87256 Federal Register / Vol. 85, No. 251 / Thursday, December 31, 2020 / Rules and Regulations 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Part 50 

[EPA–HQ–OAR–2018–0279; FRL–10019–04– 

OAR] 

RIN 2060–AU40 

Review of the Ozone National Ambient 

Air Quality Standards 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Final action. 

SUMMARY: Based on the Environmental 

Protection Agency’s (EPA’s) review of 

the air quality criteria and the national 

ambient air quality standards (NAAQS) 

for photochemical oxidants including 

ozone (O3), the EPA is retaining the 

current standards, without revision. 

DATES: This final action is effective 

December 31, 2020. 

ADDRESSES: The EPA has established a 

docket for this action under Docket ID 

No. EPA–HQ–OAR–2018–0279. 

Incorporated into this docket is a 

separate docket established for the 

Integrated Science Assessment for this 

review (Docket ID No. EPA–HQ–ORD– 

2018–0274). All documents in these 

dockets are listed on the 

www.regulations.gov website. Although 

listed in the index, some information is 

not publicly available, e.g., Confidential 

Business Information (CBI) or other 

information whose disclosure is 

restricted by statute. Certain other 

material, such as copyrighted material, 

is not placed on the internet and will be 

publicly available only in hard copy 

form. With the exception of such 

material, publicly available docket 

materials are available electronically 

through https://www.regulations.gov/. 

Out of an abundance of caution for 

members of the public and our staff, the 

EPA Docket Center and Reading Room 

are closed to the public, with limited 

exceptions, to reduce the risk of 

transmitting COVID–19. Our Docket 

Center staff will continue to provide 

remote customer service via email, 

phone, and webform. For further 

information on EPA Docket Center 

services and the current status, please 

visit us online at https://www.epa.gov/ 

dockets. 

Availability of Information Related to 

This Action 

A number of the documents that are 

relevant to this action are available 

through the EPA’s website at https://

www.epa.gov/naaqs/ozone-o3-airquality-standards. These documents 

include the Integrated Review Plan for 

the Ozone National Ambient Air Quality 

Standards (IRP [U.S. EPA, 2019b]), 

available at https://www.epa.gov/naaqs/ 

ozone-o3-standards-planningdocuments-current-review, the 

Integrated Science Assessment for 

Ozone and Related Photochemical 

Oxidants (ISA [U.S. EPA, 2020a]), 

available at https://www.epa.gov/naaqs/ 

ozone-o3-standards-integrated-scienceassessments-current-review, the Policy 

Assessment for the Review of the Ozone 

National Ambient Air Quality Standards 

(PA [U.S. EPA, 2020b]), available at 

https://www.epa.gov/naaqs/ozone-o3- 

standards-policy-assessments-currentreview. These and other related 

documents are also available for 

inspection and copying in the EPA 

docket identified above. 

FOR FURTHER INFORMATION CONTACT: Dr. 

Deirdre Murphy, Health and 

Environmental Impacts Division, Office 

of Air Quality Planning and Standards, 

U.S. Environmental Protection Agency, 

Mail Code C504–06, Research Triangle 

Park, NC 27711; telephone: (919) 541– 

0729; fax: (919) 541–0237; email: 

murphy.deirdre@epa.gov. 

SUPPLEMENTARY INFORMATION: 

Basis for Immediate Effective Date 

In accordance with section 

307(d)(1)(V), the Administrator has 

designated this action as being subject 

to the rulemaking procedures in section 

307(d) of the Clean Air Act (CAA). 

Section 307(d)(1) of the CAA states that: 

‘‘The provisions of section 553 through 

557 * * * of Title 5 shall not, except as 

expressly provided in this subsection, 

apply to actions to which this 

subsection applies.’’ Thus, section 

553(d) of the Administrative Procedure 

Act (APA), which requires publication 

of a substantive rule to be made ‘‘not 

less than 30 days before its effective 

date’’ subject to limited exceptions, does 

not apply to this action. In the 

alternative, the EPA concludes that it is 

consistent with APA section 553(d) to 

make this action effective December 31, 

2020. 

Section 553(d)(3) of the APA, 5 U.S.C. 

553(d)(3), provides that final rules shall 

not become effective until 30 days after 

publication in the Federal Register 

‘‘except . . . as otherwise provided by 

the agency for good cause found and 

published with the rule.’’ ‘‘In 

determining whether good cause exists, 

an agency should ‘balance the necessity 

for immediate implementation against 

principles of fundamental fairness 

which require that all affected persons 

be afforded a reasonable amount of time 

to prepare for the effective date of its 

ruling.’’ Omnipoint Corp. v. Fed. 

Commc’n Comm’n, 78 F.3d 620, 630 

(D.C. Cir. 1996) (quoting United States 

v. Gavrilovic, 551 F.2d 1099, 1105 (8th 

Cir. 1977)). The purpose of this 

provision is to ‘‘give affected parties a 

reasonable time to adjust their behavior 

before the final rule takes effect.’’ Id.; 

see also Gavrilovic, 551 F.2d at 1104 

(quoting legislative history). 

The EPA is determining that in light 

of the nature of this action, good cause 

exists to make this final action effective 

immediately because the Agency seeks 

to provide regulatory certainty as soon 

as possible and the Administrator’s 

decision to retain the current NAAQS 

does not change the status quo or 

impose new obligations on any person 

or entity. As a result, there is no need 

to provide parties additional time to 

adjust their behavior, and no person 

will be harmed by making the action 

immediately effective as opposed to 

delaying the effective date by 30 days. 

Accordingly, the EPA is making this 

action effective immediately upon 

publication. 

Table of Contents 

The following topics are discussed in 

this preamble: 

Executive Summary 

I. Background 

A. Legislative Requirements 

B. Related O3 Control Programs 

C. History of the Air Quality Criteria and 

O3 Standards 

D. Current Review of the Air Quality 

Criteria and O3 Standards 

E. Air Quality Information 

II. Rationale for Decision on the Primary 

Standard 

A. Introduction 

1. Background on the Current Standard 

2. Overview of Health Effects Information 

3. Overview of Exposure and Risk 

Information 

B. Conclusions on the Primary Standard 

1. Basis for the Proposed Decision 

2. Comments on the Proposed Decision 

3. Administrator’s Conclusions 

C. Decision on the Primary Standard 

III. Rationale for Decision on the Secondary 

Standard 

A. Introduction 

1. Background on the Current Standard 

2. Overview of Welfare Effects Information 

3. Overview of Air Quality and Exposure 

Information 

B. Conclusions on the Secondary Standard 

1. Basis for the Proposed Decision 

2. Comments on the Proposed Decision 

3. Administrator’s Conclusions 

C. Decision on the Secondary Standard 

IV. Statutory and Executive Order Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review and Executive 

Order 13563: Improving Regulation and 

Regulatory Review 

B. Executive Order 13771: Reducing 

Regulations and Controlling Regulatory 

Costs 


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C. Paperwork Reduction Act (PRA) 

D. Regulatory Flexibility Act (RFA) 

E. Unfunded Mandates Reform Act 

(UMRA) 

F. Executive Order 13132: Federalism 

G. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

H. Executive Order 13045: Protection of 

Children From Environmental Health 

and Safety Risks 

I. Executive Order 13211: Actions That 

Significantly Affect Energy Supply, 

Distribution or Use 

J. National Technology Transfer and 

Advancement Act 

K. Executive Order 12898: Federal Actions 

To Address Environmental Justice in 

Minority Populations and Low-Income 

Populations 

L. Determination Under Section 307(d) 

M. Congressional Review Act 

V. References 

Executive Summary 

This document presents the 

Administrator’s decisions in the current 

review of the primary (health-based) 

and secondary (welfare-based) O3

NAAQS, to retain the current standards, 

without revision. In reaching these 

decisions, the Administrator has 

considered the currently available 

scientific evidence in the ISA, 

quantitative and policy analyses 

presented in the PA, advice from the 

Clean Air Scientific Advisory 

Committee (CASAC), and public 

comments on the proposed decision. 

This document provides background 

and summarizes the rationale for these 

decisions. 

This review of the O3 standards, 

required by the Clean Air Act (CAA) on 

a periodic basis, was initiated in 2018. 

In the last review, completed in 2015, 

the EPA significantly strengthened the 

primary and secondary O3 standards by 

revising the level of both standards from 

75 parts per billion (ppb) to 70 ppb and 

retaining their indicators (O3), forms 

(annual fourth-highest daily maximum, 

averaged across three consecutive years) 

and averaging times (eight hours) (80 FR 

65291, October 26, 2015). In subsequent 

litigation on the 2015 decisions, the U.S. 

Court of Appeals for the District of 

Columbia Circuit (D.C. Circuit) upheld 

the 2015 primary standard but 

remanded the 2015 secondary standard 

to the EPA for further justification or 

reconsideration. The court’s remand of 

the secondary standard has been 

considered in reaching the decision 

described in this document on this 

standard, and in associated conclusions 

and judgments, also described here. 

Accordingly, this decision incorporates 

the EPA’s response to the judicial 

remand of the 2015 secondary standard. 

In this review as in past reviews of the 

air quality criteria and NAAQS for O3

and related photochemical oxidants, the 

health and welfare effects evidence 

evaluated in the ISA is focused on O3. 

Ozone is the most prevalent 

photochemical oxidant in the 

atmosphere and the one for which there 

is a large body of scientific evidence on 

health and welfare effects. A component 

of smog, O3 in ambient air is a mixture 

of mostly tropospheric O3 and some 

stratospheric O3. Tropospheric O3 forms 

in the atmosphere when emissions of 

precursor pollutants, such as nitrogen 

oxides and volatile organic compounds 

(VOCs), interact with solar radiation. 

Such emissions result from man-made 

sources (e.g. motor vehicles and power 

plants) and natural sources (e.g. 

vegetation and wildfires). In addition, 

O3 that is created naturally in the 

stratosphere also mixes with 

tropospheric O3 near the tropopause, 

and, less frequently can mix nearer the 

earth’s surface. 

Based on the current health effects 

evidence and quantitative information, 

as well as consideration of CASAC 

advice and public comment, the 

Administrator concludes that the 

current primary standard is requisite to 

protect public health, including the 

health of at-risk populations, with an 

adequate margin of safety, and should 

be retained, without revision. This 

decision has been informed by key 

aspects of the health effects evidence 

newly available in this review, in 

conjunction with the full body of 

evidence critically evaluated in the ISA, 

that continues to support prior 

conclusions that short-term O3 exposure 

causes and long-term O3 exposure is 

likely to cause respiratory effects. The 

strongest evidence continues to come 

from studies of short- and long-term O3

exposure and an array of respiratory 

health effects, including effects related 

to asthma exacerbation in people with 

asthma, particularly children with 

asthma. The clearest evidence comes 

from controlled human exposure 

studies, available at the time of the last 

review, of individuals exposed for 6.6 

hours during quasi-continuous exercise, 

that report an array of respiratory 

responses including lung function 

decrements and respiratory symptoms. 

Epidemiologic studies additionally 

describe consistent, positive 

associations between O3 exposures and 

hospital admissions and emergency 

department visits, particularly for 

asthma exacerbation in children. 

Populations and lifestages at risk 

include people with asthma, children, 

the elderly, and outdoor workers. The 

quantitative analyses of population 

exposure and risk, as well as policy 

considerations in the PA, summarized 

in this document and described in detail 

in the PA, also inform the decision on 

the primary standard. The general 

approach and methodology used for the 

exposure-based assessment is similar to 

that used in the last review, although a 

number of updates and improvements 

have been implemented. These include 

a more recent period (2015–2017) of 

ambient air monitoring data in which O3

concentrations in the areas assessed are 

at or near the current standard, as well 

as improvements and updates to 

models, model inputs and underlying 

databases. 

In its advice to the Administrator, the 

CASAC stated that the newly available 

health effects evidence does not differ 

substantially from that available in the 

last review when the standard was set. 

Part of CASAC concluded that the 

primary standard should be retained. 

Another part of CASAC expressed 

concern regarding the margin of safety 

provided by the current standard, 

pointing to comments from the 2014 

CASAC, who while agreeing that the 

evidence supported a standard level of 

70 ppb, additionally provided policy 

advice expressing support for a lower 

standard. In summary, the current 

evidence and quantitative analyses, 

advice from the CASAC and 

consideration of public comments have 

informed the Administrator’s judgments 

in reaching his decision that the current 

primary standard of 70 ppb O3, as the 

annual fourth-highest daily maximum 8- 

hour concentration averaged across 

three consecutive years, provides the 

requisite public health protection, with 

an adequate margin of safety. 

Based on the current welfare effects 

evidence and quantitative information, 

as well as consideration of CASAC 

advice and public comment, the 

Administrator concludes that the 

current secondary standard is requisite 

to protect the public welfare from 

known or anticipated adverse effects of 

O3 and related photochemical oxidants 

in ambient air, and should be retained, 

without revision. This decision has been 

informed by key aspects of the welfare 

effects evidence newly available in this 

review, in conjunction with the full 

body of evidence critically evaluated in 

the ISA, that supports, sharpens and 

expands somewhat on the conclusions 

reached in the last review. The currently 

available evidence describes an array of 

O3 effects on vegetation and related 

ecosystem effects, as well as the role of 

O3 in radiative forcing and subsequent 

climate-related effects. The ISA includes 

findings of causal or likely causal 


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87258 Federal Register / Vol. 85, No. 251 / Thursday, December 31, 2020 / Rules and Regulations 

1The legislative history of section 109 indicates 

that a primary standard is to be set at ‘‘the 

maximum permissible ambient air level . . . which 

will protect the health of any [sensitive] group of 

the population,’’ and that for this purpose 

‘‘reference should be made to a representative 

sample of persons comprising the sensitive group 

rather than to a single person in such a group.’’ S. 

Rep. No. 91–1196, 91st Cong., 2d Sess. 10 (1970). 

2Under CAA section 302(h) (42 U.S.C. 7602(h)), 

effects on welfare include, but are not limited to, 

‘‘effects on soils, water, crops, vegetation, manmade 

materials, animals, wildlife, weather, visibility, and 

climate, damage to and deterioration of property, 

and hazards to transportation, as well as effects on 

economic values and on personal comfort and wellbeing.’’ 

3As used here and similarly throughout this 

document, the term population (or group) refers to 

persons having a quality or characteristic in 

common, such as a specific pre-existing illness or 

a specific age or life stage. As summarized in 

section II.A.2.c below, the identification of sensitive 

groups (called at-risk groups or at-risk populations) 

involves consideration of susceptibility and 

vulnerability. 

relationships for a number of such 

effects with O3 in the ambient air. As in 

the last review, the strongest evidence, 

including quantitative characterizations 

of relationships between O3 exposure 

and occurrence and magnitude of 

effects, is for vegetation effects. The 

scales of these effects range from the 

individual plant scale to the ecosystem 

scale, with potential for impacts on the 

public welfare. 

While the welfare effects of O3 vary 

widely with regard to the extent and 

level of detail of the available 

information that describes the exposure 

circumstances that may elicit them, 

such information is most advanced for 

plant growth-related effects. For 

example, the information on exposure 

metric and relationships for these effects 

with the cumulative, concentrationweighted exposure index, W126, is 

long-standing, having been first 

described in the 1997 review. Utilizing 

this information in reviewing the public 

welfare protection provided by the 

current secondary standard, reduced 

growth has been considered as proxy or 

surrogate for a broad array of related 

vegetation effects. Quantitative analyses 

of air quality and vegetation exposure, 

including in terms of the W126 index, 

as well as policy-relevant considerations 

discussed in the PA, have also informed 

the Administrator’s decision on the 

secondary standard. These include 

analyses of air quality monitoring data 

in areas meeting the current standard 

across the U.S., as well as in Class I 

areas, updated and expanded from 

analyses conducted in the last review. 

Lastly, in its advice to the Administrator 

on the secondary standard, the full 

CASAC found the current evidence to 

support the current standard and 

concurred with the draft PA that it 

should be retained without revision. In 

summary, the current evidence and 

quantitative analyses, advice from the 

CASAC and consideration of public 

comments have informed the 

Administrator’s judgments in reaching 

his decision that the current secondary 

standard of 70 ppb O3, as the annual 

fourth-highest daily maximum 8-hour 

concentration averaged across three 

consecutive years, provides the requisite 

public welfare protection. 

I. Background 

A. Legislative Requirements 

Two sections of the CAA govern the 

establishment and revision of the 

NAAQS. Section 108 (42 U.S.C. 7408) 

directs the Administrator to identify and 

list certain air pollutants and then to 

issue air quality criteria for those 

pollutants. The Administrator is to list 

those pollutants ‘‘emissions of which, in 

his judgment, cause or contribute to air 

pollution which may reasonably be 

anticipated to endanger public health or 

welfare’’; ‘‘the presence of which in the 

ambient air results from numerous or 

diverse mobile or stationary sources’’; 

and for which he ‘‘plans to issue air 

quality criteria . . . .’’ (42 U.S.C. 

7408(a)(1)). Air quality criteria are 

intended to ‘‘accurately reflect the latest 

scientific knowledge useful in 

indicating the kind and extent of all 

identifiable effects on public health or 

welfare which may be expected from the 

presence of [a] pollutant in the ambient 

air . . . .’’ (42 U.S.C. 7408(a)(2)). 

Section 109 (42 U.S.C. 7409) directs 

the Administrator to propose and 

promulgate ‘‘primary’’ and ‘‘secondary’’ 

NAAQS for pollutants for which air 

quality criteria are issued (42 U.S.C. 

7409(a)). Section 109(b)(1) defines 

primary standards as ones ‘‘the 

attainment and maintenance of which in 

the judgment of the Administrator, 

based on such criteria and allowing an 

adequate margin of safety, are requisite 

to protect the public health.’’ 1 Under 

section 109(b)(2), a secondary standard 

must ‘‘specify a level of air quality the 

attainment and maintenance of which, 

in the judgment of the Administrator, 

based on such criteria, is requisite to 

protect the public welfare from any 

known or anticipated adverse effects 

associated with the presence of [the] 

pollutant in the ambient air.’’ 2

In setting primary and secondary 

standards that are ‘‘requisite’’ to protect 

public health and welfare, respectively, 

as provided in section 109(b), the EPA’s 

task is to establish standards that are 

neither more nor less stringent than 

necessary. In so doing, the EPA may not 

consider the costs of implementing the 

standards. See generally, Whitman v. 

American Trucking Ass’ns, 531 U.S. 

457, 465–472, 475–76 (2001). Likewise, 

‘‘[a]ttainability and technological 

feasibility are not relevant 

considerations in the promulgation of 

national ambient air quality standards.’’ 

See American Petroleum Institute v. 

Costle, 665 F.2d 1176, 1185 (D.C. Cir. 

1981); accord Murray Energy Corp. v. 

EPA, 936 F.3d 597, 623–24 (D.C. Cir. 

2019). At the same time, courts have 

clarified the EPA may consider ‘‘relative 

proximity to peak background . . . 

concentrations’’ as a factor in deciding 

how to revise the NAAQS in the context 

of considering standard levels within 

the range of reasonable values 

supported by the air quality criteria and 

judgments of the Administrator. See 

American Trucking Ass’ns, v. EPA, 283 

F.3d 355, 379 (D.C. Cir. 2002), hereafter 

referred to as ‘‘ATA III.’’ 

The requirement that primary 

standards provide an adequate margin 

of safety was intended to address 

uncertainties associated with 

inconclusive scientific and technical 

information available at the time of 

standard setting. It was also intended to 

provide a reasonable degree of 

protection against hazards that research 

has not yet identified. See Lead 

Industries Ass’n v. EPA, 647 F.2d 1130, 

1154 (D.C. Cir. 1980); American 

Petroleum Institute v. Costle, 665 F.2d at 

1186; Coalition of Battery Recyclers 

Ass’n v. EPA, 604 F.3d 613, 617–18 

(D.C. Cir. 2010); Mississippi v. EPA, 744 

F.3d 1334, 1353 (D.C. Cir. 2013). Both 

kinds of uncertainties are components 

of the risk associated with pollution at 

levels below those at which human 

health effects can be said to occur with 

reasonable scientific certainty. Thus, in 

selecting primary standards that include 

an adequate margin of safety, the 

Administrator is seeking not only to 

prevent pollution levels that have been 

demonstrated to be harmful but also to 

prevent lower pollutant levels that may 

pose an unacceptable risk of harm, even 

if the risk is not precisely identified as 

to nature or degree. The CAA does not 

require the Administrator to establish a 

primary NAAQS at a zero-risk level or 

at background concentration levels (see 

Lead Industries Ass’n v. EPA, 647 F.2d 

at 1156 n.51, Mississippi v. EPA, 744 

F.3d at 1351), but rather at a level that 

reduces risk sufficiently so as to protect 

public health with an adequate margin 

of safety. 

In addressing the requirement for an 

adequate margin of safety, the EPA 

considers such factors as the nature and 

severity of the health effects involved, 

the size of the sensitive population(s),3


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4This section of the Act requires the 

Administrator to complete these reviews and make 

any revisions that may be appropriate ‘‘at five-year 

intervals.’’ 

5Because some of these issues are not relevant to 

standard setting, some aspects of CASAC advice 

may not be relevant to EPA’s process of setting 

primary and secondary standards that are requisite 

to protect public health and welfare. Indeed, were 

the EPA to consider costs of implementation when 

reviewing and revising the standards ‘‘it would be 

grounds for vacating the NAAQS.’’ Whitman v. 

American Trucking Ass’ns, 531 U.S. 457, 471 n.4 

(2001). At the same time, the CAA directs CASAC 

to provide advice on ‘‘any adverse public health, 

welfare, social, economic, or energy effects which 

may result from various strategies for attainment 

and maintenance’’ of the NAAQS to the 

Administrator under section 109(d)(2)(C)(iv). In 

Whitman, the Court clarified that most of that 

advice would be relevant to implementation but not 

standard setting, as it ‘‘enable[s] the Administrator 

to assist the States in carrying out their statutory 

role as primary implementers of the NAAQS’’ (id. 

at 470 [emphasis in original]). However, the Court 

also noted that CASAC’s ‘‘advice concerning certain 

aspects of ‘adverse public health . . . effects’ from 

various attainment strategies is unquestionably 

pertinent’’ to the NAAQS rulemaking record and 

relevant to the standard setting process (id. at 470 

n.2). 

6The EPA has determined that air quality in the 

area including Houston has attained the 1979 1- 

hour standard (85 FR 8411, February 14, 2020). 

and the kind and degree of 

uncertainties. The selection of any 

particular approach to providing an 

adequate margin of safety is a policy 

choice left specifically to the 

Administrator’s judgment. See Lead 

Industries Ass’n v. EPA, 647 F.2d at 

1161–62; Mississippi v. EPA, 744 F.3d at 

1353. 

Section 109(d)(1) of the Act requires 

periodic review and, if appropriate, 

revision of existing air quality criteria to 

reflect advances in scientific knowledge 

concerning the effects of the pollutant 

on public health and welfare. Under the 

same provision, the EPA is also to 

periodically review and, if appropriate, 

revise the NAAQS, based on the revised 

air quality criteria.4

Section 109(d)(2) addresses the 

appointment and advisory functions of 

an independent scientific review 

committee. Section 109(d)(2)(A) 

requires the Administrator to appoint 

this committee, which is to be 

composed of ‘‘seven members including 

at least one member of the National 

Academy of Sciences, one physician, 

and one person representing State air 

pollution control agencies.’’ Section 

109(d)(2)(B) provides that the 

independent scientific review 

committee ‘‘shall complete a review of 

the criteria . . . and the national 

primary and secondary ambient air 

quality standards . . . and shall 

recommend to the Administrator any 

new . . . standards and revisions of 

existing criteria and standards as may be 

appropriate . . .’’ Since the early 

1980s, this independent review function 

has been performed by the CASAC of 

the EPA’s Science Advisory Board. A 

number of other advisory functions are 

also identified for the committee by 

section 109(d)(2)(C), which reads: 

Such committee shall also (i) advise the 

Administrator of areas in which additional 

knowledge is required to appraise the 

adequacy and basis of existing, new, or 

revised national ambient air quality 

standards, (ii) describe the research efforts 

necessary to provide the required 

information, (iii) advise the Administrator on 

the relative contribution to air pollution 

concentrations of natural as well as 

anthropogenic activity, and (iv) advise the 

Administrator of any adverse public health, 

welfare, social, economic, or energy effects 

which may result from various strategies for 

attainment and maintenance of such national 

ambient air quality standards. 

As previously noted, the Supreme 

Court has held that section 109(b) 

‘‘unambiguously bars cost 

considerations from the NAAQS-setting 

process,’’ in Whitman v. American 

Trucking Ass’ns, 531 U.S. 457, 471 

(2001). Accordingly, while some of the 

issues listed in section 109(d)(2)(C) as 

those on which Congress has directed 

the CASAC to advise the Administrator, 

are ones that are relevant to the standard 

setting process, others are not. Issues 

that are not relevant to standard setting 

may be relevant to implementation of 

the NAAQS once they are established.5

B. Related O3 Control Programs 

States are primarily responsible for 

ensuring attainment and maintenance of 

ambient air quality standards once the 

EPA has established them. Under 

sections 110 and 171 through 185 of the 

CAA, and related provisions and 

regulations, states are to submit, for the 

EPA’s approval, state implementation 

plans (SIPs) that provide for the 

attainment and maintenance of such 

standards through control programs 

directed to sources of the pollutants 

involved. The states, in conjunction 

with the EPA, also administer the 

prevention of significant deterioration of 

air quality program that covers these 

pollutants. See 42 U.S.C. 7470–7479. In 

addition, federal programs provide for 

nationwide reductions in emissions of 

O3 precursors and other air pollutants 

under Title II of the Act, 42 U.S.C. 

7521–7574, which involves controls for 

automobile, truck, bus, motorcycle, 

nonroad engine and equipment, and 

aircraft emissions; the new source 

performance standards under section 

111 of the Act, 42 U.S.C. 7411; and the 

national emissions standards for 

hazardous air pollutants under section 

112 of the Act, 42 U.S.C. 7412. 

C. History of the Air Quality Criteria and 

Standards 

Primary and secondary NAAQS were 

first established for photochemical 

oxidants in 1971 (36 FR 8186, April 30, 

1971) based on the air quality criteria 

developed in 1970 (U.S. DHEW, 1970; 

35 FR 4768, March 19, 1970). The EPA 

set both primary and secondary 

standards at 0.08 parts per million 

(ppm), as a 1-hour average of total 

photochemical oxidants, not to be 

exceeded more than one hour per year. 

Since that time, the EPA has reviewed 

the air quality criteria and standards a 

number of times, with the most recent 

review being completed in 2015. 

The EPA initiated the first periodic 

review of the NAAQS for photochemical 

oxidants in 1977. Based on the 1978 air 

quality criteria document (AQCD [U.S. 

EPA, 1978]), the EPA proposed 

revisions to the original NAAQS in 1978 

(43 FR 26962, June 22, 1978) and 

adopted revisions in 1979 (44 FR 8202, 

February 8, 1979). At that time, the EPA 

changed the indicator from 

photochemical oxidants to O3, revised 

the level of the primary and secondary 

standards from 0.08 to 0.12 ppm and 

revised the form of both standards from 

a deterministic (i.e., not to be exceeded 

more than one hour per year) to a 

statistical form. With these changes, 

attainment of the standards was defined 

to occur when the average number of 

days per calendar year (across a 3-year 

period) with maximum hourly average 

O3 concentration greater than 0.12 ppm 

equaled one or less (44 FR 8202, 

February 8, 1979; 43 FR 26962, June 22, 

1978). Several petitioners challenged 

the 1979 decision. Among those, one 

claimed natural O3 concentrations and 

other physical phenomena made the 

standard unattainable in the Houston 

area.6 The U.S. Court of Appeals for the 

District of Columbia Circuit (D.C. 

Circuit) rejected this argument, holding 

(as noted in section I.A above) that 

attainability and technological 

feasibility are not relevant 

considerations in the promulgation of 

the NAAQS (American Petroleum 

Institute v. Costle, 665 F.2d at 1185). 

The court also noted that the EPA need 

not tailor the NAAQS to fit each region 

or locale, pointing out that Congress was 

aware of the difficulty in meeting 

standards in some locations and had 

addressed it through various 

compliance-related provisions in the 

CAA (id. at 1184–86). 

The next periodic reviews of the 

criteria and standards for O3 and other 


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7The press release of this announcement is 

available at: https://archive.epa.gov/epapages/ 

newsroom_archive/newsreleases/ 

85f90b7711acb0c88525763300617d0d.html. 

8A ‘‘Call for Information’’ initiated the review (73 

FR 56581, September 29, 2008). 

9This rulemaking, completed in 2015, concluded 

the reconsideration process. 

10The ISA, as the AQCD in prior reviews, serves 

the purpose of reviewing the air quality criteria. 

11The PA presents an evaluation, for 

consideration by the Administrator, of the policy 

implications of the currently available scientific 

information, assessed in the ISA; the quantitative 

air quality, exposure or risk analyses presented in 

the PA and developed in light of the ISA findings; 

and related limitations and uncertainties. The role 

of the PA is to help ‘‘bridge the gap’’ between the 

Agency’s scientific assessment and quantitative 

technical analyses, and the judgments required of 

the Administrator in his decisions in the NAAQS 

review. 

photochemical oxidants began in 1982 

and 1983, respectively (47 FR 11561, 

March 17, 1982; 48 FR 38009, August 

22, 1983). As part of these reviews, the 

EPA published an AQCD, a Staff Paper, 

and a supplement to the AQCD (U.S. 

EPA, 1986; U.S. EPA, 1989; U.S. EPA, 

1992). The schedule for completion of 

this review was governed by court 

order. In August of 1992, the EPA 

proposed to retain the existing primary 

and secondary standards (57 FR 35542, 

August 10, 1992). In March 1993, the 

EPA concluded this review by finalizing 

its proposed decision to retain the 

standards, without revision (58 FR 

13008, March 9, 1993). 

In the next review of the air quality 

criteria and standards for O3 and other 

photochemical oxidants, for which the 

EPA had announced in August 1992 its 

intention to proceed rapidly, the EPA 

developed an AQCD and Staff Paper (57 

FR 35542, August 10, 1992; U.S. EPA, 

1996a; U.S. EPA, 1996b). Based on 

consideration of these assessments, the 

EPA proposed revisions to both the 

primary and secondary standards (61 FR 

65716, December 13, 1996). The EPA 

completed this review in 1997 by 

revising both standards to 0.08 ppm, as 

the annual fourth-highest daily 

maximum 8-hour average concentration, 

averaged over three years (62 FR 38856, 

July 18, 1997). 

In response to challenges to the EPA’s 

1997 decision revising the standards, 

the D.C. Circuit remanded the standards 

to the EPA, finding that section 109 of 

the CAA, as interpreted by the EPA, 

effected an unconstitutional delegation 

of legislative authority. See American 

Trucking Ass’ns v. EPA, 175 F.3d 1027, 

1034–1040 (D.C. Cir. 1999). The court 

also directed that, in responding to the 

remand, the EPA should consider the 

potential beneficial health effects of O3

pollution in shielding the public from 

the effects of solar ultraviolet (UV) 

radiation, as well as adverse health 

effects (id. at 1051–53). See American 

Trucking Ass’ns v. EPA, 195 F.3d 4, 10 

(D.C. Cir. 1999) (granting panel 

rehearing in part but declining to review 

the ruling on consideration of the 

potential beneficial effects of O3

pollution). After granting petitions for 

certiorari, the U.S. Supreme Court 

unanimously reversed the judgment of 

the D.C. Circuit on the constitutional 

issue, holding that section 109 of the 

CAA does not unconstitutionally 

delegate legislative power to the EPA. 

See Whitman v. American Trucking 

Ass’ns, 531 U.S. 457, 472–74 (2001). 

The Court remanded the case to the D.C. 

Circuit to consider challenges to the 

1997 O3 NAAQS that had not yet been 

addressed. On remand, the D.C. Circuit 

found the 1997 O3 NAAQS to be 

‘‘neither arbitrary nor capricious,’’ and 

so denied the remaining petitions for 

review. See ATA III, 283 F.3d at 379. 

Coincident with the continued 

litigation of the other issues, the EPA 

responded to the court’s 1999 remand to 

consider the potential beneficial health 

effects of O3 pollution in shielding the 

public from effects of UV radiation (66 

FR 57268, Nov. 14, 2001; 68 FR 614, 

January 6, 2003). In 2001, the EPA 

proposed to leave the 1997 primary 

standard unchanged (66 FR 57268, Nov. 

14, 2001). After considering public 

comment on the proposed decision, the 

EPA published its final response to this 

remand in 2003, re-affirming the 8-hour 

primary standard set in 1997 (68 FR 

614, January 6, 2003). 

The EPA initiated the fourth periodic 

review of the air quality criteria and 

standards for O3 and other 

photochemical oxidants with a call for 

information in September 2000 (65 FR 

57810, September 26, 2000). In this 

review, the EPA developed an AQCD, 

Staff Paper and related technical 

support documents and proposed 

revisions to the primary and secondary 

standards (U.S. EPA, 2006; U.S. EPA, 

2007; 72 FR 37818, July 11, 2007). The 

review was completed in March 2008 

with revision of the levels of both the 

primary and secondary standards from 

0.08 ppm to 0.075 ppm, and retention 

of the other elements of the prior 

standards (73 FR 16436, March 27, 

2008). A number of petitioners filed suit 

challenging this decision. 

In September 2009, the EPA 

announced its intention to reconsider 

the 2008 O3 standards,7 and initiated a 

rulemaking to do so. At the EPA’s 

request, the court held the consolidated 

cases in abeyance pending the EPA’s 

reconsideration of the 2008 decision. In 

January 2010, the EPA issued a notice 

of proposed rulemaking to reconsider 

the 2008 final decision (75 FR 2938, 

January 19, 2010). Later that year, in 

view of the need for further 

consideration and the fact that the 

Agency’s next periodic review of the O3

NAAQS required under CAA section 

109 had already begun (as announced in 

September 2008),8 the EPA consolidated 

the reconsideration with its statutorily 

required periodic review.9

In light of the EPA’s decision to 

consolidate the reconsideration with the 

review then ongoing, the D.C. Circuit 

proceeded with the litigation on the 

2008 O3 NAAQS decision. On July 23, 

2013, the court upheld the EPA’s 2008 

primary standard, but remanded the 

2008 secondary standard to the EPA. 

See Mississippi v. EPA, 744 F.3d 1334 

(D.C. Cir. 2013). With respect to the 

secondary standard, the court held that 

the EPA’s explanation for the setting of 

the secondary standard identical to the 

revised 8-hour primary standard was 

inadequate under the CAA because the 

EPA had not adequately explained how 

that standard provided the required 

public welfare protection. 

At the time of the court’s decision, the 

EPA had already completed significant 

portions of its next statutorily required 

periodic review of the O3 NAAQS, 

which had been formally initiated in 

2008, as summarized above. The 

documents developed for this review 

included the ISA,10 Risk and Exposure 

Assessments (REAs) for health and 

welfare, and PA (Frey, 2014a, Frey, 

2014b, Frey, 2014c, U.S. EPA, 2013, 

U.S. EPA, 2014a, U.S. EPA, 2014b, U.S. 

EPA, 2014c).11 In late 2014, the EPA 

proposed to revise the 2008 primary and 

secondary standards (79 FR 75234, 

December 17, 2014). The EPA’s final 

decision in this review established the 

now-current standards (80 FR 65292, 

October 26, 2015; 40 CFR 50.19). In this 

decision, based on consideration of the 

health effects evidence on respiratory 

effects of O3 in at-risk populations, the 

EPA revised the primary standard from 

a level of 0.075 ppm to a level of 0.070 

ppm, while retaining all other elements 

of the standard (80 FR 65292, October 

26, 2015). The EPA’s decision on the 

level for the standard was based on the 

weight of the scientific evidence and 

quantitative exposure/risk information. 

The level of the secondary standard was 

also revised from 0.075 ppm to 0.070 

ppm based on the scientific evidence of 

O3 effects on welfare, particularly the 

evidence of O3 impacts on vegetation, 

and quantitative analyses available in 

the review. The other elements of the 

standard were retained. This decision 

on the secondary standard also 

incorporated the EPA’s response to the 


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12The 2015 revisions to the NAAQS were 

accompanied by revisions to the data handling 

procedures, ambient air monitoring requirements, 

the air quality index and several provisions related 

to implementation (80 FR 65292, October 26, 2015). 

13The draft ISA and draft PA were released for 

public comment and CASAC review on September 

26, 2019 and October 31, 2019, respectively. The 

charges for the CASAC review summarized the 

overarching context for the document review 

(including reference to Pruitt [2018], and the 

CASAC’s functions under section 109(d)(2)(B) and 

(C) of the Act), as well as specific charge questions 

for review of each of the documents. 

D.C. Circuit’s remand of the 2008 

secondary standard in Mississippi v. 

EPA, 744 F.3d 1344 (D.C. Cir. 2013).12

After publication of the final rule, a 

number of industry groups, 

environmental and health organizations, 

and certain states filed petitions for 

judicial review in the D.C. Circuit. The 

industry and state petitioners argued 

that the revised standards were too 

stringent, while the environmental and 

health petitioners argued that the 

revised standards were not stringent 

enough to protect public health and 

welfare as the Act requires. On August 

23, 2019, the court issued an opinion 

that denied all the petitions for review 

with respect to the 2015 primary 

standard while also concluding that the 

EPA had not provided a sufficient 

rationale for aspects of its decision on 

the 2015 secondary standard and 

remanding that standard to the EPA. See 

Murray Energy Corp. v. EPA, 936 F.3d 

597 (D.C. Cir. 2019). The court’s 

decision on the secondary standard 

focused on challenges to particular 

aspects of EPA’s decision. The court 

concluded that EPA’s identification of 

particular benchmarks for evaluating the 

protection the standard provided against 

welfare effects associated with tree 

growth loss was reasonable and 

consistent with CASAC’s advice. 

However, the court held that EPA had 

not adequately explained its decision to 

focus on a 3-year average for 

consideration of the cumulative 

exposure, in terms of W126, identified 

as providing requisite public welfare 

protection, or its decision to not identify 

a specific level of air quality related to 

visible foliar injury. The EPA’s decision 

not to use a seasonal W126 index as the 

form and averaging time of the 

secondary standard was also challenged, 

but the court did not reach 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. See Murray Energy Corp. v. 

EPA, 936 F.3d 597, 618 (D.C. Cir. 2019). 

Accordingly, the court remanded the 

secondary standard to EPA for further 

justification or reconsideration. The 

court’s remand of the secondary 

standard has been considered in 

reaching the decision, and associated 

conclusions and judgments, described 

in section III.B.3 below. 

In the August 2019 decision, the court 

additionally addressed arguments 

regarding considerations of background 

O3 concentrations, and socioeconomic 

and energy impacts. With regard to the 

former, the court rejected the argument 

that the EPA was required to take 

background O3 concentrations into 

account when setting the NAAQS, 

holding that the text of CAA section 

109(b) precluded this interpretation 

because it would mean that if 

background O3 levels in any part of the 

country exceeded the level of O3 that is 

requisite to protect public health, the 

EPA would be obliged to set the 

standard at the higher nonprotective 

level (id. at 622–23). Thus, the court 

concluded that the EPA did not act 

unlawfully or arbitrarily or capriciously 

in setting the 2015 NAAQS without 

regard for background O3 (id. at 624). 

Additionally, the court denied 

arguments that the EPA was required to 

consider adverse economic, social, and 

energy impacts in determining whether 

a revision of the NAAQS was 

‘‘appropriate’’ under section 109(d)(1) of 

the CAA (id. at 621–22). The court 

reasoned that consideration of such 

impacts was precluded by Whitman’s 

holding that the CAA ‘‘unambiguously 

bars cost considerations from the 

NAAQS-setting process’’ (531 U.S. at 

471, summarized in section I.A above). 

Further, the court explained that section 

109(d)(2)(C)’s requirement that CASAC 

advise the EPA ‘‘of any adverse public 

health, welfare, social, economic, or 

energy effects which may result from 

various strategies for attainment and 

maintenance’’ of revised NAAQS had no 

bearing on whether costs are to be 

considered in setting the NAAQS 

(Murray Energy Corp. v. EPA, 936 F.3d 

at 622). Rather, as described in Whitman 

and discussed further in section I.A 

above, most of that advice would be 

relevant to implementation but not 

standard setting (id.). 

D. Current Review of the Air Quality 

Criteria and Standards 

In May 2018, the Administrator 

directed his Assistant Administrators to 

initiate this current review of the O3

NAAQS (Pruitt, 2018). In conveying this 

direction, the Administrator further 

directed the EPA staff to expedite the 

review, implementing an accelerated 

schedule aimed at completion of the 

review within the statutorily required 

period (Pruitt, 2018). Accordingly, the 

EPA took immediate steps to proceed 

with the review. In June 2018, the EPA 

announced the initiation of the periodic 

reviews of the air quality criteria for 

photochemical oxidants and of the O3

NAAQS and issued a call for 

information in the Federal Register (83 

FR 29785, June 26, 2018). Two types of 

information were called for: Information 

regarding significant new O3 research to 

be considered for the ISA for the review, 

and policy-relevant issues for 

consideration in this NAAQS review. 

Based in part on the information 

received in response to the call for 

information, the EPA developed a draft 

IRP, which was made available for 

consultation with the CASAC and for 

public comment (83 FR 55163, 

November 2, 2018; 83 FR 55528, 

November 6, 2018). Comments from the 

CASAC (Cox, 2018) and the public were 

considered in preparing the final IRP 

(U.S. EPA, 2019b). 

Under the plan outlined in the IRP 

and consistent with revisions to the 

process identified by the Administrator 

in his 2018 memo directing initiation of 

the review, the current review of the O3

NAAQS has progressed on an 

accelerated schedule (Pruitt, 2018). The 

EPA has incorporated a number of 

efficiencies in various aspects of the 

review process, as summarized in the 

IRP, to support the accelerated schedule 

(Pruitt, 2018). As one example of such 

an efficiency, rather than produce 

separate documents for the PA and 

associated quantitative analyses, the 

human exposure and health risk 

analyses (that inform the decision on 

the primary standard) and the air 

quality and exposure analyses (that 

inform the decision on the secondary 

standard) are included as appendices in 

the PA, along with other technical 

appendices that inform these standards 

decisions. The draft PA (including these 

analyses as appendices) was reviewed 

by the CASAC and made available for 

public comment while the draft ISA was 

also being reviewed by the CASAC and 

was available for public comment (84 

FR 50836, September 26, 2019; 84 FR 

58711, November 1, 2019).13 The 

CASAC was assisted in its review by a 

pool of consultants with expertise in a 

number of fields (84 FR 38625, August 

7, 2019). The approach employed by the 

CASAC in utilizing outside technical 

expertise represents an additional 

modification of the process from past 

reviews. Rather than join with some or 

all of the CASAC members in a CASAC 

review panel as has been common in 

other NAAQS reviews in the past, in 

this O3 NAAQS review (and also in the 

recent CASAC review of the PA for the 


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14While simultaneous review of first drafts of 

both documents has not been usual in past reviews, 

there have been occurrences of the CASAC review 

of a draft PA (or draft REA when the process 

involved a policy assessment being included within 

the REA document) simultaneous with review of a 

second (or later) draft ISA (e.g., 73 FR 19835, April 

11, 2008; 73 FR 34739, June 18, 2008; 77 FR 64335, 

October 19, 2012; 78 FR 938, January 7, 2013). 

15The ISA builds on evidence and conclusions 

from previous assessments, focusing on 

synthesizing and integrating the newly available 

evidence (ISA, section IS.1.1). Past assessments are 

generally cited when providing further, still 

relevant, details that informed the current 

assessment but are not repeated in the latest 

assessment. 

16The docket for this review, EPA–HQ–OAR– 

2018–0279, has incorporated the ISA docket (EPA– 

HQ–ORD–2018–0274) by reference. Both are 

publicly accessible at www.regulations.gov. 

17 In addition to the review’s opening ‘‘Call for 

Information’’ (83 FR 29785, June 26, 2018), 

systematic review methodologies were applied to 

identify relevant scientific findings that have 

emerged since the 2013 ISA, which included peer 

reviewed literature published through July 2011. 

Search techniques for the current ISA identified 

and evaluated studies and reports that have 

undergone scientific peer review and were 

published or accepted for publication between 

January 1, 2011 (providing some overlap with the 

cutoff date for the last ISA) and March 30, 2018. 

Studies published after the literature cutoff date for 

this ISA were also considered if they were 

submitted in response to the Call for Information or 

identified in subsequent phases of ISA 

development, particularly to the extent that they 

provide new information that affects key scientific 

conclusions (ISA, Appendix 10, section 10.2). 

References that are cited in the ISA, the references 

that were considered for inclusion but not cited, 

and electronic links to bibliographic information 

and abstracts can be found at: https://hero.epa.gov/ 

hero/index.cfm/project/page/project_id/2737. 

particulate matter NAAQS), the 

consultants comprised a pool of 

expertise that CASAC members drew on 

through the use of specific questions, 

posed in writing prior to the public 

meeting, regarding aspects of the 

documents being reviewed, obtaining 

subject matter expertise for their review 

in a focused, efficient and transparent 

manner. 

The CASAC discussed its review of 

both the draft ISA and the draft PA over 

three days at a public meeting in 

December 2019 (84 FR 58713, November 

1, 2019).14 The CASAC discussed its 

draft letters describing its advice and 

comments on the documents in a public 

teleconference in early February 2020 

(85 FR 4656; January 27, 2020). The 

letters to the Administrator conveying 

the CASAC advice and comments on the 

draft PA and draft ISA were released 

later that month (Cox, 2020a; Cox, 

2020b). 

The letters from the CASAC and 

public comment on the draft ISA and 

draft PA informed completion of the 

final documents and further informed 

development of the Administrator’s 

proposed and final decisions in this 

review. Comments from the CASAC on 

the draft ISA were considered by the 

EPA and led to a number of revisions in 

developing the final document. The 

CASAC review of the draft ISA and the 

EPA’s consideration of CASAC 

comments are described in Appendix 

10, section 10.4.5 of the final ISA. In his 

reply to the CASAC letter conveying its 

review, ‘‘Administrator Wheeler noted, 

‘for those comments and 

recommendations that are more 

significant or cross-cutting and which 

were not fully addressed, the Agency 

will develop a plan to incorporate these 

changes into future O3 ISAs as well as 

ISAs for other criteria pollutant 

reviews’ ’’ (ISA, p. 10–28; Wheeler, 

2020). The ISA was completed and 

made available to the public in April 

2020 (85 FR 21849, April 20, 2020).15

Based on the rigorous scientific 

approach utilized in its development, 

summarized in Appendix 10 of the final 

ISA, the EPA considers the final ISA to 

‘‘accurately reflect the latest scientific 

knowledge useful in indicating the kind 

and extent of all identifiable effects on 

public health or welfare which may be 

expected from the presence of [O3] in 

the ambient air, in varying quantities’’ 

as required by the CAA (42 U.S.C. 

7408(a)(2)). 

The CASAC comments additionally 

provided advice with regard to the 

primary and secondary standards, as 

well as a number of comments intended 

to improve the PA. These comments 

were considered in completing that 

document (85 FR 31182, May 22, 2020). 

The CASAC advice to the Administrator 

regarding the O3 standards has also been 

described and considered in the PA, and 

in sections II and III below. The CASAC 

advice on the primary standard is 

summarized in II.B.2 below and its 

advice on the secondary standard is 

summarized in section III.B.1.b. 

Materials upon which this proposed 

decision is based, including the 

documents described above, are 

available to the public in the docket for 

the review.16 As in prior NAAQS 

reviews, the EPA is basing its decision 

in this review on studies and related 

information included in the air quality 

criteria, which have undergone CASAC 

and public review. The studies assessed 

in the ISA 17 and PA, and the integration 

of the scientific evidence presented in 

them, have undergone extensive critical 

review by the EPA, the CASAC, and the 

public. The rigor of that review makes 

these studies, and their integrative 

assessment, the most reliable source of 

scientific information on which to base 

decisions on the NAAQS, decisions that 

all parties recognize as of great import. 

Decisions on the NAAQS can have 

profound impacts on public health and 

welfare, and NAAQS decisions should 

be based on studies that have been 

rigorously assessed in an integrative 

manner not only by the EPA but also by 

the statutorily mandated independent 

scientific advisory committee, as well as 

the public review that accompanies this 

process. Some commenters have 

referred to and discussed individual 

scientific studies on the health effects of 

O3 that were not included in the ISA 

(‘‘ ‘new’ studies’’) and that have not 

gone through this comprehensive 

review process. In considering and 

responding to comments for which such 

‘‘new’’ studies were cited in support, 

the EPA has provisionally considered 

the cited studies in the context of the 

findings of the ISA. The EPA’s 

provisional consideration of these 

studies did not and could not provide 

the kind of in-depth critical review 

described above, but rather was focused 

on determining whether they warranted 

reopening the review of the air quality 

criteria to enable the EPA, the CASAC 

and the public to consider them further. 

This approach, and the decision to 

rely on studies and related information 

included in the air quality criteria, 

which have undergone CASAC and 

public review, is consistent with the 

EPA’s practice in prior NAAQS reviews 

and its interpretation of the 

requirements of the CAA. Since the 

1970 amendments, the EPA has taken 

the view that NAAQS decisions are to 

be based on scientific studies and 

related information that have been 

assessed as a part of the pertinent air 

quality criteria, and the EPA has 

consistently followed this approach. 

This longstanding interpretation was 

strengthened by new legislative 

requirements enacted in 1977, which 

added section 109(d)(2) of the Act 

concerning CASAC review of air quality 

criteria. See 71 FR 61144, 61148 

(October 17, 2006, final decision on 

review of NAAQS for particulate matter) 

for a detailed discussion of this issue 

and the EPA’s past practice. 

As discussed in the EPA’s 1993 

decision not to revise the O3 NAAQS, 

‘‘new’’ studies may sometimes be of 

such significance that it is appropriate 

to delay a decision in a NAAQS review 

and to supplement the pertinent air 

quality criteria so the studies can be 

taken into account (58 FR at 13013– 

13014, March 9, 1993). In the present 

case, the EPA’s provisional 

consideration of ‘‘new’’ studies 

concludes that, taken in context, the 

‘‘new’’ information and findings do not 

materially change any of the broad 

scientific conclusions regarding the 

health and welfare effects of O3 in 


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18O3 monitoring seasons in each state vary from 

five months (May to September in Oregon and 

Washington) to year round (in 11 states), with 

March to October being most common (27 states). 

19A design value is a statistic that summarizes 

the air quality data for a given area in terms of the 

indicator, averaging time, and form of the standard. 

Design values can be compared to the level of the 

standard and are typically used to designate areas 

as meeting or not meeting the standard and assess 

progress towards meeting the NAAQS. 

ambient air made in the air quality 

criteria. For this reason, reopening the 

air quality criteria review would not be 

warranted. 

Accordingly, the EPA is basing the 

final decisions in this review on the 

studies and related information 

included in the O3 air quality criteria 

that have undergone rigorous review by 

the EPA, the CASAC and the public. 

The EPA will consider these ‘‘new’’ 

studies for inclusion in the air quality 

criteria for the next O3 NAAQS review, 

which the EPA expects to begin soon 

after the conclusion of this review and 

which will provide the opportunity to 

fully assess these studies through a 

more rigorous review process involving 

the EPA, the CASAC, and the public. 

E. Air Quality Information 

Ground level O3 concentrations are a 

mix of mostly tropospheric O3 and some 

stratospheric O3. Tropospheric O3 is 

formed due to chemical interactions 

involving solar radiation and precursor 

pollutants including VOCs and nitrogen 

oxides (NOX). Methane (CH4) and 

carbon monoxide (CO) are also 

important precursors, particularly at the 

regional to global scale. The precursor 

emissions leading to tropospheric O3

formation can result from both manmade sources (e.g., motor vehicles and 

electric power generation) and natural 

sources (e.g., vegetation and wildfires). 

In addition, O3 that is created naturally 

in the stratosphere also contributes to 

O3 in the troposphere. The stratosphere 

routinely mixes with the troposphere 

high above the earth’s surface and, less 

frequently, there are intrusions of 

stratospheric air that reach deep into the 

troposphere and even to the surface. 

Once formed, O3 near the surface can be 

transported by winds before eventually 

being removed from the atmosphere via 

chemical reactions or deposition to 

surfaces. In sum, O3 concentrations are 

influenced by complex interactions 

between precursor emissions, 

meteorological conditions, and 

topographical characteristics (PA, 

section 2.1; ISA, Appendix 1). 

For compliance and other purposes, 

state and local environmental agencies 

operate O3 monitors across the U.S. and 

submit the data to the EPA. At present, 

there are approximately 1,300 monitors 

across the U.S. reporting hourly O3

averages during the times of the year 

when local O3 pollution can be 

important (PA, section 2.3.1).18 Most of 

this monitoring is focused on urban 

areas where precursor emissions tend to 

be largest, as well as locations directly 

downwind of these areas. There are also 

over 100 routine monitoring sites in 

rural areas, including sites in the Clean 

Air Status and Trends Network 

(CASTNET) which is specifically 

focused on characterizing conditions in 

rural areas. Based on the monitoring 

data for the three year period from 2016 

to 2018, the EPA identified 142 

counties, in which together 

approximately 106 million Americans 

reside where O3 design values 19 were 

above 0.070 ppm, the level of the 

existing NAAQS (PA, section 2.4.1). 

Across these areas, the highest design 

values are typically observed in 

California, Texas, Denver, around Lake 

Michigan and along the Northeast 

Corridor, locations with some of the 

most densely populated areas in the 

country (e.g., PA, Figure 2–8). 

From a temporal perspective, the 

highest O3 concentrations tend to occur 

during the afternoon and within the 

warmer months of the year due to 

higher levels of solar radiation and other 

conducive meteorological conditions 

during these times. The exceptions to 

this general rule include (1) some rural 

sites where transport of O3 from upwind 

urban areas can occasionally result in 

high nighttime levels of O3, (2) highelevation sites which can be 

episodically influenced by stratospheric 

intrusions in other months of the year, 

and (3) mountain basins in the western 

U.S. where large quantities of O3

precursors emissions associated with oil 

and gas development can be trapped in 

a shallow inversion layer and form O3

under clear, calm skies with snow cover 

during the colder months (PA, section 

2.1; ISA, Appendix 1). 

Monitoring data indicate long-term 

reductions in peak O3 concentrations. 

For example, monitoring sites operating 

since 1980 indicate a 32% reduction in 

the national average annual fourth 

highest daily maximum 8-hour 

concentration from 1980 to 2018. (PA, 

Figure 2–10). This has been 

accompanied by appreciable reductions 

in peak 1-hour concentrations, as seen 

by reductions in annual second highest 

daily maximum 1-hour concentrations 

(PA, Figure 2–17). 

Concentrations of O3 in ambient air 

that result from natural and non-U.S. 

anthropogenic sources are collectively 

referred to as U.S. background O3 (USB; 

PA, section 2.5). As in the last review, 

we generally characterize O3

concentrations that would exist in the 

absence of U.S. anthropogenic 

emissions (as USB). Findings from air 

quality modeling analyses performed for 

this review to investigate patterns of 

USB in the U.S. are largely consistent 

with conclusions reached in the last 

review (PA, section 2.5.4). The current 

modeling analysis indicates spatial 

variation in USB O3 concentrations that 

is related to geography, topography and 

proximity to international borders and 

is also influenced by seasonal variation, 

with long-range international 

anthropogenic transport contributions 

peaking in the spring while U.S. 

anthropogenic contributions tend to 

peak in summer. The West is predicted 

to have higher USB concentrations than 

the East, with higher contributions from 

natural and international anthropogenic 

sources that exert influences in western 

high-elevation and near-border areas. 

The modeling predicts that for both the 

West and the East, days with the highest 

8-hour concentrations of O3 generally 

occur in summer and are likely to have 

substantially greater concentrations due 

to U.S. anthropogenic sources. While 

the USB contributions to O3

concentrations on days with the highest 

8-hour concentrations are generally 

predicted to come largely from natural 

sources, the modeling also indicates that 

some areas near the Mexico border may 

receive appreciable contributions from a 

combination of natural and 

international anthropogenic sources on 

these days. In such locations, the 

modeling suggests the potential for 

relatively infrequent events with 

substantial background contributions 

where daily maximum 8-hour O3

concentrations approach or exceed the 

level of the current NAAQS (i.e., 70 

ppb). This contrasts with most monitor 

locations in the U.S. for which 

international contributions are 

predicted to be the lowest during the 

season with the most frequent 

occurrence of daily maximum 8-hour O3

concentrations above 70 ppb. This is 

generally because, except for in nearborder areas, larger international 

contributions are associated with longdistance transport and that is most 

efficient in the springtime (PA, section 

2.5.4). 

II. Rationale for Decision on the 

Primary Standard 

This section presents the rationale for 

the Administrator’s decision to retain 

the current primary O3 standard. This 

rationale is based on the scientific 

information presented in the ISA, on 

human health effects associated with 


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20As noted in section I.A above, consideration of 

such protection is focused on the sensitive group 

of individuals and not a single person in the 

sensitive group (see S. Rep. No. 91–1196, 91st 

Cong., 2d Sess. 10 [1970]). 

21Although ppm are the units in which the level 

of the standard is defined, the units, ppb, are more 

commonly used throughout this document for 

greater consistency with the more recent literature. 

The level of the current primary standard, 0.070 

ppm, is equivalent to 70 ppb. 

22 In addition to concluding there to be a causal 

relationship between short-term O3 exposures and 

respiratory effects, and that the relationship 

between longer-term exposure and respiratory 

effects was likely to be causal, the 2013 ISA also 

concluded there likely to be a causal relationship 

between short-term exposure and mortality, as well 

as short-term exposure and cardiovascular effects, 

including related mortality, and that the evidence 

was suggestive of causal relationships between 

long-term exposures and total mortality, 

cardiovascular effects and reproductive, 

developmental effects, and between short- and longterm exposure and nervous system effects (2013 

ISA, p. 1–14, section 2.5.2). 

23Study subjects in most of the controlled human 

exposure studies are generally healthy adults. 

24The 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. 

25As used here and similarly throughout the 

document, the term population refers to persons 

having a quality or characteristic in common, such 

as, and including, a specific pre-existing illness or 

a specific age or lifestage. A lifestage refers to a 

distinguishable time frame in an individual’s life 

characterized by unique and relatively stable 

behavioral and/or physiological characteristics that 

are associated with development and growth. 

Identifying at-risk populations includes 

consideration of intrinsic (e.g., genetic or 

developmental aspects) or acquired (e.g., disease or 

smoking status) factors that increase the risk of 

health effects occurring with exposure to a 

photochemical oxidants including O3

and pertaining to the presence of these 

pollutants in ambient air. As 

summarized in section I.D above, the 

ISA was developed based on a thorough 

review of the latest scientific 

information generally published 

between January 2011 and March 2018, 

as well as more recent studies identified 

during peer review, submitted in 

response to the Call for Information, or 

public comments on the draft ISA, 

integrated with the information and 

conclusions from previous assessments 

(ISA, section IS.1.2 and Appendix 10, 

section 10.2). The Administrator’s 

rationale also takes into account: (1) The 

PA evaluation of the policy-relevant 

information in the ISA and presentation 

of quantitative analyses of air quality, 

human exposure and health risks; (2) 

CASAC advice and recommendations, 

as reflected in discussions of drafts of 

the ISA and PA at public meetings and 

in the CASAC’s letters to the 

Administrator; and (3) public comments 

on the proposed decision. 

Within this section, introductory and 

background information is presented in 

section II.A. Section II.A.1 summarizes 

the 2015 establishment of the existing 

standard, as background for this review. 

Section II.A.2 provides an overview of 

the currently available health effects 

evidence, and section II.A.3 provides an 

overview of the current exposure and 

risk information, drawing on the 

quantitative analyses presented in the 

PA. Section II.B summarizes the basis 

for the proposed decision (II.B.1), 

discusses public comments on the 

proposed decision (II.B.2), and presents 

the Administrator’s considerations, 

conclusions and decision in this review 

of the primary standard (II.B.3). The 

decision on the current primary 

standard is summarized in section II.C. 

A. Introduction 

As in prior reviews, the general 

approach to reviewing the current 

primary standard is based, most 

fundamentally, on using the Agency’s 

assessments of the current scientific 

evidence and associated quantitative 

analyses to inform the Administrator’s 

judgment regarding a primary standard 

for photochemical oxidants that is 

requisite to protect the public health 

with an adequate margin of safety. The 

EPA’s assessments are primarily 

documented in the ISA and PA, both of 

which have received CASAC review and 

public comment (84 FR 50836, 

September 26, 2019; 84 FR 58711, 

November 1, 2019; 84 FR 58713, 

November 1, 2019; 85 FR 21849, April 

20, 2020; 85 FR 31182, May 22, 2020). 

In bridging the gap between the 

scientific assessments of the ISA and the 

judgments required of the Administrator 

in his decisions on the current standard, 

the PA evaluates policy implications of 

the assessment of the current evidence 

in ISA and the quantitative exposure 

and risk analyses documented 

extensively in appendices of the PA. In 

evaluating the public health protection 

afforded by the current standard, the 

four basic elements of the NAAQS 

(indicator, averaging time, level, and 

form) are considered collectively. 

The final decision on the adequacy of 

the current primary standard is a public 

health policy judgment to be made by 

the Administrator. In reaching 

conclusions on the standard, the 

decision draws on the scientific 

information and analyses about health 

effects, population exposure and risks, 

as well as judgments about how to 

consider the range and magnitude of 

uncertainties that are inherent in the 

scientific evidence and analyses. This 

approach is based on the recognition 

that the available health effects evidence 

generally reflects a continuum, 

consisting of levels at which scientists 

generally agree that health effects are 

likely to occur, through lower levels at 

which the likelihood and magnitude of 

the response become increasingly 

uncertain. This approach is consistent 

with the requirements of the NAAQS 

provisions of the Clean Air Act and with 

how the EPA and the courts have 

historically interpreted the Act 

(summarized in section I.A. above). 

These provisions require the 

Administrator to establish primary 

standards that, in the judgment of the 

Administrator, are requisite to protect 

public health with an adequate margin 

of safety. In so doing, the Administrator 

seeks to establish standards that are 

neither more nor less stringent than 

necessary for this purpose. The Act does 

not require that primary standards be set 

at a zero-risk level, but rather at a level 

that avoids unacceptable risks to public 

health, including the health of sensitive 

groups.20

1. Background on the Current Standard 

As a result of the last O3 NAAQS 

review, completed in 2015, the level of 

the primary standard was revised from 

0.075 to 0.070 ppm,21 in conjunction 

with retaining the existing indicator, 

averaging time, and form. This revision, 

establishing the current standard, was 

based on the scientific evidence and 

quantitative exposure and risk analyses 

available at that time, as well as the 

Administrator’s judgments regarding the 

available health effects evidence, the 

appropriate degree of public health 

protection for the revised standard, and 

the available exposure and risk 

information regarding the exposures and 

risk that may be allowed by such a 

standard (80 FR 65292, October 26, 

2015). In establishing this standard, the 

Administrator considered the extensive 

body of evidence spanning several 

decades documenting the causal 

relationship between O3 exposure and a 

broad range of respiratory effects (80 FR 

65292, October 26, 2015; 2013 ISA, p. 

1–14),22 that had been augmented by 

evidence available since the prior 

review was completed in 2008. Such 

effects range from small, reversible 

changes in pulmonary function and 

pulmonary inflammation (documented 

in controlled human exposure studies 

involving exposures ranging from 1 to 8 

hours) 23 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 (2013 ISA, section 6.2).24 The 

2015 decision, which provided 

increased protection for at-risk 

populations,25 such as children and 


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substance (such as O3) as well as extrinsic, 

nonbiological factors, such as those related to 

socioeconomic status, reduced access to health care, 

or exposure. 

26 Ventilation rate (V ̇ E) is a specific technical term 

referring to breathing rate in terms of volume of air 

taken into the body per unit of time. A person 

engaged in different activities will exert themselves 

at different levels and experience different 

ventilation rates. 

27For example, the exposure concentrations 

eliciting a given level of response in subjects at rest 

are higher than those eliciting such response in 

subjects exposed while at elevated ventilation, such 

as while exercising (2013 ISA, section 6.2.1.1). 

28The studies given primary focus were those in 

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. 

29For the 70 ppb target exposure, Schelegle et al. 

(2009) reported, based on O3 measurements during 

the six 50-minute exercise periods, that the mean 

O3 concentration during the exercise portion of the 

study protocol was 72 ppb. Based on the six 

exercise period measurements, the time weighted 

average concentration across the full 6.6-hour 

exposure was 73 ppb (Schelegle et al., 2009). 

30The most recent statement from the ATS 

available at the time of the 2015 decision stated that 

‘‘[i]n drawing the distinction between adverse and 

nonadverse reversible effects, this committee 

recommended that reversible loss of lung function 

in combination with the presence of symptoms 

should be considered as adverse’’ (ATS, 2000). 

31The design values in this location during the 

study period were at or somewhat below 75 ppb 

(Wells, 2012). 

people with asthma, against an array of 

adverse health effects, drew upon the 

available scientific evidence assessed in 

the 2013 ISA, the exposure and risk 

information presented and assessed in 

the 2014 health REA (HREA), the 

consideration of that evidence and 

information in the 2014 PA, the advice 

and recommendations of the CASAC, 

and public comments on the proposed 

decision (79 FR 75234, December 17, 

2014). 

Across the different study types, the 

controlled human exposure studies, 

which were recognized to provide the 

most certain evidence indicating the 

occurrence of health effects in humans 

following specific O3 exposures, 

additionally document the roles of 

ventilation rate 26 and exposure 

duration, in addition to exposure 

concentration, in eliciting responses to 

O3 exposure (80 FR 65343, October 26, 

2015; 2014 PA, section 3.4).27 These 

aspects of the evidence were 

represented in exposure-based analyses 

developed to inform the NAAQS 

decision with estimates of exposure and 

risk associated with air quality 

conditions just meeting the thenexisting standard, and also for air 

quality conditions just meeting potential 

alternative standards (U.S. EPA, 2014a, 

hereafter 2014 HREA). The exposurebased analyses given greatest weight in 

the Administrator’s consideration of the 

HREA estimates involved comparison of 

estimates for study area populations of 

children of exposure at elevated 

exertion to exposure benchmark 

concentrations (exposures of concern). 

The benchmark concentrations (60, 70 

and 80 ppb) were identified from 

controlled human exposure studies 

(conducted with generally healthy 

adults). 

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 

then-existing standard and potential 

alternative standards considered, as 

well as judgments regarding margin of 

safety, the Administrator’s 2015 

decision considered the currently 

available information and commonly 

accepted guidelines or criteria within 

the public health community, including 

statements of the American Thoracic 

Society (ATS), an organization of 

respiratory disease specialists, advice 

from the CASAC, and public comments. 

In so doing, she recognized that the 

determination of what constitutes an 

adequate margin of safety is expressly 

left to the judgment of the EPA 

Administrator. See Lead Industries 

Ass’n v. EPA, 647 F.2d 1130, 1161–62 

(D.C. Cir 1980); Mississippi v. EPA, 744 

F.3d 1334, 1353 (D.C. Cir. 2013). In 

NAAQS reviews generally, evaluations 

of how particular primary standards 

address the requirement to provide an 

adequate margin of safety include 

consideration of such factors as the 

nature and severity of the health effects, 

the size of the sensitive population(s) at 

risk, and the kind and degree of the 

uncertainties present. Consistent with 

past practice and long-standing judicial 

precedent, the Administrator took the 

need for an adequate margin of safety 

into account as an integral part of her 

decision-making. 

In the decisions regarding adequacy of 

protection provided by the then-existing 

primary standard and on alternatives for 

a new revised standard, primary 

consideration was given to the evidence 

of respiratory effects from controlled 

human exposure studies, including 

those newly available in the review, and 

for which the exposure concentrations 

were at the lower end of those studied 

(80 FR 65342–47 and 65362–66, October 

26, 2015). This emphasis was consistent 

with comments on the strength of this 

evidence from the CASAC at that time 

(Frey, 2014b, p. 5). In placing weight on 

these studies, the Administrator at that 

time took note of the variety of 

respiratory effects reported from the 

studies of healthy adults engaged in 

quasi-continuous exercise within a 6.6- 

hour exposure to O3 concentrations of 

60 ppb and higher.28 The lowest 

exposure concentration in such studies 

for which a combination of statistically 

significant reduction in lung function 

and increase in respiratory symptoms 

was somewhat above 70 ppb,29 while 

reduced lung function and increased 

pulmonary inflammation were reported 

following such exposures to O3

concentrations as low as 60 ppb. In 

considering these findings, the 

Administrator noted that the 

combination of O3-induced lung 

function decrements and respiratory 

symptoms met ATS criteria for an 

adverse response,30 and noted CASAC 

comments, which included a caution 

regarding the potential for effects in 

some groups of people, such as people 

with asthma, at exposure concentrations 

below those affecting healthy subjects 

(Frey, 2014b, pp. 5–6; 80 FR 65343, 

October 26, 2015). With regard to the 

epidemiologic evidence, the 

Administrator noted the ISA finding 

that the pattern of effects observed 

across the range of exposures assessed 

in the controlled human exposure 

studies, increasing in severity at higher 

exposures, is coherent with (i.e., 

reasonably related to) the health 

outcomes reported to be associated with 

ambient air concentrations in 

epidemiologic studies. Additionally, 

while recognizing that most O3

epidemiologic studies reported health 

outcome associations with O3

concentrations in ambient air that 

violated the then-existing standard, the 

Administrator took note of a study that 

reported associations between shortterm O3 concentrations and asthma 

emergency department visits in children 

and adults in a U.S. location that would 

have met the then-existing standard 

over the entire 5-year study period (80 

FR 65344, October 26, 2015; Mar and 

Koenig, 2009).31 Taken together, the 

Administrator concluded that the 

scientific evidence from controlled 

human exposure and epidemiologic 

studies called into question the 

adequacy of the public health protection 

provided by the 75 ppb standard that 

had been set in 2008. 

In considering the exposure and risk 

information, the Administrator’s 2015 

decision gave particular attention to the 

exposure-based comparison-tobenchmarks analysis, focusing on the 

estimates of exposures of concern for 


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32Consideration 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). 

33 In addition to recognizing the potential for 

continued inflammation to evolve into other 

outcomes, the 2013 ISA also recognized that 

inflammation induced by a single exposure (or 

several exposures over the course of a summer) can 

resolve entirely (2013 ISA, p. 6–76; 80 FR 65331, 

October 26, 2015). 

34Although the Administrator recognized 

increased uncertainty in and placed less weight on 

the other types of HREA risk estimates, she found 

they supported her conclusion of public health 

importance on a broad national scale (80 FR 65347). 35The Administrator also noted that the CASAC 

for the prior review (2008) likewise recommended 

the standard level be revised below 75 ppb based 

on the evidence and information in the record for 

the 2008 decision (Samet, 2011; Frey and Samet, 

2012). 

36With regard to a specific concentration-based 

form, the fourth-highest daily maximum was 

selected in 1997, recognizing that a less restrictive 

form (e.g., fifth highest) would allow a larger 

percentage of sites to experience O3 peaks above the 

level of the standard, and would allow more days 

on which the level of the standard may be exceeded 

when the site attains the standard (62 FR 38868– 

38873, July 18, 1997), and there was no basis 

identified for selection of a more restrictive form 

(62 FR 38856, July 18, 1997). 

children 32 in 15 urban study areas for 

air quality conditions just meeting the 

then-current standard. Consistent with 

the finding that larger percentages of 

children than adults were estimated to 

experience exposures at or above 

benchmarks, the Administrator focused 

on the results for all children and for 

children with asthma, noting that the 

results for these two groups, in terms of 

percent of the population group, are 

virtually indistinguishable (2014 HREA, 

sections 5.3.2, 5.4.1.5 and section 5F–1). 

The Administrator placed the greatest 

weight on estimates of two or more days 

with occurrences of exposures at or 

above the benchmarks, in light of her 

increased concern about the potential 

for adverse responses with repeated 

occurrences of such exposures, noting 

that the types of effects shown to occur 

following exposures to O3

concentrations from 60 ppb to 80 ppb, 

such as inflammation, if occurring 

repeatedly as a result of repeated 

exposure, could potentially result in 

more severe effects (80 FR 65343, 65345, 

October 26, 2015; 2013 ISA, section 

6.2.3).33 The Administrator also 

considered estimates for single 

exposures at or above the higher 

benchmarks of 70 and 80 ppb (80 FR 

65345, October 26, 2015). With regard to 

the 60 ppb benchmark, while the 

Administrator recognized the effects 

reported from controlled human 

exposure studies of 60 ppb to be less 

severe than those for higher O3

concentrations, she also recognized 

there were limitations and uncertainties 

in the evidence base with regard to 

unstudied population groups. As a 

result, she judged it appropriate for the 

standard, in providing an adequate 

margin of safety, to provide some 

control of exposures at or above the 60 

ppb benchmark (80 FR 65345–65346, 

October 26, 2015). 

In considering public health 

implications of the exposure and risk 

information, the Administrator 

concluded that the exposures and risks 

projected to remain upon meeting the 

then-current (75 ppb) standard were 

reasonably judged important from a 

public health perspective. This 

conclusion was particularly based on 

her judgment that it is appropriate to set 

a standard that would be expected to 

eliminate, or almost eliminate, the 

occurrence of exposures, while at 

moderate exertion, at or above 70 and 80 

ppb (80 FR 65346, October 26, 2015). In 

addition, given that in the air quality 

scenario for the existing standard, the 

average percent of children estimated to 

experience two or more days with 

exposures at or above the 60 ppb 

benchmark approached 10% in some 

urban study areas (on average across the 

analysis years), the Administrator 

concluded that the existing standard did 

not incorporate an adequate margin of 

safety against the potentially adverse 

effects that could occur following 

repeated exposures at or above 60 ppb 

(80 FR 65345–46, October 26, 2015). 

Thus, the exposure and risk estimates 34

were judged to support a conclusion 

that the existing standard was not 

sufficiently protective and did not 

incorporate an adequate margin of 

safety. In consideration of all of the 

above, as well as the CASAC advice, 

which included the unanimous 

recommendation ‘‘that the 

Administrator revise the current 

primary ozone standard to protect 

public health’’ (Frey, 2014b, p. 5),35 the 

Administrator concluded that the thencurrent primary O3 standard (with its 

level of 75 ppb) was not requisite to 

protect public health with an adequate 

margin of safety, and should be revised 

to provide increased public health 

protection (80 FR 65346, October 26, 

2015). 

With regard to the most appropriate 

indicator for the revised standard, key 

considerations included the finding that 

O3 is the only photochemical oxidant 

(other than nitrogen dioxide) that is 

routinely monitored and for which a 

comprehensive database exists, and the 

consideration that, since the precursor 

emissions that lead to the formation of 

O3 also generally lead to the formation 

of other photochemical oxidants, 

measures leading to reductions in 

population exposures to O3 can 

generally be expected to lead to 

reductions in other photochemical 

oxidants (2013 ISA, section 3.6; 80 FR 

65347, October 26, 2015). The CASAC 

also indicated O3 to be the appropriate 

indicator ‘‘based on its causal or likely 

causal associations with multiple 

adverse health outcomes and its 

representation of a class of pollutants 

known as photochemical oxidants’’ 

(Frey, 2014b, p. ii). Based on all of these 

considerations and public comments, 

the Administrator retained O3 as the 

indicator for the primary standard (80 

FR 65347, October 26, 2015). 

With regard to averaging time, eight 

hours was the duration established in 

1997 with the replacement of the thenexisting 1-hour standard (62 FR 38856, 

July 18, 1997). The decision at that time 

was based on evidence from numerous 

controlled human exposure studies 

reporting adverse respiratory effects 

resulting from 6- to 8-hour exposures, as 

well as quantitative analyses indicating 

the control provided by an 8-hour 

averaging time of both 8-hour and 1- 

hour peak exposures and associated 

health risk (62 FR 38861, July 18, 1997; 

U.S. EPA, 1996b). The 1997 decision 

was also consistent with CASAC advice 

at that time (62 FR 38861, July 18, 1997; 

61 FR 65727, December 13, 1996). For 

similar reasons, the 8-hour averaging 

time was retained in the subsequent 

2008 review (73 FR 16436, March 27, 

2008). In 2015, the decision, based on 

then-available health effects 

information, was to again retain the 8- 

hour averaging time, as appropriate for 

addressing health effects associated 

with short-term exposures to ambient 

air O3, and based on the conclusion that 

it could effectively limit health effects 

attributable to both short- and long-term 

O3 exposures (80 FR 65348, 65350, 

October 26, 2015). 

