and ecosystem effects information
available in this review supports the need to revise the current
standard to provide increased protection from an array of
O3-related effects on sensitive vegetation and ecosystems.
EPA also agrees that the available evidence indicates that a
[[Page 16494]]
cumulative, seasonal form better reflects the scientific information on
biologically relevant exposures for vegetation. For reasons discussed
below in sections IV.C, however, EPA disagrees with aspects of these
commenters' views as to whether a standard defined in terms of a
cumulative, seasonal form is requisite to protect public welfare based
on the available scientific information.
To the extent that these and other commenters whose comments are
discussed below included ``new'' scientific studies, studies that were
published too late to be considered in the Criteria Document, in
support of their arguments for revising or not revising the standards,
EPA notes, as discussed in section I above, that as in past NAAQS
reviews, it is basing the final decisions in this review on the studies
and related information included in the O3 air quality
criteria that have undergone CASAC and public review and will consider
newly published studies for purposes of decision making in the next
O3 NAAQS review. In provisionally evaluating commenters'
arguments, as discussed in the Response to Comments document, EPA notes
that its provisional consideration of ``new'' science found that such
studies did not materially change the conclusions in the Criteria
Document.
The other main group of commenters, which included Exxon-Mobil,
UARG, API, other industry groups, The Annapolis Center for Science
Based Public Policy, individual States and other organizations
representing local energy, agriculture or business interests, expressed
the contrasting view that the limited number of studies published since
the last review and addressed in the Criteria Document provided
insufficient evidence to support a conclusion different than what was
reached in the last review. In particular, they asserted that the types
of vegetation effects evaluated in the last review have not changed,
and that the Criteria Document, Staff Paper, and CASAC have
acknowledged that the information that has become available since the
last review does not fundamentally change the conclusions reached in
the last review. As a result, they argued that the currently available
evidence fails to show that revision to the standard is requisite to
provide additional protection from these effects. In particular, Exxon-
Mobil stated that ``EPA is incorrect in concluding vegetation impacts
[occur] at or below the level of the current standard'' * * * and that
the ``newer field-based evidence EPA cites for ozone impacts on
seedlings, saplings and mature trees indicates ozone impacts but at
exposures that are likely in exceedence of the current secondary
standard.'' This commenter concluded that while these studies provide
additional support for O3-related impacts on vegetation,
including observing effects in field settings without chambers, they do
not provide support for the conclusion that ambient levels in
compliance with the current standard would result in significant
O3 impact. In addition, these commenters also generally
asserted that the evidence that has become available since the last
review does not materially reduce the uncertainties that were present
and cited by the Administrator in the last review as important factors
in her decision to set the secondary identical to the revised primary.
Those aspects of these comments that include uncertainties associated
with the exposure, risk and benefits assessments are addressed below in
section IV.2.b and in the Response to Comments document.
EPA disagrees with the commenters' assertion that the currently
available evidence has not materially reduced key uncertainties present
in the last review that factored into the Administrator's decision. For
example, there is an expansion of field-based evidence across a broad
array of vegetation effects categories, as discussed in the Criteria
Document, Staff Paper, and highlighted above in section IV.A.2. Though
in some such studies (e.g., the FACE studies) the O3
exposures are indeed at or above ambient levels, the observed
vegetation response is similar to that observed in OTC studies at
similar levels of exposure. Though these studies are still limited in
scope, it is nevertheless EPA's view that such field-based evidence
reduces the uncertainties associated with the C-R functions generated
in OTC studies that were noted by the Administrator in the last review.
Thus, the current body of evidence increases EPA's confidence in the
results from the OTC studies which demonstrate O3-related
effects below the level of the current standard. EPA has also
considered this evidence in conjunction with USDA FIA foliar injury
survey data and the Gregg et al. (2003) tree seedling biomass loss
gradient study showing effects on a sensitive tree species occurring in
the field across a range of exposure levels including levels of air
quality at to well below the level of the current secondary standard.
Taken together, EPA concludes that these studies form a coherent body
of evidence that significantly strengthens EPA's confidence that such
effects are currently occurring in the field and would continue to be
anticipated at and below the level of the current secondary standard. A
more detailed discussion of these issues can be found in the Response
to Comments document.
b. Vegetation Exposure and Risk Assessments
Section IV.A.4 above provides a summary overview of the vegetation
exposure and risk assessment information used by the Administrator to
help inform judgments about vegetation exposure and risk estimates
associated with attainment of the current and alternative standards. As
an initial matter, EPA notes that at the time of proposal, the
Administrator primarily based his conclusion on whether revision of the
secondary standard was needed primarily on evidence-based
considerations, while using the more uncertain exposure and risk
assessments in a supportive role. As discussed more fully below,
comments received on the proposal regarding these assessments and the
conclusions that could appropriately be drawn from them fell generally
into two groups. One group of commenters generally included those noted
above who supported revising the current secondary standard, while the
other group of commenters were those noted above who expressed the view
that no revision was appropriate.
The first group of commenters primarily focused on evidence-based
considerations in their support of a revised standard, while some also
referenced EPA's findings from the exposure and risk assessments in
supporting their view that the standard needed to be revised to provide
increased protection for sensitive vegetation. A few of these
commenters also provided additional exposure, risk and benefits
information from localized assessments conducted by themselves or
others in their behalf in support of their view that the standard
needed to be revised. In so doing, these commenters have generally
shown support for using such assessments to help inform a final
decision on the need to revise.
The other group of commenters expressed a number of concerns with
these assessments and generally asserted that these assessments do not
support revision of the current standard. These commenters' concerns
generally focused on (1) the method used by EPA to estimate PRB, (2)
the lack of new information since the last review that would, in their
judgment, materially reduce the uncertainties present in the
assessments conducted for the last review, and (3) EPA's interpretation
and
[[Page 16495]]
use of the results in making a judgment about the adequacy of the
current standard. These comments are addressed below.
(1) Regarding concerns related to the method used by EPA to
estimate PRB, EPA notes that this issue has been raised repeatedly
throughout the review in the context of both the primary and secondary
standards. Most generally, these commenters asserted that EPA used
unrealistically low levels of PRB that resulted in an overestimate of
risks and benefits associated with just meeting alternative standards.
EPA disagrees with this view, for the reasons discussed above in
section II.B.2.b, which addresses this and other comments related to
EPA's approach to estimating PRB and its role in exposure and risk
assessments related to the primary standard.
(2) Another concern posed by these commenters was the lack of any
new information that, in their judgment, would materially reduce the
uncertainties present in the exposure, risk and benefits assessments
conducted for the last review. For example, the Annapolis Center
asserted that ``[s]ome of the most important caveats and uncertainties
concerning the exposure and risk assessments for crop yield that were
listed in the [1996] proposal included (1) extrapolating from exposure-
response functions generated in open-top chambers to ambient
conditions; (2) the lack of a performance evaluation of the national
air quality extrapolation; (3) the methodology to adjust modeled air
quality to reflect attainment of various alternative standard options;
and (4) inherent uncertainties in models to estimate economic values
associated with attainment of alternative standard. * * * Because of
the lack of new data or substantive improvements in the risk
assessment, these same issues remain today, contributing a similar
degree of uncertainty, as was the case in the prior review.'' EPA
recognizes that important uncertainties remain in estimates of
vegetation exposure and O3-related risk to vegetation,
especially with regard to O3-related effects on crop yields.
However, EPA disagrees with comments that assert that uncertainties
have not been reduced since the last review, as discussed below.
With regard to the uncertainties associated with using the OTC C-R
functions, the Annapolis Center further stated that ``ten years have
now elapsed, and the same concentration-response functions from the OTC
studies of the 1980's are still the only viable data to use to estimate
crop loss. * * * The 1996 CASAC Panel agreed that the estimates of crop
loss at that time were highly uncertain.'' While EPA agrees that
important uncertainties continue to be associated with the use of the
C-R functions generated many years ago using OTC studies for crop yield
loss, EPA does not agree that the new information available in this
review does nothing to reduce such uncertainties identified in the last
review. As described above and in the Staff Paper and proposal, results
from the new SoyFACE and AspenFACE studies provide qualitative support
that the levels of vegetation response that have been observed in the
field are of similar magnitude as those predicted at similar exposure
levels using the OTC generated C-R functions. Therefore, EPA believes
that the uncertainties cited in the last review regarding the
appropriateness of using OTC generated C-R functions to predict
vegetation response in the field have been reduced. Providing some
further support in this regard is the limited information available in
this review on some sensitive crop species (e.g., soybean) suggesting
that O3 sensitivity has not changed significantly in the
intervening years. Taking all the above into account, EPA's level of
confidence in the applicability of the OTC generated C-R functions to
represent ambient conditions in the field has increased.
With regard to the lack of a performance evaluation of the national
air quality extrapolation, EPA notes that there have been advancements
in the tools and methods used for such extrapolations since the last
review. With respect to the generation of interpolated O3
exposure surfaces, EPA employed a different approach than that used in
the last review and undertook a quantitative assessment of the
uncertainties associated with the use of this method. This uncertainty
assessment was accomplished by sequentially dropping out of the
interpolation each monitoring site, and then recalculating the exposure
surface using the remaining monitoring sites. As discussed in the Staff
Paper, this method of evaluation may result in a slight overestimation
of error and bias for the exposure surface, since dropping out monitors
loses information that the interpolation uses in that local area. As
another point of comparison, EPA also examined the subset of rural
CASTNET sites to illustrate how the interpolation technique predicted
air quality in that rural monitoring network. For this subset, the
evaluation indicated that in general, the interpolation technique
slightly overestimated W126 exposures at relatively low levels and
underestimated W126 exposure at relatively high levels. This aspect of
the estimation method potentially resulted in an underestimation of the
more important risks associated with higher cumulative exposures in
some areas. Based on this evaluation, EPA reiterates the conclusion in
the Staff Paper that ``the calculation of error and bias metrics for
the interpolation represents a notable improvement over the 1996
assessment which did not have such an evaluation.'' EPA further
concludes that in general, the sources and likely direction of
uncertainties associated with the exposure and risk assessments have
been better accounted for and characterized than in the last review.
With regard to criticisms of the methodology used to adjust modeled
air quality to reflect attainment of various alternative standard
options, EPA notes that this issue has been raised in the context of
both the primary and secondary standards. As noted above in section
II.B.2.b, based on information in the Staff Paper (section 4.5.6) and
in more detail in a staff memorandum (Rizzo, 2006), EPA concluded that
the quadratic air quality adjustment approach used in this assessment
generally best represented the pattern of reductions across the
O3 air quality distribution observed over the last decade in
areas implementing control programs designed to attain the
O3 NAAQS. While EPA recognizes that future changes in air
quality distributions are area-specific, and will be affected by
whatever specific control strategies are implemented in the future to
attain a revised NAAQS, there is no empirical evidence to suggest that
future reductions in ambient O3 will be significantly
different from past reductions with respect to impacting the overall
shape of the O3 distribution.
With regard to comments that asserted that inherent uncertainties
in models to estimate economic values of crop loss have not been
reduced since the last review, EPA acknowledges that while an updated
state of the art model, the AGSIM benefits model, was used in this
review, substantial uncertainties remain in these estimates of economic
crop loss. Further, EPA notes that these estimates were not relied on
as a basis for reaching a decision on the need to revise the current
standard.
(3) Some commenters also asserted that the estimated exposures and
risks associated with air quality just meeting the current standard
have not appreciably changed since the last review. These commenters
used this conclusion as the basis for a claim that there is no reason
to depart from the Administrator's 1997 decision that the
[[Page 16496]]
current secondary standard is requisite to protect public welfare. EPA
believes that this claim is fundamentally flawed for three reasons.
First, it is inappropriate to compare quantitative vegetation risks
estimated in the last review with those estimated in the current
review. The 1997 risk estimates, or any comparison of the 1997 risks
estimates to the current estimates, are irrelevant for the purpose of
judging the adequacy of the current standard, as the 1997 estimates
reflect outdated analyses that have been updated in this review to
reflect the current science and as there have been significant
improvements to the modeling approaches and model inputs. Second, it is
important to take into account EPA's increased confidence in some of
the model inputs, as discussed above, since in judging the weight to
place on quantitative risk estimates it is important to examine not
only the magnitude of the estimated risks but also the degree of
confidence in those estimates. Third, quantitative vegetation risk
estimates were not the main basis for EPA's decision in setting a level
for the secondary standard in 1997, and they do not set any quantified
``benchmark'' for the Agency's decision to revise the current standard
at this time. The proposal notice made clear that decisions about the
need to revise the current standard are mainly based on an integrated
evaluation of evidence available across a broad array of vegetation
effects, while the more uncertain exposure, risk and benefits estimates
were used in a supportive role. Both the Staff Paper and proposal
clearly distinguished the roles that these different types of
information played in informing the Administrator's proposed decision.
