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EO12866_O3-NAAQS-RIN2060-AP38-NPR-Preamble_20141125.docx

 6560-50-P

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50, 51, 52, 53 and 58

[EPA-HQ-OAR-2008-0699; FRL-9918-43-OAR]

RIN 2060-AP38

National Ambient Air Quality Standards for Ozone

AGENCY: Environmental Protection Agency.

ACTION: Proposed rule.

SUMMARY: Based on its review of the air quality criteria for ozone (O3) and related 

photochemical oxidants and national ambient air quality standards (NAAQS) for O3, the EPA 

proposes to make revisions to the primary and secondary NAAQS for O3 to provide requisite

protection of public health and welfare, respectively. The EPA is proposing to revise the primary 

standard to a level within the range of 0.065 to 0.070 parts per million (ppm), and to revise the 

secondary standard to within the range of 0.065 to 0.070 ppm, which air quality analyses indicate 

would provide air quality, in terms of 3-year average W126 index values, at or below a range of 

13-17 ppm-hours. The EPA proposes to make corresponding revisions in data handling 

conventions for O3 and conforming changes to the Air Quality Index (AQI); to revise regulations 

for the prevention of significant deterioration (PSD) program to add a transition provision for 

certain applications; and to propose schedules and convey information related to implementing 

any revised standards. The EPA is proposing changes to the O3 monitoring seasons, the Federal 

Reference Method (FRM) for monitoring O3 in the ambient air, Federal Equivalent Method 
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(FEM) procedures for testing, and the Photochemical Assessment Monitoring Stations (PAMS) 

network. 

Along with proposing exceptional event schedules related to implementing any revised 

O3 standards, the EPA is proposing to apply this same schedule approach to other future revised 

NAAQS and to remove obsolete regulatory language for expired exceptional event deadlines. 

The EPA is proposing to make minor changes to the procedures and time periods for evaluating 

potential FRMs and equivalent methods (including making the requirements for nitrogen dioxide 

consistent with the requirements for O3) and to remove an obsolete requirement for the annual 

submission of documentation by manufacturers of certain particulate matter monitors. For 

additional information, see the Executive Summary, section I.A.

DATES: Written comments on this proposed rule must be received by [INSERT 90 DAYS 

AFTER DATE OF PUBLICATION IN THE FEDERAL REGISTER].

Public Hearings: The EPA intends to hold three public hearings on this proposed rule in January 

2015. These will be announced in a separate Federal Register notice that provides details, 

including specific dates, times, addresses, and contact information for these hearings.

ADDRESSES: Submit your comments, identified by Docket ID No. EPA-HQ-OAR-2008-0699,

to the EPA by one of the following methods:

 Federal eRulemaking Portal: http://www.regulations.gov. Follow the online instructions 

for submitting comments.

 Email: A-and-R-Docket@epa.gov. Include docket ID No. EPA-HQ-OAR-2008-0699 in 

the subject line of the message.

 Fax: (202) 566-9744.

 Mail: Environmental Protection Agency, EPA Docket Center (EPA/DC), Mailcode 
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28221T, Attention Docket ID No. OAR-2008-0699, 1200 Pennsylvania Ave., NW, 

Washington, DC 20460. Please include a total of two copies. 

 Hand/Courier Delivery: EPA Docket Center, Room 3334, EPA WJC West Building, 

1301 Constitution Ave., NW, Washington, DC. Such deliveries are only accepted during 

the Docket’s normal hours of operation, and special arrangements should be made for 

deliveries of boxed information.

Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-2008-0699. The 

EPA's policy is that all comments received will be included in the public docket without change 

and may be made available online at www.regulations.gov, including any personal information 

provided, unless the comment includes information claimed to be Confidential Business 

Information (CBI) or other information whose disclosure is restricted by statute. Do not submit 

information that you consider to be CBI or otherwise protected through www.regulations.gov or 

e-mail. The www.regulations.gov website is an “anonymous access” system, which means the 

EPA will not know your identity or contact information unless you provide it in the body of your 

comment. If you send an e-mail comment directly to the EPA without going through 

www.regulations.gov your e-mail address will be automatically captured and included as part of 

the comment that is placed in the public docket and made available on the Internet. If you submit 

an electronic comment, the EPA recommends that you include your name and other contact 

information in the body of your comment and with any disk or CD-ROM you submit. If the EPA 

cannot read your comment due to technical difficulties and cannot contact you for clarification, 

the EPA may not be able to consider your comment. Electronic files should avoid the use of 

special characters, any form of encryption, and be free of any defects or viruses. For additional 
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information about EPA’s public docket visit the EPA Docket Center homepage at 

http://www.epa.gov/epahome/dockets.htm.

Docket: The EPA has established dockets for these actions as discussed above. All 

documents in these dockets are listed on the www.regulations.gov website. This includes 

documents in the rulemaking docket (Docket ID No. EPA-HQ-OAR-2008-0699) and a separate 

docket, established for the Integrated Science Assessment (ISA) (Docket No. EPA-HQ-ORD2011-0050) that has have been incorporated by reference into the rulemaking docket. Although 

listed in the index, some information is not publicly available, e.g., CBI or other information 

whose disclosure is restricted by statute. Certain other material, such as copyrighted material, is 

not placed on the Internet and may be viewed, with prior arrangement, at the EPA Docket 

Center. Publicly available docket materials are available either electronically in 

www.regulations.gov or in hard copy at the Air and Radiation Docket and Information Center, 

EPA/DC, EPA WJC West Building, Room 3334, 1301 Constitution Ave., NW, Washington, DC. 

The Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday through Friday, 

excluding legal holidays. The telephone number for the Public Reading Room is (202) 566-1744 

and the telephone number for the Air and Radiation Docket and Information Center is (202) 566-

1742. For additional information about EPA’s public docket visit the EPA Docket Center 

homepage at: http://www.epa.gov/epahome/dockets.htm.

FOR FURTHER INFORMATION CONTACT: Ms. Susan Lyon Stone, 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) 541-1146; fax: (919) 541-0237; email: stone.susan@epa.gov.

SUPPLEMENTARY INFORMATION: 
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General Information

What should I consider as I prepare my comments for EPA?

1. Submitting CBI. Do not submit this information to the EPA through 

www.regulations.gov or e-mail. Clearly mark the part or all of the information that you claim to 

be CBI. For CBI information in a disk or CD ROM that you mail to EPA, mark the outside of the 

disk or CD ROM as CBI and then identify electronically within the disk or CD ROM the specific 

information that is claimed as CBI. In addition to one complete version of the comment that 

includes information claimed as CBI, a copy of the comment that does not contain the 

information claimed as CBI must be submitted for inclusion in the public docket. Information so 

marked will not be disclosed except in accordance with procedures set forth in 40 CFR part 2.

2. Tips for Preparing Your Comments. When submitting comments, remember to:

 Identify the rulemaking by docket number and other identifying information (subject 

heading, Federal Register date and page number).

 Follow directions – The agency may ask you to respond to specific questions or organize 

comments by referencing a Code of Federal Regulations (CFR) part or section number.

 Explain why you agree or disagree, suggest alternatives, and substitute language for your 

requested changes.

 Describe any assumptions and provide any technical information and/or data that you 

used.

 Provide specific examples to illustrate your concerns, and suggest alternatives.

 Explain your views as clearly as possible, avoiding the use of profanity or personal 

threats.

 Make sure to submit your comments by the comment period deadline identified.
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Availability of Related Information

A number of documents relevant to this rulemaking are available on EPA Web sites. The 

ISA for Ozone and Related Photochemical Oxidants is available on the EPA’s National Center 

for Environmental Assessment (NCEA) Web site. To obtain this document, go to 

http://www.epa.gov/ncea, and click on Ozone in the Quick Finder section. This will open a page 

with a link to the February 2013 ISA. The 2014 Policy Assessment (PA), Health and Welfare 

Risk and Exposure Assessments (HREA and WREA, respectively), and other related technical 

documents are available on EPA’s Office of Air Quality Planning and Standards (OAQPS) 

Technology Transfer Network (TTN) Web site. The final 2014 PA is available at: 

http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_2008_pa.html, and the final 2014 Health 

and Welfare Risk and Exposure Assessments and other related technical documents are available 

at: http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_2008_rea.html. These and other related 

documents are also available for inspection and copying in the EPA docket identified above.

Environmental Justice

Analyses evaluating the potential implications of a revised O3 NAAQS for environmental 

justice populations are discussed in appendix 9A of the Regulatory Impact Analysis (RIA) that 

accompanies this notice of proposed rulemaking. The RIA is available on the Web, through the 

EPA’s Technology Transfer Network website at 

http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_index.html.

Table of Contents 

The following topics are discussed in this preamble:

I. Background

A. Executive Summary

B. Legislative Requirements 

C. Related Control Programs to Implement O3 Standards

D. Review of Air Quality Criteria and Standards for O3
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E. Ozone Air Quality

