[Federal Register Volume 85, Number 158 (Friday, August 14, 2020)]
[Proposed Rules]
[Pages 49830-49917]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2020-15453]
[[Page 49829]]
Vol. 85
Friday,
No. 158
August 14, 2020
Part V
Environmental Protection Agency
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40 CFR Part 50
Review of the Ozone National Ambient Air Quality Standards; Proposed
Rule
Federal Register / Vol. 85, No. 158 / Friday, August 14, 2020 /
Proposed Rules
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ENVIRONMENTAL PROTECTION AGENCY
40 CFR Part 50
[EPA-HQ-OAR-2018-0279; FRL-10012-49-OAR]
RIN 2060-AU40
Review of the Ozone National Ambient Air Quality Standards
AGENCY: Environmental Protection Agency (EPA).
ACTION: Proposed action.
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SUMMARY: Based on the Environmental Protection Agency's (EPA's) review
of the air quality criteria and the national ambient air quality
standards (NAAQS) for photochemical oxidants including ozone
(O3), the EPA is proposing to retain the current standards,
without revision.
DATES: Comments must be received on or before October 1, 2020.
Public hearings: The EPA will hold two virtual public hearings on
Monday, August 31, 2020, and Tuesday, September 1, 2020. Please refer
to the SUPPLEMENTARY INFORMATION section for additional information on
the public hearings.
ADDRESSES: You may submit comments, identified by Docket ID No. EPA-HQ-
OAR-2018-0279, by any of the following methods:
Federal eRulemaking Portal: https://www.regulations.gov
(our preferred method). Follow the online instructions for submitting
comments.
Email: [email protected]. Include the Docket ID No.
EPA-HQ-OAR-2018-0279 in the subject line of the message.
Mail: U.S. Environmental Protection Agency, EPA Docket
Center, Air and Radiation Docket, Mail Code 28221T, 1200 Pennsylvania
Avenue NW, Washington, DC 20460.
Hand Delivery or Courier (by scheduled appointment only):
EPA Docket Center, WJC West Building, Room 3334, 1301 Constitution
Avenue NW, Washington, DC 20004. The Docket Center's hours of
operations are 8:30 a.m.-4:30 p.m., Monday-Friday (except Federal
Holidays).
Instructions: All submissions received must include the Docket ID
No. for this document. Comments received may be posted without change
to https://www.regulations.gov, including any personal information
provided. For detailed instructions on sending comments, see the
SUPPLEMENTARY INFORMATION section of this document. Out of an abundance
of caution for members of the public and our staff, the EPA Docket
Center and Reading Room are closed to the public, with limited
exceptions, to reduce the risk of transmitting COVID-19. Our Docket
Center staff will continue to provide remote customer service via
email, phone, and webform. We encourage the public to submit comments
via https://www.regulations.gov/ or email, as there may be a delay in
processing mail and faxes. Hand deliveries and couriers may be received
by scheduled appointment only. For further information on EPA Docket
Center services and the current status, please visit us online at
https://www.epa.gov/dockets.
The two virtual public hearings will be held on Monday, August 31,
2020, and Tuesday, September 1, 2020. The EPA will announce further
details on the virtual public hearing website at https://www.epa.gov/ground-level-ozone-pollution/setting-and-reviewing-standards-control-ozone-pollution. Refer to the SUPPLEMENTARY INFORMATION section below
for additional information.
FOR FURTHER INFORMATION CONTACT: For information or questions about the
public hearing, please contact Ms. Regina Chappell, U.S. Environmental
Protection Agency, Office of Air Quality Planning and Standards (OAQPS)
(Mail Code C304-03), Research Triangle Park, NC 27711; telephone: (919)
541-3650; email address: [email protected]. For information or
questions regarding the review of the O3 NAAQS, please
contact Dr. Deirdre Murphy, 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) 541-0729; fax: (919) 541-0237; email:
[email protected].
SUPPLEMENTARY INFORMATION:
General Information
Participation in Virtual Public Hearings
Please note that the EPA is deviating from its typical approach
because the President has declared a national emergency. Due to the
current Centers for Disease Control and Prevention (CDC)
recommendations, as well as state and local orders for social
distancing to limit the spread of COVID-19, the EPA cannot hold in-
person public meetings at this time. The EPA will begin pre-registering
speakers for the hearings upon publication of this document in the
Federal Register. To register to speak at a virtual hearing, please use
the online registration form available at https://www.epa.gov/ground-level-ozone-pollution/setting-and-reviewing-standards-control-ozone-pollution or contact Ms. Regina Chappell at (919) 541-3650 or by email
at [email protected] to register to speak at the virtual hearing.
The last day to pre-register to speak at one of the hearings will be
August 27, 2020. On August 28, 2020, the EPA will post a general agenda
for the hearings that will list preregistered speakers in approximate
order at: https://www.epa.gov/ground-level-ozone-pollution/setting-and-reviewing-standards-control-ozone-pollution. The EPA will make every
effort to follow the schedule as closely as possible on the day of each
hearing; however, please plan for the hearing to run either ahead of
schedule or behind schedule. Each commenter will have 5 minutes to
provide oral testimony. The EPA may ask clarifying questions during the
oral presentations but will not respond to the presentations at that
time. The EPA encourages commenters to provide the EPA with a copy of
their oral testimony electronically (via email) by emailing it to Dr.
Deirdre Murphy and Ms. Regina Chappell. The EPA also recommends
submitting the text of your oral testimony as written comments to the
rulemaking docket. Written statements and supporting information
submitted during the comment period will be considered with the same
weight as oral testimony and supporting information presented at the
public hearing. Please note that any updates made to any aspect of the
hearing will be posted online at https://www.epa.gov/ground-level-ozone-pollution/setting-and-reviewing-standards-control-ozone-pollution. While the EPA expects the hearings to go forward as set
forth above, please monitor our website or contact Ms. Regina Chappell
at (919) 541-3650 or [email protected] to determine if there are
any updates. The EPA does not intend to publish a document in the
Federal Register announcing updates. If you require the services of a
translator or a special accommodation such as audio description, please
preregister for the hearing with Ms. Regina Chappell and describe your
needs by August 21, 2020. The EPA may not be able to arrange
accommodations without advance notice.
Preparing Comments for the EPA
Follow the online instructions for submitting comments. Once
submitted to the Federal eRulemaking Portal, comments cannot be edited
or withdrawn. The EPA may publish any comment received to its public
docket. Do not submit electronically any information you consider to be
Confidential Business Information (CBI)
[[Page 49831]]
or other information whose disclosure is restricted by statute.
Multimedia submissions (audio, video, etc.) must be accompanied by a
written comment. The written comment is considered the official comment
and should include discussion of all points you wish to make. The EPA
will generally not consider comments or comment contents located
outside of the primary submission (i.e., on the web, the cloud, or
other file sharing system). For additional submission methods, the full
EPA public comment policy, information about CBI or multimedia
submissions, and general guidance on making effective comments, please
visit http://www2.epa.gov/dockets/commenting-epa-dockets.
When submitting comments, remember to:
Identify the action by docket number and other identifying
information (subject heading, Federal Register date and page number).
Explain why you agree or disagree, suggest alternatives,
and substitute language for your requested changes.
Describe any assumptions and provide any technical
information and/or data that you used.
Provide specific examples to illustrate your concerns and
suggest alternatives.
Explain your views as clearly as possible, avoiding the
use of profanity or personal threats.
Make sure to submit your comments by the comment period
deadline identified.
Availability of Information Related to This Action
All documents in the dockets pertaining to this action are listed
on the www.regulations.gov website. This includes documents in the
docket for the proposed decision (Docket ID No. EPA-HQ-OAR-2018-0279)
and a separate docket, established for the Integrated Science
Assessment (ISA) for this review (Docket ID No. EPA-HQ-ORD-2018-0274)
that has been incorporated by reference into the docket for this
proposed decision. Although listed in the index, some information is
not publicly available, e.g., CBI or other information whose disclosure
is restricted by statute. Certain other material, such as copyrighted
material, is not placed on the internet and may be viewed with prior
arrangement with the EPA Docket Center. Additionally, a number of the
documents that are relevant to this proposed decision are available
through the EPA's website at https://www.epa.gov/naaqs/ozone-o3-air-quality-standards. These documents include the Integrated Review Plan
for the Review of the Ozone National Ambient Air Quality Standards
(U.S. EPA, 2019b; hereafter IRP), available at https://www.epa.gov/naaqs/ozone-o3-standards-planning-documents-current-review, the
Integrated Science Assessment for Ozone and Related Photochemical
Oxidants (U.S. EPA, 2020a; hereafter ISA), available at https://www.epa.gov/naaqs/ozone-o3-standards-integrated-science-assessments-current-review, and the Policy Assessment for the Review of the Ozone
National Ambient Air Quality Standards (U.S. EPA, 2020b; hereafter PA),
available at https://www.epa.gov/naaqs/ozone-o3-standards-policy-assessments-current-review.
Table of Contents
The following topics are discussed in this preamble:
Executive Summary
I. Background
A. Legislative Requirements
B. Related O3 Control Programs
C. Review of the Air Quality Criteria and Standards for
O3
D. Air Quality Information
II. Rationale for Proposed Decision on the Primary Standard
A. General Approach
1. Background on the Current Standard
2. Approach for the Current Review
B. Health Effects Information
1. Nature of Effects
2. Public Health Implications and At-Risk Populations
3. Exposure Concentrations Associated With Effects
C. Summary of Exposure and Risk Information
1. Key Design Aspects
2. Key Limitations and Uncertainties
3. Summary of Exposure and Risk Estimates
D. Proposed Conclusions on the Primary Standard
1. Evidence- and Exposure/Risk-Based Considerations in the
Policy Assessment
2. CASAC Advice
3. Administrator's Proposed Conclusions
III. Rationale for Proposed Decision on the Secondary Standard
A. General Approach
1. Background on the Current Standard
2. Approach for the Current Review
B. Welfare Effects Information
1. Nature of Effects
2. Public Welfare Implications
3. Exposures Associated With Effects
C. Summary of Air Quality and Exposure Information
1. Influence of Form and Averaging Time of Current Standard on
Environmental Exposure
2. Environmental Exposures in Terms of W126 Index
D. Proposed Conclusions on the Secondary Standard
1. Evidence- and Exposure/Risk-Based Considerations in the
Policy Assessment
2. CASAC Advice
3. Administrator's Proposed Conclusions
IV. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 13563: Improving Regulation and Regulatory Review
B. Executive Order 13771: Reducing Regulations and Controlling
Regulatory Costs
C. Paperwork Reduction Act (PRA)
D. Regulatory Flexibility Act (RFA)
E. Unfunded Mandates Reform Act (UMRA)
F. Executive Order 13132: Federalism
G. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
H. Executive Order 13045: Protection of Children From
Environmental Health and Safety Risks
I. Executive Order 13211: Actions That Significantly Affect
Energy Supply, Distribution or Use
J. National Technology Transfer and Advancement Act
K. Executive Order 12898: Federal Actions To Address
Environmental Justice in Minority Populations and Low-Income
Populations
L. Determination Under Section 307(d)
V. References
Executive Summary
This document presents the Administrator's proposed decisions in
the current review of the primary (health-based) and secondary
(welfare-based) O3 NAAQS. In so doing, this document
summarizes the background and rationale for the Administrator's
proposed decisions to retain the current standards, without revision.
In reaching his proposed decisions, the Administrator has considered
the currently available scientific evidence in the ISA, quantitative
and policy analyses presented in the PA, and advice from the Clean Air
Scientific Advisory Committee (CASAC). The EPA solicits comment on the
proposed decisions described here and on the array of issues associated
with review of these standards, including judgments of public health,
public welfare and science policy inherent in the proposed decisions,
and requests commenters also provide the rationales upon which views
articulated in submitted comments are based.
This review of the O3 standards, required by the Clean
Air Act (CAA) on a periodic basis, was initiated in 2018. The last
review of the O3 NAAQS, completed in 2015 established the
current primary and secondary standards (80 FR 65291, October 26,
2015). In that review, the EPA significantly strengthened the primary
and secondary standards by revising both standards from 75 ppb to 70
ppb and retaining their indicators (O3), forms (fourth-
highest daily maximum,
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averaged across three consecutive years) and averaging times (eight
hours). These revisions to the NAAQS were accompanied by revisions to
the data handling procedures, ambient air monitoring requirements, the
air quality index and several provisions related to implementation (80
FR 65292, October 26, 2015). In the decision on subsequent litigation
on the 2015 decisions, the U.S. Court of Appeals for the District of
Columbia Circuit (D.C. Circuit) upheld the 2015 primary standard but
remanded the 2015 secondary standard to the EPA for further
justification or reconsideration. The court's remand of the secondary
standard has been considered in reaching the proposed decision, and the
associated proposed conclusions and judgments, described in this
document.
In this review as in past reviews of the NAAQS for O3
and related photochemical oxidants, the health and welfare effects
evidence evaluated in the ISA is focused on O3. Ozone is the
most prevalent photochemical oxidant in the atmosphere and the one for
which there is a large body of scientific evidence on health and
welfare effects. A component of smog, O3 in ambient air is a
mixture of mostly tropospheric O3 and some stratospheric
O3. Tropospheric O3, forms in the atmosphere when
precursor emissions of pollutants, such as nitrogen oxides and volatile
organic compounds (VOCs), interact with solar radiation. Precursor
emissions result from man-made sources (e.g., motor vehicles, and power
plants) and natural sources (e.g., vegetation and wildfires). In
addition, O3 that is created naturally in the stratosphere
also mixes with tropospheric O3 near the tropopause, and,
under more limited meteorological conditions and topographical
characteristics, nearer the earth's surface.
The proposed decision to retain the current primary standard,
without revision, has been informed by key aspects of the currently
available health effects evidence and conclusions contained in the ISA,
quantitative exposure/risk analyses and policy evaluations presented in
the PA, advice from the CASAC and public input received as part of this
ongoing review. The health effects evidence newly available in this
review, in conjunction with the full body of evidence critically
evaluated in the ISA, continues to support prior conclusions that
short-term O3 exposure causes and long-term O3
exposure likely causes respiratory effects, with evidence newly
available in this review also indicating a likely causal relationship
of short-term O3 with metabolic effects. The strongest
evidence for health effects due to ozone exposure, however, continues
to come from studies of short- and long-term ozone exposure and
respiratory health, including effects related to asthma exacerbation in
people with asthma, particularly children with asthma. The longstanding
evidence base of respiratory effects, spanning several decades,
documents the causal relationship between short-term exposure to
O3 and an array of respiratory effects. The clearest
evidence for this conclusion comes from controlled human exposure
studies, available at the time of the last review, of individuals,
exposed for 6.6 hours during quasi-continuous exercise that report an
array of respiratory responses including lung function decrements and
respiratory symptoms. Epidemiologic studies include associations
between O3 exposures and hospital admissions and emergency
department visits, particularly for asthma exacerbation in children.
People at risk include people with asthma, children, the elderly, and
outdoor workers.
The quantitative analyses of population exposure and risk, as well
as policy considerations in the PA, also inform the proposed decision
on the primary standard. The general approach and methodology for the
exposure-based assessment used in this review is similar to that used
in the last review. However, a number of updates and improvements have
been implemented in this review which result in differences from the
analyses in the prior review. These include a more recent period (2015-
2017) of ambient air monitoring data in which O3
concentrations in the areas assessed are at or near the current
standard, as well as improvements and updates to models, model inputs
and underlying databases. The analyses are summarized in this document
and described in detail in the PA.
Based on the current evidence and quantitative information, as well
as consideration of CASAC advice and public comment thus far in this
review, the Administrator proposes to conclude that the current primary
standard is requisite to protect public health, with an adequate margin
of safety, from effects of O3 in ambient air and should be
retained, without revision. In its advice to the Administrator, the
CASAC concurred with the draft PA that the currently available health
effects evidence is generally similar to that available in the last
review when the standard was set. Part of CASAC concluded that the
primary standard should be retained. Another part of CASAC expressed
concern regarding the margin of safety provided by the current
standard, pointing to comments from the 2014 CASAC, who while agreeing
that the evidence supported a standard level of 70 ppb, additionally
provided policy advice expressing support for a lower standard. The
advice from the CASAC has been considered by the Administrator in
proposing to conclude that the current standard, with its level of 70
ppb, provides the requisite public health protection, with an adequate
margin of safety. The EPA solicits comment on the Administrator's
proposed conclusion, and on the proposed decision to retain the
standard, without revision. The EPA also solicits comment on the array
of issues associated with review of this standard, including public
health and science policy judgments inherent in the proposed decision.
The proposed decision to retain the current secondary standard,
without revision, has been informed by key aspects of the currently
available welfare effects evidence and conclusions contained in the
ISA, quantitative exposure/risk analyses and policy evaluations
presented in the PA, advice from the CASAC and public input received as
part of this ongoing review. The welfare effects evidence newly
available in this review, in conjunction with the full body of evidence
critically evaluated in the ISA, supports, sharpens and expands
somewhat on the conclusions reached in the last review. Consistent with
the evidence in the last review, the currently available evidence
describes an array of O3 effects on vegetation and related
ecosystem effects, as well as the role of O3 in radiative
forcing and subsequent climate-related effects. Further, evidence newly
available in this review augments more limited previously available
evidence for some additional vegetation-related effects. As in the last
review, the strongest evidence and the associated findings of causal or
likely causal relationships with O3 in ambient air, as well
as the quantitative characterizations of relationships between
O3 exposure and occurrence and magnitude of effects, are for
vegetation effects. The scales of these effects range from the
individual plant scale to the ecosystem scale, with potential for
impacts on the public welfare. While the welfare effects of
O3 vary widely with regard to the extent and level of detail
of the available information that describes the exposure circumstances
that may elicit them, such information is most advanced for growth-
related effects such as growth and yield. For example, the information
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on exposure metric and relationships for these effects with the
cumulative, concentration-weighted exposure index, W126, is long-
standing, having been first described in the 1997 review. Utilizing
this information, reduced growth is considered as proxy or surrogate
for the broader array of vegetation effects in reviewing the public
welfare protection provided by the current standard.
Quantitative analyses of air quality and exposure, including use of
the W126 index, as well as policy considerations in the PA, also inform
the proposed decision on the secondary standard. For example, analyses
of air quality monitoring data across the U.S., as well as in Class I
areas, updated and expanded from analyses conducted in the last review,
inform EPA's understanding of vegetation exposures in areas meeting the
current standard. Based on the current evidence and quantitative
information, as well as consideration of CASAC advice and public
comment thus far in this review, the Administrator proposes to conclude
that the current secondary standard is requisite to protect the public
welfare from known or anticipated adverse effects of O3 in
ambient air, and should be retained, without revision. In its advice to
the Administrator, the full CASAC concurred with the preliminary
conclusions in the draft PA that the current evidence supports
retaining the current standard without revision. The EPA solicits
comment on the Administrator's proposed conclusion that the current
standard is requisite to protect the public welfare, and on the
proposed decision to retain the standard, without revision. The EPA
also solicits comment on the array of issues associated with review of
this standard, including public welfare and science policy judgments
inherent in the proposed decision.
I. Background
A. Legislative Requirements
Two sections of the CAA govern the establishment and revision of
the NAAQS. Section 108 (42 U.S.C. 7408) directs the Administrator to
identify and list certain air pollutants and then to issue air quality
criteria for those pollutants. The Administrator is to list those
pollutants ``emissions of which, in his judgment, cause or contribute
to air pollution which may reasonably be anticipated to endanger public
health or welfare''; ``the presence of which in the ambient air results
from numerous or diverse mobile or stationary sources''; and for which
he ``plans to issue air quality criteria. . . .'' (42 U.S.C.
7408(a)(1)). Air quality criteria are intended to ``accurately reflect
the latest scientific knowledge useful in indicating the kind and
extent of all identifiable effects on public health or welfare which
may be expected from the presence of [a] pollutant in the ambient air.
. . .'' (42 U.S.C. 7408(a)(2)).
Section 109 [42 U.S.C. 7409] directs the Administrator to propose
and promulgate ``primary'' and ``secondary'' NAAQS for pollutants for
which air quality criteria are issued [42 U.S.C. 7409(a)]. Section
109(b)(1) defines primary standards as ones ``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\ Under section 109(b)(2), a
secondary standard 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\ Under CAA section 302(h) (42 U.S.C. 7602(h)), effects on
welfare 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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In setting primary and secondary standards that are ``requisite''
to protect public health and welfare, respectively, as provided in
section 109(b), the EPA's task is to establish standards that are
neither more nor less stringent than necessary. In so doing, the EPA
may not consider the costs of implementing the standards. See
generally, Whitman v. American Trucking Ass'ns, 531 U.S. 457, 465-472,
475-76 (2001). Likewise, ``[a]ttainability and technological
feasibility are not relevant considerations in the promulgation of
national ambient air quality standards.'' See American Petroleum
Institute v. Costle, 665 F.2d 1176, 1185 (D.C. Cir. 1981); accord
Murray Energy Corp. v. EPA, 936 F.3d 597, 623-24 (D.C. Cir. 2019). At
the same time, courts have clarified the EPA may consider ``relative
proximity to peak background . . . concentrations'' as a factor in
deciding how to revise the NAAQS in the context of considering standard
levels within the range of reasonable values supported by the air
quality criteria and judgments of the Administrator. See American
Trucking Ass'ns, v. EPA, 283 F.3d 355, 379 (D.C. Cir. 2002), hereafter
referred to as ``ATA III.''
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. See Lead Industries Ass'n v. EPA, 647 F.2d 1130, 1154 (D.C.
Cir 1980); American Petroleum Institute v. Costle, 665 F.2d at 1186;
Coalition of Battery Recyclers Ass'n v. EPA, 604 F.3d 613, 617-18 (D.C.
Cir. 2010); Mississippi v. EPA, 744 F.3d 1334, 1353 (D.C. Cir. 2013).
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 include 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 Ass'n
v. EPA, 647 F.2d at 1156 n.51, Mississippi v. EPA, 744 F.3d at 1351),
but rather at a level that reduces risk sufficiently so as to protect
public health with an adequate margin of safety.
In addressing the requirement for an adequate margin of safety, the
EPA considers such factors as the nature and severity of the health
effects involved, the size of the sensitive population(s), and the kind
and degree of uncertainties. The selection of any particular approach
to providing an adequate margin of safety is a policy choice left
specifically to the Administrator's judgment. See Lead Industries Ass'n
v. EPA, 647 F.2d at 1161-62; Mississippi v. EPA, 744 F.3d at 1353.
Section 109(d)(1) of the Act requires periodic review and, if
appropriate, revision of existing air quality criteria to reflect
advances in scientific knowledge concerning the effects of the
pollutant on public health and welfare. Under the same provision, the
EPA is also to periodically review and, if appropriate,
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revise the NAAQS, based on the revised air quality criteria.\3\
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\3\ This section of the Act requires the Administrator to
complete these reviews and make any revisions that may be
appropriate ``at five-year intervals.''
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Section 109(d)(2) addresses the appointment and advisory functions
of an independent scientific review committee. Section 109(d)(2)(A)
requires the Administrator to appoint this committee, which is to be
composed of ``seven members including at least one member of the
National Academy of Sciences, one physician, and one person
representing State air pollution control agencies.'' Section
109(d)(2)(B) provides that the 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. . . .'' Since
the early 1980s, this independent review function has been performed by
the CASAC of the EPA's Science Advisory Board. A number of other
advisory functions are also identified for the committee by section
109(d)(2)(C), which reads:
Such committee shall also (i) advise the Administrator of areas
in which additional knowledge is required to appraise the adequacy
and basis of existing, new, or revised national ambient air quality
standards, (ii) describe the research efforts necessary to provide
the required information, (iii) advise the Administrator on the
relative contribution to air pollution concentrations of natural as
well as anthropogenic activity, and (iv) advise the Administrator of
any adverse public health, welfare, social, economic, or energy
effects which may result from various strategies for attainment and
maintenance of such national ambient air quality standards.
As previously noted, the Supreme Court has held that section 109(b)
``unambiguously bars cost considerations from the NAAQS-setting
process,'' in Whitman v. American Trucking Ass'ns, 531 U.S. 457, 471
(2001). Accordingly, while some of the issues listed in section
109(d)(2)(C) as those on which Congress has directed the CASAC to
advise the Administrator, are ones that are relevant to the standard
setting process, others are not. Issues that are not relevant to
standard setting may be relevant to implementation of the NAAQS once
they are established.\4\
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\4\ Because some of these issues are not relevant to standard
setting, some aspects of CASAC advice may not be relevant to EPA's
process of setting primary and secondary standards that are
requisite to protect public health and welfare. Indeed, were the EPA
to consider costs of implementation when reviewing and revising the
standards ``it would be grounds for vacating the NAAQS.'' Whitman v.
American Trucking Ass'ns, 531 U.S. 457, 471 n.4 (2001). At the same
time, the CAA directs CASAC to provide advice on ``any adverse
public health, welfare, social, economic, or energy effects which
may result from various strategies for attainment and maintenance''
of the NAAQS to the Administrator under section 109(d)(2)(C)(iv). In
Whitman, the Court clarified that most of that advice would be
relevant to implementation but not standard setting, as it
``enable[s] the Administrator to assist the States in carrying out
their statutory role as primary implementers of the NAAQS'' (id. at
470 [emphasis in original]). However, the Court also noted that
CASAC's ``advice concerning certain aspects of `adverse public
health . . . effects' from various attainment strategies is
unquestionably pertinent'' to the NAAQS rulemaking record and
relevant to the standard setting process (id. at 470 n.2).
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B. Related O3 Control Programs
States are primarily responsible for ensuring attainment and
maintenance of ambient air quality standards once the EPA has
established them. Under sections 110 and 171 through 185 of the CAA,
and related provisions and regulations, states are to submit, for the
EPA's approval, state implementation plans (SIPs) that provide for the
attainment and maintenance of such standards through control programs
directed to sources of the pollutants involved. The states, in
conjunction with the EPA, also administer the prevention of significant
deterioration of air quality program that covers these pollutants. See
42 U.S.C. 7470-7479. In addition, federal programs provide for
nationwide reductions in emissions of O3 precursors and
other air pollutants under Title II of the Act, 42 U.S.C. 7521-7574,
which involves controls for automobile, truck, bus, motorcycle, nonroad
engine and equipment, and aircraft emissions; the new source
performance standards under section 111 of the Act, 42 U.S.C. 7411; and
the national emissions standards for hazardous air pollutants under
section 112 of the Act, 42 U.S.C. 7412.
C. Review of the Air Quality Criteria and Standards for O3
Primary and secondary NAAQS were first established for
photochemical oxidants in 1971 (36 FR 8186, April 30, 1971) based on
the air quality criteria developed in 1970 (U.S. DHEW, 1970; 35 FR
4768, March 19, 1970). The EPA set both primary and secondary standards
at 0.08 parts per million (ppm), as a 1-hour average of total
photochemical oxidants, not to be exceeded more than one hour per year
based on the scientific information in the 1970 air quality criteria
document (AQCD). Since that time, the EPA has reviewed the air quality
criteria and standards a number of times, with the most recent review
being completed in 2015.
The EPA initiated the first periodic review of the NAAQS for
photochemical oxidants in 1977. Based on the 1978 AQCD (U.S. EPA,
1978), the EPA published proposed revisions to the original NAAQS in
1978 (43 FR 26962, June 22, 1978) and final revisions in 1979 (44 FR
8202, February 8, 1979). At that time, the EPA changed the indicator
from photochemical oxidants to O3, revised the level of the
primary and secondary standards from 0.08 to 0.12 ppm and revised the
form of both standards from a deterministic (i.e., not to be exceeded
more than one hour per year) to a statistical form. With these changes,
attainment of the standards was defined to occur when the average
number of days per calendar year (across a 3-year period) with maximum
hourly average O3 concentration greater than 0.12 ppm
equaled one or less (44 FR 8202, February 8, 1979; 43 FR 26962, June
22, 1978). Several petitioners challenged the 1979 decision. Among
those, one claimed natural O3 concentrations and other
physical phenomena made the standard unattainable in the Houston area.
The U.S. Court of Appeals for the District of Columbia Circuit (D.C.
Circuit) rejected this argument, holding (as noted in section I.A
above) that attainability and technological feasibility are not
relevant considerations in the promulgation of the NAAQS (American
Petroleum Institute v. Costle, 665 F.2d at 1185). The court also noted
that the EPA need not tailor the NAAQS to fit each region or locale,
pointing out that Congress was aware of the difficulty in meeting
standards in some locations and had addressed it through various
compliance-related provisions in the CAA (id. at 1184-86).
The next periodic reviews of the criteria and standards for
O3 and other photochemical oxidants began in 1982 and 1983,
respectively (47 FR 11561, March 17, 1982; 48 FR 38009, August 22,
1983). The EPA subsequently published the 1986 AQCD, 1989 Staff Paper,
and a supplement to the 1986 AQCD (U.S. EPA, 1986; U.S. EPA, 1989; U.S.
EPA, 1992). In August of 1992, the EPA proposed to retain the existing
primary and secondary standards (57 FR 35542, August 10, 1992). In
March 1993, the EPA concluded this review by finalizing its proposed
decision to retain the standards, without revision (58 FR 13008, March
9, 1993).
In the 1992 decision in that review, the EPA announced its
intention to proceed rapidly with the next review of the air quality
criteria and standards for O3 and other photochemical
oxidants
[[Page 49835]]
(57 FR 35542, August 10, 1992). The EPA subsequently published the AQCD
and Staff Paper for that next review (U.S. EPA, 1996a; U.S. EPA,
1996b). In December 1996, the EPA proposed revisions to both the
primary and secondary standards (61 FR 65716, December 13, 1996). The
EPA completed this review in 1997 by revising the primary and secondary
standards to 0.08 ppm, as the annual fourth-highest daily maximum 8-
hour average concentration, averaged over three years (62 FR 38856,
July 18, 1997).
In response to challenges to the EPA's 1997 decision, the D.C.
Circuit remanded the 1997 O3 NAAQS to the EPA, finding that
section 109 of the CAA, as interpreted by the EPA, effected an
unconstitutional delegation of legislative authority. See American
Trucking Ass'ns v. EPA, 175 F.3d 1027, 1034-1040 (D.C. Cir. 1999). The
court also directed that, in responding to the remand, the EPA should
consider the potential beneficial health effects of O3
pollution in shielding the public from the effects of solar ultraviolet
(UV) radiation, as well as adverse health effects (id. at 1051-53). See
American Trucking Ass'ns v. EPA,195 F.3d 4, 10 (D.C. Cir. 1999)
(granting panel rehearing in part but declining to review the ruling on
consideration of the potential beneficial effects of O3
pollution). After granting petitions for certiorari, the U.S. Supreme
Court unanimously reversed the judgment of the D.C. Circuit on the
constitutional issue, holding that section 109 of the CAA does not
unconstitutionally delegate legislative power to the EPA. See Whitman
v. American Trucking Ass'ns, 531 U.S. 457, 472-74 (2001). The Court
remanded the case to the D.C. Circuit to consider challenges to the
1997 O3 NAAQS that had not yet been addressed. On remand,
the D.C. Circuit found the 1997 O3 NAAQS to be ``neither
arbitrary nor capricious,'' and so denied the remaining petitions for
review. See ATA III, 283 F.3d at 379.
Coincident with the continued litigation of the other issues, the
EPA responded to the court's 1999 remand to consider the potential
beneficial health effects of O3 pollution in shielding the
public from effects of UV radiation (66 FR 57268, Nov. 14, 2001; 68 FR
614, January 6, 2003). In 2001, the EPA proposed to leave the 1997
primary standard unchanged (66 FR 57268, Nov. 14, 2001). After
considering public comment on the proposed decision, the EPA published
its final response to this remand in 2003, re-affirming the 8-hour
primary standard set in 1997 (68 FR 614, January 6, 2003).
The EPA initiated the fourth periodic review of the air quality
criteria and standards for O3 and other photochemical
oxidants with a call for information in September 2000 (65 FR 57810,
September 26, 2000). Documents developed for the review included the
2006 AQCD (U.S. EPA, 2006) and 2007 Staff Paper (U.S. EPA, 2007) and
related technical support documents. In 2007, the EPA proposed
revisions to the primary and secondary standards (72 FR 37818, July 11,
2007). The EPA completed the review in March 2008 by revising the
levels of both the primary and secondary standards from 0.08 ppm to
0.075 ppm while retaining the other elements of the prior standards (73
FR 16436, March 27, 2008). A number of petitioners filed suit
challenging this decision.
In September 2009, the EPA announced its intention to reconsider
the 2008 O3 standards,\5\ and initiated a rulemaking to do
so. At the EPA's request, the court held the consolidated cases in
abeyance pending the EPA's reconsideration of the 2008 decision. In
January 2010, the EPA issued a notice of proposed rulemaking to
reconsider the 2008 final decision (75 FR 2938, January 19, 2010).
Later that year, in view of the need for further consideration and the
fact that the Agency's next periodic review of the O3 NAAQS
required under CAA section 109 had already begun (as announced on
September 29, 2008),\6\ the EPA consolidated the reconsideration with
its statutorily required periodic review.\7\
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\5\ The press release of this announcement is available at:
https://archive.epa.gov/epapages/newsroom_archive/newsreleases/85f90b7711acb0c88525763300617d0d.html.
\6\ The ``Call for Information'' initiating the new review was
announced in the Federal Register (73 FR 56581, September 29, 2008).
\7\ This rulemaking, completed in 2015, concluded the
reconsideration process.
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In light of the EPA's decision to consolidate the reconsideration
with the review then ongoing, the D.C. Circuit proceeded with the
litigation on the 2008 O3 NAAQS decision. On July 23, 2013,
the court upheld the EPA's 2008 primary standard, but remanded the 2008
secondary standard to the EPA. See Mississippi v. EPA, 744 F.3d 1334
(D.C. Cir. 2013). With respect to the primary standard, the court
rejected petitioners' arguments, upholding the EPA's decision. With
respect to the secondary standard, the court held that the EPA's
explanation for the setting of the secondary standard identical to the
revised 8-hour primary standard was inadequate under the CAA because
the EPA had not adequately explained how that standard provided the
required public welfare protection.
At the time of the court's decision, the EPA had already completed
significant portions of its next statutorily required periodic review
of the O3 NAAQS, which had been formally initiated in 2008,
as summarized above. The documents developed for this review included
the ISA,\8\ Risk and Exposure Assessments (REAs) for health and
welfare, and PA.\9\ In late 2014, the EPA proposed to revise the 2008
primary and secondary standards (79 FR 75234, December 17, 2014; Frey,
2014a, Frey, 2014b, Frey, 2014c, U.S. EPA, 2014a, U.S. EPA, 2014b, U.S.
EPA, 2014c). The EPA's final decision in this review was published in
October 2015, establishing the now-current standards (80 FR 65292,
October 26, 2015). In this decision, based on consideration of the
health effects evidence on respiratory effects of O3 in at-
risk populations, the EPA revised the primary standard from a level of
0.075 ppm to a level of 0.070 ppm, while retaining all other elements
of the standard (80 FR 65292, October 26, 2015). The EPA's decision on
the level for the standard was based on the weight of the scientific
evidence and quantitative exposure/risk information. The level of the
secondary standard was also revised from 0.075 ppm to 0.070 ppm based
on the scientific evidence of O3 effects on welfare,
particularly the evidence of O3 impacts on vegetation, and
quantitative analyses available in the review.\10\ The other elements
of the standard were retained. This decision on the secondary standard
also incorporated the EPA's response to the D.C. Circuit's remand of
the 2008 secondary standard in Mississippi v. EPA, 744 F.3d 1344 (D.C.
Cir. 2013).\11\
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\8\ The ISA serves the same purpose, in reviewing the air
quality criteria, as the AQCD did in prior reviews.
\9\ The PA presents an evaluation, for consideration by the
Administrator, of the policy implications of the currently available
scientific information, assessed in the ISA; the quantitative air
quality, exposure or risk analyses presented in the PA and developed
in light of the ISA findings; and related limitations and
uncertainties. The role of the PA is to help ``bridge the gap''
between the Agency's scientific assessment and quantitative
technical analyses, and the judgments required of the Administrator
in his decisions in the review of the O3 NAAQS.
\10\ These standards, set in 2015, are specified at 40 CFR
50.19.
\11\ The 2015 revisions to the NAAQS were accompanied by
revisions to the data handling procedures, ambient air monitoring
requirements, the air quality index and several provisions related
to implementation (80 FR 65292, October 26, 2015).
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After publication of the final rule, a number of industry groups,
environmental and health organizations, and certain states filed
petitions for judicial review in the D.C. Circuit. The
[[Page 49836]]
industry and state petitioners argued that the revised standards were
too stringent, while the environmental and health petitioners argued
that the revised standards were not stringent enough to protect public
health and welfare as the Act requires. On August 23, 2019, the court
issued an opinion that denied all the petitions for review with respect
to the 2015 primary standard while also concluding that the EPA had not
provided a sufficient rationale for aspects of its decision on the 2015
secondary standard and remanding that standard to the EPA. See Murray
Energy Corp. v. EPA, 936 F.3d 597 (D.C. Cir. 2019). The court's
decision on the secondary standard focused on challenges to particular
aspects of EPA's decision. The court concluded that EPA's
identification of particular benchmarks for evaluating the protection
the standard provided against welfare effects associated with tree
growth loss was reasonable and consistent with CASAC's advice. However,
the court held that EPA had not adequately explained its decision to
focus on a 3-year average for consideration of the cumulative exposure,
in terms of W126, identified as providing requisite public welfare
protection, or its decision to not identify a specific level of air
quality related to visible foliar injury. The EPA's decision not to use
a seasonal W126 index as the form and averaging time of the secondary
standard was also challenged, but the court did not reach that issue,
concluding that it lacked a basis to assess the EPA's rationale on this
point because the EPA had not yet fully explained its focus on a 3-year
average W126 in its consideration of the standard. See Murray Energy
Corp. v. EPA, 936 F.3d 597, 618 (D.C. Cir. 2019). Accordingly, the
court remanded the secondary standard to EPA for further justification
or reconsideration. The court's remand of the secondary standard has
been considered in reaching the proposed decision, and associated
proposed conclusions and judgments, described in section III.D.3 below.
In the August 2019 decision, the court additionally addressed
arguments regarding considerations of background O3
concentrations, and socioeconomic and energy impacts. With regard to
the former, the court rejected the argument that the EPA was required
to take background O3 concentrations into account when
setting the NAAQS, holding that the text of CAA section 109(b)
precluded this interpretation because it would mean that if background
O3 levels in any part of the country exceeded the level of
O3 that is requisite to protect public health, the EPA would
be obliged to set the standard at the higher nonprotective level (id.
at 622-23). Thus, the court concluded that the EPA did not act
unlawfully or arbitrarily or capriciously in setting the 2015 NAAQS
without regard for background O3 (id. at 624). Additionally,
the court denied arguments that the EPA was required to consider
adverse economic, social, and energy impacts in determining whether a
revision of the NAAQS was ``appropriate'' under section 109(d)(1) of
the CAA (id. at 621-22). The court reasoned that consideration of such
impacts was precluded by Whitman's holding that the CAA ``unambiguously
bars cost considerations from the NAAQS-setting process'' (531 U.S. at
471, summarized in section 1.2 above). Further, the court explained
that section 109(d)(2)(C)'s requirement that CASAC advise the EPA ``of
any adverse public health, welfare, social, economic, or energy effects
which may result from various strategies for attainment and
maintenance'' of revised NAAQS had no bearing on whether costs are to
be considered in setting the NAAQS (Murray Energy Corp. v. EPA, 936
F.3d at 622). Rather, as described in Whitman and discussed further in
section I.A above, most of that advice would be relevant to
implementation but not standard setting (id.).
In May 2018, the Administrator directed his Assistant
Administrators to initiate this current review of the O3
NAAQS (Pruitt, 2018). In conveying this direction, the Administrator
further directed the EPA staff to expedite the review, implementing an
accelerated schedule aimed at completion of the review within the
statutorily required period (Pruitt, 2018). Accordingly, the EPA took
immediate steps to proceed with the review. In June 2018, the EPA
announced the initiation of the current periodic review of the air
quality criteria for photochemical oxidants and the O3 NAAQS
and issued a call for information in the Federal Register (83 FR 29785,
June 26, 2018). Two types of information were called for: Information
regarding significant new O3 research to be considered for
the ISA for the review, and policy-relevant issues for consideration in
this NAAQS review. Based in part on the information received in
response to the call for information, the EPA developed a draft IRP,
which was made available for consultation with the CASAC and for public
comment (83 FR 55163, November 2, 2018; 83 FR 55528, November 6, 2018).
Comments from the CASAC (Cox, 2018) and the public were considered in
preparing the final IRP (U.S. EPA, 2019b).
Under the plan outlined in the IRP and consistent with revisions to
the process identified by the administrator in his 2018 memo directing
initiation of the review, the current review of the O3 NAAQS
is progressing on an accelerated schedule (Pruitt, 2018). The EPA is
incorporating a number of efficiencies in various aspects of the review
process, as summarized in the IRP, to support completion within the
statutorily required period (Pruitt, 2018). As one example of such an
efficiency, rather than produce two separate documents, the exposure
and risk analyses for the primary standard are included as an appendix
in the PA, along with a number of other technical appendices. The draft
PA (including these analyses as appendices) was reviewed by the CASAC
and made available for public comment while the draft ISA was also
being reviewed by the CASAC and was available for public comment (84 FR
50836, September 26, 2019; 84 FR 58711, November 1, 2019).\12\ The
CASAC was assisted in its review by a pool of consultants with
expertise in a number of fields (84 FR 38625, August 7, 2019). The
approach employed by the CASAC in utilizing outside technical expertise
represents an additional modification of the process from past reviews.
Rather than join with some or all of the CASAC members in a CASAC
review panel as has been common in other NAAQS reviews in the past, in
this O3 NAAQS review (and also in the recent CASAC review of
the PA for the particulate matter NAAQS), the consultants comprised a
pool of expertise that CASAC members drew on through the use of
specific questions, posed in writing prior to the public meeting,
regarding aspects of the documents being reviewed, obtaining subject
matter expertise for its document review in a focused, efficient and
transparent manner.
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\12\ The draft ISA and draft PA were released for public comment
and CASAC review on September 26, 2019 and October 31, 2019,
respectively. The charges for the CASAC review summarized the
overarching context for the document review (including reference to
Pruitt [2018], and the CASAC's role under section 109(d)(2)(C) of
the Act), as well as specific charge questions for review of each of
the documents.
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The CASAC discussed its review of both the draft ISA and the draft
PA over three days at a public meeting in December 2019 (84 FR 58713,
November 1, 2019).\13\ The CASAC discussed its
[[Page 49837]]
draft letters describing its advice and comments on the documents in a
public teleconference in early February 2020 (85 FR 4656; January 27,
2020). The letters to the Administrator conveying the CASAC advice and
comments on the draft PA and draft ISA were released later that month
(Cox, 2020a, Cox, 2020b).
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\13\ While simultaneous review of first drafts of both documents
has not been usual in past reviews, there have been occurrences of
the CASAC review of a draft PA (or draft REA when the process
involved a policy assessment being included within the REA document)
simultaneous with review of a second (or later) draft ISA (e.g., 73
FR 19835, April 11, 2008; 73 FR 34739, June 18, 2008; 77 FR 64335,
October 19, 2020; 78 FR 938, January 7, 2013).
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The letters from the CASAC and public comment on the draft ISA and
draft PA have informed completion of the final documents and further
inform development of the Administrator's proposed decision in this
review. Comments from the CASAC on the draft ISA have been considered
by the EPA and led to a number of revisions in developing the final
document. The CASAC review and the EPA's consideration of CASAC
comments are described in Appendix 10, section 10.4.5 of the final ISA.
In his reply to the CASAC letter conveying its review, ``Administrator
Wheeler noted, `for those comments and recommendations that are more
significant or cross-cutting and which were not fully addressed, the
Agency will develop a plan to incorporate these changes into future
Ozone ISAs as well as ISAs for other criteria pollutant reviews' ''
(ISA, p. 10-28; Wheeler, 2020). The ISA was completed and made
available to the public in April 2020 (85 FR 21849, April 20, 2020).
Based on the rigorous scientific approach utilized in its development,
summarized in Appendix 10 of the final ISA, the EPA considers the final
ISA to ``accurately reflect the latest scientific knowledge useful in
indicating the kind and extent of all identifiable effects on public
health or welfare which may be expected from the presence of
[O3] in the ambient air, in varying quantities'' as required
by the CAA (42 U.S.C. 7408(a)(2)).
The CASAC comments additionally provided advice with regard to the
primary and secondary standards, as well as a number of comments
intended to improve the PA. These comments were considered in
completing that document, which was completed in May 2020 (85 FR 31182,
May 22, 2020). The CASAC advice to the Administrator regarding the
O3 standards has also been described and considered in the
PA, and in sections II and III below. The CASAC advice on the primary
standard is summarized in II.D.2 below and its advice on the secondary
standard is summarized in section III.D.2.
Materials upon which this proposed decision is based, including the
documents described above, are available to the public in the docket
for the review.\14\ Following a public comment period on the proposed
decision, a final decision in the review is projected for late in 2020.
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\14\ The docket for the current O3 NAAQS review is
identified as EPA-HQ-OAR-2018-0279. This docket has incorporated the
ISA docket (EPA-HQ-ORD-2018-0274) by reference. Both dockets are
publicly accessible at www.regulations.gov.
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D. Air Quality Information
Ground level ozone concentrations are a mix of mostly tropospheric
ozone and some stratospheric ozone. Tropospheric ozone is formed due to
chemical interactions involving solar radiation and precursor
pollutants including volatile organic compounds (VOCs) and nitrogen
oxides (NOX). Methane (CH4) and carbon monoxide
(CO) are also important precursors, particularly at the regional to
global scale. The precursor emissions leading to tropospheric
O3 formation can result from both man-made sources (e.g.,
motor vehicles and electric power generation) and natural sources
(e.g., vegetation and wildfires). In addition, O3 that is
created naturally in the stratosphere also contributes to O3
levels near the surface. The stratosphere routinely mixes with the
troposphere high above the earth's surface and, less frequently, there
are intrusions of stratospheric air that reach deep into the
troposphere and even to the surface. Once formed, O3 near
the surface can be transported by winds before eventually being removed
from the atmosphere via chemical reactions or deposition to surfaces.
In sum, O3 concentrations are influenced by complex
interactions between precursor emissions, meteorological conditions,
and topographical characteristics (PA, section 2.1; ISA, Appendix 1).
For compliance and other purposes, state and local environmental
agencies operate O3 monitors across the U.S. and submit the
data to the EPA. At present, there are approximately 1,300 monitors
across the U.S. reporting hourly O3 averages during the
times of the year when local O3 pollution can be important
(PA, section 2.3.1).\15\ Most of this monitoring is focused on urban
areas where precursor emissions tend to be largest, as well as
locations directly downwind of these areas. There are also over 100
routine monitoring sites in rural areas, including sites in the Clean
Air Status and Trends Network (CASTNET) which is specifically focused
on characterizing conditions in rural areas. Based on the monitoring
data for the most recent 3-year period (2016-2018), the EPA identified
142 counties, in which together approximately 106 million Americans
reside where O3 design values \16\ were above 0.070, the
level of the existing NAAQS (PA, section 2.4.1). Across these areas,
the highest design values are typically observed in California, Texas,
and the Northeast Corridor, locations with some of the most densely
populated areas in the country (e.g., PA, Figure 2-8).
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\15\ O3 monitoring seasons vary by state from five
months (May to September in Oregon and Washington) to all twelve
months (in 11 states), with the most common season being March to
October (in 27 states).
\16\ A design value is a statistic that summarizes the air
quality data for a given area in terms of the indicator, averaging
time, and form of the standard. Design values can be compared to the
level of the standard and are typically used to designate areas as
meeting or not meeting the standard and assess progress towards
meeting the NAAQS.
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From a temporal perspective, the highest daily peak O3
concentrations generally tend to occur during the afternoon and within
the warmer months of the year due to higher levels of solar radiation
and other conducive meteorological conditions during these times. The
exceptions to this general rule include (1) some rural sites where
transport of O3 from upwind urban areas can occasionally
result in high nighttime levels of O3, (2) high-elevation
sites which can be episodically influenced by stratospheric intrusions
in other months of the year, and (3) mountain basins in the western
U.S. where large quantities of O3 precursors emissions
associated with oil and gas development can be trapped in a shallow
inversion layer and form O3 under clear, calm skies with
snow cover during the colder months (PA, section 2.1; ISA, Appendix 1).
Monitoring data indicate long-term reductions in short-term
O3 concentrations. For example, monitoring sites operating
since 1980 indicate a 32% reduction in the national average annual
fourth highest daily maximum 8-hour concentration from 1980 to 2018.
(PA, Figure 2-10). This has been accompanied by appreciable reductions
in peak 1-hour concentrations (PA, Figure 2-17).
Concentrations of O3 in ambient air that result from
natural and non-U.S. anthropogenic sources are collectively referred to
as U.S. background O3 (USB; PA, section 2.5). As in the last
review, we generally characterize O3 concentrations that
would exist in the absence of U.S. anthropogenic emissions as U.S.
background (USB). Findings from modeling analyses performed for this
review to investigate
[[Page 49838]]
patterns of USB in the U.S. are largely consistent with conclusions
reached in the last review (PA, section 2.5.4). The current modeling
analysis indicates spatial variation in USB O3 that is
related to geography, topography and proximity to international borders
and is also influenced by seasonal variation, with long-range
international anthropogenic transport contributions peaking in the
spring while U.S. anthropogenic contributions tend to peak in summer.
The West is predicted to have higher USB concentrations than the East,
with higher contributions from natural and international anthropogenic
sources that exert influences in western high-elevation and near-border
areas. The modeling predicts that for both the West and the East, days
with the highest 8-hour concentrations of O3 generally occur
in summer and are likely to have substantially greater concentrations
due to U.S. anthropogenic sources. While the USB contributions to
O3 concentrations on days with the highest 8-hour
concentrations are generally predicted to come largely from natural
sources, the modeling also indicates that a small area near the Mexico
border may receive appreciable contributions from a combination of
natural and international anthropogenic sources on these days. In such
locations, the modeling suggests the potential for episodic and
relatively infrequent events with substantial background contributions
where daily maximum 8-hour O3 concentrations approach or
exceed the level of the current NAAQS (i.e., 70 ppb). This contrasts
with most monitor locations in the U.S. for which international
contributions are predicted to be the lowest during the season with the
most frequent occurrence of daily maximum 8-hour O3
concentrations above 70 ppb. This is generally because, except for in
near-border areas, larger international contributions are associated
with long-distance transport and that is most efficient in the
springtime (PA, section 2.5.4).
II. Rationale for Proposed Decision on the Primary Standard
This section presents the rationale for the Administrator's
proposed decision to retain the current primary O3 standard.
This rationale is based on a thorough review of the latest scientific
information generally published between January 2011 and March 2018, as
well as more recent studies identified during peer review or by public
comments (ISA, section IS.1.2),\17\ integrated with the information and
conclusions from previous assessments and presented in the ISA, on
human health effects associated with photochemical oxidants including
O3 and pertaining to their presence in ambient air. The
Administrator's rationale also takes into account: (1) The PA
evaluation of the policy-relevant information in the ISA and
presentation of quantitative analyses of air quality, human exposure
and health risks; (2) CASAC advice and recommendations, as reflected in
discussions of drafts of the ISA and PA at public meetings and in the
CASAC's letters to the Administrator; and (3) public comments received
during the development of these documents.
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\17\ In addition to the review's opening ``Call for
Information'' (83 FR 29785, June 26, 2018), systematic review
methodologies were applied to identify relevant scientific findings
that have emerged since the 2013 ISA, which included peer reviewed
literature published through July 2011. Search techniques for the
current ISA identified and evaluated studies and reports that have
undergone scientific peer review and were published or accepted for
publication between January 1, 2011 (providing some overlap with the
cutoff date for the last ISA) and March 30, 2018. Studies published
after the literature cutoff date for this ISA were also considered
if they were submitted in response to the Call for Information or
identified in subsequent phases of ISA development, particularly to
the extent that they provide new information that affects key
scientific conclusions (ISA, Appendix 10, section 10.2). References
that are cited in the ISA, the references that were considered for
inclusion but not cited, and electronic links to bibliographic
information and abstracts can be found at: https://hero.epa.gov/hero/index.cfm/project/page/project_id/2737.
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In presenting the rationale for the Administrator's proposed
decision and its foundations, section II.A provides background and
introductory information for this review of the primary O3
standard. It includes background on the establishment of the current
standard in 2015 (section II.A.1) and also describes the general
approach for the current review (section II.A.2). Section II.B
summarizes the currently available health effects evidence, focusing on
consideration of key policy-relevant aspects. Section II.C summarizes
the exposure and risk information for this review, drawing on the
quantitative analyses for O3, presented in the PA. Section
II.D presents the Administrator's proposed conclusions on the current
standard (section II.D.3), drawing on both evidence-based and exposure/
risk-based considerations (section II.D.1) and advice from the CASAC
(section II.D.2).
A. General Approach
The past and current approaches described below are both based,
most fundamentally, on using the EPA's assessments of the current
scientific evidence and associated quantitative analyses to inform the
Administrator's judgment regarding a primary standard for photochemical
oxidants that is requisite to protect the public health with an
adequate margin of safety. The EPA's assessments are primarily
documented in the ISA and PA, all of which have received CASAC review
and public comment (84 FR 50836, September 26, 2019; 84 FR 58711,
November 1, 2019; 84 FR 58713, November 1, 2019; 85 FR 21849, April 20,
2020; 85 FR 31182, May 22, 2020). In bridging the gap between the
scientific assessments of the ISA and the judgments required of the
Administrator in his decisions on the current standard, the PA
evaluates policy implications of the evaluation of the current evidence
in ISA and the quantitative exposure and risk analyses documented in
appendices of the PA. In evaluating the public health protection
afforded by the current standard, the four basic elements of the NAAQS
(indicator, averaging time, level, and form) are considered
collectively.
The final decision on the adequacy of the current primary standard
is a public health policy judgment to be made by the Administrator. In
reaching conclusions with regard to the standard, the decision will
draw on the scientific information and analyses about health effects,
population exposure and risks, as well as judgments about how to
consider the range and magnitude of uncertainties that are inherent in
the scientific evidence and analyses. This approach is based on the
recognition that the available health effects evidence generally
reflects a continuum, consisting of 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 Clean Air Act and with how
the EPA and the courts have historically interpreted the Act
(summarized in section I.A. above). These provisions require the
Administrator to establish primary standards that, in the judgment of
the Administrator, are requisite to protect public health with an
adequate margin of safety. In so doing, the Administrator seeks to
establish standards that are neither more nor less stringent than
necessary for this purpose. The Act does not require that primary
standards be set at a zero-risk level, but rather at a level that
avoids unacceptable risks to public
[[Page 49839]]
health, including the health of sensitive groups.\18\
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\18\ As noted in section I.A above, the legislative history
describes such protection for the sensitive group of individuals and
not for a single person in the sensitive group (see S. Rep. No. 91-
1196, 91st Cong., 2d Sess. 10 [1970]).
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The subsections below provide background and introductory
information. Background on the establishment of the current standard in
2015, including the rationale for that decision, is summarized in
section II.A.1. This is followed, in section II.A.2, by an overview of
the general approach for the current review of the 2015 standard.
Following this introductory section and subsections, the subsequent
sections summarize current information and analyses, including that
newly available in this review. The Administrator's proposed
conclusions on the standard set in 2015, based on the current
information, are provided in section II.D.3.
1. Background on the Current Standard
The current primary standard was set in 2015 based on the
scientific evidence and quantitative exposure and risk analyses
available at that time, and on the Administrator's judgments regarding
the available scientific evidence, the appropriate degree of public
health protection for the revised standard, and the available exposure
and risk information regarding the exposures and risk that may be
allowed by such a standard (80 FR 65292, October 26, 2015). The 2015
decision revised the level of the primary standard from 0.075 to 0.070
ppm,\19\ in conjunction with retaining the indicator (O3),
averaging time (eight hours), and form (annual fourth-highest daily
maximum 8-hour average concentration, averaged across three consecutive
years). This action provided increased protection for at-risk
populations,\20\ such as children and people with asthma, against an
array of adverse health effects. The 2015 decision drew upon the
available scientific evidence assessed in the 2013 ISA, the exposure
and risk information presented and assessed in the 2014 health REA
(HREA), the consideration of that evidence and information in the 2014
PA, the advice and recommendations of the CASAC, and public comments on
the proposed decision (79 FR 75234, December 17, 2014).
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\19\ Although ppm are the units in which the level of the
standard is defined, the units, ppb, are more commonly used
throughout this document for greater consistency with their use in
the more recent literature. The level of the current primary
standard, 0.070 ppm, is equivalent to 70 ppb.
\20\ As used here and similarly throughout the document, the
term population refers to persons having a quality or characteristic
in common, such as, and including, a specific pre-existing illness
or a specific age or lifestage. A lifestage refers to a
distinguishable time frame in an individual's life characterized by
unique and relatively stable behavioral and/or physiological
characteristics that are associated with development and growth.
Identifying at-risk populations includes consideration of intrinsic
(e.g., genetic or developmental aspects) or acquired (e.g., disease
or smoking status) factors that increase the risk of health effects
occurring with exposure to a substance (such as O3) as
well as extrinsic, nonbiological factors, such as those related to
socioeconomic status, reduced access to health care, or exposure.
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The health effects evidence base available in the 2015 review
included extensive evidence from previous reviews as well as the
evidence that had emerged since the prior review had been completed in
2008. This evidence base, spanning several decades, documents the
causal relationship between exposure to O3 and a broad range
of respiratory effects (2013 ISA, p. 1-14). Such effects range from
small, reversible changes in pulmonary function and pulmonary
inflammation (documented in controlled human exposure studies involving
exposures ranging from 1 to 8 hours) to more serious health outcomes
such as emergency department visits and hospital admissions, which have
been associated with ambient air concentrations of O3 in
epidemiologic studies (2013 ISA, section 6.2). In addition to extensive
controlled human exposure and epidemiologic studies, the evidence base
includes experimental animal studies that provide insight into
potential modes of action for these effects, contributing to the
coherence and robust nature of the evidence. Based on this evidence,
the 2013 ISA concluded there to be a causal relationship between short-
term O3 exposures and respiratory effects, and also
concluded that the relationship between longer-term exposure and
respiratory effects was likely to be causal (2013 ISA, p. 1-14).\21\
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\21\ The 2013 ISA also concluded there likely to be causal
relationship between short-term exposure and mortality, as well as
short-term exposure and cardiovascular effects, including related
mortality, and that the evidence was suggestive of causal
relationships between long-term O3 exposures and total
mortality, cardiovascular effects and reproductive and developmental
effects, and between short-term and long-term O3 exposure
and nervous system effects (2013 ISA, section 2.5.2).
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With regard to the short-term respiratory effects that were the
primary focus of the 2015 decision, the controlled human exposure
studies were recognized to provide the most certain evidence indicating
the occurrence of health effects in humans following specific
O3 exposures (80 FR 65343, October 26, 2015; 2014 PA,
section 3.4). These studies additionally illustrate the role of
ventilation rate \22\ and exposure duration in eliciting responses to
O3 exposure at the lowest studied concentrations. The
exposure concentrations eliciting a given level of response in subjects
at rest are higher than those eliciting a response in subjects exposed
while at elevated ventilation, such as while exercising (2013 ISA,
section 6.2.1.1).\23\
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\22\ Ventilation rate (VE) is a specific technical
term referring to breathing rate in terms of volume of air taken
into the body per unit of time. The units for VE are
usually liters (L) per minute (min). Another related term is
equivalent ventilation rate (EVR), which refers to VE
normalized by a person's body surface area in square meters (m\2\).
Accordingly, the units for EVR are generally L/min-m\2\. For
different activities, a person will experience different levels of
exertion and different ventilation rates.
\23\ In the controlled human exposure studies, the magnitude or
severity of the respiratory effects induced by O3 is
influenced by ventilation rate and exposure duration, as well as
exposure concentration, with physical activity increasing
ventilation and potential for effects. In studies of generally
healthy adults exposed while at rest for 2 hours, 500 ppb is the
lowest concentration eliciting a statistically significant
O3-induced reduction in group mean lung function
measures, while a much lower concentration produces such result when
the study subject ventilation rates are sufficiently increased with
exercise (2013 ISA, section 6.2.1.1). The lowest exposure
concentration found to elicit a statistically significant
O3-induced reduction in group mean lung function in an
exposure of 2 hours or less was 120 ppb after a 1-hour exposure
(continuous, very heavy exercise) of trained cyclists (2013 ISA,
section 6.2.1.1; Gong et al., 1986) and after 2-hour exposure
(intermittent heavy exercise) of young healthy adults (2013 ISA,
section 6.2.1.1; McDonnell et al., 1983).
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The exposure and risk information available in the 2015 review
included exposure and risk estimates for air quality conditions just
meeting the then-existing standard, and also for air quality conditions
just meeting potential alternative standards (U.S. EPA, 2014a,
hereafter 2014 HREA). Estimates were derived for two exposure-based
analyses, as well as for an analysis based on epidemiologic study
associations. The first of the exposure-based analyses involved
comparison of population exposure estimates at elevated exertion to
exposure benchmark concentrations (exposures of concern).\24\ These
benchmark concentrations are based on exposure concentrations from
controlled human exposure studies in which lung function changes and
other effects were measured in healthy, young adult volunteers exposed
to O3 while engaging in quasi-continuous moderate physical
activity for a defined period (generally 6.6 hours).\25\ The second
[[Page 49840]]
exposure-based analysis provided population risk estimates of the
occurrence of days with O3-attributable lung function
reductions of varying magnitudes by using the exposure-response (E-R)
information in the form of E-R functions or other quantitative
descriptions of biological processes.\26\ In the epidemiologic study-
based analysis, risk estimates were also derived from ambient air
concentrations using concentration-response (C-R) functions derived
from epidemiologic studies. These latter estimates were given less
weight by the Administrator in her decision on the standard in light of
conclusions reached in the 2014 PA and the HREA, which reflected lower
confidence in these estimates (80 FR 65316-17, October 26, 2015).
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\24\ The benchmark concentrations to which exposure
concentrations experienced while at moderate or greater exertion
were compared were 60, 70 and 80 ppb.
\25\ The studies given primary focus were those for which
O3 exposures occurred over the course of 6.6 hours during
which the subjects engaged in six 50-minute exercise periods
separated by 10-minute rest periods, with a 35-minute lunch period
occurring after the third hour (e.g., Folinsbee et al., 1988 and
Schelegle et al., 2009). Responses after O3 exposure were
compared to those after filtered air exposure.
\26\ The E-R information and quantitative models derived from it
are based on controlled human exposure studies.
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The 2014 HREA developed exposure-based estimates for several
population groups including all children and all adults. The type of
exposure-based estimates that involved comparison of exposures to
benchmarks was also derived for children with asthma and adults with
asthma. The estimates of percentages of all children with exposures at
or above benchmarks were virtually indistinguishable from the
corresponding estimates for children with asthma.\27\ When considered
in terms of the number of children (rather than percentages of the
child populations), the estimates for all children were much higher
than those for children with asthma, with the magnitude of the
differences varying based on asthma prevalence in each study area (2014
HREA, sections 5.3.2, 5.4.1.5 and section 5F-1). The estimates for
percent of children experiencing an exposure at or above the benchmarks
were higher than percent of adults due to the greater time that
children spend outdoors and engaged in activities at elevated exertion
(2014 HREA, section 5.3.2). Thus, consideration of the exposure-based
results in the 2015 decision focused on the results for all children
and children with asthma.
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\27\ This reflects use of the same time-location-activity diary
pool to construct each simulated individual's time-activity series,
which is based on the similarities observed in the available diary
data with regard to time spent outdoors and exertion levels (2014
HREA, sections 5.3.2 and 5.4.1.5).
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In weighing the 2013 ISA conclusions with regard to the health
effects evidence and making judgments regarding the public health
significance of the quantitative estimates of exposures and risks
allowed by the then-existing standard and potential alternative
standards considered, as well as judgments regarding margin of safety,
the Administrator considered the currently available information and
commonly accepted guidelines or criteria within the public health
community, including statements of the American Thoracic Society (ATS),
an organization of respiratory disease specialists,\28\ advice from the
CASAC and public comments. In so doing, she recognized that the
determination of what constitutes an adequate margin of safety is
expressly left to the judgment of the EPA Administrator. See Lead
Industries Ass'n v. EPA, 647 F.2d 1130, 1161-62 (D.C. Cir. 1980);
Mississippi v. EPA, 744 F.3d 1334, 1353 (D.C. Cir. 2013). In NAAQS
reviews generally, evaluations of how particular primary standards
address the requirement to provide an adequate margin of safety include
consideration of such factors as the nature and severity of the health
effects, the size of the sensitive population(s) at risk, and the kind
and degree of the uncertainties present. Consistent with past practice
and long-standing judicial precedent, the Administrator took the need
for an adequate margin of safety into account as an integral part of
her decision-making.
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\28\ In this regard, the 2014 PA considered statements issued by
the ATS that had also been considered in prior reviews (ATS, 2000;
ATS, 1985).