The proposal states that ``due to multiple sources of uncertainty, both
known and unknown, that continue to be associated with these analyses,
the Staff Paper put less weight on this information in drawing
conclusions on the adequacy of the current standard. However, the Staff
Paper also recognizes that some progress has been made since the last
review in better characterizing some of these associated uncertainties
and, therefore, concluded that the results of the exposure and risk
assessments continue to provide information useful to informing
judgments as to the relative changes in risks predicted to occur under
exposure scenarios associated with the different standard alternatives
considered.'' In determining the requisite level of protection, the
Staff Paper recognized that it is appropriate to weigh the importance
of the predicted risks of these effects in the overall context of
public welfare protection, along with a determination as to the
appropriate weight to place on the associated uncertainties and
limitations of this information. Thus, while the Administrator is fully
mindful of the uncertainties associated with the estimates of exposure,
risk and benefits, as discussed above, he judges that these estimates
are still useful in providing additional support for his judgment that
the current 8-hour secondary standard does not adequately protect
sensitive vegetation.
3. Conclusions Regarding the Need for Revision
Having carefully considered the public comments, discussed above,
the Administrator believes the fundamental scientific conclusions on
the effects of O3 on vegetation and sensitive ecosystems
reached in the Criteria Document and Staff Paper, as discussed above in
section IV.A, remain valid. In considering whether the secondary
O3 standard should be revised, the Administrator finds that
evidence that has become available in this review demonstrates the
occurrence of adverse vegetation effects at ambient levels of recent
O3 air quality, and that evidence and exposure- and risk-
based analyses indicate that adverse effects would be predicted to
occur under air quality scenarios that meet the current standard,
taking into consideration both the level and form of the current
standard. Ozone exposures that would be expected to remain after
meeting the current secondary standard are sufficient to cause visible
foliar injury and seedling and mature tree biomass loss in
O3-sensitive vegetation. The Administrator believes that the
degree to which such effects should be considered to be adverse depends
on the intended use of the vegetation and its significance to the
public welfare. Other O3-induced effects described in the
literature, including an impaired ability of many sensitive species and
genotypes within species to adapt to or withstand other environmental
stresses, such as freezing temperatures, pest infestations and/or
disease, and to compete for available resources, would also be
anticipated to occur. In the long run, the result of these impairments
(e.g., loss in vigor) could lead to premature plant death in
O3 sensitive species. Though effects on other ecosystem
components have only been examined in isolated cases, effects such as
those described above could have significant implications for plant
community and associated species biodiversity and the structure and
function of whole ecosystems.
The Administrator recognizes that the secondary standard is not
meant to protect against all known observed or anticipated
O3-related effects, but only those that can reasonably be
judged to be adverse to the public welfare. In considering what
constitutes a vegetation effect that is adverse from a public welfare
perspective, the Administrator believes it is appropriate to continue
to rely on the definition of ``adverse,'' discussed in section IV.A.3
of the proposal, that imbeds the concept of ``intended use'' of the
ecological receptors and resources that are affected, and applies that
concept beyond the species level to the ecosystem level.\27\ In so
doing, the Administrator has taken note of a number of actions taken by
Congress to establish public lands that are set aside for specific uses
that are intended to provide benefits to the public welfare, including
lands that are to be protected so as to conserve the scenic value and
the natural vegetation and wildlife within such areas, and to leave
them unimpaired for the enjoyment of future generations. Such public
lands that are protected areas of national interest include national
parks and forests, wildlife refuges, and wilderness areas. Because
O3-sensitive species are generally found in such areas, and
because levels of O3 allowed by the current secondary
standard are sufficient to cause known or anticipated impairment that
the Administrator judges to be adverse to sensitive vegetation and
ecosystems in such areas, the Administrator concludes that it is
appropriate to revise the secondary standard, in part, to provide
increased protection against O3-caused impairment to such
protected vegetation and ecosystems.
---------------------------------------------------------------------------
\27\ The Administrator also recognizes that other aspects of
public welfare, as welfare is defined in the CAA, may rely on
concepts other than ``intended use.''
---------------------------------------------------------------------------
The Administrator further recognizes that States, Tribes and public
interest groups also set aside areas that are intended to provide
similar benefits to the public welfare, for residents on State and
Tribal lands, as well as for visitors to those areas. Given the clear
public interest in and value of maintaining these areas in a condition
that does not impair their intended use, and the fact that many of
these areas contain O3-sensitive vegetation, the
Administrator further concludes that it is appropriate to revise the
secondary standard in part to provide increased protection against
O3-caused impairment to vegetation and ecosystems in such
specially designated areas.
[[Page 16497]]
The Administrator also recognizes that O3-related
effects on sensitive vegetation occur in areas that have not been
afforded such special protections, ranging from vegetation used for
residential or commercial ornamental purposes, such as urban/suburban
landscaping, to land use categories that are heavily managed for
commercial production of commodities such as agricultural crops,
timber, and ornamental vegetation. For vegetation used for residential
or commercial ornamental purposes, such as urban/suburban landscaping,
there are indications that impairment to the intended use of such
vegetation can occur from O3 exposures allowed by the
current standard. While the Administrator believes that there is not
adequate information at this time to establish a secondary standard
based specifically on impairment of urban/suburban landscaping and
other uses of ornamental vegetation, he notes that a secondary standard
revised to provide protection for sensitive natural vegetation and
ecosystems may also provide some degree of protection for such
ornamental vegetation.
With respect to commercial production of commodities, however, the
Administrator notes that judgments about the extent to which
O3-related effects on commercially managed vegetation are
adverse from a public welfare perspective are particularly difficult to
reach, given that what is known about the relationship between
O3 exposures and agricultural crop yield response derives
largely from data generated almost 20 years ago. The Administrator
recognizes that there is substantial uncertainty at this time as to
whether these data remain relevant to the majority of species and
cultivars of crops being grown in the field today. In addition, the
extensive management of such vegetation may to some degree mitigate
potential O3-related effects. The management practices used
on these lands are highly variable and are designed to achieve optimal
yields, taking into consideration various environmental conditions.
Thus, while the Administrator believes that a secondary standard
revised to provide protection for sensitive natural vegetation and
ecosystems may also provide some degree of additional protection for
heavily managed commercial vegetation, the need for such additional
protection is uncertain.
Based on these considerations, and taking into consideration the
advice and recommendations of CASAC, the Administrator concludes that
the protection afforded by the current secondary O3 standard
is not sufficient and that the standard needs to be revised to provide
additional protection from known and anticipated adverse effects on
sensitive natural vegetation and sensitive ecosystems, and that such a
revised standard could also be expected to provide additional
protection to sensitive ornamental vegetation. The Administrator also
concludes that there is not adequate information to establish a
separate secondary standard based on other effects of O3 on
public welfare. It is important to note that these conclusions, and the
reasoning on which they are based, do not address the question of what
specific revisions to the current secondary standard are appropriate.
Addressing that question requires looking specifically at the two
proposed options: establishing a new standard defined in terms of a
cumulative, seasonal form, or revising the current secondary standard
by making it identical to the revised primary standard. These
alternative secondary standards are discussed in the following section.
As highlighted below, the discussion of public comments above
indicates that deciding the appropriate secondary standard involves
making a difficult choice between two possible alternatives, each with
their strengths and weaknesses. EPA's decision, and the reasons for it,
are described in detail above. In reaching this decision, there has
been a robust discussion within the Administration of these same
strengths and weaknesses. As part of that process EPA received a
Memorandum on March 6, 2008 from Susan Dudley, Administrator, Office of
Information and Regulatory Affairs, Office of Management and Budget,
indicating various concerns over adopting a cumulative, seasonal
secondary standard. Deputy Administrator Marcus Peacock responded with
a Memorandum dated March 7, 2008 stating EPA's views supporting
adoption of a cumulative, seasonal secondary standard. On March 11,
2008, the President ``concluded that, consistent with Administration
policy, added protection should be afforded to public welfare by
strengthening the secondary ozone standard and setting it to be
identical to the new primary standard, the approach adopted when ozone
standards were last promulgated. This policy thus recognizes the
Administrator's judgment that the secondary standard needs to be
adjusted to provide increased protection to public welfare and avoids
setting a standard lower or higher than is necessary.'' EPA's decision
therefore also reflects the view of the Administration as to the most
appropriate secondary standard. While the Administrator fully
considered the President's views, the Administrator's decision, and the
reasons for it, are based on and supported by the record in this
rulemaking.
C. Conclusions on the Secondary O3 Standard
As an initial matter, EPA has considered the indicator for a
secondary O3 standard. As discussed above in section II.C.1
on the primary standard, in the last review, EPA focused on a standard
for O3 as the most appropriate surrogate for ambient
photochemical oxidants. In this review, while the complex atmospheric
chemistry in which O3 plays a key role has been highlighted,
no alternatives to O3 have been advanced as being a more
appropriate surrogate for ambient photochemical oxidants and their
effects on vegetation. Thus, as is the case for the primary standard,
the Administrator concludes that it is appropriate to continue to use
O3 as the indicator for a standard that is intended to
address effects associated with exposure to O3, alone and in
combination with related photochemical oxidants. In so doing, the
Administrator recognizes that measures leading to reductions in
vegetation exposures to O3 will also reduce exposures to
other photochemical oxidants.
1. Staff Paper Evaluation
The current Criteria Document and Staff Paper concluded that the
recent vegetation effects literature evaluated in this review
strengthens and reaffirms conclusions made in the last review that the
use of a cumulative exposure index that differentially weights ambient
concentrations is best able to relate ambient exposures to vegetation
response at this time (EPA, 2006a, b). The last review focused in
particular on two of these cumulative forms, the SUM06 and W126 (EPA,
1996). Given that the data available at that time were unable to
distinguish between these forms, the Administrator, based on the policy
consideration of not including O3 concentrations considered
to be within the PRB, estimated to be between 0.03 and 0.05 ppm,
concluded that the SUM06 form would be the more appropriate choice for
a cumulative, exposure index for a secondary standard, though a
cumulative form was not adopted at that time.
In this review, the Staff Paper evaluated the continued
appropriateness of the SUM06 form in
[[Page 16498]]
light of two key pieces of information: new estimates of PRB that are
lower than in the last review, and continued lack of evidence within
the vegetation effects literature of a biological threshold for
vegetation exposures of concern. On the basis of those policy and
science-related considerations, the Staff Paper concluded that the W126
form was more appropriate in the context of this review. Specifically,
the W126, by its incorporation of a sigmoidal weighting function, does
not create an artificially imposed concentration threshold, gives
proportionally more weight to the higher and typically more
biologically potent concentrations, and is not significantly influenced
by O3 concentrations within the range of estimated PRB.
The Staff Paper also considered that in the 1997 final rule, the
decision was made, on the basis of both science and policy
considerations, to make the secondary standard identical to the primary
standard (62 FR 38876). On the basis of that history, the current Staff
Paper analyzed the degree of overlap expected between alternative 8-
hour and cumulative seasonal secondary standards using recent air
quality monitoring data. Based on the results, the Staff Paper
concluded that the degree to which the current 8-hour standard form and
level would overlap with areas of concern for vegetation expressed in
terms of the 12-hour W126 standard is inconsistent from year to year
and would depend greatly on the level of the 12-hour W126 and 8-hour
standards selected and the distribution of hourly O3
concentrations within the annual and/or 3-year average period.
Thus, though the Staff Paper recognized again that meeting the
current or alternative levels of the 8-hour average standard could
result in air quality improvements that would potentially benefit
vegetation in some areas, it urged caution be used in evaluating the
likely vegetation impacts associated with a given level of air quality
expressed in terms of the 8-hour average form in the absence of
parallel W126 information. This caution is due to the concern that the
analysis in the Staff Paper may not be an accurate reflection of the
true situation in non-monitored, rural counties due to the lack of more
complete monitor coverage in many rural areas. Further, of the counties
that did not show overlap between the two standard forms, most were
located in rural/remote high elevation areas which have O3
air quality patterns that are typically different from those associated
with urban and near urban sites at lower elevations. Because the
majority of such areas are currently not monitored, it is believed
there are likely to be additional areas that have similar air quality
distributions that would lead to the same disconnect between forms.
Thus, the Staff Paper concluded that it remains problematic to
determine the appropriate level of protection for vegetation using an
8-hour average form.
2. CASAC Views
The CASAC, based on its assessment of the same vegetation effects
science, agreed with the Criteria Document and Staff Paper and
unanimously concluded that protection of vegetation from the known or
anticipated adverse effects of ambient O3 ``requires a
secondary standard that is substantially different from the primary
standard in averaging time, level, and form,'' i.e. not identical to
the primary standard for O3 (Henderson, 2007). Moreover, the
members of CASAC and a substantial majority of the CASAC Panel agreed
with Staff Paper conclusions and encouraged the Administrator to
establish an alternative cumulative secondary standard for
O3 and related photochemical oxidants that is distinctly
different in averaging time, form and level from the current or
potentially revised 8-hour primary standard (Henderson, 2006c). The
CASAC Panel also stated that ``the recommended metric for the secondary
ozone standard is the (sigmoidally weighted) W126 index'' (Henderson,
2007).