II. Rationale for Proposed Decision on the Primary Standard

A. Approach

B. Health Effects Information

1. Overview of Mechanisms

2. Nature of Effects

3. Adversity of O3 Effects 

4. Ozone-Related Impacts on Public Health

C. Human Exposure and Health Risk Assessments

1. Air Quality Adjustment

2. Exposure Assessment

3. Quantitative Health Risk Assessments

D. Conclusions on the Adequacy of the Current Primary Standard

1. Summary of Evidence-Based Considerations in the PA

2. Summary of Exposure- and Risk-Based Considerations in the PA

3. Policy Assessment Conclusions on the Current Standard

4. CASAC Advice

5. Administrator’s Proposed Conclusions Concerning the Adequacy of the 

Current Standard

E. Conclusions on the Elements of the Primary Standard

1. Indicator

2. Averaging Time

3. Form

4. Level

F. Proposed Decision on the Primary Standard 

III. Communication of Public Health Information

IV. Rationale for Proposed Decision on the Secondary Standard

A. Approach

B. Welfare Effects Information

1. Nature of Effects and Biologically Relevant Exposure Metric 

2. Potential Impacts on Public Welfare 

C. Exposure and Risk Assessment Information 

1. Air Quality Analyses

2. Tree Seedling Growth, Productivity, Carbon Storage and Associated Ecosystem 

Services

3. Crop Yield

4. Visible Foliar Injury 

D. Conclusions on Adequacy of the Current Secondary Standard

1. Evidence- and Exposure/Risk-Based Considerations in the Policy Assessment 

2. CASAC Advice 

3. Administrator’s Proposed Conclusions on Adequacy of the Current Standard 

E. Consideration of Alternative Secondary Standards

1. Indicator

2. Consideration of a Cumulative, Seasonal Exposure-based Standard in the 

Policy Assessment 

3. CASAC Advice
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4. Air Quality Analyses

5. Administrator’s Proposed Conclusions 

F. Proposed Decision on the Secondary Standard

V. Appendix U: Interpretation of the Primary and Secondary NAAQS for O3

A. Background

B. Data Selection Requirements

C. Data Reporting and Data Handling Requirements

D. Considerations for the Possibility of a Distinct Secondary Standard

E. Exceptional Events Information Submission Schedule

VI. Ambient Monitoring Related to Proposed O3 Standards

A. Background

B. Revisions to the Length of the Required O3 Monitoring Seasons

C. Revisions to the Photochemical Assessment Monitoring Stations (PAMS)

1. Network Design

2. Speciated VOC Measurements

3. Carbonyl Sampling 

4. Nitrogen Oxides Sampling

5. Meteorology Measurements

6. PAMS Season

7. Timing and Other Implementation Issues

D. Addition of a New Federal Reference Method (FRM) for O3

E. Revisions to the Procedures for Testing Performance Characteristics and Determining 

Comparability Between Candidate Methods and Reference Methods

VII. Implementation of Proposed O3 Standards

A. NAAQS Implementation Plans

1. Background

2. Timing of Rules and Guidance

3. Section 110 State Implementation Plans

4. Nonattainment Area Requirements

B. Implementing a Distinct Secondary O3 NAAQS, if One is Established

C. Designation of Areas

D. Prevention of Significant Deterioration and Nonattainment New Source Review 

Programs for the Proposed Revised Primary and Secondary O3 NAAQS

 1. Prevention of Significant Deterioration (PSD)

2. Nonattainment New Source Review

E. Transportation and General Conformity Programs

1. What are Transportation and General Conformity?

2. Why is the EPA Discussing Transportation and General Conformity in this 

Proposed Rulemaking?

3. When Would Transportation and General Conformity Apply to Areas 

Designated Nonattainment for a Revised O3 NAAQS, if One is Established?

4. Will Transportation and General Conformity Apply to a Distinct Secondary O3

NAAQS, if One is Established? 

5. What Impact Would the Implementation of a Revised O3 NAAQS Have on a 

State’s Transportation and/or General Conformity SIP?

F. How Background O3 Is Addressed in CAA Implementation Provisions
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1. Introduction

2. Exceptional Events Exclusions

3. Rural Transport Areas

4. International Transport

VIII. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: 

Improving Regulation and Regulatory Review

B. Paperwork Reduction Act 

C. Regulatory Flexibility Act 

D. Unfunded Mandates Reform Act

E. Executive Order 13132: Federalism 

F. Executive Order 13175: Consultation and Coordination with Indian Tribal 

Governments 

G. Executive Order 13045: Protection of Children from Environmental Health & Safety 

Risks

H. Executive Order 13211: Actions that Significantly Affect Energy Supply, Distribution, 

or Use 

I. National Technology Transfer and Advancement Act

J. Executive Order 12898: Federal Actions to Address Environmental Justice in Minority 

Populations and Low-Income Populations 

References

I. Background

A. Executive Summary

This section summarizes information about the purpose of this regulatory action (I.A.1), 

the major provisions of this proposal (I.A.2), and provisions related to implementation (I.A.3). 

1. Purpose of This Regulatory Action

Sections 108 and 109 of the Clean Air Act (CAA) govern the establishment, review, and 

revision, as appropriate, of the NAAQS to protect public health and welfare. The CAA requires

the EPA to periodically review the air quality criteria—the science upon which the standards are 

based—and the standards themselves. This rulemaking is being conducted pursuant to these 

statutory requirements. The schedule for completing this review is established by a federal court 

order, which requires that the EPA sign a proposal by December 1, 2014, and make a final 

determination by October 1, 2015.
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The EPA completed its most recent review of the O3 NAAQS in 2008. As a result of that 

review, EPA took four principal actions: (1) revised the level of the 8-hour primary O3 standard 

to 0.075 parts per million (ppm); (2) expressed the standard to three decimal places; (3) revised 

the 8-hour secondary O3 standard by making it identical to the revised primary standard; and (4) 

made conforming changes to the AQI for O3.

In subsequent litigation, the U.S. Court of Appeals for the District of Columbia Circuit 

upheld the EPA’s 2008 primary O3 standard, but remanded the 2008 secondary standard. State of 

Mississippi v. EPA, 744 F. 3d 1334 (D.C. Cir. 2013). With respect to the primary standard, the 

court held that the EPA reasonably determined that the existing primary standard, set in 1997, 

did not protect public health with an adequate margin of safety and required revision. In

upholding the EPA’s revised primary standard, the court dismissed arguments that the EPA 

should have adopted a more stringent standard. The court remanded the secondary standard to 

the EPA after rejecting the EPA’s explanation for setting the secondary standard identical to the 

revised 8-hour primary standard. The court held that because the EPA had failed to identify a 

level of air quality requisite to protect public welfare, the EPA’s comparison between the 

primary and secondary standards for determining if requisite protection for public welfare was 

afforded by the primary standard failed to comply with the CAA.