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In the 2015 decision, the Administrator first addressed the
adequacy of protection provided by the then-existing primary standard
and decided that the standard should be revised. Considerations related
to that decision are summarized in section II.A.1.a below. The
considerations and decisions on the revisions to the then-existing
standard in order to provide the requisite protection under the Act,
including an adequate margin of safety, are summarized in section
II.A.1.b.
a. Considerations Regarding Adequacy of the Prior Standard
In the decision that the primary standard that existed at the time
of the last review should be revised, the Administrator at that time
gave primary consideration to the evidence of respiratory effects from
controlled human exposure studies, including those newly available in
the review, and for which the exposure concentrations were at the lower
end of those studied (80 FR 65343, October 26, 2015). This emphasis was
consistent with comments from the CASAC at that time on the strength of
this evidence (Frey, 2014b, p. 5). In placing weight on these studies,
the Administrator took note of the variety of respiratory effects
reported from the studies of healthy adults engaged in six 50-minute
periods of moderate exertion within a 6.6-hour exposure to
O3 concentrations of 60 ppb and higher. The lowest exposure
concentration in such studies for which a combination of statistically
significant reduction in lung function and increase in respiratory
symptoms was reported was 72 ppb (during the exercise periods),\29\
while reduced lung function and increased pulmonary inflammation were
reported following such exposures to O3 concentrations as
low as 60 ppb. In considering these findings, the Administrator noted
that the combination of O3-induced lung function decrements
and respiratory symptoms met ATS criteria for an adverse response.\30\
She additionally noted the CASAC comments on this point and also its
caution that these study findings were for healthy adults and thus
indicated the potential for such effects in some groups of people, such
as people with asthma, at lower exposure concentrations (Frey, 2014b,
pp. 5-6; 80 FR 65343, October 26, 2015).
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\29\ For the 70 ppb target exposure, Schelegle et al. (2009)
reported, based on O3 measurements during the six 50-
minute exercise periods, that the mean O3 concentration
during the exercise portion of the study protocol was 72 ppb. Based
on the measurements for the six exercise periods, the time weighted
average concentration across the full 6.6-hour exposure was 73 ppb
(Schelegle et al., 2009).
\30\ The most recent statement from the ATS available at the
time of the 2015 decision stated that ``[i]n drawing the distinction
between adverse and nonadverse reversible effects, this committee
recommended that reversible loss of lung function in combination
with the presence of symptoms should be considered as adverse''
(ATS, 2000).
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The 2013 ISA indicated that the pattern of effects observed across
the range of exposures assessed in the controlled human exposure
studies, increasing with severity at higher exposures, is coherent with
(i.e., reasonably related to) the health outcomes reported to be
associated with ambient air concentrations in epidemiologic studies
(e.g., respiratory-related hospital admissions, emergency department
visits). With regard to the available epidemiologic studies, while
analyses of O3 air quality in the 2014 PA indicated that
most O3 epidemiologic studies reported health effect
associations with O3 concentrations in ambient air that
violated the then-current (75 ppb) standard, the Administrator took
particular note of a study that reported associations
[[Page 49841]]
between short-term O3 concentrations and asthma emergency
department visits in children and adults in a U.S. location that would
have met the then-current standard over the entire 5-year study period
(80 FR 65344, October 26, 2015; Mar and Koenig, 2009).\31\ While
uncertainties limited the Administrator's conclusions on air quality in
locations of multicity epidemiologic studies,\32\ in looking across the
body of epidemiologic evidence, the Administrator reached the
conclusion that analyses of air quality in some study locations
supported the occurrence of adverse O3-associated effects at
O3 concentrations in ambient air that met, or are likely to
have met, the then-current standard (80 FR 65344, October 26, 2016).
Taken together, the Administrator concluded that the scientific
evidence from controlled human exposure and epidemiologic studies
called into question the adequacy of the public health protection
provided by the 75 ppb standard that had been set in 2008.
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\31\ The design values in this location over the study period
were at or somewhat below 75 ppb (Wells, 2012).
\32\ Compared to the single-city epidemiologic studies, the
Administrator noted additional uncertainty that applied specifically
to interpreting air quality analyses within the context of multicity
effect estimates for short-term O3 concentrations, where
effect estimates for individual study cities are not presented (80
FR 65344; October 26, 2015).
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In considering the exposure and risk information, the Administrator
gave particular attention to the exposure-based comparison-to-
benchmarks analysis, focusing on the estimates of exposures of concern
for children, in 15 urban study areas for air quality conditions just
meeting the then-current standard. Consistent with the finding that
larger percentages of children than adults were estimated to experience
exposures at or above benchmarks, the Administrator focused on the
results for all children and for children with asthma, noting that the
results for these two groups, in terms of percent of the population
group, are virtually indistinguishable (2014 HREA, sections 5.3.2,
5.4.1.5 and section 5F-1). In considering these estimates, she placed
the greatest weight on estimates of two or more days with occurrences
of exposures at or above the benchmarks, in light of her increased
concern about the potential for adverse responses with repeated
occurrences of such exposures. In particular, she noted that the types
of effects shown to occur following exposures to O3
concentrations from 60 ppb to 80 ppb, such as inflammation, if
occurring repeatedly as a result of repeated exposure, could
potentially result in more severe effects based on the ISA conclusions
regarding mode of action (80 FR 65343, 65345, October 26, 2015; 2013
ISA, section 6.2.3).\33\ While generally placing the greatest weight on
estimates of repeated exposures, the Administrator also considered
estimates for single exposures at or above the higher benchmarks of 70
and 80 ppb (80 FR 65345, October 26, 2015). Further, while the
Administrator recognized the effects documented in the controlled human
exposure studies for exposures to 60 ppb to be less severe than those
associated with exposures to higher O3 concentrations, she
also recognized there to be limitations and uncertainties in the
evidence base with regard to unstudied population groups. As a result,
she judged it appropriate for the standard, in providing an adequate
margin of safety, to provide some control of exposures at or above the
60 ppb benchmark (80 FR 65345-65346, October 26, 2015).
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\33\ In addition to recognizing the potential for continued
inflammation to evolve into other outcomes, the 2013 ISA also
recognized that inflammation induced by a single exposure (or
several exposures over the course of a summer) can resolve entirely
(2013 ISA, p. 6-76; 80 FR 65331, October 26, 2015).
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In considering the exposure estimates from the 2014 HREA with
regard to public health implications, the Administrator concluded that
the exposures and risks projected to remain upon meeting the then-
current (75 ppb) standard could reasonably be judged to be important
from a public health perspective. In particular, this conclusion was
based on her judgment that it is appropriate to set a standard that
would be expected to eliminate, or almost eliminate, the occurrence of
exposures, while at moderate exertion, at or above 70 and 80 ppb (80 FR
65346, October 26, 2015). In addition, given that the average percent
of children estimated to experience two or more days with exposures at
or above the 60 ppb benchmark approaches 10% in some urban study areas
(on average across the analysis years), the Administrator concluded
that the then-current standard did not incorporate an adequate margin
of safety against the potentially adverse effects that could occur
following repeated exposures at or above 60 ppb (80 FR 65345-46,
October 26, 2015). Further, although the Administrator recognized
increased uncertainty in and placed less weight on the HREA estimates
for lung function risk and for the epidemiologic-study-based risk
analyses, she found them supportive of a conclusion that the
O3-associated health effects estimated to remain upon just
meeting the then-current standard are an issue of public health
importance on a broad national scale. Thus, she concluded that
O3 exposure and risk estimates, taken together, supported a
conclusion that the exposures and health risks associated with just
meeting the then-current standard could reasonably be judged to be of
public health significance, such that the then-current standard was not
sufficiently protective and did not incorporate an adequate margin of
safety.
In consideration of all of the above, as well as the CASAC advice,
which included the unanimous recommendation ``that the Administrator
revise the current primary ozone standard to protect public health''
(Frey, 2014b, p. 5),\34\ the Administrator concluded that the then-
current primary O3 standard (with its level of 75 ppb) was
not requisite to protect public health with an adequate margin of
safety, and that it should be revised to provide increased public
health protection. This decision was based on the Administrator's
conclusions that the available evidence and exposure and risk
information clearly called into question the adequacy of public health
protection provided by the then-current primary standard such that it
was ``not appropriate, within the meaning of section 109(d)(1) of the
CAA, to retain the current standard'' (80 FR 65346, October 26, 2015).
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\34\ The Administrator also noted that CASAC for the prior,
2008, review likewise recommended revision of the standard to one
with a level below 75 ppb. This earlier recommendation was based
entirely on the evidence and information in the record for the 2008
decision, which had been expanded in the 2015 review (Samet, 2011;
Frey and Samet, 2012).
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b. Considerations for the Revised Standard
With regard to the most appropriate indicator for a revised
standard, the Administrator considered findings and assessments in the
2013 ISA and 2014 PA, as well as advice from the CASAC and public
comment. These include the finding that O3 is the only
photochemical oxidant (other than nitrogen dioxide) that is routinely
monitored and for which a comprehensive database exists, and the
consideration that, since the precursor emissions that lead to the
formation of O3 also generally 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 other photochemical oxidants (2013 ISA, section 3.6;
80 FR
[[Page 49842]]
65347, October 26, 2015). The CASAC indicated its view that
O3 is the appropriate indicator ``based on its causal or
likely causal associations with multiple adverse health outcomes and
its representation of a class of pollutants known as photochemical
oxidants'' (Frey, 2014c, p. ii). Based on all of these considerations
and public comments, the Administrator concluded that O3
remained the most appropriate indicator for a standard meant to provide
protection against photochemical oxidants in ambient air, and she
retained O3 as the indicator for the primary standard (80 FR
65347, October 26, 2015).
The 8-hour averaging time for the primary O3 standard
was established in 1997 with the decision to replace the then-existing
1-hour standard with an 8-hour standard (62 FR 38856, July 18, 1997).
The decision in that review was based on evidence from numerous
controlled human exposure studies of healthy adults of adverse
respiratory effects resulting from 6- to 8-hour exposures, as well as
quantitative analyses indicating the control provided by an 8-hour
averaging time of both 8-hour and 1-hour peak exposures and associated
health risk (62 FR 38861, July 18, 1997; U.S. EPA, 1996b). The 1997
decision was also consistent with advice from the CASAC (62 FR 38861,
July 18, 1997; 61 FR 65727, December 13, 1996). The EPA reached similar
conclusions in the subsequent 2008 review in which the 8-hour averaging
time was retained (73 FR 16436, March 27, 2008). In the review
completed in 2015, the Administrator concluded, in consideration of the
then-available health effects information, that an 8-hour averaging
time remained appropriate for addressing health effects associated with
short-term exposures to ambient air O3 and that it could
effectively limit health effects attributable to both short- and long-
term O3 exposures (80 FR 65348, October 26, 2015). Thus, she
found it appropriate to retain this averaging time (80 FR 65350,
October 26, 2015).
While giving foremost consideration to the adequacy of public
health protection provided by the combination of all elements of the
standard, including the form, the Administrator additionally considered
the appropriateness of retaining the nth-high metric as the form for
the revised standard (80 FR 65350-65352, October 26, 2015). In so
doing, she considered findings from prior reviews, including the 1997
review, in which it was recognized that a concentration-based form, by
giving proportionally more weight to years when 8-hour O3
concentrations are well above the level of the standard than years when
concentrations are just above the level, better reflects the continuum
of health effects associated with increasing O3
concentrations than does an expected exceedance form, which had been
the form of the standard prior to 1997.\35\ Although the subsequent
2008 review considered the potential value of a percentile-based form,
the EPA concluded at that time that, because of the differing lengths
of the monitoring season for O3 across the U.S., a
percentile-based statistic would not be effective in ensuring the same
degree of public health protection across the country (73 FR 16474-75,
March 27, 2008). The 2008 review additionally recognized the importance
of a form that provides stability to ongoing control programs and
insulation from the impacts of extreme meteorological events that are
conducive to O3 occurrence (73 FR 16474-16475, March 27,
2008). Based on all of these considerations, and including advice from
the CASAC, which stated that this form ``provides health protection
while allowing for atypical meteorological conditions that can lead to
abnormally high ambient ozone concentrations which, in turn, provides
programmatic stability'' (Frey, 2014b, p. 6), the 2015 decision was to
retain the existing form (the annual fourth-highest daily maximum 8-
hour O3 average concentration, averaged over three
consecutive years), without revision (80 FR 65352, October 26, 2015).
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\35\ With regard to a specific concentration-based form, the
fourth-highest daily maximum was selected in 1997, recognizing that
a less restrictive form (e.g., fifth highest) would allow a larger
percentage of sites to experience O3 peaks above the
level of the standard, and would allow more days on which the level
of the standard may be exceeded when the site attains the standard
(62 FR 38868-38873, July 18, 1997), and there was no basis
identified for selection of a more restrictive form (62 FR 38856,
July 18, 1997).
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The 2015 decision to set the level of the revised primary
O3 standard at 70 ppb built upon the Administrator's
conclusion (summarized in section II.A.1.a above) that the overall body
of scientific evidence and exposure/risk information called into
question the adequacy of the public health protection afforded by the
then-current standard, particularly for at-risk populations and
lifestages (80 FR 65362, October 26, 2015). In her decision on level,
the Administrator placed the greatest weight on the results of
controlled human exposure studies and on quantitative analyses based on
information from these studies, particularly analyses of O3
exposures of concern.\36\ In so doing, the Administrator noted that
controlled human exposure studies provide the most certain evidence
indicating the occurrence of health effects in humans following
specific O3 exposures, noting in particular that the effects
reported in the controlled human exposure studies are due solely to
O3 exposures, and are not complicated by the presence of co-
occurring pollutants or pollutant mixtures (as is the case in
epidemiologic studies). The Administrator's emphasis on the information
from the controlled human exposure studies was consistent with the
CASAC's advice and interpretation of the scientific evidence (80 FR
65362, October 26, 2015; Frey, 2014b). In this regard, the
Administrator recognized that: (1) The largest respiratory effects, and
the broadest range of effects, have been studied and reported following
exposures to 80 ppb O3 or higher (i.e., decreased lung
function, increased airway inflammation, increased respiratory
symptoms, airway hyperresponsiveness, and decreased lung host defense);
(2) exposures to O3 concentrations somewhat above 70 ppb
have been shown to both decrease lung function and to result in
respiratory symptoms; and (3) exposures to O3 concentrations
as low as 60 ppb have been shown to decrease lung function and to
increase airway inflammation (80 FR 65363, October 26, 2015). The
Administrator also considered both ATS recommendations and CASAC advice
to inform her judgments on the potential adversity to public health
associated with O3 effects reported in controlled human
exposure studies (80 FR 65363, October 26, 2015).\37\
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\36\ The Administrator viewed the results of the lung function
risk assessment, analyses of O3 air quality in locations
of epidemiologic studies, and epidemiologic-study-based quantitative
health risk assessment as being of less utility for selecting a
particular standard level among a range of options (80 FR 65362,
October 26, 2015).
\37\ In so doing, the Administrator recognized that a standard
level of 70 ppb would be well below the O3 exposure
concentration documented to result in the widest range of
respiratory effects (i.e., 80 ppb), and below the lowest
O3 exposure concentration shown to result in the adverse
combination of lung function decrements and respiratory symptoms (80
FR 65363, October 26, 2015).
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In considering the degree of protection provided by a revised
primary O3 standard, and the extent to which that standard
would be expected to limit population exposures to the broad range of
O3 exposures shown to result in health effects, the
Administrator considered the exposure estimates from the HREA, focusing
particularly on the estimates of two or more exposures of concern. In
so doing,
[[Page 49843]]
she placed the most emphasis on setting a standard that appropriately
limits repeated occurrences of exposures at or above the 70 and 80 ppb
benchmarks, while at elevated ventilation. She noted that a revised
standard with a level of 70 ppb was estimated to eliminate the
occurrence of two or more days with exposures at or above 80 ppb and to
virtually eliminate the occurrence of two or more days with exposures
at or above 70 ppb for all children and children with asthma, even in
the worst-case year and location evaluated.\38\ Given the considerable
protection provided against repeated exposures of concern for all
benchmarks evaluated in the HREA, the Administrator judged that a
standard with a level of 70 ppb incorporated a margin of safety against
the adverse O3-induced effects shown to occur in the
controlled human exposure studies (80 FR 65364, October 26, 2015).\39\
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\38\ Under conditions just meeting an alternative standard with
a level of 70 ppb across the 15 urban study areas, the estimate for
two or more days with exposures at or above 70 ppb was 0.4% of
children, in the worst year and worst area (80 FR 65313, Table 1,
October 26, 2015).
\39\ In so judging, she noted that the CASAC had recognized the
choice of a standard level within the range it recommended based on
the scientific evidence (which is inclusive of 70 ppb) to be a
policy judgment (80 FR 65355, October 26, 2015; Frey, 2014).
---------------------------------------------------------------------------
While she was less confident that adverse effects would occur
following exposures to O3 concentrations as low as 60
ppb,\40\ as discussed above, the Administrator also considered
estimates of exposures (while at moderate or greater exertion) for the
60 ppb benchmark (80 FR 65363-64, October 26, 2015). In so doing, she
recognized that while CASAC advice regarding the potential adversity of
effects observed in studies of 60 ppb was less definitive than for
effects observed at the next higher concentration studied, the CASAC
did clearly advise the EPA to consider the extent to which a revised
standard is estimated to limit the effects observed in studies of 60
ppb exposures (80 FR 65364, October 26, 2015; Frey, 2014b). The
Administrator's consideration of exposures at or above the 60 ppb
benchmark, and particularly consideration of multiple occurrences of
such exposures, was primarily in the context of considering the extent
to which the health protection provided by a revised standard included
a margin of safety against the occurrence of adverse O3-
induced effects (80 FR 65464, October 26, 2015). In this context, the
Administrator noted that a revised standard with a level of 70 ppb was
estimated to protect the vast majority of children in urban study areas
(i.e., about 96% to more than 99% of children in individual areas) from
experiencing two or more days with exposures at or above 60 ppb (while
at moderate or greater exertion). Compared to the estimates for the
then-current standard (with its level of 75 ppb), this represented a
reduction in repeated exposures of more than 60%. Given the
considerable protection provided against repeated exposures of concern
for all of the benchmarks evaluated, including the 60 ppb benchmark,
the Administrator judged that a standard with a level of 70 ppb would
incorporate a margin of safety against the adverse O3-
induced effects shown to occur following exposures (while at moderate
or greater exertion) to a somewhat higher concentration. The
Administrator also judged the HREA results for one or more exposures at
or above 60 ppb to provide further support for her somewhat broader
conclusion that ``a standard with a level of 70 ppb would incorporate
an adequate margin of safety against the occurrence of O3
exposures that can result in effects that are adverse to public
health'' (80 FR 65364, October 26, 2015).\41\
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\40\ The Administrator was ``notably less confident in the
adversity to public health of the respiratory effects that have been
observed following exposures to O3 concentrations as low
as 60 ppb,'' based on her consideration of the ATS recommendation on
judging adversity from transient lung function decrements alone, the
uncertainty in the potential for such decrements to increase the
risk of other, more serious respiratory effects in a population (per
ATS recommendations on population-level risk), and the less clear
CASAC advice regarding potential adversity of effects at 60 ppb
compared to higher concentrations studied (80 FR 65363, October 26,
2015).
\41\ While the Administrator was less concerned about single
occurrences of O3 exposures of concern, especially for
the 60 ppb benchmark, she judged that estimates of one or more
exposures of concern can provide further insight into the margin of
safety provided by a revised standard. In this regard, she noted
that ``a standard with a level of 70 ppb is estimated to (1)
virtually eliminate all occurrences of exposures of concern at or
above 80 ppb; (2) protect the vast majority of children in urban
study areas from experiencing any exposures of concern at or above
70 ppb (i.e., >= about 99%, based on mean estimates; Table 1); and
(3) to achieve substantial reductions, compared to the then-current
standard, in the occurrence of one or more exposures of concern at
or above 60 ppb (i.e., about a 50% reduction; Table 1)'' (80 FR
65364, October 26, 2015).
---------------------------------------------------------------------------
In the context of considering a standard with a level of 70 ppb,
the Administrator additionally considered the lung function risk
estimates, epidemiologic evidence and quantitative estimates based on
information from the epidemiologic studies. Although she placed less
weight on these estimates and information in light of associated
uncertainties,\42\ she judged that a standard with a level of 70 ppb
would be expected to result in important reductions in the population-
level risk of endpoints on which these types of information are focused
and provide associated additional public health protection, beyond that
provided by the then-current standard (80 FR 65364, October 26, 2015).
---------------------------------------------------------------------------
\42\ The Administrator noted important uncertainties in using
lung function risk estimates as a basis for considering the
occurrence of adverse effects in the population (also recognized in
the prior review) that limited her reliance on these estimates in
reaching judgments on health protection of a standard level of 70
ppb versus lower levels. Additionally, with regard to epidemiologic
studies, while the Administrator recognized there to be support for
a standard level at least as low as 70 ppb from a single-
epidemiologic study (Mar and Koenig, 2009) that reported health
effect associations in a location that met the then-current standard
over the entire study period but that would have violated a revised
standard with a level of 70 ppb, she found these studies to be of
more limited utility for distinguishing between the appropriateness
of health protection estimated for a standard level of 70 ppb and
that estimated for lower levels (80 FR 65364, October 26, 2015).
---------------------------------------------------------------------------
In summary, given her consideration of the evidence, exposure and
risk information, advice from the CASAC, and public comments, the
Administrator in 2015 judged a revised primary standard of 70 ppb, in
terms of the 3-year average of annual fourth-highest daily maximum 8-
hour average O3 concentrations, to be requisite to protect
public health, including the health of at-risk populations, with an
adequate margin of safety (80 FR 65365, October 26, 2015).
2. Approach for the Current Review
To evaluate whether it is appropriate to consider retaining the
current primary O3 standard, or whether consideration of
revision is appropriate, the EPA has adopted an approach in this review
that builds upon the general approach used in the last review and
reflects the body of evidence and information now available.
Accordingly, the approach in this review takes into consideration the
approach used in the last review, addressing key policy-relevant
questions in light of currently available scientific and technical
information. As summarized above, the Administrator's decisions in the
prior review were based on an integration of O3 health
effects information with judgments on the adversity and public health
significance of key health effects, policy judgments as to when the
standard is requisite to protect public health with an adequate margin
of safety, consideration of CASAC advice, and consideration of public
comments.
Similarly, in this review, we draw on the current evidence and
quantitative assessments of exposure pertaining to
[[Page 49844]]
the public health risk of O3 in ambient air. In considering
the scientific and technical information here, we consider both the
information available at the time of the last review and information
newly available since the last review, including that which has been
critically analyzed and characterized in the current ISA. The
quantitative exposure and risk analyses provide a context for
interpreting the evidence of respiratory effects in people breathing at
elevated rates and the potential public health significance of
exposures associated with air quality conditions that just meet the
current standard. The overarching purpose of these analyses is to
inform the Administrator's conclusions on the public health protection
afforded by the current primary standard, with an important focus on
the potential for exposures and risks beyond those indicated by the
information available at the time the standard was established.
B. Health Effects Information
The information summarized here is based on our scientific
assessment of the health effects evidence available in this review;
this assessment is documented in the ISA and its policy implications
are further discussed in the PA. In this review, as in past reviews,
the health effects evidence evaluated in the ISA for O3 and
related photochemical oxidants is focused on O3 (ISA,
section IS.1.1). Ozone is concluded to be the most prevalent
photochemical oxidant present in the atmosphere and the one for which
there is a very large, well-established evidence base of its health and
welfare effects. Further, ``the primary literature evaluating the
health and ecological effects of photochemical oxidants includes ozone
almost exclusively as an indicator of photochemical oxidants'' (ISA,
section IS.1.1). Thus, the current health effects evidence and the
Agency's review of the evidence, including the evidence newly available
in this review, continues to focus on O3.
More than 1600 studies are newly available and considered in the
ISA, including more than 1000 health studies (ISA, Appendix 10, Figure
10-2). As in the last review, the key evidence comes from the body of
controlled human exposure studies that document respiratory effects in
people exposed for short periods (6.6 to 8 hours) during quasi-
continuous exercise. Policy implications of the currently available
evidence are discussed in the PA (as summarized in section II.D.1
below). The subsections below briefly summarize the following aspects
of the evidence: The nature of O3-related health effects
(section II.B.1), the potential public health implications and
populations at risk (section II.B.2), and exposure concentrations
associated with health effects (section II.B.3).
1. Nature of Effects
The evidence base available in the current review includes decades
of extensive evidence that clearly describes the role of O3
in eliciting an array of respiratory effects and recent evidence
suggests the potential for relationships between O3 exposure
and other effects. As was established in prior reviews, the most
commonly observed effects, and those for which the evidence is
strongest, are transient decrements in pulmonary function and
respiratory symptoms, such as coughing and pain on deep inspiration, as
a result of short-term exposures (ISA, section IS.4.3.1; 2013 ISA, p.
2-26). These effects are demonstrated in the large, long-standing
evidence base of controlled human exposure studies \43\ (1978 AQCD,
1986 AQCD, 1996 AQCD, 2006 AQCD, 2013 ISA, ISA). The lung function
effects are also positively associated with ambient air O3
concentrations in epidemiologic panel studies, available in past
reviews, that describe these associations for outdoor workers and
children attending summer camps in the 1980s and 1990s (2013 ISA,
section 6.2.1.2; ISA, Appendix 3, section 3.1.4.1.3). The epidemiologic
evidence base additionally documents associations of O3
concentrations in ambient air with more severe health outcomes,
including asthma-related emergency department visits and hospital
admissions (2013 ISA, section 6.2.7; ISA, Appendix 3, sections 3.1.5.1
and 3.1.5.2). Extensive experimental animal evidence informs a detailed
understanding of mechanisms underlying the respiratory effects of
short-term exposures (ISA, Appendix 3, section 3.1.11), and studies in
animal models also provide evidence for effects of longer-term
O3 exposure on the developing lung (ISA, Appendix 3, section
3.2.6).
---------------------------------------------------------------------------
\43\ The vast majority of the controlled human exposure studies
(and all of the studies conducted at the lowest exposures) involved
young healthy adults (typically 18-13 years old) as study subjects
(2013 ISA, section 6.2.1.1). There are also some controlled human
exposure studies of one to eight hours duration in older adults and
adults with asthma, and there are still fewer controlled human
exposure studies in healthy children (i.e., individuals aged younger
than 18 years) or children with asthma (See, for example, PA,
Appendix 3A, Table 3A-3).
---------------------------------------------------------------------------
The current evidence continues to support our prior conclusion that
short-term O3 exposure causes respiratory effects.
Specifically, the full body of evidence continues to support the
conclusion of a causal relationship of respiratory effects with short-
term O3 exposures and the conclusion that the relationship
of respiratory effects with longer-term exposures is likely to be
causal (ISA, sections IS.4.3.1 and IS.4.3.2). The current evidence base
for short-term O3 exposure and metabolic effects,\44\ which
was not evaluated as a separate category of effects in the last review
when less evidence was available, is expanded by evidence newly
available in this review. The ISA determines the current evidence
sufficient to conclude that the relationship between short-term
O3 exposure and metabolic effects is likely to be causal
(ISA, section IS.4.3.3). The newly available evidence is primarily from
experimental animal research. For other types of health effects, new
evidence has led to different conclusions from those reached in the
prior review. Specifically, the current evidence, particularly in light
of the additional controlled human exposure studies, is less consistent
than what was previously available and less indicative of
O3-induced cardiovascular effects. This evidence has altered
conclusions from the last review with regard to relationships between
short-term O3 exposures and cardiovascular effects and
mortality, such that the evidence is no longer concluded to indicate
that the relationships are likely to be causal.\45\ Thus, while
conclusions have changed for some effects based on the new evidence,
the conclusions reached in the last review on respiratory effects are
supported by the current evidence, and conclusions are also newly
reached for an additional category of health effects.
---------------------------------------------------------------------------
\44\ The term metabolic effects is used in the ISA to refer
metabolic syndrome (a collection of risk factors including high
blood pressure, elevated triglycerides and low high density
lipoprotein cholesterol), diabetes, metabolic disease mortality, and
indicators of metabolic syndrome that include alterations in glucose
and insulin homeostasis, peripheral inflammation, liver function,
neuroendocrine signaling, and serum lipids (ISA, section IS.4.3.3).
\45\ The currently available evidence for cardiovascular,
reproductive and nervous system effects, as well as mortality, is
``suggestive of, but not sufficient to infer'' a causal relationship
with short- or long-term O3 exposures (ISA, Table IS-1).
The evidence is inadequate to infer the presence or absence of a
causal relationship between long-term O3 exposure and
cancer (ISA, section IS.4.3.6.6).
---------------------------------------------------------------------------
a. Respiratory Effects
As in the last review, the currently available evidence in this
review supports the conclusion of a causal relationship between short-
term O3 exposure and respiratory effects (ISA, section
IS.1.3.1). The strongest evidence for this comes from controlled human
[[Page 49845]]
exposure studies, also available in the last review, demonstrating
O3-related respiratory effects in generally healthy
adults.\46\ Experimental studies in animals also document an array of
respiratory effects resulting from short-term O3 exposure
and provide information related to underlying mechanisms (ISA, Appendix
3, section 3.1). The potential for O3 exposure to elicit
health outcomes more serious than those assessed in the controlled
human exposure studies continues to be indicated by the epidemiologic
evidence of associations of O3 concentrations in ambient air
with increased incidence of hospital admissions and emergency
department visits for an array of health outcomes, including asthma
exacerbation, COPD exacerbation, respiratory infection, and
combinations of respiratory diseases (ISA, Appendix 3, sections 3.1.5
and 3.1.6). The strongest such evidence is for asthma-related outcomes
and specifically asthma-related outcomes for children, indicating an
increased risk for people with asthma and particularly children with
asthma (ISA, Appendix 3, section 3.1.5.7).
---------------------------------------------------------------------------
\46\ The phrases ``healthy adults'' or ``healthy subjects'' are
used to distinguish from subjects with asthma or other respiratory
diseases, for which there are many fewer controlled human exposure
studies. For studies of healthy subjects ``the study design
generally precludes inclusion of subjects with serious health
conditions,'' such as individuals with severe respiratory diseases
(2013 ISA, p. lx).
---------------------------------------------------------------------------
Respiratory responses observed in human subjects exposed to
O3 for periods of 8 hours or less, while intermittently or
quasi-continuously, exercising, include reduced lung function,\47\
respiratory symptoms, increased airway responsiveness, mild
bronchoconstriction (measured as an increase in specific airway
resistance [sRaw]), and pulmonary inflammation, with associated injury
and oxidative stress (ISA, Appendix 3, section 3.1.4; 2013 ISA,
sections 6.2.1 through 6.2.4). The available mechanistic evidence,
discussed in greater detail in the ISA, describes pathways involving
the respiratory and nervous systems by which O3 results in
pain-related respiratory symptoms and reflex inhibition of maximal
inspiration (inhaling a full, deep breath), commonly quantified by
decreases in forced vital capacity (FVC) and total lung capacity. This
reflex inhibition of inspiration combined with mild bronchoconstriction
contributes to the observed decrease in FEV1, the most
common metric used to assess O3-related lung function
effects. The evidence also indicates that the additionally observed
inflammatory response is correlated with mild airway obstruction,
generally measured as an increase in sRaw (ISA, Appendix 3, section
3.1.3). As described in section II.B.3 below, the prevalence and
severity of respiratory effects in controlled human exposure studies,
including symptoms (e.g., pain on deep inspiration, shortness of
breath, and cough), increases with increasing O3
concentration, exposure duration, and ventilation rate of exposed
subjects (ISA, Appendix 3, sections 3.1.4.1 and 3.1.4.2).
---------------------------------------------------------------------------
\47\ In summarizing FEV1 responses from controlled
human exposure studies, an O3-induced change in
FEV1 is typically the difference between the change
observed with O3 exposure (post-exposure FEV1
minus pre-exposure FEV1) and what is generally an
improvement observed with filtered air (FA) exposure (post-exposure
FEV1 minus pre-exposure FEV1). As explained in
the 2013 ISA, ``[n]oting that some healthy individuals experience
small improvements while others have small decrements in
FEV1 following FA exposure, investigators have used the
randomized, crossover design with each subject serving as their own
control (exposure to FA) to discern relatively small effects with
certainty since alternative explanations for these effects are
controlled for by the nature of the experimental design'' (2013 ISA,
pp. 6-4 to 6-5).
---------------------------------------------------------------------------
Within the evidence base from controlled human exposure studies,
the majority of studies involve healthy adult subjects (generally 18 to
35 years), although there are studies involving subjects with asthma,
and a limited number of studies, generally of durations shorter than
four hours, involving adolescents and adults older than 50 years. A
summary of salient observations of O3 effects on lung
function, based on the controlled human exposure study evidence
reviewed in the 1996 and 2006 AQCDs, and recognized in the 2013 ISA,
continues to pertain to this evidence base as it exists today: ``(1)
young healthy adults exposed to >=80 ppb ozone develop significant
reversible, transient decrements in pulmonary function and symptoms of
breathing discomfort if minute ventilation (Ve) or duration of exposure
is increased sufficiently; (2) relative to young adults, children
experience similar spirometric responses [i.e., as measured by
FEV1 and/or FVC] but lower incidence of symptoms from
O3 exposure; (3) relative to young adults, ozone-induced
spirometric responses are decreased in older individuals; (4) there is
a large degree of inter-subject variability in physiologic and
symptomatic responses to O3, but responses tend to be
reproducible within a given individual over a period of several months;
and (5) subjects exposed repeatedly to O3 for several days
experience an attenuation of spirometric and symptomatic responses on
successive exposures, which is lost after about a week without
exposure'' (ISA, Appendix 3, section 3.1.4.1.1, p. 3-11).\48\
---------------------------------------------------------------------------
\48\ A spirometric response refers to a change in the amount of
air breathed out of the body (forced expiratory volumes) and the
associated time to do so (e.g., FEV1).
---------------------------------------------------------------------------
The evidence is most well established with regard to the effects,
reversible with the cessation of exposure, that are associated with
short-term exposures of several hours. For example, the evidence
indicates a rapid recovery from O3-induced lung function
decrements (e.g., reduced FEV1) and respiratory symptoms
(2013 ISA, section 6.2.1.1). However, in some cases, such as after
exposure to higher concentrations such as 300 ppb, the recovery phase
may be slower and involve a longer time period (e.g., at least 24
hours). Repeated daily exposure studies at such higher concentrations
also have found FEV1 response to be enhanced on the second
day of exposure. This enhanced response is absent, however, with
repeated exposure at lower concentrations, perhaps as a result of a
more complete recovery or less damage to pulmonary tissues (2013 ISA,
section pp. 6-13 to 6-14; Folinsbee et al., 1994).
With regard to airway inflammation and the potential for repeated
occurrences to contribute to further effects, 2013 ISA indicates that
O3-induced respiratory tract inflammation ``can have several
potential outcomes: (1) Inflammation induced by a single exposure (or
several exposures over the course of a summer) can resolve entirely;
(2) continued acute inflammation can evolve into a chronic inflammatory
state; (3) continued inflammation can alter the structure and function
of other pulmonary tissue, leading to diseases such as fibrosis; (4)
inflammation can alter the body's host defense response to inhaled
microorganisms, particularly in potentially at-risk populations such as
the very young and old; and (5) inflammation can alter the lung's
response to other agents such as allergens or toxins'' (2013 ISA, p. 6-
76). With regard to O3-induced increases in airway
responsiveness, the controlled human exposure study evidence for
healthy adults generally indicates resolution within 18 to 24 hours
after exposure (ISA, Appendix 3, section 3.1.4.3.1).
The extensive evidence base for O3 health effects,
compiled over several decades, continues to indicate respiratory
responses to short exposures as the most sensitive effects of
O3. Such
[[Page 49846]]
effects are well documented in controlled human exposure studies, most
of which involve healthy adult study subjects. These studies have
documented an array of respiratory effects, including reduced lung
function, respiratory symptoms, increased airway responsiveness, and
inflammation, in study subjects following 1- to 8-hour exposures,
primarily while exercising. Such effects are of increased significance
to people with asthma given aspects of the disease that contribute to a
baseline status that includes chronic airway inflammation and greater
airway responsiveness than people without asthma (ISA, section 3.1.5).
For example, due to the latter characteristic, O3 exposure
of a magnitude that increases airway responsiveness may put such people
at potential increased risk for prolonged bronchoconstriction in
response to asthma triggers (ISA, p. IS-22; 2013 ISA, section 6.2.9;
2006 AQCD, section 8.4.2). Further, children are the age group most
likely to be outdoors at activity levels corresponding to those that
have been associated with respiratory effects in the human exposure
studies (as recognized below in sections II.B.2 and II.C). The
increased significance of effects in people with asthma and risk of
increased exposure for children is illustrated by the epidemiologic
findings of positive associations between O3 exposure and
asthma-related ED visits and hospital admissions for children with
asthma. Thus, the evidence indicates O3 exposure to increase
the risk of asthma exacerbation, and associated outcomes, in children
with asthma.
With regard to an increased susceptibility to infectious diseases,
the experimental animal evidence continues to indicate, as described in
the 2013 ISA and past AQCDs, the potential for O3 exposures
to increase susceptibility to infectious diseases through effects on
defense mechanisms of the respiratory tract (ISA, section 3.1.7.3; 2013
ISA, section 6.2.5). The evidence base regarding respiratory infections
and associated effects has been augmented in this review by a number of
epidemiologic studies reporting positive associations between short-
term O3 concentrations and emergency department visits for a
variety of respiratory infection endpoints (ISA, Appendix 3, section
3.1.7).
Although the long-term exposure conditions that may contribute to
further respiratory effects are less well understood, the conclusion
based on the current evidence base remains that the relationship for
such exposure conditions with respiratory effects is likely to be
causal (ISA, section IS.4.3.2). Most notably, experimental studies,
including with nonhuman infant primates, have provided evidence
relating O3 exposure to asthma-like effects, and
epidemiologic cohort studies have reported associations of
O3 concentrations in ambient air with asthma development in
children (ISA, Appendix 3, sections 3.2.4.1.3 and 3.2.6). The
biological plausibility of such a role for O3 has been
indicated by animal toxicological evidence on biological mechanisms
(ISA, Appendix 3, sections 3.2.3 and 3.2.4.1.2). Specifically, the
animal evidence, including the nonhuman primate studies of early life
O3 exposure, indicates that such exposures can cause
``structural and functional changes that could potentially contribute
to airway obstruction and increased airway responsiveness,'' which are
hallmarks of asthma (ISA, Appendix 3, section 3.2.6, p. 3-113).
Overall, the respiratory effects evidence newly available in this
review is generally consistent with the evidence base in the last
review (ISA, Appendix 3, section 3.1.4). A few recent studies provide
insights in previously unexamined areas, both with regard to human
study groups and animal models for different effects, while other
studies confirm and provide depth to prior findings with updated
protocols and techniques (ISA, Appendix 3, sections 3.1.11 and 3.2.6).
Thus, our current understanding of the respiratory effects of
O3 is similar to that in the last review.
One aspect of the evidence that has been augmented concerns
pulmonary function in adults older than 50 years of age. Previously
available evidence in this age group indicated smaller O3-
related decrements in middle-aged adults (35 to 60 years) than in
adults 35 years of age and younger (2006 AQCD, p. 6-23; 2013 ISA, p. 6-
22; ISA, Appendix 3, section 3.1.4.1.1.2). A recent multicenter study
of 55- to 70-year old subjects (average age of 60 years), conducted for
a 3-hour duration involving alternating 15-minute rest and exercise
periods and a 120 ppb exposure concentration, reported a statistically
significant O3 FEV1 response (ISA, Appendix 3,
section 3.1.4.1.1.2; Arjomandi et al., 2018). While there is not a
study in younger adults of precisely comparable design, the mean
response for the 55- to 70-year olds, 1.2% O3-related
FEV1 decrement, is lower than results for somewhat
comparable exposures in adults aged 18 to 35 years, suggesting somewhat
reduced responses to O3 exposure in this older age group
(ISA, Appendix 3, section 3.1.4.1.1.2; Arjomandi et al., 2018; Adams,
2000; Adams, 2006b).\49\ Such a reduced response in middle-aged and
older adults compared to young adults is consistent with conclusions in
previous reviews (2013 ISA, section 6.2.1.1; 2006 AQCD, section 6.4).
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\49\ For the same exposure concentration of 120 ppb, Adams
(2006b) observed an average 3.2%, statistically significant,
O3-related FEV1 decrement in young adults
(average age 23 years) at the end of the third hour of an 8-hour
protocol that alternated 30 minutes of exercise and rest, with the
equivalent ventilation rate (EVR) averaging 20 L/min-m\2\ during the
exercise periods (versus 15 to 17 L/min-m\2\ in.Arjomandi et
al.[2018]). For the same concentration with a lower EVR during
exercise (17 L/min-m\2\), although with more exercise, Adams (2000)
observed a 4%, statistically significant, O3-related
FEV1 decrement in young adults (average age 22 years)
after the third hour of a 6.6-hour protocol (alternating 50 minutes
exercise and 10 minutes rest).
---------------------------------------------------------------------------
The strongest evidence of O3-related health effects, as
was the case in the last review, continues to be that for respiratory
effects of O3 (ISA, section ES.4.1). Among the newly
available studies, there are several controlled human exposure studies
that investigated lung function effects of higher exposure
concentrations (e.g., 100 to 300 ppb) in healthy individuals younger
than 35 years old, with findings generally consistent with previous
studies (ISA, Appendix 3, section 3.1.4.1.1.2, p. 3-17). No studies are
newly available in this review of 6.6-hour controlled human exposures
(with exercise) to O3 concentrations below those previously
studied.\50\ The newly available animal toxicological studies augment
the previously available information concerning mechanisms underlying
the effects documented in experimental studies. Newly available
epidemiologic studies of hospital admissions and emergency department
visits for a variety of respiratory outcomes supplement the previously
available evidence with additional findings of consistent associations
with O3 concentrations across a number of study locations
(ISA, Appendix 3, sections 3.1.4.1.3, 3.1.5, 3.1.6.1.1, 3.1.7.1 and
3.1.8). These studies include a number that report positive
associations for asthma-related outcomes, as well as a few for COPD-
related outcomes. Together these studies in the current epidemiologic
evidence base continue to indicate the potential for O3
exposures to contribute to such serious health outcomes, particularly
for people with asthma.
---------------------------------------------------------------------------
\50\ The recent 3-hour study of 55- to 70-year old subjects
included a target exposure of 70 ppb, as well as 120 ppb, with only
the latter eliciting a statistically significant FEV1
decrement in this age group of subjects (ISA, Appendix 3, section
3.1.4.1.1.2).
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[[Page 49847]]
b. Other Effects
As was the case for the evidence available in the last review, the
currently available evidence for health effects other than those of
O3 exposures on the respiratory system is more uncertain
than that for respiratory effects. For some of these other categories
of effects, the evidence now available has contributed to changes in
conclusions reached in the last review. For example, the current
evidence for cardiovascular effects and mortality, expanded from that
in the last review, is no longer considered sufficient to conclude that
the relationships of short-term exposure with these effects are likely
to be causal (ISA, sections IS.4.3.4 and IS.4.3.5). These changes stem
from newly available evidence in combination with the uncertainties
recognized for the evidence available in the last review. Additionally,
newly available evidence has also led to conclusions for another
category, metabolic effects, for which formal causal determinations
were previously not articulated.
The ISA finds the evidence for metabolic effects sufficient to
conclude that the relationship with short-term O3 exposures
is likely to be causal (ISA, section IS.4.3.3). The evidence of
metabolic effects of O3 comes primarily from experimental
animal study findings that short-term O3 exposure can impair
glucose tolerance, increase triglyceride levels and elicit fasting
hyperglycemia, and increase hepatic gluconeogenesis (ISA, Appendix 5,
section 5.1.8 and Table 5-3). The exposure conditions from these
studies generally involve much higher O3 concentrations than
those commonly occurring in areas of the U.S. where the current
standard is met. For example, the animal studies include 4-hour
concentrations of 400 to 800 ppb (ISA, Appendix 5, Tables 5-8 and 5-
10). The concentration in the available controlled human exposure study
is similarly high, at 300 ppb; this study reported increases in two
biochemicals suggestive of some liver biomarkers and no change in a
number of other biochemicals associated with metabolic effects (ISA,
sections 5.1.3, 5.1.5 and 5.1.8, Table 5-3). A limited number of
epidemiologic studies is also available (ISA, section IS.4.3.3;
Appendix 5, sections 5.1.3 and 5.1.8).
The ISA additionally concludes that the evidence is suggestive of,
but not sufficient to infer, a causal relationship between long-term
O3 exposures and metabolic effects (ISA, section
IS.4.3.6.2). As with metabolic effects and short-term O3,
the primary evidence is from experimental animal studies in which the
exposure concentrations are appreciably higher than those commonly
occurring in the U.S. For example, the animal studies include exposures
over several weeks to concentrations of 250 ppb and higher (ISA,
Appendix 5, section 5.2.3.1.1). The somewhat limited epidemiologic
evidence related to long-term O3 concentrations and
metabolic effects includes studies reporting increased odds of being
overweight or obese or having metabolic syndrome and increased hazard
ratios for diabetes incidence with increased O3
concentrations (ISA, Appendix 5, sections 5.2.3.4.1, 5.2.5 and 5.2.9,
Tables 5-12 and 5-15).
With regard to cardiovascular effects and total (nonaccidental)
mortality and short-term O3 exposures, the conclusions
regarding the potential for a causal relationship have changed from
what they were in the last review after integrating the previously
available evidence with newly available evidence. The relationships are
now characterized as suggestive of, but not sufficient to infer, a
causal relationship (ISA, Appendix 4, section 4.1.17; Appendix 6,
section 6.1.8). This reflects several aspects of the current evidence
base: (1) A now-larger body of controlled human exposure studies
providing evidence that is not consistent with a cardiovascular effect
in response to short-term O3 exposure; (2) a paucity of
epidemiologic evidence indicating more severe cardiovascular morbidity
endpoints (e.g., emergency department visits and hospital visits for
cardiovascular endpoints including myocardial infarctions, heart
failure or stroke) that could connect the evidence for impaired
vascular and cardiac function from animal toxicological studies with
the evidence from epidemiologic studies of cardiovascular mortality;
and (3) the remaining uncertainties and limitations recognized in the
2013 ISA (e.g., lack of control for potential confounding by
copollutants in epidemiologic studies) that still remain. Although
there exists consistent or generally consistent evidence for a limited
number of O3-induced cardiovascular endpoints in animal
toxicological studies and cardiovascular mortality in epidemiologic
studies, there is a general lack of coherence between these results and
findings in controlled human exposure and epidemiologic studies of
cardiovascular health outcomes (ISA, section IS.1.3.1, Appendix 6,
section 6.1.8). Related to the updated evidence for cardiovascular
effects, the evidence for short-term O3 concentrations and
mortality is also updated (ISA, section 4.3.5 and Appendix 6, section
6.1.8). While epidemiologic studies show positive associations between
short-term O3 concentrations and total (nonaccidental) and
cardiovascular mortality (and there are some studies reporting
associations that remain after controlling for PM10 and
NO2), the full evidence base does not describe a continuum
of effects that could lead to cardiovascular mortality.\51\ The
category of total mortality includes all contributions to mortality,
including both respiratory and cardiovascular mortality, as well as
other causes of death, such as cancer or other chronic diseases. The
evidence base supporting a continuum of effects of short-term
O3 concentrations that could potentially lead to respiratory
mortality is more consistent and coherent as compared to that for
cardiovascular mortality (ISA, sections 3.1.11 and 4.1.17; 2013 ISA,
section 6.2.8). However, because cardiovascular mortality is the
largest contributor to total mortality, the relatively limited
biological plausibility and coherence within and across disciplines for
cardiovascular effects (including mortality) is the dominant factor
which contributes to a revised causality determination for total
mortality (ISA, section IS.4.3.5). The ISA concludes that the currently
available evidence for cardiovascular effects and total mortality is
suggestive of, but not sufficient to infer, a causal relationship with
short-term (as well as long-term) O3 exposures (ISA,
sections IS.4.3.4 and IS.4.3.5).
---------------------------------------------------------------------------
\51\ Due to findings from controlled human exposure studies
examining clinical endpoints (e.g., blood pressure) that do not
indicate an O3 effect and from epidemiologic studies
examining cardiovascular-related hospital admissions and ED visits
that do not find positive associations, a continuum of effects that
could lead to cardiovascular mortality is not apparent (ISA,
Appendices 4 and 6).
---------------------------------------------------------------------------
For other health effect categories, conclusions in this review are
largely unchanged from those in the last review. The available evidence
for reproductive and developmental effects, as well as for effects on
the nervous system, is suggestive of, but not sufficient to infer, a
causal relationship, as was the case in the last review (ISA, section
IS.4.3.6.5 and Table IS-1). Additionally, the evidence is inadequate to
determine if a causal relationship exists between O3
exposure and cancer (ISA, section IS.4.3.6.6 and Table IS-1).
2. Public Health Implications and At-Risk Populations
The public health implications of the evidence regarding
O3-related health
[[Page 49848]]
effects, as for other effects, are dependent on the type and severity
of the effects, as well as the size of the population affected. Such
factors are discussed here in the context of our consideration of the
health effects evidence related to O3 in ambient air.
Additionally, we summarize the currently available information related
to judgments or interpretative statements developed by public health
experts, particularly experts in respiratory health. This section also
summarizes the current information on population groups at increased
risk of the effects of O3 in ambient air.
With regard to O3 in ambient air, the potential public
health impacts relate most importantly to the role of O3 in
eliciting respiratory effects, the category of effects that the ISA
concludes to be causally related to O3 exposure (short-
term). Controlled human exposure studies have documented reduced lung
function, respiratory symptoms, increased airway responsiveness, and
inflammation, among other effects, in healthy adults exposed while at
elevated ventilation, such as while exercising. Ozone effects in
individuals with compromised respiratory function, such as individuals
with asthma, are plausibly related to emergency department visits and
hospital admissions for asthma which have been associated with ambient
air concentrations of O3 in epidemiologic studies (as
summarized in section II.B.1 above; 2013 ISA, section 6.2.7; ISA,
Appendix 3, sections 3.1.5.1 and 3.1.5.2).
The clinical significance of individual responses to O3
exposure depends on the health status of the individual, the magnitude
of the changes in pulmonary function, the severity of respiratory
symptoms, and the duration of the response. With regard to pulmonary
function, the greater impact of larger decrements on affected
individuals can be described. For example, moderate effects on
pulmonary function, such as transient FEV1 decrements
smaller than 20% or transient respiratory symptoms, such as cough or
discomfort on exercise or deep breath, would not be expected to
interfere with normal activity for most healthy individuals, while
larger effects on pulmonary function (e.g., FEV1 decrements
of 20% or larger lasting longer than 24 hours) and/or more severe
respiratory symptoms are more likely to interfere with normal activity
for more of such individuals (e.g., 2014 PA, p. 3-53; 2006 AQCD, Table
8-2).
In addition to the difference in severity or magnitude of specific
effects in healthy people, the same reduction in FEV1 or
increase in inflammation or airway responsiveness in a healthy group
and a group with asthma may increase the risk of a more severe effect
in the group with asthma. For example, the same increase in
inflammation or airway responsiveness in individuals with asthma could
predispose them to an asthma exacerbation event triggered by an
allergen to which they may be sensitized (e.g., 2013 ISA, sections
6.2.3 and 6.2.6). Duration and frequency of documented effects is also
reasonably expected to influence potential adversity and interference
with normal activity. In summary, consideration of differences in
magnitude or severity, and also the relative transience or persistence
of such FEV1 changes and respiratory symptoms, as well as
pre-existing sensitivity to effects on the respiratory system, and
other factors, are important to characterizing implications for public
health effects of an air pollutant such as O3 (ATS, 2000;
Thurston et al., 2017).
Decisions made in past reviews of the O3 primary
standard and associated judgments regarding adversity or health
significance of measurable physiological responses to air pollutants
have been informed by guidance, criteria or interpretative statements
developed within the public health community, including the ATS, an
organization of respiratory disease specialists, as well as the CASAC.
The ATS released its initial statement (titled Guidelines as to What
Constitutes an Adverse Respiratory Health Effect, with Special
Reference to Epidemiologic Studies of Air Pollution) in 1985 and
updated it in 2000 (ATS, 1985; ATS, 2000). The ATS described its 2000
statement, considered in the last review of the O3 standard,
as being intended to ``provide guidance to policy makers and others who
interpret the scientific evidence on the health effects of air
pollution for the purposes of risk management'' (ATS, 2000). The ATS
described the statement as not offering ``strict rules or numerical
criteria,'' but rather proposing ``principles to be used in weighing
the evidence and setting boundaries,'' and stated that ``the placement
of dividing lines should be a societal judgment'' (ATS, 2000).
Similarly, the most recent policy statement by the ATS, which once
again broadens its discussion of effects, responses and biomarkers to
reflect the expansion of scientific research in these areas, reiterates
that concept, conveying that it does not offer ``strict rules or
numerical criteria, but rather proposes considerations to be weighed in
setting boundaries between adverse and nonadverse health effects,''
providing a general framework for interpreting evidence that proposes a
``set of considerations that can be applied in forming judgments'' for
this context (Thurston et al., 2017).
With regard to pulmonary function decrements, the earlier ATS
statement concluded that ``small transient changes in forced expiratory
volume in 1 s[econd] (FEV1) alone were not necessarily
adverse in healthy individuals, but should be considered adverse when
accompanied by symptoms'' (ATS, 2000). The more recent ATS statement
continues to support this conclusion and also gives weight to findings
of such lung function changes in the absence of respiratory symptoms in
individuals with pre-existing compromised function, such as that
resulting from asthma (Thurston et al., 2017). More specifically, the
recent ATS statement expresses the view that the occurrence of ``small
lung function changes'' in individuals with pre-existing compromised
function, such as asthma, ``should be considered adverse . . . even
without accompanying respiratory symptoms'' (Thurston et al., 2017). In
keeping with the intent of these statements to avoid specific criteria,
neither statement provides more specific descriptions of such
responses, such as with regard to magnitude, duration or frequency, for
consideration of such conclusions. The earlier ATS statement, in
addition to emphasizing clinically relevant effects, also emphasized
both the need to consider changes in ``the risk profile of the exposed
population,'' and effects on the portion of the population that may
have a diminished reserve that puts its members at potentially
increased risk if affected by another agent (ATS, 2000). These
concepts, including the consideration of the magnitude of effects
occurring in just a subset of study subjects, continue to be recognized
as important in the more recent ATS statement (Thurston et al., 2017)
and continue to be relevant to the evidence base for O3.
The information newly available in this review has not altered our
understanding of human populations at particular risk of health effects
from O3 exposures (ISA, section IS.4.4). For example, as
recognized in prior reviews, people with asthma are the key population
at risk of O3-related effects. The respiratory effects
evidence, extending decades into the past and augmented by new studies
in this review, supports this conclusion (ISA, sections IS.4.3.1). For
example, numerous epidemiological studies document associations with
O3 with asthma exacerbation. Such studies
[[Page 49849]]
indicate the associations to be strongest for populations of children
which is consistent with their generally greater time outdoors while at
elevated exertion. Together, these considerations indicate people with
asthma, including particularly children with asthma, to be at
relatively greater risk of O3-related effects than other
members of the general population (ISA, section IS.4.4.2 and Appendix
3).\52\
---------------------------------------------------------------------------
\52\ Populations or lifestages can be at increased risk of an
air pollutant-related health effect due to one or more of a number
of factors. These factors can be intrinsic, such as physiological
factors that may influence the internal dose or toxicity of a
pollutant, or extrinsic, such as sociodemographic, or behavioral
factors.
---------------------------------------------------------------------------
With respect to people with asthma, the limited evidence from
controlled human exposure studies (which are primarily in adult
subjects) indicates similar magnitude of FEV1 decrements as
in people without asthma (ISA, Appendix 3, section 3.1.5.4.1). Across
other respiratory effects of O3 (e.g., increased respiratory
symptoms, increased airway responsiveness and increased lung
inflammation), the evidence has also found the observed responses to
generally not differ due to the presence of asthma, although the
evidence base is more limited with regard to study subjects with asthma
(ISA, Appendix 3, section 3.1.5.7). However, the features of asthma
(e.g., increased airway responsiveness) contribute to a risk of asthma-
related responses, such as asthma exacerbation in response to asthma
triggers, which may increase the risk of more severe health outcomes
(ISA, section 3.1.5). For example, a particularly strong and consistent
component of the epidemiologic evidence is the appreciable number of
epidemiologic studies that demonstrate associations between ambient
O3 concentrations and hospital admissions and emergency
department visits for asthma (ISA, section IS.4.4.3.1). \53\ We
additionally recognize that in these studies, the strongest
associations (e.g., highest effect estimates) or associations more
likely to be statistically significant are those for childhood age
groups, which are recognized in section II.C.1 as age groups most
likely to spend time outdoors during afternoon periods (when
O3 may be highest) and at activity levels corresponding to
those that have been associated with respiratory effects in the human
exposure studies (ISA, Appendix 3, sections 3.1.4.1 and 3.1.4.2).\54\
The epidemiologic studies of hospital admissions and emergency
department visits are augmented by a large body of individual-level
epidemiologic panel studies that demonstrated associations of short-
term ozone concentrations with respiratory symptoms in children with
asthma. Additional support comes from epidemiologic studies that
observed ozone-associated increases in indicators of airway
inflammation and oxidative stress in children with asthma (ISA, section
IS.4.3.1). Together, this evidence continues to indicate the increased
risk of population groups with asthma (ISA, Appendix 3, section
3.1.5.7).
---------------------------------------------------------------------------
\53\ In addition to asthma exacerbation, the epidemiologic
evidence also includes findings of positive associations of
increased O3 concentrations with hospital admissions or
emergency department visits for COPD exacerbation and other
respiratory diseases (ISA, Appendix 3, sections 3.1.6.1.3 and
3.1.8).
\54\ There is limited data on activity patterns by health
status. An analysis in the 2014 HREA indicated that asthma status
had little to no impact on the percent of people participating in
outdoor activities during afternoon hours, the amount of time spent,
and whether they performed activities at elevated exertion levels
(2014 HREA, section 5.4.1.5). Based on an updated evaluation of
recent activity pattern data we found children, for days having some
time spent outdoors spend, on average, approximately 2\1/4\ hours of
afternoon time outdoors, 80% of which is at a moderate or greater
exertion level, regardless of their asthma status (see Appendix 3D,
section 3D.2.5.3). Adults, for days having some time spent outdoors,
also spend approximately 2\1/4\ hours of afternoon time outdoors
regardless of their asthma status but the percent of afternoon time
at moderate or greater exertion levels for adults (about 55%) is
lower than that observed for children.
---------------------------------------------------------------------------
Children, and also outdoor adult workers, are at increased risk
largely due to their generally greater time spent outdoors while at
elevated exertion rates (including in the summer when O3
levels may be higher). This behavior makes them more likely to be
exposed to O3 in ambient air, under conditions contributing
to increased dose due to greater air volumes taken into the lungs (2013
ISA, section 5.2.2.7). In light of the evidence summarized in the prior
paragraph, children and outdoor workers with asthma may be at increased
risk of more severe outcomes, such as asthma exacerbation. Further,
there is experimental evidence from early life exposures of nonhuman
primates that indicates potential for effects in childhood when human
respiratory systems are under development (ISA, section IS.4.4.4.1).
Overall, the evidence available in the current review, while not
increasing our knowledge about susceptibility of these population
groups, is consistent with that in the last review.
Older adults have also been identified as being at increased risk.
That identification, based on the assessment in the 2013 ISA, was based
largely on studies of short-term O3 exposure and mortality,
which are part of the larger evidence base that is now concluded to be
suggestive, but not sufficient to infer a causal relationship (ISA,
sections IS.4.3.5 and IS.4.4.4.2, Appendix 4, section 4.1.16.1 and
4.1.17).\55\ Other evidence available in the current review adds little
to the evidence available at the time of the last review for older
adults (ISA, sections IS.4.4.2 and IS.4.4.4.2).
---------------------------------------------------------------------------
\55\ As noted in the ISA, ``[t]he majority of evidence for older
adults being at increased risk of health effects related to ozone
exposure comes from studies of short-term ozone exposure and
mortality evaluated in the 2013 Ozone ISA'' (ISA, p. IS-52).
---------------------------------------------------------------------------
The ISA in the last review concluded that the information available
at the time for low socioeconomic status (SES) as a factor associated
with the risk of O3-related health effects, provided
suggestive evidence of potentially increased risk (2013 ISA, section
8.3.3 and p. 8-37). The 2013 ISA concluded that ``[o]verall, evidence
is suggestive of SES as a factor affecting risk of O3-
related health outcomes based on collective evidence from epidemiologic
studies of respiratory hospital admissions but inconsistency among
epidemiologic studies of mortality and reproductive outcomes,''
additionally stating that ``[f]urther studies are needed to confirm
this relationship, especially in populations within the U.S.'' (2013
ISA, p. 8-28). The evidence available in the current review adds little
to the evidence available at the time of the last review in this area
(ISA, section IS.4.4.2 and Table IS-10). The ISA in the last review
additionally identified a role for dietary anti-oxidants such as
vitamins C and E in influencing risk of O3-related effects,
such as inflammation, as well as a role for genetic factors to also
confer either an increased or decreased risk (2013 ISA, sections 8.1
and 8.4.1). No newly available evidence has been evaluated that would
inform or change these prior conclusions (ISA, section IS.4.4 and Table
IS-10).
The magnitude and characterization of a public health impact is
dependent upon the size and characteristics of the populations
affected, as well as the type or severity of the effects. As summarized
above, a key population most at risk of health effects associated with
O3 in ambient air is people with asthma. The National Center
for Health Statistics data for 2017 indicate that approximately 7.9% of
the U.S. populations has asthma (CDC, 2019; PA, Table 3-1). This is one
of the principal populations that the primary O3 NAAQS is
designed to protect (80 FR 65294, October 26, 2015).
The age group for which the prevalence documented by these data is
greatest is children aged five to 19 years old, with 9.7% of children
aged five to
[[Page 49850]]
14 and 9.4% of children aged 15 to 19 years old having asthma (CDC,
2019, Tables 3-1 and 4-1; PA, Table 3-1). In 2012 (the most recent year
for which such an evaluation is available), asthma was the leading
chronic illness affecting children (Bloom et al., 2013). The prevalence
is greater for boys than girls (for those less than 18 years of age).
Among populations of different races or ethnicities, black non-Hispanic
children aged five to 14 have the highest prevalence, at 16.1%. Asthma
prevalence is also increased among populations in poverty. For example,
11.7% of people living in households below the poverty level have
asthma compared to 7.3%, on average, of those living above it (CDC,
2019, Tables 3-1 and 4-1; PA, Table 3-1). Population groups with
relatively greater asthma prevalence might be expected to have a
relatively greater potential for O3-related health
impacts.\56\
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\56\ As summarized in section II.A.1 above, the current standard
was set to protect at-risk populations, which include people with
asthma. Accordingly, populations with asthma living in areas not
meeting the standard would be expected to be at increased risk of
effects than others in those areas.
---------------------------------------------------------------------------
Children under the age of 18 account for 16.7% of the total U.S.
population, with 6.2% of the total population being children under 5
years of age (U.S. Census Bureau, 2019). Based on a prior analysis of
data from the Consolidated Human Activity Database (CHAD) \57\ in the
2014 HREA, children ages 4-18 years old, for days having some time
spent outdoors, were found to more frequently spend time outdoors
compared to other age groups (e.g., adults aged 19-34) spending more
than 2 hours outdoors, particularly during the afternoon and early
evening (e.g., 12:00 p.m. through 8:00 p.m.) (2014 HREA, section 5G-
1.2). These results were confirmed by additional analyses of CHAD data
reported in the ISA, noting greater participation in afternoon outdoor
events for children ages 6-19 years old during the warm season months
compared to other times of the day (ISA, Appendix 2, section 2.4.1,
Table 2-1). The 2014 HREA also found that children ages 4-18 years old
spent 79% of their outdoor time at moderate or greater exertion (2014
HREA, section 5G-1.4). Further analyses performed for this review using
the most recent version of CHAD generated similar results (PA, Appendix
3D, section 3D.2.5.3 and Figure 3D-9). Each of these analyses indicate
children participate more frequently and spend more afternoon time
outdoors than all other age groups while at elevated exertion, and
consistently do so when considering the most important influential
factors such as day-of-week and outdoor temperature. Given that
afternoon time outdoors and elevated exertion were determined most
important in understanding the fraction of the population that might
experience O3 exposures of concern (e.g., 2014 HREA, section
5.4.2), they may be at greater risk of effects due to increased
exposure to O3 in ambient air.
---------------------------------------------------------------------------
\57\ The CHAD provides time series data on human activities
through a database system of collected human diaries, or daily time
location activity logs.
---------------------------------------------------------------------------
About one third of workers were required to perform outdoor work in
2018 (Bureau of Labor Statistics, 2019). Jobs in construction and
extraction occupations and protective service occupations required more
than 90% of workers to spend at least part of their workday outdoors
(Bureau of Labor Statistics, 2017). Other employment sectors, including
installation, maintenance and repair occupations and building and
grounds cleaning and maintenance operations, also had a high percentage
of employees who spent part of their workday outdoors (Bureau of Labor
Statistics, 2017). These occupations often include physically demanding
tasks and involve increased ventilation rates which when combined with
exposure to O3, may increase the risk of health effects.