3. Administrator's Proposed Conclusions
In EPA's proposal, the Administrator agreed with the conclusions
drawn in the Criteria Document, Staff Paper and by CASAC that the
scientific evidence available in the current review continues to
demonstrate the cumulative nature of O3-induced plant
effects and the need to give greater weight to higher concentrations.
Thus, the Administrator proposed that a cumulative exposure index that
differentially weights O3 concentrations could represent a
reasonable policy choice for a seasonal secondary standard to protect
against the effects of O3 on vegetation. The Administrator
further agreed with both the Staff Paper and CASAC that the most
appropriate cumulative, concentration-weighted form to consider in this
review is the sigmoidally weighted W126 form, due to his recognition
that there is no evidence in the literature for an exposure threshold
that would be appropriate across all O3-sensitive vegetation
and that this form is unlikely to be significantly influenced by
O3 air quality within the range of PRB levels identified in
this review. Thus, the Administrator proposed as one option to replace
the current 8-hour average secondary standard form with the cumulative,
seasonal W126 form.
The Administrator also proposed to revise the current secondary
standard by making it identical to the proposed 8-hour primary
standard, which was proposed to be within the range of 0.070 to 0.075
ppm. For this option, EPA also solicited comment on a wider range of 8-
hour standard levels, including levels down to 0.060 ppm and up to the
current standard (i.e., effectively 0.084 ppm with the current rounding
convention). In putting forward such a proposal, the Administrator
focused on the decision made in the last review, and the rationale for
that decision that made the revised secondary standard identical to the
revised primary standard.
4. Comments on the Secondary Standard Options
Comments received following proposal regarding revising the
secondary standard either to reflect a new, cumulative form or by
remaining equal to a revised primary standard generally fell into two
groups. These comments were similar to those raised prior to the
proposal during earlier phases of the NAAQS review, as summarized in
the proposal notice and highlighted below.
One group of commenters, including the National Park Service,
Environmental Defense, NESCAUM, NACAA, individual States, Tribal
Associations, and local environmental organizations, asserted that the
weight of scientific evidence was unambiguous with regard to the need
for a cumulative form, and specifically supported the proposed W126
exposure index. For example, New York State DEC explained that
``scientific research recognizes that exposure-based indices
considering seasonal time period, exposure duration, diurnal dynamics,
peak hourly ozone concentrations, and cumulative effects are important
when assessing vegetation effects of ozone exposure (Musselman et al.,
2006). The W126 exposure index has long been recognized as a
biologically meaningful and useful way to summarize hourly ozone data
as a measure of ozone exposure to vegetation (Lefohn et al., 1989)''.
Similarly, Environmental Defense stated ``[f]or reasons amply explained
by CASAC and the Staff, neither the existing secondary standard for
ozone nor the proposed primary standards are requisite to protect
against
[[Page 16499]]
adverse welfare effects on vegetation and forested ecosystems. CASAC
and Staff further amply justified the need for a separate cumulative
seasonal welfare standard to protect against these effects, rather than
relying solely on the primary standards to provide such protection.''
The National Park Service (NPS) comment provided additional support to
this view and more specifically stated that ``the NPS supports both the
conclusion that a seasonal, cumulative metric is needed to protect
vegetation, and that the W126 is a more appropriate metric than the
SUM06.'' EPA agrees with these comments for the reasons discussed above
in sections IV.A.3 and IV.B.2.a).
In addition to expressing strong support for the W126 cumulative
seasonal form, commenters in this group also expressed serious concerns
with EPA's other proposed option of setting the secondary standard
equal to a revised primary standard. For example, NPS agreed with CASAC
that ``retaining the current form of the 8-hour standard for the
secondary NAAQS is inappropriate and inadequate for characterizing
ozone exposures to vegetation.'' NESCAUM stated ``we also strongly
encourage EPA to avoid the flawed rationale employed in the previous
1997 ozone NAAQS review, i.e., that many of the benefits of a secondary
NAAQS would be achieved if the primary NAAQS were attained. This
rationale is flawed in at least two ways: first, ozone damage to
vegetation persists in areas that attain the primary NAAQS; and second,
the relationship between short-term 8-hour peak concentrations and
longer-term seasonal aggregations is not constant, but varies over
space and time * * * as EPA notes at 72 FR 37904. * * * EPA should set
a secondary NAAQS on its own independent merits based on adverse
welfare effects. Real or perceived relationships between primary and
secondary nonattainment areas are irrelevant to setting the appropriate
form and level of the secondary NAAQS.'' Environmental Defense made the
argument that ``[b]ecause there is no rational connection between the
proposed primary standards and the level of protection needed to
protect vegetation against adverse ozone-induced welfare effects, any
EPA finding that the primary standards would be sufficient for
secondary standards purposes would be arbitrary.* * * The mere fact
that the primary might provide ancillary welfare benefits does not
satisfy the statute and does not provide a rational basis for
concluding that the primary standards are also requisite to protect to
[sic] any adverse welfare effects.''
The other set of commenters, including UARG, API, Exxon-Mobil, The
Annapolis Center, ASL and Associates, and AAM, did not support adopting
an alternative, cumulative form for the secondary standard. Some of
these commenters, while agreeing that ``directionally a cumulative form
of the standard may better match the underlying data,'' believe that
further work is needed to determine whether a cumulative exposure index
for the form of the secondary standard is requisite to protect public
welfare. These commenters also restated concerns that have been
described above in section IV.B.2 regarding the remaining uncertainties
associated with the vegetation effects evidence and/or the exposure,
risk and benefits assessments. They point to the uncertainties cited by
the Administrator in the 1997 review as part of her rationale for
deciding it was not appropriate to move forward with a seasonal
secondary, and state that these same uncertainties have not been
materially reduced in the current review. These commenters also
asserted that EPA's analysis of the impact of the nation's
O3 control program for the 8-hour standard on W126 exposures
is not scientifically sound due to the use of low estimates of PRB and
an arbitrary rollback method that is uninformed by atmospheric
chemistry from photochemical models. They argue that EPA must first
realistically evaluate the total O3 reductions that would
occur by using a state-of-the-art photochemical model and perform an
analysis of the exposure-response data to determine if effects are
observed for exposures which do not exceed the 8-hour standard. These
commenters also stated that without producing C-R functions for the 8-
hour form of the standard, EPA has failed to show that the current 8-
hour standard would provide less than requisite protection. These
commenters asserted that substantial uncertainties remain in this
review, and that the benefits of changing to a W126 form are too
uncertain to warrant revising the form of the standard at this time.
This group of commenters also addressed limitations associated with
selection of the W126 cumulative form. Commenters asserted that: (1)
The W126 form lacks a biological basis, since it is merely a
mathematical expression of exposure that has been fit to specific
responses in OTC studies, such that its relevance for real world
biological responses is unclear; (2) a flux-based model would be a
better choice than a cumulative metric because it is an improvement
over the many limitations and simplifications associated with the
cumulative form; however, there is insufficient data to apply such a
model at present; (3) the European experience with cumulative
O3 metrics has been disappointing and now Europeans are
working on their second level approach, which will be flux-based; and
(4) the W126 form cannot provide nationally uniform protection, as the
same value of an exposure index may relate to different vegetation
responses; some commenters support adding a second index that reflects
the accumulation of peaks at or above 0.10 ppm (called N100).
5. Administrator's Final Conclusions
In considering the appropriateness of establishing a new standard
defined in terms of a cumulative, seasonal form, or revising the
current secondary standard by making it identical to the revised
primary standard, the Administrator took into account the approach used
by the Agency in the last review, the conclusions of the Staff Paper,
CASAC advice, and the views of public commenters. In giving careful
consideration to the approach taken in the last review, the
Administrator first considered the Staff Paper analysis of the
projected degree of overlap between counties with air quality expected
to meet the revised 8-hour primary standard, set at a level of 0.075
ppm, and alternative levels of a W126 standard based on currently
monitored air quality data. This analysis showed significant overlap
between the revised 8-hour primary standard and selected levels of the
W126 standard form being considered, with the degree of overlap between
these alternative standards depending greatly on the W126 level
selected and the distribution of hourly O3 concentrations
within the annual and/or 3-year average period.\28\ On this basis, as
an initial matter, the Administrator recognizes that a secondary
standard set identical to the proposed primary standard would provide a
significant degree of additional protection for vegetation as compared
to that provided by the current secondary standard. In further
considering the significant uncertainties that remain in the available
body of evidence of O3-related vegetation effects and in the
exposure and risk analyses conducted for this review, and the
difficulty in determining at what point various types of vegetation
effects become adverse for sensitive vegetation and ecosystems, the
Administrator focused his consideration on a level for
[[Page 16500]]
an alternative W126 standard at the upper end of the proposed range
(i.e., 21 ppm-hours). The Staff Paper analysis shows that at that W126
standard level, there would be essentially no counties with air quality
that would be expected both to exceed such an alternative W126 standard
and to meet the revised 8-hour primary standard--that is, based on this
analysis of currently monitored counties, a W126 standard would be
unlikely to provide additional protection in any areas beyond that
likely to be provided by the revised primary standard.
---------------------------------------------------------------------------
\28\ EPA has done further analysis of the degree of overlap, and
that analysis is in the docket.
---------------------------------------------------------------------------
The Administrator also recognizes that the general lack of rural
monitoring data makes uncertain the degree to which the revised 8-hour
standard or an alternative W126 standard would be protective, and that
there would be the potential for not providing the appropriate degree
of protection for vegetation in areas with air quality distributions
that result in a high cumulative, seasonal exposure but do not result
in high 8-hour average exposures. While this potential for under-
protection is clear, the number and size of areas at issue and the
degree of risk is hard to determine. However, such a standard would
also tend to avoid the potential for providing more protection than is
necessary, a risk that would arise from moving to a new form for the
secondary standard despite significant uncertainty in determining the
degree of risk for any exposure level and the appropriate level of
protection, as well as uncertainty in predicting exposure and risk
patterns.
The Administrator also considered the views and recommendations of
CASAC, and agrees that a cumulative, seasonal standard is the most
biologically relevant way to relate exposure to plant growth response.
However, as reflected in the public comments, the Administrator also
recognizes that there remain significant uncertainties in determining
or quantifying the degree of risk attributable to varying levels of
O3 exposure, the degree of protection that any specific
cumulative, seasonal standard would produce, and the associated
potential for error in determining the standard that will provide a
requisite degree of protection--i.e., sufficient but not more than what
is necessary. Given these significant uncertainties, the Administrator
concludes that establishing a new secondary standard with a cumulative,
seasonal form at this time would result in uncertain benefits beyond
those afforded by the revised primary standard and therefore may be
more than necessary to provide the requisite degree of protection.
Based on his consideration of the full range of views as described
above, the Administrator judges that the appropriate balance to be
drawn is to revise the secondary standard to be identical in every way
to the revised primary standard. The Administrator believes that such a
standard would be sufficient to protect public welfare from known or
anticipated adverse effects, and does not believe that an alternative
cumulative, seasonal standard is needed to provide this degree of
protection. This judgment by the Administrator appropriately considers
the requirement for a standard that is neither more nor less stringent
than necessary for this purpose.
D. Final Decision on the Secondary O3 Standard
For the reasons discussed above, and taking into account
information and assessments presented in the Criteria Document and
Staff Paper, the advice and recommendations of the CASAC Panel, and the
public comments to date, the Administrator has decided to revise the
existing 8-hour secondary standard. Specifically, the Administrator is
revising the current standard by making it identical to the revised
primary standard. Data handling conventions for the secondary standard
are the same as for the primary standard, and are specified in the new
Appendix P that is adopted, as discussed in section V below. Issues
related to the monitoring requirements for the revised O3
secondary standard are discussed below in section VI.
V. Creation of Appendix P--Interpretation of the NAAQS for
O3
This section presents EPA's final decisions regarding the addition
of Appendix P to 40 CFR part 50 on interpreting the primary and
secondary NAAQS for O3. EPA did not propose to address
revocation of the existing 8-hour standard in this rulemaking.
Therefore, EPA is retaining Appendix I to 40 CFR part 50 in its current
form. A new Appendix P explains the computations necessary for
determining when the new 8-hour primary and secondary standards are
met. More specifically, Appendix P addresses data completeness
requirements, data reporting and handling conventions, and rounding
conventions, and provides example calculations.
In the proposal, two alternative secondary standards were proposed:
a 3-month secondary standard expressed as a cumulative peak-weighted
index form; or a standard set to be identical to the primary standard.
For reasons stated above, the Administrator has decided to set the
secondary standard to be identical in all respects to the primary
standard. Therefore, the portions of the proposed Appendix P providing
data handling procedures for a non-identical secondary standard are not
included in the final rule.