With regard to the form for the 

standard, the existing nth-high metric 

form had been established in the 1997 

review, when the form was revised from 

an expected exceedance form. At that 

time, it was recognized that a 

concentration-based form, by giving 

proportionally more weight to years 

when 8-hour O3 concentrations are well 

above the level of the standard than 

years when concentrations are just 

above the level, better reflects the 

continuum of health effects associated 

with increasing O3 concentrations than 

does an expected exceedance form (80 

FR 65350–65352, October 26, 2015).36

The subsequent 2008 review also 


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37The Administrator viewed the results of other 

quantitative analyses in this 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). 

38Under 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). 

39The Administrator was ‘‘notably less confident 

in the adversity to public health of the respiratory 

effects that have been observed following exposures 

to O3 concentrations as low as 60 ppb,’’ based on 

her consideration of the ATS statement on judging 

adversity from transient lung function decrements 

alone, the uncertainty in the potential for such 

decrements to increase the risk of other, more 

serious respiratory effects in a population (per ATS 

recommendations on population-level risk), and the 

less clear CASAC advice regarding potential 

adversity of effects at 60 ppb compared to higher 

concentrations studied (80 FR 65363, October 26, 

2015). 

40 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). 

41While the Administrator was less concerned 

about single exposures, especially for the 60 ppb 

benchmark, she judged the HREA 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). 

considered the potential value of a 

percentile-based form, but the EPA 

concluded that, because of the differing 

lengths of the monitoring season for O3

across the U.S., such a form would not 

be effective in ensuring the same degree 

of public health protection across the 

country (73 FR 16474–75, March 27, 

2008). Additionally, the EPA recognized 

the importance of a form that provides 

stability to ongoing control programs 

and insulation from the impacts of 

extreme meteorological events that are 

conducive to O3 occurrence (73 FR 

16474–16475, March 27, 2008). In the 

2015 decision, based on all of these 

considerations, and including advice 

from the CASAC, which stated that this 

form ‘‘provides health protection while 

allowing for atypical meteorological 

conditions that can lead to abnormally 

high ambient ozone concentrations 

which, in turn, provides programmatic 

stability’’ (Frey, 2014b, p. 6), the 

existing form (the annual fourth-highest 

daily maximum 8-hour O3 average 

concentration, averaged over three 

consecutive years) was retained (80 FR 

65352, October 26, 2015). 

As for the decision on adequacy of 

protection provided by the combination 

of all elements of the existing standard, 

the 2015 decision to set the level of the 

revised standard at 70 ppb placed the 

greatest weight on the results of 

controlled human exposure studies and 

on quantitative analyses based on 

information from these studies, 

particularly analyses of O3 exposures of 

concern, consistent with CASAC advice 

and interpretation of the scientific 

evidence (80 FR 65362, October 26, 

2015; Frey, 2014b).37 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 complicated by the presence of cooccurring pollutants or pollutant 

mixtures (as is the case in epidemiologic 

studies) (80 FR 65362–65363, October 

26, 2015). With regard to this evidence, 

the Administrator at that time 

recognized that: (1) The largest 

respiratory effects, and the broadest 

range of effects, have been studied and 

reported following exposures to 80 ppb 

O3 or higher (i.e., decreased lung 

function, increased airway 

inflammation, increased respiratory 

symptoms, airway hyperresponsiveness, 

and decreased lung host defense); (2) 

exposures to O3 concentrations 

somewhat above 70 ppb have been 

shown to both decrease lung function 

and to result in respiratory symptoms; 

and (3) exposures to O3 concentrations 

as low as 60 ppb have been shown to 

decrease lung function and to increase 

airway inflammation (80 FR 65363, 

October 26, 2015). The Administrator 

also noted that 70 ppb was well below 

the O3 exposure concentration 

documented to result in the widest 

range of respiratory effects (i.e., 80 ppb), 

and below the lowest O3 exposure 

concentration shown in 6.6 hour 

exposures with quasi-continuous 

exercise to result in the combination of 

lung function decrements and 

respiratory symptoms (80 FR 65363, 

October 26, 2015). 

In considering 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 HREA 

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 revised standard with a 

level of 70 ppb was estimated to 

eliminate the occurrence of two or more 

days with exposures at or above 80 ppb 

and to virtually eliminate the 

occurrence of two or more days with 

exposures at or above 70 ppb for all 

children and children with asthma, 

even in the worst-case year and location 

evaluated (80 FR 65363–65364, October 

26, 2015).38 The Administrator’s 

consideration of exposure estimates at 

or above the 60 ppb benchmark (focused 

most particularly on multiple 

occurrences), an exposure to which the 

Administrator was less confident would 

result in adverse effects,39 as discussed 

above, was primarily in the context of 

considering the extent to which the 

health protection provided by a revised 

standard included a margin of safety 

against the occurrence of adverse O3- 

induced effects (80 FR 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 experiencing two 

or more days with exposures at or above 

60 ppb (while at moderate or greater 

exertion). This represented a more than 

60% reduction in repeated exposures 

over the estimates for the then-existing 

standard, with its level of 75 ppb. 

Given the considerable protection 

provided against repeated exposures of 

concern for all three benchmarks, 

including the 60 ppb benchmark, the 

Administrator judged that a standard 

with a level of 70 ppb would 

incorporate a margin of safety against 

the adverse O3-induced effects shown to 

occur 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).40

The Administrator also judged the 

HREA 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).41 Although she placed less 


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42More than 1600 studies are newly available and 

considered in the ISA, including more than 1000 

health studies (ISA, Appendix 10, Figure 10–2). 

43The 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, PA, Appendix 3A, 

Table 3A–3). 

44The term metabolic effects is used in the ISA 

to refer metabolic syndrome (a collection of risk 

factors including alterations in glucose and insulin 

homeostasis, high blood pressure, adiposity, 

elevated triglycerides and low high density 

lipoprotein cholesterol), diabetes, metabolic disease 

mortality, and indicators of metabolic syndrome 

that include peripheral inflammation, liver 

function, neuroendocrine signaling, and serum 

lipids (ISA, section IS.4.3.3). 

45The currently available evidence for 

cardiovascular, reproductive and nervous system 

effects, as well as mortality, is ‘‘suggestive of, but 

not sufficient to infer’’ a causal relationship with 

short- or long-term O3 exposures (ISA, Table IS–1). 

The evidence is inadequate to infer the presence or 

absence of a causal relationship between long-term 

O3 exposure and cancer (ISA, section IS.4.3.6.6). 

46The phrases ‘‘healthy adults’’ or ‘‘healthy 

subjects’’ are used to distinguish from subjects with 

asthma or other respiratory diseases because the 

‘‘the study design generally precludes inclusion of 

subjects with serious health conditions,’’ such as 

individuals with severe respiratory diseases (2013 

ISA, p. lx). 

47A quasi-continuous exercise protocol is 

common to these controlled exposure studies where 

study subjects complete six 50-minute periods of 

exercise, each followed by 10-minute periods of rest 

(e.g., ISA, Appendix 3, section 3.1.4.1.1, and p. 3– 

11; 2013 ISA, section 6.2.1.1). 

48 In summarizing FEV1 responses from 

controlled human exposure studies as 

weight on the other HREA risk estimates 

and epidemiologic evidence for 

considering the standard level, in light 

of associated uncertainties, the 

Administrator judged that a standard 

with a level of 70 ppb would be 

expected to result in important 

reductions in the population-level risk 

of endpoints on which these types of 

information are focused and provide 

associated additional public health 

protection, beyond that provided by the 

then-existing standard (80 FR 65364, 

October 26, 2015). In summary, based 

on the evidence, exposure and risk 

information, advice from the CASAC, 

and public comments, the 2015 decision 

was to revise the primary standard to be 

70 ppb, in terms of the 3-year average 

of annual fourth-highest daily maximum 

8-hour average O3 concentrations, to 

provide the requisite protection of 

public health, including the health of atrisk populations, with an adequate 

margin of safety (80 FR 65365, October 

26, 2015). 

2. Overview of Health Effects 

Information 

The information summarized in this 

section is an overview of the scientific 

assessment of the health effects 

evidence available in this review; the 

assessment is documented in the ISA 

and its policy implications are further 

discussed in the PA. In this review, as 

in past reviews, the health effects 

evidence evaluated in the ISA for O3

and related photochemical oxidants is 

focused on O3 (ISA, section IS.1.1). 

Ozone is concluded to be the most 

prevalent photochemical oxidant 

present in the atmosphere and the one 

for which there is a very large, wellestablished evidence base of its health 

and welfare effects (ISA, section IS.1.1). 

Thus, the current health effects 

evidence and the Agency’s review of the 

evidence, including the evidence newly 

available in this review,42 continues to 

focus on O3. The subsections below 

briefly summarize the following aspects 

of the evidence: The nature of O3-related 

health effects, the potential public 

health implications and populations at 

risk, and exposure concentrations 

associated with health effects. 

a. Nature of Effects 

The evidence base available in the 

current review includes decades of 

extensive evidence that clearly 

describes the role of O3 in eliciting an 

array of respiratory effects and recent 

evidence indicates 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 pulmonary function and 

respiratory symptoms, such as coughing 

and pain on deep inspiration, as a result 

of short-term exposures particularly 

when breathing at elevated rates (ISA, 

section IS.4.3.1; 2013 ISA, p. 2–26). 

These effects are demonstrated in the 

large, long-standing evidence base of 

controlled human exposure studies 43

(1978 AQCD, 1986 AQCD, 1996 AQCD, 

2006 AQCD, 2013 ISA, ISA). The 

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; ISA, Appendix 3, 

section 3.1.4.1.3), and with more severe 

health outcomes, including asthmarelated emergency department visits and 

hospital admissions (2013 ISA, section 

6.2.7; ISA, Appendix 3, sections 3.1.5.1 

and 3.1.5.2). Extensive experimental 

animal evidence informs a detailed 

understanding of mechanisms 

underlying the short-term respiratory 

effects, and studies in animal models 

describe effects of longer-term O3

exposure on the developing lung (ISA, 

Appendix 3, sections 3.1.11 and 3.2.6). 

The full body of evidence continues 

to support the conclusions of a causal 

relationship of respiratory effects with 

short-term O3 exposures and of a 

relationship of respiratory effects with 

longer-term exposures that is likely to 

be causal (ISA, sections IS.4.3.1 and 

IS.4.3.2). Further, the ISA determines 

that the relationship between short-term 

O3 exposure and metabolic effects 44 is 

likely to be causal, based primarily on 

newly available experimental animal 

evidence (ISA, section IS.4.3.3). The 

newly available evidence, particularly 

from controlled human exposure studies 

of cardiovascular endpoints, has altered 

conclusions from the last review with 

regard to relationships between shortterm O3 exposures and cardiovascular 

effects and mortality, such that the 

evidence no longer supports 

conclusions that the relationships are 

likely to be causal.45

With regard to respiratory effects from 

short-term O3 exposure, the strongest 

evidence comes from controlled human 

exposure studies, also available in the 

last review, demonstrating O3-related 

respiratory effects in generally healthy 

adults (ISA, section IS.1.3.1).46 As in the 

last review, the key evidence comes 

from the body of controlled human 

exposure studies that document 

respiratory effects in people exposed for 

short periods (6.6 to 8 hours) during 

quasi-continuous exercise.47 The 

potential for O3 exposure to elicit health 

outcomes more serious than those 

assessed in the controlled human 

exposure studies continues to be 

indicated by the epidemiologic evidence 

of associations of O3 concentrations in 

ambient air with increased incidence of 

hospital admissions and emergency 

department visits for an array of health 

outcomes, including asthma 

exacerbation, chronic obstructive 

pulmonary disease (COPD) 

exacerbation, respiratory infection, and 

combinations of respiratory diseases 

(ISA, Appendix 3, sections 3.1.5 and 

3.1.6). The strongest such evidence is 

for asthma-related outcomes and 

specifically asthma-related outcomes for 

children, indicating an increased risk 

for people with asthma and particularly 

children with asthma (ISA, Appendix 3, 

section 3.1.5.7). 

Respiratory responses observed in 

human subjects exposed to O3 for 

periods of 8 hours or less, while 

intermittently or quasi-continuously 

exercising, include lung function 

decrements (e.g., based on forced 

expiratory volume in one second [FEV1] 

measurements),48 respiratory symptoms, 


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‘‘decrements’’, an O3-induced change in FEV1 is 

typically the difference between the change 

observed with O3 exposure ([post-exposure FEV1

minus pre-exposure FEV1] divided by pre-exposure 

FEV1) and what is generally an improvement 

observed with filtered air (FA) exposure ([postexposure FEV1 minus pre-exposure FEV1] divided 

by pre-exposure FEV1). 

49A spirometric response refers to a change in the 

amount of air breathed out of the body (forced 

expiratory volumes) and the associated time to do 

so (e.g., FEV1). 

50Children are the age group most likely to be 

outdoors at activity levels corresponding to those 

that have been associated with respiratory effects in 

the human exposure studies (PA, Appendix 3D, 

section 3D.2.5.3), as recognized in section II.A.2.b 

below. 

increased airway responsiveness, mild 

bronchoconstriction (measured as an 

increase in specific airway resistance 

[sRaw]), and pulmonary inflammation, 

with associated injury and oxidative 

stress (ISA, Appendix 3, section 3.1.4; 

2013 ISA, sections 6.2.1 through 6.2.4). 

The available mechanistic evidence, 

discussed in greater detail in the ISA, 

describes pathways involving the 

respiratory and nervous systems by 

which O3 results in pain-related 

respiratory symptoms and reflex 

inhibition of maximal inspiration 

(inhaling a full, deep breath), commonly 

quantified by decreases in forced vital 

capacity (FVC) and total lung capacity. 

This reflex inhibition of inspiration 

combined with mild 

bronchoconstriction contributes to the 

observed decrease in FEV1, the most 

common metric used to assess O3- 

related lung function effects. The 

evidence also indicates that the 

additionally observed inflammatory 

response is correlated with mild airway 

obstruction, generally measured as an 

increase in sRaw (ISA, Appendix 3, 

section 3.1.3). As described below, the 

prevalence and severity of respiratory 

effects in controlled human exposure 

studies, including symptoms (e.g., pain 

on deep inspiration, shortness of breath, 

and cough), increases with increasing 

O3 concentration, exposure duration, 

and ventilation rate of exposed subjects 

(ISA, Appendix 3, sections 3.1.4.1 and 

3.1.4.2). 

Within the evidence base from 

controlled human exposure studies, the 

majority of studies involve healthy adult 

subjects (generally 18 to 35 years), 

although there are studies involving 

subjects with asthma, and a limited 

number of studies, generally of 

durations shorter than four hours, 

involving adolescents and adults older 

than 50 years. A summary of salient 

observations of O3 effects on lung 

function, based on the controlled 

human exposure study evidence 

reviewed in the 1996 and 2006 AQCDs, 

and recognized in the 2013 ISA, 

continues to pertain to this evidence 

base as it exists today: ‘‘(1) young 

healthy adults exposed to ≥80 ppb 

ozone develop significant reversible, 

transient decrements in pulmonary 

function and symptoms of breathing 

discomfort if minute ventilation (Ve) or 

duration of exposure is increased 

sufficiently; (2) relative to young adults, 

children experience similar spirometric 

responses [i.e., as measured by FEV1

and/or FVC] but lower incidence of 

symptoms from O3 exposure; (3) relative 

to young adults, ozone-induced 

spirometric responses are decreased in 

older individuals; (4) there is a large 

degree of inter-subject variability in 

physiologic and symptomatic responses 

to O3, but responses tend to be 

reproducible within a given individual 

over a period of several months; and (5) 

subjects exposed repeatedly to O3 for 

several days experience an attenuation 

of spirometric and symptomatic 

responses on successive exposures, 

which is lost after about a week without 

exposure’’ (ISA, Appendix 3, section 

3.1.4.1.1, p. 3–11).49 Repeated daily 

exposure studies at higher 

concentrations, such as 300 ppb, have 

found FEV1 responses to be enhanced 

on the second day of exposure. This 

enhanced response is absent, however, 

with repeated exposure at lower 

concentrations, perhaps as a result of a 

more complete recovery or less damage 

to pulmonary tissues (2013 ISA, section 

pp. 6–13 to 6–14; Folinsbee et al., 1994). 

With regard to airway inflammation 

and the potential for repeated 

occurrences to contribute to further 

effects, O3-induced respiratory tract 

inflammation ‘‘can have several 

potential outcomes: (1) Inflammation 

induced by a single exposure (or several 

exposures over the course of a summer) 

can resolve entirely; (2) continued acute 

inflammation can evolve into a chronic 

inflammatory state; (3) continued 

inflammation can alter the structure and 

function of other pulmonary tissue, 

leading to diseases such as fibrosis; (4) 

inflammation can alter the body’s host 

defense response to inhaled 

microorganisms, particularly in 

potentially at-risk populations such as 

the very young and old; and (5) 

inflammation can alter the lung’s 

response to other agents such as 

allergens or toxins’’ (2013 ISA, p. 6–76; 

ISA Appendix 3, section 3.1.5.6). With 

regard to O3-induced increases in 

airway responsiveness, the controlled 

human exposure study evidence for 

healthy adults generally indicates 

resolution within 18 to 24 hours after 

exposure, with slightly longer 

persistence in some individuals (ISA, 

Appendix 3, section 3.1.4.3.1; 2013 ISA, 

p. 6–74; Folinsbee and Hazucha, 2000). 

The array of O3-associated respiratory 

effects, including reduced lung 

function, respiratory symptoms, 

increased airway responsiveness, and 

inflammation are of increased 

significance to people with asthma 

given aspects of the disease that 

contribute to a baseline status that 

includes chronic airway inflammation 

and greater airway responsiveness than 

people without asthma (ISA, section 

3.1.5). For example, O3 exposure of a 

magnitude that increases airway 

responsiveness may put such people at 

potential increased risk for prolonged 

bronchoconstriction in response to 

asthma triggers (ISA, Appendix 3, p. 3– 

7, 3–28; 2013 ISA, section 6.2.9; 2006 

AQCD, section 8.4.2). The increased 

significance of effects in people with 

asthma and risk of increased exposure 

for children (from greater frequency of 

outdoor exercise) 50 is illustrated by the 

epidemiologic findings of positive 

associations between O3 exposure and 

asthma-related emergency department 

visits and hospital admissions for 

children with asthma. Thus, the 

evidence indicates O3 exposure to 

increase the risk of asthma exacerbation, 

and associated outcomes, in children 

with asthma. 

With regard to an increased 

susceptibility to infectious diseases, the 

experimental animal evidence continues 

to indicate, as described in the 2013 ISA 

and past AQCDs, the potential role for 

O3 exposures through effects on defense 

mechanisms of the respiratory tract 

(ISA, section 3.1.7.3; 2013 ISA, section 

6.2.5). The evidence base regarding 

respiratory infections and associated 

effects has been augmented in this 

review by a number of epidemiologic 

studies reporting positive associations 

between short-term O3 concentrations 

and emergency department visits for a 

variety of respiratory infection 

endpoints (ISA, Appendix 3, section 

3.1.7). 

Although the long-term exposure 

conditions that may contribute to 

further respiratory effects are less well 

understood, experimental studies, 

including with nonhuman infant 

primates, have provided evidence 

relating O3 exposure to asthma-like 

effects, and epidemiologic cohort 

studies have reported associations of O3

concentrations in ambient air with 

asthma development in children (ISA, 

IS.4.3.2 and Appendix 3, sections 

3.2.4.1.3 and 3.2.6). The biological 

plausibility of such a role for O3 has 

been indicated by animal toxicological 


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51For example, the available evidence for 

reproductive and developmental effects, as well as 

for effects on the nervous system, is suggestive of, 

but not sufficient to infer, a causal relationship 

(ISA, section IS.4.3.6.5 and Table IS–1). 

52These aspects of the current evidence base 

include: (1) A now-larger body of controlled human 

exposure studies providing evidence that is not 

consistent with a cardiovascular effect in response 

to short-term O3 exposure; (2) a paucity of 

epidemiologic evidence indicating more severe 

cardiovascular morbidity endpoints (e.g., 

emergency department visits and hospital visits for 

cardiovascular endpoints including myocardial 

infarctions, heart failure or stroke) that could 

connect the evidence for impaired vascular and 

cardiac function from animal toxicological studies 

with the evidence from epidemiologic studies of 

cardiovascular mortality; and (3) the remaining 

uncertainties and limitations recognized in the 2013 

ISA (e.g., lack of control for potential confounding 

by copollutants in epidemiologic studies) still 

remain. 

53For example, for most healthy individuals 

moderate effects on pulmonary function, such as 

transient FEV1 decrements smaller than 20% or 

transient respiratory symptoms, such as cough or 

discomfort on exercise or deep breath, would not 

be expected to interfere with normal activity, while 

larger effects on pulmonary function (e.g., FEV1

decrements of 20% or larger lasting longer than 24 

hours) and/or more severe respiratory symptoms are 

more likely to interfere with normal activity (e.g., 

PA, p. 3–30; 2006 AQCD, Table 8–2). 

evidence on biological mechanisms 

(ISA, Appendix 3, sections 3.2.3 and 

3.2.4.1.2). 

Overall, the respiratory effects 

evidence newly available in this review 

is consistent with the evidence base in 

the last review, supporting a generally 

similar understanding of the respiratory 

effects of O3 (ISA, Appendix 3, section 

3.1.4). A few recent studies provide 

insights in previously unexamined 

areas, both with regard to human study 

groups and animal models for different 

effects, while other studies confirm and 

provide depth to prior findings with 

updated protocols and techniques (ISA, 

Appendix 3, sections 3.1.11 and 3.2.6). 

Newly available epidemiologic studies 

of hospital admissions and emergency 

department visits for a variety of 

respiratory outcomes supplement the 

previously available evidence with 

additional findings of consistent 

associations with O3 concentrations 

across a number of study locations (ISA, 

Appendix 3, sections 3.1.4.1.3, 3.1.5, 

3.1.6.1.1, 3.1.7.1 and 3.1.8). These 

studies include a number that report 

positive associations for asthma-related 

outcomes, as well as a few for COPDrelated outcomes. Together these 

epidemiologic studies continue to 

indicate the potential for O3 exposures 

to contribute to such serious health 

outcomes, particularly for people with 

asthma. 

As was the case for the evidence 

available in the last review, the 

currently available evidence for health 

effects other than those of O3 exposures 

on the respiratory system is more 

uncertain than that for respiratory 

effects.51 Further, the evidence now 

available has contributed to changes in 

conclusions for some of these effects. 

For example, the current evidence for 

cardiovascular effects and mortality, 

expanded from that in the last review, 

is no longer considered sufficient to 

conclude that the relationships of shortterm exposure with these effects are 

likely to be causal (ISA, sections IS.4.3.4 

and IS.4.3.5). These changes stem from 

newly available evidence in 

combination with the uncertainties 

recognized for the evidence available in 

the last review.52 Although there exists 

largely consistent evidence for a limited 

number of O3-induced cardiovascular 

endpoints in animal toxicological 

studies and cardiovascular mortality in 

epidemiologic studies, there is a general 

lack of coherence between these results 

and findings in controlled human 

exposure and epidemiologic studies of 

cardiovascular health outcomes (ISA, 

section IS.1.3.1, Appendix 6, section 

6.1.8). The relationships are now 

characterized as suggestive of, but not 

sufficient to infer, a causal relationship 

(ISA, Appendix 4, section 4.1.17; 

Appendix 6, section 6.1.8). 

With regard to metabolic effects of 

short-term O3 exposures, the evidence 

comes primarily from experimental 

animal study findings, with a limited 

number of epidemiologic studies (ISA, 

section IS.4.3.3 and Appendix 5, section 

5.1.8 and Table 5–3). The exposure 

conditions from the animal studies 

generally involve much higher O3

concentrations (e.g., 4-hour 

concentrations of 400 to 800 ppb [ISA, 

Appendix 5, Tables 5–8 and 5–10]) than 

those commonly occurring in areas of 

the U.S. where the current standard is 

met, and the concentration in the 

available controlled human exposure 

study is similarly high, at 300 ppb (ISA, 

sections 5.1.3, 5.1.5 and 5.1.8, Table 5– 

3). The evidence for metabolic effects 

and long-term exposures is concluded to 

be suggestive of, but not sufficient to 

infer, a causal relationship (ISA, section 

IS.4.3.6.2). 

b. Public Health Implications and AtRisk Populations 

The public health implications of the 

evidence regarding O3-related health 

effects, as for other effects, are 

dependent on the type and severity of 

the effects, as well as the size of the 

population affected. Judgments or 

interpretative statements developed by 

public health experts, particularly 

experts in respiratory health, also 

inform consideration of public health 

implications. 

With regard to O3 in ambient air, the 

potential public health impacts relate 

most importantly to respiratory effects. 

Controlled human exposure studies 

have documented reduced lung 

function, respiratory symptoms, 

increased airway responsiveness, and 

inflammation, among other effects, in 

healthy adults exposed while at 

elevated ventilation, such as while 

exercising. Ozone effects in individuals 

with compromised respiratory function, 

such as individuals with asthma, are 

plausibly related to emergency 

department visits and hospital 

admissions for asthma which have been 

associated with ambient air 

concentrations of O3 in epidemiologic 

studies (as summarized in section 

II.A.2.a above; 2013 ISA, section 6.2.7; 

ISA, Appendix 3, sections 3.1.5.1 and 

3.1.5.2). 

The clinical significance of individual 

responses to O3 exposure depends on 

the health status of the individual, the 

magnitude of the responses, the severity 

of respiratory symptoms, and the 

duration of the response. While a 

particular reduction in FEV1 or increase 

in inflammation or airway 

responsiveness may not be of concern 

for a healthy group, it may increase the 

risk of a more severe effect in a group 

with asthma. As a more specific 

example, the same increase in 

inflammation or airway responsiveness 

in individuals with asthma could 

predispose them to an asthma 

exacerbation event triggered by an 

allergen to which they may be 

sensitized (e.g., ISA, Appendix 3, 

section 3.1.5.6.1; 2013 ISA, sections 

6.2.3 and 6.2.6). Duration and frequency 

of documented effects is also reasonably 

expected to influence potential 

adversity and interference with normal 

activity.53 In summary, consideration of 

differences in magnitude or severity, 

and also the relative transience or 

persistence of the responses (e.g., FEV1

changes) and respiratory symptoms, as 

well as pre-existing sensitivity to effects 

on the respiratory system, and other 

factors, are important to characterizing 

implications for public health effects of 

an air pollutant such as O3 (ATS, 2000; 

Thurston et al., 2017). 

Decisions made in past reviews of the 

O3 primary standard and associated 

judgments regarding adversity or health 

significance of measurable physiological 

responses to air pollutants have been 

informed by guidance, criteria or 

interpretative statements developed 

within the public health community, 

including the ATS, an organization of 

respiratory disease specialists, as well as 


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54Populations or lifestages can be at increased 

risk of an air pollutant-related health effect due to 

one or more of a number of factors. These factors 

can be intrinsic, such as physiological factors that 

may influence the internal dose or toxicity of a 

pollutant, or extrinsic, such as sociodemographic, 

or behavioral factors. 

55Evaluations of activity pattern data in current 

and last review indicate children to more frequently 

spend time outdoors during afternoon and early 

evening hours, while at moderate or greater exertion 

level, than other age groups (PA, Appendix 3D, 

section 3D.2.5.3, including Figure 3D–9; 2014 

HREA, section 5.4.1.5 and Appendix 5G, section 

5G–1.4). For example, for days with some time 

spent outdoors, children spend, on average, 

approximately 21⁄4 hours of afternoon time 

outdoors, 80% of which is at a moderate or greater 

exertion level, regardless of their asthma status (PA, 

Appendix 3D, section 3D.2.5.3). Adults, for days 

having some time spent outdoors, also spend 

approximately 21⁄4 hours of afternoon time outdoors 

regardless of their asthma status but the percent of 

afternoon time at moderate or greater exertion levels 

for adults (about 55%) is lower than that observed 

for children. Such analyses also note greater 

participation in outdoor events during the 

afternoon, compared to other times of day, for 

children ages 6 through 19 years old during the 

warm season months (ISA, Appendix 2, section 

2.4.1, Table 2–1). Analyses of the limited activity 

pattern data by health status do not indicate asthma 

status to have appreciable impact (PA, Appendix 

3D, section 3D.2.5.3; 2014 HREA, section 5.4.1.5). 

the advice from the CASAC. The ATS 

released its initial statement (titled 

Guidelines as to What Constitutes an 

Adverse Respiratory Health Effect, with 

Special Reference to Epidemiologic 

Studies of Air Pollution) in 1985 and 

updated it in 2000 (ATS, 1985; ATS, 

2000). The ATS described its 2000 

statement, considered in the last review 

of the O3 standard, as being intended to 

‘‘provide guidance to policy makers and 

others who interpret the scientific 

evidence on the health effects of air 

pollution for the purposes of risk 

management’’ (ATS, 2000). The recent 

statement further notes that it does not 

offer ‘‘strict rules or numerical criteria, 

but rather proposes considerations to be 

weighed in setting boundaries between 

adverse and nonadverse health effects,’’ 

providing a general framework for 

interpreting evidence that proposes a 

‘‘set of considerations that can be 

applied in forming judgments’’ for this 

context (Thurston et al., 2017). 

Similarly, in the 2000 statement, the 

ATS describes it as proposing 

‘‘principles to be used in weighing the 

evidence and setting boundaries’’ and 

states that ‘‘the placement of dividing 

lines should be a societal judgment’’ 

(ATS, 2000). The ATS explicitly states 

that it does ‘‘not attempt to provide an 

exact definition or fixed list of health 

impacts that are, or are not, adverse,’’ 

providing instead ‘‘a number of 

generalizable ‘considerations’’’ (ATS, 

2000). The ATS state there ‘‘cannot be 

precise numerical criteria, as broad 

clinical knowledge and scientific 

judgments, which can change over time, 

must be factors in determining 

adversity’’ (ATS, 2000). 

With regard to pulmonary function 

decrements, the earlier ATS statement 

concluded that ‘‘small transient changes 

in forced expiratory volume in 1 

s[econd] (FEV1) alone were not 

necessarily adverse in healthy 

individuals but should be considered 

adverse when accompanied by 

symptoms’’ (ATS, 2000). The more 

recent ATS statement continues to 

support this conclusion and also gives 

weight to findings of small lung 

function changes in the absence of 

respiratory symptoms in individuals 

with pre-existing compromised 

function, such as that resulting from 

asthma (Thurston et al., 2017). In 

keeping with the intent of these 

statements to avoid specific criteria, 

neither statement provides more 

specific descriptions of such responses, 

such as with regard to magnitude, 

duration or frequency, for consideration 

of such conclusions. The earlier ATS 

statement, in addition to emphasizing 

clinically relevant effects, also 

emphasized both the need to consider 

changes in ‘‘the risk profile of the 

exposed population,’’ and effects on the 

portion of the population that may have 

a diminished reserve that puts its 

members at potentially increased risk if 

affected by another agent (ATS, 2000). 

These concepts, including the 

consideration of the magnitude of 

effects occurring in just a subset of 

study subjects, continue to be 

recognized as important in the more 

recent ATS statement (Thurston et al., 

2017) and continue to be relevant to the 

evidence base for O3. 

The information newly available in 

this review regarding O3 exposure and 

health effects among sensitive 

populations, thoroughly evaluated in 

the ISA, has not altered our 

understanding of human populations at 

particular risk of health effects from O3

exposures (ISA, section IS.4.4). The 

respiratory effects evidence, extending 

decades into the past and augmented by 

new studies in this review, supports the 

conclusion that ‘‘individuals with preexisting asthma are at greater risk of 

ozone-related health effects based on the 

substantial and consistent evidence 

within epidemiologic studies and the 

coherence with toxicological studies’’ 

(ISA, p. IS–57). Numerous 

epidemiologic studies document 

associations of O3 with asthma 

exacerbation. Such studies indicate the 

associations to be strongest for 

populations of children which is 

consistent with their generally greater 

time outdoors while at elevated 

exertion. Together, these considerations 

indicate people with asthma, including 

particularly children with asthma, to be 

at relatively greater risk of O3-related 

effects than other members of the 

general population (ISA, section IS.4.4.2 

and Appendix 3).54

With respect to people with asthma, 

the limited evidence from controlled 

human exposure studies (which are 

primarily in adult subjects) indicates 

similar magnitude of FEV1 decrements 

as in people without asthma (ISA, 

Appendix 3, section 3.1.5.4.1). Across 

studies of other respiratory effects of O3

(e.g., increased respiratory symptoms, 

increased airway responsiveness and 

increased lung inflammation), the 

responses observed in study subjects 

generally do not differ due to the 

presence of asthma, although the 

evidence base is more limited with 

regard to study subjects with asthma 

(ISA, Appendix 3, section 3.1.5.7). 

However, the features of asthma (e.g., 

increased airway responsiveness) 

contribute to a risk of asthma-related 

responses, such as asthma exacerbation 

in response to asthma triggers, which 

may increase the risk of more severe 

health outcomes (ISA, section 3.1.5). For 

example, a particularly strong and 

consistent component of the 

epidemiologic evidence is the 

appreciable number of epidemiologic 

studies that demonstrate associations 

between ambient O3 concentrations and 

hospital admissions and emergency 

department visits for asthma (ISA, 

section IS.4.4.3.1). The strongest 

associations (e.g., highest effect 

estimates) or associations more likely to 

be statistically significant are those for 

childhood age groups, which are age 

groups most likely to spend time 

outdoors during afternoon periods 

(when O3 may be highest) and at activity 

levels corresponding to those that have 

been associated with respiratory effects 

in the human exposure studies (ISA, 

Appendix 3, sections 3.1.4.1 and 

3.1.4.2).55 The epidemiologic studies of 

hospital admissions and emergency 

department visits are augmented by a 

large body of individual-level 

epidemiologic panel studies that 

demonstrated associations of short-term 

ozone concentrations with respiratory 

symptoms in children with asthma. 

Additional support comes from 

epidemiologic studies that observed O3- 

associated increases in indicators of 

airway inflammation and oxidative 

stress in children with asthma (ISA, 

section IS.4.3.1). Together, this evidence 

continues to indicate the increased risk 

of population groups with asthma, 


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56Additionally, compared to adults, children 

have higher ventilation rates relative to their lung 

volume which tends to increase the dose 

normalized to lung surface area. (ISA, p. IS–60). 

57Human lung development begins during the 

fetal period and continues into early adulthood. 

This continued development comprises an 

extended window of potential vulnerability to O3

(ISA, p. 3–99). 

58Evidence available in the current review for 

older adults, a population identified as at risk in the 

last review, adds little to the evidence previously 

available (ISA, sections IS.4.4.2 and IS.4.4.4.2). The 

ISA notes, however, that ‘‘[t]he majority of evidence 

for older adults being at increased risk of health 

effects related to ozone exposure comes from 

studies of short-term ozone exposure and mortality 

evaluated in the 2013 Ozone ISA’’ (ISA, p. IS–52). 

Such studies are part of the larger evidence base 

that is now concluded to be suggestive, but not 

sufficient to infer a causal relationship of O3 with 

mortality (ISA, sections IS.4.3.5 and IS.4.4.4.2, 

Appendix 4, section 4.1.16.1 and 4.1.17). 

59The 2013 ISA concluded that the overall 

evidence is suggestive of socioeconomic economic 

status (SES) as a factor affecting risk of O3-related 

health outcomes ‘‘based on collective evidence from 

epidemiologic studies of respiratory hospital 

admissions but inconsistency among epidemiologic 

studies of mortality and reproductive outcomes,’’ 

additionally stating that ‘‘[f]urther studies are 

needed to confirm this relationship, especially in 

populations within the U.S.’’ (2013 ISA, p. 8–28). 

The evidence available in the current review adds 

little to the evidence available at the time of the last 

review in this area (ISA, section IS.4.4.2 and Table 

IS–10). Other factors for which the evidence 

remains suggestive of an influence on risk status are 

being male or being female and pre-existing obesity 

(ISA, Table IS–10). 

60For example, jobs in construction and 

extraction occupations and protective service 

occupations, as well as installation, maintenance 

and repair occupations and building and grounds 

cleaning and maintenance operations, had high 

percentages of employees who spent part of their 

workday outdoors (Bureau of Labor Statistics, 

2017). Such jobs often include physically 

demanding tasks and involve increased ventilation 

rates, increasing the potential for exposure to O3. 

61 In 2017 and 2018, the prevalence of asthma in 

children 0 to 17 years old was 8.4% and 7.5% 

respectively (CDC, 2019). 

62As the current standard was set to protect atrisk populations, such as people with asthma, 

populations with asthma living in areas not meeting 

the standard would be expected to be at greater risk 

of effects than others in those areas. 

63The risk of more severe health outcomes 

associated with such effects is increased in people 

with asthma as illustrated by the epidemiologic 

findings of positive associations between O3

exposure and asthma-related ED visits and hospital 

admissions. 

64The newly available 3-hour controlled human 

exposure studies (involving intermittent exercise) 

reported statistically significant respiratory 

response at 120 ppb in adults 55 to 70 years old 

(ISA, Appendix 3, section 3.1.4; PA, Appendix 3A, 

Table 3A–3). 

including particularly, children (ISA, 

Appendix 3, section 3.1.5.7). 

Children, and also outdoor adult 

workers, are at increased risk largely 

due to their generally greater time spent 

outdoors while at elevated exertion rates 

(including in summer afternoons and 

early evenings when O3 levels may be 

higher). This behavior makes them more 

likely to be exposed to O3 in ambient 

air, under conditions contributing to 

increased dose, e.g., elevated ventilation 

taking greater air volumes into the 

lungs 56 (2013 ISA, section 5.2.2.7). In 

light of the evidence summarized in the 

prior paragraph, children and outdoor 

workers with asthma may be at 

increased risk of more severe outcomes, 

such as asthma exacerbation. Further, 

there is experimental evidence from 

early life exposures of nonhuman 

primates that indicates potential for 

effects in childhood when human 

respiratory systems are under 

development 57 (ISA, section IS.4.4.4.1). 

Overall, the evidence available in the 

current review, while not increasing our 

knowledge about susceptibility or atrisk status of these population groups, is 

consistent with that in the last review 

(ISA, section IS.4.4).58

The ISA also expressly considered the 

evidence regarding O3 exposure and 

health effects among populations with 

several other potential risk factors. As in 

the last review, the evidence for low 

income and minority populations, 

remains ‘‘suggestive’’ of increased risk, 

and includes several inconsistencies 

(ISA, Tables IS–9 and IS–10).59 The ISA 

in the last review additionally identified 

a role for dietary anti-oxidants such as 

vitamins C and E in influencing risk of 

O3-related effects, such as inflammation, 

as well as a role for genetic factors to 

also confer either an increased or 

decreased risk (2013 ISA, sections 8.1 

and 8.4.1). No newly available evidence 

has been evaluated that would inform or 

change these prior conclusions (ISA, 

section IS.4.4 and Table IS–10). 

The magnitude and characterization 

of a public health impact is dependent 

upon the size and characteristics of the 

populations affected, as well as the type 

or severity of the effects. As summarized 

above, a population most at risk of 

health effects associated with O3 in 

ambient air is people with asthma. The 

National Center for Health Statistics 

data for 2017 indicate that 

approximately 7.9% of the U.S. 

populations has asthma (CDC, 2019; PA, 

Table 3–1) and this is one of the 

principal populations that the primary 

O3 NAAQS is designed to protect (80 FR 

65294, October 26, 2015). Children 

under the age of 18 account for 16.7% 

of the total U.S. population, with 6.2% 

of the total population being children 

under 5 years of age (U.S. Census 

Bureau, 2019). Another at-risk 

population group, also due to time and 

activity outdoors, is outdoor workers.60

Population groups with relatively 

greater asthma prevalence, such as 

populations in poverty and children 61

(CDC, 2019, Tables 3–1 and 4–1; PA, 

Table 3–1), might be expected to have 

a relatively greater potential for O3- 

related health impacts.62

c. Exposure Concentrations Associated 

With Effects 

The extensive evidence base for O3

health effects, compiled over several 

decades, continues to indicate 

respiratory responses to short-term 

exposures as the most sensitive effects. 

As at the time of the last review, our 

conclusions regarding O3 exposure 

concentrations associated with 

respiratory effects reflect the extensive 

longstanding evidence base of 

controlled human exposure studies of 

short-term exposures of people with and 

without asthma (ISA, Appendix 3). As 

summarized in section II.A.2.a above, 

these studies have documented an array 

of respiratory effects, including reduced 

lung function, respiratory symptoms, 

increased airway responsiveness, and 

inflammation, in study subjects 

following 1- to 8-hour exposures, 

primarily while exercising.63

The current evidence, including that 

newly available in this review, does not 

alter the scientific conclusions reached 

in the last review on exposure duration 

and concentrations associated with O3- 

related health effects. These conclusions 

were largely based on the body of 

evidence from the controlled human 

exposure studies. A limited number of 

controlled human exposure studies are 

newly available in the current review, 

with none involving lower exposure 

concentrations than those previously 

studied or finding effects not previously 

reported (ISA, Appendix 3, section 

3.1.4).64

The severity of observed responses, 

the percentage of individuals 

responding, and strength of statistical 

significance at the study group level 

have been found to increase with 

increasing exposure (ISA; 2013 ISA; 

2006 AQCD). For example, the 

magnitude of respiratory response (e.g., 

size of lung function reductions and 

magnitude of symptom scores) 

documented in the controlled human 

exposure studies is influenced by 

ventilation rate, exposure duration, and 

exposure concentration. When 

performing physical activities requiring 

elevated exertion, ventilation rate is 

increased, leading to greater potential 

for health effects due to an increased 

internal dose (2013 ISA, section 6.2.1.1, 

pp. 6–5 to 6–11). Accordingly, the 

exposure concentrations eliciting a 

given level of response after a given 

exposure duration is lower for subjects 

exposed while at elevated ventilation, 

such as while exercising (2013 ISA, pp. 


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65The lowest exposure concentration that has 

elicited a statistically significant O3-induced 

reduction in group mean lung function in an 

exposure of 2 hours or less is 120 ppb, occurring 

in trained cyclists after a 1-hour exposure during 

continuous, very heavy exercise (2013 ISA, section 

6.2.1.1; Gong et al., 1986) and in young healthy 

adults after a 2-hour exposure during intermittent 

heavy exercise (2013 ISA, section 6.2.1.1; 

McDonnell et al., 1983). 

66 Ventilation rate (V ̇ E) is a specific technical term 

referring to breathing rate in terms of volume of air 

taken into the body per unit of time. The units for 

V ̇ E are usually liters (L) per minute (min). Another 

related term is equivalent ventilation rate (EVR), 

which refers to V ̇ E normalized by a person’s body 

surface area in square meters (m2). Accordingly, the 

units for EVR are generally L/min per m2. 

67A few studies have involved exposures by 

facemask rather than freely breathing in a chamber. 

To date, there is little research differentiating 

between exposures conducted with a facemask and 

in a chamber since the pulmonary responses of 

interest do not seem to be influenced by the 

exposure mechanism. However, similar responses 

have been seen in studies using both exposure 

methods at higher O3 concentrations (Adams, 2002; 

Adams, 2003). In the facemask designs, there is a 

short period of zero O3 exposure, such that the total 

period of exposure is closer to 6 hours than 6.6 

(Adams, 2000; Adams, 2002; Adams, 2003). 

68 In these studies, the exposure concentration 

changes for each of the six hours in which there is 

exercise and the concentration during the 35- 

minute lunch is the same as in the prior (third) hour 

with exercise. For example, in the study by Adams 

(2006), the protocol for the 6.6-hour period is as 

follows: 60 minutes at 40 ppb, 60 minutes at 70 

ppb, 95 minutes at 90 ppb, 60 minutes at 70 ppb, 

60 minutes at 50 ppb and 60 minutes at 40 ppb. 

69The relationship also exists for size of FEV1

decrement with alternative exposure or dose 

metrics, including total inhaled O3 and intake 

volume averaged concentration (ISA, Appendix 3). 

70The design for the study on which the 70 ppb 

benchmark concentration is based, Schelegle et al. 

(2009), involved varying concentrations across the 

full exposure period, with a 35-minute lunch period 

following the third exposure hour during which the 

exposure concentration remains the same as in the 

third hour. The study reported the average O3

concentration measured during each of the six 

exercise periods. The mean concentration across 

these six values is 72 ppb. The time weighted 

average for the full 6.6-hour exposure period, based 

on the six reported measurements and the study 

design, is 73 ppb (Schelegle et al., 2009). Other 6.6- 

hour studies have not reported measured 

concentrations for each exposure, but have 

generally reported an exposure concentration 

precision at or tighter than 3 ppb (e.g., Adams 

2006). 

71Consistent with the ISA and 2013 ISA, the 

phrase ‘‘O3-induced’’ decrement or reduction in 

lung function or FEV1 refers to the percent change 

from pre-exposure measurement of the O3 exposure 

minus the percent change from pre-exposure 

measurement of the filtered air exposure (2013 ISA, 

p. 6–4). 

72For these four experiments, the average 

concentration across the 6.6 hour period ranged 

from 60 to 63 ppb (PA, Appendix 3A, Table 3A– 

2). 

73With regard to decrements at or above 10%, the 

percentages of study subjects with such a response 

Continued 

6–5 to 6–6; ISA Appendix 3, section 

3.1.4.2). For example, in studies of 

healthy young adults exposed while at 

rest for 2 hours, 500 ppb is the lowest 

concentration eliciting a statistically 

significant O3-induced group mean lung 

function decrement, while a 1- to 2-hour 

exposure to 120 ppb produces a 

statistically significant response in lung 

function when the ventilation rate of the 

group of study subjects is sufficiently 

increased with exercise (2013 ISA, pp. 

6–5 to 6–6).65

The exposure conditions (e.g., 

duration and exercise) given primary 

focus in the past several O3 NAAQS 

reviews are those of the 6.6-hour study 

design, which involves six 50-minute 

exercise periods during which subjects 

maintain a moderate level of exertion to 

achieve a ventilation rate of 

approximately 20 L/min per m2 body 

surface area while exercising.66 The 6.6 

hours of exposure in these quasicontinuous exercise studies has 

generally occurred in an enclosed 

chamber and the study design includes 

three hours in each of which is a 50- 

minute exercise period and a 10-minute 

rest period, followed by a 35-minute 

lunch (rest) period, which is followed 

by three more hours of exercise and rest, 

as before lunch.67 Most of these studies 

performed to date involve exposure 

maintained at a constant (unchanging) 

concentration for the full duration, 

although a subset of studies have 

concentrations that vary (generally in a 

stepwise manner) across the exposure 

period and are selected so as to achieve 

a specific target concentration as the 

exposure average.68

Evidence from studies with similar 

duration and quasi-continuous exercise 

aspects (6.6-hour duration with six 50- 

minute exercise periods) demonstrates 

an exposure-response (E–R) relationship 

for O3-induced reduction in lung 

function (Table 1; ISA, Appendix 3, 

Figure 3–3 PA, Figure 3–2).69 No studies 

of the 6.6-hour design are newly 

available in this review. The previously 

available studies of this design 

document statistically significant O3- 

induced reduction in lung function 

(FEV1) and increased pulmonary 

inflammation in young healthy adults 

exposed to O3 concentrations as low as 

60 ppb. Statistically significant group 

mean changes in FEV1, also often 

accompanied by statistically significant 

increases in respiratory symptoms, 

become more consistent across such 

studies of exposures to higher O3

concentrations, such as somewhat above 

70 ppb (73 ppb),70 and 80 ppb (Table 1 

and Appendix 3A, Table 3A–1). The 

lowest exposures concentration for 

which these studies document a 

statistically significant increase in 

respiratory symptoms is somewhat 

above 70 ppb, at 73 ppb (Schelegle et 

al., 2009). In the 6.6-hour studies, the 

group means of O3-induced 71 FEV1

reductions for target exposure 

concentrations at or below 70 ppb are 

approximately 6% or lower (Table 1). 

For example, the group means of O3- 

induced FEV1 decrements reported in 

these studies that are statistically 

significantly different from the 

responses in filtered air are 6.1% for 70 

ppb and 1.7% to 3.5% for 60 ppb (Table 

1). 

The group mean O3-induced FEV1

decrements generally increase with 

increasing O3 exposures, reflecting 

increases in both the number of the 

individuals experiencing FEV1

reductions and the magnitude of the 

FEV1 reduction (Table 1; ISA, Appendix 

3, Figure 3–3; PA, Figure 3–2). For 

example, following 6.6-hour exposures 

to a lower concentration (40 ppb), for 

which decrements were not statistically 

significant at the group mean level, 

none of 60 subjects across two separate 

studies experienced an O3-induced 

FEV1 reduction as large as 15% or more 

(Table 1; PA, Appendix 3D, Table 3D– 

19). The group mean O3-induced FEV1

decrements generally increase with 

increasing O3 exposures, reflecting 

increases in both the number of the 

individuals experiencing FEV1

reductions and the magnitude of the 

FEV1 reduction (Table 1; ISA, Appendix 

3, Figure 3–3; PA, Figure 3–2). For 

example, following 6.6-hour exposures 

to a lower concentration (40 ppb), for 

which decrements were not statistically 

significant at the group mean level, 

none of 60 subjects across two separate 

studies experienced an O3-induced 

FEV1 reduction as large as 15% or more 

(Table 1; PA, Appendix 3D, Table 3D– 

19). Across the four experiments (with 

number of subjects ranging from 30 to 

59) that have reported results for a 60 

ppb target exposure,72 the number of 

subjects experiencing this magnitude of 

FEV1 reduction (at or above 15%) varied 

(zero of 30, one of 59, two of 31 and two 

of 30 exposed subjects), while, together, 

they represent 3% of all 150 subjects. 

This percentage of subjects (with 

reductions of 15% or more) increased to 

10% (three of 31 subjects) for the study 

at 73 ppb (70 ppb target) (PA, Appendix 

3D, Table 3D–19; Schelegle et al., 2009), 

and is higher still (16%) in a variable 

exposure study at 80 ppb (PA, 

Appendix 3D, Table 3D–20; Schelegle et 

al., 2009). In addition to illustrating the 

E–R relationship, these findings also 

illustrate the considerable variability in 

magnitude of responses observed among 

study subjects (ISA, Appendix 3, section 

3.1.4.1.1; 2013 ISA, p. 6–13).73


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increased from 7% of the 150 subjects of the four 

studies with target exposures of 60 ppb (average 

exposure ranged from 60 to 63) to 19% for the study 

at 73 ppb to more than 32% in one variable 

exposure study of 80 ppb (PA, Appendix 3D, Table 

3D–20). 

TABLE 1—SUMMARY OF 6.6-HOUR CONTROLLED HUMAN EXPOSURE STUDY-FINDINGS, HEALTHY ADULTS

Endpoint O3 target exposure 

concentration A

Statistically 

significant 

effect B

O3-induced group 

mean 

response B

Study 

FEV1 Reduction ........................... 120 ppb .................... Yes ............... ¥10.3% to ¥15.9% C Horstman et al. 1990; Adams 2002; 

Folinsbee et al. (1988); Folinsbee et al. 

(1994); Adams, 2002; Adams 2000; 

Adams and Ollison 1997.D

100 ppb .................... Yes ............... ¥8.5% to ¥13.9% C Horstman et al., 1990; McDonnell et al., 

1991.D

87 ppb ...................... Yes ............... ¥12.2% .................... Schelegle et al., 2009. 

80 ppb ...................... Yes ............... ¥7.5% ...................... Horstman et al., 1990. 

¥7.7% ...................... McDonnell et al., 1991. 

¥6.5% ...................... Adams, 2002. 

¥6.2% to ¥5.5% C .. Adams, 2003. 

¥7.0% to ¥6.1% C .. Adams, 2006. 

¥7.8% ...................... Schelegle et al., 2009. 

ND E ............. ¥3.5% ...................... Kim et al., 2011.F

70 ppb ...................... Yes ............... ¥6.1% ...................... Schelegle et al., 2009. 

60 ppb ...................... Yes G ............ ¥2.9% ......................

¥2.8% 

Adams, 2006; Brown et al., 2008. 

Yes ............... ¥1.7% ...................... Kim et al., 2011. 

No ................ ¥3.5% ...................... Schelegle et al., 2009. 

40 ppb ...................... No ................ ¥1.2% ...................... Adams, 2002. 

No ................ ¥0.2% ...................... Adams, 2006. 

Increased Respiratory Symptoms 120 ppb .................... Yes ............... Increased symptom 

scores.

Horstman et al. 1990; Adams 2002; 

Folinsbee et al. 1988; Folinsbee et al. 

1994; Adams, 2002; Adams 2000; Adams 

and Ollison 1997; Horstman et al., 1990; 

McDonnell et al., 1991; Schelegle et al., 

2009; Adams, 2003; Adams, 2006.H

100 ppb .................... Yes ...............

87 ppb ...................... Yes ...............

80 ppb ...................... Yes ...............

70 ppb ...................... Yes ...............

60 ppb ......................

40 ppb ......................

No ................

No 

................................... Adams, 2006; Kim et al., 2011; Schelegle 

et al., 2009; Adams, 2002.H

Airway Inflammation .................... 80 ppb ...................... Yes ............... Multiple indicators I .... Devlin et al., 1991; Alexis et al., 2010. 

60 ppb ...................... Yes ............... Increased neutrophils Kim et al., 2011. 

Increased Airway Resistance and 

Responsiveness.

120 ppb .................... Yes ............... Increased .................. Horstman et al., 1990; Folinsbee et al., 

1994 (O3 induced sRaw not reported). 

100 ppb .................... Yes ............... Horstman et al., 1990. 

80 ppb ...................... Yes ............... Horstman et al., 1990. 

A This refers to the average concentration across the six exercise periods as targeted by authors. This differs from the time-weighted average 

concentration for the full exposure periods (targeted or actual). For example, as shown in Appendix 3A, Table 3A–2, in chamber studies implementing a varying concentration protocol with targets of 0.03, 0.07, 0.10, 0.15, 0.08 and 0.05 ppm, the exercise period average concentration is 

0.08 ppm while the time weighted average for the full exposure period (based on targets) is 0.082 ppm due to the 0.6 hour lunchtime exposure 

between periods 3 and 4. In some cases this also differs from the exposure period average based on study measurements. For example, based 

on measurements reported in Schelegle et al., (2009), the full exposure period average concentration for the 70 ppb target exposure is 73 ppb, 

and the average concentration during exercise is 72 ppb. B Statistical significance based on the O3 compared to filtered air response at the study group mean (rounded here to decimal). C Ranges reflect the minimum to maximum FEV1 decrements across multiple exposure designs and studies. Study-specific values and exposure details provided in the PA, Appendix 3A, Tables 3A–1 and 3A–2, respectively. D Citations for specific FEV1 findings for exposures above 70 ppb are provided in PA, Appendix 3A, Table 3A–1. E ND (not determined) indicates these data have not been subjected to statistical testing. 

F The data for 30 subjects exposed to 80 ppb by Kim et al. (2011) are presented in Figure 5 of McDonnell et al. (2012). 

G Adams (2006) reported FEV1 data for 60 ppb exposure by both constant and varying concentration designs. Subsequent analysis of the 

FEV1 data from the former found the group mean O3 response to be statistically significant (p < 0.002) (Brown et al., 2008; 2013 ISA, section 

6.2.1.1). The varying-concentration design data were not analyzed by Brown et al., 2008. H Citations for study-specific respiratory symptoms findings are provided in the PA, Appendix 3A, Table 3A–1. 

I Increased numbers of bronchoalveolar neutrophils, permeability of respiratory tract epithelial lining, cell damage, production of 

proinflammatory cytokines and prostaglandins (ISA, Appendix 3, section 3.1.4.4.1; 2013 ISA, section 6.2.3.1). 

For shorter exposure periods (e.g., one 

to two hours), with heavy intermittent 

or very heavy continuous exercise, 

higher exposure concentrations, ranging 

up from 80 ppb up to 400 ppb, have 

been studied (ISA, section 3.1; 2013 

ISA, section 6.2.1.1; 2006 AQCD, 

chapter 6; PA, Appendix 3A, Table 3A– 

3). Across these shorter-duration studies 

(which involved ventilation rates 2–3 

times greater than in the prolonged [6.6- 

or 8-hour] exposure studies) the lowest 

exposure concentration for which 

statistically significant respiratory 

effects were reported is 120 ppb, for a 

1-hour exposure combined with 

continuous very heavy exercise and a 2- 

hour exposure with intermittent heavy 

exercise. As recognized above, the 

increased ventilation rate associated 

with increased exertion increases the 


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74Combined with the coherent evidence from 

experimental studies, the epidemiologic studies 

‘‘can support and strengthen determinations of the 

causal nature of the relationship between health 

effects and exposure to ozone at relevant ambient 

air concentrations’’ (ISA, p. ES–17). 

75For example, these studies generally do not 

measure personal exposures of the study population 

or track individuals in the population with a 

defined exposure to O3 alone. 

76Consistent with the evaluation of the 

epidemiologic evidence of associations between O3

exposure and respiratory health effects in the ISA, 

this focuses on those studies conducted in the U.S. 

and Canada as including populations and air 

quality characteristics that may be most relevant to 

circumstances in the U.S. (ISA, Appendix 3, section 

3.1.2). Among the epidemiologic studies finding a 

statistically significant positive relationship of 

short- or long-term O3 concentrations with 

respiratory effects, there are no single-city studies 

conducted in the U.S. in locations with ambient air 

O3 concentrations that would have met the current 

standard for the entire duration of the study (ISA, 

Appendix 3, Tables 3–13, 3–14, 3–39, 3–41, 3–42 

and Appendix 6, Tables 6–5 and 6–8; PA, Appendix 

3B, Table 3B–1). There are two single city studies 

conducted in Canada that include locations for 

which the highest-monitor design values calculated 

in the PA fell below 70 ppb, at 65 and 69 ppb (PA, 

Appendix 3B, Table 3B–1; Kousha and Rowe, 2014; 

Villeneuve et al., 2007). These studies did not 

include analysis of correlations with other cooccurring pollutants or of the strength of the 

associations when accounting for effects of 

copollutants in copollutant models (ISA, Appendix 

3, Tables 3–14 and 3–39). 

77These studies indicate that sufficient early-life 

O3 exposure can cause structural and functional 

changes that could potentially contribute to airway 

obstruction and increased airway responsiveness 

(ISA, Table IS–10, p. 3–92 and p.3–113). 

78For example, the evidence base for metabolic 

effects is comprised primarily of experimental 

animal studies, and generally involve much higher 

O3 concentrations (400–800 ppb, [ISA, Appendix 5, 

Table 5–87]) than those examined in the controlled 

human exposure studies of respiratory effects (and 

much higher than concentrations commonly 

occurring in ambient air in areas of the U.S. where 

the current standard is met). There are only two 

epidemiologic studies reporting statistically 

significant positive associations of O3 with 

metabolic effects (e.g., changes in glucose, insulin, 

metabolic clearance), both based in Asian countries, 

in which there is a potential for appreciable 

differences from the U.S. in air quality patterns, 

limiting their usefulness for informing our 

understanding of exposure concentrations and 

conditions eliciting such effects in the U.S. (ISA, 

Appendix 5, section 5.1). 

amount of O3 entering the lung, where 

depending on dose and the individual’s 

susceptibility, it may cause respiratory 

effects (2013 ISA, section 6.2.1.1). Thus, 

for exposures involving a lower exertion 

level, a comparable response would not 

be expected to occur without a longer 

exposure duration (ISA, Appendix 3, 

Figure 3–3; PA, Appendix 3A, Table 

3A–1). 

With regard to the epidemiologic 

studies reporting associations between 

O3 and respiratory health outcomes 

such as asthma-related emergency 

department visits and hospitalizations, 

these studies are generally focused on 

investigating the existence of a 

relationship between O3 occurring in 

ambient air and specific health 

outcomes. Accordingly, while as a 

whole, this evidence base of 

epidemiologic studies provides strong 

support for the conclusions of 

causality,74 these studies provide less 

information on details of the specific O3

exposure circumstances that may be 

eliciting health effects associated with 

such outcomes, and whether these occur 

under air quality conditions that meet 

the current standard.75 Further, the vast 

majority of these studies were 

conducted in locations and during time 

periods that would not have met the 

current standard.76 The extent to which 

reported associations with health 

outcomes in the resident populations in 

these studies are influenced by the 

periods of higher concentrations during 

times that did not meet the current 

standard is unknown. While this does 

not lessen their importance in the 

evidence base documenting the causal 

relationship between O3 and respiratory 

effects, it means they are less 

informative in considering O3 exposure 

concentrations occurring under air 

quality conditions allowed by the 

current standard. 

With regard to the experimental 

animal evidence (largely in rodents) and 

exposure conditions associated with 

respiratory effects, the exposure 

concentrations are generally much 

greater than those examined in the 

controlled human exposure studies 

(summarized above), and higher than 

concentrations commonly occurring in 

ambient air in areas of the U.S. where 

the current standard is met. This is also 

true for the small number of early life 

studies in nonhuman primates that 

reported O3 to contribute to asthma-like 

effects in infant primates.77 The 

exposures eliciting the effects in these 

studies included multiple 5-day periods 

with O3 concentrations of 500 ppb over 

8-hours per day (ISA, Appendix 3, 

section 3.2.4.1.2). 

Thus, as in the last review the 

exposures given greatest attention in 

this review, particularly with regard to 

considering O3 exposures expected 

under air quality conditions that meet 

the current standard, are those informed 

by the controlled human exposure 

studies. The full body of evidence 

continues to indicate respiratory effects 

as the effects associated with lowest 

exposures, with conditions of exposure 

(duration, ventilation rate, as well as 

concentration) influencing dose and 

associated response. Evidence for other 

categories of effects does not indicate 

effects at comparably low exposures.78

3. Overview of Exposure and Risk 

Information 

Consideration of the scientific 

evidence available in the current 

review, as at the time of the last review, 

is informed by results from quantitative 

analyses of estimated population 

exposure and consequent risk of 

respiratory effects. These analyses in 

this review have focused on exposurebased risk analyses, producing two 

types of risk metrics. The first metric 

estimates population occurrences of 

daily maximum 7-hour average 

exposure concentrations (during periods 

of elevated breathing rates) at or above 

concentrations of potential concern 

(benchmark concentrations). The second 

metric (lung function risk) uses E–R 

information for O3 exposures and FEV1

decrements to estimate the portion of 

the simulated at-risk population 

expected to experience one or more 

days with an O3-related FEV1 decrement 

of at least 10%, 15% or 20%. Both of 

these metrics were used to characterize 

health risk associated with O3 exposures 

among the simulated population during 

periods of elevated breathing rates. 

Similar risk metrics were also derived in 

the 2014 HREA for the last review and 

the associated estimates informed the 

Administrator’s 2015 decision on the 

current standard (80 FR 65292, October 

26, 2015). 

The currently available evidence in 

this review continues to demonstrate a 

causal relationship between short-term 

O3 exposures and respiratory effects, 

with the current evidence base for 

respiratory effects largely consistent 

with that for the last review, as 

summarized in section II.A.2 above. 

Accordingly, the exposure-based 

analyses performed in this review, 

summarized below, are conceptually 

similar to those in the last review while 

also incorporating a number of updates 

that contribute to reduced uncertainty. 

Drawing on the summary in section II.C 

of the proposal, while giving relatively 

greater focus on the comparison-tobenchmarks analysis, the short sections 

below provide an overview of key 

aspects of the assessment design 

(II.A.3.a), key limitations and 

uncertainties (II.A.3.b), and exposure/ 

risk estimates (II.A.3.c). 

a. Key Design Aspects 

Exposure and risk estimates were 

derived for air quality conditions just 

meeting the current primary O3

standard, and for two additional 

scenarios reflecting conditions just 

meeting design values just lower and 

just higher than the level of the current 


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79All analyses are summarized more fully in the 

PA section 3.4 and Appendices 3C and 3D. 

80A broad variety of spatial and temporal patterns 

of O3 concentrations can exist when ambient air 

concentrations just meet the current standard. 

These patterns will vary due to many factors 

including the types, magnitude, and timing of 

emissions in a study area, as well as local factors, 

such as meteorology and topography. We focused 

our current assessment on specific study areas 

having ambient air concentrations close to 

conditions that reflect air quality that just meets the 

current standard. Accordingly, assessment of these 

study areas is more informative to evaluating the 

health protection provided by the current standard 

than would be an assessment that included areas 

with much higher and much lower concentrations. 

81Limited exploratory analyses of a hypothetical 

outdoor worker population in the 2014 HREA 

(single study area, single year) for the 75 ppb air 

quality scenario estimated an appreciably greater 

portion of this population to experience exposures 

at or above benchmark concentrations than the full 

adult or child populations simulated, although 

there are a number of uncertainties associated with 

the estimates due to appreciable limitations in the 

data underlying the analyses (2014 HREA, section 

5.4.3.2). It is expected that if an approach similar 

to that used in the 2014 HREA had been used for 

this assessment a generally similar pattern might be 

observed, although with somewhat lower overall 

percentages based on the comparison of current 

estimates with estimates from the 2014 HREA (PA, 

Appendix 3D, section 3D.3.2.4). 

82The APEX model has a history of application, 

evaluation, and progressive model development in 

estimating human exposure, dose, and risk for 

reviews of NAAQS for gaseous pollutants, 

including the last review of the O3 NAAQS (U.S. 

EPA, 2008; U.S. EPA, 2009; U.S. EPA, 2010; U.S. 

EPA, 2014a; U.S. EPA, 2018). 83The APEX model generates each simulated 

person or profile by probabilistically selecting 

values for a set of profile variables, including 

demographic variables, health status and physical 

attributes (e.g., residence with air conditioning, 

height, weight, body surface area), and activityspecific ventilation rate (PA, Appendix 3D, section 

3D.2). 

standard (65 and 75 ppb).79 The 

analyses estimated population exposure 

and risk for simulated populations in 

eight urban study areas which represent 

a variety of circumstances with regard to 

population exposure to short-term 

concentrations of O3 in ambient air. The 

areas (Atlanta, Boston, Dallas, Detroit, 

Philadelphia, Phoenix, Sacramento and 

St. Louis) range in total population size 

from approximately two to eight million 

and are distributed across seven regions 

of the U.S.: Northeast, Southeast, 

Central, East North Central, South, 

Southwest and West (PA, Appendix 3D, 

Table 3D–1). Study-area-specific 

characteristics contribute to variation in 

the estimated magnitude of exposure 

and associated risk across the urban 

study areas that reflect an array of air 

quality, meteorological, and population 

exposure conditions. The current set of 

study areas, streamlined compared to 

the 15-area set in the last review, was 

chosen to ensure it reflects the full range 

of air quality and exposure variation 

expected in major urban areas in the 

U.S. with air quality that just meets the 

current standard. Seven of the eight 

study areas were also included in the 

2014 HREA; the eighth study area 

(Phoenix) is newly added in the current 

assessment to insure representation of a 

large city in the southwest. 

Additionally, the O3 concentrations 

simulated in these areas are somewhat 

nearer the current standard than was the 

case for the 2014 HREA (PA, Appendix 

3C, Table 3C and 2014 HREA, Table 4– 

1). This contributes to a reduction in the 

uncertainty associated with 

development of the air quality scenarios 

of interest, particularly the one 

reflecting air quality conditions that just 

meet the current standard. 

With regard to the objectives for the 

analysis approach, the analyses and the 

use of a case study approach are 

intended to provide assessments of air 

quality scenarios, including particularly 

one just meeting the current standard, 

for a diverse set of areas and associated 

exposed populations. These analyses are 

not intended to provide a 

comprehensive national assessment 

(PA, section 3.4.1). Nor is the objective 

to present an exhaustive analysis of 

exposure and risk in the areas that 

currently just meet the current standard 

and/or of exposure and risk associated 

with air quality adjusted to just meet the 

current standard in areas that currently 

do not meet the standard. Rather, the 

purpose is to assess, based on current 

tools and information, the potential for 

exposures and risks beyond those 

indicated by the information available at 

the time the standard was established. 

Accordingly, use of this approach 

recognizes that capturing an appropriate 

diversity in study areas and air quality 

conditions 80 is an important aspect of 

the role of the exposure and risk 

analyses in informing the 

Administrator’s conclusions on the 

public health protection afforded by the 

current standard. 

Consistent with the health effects 

evidence in this review (summarized in 

section II.A.2 above), the focus of the 

quantitative assessment is on short-term 

exposures of individuals in the 

population during times when they are 

breathing at an elevated rate. Exposure 

and risk are characterized for four 

population groups. Two are populations 

of school-aged children, aged 5 to 18 

years: All children and children with 

asthma; two are populations of adults: 

All adults and adults with asthma. 

Estimates for adults, in terms of 

percentages, are generally lower due to 

the lesser amount and frequency of time 

spent outdoors at elevated exertion (PA, 

Appendix 3D, section 3D.3.2). The 

exception is outdoor workers who, due 

to the requirements of their job, spend 

more time outdoors at elevated exertion. 

For a number of reasons, including the 

appreciable data limitations (e.g., 

related to specific durations of time 

spent outdoors and activity data), and 

associated uncertainties summarized in 

Table 3D–64 of Appendix 3D of the PA, 

the group was not simulated in these 

analyses, a decision also made for past 

exposure assessments.81 Asthma 

prevalence estimates for the full 

populations in the eight study areas 

range from 7.7 to 11.2%; the rates for 

children in these areas range from 9.2 to 

12.3% (PA, Appendix 3D, section 

3D.3.1). 

The approach for this analysis 

incorporates an array of models and 

data (PA, section 3.4.1). Ambient air O3

concentrations were estimated in each 

study area for the air quality conditions 

of interest by adjusting hourly ambient 

air concentrations, from monitoring data 

for the years 2015–2017, using a 

photochemical model-based approach 

and then applying a spatial 

interpolation technique to produce air 

quality surfaces with high spatial and 

temporal resolution (PA, Appendix 3C). 

The final products were datasets of 

ambient air O3 concentration estimates 

with high temporal and spatial 

resolution (hourly concentrations in 500 

to 1,700 census tracts) for each of the 

eight study areas (PA, section 3.4.1 and 

Appendix 3C, section 3C.7) representing 

the three air quality scenarios assessed. 

Population exposures were estimated 

using the EPA’s Air Pollutant Exposure 

model (APEX) version 5, which 

probabilistically generates a large 

sample of hypothetical individuals from 

population demographic and activity 

pattern databases and simulates each 

individual’s movements through time 

and space to estimate their time series 

of O3 exposures occurring within 

indoor, outdoor, and in-vehicle 

microenvironments (PA, Appendix 3D, 

section 3D.2).82 The APEX model 

accounts for the most important factors 

that contribute to human exposure to O3

from ambient air, including the 

temporal and spatial distributions of 

people and ambient air O3

concentrations throughout a study area, 

the variation of ambient air-related O3

concentrations within various 

microenvironments in which people 

conduct their daily activities, and the 

effects of activities involving different 

levels of exertion on breathing rate (or 

ventilation rate) for the exposed 

individuals of different sex, age, and 

body mass in the study area (PA, 

Appendix 3D, section 3D.2).83 By 

incorporating individual activity 


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84To represent personal time-location-activity 

patterns of simulated individuals, the APEX model 

draws from the consolidated human activity 

database (CHAD) developed and maintained by the 

EPA (McCurdy, 2000; U.S. EPA, 2019a). The CHAD 

provides data on human activities through a 

database system of human diaries or daily time 

series or daily time location activity logs collected 

in surveys at city, state, and national levels. 

Included are personal attributes of survey 

participants (e.g., age, sex), along with the locations 

they visited, activities performed throughout a day, 

time-of-day the activities occurred and activity 

duration (PA, Appendix 3D, section 3D.2.5.1). 

patterns, and estimating physical 

exertion for each exposure event, the 

model addresses an important 

determinant of their exposure (2013 

ISA, section 4.4.1).84 For each exposure 

event, the APEX model tracks activity 

performed, ventilation rate, exposure 

concentration, and duration for all 

simulated individuals throughout the 

assessment period, and then utilizes the 

time-series of exposure events in 

derivation of the exposure and risk 

estimates. 

The general approach and 

methodology for the exposure-based 

assessment used in this review is 

similar to that used in the last review, 

although a number of updates and 

improvements, related to the air quality, 

exposure, and risk aspects of the 

assessment, have been implemented 

(Appendices 3C and 3D). These include 

(1) a more recent period (2015–2017) of 

ambient air monitoring data in which O3

concentrations in the eight study areas 

are at or near the current standard; (2) 

the most recent version of the 

photochemical air quality model, CAMx 

(comprehensive air quality model with 

extensions), with updates to the 

treatment of atmospheric chemistry and 

physics within the model; (3) a 

significantly expanded CHAD, that now 

has nearly 180,000 diaries, with over 

25,000 school aged children; (4) 

updated National Health and Nutrition 

Examination Survey data (2009–2014), 

which are the basis for the age- and sexspecific body weight distributions used 

to specify the individuals in the 

modeled populations; (5) updated 

algorithms used to estimate age- and 

sex-specific resting metabolic rate, a key 

input to estimating a simulated 

individual’s activity-specific ventilation 

(or breathing) rate; (6) updates to the 

ventilation rate algorithm itself; and (7) 

an approach that better matches the 

simulated exposure estimates with the 

6.6-hour duration of the controlled 

human exposure studies and with the 

study subject ventilation rates. Further, 

the current APEX model uses the most 

recent U.S. Census demographic and 

commuting data (2010), NOAA 

Integrated Surface Hourly 

meteorological data to reflect the 

assessment years studied (2015–2017), 

and updated estimates of asthma 

prevalence for all census tracts in all 

study areas based on 2013–2017 

National Health Interview Survey and 

Behavioral Risk Factor Surveillance 

System data. Additional details are 

described in the PA (e.g., PA, section 

3.4.1, Appendices 3C and 3D). 

The comparison-to-benchmarks 

analysis characterizes the extent to 

which individuals in at-risk populations 

could experience O3 exposures, while 

engaging in their daily activities, with 

the potential to elicit the effects 

reported in controlled human exposure 

studies for concentrations at or above 

specific benchmark concentrations. 

Results are characterized through 

comparison of exposure concentrations 

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, with quasi-continuous 

exercise, and that yielded different 

occurrences, of statistical significance, 

and severity of respiratory effects, as 

summarized in section II.A.2.c above 

and section II.C.1 of the proposal (PA, 

section 3.3.3; PA, Appendix 3A, section 

3A.1; PA, Appendix 3D, section 

3D.2.8.1). The lowest benchmark, 60 

ppb, represents the lowest exposure 

concentration for which controlled 

human exposure studies have reported 

statistically significant respiratory 

effects, as summarized in section 

II.A.2.c above. Exposure to 

approximately 70 ppb averaged over 6.6 

hours resulted in a larger group mean 

lung function decrement, as well as a 

statistically significant increase in 

prevalence of respiratory symptoms 

(Table 1; ISA, Appendix 3, Figure 3–3 

and section 3.1.4.1.1; Schelegle et al., 

2009). Studies of exposures to 

approximately 80 ppb have reported 

larger lung function decrements at the 

study group mean than following 

exposures to 60 or 70 ppb, in addition 

to an increase in airway inflammation, 

increased respiratory symptoms, 

increased airway responsiveness, and 

decreased resistance to other respiratory 

effects (ISA, Appendix 3, sections 

3.1.4.1—3.1.4.4; PA, Figure 3–2 and 

section 3.3.3). 

The APEX-generated exposure 

concentrations for comparison to these 

benchmark concentrations is the average 

of concentrations encountered by an 

individual while at an activity level that 

elicits the specified elevated ventilation 

rate. The incidence of such exposures 

above the benchmark concentrations are 

summarized for each simulated 

population, study area, and air quality 

scenario in Appendix 3D of the PA. 