3. Exposure Concentrations Associated With Effects
As at the time of the last review, the EPA's conclusions regarding
exposure concentrations of O3 associated with respiratory
effects reflect the extensive longstanding evidence base of controlled
human exposure studies of short-term O3 exposures of people
with and without asthma (ISA, Appendix 3). These studies have
documented an array of respiratory effects, including reduced lung
function, respiratory symptoms, increased airway responsiveness, and
inflammation, in study subjects following 1- to 8-hour exposures,
primarily while exercising. The severity of observed responses, the
percentage of individuals responding, and strength of statistical
significance at the study group level have been found to increase with
increasing exposure (ISA; 2013 ISA; 2006 AQCD). Factors influencing
exposure include activity level or ventilation rate, exposure
concentration, and exposure duration (ISA; 2013 ISA; 2006 AQCD). For
example, evidence from studies with similar duration and exercise
aspects (6.6-hour duration with six 50-minute exercise periods)
demonstrates an exposure-response relationship for O3-
induced reduction in lung function (ISA, Appendix 3, Figure 3-3; PA,
Figure 3-2).58 59
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\58\ For a subset of the studies included in PA, Figure 3-2
(those with face mask rather than chamber exposures), there is no
O3 exposure during some of the 6.6-hour experiment (e.g.,
during the lunch break). Thus, while the exposure concentration
during the exercise periods is the same for the two types of
studies, the time-weighted average (TWA) concentration across the
full 6.6-hour period differs slightly. For example, in the facemask
studies of 120 ppb, the TWA across the full 6.6-hour experiment is
109 ppb (PA, Appendix 3A, Table 3A-2).
\59\ The relationship also exists for size of FEV1
decrement with alternative exposure or dose metrics, including total
inhaled O3 and intake volume averaged concentration.
---------------------------------------------------------------------------
The current evidence, including that newly available in this
review, does not alter the scientific conclusions reached in the last
review on exposure duration and concentrations associated with
O3-related health effects. These conclusions were largely
based on the body of evidence from the controlled human exposure
studies. A limited number of controlled human exposure studies are
newly available in the current review, with none involving lower
exposure concentrations than those previously studied or finding
effects not previously reported (ISA, Appendix 3, section 3.1.4).\60\
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\60\ No 6.6-hour studies are newly available in this review
(ISA, Appendix 3, section 3.1.4.1.1). Rather, the newly available
controlled human exposure studies are generally for exposures of
three hours or less, and in nearly all instances involve exposure
(while at elevated exertion) to concentrations above 100 ppb (ISA,
Appendix 3, section 3.1.4).
---------------------------------------------------------------------------
The extensive evidence base for O3 health effects,
compiled over several decades, continues to indicate respiratory
responses to short-term exposures as the most sensitive effects of
O3. As summarized in section II.B.1 above, an array of
respiratory effects is well documented in controlled human exposure
studies of subjects exposed for 1 to 8 hours, primarily while
exercising. The risk of more severe health outcomes associated with
such effects is increased in people with asthma as illustrated by the
epidemiologic findings of positive associations between O3
exposure and asthma-related ED visits and hospital admissions.
The magnitude of respiratory response (e.g., size of lung function
reductions and magnitude of symptom scores) documented in the
controlled human exposure studies is influenced by ventilation rate,
exposure duration, and exposure concentration. When performing physical
activities requiring elevated exertion, ventilation rate is increased,
leading to greater potential for health effects due to an increased
internal dose (2013 ISA, section 6.2.1.1, pp. 6-5 to 6-11).
Accordingly, the exposure concentrations eliciting a
[[Page 49851]]
given level of response after a given exposure duration is lower for
subjects exposed while at elevated ventilation, such as while
exercising (2013 ISA, pp. 6-5 to 6-6). For example, in studies of
healthy young adults exposed while at rest for 2 hours, 500 ppb is the
lowest concentration eliciting a statistically significant
O3-induced group mean lung function decrement, while a 1- to
2-hour exposure to 120 ppb produces a statistically significant
response in lung function when the ventilation rate of the group of
study subjects is sufficiently increased with exercise (2013 ISA, pp.
6-5 to 6-6).
The exposure conditions (e.g., duration and exercise) given primary
focus in the past several reviews are those of the 6.6-hour study
design, which involves six 50-minute exercise periods during which
subjects maintain a moderate level of exertion to achieve a ventilation
rate of approximately 20 L/min per m\2\ body surface area while
exercising. The 6.6 hours of exposure in these studies has generally
occurred in an enclosed chamber and the study design includes three
hours in each of which is a 50-minute exercise period and a 10-minute
rest period, followed by a 35-minute lunch (rest) period, which is
followed by three more hours of exercise and rest, as before lunch.\61\
Most of these studies performed to date involve exposure maintained at
a constant (unchanging) concentration for the full duration, although a
subset of studies have concentrations that vary (generally in a
stepwise manner) across the exposure period and are selected so as to
achieve a specific target concentration as the exposure average.\62\ No
studies of the 6.6-hour design are newly available in this review. The
previously available studies of this design document statistically
significant O3-induced reduction in lung function
(FEV1) and increased pulmonary inflammation in young healthy
adults exposed to O3 concentrations as low as 60 ppb.
Statistically significant group mean changes in FEV1, also
often accompanied by statistically significant increases in respiratory
symptoms, become more consistent across such studies of exposures to
higher O3 concentrations, such as 70 ppb and 80 ppb (Table
1; PA, Appendix 3A, Table 3A-1). The lowest exposures concentration for
which these studies document a statistically significant increase in
respiratory symptoms is somewhat above 70 ppb (Schelegle et al.,
2009).\63\
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\61\ A few studies have involved exposures by facemask rather
than freely breathing in a chamber. To date, there is little
research differentiating between exposures conducted with a facemask
and in a chamber since the pulmonary responses of interest do not
seem to be influenced by the exposure mechanism. However, similar
responses have been seen in studies using both exposure methods at
higher O3 concentrations (Adams, 2002; Adams, 2003). In
the facemask designs, there is a short period of zero O3
exposure, such that the total period of exposure is closer to 6
hours than 6.6 (Adams, 2000; Adams, 2002; Adams, 2003).
\62\ In these studies, the exposure concentration changes for
each of the six hours in which there is exercise and the
concentration during the 35-minute lunch is the same as in the prior
(third) hour with exercise. For example, in the study by Adams,
2006a), the protocol for the 6.6-hour period is as follows: 60
minutes at 40 ppb, 60 minutes at 70 ppb, 95 minutes at 90 ppb, 60
minutes at 70 ppb, 60 minutes at 50 ppb and 60 minutes at 40 ppb.
\63\ Measurements are reported in this study for each of the six
50-minute exercise periods, for which the mean is 72 ppb (Schelegle
et al., 2009). Based on these data, the time-weighted average
concentration across the full 6.6-hour duration was 73 ppb
(Schelegle et al., 2009). The study design includes a 35-minute
lunch period following the third exposure hour during which the
exposure concentration remains the same as in the third hour.
---------------------------------------------------------------------------
In the 6.6-hour studies, the group means of O3-induced
\64\ FEV1 reductions for exposure concentrations below 80
ppb are at or below 6% (Table 1). For example, the group means of
O3-induced FEV1 decrements reported in these
studies that are statistically significantly different from the
responses in filtered air are 6.1% for 70 ppb and 1.7% to 3.5% for 60
ppb (Table 1). The group mean O3-induced FEV1
decrements generally increase with increasing O3 exposures,
reflecting increases in both the number of the individuals experiencing
FEV1 reductions and the magnitude of the FEV1
reduction (Table 1; ISA, Figure 3-3; PA, Figure 3-2). For example,
following 6.6-hour exposures to a lower concentration (40 ppb), for
which decrements were not statistically significant at the group mean
level, none of 60 subjects across two separate studies experienced an
O3-induced FEV1 reduction as large as 15% or more
(Table 1; PA, Appendix 3D, Table 3D-19). Across the four experiments
(with number of subjects ranging from 30 to 59) that have reported
results for 60 ppb target exposure, the number of subjects experiencing
this magnitude of FEV1 reduction (at or above 15%) varied
(zero of 30, one of 59, two of 31 and two of 30 exposed subjects). This
response increased to three of 31 subjects for the study with a 70 ppb
target concentration (PA, Appendix 3D, Table 3D-19; Schelegle et al.,
2009). In addition to illustrating the E-R relationship, these findings
also illustrate the considerable variability in magnitude of responses
observed among study subjects (ISA, Appendix 3, section 3.1.4.1.1; 2013
ISA, p. 6-13).
---------------------------------------------------------------------------
\64\ Consistent with the ISA and 2013 ISA, the phrase
``O3-induced'' decrement or reduction in lung function or
FEV1 refers to the percent change from pre-exposure
measurement of the O3 exposure minus the percent change
from pre-exposure measurement of the filtered air exposure (2013
ISA, p. 6-4).
Table 1--Summary of 6.6-Hour Controlled Human Exposure Study-Findings, Healthy Adults
--------------------------------------------------------------------------------------------------------------------------------------------------------
O3 target exposure concentration Statistically O3-induced group mean
Endpoint \A\ significant effect \B\ response \B\ Study
--------------------------------------------------------------------------------------------------------------------------------------------------------
FEV1 Reduction...................... 120 ppb........................... Yes..................... -10.3% to -15.9% \C\... Horstman et al. (1990);
Adams (2002); Folinsbee et
al. (1988); Folinsbee et
al. (1994); Adams, 2002;
Adams (2000); Adams and
Ollison (1997).\D\
100 ppb........................... Yes..................... -8.5% to -13.9% \C\.... Horstman et al., 1990;
McDonnell et al., 1991.\D\
87 ppb............................ Yes..................... -12.2%................. Schelegle et al., 2009.
80 ppb............................ Yes..................... -7.5%.................. Horstman et al., 1990.
-7.7%.................. McDonnell et al., 1991.
-6.5%.................. Adams, 2002.
-6.2% to -5.5% \C\..... Adams, 2003.
-7.0% to -6.1% \C\..... Adams, 2006a.
-7.8%.................. Schelegle et al., 2009.
ND \E\.................. -3.5%.................. Kim et al., 2011.\F\
70 ppb............................ Yes..................... -6.1%.................. Schelegle et al., 2009.
[[Page 49852]]
60 ppb............................ Yes \G\................. -2.9%.................. Adams, 2006a; Brown et al.,
-2.8%.................. 2008.
Yes..................... -1.7%.................. Kim et al., 2011.
No...................... -3.5%.................. Schelegle et al., 2009.
40 ppb............................ No...................... -1.2%.................. Adams, 2002.
No...................... -0.2%.................. Adams, 2006a.
Increased Respiratory Symptoms...... 120 ppb........................... Yes..................... Increased symptom Horstman et al. (1990);
100 ppb........................... Yes..................... scores. Adams (2002); Folinsbee et
87 ppb............................ Yes..................... al. (1988); Folinsbee et
80 ppb............................ Yes..................... al. (1994); Adams, 2002;
70 ppb............................ Yes..................... Adams (2000); Adams and
Ollison (1997); Horstman
et al., 1990; McDonnell et
al., 1991; Schelegle et
al., 2009; Adams, 2003;
Adams, 2006a.\H\
60 ppb............................ No...................... ....................... Adams, 2006a; Kim et al.,
40 ppb............................ No...................... 2011; Schelegle et al.,
2009; Adams, 2002.\H\
Airway Inflammation................. 80 ppb............................ Yes..................... Multiple indicators \H\ Devlin et al., 1991; Alexis
60 ppb............................ Yes..................... Increased neutrophils.. et al., 2010.
Kim et al., 2011.
Increased Airway Resistance and 120 ppb........................... Yes..................... Increased.............. Horstman et al., 1990;
Responsiveness. Folinsbee et al., 1994 (O3
induced sRaw not
reported).
100 ppb........................... Yes..................... ....................... Horstman et al., 1990.
80 ppb............................ Yes..................... ....................... Horstman et al., 1990.
--------------------------------------------------------------------------------------------------------------------------------------------------------
\A\ This refers to the average concentration across the six exercise periods as targeted by authors. This differs from the time-weighted average
concentration for the full exposure periods (targeted or actual). For example, as shown in Appendix 3A, Table 3A-2, in chamber studies implementing a
varying concentration protocol with targets of 0.03, 0.07, 0.10, 0.15, 0.08 and 0.05 ppm, the exercise period average concentration is 0.08 ppm while
the time weighted average for the full exposure period (based on targets) is 0.082 ppm due to the 0.6 hour lunchtime exposure between periods 3 and 4.
In some cases this also differs from the exposure period average based on study measurements. For example, based on measurements reported in Schelegle
at al (2009), the full exposure period average concentration for the 70 ppb target exposure is 73 ppb, and the average concentration during exercise
is 72 ppb.
\B\ Statistical significance based on the O3 compared to filtered air response at the study group mean (rounded here to decimal).
\C\ Ranges reflect the minimum to maximum FEV1 decrements across multiple exposure designs and studies. Study-specific values and exposure details
provided in the PA, Appendix 3A, Tables 3A-1 and 3A-2, respectively.
\D\ Citations for specific FEV1 findings for exposures above 70 ppb are provided in PA, Appendix 3A, Table 3A-1.
\E\ ND (not determined) indicates these data have not been subjected to statistical testing.
\F\ The data for 30 subjects exposed to 80 ppb by Kim et al. (2011) are presented in Figure 5 of McDonnell et al. (2012).
\G\ Adams (2006a) reported FEV1 data for 60 ppb exposure by both constant and varying concentration designs. Subsequent analysis of the FEV1 data from
the former found the group mean O3 response to be statistically significant (p <0.002) (Brown et al., 2008; 2013 ISA, section 6.2.1.1). The varying-
concentration design data were not analyzed by Brown et al., 2008.
\H\ Citations for study-specific respiratory symptoms findings are provided in the PA, Appendix 3A, Table 3A-1.
\I\ Increased numbers of bronchoalveolar neutrophils, permeability of respiratory tract epithelial lining, cell damage, production of proinflammatory
cytokines and prostaglandins (ISA, Appendix 3, section 3.1.4.4.1; 2013 ISA, section 6.2.3.1).
For shorter exposure periods, ranging from one to two hours, higher
exposure concentrations, ranging up from 80 ppb up to 400 ppb, have
been studied (ISA, section 3.1; 2013 ISA, section 6.2.1.1; 2006 AQCD;
PA, Appendix 3A, Table 3A-3). In these studies, some exposure protocols
have included heavy intermittent or very heavy continuous exercise,
which results in 2-3 times greater ventilation rate than in the
prolonged (6.6- or 8-hour) exposure studies, which only incorporate
moderate quasi-continuous exercise.\65\ Across these shorter-duration
studies, the lowest exposure concentration for which statistically
significant respiratory effects were reported is 120 ppb, for a 1-hour
exposure combined with continuous very heavy exercise and a 2-hour
exposure with intermittent heavy exercise. As recognized above, the
increased ventilation rate associated with increased exertion increases
the amount of O3 entering the lung, where depending on dose
and the individual's susceptibility, it may cause respiratory effects
(2013 ISA, section 6.2.1.1). Thus, for exposures involving a lower
exertion level, a comparable response would not be expected to occur
without a longer duration at this concentration (120 ppb), as is
illustrated by the 6.6-hour study results for this concentration (ISA,
Appendix 3, Figure 33; PA, Appendix 3A, Table 3A-1).
---------------------------------------------------------------------------
\65\ A quasi-continuous exercise protocol is common to the
prolonged exposure studies where study subjects complete six 50-
minute periods of exercise, each followed by 10-minute periods of
rest (e.g., ISA, Appendix 3, section 3.1.4.1.1, and p. 3-11; 2013
ISA, section 6.2.1.1).
---------------------------------------------------------------------------
With regard to the epidemiologic studies reporting associations
between O3 and respiratory health outcomes such as asthma-
related emergency department visits and hospitalizations, these studies
are generally focused on investigating the existence of a relationship
between O3 occurring in ambient air and specific health
outcomes. Accordingly, while as a whole, this evidence base of
epidemiologic studies provides strong support for the conclusions of
causality, as summarized in section II.B.1 above,\66\ these studies
provide less information on details of the specific O3
exposure circumstances that may be eliciting health effects associated
with such outcomes, and whether these occur under conditions that meet
the current standard. For example, these studies generally do not
measure personal exposures of the study population or track individuals
in the population with a defined exposure to O3 alone.
Further, the vast majority of these studies were conducted in locations
and during time periods that would not have met the current
standard.\67\ While this does not
[[Page 49853]]
lessen their importance in the evidence base documenting the causal
relationship between O3 and respiratory effects, it means
they are less informative in considering O3 exposure
concentrations occurring under air quality conditions allowed by the
current standard.
---------------------------------------------------------------------------
\66\ Combined with the coherent evidence from experimental
studies, the epidemiologic studies ``can support and strengthen
determinations of the causal nature of the relationship between
health effects and exposure to ozone at relevant ambient air
concentrations'' (ISA, p. ES-17).
\67\ Consistent with the evaluation of the epidemiologic
evidence of associations between O3 exposure and
respiratory health effects in the ISA, this summary focuses on those
studies conducted in the U.S. and Canada to provide a focus on study
populations and air quality characteristics that may be most
relevant to circumstances in the U.S. (ISA, Appendix 3, section
3.1.2).
---------------------------------------------------------------------------
Among the epidemiologic studies finding a statistically significant
positive relationship of short- or long-term O3
concentrations with respiratory effects, there are no single-city
studies conducted in the U.S. in locations with ambient air
O3 concentrations that would have met the current standard
for the entire duration of the study (ISA, Appendix 3, Tables 3-13, 3-
14, 3-39, 3-41, 3-42 and Appendix 6, Tables 6-5 and 6-8; PA, Appendix
3B, Table 3B-1). There are (among this large group of studies) two
single city studies conducted in western Canada that include locations
for which the highest-monitor design values calculated in the PA fell
below 70 ppb, at 65 and 69 ppb (PA, Appendix 3B, Table 3B-1; Kousha and
Rowe, 2014; Villeneuve et al., 2007). These studies did not include
analysis of correlations with other co-occurring pollutants or of the
strength of the associations when accounting for effects of
copollutants in copollutant models (ISA, Tables 3-14 and 3-39). Thus,
the studies pose significant limitations with regard to informing
conclusions regarding specific O3 exposure concentrations
and elicitation of such effects. There is also a handful of multicity
studies conducted in the U.S. or Canada in which the O3
concentrations in a subset of the study locations and for a portion of
the study period appear to have met the current standard (PA, Appendix
3B). Concentrations in other portions of the study area or study
period, however, do not meet the standard, or data were not available
in some cities for the earlier years of the study period when design
values for other cities in the study were well above 70 ppb. The extent
to which reported associations with health outcomes in the resident
populations in these studies are influenced by the periods of higher
concentrations during times that did not meet the current standard is
unknown. Additionally, with regard to multicity studies, the reported
associations were based on the combined dataset from all cities,
complicating interpretations regarding the contribution of
concentrations in the small subset of locations that would have met the
current standard compared to that from the larger number of locations
that would have violated the standard (Appendix 3B).\68\ Further, given
that populations in the single city or multicity studies may have also
experienced longer-term, variable and uncharacterized exposure to
O3 (as well as to other ambient air pollutants),
``disentangling the effects of short-term ozone exposure from those of
long-term ozone exposure (and vice-versa) is an inherent uncertainty in
the evidence base'' (ISA, p. IS-87 [section IS.6.1]). While given the
depth and breadth of the evidence base for O3 respiratory
effects, such uncertainties do not change our conclusions regarding the
causal relationship between O3 and respiratory effects, they
affect the extent to which the two studies mentioned here (conducted in
conditions that may have met the current standard) can inform our
conclusions regarding the potential for O3 concentrations
allowed by the current standard to contribute to health effects.
---------------------------------------------------------------------------
\68\ As recognized in the last review, ``multicity studies do
not provide a basis for considering the extent to which reported
O3 health effects associations are influenced by
individual locations with ambient [air] O3 concentrations
low enough to meet the current O3 standard versus
locations with O3 concentrations that violate this
standard'' (80 FR 64344, October 26, 2015).
---------------------------------------------------------------------------
With regard to the experimental animal evidence and exposure
conditions associated with respiratory effects, concentrations are
generally much greater than those examined in the controlled human
exposure studies, summarized in section II.B.1 above, and higher than
concentrations commonly occurring in ambient air in areas of the U.S.
where the current standard is met. In addition to being true for the
various rodent studies, this is also true for the small number of early
life studies in nonhuman primates that reported O3 to
contribute to asthma-like effects in infant primates. The exposures
eliciting the effects in these studies included multiple 5-day periods
with O3 concentrations of 500 ppb over 8-hours per day (ISA,
Appendix 3, section 3.2.4.1.2).
With regard to short-term O3 and metabolic effects, the
category of effects for which the ISA concludes there likely to be a
causal relationship with O3, the evidence base is comprised
primarily of experimental animal studies, as summarized in section
II.B.1 above (ISA, Appendix 5, section 5.1). The exposure conditions
from these animal studies generally involve much higher O3
concentrations than those examined in the controlled human exposure
studies of respiratory effects (and much higher than concentrations
commonly occurring in ambient air in areas of the U.S. where the
current standard is met). For example, the animal studies include 4-
hour concentrations of 400 to 800 ppb (ISA, Appendix 5, Table 5-
87).\69\ The two epidemiologic studies reporting statistically
significant positive associations of O3 with metabolic
effects (e.g., changes in glucose, insulin, metabolic clearance) are
based in Taiwan and South Korea, respectively.\70\ Given the potential
for appreciable differences in air quality patterns between Taiwan and
South Korea and the U.S., as well as differences in other factors that
might affect exposure (e.g., activity patterns), those studies are of
limited usefulness for informing our understanding of exposure
concentrations and conditions eliciting such effects in the U.S. (ISA,
Appendix 5, section 5.1).
---------------------------------------------------------------------------
\69\ Resting rats and resting human subjects exposed to the same
concentration receive similar O3 doses (ISA, section
3.1.4.1.2; Hatch et al., 2013). Further, the exposure concentration
in the single controlled human exposure study of metabolic effects
(e.g., 300 ppb for two hours of intermittent moderate to heavy
exercise [Miller et al., 2016]) is also well above exposures
examined in the 6.6- to 8-hour respiratory effect studies (ISA,
Appendix 5, Table 5-7).
\70\ Of the epidemiologic studies discussed in the ISA that
investigate associations between short-term O3 exposure
and metabolic effects, two are conducted in the U.S. and they report
either a null or negative association of metabolic markers with
O3 concentration (ISA, Appendix 5, Tables 5-6 and 5-9).
---------------------------------------------------------------------------
C. Summary of Exposure and Risk Information
Our consideration of the scientific evidence available in the
current review, as at the time of the last review, is informed by
results from quantitative analyses of estimated population exposure and
consequent risk of respiratory effects. These analyses in this review
have focused on exposure-based risk analyses. Estimates from such
analyses, particularly the comparison of daily maximum exposures to
benchmark concentrations reflecting exposures at which respiratory
effects have been observed in controlled human exposure studies, were
most informative to the Administrator's decision in the last review (as
summarized in section II.A.1 above). This largely reflected the
conclusion that ``controlled human exposure studies provide the most
certain evidence indicating the occurrence of health effects in humans
following specific O3 exposures,'' and recognition that
``effects reported in controlled human exposure studies are due solely
to O3 exposures, and interpretation of
[[Page 49854]]
study results is not complicated by the presence of co-occurring
pollutants or pollutant mixtures (as is the case in epidemiologic
studies)'' (80 FR 65343, October 26, 2015).\71\ The focus in this
review on exposure-based analyses reflects both the emphasis given to
these types of analyses and the characterization of their uncertainties
in the last review, and also the availability of new or updated
information, models, and tools that address those uncertainties (IRP,
Appendix 5A).
---------------------------------------------------------------------------
\71\ In the last review, the Administrator placed relatively
less weight on the air quality epidemiologic-based risk estimates,
in recognition of an array of uncertainties, including, for example,
those related to exposure measurement error (80 FR 65316, 65346,
October 26, 2015; 79 FR 75277-75279, December 17, 2014; 2014 HREA,
sections 3.2.3.2 and 9.6). Further, importantly in this review, the
causal determinations for short-term O3 with mortality in
the current ISA differ from the 2013 ISA. The current determinations
for both short-term and long-term O3 exposure (as
summarized in section II.B.1 above) are that the evidence is
``suggestive'' but not sufficient to infer causal relationships for
O3 with mortality (ISA, Table IS-1).
---------------------------------------------------------------------------
The longstanding evidence continues to demonstrate a causal
relationship between short-term O3 exposures and respiratory
effects, with the current evidence base for respiratory effects is
largely consistent with that for the last review, as summarized in
section II.B above. Accordingly, the exposure-based analyses performed
in this review, summarized below, are conceptually similar to those in
the last review. Section II.C.1 summarizes key aspects of the
assessment design, including the study areas, populations simulated,
the conceptual approach, modeling tools, benchmark concentrations and
exposure and risk metrics derived. Key limitations and uncertainties
associated with the assessment are identified in section II.C.2 and the
exposure and risk estimates are summarized in section II.C.3. An
overarching focus of these analyses is whether the current exposure and
risk information alters overall conclusions reached in the last review
regarding health risk estimated to result from exposure to
O3 in ambient air, and particularly for air quality
conditions that just meet the current standard.
1. Key Design Aspects
The analyses of O3 exposures and risk summarized here
inform our understanding of the protection provided by the current
standard from effects that the health effects evidence indicates to be
elicited in some portion of exercising people exposed for several hours
to elevated O3 concentrations. The analyses estimated
population exposure and risk for simulated populations in eight urban
study areas: Atlanta, Boston, Dallas, Detroit, Philadelphia, Phoenix,
Sacramento and St. Louis. In addition to deriving exposure and risk
estimates for air quality conditions just meeting the current primary
O3 standard, estimates were also derived for two additional
scenarios reflecting conditions just meeting design values just lower
and just higher than the level of the current standard (65 and 75
ppb).\72\
---------------------------------------------------------------------------
\72\ All analyses are summarized more fully in the PA section
3.4 and Appendices 3C and 3D.
---------------------------------------------------------------------------
The eight study areas represent a variety of circumstances with
regard to population exposure to short-term concentrations of
O3 in ambient air. The areas range in total population size
from approximately two to eight million and are distributed across
seven of the nine climate regions of the U.S.: Northeast, Southeast,
Central, East North Central, South, Southwest and West (PA, Appendix
3D, Table 3D-1). The set of eight study areas is streamlined compared
to the 15-area set in the last review and was chosen to ensure it
reflects the full range of air quality and exposure variation expected
in major urban areas in the U.S. with air quality that just meets the
current standard (2014 HREA, section 3.5). Accordingly, while seven of
the eight study areas were also included in the 2014 HREA, the eighth
study area is newly added in the current assessment to insure
representation of a large city in the southwest. Additionally, the
years simulated reflect more recent emissions and atmospheric
conditions subsequent to data used in the 2014 HREA, and therefore
represent O3 concentrations somewhat nearer the current
standard than was the case for study areas included in the 2014 HREA
(Appendix 3C, Table 3C and 2014 HREA, Table 4-1). This contributes to a
reduction in the uncertainty associated with development of the air
quality scenarios of interest, particularly the one reflecting air
quality conditions that just meet the current standard. Study-area-
specific characteristics contribute to variation in the estimated
magnitude of exposure and associated risk across the urban study areas
(e.g., combined statistical areas that include urban and suburban
populations) that reflect an array of air quality, meteorological, and
population exposure conditions.
With regard to the objectives for the analysis approach, the
analyses and the use of a case study approach are intended to provide
assessments of an air quality scenario just meeting the current
standard for a diverse set of areas and associated exposed populations.
These analyses are not intended to provide a comprehensive national
assessment (PA, section 3.4.1). Nor is the objective to present an
exhaustive analysis of exposure and risk in the areas that currently
just meet the current standard and/or of exposure and risk associated
with air quality adjusted to just meet the current standard in areas
that currently do not meet the standard. Rather, the purpose is to
assess, based on current tools and information, the potential for
exposures and risks beyond those indicated by the information available
at the time the standard was established. Accordingly, use of this
approach recognizes that capturing an appropriate diversity in study
areas and air quality conditions (that reflect the current standard
scenario) \73\ is an important aspect of the role of the exposure and
risk analyses in informing the Administrator's conclusions on the
public health protection afforded by the current standard.
---------------------------------------------------------------------------
\73\ A broad variety of spatial and temporal patterns of
O3 concentrations can exist when ambient air
concentrations just meet the current standard. These patterns will
vary due to many factors including the types, magnitude, and timing
of emissions in a study area, as well as local factors, such as
meteorology and topography. We focused our current assessment on
specific study areas having ambient air concentrations close to
conditions that reflect air quality that just meets the current
standard. Accordingly, assessment of these study areas is more
informative to evaluating the health protection provided by the
current standard than would be an assessment that included areas
with much higher and much lower concentrations.
---------------------------------------------------------------------------
Consistent with the health effects evidence in this review
(summarized in section II.B.1 above), the focus of the quantitative
assessment is on short-term exposures of individuals in the population
during times when they are breathing at an elevated rate. Exposure and
risk are characterized for four population groups. Two are populations
of school-aged children, aged 5 to 18 years: \74\ All children and
children with asthma; two are populations of adults: All adults and
adults with asthma. Asthma prevalence in each study area is estimated
using regional, national, and state level prevalence information, as
well as U.S. census tract-level population data and demographic
information related to age, sex, and family income to represent
expected spatial variability in asthma prevalence within and across the
eight study areas. Asthma prevalence estimates for the full populations
in the eight study areas
[[Page 49855]]
range from 7.7 to 11.2%; the rates for children in these areas range
from 9.2 to 12.3% (PA, Appendix 3D, section 3D.3.1).
---------------------------------------------------------------------------
\74\ The child population group focuses on ages 5 to 18 in
recognition of data limitations and uncertainties, including those
related to accurately simulating activities performed and estimating
physiological attributes, as well as challenges in asthma diagnoses
for children younger than 5 years old.
---------------------------------------------------------------------------
The approach for this analysis incorporates an array of models and
data (PA, section 3.4.1). Ambient air O3 concentrations were
estimated using an approach that relies on a combination of ambient air
monitoring data, atmospheric photochemical modeling, and statistical
methods (PA, Appendix 3C). Population exposure and risk modeling is
employed to estimate exposures and related lung function risk resulting
from the estimated ambient air O3 concentrations (PA,
Appendix 3D). While the lung function risk analysis focuses only on the
specific O3 effect of FEV1 reduction, the
comparison-to-benchmark approach, with its use of multiple benchmark
concentrations, provides for risk characterization of the array of
respiratory effects elicited by O3 exposure, the type and
severity of which increase with increased exposure concentration.
Ambient air O3 concentrations were estimated in each
study area for the air quality conditions of interest by adjusting
hourly ambient air concentrations, from monitoring data for the years
2015-2017, using a photochemical model-based approach and then applying
a spatial interpolation technique to produce air quality surfaces with
high spatial and temporal resolution (PA, Appendix 3C).\75\ The final
product were datasets of ambient air O3 concentration
estimates with high temporal and spatial resolution (hourly
concentrations in 500 to 1,700 census tracts) for each of the eight
study areas (PA, section 3.4.1 and Appendix 3C, section 3C.7)
representing the three air quality scenarios (just meeting the current
standard, and the 65 ppb and 75 ppb scenarios).
---------------------------------------------------------------------------
\75\ A similar approach was used to develop the air quality
scenarios for the 2014 HREA.
---------------------------------------------------------------------------
Population exposures were estimated using the EPA's Air Pollutant
Exposure model (APEX) version 5, which probabilistically generates a
large sample of hypothetical individuals from population demographic
and activity pattern databases and simulates each individual's
movements through time and space to estimate their time series of
O3 exposures occurring within indoor, outdoor, and in-
vehicle microenvironments (PA, Appendix 3D, section 3D.2).\76\ The APEX
model accounts for the most important factors that contribute to human
exposure to O3 from ambient air, including the temporal and
spatial distributions of people and ambient air O3
concentrations throughout a study area, the variation of ambient air-
related O3 concentrations within various microenvironments
in which people conduct their daily activities, and the effects of
activities involving different levels of exertion on breathing rate (or
ventilation rate) for the exposed individuals of different sex, age,
and body mass in the study area (PA, Appendix 3D, section 3D.2). The
APEX model generates each simulated person or profile by
probabilistically selecting values for a set of profile variables,
including demographic variables, health status and physical attributes
(e.g., residence with air conditioning, height, weight, body surface
area), and activity-specific ventilation rate (PA, Appendix 3D, section
3D.2).
---------------------------------------------------------------------------
\76\ The APEX model estimates population exposure using a
stochastic, event-based microenvironmental approach. This model has
a history of application, evaluation, and progressive model
development in estimating human exposure, dose, and risk for reviews
of NAAQS for gaseous pollutants, including the last review of the
O3 NAAQS (U.S. EPA, 2008; U.S. EPA, 2009; U.S. EPA, 2010;
U.S. EPA, 2014a; U.S. EPA, 2018).
---------------------------------------------------------------------------
The activity patterns of individuals are an important determinant
of their exposure (2013 ISA, section 4.4.1). By incorporating
individual activity patterns,\77\ the model estimates physical exertion
associated with each exposure event. This aspect of the exposure
modeling is critical in estimating exposure, ventilation rate,
O3 intake (dose), and health risk resulting from ambient air
concentrations of O3.\78\ Because of variation in
O3 concentrations among the different microenvironments in
which individuals are active, the amount of time spent in each
location, as well as the exertion level of the activity performed, will
influence an individual's exposure to O3 from ambient air
and potential for adverse health effects. Activity patterns vary both
among and within individuals, resulting in corresponding variations in
exposure across a population and over time (2013 ISA, section 4.4.1;
2020 ISA, Appendix 2, section 2.4). For each exposure event, the APEX
model tracks activity performed, ventilation rate, exposure
concentration, and duration for all simulated individuals throughout
the assessment period. The time-series of exposure events serves as the
basis for calculating exposure and risk metrics of interest.
---------------------------------------------------------------------------
\77\ To represent personal time-location-activity patterns of
simulated individuals, the APEX model draws from the consolidated
human activity database (CHAD) developed and maintained by the EPA
(McCurdy, 2000; U.S. EPA, 2019a). The CHAD is comprised of data from
several surveys that collected activity pattern data at city, state,
and national levels. Included are personal attributes of survey
participants (e.g., age, sex), along with the locations they
visited, activities performed throughout a day, time-of-day the
activities occurred and activity duration (PA, Appendix 3D, section
3D.2.5.1).
\78\ Indoor sources are generally minor in comparison to
O3 from ambient air (ISA, Appendix 2, section 2.1) and
are not accounted for by the exposure modeling in this assessment.
---------------------------------------------------------------------------
As in the last review, the quantitative analyses for this review
uses the APEX model estimates of population exposures for simulated
individuals breathing at elevated rates \79\ to characterize health
risk based on information from the controlled human exposure studies on
the incidence of lung function decrements in study subjects who are
exposed over multiple hours while intermittently or quasi-continuously
exercising (PA, Appendix 3D, section 3D.2.8). In drawing on this
evidence base for this purpose, the assessment has given primary focus
to the well-documented controlled human exposure studies for 6.6-hour
average exposure concentrations ranging from 40 ppb to 120 ppb (ISA,
Appendix 3, Figure 3-3; PA, Figure 3-2 and Appendix 3A, Table 3A-1).
Health risk is characterized in two ways, producing two types of risk
metrics: One that compares population exposures involving elevated
exertion to benchmark concentrations (that are specific to elevated
exertion exposures), and the second that estimates population
occurrences of ambient air O3-related lung function
decrements. The first risk metric is based on comparison of estimated
daily maximum 7-hour average exposure concentrations for individuals
breathing at elevated rates to concentrations of potential concern
(benchmark concentrations). The second metric (lung function risk) uses
E-R information for O3 exposures and FEV1
decrements to estimate the portion of the simulated at-risk population
expected to experience one or more days with an O3-related
FEV1 decrement of at least 10%, 15% and 20%. Both of these
metrics are used to characterize health risk associated with
O3 exposures among the simulated population during periods
of elevated breathing rates. Similar risk metrics were also derived in
the 2014 HREA for the last review and the associated estimates informed
the Administrator's 2015 decision on the current standard (80 FR 65292,
October 26, 2015).
---------------------------------------------------------------------------
\79\ Based on minute-by-minute activity levels, and
physiological characteristics of the simulated person, APEX
estimates an equivalent ventilation rate, by normalizing the
simulated individuals' activity-specific ventilation rate to their
body surface area (PA, Appendix 3D, section 3D.2.2.3.3).
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[[Page 49856]]
The general approach and methodology for the exposure-based
assessment used in this review is similar to that used in the last
review. However, a number of updates and improvements, related to the
air quality, exposure, and risk aspects of the assessment, have been
implemented in this review which result in differences from the
analyses in the prior review (Appendices 3C and 3D). These include (1)
a more recent period (2015-2017) of ambient air monitoring data in
which O3 concentrations in the eight study areas are at or
near the current standard; (2) the most recent CAMx model, with updates
to the treatment of atmospheric chemistry and physics within the model;
(3) a significantly expanded CHAD, that now has nearly 180,000 diaries,
with over 25,000 school aged children; (4) updated National Health and
Nutrition Examination Survey data (2009-2014), which are the basis for
the age- and sex-specific body weight distributions used to specify the
individuals in the modeled populations; (5) updated algorithms used to
estimate age- and sex-specific resting metabolic rate, a key input to
estimating a simulated individual's activity-specific ventilation (or
breathing) rate; (6) updates to the ventilation rate algorithm itself;
and (7) an approach that better matches the simulated exposure
estimates with the 6.6-hour duration of the controlled human exposure
studies and with the study subject ventilation rates. Further, the
current APEX model uses the most recent U.S. Census demographic and
commuting data (2010), NOAA Integrated Surface Hourly meteorological
data to reflect the assessment years studied (2015-2017), and updated
estimates of asthma prevalence for all census tracts in all study areas
based on 2013-2017 National Health Interview Survey and Behavioral Risk
Factor Surveillance System data. Additional details are described in
the PA (e.g., PA, section 3.4.1, Appendices 3C and 3D).
The exposure-to-benchmark comparison characterizes the extent to
which individuals in at-risk populations could experience O3
exposures, while engaging in their daily activities, with the potential
to elicit the effects reported in controlled human exposure studies for
concentrations at or above specific benchmark concentrations. Results
are characterized using three benchmark concentrations of
O3: 60, 70, and 80 ppb. These are based on the three lowest
concentrations targeted in studies of 6- to 6.6-hour exposures, with
quasi-continuous exercise, and that yielded different occurrences, of
statistical significance, and severity of respiratory effects (PA,
section 3.3.3; PA, Appendix 3A, section 3A.1; PA, Appendix 3D, section
3D.2.8.1). The lowest benchmark, 60 ppb, represents the lowest exposure
concentration for which controlled human exposure studies have reported
statistically significant respiratory effects. At this concentration,
there is evidence of a statistically significant decrease in lung
function and increase in markers of airway inflammation (ISA, Appendix
3, section 3.1.4.1.1; Brown et al., 2008; Adams, 2006a). Exposure to
approximately 70 ppb \80\ averaged over 6.6 hours resulted in a larger
group mean lung function decrement, as well as an increase in
prevalence of respiratory symptoms over what was observed for 60 ppb
(Table 1; ISA, Appendix 3, Figure 3-3 and section 3.1.4.1.1; Schelegle
et al., 2009). Studies of exposures to approximately 80 ppb have
reported larger lung function decrements at the study group mean than
following exposures to 60 or 70 ppb, in addition to an increase in
airway inflammation, increased respiratory symptoms, increased airway
responsiveness, and decreased resistance to other respiratory effects
(Table 1; ISA, Appendix 3, sections 3.1.4.1 through 3.1.4.4; PA, Figure
3-2 and section 3.3.3;). The APEX-generated exposure concentrations for
comparison to these benchmark concentrations is the average of
concentrations encountered by an individual while at an activity level
that elicits the specified elevated ventilation rate.\81\ The incidence
of such exposures above the benchmark concentrations are summarized for
each simulated population, study area, and air quality scenario as
discussed in section II.C.3 below.
---------------------------------------------------------------------------
\80\ The design for the study on which the 70 ppb benchmark
concentration is based, Schelegle et al. (2009), involved varying
concentrations across the full exposure period. The study reported
the average O3 concentration measured during each of the
six exercise periods. The mean concentration across these six values
is 72 ppb. The 6.6-hour time weighted average based on the six
reported measurements and the study design is 73 ppb (Schelegle et
al., 2009). Other 6.6-hour studies have not reported measured
concentrations for each exposure, but have generally reported an
exposure concentration precision at or tighter than 3 ppb (e.g.,
Adams, 2006a).
\81\ For this assessment, the APEX model averages the
ventilation rate (VE) and simultaneously occurring
exposure concentration for every simulated individual (based on the
activities performed) over 7-hour periods using their time-series of
exposure events. To reasonably extrapolate the VE of the
controlled human study subjects (i.e., adults having a specified
body size and related lung capacity), who were engaging in quasi-
continuous exercise during the study period, to individuals having
varying body sizes (e.g., children with smaller size and related
lung capacity), an equivalent ventilation rate (EVR) was calculated
by normalizing the VE (L/min) by body surface area
(m\2\). Then, daily maximum 7-hour exposure concentrations
associated with 7-hour average EVR at or above the target of 17.3
1.2 L/min-m\2\ (i.e., the value corresponding to
average EVR across the 6.6-hour study duration in the controlled
human exposure studies) are compared to the benchmark concentrations
(PA, Appendix 3D, section 3D.2.8.1).
---------------------------------------------------------------------------
The lung function risk analysis provides estimates of the extent to
which individuals in the populations could experience decrements in
lung function. Estimates were derived for risk of experiencing a day
with a lung function decrement at or above three different magnitudes,
i.e., FEV1 reductions of at least 10%, 15%, and 20%. Lung
function decrement risk was estimated by two different approaches,
which utilize the evidence from the 6.6-hour controlled human exposure
studies in different ways.\82\ One, the population-based E-R function
risk approach, uses quantitative descriptions of the E-R relationships
for study group incidence of the different magnitudes of lung function
decrements based on the individual study subject observations (PA,
Appendix 3D, section 3D.2.8.2.1). The second, the individual-based
McDonnell-Smith-Stewart model (MSS; McDonnell et al., 2013), uses
quantitative descriptions of biological processes identified as
important in eliciting the different sizes of decrements at the
individual level, with a factor that also provides a representation of
intra- and inter-individual response variability (PA, Appendix 3D,
section 3D.2.8.2.2). These two approaches involve different uses of the
health effects evidence, with each accordingly, differing in their
strengths, limitations and uncertainties.
---------------------------------------------------------------------------
\82\ In so doing, the approaches also estimate responses
associated with unstudied exposure circumstances and population
groups in different ways.
---------------------------------------------------------------------------
The E-R functions used for estimating the risk of lung function
decrements at or above three sizes were developed from the individual
study subject measurements of O3-related FEV1
decrements from the 6.6-hour controlled human exposure studies
targeting mean exposure concentrations from 120 ppb down to 40 ppb (PA,
Appendix 3D, Table 3D-19; PA, Appendix 3A, Figure 3A-1). Functions were
developed from the study results in terms of percent of study subjects
experiencing O3-related decrements equal to at least 10%,
15% or 20%.\83\ The functions indicate the
[[Page 49857]]
fraction of the population experiencing a particular decrement as a
function of the exposure concentration experienced while at the target
ventilation rate. This type of risk model, which has been used in risk
assessments since the 1997 O3 NAAQS review, was last updated
with the recently available study data (PA, Appendix 3D, section
3D.2.8.2.1). In this review, the E-R functions are applied to the APEX
estimates of daily maximum 7-hour average exposure concentrations
concomitant with the target ventilation level estimated by APEX, with
the results presented in terms of number of individuals in the
simulated populations (and percent of the population) estimated to
experience a day (or more) with a lung function decrement at or above
10%, 15% or 20%.
---------------------------------------------------------------------------
\83\ Across the exposure range from 40 to 120 ppb, the
percentage of exercising study subjects with asthma estimated to
have at least a 10% O3 related FEV1 decrement
increases from 0 to 7% (a statistically non-significant response at
exposures of 40 ppb) up to approximately 50 to 70% at exposures of
120 ppb (PA, Appendix 3D, Section 3D.2.8.2.1, Table 3D-19).
---------------------------------------------------------------------------
The MSS model, also used for estimating the risk of lung function
decrements, was developed using the extensive database from controlled
human exposure studies that has been compiled over the past several
decades, and biological concepts based on that evidence (McDonnell et
al., 2012; McDonnell et al., 2013). The model mathematically estimates
the magnitude of FEV1 decrement as a function of inhaled O3
dose (based on concentration & ventilation rate) over the time period
of interest (PA, Appendix 3D, section 3D.2.8.2.2). The simulation of
decrements is dynamic, based on a balance between predicted development
of the decrement in response to inhaled dose and predicted recovery
(using a decay factor). This model was first applied in combination
with the APEX model to generate lung function risk estimates in the
last review (80 FR 65314, October 26, 2015) and has been updated since
then based on the most recent study by its developers (McDonnell et
al., 2013). In this review, the model is applied to the APEX estimates
of exposure concentration and ventilation for every exposure event
experienced by each simulated individual. The model then utilizes its
mathematical predictions of lung function response to inhaled dose and
predicted recovery to estimate the magnitude of O3 response
across the sequence of exposure events in each individual's day. Each
occurrence of decrements reaching magnitudes of interest (e.g., 10%,
15% and 20%) is tallied. Thus, results are reported using the same
metrics as for the E-R function, i.e., number of individuals in the
simulated populations (and percent of the population) estimated to
experience a day (or more) per simulation period with a lung function
decrement at or above 10%, 15% and 20%.
The comparison-to-benchmark analysis (involving comparison of daily
maximum 7-hour average exposure concentrations that coincide with 7-
hour average elevated ventilation rates at or above the target to
benchmark concentrations) provides perspective on the extent to which
the air quality being assessed could be associated with discrete
exposures to O3 concentrations reported to result in
respiratory effects. For example, estimates of such exposures can
indicate the potential for O3-related effects in the exposed
population, including effects for which we do not have E-R functions
that could be used in quantitative risk analyses (e.g., airway
inflammation). Thus, the comparison-to-benchmark analysis provides for
a broader risk characterization with consideration of the array of
O3-related respiratory effects. For this reason, as well as
the uncertainties associated with the lung function risk estimates, as
summarized below, the summary of estimates in section II.C.3 below
focuses primarily on results for the comparison-to-benchmark analysis.
2. Key Limitations and Uncertainties
Uncertainty in the current exposure and risk analyses was
characterized using a largely qualitative approach adapted from the
World Health Organization (WHO) approach for characterizing uncertainty
in exposure assessment (WHO, 2008) augmented by several quantitative
sensitivity analyses for key aspects of the assessment approach
(described in detail in Appendix 3D of the PA).\84\ This
characterization and associated analyses builds on information
generated from a previously conducted quantitative uncertainty analysis
of population-based O3 exposure modeling (Langstaff, 2007).
In so doing, the characterization considers the various types of data,
algorithms, and models that together yield exposure and risk estimates
for the eight study areas. In this way, the limitations and
uncertainties underlying these data, algorithms, and models and the
extent of their influence on the resultant exposure/risk estimates are
considered. Consistent with the WHO (2008) uncertainty guidance, the
overall impact of the uncertainty is scaled by qualitatively assessing
the extent or magnitude of the impact of the uncertainty as implied by
the relationship between the source of the uncertainty and the exposure
and risk output. The characterization in the current assessment also
evaluates the direction of influence, indicating how the source of
uncertainty was judged to affect the exposure and risk estimates, e.g.,
likely to over- or under-estimate (PA, Appendix 3D, section 3D.3.4.1).
---------------------------------------------------------------------------
\84\ The approach used has been applied in REAs for past NAAQS
reviews for O3, NOX, CO and sulfur oxides
(U.S. EPA, 2008; U.S. EPA, 2010; U.S. EPA, 2014a; U.S. EPA, 2018).
---------------------------------------------------------------------------
Several areas of uncertainty are identified as particularly
important to considering the exposure and risk estimates. There are
also several areas where new or updated information have reduced
uncertainties since the last review. Some of these areas pertain to
estimates for both types of risk metrics, and some pertain more to one
type of estimate versus the other. There are also differences in the
uncertainties that pertain to each of the two approaches used for the
lung function risk metric.
An overarching and important area of uncertainty, which remains
from the last review, and is important to our consideration of the
exposure and risk analysis results relates to the underlying health
effects evidence base. This analysis focuses on the evidence base
described as providing the ``strongest evidence'' of O3
respiratory effects (ISA, p. IS-1), the controlled human exposure
studies, and on the array of respiratory responses documented in those
studies (e.g., lung function decrements, respiratory symptoms,
increased airway responsiveness and inflammation). However, we
recognize the lack of evidence from controlled human exposure studies
at the lower concentrations of greatest interest (e.g., 60, 70 and 80
ppb) for children and for people of any age with asthma. While the
limited evidence that informs our understanding of potential risk to
people with asthma is uncertain, it indicates some potential for them
to have lesser reserve to protect against such effects than other
population groups under similar exposure circumstances, as summarized
in section II.B above. Thus, the health effects reported in controlled
human exposure studies of healthy adults may be contribute to more
severe outcomes in people with asthma. Such a conclusion is consistent
with the epidemiologic study findings of positive associations of
O3 concentrations with asthma-related ED visits and hospital
admissions (and the higher effect estimates from these studies), as
referenced in section II.B. above and presented in detail in the ISA.
Further, with regard to lung function decrements, information is
lacking on the factors contributing to increased
[[Page 49858]]
susceptibility to O3-induced lung function decrements among
some people. Thus, there is uncertainty regarding the interpretation of
the exposure and risk estimates and the extent to which they represent
the populations at greatest risk of O3-related respiratory
effects.
Aspects of the analytical design that pertain to both exposure-
based risk metrics include the estimation of ambient air O3
concentrations for the assessed air quality scenarios, as well as the
main components of the exposure modeling. Key uncertainties include the
modeling approach used to adjust ambient air concentrations to meet the
air quality scenarios of interest and the method used to interpolate
monitor concentrations to census tracts. While the adjustment to
conditions near, just above, or just below the current standard is an
important area of uncertainty, the approach used has taken into account
the currently available information and selected study areas having
design values near the level of the current standard to minimize the
size of the adjustment needed to meet a given air quality scenario. The
approach also uses more recent data as inputs for the air quality
modeling, such as more recent O3 concentration data (2015-
2017), meteorological data (2016) and emissions data (2016), as well as
a recently updated air quality photochemical model which includes
state-of-the-science atmospheric chemistry and physics (PA, Appendix
3C). Further, the number of ambient monitors sited in each of the eight
study areas provides a reasonable representation of spatial and
temporal variability in those areas for the air quality conditions
simulated. Among other key aspects, there is uncertainty associated
with the simulation of study area populations (and at-risk
populations), including those with particular physical and personal
attributes. As also recognized in the 2014 HREA, exposures could be
underestimated for some population groups that are frequently and
routinely outdoors during the summer (e.g., outdoor workers, children).
In addition, longitudinal activity patterns do not exist for these and
other potentially important population groups (e.g., those having
respiratory conditions other than asthma), thus limiting the extent to
which the exposure model outputs reflect information that may be
particular to these groups. Important uncertainties in the approach
used to estimate energy expenditure (i.e., metabolic equivalents of
work or METs), which are ultimately used to estimate ventilation rates,
include the use of longer-term average MET distributions to derive
short-term estimates, along with extrapolating adult observations to
children. Both of these approaches are reasonable based on the
availability of relevant data and appropriate evaluations conducted to
date, and uncertainties associated with these steps are somewhat
reduced in the current analyses (compared to the 2014 HREA) because of
the added specificity and redevelopment of METs distributions, based on
information newly available in this review, is expected to more
realistically estimate activity-specific energy expenditure.
With regard to the aspects of the two risk metrics, there are some
uncertainties that apply to the estimation of lung function risk and
not to the comparison-to-benchmarks analysis. Both lung function risk
approaches utilized in the risk analyses incorporate some degree of
extrapolation beyond the exposure circumstances evaluated in the
controlled human exposure studies. This is the case in different ways
and with differing impacts for the two approaches. One way in which
both approaches extrapolate beyond the exposure studies concerns
estimates of lung function risk derived for exposure concentrations
below those represented in the evidence base. The approaches provide
this in recognition of the potential for lung function decrements to be
greater in unstudied at-risk population groups than is evident from the
available studies. Accordingly, the uncertainty in the lung function
risk estimates increases with decreasing exposure concentration and is
particularly increased for concentrations below those evaluated in
controlled human exposure studies.
There are differences between the two lung function risk approaches
in how they extrapolate beyond the controlled human exposure study
conditions and in the impact on the estimates (with somewhat smaller
differences for multiple day estimates).\85\ The E-R function approach
generates nonzero predictions from the full range of nonzero
concentrations for 7-hour average durations in which the average
exertion levels meets or exceeds the target. The MSS model, which draws
on evidence-based concepts of how human physiological processes respond
to O3, extrapolates beyond the controlled experimental
conditions with regard to exposure concentration, duration and
ventilation rate (both magnitude and duration). The difference between
the two models in the impact of the differing extents of extrapolation
is illustrated by differences in the percent of the risk estimates for
days for which the highest 7-hour average concentration is below the
lowest 6.6-hour exposure concentration tested (PA, Tables 3-6 and 3-7).
For example, with the E-R model, 3 to 6% of the risk to children of
experiencing at least one day with decrements greater than 20% (for
single years in three study areas) is associated with exposure
concentrations below 40 ppb (the lowest concentration studied in the
controlled human exposure studies, and at which no decrements of this
severity occurred in any study subjects). This is in comparison to 25%
to nearly 40% of MSS model estimates of decrements greater than 20%
deriving from exposures below 40 ppb. The MSS model also used
ventilation rates lower than those used for the E-R function risk
approach (which are based on the controlled human exposure study
conditions), contributing to relatively greater risks estimated by the
MSS model.\86\
---------------------------------------------------------------------------
\85\ This is largely because the percent contribution to low-
concentration risk for two or more decrement days predicted by the
E-R approach is, by design, greater than the corresponding
contribution to low-concentration risk for one or more days. This
also occurs because the MSS model estimates risk from a larger
variety of exposure and ventilation conditions (PA, Tables 3-6 and
3-7, Appendix 3D, sections 3D.3.4.2.3 and 3D.3.4.2.4).
\86\ Limiting the MSS model results to estimates for individuals
with at least the same exertion level achieved by study subjects
(>=17.3 L/min-m\2\), reduces the risks of experiencing at least one
lung function decrement by an amount between 24 to 42%. (PA,
Appendix 3D, Table 3D-69).
---------------------------------------------------------------------------
Many of the uncertainties previously identified as part of the 2014
HREA as unique to the MSS model also remain as important uncertainties
in the current assessment. For example, the extrapolation of the MSS
model age parameter down to age 5 (from the age range of the 18- to 35-
year old study subjects to which the model was fit) is an important
uncertainty given that children are an at-risk population in this
assessment. There is also uncertainty in estimating the frequency and
magnitude of lung function decrements as a result of the statistical
form and parameters used for the MSS model inter- and intra-individual
variability terms (PA, Appendix 3D, section 3D.3.4). As a whole, the
differences between the two lung function risk approaches and the
estimates generated by these approaches indicate appreciably greater
uncertainty for the MSS model estimates than the E-R function estimates
(PA, section 3.4.4
[[Page 49859]]
and Tables 3-6 and 3-7).\87\ In light of the uncertainties summarized
here for the MSS model (and discussed in detail in Appendix 3D, section
3D.3.4 of the PA), the lung function risk estimates summarized in
section II.C.3 below are those derived using the E-R approach.
---------------------------------------------------------------------------
\87\ The E-R function risk approach conforms more closely to the
circumstances of the 6.6-hour controlled human exposure studies,
such that the 7-hour duration and moderate or greater exertion level
are necessary for nonzero risk. This approach does, however, use a
continuous function which predicts responses for exposure
concentrations below those studied down to zero. As a result,
exposures below those studied in the controlled human exposures will
result in a fraction of the population being estimated by the E-R
function to experience a lung function decrement (albeit to an
increasingly small degree with decreasing exposures). The MSS model,
which has been developed based on a conceptualization intended to
reflect a broader set of controlled human exposure studies (e.g.,
including studies of exposures to higher concentrations for shorter
durations), does not require a 7-hour duration for estimation of a
response, and lung function decrements are estimated for exertion
below moderate or greater levels, as well as for exposure
concentrations below those studied (PA, Appendix 3D, section
3D.3.4.2; 2014 HREA section 6.3.3). These differences in the models,
accordingly, result in differences in the extent to which they
reflect the particular conditions of the available controlled human
exposure studies and the frequency and magnitude of the measured
responses.
---------------------------------------------------------------------------
Two updates to the analysis approach since the 2014 HREA reduce
uncertainty in the results. The first is related to the approach to
identifying when simulated individuals may be at moderate or greater
exertion. The approach used in the current review reduces the potential
for overestimation of the number of people achieving the associated
ventilation rate, an important uncertainty identified in the 2014 HREA.
Additionally, the current analysis focuses on exposures of 7 hours
duration to better represent the 6.6-hour exposures from the controlled
human exposure studies (than the 8-hour exposure durations used for the
2014 HREA and prior assessments).
In summary, among the multiple uncertainties and limitations in
data and tools that affect the quantitative estimates of exposure and
risk and their interpretation in the context of considering the current
standard, several are particularly important, some of which are similar
to those recognized in the last review. These include uncertainty
related to estimation of the concentrations in ambient air for the
current standard and the additional air quality scenarios; lung
function risk approaches that rely, to varying extents, on
extrapolating from controlled human exposure study conditions to lower
exposure concentrations, lower ventilation rates, and shorter
durations; and characterization of risk for particular population
groups that may be at greatest risk, particularly for people with
asthma, and particularly children. Areas in which uncertainty has been
reduced by new or updated information or methods include the use of
more refined air quality modeling based on selection of study areas
with design values near the current standard and a more recent model
and model inputs, as well as updates to several inputs to the exposure
model including changes to the exposure duration to better match those
in the controlled human exposure studies and an alternate approach to
characterizing periods of activity while at moderate or greater
exertion for simulated individuals.
3. Summary of Exposure and Risk Estimates
Exposure and risk estimates for the eight urban study areas are
summarized here, with a focus on the estimates for air quality
conditions adjusted to just meet the current standard. The analyses in
this review include two types of risk estimates for the 3-year
simulation in each study area: (1) The number and percent of simulated
people experiencing exposures at or above the particular benchmark
concentrations of interest in a year, while breathing at elevated
rates; and (2) the number and percent of people estimated to experience
at least one O3-related lung function decrement
(specifically, FEV1 reductions of a magnitude at or above
10%, 15% or 20%) in a year and the number and percent of people
estimated to experience multiple lung function decrements associated
with O3 exposures.
The benchmark-based risk metric results are summarized in terms of
the percent of the simulated populations of all children and children
with asthma estimated to experience at least one day per year \88\ with
a 7-hour average exposure concentration at or above the different
benchmark concentrations while breathing at elevated rates under air
quality conditions just meeting the current standard (Table 2).
Estimates for adults, in terms of percentages, are generally lower due
to the lesser amount and frequency of time spent outdoors at elevated
exertion (PA, Appendix 3D, section 3D.3.2). The exception is outdoor
workers who, due to the requirements of their job, spend more time
outdoors. Targeted analyses of outdoor workers in the 2014 HREA (single
study area, single year) estimated an appreciably greater portion of
this population to experience exposures at or above benchmark
concentration than the full adult or child populations (2014 HREA,
section 5.4.3.2) although there are a number of uncertainties
associated with these estimates due to appreciable limitations in the
data underlying the analyses. For a number of reasons, including the
appreciable data limitations (e.g., related to specific durations of
time spent outdoors and activity data), and associated uncertainties
summarized in Table 3D-64 of Appendix 3D of the PA, the group was not
simulated in the current analyses.\89\
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\88\ While the duration of an O3 season for each year
may vary across the study areas, for the purposes of the exposure
and risk analyses, the O3 season in each study area is
considered synonymous with a year. These seasons capture the times
during the year when concentrations are elevated (80 FR 65419-65420,
October 26, 2015).
\89\ It is expected that if an approach similar to that used in
the 2014 HREA were used for this assessment the distribution of
exposures (single day and multiday) would be similar to that
estimated in the 2014 HREA (e.g., 2014 HREA, Figure 5-14), although
with slightly lower overall percentages (and based on the comparison
of current estimates with estimates from the 2014 HREA) (PA,
Appendix 3D, section 3D.3.2.4).
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Given the recognition of people with asthma as an at-risk
population and the relatively greater amount and frequency of time
spent outdoors at elevated exertion of children, we focus here on the
estimates for children, including children with asthma. Under air
quality conditions just meeting the current standard, approximately
less than 0.1% of any area's children with asthma, on average, were
estimated to experience any days per year with a 7-hour average
exposure at or above 80 ppb, while breathing at elevated rates (Table
2). With regard to the 70 ppb benchmark, the study areas' estimates for
children with asthma are as high as 0.7 percent (0.6% for all
children), on average across the 3-year period, and range up to 1.0% in
a single year. Approximately 3% to nearly 9% of each study area's
simulated children with asthma, on average across the 3-year period,
are estimated to experience one or more days per year with a 7-hour
average exposure at or above 60 ppb. This range is very similar for the
populations of all children.
Regarding multiday occurrences, the analyses indicate that no
children would be expected to experience more than a single day with a
7-hour average exposure at or above 80 ppb in any year simulated in any
location (Table 2). For the 70 ppb benchmark, the estimate is less than
0.1% of any area's children (on average across 3-year period), both
those with asthma and all children. The estimates for the 60 ppb
benchmark are slightly higher, with up to 3% of
[[Page 49860]]
children estimated to experience more than a single day with a 7-hour
average exposure at or above 60 ppb, on average (and more than 4% in
the highest year across all eight study area locations).
These estimates for the analyses in the current review, while based
on conceptually similar approaches to those used in the 2014 HREA, also
reflect the updates and revisions to those approaches that have been
implemented since that time. The range of estimates across the study
areas from the current assessment for air quality conditions simulated
to just meet the current standard are similar, although the upper end
of the ranges is slightly lower in some cases, to the estimates for
these same populations in the 2014 HREA. For example, for air quality
conditions just meeting the now-current standard, the 2014 HREA
estimated 0.1 to 1.2% of all children across the study areas to
experience, on average, at least one day with exposure at or above 70
ppb, while at elevated ventilation, compared to the comparable
estimates of 0.2 to 0.6% from the current analyses (PA, Appendix 3D,
section 3D.3.2.4, Table 3D-38). There are a number of differences
between the quantitative modeling and analyses performed in the current
assessment and the 2014 HREA that likely contribute to the small
differences in estimates between the two assessments (e.g., 2015-2017
vs. 2006-2010 distribution of ambient air O3 concentrations,
better matching of simulated exposure estimates with the 6.6-hour
duration of the controlled human exposure studies and with the study
subject ventilation rates).
Table 2--Percent and Number of Simulated Children and Children With Asthma Estimated To Experience at Least One or More Days per Year With a 7-Hour
Average Exposure at or Above Indicated Concentration While Breathing at an Elevated Rate in Areas Just Meeting the Current Standard
--------------------------------------------------------------------------------------------------------------------------------------------------------
One or more days Two or more days Four or more days
-----------------------------------------------------------------------------------------------
Exposure concentration (ppb) Average per Highest in a Average per Highest in a Average per Highest in a
year single year year single year year single year
--------------------------------------------------------------------------------------------------------------------------------------------------------
Children With Asthma--Percent of Simulated Population \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=80.................................................... 0 \B\-<0.1 \C\ 0.1% 0 0 0 0
>=70.................................................... 0.2-0.7 1.0% <0.1 0.1 0 0
>=60.................................................... 3.3-8.8 11.2 0.6-3.2 4.9 <0.1-0.8 1.3
--------------------------------------------------------------------------------------------------------------------------------------------------------
Children With Asthma--Number of Individuals \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=80.................................................... 0-67 202 0 0 0 0
>=70.................................................... 93-1,145 1,616 3-39 118 0 0
>=60.................................................... 1,517-8,544 11,776 282--2,609 3,977 23-637 1,033
--------------------------------------------------------------------------------------------------------------------------------------------------------
All Children--Percent of Simulated Population \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=80.................................................... 0 \B\-<0.1 0.1 0 0 0 0
>=70.................................................... 0.2-0.6 0.9 <0.1 0.1 0-<0.1 <0.1
>=60.................................................... 3.2-8.2 10.6 0.6-2.9 4.3 <0.1-0.7 1.1
--------------------------------------------------------------------------------------------------------------------------------------------------------
All Children--Number of Individuals \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=80.................................................... 0-464 1,211 0 0 0 0
>=70.................................................... 727-8,305 11,923 16-341 660 0-5 14
>=60.................................................... 14,928-69,794 96,261 2,601-24,952 36,643 158-5,997 9,554
--------------------------------------------------------------------------------------------------------------------------------------------------------
\A\ Estimates for each study area were averaged across the 3-year assessment period. Ranges reflect the ranges of averages.
\B\ A value of zero (0) means that there were no individuals estimated to have the selected exposure in any year.
\C\ An entry of <0.1 is used to represent small, non-zero values that do not round upwards to 0.1 (i.e., <0.05).
In framing these same exposure estimates from the perspective of
estimated protection provided by the current standard, these results
indicate that, in the single year with the highest concentrations
across the 3-year period, 99% of the population of children with asthma
would not be expected to experience such a day with an exposure at or
above the 70 ppb benchmark; 99.9% would not be expected to experience
such a day with exposure at or above the 80 ppb benchmark. The
estimates, on average across the 3-year period, indicate that over
99.9%, 99.3% and 91.2% of the population of children with asthma would
not be expected to experience a day with a 7-hour average exposure
while at elevated ventilation that is at or above 80 ppb, 70 ppb and 60
ppb, respectively (Table 2, above). Further, more than approximately
97% of all children or children with asthma are estimated to be
protected against multiple days of exposures at or above 60 ppb. These
estimates are of a magnitude roughly consistent with the level of
protection that was described in establishing the current standard in
2015 (PA, section 3.1).
