Plan for Review of the National Ambient
Air Quality Standards for Ozone
Office of Air Quality Planning and Standards
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
March 2005
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DISCLAIMER
This plan for the review of the national ambient air quality standards (NAAQS) for ozone
(O3) is an informational document that summarizes background information on EPA's NAAQS
review process and the schedule for the ongoing review of the O3 NAAQS This document also
includes staff views as to the planned organization and content of a key document, the Ozone
Staff Paper, that will be prepared by OAQPS staff as part of this review As such, some
elements of this plan may be modified to reflect information developed during this review and to
address advice and comments received from the Clean Air Scientific Advisory Committee and
the public throughout this review.
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Table of Contents
1 INTRODUCTION 1
1 1 Overview of Review Process . 1
12 Scope of O3 NAAQS Review Plan 3
1.3 Summary of Past O3 NAAQS Reviews 3
2 OZONE NAAQS REVIEW SCHEDULE AND STATUS .... 5
3 OZONE STAFF PAPER DEVELOPMENT 7
31 Organization and Content . 7
3 2 Air Quality Characterization and Analyses 8
3 3 Ozone-Related Health Effects and Primary Standards 8
331 Policy-Relevant Assessment of Health Effects Evidence 9
33.2 Human Exposure Analysis 10
333 Human Health Risk Assessment 11
334 Approach to Staff Review of Primary Standards . 12
3 4 Ozone-Related Environmental Effects and Secondary Standards 14
3 4.1 Policy-Relevant Assessment of Ozone Environmental Impacts 14
342 Environmental Effects Analysis .... . 15
343 Approach to Staff Review of Secondary Standards ..16
REFERENCES ... 18
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Plan for Review of the National Ambient Air
Quality Standards for Ozone
1 INTRODUCTION
The U.S Environmental Protection Agency (EPA) is presently conducting a review of
the national ambient air quality standards (NAAQS) for ozone (O3) This plan for the O3
NAAQS review presents the schedule for key milestones in this review, and provides
background information on the NAAQS review process, a brief summary of past O3 NAAQS
reviews, and the status of current review activities. This document also focuses on the
development of a key document in the review process, the Ozone Staff Paper. Policy
Assessment of Scientific and Technical Information in the Review of the Ozone NAAQS (O3 Staff
Paper), and discusses the planned organization and content of that document As such, this plan
is intended to serve as an informational document to help interested parties understand the status
and plans for EPA's ongoing O3 NAAQS review
1.1 Overview of Review Process
Sections 108 and 109 of the Clean Air Act (Act) govern the establishment and periodic
review of the NAAQS These standards are established for pollutants that may reasonably be
anticipated to endanger public health and welfare, and whose presence in the ambient air results
from numerous or diverse mobile or stationary sources. The NAAQS are to be based on air
quality criteria, which 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 the pollutant in ambient air ' The EPA Administrator is to promulgate and
periodically review, at five-year intervals, "primary" (health-based) and "secondary" (welfare-
based) NAAQS for such pollutants. Section 109(b)(l) of the Act 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." Section 109(b)(2) of the Act directs that a secondary standard is to "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
Based on periodic reviews of the air quality criteria and standards, the Administrator is to
make revisions in the criteria and standards and promulgate any new standards, as may be
1 The cost of attaining the NAAQS is not to be taken into account in setting the standards, but rather is
considered in the development of control strategies designed to implement the standards
2 Welfare effects, as defined in section 302(h) of the Act include, but are not limited to, "effects on soils,
water, crops, vegetation, man-made 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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appropriate The Act also requires that an independent scientific review committee advise the
Administrator on the catena and standards as part of this NAAQS review process Since the
early 1980's, this independent review function has been performed by the Clean Air Scientific
Advisory Committee (CASAC), a standing committee of EPA's Science Advisory Board
The process generally used by EPA for conducting periodic reviews of the criteria and
NAAQS for a given pollutant includes the preparation of two key documents, an Air Quality
Criteria Document (AQCD) and a Staff Paper, followed by the proposal and promulgation of
decisions as to whether to retain or revise the existing standards. The AQCD, prepared by
EPA's National Center for Environmental Assessment in Research Triangle Park (NCEA-RTP)
within the Office of Research and Development (ORD), provides a critical assessment of the
latest available scientific information upon which the NAAQS are to be based Drawing upon
the AQCD, staff in EPA's Office of Air Quality Planning and Standards (OAQPS) within the
Office of Air and Radiation (OAR) prepares a Staff Paper that evaluates policy implications of
the key studies and scientific information contained in the AQCD and presents the conclusions
and recommendations of the staff for standard-setting options for the EPA Administrator to
consider. The Staff Paper is intended to help "bndge the gap" between the scientific assessment
contained in the AQCD and the judgments required of the Administrator in determining whether
it is appropriate to retain or to revise the primary and secondary NAAQS In conjunction with
the Staff Paper, OAQPS staff conducts various policy-relevant assessments of air quality data
and health and/or environmental effects to help inform staffs conclusions and recommendations
Drafts of the AQCD and the Staff Paper and related assessments are made available for public
comment and CASAC review. The final versions of these documents incorporate changes made
in response to CASAC advice and recommendations and public comments
Based on the information in these documents, the Administrator proposes decisions on
whether to retain or revise the NAAQS, taking into account CASAC advice and
recommendations and public comments The Administrator's proposed decisions are published
in the Federal Register, with a preamble that presents the rationale for the decisions and solicits
public comment. The Administrator makes final decisions after considering comments received
on the proposed decisions The Administrator's final decisions are promulgated in a Federal
Register notice that addresses significant comments received on the proposal
NAAQS decisions involve consideration of the four basic elements of a standard.