This proposal reflects the Administrator’s proposed conclusions based on a review of the 

O3 NAAQS that began in September 2008. In conducting this review, the EPA has carefully 

evaluated the currently available scientific literature on the health and welfare effects of ozone, 

focusing particularly on the new literature available since the conclusion of the previous review 

in 2008. In addition, the EPA has also addressed the remand of the Agency’s 2008 decision on 

the secondary standard. Between 2008 and 2014, the EPA prepared draft and final versions of 
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the Integrated Science Assessment, the Health and Welfare Risk and Exposure Assessments, and 

the Policy Assessment. Multiple drafts of these documents were available for public review and 

comment, and as required by the CAA, were peer-reviewed by the Clean Air Scientific Advisory 

Committee (CASAC), an independent scientific advisory committee established by the CAA and 

charged with providing advice to the Administrator. The final documents reflect the EPA staff’s 

consideration of the comments and recommendations made by CASAC and the public on draft 

versions of these documents. 

2. Summary of Major Provisions

The EPA is proposing that the current primary O3 standard set at a level of 0.075 ppm is 

not requisite to protect public health with an adequate margin of safety, and that it should be 

revised to provide increased public health protection. Specifically, the EPA is proposing to retain 

the indicator (ozone), averaging time (8-hour) and form (annual fourth-highest daily maximum, 

averaged over 3 years) of the existing primary O3 standard and is proposing to revise the level of 

that standard to within the range of 0.065 ppm to 0.070 ppm. The EPA is proposing this revision 

to increase public health protection, including for “at-risk” populations such as children, older 

adults, and people with asthma or other lung diseases, against an array of O3-related adverse 

health effects. For short-term O3 exposures, these effects include decreased lung function, 

increased respiratory symptoms and pulmonary inflammation, effects that result in serious 

indicators of respiratory morbidity such as emergency department visits and hospital admissions, 

and all-cause (total nonaccidental) mortality. For long-term O3 exposures, these health effects 

include a variety of respiratory morbidity effects and respiratory mortality. Recognizing that the 

CASAC recommended a range of levels from 0.060 ppm to 0.070 ppm, and that levels as low as 

0.060 ppm could potentially be supported, the Administrator solicits comment on alternative 
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standard levels below 0.065 ppm, and as low as 0.060 ppm. However, the Administrator notes 

that setting a standard below 0.065 ppm, down to 0.060 ppm, would inappropriately place very 

little weight on the uncertainties in the health effects evidence and exposure/risk information. 

Given alternative views of the currently available evidence and information expressed by some

commenters, the EPA is taking comment on both the Administrator’s proposed decision to revise 

the current primary O3 standard and the option of retaining that standard. 

In addition to proposing changes to the level of the standard, the EPA is proposing 

conforming changes to the Air Quality Index (AQI) by proposing to set an AQI value of 100 

equal to the level of the 8-hour primary O3 standard, and proposing adjustments to the AQI 

values of 50, 150, 200 and 300.

The EPA also proposes to revise the secondary standard to provide increased protection 

against vegetation-related effects on public welfare. As an initial matter, the Administrator 

proposes to conclude that air quality in terms of a three-year average seasonal W126 index value, 

based on the three consecutive month period within the O3 season with the maximum index 

value, with daily exposures cumulated for the 12-hour period from 8:00 am to 8:00 pm, within 

the range from 13 ppm-hrs to 17 ppm-hrs would provide the requisite protection against known 

or anticipated adverse effects to the public welfare. The EPA solicits comment on this proposed 

conclusion. In considering how to achieve that level of air quality, the Administrator recognizes 

that air quality data analyses suggest that air quality in terms of three-year average W126 index 

values of a range at or below 13 to 17 ppm-hrs would be provided by a secondary standard level 

within the range of 0.065 to 0.070 ppm, and that to the extent areas need to take action to attain a 

standard in the range of 0.065 to 0.070 ppm, those actions would also improve air quality as 

measured by the W126 metric. Thus, the Administrator proposes to revise the level of the current 
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secondary standard to within the range of 0.065 to 0.070 ppm. The EPA solicits comments on 

this proposed revision of the secondary standard. 

The EPA also solicits comments on the alternative approach of revising the secondary 

standard to a W126-based form, averaged over three years, with a level within the range of 13

ppm-hrs to 17 ppm-hrs. The EPA additionally solicits comments on such a distinct secondary 

standard with a level within the range extending below 13 ppm-hrs down to 7 ppm-hrs. Further, 

the EPA solicits comments on retaining the current secondary standard without revision, along 

with the alternative views of the evidence that would support retaining the current standard.

3. Provisions Related to Implementation

As directed by the CAA, reducing pollution to meet national air quality standards always 

has been a shared task, one involving the federal government, states, tribes and local air

agencies. This partnership has proved effective since the EPA first issued O3 standards more than 

three decades ago, and is evidenced by significantly lower O3 levels throughout the country. To 

provide a foundation that helps air agencies build successful strategies for attaining new O3

standards, the EPA will continue to move forward with federal regulatory programs, such as the 

proposed Clean Power Plan and the final Tier 3 motor vehicle emissions standards. To facilitate 

the development of CAA-compliant implementation plans and strategies to attain new standards,

the EPA intends to issue timely and appropriate implementation guidance and, where appropriate 

and consistent with the law, new rulemakings to streamline regulatory burdens and provide 

flexibility in implementation. In addition, given the regional nature of O3 air pollution, the EPA 

will continue to work with states to address interstate transport of O3 and O3 precursors.

