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Integrated Review Plan for the National
Ambient Air Quality Standards for Ozone
and Related Photochemical Oxidants
Volume 1: Background Document
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EPA-452/R-24-001 a
December 2024
Integrated Review Plan for the National Ambient Air
Quality Standards for Ozone and Related
Photochemical Oxidants.
Volume 1: Background Document
U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Health and Environmental Impacts Division
and
Center for Public Health and Environmental Assessment
Office of Research and Development
Research Triangle Park, NC
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DISCLAIMER
This document serves as a public information document and as a management
tool for the U.S. Environmental Protection Agency's (EPA's) Center for Public Health and
Environmental Assessment and the Office of Air Quality Planning and Standards in
conducting the review of air quality criteria and the primary national ambient air quality
standards for ozone and related photochemical oxidants. It does not represent and
should not be construed to represent an Agency determination or policy. Mention of
trade names or commercial products does not constitute endorsement or
recommendation for use.
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TABLE OF CONTENTS
1 BACKGROUND ON THE OZONE NAAQS 1-1
1.1 History of Air Quality Criteria and Standards for Photochemical Oxidants Including
Ozone 1-1
1.2 The Primary Standard 1-9
1.2.1 Indicator 1-10
1.2.2 Averaging Time 1-11
1.2.3 Form 1-11
1.2.4 Level 1-13
1.3 The Secondary Standard 1-25
2 THE CURRENT OZONE NAAQS REVIEW: MILESTONES AND TIMELINE 2-1
3 REFERENCES 3-1
Appendix: Ambient Air Monitoring and Data Handling A-1
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The planning phase of the U.S. Environmental Protection Agency's (EPA's) reviews
of the air quality criteria and the national ambient air quality standards (NAAQS)
includes development of an integrated review plan (IRP), which is composed of three
volumes. Volume 1 (this document) provides background information and serves as a
reference for the public and the Clean Air Scientific Advisory Committee (CASAC).
Volume 2 addresses the general approach for the review, identifies policy-relevant
issues in the review and describes key considerations in the EPA's development of the
ISA. This document is the subject of CASAC consultation and public comment. Volume 3
describes key considerations in the EPA's planning with regard to any quantitative risk
and exposure analyses to be considered for the review. In order that consideration of
the availability of new scientific evidence in the review inform these plans, the
development and public release of Volume 3 will generally coincide with the availability
of the draft ISA. At that time, Volume 3 is the subject of CASAC consultation and public
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1 BACKGROUND ON THE OZONE NAAQS
1.1 HISTORY OF AIR QUALITY CRITERIA AND STANDARDS FOR
PHOTOCHEMICAL OXIDANTS INCLUDING OZONE
Air quality criteria were developed for photochemical oxidants in 1970 (U.S.
DHEW, 1970; 35 FR4768, March 19, 1970), and primary and secondary NAAQS were first
established in 1971 (36 FR 8186, April 30, 1971). Based on the scientific information in
the 1970 air quality criteria document (AQCD), 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.
The EPA initiated the first periodic review of the NAAQS for photochemical
oxidants in 1977, and proposed revisions in 1979, based on the 1978 AQCD (U.S. EPA,
1978; 43 FR 26962, June 22, 1978). In 1979, the EPA published its final decision in the
review (44 FR 8202, February 8, 1979). With this decision, 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).
Following the EPA's decision in the 1979 review, several petitioners sought
judicial review. Among those, the city of Houston challenged the Administrator's
decision arguing that the standard was arbitrary and capricious because natural O3
concentrations and other physical phenomena in the Houston area made the standard
unattainable in that area. The U.S. Court of Appeals for the District of Columbia Circuit
(D.C. Circuit) rejected this argument, holding 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
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of the difficulty in meeting standards in some locations and had addressed this difficulty
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
(U.S. EPA, 1986) and the 1989 Staff Paper (U.S. EPA, 1989). A 1992 supplement to the
1986 AQCD described additional important scientific studies on potential health and
welfare effects (U.S. EPA, 1992). In August 1992, the EPA proposed to retain the existing
primary and secondary standards based on the health and welfare effects information
contained in the 1986 AQCD and its 1992 Supplement (57 FR 35542, August 10, 1992).
In March 1993, the EPA announced its decision to conclude this review by affirming its
proposed decision to retain the standards, without revision (58 FR 13008, March 9,
1993).
In the 1992 notice of its proposed decision in that review, the EPA announced its
intention to proceed as rapidly as possible with the next review of the air quality criteria
and standards for O3 and other photochemical oxidants in light of emerging evidence of
health effects related to 6- to 8-hour O3 exposures (57 FR 35542, August 10, 1992). The
EPA subsequently published the AQCD and Staff Paper for that next review (U.S. EPA,
1996a, b). In December 1996, the EPA proposed revisions to both the primary and
secondary standards (61 FR 65716, December 13, 1996). With regard to the primary
standard, the EPA proposed to replace the then-existing 1-hour primary standard with
an 8-hour standard set at a level of 0.08 ppm (equivalent to 0.084 ppm based on the
proposed data handling convention) as a 3-year average of the annual third-highest
daily maximum 8-hour concentration. The EPA proposed to revise the secondary
standard either by setting it identical to the proposed new primary standard or by
setting it as a new seasonal standard using a cumulative form. The EPA completed this
review in 1997 by setting the primary standard at a level of 0.08 ppm, based on the
annual fourth-highest daily maximum 8-hour average concentration, averaged over
three years, and setting the secondary standard identical to the revised primary
standard (62 FR 38856, July 18, 1997).
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On May 14, 1999, in response to challenges by industry and others to the EPA's
1997 decision, the D.C. Circuit remanded the O3 NAAQS to the EPA, finding that section
109 of the CAA, as interpreted by the EPA, effected an unconstitutional delegation of
legislative authority {American Trucking Assoc. v. EPA, 175 F.3d 1027, 1034-1040 [D.C.
Cir. 1999]). In addition, the court 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). In 1999, the EPA petitioned for rehearing en banc on several
issues related to that decision. The court granted the request for rehearing in part and
denied it in part, but declined to review its ruling with regard to the potential beneficial
effects of O3 pollution {American Trucking Assoc. v. EPA195 F.3d 4, 10 [D.C Cir., 1999]).
On January 27, 2000, the EPA petitioned the U.S. Supreme Court for certiorari on the
constitutional issue (and two other issues), but did not request review of the ruling
regarding the potential beneficial health effects of O3. On February 27, 2001, the U.S.
Supreme Court unanimously reversed the judgment of the D.C. Circuit on the
constitutional issue. Whitman v. American Trucking Assoc., 531 U. S. 457, 472-74 (2001)
(holding that section 109 of the CAA does not delegate legislative power to the EPA in
contravention of the Constitution). The Court remanded the case to the D.C. Circuit to
consider challenges to the O3 NAAQS that had not been addressed by that court's
earlier decisions. On March 26, 2002, the D.C. Circuit issued its final decision on the
remand, finding the 1997 O3 NAAQS to be "neither arbitrary nor capricious," and so
denying the remaining petitions for review. See ATA III, 283 F.3d at 379.
Specifically, in ATA III, the D.C. Circuit upheld the EPA's decision on the 1997 O3
standard as the product of reasoned decision making. With regard to the primary
standard, the court made clear that the most important support for the EPA's decision
to revise the standard was the health evidence of insufficient protection afforded by the
then-existing standard ("the record [is] replete with references to studies demonstrating
the inadequacies of the old one-hour standard"), as well as extensive information
supporting the change to an 8-hour averaging time {id. at 378). The court further upheld
the EPA's decision not to select a more stringent level for the primary standard noting
"the absence of any human clinical studies at ozone concentrations below 0.08 [ppm]"
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which supported the EPA's conclusion that "the most serious health effects of ozone are
'less certain' at low concentrations, providing an eminently rational reason to set the
primary standard at a somewhat higher level, at least until additional studies become
available" {id. at 379, emphasis in original, internal citations omitted). The court also
pointed to the significant weight that the EPA properly placed on the advice it received
from the CASAC {id. at 379). In addition, the court noted that "although relative
proximity to peak background O3 concentrations did not, in itself, necessitate a level of
0.08 [ppm], EPA could consider that factor when choosing among the three alternative
levels" {id. 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). The EPA provisionally determined that the
information linking changes in patterns of ground-level O3 concentrations to changes in
relevant patterns of exposures to UV radiation of concern to public health was too
uncertain, at that time, to warrant any relaxation in 1997 O3 NAAQS. The EPA also
expressed the view that any plausible changes in UV-B radiation exposures from
changes in patterns of ground-level O3 concentrations would likely be very small from a
public health perspective. In view of these findings, 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). In 2007, the EPA proposed to
revise the level of the primary standard within a range of 0.075 to 0.070 ppm (72 FR
37818, July 11, 2007). The EPA proposed to revise the secondary standard either by
setting it identical to the proposed new primary standard or by setting it as a new
seasonal standard using a cumulative form. Documents supporting these proposed
decisions included the 2006 AQCD (U.S. EPA, 2006) and 2007 Staff Paper (U.S EPA, 2007)
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and related technical support documents. 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).
In May 2008, state, public health, environmental, and industry petitioners filed
suit challenging the EPA's final decision on the 2008 O3 standards. On September 16,
2009, the EPA announced its intention to reconsider the 2008 O3 standards-1 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). In that notice, the EPA proposed
that further revisions of the primary and secondary standards were necessary to provide
a requisite level of protection to public health and welfare. The EPA proposed to revise
the level of the primary standard from 0.075 ppm to a level within the range of 0.060 to
0.070 ppm, and to revise the secondary standard to one with a cumulative, seasonal
form. At the EPA's request, the CASAC reviewed the proposed rule at a public
teleconference on January 25, 2010 and provided additional advice in early 2011 (Samet,
2010, 2011). 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), the EPA decided to consolidate the
reconsideration with its statutorily required periodic review.2
In light of the EPA's decision to consolidate the reconsideration with the next
review, the D.C. Circuit proceeded with the litigation on the 2008 final decision. On July
23, 2013, the court upheld the EPA's 2008 primary O3 standard, but remanded the 2008
secondary standard to the EPA (Mississippi v. EPA, 744 F. 3d 1334 [D.C. Cir. 2013]). With
respect to the primary standard, the court first rejected arguments that the EPA should
not have lowered the level of the existing primary standard, holding that the EPA
reasonably determined that the existing primary standard was not requisite to protect
1 The press release of this announcement is available at: https://archive.epa.gov/epapaaes/newsroom
archive/newsreleases/85f90b7711 acb0c88525763300617d0d.html.
2 This rulemaking, completed in 2015, concluded the reconsideration process.
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public health with an adequate margin of safety, and consequently required revision.
The court went on to reject arguments that the EPA should have adopted a more
stringent primary standard. 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. This review
had been formally initiated in 2008 with a call for information in the Federal Register (73
FR 56581, September 29, 2008). In late 2014, based on the Integrated Science
Assessment (ISA), Risk and Exposure Assessments (REAs) for health and welfare, and PA3
developed for this review, the EPA proposed to revise the 2008 primary and secondary
standards by reducing the level of both standards to within the range of 0.070 to 0.065
ppm (79 FR 75234, December 17, 2014).
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 the 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.4 The other elements of the
standard were retained. This decision on the secondary standard also incorporated the
3 The final versions of these documents, released in August 2014, were developed with consideration of
the comments and recommendations from the CASAC, as well as comments from the public on the
draft documents (U.S. EPA 2014a; U.S. EPA, 2014b; U.S. EPA, 2014c; Frey, 2014a; Frey, 2014b; Frey,
2014c).
4 The standards set in 2015 (generally referred to as the current standards herein) are specified at 40 CFR
50.19.
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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). The 2015 revisions to the NAAQS were
accompanied by revisions to the data handling procedures, and the ambient air
monitoring requirements5 (80 FR 65292, October 26, 2015).6 The Appendix to this
volume summarizes the current ambient air monitoring and data handling requirements.
After publication of the final rule, a number of industry groups, environmental
and public health organizations, and certain states filed petitions for judicial review in
the D.C. Circuit. The industry and state petitioners filed briefs arguing that the revised
standards are too stringent, while the environmental and health petitioners' brief argued
that the revised standards are 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 (Murray Energy v.
EPA, 936 F.3d 597 (D.C. Cir. 2019)).