indicator, averaging time, form, and level. The indicator defines the pollutant to be measured in
the ambient air for the purpose of determining compliance with the standard The averaging
time defines the time period over which air quality measurements are to be obtained and
averaged, considering evidence of effects associated with various time penods of exposure The
form of a standard defines the air quality statistic that is to be compared to the level of the
standard (i e , an ambient concentration of the indicator pollutant) in determining whether an
area attains the standard. The form of the standard specifies the air quality measurements that
are to be used for compliance purposes (e g, the annual 4th highest daily maximum 8-hour
concentration, averaged over three years), the monitors from which the measurements are to be
obtained (e.g., one or more population-oriented monitors in an area), and whether the statistic is
to be averaged across multiple years These basic elements of a standard are the primary focus
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of the staff conclusions and recommendations in the Staff Paper and in the subsequent
rulemakmg These four elements taken together determine the degree of public health and
welfare protection afforded by the NAAQS
1.2 Scope of O3 NAAQS Review Plan
The review of the O3 criteria was initiated in September 2000 by NCEA-RTP with a
general call for information published in the Federal Register In November 2002, NCEA-RTP
released a work plan for the review and revision of the O3 AQCD (EPA, 2002). This O3 NAAQS
review plan is intended to update and go beyond the scope of the earlier O3 AQCD work plan
That work plan focused on key issues to be addressed in the preparation of a revised O3 AQCD,
the organization and content of the revised AQCD, and the schedule for its preparation. Since
the release of the O3 AQCD work plan, a lawsuit was filed in March 2003 to compel EPA to
complete the O3 NAAQS review.3 In resolving that lawsuit, EPA entered into a consent decree
The schedule for completion of the revised O3 AQCD and the O3 NAAQS review is now
governed by that consent decree, as reflected m this plan
Beyond updating the schedule for the preparation of the O3 AQCD, this plan addresses
the preparation of an O3 Staff Paper and the subsequent rule making. Following a summary of
past 03 NAAQS reviews, this plan presents an updated schedule for key milestones through
completion of the O3 NAAQS review and the status of current review activities The remainder
of this plan then focuses on the development of the O3 Staff Paper and related health and
environmental assessments. The planned organization and content of the 03 Staff Paper are
outlined; key policy-relevant issues are identified; plans for preparing health and environmental
assessments of scientific and technical information contained in the O3 AQCD are briefly
summarized, and general approaches are discussed for drawing upon the available evidence and
assessments to develop staff recommendations on whether, and if so, how, it may be appropriate
to revise the primary and secondary O3 NAAQS
Since this plan is being prepared prior to completion of the O3 AQCD, it anticipates
various issues that might emerge during the review of that document and allows for various
approaches that might be appropriate for the health and environmental assessments planned in
conjunction with the preparation of the O3 Staff Paper. Thus, this plan represents current staff
views and may be modified to reflect information developed during this review and to address
advice and recommendations received from the CASAC and public comments.
1.3 Summary of Past O3 NAAQS Reviews
On April 30,1971, the EPA initially established primary and secondary NAAQS for
photochemical oxidants under section 109 of the Act (36 FR 8186). Both primary and secondary
3 The lawsuit filed by a group of plaintiffs representing national environmental organizations alleged that
EPA had failed to perform its mandatory duty, under section 109(d)(l) of the Act, of completing the current review
within the penod provided by statute American Lung Association v. Whitman (No 1 03CV00778, D D C 2003)
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standards were set at a level of 0 08 parts per million (ppm), 1-hour average, total photochemical
oxidants, not to be exceeded more than one hour per year
On February 8,1979, the first periodic review of the 03 air quality criteria and standards
was completed with the promulgation of revised standards (44 FR 8202) The level of the
primary and secondary NAAQS was changed to 0 12 ppm, the indicator was changed to 03, and
the form of the standards was changed to be based on the expected number of days per calendar
year with a maximum hourly average concentration above 012 ppm (i e , attainment of the
standard occurs when that number is equal to or less than one)
On March 9,1993, EPA concluded its second periodic O3 NAAQS review by deciding
that revisions to the standards were not warranted at that time (58 FR 13008) The timing of this
decision was required by a court order issued to resolve a lawsuit filed to compel EPA to
complete its review of the catena and standards for O3 in accordance with the Act This
decision reflected EPA's review of relevant scientific information assembled since the last
review, as contained in the 1986 O3 AQCD and its Supplement and the 1989 O3 Staff Paper,
although it did not take into consideration a large number of more recently published studies on
the health and welfare effects of O3 The final decision emphasized the Administrator's intention
to proceed as rapidly as possible with the next periodic review of the air quality criteria and
standards to consider the more recent information.
Under a highly accelerated review process, EPA completed the last 03 NAAQS review
on July 18, 1997, revising the primary and secondary standards on the basis of the latest
scientific evidence linking exposures to ambient O3 to adverse health and welfare effects at
levels allowed by the existing standards (62 FR 38856) The O3 standards were revised by
replacing the then existing primary 1-hour standard with an 8-hour average O3 standard set at a
level of 0 08 ppm The form of the primary standard was changed to the annual fourth-highest
daily maximum 8-hour average concentration, averaged over three years The secondary O3
standard was changed by making it identical in all respects to the revised primary standard
On May 14,1999, in response to challenges to EPA's 1997 decision filed by industry and
others," the U.S. Court of Appeals for the District of Columbia Circuit (D C Circuit Court)
remanded the O3 NAAQS to EPA, finding that section 109 of the Act, as interpreted by EPA,
effected an unconstitutional delegation of legislative authority.5 In addition, the D C Circuit
Court directed that, in responding to the remand, EPA should consider the potential beneficial
health effects of O3 pollution in shielding the public from the effects of solar ultraviolet (UV)
radiation. On January 27, 2000, EPA petitioned the U S. Supreme Court for certioran on the
constitutional issue (and two other issues) but did not request review of the D C. Circuit Court
ruling regarding the potential beneficial health effects of O3 On February 27, 2001, the U.S.