This notice contains several proposed provisions related to implementation of the 

proposed standards. In addition to revisions to the primary and secondary NAAQS, the EPA is 
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proposing to make corresponding revisions in data handling conventions for O3; to revise 

regulations for the Prevention of Significant Deterioration (PSD) permitting program to add a 

provision grandfathering certain pending permits from certain requirements with respect to the 

proposed revisions to the O3 NAAQS; and to convey schedules and information related to 

implementing any revised standards.

In conjunction with proposing exceptional event schedules related to implementing any 

revised O3 standards, the EPA is also proposing to extend the new schedule approach to other 

future revised NAAQS and to remove obsolete regulatory language associated with expired 

exceptional event deadlines for historical standards for both O3 and other NAAQS pollutants. 

The EPA is also proposing to make minor changes to the procedures and time periods for 

evaluating potential FRMs and equivalent methods, including making the requirements for 

nitrogen dioxide consistent with the requirements for O3, and removing an obsolete requirement 

for the annual submission of documentation by manufacturers of certain particulate matter 

monitors. 

B. Legislative Requirements

Two sections of the CAA govern the establishment and revision of the NAAQS. Section 

108 (42 U.S.C. section 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 air 

pollutants that in her “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 . . . [the 

Administrator] plans to issue air quality criteria....” Air quality criteria are intended to 

“accurately reflect the latest scientific knowledge useful in indicating the kind and extent of all 
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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(b). 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. Section 109(b)(1) defines a primary standard as one “the 

attainment and maintenance of which in the judgment of the Administrator, based on such 

criteria and allowing an adequate margin of safety, are requisite to protect the public health.”1 A 

secondary standard, as defined in section 109(b)(2), must “specify a level of air quality the 

attainment and maintenance of which, in the judgment of the Administrator, based on such 

criteria, is requisite to protect the public welfare from any known or anticipated adverse effects 

associated with the presence of [the] pollutant in the ambient air.”2

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 State of Mississippi 

v. EPA, 744 F. 3d 1334, 1353 (D.C. Cir. 2013) (“By requiring an ‘adequate margin of safety’, 

Congress was directing EPA to build a buffer to protect against uncertain and unknown dangers 

to human health”); see also Lead Industries Association v. EPA, 647 F.2d 1130, 1154 (D.C. Cir 

1980); American Petroleum Institute v. Costle, 665 F.2d 1176, 1186 (D.C. Cir. 1981); American 

 

1 The legislative history of section 109 indicates that a primary standard is to be set at “the 

maximum permissible ambient air level . . . which will protect the health of any [sensitive] group 

of the population,” and that, for this purpose, “reference should be made to a representative 

sample of persons comprising the sensitive group rather than to a single person in such a group.” 

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

2 Welfare effects as defined in section 302(h) (42 U.S.C. 7602(h)) include, but are not limited to, 

“effects on soils, water, crops, vegetation, man-made materials, animals, wildlife, weather, 

visibility and climate, damage to and deterioration of property, and hazards to transportation, as 

well as effects on economic values and on personal comfort and well-being.”
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Farm Bureau Federation v. EPA, 559 F. 3d 512, 533 (D.C. Cir. 2009); Association of Battery

Recyclers v. EPA, 604 F. 3d 613, 617-18 (D.C. Cir. 2010). Both kinds of uncertainties are 

components of the risk associated with pollution at levels below those at which human health 

effects can be said to occur with reasonable scientific certainty. Thus, in selecting primary 

standards that provide an adequate margin of safety, the Administrator is seeking not only to 

prevent pollution levels that have been demonstrated to be harmful but also to prevent lower 

pollutant levels that may pose an unacceptable risk of harm, even if the risk is not precisely 

identified as to nature or degree. The CAA does not require the Administrator to establish a 

primary NAAQS at a zero-risk level or at background concentrations, see Lead Industries v. 

EPA, 647 F.2d at 1156 n.51; State of 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, the size of sensitive population(s)3at risk, 

and the kind and degree of the uncertainties that must be addressed. The selection of any 

particular approach for providing an adequate margin of safety is a policy choice left specifically 

to the Administrator’s judgment. See Lead Industries Association v. EPA, 647 F.2d at 1161-62; 

State of Mississippi, 744 F. 3d at 1353.

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 for these purposes. In so doing, the EPA 

may not consider the costs of implementing the standards. See generally, Whitman v. American 

 

3 As used here and similarly throughout this document, the term “population” refers to people

having a quality or characteristic in common, including a specific pre-existing illness or a 

specific age or life stage.
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Trucking Associations, 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 at 1185.

Section 109(d)(1) requires that “not later than December 31, 1980, and at 5-year intervals 

thereafter, the Administrator shall complete a thorough review of the criteria published under 

section 108 and the national ambient air quality standards . . . and shall make such revisions in 

such criteria and standards and promulgate such new standards as may be appropriate . . . .” 

Section 109(d)(2) requires that an independent scientific review committee “shall complete a 

review of the criteria . . . and the national primary and secondary ambient air quality standards . . 