The lung function risk analysis 

estimates the extent to which 

individuals in exposed populations 

could experience O3-induced lung 

function decrements of different sizes in 

two different ways. The populationbased E–R function approach uses 

quantitative descriptions of the E–R 

relationships for study group incidence 

of different magnitudes of lung function 

decrements based on individual study 

subject observations (PA, Appendix 3D, 

section 3D.2.8.2.1). The individualbased McDonnell-Smith-Stewart (MSS) 

model uses quantitative estimates of 

biological processes identified as 

important in eliciting the different sizes 

of decrements at the individual level, 

with a factor that also provides a 

representation of intra- and interindividual response variability (PA, 

Appendix 3D, section 3D.2.8.2.2; 

McDonnell et al., 2013). The two 

approaches, summarized in sections II.C 

and II.D.1 of the proposal and described 

in detail in Appendix 3D of the PA, 

utilize evidence from the 6.6-hour 

controlled human exposure studies in 

different ways, and accordingly, differ 

in strengths, limitations and 

uncertainties. 

While the lung function risk analysis 

focuses only on the specific O3 effect of 

FEV1 reduction, the comparison-tobenchmark analysis, with its use of 

multiple benchmark concentrations, 

provides for risk characterization of the 

array of respiratory effects elicited by O3

exposure, the type and severity of which 

increase with increased exposure 

concentration. In this way, the 

comparison-to-benchmark analysis 

(involving comparison of daily 

maximum 7-hour average exposure 

concentrations that coincide with 7- 

hour average elevated ventilation rates 

at or above the target rate to benchmark 

concentrations) provides perspective on 

the extent to which the air quality being 

assessed could be associated with 

discrete exposures to O3 concentrations 

reported to result in an array of 

respiratory effects. For example, 

estimates of such exposures can indicate 

the potential for O3-related effects in the 

exposed population, including effects 

for which we do not have E–R functions 

that could be used in quantitative risk 

analyses. Thus, the comparison-tobenchmark analysis provides for a 

broader risk characterization with 

consideration of the array of O3-related 

respiratory effects. 

b. Key Limitations and Uncertainties 

Uncertainty in the exposure and risk 

analyses was characterized using a 


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largely qualitative approach adapted 

from the World Health Organization 

approach for characterizing uncertainty 

in exposure assessment (WHO, 2008) 

augmented by several quantitative 

sensitivity analyses for key aspects of 

the assessment approach (PA, section 

3.4.4 and Appendix 3D, section 3D.3.4). 

This characterization and associated 

analyses build on information generated 

from a previously conducted 

quantitative uncertainty analysis of 

population-based O3 exposure modeling 

(Langstaff, 2007), considering the 

various types of data, algorithms, and 

models that together yield exposure and 

risk estimates for the eight study areas. 

In this way, we considered the 

limitations and uncertainties underlying 

these data, algorithms, and models and 

the extent of their influence on the 

resultant exposure/risk estimates using 

the general approach applied in past 

risk and exposure assessments for O3, 

nitrogen dioxide, carbon monoxide, and 

sulfur dioxide (U.S. EPA, 2008; U.S. 

EPA, 2010; U.S. EPA, 2014a; U.S. EPA, 

2018). 

Key uncertainties and limitations in 

data and tools that affect the 

quantitative estimates of exposure and 

risk and their interpretation in the 

context of considering the current 

standard are summarized here. These 

include uncertainty related to 

estimation of the concentrations in 

ambient air for the current standard and 

the additional air quality scenarios; lung 

function risk approaches that rely, to 

varying extents, on extrapolating from 

controlled human exposure study 

conditions to lower exposure 

concentrations, lower ventilation rates, 

and shorter durations; and 

characterization of risk for particular 

population groups that may be at 

greatest risk, particularly for people 

with asthma, and particularly children 

with asthma. Areas in which 

uncertainty has been reduced by new or 

updated information or methods 

include the use of updated air quality 

modeling, with a more recent model 

version and model inputs, applied to 

study areas with design values near the 

current standard, as well as updates to 

several inputs to the exposure model, 

including changes to the exposure 

duration to better match those in the 

controlled human exposure studies and 

an alternate approach to characterizing 

periods of activity while at moderate or 

greater exertion for simulated 

individuals. 

With regard to the analysis approach 

overall, two updates since the 2014 

HREA reduce uncertainty in the results. 

The first relates to identifying when 

simulated individuals may be at 

moderate or greater exertion, with the 

new approach reducing the potential for 

overestimation of the number of people 

achieving the associated ventilation 

rate, which was an important 

uncertainty in the 2014 HREA. 

Additionally, the current analysis focus 

on exposures of 7 hours duration better 

represents the 6.6-hour exposures from 

the controlled human exposure studies 

(than the 8-hour exposure durations 

used for the 2014 HREA and prior 

assessments). 

Additional aspects of the analytical 

design pertaining to both exposurebased risk metrics include the 

estimation of ambient air O3

concentrations for the air quality 

scenarios, and main components of the 

exposure modeling. Uncertainties 

include the modeling approach used to 

adjust ambient air concentrations to 

meet the air quality scenarios of interest 

and the method used to interpolate 

monitor concentrations to census tracts. 

While the adjustment to conditions 

near, just above, or just below the 

current standard is an important area of 

uncertainty, the size of the adjustment 

needed to meet a given air quality 

scenario is minimized with the selection 

of study areas for which recent O3

design values were near the level of the 

current standard. Also, more recent data 

are used as inputs for the air quality 

modeling, such as more recent O3

concentration data (2015–2017), 

meteorological data (2016) and 

emissions data (2016), as well as a 

recently updated air quality 

photochemical model which includes 

state-of-the-science atmospheric 

chemistry and physics (PA, Appendix 

3C). Further, the number of ambient 

monitors sited in each of the eight study 

areas provides a reasonable 

representation of spatial and temporal 

variability for the air quality conditions 

simulated in those areas. Among other 

key aspects, there is uncertainty 

associated with the simulation of study 

area populations (and at-risk 

populations), including those with 

particular physical and personal 

attributes. As also recognized in the 

2014 HREA, exposures could be 

underestimated for some population 

groups that are frequently and routinely 

outdoors during the summer (e.g., 

outdoor workers, children). In addition, 

longitudinal activity patterns do not 

exist for these and other potentially 

important population groups (e.g., those 

having respiratory conditions other than 

asthma), limiting the extent to which 

the exposure model outputs reflect 

information that may be particular to 

these groups. Important uncertainties in 

the approach used to estimate energy 

expenditure (i.e., metabolic equivalents 

of work or METs used to estimate 

ventilation rates), include the use of 

longer-term average MET distributions 

to derive short-term estimates, along 

with extrapolating adult observations to 

children. Both of these approaches are 

reasonable based on the availability of 

relevant data and appropriate 

evaluations conducted to date, and 

uncertainties associated with these steps 

are somewhat reduced in the current 

analyses (compared to the 2014 HREA) 

because of the added specificity, and 

use of redeveloped METs distributions 

(based on newly available information), 

which is expected to more realistically 

estimate activity-specific energy 

expenditure. 

There are some uncertainties that 

apply to the estimation of lung function 

risk and not to the comparison-tobenchmarks analysis. For example, both 

lung function risk approaches utilized 

in the risk analyses incorporate some 

degree of extrapolation beyond the 

exposure circumstances evaluated in the 

controlled human exposure studies. 

Accordingly, the uncertainty in the lung 

function risk estimates increases with 

decreasing exposure concentration and 

is particularly increased for 

concentrations below those evaluated in 

controlled human exposure studies (85 

FR 49857–49859, PA, section 3.4.4 and 

Appendix 3D, section 3D.3.4). The two 

lung function risk approaches differ in 

how they extrapolate beyond the 

controlled human exposure study 

conditions and in the impact on the 

estimates. The E–R function approach 

generates nonzero predictions from the 

full range of nonzero concentrations for 

7-hour average durations in which the 

average exertion levels meets or exceeds 

the target. The MSS model, which 

draws on evidence-based concepts of 

how human physiological processes 

respond to O3, extrapolates beyond the 

controlled experimental conditions with 

regard to exposure concentration, 

duration and ventilation rate (both 

magnitude and duration). Differences in 

percent of the risk estimates for days for 

which the highest 7-hour average 

concentration is below the lowest 6.6- 

hour exposure concentration tested, as 

presented in the PA, Tables 3–6 and 3– 

7, illustrate the impact. 

An overarching area of uncertainty, 

remaining from the last review and 

important to consideration of the 

exposure and risk analysis results, 

relates to the underlying health effects 

evidence base. Although the 

quantitative analysis focuses on the 

evidence providing the ‘‘strongest 

evidence’’ of O3 respiratory effects (ISA, 


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85While the duration of an O3 season for each 

year may vary across the study areas, for the 

purposes of the exposure and risk analyses, the O3

season in each study area is considered 

synonymous with a year. These seasons capture the 

times during the year when concentrations are 

elevated (80 FR 65419–65420, October 26, 2015). 

p. IS–1), the controlled human exposure 

studies, and on the array of respiratory 

responses documented in those studies, 

evidence is lacking from controlled 

human exposure studies at the lower 

concentrations (e.g., 60, 70 and 80 ppb) 

for children and for people of any age 

with asthma. While the limited 

evidence informing our understanding 

of potential risk to people with asthma 

is uncertain, it indicates the potential 

for this group, given their disease status, 

to be at great risk, as summarized in 

section II.A.2 above. Such a conclusion 

is consistent with the epidemiologic 

study findings of positive associations 

of O3 concentrations with asthmarelated ED visits and hospital 

admissions (and the higher effect 

estimates from these studies). 

c. Summary of Exposure and Risk 

Estimates 

The benchmark-based risk metric 

results are summarized in terms of the 

percent of the simulated populations of 

all children and children with asthma 

estimated to experience at least one day 

per year 85 with a 7-hour average 

exposure concentration at or above the 

different benchmark concentrations 

while breathing at elevated rates under 

air quality conditions just meeting the 

current standard (Table 2). Given the 

recognition of people with asthma as an 

at-risk population and the relatively 

greater amount and frequency of time 

spent outdoors at elevated exertion of 

children, this summary focuses on the 

estimates from the comparison-tobenchmarks analysis for children, 

including children with asthma, which 

were the focus of the Administrator’s 

proposed decision. Under air quality 

conditions just meeting the current 

standard, less than 0.1% of any study 

area’s children with asthma, on average, 

were estimated to experience any days 

per year with a 7-hour average exposure 

at or above 80 ppb, while breathing at 

elevated rates (Table 3; PA, section 3.4 

and Appendix 3D). With regard to the 

70 ppb benchmark, the study areas’ 

estimates for children with asthma 

range up to 0.7 percent (0.6% for all 

children), on average across the 3-year 

period, and range up to 1.0% in a single 

year. Approximately 3% to nearly 9% of 

each study area’s simulated children 

with asthma, on average across the 3- 

year period, are estimated to experience 

one or more days per year with a 7-hour 

average exposure at or above 60 ppb. 

This range is very similar for the 

populations of all children. 

Regarding multiday occurrences, the 

analyses indicate that no children 

would be expected to experience more 

than a single day with a 7-hour average 

exposure at or above 80 ppb in any year 

simulated in any location (Table 2). For 

the 70 ppb benchmark, the estimate is 

less than 0.1% of any area’s children (on 

average across 3-year period), both those 

with asthma and all children. The 

estimates for the 60 ppb benchmark are 

slightly higher, with up to 3% of 

children estimated to experience more 

than a single day with a 7-hour average 

exposure at or above 60 ppb, on average 

(and more than 4% in the highest year 

across all eight study area locations). 

Framed from the perspective of 

estimated protection provided by the 

current standard, these results indicate 

that, in the single year with the highest 

concentrations across the 3-year period, 

99% of the population of children with 

asthma would not be expected to 

experience such a day with an exposure 

at or above the 70 ppb benchmark; 

99.9% would not be expected to 

experience such a day with exposure at 

or above the 80 ppb benchmark. The 

estimates, on average across the 3-year 

period, indicate that over 99.9%, 99.3% 

and 91.2% of the population of children 

with asthma would not be expected to 

experience a day with a 7-hour average 

exposure while at elevated ventilation 

that is at or above 80 ppb, 70 ppb and 

60 ppb, respectively (Table 1). Further, 

more than approximately 97% of all 

children or children with asthma are 

estimated to be protected against 

multiple days of exposures at or above 

60 ppb. 

TABLE 2—PERCENT AND NUMBER OF SIMULATED CHILDREN AND CHILDREN WITH ASTHMA ESTIMATED TO EXPERIENCE AT

LEAST ONE OR MORE DAYS PER YEAR WITH A 7-HOUR AVERAGE EXPOSURE AT OR ABOVE INDICATED CONCENTRATION WHILE BREATHING AT AN ELEVATED RATE IN AREAS JUST MEETING THE CURRENT STANDARD

Exposure concentration 

(ppb) 

One or more days Two or more days Four or more days 

Average per 

year 

Highest in a 

single year 

Average per 

year 

Highest in a 

single year 

Average per 

year 

Highest in a 

single year 

Children with asthma—percent of simulated population A

≥80 ........................................................... 0 B–<0.1 C 0.1 0 0 0 0 

≥70 ........................................................... 0.2–0.7 1.0 <0.1 0.1 0 0 

≥60 ........................................................... 3.3–8.8 11.2 0.6–3.2 4.9 <0.1–0.8 1.3 

—number of individuals A

≥80 ........................................................... 0–67 202 0 0 0 0 

≥70 ........................................................... 93–1145 1616 3–39 118 0 0 

≥60 ........................................................... 1517–8544 11776 282–2609 3977 23–637 1033 

All children—percent of simulated population A

≥80 ........................................................... 0 B–<0.1 0.1 0 0 0 0 

≥70 ........................................................... 0.2–0.6 0.9 <0.1 0.1 0–<0.1 <0.1 

≥60 ........................................................... 3.2–8.2 10.6 0.6–2.9 4.3 <0.1–0.7 1.1 

—number of individuals A

≥80 ........................................................... 0–464 1211 0 0 0 0 

≥70 ........................................................... 727–8305 11923 16–341 660 0–5 14 


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86For example, the 2015 decision to set the 

standard level at 70 ppb noted that ‘‘a revised 

standard with a level of 70 ppb is estimated to 

eliminate the occurrence of two or more exposures 

of concern to O3 concentrations at or above 80 ppb 

and to virtually eliminate the occurrence of two or 

more exposures of concern to O3 concentrations at 

or above 70 ppb for all children and children with 

asthma, even in the worst-case year and location 

evaluated’’ (80 FR 65363, October 26, 2015). This 

statement remains true for the current assessment 

(Table 3). For the 60 ppb benchmark, on which the 

2015 decision placed relatively greater weight for 

multiple (versus single) occurrences of exposures at 

or above it, the Administrator at that time noted the 

2014 HREA estimates for the 70 ppb air quality 

scenario that estimated 0.5 to 3.5% of children to 

experience multiple such occurrences on average 

across the study areas, stating that the now-current 

standard ‘‘is estimated to protect the vast majority 

of children in urban study areas . . . from 

experiencing two or more exposures of concern at 

or above 60 ppb’’ (80 FR 65364, October 26, 2015). 

The corresponding estimates, on average across the 

3-year period in the current assessments, are 

remarkably similar at 0.6 to 2.9% (Table 3). 

TABLE 2—PERCENT AND NUMBER OF SIMULATED CHILDREN AND CHILDREN WITH ASTHMA ESTIMATED TO EXPERIENCE AT

LEAST ONE OR MORE DAYS PER YEAR WITH A 7-HOUR AVERAGE EXPOSURE AT OR ABOVE INDICATED CONCENTRATION WHILE BREATHING AT AN ELEVATED RATE IN AREAS JUST MEETING THE CURRENT STANDARD—Continued 

Exposure concentration 

(ppb) 

One or more days Two or more days Four or more days 

Average per 

year 

Highest in a 

single year 

Average per 

year 

Highest in a 

single year 

Average per 

year 

Highest in a 

single year 

≥60 ........................................................... 14928–69794 96261 2601–24952 36643 158–5997 9554 

A Estimates for each study area were averaged across the 3-year assessment period. Ranges reflect the ranges of averages. 

B A value of zero (0) means that there were no individuals estimated to have the selected exposure in any year. 

C An entry of <0.1 is used to represent small, non-zero values that do not round upwards to 0.1 (i.e., <0.05). 

These estimates are of generally 

similar magnitude to those which were 

the focus in the 2015 decision 

establishing the current standard (Table 

3; PA, sections 3.1 and 3.4, Appendix 

3D, section 3D.3.2.4, Table 3D–38).86

The differences observed are generally 

slight, likely reflecting influences of a 

number of the differences in the 

quantitative modeling and analyses 

performed in the current assessment 

from those for the 2014 HREA, 

summarized in section II.A.3.a above 

(e.g., 2015–2017 vs. 2006–2010 

distribution of ambient air O3

concentrations, better matching of 

simulated exposure estimates with the 

6.6-hour duration of the controlled 

human exposure studies and with the 

study subject ventilation rates). Much 

larger differences are seen between 

different air quality scenario results for 

the same benchmark. For example, for 

the 70 ppb benchmark, the differences 

between the 75 ppb and current 

standard scenario (or between the 65 

ppb and current standard scenarios) in 

either assessment are appreciably larger 

than the slight differences between the 

two assessments for any one air quality 

scenario. 

TABLE 3—COMPARISON OF CURRENT ASSESSMENT AND 2014 HREA (ALL STUDY AREAS) FOR PERCENT OF CHILDREN

ESTIMATED TO EXPERIENCE AT LEAST ONE, OR TWO, DAYS WITH AN EXPOSURE AT OR ABOVE BENCHMARKS WHILE

AT MODERATE OR GREATER EXERTION

Air Quality 

Scenario 

(DV,C ppb) 

Estimated average % of 

simulated children with at 

least one day per year at or 

above benchmark 

(highest in single season) 

Estimated average % of 

simulated children with at 

least two days per year at or 

above benchmark 

(highest in single season) 

Current PAA 2014 HREAB Current PAA 2014 HREAB

Benchmark Exposure Concentration of 80 ppb 

75 ..................................................................................................... <0.1 A–0.3 (0.6) 0–0.3 (1.1) 0–<0.1 (<0.1) 0 (0.1) 

70 ..................................................................................................... 0–<0.1 (0.1) 0–0.1 (0.2) 0 (0) 0 (0) 

65 ..................................................................................................... 0–<0.1 (<0.1) 0 (0) 0 (0) 0 (0) 

Benchmark Exposure Concentration of 70 ppb 

75 ..................................................................................................... 1.1–2.0 (3.4) 0.6–3.3 (8.1) 0.1–0.3 (0.7) 0.1–0.6 (2.2) 

70 ..................................................................................................... 0.2–0.6 (0.9) 0.1–1.2 (3.2) <0.1 (0.1) 0–0.1 (0.4) 

65 ..................................................................................................... 0–0.2 (0.2) 0–0.2 (0.5) 0–<0.1 (<0.1) 0 (0) 

Benchmark Exposure Concentration of 60 ppb 

75 ..................................................................................................... 6.6–15.7 (17.9) 9.5–17.0 (25.8) 1.7–8.0 (9.9) 3.1–7.6 (14.4) 

70 ..................................................................................................... 3.2–8.2 (10.6) 3.3–10.2 (18.9) 0.6–2.9 (4.3) 0.5–3.5 (9.2) 

65 ..................................................................................................... 0.4–2.3 (3.7) 0–4.2 (9.5) <0.1–0.3 (0.5) 0–0.8 (2.8) 

A For the current analysis, calculated percent is rounded to the nearest tenth decimal using conventional rounding. Values equal to zero are 

designated by ‘‘0’’ (there are no individuals exposed at that level). Small, non-zero values that do not round upwards to 0.1 (i.e., <0.05) are given 

a value of ‘‘<0.1’’. B For the 2014 HREA. calculated percent was rounded to the nearest tenth decimal using conventional rounding. Values that did not round upwards to 0.1 (i.e., <0.05) were given a value of ‘‘0’’. C The monitor location with the highest concentrations in each area had a design value just equal to the indicated value. 


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B. Conclusions on the Primary Standard 

In drawing conclusions on the 

adequacy of the current primary 

standard, in view of the advances in 

scientific knowledge and additional 

information now available, the 

Administrator has considered the 

currently available health effects 

evidence and exposure/risk information. 

He additionally has considered the 

evidence base, information, and policy 

judgments that were the foundation of 

the last review, to the extent they 

remain relevant in light of the currently 

available information. The 

Administrator has taken into account 

both evidence-based and exposure- and 

risk-based considerations discussed in 

the PA, as well as advice from the 

CASAC and public comments. 

Evidence-based considerations draw 

upon the EPA’s assessment and 

integrated synthesis of the scientific 

evidence, particularly that from 

controlled human exposure studies and 

epidemiologic studies evaluating health 

effects related to O3 exposures as 

presented in the ISA, with a focus on 

policy-relevant considerations as 

discussed in the PA (summarized in 

sections II.B and II.D.1 of the proposal 

and section II.A.2 above). The exposureand risk-based considerations draw 

from the results of the quantitative 

analyses presented and considered in 

the PA (as summarized in section II.C of 

the proposal and section II.A.3 above). 

The consideration of the evidence and 

exposure/risk information in the PA 

informed the Administrator’s proposed 

conclusions and judgments in this 

review, and his associated proposed 

decision. Section II.B.1 below briefly 

summarizes the basis for the 

Administrator’s proposed decision, 

drawing from section II.D of the 

proposal. Section II.B.1.a provides a 

brief overview of key aspects of the 

policy evaluations presented in the PA, 

and the advice and recommendations of 

the CASAC are summarized in section 

II.B.1.b. An overview of the 

Administrator’s proposed conclusions is 

presented in section II.B.1.c. Public 

comments on the proposed decision are 

addressed in section II.B.2, and the 

Administrator’s conclusions and 

decision in this review regarding the 

adequacy of the current primary 

standard and whether any revisions are 

appropriate are described in section 

II.B.3. 

1. Basis for Proposed Decision 

a. Policy-Relevant Evaluations in the PA 

The main focus of the policy-relevant 

considerations in the PA is 

consideration of the question: Does the 

currently available scientific evidenceand exposure/risk-based information 

support or call into question the 

adequacy of the protection afforded by 

the current primary O3 standard? The 

PA response to this overarching 

question takes into account discussions 

that address the specific policy-relevant 

questions for this review, focusing first 

on consideration of the evidence, as 

evaluated in the ISA, including that 

newly available in this review, and the 

extent to which it alters key conclusions 

supporting the current standard. The PA 

also considers the quantitative exposure 

and risk estimates drawn from the 

exposure/risk analyses (presented in 

detail in Appendices 3C and 3D of the 

PA), including associated limitations 

and uncertainties, and the extent to 

which they may indicate different 

conclusions from those in the last 

review regarding the magnitude of risk, 

as well as level of protection from 

adverse effects, associated with the 

current standard. The PA additionally 

considers the key aspects of the 

evidence and exposure/risk estimates 

that were emphasized in establishing 

the current standard, as well as the 

associated public health policy 

judgments and judgments about the 

uncertainties inherent in the scientific 

evidence and quantitative analyses that 

are integral to the Administrator’s 

consideration of whether the currently 

available information supports or calls 

into question the adequacy of the 

current primary O3 standard (PA, 

section 3.5). 

As summarized in section II.D.1 of the 

proposal, based on the evidence in the 

ISA, the PA concludes that the 

respiratory effects evidence newly 

available in this review is consistent 

with the evidence base in the last 

review, supporting a generally similar 

understanding of the respiratory effects 

of O3 (PA, section 3.5.4; ISA, Appendix 

3). As was the case for the evidence 

available in the last review, the 

currently available evidence for health 

effects other than those of O3 exposures 

on the respiratory system is more 

uncertain than that for respiratory 

effects. Such effects include metabolic 

effects, for which the evidence available 

in this review is sufficient to conclude 

there to likely be a causal relationship 

with short-term O3 exposures and 

suggestive of, but not sufficient to infer, 

such a relationship between long-term 

O3 exposure (ISA, section IS.1.3.1). 

These new determinations are based on 

evidence largely from experimental 

animal studies, that is newly available 

in this review (ISA, Appendix 5). 

Additionally, newly available evidence 

regarding cardiovascular effects and 

mortality, in combination with 

uncertainties in the previously available 

evidence that had been identified in the 

last review, contributes to conclusions 

that the evidence is suggestive of, but 

not sufficient to infer, causal 

relationships with O3 exposures (ISA, 

Appendix 4, section 4.1.17 and 

Appendix 6, section 6.1.8). As in the 

last review, the evidence is also 

suggestive of such relationships for 

reproductive and developmental effects, 

and nervous system effects (ISA, section 

IS.1.3.1). 

In evaluating the policy implications 

of the current evidence, the PA observes 

that within the respiratory effects 

evidence base, the most certain 

evidence comes from controlled human 

exposure studies, the majority of which 

involve healthy adult subjects (generally 

18 to 35 years), although there are 

studies (generally not at the lowest 

studied exposures) involving subjects 

with asthma, and a limited number of 

studies, generally of durations shorter 

than four hours, involving adolescents 

and adults older than 50 years. 

Respiratory responses observed in 

human subjects exposed to O3 for 

periods of 8 hours or less, while 

intermittently or quasi-continuously 

exercising, include lung function 

decrements (e.g., based on FEV1

measurements), respiratory symptoms, 

increased airway responsiveness, mild 

bronchoconstriction (measured as an 

increase in sRaw), and pulmonary 

inflammation, with associated injury 

and oxidative stress (ISA, Appendix 3, 

section 3.1.4; 2013 ISA, sections 6.2.1 

through 6.2.4). Newly available 

epidemiologic studies of hospital 

admissions and emergency department 

visits for a variety of respiratory 

outcomes supplement the previously 

available evidence with additional 

findings of consistent associations with 

O3 concentrations across a number of 

study locations (ISA, Appendix 3, 

sections 3.1.4.1.3, 3.1.5, 3.1.6.1.1, 

3.1.7.1 and 3.1.8). Together, the clinical 

and epidemiological bodies of evidence, 

in combination with the insights gained 

from the experimental animal evidence, 

continue to indicate the potential for O3

exposures to contribute to serious health 

outcomes and to indicate the increased 

risk of population groups with asthma, 

including particularly, children (ISA, 

Appendix 3, section 3.1.5.7). 

The PA concludes that the newly 

available evidence in this review does 

not alter conclusions from the last 

review on exposure duration and 

concentrations associated with O3- 

related effects, observing that the 6.6- 

hour controlled human exposure studies 


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87 In the last review, the advice from the prior 

CASAC included a range of recommended levels for 

the standard, with the CASAC concluding that 

‘‘there is adequate scientific evidence to 

recommend a range of levels for a revised primary 

ozone standard from 70 ppb to 60 ppb’’ (Frey, 

2014b, p. ii). In so doing, the prior CASAC noted 

that ‘‘[i]n reaching its scientific judgment regarding 

a recommended range of levels for a revised ozone 

primary standard, the CASAC focused on the 

scientific evidence that identifies the type and 

extent of adverse effects on public health’’ and 

further acknowledged ‘‘that the choice of a level 

within the range recommended based on scientific 

evidence is a policy judgment under the statutory 

mandate of the Clean Air Act’’ (Frey, 2014b, p. ii). 

The prior CASAC then described that its ‘‘policy 

advice [emphasis added] is to set the level of the 

standard lower than 70 ppb within a range down 

to 60 ppb, taking into account [the Administrator’s] 

judgment regarding the desired margin of safety to 

protect public health, and taking into account that 

lower levels will provide incrementally greater 

margins of safety’’ (Frey, 2014b, p. ii). 

of respiratory effects remain the focus 

for our consideration of exposure 

circumstances associated with O3 health 

effects. The PA additionally recognizes 

that while the evidence clearly 

demonstrates that short-term O3

exposures cause respiratory effects, as 

was the case in the last review, 

uncertainties remain in several aspects 

of our understanding of these effects. 

These include uncertainties related to 

exposures likely to elicit effects (and the 

associated severity and extent) in 

population groups not studied, or less 

well studied (including individuals 

with asthma and children) and also the 

severity and prevalence of responses to 

short (e.g., 6.6- to 8-hour) O3 exposures, 

at and below 60 ppb, while at increased 

exertion levels. 

The PA additionally includes 

exposure/risk analyses of air quality 

scenarios in eight study areas, with a 

focus on the scenario for air quality that 

just meets the current standard, as 

described in section II.C of the proposal 

and summarized in section II.A.3 above. 

In considering the results of these 

analyses, the PA gives particular 

emphasis to the comparison-tobenchmarks analysis, which provides a 

characterization of the extent to which 

population exposures to O3

concentrations, similar to those 

evaluated in controlled human exposure 

studies, have the potential to occur in 

areas of the U.S. when air quality just 

meets the current standard (PA, section 

3.4). The policy evaluations of the 

exposure/risk analyses focus on 

children and children with asthma as 

key at-risk populations, and 

consideration of the potential for one 

versus multiple exposures to occur. The 

PA recognizes that consideration of 

differences in magnitude or severity of 

responses (e.g., FEV1 changes) including 

the relative transience or persistence of 

the responses and respiratory 

symptoms, as well as pre-existing 

sensitivity to effects on the respiratory 

system, and other factors, are important 

to characterizing implications for public 

health effects of an air pollutant such as 

O3 (PA, sections 3.3.2, 3.4.5 and 3.5). 

In summary, the PA concludes that 

the newly available health effects 

evidence, critically assessed in the ISA 

as part of the full body of evidence, 

reaffirms conclusions on the respiratory 

effects recognized for O3 in the last 

review on which the current standard is 

based. The PA additionally draws on 

the quantitative exposure and risk 

estimates for conditions just meeting the 

current standard (PA, sections 3.4 and 

3.5.2). Limitations and uncertainties 

associated with the available 

information remain (PA, sections 3.5.1 

and 3.5.2). The PA recognizes that the 

newly available quantitative exposure/ 

risk estimates for conditions just 

meeting the current standard indicate a 

generally similar level of protection for 

at-risk populations from respiratory 

effects, as that described in the last 

review for the now-current standard 

(section II.A.3, Table 3, above; PA, 

sections 3.1 and 3.4, Appendix 3D, 

section 3D.3.2.4, Table 3D–38). 

Collectively, in consideration of the 

evidence and quantitative exposure/risk 

information available in the current 

review, as well as advice from the 

CASAC, the PA concludes that it is 

appropriate to consider retaining the 

current primary standard of 0.070 ppm 

O3, as the fourth-highest daily 

maximum 8-hour concentration 

averaged across three years, without 

revision. 

b. CASAC Advice in This Review 

In comments on the draft PA, the 

CASAC agreed with the draft PA 

findings that the health effects evidence 

newly available in this review does not 

substantially differ from that available 

in the 2015 review, stating that, ‘‘[t]he 

CASAC agrees that the evidence newly 

available in this review that is relevant 

to setting the ozone standard does not 

substantially differ from that of the 2015 

Ozone NAAQS review’’ (Cox, 2020a, 

Consensus Responses to Charge 

Questions p. 12). With regard to the 

adequacy of the current standard, views 

of individual CASAC members differed. 

Part of the CASAC ‘‘agree with the EPA 

that the available evidence does not call 

into question the adequacy of protection 

provided by the current standard, and 

thus support retaining the current 

primary standard’’ (Cox, 2020a, p. 1). 

Another part of the CASAC indicated its 

agreement with the previous CASAC’s 

advice, based on review of the 2014 

draft PA, that a primary standard with 

a level of 70 ppb may not be protective 

of public health with an adequate 

margin of safety, including for children 

with asthma (Cox, 2020a, p. 1 and 

Consensus Responses to Charge 

Questions p. 12).87 Additional 

comments from the CASAC in the 

‘‘Consensus Responses to Charge 

Questions’’ on the draft PA attached to 

the CASAC letter provide 

recommendations on improving the 

presentation of the information on 

health effects and exposure and risk 

estimates in completing the final PA. 

The EPA considered these comments, 

making a number of revisions to address 

them in completing the PA. The 

comments from the CASAC also took 

note of uncertainties that remain in this 

review of the primary standard and 

identified a number of additional areas 

for future research and data gathering 

that would inform the next review of the 

primary O3 NAAQS (Cox, 2020a, 

Consensus Responses to Charge 

Questions p. 14). The recommendations 

from the CASAC were considered in the 

proposed decision and have been 

considered by the Administrator in his 

decision in this review, summarized in 

section II.B.3 below. 

c. Administrator’s Proposed 

Conclusions 

In reaching conclusions on the 

adequacy and appropriateness of 

protection provided by the current 

primary standard and his proposed 

decision to retain the standard, the 

Administrator carefully considered: (1) 

The assessment of the current evidence 

and conclusions reached in the ISA; (2) 

the currently available exposure and 

risk information, including associated 

limitations and uncertainties, described 

in detail in the PA; (3) the 

considerations and staff conclusions 

and associated rationales presented in 

the PA, including consideration of 

commonly accepted guidelines or 

criteria within the public health 

community, including the ATS, an 

organization of respiratory disease 

specialists; (4) the advice and 

recommendations from the CASAC; and 

(5) public comments that had been 

offered up to that point (85 FR 49830, 

August 14, 2020). In so doing, he 

considered the evidence base on health 

effects associated with exposure to 

photochemical oxidants, including O3, 

in ambient air, noting the health effects 

evidence newly available in this review, 

and the extent to which it alters key 

scientific conclusions in the last review. 

He additionally considered the 

quantitative exposure and risk estimates 


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88With the 2015 decision, the prior 

Administrator judged there to be uncertainty in the 

adversity of the effects shown to occur following 

exposures to 60 ppb O3, including the inflammation 

reported by the single study at the level, and 

accordingly placed greater weight on estimates of 

multiple, versus single, exposures for the 60 ppb 

benchmark, particularly when considering the 

extent to which the current and revised standards 

incorporate a margin of safety (80 FR 65344–45, 

October 26, 2015). She based this, at least in part, 

on consideration of effects at this exposure level, 

the evidence for which remains the same in the 

current review, and she considered this information 

in judgments regarding the 2014 HREA estimates 

for the 60 ppb benchmark. 

developed in this review, including 

associated limitations and uncertainties, 

and what they indicate regarding the 

magnitude of risk, as well as level of 

protection from adverse effects, 

associated with the current standard. 

The Administrator also considered the 

key aspects of the evidence and 

exposure/risk estimates from the 2015 

review that were emphasized in 

establishing the standard at that time. 

Further, he considered uncertainties in 

the current evidence and the exposure/ 

risk information, as a part of public 

health judgments that are essential and 

integral to his decision on the adequacy 

of protection provided by the standard, 

similar to the judgments made in 

establishing the current standard. Such 

judgments include public health policy 

judgments and judgments about the 

uncertainties inherent in the scientific 

evidence and quantitative analyses. The 

Administrator drew on the 

considerations and conclusions in the 

current PA, taking note of key aspects of 

the associated rationale, and he 

considered the advice and conclusions 

of the CASAC, including particularly its 

overall agreement that the currently 

available evidence does not 

substantially differ from that which was 

available in the 2015 review when the 

current standard was established. 

As an initial matter, the Administrator 

recognized the continued support in the 

current evidence for O3 as the indicator 

for photochemical oxidants, taking note 

that no newly available evidence has 

been identified in this review regarding 

the importance of photochemical 

oxidants other than O3 with regard to 

abundance in ambient air, and potential 

for health effects. For such reasons, 

described with more specificity in the 

ISA and PA and summarized in the 

proposal, he proposed to conclude it is 

appropriate for O3 to continue to be the 

indicator for the primary standard for 

photochemical oxidants and focused on 

the current information for O3 (85 FR 

49830, August 14, 2020). 

With regard to O3 health effects, the 

Administrator recognized the longstanding evidence that has established 

there to be a causal relationship 

between respiratory effects and shortterm O3 exposures. He recognized that 

the strongest and most certain evidence 

for this conclusion, as in the last review, 

is that from controlled human exposure 

studies that report an array of 

respiratory effects in study subjects 

(which are largely generally healthy 

adults) engaged in quasi-continuous or 

intermittent exercise. He also 

recognized the supporting experimental 

animal and epidemiologic evidence, 

including the epidemiologic studies 

reporting positive associations for 

asthma-related hospital admissions and 

emergency department visits, which are 

strongest for children, with short-term 

O3 exposures (85 FR 49830, August 14, 

2020). 

Regarding the current evidence and 

EPA conclusions for populations at 

increased risk of O3-related health 

effects (ISA, section 4.4), the 

Administrator took particular note of 

the robust evidence that continues to 

identify people with asthma as being at 

increased risk of O3 related respiratory 

effects, including specifically asthma 

exacerbation and associated health 

outcomes, and also children, 

particularly due to their generally 

greater time outdoors while at elevated 

exertion (PA, section 3.3.2; ISA, sections 

IS.4.3.1, IS.4.4.3.1, and IS.4.4.4.1, 

Appendix 3, section 3.1.11). Based on 

this evidence and related factors, the 

Administrator proposed to conclude it 

appropriate to give particular focus to 

people with asthma and children 

(population groups for which the 

evidence of increased risk is strongest) 

in evaluating whether the current 

standard provides requisite protection 

based on the judgment that such a focus 

will also provide protection of other 

population groups, identified in the 

ISA, for which the current evidence is 

less robust and clear as to the extent and 

type of any increased risk, and the 

exposure circumstances that may 

contribute to it. 

The Administrator additionally 

recognized newly available evidence 

and conclusions regarding O3 exposures 

and metabolic effects. In so doing, he 

also noted that the basis for the 

conclusions is largely experimental 

animal studies in which the exposure 

concentrations were well above those in 

the controlled human exposure studies 

for respiratory effects, and also above 

those likely to occur in areas of the U.S. 

that meet the current standard. In light 

of these considerations, he further 

proposed to judge the current standard 

to be protective of such circumstances, 

leading him to continue to focus on 

respiratory effects in evaluating whether 

the current standard provides requisite 

protection (85 FR 49830, August 14, 

2020). 

With regard to exposure 

circumstances of interest for respiratory 

effects, the Administrator focused 

particularly on the 6.6-hour controlled 

human exposure studies involving 

exposure, with quasi-continuous 

exercise, that examine exposures from 

60 to 80 ppb. In so doing, he recognized 

that this information on exposure 

concentrations that have been found to 

elicit effects in exercising study subjects 

is unchanged from what was available 

in the last review. He additionally 

recognized that while, as a whole, the 

epidemiologic studies of associations 

between O3 and respiratory effects and 

health outcomes (e.g., asthma-related 

hospital admission and emergency 

department visits) provide strong 

support for the conclusions of causality, 

they are less useful for his consideration 

of the potential for O3 exposures 

associated with air quality conditions 

allowed by the current standard to 

contribute to such health outcomes, 

taking note of the scarcity of U.S. 

studies conducted in locations in which 

and during time periods when the 

current standard would have been met 

(85 FR 49830, August 14, 2020). 

In reaching his proposed decision to 

retain the 2015 standard, the 

Administrator took note of several 

aspects of the rationale by which it was 

established, giving weight to the 

considerations summarized here. The 

2015 decision considered the breadth of 

the O3 respiratory effects evidence, 

recognizing the relatively greater 

significance of effects reported for 

exposures while at elevated exertion to 

average O3 concentrations at and above 

80 ppb, as well as to the greater array 

of effects elicited. The decision also 

recognized the significance of effects 

observed at the next lower studied 

exposures (slightly above 70 ppb) that 

included both lung function decrements 

and respiratory symptoms. The standard 

level was set to provide a high level of 

protection from such exposures. The 

decision additionally emphasized 

consideration of lower exposures down 

to 60 ppb, particularly with regard to 

consideration of a margin of safety in 

setting the standard. In this context, the 

2015 decision identified the 

appropriateness of a standard that 

provided a degree of control of multiple 

or repeated occurrences of exposures, 

while at elevated exertion, at or above 

60 ppb (80 FR 65365, October 26, 

2015).88 The controlled human 

exposure study evidence as a whole 

provided context for consideration of 

the 2014 HREA estimates for the 


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comparison-to-benchmarks analysis (80 

FR 65363, October 26, 2015). The 

current Administrator proposed to 

similarly consider the currently 

available exposure and risk analyses in 

this review (85 FR 49830, August 14, 

2020). 

The Administrator also recognized 

some uncertainty, reflecting limitations 

in the evidence base, with regard to the 

exposure levels eliciting effects (as well 

as the severity of the effects) in some 

population groups not well represented 

in the available controlled human 

exposure studies, such as children and 

individuals with asthma. In so doing, 

the Administrator recognizes that the 

controlled human exposure studies, 

primarily conducted in healthy adults, 

on which the depth of our 

understanding of O3-related health 

effects is based, provide limited, but 

nonetheless important information with 

regard to responses in people with 

asthma or in children. Additionally, 

some aspects of our understanding 

continue to be limited, as in the 2015 

review; among these aspects are the risk 

posed to these less studied population 

groups by 7-hour exposures with 

exercise to concentrations as low as 60 

ppb that are estimated in the exposure 

analyses. Collectively, these aspects of 

the evidence and associated 

uncertainties contribute to a recognition 

that for O3, as for other pollutants, the 

available evidence base in a NAAQS 

review generally reflects a continuum, 

consisting of ambient levels at which 

scientists generally agree that health 

effects are likely to occur, through lower 

levels at which the likelihood and 

magnitude of the response become 

increasingly uncertain. 

As in the 2015 decision, the 

Administrator’s proposed decision in 

this review recognized that the exposure 

and risk estimates developed from 

modeling exposures to O3 in ambient air 

are critically important to consideration 

of the potential for exposures and risks 

of concern under air quality conditions 

of interest, and consequently are 

critically important to judgments on the 

adequacy of public health protection 

provided by the current standard. Thus 

taking into consideration related 

information, limitations and 

uncertainties recognized in the 

proposal, the Administrator considered 

the exposure and risk estimates across 

the eight study areas (with their array of 

exposure conditions) for air quality 

conditions just meeting the current 

standard. In light of factors recognized 

above and summarized in section II.D.4 

of the proposal, the Administrator, in 

his consideration of the exposure and 

risk analyses, focused in the proposal on 

the results for children and children 

with asthma. In considering the public 

health implications of estimated 

occurrences of exposures, while at 

increased exertion, at or above the three 

benchmark concentrations (60, 70, and 

80 ppb), the Administrator considered 

the effects reported in controlled human 

exposure studies of this range of 

concentrations during 6.6 hours of 

quasi-continuous exercise. While the 

Administrator noted reduced 

uncertainty in several aspects of the 

exposure and risk approaches as 

compared to the analyses in the last 

review, he recognized the relatively 

greater uncertainty associated with the 

lung function risk estimates compared 

to the results of the comparison-tobenchmarks analysis. In light of these 

uncertainties, as well as the recognition 

that the comparison-to-benchmarks 

analysis provides for characterization of 

risk for the broad array of respiratory 

effects compared to a narrower focus 

limited to lung function decrements, the 

Administrator focused in the proposal 

primarily on the estimates of exposures 

at or above different benchmark 

concentrations that represent different 

levels of significance of O3-related 

effects, both with regard to the array of 

effects and severity of individual effects 

(85 FR 49830, August 14, 2020). 

In his consideration of the exposure 

analysis estimates for exposures at or 

above the different benchmark 

concentrations (with reduced associated 

uncertainty compared to the analysis 

available in 2015) and based on the 

greater severity of responses reported in 

controlled human exposures, with 

quasi-continuous exercise, at and above 

73 ppb, the Administrator focused in 

the proposal first on the higher two 

benchmark concentrations (which at 70 

and 80 ppb are, respectively, slightly 

below and above this level) and the 

estimates for one-or-more-day 

occurrences. In this context, he 

proposed to judge it desirable that the 

standard provide a high level of 

protection against one or more 

occurrences of days with exposures, 

while breathing at an elevated rate, to 

concentrations at or above 70 ppb. With 

regard to the 60 ppb benchmark, the 

Administrator gave greater weight to 

estimates of occurrences of two or more 

(rather than one or more) days with an 

exposure at or above that benchmark, 

taking note of the lesser severity of 

responses observed in studies of the 

lowest benchmark concentration of 60 

ppb and other considerations 

summarized in the proposal, including 

potential risks for at-risk populations. 

Based on this weighting of the exposure 

analysis results for the eight urban study 

areas, the Administrator noted what was 

indicated by the exposure estimates for 

air quality conditions just meeting the 

current standard with regard to 

protection for the simulated at-risk 

populations. Some 97% to more than 

99% of all children (including those 

with asthma), on average, and more than 

95% in the single highest year, are 

estimated to be protected from 

experiencing two or more days with 

exposures at or above 60 ppb while at 

elevated exertion. More than 99% of 

children with asthma (and of all 

children), on average per year, are 

estimated to be protected from a day or 

more with an exposure at or above 70 

ppb. Lastly, the percentage (for both 

population groups) for at least one day 

with such an exposure at or above 80 

ppb is 99.9% or more in each of the 

three years simulated, with no 

simulated children estimated to 

experience more than a single such day. 

The Administrator proposed to judge 

that protection from this set of 

exposures provides a strong degree of 

protection to at-risk populations, such 

as children with asthma. In so doing, he 

found that the updated exposure and 

risk analyses continue to support a 

conclusion of a high level of protection, 

including for at-risk populations, from 

O3-related effects of exposures that 

might be expected with air quality 

conditions that just meet the current 

standard (85 FR 49830, August 14, 

2020). 

In reaching his proposed conclusion, 

the Administrator additionally took note 

of the comments and advice from the 

CASAC, including the CASAC 

conclusion that the newly available 

evidence does not substantially differ 

from that available in the last review, 

and the conclusion expressed by part of 

the CASAC, that the currently available 

evidence supports retaining the current 

standard (85 FR 49873, August 14, 

2020). He also noted that another part of 

the CASAC indicated its agreement with 

the prior CASAC comments on the 2014 

draft PA, in which the prior CASAC 

opined that a standard set at 70 ppb may 

not provide an adequate margin of 

safety (Cox, 2020a, p. 1). With regard to 

the latter view (that referenced 2014 

comments from the prior CASAC), the 

Administrator additionally noted that 

the 2014 advice from the prior CASAC 

also concluded that the scientific 

evidence supported a range of standard 

levels that included 70 ppb and 

recognized the choice of a level within 

its recommended range to be ‘‘a policy 

judgment under the statutory mandate 


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89This 2014 advice was considered in the last 

review’s decision to establish the current standard 

with a level of 70 ppb (80 FR 65362, October 26, 

2015). 

of the Clean Air Act’’ (Frey, 2014b, p. 

ii).89

In reflecting on all of the information 

currently available, the Administrator 

also considered the extent to which the 

currently available information might 

indicate support for a less stringent 

standard, noting that the CASAC advice 

did not convey support for such a 

standard. He additionally considered 

the current exposure and risk estimates 

for the air quality scenario for a design 

value just above the level of the current 

standard (at 75 ppb), in comparison to 

the scenario for the current standard, 

with its level of 70 ppb. In so doing, he 

found the markedly increased estimates 

of exposures to the higher benchmarks 

under air quality for a higher standard 

level to be of concern and indicative of 

less than the requisite protection. Thus, 

in light of considerations raised in the 

proposal, including the need for an 

adequate margin of safety, the 

Administrator proposed to judge that a 

less stringent standard would not be 

appropriate to consider (85 FR 49830, 

August 14, 2020). 

Similarly, the Administrator also 

considered whether it would be 

appropriate to consider a more stringent 

standard that might be expected to 

result in reduced O3 exposures. As an 

initial matter in this regard, he 

considered the advice from the CASAC 

(summarized in section II.B.1.b above). 

With regard to the CASAC advice, he 

noted that while part of the Committee 

concluded that the evidence supported 

retaining the current standard without 

revision, another part of the Committee 

reiterated advice from the prior CASAC, 

which while including the current 

standard level among the range of 

recommended standard levels, also 

provided policy advice to set the 

standard at a lower level (85 FR 49873, 

August 14, 2020). In considering the 

reference to the 2014 CASAC advice, the 

Administrator noted the slight 

differences of the current exposure and 

risk estimates from the 2014 HREA 

estimates considered by the prior 

CASAC. The Administrator additionally 

recognized the PA finding that the 

factors contributing to these differences, 

which include the use of air quality data 

reflecting concentrations much closer to 

the now-current standard than was the 

case in the 2015 review, also contribute 

to a reduced uncertainty in the 

estimates. Thus, he noted that the 

current exposure analysis estimates 

indicate the current standard to provide 

appreciable protection against multiple 

days with a maximum exposure at or 

above 60 ppb. He considered this in the 

context of the adequacy of protection 

provided by the standard and of the 

CAA requirement that the standard 

protect public health, including the 

health of at-risk populations, with an 

adequate margin of safety, and proposed 

to conclude that the current standard 

provides an adequate margin of safety, 

and that a more stringent standard is not 

needed (85 FR 49873, August 14, 2020). 

In light of all of the above, including 

advice from the CASAC, the 

Administrator proposed to judge the 

current exposure and risk analysis 

results to describe appropriately strong 

protection of at-risk populations from 

O3-related health effects. Thus, based on 

his consideration of the evidence and 

exposure/risk information, including 

that related to the lowest exposures 

studied and the associated 

uncertainties, the Administrator 

proposed to judge that the current 

standard provides the requisite 

protection, including an adequate 

margin of safety, and thus should be 

retained, without revision (85 FR 49874, 

August 14, 2020). In so doing, he 

recognized that the protection afforded 

by the current standard can only be 

assessed by considering its elements 

collectively, including the standard 

level of 70 ppb, the averaging time of 

eight hours and the form of the annual 

fourth-highest daily maximum 

concentration averaged across three 

years. The Administrator proposed to 

judge that the current evidence 

presented in the ISA and considered in 

the PA, as well as the current air 

quality, exposure and risk information 

presented and considered in the PA, 

provide continued support to these 

elements, as well as to the current 

indicator. 

In summary, in the proposal the 

Administrator recognized that the ISA 

found the newly available health effects 

evidence, critically assessed in the ISA 

as part of the full body of evidence, 

consistent with the conclusions on the 

respiratory effects recognized for O3 in 

the last review. He additionally noted 

that the evidence newly available in this 

review, such as that related to metabolic 

effects, does not include information 

indicating a basis for concern for 

exposure conditions associated with air 

quality conditions meeting the current 

standard. Further, the Administrator 

noted the quantitative exposure and risk 

estimates for conditions just meeting the 

current standard that indicate a high 

level of protection for at-risk 

populations from respiratory effects. 

Collectively, these considerations 

(including those discussed more 

completely in the proposal) provided 

the basis for the Administrator’s 

proposed judgments regarding the 

public health protection provided by the 

current primary standard of 0.070 ppm 

O3, as the fourth-highest daily 

maximum 8-hour concentration 

averaged across three years. On this 

basis, the Administrator proposed to 

conclude that the current standard is 

requisite to protect the public health 

with an adequate margin of safety, and 

that it is appropriate to retain the 

standard without revision (85 FR 49874, 

August 14, 2020). 

2. Comments on the Proposed Decision 

Over 50,000 individuals and 

organizations indicated their views in 

public comments on the proposed 

decision. Most of these are associated 

with mass mail campaigns or petitions. 

Approximately 40 separate submissions 

were also received from individuals, 

and 75 from organizations and groups of 

organizations; forty elected officials also 

submitted comments. Among the 

organizations commenting were state 

and local agencies and organizations of 

state agencies, organizations of health 

professionals and scientists, 

environmental and health protection 

advocacy organizations, industry 

organizations and regulatory policyfocused organizations. The comments 

on the proposed decision to retain the 

current primary standard are addressed 

here. Those in support of the proposed 

decision are addressed in section 

II.B.2.a and those in disagreement are 

addressed in section II.B.2.b. Comments 

related to aspects of the process 

followed in this review of the O3

NAAQS (described in section I.D 

above), as well as comments related to 

other legal, procedural or administrative 

issues, and those related to issues not 

germane to this review are addressed in 

the separate Response to Comments 

document. 

a. Comments in Support of Proposed 

Decision 

Of the commenters supporting the 

Administrator’s proposed decision to 

retain the current primary standard, 

without revision, all generally note their 

agreement with the rationale provided 

in the proposal, with the CASAC 

conclusion that the current evidence is 

generally consistent with that available 

in the last review, and with the CASAC 

members that conclude the evidence 

does not call into question the adequacy 

of the current standard. Some 

commenters further remarked that the 

primary standard was upheld in the 

litigation following its 2015 


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90Comments related to implementation programs 

are not addressed here because, as described in 

section I.A above, this action is being taken 

pursuant to CAA section 109(d)(1) and relevant case 

law. Accordingly, concerns related to 

implementation of the existing or an alternate 

standard are outside the scope of this action. 

establishment (Murray Energy Corp. v. 

EPA, 936 F.3d 597 [D.C. Cir. 2019]) and 

that this review is based largely on the 

same body of respiratory effects 

evidence. These commenters all find the 

process for the review to conform to 

Clean Air Act requirements and the 

proposed decision to retain the current 

standard to be well supported, noting 

that the there are no new controlled 

human exposure studies (of the type 

given primary focus in the 

establishment of the current standard) 

and concurring with the proposed 

judgment that at-risk populations are 

protected with an adequate margin of 

safety. Some commenters also variously 

cited EPA statements that the recent 

metabolic studies, as well as the 

epidemiologic and toxicological studies 

newly available in this review for other 

health endpoints, do not demonstrate 

effects of O3 when the current standard 

is met and thus do not call into question 

the protection provided by the standard. 

The EPA agrees with these commenters’ 

conclusion on the current standard. 

Further, these comments concur with 

the EPA’s consideration of 

epidemiologic and toxicological studies 

of respiratory effects, and with the 

weight the proposed decision placed on 

the evidence for other effects, including 

metabolic and cardiovascular effects, 

and total mortality. Some of these 

comments also express the view that 

health benefits of a more restrictive O3

standard are highly uncertain, while 

such a standard would likely cause an 

increase in nonattainment areas and 

socioeconomic impacts that the EPA 

should consider and find to outweigh 

the uncertain benefits. While, as 

discussed in section II.B.3 below, the 

Administrator does not find a more 

stringent standard necessary to provide 

requisite public health protection, he 

does not consider the number of 

nonattainment areas or economic 

impacts of alternate standards in 

reaching this judgment.90 As 

summarized in section I.A. above, in 

setting primary and secondary standards 

that are ‘‘requisite’’ to protect public 

health and welfare, respectively, as 

provided in section 109(b), the EPA may 

not consider the costs of implementing 

the standards. See generally, Whitman 

v. American Trucking Ass’ns, 531 U.S. 

457, 465–472, 475–76 (2001). Likewise, 

‘‘[a]ttainability and technological 

feasibility are not relevant 

considerations in the promulgation of 

national ambient air quality standards’’ 

(American Petroleum Institute v. Costle, 

665 F.2d 1176, 1185 [D.C. Cir. 1981]; 

accord Murray Energy Corp. v. EPA, 936 

F.3d 597, 623–24 [D.C. Cir. 2019]). 

Arguments such as the views on 

socioeconomic impacts expressed by 

these commenters have been rejected by 

the courts, as summarized in section I.A 

above, including in Murray Energy, with 

the reasoning that consideration of such 

impacts was precluded by Whitman’s 

holding that the ‘‘plain text of the Act 

‘unambiguously bars cost considerations 

from the NAAQS-setting process’ ’’ 

(Murray Energy Corp. v. EPA, 936 F.3d 

at 621, quoting Whitman, 531 U.S. at 

471). 

We also note that some commenters 

that stated their support for retaining 

the current standard without revision 

additionally claimed that, based on the 

results of the exposure and risk analyses 

in this review, the current standard 

provides somewhat more public health 

protection than the EPA recognized in 

the 2015 decision establishing it. As 

support for this view, these commenters 

cite conclusions (including those in the 

PA) that the exposure and risk estimates 

are equivalent or slightly lower than 

those from the 2014 HREA. In generally 

agreeing with the commenters’ 

observation with regard to the 

differences in exposure/risk estimates 

from analyses in this review compared 

to those from 2014, we note that the 

current exposure/risk estimates, while 

based on conceptually similar 

approaches to those used in the 2014 

HREA, reflect a number of 

improvements to input data and 

modeling approaches, summarized in 

section II.A.3 above, which have 

reduced uncertainties. These updated 

analyses inform the Administrator’s 

judgments in this review. 

b. Comments in Disagreement With 

Proposed Decision 

Of the commenters that disagreed 

with the proposal to retain the current 

standard, some recommend tightening 

the standard, while one submission 

recommends a less stringent standard. 

The commenters supporting a less 

stringent standard generally assert that 

the current standard is overprotective, 

stating that information they provide 

supports returning to the pre-2015 

standard of 75 ppb and/or revising the 

form from the 4th highest daily 

maximum to the seventh highest daily 

maximum. The commenters that 

recommended a more stringent standard 

describe a need for revision to provide 

greater public health protection, 

generally claiming that the current 

standard is inadequate and does not 

provide an adequate margin of safety for 

potentially vulnerable groups. We 

address these sets of comments in turn 

below. 

(i) Comments in Disagreement With 

Proposed Decisions—Calling for Less 

Stringent Standard 

The commenters recommending 

revision to a less stringent standard 

generally expressed the view that the 

current standard is more stringent than 

necessary to protect public health. In 

support of this view the commenters 

argue (1) that in this review the EPA 

‘‘discredited’’ a cardiovascular mortality 

study on which commenters assert the 

2015 decision had placed especially 

heavy weight; (2) that in light of 

limitations they assert for the exposure 

and risk estimate analyses conducted in 

this review, a 75 ppb standard would 

meet 2015 objectives; and, (3) that 

additional factors they identify indicate 

that the current standard of 70 ppb is 

too close to background levels while a 

standard of 75 ppb or one with a form 

that uses the seventh (versus fourth) 

highest daily maximum 8-hour O3

concentration would not be. 

With regard to the first argument, the 

EPA knows of no cardiovascular 

mortality study, much less any health 

study, that was relied on in the 2015 

review that has been discredited, and 

the commenters provide no citation for 

such a study. To the extent that the 

commenter may be intending to refer to 

the difference of the current review from 

the 2015 review with regard to the 

Agency’s causality determinations for 

cardiovascular effects and all-cause 

mortality, we note that these changes 

did not involve ‘‘discrediting’’ of any 

studies in the 2013 ISA. Rather, as 

summarized in section II.A.2.a above, 

since the time of the last review the 

controlled human exposure study 

evidence base has been appreciably 

expanded from one study to several, 

none of which report O3-induced 

cardiovascular endpoints. This update 

to the evidence base for cardiovascular 

effects, which also includes 

epidemiologic studies, has contributed 

to a change in the weight of evidence 

that supports the Agency’s causality 

determinations for both cardiovascular 

effects and mortality. To the extent that 

the commenters intend to suggest that 

these changes in causality 

determinations indicate that the current 

standard is more stringent than 

necessary to protect public health, the 

Agency disagrees. The Administrator’s 

reasons for concluding that the current 

standard provides the requisite public 


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91The EPA’s exposure and risk analyses estimate 

<.1 to 0.3% of children with asthma might be 

expected to experience at least one exposure, while 

at increased exertion, at or above 80 ppb, on average 

across a 3-year period in areas just meeting a 

potential alternative standard of 75 ppb (85 FR 

49865,Table 4, August 14, 2020). For the 70 ppb 

benchmark, these percentages are 1.1 to 2.0%. 

92Taken together, the EPA generally understands 

prior court decisions addressing consideration of 

background O3 in NAAQS reviews to hold that 

while the Agency may not establish a NAAQS that 

is outside the range of reasonable values supported 

by the air quality criteria and the judgments of the 

Administrator because of proximity to background 

concentrations, it is not precluded from considering 

relative proximity to background O3 as one factor 

in selecting among standards that are within that 

range (American Trucking Ass’ns v. EPA, 283 F.3d 

355, 379 [D.C. Cir. 2002]; Murray Energy v. EPA, 

936 F.3d at 622–624; American Petroleum Institute 

v. Costle, 665 F.2d 1176, 1185 [D.C. Cir. 1982]). 

93Comments related to implementation programs 

are not addressed here because, as described in 

section I.A above, this action is being taken 

pursuant to CAA section 109(d)(1) and relevant case 

law. Furthermore, leaving the NAAQS unaltered 

will not require the EPA to make new air quality 

designations, nor require States or authorized tribes 

to undertake new planning or control efforts. 

Accordingly, concerns related to implementation of 

the existing or an alternate standard are outside the 

scope of this action. 

health protection are explained in 

section II.B.3 below. 

With regard to the risk and exposure 

analyses, the comment argues that 2019 

O3 ambient air monitoring data for 

locations meeting a design value of 75 

ppb indicate that a 75 ppb standard 

could achieve comparable exposure 

estimates to those derived for air quality 

just meeting the current standard by the 

EPA’s exposure/risk analyses. The 

comment also asserts that uncertainty in 

the controlled human exposure 

evidence base with regard to children 

with asthma suggests ‘‘some latitude’’ is 

needed in the risk calculations. The 

analysis provided in the comment 

appears to focus on counties in 

designated nonattainment areas with 

2019 design values ranging from 71 to 

75. For these counties, the commenters’ 

analysis appears to sum the population 

of the subset of these counties with at 

least one daily maximum 8-hour average 

concentration in 2019 falling in the 

range from 73 to 79 ppb (and, 

separately, the population of counties 

with at least one such value above 80 

ppb). From these population counts, the 

analysis derives estimates of the 

subpopulations of children with asthma 

spending afternoons outdoors (using 

national estimates for representation of 

children in the total population, of 

children with asthma in the total child 

population, and of children in asthma 

spending afternoons outdoors using 

analysis of CHAD diaries for children). 

The analysis divides the two values by 

the commenters’ estimate of children 

with asthma in the U.S. (304 million 

[total population of the U.S.] × 10.5% 

[percentage representing children] × 

9.7% [percentage representing children 

with asthma]). 

There are many aspects of the analysis 

submitted with the comment that are 

not focused on the objective of 

estimating exposures of concern that 

might be expected to be experienced by 

at-risk populations in U.S. areas that 

just meet a standard with an alternative 

level of 75 ppb. As just one example of 

these aspects, the denominator in the 

final step of the commenters’ 

calculation is inflated by population 

counts for areas of the U.S. excluded 

from the commenters’ analysis (with 

this larger population multiplied by a 

national estimates of percent that are 

children, 10.5%, and a national estimate 

of percent of children that have asthma, 

9.7%), yielding a percentage of unclear 

relevance to consideration of exposures 

occurring in areas just meeting an 

alternative standards of 75 ppb. If the 

population of the nonattainment areas 

on which the commenters’ focus is 

substituted in the calculation for the 

total population of the U.S. as the 

denominator (29.5 million × 10.5% × 

9.7% = 146,664), with the commenters’ 

estimates of children in those areas that 

may experience an exposure at or above 

80 ppb (4,788) or below 80 ppb and at 

or above 73 ppb (12,641), the 

percentages are 3.3% and 8.6%, 

respectively (and the percentage for at 

or above 73 ppb would be 5.8%).91

Thus, contrary to the commenters’ 

assertion, their analytical approach, 

with use of a denominator that reflects 

the commenters’ focus areas, results in 

higher estimates of the percentage of atrisk children that may experience 

particular exposures of concern in areas 

meeting a 75 ppb standard than does the 

EPA’s analysis, which takes into 

account a number of factors in much 

greater detail (e.g., through the use of 

exposure modeling and human activity 

data to estimate time series contributing 

to 7-hour exposure periods with average 

O3 concentrations at or above 

benchmarks), and focuses on temporal 

and spatial patterns of air quality in 

areas just meeting a standard of 75 ppb. 

The commenters analysis is not focused 

on the factors that are key determinants 

of population exposures of concern, 

leading to results that are inconsistent 

with and less informative than the 

findings of EPA’s more detailed, 

extensive and technically sound 

exposure and risk analyses (summarized 

in section II.A.3 above and Appendices 

3C and 3D of the PA). Based on 

consideration of these analyses, among 

other factors, as described in section 

II.B.3 below, the EPA disagrees that the 

available evidence and quantitative 

analyses supports the conclusion that 

the current standard is overprotective 

and that a standard of 75 ppb would 

protect public health with an adequate 

margin of safety. 

In support of the commenters’ 

additional argument that the current 

standard is too close to background and 

that a 75 ppb standard (or a standard 

using the seventh highest form) would 

not be, the commenters (1) state that just 

because a D.C. Circuit decision has 

stated that EPA is not required to take 

U.S. background O3 (USB) into 

consideration in NAAQS decisions does 

not mean that such considerations are 

precluded; (2) cite the lower number of 

counties (and associated population) 

that would be in nonattainment for a 75 

ppb standard as compared to the current 

standard (while also suggesting that 

revision of the form to a seventh highest 

would appropriately allow for 

additional high O3 days due to 

wildfires); and (3) suggest that the EPA 

is underestimating USB by a factor of 

three. 

With regard to the legal point, the 

EPA agrees that while it is not required 

to take USB into account in NAAQS 

decisions, it may do so when such 

consideration is consistent with the 

Clean Air Act and prior court decisions. 

The EPA is not relying on consideration 

of background O3 levels to support its 

decision in this review. Moreover, given 

the differences in public health 

protection, as noted in the 

Administrator’s proposed conclusions 

and described in his conclusions in 

section II.B.3 below, we do not believe 

that we could use proximity to 

background concentrations as a basis for 

revising the current 70 ppb standard to 

a potential 75 ppb standard.92 On the 

commenters’ second point, the EPA 

notes that the number of counties that 

would or would not be in 

nonattainment, the size of population 

living in them, and the increasing 

number of days for high O3 due to 

wildfires are not relevant factors in 

judging whether a particular standard is 

requisite under the Clean Air Act. 

Regardless of such implications of a 

decision to retain or revise a NAAQS, 

the key consideration for the review of 

a primary standard is whether the 

standard is judged to provide the 

requisite protection of public health 

with an adequate margin of safety.93 The 

commenters have provided no evidence 

suggesting that the current standard 

provides more than the requisite public 

health protection under the CAA or 

indicating that an alternate standard 


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94https://covid19.apple.com/mobility. 

95We also note, contrary to the commenters’ 

premise, NO2 and/or NOX are not conserved over 

a day. Rather, the overall lifetime of NOX is on the 

order of six hours. Further, while the commenter 

describes the ‘‘local’’ nature of O3, it is well 

established that O3 has a large transport component. 

The diurnal pattern of O3 concentrations 

highlighted on this point is likely illustrating O3

concentrations subject to local NOX-titration rather 

than purely local formation as suggested by the 

commenters. 

96We note that comments raised in the prior 

review were fully considered in reaching the 

decision in that review. Such comments are 

addressed in the decision and associated Response 

to Comments (80 FR 65292, October 26, 2020; U.S. 

EPA, 2015). To the extent that commenters are 

raising similar issues in support of their comments 

on the proposed decision in this review, we have 

addressed them in the current decision, based on 

the information now available. 

with a level of 75 ppb or with a seventh 

highest form would provide requisite 

protection. For these reasons, we do not 

find these comments persuasive in 

supporting consideration of revising the 

current standard to an alternate 

standard with a level of 75 ppb or with 

a seventh highest form. 

With regard to USB, the commenters 

present an argument focused on an 

urban/‘‘rural’’ comparison and one 

focused on a 1-month analysis of O3

concentrations in response to 

population mobility changes attributed 

to restrictions placed to manage 

infections of Corona virus 19 disease 

(COVID–19). We find there to be 

limitations in both arguments that 

undercut the conclusions reached by the 

commenter. As a result, we disagree that 

the observations made by the 

commenters support their statements 

regarding USB and with the implication 

that they contradict the EPA’s findings 

from the detailed and extensive analyses 

presented in the PA (PA, section 2.5 and 

Appendix 2B). 

With regard to the urban/‘‘rural’’ 

comparison, the commenters’ first cite 

EPA’s analysis in the PA which 

indicated, based on daily maximum 8- 

hour (MDA8) concentrations for the 

nation as a whole, that from one quarter 

(10 out of 42 ppb) to one third (14 out 

of 45 ppb) of average MDA8 

concentrations in spring and summer, 

respectively, are derived from 

anthropogenic sources. They then state 

that differences in monthly mean MDA8 

concentration between two sets of 

monitoring sites in the Philadelphia 

metropolitan area that they identify as 

the three highest and the three most 

rural was 3.3 ppb in April 2020. The 

commenters suggest that this amount is 

much smaller than the 10 to 14 ppb that 

EPA estimated to be from anthropogenic 

sources. Based on these two statements, 

they contend that USB is being 

underestimated by a factor of three. 

We find the commenters’ analysis to 

have several flaws that undercut their 

conclusion. First, the difference 

between the two sets of sites, all of 

which fall in the Philadelphia 

metropolitan area, are not indicative of 

either USA (i.e., U.S. anthropogenic) or 

USB contributions. There is no evidence 

that this difference is indicative of 

either USB or USA, and it is especially 

anomalous given that the commenters’ 

analysis is based on 2020 data (affected 

by reduced emissions during the 

reduced travel during the initial months 

of the COVID–19 epidemic in the U.S.) 

while EPA’s is based on 2016 data. 

Second, the authors cite a country-wide 

seasonal average despite the fact that the 

U.S. anthropogenic contributions are 

clearly higher in the nonattainment area 

(than a U.S. average) being referenced. 

Further, the conclusions about USB 

underestimation appear quantitatively 

incorrect and to perhaps confuse USA 

and USB in the calculations. Even if all 

USA anthropogenic contributions cited 

(10 USA and 30 USB of total 40 ppb) in 

spring of 2016 were actually USB, the 

underestimation of USB would be 25% 

at most (0 USA and 40 USB of total 40 

ppb; (40¥30)/40 = 25%), thus it is 

unclear how the commenter concluded 

a factor of three (300%) underestimation of USB. In addition, the 

commenter’s dataset is for the 

Philadelphia-Wilmington-Atlantic City 

CSA, where O3 more frequently exceeds 

the level of the standard in May through 

September (e.g., PA, Appendix 3C, 

Figure 3C–79), months that have lower 

USB and higher US anthropogenic than 

month of April, which the commenters 

analyzed. Finally, the commenter has 

focused on low concentration days 

(averaging ∼40–45ppb) that the PA 

shows tend to be different than high 

days (PA, section 2.5 and Appendix 2B). 

The second argument is based on data 

on Apple Mobility data 94 and O3 and 

NO2 concentrations for the period from 

3/22/2020 to 4/20/2020 (when 

transportation activity was affected by 

the behavioral changes in response to 

COVID–19) and differences from the 

same period in prior years. Based on the 

differences, the commenters conclude 

that O3 concentrations were less 

responsive to the 40 to 60% reduction 

in mobility than were NO2

concentrations (7% vs 22% difference), 

indicating to the commenters that 

society is reaching a period of 

diminishing returns of actions to control 

O3 concentrations. We note, however, 

that the period of the commenters’ 

analysis is April, while the majority of 

days with MDA8 greater than 70 ppb in 

the Philadelphia nonattainment area 

occur in May to September. In the mid 

to late summer period, local production 

of O3 is increased (see PA section 

2.5.3.2) and MDA8 concentrations in the 

Philadelphia nonattainment area more 

frequently are above the level of the 

standard. Thus, the analysis does not 

support the commenters’ argument for a 

less stringent standard.95

(ii) Comments in Disagreement With 

Proposed Decision and Calling for More 

Stringent Standard 

Among the commenters that disagree 

with the proposed decision and call for 

a more stringent standard, most express 

concerns regarding the process for 

reviewing the criteria and standards in 

this review and assert that the proposal 

must be withdrawn, and a new review 

conducted. The commenters expressing 

the view that a more stringent standard 

is needed variously cite a number of 

concerns. Some state that EPA cannot, 

as some commenters imply it does, 

simply base its decision on a judgment 

that the available evidence is similar to 

that when the standard was established 

in a prior review, and some argue that 

the available health effects evidence 

indicates that adverse health effects 

occur from exposures allowed by the 

current standard. Further some 

commenters express their views that the 

combined consideration of the complete 

evidence base indicates that sensitive or 

vulnerable populations are not 

protected by the current standard; and/ 

or that the standard does not provide an 

adequate margin of safety. Additionally, 

in support of their view that the 

standard should be made more 

stringent, some commenters disagree 

with the conclusions of the exposure 

and risk analyses, characterizing the 

analyses as deficient, and contending 

that other quantitative analyses they cite 

indicate health impacts that would be 

avoided by a lower standard level. Most 

of the commenters advocating a more 

stringent standard recommend revision 

of the level to a value at or below 60 ppb 

and others support a level at or below 

65 ppb. Some of these commenters 

additionally note they had raised 

similar concerns during the 2015 

review.96 Some commenters also 

express the view that the EPA should 

establish a separate long-term standard. 

With regard to the process by which 

this review has been conducted, we 

disagree with the commenters that it is 

arbitrary and capricious or that it does 

not comport with legislative 

requirements. The review process, 

summarized in section I.D, 

implemented a number of features, 

some of which have been employed in 

past reviews and others which have not, 


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97As just one example, the causal determinations 

for cardiovascular effects and total mortality in this 

review differ from those made in the last review, 

as described in section II.A.2.a. 

98 In so doing, to the extent the current evidence 

before the Administrator continues to support or 

reinforce conclusions reached in prior reviews, he 

may reasonably reach those same conclusions. 

99With regard to effects other than respiratory 

effects, studies cited by these commenters include 

studies of cardiovascular effects (Day et al., 2017; 

Shin et al., 2019; Wang et al., 2019b), all-cause 

mortality (Bell et al., 2014; Cohen et al., 2017; Di 

et al., 2017a, b), neurological effects (Cleary et al., 

2018), and reproductive and developmental effects 

(Wallace et al., 2016; Lavigne et al., 2016; Salam et 

al., 2005; Steib et al, 2019; Morello-Frosch et al., 

2010). 

100 In updating the air quality criteria in the 

current review, the current ISA evaluates relevant 

scientific literature published since the 2013 ISA, 

integrating with key information and judgments 

contained in the 2013 Ozone ISA and previous 

assessments (ISA, p. lxix; 2013 ISA; U.S. EPA, 2006; 

U.S. EPA, 1996a; U.S. EPA, 1982; U.S. EPA, 1986; 

U.S. EPA 1978; NAPCA, 1969). 

101This commenter cited a epidemiologic study 

(Day et al., 2017) that had been among studies of 

short term O3 and cardiovascular effects excluded 

from the draft ISA due to location, however this 

study was considered by the EPA in response to 

advice from the CASAC on the draft ISA (Luben, 

2020). This consideration of these studies did not 

change EPA’s analysis of the weight of evidence 

from that described in the draft ISA, thus 

supporting the causality determination for 

cardiovascular effects described in the final ISA 

(ISA, section IS.4.3). 

102We note that one study identified by a 

commenter to support their view that O3

concentrations allowed by the current standard is 

causing health effects does not include O3 among 

the pollutants it examines (Gan et al., 2014). 

Accordingly, we do not find the study to provide 

support to the commenter’s point. 

103As discussed in section I.D above, the ‘‘new’’ 

studies identified by commenters have not been 

through the comprehensive CASAC and public 

review process that the air quality criteria went 

through. To address these comments, we have 

provisionally considered these studies, as discussed 

in I.D above, and found they do not materially 

Continued 

and several which represent efficiencies 

in consideration of the statutorily 

required time frame for completion of 

the review. The comments that raise 

concerns regarding specific aspects of 

the process are addressed in the 

separate Response to Comments 

document. As indicated there, the EPA 

disagrees with these comments. The 

EPA finds the review to have been 

lawfully conducted, the process 

reasonably explained, and thus finds no 

reason to withdraw the proposal. 

We disagree with some commenters’ 

contention that the EPA based its 

proposed decision simply on the 

similarity of the health effects evidence 

to that available in the last review. 

While the health effects information is 

generally similar to that available in the 

last review, particularly with regard to 

respiratory effects (the effects causally 

related to O3 exposure), the current 

health effects evidence base includes 

hundreds of new health studies. Based 

on consideration of the full evidence 

base, including that the newly available 

in the current review, the EPA has 

reached different conclusions regarding 

some categories of effects (as 

summarized in II.A.2.a above). The 

EPA’s observation that the nature of the 

evidence has not substantially changed 

with regard to effects causally related to 

O3 exposure, was not, as implied by the 

comment, the primary consideration in 

the Administrator’s proposed decision. 