The EPA announced its initiation of the fifth periodic review of the air quality
criteria for photochemical oxidants and the O3 NAAQS in June 2018, issuing a call for
information in the Federal Register (83 FR 29785, June 26, 2018). Under the plan outlined
in the final IRP (U.S. EPA 2019) and as directed by the Administrator in initiating the
review, this O3 NAAQS review progressed on an accelerated schedule (Pruitt, 2018). In a
divergence from past practice in recent history, a pollutant-specific O3 review panel was
not assembled to assist the CASAC in its review. Rather, the CASAC was assisted in its
review by a pool of consultants with expertise in a number of fields (84 FR 38625,
5 The current federal regulatory measurement methods for O3 are specified in 40 CFR 50, Appendix D and
40 CFR Part 53. Consideration of ambient air measurements with regard to judging attainment of the
standards is specified in 40 CFR 50, Appendix U. The O3 monitoring network requirements are specified
in 40 CFR 58.
5 This decision additionally announced revisions to the exceptional events scheduling provisions, as well as
changes to the air quality index and the regulations for the prevention of significant deterioration
permitting program.
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August 7, 2019).7 On August 14, 2020, based on the current evidence in the 2020 ISA,
the PA, with associated air quality, risk and exposure analyses, and CASAC advice, the
EPA proposed to retain both the primary and secondary O3 standards, without revision
(85 FR 49830, August 14, 2020). In December 2020, the EPA issued its final decision to
retain the existing standards without revision (85 FR 87256, December 31, 2020).8
Following publication of the 2020 decision, three petitions were filed seeking
review in the D.C. Circuit and the court consolidated the cases. The EPA also received
two petitions for reconsideration of the 2020 decision. On October 29, 2021, the Agency
announced its decision to reconsider the 2020 decision and filed a motion with the
court which explained that decision.9The consolidated cases were put in abeyance.
The EPA's approach for the reconsideration included establishment of an O3
Review Panel to assist the CASAC in its role. In a series of public meetings from 2021 to
2023, the EPA briefed the Panel on the 2020 ISA, and a small set of more recent
provisionally considered health studies (2020 ISA; Luben et al., 2020; Duffney et al., 2022;
87 FR 41309, July 12, 2022), and also engaged the Panel in review of two versions of a
draft PA for the reconsideration (U.S. EPA, 2022; 87 FR 19501, April 4, 2022; U.S. EPA
2023; 88 FR 9275, February 13, 2023; 88 FR 17840, March 24, 2023). During this period,
the O3 Review Panel and CASAC also held discussions in consideration of the ISA and
more recent studies (87 FR41309, July 12, 2022; 87 FR 60394, October 5, 2022;
Sheppard, 2022a, b). Based on these discussions, the CASAC determined "that the
existing scientific evidence summarized in the 2020 ISA provides a scientifically sound
foundation for the Agency's reconsideration of the 2020 Ozone NAAQS decision" and
stated "that the CASAC was not recommending that the 2020 ISA be reopened or
7 Rather than join with some or all of the CASAC members in a pollutant specific review panel as had been
common in previous NAAQS reviews, 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, as a means of obtaining subject matter expertise
for its document review.
8 The decision on the secondary standard also considered and addressed the 2019 remand of
the secondary standard by the D.C. Circuit such that the 2020 decision incorporated the EPA's response
to that remand.
9 The Agency's October 29, 2021 announcement is available at https://www.epa.aov/around-level-ozone-
pollution/epa-reconsider-previous-administrations-decision-retain-2015-ozone.
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revised" (Sheppard, 2022c).10 In June 2023, based on its additional consideration of the
second version of the draft PA for the reconsideration (U.S. EPA, 2023), the CASAC
conveyed its comments on the draft document, and its recommendations on the
standards (Sheppard, 2023). Relying in part on several new studies published
subsequent to the 2020 ISA, the majority of the CASAC recommended that the primary
and secondary O3 NAAQS be revised (Sheppard, 2023). On August 21, 2023, after
consideration of the advice received from the CASAC, the EPA announced its decision to
initiate a new, full statutory review of the O3 NAAQS and the underlying air quality
criteria and to incorporate the ongoing reconsideration of the 2020 O3 NAAQS decision
into the new review (Regan, 2023).11
With regard to the consolidated cases before the D.C. Circuit seeking review of
EPA's 2020 decision, on January 3, 2024, the EPA filed an unopposed motion for
voluntary remand without vacatur. The court granted the motion on February 2, 2024.
See New York et at. v. EPA, No. 21-1028, Order (Doc. No. 2038660, D.C. Cir. Feb. 2, 2024).
1.2 THE PRIMARY STANDARD
The current primary O3 standard of 0.070 ppm,12 as the annual fourth-highest
daily maximum 8-hour average concentration, averaged across three consecutive years,
was set in 2015 and retained without revision in 2020 (80 FR 65292, October 26, 2015;
85 FR 87256, December 31, 2020). Establishment of this standard, and its retention in
2020, were based on the extensive body of evidence most prominently documenting the
causal relationship between O3 exposure and a broad range of respiratory effects, as
well as the Administrator's judgments regarding the appropriate degree of public health
10 The CASAC additionally noted that "[regarding the Agency's judgments, in some instances the CASAC
does have differing opinions," and also offered comments and advice on several issues and areas for
improvement in future O3 ISAs (Sheppard, 2022c).
11 The Agency's August 21, 2023, announcement is available at https://www.epa.gov/newsreleases/epa-
initiates-new-review-ozone-national-ambient-air-qualitv-standards-reflect-latest. As noted in this
announcement and in the Administrator's response letter to CASAC, the new review will also consider
the advice and recommendations of the CASAC (Regan, 2023)).
12 Although ppm are the units in which the level of the standard is defined, the units, ppb, are more
commonly used throughout this IRP 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.
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protection for the 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; 85 FR 87263, December 31, 2020). The respiratory effects associated with O3
exposure range from small, reversible changes in lung 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 asthma-related
emergency department visits and hospital admissions, which have been associated with
ambient air concentrations of O3 in epidemiologic studies (2020 ISA, section 3.1; 2013
ISA, section 6.2).13 The EPA's establishment of the standard in 2015, and its retention in
2020, focused particularly on implications of the effects evidence to ensure protection of
at-risk populations,14 such as people with asthma, and particularly children with asthma
(80 FR 65343, October 26, 2015; 85 FR 87305, December 31, 2020). Key aspects of the
decisions in 2015 and 2020, are summarized below for each of the four basic elements
of the NAAQS (indicator, averaging time, form, and level), in turn.
1.2.1 Indicator
In 1979, O3 was established as the indicator for a standard meant to provide
protection against photochemical oxidants in ambient air (44 FR 8202, February 8, 1979).
In setting the current standard in 2015 and reviewing it in 2020, the Administrator
considered the available information presented in the ISA and PA, along with advice
from the CASAC and public comment. Both the 2013 and 2020 ISAs specifically noted
that O3 is the only photochemical oxidant (other than nitrogen dioxide) that is routinely
monitored and for which a comprehensive database exists (2013 ISA, section 3.6; 80 FR
65347, October 26, 2015; 2020 ISA, p. IS-3; 85 FR 87301, December 31, 2020). The 2020
13 The evidence base also includes experimental animal studies that provide insight into potential modes
of action, contributing to the coherence and robust nature of the evidence.
14 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 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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ISA further noted that "the primary literature evaluating the health and ecological effects
of photochemical oxidants includes ozone almost exclusively as an indicator of
photochemical oxidants" (2020 ISA, p. IS-3). In both reviews, the CASAC indicated its
support for O3 as the appropriate indicator. Based on these considerations and public
comments, the Administrators in both reviews concluded that O3 remains the most
appropriate indicator for a standard meant to provide protection against photochemical
oxidants in ambient air, and they retained O3 as the indicator for the primary standard
(80 FR 65347, October 26, 2015; 85 FR 87306; December 31, 2020).
1.2.2 Averaging Time
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). This decision was based on the then newly available
evidence from numerous controlled human exposure studies in healthy adults of
adverse respiratory effects resulting from 6- to 8-hour exposures, as well as quantitative
analyses indicating the air quality 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, 1996). In the establishment of the existing standard in 2015 and its
review in 2020, the averaging time was retained in light of both the strong evidence for
03-associated respiratory effects following short-term exposures and the available
evidence related to effects following longer-term exposures (80 FR 65347-50, October
26, 2015). Based on the health effects evidence and quantitative exposure/risk
information in the 2015 review, along with CASAC advice and public comments, the
Administrator concluded that a standard with an 8-hour averaging time (and the newly
revised level) could effectively limit health effects attributable to both short- and long-
term O3 exposures and that it was appropriate to retain the 8-hour averaging time (80
FR 65350, October 26, 2015). The EPA reached similar conclusions in the 2020 review
and retained the 8-hour averaging time (85 FR 87306; December 31, 2020).
1.2.3 Form
The concentration-based form (e.g., the nth-high metric) of the existing standard
was established in the 1997 review when it was recognized that such a form better
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reflects the continuum of health effects associated with increasing O3 concentrations
than an expected exceedance form.15 Unlike an expected exceedance form, a
concentration-based form gives proportionally more weight to years when 8-hour O3
concentrations are well above the level of the standard than years when 8-hour O3
concentrations are just above the level of the standard. 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 not a basis
identified for selection of a more restrictive form (62 FR 38856, July 18, 1997). In
subsequent reviews, while the potential value of a percentile-based form16 was
considered, the EPA concluded that, because of the differing lengths of the monitoring
season for O3 across the U.S., such a statistic would not be effective in ensuring the
same degree of public health protection across the country (73 FR 16474-75, March 27,
2008).17 The form includes averaging across three years in recognition of the importance
of a form that provides stability to ongoing control programs.18 In establishing the
15 The first O3 standard, set in 1979 as an hourly standard, had an expected exceedance form, such that
attainment was defined as when the expected number of days per calendar year, with maximum hourly
average concentration greater than 0.12 ppm, was equal to or less than 1 (44 FR 8202, February 8,
1979).
15 It is noted that such statistic allows comparison among datasets of varying length because it samples
approximately the same place in the distribution of air quality values, whether the dataset is several
months or several years long.
17 Specifically, a percentile-based form would allow more days with higher air quality values (i.e., higher O3
concentrations) in locations with longer O3 seasons relative to locations with shorter O3 seasons.
18 For example, it was noted that it was important to have a form that provides stability and insulation
from the impacts of extreme meteorological events that are conducive to O3 occurrence. Such events
could have the effect of reducing public health protection, to the extent they result in frequent shifts in
and out of attainment due to meteorological conditions because such frequent shifting could disrupt
an area's ongoing implementation plans and associated control programs (73 FR 16475, March 27,
2008). Advice from the CASAC in the 2015 review supported this, stating that this concentration-based
form that is averaged over three years "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, 2014, p. 6; 80 FR 65352, October 26, 2015).
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existing standard in 2015 and in retaining it in 2020, the fourth-high form (i.e., the
annual fourth-highest daily maximum 8-hour O3 average concentration, averaged over 3
years) was retained (80 FR 65352, October 26, 2015; 85 FR 87306; December 31, 2020).
1.2.4 Level
In establishing the level of the standard in 2015 and in the decision to retain it in
2020, the Administrator at each time carefully considered: (1) the assessment of the
health effects evidence and conclusions reached in the ISA; (2) the available quantitative
exposure/risk analyses, including associated limitations and uncertainties, described in
detail in the Health Risk and Exposure Assessment (HREA, in the 2015 review or
appendices of the 2020 PA in the 2020 review); (3) considerations and staff conclusions
and associated rationales in the PA; (4) advice and comments from the CASAC; and (5)
public comments (80 FR 65362, October 26, 2015; 85 FR 37300, December 31, 2020).
In weighing the health effects evidence and making judgments regarding the public
health significance of the quantitative estimates of exposures and risks allowed by the existing
standard and potential alternative standards considered, as well as judgments regarding margin
of safety, both of the decisions, in 2015 and 2020, considered the currently available
information, including EPA judgments in prior reviews, advice from the CASAC, statements of
the American Thoracic Society (ATS, an organization of respiratory disease specialists), and
public comments. Such statements from the ATS (ATS, 2000; Thurston et al., 2017), as well as
judgments made by the EPA in considering similar health effects in previous NAAQS reviews,
were considered when the standard was set in 2015 and reviewed in 2020 (85 FR 87270-72,
87302-87305, December 31, 2020; 80 FR 65343, October 26, 2015). The 2020 review included a
newly available ATS statement (Thurston et al., 2017), which is generally consistent with the
prior statement (ATS, 2000) including the attention that it gives 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. In 2020, the Administrator
recognized the role of such statements, as described by the ATS, as proposing principles or
considerations for weighing the evidence rather than offering "strict rules or numerical
criteria" (ATS, 2000, Thurston et al., 2017).