. and shall recommend to the Administrator any new . . . standards and revisions of existing 

criteria and standards as may be appropriate . . . .” Since the early 1980's, the Clean Air 

Scientific Advisory Committee (CASAC) has performed this independent review function. 4

C. Related Control Programs to Implement O3 Standards

States are primarily responsible for ensuring attainment and maintenance of ambient air 

quality standards once the EPA has established them. Under section 110 of the CAA, and related 

provisions, 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 

PSD program (CAA sections 160 to 169). In addition, federal programs provide for nationwide 

reductions in emissions of O3 precursors and other air pollutants through the federal motor 

 

4 Lists of CASAC members and of members of the CASAC Ozone Review Panel are available 

at: 

http://yosemite.epa.gov/sab/sabpeople.nsf/WebCommitteesSubCommittees/Ozone%20Review%

20Panel.
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vehicle and motor vehicle fuel control program under title II of the CAA (sections 202 to 250) 

which involves controls for emissions from mobile sources and controls for the fuels used by 

these sources, and new source performance standards for stationary sources under section 111 of 

the CAA. For some stationary sources, the national emissions standards for hazardous air 

pollutants under section 112 of the CAA may provide ancillary reductions in O3 precursors.

After the EPA establishes a new or revised NAAQS, the CAA directs the EPA and the 

states to take steps to ensure that the new or revised NAAQS is met. One of the first steps, 

known as the initial area designations, involves identifying areas of the country that either are 

attaining or not attaining the new or revised NAAQS along with the nearby areas that contribute 

to the violations. Upon designation of nonattainment areas, certain states would then be required 

to develop SIPs to attain the standards. In developing their attainment plans, states would first 

take into account projected emission reductions from federal and state rules that have been 

already adopted at the time of plan submittal. A number of significant emission reduction 

programs that will lead to reductions of O3 precursors are in place today or are expected to be in 

place by the time any new SIPs will be due. Examples of such rules include the Nitrogen Oxides 

(NOX) SIP Call, Clean Air Interstate Rule (CAIR), and Cross-State Air Pollution Rule 

(CSAPR),5regulations controlling onroad and nonroad engines and fuels, the utility and 

industrial boilers hazardous air pollutant rules, and various other programs already adopted by 

states to reduce emissions from key emissions sources. States would then evaluate the level of 

additional emission reductions needed for each nonattainment area to attain the O3 standards “as 

 

5 The Cross-State Air Pollution Rule was recently upheld by the Supreme Court in 

Environmental Protection Agency v. EME Homer City Generation, U.S. (2014). The D.C. 

Circuit has since lifted the stay of the rule. Order, Document #1518738, EME Homer City 

Generation, L.P. v. EPA, Case #11-1302 (D.C. Cir. Oct. 23, 2014).
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expeditiously as practicable,” and adopt new state regulations as appropriate. Section VII of this 

preamble includes additional discussion of designation and implementation issues associated 

with any revised O3 NAAQS. 

D. Review of Air Quality Criteria and Standards for O3

The EPA first established primary and secondary NAAQS for photochemical oxidants in 

1971 (36 FR 8186, April 30, 1971). The EPA set both primary and secondary standards at a level 

of 0.08 parts per million (ppm), 1-hr average, total photochemical oxidants, not to be exceeded 

more than one hour per year. The EPA based the standards on scientific information contained in 

the 1970 Air Quality Criteria for Photochemical Oxidants (U.S. DHEW, 1970). The EPA

initiated the first periodic review of the NAAQS for photochemical oxidants in 1977. Based on 

the 1978 Air Quality Criteria for Ozone and Other Photochemical Oxidants (U.S. EPA, 1978), 

the EPA published proposed revisions to the original NAAQS in 1978 (43 FR 16962) and final 

revisions in 1979 (44 FR 8202). At that time, the EPA revised the level of the primary and 

secondary standards from 0.08 to 0.12 ppm and changed the indicator from photochemical 

oxidants to O3, and the form of the standards from a deterministic (i.e., not to be exceeded more 

than one hour per year) to a statistical form. This statistical form defined attainment of the 

standards as occurring when the expected number of days per calendar year with maximum 

hourly average concentration greater than 0.12 ppm equaled one or less. 

Following the final decision in the 1979 review, the City of Houston challenged the 

Administrator’s decision arguing that the standard was arbitrary and capricious because natural 

O3 concentrations and other physical phenomena in the Houston area made the standard 

unattainable in that area. The U.S. Court of Appeals for the District of Columbia Circuit (D.C. 

Circuit) rejected this argument, holding (as noted above) that attainability and technological 
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feasibility are not relevant considerations in the promulgation of the NAAQS. 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 

this difficulty through various compliance related provisions in the CAA. See API v. Costle, 665 

F.2d 1176, 1184-6 (D.C. Cir. 1981). 

In 1982, the EPA announced plans to revise the 1978 Air Quality Criteria document (47 

FR 11561), and in 1983, the EPA initiated the second periodic review of the O3 NAAQS (48 FR 

38009). The EPA subsequently published the 1986 Air Quality Criteria for Ozone and Other 

Photochemical Oxidants (U.S. EPA, 1986) and the 1989 Staff Paper (U.S. EPA, 1989). 