The Administrator considered a number 

of factors in reaching his proposed 

decision, including the full extent of the 

currently available health effects 

evidence, and the details in which it is, 

and is not, similar to the last review, 

which has led to conclusions similar to 

prior conclusions for some categories of 

O3 effects and resulted in changes to 

others (85 FR 49868–49874, August 14, 

2020).97 Further, in reaching his final 

decision in this review, as described in 

section II.B.3 below, he has again 

considered the currently available 

information, now in light of the public 

comments received on the proposal, 

among other factors.98 In sum, while we 

have noted the similarities in the health 

effects information between this review 

and the last review (particularly for 

respiratory effects), we have engaged in 

independent analysis and assessment of 

the health effects information in this 

review, and the Administrator has 

exercised his independent judgment 

based on the current health effects 

assessment, in combination with current 

exposure/risk information, advice from 

the CASAC and public comment. Thus, 

contrary to the suggestion by these 

commenters, the decision on the 

primary standard has been made in 

consideration of the current health 

effects evidence, current analyses of air 

quality, exposure and risk, advice from 

the CASAC, and public comments, 

consistent with requirements under the 

CAA. 

In support of their position that the 

available health effects evidence 

indicates that O3 exposures occurring in 

areas that meet the current standard are 

causing adverse effects, some 

commenters cite studies that investigate 

associations of O3 concentrations and 

effects, such as respiratory effects, 

mortality, and preterm birth.99 These 

studies include some already evaluated 

in the air quality criteria,100 101 some 

published subsequent to the literature 

cutoff date for the ISA, and some which 

some commenters claim the EPA 

arbitrarily dismissed or inconsistently 

weighed in reaching the proposed 

decision.102 As discussed in I.D above, 

we have provisionally considered these 

‘‘new’’ studies that have not already 

been evaluated in the air quality criteria 

and that were cited by commenters in 

support of their comments on the 

proposed decision (Luben et al., 2020). 

Based on this consideration, we 

conclude that these studies do not 

materially change the broad conclusions 

of the ISA with regard to these health 

effects, including the conclusions that 

there is a causal relationship of shortterm respiratory effects with O3

exposures; a relationship of long-term 

respiratory effects with O3 exposure that 

is likely to be causal; evidence that is 

suggestive of, but not sufficient to infer, 

causal relationships of cardiovascular 

effects and total mortality with short- or 

long-term O3 exposure; evidence that is 

suggestive of, but not sufficient to infer, 

causal relationships of central nervous 

system effects with short- or long-term 

O3 exposure; and, evidence that is 

suggestive of, but not sufficient to infer, 

causal relationships of reproductive and 

developmental effects with long-term O3

exposure (ISA, section IS.1.3.1). Nor do 

we find that these studies warrant 

reopening the air quality criteria for 

further review (Luben et al., 2020). 

Thus, we do not find these publications 

to be contrary to the discussions and 

associated conclusions in the PA and 

proposal or to indicate the current 

standard to be inadequate. We disagree 

that studies cited by commenters show 

these categories of effects to be caused 

by O3 exposures associated with O3 air 

quality that meets the current standard. 

We continue to focus on the studies of 

respiratory effects as most important to 

the Administrator’s judgments 

concerning the public health protection 

provided by the current standard. 

The epidemiologic studies of 

respiratory effects identified by the 

commenters include some investigating 

associations of O3 exposure with 

hospital admissions or emergency 

department visits for respiratory 

outcomes, or with various respiratory 

effects for selected population groups. 

Studies of O3 and respiratory effects 

cited by these commenters in support of 

their comment include studies that have 

already been evaluated in the air quality 

criteria (Goodman et al., 2017; O’Lenick 

et al., 2017; Jerrett et al., 2009; Lin et al., 

2008; Islam et al., 2009; Galizia et al., 

1999; Peters et al., 1999; Wendt et al., 

2014), and also several ‘‘new’’ studies, 

including four that investigate a 

relationship between O3 and COVID–19 

(Ware et al., 2016; Strosnider et al., 

2019; Wang et al., 2019a; Adhikari and 

Yin, 2020; Zhu et al., 2020; Zoran et al., 

2020; Petroni et al., 2020).103 We do not 


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change the broad scientific conclusions of the ISA 

with regard to respiratory effects, or warrant reopening the air quality criteria for further review 

(Luben et al., 2020). 

104Ecologic fallacy is a specific type of bias that 

results when group- or population-level data are 

used to estimate individual-level risks in an 

epidemiologic study 

105This study uses 2016 summertime average O3

as a surrogate for O3 from 3/1/2020 to 7/11/2020 

(Petroni et al., 2020). Yet COVID–19 cases did not 

surge in many parts of the U.S. until late summer 

or fall 2020. To the extent these areas (e.g., rural 

upper midwest) have lower O3 concentrations than 

areas of the country where COVID–19 cases surged 

earlier (e.g., New York City), a correlation between 

O3 concentrations and COVID–19 deaths would be 

overestimated. 

106While there may be correlations between O3

concentrations and COVID–19 cases and deaths, 

they could be explained by coincidental timing of 

the COVID–19 community transmission period in 

New York City and Milan with the early part of the 

O3 seasons in those areas, and neither the 

investigators or commenters provide evidence 

supporting an alternative plausible basis (Adhikari 

and Yin, 2020; Zoran et al., 2020). 

107While the full evidence base indicates the 

potential for O3 to increase susceptibility to some 

respiratory infections, the studies cited by 

commenters do not provide evidence that shortterm or long-term O3 exposure increases 

susceptibility to COVID–19. 

108Locations and time periods analyzed in these 

studies include three large metropolitan areas in 

Texas before 2012 (Goodman et al., 2017); Atlanta, 

Dallas and St. Louis from 2002 to 2008 (O’Lenick 

et al., 2017); large cities across the U.S. from late 

1970s through 2000 (Jerrett et al., 2009); New York 

State, primarily during the 1990s (Lin et al., 2008); 

U.S. location chosen for O3 concentrations not 

meeting the standard (Galizia and Kinney, 1999); a 

set of southern California communities during 

period (1990s) recognized to be exceeding the 

NAAQS (Peters et al., 1999; Islam et al., 2009); 

Houston metropolitan area during 2005 to 2007 

(Wendt et al., 2014); multiple locations including 

St. Louis, Memphis and Atlanta 2003 through 2012 

(Ware et al., 2016); six U.S. metropolitan areas, 

including Los Angeles, Baltimore and New York 

City, from 1999 thru 2018 (Wang et al., 2019a); and 

894 U.S. counties, including those for New York 

City and Los Angeles, 2001 to 2014 (Strosnider et 

al., 2019). Air quality data and design values 

derived by the U.S. indicate that the current 70 ppb 

standard was not met throughout the study period, 

or, for multicity studies for which single-city 

analyses not performed, was not met in all cities 

throughout the study (PA, Appendix 3B and Excell 

files available at: https://www.epa.gov/air-trends/ 

air-quality-design-values). 

109This uncertainty applies specifically to 

interpreting air quality analyses within the context 

of multicity effect estimates for short-term O3

concentrations, where effect estimates for 

individual study cities are not presented, as is the 

case for some of the multicity studies identified by 

commenters (85 FR 49870, August 14, 2020). 

find these studies to contradict any of 

the scientific conclusions on respiratory 

effects described in the ISA. 

With regard to the four studies on 

COVID–19, we disagree with the 

commenters that they provide evidence 

that O3 exposure contributes to COVID– 

19 incidence, much less that they 

indicate that O3 concentrations 

occurring when the current standard is 

met would do so. These studies 

investigate an association between O3

and COVID–19 cases or deaths. We note, 

however, that the time-series study 

design used in three of these studies 

(Zhu et al., 2020 [incorrectly cited by 

some commenters as Yongiian et al, 

2020]; Adhikari and Yin, 2020; Zoran et 

al., 2020) is not appropriate for 

infectious disease cases, which do not 

follow a Poisson distribution, as they 

increase exponentially with community 

spread. The fourth study, an ecological 

study (Petroni et al. 2020), is also 

limited by its study design, which is 

susceptible to confounding or other 

biases related to ecologic fallacy,104 as 

well as its manner of assigning exposure 

to the population.105 Further, the time 

periods in none of the four studies is 

long enough to rule out a coincidental 

increase in the community spread of 

COVID–19 with the increased O3

concentrations expected with the 

beginning of O3 season in these areas 

(e.g., March–April). Lastly, the 

biological basis by which a gaseous 

pollutant such as O3 would be expected 

to contribute to incidence of this disease 

is unclear.106 Thus, we do not find these 

studies to support a conclusion that O3

exposure causes COVID–19 morbidity or 

mortality.107

With regard to the commenters’ 

claims that effects other than respiratory 

effects (see above) are occurring as a 

result of O3 concentrations allowed by 

the current standard, we note that the 

standard is exceeded in nearly all of the 

locations and time periods analyzed in 

these studies.108 Although some studies 

analyzed multiple cities or locations in 

which the current standard was met 

during some time periods, air quality 

during other time periods or locations in 

the dataset does not meet the current 

standard. As noted in past reviews, 

compared to single-city studies, there is 

additional uncertainty in interpreting 

relationships between O3 air quality in 

individual study cities and reported O3

multicity effect estimates. Specifically, 

as recognized in section II.A.2.c above, 

the available multicity effect estimates 

in studies of short-term O3 do not 

provide a basis for considering the 

extent to which O3 health effect 

associations are influenced by 

individual locations with ambient O3

concentrations low enough to meet the 

current O3 standards versus locations 

with O3 concentrations that violate this 

standard (85 FR 49853, August 14, 2020; 

80 FR 65344, October 26, 2015).109

Thus, based on this information and the 

full health effects evidence base for O3,

we disagree with commenters about the 

implications of the cited epidemiologic 

studies regarding health risks of O3

exposures resulting from the O3

concentrations in ambient air allowed 

by the current standard. 

Protection of Sensitive Groups: 

Commenters expressing the view that 

the current standard does not protect 

sensitive or at-risk populations, 

variously state that the EPA does not 

consider risks to a number of population 

groups the commenters identify as at 

higher risk for O3-related health effects, 

and that retaining the current standard 

‘‘creates additional and unacceptable 

risks’’ for Black and low-income 

communities. Further, some 

commenters express the views that 

together the evidence from controlled 

human exposure studies and from 

epidemiologic studies indicates adverse 

effects associated with exposures 

allowed by the current standard; and 

that the EPA has not appropriately 

considered a number of aspects of the 

evidence related to risks to people with 

asthma. 

Some commenters, in addition to 

contending that the current standard 

will not protect populations for which 

the EPA has concluded there is 

adequate evidence for identification of 

increased risk (e.g., people with asthma, 

children, and outdoor workers), 

additionally assert that the current 

standard will not protect populations of 

color, American Indian/American 

Native groups, low SES communities, 

people of any age with respiratory 

issues other than asthma, diabetes or 

atrial fibrillation and pregnant women. 

As described in section I.A. above, 

primary NAAQS are intended to protect 

the public health, including at-risk 

populations, with an adequate margin of 

safety. Accordingly, in reviewing the air 

quality criteria, the EPA evaluates the 

evidence with regard to factors that 

place some populations at increased risk 

of harm from the subject pollutant. In 

this review, the populations for which 

the evidence indicates increased risk 

include people with asthma, children 

and outdoor workers, among other 

groups, as summarized in section 

II.A.2.b above (ISA, section IS.4.4). 

In support of their argument that 

individuals with atrial fibrillation are at 

increased risk of O3-related health 

effects, the commenter cited a study of 

O3 exposure and total mortality that has 

been evaluated in the ISA (MedinaRamon and Schwartz, 2008). It was 

initially evaluated in the last review and 

explicitly discussed again as part of the 

evidence base available in the current 

review (ISA, section 6.1.5.2 and Table 

IS–10; 2013 ISA, sections 6.6.2.2 and 

8.2.4). Based on consideration of that 

study and others investigating a 

potential for increased risk among 

populations with cardiovascular disease 


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110Further, we note that a more recent study than 

that cited by the commenters investigated the 

potential for an association of O3-related mortality 

risk with individuals with atrial fibrillation and 

observed no evidence of an association (ISA, 

Appendix 6, p. 6–11). 

111 In support of their view that O3-related risk is 

increased in Black populations, some commenters 

cite a study published after the ISA (Gharibi et al., 

2019). We have provisionally considered this study, 

as described in section I.D. above, and found that 

it does not materially affect the broad conclusions 

in the ISA, including those regarding the adequacy 

of evidence for finding an influence on O3-related 

risk of different categories of population status, or 

warrant reopening the air quality criteria for further 

review (Luben et al., 2020). 

112We note that two studies described by one 

commenter as indicating that those with low SES 

or who live in low SES communities face higher 

risk of hospital admissions and emergency 

department visits related to O3 pollution have been 

evaluated by the EPA and found not to report such 

findings (2013 ISA, section 8.3.3; ISA, Table IS–10). 

In the first, a study of O3 exposure and respiratory 

hospital admissions in 10 Canadian cities (Cakmak 

et al., 2006) ‘‘no consistent trend in the effect was 

seen across quartiles of income,’’ and the second, 

a study of O3 exposure and asthma hospital 

admissions and emergency visits (Burra et al., 

2009), ‘‘reported inverse effects for all levels of 

SES’’ (2013 ISA, p. 8–27; ISA, Table IS–10). 

113This is noted in the PA and proposal with 

regard to Black non-Hispanic and several Hispanic 

population groups (PA, Table 3–1). As some 

commenters note, this is also the case for American 

Indian and Native American population groups. 

Based on the recently available, 2016–2018 

National Health Interview Survey, while just under 

8% of the U.S. population is estimated to have 

asthma, the estimate is more than 10% for 

American Indian or Native American populations 

in the U.S. (https://www.cdc.gov/asthma/most_

recent_national_asthma_data.htm; document 

identifier EPA–HQ–OAR–2018–0279–0086). 

114The commenter cites Price-Haywood et al. 

(2020), Stokes et al. (2020), Millett et al. (2020), 

Killerby et al. (2020), and Gold et al. (2020). These 

studies present information regarding COVID–19 

cases, hospitalizations and/or deaths among various 

population groups, but they do not investigate 

association of those occurrences with O3. 

115 In making their argument, these commenters 

do not provide any explanation for why retaining 

the existing standard (i.e., making no regulatory 

change) would create additional risk for these 

populations. Rather, these commenters seem to be 

describing differences in predicted risk or mortality 

of air quality associated with a lower standard level 

and that of the current standard. In that way, they 

are claiming that retaining the current standard 

‘‘creates’’ additional risk. We address comments 

advocating a lower standard based on commentercited risk estimates (e.g., mortality) further below. 

(CVD), the 2013 ISA concluded that the 

evidence was ‘‘inadequate to classify 

pre-existing CVD as a potential at-risk 

factor for O3-related health effects’’ 

(2013 ISA, sections 8.2.4). In the current 

review, while a limited number of 

recent studies add to the evidence 

available in the 2013 ISA,110

collectively the evidence remains 

inadequate to conclude whether 

individuals with pre-existing CVD are at 

greater risk of O3-related health effects 

(ISA, Table IS–10, section IS.4.4.3.5). 

Thus, the evidence does not support the 

commenters assertion that populations 

with atrial fibrillation are at increased 

risk of O3-related effects and that the 

current standard does not protect these 

groups. 

The commenters who contend 

pregnant women are at increased risk do 

not provide supporting evidence, and 

the ISA does not reach such a 

conclusion based on the currently 

available evidence. Further, the ISA 

determined the evidence to be 

suggestive of, but not sufficient to infer, 

a causal relationship between O3

exposure and reproductive effects (ISA, 

section IS.4.3.6.3). Thus, we disagree 

with the commenters that pregnant 

women may be at increased risk of O3- 

related effects and disagree that the 

current standard does not protect these 

groups. 

With regard to a potential for 

increased risk of O3-related health 

effects based upon race or ethnicity, 

including American Indians or Native 

Americans), the available evidence is 

inadequate to make such a 

determination (ISA, section IS.4.4, 

Tables IS–9 and IS–10).111 Additionally, 

the evidence of increased O3 risk based 

on SES has been evaluated in the ISA 

and concluded to be ‘‘suggestive,’’ but 

the evidence is limited by 

inconsistencies (ISA, section IS.4.4). 

Thus, contrary to the view expressed by 

some commenters, the EPA has 

considered this factor in this review and 

the evidence was not adequate to 

identify SES as a risk factor for O3

related health effects. As noted by the 

commenters, the evidence for low SES 

populations is ‘‘suggestive’’ of increased 

risk (ISA, section IS.4.4), in part because 

it includes several inconsistencies (as 

summarized in section II.A.2.b above), 

including studies that did not find O3- 

related risk to be higher in lower SES 

communities.112 While we agree with 

the commenters that populations of 

some particular races or ethnic 

backgrounds or with low SES have 

higher rates of some health conditions, 

including asthma,113 the available 

evidence is not adequate to conclude an 

increased risk status based solely on 

racial, ethnic or income variables alone 

(ISA, section IS.4.4). Thus, we disagree 

with commenters that EPA has 

arbitrarily not considered such factors 

in reaching the decision on the primary 

standard. 

Some commenters further claim that 

tribal populations and communities of 

color are at increased risk of O3-related 

health effects due to increased impacts 

of COVID–19. We disagree with 

commenters that the studies they cite 

provide support for the role of O3

exposure in the observed increase in 

prevalence. The studies cited simply 

describe greater prevalence of COVID– 

19 among such communities and do not 

investigate and therefore do not provide 

evidence for a role for O3 exposure.114

An additional study cited by one 

commenter in support of their statement 

that people with COVID–19 are more 

susceptible to effects of O3, does not 

include any analyses with O3 among its 

analyses (Wu et al, 2020). With regard 

to diabetes, we note that the evidence 

related to a potential for this to affect 

risk of O3-related effects has been 

explicitly evaluated and found to be 

inadequate, thus indicating a lack of 

basis in the evidence for the statement 

by some commenters that diabetes 

prevalence in a community increases 

the risk of O3-related effects (ISA, Table 

IS–10). 

Additionally, commenters that 

contend that retaining the current 

standard ‘‘creates additional and 

unacceptable risks’’ for minority and 

low-income populations variously cite 

higher rates of asthma and other 

preexisting conditions in these 

populations and higher levels of 

pollution.115 In making this claim, these 

commenters state that non-Hispanic 

Blacks have been found to be more 

likely to live in counties with higher O3

pollution. To the extent that such 

patterns in the distribution of certain 

population groups and O3

concentrations result in these 

populations residing in areas that do not 

currently meet the current standard, we 

note that they are at greater risk than 

populations residing in areas that meet 

the current standard, and implementing 

the standard will reduce their risks. But 

we disagree with the commenters’ 

conclusion that retaining the current 

standard, without any change, creates 

additional risks for these populations. 

Thus, contrary to statements by some 

commenters, the EPA’s proposed 

decision to retain the current standard 

did consider evidence regarding risk to 

and thus protection of specific 

populations, such as those of particular 

races or ethnicities or low-income 

populations. The proposed decision, 

and the Administrator’s decision 

described in section II.B.3 below, are 

based on consideration of the currently 

available evidence, particularly that 

with regard to populations that may be 

at greater risk of O3-related health 

effects than the general population. As 

described in section II.B.3 below, the 

Administrator judges that by basing his 

decision on consideration of these 

populations, including adults and 

children with asthma, the at-risk 

population groups for which the 


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116The sentence in the 2017 statement of which 

one commenter quoted only a part, ‘‘As discussed 

in the previous ATS statement, a small but 

statistically significant mean reduction in FEV1 in 

a population means that some people had larger 

reductions, with the likelihood that reductions in 

a subset of susceptible subjects can have passed a 

threshold for clinical importance’’ This paragraph 

goes on to note that a study in which the mean 

decrement is about 3%, included two subjects with 

decrements greater than 10% (Thurston et al., 

2017). 

evidence is strongest and most 

extensive, will also provide protection 

for other at-risk populations for which 

the evidence is less certain and less 

complete. 

The commenters who express the 

view that the current standard does not 

provide sufficient protection of people 

with asthma raise concerns with the 

EPA’s consideration of this group and 

O3-related effects. Further, some 

commenters state that the EPA has not 

adequately explained how its approach 

for decision-making in this review 

protects at-risk populations, such as 

people with asthma. Such commenters 

state that the EPA does not explain how 

the proposed decision accounts for the 

greater vulnerability of people with 

asthma, given the attention to evidence 

from controlled human exposure studies 

of largely healthy subjects. Some 

commenters contend that the EPA 

arbitrarily focuses on lung function 

decrements and respiratory symptoms 

ahead of lung inflammation, and/or that 

the EPA has not rationally considered 

the most recent ATS statement with 

regard to consideration of effects in 

people with respiratory disease, such as 

asthma (which the commenters describe 

as a difference from past reviews). 

We disagree with these commenters. 

In this review, as in past reviews, the 

EPA has fully considered the health 

effects evidence in this review, 

including for sensitive populations, 

such as people with asthma, and 

explained its conclusions regarding the 

adequacy of public health protection 

offered by the current standard, 

including for such populations. Thus, 

the decision in this review, as described 

in section II.B.3 below, is based on the 

current scientific information. Further, 

our approach in this review does not 

differ appreciably from our approach in 

the last review. This approach is 

consistent with the applicable legal 

requirements for this review, including 

with provisions of the CAA related to 

the review of the NAAQS, and with how 

the EPA and the courts have historically 

interpreted the CAA. The approach is 

based fundamentally on the current 

health effects evidence in the ISA and 

quantitative analyses of exposure and 

risk in the PA. The policy implications 

of this information, along with 

guidance, criteria or interpretive 

statements developed within the public 

health community, including, also, 

statements from the ATS, in addition to 

advice from the CASAC are evaluated in 

the PA for consideration by the 

Administrator. The PA evaluations 

inform the Administrator’s public 

health policy judgments and 

conclusions. Thus, as in past reviews, 

the Administrator’s decision on the 

adequacy of the current primary 

standard draws upon the scientific 

evidence for health effects, quantitative 

analyses of population exposures and 

health risks, CASAC advice, and 

judgments about how to consider the 

uncertainties and limitations that are 

inherent in the scientific evidence and 

quantitative analyses, as well as public 

comments on the proposed decision. 

As described in section II.B.3 below, 

key aspects of the evidence informing 

the Administrator’s decision-making in 

this review include: (1) The causal 

relationship of O3 with respiratory 

effects, based on the full health effects 

evidence base, including both the 

controlled human exposure studies 

conducted primarily in largely healthy 

adult subjects, and the epidemiologic 

studies of health outcomes for people 

with asthma, and particularly children 

with asthma; (2) the increased risk to 

children and people with asthma, 

among other groups (3) the respiratory 

effects reported at the lowest exposures 

in the controlled human exposure 

studies; and (4) features of asthma that 

contribute to the susceptibility of people 

with asthma to O3-related effects. As a 

whole, the evidence base in this 

NAAQS review generally reflects a 

continuum, consisting of exposure 

levels at which scientists generally agree 

that health effects are likely to occur, 

through lower levels at which the 

likelihood and magnitude of the 

response become increasingly uncertain. 

As summarized in section I.A above, the 

CAA does not require the Administrator 

to establish a primary NAAQS at a zerorisk level or at background 

concentration levels (see Lead 

Industries Ass’n v. EPA, 647 F.2d at 

1156 n.51, Mississippi v. EPA, 744 F.3d 

at 1351), but rather at a level that 

reduces risk sufficiently so as to protect 

public health with an adequate margin 

of safety. The Administrator’s 

consideration of the scientific evidence 

is informed by the quantitative 

estimates of exposure and risk for air 

quality allowed by the current standard, 

and associated judgments on the 

adequacy of public health protection 

provided by the current standard are 

informed by advice from the CASAC 

and statements from ATS on adversity. 

With regard to the most recent ATS 

statement, the commenters’ claim that 

the EPA does not adequately consider 

the implications of the sentence that 

‘‘small lung function changes should be 

considered adverse in individuals with 

extant compromised function, such as 

that resulting from asthma, even 

without accompanying respiratory 

symptoms’’ and to consider the 

importance of examining effects in 

susceptible subsets of broader 

populations (Thurston et al., 2017). We 

disagree. The ATS statements (from the 

initial statement in 1985 to the recent 

2017 statement) and their role in 

primary O3 standard reviews, 

summarized in section II.A.2.b above, 

occupy a prominent role in 

consideration of public health 

implications in the PA and the proposal 

(PA, section 3.3.2; 85 FR 49848, 49866, 

49871, August 14, 2020), and the 

Administrator considers them in his 

decision, as described in section II.B.3 

below. The PA presentation includes 

summaries of the purpose and 

intentions articulated by the ATS, and 

of the evolution and areas of 

consistency across the statements. The 

PA gave particular attention to the ATS 

emphasis on consideration of the 

significance or adversity of effects, 

particularly for more susceptible 

individuals. It recognized both the 2000 

ATS statement concluding that ‘‘small 

transient changes in forced expiratory 

volume in 1 s[econd] (FEV1) alone were 

not necessarily adverse in healthy 

individuals, but should be considered 

adverse when accompanied by 

symptoms’’ (ATS, 2000), and also the 

more recent statement that also gives 

weight to findings of such lung function 

changes in the absence of respiratory 

symptoms in individuals with preexisting compromised function, such as 

that resulting from asthma (Thurston et 

al., 2017). With regard to population 

risk (another aspect of the ATS 

statement cited by commenters), the PA 

and proposal summarized the 2000 and 

2017 ATS statements, recognizing that 

the 2017 statement references and 

further describes concepts described in 

the 2000 statement, such as its 

discussion of considering effects on the 

portion of the population that may have 

a diminished reserve that puts its 

members at potentially increased risk if 

affected by another agent (ATS, 

2000).116 As described in section II.B.3 

below, the Administrator considers the 

ATS statements in reaching his 

conclusions in this review. 

In support of their claim that the EPA 

has not appropriately considered the 

ATS statements, some commenters 


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117With regard to 10% as a magnitude decrement, 

the prior ATS statement noted that the EPA had 

graded this ‘‘mild’’ in a prior review, while noting 

that such a grading has not been evaluated against 

other measures (ATS, 2000). In this review, as in 

past reviews, the EPA has summarized study results 

with regard to multiple magnitudes of lung function 

decrement, including 10%, recognizing that 10% 

has been used in clinical settings to detect a FEV1

change likely indicative of a response rather than 

intrasubject variability, e.g., for purposes of 

identifying subjects with responses to increased 

ventilation (Dryden, 2010). For example, the PA in 

the current review provides such a summary (PA, 

Appendix 3D, p. 3D–77). 

118Contrary to this claim, the lung function risk 

analysis in the current review (which is an update 

of the very same analysis in the 2014 HREA to 

which the commenters cite) presents the results for 

exactly the same categories of lung function 

decrement (at/above 10%, at/above 15% and at/ 

above 20%) as in the 2014 HREA (e.g., PA, Table 

3–4). 

119The citation provided by the commenters is 

the CASAC letter on the draft PA; in this letter the 

CASAC cites the ATS statement in making a 

comment on the draft PA indicating that the 

concept that lung function decrements in the 

absence of symptoms do not represent an adverse 

health effect should not apply to the susceptible 

group of children with asthma (Cox, 2020a, 

Consensus Responses to Charge Questions, pp. 8– 

9). 

additionally take issue with the EPA’s 

use of the number of subjects 

experiencing at least a 15% FEV1

decrement in its description in the 

proposal of the increased response 

evident by comparing from the lowest 

exposure levels studied (40 ppb) up to 

70 ppb (85 FR 49851, August 14, 2020). 

These comments also state that EPA did 

not discuss the clinical significance of 

FEV1 decrements of 10% or higher for 

people with existing lung disease, while 

stating that the ATS statement mentions 

this magnitude of decrement. The ATS 

statement references decrements at or 

above 10% in illustrating a point about 

variation of subject responses beyond a 

group mean, noting that while the mean 

of an exposed group of study subjects 

may be small, some group members 

have larger reductions and can have 

passed a threshold for clinical 

importance. It does not provide a 

discussion of thresholds of clinical 

importance.117 In claiming that EPA’s 

discussion on this represents a 

difference from the last review, the 

commenters cite the 2014 HREA and 

state that we have not considered FEV1

decrements at or above 10% in the 

current review, however this is not the 

case.118 Furthermore, the PA states that 

the mid- to upper-end of the range of 

moderate levels of functional responses 

and higher (i.e., FEV1 decrements ≥15% 

and ≥20%) are included to generally 

represent potentially adverse lung 

function decrements in active healthy 

adults, while for people with asthma or 

lung disease, a focus on moderate 

functional responses (FEV1 decrements 

down to 10%) may be appropriate (PA, 

Appendix 3D, p. 3D–76). 

In objecting to the EPA’s approach to 

considering the ATS statement, these 

commenters cite a reference to the ATS 

statement in CASAC’s advice as 

additional evidence that the EPA 

approach to considering the ATS 

statement is arbitrary.119 This comment 

was made within the context of the 

CASAC comments on the draft PA that 

emphasized the need to improve 

discussion of the susceptibility of 

people with asthma, including giving 

attention to the occurrence of lung 

function decrements in susceptible 

groups, specifically children with 

asthma. This section of the CASAC 

letter also cautions against too great a 

focus on lung function decrements and 

emphasizes the need for fuller 

consideration of respiratory effects that 

are likely to be important in people with 

asthma due to features of that disease. 

In consideration of these comments, the 

final PA includes an improved 

discussion on the unique vulnerability 

of people with asthma (PA, sections 

3.3.1.1, 3.3.2, 3.3.4, and 3.5.1) that 

contributes to due consideration of this 

population group in decision-making on 

the primary O3 standard. Further, in 

considering the exposure and risk 

analysis results, we recognize the 

comparison-to-benchmarks analysis as 

providing a more robust consideration 

of risk to sensitive groups as it provides 

the ability to consider O3 effects more 

broadly, with each benchmark 

representing the array of effects, at 

different severities, associated with that 

exposure level. This is one of the 

reasons (consistent with the CASAC 

advice) that this analysis (rather than 

the lung function risk analysis) receives 

greater emphasis in the PA, consistent 

with the CASAC advice in this area. 

In light of the above discussion, we 

note that the PA, the proposal, and the 

decision described in section II.B.3 

below, focus specifically on 

consideration of people with asthma, 

and particularly children with asthma. 

While the evidence regarding the 

susceptibility of people with asthma to 

the effects of O3 is robust, our 

understanding of the exposures at 

which various effects (of varying 

severity) would be elicited is less 

defined. For example, the inherent 

characteristics of asthma contribute to a 

risk of asthma-related responses, such as 

asthma exacerbation in response to 

asthma triggers, which may increase the 

risk of more severe health outcomes 

(ISA, section 3.1.5). This is supported 

by the strong and consistent 

epidemiologic evidence that 

demonstrates associations between 

ambient O3 concentrations and hospital 

admissions and emergency department 

visits for asthma (ISA, section 

IS.4.4.3.1). In moving to consideration 

of the potential specific exposure 

scenarios (e.g., multiple-hour exposures 

to 60 to 80 ppb O3 during quasicontinuous exercise), we note that the 

evidence is for largely healthy adult 

subjects. With regard to lung function 

decrements, the limited evidence from 

controlled human exposure studies 

(primarily at higher exposures and in 

adult subjects) indicates similar 

magnitude of O3-related FEV1

decrements for people with as for 

people without asthma (ISA, Appendix 

3, section 3.1.5.4.1). Further, across 

other respiratory effects of O3 (e.g., 

increased respiratory symptoms, 

increased airway responsiveness and 

increased lung inflammation), the 

evidence has also found the observed 

responses to generally not differ due to 

the presence of asthma, although the 

evidence base is more limited with 

regard to study subjects with asthma 

(ISA, Appendix 3, section 3.1.5.7). 

Thus, in light of the uncertainties in the 

evidence base with respect to people 

with asthma and exposures eliciting 

effects and the severity of those effects, 

other aspects of the evidence are 

informative to the necessary judgments. 

Accordingly, the advice from the 

CASAC and the statements from the 

ATS are important to the judgments 

made by the Administrator in basing his 

decision on the current evidence and 

ensuring a primary standard that 

protects at-risk populations, such as 

people with asthma. 

Contrary to the claim from some 

commenters, our consideration of effects 

in people with asthma did not focus 

solely on lung function responses. As 

noted above, we recognize that the 

inherent characteristics of asthma as a 

disease provide the potential for O3

exposures to trigger asthmatic 

responses, such as through causing an 

increase in airway responsiveness. 

Based on the available evidence, we 

consider the potential for such a 

response to be greater, in general, at 

relatively higher, versus lower, exposure 

concentrations, noting 80 ppb to be the 

lowest exposure concentration at which 

increased airway responsiveness has 

been reported in generally healthy 

adults. We recognize that this evidence 

and the evidence represented by the 

three benchmark concentrations used in 

the exposure/risk analyses (60, 70 and 

80 ppb) is for largely healthy adults and 

does not include data for people with 


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120One commenter contends that inflammation is 

apparent from short-term O3 exposures ranging 

from 12 to 35 ppb, based on air quality metrics 

reported in some epidemiologic studies, such as 

mean 24-hour averages or monthly averages of 8- 

hour concentrations (ISA, Table 4–28). The 

commenter implies that such values for these 

metrics are lower than the level of the standard (70 

ppb) means that exposures allowed by the standard 

are causing outcomes analyzed in the study. 

However, none of the metrics for which values are 

cited by the commenter are in terms of design 

values for the current standard, such that a direct 

comparison of the values is not meaningful. 

121The currently available evidence does not 

support the implication of the commenters that the 

inflammatory response reported in some 

individuals after a 6.6-hour exposure to 60 ppb, 

during quasi-continuous exercise (as in Kim et al., 

2011), causes permanent lung damage or 

development of severe lung disease. While the 

experimental animal evidence indicates the 

potential for repeated exposures to elevated 

concentrations (e.g., at or above 500 ppb over 

multiple days) can contribute to other effects in 

animal models or to other asthmatic responses in 

animal models of asthma, the full evidence base for 

single exposures to lower concentrations does not 

provide such a finding (ISA, sections 3.1.4.4, 

3.1.4.4.2 and 3.1.5.6.2; 2013 ISA, section 6.2.3). 

Thus, the potential for effects reported from 6.6- 

hour exposures to 60 ppb O3, during quasicontinuous exercise, including the inflammation 

reported by Kim et al. (2011) to contribute to 

adverse health effects is uncertain. Newly available 

evidence in this review does not reduce this 

uncertainty or provide a contradiction to 

conclusion regarding the implications of 

inflammation induced by single or isolated 

exposures (ISA, Appendix 3). 

asthma. In reaching his decision in this 

review, the Administrator gives 

additional consideration to the effects of 

particular concern for people with 

asthma, such as asthma exacerbation, in 

light of the limitations of the evidence 

represented by the benchmarks in this 

regard, as discussed in section II.B.3 

below. 

In support of their view that the EPA 

gives too little weight to effects reported 

in studies of 60 ppb, some commenters 

assert that the EPA arbitrarily focused 

on the evidence for lung function 

decrements and respiratory symptoms, 

and does not explain how the proposed 

decision protects against the harm 

posed by inflammatory responses to O3. 

In making this statement they cite the 

study by Kim et al. (2011) and 

discussions in the ISA regarding studies 

documenting the role of O3 in eliciting 

inflammatory responses and regarding 

possible conceptual mechanisms by 

which inflammatory responses can 

contribute to other effects (including 

cardiovascular effects). In so doing, they 

contend that exposures lower than those 

for which the current standard is 

intended can cause inflammation 

resulting in permanent lung damage and 

the development of severe lung disease. 

They additionally state that airway 

inflammation of O3 is of particular 

concern for people with asthma as 

airway inflammation is a feature in the 

definition of asthma. 

Contrary to the view of some 

commenters, the Administrator has 

given significant consideration (in the 

proposal and in section II.B.3 below) to 

the exposure estimates for the 60 ppb 

benchmark. In considering the O3

inflammatory response, we note that 

inflammation induced by a single 

exposure (or several exposures over the 

course of a summer) can resolve entirely 

(2013 ISA, p. 6–76). Thus, the 

inflammatory response observed 

following the single exposure to 60 ppb 

in the study by Kim et al. (2011) of 

largely healthy subjects is not 

necessarily an adverse response.120 We 

further consider the comments from the 

CASAC regarding airway inflammation 

as an important aspect of asthma, 

including the CASAC’s description of 

increased airway inflammation in 

people with asthma as having the 

potential to increase the risk of an 

asthma exacerbation. As described in 

section II.B.3 below, the Administrator 

also considers this, while noting also 

the lack of evidence from studies of 

people with asthma at 60 ppb. In so 

doing, he recognizes that, due to 

interindividual variability in 

responsiveness, both in regard to O3 and 

in regard to asthma exacerbation 

triggering events, not every occurrence 

of an exposure considered to have the 

potential to increase airway 

inflammation will result in such an 

adverse effect. We find it important to 

note, however, that continued acute 

inflammation can contribute to a 

chronic inflammatory state, with the 

potential to affect the structure and 

function of the lung (2013 ISA, p. 6–76; 

ISA, sections 3.1.4.4.2 and 3.1.5.6.2).121

In light of this evidence, the 

Administrator, in his consideration of 

the exposure/risk estimates of exposures 

at or above the 60 ppb benchmark 

(described in section II.B.3 below), is 

less concerned about such estimates 

representing a single occurrence, and 

gives weight to estimates of multiple 

occurrences and their associated greater 

risk. Thus, rather than a sole focus on 

a single exposure level or type of effect 

(such as lung function decrements), the 

Administrator considers the quantitative 

estimates for all three benchmarks (with 

regard to single and multiple 

occurrences), recognizing that they 

represent differing levels of significance 

and severity of O3-related effects, both 

with regard to the array of effects and 

severity of each type of effects, as well 

as the implications for the at-risk 

populations, including people with 

asthma. The comparison-to-benchmarks 

analysis provides for this full 

characterization of risk for the broad 

array of respiratory effects, including 

inflammation and airway 

responsiveness, thus avoiding an 

inadequate and narrower focus, e.g., 

limited to lung function decrements (85 

FR 49872, August 14, 2020). 

Contrary to the commenters’ claims, 

the Administrator, in reaching his 

proposed decision, and in his final 

decision, as described in section II.B.3 

below, placed primary focus on what 

the evidence indicates with regard to 

health effects in the at-risk population 

of people with asthma, particularly 

children with asthma, and on results of 

the exposure and risk analysis for this 

population. In so doing, he recognizes 

key aspects of the evidence, as 

summarized in section II.A.2.a above, 

that indicate the array of O3-associated 

respiratory effects to be of increased 

significance to people with asthma 

given aspects of the disease that may 

put such peoples at increased risk for 

prolonged bronchoconstriction in 

response to asthma triggers. The 

increased significance of effects in 

people with asthma and risk of 

increased exposure for children (from 

greater frequency of outdoor exercise) is 

illustrated by the epidemiologic 

findings of positive associations 

between O3 exposure and asthmarelated emergency department visits and 

hospital admissions for children with 

asthma. In this context, the 

Administrator focuses on the breadth of 

O3 respiratory effects evidence at the 

lowest exposures tested in the 

controlled human exposure studies 

which provide the most certain 

evidence, considering this in light of the 

fuller evidence base which provides a 

foundation for necessary judgments in 

light of uncertainties. 

Thus, we disagree with commenters 

that we have not considered the full 

body of evidence and quantitative 

information available in this review 

with regard to exposures that might be 

expected to elicit effects in at-risk 

populations. In so doing, as summarized 

in section II.A.2.a above, section II.B.1.a 

of the proposal, and the PA, we 

recognize that the currently available 

evidence supports the conclusion of a 

causal relationship between short-term 

O3 exposure and respiratory effects, 

with the strongest evidence coming 

from controlled human exposure studies 

that document subtle reversible effects 

in 6.6-hour exposures of largely healthy 

adult subjects, engaged in quasicontinuous exercise, to average 

concentrations as low as 60 ppb. The 

epidemiologic evidence of associations 

of O3 concentrations in ambient air with 


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122As described in section II.A.2.c above and in 

the PA, these studies generally do not detail the 

specific exposure circumstances eliciting such 

effects. 

123Accordingly, uncertainties remain with regard 

to the independent role of O3 exposures in eliciting 

the reported health outcomes analyzed, and in the 

absence of analyses that might reduce such 

uncertainties (e.g., analyses of the presence and 

effects of co-occurring pollutants). 

124Contrary to implications of some commenters, 

this judgment by the current Administrator is 

Continued 

increased incidence of hospital 

admissions and emergency department 

visits for an array of respiratory health 

outcomes further indicates the potential 

for O3 exposures to elicit health 

outcomes more serious than those 

assessed in the experimental studies, 

particularly for children with asthma, 

and the evidence base of such 

epidemiologic studies as a whole 

provides strong support for the 

conclusion of causality for respiratory 

effects.122 Further as described in the 

PA and the proposal and summarized in 

section II.A.2.a above, very few of these 

studies were conducted in locations 

during periods when the current 

standard was met. While some 

commenters cite the low values of some 

of the air quality metrics analyzed in 

such studies, those metrics are not in 

the form of the design value for the 

current standard and so, contrary to 

commenters’ assertion, cannot show 

that serious health effects are occurring 

under air quality conditions allowed by 

the current 70 ppb standard. 

Protection With an Adequate Margin 

of Safety: Some commenters expressed 

the view that the current standard does 

not provide an adequate margin of 

safety variously argue that the EPA is 

ignoring precedent and CAA 

requirements for considering scientific 

uncertainty in its judgments regarding 

an adequate margin of safety, and that 

statements from the prior CASAC and 

new evidence suggests that the current 

70 ppb standard provides little margin 

of safety for protection of sensitive 

subpopulations from harm. These 

commenters generally advocate revision 

to a 60 ppb standard to address this 

concern. In support of their views, some 

state that the EPA is ignoring findings 

of a statistically significant lung 

function response to 6.6-hour exposure 

to 60 ppb during quasi-continuous 

exercise while others cite the EPA 

consideration of epidemiologic 

evidence, claiming that the EPA is 

inappropriately using identified 

uncertainties as a basis for not revising 

the standard. We disagree with these 

characterizations. 

As an initial matter, we note that, 

contrary to the statements made by 

these commenters, the Administrator, in 

reaching his proposed decision, as in 

reaching his final decision, has 

considered legal precedent and CAA 

requirements for a primary standard that 

protects public health, including the 

health of sensitive groups, with an 

adequate margin of safety. With regard 

to scientific uncertainty, as summarized 

in section I.A above, the CAA 

requirement that primary standards 

provide an adequate margin of safety 

was intended to address uncertainties 

associated with inconclusive scientific 

and technical information available at 

the time of standard setting. It was also 

intended to provide a reasonable degree 

of protection against hazards that 

research has not yet identified. See Lead 

Industries Ass’n v. EPA, 647 F.2d 1130, 

1154 (D.C. Cir. 1980); American 

Petroleum Institute v. Costle, 665 F.2d at 

1186; Coalition of Battery Recyclers 

Ass’n v. EPA, 604 F.3d 613, 617–18 

(D.C. Cir. 2010); Mississippi v. EPA, 744 

F.3d 1334, 1353 (D.C. Cir. 2013). Thus, 

in considering whether the primary 

standard includes an adequate margin of 

safety, the Administrator is seeking to 

ensure that the standard not only 

prevents pollution levels that have been 

demonstrated to be harmful but also 

prevents lower pollutant levels that may 

pose an unacceptable risk of harm, even 

if the risk is not precisely identified as 

to nature or degree. In so doing, 

however, the CAA does not require the 

Administrator to establish a primary 

NAAQS at a zero-risk level or at 

background concentration levels (see 

Lead Industries Ass’n v. EPA, 647 F.2d 

at 1156 n.51, Mississippi v. EPA, 744 

F.3d at 1351), but rather at a level that 

reduces risk sufficiently so as to protect 

public health with an adequate margin 

of safety. 

In the proposed decision, as in the 

decision described in section II.B.3 

below, the Administrator’s 

consideration of the kind and degree of 

uncertainties associated with the 

current information (as some of the 

factors the EPA considers in addressing 

the requirement for an adequate margin 

of safety) involved a number of 

judgments. With regard to his 

consideration of the epidemiologic 

evidence, for example, the 

Administrator recognizes that, as a 

whole, investigations of associations 

between O3 and respiratory effects and 

health outcomes (e.g., asthma-related 

hospital admission and emergency 

department visits) provide strong 

support for the overarching conclusion 

of that O3 causes respiratory effects, and 

its risks to people with asthma. In his 

consideration of O3 exposures of 

concern, the Administrator, agrees with 

staff evaluations in the PA, that such 

studies available in this review are less 

informative to his judgments related to 

air quality conditions allowed by the 

current standard (85 FR 49870, August 

14, 2020). For example, as summarized 

in section II.A.2.c above, none of the 

U.S. studies that show associations 

between O3 and the clearly adverse 

health outcomes of hospital admissions 

or emergency department visits for 

respiratory causes were based in 

locations during time periods when the 

current standard was always met (PA, 

section 3.3.3). While there were two 

such studies based in single cities in 

Canada, as discussed above, the 

interpretation of individual single-city 

results is complicated by the presence of 

co-occurring pollutants or pollutant 

mixtures (PA, section 3.3.3).123 Thus, as 

in reaching his decision in this review, 

the Administrator has fully considered 

conclusions reached in the ISA 

regarding the epidemiologic evidence 

and the policy evaluations in the PA, 

and does not find the currently available 

epidemiologic studies to provide 

insights regarding exposure 

concentrations associated with health 

outcomes that might be expected under 

air quality conditions that meet the 

current standard (85 FR 49870, August 

14, 2020). Thus, the EPA’s decision on 

the standard in this review fully and 

appropriately considers the full 

evidence base, including the 

epidemiologic evidence, and associated 

uncertainties and limitations. 

With regard to the controlled human 

exposure studies, and the nature and 

degree of effects that might be expected 

at exposures lower than those studied or 

in unstudied population groups, the 

Administrator has considered first what 

the evidence base indicates with regard 

to the lowest exposures as well as 

differences and similarities between the 

studied populations and the less well 

studied population groups recognized to 

be at increased risk. In so doing, he 

considers the findings of statistically 

significant respiratory responses in the 

studies of 60 ppb exposures in largely 

healthy subjects, particularly in his 

consideration of the exposure and risk 

estimates. For example, in reaching his 

decision in section II.B.3 below, as for 

his proposed decision, he finds it 

appropriate to consider the level of 

protection provided by the current 

standard from single exposures, but to 

give greater weight to multiple 

exposures, in judging adequacy of the 

margin of safety provided by the current 

standard.124 Such considerations 


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consistent with that made by the prior 

Administrator in establishing the current standard, 

as seen from the summary of the prior 

Administrator’s judgment in that regard that was 

summarized in the proposal and that these 

commenters cite: 

Further, while the Administrator recognized the 

effects documented in the controlled human 

exposure studies for exposures to 60 ppb to be less 

severe than those associated with exposures to 

higher O3 concentrations, she also recognized there 

to be limitations and uncertainties in the evidence 

base with regard to unstudied population groups. 

As a result, she judged it appropriate for the 

standard, in providing an adequate margin of safety, 

to provide some control of exposures at or above 

the 60 ppb benchmark (80 FR 65345–65346, 

October 26, 2015). [85 FR 49841, August 14, 2020] 

125The context for this statement is in 

considering the benchmark concentrations utilized 

in the exposure-to-benchmarks analysis of the 2014 

HREA and reflecting on responses reported in 

controlled human exposure studies of healthy 

subjects exposed for 6.6 hours with quasicontinuous exercise. With regard to the responses 

reported from exposure to 72 ppb, on average across 

the exercise periods, the prior CASAC stated its 

view ‘‘that these effects almost certainly occur in 

some people, including asthmatics and others with 

low lung function . . . at levels of 70 ppb and 

below’’ (Frey, 2014b, p. 6). 

126 In their 2014 advice, the prior CASAC 

concluded by explicitly stating ‘‘our policy advice 

is to set the level of the standard lower than 70 ppb 

within a range down to 60 ppb, taking into account 

your judgment regarding the desired margin of 

safety to protect public health.’’ 

127The legislative history of section 109 indicates 

that a primary standard is to be set at ‘‘the 

maximum permissible ambient air level . . . which 

will protect the health of any [sensitive] group of 

the population,’’ and that for this purpose 

‘‘reference should be made to a representative 

sample of persons comprising the sensitive group 

rather than to a single person in such a group.’’ 

contributed to the Administrator’s 

proposed judgments with regard to the 

requisite level of protection needed to 

protect at-risk populations with an 

adequate margin of safety, as required 

by the Act and consistent with the 

factors recognized in the relevant case 

law. Thus, consistent with the CAA 

requirements and prior judicial 

decisions, the Administrator based his 

proposed decision, and bases his final 

decision (as summarized in II.B.4 

below) on the scientific evidence, our 

current understanding of it, and his 

judgments concerning associated 

uncertainties, both those associated 

with inconclusive scientific and 

technical information, and those 

associated with hazards that research 

has not yet identified. These judgments, 

along with the factors recognized above 

that the EPA generally considers in each 

NAAQS review, contribute to his 

reasoned decision making in this 

review, as described in section II.B.3 

below. 

With regard to advice provide by 

CASAC in the last review as a general 

matter, we disagree with the 

commenters’ presumption that it is 

necessary for EPA to address in this 

review each statement a prior CASAC 

made in a prior review. The Clean Air 

Act does not impose such a 

requirement. We further note that a 

prior CASAC’s advice would be based 

on review of the prior air quality 

criteria, exposure/risk analyses and 

standard, as well as considerations 

pertinent in the prior review (which 

may, depending on the issue, differ from 

the pertinent evidence, information and 

considerations before a current CASAC). 

We note, however, that this specific 

advice from the prior CASAC on the 

adequacy of the margin of safety was 

cited by part of the CASAC in the 

current review. As summarized in the 

proposal and in section II.B.1.b above, 

while the prior CASAC advised that the 

size of the margin of safety provided 

varied across different standard levels 

within the range from 70 to 60 ppb that 

the prior CASAC recommended, it 

found a level of 70 ppb could be 

supported by the scientific evidence. 

Further, the prior CASAC recognized, as 

summarized in section II.B.1.b above, 

that with regard to the ‘‘size’’ of the 

margin of safety, the selection of any 

particular approach to providing an 

adequate margin of safety is a policy 

choice left specifically to the 

Administrator’s judgment (Lead 

Industries Ass’n v. EPA, 647 F.2d at 

1161–62; Mississippi v. EPA, 744 F.3d at 

1353; section I.A above). Thus, in 

reaching his proposed decision, the 

Administrator explicitly considered the 

advice provided by the prior CASAC to 

the extent it was represented in advice 

from the current CASAC as emphasized 

by part of the current CASAC (85 FR 

49873, August 14, 2020), and he has 

similarly again considered it reaching 

his final decision, as described in 

section II.B.3 below, in light of public 

comments raising it. 

Some commenters also express the 

view that EPA is using limited data in 

sensitive population groups as an 

excuse for not establishing a level at 

which there is ‘‘an absence of adverse 

effect’’ in sensitive groups. In support of 

their view, some commenters claim that 

the EPA has not addressed a statement 

of the prior CASAC regarding the 

potential for lung function decrements 

and respiratory symptoms to occur in 

people with asthma with exposure to 70 

ppb (while at elevated exertion). 

Contrary to this claim,125 the EPA 

considered in the last review the point 

made by the prior CASAC in the 

statement highlighted by the 

commenters. The statement from the 

prior CASAC that the commenters 

reference was provided in the CASAC 

review of a draft PA in the last review, 

fully considered in completing the final 

2014 PA, and, along with the totality of 

the prior CASAC advice, taken into 

account in establishing the current 

primary standard (80 FR 65292, October 

26, 2020). It is not necessary for the EPA 

to address in this review each statement 

a prior CASAC made in a prior review. 

We agree with commenters who 

express the view that to protect 

sensitive populations from effects 

reported in some largely healthy 

subjects from the 6.6 hour exposure to 

73 ppb (with quasi-continuous 

exercise), the standard should provide 

protection against somewhat lower 

exposures. As summarized in section 

II.B.3 below, this is an objective the 

Administrator identifies for the standard 

and, based on the exposure/risk 

estimates, he finds the standard to 

provide strong protection from such 

exposures (and associated risk of such 

effects). In addition, in highlighting this 

isolated statement from the last review, 

the commenters fail to distinguish 

CASAC advice on the primary standard 

from consideration of the exposure 

benchmark for comparison to a multihour exposure while engaged in quasicontinuous exercise. 

With regard to the prior CASAC’s 

scientific and policy advice on the 

primary standard,126 the prior CASAC 

concluded that the scientific evidence 

supported a range of standard levels that 

included 70 ppb, and also recognized 

the choice of a level within that range 

to be ‘‘a policy judgment under the 

statutory mandate of the Clean Air Act’’ 

(85 FR 49873).127 We further note that 

the current CASAC concludes in this 

review that newly available evidence 

relevant to standard setting does not 

substantially differ from that available 

in the last review (Cox, 2020a, 

Consensus Responses to Charge 

Questions p. 12; 85 FR 49873, August 

14, 2020). As discussed further below, 

we note that the CAA does not require 

the Administrator to establish a primary 

NAAQS at a zero-risk level or at 

background concentration levels (see 

Lead Industries Ass’n v. EPA, 647 F.2d 

at 1156 n.51, Mississippi v. EPA, 744 

F.3d at 1351), but rather at a level that 

reduces risk sufficiently so as to protect 

public health with an adequate margin 

of safety. 

Some commenters also state that the 

primary NAAQS must be set at a level 

at which there is an absence of adverse 

effects in sensitive populations. While 

the EPA agrees that the NAAQS must be 

set to protect sensitive populations with 

an adequate margin of safety, it is well 

established that the NAAQS are not 


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128The legislative history of the Clean Air Act 

provides further support for these holdings, as do 

the statutory deadlines for attainment. See H. Rep. 

95–294, 95th Cong. 1st sess. 127, 123 Cong. Rec. 

S9423 (daily ed. June 10, 1977) (statement of 

Senator Muskie during the floor debates on the 

1977 Amendments that ‘‘there is no such thing as 

a threshold for health effects. Even at the national 

primary standard level, which is the health 

standard, there are health effects that are not 

protected against.’’ 

129Contrary to the commenters’ assertion of a lack 

of explanation for the study areas included in the 

analyses, the PA describes the study area selection 

criteria and process, including steps taken to 

include adequate representation of diverse 

conditions. As observed in the PA, seven of the 

eight study areas were also included in the 2014 

HREA, and the eighth study area (Sacramento) was 

newly added in the current review to insure 

representation of a large city in the southwest (PA, 

section 3.4.1 and Appendix 3D, section 3D.2.1). 

Clarification on this point in the final PA was 

responsive to the only CASAC comment on 

completeness of the description of study area 

selection (Cox, 2020a). We disagree with the 

implication by some commenters that each review’s 

analyses must focus on the same areas. There is no 

such requirement under the Act, and such a view 

ignores the need to consider the current information 

in each review in planning appropriate analyses. 

130For example, the exposure assessment for the 

1997 O3 NAAQS review included nine urban study 

areas, for which the combined population 

simulated was 41.7 million. The exposure 

assessment for the current review included eight 

urban study areas with a combined simulated 

population size of approximately 39 million (PA, p. 

3D–96; U.S. EPA, 1996b, p. 76). We additionally 

note the focus on analysis results in terms of 

population percentages rather than population 

counts. 

131A broad variety of spatial and temporal 

patterns of O3 concentrations can exist when 

ambient air concentrations just meet the current 

standard. These patterns will vary due to many 

factors including the types, magnitude, and timing 

of emissions in a study area, as well as local factors, 

such as meteorology and topography. 

meant to be zero risk standards. See 

Lead Industries v. EPA, 647 F.2d at 1156 

n.51; ATA III, 283 F. 3d at 360 (‘‘[t]he 

lack of a threshold concentration below 

which these pollutants are known to be 

harmless makes the task of setting 

primary NAAQS difficult, as EPA must 

select standard levels that reduce risks 

sufficiently to protect public health 

even while recognizing that a zero-risk 

standard is not possible’’); Mississippi, 

744 F. 3d at 1351 (same); see also id. at 

1343 (‘‘[d]etermining what is ‘requisite’ 

to protect the ‘public health’ with an 

‘adequate’ margin of safety may indeed 

require a contextual assessment of 

acceptable risk. See Whitman, 531 U.S. 

at 494–95 (Breyer J. concurring)’’). As 

the Court of Appeals for the D.C. Circuit 

said in reviewing the 2015 O3 NAAQS, 

‘‘the primary standard for a nonthreshold pollutant like ozone is not 

required to produce zero risk, and ‘[t]he 

task of determining what standard is 

‘requisite’ to protect the qualitative 

value of public health or what margin of 

safety is ‘adequate’ to protect sensitive 

subpopulations necessarily requires the 

exercise of policy judgment.’ ’’ Murray 

Energy, 936 F.3d at 610 (quoting 

Mississippi v. EPA).128 The 

Administrator’s judgments in this 

review are rooted in his evaluation of 

the evidence, which reflects the 

scientific uncertainty as to the O3

concentrations at which sensitive 

subpopulations would experience 

adverse health effects, and his 

judgments weigh both the risks and the 

uncertainties. This is a legitimate, and 

well recognized, exercise of ‘‘reasoned 

decision-making.’’ ATA III. 283 F. 3d at 

370; see also id. at 370 (‘‘EPA’s inability 

to guarantee the accuracy or increase the 

precision of the . . . NAAQS in no way 

undermines the standards’ validity. 

Rather, these limitations indicate only 

that significant scientific uncertainty 

remains about the health effects of fine 

particulate matter at low atmospheric 

concentration. . . .’’); Mississippi, 744 

F. 3d at 1352–53 (appropriate for EPA 

to balance scientific uncertainties in 

determining level of revised O3

NAAQS). 

Exposure/risk Analyses: In expressing 

the view that the standard should be 

made more stringent, some commenters 

disagree with EPA conclusions based on 

the exposure/risk analyses, and point to 

other analyses that they state show that 

a lower standard level (e.g. 65 ppb or 

lower) would avoid important health 

effects. These commenters’ claims of 

deficiencies with the exposure/risk 

analyses include claims that the study 

area selection is not explained, that 

population size of the study areas 

analyzed is too small to support 

conclusions and does not include 

particular areas; that the analysis does 

not include adults, and other groups of 

interest, and that selection of study 

areas with air quality close to the 

current standard contributed to 

underestimates of population exposures. 

We disagree with these commenters’ 

claims. 

With regard to study area selection 

and population size for the analysis, we 

note that an exposure and risk analysis 

based on eight study areas, all of which 

are major metropolitan areas provides a 

robust foundation for population 

exposure estimates. The eight study 

areas included reflect the full range of 

air quality and exposure variation 

expected across major urban areas in the 

U.S and seven different NOAA climate 

regions (PA, section 3.4.1).129 This 

number of areas (8) and combined 

population size (more than 45 million in 

the combined metropolitan areas [PA, 

Appendix 3D, Table 3D–1]) are much 

larger than similar analyses in recent 

NAAQS reviews for other pollutants 

(e.g., sulfur dioxide [U.S. 2018]), and 

not that dissimilar to similar analyses in 

past O3 NAAQS reviews.130 Some 

commenters claim that the exclusion of 

specific urban areas in which O3

concentrations are much higher than 

those analyzed resulted in 

underestimates of exposure. We 

disagree with this claim as the air 

quality analyzed across all study areas 

was adjusted to just meet the current 

standard (or alternative scenarios). 

Thus, an urban area that currently has 

O3 concentrations well in exceedance of 

the current standard would not 

necessarily have been found to have 

higher exposure estimates if it were 

simulated to have air quality just 

meeting the current standard. Such 

estimates would, however, have greater 

uncertainty, which is the reason such 

study areas as those identified by 

commenters (e.g., Los Angeles) were 

excluded. Areas included were those for 

which only small adjustments were 

required for the air quality to just meet 

the current standard (and alternative 

scenarios), yielding reduced uncertainty 

(e.g., given the need for larger air quality 

adjustments to achieve conditions that 

just meet the current standard) in these 

estimates compared to those from the 

2014 HREA (PA, sections 3.4.1 and 

3.4.4, and Appendix 3D). In selecting 

such areas, however, we considered a 

number of other characteristics in order 

to achieve a varied set of study areas, 

including with regard to air quality 

patterns.131 This variation contributed 

to variation in exposure estimates, even 

for the same air quality scenario (PA, 

Appendix 3D, Tables 3D–26, 3D–28 and 

3D–30). Thus, in addition to focusing on 

study areas with ambient air 

concentrations close to conditions that 

reflect air quality that just meets the 

current standard that would be more 

informative to evaluating the health 

protection provided by the current 

standard than areas with much higher 

(or much lower) concentrations, the 

approach employed recognizes that 

capturing an appropriate diversity in 

study areas and air quality conditions 

(that reflect the current standard 

scenario) is an important aspect of the 

role of the exposure and risk analyses in 

informing the Administrator’s 

conclusions on the public health 

protection afforded by the current 

standard. 

Contrary to one commenter’s assertion 

that adults were not included in the 

exposure assessment, the populations 

assessed included two adult 

populations groups: All adults and 


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132Further, contrary to the implication of one 

comment, the exposure/risk analyses did not 

exclude athletes, hikers and others who exercise 

outdoors, using their full lung capacity, a group the 

commenter characterizes as at increased risk. In 

fact, it is just such individuals who are most likely, 

depending on their locations, to experience 

exposures of concern due to their high exertion 

levels. As described in the PA, the comparison to 

benchmarks analysis identifies the portion of the 

exposed population whose 7-hour average 

concentration, while at moderate or greater 

exertion, is at or above the benchmarks (PA, section 

3.4 and Appendix 3D). 

133With regard to the other two groups, we note 

the ISA explicitly evaluated evidence for people 

with the lung disease, COPD, and concluded the 

evidence was inadequate to determine whether this 

lung impairment confers increased risk of O3

related effects (ISA, Table IS–10). With regard to 

children at summer camp, we note that to the extent 

that the behaviors of such children (e.g., exercising 

outdoors) are represented in the CHAD, they are 

represented among the at-risk populations of 

children and children with asthma that were 

simulated in the exposure/risk analyses. 

134Similarly, the EPA also did not conduct an 

exposure analysis for outdoor workers in the 2008 

review and instead focused on children since it was 

judged that school aged children presented the 

greatest likelihood of being outdoors and exposed 

under moderate exertion averaged over the critical 

time period based on prior analysis findings. Thus, 

while as recognized in multiple reviews, outdoor 

workers are also at risk, the EPA has focused, in 

past reviews as in the current one, on children, the 

population group for which the analysis estimates 

in terms of percentage of population are greatest 

(PA, section 3.4.2). Accordingly, providing 

protection for this population group will provide 

protection for other at-risk populations as well. 

135 In support of their view that estimates should 

have been derived for outdoor workers, one group 

of commenters cites a study on research priorities 

for assessing climate change impacts on outdoor 

workers (Moda et al., 2019). We note, that other 

than being focused on outdoor workers and 

recognizing there to be significant research needed 

for impacts assessment, this paper has little 

relevance in this review. The paper is focused on 

climate change impacts in tropical developing 

countries with a focus on sub-Saharan Africa and 

does not discuss exposure modeling of outdoor 

workers or O3. 

136The analyses cited by these commenters 

include Cromar et al (2019) and OTC (2020). To 

address these comments, we have provisionally 

considered the documents, as discussed in I.D 

above, and found they do not materially change the 

broad scientific conclusions of the ISA with regard 

to respiratory effects, or warrant re-opening the air 

quality criteria for further review (Luben et al., 

2020). Further, some of these commenters reference 

epidemiologic study based risk, analyses in the 

2014 HREA. 

adults with asthma.132 The results for 

these groups and all of the populations 

assessed are presented in detail in the 

PA (PA, Appendix 3D). As described in 

the PA and proposal, the estimates for 

adults as a percentage of the study 

populations were generally lower than 

those for children. Thus, we focused 

discussion on the estimates for children, 

including particularly children with 

asthma. As recognized by the 

Administrator in section II.B.3 below, 

his judgments on the adequacy of 

protection provided by the current 

standard takes into account protection 

provided to the U.S. population, 

including those population groups at 

increased risk, which includes children 

and people of all ages with asthma, 

among other groups. 

Some commenters claimed that the 

EPA should have separately assessed 

exposure for certain additional 

population subgroups, such as children 

at summer camps, adults with lung 

impairments other than asthma or 

outdoor workers. As an initial matter, 

we recognize appreciably increased 

uncertainty regarding key aspects of the 

information necessary for such 

simulations for all of these groups. Of 

the three groups, only outdoor workers 

are identified as an at-risk population in 

this review (ISA, Table IS–10), and 

accordingly this group was explicitly 

considered in designing the exposure 

analyses.133 The information available, 

however, was considered to be too 

uncertain to produce estimates for this 

population, as a separate group, with 

confidence. As described in the PA, 

important uncertainties exist in 

generating the simulated activity 

patterns for this group, including the 

limited number of CHAD diary days 

available for outdoor workers, 

assignment of diaries to proper 

occupation categories, and in 

approximating number of days/week 

and hours/day outdoors, among other 

pertinent aspects (PA, Appendix 3D, 

Table 3D–64). We note that these 

appreciable data limitations and 

associated uncertainties were also 

recognized in the 2014 HREA in which 

a limited sensitivity analyses was 

conducted for this subgroup. Those 

limited analyses, conducted for a single 

area with air quality just meeting the 

prior 75 ppb standard, indicated that 

when diaries were selected to mimic 

exposures that could be experienced by 

outdoor workers, the percentages of 

modeled individuals estimated to 

experience exposures of concern were 

generally similar to the percentages 

estimated for children (i.e., using the 

full database of diary profiles) in the 

urban study areas and years with the 

largest exposure estimates (2014 HREA, 

section 5.4.3.2, Figure 5–14).134

Accordingly, in this review, in 

recognition of the data limitations that 

remain in the current review,135 outdoor 

workers were not assessed as a separate 

population group, and in light of our 

consideration of conclusions from the 

sensitivity analyses in the last review, 

we have generally given primary focus 

to the estimates for the populations of 

children. 

In summary, we disagree with 

comments stating that the exposure/risk 

analyses were deficient and do not 

provide support for their conclusions. 

As summarized above, in planning and 

conducting the exposure/risk analyses, 

we have appropriately considered issues 

raised by the commenters, such that the 

analyses reasonably reflect current 

understanding, information, tools and 

methodologies. Further, in presenting 

the analyses in the PA, we have 

recognized any associated limitations 

and uncertainties in an uncertainty 

characterization that utilized a largely 

qualitative approach adapted from the 

World Health Organization approach 

(and commonly utilized in NAAQS 

exposure/risk assessments, as discussed 

in the PA and proposal [85 FR 49857, 

August 14, 2020]), accompanied by a 

number of quantitative sensitivity 

analyses. This characterization and 

accompanying analyses build on 

previously conducted work in the 2014 

HREA and provide a transparent and 

explicit recognition of strengths, 

limitations and uncertainties of the 

current exposure/risk analysis that were 

described the PA, considered in the 

proposal and also in the Administrator’s 

decision described in section II.B.3 

below. Thus, the exposure/risk analyses 

conducted for this review appropriately 

and soundly reflect current information 

and methodologies; and we have 

interpreted their results appropriately in 

light of any associated limitations and 

uncertainties. 

With regard to other quantitative 

analyses identified by some commenters 

and described as showing health 

impacts that would be avoided by a 

more stringent standard (e.g. with a 

level of 65 ppb or lower), we note that 

these analyses utilize epidemiologic 

study effect estimates as concentrationresponse functions to generate 

predictions of the occurrence of health 

outcomes, primarily mortality, under 

different air quality conditions 

(characterized by the metric used in the 

epidemiologic study).136 As an initial 

matter, we note that our understanding 

of the relationship between O3

exposures and total mortality is 

different in this review than it was in 

the last review, based on the more 

extensive evidence base now available. 

As summarized in section II.A.2.a 

above, and noted earlier in this section, 

while our conclusion in the last review 

was that the relationship of O3 exposure 

with mortality was likely to be causal, 

the current evidence base does not 

support that conclusion because of 

limited evidence for cardiovascular 

mortality, which is by far the largest 

contributor to total mortality. Rather, 

the EPA has concluded the evidence in 


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137These commenters also assert that some other 

studies published after the ISA cut-off date were 

arbitrarily included in the ISA, citing just a single 

study (Garcia et al., 2019). Contrary to implication 

by the commenters, such an occurrence is clearly 

described in the ISA, which states ‘‘[s]tudies 

published after the literature cutoff date for this 

review were also considered if they were submitted 

in response to the Call for Information or identified 

in subsequent phases of ISA development . . ., 

particularly to the extent that they provide new 

information that affects key scientific conclusions’’ 

(ISA, Appendix 10, p. 10–1). 

138Although the commenters submitted a 

document that appears to be an unpublished draft 

of an earlier manuscript of this paper, to which they 

assigned a 2019 publication date and a very slightly 

different title (rather than the published paper, it is 

the published study, Paulin et al., (2020) that we 

have provisionally considered (Luben et al., 2020). 

139Some commenters imply that projections of 

increasing O3 concentrations in response to climate 

change in the future will ‘‘heighten’’ long-term O3

concentrations and chronic exposures and indicate 

a need for a long-term standard. In making this 

claim, they cite an analysis of air quality projected 

in 2045 through 2055 (Nassikas et al., 2020) and an 

evaluation of the effects of climate change on air 

quality including O3 concentrations. (Archer et al., 

2019). The former ‘‘new’’ study has been 

provisionally considered and found not to 

materially affect the broad scientific conclusions 

regarding the air quality criteria documented in the 

ISA or to warrant reopening the air quality criteria 

(Luben et al., 2020) As neither is evaluating health 

effects associated with air quality under the current 

standard, we do not find these studies informative 

to consideration of a need for a long-term standard 

to protect public health. 

140Two others (Dedoussi et al 2020; Seltzer et al, 

2020) are quantitative assessments that estimate O3

impacts based on use of effect estimates from 

previously published studies that are included in 

the ISA, another (Dominici et al., 2019) is the full 

technical report from the Health Effects Institute, 

the main results of which were previously 

published in studies that are included in the ISA, 

and a fourth (Limaye and Knowlton., 2020) is 

commentary on a previously published study that 

is included in the ISA. One other study cited by the 

commenters is focused on short-term O3 exposures, 

not long-term O3 exposure as indicated by the 

commenters (Strosnider et al., 2019) 

141While studies by Paulin et al. (2020) and Rhee 

et al. (2019) provide evidence for a novel 

population sub-group (smokers) or endpoint (e.g., 

acute respiratory distress syndrome, ARDS), each 

study has limitations. For example, the crosssectional design of Paulin et al. (2020) is a major 

limitation, while limitations associated with Rhee 

et al. (2019) relate to linking long-term exposure 

with hospital admissions for ARDS based on 

exposure timing and the mechanism for acute vs. 

chronic development of disease, and to power in 

the study (e.g., very low hospital admission counts 

per year per ZIP code [Rhee et al., 2019, Table 2]). 

this review to be suggestive of, but not 

sufficient to infer, causal relationships 

of total mortality with short- or longterm O3 exposure, as summarized in 

section II.A.2.a above (ISA, Appendix 

6). Thus, we do not find the weight the 

commenter is suggesting we place on 

predictions of total mortality from the 

epidemiologic study based risk analyses 

cited by commenters to appropriately 

reflect the current evidence base for O3

and mortality, or the evidence base for 

O3 and cardiovascular effects (the 

primary contributor to mortality in the 

U.S.). 

With regard to estimates of avoided 

respiratory mortality from the analyses 

cited by these commenters, we note 

that, while the epidemiologic studies 

that are inputs to the quantitative 

analyses cited by the commenters are 

part of the evidence base that supports 

our conclusion of a causal relationship 

between short-term O3 exposure and 

respiratory effects, there are 

uncertainties inherent in the derivation 

of estimates of mortality ascribed to O3

exposures using effect estimates from 

these studies. For example, in planning 

for analyses in this review, the IRP 

recognized several important 

uncertainties associated with aspects of 

the O3 epidemiologic study-based 

approach used in the 2014 HREA (one 

of the analyses cited by commenters), 

and similar to the approach used in 

other analyses cited by commenters, 

that the EPA concluded to have a 

moderate or greater impact on risk 

estimates (IRP, Appendix 5A). Such 

uncertainties include complications 

posed by the presence of co-occurring 

pollutants or pollutant mixtures, as well 

as those involving the correlation of 

population O3 exposures and ambient 

air monitor concentrations (including 

the use of area wide average O3

concentrations) and uncertainties in the 

derived concentration-response 

functions (IRP, Appendix 5A; PA, 

Appendix 34D, section 3D.1.4). 

Specifically with regard to the 2014 

HREA estimates of respiratory mortality, 

the EPA has recognized uncertainty 

about the extent to which mortality 

estimates based on the long-term metric 

in Jerrett et al. (2009) (i.e., seasonal 

average of 1-hour daily maximum 

concentrations) reflect associations with 

long-term average O3 versus repeated 

occurrences of elevated short-term 

concentrations; and given potential 

nonlinearity of the C–R function to 

reflect a threshold of the mortality 

response, these estimates should be 

viewed with caution (IRP, Appendix 

5A). Accordingly, the 2014 HREA 

concluded that lower confidence should 

be placed in the results of the 

assessment of respiratory mortality risks 

associated with long-term O3, primarily 

because that analysis relies on just one 

study (Jerrett et al., 2009), and because 

of the uncertainty in that study about 

the existence and identification of a 

potential threshold in the concentrationresponse function (2014 HREA, section 

9.6; 80 FR 65316, October 26, 2015). The 

other analysis cited by the commenters 

for predictions of respiratory mortality 

is also based its estimates on Jerrett et 

al. (2009). Thus, we find the 

conclusions regarding uncertainty and 

low confidence recognized for the 2014 

HREA estimates to also apply to the 

other analysis by commenters and 

disagree with the conclusions reached 

by these commenters on this analysis. 

Further, we do not find that the 2014 

HREA or other analyses cited by the 

commenters, in combination with the 

full body of evidence currently 

available, support a conclusion of 

significant health outcomes for O3 air 

quality that meets the current standard. 

Long-term Standard Consideration: In 

support of their view that EPA should 

establish an additional primary standard 

that targets long-term exposure, some 

commenters stated that recent 

epidemiologic studies indicate causal 

linkages between long-term exposures 

and adverse health outcomes, while also 

suggesting there was support for such a 

standard in a statement made by the 

CASAC in reviewing the draft PA. With 

regard to the epidemiologic studies, 

these commenters cite several studies 

published after the literature cut-off date 

for the ISA 137 that they describe as 

showing linkages of long-term O3

exposure to a number of outcomes, 

including mortality, smokers 

progression to COPD, hospital 

admissions for acute respiratory disease 

syndrome and emergency department 

visits (Dominici et al., 2019; Seltzer et 

al., 2020; Limaye and Knowlton, 2020; 

Dedoussi et al., 2020; Lim et al., 2019; 

Paulin et al,. 2020; 138 Rhee et al., 2019; 

Strosnider et al., 2019).139 We have 

provisionally considered these ‘‘new’’ 

studies in addressing these comments 

consistent with section I.D above (Luben 

et al., 2020). Of the studies focused on 

mortality that these commenters cite as 

providing new information in support of 

a long-term standard, just three 

represent new evidence related to 

investigation of associations of longterm O3 exposure with mortality (Lim et 

al., 2019) or respiratory morbidity 

(Paulin et al., 2020 and Rhee et al., 

2019).140 The study by Lim et al. (2019) 

analyzes associations between long-term 

O3 exposure and respiratory mortality in 

a U.S. population of older adults in the 

U.S., reporting a positive association 

with an effect estimate lower than a 

previously published study included in 

the ISA. These results contribute to the 

evidence base for respiratory effects, 

e.g., with an additional high-quality 

study of a previously studied 

population group (Lim et al., 2019) or 

with studies investigating additional 

populations and respiratory outcomes 

(Paulin et al., 2020; Rhee et al., 2019), 

albeit with limitations,141 without 

reducing uncertainties in the evidence 

base as a whole. These studies are 


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142The studies by Lim et al (2019) and Rhee et 

al (2019) include zip codes across the entire U.S., 

while Paulin et al (2020) includes the cities of 

Baltimore, Maryland, New York City, New York, 

Los Angeles and San Francisco, California, Ann 

Arbor, Michigan, Salt Lake City, Utah and WinstonSalem, North Carolina. The study time periods 

include ten or more years extending from the early 

2000s to the late 2010s; a period within which the 

design values for most of those identified cities and 

many other U.S. metropolitan areas exceeded the 

level of the current standard (as seen by the design 

values presented for those areas during those time 

periods at https://www.epa.gov/air-trends/airquality-design-values). 

