Document ID: EPA-HQ-OAR-2005-0172-7205
Agency: epa
Document Type: Rule
Title: National Ambient Air Quality Standards for Ozone
Posted Date: 2008-03-27T04:00Z

[Federal Register: March 27, 2008 (Volume 73, Number 60)]
[Rules and Regulations]               
[Page 16435-16514]
From the Federal Register Online via GPO Access [wais.access.gpo.gov]
[DOCID:fr27mr08-8]                         

[[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

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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: mckee.dave@epa.gov.

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.

[[Page 16437]]

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

[[Page 16438]]

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

[[Page 16439]]

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.''
---------------------------------------------------------------------------

    \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.
---------------------------------------------------------------------------

    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\
---------------------------------------------------------------------------

    \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).
---------------------------------------------------------------------------

    (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.
---------------------------------------------------------------------------

    \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).
---------------------------------------------------------------------------

    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.
---------------------------------------------------------------------------

    \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.
---------------------------------------------------------------------------

    (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\
---------------------------------------------------------------------------

    \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).
---------------------------------------------------------------------------

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.

References

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Selected Urban Areas. Prepared for Office of Air Quality Planning 
and Standards, U.S. Environmental Protection Agency, Research 
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 
for Office of Air Quality Planning and Standards, U.S. Environmental 
Protection Agency, Research Triangle Park, NC. January 2007; EPA 
report no. EPA-452/R-07-002. Available online at: http://
www.epa.gov/ttn/naaqs/standards/ozone/s_o3_cr_td.html.
Adams, W. C. (2002) Comparison of chamber and face-mask 6.6-hour 
exposures to ozone on pulmonary function and symptoms responses. 
Inhalation Toxicol. 14: 745-764.
Adams, W. C. (2003a) Comparison of chamber and face mask 6.6-hour 
exposure to 0.08 ppm ozone via square-wave and triangular profiles 
on pulmonary responses. Inhalation Toxicol. 15: 265-281.
Adams, W. C. (2003b) Relation of pulmonary responses induced by 6.6 
hour exposures to 0.08 ppm ozone and 2-hour exposures to 0.30 ppm 
via chamber and face-mask inhalation. Inhalation Toxicol. 15: 745-
759.
Adams, W. C. (2006) Comparison of chamber 6.6 hour exposures to 
0.04-0.08 ppm ozone via square-wave and triangular profiles on 
pulmonary responses. Inhalation Toxicol. 18: 127-136.
Alliance of Automobile Manufacturers (AAM) (2007) Letter and 
Document Sent to Stephen L. Johnson re: Proposed Rule--National 
Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172-
4191. October 9, 2007.
American Academy of Pediatrics (2007) Letter and Comments Sent to 
Docket No. OAR-2005-0172 re: Proposed Rule--National Ambient Air 
Quality Standards for Ozone. Docket No. OAR-2005-0172-4570. October 
10, 2007.
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1288-1291.

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North Carolina Department of Environment and Natural Resources 
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Docket No. OAR-2005-0172 re: Proposed Rule--National Ambient Air 
Quality Standards for Ozone. Docket No. OAR-2005-0172-4428. October 
9 2007.
Sarnat, J. A.; Schwartz, J.; Catalano, P. J.; Suh, H. H. (2001) 
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Kress, L. W. (1991) Carryover effects of acid rain and ozone on the 
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Care Med. 171: 627-631.
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Koenig, J. Q.; Shapiro, G. G. (2006) Ambient air pollution and 
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59-66.
Texas Commission on Environmental Quality (2007) Letter and Comments 
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[[Page 16511]]

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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