Following publication of the 1986 Air Quality Criteria Document (AQCD), a number of 

scientific abstracts and articles were published that appeared to be of sufficient importance 

concerning potential health and welfare effects of O3 to warrant preparation of a Supplement 

(U.S. EPA, 1992). On August 10, 1992, under the terms of a court order, the EPA published a 

proposed decision to retain the existing primary and secondary standards (57 FR 35542). The 

notice explained that the proposed decision would complete the EPA’s review of information on 

health and welfare effects of O3 assembled over a 7-year period and contained in the 1986 

AQCD and its 1992 Supplement. The proposal also announced the EPA’s intention to proceed as 

rapidly as possible with the next review of the air quality criteria and standards for O3 in light of 

emerging evidence of health effects related to 6- to 8-hour O3 exposures. On March 9, 1993, the 

EPA concluded the review by affirming its proposed decision to retain the existing primary and 

secondary standards (58 FR 13008). 

In August 1992, the EPA announced plans to initiate the third periodic review of the air 

quality criteria and O3 NAAQS (57 FR 35542). In December 1996, the EPA proposed to replace 
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the then-existing 1-hour primary and secondary standards with 8-hour average O3 standards set 

at a level of 0.08 ppm (equivalent to 0.084 ppm using standard rounding conventions) (61 FR 

65716). The EPA also proposed to establish a new distinct secondary standard using a 

biologically based cumulative, seasonal form. The EPA completed this review on July 18, 1997 

(62 FR 38856) by setting the primary standard at a level of 0.08 ppm, based on the annual fourthhighest daily maximum 8-hr average concentration, averaged over three years, and setting the 

secondary standard identical to the revised primary standard. In reaching this decision, the EPA 

identified several reasons supporting its decision to reject a potential alternate standard set at 

0.07 ppm. Most importantly, the EPA pointed out the scientific uncertainty at lower 

concentrations and placed significant weight on the fact that no CASAC panel member 

supported a standard level set lower than 0.08 ppm (62 FR 38868). In addition to noting the 

uncertainties in the health evidence for exposure concentrations below 0.08 ppm and the advice 

of CASAC, the EPA noted that a standard set at a level of 0.07 ppm would be closer to peak 

background concentrations that infrequently occur in some areas due to nonanthropogenic 

sources of O3 precursors (62 FR 38856, 38868; July 18, 1997). 

On May 14, 1999, in response to challenges by industry and others to the EPA’s 1997 

decision, the D.C. Circuit remanded the O3 NAAQS to the EPA, finding that section 109 of the 

CAA, as interpreted by the EPA, effected an unconstitutional delegation of legislative authority. 

American Trucking Assoc. vs. EPA, 175 F.3d 1027, 1034-1040 (D.C. Cir. 1999) (“ATA I”). In 

addition, the court directed that, in responding to the remand, the EPA should consider the 

potential beneficial health effects of O3 pollution in shielding the public from the effects of solar 

ultraviolet (UV) radiation, as well as adverse health effects. Id. at 1051-53. In 1999, the EPA 

petitioned for rehearing en banc on several issues related to that decision. The court granted the 
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request for rehearing in part and denied it in part, but declined to review its ruling with regard to 

the potential beneficial effects of O3 pollution. 195 F. 3d 4, 10 (D.C Cir., 1999) (“ATA II”). On 

January 27, 2000, the EPA petitioned the U.S. Supreme Court for certiorari on the constitutional 

issue (and two other issues), but did not request review of the ruling regarding the potential 

beneficial health effects of O3. On February 27, 2001, the U.S. Supreme Court unanimously 

reversed the judgment of the D.C. Circuit on the constitutional issue. Whitman v. American 

Trucking Assoc., 531 U. S. 457, 472-74 (2001) (holding that section 109 of the CAA does not 

delegate legislative power to the EPA in contravention of the Constitution). The Court remanded 

the case to the D.C. Circuit to consider challenges to the O3 NAAQS that had not been addressed 

by that court’s earlier decisions. On March 26, 2002, the D.C. Circuit issued its final decision on 

remand, finding the 1997 O3 NAAQS to be “neither arbitrary nor capricious,” and so denying the 

remaining petitions for review. American Trucking Associations, Inc. v EPA, 283 F.3d 355, 379 

(D.C Cir., 2002)(“ATA III”).

Specifically, in ATA III, the D.C. Circuit upheld the EPA’s decision on the 1997 O3

standard as the product of reasoned decision-making. With regard to the primary standard, the 

court made clear that the most important support for EPA’s decision to revise the standard was 

the health evidence of insufficient protection afforded by the then-existing standard (“the record 

is replete with references to studies demonstrating the inadequacies of the old one-hour 

standard”), as well as extensive information supporting the change to an 8-hour averaging time. 

283 F. 3d at 378. The court further upheld the EPA’s decision not to select a more stringent level 

for the primary standard noting “the absence of any human clinical studies at ozone 

concentrations below 0.08 [ppm]” which supported EPA’s conclusion that “the most serious 

health effects of ozone are ‘less certain’ at low concentrations, providing an eminently rational 
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reason to set the primary standard at a somewhat higher level, at least until additional studies 

become available.” Id. (internal citations omitted). The Court also pointed to the significant 

weight that the EPA properly placed on the advice it received from CASAC. Id. at 379. In 

addition, the court noted that “although relative proximity to peak background O3 concentrations 

did not, in itself, necessitate a level of 0.08 [ppm], EPA could consider that factor when choosing 

among the three alternative levels.” Id.