Supreme Court unanimously reversed the judgment of the D C. Circuit Court on the
constitutional issue, holding that section 109 of the CAA does not delegate legislative power to
the EPA in contravention of the Constitution, and remanded the case to the D C. Circuit Court to
4 American Trucking Associations v. EPA, No 96-1441
5 American Trucking Associations v EPA, 175 F 3d 1027 (DC Cir , 1999)
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consider challenges to the 03 NAAQS that had not been addressed by that Court's earlier
decisions.6 On March 26, 2002, the D C. Circuit Court issued its final decision, finding the 1997
O3 NAAQS to be "neither arbitrary nor capricious," and denied the remaining petitions for
review.7
On November 14, 2001, EPA proposed to respond to the Court's remand to consider the
potential beneficial health effects of O3 pollution in shielding the public from the effects of solar
(UV) radiation by leaving the 1997 8-hour NAAQS unchanged (66 FR 52768). Taking into
account public comment on the proposed decision, EPA published its final response to this
remand on January 6, 2003, reaffirming the 8-hour O3 NAAQS set in 1997 (68 FR 614)
2 OZONE NAAQS REVIEW SCHEDULE AND STATUS
Key milestones in the ongoing O3 catena and standards review are summarized below in
Table 1 As noted above, the schedule for this review is now governed by a consent decree, as
modified and entered by the court on December 16, 2004. The consent decree provides that EPA
will meet the dates for the three milestones listed in Table 1 in bold type, which are premised on
the expectation that the dates for other listed interim milestones will be met as well.
As shown in Table 1, EPA initiated this review in September 2000 with a call for
information A work plan for the preparation of the O3 AQCD was released in November 2002
for CASAC and public review EPA held a series of workshops on several draft chapters of the
O3 AQCD to obtain broad input from the relevant scientific communities in mid-2003 These
workshops helped to inform the preparation of the first draft O3 AQCD (EPA, 2005), which was
released for CASAC and public review on January 31, 2005
During the process of preparing the first draft O3 AQCD, NCEA-RTP decided to make
some revisions to the planned format of the O3 AQCD that was described in the 2002 work plan
These decisions were made as part of a collaborative effort with OAQPS staff to modify the
review process so as to enhance the Agency's ability to meet this and future NAAQS review
schedules As described in Chapter 1 of the first draft O3 AQCD, emphasis is placed on
interpretative evaluation and integration of evidence in the mam body of the document, with
more detailed descriptions of individual studies being provided in a series of accompanying
annexes. This change is intended to streamline the document so as to facilitate timely CASAC
and public review and to focus more clearly on issues most relevant to the policy assessment to
be developed in the Staff Paper. The modified review process envisions that key policy-relevant
issues will be identified earlier in the review process through enhanced collaboration between
NCEA-RTP and OAQPS staff, leading to a more efficient linkage between the AQCD and the
Staff Paper Since this modified process was evolving during the later stages of the preparation
of the first draft O3 AQCD, the document does not fully reflect the revised format, especially
those chapters that deal with welfare effects It is intended that following the CASAC and public
6 Whitman v American Trucking Associations, 531 U S 457 (2001)
1 American Trucking Associations v EPA, 283 F 3d 355, (D C Cir 2002)
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review meeting in early May, the revised format will be more consistently used throughout the
second draft of the document
Table 1. Key Milestones in the O3 NAAQS review
Criteria Document and Staff Paper:
• Call for information published in Federal Register Sept 2000
• Release draft O3 AQCD Work Plan Nov. 2002
• CASAC/public review and meeting on O3 AQCD Work Plan Feb 2003
• Peer Review Workshops on draft chapters of O3 AQCD Mar. - Oct 2003
• Release 1" draft O3 AQCD Jan 31, 2005
• Release draft Health Assessment Plan (Exposure and Risk Assessment) Apr 2005
• CASAC/public review and meeting on 1st draft O3 AQCD and CASAC May 4-5, 2005
consultation on draft Health Assessment Plan
• Release draft Environmental Assessment Plan May 2005
• CASAC consultation on draft Environmental Assessment Plan June 2005
• Release 2nd draft O3 AQCD Aug /Sept. 2005
• Release 1st drafts of O3 Staff Paper and Health/Environmental Assessments Sept 2005
• CASAC/public review and meeting on 2nd draft O3 AQCD and Dec 2005
1" drafts of O3 Staff Paper and Health/Environmental Assessments
• Final O, AQCD Feb 28,2006
• Release 2nd drafts of O3 Staff Paper and Health/Environmental Assessments Apr 2006
• CASAC/pubhc review and meeting on 2nd drafts of O3 Staff Paper and July 2006
Health/Environmental Assessments
• Final O3 Staff Paper and Health/Environmental Assessments Sept. 2006
Rulemaking:
• Federal Register Notice of Proposed Rulemaking Mar 28, 2007
• Federal Register Notice of Final Rulemaking Dec 19, 2007
Consistent with the above schedule, OAQPS staff is now preparing plans for the health
and environmental assessments to be done in conjunction with the preparation of the 03 Staff
Paper As discussed below, the health-related assessment plan will include discussions of the
planned scope and methods to be used in conducting an exposure analysis and health nsk
assessment. The environmental-related assessment plan will focus on the scope and methods
that could be used to conduct analyses of O3-related impacts on vegetation, considering both
agricultural crops and tree species. After consulting with the CASAC and considering public
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comments on these plans, OAQPS staff will conduct the assessments and incorporate initial
results into the first draft O3 Staff Paper The first draft O3 Staff Paper will be released for
CASAC review and public comment shortly after release of the second draft O3 AQCD, and will
be based on information in the second draft O3 AQCD to the extent possible. The final O3 Staff
Paper and related assessments are not scheduled to be completed until several months following
completion of the final O3 AQCD, so as to ensure that those documents are based on information
in the final O3 AQCD
3 OZONE STAFF PAPER DEVELOPMENT
3.1 Organization and Content
The policy assessment to be presented in the O3 Staff Paper will be based on staffs
evaluation of the policy implications of the scientific evidence contained in the O3 AQCD and
the results of quantitative analyses based on that evidence. Taken together, this information will
inform staff conclusions and recommendations on the elements of the O3 standards under review.