In keeping with this intent of the ATS statements (to avoid specific criteria), the
statements, in discussing what constitutes an adverse health effect, do not
comprehensively describe all the biological responses raised, e.g., with regard to
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magnitude, duration or frequency of small pollutant-related changes in pulmonary
function. While recognizing the limitations in the available evidence base with regard to
our understanding of these aspects of such changes that may be associated with
exposure concentrations of interest (e.g., as estimated in the exposure analysis), the
Administrator, in both reviews, considered individuals with preexisting compromised
function, such as that resulting from asthma, important to judgments on the adequacy
of protection provided for at-risk populations. The Administrator in each review also
recognized that the controlled human exposure studies, primarily conducted in healthy
adults, on which the depth of our understanding of 03-related health effects is based, in
combination with the larger evidence base, informs our conceptual understanding of O3
responses in people with asthma and in children (85 FR 87303, December 31, 2020). In
so doing, each decision 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, in both the 2015 and 2020 decisions, the Administrator took into account the
need for an adequate margin of safety as an integral part of their decision-making.
The evidence base available in the 2020 review included decades of extensive
evidence that clearly describes the role of O3 in eliciting an array of respiratory effects
and more recent evidence indicating the potential for relationships between O3
exposure and metabolic effects. As was established in prior reviews, the effects for which
the evidence is strongest are transient decrements in lung function and respiratory
symptoms as a result of short-term exposures particularly when breathing at elevated
rates (2020 ISA, section IS.4.3.1; 2013 ISA, p. 2-26). These effects are demonstrated in the
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large, long-standing evidence base of controlled human exposure studies19 (1978
AQCD, 1986 AQCD, 1996 AQCD, 2006 AQCD, 2013 ISA, 2020 ISA). The epidemiologic
evidence base documents consistent, positive associations of O3 concentrations in
ambient air with lung function effects in panel studies (2013 ISA, section 6.2.1.2; 2020
ISA, Appendix 3, section 3.1.4.1.3), and with more severe health outcomes, including
asthma-related emergency department visits and hospital admissions (2013 ISA, section
6.2.7; 2020 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 short-term
respiratory effects, and studies in animal models describe effects of longer-term O3
exposure on the developing lung (2020 ISA, Appendix 3, sections 3.1.11 and 3.2.6).
Although less influential to considering the standard than the respiratory effects
evidence, the available evidence when the standard was set and when it was reviewed in
2020 also included evidence for effects other than respiratory effects. Most prominent
was evidence regarding O3 exposure and cardiovascular effects and associated
mortality, conclusions regarding which changed across the two reviews (2013 ISA, Table
1-1; 2020 ISA, Table ES-1). For example, while the evidence available in the 2015 review
was sufficient to conclude that the relationships for short-term O3 exposure with
cardiovascular health effects and mortality were likely to be causal, that conclusion was
no longer supported by the more expansive evidence base which the 2020 ISA
determined to be suggestive of, but not sufficient to infer, a causal relationship for these
health effect categories (2020 ISA, Appendix 4, section 4.1.17; Appendix 6, section 6.1.8).
Further, newly available evidence in the 2020 review, largely experimental animal
studies, with exposure concentrations well above those at which respiratory effects
occur, was judged sufficient to conclude there to be a causal relationship between
short-term O3 exposure and metabolic effects (2020 ISA, section IS.4.3.3; 85 FR 87270,
December 31, 2020).
19 The vast majority of the controlled human exposure studies (and all of the studies conducted at the
lowest exposures) involved young healthy adults (typically 18-35 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,
2020 PA, Appendix 3A, Table 3A-3).
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The 2015 decision to set the level of the revised primary O3 standard at 70 ppb
and the 2020 decision to retain this standard, without revision, placed the greatest
weight on the results of controlled human exposure studies and on quantitative
analyses based on information from these studies, particularly the comparison-to-
benchmarks analysis comparing exposure estimates for study area populations of
children at elevated exertion20 to exposure benchmark concentrations (exposures of
concern) under air quality conditions just meeting the current standard (80 FR 65362,
October 26, 2015; 85 FR 87284, December 31, 2020).21 In considering the epidemiologic
studies in the 2015 review, the Administrator concluded that a revised standard with a
level of 70 ppb would result in improvements in public health, beyond the protection
provided by the current standard, against the clearly adverse effects reported in
epidemiologic studies.22 In further evaluating information from epidemiologic studies,
the Administrator considered the epidemiologic-based risk estimates for 03-associated
morbidity or mortality and noted relatively less confidence in these estimates than in the
estimates of exposures of concern and lung function risks (80 FR 65364-65365, October
26, 2015). This weighting reflected the recognition that controlled human exposure
studies provide the most certain evidence indicating the occurrence of health effects in
humans following specific O3 exposures, and, in particular, that the effects reported in
the controlled human exposure studies are due solely to O3 exposures, and are not
20 Consideration focused on estimates for children, reflecting the finding that 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 children spend outdoors engaged in activities at elevated exertion (2014 HREA,
section 5.3.2).
21 The Administrator viewed the results of other quantitative analyses in the 2015 review - 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).
22 This included consideration of single-city epidemiologic studies reporting significant positive
associations of O3 with health effects in areas where the existing standard of 75 ppb was met, as well as
the epidemiology-based risk estimates of reductions in mean premature mortality associated with
ozone levels lower than the current standard (80 FR 65364-65365, October 26, 2015).
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complicated by the presence of co-occurring pollutants or pollutant mixtures, as is the
case in epidemiologic studies (80 FR 65362-65363, October 26, 2015).23
The Administrator's judgment in establishing the standard in 2015, and in
retaining it in 2020, included a focus on the public health implications of the exposure
and risk analyses conducted in each review. The comparison-to-benchmarks analysis,
which included a focus on the at-risk populations of children and children with asthma,
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. The analysis conducted for the 2020 review reflected a
number of updates and improvements and provided estimates with reduced uncertainty
compared to those from the 2015 review. The results for analyses in both reviews are
characterized through comparison of exposure concentration estimates to 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 of generally
healthy adults engaging in quasi-continuous exercise (at a moderate level of exertion),
and that yielded different occurrences of statistical significance and severity of
respiratory effects (80 FR 65312, October 26, 2015; 85 FR 87277; December 31, 2020;
2020 PA, section 3.3.3).24 Such study data were further recognized to be lacking at these
exposure levels for children and people with asthma, and the evidence indicates that
such responses, if repeated or sustained, particularly in people with asthma, pose risks
23 Other quantitative exposure/risk analyses (e.g., the lung function risk assessment, analyses of 03 air
quality in locations of epidemiologic studies, and epidemiologic-study-based quantitative health risk
assessment) were viewed as providing information in support of the 2015 decision to revise the then-
current standard level of 75 ppb, but of less utility for selecting a particular standard level among a
range of options (80 FR 65362, October 26, 2015). For example, with regard to the epidemiologic
studies, the Administrator noted that most of the studies were conducted in locations likely to have
violated the then current standard during all or part of the study period.
24 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.
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of effects of greater concern, including asthma exacerbation, as cautioned by the CASAC
(85 FR 87302, December 31, 2020).25
The three benchmark concentrations (60, 70 and 80 ppb) were recognized to
represent exposure conditions (during quasi-continuous exercise) associated with
different levels of respiratory response (both with regard to the array of effects and
severity of individual effects) in the subjects studied, and also to inform the
Administrators'judgments in both reviews regarding different levels of risk that might
be posed to unstudied members of at-risk populations. The highest benchmark
concentration (80 ppb) represented an exposure where multiple controlled human
exposure studies involving 6.6-hour exposures during quasi-continuous exercise
demonstrate a range of 03-related respiratory effects including inflammation and airway
responsiveness, as well as respiratory symptoms and lung function decrements in
healthy adult subjects. The second benchmark (70 ppb) represented an exposure level
below the lowest exposures that have reported both statistically significant lung
function decrements and increased respiratory symptoms (reported at 73 ppb,26
Schelegle et al., 2009) or statistically significant increases in airway resistance and
responsiveness (reported at 80 ppb, Horstman et al., 1990).27 The lowest benchmark (60
ppb) represents still lower exposure, and a level for which findings from controlled
human exposure studies of largely healthy subjects have included: statistically significant
25 In the 2020 review, the CASAC noted that "[a]rguably the most important potential adverse effect of
acute ozone exposure in a child with asthma is not whether it causes a transient decrement in lung
function, but whether it causes an asthma exacerbation" and that CV'has respiratory effects beyond its
well-described effects on lung function," including increases in airway inflammation which also have the
potential to increase the risk for an asthma exacerbation. (Cox, 2020, Consensus Responses to Charge
Questions pp. 7-8).
25 For the 70 ppb target exposure, the time weighted average concentration across the full 6.6-hour
exposure was 73 ppb and the mean O3 concentration during the exercise portion of the study protocol
was 72 ppb, based on O3 measurements during the six 50-minute exercise periods (Schelegle et al.,
2009).
27 The study group mean lung function decrement for the 73 ppb exposure was 6%, with individual
decrements of 15% or greater (moderate or greater) in about 10% of subjects and decrements of 10%
or greater in 19% of subjects. Decrements of 20% or greater were reported in 6.5% of subjects
(Schelegle et al., 2009; 2020 PA, Table 3-2 and Appendix 3D, Table 3D-20). In studies of 80 ppb
exposure, the percent of study subjects with individual FEV1 decrements of this size ranged up to nearly
double this (2020 PA, Appendix 3D, Table 3D-20).
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decrements in lung function (with mean decrements ranging from 1.7% to 3.5% across
the four studies with average exposures of 60 to 63 ppb), but not respiratory symptoms;
and a statistically significant increase in a biomarker of airway inflammatory response
relative to filtered air exposures in one study (Kim et al., 2011).
In placing greater weight and giving primary attention to the comparison-to-
benchmarks analysis, the Administrators in both reviews recognized that this analysis
provides for characterization of risk for the broad array of respiratory effects
documented in the controlled human exposure studies, facilitating consideration of an
array of respiratory effects, including but not limited to lung function decrements (80 FR
65363, October 26, 2015; 85 FR 87303, December 31, 2020). As in the 2015 decision, the
Administrator in 2020 noted that due to differences among individuals in
responsiveness, not all people experiencing exposures (e.g., to 73 ppb), experience a
response, such as a lung function decrement, and among those experiencing a
response, not all will experience an adverse effect (85 FR 87304, December 31, 2020).
Accordingly, the Administrators in the two reviews noted that not all people estimated
to experience an exposure of 7-hour duration while at elevated exertion above even the
highest benchmark would be expected to experience an adverse effect, even members
of at-risk populations (80 FR 65345, October 26, 2015; 85 FR 87304, December 31, 2020).
With these considerations in mind, the Administrators in the two reviews noted that
while single occurrences could be adverse for some people, particularly for the higher
benchmark concentration where the evidence base is stronger, the potential for adverse
response and greater severity increased with repeated occurrences (as cautioned by the
CASAC) (80 FR 65345, October 26, 2015; 85 FR 87305, December 31, 2020). The
Administrators also noted that while the exposure/risk analyses provide estimates of
exposures of the at-risk population to concentrations of potential concern, they do not
provide information on how many of such populations will have an adverse health
outcome. Accordingly, in considering the exposure/risk analysis results, while giving due
consideration to occurrences of one or more days with an exposure at or above a
benchmark, particularly the higher benchmarks, both Administrators judged multiple
occurrences to be of greater concern than single occurrences (80 FR 65364, October 26,
2015; 85 FR 87304, December 31, 2020).
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The Administrators'judgments in considering the exposure analysis results for
each of the three benchmarks are briefly summarized below, first in the context of
setting the standard level of 70 ppb in 2015, and then in the context of the decision to
retain this standard in 2020.
2015 Decision: In the 2015 considerations of the degree of protection to be
provided by a revised 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 focused particularly on the exposure analysis
estimates of two or more exposures of concern. Placing the most emphasis on a
standard that limits repeated occurrences of exposures at or above the 70 and 80 ppb
benchmarks, while at elevated ventilation, the Administrator noted that a standard of
the existing form and averaging time with a revised 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 (80 FR 65363-65364, October 26, 2015).28 The Administrator's consideration
of exposure estimates at or above the 60 ppb benchmark, an estimated exposure to
which the Administrator was less confident would result in adverse effects,29 focused
most particularly on multiple occurrences and 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 03-induced effects (80 FR
65364, 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
28 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).
29 The 2015 decision noted that "the Administrator is 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," citing, among other considerations, "uncertainty in the extent to which short-term,
transient population-level decrease in FEV1 would increase the risk of other, more serious respiratory
effects in that population" (80 FR 54363, October 26, 2015). Note: FEV1 (a measure of lung function
response) is the forced expiratory volume in one second.