143 In its comments regarding the 2015 statement, 

the CASAC and its consultants stated that controls 

that reduce peak O3 concentrations will not 

consistently reduce mean O3 concentrations. We 

don’t disagree with this statement, and we note that 

we did not make a statement to the contrary in 

either the proposal or this final decision document. 

generally consistent with the evidence 

assessed in the ISA, and they do not 

materially change the broad conclusions 

in the ISA regarding the scientific 

evidence. 

We additionally note that the O3

concentrations did not meet the current 

standard in all locations and time 

periods analyzed in these three 

multicity studies. Although two of these 

studies include some locations across 

the U.S. in which the current standard 

was likely met during some portions of 

the study period, air quality during 

other time periods of locations in the 

dataset did not meet the current 

standard.142 Further, the multicity effect 

estimates in these studies do not 

provide a basis for considering the 

extent to which O3 health effect 

associations are influenced by 

individual locations with ambient O3

concentrations low enough to meet the 

current O3 standards versus locations 

with O3 concentrations that violate this 

standard. Thus, while these more recent 

studies may be consistent with the 

existing evidence base evaluated in the 

ISA, they do not provide a basis for 

conclusions regarding whether the O3

exposures occurring under air quality 

conditions allowed by the current 

standard may be eliciting the effects 

analyzed. 

We additionally note that while 

epidemiologic studies evaluate the 

relationship between health effects and 

specific O3 concentrations during a 

defined study period and the generally 

consistent and coherent associations 

observed in these epidemiologic studies 

contribute to the causality 

determinations and conclusions 

regarding the causal nature of the effect 

of O3 exposure on health effects, ‘‘they 

do not provide information about which 

averaging times or exposure metrics 

may be eliciting the health effects under 

study’’ (ISA, section IS.6.1). As noted in 

the ISA, ‘‘disentangling the effects of 

short-term ozone exposure from those of 

long-term ozone exposure (and viceversa) is an inherent uncertainty in the 

evidence base,’’ as ‘‘the populations 

included in epidemiologic studies have 

long-term, variable, and uncharacterized 

exposures to ozone and other ambient 

pollutants’’ (ISA, section IS.6.1). As 

summarized in the proposal, however, 

we have also considered the 

toxicological studies of effects 

associated with long-term exposures 

and note that they involve much higher 

exposures than those occurring at the 

current standard (85 FR, 49853, August 

14, 2020). 

Lastly, we disagree with the 

commenters’ implication that the EPA 

has not addressed a CASAC 

recommendation. The commenters 

appear to be asserting that the CASAC 

recommended that EPA consider a longterm standard. However, the CASAC did 

not make such a recommendation (Cox, 

2020a). In making this assertion, the 

commenter cites a comment the CASAC 

makes with regard to a sentence in the 

draft PA that is drawn from the 

Administrator’s conclusions section of 

the 2015 decision. Rather than ignoring 

this CASAC comment, as asserted by the 

commenters, we made a revision to that 

section of the PA (moving the statement 

from the draft PA to a footnote in the 

final PA with the objective of retaining 

an accurate description of a 

consideration related to that 2015 

decision, while lessening the potential 

for confusion of a 2015 consideration 

with considerations in the current 

review).143 Notwithstanding sentences 

pertaining to the last review, we note 

the PA evaluates the information in the 

current review with regard to the 

protection offered by the current 

standard (and that the Administrator 

considered the PA evaluation, as well as 

the CASAC advice in his proposed 

decision [summarized in section II.B.1.c 

above] as in his final decision below). 

We further note that the description of 

the Administrator’s conclusion in the 

last review, which is also summarized 

in the proposal (and in section II.A.1 

above), does not describe health effects 

associated with long-term average 

concentrations likely under the current 

standard. 

Further, in considering an implication 

of the commenters’ claim that a ‘‘longterm standard’’ is needed in order to 

provide protection from health effects 

that may be elicited by long-term 

exposures to O3, we note that the impact 

of standards with short averaging times, 

such as eight hours, is not limited to 

reducing short-term exposures. This is 

because a reduction in magnitude of 

short-term exposure concentrations 

(e.g., daily maximum 8-hour 

concentrations) is also a reduction in 

exposure to such concentrations over 

the longer term. For example, a 

standard, such as the current one, that 

limits daily maximum 8-hour 

concentrations to not exceed 70 ppb as 

a 3-year average of the annual fourth 

highest value, in addition to limiting the 

magnitude of concentrations to which a 

population is exposed in eight hour 

periods, also limits the frequency to 

which the population is exposed to such 

concentrations over the long term. That 

is, the reduction in frequency of the 

higher concentrations reduces 

exposures to such concentrations over 

the short and long term. Thus, given 

that, as indicated by the current and 

established evidence, the O3

concentrations most likely to contribute 

to health effects are the higher 

concentrations, the current standard 

provides control of exposures to such 

concentrations over both the short and 

long term. In light of all of the 

considerations raised here, we disagree 

with the commenters assertion of the 

need for a long-term O3 standard. 

4. Administrator’s Conclusions 

Having carefully considered the 

public comments, as discussed above, 

the Administrator believes that the 

fundamental scientific conclusions on 

effects of photochemical oxidants, 

including O3, in ambient air that were 

reached in the ISA and summarized in 

the PA, and estimates of potential O3

exposures and risks described in the PA, 

and summarized above and in sections 

II.B and II.C of the proposal, remain 

valid. Additionally, the Administrator 

believes the judgments he proposed to 

reach in the proposal (section II.D.3) 

with regard to the evidence and the 

quantitative exposure/risk information 

remain appropriate. Thus, as described 

below, the Administrator concludes that 

the current primary O3 standard 

provides the requisite protection of the 

public health with an adequate margin 

of safety, including for at-risk 

populations, and should be retained. In 

considering the adequacy of the current 

primary O3 standard, the Administrator 

has carefully considered the assessment 

of the available health effects evidence 

and conclusions contained in the ISA; 

the evaluation of policy-relevant aspects 

of the evidence and quantitative 

analyses in the PA (summarized in 

sections II.A.2 and II.A.3 above); the 

advice and recommendations from the 

CASAC (summarized in section II.B.1.b 

above); and public comments (discussed 

in section II.B.2 above and in the 

separate RTC document). 


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In the discussion below, the 

Administrator considers the key aspects 

of the evidence and exposure/risk 

estimates important to his judgment 

regarding the adequacy of protection 

provided by the current standard. First, 

the Administrator considers the 

evidence base on health effects 

associated with exposure to 

photochemical oxidants, including O3, 

in ambient air. He additionally 

considers the quantitative exposure and 

risk estimates developed in this review, 

including associated limitations and 

uncertainties, and what they indicate 

regarding the magnitude of risk, as well 

as degree of protection from adverse 

health effects, associated with the 

current standard. He additionally 

considers uncertainties in the evidence 

and the exposure/risk information, as a 

part of public health judgments that are 

essential and integral to his decision on 

the adequacy of protection provided by 

the standard. Such judgments include 

public health policy judgments and 

judgments about the implications of the 

uncertainties inherent in the scientific 

evidence and quantitative analyses. The 

Administrator draws on the PA 

considerations, and PA conclusions in 

the current review, taking note of key 

aspects of the rationale presented for 

those conclusions. Further, the 

Administrator considers the advice and 

conclusions of the CASAC, including 

particularly its overall agreement that 

the currently available evidence does 

not substantially differ from that which 

was available in the 2015 review when 

the current standard was established. 

As an initial matter, the Administrator 

recognizes the continued support in the 

current evidence for O3 as the indicator 

for photochemical oxidants (as 

recognized in section II.D.1 of the 

proposal). As recognized in the 

proposal, no newly available evidence 

has been identified in this review 

regarding the importance of 

photochemical oxidants other than O3

with regard to abundance in ambient 

air, and potential for health effects, and 

the ‘‘the primary literature evaluating 

the health and ecological effects of 

photochemical oxidants includes ozone 

almost exclusively as an indicator of 

photochemical oxidants’’ (ISA, p. IS–3). 

Accordingly, the information relating 

health effects to photochemical oxidants 

in ambient air is also focused on O3. 

Thus, the Administrator concludes it is 

appropriate for O3 to continue to be the 

indicator for the primary standard for 

photochemical oxidants. 

With regard to the extensive evidence 

base for health effects of O3, the 

Administrator gives particular attention 

to the longstanding evidence of 

respiratory effects causally related to 

short-term O3 exposures (summarized in 

section II.A.2.a above). He recognizes 

that the strongest and most certain 

evidence for this conclusion, as in the 

last review, is that from controlled 

human exposure studies that report an 

array of respiratory effects in study 

subjects (largely generally healthy 

adults) engaged in quasi-continuous or 

intermittent exercise. He additionally 

recognizes the supporting experimental 

animal and epidemiologic evidence. In 

so doing, he takes note of the 

epidemiologic evidence of positive 

associations for increased incidence of 

hospital admissions and emergency 

department visits for an array of 

respiratory outcomes, with the strongest 

such evidence being for asthma-related 

outcomes and specifically asthmarelated outcomes for children, with 

short-term O3 exposures. As a whole, 

this strong evidence base continues to 

demonstrate a causal relationship 

between short-term O3 exposures and 

respiratory effects, including in people 

with asthma. The Administrator also 

notes the ISA conclusion that the 

relationship between long-term 

exposures and respiratory effects is 

likely to be causal. These conclusions 

are also consistent with the conclusions 

in the last review and reflect a general 

similarity in the underlying evidence 

base for such effects. 

With regard to conclusions regarding 

the health effects evidence that differ 

from those in the last review, the 

Administrator recognizes the new 

conclusions regarding metabolic effects, 

cardiovascular effects and mortality (as 

summarized in section II.A.2.a above; 

ISA, Table ES–1). As an initial matter, 

he takes note of the fact that while the 

2013 ISA considered the evidence 

available in the last review sufficient to 

conclude that the relationships for 

short-term O3 exposure with 

cardiovascular health effects and 

mortality were likely to be causal, that 

conclusion is no longer supported by 

the now more expansive evidence base 

which the current ISA determines to be 

suggestive of, but not sufficient to infer, 

a causal relationship for these health 

effect categories (ISA, Appendix 4, 

section 4.1.17; Appendix 6, section 

6.1.8). Further, the Administrator 

recognizes the new ISA determination 

that the relationship between short-term 

O3 exposure and metabolic effects is 

likely to be causal (ISA, section 

IS.4.3.3). In so doing, he takes note that 

the basis for this conclusion is largely 

experimental animal studies in which 

the exposure concentrations were well 

above those in the controlled human 

exposure studies for respiratory effects 

as well as above those likely to occur in 

areas of the U.S. that meet the current 

standard (as summarized in section 

II.A.2.c above). Thus, while recognizing 

the ISA’s conclusion regarding this 

potential hazard of O3, he also 

recognizes that the evidence base is 

largely focused on circumstances of 

elevated concentrations above those 

occurring in areas that meet the current 

standard. In light of these 

considerations, he judges the current 

standard to be protective of such 

circumstances leading him to continue 

to focus on respiratory effects in his 

evaluation of whether the current 

standard provides requisite protection. 

With regard to populations at 

increased risk of O3-related health 

effects, the Administrator notes the 

populations and lifestages identified in 

the ISA and summarized in section 

II.A.2.b above. In so doing, he takes note 

of the longstanding and robust evidence 

that supports identification of people 

with asthma as being at increased risk 

of O3-related respiratory effects, 

including specifically asthma 

exacerbation and associated health 

outcomes, and also children, 

particularly due to their generally 

greater time outdoors while at elevated 

exertion (PA, section 3.3.2; ISA, sections 

IS.4.3.1, IS.4.4.3.1, and IS.4.4.4.1, 

Appendix 3, section 3.1.11). This 

tendency of children to spend more 

time outdoors while at elevated exertion 

than other age groups, including in the 

summer when O3 levels may be higher, 

makes them more likely to be exposed 

to O3 in ambient air under conditions 

contributing to increased dose due to 

greater air volumes taken into the lungs. 

Based on these considerations, the 

Administrator concludes it is 

appropriate to give particular focus to 

people with asthma and children, 

population groups for which the 

evidence of increased risk is strongest, 

in evaluating whether the current 

standard provides requisite protection. 

He judges that such a focus will also 

provide protection of other potentially 

at-risk population groups, identified in 

the ISA, for which the current evidence 

is less robust and clear as to the extent 

and type of any increased risk, and the 

exposure circumstances that may 

contribute to it. 

With regard to exposures of interest 

for respiratory effects, the Administrator 

refers to the controlled human exposure 

studies of 6.6-hour exposures, with 


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144These studies employ a 6.6-hour protocol that 

includes six 50-minute periods of exercise at 

moderate or greater exertion. 

145Consistent with the evaluation of the 

epidemiologic evidence of associations between 

short-term O3 exposure and respiratory health 

effects in the ISA, we focus on those studies 

conducted in the U.S. and Canada, and most 

particularly in the U.S., to provide a focus on study 

populations and air quality characteristics that are 

most relevant to circumstances in the U.S. (PA, p. 

3–45). 

146Among the epidemiologic studies finding a 

statistically significant positive relationship of 

short- or long-term O3 concentrations with 

respiratory effects, there are no single-city studies 

conducted in the U.S. in locations with ambient air 

O3 concentrations that would have met the current 

standard for the entire duration of the study. Nor 

is there a U.S. multicity study for which all cities 

met the standard for the entire study period. The 

extent to which reported associations with health 

outcomes in the resident populations in these 

studies are influenced by the periods of higher 

concentrations during times that did not meet the 

current standard is unknown. These and additional 

considerations are summarized in section II.A.2.c 

above and in the PA. 

147The 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 

increases in airway inflammation also have the 

potential to increase the risk for an asthma 

exacerbation. The CASAC further cautioned with 

regard to repeated episodes of such responses, e.g., 

airway inflammation, indicating that they have the 

potential to contribute to irreversible reductions in 

lung function (Cox, 2020a, Consensus Responses to 

Charge Questions pp. 7–8). 

quasi-continuous exercise,144 to 

concentrations ranging from as low as 

approximately 40 ppb to 120 ppb (as 

considered in the PA, and summarized 

in sections II.A.2.c above). He also notes 

that, as in the last review, these studies, 

and particularly those that examine 

exposures from 60 to 80 ppb, are the 

primary focus of the PA consideration of 

exposure circumstances associated with 

O3 health effects important to the 

Administrator’s judgments regarding the 

adequacy of the current standard. The 

Administrator further recognizes that 

this information on exposure 

concentrations that have been found to 

elicit effects in exercising study subjects 

is unchanged from what was available 

in the last review. 

With regard to the epidemiologic 

studies of respiratory effects, the 

Administrator recognizes 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) provide 

strong support for the conclusions of 

causality (as summarized in section 

II.A.2.a above). He additionally takes 

note of the PA observation that these 

studies are generally focused on 

investigating the existence of 

relationships between O3 in ambient air 

and specific health outcomes and not on 

detailing the specific exposure 

circumstances eliciting such effects (PA, 

section 3.3.3). In so doing, he takes note 

of the PA conclusions in this regard, 

including the scarcity of U.S. studies 145

conducted in locations in which and 

during time periods when the current 

standard would have been met (as 

summarized in sections II.A.2.c 

above).146 He also recognizes the 

additional considerations raised in the 

PA and summarized in section II.A.2.c 

above regarding information on 

exposure concentrations in these studies 

during times and locations that would 

not have met the current standard, 

including considerations such as 

complications in disentangling specific 

O3 exposures that may be eliciting 

effects (PA, section 3.3.3; ISA, p. IS–86 

to IS–88). He takes note that such 

considerations do not lessen the 

importance of these studies in the 

evidence base documenting the causal 

relationship between O3 and respiratory 

effects. With regard to his consideration 

of exposure concentrations associated 

with O3 air quality conditions that meet 

the current standard, based on 

information cited here and discussed in 

the PA and section II.B.2.b(ii) above, he 

judges these studies that are available in 

the current review to be less 

informative. Thus, the Administrator 

agrees with the PA conclusions in 

consideration of this evidence from 

controlled human exposure and 

epidemiologic studies (as assessed in 

the ISA and summarized in the PA), and 

in consideration of public comments in 

section II.B.2.b(ii) above, that the 

evidence base in this review does not 

include new evidence of respiratory 

effects associated with appreciably 

different exposure circumstances than 

the evidence available in the last 

review, including particularly any 

circumstances that would also be 

expected to be associated with air 

quality conditions likely to occur under 

the current standard. In light of these 

considerations, he finds it appropriate 

to focus on the studies of 6.6-hour 

exposures with quasi-continuous 

exercise, and particularly on study 

results for concentrations ranging from 

60 to 80 ppb. 

In considering the significance of 

responses documented in these studies 

and the full evidence base for his 

purposes in judging implications of the 

current information on public health 

protection provided by the current 

standard, notes that the responses 

reported from exposures ranging from 

60 to 80 ppb are transient and reversible 

in the study subjects. In so doing, he 

also notes that these studies are in 

largely healthy adult subjects, that such 

data are lacking at these exposure levels 

for children and people with asthma, 

and that the evidence indicates that 

such responses, if repeated or sustained, 

particularly in people with asthma, pose 

risks of effects of greater concern, 

including asthma exacerbation, as 

cautioned by the CASAC.147 The 

Administrator also takes note of 

statements from the ATS (summarized 

in section II.A.2.b above), as well as 

judgments made by the EPA in 

considering similar effects (and ATS 

statements) in previous NAAQS reviews 

(80 FR 65343, October 26, 2015). With 

regard to the ATS statements, including 

the one newly available in this review 

(Thurston et al., 2017), the 

Administrator recognizes 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). 

The more recent ATS statement is 

generally consistent with the prior 

statement (that was considered in the 

last O3 NAAQS review) and the 

attention that statement 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 this way, the 

most recent statement updates the prior 

statement, while retaining previously 

identified considerations, including, for 

example, its emphasis on consideration 

of vulnerable populations, thus 

expanding upon (e.g., with some 

increased specificity), while retaining 

core consistency with, the earlier ATS 

statement (Thurston et al., 2017; ATS, 

2000). One example of this increased 

specificity that was raised in public 

comments and discussed in section 

II.B.2 above, is in the discussion of 

small changes in lung function (in terms 

of FEV1) in people with compromised 

function, such as people with asthma 

(Thurston et al., 2017). In considering 

these statements, the Administrator 

notes that, in keeping with the intent of 

these 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 magnitude, duration or 

frequency of small pollutant-related 

changes in lung function. In so doing, 

he also recognizes the limitations in the 

current evidence base with regard to our 


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understanding of these aspects of such 

changes that may be associated with 

exposure concentrations of interest. 

Notwithstanding these limitations and 

associated uncertainties, he takes note 

of the emphasis of the ATS statement on 

consideration of individuals with preexisting compromised function, such as 

that resulting from asthma (an emphasis 

which is reiterated and strengthened in 

the current statement), and agrees that 

these are important considerations in 

his judgment on the adequacy of 

protection provided by the current 

standard for at-risk populations, as 

recognized below. 

The Administrator recognizes some 

uncertainty, reflecting limitations in the 

evidence base, with regard to the 

exposure levels eliciting effects (as well 

as the severity of the effects) in some 

population groups not included in the 

available controlled human exposure 

studies, such as children and 

individuals with asthma. In so doing, 

the Administrator recognizes that the 

controlled human exposure studies, 

primarily conducted in healthy adults, 

on which the depth of our 

understanding of O3-related health 

effects is based, in combination with the 

larger evidence base, informs our 

conceptual understanding of O3

responses in people with asthma and in 

children. Aspects of our understanding 

continue to be limited, however, 

including with regard to the risk of 

particular effects and associated severity 

for these less studied population groups 

that may be posed by 7-hour exposures 

with exercise to concentrations as low 

as 60 ppb that are estimated in the 

exposure analyses. Collectively, these 

aspects of the evidence and associated 

uncertainties contribute to the 

Administrator’s recognition that for O3, 

as for other pollutants, the available 

evidence base in a NAAQS review 

generally reflects a continuum, 

consisting of levels at which scientists 

generally agree that health effects are 

likely to occur, through lower levels at 

which the likelihood and magnitude of 

the response become increasingly 

uncertain. 

In light of these uncertainties in the 

evidence, as well as those associated 

with the exposure and risk analyses, the 

Administrator notes that, as is the case 

in NAAQS reviews in general, his 

decision regarding the primary O3

standard in this review depends on a 

variety of factors, including his science 

policy judgments and public health 

policy judgments. These factors include 

judgments regarding aspects of the 

evidence and exposure/risk estimates, 

such as judgments concerning his 

interpretation of the different 

benchmark concentrations, in light of 

the available evidence and of associated 

uncertainties, as well as judgments on 

the public health significance of the 

effects that have been observed at the 

exposures evaluated in the health effects 

evidence. These judgments are rooted in 

his interpretation of the evidence, 

which reflects a continuum of healthrelevant exposures, with less confidence 

and greater uncertainty in the existence 

of adverse health effects as one 

considers lower O3 exposures. The 

factors relevant to judging the adequacy 

of the standards also include the 

interpretation of, and decisions as to the 

relative weight to place on, different 

aspects of the results of the exposure 

and risk assessment for the eight areas 

studied and the associated 

uncertainties. Together, factors 

described here inform the 

Administrator’s judgment about the 

degree of protection that is requisite to 

protect public health with an adequate 

margin of safety, including the health of 

sensitive groups, and, accordingly, his 

conclusion that the current standard is 

requisite to protect public health with 

an adequate margin of safety. 

As in prior O3 NAAQS reviews, the 

Administrator considers the exposure 

estimates developed from modeling 

exposures to O3 in ambient air in this 

review to be critically important to 

consideration of the potential for 

exposures and risks of concern under air 

quality conditions of interest, and 

consequently important to his 

judgments on the adequacy of public 

health protection provided by the 

current standard. The exposure/risk 

analysis provides a framework within 

which to consider implications of the 

health effects evidence with regard to 

protection afforded by the current 

standard. In his consideration of the 

exposure/risk estimates, the 

Administrator places greater weight and 

gives primary attention to the 

comparison-to-benchmarks analysis. 

This focus reflects his recognition of 

multiple factors, including the relatively 

greater uncertainty associated with the 

lung function risk estimates compared 

to the results of the comparison-tobenchmarks analysis. Additionally, he 

recognizes that, as noted in the PA, the 

comparison-to-benchmarks analysis 

provides for characterization of risk for 

the broad array of respiratory effects 

documented in the controlled human 

exposure studies. Accordingly, this 

analysis facilitates consideration of an 

array of respiratory effects, including 

but not limited to lung function 

decrements. Accordingly, the 

Administrator focuses primarily on the 

estimates of exposures at or above 

different benchmark concentrations that 

represent different levels of significance 

of O3-related effects, both with regard to 

the array of effects and severity of 

individual effects. In so doing, he notes 

that this assures his consideration of the 

protection provided by the standard 

from the array of respiratory effects 

documented in the currently available 

evidence base. 

In considering the public health 

implications of estimated occurrences of 

exposures (while at increased exertion) 

to the three benchmark concentrations 

(60, 70 and 80 ppb), the Administrator 

considers the respiratory effects 

reported in controlled human exposure 

studies of this range of concentrations 

(during quasi-continuous exercise). 

Accordingly, the controlled human 

exposure study evidence base, as a 

whole, provides context for 

consideration of the exposure/risk 

estimates. The Administrator recognizes 

the three benchmarks to represent 

exposure conditions associated with 

different levels of respiratory response 

in the subjects studied and to inform his 

judgments on different levels of risk that 

might be posed to unstudied members 

of at-risk populations. The highest 

benchmark concentration (80 ppb) 

represents an exposure where multiple 

controlled human exposure studies 

involving 6.6-hour exposures during 

quasi-continuous exercise demonstrate a 

range of O3-related respiratory effects 

including inflammation and airway 

responsiveness, as well as respiratory 

symptoms and lung function 

decrements in healthy adult subjects. 

Findings for this O3 exposure include: A 

statistically significant increase in 

multiple types of respiratory 

inflammation indicators in multiple 

studies; statistically significantly 

increased airway resistance and 

responsiveness; statistically significant 

FEV1 decrements; and statistically 

significant increases in respiratory 

symptoms (Table 1). In one variable 

exposure study for which this (80 ppb) 

was the exposure period average 

concentration, the study subject mean 

FEV1 decrement was nearly 8%, with 

individual decrements of 15% or greater 

(moderate or greater) in 16% of subjects 

and decrements of 10% or greater in 

32% of subjects (Schelegle et al 2009); 

the percentages of individual subjects 

with decrements great than 10 or 15% 

were lower in other studies for this 

exposure. The second benchmark (70 

ppb) represents an exposure level below 

the lowest exposures that have reported 

both statistically significant FEV1


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148The 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; 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 (PA, Appendix 3D, Table 3D–20). 149Among subjects in all four of these studies, 

individual FEV1 decrements of at least 15% were 

reported in 3% of subjects, with 7% of subjects 

reported to have decrements at or above a lower 

value of 10% (PA, Appendix 3D, Table 3D–20). 150For example, for people with asthma, the risk 

of an asthma exacerbation event may be expected 

to increase with repeated occurrences of lung 

function decrements of 10% or 15% as compared 

to a single occurrence. 

151This finding relates to children’s greater 

frequency and duration of outdoor activity, as well 

as their greater activity level while outdoors (PA, 

section 3.4.3). 

152The response for the 60 ppb studies is also 

somewhat lower than that for the 63 ppb study 

(Table 1; PA, Appendix 3D, Table 3D–20). 

decrements 148 and increased respiratory 

symptoms (reported at 73 ppb, 

Schelegle et al 2009) or statistically 

significant increases in airway 

resistance and responsiveness (reported 

at 80 ppb, Horstman et al., 1990). 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 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 149), 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 turning to the exposure/risk 

analysis results, the Administrator 

considers the evidence represented by 

these benchmarks noting that due to 

differences among individuals in 

responsiveness, not all people 

experiencing such exposures experience 

a response, such as a lung function 

decrement, as illustrated by the 

percentages cited above. Further, among 

those that experience a response, not all 

will experience an adverse effect. 

Accordingly, the Administrator notes 

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. With these considerations 

in mind, he notes 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 increases with 

repeated occurrences (as cautioned by 

the CASAC).150 In so doing, he also 

notes 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, he 

judges multiple occurrences to be of 

greater concern than single occurrences. 

In this context, the Administrator 

considers the exposure risk estimates, 

focusing first on the results for the 

highest benchmark concentration (80 

ppb), which represents an exposure well 

established to elicit an array of 

responses in sensitive individuals 

among study groups of largely healthy 

adult subjects, exposed while at 

elevated exertion. Similar to judgments 

of past Administrators, the current 

Administrator judges these effects in 

combination and severity to represent 

adverse effects for individuals in the 

population group studied, and to pose 

risk of adverse effects for individuals in 

at-risk populations, most particularly 

people with asthma, as noted above. 

Accordingly, he judges that the primary 

standard should provide protection 

from such exposures. In considering the 

exposure/risk estimates, he focuses on 

the results for children, and children 

with asthma, given the higher frequency 

of exposures of potential concern for 

children compared to adults, in terms of 

percent of the population groups.151 The 

exposure/risk estimates indicate 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 this level; the estimate is 99.9% 

of children with asthma and of all 

children for the highest year and study 

area (Table 2). Further, no children in 

the simulated populations (zero percent) 

are 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 (Table 2). These estimates 

indicate strong protection against 

exposures of at-risk populations that 

have been demonstrated to elicit a wide 

array of respiratory responses in 

multiple studies. 

The Administrator next considers the 

results for the second benchmark 

concentration (70 ppb), which is just 

below the lowest exposure 

concentration (73 ppb) for which a 

study has reported a combination of a 

statistically significant increase in 

respiratory 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). Recognizing 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 judges it appropriate that 

the standard protect against exposure, 

particularly multiple occurrences of 

exposure, to somewhat lower levels. In 

so doing, he notes that the exposure/risk 

estimates indicate more than 99% of 

children with asthma, and of all 

children, 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 

(Table 2). The estimate is 99% of 

children with asthma for the highest 

year and study area (Table 2). Further, 

he notes that 99.9% of these groups are 

estimated to be protected from two or 

more such occasions, and 100% from 

still more occasions. These estimates 

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. 

In consideration of the exposure/risk 

results for the lowest benchmark (60 

ppb), the Administrator notes 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, 

are much reduced from that observed in 

the next higher studied concentration 

(73 ppb), both at the mean and 

individual level, and are not reported to 

be associated with increased respiratory 

symptoms in healthy subjects.152 In 

light of these results and the transient 

nature of the responses, the 

Administrator does not judge these 

responses to represent adverse effects 

for generally healthy individuals. 

However, he further considers these 

findings specifically with regard to 

protection of at-risk populations, such 

as people with asthma. In so doing, he 

notes that such data are lacking for atrisk groups, such as people with asthma, 

and considers the evidence and 


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153As noted in section I.A above, consideration 

of such protection is focused on the sensitive group 

of individuals and not a single person in the 

sensitive group (see S. Rep. No. 91–1196, 91st 

Cong., 2d Sess. 10 [1970]). 

comments from the CASAC regarding 

the need to consider endpoints of 

particular importance for this 

population group, such as risk of asthma 

exacerbation and prolonged 

inflammation. He takes note of 

comments from the CASAC (and also 

noted in the ATS statement) that small 

lung function decrements in this at-risk 

group may contribute to a risk of asthma 

exacerbation, an outcome described by 

the CASAC as ‘‘arguably the most 

important potential adverse effect’’ of O3

exposure for a child with asthma. Thus, 

he judges it important for the standard 

to provide protection that reduces such 

risks. However, he recognizes gaps in 

our ability to predict risk of such events 

at the low concentrations such as those 

represented by the lowest benchmark in 

the exposure/risk analysis. With regard 

to the inflammatory response he notes 

the evidence, discussed in section II.B.2 

above, indicating the role of repeated 

occurrences of inflammation in 

contributing to severity of response. 

Thus, he finds repeated occurrences of 

exposure events of potential concern to 

pose greater risk than single events, 

leading him to place greater weight to 

exposure/risk estimates for multiple 

occurrences. 

In light of the uncertainties associated 

with the lack of controlled human 

exposure data for people with asthma, 

particularly with regard to the extent to 

which the lower exposure 

concentrations studied in generally 

healthy adults might be expected to 

elicit asthmatic responses in this at-risk 

population, the Administrator notes that 

the CASAC also recognized this, 

describing the gap in clinical studies to 

be a ‘‘key knowledge gap’’ important to 

considerations of margin of safety for 

the standard. The Administrator further 

notes that the CAA requirement that 

primary standards provide an adequate 

margin of safety was intended to 

address uncertainties associated with 

inconclusive scientific and technical 

information available at the time of 

standard setting. This approach is 

consistent with the requirements of the 

NAAQS provisions of the CAA and with 

how the EPA and the courts have 

historically interpreted the Act 

(summarized in section I.A. above). 

These provisions require the 

Administrator to establish primary 

standards that, in the judgment of the 

Administrator, are requisite to protect 

public health with an adequate margin 

of safety. In so doing, the Administrator 

seeks to establish standards that are 

neither more nor less stringent than 

necessary for this purpose. The Act does 

not require that primary standards be set 

at a zero-risk level, but rather at a level 

that avoids unacceptable risks to public 

health, including the health of sensitive 

groups.153

Thus, in this context, and given that 

the 70 ppb benchmark represents an 

exposure level somewhat below the 

lowest exposure concentration for 

which both statistically significant lung 

function decrements and increased 

respiratory symptoms have been 

reported in largely healthy adult 

subjects, the Administrator considers 

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 that 

context, the Administrator turns to the 

third benchmark concentration (60 ppb). 

In so doing, he takes note that these 

estimates indicate more than 96% to 

more than 99% of children with asthma 

to be protected from more than one 

occasion in a year (two or more), on 

average, of 7-hour exposures to 

concentrations at or above this level, 

while at elevated exertion (Table 2). 

Additionally, the analysis estimates 

more than 90% of all children, on 

average across the three years, to be 

protected from one or more occasions of 

exposure at or above this level. The 

Administrator finds this to indicate an 

appropriate degree of protection from 

such exposures. 

The Administrator additionally takes 

note of the new finding in this review 

of evidence of a likely to be causal 

relationship between O3 and metabolic 

effects. In so doing, he notes the lack of 

evidence that would suggest such effects 

to be associated with exposures likely to 

occur with air quality conditions 

meeting the current standard, as 

discussed in section II.A.2.c above. 

Thus, he judges the current standard to 

provide protection from effects other 

than respiratory effects, for which the 

evidence is less certain. Accordingly, 

the Administrator concludes that the 

standard does not need to be revised to 

provide additional protection from such 

effects. 

In reflecting on all of the information 

currently available, the Administrator 

considers the extent to which the 

currently available information might 

indicate support for a less stringent 

standard. He recognizes the advice from 

the CASAC, which generally indicates 

support for retaining the current 

standard without revision or for revision 

to a more stringent level based on 

additional consideration of the margin 

of safety for at-risk populations. He 

notes that the CASAC advice did not 

convey support for a less stringent 

standard. He additionally considers the 

current exposure and risk estimates for 

the air quality scenario for a design 

value just above the level of the current 

standard (at 75 ppb), in comparison to 

the scenario for the current standard, as 

summarized in section II.A.3 above. In 

so doing, he finds the markedly 

increased estimates of exposures to the 

higher benchmarks under air quality for 

a higher standard level to be of concern 

and indicative of less than the requisite 

protection (Table 2). Thus, in light of 

the considerations raised here, 

including the need for an adequate 

margin of safety, the Administrator 

judges that a less stringent standard 

would not be appropriate. 

The Administrator additionally 

considers whether it would be 

appropriate to consider a more stringent 

standard that might be expected to 

result in reduced O3 exposures. As an 

initial matter, he considers the advice 

from the CASAC. With regard to the 

CASAC advice, while part of the 

Committee concluded the evidence 

supported retaining the current standard 

without revision, another part of the 

Committee reiterated advice from the 

prior CASAC, which while including 

the current standard level among the 

range of recommended standard levels, 

also provided policy advice to set the 

standard at a lower level. In considering 

this advice now in this review, as it was 

raised by part of the current CASAC, the 

Administrator notes the slight 

differences of the current exposure and 

risk estimates from the 2014 HREA 

estimates for the lowest benchmark, 

which were those considered by the 

prior CASAC (Table 4). For example, 

while the 2014 HREA estimated 3.3 to 

10.2% of children, on average, to 

experience one or more days with an 

exposures at or above 60 ppb (and as 

many as 18.9% in a single year), the 

comparable estimates for the current 

analyses are lower, particularly at the 

upper end (3.2 to 8.2% and 10.6%). 

While the estimates for two or more 

days with occurrences at or above 60 

ppb, on average across the assessment 

period, are more similar between the 

two assessments, the current estimate 

for the single highest year is much lower 

(9.2 versus 4.3%). The Administrator 

additionally recognizes the PA finding 

that the factors contributing to these 

differences, which includes the use of 

air quality data reflecting concentrations 

much closer to the now-current 

standard than was the case in the 2015 

review, also contribute to a reduced 


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uncertainty in the current estimates, as 

summarized in section II.A.3 above (PA, 

sections 3.4 and 3.5). Thus, he notes 

that the current exposure analysis 

estimates indicate the current standard 

to provide appreciable protection 

against multiple days with a maximum 

exposure at or above 60 ppb. In the 

context of his consideration of the 

adequacy of protection provided by the 

standard and of the CAA requirement 

that the standard protect public health, 

including the health of at-risk 

populations, with an adequate margin of 

safety, the Administrator concludes, in 

light of all of the considerations raised 

here, that the current standard provides 

appropriate protection, and that a more 

stringent standard would be more than 

requisite to protect public health. 

In light of all of the above, including 

advice from the CASAC, the 

Administrator finds the current 

exposure and risk analysis results to 

describe appropriately strong protection 

of at-risk populations from exposures 

associated with O3-related health 

effects. 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 judges 

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 concludes that 

a more stringent standard is not needed 

to provide requisite protection and that 

the current standard provides the 

requisite protection of public health 

under the Act. With regard to key 

aspects of the specific elements of the 

standard, the Administrator recognizes 

the support in the current evidence base 

for O3 as the indicator for 

photochemical oxidants. In so doing, he 

notes the ISA conclusion that O3 is the 

most abundant of the photochemical 

oxidants in the atmosphere and the one 

most clearly linked to human health 

effects. He additionally recognizes the 

control exerted by the 8-hour averaging 

time on associated exposures of 

importance for O3-related health effects. 

Lastly, with regard to form and level of 

the standard, the Administrator takes 

note of the exposure and risk results as 

discussed above and the level of 

protection that they indicate the 

elements of the current standard to 

provide. Beyond his recognition of this 

support in the available information for 

the elements of the current standard, the 

Administrator has considered the 

elements collectively in evaluating the 

health protection afforded by the 

current standard. For all of the reasons 

discussed above, the Administrator 

concludes that the current primary O3

standard (in all of its elements) is 

requisite to protect public health with 

an adequate margin of safety, including 

the health of at-risk populations, and 

thus should be retained, without 

revision. 

C. Decision on the Primary Standard 

For the reasons discussed above and 

taking into account information and 

assessments presented in the ISA and 

PA, the advice from the CASAC, and 

consideration of public comments, the 

Administrator concludes that the 

current primary O3 standard is requisite 

to protect public health with an 

adequate margin of safety, including the 

health of at-risk populations, and is 

retaining the current standard without 

revision. 

III. Rationale for Decision on the 

Secondary Standard 

This section presents the rationale for 

the Administrator’s decision to retain 

the current secondary O3 standard. This 

rationale is based on the scientific 

information presented in the ISA, on 

welfare effects associated with 

photochemical oxidants including O3

and pertaining to the presence of these 

pollutants in ambient air. As 

summarized in section I.D above, the 

ISA was developed based on a thorough 

review of the latest scientific 

information generally published 

between January 2011 and March 2018, 

as well as more recent studies identified 

during peer review or by public 

comments on the draft ISA integrated 

with the information and conclusions 

from previous assessments (ISA, section 

IS.1.2 and Appendix 10, section 10.2). 

The Administrator’s rationale also takes 

into account: (1) The PA evaluation of 

the policy-relevant information in the 

ISA and presentation of quantitative 

analyses of air quality, exposure, and 

risk; (2) CASAC advice and 

recommendations, as reflected in 

discussions of drafts of the ISA and PA 

at public meetings, and in the CASAC’s 

letters to the Administrator; (3) public 

comments on the proposed decision; 

and also (4) the August 2019 decision of 

the D.C. Circuit remanding the 

secondary standard established in the 

last review to the EPA for further 

justification or reconsideration. See 

Murray Energy Corp. v. EPA, 936 F.3d 

597 (D.C. Cir. 2019). 

Within this section, introductory and 

background information is presented in 

section III.A. Section III.A.1 summarizes 

the 2015 establishment of the existing 

standard, as background for this review. 

Sections III.A.2 and III.A.3 provide 

overviews of the currently available 

welfare effects evidence and current air 

quality and environmental exposure 

information, respectively. Section III.B 

summarizes the basis for the proposed 

decision (III.B.1), including CASAC 

advice, discusses public comments on 

the proposed decision (III.B.2), and 

presents the Administrator’s 

considerations, conclusions and 

decision in this review of the secondary 

standard (III.B.3). The decision is 

summarized in section III.C. 

A. Introduction 

As in prior reviews, the general 

approach to reviewing the current 

secondary standard is based, most 

fundamentally, on using the Agency’s 

assessments of the current scientific 

evidence and associated quantitative 

analyses to inform the Administrator’s 

judgment regarding a secondary 

standard for photochemical oxidants 

that is requisite to protect the public 

welfare from known or anticipated 

adverse effects associated with the 

pollutant’s presence in the ambient air. 

The EPA’s assessments are primarily 

documented in the ISA and PA, both of 

which have received CASAC review and 

public comment (84 FR 50836, 

September 26, 2019; 84 FR 58711, 

November 1, 2019; 84 FR 58713, 

November 1, 2019; 85 FR 21849, April 

20, 2020; 85 FR 31182, May 22, 2020). 

In bridging the gap between the 

scientific assessments of the ISA and the 

judgments required of the Administrator 

in his decisions on the current standard, 

the PA evaluates policy implications of 

the assessment of the current evidence 

in the ISA and the quantitative air 

quality, exposure and risk analyses and 

information documented in the PA. In 

evaluating the public welfare protection 

afforded by the current standard, the 

four basic elements of the NAAQS 

(indicator, averaging time, level, and 

form) are considered collectively. 

The final decision on the adequacy of 

the current secondary standard is a 

public welfare policy judgment to be 

made by the Administrator. In reaching 

conclusions on the standard, the 

decision draws on the scientific 

information and analyses about welfare 

effects, environmental exposure and 

risks, and associated public welfare 

significance, as well as judgments about 

how to consider the range and 

magnitude of uncertainties that are 

inherent in the scientific evidence and 

analyses. This approach is based on the 

recognition that the available evidence 

generally reflects a continuum that 

includes ambient air exposures at which 


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154The EPA’s decision not to use a seasonal 

W126 index as the form and averaging time of the 

secondary standard was also challenged in this 

case, but the court did not reach a decision on that 

issue, concluding that it lacked a basis to assess the 

EPA’s rationale on this point because the EPA had 

not yet fully explained its focus on a 3-year average 

W126 in its consideration of the standard. See 

Murray Energy Corp. v. EPA, 936 F.3d 597, 618 

(D.C. Cir. 2019). 

155 In addition to concluding there to be causal 

relationships between O3 and visible foliar injury, 

reduced vegetation growth, reduced productivity, 

reduced growth and yield of agricultural crops, and 

alteration of below-ground biogeochemical cycles, 

the 2013 ISA also concluded there likely to be a 

causal relationships between O3 and reduced 

carbon sequestration in terrestrial ecosystems, 

alteration of terrestrial ecosystem water cycling and 

alteration of terrestrial community composition 

(2013 ISA, p. lxviii and Table 9–19). The 2013 ISA 

also found there to be a causal relationship between 

changes in tropospheric O3 concentrations and 

radiative forcing, and likely to be a causal 

relationship between tropospheric O3

concentrations and effects on climate as quantified 

through surface temperature response (2013 ISA, 

section 10.5). 

156The 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). 

157These functions for RBL estimate the 

reduction in a year’s growth as a percentage of that 

expected in the absence of O3 (2013 ISA, section 

9.6.2; 2014 WREA, section 6.2). 

scientists generally agree that effects are 

likely to occur through lower levels at 

which the likelihood and magnitude of 

responses become increasingly 

uncertain. This approach is consistent 

with the requirements of the provisions 

of the Clean Air Act related to the 

review of NAAQS and with how the 

EPA and the courts have historically 

interpreted the Act. These provisions 

require the Administrator to establish 

secondary standards that, in the 

judgment of the Administrator, are 

requisite to protect the public welfare 

from known or anticipated adverse 

effects associated with the presence of 

the pollutant in the ambient air. In so 

doing, the Administrator seeks to 

establish standards that are neither more 

nor less stringent than necessary for this 

purpose. The Act does not require that 

standards be set at a zero-risk level, but 

rather at a level that reduces risk 

sufficiently so as to protect the public 

welfare from known or anticipated 

adverse effects. 

This decision on the secondary O3

standard also considers the August 2019 

decision by the D.C. Circuit and issues 

raised by the court in its remand of the 

2015 standard to the EPA such that the 

decision in this review incorporates the 

EPA’s response to the court’s remand. 

The opinion issued by the court 

concluded, in relevant part, that EPA 

had not provided a sufficient rationale 

for aspects of its 2015 decision on the 

secondary standard. See Murray Energy 

Corp. v. EPA, 936 F.3d 597 (D.C. Cir. 

2019). Accordingly, the court remanded 

that standard to EPA for further 

justification or reconsideration, 

particularly in relation to its decision to 

focus on a 3-year average for 

consideration of the cumulative 

exposure for vegetation, in terms of 

W126, identified as providing requisite 

public welfare protection, and its 

decision to not identify a specific level 

of air quality related to visible foliar 

injury.154 Thus, in addition to 

considering the currently available 

welfare effects evidence and 

quantitative air quality, exposure and 

risk information, the decision described 

here, and the associated conclusions 

and judgments, also consider the court’s 

remand. In consideration of the court 

remand, for example, certain analyses in 

this review are expanded compared 

with those conducted in the last review, 

issues raised in the remand have been 

discussed, and additional explanation of 

rationales for conclusions on these 

points is provided in this review. 

1. Background on the Current Standard 

As a result of the last O3 review, 

completed in 2015, the level of the 

secondary standard was revised to 0.070 

ppm, in conjunction with retaining the 

indicator, averaging time and form. This 

revision, establishing the current 

standard, was based on 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 that may 

be allowed by such a standard (80 FR 

65292, October 26, 2015). In 

establishing this standard, the 

Administrator considered the extensive 

welfare effects evidence base compiled 

from more than fifty years of extensive 

research on the phytotoxic effects of O3, 

conducted both in and outside of the 

U.S., that documents the impacts of O3

on plants and their associated 

ecosystems (U.S. EPA, 1978, 1986, 1996, 

2006, 2013). As was established in prior 

reviews, O3 can interfere with carbon 

gain (photosynthesis) and allocation of 

carbon within the plant, making fewer 

carbohydrates available for plant 

growth, reproduction, and/or yield (U.S. 

EPA, 1996b, pp. 5–28 and 5–29).155 The 

2015 decision drew upon: (1) The 

available scientific evidence assessed in 

the 2013 ISA; (2) assessments in the 

2014 PA of the most policy-relevant 

information in the 2013 ISA regarding 

evidence of adverse effects of O3 to 

vegetation and ecosystems, information 

on biologically-relevant exposure 

metrics, 2014 welfare REA (WREA) 

analyses of air quality, exposure, and 

ecological risks and associated 

ecosystem services, and staff analyses of 

relationships between levels of a W126- 

based exposure index 156 and potential 

alternative standard levels in 

combination with the form and 

averaging time of the existing standard; 

(3) additional air quality analyses of the 

W126 index and design values based on 

the form and averaging time of the 

existing standard; (4) CASAC advice 

and recommendations; and (5) public 

comments received during the 

development of these documents and on 

the 2014 proposal (80 FR 65292, 

October 26, 2015). In addition to 

reviewing the most recent scientific 

information as required by the CAA, the 

2015 rulemaking also incorporated the 

EPA’s response to the judicial remand of 

the 2008 secondary O3 standard in 

Mississippi v. EPA, 744 F.3d 1334 (D.C. 

Cir. 2013) and, in light of the court’s 

decision in that case, explained the 

Administrator’s conclusions as to the 

level of air quality judged to provide the 

requisite protection of public welfare 

from known or anticipated adverse 

effects. 

Across the different types of studies, 

the strongest evidence for effects from 

O3 exposure on vegetation was from 

controlled exposure studies of many 

species of vegetation (2013 ISA, p. 1– 

15). Primary consideration in the 

decision was given to the studies of O3

exposures that reduced growth in tree 

seedlings from which E–R functions of 

seasonal relative biomass loss (RBL) 

have been established (80 FR 65385–86, 

65389–90, October 26, 2015). The 

Administrator considered the effects of 

O3 on tree seedling growth, as suggested 

by the CASAC, as a surrogate or proxy 

for the broader array of vegetationrelated effects of O3, ranging from effects 

on sensitive species to broader 

ecosystem-level effects (80 FR 65369, 

65406, October 26, 2015). The metric 

used for quantifying effects on tree 

seedling growth in the review was RBL, 

with the evidence base providing robust 

and established E–R functions for 

seedlings of 11 tree species (80 FR 

65391–92, October 26, 2015; 2014 PA, 

Appendix 5C).157 The Administrator 

used this metric in her judgments on O3

effects on the public welfare. In this 

context, exposure was evaluated in 

terms of the W126 cumulative seasonal 


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158Areas designated as Class I include all 

international parks, national wilderness areas 

which exceed 5,000 acres in size, national memorial 

parks which exceed 5,000 acres in size, and 

national parks which exceed 6,000 acres in size, 

provided the park or wilderness area was in 

existence on August 7, 1977. Other areas may also 

be Class I if designated as Class I consistent with 

the CAA. 

159This emphasis on such lands was consistent 

with a similar emphasis in the 2008 review of the 

standard (73 FR 16485, March 27, 2008). 

160The Administrator focused on the median RBL 

estimate across the eleven tree species for which 

robust established E–R functions were available and 

took note of the CASAC’s consideration of RBL 

estimates presented in the 2014 draft PA, in which 

it characterized an estimate of 6% RBL in the 

median studied species as being ‘‘unacceptably 

high,’’ (Frey, 2014b). 

161As described in the ISA, ‘‘[t]ypical types of 

visible injury to broadleaf plants include stippling, 

flecking, surface bleaching, bifacial necrosis, 

pigmentation (e.g., bronzing), and chlorosis or 

premature senescence’’ and ‘‘[t]ypical visible injury 

symptoms for conifers include chlorotic banding, 

tip burn, flecking, chlorotic mottling, and 

premature senescence of needles’’ (ISA, Appendix 

8, p. 8–13). 

162The Administrator additionally recognized 

that providing protection for this purpose will also 

provide a level of protection for other vegetation 

that is used by the public and potentially affected 

by O3 including timber, produce grown for 

consumption, and horticultural plants used for 

landscaping (80 FR 65403, October 26, 2015). 

163The CAA does not require that a secondary 

standard be protective of all effects associated with 

a pollutant in the ambient air but rather those 

known or anticipated effects judged adverse to the 

public welfare (CAA section 109). 

exposure index, an index supported by 

the evidence in the 2013 ISA for this 

purpose and that was consistent with 

advice from the CASAC (2013 ISA, 

section 9.5.3, p. 9–99; 80 FR 65375, 

October 26, 2015). 

The 2015 decision was a public 

welfare policy judgment made by the 

Administrator, that drew upon the 

available scientific evidence for O3- 

attributable welfare effects and on 

quantitative analyses of exposures and 

public welfare risks, as well as 

judgments about the appropriate weight 

to place on the range of uncertainties 

inherent in the evidence and analyses. 

Included in this decision were 

judgments on the weight to place on the 

evidence of specific vegetation-related 

effects estimated to result across a range 

of cumulative seasonal concentrationweighted O3 exposures; on the weight to 

give associated uncertainties, including 

uncertainties of predicted 

environmental responses (based on 

experimental study data); variability in 

occurrence of the specific effects in 

areas of the U.S., especially in areas of 

particular public welfare significance; 

and on the extent to which such effects 

in such areas may be considered adverse 

to public welfare. For example, in 

considering the public welfare 

protection provided by the then-existing 

standard, the Administrator gave 

primary consideration to an analysis of 

cumulative seasonal exposures in or 

near Class I areas,158 which are lands 

that Congress set aside for specific uses 

intended to provide benefits to the 

public welfare, including lands that are 

to be protected so as to conserve the 

scenic value and the natural vegetation 

and wildlife within such areas, and to 

leave them unimpaired for the 

enjoyment of future generations.159 The 

decision additionally recognized that 

states, tribes and public interest groups 

also set aside areas that are intended to 

provide similar benefits to the public 

welfare for residents on those lands, as 

well as for visitors to those areas (80 FR 

65390, October 26, 2015). In recognizing 

that her judgments regarding effects that 

are adverse to the public welfare 

consider the intended use of the natural 

resources and ecosystems affected, the 

Administrator 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). 

In the Administrator’s consideration 

of the adequacy of public welfare 

protection afforded by the existing 

standard, she gave particular attention 

to the air quality analysis for Class I 

areas that estimated cumulative 

exposures, in terms of 3-year average 

W126 index values, at and above 19 

ppm-hrs, to have occurred under the 

standard in nearly a dozen areas 

distributed across two NOAA climatic 

regions of the U.S (80 FR 65385–86, 

October 26, 2015). The Administrator 

took note of these occurrences of 

exposures in Class I areas during 

periods when the existing standard was 

met, for which the associated estimates 

of growth effects across the species with 

E–R functions extend above a 

magnitude considered to be 

‘‘unacceptably high’’ by the CASAC (80 

FR 65385–65386, 65389–65390, October 

26, 2015).160 Based on this analysis and 

the considerations summarized above, 

including consideration of CASAC 

advice and public comment, the 

Administrator concluded that the 

protection afforded by the then-existing 

standard was not sufficient and that the 

standard needed to be revised to 

provide additional protection from 

known and anticipated adverse effects 

to public welfare, related to effects on 

sensitive vegetation and ecosystems, 

most particularly those occurring in 

Class I areas, and also in other areas set 

aside by states, tribes and public interest 

groups to provide similar benefits to the 

public welfare. In so doing, she further 

noted that a revised standard would 

provide increased protection for other 

growth-related effects, including relative 

yield loss (RYL) of crops, reduced 

carbon storage, and types of effects for 

which it is more difficult to determine 

public welfare significance, as well as 

other welfare effects of O3, such as 

visible foliar injury 161 (80 FR 65390, 

October 26, 2015). 

In light of the judicial remand of the 

2008 secondary O3 standard referenced 

above, the 2015 decision on selection of 

a revised secondary standard first 

considered the available evidence and 

quantitative analyses in the context of 

an approach for considering and 

identifying public welfare objectives for 

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 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.162

In reaching a conclusion on the 

amount of public welfare protection 

from the presence of O3 in ambient air 

that is appropriate to be afforded by a 

revised secondary standard, the 

Administrator gave particular 

consideration to the following: (1) The 

nature and degree of effects of O3 on 

vegetation, including her judgments as 

to what constitutes an adverse effect to 

the public welfare; (2) the strengths and 

limitations of the available and relevant 

information; (3) comments from the 

public on the Administrator’s proposed 

decision, including comments related to 

identification of a target level of 

protection; and (4) the CASAC’s views 

regarding the strength of the evidence 

and its adequacy to inform judgments 

on public welfare protection. The 

Administrator recognized that such 

judgments should neither overstate nor 

understate the strengths and limitations 

of the evidence and information nor the 

appropriate inferences to be drawn as to 

risks to public welfare, and that the 

choice of the appropriate level of 

protection is a public welfare policy 

judgment entrusted to the Administrator 

under the CAA taking into account both 

the available evidence and the 

uncertainties (80 FR 65404–05, October 

26, 2015).163


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164These 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 O3-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). 

165With respect to commercial production of 

commodities, the Administrator noted the difficulty 

in discerning the extent to which O3-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 O3- 

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). 

166When stated to the first decimal place, the 

median RBL was 6.0% for a cumulative seasonal 

W126 exposure index of 19 ppm-hrs. For 18 ppmhrs, the median RBL estimate was 5.7%, which 

rounds to 6%, and for 17 ppm-hrs, the median RBL 

estimate was 5.3%, which rounds to 5% (80 FR 

65407, October 26, 2015). 

167Based on a number of considerations, the 

Administrator recognized greater confidence in 

judgments related to public welfare impacts based 

on a 3-year average metric than a single-year metric, 

and consequently concluded it to be appropriate to 

use a seasonal W126 index averaged across three 

years for judging public welfare protection afforded 

by a revised secondary standard. 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 potential for adverse effects to public 

welfare that may relate to shorter exposure periods, 

including a single year (80 FR 65404, October 26, 

2015). 

With regard to the extensive evidence 

of welfare effects of O3, including 

visible foliar injury and crop RYL, 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 

(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).164 Similarly, while O3-related 

growth effects on agricultural and 

commodity crops had been extensively 

studied and robust E–R functions 

developed for a number of species, the 

Administrator found this information 

less useful in informing her judgments 

regarding an appropriate level of public 

welfare protection (80 FR 65405, 

October 26, 2015).165 Thus, and in light 

of the extensive evidence base in this 

regard, the Administrator focused on the 

information related to trees and growth 

impacts in identifying the public 

welfare objectives for the revised 

secondary standard. 

Accordingly, consideration of the 

appropriate public welfare protection 

objective for a revised standard focused 

on 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). 

The Administrator also incorporated 

into her considerations the broader 

evidence base associated with forest tree 

seedling biomass loss, including other 

less quantifiable effects of potentially 

greater public welfare significance. That 

is, in drawing on these RBL estimates, 

the Administrator was not simply 

making judgments about a specific 

magnitude of growth effect in seedlings 

that would be acceptable or 

unacceptable in the natural 

environment. Rather, though mindful of 

associated uncertainties, the 

Administrator used the RBL estimates as 

a surrogate or proxy for consideration of 

the broader array of related vegetationrelated effects of potential public 

welfare significance, which included 

effects on individual species and 

extending to ecosystem-level effects (80 

FR 65406, October 26, 2015). This 

broader array of vegetation-related 

effects included those for which public 

welfare implications are more 

significant but for which the tools for 

quantitative estimates were more 

uncertain. 

In using the RBL estimates as a proxy, 

the Administrator 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 robust and established E–R 

functions would be somewhat below 

6% (80 FR 65406–07, October 26, 2015). 

In so doing, 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).166 Given the 

information on median RBL at different 

W126 exposure levels, using a 3-year 

cumulative exposure index for assessing 

vegetation effects,167 the potential for 

single-season effects of concern, and 

CASAC comments on the 

appropriateness of a lower value for a 3- 

year average W126 index, the 

Administrator concluded it was 

appropriate to identify a standard that 

would restrict cumulative seasonal 

exposures to 17 ppm-hrs or lower, in 

terms of a 3-year W126 index, in nearly 

all instances (80 FR 65407, October 26, 

2015). Based on such information, 

available at that time, to inform 

consideration of vegetation effects and 

their potential adversity to public 

welfare, the Administrator additionally 

judged that the RBL estimates associated 

with marginally higher exposures in 

isolated, rare instances 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 

Administrator’s decision regarding a 

revised standard was based on extensive 

air quality analyses that included the 

most recently available data (monitoring 

year 2013) and 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. 

Based on these analyses, the 

Administrator judged that the desired 

level of public welfare protection, 

considered in terms of cumulative 

exposure (quantified as the W126 

index), could be achieved by a standard 

with a revised level in combination with 

the existing form and averaging time (80 

FR 65408, October 26, 2015). 

In the most recent period of air 

quality data (2011–2013), across the 

more than 800 monitor locations 

meeting the existing standard (with its 

level of 75 ppb), the 3-year average 

W126 index values were above 17 ppmhrs in 25 sites distributed across 

different NOAA climatic regions, and 


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168More than 1600 studies are newly available 

and considered in the ISA, including nearly 600 

studies on welfare effects (ISA, Appendix 10, Figure 

10–2). 

169Evidence continues to indicate that ‘‘visible 

foliar injury usually occurs when sensitive plants 

are exposed to elevated ozone concentrations in a 

predisposing environment,’’ with a major factor for 

such an environment being the amount of soil 

moisture available to the plant (ISA, Appendix 8, 

p. 8–23; 2013 ISA, section 9.4.2). 

170The 2013 ISA did not include a separate 

causality determination for reduced plant 

reproduction. Rather, it was included with the 

above 19 ppm-hrs at nearly half of these 

sites, with some well above (Wells, 

2015). In comparison, among the more 

than 500 sites meeting an alternative 

standard of 70 ppb across 46 of the 50 

states, there were no occurrences of a 

W126 value above 17 ppm-hrs and 

fewer than five occurrences that equaled 

17 ppm-hrs (Wells, 2015 and associated 

dataset [document identifier, EPA–HQ– 

OAR–2008–0699–4325]). For the full air 

quality dataset (extending back to 2001), 

among the nearly 4000 instances where 

a monitoring site met a standard level of 

70 ppb, the Administrator noted that 

there was only ‘‘a handful of isolated 

occurrences’’ of 3-year W126 index 

values above 17 ppm-hrs, ‘‘all but one 

of which were below 19 ppm-hrs’’ (80 

FR 65409, October 26, 2015). The 

Administrator concluded that that 

single value of 19.1 ppm-hrs (just 

equaling 19, when rounded), observed 

at a monitor for the 3-year period of 

2006–2008, was reasonably regarded as 

an extremely rare and isolated 

occurrence, and, as such, it was unclear 

whether it would recur, particularly as 

areas across the U.S. took further steps 

to reduce O3 to meet revised primary 

and secondary standards. Further, based 

on all of the then available information, 

as noted above, the Administrator did 

not judge RBL estimates associated with 

marginally higher exposures in isolated, 

rare instances to be indicative of adverse 

effects to the public welfare. The 

Administrator concluded that a 

standard with a level of 70 ppb and the 

existing form and averaging time would 

be expected to limit cumulative 

exposures, in terms of a 3-year average 

W126 exposure index, to values at or 

below 17 ppm-hrs, in nearly all 

instances, and accordingly, to eliminate 

or virtually eliminate cumulative 

exposures associated with a median 

RBL of 6% or greater (80 FR 65409, 

October 26, 2015). Thus, using RBL as 

a proxy in judging effects to public 

welfare, the Administrator judged that 

such a standard with a level of 70 ppb 

would provide the requisite protection 

from adverse effects to public welfare by 

limiting cumulative seasonal exposures 

to 17 ppm-hrs or lower, in terms of a 3- 

year W126 index, in nearly all 

instances, and decided to revise the 

standard level to 70 ppb. 

In summary, the Administrator judged 

that the revised standard would protect 

natural forests in Class I and other 

similarly protected areas against an 

array of adverse vegetation effects, most 

notably including those related to 

effects on growth and productivity in 

sensitive tree species. The 

Administrator additionally judged that 

the revised standard would be sufficient 

to protect public welfare from known or 

anticipated adverse effects. This 

judgment by the Administrator 

appropriately recognized that the CAA 

does not require that standards be set at 

a zero-risk level, but rather at a level 

that reduces risk sufficiently so as to 

protect the public welfare from known 

or anticipated adverse effects. Thus, 

based on the conclusions drawn from 

the air quality analyses which 

demonstrated a strong, positive 

relationship between the 8-hour and 

W126 metrics and the findings that 

indicated the significant amount of 

control provided by the fourth-high 

metric, the evidence base of O3 effects 

on vegetation and her public welfare 

policy judgments, as well as public 

comments and CASAC advice, the 

Administrator decided to retain the 

existing form and averaging time and 

revise the level to 0.070 ppm, judging 

that such a standard would provide the 

requisite protection to the public 

welfare from any known or anticipated 

adverse effects associated with the 

presence of O3 in ambient air (80 FR 

65409–10, October 26, 2015). 

2. Overview of Welfare Effects 

Information 

The information summarized here is 

an overview of the scientific assessment 

of the welfare effects evidence available 

in this review; this assessment is 

documented in the ISA and its policy 

implications are further discussed in the 

PA. As in past reviews, the welfare 

effects evidence evaluated in the ISA for 

O3 and related photochemical oxidants 

is focused on O3 (ISA, p. IS–3). Ozone 

is the most prevalent photochemical 

oxidant present in the atmosphere and 

the one for which there is a very large, 

well-established evidence base of its 

health and welfare effects (ISA, p. IS–3). 

Thus, the current welfare effects 

evidence and the Agency’s review of the 

evidence, including the evidence newly 

available in this review,168 continues to 

focus on O3. The subsections below 

briefly summarize the following aspects 

of the evidence: the nature of O3-related 

welfare effects, the potential public 

welfare implications, and exposure 

concentrations associated with effects. 

a. Nature of Effects 

The welfare effects evidence base 

available in the current review includes 

more than sixty years of extensive 

research on the phytotoxic effects of O3

and subsequent effects on associated 

ecosystems (1978 AQCD, 1986 AQCD, 

1996 AQCD, 2006 AQCD, 2013 ISA, 

2020 ISA). As described in past reviews, 

O3 can interfere with carbon gain 

(photosynthesis) and allocation of 

carbon within the plant, making fewer 

carbohydrates available for plant 

growth, reproduction, and/or yield 

(2013 ISA, p. 1–10; 1996 AQCD, pp. 5– 

28 and 5–29). As described in the 2013 

ISA, the strongest evidence for effects 

from O3 exposure on vegetation is from 

controlled exposure studies, which 

‘‘have clearly shown that exposure to O3

is causally linked to visible foliar injury, 

decreased photosynthesis, changes in 

reproduction, and decreased growth’’ in 

many species of vegetation (2013 ISA, p. 

1–15). Such effects at the plant scale can 

also be linked to an array of effects at 

larger spatial scales (and higher levels of 

biological organization), with the 

evidence available in the last review 

indicating that ‘‘O3 exposures can affect 

ecosystem productivity, crop yield, 

water cycling, and ecosystem 

community composition’’ (2013 ISA, p. 

1–15, Chapter 9, section 9.4). Beyond its 

effects on plants, the 2013 ISA also 

recognized O3 in the troposphere as a 

major greenhouse gas (ranking behind 

carbon dioxide and methane in 

importance), with associated radiative 

forcing and effects on climate, and 

recognized the accompanying ‘‘large 

uncertainties in the magnitude of the 

radiative forcing estimate . . . making 

the impact of tropospheric O3 on 

climate more uncertain than the effect of 

the longer-lived greenhouse gases’’ 

(2013 ISA, sections 10.3.4 and 10.5.1 [p. 

10–30]). 

The evidence newly available in this 

review supports, sharpens and expands 

somewhat on the conclusions reached 

in the last review (ISA, Appendices 8 

and 9). Consistent with the evidence in 

the last review, the currently available 

evidence describes an array of O3 effects 

on vegetation and related ecosystem 

effects, as well as the role of O3 in 

radiative forcing and subsequent 

climate-related effects. The ISA 

concludes there to be causal 

relationships between O3 and visible 

foliar injury,169 reduced vegetation 

growth and reduced plant 

reproduction,170 as well as reduced 


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conclusion of a causal relationship with reduced 

vegetation growth (ISA, Table IS–12). 171The 2013 ISA had concluded alteration of 

terrestrial community composition to be likely 

causally related to O3 based on the then available 

information (ISA, Table IS–12). 

172Radiative forcing is a metric used to quantify 

the change in balance between radiation coming 

into and going out of the atmosphere caused by the 

presence of a particular substance (ISA, Appendix 

9, section 9.1.3.3). 173Effects on temperature, precipitation, and 

related climate variables were referred to as 

‘‘climate change’’ or ‘‘effects on climate’’ in the 

2013 ISA (ISA, p. IS–82; 2013 ISA, pp. 1–14 and 

10–31). 

174For example, the fundamental purpose of 

parks in the National Park System ‘‘is to conserve 

the scenery, natural and historic objects, and wild 

life in the System units and to provide for the 

enjoyment of the scenery, natural and historic 

objects, and wild life in such manner and by such 

means as will leave them unimpaired for the 

enjoyment of future generations’’ (54 U.S.C. 

100101). Additionally, the Wilderness Act of 1964 

defines designated ‘‘wilderness areas’’ in part as 

areas ‘‘protected and managed so as to preserve 

[their] natural conditions’’ and requires that these 

areas ‘‘shall be administered for the use and 

enjoyment of the American people in such manner 

as will leave them unimpaired for future use and 

enjoyment as wilderness, and so as to provide for 

the protection of these areas, [and] the preservation 

of their wilderness character . . .’’ (16 U.S.C. 

1131(a) and (c)). Other lands that benefit the public 

welfare include national forests which are managed 

for multiple uses including sustained yield 

management in accordance with land management 

plans (see 16 U.S.C. 1600(1)–(3); 16 U.S.C. 

1601(d)(1)). 

yield and quality of agricultural crops, 

reduced productivity in terrestrial 

ecosystems, alteration of terrestrial 

community composition,171 and 

alteration of belowground 

biogeochemical cycles (ISA, section 

IS.5). The current ISA also concludes 

there likely to be a causal relationship 

between O3 and alteration of ecosystem 

water cycling, reduced carbon 

sequestration in terrestrial ecosystems, 

and with increased tree mortality (ISA, 

section IS.5). Additionally, evidence 

newly available in this review augments 

more limited previously available 

evidence related to insect interactions 

with vegetation, contributing to the ISA 

conclusion that the evidence is 

sufficient to infer that there are likely to 

be causal relationships between O3

exposure and alteration of plant-insect 

signaling (ISA, Appendix 8, section 8.7) 

and of insect herbivore growth and 

reproduction (ISA, Appendix 8, section 

8.6). Thus, conclusions reached in the 

last review continue to be supported by 

the current evidence base and 

conclusions are also reached in a few 

new areas based on the now expanded 

evidence. 

As in the last review, the strongest 

evidence and the associated findings of 

causal or likely causal relationships 

with O3 in ambient air, and the 

quantitative characterizations of 

relationships between O3 exposure and 

occurrence and magnitude of effects are 

for vegetation effects. Visible foliar 

injury has long been used as a 

bioindicator of O3 exposure, although it 

is not always a reliable indicator of 

other negative effects on vegetation 

(ISA, sections IS.5.1.2 and 8.2). Effects 

of O3 on physiology of individual plants 

at the cellular level, such as through 

photosynthesis and carbon allocation, 

can impact plant growth and 

reproduction (ISA, section IS.5.1.2). The 

scales of these effects range from the 

individual plant scale to the ecosystem 

scale, with potential for impacts on the 

public welfare (as discussed in section 

III.A.2.b below). The effects of O3 on 

plants and plant populations have 

implications for ecosystems. Effects at 

the ecosystem scale include reduced 

terrestrial productivity and carbon 

storage, and altered terrestrial 

community composition, as well as 

impacts on ecosystem functions, such as 

belowground biogeochemical cycles and 

ecosystem water cycling (ISA, Appendix 

8, sections 8.11 and 8.9). 

Ozone welfare effects also extend 

beyond effects on vegetation and 

associated biota due to it being a major 

greenhouse gas and radiative forcing 

agent.172 The current evidence, 

augmented since the 2013 ISA, 

continues to support a causal 

relationship between the global 

abundance of O3 in the troposphere and 

radiative forcing, and a likely causal 

relationship between the global 

abundance of O3 in the troposphere and 

effects on temperature, precipitation, 

and related climate variables 173 (ISA, 

section IS.5.2 and Appendix 9; Myhre et 

al., 2013). Uncertainty in the magnitude 

of radiative forcing estimated to be 

attributed to tropospheric O3 contributes 

to the relatively greater uncertainty 

associated with climate effects of 

tropospheric O3 compared to such 

effects of the well mixed greenhouse 

gases, such as carbon dioxide and 

methane (ISA, section IS.6.2.2). 

Lastly, the evidence regarding 

tropospheric O3 and UV–B shielding 

(shielding of ultraviolet radiation at 

wavelengths of 280 to 320 nanometers) 

was evaluated in the 2013 ISA and 

determined to be inadequate to draw a 

causal conclusion (2013 ISA, section 

10.5.2). The current ISA concludes there 

to be no new evidence since the 2013 

ISA relevant to the question of UV–B 

shielding by tropospheric O3 (ISA, 

IS.1.2.1 and Appendix 9, section 

9.1.3.4). 

b. Public Welfare Implications 

The secondary standard is to ‘‘specify 

a level of air quality the attainment and 

maintenance of which in the judgment 

of the Administrator . . . is requisite to 

protect the public welfare from any 

known or anticipated adverse effects 

associated with the presence of such air 

pollutant in the ambient air’’ (CAA, 

section 109(b)(2)). As recognized in 

prior reviews of secondary standards, 

the secondary standard is not meant to 

protect against all known or anticipated 

O3-related welfare effects, but rather 

those that are judged to be adverse to 

the public welfare, and a bright line 

determination of adversity is not 

required in judging what is requisite (78 

FR 3212, January 15, 2013; 80 FR 65376, 

October 26, 2015; see also 73 FR 16496, 

March 27, 2008). The significance of 

each type of welfare effect with regard 

to potential effects on the public welfare 

depends on the type and severity of 

effects, as well as the extent of such 

effects on the affected environmental 

entity, and on the societal use of the 

affected entity and the entity’s 

significance to the public welfare. Such 

factors have been considered in the 

context of judgments and conclusions 

made in some prior reviews regarding 

public welfare effects. For example, 

judgments regarding public welfare 

significance in the last two O3 NAAQS 

decisions gave particular attention to O3

effects in areas with special federal 

protections (such as Class I areas), and 

lands set aside by states, tribes and 

public interest groups to provide similar 

benefits to the public welfare (73 FR 

16496, March 27, 2008; 80 FR 65292, 

October 26, 2015).174 In the 2015 

review, the EPA recognized the ‘‘clear 

public interest in and value of 

maintaining these areas in a condition 

that does not impair their intended use 

and the fact that many of these lands 

contain O3-sensitive species’’ (73 FR 

16496, March 27, 2008). 

Judgments regarding effects on the 

public welfare can depend on the 

intended use for, or service (and value) 

of, the affected vegetation, ecological 

receptors, ecosystems and resources and 

the importance of that use to the public 

welfare (73 FR 16496, March 27, 2008; 

80 FR 65377, October 26, 2015). Uses or 

services provided by areas that have 

been afforded special protection can 

flow in part or entirely from the 

vegetation that grows there. Ecosystem 

services range from those directly 

related to the natural functioning of the 

ecosystem to ecosystem uses for human 

recreation or profit, such as through the 

production of lumber or fuel (Costanza 

et al., 2017; ISA, section IS.5.1). 

Services of aesthetic value and outdoor 

recreation depend, at least in part, on 


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175Authors of studies presenting USFS 

biomonitoring program data have suggested what 

might be ‘‘assumptions of risk’’ (e.g., for the forest 

resource) related to scores in these categories, e.g., 

none, low, moderate and high for BI scores of zero 

to five, five to 15, 15 to 25 and above 25, 

respectively (e.g., Smith et al., 2003; Smith et al., 

2012. For example, maps of localized moderate to 

high risk areas may be used to identify areas where 

more detailed evaluations are warranted (Smith et 

al., 2012). 

176While carbon sequestration or storage also 

occurs for vegetated ecosystems other than forests, 

it is relatively larger in forests given the relatively 

greater biomass for trees compared to other plants. 

the perceived scenic beauty of the 

environment. Additionally, public 

surveys have indicated that Americans 

rank as very important the existence of 

resources, the option or availability of 

the resource and the ability to bequest 

or pass it on to future generations 

(Cordell et al., 2008). 

The different types of O3 effects on 

vegetation recognized in section 

III.A.2.a above differ with regard to 

aspects important to judging their 

public welfare significance. For 

example, in the case of effects on crop 

yield, such judgments may consider 

aspects such as the heavy management 

of agriculture in the U.S., while 

judgments for other categories of effects 

may generally relate to considerations 

regarding natural areas, including 

specifically those areas that are not 

managed for harvest. In this context, it 

may be important to consider that O3

effects on tree growth and reproduction 

could, depending on severity, extent 

and other factors, lead to effects on a 

larger scale including reduced 

productivity, altered forest and forest 

community (plant, insect and microbe) 

composition, reduced carbon storage 

and altered ecosystem water cycling 

(ISA, section IS.5.1.8.1; 2013 ISA, 

Figure 9–1, sections 9.4.1.1 and 9.4.1.2). 

For example, the composition of 

vegetation or of terrestrial community 

composition can be affected through O3

effects on growth and reproductive 

success of sensitive species in the 

community, with the extent of 

compositional changes dependent on 

factors such as competitive interactions 

(ISA, section IS.5.1.8.1; 2013 ISA, 

sections 9.4.3 and 9.4.3.1). Impacts on 

some of these characteristics (e.g., forest 

or forest community composition) may 

be considered of greater public welfare 

significance when occurring in Class I 

or other protected areas, due to value for 

particular services that the public places 

on such areas. 

Agriculture and silviculture provide 

ecosystem services with clear public 

welfare benefits. With regard to 

agriculture-related effects of O3, 

however, there are complexities in this 

consideration related to areas and plant 

species that are heavily managed to 

obtain a particular output (such as 

commodity crops or commercial timber 

production). In light of this, the degree 

to which O3 impacts on agriculturally 

important vegetation would impair the 

intended use at a level that might be 

judged adverse to the public welfare has 

been less clear (80 FR 65379, October 

26, 2015; 73 FR 16497, March 27, 2008). 