While the O3 AQCD focuses on new scientific information available since the last review, it
appropriately integrates that information with scientific criteria from previous reviews The
quantitative analyses to be presented in the O3 Staff Paper (and to be descnbed in more detail in
a number of technical support documents) are based on the most recently available air quality
information, so as to provide current characterizations of 03 air quality patterns and estimated
health and welfare effects risks related to exposure to ambient O3 concentrations
Following an introductory chapter, the O3 Staff Paper will be organized into three main
parts the characterization of ambient O3 air quality data, 03-related health effects and primary
O3 NAAQS, and O3-related welfare effects and secondary O3 NAAQS The content of these
parts is summarized here and discussed more fully below
• The characterization of ambient O3 air quality data will include information on O3
properties, current O3 air quality patterns, historic trends, and background levels, as well
as providing a frame of reference for subsequent discussion of current and alternative O
NAAQS and alternative forms of O3 standards.
'3
Health-based information will include an overview of key policy-relevant health effects
evidence, major health-related conclusions from the O3 AQCD, and an examination of
issues related to the quantitative assessment of evidence from controlled human exposure
and epidemiological studies Results from the planned health assessment (i.e, an
exposure analysis and risk assessment) will be presented This part will conclude with a
discussion of the adequacy of the current primary standard, staff conclusions as to
potential alternative indicators, averaging times, levels, and forms, and staff
recommendations on ranges of alternative primary standards for consideration by the
Administrator
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• Welfare-based information will include an overview of key policy-relevant welfare
effects evidence and major welfare-related conclusions from the 03 AQCD Results from
the planned environmental assessment (i.e., exposure analyses and risk/benefit
assessment) will be presented This part will conclude with a discussion of the adequacy
of the current secondary standard; staff conclusions as to potential alternative indicators,
averaging times, levels, and forms, and staff recommendations on ranges of alternative
secondary standards for consideration by the Administrator
3.2 Air Quality Characterization and Analyses
Ambient O3 air quality information, generally based on air quality data through 2004
available from EPA's Air Quality System database, and information in Chapters 2,3, and 10 of
the O3 AQCD will be presented in Chapter 2 This chapter will summarize the chemical and
physical properties of ambient ground-level O3, including discussions of atmospheric processes
that lead to the formation, removal, and transport of O3 in the ambient air and radiative
properties that affect the transmission of ultraviolet radiation to the earth's surface and global
climate change processes. Urban and rural trends in O3 concentrations and precursor emissions
will be presented The distributions of specific O3 measures, including health-based indices such
as daily maximum 1 - and 8-hour averages, and vegetation-based indices such as the cumulative,
seasonal SUM06 index,8 will be characterized Spatial patterns of O3 over different geographic
scales and temporal patterns over seasonal and diurnal time penods also will be char act en zed
Finally, background O3 levels will be characterized, and issues pertaining to the international
transport of O3 and precursors, and various estimates of the impact of international transport will
be discussed
A key issue to be addressed in this chapter is the characterization of policy-relevant
background9 O3 levels in the U S. Policy-relevant background is an important input to the
assessment of human health and environmental risks, since those assessments will focus on
estimating nsk associated with pollutant levels that can be controlled by U S regulations or
through international agreements with border countries Evaluation of the assessment provided
in the draft O3 AQCD concerning this issue and consideration of the results from 3-D global
troposphenc O3 model simulations will inform estimates of policy-relevant background,
including consideration of regional and season differences in these estimates
3.3 Ozone-Related Health Effects and Primary Standards
In presenting staffs review of the primary 03 NAAQS, Chapter 3 will present a policy-
relevant assessment of the health effects evidence evaluated in the O3 AQCD. To put this
information into a public health perspective, staff plans to conduct a quantitative assessment of
public health impacts attributable to 03, including an exposure analysis and health risk
8 SUM06 is an index which sums all hourly O3 concentrations at and above 0 06 ppm over a specified
penod of tune, this index was proposed for consideration in the last review of the O, secondary standard
9 Policy-relevant background is defined as the distribution of O, concentrations that would be observed in
the U.S in the absence of anthropogenic (man-made) emissions of O, precursors in the U S, Canada, and Mexico
8
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assessment, to be presented in Chapters 4 and 5, respectively. This assessment will provide
quantitative estimates of human exposure to ambient O3 and of the risk to public health
associated with current O3 levels, with attainment of the current standard, and with attainment of
alternative O3 standards A Health Assessment Plan is being prepared that outlines the scope and
methods being considered for use, staff intends to modify this plan, as appropnate, based on
input received through a consultation with CASAC and from public comments. The complete
assessment will be documented in an Exposure Analysis Report and a Risk Assessment Report,
these technical support documents will include detailed descriptions of the assessment methods
and results. Chapter 6 will present staff conclusions and recommendations on the various
elements of the primary O3 NAAQS and will also include a summary of key uncertainties and
related staff research recommendations
3.3.1 Policy-Relevant Assessment of Health Effects Evidence
An assessment of key policy-relevant health evidence on the known and potential health
effects associated with exposure to ambient O3, alone and in combination with other pollutants
that are routinely present in ambient air, will be presented in Chapter 3. This chapter will
discuss key policy-relevant findings on Cyrelated health effects evaluated in Chapters 4 through
7 of the O3 AQCD, placing particular emphasis on the integrative synthesis presented in Chapter
8 of that document. Various factors shown to modify human responses to O3 inhalation will be
identified, as will population groups that show increased sensitivity to O3 exposure. The nature
of identified physiological effects will be discussed, including consideration of when such
effects might be judged to be adverse to the health of an individual. As in the last review, this
assessment will draw upon the latest American Thoracic Society (ATS) guidelines as to what
constitutes an adverse health effect. This issue of adversity of effects will be considered for both
healthy individuals as well as for individuals with impaired respiratory systems. Consideration
will be given to how these findings may change our understandings from the last review of the
nature and/or significance of O3 health effects and the O3 exposure levels associated with such
effects In addition, based on information in Chapter 10 of the O3 AQCD, indirect health
effects associated with the role of changes in ground level O3 m altering the flux of solar
ultraviolet radiation and climate change processes will also be addressed.