With regard to lung function risk estimated using the population-
based E-R function approach, the estimates for children with asthma are
similar to those for all children, but with the higher end of the
ranges for the eight study areas being just slightly higher in some
cases (Table 3). For example, on average between 0.5 to 0.9% (and at
most 1.0%) of children with asthma are estimated to have at least one
day per year with a 15% (or larger) FEV1 decrement. When
considering the same decrement for all children, on average the
estimate is between 0.5 to 0.8% (and at most 0.9%). Somewhat larger
differences are seen when comparing single-day occurrences of 10% (or
larger) FEV1 decrements for the two population groups, but
again, differing by only a few tenths of a percent (e.g., at most, 3.6%
percent of children with asthma versus 3.3% of all children).
Regarding multi-day occurrences, the analyses find that very few
children are estimated to experience 15% (or larger) FEV1
decrements (i.e., on the order of a few tenths of a percent). For
example, at most 0.6% and 0.2% of all children (and children with
asthma) are estimated to
[[Page 49861]]
experience 15% (or larger) and 20% (or larger) FEV1
decrements, respectively, for two or more days, and at most, about 2.5%
of children are estimated to experience two or more days with a 10%
FEV1 decrement.
Table 3--Percent of Simulated Children and Children With Asthma Estimated To Experience at Least One or More Days per Year With a Lung Function
Decrement at or Above 10, 15 or 20% While Breathing at an Elevated Rate in Areas Just Meeting the Current Standard
--------------------------------------------------------------------------------------------------------------------------------------------------------
One or more days Two or more days Four or more days
-----------------------------------------------------------------------------------------------
Lung function decrement \A\ Average per Highest in a Average per Highest in a Average per Highest in a
year single year year single year year single year
--------------------------------------------------------------------------------------------------------------------------------------------------------
E-R Function
--------------------------------------------------------------------------------------------------------------------------------------------------------
Percent of Simulated Children With Asthma \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=20%................................................... 0.2-0.3 0.4 0.1-0.2 0.2 <0.1 \B\-0.1 0.1
>=15%................................................... 0.5-0.9 1.0 0.3-0.6 0.6 0.2-0.4 0.4
>=10%................................................... 2.3-3.3 3.6 1.5-2.4 2.6 0.9-1.7 1.8
--------------------------------------------------------------------------------------------------------------------------------------------------------
Percent of All Simulated Children \A\
--------------------------------------------------------------------------------------------------------------------------------------------------------
>=20%................................................... 0.2-0.3 0.4 0.1-0.2 0.2 <0.1-0.1 0.1
>=15%................................................... 0.5-0.8 0.9 0.3-0.5 0.6 0.2-0.4 0.4
>=10%................................................... 2.2-3.1 3.3 1.3-2.2 2.4 0.8-1.6 1.7
--------------------------------------------------------------------------------------------------------------------------------------------------------
\A\ Estimates for each urban case study area were averaged across the 3-year assessment period. Ranges reflect the ranges across urban study area
averages.
\B\ An entry of <0.1 is used to represent small, non-zero values that do not round upwards to 0.1 (i.e., <0.05).
D. Proposed Conclusions on the Primary Standard
In reaching proposed conclusions on the current O3
primary standard (presented in section II.D.3), the Administrator has
taken into account the current evidence and associated conclusions in
the ISA, in light of the policy-relevant evidence-based and exposure-
and risk-based considerations discussed in the PA (summarized in
section II.D.1), as well as advice from the CASAC, and public comment
received on the standard thus far in the review (section II.D.2). In
general, the role of the PA is to help ``bridge the gap'' between the
Agency's assessment of the current evidence and quantitative analyses
(of air quality, exposure and risk), and the judgments required of the
Administrator in determining whether it is appropriate to retain or
revise the NAAQS. Evidence-based considerations draw upon the EPA's
integrated assessment of the scientific evidence of health effects
related to O3 exposure presented in the ISA (summarized in
section II.B above) to address key policy-relevant questions in the
review. Similarly, the exposure- and risk-based considerations draw
upon our assessment of population exposure and associated risk
(summarized in section II.C above) in addressing policy-relevant
questions focused on the potential for O3 exposures
associated with respiratory effects under air quality conditions
meeting the current standard.
The approach to reviewing the primary standard is consistent with
requirements of the provisions of the CAA related to the review of the
NAAQS and with how the EPA and the courts have historically interpreted
the CAA. As discussed in section I.A above, these provisions require
the Administrator to establish primary standards that, in the
Administrator's judgment, are requisite (i.e., neither more nor less
stringent than necessary) to protect public health with an adequate
margin of safety. Consistent with the Agency's approach across all
NAAQS reviews, the EPA's approach to informing these judgments is based
on a recognition that the available health effects evidence generally
reflects a continuum that includes ambient air exposures for which
scientists generally agree that health effects are likely to occur
through lower levels at which the likelihood and magnitude of response
become increasingly uncertain. The CAA does not require the
Administrator to establish a primary standard at a zero-risk level or
at background concentration levels, but rather at a level that reduces
risk sufficiently so as to protect public health, including the health
of sensitive groups, with an adequate margin of safety.
The proposed decision on the adequacy of the current primary
standard described below is a public health policy judgment by the
Administrator that draws on the scientific evidence for health effects,
quantitative analyses of population exposures and/or health risks, and
judgments about how to consider the uncertainties and limitations that
are inherent in the scientific evidence and quantitative analyses. The
four basic elements of the NAAQS (i.e., indicator, averaging time,
form, and level) have been considered collectively in evaluating the
health protection afforded by the current standard. The Administrator's
final decision will additionally consider public comments received on
this proposed decision.
1. Evidence- and Exposure/Risk-Based Considerations in the Policy
Assessment
The main focus of the policy-relevant considerations in the PA is
consideration of the question: Does the currently available scientific
evidence- and exposure/risk-based information support or call into
question the adequacy of the protection afforded by the current primary
O3 standard? The PA response to this overarching question
takes into account discussions that address the specific policy-
relevant questions for this review, focusing first on consideration of
the evidence, as evaluated in the ISA, including that newly available
in this review, and the extent to which it alters key conclusions
supporting the current standard. The PA also considers the quantitative
exposure and risk estimates drawn from the exposure/risk analyses
(presented in detail in Appendices 3C and 3D of the PA), including
associated limitations and uncertainties, and the extent to which they
may indicate different conclusions from those in the last review
regarding the magnitude of risk,
[[Page 49862]]
as well as level of protection from adverse effects, associated with
the current standard. The PA additionally considers the key aspects of
the evidence and exposure/risk estimates that were emphasized in
establishing the current standard, as well as the associated public
health policy judgments and judgments about the uncertainties inherent
in the scientific evidence and quantitative analyses that are integral
to consideration of whether the currently available information
supports or calls into question the adequacy of the current primary
O3 standard (PA, section 3.5).
With regard to the support in the current evidence for
O3 as the indicator for photochemical oxidants, no newly
available evidence has been identified in this review regarding the
importance of photochemical oxidants other than O3 with
regard to abundance in ambient air, and potential for health
effects.\90\ As summarized in section 2.1 of the PA, O3 is
one of a group of photochemical oxidants formed by atmospheric
photochemical reactions of hydrocarbons with NOX in the
presence of sunlight, with O3 being the only photochemical
oxidant other than nitrogen dioxide that is routinely monitored in
ambient air. Data for other photochemical oxidants are generally
derived from a few focused field studies such that national-scale data
for these other oxidants are scarce (ISA, Appendix 1, section 1.1; 2013
ISA, sections 3.1 and 3.6). Moreover, few studies of the health impacts
of other photochemical oxidants beyond O3 have been
identified by literature searches conducted for the 2013 ISA or 2006
AQCD (ISA, Appendix 1, section 1.1). As stated in the ISA, ``the
primary literature evaluating the health . . . effects of photochemical
oxidants includes ozone almost exclusively as an indicator of
photochemical oxidants'' (ISA, section IS.1.1, p. IS-3). Thus, as was
the case for previous reviews, the PA finds that the evidence base for
health effects of photochemical oxidants does not indicate an
importance of any other photochemical oxidants such that O3
continues to be appropriately considered for the primary standard's
indicator.
---------------------------------------------------------------------------
\90\ Close agreement between past O3 measurements and
photochemical oxidant measurements indicated the very minor
contribution of other oxidant species in comparison to O3
(U.S. DHEW, 1970).
---------------------------------------------------------------------------
The currently available evidence on the health effects of
O3, including that newly available in this review, is
largely consistent with the conclusions reached in the last review
regarding health effects causally related to O3 exposures
(i.e., respiratory effects). Specifically, as in the last review,
respiratory effects are concluded to be causally related to short-term
exposures to O3. Also, as in the last review, the evidence
is sufficient to conclude that the relationship between longer-term
O3 exposures and respiratory effects is likely to be causal
(ISA, section IS.1.3.1, Appendix 3). Further, while a causal
determination was not made in the last review regarding metabolic
effects, the ISA for this review finds there to be sufficient evidence
to conclude there to likely be a causal relationship of short-term
O3 exposures and metabolic effects and finds the evidence to
be suggestive of, but not sufficient to infer, such a relationship
between long-term O3 exposure and metabolic effects (ISA,
section IS.1.3.1). These new determinations are based on evidence on
this category of effects, largely from experimental animal studies,
that is newly available in this review (ISA, Appendix 5). Additionally,
conclusions reached in the current review differ with regard to
cardiovascular effects and mortality, based on newly available evidence
in combination with uncertainties in the previously available evidence
that had been identified in the last review (ISA, Appendix 4, section
4.1.17 and Appendix 6, section 6.1.8). The current evidence base is
concluded to be suggestive of, but not sufficient to infer, causal
relationships between O3 exposures (short- and long-term)
and cardiovascular effects, mortality, reproductive and developmental
effects, and nervous system effects (ISA, section IS.1.3.1). As in the
last review, the strongest evidence, including with regard to
characterization of relationships between O3 exposure and
occurrence and magnitude of effects, is for respiratory effects, and
particularly for effects such as lung function decrements, respiratory
symptoms, airway responsiveness, and respiratory inflammation.
The current evidence does not alter our understanding of
populations at increased risk from health effects of O3
exposures. As in the last review, people with asthma, and particularly
children, are the at-risk population groups for which the evidence is
strongest. In addition to populations with asthma, groups with
relatively greater exposures, particularly those who spend more time
outdoors during times when ambient air concentrations of O3
are highest and while engaged in activities that result in elevated
ventilation, are recognized as at increased risk. Such groups include
outdoor workers and children. Other groups identified as at risk, and
for which the recent evidence is less clear, include older adults (in
light of changes in causality determinations, as discussed in section
II.B.2 above), and recent evidence regarding individuals with reduced
intake of certain nutrients and individuals with certain genetic
variants does not provide additional information for these groups
beyond the evidence available at the time of the last review (ISA,
section IS.4.4).
As in the last review, the most certain evidence of health effects
in humans elicited by specific O3 exposure concentrations is
provided by controlled human exposure studies (largely with generally
healthy adults). This category of short-term studies includes an
extensive evidence base of 1- to 3-hour studies, conducted with
continuous or intermittent exercise and generally involving relatively
higher exposure concentrations, e.g., greater than 120 ppb (as
summarized in the PA, Appendix 3A, Table 3A-3, based on assessments of
the studies in the 1996 and 2006 AQCDs, as well as the 2013 and current
ISA). Given the lack of ambient air concentrations of this magnitude in
areas meeting the current standard (as documented in section 2.4.1 of
the PA), the focus in reviewing the current standard continues to
primarily be on a second group of somewhat longer-duration studies of
much lower exposure concentrations. These studies employ a 6.6-hour
protocol that includes six 50-minute periods of exercise at moderate or
greater exertion.
Respiratory effects continue to be the effects for which the
experimental information regarding exposure concentrations eliciting
effects is well established, as summarized here and in section II.B.3
above. Such information allows for characterization of potential
population risk associated with O3 in ambient air under
conditions allowed by the current standard. The respiratory effects
evidence includes support from a large number of epidemiologic studies
that report positive associations of O3 with severe
respiratory health outcomes, such as asthma-related hospital admissions
and emergency department visits, coherent with findings from the
controlled human exposure and experimental animal studies. However, as
summarized in section II.B.3 above, all but a few of these short- and
long-term studies (and all U.S. studies) include areas and periods in
which O3 exceeds the current standard, making them less
useful with regard to indication of effects of exposures that would
occur with air quality allowed by the current standard.
[[Page 49863]]
Within the evidence base for the newly identified category of
metabolic effects, the evidence derives largely from experimental
animal studies of exposures appreciably higher than those for the 6.6-
hour human exposure studies along with a small number of epidemiologic
studies. The PA notes that, as discussed in section II.B.3 above, these
studies do not prove to be informative to our consideration of exposure
circumstances likely to elicit health effects.
Thus, the PA finds that the currently available evidence regarding
O3 exposures associated with health effects is largely
similar to that available at the time of the last review and does not
indicate effects attributable to exposures of shorter duration or lower
concentrations than previously understood. The 6.6-hour controlled
human exposure studies of respiratory effects remain the focus for our
consideration of exposure circumstances associated with O3
health effects. Based on these studies, the exposure concentrations
investigated range from as low as approximately 40 ppb to 120 ppb. This
information on concentrations that have been found to elicit effects
for 6.6-hour exposures while exercising is unchanged from what was
available in the last review. The lowest concentration for which lung
function decrements have been found to be statistically significantly
increased over responses to filtered air remains approximately 60 ppb
\91\ (target concentration, as average across exercise periods), at
which group mean O3-related FEV1 decrements on
the order of 2% to 3.5% have been reported (with decrements on the
order of 2% to 3% of statistically significance), with associated
individual study subject variability in decrement size; these results
were not accompanied by a statistically significant increase in
respiratory symptoms (Table 1).\92\ In the single study assessing the
next highest exposure concentration (73 ppb as the 6.6-hour average
based on study-reported measurements), the group mean FEV1
decrement was higher (6%) and was also statistically significant, as
were respiratory symptom scores, as summarized in section II.B.3 above.
At still higher exposure concentrations (80 ppb and above), the
reported incidence of both respiratory symptom scores and
O3-related lung function decrements in the study subjects is
increased and the incidence of decrements at or above 15% is larger.
Other respiratory effects, such as inflammatory response and airway
resistance, are also increased at higher exposures (ISA; 2013 ISA).
---------------------------------------------------------------------------
\91\ Two studies have assessed exposure concentrations at the
lower concentration of 40 ppb, with no statistically significant
finding of O3-related FEV1 decrement for the
group mean in either study, which is just above 1% in one study and
well below 1% in the second (Table 1).
\92\ A statistically significant, small increase in a marker of
airway inflammation was observed in one controlled human exposure
study following 6.6-hour exposures to 60 ppb (Table 1). An increase
in respiratory symptoms has not been reported with this exposure
level.
---------------------------------------------------------------------------
The PA concludes that important uncertainties identified in the
health effects evidence at the time of the last review generally remain
in the current evidence. Although the evidence clearly demonstrates
that short-term O3 exposures cause respiratory effects, as
was the case in the last review, uncertainties remain in several
aspects of our understanding of these effects. These include
uncertainties related to exposures likely to elicit effects (and the
associated severity and extent) in population groups not studied, or
less well studied (including individuals with asthma and children) and
also the severity and prevalence of responses to short (e.g., 6.6- to
8-hour) O3 exposures at and below 60 ppb. The PA
additionally recognizes uncertainties associated with the epidemiologic
studies concerning the potential influence of exposure history and co-
exposure to other pollutants (including complications of prior
population exposures) on the relationship between short-term
O3 exposure and respiratory effects. In so doing, however,
the PA notes the appreciably greater strength in the epidemiologic
evidence in its support for determination of a causal relationship for
respiratory effects than that related to other categories, such as
metabolic effects, for the current ISA newly determines there likely to
be a causal relationship with short-term O3 exposures (as
summarized in section II.B.3 above), and recognizes the greater
uncertainty with regard relationships between O3 exposures
and health effects other than respiratory effects. The array of
important areas of uncertainty related to the current health evidence,
including the evidence newly available in this review, is summarized
below.
With regard to less well studied population groups, the PA notes
that the majority of the available studies have generally involved
healthy young adult subjects, although there are some studies involving
subjects with asthma, and a limited number of studies, generally of
very short durations (i.e., less than four hours), involving
adolescents and adults older than 50 years. For example, the only
controlled human exposure study of 6.6- to 8-hour duration (7.6 hours
with quasi-continuous light exercise) conducted in people with asthma
was for an exposure concentration of 160 ppb (PA, Appendix 3A, Table
3A-2). Given a general lack of studies using subjects that have asthma,
particularly those at exposure concentrations likely to occur under
conditions meeting the current standard, uncertainties remain with
regard to characterizing the response in people with asthma while at
elevated ventilation to lower exposure concentrations, e.g., below 80
ppb. The extent to which the epidemiologic evidence, including that
newly available, can inform this specific area of uncertainty also may
be limited.\93\ As discussed in section II.B.2 above, given the effects
of asthma on the respiratory system, exposures associated with
significant respiratory responses in healthy people may pose an
increased risk of more severe responses, including asthma exacerbation,
in people with asthma. Thus, uncertainty remains with regard to the
responses of the populations, such as children with asthma, that may be
most at risk of O3-related respiratory effects (e.g.,
through an increased likelihood of severe responses, or greatest
likelihood of response) to short-term (e.g., 6.6 hr) exposures with
exercise to concentrations at or below 80 ppb.
---------------------------------------------------------------------------
\93\ Associations of health effects with O3 that are
reported in the epidemiologic analyses are based on air quality
concentration metrics used as surrogates for the actual pattern of
O3 exposures experienced by study population individuals
over the period of a particular study. Accordingly, the studies are
limited in what they can convey regarding the specific patterns of
exposure circumstances (e.g., magnitude of concentrations over
specific duration and frequency) that might be eliciting reported
health outcomes.
---------------------------------------------------------------------------
Other areas of uncertainty concerning the potential influence of
O3 exposure history and co-exposure to other pollutants on
the relationship between O3 exposures and respiratory
effects in epidemiologic studies also remain from the last review. As
in the epidemiologic evidence in the last review, there is a limited
number of studies that include copollutant analyses for a small set of
pollutants (e.g., PM or NO2). Recent studies with such
analyses suggest that observed associations between O3
concentrations and respiratory effects are independent of co-exposures
to correlated pollutants or aeroallergens (ISA, sections IS.4.3.1 and
IS.6.1; Appendix 3, sections 3.1.10.1 and 3.1.10.2). Despite the
increased prevalence of copollutant modeling in recent epidemiologic
studies, uncertainty still exists with regard to the independent effect
of O3 given the high correlations observed for some
copollutants in some studies and the small fraction of all atmospheric
[[Page 49864]]
pollutants included in these analyses (ISA, section IS.4.3.1; Appendix
2, section 2.5).
Further, although there remains uncertainty in the evidence with
regard to the potential role of exposures to O3 in eliciting
health effects other than respiratory effects, the evidence has been
strengthened since the last review with regard to metabolic effects. As
noted in section II.B.1 above, the ISA newly identifies metabolic
effects as likely to be causally related to short-term O3
exposures. The evidence supporting this relationship is limited and not
without its own uncertainties, such as the fact that the conclusion for
this relationship is based primarily on animal toxicological studies
conducted at much higher O3 concentrations than those common
in ambient air in the U.S. Only a handful of epidemiologic studies of
short-term O3 exposure and metabolic effects, with some
inconsistencies, are available, ``many of these did not control for
copollutant confounding,'' and the two U.S. studies in the group did
not find a statistically significant association (ISA, p. 5-29 and
Appendix 5, section 5.1; PA, section 3.3).
With regard to the evidence for other categories of health effects,
its support for a causal relationship with O3 in ambient air
is appreciably more uncertain. For example, as noted in section II.B.1
above, the ISA has determined the evidence to be suggestive of, but not
sufficient to infer, a causal relationship between long-term
O3 exposures and metabolic effects, and between
O3 exposures and several other categories of health effects,
including effects on the cardiovascular, reproductive and nervous
systems, and mortality (ISA, section IS.4.3).\94\ Additionally, the ISA
finds the evidence to be inadequate to determine if a causal
relationship exists with O3 and cancer (ISA, section
IS.4.3).
---------------------------------------------------------------------------
\94\ An evidence base determined to be ``suggestive of, but not
sufficient to infer, a causal relationship'' is described as
``limited, and chance, confounding, and other biases cannot be ruled
out'' (U.S. EPA, 2015, p. 23).
---------------------------------------------------------------------------
As at the time of the last review, consideration of the scientific
evidence in the current review is informed by results from a newly
performed quantitative analysis of estimated population exposure and
associated risk. The overarching PA consideration regarding these
results is whether they alter the overall conclusions from the previous
review regarding health risk associated with exposure to O3
in ambient air and associated judgments on the adequacy of public
health protection provided by the now-current standard. The
quantitative exposure and risk analyses completed in this review update
and in many ways improve upon analyses completed in the last review (as
summarized in section II.C.1 above).
The exposure and risk analyses conducted for this review, as was
true for those conducted for the last review, develop exposure and risk
estimates for study area populations of children with asthma, as well
as the populations of all children in each study area. The primary
analyses focus on exposure and risk associated with air quality that
might occur in an area under conditions that just meet the current
standard. These study areas reflect different combinations of different
types of sources of O3 precursor emissions, and also
illustrate different patterns of exposure to O3
concentrations in a populated area in the U.S. (PA, Appendix 3C,
section 3C.2). While the same conceptual air quality scenario is
simulated in all eight study areas (i.e., conditions that just meet the
existing standard), variability in emissions patterns of O3
precursors, meteorological conditions, and population characteristics
in the study areas contribute to variability in the estimated magnitude
of exposure and associated risk across study areas. In this way, the
eight areas provide a variety of examples of exposure patterns that can
be informative to the Administrator's consideration of potential
exposures and risks that may be associated with air quality conditions
occurring under the current O3 standard.
In considering the exposure and risk analyses available in this
review, the PA notes that there are a number of ways in which the
current analyses update and improve upon those available in the last
review. These include a number of improvements to input data and
modeling approaches summarized in section II.C.1 above. As in prior
reviews, exposure and risk are estimated from air quality scenarios
designed to just meet an O3 standard in all its elements.
That is, the air quality scenarios are defined by the highest design
value in the study area, which is the monitor location with the highest
3-year average of annual fourth highest daily maximum 8-hour
O3 concentrations (e.g., equal to 70 ppb for the current
standard scenario). The current risk and exposure analyses include air
quality simulations based on more recent ambient air quality data that
include O3 concentrations closer to the current standard
than was the case for the development of the air quality scenarios in
the last review. As a result of this and the use of updated
photochemical modeling, there is reduced uncertainty associated with
the spatial and temporal patterns of O3 concentrations that
define these scenarios across all eight study areas. Additionally, the
approach for deriving population exposure estimates, both for
comparison to benchmark concentrations and for use in deriving lung
function risk using the E-R function approach, has been modified to
provide for a better match of the simulated population exposure
estimates with the 6.6-hour duration of the controlled human exposure
studies and with the study subject ventilation rates. Together, these
differences, as well as a variety of updates to model inputs, are
believed to reduce uncertainty associated with interpretation of the
analysis results.
The PA also notes the array of air quality and exposure
circumstances represented by the eight study areas. As summarized in
section II.C.1 above, the areas fall into seven of the nine climate
regions in the continental U.S. The population sizes of the associated
metropolitan areas range in size from approximately 2.4 to 8 million
and vary in population demographic characteristics. While there are
uncertainties and limitations associated with the exposure and risk
estimates, as noted in II.C.2, the PA considers the factors recognized
here to contribute to their usefulness in informing the current review.
The PA gives primary attention to results for the comparison-to-
benchmarks analysis in recognition of the relatively lesser uncertainty
of these results (than the lung function risk estimates), and also of
the broader characterization of respiratory effects that they can
inform, as noted in section II.C above. Similarly, the results for this
risk metric also received greater emphasis in the last review and were
a focus in establishing the current standard in 2015. The estimates
across all study areas from the current review are generally similar to
those reported across all study areas assessed in the last review,
particularly for estimates for two or more occurrences at or above a
benchmark, and for the 80 ppb benchmark (Table 4). For consistency with
the estimates highlighted in the 2015 review (e.g., 80 FR 65313-65315,
October 26, 2015), the PA comparison, summarized in Table 4 below,
focuses on the simulated population of all children. We additionally
note, however, the similarity of the estimates for all children to the
estimates for the simulated population of children with asthma (Table
2). For example, for urban study areas with air quality that just meets
the current standard, as many as 0.7% of children with asthma, on
[[Page 49865]]
average across the 3-year period, and up to 1.0% in a single year might
be expected to experience, while at elevated exertion, at least one day
with a 7-hour average O3 exposure concentration at or above
70 ppb (Table 2). The corresponding estimates for the simulated
population of all children are as many as 0.6% of all children, on
average across the 3-year period, and up to 0.9% in a single year
(Table 2). For the benchmark concentration of 80 ppb (which reflects
the potential for more severe effects), a much lower percentage (0.1%)
of children with asthma, on average across the 3-year period or in any
single year (compared to less than 0.1% on average and as many as 0.1%
in a single year for all children), might be expected to experience,
while at elevated exertion, at least one day with such a concentration
(Table 2). Regarding estimates for multiple days, the percent of
children with asthma (as well as the percent of all children) estimated
to experience two or more days with an exposure at or above 70 ppb is
less than 0.1%, on average across three years, and up to 0.1% in a
single year period. There are no children estimated to experience more
than a single day per year with a 7-hour average O3
concentration at or above 80 ppb. With regard to the lowest benchmark
concentration of 60 ppb, the percentages for the simulated population
of children with asthma for more than a single day occurrence are 3%,
on average across the three years, and just below 5% in a single year
period, with just slightly lower percentages (2.9 and 4.3%) for the
population of all children (Table 2).
The PA additionally compares the estimates derived in the current
analyses with those from the 2014 HREA in the last review, finding them
to be quite similar.\95\ For example, with regard to the 80 ppb
benchmark and air quality conditions just meeting the current standard,
the percentage of children estimated to experience a day or more with
such an exposure, ranges from zero (in both assessments) up to 0.1%
(2014 HREA) and a nonzero value less than 0.1% (current assessment), on
average across the three year period (Table 4). The estimates for the
highest year (0.2 and 0.1%, for the 2014 and current assessments,
respectively) are within 0.1% of each other. Both assessments estimate
zero children to experience two or more days with an exposure at or
above 80 ppb. The differences observed, which are particularly evident
for the lower benchmarks and in the estimates for the highest year, are
generally slight. Much larger differences are seen in comparing
different air quality scenario results for the same benchmark. For
example, for the 70 ppb benchmark, the differences between the 75 ppb
scenario and the current standard (or between the 65 ppb scenario and
the current standard) in either assessment are appreciably larger than
are the slight differences observed between the two assessments for any
air quality scenario. The factors likely contributing to the slight
differences, e.g., for the lowest benchmark, include greater variation
in ambient air concentrations in some of the study areas in the 2014
HREA, as well as the lesser air quality adjustments required in study
areas for the current assessment due to closer proximity of conditions
to meeting the current standard (70 ppb).\96\ Other important
differences between the two assessments are the updates made to the
ventilation rates used for identifying when a simulated individual is
at moderate or greater exertion and the use of 7 hours for the exposure
duration. Both of these changes were made to provide closer linkages to
the conditions of the controlled human exposure studies which are the
basis for the benchmark concentrations. Thus, the PA recognizes there
to be reduced uncertainty associated with the current estimates.
---------------------------------------------------------------------------
\95\ In this comparison, the PA focuses on the full array of
study areas assessed in each analysis given the purpose of each in
providing estimates across a range of study areas to inform decision
making with regard to the exposures and risks that may occur across
the U.S. in areas that just meet the current standard.
\96\ The 2014 HREA air quality scenarios involved adjusting
2006-2010 ambient air concentrations, and some study areas had
design values in that time period that were well above the then-
existing standard (and more so for the current standard). Study
areas included the current exposure analysis had 2015-2017 design
values close to the current standard, requiring less of an
adjustment for the current standard (70 ppb) air quality scenario.
Table 4--Comparison of Current Assessment and 2014 HREA (All Study Areas) for Percent of Children Estimated To
Experience at Least One, or Two, Days With an Exposure at or Above Benchmarks While at Moderate or Greater
Exertion
----------------------------------------------------------------------------------------------------------------
Estimated average % of simulated Estimated average % of simulated
children with at least one day per children with at least two days
year at or above benchmark per year at or above benchmark
Air quality scenario (DV, ppb) (highest in single season) (highest in single season)
-----------------------------------------------------------------------
Current PA \A\ 2014 HREA \B\ Current PA \A\ 2014 HREA \B\
----------------------------------------------------------------------------------------------------------------
Benchmark Exposure Concentration of 80 ppb
----------------------------------------------------------------------------------------------------------------
75...................................... <0.1 \A\-0.3 0-0.3 (1.1) 0-<0.1 (<0.1) 0 (0.1)
(0.6)
70...................................... 0-<0.1 (0.1) 0-0.1 (0.2) 0 (0) 0 (0)
65...................................... 0-<0.1 (<0.1) 0 (0) 0 (0) 0 (0)
----------------------------------------------------------------------------------------------------------------
Benchmark Exposure Concentration of 70 ppb
----------------------------------------------------------------------------------------------------------------
75...................................... 1.1-2.0 (3.4) 0.6-3.3 (8.1) 0.1-0.3 (0.7) 0.1-0.6 (2.2)
70...................................... 0.2-0.6 (0.9) 0.1-1.2 (3.2) <0.1 (0.1) 0-0.1 (0.4)
65...................................... 0-0.2 (0.2) 0-0.2 (0.5) 0-<0.1 (<0.1) 0 (0)
----------------------------------------------------------------------------------------------------------------
Benchmark Exposure Concentration of 60 ppb
----------------------------------------------------------------------------------------------------------------
75...................................... 6.6-15.7 (17.9) 9.5-17.0 (25.8) 1.7-8.0 (9.9) 3.1-7.6 (14.4)
70...................................... 3.2-8.2 (10.6) 3.3-10.2 (18.9) 0.6-2.9 (4.3) 0.5-3.5 (9.2)
[[Page 49866]]
65...................................... 0.4-2.3 (3.7) 0-4.2 (9.5) <0.1-0.3 (0.5) 0-0.8 (2.8)
----------------------------------------------------------------------------------------------------------------
\A\ For the current analysis, calculated percent is rounded to the nearest tenth decimal using conventional
rounding. Values equal to zero are designated by ``0'' (there are no individuals exposed at that level).
Small, non-zero values that do not round upwards to 0.1 (i.e., <0.05) are given a value of ``<0.1''.
\B\ For the 2014 HREA. calculated percent was rounded to the nearest tenth decimal using conventional rounding.
Values that did not round upwards to 0.1 (i.e., <0.05) were given a value of ``0''.
Overall, the comparison-to-benchmarks estimates are generally
similar to those which were the focus in the 2015 decision on
establishing the current standard. For example, in the 2015 decision to
set the standard level at 70 ppb, the Administrator took note of
several findings for the air quality scenarios for this level, noting
that ``a revised standard with a level of 70 ppb is estimated to
eliminate the occurrence of two or more exposures of concern to
O3 concentrations at or above 80 ppb and to virtually
eliminate the occurrence of two or more exposures of concern to
O3 concentrations at or above 70 ppb for all children and
children with asthma, even in the worst-case year and location
evaluated'' (80 FR 65363, October 26, 2015). This statement remains
true for the results of the current assessment (Table 4). With regard
to the 60 ppb benchmark, for which the 2015 decision placed relatively
greater weight on multiple (versus single) occurrences of exposures at
or above it, the Administrator at that time noted the 2014 HREA
estimates for the 70 ppb air quality scenario that estimated 0.5 to
3.5% of children to experience multiple such occurrences on average
across the study areas, stating that the now-current standard ``is
estimated to protect the vast majority of children in urban study areas
. . . from experiencing two or more exposures of concern at or above 60
ppb'' (80 FR 65364, October 26, 2015). The corresponding estimates, on
average across the 3-year period in the current assessments, are
remarkably similar at 0.6 to 2.9% (Table 4).
In considering the public health implications of the estimated
occurrence of exposures of different magnitudes, the PA considers the
magnitude or severity of the effects associated with the estimated
exposures as well as their adversity, the size of the population
estimated to experience exposures associated with such effects, as well
as consideration for such implications in previous NAAQS decisions and
ATS policy statements (as summarized in section II.B.2 above). As an
initial matter, the PA considers the severity of responses associated
with the exposure and risk estimates, taking note of the health effects
evidence for the different benchmark concentrations and judgments made
with regard to the severity of these effects in the last review. As in
the last review, the PA recognizes the greater prevalence of more
severe lung function decrements among study subjects exposed to 80 ppb
or higher concentrations compared to 60 or 70 ppb exposure
concentrations, as well as the prevalence of other effects such as
respiratory symptoms. In so doing, the PA notes that such exposures are
appropriately considered to be associated with adverse respiratory
effects consistent with past and recent ATS position statements.
Studies of 6.6-hour controlled human exposures, with quasi-continuous
exercise, to the lowest benchmark concentration of 60 ppb have found
small but statistically significant O3-related decrements in
lung function (specifically reduced FEV1) and airway
inflammation. Somewhat above 70 ppb,\97\ statistically significant
increases in lung function decrements, of a somewhat greater magnitude
(e.g., approximately 6% increase, as study group average, versus 2 to
3% [Table 1]), and respiratory symptoms have been reported, which has
led to characterization of these exposure conditions as also being
associated with adverse responses, consistent with past ATS statements
as summarized in section II.B.1 above (e.g., 80 FR 65343, 65345,
October 26, 2015).
---------------------------------------------------------------------------
\97\ As noted in sections II.A.1 and II.B.3 above, the 70 ppb
target exposure concentration comes from Schelegle et al. (2009).
That study reported, based on O3 measurements during the
six 50-minute exercise periods, that the mean O3
concentration during the exercise portion of the study protocol was
72 ppb. Based on the measurements for the six exercise periods, the
time weighted average concentration across the full 6.6-hour
exposure was 73 ppb (Schelegle et al., 2009).
---------------------------------------------------------------------------
The PA additionally takes note of the greater significance of
estimates for multiple occurrences of exposures at or above these
benchmarks consistent with the evidence, as has been recognized in
multiple past O3 NAAQS reviews. The role of such a
consideration has also differed across the three benchmarks. More
specifically, while estimates of one or more exposures at or above the
higher benchmark concentrations (70 ppb and 80 ppb) was an important
consideration in the decision on the current standard, estimates of
multiple exposures at or above the lowest benchmark concentration of 60
ppb were given greater weight than estimates for one or more such
exposures. More specifically, in the 2015 decision leading to
establishment of the current standard, a greater emphasis on protection
against multiple (versus single) occurrences of exposures at or above
60 ppb last was based in part on a recognition of the lesser severity
of the effects at this exposure level in combination with the
recognition that for effects such as inflammation (even when occurring
to a small extent). This greater emphasis reflected a recognition that,
while isolated occurrences can resolve entirely, repeated occurrences
from repeated exposure could potentially result in more severe effects
(2013 ISA, section 6.2.3 and p. 6-76). Additionally, while even
multiple occurrences of such effects of lesser severity to otherwise
healthy individuals may not result in severe effects, they may
contribute to more important effects in individuals with compromised
respiratory function, such as those with asthma. The ascribing of
greater significance to repeated occurrences of exposures of potential
concern is also consistent with public
[[Page 49867]]
health judgments in NAAQS reviews for other pollutants, such as sulfur
oxides and CO (84 FR 9900, March 18, 2019; 76 FR 54307, August 31,
2011).
As in the last review, while the exposure-based analyses include
two types of metrics, the quantitative exposure and risk analyses
results in which the PA expresses the greatest confidence are estimates
from the comparison-to-benchmarks analysis, as discussed in section
II.C above. In light of the conclusions that people with asthma and
children are at-risk populations for O3-related health
effects (summarized in section II.B.2 above) and the exposure and risk
analysis findings of higher exposures and risks for children (in terms
of percent of that population), the PA focused its consideration of the
analysis results on children (and also specifically children with
asthma). The exposure and risk estimates indicate that in some areas of
the U.S. where O3 concentrations just meet the current
standard, on average across the 3-year period simulated, less than 1%,
and less than 0.1% of the simulated population of children with asthma
might be expected to experience a single day per year with a maximum 7-
hour exposure at or above 70 ppb and 80 ppb, respectively, while
breathing at an elevated rate (Table 2). With regard to the lowest
benchmark considered (60 ppb), the corresponding percentage is less
than approximately 9%, on average across the 3-year period (Table 2).
The corresponding estimates for the 75 ppb air quality scenario are
notably higher, e.g., 1.1 to 2.1% of children with asthma, on average
across the 3-year design period, for the 70 ppb benchmark, with as many
as 3.9% in a single year (PA, Table 3-5). The estimates for the 65 ppb
scenario are appreciably lower (PA, Table 3-5).
While recognizing greater uncertainty and accordingly less
confidence in the lung function risk estimates, the PA noted the
results based on the E-R model that estimated 0.2 to 0.3% of children
with asthma, on average across the 3-year design period are estimated
to experience one or more days with a lung function decrement at or
above 20%, and 0.5 to 0.9% to experience one or more days with a
decrement at or above 15% (Table 3). In a single year, the highest
estimate is 1.0% of this at-risk population expected to experience one
or more days with a decrement at or above 15%. The corresponding
estimate for two or more days is 0.6% (Table 3).
As summarized in section II.B.2 above, the size of the at-risk
population (people with asthma, particularly children) in the U.S. is
substantial. Nearly 8% of the total U.S. population and 8.4% of U.S.
children have asthma.\98\ The asthma prevalence in U.S. child
populations (younger than 18 years) of different races or ethnicities
ranges from 6.2% for Hispanic, Mexican or Mexican-American children to
12.6% for black non-Hispanic children (PA, Table 3-1). This is well
reflected in the exposure and risk analysis study areas in which the
asthma prevalence ranged from 7.7% to 11.2% of the total populations
and 9.2% to 12.3% of the children. In each study area, the prevalence
varies among census tracts, with the highest tract having a prevalence
in boys of 25.5% and a prevalence in girls of 17.1% (PA, Appendix 3D,
Table 3D-3).
---------------------------------------------------------------------------
\98\ The number of people in the US with asthma is estimated to
be about 25 million. As shown in the PA, Table 3-1 the estimated
number of people with asthma was 25,191,000 in 2017. The updated
estimate from the 2018 National Health Interview Survey is
24,753,000 (CDC, 2020). For children (younger than 18 years), the
2017 estimate is approximately 6,182,000, while the estimate for
2018 is slightly lower at 5,530,131 (PA, Table 3-1).
---------------------------------------------------------------------------
The exposure and risk analyses inherently recognize that
variability in human activity patterns (where people go and what they
do) is key to understanding the magnitude, duration, pattern, and
frequency of population exposures. For O3 in particular, the
amount and frequency of afternoon time outdoors at moderate or greater
exertion is an important factor for understanding the fraction of the
population that might experience O3 exposures that have
elicited respiratory effects in experimental studies (2014 HREA,
section 5.4.2). In considering the available information regarding
prevalence of behavior (time outdoors and exertion levels) and daily
temporal pattern of O3 concentrations, the PA notes the
findings of evaluations of the data in the CHAD. Based on these
evaluations of human activity pattern data, it appears that children
and adults both, for days having some time spent outdoors spend, on
average, about 2 hours of afternoon time outdoors per day, but differ
substantially in their participation in these events at elevated
exertion levels (rates of about 80% versus 60%, respectively) (2014
HREA, section 5.4.1.5), indicating children are more likely to
experience exposures that may be of concern. This is one basis for
their identification as an at-risk population for O3-related
health effects. The human activity pattern evaluations have also shown
there is little to no difference in the amount or frequency of
afternoon time outdoors at moderate or greater exertion for people with
asthma compared with those who do not have asthma (2014 HREA, section
5.4.1.5). Further, recent CHAD analyses indicate that while 46-73% of
people do not spend any afternoon time outdoors at moderate or greater
exertion, a fraction of the population (i.e., between 5.5-6.8% of
children) spend more than 4 hours per day outdoors at moderate or
greater exertion and may have greater potential to experience exposure
events of concern than adults (PA, Appendix 3D, section 3D.2.5.3 and
Figure 3D-9). It is this potential that contributes importance to
consideration of the exposure and risk estimates.
In considering the public health implications of the exposure and
risk estimates across the eight study areas, the PA notes that the
purpose for the study areas is to illustrate exposure circumstances
that may occur in areas that just meet the current standard, and not to
estimate exposure and risk associated with conditions occurring in
those specific locations today. To the extent that concentrations in
the specific areas simulated may differ from others across the U.S.,
the exposure and risk estimates for these areas are informative to
consideration of potential exposures and risks in areas existing across
the U.S. that have air quality and population characteristics similar
to the study areas assessed, and that have ambient concentrations of
O3 that just meet the current standard today or that will be
reduced to do so at some period in the future. We note that numerous
areas across the U.S. have air quality for O3 that is near
or above the existing standard.\99\ Thus, the air quality and exposure
circumstances assessed in the eight study areas are of particular
importance in considering whether the currently available information
calls into question the adequacy of public health protection afforded
by the current standard.
---------------------------------------------------------------------------
\99\ Based on the most recently available data from 2016-2018,
142 counties have O3 concentrations that exceed the
current standard. Population size in these counties ranges from
approximately 20,000 to more than ten million, with a total
population of over 112 million living in counties that exceed the
current standard. Air quality data are from Table 4. Monitor Status
in the Excel file named
ozone_designvalues_20162018_final_06_28_19.xlsx downloaded from
https://www.epa.gov/air-trends/air-quality-design-values. Population
sizes are based on 2017 estimates from the U.S. Census Bureau
(https://www.census.gov/programs-surveys/popest.html).
---------------------------------------------------------------------------
The exposure and risk estimates for the study areas assessed for
this review reflect differences in exposure circumstances among those
areas and illustrate the exposures and risks that might be expected to
occur in other areas with such circumstances under air quality
conditions that just meet the current standard (or the alternate
[[Page 49868]]
conditions assessed). Thus, the exposure and risk estimates indicate
the magnitude of exposure and risk that might be expected in many areas
of the U.S. with O3 concentrations at or near the current
standard. Although the methodologies and data used to estimate
population exposure and lung function risk in this review differ in
several ways from what was used in the last review, the findings and
considerations summarized here present a pattern of exposure and risk
that is generally similar to that considered in the last review (as
described above), and indicate a level of protection from respiratory
effects that is generally consistent with that described in the 2015
decision.
Collectively, the PA finds that the evidence and exposure and risk-
based considerations provide the basis for its conclusion that
consideration should be given to retaining the current primary
standard, without revision (PA, section 3.5.4). Accordingly, and in
light of this conclusion that it is appropriate to consider the current
primary standard to be adequate, the PA did not identify any potential
alternative primary standards for consideration in this review (PA,
section 3.5.4). In reaching these conclusions, the PA additionally
notes that considerations raised in the PA are important to conclusions
and judgments to be made by the Administrator concerning the public
health significance of the evidence and of the exposure and risk
estimates. Such judgments that are common to NAAQS decisions include
those related to public health implications of effects of differing
severity (75 FR 355260 and 35536, June 22, 2010; 76 FR 54308, August
31, 2011; 80 FR 65292, October 26, 2015). Such judgments also include
those concerning the public health significance of effects at exposures
for which evidence is limited or lacking, such as effects at the lower
benchmark concentrations considered and lung function risk estimates
associated with exposure concentrations lower than those tested or for
population groups not included in the controlled exposure studies. The
PA recognizes that such public health policy judgments will weigh in
the Administrator's decision in this review with regard to the adequacy
of protection afforded by the current standard.
2. CASAC Advice
The CASAC has provided advice on the adequacy of the current
primary O3 standard in the context of its review of the
draft PA.\100\ In this context, the CASAC agreed with the draft PA
findings that the evidence newly available in this review does not
substantially differ from that available in the 2015 review, stating
that, ``[t]he CASAC agrees that the evidence newly available in this
review that is relevant to setting the ozone standard does not
substantially differ from that of the 2015 Ozone NAAQS review'' (Cox,
2020a, p. 12 of the Consensus Responses). With regard to the adequacy
of the current standard, views of individual CASAC members differed.
Part of the CASAC ``agree with the EPA that the available evidence does
not call into question the adequacy of protection provided by the
current standard, and thus support retaining the current primary
standard'' (Cox, 2020a, p. 1 of letter). Another part of the CASAC
indicated its agreement with the previous CASAC's advice, based on
review of the 2014 draft PA, that a primary standard with a level of 70
ppb may not be protective of public health with an adequate margin of
safety, including for children with asthma (Cox, 2020a, p. 1 of letter
and p. 12 of the enclosed Consensus Responses).\101\ Additional
comments from the CASAC in the ``Consensus Responses to Charge
Questions'' on the draft PA attached to the CASAC letter provide
recommendations on improving the presentation of the information on
health effects and exposure and risk estimates in completing the final
PA. The EPA considered these comments in completing the PA and in
presentations of the information in prior sections of this proposal
document.
---------------------------------------------------------------------------
\100\ A limited number of public comments have also been
received in this review to date, including comments focused on the
draft IRP or draft PA. Of the public comment that addressed adequacy
of the current primary O3 standard, some expressed
agreement with staff conclusions in the draft PA, while others
expressed the view that the standard should be more restrictive. In
support of this latter view, commenters largely cited advice from,
and considerations raised by, the previous CASAC in the last review
regarding adequacy of the margin of safety.
\101\ In the last review, the advice from the prior CASAC
included a range of recommended levels for the standard, with the
CASAC concluding that ``there is adequate scientific evidence to
recommend a range of levels for a revised primary ozone standard
from 70 ppb to 60 ppb'' (Frey, 2014, p. ii). In so doing, the prior
CASAC noted that ``[i]n reaching its scientific judgment regarding a
recommended range of levels for a revised ozone primary standard,
the CASAC focused on the scientific evidence that identifies the
type and extent of adverse effects on public health'' and further
acknowledged ``that the choice of a level within the range
recommended based on scientific evidence is a policy judgment under
the statutory mandate of the Clean Air Act'' (Frey, 2014, p. ii).
The prior CASAC then described that its ``policy advice [emphasis
added] is to set the level of the standard lower than 70 ppb within
a range down to 60 ppb, taking into account [the Administrator's]
judgment regarding the desired margin of safety to protect public
health, and taking into account that lower levels will provide
incrementally greater margins of safety'' (Frey, 2014, p. ii).
---------------------------------------------------------------------------
The comments from the CASAC also took note of uncertainties that
remain in this review of the primary standard and identified a number
of additional areas for future research and data gathering that would
inform the next review of the primary O3 NAAQS (Cox, 2020a,
p. 14 of the Consensus Responses).
3. Administrator's Proposed Conclusions
Based on the large body of evidence concerning the health effects
and potential public health impacts of exposure to O3 in
ambient air, and taking into consideration the attendant uncertainties
and limitations of the evidence, the Administrator proposes to conclude
that the current primary O3 standard provides the requisite
protection of public health, including an adequate margin of safety,
and should therefore be retained, without revision. In reaching these
proposed conclusions, the Administrator has carefully considered the
assessment of the available health effects evidence and conclusions
contained in the ISA; the evaluation of policy-relevant aspects of the
evidence and quantitative analyses in the PA (summarized in section
II.D.1 above); the advice and recommendations from the CASAC
(summarized in section II.D.2 above); and public comments received to
date in this review.
In the discussion below, the Administrator considers first the
evidence base on health effects associated with exposure to
photochemical oxidants, including O3, in ambient air. In so
doing, he considers that health effects evidence newly available in
this review, and the extent to which it alters key scientific
conclusions in the last review. The Administrator additionally
considers the quantitative exposure and risk estimates developed in
this review, including associated limitations and uncertainties, and
what they indicate regarding the magnitude of risk, as well as level of
protection from adverse effects, associated with the current standard.
Further, the Administrator considers the key aspects of the evidence
and exposure/risk estimates emphasized in establishing the current
standard. He additionally considers uncertainties in the evidence and
the exposure/risk information, as a part of public health judgments
that are essential and integral to his decision on the adequacy of
protection provided by the standard, similar to the judgments made in
establishing the current
[[Page 49869]]
standard. Such judgments include public health policy judgments and
judgments about the uncertainties inherent in the scientific evidence
and quantitative analyses. The Administrator draws on the PA
considerations, and PA conclusions in the current review, taking note
of key aspects of the rationale presented for those conclusions.
Further, the Administrator considers the advice and conclusions of the
CASAC, including particularly its overall agreement that the currently
available evidence does not substantially differ from that which was
available in the 2015 review when the current standard was established.
With attention to such factors as these, the Administrator considers
the information currently available in this review with regard to the
adequacy and appropriateness of the protection provided by the current
standard.
As an initial matter, the Administrator recognizes the continued
support in the current evidence for O3 as the indicator for
photochemical oxidants (as recognized in section II.D.1 above). He
takes note of the PA conclusion that no newly available evidence has
been identified in this review regarding the importance of
photochemical oxidants other than O3 with regard to
abundance in ambient air, and potential for health effects, and of the
ISA observation that ``the primary literature evaluating the health and
ecological effects of photochemical oxidants includes ozone almost
exclusively as an indicator of photochemical oxidants'' (ISA, p. IS-3).
Accordingly, the information relating health effects to photochemical
oxidants in ambient air is also focused on O3. Thus, he
proposes to conclude it is appropriate for O3 to continue to
be the indicator for the primary standard for photochemical oxidants.
With regard to the extensive evidence base for health effects of
O3, the Administrator gives particular attention to the
longstanding evidence of respiratory effects causally related to short-
term O3 exposures. This array of effects, and the underlying
evidence base, was integral to the basis for setting the current
standard. The Administrator takes note of the ISA conclusion that this
evidence base of studies on O3 exposure and respiratory
health is the ``strongest evidence for health effects due to ozone
exposure'' (ISA p. IS-8). While the overall health effects evidence
base has been augmented somewhat since the time of the last review, the
Administrator notes that, as summarized in section II.B.1 above, the
newly available evidence does not lead to different conclusions
regarding the respiratory effects of O3 in ambient air or
regarding exposure concentrations associated with those effects; nor
does it identify different populations at risk of O3-related
effects, than in the last review.
The Administrator recognizes that this strong evidence base
continues to demonstrate a causal relationship between short-term
O3 exposures and respiratory effects, including in people
with asthma. He also recognizes that the strongest and most certain
evidence for this conclusion, as in the last review, is that from
controlled human exposure studies that report an array of respiratory
effects in study subjects (largely generally healthy adults) engaged in
quasi-continuous or intermittent exercise. He additionally notes the
supporting experimental animal and epidemiologic evidence, including
the epidemiologic studies reporting positive associations for asthma-
related hospital admissions and emergency department visits, which are
strongest for children, with short-term O3 exposures. The
Administrator also notes the ISA conclusion that the relationship
between long-term exposures and respiratory effects is likely to be
causal, a conclusion that is consistent with the conclusion in the last
review and that reflects a general similarity in the underlying
evidence base.
With regard to populations at increased risk of O3-
related health effects, the Administrator notes the populations and
lifestages identified in the ISA and summarized in section II.B.2
above. In so doing, he takes note of the longstanding and robust
evidence that supports identification of people with asthma as being at
increased risk of O3 related respiratory effects, including
specifically asthma exacerbation and associated health outcomes, and
also children, particularly due to their generally greater time
outdoors while at elevated exertion (PA, section 3.3.2; ISA, sections
IS.4.3.1, IS.4.4.3.1, and IS.4.4.4.1, Appendix 3, section 3.1.11). This
tendency of children to spend more time outdoors while at elevated
exertion than other age groups, including in the summer when
O3 levels may be higher, makes them more likely to be
exposed to O3 in ambient air under conditions contributing
to increased dose due to greater air volumes taken into the lungs (2013
ISA, section 5.2.2.7). These factors and the strong evidence (briefly
summarized in section II.B.2 above, and section 3.3.2 of the PA, based
on evidence described in detail in the ISA), indicate people with
asthma, including children, to be at increased risk of O3
related respiratory effects, including specifically asthma exacerbation
and associated health outcomes. Based on these considerations, the
Administrator proposes to conclude it is appropriate to give particular
focus to people with asthma and children, population groups for which
the evidence of increased risk is strongest, in evaluating whether the
current standard provides requisite protection. He proposes to judge
that such a focus will also provide protection of other population
groups, identified in the ISA, for which the current evidence is less
robust and clear as to the extent and type of any increased risk, and
the exposure circumstances that may contribute to it.
With regard to ISA conclusions that differ from those in the last
review, the Administrator recognizes the new conclusions regarding
metabolic effects, cardiovascular effects and mortality (as summarized
in section II.B.1 above; ISA, Table ES-1). As an initial matter, he
takes note of the fact that while the 2013 ISA considered the evidence
available in the last review sufficient to conclude that the
relationships for short-term O3 exposure with cardiovascular
effects and mortality were likely to be causal, that conclusion is not
supported by the now more expansive evidence base which the ISA now
determines to be suggestive of, but not sufficient to infer, a causal
relationship for these health effect categories. Further, the
Administrator recognizes the new ISA determination that the
relationship between short-term O3 exposure and metabolic
effects is likely to be causal. In so doing, he takes note that the
basis for this conclusion is largely experimental animal studies in
which the exposure concentrations were well above those in the
controlled human exposure studies for respiratory effects as well as
above those likely to occur in areas of the U.S. that meet the current
standard (as summarized in section II.B.3 and II.D.1 above). Thus,
while recognizing the ISA's conclusion regarding this potential hazard
of O3, he also recognizes that the evidence base is largely
focused on circumstances of elevated concentrations above those
occurring in areas that meet the current standard. In light of these
considerations, he proposes to judge the current standard to be
protective of such circumstances leading him to continue to focus on
respiratory effects in evaluating whether the current standard provides
requisite protection.
With regard to exposures of interest for respiratory effects, the
Administrator notes the 6.6 hour controlled human exposure studies
involving exposure,
[[Page 49870]]
with quasi-continuous exercise,\102\ to concentrations ranging from as
low as approximately 40 ppb to 120 ppb (as considered in the PA, and
summarized in sections II.B.3 and II.D.1 above). He also notes that, as
in the last review, these studies, and particularly those that examine
exposures from 60 to 80 ppb, are the primary focus of the PA
consideration of exposure circumstances associated with O3
health effects important to Administrator judgments regarding the
adequacy of the current standard. The Administrator further recognizes
that this information on exposure concentrations that have been found
to elicit effects in exercising study subjects is unchanged from what
was available in the last review. With regard to the epidemiologic
studies, the Administrator recognizes that while, as a whole, these
investigations of associations between O3 and respiratory
effects and health outcomes (e.g., asthma-related hospital admission
and emergency department visits) provide strong support for the
conclusions of causality (as summarized in section II.B.1 above), these
studies are less useful for his consideration of the potential for
O3 exposures associated with air quality conditions allowed
by the current standard to contribute to such health outcomes. The
Administrator takes note of the PA conclusions in this regard,
including the scarcity of U.S. studies conducted in locations in which
and during time periods when the current standard would have been met
(as summarized in sections II.B.3 and II.D.1 above).\103\ He also
recognizes the additional considerations raised in the PA and
summarized in section II.B.3 above regarding information on exposure
concentrations in these studies during times and locations that would
not have met the current standard, and also including considerations
such as complications in disentangling specific O3 exposures
that may be eliciting effects (PA, section 3.3.3; ISA, p. IS-86 to IS-
88). While he notes that such considerations do not lessen their
importance in the evidence base documenting the causal relationship
between O3 and respiratory effects, he concurs with the PA
that these studies are less informative in considering O3
exposure concentrations occurring under air quality conditions allowed
by the current standard. Thus, the Administrator does not find the
available epidemiologic studies to provide insights regarding exposure
concentrations associated with health outcomes that might be expected
under air quality conditions that meet the current standard. In
consideration of this evidence from controlled human exposure and
epidemiologic studies, as assessed in the ISA and summarized in the PA,
the Administrator notes that the evidence base in this review does not
include new evidence of respiratory effects associated with appreciably
different exposure circumstances than the evidence available in the
last review, including particularly any circumstances that would also
be expected to be associated with air quality conditions likely to
occur under the current standard. In light of these considerations, he
finds it appropriate to give particular focus to the studies of 6.6-
hour exposures with quasi-continuous exercise to concentrations
generally ranging from 60 to 80 ppb.
---------------------------------------------------------------------------
\102\ These studies employ a 6.6-hour protocol that includes six
50-minute periods of exercise at moderate or greater exertion.
\103\ Among the epidemiologic studies finding a statistically
significant positive relationship of short- or long-term
O3 concentrations with respiratory effects, there are no
single-city studies conducted in the U.S. in locations with ambient
air O3 concentrations that would have met the current
standard for the entire duration of the study. Nor is there a U.S.
multicity study for which all cities met the standard for the entire
study period. The extent to which reported associations with health
outcomes in the resident populations in these studies are influenced
by the periods of higher concentrations during times that did not
meet the current standard is unknown. These and additional
considerations are summarized in section II.B.3 above and in the PA.
---------------------------------------------------------------------------
With regard to these 6.6-hour controlled human exposure studies,
although two such studies have assessed exposures at the lower
concentration of 40 ppb, statistically significant responses have not
been reported from those exposures. Studies at the next highest
concentration studied (a 60 ppb target) have reported decrements in
lung function (assessed by FEV1) that are statistically
significantly increased over the decrements occurring with filtered
air, with group mean O3-related decrements on the order of 2
to 3% (and associated individual study subject variability in decrement
size). A statistically significant, small increase in a marker of
airway inflammation has also been reported in one of these 60 ppb
studies. Exposure with the same study protocol to a concentration
slightly above 70 ppb (73 ppb as the 6.6-hour average and 72 ppb as the
exercise period average, based on study-reported measurements) has been
reported to elicit statistically significant increases in both lung
function decrements (group mean of 6%) and respiratory symptom scores,
as summarized in section II.B.3 above. Further increases in
O3-related lung function decrements and respiratory symptom
scores, as well as inflammatory response and airway responsiveness, are
reported for exposure concentrations of 80 ppb and higher (ISA; 2013
ISA; 2006 AQCD).
In this review, as in the last review, the Administrator recognizes
some uncertainty, reflecting limitations in the evidence base, with
regard to the exposure levels eliciting effects (as well as the
severity of the effects) in some population groups not included in the
available controlled human exposure studies, such as children and
individuals with asthma. In so doing, the Administrator recognizes that
the controlled human exposure studies, primarily conducted in healthy
adults, on which the depth of our understanding of O3-
related health effects is based, provide limited, but nonetheless
important information with regard to responses in people with asthma or
in children. Additionally, some aspects of our understanding continue
to be limited; among these aspects are the risk posed to these less
studied population groups by 7-hour exposures with exercise to
concentrations as low as 60 ppb that are estimated in the exposure
analyses. Collectively, these aspects of the evidence and associated
uncertainties contribute to a recognition that for O3, as
for other pollutants, the available evidence base in a NAAQS review
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.
In light of these uncertainties, as well as those associated with
the exposure and risk analyses, the Administrator notes that, as is the
case in NAAQS reviews in general, the extent to which the current
primary O3 standard is judged to be adequate will depend on
a variety of factors, including his science policy judgments and public
health policy judgments. These factors include judgments regarding
aspects of the evidence and exposure/risk estimates, such as judgments
concerning the appropriate benchmark concentrations on which to place
weight, in light of the available evidence and of associated
uncertainties, as well as judgments on the public health significance
of the effects that have been observed at the exposures evaluated in
the health effects evidence. The factors relevant to judging the
adequacy of the standards also include the interpretation of, and
decisions as to the weight to place on, different aspects of the
results of the exposure and risk assessment for the eight areas studied
and the associated
[[Page 49871]]
uncertainties. Together, these and related factors will inform the
Administrator's judgment about the degree of protection that is
requisite to protect public health with an adequate margin of safety,
and, accordingly, his conclusion regarding the adequacy of the current
standard.
As at the time of the last review, the exposure and risk estimates
developed from modeling exposures to O3 in ambient air are
critically important to consideration of the potential for exposures
and risks of concern under air quality conditions of interest, and
consequently are critically important to judgments on the adequacy of
public health protection provided by the current standard. In
considering the public health implications of estimated occurrences of
exposures, while at increased exertion, to the three benchmark
concentrations, the Administrator considers the effects reported in
controlled human exposure studies of this range of concentrations
during quasi-continuous exercise. In so doing, he notes the statements
from the ATS, as well as judgments made by the EPA in considering
similar effects in previous NAAQS reviews and the extent to which they
may be adverse to health (80 FR 65343, October 26, 2015). In
considering the ATS statements, including the most recent one which is
newly available in the current review (Thurston et al., 2017), the
Administrator recognizes the role of such statements, as described by
the ATS, and as summarized in section II.B.2 above, as providing
principles or considerations for weighing the evidence rather than
offering ``strict rules or numerical criteria'' (ATS, 2000, Thurston et
al., 2017). The more recent statement is generally consistent with the
prior statement (that was considered in the last O3 NAAQS
review) and the attention of that statement to at-risk or vulnerable
population groups, while also broadening the discussion of effects,
responses and biomarkers to reflect the expansion of scientific
research in these areas, as summarized in section II.B.2 above. In this
way, the most recent statement updates the prior statement, while
retaining previously identified considerations, including, for example,
its emphasis on consideration of vulnerable populations, thus expanding
upon (e.g., with some increased specificity), while retaining core
consistency with, the earlier ATS statement. In considering these
statements, the Administrator notes that, in keeping with the intent of
avoiding specific criteria, the statements do not provide specific
descriptions of responses, such as with regard to magnitude, duration
or frequency of small pollutant-related changes in lung function, and
also takes note of the broader ATS emphasis on consideration of
individuals with pre-existing compromised function, such as that
resulting from asthma, recognizing such a focus to be important in his
judgment on the adequacy of protection provided by the current standard
for at-risk populations.
In this review of the 2015 standard, the Administrator takes note
of several aspects of the rationale by which it was established. As
summarized in section II.A.1 above, the decision in the last review
considered the breadth of the O3 respiratory effects
evidence, recognizing the relatively greater significance of effects
reported for exposures while at elevated exertion to average
O3 concentrations at and above 80 ppb, as well as to the
greater array of effects elicited. The decision also recognized the
significance of effects observed at the next lower studied exposures
(slightly above 70 ppb) that included both lung function decrements and
respiratory symptoms. The standard level was set to provide a high
level of protection from such exposures. The decision additionally
emphasized consideration of lower exposures down to 60 ppb,
particularly with regard to consideration of a margin of safety in
setting the standard. In this context, the decision identified the
appropriateness of a standard that provided a degree of control of
multiple or repeated occurrences of exposures, while at elevated
exertion, at or above 60 ppb (80 FR 65365, October 26, 2015).\104\ The
controlled human exposure study evidence as a whole provided context
for consideration of the 2014 HREA results for the exposures of
concern, i.e., the comparison-to-benchmarks analysis (80 FR 65363,
October 26, 2015). The Administrator proposes to similarly consider the
exposure and risk analyses for this review.
---------------------------------------------------------------------------
\104\ With the 2015 decision, the prior Administrator judged
there to be uncertainty in the adversity of the effects shown to
occur following exposures to 60 ppb O3, including the
inflammation reported by the single study at the level, and
accordingly placed greater weight on estimates of multiple exposures
for the 60 ppb benchmark, particularly when considering the extent
to which the current and revised standards incorporate a margin of
safety (80 FR 65344-45, October 26, 2015). She based this, at least
in part, on consideration of effects at this exposure level, the
evidence for which remains the same in the current review. In one
such consideration in 2015, the EPA noted that ``inflammation
induced by a single exposure (or several exposures over the course
of a summer) can resolve entirely. Thus, the inflammatory response
observed following the single exposure to 60 ppb in the study by Kim
et al. (2011) is not necessarily a concern. However, the EPA notes
that it is also important to consider the potential for continued
acute inflammatory responses to evolve into a chronic inflammatory
state and to affect the structure and function of the lung'' (80 FR
65344, October 26, 2015; 2013 ISA, p. 6-76). The prior Administrator
considered this information in judgments regarding the 2014 HREA
estimates for the 60 ppb benchmark.
---------------------------------------------------------------------------
As recognized above, people with asthma, and children, are key
populations at increased risk of respiratory effects related to
O3 in ambient air. Children with asthma, which number
approximately six million in the U.S., may be particularly at risk.
While there are more adults in the U.S. with asthma than children with
asthma, the exposure and risk analysis results in terms of percent of
the simulated at-risk populations, indicate higher frequency of
exposures of potential concern and risks for children as compared to
adults. This finding relates to children's greater frequency and
duration of outdoor activity, as well as their greater activity level
while outdoors (PA, section 3.4.3). In light of these factors and those
recognized above, the Administrator is focusing his consideration of
the exposure and risk analyses here on children and children with
asthma.
In considering the exposure and risk analyses available in this
review, the Administrator first notes that there are a number of ways
in which the current analyses update and improve upon those available
in the last review (as summarized in sections II.C.1 and II.D.1 above).
For example, the Administrator notes that the air quality scenarios in
the current assessment are based on the combination of updated
photochemical modeling with more recent air quality data that include
O3 concentrations closer to the current standard than was
the case for the development of the air quality scenarios in the last
review. As a result of this and the use of updated photochemical
modeling, there is reduced uncertainty with the resulting exposure and
risk estimates. Additionally, two modifications have been made to the
exposure and risk analysis in light of comments received in past
reviews that provide for a better match of the exposure modeling
estimates with the 6.6-hour duration of the controlled human exposure
studies and with the study subject ventilation rates. The Administrator
notes, as summarized in section II.C.2 above, that these and other
updates have reduced the uncertainty associated with interpretation of
the analysis results from that associated with results in the last
review (PA, sections 3.4 through 3.6).