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experiencing two or more days with exposures at or above 60 ppb (while at moderate or
greater exertion).30
Given the considerable protection provided against repeated exposures of
concern for all three benchmarks, including the 60 ppb benchmark, the Administrator in
2015 judged that a standard with a level of 70 ppb would incorporate a margin of safety
against the adverse 03-induced effects shown to occur in the controlled human
exposure studies following exposures (while at moderate or greater exertion) to a
concentration somewhat higher than 70 ppb (80 FR 65364, October 26, 2015).31 The
Administrator also judged the estimates of one or more exposures (while at moderate or
greater exertion) at or above 60 ppb to also provide 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).32
30 The 2015 decision also noted the Administrator's consideration of the extent to which she judged that
adverse effects could occur following specific O3 exposures related to each of the three benchmarks.
The Administrator recognized the interindividual variability in responsiveness in her interpretation of
the exposure analysis results noting noted "that not everyone who experiences an exposure of concern,
including for the 70 ppb benchmark, is expected to experience an adverse response," further judging
"that the likelihood of adverse effects increases as the number of occurrences of O3 exposures of
concern increases." And "[i]n making this judgment, she note[d] that the types of respiratory effects that
can occur following exposures of concern, particularly if experienced repeatedly, provide a plausible
mode of action by which O3 may cause other more serious effects. Therefore, her decisions on the
primary standard emphasize[d] the public health importance of limiting the occurrence of repeated
exposures to O3 concentrations at or above those shown to cause adverse effects in controlled human
exposure studies" (80 FR 65331, October 26, 2015).
31 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 was inclusive of 70 ppb) to be a policy
judgment (80 FR 65355, October 26, 2015; Frey, 2014b).
32 While the Administrator was less concerned about single exposures, especially for the 60 ppb
benchmark, she judged the HREA of one-or-more estimates informative to margin of safety
considerations. 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).
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2020 Decision: The 2020 review of the standard established in 2015 also focused
on the exposure-based comparison-to-benchmark analyses in the context of results
from the controlled human exposure studies of exposures from 60 to 80 ppb,
recognizing this information on exposure concentrations found to elicit respiratory
effects in exercising study subjects to be unchanged from what was available in the 2015
review (2020 PA, section 3.3.1; 85 FR 87302, December 31, 2020).33
The Administrator in 2020, similar to Administrators in prior reviews, judged the
array of effects associated with exposure at or above the highest benchmark
concentration (80 ppb), in combination and severity, to represent adverse effects for
individuals in the population group studied, and to pose a risk of adverse effects for
individuals in at-risk populations, most particularly people with asthma. With this in
mind, he considered the exposure/risk estimates for this benchmark, particularly the
results for children and children with asthma,34 and found them to indicate strong
protection against exposures of at-risk populations that have been demonstrated to
elicit a wide array of respiratory responses in multiple studies (85 FR 87304, December
31, 2020).
With regard to the second benchmark concentration (70 ppb), the Administrator
recognized it to be just below the lowest exposure concentration (73 ppb) for which a
study has reported a combination of a statistically significant increase in respiratory
33 With regard to the epidemiologic studies of respiratory effects, the Administrator recognized that, 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) provided strong support for
the conclusions of causality but the studies were less informative regarding exposure concentrations
associated with O3 air quality conditions that meet the current standard. He noted that the evidence
base in the 2020 review did not include new evidence of respiratory effects associated with appreciably
different exposure circumstances than the evidence available in the 2015 review, including particularly
any circumstances that would also be expected to be associated with air quality conditions likely to
occur under the current standard.
34 For the current standard, the exposure/risk estimates indicated more than 99.9% to 100% of children
and children with asthma, on average across the three years, to be protected from one or more
occasions of exposure at or above 80 ppb; the estimate is 99.9% of children with asthma and of all
children for the highest year and study area (85 FR 87279, Table 2, December 31, 2020). Further, no
children in the simulated populations (zero percent) were estimated to be exposed more than once
(two or more occasions) in the 3-year simulation to 7-hr concentrations, while at elevated exertion, at
or above 80 ppb (85 FR 87279, Table 2, December 31, 2020).
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symptoms and statistically significant lung function decrements in sensitive individuals
in a study group of largely healthy adult subjects, exposed while at elevated exertion
(Schelegle et al., 2009). However, in light of the lack of evidence for people with asthma
from studies at 80 ppb and 73 ppb, as well as the emphasis in the ATS statement on the
vulnerability of people with compromised respiratory function, such as people with
asthma, the Administrator judged it appropriate that the standard protect against
exposure, particularly multiple occurrences of exposure, to levels somewhat below 73
ppb. In this context, the Administrator considered the exposure/risk estimates, finding
them to indicate more than 99% of all children, including all children with asthma, to be
protected from one or more occasions in a year, on average, of 7-hour exposures to
concentrations at or above 70 ppb, while at elevated exertion; 99.9% of both groups to
be protected from two or more such occasions; and 100% from still more occasions (85
FR 87279, Table 2, December 31, 2020). Accordingly, he judged these estimates to also
indicate strong protection of at-risk populations against exposures similar to those
demonstrated to elicit lung function decrements and increased respiratory symptoms in
healthy subjects, a response described as adverse by the ATS (85 FR 87304, December
31, 2020).
As in 2015, the Administrator in 2020 considered the exposure/risk estimates for
the third benchmark of 60 ppb to be informative most particularly to his judgments on
an adequate margin of safety. In so doing, he noted that the lung function decrements
in controlled human exposure studies of largely healthy adult subjects exposed while at
elevated exertion to concentrations of 60 ppb, although statistically significant, were
much reduced from that observed in the next higher studied concentration (73 ppb),
both at the mean and individual level, and were not reported to be associated with
increased respiratory symptoms in healthy subjects (85 FR 87274, Table 1, December 31,
2020.35 While the Administrator did not judge these responses to represent adverse
effects for generally healthy individuals, he recognized that such data are lacking for at-
risk groups, such as people with asthma, and in consideration of comments from the
35 The response for the 60 ppb studies is also somewhat lower than that for a 63 ppb study (Table 1; 2020
PA, Appendix 3D, Table 3D-20).
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CASAC (and the ATS statement), he judged it important for the standard to provide
protection that reduces the potential risk of asthma exacerbation in this at-risk group.
Further, in consideration of the potential risk of inflammatory response in this group, he
noted evidence indicating the role of repeated occurrences of inflammation in
contributing to severity of response. In consideration of these factors, he placed greater
weight on exposure/risk estimates for multiple occurrences (85 FR 87304-87305,
December 31, 2020). The Administrator found the 2020 estimates of children with
asthma protected from 7-hour exposures to concentrations at or above this level (60
ppb), while at elevated exertion (more than 96% to more than 99% for multiple
occasions and more than 90% for one or more exposures on average across the 3-year
assessment period), to indicate an appropriate degree of protection from such
exposures (85 FR 87305, December 31, 2020).
In the 2020 review, the Administrator additionally considered the slight
differences of the 2020 exposure and risk estimates from the corresponding estimates in
the 2015 review for the 60 ppb benchmark (85 FR 87280, Table 3, December 31, 2020).
The Administrator recognized that the factors contributing to these differences, which
includes the use of air quality data reflecting concentrations much closer to the existing
standard than was the case in the 2015 review, also contribute to a reduced uncertainty
in the current estimates (85 FR 87275-87279, December 31, 2020; 2020 PA, sections 3.4
and 3.5). Thus, he noted that the exposure analysis estimates in the 2020 review indicate
the current standard to provide appreciable protection against multiple days with a
maximum exposure at or above 60 ppb. Therefore, based on his consideration of the
evidence and exposure/risk information, including that related to the lowest exposures
studied in controlled human exposure studies, and the associated uncertainties, the
Administrator judged that the current standard provides the requisite protection of
public health, including an adequate margin of safety, and thus should be retained,
without revision. Accordingly, he also concluded that a more stringent standard was not
needed to provide requisite protection and that the current standard provides the
requisite protection of public health under the Act (85 FR 87306, December 31, 2020).
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1.3 THE SECONDARY STANDARD
The current secondary O3 standard is 0.070 ppm,36 as the annual fourth-highest
daily maximum 8-hour average concentration, averaged across three consecutive years.
The establishment of this standard in 2015, and its retention in 2020, is based primarily
on consideration of the extensive welfare effects evidence base compiled from more
than fifty years of 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, 2020). Key considerations when the
standard was established in 2015, and when it was retained in 2020, were the scientific
evidence and technical analyses 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 (in terms of the W126 exposure index37) that may be
allowed by such a standard (80 FR 65292, October 26, 2015; 85 FR 87256, December 31,
2020).
The 2020 decision to retain the standard, without revision, additionally took into
account updates to the evidence base since the 2015 review, and associated conclusions
regarding welfare effects; updated and expanded quantitative analyses of air quality
data, including the frequency of cumulative exposures of potential concern and of
elevated hourly concentrations in areas with air quality meeting the standard; and also
the August 2019 decision of the D.C. Circuit remanding the 2015 secondary standard to
the EPA for further justification or reconsideration, as mentioned earlier in Section 1.1
{Murray Energy Corp. v. EPA, 936 F.3d 597 [D.C. Cir. 2019]). In the August 2019 decision,
the court held that the EPA had not adequately explained its decision to focus on a 3-
35 Although ppm are the units in which the level of the standard is defined, the units, ppb, are more
commonly used throughout this IRP 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.
37 The W126 index is a cumulative seasonal metric described as the sigmoidally weighted sum of all hourly
O3 concentrations during a specified daily and seasonal time window, with each hourly O3
concentration given a weight that increases from zero to one with increasing concentration (80 FR
65373-74, October 26, 2015). The units for W126 index values are ppm-hours (ppm-hrs). More detail is
provided in section 4.3.3.1.1 below.
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year average for consideration of the cumulative exposure, in terms ofW126, 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 a decision on that issue, concluding that it
lacked a basis to assess the EPA's rationale because the EPA had not yet fully explained
its focus on a 3-year average W126 in its consideration of the standard. Accordingly, the
2020 decision included discussion of these areas to address these aspects of the court's
decision.
The extensive evidence base considered in the 2015 and 2020 decisions
documents an array of vegetation and vegetation-related effects, ranging from the
organism scale to larger-scale impacts, such as those on populations, communities, and
ecosystems. These categories of effects which the 2013 and 2020 ISAs identified as
causally or likely causally related to O3 in ambient air include: reduced vegetation
growth, reproduction, crop yield, productivity and carbon sequestration in terrestrial
systems; alteration of terrestrial community composition, belowground biogeochemical
cycles and ecosystem water cycling; and visible foliar injury (2013 ISA, Appendix 9; 2020
ISA, Appendix 8).38 Across the different types of studies, the strongest quantitative
evidence available at the times of both the 2015 and 2020 decisions for effects from O3
exposure on vegetation comes from controlled exposure studies of growth effects in a
number of species (2013 ISA, p. 1-15). Of primary importance in considering the
appropriate level of protection for the standard, both in the 2015 decision establishing it
and in its 2020 retention, were the studies of O3 exposures that reduced growth in tree
seedlings from which E-R functions of seasonal relative biomass loss (RBL)39 have been
38 The 2020 ISA also newly determined the evidence sufficient to infer likely causal relationships of O3 with
increased tree mortality, which was not causally assessed in 2013, although it does not indicate a
potential for O3 concentrations that occur in locations that meet the current standard to cause this
effect (85 FR 87319, December 31, 2020; 2020 PA, section 4.3.1).
39 These functions were developed to quantify 03-related reduced growth in tree seedlings relative to
control treatments (without O3). In this way, RBL is the percentage by which the O3 treatment growth in
a growing season differs from the control seedlings over the same period, and the functions provide a
quantitative estimate of the reduction in a year's growth as a percentage of that expected in the
absence of O3 (2013 ISA, section 9.6.2; 2020 PA, Appendix 4A).
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established (80 FR 65385-86, 65389-90, October 26, 2015, 85 FR 87256, December 31,
2020). Consistent with advice from the CASAC in both reviews, the Administrators
considered the effects of O3 on tree seedling growth 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; 85 FR
87319, 87399, December 31, 2020).
In their consideration of O3 effects on tree seedling growth, the Administrators in
both the 2015 and 2020 decisions ascribed importance to the intended use of the
natural resources and ecosystems potentially affected. For example, the 2015 decision
considered the available evidence and quantitative analyses in the context of an
approach for considering and identifying public welfare objectives for the revised
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, in
both decisions, 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, and areas
similarly set aside by states, tribes and public interest groups, with the intention of
providing benefits to the public welfare for current and future generations (80 FR 65405,
October 26, 2015; 85 FR 87344, December 31, 2020).