This assessment will also address a number of key issues relevant to staffs interpretation
and quantitative assessment of available toxicologic, controlled human exposure, and
epidemiological evidence, so as to provide a foundation for a quantitative exposure analysis and
health nsk assessment Such issues include, for example, considerations related to air quality
measurements and data used in the health studies, interpretation and relevance of the wide range
of inhalation effects identified in laboratory animal and controlled human exposure studies;
judgments as to the adversity of health effects reported in these studies, and interpretation of
epidemiological studies reporting associations between adverse health effects and ambient O3
concentrations In considering the epidemiological evidence, additional issues will be addressed,
such as the specification of models used in epidemiologic studies; approaches used to evaluate
the role of co-pollutants and potential confounding in O3-effects associations; questions of
temporality in associations between air quality and health effects, including lag periods used in
short-term studies and the selection of time periods used to represent exposures in long-term
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exposure studies; and questions related to the form of concentration-response relationships and
potential threshold levels.
3.3.2 Human Exposure Analysis
Characterization of human exposures to ambient O3 will be discussed in Chapter 4,
drawing from information generally presented in Chapter 3 of the O3 AQCD This chapter will
include discussions of factors that affect exposure to ambient O3 and the use of central
measurements of O3 concentrations as a surrogate for population exposure in epidemiological
studies. The central focus of this chapter will be on the exposure analysis being designed to
estimate population exposure to ambient O3 in a number of generally representative urban areas
across the U S This analysis will build upon the exposure analysis done in the last review and
will incorporate current air quality data (i e, 2002 through 2004) and enhancements made to
exposure models and model inputs since the last review Estimates will be generated for
population exposures associated with current O3 levels and with attainment of the current O3
standard and potential alternative standards Exposure estimates will be used as an input to the
risk assessment for health endpoints for which exposure-response functions are available and
will provide information on population exposures exceeding levels of concern that may be
identified for various other health endpoints
As will be discussed more fully in the Health Assessment Plan, staff is planning to model
exposures in approximately 12 urban areas located throughout the U.S These areas are being
selected to represent a wide variety in population, geographic area, demographic makeup,
climatology, and O3 air quality. In additon, selection of urban areas will take into consideration
the location of O3 field and epidemiological studies reporting significant health effects that are to
be included in the O3 health nsk assessment. Staff is now planning to develop exposure
estimates for the general population as well as for subpopulations including school-age children
and children with asthma,
A new version of EPA's Air Pollutants Exposure (APEX) model (also referred to as the
Total Risk Integrated Methodology/Exposure (TRIM Expo) model) will be used in this analysis
APEX simulates the movement of individuals through time and space and their exposure to O3 in
indoor, outdoor, and in-vehicle rmcroenvironments It is a Monte Carlo simulation model that
will be used to simulate a large number of randomly sampled individuals within each urban area
(e.g , 100,000) to represent area-wide population exposures. The development of appropriate
distributions representing variability and uncertainty in various model inputs (e g, air exchange
rates, O3 decay rates, physiological parameters) will be a key aspect of this modeling effort.
As part of this analysis, it is necessary to adjust recent O3 air quality data to simulate just
attaining alternative O3 standards in each area In the last review, EPA evaluated several
procedures for simulating changes in O3 air quality likely to result from attainment of the current
or alternative standards based on analyzing changes in O3 levels that have been observed
historically. Staff and others are now giving further consideration to alternative air quality
adjustment procedures for use in this analysis.
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Human activity data needed for this analysis will be drawn from the Consolidated Human
Activity Database (CHAD) developed and maintained by ORD's National Exposure Research
Laboratory (NERL) Another key issue in this analysis is the development of an approach for
creating O3-season or year-long activity sequences for individuals based on a cross-sectional
activity data base that includes 24-hour records.