While the Administrator notes reduced uncertainty in several
aspects
[[Page 49872]]
of the exposure and risk analysis approach as compared to the analyses
in the last review, he recognizes the relatively greater uncertainty
associated with the lung function risk estimates compared to the
results of the comparison-to-benchmarks analysis. In so doing, he notes
the PA analyses of uncertainty associated with the lung function risk
estimates (and relatively greater uncertainty with estimates derived
using the MSS model, versus the E-R models approach), as summarized in
section II.C.2 above. In light of these uncertainties, as well as the
recognition that the comparison-to-benchmarks analysis provides for
characterization of risk for the broad array of respiratory effects
compared to a narrower focus limited to lung function decrements, the
Administrator focuses primarily on the estimates of exposures at or
above different benchmark concentrations that represent different
levels of significance of O3-related effects, both with
regard to the array of effects and severity of individual effects.
In considering the exposure and risk estimates, the Administrator
also notes that the eight study areas assessed represent an array of
air quality and exposure circumstances reflecting such variation that
occurs across the U.S. The areas fall into seven of the nine climate
regions represented in the continental U.S., with populations of the
associated metropolitan areas ranging in size from approximately 2.4 to
8 million and varying in demographic characteristics. The Administrator
considers such factors as those identified here to contribute to their
usefulness in informing the current review. As a result of such
variation in exposure-related factors, the eight study areas represent
an array of exposure circumstances, and accordingly, illustrate the
magnitude of exposures and risks that may be expected in areas of the
U.S. that just meet the current standard but that may differ in ways
affecting population exposures of interest. The Administrator finds the
estimates from these analyses to be informative to consideration of
potential exposures and risks associated with the current standard and
to his judgment on the adequacy of protection provided by the current
standard.
Taking into consideration related information, limitations and
uncertainties, such as those recognized above, the Administrator
considers the exposure estimates across the eight study areas (with
their array of exposure conditions) for air quality conditions just
meeting the current standard. Given the greater severity of responses
reported in controlled human exposures, with quasi-continuous exercise,
at and above 73 ppb, the Administrator finds it appropriate to focus
first on the higher two benchmark concentrations (which at 70 and 80
ppb are, respectively, slightly below and above this level) and the
estimates for one-or-more-day occurrences. In so doing, he notes that
across all eight study areas, less than 1% of children with asthma (and
also of all children) are estimated to experience, while breathing at
an elevated rate, a daily maximum 7-hour exposure per year at or above
70 ppb, on average across the 3-year period, with a maximum of about 1%
for the study area with the highest estimates in the highest single
year (Table 2). Further, the percentage (for both population groups)
for at least one day with such an exposure at or above 80 ppb is less
than 0.1%, as an average across the 3-year period (and 0.1% or less in
each of the three years simulated across the eight study areas). No
simulated children were estimated to experience more than a single such
day with an exposure at or above the 80 ppb benchmark (Table 2). The
Administrator recognizes these estimates to indicate a very high level
of protection from exposures that been found in controlled human
exposure studies to elicit lung function decrements of notable
magnitude (e.g., 6% at the study group mean for exposure to 73 ppb)
accompanied by increases in respiratory symptom scores, as summarized
in section II.B.3.
The Administrator additionally considers the estimated occurrences
of days that include lower 7-hour exposures, while at elevated exertion
(i.e., daily maximum exposures at or above 60 ppb). In so doing, the
Administrator takes note of the lesser severity of effects observed in
controlled human exposure studies to 60 ppb (while at increased
exertion) compared to the effects at the higher concentrations that
have been studied (e.g., statistically significant O3-
related decrements on the order of 2 to 3% at the study group mean
compared to 6%). He notes the finding of statistically significant
increased respiratory symptom scores with exposures targeted at an
exposure concentration of 70 ppb (and averaging 73 ppb across the
exposure period), and the lack of such finding for any lower exposure
concentrations that have been studied. In light of these
considerations, he finds occurrences of exposures at or above the
lowest benchmark of 60 ppb to be of lesser concern than occurrences for
the next higher benchmark of 70 ppb. As described above for the higher
exposure concentrations, he additionally recognizes that the studies of
60 ppb were of generally healthy adults. While he notes the uncertainty
regarding the risk that may be posed by this exposure concentration to
at-risk populations, such as people with asthma, he additionally notes
that the limited evidence available at higher exposure concentrations
indicates lung function responses for this group that are similar to
those for the generally healthy subjects, as well as the evidence of
the transience of the responses in controlled human exposure studies.
Further, he considers that due to the inherent characteristics of
asthma as a disease, there is a potential, as summarized in section
II.B.2 above, for O3 exposures to trigger asthmatic
responses, such as through causing an increase in airway
responsiveness. In this context, he additionally recognizes the
potential for such a response to be greater, in general, at relatively
higher, versus lower, exposure concentrations, noting 80 ppb to be the
lowest exposure concentration at which increased airway responsiveness
has been reported in generally healthy adults. In recognizing that the
finding for this exposure concentration is for generally healthy adults
and does not directly relate to people with asthma, he finds it
appropriate to give additional consideration to the two lower
benchmarks. In so doing, he judges that a high level of protection is
desirable against one or more occurrences of days with exposures while
breathing at an elevated rate to concentrations at or above 70 ppb.
Additionally, he takes note of the lesser severity of responses
observed in studies of the lowest benchmark concentration of 60 ppb,
while considering the exposure analysis estimates of occurrences of
daily maximum exposures at or above this benchmark, while also
recognizing there to be greater risk for occurrence of a more serious
effect with greater frequency of such exposure occurrence. Thus, based
on the considerations recognized here, including potential risks for
at-risk populations, the Administrator considers it appropriate to give
greater weight to the exposure analysis estimates of occurrences of two
or more days (rather than one or more) with an exposure at or above the
60 ppb benchmark.
The exposure analysis estimates indicate fewer than 1% to just over
3% of children with asthma (just under 3% of all children), on average
across the 3-year period to be expected to experience two or more days
with an exposure at
[[Page 49873]]
or above 60 ppb, while at elevated ventilation. The Administrator notes
this to indicate that some 97% to more than 99% of children, on
average, and more than 95% in the single highest year, are protected
from experiencing two or more days with exposures at or above 60 ppb
while at elevated exertion. He also considers this in combination with
the high level of protection indicated by the exposure estimates for
the higher benchmark concentration of 70 ppb, which is slightly below
the exposure level at which increases in FEV1 decrement (6%
at the study group mean) accompanied by respiratory symptoms have been
demonstrated. The current exposure analysis, with reduced uncertainty
compared to the analysis available in the last review for air quality
conditions in areas that just meet the current standard, indicates more
than 99% of children with asthma (and of all children), on average per
year, to be protected from a day or more with an exposure at or above
70 ppb. In light of all of the considerations summarized above, the
Administrator proposes to judge that protection from these exposures,
as described here, provides a strong degree of protection to at-risk
populations such as children with asthma. In light of all of the above,
the Administrator finds the updated exposure and risk analyses based on
updated and improved information, including air quality concentrations
closer to the current standard, to continue to support a conclusion of
a high level of protection, including for at-risk populations, from
O3-related effects of exposures that might be expected with
air quality conditions that just meet the current standard.
In reaching his proposed conclusion, the Administrator additionally
takes note of the comments and advice from the CASAC, including the
CASAC conclusion that the newly available evidence does not
substantially differ from that available in the last review, and the
associated conclusion expressed by part of the CASAC, that the current
evidence supports retaining the current standard. He also notes that
another part of the CASAC indicated its agreement with the prior CASAC
comments on the 2014 draft PA, in which the prior CASAC opined that a
standard set at 70 ppb may not provide an adequate margin of safety
(Cox, 2020, p. 1). With regard to the latter view (that referenced 2014
comments from the prior CASAC), the Administrator additionally notes
that the 2014 advice from the prior CASAC also concluded that the
scientific evidence supported a range of standard levels that included
70 ppb and recognized the choice of a level within its recommended
range to be ``a policy judgment under the statutory mandate of the
Clean Air Act'' (Frey, 2014, p. ii). The Administrator considers these
points to provide additional context for the comments of the prior
CASAC that were cited by part of the current CASAC in its review of the
draft PA in this review, as noted above.\105\
---------------------------------------------------------------------------
\105\ This 2014 advice was considered in the last review's
decision to establish the current standard with a level of 70 ppb
(80 FR 65362, October 26, 2015).
---------------------------------------------------------------------------
In reflecting on all of the information currently available, the
Administrator considers the extent to which the currently available
information might indicate support for a less stringent standard. He
recognizes the advice from the CASAC, which generally indicates support
for retaining the current standard without revision or for revision to
a more stringent level based on additional consideration of the margin
of safety for at-risk populations. He notes that the CASAC advice did
not convey support for a less stringent standard. He additionally
considers the current exposure and risk estimates for the air quality
scenario for a design value just above the level of the current
standard (at 75 ppb), in comparison to the scenario for the current
standard, as summarized in section II.D.1 above. In so doing, he finds
the markedly increased estimates of exposures to the higher benchmarks
under air quality for a higher standard level to be of concern and
indicative of less than the requisite protection (Table 2). Thus, in
light of the considerations raised here, including the need for an
adequate margin of safety, the Administrator proposes to judge that a
less stringent standard would not be appropriate to consider.
The Administrator additionally considers whether it would be
appropriate to consider a more stringent standard that might be
expected to result in reduced O3 exposures. As an initial
matter, he considers the advice from the CASAC. With regard to the
CASAC advice, while part of the Committee concluded the evidence
supported retaining the current standard without revision, another part
of the Committee reiterated advice from the prior CASAC, which while
including the current standard level among the range of recommended
standard levels, also provided policy advice to set the standard at a
lower level. In considering this advice now in this review, the
Administrator notes the slight differences of the current exposure and
risk estimates from the 2014 HREA estimates for the lowest benchmark,
which were those considered by the prior CASAC (Table 4). For example,
while the 2014 HREA estimated 3.3 to 10.2% of children, on average, to
experience one or more days with an exposures at or above 60 ppb (and
as many as 18.9% in a single year), the comparable estimates for the
current analyses are lower, particularly at the upper end (3.2 to 8.2%
and 10.6%). While the estimates for two or more days with occurrences
at or above 60 ppb, on average across the assessment period, are more
similar between the two assessments, the current estimate for the
single highest year is much lower (9.2 versus 4.3%). The Administrator
additionally recognizes the PA finding (summarized in section II.D.1
above) that the factors contributing to these differences, which
includes the use of air quality data reflecting concentrations much
closer to the now-current standard than was the case in the 2015
review, also contribute to a reduced uncertainty in the estimates.
Thus, he notes that the current exposure analysis estimates indicate
the current standard to provide appreciable protection against multiple
days with a maximum exposure at or above 60 ppb. He considers this in
the context of his consideration of the adequacy of protection provided
by the standard and of the CAA requirement that the standard protect
public health, including the health of at-risk populations, with an
adequate margin of safety, and proposes to conclude, in light of all of
the considerations raised here, that the current standard provides an
adequate margin of safety, and that a more stringent standard is not
needed.
In light of all of the above, including advice from the CASAC, the
Administrator finds the current exposure and risk analysis results to
describe appropriately strong protection of at-risk populations from
O3-related health effects. Thus, based on his consideration
of the evidence and exposure/risk information, including that related
to the lowest exposures studied and the associated uncertainties, the
Administrator proposes to judge that the current standard provides the
requisite protection, including an adequate margin of safety, and thus
should be retained, without revision.
As recognized above, the protection afforded by the current
standard can only be assessed by considering its elements collectively,
including the standard level of 70 ppb, the averaging time of eight
hours and the form of the annual fourth-highest daily maximum
[[Page 49874]]
concentration averaged across three years. The Administrator finds that
the current evidence presented in the ISA and considered in the PA, as
well as the current air quality, exposure and risk information
presented and considered in the PA provide continued support to these
elements, as well as to the current indicator, as discussed above. In
summary, the Administrator recognizes the newly available health
effects evidence, critically assessed in the ISA as part of the full
body of evidence, to reaffirm conclusions on the respiratory effects
recognized for O3 in the last review. He additionally notes
that the evidence newly available in this review, such as that related
to metabolic effects, does not include information indicating a basis
for concern for exposure conditions associated with air quality
conditions meeting the current standard. Further, the Administrator
notes the quantitative exposure and risk estimates for conditions just
meeting the current standard that indicate a high level of protection
for at-risk populations from respiratory effects. Collectively, these
considerations (including those discussed above) provide the basis for
the Administrator's judgments regarding the public health protection
provided by the current primary standard of 0.070 ppm O3, as
the fourth-highest daily maximum 8-hour concentration averaged across
three years. On this basis, the Administrator proposes to conclude that
the current standard is requisite to protect the public health with an
adequate margin of safety, and that it is appropriate to retain the
standard without revision. The Administrator solicits comment on these
proposed conclusions.
Having reached the proposed decision described here based on
interpretation of the health effects evidence, as assessed in the ISA,
and the quantitative analyses presented in the PA; the evaluation of
policy-relevant aspects of the evidence and quantitative analyses in
the PA; the advice and recommendations from the CASAC; public comments
received to date in this review; and the public health policy judgments
described above, the Administrator recognizes that other
interpretations, assessments and judgments might be possible.
Therefore, the Administrator solicits comment on the array of issues
associated with review of this standard, including public health and
science policy judgments inherent in the proposed decision, as
described above, and the rationales upon which such views are based.
III. Rationale for Proposed Decision on the Secondary Standard
This section presents the rationale for the Administrator's
proposed decision to retain the current secondary O3
standard. This rationale is based on a thorough review of the latest
scientific information generally published between January 2011 and
March 2018, as well as more recent studies identified during peer
review or by public comments (ISA, section IS.1.2),\106\ integrated
with the information and conclusions from previous assessments and
presented in the ISA on welfare effects associated with photochemical
oxidants including O3 and pertaining to their presence in
ambient air. The Administrator's rationale also takes into account: (1)
The PA evaluation of the policy-relevant information in the ISA and
presentation of quantitative analyses of air quality, exposure, and
risk; (2) CASAC advice and recommendations, as reflected in discussions
of drafts of the ISA and PA at public meetings and in the CASAC's
letters to the Administrator; (3) public comments received during the
development of these documents; and also (4) the August 2019 decision
of the D.C. Circuit remanding the secondary standard established in the
last review to the EPA for further justification or reconsideration.
See Murray Energy Corp. v. EPA, 936 F.3d 597 (D.C. Cir. 2019).
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\106\ In addition to the review's opening ``Call for
Information'' (83 FR 29785, June 26, 2018), systematic review
methodologies were applied to identify relevant scientific findings
that have emerged since the 2013 ISA, which included peer reviewed
literature published through July 2011. Search techniques for the
current ISA identified and evaluated studies and reports that have
undergone scientific peer review and were published or accepted for
publication between January 1, 2011 (providing some overlap with the
cutoff date for the last ISA) and March 30, 2018. Studies published
after the literature cutoff date for this ISA were also considered
if they were submitted in response to the Call for Information or
identified in subsequent phases of ISA development, particularly to
the extent that they provide new information that affects key
scientific conclusions (ISA, Appendix 10, section 10.2). References
that are cited in the ISA, the references that were considered for
inclusion but not cited, and electronic links to bibliographic
information and abstracts can be found at: https://hero.epa.gov/hero/index.cfm/project/page/project_id/2737.
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In presenting the rationale for the Administrator's proposed
decision and its foundations, section III.A provides background and
introductory information for this review of the secondary O3
standard. It includes background on the establishment of the current
standard in 2015 (section III.A.1) and also describes the general
approach for its current review (section III.A.2). Section III.B
summarizes the currently available welfare effects evidence, focusing
on consideration of key policy-relevant aspects. Section III.C
summarizes current air quality and environmental exposure information,
drawing on the quantitative analyses presented in the PA. Section III.D
presents the Administrator's proposed conclusions on the current
standard (section III.D.3), drawing on both evidence-based and air
quality, exposure and risk-based considerations (section III.D.1) and
advice from the CASAC (section III.D.2).
A. General Approach
As is the case for all such reviews, this review of the current
secondary O3 standard is based, most fundamentally, on using
the EPA's assessments of the current scientific evidence and associated
quantitative analyses to inform the Administrator's judgment regarding
a secondary standard that is requisite to protect the public welfare
from known or anticipated adverse effects associated with the
pollutant's presence in the ambient air. The EPA's assessments are
primarily documented in the ISA and PA, both of which have received
CASAC review and public comment (84 FR 50836, September 26, 2019; 84 FR
58711, November 1, 2019; 84 FR 58713, November 1, 2019; 85 FR 21849,
April 20, 2020; 85 FR 31182, May 22, 2020). In bridging the gap between
the scientific assessments of the ISA and the judgments required of the
Administrator in determining whether the current standard provides the
requisite public welfare protection, the PA evaluates policy
implications of the evaluation of the current evidence in the ISA and
the quantitative air quality, exposure and risk analyses and
information documented in the PA. In evaluating the public welfare
protection afforded by the current standard, the four basic elements of
the NAAQS (indicator, averaging time, level, and form) are considered
collectively.
The final decision on the adequacy of the current secondary
standard is a public welfare policy judgment to be made by the
Administrator. In reaching conclusions with regard to the standard, the
decision will draw on the scientific information and analyses about
welfare effects, environmental exposure and risks, and associated
public welfare significance, as well as judgments about how to consider
the range and magnitude of uncertainties that are inherent in the
scientific evidence and analyses. This approach is based on the
recognition that the available evidence generally reflects a continuum
that includes ambient air exposures at which scientists generally agree
that effects are
[[Page 49875]]
likely to occur through lower levels at which the likelihood and
magnitude of responses become increasingly uncertain. This approach is
consistent with the requirements of the provisions of the Clean Air Act
related to the review of NAAQS and with how the EPA and the courts have
historically interpreted the Act. These provisions require the
Administrator to establish secondary standards that, in the judgment of
the Administrator, are requisite to protect the public welfare from
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 Act does not require that standards be
set at a zero-risk level, but rather at a level that reduces risk
sufficiently so as to protect the public welfare from known or
anticipated adverse effects.
The subsections below provide background and introductory
information. Background on the establishment of the current standard in
2015, including the rationale for that decision, is summarized in
section III.A.1. This is followed, in section III.A.2, by an overview
of the general approach for the current review of the 2015 standard.
Following this introductory section and subsections, the subsequent
sections summarize current information and analyses, including that
newly available in this review. The Administrator's proposed
conclusions on the standard set in 2015, based on the current
information, are provided in section III.D.3
1. Background on the Current Standard
The current standard was set in 2015 based on the scientific and
technical information available at that time, as well as the
Administrator's judgments regarding the available welfare effects
evidence, the appropriate degree of public welfare protection for the
revised standard, and available air quality information on seasonal
cumulative exposures that may be allowed by such a standard (80 FR
65292, October 26, 2015). With the 2015 decision, the Administrator
revised the level of the secondary standard for photochemical oxidants,
including O3, to 0.070 ppm, in conjunction with retaining
the indicator (O3), averaging time (8 hours) and form
(fourth-highest annual daily maximum 8-hour average concentration,
averaged across three years).
The welfare effects evidence base available in the 2015 review
included more than fifty years of extensive research on the phytotoxic
effects of O3, conducted both in and outside of the U.S.
that documents the impacts of O3 on plants and their
associated ecosystems (U.S. EPA, 1978, 1986, 1996, 2006, 2013). As was
established in prior reviews, O3 can interfere with carbon
gain (photosynthesis) and allocation of carbon within the plant, making
fewer carbohydrates available for plant growth, reproduction, and/or
yield (U.S. EPA, 1996, pp. 5-28 and 5-29). The strongest evidence for
effects from O3 exposure on vegetation is from controlled
exposure studies, which ``have clearly shown that exposure to
O3 is causally linked to visible foliar injury, decreased
photosynthesis, changes in reproduction, and decreased growth'' in many
species of vegetation (2013 ISA, p. 1-15).\107\ Such effects at the
plant scale can also be linked to an array of effects at larger
organizational (e.g., population, community, system) and spatial
scales, with the evidence available in the last review supporting
conclusions of causal relationships between O3 and
alteration of below-ground biogeochemical cycles, in addition to likely
to be a causal relationships between O3 and reduced carbon
sequestration in terrestrial ecosystems, alteration of terrestrial
ecosystem water cycling and alteration of terrestrial community
composition (2013 ISA, p. lxviii and Table 9-19). Further, the 2013 ISA
also found there to be a causal relationship between changes in
tropospheric O3 concentrations and radiative forcing, and
likely to be a causal relationship between tropospheric O3
concentrations and effects on climate as quantified through surface
temperature response (2013 ISA, section 10.5).
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\107\ Visible foliar injury includes leaf or needle changes such
as small dots or bleaching (2013 ISA, p. 9-38).
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The 2015 decision was a public welfare policy judgment made by the
Administrator, which drew upon the available scientific evidence for
O3-attributable welfare effects and on quantitative analyses
of exposures and public welfare risks, as well as judgments about the
appropriate weight to place on the range of uncertainties inherent in
the evidence and analyses. The analyses utilized cumulative,
concentration-weighted exposure indices for O3. Use of this
metric was based on conclusions in the 2013 ISA that exposure indices
that cumulate hourly O3 concentrations, giving greater
weight to the higher concentrations (such as the W126 index), perform
well in describing exposure-response relationships documented in crop
and tree seedling studies (2013 ISA, section 9.5). Included in this
decision were judgments on the weight to place on the evidence of
specific vegetation-related effects estimated to result across a range
of cumulative seasonal concentration-weighted O3 exposures;
on the weight to give associated uncertainties, including uncertainties
of predicted environmental responses (based on experimental study
data); variability in occurrence of the specific effects in areas of
the U.S., especially in areas of particular public welfare
significance; and on the extent to which such effects in such areas may
be considered adverse to public welfare.
The decision was based on a thorough review in the 2013 ISA of the
scientific information on O3-induced environmental effects.
The decision also took into account: (1) Assessments in the 2014 PA of
the most policy-relevant information in the 2013 ISA regarding evidence
of adverse effects of O3 to vegetation and ecosystems,
information on biologically-relevant exposure metrics, 2014 welfare REA
(WREA) analyses of air quality, exposure, and ecological risks and
associated ecosystem services, and staff analyses of relationships
between levels of a W126-based exposure index \108\ and potential
alternative standard levels in combination with the form and averaging
time of the then-current standard; (2) additional air quality analyses
of the W126 index and design values based on the form and averaging
time of the then-current standard; (3) CASAC advice and
recommendations; and (4) public comments received during the
development of these documents and on the proposal document. In
addition to reviewing the most recent scientific information as
required by the CAA, the 2015 rulemaking also incorporated the EPA's
response to the judicial remand of the 2008 secondary O3
standard in Mississippi v. EPA, 744 F.3d 1334 (D.C. Cir. 2013) and, in
light of the court's decision in that case, explained the
Administrator's conclusions as to the level of air quality judged to
provide the requisite protection of public welfare from known or
anticipated adverse effects.
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\108\ The W126 index is a cumulative seasonal metric described
as the sigmoidally weighted sum of all hourly O3
concentrations observed during a specified daily and seasonal time
window, where each hourly O3 concentration is given a
weight that increases from zero to one with increasing concentration
(80 FR 65373-74, October 26, 2015). Accordingly, W126 index values
are in the units of ppm-hours (ppm-hrs).
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Consistent with the general approach routinely employed in NAAQS
reviews, the initial consideration in the 2015 review of the secondary
standard was
[[Page 49876]]
with regard to the adequacy of protection provided by the existing
standard, that was set in 2008 (0.075 ppm, as annual fourth-highest
daily maximum 8-hour average concentration averaged over three
consecutive years). In her decision making, the Administrator
considered the effects of O3 on tree seedling growth, as
suggested by the CASAC, as a surrogate or proxy for the broader array
of vegetation-related effects of O3, ranging from effects on
sensitive species to broader ecosystem-level effects (80 FR 65369,
65406, October 26, 2015). The metric used for quantifying effects on
tree seedling growth in the review was relative biomass loss (RBL),
with the evidence base providing robust and established exposure-
response (E-R) functions for seedlings of 11 tree species (80 FR 65391-
92, October 26, 2015; 2014 PA, Appendix 5C).\109\ The Administrator
used this surrogate or proxy in making her judgments on O3
effects to the public welfare. In this context, exposure was evaluated
in terms of the W126 cumulative seasonal exposure index, an index
supported by the evidence in the 2013 ISA for this purpose and that was
consistent with advice from the CASAC (2013 ISA, section 9.5.3, p. 9-
99; 80 FR 65375, October 26, 2015).
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\109\ These functions for RBL estimate the reduction in a year's
growth as a percentage of that expected in the absence of
O3 (2013 ISA, section 9.6.2; 2014 WREA, section 6.2).
---------------------------------------------------------------------------
In considering the public welfare protection provided by the then-
current standard, the Administrator gave primary consideration to an
analysis of cumulative seasonal exposures in or near Class I areas
\110\ during periods when the then-current standard was met, and the
associated estimates of growth effects in well-studied species of tree
seedlings, in terms of the O3 attributable reductions in RBL
in the median species for which E-R functions have been established (80
FR 65385-65386, 65389-65390, October 26, 2015).\111\ The Administrator
noted the occurrence of exposures for which the associated median
estimates of growth effects across the species with E-R functions
extend above a magnitude considered to be ``unacceptably high'' by the
CASAC.\112\ This analysis estimated cumulative exposures, in terms of
3-year average W126 index values, at and above 19 ppm-hrs, occurring
under the then-current standard for nearly a dozen areas, distributed
across two NOAA climatic regions of the U.S. (80 FR 65385-86, October
26, 2015). The Administrator gave particular weight to this analysis
because of its focus on exposures in Class I areas, which are lands
that Congress set aside for specific uses 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. This emphasis on lands afforded special government
protections, such as national parks and forests, wildlife refuges, and
wilderness areas, some of which are designated Class I areas under the
CAA, was consistent with a similar emphasis in the 2008 review of the
standard (73 FR 16485, March 27, 2008). The Administrator additionally
recognized 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 those lands, as well as for visitors to those
areas (80 FR 65390, October 26, 2015).
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\110\ Areas designated as Class I include all international
parks, national wilderness areas which exceed 5,000 acres in size,
national memorial parks which exceed 5,000 acres in size, and
national parks which exceed 6,000 acres in size, provided the park
or wilderness area was in existence on August 7, 1977. Other areas
may also be Class I if designated as Class I consistent with the
CAA.
\111\ In specifically evaluating exposure levels in terms of the
W126 index as to potential for impacts on vegetation, the
Administrator focused on the median RBL estimate across the eleven
tree species for which robust established E-R functions were
available. The presentation of these E-R functions for growth
effects on tree seedlings (and crops) included estimates of RBL (and
relative yield loss [RYL]) at a range of W126-based exposure levels
(2014 PA, Tables 5C-1 and 5C-2). The median tree species RBL or crop
RYL was presented for each W126 level (2014 PA, Table 5C-3; 80 FR
65391 [Table 4], October 26, 2015). The Administrator focused on RBL
as a surrogate or proxy for the broader array of vegetation-related
effects of potential public welfare significance, which include
effects on growth of individual sensitive species and extend to
ecosystem-level effects, such as community composition in natural
forests, particularly in protected public lands, as well as forest
productivity (80 FR 65406, October 26, 2015).
\112\ In the CASAC's consideration of RBL estimates presented in
the 2014 draft PA, it characterized an estimate of 6% RBL in the
median studied species as being ``unacceptably high,'' (Frey,
2014b).
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As noted across past reviews of O3 secondary standards,
the Administrator's judgments regarding effects that are adverse to
public welfare consider the intended use of the ecological receptors,
resources and ecosystems affected (80 FR 65389, October 26, 2015; 73 FR
16496, March 27, 2008). Thus, in the 2015 review, the Administrator
utilized the median RBL estimate for the studied species as a
quantitative tool within a larger framework of considerations
pertaining to the public welfare significance of O3 effects.
She recognized such considerations to include effects that are
associated with effects on growth and that the 2013 ISA determined to
be causally or likely causally related to O3 in ambient air,
yet for which there are greater uncertainties affecting estimates of
impacts on public welfare. These other effects included reduced
productivity in terrestrial ecosystems, reduced carbon sequestration in
terrestrial ecosystems, alteration of terrestrial community
composition, alteration of below-ground biogeochemical cycles, and
alteration of terrestrial ecosystem water cycles. Thus, in giving
attention to the CASAC's characterization of a 6% estimate for tree
seedling RBL in the median studied species as ``unacceptably high'',
the Administrator, while mindful of uncertainties with regard to the
magnitude of growth impact that might be expected in the field and in
mature trees, was also mindful of related, broader, ecosystem-level
effects for which the available tools for quantitative estimates are
more uncertain and those for which the policy foundation for
consideration of public welfare impacts is less well established. As a
result, the Administrator considered tree growth effects of
O3, in terms of RBL ``as a surrogate for the broader array
of O3 effects at the plant and ecosystem levels'' (80 FR
65389, October 26, 2015).
Based on all of these considerations, and taking into consideration
CASAC advice and public comment, the Administrator concluded that the
protection afforded by the then-current standard was not sufficient and
that the standard needed to be revised to provide additional protection
from known and anticipated adverse effects to public welfare, related
to effects on sensitive vegetation and ecosystems, most particularly
those occurring in Class I areas, and also in other areas set aside by
states, tribes and public interest groups to provide similar benefits
to the public welfare for residents on those lands, as well as for
visitors to those areas. In so doing, she further noted that a revised
standard would provide increased protection for other growth-related
effects, including relative yield loss (RYL) of crops, reduced carbon
storage, and types of effects for which it is more difficult to
determine public welfare significance, as well as other welfare effects
of O3, such as visible foliar injury (80 FR 65390, October
26, 2015).
Consistent with the approach employed for considering the adequacy
of the then-current secondary standard, the approach for considering
revisions
[[Page 49877]]
that would result in a standard providing the requisite protection
under the Act also focused on growth-related effects of O3,
using RBL as a surrogate for the broader array of vegetation-related
effects and included judgments on the magnitude of such effects that
would contribute to public welfare impacts of concern. In considering
the adequacy of potential alternative standards to provide protection
from such effects, the approach also focused on considering the
cumulative seasonal O3 exposures likely to occur with
different alternative standards.
In light of the judicial remand of the 2008 secondary O3
standard referenced above, the 2015 decision on selection of a revised
secondary standard first considered the available evidence and
quantitative analyses in the context of an approach for considering and
identifying public welfare objectives for such a standard (80 FR 65403-
65408, October 26, 2015). In light of the extensive evidence base of
O3 effects on vegetation and associated terrestrial
ecosystems, the Administrator focused on protection against adverse
public welfare effects of O3-related effects on vegetation,
giving particular attention to such effects in natural ecosystems, such
as those in areas with protection designated by Congress for current
and future generations, as well as areas similarly set aside by states,
tribes and public interest groups with the intention of providing
similar benefits to the public welfare. The Administrator additionally
recognized that providing protection for this purpose will also provide
a level of protection for other vegetation that is used by the public
and potentially affected by O3 including timber, produce
grown for consumption and horticultural plants used for landscaping (80
FR 65403, October 26, 2015).
As mentioned above, the Administrator considered the use of a
cumulative seasonal exposure index (the W126 index) for purposes of
assessing potential public welfare risks, and similarly, for assessing
potential protection achieved against such risks on a national scale.
In consideration of conclusions of the 2013 ISA and 2014 PA, as well as
advice from the CASAC and public comments, this W126 index was defined
as a maximum, seasonal (3-month), 12-hour index (80 FR 65404, October
26, 2015).\113\ While recognizing that no one definition of an exposure
metric used for the assessment of protection for multiple effects at a
national scale will be exactly tailored to every species or each
vegetation type, ecosystem and region of the country, the Administrator
judged that on balance, a W126 index derived in this way, and averaged
over three years would be appropriate for such purposes (80 FR 65403,
October 26, 2015).
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\113\ As also described in section III.B.3.a below, this index
is defined by the 3-consecutive-month period within the
O3 season with the maximum sum of W126-weighted hourly
O3 concentrations during the period from 8:00 a.m. to
8:00 p.m. each day.
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Based on a number of considerations, the Administrator recognized
greater confidence in judgments related to public welfare impacts based
on a 3-year average metric than a single-year metric, and consequently
concluded it to be appropriate to use a seasonal W126 index averaged
across three years for judging public welfare protection afforded by a
revised secondary standard (80 FR 65404, October 26, 2015). For
example, the Administrator was mindful of both the strengths and
limitations of the evidence and of the information on which to base her
judgments with regard to adversity of effects on the public
welfare.\114\ While the Administrator recognized the scientific
information and interpretations, as well as CASAC advice, with regard
to a single-year exposure index, she also took note of uncertainties
associated with judging the degree of vegetation impacts for single-
year effects that would be adverse to public welfare. The Administrator
was also mindful of the variability in ambient air O3
concentrations from year to year, as well as year-to-year variability
in environmental factors, including rainfall and other meteorological
factors, that influence the occurrence and magnitude of O3-
related effects in any year, and contribute uncertainties to
interpretation of the potential for harm to public welfare over the
longer term (80 FR 65404, October 26, 2015).
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\114\ In this regard, she recognized uncertainties associated
with interpretation of the public welfare significance of effects
resulting from a single-year exposure, and that the public welfare
significance of effects associated with multiple years of critical
exposures are potentially greater than those associated with a
single year of such exposure. The Administrator concluded that use
of a 3-year average metric could address the potential for adverse
effects to public welfare that may relate to shorter exposure
periods, including a single year (80 FR 65404, October 26, 2015).
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In reaching a conclusion on the amount of public welfare protection
from the presence of O3 in ambient air that is appropriate
to be afforded by a revised secondary standard, the Administrator gave
particular consideration to the following: (1) The nature and degree of
effects of O3 on vegetation, including her judgments as to
what constitutes an adverse effect to the public welfare; (2) the
strengths and limitations of the available and relevant information;
(3) comments from the public on the Administrator's proposed decision,
including comments related to identification of a target level of
protection; and (4) the CASAC's views regarding the strength of the
evidence and its adequacy to inform judgments on public welfare
protection. The Administrator recognized that such judgments should
neither overstate nor understate the strengths and limitations of the
evidence and information nor the appropriate inferences to be drawn as
to risks to public welfare, and that the choice of the appropriate
level of protection is a public welfare policy judgment entrusted to
the Administrator under the CAA taking into account both the available
evidence and the uncertainties (80 FR 65404-05, October 26, 2015).\115\
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\115\ The CAA does not require that a secondary standard be
protective of all effects associated with a pollutant in the ambient
air but rather those known or anticipated effects judged ``adverse
to the public welfare'' (CAA section 109).
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With regard to the extensive evidence of welfare effects of
O3, including visible foliar injury and crop RYL, the
information available for tree species was judged to be more useful in
informing judgments regarding the nature and severity of effects
associated with different air quality conditions and associated public
welfare significance. Accordingly, the Administrator gave particular
attention to the effects related to native tree growth and
productivity, including forest and forest community composition,
recognizing the relationship of tree growth and productivity to a range
of ecosystem services, (80 FR 65405-06, October 26, 2015). In making
this judgment, the Administrator recognized that among the broad array
of O3-induced vegetation effects were the occurrence of
visible foliar injury and growth and/or yield loss in O3-
sensitive species, including crops and other commercial species (80 FR
65405, October 26, 2015). In regard to visible foliar injury, the
Administrator recognized the potential for this effect to affect the
public welfare in the context of affecting value ascribed to natural
forests, particularly those afforded special government protection,
with the significance of O3-induced visible foliar injury
depending on the extent and severity of the injury (80 FR 65407,
October 26, 2015). In so doing, however, the Administrator also took
note of limitations in the available visible foliar injury information,
including the lack of established E-R functions that would allow
prediction of
[[Page 49878]]
visible foliar injury severity and incidence under varying air quality
and environmental conditions, a lack of consistent quantitative
relationships linking visible foliar injury with other O3-
induced vegetation effects, such as growth or related ecosystem
effects, and a lack of established criteria or objectives that might
inform consideration of potential public welfare impacts related to
this vegetation effect (80 FR 65407, October 26, 2015). Similarly,
while O3-related growth effects on agricultural and
commodity crops had been extensively studied and robust E-R functions
developed for a number of species, the Administrator found this
information less useful in informing her judgments regarding an
appropriate level of public welfare protection (80 FR 65405, October
26, 2015).\116\
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\116\ With respect to commercial production of commodities, the
Administrator noted 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 the extensive management of such vegetation
(which, as the CASAC noted, may reduce yield variability) may also
to some degree mitigate potential O3-related effects. The
management practices used on such vegetation are highly variable and
are designed to achieve optimal yields, taking into consideration
various environmental conditions. In addition, changes in yield of
commercial crops and commercial commodities, such as timber, may
affect producers and consumers differently, further complicating the
question of assessing overall public welfare impacts (80 FR 65405,
October 26, 2015).
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Thus, and in light of the extensive evidence base in this regard,
the Administrator focused on trees and associated ecosystems in
identifying the appropriate level of protection for the secondary
standard. Accordingly, the Administrator found the estimates of tree
seedling growth impacts (in terms of RBL) associated with a range of
W126-based index values developed from the E-R functions for 11 tree
species (referenced above) to be appropriate and useful for considering
the appropriate public welfare protection objective for a revised
standard (80 FR 65391-92, Table 4, October 26, 2015). The Administrator
also incorporated into her considerations the broader evidence base
associated with forest tree seedling biomass loss, including other less
quantifiable effects of potentially greater public welfare
significance. That is, in drawing on these RBL estimates, the
Administrator recognized she was not simply making judgments about a
specific magnitude of growth effect in seedlings that would be
acceptable or unacceptable in the natural environment. Rather, though
mindful of associated uncertainties, the Administrator used the RBL
estimates as a surrogate or proxy for consideration of the broader
array of related vegetation and ecosystem effects of potential public
welfare significance that include effects on growth of individual
sensitive species and extend to ecosystem-level effects, such as
community composition in natural forests, particularly in protected
public lands, as well as forest productivity (80 FR 65406, October 26,
2015). This broader array of vegetation-related effects included those
for which public welfare implications are more significant but for
which the tools for quantitative estimates were more uncertain.
In using the RBL estimates as a proxy, and in consideration of
CASAC advice; strengths, limitations and uncertainties in the evidence;
and the linkages of growth effects to larger population, community and
ecosystem impacts, the Administrator considered it appropriate to focus
on a standard that would generally limit cumulative exposures to those
for which the median RBL estimate for seedlings of the 11 species with
robust and established E-R functions would be somewhat below 6% (80 FR
65406-07, October 26, 2015). In focusing on cumulative exposures
associated with a median RBL estimate somewhat below 6%, the
Administrator considered the relationships between W126-based exposure
and RBL in the studied species (presented in the final PA and proposal
document), noting that the median RBL estimate was 6% for a cumulative
seasonal W126 exposure index of 19 ppm-hrs (80 FR 65391-92, Table 4,
October 26, 2015).\117\ Given the information on median RBL at
different W126 exposure levels, using a 3-year cumulative exposure
index for assessing vegetation effects, the potential for single-season
effects of concern, and CASAC comments on the appropriateness of a
lower value for a 3-year average W126 index, the Administrator
concluded it was appropriate to identify a standard that would restrict
cumulative seasonal exposures to 17 ppm-hrs or lower, in terms of a 3-
year W126 index, in nearly all instances (80 FR 65407, October 26,
2015). Based on such information, available at that time, to inform
consideration of vegetation effects and their potential adversity to
public welfare, the Administrator additionally judged that the RBL
estimates associated with marginally higher exposures in isolated, rare
instances are not indicative of effects that would be adverse to the
public welfare, particularly in light of variability in the array of
environmental factors that can influence O3 effects in
different systems and uncertainties associated with estimates of
effects associated with this magnitude of cumulative exposure in the
natural environment (80 FR 65407, October 26, 2015).
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\117\ When stated to the first decimal place, the median RBL was
6.0% for a cumulative seasonal W126 exposure index of 19 ppm-hrs.
For 18 ppm-hrs, the median RBL estimate was 5.7%, which rounds to
6%, and for 17 ppm-hrs, the median RBL estimate was 5.3%, which
rounds to 5% (80 FR 65407, October 26, 2015).
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The Administrator's decisions regarding the revisions to the then-
current standard that would appropriately achieve these public welfare
protection objectives were based on extensive air quality analyses that
extended from the then most recently available data (monitoring year
2013) back more than a decade (80 FR 65408, October 26, 2015; Wells,
2015). These analyses evaluated the cumulative seasonal exposure levels
in locations meeting different alternative levels for a standard of the
existing form and averaging time, indicating reductions in cumulative
exposures associated with air quality meeting lower levels of a
standard of the existing form and averaging time. Based on these
analyses, the Administrator judged that the desired level of public
welfare protection could be achieved with a secondary standard having a
revised level in combination with the existing form and averaging time
(80 FR 65408, October 26, 2015).
The air quality analyses described the occurrences of 3-year W126
index values of various magnitudes at monitor locations where
O3 concentrations met potential alternative standards; the
alternative standards were different levels for the current form and
averaging time (annual fourth-highest daily maximum 8-hour average
concentration, averaged over three consecutive years) (Wells, 2015). In
the then-most recent period, 2011-2013, across the more than 800
monitor locations meeting the then-current standard (with a level of 75
ppb), the 3-year W126 index values were above 17 ppm-hrs in 25 sites
distributed across different NOAA climatic regions, and above 19 ppm-
hrs at nearly half of these sites, with some well above. In comparison,
among sites meeting an alternative standard of 70 ppb, there were no
occurrences of a W126 value above 17 ppm-hrs and fewer than a handful
of occurrences that equaled 17 ppm-hrs.\118\ For the longer
[[Page 49879]]
time period (extending back to 2001), among the nearly 4000 instances
where a monitoring site met a standard level of 70 ppb, the
Administrator noted that there was only ``a handful of isolated
occurrences'' of 3-year W126 index values above 17 ppm-hrs, ``all but
one of which were below 19 ppm-hrs'' (80 FR 65409, October 26, 2015).
The Administrator concluded that that single value of 19.1 ppm-hrs
(just equaling 19, when rounded), observed at a monitor for the 3-year
period of 2006-2008, was reasonably regarded as an extremely rare and
isolated occurrence, and, as such, it was unclear whether it would
recur, particularly as areas across the U.S. took further steps to
reduce O3 to meet revised primary and secondary standards.
Further, based on all of the then available information, as noted
above, the Administrator did not judge RBL estimates associated with
marginally higher exposures in isolated, rare instances to be
indicative of adverse effects to the public welfare. The Administrator
concluded that a standard with a level of 70 ppb and the existing form
and averaging time would be expected to limit cumulative exposures, in
terms of a 3-year average W126 exposure index, to values at or below 17
ppm-hrs, in nearly all instances, and accordingly, to eliminate or
virtually eliminate cumulative exposures associated with a median RBL
of 6% or greater (80 FR 65409, October 26, 2015). Thus, using RBL as a
proxy in judging effects to public welfare, the Administrator judged
that such a standard with a level of 70 ppb would provide the requisite
protection from adverse effects to public welfare by limiting
cumulative seasonal exposures to 17 ppm-hrs or lower, in terms of a 3-
year W126 index, in nearly all instances.
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\118\ The more than 500 monitors that would meet an alternative
standard of 70 ppb during the 2011-2013 period were distributed
across all nine NOAA climatic regions and 46 of the 50 states
(Wells, 2015 and associated dataset in the docket [document
identifier, EPA-HQ-OAR-2008-0699-4325]).
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In summary, the Administrator judged that the revised standard
would protect natural forests in Class I and other similarly protected
areas against an array of adverse vegetation effects, most notably
including those related to effects on growth and productivity in
sensitive tree species. The Administrator additionally judged that the
revised standard would be sufficient to protect public welfare from
known or anticipated adverse effects. This judgment by the
Administrator appropriately recognized that the CAA does not require
that standards be set at a zero-risk level, but rather at a level that
reduces risk sufficiently so as to protect the public welfare from
known or anticipated adverse effects. Thus, based on the conclusions
drawn from the air quality analyses which demonstrated a strong,
positive relationship between the 8-hour and W126 metrics and the
findings that indicated the significant amount of control provided by
the fourth-high metric, the evidence base of O3 effects on
vegetation and her public welfare policy judgments, as well as public
comments and CASAC advice, the Administrator decided to retain the
existing form and averaging time and revise the level to 0.070 ppm,
judging that such a standard would provide the requisite protection to
the public welfare from any known or anticipated adverse effects
associated with the presence of O3 in ambient air (80 FR
65409-10, October 26, 2015).
2. Approach for the Current Review
To evaluate whether it is appropriate to consider retaining the now
current secondary O3 standard, or whether consideration of
revision is appropriate, the EPA has adopted an approach in this review
that builds upon the general approach used in the last review and
reflects the body of evidence and information now available.
Accordingly the approach in this review takes into consideration the
approach used in the last review, including the substantial assessments
and evaluations performed over the course of that review, and also
taking into account the more recent scientific information and air
quality data now available to inform understanding of the key policy-
relevant issues in the current review. As summarized above, the
Administrator's decisions in the prior review were based on an
integration of O3 welfare effects information with judgments
on the public welfare significance of key effects, policy judgments as
to when the standard is requisite, consideration of CASAC advice, and
consideration of public comments.
Similarly, in this review we draw on the current evidence and
quantitative analyses of air quality and exposure pertaining to the
welfare effects of O3 in ambient air. In so doing, we
consider both the information available at the time of the last review
and information more recently available, including that which has been
critically analyzed and characterized in the current ISA. The
evaluations in the PA, of the potential implications of various aspects
of the scientific evidence assessed in the ISA (building on prior such
assessments), augmented by the quantitative air quality, exposure or
risk-based information, are also considered along with the associated
uncertainties and limitations.
This review of the secondary O3 standard also considers
the August 2019 decision by the D.C. Circuit on the secondary standard
established in 2015 and issues raised by the court in its remand of
that standard to the EPA such that the decision in this review will
incorporate the EPA's response to this remand. The opinion issued by
the court concluded, in relevant part, that EPA had not provided a
sufficient rationale for aspects of its decision on the 2015 secondary
standard. See Murray Energy Corp. v. EPA, 936 F.3d 597 (D.C. Cir.
2019). Accordingly, the court remanded the secondary standard to EPA
for further justification or reconsideration, particularly in relation
to its decision to focus on a 3-year average for consideration of the
cumulative exposure, in terms of W126, identified as providing
requisite public welfare protection, and its decision to not identify a
specific level of air quality related to visible foliar injury.\119\
Thus, in addition to considering the currently available welfare
effects evidence and quantitative air quality, exposure and risk
information, this proposed decision on the secondary standard that was
established in 2015, and the associated proposed conclusions and
judgments, also consider the court's remand. In so doing, we have, for
example, expanded certain analyses in this review compared with those
conducted in the last review, included discussion on issues raised in
the remand, and provided additional explanation of rationales for
proposed conclusions on these points in this review. Together, the
information, evaluations and considerations recognized here inform the
Administrator's public welfare policy judgments and conclusions,
including his decision as to whether to retain or revise this standard.
---------------------------------------------------------------------------
\119\ The EPA's decision not to use a seasonal W126 index as the
form and averaging time of the secondary standard was also
challenged in this case, but the court did not reach that issue,
concluding that it lacked a basis to assess the EPA's rationale on
this point because the EPA had not yet fully explained its focus on
a 3-year average W126 in its consideration of the standard. See
Murray Energy Corp. v. EPA, 936 F.3d 597, 618 (D.C. Cir. 2019).
---------------------------------------------------------------------------
B. Welfare Effects Information
The information summarized here is based on our scientific
assessment of the welfare effects evidence available in this review;
this assessment is documented in the ISA \120\ and its policy
[[Page 49880]]
implications are further discussed in the PA. In this review, as in
past reviews, the health effects evidence evaluated in the ISA for
O3 and related photochemical oxidants is focused on
O3 (ISA, p. IS-3). Ozone is concluded to be the most
prevalent photochemical oxidant present in the atmosphere and the one
for which there is a very large, well-established evidence base of its
health and welfare effects. Further, ``the primary literature
evaluating the health and ecological effects of photochemical oxidants
includes ozone almost exclusively as an indicator of photochemical
oxidants'' (ISA, section IS.1.1). Thus, the current welfare effects
evidence and the Agency's review of the evidence, including the
evidence newly available in this review, continues to focus on
O3.
---------------------------------------------------------------------------
\120\ The ISA builds on evidence and conclusions from previous
assessments, focusing on synthesizing and integrating the newly
available evidence (ISA, section IS.1.1). Past assessments are cited
when providing further details not repeated in newer assessments.
---------------------------------------------------------------------------
More than 1600 studies are newly available and considered in the
ISA, including more than 500 studies on welfare effects (ISA, Appendix
10, Figure 10-2). While expanding the evidence for some effect
categories, studies on growth-related effects, a key group of effects
from the last review, are largely consistent with the evidence that was
previously available. Policy implications of the currently available
evidence are discussed in the PA (as summarized in section III.D.1
below). The subsections below briefly summarize the following aspects
of the evidence: The nature of O3-related welfare effects
(section III.B.1), the potential public welfare implications (section
III.B.2), and exposure concentrations associated with effects (section
III.B.3).
1. Nature of Effects
The welfare effects evidence base available in the current review
includes more than fifty years of extensive research on the phytotoxic
effects of O3, conducted both in and outside of the U.S.,
that documents the impacts of O3 on plants and their
associated ecosystems (1978 AQCD, 1986 AQCD, 1996 AQCD, 2006 AQCD, 2013
ISA, 2020 ISA). As was established in prior reviews, O3 can
interfere with carbon gain (photosynthesis) and allocation of carbon
within the plant, making fewer carbohydrates available for plant
growth, reproduction, and/or yield (1996 AQCD, pp. 5-28 and 5-29). For
seed-bearing plants, reproductive effects can include reduced seed or
fruit production or yield. The strongest evidence for effects from
O3 exposure on vegetation was recognized at the time of the
last review to be from controlled exposure studies, which ``have
clearly shown that exposure to O3 is causally linked to
visible foliar injury, decreased photosynthesis, changes in
reproduction, and decreased growth'' in many species of vegetation
(2013 ISA, p. 1-15). Such effects at the plant scale can also be linked
to an array of effects at larger spatial scales (and higher levels of
biological organization), with the evidence available in the last
review indicating that ``O3 exposures can affect ecosystem
productivity, crop yield, water cycling, and ecosystem community
composition'' (2013 ISA, p. 1-15, Chapter 9, section 9.4). Beyond its
effects on plants, the evidence in the last review also recognized
O3 in the troposphere as a major greenhouse gas (ranking
behind carbon dioxide and methane in importance), with associated
radiative forcing and effects on climate, and recognized the
accompanying ``large uncertainties in the magnitude of the radiative
forcing estimate . . . making the impact of tropospheric O3
on climate more uncertain than the effect of the longer-lived
greenhouse gases'' (2013 ISA, sections 10.3.4 and 10.5.1 [p. 10-30]).
The evidence newly available in this review supports, sharpens and
expands somewhat on the conclusions reached in the last review (ISA,
Appendices 8 and 9). Consistent with the evidence in the last review,
the currently available evidence describes an array of O3
effects on vegetation and related ecosystem effects, as well as the
role of O3 in radiative forcing and subsequent climate-
related effects. Evidence newly available in this review augments more
limited previously available evidence related to insect interactions
with vegetation, contributing to conclusions regarding O3
effects on plant-insect signaling (ISA, Appendix 8, section 8.7) and on
insect herbivores (ISA, Appendix 8, section 8.6), as well as for ozone
effects on tree mortality (Appendix 8, section 8.4). Thus, conclusions
reached in the last review are supported by the current evidence base
and conclusions are also reached in a few new areas based on the now
expanded evidence.
The current evidence base, including a wealth of longstanding
evidence, supports the conclusion of causal relationships between
O3 and visible foliar injury, reduced vegetation growth and
reduced plant reproduction,\121\ as well as reduced yield and quality
of agricultural crops, reduced productivity in terrestrial ecosystems,
alteration of terrestrial community composition,\122\ and alteration of
belowground biogeochemical cycles (ISA, section IS.5). Based on the
current evidence base, the ISA also concluded there likely to be a
causal relationship between O3 and alteration of ecosystem
water cycling, reduced carbon sequestration in terrestrial ecosystems,
and with increased tree mortality (ISA, section IS.5). Additional
evidence newly available in this review is concluded by the ISA to
support conclusions on two additional plant-related effects: The body
of evidence is concluded to be sufficient to infer that there is likely
to be a causal relationship between O3 exposure and
alteration of plant-insect signaling, and to infer that there is likely
to be a causal relationship between O3 exposure and altered
insect herbivore growth and reproduction (ISA, Table IS-12).
---------------------------------------------------------------------------
\121\ The 2013 ISA did not include a separate causality
determination for reduced plant reproduction. Rather, it was
included with the conclusion of a causal relationship with reduced
vegetation growth (ISA, Table IS-12).
\122\ The 2013 ISA concluded alteration of terrestrial community
composition to be likely causally related to O3 based on
the then available information (ISA, Table IS-12).
---------------------------------------------------------------------------
As in the last review, the strongest evidence and the associated
findings of causal or likely causal relationships with O3 in
ambient air, and the quantitative characterizations of relationships
between O3 exposure and occurrence and magnitude of effects
are for vegetation effects. The scales of these effects range from the
individual plant scale to the ecosystem scale, with potential for
impacts on the public welfare (as discussed in section III.B.2 below).
The following summary addresses the identified vegetation-related
effects of O3 across these scales.
The current evidence, consistent with the decades of previously
available evidence, documents and characterizes visible foliar injury
in many tree, shrub, herbaceous, and crop species as an effect of
exposure to O3 (ISA, Appendix 8, section 8.2; 2013 ISA,
section 9.4.2; 2006 AQCD, 1996 AQCD, 1986 AQCD, 1978 AQCD). As was also
stated in the last scientific assessment, ``[r]ecent experimental
evidence continues to show a consistent association between visible
injury and ozone exposure'' (ISA, Appendix 8, section 8.2, p. 8-13;
2013 ISA, section 9.4.2, p. 9-41). Ozone-induced visible foliar injury
symptoms on certain tree and herbaceous species, such as black cherry,
yellow-poplar and common milkweed, have long been considered diagnostic
of exposure to elevated O3 based on the consistent
association established with experimental evidence (ISA, Appendix 8,
section 8.2; 2013 ISA, p. 1-10).\123\
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\123\ As described in the ISA, ``[t]ypical types of visible
injury to broadleaf plants include stippling, flecking, surface
bleaching, bifacial necrosis, pigmentation (e.g., bronzing), and
chlorosis or premature senescence'' and ``[t]ypical visible injury
symptoms for conifers include chlorotic banding, tip burn, flecking,
chlorotic mottling, and premature senescence of needles'' (ISA,
Appendix 8, p. 8-13).
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[[Page 49881]]
The currently available evidence, consistent with that in past
reviews, indicates that ``visible foliar injury usually occurs when
sensitive plants are exposed to elevated ozone concentrations in a
predisposing environment,'' with a major factor for such an environment
being the amount of soil moisture available to the plant (ISA, Appendix
8, p. 8-23; 2013 ISA, section 9.4.2). Further, the significance of
O3 injury at the leaf and whole plant levels also depends on
an array of factors that include the amount of total leaf area
affected, age of plant, size, developmental stage, and degree of
functional redundancy among the existing leaf area (ISA, Appendix 8,
section 8.2; 2013 ISA, section 9.4.2). In this review, as in the past,
such modifying factors contribute to the difficulty in quantitatively
relating visible foliar injury to other vegetation effects (e.g.,
individual tree growth, or effects at population or ecosystem levels),
such that visible foliar injury ``is not always a reliable indicator of
other negative effects on vegetation'' (ISA, Appendix 8, section 8.2,
p. 8-24; 2013 ISA, p. 9-39).\124\
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\124\ Similar to the 2013 ISA, the ISA for the current review
states the following (ISA, pp. 8-24).
Although visible injury is a valuable indicator of the presence
of phytotoxic concentrations of ozone in ambient air, it is not
always a reliable indicator of other negative effects on vegetation
[e.g., growth, reproduction; U.S. EPA (2013)]. The significance of
ozone injury at the leaf and whole-plant levels depends on how much
of the total leaf area of the plant has been affected, as well as
the plant's age, size, developmental stage, and degree of functional
redundancy among the existing leaf area (U.S. EPA, 2013). Previous
ozone AQCDs have noted the difficulty in relating visible foliar
injury symptoms to other vegetation effects, such as individual
plant growth, stand growth, or ecosystem characteristics (U.S. EPA,
2006, 1996). Thus, it is not presently possible to determine, with
consistency across species and environments, what degree of injury
at the leaf level has significance to the vigor of the whole plant.
---------------------------------------------------------------------------
Consistent with conclusions in past reviews, the evidence,
extending back several decades, continues to document the detrimental
effects of O3 on plant growth and reproduction (ISA,
Appendix 8, sections 8.3 and 8.4; 2013 ISA, p. 9-42). The available
studies come from a variety of different study types that cover an
array of different species, effects endpoints, and exposure methods and
durations. In addition to studies on scores of plant species that have
found O3 to reduce plant growth, the evidence accumulated
over the past several decades documents O3 alteration of
allocation of biomass within the plant and plant reproduction (ISA,
Appendix 8, sections 8.3 and 8.4; 2013 ISA, p. 1-10). The biological
mechanisms underlying the effect of O3 on plant reproduction
include ``both direct negative effects on reproductive tissues and
indirect negative effects that result from decreased photosynthesis and
other whole plant physiological changes'' (ISA, p. IS-71). A newly
available meta-analysis of more than 100 studies published between 1968
and 2010 summarizes effects of O3 on multiple measures of
reproduction (ISA, Appendix 8, section 8.4.1).
Studies involving experimental field sites have also reported
effects on measures of plant reproduction, such as effects on seeds
(reduced weight, germination, and starch levels) that could lead to a
negative impact on species regeneration in subsequent years, and bud
size that might relate to a delay in spring leaf development (ISA,
Appendix 8, section 8.4; 2013 ISA, section 9.4.3; Darbah et al., 2007,
Darbah et al., 2008). A more recent laboratory study reported 6-hour
daily O3 exposures of flowering mustard plants to 100 ppb
during different developmental stages to have mixed effects on
reproductive metrics. While flowers exposed early versus later in
development produced shorter fruits, the number of mature seeds per
fruit was not significantly affected by flower developmental stage of
exposure (ISA, Appendix 8, section 8.4.1; Black et al., 2012). Another
study assessed seed viability for a flowering plant in laboratory and
field conditions, finding effects on seed viability of O3
exposures (90 and 120 ppb) under laboratory conditions but less clear
effects under more field-like conditions (ISA, Appendix 8, section
8.4.1; Landesmann et al., 2013).
With regard to agricultural crops, the current evidence base, as in
the last review, is sufficient to infer a causal relationship between
O3 exposure and reduced yield and quality (ISA, section
IS.5.1.2). The current evidence is augmented by new research in a
number of areas, including studies on soybean, wheat and other nonsoy
legumes. The new information assessed in the ISA remains consistent
with the conclusions reached in the 2013 ISA (ISA, section IS.5.1.2).
The evidence base for trees includes a number of studies conducted
at the Aspen free-air carbon-dioxide and ozone enrichment (FACE)
experiment site in Wisconsin (that operated from 1998 through 2011) and
also available in the last review (ISA, IS.5.1 and Appendix 8, section
8.1.2.1; 2013 ISA, section 9.2.4). These studies, which occurred in a
field setting (more similar to natural forest stands than open-top-
chamber studies), reported reduced tree growth when grown in single or
three species stands within 30-m diameter rings and exposed over one or
more years to elevated O3 concentrations (hourly
concentrations 1.5 times concentrations in ambient air at the site)
compared to unadjusted ambient air concentrations (2013 ISA, section
9.4.3; Kubiske et al., 2006, Kubiske et al., 2007).\125\
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\125\ Seasonal (90-day) W126 index values for unadjusted
O3 concentrations over six years of the Aspen FACE
experiments ranged from 2 to 3 ppm-hrs, while the elevated exposure
concentrations (reflecting addition of O3 to ambient air
concentrations) ranged from somewhat above 20 to somewhat above 35
ppm-hrs (ISA, Appendix 8, Figure 8-17).
---------------------------------------------------------------------------
With regard to tree mortality, the 2013 ISA did not include a
determination of causality (ISA, Appendix 8, section 8.4). While the
then-available evidence included studies identifying ozone as a
contributor to tree mortality, which contributed to the 2013 conclusion
regarding O3 and alteration of community composition (2013
ISA, section 9.4.7.4), a separate causality determination regarding
O3 and tree mortality was not assessed (ISA, Appendix 8,
section 8.4; 2013 ISA, Table 9-19). The evidence assessed in the 2013
ISA (and 2006 AQCD) was largely observational, including studies that
reported declines in conifer forests for which elevated O3
was identified as contributor but in which a variety of environmental
factors may have also played a role (2013 ISA, section 9.4.7.1; 2006
AQCD, sections AX9.6.2.1, AX9.6.2.2, AX9.6.2.6, AX9.6.4.1 and
AX9.6.4.2). Since the last review, three additional studies are
available (ISA, Appendix 8, Table 8-9). Two of these are analyses of
field observations, one of which is set in the Spanish Pyrenees.\126\ A
second study is a large-scale empirical statistical analysis of factors
potentially contributing to tree mortality in eastern and central U.S.
forests during the 1971-2005 period, which reported O3
(county-level 11-year [1996-2006] average 8 hour metric) \127\ to be
ninth among the 13 potential factors assessed \128\ and to have a
[[Page 49882]]
significant positive correlation with tree mortality (ISA, section
IS.5.1.2, Appendix 8, section 8.4.3; Dietze and Moorcroft, 2011). A
newly available experimental study also reported increased mortality in
two of five aspen genotypes grown in mixed stands under elevated
O3 concentrations (ISA, section IS.5.1.2; Moran and Kubiske,
2013). Coupled with the plant-level evidence of phytotoxicity discussed
above, as well as consideration of community composition effects, this
evidence was concluded to indicate the potential for elevated
O3 concentrations to contribute to tree mortality (ISA,
section IS.5.1.2 and Appendix 8, sections 8.4.3 and 8.4.4). Based on
the current evidence, the ISA concludes there is likely to be a causal
relationship between O3 and increased tree mortality (ISA,
Table IS-2, Appendix 8, section 8.4.4). A variety of factors in natural
environments can either mitigate or exacerbate predicted O3-
plant interactions and are recognized sources of uncertainty and
variability. Such factors at the plant level include multiple
genetically influenced determinants of O3 sensitivity,
changing sensitivity to O3 across vegetative growth stages,
co-occurring stressors and/or modifying environmental factors (ISA,
Appendix 8, section 8.12).
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\126\ The concentration gradient with altitude in the Spanish
study, includes--at the highest site--annual average April-to-
September O3 concentrations for the 2004 to 2007 period
that range up to 74 ppb (Diaz-de-Quijano et al., 2016).
\127\ Annual fourth highest daily maximum 8-hour O3
concentrations in these regions were above 80 ppb in the early 2000s
and median design values at national trend sites were nearly 85 ppb
(PA, Figures 2-11 and 2-12).
\128\ This statistical analysis, which utilized datasets from
within the 1971-2005 period, included an examination of the
sensitivity of predicted mortality rate to 13 different covariates.
On average across the predictions for 10 groups of trees (based on
functional type and major representative species), the order of
mortality rate sensitivity to the covariates, from highest to
lowest, was: Sulfate deposition, tree diameter, nitrate deposition,
summer temperature, tree age, elevation, winter temperature,
precipitation, O3 concentration, tree basal area,
topographic moisture index, slope and topographic radiation index
(Dietze and Moorcroft, 2011).
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Ozone-induced effects at the scale of the whole plant have the
potential to translate to effects at the ecosystem scale, such as
reduced terrestrial productivity and carbon storage, and altered
terrestrial community composition, as well as impacts on ecosystem
functions, such as belowground biogeochemical cycles and ecosystem
water cycling. For example, under the relevant exposure conditions,
O3-related reduced tree growth and reproduction, as well as
increased mortality, could lead to reduced ecosystem productivity.
Recent studies from the Aspen FACE experiment and modeling simulations
indicate that O3-related negative effects on ecosystem
productivity may be temporary or may be limited in some systems (ISA,
Appendix 8, section 8.8.1). Previously available studies had reported
impacts on productivity in some forest types and locations, such as
ponderosa pine in southern California and other forest types in the
mid-Atlantic region (2013 ISA, section 9.4.3.4). Through reductions in
sensitive species growth, and related ecosystem productivity,
O3 could lead to reduced ecosystem carbon storage (ISA,
IS.5.1.4; 2013 ISA, section 9.4.3). With regard to forest community
composition, available studies have reported changes in tree
communities composed of species with relatively greater and relatively
lesser sensitivity to O3 (ISA, section IS.5.1.8.1, Appendix
8, section 8.10; 2013 ISA, section 9.4.3; Kubiske et al., 2007). As the
ISA concludes, ``[t]he extent to which ozone affects terrestrial
productivity will depend on more than just community composition, but
other factors, which both directly influence [net primary productivity]
(i.e., availability of N and water) and modify the effect of ozone on
plant growth'' (ISA, Appendix 8, section 8.8.1). Thus, the magnitude of
O3 impact on ecosystem productivity, as on forest
composition, can vary among plant communities based on several factors,
including the type of stand or community in which the sensitive species
occurs (e.g., single species versus mixed canopy), the role or position
of the species in the stand (e.g., dominant, sub-dominant, canopy,
understory), and the sensitivity of co-occurring species and
environmental factors (e.g., drought and other factors).