Another category of effects considered in both reviews is climate-related effects
(2013 ISA, Appendix 10, Section 10.3; 2020 ISA, Appendix 9, Section 9.2 and 9.3). In
2020, as was the case when the standard was set in 2015, the evidence documented
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 2020, as in 2015, limitations and uncertainties in the evidence base
affected characterization of the extent of any relationships between ground-level O3
concentrations in ambient air in the U.S. and climate-related effects and precluded
quantitative characterization of climate responses to changes in ground-level O3
concentrations in ambient air at regional or national (vs global) scales (80 FR 65405,
October 26, 2015; 80 FR 65370, October 26, 2015; 85 FR 87337-87339, December 31,
2020). The 2020 review also identified two other types of effects - alterations in plant-
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insect signaling and insect herbivore growth and reproduction - as likely causally related
to O3, although uncertainties in the evidence for the effects precluded a full
understanding of the effects, the air quality conditions that might elicit them and the
potential for impacts in a natural system (2020 ISA, sections 8.6 and 8.7). Thus, as for
climate-related effects (in 2015 and 2020), the evidence for insect-related effects was
not a primary consideration in the 2020 decision to retain the existing standard (80 FR
65292, October 26, 2015; 85 FR 87256, December 31, 2020).
In both 2015 and 2020, effects on tree seedling growth, quantified in terms of
RBL, were used as a surrogate or proxy for a broader array of vegetation-related effects
and were quantified using the RBL metric and a set of established E-R functions for
seedlings of 11 tree species (80 FR 65391-92, October 26, 2015; 2014 PA, Appendix 5C;
85 FR 87307-9, 87313-4, December 31, 2020; 2020 PA, Appendix 4A). Cumulative O3
exposure was evaluated in terms of the W126 cumulative seasonal exposure index, an
index supported by the evidence in the 2013 and 2020 ISAs for this purpose and
consistent with advice from the CASAC in both reviews (2013 ISA, section 9.5.3, p. 9-99;
80 FR 65375, October 26, 2015; 2020 ISA, section 8.13; 85 FR 87307-8, December 31,
2020). In judgments regarding effects that are adverse to the public welfare, the decision
setting the standard in 2015, and the decision retaining it in 2020, both utilized the RBL
as a quantitative tool within a larger framework of considerations pertaining to the
public welfare significance of O3 effects (80 FR 65389, October 26, 2015; 73 FR 16496,
March 27, 2008; 85 FR 87339-41, December 31, 2020).
Accordingly, in both the 2015 and 2020 decisions, consideration of the
appropriate public welfare protection objective for the secondary standard gave
prominence to the estimates of tree seedling growth impacts (in terms of RBL) for a
range of W126 index values, developed from the E-R functions for 11 tree species (80 FR
65391-92, Table 4, October 26, 2015; 85 FR 87339-41, December 31, 2020). The
Administrators also incorporated into their 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 Administrators noted they were not simply making judgments about a
specific magnitude of growth effect in seedlings that would be acceptable or
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unacceptable in the natural environment. Rather, mindful of associated uncertainties,
the RBL estimates were used as a surrogate or proxy for consideration of the broader
array of related vegetation-related effects of potential public welfare significance, which
included effects on individual species and extending to ecosystem-level effects (80 FR
65406, October 26, 2015; 85 FR 87304, December 31, 2020). 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 the 2015 decision to revise the standard level to 70 ppb and the 2020 decision
to retain that standard, without revision, air quality analyses played an important role in
the Administrators'judgments. Such judgments of the Administrator in setting the
revised standard in 2015 are briefly summarized below. These are followed by a
summary of additional key aspects of the considerations and judgments associated with
the decision to retain this standard in 2020.
2015 Review: In using the RBL estimates as a proxy, the Administrator in 2015
focused her attention on a revised standard that would generally limit cumulative
exposures to those for which the median RBL estimate for seedlings of the 11 species
with established E-R functions would be somewhat below 6% (80 FR 65406-07, October
26, 2015).40 She noted 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). Given
the information on median RBL at different W126 exposure levels, using a 3-year
cumulative exposure index for assessing vegetation effects,41 the potential for single-
40 In her focus on 6%, the Administrator noted the CASAC view regarding 6%, most particularly the
CASAC's characterization of this level of effect in the median studied species as "unacceptably high"
(Frey, 2014b, pp. iii, 13, 14). These comments were provided in the context of CASAC's considering the
significance of effects associated with a range of alternatives for the secondary standard (80 FR 65406,
October 26, 2015).
41 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
forjudging public welfare protection afforded by a revised secondary standard. For example, 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. She additionally concluded that use of a 3-year average metric could address the
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season effects of concern, and CASAC comments on the appropriateness of a lower
value for a 3-year average W126 index, the Administrator judged it 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
additionallyjudged that the RBL estimates associated with marginally higher exposures
in isolated, rare instances were 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).
Using these objectives, the 2015 decision regarding a standard revised from the
then-existing (2008) standard was based on extensive air quality analyses that included
the most recently available data as well as air monitoring data that extended 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. These analyses supported the
Administrator's judgment that a standard with a revised level in combination with the
existing form and averaging time could achieve the desired level of public welfare
protection, considered in terms of cumulative exposure, quantified as the W126 index
(80 FR 65408, October 26, 2015). Based on the extensive air quality analyses and
consideration of the W126 index value associated with a median RBL of 6%, and the
W126 index values at monitoring sites that met different levels for a revised standard of
the existing form and averaging time, the Administrator additionallyjudged that a
standard level of 70 ppb would provide the requisite protection. The Administrator
noted that such a standard 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
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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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).
The 2015 decision also took note of the well-recognized evidence for visible foliar
injury and crop yield effects. However, the RBL information available for seedlings of a
set of 11 tree species was judged to be more useful (particularly in a role as surrogate
for the broader array of vegetation-related effects) in informing judgments regarding
the nature and severity of effects associated with different air quality conditions and
associated public welfare significance than the available information on visible foliar
injury and crop yield effects (80 FR 65405-06, October 26, 2015). With regard to visible
foliar injury, while 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, she also recognized limitations in the
available information that might inform consideration of potential public welfare
impacts related to this vegetation effect noting the significant challenges in judging the
specific extent and severity at which such effects should be considered adverse to public
welfare (80 FR 65407, October 26, 2015).42 Similarly, while 03-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 judgments regarding an appropriate level of public welfare
protection (80 FR 65405, October 26, 2015).43
42 These limitations included the lack of established E-R functions that would allow prediction of 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 03-induced vegetation
effects, such as growth or related ecosystem effects, and a lack of established criteria or objectives
relating reports of foliar injury with public welfare impacts (80 FR 65407, October 26, 2015).
43 With respect to commercial production of commodities, the Administrator noted the difficulty in
discerning the extent to which 03-related effects on commercially managed vegetation are adverse
from a public welfare perspective, given that the extensive management of such vegetation (which, as
the CASAC noted, may reduce yield variability) may also to some degree mitigate potential 03-related
effects. Management practices are highly variable and are designed to achieve optimal yields, taking
into consideration various environmental conditions. Further, changes in yield of commercial crops and
commercial commodities, such as timber, may affect producers and consumers differently, complicating
the assessment of overall public welfare effects still further (80 FR 65405, October 26, 2015).
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In summary, the 2015 decision focused primarily on the information related to
trees and growth impacts in identifying the public welfare objectives for the revised
secondary standard (80 FR 65409-65410, October 26, 2015). In this context, the
Administrator in 2015 judged that the 70 ppb 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. She additionallyjudged that the new standard would be sufficient to
protect public welfare from known or anticipated adverse effects. These judgments by
the Administrator at that time 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.
As noted in Section 1.1 above and earlier in this section, the D.C. Circuit
remanded the 2015 secondary standard to the EPA for further justification or
reconsideration {Murray Energy Corp. v. EPA, 936 F.3d 597 [D.C. Cir. 2019]), and the 2020
review incorporated EPA's response to that remand, as discussed further below.
2020 Review: Regarding the appropriate O3 exposure metric to employ in
assessing adequacy of air quality control in protecting against RBL, in addition to finding
it appropriate to continue to consider the seasonal W126 index averaged over a 3-year
period to estimate median RBL (as was concluded in 2015), the Administrator in 2020
judged it appropriate to also consider other metrics including peak hourly
concentrations44 (85 FR 87344, December 2020). With regard to his consideration of the
W126 index averaged over three years (as described below), he recognized conceptual
44 Both the 2020 and 2013 ISAs reference the longstanding recognition of the risk posed to vegetation of
peak hourly O3 concentrations (e.g., "[h]igher concentrations appear to be more important than lower
concentrations in eliciting a response" [2020 ISA, p. 8-180]; "higher hourly concentrations have greater
effects on vegetation than lower concentrations" [2013 ISA, p. 91 -4] "studies published since the 2006
O3 AQCD do not change earlier conclusions, including the importance of peak concentrations,... in
altering plant growth and yield" [2013 ISA, p. 9-117]). While the evidence does not indicate a particular
threshold number of hours at or above 100 ppb (or another reference point for elevated
concentrations), the evidence of greater impacts from higher concentrations (particularly with increased
frequency) and the air quality analyses that document variability in such concentrations for the same
W126 index value led the Administrator to judge such a multipronged approach to be needed to
ensure appropriate consideration of exposures of concern and the associated protection from them
afforded by the secondary standard (85 FR 87340, December 31, 2020).
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similarities of the 3-year average W126 index to some aspects of the derivation
approach for the established E-R functions, and his use of the RBL as a proxy for other
effects (as recognized above). His consideration of peak hourly concentration metrics
(described below) related to his recognition of limitations associated with a reliance
solely on W126 index as a metric to control exposures that might be termed "unusually
damaging"45 (85 FR 877339-40, December 31, 2020).
In describing the focus on a 3-year average W126 index, the 2020 review
recognized that several factors associated with the derivation and application of the
established E-R functions contributed uncertainty and some resulting imprecision or
inexactitude to RBL estimated from single-year seasonal W126 index values, and that
our understanding, in many cases, of relationships of O3 effects on plant growth and
productivity with larger-scale impacts, such as those on populations, communities and
ecosystems is largely of a qualitative and conceptual nature (85 FR 49900-01, August 14,
2020; 2020 PA sections 4.5.1.2 and 4.5.3).46 Accordingly, the Administrator judged that
45 In its discussion regarding the EPA's use of a 3-year average W126 index, the 2019 court decision
remanding the 2015 standard back to the EPA referenced advice from the CASAC in the 2015 review on
protection against "unusually damaging years." Use of this term occurs in the 2014 CASAC letter on the
second draft PA (Frey, 2014b). Most prominently, the CASAC defined as damage "injury effects that
reach sufficient magnitude as to reduce or impair the intended use or value of the plant to the public,
and thus are adverse to public welfare" (Frey, 2014b, p. 9). We also note that the context for the
CASAC's use of the phrase "unusually damaging years" in the 2015 review is in considering the form
and averaging time for a revised secondary standard in terms of a W126 index (Frey, 2014b, p. 13),
which as discussed below is relatively less controlling of high-concentration years (whether as a single
year index or averaged over three years) than the current secondary standard and its fourth highest
daily maximum 8-hour metric (85 FR 87327, December 31, 2020).
45 The E-R functions were derived mathematically from studies of different exposure durations (varying
from shorter than one to multiple growing seasons) by applying adjustments so that they would yield
estimates normalized to the same period of time (season). Accordingly, the estimates may represent
average impact for a season, and have compatibility with W126 index averaged over multiple growing
seasons or years (85 FR 87326, December 31, 2020; 2020 PA, section 4.5.1.2, Appendix 4A, Attachment
1). The available information also indicated that the patterns of hourly concentrations (and frequency of
peak concentrations, e.g., at/above 100 ppb) in O3 treatments on which the E-R functions are based
differ from the patterns in ambient air meeting the current standard across the U.S. today (85 FR 87327,
December 31, 2020). Additionally noted was the year-to-year variability of factors other than O3
exposures that affect tree growth in the natural environment (e.g., related to variability in soil moisture,
meteorological, plant-related and other factors), that have the potential to affect O3 E-R relationships
(2020 ISA, Appendix 8, section 3.12; 2013 ISA section 9.4.8.3; PA, sections 4.3 and 4.5). All of these
considerations contributed to the finding of a consistency of the use of W126 index averaged over
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use of a seasonal RBL averaged over multiple years (e.g., 3-year average) is reasonable,
and provides a more stable and well-founded RBL estimate for its use as a proxy for the
array of vegetation-related effects identified above. More specifically, the Administrator
concluded that the use of an average seasonal W126 index derived from multiple years
(with their representation of variability in environmental factors) provides an appropriate
representation of the evidence and attention to the identified considerations, and that a
sole 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 evidence. Thus, the Administrator in 2020 found it appropriate, for purposes of
considering public welfare protection from effects for which RBL is used as a proxy, to
primarily consider W126 index in terms of a 3-year average metric (85 FR 87339-87340,
December 31, 2020).