Key elements of Appendix P are outlined below.
A. General
As proposed, EPA is adding several new definitions to section 1.0
and using these definitions throughout Appendix P.
B. Data Completeness
EPA proposed data completeness requirements for the new Appendix P
for the revised 8-hour primary standard that would be the same as those
in Appendix I applicable to the pre-existing standard. To satisfy the
data completeness requirement, Appendix P as proposed would require 90%
data completeness, on average, for the 3-year period at a monitoring
site, with no single year within the period having less than 75% data
completeness. This data completeness requirement applies only during
the required O3 monitoring season and must be satisfied in
order to determine that the standard has been met at a monitoring site.
A site could be found to violate the standard with less than complete
data. EPA concluded in adopting these same data completeness
requirements in Appendix I in 1997 that these proposed requirements are
reasonable based on its earlier analysis of available air quality data
that showed that 90% of all monitoring sites that are operated on a
continuous basis routinely meet this objective. EPA received no
comments on these requirements, and the final Appendix P includes them
as proposed.
Appendix I and the proposed Appendix P allow missing days to be
counted for the purpose of meeting the data completeness requirements
if meteorological conditions on these missing days were not conducive
to concentrations above the level of the standard. Such determinations
under Appendix I and the proposed Appendix P would be made on a case-
by-case basis using available evidence. In the proposal, EPA
specifically requested comment on whether meteorological data could
provide an objective basis for determining, for a day for which there
is missing data, that the meteorological conditions were not conducive
to high O3 concentrations, and therefore, that the day could
be assumed to have an O3 concentration less than the level
of the
[[Page 16501]]
NAAQS. Further, the proposal requested comments on whether days assumed
less than the level of the standard should be counted as non-missing
when computing whether the data completeness requirements have been met
at the site. The proposal pointed out that this could allow a
determination of attainment which would otherwise be precluded by the
75% and/or 90% completeness tests. Most commenters supported the use of
meteorological data to establish that missing days could be assumed to
have low O3 levels. However, no commenter suggested any
particular objective criteria or formula for making such
determinations. Based on these comments, EPA will continue to use the
current case-by-case approach as proposed in Appendix P, as is the
current approach in Appendix I, to count missing days when computing
whether the data completeness requirement has been met for the primary
standard.
As noted above, because the Administrator has decided to set the
secondary standard identical in all respects to the primary standard,
the final Appendix P provides that its data completeness requirements
apply to both standards.
C. Data Reporting and Handling and Rounding Conventions
For reasons discussed above, the Administrator has set the level of
the revised 8-hour primary and secondary standards at 0.075 ppm. As
explained in the proposal, the level of the 8-hour standard is
expressed to the third decimal place. Almost all State agencies now
report hourly O3 concentrations to three decimal places, in
ppm, or in a format easily convertible to ppm, since the typical
incremental sensitivity of currently used O3 monitors is
0.001 ppm. Consistent with the current approach for computing 8-hour
averages, in calculating 8-hour average O3 concentrations
from hourly data, any calculated digits beyond the third decimal place
would be truncated, preserving the number of digits in the reported
data. In calculating 3-year averages of the fourth highest maximum 8-
hour average concentrations, digits to the right of the third decimal
place would also be truncated, preserving the number of digits in the
reported data. Analyses discussed in the Staff Paper demonstrated that
taking into account the precision and bias in 1-hour O3
measurements, the 8-hour design value has an uncertainty of
approximately 0.001 ppm. Truncating both the individual 8-hour averages
used to determine the annual fourth maximum as well as the 3-year
average of the fourth maxima to the third decimal place is consistent
with the approach used in Appendix I for the previous 8-hour
O3 standard. In the proposal, EPA sought comment on the
appropriateness of rounding rather than truncating to the third decimal
place as well as the scientific validity of truncating the 3-year
average and the policy reasons behind either truncating or rounding the
3-year average to the third decimal place. Many of the comments EPA
received on the rounding/truncation issue in effect were comments that
supported expressing the level of the NAAQS to either the second or
third decimal place. These comments are addressed in the Response to
Comment document. EPA continues to believe the conclusions from the
Staff paper regarding monitor precision and error propagation when
calculating 8-hour O3 averages are appropriate. EPA has
decided to continue to truncate, as done in Appendix I, and this
approach is included in the final Appendix P.
As discussed above in section II.C.3, EPA is setting an 8-hour
standard extending to three decimal places. Given that both the
standard and the calculated value of the 3-year average of the fourth
highest maximum 8-hour O3 concentration are expressed to
three decimal places, the two values can be compared directly.
As noted above, because the Administrator has decided to set the
secondary standard identical in all respects to the primary standard,
the same data reporting and handling and rounding conventions will
apply to both.
VI. Ambient Monitoring Related to Revised O3 Standards
As noted in the O3 NAAQS proposal (see 72 FR 37906), EPA
did not propose any specific changes to existing requirements for
monitoring of O3 in the ambient air. However, comment was
invited on a number of specific issues which were expected to be of
significance in the event that one or more of the O3 NAAQS
was revised. Comments were received from Federal agencies, State
monitoring agencies, State organizations, environmental organizations,
and industrial trade associations. As noted elsewhere in this
rulemaking, EPA is finalizing changes to both the primary and secondary
O3 NAAQS. In light of these revisions, EPA intends to issue
a monitoring rule to address the issues identified in the proposal, as
well as other issues raised in the comments. EPA intends to issue a
proposed monitoring rule in June 2008 and a final rule by March 2009.
In recognition of the comments received on the proposed O3
standards and to provide EPA's initial thinking on O3
specific monitoring rule amendments, we offer the following
observations. The following paragraphs also point out one way in which
some State/local monitoring agencies might need to make changes to
their O3 monitoring network as a result of the revision to
the primary and secondary O3 NAAQS, based on the existing
minimum monitoring requirements including a factor based on the
comparison of design value to the O3 NAAQS (see 71 FR
61318). The following text explains why an amendment to the monitoring
regulations is not required to trigger these increased O3
monitoring requirements.
Presently, States (including the District of Columbia, Puerto Rico,
and the Virgin Islands, and including local agencies when so delegated
by the State) are required to operate minimum numbers of EPA-approved
O3 monitors based on the population of each of their
Metropolitan Statistical Areas (MSA) and the most recently measured
O3 levels in each area. These requirements are contained in
40 CFR part 58 Appendix D, Network Design Criteria for Ambient Air
Quality Monitoring, Table D-2. These requirements were last revised on
October 17, 2006 as part of a comprehensive review of ambient
monitoring requirements for all criteria pollutants. (See 71 FR 61236).
The minimum number of monitors required in an MSA ranges from zero
(for an area with population under 350,000 and no recent history of an
O3 design value greater than 85 percent of the NAAQS) to
four (for an area with population greater than 10 million and an
O3 design value greater than 85 percent of the NAAQS).
Because these requirements apply at the MSA level, large urban areas
consisting of multiple MSAs can require more than four monitors. In
total, about 400 monitors are required in MSAs, but about 1100 are
actually operating in MSAs because most States operate more than the
minimum required number of monitors.
As noted above, the requirements listed in Table D-2 of 40 CFR part
58 Appendix D are based on the percentage of the O3 NAAQS,
with a design value breakpoint at 85 percent of the NAAQS. For an MSA
of a given population size, there are a greater number of required
monitors when the design value is greater than or equal to 85 percent
of the O3 NAAQS compared with MSAs that have a design value
of less than 85 percent of the O3 NAAQS. At the pre-existing
level of 0.084 ppm for the 8-hour primary and secondary standards,
[[Page 16502]]
an 8-hour O3 design value of 0.068 ppm would trigger such
increased minimum monitoring requirements for an MSA.\29\ With the
decision to revise the 8-hour primary and secondary standards to a
level of 0.075 ppm, the 8-hour O3 design value that will
trigger increased minimum monitoring requirements for an MSA has
decreased from 0.068 ppm to 0.064 ppm. Therefore, MSAs with 8-hour
design values between 0.064 ppm and 0.067 ppm are now required to
increase the number of monitors operating to meet minimum requirements
based on existing monitoring requirements.\30\ In practice, however,
virtually all of these areas already are operating at least as many
monitors as required based on the revised primary standard, so the
number of new monitors that are needed (or needed to be moved from a
location of excess monitors) is negligible to meet the existing minimum
requirements.
---------------------------------------------------------------------------
\29\ Calculated as 85 percent of 0.08 ppm, per the stated level
of the pre-existing 8-hour primary and secondary standards.
\30\ Approximately 16 MSAs that are subject to minimum
monitoring requirements have 8-hour design values between 0.064 ppm
and 0.067 ppm based on an analysis of 2004-2006 ambient
O3 data.
---------------------------------------------------------------------------
About 100 MSAs with populations less than 350,000 presently are
without any O3 monitors, and hence they do not have an
O3 design value for use with Table D-2. These unmonitored
MSAs are not required to add monitors. Commenters from State monitoring
agencies and State organizations expressed concern that these current
requirements ignore the needs that States and localities will have for
additional monitors to measure O3 levels in currently under-
monitored areas and, in particular, in unmonitored areas with
populations under 350,000. They stated that unless this deficiency is
corrected, the health benefits of EPA's O3 NAAQS revision
would likely be limited to those living in Metropolitan Statistical
Areas (MSAs) having populations of more than 350,000. Other commenters
noted the difficulty in defining the boundaries of new attainment/non-
attainment areas without additional monitoring in the MSAs below
350,000.
EPA recognizes that the issues raised by the commenters are
important. EPA intends to address these issues as part of its proposed
monitoring rule.
In relation to the proposed secondary standard options, EPA invited
comment on whether, where, and how monitoring in rural areas
specifically focused on the secondary NAAQS should be required. As
noted in the O3 NAAQS proposal and described earlier in this
section, existing O3 monitoring requirements and current
State monitoring practices are primarily oriented towards protecting
against health effects in people and therefore the primary NAAQS. This
accounts for the current focus of the monitoring requirements on urban
areas, where large populations reside, in which significant emissions
of O3 -forming precursors are found, and where O3
concentrations of concern are likely to occur.
There are no EPA requirements for O3 monitoring in less
populated areas outside of MSA boundaries or in rural areas. However,
at present there are about 250 O3 monitors in counties that
are not part of MSAs. These monitors are operated by State, local, and
tribal monitoring agencies for a variety of objectives including the
assessment of O3 transport and the support of research
programs including studies of atmospheric chemistry and ecosystem
impacts. Additionally, EPA operates a network of about 56 O3
monitors as part of its Clean Air Status and Trends Network (CASTNET).
The National Park Service (NPS) operates about 27 monitors at other
CASTNET sites. On an overall basis, the spatial density of non-urban
O3 monitors is relatively high in the eastern one-third of
the U.S. and in California, with significant gaps in coverage elsewhere
across the country.
Some commenters expressed concern about the quality assurance
practices at CASTNET sites with regard to certain aspects of
O3 monitoring. They recommended that EPA upgrade such
practices to meet the 40 CFR part 58 Appendix A quality assurance
requirements already followed by the States so that the resulting data
could be used in assessing compliance with the revised secondary
standard. EPA notes that such upgrades have been completed at some of
the CASTNET sites, and that such upgrades will be completed at all
CASTNET sites by 2009. EPA notes that the resulting O3
ambient data from the upgraded sites will meet Appendix A requirements
as is presently the case for O3 data from State operated
monitors and NPS monitors. These data will be deemed acceptable for
NAAQS-comparison objectives and available in the AQS database beginning
in 2008.
Most commenters noted the relative lack of rural O3
monitors, stating that EPA should consider adding monitoring
requirements that support a revised secondary O3 standard by
requiring O3 monitors in locations that contain
O3-sensitive plants or ecosystems. These commenters also
noted that the placement of current O3 monitors may not be
appropriate for evaluating vegetation exposure since many of these
monitors were likely located to meet other objectives.
In light of the Administrator's decision to revise the 8-hour
secondary standard, EPA believes that it is appropriate to consider
whether the existing urban-based monitoring requirements described
elsewhere in this section are adequate and appropriate to characterize
the exposure in more rural areas where O3-sensitive plant
species and more sensitive ecosystems exist and where resulting
vegetation damage would adversely affect land usage. Such areas would
likely include public lands that are protected areas of national
interest (e.g., national parks, wilderness areas).
In consideration of the spatial gaps that currently exist in the
rural ozone monitoring network, and to the extent that the existence of
such gaps has contributed to the overall uncertainty that exists in the
level of protection that would be provided by the revised secondary
standard, EPA believes that there is merit in considering whether
additional monitoring requirements in certain rural areas would help
support ongoing ecosystem research studies as well as future reviews of
the O3 NAAQS by providing a more robust data set with which
to assess the relationship of vegetation damage to O3
concentrations.