The effects of O3 on plants and plant populations have
implications for ecosystem functions. Two such functions, effects with
which O3 is concluded to be likely causally or causally
related, are ecosystem water cycling and belowground biogeochemical
cycles, respectively (ISA, Appendix 8, sections 8.11 and 8.9). With
regard to the former, the effects of O3 on plants (e.g., via
stomatal control, as well as leaf and root growth and changes in wood
anatomy associated with water transport) can affect ecosystem water
cycling through impacts on root uptake of soil moisture and groundwater
as well as transpiration through leaf stomata to the atmosphere (ISA,
Appendix 8, section 8.11.1). These ``impacts may in turn affect the
amount of water moving through the soil, running over land or through
groundwater and flowing through streams'' (ISA, Appendix 8, p. 8-161).
Evidence newly available in this review is supportive of previously
available evidence in this regard (ISA, Appendix 8, section 8.11.6).
The current evidence, including that newly available, indicates the
extent to which the effects of O3 on plant leaves and roots
(e.g., through effects on chemical composition and biomass) can impact
belowground biogeochemical cycles involving root growth, soil food web
structure, soil decomposer activities, soil microbial respiration, soil
carbon turnover, soil water cycling and soil nutrient cycling (ISA,
Appendix 8, section 8.9).
Additional vegetation-related effects with implications beyond
individual plants include the effects of O3 on insect
herbivore growth and reproduction and plant-insect signaling (ISA,
Table IS-12, Appendix 8, sections 8.6 and 8.7). With regard to insect
herbivore growth and reproduction, the evidence includes multiple
effects in an array of insect species, although without a consistent
pattern of response for most endpoints (ISA, Appendix 8, Table 8-11).
As was also the case with the studies available at the time of the last
review,\129\ in the newly available studies individual-level responses
are highly context- and species-specific and not all species tested
showed a response (ISA, section IS.5.1.3 and Appendix 8, section 8.6).
Evidence on plant-insect signaling that is newly available in this
review comes from laboratory, greenhouse, open top chambers (OTC) and
FACE experiments (ISA, section IS.5.1.3 and Appendix 8, section 8.7).
The available evidence indicates a role for elevated O3 in
altering and degrading emissions of chemical signals from plants and
reducing detection of volatile plant signaling compounds (VPSCs) by
insects, including pollinators. Elevated O3 concentrations
degrade some VPSCs released by plants, potentially affecting ecological
processes including pollination and plant defenses against herbivory.
Further, the available studies report elevated O3 conditions
to be associated with plant VPSC emissions that may make a plant either
more attractive or more repellant to herbivorous insects, and to
predators and parasitoids that target phytophagous (plant-eating)
insects (ISA, section IS.5.1.3 and Appendix 8, section 8.7).
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\129\ During the last review, the 2013 ISA stated with regard to
O3 effects on insects and other wildlife that ``there is
no consensus on how these organisms respond to elevated
O3'' (2013 ISA, section 9.4.9.4, p. 9-98).
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Ozone welfare effects also extend beyond effects on vegetation and
associated biota due to it being a major greenhouse gas and radiative
forcing agent.\130\ As in the last review, the
[[Page 49883]]
current evidence, augmented since the 2013 ISA, continues to support a
causal relationship between the global abundance of O3 in
the troposphere and radiative forcing, and a likely causal relationship
between the global abundance of O3 in the troposphere and
effects on temperature, precipitation, and related climate variables
\131\ (ISA, section IS.5.2 and Appendix 9; Myhre et al., 2013). As was
also true at the time of the last review, tropospheric O3
has been ranked third in importance for global radiative forcing, after
carbon dioxide and methane, with the radiative forcing of O3
since pre-industrial times estimated to be about 25 to 40% of the total
warming effects of anthropogenic carbon dioxide and about 75% of the
effects of anthropogenic methane (ISA, Appendix 9, section 9.1.3.3).
Uncertainty in the magnitude of radiative forcing estimated to be
attributed to tropospheric O3 is a contributor to the
relatively greater uncertainty associated with climate effects of
tropospheric O3 compared to such effects of the well mixed
greenhouse gases, such as carbon dioxide and methane (ISA, section
IS.6.2.2).
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\130\ Radiative forcing is a metric used to quantify the change
in balance between radiation coming into and going out of the
atmosphere caused by the presence of a particular substance. The ISA
describes it more specifically as ``a perturbation in net radiative
flux at the tropopause (or top of the atmosphere) caused by a change
in radiatively active forcing agent(s) after stratospheric
temperatures have readjusted to radiative equilibrium
(stratospherically adjusted RF)'' (ISA, Appendix 9, section
9.1.3.3).
\131\ Effects on temperature, precipitation, and related climate
variables were referred to as ``climate change'' or ``effects on
climate'' in the 2013 ISA (ISA, p. IS-82; 2013 ISA, pp. 1-14 and 10-
31).
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Lastly, the evidence regarding tropospheric O3 and UV-B
shielding (shielding of ultraviolet radiation at wavelengths of 280 to
320 nanometers) was evaluated in the 2013 ISA and determined to be
inadequate to draw a causal conclusion (2013 ISA, section 10.5.2). The
current ISA concludes there to be no new evidence since the 2013 ISA
relevant to the question of UV-B shielding by tropospheric
O3 (ISA, IS.1.2.1 and Appendix 9, section 9.1.3.4).
2. Public Welfare Implications
The secondary standard is to ``specify a level of air quality the
attainment and maintenance of which in the judgment of the
Administrator . . . is requisite to protect the public welfare from any
known or anticipated adverse effects associated with the presence of
such air pollutant in the ambient air'' (CAA, section 109(b)(2)). As
recognized in prior reviews, the secondary standard is not meant to
protect against all known or anticipated O3-related welfare
effects, but rather those that are judged to be adverse to the public
welfare, and a bright-line determination of adversity is not required
in judging what is requisite (78 FR 3212, January 15, 2013; 80 FR
65376, October 26, 2015; see also 73 FR 16496, March 27, 2008). Thus,
the level of protection from known or anticipated adverse effects to
public welfare that is requisite for the secondary standard is a public
welfare policy judgment to be made by the Administrator. In each
review, the Administrator's judgment regarding the currently available
information and adequacy of protection provided by the current standard
is generally informed by considerations in prior reviews and associated
conclusions.
The categories of effects identified in the CAA to be included
among welfare effects are quite diverse,\132\ and among these
categories, any single category includes many different types of
effects that are of broadly varying specificity and level of
resolution. For example, effects on vegetation, is a category
identified in CAA section 302(h), and the ISA recognizes numerous
vegetation-related effects of O3 at the organism,
population, community and ecosystem level, as summarized in section
III.B.1 above (ISA, Appendix 8). The significance of each type of
vegetation-related effect with regard to potential effects on the
public welfare depends on the type and severity of effects, as well as
the extent of such effects on the affected environmental entity, and on
the societal use of the affected entity and the entity's significance
to the public welfare. Such factors are generally considered in light
of judgments and conclusions made in prior reviews regarding effects on
the public welfare. For example, a key consideration with regard to
public welfare implications in prior reviews of the O3
secondary standard was the intended use of the affected or sensitive
vegetation and the significance of the vegetation to the public welfare
(73 FR 16496, March 27, 2008; 80 FR 65292, October 26, 2015).
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\132\ Section 302(h) of the CAA states that language referring
to ``effects on welfare'' in the CAA ``includes, but is 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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More specifically, judgments regarding public welfare significance
in the last two O3 NAAQS decisions gave particular attention
to O3 effects in areas with special federal protections, and
lands set aside by states, tribes and public interest groups to provide
similar benefits to the public welfare (73 FR 16496, March 27, 2008; 80
FR 65292, October 26, 2015). For example, in the decision to revise the
secondary standard in the 2008 review, the Administrator took 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'' (73 FR 16496, March 27, 2008).\133\ Such areas
include Class I areas \134\ which are federally mandated to preserve
certain air quality related values. Additionally, as the Administrator
recognized, ``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'' (73 FR 16496, March 27, 2008). The
Administrator took note of 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 lands contain
O3-sensitive species'' (73 FR 16496, March 27, 2008).
Similarly, in the 2015 review, the Administrator indicated particular
concern for O3-related effects on plant function and
productivity and associated ecosystem effects in natural ecosystems
``such as those in areas with protection designated by Congress for
current and future generations, as well
[[Page 49884]]
as areas similarly set aside by states, tribes and public interest
groups with the intention of providing similar benefits to the public
welfare'' (80 FR 65403, October 26, 2015).
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\133\ For example, the fundamental purpose of parks in the
National Park System ``is to conserve the scenery, natural and
historic objects, and wild life in the System units and to provide
for the enjoyment of the scenery, natural and historic objects, and
wild life in such manner and by such means as will leave them
unimpaired for the enjoyment of future generations'' (54 U.S.C.
100101). Additionally, the Wilderness Act of 1964 defines designated
``wilderness areas'' in part as areas ``protected and managed so as
to preserve [their] natural conditions'' and requires that these
areas ``shall be administered for the use and enjoyment of the
American people in such manner as will leave them unimpaired for
future use and enjoyment as wilderness, and so as to provide for the
protection of these areas, [and] the preservation of their
wilderness character . . .'' (16 U.S.C. 1131 (a) and (c)). Other
lands that benefit the public welfare include national forests which
are managed for multiple uses including sustained yield management
in accordance with land management plans (see 16 U.S.C. 1600(1)-(3);
16 U.S.C. 1601(d)(1)).
\134\ Areas designated as Class I include all international
parks, national wilderness areas which exceed 5,000 acres in size,
national memorial parks which exceed 5,000 acres in size, and
national parks which exceed 6,000 acres in size, provided the park
or wilderness area was in existence on August 7, 1977. Other areas
may also be Class I if designated as Class I consistent with the CAA
(as described in the PA, Appendix 4D, section 4D.2.4).
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The 2008 and 2015 decisions recognized that the degree to which
effects on vegetation in specially protected areas, such as those
identified above, may be judged adverse involves considerations from
the species level to the ecosystem level, such that judgments can
depend on the intended use for, or service (and value) of, the affected
vegetation, ecological receptors, ecosystems and resources and the
significance of that use to the public welfare (73 FR 16496, March 27,
2008; 80 FR 65377, October 26, 2015). Uses or services provided by
areas that have been afforded special protection can flow in part or
entirely from the vegetation that grows there. For example, ecosystem
services are the ``benefits that people derive from functioning
ecosystems'' (Costanza et al., 2017; ISA, section IS.5.1).\135\
Ecosystem services range from those directly related to the natural
functioning of the ecosystem to ecosystem uses for human recreation or
profit, such as through the production of lumber or fuel (Costanza et
al., 2017). Aesthetic value and outdoor recreation depend, at least in
part, on the perceived scenic beauty of the environment. Further, there
have been analyses that report the American public values--in monetary
as well as nonmonetary ways--the protection of forests from air
pollution damage (Haefele et al., 1991). In fact, public surveys have
indicated that Americans rank as very important the existence of
resources, the option or availability of the resource and the ability
to bequest or pass it on to future generations (Cordell et al., 2008).
The spatial, temporal and social dimensions of public welfare impacts
are also influenced by the type of service affected. For example, a
national park can provide direct recreational services to the thousands
of visitors that come each year, but also provide an indirect value to
the millions who may not visit but receive satisfaction from knowing
that it exists and is preserved for the future (80 FR 65377, October
26, 2015).
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\135\ Ecosystem services analyses were one of the tools used in
the last review of the secondary standards for oxides of nitrogen
and sulfur to inform the decisions made with regard to adequacy of
protection provided by the standards and as such, were used in
conjunction with other considerations in the discussion of adversity
to public welfare (77 FR 20241, April 3, 2012).
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The different types of effects on vegetation discussed in section
III.B.1 above differ with regard to aspects important to judging their
public welfare significance. In the case of crop yield loss, such
judgments depend on considerations related to the heavy management of
agriculture in the U.S. Judgments for other categories of effects may
generally relate to considerations regarding forested areas, including
specifically those forested areas that are not managed for harvest. For
example, effects on tree growth and reproduction, and also visible
foliar injury, have the potential to be significant to the public
welfare through impacts in Class I and other areas given special
protection in their natural/existing state, although they differ in how
they might be significant. Additionally, as described in section
III.B.1 above, O3 effects on tree growth and reproduction
could, depending on severity, extent and other factors, lead to effects
on a larger scale including reduced productivity, altered forest and
forest community (plant, insect and microbe) composition, reduced
carbon storage and altered ecosystem water cycling (ISA, section
IS.5.1.8.1; 2013 ISA, Figure 9-1, sections 9.4.1.1 and 9.4.1.2). For
example, forest or forest community composition can be affected through
O3 effects on growth and reproductive success of sensitive
species in the community, with the extent of compositional changes
dependent on factors such as competitive interactions (ISA, section
IS.5.1.8.1; 2013 ISA, sections 9.4.3 and 9.4.3.1). Impacts on some of
these characteristics (e.g., forest or forest community composition)
may be considered of greater public welfare significance when occurring
in Class I or other protected areas, due to value for particular
services that the public places on such areas.
Depending on the type and location of the affected ecosystem,
however, a broader array of services benefitting the public can be
affected in a broader array of areas as well. For example, other
services valued by people that can be affected by reduced tree growth,
productivity and associated forest effects include aesthetic value,
food, fiber, timber, other forest products, habitat, recreational
opportunities, climate and water regulation, erosion control, air
pollution removal, and desired fire regimes (PA, Figure 4-2; ISA,
section IS.5.1; 2013 ISA, sections 9.4.1.1 and 9.4.1.2). In considering
such services in past reviews, the Agency has given particular
attention to effects in natural ecosystems, indicating that a
protective standard, based on consideration of effects in natural
ecosystems in areas afforded special protection, would also ``provide a
level of protection for other vegetation that is used by the public and
potentially affected by O3 including timber, produce grown
for consumption and horticultural plants used for landscaping'' (80 FR
65403, October 26, 2015). For example, locations potentially vulnerable
to O3-related impacts might include forested lands, both
public and private, where trees are grown for timber production.
Forests in urbanized areas also provide a number of services that are
important to the public in those areas, such as air pollution removal,
cooling, and beautification. There are also many other tree species,
such as various ornamental and agricultural species (e.g., Christmas
trees, fruit and nut trees), that provide ecosystem services that may
be judged important to the public welfare.
Depending on its severity and spatial extent, visible foliar
injury, which affects the physical appearance of the plant, also has
the potential to be significant to the public welfare through impacts
in Class I and other similarly protected areas. In cases of widespread
and severe injury during the growing season (particularly when
sustained across multiple years, and accompanied by obvious impacts on
the plant canopy), O3-induced visible foliar injury might be
expected to have the potential to impact the public welfare in scenic
and/or recreational areas, particularly in areas with special
protection, such as Class I areas.\136\ The ecosystem services most
likely to be affected by O3-induced visible foliar injury
(some of which are also recognized above for tree growth-related
effects) are cultural services, including aesthetic value and outdoor
recreation.
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\136\ For example, although analyses specific to visible foliar
injury are of limited availability, there have been analyses
developing estimates of recreation value damages of severe impacts
related to other types of forest effects, such as tree mortality due
to bark beetle outbreaks (e.g., Rosenberger et al., 2013). Such
analyses estimate reductions in recreational use when the damage is
severe (e.g., reductions in the density of live, robust trees). Such
damage would reasonably be expected to also reflect damage
indicative of injury with which a relationship with other plant
effects (e.g., growth and reproduction) would be also expected.
Similarly, a couple of studies from the 1970s and 1980s indicated
likelihood for reduced recreational use in areas with stands of pine
in which moderate to severe injury was apparent from 30 or 40 feet
(PA, section 4.3.2).
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The geographic extent of protected areas that may be vulnerable to
public welfare effects of O3, such as impacts to outdoor
recreation, is potentially appreciable. For example, biomonitoring
surveys that were routinely administered by the U.S.
[[Page 49885]]
Forest Service (USFS) as far back as 1994 in the eastern U.S. and 1998
in the western U.S. include many field sites at which there are plants
sensitive to O3-related visible foliar injury; there are 450
field sites across 24 states in the North East and North Central
regions (Smith, 2012).\137\ Since visible foliar injury is a visible
indication of O3 exposure in species sensitive to this
effect, a number of such species have been established as bioindicator
species, and such surveys have been used by federal land managers as
tools in assessing potential air quality impacts in Class I areas (U.S.
Forest Service, 2010). Additionally, the USFS has developed categories
for the scoring system that it uses for purposes of describing and
comparing injury severity at biomonitoring sites. The sites are termed
biosites and the scoring system involves deriving biosite index (BI)
scores that may be described with regard to one of several categories
ranging from little or no foliar injury to severe injury (e.g., Smith
et al., 2003; Campbell et al., 2007; Smith et al., 2007; Smith,
2012).\138\ As noted in section III.B.1 above, there is not an
established quantitative relationship between visible foliar injury and
other effects, such as reduced growth and productivity as visible
foliar injury ``is not always a reliable indicator of other negative
effects'' (ISA, Appendix 8, section 8.2).
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\137\ This aspect of the USFS biomonitoring surveys has
apparently been suspended, with the most recent surveys conducted in
2011 (USFS, 2013, USFS, 2017).
\138\ Studies presenting USFS biomonitoring program data have
suggested what might be ``assumptions of risk'' related to scores in
these categories, e.g., none, low, moderate and high for BI scores
of zero to five, five to 15, 15 to 25 and above 25, respectively
(e.g., Smith et al., 2003; Smith et al., 2012).
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Public welfare implications associated with visible foliar injury
might further be considered to relate largely to effects on scenic and
aesthetic values. The available information does not yet address or
describe the relationships expected to exist between some level of
injury severity (e.g., little, low/light, moderate or severe) and/or
spatial extent affected and scenic or aesthetic values. This gap
impedes consideration of the public welfare implications of different
injury severities, and accordingly judgments on the potential for
public welfare significance. That notwithstanding, while minor spotting
on a few leaves of a plant may easily be concluded to be of little
public welfare significance, some level of severity and widespread
occurrence of visible foliar injury, particularly if occurring in
specially protected areas, such as Class I areas, where the public can
be expected to place value (e.g., for recreational uses), might
reasonably be concluded to impact the public welfare. Accordingly, key
considerations for public welfare significance of this endpoint would
relate to qualitative consideration of the potential for such effects
to affect the aesthetic value of plants in protected areas, such as
Class I areas (73 FR 16490, March 27, 2008).
While, as noted above, public welfare benefits of forested lands
can be particular to the type of area in which the forest occurs, some
of the potential public welfare benefits associated with forest
ecosystems are not location dependent. A potentially extremely valuable
ecosystem service provided by forested lands is carbon sequestration or
storage (ISA, section IS.5.1.4 and Appendix 8, section 8.8.3; 2013 ISA,
section 2.6.2.1 and p. 9-37).\139\ As noted above, the EPA has
concluded that effects on this ecosystem service are likely causally
related to O3 in ambient air (ISA, Table IS-12). The
importance of carbon sequestration to the public welfare relates to its
role in counteracting the impact of greenhouse gases on radiative
forcing and related climate effects. As summarized in section III.B.1
above, O3 is also a greenhouse gas and O3
abundance in the troposphere is causally related to radiative forcing
and likely causally related to subsequent effects on temperature,
precipitation and related climate variables (ISA, section IS.6.2.2).
Accordingly, such effects also have important public welfare
implications, although their quantitative evaluation in response to
O3 concentrations in the U.S. is complicated by ``[c]urrent
limitations in climate modeling tools, variation across models, and the
need for more comprehensive observational data on these effects'' (ISA,
section IS.6.2.2). The service of carbon storage is of paramount
importance to the public welfare no matter in what location the trees
are growing or what their intended current or future use (e.g., 2013
ISA, section 9.4.1.2). In other words, the benefit exists as long as
the trees are growing, regardless of what additional functions and
services it provides.
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\139\ While carbon sequestration or storage also occurs for
vegetated ecosystems other than forests, it is relatively larger in
forests given the relatively greater biomass for trees compared to
other plants.
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With regard to agriculture-related effects, the EPA has recognized
other complexities related to areas and plant species that are heavily
managed to obtain a particular output (such as commodity crops or
commercial timber production). For example, the EPA has recognized that
the degree to which O3 impacts on vegetation that could
occur in such areas and on such species would impair the intended use
at a level that might be judged adverse to the public welfare has been
less clear (80 FR 65379, October 26, 2015; 73 FR 16497, March 27,
2008). While having sufficient crop yields is of high public welfare
value, important commodity crops are typically heavily managed to
produce optimum yields. Moreover, based on the economic theory of
supply and demand, increases in crop yields would be expected to result
in lower prices for affected crops and their associated goods, which
would primarily benefit consumers. These competing impacts on producers
and consumers complicate consideration of these effects in terms of
potential adversity to the public welfare (2014 WREA, sections 5.3.2
and 5.7). When agricultural impacts or vegetation effects in other
areas are contrasted with the emphasis on ecosystem effects in Class I
and similarly protected areas, the EPA most recently has judged the
significance to the public welfare of O3-induced effects on
sensitive vegetation growing within the U.S. to differ depending on the
nature of the effect, the intended use of the sensitive plants or
ecosystems, and the types of environments in which the sensitive
vegetation and ecosystems are located, with greater significance
ascribed to areas identified for specific uses and benefits to the
public welfare, such as Class I areas, than to areas for which such
uses have not been established (80 FR 65292, October 26, 2015; FR 73
16496-16497, March 27, 2008).
Categories of effects newly identified as likely causally related
to O3 in ambient air, such as alteration of plant-insect
signaling and insect herbivore growth and reproduction, also have
potential public welfare implications. For example, given the role of
plant-insect signaling in such important ecological processes as insect
herbivore growth and reproduction. The potential to contribute to
adverse effects to the public welfare, e.g., given the role of the
plant-insect signaling process in pollination and seed dispersal, as
well as natural plant defenses against predation and parasitism,
particular effects on particular signaling processes can be seen to
have the potential for adverse effects on the public welfare (ISA,
section IS.5.1.3). However, uncertainties and limitations in the
current evidence (e.g., summarized in sections III.B.3 and III.D.1
below) preclude an assessment of the extent
[[Page 49886]]
and magnitude of O3 effects on these endpoints, which thus
also precludes an evaluation of the potential for associated public
welfare implications, particularly under exposure conditions expected
to occur in areas meeting the current standard.
In summary, several considerations are recognized as important to
judgments on the public welfare significance of the array of welfare
effects of different O3 exposure conditions. There are
uncertainties and limitations associated with the consideration of the
magnitude of key welfare effects that might be concluded to be adverse
to ecosystems and associated services. There are numerous locations
where the presence of O3-sensitive tree species may
contribute to a vulnerability to impacts from O3 on tree
growth, productivity and carbon storage and their associated ecosystems
and services. Exposures that may elicit effects and the significance of
the effects in specific situations can vary due to differences in
exposed species sensitivity, the severity and associated significance
of the observed or predicted O3-induced effect, the role
that the species plays in the ecosystem, the intended use of the
affected species and its associated ecosystem and services, the
presence of other co-occurring predisposing or mitigating factors, and
associated uncertainties and limitations.
3. Exposures Associated With Effects
The welfare effects identified in section III.B.1 above vary widely
with regard to the extent and level of detail of the available
information that describes the O3 exposure circumstances
that may elicit them. As recognized in the 2013 ISA and in the ISA for
this review, such information is most advanced for growth-related
effects such as growth and yield. For example, the information on
exposure metric and E-R relationships for these effects is long-
standing, having been first described in the 1997 review. The current
information regarding exposure metrics and relationships between
exposure and the occurrence and severity of visible foliar injury,
summarized in section III.B.3.b below, is much less advanced or well
established. The evidence base for other categories of effects is still
more lacking in information that might support characterization of
potential impacts related to these effects of changes in O3
concentrations.
a. Growth-Related Effects
(i) Exposure Metric
The long-standing body of vegetation effects evidence includes a
wealth of information on aspects of O3 exposure that are
important in influencing effects on plant growth and yield that has
been described in the scientific assessments across the last several
decades (1996 AQCD; 2006 AQCD; 2013 ISA; 2020 ISA). A variety of
factors have been investigated, including ``concentration, time of day,
respite time, frequency of peak occurrence, plant phenology,
predisposition, etc.'' (2013 ISA, section 9.5.2), and the importance of
the duration of the exposure as well as the relatively greater
importance of higher concentrations over lower concentrations have been
consistently well documented (2013 ISA, section 9.5.3). Based on the
associated improved understanding of the biological basis for plant
response to O3 exposure, a number of mathematical approaches
have been developed for summarizing O3 exposure for the
purpose of assessing effects on vegetation, including those that
cumulate exposures over some specified period while weighting higher
concentrations more than lower (2013 ISA, sections 9.5.2 and 9.5.3;
ISA, Appendix 8, section 8.2.2.2).
In the last several reviews, based on the then-available evidence,
as well as advice from the CASAC, the EPA's scientific assessments have
focused on the use of a cumulative, seasonal \140\ concentration-
weighted index for considering the growth-related effects evidence and
in quantitative exposure analyses for purposes of reaching conclusions
on the secondary standard. More specifically, the Agency used the W126-
based cumulative, seasonal metric (80 FR 65404, October 26, 2015; ISA,
section IS.3.2, Appendix 8, section 8.13). This metric, commonly called
the W126 index, is a non-threshold approach described as the
sigmoidally weighted sum of all hourly O3 concentrations
observed during a specified daily and seasonal time window, where each
hourly O3 concentration is given a weight that increases
from zero to one with increasing concentration (2013 ISA, pp. 9-101, 9-
104).
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\140\ The ``seasonal'' descriptor refers to the duration of the
period quantified (3 months) rather than a specific season of the
year.
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Across the last several decades, several different exposure metrics
have been evaluated, primarily for their ability to summarize ambient
air O3 concentrations into a metric that best describes
quantitatively the relationship of O3 in ambient air with
the occurrence and/or extent of effects on vegetation, particularly
growth-related effects. More specifically, an important objective has
been to identify the metric that summarizes O3 exposure in a
way that is most predictive of the effect of interest (e.g., reduced
growth). Along with the continuous weighted, W126 index, the two other
cumulative indices that have received greatest attention across the
past several O3 NAAQS reviews are the threshold weighted
indices, AOT60 \141\ and SUM06.\142\ Accordingly, some studies of
O3 vegetation effects have reported exposures using these
metrics. Alternative methods for characterizing O3 exposure
to predict various plant responses (particularly those related to
photosynthesis, growth and productivity) have, in recent years, also
included flux models (models that are based on the amount of
O3 that enters the leaf). However, as was the case in the
last review, there remain a variety of complications, limitations and
uncertainties associated with this approach. For example, ``[w]hile
some efforts have been made in the U.S. to calculate ozone flux into
leaves and canopies, little information has been published relating
these fluxes to effects on vegetation'' (ISA, section IS.3.2). Further,
as flux of O3 into the plant under different conditions of
O3 in ambient air is affected by several factors including
temperature, vapor pressure deficit, light, soil moisture, and plant
growth stage, use of this approach to quantify the vegetation impact of
O3 would require information on these various types of
factors (ISA, section IS.3.2). In addition to these data requirements,
each species has different amounts of internal detoxification potential
that may protect species to differing degrees. The lack of detailed
species- and site-specific data required for flux modeling in the U.S.
and the lack of understanding of detoxification
[[Page 49887]]
processes continues to make this technique less viable for use in risk
assessments in the U.S. (ISA, section IS.3.2).
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\141\ The AOT60 index is the seasonal sum of the difference
between an hourly concentration above 60 ppb, minus 60 ppb (2006
AQCD, p. AX9-161). More recently, some studies have also reported
O3 exposures in terms of AOT40, which is conceptually
similar but with 40 substituted for 60 in its derivation (ISA,
Appendix 8, section 8.13.1).
\142\ The SUM06 index is the seasonal sum of hourly
concentrations at or above 0.06 ppm during a specified daily time
window (2006 AQCD, p. AX9-161; 2013 ISA, section 9.5.2). This may
sometimes be referred to as SUM60, e.g., when concentrations are in
terms of ppb. There are also variations on this metric that utilize
alternative reference points above which hourly concentrations are
summed. For example, SUM08 is the seasonal sum of hourly
concentrations at or above 0.08 ppm and SUM0 is the seasonal sum of
all hourly concentrations.
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Based on extensive review of the published literature on different
types of E-R metrics, including comparisons between metrics, the EPA
has generally focused on cumulative, concentration-weighted indices of
exposure, recognizing them as the most appropriate biologically based
metrics to consider in this context (1996 AQCD; 2006 AQCD; 2013 ISA).
Quantifying exposure in this way has been found to improve the
explanatory power of E-R models for growth and yield over using indices
based only on mean and peak exposure values (2013 ISA, section 2.6.6.1,
p. 2-44). The most well-analyzed datasets in such evaluations are two
detailed datasets established two decades ago, one for seedlings of 11
tree species and one for 10 crops, described further in section
III.B.3.a(ii) below (e.g., Lee and Hogsett, 1996, Hogsett et al.,
1997). These datasets, which include species-specific seedling growth
and crop yield response information across multiple seasonal cumulative
exposures, were used to develop robust quantitative E-R functions to
predict growth reduction relative to a zero-O3 setting
(termed relative biomass loss or RBL) in seedlings of the tree species
and E-R functions for RYL for a set of common crops (ISA, Appendix 8,
section 8.13.2; 2013 ISA, section 9.6.2).
Among the studies newly available in this review, no new exposure
indices for assessing effects on vegetation growth or other
physiological process parameters have been identified. The SUM06, AOTx
(e.g., AOT60) and W126 exposure metrics remain the cumulative metrics
that are most commonly discussed (ISA, Appendix 8, section 8.13.1). The
ISA notes that ``[c]umulative indices of exposure that differentially
weight hourly concentrations [which would include the W126 index] have
been found to be best suited to characterize vegetation exposure to
ozone with regard to reductions in vegetation growth and yield'' (ISA,
section ES.3). Accordingly, in this review, as in the last two reviews,
the seasonal W126-based cumulative, concentration-weighted metric
receives primary attention in considering the effects evidence and
exposure analyses, particularly related to growth effects (e.g., in
sections III.C and III.D below).
The first step in calculating the seasonal W126 index for a
specific year, as described and considered in this review, is to sum
the weighted hourly O3 concentrations in ambient air during
daylight hours (defined as 8:00 a.m. to 8:00 p.m. local standard time)
within each calendar month, resulting in monthly index values. The
monthly W126 index values are calculated from hourly O3
concentrations as follows.\143\
---------------------------------------------------------------------------
\143\ In situations where data are missing, an adjustment is
factored into the monthly index (PA, Appendix 4D, section 4D.2.2).
[GRAPHIC] [TIFF OMITTED] TP14AU20.001
---------------------------------------------------------------------------
where,
N is the number of days in the month
d is the day of the month (d = 1, 2, . . ., N)
h is the hour of the day (h = 0, 1, . . ., 23)
Cdh is the hourly O3 concentration observed on
day d, hour h, in parts per million
The W126 index value for a specific year is the maximum sum of the
monthly index values for three consecutive months within a calendar
year (i.e., January to March, February to April, . . . October to
December). Three-year average W126 index values are calculated by
taking the average of seasonal W126 index values for three consecutive
years (e.g., as described in the PA, Appendix 4D, section 4D.2.2).
(ii) Relationships Between Exposure Levels and Effects
Across the array of O3-related welfare effects,
consistent and systematically evaluated information on E-R
relationships across multiple exposure levels is limited. Most
prominent is the information on E-R relationships for growth effects on
tree seedlings and crops,\144\ which has been available for the past
several reviews. The information on which these functions are based
comes primarily from the U.S. EPA's National Crop Loss Assessment
Network (NCLAN) \145\ project for crops and the NHEERL-WED project for
tree seedlings, projects implemented primarily to define E-R
relationships for major agricultural crops and tree species, thus
advancing understanding of responses to O3 exposures (ISA,
Appendix 8, section 8.13.2). These projects included a series of
experiments that used OTCs to investigate tree seedling growth response
and crop yield over a growing season under a variety of O3
exposures and growing conditions (2013 ISA, section 9.6.2; Lee and
Hogsett, 1996). These experiments have produced multiple studies that
document O3 effects on tree seedling growth and crop yield
across multiple levels of exposure. Importantly, the information on
exposure includes hourly concentrations across the season-long (or
longer) exposure period which can then be summarized in terms of the
various seasonal metrics.\146\ In the initial analyses of these data,
exposure was characterized in terms of several metrics, including
seasonal SUM06 and W126 indices (Lee and Hogsett, 1996; 1997 Staff
Paper, sections IV.D.2 and IV.D.3; 2007 Staff Paper, section 7.6),
while use of these functions more recently has focused on their
implementation in terms of seasonal W126 index (2013 ISA, section 9.6;
80 FR 65391-92, October 26, 2015).
---------------------------------------------------------------------------
\144\ The E-R functions estimate O3-related reduction
in a year's tree seedling growth or crop yield as a percentage of
that expected in the absence of O3 (ISA, Appendix 8,
section 8.13.2).
\145\ The NCLAN program, which was undertaken in the early to
mid-1980s, assessed multiple U.S. crops, locations, and
O3 exposure levels, using consistent methods, to provide
the largest, most uniform database on the effects of O3
on agricultural crop yields (1996 AQCD, 2006 AQCD, 2013 ISA,
sections 9.2, 9.4, and 9.6; ISA, Appendix 8, section 8.13.2).
\146\ This underlying database for the exposure is a key
characteristic that sets this set of studies (and their associated
E-R analyses) apart from other available studies.
---------------------------------------------------------------------------
The 11 tree species for which robust and well-established E-R
functions for RBL are available are black cherry, Douglas fir, loblolly
pine, ponderosa pine, quaking aspen, red alder, red maple, sugar maple,
tulip poplar, Virginia pine, and white pine (PA, Appendix 4A; 2013 ISA,
section 9.6).\147\ While these 11 species represent only a small
fraction of the total number of native tree species in the contiguous
U.S., this small subset includes eastern and western species, deciduous
and coniferous species, and species that
[[Page 49888]]
grow in a variety of ecosystems and represent a range of tolerance to
O3 (PA, Appendix 4B; 2013 ISA, section 9.6.2). The
established E-R functions for most of the 11 species were derived using
data from multiple studies or experiments involving a wide range of
exposure and/or growing conditions. From the available data, separate
E-R functions were developed for each combination of species and
experiment (2013 ISA, section 9.6.1; Lee and Hogsett, 1996). From these
separate species-experiment-specific E-R functions, species-specific
composite E-R functions were developed (PA, Appendix 4A).
---------------------------------------------------------------------------
\147\ A quantitative analysis of E-R information for an
additional species was considered in the 2014 WREA. But the
underlying study, rather than being a controlled exposure study,
involves exposure to ambient air along an existing gradient of
O3 concentrations in the New York City metropolitan area,
such that O3 and climate conditions were not controlled
(2013 ISA, section 9.6.3.3). Based on recognition that this dataset
is not as strong as those for the 11 species for which E-R functions
are based on controlled ozone exposure, this study is not included
with the established E-R functions for the 11 species (PA, section
4.3.3).
---------------------------------------------------------------------------
In total, the 11 species-specific composite E-R functions are based
on 51 tree seedling studies or experiments (PA, Appendix 4A, section
4A.1.1). For six of the 11 species, this function is based on just one
or two studies (e.g., red maple and black cherry), while for other
species there were as many as 11 studies available (e.g., ponderosa
pine). A stochastic analysis drawing on the experiment-specific
functions provides a sense of the variability and uncertainty
associated with the estimated E-R relationships among and within
species (PA, Appendix 4A, section 4A.1.1, Figure 4A-13). Based on the
species-specific E-R functions, growth of the studied tree species at
the seedling stage appears to vary widely in sensitivity to
O3 exposure (PA, Appendix 4A, section 4A.1.1). Since the
initial set of studies were completed, several additional studies,
focused on aspen, have been published based on the Aspen FACE
experiment in a planted forest in Wisconsin; the findings were
consistent with many of the earlier OTC studies (ISA, Appendix 8,
section 8.13.2).
With regard to crops, established E-R functions are available for
10 crops: Barley, field corn, cotton, kidney bean, lettuce, peanut,
potato, grain sorghum, soybean and winter wheat (PA, Appendix 4A,
section 4A.1; ISA, Appendix 8, section 8.13.2). Studies available since
the last review for seven soybean cultivars support conclusions from
prior studies that of similarity of current soybean cultivar
sensitivity compared to the earlier genotypes from which the soybean E-
R functions were (ISA, Appendix 8, section 8.13.2).
Newly available studies that investigated growth effects of
O3 exposures are also consistent with the existing evidence
base, and generally involve particular aspects of the effect rather
than expanding the conditions under which plant species, particularly
trees, have been assessed (ISA, section IS.5.1.2). These include a
compilation of previously available studies on plant biomass response
to O3 (in terms of AOT40); the compilation reports linear
regressions conducted on the associated varying datasets (ISA, Appendix
8, section 8.13.2; van Goethem et al., 2013). Based on these
regressions, this study describes distributions of sensitivity to
O3 effects on biomass across nearly 100 plant species (trees
and grasslands) including 17 species native to the U.S. and 65
additional species that have been introduced to the U.S. (ISA, Appendix
8, section 8.13.2; van Goethem et al., 2013). Additional information is
needed to more completely describe O3 exposure response
relationships for these species in the U.S.\148\
---------------------------------------------------------------------------
\148\ The set of studies included in this compilation were
described as meeting a set of criteria, such as: Including
O3 only exposures in conditions described as ``close to
field'' exposures (which were expressed as AOT40); including at
least 21 days exposure above 40 ppb O3; and having a
maximum hourly concentration that was no higher than 100 ppb (van
Goethem et al., 2013). The publication does not report exposure
duration for each study or details of biomass response measurements,
making it less useful for the purpose of describing E-R
relationships that might provide for estimation of specific impacts
associated with air quality conditions meeting the current standard
(e.g., 2013 ISA, p. 9-118).
---------------------------------------------------------------------------
b. Visible Foliar Injury
With regard to visible foliar injury, as with the evidence
available in the last review, the current evidence ``continues to show
a consistent association between visible injury and ozone exposure,''
while also recognizing the role of modifying factors such as soil
moisture and time of day (ISA, section IS.5.1.1). The current ISA, in
concluding that the newly available information is consistent with
conclusions of the 2013 ISA, also summarizes several recently available
studies that continue to document that O3 elicits visible
foliar injury in many plant species. These include a synthesis of
previously published studies that categorizes studied species (and
their associated taxonomic classifications) as to whether or not
O3-related foliar injury has been reported to occur in the
presence of elevated O3,\149\ while not providing
quantitative information regarding specific exposure conditions or
analyses of E-R relationships (ISA, Appendix 8, section 8.2). The
evidence in the current review, as was the case in the last review,
while documenting that elevated O3 conditions in ambient air
generally results in visible foliar injury in sensitive species (when
in a predisposing environment),\150\ does not include a quantitative
description of the relationship of incidence or severity of visible
foliar injury in sensitive species in natural locations in the U.S.
with specific metrics of O3 exposure.
---------------------------------------------------------------------------
\149\ The publication identifies 245 species across 28 plant
genera, many native to the U.S., in which O3-related
visible foliar injury has been reported (ISA, Appendix 8, Table 8-
3).
\150\ As noted in the 2013 ISA and the ISA for the current
review, visible foliar injury usually occurs when sensitive plants
are exposed to elevated ozone concentrations in a predisposing
environment, with a major modifying factor being the amount of soil
moisture available to a plant. Accordingly, dry periods are
concluded to decrease the incidence and severity of ozone-induced
visible foliar injury, such that the incidence of visible foliar
injury is not always higher in years and areas with higher ozone,
especially with co-occurring drought (ISA, Appendix 8, p. 8-23;
Smith, 2012; Smith et al., 2003).
---------------------------------------------------------------------------
Several studies of the extensive USFS field-based dataset of
visible foliar injury incidence in forests across the U.S.\151\
illustrate the extent to which our current understanding of this
relationship is limited. For example, a study that was available in the
last review presents a trend analysis of these data for sites located
in 24 states of the northeast and north central U.S. for the 16-year
period from 1994 through 2009 that provides some insight into the
influence of changes in air quality and soil moisture on visible foliar
injury and the difficulty inherent in predicting foliar injury response
under different air quality and soil moisture scenarios (Smith, 2012,
Smith et al., 2012; ISA, Appendix 8, section 8.2). This study, like
prior analyses of such data, shows the dependence of foliar injury
incidence and severity on local site conditions for soil moisture
availability and O3 exposure. For example, while the authors
characterize the ambient air O3 concentrations to be the
``driving force'' behind incidence of injury and its severity, they
state that ``site moisture conditions are also a very strong influence
on the biomonitoring data'' (Smith et al., 2003). In general, the USFS
data analyses have found foliar injury prevalence and severity to be
higher during seasons and sites that have experienced the highest
O3 than during other periods (e.g., Campbell et al., 2007;
Smith, 2012).
---------------------------------------------------------------------------
\151\ These data were collected as part of the U.S. Forest
Service Forest Health Monitoring/Forest Inventory and Analysis (USFS
FHM/FIA) biomonitoring network program (2013 ISA, section 9.4.2.1;
Campbell et al., 2007, Smith et al., 2012).
---------------------------------------------------------------------------
Although studies of the incidence of visible foliar injury in
national forests, wildlife refuges, and similar areas have often used
cumulative indices (e.g., SUM06) to investigate variations in incidence
of foliar injury, studies also suggest an additional role for metrics
focused on peak concentrations (ISA; 2013 ISA; 2006 AQCD; Hildebrand et
al., 1996; Smith, 2012). For example, a
[[Page 49889]]
study of six years of USFS biosite \152\ data (2000-2006) for three
western states found that the biosites with the highest O3
exposure (SUM06 at or above 25 ppm-hrs) had the highest percentage of
biosites with injury and the highest mean BI, with little discernable
difference among the lower exposure categories; this study also
identified ``better linkage between air levels and visible injury'' as
an O3 research need (Campbell et al., 2007).\153\ More
recent studies of the complete 16 years of data in 24 northeast and
north central states have suggested that a cumulative exposure index
alone may not completely describe the O3-related risk of
this effect at USFS sites (Smith et al., 2012; Smith, 2012). For
example, Smith (2012) observed there to be a declining trend in the 16-
year dataset, ``especially after 2002 when peak ozone concentrations
declined across the entire region'' thus suggesting a role for peak
concentrations.
---------------------------------------------------------------------------
\152\ As described in section III.B.2 above, biosites are
biomonitoring sites where the USFS applies a scoring system for
purposes of categorizing areas with regard to severity of visible
foliar injury occurrence (U.S. Forest Service, 2010).
\153\ In considering their findings, the authors expressed the
view that ``[a]lthough the number of sites or species with injury is
informative, the average biosite injury index (which takes into
account both severity and amount of injury on multiple species at a
site) provides a more meaningful measure of injury'' for their
assessment at a statewide scale (Campbell et al., 2007).
---------------------------------------------------------------------------
Some studies of visible foliar injury incidence data have
investigated the role of peak concentrations quantified by an
O3 exposure index that is a count of hourly concentrations
(e.g., in a growing season) above a threshold 1-hour concentration of
100 ppb, N100 (e.g., Smith, 2012; Smith et al., 2012). For example, the
study by Smith (2012) discussed injury patterns at biosites in 24
states in the Northeast and North Central regions in the context of the
SUM06 index and N100 metrics (although not via a statistical
model).\154\ That study of 16 years of biomonitoring data from these
sites suggested that there may be a threshold exposure needed for
injury to occur, and the number of hours of elevated O3
concentrations during the growing season (such as what is captured by a
metric like N100) may be more important than cumulative exposure in
determining the occurrence of foliar injury (Smith, 2012).\155\ The
study's authors noted this finding to be consistent with findings
reported by a study of statistical analyses of seven years of visible
foliar injury data from a wildlife refuge in the mid-Atlantic (Davis
and Orendovici, 2006, Smith et al., 2012). The latter study
investigated the fit of multiple models that included various metrics
of cumulative O3 (SUM06, SUM0, SUM08), alone and in
combination with some other variables (Davis and Orendovici, 2006).
Among the statistical models investigated (which did not include one
with either W126 index or N100 alone), the model with the best fit to
the visible foliar injury incidence data was found to be one that
included the cumulative metric, W126, and the N100 index, as well as
drought index (Davis and Orendovici, 2006).\156\
---------------------------------------------------------------------------
\154\ The current ISA, 2013 ISA and prior AQCDs have not
described extensive evaluation of specific peak-concentration
metrics such as the N100 that might assist in identifying the one
best suited for such purposes.
\155\ In summarizing this study in the last review, the ISA
observed that ``[o]verall, there was a declining trend in the
incidence of foliar injury as peak O3 concentrations
declined'' (2013 ISA, p. 9-40).
\156\ The models evaluated included several with cumulative
exposure indices alone. These included SUM60, SUM0, and SUM80, but
not W126. They did not include a model with W126 that did not also
include N100. Across all of the models evaluated, the model with the
best fit to the data was found to be the one that included N100 and
W126, along with the drought index (Davis and Orendovici, 2006).
---------------------------------------------------------------------------
The established significant role of higher or peak O3
concentrations, as well as pattern of their occurrence, in plant
responses has been noted in prior ISAs or AQCDs. In identifying support
with regard to foliar injury as the response, the 2013 ISA and 2006
AQCD both cite studies that support the ``important role that peak
concentrations, as well as the pattern of occurrence, plays in plant
response to O3'' (2013 ISA, p. 9-105; 2006 AQCD, p. AX9-
169). For example, a study of European white birch saplings reported
that peak concentrations and the duration of the exposure event were
important determinants of foliar injury (2013 ISA, section 9.5.3.1;
Oksanen and Holopainen, 2001). This study also evaluated tree growth,
which was found to be more related to cumulative exposure (2013 ISA, p.
9-105).\157\ A second study that was cited by both assessments that
focused on aspen, reported that ``the variable peak exposures were
important in causing injury, and that the different exposure
treatments, although having the same SUM06, resulted in very different
patterns of foliar injury'' (2013 ISA, p. 9-105; 2006 AQCD, p. AX9-169;
Yun and Laurence, 1999). As noted in the 2006 AQCD, the cumulative
exposure indices (e.g., SUM06, W126) were ``originally developed and
tested using only growth/yield data, not foliar injury'' and ``[t]his
distinction is critical in comparing the efficacy of one index to
another'' (2006 AQCD, p. AX9-173). It is also recognized that where
cumulative indices are highly correlated with the frequency or
occurrence of higher hourly average concentrations, they could be good
predictors of such effects (2006 AQCD, section AX9.4.4.3).
---------------------------------------------------------------------------
\157\ The study authors concluded that ``high peak
concentrations were important for visible injuries and stomatal
conductance, but less important for determining growth responses''
(Oksanen and Holopainen, 2001).
---------------------------------------------------------------------------
In a more recent study (by Wang et al. [2012]) that is cited in the
current ISA, a statistical modeling analysis was performed on a subset
of the years of data that were described in Smith (2012). This
analysis, which involved 5,940 data records from 1997 through 2007 from
the 24 northeast and north central states, tested a number of models
for their ability to predict the presence of visible foliar injury (a
nonzero biosite score), regardless of severity, and generally found
that the type of O3 exposure metric (e.g., SUM06 versus
N100) made only a small difference, although the models that included
both a cumulative index (SUM06) and N100 had a just slightly better fit
(Wang et al., 2012). Based on their investigation of 15 different
models, using differing combination of several types of potential
predictors, the study authors concluded that they were not able to
identify environmental conditions under which they ``could reliably
expect plants to be damaged'' (Wang et al., 2012). This is indicative
of the current state of knowledge, in which there remains a lack of
established quantitative functions describing E-R relationships that
would allow prediction of visible foliar injury severity and incidence
under varying air quality and environmental conditions.
The available information related to O3 exposures
associated with visible foliar injury of varying severity also includes
the dataset developed by the EPA in the last review from USFS BI
scores, collected during the years 2006 through 2010 at locations in 37
states. In developing this dataset, the BI scores were combined with
estimates of soil moisture \158\ and estimates of seasonal cumulative
O3 exposure in terms of
[[Page 49890]]
W126 index \159\ (Smith and Murphy, 2015; PA, Appendix 4C). This
dataset includes more than 5,000 records of which more than 80 percent
have a BI score of zero (indicating a lack of visible foliar injury).
While the estimated W126 index assigned to records in this dataset
ranges from zero to somewhat above 50 ppm-hrs, more than a third of all
the records (and also of records with BI scores above zero or five)
\160\ are at sites with W126 index estimates below 7 ppm-hrs.
---------------------------------------------------------------------------
\158\ Soil moisture categories (dry, wet or normal) were
assigned to each biosite record based on the NOAA Palmer Z drought
index values obtained from the NCDC website for the April-through-
August periods, averaged for the relevant year; details are provided
in the PA, Appendix 4C, section 4C.2. There are inherent
uncertainties in this assignment, including the substantial spatial
variation in soil moisture and large size of NOAA climate divisions
(hundreds of miles). This dataset, including associated
uncertainties and limitations, is described in the PA, Appendix 4C,
section 4C.5.
\159\ The W126 index values assigned to the biosite locations
are estimates developed for 12 kilometer (km) by 12 km cells in a
national-scale spatial grid for each year. The grid cell estimates
were derived from applying a spatial interpolation technique to
annual W126 values derived from O3 measurements at
ambient air monitoring locations for the years corresponding to the
biosite surveys (details in the PA, Appendix 4C, sections 4.C.2 and
4C.5).
\160\ One third (33%) of scores above 15 are at sites with W126
below 7 ppm-hrs (PA, Appendix 4C, Table 4C-3).
---------------------------------------------------------------------------
In an extension of analyses of this dataset developed in the last
review, the presentation in the PA \161\ describes the BI scores for
the records in the dataset in relation to the W126 index estimate for
each record, using bins of increasing W126 index values. The PA
presentation utilizes the BI score breakpoints in the scheme used by
the USFS to categorize severity. The lowest USFS category encompasses
BI scores from zero to just below 5; scores of this magnitude are
described as ``little or no foliar injury'' (Smith et al., 2012). The
next highest category encompasses scores from five to just below 15 and
is described as ``light to moderate foliar injury,'' BI scores of 15 up
to 25 are described as ``moderate'' and above 25 is described as
``severe'' (Smith et al., 2012). The PA presentation indicates that
across the W126 bins, there is variation in both the incidence of
particular magnitude BI scores and in the average score per bin. In
general, however the greatest incidence of records with BI scores above
zero, five, or higher--and the highest average BI score--occurs with
the highest W126 bin, i.e., the bin for W126 index estimates greater
than 25 ppm-hrs (PA, Appendix 4C, Table 4C-6).
---------------------------------------------------------------------------
\161\ Beyond the presentation of a statistical analysis
developed in the last review, the PA presentations are primarily
descriptive (as compared to statistical) in recognition of the
limitations and uncertainties of the dataset (PA, Appendix 4C,
section 4C.5).
---------------------------------------------------------------------------
While recognizing limitations in the dataset,\162\ the PA makes
several observations, focusing particularly on records in the normal
soil category (PA, section 4.5.1). For records categorized as wet soil
moisture, the sample size for the W126 bins above 13 ppm-hrs is quite
small (including only 18 of the 1,189 records in that soil moisture
category), precluding meaningful interpretation.\163\ For the normal
soil category, the percentages of records in the greater than 25 ppm-
hrs bin that have BI scores above 15 (``moderate'' and ``severe''
injury) or above 5 (``little,'' ``moderate'' and ``severe'' injury) are
both more than three times greater than such percentages in any of the
lower W126 bins.\164\ For example, the proportion of records with BI
above five fluctuates between 5% and 13% across all but the highest
W126 bin (>25 ppm-hrs) for which the proportion is 41% (PA, Appendix
4C, Table 4C-6). The same pattern is observed for BI scores above 15 at
sites with normal and dry soil moisture conditions, albeit with lower
incidences. For example, the incidence of normal soil moisture records
with BI score above 15 in the bin for W126 index values above 25 ppm-
hrs was 20% but fluctuates between 1% and 4% in the bin for W126 index
values at or below 25 ppm-hrs (PA, Appendix 4C, Table 4C-6). The
average BI of 7.9 in the greater-then-25-ppm-hrs bin is more than three
times the next highest W126 bin average. The average BI in each of the
next two lower W126 bins is just slightly higher than average BIs for
the rest of the bins, and the average BI for all bins at or below 25
ppm-hrs are well below 5 (PA, Appendix 4C).
---------------------------------------------------------------------------
\162\ For example, the majority of records have W126 index
estimates at or below 9 ppm-hrs, and fewer than 10% have W126
estimates above 15 ppm-hrs. Further, the BI scores are quite
variable across the range of W126 bins, with even the lowest W126
bin (estimates below 7 ppm-hrs) including BI scores well above 15
(PA, Appendix 4C, section 4C.4.2). The records for the wet soil
moisture category in the higher W126 bins are more limited that the
other categories, with nearly 90% of the wet soil moisture records
falling into the bins for W126 index at or below 9 ppm-hrs, limiting
interpretations for higher W126 bins (PA, Appendix 4C, Table 4C.4
and section 4C.6). Accordingly, the PA observations focused
primarily on the records for the normal or dry soil moisture
categories, for which W126 index above 13 ppm-hrs is better
represented.
\163\ The full database includes only 18 records at sites in the
wet soil moisture category with estimated W126 index above 13 ppm-
hrs, with 9 or fewer (less than 1%) in each of the W126 bins above
13 ppm-hrs (PA, Appendix 4C, Table 4C-3). Among the bins for W126 at
or below 13 ppm-hrs, the average BI score is less than 2 (PA,
Appendix 4C, Table 4C-5).
\164\ When scores characterized as ``little injury'' by the USFS
classification scheme are also included (i.e., when considering all
scores above zero), there is a suggestion of increased frequency of
records for the W126 bins above 19 or 17 ppm-hrs, although
difference from lower bins is less than a factor of two (PA,
Appendix 4C).
---------------------------------------------------------------------------
Overall, the dataset described in the PA generally indicates the
risk of injury, and particularly injury considered at least light,
moderate or severe, to be higher at the highest W126 index values, with
appreciable variability in the data for the lower bins (PA, Appendix
4C). This appears to be consistent with the conclusions of the studies
of detailed quantitative analyses, summarized above, that the pattern
is stronger at higher O3 concentrations. A number of factors
may contribute to the observed variability in BI scores and lack of a
clear pattern with W126 index bin; among others, these may include
uncertainties in assignment of W126 estimates and soil moisture
categories to biosite locations, variability in biological response
among the sensitive species monitored, and the potential role of other
aspects of O3 air quality not captured by the W126 index.
Thus, the dataset has limitations affecting associated conclusions and
uncertainty remains regarding the tools for and the appropriate metric
(or metrics) for quantifying O3 exposures, as well as
perhaps soil moisture conditions, with regard to their influence on
extent and/or severity of injury in sensitive species in natural areas
(Davis and Orendovici, 2006, Smith et al., 2012; Wang et al., 2012).
Dose modeling or flux models (referenced in section III.B.3.a(i)
above, have also been considered for quantifying O3 dose
that may be related to plant leaf injury. Among the newly available
evidence is a study examining relationships between short-term flux and
leaf injury on cotton plants that described a sensitivity parameter
that might characterize the influence on the flux-injury relationship
of diel and seasonal variability in plant defenses (among other
factors) and suggested additional research might provide for such a
sensitivity parameter to ``function well in combination with a
sigmoidal weighting of flux, analogous to the W126 weighting of
concentration'', and perhaps an additional parameter (Grantz et al.,
2013, p. 1710; ISA, Appendix 8, section 8.13.1). However, the ISA
recognizes there is ``much unknown'' with regard to the relationship
between O3 uptake and leaf injury, and relationships with
detoxification processes (ISA, Appendix 8, section 8.13.1 and p. 8-
184). These uncertainties have made this technique less viable for
assessments in the U.S., precluding use of a flux-based approach at
this time (ISA, Appendix 8, section 8.13.1 and p. 8-184).
c. Other Effects
With regard to radiative forcing and subsequent climate effects
associated with the global tropospheric abundance of O3, the
newly available evidence in this review does not provide more detailed
quantitative information
[[Page 49891]]
regarding O3 concentrations at the national scale. For
example, tropospheric O3 continues to be recognized as
having a causal relationship with radiative forcing, although
``uncertainty in the magnitude of radiative forcing estimated to be
attributed to tropospheric ozone is a contributor to the relatively
greater uncertainty associated with climate effects of tropospheric
ozone compared to such effects of the well mixed greenhouse gases
(e.g., carbon dioxide and methane)'' (ISA, section IS.6.2.2).
While tropospheric O3 also continues to be recognized as
having a likely causal relationship with subsequent effects on
temperature, precipitation and related climate variables, the non-
uniform distribution of O3 within the troposphere (spatially
and temporally) makes the development of quantitative relationships
between the magnitude of such effects and differing O3
concentrations in the U.S. challenging (ISA, Appendix 9). Additionally,
``the heterogeneous distribution of ozone in the troposphere
complicates the direct attribution of spatial patterns of temperature
change to ozone induced [radiative forcing]'' and there are ``ozone
climate feedbacks that further alter the relationship between ozone
[radiative forcing] and temperature (and other climate variables) in
complex ways'' (ISA, Appendix 9, section 9.3.1, p. 9-19). Thus, various
uncertainties ``render the precise magnitude of the overall effect of
tropospheric ozone on climate more uncertain than that of the well-
mixed GHGs'' and ``[c]urrent limitations in climate modeling tools,
variation across models, and the need for more comprehensive
observational data on these effects represent sources of uncertainty in
quantifying the precise magnitude of climate responses to ozone
changes, particularly at regional scales'' (ISA, section IS.6.2.2,
Appendix 9, section 9.3.3, p. 9-22). For example, current limitations
in modeling tools include ``uncertainties associated with simulating
trends in upper tropospheric ozone concentrations'' (ISA, section
9.3.1, p. 9-19), and uncertainties such as ``the magnitude of
[radiative forcing] estimated to be attributed to tropospheric ozone''
(ISA, section 9.3.3, p. 9-22). Further, ``precisely quantifying the
change in surface temperature (and other climate variables) due to
tropospheric ozone changes requires complex climate simulations that
include all relevant feedbacks and interactions'' (ISA, section 9.3.3,
p. 9-22). For example, an important limitation in current climate
modeling capabilities for O3 is representation of important
urban- or regional-scale physical and chemical processes, such as
O3 enhancement in high-temperature urban situations or
O3 chemistry in city centers where NOx is abundant. Such
limitations impede our ability to quantify the impact of incremental
changes in O3 concentrations in the U.S. on radiative
forcing and subsequent climate effects.
With regard to tree mortality (the evidence for which the 2013 ISA
did not assess with regard to its support for inference of a causal
relationship with O3 exposure), the evidence available in
the last several reviews included field studies of pollution gradients
that concluded O3 damage to be an important contributor to
tree mortality although several confounding factors such as drought,
insect outbreak and forest management were identified as potential
contributors (2013 ISA, section 9.4.7.1). Although three newly
available studies contribute to the ISA conclusion of sufficient
evidence to infer a likely causal relationship for O3 with
tree mortality (ISA, Appendix 8, section 8.4), there is only limited
experimental evidence that isolates the effect of O3 on tree
mortality and might be informative regarding O3
concentrations of interest in the review. This evidence, primarily from
an Aspen FACE study of aspen survival, involves cumulative seasonal
exposure to W126 index levels above 30 ppm-hrs during the first half of
the 11-year study period (ISA, Appendix 8, Tables 8-8 and 8-9).
Evidence is lacking regarding exposure conditions closer to those
occurring under the current standard and any contribution to tree
mortality.
With regard to the two categories of welfare effects involving
insects (for which there are new causal determinations in this review),
there are multiple limitations and uncertainties regarding
characterization of exposure conditions that might elicit effects and
the comprehensive characterization of the effects (ISA, p. IS-91,
Appendix 8, section 8.6.3). For example, with regard to alteration of
herbivore growth and reproduction, although ``[t]here are multiple
studies demonstrating ozone effects on fecundity and growth in insects
that feed on ozone-exposed vegetation'', ``no consistent directionality
of response is observed across studies and uncertainties remain in
regard to different plant consumption methods across species and the
exposure conditions associated with particular severities of effects ''
(ISA, pp. ES-18). The ISA also notes the variation in study designs and
endpoints used to assess O3 response (ISA, IS.6.2.1 and
Appendix 8, section 8.6). Thus, while the evidence describes changes in
nutrient content and leaf chemistry following O3 exposure
(ISA, p. IS-73), the effect of these changes on herbivores consuming
the leaves is not well characterized, and factors such as identified
here preclude broader characterization, as well as quantitative
analysis related to air quality conditions meeting the O3
standard.
The evidence for the second category, alteration of plant-insect
signaling, draws on new research that has provided clear evidence of
O3 modification of VPSCs and behavioral responses of insects
to these modified chemical signals (ISA, section IS.6.2.1). The
available evidence involves a relatively small number of plant species
and plant-insect associations. While the evidence documents effects on
plant production of signaling chemicals and on the atmospheric
persistence of signaling chemicals, as well as on the behaviors of
signal-responsive insects, it is limited with regard to
characterization of mechanisms and the consequences of any modification
of VPSCs by O3 (ISA, p. ES-18; sections ES.5.1.3 and
IS.6.2.1). Further, the available studies vary with regard to the
experimental exposure circumstances in which the different types of
effects have been reported (most of the studies have been carried out
in laboratory conditions rather than in natural environments), and many
of the studies involve quite short controlled exposures (hours to days)
to elevated concentrations, posing limitations for our purposes of
considering the potential for impacts associated with the studied
effects to be elicited by air quality conditions that meet the current
standard (ISA, section IS.6.2.1 and Appendix 8, section 8.7).
With regard to previously recognized categories of vegetation-
related effects, other than growth and visible foliar injury, such as
reduced plant reproduction, reduced productivity in terrestrial
ecosystems, alteration of terrestrial community composition and
alteration of below-ground biogeochemical cycles, the newly available
evidence includes a variety of studies, as identified in the ISA (ISA,
Appendix 8, sections 8.4, 8.8 and 8.10). Across the studies, a variety
of metrics (including AOT40, 4- to 12-hour mean concentrations, and
others) are used to quantify exposure over varying durations and
various countries. The ISA additionally describes publications that
summarize previously published studies in several ways. For example, a
meta-analysis of reproduction studies categorized the reported
O3 exposures into bins of differing magnitude,
[[Page 49892]]
grouping differing concentration metrics and exposure durations
together, and performed statistical analyses to reach conclusions
regarding the presence of an O3-related effect (ISA,
Appendix 8, section 8.4.1). While such studies continue to support
conclusions of the ecological hazards of O3, they do not
improve capabilities for characterizing the likelihood of such effects
under varying patterns of environmental O3 concentrations
that occur with air quality conditions that meet the current standard.
As at the time of the last review, growth impacts, most
specifically as evaluated by RBL for tree seedlings and RYL for crops,
remain the type of vegetation-related effects for which we have the
best understanding of exposure conditions likely to elicit them. Thus,
as was the case in the decision for the last review, the quantitative
analyses of exposures occurring under air quality that meets the
current standard, summarized below, are focused primarily on the W126
index, given its established relationship with growth effects.
C. Summary of Air Quality and Exposure Information
The air quality and exposure analyses developed in this review,
like those in the last review, are of two types: (1) W126-based
cumulative exposure estimates in Class I areas; and (2) analyses of
W126-based exposures and their relationship with the current standard
for all U.S. monitoring locations (PA, Appendix 4D). As summarized in
the IRP, we identified these analyses to be updated in this review in
recognition of the relatively reduced uncertainty associated with the
use of these types of analyses (compared to the national or regional-
scale modeling analyses performed in the last review) to inform a
characterization of cumulative O3 exposure (in terms of the
W126 index) associated with air quality just meeting the current
standard (IRP, section 5.2.2). As in the last review, the lesser
uncertainty of these air quality monitoring-based analyses contributes
to their value in informing the current review. The sections below
present findings of the updated analyses that have been performed in
the current review using recently available information.
As in the last review, the analyses focus on both the most recent
3-year period (2016 to 2018) for which data were available when the
analyses were performed, and also across the full historical period
back to 2000, which is now expanded from that available in the last
review.\165\ Design values (3-year average annual fourth-highest 8-hour
daily maximum concentration, also termed ``4th max metric'' in this
analysis) and W126 index values (in terms of the 3-year average) were
calculated at each site where sufficient data were available.\166\
Across the seventeen 3-year periods from 2000-2002 to 2016-2018, the
number of monitoring sites with sufficient data for calculation of
valid design values and W126 index values (across the 3-year design
value period) ranged from a low of 992 in 2000-2002 to a high of 1119
in 2015-2017. The specific monitoring sites differed somewhat across
the 19 years. There were 1,557 sites with sufficient data for
calculation of valid design values and W126 index values for at least
one 3-year period between 2000 and 2018, and 543 sites had such data
for all seventeen 3-year periods. Analyses in the current review are
based on the expanded set of air monitoring data now available \167\
(PA, Appendix 4D, section 4D.2.2).
---------------------------------------------------------------------------
\165\ In the last review, the dataset analyzed included data
from 2000 through 2013, with the most recent period being 2011 to
2013 (Wells, 2015).