While having sufficient crop yields is of 

high public welfare value, important 

commodity crops are typically heavily 

managed to produce optimum yields. 

Moreover, based on the economic theory 

of supply and demand, increases in crop 

yields would be expected to result in 

lower prices for affected crops and their 

associated goods, which would 

primarily benefit consumers. Analyses 

in past reviews have described how 

these competing impacts on producers 

and consumers complicate 

consideration of these effects in terms of 

potential adversity to the public welfare 

(2014 WREA, sections 5.3.2 and 5.7). 

Other ecosystem services valued by 

people that can be affected by reduced 

tree growth, productivity and associated 

forest effects include aesthetic value; 

provision of food, fiber, timber, other 

forest products, habitat, and recreational 

opportunities; climate and water 

regulation; erosion control; air pollution 

removal, and desired fire regimes (PA, 

Figure 4–2; ISA, section IS.5.1; 2013 

ISA, sections 9.4.1.1 and 9.4.1.2). In 

considering such services in past 

reviews, the Agency has given particular 

attention to effects in natural 

ecosystems, indicating that a protective 

standard, based on consideration of 

effects in natural ecosystems in areas 

afforded special protection, would also 

‘‘provide a level of protection for other 

vegetation that is used by the public and 

potentially affected by O3 including 

timber, produce grown for consumption 

and horticultural plants used for 

landscaping’’ (80 FR 65403, October 26, 

2015). For example, locations 

potentially vulnerable to O3-related 

impacts might include forested lands, 

both public and private, where trees are 

grown for timber production. Forests in 

urbanized areas also provide a number 

of services that are important to the 

public in those areas, such as air 

pollution removal, cooling, and 

beautification. There are also many 

other tree species, such as various 

ornamental and agricultural species 

(e.g., Christmas trees, fruit and nut 

trees), that provide ecosystem services 

that may be judged important to the 

public welfare. 

With its effect on the physical 

appearance of plants, visible foliar 

injury has the potential to be significant 

to the public welfare, depending on its 

severity and spatial extent, by impacting 

aesthetic or scenic values and outdoor 

recreation in Class I and other similarly 

protected areas valued by the public. To 

assess evidence of injury to plants in 

forested areas on national and regional 

scales, the U.S. Forest Service (USFS) 

conducted surveys of the occurrence 

and severity of visible foliar injury on 

sensitive (bioindicator) species at 

biomonitoring sites across most of the 

U.S., beginning in 1994 (in eastern U.S.) 

and extending through 2011 (Smith et 

al., 2003; Coulston et al., 2003). At these 

sites (biosites), a national protocol, 

including verification and quality 

assurance procedures and a scoring 

system, was implemented. The resultant 

biosite index (BI) scores may be 

described with regard to one of several 

categories ranging from little or no foliar 

injury to severe injury (e.g., Smith et al., 

2003; Campbell et al., 2007; Smith et al., 

2007; Smith, 2012).175 However, the 

available information does not yet 

address or describe the relationships 

expected to exist between some level of 

injury severity (e.g., little, low/light, 

moderate or severe) and/or spatial 

extent affected and scenic or aesthetic 

values. This gap impedes consideration 

of the public welfare implications of 

different injury severities, and 

accordingly judgments on the potential 

for public welfare significance. That 

notwithstanding, while minor spotting 

on a few leaves of a plant may easily be 

concluded to be of little public welfare 

significance, some level of severity and 

widespread occurrence of visible foliar 

injury, particularly if occurring in 

specially protected areas, where the 

public can be expected to place value 

(e.g., for recreational uses), might 

reasonably be concluded to impact the 

public welfare. 

The tropospheric O3-related effects of 

radiative forcing and subsequent effects 

on temperature, precipitation and 

related climate also have important 

public welfare implications, although 

their quantitative evaluation in response 

to O3 concentrations in the U.S. is 

complicated by ‘‘[c]urrent limitations in 

climate modeling tools, variation across 

models, and the need for more 

comprehensive observational data on 

these effects’’ (ISA, section IS.6.2.2). An 

ecosystem service provided by forested 

lands is carbon sequestration or storage 

(ISA, section IS.5.1.4 and Appendix 8, 

section 8.8.3; 2013 ISA, section 2.6.2.1 

and p. 9–37) 176, which has an extremely 

valuable role in counteracting the 

impact of greenhouse gases on radiative 

forcing and related climate effects on 

the public welfare. Accordingly, the 


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177The ‘‘seasonal’’ descriptor refers to the 

duration of the period quantified (3 months) rather 

than a specific season of the year. 

178The SUM06 index received attention across 

past O3 NAAQS reviews. It is the seasonal sum of 

hourly concentrations at or above 0.06 ppm during 

a specified daily time window (2006 AQCD, p. 

AX9–161; 2013 ISA, section 9.5.2). 

179The W126 index is described in section 

III.B.3.a(i) of the proposal (85 FR 49887, August 14, 

2020) and in the PA (PA, Appendix 4D, section 

4D.2.2). 

180 In total, the 11 species-specific composite E– 

R functions are based on 51 tree seedling studies 

or experiments, many of which employed open top 

chambers, an established experimental approach 

(PA, Appendix 4A, section 4A.1.1; ISA, section 

8.1.2.1.2). For six of the 11 species, this function 

is based on just one or two studies, while for other 

species there were as many as 11 studies available. 

181While the 11 species represent only a small 

fraction of the total number of native tree species 

in the contiguous U.S., this subset includes eastern 

and western species, deciduous and coniferous 

species, and species that grow in a variety of 

ecosystems and represent a range of tolerance to O3

(PA, Appendix 4B; 2013 ISA, section 9.6.2). 

182Across the experiments for the 11 tree species, 

the exposure levels assessed are more extensive for 

relatively higher seasonal exposures (e.g., at/above 

a SUM06 of 30 ppm-hrs). Across these experiments, 

there is more limited representation of lower 

cumulative exposure levels, such as SUM06 values 

below those that may correspond to a W126 index 

of 20 ppm-hrs. These lowest levels did not always 

yield a statistically significant effect (PA, section 

4.5.1.2 and Appendix 4A; 85 FR 49901, August 14, 

2020). 

183The exposure durations varied from periods of 

82 to 140 days over a single year to periods of 180 

to 555 days across two years (Lee and Hogsett, 1996; 

PA, Appendix 4A, Table 4A–5). 

184Underlying the adjustment is a simplifying 

assumption of uniform W126 distribution across the 

exposure periods and of a linear relationship 

between duration of cumulative exposure in terms 

of the W126 index and plant growth response (85 

FR 49901; August 14, 2020; PA). Some functions for 

experiments that extended over two seasons were 

derived by distributing responses observed at the 

end of two seasons of varying exposures equally 

across the two seasons (e.g., essentially applying the 

average to both seasons). 

service of carbon storage can be of 

paramount importance to the public 

welfare no matter in what location the 

trees are growing or what their intended 

current or future use (e.g., 2013 ISA, 

section 9.4.1.2). This benefit exists as 

long as the trees are growing, regardless 

of what additional functions and 

services it provides. 

Categories of effects newly identified 

as likely causally related to O3 in 

ambient air, such as alteration of plantinsect signaling and insect herbivore 

growth and reproduction, also have 

potential public welfare implications 

(e.g., given the role of the plant-insect 

signaling process in pollination and 

seed dispersal). Uncertainties and 

limitations in the current evidence (e.g., 

summarized in sections III.B.3.c and 

III.D.1 of the proposal) preclude an 

assessment of the extent and magnitude 

of O3 effects on these endpoints, which 

thus also precludes an evaluation of the 

potential for associated public welfare 

implications. 

In summary, several considerations 

are recognized as important to 

judgments on the public welfare 

significance of the array of welfare 

effects of different O3 exposure 

conditions. These include uncertainties 

and limitations associated with the 

magnitude of key welfare effects that 

might be concluded to be adverse to 

ecosystems and associated services. 

Additionally, the presence of O3- 

sensitive tree species may contribute to 

a vulnerability of numerous locations to 

public welfare impacts from O3 related 

to tree growth, productivity and carbon 

storage and their associated ecosystems 

and services. Other important 

considerations include the exposure 

circumstances that may elicit effects and 

the potential for the significance of the 

effects to vary in specific situations due 

to differences in sensitivity of the 

exposed species, the severity and 

associated significance of the observed 

or predicted O3-induced effect, the role 

that the species plays in the ecosystem, 

the intended use of the affected species 

and its associated ecosystem and 

services, the presence of other cooccurring predisposing or mitigating 

factors, and associated uncertainties and 

limitations. 

c. Exposures Associated With Effects 

The welfare effects identified in 

section III.A.2.a above vary widely with 

regard to the extent and level of detail 

of the available information that 

describes the O3 exposure 

circumstances that may elicit the effects. 

The information on exposure metric and 

E–R relationships for effects related to 

vegetation growth is long-standing, 

having been first described in the 1997 

review, while such information is much 

less established for visible foliar injury. 

The evidence base for other categories of 

effects is also lacking in information 

that might support characterization of 

potential impacts of changes in O3

concentrations. 

(i) Growth-Related Effects 

The long-standing body of vegetation 

effects evidence includes a wealth of 

information on aspects of O3 exposure 

that influence its effects on plant growth 

and yield, and that has been described 

in the scientific assessments across the 

last several decades (1996 AQCD; 2006 

AQCD; 2013 ISA; 2020 ISA). A variety 

of factors have been investigated, and a 

number of mathematical approaches 

have been developed for summarizing 

O3 exposure for the purpose of assessing 

effects on vegetation, including several 

that cumulate exposures over some 

specified period while weighting higher 

more than lower concentrations (2013 

ISA, sections 9.5.2 and 9.5.3; ISA, 

Appendix 8, section 8.2.2.2). Over this 

period, the EPA’s scientific assessments 

have focused on the use of a cumulative, 

seasonal 177 concentration-weighted 

index when considering the growthrelated effects evidence and when 

analyzing exposures for purposes of 

reaching conclusions on the secondary 

standard. Such metrics have included 

SUM06,178 in the past, and more 

recently (since the 2008 review), the 

focus has been on the W126-based, 

seasonal metric, termed the ‘‘W126 

index’’ 179 (ISA, section IS.3.2, 

Appendix 8, sections 8.1 and 8.13). 

Quantifying exposure using 

cumulative, concentration-weighted 

indices of exposure, such as the W126 

index, has been found to improve the 

explanatory power of E–R models for 

growth and yield over using indices 

based only on mean and peak exposure 

values (ISA, section IS.5.1.9, p. IS–79; 

2013 ISA, section 2.6.6.1, p. 2–44). The 

most well-analyzed datasets in such 

evaluations are two detailed datasets 

established two decades ago, one for 

seedlings of 11 tree species 180 and one 

for 10 crops (e.g., Lee and Hogsett, 1996, 

Hogsett et al., 1997). These datasets, 

which include species-specific seedling 

growth and crop yield response 

information across multiple seasonal 

cumulative exposures, were used to 

develop robust quantitative E–R 

functions to predict growth reduction 

relative to a zero-O3 setting (RBL) in 

seedlings of the tree species 181 and 

similarly, E–R functions for predicting 

RYL for a set of 10 common crops (ISA, 

Appendix 8, section 8.13.2; 2013 ISA, 

section 9.6.2). 

The tree seedling E–R functions were 

derived from data for multiple studies 

documenting effects on tree seedling 

growth under a variety of O3

exposures 182 and growing conditions. 

Importantly the data included hourly 

concentrations recorded across the 

duration of the exposure, which allowed 

for derivation of various metrics that 

were analyzed for association with 

reduced growth (2013 ISA, section 9.6.2; 

Lee and Hogsett, 1996). In producing E– 

R functions of consistent duration 

across the experiments, the E–R 

functions were derived first based on 

the exposure duration of the 

experiment 183 and then normalized to 

3-month (seasonal) periods 184 (see Lee 

and Hogsett, 1996, section I.3; PA, 

Appendix 4A). The species-specific 

composite E–R functions developed 

from the experiment-specific functions 

indicate the wide variation in growth 


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185These studies included experiments that used 

OTCs to investigate tree seedling growth response 

and crop yield over a growing season under a 

variety of O3 exposures and growing conditions 

(2013 ISA, section 9.6.2; Lee and Hogsett, 1996). 

186The studies compiled in this publication 

included at least 21 days exposure above 40 ppb O3

(expressed as AOT40 [seasonal sum of the 

difference between an hourly concentration above 

40 ppb and 40 ppb]); and had a maximum hourly 

concentration that was no higher than 100 ppb (van 

Goethem et al., 2013). The publication does not 

report study-specific exposure durations, details of 

biomass response measurements or hourly O3

concentrations, making it less useful for describing 

E–R relationships that might support estimation of 

specific impacts associated with air quality 

conditions meeting the current standard (e.g., 2013 

ISA, p. 9–118). 

187As a major modifying factor is the amount of 

soil moisture available to a plant, dry periods 

decrease the incidence and severity of ozoneinduced visible foliar injury, such that the 

incidence of visible foliar injury is not always 

higher in years and areas with higher ozone, 

especially with co-occurring drought (ISA, 

Appendix 8, p. 8–23; Smith, 2012; Smith et al., 

2003). 

188 In considering their findings, the authors 

expressed the view that ‘‘[a]lthough the number of 

sites or species with injury is informative, the 

average biosite injury index (which takes into 

account both severity and amount of injury on 

multiple species at a site) provides a more 

meaningful measure of injury’’ for their assessment 

at a statewide scale (Campbell et al., 2007). 

189Although the ISA and past assessments have 

not described extensive evaluations of specific peak 

concentration metrics such as the N100, in 

summarizing this study in the last review, the ISA 

observed that ‘‘[o]verall, there was a declining trend 

in the incidence of foliar injury as peak O3 

concentrations declined’’ (2013 ISA, p. 9–40). 

190The models evaluated included several with 

cumulative exposure indices alone. These included 

SUM60 (i.e., SUM06 in ppb), SUM0, and SUM80 

(SUM08 in ppb), but not W126. They did not 

include a model with W126 that did not also 

include N100. Across all of the models evaluated, 

the model with the best fit to the data was found 

to be the one that included N100 and W126, along 

with the drought index (Davis and Orendovici, 

2006). 

sensitivity of the studied tree species at 

the seedling stage (PA, Appendix 4A, 

section 4A.1.1). 

Since the initial set of tree seedling 

growth studies were completed, several 

additional studies, focused on aspen, 

have been published based on the 

Aspen FACE experiment in a planted 

forest in Wisconsin; the findings were 

consistent with earlier open top 

chamber (OTC) studies 185 (ISA, 

Appendix 8, section 8.13.2). Newly 

available studies that investigated 

growth effects of O3 exposures are also 

consistent with the existing evidence 

base, and generally involve particular 

aspects of the effect rather than 

expanding the conditions under which 

plant species, particularly tree species, 

have been assessed (ISA, section 

IS.5.1.2). These publications include a 

compilation of previously available 

studies on plant biomass response to O3; 

the compilation reports linear 

regressions conducted on the associated 

varying datasets. Based on these 

regressions, this study describes 

distributions of sensitivity to O3 effects 

on biomass across many tree and 

grassland species, including 17 species 

native to the U.S. and 65 introduced 

species (ISA, Appendix 8, section 

8.13.2; van Goethem et al., 2013). 

Additional information is needed to 

more completely describe O3 exposure 

response relationships for these species 

in the U.S.186

(ii) Visible Foliar Injury 

Current evidence ‘‘continues to show 

a consistent association between visible 

injury and ozone exposure,’’ while also 

recognizing the role of modifying factors 

such as soil moisture and time of day 

(ISA, section IS.5.1.1). The ISA 

summarizes several recently available 

studies that continue to document that 

O3 elicits visible foliar injury in many 

plant species. As in the prior review, the 

evidence in the current review, while 

documenting that elevated O3

conditions in ambient air generally 

results in visible foliar injury in 

sensitive species (when in a 

predisposing environment),187 does not 

include a quantitative description of the 

relationship of incidence or severity of 

visible foliar injury in natural areas of 

the U.S. with specific metrics of 

seasonal O3 exposure. 

Although studies of the incidence of 

visible foliar injury in national forests, 

wildlife refuges, and similar areas have 

often used cumulative indices (e.g., 

SUM06) to investigate variations in 

incidence of foliar injury, studies also 

suggest an additional role for metrics 

focused on peak concentrations (ISA; 

2013 ISA; 2006 AQCD; Hildebrand et 

al., 1996; Smith, 2012). Other studies 

have indicated this uncertainty 

regarding the influential metric(s), e.g., 

by recognizing the need for research to 

help develop a ‘‘better linkage between 

air levels and visible injury’’ (Campbell 

et al., 2007).188 Some studies of visible 

foliar injury incidence data have 

investigated such a role for peak 

concentrations quantified by an O3

exposure index that is a count of hourly 

concentrations (e.g., in a growing 

season) above a threshold concentration 

of 100 ppb, N100 (e.g., Smith, 2012; 

Smith et al., 2012). For example, a study 

describing injury patterns over 16 years 

at USFS biosites in 24 states in the 

Northeast and North Central regions, in 

the context of the SUM06 index and 

N100 metrics, suggested that there may 

be a threshold exposure needed for 

injury to occur, and that the number of 

hours of elevated O3 concentrations 

during the growing season (such as what 

is captured by a metric like N100) may 

be more important than cumulative 

exposure in determining the occurrence 

of foliar injury (Smith, 2012).189 This 

finding is consistent with statistical 

analyses of seven years of visible foliar 

injury data from a wildlife refuge in the 

mid-Atlantic area (Davis and 

Orendovici, 2006).190

The established significant role of 

higher or peak O3 concentrations, as 

well as pattern of their occurrence, in 

plant responses has also been noted in 

prior ISAs or AQCDs. The evidence has 

included studies that use indices to 

summarize the incidence of injury on 

bioindicator species present at specific 

monitored sites, as well as experimental 

studies that assess varying O3 treatments 

on cultured stands of different tree 

species (2013 ISA, section 9.5.3.1; 2006 

AQCD, p. AX9–169; Oksanen and 

Holopainen, 2001; Yun and Laurence, 

1999). In identifying support for such O3

metrics with regard to foliar injury as 

the response, the 2013 ISA and 2006 

AQCD both cite studies that support the 

‘‘important role that peak 

concentrations, as well as the pattern of 

occurrence, plays in plant response to 

O3’’ (2013 ISA, p. 9–105; 2006 AQCD, p. 

AX9–169). 

A recent study (by Wang et al. [2012]) 

involved a statistical modeling analysis 

on a subset of the years of USFS BI data 

that were described in Smith (2012). 

This analysis tested a number of models 

for their ability to predict the presence 

of visible foliar injury (a nonzero biosite 

score), regardless of severity, and 

generally found that the type of O3

exposure metric (e.g., SUM06 versus 

N100) made only a small difference, 

although the models that included both 

a cumulative index (SUM06) and N100 

had a just slightly better fit (Wang et al., 

2012). Based on their investigation of 15 

different models, using differing 

combinations of several types of 

potential predictors, the study authors 

concluded that they were not able to 

identify environmental conditions 

under which they ‘‘could reliably expect 

plants to be damaged’’ (Wang et al., 

2012). This is indicative of the current 

state of knowledge, in which there 

remains a lack of established 

quantitative functions describing E–R 

relationships that would allow 

prediction of visible foliar injury 

severity and incidence under varying air 

quality and other environmental 

conditions. 

The information related to O3

exposures associated with visible foliar 

injury of varying severity available in 

this review also includes quantitative 


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191This dataset, including associated 

uncertainties and limitations in the assignment of 

soil moisture categories (dry, wet or normal), such 

as the substantial spatial variation in soil moisture 

and large size of NOAA climate divisions, is 

described in the PA, Appendix 4C. 

192The W126 index estimates assigned to the 

biosite locations were developed for 12 kilometer 

(km) by 12 km cells in a national-scale spatial grid 

for each year. A spatial interpolation technique was 

applied to annual W126 values derived from O3

measurements at ambient air monitoring locations 

for the years of the BI data (PA, Appendix 4C, 

sections 4.C.2 and 4C.5). 

193One third (33%) of scores above 15 are at sites 

with W126 below 7 ppm-hrs (PA, Appendix 4C, 

Table 4C–3). 

194Beyond the presentation of a statistical 

analysis developed in the last review, the PA 

presentations are primarily descriptive (as 

compared to statistical) in recognition of the 

limitations and uncertainties of the dataset (PA, 

Appendix 4C, section 4C.5). 

195Of the three new studies on tree mortality 

described in the ISA is another field study of a 

pollution gradient that, like such studies in prior 

reviews, recognizes O3 exposures as one of several 

contributing environmental and anthropogenic 

stressors (ISA, p. 8–55). 

presentations of the dataset (developed 

by the EPA in the last review) of USFS 

BI scores, collected during the years 

2006 through 2010 at locations in 37 

states. In developing this dataset, the BI 

scores were combined with estimates of 

soil moisture 191 and estimates of 

seasonal cumulative O3 exposure in 

terms of W126 index 192 (PA, Appendix 

4C). This dataset includes more than 

5,000 records of which more than 80 

percent have a BI score of zero 

(indicating a lack of visible foliar 

injury). While the estimated W126 

index assigned to records in this dataset 

ranges from zero to somewhat above 50 

ppm-hrs, more than a third of all the 

records (and also of records with BI 

scores above zero or five) 193 are at sites 

with W126 index estimates below 7 

ppm-hrs. In an extension of analyses 

developed in the last review, the 

presentation in the PA 194 describes the 

BI scores for the records in this dataset 

in relation to the W126 index estimate 

for each record, using ‘‘bins’’ of 

increasing W126 index values. The PA 

presentation utilizes the BI score 

breakpoints in the scheme used by the 

USFS to categorize severity. This 

presentation indicates that, across the 

W126 bins, there is variation in both the 

incidence of particular magnitude BI 

scores and in the average score per bin. 

In general, however, the greatest 

incidence of records with BI scores 

above zero, five, or higher—and the 

highest average BI score—occurs with 

the highest W126 bin, i.e., the bin for 

W126 index estimates greater than 25 

ppm-hrs (PA, Appendix 4C, Table 4C– 

6). 

Overall, the dataset described in the 

PA generally indicates the risk of injury, 

and particularly injury considered at 

least light, moderate or severe, to be 

higher at the highest W126 index 

values, with appreciable variability in 

the data for the lower bins (PA, 

Appendix 4C). This appears to be 

consistent with the conclusions of the 

detailed quantitative analysis studies, 

summarized above, that the pattern is 

stronger at higher O3 concentrations. A 

number of factors may contribute to the 

observed variability in BI scores and 

lack of a clear pattern with W126 index 

bin; among other factors, these may 

include uncertainties in assignment of 

W126 estimates and soil moisture 

categories to biosite locations, 

variability in biological response among 

the sensitive species monitored, and the 

potential role of other aspects of O3 air 

quality not captured by the W126 index. 

Thus, the dataset has limitations 

affecting associated conclusions, and 

uncertainty remains regarding the tools 

for and the appropriate metric (or 

metrics) for quantifying O3 exposures, as 

well as perhaps for quantifying soil 

moisture conditions, with regard to their 

influence on extent and/or severity of 

injury in sensitive species in natural 

areas, as quantified via BI scores (Davis 

and Orendovici, 2006, Smith et al., 

2012; Wang et al., 2012). 

(iii) Other Effects 

With regard to radiative forcing and 

subsequent climate effects associated 

with the global tropospheric abundance 

of O3, the newly available evidence in 

this review does not provide more 

detailed quantitative information 

regarding response to O3 concentrations 

at the national scale. Rather, it is noted 

that ‘‘the heterogeneous distribution of 

ozone in the troposphere complicates 

the direct attribution of spatial patterns 

of temperature change to ozone induced 

[radiative forcing]’’ and there are ‘‘ozone 

climate feedbacks that further alter the 

relationship between ozone [radiative 

forcing] and temperature (and other 

climate variables) in complex ways’’ 

(ISA, Appendix 9, section 9.3.1, p. 9– 

19). Further, ‘‘precisely quantifying the 

change in surface temperature (and 

other climate variables) due to 

tropospheric ozone changes requires 

complex climate simulations that 

include all relevant feedbacks and 

interactions’’ (ISA, section 9.3.3, p. 9– 

22). Yet, there are limitations in current 

climate modeling capabilities for O3; an 

important one is representation of 

important urban- or regional-scale 

physical and chemical processes, such 

as O3 enhancement in high-temperature 

urban situations or O3 chemistry in city 

centers where NOX is abundant. Such 

limitations impede our ability to 

quantify the impact of incremental 

changes in O3 concentrations in the U.S. 

on radiative forcing and subsequent 

climate effects. 

With regard to tree mortality, the 

evidence available in the last several 

reviews included field studies of 

pollution gradients that concluded O3

damage to be an important contributor 

to tree mortality although ‘‘several 

confounding factors such as drought, 

insect outbreak and forest management’’ 

were identified as potential contributors 

(2013 ISA, p. 9–81, section 9.4.7.1). 

Among the newly available studies, 

there is only limited experimental 

evidence that isolates the effect of O3 on 

tree mortality 195 and might be 

informative regarding O3 concentrations 

of interest in the review, and evidence 

is lacking regarding exposure conditions 

closer to those occurring under the 

current standard and any contribution 

to tree mortality. 

With regard to alteration of herbivore 

growth and reproduction, although 

‘‘[t]here are multiple studies 

demonstrating ozone effects on 

fecundity and growth in insects that 

feed on ozone-exposed vegetation’’, ‘‘no 

consistent directionality of response is 

observed across studies and 

uncertainties remain in regard to 

different plant consumption methods 

across species and the exposure 

conditions associated with particular 

severities of effects’’ (ISA, pp. ES–18). 

The evidence for alteration of plantinsect signaling draws on new research 

yielding clear evidence of O3

modification of volatile plant signaling 

compounds and behavioral responses of 

insects to the modified chemical signals 

(ISA, section IS.6.2.1). The evidence 

includes a relatively small number of 

plant species and plant-insect 

associations and is limited to short 

controlled exposures, posing limitations 

for consideration of the potential for 

associated impacts to be elicited by air 

quality conditions that meet the current 

standard (ISA, section IS.6.2.1 and 

Appendix 8, section 8.7). 

For categories of vegetation-related 

effects that were recognized in past 

reviews, other than growth and visible 

foliar injury (e.g., reduced plant 

reproduction, reduced productivity in 

terrestrial ecosystems, alteration of 

terrestrial community composition and 

alteration of below-ground 

biogeochemical cycles), the newly 

available evidence includes a variety of 

studies that quantify exposure of 

varying duration in various countries 

using a variety of metrics (ISA, 

Appendix 8, sections 8.4, 8.8 and 8.10). 


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196Across the seventeen 3-year periods from 

2000–2002 to 2016–2018, the number of monitoring 

sites with sufficient data for calculation of valid 

design values and W126 index values (across the 3- 

year design value period) ranged from a low of 992 

in 2000–2002 to a high of 1119 in 2015–2017 (PA, 

Section 4.3). 

197 In 2015 the Administrator concluded that, 

with revision of the standard level, the existing 

form and averaging time provided the control of 

cumulative seasonal exposure circumstances 

needed for the public welfare protection desired (80 

FR 65408, October 26, 2015). 

198This evaluation, performed for all U.S. 

monitoring sites with sufficient data available in the 

most recent 3-year period, 2016 to 2018, indicates 

the extent to which the three single-year W126 

index values within a 3-year period deviate from 

the average for the period. Across the full set of 

sites, regardless of W126 index magnitude (or 

whether or not the current standard is met), singleyear W126 index values differ less than 15 ppm-hrs 

from the average for the 3-year period (PA, 

Appendix 4D, Figure 4D–6). For the approximately 

850 sites meeting the current standard, over 99% 

of single-year W126 index values differ from the 3- 

year average by no more than 5 ppm-hrs, and 87% 

by no more than 2 ppm-hrs (PA, Appendix 4D, 

Figure 4D–7). 

The ISA also describes publications that 

analyze and summarize previously 

published studies. For example, a metaanalysis of reproduction studies 

categorized the reported O3 exposures 

into bins of differing magnitude, 

grouping differing concentration metrics 

and exposure durations together, and 

performed statistical analyses 

investigating associations with an O3- 

related effect (ISA, Appendix 8, section 

8.4.1). While such studies continue to 

support conclusions of O3 ecological 

hazards, they do not improve 

capabilities for characterizing the 

likelihood of such effects under patterns 

of environmental O3 concentrations 

occuring with air quality conditions that 

meet the current standard (e.g., factors 

such as variation in exposure 

assessments and limitations in response 

information preclude detailed analysis 

for such conditions), as discussed 

further in the PA. 

As at the time of the last review, 

growth impacts, most specifically as 

evaluated by RBL for tree seedlings and 

RYL for crops, remain the type of 

vegetation-related effects for which we 

have the best understanding of exposure 

conditions likely to elicit them. 

Accordingly, as was the case in the last 

review, the quantitative analyses of 

exposures occurring under air quality 

that meets the current standard, 

summarized below, are focused 

primarily on the W126 index, given its 

established relationship with growth 

effects. 

3. Overview of Air Quality and 

Exposure Information 

The air quality and exposure analyses 

developed in this review, like those in 

the last review, are of two types: (1) 

W126-based cumulative exposure 

estimates in Class I areas; and (2) 

analyses of W126-based exposures and 

their relationship with the current 

standard for all U.S. monitoring 

locations (PA, Appendix 4D). We 

recognize relatively lower uncertainty 

associated with the use of these types of 

analyses (compared to the national or 

regional-scale modeling analyses 

performed in the last review) to inform 

a characterization of cumulative O3

exposure (in terms of the W126 index) 

associated with air quality just meeting 

the current standard (IRP, section 5.2.2). 

As in the last review, the lower 

uncertainty of these air quality 

monitoring-based analyses contributes 

to their value in informing the current 

review. 

The analyses conducted in this review 

focus on design values (3-year average 

annual fourth-highest 8-hour daily 

maximum concentration, also termed 

the ‘‘4th max metric’’) and W126 index 

values (in terms of the 3-year average) 

for the recent 2016 to 2018 period and 

across the historical record back to 2000 

(PA, Section 4.3). These analyses are 

based primarily on the hourly air 

monitoring data that were reported to 

EPA from O3 monitoring sites 

nationwide and in or near Class 1 

areas.196

a. Influence of Form and Averaging 

Time of Current Standard on 

Environmental Exposure 

The findings of the quantitative 

analyses in this review of relationships 

between air quality in terms of the form 

and averaging time of the current 

standard and environmental exposures 

in terms of the W126 index are similar 

to those based on the data available 

during the last review (PA, Appendix 

4D, section 4D.2.2).197 As previously, 

the current analysis of data spanning 19 

years and including seventeen 3-year 

periods documented a positive 

nonlinear relationship between 

cumulative seasonal exposure 

(quantified using the W126 index) and 

design values (based on the form and 

averaging time of the current standard). 

In the current analysis, which revealed 

the variability in the annual W126 index 

values across a 3-year period to be 

relatively low,198 the positive nonlinear 

relationship is shown for both the 

average W126 index across the 3-year 

design value period and for W126 index 

values for individual years within the 

period (PA, Figure 4–7; Appendix 4D, 

section 4D.3.1.2). That is, W126 index 

values (in a single year or averaged 

across years) are lower at monitoring 

sites with lower design values. This is 

seen both for design values above the 

standard and across lower design 

values, indicating the effectiveness of 

the averaging time and form of the 

current standard at controlling W126- 

based cumulative exposures. 

Further, analysis of the relationship 

between trends or long-term changes in 

design value and long-term changes in 

W126 index shows there to be a 

positive, linear relationship at 

monitoring sites across the U.S. (PA, 

Appendix 4D, section 4D.3.2.3). The 

existence of this relationship means that 

a change in the design value at a 

monitoring site was generally 

accompanied by a similar change in the 

W126 index. The relationship varies 

across the NOAA climate regions, with 

the greatest change in W126 index per 

unit change in design value observed in 

the Southwest and West regions, the 

regions which had the highest W126 

index values at sites meeting the current 

standard (PA, Appendix 4D, Figures 

4D–6 and 4D–14, Table 4D–12). Thus, 

this analysis indicates that going 

forward, as design values are reduced in 

areas that are presently not meeting the 

current standard, the W126 index in 

those areas would also be expected to 

decline and the greatest improvement in 

W126 index per unit decrease in design 

value would be expected in the 

Southwest and West regions (PA, 

Appendix 4D, section 4D.3.2.3 and 

4D.5). The overall trend showing 

reductions in the W126 index 

concurrent with reductions in the 

design value metric for the current 

standard is positive whether the W126 

index is expressed in terms of the 

average across the 3-year design value 

period or the annual value (PA, 

Appendix 4D, section 4D.3.2.3). 

The available air quality information 

also indicates that the current standard’s 

form and averaging time exerts control 

on other vegetation exposures of 

potential concern, such as days with 

particularly high O3 concentrations that 

may contribute to visible foliar injury. 

The current form and averaging time, by 

their very definition, limit occurrences 

of such concentrations. This is 

demonstrated by reductions in daily 

maximum 8-hour concentrations, as 

well as in the frequency of elevated 1- 

hour concentrations, including 

concentrations at or above 100 ppb, 

with decreasing design values (PA, 

Figure 2–11, Appendix 2A, section 

2A.2). As the form and averaging time 

of the secondary standard have not 

changed since 1997, the analyses have 

been able to assess the amount of 

control exerted by these aspects of the 

standard, in combination with 

reductions in the standard level (i.e., 


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199This includes monitors sited within Class I 

areas or the closest monitoring site within 15 km 

of the area boundary. 

200Rounding conventions are described in detail 

in the PA, Appendix 4D, section 4D.2.2. 

from 0.08 ppm in 1997 to 0.075 ppm in 

2008 to 0.070 ppm in 2015), on 

cumulative seasonal exposures in terms 

of W126 index, and on the magnitude of 

short-term peak concentrations. These 

analyses indicate that the long-term 

reductions in the design values, 

presumably associated with 

implementation of the revised 

standards, were accompanied by 

reductions in W126 index, as well as in 

short-term peak concentrations. 

b. Environmental Exposures in Terms of 

W126 Index 

The analyses summarized here 

describe the nature and magnitude of 

vegetation exposures associated with 

conditions meeting the current standard 

at sites across the U.S., particularly in 

specially protected areas, such as Class 

I areas. Given the evidence indicating 

the W126 index to be strongly related to 

growth effects and its use in the E–R 

functions for tree seedling RBL (as 

summarized in section III.A.2.c above), 

exposure is quantified using the W126 

metric. These analyses include a 

particular focus on monitoring sites in 

or near Class I areas,199 in light of the 

greater public welfare significance of 

many O3 related impacts in such areas, 

as described in section III.A.2.b above, 

and consider both recent air quality 

(2016–2018) and the air quality record 

since 2000 (PA, Appendix 4D). As was 

the case in the last review, the currently 

available quantitative information 

continues to indicate appreciable 

control of seasonal W126 index-based 

cumulative exposure at all sites with air 

quality meeting the current standard. 

Among sites nationwide meeting the 

current standard in the recent period of 

2016 to 2018, there are none with a 

W126 index, based on the 3-year 

average, above 19 ppm-hrs, and just one 

with such a value above 17 ppm-hrs 

(Table 4).200 Additionally, the full 

historical dataset includes no 

occurrences of a 3-year average W126 

index above 19 ppm-hrs for sites 

meeting the current standard, and just 

eight occurrences of a W126 index 

above 17 ppm-hrs (less than 0.1% of the 

dataset), with the highest such 

occurrence just equaling 19 ppm-hrs 

(Table 4; PA, Appendix 4D, section 

4D.3.2.1). 

With regard to Class I areas, the 

updated air quality analyses include 

data from sites in or near 65 Class I 

areas. The findings for these sites, 

which are distributed across all nine 

NOAA climate regions in the contiguous 

U.S., as well as Alaska and Hawaii, 

mirror the findings for the analysis of all 

U.S. sites in the dataset. Among the 

Class I area sites meeting the current 

standard (i.e., having a design value at 

or below 70 ppb) in the most recent 

period of 2016 to 2018, there are none 

with a W126 index (averaged over the 

design value period) above 17 ppm-hrs 

(Table 4). The historical dataset 

includes just seven occurrences (all 

dating from the 2000–2010 period) of a 

Class I area site meeting the current 

standard and having a 3-year average 

W126 index above 17 ppm-hrs, and no 

such occurrences above 19 ppm-hrs 

(Table 4). 

The W126 index values at sites that 

do not meet the current standard are 

much higher, with values at such sites 

ranging as high as approximately 60 

ppm-hrs (PA, Appendix 4D, Figure 4D– 

3). Among all sites across the U.S. that 

do not meet the current standard in the 

2016 to 2018 period, more than a 

quarter have average W126 index values 

above 19 ppm-hrs and a third exceed 17 

ppm-hrs (Table 4). A similar situation 

exists for Class I area sites (Table 4). For 

example, out of the 11 Class I area sites 

with design values above 70 ppb during 

the most recent period, eight sites had 

a 3-year average W126 index above 19 

ppm-hrs (with a maximum value of 47 

ppm-hrs) and for nine, it was above 17 

ppm-hrs (Table 4; PA, Appendix 4D, 

Table 4D–17). 

TABLE 4—DISTRIBUTION OF 3-YR AVERAGE SEASONAL W126 INDEX FOR SITES IN CLASS I AREAS AND ACROSS U.S. 

THAT MEET THE CURRENT STANDARD AND FOR THOSE THAT DO NOT

3-year periods 

Number of occurrences or site-DVs A

In Class I 

areas 

Across all monitoring sites 

(urban and rural) 

Total 

W126 

(ppm-hrs) Total 

W126 

(ppm-hrs) 

>19 >17 ≤17 >19 >17 ≤17 

At sites that meet the current standard (design value at or below 70 ppb) 

2016–2018 ....................... 47 0 0 47 849 0 1 848 

All from 2000 to 2018 ...... 498 0 7 491 8,292 0 8 8,284 

At sites that exceed the current standard (design value above 70 ppb) 

2016–2018 ....................... 11 8 9 2 273 78 91 182 

All from 2000 to 2018 ...... 362 159 197 165 10,695 2,317 3,174 7,521 

A Counts presented here are drawn from the PA, Appendix D, Tables 4D–1, 4D–4, 4D–5, 4D–6, 4D–9, 4D–10 and 4D–13 through 16. 

B. Conclusions on the Secondary 

Standard 

In drawing conclusions on the 

adequacy of the current secondary 

standard, in view of the advances in 

scientific knowledge and additional 

information now available, the 

Administrator has considered the 

currently available welfare effects 

evidence and air quality and ecological 

exposure information. He additionally 

has considered the evidence base, 

information, and policy judgments that 

were the foundation of the last review, 

to the extent they remain relevant in 

light of the currently available 

information. The Administrator has 

taken into account both evidence-based 

and air quality and exposure-based 

considerations discussed in the PA, as 

well as advice from the CASAC and 


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201As recognized in the ISA, ‘‘[c]urrent 

limitations in climate modeling tools, variation 

across models, and the need for more 

comprehensive observational data on these effects 

represent sources of uncertainty in quantifying the 

precise magnitude of climate responses to ozone 

changes, particularly at regional scales’’ (ISA, 

section IS.6.2.2, Appendix 9, section 9.3.3, p. 9–22). 

These complexities impede our ability to consider 

specific O3 concentrations in the U.S. with regard 

to specific magnitudes of impact on radiative 

forcing and subsequent climate effects. 

public comments. Evidence-based 

considerations draw upon the EPA’s 

assessment and integrated synthesis of 

the scientific evidence as presented in 

the ISA, with a focus on policy-relevant 

considerations as discussed in the PA 

(summarized in sections III.B and III.D.1 

of the proposal and section III.A.2 

above). The air quality and exposurebased considerations draw from the 

results of the quantitative air quality 

analyses presented and considered in 

the PA (as summarized in section III.C 

of the proposal and section III.A.3 

above). The Administrator additionally 

considered the August 2019 decision of 

the D.C. Circuit remanding the 2015 

secondary standard for further 

justification or reconsideration. 

The consideration of the evidence and 

air quality/exposure information in the 

PA informed the Administrator’s 

proposed conclusions and judgments in 

this review, and his associated proposed 

decision. Section III.B.1 below briefly 

summarizes the basis for the 

Administrator’s proposed decision, 

drawing from section III.D of the 

proposal. Section III.B.1.a provides a 

brief overview of key aspects of the 

policy evaluations presented in the PA, 

and the advice and recommendations of 

the CASAC are summarized in III.B.1.b. 

An overview of the Administrator’s 

proposed conclusions is presented in 

section III.B.1.c. Public comments on 

the proposed decision are addressed 

below in sections III.B.2 and the 

Administrator’s conclusions and 

decision in this review regarding the 

adequacy of the current secondary 

standard and whether any revisions are 

appropriate are described in section 

III.B.3. 

1. Basis for Proposed Decision 

a. Policy-Relevant Evaluations in the PA 

The main focus of the policy-relevant 

considerations in the PA is 

consideration of the question: Does the 

currently available scientific evidenceand air quality and environmental 

exposure-based information support or 

call into question the adequacy of the 

protection afforded by the current 

secondary O3 standard? The PA 

response to this overarching question 

takes into account discussions that 

address the specific policy-relevant 

questions for this review, focusing first 

on consideration of the evidence, as 

evaluated in the ISA, including that 

newly available in this review. The PA 

also considers the quantitative 

information available in this review that 

relates O3 environmental exposures to 

vegetation responses (presented in 

Appendices 4A and 4C of the PA) and 

the air quality analyses that investigate 

relationships between air quality that 

meets the current standard and 

cumulative and peak exposures 

(presented in detail in Appendix 4D of 

the PA). The PA additionally discusses 

the key aspects of the evidence and 

exposure/risk estimates that were 

emphasized in establishing the current 

standard, and key aspects of the 2019 

court remand on the standard. In so 

doing, the PA also considers associated 

public welfare policy judgments and 

judgments about the uncertainties 

inherent in the scientific evidence and 

quantitative analyses that are integral to 

the Administrator’s consideration of 

whether the currently available 

information supports or calls into 

question the adequacy of the current 

secondary O3 standard (PA, section 3.5). 

Key policy-relevant considerations 

identified by the PA included the 

following. The new information 

available is consistent with that 

available in the last review for the 

principal effects for which the evidence 

is strongest (e.g., growth, reproduction, 

and related larger-scale effects, as well 

as, visible foliar injury) and for key 

aspects of the decision in that review. 

The currently available information 

does not provide established 

quantitative relationships and tools for 

estimating incidence and severity of 

visible foliar injury in protected areas 

across the U.S. or provide information 

linking extent and severity of injury to 

aesthetic values that might be useful for 

considering public welfare implications. 

Further, the currently available 

evidence for forested locations across 

the U.S., such as studies of USFS 

biosites, does not indicate widespread 

incidence of significant visible foliar 

injury. Additionally, the evidence 

regarding RBL and air quality in areas 

meeting the current standard does not 

appear to call into question the 

adequacy of protection. For other 

vegetation-related effects that the ISA 

newly concludes likely to be causally 

related to O3, the new information does 

not provide us an indication of the 

extent to which such effects might be 

anticipated to occur in areas that meet 

the current standard of a significance 

reasonably judged significant to public 

welfare. Thus, the PA does not find the 

current information for these newly 

identified categories to call into 

question the adequacy of the current 

standard. Similarly, the current 

information regarding O3 contribution to 

radiative forcing or effects on 

temperature, precipitation and related 

climate variables is not strengthened 

from that available in the last review, 

including with regard to uncertainties 

that limit quantitative evaluations. 

Based on such considerations, discussed 

in detail in the PA, it concludes that the 

currently available evidence and 

quantitative exposure/risk information 

does not call into question the adequacy 

of the current secondary standard such 

that it is appropriate to consider 

retaining the current standard without 

revision. In so doing, it recognized that, 

as is the case in NAAQS reviews in 

general, the extent to which the 

Administrator judges the current 

secondary O3 standard to be adequate 

will depend on a variety of factors, 

including science policy judgments and 

public welfare policy judgments. 

b. CASAC Advice in This Review 

In comments on the draft PA, the 

CASAC concurred with the PA 

conclusions, stating that it ‘‘finds, in 

agreement with the EPA, that the 

available evidence does not reasonably 

call into question the adequacy of the 

current secondary ozone standard and 

concurs that it should be retained’’ (Cox, 

2020a, p. 1). The CASAC additionally 

stated that it ‘‘commends the EPA for 

the thorough discussion and rationale 

for the secondary standard’’ (Cox, 

2020a, p. 2). The CASAC also provided 

comments particular to the 

consideration of climate and growthrelated effects. 

With regard to O3 effects on climate, 

the CASAC recommended quantitative 

uncertainty and variability analyses, 

with associated discussion (Cox, 2020a, 

p. 2 and Consensus Responses to Charge 

Questions p. 22).201 With regard to 

growth-related effects and consideration 

of the evidence in quantitative exposure 

analyses, it stated that the W126 index 

‘‘appears reasonable and scientifically 

sound,’’ ‘‘particularly [as] related to 

growth effects’’ (Cox, 2020a, Consensus 

Responses to Charge Questions p. 16). 

Additionally, with regard to the prior 

Administrator’s expression of greater 

confidence in judgments related to 

public welfare impacts based on a 

seasonal W126 index estimated by a 

three-year average and accordingly 

relying on that metric the CASAC 

expressed the view that this ‘‘appears of 

reasonable thought and scientifically 


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sound’’ (Cox, 2020a, Consensus 

Responses to Charge Questions p. 19). 

Further, the CASAC stated that ‘‘RBL 

appears to be appropriately considered 

as a surrogate for an array of adverse 

welfare effects and based on 

consideration of ecosystem services and 

potential for impact to the public as 

well as conceptual relationships 

between vegetation growth effects and 

ecosystem scale effects’’ and that it 

agrees ‘‘that biomass loss, as reported in 

RBL, is a scientifically-sound surrogate 

of a variety of adverse effects that could 

be exerted to public welfare,’’ 

concurring that this approach is not 

called into question by the current 

evidence which continues to support 

‘‘the use of tree seedling RBL as a proxy 

for the broader array of vegetation 

related effects, most particularly those 

related to growth that could be impacted 

by ozone’’ (Cox, 2020a, Consensus 

Responses to Charge Questions p. 21). 

The CASAC additionally concurred that 

the strategy of a secondary standard that 

generally limits 3-year average W126 

index values somewhat below those 

associated with a 6% RBL in the median 

species is ‘‘scientifically reasonable’’ 

and that, accordingly, a W126 index 

target value of 17 ppm-hrs for generally 

restricting cumulative exposures ‘‘is still 

effective in particularly protecting the 

public welfare in light of vegetation 

impacts from ozone’’ (Cox, 2020a, 

Consensus Responses to Charge 

Questions p. 21). 

With regard to the court’s remand of 

the 2015 secondary standard to the EPA 

for further justification or 

reconsideration (‘‘particularly in 

relation to its decision to focus on a 3- 

year average for consideration of the 

cumulative exposure, in terms of W126, 

identified as providing requisite public 

welfare protection, and its decision to 

not identify a specific level of air quality 

related to visible foliar injury’’), while 

the CASAC stated that it was not clear 

whether the draft PA had fully 

addressed this concern (Cox, 2020a, 

Consensus Responses to Charge 

Questions p. 21), it described there to be 

a solid scientific foundation for the 

current secondary standard and also 

commented on areas related to the 

remand. With regard to support in the 

information available in the current 

review for the focus on the 3-year 

average W126 index in assessing 

different patterns of air quality using 

median tree seedling RBL, in addition to 

the comments summarized above, the 

CASAC concluded, in considering the 

approach used in the last review, that 

reliance on the 3-year average and 

associated judgments in doing so 

‘‘appears of reasonable thought and 

scientifically sound’’ (Cox, 2020a, 

Consensus Responses to Charge 

Questions p. 19). Further, while 

recognizing the existence of established 

E–R functions that relate cumulative 

seasonal exposure of varying 

magnitudes to various incremental 

reductions in expected tree seedling 

growth (in terms of RBL) and in 

expected crop yield, the CASAC letter 

also noted that while decades of 

research also recognizes visible foliar 

injury as an effect of O3, ‘‘uncertainties 

continue to hamper efforts to 

quantitatively characterize the 

relationship of its occurrence and 

relative severity with ozone exposures’’ 

(Cox, 2020a, Consensus Responses to 

Charge Questions p. 20). In summary, 

the CASAC stated that the approach 

described in the draft PA to considering 

the evidence for welfare effects ‘‘is laid 

out very clearly, thoroughly discussed 

and documented, and provided a solid 

scientific underpinning for the EPA 

conclusion leaving the current 

secondary standard in place’’ (Cox, 

2020a, Consensus Responses to Charge 

Questions p. 22). 

c. Administrator’s Proposed 

Conclusions 

In reaching conclusions on the 

adequacy and appropriateness of 

protection provided by the current 

secondary standard and his proposed 

decision to retain the standard, the 

Administrator carefully considered: (1) 

The assessment of the available welfare 

effects evidence and conclusions 

contained in the ISA, with supporting 

details in the 2013 ISA and past AQCDs; 

(2) the evaluation of policy-relevant 

aspects of the evidence and quantitative 

analyses in the PA; (3) the advice and 

recommendations from the CASAC; (4) 

the August 2019 decision of the D.C. 

Circuit remanding the secondary 

standard established in the last review 

to the EPA for further justification or 

reconsideration; and (5) public 

comments that had been received up to 

that point (85 FR 49830, August 14, 

2020). In considering the evidence base 

on welfare effects associated with 

exposure to photochemical oxidants, 

including O3, in ambient air, he noted 

the newly available evidence, and the 

extent to which it alters key scientific 

conclusions from the last review. He 

additionally considered the quantitative 

analyses developed in this review, and 

their associated limitations and 

uncertainties, with regard to what they 

indicate regarding the protection 

provided by the current standard. Key 

aspects of the evidence and air quality 

and exposure information emphasized 

in establishing the current standard 

were also considered. Further, he 

considered uncertainties in the evidence 

and quantitative information as a part of 

public welfare policy judgments that are 

essential and integral to his decision on 

the adequacy of protection provided by 

the standard. In considering the CASAC 

advice, he noted the CASAC 

characterization of the ‘‘thorough 

discussion and rationale for the 

secondary standard’’ presented in the 

PA (Cox, 2020a, p. 2), and also 

considered the Committee’s overall 

agreement that the currently available 

evidence does not call into question the 

adequacy of the current standard and 

that it should be retained (Cox, 2020a, 

p. 1). 

As an initial matter, the Administrator 

recognized the continued support in the 

current evidence for O3 as the indicator 

for photochemical oxidants, noting that 

no newly available evidence has been 

identified in this review on the 

importance of photochemical oxidants 

other than O3 with regard to abundance 

in ambient air and potential for welfare 

effects. For such reasons, described with 

more specificity in the ISA and PA and 

summarized in the proposal, he 

proposed to conclude it to be 

appropriate to retain O3 as the indicator 

for the secondary NAAQS for 

photochemical oxidants and he focused 

on the current information for O3. 

With regard to the currently available 

welfare effects evidence, the 

Administrator recognized that, 

consistent with the evidence in the last 

review, the currently available evidence 

describes an array of effects on 

vegetation and related ecosystem effects 

causally or likely to be causally related 

to O3 in ambient air, as well as the 

causal relationship of tropospheric O3 in 

radiative forcing and subsequent likely 

causally related effects on temperature, 

precipitation and related climate 

variables. The evidence for three 

additional categories of effects was 

newly determined in this review to be 

sufficient to infer likely causal 

relationships with O3. However, the 

Administrator did not find the evidence 

for these effects to be informative to his 

proposed decision in review of the 

standard. For example, the 

Administrator noted the PA did not find 

the current evidence to indicate air 

quality under the current standard to 

cause increased tree mortality, and, 

accordingly, he found it appropriate to 

focus on more sensitive effects, such as 

tree seedling growth, in his review of 

the standard. With regard to the two 

insect-related categories of effects with 

new ISA determinations (alteration of 

plant-insect signaling and alteration of 


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202The E–R functions for the 11 species were 

derived in terms of a seasonal W126 index from 

experiments that varied in duration from less than 

three months to many more. Underlying the 

adjustments made to derive the functions for a 3- 

month season duration are simplifying assumptions 

of uniform W126 distribution over the exposure 

period and linear relationship between cumulative 

exposure duration and response. Averaging of 

seasonal W126 across three years, with its reduction 

of the influence of annual variations in seasonal 

W126, would give less influence to RBL estimates 

derived from such potentially variable 

representations of W126, thus providing an estimate 

of W126 considered more suitably paired with the 

E–R functions. 

insect herbivore growth and 

reproduction), the Administrator noted 

the associated uncertainties in the 

evidence that preclude a full 

understanding of key aspects of the 

effects and indicate there to be 

insufficient information to judge the 

current standard inadequate based on 

these effects as described in the 

proposal. 

In considering the evidence 

documenting tropospheric O3 as a 

greenhouse gas causally related to 

radiative forcing, and likely causally 

related to subsequent effects on 

variables such as temperature and 

precipitation, the Administrator took 

note of the limitations and uncertainties 

in the evidence base that affect 

characterization of the extent of any 

relationships between O3 concentrations 

in ambient air in the U.S. and climaterelated effects. He found this to 

preclude quantitative characterization of 

climate responses to changes in O3

concentrations in ambient air at regional 

(versus global) scales. This lack of 

quantitative tools precluding important 

analyses and the resulting uncertainty 

led the Administrator to conclude there 

to be insufficient information available 

for these effects in the current review to 

support judging the existing standard 

inadequate or to identify an appropriate 

revision. 

With regard to visible foliar injury, 

the Administrator recognized that, 

depending on its severity and spatial 

extent, as well as the location(s) and 

intended use(s), the impact of visible 

foliar injury on the physical appearance 

of plants has the potential to be 

significant to the public welfare. For 

example, depending on its extent and 

severity, its occurrence in specially 

protected natural areas may affect 

aesthetic and recreational values, such 

as the aesthetic value of scenic vistas in 

protected natural areas (e.g., national 

parks and wilderness areas). While 

recognizing there to be a paucity of 

information that relates incidence or 

severity of injury on vegetation in 

public lands to impacts on the public 

welfare (e.g., related to recreational 

services), the Administrator noted the 

USFS BI scoring scheme, and proposed 

to judge that occurrence of the lower 

categories of BI scores does not pose 

concern for the public welfare, but that 

findings of BI scores categorized as 

‘‘moderate to severe’’ injury by the 

USFS scheme would be an indication of 

visible foliar injury occurrence that, 

depending on extend and severity, may 

raise public welfare concerns. 

While recognizing that important 

uncertainties remain in the 

understanding of the O3 exposure 

conditions that will elicit visible foliar 

injury of varying severity and extent in 

natural areas, the Administrator took 

note of the evidence indicating a general 

association of injury incidence and 

severity with cumulative exposure 

metrics, including the W126 index, and 

also an influence of peak 

concentrations, as well as the 

quantitative analyses in the PA of USFS 

biosite data and of air quality 

monitoring data. In the PA analysis of 

biosite scores, the incidence of nonzero 

BI scores, and particularly of relatively 

higher scores, such as those indicative 

of ‘‘moderate to severe’’ injury in the 

USFS scheme, appear to markedly 

increase only with W126 index values 

above 25 ppm-hrs. The Administrator 

noted that such a magnitude of W126 

index (either as a 3-year average or in 

a single year) is not seen to occur at 

monitoring locations in or near Class I 

areas where the current standard is met 

(and such a W126 index, in a single 

year, has occurred only once in 19 years 

of monitoring data at sites across the 

U.S.), and that values above 17 or 19 

ppm-hrs are rare (PA, Appendix 4C, 

section 4C.3; Appendix 4D, section 

4D.3.2.3; 85 FR 49911, August 14, 2020). 

The Administrator further took note of 

the PA consideration of the USFS 

publications that identify an influence 

of peak concentrations on BI scores 

(beyond an influence of cumulative 

exposure) and the PA observation of the 

appreciable control of peak 

concentrations exerted by the form and 

averaging time of the current standard, 

as evidenced by the air quality analyses 

which document reductions in 1-hour 

daily maximum concentrations with 

declining design values. Based on these 

considerations, the Administrator 

agreed with the PA finding that the 

current standard provides control of air 

quality conditions that contribute to 

increased BI scores and to scores of a 

magnitude indicative of ‘‘moderate to 

severe’’ foliar injury. Based on his 

consideration of PA findings that areas 

that meet the current standard are 

unlikely to have BI scores reasonably 

considered to be impacts of public 

welfare significance, the Administrator 

further proposed to conclude that the 

current standard provides sufficient 

protection of natural areas, including 

particularly protected areas such as 

Class I areas, from O3 concentrations in 

the ambient air that might be expected 

to elicit visible foliar injury of such an 

incidence and severity as would 

reasonably be judged adverse to the 

public welfare. 

With regard to the welfare effects of 

reduced plant growth or yield, the 

Administrator recognized that the 

evidence base continues to indicate 

growth-related effects as sensitive 

welfare effects, with the potential for 

ecosystem-scale ramifications. While 

recognizing associated uncertainties, the 

Administrator took note of the PA 

conclusion and CASAC advice that the 

approach taken in the last review of 

using estimates of O3 impacts on tree 

seedling growth (in terms of RBL) as a 

surrogate for comparable information on 

other species and lifestages, as well as 

a proxy or surrogate for other 

vegetation-related effects, including 

larger-scale effects, continues to appear 

to be a reasonable judgment in the 

current review (85 FR 49910, August 14, 

2020; PA, section 4.5.3). These estimates 

were medians based on the established 

E–R functions for 11 tree species. In 

light of this and the lack of an 

alternative metric or approach being 

indicated by the current evidence, the 

Administrator found it appropriate to 

adopt this approach in the current 

review. 

The Administrator additionally took 

note of considerations in the PA 

regarding aspects of the derivation of 

the tree seedling E–R functions that he 

found informative to his consideration 

of issues discussed in the court’s 

remand of the 2015 secondary standard 

with respect to use of a 3-year average 

W126. In this context, the Administrator 

considered whether aspects of this 

evidence support making judgments 

using the E–R functions with W126 

index derived as an average across 

multiple years. He noted that such 

averaging would have some conceptual 

similarity to the assumptions 

underlying the adjustment made to 

develop seasonal W126 E–R functions 

from exposures that extended over 

multiple seasons (or less than a single 

season).202 The Administrator also 

noted uncertainties in regard to 

estimated RBL at lower cumulative 

exposure levels, given the more limited 

data and fewer findings of statistical 

significance supporting the functions at 

the relatively lower cumulative 

exposure levels most commonly 


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203For example, there is variability associated 

with tree growth in the natural environment (e.g., 

related to variability in plant, soil, meteorological 

and other factors), as well as variability associated 

with plant responses to O3 exposures in the natural 

environment (85 FR 49910, August 14, 2020). 

associated with the current standard 

(e.g., at or below 17 ppm-hrs). The 

Administrator additionally took note of 

the PA summary of different 

comparisons that had been performed in 

the 2013 ISA and the current ISA of RBL 

estimated via the aspen E–R function 

using either a cumulative average multiyear W126 index (2013 ISA) or a singleyear W126 index (current ISA) with RBL 

estimates derived directly from aspen 

growth information in a multi-year O3

exposure study. In this context, he 

noted the PA finding that consideration 

of these two different comparisons 

illustrate the variability inherent in the 

magnitude of growth impacts of O3 and 

in the quantitative relationship of O3

exposure and RBL,203 while also 

providing general agreement of 

predictions (based on either metric) 

with observations. In light of these 

considerations, the Administrator 

recognized that such factors as 

identified in the proposal, including the 

currently available evidence and its 

recognized limitations, variability and 

uncertainties, support a conclusion that 

it is reasonable to use a seasonal RBL 

averaged over multiple years, such as a 

3-year average (85 FR 49910, August 14, 

2020). The Administrator additionally 

took note of the CASAC advice 

reaffirming the EPA’s focus on a 3-year 

average W126, concluding such a focus 

to be reasonable and scientifically 

sound. In light of these considerations, 

the Administrator found there to be 

support for use of an average seasonal 

W126 index derived from multiple years 

(with their representation of variability 

in environmental factors), concluding 

the use of such averaging to provide an 

appropriate representation of the 

evidence and attention to considerations 

summarized above. In so doing, he 

found that a reliance on single year 

W126 estimates for reaching judgments 

with regard to magnitude of O3-related 

RBL and associated judgments of public 

welfare protection would ascribe a 

greater specificity and certainty to such 

estimates than supported by the current 

evidence. Thus, he proposed to 

conclude that it is appropriate to use a 

seasonal W126 averaged over a 3-year 

period, which is the design value period 

for the current standard, to estimate 

median RBL using the established E–R 

functions for purposes in this review of 

considering the public welfare 

protection provided by the standard. 

In reaching his proposed conclusions 

and judgments related to the use of RBL 

as a surrogate for the broad array of 

vegetation-related effects, the 

Administrator recognized a number of 

important public welfare policy 

judgments. The Administrator proposed 

to conclude that the current evidence 

base and available information 

(qualitative and quantitative) continue 

to support consideration of the potential 

for O3-related vegetation impacts in 

terms of the RBL estimates from 

established E–R functions as a 

quantitative tool within a larger 

framework of considerations pertaining 

to the public welfare significance of O3

effects. He judged the framework to 

include consideration of effects that are 

associated with effects on vegetation, 

and particularly those that conceptually 

relate to growth, and that are causally or 

likely causally related to O3 in ambient 

air, yet for which there are greater 

uncertainties affecting estimates of 

impacts on public welfare. In his 

consideration of the adequacy of 

protection provided by the current 

standard, the Administrator also noted 

judgments of the prior Administrator in 

considering the public welfare 

significance of small magnitude 

estimates of RBL and associated 

unquantified potential for larger-scale 

related effects. In light of CASAC advice 

and based on the current evidence as 

evaluated in the PA, the Administrator 

proposed to conclude that the approach 

or framework initially described with 

the 2015 decision, with its focus on 

controlling air quality such that 

cumulative exposures at or above 19 

ppm-hrs, in terms of a 3-year average 

W126 index, are isolated and rare, is 

appropriate for a secondary standard 

that provides the requisite public 

welfare protection and proposed to use 

such an approach in this review (85 FR 

49911, August 14, 2020). 

With this approach and protection 

target in mind, the Administrator 

considered the analyses of air quality at 

sites across the U.S., particularly 

including those sites in or near Class I 

areas. In virtually all design value 

periods and all locations at which the 

current standard was met (i.e., in more 

than 99.9% of such instances) across the 

19 years of the data analyzed, the 3-year 

average W126 metric was at or below 17 

ppm-hrs. Further, in all such design 

value periods and locations the 3-year 

average W126 index was at or below 19 

ppm-hrs. The Administrator 

additionally considered the protection 

provided by the current standard from 

the occurrence of O3 exposures within 

a single year with potentially damaging 

consequences, such as a significantly 

increased incidence of areas with visible 

foliar injury that might be judged 

moderate to severe. In so doing, he 

noted the PA findings that incidence of 

sites with BI scores above 15 (termed 

‘‘moderate to severe injury’’ by the 

USFS categorization scheme) markedly 

increases with W126 index estimates 

above 25 ppm-hrs, and the scarcity of 

single-year W126 index values above 25 

ppm-hrs at sites that meet the current 

standard, with just a single occurrence 

across all U.S. sites with design values 

meeting the current standard in the 19- 

year historical dataset dating back to 

2000 (PA, section 4.4 and Appendix 

4D). 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 recognized 

the control of peak 1-hour 

concentrations provided by the form 

and averaging time of the current 

standard and noted there to be less than 

one day per site with a maximum 

hourly concentration at or above 100 

ppb (PA, Appendix 2A, section 2A.2). In 

consideration of these findings, the 

Administrator proposed to judge that 

the current standard provides adequate 

protection from air quality conditions 

with the potential to be adverse to the 

public welfare (85 FR 49912, August 14, 

2020). 

In reaching his proposed decision, the 

Administrator gave primary attention to 

the principal effects of O3 as recognized 

in the current ISA, the 2013 ISA and 

past AQCDs, and for which the evidence 

is strongest (e.g., growth, reproduction, 

and related larger-scale effects, as well 

as visible foliar injury). With respect to 

the currently available information 

related to O3-related visible foliar injury, 

the Administrator considered air quality 

analyses that may be informative with 

regard to air quality conditions 

associated with appreciably increased 

incidence and severity of BI scores at 

USFS biomonitoring sites, noting that 

this information does not indicate a 

potential for public welfare impacts of 

concern under air quality conditions 

that meet the current standard. In light 

of these and other considerations 

discussed more completely in the 

proposal, and with particular attention 

to Class I and other areas afforded 

special protection, the Administrator 

proposed to conclude that the evidence 

regarding visible foliar injury and air 

quality in areas meeting the current 

standard indicates that the current 

standard provides adequate protection 

for this effect. 


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The Administrator additionally 

considered O3 effects on crop yield, 

taking note of the long-standing 

evidence, qualitative and quantitative, 

of the reducing effect of O3 on the yield 

of many crops, as summarized in the PA 

and current ISA and characterized in 

detail in past reviews (e.g., 2013 ISA, 

2006 AQCD, 1997 AQCD, 2014 WREA). 

In so doing, he recognized that not every 

effect on crop yield will be adverse to 

public welfare and in the case of crop 

yield effects in particular there are a 

number of complexities related to the 

heavy management of many crops to 

obtain a particular output for 

commercial purposes, and to other 

factors, that contribute uncertainty to 

predictions of potential O3-related 

public welfare impacts, as summarized 

in sections III.B.2 and III.D.1 of the 

proposal (PA, sections 4.5.1.3 and 

4.5.3). Thus, in judging the extent to 

which the median RYL estimated for the 

W126 index values generally occurring 

in areas meeting the current standard 

would be expected to be of public 

welfare significance, he recognized the 

potential for a much larger influence of 

extensive management of such crops, 

and also considered other factors 

recognized in the PA and proposal, 

including similarities in median 

estimates of RYL and RBL (PA, sections 

4.5.1.3 and 4.5.3). With this context, the 

information for crop yield effects did 

not lead the Administrator to identify 

this endpoint as requiring separate 

consideration or to provide a more 

appropriate focus for the standard than 

RBL, in its role as a proxy or surrogate 

for the broader array of vegetationrelated effects, as discussed above. 

Rather, in light of these considerations, 

he proposed to judge that a decision 

based on RBL as a proxy for other 

vegetation-related effects will provide 

adequate protection against crop related 

effects. In light of the current 

information and considerations 

discussed more completely in the 

proposal, the Administrator further 

proposed to conclude that the evidence 

regarding RBL, and its use as a proxy or 

surrogate for the broader array of 

vegetation-related effects, in 

combination with air quality in areas 

meeting the current standard, provide 

adequate protection for these effects (85 

FR 49912, August 14, 2020). 

In reaching his proposed conclusion 

on the current standard, the 

Administrator also considered the 

extent to which the current information 

may provide support for an alternative 

standard, proposing to conclude that the 

appreciably greater occurrence of higher 

levels of cumulative exposure, in terms 

of the W126 index, as well as an 

appreciably greater occurrence of peak 

concentrations (both hourly and 8-hour 

average concentrations) in areas that do 

not meet the current standard (e.g., areas 

meeting a higher standard level), would 

not provide the appropriate protection 

of public welfare in light of the potential 

for adverse effects on the public welfare. 

The Administrator also considered an 

alternative based solely on the W126 

metric, as was considered in the last 

review, based on such a concentrationweighted, cumulative exposure metric 

having been identified as quantifying 

exposure in a way that relates to 

reduced plant growth (ISA, Appendix 8, 

section 8.13.1). While recognizing a role 

for W126 index in quantifying exposure 

to develop estimates of RBL that the 

Administrator considers appropriately 

used as a proxy or surrogate for the 

broader array of vegetation-related 

effects, he notes that the evidence 

indicates there to be aspects of O3 air 

quality not captured by measures of 

cumulative exposure like W126 index 

that may pose a risk of harm to the 

public welfare (e.g., risk of visible foliar 

injury related to peak concentrations). 

Thus, in light of the information 

available in this review, the 

Administrator proposed to conclude 

that such an alternative standard in 

terms of a W126 index would be less 

likely to provide sufficient protection 

against such occurrences and 

accordingly would not provide the 

requisite control of aspects of air quality 

that pose risk to the public welfare. 

In summary, the Administrator 

recognized that his proposed decision 

on the public welfare protection 

afforded by the current secondary O3

standard from identified O3-related 

welfare effects, and from their potential 

to present adverse effects to the public 

welfare, is based in part on judgments 

regarding uncertainties and limitations 

in the available information, such as 

those identified above. In this context, 

he considered what the available 

evidence and quantitative information 

indicated with regard to the protection 

provided from the array of O3 welfare 

effects, finding it to not indicate the 

current standard to allow air quality 

conditions with implications of concern 

for the public welfare. He additionally 

took note of the advice from the CASAC 

in this review. Based on all of the above 

considerations, described in more detail 

in the proposal, including his 

consideration of the currently available 

evidence and quantitative exposure/risk 

information, the Administrator 

proposed to conclude that the current 

secondary standard provides the 

requisite protection against known or 

anticipated effects to the public welfare, 

and thus that the current standard 

should be retained, without revision. 

3. Comments on the Proposed Decision 

Over 50,000 individuals and 

organizations indicated their views in 

public comments on the proposed 

decision. Most of these are associated 

with mass mail campaigns or petitions. 

Approximately 40 separate submissions 

were also received from individuals, 

and 75 from organizations and groups of 

organizations; 40 elected officials also 

submitted comments. Among the 

organizations commenting were state 

and local agencies and organizations of 

state agencies, organizations of health 

professionals and scientists, 

environmental and health protection 

advocacy organizations, industry 

organizations and regulatory policyfocused organizations. The comments 

on the proposed decision to retain the 

current secondary standard are 

addressed here. Those in support of the 

proposed decision are addressed in 

section III.B.2.a and those in 

disagreement are addressed in section 

III.B.2.b. Comments related to aspects of 

the process followed in this review of 

the O3 NAAQS (described in section I.D 

above), as well as comments related to 

other legal, procedural or administrative 

issues, and those related to issues not 

germane to this review are addressed in 

the separate Response to Comments 

document. 

a. Comments in Support of Proposed 

Decision 

Of the comments supporting the 

Administrator’s proposed decision to 

retain the current secondary standard 

without revision, all generally state that 

the record supports the proposed 

decision, and note the CASAC 

conclusion that the current evidence is 

generally consistent with that available 

in the last review, and the CASAC 

conclusion that the evidence does not 

call into question the adequacy of the 

current standard and should be 

retained. In support of their views, some 

of these commenters state that new 

evidence is lacking that might call into 

question the objective for the standard 

to generally protect against cumulative 

exposures associated with median RBL 

estimates above 6%. They additionally 

state that the proposed decision 

appropriately addresses the Murray 

Energy remand issues. Further, these 

commenters conclude that the available 

evidence with regard to areas meeting 

the current standard does not call into 

question the adequacy of protection 

provided by the current standard from 


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204The term design value, defined above, is used 

in this discussion to refer to the metric for the 

standard. 

the array of vegetation effects, including 

in Class I areas. Lastly, these 

commenters find the EPA’s proposed 

judgments regarding the uncertainties 

associated with predicting responses of 

climate-related effects to changes in O3

concentrations across the U.S., as well 

as the limitations in the availability of 

tools for such purposes, to be 

appropriate and well supported. The 

EPA agrees with these comments. 

Some of these comments also express 

the view that welfare benefits of a more 

restrictive O3 standard are highly 

uncertain, while such a standard would 

likely cause socioeconomic impacts that 

the EPA should consider and find to 

outweigh the uncertain benefits. While 

as discussed in section III.B.3 below, the 

Administrator does not find a more 

stringent secondary standard requisite 

to protect the public welfare, he does 

not consider economic impacts of 

alternate standards in reaching this 

judgment. As summarized in section 

I.A. above, in setting primary and 

secondary standards that are ‘‘requisite’’ 

to protect public health and welfare, 

respectively, as provided in section 

109(b), the EPA may not consider the 

costs of implementing the standards. 

See generally Whitman v. American 

Trucking Ass’ns, 531 U.S. 457, 465–472, 

475–76 (2001). Likewise, ‘‘[a]ttainability 

and technological feasibility are not 

relevant considerations in the 

promulgation of national ambient air 

quality standards.’’ See American 

Petroleum Institute v. Costle, 665 F.2d 

1176, 1185 (D.C. Cir. 1981); accord 

Murray Energy Corp. v. EPA, 936 F.3d 

597, 623–24 (D.C. Cir. 2019). Arguments 

such as the views on socioeconomic 

impacts expressed by these commenters 

have been rejected by the courts, as 

summarized in section I.A above, 

including in the recent Murray Energy 

decision, with the reasoning that 

consideration of such impacts was 

precluded by Whitman’s holding that 

the ‘‘plain text of the Act 

unambiguously bars cost considerations 

from the NAAQS-setting process’’ 

(Murray Energy Corp. v. EPA, 936 F.3d 

at 621, quoting Whitman [531 U.S. at 

471]). 

b. Comments in Disagreement With 

Proposed Decision 

Among those submitting comments 

that disagreed with the proposed 

decision to retain the current secondary 

standard, or that raised concerns with 

the basis for the decision, most of these 

commenters expressed concerns 

regarding the process for reviewing the 

criteria and standards and state that the 

proposal must be withdrawn, and a new 

review conducted. Most of these 

commenters also disagree with the 

EPA’s proposed conclusion that the 

current standard, with its current 

averaging time and form, provides the 

requisite public welfare protection from 

known or anticipated adverse public 

welfare effects associated with the array 

of O3-related effects, and generally state 

that the standard should be revised to be 

in terms of a single-year W126 index. 

Among the claims made in describing 

the basis for their view, these 

commenters claim that EPA failed to 

describe the basis for its proposed 

conclusion; to explain why a standard 

using the W126 index was not 

proposed, consistent with 2014 advice 

from the former CASAC, and to address 

the issues raised by court remand of the 

2015 standard. Some commenters 

expressing the view that the standard 

should be revised also express the view 

that an additional standard should be 

established to protect from O3 effects on 

climate. 

With regard to the process by which 

this review has been conducted, we 

disagree with the commenters that claim 

that it is arbitrary and capricious or that 

it does not comport with legislative 

requirements. The review process, 

summarized in section I.D, 

implemented a number of features, 

some of which have been employed in 

past reviews and others which have not, 

and several which represent efficiencies 

in consideration of the statutorily 

required time frame for completion of 

the review. The comments received that 

raise concerns regarding specific aspects 

of the process are addressed in the 

separate Response to Comments 

document. As indicated there, the EPA 

disagrees with these comments. The 

EPA finds the review to have been 

lawfully conducted and the process 

reasonably explained. Accordingly, the 

EPA is not withdrawing the proposal 

and restarting the review. 

(i) Metric for Standard 

The premise of many of the comments 

expressing disagreement with the 

proposed decision is that the secondary 

standard must be a ‘‘biologically 

relevant’’ metric, which they identify to 

be the W126 index. Similarly, some 

commenters assert that EPA cannot 

lawfully or rationally set a secondary 

standard using the metric of the current 

standard, which is also the metric used 

for the primary standard, claiming that 

this contradicts EPA’s recognition of the 

relevance of the W126 index as an 

exposure metric for assessing the level 

of protection from welfare effects, such 

as RBL. These commenters also claim 

that this approach arbitrarily disregards 

the recommendations of the prior 

CASAC, and, in doing so, imply that 

EPA must establish a W126 based 

standard because of prior CASAC 

advice. 

We disagree with these commenters. 

The Clean Air Act includes no 

requirements with respect to what 

metrics should be used to establish the 

secondary standards. As is clear from 

the text of Section 109(b)(2) of the CAA, 

the critical test for NAAQS is whether 

they achieve the requisite protection. In 

so doing, it is not uncommon for the 

form and averaging time of a NAAQS to 

differ from exposure metrics most 

relevant to assessment of particular 

effects. These exposure metrics are 

based on the health or welfare effects 

evidence for the specific pollutant and 

commonly, in assessments for primary 

standards, on established exposureresponse relationships or health-based 

benchmarks (doses or exposures of 

concern) for effects associated with 

specific exposure circumstances. 