In the context of his primary focus on RBL in its role as proxy for the broader
array of vegetation-related effects of O3, the Administrator further considered the
available analyses of air quality data at sites across the U.S., particularly including those
sites in or near Class I areas, which were consistent with the air quality analyses available
in the 2015 review.47 In virtually all design value periods between 2000 and 2018 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 (85 FR 87344, December 31,
2020).
In using a 3-year average W126 index to assess protection from RBL, the 2020
decision additionally took into account the 2019 court remand on this issue, including
the remand's reference to protection against "unusually damaging years" (85 FR 87325-
multiple years with the approach used in deriving the E-R function, and with other factors that may
affect growth in the natural environment (85 FR 87340, December 31, 2020).
47 These data 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 (2020 PA, Appendix 4D).
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87328, December 31, 2020). In this context, the 2020 decision also relied on
consideration of air quality analyses of peak hourly concentrations in the context of
controlling exposure circumstances of concern (e.g., for growth effects, among others).
More specifically, the EPA considered air quality analyses that investigated the annual
occurrence of elevated hourly O3 concentrations which may contribute to vegetation
exposures of concern (2020 PA, Appendix 2A, section 2A.2; Wells, 2020). In illustrating
limitations of the W126 index (whether in terms of a 3-year average or a single year) for
the purpose of controlling peak concentrations,48 and also the strengths of the current
standard in this regard, the air quality analyses show that the form and averaging time
of the existing standard controls cumulative exposures in terms of W126 and also is
much more effective than the W126 index in limiting peak concentrations (e.g., hourly
O3 concentrations at or above 100 ppb)49 and in limiting number of days with any such
hours (Wells, 2020, e.g., Figures 4, 5, 8, 9 compared to Figures 6, 7, 10 and 11).50 Thus,
the 2020 review found that the W126 index, by its very definition, and as illustrated by
the air quality data analyses, does not provide specificity with regard to year-to-year
variability in elevated hourly O3 concentrations with the potential to contribute to the
"unusually damaging years" that the CASAC had identified for increased concern in the
2015 review. As a result, the 2020 decision found that a standard based on a W126
index (either a 3-year or a single-year index) would not be expected to provide effective
control of the peak concentrations that may contribute to "unusually damaging years"
for vegetation, while control of such years is a characteristic of the existing standard.51 In
48 The W126 index cannot, by virtue of its definition, always differentiate between air quality patterns with
high peak concentrations and those without such concentrations.
49 As described in section 4.3.3 below, the occurrence of high concentrations (including those at or above
100 ppb [e.g., Smith, 2012; Smith et al., 2012]), as well as cumulative exposures influence the effects of
O3 on plants.
50 With regard to the existing standard, historical air quality data extending back to 2000 additionally
show the appreciable reductions in peak concentrations that have been achieved in the U.S. as air
quality has improved under O3 standards of the existing form and averaging time (Wells, 2020, Figures
12 and 13).
51 From these analyses, the Administrator concluded that the form and averaging time of the current
standard is effective in controlling peak hourly concentrations and that a W126 index based standard
would be much less effective in providing the needed protection against years with such elevated and
potentially damaging hourly concentrations.
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light of the air quality analyses and evidence of short-term risks to vegetation, the 2020
decision concluded that for considering the public welfare protection provided by the
standard, it is appropriate to consider use of a seasonal W126 averaged over a 3-year
period to estimate median RBL using the established E-R functions, in combination with
a broader consideration of the air quality pattern of peak hourly concentrations (85 FR
87340-87341, December 31, 2020).
Additionally, the Administrator concluded that the 0.07 ppm standard provides
adequate protection of the public welfare related to crop yield loss (85 FR 87342,
December 31, 2020). Key considerations in this conclusion included the established E-R
functions for 10 crops and the estimates of RYL derived from them (2020 ISA, 2020 PA,
Appendix 4A, section 4A.1, Table 4A-5), as well as the existence of 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 (85 FR 87341-87342,
December 31, 2020). With regard to RYL estimates for the 10 crops with established E-R
functions, the air quality analysis indicated that the current standard generally maintains
air quality at a W126 index below 17 ppm-hrs, with few exceptions, which would
accordingly limit the associated estimates of median RYL below 5.1% (based on
experimental O3 exposures), a level which the Administrator judged would not
constitute an adverse effect on public welfare. Therefore, the Administrator concluded
that the current standard provides adequate protection of public welfare related to crop
yield loss and did not need to be revised to provide additional protection against this
effect (85 FR 87342, December 31, 2020).
With regard to visible foliar injury and the question of a level of air quality that
would provide protection against visible foliar injury related effects known or
anticipated to cause adverse effects to the public welfare, the Administrator recognized
that there was a paucity of established approaches for interpreting specific levels of
severity and extent of foliar injury in natural areas with regard to impacts on the public
welfare (e.g., related to recreational services). The Administrator recognized that the
available information did not provide for specific characterization of the incidence and
severity that would not be expected to be apparent to the casual observer, nor for clear
identification of the pattern of O3 concentrations that would provide for such a
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situation. In 2020, the Administrator further considered the USFS system for interpreting
visible foliar injury impacts in its surveys across the U.S. More specifically, he concluded
that scores in the USFS system categorized as "moderate to severe" injury would be an
indication of visible foliar injury occurrence that, depending on extent and severity, may
raise public welfare concerns. In this framework, the Administrator noted the findings of
the 2020 PA evaluations that, the incidence of USFS scores classified as indicative of
"moderate to severe "injury in the USFS scheme appear to markedly increase only with
W126 index values above 25 ppm-hrs. He further took note of the multiple published
studies analyzing the USFS data across multiple years and multiple U.S. regions with
regard to metrics intended to quantify influential aspects of O3 air quality, which
indicated a potential role for an additional metric related to the occurrence of days with
relatively high hourly concentrations (e.g., number of days with a 1 -hour concentration
at or above 100 ppb [2020 PA, section 4.5.1.2]). In light of this evidence and the 2020 PA
analyses of these data, the Administrator judged that W126 index values at or below 25
ppm-hrs, when in combination with infrequent occurrences of hourly concentrations at
or above 100 ppb, would not be anticipated to pose risk of visible foliar injury of an
extent and severity so as to be adverse to the public welfare (85 FR 87343, December 31,
2020).
The Administrator further noted that the available air quality analyses that a
W126 index above 25 ppm-hrs (either as a 3-year average or in a single year) was not
seen to occur at monitoring locations where the current standard is met (including in or
near Class I areas), and that, in fact, values above 17 or 19 ppm-hrs are rare and that
days with any hourly concentrations at or above 100 ppb at monitoring sites that meet
the current standard are uncommon (85 FR 87316-18, December 31, 2020; 2020 PA,
Appendix 4C, section 4C.3; Appendix 4D; Wells, 2020). Based on these findings, the
Administrator concluded that the current standard provides control of air quality
conditions that contribute to USFS scores of a magnitude indicative of "moderate to
severe" foliar injury. In so doing, he also noted the 2020 PA finding that the information
from the USFS monitoring program, 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 or specific impacts on recreational
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or related services for areas, such as wilderness areas or national parks, such that, as
concluded by the 2020 PA the evidence indicates that areas that meet the current
standard are unlikely to have scores reasonably considered to be impacts of public
welfare significance (85 FR 87344, December 31, 2020).
With regard to the protection provided by the current standard from the
occurrence of O3 exposures within a single year with potentially damaging
consequences, including a significantly increased incidence of areas with visible foliar
injury that might be judged moderate to severe, the Administrator gave particular focus
to USFS scores termed "moderate to severe injury" (85 FR 87344, December 31, 2020;
2020 PA, sections 4.3.3.2, 4.5.1.2 and Appendix AC). As discussed above, the incidence of
USFS sites with scores above 15 markedly increases with W126 index estimates above 25
ppm-hrs, a magnitude of W126 index indicated by the air quality analysis to be scarce 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 (2020 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 took note of the air quality analyses of hourly concentrations (2020 PA,
Appendix 2A; Wells 2020). These analyses of data from the past 20 years show a
declining trend in 1-hour daily maximum concentrations mirroring the declining trend in
design values, and indicate that sites meeting the current standard had few days with
hourly concentrations at or above 100 ppb, supporting the 2020 PA conclusion that the
form and averaging time of the current standard provides appreciable control of peak 1 -
hour concentrations. In light of these findings from the air quality analyses and
considerations in the 2020 PA, 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 judged
that the current standard provides adequate protection from air quality conditions with
the potential to be adverse to the public welfare (85 FR 87344, December 31, 2020).
1-38
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2 THE CURRENT OZONE NAAQS REVIEW:
MILESTONES AND TIMELINE
In August 2023, EPA announced the initiation of the current periodic review of
the air quality criteria for O3 and related photochemical oxidants, and the O3 NAAQS and
issued a call for information in the Federal Register (88 FR 58264). The current review of
the O3 standards builds on the substantial body of work done during the course of prior
reviews, represented both in comprehensive science assessments (ISAs) and past
quantitative exposure and risk analyses. These different types of information, evaluated
in past policy assessments, provided the basis for decisions on the existing O3 NAAQS.
A wide range of external experts, as well as EPA staff representing a variety of
areas of expertise (e.g., epidemiology, controlled human exposure studies, animal
toxicology, ecology, statistics, biological, environmental, and physical sciences,
atmospheric and climate science, human exposure science, and risk analysis),
participated in a virtual workshop held by the EPA on May 13-16, 2024. The workshop
provided an opportunity for a public discussion of the key policy-relevant issues
associated with the review of the O3 NAAQS and the new science available to inform our
understanding of these issues52.
The timeline projected for the remainder of the current review is presented in
Table 4-1. Concurrent with the release of this background document (Volume 1 of the
IRP),53 the EPA is releasing the planning document for the review and the ISA, as Volume
2 of the IRP (U.S. EPA, 2024). Volume 2 identifies policy-relevant science issues
important to guiding the evaluation of the air quality criteria for O3 and the reviews of
the primary and secondary O3 NAAQS. It will be subject of a consultation with CASAC.
Based on consideration of input received during this consultation, the EPA will develop a
draft ISA for external review.
52 The proceedings document from the workshop are available at: https://assessments.epa.gov/risk/
document/&deid%3D362873.
53 In addition to providing an overview of the history of the criteria and standards for ozone and related
photochemical oxidants (chapter 1), this document also includes a summary of the monitoring and data
handling regulations, as well an overview of recent air quality and trends in the Appendix.
2-1
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With consideration of the newly available evidence identified in the draft ISA, the
EPA will develop the planning document for quantitative analyses, including
exposure/risk analyses, that might be warranted to inform decisions in the current
review. This planning document for quantitative analyses will comprise the third volume
of the IRP. With consideration of the CASAC review of the draft ISA and consultation
discussion on Volume 3 of the IRP, the EPA will develop a draft of the PA (with
associated policy evaluations and quantitative analyses) for public and CASAC review.
The timeline projects completion of the final ISA in 2027 and the final PA in 2028,
followed by proposed and final decisions in 2029.
Table 2-1. Projected timeline for the review of ambient air quality criteria and
NAAQS for Ozone.
Stage of
Review
Major Milestone
Target Dates*
Planning
Federal Register Call for Information
Workshop To Inform Review of the O3 NAAQS
Integrated Review Plan (IRP), volumes 1 and 2
CASAC consultation on IRP, volume 2
IRP, volume 3
CASAC consultation on IRP, volume 3
August 25, 2023
May 13-16, 2024
December 2024
February/March 2025
Spring 2027
Spring 2027
Science
Assessment
External review draft of ISA
CASAC public meeting for review of draft ISA
Final ISA
Early 2027
Spring 2027
Late 2027
Quantitative
Exposure/Risk
Analyses and
Policy
Assessment
External draft of PA (including quantitative air quality,
exposure and/or risk analyses, as warranted)
CASAC public meeting for review of draft PA
Final PA
Summer 2028
Summer 2028
Early 2029
Regulatory
Process
Notice of proposed decision
Notice of final decision
2029
2030
* Exact dates are given for milestones that have already occurred.
2-2
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Luben, T, Lassiter, M and Herrick, J (2020). Memorandum to Ozone NAAQS Review
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Samet, JM (2011). Letter from Jonathan Samet, Chair, Clean Air Scientific Advisory
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Schelegle, ES, Morales, CA, Walby, WF, Marion, S and Allen, RP. (2009). 6.6-hour
inhalation of ozone concentrations from 60 to 87 parts per billion in healthy
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Sheppard, EA (2022a). Letter from Elizabeth A. Sheppard Chair, Clean Air Scientific
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/sab/f?p=105:19:17341438189034::: 19:PI9 ID:972#materials.
Sheppard, EA (2022b). Letter from Elizabeth A. Sheppard Chair, Clean Air Scientific
Advisory Committee, to Administrator Michael S. Regan. Re: CASAC Review
Process for the National Ambient Air Quality Standards for Ozone. June 15, 2022.