\166\ Data adequacy requirements and methods for these
calculations are described in Appendix 4D, section 4D.2 of the PA.
\167\ In addition to being expanded with regard to data for more
recent time periods than were available during the last review, the
current dataset also includes a small amount of newly available
older data for some rural monitoring sites that are now available in
the AQS.
---------------------------------------------------------------------------
These analyses are based primarily on the hourly air monitoring
data that were reported to EPA from O3 monitoring sites
nationwide. In the recent and historical datasets, the O3
monitors (more than 1000 in the most recent period) are distributed
across the U.S., covering all nine NOAA climate regions and all 50
states (PA, Figure 4-6 and Appendix 4D, Table 4D-1). Some geographical
areas within these regions and states are more densely covered and well
represented by monitoring sites, while others may have sparse or no
data. Given that there has been a longstanding emphasis on urban areas
in the EPA's monitoring regulations, urban areas are generally well
represented in the U.S. dataset, with the effect being that the current
dataset is more representative of locations where people live than of
complete spatial coverage for all areas in the U.S., (i.e., the current
dataset is more population weighted than geographically weighted). As
O3 precursor sources are also generally more associated with
urban areas, one impact of this may be a greater representation of
relatively higher concentration sites (PA, section 4.4.3 and Appendix
4D, section 4D.4).
With regard to Class I areas, of the 158 mandated federal Class I
areas, 65 (just over 40%) have or have had O3 monitors
within 15 km with valid design values, thus allowing inclusion in the
Class I area analysis. Even so, the Class I areas dataset includes
monitoring sites in 27 states distributed across all nine NOAA climatic
regions across the contiguous U.S, as well as Hawaii and Alaska. Some
NOAA regions have far fewer numbers of Class I areas with monitors than
others. For instance, the Central, Northeast, East North Central, and
South regions all have three or fewer Class I areas in the dataset.
However, these areas also have appreciably fewer Class I areas in
general when compared to the Southwest, Southeast, West, and West North
Central regions, which are more well represented in the dataset. The
West and Southwest regions are identified as having the largest number
of Class I areas, and they have approximately one third of those areas
represented with monitors, which include locations where W126 index
values are generally higher, thus playing a prominent role in the
analysis (PA, section 4.4.3 and Appendix 4D, section 4D.4).
These updated air quality analyses, and what they indicate
regarding environmental exposures of interest in this review, are
summarized in the following two subsections which differ in their areas
of focus. The first subsection (section III.C.1) summarizes information
regarding relationships between air quality in terms of the form and
averaging time of the current standard and environmental exposures in
terms of the W126 index. The second subsection (section III.C.2)
summarizes findings of the analyses of the currently available
monitoring data with regard to the magnitude of environmental
exposures, in terms of the W126 index, in areas across the U.S., and
particularly in Class I areas, during periods in which air quality met
the current standard.
1. Influence of Form and Averaging Time of Current Standard on
Environmental Exposure
In revising the standard in 2015 to the now-current standard, the
Administrator concluded that, with revision of the standard level, the
existing form and averaging time provided the control of cumulative
seasonal exposure circumstances needed for the public welfare
protection desired (80 FR 65408, October 26, 2015). The focus on
cumulative seasonal exposure as the type of exposure metric of interest
primarily reflects the
[[Page 49893]]
evidence on E-R relationships for plant growth (summarized in section
III.B.3 above). The 2015 conclusion was based on the air quality data
analyzed at that time (80 FR 65408, October 26, 2015). Analyses in the
current review of the now expanded set of air monitoring data, which
now span 19 years and 17 3-year periods, document similar findings as
from the analysis of data from 2000-2013 described in the last review
(PA, Appendix 4D, section 4D.2.2).
Among the analyses performed is an evaluation of the variability in
the annual W126 index values across a 3-year period (PA, Appendix 4D,
section 4D.3.1.2). This evaluation was performed for all U.S.
monitoring sites with sufficient data available in the most recent 3-
year period, 2016 to 2018. This analysis indicates the extent to which
the three single-year W126 index values within a 3-year period deviate
from the average for the period. Across the full set of sites,
regardless of W126 index magnitude (or whether or not the current
standard is met), single-year W126 index values differ less than 15
ppm-hrs from the average for the 3-year period (PA, Appendix 4D, Figure
4D-6). Focusing on the approximately 850 sites meeting the current
standard (i.e., sites with a design value at or below 70 ppb), over 99%
of single-year W126 index values in this subset differ from the 3-year
average by no more than 5 ppm-hrs, and 87% by no more than 2 ppm-hrs
(PA, Appendix 4D, Figure 4D-7).
Another air quality analysis performed for the current review
documents the positive nonlinear relationship that is observed between
cumulative seasonal exposure, quantified using the W126 index, and
design values, based on the form and averaging time of the current
standard. This relationship is shown for both the average W126 index
across the 3-year design value period and for W126 index values for
individual years within the period (PA, Figure 4-7). From this
presentation, it is clear that cumulative seasonal exposures, assessed
in terms of W126 index (in a year or averaged across years), are lower
at monitoring sites with lower design values. This is seen both for
design values above the level of the current standard (70 ppb), where
the slope is steeper (due to the sigmoidal weighting of higher
concentrations by the W126 index function), as well as for lower design
values that meet the current standard (PA, Figure 4-7). This
presentation also indicates some regional differences in the
relationship. For example, for the 2016-2018 period, at sites meeting
the current standard in the regions outside of the West and Southwest
regions, all 3-year average W126 index values are at or below 12 ppm-
hrs and all single-year values are at or below 16 ppm-hrs (PA, Figures
4-6 and 4-7). The W126 index values are generally higher in the West
and Southwest regions. However, the positive relationship between the
W126 index and the design value is evident in all nine regions (PA,
Figure 4-7).
An additional analysis assesses the relationship between long-term
changes in design value and long-term changes in the W126 index. This
analysis is presented in detail in the PA and focuses on the
relationship between changes (at each monitoring site) in the 3-year
design value across the 16 design value periods from 2000-2002 to 2016-
2018 and changes in the W126 index over the same period (PA, Appendix
4D, section 4D.3.2.3).\168\ This analysis, performed using either the
3-year average W126 index or values for individual years, shows there
to be a positive, linear relationship between the changes in the W126
index and the changes in the design value at monitoring sites across
the U.S. (PA, Appendix 4D, Figure 4D-11). The existence of this
relationship means that a change in the design value at a monitoring
site was generally accompanied by a similar change in the W126 index.
Nationally, the W126 index (in terms of 3-year average) decreased by
approximately 0.62 ppm-hrs per ppb decrease in design value over the
full period from 2000 to 2018 (PA, Appendix 4D, Table 4D-12). This
relationship varies across the NOAA climate regions, with the greatest
change in the W126 index per unit change in design value observed in
the Southwest and West regions. Thus, the regions which had the highest
W126 index values at sites meeting the current standard (PA, Figure 4D-
6) also showed the greatest improvement in the W126 index per unit
decrease in their design values over the past 19 years (PA, Appendix
4D, Table 4D-12 and Figure 4D-14).
---------------------------------------------------------------------------
\168\ At each site, the trend in values of a metric (W126 or
design value), in terms of a per-year change in metric value, is
calculated using the Theil-Sen estimator, a type of linear
regression method that chooses the median slope among all lines
through pairs of sample points. For example, if applying this method
to a dataset with metric values for four consecutive years (e.g.,
W1261, W1262, W1263,
W1264), the trend would be the median of the different
per-year changes observed in the six possible pairs of values
([W1264-W1263]/1, [W1263-
W1262]/1, [W1262-W1261]/1,
[W1264-W1262]/2, [W1263-
W1261]/2, [W1264-W1261]/3).
---------------------------------------------------------------------------
The trends analyses indicate that going forward as design values
are reduced in areas that are presently not meeting the current
standard, the W126 index in those areas would also be expected to
decline (PA, Appendix 4D, section 4D.3.2.3 and 4D.5). The overall trend
showing reductions in the W126 index concurrent with reductions in the
design value metric for the current standard is positive whether the
W126 index is expressed in terms of the average across the 3-year
design value period or the annual value (PA, Appendix 4D, section
4D.3.2.3). This similarity is consistent with the strong positive
relationship that exists between the W126 index and the design value
metric for the current standard summarized above.
With regard to the control of the current form and averaging time
on vegetation exposures of potential concern, the PA also describes air
quality information pertinent to the evidence discussed in section
III.B.3 above regarding the potential for days with particularly high
O3 concentrations to play a contributing role in visible
foliar injury. In so doing, the PA notes that the current standard's
form and averaging time, by their very definition, limit occurrences of
such concentrations. For example, the peak 8-hour average
concentrations are lower at sites with lower design values, as
illustrated by the declining trends in annual fourth highest MDA8
concentrations that accompany the declining trend in design values (PA,
Figure 2-11). Additionally, the frequency of elevated 1-hour
concentrations, including concentrations at or above 100 ppb, decrease
with decreasing design values (PA, Appendix 2A, section 2A.2). For
example, in the most recent design value period (2016-2018) across all
sites with adequate data to derive design values, the mean number of
daily maximum 1-hour observations per site at or above 100 ppb was well
below one (0.19) for sites that meet the current standard, compared to
well above one (8.09) for sites not meeting the current (PA, Appendix
2A, Table 2A-2).
In summary, monitoring sites with lower O3
concentrations as measured by the design value metric (based on the
current form and averaging time of the secondary standard) have lower
cumulative seasonal exposures, as quantified by the W126 index, as well
as lower short-term peak concentrations. As the form and averaging time
of the secondary standard have not changed since 1997, the analyses
performed have been able to assess the amount of control exerted by
these aspects of the standard, in combination with reductions in the
standard level (i.e., from 0.08 ppm in 1997 to 0.075 ppm in 2008 to
0.070 ppm in 2015) on
[[Page 49894]]
cumulative seasonal exposures in terms of W126 index (and on the
magnitude of short-term peak concentrations). The analyses have found
that the long-term reductions in the design values, presumably
associated with implementation of the revised standards, have been
accompanied by reductions in cumulative seasonal exposures in terms of
W126 index, as well as reductions in short-term peak concentrations.
2. Environmental Exposures in Terms of W126 Index
The following presentation is framed by the question: What are the
nature and magnitude of vegetation exposures associated with conditions
meeting the current standard at sites across the U.S., particularly in
specially protected areas, such as Class I areas, and what do they
indicate regarding the potential for O3-related vegetation
impacts? Given the evidence indicating the W126 index to be strongly
related to growth effects and its use in the E-R functions for tree
seedling RBL (as summarized in section III.B above), exposure is
quantified using the W126 metric. The potential for impacts of interest
is assessed through considering the magnitude of estimated exposure, in
light of current information and, in comparison to levels given
particular focus in the 2015 decision on the current standard (80 FR
65292; October 26, 2015). The updated analyses summarized here, while
including assessment of all monitoring sites nationally, include a
particular focus on monitoring sites in or near Class I areas \169\, in
light of the greater public welfare significance of many O3
related impacts in such areas, as described in section III.B.2 above.
---------------------------------------------------------------------------
\169\ This includes monitors sited within Class I areas or the
closest monitoring site within 15 km of the area boundary.
---------------------------------------------------------------------------
The analyses summarized here consider both recent air quality
(2016-2018) and air quality since 2000 (PA, Appendix 4D). These air
quality analyses of cumulative seasonal exposures associated with
conditions meeting the current standard nationally provide conclusions
generally similar to those based on the data available at the time of
the last review when the current standard was set, when the most recent
data were available for 2011 to 2013 (Wells, 2015). Such conclusions
are with regard to regional differences as well as the rarity of W126
index values at or above 19 ppm-hrs in areas with air quality meeting
the current standard.\170\
---------------------------------------------------------------------------
\170\ Rounding conventions are described in detail in the PA,
Appendix 4D, section 4D.2.2.
---------------------------------------------------------------------------
Cumulative exposures vary across the U.S. with the highest W126
index values for sites that met the current standard being located
exclusively in Southwest and West climate regions (PA, Figure 4-6). At
sites meeting the current standard in all other NOAA climate regions,
W126 index values, averaged over the 3-year design value period are at
or below 13 ppm-hrs (PA, Figure 4-6 and Appendix 4D, Figure 4D-2). At
Southwest and West region sites that met the current standard, W126
index values, averaged across the 3-year design value period, are at or
below 17 ppm-hrs in virtually all cases in the most recent 3-year
period and across all of the seventeen 3-year periods in the full
dataset evaluated (i.e., all but one site out of 147 for recent period
and all but eight out of over 1,800 cases across full dataset). Across
all U.S. sites with valid design values at or below 70 ppb in the full
2000 to 2018 dataset, the W126 index, averaged over three years, was at
or below 17 ppm-hrs on 99.9% of all occasions, and at or below 13 ppm-
hrs on 97% of all occasions. All but one of the eight occasions when
the 3-year W126 index was above 17 ppm-hrs (including the highest
occasion at 19 ppm-hrs) occurred in the Southwest region during a
period before 2011. The most recent occasion occurred in 2018 at a site
in the West region when the 3-year average W126 index value was 18 ppm-
hrs (PA, Appendix 4D, section 4D.3.2).
In summary, among sites meeting the current standard in the most
recent period of 2016 to 2018, there are none with a W126 index, based
on the 3-year average, above 19 ppm-hrs, and just one with such a value
above 17 ppm-hrs (Table 5). Additionally, the full historical dataset
includes no occurrences of a 3-year average W126 index above 19 ppm-hrs
for sites meeting the current standard, and just eight occurrences of a
W126 index above 17 ppm-hrs, with the highest such occurrence just
equaling 19 ppm-hrs (Table 5; PA, Appendix 4D, section 4D.3.2.1).
With regard to Class I areas, the updated air quality analyses
include data at sites in or near 65 Class I areas. The findings for
these sites, which are distributed across all nine NOAA climate regions
in the contiguous U.S., as well as Alaska and Hawaii, mirror the
findings for the analysis of all U.S. sites. Among the Class I area
sites meeting the current standard (i.e., having a design value at or
below 70 ppb) in the most recent period of 2016 to 2018, there are none
with a W126 index (as average over design value period) above 17 ppm-
hrs (Table 5). The historical dataset includes just seven occurrences
(all dating from the 2000-2010 period) of a Class I area site meeting
the current standard and having a 3-year average W126 index above 17
ppm-hrs, and no such occurrences above 19 ppm-hrs (Table 5).
The W126 exposures at sites with design values above 70 ppb range
up to approximately 60 ppm-hrs (Table 5). Among all sites across the
U.S. that do not meet the current standard in the 2016 to 2018 period,
more than a quarter have average W126 index values above 19 ppm-hrs and
a third exceed 17 ppm-hrs (Table 5). A similar situation exists for
Class I area sites (Table 5). Thus, as was the case in the last review,
the currently available quantitative information continues to indicate
appreciable control of seasonal W126 index-based cumulative exposure at
all sites with air quality meeting the current standard.
[[Page 49895]]
Table 5--Distribution of 3-Yr Average Seasonal W126 Index for Sites in Class I Areas and Across U.S. That Meet the Current Standard and for Those That
Do Not
--------------------------------------------------------------------------------------------------------------------------------------------------------
Number of occurrences or site-DVs \A\
-------------------------------------------------------------------------------------------------------
In Class I areas Across all monitoring sites (urban and rural)
3-year periods -------------------------------------------------------------------------------------------------------
W126 (ppm-hrs) W126 (ppm-hrs)
Total --------------------------------------- Total --------------------------------------
>19 >17 <=17 >19 >17 <=17
--------------------------------------------------------------------------------------------------------------------------------------------------------
At Sites That Meet the Current Standard (Design Value at or Below 70 ppb)
--------------------------------------------------------------------------------------------------------------------------------------------------------
2016-2018....................................... 47 0 0 47 849 0 1 848
All from 2000 to 2018........................... 498 0 7 491 8,292 0 8 8,284
--------------------------------------------------------------------------------------------------------------------------------------------------------
At Sites That Exceed the Current Standard (Design Value Above 70 ppb)
--------------------------------------------------------------------------------------------------------------------------------------------------------
2016-2018....................................... 11 8 9 2 273 78 91 182
All from 2000 to 2018........................... 362 159 197 165 10,695 2,317 3,174 7,521
--------------------------------------------------------------------------------------------------------------------------------------------------------
\A\ Counts presented here are drawn from the PA, Appendix D, Tables 4D-1, 4D-4, 4D-5, 4D-6, 4D-9, 4D-10 and 4D-13 through 16.
As summarized above, the information available in this review
continues to indicate that average cumulative seasonal exposure levels
at virtually all sites and 3-year periods with air quality meeting the
current standard fall at or below the level of 17 ppm-hrs that was
identified when the current standard was established (80 FR 65393;
October 26, 2015). Additionally, the full dataset indicates that at
sites meeting the current standard, annual W126 index values were less
than or equal to 19 ppm-hrs well over 99% of the time (PA, Appendix 4D,
section 4D.3.2.1). Additionally, the average W126 index in Class I
areas that meet the current standard for the most recent 3-year period
is below 17 ppm-hrs at all areas which have a monitor within or near
their borders (PA, Appendix 4D, Table 4D-16). Further, with the
exception of seven values that occurred prior to 2011, cumulative
seasonal exposures, in terms of average 3-year W126, in all Class I
areas during periods that met the current standard were no higher than
17 ppm-hrs. This contrasts with the occurrence of much higher W126
index values at sites when the current standard was not met. For
example, out of the 11 Class I area sites with design values above 70
ppb during the most recent period, eight sites had a 3-year average
W126 index above 19 ppm-hrs (ranging up to 47 ppm-hrs) and for nine, it
was above 17 ppm-hrs (Table 5; PA, Appendix 4D, Table 4D-17).
D. Proposed Conclusions on the Secondary Standard
In reaching proposed conclusions on the current secondary
O3 standard (presented in section III.D.3), the
Administrator has taken into account policy-relevant evidence-based and
air quality-, exposure- and risk-based considerations discussed in the
PA (summarized in section III.D.1), as well as advice from the CASAC,
and public comment on the standard received thus far in the review
(section III.D.2). In general, the role of the PA is to help ``bridge
the gap'' between the Agency's assessment of the current evidence and
quantitative analyses (of air quality, exposure and risk), and the
judgments required of the Administrator in determining whether it is
appropriate to retain or revise the NAAQS. Evidence-based
considerations draw upon the EPA's integrated assessment of the
scientific evidence of welfare effects related to O3
exposure presented in the ISA (summarized in section III.B above) to
address key policy-relevant questions in the review. Similarly, the air
quality-, exposure- and risk-based considerations draw upon our
assessment of air quality, exposure and associated risk (summarized in
section III.C above) in addressing policy-relevant questions focused on
the potential for O3 exposures associated with welfare
effects under air quality conditions meeting the current standard.
This approach to reviewing the secondary standard is consistent
with requirements of the provisions of the CAA related to the review of
the NAAQS and with how the EPA and the courts have historically
interpreted the CAA. As discussed in section I.A above, these
provisions require the Administrator to establish secondary standards
that, in the Administrator's judgment, are requisite (i.e., neither
more nor less stringent than necessary) to protect the public welfare
from known or anticipated adverse effects associated with the presence
of the pollutant in the ambient air. Consistent with the Agency's
approach across all NAAQS reviews, the EPA's approach to informing
these judgments is based on a recognition that the available welfare
effects evidence generally reflects a continuum that includes ambient
air exposures for which scientists generally agree that effects are
likely to occur through lower levels at which the likelihood and
magnitude of response become increasingly uncertain. The CAA does not
require the Administrator to establish a secondary standard at a zero-
risk level, but rather at a level that reduces risk sufficiently so as
to protect the public welfare from known or anticipated adverse
effects.
The proposed decision on the adequacy of the current secondary
standard described below is a public welfare policy judgment by the
Administrator that draws upon the scientific evidence for welfare
effects, quantitative analyses of air quality, exposure and risks, as
available, and judgments about how to consider the uncertainties and
limitations that are inherent in the scientific evidence and
quantitative analyses. This proposed decision has additionally
considered the August 2019 remand of the secondary standard. The four
basic elements of the NAAQS (i.e., indicator, averaging time, form, and
level) have been considered collectively in evaluating the public
welfare protection afforded by the current standard. The
Administrator's final decision will additionally consider public
comments received on this proposed decision.
1. Evidence- and Exposure/Risk-Based Considerations in the Policy
Assessment
Based on its evaluation of the evidence and quantitative analyses
of
[[Page 49896]]
air quality, exposure and potential risk, the PA for this review
reaches the conclusion that consideration should be given to retaining
the current secondary standard, without revision (PA, section 4.5.3).
Accordingly, and in light of this conclusion that it is appropriate to
consider the current secondary standard to be adequate, the PA did not
identify any potential alternative secondary standards for
consideration in this review (PA, section 4.5.3). The PA additionally
recognized that, as is the case in NAAQS reviews in general, the extent
to which the Administrator judges the current secondary O3
standard to be adequate will depend on a variety of factors, including
science policy judgments and public welfare policy judgments. These
factors include public welfare policy judgments concerning the
appropriate benchmarks on which to place weight, as well as judgments
on the public welfare significance of the effects that have been
observed at the exposures evaluated in the welfare effects evidence.
The factors relevant to judging the adequacy of the standard also
include the interpretation of, and decisions as to the weight to place
on, different aspects of the quantitative analyses of air quality and
cumulative O3 exposure and any associated uncertainties.
Thus, the Administrator's conclusions regarding the adequacy of the
current standard will depend in part on public welfare policy
judgments, science policy judgments regarding aspects of the evidence
and exposure/risk estimates, as well as judgments about the level of
public welfare protection that is requisite under the Clean Air Act.
The subsections below summarize key considerations and conclusions
from the PA. The main focus of the policy-relevant considerations in
the PA is the question: Does the currently available scientific
evidence- and exposure/risk-based information support or call into
question the adequacy of the protection afforded by the current
secondary O3 standard? In addressing this overarching
question, the PA focuses first on consideration of the evidence, as
evaluated in the ISA (and supported by the prior ISA and AQCDs),
including that newly available in this review, and the extent to which
it alters the EPA's overall conclusions regarding welfare effects
associated with photochemical oxidants, including O3, in
ambient air. The PA also considers questions related to the general
approach or framework in which to evaluate public welfare protection of
the standard. Additionally, the PA considers the currently available
quantitative information regarding environmental exposures likely to
occur in areas of the U.S. where the standard is met, including
associated limitations and uncertainties, and the significance of these
exposures with regard to the potential for O3-related
vegetation effects, their potential severity and any associated public
welfare implications and judgments about the uncertainties inherent in
the scientific evidence and quantitative analyses that are integral to
consideration of whether the currently available information supports
or calls into question the adequacy of the current secondary
O3 standard.
a. Welfare Effects Evidence
With regard to the support in the current evidence for
O3 as the indicator for photochemical oxidants, no newly
available evidence has been identified in this review regarding the
importance of photochemical oxidants other than O3 with
regard to abundance in ambient air, and potential for welfare
effects.\171\ Data for photochemical oxidants other than O3
are generally derived from a few special field studies; such that
national-scale data for these other oxidants are scarce (ISA, Appendix
1, section 1.1; 2013 ISA, sections 3.1 and 3.6). Moreover, few studies
of the welfare effects of other photochemical oxidants beyond
O3 have been identified by literature searches conducted for
the 2013 ISA and prior AQCDs, such that ``the primary literature
evaluating the . . . ecological effects of photochemical oxidants
includes ozone almost exclusively as an indicator of photochemical
oxidants'' (ISA, section IS.1.1, Appendix 1, section 1.1). Thus, as was
the case for previous reviews, the PA finds that the evidence base for
welfare effects of photochemical oxidants does not indicate an
importance of any other photochemical oxidants such that O3
continues to be appropriately considered for the secondary standard's
indicator.
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\171\ Close agreement between past ozone measurements and the
photochemical oxidant measurements upon which the early NAAQS (for
photochemical oxidants including O3) was based indicated
the very minor contribution of other oxidant species in comparison
to O3 (U.S. DHEW, 1970).
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(i) Nature of Effects
Across the full array of welfare effects, summarized in section
III.B.1 above, the evidence newly available in this review strengthens
previous conclusions, provides further mechanistic insights and
augments current understanding of varying effects of O3
among species, communities and ecosystems (ISA, sections IS.1.3.2, IS.5
and IS.6.2, and Appendices 8 and 9). The current evidence, including
the wealth of long-standing evidence, continues to support conclusions
of causal relationships between O3 and visible foliar
injury, reduced yield and quality of agricultural crops, reduced
vegetation growth and plant reproduction, reduced productivity in
terrestrial ecosystems, and alteration of belowground biogeochemical
cycles. The current evidence additionally continues to support
conclusions of likely causal relationships between O3 and
reduced carbon sequestration in terrestrial systems, and alteration of
terrestrial ecosystem water cycling (ISA, section IS.I.3.2). Also as in
the last review, the current ISA determines there to be a causal
relationship between tropospheric O3 and radiative forcing
and a likely causal relationship between tropospheric O3 and
temperature, precipitation and related climate variables (ISA, section
IS.1.3.3). The current evidence has led to an updated conclusion on the
relationship of O3 with alteration of terrestrial community
composition to causal (ISA, sections IS.I.3.2). Lastly, the current ISA
concludes the current evidence sufficient to infer likely causal
relationships of O3 with three additional categories of
effects (ISA, sections IS.I.3.2). For example, while previous
recognition of O3 as a contributor to tree mortality in a
number of field studies was a factor in the 2013 conclusion of a likely
causal relationship between O3 and alterations in community
composition, tree mortality has been separately assessed in this
review. Additionally, newly available evidence on two additional plant
related effects augments more limited previously available evidence
related to insect interactions with vegetation, contributing to
additional conclusions that the body of evidence is sufficient to infer
likely causal relationships between O3 and alterations of
plant-insect signaling and insect herbivore growth and reproduction
(ISA, Appendix 8, sections 8.6 and 8.7).\172\
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\172\ As in the last review, the ISA again concludes that the
evidence is inadequate to determine if a causal relationship exists
between changes in tropospheric ozone concentrations and UV-B
effects (ISA, Appendix 9, section 9.1.3.4; 2013 ISA, section
10.5.2).
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As in the last review, the strongest evidence and the associated
findings of causal or likely causal relationships with O3 in
ambient air, and quantitative characterizations of relationships
between O3 exposure and occurrence and magnitude of effects
are for vegetation-related effects. With regard to uncertainties and
limitations associated with the current welfare effects
[[Page 49897]]
evidence, the PA recognized that the type of uncertainties for each
category of effects tends to vary, generally in relation to the
maturity of the associated evidence base, from those associated with
overarching characterizations of the effects to those associated with
quantification of the cause and effect relationships. For example,
given the longstanding nature of the evidence for many of the
vegetation effects identified in the ISA as causally or likely causally
related to O3 in ambient air, the key uncertainties and
limitations in our understanding of these effects relate largely to the
implications or specific aspects of the evidence, as well as to current
understanding of the quantitative relationships between O3
concentrations in the environment and the occurrence and severity (or
relative magnitude) of such effects or understanding of key influences
on these relationships. For more newly identified categories of
effects, the evidence may be less extensive, and accordingly, the areas
of uncertainty greater, thus precluding consideration of quantitative
details related to risk of such effects under varying air quality
conditions that would inform review of the current standard.
The evidence bases for the three newly identified categories
provide examples of such gaps in relevant information. For example, the
evidence for increased tree mortality includes previously available
studies with field observations from locations and periods of
O3 concentrations higher than are common today and three
more recently available publications assessing O3 exposures
not expected under conditions meeting the current standard, as
summarized in section III.B.1 above. The information available
regarding the newly identified categories of plant-insect signaling and
insect herbivore growth and reproduction additionally does not provide
for a clear understanding of the specific environmental effects that
may occur in the natural environment under specific exposure
conditions, as summarized in sections III.B.1 and III.B.3 above (PA,
section 4.5.1.1). Accordingly, the PA does not find the current
evidence for these newly identified categories to call into question
the adequacy of the current standard.
With regard to tropospheric O3 as a greenhouse gas at
the global scale, and associated effects on climate, the PA notes that
while additional characterizations of tropospheric O3 and
climate have been completed since the last review, uncertainties and
limitations in the evidence that were also recognized in the last
review remain (PA, section 4.5.1.1). As summarized in section III.B.3
above, there is appreciable uncertainty associated with understanding
quantitative relationships involving regional O3
concentrations near the earth's surface and climate effects of
tropospheric O3 on a global scale. Further, there are
limitations in our modeling tools and associated uncertainties in
interpretations related to capabilities for quantitatively estimating
effects of regional-scale lower tropospheric O3
concentrations on climate. These uncertainties and limitations affect
our ability to make a quantitative characterization of the potential
magnitude of climate response to changes in O3
concentrations in ambient air, particularly at regional (vs global)
scales, and thus our ability to assess the impact of changes in ambient
air O3 concentrations in regions of the U.S. on global
radiative forcing or temperature, precipitation and related climate
variables. Consequently, the PA finds that current evidence in this
area is not informative to consideration of the adequacy of public
welfare protection of the current standard (PA, section 4.5.1.1).
(ii) E-R Information
The category of O3 welfare effects for which current
understanding of quantitative relationships is strongest continues to
be reduced plant growth. While the ISA describes studies of welfare
effects associated with O3 exposures newly identified since
the last review, the established E-R functions for tree seedling growth
and crop yield that have been available in the last several reviews
continue to be the most robust descriptions of E-R relationships for
welfare effects. These well-established E-R functions for seedling
growth reduction in 11 tree species and yield loss in 10 crop species
are based on response information across multiple levels of cumulative
seasonal exposure (estimated from extensive records of hourly
O3 concentrations across the exposure periods). Studies of
some of the same species, conducted since the derivation of these
functions, provide supporting information (ISA, Appendix 8, section
8.13.2; 2013 ISA, sections 9.6.3.1 and 9.6.3.2). The E-R functions
provide for estimation of the growth-related effect, RBL, for a range
of cumulative seasonal exposures.
The evidence newly available in this review does not include
studies that assessed reductions in tree growth or crop yield responses
across multiple O3 exposures and for which sufficient data
are available for analyses of the shape of the E-R relationship across
a range of cumulative exposure levels (e.g., in terms of W126 index)
relevant to conditions associated with the current standard. While
there are several newly available studies that summarize previously
available studies or draw from them, such as for linear regression
analyses, these do not provide robust E-R functions or cumulative
seasonal exposure levels associated with important vegetation effects,
such as reduced growth, that define the associated exposure
circumstances in a consistent manner (as summarized in section III.B.3
above).\173\ This limits their usefulness for considering the potential
for occurrence of welfare effects in air quality conditions that meet
the current standard. Thus, the PA concludes that robust E-R functions
are not available for growth or yield effects on any additional tree
species or crops in this review.
---------------------------------------------------------------------------
\173\ For example, among the newly available publications cited
in the ISA is a study that compiles EC10 values
(estimated concentration at which 10% lower biomass [compared to
zero O3] is predicted) derived for trees and grassland
species (including 17 native to the U.S. [ISA, Table 8-26]) using
linear regression of previously published data on plant growth
response and O3 concentration quantified as AOT40. The
data were from studies of various experimental designs, that
involved various durations ranging up from 21 days, and involving
various concentrations no higher than 100 ppb as a daily maximum
hourly concentration. More detailed analyses of exposure and
response information across a relevant range of seasonal exposure
levels (e.g., accompanied by detailed records of O3
concentrations) that would support derivation of robust E-R
functions for purposes discussed here are not available.
---------------------------------------------------------------------------
In considering the E-R functions and their use in informing
judgments regarding such effects in areas with air quality of interest,
the PA additionally recognized a number of limitations, and associated
uncertainties, that remain in the current evidence base, and that
affect characterization of the magnitude of cumulative exposure
conditions eliciting growth reductions in U.S. forests (PA, section
4.3.4). For example, there are uncertainties in the extent to which the
11 tree species for which there are established E-R functions encompass
the range of O3 sensitive species in the U.S., and also the
extent to which they represent U.S. vegetation as a whole. These 11
species include both deciduous and coniferous trees with a wide range
of sensitivities and species native to every NOAA climate region across
the U.S. and in most cases are resident across multiple states and
regions. Thus, they may provide a range that encompasses species
without E-R
[[Page 49898]]
functions.\174\ The PA additionally recognizes important uncertainties
in the extent to which the E-R functions for reduced growth in tree
seedlings are also descriptive of such relationships during later
lifestages, for which there is a paucity of established E-R
relationships. Although such information is limited with regard to
mature trees, analyses in the 2013 ISA indicated that reported growth
response of young aspen over six years was similar to the reported
growth response of seedlings (ISA, Appendix 8, section 8.13.2; 2013
ISA, section 9.6.3.2). Additionally, there are uncertainties with
regard to the extent to which various factors in natural environments
can either mitigate or exacerbate predicted O3-plant
interactions and contribute variability in vegetation-related effects,
including reduced growth. Such factors include multiple genetically
influenced determinants of O3 sensitivity, changing
sensitivity to O3 across vegetative growth stages, co-
occurring stressors and/or modifying environmental factors (PA, section
4.3.4).
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\174\ This was the view of the CASAC in the 2015 review (Frey,
2014b, p. 11).
---------------------------------------------------------------------------
The PA additionally considered the quantitative information for
other long-recognized effects of O3 (PA, section 4.3.4). For
example, with regard to crop yield effects, as at the time of the last
review, the PA recognized the potential for greater uncertainty in
estimating the impacts of O3 exposure on agricultural crop
production than that associated with O3 impacts on
vegetation in natural forests. This relates to uncertainty in the
extent to which agricultural management methods influence potential for
O3-related effects and accordingly, the applicability of the
established E-R functions for RYL in current agricultural areas (PA,
section 4.3.4).
With regard to visible foliar injury, the PA finds that, as in the
last review, there remains a lack of established E-R functions that
would quantitatively describe relationships between the occurrence and
severity of visible foliar injury and O3 exposure, as well
as factors influential in those relationships, such as soil moisture
conditions (PA, section 4.5.1.1). While the currently available
information continues to include studies that document foliar injury in
sensitive plant species in response to specific O3
exposures, investigations of a quantitative relationship between
environmental O3 exposures and visible foliar injury
occurrence/severity have not yielded a predictive result. In addition
to experimental studies, the evidence includes multiple studies that
have analyzed data collected as part of the USFS biosite biomonitoring
program (e.g., Smith, 2012). These analyses continue to indicate the
limitations in capabilities for predicting the exposure circumstances
under which visible foliar injury would be expected to occur, as well
as the circumstances contributing to increased injury severity. As
noted in section III.B.3.b above, expanded summaries of the dataset
compiled in the 2015 review from several years of USFS biosite records
also does not clearly and consistently describe a relationship between
incidence of foliar injury or severity (based on individual site
scores) and W126 index estimates across the range of exposures.
Overall, however, the dataset indicates that the proportion of records
having different levels of severity score is generally highest in the
records at sites with the highest W126 index (e.g., greater than 25
ppm-hrs for the normal and dry soil moisture categories). This analysis
does not provide for identification of air quality conditions, in terms
of O3 concentrations associated with the relatively lower
environmental exposures most common in the USFS dataset that would
correspond to a specific magnitude of injury incidence or severity
scores across locations.
As discussed in section III.B.3 above, a number of analyses of the
USFS biosite data (as well as several experimental studies), while
often using cumulative exposure metrics to quantify O3
exposures have additionally reported there to be a role for a metric
that quantifies the incidence of ``high'' O3 days (2013 ISA,
p. 9-10; Smith, 2012; Wang et al., 2012). Such analyses have not,
however, established specific air quality metrics and associated
quantitative functions for describing the influence of ambient air
O3 on incidence and severity of visible foliar injury. As a
result, the PA concludes that limitations recognized in the last review
remain in our ability to quantitatively estimate incidence and severity
of visible foliar injury likely to occur in areas across the U.S. under
different air quality conditions over a year, or over a multi-year
period.
In looking across the full array of O3 welfare effects,
the PA recognizes that the E-R functions for growth-related effects
that were available in the last review continue to be the most robust
E-R information available. The currently available evidence for growth-
related effects, including that newly available in this review, does
not indicate the occurrence of growth-related responses attributable to
cumulative O3 exposures lower than was established at the
time of the last review. With regard to visible foliar injury, the
available information that would support estimates of occurrence and
severity across a range of air quality conditions continues to be
limited, affecting the nature of conclusions that may be reached
related to potential occurrence and/or severity for conditions. The
quantitative information for other effects is more limited, as
recognized earlier in this section and in section III.B.3 above. Thus,
the PA concludes that the newly available evidence does not appreciably
address key limitations or uncertainties as would be needed to expand
capabilities for estimating welfare impacts that might be expected as a
result of differing patterns of O3 concentrations in the
U.S.
(iii) W126 Index as Exposure Metric
With regard to exposure metric the currently available evidence
continues to support a cumulative, seasonal exposure index as a
biologically relevant and appropriate metric for assessment of the
evidence of exposure/risk information for vegetation, most particularly
for growth-related effects. The most commonly used such metrics are the
SUM06, AOT40 (or AOT60) and W126 indices (ISA, section IS.3.2).\175\
The evidence for growth-related effects continues to support important
roles for cumulative exposure and for weighting higher concentrations
over lower concentrations. Thus, among the various such indices
considered in the literature, the cumulative, concentration-weighted
metric, defined by the W126 function, continues to be best supported
for purposes of relating O3 air quality to growth-related
effects. Accordingly, the PA continues to find the W126 index
appropriate for consideration of the potential for vegetation-related
effects to occur under air quality conditions (PA, section 4.5.1.1).
The PA also recognizes, as recognized in the past, the lack of support
for E-R functions for incidence and severity of visible foliar injury
with W126 index as the descriptor of exposure, particularly in
environmental settings where exposures are below a
[[Page 49899]]
W126 index of 25 ppm-hrs. While the PA analysis of the dataset of USFS
biosite scores indicates appreciable increases in incidence and
severity at and above 25 ppm-hrs, a pattern is unclear at lower W126
index estimates across which the dataset does not support a predictive
relationship. As summarized in section III.3.b above, while the overall
evidence also indicates an important role for peak concentrations
(e.g., N100) in influencing the occurrence and severity of visible
foliar injury, the current evidence does not include an established
predictive relationship based on such an additional metric (PA, section
4.5.1.1).
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\175\ The evidence includes some studies reporting
O3-reduced soybean yield and perennial plant biomass loss
using AOT40 (as well as W126) as the exposure metric, however, no
newly available analyses are available that compare AOT40 to W126 in
terms of the strength of association with such responses. Nor are
studies available that provide analyses of E-R relationships for AOT
with reduced growth or RBL with such extensiveness as the analyses
supporting the established E-R functions for W126 with RBL and RYL.
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b. General Approach for Considering Public Welfare Protection
This section summarizes PA consideration of the current evidence
and air quality information with regard to key aspects of the general
approach and risk management framework for making judgments and
reaching conclusions regarding the adequacy of public welfare
protection provided by the secondary standard that was applied in 2015
(summarized in section III.A.1 above). Key aspects of the approach
include the use of RBL as a proxy for the broad array of O3
vegetation-related effects, E-R relationships for this endpoint with
the W126 index, and the focus on this index averaged across a 3-year
period.
(i) RBL as Proxy or Surrogate
In the last review, the Administrator used RBL as a proxy or
surrogate for an array of adverse welfare effects based on
consideration of ecosystem services and potential for impacts to the
public, as well as conceptual relationships between vegetation growth
effects and ecosystem-scale effects. Such a use was supported by the
CASAC at that time (80 FR 65406, October 26, 2015; Frey, 2014b, pp.
iii, 9-10).\176\ In consideration of the broader evidence base and
public welfare implications, including associated strengths,
limitations and uncertainties, the Administrator focused on RBL, not
simply in making judgments specific to a magnitude of growth effect in
seedlings that would be acceptable or unacceptable in the natural
environment, but as a surrogate or proxy for consideration of the
broader array of vegetation-related effects of potential public welfare
significance, that included effects on growth of individual sensitive
species and extended to ecosystem-level effects, such as community
composition in natural forests, particularly in protected public lands
(80 FR 65406, October 26, 2015).
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\176\ The CASAC letter on the second draft PA in that review
stated the following (Frey, 2014b, p. 9-10):
For example, CASAC concurs that trees are important from a
public welfare perspective because they provide valued services to
humans, including aesthetic value, food, fiber, timber, other forest
products, habitat, recreational opportunities, climate regulation,
erosion control, air pollution removal, and hydrologic and fire
regime stabilization. Damage effects to trees that are adverse to
public welfare occur in such locations as national parks, national
refuges, and other protected areas, as well as to timber for
commercial use. The CASAC concurs that biomass loss in trees is a
relevant surrogate for damage to tree growth that affects ecosystem
services such as habitat provision for wildlife, carbon storage,
provision of food and fiber, and pollution removal. Biomass loss may
also have indirect process-related effects such as on nutrient and
hydrologic cycles. Therefore, biomass loss is a scientifically valid
surrogate of a variety of adverse effects to public welfare.
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The currently available evidence related to conceptual
relationships between plant growth impacts and the broader array of
vegetation effects (e.g., that supported the use of RBL as a surrogate
or proxy) is largely consistent with that available in the last review.
In fact, the ISA for the current review describes (or relies on) such
relationships in considering causality determinations for ecosystem-
scale effects such as altered terrestrial community composition and
reduced productivity, as well as reduced carbon sequestration, in
terrestrial ecosystems (ISA, Appendix 8, sections 8.8 and 8.10). Thus,
the PA concludes that the current evidence does not call into question
conceptual relationships between plant growth impacts and the broader
array of vegetation effects. Rather, the current evidence continues to
support the use of tree seedling RBL as a proxy for the broad array of
vegetation-related effects, most particularly those conceptually
related to growth (PA, sections 4.5.1.2 and 4.5.3).
Beyond tree seedling growth, on which RBL is specifically based,
two other vegetation effect categories with extensive evidence bases,
crop yield and visible foliar injury, were also given attention in
considering the public welfare protection provided by the standard in
2015. Based on the available information for these endpoints, along
with associated limitations and uncertainties, the Administrator at
that time concluded there was not support for giving a primary focus,
in selecting a revised secondary standard, to these two types of
effects. With regard to crop yield, the Administrator recognized the
significant role of agricultural management practices in agricultural
productivity, as well as market variability, concluding that, in
describing her public welfare protection objectives, additional
attention to this endpoint was not necessary. The rough similarities in
estimated W126 levels of median crops and tree species are also
noteworthy. With regard to foliar injury, the lack of clear
quantitative relationships that would support predictive E-R functions
was recognized. In light of such considerations, the Administrator
focused on RBL estimates in identifying the requisite standard, and
judged that a standard set based on public welfare protection
objectives described in terms of cumulative exposures and relationships
with tree seedling RBL was an appropriate means to, and would, provide
appropriate protection for the array of vegetation-related effects.
With regard to the information available in the current review, the PA
concludes it does not call into question the basis for such judgments
and continues to be supportive of the use of tree seedling RBL as a
proxy for the broad array of vegetation-related effects (PA, section
4.5.1.2).
In considering the magnitude of estimated RBL on which to focus in
its role as a surrogate or proxy for the full array of vegetation
effects in the last review, the Administrator endeavored to identify a
secondary standard that would limit 3-year average O3
exposures somewhat below W126 index values associated with a 6% RBL
median estimate from the established species-specific E-R functions.
This led to identification of a seasonal W126 index value of 17 ppm-hrs
that the Administrator concluded appropriate as a target at or below
which the new standard would generally restrict cumulative seasonal
exposures (80 FR 65407, October 26, 2015). In identifying this exposure
level as a target, the Administrator, recognizing limitations and
uncertainties in the evidence and variability in biota and ecosystems
in the natural environment, additionally judged that RBL estimates
associated with isolated rare instances of marginally higher cumulative
exposures (in terms of a 3-year average W126 index), e.g., those that
round to 19 ppm-hrs (which corresponds to 6% RBL as median from 11
established E-R functions), were not indicative of adverse effects to
the public welfare (80 FR 65409, October 26, 2015).
The PA concludes that the information newly available in this
review does not differ from that available in the last review with
regard to a magnitude of RBL in the median species appropriately
considered a reference for judgments concerning
[[Page 49900]]
potential vegetation-related impacts to the public welfare (PA, section
4.5.1.2). The currently available evidence continues to indicate
conceptual relationships between reduced growth and the broader array
of vegetation-related effects, and limitations and uncertainties remain
with regard to quantitation. The PA notes that consideration of the
magnitude of tree growth effects that might cause or contribute to
adverse effects for trees, forests, forested ecosystems or the public
welfare is complicated by various uncertainties or limitations in the
evidence base, including those associated with relating magnitude of
tree seedling growth reduction to larger-scale forest ecosystem
impacts. Further, other factors can influence the degree to which
O3-induced growth effects in a sensitive species affect
forest and forest community composition and other ecosystem service
flows (e.g., productivity, belowground biogeochemical cycles and
terrestrial ecosystem water cycling) from forested ecosystems. These
include (1) the type of stand or community in which the sensitive
species is found (i.e., single species versus mixed canopy); (2) the
role or position the species has in the stand (i.e., dominant, sub-
dominant, canopy, understory); (3) the O3 sensitivity of the
other co-occurring species (O3 sensitive or tolerant); and
(4) environmental factors, such as soil moisture and others. The lack
of such established relationships with O3 complicates
consideration of the extent to which different estimates of impacts on
tree seedling growth would indicate significance to the public welfare.
Further, efforts to estimate O3 effects on carbon
sequestration are handicapped by the large uncertainties involved in
attempting to quantify the additional carbon uptake by plants as a
result of avoided O3-related growth reductions. Such
analyses require complex modeling of biological and ecological
processes with their associated sources of uncertainty.
Quantitative representations of such relationships have been used
to study potential impacts of tree growth effects on such larger-scale
effects as community composition and productivity with the results
indicating the array of complexities involved (e.g., ISA, Appendix 8,
section 8.8.4). Given their purpose in exploring complex ecological
relationships and their responses to environmental variables, as well
as limitations of the information available for such work, these
analyses commonly utilize somewhat general representations. The PA
notes that this work indicates how established the existence of such
relationships is, while also identifying complexities inherent in
quantitative aspects of such relationships and interpretation of
estimated responses. Thus, the PA finds the currently available
evidence to be little changed from the last review with regard to
informing identification of an RBL reference point reflecting
ecosystem-scale effects with public welfare impacts elicited through
such linkages (PA, section 4.5.1.2).
(ii) Focus on 3-Year Average W126 Index
In setting the current standard, as described in section III.A.1
above, the Administrator focused on control of seasonal cumulative
exposures in terms of a 3-year average W126 index. The evaluations in
the PA for that review recognized there to be limited information to
discern differences in the level of protection afforded for cumulative
growth-related effects by a standard focused on a single-year W126
index as compared to a 3-year W126 index (80 FR 65390, October 26,
2015). Accordingly, 3-year average was identified for considering the
seasonal W126 index based on the recognition that there was year-to-
year variability not just in O3 concentrations, but also in
environmental factors, including rainfall and other meteorological
factors, that influence the occurrence and magnitude of O3-
related effects in any year (e.g., through changes in soil moisture),
contributing uncertainties to projections of the potential for harm to
public welfare (80 FR 65404 October 26, 2015). Given this recognition,
as well as other considerations, the Administrator expressed greater
confidence in judgments related to projections of public welfare
impacts based on seasonal W126 index estimated by a 3-year average and
accordingly, relied on that metric.
A general area of uncertainty that remains in the current evidence
continues to affect interpretation of the potential for harm to public
welfare over multi-year periods of air quality that meet the current
standard (PA, section 4.3.4). As recognized in the last review, there
is variability in ambient air O3 concentrations from year to
year, as well as year-to-year variability in environmental factors,
including rainfall and other meteorological factors that affect plant
growth and reproduction, such as through changes in soil moisture.
Accordingly, these variabilities contribute uncertainties to estimates
of the occurrence and magnitude of O3-related effects in any
year, and to such estimates over multi-year periods. The PA recognizes
that limitations in our ability to estimate the effects on growth over
tree lifetimes of year-to-year variation in O3
concentrations, particularly those associated with conditions meeting
the current standard, contribute uncertainty to estimates of cumulative
growth (biomass) effects over multi-year periods in the life of
individual trees and associated populations, as well as related effects
in associated communities and ecosystems (PA, section 4.3.4).
As summarized in section III.B.3 above, the longstanding evidence
on O3 effects on plant growth includes the established and
robust E-R functions for 11 species of tree seedlings (ISA, Appendix 8,
Table 8-24; PA, Appendix 4A, Table 4A-1,). The PA recognized the
strength of these functions in describing tree seedling response across
a broad range of W126 index values, concluding that the evidence
continues to support their use in estimating the median RBL across
species in this review. In considering the appropriate representation
of seasonal W126 for use of these functions with air quality data, the
PA additionally considered the available information underlying the E-R
functions and the extent to which the information is specific to a
single seasonal exposure, e.g., as compared to providing representation
for an average W126 index across multiple seasons (PA, section
4.5.1.2). In so doing, the PA took note of aspects of the evidence that
reflect variability in organism response under different experimental
conditions and the extent to which this variability is represented in
the available data. This might indicate an appropriateness of assessing
environmental conditions using a mean across seasons in recognition of
the existence of such year-to-year variability in conditions and
responses. An additional aspect of the information underlying the E-R
functions that was identified as relevant to consider is the extent to
which the exposure conditions represented include those associated with
O3 concentrations that meet the current standard, and the
extent to which tree seedling growth responses to such conditions may
have been found to not be significantly different from responses to the
control (e.g., zero O3) conditions. The extent to which E-R
predictions are extrapolated beyond the tested exposure conditions also
contributes to uncertainty which the PA indicated may argue for a less
precise interpretation, such as an average across multiple seasons.
The experiments from which the functions were derived vary in
duration
[[Page 49901]]
from periods of 82 to 140 days over a single year to periods of 180 to
555 days across two years, and in whether measurements were made
immediately following exposure period or in the subsequent season (PA,
section 4.5.1.2, Appendix 4A, Table 4A-5; Lee and Hogsett, 1996). In
producing E-R functions of consistent duration across the experiments,
the E-R functions were derived first based on the exposure duration of
the experiment and then normalized to 3-month (seasonal) periods (see
Lee and Hogsett, 1996, section I.3; PA, Appendix 4A). Underlying the
adjustment is a simplifying assumption of uniform W126 distribution
across the exposure periods and of a linear relationship between
duration of cumulative exposure in terms of the W126 index and plant
growth response. Some functions for experiments that extended over two
seasons were derived by distributing responses observed at the end of
two seasons of varying exposures equally across the two seasons (e.g.,
essentially applying the average to both seasons).
The PA additionally recognizes that the experiment-specific E-R
functions for both aspen and ponderosa pine illustrate appreciable
variability in response across experiments (PA, Appendix 4A, Figure 4A-
10). The PA suggested that reasons for this variability may relate to a
number of factors, including variability in seasonal response related
to variability in non-O3 related environmental influences on
growth, such as rainfall, temperature and other meteorological
variables, as well as biological variability across individual
seedlings, in addition to potentially variability in the pattern of
O3 concentrations contributing to similar cumulative
exposures (PA, section 4.5.1.2). In recognition of some of the
variability in both seasonal environmental conditions in the studies
and the associated experimental data, the 11 species-specific E-R
functions are based on median responses (derived from experiment-
specific functions) across an array of W126 index values (PA, Appendix
4A; Lee and Hogsett, 1996).\177\ The number of experiments used in
deriving the E-R functions for each species varies. For example, there
are 7 experimental studies for wild aspen and 11 for ponderosa pine
(PA, Appendix 4A, Table 4A-5), and only two or three for the three
species (black cherry, sugar maple and tulip poplar) that exhibit
greater sensitivity than aspen and ponderosa pine (PA, Appendix 4A,
section 4A-2, Table 4A-5; 1996 AQCD, Table 5-28; Lee and Hogsett,
1996). Regarding the extent or strength of the database underlying the
E-R functions for cumulative exposure levels of interest in the current
review, the PA also notes that the data generally appear to be more
extensive for relatively higher (e.g., at/above a SUM06 of 30 ppm-hrs),
versus lower, seasonal exposures (PA, Appendix 4A, Table 4A-6).
Additionally, while the evidence is long-standing and robust for growth
effects of O3, the studies available for some species appear
to be somewhat limited in the extent to which they include cumulative
O3 exposures commonly occuring with air quality conditions
that meet the current standard (e.g., W126 index values below 20 ppm-
hrs).\178\ The PA concludes the factors identified here to contribute
to uncertainty or inexactitude in estimates based on the E-R functions.
---------------------------------------------------------------------------
\177\ This median-based approach is expected to guard against
statistical bias in parameter values.
\178\ The evidence is unclear on the extent to which six of the
11 species include exposure treatments likely to correspond to W126
index values at or below 20 ppm-hrs (PA, Appendix 4A, Table 4A-5).
For five of the species in Table 4A-5 in Appendix 4A, SUM06 index
values below 25 ppm-hrs range from 12 to 21.7. In considering these
values, we note that an approach used in the 2007 Staff Paper on
specific temporal patterns of O3 concentrations concluded
that a SUM06 index value of 25 ppm-hrs would be estimated to
correspond to a W126 index value of approximately 21 ppm-hrs (U.S.
EPA, 2007, Appendix 7B, p. 7B-2). Accordingly, a SUM06 value of 21
ppm-hrs might be expected to correspond to a W126 index value below
20 ppm-hrs. The PA further notes that for one of the species for
which lower exposures were studied, black cherry, the findings for
at least one study reported statistical significance only for
effects observed for higher exposures (PA, section 4.3.4, Appendix
4A, Table 4A-6).
---------------------------------------------------------------------------
The PA recognizes that the evidence that allows for specific
evaluation of the predictability of growth impacts from single-year
versus multiple-year average exposure estimates is quite limited. Such
evidence would include multi-year studies reporting results for each
year of the study, which are the most informative to the question of
plant annual and cumulative responses to individual years (high and
low) over multiple-year periods. The evidence is quite limited with
regard to studies of O3 effects that report seasonal
observations across multi-year periods and that also include detailed
hourly O3 concentration records (to allow for derivation of
exposure index values). Such a limitation contributes uncertainty and
accordingly a lack of precision to our understanding of the
quantitative impacts of seasonal O3 exposure, including its
year-to-year variability on tree growth and annual biomass accumulation
(PA, section 4.3.4). The PA finds this uncertainty to limit our
understanding of the extent to which tree biomass would be expected to
appreciably differ at the end of multi-year exposures for which the
overall average exposure is the same, yet for which the individual year
exposures varied in different ways (e.g., as analyzed in Appendix 4D of
the PA). Thus, the PA notes that the extent of any differences in tree
biomass for two multi-year scenarios with the same 3-year average W126
index but differing single-year indices is not clear, including for
exposures associated with O3 concentrations that would meet
the current standard (PA, section 4.3.4).\179\
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\179\ Variation in annual W126 index values indicates that for
the period, 2016-2018, the amount by which annual W126 index values
at a site differ from the 3-year average varies is generally below
10 ppm-hrs across all sites and generally below 5 ppm-hrs at sites
with design values at or below 70 ppb (PA, Appendix 4D, Figure 4D-
7).
---------------------------------------------------------------------------
One such study, which tracked exposures across six years, is
available for aspen (King et al., 2005; 2013 ISA, section 9.6.3.2; ISA,
Appendix 8, section 8.13.2).\180\ This study was used in a presentation
of the 2013 ISA that compared the observed growth response to that
predicted from the E-R function for aspen. Specifically, the observed
aboveground biomass (and RBL) after each of the six growing seasons was
compared to estimates derived from the aspen E-R function based on the
cumulative multiple-year average seasonal W126 index values for each
year \181\ (2013 ISA, section 9.6.3.2). The conclusions reached were
that the agreement between the set of predictions and the Aspen FACE
observations were ``very close'' and that ``the function based on one
year of growth was shown to be applicable to subsequent years'' (2013
ISA, p. 9-135). The PA observes that such results indicate that when
considering O3 impacts on growing trees across multiple
years, a multi-year average index yields predictions close to observed
measurements across the multi-year time period (2013 ISA, section
9.6.3.2 and Figure 9-20; PA, Appendix 4A, section 4.A.3). The PA also
includes example analyses that use biomass measurements from the multi-
year study (King et al., 2005) to estimate aboveground aspen biomass
over a multi-year period using the established
[[Page 49902]]
E-R function for aspen with a constant single-year W126 index, e.g., of
17 ppm-hrs, or with varying annual W126 index values (10, 17 and 24
ppm-hrs) for which the 3-year average is 17 ppm-hrs, and that yield
somewhat similar total biomass estimates after multiple years (PA,
Appendix 4A, section 4A.3).\182\
---------------------------------------------------------------------------
\180\ A similar comparison is presented in the current ISA (ISA,
Appendix 8).
\181\ Although not emphasized or explained in detail in the 2013
ISA, the W126 estimates used to generate the predicted growth
response were cumulative average. To clarify, the cumulative average
W126 for year 1 is simply the W126 index for that year (e.g., based
on highest 3 months). For year 2, it is the average of the year 1
seasonal W126 and year 2 seasonal W126, and so on. For year 6, it is
the average of each of the six year's seasonal W126 index values.
\182\ This example, while simplistic in nature, and with
inherent uncertainties, including with regard to broad
interpretation given the reliance on data available for the single
study, quantitatively illustrates potential differences in growth
impacts of W126 index, as a 3-year average, for which individual
year values vary while still meeting the value specified for the
average, from such impacts from exposure controlled to the same W126
index value annually. The PA suggests that this example indicates
based on the magnitude of variation documented for annual W126 index
values occurring under the current standard, a quite small magnitude
of differences in tree biomass between single-year and multi-year
average approaches to controlling cumulative exposure (PA, Appendix
4A, section 4A.3).
---------------------------------------------------------------------------
Thus, the PA finds that, while the E-R functions are based on
strong evidence of seasonal and cumulative seasonal O3
exposure reducing tree growth, and while they provide for quantitative
characterization of the extent of such effects across O3
exposure levels of appreciable magnitude, there is uncertainty
associated with the resulting RBL predictions. Further, the current
evidence does not indicate single-year seasonal exposure in combination
with the established E-R functions to be a better predictor of RBL than
a seasonal exposure based on a multi-year average, or vice versa
(Appendix 4A, section 4A.3.1). Rather, associated uncertainty
contributes or implies an imprecision or inexactitude in the resulting
predictions, particularly for the lower W126 index estimates of
interest in this review. In light of this, the current evidence does
not support concluding there to be an appreciable difference in the
effect of three years of exposure held at 17 ppm-hrs compared to a 3-
year exposure that averaged 17 ppm-hrs yet varied by 5 to 10 ppm (e.g.,
7 ppm-hrs) from 17 ppm-hrs in any of the three years for tree RBL over
such multiple-year periods. The PA considered all of the factors
identified here, the currently available evidence and recognized
limitations, variability and uncertainties, to contribute uncertainty
and resulting imprecision or inexactitude to RBL estimates of single-
year seasonal W126 index values. The PA found these considerations to
indicate there to be no lesser support for use of an average seasonal
W126 index derived from multiple years (with their representation of
variability in environmental factors), such as for a 3-year period, for
estimating median RBL using the established E-R functions than for use
of a single-year index.
(iii) Visible Foliar Injury
In considering a public welfare protection approach related to
visible foliar injury, the PA first notes that some level of visible
foliar injury can impact public welfare and thus might reasonably be
judged adverse to public welfare.\183\ As summarized in section III.B.2
above, depending on its spatial extent and severity, there are many
situations or locations in which visible foliar injury can adversely
affect the public welfare. For example, significant, readily
perceivable and widespread injury in national parks and wilderness
areas can adversely affect the perceived scenic beauty of these areas,
harming the aesthetic experience for both outdoor enthusiasts and the
occasional park visitor. Such considerations have also been recognized
by the Agency in past reviews, in which decisions to revise the
O3 secondary standard emphasized protection of Class I
areas, which are areas such as national wilderness areas and national
parks given special protections by the Congress (e.g., 73 FR 16496,
March 27, 2008, ``the Administrator concludes it is appropriate to
revise the secondary standard, in part, to provide increased protection
against O3-caused impairment to such protected vegetation
and ecosystems'').\184\
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\183\ As stated in the 2015 decision notice: ``both tree growth-
related effects and visible foliar injury have the potential to be
significant to the public welfare'' (80 FR 65377, October 26, 2015);
``O3-induced visible foliar injury also has the potential
to be significant to the public welfare through impacts in Class I
and other similarly protected areas'' (80 FR 65378, October 26,
2015); ``[d]epending on the extent and severity, O3-
induced visible foliar injury might be expected to have the
potential to impact the public welfare in scenic and/or recreational
areas during the growing season, particularly in areas with special
protection, such as Class I areas. (80 FR 65379, October 26, 2015);
``[t]he Administrator also recognizes the potential for this effect
to affect the public welfare in the context of affecting values
pertaining to natural forests, particularly those afforded special
government protection (80 FR 65407, October 26, 2015).
\184\ In the discussion of the need for revision of the 1997
secondary standard, the 2008 decision noted that ``[i]n considering
what constitutes a vegetation effect that is adverse from a public
welfare perspective, . . . 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'' (73 FR 16496,
March 27, 2008). This passage of the 2008 decision notice clarified
that ``[s]uch public lands that are protected areas of national
interest include national parks and forests, wildlife refuges, and
wilderness areas'' (73 FR 16496, March 27, 2008).
---------------------------------------------------------------------------
In establishing the current secondary standard and describing its
underlying public welfare protection objectives (as summarized in
section III.A.1, above), the Administrator at that time focused
primarily on RBL in tree seedlings as a proxy or surrogate for the full
array of vegetation related effects of O3, while
additionally concluding that the then-available information on visible
foliar injury provided some support for establishing a strengthened
standard. In so doing, she took note of the indication of the evidence
of the association between O3 and visible foliar injury, as
well as in the declines generally observed in USFS BI scores with
reductions in W126 index from well above 20 ppm-hrs to lower levels (80
FR 65407-65408, October 26, 2015). She recognized, however, that the
evidence was not conducive to use in identifying a quantitative public
welfare protection objective focused specifically on visible foliar
injury (based on judgment of the specific extent and severity at which
such effects should be considered adverse to the public welfare) due to
uncertainties and complexities associated with the available
information. In related manner, she specifically recognized significant
challenges posed by the lack of clear quantitative relationships
(including robust exposure-response functions that addressed the
variability observed in the available data, likely associated with the
variables creating a predisposing environment), that would allow
prediction of visible foliar injury severity and incidence under
varying air quality and environmental conditions, as well as the lack
of established criteria or objectives that might inform consideration
of potential public welfare impacts related to this vegetation effect
(80 FR 65407, October 26, 2015).
The PA finds that these challenges are not addressed by the
information available in the current review. Beyond the lack of
established descriptive quantitative relationships for O3
concentrations or exposure metrics with incidence or severity of
visible foliar injury, summarized in sections III.D.1.a and III.B.3
above, there is a paucity of information clearly relating differing
levels of severity and extent of location affected to scenic or
aesthetic values (e.g., reflective of visitor enjoyment and likelihood
of frequenting such areas) that might inform judgments of public
welfare protection from adversity (PA, section 4.5.1). Thus, there
remain appreciable limitations of the current information for the
purpose of providing a foundation for judgments on public
[[Page 49903]]
welfare protection objectives specific to visible foliar injury.
Notwithstanding these limitations with regard to a detailed
approach or framework for judging public welfare protection related to
impacts of visible foliar injury, the current evidence and analyses are
informative to such considerations. For example, the published studies
and EPA analyses of the USFS biosite data indicate that incidence and
severity of injury are increased at the highest exposures. With regard
to the dataset analyzed in the PA, while clear trends in incidence and
severity related to increasing W126 index are not evident across the
W126 bins below 25 ppm-hrs, the incidence of sites with the more severe
classification of injury (e.g., BI score above 15 [``moderate'' or
``severe''] or 5 [``light,'' ``moderate,'' or ``severe'']) is
appreciably lower at sites with W126 index values below 25 ppm-hrs than
at sites with higher values (e.g., PA, Appendix 4C, Figures 4C-5 and
4C-6 and Table 4C-5). This observation is based primarily on records
for the normal soil moisture category, for which is sufficient sample
size across the full range of W126 and the largest differences in
incidence and average score are observed.\185\ Based on these
observations and the full analysis, the PA concludes that the currently
available information does not support precise conclusions as to the
severity and extent of such injury associated with the lower values of
W126 index most common at USFS sites during the years of the dataset,
2006-2010.\186\ Based on the general pattern observed, however, the PA
suggests a reduced severity (average BI score below 5) and incidence of
visible foliar injury, as quantified by BI scores, to be expected under
conditions that maintain W126 index values below 25 ppm-hrs, (PA,
section 4.5.1.3).
---------------------------------------------------------------------------
\185\ Across W126 bins in which at least 1% of the wet soil
moisture records are represented, differences of highest bin from
lower bins for injury incidence or average score is less than a
factor of two (PA, section 4.3.3).
\186\ Factors that may contribute to the observed variability in
BI scores and lack of a clear pattern with W126 index bin may
include uncertainties in assignment of W126 estimates and soil
moisture categories to biosite locations, variability in biological
response among the sensitive species monitored, and potential role
of other aspects of O3 air quality not captured by the
W126 index.