Evidence for this is found in the 

common use, in assessments conducted 

for NAAQS reviews, of exposure metrics 

that differ in a variety of ways from the 

ambient air concentration metrics of 

those standards.204 Across reviews for 

the various NAAQS pollutants over the 

years, the EPA has used a variety of 

exposure metrics to evaluate the 

protection afforded by the standards 

(see examples identified at 80 FR 

65399–65400, October 26, 2015). 

Further, a single standard may provide 

protection from multiple different 

effects, the protection for which may be 

assessed using different exposure 

metrics. One standard may also provide 

protection from multiple pathways of 

exposure. Both the primary and 

secondary Pb standards provide 

examples of this. While these standards 

are expressed in terms of the 

concentration of lead in particles 

suspended in air, different exposure 

metrics have been used to evaluate the 

protection provided by the Pb 

standards. The salient exposure metric 

for assessment of protection provided by 

the primary standard has been blood Pb, 

while for the secondary standard, 

concentrations of lead in soil, surface 

water and sediment are pertinent, and 

have been evaluated to assess the 

potential for welfare effects related to 

lead deposition from air (73 FR 67009, 

November 12, 2008). In somewhat 

similar manner, the exposure metric 

used to evaluate health impacts in the 

primary sulfur dioxide standard review 

includes a 5-minute exposure 


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205 In fact, the D.C. Circuit has upheld secondary 

NAAQS that were identical to the corresponding 

primary standard for the pollutant (e.g., ATA III, 

283 F.3d at 375, 380 [D.C. Cir. 2002, upholding 

secondary standards for PM2.5 and O3 that were 

identical to primary standards]). 

206See CAA sections 307(d)(3) and 307(d)(6)(A); 

see also Mississippi v. EPA, 744 F.3d 1334, 1354 

(D.C. Cir. 2013) (‘‘Although EPA is not bound by 

CASAC’s recommendations, it must fully explain 

its reasons for any departure from them’’); id. at 

1358 (noting CASAC, like EPA, exercises both 

scientific judgment and public health policy 

judgment). Selection of a metric for the standard is 

a public health or public welfare policy judgment 

about what standards will control air quality to the 

extent judged requisite to protect from adverse 

public health or welfare effects. 

207This analysis focuses on the relationship 

between changes (at each monitoring site) in the 3- 

year design value across the 17 design value periods 

from 2000–2002 to 2016–2018 and changes in the 

W126 index over the same period (PA, Appendix 

4D, section 4D.3.2.3). 

concentration. In contrast, the healthbased standard for this pollutant is the 

average across three years of the 99th 

percentile of 1-hour daily maximum 

concentration of sulfur dioxide in 

ambient air (75 FR 35520, June 22, 2010; 

84 FR 9866, March 18, 2019). 

We disagree with the comment that a 

secondary standard with the same form 

and averaging time as the primary 

standard does not comply with the 

CAA. The CAA does not require that the 

secondary standard be established in a 

specific form or averaging time. The 

Act, at Section 109(b)(2), provides only 

that any secondary NAAQS ‘‘shall 

specify a level of air quality the 

attainment and maintenance of which in 

the judgment of the Administrator, 

based on [the air quality] criteria, is 

requisite to protect the public welfare 

from any known or anticipated adverse 

effects associated with the presence of 

such air pollutant in the ambient 

air. . . . [S]econdary standards may be 

revised in the same manner as 

promulgated.’’ The EPA interprets this 

provision to leave it considerable 

discretion to determine whether a 

particular form and averaging time are 

appropriate, in combination with the 

other aspects of the standard (level and 

indicator), for specifying the air quality 

that provides the requisite protection, 

and to determine whether, once a 

standard has been established in a 

particular form, that form must be 

revised. Moreover, nothing in the Act or 

the relevant case law precludes the EPA 

from establishing a secondary standard 

equivalent to the primary standard in 

some or all respects, as long as the 

Agency has engaged in reasoned 

decision-making.205

Thus, we note that particular metrics 

may logically, reasonably, and for 

technically or scientifically sound 

reasons, be used in assessing exposures 

of concern or characterizing risk. The 

purpose, and use, of exposure metrics is 

different from the purpose, and use, of 

metrics for the standard, and as a result 

the metrics may differ from one use to 

the other. Exposure metrics are used to 

assess the likely occurrence and/or 

frequency and extent of effects under 

different air quality conditions, while 

the air quality standards are intended to 

control air quality to the extent requisite 

to protect from the occurrence of public 

health or welfare effects judged to be 

adverse. In this review of the O3

secondary standard, the EPA agrees that 

based on evidence summarized in 

section III.A above, metrics such as the 

W126 index are appropriate for 

assessing exposures of concern for 

vegetation, characterizing risk to public 

welfare, and evaluating what air quality 

conditions might provide the 

appropriate degree of public welfare 

protection. We disagree, however, that 

the secondary standard must be 

established using those same metrics. 

Rather, when the Administrator judges 

that a standard using a different metric 

provides the requisite protection, in 

light of his consideration of all the 

elements of the standard together, he 

may reasonably establish or retain such 

a standard. 

With regard to the commenter’s 

emphasis on recommendations from the 

CASAC on the form of the secondary 

standard, the EPA generally agrees with 

the importance of giving such 

recommendations careful consideration. 

However, it is not necessary for EPA to 

address in this review each statement a 

prior CASAC made in a prior review. In 

addition, if a recommendation of a prior 

CASAC is raised in a subsequent review 

(e.g., in public comments or as a focus 

in court decision being addressed), it is 

reasonable for the Agency to consider it 

in the context both of the current review 

and of consideration of all the other 

now available scientific, technical and 

policy-relevant information, including 

advice from the current CASAC. We 

note that in this review of the secondary 

standard, the current CASAC, based on 

its review of the information and 

analyses available in the current review, 

concurs with retention of a secondary 

standard with a metric that differs from 

commonly used vegetation exposure 

metrics, such as the W126 index (Cox, 

2020a). We further note, under the 

relevant provisions of the CAA and case 

law interpreting them, the 

Administrator is never bound by the 

CASAC’s conclusions but rather may 

depart from them when he has provided 

an explanation of the reasons for such 

differences.206 While the EPA does not 

interpret the requirements of CAA 

sections 307(d)(3) and 307(d)(6)(A) to 

apply to every recommendation it has 

received from a prior CASAC, even 

assuming there are some circumstances 

in which EPA were required to comply 

with the requirements of CAA section 

307(d)(3) and (6)(A) with respect to 

particular recommendations from a 

prior CASAC, these same principles 

would apply. Thus, the Administrator 

would not be bound to follow those 

recommendations, but rather could 

depart from them when he had 

explained his reasons for doing so. 

Accordingly, in reaching conclusions on 

the revised secondary standard in this 

review, the Administrator has given 

careful consideration to the current 

CASAC advice in this review and to 

issues raised by the prior CASAC that 

are subject to the Murray Energy 

remand. When he has differed from 

those CASAC recommendations, the 

reasons and judgments that led to a 

different conclusion are explained, as 

summarized in this section and in 

section III.B.3 below. Consistent with 

his consideration of all significant 

issues raised in public comments, the 

Administrator has also considered the 

issues raised by commenters that have 

also been raised by a prior CASAC, 

together with the Agency’s responses to 

those comments, as summarized in this 

section and in section III.B.3 below. 

The current air quality analyses 

demonstrate the successfulness of the 

current form and averaging time in 

controlling cumulative exposures, in 

terms of W126. These extensive air 

quality analyses, presented in the PA 

and summarized in the proposal, are 

based on data collected across the U.S. 

over a time span of nearly 20 years (85 

FR 49892–49895, 49903–49904, August 

14, 2020). One of these analyses 

describes the positive, linear 

relationship between long-term changes 

in the O3 design value and long-term 

changes in the W126 index at 

monitoring sites across the U.S.207 This 

positive, linear relationship exists for 

the O3 design value with both a 3-year 

average and single-year W126 index 

(PA, Appendix 4D, Figure 4D–11). The 

existence of this relationship means that 

a change (e.g., reduction) in the design 

value at a monitoring site was generally 

accompanied by a similar change (e.g., 

reduction) in the W126 index, both in 

the 3-year average and in the single-year 

values. As the form and averaging time 

of the secondary standard have not 

changed since 1997, the analyses 

performed have been able to assess the 

amount of control exerted by these 

aspects of the standard, in combination 


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208The prior CASAC comments on this matter 

were in the context of its recommendation for a 

secondary standard in the form of a single-year 

W126 index, which as discussed below would be 

expected to provide relatively less control against 

high-concentration years compared with the current 

secondary standard. The prior CASAC additionally 

commented that it ‘‘favor[ed] a single-year period’’ 

which it stated would ‘‘provide more protection for 

annual crops and for the anticipated cumulative 

effects on perennial species.’’ The prior CASAC 

continued on to state that if the Administrator 

preferred, instead, to establish a secondary standard 

as a 3-year average W126 index, as a policy matter, 

the level should be revised downward (Frey, 2014b, 

p. iii). The prior CASAC stated the purpose for this 

step would be to be protecting ‘‘against single 

unusually damaging years that will be obscured in 

the average’’ (Frey, 2014b, p. 13). 

209The Agency intends this decision, associated 

analyses conducted for this review in consideration 

of issues raised by the court’s remand, and the 

discussions herein to constitute its response to the 

Murray Energy remand on this issue. 

with reductions in the level (i.e., from 

80 ppb in 1997 to 75 ppb in 2008 to 70 

ppb in 2015) on cumulative seasonal 

exposures in terms of W126 index. The 

analyses have found that the reductions 

in design value, presumably associated 

with implementation of the revised 

standards, have been accompanied by 

reductions in cumulative seasonal 

exposures in terms of W126 index (PA, 

section 4.4.1). Further, while the 

formulation of the W126 metric gives 

more weight to higher concentrations 

(in the context of its focus on 

cumulative exposure), it is much less 

effective at curbing elevated hourly 

concentrations (that can be important in 

altering plant growth and yield) than the 

current design value metric, as 

discussed in section III.B.2.b(ii) below. 

In expressing the view that the 

secondary standard should be in terms 

of a W126 index, some commenters 

describe the EPA’s statements regarding 

the protection from cumulative 

exposures that is provided by the 

current form and averaging time to be 

‘‘incidental’’ and ‘‘happenstance,’’ 

which leads them to claim the EPA’s 

findings of protection to be arbitrary. In 

support of their view, the commenters 

quote a statement of the prior CASAC 

cautioning against interpreting the 

W126 index levels in the W126 index 

scenario created for the 2014 WREA, by 

first adjusting air quality to meet the 

then-existing fourth maximum standard 

of 75 ppb, to be representative of 

implementation of a W126 index 

standard. The issue described by the 

prior CASAC related to the application 

to all monitoring sites of the precursor 

reduction necessary for the highest 

monitoring site in a region to just meet 

the scenario target; the prior CASAC’s 

concern was that actual implementation 

of the target as a standard would not 

necessarily yield such reductions. We 

disagree with the commenters that this 

is relevant to the air quality analysis in 

the current review, in which we simply 

observe the W126 index values that 

exist in reality at sites that have met the 

existing secondary standard. Contrary to 

the context for the prior CASAC’s 

caution, the analysis in the current 

review is not showing the results of a 

theoretical scenario created by modeling 

theoretical precursor reductions 

estimated for attaining a particular 

W126-based or fourth high standard. 

Rather, we are observing what the 

W126-based cumulative exposure is at 

ambient air monitoring sites that meet 

the current secondary standard. Thus, 

regardless of the labels assigned by the 

commenter to the findings of the air 

quality analyses in the current review, 

these analyses clearly document the 

success of the existing standard (with its 

fourth maximum form and 8-hour 

averaging time) in controlling exposure 

in terms of the W126 index. 

Thus, in light of this evidence, the 

EPA disagrees with the commenters 

who express the view that to provide 

the requisite protection the secondary 

standard must be a W126 index 

standard. In assessing the air quality 

necessary to provide the requisite 

degree of protection, particularly for 

growth and related vegetation and 

ecosystem effects, the Agency has 

recognized the importance of 

cumulative exposures, but also the 

significance of higher peak exposures 

(as summarized in section III.B.2.b(ii) 

below) that can be characterized 

through other metrics (e.g., N100). As a 

result, in assessing the protection 

provided by the current standard, the 

Agency has focused on the W126 index, 

expressed in terms of the average of 

three consecutive years (in light of 

considerations discussed below), as a 

metric for cumulative exposure, but has 

also considered the frequency and 

magnitude of elevated single-year W126 

index values, and of elevated hourly O3

concentrations (as discussed further 

below). 

(ii) Protection Against Unusually 

Damaging Years 

In the last review, the Administrator 

relied on the 70 ppb standard (as the 

fourth highest daily maximum 8-hour 

average concentration averaged over 

three consecutive years) to achieve a 

level of air quality that would restrict 

cumulative seasonal exposures to 17 

ppm-hrs or lower, in terms of a 3-year 

average W126 value, in nearly all 

instances. The Murray Energy court 

found in relevant part that the EPA had 

not explained why that level of 

protection was requisite, in light of 

certain comments from the CASAC in 

2014 recommending that EPA base a 

standard on a one-year W126 metric, in 

part to limit exposures in single 

unusually damaging years.208 In 

responding to the remand,209 we are 

explaining in this document that the 

EPA is looking to prevent the damaging 

effects of O3 on tree growth as a proxy 

for public welfare effects related to the 

broad array of O3’s vegetation-related 

effects conceptually related to growth 

effects, including ecosystem-level 

effects (as discussed in section 

III.B.2.b(v) below). In this review, in 

assessing the air quality requisite to 

prevent adverse effects on public 

welfare from these effects, the EPA is 

not relying solely on maintaining a 

particular 3-year W126 value. Rather, 

we are considering air quality patterns 

that are associated with meeting the 

current standard, including control of 

peak hourly concentrations, and the 

exposures that would be expected under 

the current standard, including in terms 

of W126 values, particularly those 

averaged over a 3-year period. The EPA 

is explaining the grounds for our 

conclusion that use of the 3-year average 

W126 index is a reasonable basis for 

assessing protection from RBL, but also 

that the Administrator is using other 

exposure information in reaching the 

conclusion that retention of the existing 

standard (with its form and averaging 

time of the fourth highest annual daily 

maximum 8-hour average concentration, 

averaged over three years) provides the 

needed protection of RBL, including 

from what the Murray Energy court 

noted that the prior CASAC termed 

‘‘unusually damaging years.’’ 

In disagreeing with the EPA’s 

proposed decision, some commenters 

object to the EPA’s use of a 3-year 

average W126 index in assessing 

different patterns of air quality using 

median tree seedling RBL as a surrogate 

for an array of vegetation-related effects, 

particularly those related to growth and 

productivity. In so doing, these 

commenters variously claim that this 

use of a 3-year average W126 index 

(rather than a single-year W126 index) 

is inconsistent with recommendations 

from the prior CASAC, does not address 

the court remand on this point, and that 

it is inadequate to protect vegetation 

from high years or years with hourly O3

concentrations that can be most 

important in eliciting adverse effects. 

The EPA disagrees with these 

commenters and notes that it has taken 

such concerns, as well as the court’s 

remand, into account in the final 

decision. In evaluating the air quality 


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210Additionally, as described in section III.B.1.c 

above and III.B.2.b(v) below, the EPA’s 

identification of 17 ppm-hrs for a target W126 index 

of 17 ppm-hrs (e.g., versus 18 ppm-hrs) was in 

consideration of the prior CASAC recommendation 

for considering a ‘‘lower’’ level ppm-hrs. 

211For example, the growth impact estimate for 

year 1 used the W126 index for year 1; the estimate 

for year 2 used the average of W126 index in year 

1 and W126 index in year 2; the estimate for year 

3 used the average of W126 index in years 1, 2 and 

3; and so on. 

212One finding of this evaluation was that ‘‘the 

function based on one year of growth was shown 

to be applicable to subsequent years’’ (2013 ISA, p. 

9–135). 

213Based on information drawn from Figure 8–17 

in the 2020 ISA, the correlation metric (r2) for the 

percent difference (estimated vs observed biomass) 

and year of growth can be estimated to be 

approximately 0.7, while using values reported in 

Table 9–15 of the 2013 ISA (which are plotted in 

Figure 9–20), the r2 for predicted O3 impact versus 

observed impact is 0.99 and for the percent 

difference versus year is approximately 0.85. 

conditions allowed by the current 

standard, the EPA has focused on the 

W126 metric averaged over 3 years as 

the most appropriate measure of 

cumulative exposure for consideration 

of adverse effects on public welfare, but 

EPA has also considered other relevant 

exposure information, including higher 

exposures that might be expected to 

occur in an ‘‘unusually damaging year.’’ 

The Administrator’s decision on the 

adequacy of protection provided by the 

current standards is based on the full 

scope of exposure information he has 

considered. 

The EPA concludes that the 3-year 

average W126 index is a reasonable 

metric for assessing the level of 

protection provided by the current 

standard from cumulative seasonal 

exposures related to RBL, while noting 

that our evaluation for the protection 

provided by the current standard has 

also been informed by our consideration 

of other metrics (as described further 

below). In reaching this conclusion, we 

have taken into account the available 

evidence base and air quality analyses, 

with a focus on two types of 

considerations, as well as consideration 

of the context for RBL as a proxy for an 

array of other vegetation effects 

(discussed in section III.B.2.b(v) below). 

The first of the two consideration types 

concerns the E–R functions and their 

use with a 3-year average W126 index, 

and the second concerns the control by 

the W126 index metric of exposures that 

might be termed ‘‘unusually damaging.’’ 

With regard to the first, we find our use 

of the 3-year average W126 index 

appropriate in light of the approach 

used in deriving the E–R functions from 

the underlying data (from exposures of 

varying durations, including of multiple 

years), and the evidence available for 

evaluating these functions across 

multiyear exposures.210 Additionally, 

with regard to the second consideration, 

we recognize limitations associated with 

a reliance solely on W126 index as a 

metric to control exposures that might 

be termed ‘‘unusually damaging.’’ For 

example, two different air quality 

patterns for which the associated W126 

index is the same may have very 

different incidence of elevated O3

concentrations, and accordingly pose 

different risks to vegetation. As 

discussed below, however, the 

occurrence of such concentrations (and 

any associated risk of damage) are 

controlled by the current secondary 

standard. 

In light of this evidence, and 

recognizing the role for both peak and 

cumulative exposures in eliciting 

growth and related vegetation and 

ecosystem effects, the EPA concludes 

that focusing solely on W126 index 

(either in terms of a single year or 3-year 

average) in considering the public 

welfare protection provided by the 

current standard would not be 

considering all the relevant scientific 

information. To the extent that the prior 

CASAC advised that the EPA should 

focus solely on single-year W126 index 

values in evaluating the protection 

provided by the secondary standard, the 

EPA disagrees that this would provide 

the needed protection, for the reasons 

explained more fully below. In this 

regard, we additionally note that the 

current CASAC concluded that focusing 

on three-year average W126 index 

values in considering the public welfare 

protection offered by the secondary 

standard ‘‘appears of reasonable thought 

and scientifically sound’’ (Cox, 2020a, 

p. 19). 

With regard to the established tree 

seedling E–R functions, we note there 

are aspects of the datasets and 

methodology on which the E–R 

functions are based which provide 

support for a 3-year average approach. 

As summarized in section III.A.2.c(i) 

above, in deriving the E–R functions 

from studies of durations that varied 

from shorter than 90 days to multiple 

years or growing seasons, the results 

were normalized to the duration of a 

single 90-day seasonal period (PA, 

section 4.5.1.2 and Appendix 4A, pp. 

4A–28 to 4A–29 and footnote 17). 

Inherent in this approach is an 

assumption that the growth impacts 

relate generally to the cumulative O3

exposure across the multiple growing 

seasons, i.e., with little additional 

influence related to any year to year 

differences in the exposures. As 

discussed in the proposal, the use of a 

3-year average in assessing RBL using 

the established tree seedling E–R 

functions is compatible with the 

normalization step taken to derive 

functions for a seasonal 90-day period 

from the underlying data with its 

varying exposure durations (85 FR 

49901, August 14, 2020). 

This concept of the importance of 

cumulative multiyear O3 exposure to 

multiyear impacts, and its 

representation as an average, is also 

reflected in the evaluation of the 

predicted growth impacts compared to 

observations from the multiyear study of 

O3 impacts on aspen by King et al 

(2005), as presented in the 2013 and 

2020 ISAs and summarized in the PA 

(PA, Section 4.5.1.2). The ISAs 

considered the 6-year experimental 

dataset of O3 exposures and aspen 

growth effects with regard to 

correspondence of E–R function 

predictions with study observations 

(2020 ISA, Appendix 8, section 8.13.2 

and Figure 8–17; 2013 ISA, section 

9.6.3.2, Table 9–15, Figure 9–20). The 

analysis in the 2013 ISA compared 

observed reductions in growth for each 

of the six years to those predicted by 

applying the established E–R function 

for Aspen to cumulative multi-year 

average W126 index values (2013 ISA, 

section 9.6.3.2).211 212 The evaluation in 

the 2020 ISA applied the E–R functions 

to the single-year W126 index for each 

year rather than the cumulative multiyear W126 (2020 ISA, Appendix 8, 

Figure 8–17), with this approach 

indicating a somewhat less tight fit to 

the experimental observations (2020 

ISA, Appendix 8, p. 8–192),213 Both 

ISAs reach similar conclusions 

regarding general support for the E–R 

functions across a multiyear study of 

trees in naturalistic settings (ISA, 

Appendix 8, section 8.13.3 and p. 8– 

192; 2013 ISA, p. 9–135). 

Based on all of the above 

considerations, the EPA finds the 

evidence to support a 3-year average 

W126 index for use in assessing the 

level of protection provided by the 

current standard from cumulative 

seasonal exposures related to RBL of 

concern based on the established E–R 

functions. As discussed in section 

III.B.3 below, the EPA additionally finds 

the 3-year average metric to be 

reasonable in the context of the use of 

RBL as a proxy to represent an array of 

vegetation-related effects. In the 

discussion immediately below, we 

additionally and specifically address the 

issue of protection from ‘‘unusually 

damaging years’’ of vegetation exposure. 

With regard to the comment that cited 

a recommendation from the prior 

CASAC on protection of vegetation 


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214For example, many of the experimental 

exposures of elevated O3 on which the established 

E–R functions for the 11 tree seedling species are 

based, had hundreds of hours of O3 concentrations 

above 100 ppb, far more than are common in 

(unadjusted) ambient air, including in areas that 

meet the current standard (Lefohn et al. 1997; PA, 

Appendix 2A, section 2A.2; Wells, 2020). Similarly, 

the experimental exposures in studies supporting 

some of the established E–R functions for 10 crop 

species also include many hours with hourly O3

concentrations at or above 100 ppb (Lefohn and 

Foley, 1992). 

215The value of 100 ppb is used here as it has 

been in some studies focused on O3 effects on 

vegetation, simply as an indicator of elevated or 

peak hourly O3 concentrations (e.g., Lefohn et al. 

1997, Smith, 2012; Davis and Orendovici, 2006; 

Kohut, 2007a). Values of 95 ppb and 110 ppb have 

also been considered in this way (2013 ISA, section 

9.5.3.1). 

against ‘‘unusually damaging years’’ and 

the part of the court remand referencing 

that CASAC recommendation, we have 

considered the CASAC discussion using 

this term, in the context of the court 

remand. Use of this term by the prior 

CASAC occurs in the 2014 letter on the 

second draft PA in the 2015 review 

(Frey, 2014b). Most prominently, the 

prior 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). The prior CASAC 

additionally provided advice with 

regard to surrogate metrics for judging 

such ‘‘damage,’’ e.g., use of RBL for 

judging effects on trees and their related 

functions and ecosystem services, use of 

crop RYL for judging public welfare 

effects of crop effects (Frey, 2014b, p. 

10). We also note that the context for the 

prior CASAC’s use of the phrase 

‘‘unusually damaging years’’ 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 highconcentration years, rather than in the 

context of the current secondary 

standard and its fourth highest daily 

maximum 8-hour metric. 

While the prior CASAC did not 

provide any specificity or details as to 

the exposure circumstances and damage 

intended by its more general phrasing, 

nor did it cite to specific evidence in 

scientific publications, we agree with 

the general concept that particular air 

quality patterns in a year may pose 

particular risk of vegetation damage, in 

terms of both or either growth-related 

effects or visible foliar injury (discussed 

in section III.B.2(iii) below). Across past 

O3 NAAQS reviews, the air quality 

criteria for vegetation effects have 

emphasized the risk posed to vegetation 

from higher hourly average O3

concentrations (e.g., ‘‘[h]igher 

concentrations appear to be more 

important than lower concentrations in 

eliciting a response’’ [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]). In fact, the EPA has recognized 

the W126 index for E–R models for 

growth and yield (in the current and 

prior ISA and prior AQCD) in part due 

to its preferential weighting of higher 

concentrations (ISA, p. 8–130). 

We note, however, that while the 

W126 index weights higher hourly 

concentrations, it cannot, given its 

definition as an index that sums three 

months of weighted hourly 

concentrations into a single value, 

always differentiate between air quality 

patterns with high peak concentrations 

and those without such concentrations. 

This is illustrated by the following two 

hypothetical examples. In the first 

example, two air quality monitors have 

a similar pattern of generally lower 

average hourly concentrations, but differ 

in the occurrence of higher 

concentrations (e.g., hourly 

concentrations at or above 100 ppb). 

The W126 index describing these two 

monitors would differ. In the second 

example, one monitor has appreciably 

more hourly concentrations above 100 

ppb compared to a second monitor; but 

the second monitor has higher average 

hourly concentrations than the first. In 

the second example, the two monitors 

may have the same W126 index, even 

though the air quality patterns observed 

at those monitors are quite different, 

particularly with regard to the higher 

concentrations, which have been 

recognized to be important in eliciting 

responses (as noted above). 

Thus, the EPA disagrees with a view 

implied by many of the commenters 

(who object to the EPA’s proposed 

decision) that the sole focus for 

assessing public welfare protection, 

related to vegetation damage, and air 

quality control provided by the 

secondary standard should be on the 

W126 index. This view ignores both the 

limitations of the W126 index itself in 

distinguishing among different patterns 

of hourly O3 concentrations and the fact 

that the current secondary standard has, 

by virtue of its form, a metric that does. 

With regard to these limitations of the 

W126 index, as described above, two 

different locations or years may have 

different patterns of hourly 

concentrations but the same W126 

index value. This was recognized in the 

study by Lefohn et al. (1997), which 

observed the appreciable differences 

between the prevalence of hourly 

concentrations at or above 100 ppb in 

exposures on which the E–R functions 

are based and those common in ambient 

air.214

This potential for such a difference in 

peak concentrations between two 

different locations with the same W126 

index was noted by one commenter who 

objected to the EPA’s focus on a 3-year 

average W126 index in assessing RBL 

and advocated use of a single-year W126 

index. This commenter stated that the 

same 3-year average could be 

maintained in two different locations in 

which the annual exposure may differ 

due to ‘‘variability of the higher hourly 

average concentrations associated with 

vegetation effects.’’ In emphasizing the 

higher hourly average concentrations 

associated with effects, the commenter 

cited the support provided by the 

evidence for the San Bernardino 

National Forest, described in the 2013 

ISA and prior CDs (e.g., 2013 ISA, 

section 9.5.3.1). We agree with this 

point and additionally note that this 

point also applies to two locations with 

the same single-year W126 index, given 

its definition (as noted above). 

Given the mathematics inherent in 

calculation of the W126 index, while 

the metric is useful for comparing 

cumulative exposures, it can conceal 

peak concentrations that can be of 

concern (as described above). More 

specifically, one year or location could 

have few, or even no, hourly 

concentrations above 100 ppb 215 and 

the second could have many such 

concentrations; yet each of the two years 

or locations could have the identical 

W126 index (e.g., equal to 25 or 17 or 

10 ppm-hrs, or some other value). 

However, as can be seen by the 

historical ambient air monitoring 

dataset of O3 concentrations, the form of 

the current standard limits the 

occurrence of such elevated 

concentrations, e.g., at or above 100 ppb 

(PA, Appendix 2A, section 2A.2; Wells, 

2020). 

Analyses of hourly concentrations for 

different air quality scenarios developed 

in consideration of the remand and such 

comments (and documented in a 

technical memorandum to the docket) 

show the form and averaging time of the 

existing standard to be much more 

effective than the W126 index in 

limiting the number of hours with O3

concentrations at or above 100 ppb 

(N100) and in limiting the number of 

days with any such hours (Wells, 


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216The impact of the current form of the standard 

on occurrence of elevated hourly concentrations is 

also seen by a recent study submitted with 

comments (Neufeld et al., 2019). For example, the 

frequency of episodes defined by three consecutive 

hours at or above 60 ppb, as well as the magnitude 

of W126 index, has appreciably declined at 

locations within and immediately adjacent to the 

Smoky Mountains National Park, and the periods of 

respite from elevated episodes has appreciably 

increased (Neufeld et al., 2019). This was found for 

low elevation sites, and also high elevation Park 

sites, which generally have higher levels (Neufeld 

et al., 2019). 

217 In these analyses the N100 and D100 metrics 

are based on counts of hourly O3 concentrations at 

or above 100 ppb across the consecutive 3-month 

period with the highest total (Wells, 2020). The 

metric D100 is the count of days with an hour at 

or above 100 ppb. 

218We note that we are not intending to ascribe 

specific significance to five days with an hour at or 

above 100 ppb or ten hours such, per se. Rather, 

these are used simply as reference points to 

facilitate comparison to illustrate the point that 

such high concentrations, which based on 

toxicological principles, pose greater risk to biota 

than lower concentrations (e.g., ‘‘[h]igher 

concentrations appear to be more important than 

lower concentrations in eliciting a response’’ [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]). 

219We also note the higher percentages of sites 

with an N100 above five among sites meeting a 

single-year W126 index of 7 ppm-hrs than sites 

meeting the current standard (Wells, 2020, Table 2). 

Sites with an annual W126 index of 7 ppm-hrs also 

record a greater percentage of sites with more than 

two days with an hour at or above 100 ppb (Wells, 

2020, Table 2). 

220The first paragraph, conveying scientific 

judgment provides a range of levels for a revised 

standard (Frey, 2014b, p. iii). The second begins by 

noting that the ‘‘scientific judgment’’ regarding a 

revised secondary standard, in prior paragraph, are 

based on the scientific evidence. Midway through 

that paragraph, as shown below, the prior CASAC 

turns to its policy recommendations, in which it 

relates various W126 index values in different ways 

to various effect categories, including crop yield 

loss, foliar injury, and relative biomass loss (Frey, 

2014b, p. iii). Given that the prior CASAC 

recommended multiple times in this letter a 

standard level range that extends higher than 10 

ppm-hrs (to 15 ppm-hrs), the fact that the sentence 

regarding visible foliar injury in the version of this 

second paragraph that appears within the 

attachment to the letter begins with the phrase 

‘‘[b]ased on its scientific judgment’’ cannot 

reasonably be interpreted to be overriding the 

Committee’s scientific advice on the standard. 

Rather, the prior CASAC appears to be implying 

that to the extent the Administrator judges, as a 

matter of public welfare policy, it important to 

consider such a focus on foliar injury, the prior 

CASAC’s scientific judgment is that 10 ppm-hrs is 

required to reduce it (Frey, 2014b, pp. iii and 15). 

In relevant part, the second paragraph reads: 

In reaching its scientific judgment regarding the 

indicator, form, summation time, and range of 

levels for a revised secondary standard, the CASAC 

has focused on the scientific evidence for the 

identification of the kind and extent of adverse 

effects on public welfare. The CASAC 

acknowledges that the choice of a level within the 

range recommended based on scientific evidence is 

a policy judgment under the statutory mandate of 

the Clean Air Act. . . . As a policy 

recommendation, separate from its advice above 

regarding scientific findings, the CASAC advises 

2020).216 For example, during the recent 

design value period (2016–2018), across 

all sites that met the current standard, 

few sites had any hours at or above 100 

ppb in a year (6% in the highest year, 

Wells, 2020, Table 2).217 Among the 

sites with any such hours, the vast 

majority had fewer than five such hours 

(99.5% in the highest year, Wells, 2020, 

Table 2), with none having more than 

ten such hours,218 and no site having 

more than three days in any one year 

with any such concentrations (Wells, 

2020, Figures 4 and 5). In comparison, 

sites with an annual W126 index below 

15 ppm-hrs recorded nearly 40 hourly 

concentrations at or above 100 ppb, and 

as many as seven days with such a 

concentration (Wells, 2020, e.g., Figures 

10 and 11).219 A similar pattern is seen 

using the historical dataset extending 

back to 2000. This historical dataset also 

shows the appreciable reductions in 

peak concentrations (via either the N100 

or D100 metric) 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). Thus, based on the findings 

of both the analyses in the PA (PA, 

Appendix 2A) and the additional 

analyses (Wells, 2020), the EPA 

disagrees with the commenter that the 

proposed decision ignores the 

importance of elevated hourly O3

concentrations in eliciting effects on 

vegetation. Rather, the proposed 

decision, and final decision to retain the 

existing standard, which controls peak 

concentrations and also cumulative 

seasonal exposure in terms of W126 

index, explicitly considers this 

importance and address it in a way that 

is more effective than a standard 

expressed in terms of the W126 index 

would be, even based on a single-year 

W126 well below 17 ppm-hrs (as shown 

in the additional air quality analyses 

[Wells, 2020]). 

In summary, we find that a 3-year 

average is appropriate for use in 

assessing protection for RBL based on 

the established tree seedling E–R 

functions, in light of the discussion 

above, while also finding it important to 

consider additional aspects of O3 air 

quality, that influence vegetation 

exposures of potential concern, in 

reaching conclusions about the 

adequacy of the current standard. We 

disagree with the commenters and the 

prior CASAC that focus on a single year 

W126 index is needed to protect against 

years with O3 concentrations with the 

potential to be ‘‘unusually damaging,’’ 

Rather, as described here, the metric of 

the current standard provides strong 

protection against elevated hourly 

concentrations that might contribute to 

‘‘unusually damaging’’ years with the 

potential to be adverse to the public 

welfare, as well as providing protection 

against effects of cumulative exposures 

seen in experimental studies. 

Accordingly, we disagree with those 

commenters that express the view that 

the current standard does not provide 

such protection. 

(iii) Visible Foliar Injury 

In support of their disagreement with 

the EPA’s proposed decision, some 

commenters express the view that the 

EPA’s proposed conclusion that the 

current standard provides sufficient 

protection from an incidence and 

severity of visible foliar injury that 

would reasonably be judged adverse to 

the public welfare is unlawful. These 

commenters variously claim that EPA 

analyses are flawed, arbitrary, and 

ignore conclusions and judgments of the 

prior CASAC; cite some studies that 

they state indicate a threshold for foliar 

injury lower than 25 or 17 ppm-hrs; 

claim that the EPA must, yet does not, 

identify a level of injury that is adverse; 

state that the EPA does not explain its 

use of USFS biosite scores in this 

regard, and state that the EPA does not 

adequately address the Murray Energy 

remand related to these effects. With 

regard to the latter, the Agency intends 

this decision, associated analyses 

conducted for this review in 

consideration of issues raised by the 

court remand, and the discussions 

herein to constitute its response to the 

Murray Energy remand on these effects. 

With regard to EPA’s analyses of the 

current information on O3-related 

visible foliar injury, some commenters 

claim that the EPA needs to and has not 

adequately explained why it disagrees 

with the conclusions and judgments of 

the prior CASAC in comments on the 

2014 draft PA regarding a W126 index 

value of 10 ppm-hrs. As an initial 

matter, we note that in discussing this 

topic, these commenters conflate the 

prior CASAC’s scientific evidence-based 

recommendations on the secondary 

standard with its judgments of scientific 

information in the context of its policy 

recommendations. In its letter on the 

draft PA, the prior CASAC explicitly 

separates into two separate paragraphs 

its scientific judgment based 

recommendations to the Administrator 

on the standard from its additional 

policy recommendations, with this 

statement regarding visible foliar injury 

occurring in the second paragraph (that 

addresses policy recommendations) 

(Frey, 2014b, p. iii).220 Thus, we 


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that a level . . . below 10 ppm-hrs is required to 

reduce foliar injury. A level of 7 ppm-hrs . . . offers 

additional protection against crop yield loss and 

foliar injury. . . . Thus, lower levels within the 

recommended range offer a greater degree of 

protection of more endpoints than do higher levels 

within the range. (Frey, 2014b, p. iii, [emphasis 

added]). 221 In reference to the 2013 draft WREA 

cumulative frequency analysis (e.g., 2013 draft 

WREA, Figures 7–9 to 7–12), a 2014 CASAC 

comment cited by commenters states that ‘‘W126 

values below 10 ppm-hrs [are] required to reduce 

the number of sites showing visible foliar 

symptoms’’ (Frey, 2014b, p. 14). 222We note that in light of, and subsequent to, the 

prior CASAC’s 2014 letter in the last review, the 

EPA had considered the extensive evidence 

documented in the 2013 ISA, as well as analyses 

of USFS data in the 2008 and 2015 reviews, 

including technical memos developed after the 

prior CASAC provided its 2014 advice (80 FR 

65376, 65395–96, October 26, 2015). In the current 

review, the now expanded available data and 

analyses augment the support for EPA’s conclusions 

in this regard. 

reasonably interpreted the statement by 

the prior CASAC as simply indicating a 

consideration of the prior CASAC in 

reaching its decision on the 

recommended range of levels, stated 

multiple times in the same letter and 

including levels higher than 10 ppmhrs, that the Committee thought might 

be useful (e.g., as a ‘‘policy 

recommendation’’) to the Administrator 

in exercising the discretion granted him 

under the Act for specifying a secondary 

standard (Frey, 2014b, p. iii). The prior 

CASAC statement regarding a W126 

index value of 10 ppm-hrs, is related to 

visible foliar injury at biosites, and, 

more specifically, is based on its 

consideration of an EPA cumulative 

analysis of a biomonitoring dataset 

presented in the 2013 draft WREA.221

This analysis, the dataset for which is 

further described in Appendix 3C of the 

PA for the current review, does not 

show, as implied by the 2014 CASAC 

comments, that, in considering sites 

with W126 index values from highest to 

lowest, there is no reduction in 

prevalence of sites with visible foliar 

injury above a W126 index of 10 ppmhrs (i.e., there are not differences in the 

occurrence of injury across higher 

values).222 The 2014 WREA analysis 

could not and was not addressing this 

issue. 

The 2014 WREA analysis is a 

cumulative analysis of the proportion of 

records with nonzero BI scores; each 

point graphed in the analysis includes 

the records for the same and lower 

W126 index values. Not only is the 

analysis silent with regard to severity of 

injury, but it also does not compare the 

incidence of visible foliar injury for 

records of differing W126 index values. 

Rather, each point in the cumulative 

frequency figure represents all the 

records included in the group (thus far), 

which increase by one with each new 

point (moving through dataset). Where 

the record added to the group has the 

same W126 index value as the prior 

included record, the point is at the same 

location along the x-axis, but at a 

slightly higher location along the y-axis 

(if it has a nonzero BI), thus contributing 

to an increase in the proportion of sites 

(the metric assessed on the y-axis). 

Thus, where there are many records 

with quite similar W126 index values, 

the points do not appreciably move 

along the x-axis, yet when they have a 

nonzero BI score, they are placed higher 

along the y-axis (as each represents 

another nonzero record in the dataset, 

thus increasing the proportion of 

records). At such a location along the xaxis, an inflection occurs (i.e., a location 

along the x-axis for which each 

additional record had the same or quite 

similar W126 index as the prior record 

such that the point is at a similar 

location on the x-axis but contributes to 

increasing values along the y-axis). As 

the addition of each new record makes 

the dataset larger, such increases (or 

decreases for zero BI records) become 

progressively smaller (along the y-axis), 

making such changes or inflections less 

pronounced at higher W126 index 

values. Accordingly, given the much 

greater representation in the dataset of 

relatively lower W126 index records 

(some two thirds of the dataset has 

W126 index values at/below 11 ppmhrs), the prominent inflection point 

noted by the prior CASAC on the 

cumulative frequency graph occurs 

around 11 ppm-hrs, and the figure from 

the 2014 WREA shows only small 

changes in the height of the line with 

increasing W126 index. This does not 

mean that records with higher W126 

index values have no greater occurrence 

of foliar injury than values below 11 

ppm-hrs; in fact, they do, most 

particularly the records with W126 

index values above 25 ppm-hrs (PA, 

Figure 4–5). Thus, we disagree with the 

prior CASAC statement that W126 index 

values below 10 ppm-hrs are required 

for any reduction in visible foliar injury 

and with the suggestion that the WREA 

cumulative analysis supports such a 

conclusion. Given that the statement by 

the prior CASAC did not provide any 

information to indicate another basis for 

its statement and because the 2014 

WREA analysis cannot and does not 

address this issue, we conclude that the 

prior CASAC’s statement lacks scientific 

support. Based on this conclusion, the 

Administrator does not find this 

statement from the prior CASAC 

informative to his consideration of the 

adequacy of the protection provided by 

the current standard for adverse public 

welfare effects related to visible foliar 

injury (discussed in section III.B.3 

below). 

Unlike the 2014 WREA cumulative 

frequency analysis, the presentations in 

the PA for this review allow for 

comparison of injury incidence, and 

severity, at distinctly different 

exposures. As can be seen by graphs of 

the distribution of nonzero BI scores for 

bins of increasing W126 index 

estimates, the greatest representation of 

nonzero BI scores occurs in the bin with 

the highest W126 index estimates, 

which for the normal soil moisture 

category is above 25 ppm-hrs (PA, 

Figure 4–5). In disagreeing with the 

EPA’s observations from this analysis, 

these commenters express the view that 

the higher percentage at the higher 

W126 index level is not meaningful 

because there are fewer records for the 

higher W126 index levels. While we 

agree that there are fewer records in the 

higher W126 index bins, as noted above, 

we disagree that there are too few 

records in those bins to support some 

interpretation for some soil moisture 

categories (such as the normal or dry 

categories), although for other soil 

moisture categories (i.e., wet), the small 

sample size does limit interpretation. 

Sample size in each bin was considered 

in the PA analysis and was recognized 

as placing a limitation on interpretation 

of patterns for the wet soil moisture 

category. Contrary to these commenters’ 

view that EPA provides no reason for 

giving little focus to the higher W126 

index bins for the wet soil moisture 

category, the PA explains that 

interpretations of patterns across the 

higher W126 bins are limited for the wet 

soil moisture category, noting that the 

number of records in each of the W126 

bins above 13 ppm-hrs comprise less 

than 1% of the records available for that 

soil moisture category (PA, Appendix 

4C, section 4C.6). Thus, we agree with 

these commenters that sample size is an 

important consideration in reaching 

conclusions from this dataset, and, 

contrary to the commenters’ assertion of 

providing no valid reasons with regard 

to the EPA’s lesser emphasis on the wet 

soil moisture category, the proposal 

stated that the PA observations focused 

primarily on the records for the normal 

or dry soil moisture categories explicitly 

in recognition of those categories having 

adequate sample size which the bins 

above 13 ppm-hrs did not for the wet 

soil moisture category (85 FR 49890, 

August 14, 2020). While the dataset 

includes an extremely small number of 

records in the wet soil moisture category 

that fall into the higher W126 index bins 


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223The records for the wet soil moisture category 

in the higher W126 bins are more limited than the 

other categories, with nearly 90% of the wet soil 

moisture records falling into the bins for W126 

index at or below 9 ppm-hrs, limiting 

interpretations for higher W126 bins (PA, Appendix 

4C, Table 4C.4 and section 4C.6). The number of 

records in each of the W126 bins above 13 ppmhrs (sample size ranging from zero to 9) comprise 

less than 1% of the wet soil moisture category. 

Accordingly, the PA observations focused primarily 

on the records for the normal or dry soil moisture 

categories, for which all W126 index in the 

analysis, including those above 13 ppm-hrs, are 

better represented (85 FR 49890, August 14, 2020). 

For the wet soil moisture category, we agree with 

the commenter’s statement that ‘‘higher percentage 

at higher levels isn’t necessarily meaningful, 

because there are fewer sites with any data at those 

levels,’’ however note that there is much greater 

representation of the normal and dry soil moisture 

categories in each of the higher bins, extending to 

the highest bins, than is the case for the wet soil 

moisture category bins. 

224Such information informs the Administrator’s 

consideration of the currently available evidence 

and the extent to which it can inform his judgments 

on O3 air quality associated with visible foliar 

injury of such an extent and severity in the 

environment as to indicate adverse effects to the 

public welfare. Such judgments, as discussed 

further below, rely on information on relationships 

between different O3 air quality metrics and injury 

incidence and severity as well as factors influencing 

the public welfare significance of different 

incidence and severity of foliar injury in vegetated 

areas valued by the public (e.g., as summarized in 

section III.A.2.b). 

225This characterization was made in the 2014 

letter providing the prior CASAC’s review of the 

second draft WREA. As noted by some commenters, 

the letter goes on to state, ‘‘[b]ased on this E–R 

slope change, 10 ppm-hrs is a reasonable candidate 

level for consideration in the WREA, along with 

other levels’’ (Frey, 2014c, p. 7). Although the EPA 

did not examine the specific value of 10 ppm-hrs 

in the 2014 WREA, as observed by these 

commenters, the EPA did consider this 

recommendation in the 2015 decision, contrary to 

the claim of the commenters (80 FR 65395–96, 

October 26, 2020). 

(just 18 distributed across the three 

W126 index bins above 13 ppm-hrs),223

there are more than 550 records 

categorized as normal soil moisture 

distributed across all five bins for W126 

index above 13 ppm-hrs, more than 40 

in each bin (PA, Appendix 4C, Table 

4C–4). To the extent that the 

commenters are suggesting that the EPA 

is disregarding data for sites categorized 

as wet soil moisture, we disagree. In 

recognition of the role of soil moisture 

in contributing to a condition 

‘‘necessary for visible foliar injury to 

occur,’’ the PA analysis presents BI 

scores separated into groups based on 

categorization related to soil moisture 

(ISA, Appendix 8, p. 8–13; 85 FR 49881; 

PA, pp. 4–40 to 4–41). The EPA thus 

considered the available evidence for all 

of the soil moisture categories, but with 

regard to any patterns evidenced for the 

higher W126 index bins (above 13 ppmhrs), the EPA reasonably explained its 

focus on two of the three categories (the 

normal or dry soil moisture categories), 

and lesser attention to the third category 

(wet soil moisture) due to the extremely 

small number of records in that category 

that fall into the higher W126 index 

bins. 

Further, in addition to incidence of 

sites with any injury, the PA 

presentations indicate that the severity 

of injury is also highest in records for 

the highest W126 index values, 

appreciably higher that it is in all of the 

lower W126 index bins. For normal soil 

moisture category, the median BI score 

across the nonzero records in the 

highest W126 bin (greater than 25 ppmhrs) is just over 10 (with an average over 

15), compared to well below 5 (averages 

below 7) for each of the lower W126 

bins (PA, Figure 4–5, Appendix 4C, 

Table 4C–5). Both of these observations 

are consistent with an E–R relationship 

of O3 with visible foliar injury, while 

the variability observed across the full 

dataset, in addition to perhaps 

indicating limitations in some aspects of 

the dataset (e.g., categorization by soil 

moisture, among others [PA, Appendix 

4C, section 4C.5]), no doubt also 

indicates the role of other factors that 

have not been completely accounted for. 

Given the evidence from controlled 

experiments documented across many 

years, the lack of noticeable change in 

incidence or severity across lower W126 

index values may, as recognized in the 

PA, relate to a number of factors, 

including uncertainties in the 

assignment of W126 index estimates to 

the biosite locations and the soil 

moisture categorization of sites, as well 

as potential for differences in individual 

plant responses in controlled 

experiments from plant communities in 

natural environmental settings. 

Although such factors may contribute to 

an unclear pattern at lower exposures, 

precluding reaching conclusions 

regarding O3-related response across the 

lower W126 index bins, the observed 

response for the highest bin clearly 

indicates an O3-related response for 

W126 index values above 25 ppm-hrs. 

Some commenters question the 

significance EPA ascribes to its 

observation that the BI scores are 

appreciably higher for records in the 

highest W126 index bin, cryptically 

characterizing the observation as 

describing a ‘‘derivative of a derivative.’’ 

Yet, this observation is simply focused 

on the response (e.g., incidence of BI 

score greater than 0 or 5 or 15) exhibited 

across the range of exposure levels 

evaluated. The EPA makes this 

observation in assessing the dataset as to 

whether an E–R relationship is 

exhibited and if so, at what part of the 

exposure range is there a noticeable 

increase in response. This assessment, 

in combination with related evidence, 

then informs the Agency’s conclusions 

regarding O3 exposure circumstances 

that influence BI scores, as well as 

levels of W126 for which such an 

influence is indicated.224 The 

commenters quote the prior CASAC as 

characterizing the 2014 WREA analysis 

as ‘‘a change in the E–R slope,’’ 225 but, 

as discussed in detail above, the 2014 

WREA figure is presenting a cumulative 

frequency analysis, which, by its design, 

does not show ‘‘a change in the E–R 

slope.’’ Such an analysis, because 

responses are not compared among 

distinct and discrete exposures, as 

explained above, is not well described 

as an exposure-response assessment 

(i.e., an analysis of responses occurring 

across a range of different exposures). 

This is in contrast to the current PA 

presentation of BI scores across bins of 

increasing W126 index, which presents 

the occurrence of responses, quantified 

by magnitude of BI score, associated 

with multiple different exposures 

(presented as bins). Thus, the EPA finds 

the current analyses in the PA, and not 

the cumulative frequency analysis in the 

2014 WREA, to be informative to the 

consideration of relationships between 

extent of visible foliar injury and W126 

index, and finds the 2014 WREA 

analysis to be mistakenly interpreted by 

the commenters. 

Further some commenters, who object 

to the Administrator’s proposed focus 

on BI scores above 15 for his 

consideration of visible foliar injury that 

may be adverse to the public welfare, 

additionally suggest that EPA should 

give weight to all nonzero BI scores in 

considering the appropriate protection 

against this effect for the standard. As 

an initial matter, contrary to the 

implication of the commenters that any 

amount of visible foliar injury is adverse 

to the affected plant, we note the longstanding conclusions that visible foliar 

injury ‘‘is not always a reliable indicator 

of other negative effects on vegetation,’’ 

such as growth and reproduction, and 

the ‘‘significance of ozone injury at the 

leaf and whole-plant levels depends on 

how much of the total leaf area of the 

plant has been affected, as well as the 

plant’s age, size, developmental stage, 

and degree of functional redundancy, 

among the existing leaf area’’ (ISA, p. 8– 

24; 2013 ISA, section 9.4.2). Further, we 

disagree with the further implication of 

these commenters that any occurrence 

of a nonzero BI score in the PA dataset 

can be used to identify O3 exposure 

conditions that are adverse to the public 


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226For example, valid design values include: (1) 

73 (2002) and 72 (2003) at monitoring site 

450190046, (2) 91 (2002), 94 (2003), and 88 (2004) 

at 230090102; (3) 77 ppb (2004) at 261530001, and 

(4) 90 (2002 and 2003) at 340010005. 

welfare. As discussed in section 

III.A.2.b above, a number of factors 

influence the public welfare 

implications of visible foliar injury, and 

as discussed further below, the 

Administrator has taken these into 

account in his decision making 

regarding the protection from such 

effects that should be afforded by the 

secondary standard. 

These commenters additionally claim 

that the USFS dataset indicates a clear 

relationship between the W126 metric 

and foliar injury. While we agree that 

the dataset provides some support for 

the conclusion of a greater incidence of 

nonzero BI scores and higher scores for 

the highest W126 bin, a change in 

response is not evident across the full 

range of W126 index levels (for records 

of similar soil moisture category), thus 

suggesting a limitation of the dataset in 

its ability to describe the E–R 

relationship of BI scores with W126 

index. As discussed in the PA, 

limitations in the dataset (e.g., with 

regard to assignment of W126 index 

estimates to biosite records and the 

approach for accounting for the role of 

soil moisture) may be contributing to 

the lack of a clearly delineated E–R 

relationship of injury occurrence and BI 

score with W126 index across a range of 

W126 index values, such that a clear 

shape for a relationship between these 

variables is not evident with this 

dataset, and may be contributing to 

uncertainties in this regard. It is with 

the increase in W126 for the last bin 

(>25 ppm-hrs) that the accompanying 

noticeable increase in response provides 

increased confidence in that response 

(BI scores) being related to a particular 

magnitude of the O3 metric. It is this 

consideration which leads to the 

emphasis that EPA’s conclusions from 

this analysis place on W126 index above 

25 ppm-hrs, albeit with a recognition of 

some associated uncertainty. 

Regarding the Administrator’s 

judgment of the extent and severity of 

visible foliar injury that may be adverse 

to the public welfare, some commenters 

state that the EPA must, and has not, 

considered the full USFS dataset, 

including records for which the BI 

scores are below 5, and they express the 

view that the USFS data indicate injury 

(i.e., a nonzero BI score) to be occurring 

at W126 index values as low as 3 ppmhrs. In so doing, they note the 

occurrence of scores above 15 in the 

lowest bin (W126 index below 7 ppmhrs). These commenters note that a third 

of all records with a BI above 15 are in 

the lowest W126 index bin (W126 <7 

ppm-hrs) and more than 500 records 

with nonzero BI are in higher bins, 

seemingly intending this as support of 

their view that the EPA should identify 

a W126 of 7 ppm-hrs as a target level for 

visible foliar protection. However, this 

line of logic seems to ignore the fact that 

this bin also has over a third of the 

records with a BI above zero (PA, Table 

4C–4), a fact which would seem 

contrary to these commenters’ position 

that 7 ppm-hrs would protect against 

such scores. All three of these 

observations are likely due to the fact 

that this bin contains 42% of all records 

and the most records of any bin, by far 

(PA, Appendix 4C, Table 4C–4). 

Accordingly, the more important 

observation with regard to the extent of 

conclusions supported by the dataset on 

the role of W126 index in influencing BI 

scores is that the proportion of records 

in the lowest W126 bin that have scores 

above 15, 5 or 0 is appreciably less that 

in the highest W126 index bin (PA, 

Appendix 4C, Table 4C–6). The fact that 

there is not a clear pattern of increasing 

proportion across the intervening (and 

full set of) bins indicates there to be 

factors unaccounted-for in this dataset 

with regard to the O3 exposure 

circumstances and the environmental 

circumstances that together elicit 

increased scores in vegetated areas. 

In considering the PA analyses of the 

biosite dataset in light of these 

comments, we first note that, as 

described in the PA, the USFS dataset 

includes a broad assortment of BI 

scores, extending down to zero, 

occurring across the range of W126 

estimates applied to the records (PA, 

Appendix 4C, Figure 4C–3). Contrary to 

the statement by these commenters, the 

EPA has considered the full dataset. The 

PA documents the various ways in 

which this is done, and the proposal 

discusses key observations from this 

dataset to inform the Administrator’s 

judgment on adversity to public welfare 

(PA, section 4.3.3.2, 4.5.1.2 and 

Appendix 4C; 85 FR 49889–90, 49903, 

August 14, 2020). For example, the lack 

of clear BI score response to W126 

across the range of lower values is 

consistent with findings of published 

studies of the USFS biomonitoring data 

which find that W126 index alone may 

not be sufficient to characterize the O3

conditions contributing to injury levels 

that may be of interest (e.g., Smith et al., 

2012; Smith, 2012; 85 FR 49888–49889, 

August 14, 2020). Similar to the 

discussion above, these studies suggest 

a role for the occurrence of elevated 

hourly concentrations and a focus solely 

on W126 index may miss this. This 

consideration of the larger evidence 

base for visible foliar injury and 

associated USFS biomonitoring findings 

is important to judging the findings of 

analyses of the BI dataset and their 

informativeness to the Administrator’s 

needs in judging public welfare 

adversity. Based on a detailed 

evaluation of the currently available 

record regarding such data, the EPA 

recognizes the need to consider factors 

beyond just W126 index in considering 

O3 conditions most influential in the 

incidence and extent of visible foliar 

injury. 

With regard to lower ‘‘thresholds,’’ 

the commenters simply cite a set of 

studies that describe visible foliar injury 

observations in bioindicator species and 

for which estimates of W126 index for 

a prior time period are below 25 ppmhrs. The first group of these studies 

focus on naturally occurring plants in 

locations during which the current 

standard (with its level of 70 ppb) is not 

met.226 As discussed above, the current 

standard limits the occurrence of 

elevated concentrations which, as 

discussed above, is suggested to be 

important in the occurrence of visible 

foliar injury in sites of the USFS biosite 

monitoring program, and such elevated 

concentrations are much more prevalent 

in areas that do not meet the current 

standard (e.g., PA, Appendix 4A, 

section 2A.2; Wells [2020]). Thus, this 

group of studies do not provide 

sufficient information to characterize 

the O3 exposure circumstances that may 

be eliciting the observed responses. Nor 

are they informative with regard to 

consideration of the incidence and 

extent or severity of injury that may 

occur under air quality conditions 

allowed by the current standard. Two 

other examples raised by commenters 

(but without complete study citations), 

appear to relate to leaf injury assessed 

in potted plants either outdoors but 

watered daily or maintained in 

greenhouse conditions. The injury 

assessed is at the individual plant level, 

making implications with regard to 

natural vegetation communities unclear, 

and the extent to which either finding 

in artificial conditions might represent 

such plant responses in natural 

environmental conditions is unknown. 

These commenters additionally note 

what they describe as ‘‘threshold 

values’’ reported in a National Park 

Service publication (Kohut, 2020). This 

publication includes three ‘‘injury 

thresholds’’ in terms of three assessment 

metrics, with one being a 3-month W126 

index and a second in terms of 


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87332 Federal Register / Vol. 85, No. 251 / Thursday, December 31, 2020 / Rules and Regulations 

227We note that the third assessment approach 

utilizes a combination of a W126 index metric with 

the N100 metric, illustrating the consideration by 

the National Park Service of the role of peak 

concentrations in posing risk of visible foliar injury 

(Kohut, 2020). 

228Studies that consider such data for purposes 

of identifying areas of potential impact to the forest 

resource suggest this category corresponds to 

‘‘none’’ with regard to ‘‘assumption of risk’’ (Smith 

et al., 2007; Smith et al., 2012). 

SUM06.227 For each metric, three ranges 

of ‘‘thresholds’’ are presented (for 

different purposes). The ranges for 

SUM06 come from a 1996 workshop 

report (Heck and Cowling, 1997). The 

ranges for W126 index are based on a 

W126 index conversion of the SUM06 

ranges. One of the ranges is labeled as 

pertaining to foliar injury as a response, 

yet, the publication cited does not 

provide data on foliar injury in relation 

to that range, nor do publications cited 

by the former publication. As we can 

best discern based on cited and related 

publications, it appears to at the lower 

end relate to a benchmark derived for 

growth effects (10% RBL) in the highly 

sensitive species, black cherry, rather 

than visible foliar injury (Kohut, 2007b; 

Lefohn et al., 1997; 80 FR 65378, 

October 26, 2015). Thus, contrary to the 

commenters’ assertion, the range for 

W126 index (labeled as pertaining to 

foliar injury) does not appear to provide 

a threshold based on evidence for 

visible foliar injury. 

Some commenters (citing page 4C–18 

of the PA), express confusion over how 

EPA can state there to be an incomplete 

understanding of the relationships 

influencing severity of visible foliar 

injury while also using the USFS scores 

to inform the Administrator’s judgments 

regarding conditions that may be 

adverse to the public welfare. We see no 

contradiction in this. Rather, it is this 

recognition of an incomplete 

understanding, including the 

recognition of uncertainty in ‘‘specific 

aspects of [the influences of 

environmental/genetic factors] on the 

relationship between O3 exposures, the 

most appropriate exposure metrics, and 

the occurrence or severity of visible 

foliar injury’’ (PA, Appendix 4C, p. 4C– 

18), that leads the EPA to place greatest 

weight on the most clear findings from 

the USFS data. With regard to the PA 

presentation, with its recognized 

uncertainties and limitations, such a 

finding is the obviously increased 

prevalence and severity of visible foliar 

injury for records with W126 index 

estimates above 25 ppm-hrs. 

Further, in considering public welfare 

implications of O3 related visible foliar 

injury, the EPA continues to recognize 

that the occurrence of visible foliar 

injury has the potential to be adverse to 

the public welfare (e.g., as summarized 

in section III.A.2.b above and section 

III.B.2 of the proposal). However, as 

noted in the proposal, the EPA does not 

find that any small discoloring on a 

single leaf of a plant (which might yield 

a quite low, nonzero BI score in the 

USFS system) is reasonably considered 

adverse to the public welfare. Thus, 

findings such as those raised by 

commenters of injury on individual 

plants in controlled conditions, while 

providing support to the conclusion of 

a causal relationship between O3

exposure and visible foliar injury (ISA, 

Appendix 8, Table 8–3), are less 

informative to the Administrator’s 

judgment on adequacy of the protection 

provided by the current standard from 

adverse effects to the public welfare. 

Rather, the USFS biosite monitoring 

data provide information that is more 

useful for such a judgment because this 

monitoring program, as summarized in 

section III.A.2.b above (and III.B.3.b of 

the proposal), and the scale of its 

objectives which focus on natural 

settings in the U.S. and forests as 

opposed to individual plants is better 

suited for the Administrator’s 

consideration with regard to the public 

welfare protection afforded by the 

current standard. In this context, as 

described in section III.B.3 below, the 

Administrator judges that very low BI 

scores, such as those less than 5, 

described by the USFS scheme as ‘‘little 

or no foliar injury’’ do not pose concern 

for the public welfare.228

Lastly, we disagree with the comment 

that the Act requires the EPA to specify 

‘‘a level’’ of injury that is adverse. The 

Court of Appeals for the D.C. Circuit has 

held that ‘‘the Agency may sometimes 

need to articulate the level of threat to 

the population it considers tolerable; 

but there is no separate methodological 

requirement under § 109 that the 

Administrator establish a measure of the 

risk to safety it considers adequate to 

protect public health every time it 

establishes a standard pursuant to 

§ 109.’’ See Nat. Res. Def. Council, Inc. 

v. EPA, 902 F.2d 962, 973 (D.C. Cir. 

1990), opinion vacated in part on other 

grounds, 921 F.2d 326 (D.C. Cir. 1991). 

The same principle applies for 

consideration of the protection of public 

welfare in the context of establishing or 

reviewing secondary standards. The 

court later confirmed that it ‘‘expressly 

rejected the notion that the Agency must 

‘establish a measure of the risk to safety 

it considers adequate to protect public 

health every time it establishes a 

[NAAQS].’’ See ATA III, 283 F.3d at 369 

(D.C. Cir. 2002) (quoting Natural Res. 

Def. Council, Inc. v. EPA, 902 F.2d 962, 

973 [D.C. Cir. 1990]). As is recognized 

by the courts and by EPA and CASAC 

across NAAQS reviews, the judgment of 

the Administrator, in addition to being 

based on the scientific evidence, 

depends on a variety of factors, 

including science policy judgments and 

public welfare policy judgments. As 

noted by the case law and also in 

section III.B.2.b(iv) below, the EPA is 

not required under the Act to identify 

individual levels of adversity or set 

separate standards for every type of 

effect that may be caused by a pollutant 

in ambient air, as long as it has engaged 

in reasoned decision making in 

determining that a particular standard 

provides the requisite protection. Thus, 

it is common for one NAAQS to provide 

protection for multiple effects, with the 

most sensitive effect influencing the 

stringency of the standard and 

accordingly leading to protection that is 

adequate for other, less sensitive effects. 

Given the significant uncertainties 

which are present in every NAAQS 

review, it is enough for the 

Administrator to set standards that 

specify a level of air quality that will be 

‘‘tolerable,’’ (NRDC, 902 F.2d at 973), 

and ‘‘qualitatively to describe the 

standard governing its selection of 

particular NAAQS’’ (ATA III, 283 F.3d 

at 369). In reviewing each standard, the 

EPA gives due consideration to each of 

the effects that are relevant for that 

standard in considering whether the 

standard provides adequate protection 

from the type, magnitude or extent of 

such effects known or anticipated to be 

adverse to the public welfare. In the 

case of visible foliar injury, as discussed 

in section III.B.3 below, the 

Administrator has considered the 

available scientific evidence, with 

associated uncertainties and limitations, 

in reaching his decision that the current 

secondary standard provides adequate 

public welfare protection for this effect. 

(iv) Crop Yield Effects 

Some commenters object to the 

proposed conclusions with regard to the 

protection provided by the existing 

secondary standard from adverse effects 

on the public welfare related to O3

effects on crop yield, expressing the 

view that the EPA must specify ‘‘a 

level’’ to protect the public welfare 

against crop yield reductions and that 

not doing so is unlawful and arbitrary. 

These commenters’ additionally object 

to the Administrator’s proposed 

judgment that a decision based on RBL 

as a proxy for other vegetation-related 

effects will also provide adequate 

protection against crop related effects, 

indicating their view that EPA does not 


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229Section 109(b)(2) of the CAA provides only 

that any secondary standard ‘‘shall specify a level 

of air quality the attainment and maintenance of 

which in the judgment of the Administrator, based 

on [the air quality ] criteria, is requisite to protect 

the public welfare from any known or anticipated 

adverse effects associated with the presence of such 

air pollutant in the ambient air.’’ 

230The first reference to 5% RYL by the prior 

CASAC (in the 2015 O3 NAAQS review) appears to 

be in its letter on the first draft PA (Frey and Samet, 

2012). In that letter, the prior CASAC identifies 5% 

RYL as a factor on which levels for a W126 index 

secondary standard should be based, although no 

rationale is provided for this recommendation. In a 

letter attachment, comments from an individual 

member point to a 1996 workshop (2014 PA, pp. 

6–15 through 6–17; Heck and Cowling, 1997). As 

summarized in the 2015 O3 decision, the 1996 

workshop participants (16 leading scientists, 

discussing their views for a secondary O3 standard) 

indicated an interest in protecting against crop 

yield reductions of 5% yet noted uncertainties 

surrounding such a percentage which led them to 

identify 10% RYL (80 FR 65378, October 26, 2015). 

In their emphasis on 5%, the 2012 comments from 

the individual prior CASAC member expressed the 

view that the ability to estimate 5% RYL has 

improved (Frey and Samet, 2012, p. A–54). Neither 

the individual prior CASAC member nor the 1997 

workshop report provide any explicit rationale for 

the percentages identified or any description of 

their connection to ecosystem impacts of a specific 

magnitude or type, or to judgments on significance 

of the identified effects for public welfare (80 FR 

65378, October 26, 2015; Heck and Cowling, 1997). 

adequately explain the basis for this 

judgment. These commenters 

additionally claim that the prior CASAC 

described 5.1% RYL as constituting an 

adverse welfare effect and express the 

view that the EPA arbitrarily and 

unlawfully does not ‘‘give effect to’’ the 

prior CASAC’s recommendation. 

We disagree with the implication of 

these commenters that, in judging 

adequacy of protection provided by the 

current standard for a particular effect, 

it is per se unlawful to conclude that the 

air quality achieved by the current 

standard provides adequate protection 

for that particular effect, even if the 

greater attention in reviewing the 

current standard is on another effect. 

The EPA is not precluded from reaching 

such a conclusion as long as the Agency 

has engaged in reasoned decisionmaking in doing so.229 In reaching his 

proposed conclusions regarding the 

extent to which the current standard 

provides appropriate protection from O3

effects on crop yield that may be 

adverse to the public welfare, as in his 

conclusions described in section III.B.3 

below, the Administrator recognizes the 

long-standing evidence of O3 effects on 

crop yield and the established E–R 

functions for which RYL estimates for 

the median crop species are presented 

in the PA (PA, Appendix 3A). He also 

considers factors that might be 

important to his judgments related to 

the requisite protection for a secondary 

standard that protects against adverse 

effects to the public welfare. In this 

context he judges that the median RYL 

estimated for air quality that achieves 

his RBL-related objectives for the 

current standard does not constitute an 

adverse effect on public welfare and 

thus concludes that the current standard 

also provides adequate protection for 

crop yield-related effects. Given that the 

decision on adequacy of protection is a 

judgment of the Administrator and that 

the Clean Air Act does not require a 

particular approach for reaching such 

judgments, we disagree with the 

commenters to the extent that they 

suggest that it is per se unlawful for the 

Administrator to use such an approach. 

The circumstances for his use of this 

approach include particular aspects of 

the information available on O3-related 

crop yield effects and other factors 

important to judgments on public 

welfare effects related to crop yield 

effects. 

In reaching his decision in this 

review, as described in section III.B.3 

below, the Administrator has also 

considered public comments on these 

issues, including that regarding a prior 

CASAC statement. The comment 

regarding the prior CASAC appears to 

draw on a judgment of the prior CASAC 

that a median RYL of 5% ‘‘represents an 

adverse impact’’ (Frey, 2014b, p. 14). 

The prior CASAC provided no clear 

scientific foundation for this judgment. 

While we infer this judgment to draw on 

discussion at a 1996 workshop,230

neither the prior CASAC nor the 

workshop summary provides any 

explicit rationale for identification of 

5% (with regard to RYL), or any 

description of a connection of an 

estimated 5% RYL to broader impacts of 

a specific magnitude or type, or to 

judgments on significance of a 5% RYL 

to the public welfare. Thus the EPA 

disagrees with the commenters 

regarding the weight to give the prior 

CASAC statement and, as described 

below, respectfully disagrees with the 

prior CASAC on this statement. 

In reaching his judgment regarding 

whether the current standard provides 

the requisite public welfare protection, 

as described in section III.B.3 below, the 

Administrator considers the extent to 

which a specific estimate of RYL may be 

indicative of adverse effects to the 

public welfare. In so doing, he notes 

that the secondary standard is not 

intended to protect against all known or 

anticipated O3-related effects, but rather 

those that are judged to be adverse to 

the public welfare, and that a bright-line 

determination of adversity is not 

required in judging what is requisite. In 

his decision described below, the 

Administrator also notes that the 

determination of the extent of RYL 

estimated from experimental O3

exposures that should be judged adverse 

to the public welfare is not clear, in 

light of the extensive management of 

agricultural crops that occurs to elicit 

optimum yields (e.g., through irrigation 

and usage of soil amendments, such as 

fertilizer). Further, in considering effects 

on the public welfare that may relate to 

agricultural markets, we note that 

detrimental impacts on crops, as well as 

beneficial impacts, can be unevenly 

distributed between producers and 

consumers, complicating conclusions 

with regard to relative adversity. In light 

of such considerations, the 

Administrator, while finding 

consideration of the RYL estimate for 

the median crop species informative to 

his judgment on the adequacy of the 

protection provided by the current 

standard for this effect, does not find 

such an RYL estimate of 5% to represent 

an adverse effect to the public welfare, 

as described more fully in section III.B.3 

below. For these reasons, and for the 

reasons discussed earlier in this section, 

including those regarding advice from a 

prior CASAC, the EPA also disagrees 

with the commenters’ assertion that the 

Administrator is arbitrarily and 

unlawfully failing to ‘‘give effect to’’ the 

prior CASAC’s recommendation. 

Further, we disagree with the 

comment that the Act requires the EPA 

to specify ‘‘a level’’ to protect the public 

welfare against crop yield reductions. 

As discussed in greater detail in section 

III.B.2.b(iii) above, the EPA is not 

required under the Act to set separate 

standards for every type of effect that 

may be caused by a pollutant in ambient 

air, as long as it has engaged in reasoned 

decision making in determining that a 

particular standard provides the 

requisite protection. Thus, it is common 

for one NAAQS to provide protection 

for multiple effects, with the most 

sensitive effect influencing the 

stringency of the standard and 

accordingly leading to protection that is 

adequate for other less sensitive effects. 

As discussed further in section 

III.B.2.b(iii) above, in reviewing each 

standard, the EPA gives due 

consideration to each effect relevant for 

that standard in considering whether 

the standard provides adequate 

protection from the type, magnitude or 

extent of such effects known or 

anticipated to be adverse to the public 

welfare. In the case of crop yield loss, 

as discussed in section III.B.3 below, the 

Administrator has considered the 

magnitude of RYL that may be 


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231Additionally, an explicit scientific rationale 

for 2% is not provided by the former CASAC. Nor 

is it provided in the workshop report referenced by 

the prior CASAC in its discussion, as further 

discussed in the 2015 decision (80 FR 65394, 

October 26, 2015; Frey, 2014b, p. 14). 

232 It is unclear by what logic the commenters 

conclude that RBL, a metric describing the effect of 

associated with W126 index values that 

occur under the current standard and, 

based on the current information with 

regard to the RYL estimates, notes that 

these estimates are generally no higher 

than 5.1% and predominantly well 

below that. In so doing, he has also 

considered factors such as those raised 

above, and in light of all of these 

considerations, he judges that a RYL of 

5.1% does not represent an adverse 

effect to the public welfare. Thus, the 

Administrator judges that the current 

standard provides adequate protection 

of the public welfare for crop yield loss 

related effects. 

(v) RBL 

In objecting to the EPA’s proposed 

decision, some commenters disagree 

with the target level of protection 

identified based on use of RBL. In so 

doing, such commenters variously claim 

that a 3-year average of 17 ppm-hrs is 

‘‘ill-suited’’ to protect against adverse 

impacts to the public welfare; that 6% 

RBL is too high to protect the public 

welfare; that use of a 3-year average 

instead of a single year W126 index is 

needed; and, that EPA must focus a 

target on exposures that would avoid 

2% RBL, citing comments from the prior 

CASAC on the second draft PA in the 

2015 review, and claiming that a focus 

on a W126 index of 7 ppm-hrs is needed 

for that. With regard to the EPA’s use of 

6% in considering the adequacy of 

protection related to RBL, these 

commenters recognize that Murray 

Energy rejected an argument that EPA’s 

prior reliance on 6% (in the 2015 

decision) was arbitrary based on the 

record in that case (Murray Energy, 936 

F.3d at 615–16). In pressing their views, 

however, the commenters state that 

nothing in Murray Energy prevents EPA 

from revising its prior determination 

based on the scientific evidence and 

CASAC advice. 

With respect to the latter point, the 

EPA agrees that the Administrator’s 

decision in this review must take into 

account the currently available 

scientific evidence and advice from the 

CASAC, and that the Agency is not 

bound by the Administrator’s 

conclusions in the prior review. As 

summarized in the proposal for the 

current review, in the proposal, the 

Administrator took the currently 

available scientific evidence and advice 

from the CASAC into account, while 

also choosing to consider the judgments 

and decision made by the prior 

Administrator in that Administrator’s 

consideration of RBL related targets for 

cumulative seasonal exposure. He did 

so, in light of the welfare effects 

evidence and air quality information 

now available, as well as the advice 

from the current CASAC reflecting its 

concurrence that implementation of the 

prior Administrator’s approach or 

framework is ‘‘still effective’’ in 

protecting the public welfare from 

vegetation effects of O3 (Cox, 2020a, 

Consensus Responses to Charge 

Questions p. 21). As described in 

section III.B.3 below, after considering 

the public comments on this point, he 

is taking a similar approach in reaching 

his decision in this review. 

With regard to the commenters’ 

objection to the EPA’s use of a 3-year 

average in assessing RBL, we note, as an 

initial matter, that the EPA’s focus on a 

3-year average of 17 ppm-hrs as a target 

level relates to an RBL estimate of 5.3%, 

a value that was also chosen in 2015 in 

recognition of the prior CASAC advice 

both with regard to 6% RBL and about 

considering a lower W126 index target 

for a 3-year average due to the prior 

CASAC’s concern about ‘‘unusually 

damaging years.’’ In the current review, 

the CASAC has explicitly considered 

the EPA’s interpretation of 6% in 

identifying a target of 17 ppm-hrs as a 

3-year average, and expressed its view 

that this target ‘‘is still effective in 

particularly protecting the public 

welfare in light of vegetation impacts 

from ozone’’ (Cox, 2020a, Consensus 

Responses to Charge Questions p. 21). 