Accordingly, as part of its separate monitoring rulemaking, EPA
intends to consider specific requirements for a minimum number of rural
monitors per State, with detailed rule language to ensure that States
locate such monitors in appropriate areas. For example, these areas
could include Federal, State, or Tribal lands characterized by areas of
sensitive vegetation species subject to visible foliar injury, seedling
and mature tree biomass loss, and other adverse impacts to a degree
that could be considered adverse depending on the intended use of the
plant and its significance to the public welfare. EPA is also
considering recommending that States and Tribes employ other
quantitative tools, such as photochemical modeling and/or the spatial
interpolation of ambient data from existing O3 monitors, to
determine the adequacy of existing locations of rural monitors and to
inform the locations of new or relocated monitors that might be
required to meet revised rural minimum monitoring requirements.
Finally, EPA solicited comment on the issue of O3
monitoring seasons. Unlike the year-round monitoring required for other
criteria pollutants, the
[[Page 16503]]
required O3 monitoring seasons \31\ vary in length due to
the inter-relationship of O3-forming photochemical activity
with ambient temperature, strength of solar insolation, and length of
day. For example, in States with colder climates such as Montana and
South Dakota, the O3 season has a length of 4 months. In
States with warmer climates such as California, Nevada, and Arizona,
the O3 season has a length of 12 months.
With the decision to revise the 8-hour primary standard to a level
of 0.075 ppm, and to set the secondary standard identical in all
respects to the primary standard, the issue arises of whether in some
areas the required O3 monitoring season should be made
longer. EPA notes that under the existing regulations, the Regional
Administrator may approve State-requested deviations from the
established O3 monitoring season, but EPA may not increase
the length of the season for an area at EPA's own initiative other than
by notice and comment rulemaking.
EPA has done a preliminary analysis of 2004-2006 ambient data to
address the issue of whether extensions of currently required
O3 monitoring seasons are appropriate in light of the
revised level for the primary and secondary O3 standards and
the revised breakpoints for the AQI. The results of the analysis
demonstrated that out-of-season exceedances of the revised level
occurred in eight States during the study period. Additionally, the
frequency of days with O3 concentrations that reached the
revised Moderate AQI category (based on a breakpoint of 0.060 ppm) was
much greater compared with the frequency of days with concentrations
that reached the pre-existing Moderate AQI category (based on a
breakpoint of 0.065 ppm). This increased frequency of days with
Moderate AQI levels was noted to occur during periods before and after
the currently required O3 seasons.
Based on these preliminary analyses, EPA intends to consider
changes to the length of the required O3 season for the
coming monitoring rulemaking. Such changes could be based solely on the
frequency of exceedances of the revised primary and secondary
standards, or could also consider the frequency of concentrations in
the Moderate category of the AQI.
VII. Implementation and Related Control Requirements
A. Future Implementation Steps
In today's rule, EPA is replacing the existing (1997) standards
with revised primary and secondary O3 standards. However,
the 1997 standards--and the implementation rules for those standards--
will remain in place for implementation purposes as EPA undertakes
rulemaking to address the transition from the 1997 O3
standards to the 2008 O3 standards. States are required to
continue to develop and implement their State Implementation Plans
(SIPs) for the 1997 standards as they begin the process of recommending
designations for the 2008 standards.
1. Designations
After EPA establishes or revises a NAAQS, the CAA requires EPA and
States to begin taking steps to ensure that the new or revised
standards are met. The first step is to identify areas of the country
that do not attain the new or revised standards, or that contribute to
violations of the new or revised standards. Section 107(d)(1) provides
``By such date as the Administrator may reasonably require, but not
later than 1 year after promulgation of a new or revised national
ambient air quality standard for any pollutant under section 109, the
Governor of each State shall * * * submit to the Administrator a list
of all areas (or portions thereof) in the State'' that designates those
areas as non-attainment, attainment, or unclassifiable. Section
107(d)(1)(B)(i) further provides, ``Upon promulgation or revision of a
national ambient air quality standard, the Administrator shall
promulgate the designations of all areas (or portions thereof) * * * as
expeditiously as practicable, but in no case later than 2 years from
the date of promulgation. Such period may be extended for up to one
year in the event the Administrator has insufficient information to
promulgate the designations.''
The term ``promulgation'' has been interpreted by the courts to be
signature and dissemination of a rule.\32\ As noted above, the CAA
requires EPA to establish a deadline for the States' submission of the
designation recommendations, but under the CAA, it can be no later than
March 12, 2009, one year after the promulgation of this rule.
Therefore, Governors of States should submit their designation
recommendations to EPA no later than March 12, 2009. EPA's promulgation
of designations must occur no later than March 12, 2010, although that
date may be extended by up to one year under the CAA (no later than
March 12, 2011) if EPA has insufficient information to promulgate the
designations.
EPA intends to provide additional guidance to the States concerning
the technical considerations for establishing boundaries for designated
areas. For the revised primary and secondary standards, we anticipate
relying on past O3 designation guidance issued by EPA prior
to the designations for the 1997 O3 standards.\33\ We
anticipate working closely with State air agencies and Tribes on
establishing new guidance on designations, if needed.
2. State Implementation Plans
CAA section 110 provides the general requirements for SIPs. Within
3 years after the promulgation of new or revised NAAQS (or such shorter
period as the Administrator may prescribe) each State must adopt and
submit ``infrastructure'' SIPs to EPA to address the requirements of
section 110(a)(1). Thus, States should submit these SIPs no later than
March 12, 2011. These ``infrastructure SIPs'' provide assurances of
State resources and authorities, and establish the basic State
programs, to implement, maintain, and enforce new or revised standards.
In addition to the infrastructure SIPs, which apply to all States,
CAA title I, part D outlines the State requirements for achieving clean
air in designated nonattainment areas. These requirements include
timelines for when designated nonattainment areas must attain the
standards, deadlines for developing SIPs that demonstrate how the State
will ensure attainment of the standards, and specific emissions control
requirements. EPA plans to address how these requirements, such as
attainment demonstrations and attainment dates, reasonable further
progress, new source review, conformity, and other implementation
requirements, apply to the revised O3 NAAQS in a proposed
rulemaking in Fall 2008. Also in that rulemaking EPA will establish
deadlines for submission of nonattainment area SIPs but anticipates
that the deadlines will be no later than 3 years after final
designation. Depending on the classification of an area, the SIP must
provide for attainment within 3 years (for areas classified marginal)
to 20 years (for areas classified extreme) after final designations.
3. Trans-boundary Emissions
Cross border O3 contributions from within North America
(Canada and Mexico) entering the U.S. are generally thought to be
small. Section 179B of the
[[Page 16504]]
Clean Air Act allows designated nonattainment areas to petition EPA to
consider whether such a locality might have met a clean air standard
``but for'' cross border contributions. To date, few areas have
petitioned EPA under this authority. The impact of foreign emissions on
domestic air quality in the United States is a challenging and complex
problem to assess. EPA is engaged in a number of activities to improve
our understanding of international transport. As work progresses on
these activities, EPA will be able to better address the uncertainties
associated with trans-boundary flows of air pollution and their
impacts.
4. Monitoring Requirements
As discussed more fully in section VI, EPA intends, in light of the
revisions of the O3 standards, to issue a monitoring rule to
address a variety of monitoring-related issues identified in the
preamble to the proposed rule or in comments received by the Agency on
the proposal. EPA intends to issue a proposed monitoring rule in June
2008 and a final rule by March 2009.
---------------------------------------------------------------------------
\31\ See 40 CFR Part 58 Appendix D, section 2.5 for a table of
required O3 seasons.
\32\ American Petroleum Institute v. Costle, 609 F.2d 20 (D.C.
Cir. 1979).
\33\ Memorandum of March 28, 2000 from John Seitz, ``Boundary
Guidance on Air Quality Designations for the 8-Hour Ozone National
Ambient Air Quality Standards (NAAQS or Standard).''
---------------------------------------------------------------------------
B. Related Control Requirements
The man-made oxides of nitrogen (NOX) and volatile
organic carbon (VOC) emissions that contribute to O3
formation in the United States come from a variety of source
categories, including mobile sources, industrial processes, area-wide
sources (which include consumer and commercial products), and the
electric power industry.\34\ Emissions from natural sources, such as
trees and wildfires can also constitute a significant portion of total
VOC emissions in certain regions of the country, especially during the
O3 season. Natural sources such as wildfires, lightning, and
soils also emit NOX. Emissions of VOCs and NOX
from these sources are considered natural background emissions.\35\
---------------------------------------------------------------------------
\34\ National Emission Inventory posted at the following Web
site: http://www.epa.gov/ttn/chief/trends/index.html.
\35\ In some cases natural emissions may cause or significantly
contribute to violations of the ozone standard. EPA has issued rules
that address how these ``exceptional events'' can be discounted in
regulatory determinations. The Exceptional Events Rule (72 FR 13560
(March 22, 2007) implements CAA section 319(b)(3)(B) and section
107(d)(3) authority to exclude air quality monitoring data from
regulatory determinations related to exceedances or violations of
the National Ambient Air Quality Standards (NAAQS). If an event is
determined by EPA to be a qualifying exceptional event, the affected
area may avoid being designated as nonattainment, being redesignated
as nonattainment, or being reclassifed to a higher classification.
The requirements for demonstrating that elevated ozone levels are
the result of a qualifying exceptional event are provided in the
Exceptional Events Rule.
---------------------------------------------------------------------------
EPA has developed new emissions standards for many types of
stationary sources and for nearly every class of mobile sources in the
last decade to reduce O3 by decreasing emissions of
NOX and VOC. These programs complement State and local
efforts to improve air quality and to meet the national O3
standards. Under the Federal Motor Vehicle Control Program (FMVCP, see
title II of the CAA, 42 U.S.C. 7521-7574), EPA has established new
emissions standards for nearly every type of automobile, truck, bus,
motorcycle, earth mover, and aircraft engine, and for the fuels used to
power these engines. Also, EPA established new standards for the
smaller engines used in small watercraft, lawn and garden equipment.
Recently, EPA proposed new standards for locomotive and marine diesel
engines. Vehicles and engines are replaced over time with newer,
cleaner models. In time, these programs will yield substantial
emissions reductions. Emissions reductions associated with fuel
programs generally begin as soon as a new fuel is available.
The reduction of VOC emissions from industrial processes and
consumer and commercial product categories has been achieved either
directly or indirectly through implementation of control technology
standards, including reasonably available control technology, best
available control technology, and maximum achievable control technology
standards; or is anticipated due to proposed or upcoming proposals
based on generally available control technology or best available
controls under provisions related to consumer and commercial products.
These standards have resulted in VOC emissions reductions of almost a
million tons per year accumulated starting in 1997 from a variety of
sources including combustion sources, coating categories, and chemical
manufacturing. In 2006 and 2007, EPA issued national rules and control
techniques guidelines for control of VOC emissions from 10 categories
of consumer and commercial products. EPA is currently working to
finalize new Federal rules, or amendments to existing rules, intended
to establish new nationwide VOC content limits for several categories
of consumer and commercial products, including aerosol coatings,
architectural and industrial maintenance coatings, and household and
institutional commercial products. EPA anticipates that final rules
addressing emissions from these sources will take effect in 2009.
Fuel combustion is one of the largest anthropogenic sources of
emissions of NOX in the United States. Power industry
emission sources include large electric generating units and some large
industrial boilers and turbines. The EPA's landmark Clean Air
Interstate Rule (CAIR), issued on March 10, 2005, permanently caps
power industry emissions of NOX in the eastern United
States. The first phase of the cap begins in 2009, and a lower second
phase cap begins in 2015. By 2015, EPA projects that the CAIR and other
programs in the Eastern U.S. will reduce power industry annual
NOX emissions in that region by about 60 percent from 2003
levels.
With respect to agricultural sources, the U.S. Department of
Agriculture (USDA) has recommended conservation systems and activities
that can reduce agricultural emissions of NOX and VOC.
Current practices that may reduce emissions of NOX and VOC
include engine replacement programs, management of pesticide
applications, and manure management techniques. The EPA recognizes that
USDA has been working with the agricultural community to plan
conservation systems and activities to manage emissions of
O3 precursors.
These conservation systems and activities can be voluntarily
adopted in areas where mitigation of O3 precursors have been
identified as an air quality concern through the use of incentives
provided to the agricultural producer. In cases where the States need
these measures to attain the O3 standards, agricultural
producers could choose to adopt these measures. The EPA will continue
to work with USDA on planning the implementation of these conservation
systems and activities in order to identify and/or improve mitigation
efficiencies, prioritize their adoption, and ensure that appropriate
criteria are used for identifying the most effective application of
conservation systems and activities.