This analysis will take into account several important factors including the magnitude
and duration of exposures, frequency of repeated high exposures, and breathing rate of
individuals at the time of exposure. Estimates will be developed for several indicators of
exposure to various levels of O3 air quality, including counts of people exposed one or more
times to a given O3 concentration while at a specified breathing rate, and counts of person-
occurrences which accumulate occurrences of specific exposure conditions over all people in the
population of interest The complete set of results and a detailed description of the methods used
in this analysis will be presented in a separate Exposure Analysis Report
3.3.3 Human Health Risk Assessment
The characterization of human health risks attributable to exposure to ambient O3 levels
will be presented in Chapter 5, based primarily on controlled human exposure and
epidemiological studies evaluated in Chapters 6 and 7 of the O3 AQCD. The human health risk
assessment that will be presented in this chapter is now being designed to estimate population
risks in a number of generally representative urban areas across the U.S., consistent with the
scope of the exposure analysis descnbed above. This risk assessment will build upon the
assessment done in the last review, and will include additional health endpoints for which newly
available studies have shown associations with exposure to ambient O3. Risk estimates will be
generated for public health risks associated with current O3 levels and with attainment of the
current O3 standard and potential alternative standards Particular attention will be given to
providing a clear and quantitative characterization of the uncertainty and variability inherent in
the assessment Public health risks associated with health endpoints for which the available
evidence is judged to be inadequate to support quantitative risk assessment will be characterized
qualitatively
As will be discussed more fully in the Health Assessment Plan, this assessment will
include risk estimates based on both controlled human exposure studies and epidemiological and
field studies. Staff is now planning to generate Cyrelated risk estimates for lung function
decrements based on probabilistic exposure-response relationships developed in controlled
human exposure studies, together with probabilistic exposure estimates from the exposure
analysis For various other health endpoints, staff is now planning to generate O3-related risk
estimates based on concentration-response relationships developed in epidemiological or field
studies, together with ambient air quality concentrations, baseline health incidence rates and
population data for the various locations being included in the assessment At this time, staff is
considering inclusion of the following health endpoints in the assessment based on
epidemiological or field studies respiratory symptoms in asthmatic children, respiratory-related
school absences, emergency department visits and hospital admissions for respiratory illness,
and premature mortality. The inclusion of any particular health endpomt will depend in part on
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the extent to which the O3 AQCD infers the likelihood of a causal relationship between O3
exposure and a given endpoint
A number of issues related to the selection and application of appropriate concentration-
response functions for use in this assessment will need to be addressed. For example,
consideration will be given to the appropriate use of functions based on single- and multi-city
studies, single- and multi-pollutant models, and models with different lag structures Another
important issue relates to the development of appropnate approaches for estimating risk in
excess of policy-relevant background O3 levels, consistent with the risk assessments that have
been conducted in past NAAQS reviews Particular attention will also be given to plans to
conduct sensitivity analyses to characterize uncertainties in the assessment and the influence of
various assumptions made to conduct the assessment The complete set of results and a detailed
description of the methods used in this nsk assessment will be presented in a separate Risk
Assessment Report.
3.3.4 Approach to Staff Review of Primary Standards
Chapter 6 will present staff conclusions and recommendations for the Administrator to
consider in deciding whether the existing primary O3 standard should be revised and, if so, what
revised standards would be appropriate. Staff conclusions and recommendations on the primary
standard will be based on the information contained in the O3 AQCD, focusing particularly on
the assessment and integrative synthesis of information presented in Chapter 8 of that document,
and on the staff evaluations and assessments discussed in the preceding chapters of the O3 Staff
Paper.
In the last review, EPA's general approach to evaluating the primary standard focused on
three areas First, EPA examined the scientific literature to assess which acute and chronic
health effects are associated with O3, and where possible, identified exposure levels at which
those effects occur. Second, EPA made judgments, based on advice from medical experts, as to
when physiological effects become significant enough to be considered "adverse" to the health
of individuals. Finally, EPA made public health policy judgments, informed by air quality,
exposure, and risk analyses when possible, concerning the point at which risks would be reduced
sufficiently to protect public health with an adequate margin of safety Based on these
considerations, EPA revised the primary standard to focus on acute adverse effects to public
health associated with prolonged exposures to ambient 03, based on an 8-hour averaging time
In recommending a range of primary standard options for the Administrator to consider,
it is recognized that the final decision will be largely a public health policy judgment A final
decision must draw upon scientific information and analyses about health effects and nsks, as
well as judgments about how to deal with the range of uncertainties that are inherent in the
scientific evidence and analyses Staffs approach to informing these judgments is based on a
recognition that the available health effects evidence generally reflects a continuum consisting of
ambient levels at which scientists generally agree that health effects are likely to occur through
lower levels at which the likelihood and magnitude of the response become increasingly
uncertain. This approach is consistent with the requirements of the NAAQS provisions of the
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Act and with how EPA and the courts have historically interpreted the Act. These provisions
require the Administrator to establish primary standards that are requisite to protect public health
and are neither more nor less stringent than necessary for this purpose. The provisions do not
require that primary standards be set at a zero-nsk level, but rather at a level that avoids
unacceptable risks to public health
In this review, a series of questions will frame staffs approach to reaching conclusions
and recommendations, based on available evidence and information, as to whether consideration
should be given to retaining or revising the current primary O3 NAAQS. Staffs review of the
adequacy of the current primary standard begins by considering whether the currently available
body of evidence assessed in the O3 AQCD suggests that revision of any of the basic elements of
the standard would be appropriate This evaluation of the adequacy of the current standard will
involve addressing questions such as the following
• To what extent does newly available information reinforce or call into question evidence
of associations with effects identified in the last review?
• To what extent does newly available information reinforce or call into question any of the
basic elements of the current O3 standard?
• To what extent have important uncertainties identified in the last review been reduced
and have new uncertainties emerged?
To the extent that the evidence suggests that revision of the current standard would be
appropriate, staff will then consider whether the currently available body of evidence supports
consideration of standards that are either more or less protective by addressing the following
questions
Is there evidence that associations, especially likely causal associations, extend to air
quality levels that are as low as or lower than had previously been observed, and what are
the important uncertainties associated with that evidence?
Are health risks estimated to occur in areas that meet the current standard, are they
important from a public health perspective, and what are the important uncertainties
associated with estimated risks?
To the extent that there is support for consideration of revised standards, staff will then identify
ranges of standards (in terms of averaging times, levels and forms) that would reflect a range of
alternative public health policy judgments, based on the currently available evidence, as to the
degree of protection that is requisite to protect public health with an adequate margin of safety
In so doing, staff will address the following questions
Does the evidence provide support for considering different exposure indices or
averaeme tmes?
averaging times?
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• What range of levels and forms of alternative standards is supported by the evidence, and
what are the uncertainties and limitations in that evidence*7
• To what extent do specific levels and forms of alternative standards reduce the estimated
risks attributable to O3, and what are the uncertainties in the estimated risk reductions'?
Based on the evidence, estimated risk reductions, and related uncertainties, the staff will then
make recommendations as to ranges of alternative standards for the Administrator's
consideration in reaching decisions as to whether to retain or revise the primary 03 NAAQS.