Accordingly, the EPA disagrees with the 

comments that 6% RBL and a 3-year 

average W126 index target of 17 ppmhrs are too high to inform the 

Administrator’s judgments on O3 air 

quality that protects the public welfare; 

rather, the Administrator continues to 

find this useful in informing his 

judgments regarding the public welfare 

protection provided by the standard, 

together with a broader consideration of 

air quality patterns associated with 

meeting the current standard, such as 

control of peak hourly concentrations, 

as described in section III.B.3 below. 

Further, we refer to the discussion 

above of how the existing standard, with 

its current averaging time and form 

provides the protection from the 

occurrence of elevated hourly 

concentrations that may characterize 

what the prior CASAC described as 

‘‘unusually damaging years.’’ As 

discussed above, the available air 

quality data demonstrate the strong 

protection provided by the current 

standard from elevated concentrations 

that may occur in some years. As noted 

above, these analyses indicate that 

while the current form and averaging 

time of the existing standard provides 

control of these concentrations and the 

associated peak exposures, reliance 

solely on a standard in the form of the 

W126 index based standard, as 

advocated by the commenters, even 

with a level as low as 7 ppm-hrs cannot 

be relied on to provide it. 

In support of their view that the EPA 

must focus on avoiding 2% RBL with a 

W126 index of 7 ppm-hrs, these 

commenters provide little rationale 

beyond citing a comment by the prior 

CASAC made in the last review. In so 

doing, the commenters assert that 

because the prior CASAC had noted that 

7 ppm-hrs was the only W126 index 

level for which the E–R functions 

yielded a RBL for the median tree 

species that was less than or equal to 

2%, the EPA must protect against 2% 

RBL and adopt a W126 index level of 7 

ppm-hrs. We disagree. As an initial 

matter, we note our discussion above 

regarding the EPA’s consideration in 

this review of advice from a prior 

CASAC, including prior CASAC 

statements that are raised by 

commenters, such as those noted here. 

Further, in making the statement that 

the commenters’ cite, the prior CASAC 

did not reach the same conclusion as 

the commenters with regard to the 

extent to which a revised secondary 

standard should limit cumulative 

exposures and associated estimates of 

RBL, such that the prior CASAC did not 

recommend that the EPA consider only 

W126 index levels associated with 

median RBL estimates at or below 

2%.231 See Murray Energy, 936 F.36 at 

615–16 (noting that ‘‘CASAC did not 

identify 2% growth loss as the only 

sufficiently protective level’’ but merely 

recommended ‘‘2% as the lower end of 

a range of permissible target levels’’ to 

be considered). In fact, seven of the nine 

W126 index levels in the range 

recommended by the prior CASAC (7 to 

15 ppm-hrs [Frey, 2014b]) are associated 

with RBL estimates higher than 2% (PA, 

Appendix 4A). As a basis for their 

assertion that the secondary standard 

should protect against a median RBL of 

2%, these commenters additionally 

oddly declare that after three years, a 

2% RBL per year ‘‘becomes 6%.’’ There 

is no evidence in the record, and the 

commenter provides no evidence, that 

would support their declaration that 

without a tripling in exposure, the O3- 

attributable reduction in annual growth 

(the RBL) would triple.232 Nor is there 


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the O3 exposure in a single year, can be modified 

by the RBL in a prior year. 

233The fallacy of such interpretations can be seen 

in the presentation of above-ground biomass from 

a multiyear study of O3 exposure of aspen that 

varies little over six years. Across the six years, the 

above-ground biomass of the trees receiving 

elevated O3 exposure is 25%, 30%, 29%, 29%, 31% 

and 29% lower than the reference trees (2013 ISA, 

Table 9–14; 2020 ISA, Figure 8–17). 

234As summarized in the ISA, O3 can mediate 

changes in plant carbon budgets (affecting carbon 

allocation to leaves, stems, roots and other biomass 

pools) contributing to growth impacts, and altering 

ecosystem properties such as productivity, carbon 

sequestration and biogeochemical cycling. In this 

way, O3 mediated changes in carbon allocation can 

‘‘scale up’’ to population, community and 

ecosystem-level effects including changes in soil 

biogeochemical cycling, increased tree mortality, 

shifts in community composition, changes in 

species interactions, declines in ecosystem 

productivity and carbon sequestration and 

alteration of ecosystem water cycling (ISA, section 

8.1.3). 

235The prior CASAC 2014 letter on the second 

draft PA in that review stated the following (Frey, 

2014b, p. 9–10): 

For example, CASAC concurs that trees are 

important from a public welfare perspective 

because they provide valued services to humans, 

including aesthetic value, food, fiber, timber, other 

forest products, habitat, recreational opportunities, 

climate regulation, erosion control, air pollution 

removal, and hydrologic and fire regime 

stabilization. Damage effects to trees that are 

adverse to public welfare occur in such locations 

as national parks, national refuges, and other 

protected areas, as well as to timber for commercial 

use. The CASAC concurs that biomass loss in trees 

is a relevant surrogate for damage to tree growth 

that affects ecosystem services such as habitat 

provision for wildlife, carbon storage, provision of 

food and fiber, and pollution removal. Biomass loss 

may also have indirect process-related effects such 

as on nutrient and hydrologic cycles. Therefore, 

biomass loss is a scientifically valid surrogate of a 

variety of adverse effects to public welfare. 

236The CASAC letter on the draft PA in the 

current review stated the following (Cox, 2020a, 

Consensus Responses to Charge Questions p. 18): 

The RBL appears to be appropriately considered 

as a surrogate for an array of adverse welfare effects 

and based on consideration of ecosystem services 

and potential for impacts to the public as well as 

conceptual relationships between vegetation growth 

effects and ecosystem scale effects. Biomass loss is 

a scientifically sound surrogate of a variety of 

adverse effects that could be exerted to public 

welfare. . . . In the previous review, the 

Administrator used RBL as a surrogate for 

consideration of the broader array of vegetation 

related effects of potential welfare significance that 

included effects of growth of individual sensitive 

species and extended to ecosystem level effects 

such as community composition in natural forests, 

particularly in protected public lands (80 FR 65406, 

October 26, 2015). The EPA believes, and the 

CASAC concurs, that information available in the 

present review does not call into question this 

approach, indicating there continues to be support 

for the use of tree seedling RBL as a proxy for the 

broader array of vegetation-related effects, most 

particularly those related to growth. 

237Further, the EPA lacks sufficient information 

in the air quality criteria to identify requisite air 

quality for these effects. 

evidence that would support an 

alternative interpretation of the 

commenters’ statement as a claim that a 

tree experiencing a 2% RBL per year is 

reduced in absolute biomass by 6% after 

three years.233

Some commenters who disagree with 

the proposed decision also express the 

view that the EPA has ‘‘proposed’’ to 

use RBL functions for trees as a proxy 

for all vegetation effects. Based on this 

view, these commenters variously assert 

that the EPA is failing to comply with 

its obligation under the Clean Air Act 

that a secondary standard protect the 

public welfare from ‘‘any known or 

anticipated adverse effects’’; that the 

EPA’s approach is not the same as the 

prior CASAC’s discussion of RBL as a 

surrogate; that the EPA is contravening 

its statutory obligation by using one 

adverse effect as a surrogate for another 

without showing that prevention of the 

former will prevent the latter; and that, 

based on the commenters’ interpretation 

of a statement made by the prior 

CASAC, a standard that allows tree 

growth loss above 2% cannot protect 

against visible foliar injury. As an initial 

matter, we note that the citation 

provided by the commenters for their 

statement that the ‘‘EPA proposes’’ to 

use RBL functions as a proxy for the 

broad array of O3 vegetation-related 

effects does not include such a 

‘‘proposal.’’ Rather the commenters’ 

citation points to the background 

section of the proposal which simply 

summarizes the concept of RBL as a 

proxy or surrogate which was employed 

in the last review and which was 

described by the prior CASAC (85 FR 

49899, August 14, 2020). In describing 

use of RBL as a proxy or surrogate, the 

proposal (and the PA) use several 

phrases, ranging from ‘‘for consideration 

of the broader array of vegetation-related 

effects of potential public welfare 

significance, that included effects on 

growth of individual sensitive species 

and extended to ecosystem-level effects, 

such as community composition in 

natural forests, particularly in protected 

public lands, as well as forest 

productivity’’ (85 FR 49878, August 14, 

2020), to shorter phrases, such as ‘‘for 

the broad array of vegetation related 

effects that extend to the ecosystem 

scale’’ (85 FR 49911, August 14, 2020). 

We disagree with these commenters 

that the way the EPA uses RBL as a 

‘‘proxy’’ or ‘‘surrogate’’ is contrary to 

law, and with their contention that the 

EPA uses one adverse effect as a 

surrogate for another without showing 

that prevention of the former will 

prevent the latter. As described in the 

Administrator’s decision below, the 

most precise use of RBL as a surrogate 

or proxy is in the target level of 

protection for cumulative seasonal 

exposure (17 ppm-hrs as a 3-year 

average W126 index). This use relates 

specifically to public welfare effects 

related to O3 effects on growth of 

individual sensitive species and related 

effects, including ecosystem-level 

effects, such as community composition 

in natural forests, particularly in 

protected public lands, as well as forest 

productivity (as discussed in the PA, 

section 4.5.1.2). In fact, the ISA 

describes (or relies on) conceptual 

relationships among such effects in 

considering causality determinations for 

ecosystem-scale effects such as altered 

terrestrial community composition and 

reduced productivity, as well as 

reduced carbon sequestration, in 

terrestrial ecosystems (ISA, Appendix 8, 

sections 8.8 and 8.10). Beyond these 

relationships of plant-level effects and 

ecosystem-level effects,234 RBL can be 

appropriately described as a 

scientifically valid surrogate of a variety 

of welfare effects based on consideration 

of ecosystem services and the potential 

for adverse impacts on public welfare, 

as well as conceptual relationships 

between vegetation growth-related 

effects (including carbon allocation) and 

ecosystem-scale effects (PA, pp. 4–75 

and 4–76). Both the prior CASAC and 

the current CASAC recognized this 

(Frey, 2014b, pp. iii, 9–10; 235 Cox, 

2020a, Consensus Responses to Charge 

Questions pp. 18 and 21 236). As was 

discussed in the proposal, the 

information available in this review 

provides continued support for the use 

of tree seedling RBL as a proxy for the 

broad array of vegetation-related effects 

conceptually related to growth effects, a 

conclusion with which the CASAC 

agreed (85 FR 49899,49906, August 14, 

20202).237

As recognized in the proposal (and 

PA) there are two other vegetation effect 

categories with extensive evidence bases 

(which include analyses that assess the 

influence of cumulative seasonal 

exposure); these are crop yield loss and 

visible foliar injury. As discussed above, 

the consideration of protection provided 

by the current standard for the former 

goes beyond the target focused on RBL 

and includes aspects of the evidence 

specific to those effects. As described 

above and in section III.B.3 below, the 

EPA is concluding that the level of 

protection is adequate to protect the 

public welfare from effects related to 

crop yield loss. With regard to the latter, 

contrary to the commenter’s assertion, 

the EPA is not claiming that protection 

focused on RBL provides protection for 

visible foliar injury. The EPA’s 

consideration of visible foliar injury is 

described earlier in this section and in 

section III.B.3 below. 


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With regard to the two newly 

identified categories of insect-related 

effects, the Administrator finds there to 

be insufficient information to judge the 

current standard inadequate based on 

these effects, as discussed in section 

III.B.3 below. He does not claim that 

RBL provides a surrogate for these 

effects. However, he notes that the 

available information in the air quality 

criteria does not indicate a greater 

sensitivity of such effects as compared 

to O3 effects on vegetation growth, and 

that he lacks sufficient information in 

the air quality criteria to identify 

requisite air quality for these effects. 

(vi) W126 Index in Areas Meeting 

Current Standard 

In objecting to the proposed decision, 

one group of commenters disagree with 

EPA’s findings regarding the W126 

index levels in areas that meet the 

current standard. In so doing, these 

commenters claim that the EPA is 

mistaken to claim that in virtually all 

design value periods and locations at 

which the current standard was met 

across the period covered by the 

historical dataset the 3-year W126 index 

was at or below 17 ppm-hrs because 

they variously assert there are either 25 

or 21 such occurrences, and they further 

assert there to be either 50 occurrences 

of a single-year W126 index at or above 

19 ppm-hrs or 52 occurrences of a 

single-year W126 index above 19 ppmhrs. These counts are in disagreement 

with the air quality analyses 

documented in Appendix 4D of the PA. 

For example, out of 8,292 values across 

nearly 20 years for U.S. ambient air 

monitoring sites, distributed across all 

nine climate regions, with air quality 

that meets the current standard, there 

are just eight occurrences of a 3-year 

W126 index value above 17 ppm-hrs 

(PA, Appendix 4D, Tables 4D–10 and 

4D–7). This means that 99.9% of the 

records (virtually all) were at or below 

17 ppm-hrs. While the details of each 

step of the analyses in the PA are 

extensively documented, including data 

handling, rounding conventions and 

data acceptability criteria (PA, 

Appendix 4D, section4D.2), the lack of 

documentation provided by the 

commenters and their conflicting claims 

(indicated above) leave the EPA to 

hypothesize that the reason for the 

disagreements include differences with 

regard to these details, such as those 

regarding rounding conventions. As 

described in the PA, W126 values ‘‘were 

rounded to the nearest unit ppm-hr for 

applications requiring direct 

comparison to a W126 level,’’ a 

convention intended to provide 

consistency in the precision of the 

comparison as the W126 levels for 

comparison were also in whole numbers 

(PA, Appendix 4D, section 4D.2.2). With 

the rounding conventions applied in the 

PA, there are eight 3-year W126 index 

values greater than 17 ppm-hrs (i.e., 

equal to 18 or 19). It may be that the 

commenters counted unrounded 3-year 

W126 index values as low as 17.01 as 

being greater than 17 ppm-hrs, although 

the reason for them providing two 

conflicting counts is unclear. Similarly 

with regard to the counts for single-year 

W126 index values above 19 ppm-hrs, 

the commenters may have counted 

unrounded single-year index values as 

low as 19.01 ppm-hrs as being greater 

than 19 ppm-hrs. Thus, we find the 

commenters criticism of the EPA’s 

characterization of the findings of the 

air quality analyses, as well as the 

commenters’ counts, to be unfounded. 

Some commenters claim EPA pays 

inadequate attention to the relatively 

few occurrences of single-year W126 

index values at or above 19 ppm-hrs 

that have occurred at sites meeting the 

current standard since 2002 and that the 

standard must be set to avoid such 

occurrences. The EPA disagrees with 

these commenters, as described below, 

after carefully considering the relatively 

few occurrences of W126 index values 

at or above 19 ppm-hrs, including 

single-year values. In so doing, we have 

given particular focus on Class I areas, 

recognizing the attention given to such 

areas by the Administrator in judging 

the potential for effects adverse to the 

public welfare, a focus recognized by 

the CASAC and with which the prior 

CASAC explicitly concurred (Cox, 

2020a; Frey, 2014b, p. 9). 

Among the nearly 500 values for 

monitoring sites in or near Federal Class 

I areas across the U.S., during periods 

from 2000 through 2018 when the 

current standard was met, there are no 

occurrences of a 3-year average W126 

index above 19 ppm-hrs (PA, Table 4– 

1). Across this same period in the same 

Class I locations, there are just 15 

occurrences of single-year W126 index 

values above 19 ppm-hrs, all of which 

date prior to 2013 (PA, Appendix 4D, 

section 4D.3.2.4). All of these 

occurrences are below 25 ppm-hrs. 

Thus, in addition to their being 

relatively few occurrences of a singleyear W126 above 19 ppm-hrs in/near a 

Class I area in the 19-year dataset, none 

of them (the most recent of which was 

in 2012) is higher than 25 ppm-hrs; in 

fact, the highest is 23 ppm-hrs (PA, 

Appendix 4D, section 4D.3.2.4). 

We have also considered the full 19- 

year dataset for locations beyond those 

in or near Class I areas, noting that, at 

other sites across the U.S., occurrences 

of single-year W126 index above 19 

ppm-hrs (22) were predominantly in 

urban areas, including Los Angeles, and 

the highest values were just equal to 25 

ppm-hrs, or, in one instance, just equal 

to 26 ppm-hrs, when rounded (85 FR 

49895, 49904, August 14, 2020; PA, 

sections 4.4 and 4.5, Appendix 4D). In 

considering the potential risk posed by 

these scattered occurrences, largely in 

urban areas, with none since 2012 in or 

near a Class I areas, we additionally 

consider the data on peak hourly 

concentrations also discussed above 

(Wells, 2020). Together, these data 

indicate the control provided by the 

current standard in areas that are of 

particular focus in protecting the public 

welfare, on the extent and frequency of 

occurrence of cumulative exposures in 

terms of the W126 index (and of peak 

concentrations) of a magnitude of 

potential concern. As discussed in 

section III.B.3 below, the Administrator 

does not find the air quality patterns 

allowed by the current standard, as 

indicated by these analyses, to pose a 

risk of adverse effects to the public 

welfare. 

In their criticism of the EPA’s air 

quality analyses, one commenter claims 

that the analyses are difficult to evaluate 

for California and other West region 

states and suggest that California sites 

brought into compliance with the 

existing standard would still have 

elevated W126 index values, similar to 

sites in the Southwest region. We 

disagree with the commenter’s claim 

that the air quality analyses suggest that 

compliance with the existing standard 

would not reduce the W126 index 

values at California sites. In making 

their claim, the commenters cite Figures 

4D–4 and 4D–5 of the PA. These figures, 

however, simply document W126 index 

at sites with various design values at 

one point in time (2016–2018). They do 

not describe analyses of trends over 

time, with changes in air quality. Yet, 

that very issue was the subject of 

separate regression analyses in the PA 

(PA, Appendix 4D, section 4D.3.2.3). 

These analyses show that the Southwest 

region, which had highest W126 index 

values at sites meeting the current 

standard, also exhibited the greatest 

improvement in the W126 metric values 

per unit decrease in their design value 

(slope of 0.93) over the nearly 20 year 

period analyzed. The pattern is very 

similar for the West region (with a slope 

of 0.80), with the exception of three 

sites (in downtown LA); however, the 

design values for these sites are above 

100 ppb, making such projections quite 

uncertain (PA, Appendix 4D, section 

4D.3.2.3). 


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238 In raising EPA’s conclusions on a carbon 

storage analysis in the last review, some 

commenters repeat their comments in the last 

review that claimed that the relatively lesser weight 

the EPA placed on the 2014 WREA estimates of 

carbon storage (in terms of CO2) was inconsistent 

with the emphasis the EPA placed on CO2

emissions reductions estimated for another 

regulatory action. The commenters overlook, 

however, key distinctions between the two types of 

estimates in the two different analyses which 

appropriately led the EPA to recognize much 

greater uncertainty in the WREA estimates and 

accordingly give them less weight. While the WREA 

estimates were for amounts of CO2 removed from 

the air and stored in vegetation as a result of plant 

photosynthesis occurring across the U.S., the 

estimates for the other action were for reductions 

in CO2 produced and emitted from power plants (79 

FR 34830, 34931–33). The potentially transient 

nature of carbon storage in vegetation makes a ton 

of additional carbon uptake by plants in the former 

arguably unequal to a ton of reduced emissions 

from fossil fuels. Further, there are appreciably 

larger uncertainties involved in attempting to 

quantify the additional carbon uptake by plants 

which requires complex modeling of biological and 

ecological processes and their associated sources of 

uncertainty, and there is no new information 

available in the current review that would reduce 

such uncertainties in quantitative estimates of 

carbon storage benefits to climate. In recognizing 

the public welfare value of ecosystem carbon 

storage, we additionally note, however, that 

protection provided by the current standard from 

vegetation effects (and RBL) also provides a degree 

of protection in terms of carbon storage. 

(vii) Climate Effects 

In support of their disagreement with 

the EPA’s proposed decision, some 

commenters claim that EPA needs to 

establish a standard to protect from 

radiative forcing and related climate 

effects. In so doing, they stated that EPA 

cannot rely on uncertainty by retaining 

the existing standard and instead, given 

the uncertainties recognized in the ISA, 

which they suggest could mean current 

information underestimates O3 climate 

related impacts, the Administrator 

should strengthen the existing standard 

or establish an additional standard. 

Some commenters additionally assert 

that the EPA has failed to address a 

recommendation from CASAC regarding 

a quantitative analysis, while also 

criticizing EPA conclusions regarding a 

carbon storage analysis in the last 

review. The EPA disagrees with the 

commenters that the available 

information is sufficient to identify such 

a standard that could be judged to 

provide the requisite protection under 

the Act, and notes that the commenters 

do not submit or describe such 

information; nor do the commenters 

identify a standard that they claim 

would provide such protection. 

With regard to the CASAC 

recommendation cited by some 

commenters, we note in its review of the 

draft PA, the CASAC recommended 

changes to the PA to ‘‘more thoroughly 

address effects of ozone on climate 

change,’’ that would include some 

quantitation, such as estimates of 

climate change related to a change in O3

(Cox 2020a, Consensus Responses to 

Charge Questions p. 22). In 

consideration of this advice, the final 

PA included additional discussion on 

the available information and tools 

related to such estimates. As discussed 

below, we conclude that this 

information, as documented in the ISA, 

does not provide a foundation with 

which to derive such estimates as might 

pertain to O3 and public health and 

welfare considerations relevant to 

decisions on the NAAQS.238

As recognized in the proposal and 

summarized in section III.A.2 above, 

there are a number of limitations and 

uncertainties that affect our ability to 

characterize the extent of any 

relationships between O3 concentrations 

in ambient air in the U.S. and climaterelated effects, thus precluding a 

quantitative characterization of climate 

responses to changes in O3

concentrations in ambient air at regional 

(vs global) scales. While evidence 

supports a causal relationship between 

the global abundance of O3 in the 

troposphere and radiative forcing, and a 

likely causal relationship between the 

global abundance of O3 in the 

troposphere and effects on temperature, 

precipitation, and related climate 

variables (ISA, section IS.5.2 and 

Appendix 9; Myhre et al., 2013), the 

non-uniform distribution of O3

(spatially and temporally) makes the 

development of quantitative 

relationships between the magnitude of 

such effects and differing O3

concentrations in the U.S. challenging 

(ISA, Appendix 9). Additionally, ‘‘the 

heterogeneous distribution of ozone in 

the troposphere complicates the direct 

attribution of spatial patterns of 

temperature change to ozone induced 

[radiative forcing]’’ and there are ‘‘ozone 

climate feedbacks that further alter the 

relationship between ozone [radiative 

forcing] and temperature (and other 

climate variables) in complex ways’’ 

(ISA, Appendix 9, section 9.3.1, p. 9– 

19). Thus, various uncertainties ‘‘render 

the precise magnitude of the overall 

effect of tropospheric ozone on climate 

more uncertain than that of the wellmixed GHGs’’ and ‘‘[c]urrent limitations 

in climate modeling tools, variation 

across models, and the need for more 

comprehensive observational data on 

these effects represent sources of 

uncertainty in quantifying the precise 

magnitude of climate responses to ozone 

changes, particularly at regional scales’’ 

(ISA, section IS.6.2.2, Appendix 9, 

section 9.3.3, p. 9–22). For example, 

current limitations in modeling tools 

include ‘‘uncertainties associated with 

simulating trends in upper tropospheric 

ozone concentrations’’ (ISA, section 

9.3.1, p. 9–19), and uncertainties such 

as ‘‘the magnitude of [radiative forcing] 

estimated to be attributed to 

tropospheric ozone’’ (ISA, section 9.3.3, 

p. 9–22). Further, ‘‘precisely quantifying 

the change in surface temperature (and 

other climate variables) due to 

tropospheric ozone changes requires 

complex climate simulations that 

include all relevant feedbacks and 

interactions’’ (ISA, section 9.3.3, p. 9– 

22). For example, an important 

limitation in current climate modeling 

capabilities for O3 is representation of 

important urban- or regional-scale 

physical and chemical processes, such 

as O3 enhancement in high-temperature 

urban situations or O3 chemistry in city 

centers where NOx is abundant. 

Because of such limitations we cannot 

quantify or judge the impact of 

incremental changes in O3

concentrations in the U.S. on radiative 

forcing and subsequent climate effects. 

Thus, as discussed in section III.B.3 

below, the significant limitations and 

uncertainties summarized here together 

preclude identification of an O3

standard that could be judged to provide 

requisite protection of the public 

welfare from adverse effects linked to O3

influence on radiative forcing, and 

related climate effects. Contrary to the 

commenters’ charge that the lack of a 

quantitative analysis of climate-related 

effects due to recognition of such 

limitations and uncertainties is 

unlawful and arbitrary, the information 

available in this review is insufficient to 

judge the existing standard inadequate 

or to identify an appropriate revision 

based on O3-related climate effects. In 

the face of insufficient evidence for such 

conclusions, it might, on the contrary, 

be judged unlawful and arbitrary for the 

Agency to perform guesswork to assert 

a particular new standard provided 

requisite protection for this category of 

effects. The EPA agrees with the 

commenters that ‘‘perfect information’’ 

is not required. However, information 

that provides for assessment of how the 

current and potential alternative or 

additional standards would affect O3- 

related climate impacts is lacking. As 

noted in the ISA, few studies have even 

attempted to estimate ‘‘climate response 

to changes in tropospheric ozone 

concentrations alone in the future 

atmosphere,’’ and effects of O3 on 

radiative forcing and climate depend on 

many factors other than tropospheric 

ozone concentrations, including 

‘‘changes in a suite of climate-sensitive 


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factors, such as the water vapor content 

of the atmosphere’’ (ISA, p. 9–20; Myhre 

et al., 2013). Thus, as discussed in 

section III.B.3 below, while the 

Administrator recognizes that the 

evidence supports a relationship of 

tropospheric O3 with climate effects, he 

judges the quantitative uncertainties to 

be too great to support identification of 

a standard specific to such effects. 

4. Administrator’s Conclusions 

Based on the large body of evidence 

concerning the welfare effects, and 

potential for public welfare impacts, of 

exposure to O3 in ambient air, and 

taking into consideration the attendant 

uncertainties and limitations of the 

evidence, the Administrator concludes 

that the current secondary O3 standard 

provides the requisite protection against 

known or anticipated adverse effects to 

the public welfare, and should therefore 

be retained, without revision. In 

reaching these conclusions, the 

Administrator has carefully considered 

the assessment of the available welfare 

effects evidence and conclusions 

contained in the ISA, with supporting 

details in the 2013 ISA and past AQCDs; 

the evaluation of policy-relevant aspects 

of the evidence and quantitative 

analyses in the PA (summarized in 

sections III.A.2 and III.A.3 above); the 

advice and recommendations from the 

CASAC (summarized in section III.B.1.b 

above); and public comments (as 

discussed in section III.B.2 above and 

the separate Response to Comments 

document), as well as the August 2019 

decision of the D.C. Circuit remanding 

the secondary standard established in 

the last review to the EPA for further 

justification or reconsideration. 

In considering the currently available 

information in this review of the O3

secondary standard, the Administrator 

recognizes the longstanding evidence 

base for vegetation-related effects, 

augmented in some aspects since the 

last review, described in section 

III.A.2.a above. The currently available 

evidence describes an array of effects on 

vegetation and related ecosystem effects 

causally or likely to be causally related 

to O3 in ambient air, as well as the 

causal relationship of tropospheric O3 in 

radiative forcing and subsequent likely 

causally related effects on temperature, 

precipitation and related climate 

variables. The Administrator also takes 

note of the quantitative analyses and 

policy evaluations documented in the 

PA that, with CASAC advice and 

consideration of public comment, 

inform the judgments required of him in 

reaching his decision on a secondary 

standard that provides the requisite 

protection under the Act. 

As an initial matter, the Administrator 

recognizes the continued support in the 

current evidence for O3 as the indicator 

for photochemical oxidants (as 

recognized in section III.B.1.c above). In 

so doing, he notes that no newly 

available evidence has been identified 

in this review regarding the importance 

of photochemical oxidants other than O3

with regard to abundance in ambient 

air, and potential for welfare effects, and 

that, as stated in the current ISA, ‘‘the 

primary literature evaluating the health 

and ecological effects of photochemical 

oxidants includes ozone almost 

exclusively as an indicator of 

photochemical oxidants’’ (ISA, section 

IS.1.1). Thus, the Administrator 

recognizes that, as was the case for 

previous reviews, the evidence base for 

welfare effects of photochemical 

oxidants does not indicate an 

importance of any other photochemical 

oxidants. For these reasons, described 

with more specificity in the ISA and PA, 

he proposes to conclude it is 

appropriate to retain O3 as the indicator 

for the secondary NAAQS for 

photochemical oxidants (85 FR 49896, 

August 14, 2020). 

In his review of the existing 

secondary O3 standard, in light of the 

evidence base and quantitative analyses 

available today, the Administrator has 

given particular attention to 

consideration of the issues raised by the 

August 2019 court remand, and related 

issues raised in public comment, as well 

as analyses that were conducted or 

updated in this review in consideration 

of the remand and related public 

comment. In so doing, he has also given 

careful consideration of the form and 

averaging time of the current standard 

and its ability to control the patterns of 

O3 concentrations that contribute to 

environmental exposures of potential 

concern to the public welfare. Further, 

he has considered what is indicated by 

the information currently available with 

regard to exposure metrics, supported 

by the current evidence, for assessing 

potential risks posed to vegetation, and 

protection provided from such 

exposures. Additionally, with regard to 

visible foliar injury, he has considered 

the current evidence in the ISA in 

combination with quantitative 

information and policy evaluations in 

the PA, advice from the CASAC and 

public comment, in making judgments 

regarding adequacy of the protection 

provided by the current standard from 

adverse effects to the public welfare 

related to this effect. Before turning to 

these issues, discussed, in turn, below 

in the context of the EPA’s 

understanding of the information now 

available in the current review, he 

addresses two endpoints newly 

identified in this review, as well as 

tropospheric O3 effects related to 

climate. 

With regard to the two insect-related 

categories of effects with new ISA 

determinations in this review, the 

Administrator takes note of the 

conclusions that the current evidence is 

sufficient to infer likely causal 

relationships of O3 with alterations of 

plant-insect signaling and insect 

herbivore growth and reproduction (as 

summarized in section III.A.2.a above). 

He additionally recognizes the PA 

finding that uncertainties in the current 

evidence, as summarized in section 

III.A.2 above, preclude a full 

understanding of such effects, the air 

quality conditions that might elicit 

them, the potential for impacts in a 

natural ecosystem. Accordingly, the 

Administrator notes a lack of clarity in 

the characterization of these effects, and 

a lack of important quantitative 

information to consider such effects in 

this context such that it is not feasible 

to relate different patterns of O3

concentrations with specific risks of 

such alterations. As a result, the 

Administrator concludes there is 

insufficient information to judge how 

particular ambient air concentrations of 

O3 relate to the degree of impacts on 

public welfare related to these effects. 

Thus, he concludes there is insufficient 

information to judge the current 

standard inadequate or to identify an 

appropriate revision based on these 

effects. 

Before focusing further on the key 

vegetation-related effects identified 

above, the Administrator first considers 

the strong evidence documenting 

tropospheric O3 as a greenhouse gas 

causally related to radiative forcing, and 

likely causally related to subsequent 

effects on variables such as temperature 

and precipitation. In so doing, he takes 

note of the limitations and uncertainties 

in the evidence base that affect 

characterization of the extent of any 

relationships between O3 concentrations 

in ambient air in the U.S. and climaterelated effects, and preclude 

quantitative characterization of climate 

responses to changes in O3

concentrations in ambient air at regional 

or national (vs global) scales, as 

summarized in sections III.A.2 above. 

As a result, he recognizes the lack of 

important quantitative tools with which 

to consider such effects in this context 

such that it is not feasible to relate 

different patterns of O3 concentrations 

at the regional (or national) scale in the 

U.S. with specific risks of alterations in 

temperature, precipitation and other 


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climate-related variables. The 

Administrator finds that these 

significant limitations and uncertainties 

together preclude his identification of 

an O3 standard reasonably judged to 

provide requisite protection of the 

public welfare from adverse effects 

linked to O3 influence on radiative 

forcing, and related climate effects. 

Thus, the Administrator concludes that 

the information available in this review 

is insufficient to judge the existing 

standard inadequate or to identify an 

appropriate revision based on O3-related 

climate effects. 

The Administrator turns now to 

vegetation-related effects, the evidence 

for which as a whole is extensive, spans 

several decades, and supports the 

Agency’s conclusions of causal or likely 

to be causal relationship for O3 in 

ambient air with an array of effect 

categories. These categories include 

reduced vegetation growth, 

reproduction, crop yield, productivity 

and carbon sequestration in terrestrial 

systems; increased tree mortality; 

alteration of terrestrial community 

composition, belowground 

biogeochemical cycles and ecosystem 

water cycling; and visible foliar injury 

(ISA, Appendix 8). As an initial matter, 

the Administrator notes the new ISA 

determination that the current evidence 

is sufficient to infer likely causal 

relationships of O3 with increased tree 

mortality. With regard to the current 

evidence for this effect, the 

Administrator notes that the evidence 

does not indicate a potential for O3

concentrations that occur in locations 

that meet the current standard to cause 

increased tree mortality, as summarized 

in section III.A.2.a above (PA, section 

4.3.1). Accordingly, he finds it 

appropriate to focus on more sensitive 

effects, such as tree seedling growth, in 

his review of the standard. Thus, in 

considering the adequacy of protection 

provided by the current standard from 

adverse effects to the public welfare 

related to these effects, the 

Administrator begins by considering 

vegetation growth and conceptually 

related effects with a focus on RBL 

(described in section III.B.2 above), then 

turns to a specific consideration of crop 

yield loss and lastly, to consideration of 

visible foliar injury. 

With regard to vegetation growth and 

related effects, the Administrator has 

considered discussions in the PA and in 

response to public comments in section 

III.B.2 above, and finds it appropriate 

for identification of the requisite 

protection to extend beyond 

consideration of a magnitude of growth 

effects, per se, that he may judge 

adverse to the public welfare. Rather, 

the Administrator extends his 

consideration beyond that, judging it 

appropriate to consider reduced growth 

(i.e., RBL) as a proxy for an array of 

other vegetation-related effects to the 

public welfare. As discussed in section 

III.B.2 above, these categories of effects 

include reduced vegetation growth, 

reproduction, productivity and carbon 

sequestration in terrestrial systems, and 

also alteration of terrestrial community 

composition, belowground 

biogeochemical cycles and ecosystem 

water cycling. In adopting RBL as a 

proxy for this array of effects, the 

Administrator notes that such a use is 

consistent with advice from CASAC, 

and that RBL was also adopted as a 

proxy for this array of effects by the 

prior Administrator, in consideration of 

advice from the prior CASAC. 

In assessments of RBL estimated from 

O3 exposure, the Administrator takes 

note of the PA consideration of the 

established E–R relationships for RBL in 

tree seedlings of 11 species with O3

exposures in terms of W126 index (PA, 

Appendix 4A). In so doing, he agrees 

with the PA conclusion regarding 6% 

RBL, with which the CASAC concurred, 

as described in sections III.B.1.b and 

III.B.2 above), and judges that for his use 

of RBL as a proxy, maintaining O3

concentrations such that associated 

estimates of RBL fall below 6%, as a 

median across the 11 species 

represented by the established E–R 

relationships would assure the 

appropriate protection. In making these 

judgments, he observes that they were 

also adopted by the prior Administrator, 

with consideration of advice from the 

prior CASAC. 

Further, based on considerations 

discussed in the PA, advice from 

CASAC and discussion in section III.B.2 

above, Administrator has considered the 

use of RBL in his judgment of the public 

welfare protection provided by the 

secondary standard. Based on those 

considerations, including uncertainties 

in the E–R relationships and their use in 

the way described here, the 

Administrator judges it appropriate for 

the standard to protect against W126 

index values associated with a median 

RBL at or above 6% (while also 

controlling peak hourly concentrations, 

as discussed below). Based on this 

judgment, in addition to a recognition of 

uncertainty in these estimates (in light 

of the discussion in section III.B.2.b(ii) 

above regarding the appropriate 

duration or averaging for the W126 

index metric) he concludes it 

appropriate for the standard to generally 

control exposures in terms of W126 

index to a level of 17 ppm-hrs, 

recognizing that the RBL estimated for 

such a W126 index value is 5.3%, a 

value appreciably below 6%. 

With regard to the appropriate O3

exposure metric to employ in assessing 

adequacy of air quality control in 

protecting against RBL, the 

Administrator has considered the 

discussions in the PA, and in response 

to public comments in section III.B.2 

above regarding the available evidence 

and air quality analyses. He has also 

considered this in the context of the 

court remand with regard to the EPA’s 

use of a 3-year average W126 index to 

assess protection from RBL and the 

court’s reference to advice from the 

prior CASAC on protection against 

‘‘unusually damaging years’’ (described 

in section III.B.2 above). In so doing, the 

Administrator considers below the 

extent of conceptual similarities of the 

3-year average W126 index with some 

aspects of the derivation approach for 

the established E–R functions, the 

context of RBL as a proxy (as recognized 

above), and limitations associated with 

a reliance solely on W126 index as a 

metric to control exposures that might 

be termed ‘‘unusually damaging.’’ 

With regard to the established E–R 

functions used to describe the 

relationship of RBL with O3 in terms of 

a seasonal W126 index, the 

Administrator recognizes that 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), such that 

the estimates may represent average 

impact for a season, as summarized in 

section III.A.1.c(ii) above (PA, section 

4.5.1.2, Appendix 4A, Attachment 1). 

He notes the compatibility of W126 

index averaged over multiple growing 

seasons or years with these adjustments. 

He also notes the exposure levels 

represented in the data underlying the 

E–R functions are somewhat limited 

with regard to the relatively lower 

cumulative exposure levels most 

commonly associated with the current 

standard (e.g., at or below 17 ppm-hrs), 

indicating additional uncertainty for 

application to such levels. Further, he 

notes the PA observation that some of 

the underlying studies did not find 

statistically significant effects of O3 at 

the lower exposure levels, indicating 

some uncertainty in predictions of an 

O3-related RBL at those levels, as 

summarized in section III.A.1.c(ii) above 

(PA, section 4.5.1.2). He additionally 

notes the differing patterns of hourly 

concentrations of the elevated exposure 

levels in the datasets from which the E– 

R functions from the patterns in ambient 


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239The ISA references 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’’ [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]). 

air meeting the current standard across 

the U.S. today, as summarized in 

section III.B.2.b(ii). With these 

considerations regarding the E–R 

functions and their underlying datasets 

in mind, he also takes note of year-toyear 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 (ISA, Appendix 8, 

section 3.12; 2013 ISA section 9.4.8.3; 

PA, sections 4.3 and 4.5). Based on 

these considerations, the Administrator 

finds there to be a consistency of his use 

of the W126 index averaged over 

multiple years with the approach used 

in deriving the E–R function, and with 

other factors that may affect growth in 

the natural environment. 

In light of such considerations, the 

Administrator agrees with the PA 

finding that several factors contribute 

uncertainty and some resulting 

imprecision or inexactitude to RBL 

estimated from single-year seasonal 

W126 index values, as discussed in 

section III.D.1.b(ii) of the proposal (85 

FR 49900–01, August 14, 2020; PA 

sections 4.5.1.2 and 4.5.3). The 

Administrator additionally recognizes 

the qualitative and conceptual nature of 

our understanding, in many cases, of 

relationships of O3 effects on plant 

growth and productivity with largerscale impacts, such as those on 

populations, communities and 

ecosystems. From these considerations, 

he judges that use of a seasonal RBL 

averaged over multiple years, such as a 

3-year average, is reasonable, and 

provides a more stable and wellfounded RBL estimate for his purposes 

as a proxy to represent the array of 

vegetation-related effects identified 

above. The Administrator additionally 

takes note of the CASAC advice agreeing 

with the EPA’s focus on a 3-year average 

W126 for this purpose, concluding such 

a focus to be reasonable and 

scientifically sound, as summarized in 

section III.B.1.b above. In light of these 

considerations, the Administrator finds 

there to be support for use of an average 

seasonal W126 index derived from 

multiple years (with their representation 

of variability in environmental factors), 

concluding the use of such averaging to 

provide an appropriate representation of 

the evidence and attention to 

considerations summarized above. In so 

doing, he finds that 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 current evidence. 

Rather, he finds 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. 

In his consideration of the 

appropriateness of using a 3-year 

average W126 metric, the Administrator 

additionally takes note of the discussion 

in section III.B.2 above with regard to 

protection against ‘‘unusually damaging 

years,’’ a caution raised by the prior 

CASAC in considering a secondary 

standard in terms of a 3-year average 

W126 index (and an issue raised in the 

court remand). With regard to this 

caution, the Administrator finds 

informative the discussion in section 

III.B.2 above regarding the extent to 

which a standard in terms of a W126 

metric might be expected to control 

exposure circumstances of concern (e.g., 

for growth effects, among others). This 

discussion and its focus on air quality 

analyses in the PA and additional 

analyses conducted in consideration of 

public comment investigate the annual 

occurrence of elevated hourly O3

concentrations which may contribute to 

vegetation exposures of concern (PA, 

Appendix 2A, section 2A.2; Wells, 

2020).239

These air quality analyses illustrate 

limitations of the W126 index for 

purposes of controlling peak 

concentrations, and also the strengths of 

the current standard in this regard. As 

discussed more fully in section 

III.B.2.b(ii) above, the W126 index 

cannot, by virtue of its definition, 

always differentiate between air quality 

patterns with high peak concentrations 

and those without such concentrations. 

This is demonstrated in the air quality 

analyses which show that the form and 

averaging time of the existing standard 

is much more effective than the W126 

index in limiting peak concentrations 

(e.g., hourly O3 concentrations at or 

above 100 ppb) 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). A similar 

finding is evidence in the historical data 

extending back to 2000. These data 

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). From these analyses, the 

Administrator concludes 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. Thus, in light of 

the current evidence and quantitative 

air quality analyses, the Administrator 

notes that the W126 index, by its very 

definition, 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 

prior CASAC identified for increased 

concern. In so doing, he disagrees with 

the statement of the prior CASAC that 

a single-year W126 index would 

necessarily provide protection from 

such years. Further, he judges that a 

standard based on either a 3-year or a 

single-year W126 index would not be 

expected to provide effective control of 

the peak concentrations that may 

contribute to ‘‘unusually damaging 

years’’ for vegetation. 

Thus, in considering the extent of 

protection provided by the current 

standard, in addition to considering 

seasonal W126 averaged over a 3-year 

period to estimate median RBL using 

the established E–R functions, the 

Administrator finds it appropriate to 

also consider other metrics, including 

peak hourly concentrations. While he 

recognizes that the evidence does not 

indicate a particular threshold number 

of hours at or above 100 ppb (or another 

reference point for elevated 

concentrations), he takes particular note 

of the evidence of greater impacts from 

higher concentrations (particularly with 

increased frequency) and of the air 

quality analyses that document 

variability in such concentrations for 

the same W126 index value. In light of 

these considerations, he judges 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. Thus, the 

Administrator concludes that use of a 

seasonal W126 averaged over a 3-year 

period, which is the design value period 

for the current standard, to estimate 

median RBL using the established E–R 

functions, in combination with a 


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broader consideration of air quality 

patterns, such as peak hourly 

concentrations, is appropriate for 

considering the public welfare 

protection provided by the standard. 

In the discussion above, the 

Administrator recognizes a number of 

public welfare policy judgments 

important to his review of the current 

standard that include the 

appropriateness of the W126 index, 

averaged across a 3-year period, for 

assessing the extent of protection 

afforded by the standard from 

cumulative seasonal O3 exposures. In 

reflecting on these judgments, the 

current evidence presented in the ISA 

and the associated evaluations in the 

PA, the Administrator concludes that 

the currently available information 

supports such judgments, additionally 

noting the CASAC concurrence with 

regard to the scientific support for these 

judgments (Cox 2020a, Consensus 

Responses to Charge Questions p. 21). 

Accordingly, the Administrator 

concludes that the current evidence 

base and available information 

(qualitative and quantitative) continues 

to support consideration of the potential 

for O3-related vegetation impacts in 

terms of the RBL estimates from 

established E–R functions as a 

quantitative tool within a larger 

framework of considerations pertaining 

to the public welfare significance of O3

effects. Such consideration includes 

effects that are associated with effects 

on vegetation, and particularly those 

that conceptually relate to growth, and 

that are causally or likely causally 

related to O3 in ambient air, yet for 

which there are greater uncertainties 

affecting estimates of impacts on public 

welfare. The Administrator additionally 

notes that this approach to weighing the 

available information in reaching 

judgments regarding the secondary 

standard additionally takes into account 

uncertainties regarding the magnitude of 

growth impact that might be expected in 

mature trees (e.g., compared to 

seedlings), and of related, broader, 

ecosystem-level effects for which the 

available tools for quantitative estimates 

are more uncertain and those for which 

the policy foundation for consideration 

of public welfare impacts is less well 

established. 

In his consideration of the adequacy 

of protection provided by the current 

standard, the Administrator does not 

consider every possible instance of an 

effect on vegetation growth from O3 to 

be adverse to public welfare, although 

he recognizes that, depending on factors 

including extent and severity, such 

vegetation-related effects have the 

potential to be adverse to public 

welfare. Comments from the current 

CASAC, in the context of its review of 

the draft PA, expressed the view that the 

strategy described by the prior 

Administrator for the secondary 

standard established in 2015 with its 

focus on limiting 3-year average W126 

index values somewhat below those 

associated with a 6% RBL in the median 

species and associated W126 index 

target of 17 ppm-hrs (in terms of a 3- 

year average), at or below which the 

2015 standard was expected to generally 

restrict cumulative seasonal exposure, is 

‘‘still scientifically reasonable’’ and 

‘‘still effective in particularly protecting 

the public welfare in light of vegetation 

impacts from ozone’’ (Cox, 2020a, 

Consensus Responses to Charge 

Questions p. 21). In light of this advice 

and based on the current evidence as 

evaluated in the PA, the Administrator 

judges that this approach or framework, 

with its focus on controlling cumulative 

seasonal exposures associated with an 

RBL of 6% or greater, by limiting air 

quality in terms of a 3-year average 

W126 index, to or below a target of 17 

ppm-hrs, in combination with a broader 

consideration of air quality patterns, 

such as control of peak hourly 

concentrations, associated with meeting 

the current standard, is appropriate for 

his use in this review. In so doing, he 

additional notes the isolated, rare 

occurrences in locations meeting the 

current standard of such exposures at 19 

ppm-hrs. Based on the current 

information to inform consideration of 

vegetation effects and their potential 

adversity to public welfare, he 

additionally judges that the RBL 

estimates associated with such 

marginally higher exposures in isolated, 

rare instances are not indicative of 

effects that would be adverse to the 

public welfare, particularly in light of 

variability in the array of environmental 

factors that can influence O3 effects in 

different systems and uncertainties 

associated with estimates of effects 

associated with this magnitude of 

cumulative exposure in the natural 

environment. 

With regard to O3 effects on crop 

yield, the Administrator, as an initial 

matter, takes note of the long-standing 

evidence, qualitative and quantitative, 

of the reducing effect of O3 on the yield 

of many crops, as summarized in the PA 

and current ISA and characterized in 

detail in past reviews (e.g., 2013 ISA, 

2006 AQCD, 1997 AQCD, 2014 WREA). 

He additionally notes the established E– 

R functions for 10 crops and the 

estimates of RYL derived from them, as 

presented in the PA (PA, Appendix 4A, 

section 4A.1, Table 4A–4), and the 

potential public welfare significance of 

reductions in crop yield, as summarized 

in section III.A.2.b above. In so doing, 

however, he additionally recognizes that 

not every effect on crop yield will be 

adverse to public welfare. In the case of 

crops in particular there are a number 

of complexities related to the heavy 

management of many crops to obtain a 

particular output for commercial 

purposes, and related to other factors, 

that the Administrator takes into 

consideration in evaluating potential 

O3-related public welfare impacts, as 

summarized in section III.B.2.b(iv) 

above (PA, sections 4.5.1.3 and 4.5.3). 

Similarly, the Administrator 

concludes that the extensive 

management of agricultural crops that 

occurs to elicit optimum yields (e.g., 

through irrigation and usage of soil 

amendments, such as fertilizer) is 

relevant in evaluating the extent of RYL 

estimated from experimental O3

exposures that should be judged adverse 

to the public welfare. He considers these 

opportunities in crop management for 

market objectives, as well as 

complications in judging relative 

adversity that relate to market responses 

and their effects on producers and 

consumers in evaluating the potential 

impact on public welfare of estimated 

crop yield losses. Further, the 

Administrator takes note of the 

conclusion of the CASAC that the 

available evidence does not call into 

question the adequacy of the current 

secondary standard and that it should 

be retained (Cox 2020a, p.1). 

The Administrator also considered 

the public comments, discussed in 

section III.B.2.b(iv) above, suggesting 

that the proposed decision was not 

giving adequate consideration to crop 

yield effects and that his decision 

should consider a statement by the prior 

CASAC, raised in public comments, that 

a 5% RYL estimate, as the median based 

on the 10 E–R functions, ‘‘represents an 

adverse impact.’’ With regard to the 

prior CASAC statement, he notes the 

discussion in section III.B.2.b(iv) above 

regarding the unclear basis for the prior 

CASAC judgment, both with regard to a 

connection of an estimated 5% RYL to 

broader impacts and to judgments on 

significance of a 5% RYL to the public 

welfare. In considering the adequacy of 

protection of the public welfare from 

effects related to crop yield loss, the 

Administrator considers the air quality 

analyses and the W126 index levels 

commonly occurring in areas that meet 

the current standard. In so doing, he 

notes that W126 index values (3-year 

average) were at or below 17 ppm-hrs in 

virtually all monitoring sites with air 

quality meeting the current standard. 


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240 During the period from 1994 (beginning in 

eastern U.S.) through 2011, the USFS conducted 

surveys of the occurrence and severity of visible 

foliar injury on sensitive species at sites across most 

of the U.S. following a national protocol. 

Based on the established E–R functions, 

the median RYL estimate corresponding 

to 17 ppm-hrs is 5.1%. In considering 

single-year index values, as discussed in 

section III.B.2.b(vi), the vast majority are 

similarly low (with more than 99% less 

than or equal to 17 ppm-hrs), and the 

higher values predominantly occur in 

urban areas. The Administrator 

additionally takes note of the discussion 

in section III.B.2.b(ii) above regarding 

the role of elevated hourly 

concentrations in effects on vegetation 

growth and yield. In so doing, in 

addition to his consideration of W126 

index occurring in areas that meet the 

current standard, he also takes note of 

the control of elevated hourly O3

concentrations that is exerted by the 

current standard. 

In light of all of the above, in reaching 

his judgment regarding public welfare 

implications of the W126 index values 

summarized here (and associated 

estimated RYL), including the isolated 

and rare occurrence of somewhat higher 

values, the Administrator notes that the 

secondary standard is not intended to 

protect against all known or anticipated 

O3-related effects, but rather those that 

are judged to be adverse to the public 

welfare. He also takes into consideration 

the extensive management of 

agricultural crops, and the complexities 

associated with identifying adverse 

public welfare effects for market-traded 

goods (where producers and consumers 

may be impacted differently). Based on 

all of these factors, the Administrator 

disagrees with the prior CASAC 

statement that an estimated median RYL 

of 5% represents an adverse impact and 

further judges that an estimated median 

RYL of 5.1%, based on experimental 

exposures, would not constitute an 

adverse effect on public welfare. 

Accordingly, the Administrator notes 

that the current standard generally 

maintains air quality at a W126 index 

below 17 ppm-hrs, with few exceptions, 

and accordingly would limit the 

estimated RYL (based on experimental 

O3 exposures) to this degree. Therefore, 

he concludes that the current standard 

provides adequate protection of public 

welfare related to crop yield loss and 

does not need to be revised to provide 

additional protection against this effect. 

In so doing, the Administrator notes the 

conclusions by the current CASAC that 

the evidence supports retaining the 

current standard, without revision. 

Turning to consideration of visible 

foliar injury and protection afforded by 

the secondary standard from associated 

impacts to the public welfare, the 

Administrator takes note of the longstanding and well-established evidence 

base, updated in the ISA for this review, 

and of policy-relevant analyses 

presented in the PA to inform his 

judgments regarding a secondary 

standard that provides appropriate 

protection of the public welfare from 

this effect. In so doing, he has also taken 

into account issues raised by public 

comments, both with regard to our 

understanding of relationships between 

O3 exposure circumstances and extent 

and severity of injury in natural areas 

across the U.S., and with regard to the 

extent of our understanding of the 

relationship of injury extent and 

severity to public welfare effects 

anticipated to be adverse, and the 

Murray Energy remand. 

In considering public welfare 

implications of this effect, he notes the 

potential for this effect, when of a 

significant extent and severity, to reduce 

aesthetic and recreational values, such 

as the aesthetic value of scenic vistas in 

protected natural areas including 

national parks and wilderness areas, as 

well as other areas similarly protected 

by state and local governments for 

similar public uses. Based on these 

considerations, the Administrator 

recognizes that, depending on its 

severity and spatial extent, as well as 

the location(s) and the associated 

intended use, the impact of visible foliar 

injury on the physical appearance of 

plants has the potential to be significant 

to the public welfare. In this regard, he 

agrees with the PA statement that cases 

of widespread and relatively severe 

injury during the growing season 

(particularly when sustained across 

multiple years and accompanied by 

obvious impacts on the plant canopy) 

might reasonably be expected to have 

the potential to adversely impact the 

public welfare in scenic and/or 

recreational areas, particularly in areas 

with special protection, such as Class I 

areas (PA, sections 4.3.2 and 4.5.1). In 

so doing, the Administrator notes that 

the secondary standard is not meant to 

protect against all known or anticipated 

O3-related welfare effects, but rather 

those that are judged to be adverse to 

the public welfare, and further notes 

that there are not established measures 

for when such welfare effects should be 

judged adverse to the public welfare. 

Rather, the level of protection from 

known or anticipated adverse effects to 

public welfare that is requisite for the 

secondary standard is a public welfare 

policy judgment to be made by the 

Administrator. 

While recognizing there to be 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 

recognizes that injury to whole stands of 

trees of a severity apparent to the casual 

observer (e.g., when viewed as a whole 

from a distance) would reasonably be 

expected to affect recreational values 

and thus pose a risk of adverse effects 

to the public welfare. He further notes 

that the available information does 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 situation. 

In this context, the Administrator 

takes note of the system developed by 

the USFS for its monitoring program 240

to categorize BI scores of visible foliar 

injury at biosites (sites with O3-sensitive 

vegetation assessed for visible foliar 

injury) in natural vegetated areas by 

severity levels (described in section 

III.A.2.c(ii) above). While recognizing 

that quantitative analyses and evidence 

are lacking that might support a more 

precise conclusion with regard to a 

magnitude of BI score coupled with an 

extent of occurrence that might be 

specifically identified as adverse to the 

public welfare, the Administrator also 

takes note of the D.C. Circuit’s holding 

that substantial uncertainty about the 

level at which visible foliar injury may 

become adverse to public welfare does 

not necessarily provide a basis for 

declining to evaluate whether the 

existing standard provides requisite 

protection against such effects. See 

Murray Energy Corp. v. EPA, 936 F.3d 

597, 619–20 (D.C. Cir. 2019). In this 

context, he considers the discussion in 

the PA and in sections III.A.2.b, III.A.2.c 

and III.B.2 above regarding the USFS 

biosite monitoring program. He finds 

the scale of this program’s objectives, 

which focus on natural settings in the 

U.S. and forests as opposed to 

individual plants, to be suited for his 

consideration with regard to the public 

welfare protection afforded by the 

current standard, and consequently, he 

finds the data and analyses generated by 

the program informative in such 

considerations. 

In this context, he takes note of the 

USFS system, including its descriptors 

for BI scores of differing magnitude 

intended for that Agency’s 

consideration in identifying areas of 

potential impact to forest resources. As 

described in section III.A.2.b(iii) above, 

very low BI scores (at or below 5) are 


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241 Studies that consider such data for purposes 

of identifying areas of potential impact to the forest 

resource suggest this category corresponds to 

‘‘none’’ with regard to ‘‘assumption of risk’’ (Smith 

et al., 2007; Smith et al., 2012). 

242For example, the PA describes findings from 

USFS studies that have concluded a ‘‘declining risk 

of probable impact’’ over the 16-year period of the 

program, especially after 2002 (e.g., Smith, 2012), 

and the parallel national reductions in O3

concentrations from 2000 through 2018 in terms of 

cumulative seasonal exposures, as well as in peak 

O3 concentrations such as the annual fourth highest 

daily maximum 8-hour concentration and also the 

occurrence of 1-hour concentrations above 100 ppb 

(PA, Figure 2–11, Appendix 2A, Tables 2A–2 to 

2A–4, and Appendix 4D, Figure 4D–9). 

243As discussed in section III.B.2.b, the 

cumulative frequency graph relied on by the 

CASAC does not present biosite scores for 

comparison at different cumulative exposure levels. 

Accordingly, it does not provide the type of 

analysis that is needed for the EPA to reach a 

conclusion about the extent of protection that 

different patterns of O3 concentrations would 

provide against visible foliar injury of an extent and 

severity as to pose risk of adverse effects to the 

public welfare. 

described by the USFS scheme as ‘‘little 

or no foliar injury’’ (Smith et al., 2007; 

Smith et al., 2012).241 The 

Administrator notes that BI scores above 

15 are categorized as moderate to severe 

(and scores above 25 as severe). In so 

doing, in light of considerations raised 

in the PA and consideration of public 

comment, he recognizes the lower 

categories of BI scores as indicative of 

injury of generally lesser risk to the 

natural area or to public enjoyment, 

which he judges unlikely to be 

indicative of injury of such a magnitude 

or extent as to pose risk of adverse 

effects to the public welfare. Thus, the 

Administrator reaches the conclusion 

that occurrence of the lower categories 

of BI scores does not pose concern for 

the public welfare, but that findings of 

BI scores categorized as ‘‘moderate to 

severe’’ injury by the USFS scheme 

would be an indication of visible foliar 

injury occurrence that, depending on 

extent and severity, may raise public 

welfare concerns. In this framework, the 

Administrator considers the PA 

evaluations of the currently available 

information and what it indicates with 

regard to patterns of air quality of 

concern for such an occurrence, and the 

extent to which they are expected to 

occur in areas that meet the current 

standard. 

In so doing, the Administrator takes 

particular note of the USFS biosite 

monitoring program studies of the 

occurrence, extent and severity of 

visible foliar injury in indicator species 

in defined plots or biosites in natural 

areas across the U.S. These studies of 

data for USFS biosites (sites with O3- 

sensitive vegetation assessed for visible 

foliar injury) have often summarized O3

concentrations in terms of cumulative 

exposure metrics (e.g., SUM06 or W126 

index). Some of these studies, 

particularly those examining such data 

across multiple years and multiple 

regions of the U.S., have reported that 

variation in cumulative O3 exposure, in 

terms of such metrics, does not 

completely explain the patterns of 

occurrence and severity of injury 

observed. Although the availability of 

detailed analyses that have explored 

multiple exposure metrics and other 

influential variables is limited, multiple 

studies have indicated a potential role 

for an additional metric, one related to 

the occurrence of days with relatively 

high concentrations (e.g., number of 

days with a 1-hour concentration at or 

above 100 ppb), as summarized in 

section III.A.2.c above (PA, section 

4.5.1.2). Thus, the Administrator takes 

note of this evidence indicating an 

influence of peak concentrations on BI 

scores (beyond an influence of 

cumulative exposure). He also finds 

noteworthy the extensive evidence of 

trends across the past nearly 20 years 

that indicate reductions in severity of 

visible foliar injury that parallel 

reductions in peak concentrations that 

have been suggested to be influential in 

the severity of visible foliar injury.242

Further, the Administrator considers 

the PA analysis of a dataset developed 

from USFS biosite index scores, 

combined with W126 estimates and soil 

moisture categories, summarized in 

section III.A.2.c above. In so doing, he 

takes note of the PA observation that 

important uncertainties remain in the 

understanding of the O3 exposure 

conditions that will elicit visible foliar 

injury of varying severity and extent in 

natural areas, and particularly in light of 

the other environmental variables that 

influence its occurrence, and of the 

recognition by the CASAC that 

‘‘uncertainties continue to hamper 

efforts to quantitatively characterize the 

relationship of [visible foliar injury] 

occurrence and relative severity with 

ozone exposures’’ (Cox 2020a, 

Consensus Responses to Charge 

Questions, p. 20). Notwithstanding, and 

while being mindful of, such 

uncertainties with regard to predictive 

O3 metric or metrics and a quantitative 

function relating them to incidence and 

severity of visible foliar injury in natural 

areas (as also noted in the USFS studies 

referenced above), the Administrator 

takes note of the PA finding that the 

incidence of nonzero BI scores, and, 

particularly of relatively higher scores 

(such as scores above 15 which are 

indicative of ‘‘moderate to severe’’ 

injury in the USFS scheme) appears to 

markedly increase only with W126 

index values above 25 ppm-hrs, as 

summarized in section III.B.2.b above 

(PA, section 4.3.3 and Appendix 4C). 

In light of these observations, the 

Administrator finds the current 

evidence to be incomplete with regard 

to information to support a quantitative 

characterization of air quality that 

would be anticipated to result in visible 

foliar injury of an extent and severity to 

cause adverse effects to the public 

welfare. The Administrator also 

considers discussion in the court’s 

remand of the 2015 standard with 

regard to visible foliar injury (Murray 

Energy Corp. v EPA, 936 F.3d at 619– 

20). The court concluded that the EPA 

had failed to offer a reasoned 

explanation for deciding not to specify 

a level of air quality to protect against 

adverse effects related to visible foliar 

injury. In particular, the court stated 

that the EPA had not explained why it 

was unable to choose such a level 

although the prior CASAC had provided 

advice with regard to a specific level. 

The EPA’s disagreement with the prior 

CASAC on its identified level is 

explained in section III.B.2 above, as is 

the reason why the EPA did not find the 

analysis on which the prior CASAC 

based its advice to be appropriate for 

such a conclusion.243 This and other 

associated issues raised by the court 

have been raised in public comments on 

the proposal for this action and are 

addressed in section III.B.2 above. 

Based on the evidence and 

quantitative analyses available in the 

present review, and advice from the 

current CASAC, the Administrator 

considers 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. 

Based on the evidence and associated 

quantitative analyses in this review, the 

Administrator’s judgment reflects his 

recognition of less confidence and 

greater uncertainty in the existence of 

adverse public welfare effects with 

lower O3 exposures. In this context, the 

Administrator judges 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. 

With these conclusions in mind, the 

Administrator considers the air quality 

analyses presented in the PA and the 

additional analyses developed in 

response to public comment. In so 

doing, he notes that a W126 index above 


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25 ppm-hrs (either as a 3-year average 

or in a single year) is not seen to occur 

at monitoring locations (including in or 

near Class I areas) where the current 

standard is met, and that, in fact, values 

above 17 or 19 ppm-hrs are rare, as 

summarized in section III.A.3 above 

(PA, Appendix 4C, section 4C.3; 

Appendix 4D, section 4D.3.2.3). Further, 

the Administrator takes note of the PA 

consideration of the USFS publications 

that identify an influence of peak 

concentrations on BI scores (beyond an 

influence of cumulative exposure) and 

the PA observation of the appreciable 

control of peak concentrations exerted 

by the form and averaging time of the 

current standard, as evidenced by the 

air quality analyses which document 

reductions in 1-hour daily maximum 

concentrations with declining design 

values. He also notes, as discussed 

above, the uncommonness of days with 

any hours at or above 100 ppb at 

monitoring sites that meet the current 

standard, as well as the minimal 

number of hours on any such days 

(Wells, 2020). Based on these 

considerations, the Administrator 

concludes that the current standard 

provides control of air quality 

conditions that contribute to increased 

BI scores and to scores of a magnitude 

indicative of ‘‘moderate to severe’’ foliar 

injury. 

The Administrator further takes note 

of the PA finding that the current 

information, particularly in locations 

meeting the current standard or with 

W126 index estimates likely to occur 

under the current standard, does not 

indicate a significant extent and degree 

of injury (e.g., based on analyses of BI 

scores in the PA, Appendix 4C) or 

specific impacts on recreational or 

related services for areas, such as 

wilderness areas or national parks. 

Thus, he gives credence to the 

associated PA conclusion that the 

evidence indicates that areas that meet 

the current standard are unlikely to 

have BI scores reasonably considered to 

be impacts of public welfare 

significance. Based on all of the 

considerations raised here, the 

Administrator concludes that the 

current standard provides sufficient 

protection of natural areas, including 

particularly protected areas such as 

Class I areas, from O3 concentrations in 

the ambient air that might be expected 

to elicit visible foliar injury of such an 

incidence and severity as would 

reasonably be judged adverse to the 

public welfare. 

With a primary focus on RBL in its 

role as proxy, the Administrator further 

considers the analyses available in this 

review of recent air quality at sites 

across the U.S., particularly including 

those sites in or near Class I areas, and 

also the analyses of historical air 

quality. In so doing, the Administrator 

recognizes that these analyses are 

distributed across all nine NOAA 

climate regions and 50 states, although 

some geographic areas within specific 

regions and states may be more densely 

covered and represented by monitors 

than others, as summarized in section 

III.C of the proposal (PA, Appendix 4D). 

The Administrator notes that the 

findings from both the analysis of the air 

quality data from the most recent period 

and from the larger analysis of historical 

air quality data extending back to 2000, 

as presented in the PA and summarized 

in section III.A.3 above, are consistent 

with the air quality analyses available in 

the last review. That is, in virtually all 

design value periods and all locations at 

which the current standard was met 

across the 19 years and 17 design value 

periods (in more than 99.9% of such 

observations), the 3-year average W126 

metric was at or below 17 ppm-hrs. 

Further, in all such design value periods 

and locations the 3-year average W126 

index was at or below 19 ppm-hrs. The 

Administrator additionally considers 

the protection provided by the current 

standard from the occurrence of O3

exposures within a single year with 

potentially damaging consequences, 

such as a significantly increased 

incidence of areas with visible foliar 

injury that might be judged moderate to 

severe, as discussed in section III.B.2 

above. In so doing, he takes notes of the 

PA analyses, summarized in section 

III.A.2.c above, of USFS BI scores, 

giving particular focus to scores above 

15, termed ‘‘moderate to severe injury’’ 

by the USFS categorization scheme, as 

described in section III.A.2.b above (PA, 

sections 4.3.3.2, 4.5.1.2 and Appendix 

4C). He notes the PA finding that 

incidence of sites with BI scores above 

15 markedly increases with W126 index 

estimates above 25 ppm-hrs. In this 

context, he additionally takes note of 

the air quality analysis finding of a 

scarcity of single-year W126 index 

values above 25 ppm-hrs at sites that 

meet the current standard, with just a 

single occurrence across all U.S. sites 

with design values meeting the current 

standard in the 19-year historical 

dataset dating back to 2000 (PA, section 

4.4 and Appendix 4D). Further, in light 

of the evidence indicating that peak 

short-term concentrations (e.g., of 

durations as short as one hour) may also 

play a role in the occurrence of visible 

foliar injury, the Administrator 

additionally takes note of the air quality 

analyses in the PA and in the additional 

analysis documented in 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, supporting the PA conclusion 

that the form and averaging time of the 

current standard provides appreciable 

control of peak 1-hour concentrations. 

Furthermore, these analyses indicate 

there to be only a few days among sites 

meeting the current standard, with 

hourly concentrations at or above 100 

ppb (just seven in the period from 2000 

through 2018) (Wells, 2020). In light of 

these findings from the air quality 

analyses and considerations in the 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 

judges that the current standard 

provides adequate protection from air 

quality conditions with the potential to 

be adverse to the public welfare. 

In reaching his conclusion on the 

current secondary O3 standard, the 

Administrator recognizes, as is the case 

in NAAQS reviews in general, his 

decision depends on a variety of factors, 

including science policy judgments and 

public welfare policy judgments, as well 

as the currently available information. 

With regard to the current review, the 

Administrator gives primary attention to 

the principal effects of O3 as recognized 

in the current ISA, the 2013 ISA and 

past AQCDs, and for which the evidence 

is strongest (e.g., growth, reproduction, 

and related larger-scale effects, as well 

as, visible foliar injury). With regard to 

growth and the categories of effects 

identified above for which RBL has been 

identified for use as a proxy, based on 

all of the considerations above, 

including the discussion of air quality 

immediately above, the Administrator 

judges the current standard to provide 

adequate protection for air quality 

conditions with the potential to be 

adverse to the public welfare. Further, 

with regard to visible foliar injury, the 

Administrator concludes that the 

currently available information on 

visible foliar injury and with regard to 

air quality analyses that may be 

informative with regard to air quality 

conditions associated with appreciably 

increased incidence and severity of BI 

scores at USFS biomonitoring sites, and 

with particular attention to Class I and 

other areas afforded special protection, 


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indicates the current standard to 

provide adequate protection from 

visible foliar injury of an extent or 

severity that might be anticipated to be 

adverse to the public welfare. 

In summary, the Administrator has 

based his decision on the public welfare 

protection afforded by the secondary O3

standard from identified O3-related 

welfare effects, and from their potential 

to present adverse effects to the public 

welfare, on judgments regarding what 

the available evidence, quantitative 

information, and associated 

uncertainties and limitations (such as 

those identified above) indicate with 

regard to the protection provided from 

the array of O3 welfare effects. He finds 

that, as a whole, this information, as 

summarized above, and presented in 

detail in the ISA and PA, does not 

indicate the current standard to allow 

air quality conditions with implications 

of concern for the public welfare. He has 

additionally considered the advice from 

the CASAC in this review, including its 

finding ‘‘that the available evidence 

does not reasonably call into question 

the adequacy of the current secondary 

ozone standard and concurs that it 

should be retained’’ (Cox, 2020a, p. 1), 

and well as public comment on the 

proposed decision. Based on all of the 

above considerations, including his 

consideration of the currently available 

evidence and quantitative exposure/risk 

information, the Administrator 

concludes that the current secondary 

standard is requisite to protect the 

public welfare from known or 

anticipated adverse effects of O3 and 

related photochemical oxidants in 

ambient air, and thus that the current 

standard should be retained, without 

revision. 

C. Decision on the Secondary Standard 

For the reasons discussed above and 

taking into account information and 

assessments presented in the ISA and 

PA, the advice from the CASAC, and 

consideration of public comments, the 

Administrator concludes that the 

current secondary O3 standard is 

requisite to protect the public welfare 

from known or anticipated adverse 

effects, and is retaining the current 

standard without revision. 

IV. Statutory and Executive Order 

Reviews 

Additional information about these 

statutes and Executive Orders can be 

found at http://www2.epa.gov/lawsregulations/laws-and-executive-orders. 

A. Executive Order 12866: Regulatory 

Planning and Review and Executive 

Order 13563: Improving Regulation and 

Regulatory Review 

The Office of Management and Budget 

(OMB) has determined that this action 

is a significant regulatory action and it 

was submitted to OMB for review. Any 

changes made in response to OMB 

recommendations have been 

documented in the docket. Because this 

action does not change the existing O3

NAAQS, it does not impose costs or 

benefits relative to the baseline of 

continuing with the current NAAQS in 

effect. EPA has thus not prepared a 

Regulatory Impact Analysis for this 

action. 

B. Executive Order 13771: Reducing 

Regulations and Controlling Regulatory 

Costs 

This action is not an Executive Order 

13771 regulatory action. There are no 

quantified cost estimates for this action 

because EPA is retaining the current 

standards. 

C. Paperwork Reduction Act (PRA) 

This action does not impose an 

information collection burden under the 

PRA. There are no information 

collection requirements directly 

associated with a decision to retain a 

NAAQS without any revision under 

section 109 of the CAA, and this action 

retains the existing O3 NAAQS without 

any revisions. 

D. Regulatory Flexibility Act (RFA) 

I certify that this action will not have 

a significant economic impact on a 

substantial number of small entities 

under the RFA. This action will not 

impose any requirements on small 

entities. Rather, this action retains, 

without revision, existing national 

standards for allowable concentrations 

of O3 in ambient air as required by 

section 109 of the CAA. See also 

American Trucking Associations v. 

EPA, 175 F.3d 1027, 1044–45 (D.C. Cir. 

1999) (NAAQS do not have significant 

impacts upon small entities because 

NAAQS themselves impose no 

regulations upon small entities), rev’d in 

part on other grounds, Whitman v. 

American Trucking Associations, 531 

U.S. 457 (2001). 

E. Unfunded Mandates Reform Act 

(UMRA) 

This action does not contain any 

unfunded mandate as described in the 

UMRA, 2 U.S.C. 1531–1538, and does 

not significantly or uniquely affect small 

governments. This action imposes no 

enforceable duty on any state, local, or 

tribal governments or the private sector. 

F. Executive Order 13132: Federalism 

This action does not have federalism 

implications. It will not have substantial 

direct effects on the states, on the 

relationship between the national 

government and the states, or on the 

distribution of power and 

responsibilities among the various 

levels of government. 

G. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

This action does not have tribal 

implications, as specified in Executive 

Order 13175. It does not have a 

substantial direct effect on one or more 

Indian Tribes. This action does not 

change existing regulations; it retains 

the existing O3 NAAQS, without 

revision. Executive Order 13175 does 

not apply to this action. 

H. Executive Order 13045: Protection of 

Children From Environmental Health 

and Safety Risks 

This action is not subject to Executive 

Order 13045 because it is not 

economically significant as defined in 

Executive Order 12866. The health 

effects evidence and risk assessment 

information for this action, which 

focuses on children and people (of all 

ages) with asthma as key at-risk 

populations, is summarized in section 

II.A.2 and II.A.3 above and described in 

the ISA and PA, copies of which are in 

the public docket for this action. 

I. Executive Order 13211: Actions That 

Significantly Affect Energy Supply, 

Distribution or Use 

This action is not a ‘‘significant 

energy action’’ for purposes of Executive 

Order 13211. The action is not likely to 

have a significant adverse effect on the 

supply, distribution, or use of energy. 

This action retains the current O3

NAAQS. This decision does not change 

existing requirements. The 

Administrator of the Office of 

Information and Regulatory Affairs has 

not otherwise designated this action as 

a significant energy action. Thus, this 

decision does not constitute a 

significant energy action as defined in 

Executive Order 13211. 

J. National Technology Transfer and 

Advancement Act 

This action does not involve technical 

standards. 

K. Executive Order 12898: Federal 

Actions To Address Environmental 

Justice in Minority Populations and 

Low-Income Populations 

The EPA believes that this action does 

not have disproportionately high and 


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adverse human health or environmental 

effects on minority, low-income 

populations and/or indigenous peoples, 

as specified in Executive Order 12898 

(59 FR 7629, February 16, 1994). The 

action described in this document is to 

retain without revision the existing O3

NAAQS based on the Administrator’s 

conclusions that the existing primary 

standard protects public health, 

including the health of sensitive groups, 

with an adequate margin of safety, and 

that the existing secondary standard 

protects public welfare from known or 

anticipated adverse effects. As 

discussed in section II above, the EPA 

expressly considered the available 

information regarding health effects 

among at-risk populations in reaching 

the decision that the existing standard is 

requisite. 

L. Determination Under Section 307(d) 

Section 307(d)(1)(V) of the CAA 

provides that the provisions of section 

307(d) apply to ‘‘such other actions as 

the Administrator may determine.’’ 

Pursuant to section 307(d)(1)(V), the 

Administrator determines that this 

action is subject to the provisions of 

section 307(d). 

M. Congressional Review Act 

The EPA will submit a rule report to 

each House of the Congress and to the 

Comptroller General of the United 

States. This action is not a ‘‘major rule’’ 

as defined by 5 U.S.C. 804(2). 

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Federal Register / Vol. 85, No. 251 / Thursday, December 31, 2020 / Rules and Regulations 87351 

List of Subjects in 40 CFR Part 50 

Environmental protection, Air 

pollution control, Carbon monoxide, 

Lead, Nitrogen dioxide, Ozone, 

Particulate matter, Sulfur oxides. 

Andrew Wheeler, 

Administrator. 

[FR Doc. 2020–28871 Filed 12–30–20; 8:45 am] 

BILLING CODE 6560–50–P 


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