The EPA will work together with USDA and with States to identify
appropriate measures to meet the primary and secondary standards,
including site-specific conservation systems and activities. Based on
prior experience identifying conservation measures and practices to
meet the PM NAAQS requirements, the EPA will use a similar process to
identify measures that could meet the O3 requirements. The
EPA anticipates that certain USDA-approved conservation systems and
activities that reduce agricultural emissions of NOX and VOC
may be able to satisfy the requirements for
[[Page 16505]]
applicable sources to implement reasonably available control measures
for purposes of attaining the primary and secondary O3
NAAQS.
VIII. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review
Under section 3(f)(1) of Executive Order (EO) 12866 (58 FR 51735,
October 4, 1993), this action is an ``economically significant
regulatory action'' because it is likely to have an annual effect on
the economy of $100 million or more. Accordingly, EPA submitted this
action to the Office of Management and Budget (OMB) for review under EO
12866 and any changes made in response to OMB recommendations have been
documented in the docket for this action. In addition, EPA prepared an
analysis of the potential costs and benefits associated with this
action. This analysis is contained in the Final Ozone NAAQS Regulatory
Impact Analysis, March 2008 (henceforth, ``RIA''). A copy of the
analysis is available in the RIA docket (EPA-HQ-OAR-2007-0225) and the
analysis is briefly summarized here. The RIA estimates the costs and
monetized human health and welfare benefits of attaining three
alternative O3 NAAQS nationwide. Specifically, the RIA
examines the alternatives of 0.079 ppm, 0.075 ppm, 0.070 ppm, and 0.065
ppm. The RIA contains illustrative analyses that consider a limited
number of emissions control scenarios that States and Regional Planning
Organizations might implement to achieve these alternative
O3 NAAQS. However, the CAA and judicial decisions make clear
that the economic and technical feasibility of attaining ambient
standards are not to be considered in setting or revising NAAQS,
although such factors may be considered in the development of State
plans to implement the standards. Accordingly, although a RIA has been
prepared, the results of the RIA have not been considered in issuing
this final rule.
B. Paperwork Reduction Act
This action does not impose an information collection burden under
the provisions of the Paperwork Reduction Act, 44 U.S.C. 3501 et seq.
There are no information collection requirements directly associated
with the establishment of a NAAQS under section 109 of the CAA.
Burden means the total time, effort, or financial resources
expended by persons to generate, maintain, retain, or disclose or
provide information to or for a Federal agency. This includes the time
needed to review instructions; develop, acquire, install, and utilize
technology and systems for the purposes of collecting, validating, and
verifying information, processing and maintaining information, and
disclosing and providing information; adjust the existing ways to
comply with any previously applicable instructions and requirements;
train personnel to be able to respond to a collection of information;
search data sources; complete and review the collection of information;
and transmit or otherwise disclose the information.
An agency may not conduct or sponsor, and a person is not required
to respond to a collection of information unless it displays a
currently valid OMB control number. The OMB control numbers for EPA's
regulations in 40 CFR are listed in 40 CFR part 9.
C. Regulatory Flexibility Act
The Regulatory Flexibility Act (RFA) generally requires an agency
to prepare a regulatory flexibility analysis of any rule subject to
notice and comment rulemaking requirements under the Administrative
Procedure Act or any other statute unless the agency certifies that the
rule will not have a significant economic impact on a substantial
number of small entities. Small entities include small businesses,
small organizations, and small governmental jurisdictions.
For purposes of assessing the impacts of this rule on small
entities, small entity is defined as: (1) A small business that is a
small industrial entity as defined by the Small Business
Administration's (SBA) regulations at 13 CFR 121.201; (2) a small
governmental jurisdiction that is a government of a city, county, town,
school district or special district with a population of less than
50,000; and (3) a small organization that is any not-for-profit
enterprise which is independently owned and operated and is not
dominant in its field.
After considering the economic impacts of this final rule on small
entities, I certify that this action will not have a significant
economic impact on a substantial number of small entities. This final
rule will not impose any requirements on small entities. Rather, this
rule establishes national standards for allowable concentrations of
O3 in ambient air as required by section 109 of the CAA.
American Trucking Ass'ns v. EPA, 175 F. 3d 1027, 1044-45 (D.C. cir.
1999) (NAAQS do not have significant impacts upon small entities
because NAAQS themselves impose no regulations upon small entities).
D. Unfunded Mandates Reform Act
Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Public
Law 104-4, establishes requirements for Federal agencies to assess the
effects of their regulatory actions on State, local, and Tribal
governments and the private sector. Under section 202 of the UMRA, EPA
generally must prepare a written statement, including a cost-benefit
analysis, for proposed and final rules with ``Federal mandates'' that
may result in expenditures to State, local, and Tribal governments, in
the aggregate, or to the private sector, of $100 million or more in any
one year. Before promulgating an EPA rule for which a written statement
is needed, section 205 of the UMRA generally requires EPA to identify
and consider a reasonable number of regulatory alternatives and to
adopt the least costly, most cost-effective or least burdensome
alternative that achieves the objectives of the rule. The provisions of
section 205 do not apply when they are inconsistent with applicable
law. Moreover, section 205 allows EPA to adopt an alternative other
than the least costly, most cost-effective or least burdensome
alternative if the Administrator publishes with the final rule an
explanation why that alternative was not adopted. Before EPA
establishes any regulatory requirements that may significantly or
uniquely affect small governments, including Tribal governments, it
must have developed under section 203 of the UMRA a small government
agency plan. The plan must provide for notifying potentially affected
small governments, enabling officials of affected small governments to
have meaningful and timely input in the development of EPA regulatory
proposals with significant Federal intergovernmental mandates, and
informing, educating, and advising small governments on compliance with
the regulatory requirements.
This final rule contains no Federal mandates (under the regulatory
provisions of Title II of the UMRA) for State, local, or Tribal
governments or the private sector. The rule imposes no new expenditure
or enforceable duty on any State, local or Tribal governments or the
private sector, and EPA has determined that this rule contains no
regulatory requirements that might significantly or uniquely affect
small governments. Furthermore, as indicated previously, in setting a
NAAQS EPA cannot consider the economic or technological feasibility of
attaining ambient air quality standards, although such factors may be
considered to a degree in the development of State
[[Page 16506]]
plans to implement the standards. See also American Trucking Ass'ns v.
EPA, 175 F. 3d at 1043 (noting that because EPA is precluded from
considering costs of implementation in establishing NAAQS, preparation
of a Regulatory Impact Analysis pursuant to the Unfunded Mandates
Reform Act would not furnish any information which the court could
consider in reviewing the NAAQS). Thus, this rule is not subject to the
requirements of sections 202 and 205 of the UMRA. EPA has determined
that this rule contains no regulatory requirements that might
significantly or uniquely affect small governments.
E. Executive Order 13132: Federalism
Executive Order 13132, entitled ``Federalism'' (64 FR 43255, August
10, 1999), requires EPA to develop an accountable process to ensure
``meaningful and timely input by State and local officials in the
development of regulatory policies that have federalism implications.''
``Policies that have federalism implications'' is defined in the
Executive Order to include regulations that have ``substantial direct
effects on the States, on the relationship between the national
government and the States, or on the distribution of power and
responsibilities among the various levels of government.''
This final rule does not have federalism implications. It will not
have substantial direct effects on the States, on the relationship
between the national government and the States, or on the distribution
of power and responsibilities among the various levels of government,
as specified in Executive Order 13132. The rule does not alter the
relationship between the Federal government and the States regarding
the establishment and implementation of air quality improvement
programs as codified in the CAA. Under section 109 of the CAA, EPA is
mandated to establish NAAQS; however, CAA section 116 preserves the
rights of States to establish more stringent requirements if deemed
necessary by a State. Furthermore, this rule does not impact CAA
section 107 which establishes that the States have primary
responsibility for implementation of the NAAQS. Finally, as noted in
section E (above) on UMRA, this rule does not impose significant costs
on State, local, or Tribal governments or the private sector. Thus,
Executive Order 13132 does not apply to this rule.
F. Executive Order 13175: Consultation and Coordination With Indian
Tribal Governments
Executive Order 13175, entitled ``Consultation and Coordination
with Indian Tribal Governments'' (65 FR 67249, November 9, 2000),
requires EPA to develop an accountable process to ensure ``meaningful
and timely input by tribal officials in the development of regulatory
policies that have tribal implications.'' This final rule does not have
Tribal implications, as specified in Executive Order 13175. It does not
have a substantial direct effect on one or more Indian Tribes, since
Tribes are not obligated to adopt or implement any NAAQS. Thus,
Executive Order 13175 does not apply to this rule.
Although Executive Order 13175 does not apply to this rule, EPA
contacted Tribal environmental professionals during the development of
this rule. EPA staff participated in the regularly scheduled Tribal Air
call sponsored by the National Tribal Air Association during the spring
of 2007 as the proposal was under development. EPA specifically
solicited additional comment on the proposed rule from Tribal
officials. Comments from Tribal officials on the proposed rule are
summarized in the Response to Comments document.
G. Executive Order 13045: Protection of Children From Environmental
Health & Safety Risks
Executive Order 13045, ``Protection of Children from Environmental
Health Risks and Safety Risks'' (62 FR 19885, April 23, 1997) applies
to any rule that: (1) Is determined to be ``economically significant''
as defined under Executive Order 12866, and (2) concerns an
environmental health or safety risk that EPA has reason to believe may
have a disproportionate effect on children. If the regulatory action
meets both criteria, the Agency must evaluate the environmental health
or safety effects of the planned rule on children, and explain why the
planned regulation is preferable to other potentially effective and
reasonably feasible alternatives considered by the Agency.
This final rule is subject to Executive Order 13045 because it is
an economically significant regulatory action as defined by Executive
Order 12866, and we believe that the environmental health risk
addressed by this action may have a disproportionate effect on
children. Accordingly, we have evaluated the environmental health or
safety effects of exposure to O3 pollution among children.
These effects and the size of the population affected are summarized in
section 8.7 of the Criteria Document and section 3.6 of the Staff
Paper, and the results of our evaluation of the effects of
O3 pollution on children are discussed in sections II.A-C of
this preamble.
H. Executive Order 13211: Actions That Significantly Affect Energy
Supply, Distribution or Use
Executive Order 13211, ``Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution, or Use'' (66 FR 28355
(May 22, 2001)), requires EPA to prepare and submit a Statement of
Energy Effects to the Administrator of the Office of Information and
Regulatory Affairs, Office of Management and Budget, for certain
actions identified as ``significant energy actions.'' Section 4(b) of
Executive Order 13211 defines ``significant energy actions'' as ``any
action by an agency (normally published in the Federal Register) that
promulgates or is expected to lead to the promulgation of a final rule
or regulation, including notices of inquiry, advance notices of
proposed rulemaking, and notices of proposed rulemaking: (1)(i) That is
a significant regulatory action under Executive Order 12866 or any
successor order, and (ii) is likely to have a significant adverse
effect on the supply, distribution, or use of energy; or (2) that is
designated by the Administrator of the Office of Information and
Regulatory Affairs as a significant energy action.'' The U.S. Office of
Management and Budget has designated this rulemaking as a significant
energy action. Accordingly, EPA has prepared a Statement of Energy
Effects for this action which appears in Chapter 9 of the RIA conducted
for this rulemaking. A copy of the RIA is available in the RIA docket
(EPA-HQ-OAR-2007-0225) and the energy analysis is briefly summarized
here. The analysis estimates potential impacts of an illustrative
control strategy for the 0.070 ppm primary standard alternative on the
production of coal, crude oil, natural gas, and electricity; on energy
prices; on control technologies adopted by the electricity generating
sector; and on the mix of electricity generation. EPA believes that the
energy impacts estimated for this illustrative control strategy for the
0.070 ppm primary standard alternative are higher than those that would
be estimated for an illustrative control strategy for the primary
standard level of 0.075 ppm which was selected by the Administrator.
However, due to modeling limitations, EPA did not generate separate
estimates of the energy impacts associated specifically with an
[[Page 16507]]
illustrative control strategy designed for a primary standard of 0.075
ppm. It is important to note that the CAA make clear that the economic
impacts associated with attaining ambient standards are not to be
considered in setting or revising the NAAQS. Accordingly, although the
Statement of Energy Effects has been prepared, the results of EPA's
energy analysis have not been considered in issuing this final rule.
I. National Technology Transfer and Advancement Act
As noted in the proposed rule, section 12(d) of the National
Technology Transfer and Advancement Act of 1995 (NTTAA), Public Law
104-113, section 12(d) (15 U.S.C. 272 note) directs EPA to use
voluntary consensus standards in its regulatory activities unless to do
so would be inconsistent with applicable law or otherwise impractical.
Voluntary consensus standards are technical standards (e.g., materials
specifications, test methods, sampling procedures, and business
practices) that are developed or adopted by voluntary consensus
standards bodies. The NTTAA directs EPA to provide Congress, through
OMB, explanations when the Agency decides not to use available and
applicable voluntary consensus standards.
This action does not involve technical standards. Therefore, EPA
did not consider the use of any voluntary consensus standards.