EPA-CASAC-22-004. Available at: https://casac.epa.gov/ords/sab/f7p = 105:
18:15745403206599:::RP.18:P18 ID:2624#report.
Sheppard, EA. (2022c). Letter from Elizabeth A. Sheppard Chair, Clean Air Scientific
Advisory Committee, to Administrator Michael S. Regan. Re: CASAC Review of the
EPA's Integrated Science Assessment (ISA) for Ozone and Related Photochemical
Oxidants (Final Report - April 2020). November 22, 2022. EPA-CASAC-23-001.
Available at: https://casac.epa.gov/ords/sab/f?p = 105:18:8476900499267:::RP,
18:P18 ID:2614.
Sheppard, EA. (2023). Letter from Elizabeth A. Sheppard Chair, Clean Air Scientific
Advisory Committee, to Administrator Michael S. Regan. Re: CASAC Review of the
EPA's Policy Assessment (PA) for the Reconsideration of the Ozone National
Ambient Air Quality Standards (External Review Draft Version 2). June 9, 2023.
EPA-CASAC-23-002. Available at: https://casac.epa.goV/ords/sab/r/sab
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Smith, G (2012). Ambient ozone injury to forest plants in Northeast and North Central
USA: 16 years of biomonitoring. Environ Monit Assess(184): 4049-4065.
Smith, GC, Morin, RS and McCaskill, GL. (2012). Ozone injury to forests across the
Northeast and North Central United States, 1994-2010. General Technical Report
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Thurston, GD, Kipen, H, Annesi-Maesano, I, Balmes, J, Brook, RD, Cromar, K, De Matteis,
S, Forastiere, F, Forsberg, B, Frampton, MW, Grigg, J, Heederik, D, Kelly, FJ,
Kuenzli, N, Laumbach, R, Peters, A, Rajagopalan, ST, Rich, D, Ritz, B, Samet, JM,
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U.S. EPA (1978). Air Quality Criteria for Ozone and Other Photochemical Oxidants
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600/8-78-004. April 1978. Available at: https://nepis.epa.aov/Exe/ZvPURL.cai?
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U.S. EPA (1986). Air Quality Criteria for Ozone and Other Photochemical Oxidants
(Volume I - V). Environmental Criteria and Assessment Office. Research Triangle
Park, NC. U.S. EPA. EPA-600/8-84-020aF, EPA-600/8-84-020bF, EPA-600/8-84-
020cF, EPA-600/8-84-020dF, EPA-600/8-84-020eF. August 1986. Available at:
https://nepis.epa.gov/Exe/ZvPU RL.cgi?Dockev=30001 D3J.txt
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Assessment of Scientific and Technical Information. OAQPS Staff Paper. Office of
Air Quality Planning and Standards. Research Triangle Park, NC U.S. EPA.
U.S. EPA (1992). Summary of Selected New Information on Effects of Ozone on Health
and Vegetation: Supplement to 1986 Air Quality Criteria for Ozone and Other
Photochemical Oxidants. Office of Research and Development. Washington, DC.
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Volume I - III. Office of Research and Development Research Triangle Park, NC.
U.S. EPA. EPA-600/P-93-004aF, EPA-600/P-93-004bF, EPA-600/P-93-004cF. July
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U.S. EPA (1996b). Review of national ambient air quality standards for ozone:
Assessment of scientific and technical information: OAQPS staff paper. Office of
Air Quality Planning and Standards. Research Triangle Park, NC. U.S. EPA. EPA-
452/R-96-007. June 1996. Available at: http://nepis.epa.gov/Exe/ZvPDF.cgi?
Dockev=2000DKJT.PDF.
U.S. EPA (2006). Air Quality Criteria for Ozone and Related Photochemical Oxidants
(Volume I - III). Office of Research and Development U.S. EPA. EPA-600/R-05-
004aF, EPA-600/R-05-004bF, EPA-600/R-05-004cF February 2006. Available at:
https://cfpub.epa.gov/ncea/risk/recordisplav.cfm7deid = 149923.
U.S. EPA (2007). Review of the National Ambient Air Quality Standards for Ozone: Policy
Assessment of Scientific and Technical Information: OAQPS Staff Paper. Office of
Air Quality Planning and Standards. Research Triangle Park, NC. U.S. EPA. EPA-
452/R-07-003. January 2007. Available at: https://nepis.epa.gov/Exe/ZvPURL.cgi?
Dockev= P10083VX.txt.
U.S. EPA (2013). Integrated Science Assessment of Ozone and Related Photochemical
Oxidants (Final Report). Office of Research and Development, National Center for
Environmental Assessment. Research Triangle Park, NC. U.S. EPA. EPA-600/R-10-
076F. February 2013. Available at: https://nepis.epa.gov/Exe/ZvPURL.cgi?
Dockev= P100KETF.txt.
U.S. EPA (2014a). Policy Assessment for the Review of National Ambient Air Quality
Standards for Ozone (Final Report). Office of Air Quality Planning and Standards,
Health and Environmental Impacts Divison. Research Triangle Park, NC. U.S. EPA.
EPA-452/R-14-006 August 2014. Available at: https://nepis.epa.gov/Exe/ZvPDF.
cgi?Dockev= P100KCZ5.txt.
U.S. EPA (2014b). Welfare Risk and Exposure Assessment for Ozone (Final). Office of Air
Quality Planning and Standards. Research Triangle Park, NC. U.S. EPA. EPA-452/P-
14-005a August 2014. Available at: https://nepis.epa.gov/Exe/ZvPURL.cgi?
Dockev= P100KB9D.txt.
U.S. EPA (2014c). Health Risk and Exposure Assessment for Ozone. (Final Report). Office
of Air Quality Planning and Standards. Research Triangle Park, NC. U.S. EPA. EPA-
452/R-14-004a. August 2014. Available at: https://nepis.epa.gov/Exe/ZvPURL.
cgi?Dockev= P100KBUF.txt.
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U.S. EPA (2019). Integrated Review Plan for the Ozone National Ambient Air Quality
Standards. Office of Air Quality Planning and Standards. Research Triangle Park,
NC. U.S. EPA. EPA-452/R-19-002. Available at: https://www.epa.gov/sites/
production/files/2019-08/documents/o3-irp-aug27-2019 final.pdf.
U.S. EPA (2020a). Integrated Science Assessment for Ozone and Related Photochemical
Oxidants. U.S. Environmental Protection Agency. Washington, DC. Office of
Research and Development. EPA/600/R-20/012. Available at: https://www.epa.
gov/isa/integrated-science-assessment-isa-ozone-and-related-photochemical-
oxidants.
U.S. EPA (2020b). Policy Assessment for the Review of the Ozone National Ambient Air
Quality Standards. Office of Air Quality Planning and Standards, Health and
Environmental Impacts Division. Research Triangle Park, NC. U.S. EPA. EPA-
452/R-20-001. 2020 Available at: https://www.epa.gov/ naaqs/ozone-o3-
standards-policvassessments-current-review.
U.S. EPA (2022). Policy Assessment for the Reconsideration of the Ozone National
Ambient Air Quality Standards, External Review Draft. Office of Air Quality
Planning and Standards Health and Environmental Impacts Division Research
Triangle Park, NC. U.S. EPA. EPA-452/D-22-002. Available at: https://www.epa.
gov/naaqs/ozone-o3-standards-policv-assessments-current-review.
U.S. EPA (2023). Policy Assessment for the Reconsideration of the Ozone National
Ambient Air Quality Standards, External Review Draft Version 2. Office of Air
Quality Planning and Standards Health and Environmental Impacts Division
Research Triangle Park, NC. U.S. EPA. EPA-452/P-23-002. 2022 Available at:
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PA Marl-2023 ERDcmp O.pdf.
U.S. EPA (2024). Integrated Review Plan for the Ozone National Ambient Air Quality
Standards, Volume 2. Office of Air Quality Planning and Standards. Research
Triangle Park, NC. EPA-452/R-24-001b. Available at: https://www.epa.gov/
naaqs/ozone-o3-air-qualitv-standards
Wells, B. (2015). Memorandum to Ozone NAAQS Review Docket (EPA-HQ-OAR-2008-
0699). Expanded Comparison of Ozone Metrics Considered in the Current NAAQS
Review. September 28, 2015. Docket Document Identifier EPA-HQ-OAR-2008-
0699-0163. Available at: https://www.regulations.gov/contentStreamer?
documentld = EPA-HQ-QAR-2008-0699-4325&contentTvpe=pdf.
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Wells, B. (2020). Memorandum to Ozone NAAQS Review Docket (EPA-HQ-OAR-2018-
0279). Additional Analyses of Ozone Metrics Related to Consideration of the
Ozone Secondary Standard. December 2020. Docket Document Identifier EPA-
HQ-OAR-2018-0279-0557. Available at: https://www.reaulations.gov/document/
EPA-HQ-QAR-2018-0279-0557.
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APPENDIX AMBIENT AIR MONITORING AND DATA
HANDLING
Ozone is a gas composed of three oxygen atoms (O3). It is naturally present in the
Earth's atmosphere, both in the stratospheric layer occurring roughly 10 to 30 miles
above the Earth's surface as well as in the closer tropospheric layer. The stratosphere
contains a large reservoir of O3 (i.e. the "ozone layer") that results naturally from
photochemical reactions between ultraviolet light (UV) and molecular oxygen (O2).1
Under specific meteorological conditions, this reservoir can contribute to O3
concentrations at the Earth's surface (Langford et al., 2017). Ozone is also produced near
the earth's surface due to chemical interactions involving solar radiation and pollution
resulting from human activity. These chemical reactions involve specific O3 precursors,
such as nitrogen oxides (NOx), volatile organic compounds (VOCs), carbon monoxide
(CO) and methane (CH4), which can be emitted from both natural and anthropogenic
sources.2
The EPA established O3 as the indicator for the NAAQS for photochemical
oxidants in 1979. Prior to 1979, the indicator for the NAAQS for photochemical oxidants
was total photochemical oxidants. Early ambient air monitoring indicated similarities
between O3 measurements and the photochemical oxidant measurements, as well as
reduced precision and accuracy of the latter. Ozone is currently the only photochemical
oxidant other than nitrogen dioxide that is routinely monitored in a national ambient air
monitoring network.
The EPA and State and local agencies have been measuring O3 in the atmosphere
for decades. Ambient air O3 concentrations are measured in several national networks.
These include the state and local air monitoring stations (SLAMS network) intended for
O3 NAAQS surveillance, the photochemical assessment monitoring stations (PAMs),
1 This layer of O3 in the upper atmosphere helps to protect the earth's populations and ecosystems from
the damaging effects of UV radiation (Norval et al., 2011; Bais et al., 2017).
2 Impacts from methane emissions on O3 formation are generally observed at the global scale over longer
time periods (e.g., decadal scale) while impacts from NOx and VOCs may occur over shorter temporal
timescales (days to weeks) and over a variety of spatial scales (urban up to global).
A-1
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national core (NCore) monitoring sites, the clean air status and trends network
(CASTNET) monitors, and special purpose monitoring. The data from these networks are
accessible via EPA's Air Quality System (AQS): http://www.epa.gov/ttn/airs/airsaqs/.
There were 1,287 monitoring sites reporting hourly O3 concentration data to the
EPA during the 2021-2023 period (Figure A-1). Nearly 80% of this network are SLAMS
monitors operated by state and local governments to meet regulatory requirements and
provide air quality information to public health agencies; these sites are largely focused
on urban and suburban areas.
Federal regulations specify requirements for the data collection and calculations
performed to assess whether the O3 NAAQS are met. This appendix describes the
ambient air O3 measurement methods, the sites and networks where these
measurements are made, and the data handling conventions and computations.
A.1. STATE AND LOCAL AIR MONITORING STATIONS NETWORK
This section describes the monitoring O3 monitoring requirements for the SLAMS
network, the main purpose of which is surveillance for the O3 NAAQS. The EPA regulates
how this monitoring is conducted to ensure accurate and comparable data for
determining compliance with the NAAQS. The code of federal regulations (CFR) at parts
50, 53, and 58 specifies required aspects of the ambient air monitoring program for
NAAQS pollutants.3
A.1.1.Sampling and Analysis Methods
In order to be used in NAAQS attainment designations, ambient air O3
concentration data must be obtained using either the Federal Reference Method (FRM)
or a Federal Equivalent Method (FEM). In recent years, about 99% of the state, local, and
tribal air monitoring stations that report data to the EPA use ultraviolet FEMs. The FRM
was revised in 2015 to include a new chemiluminescence by nitric oxide (NO-CL)
method (40 CFR Part 50, Appendix D). The previous ethylene (ET-CL) method, while still
3 The Federal Reference Methods (FRMs) for sample collection and analysis are specified in 40 CFR Part 50,
the procedures for approval of FRMs and Federal Equivalent Methods (FEMs) are specified in 40 CFR
Part 53, and the rules specifying requirements for the planning and operations of the ambient
monitoring network are specified in 40 CFR Part 58.