3.4 Ozone-Related Environmental Effects and Secondary Standards
In presenting staffs review of the secondary 03 NAAQS, Chapter 7 will first discuss key
policy-relevant findings on O3-related welfare effects evaluated in the draft 03 AQCD, including
environmental effects on vegetation and ecosystems, effects on man-made materials, and indirect
effects associated with O3's role in altering the flux of solar ultraviolet radiation and climate
change processes. Chapter 8 will then present staffs quantitative assessment of environmental
impacts attributable to O3, now being planned to include exposure and risk/benefits analyses for
agricultural crops and possibly commercial forest tree species An Environmental Assessment
Plan is being prepared that outlines the scope and methods being considered for use, staff will
finalize this plan based on input received through a consultation with CASAC and from public
comments. The complete assessment will be documented in an Environmental Assessment
Report, this technical support document will include a detailed description of the assessment
methods and results Chapter 9 will present staff conclusions and recommendations on the
various elements of the secondary 03 NAAQS, and will also include a summary of key
uncertainties and related staff research recommendations.
3.4.1 Policy-Relevant Assessment of Ozone Environmental Impacts
An assessment of key policy-relevant evidence on the known and potential environmental
effects associated with exposure to ambient O3, alone and in combination with other pollutants
and stressors that are routinely present in ambient air, will be presented in Chapter 7 This
chapter will discuss key policy-relevant findings on Cyrelated welfare effects evaluated in
Chapters 9 through 11 of the O3 AQCD. Various factors that modify plant responses to O3 will
be discussed, and species likely to have increased sensitivity to O3 exposure will be identified
Consideration will be given to how newly available information change, if at all, our
understandings from the last review of the nature and/or significance of O3 welfare effects and
the O3 exposure levels associated with such effects. In addition, based on information in Chapter
10 of the O3 AQCD, indirect environmental effects associated with the role of ground level O3 in
altering the flux of solar ultraviolet radiation and climate change processes will also be
addressed.
Since the last review, research on O3 effects on vegetation has continued to be published
in the scientific literature as discussed in the draft 03 AQCD The majority of these studies have
continued to use open-top chamber systems (OTCs) that were the standard 03 exposure method
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used in studies available in the last review. An alternate technology, FACE (Free Air CO2
Enrichment), originally developed to expose vegetation to elevated levels of CO2 in the field
without using chambers, has more recently been employed to expose vegetation to 03 Two
major research studies ongoing in the U S. are currently using this approach for O3 exposure
One such study, referred to as SOY FACE, located in Illinois, is focusing on the effects of O3 on
soybeans. The other study, referred to as ASPEN FACE, is located in Wisconsin and includes
effects of O3 on aspen, birch, and red maple seedlings/saplings. These FACE studies report O3
responses that are similar to those reported previously in OTC studies, without the alterations of
microclimate that are an artifact of the use of such chambers
In addition to new exposure techniques, the draft O3 AQCD identifies several other
important areas of research that will be discussed in this chapter. A number of new European
studies have explored the application of the critical level concept with regard to O3 exposures
and vegetation response, and have evaluated the practicality of measuring O3 flux as a way to
establish dose-response functions for vegetation These concepts will be considered by staff in
the context of identifying appropriate forms for a distinct secondary standard for protection of
vegetation
3.4.2 Environmental Effects Analysis
Staff is planning a quantitative assessment of key policy-relevant information on the
known and potential environmental effects associated with ambient O3 exposure This
assessment, to be presented in Chapter 8, will draw upon the most relevant information
contained in the O3 AQCD and other significant research evaluated therein and will build upon
the quantitative assessment performed during the last review As will be discussed more fully in
the Environmental Assessment Plan, staff is planning to focus on soybean crops based on their
economic importance, O3 sensitivity, and the availability of new information from recent studies
using FACE technology. Other plant species, such as economically important trees, will be
assessed if staff determines that sufficient new data are available to warrant updating the
assessment of tree species conducted in the last review. Recent monitoring and atmospheric
modeling data will be used to update current estimates of vegetation exposure to O3 In addition
to a quantitative assessment, the staff will discuss the potential qualitative nsks to vegetation
including nsks to biodiversity, health of forest ecosystems, Class I areas, and aesthetic values.
Using 03 monitoring data and the latest EPA models, exposure estimates will be modeled
for soybean growing areas throughout the U S for several recent growing seasons. If other plant
species are included, exposure estimates will be modeled for areas to cover the growing areas for
those plants. The exposure estimates together with exposure-response relationships for soybean
will be used to estimate yield losses based on yearly reports of county-level crop yield.
Economic models, such as AgSim and other approaches discussed in the draft O3 AQCD, will be
evaluated for estimating economic losses due to current ambient O3 levels Additionally, the
impact of attaining the current 8-hour O3 NAAQS and alternative O3 standards on economic
losses will also be evaluated.