J. Executive Order 12898: Federal Actions To Address Environmental
Justice in Minority Populations and Low-Income Populations
Executive Order 12898 (59 FR 7629; Feb. 16, 1994) establishes
Federal executive policy on environmental justice. Its main provision
directs Federal agencies, to the greatest extent practicable and
permitted by law, to make environmental justice part of their mission
by identifying and addressing, as appropriate, disproportionately high
and adverse human health or environmental effects of their programs,
policies, and activities on minority populations and low-income
populations in the United States.
EPA has determined that this final rule will not have
disproportionately high and adverse human health or environmental
effects on minority or low-income populations because it increases the
level of environmental protection for all affected populations without
having any disproportionately high and adverse human health or
environmental effects on any population, including any minority or low-
income population. This final rule will establish uniform national
standards for O3 air pollution.
K. Congressional Review Act
The Congressional Review Act, 5 U.S.C. 801 et seq., as added by the
Small Business Regulatory Enforcement Fairness Act of 1996, generally
provides that before a rule may take effect, the agency promulgating
the rule must submit a rule report, which includes a copy of the rule,
to each House of the Congress and to the Comptroller General of the
United States. EPA submitted a report containing this rule and other
required information to the U.S. Senate, the U.S. House of
Representatives, and the Comptroller General of the United States prior
to publication of the rule in the Federal Register. A major rule cannot
take effect until 60 days after it is published in the Federal
Register. This action is a ``major rule'' as defined by 5 U.S.C.
804(2). This rule will be effective May 27, 2008.
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Triangle Park, NC. July 2007; EPA report no. EPA-452/R-07-009.
Available online at: http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_cr_td.html.
Abt Associates Inc. (2007b) Technical Report on Ozone Exposure,
Risk, and Impacts Assessments for Vegetation: Final Report. Prepared
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4059. October 5, 2007.
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National Ambient Air Quality Standards for Ozone. Docket No. OAR-
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Docket No. OAR-2005-0172 re: Proposed Rule--National Ambient Air
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5, 2007.
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Medicine/American College of Occupational and Environmental
Medicine/National Association for the Medical Direction of
Respiratory Care (ATS et al.) (2007) Letter and Comments Sent to
Stephen Johnson, Administrator re: Proposed Rule--National Ambient
Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4305.
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Temple, P. J.; Riechers, G. H.; Miller, P. R.; Lennox, R. W. (1993)
Growth responses of ponderosa pine to longterm exposure to ozone,
wet and dry acidic deposition, and drought. Can. J. For. Res. 23:
59-66.
Texas Commission on Environmental Quality (2007) Letter and Comments
Sent to Docket No. OAR-2005-0172 re: Proposed Rule--National Ambient
Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4435.
October 9, 2007.
Texas Department of Transportation (2007) Letter and Comments Sent
to Docket No. OAR-2005-0172 re: Proposed Rule--National Ambient Air
Quality Standards for Ozone. Docket No. OAR-2005-0172-4409. October
9, 2007.
Tingey, D. T.; Laurence, J. A.; Weber, J. A.; Greene, J.; Hogsett,
W. E.; Brown, S.; Lee, E. H. (2001) Elevated CO2 and
temperature alter the response of Pinus ponderosa to ozone: A
simulation analysis. Ecol. Appl.11: 1412-1424.
U.S. Department of Agriculture, 2006. The PLANTS Database (http://plants.usda.gov,
[[Page 16511]]
December 2006). National Plant Data Center, Baton Rouge, LA.
Utah Department of Environmental Quality, Division of Air Quality
(2007) Letter and Comments Sent to Docket No. OAR-2005-0172 re:
Proposed Rule--National Ambient Air Quality Standards for Ozone.
Docket No. OAR-2005-0172-4138. October 9, 2007.
Utility Air Regulatory Group (2007) Letter and Comments Sent to
Docket No. OAR-2005-0172 re: Proposed Rule--National Ambient Air
Quality Standards for Ozone. Docket No. OAR-2005-0172-4183. October
9, 2007.
Union of Concerned Scientists (UCS) (2007) Letter and Comments Sent
to Docket No. OAR-2005-0172 re: Proposed Rule--National Ambient Air
Quality Standards for Ozone. Docket No. OAR-2005-0172-4768. October
9, 2007.
Vedal, S.; Brauer, M.; White, R.; Petkau, J. (2003) Air pollution
and daily mortality in a city with low levels of pollution. Environ.
Health Perspect. 111: 45-51.
Washington State Department of Transportation (2007) Letter and
Comments Sent to Docket No. OAR-2005-0172 re: Proposed Rule--
National Ambient Air Quality Standards for Ozone. Docket No. OAR-
2005-0172-4157. October 8, 2007.
Washington State Department of Ecology (2007) Letter and Comments
Sent to Docket No. OAR-2005-0172 re: Proposed Rule--National Ambient
Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4267.
October 9, 2007.
Weinstein, D.A., Beloin, R.M., R.D. Yanai (1991) ``Modeling changes
in red spruce carbon balance and allocation in response to
interacting ozone and nutrient stress.'' Tree Physiology 9: 127-146.
Weinstein, D.A., J.A. Laurence, W.A. Retzlaff, J.S. Kern, E.H. Lee,
W.E. Hogsett, J. Weber (2005) Predicting the effects of tropospheric
ozone on regional productivity of ponderosa pine and white fir.
Forest Ecology and Management 205: 73-89.
Wisconsin Department of Natural Resources (2007) Letter and Comments
Sent to Docket No. OAR-2005-0172 re: Proposed Rule--National Ambient
Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4358.
October 9, 2007.
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Probabilistic Assessment of Health Risks Associated with Short- and
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Associated with Short-term Exposure to Tropospheric Ozone: A
Supplement. Argonne National Laboratory, Argonne, IL.
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of Ozone Human Exposure and Health Risk Analyses Used in the U.S.
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Schneider, ed. Air Pollution in the 21st Century: Priority Issues
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Wolff, G.T. (1995) Letter from Chairman of Clean Air Scientific
Advisory Committee to the EPA Administrator, dated November 30,
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Wolff, G.T. (1996) Letter from Chairman of Clean Air Scientific
Advisory Committee to the EPA Administrator, dated April 4, 1996.
EPA-SAB-CASAC-LTR-96-006.
Young, T. F.; Sanzone, S., eds. (2002) A framework for assessing and
reporting on ecological condition: an SAB report. Washington, DC:
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yosemite.epa.gov/sab/sabproduct.nsf/
C3F89E598D843B58852570CA0075717E/$File/epec02009a.pdf.
List of Subjects
40 CFR Part 50
Environmental protection, Air pollution control, Carbon monoxide,
Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.
40 CFR Part 58
Environmental protection, Air pollution control, Reporting and
recordkeeping requirements.
Dated: March 12, 2008.
Stephen L. Johnson,
Administrator.
0
For the reasons stated in the preamble, title 40, chapter I of the code
of Federal regulations is to be amended as follows:
PART 50--NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY
STANDARDS
0
1. The authority citation for part 50 continues to read as follows:
Authority: 42 U.S.C. 7401, et seq.
0
2. Section 50.15 is added to read as follows:
Sec. 50.15 National primary and secondary ambient air quality
standards for ozone.
(a) The level of the national 8-hour primary and secondary ambient
air quality standards for ozone (O3) is 0.075 parts per million (ppm),
daily maximum 8-hour average, measured by a reference method based on
Appendix D to this part and designated in accordance with part 53 of
this chapter or an equivalent method designated in accordance with part
53 of this chapter.
(b) The 8-hour primary and secondary O3 ambient air quality
standards are met at an ambient air quality monitoring site when the 3-
year average of the annual fourth-highest daily maximum 8-hour average
O3 concentration is less than or equal to 0.075 ppm, as determined in
accordance with Appendix P to this part.
0
3. Appendix P is added to read as follows:
Appendix P to Part 50--Interpretation of the Primary and Secondary
National Ambient Air Quality Standards for Ozone
1. General
(a) This appendix explains the data handling conventions and
computations necessary for determining whether the national 8-hour
primary and secondary ambient air quality standards for ozone (O3)
specified in Sec. 50.15 are met at an ambient O3 air quality
monitoring site. Ozone is measured in the ambient air by a reference
method based on Appendix D of this part, as applicable, and
designated in accordance with part 53 of this chapter, or by an
equivalent method designated in accordance with part 53 of this
chapter. Data reporting, data handling, and computation procedures
to be used in making comparisons between reported O3 concentrations
and the levels of the O3 standards are specified in the following
sections. Whether to exclude, retain, or make adjustments to the
data affected by exceptional events, including stratospheric O3
intrusion and other natural events, is determined by the
requirements under Sec. Sec. 50.1, 50.14 and 51.930.
(b) The terms used in this appendix are defined as follows:
8-hour average is the rolling average of eight hourly
O3 concentrations as explained in section 2 of this
appendix.
Annual fourth-highest daily maximum refers to the fourth highest
value measured at a monitoring site during a particular year.
Daily maximum 8-hour average concentration refers to the maximum
calculated 8-hour average for a particular day as explained in
section 2 of this appendix.
Design values are the metrics (i.e., statistics) that are
compared to the NAAQS levels to determine compliance, calculated as
shown in section 3 of this appendix.
O3 monitoring season refers to the span of time
within a calendar year when individual States are required to
measure ambient O3 concentrations as listed in part 58
Appendix D to this chapter.
Year refers to calendar year.
2. Primary and Secondary Ambient Air Quality Standards for Ozone
2.1 Data Reporting and Handling Conventions
Computing 8-hour averages. Hourly average concentrations shall
be reported in parts per million (ppm) to the third decimal place,
with additional digits to the right of the third decimal place
truncated. Running 8-hour averages shall be computed from the hourly
O3 concentration data for each hour of the year and shall
be stored in the first, or start, hour of the 8-hour period. An 8-
hour average shall be considered valid if at least 75% of the hourly
averages for the 8-hour period are available. In the event that only
6 or 7 hourly averages are available, the 8-hour average shall be
computed on the basis of the hours available using 6 or 7 as the
divisor. 8-hour periods with three or more missing hours shall be
considered valid also, if, after substituting one-half the minimum
detectable limit for the missing hourly concentrations, the 8-hour
average concentration is greater
[[Page 16512]]
than the level of the standard. The computed 8-hour average
O3 concentrations shall be reported to three decimal
places (the digits to the right of the third decimal place are
truncated, consistent with the data handling procedures for the
reported data).
Daily maximum 8-hour average concentrations. (a) There are 24
possible running 8-hour average O3 concentrations for
each calendar day during the O3 monitoring season. The
daily maximum 8-hour concentration for a given calendar day is the
highest of the 24 possible 8-hour average concentrations computed
for that day. This process is repeated, yielding a daily maximum 8-
hour average O3 concentration for each calendar day with
ambient O3 monitoring data. Because the 8-hour averages
are recorded in the start hour, the daily maximum 8-hour
concentrations from two consecutive days may have some hourly
concentrations in common. Generally, overlapping daily maximum 8-
hour averages are not likely, except in those non-urban monitoring
locations with less pronounced diurnal variation in hourly
concentrations.
(b) An O3 monitoring day shall be counted as a valid
day if valid 8-hour averages are available for at least 75% of
possible hours in the day (i.e., at least 18 of the 24 averages). In
the event that less than 75% of the 8-hour averages are available, a
day shall also be counted as a valid day if the daily maximum 8-hour
average concentration for that day is greater than the level of the
standard.
2.2 Primary and Secondary Standard-related Summary Statistic
The standard-related summary statistic is the annual fourth-
highest daily maximum 8-hour O3 concentration, expressed
in parts per million, averaged over three years. The 3-year average
shall be computed using the three most recent, consecutive calendar
years of monitoring data meeting the data completeness requirements
described in this appendix. The computed 3-year average of the
annual fourth-highest daily maximum 8-hour average O3
concentrations shall be reported to three decimal places (the digits
to the right of the third decimal place are truncated, consistent
with the data handling procedures for the reported data).
2.3 Comparisons with the Primary and Secondary Ozone Standards
(a) The primary and secondary O3 ambient air quality
standards are met at an ambient air quality monitoring site when the
3-year average of the annual fourth-highest daily maximum 8-hour
average O3 concentration is less than or equal to 0.075
ppm.
(b) This comparison shall be based on three consecutive,
complete calendar years of air quality monitoring data. This
requirement is met for the 3-year period at a monitoring site if
daily maximum 8-hour average concentrations are available for at
least 90% of the days within the O3 monitoring season, on
average, for the 3-year period, with a minimum data completeness
requirement in any one year of at least 75% of the days within the
O3 monitoring season. When computing whether the minimum
data completeness requirements have been met, meteorological or
ambient data may be sufficient to demonstrate that meteorological
conditions on missing days were not conducive to concentrations
above the level of the standard. Missing days assumed less then the
level of the standard are counted for the purpose of meeting the
data completeness requirement, subject to the approval of the
appropriate Regional Administrator.