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included in the CFR as an acceptable method, is no longer used due to lack of
availability and safety concerns with ethylene.
In 2023, the EPA updated a standard parameter used to measure concentrations
of O3 in ambient air (40 CFR Part 50, Appendix D). This parameter, called the absorption
cross-section value, is used in ultraviolet-based O3 analyzers and Standard Reference
Photometers (SRPs). The new value reflects advances in science and measurement
technology and is more accurate and precise than the value established in 1961. An
international group reviewed absorption cross-section measurements in 2019 and
reached consensus on an updated value, which will be implemented worldwide
beginning in 2025. The new absorption cross-section value will improve the accuracy of
surface O3 monitoring measurements and reduce the uncertainty in measured O3
concentrations.
A. 1.2. Network Requirements
The requirements for the SLAMS network depend on the population and most
recent O3 design values4 in an area. The minimum number of O3 monitors required in a
metropolitan statistical area (MSA) ranges from zero for areas with a population less
than 350,000 and no recent history of an O3 design value greater than 85 percent of the
level of the standard, to four monitors for areas with a population greater than 10
million and an O3 design value greater than 85 percent of the standard level.5 At least
one monitoring site for each MSA must be situated to record the maximum
concentration for that particular metropolitan area.
Siting criteria for SLAMS includes horizontal and vertical inlet probe placement;
spacing from minor sources, obstructions, trees, and roadways; inlet probe material; and
4 A design value is a statistic that summarizes the air quality status of a given area relative to the level of
the standard, taking the averaging time and form into account, as well as any data handling
requirements (e.g., for the 2015 O3 NAAQS, these requirements are specified in Appendix U to 40 CFR
Part 50). Design values are typically used to classify nonattainment areas as meeting or not meeting the
standard, to assess progress towards meeting the NAAQS, and to develop control strategies.
5 The SLAMS minimum monitoring requirements to meet the O3 design criteria are specified in 40 CFR
Part 58, Appendix D. The minimum O3 monitoring network requirements for urban areas are listed in
Table D-2 of Appendix D to 40 CFR Part 58 (accessible at https://www.ecfr.gov).
A-3
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sample residence times.6 Adherence to these criteria ensures uniform collection and
comparability of O3 data. Since the highest O3 concentrations tend to be associated with
a particular season for various locations, the EPA requires O3 monitoring during specific
O3 monitoring seasons (shown in Figure A-2) which 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).7
Ambient air quality data and associated quality assurance (QA) data are reported
to the EPA via the AQS, as required by 40 CFR 58.16 and summarized here. Data are
reported quarterly and must be submitted to AQS within 90 days after the end of the
quarterly reporting period. Each monitoring agency is required to certify data that is
submitted to AQS from the previous year. The data are certified, taking into
consideration any QA findings, and a data certification letter is sent to the EPA Regional
Administrator. Data must be certified by May 1st of the following year. Data collected by
FRM or FEM monitors that meet the QA requirements must be certified as meeting the
QA criteria for use in assessing NAAQS attainment (40 CFR 58.15). The estimates of both
precision and bias are derived from quality control (QC) checks using calibration gas,
performed at each site by the monitoring agency. The data quality goal for precision
and bias is 7 percent.8
5 The probe and monitoring path siting criteria for ambient air quality monitoring are specified in 40 CFR,
Part 58, Appendix E.
7 The required O3 monitoring seasons for each state are listed in 40 CFR Part 58, Appendix D, Table D-3.
8 Quality assurance requirements for monitors used in evaluations of the NAAQS are provided in 40 CFR
Part 58, Appendix A. Annual summary reports of precision and bias can be obtained for each
monitoring site at the EPA's Air Data website: https://www.epa.aov/outdoor-air-quality-data/sinale-
point-precision-and-bias-report.
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O SUMS (926)
Monitoring Network (# Sites)
NCORE/PAMS (123) ~ CASTNET (86) A SPM/OTHER (112) T TRIBAL (40)
Figure A-1. Map of U.S. O3 monitoring sites reporting data to the EPA during the
2021-2023 period. Source: AQS.
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1
2
3 Figure A-2. Current O3 monitoring seasons in the U.S. Numbers in each state indicate the months of the year the state
4 is required to monitor for O3 (e.g., 3-10 means O3 monitoring is required from March through October).
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Two important subsets of SLAMS are the NCore stations and PAMS. The NCore
sites feature co-located measurements of chemical species such as nitrogen oxide (NOx)
and total reactive nitrogen (NOy), along with meteorological measurements. The
additional data collected at the PAMS sites include measurements of NOx, a target set
of VOCs, and meteorological measurements. The enhanced monitoring at sites in these
two networks informs our understanding of local O3 formation.
A.2. OTHER NETWORKS MONITORING Os
While the SLAMS network has a largely urban and population-based focus, there
are monitoring sites in other networks that can be used to track compliance with the
NAAQS in rural areas. For example, the Clean Air Status and Trends Network (CASTNET)
monitors are located in rural areas. There were 86 CASTNET monitors operating during
the 2021-2023 period, with most of the sites in the eastern U.S. being operated by the
EPA, and most of the sites in the western U.S. being operated by the National Park
Service (NPS).
Additionally, there are also a number of Special Purpose Monitoring Stations
(SPMs), which are not required but are often operated by air agencies for short periods
of time (less than 3 years) to collect data for human health and welfare studies, as well
as other types of monitoring sites, including monitors operated by tribes and industrial
sources. The SPMs are typically not used to assess compliance with the NAAQS.62
A.3. DATA HANDLING CONVENTIONS AND COMPUTATIONS FOR
DETERMINING WHETHER STANDARDS ARE MET
To assess whether a monitoring site or geographic area (usually a county or
urban area) meets or exceeds a NAAQS, the monitoring data are analyzed consistent
with the established regulatory requirements for the handling of monitoring data for the
purposes of deriving a design value. A design value summarizes ambient air
concentrations for an area in terms of the indicator, averaging time, and form for a
52 However, SPMs that use FEMs or FRMs, meet all applicable requirements in 40 CFR Part 58, and operate
continuously for more than 24 months may be used to assess compliance with the NAAQS (40 CFR
58.20(c)). If an SPM using an FRM or FEM is discontinued within 24 months of start-up, a NAAQS
violation determination for O3 NAAQS will not be based solely on data from the SPM (40 CFR 58.20(d)).
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given standard, such that its comparison to the level of the standard indicates whether
the area meets or exceeds the standard. The procedures for calculating design values for
the current O3 NAAQS (established in 2015) are detailed in Appendix U to 40 CFR Part
50 and are summarized below.
Hourly average O3 concentrations at the monitoring sites used for assessing
whether an area meets or exceeds the NAAQS are required to be reported in ppm to the
third decimal place, with additional digits truncated, consistent with the typical
measurement precision associated with most O3 monitoring instruments. The hourly
concentrations are used to compute moving 8-hour averages, which are stored in the
first hour of each 8-hour period (e.g., the 8-hour average for the 7:00 AM to 3:00 PM
period is stored in the 7:00 AM hour), and digits to the right of the third decimal place
are truncated. Each 8-hour average is considered valid if 6 or more hourly
concentrations are available for the 8-hour period.
Next, the daily maximum 8-hour average (MDA8) concentration for each day is
identified as the highest of the 17 consecutive, valid 8-hour average concentrations
beginning at 7:00 AM and ending at 11:00 PM (which includes hourly O3 concentrations
from the subsequent day). MDA8 values are considered valid if at least 13 valid 8-hour
averages are available for the day, or if the M DA8 value is greater than the level of the
NAAQS. Finally, the O3 design value is calculated as the annual fourth highest MDA8
value averaged over three consecutive years63. An O3 design value less than or equal to
the level of the NAAQS is considered to be valid if valid MDA8 values are available for at
least 90% of the days in the O3 monitoring season (as defined for each state and shown
in Figure A-1) on average over the 3 years, with a minimum of 75% data completeness
in any individual year. Design values greater than the level of the NAAQS are always
considered to be valid.
An O3 monitoring site meets the NAAQS if it has a valid design value less than or
equal to the level of the standard, and it exceeds the NAAQS if it has a design value
greater than the level of the standard. A geographic area meets the NAAQS if all
63 Design values are reported in ppm to the third decimal place, with additional digits truncated. This
truncation step also applies to the initially calculated 8-hour average concentrations (2020 PA,
Appendix 2A, section 2A.1).
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ambient air monitoring sites in the area have valid design values meeting the standard.
Conversely, if one or more monitoring sites has a design value exceeding the standard,
then the area exceeds the NAAQS.
As discussion in section A.1, to assess O3 concentrations across the U.S., state and
local environmental agencies submit the monitoring data to the EPA for analyses. Each
year EPA calculates and makes available the air quality design values to the public
(available here: https://www.epa.aov/air-trends/air-qualitv-design-values).64 Figure A-3
is a map of the most recent O3 design values at U.S. ambient air monitoring sites based
on data from the 2021-2023 period, that shows many monitoring sites with design
values exceeding the current NAAQS, with most of these located in or near urban areas.
Overall, concentrations of O3 in the U.S. have trended downward over the past several
decades. The U.S. median design value decreased by 23% from 2000 (86 ppb) to 2023
(66 ppb) (Figure A-4).
54 EPA also publishes an overview of O3 air quality in the U.S. with up-to-date graphical summaries of air
quality information that supports the review of the NAAQS for O3 (available here:
https://www.epa.aov/air-qualitv-analvsis/ozone-naaqs-review-analvses-and-data-sets).
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• 40-60 ppb (171 sites) O 66 -70 ppb (339 sites) • 76-84 ppb (75 sites)
O 61-65 ppb (334 sites) O 71 - 75 ppb (154 sites) ® 85- 106 ppb (20 sites)
Figure A-3. O3 design values in ppb for the 2021-2023 period. Source: AQS.
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O^CMrttmtONCOO)OT-CM(OtK)lDNCOO)OT-(Mr)
OOOOOOOOOO^- ¦"!— t— -^r— r— t— CvJCMCvJCM
OOOOOOOOOOOOOOOOOOOOOOOO
(MCMOJCMCMCNJCMCMCNICMCMCMCMCMCvICMCMCMCMCMCnJCMCMCM
Figure A-4. National trend in O3 design values in ppb, 2000 to 2023. Source: AQS.
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A.4. REFERENCES
Bais AF, Lucas RM, Bornman JF, Williamson CE, Sulzberger B, Austin AT, Wilson SR,
Andrady AL, Bernhard G, McKenzie RL, Aucamp PJ, Madronich S, Neale RE, Yazar
S, Young AR, de Gruijl FR, Norval M, Takizawa Y, Barnes PW, Robson TM,
Robinson SA, Ballare CL, Flint SD, Neale PJ, Hylander S, Rose KC, Wangberg SA,
Hader DP, Worrest RC, Zepp RG, Paul ND, Cory RM, Solomon KR, Longstreth J,
Pandey KK, Redhwi HH, Torikai A, and AM Heikkila. (2018). Environmental effects
of ozone depletion, UV radiation and interactions with climate change: UNEP
Environmental Effects Assessment Panel, update 2017. Photochem Photobiol Sci.
17:127-179.
Langford AO, Alvarez RJ, Brioude J, Fine R, Gustin MS, Lin MY, Marchbanks RD, Pierce
RB, Sandberg SP, Senff CJ, Weickmann AM, and EJ Williams. (2017). Entrainment
of stratospheric air and Asian pollution by the convective boundary layer in the
southwestern U.S. J Geophys Res Atmos. 122:1312-1337.
Norval M, Lucas RM, Cullen AP, de Gruijl FR, Longstreth J, Takizawa Y and JC van der
Leun. (2011). The human health effects of ozone depletion and interactions with
climate change. Photochem Photobiol Sci. 10:199-225.
U.S. EPA (2020). Policy Assessment for the Review of the Ozone National Ambient Air
Quality Standards. Office of Air Quality Planning and Standards, Health and
Environmental Impacts Division. Research Triangle Park, NC. U.S. EPA. EPA-
452/R-20-001. 2020 Available at: https://www.epa.gov/ naaqs/ozone-o3-standards-
policyassessments-current-review.
U.S. EPA (2024). Overview of Ozone (O3) Air Quality in the United States, based on data
through 2023. Available at: https://www.epa.gov/air-qualitv-analysis/ozone-naaqs-
review-analyses-and-data-sets/.
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United States Office of Air Quality Planning and Standards Publication No. EPA-452/R-24-001a
Environmental Protection Health and Environmental Impacts Division December 2024
Agency Research Triangle Park, NC
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