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3.4.3 Approach to Staff Review of Secondary Standards
Staff conclusions and recommendations for the Administrator to consider in deciding
whether the existing secondary O3 standard should be revised and, if so, what revised standard
would be appropriate will be presented in Chapter 9 Staff conclusions and recommendations on
the secondary standard will be based on the information presented in chapters 9,10, and 11 of
the O3 AQCD and on staff analyses and evaluations discussed in the preceding chapters of the O3
Staff Paper
In the last review, EPA concluded that under ambient O3 exposure conditions existing at
the time, adverse O3 related effects were occurring to a number of commercially important crop
species and tree species in the seedling stage and that additional protection against these effects
was warranted In considering what was known about the relationships between ambient O3
concentrations and plant response, staff concluded that the best predictor of plant response to
ambient O3 levels would be measuring the amount of O3 taken up by the plant as a result of
stomatal conductance. Staff recognized, however, that using such a flux-based index as a basis
for a national standard at that time was impractical, given the state of the science available, and
focused instead on a number of biologically relevant seasonal cumulative, peak-weighted
exposure indices as surrogates for a flux-based standard Staff also considered the impact that
alternative primary standard options being considered might have on improving ambient O3
distributions in non-urban, agricultural and forested areas Based on these considerations, EPA
proposed to revise the secondary O3 N AAQS by replacing the existing standard with either a
standard defined in terms of a biologically relevant index (i.e, the SUM06 index), or a standard
equal to a revised primary standard with an 8-hour averaging time The final Agency decision
made the secondary standard identical to the revised 8-hour primary standard
In recommending a range of secondary standard options for the Administrator to
consider, staff recognizes that the final decision will be largely a public policy judgment A final
decision must draw upon scientific evidence and analyses about effects on public welfare, as
well as judgments about how to deal with the range of uncertainties that are inherent in the
relevant information. Staffs approach to informing these judgements is based on a recognition
that the effects of O3 on vegetation are the most widely recognized and thoroughly studied of the
public welfare effects categories, that plants exhibit a wide range of O3 sensitivities, both
between and within species, and that O3 impacts to vegetation are influenced by numerous and
varied coexisting biotic and abiotic environmental stressors. Staff also recognizes that
determinations of what is an adverse impact to vegetation may vary, depending on the associated
value or intended use of the impacted species. This approach is consistent with the requirements
of the NAAQS provisions of the Act and with how EPA and the courts have historically
interpreted the Act These provisions require the Administrator to establish secondary standards
that are requisite to protect public welfare from any known or anticipated adverse effects
associated with the presence of the pollutant in the ambient air. In so doing, the Administrator
seeks to establish standards that are neither more nor less stringent than necessary for this
purpose. The provisions do not require that secondary standards be set to eliminate all welfare
effects, but rather at a level that protects public welfare from those effects that are judged to be
adverse.
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In this review, a series of questions will frame staffs approach to reaching conclusions
and recommendations, based on available evidence and information, as to whether consideration
should be given to retaining or revising the current secondary O3 NAAQS. The staffs review of
the adequacy of the current standard begins by considering whether the currently available body
of evidence assessed in the O3 AQCD suggests that revision of any of the basic elements of the
NAAQS would be appropriate This evaluation is done for each category of O3-related welfare
effects identified in the O3 AQCD as being associated with the presence of O3 in the ambient air.
Staffs review of the adequacy of the current O3 standard for each effects category involves
addressing questions such as:
• To what extent does the available information demonstrate or suggest that O3-related
effects are occurring at current ambient conditions or at levels that would meet the
current standard7
• To what extent does the available information inform judgments as to whether any
observed or anticipated effects are adverse to public welfare?
To what extent is the current secondary standard likely to be effective in achieving
protection against any identified adverse effects'?
To the extent that the evidence suggests that revision of the current secondary O3
NAAQS would be appropriate, staff then identifies ranges of standards (in terms of exposure
indices, averaging times, levels, and forms) that would reflect a range of alternative policy
judgments as to the degree of protection that is requisite to protect public welfare from known or
anticipated adverse effects In so doing, staff addresses questions such as
Does the available information provide support for considering different O3 exposure
indices?
• Does the available information provide support for considering different averaging
times?
What range of levels and forms of alternative standards is supported by the information,
and what are the uncertainties and limitations in that information1?
• To what extent do specific levels and forms of alternative standards reduce adverse
impacts attributable to O3, and what are the uncertainties in the estimated reductions?
Based on the available information, estimated reductions in adverse impacts, and related
uncertainties, staff will make recommendations as to ranges of alternative standards for the
Administrator's consideration in reaching decisions as to whether to retain or revise the
secondary O3 NAAQS
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REFERENCES
Federal Register (1971) National primary and secondary ambient air quality standards for
photochemical oxidants, Final rule 40 CFR 50, Federal Register 36. 8186
Federal Register (1979) National primary and secondary ambient air quality standards revisions
to the National Ambient Air Quality Standards for photochemical oxidants, Final Rule
40 CFR 50, Federal Register 44 8202
Federal Register (1993) National Ambient Air Quality Standards for Ozone, Final rule. 40 CFR
50, Federal Register 58 13008.
Federal Register (1997) National Ambient Air Quality Standards for Ozone, Final Rule 40 CFR
50, Federal Register 62- 38856.
Federal Register (2001) National Ambient Air Quality Standards for Ozone, Proposed Response
to Remand; Proposed Rule. Federal Register 66 57368.
Federal Register (2003) National Ambient Air Quality Standards for Ozone, Proposed Response
Remand. Final Rule Federal Register 68 614.
U S. Environmental Protection Agency. (1986) Air quality criteria for ozone and other
photochemical oxidants Research Triangle Park, NC Office of Health and
Environmental Assessment, Environmental Criteria and Assessment Office; EPA report
nos EPA-600/8-84-020aF-eF. Available from NTIS, Springfield, VA, PB87-142949.
U S Environmental Protection Agency (1992) Summary of selected new information on effects
of ozone on health and vegetation supplement to 1986 air quality criteria for ozone and
other photochemical oxidants. Research Triangle Park, NC: Office of Health and
Environmental Assessment, Environmental Criteria and Assessment Office, EPA report
no EPA/600/8-88/105F. Available from NTIS, Springfield, VA; PB92-235670
U S. Environmental Protection Agency (1996) Air quality criteria for ozone and related
photochemical oxidants. Research Triangle Park, NC Office of Research and
Development, report nos EPA/600/AP-93/004aF-cF 3v Available from NTIS,
Springfield, VA, PB96-185582, PB96-185590, and PB96-185608 Available online at
www epa gov/ncea/ozone.htm
U S Environmental Protection Agency (2002) Project Work Plan for Revised Air Quality
Criteria for Ozone and Related Photochemical Oxidants. Research Triangle Park, NC
National Center for Environmental Assessment-RTP Report no. NCEA-R-1068
U S. Environmental Protection Agency (2005) Air Quality Criteria for Ozone and Related
Photochemical Oxidants (First External Review Draft) Washington, DC, EPA/600/R-
05/004aA-cA
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