---------------------------------------------------------------------------
Given the evidence regarding the role of peak O3
concentrations as an influence on occurrence of visible foliar injury
separate from that of the cumulative, concentration-weighted, W126
index (summarized in section III.B.3.b above), the PA additionally
finds that the conditions associated with visible foliar injury in
locations with sensitive species appear to relate to peak concentration
as well as cumulative exposure to generally higher concentrations over
the growing season (PA. section 4.5.1.2). Accordingly, the PA also
considered the current information with regard to peak concentration
metrics. Such information includes the 2007 Staff Paper comparison
based on the less extensive USFS dataset of counties grouped by fourth
highest annual daily maximum 8-hour concentration. This analysis found
a smaller incidence of nonzero BI biosites in counties with a fourth-
high metric at or below 74 ppb as compared to counties limited to
metric values at or below 84 ppb (U.S. EPA 2007, pp. 7-63 to 7-64). The
indication of this finding that the averaging time and form of the
current standard, which emphasizes peak concentrations through a short
(8-hour) averaging time and a rare-occurrence form (annual fourth
highest daily maximum), exert some control on the incidence of sites
with visible foliar injury has a conceptual similarity to the finding
of the most extensive study of USFS data (1994-2009) that reductions in
peak 1-hour concentrations have influenced the declining trend observed
in visible foliar injury since 2002 (Smith, 2012).
(iv) Climate Effects
In considering the currently available information for the effects
of the global tropospheric abundance of O3 on radiative
forcing, and temperature, precipitation and related climate variables,
the PA recognized there to be limitations and uncertainties in the
associated evidence bases with regard to assessing potential for
occurrence of climate-related effects as a result of varying
O3 concentrations in ambient air of locations in the U.S (as
summarized in III.B.3 above). The current evidence is limited with
regard to support for such quantitative analyses that might inform
considerations related to the current standard. For example, as stated
in the ISA, ``[c]urrent limitations in climate modeling tools,
variation across models, and the need for more comprehensive
observational data on these effects represent sources of uncertainty in
quantifying the precise magnitude of climate responses to ozone
changes, particularly at regional scales'' (ISA, section 9.3.1). These
are ``in addition to the key sources of uncertainty in quantifying
ozone RF changes, such as emissions over the time period of interest
and baseline ozone concentrations during preindustrial times'' (ISA,
section IS.9.3.1). Together such uncertainties limit development of
quantitative estimates of climate-related effects in response to earth
surface O3 concentrations at the regional scale, such as in
the U.S. While these complexities inhibit our ability to consider
tropospheric O3 effects, such as radiative forcing, we note
that our consideration of O3 growth-related impacts on trees
inherently encompasses consideration of the potential for O3
to reduce carbon sequestration in terrestrial ecosystems (e.g., through
reduced tree biomass as a result of reduced growth). That is, limiting
the extent of O3-related effects on growth would be expected
to also limit reductions in carbon sequestration, a process that can
reduce the tropospheric abundance of CO2, the greenhouse gas
ranked highest in importance as a greenhouse gas and radiative forcing
agent (section III.B.3 above; ISA, section 9.1.1).
c. Public Welfare Implications of Air Quality Under the Current
Standard
In considering the potential for effects and related public welfare
implications of air quality conditions and associated exposures
indicated to occur under the current standard, the PA first looked to
the air quality analyses particular to cumulative O3
exposures, in terms of the W126 index, given its established
relationship with growth-related effects and specifically RBL as the
identified proxy or surrogate for the full array of such effects (PA,
section 4.5.1.3, Appendix 4D). In that context, the PA gave relatively
greater emphasis to air quality in Class I areas in recognition of the
increased significance of effects in such areas that have been accorded
special protection, as discussed in section III.B.2 above. In
evaluating the extent and magnitude of O3 exposures, in
terms of W126, in such areas that meet the current standard, the PA
also considered year to year variability in the index, while
recognizing that, with regard to W126 index relationships with RBL,
there was uncertainty associated with RBL predictions from a single
year W126 estimate (PA, sections 4.3.4 and 4.5.1, Appendix 4A). As
discussed in section III.D.1.b above, the evidence does not indicate
estimates based on an average of seasonal W126 across three years to be
less, or more, predictive of RBL or resulting total plant biomass (PA,
sections 4.3.4 and 4.5.1.2). The PA considered the magnitude of W126
index occurring in areas nationwide, and particularly in Class I areas,
that meet the current standard, as well as the frequency of the
relatively higher index values. Further, the PA evaluated the extent of
control of such index values exerted by the current standard, as
[[Page 49904]]
evidence by comparisons of sites with design values at or below the
current standard level and sites with higher design values (PA, section
4.4). Lastly, the PA also considered what the currently available
information indicated with regard to the incidence and severity of
visible foliar injury that might be expected to occur under air quality
conditions that meet the current standard, and the potential for
impacts on public welfare (PA, sections 4.5.1.2, 4.5.1.3 and 4.5.3).
The air quality analyses of monitoring data at sites across the
U.S. that meet the current standard in the most recent 3-year period
find that the seasonal W126 index, as assessed by the 3-year average,
is at or below 17 ppm-hrs, with just one exception, among 849
locations, where it equaled 18 ppm-hrs. No 3-year average W126 index
values exceeded 17 ppm-hrs in or near Class I areas. Further, such W126
exposures are generally well below 17 ppm-hrs across most of the U.S.
These findings for sites meeting the current standard, differ
dramatically from sites with higher design values. For example, a third
of all U.S. sites with design values above 70 ppb in the recent period,
and more than 80% of Class I area sites with design values above 70
ppb, have average W126 index values above 17 ppm-hrs. Looking back
across the 19 years covered by the full historical dataset, the
cumulative exposure estimates, averaged over the design value periods,
were virtually all at or below 17 ppm-hrs, with most of the W126 index
values below 13 ppm-hrs (PA, Appendix 4D, Table 4D-9).\187\
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\187\ Based on the established E-R functions for tree seedlings
of 11 species, the median RBL estimates for such W126 index values
are 3.8% or less (PA, Appendix 4A).
---------------------------------------------------------------------------
The PA also considered the general occurrence and distribution of
relatively higher single-year W126 index values, finding a generally
similar pattern to that for averages over the design value period. For
example, fewer than two dozen of the 849 sites meeting the current
standard in the recent period had a single-year index above 17 ppm-hrs;
about a dozen of these sites fall above 19 ppm- hrs, the highest of
which just reaches 25 ppm-hrs in downtown Denver, CO.\188\ The
frequency of such occurrences is still lower for the Class I area
monitors. For example, during the most recent three years, when the
average seasonal W126 index is at or below 17 ppm-hrs in all Class I
areas meeting the current standard, there were just three single-year
W126 index values above 17 ppm-hrs and none above 19 ppm-hrs (PA,
Appendix 4D, Table 4D-15).\189\ The PA additionally notes that single-
year W126 index values in Class I areas over the 19-year dataset
evaluated were generally at or below 19 ppm-hrs, particularly in the
more recent years (PA, Appendix 4D, section 4D.3.2.3).
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\188\ These highest W126 index values occur in the South West
and West regions in which there are nearly 150 monitor locations
meeting the current standard (PA, Figure 4-6, Appendix 4D, Figure
4D-5, Table 4D-1). Across the full 19-year dataset, the downtown
Denver site value is just one of six instances in the more than
8,000 design value periods meeting the current standard of a single-
year W126 index value at or above 25 ppm-hrs. All but one of these
instances were equal to 25 ppm-hrs; the single higher occurrence was
equal to 26 ppm-hrs.
\189\ Across the full 19-year dataset for Class I area monitors
meeting the current standard (58 monitors with at least one such
occurrence and approximately 500 total occurrences), there are no
more than 15 occurrences of single-year W126 index values above 19
ppm-hrs, all of which date prior to 2013 (PA, Appendix 4D, section
4D.3.2.4).
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In reflecting on the air quality analysis findings summarized here,
the PA additionally recognized limitations and uncertainties of the
underlying database, noting there to be inherent limitations in any air
monitoring network. The monitors for O3 are distributed
across the U.S., covering all NOAA regions and all states although some
geographical areas are more densely covered than others, which may have
sparse or no data. For example, only about 40% of all Federal Class I
Areas have or have had O3 monitors (with valid design
values) within 15 km, thus allowing inclusion in the Class I area
analysis. Even so, the dataset for that analysis includes sites in 27
states distributed across all nine NOAA climatic regions across the
contiguous U.S, as well as Hawaii and Alaska. While some NOAA regions
have far fewer numbers of Class I areas with monitors than others
(e.g., the Central, North East, East North Central, and South regions
versus other regions), these areas also have appreciably fewer Class 1
areas in general. Thus, the regions with relatively more Class I area
are also more well represented in the dataset. For example, the West
and Southwest regions (with the largest number of Class I areas) have
approximately a third of those areas represented with monitors, which
include locations where W126 index values are generally higher, thus
playing a prominent role in the analysis.
Another inherent uncertainty is with regard to the extent to which
the results will prove to reflect conditions far out into the future as
air quality and patterns of O3 concentrations in ambient air
continue to change in response to changing circumstances, such as
changes in precursor emissions to meet the current standard across the
U.S. However, findings from these analyses in the current review are
largely consistent with those from analyses of the data available in
the last review. Further, the analysis of how changes in O3
patterns in the past have affected the relationship between W126 index
and the averaging time and form of the current standard finds a
positive, linear relationship between trends in design values and
trends in the W126 index (both in terms of single-year W126 index and
averages over 3-year design value period), as was also the case for
similar analyses conducted for the data available at the time of the
last review (Wells, 2015). While this relationship varied across NOAA
regions, the regions showing the greatest potential for exceeding W126
index values of interest (e.g., with 3-year average values above 17
and/or 19 ppm-hrs) also showed the greatest improvement in the W126
index per unit decrease in design value over the historical period
assessed (PA, Appendix 4D, section 4D.3.2.3). Thus, the available data
and this analysis appear to indicate that as design values are reduced
to meet the current standard in areas that presently do not, W126
values in those areas would also be expected to decline (PA, Appendix
4D, section 4D.4).
In the last review, the Administrator focused on cumulative
exposure estimates derived as the average W126 index over the 3-year
design value period, concluding variations of single-year W126 index
from the average to be of little significance in assessing public
welfare protection. This focus generally reflected the judgment that
estimates based on the average adequately, and appropriately reflected
the precision of current understanding of O3-related growth
reductions, given the various limitations and uncertainties in such
predictions, that have been further evaluated in the current review (as
summarized in section III.D.1.b above). Based on the information
available in the current review, the PA concludes that, with the year-
to-year variation observed in areas meeting the current standard,\190\
differences in year-to-year tree growth in response to each year's
seasonal exposure from the tree growth estimated from the 3-year
average of the single-year values would, given the offsetting impacts
of seasonal exposures above and below the average, reasonably be
expected to generally be small over
[[Page 49905]]
tree lifetimes (PA, section 4.5.1.2). In so doing, the PA takes note of
limitations in aspects of the data underlying the E-R functions that
contribute to imprecision or inexactitude to estimates of growth
impacts associated with multi-year exposures in the relatively lower
W126 index values pertinent to air quality under the current standard.
The information newly available in the current review does not
appreciably address such limitations and uncertainties or improve the
certainty or precision in RBL estimates for such exposures (PA,
sections 4.3.4, 4.5.1).
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\190\ The current air quality data indicates single-year W126
index values generally to vary by less than 5 ppm-hrs from the 3-
year average when the 3-year average is below 20 ppm-hrs, which is
the case for locations meeting the current standard (PA, Appendix
4D).
---------------------------------------------------------------------------
Combining the findings of W126 index values (averaged over design
value period) likely under the current standard with the established E-
R functions for reduced growth in 11 tree seedling species yields a
median species RBL for tree seedlings at or below 5.3% for the recent
period, with very few exceptions, with the highest estimates occurring
in areas not near or within Class I areas. This general pattern is
confirmed over the longer time period (2000-2018) for the vast majority
of the data, with virtually all RBL estimates below 6%.\191\ Further,
given the variability and uncertainty associated with the data
underlying the E-R functions (as summarized in section III.D.1.a
above), the few higher single-year occurrences are reasonably
considered to be of less significance than 3-year average values.
Judgments in the last review (in the context of the framework
summarized in section III.D.1.b above) concluded isolated rare
occurrences of exposures for which median RBL estimates might be at or
just above 6% to not be indicative of conditions adverse to the public
welfare, particularly considering the variability in the array of
environmental factors that can influence O3 effects in
different systems, and the uncertainties associated with estimates of
effects in the natural environment.
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\191\ Although potential for effects on crop yield was not given
particular emphasis in the last review (for reasons similar to those
summarized earlier), we additionally note that combining the
exposure levels summarized for areas across the U.S. where the
current standard is met with the E-R functions established for 10
crop species indicates a median RYL across crops to be at or below
5.1%, on average, with very few exceptions. Further, estimates based
on W126 index at the great majority of the areas are below 5% (PA,
Appendices 4A and 4D).
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With regard to visible foliar injury, the PA observes that the
available evidence does not include an approach for characterizing
natural areas experiencing some severity or extent injury (e.g., via
USFS BI score) with regard to public perception and potential impacts
on public enjoyment; nor does it address this in combination with
information on whether air quality conditions in sites with scores of a
particular severity level do or do not meet the current standard (PA,
section 4.5.1). As summarized in section III.B.2 above, public welfare
implications relate largely to effects on scenic and aesthetic values.
Accordingly, key considerations of this endpoint in past reviews have
generally related to qualitative consideration of potential impacts
related to the plant's aesthetic value in protected forested areas and
the somewhat general, nonspecific judgment that a more restrictive
standard is likely to provide increased protection. The currently
available information does not yet address or describe the
relationships expected to exist for some level of visible foliar injury
severity (below that at which broader physiological effects on plant
growth and survival might also be expected) and/or extent of location
or site injury (e.g., BI) scores with values held by the public and
associated impacts on public uses of the locations.\192\ Additionally,
no criteria have been established regarding a level or prevalence of
visible foliar injury considered to be adverse to the affected
vegetation as the current evidence does not provide for determination
of a degree of leaf injury that would have significance to the vigor of
the whole plant (ISA, Appendix 8, p. 8-24). Nevertheless, while minor
spotting on a few leaves of a plant may easily be concluded to be of
little public welfare significance, it is reasonable to conclude that
cases of widespread and relatively severe injury during the growing
season (particularly when sustained across multiple years, and
accompanied by obvious impacts on the plant canopy) would likely impact
the public welfare in scenic and/or recreational areas, particularly in
areas with special protection, such as Class I areas. However, the gaps
in our information and tools, as summarized in prior sections, restrict
our ability to identify air quality conditions that might be expected
to provide a specific level of protection from public welfare effects
of this endpoint.
---------------------------------------------------------------------------
\192\ Information with some broadly conceptual similarity to
this has been used for judging public welfare implications of
visibility effects of PM in setting the PM secondary standard (78 FR
3086, January 15, 2012).
---------------------------------------------------------------------------
Assessment of any public welfare implications of air quality
occurring under the current standard with regard to visible foliar
injury is further hampered by the lack of an established quantitative
description of the relationship between O3 concentrations
(or exposure metrics) and injury extent or incidence, as well as
severity, that would support estimates of potential injury for varying
air quality and environmental conditions (e.g., moisture), most
particularly for situations that meet the current standard. Although no
such relationship or pertinent metrics for describing exposure are
established, the available information, indicates a role for both a
cumulative metric of exposure as well as the occurrence of relatively
higher concentrations. More specifically, the PA notes the information
indicating potential for increased incidence and severity of injury in
locations with W126 index above 25 ppm-hrs and with increased
occurrence of peak (1-hour) concentrations such as above 100 ppb (PA,
section 4.5.1).
The analyses of recent and historical air quality at monitoring
sites where the current standard is met do not indicate a tendency for
such occurrence of cumulative exposures or peak concentrations (PA,
sections 2.4.5 and 4.4, Appendices 2A and 4D). In these analyses, all
3-year average W126 index values are below 25 ppm-hrs, and values above
17 ppm-hrs are rare. In addition, all single-year, W126 index values at
Class I area locations meeting the current standard (and virtually all
sites across the U.S.) are at or below 25 ppm-hr; even, and values
above 19 ppm-hrs are rare, and mores so in more recent years (PA,
section 4.4.2, Appendix 4D). Accordingly, while the current evidence is
limited for the purposes of identifying public welfare protection
objectives related to visible foliar injury in terms of specific air
quality metrics, the PA notes that the current information indicates
that the occurrence of injury categorized as more severe than
``little'' by the USFS categorization (i.e., a BI scores above 5 or
above 15) would be expected to be infrequent in areas that meet the
current standard.
In light of the evidence regarding a role for peak concentrations,
the PA additionally took note of the control of peak concentrations
exerted by the form and averaging time of the current standard. For
example, daily maximum 1-hour, as well as 8-hour average O3
concentrations have declined over the past 15 years, a period in which
there have been two revisions of the level of the secondary standard,
each providing greater stringency, while retaining the same averaging
time and form as the current standard (e.g., PA, Figures 2-10, 2-12 and
2-17). Further, during periods when the current standard is met, there
is less than one day per site, on average
[[Page 49906]]
with a maximum hourly concentration at or above 100 ppb. This compares
with roughly 40 times as many such days, on average, for sites with
design values above the current standard level (PA, Appendix 2A,
section 2A.2). The currently available information indicates that the
current standard provides appreciable control of peak 1-hour
concentrations, as well as W126 index values, and thus, to the extent
that such metrics play a role in the occurrence and severity of visible
foliar injury, the current standard also provides appreciable control
of these.
Thus, although the current information does not establish a metric
or combination of metrics that well describes the relationship between
occurrence and severity of visible foliar injury across a broad range
of O3 concentration patterns from those more common in the
past to those in areas recently meeting the current standard, the PA
concludes that the currently available information does not indicate
that a situation of widespread and relatively severe visible foliar
injury, with apparent implications for the public welfare, is likely
associated with air quality that meets the current standard. Based on
the USFS dataset presentations as well as the air quality analyses of
W126 index values and frequency of 1-hour observations at or above 100
ppb, the prevalence of injury scores categorized as severe, or even
moderate, which, depending on spatial extent, might reasonably be
concluded to have potential to be adverse to the public welfare do not
appear likely to occur under air quality conditions that meet the
current standard. Thus, the PA finds, based on the current evidence and
currently available air quality information, that the exposure
conditions associated with air quality meeting the current standard are
not those that might reasonably be concluded to result in the
occurrence of significant foliar injury (with regard to severity and
extent).
With regard to other vegetation-related effects, including those at
the ecosystem scale, such as alteration in community composition or
reduced productivity in terrestrial ecosystems, as recognized in
section III.D.1.a above, the available evidence is not clear with
regard to the risk of such impacts (and their magnitude or severity)
associated with the environmental O3 exposures estimated to
occur under air quality conditions meeting the current standard, which
primarily include W126 index at or below 17 ppm-hrs. In considering
effects on crop yield, the air quality analyses at monitoring locations
that meet the current standard indicate estimates of RYL for such
conditions to be at and below 5.1%, based on the median estimate
derived from the established E-R functions for 10 crops (PA, Appendix
4A, Table 4A-5). We additionally recognize there to be complexities
involved in interpreting the significance of such small RYL estimates
in light of the factors also recognized in the last review. These
included the extensive management of crops in agricultural areas that
may to some degree mitigate potential O3-related effects, as
well as the use of variable management practices to achieve optimal
yields, while taking into consideration various environmental
conditions. We also recognize that changes in yield of commercial crops
and commercial commodities may affect producers and consumers
differently, further complicating the question of assessing overall
public welfare impacts for such RYL estimates (80 FR 65405, October 26,
2015).
2. CASAC Advice
The CASAC provided its advice regarding the current secondary
standard in the context of its review of the draft PA (Cox,
2020a).\193\ In so doing, the CASAC concurred with the PA conclusions,
stating that it ``finds, in agreement with the EPA, that the available
evidence does not reasonably call into question the adequacy of the
current secondary ozone standard and concurs that it should be
retained'' (Cox, 2020a, p. 1). The CASAC additionally stated that it
``commends the EPA for the thorough discussion and rationale for the
secondary standard'' (Cox, 2020, p. 2). The CASAC also provided
comments particular to the consideration of climate and growth-related
effects.
---------------------------------------------------------------------------
\193\ A limited number of public comments have been received in
this review to date, including comments focused on the draft IRP,
draft ISA or draft PA. Of the commenters that addressed adequacy of
the current secondary O3 standard, most expressed
agreement with staff conclusions in the draft PA, while some
expressed the view that the standard should be revised to a W126-
based form or that articulation of its rationale should more
explicitly address the protection the standard provides for public
welfare effects.
---------------------------------------------------------------------------
With regard to O3 effects on climate, the CASAC
recommended quantitative uncertainty and variability analyses, with
associated discussion (Cox, 2020a, pp. 2, 22).\194\ With regard to
growth-related effects and consideration of the evidence in
quantitative exposure analyses, it stated that the W126 index ``appears
reasonable and scientifically sound,'' ``particularly [as] related to
growth effects'' (Cox, 2020a, p. 16). Additionally, with regard to the
prior Administrator's expression of greater confidence in judgments
related to public welfare impacts based on a seasonal W126 index
estimated by a three-year average and accordingly relying on that
metric the CASAC expressed the view that this ``appears of reasonable
thought and scientifically sound'' (Cox, 2020, p. 19). Further, the
CASAC stated that ``RBL appears to be appropriately considered as a
surrogate for an array of adverse welfare effects and based on
consideration of ecosystem services and potential for impact to the
public as well as conceptual relationships between vegetation growth
effects and ecosystem scale effects'' and that it agrees ``that biomass
loss, as reported in RBL, is a scientifically-sound surrogate of a
variety of adverse effects that could be exerted to public welfare,''
concurring that this approach is not called into question by the
current evidence which continues to support ``the use of tree seedling
RBL as a proxy for the broader array of vegetation related effects,
most particularly those related to growth that could be impacted by
ozone'' (Cox, 2020a, p. 21). The CASAC additionally concurred that the
strategy of a secondary standard that generally limits 3-year average
W126 index values somewhat below those associated with a 6% RBL in the
median species is ``scientifically reasonable'' and that, accordingly,
a W126 index target value of 17 ppm-hrs for generally restricting
cumulative exposures ``is still effective in particularly protecting
the public welfare in light of vegetation impacts from ozone'' (Cox,
2020a, p 21.).
---------------------------------------------------------------------------
\194\ As recognized in the ISA, ``[c]urrent limitations in
climate modeling tools, variation across models, and the need for
more comprehensive observational data on these effects represent
sources of uncertainty in quantifying the precise magnitude of
climate responses to ozone changes, particularly at regional
scales'' (ISA, section IS.6.2.2, Appendix 9, section 9.3.3, p. 9-
22). These complexities impede our ability to consider specific
O3 concentrations in the U.S. with regard to specific
magnitudes of impact on radiative forcing and subsequent climate
effects.
---------------------------------------------------------------------------
With regard to the court's remand of the 2015 secondary standard to
the EPA for further justification or reconsideration (``particularly in
relation to its decision to focus on a 3-year average for consideration
of the cumulative exposure, in terms of W126, identified as providing
requisite public welfare protection, and its decision to not identify a
specific level of air quality related to visible foliar injury''),
while the CASAC stated that it was not clear whether the draft PA had
fully addressed this concern (Cox, 2020a, p. 21), it described there to
be a solid
[[Page 49907]]
scientific foundation for the current secondary standard and also
commented on areas related to the remand. With regard to the focus on
the 3-year average W126 index, in addition to the comments summarized
above, the CASAC concluded, as noted above, that the EPA
Administrator's focus on the 3-year average and her judgments in doing
so ``appears of reasonable thought and scientifically sound'' (Cox,
2020a, p. 19). Further, while recognizing the existence of established
E-R functions that relate cumulative seasonal exposure of varying
magnitudes to various incremental reductions in expected tree seedling
growth (in terms of RBL) and in expected crop yield, the CASAC letter
also noted that while decades of research also recognizes visible
foliar injury as an effect of O3, ``uncertainties continue
to hamper efforts to quantitatively characterize the relationship of
its occurrence and relative severity with ozone exposures'' (Cox,
2020a, p 20). In summary, the CASAC stated that the approach described
in the draft PA to considering the evidence for welfare effects ``is
laid out very clearly, thoroughly discussed and documented, and
provided a solid scientific underpinning for the EPA conclusion leaving
the current secondary standard in place'' (Cox, 2020a, p. 22).
3. Administrator's Proposed Conclusions
Based on the large body of evidence concerning the welfare effects,
and potential for public welfare impacts, of exposure to O3
in ambient air, and taking into consideration the attendant
uncertainties and limitations of the evidence, the Administrator
proposes to conclude that the current secondary O3 standard
provides the requisite protection against known or anticipated adverse
effects to the public welfare, and should therefore be retained,
without revision. In reaching these proposed conclusions, the
Administrator has carefully considered the assessment of the available
welfare effects evidence and conclusions contained in the ISA, with
supporting details in the 2013 ISA and past AQCDs; the evaluation of
policy-relevant aspects of the evidence and quantitative analyses in
the PA (summarized in section III.D.1 above); the advice and
recommendations from the CASAC (summarized in section III.D.2 above);
and public comments received to date in this review, as well as the
August 2019 decision of the D.C. Circuit remanding the secondary
standard established in the last review to the EPA for further
justification or reconsideration.
In the discussion below, the Administrator considers first the
evidence base on welfare effects associated with exposure to
photochemical oxidants, including O3, in ambient air. In so
doing, he considers the welfare effects evidence newly available in
this review, and the extent to which it alters key scientific
conclusions. The Administrator additionally considers the quantitative
analyses available in this review, including associated limitations and
uncertainties, and the extent to which they indicate differing
conclusions regarding level of protection indicated to be provided by
the current standard from adverse effects to the public welfare.
Further, the Administrator considers the key aspects of the evidence
and air quality and exposure information emphasized in establishing the
now-current standard. He additionally considers uncertainties in the
evidence and quantitative information, as part of public welfare policy
judgments that are essential and integral to his decision on the
adequacy of protection provided by the standard. The Administrator
draws on the considerations and conclusions in the PA, taking note of
key aspects of the rationale presented for those conclusions. In so
doing, he notes the CASAC characterization of the ``thorough discussion
and rationale for the secondary standard'' presented in the PA (Cox,
2020a, p. 2). Further, the Administrator considers the advice of the
CASAC regarding the secondary standard, including particularly its
overall agreement that the currently available evidence does not call
into question the adequacy of the current standard and that it should
be retained (Cox, 2020a, p. 1). With attention to all of the above, the
Administrator considers the information currently available in this
review with regard to the appropriateness of the protection provided by
the current standard.
As an initial matter, the Administrator recognizes the continued
support in the current evidence for O3 as the indicator for
photochemical oxidants (as recognized in section III.D.1 above). In so
doing, he notes that no newly available evidence has been identified in
this review regarding the importance of photochemical oxidants other
than O3 with regard to abundance in ambient air, and
potential for welfare effects, and that, as stated in the current ISA,
``the primary literature evaluating the health and ecological effects
of photochemical oxidants includes ozone almost exclusively as an
indicator of photochemical oxidants'' (ISA, section IS.1.1). Thus, the
Administrator recognizes that, as was the case for previous reviews,
the evidence base for welfare effects of photochemical oxidants does
not indicate an importance of any other photochemical oxidants. For
these reasons, described with more specificity in the ISA and PA, he
proposes to conclude it is appropriate to retain the O3 as
the indicator for the secondary NAAQS for photochemical oxidants.
In considering the currently available welfare effects evidence for
O3, the Administrator recognizes the longstanding evidence
base for vegetation-related effects, augmented in some aspects since
the last review, described in section III.B.1 above. Consistent with
the evidence in the last review, the currently available evidence
describes an array of effects on vegetation and related ecosystem
effects causally or likely to be causally related to O3 in
ambient air, as well as the causal relationship of tropospheric
O3 in radiative forcing and subsequent likely causally
related effects on temperature, precipitation and related climate
variables. The Administrator also notes the Agency conclusions on three
categories of effects with new ISA determinations that the current
evidence is sufficient to infer likely causal relationships of
O3 with increased tree mortality, alteration of plant-insect
signaling and alteration of insect herbivore growth and reproduction
(as summarized in section III.B.1 above). With regard to the current
evidence for increased tree mortality, the Administrator notes the PA
finding that the evidence does not indicate a potential for
O3 concentrations that occur in locations that meet the
current standard to cause increased tree mortality. Accordingly,
consistent with the approach in the PA, he finds it appropriate to
focus on more sensitive effects, such as tree seedling growth, in his
review of the standard. With regard to the two insect-related
categories of effects with new ISA determinations in this review, the
Administrator takes note of the PA finding that uncertainties in the
current evidence, as summarized in section III.B and III.D.1 above,
preclude a full understanding of such effects, the air quality
conditions that might elicit them, the potential for impacts in a
natural ecosystem and, consequently, the potential for such impacts
under air quality conditions associated with meeting the current
standard; thus, there is insufficient information to judge the current
[[Page 49908]]
standard inadequate based on these effects.
In considering the evidence with regard to support for quantitative
description of relationships between air quality conditions and
response to inform his judgments on the current standard, the
Administrator recognizes the supporting evidence for plant growth and
yield. The evidence base continues to indicate growth-related effects
as sensitive welfare effects, with the potential for ecosystem-scale
ramifications. For this category of effects, there are established E-R
functions that relate cumulative seasonal exposure of varying
magnitudes to various incremental reductions in expected tree seedling
growth (in terms of RBL) and in expected crop yield (in terms of RYL).
Many decades of research also recognize visible foliar injury as an
effect of O3, although uncertainties continue to hamper
efforts to quantitatively characterize the relationship of its
occurrence and relative severity with O3 exposures, as
discussed further below (and summarized in sections III.B.3.b and
III.D.1.b above).
Before focusing further on the key vegetation-related effects
identified above, the Administrator first considers the strong evidence
documenting tropospheric O3 as a greenhouse gas causally
related to radiative forcing, and likely causally related to subsequent
effects on variables such as temperature and precipitation. In so
doing, he takes note of the limitations and uncertainties in the
evidence base that affect characterization of the extent of any
relationships between O3 concentrations in ambient air in
the U.S. and climate-related effects, and preclude quantitative
characterization of climate responses to changes in O3
concentrations in ambient air at regional (vs global) scales, as
summarized in sections III.D.1 and II.B.3 above. As a result, he
recognizes the lack of important quantitative tools with which to
consider such effects in this context such that it is not feasible to
relate different patterns of O3 concentrations at the
regional scale in the U.S. with specific risks of alterations in
temperature, precipitation and other climate-related variables. The
resulting uncertainty leads the Administrator to conclude that, with
respect to radiative forcing and related effects, there is insufficient
information available in the current review to judge the existing
standard inadequate or to identify an appropriate revision.
The Administrator turns next to consideration of visible foliar
injury. In so doing, he considers both the conclusions of the ISA and
the examination and analysis in the PA of the currently available
information as to what it indicates and supports with regard to
adequacy of protection provided by the current standard, as summarized
in section III.D.1 above. As an initial matter, he takes note of the
long-standing documentation of visible foliar injury as an effect of
O3 in ambient air under certain conditions. Further, as
summarized in section III.B.2 above, the public welfare significance of
visible foliar injury of vegetation in areas not closely managed for
harvest, particularly specially protected natural areas, has generally
been considered in the context of potential effects on aesthetic and
recreational values, such as the aesthetic value of scenic vistas in
protected natural areas such as national parks and wilderness areas
(e.g., 73 FR 16496, March 27, 2008). Based on these considerations, the
Administrator recognizes that, depending on its severity and spatial
extent, as well as the location(s) and the associated intended use, the
impact of visible foliar injury on the physical appearance of plants
has the potential to be significant to the public welfare. In this
regard, he notes the PA statement that cases of widespread and
relatively severe injury during the growing season (particularly when
sustained across multiple years and accompanied by obvious impacts on
the plant canopy) might reasonably be expected to have the potential to
adversely impact the public welfare in scenic and/or recreational
areas, particularly in areas with special protection, such as Class I
areas, summarized in section III.D.1 above (PA, sections 4.3.2 and
4.5.1). Thus, he considers the PA evaluation of the currently available
information with regard to the potential for such an occurrence with
air quality conditions that meet the current standard.
In considering the PA evaluations, the Administrator takes note of
the PA observation that important uncertainties remain in the
understanding of the O3 exposure conditions that will elicit
visible foliar injury of varying severity and extent in natural areas,
and particularly in light of the other environmental variables that
influence its occurrence, as summarized in sections III.B.3 and III.D.1
above. In so doing, he notes the recognition by the CASAC that
``uncertainties continue to hamper efforts to quantitatively
characterize the relationship of [visible foliar injury] occurrence and
relative severity with ozone exposures,'' as summarized in section
III.D.2 above.
Notwithstanding, and while being mindful of, such uncertainties
with regard to predictive O3 metric or metrics and a
quantitative function relating them to incidence and severity of
visible foliar injury in natural areas, as well as interpretation of
such incidence and severity in the context of considering protection
from such impacts that might reasonably be considered adverse to the
public welfare, the Administrator takes note of several findings of the
PA. First, he notes that the evidence for visible foliar injury, as
well as analyses of data for USFS biosites (sites with O3-
sensitive vegetation assessed for visible foliar injury) indicate there
to be associations with cumulative exposure metrics (e.g., SUM06 or
W126 index), such metrics do not completely explain the occurrence and
severity of injury. Although the availability of detailed analyses that
have explored multiple exposure metrics and other influential variables
is limited, multiple studies also have indicated a potential role for
an additional metric related to the occurrence of days with relatively
high concentrations (e.g., number of days with a 1-hour concentration
at or above 100 ppb), as summarized in section III.B.3 above (PA,
section 4.5.1.2).
The Administrator also notes the PA observation that publications
related to the evidence base for the USFS biosite monitoring program
document reductions in the incidence of the higher BI scores over the
16-year period of the program (1994 through 2010), especially after
2002, leading to researcher conclusions of a ``declining risk of
probable impact'' on the monitored forests over this period (e.g.,
Smith, 2012). The PA observes that these reductions parallel the
O3 concentration trend information nationwide that shows
clear reductions in cumulative seasonal exposures, as well as in peak
O3 concentrations such as the annual fourth highest daily
maximum 8-hour concentration, from 2000 through 2018 (PA, Figure 2-11
and Appendix 4D, Figure 4D-9). These USFS BI score reductions also
parallel reductions in the occurrence of 1-hour concentrations above
100 ppb (PA, Appendix 2A, Tables 2A-2 to 2A-4). Thus, the extensive
evidence of trends across the past nearly 20 years indicate reductions
in severity of visible foliar injury in addition to reductions in peak
concentrations that some studies have suggested to be influential in
the severity of visible foliar injury, as summarized in section III.D.1
above (PA, section 4.5.1).
The Administrator additionally takes note of the PA recognition of
a paucity of established approaches for
[[Page 49909]]
interpreting specific levels of severity and extent of foliar injury in
protected forests with regard to impacts on public welfare effects,
e.g., related to recreational services. The PA notes that injury to
whole stands of trees of a severity apparent to the casual observer
(e.g., when viewed as a whole from a distance) would reasonably be
expected to affect recreational values. However, the available
information does not provide for specific characterization of the
incidence and severity that would not be expected to have such an
impact, nor for clear identification of the pattern of O3
concentrations that would provide for such a situation. In this
context, the Administrator notes the PA description of the scheme
developed by the USFS to categorize biosite scores of injury in natural
vegetated areas by severity levels (as summarized in section III.B.2
above). He notes the USFS description of scores above 15 as ``moderate
to severe,'' as well as the USFS categorization of lower scores, such
as those from zero to just below 5, which are described as ``little to
no foliar injury'' and 5 to just below 10 as ``light to moderate.'' In
so doing, he recognizes the PA consideration of such lower scores as
being unlikely to be indicative of injury of such a magnitude or extent
that would reasonably be considered significant risks to the public
welfare. In light of these considerations, the Administrator takes note
of the PA finding that quantitative analyses and evidence are lacking
that might support a more precise conclusion with regard to a magnitude
of BI score coupled with an extent of occurrence that might be
specifically identified as adverse to the public welfare, but that the
lower categories of BI scores are indicative of injury of generally
lesser risk to the natural area or to public enjoyment. The
Administrator also takes note of the D.C. Circuit's holding that
substantial uncertainty about the level at which visible foliar injury
may become adverse to public welfare does not necessarily provide a
basis for declining to evaluate whether the existing standard provides
requisite protection against such effects. See Murray Energy Corp. v.
EPA, 936 F.3d 597, 619-20 (D.C. Cir. 2019). Consequently, he proposes
to judge that occurrence of the lower categories of BI scores does not
pose concern for the public welfare, but that findings of BI scores
categorized as ``moderate to severe'' injury by the USFS scheme would
be an indication of visible foliar injury occurrence that, depending on
extent and severity, may raise public welfare concerns.
With regard to the PA presentations of the USFS data combined with
W126 estimates and soil moisture categories, summarized in section
III.B.3 above, the Administrator takes note of the PA finding that the
incidence of nonzero BI scores, and, particularly of relatively higher
scores (such as scores above 15 which are indicative of ``moderate to
severe'' injury in the USFS scheme) appears to markedly increase only
with W126 index values above 25 ppm-hrs, as summarized in section
III.B.3.b above (PA, section 4.3.3 and Appendix 4C). In so doing, he
notes that such a magnitude of W126 index (either as a 3-year average
or in a single year) is not seen to occur at monitoring locations
(including in or near Class I areas) where the current standard is met,
and that values above 17 or 19 ppm-hrs are rare, as summarized in
section III.D.1.c above (PA, Appendix 4C, section 4C.3; Appendix 4D,
section 4D.3.2.3). Further, the Administrator takes note of the PA
consideration of the USFS publications that identify an influence of
peak concentrations on BI scores (beyond an influence of cumulative
exposure) and the PA observation of the appreciable control of peak
concentrations exerted by the form and averaging time of the current
standard, as evidenced by the air quality analyses which document
reductions in 1-hour daily maximum concentrations with declining design
values. For example, the PA finds the average number of 1-hour daily
maximum concentrations across monitored sites to be some 40 times lower
for sites meeting the current standards compared to sites that do not,
as summarized in section III.D.1 above. Based on these considerations,
the Administrator agrees with the PA finding that the current standard
provides control of air quality conditions that contribute to increased
BI scores and to scores of a magnitude indicative of ``moderate to
severe'' foliar injury.
The Administrator further takes note of the PA finding that the
current information, particularly in locations meeting the current
standard or with W126 index estimates likely to occur under the current
standard, does not indicate a significant extent and degree of injury
(e.g., based on analyses of BI scores in the PA, Appendix 4C) or
specific impacts on recreational or related services for areas, such as
wilderness areas or national parks. Thus, he gives credence to the
associated PA conclusion that the evidence indicates that areas that
meet the current standard are unlikely to have BI scores reasonably
considered to be impacts of public welfare significance. Based on all
of the considerations raised here, the Administrator proposes to
conclude that the current standard provides sufficient protection of
natural areas, including particularly protected areas such as Class I
areas, from O3 concentrations in the ambient air that might
be expected to elicit visible foliar injury of such an incidence and
severity as would reasonably be judged adverse to the public welfare.
In turning to consideration of the remaining array of vegetation-
related effects, the Administrator first takes note of uncertainties in
the details and quantitative aspects of relationships between plant-
level effects such as growth and reproduction, and ecosystem impacts,
the occurrence of which are influenced by many other ecosystem
characteristics and processes. These examples illustrate the role of
public welfare policy judgments, both with regard to the extent of
protection that is requisite and concerning the weighing of
uncertainties and limitations of the underlying evidence base and
associated quantitative analyses. The Administrator notes that such
judgments will inform his decision in the current review, as is common
in NAAQS reviews. Public welfare policy judgments play an important
role in each review of a secondary standard, just as public health
policy judgments have important roles in primary standard reviews. One
type of public welfare policy judgment focuses on how to consider the
nature and magnitude of the array of uncertainties that are inherent in
the scientific evidence and analyses. These judgments are traditionally
made with a recognition that current understanding of the relationships
between the presence of a pollutant in ambient air and associated
welfare effects is based on a broad body of information encompassing
not only more established aspects of the evidence but also aspects in
which there may be substantial uncertainty. This may be true even of
the most robust aspect of the evidence base. In the case of the
secondary O3 standard review, as an example, while
recognizing the strength of the established and well-founded E-R
functions in predicting the relationship of O3 in terms of
the W126 index cumulative exposure metric across a wide array of
exposure levels, the Administrator additionally recognizes increased
uncertainty, and associated imprecision or inexactitude in application
of the E-R functions with lower cumulative exposures, and in the
current understanding of aspects of
[[Page 49910]]
relationships of such estimated effects with larger-scale impacts, such
as those on populations, communities and ecosystems, as discussed in
the PA and summarized in sections III.D.1 above.
The Administrator now turns to the welfare effects of reduced plant
growth or yield. In so doing, he takes note of the well-established E-R
functions for seedlings of 11 tree species that relate cumulative
seasonal O3 exposures of varying magnitudes to various
incremental reductions in expected tree seedling growth (in terms of
RBL) and in expected crop yield, that have been recognized across
multiple O3 NAAQS reviews. In so doing, he additionally
takes note of uncertainties recognized in the PA, as summarized in
section III.D.1.a above, that include the limited information that can
address the extent to which the E-R functions for tree seedlings
reflect growth impacts in mature trees, and the fact that the 11
species represent a very small portion of the tree species across the
U.S. (PA, sections 4.3.4 and 4.5.3). While recognizing these and other
uncertainties, RBL estimates based on the median of the 11 species were
used as a surrogate in the last review for comparable information on
other species and lifestages, as well as a proxy or surrogate for other
vegetation-related effects, including larger-scale effects. The
Administrator takes note of the PA conclusion and CASAC advice that use
of this approach continues to appear to be a reasonable judgment in
this review (PA, section 4.5.3). More specifically, the PA concludes
that the currently available information continues to support (and does
not call into question) the use of RBL as a useful and evidence-based
approach for consideration of the extent of protection from the broad
array of vegetation-related effects associated with O3 in
ambient air, as summarized in section III.D.1.b above. The
Administrator also takes note of the PA conclusions that the currently
available evidence, while somewhat expanded since the last review does
not indicate an alternative metric for such a use; nor is an
alternative approach evident. He further notes the CASAC concurrence
that the current evidence continues to support this approach, as
summarized in section III.D.2 above. Thus, he finds it appropriate to
adopt this approach in the current review.
With regard to the use of RBL and the median RBL estimate based on
the established E-R functions for 11 species of tree seedlings, the
Administrator takes note of considerations in the PA. For example,
while the E-R functions for the 11 species have been derived in terms
of a seasonal W126 index, the experiments from which they were derived
vary in duration from less than three months to many more, such that,
the adjustment to a 3-month season duration, with its underlying
simplifying assumptions of uniform W126 distribution over the exposure
period and relationship between duration and response, contributes some
imprecision or inexactitude to the resulting functions and estimates
derived using it, as discussed in section III.D.1.b above.
Additionally, there is greater uncertainty with regard to estimated RBL
at lower cumulative exposure levels, as the exposure levels represented
in the data underlying the E-R functions are somewhat limited with
regard to the relatively lower cumulative exposure levels, such as
those most commonly associated with the current standard (e.g., at or
below 17 ppm-hrs). Further, he notes the PA observation that some of
the underlying studies did not find statistically significant effects
of O3 at the lower exposure levels, indicating some
uncertainty in predictions of an O3-related RBL at those
levels. With these considerations regarding the E-R functions and their
underlying datasets in mind, he also takes note of variability
associated with tree growth in the natural environment (e.g., related
to variability in plant, soil, meteorological and other factors), as
well as variability associated with plant responses to O3
exposures in the natural environment, as summarized in section III.D.1
above. The Administrator also considers the issues discussed in the
court's remand of the 2015 secondary standard with respect to use of a
3-year average. See Murray Energy Corp. v. EPA, 936 F.3d at 617-18. In
light of these considerations, the Administrator considers whether
aspects of this evidence support making judgments using the E-R
functions with W126 index derived as an average across multiple years.
The Administrator notes that such averaging would have some conceptual
similarity to the assumptions underlying the adjustment made to develop
seasonal W126 E-R functions from exposures that extended over multiple
seasons (or less than a single). Such averaging, with its reduction of
the influence of annual variations in seasonal W126, would give less
influence to RBL estimates derived from such potentially variable
representations of W126, thus providing an estimate of W126 more
suitably paired with the E-R functions. The Administrator additionally
takes note of the PA summary of comparisons performed in the 2013 ISA
and current ISA of RBL estimates based on either cumulative average
multi-year W126 index or single-year W126 with estimates derived from
information in a multi-year O3 exposure study, summarized in
section III.D.1.b(ii) above (PA, section 4.5.1 and Appendix 4A, section
4A.3.1). He notes the PA finding that these comparisons illustrate the
variability inherent in the magnitude of growth impacts of
O3 and in the quantitative relationship of O3
exposure and RBL, while also providing general agreement of predictions
(based on either metric) with observations. The Administrator finds
these considerations particularly informative in considering the
evidence with regard to the appropriateness of a focus on a multi-year
(e.g., 3-year) average seasonal W126 index in assessing protection
using RBL as a proxy or surrogate of the broader array of effects to
obscure cumulative seasonal exposures of concern, a point discussed by
the court in its 2019 remand of the 2015 secondary standard to EPA
(Murray Energy Corp. v. EPA, 936 F.3d at 617-18).
In light of the above considerations, the Administrator agrees with
the PA finding that such factors as those identified here (also
summarized in section III.D.1.b(ii) above), and discussed in the PA
(PA, sections 4.5.1.2 and 4.5.3), including the currently available
evidence and its recognized limitations, variability and uncertainties,
contribute uncertainty and resulting imprecision or inexactitude to RBL
estimates of single-year seasonal W126 index values, thus supporting a
conclusion that it is reasonable to use a seasonal RBL averaged over
multiple years, such as a 3-year average. The Administrator
additionally takes note of the CASAC advice reaffirming the EPA's focus
on a 3-year average W126, concluding such a focus to be reasonable and
scientifically sound, as summarized in section III.D.2 above. In light
of these considerations, the Administrator finds there to be support
for use of an average seasonal W126 index derived from multiple years
(with their representation of variability in environmental factors),
concluding the use of such averaging to provide an appropriate
representation of the evidence and attention to considerations
summarized above. In so doing, he finds that a reliance on single year
W126 estimates for reaching judgments with regard to magnitude of
O3 related RBL and associated judgments of public welfare
protection would ascribe a greater specificity and certainty to such
estimates than supported by the current
[[Page 49911]]
evidence. Thus, the Administrator proposes to conclude that it is
appropriate to use a seasonal W126 averaged over a 3-year period, which
is the design value period for the current standard, to estimate median
RBL using the established E-R functions for purposes in this review of
considering the public welfare protection provided by the standard.
Thus, the Administrator recognizes a number of public welfare
policy judgments important to his review of the current standard. Those
judgments include adoption of the median tree seedling RBL estimate for
the studied species as a surrogate for the broad array of vegetation
related effects that extend to the ecosystem scale, and identification
of cumulative seasonal exposures (in terms of the average W126 index
across the 3-year design period for the standard) for assessing
O3 concentrations in areas that meet the standard with
regard to the extent of protection afforded by the standard. In
reflecting on these judgments, the current evidence presented in the
ISA and the associated evaluations in the PA, the Administrator
proposes to conclude that the currently available information supports
such judgments, additionally noting the CASAC concurrence with regard
to the scientific support for these judgments (Cox 2020, p. 21).
Accordingly, the Administrator proposes to conclude that the current
evidence base and available information (qualitative and quantitative)
continues to support consideration of the potential for O3-
related vegetation impacts in terms of the RBL estimates from
established E-R functions as a quantitative tool within a larger
framework of considerations pertaining to the public welfare
significance of O3 effects. Such consideration includes
effects that are associated with effects on vegetation, and
particularly those that conceptually relate to growth, and that are
causally or likely causally related to O3 in ambient air,
yet for which there are greater uncertainties affecting estimates of
impacts on public welfare. The Administrator additionally notes that
this approach to weighing the available information in reaching
judgments regarding the secondary standard additionally takes into
account uncertainties regarding the magnitude of growth impact that
might be expected in mature trees, and of related, broader, ecosystem-
level effects for which the available tools for quantitative estimates
are more uncertain and those for which the policy foundation for
consideration of public welfare impacts is less well established.
In his consideration of the adequacy of protection provided by the
current standard, the Administrator also notes judgments of the prior
Administrator in considering the public welfare significance of small
magnitude estimates of RBL and associated unquantified potential for
larger-scale related effects. As with visible foliar injury, the
Administrator does not consider every possible instance of an effect on
vegetation growth from O3 to be adverse to public welfare,
although he recognizes that, depending on factors including extent and
severity, such vegetation-related effects have the potential to be
adverse to public welfare. In this context, the Administrator notes
that the 2015 decision set the standard with an ``underlying objective
of a revised secondary standard that would limit cumulative exposures
in nearly all instances to those for which the median RBL estimate
would be somewhat lower than 6%'' (80 FR 65407, October 26, 2015). With
this objective, the prior Administrator did not additionally find that
a cumulative seasonal exposure, for which such a magnitude of median
species RBL was estimated, represented conditions that were adverse to
the public welfare. Rather, the 2015 decision noted that ``the
Administrator does not judge RBL estimates associated with marginal
higher exposures [at or above 19 ppm-hrs] in isolated, rare instances
to be indicative of adverse effects to the public welfare'' (80 FR
65407, October 26, 2015). Comments from the current CASAC, in the
context of its review of the draft PA, expressed the view that the
strategy described by the prior Administrator for the secondary
standard established in 2015 with its W126 index target of 17 ppm-hrs
(in terms of a 3-year average), at or below which the 2015 standard was
expected to generally restrict cumulative seasonal exposure, is ``still
effective in particularly protecting the public welfare in light of
vegetation impacts form ozone'' (Cox, 2020, p. 21). In light of this
advice and based on the current evidence as evaluated in the PA, the
Administrator proposes to conclude that this approach or framework,
with its focus on controlling air quality such that cumulative
exposures at or above 19 ppm-hrs, in terms of a 3-year average W126
index, are isolated and rare, is appropriate for a secondary standard
that provides the requisite public welfare protection and proposes to
use such an approach in this review.
With this approach and protection target in mind, the Administrator
further considers the analyses available in this review of recent air
quality at sites across the U.S., particularly including those sites in
or near Class I areas, and also the analyses of historical air quality.
In so doing, the Administrator recognizes that these analyses are
distributed across all nine NOAA climate regions and 50 states,
although some geographic areas within specific regions and states may
be more densely covered and represented by monitors than others, as
summarized in section III.C above. The Administrator notes that the
findings from both the analysis of the air quality data from the most
recent period and from the larger analysis of historical air quality
data extending back to 2000, as presented in the PA and summarized in
section III.C above, are consistent with the air quality analyses
available in the last review. That is, in virtually all design value
periods and all locations at which the current standard was met across
the 19 years and 17 design value periods (in more than 99.9% of such
observations), the 3-year average W126 metric was at or below 17 ppm-
hrs. Further, in all such design value periods and locations the 3-year
average W126 index was at or below 19 ppm-hrs. The Administrator
additionally considers the protection provided by the current standard
from the occurrence of O3 exposures within a single year
with potentially damaging consequences, such as a significantly
increased incidence of areas with visible foliar injury that might be
judged moderate to severe. In so doing, he takes notes of the PA
analyses, summarized in section III.D.1 above, of USFS BI scores,
giving particular focus to scores above 15 (termed ``moderate to severe
injury'' by the USFS categorization scheme). He notes the PA finding
that incidence of sites with BI scores above 15 markedly increases with
W126 index estimates above 25 ppm-hrs. In this context, he additionally
takes note of the air quality analysis finding of a scarcity of single-
year W126 index values above 25 ppm-hrs at sites that meet the current
standard, with just a single occurrence across all U.S. sites with
design values meeting the current standard in the 19-year historical
dataset dating back to 2000 (PA, section 4.4 and Appendix 4D). Further,
in light of the evidence indicating that peak short-term concentrations
(e.g., of durations as short as one hour) may also play a role in the
occurrence of visible foliar injury, the Administrator additionally
takes note of the PA presentation of air quality data over the past 20
years, as summarized in section III.D.1 above, that shows a declining
trend in 1-hour daily maximum concentrations
[[Page 49912]]
mirroring the declining trend in design values, and the associated PA
conclusion that the form and averaging time of the current standard
provides appreciable control of peak 1-hour concentrations. As further
evidence of the level of control exerted, the PA notes there to be less
than one day per site, on average (among sites meeting the current
standard), with a maximum hourly concentration at or above 100 ppb,
compared to roughly 40 times as many such days, on average, for sites
with design values above the current standard level (PA, Appendix 2A,
section 2A.2). In light of these findings from the air quality analyses
and considerations in the PA, summarized in section III.D.1 above, both
with regard to 3-year average W126 index values at sites meeting the
current standard and the rarity of such values at or above 19 ppm-hrs,
and with regard to single-year W126 index values at sites meeting the
current standard, and the rarity of such values above 25 ppm-hrs, as
well as with regard to the appreciable control of 1-hour daily maximum
concentrations, the Administrator proposes to judge that the current
standard provides adequate protection from air quality conditions with
the potential to be adverse to the public welfare.
In reaching his proposed conclusion on the current secondary
O3 standard, the Administrator recognizes, as is the case in
NAAQS reviews in general, his decision depends on a variety of factors,
including science policy judgments and public welfare policy judgments,
as well as the currently available information. With regard to the
current review, the Administrator gives primary attention to the
principal effects of O3 as recognized in the current ISA,
the 2013 ISA and past AQCDs, and for which the evidence is strongest
(e.g., growth, reproduction, and related larger-scale effects, as well
as, visible foliar injury). As discussed above, the Administrator notes
that the currently available information on visible foliar injury and
with regard to air quality analyses that may be informative with regard
to air quality conditions associated with appreciably increased
incidence and severity of BI scores at USFS biomonitoring sites
indicates a sufficient degree of protection from such conditions.
Further, the currently available evidence for natural areas across the
U.S., such as studies of USFS biosites, does not indicate widespread
incidence of significant visible foliar injury, and analyses of USFS
biosite scores in the PA do not indicate marked increases in scores
categorized by the USFS as ``moderate'' or ``severe'' for W126 index
values generally occurring at sites that meet the current standard. The
Administrator finds this information does not indicate a potential for
public welfare impacts of concern under air quality conditions that
meet the current standard. In light of these and other considerations
discussed more completely above, and with particular attention to Class
I and other areas afforded special protection, the Administrator
proposes to conclude that the evidence regarding visible foliar injury
and air quality in areas meeting the current standard indicates that
the current standard provides adequate protection for this effect.
The Administrator additionally considers O3 effects on
crop yield. In so doing, he takes note of the long-standing evidence,
qualitative and quantitative, of the reducing effect of O3
on the yield of many crops, as summarized in the PA and current ISA and
characterized in detail in past reviews (e.g., 2013 ISA, 2006 AQCD,
1997 AQCD, 2014 WREA). He additionally notes the established E-R
functions for 10 crops and the estimates of RYL derived from them, as
presented in the PA (PA, Appendix 4A, section 4A.1, Table 4A-4), and
the potential public welfare significance of reductions in crop yield,
as summarized in section III.B.2 above. However, he additionally
recognizes that not every effect on crop yield will be adverse to
public welfare and in the case of crops in particular there are a
number of complexities related to the heavy management of many crops to
obtain a particular output for commercial purposes, and related to
other factors, that contribute uncertainty to predictions of potential
O3-related public welfare impacts, as summarized in sections
III.B.2 and III.D.1 above (PA, sections 4.5.1.3 and 4.5.3). Thus, in
judging the extent to which the median RYL estimated for the W126 index
values generally occurring in areas meeting the current standard would
be expected to be of public welfare significance, he recognizes the
potential for a much larger influence of extensive management of such
crops, and also considers other factors recognized in the PA and
summarized in section III.D.1 above, including similarities in median
estimates of RYL and RBL (PA, sections 4.5.1.3 and 4.5.3). With this in
mind, the Administrator does not find that the information for crop
yield effects leads him to identify this endpoint as requiring separate
consideration or to provide a more appropriate focus for the standard
than RBL, in its role as a proxy or surrogate for the broader array of
vegetation-related effects, as discussed above. Rather, in light of
these considerations, he proposes to judge that a decision based on RBL
as a proxy for other vegetation-related effects will provide adequate
protection against crop related effects. In light of the current
information and considerations discussed more completely above, the
Administrator further proposes to conclude that the evidence regarding
RBL, and its use as a proxy or surrogate for the broader array of
vegetation-related effects, in combination with air quality in areas
meeting the current standard, provide adequate protection for these
effects.
In reaching his proposed conclusion on the current standard, the
Administrator also considers the extent to which the current
information may provide support for an alternative standard. In so
doing, he notes the longstanding evidence documenting the array of
welfare effects associated with O3 in ambient air, as
summarized in section III.B.1 above. He additionally recognizes the
robust quantitative evidence for growth-related effects and the E-R
functions for RBL, which he considers as a proxy for the broader array
of effects in reaching his proposed decision. He takes note of the air
quality analyses that show an appreciably greater occurrence of higher
levels of cumulative exposure, in terms of the W126 index, as well as
an appreciably greater occurrence of peak concentrations (both hourly
and 8-hour average concentrations) in areas that do not meet the
current standard, as summarized in section III.C above for areas with
design values above 70 ppb. He proposes to conclude that such
occurrences contribute to air quality conditions that would not provide
the appropriate protection of public welfare in light of the potential
for adverse effects on the public welfare.
Further, the Administrator recognizes that public comments thus far
in this review have suggested that an alternative standard, such as one
based solely on the W126 metric, is required to provide adequate
protection of the public welfare. Such a point was raised in the
litigation challenging the 2015 secondary standard, although the court
did not resolve this issue in its decision. In considering this issue,
the Administrator recognizes that, as summarized in section III.B.3.a
above, concentration-weighted, cumulative exposure metrics, including
the W126 index, have been identified as quantifying exposure in a way
that relates to reduced plant growth (ISA, Appendix 8, section 8.13.1).
The W126 index is the metric used with the 11
[[Page 49913]]
established E-R functions discussed above, which provide estimates of
RBL that the Administrator considers appropriately used as a proxy or
surrogate for the broader array of vegetation-related effects. The
Administrator additionally notes, however, that the evidence indicates
there to be aspects of O3 air quality not captured by
measures of cumulative exposure, such as W126 index, that may pose a
risk of harm to the public welfare. For example, as discussed above,
the current evidence indicates a role for peak concentrations in the
occurrence of visible foliar injury. With this in mind, the
Administrator notes that an ambient air quality standard established in
terms of the W126 index, while giving greater weight to generally
higher concentrations, would not explicitly limit the occurrence of
hourly concentrations at or above specific magnitudes. For example, two
records of air quality may have the same W126 index while differing
appreciably in patterns of hourly concentrations, including in the
frequency of occurrence of peak concentrations (e.g., number of hours
above 100 ppb). The Administrator notes, however, as discussed above,
that the current standard, with its 8-hour averaging time and fourth-
highest daily maximum form (averaged over three years), can provide
control of both peak concentrations and concentration-weighted
cumulative exposures, as illustrated by the substantially limited
occurrence of hourly concentrations of magnitudes at or above 100 ppb
and of cumulative exposures at or above 19 ppm-hrs in areas that meet
the current standard (PA, section 2.4.5, Appendix 2A, section 2A.2 and
Appendix 4D). Thus, in light of the information available in this
review, summarized in the sections above and including that related to
a role of peak concentrations in posing risk of visible foliar injury
to sensitive vegetation, the Administrator proposes to conclude that
such an alternative standard in terms of a W126 index would be less
likely to provide sufficient protection against such occurrences and
accordingly would not provide the requisite control of aspects of air
quality that pose risk to the public welfare. As indicated above, he
proposes to judge that the current information indicates that the
requisite control of such aspects of air quality is provided by the
current standard.
In summary, the Administrator recognizes that his proposed decision
on the public welfare protection afforded by the secondary
O3 standard from identified O3-related welfare
effects, and from their potential to present adverse effects to the
public welfare, is based in part on judgments regarding uncertainties
and limitations in the available information, such as those identified
above. In this context, he has considered what the available evidence
and quantitative information indicate with regard to the protection
provided from the array of O3 welfare effects. He finds that
the information, as summarized above, and presented in detail in the
ISA and PA, does not indicate the current standard to allow air quality
conditions with implications of concern for the public welfare. He
additionally takes note of the advice from the CASAC in this review,
including its finding ``that the available evidence does not reasonably
call into question the adequacy of the current secondary ozone standard
and concurs that it should be retained'' (Cox, 2020a, p. 1). Based on
all of the above considerations, including his consideration of the
currently available evidence and quantitative exposure/risk
information, the Administrator proposes to conclude that the current
secondary standard provides the requisite protection against known or
anticipated effects to the public welfare, and thus that the current
standard should be retained, without revision. The Administrator
solicits comment on this proposed conclusion.
Having reached the proposed decision described here based on
interpretation of the welfare effects evidence, as assessed in the ISA,
and the quantitative analyses presented in the PA; the evaluation of
policy-relevant aspects of the evidence and quantitative analyses in
the PA; the advice and recommendations from the CASAC; public comments
received to date in this review; and the public welfare policy
judgments described above, the Administrator recognizes that other
interpretations, assessments and judgments might be possible.
Therefore, the Administrator solicits comment on the array of issues
associated with review of this standard, including public welfare and
science policy judgments inherent in the proposed decision, as
described above, and the rationales upon which such views are based.
IV. Statutory and Executive Order Reviews
Additional information about these statutes and Executive Orders
can be found at http://www2.epa.gov/laws-regulations/laws-and-executive-orders.
A. Executive Order 12866: Regulatory Planning and Review and Executive
Order 13563: Improving Regulation and Regulatory Review
The Office of Management and Budget (OMB) has determined that this
action is a significant regulatory action and it was submitted to OMB
for review. Any changes made in response to OMB recommendations have
been documented in the docket. Because this action does not propose to
change the existing NAAQS for O3, it does not impose costs
or benefits relative to the baseline of continuing with the current
NAAQS in effect. EPA has thus not prepared a Regulatory Impact Analysis
for this action.
B. Executive Order 13771: Reducing Regulations and Controlling
Regulatory Costs
This action is not expected to be an Executive Order 13771
regulatory action. There are no quantified cost estimates for this
proposed action because EPA is proposing to retain the current
standards.
C. Paperwork Reduction Act (PRA)
This action does not impose an information collection burden under
the PRA. There are no information collection requirements directly
associated with a decision to retain a NAAQS without any revision under
section 109 of the CAA, and this action proposes to retain the current
O3 NAAQS without any revisions.
D. Regulatory Flexibility Act (RFA)
I certify that this action will not have a significant economic
impact on a substantial number of small entities under the RFA. This
action will not impose any requirements on small entities. Rather, this
action proposes to retain, without revision, existing national
standards for allowable concentrations of O3 in ambient air
as required by section 109 of the CAA. See also American Trucking
Associations 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), rev'd in part on
other grounds, Whitman v. American Trucking Associations, 531 U.S. 457
(2001).
E. Unfunded Mandates Reform Act (UMRA)
This action does not contain any unfunded mandate as described in
the UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely
affect small governments. This action imposes no
[[Page 49914]]
enforceable duty on any state, local, or tribal governments or the
private sector.
F. Executive Order 13132: Federalism
This action 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.
G. Executive Order 13175: Consultation and Coordination With Indian
Tribal Governments
This action 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. This action does not change existing
regulations; it proposes to retain the current O3 NAAQS,
without revision. Executive Order 13175 does not apply to this action.
H. Executive Order 13045: Protection of Children From Environmental
Health and Safety Risks
This action is not subject to Executive Order 13045 because it is
not economically significant as defined in Executive Order 12866. The
health effects evidence and risk assessment information for this
action, which focuses on children and people (of all ages) with asthma
as key at-risk populations, is summarized in sections II.B and II.C
above and described in the ISA and PA, copies of which are in the
public docket for this action.
I. Executive Order 13211: Actions That Significantly Affect Energy
Supply, Distribution or Use
This action is not subject to Executive Order 13211, because it is
not likely to have a significant adverse effect on the supply,
distribution, or use of energy. The purpose of this document is to
propose to retain the current O3 NAAQS. This proposal does
not change existing requirements. Thus, the EPA concludes that this
proposal does not constitute a significant energy action as defined in
Executive Order 13211.
J. National Technology Transfer and Advancement Act
This action does not involve technical standards.
K. Executive Order 12898: Federal Actions To Address Environmental
Justice in Minority Populations and Low-Income Populations
The EPA believes that this action does not have disproportionately
high and adverse human health or environmental effects on minority,
low-income populations and/or indigenous peoples, as specified in
Executive Order 12898 (59 FR 7629, February 16, 1994). The action
proposed in this document is to retain without revision the existing
O3 NAAQS based on the Administrator's proposed conclusions
that the existing primary standard protects public health, including
the health of sensitive groups, with an adequate margin of safety, and
that the existing secondary standard protects public welfare from known
or anticipated adverse effects. As discussed in section II above, the
EPA expressly considered the available information regarding health
effects among at-risk populations in reaching the proposed decision
that the existing standard is requisite.
L. Determination Under Section 307(d)
Section 307(d)(1)(V) of the CAA provides that the provisions of
section 307(d) apply to ``such other actions as the Administrator may
determine.'' Pursuant to section 307(d)(1)(V), the Administrator
determines that this action is subject to the provisions of section
307(d).
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List of Subjects in 40 CFR Part 50
Environmental protection, Air pollution control, Carbon monoxide,
Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.
Andrew Wheeler,
Administrator.
[FR Doc. 2020-15453 Filed 8-13-20; 8:45 am]
BILLING CODE 6560-50-P