(c) Years with concentrations greater than the level of the
standard shall be included even if they have less than complete
data. Thus, in computing the 3-year average fourth maximum
concentration, calendar years with less than 75% data completeness
shall be included in the computation if the 3-year average fourth-
highest 8-hour concentration is greater than the level of the
standard.
(d) Comparisons with the primary and secondary O3
standards are demonstrated by examples 1 and 2 in paragraphs (d)(1)
and (d)(2) respectively as follows:
Example 1.--Ambient Monitoring Site Attaining the Primary and Secondary O3 Standards
--------------------------------------------------------------------------------------------------------------------------------------------------------
Percent valid
days (within 1st Highest 2nd Highest 3rd Highest 4th Highest 5th Highest
Year the required daily max 8- daily max 8- daily max 8- daily max 8- daily max 8-
monitoring hour Conc. hour Conc. hour Conc. hour Conc. hour Conc.
season) (ppm) (ppm) (ppm) (ppm) (ppm)
--------------------------------------------------------------------------------------------------------------------------------------------------------
2004.................................................... 100 0.092 0.090 0.085 0.079 0.078
2005.................................................... 96 0.084 0.083 0.075 0.072 0.070
2006.................................................... 98 0.080 0.079 0.077 0.076 0.060
-----------------------------------------------------------------------------------------------
Average............................................. 98 .............. .............. .............. 0.075 ..............
--------------------------------------------------------------------------------------------------------------------------------------------------------
(1) As shown in Example 1, this monitoring site meets the
primary and secondary O3 standards because the 3-year
average of the annual fourth-highest daily maximum 8-hour average
O3 concentrations (i.e., 0.075666 * * * ppm, truncated to
0.075 ppm) is less than or equal to 0.075 ppm. The data completeness
requirement is also met because the average percent of days within
the required monitoring season with valid ambient monitoring data is
greater than 90%, and no single year has less than 75% data
completeness. In Example 1, the individual 8-hour averages used to
determine the annual fourth maximum have also been truncated to the
third decimal place.
Example 2.--Ambient Monitoring Site Failing to Meet the Primary and Secondary O3 Standards
--------------------------------------------------------------------------------------------------------------------------------------------------------
Percent valid
days (within 1st Highest 2nd Highest 3rd Highest 4th Highest 5th Highest
Year the required daily max 8- daily max 8- daily max 8- daily max 8- daily max 8-
monitoring hour Conc. hour Conc. hour Conc. hour Conc. hour Conc.
season) (ppm) (ppm) (ppm) (ppm) (ppm)
--------------------------------------------------------------------------------------------------------------------------------------------------------
2004.................................................... 96 0.105 0.103 0.103 0.103 0.102
2005.................................................... 74 0.104 0.103 0.092 0.091 0.088
2006.................................................... 98 0.103 0.101 0.101 0.095 0.094
-----------------------------------------------------------------------------------------------
Average............................................. 89 .............. .............. .............. 0.096 ..............
--------------------------------------------------------------------------------------------------------------------------------------------------------
As shown in Example 2, the primary and secondary O3
standards are not met for this monitoring site because the 3-year
average of the fourth-highest daily maximum 8-hour average
O3 concentrations (i.e., 0.096333 * * * ppm, truncated to
0.096 ppm) is greater than 0.075 ppm, even though the data capture
is less than 75% and the average data capture for the 3 years is
less than 90% within the required monitoring season. In Example 2,
the individual 8-hour averages used to determine the annual fourth
maximum have also been truncated to the third decimal place.
[[Page 16513]]
3. Design Values for Primary and Secondary Ambient Air Quality
Standards for Ozone
The air quality design value at a monitoring site is defined as
that concentration that when reduced to the level of the standard
ensures that the site meets the standard. For a concentration-based
standard, the air quality design value is simply the standard-
related test statistic. Thus, for the primary and secondary
standards, the 3-year average annual fourth-highest daily maximum 8-
hour average O3 concentration is also the air quality
design value for the site.
PART 58--AMBIENT AIR QUALITY SURVEILLANCE
0
4. The authority citation of part 58 continues to read as follows:
Authority: 42 U.S.C. 7403, 7410, 7601(a), 7611, and 7619.
0
5. Appendix G to Part 58 is amended as follows:
0
a. By revising section 9.
0
b. By revising section 10.
0
c. By revising section 12.
0
d. By revising section 13.
Appendix G to Part 58--Uniform Air Quality Index (AQI) and Daily
Reporting
* * * * *
9. How Does the AQI Relate to Air Pollution Levels?
For each pollutant, the AQI transforms ambient concentrations to
a scale from 0 to 500. The AQI is keyed as appropriate to the
national ambient air quality standards (NAAQS) for each pollutant.
In most cases, the index value of 100 is associated with the
numerical level of the short-term standard (i.e., averaging time of
24-hours or less) for each pollutant. A different approach is taken
for NO2, for which no short-term standard has been
established. The index value of 50 is associated with the numerical
level of the annual standard for a pollutant, if there is one, at
one-half the level of the short-term standard for the pollutant, or
at the level at which it is appropriate to begin to provide guidance
on cautionary language. Higher categories of the index are based on
increasingly serious health effects and increasing proportions of
the population that are likely to be affected. The index is related
to other air pollution concentrations through linear interpolation
based on these levels. The AQI is equal to the highest of the
numbers corresponding to each pollutant. For the purposes of
reporting the AQI, the sub-indexes for PM10 and
PM2.5 are to be considered separately. The pollutant
responsible for the highest index value (the reported AQI) is called
the ``critical'' pollutant.
10. What Monitors Should I Use To Get the Pollutant Concentrations for
Calculating the AQI?
You must use concentration data from population-oriented State/
Local Air Monitoring Station (SLAMS) or parts of the SLAMS required
by 40 CFR 58.10 for each pollutant except PM. For PM, calculate and
report the AQI on days for which you have measured air quality data
(e.g., from continuous PM2.5 monitors required in
Appendix D to this part). You may use PM measurements from monitors
that are not reference or equivalent methods (for example,
continuous PM10 or PM2.5 monitors). Detailed
guidance for relating non-approved measurements to approved methods
by statistical linear regression is referenced in section 13 below.
* * * * *
12. How Do I Calculate the AQI?
i. The AQI is the highest value calculated for each pollutant as
follows:
a. Identify the highest concentration among all of the monitors
within each reporting area and truncate the pollutant concentration
to one more than the significant digits used to express the level of
the NAAQS for that pollutant. This is equivalent to the rounding
conventions used in the NAAQS.
b. Using Table 2, find the two breakpoints that contain the
concentration.
c. Using Equation 1, calculate the index.
d. Round the index to the nearest integer.
Table 2.--Breakpoints for the AQI
--------------------------------------------------------------------------------------------------------------------------------------------------------
These breakpoints Equal these AQI's
--------------------------------------------------------------------------------------------------------------------------------------------------------
PM10
O3 (ppm) 8-hour O3 (ppm) 1- PM2.5 ([mu]g/ ([mu]g/ CO (ppm) SO2 (ppm) NO2 (ppm) AQI Category
hour \1\ m\3\) m\3\)
--------------------------------------------------------------------------------------------------------------------------------------------------------
0.000-0.059..................... .............. 0.0-15.4 0-54 0.0-4.4 0.000-0.034 (\3\) 0-50 Good.
0.060-0.075..................... .............. 15.5-40.4 55-154 4.5-9.4 0.035-0.144 (\3\) 51-100 Moderate.
0.076-0.095..................... 0.125-0.164 40.5-65.4 155-254 9.5-12.4 0.145-0.224 (\3\) 101-150 Unhealthy for
Sensitive Groups.
0.096-0.115..................... 0.165-0.204 \4\ 65.5-150.4 255-354 12.5-15.4 0.225-0.304 (\3\) 151-200 Unhealthy.
0.116-0.374..................... 0.205-0.404 \4\ 150.5- 355-424 15.5-30.4 0.305-0.604 0.65-1.24 201-300 Very Unhealthy.
250.4
(\2\)........................... 0.405-0.504 \4\ 250.5- 425-504 30.5-40.4 0.605-0.804 1.25-1.64 301-400 ..................
350.4
(\2\)........................... 0.505-0.604 \4\ 350.5- 505-604 40.5-50.4 0.805-1.004 1.65-2.04 401-500 Hazardous.
500.4
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Areas are generally required to report the AQI based on 8-hour ozone values. However, there are a small number of areas where an AQI based on 1-hour
ozone values would be more precautionary. In these cases, in addition to calculating the 8-hour ozone index value, the 1-hour ozone index value may be
calculated, and the maximum of the two values reported.
\2\ 8-hour O3 values do not define higher AQI values (>= 301). AQI values of 301 or greater are calculated with 1-hour O3 concentrations.
\3\ NO2 has no short-term NAAQS, and can generate an AQI only above the value of 200.
\4\ If a different SHL for PM2.5 is promulgated, these numbers will change accordingly.
ii. If the concentration is equal to a breakpoint, then the
index is equal to the corresponding index value in Table 2. However,
Equation 1 can still be used. The results will be equal. If the
concentration is between two breakpoints, then calculate the index
of that pollutant with Equation 1. You must also note that in some
areas, the AQI based on 1-hour O3 will be more
precautionary than using 8-hour values (see footnote 1 to Table 2).
In these cases, you may use 1-hour values as well as 8-hour values
to calculate index values and then use the maximum index value as
the AQI for O3.
[GRAPHIC] [TIFF OMITTED] TR27MR08.001
Where:
Ip = the index value for pollutantp
Cp = the truncated concentration of pollutantp
[[Page 16514]]
BPHi = the breakpoint that is greater than or equal to Cp
BPLo = the breakpoint that is less than or equal to Cp
IHi = the AQI value corresponding to BPHi
Ilo = the AQI value corresponding to BPLo.
iii. If the concentration is larger than the highest breakpoint
in Table 2 then you may use the last two breakpoints in Table 2 when
you apply Equation 1.
Example
iv. Using Table 2 and Equation 1, calculate the index value for
each of the pollutants measured and select the one that produces the
highest index value for the AQI. For example, if you observe a
PM10 value of 210 [mu]g/m\3\, a 1-hour O3
value of 0.156 ppm, and an 8-hour O3 value of 0.130 ppm,
then do this:
a. Find the breakpoints for PM10 at 210 [mu]g/m\3\ as
155 [mu]g/m\3\ and 254 [mu]g/m\3\, corresponding to index values 101
and 150;
b. Find the breakpoints for 1-hour O3 at 0.156 ppm as
0.125 ppm and 0.164 ppm, corresponding to index values 101 and 150;
c. Find the breakpoints for 8-hour O3 at 0.130 ppm as
0.116 ppm and 0.374 ppm, corresponding to index values 201 and 300;
d. Apply Equation 1 for 210 [mu]g/m\3\, PM10:
[GRAPHIC] [TIFF OMITTED] TR27MR08.002
e. Apply Equation 1 for 0.156 ppm, 1-hour O3:
[GRAPHIC] [TIFF OMITTED] TR27MR08.003
f. Apply Equation 1 for 0.130 ppm, 8-hour O3:
[GRAPHIC] [TIFF OMITTED] TR27MR08.004
g. Find the maximum, 206. This is the AQI. The minimal AQI
report would read:
v. Today, the AQI for my city is 206 which is Very Unhealthy,
due to ozone. Children and people with asthma are the groups most at
risk.
13. What Additional Information Should I Know?
The EPA has developed a computer program to calculate the AQI
for you. The program prompts for inputs, and it displays all the
pertinent information for the AQI (the index value, color, category,
sensitive group, health effects, and cautionary language). The EPA
has also prepared a brochure on the AQI that explains the index in
detail (The Air Quality Index), Reporting Guidance (Guideline for
Public Reporting of Daily Air Quality) that provides associated
health effects and cautionary statements, and Forecasting Guidance
(Guideline for Developing an Ozone Forecasting Program) that
explains the steps necessary to start an air pollution forecasting
program. You can download the program and the guidance documents at
www.airnow.gov. Reference for relating non-approved PM measurements
to approved methods (Eberly, S., T. Fitz-Simons, T. Hanley, L.
Weinstock., T. Tamanini, G. Denniston, B. Lambeth, E. Michel, S.
Bortnick. Data Quality Objectives (DQOs) For Relating Federal
Reference Method (FRM) and Continuous PM2.5 Measurements to Report
an Air Quality Index (AQI). U.S. Environmental Protection Agency,
research Triangle Park, NC. EPA-454/B-02-002, November 2002) can be
found on the Ambient Monitoring Technology Information Center
(AMTIC) Web site, http://www.epa.gov/ttnamti1/.
[FR Doc. E8-5645 Filed 3-26-08; 8:45 am]
BILLING CODE 6560-50-P