Independently of the litigation, the EPA responded to the court’s remand to consider the 

potential beneficial health effects of O3 pollution in shielding the public from effects of UV

radiation. The EPA provisionally determined that the information linking changes in patterns of 

ground-level O3 concentrations to changes in relevant patterns of exposures to UV radiation of 

concern to public health was too uncertain, at that time, to warrant any relaxation in 1997 O3

NAAQS. The EPA also expressed the view that any plausible changes in UV-B radiation 

exposures from changes in patterns of ground-level O3 concentrations would likely be very small 

from a public health perspective. In view of these findings, the EPA proposed to leave the 1997 

8-hour NAAQS unchanged (66 FR 57268, Nov. 14, 2001). After considering public comment on 

the proposed decision, the EPA published its final response to this remand on January 6, 2003, 

re-affirming the 8-hour O3 NAAQS set in 1997 (68 FR 614). 

The EPA initiated the fourth periodic review of the air quality criteria and O3 standards in 

September 2000 with a call for information (65 FR 57810). The schedule for completion of that 

review was ultimately governed by a consent decree resolving a lawsuit filed in March 2003 by 

plaintiffs representing national environmental and public health organizations, who maintained 

that the EPA was in breach of a mandatory legal duty to complete review of the O3 NAAQS 

within a statutorily mandated deadline. On July 11, 2007, the EPA proposed to revise the level of 
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the primary standard within a range of 0.075 to 0.070 ppm (72 FR 37818). Documents 

supporting this proposed decision included the Air Quality Criteria for Ozone and Other 

Photochemical Oxidants (U.S. EPA, 2006a) and the Staff Paper (U.S EPA, 2007) and related 

technical support documents. The EPA also proposed two options for revising the secondary 

standard: (1) replace the current standard with a cumulative, seasonal standard, expressed as an 

index of the annual sum of weighted hourly concentrations cumulated over 12 daylight hours 

during the consecutive 3-month period within the O3 season with the maximum index value, set 

at a level within the range of 7 to 21 ppm-hrs, or (2) set the secondary standard identical to the 

proposed primary standard. The EPA completed the review with publication of a final decision 

on March 27, 2008 (73 FR 16436). In that final rule, the EPA revised the NAAQS by lowering 

the level of the 8-hour primary O3 standard from 0.08 ppm to 0.075 ppm, not otherwise revising 

the primary standard, and adopting a secondary standard identical to the revised primary 

standard. In May 2008, state, public health, environmental, and industry petitioners filed suit 

challenging the EPA’s final decision on the 2008 O3 standards. On September 16, 2009, the EPA 

announced its intention to reconsider the 2008 O3 standards, 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. 

On January 19, 2010 (75 FR 2938), the EPA issued a notice of proposed rulemaking to 

reconsider the 2008 final decision. In that notice, the EPA proposed that further revisions of the 

primary and secondary standards were necessary to provide a requisite level of protection to 

public health and welfare. The EPA proposed to decrease the level of the 2008 8-hour primary 

standard from 0.075 ppm to a level within the range of 0.060 to 0.070 ppm, and to change the 

secondary standard to a new cumulative, seasonal standard expressed as an annual index of the 
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sum of weighted hourly concentrations, cumulated over 12 hours per day (8 am to 8 pm), during 

the consecutive 3-month period within the O3 season with a maximum index value, set at a level 

within the range of 7 to 15 ppm-hours. The Agency also solicited CASAC review of the 

proposed rule on January 25, 2010 and solicited additional CASAC advice on January 26, 2011. 

After considering comments from CASAC and the public, the EPA prepared a draft final rule, 

which was submitted for interagency review pursuant to Executive Order 12866. On September 

2, 2011, consistent with the direction of the President, the Administrator of the Office of 

Information and Regulatory Affairs (OIRA), Office of Management and Budget (OMB), 

returned the draft final rule to the EPA for further consideration. In view of this return and the 

fact that the Agency’s next periodic review of the O3 NAAQS required under CAA section 109 

had already begun (as announced on September 29, 2008), the EPA deferred the decisions 

involved in the reconsideration until it completed its statutorily required periodic review.

In light of EPA’s decision to consolidate the reconsideration with the current review, the 

D.C. Circuit proceeded with the litigation on the 2008 final decision. On July 23, 2013, the Court

upheld the EPA’s 2008 primary O3 standard, but remanded the 2008 secondary standard to the 

EPA. State of Mississippi v. EPA, 744 F. 3d 1334. With respect to the primary standard, the court 

first held that the EPA reasonably determined that the existing standard was not requisite to 

protect public health with an adequate margin of safety, and consequently required revision. 

Specifically, the court noted that there were “numerous epidemiologic studies linking health 

effects to exposure to ozone levels below 0.08 ppm and clinical human exposure studies finding 

a causal relationship between health effects and exposure to ozone levels at and below 0.08 

ppm.” 744 F. 3d at 1345. The court also specifically endorsed the weight of evidence approach 

utilized by the EPA in its deliberations. Id. at 1344. 
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The court went on to reject arguments that the EPA should have adopted a more stringent 

primary standard. Dismissing arguments that a clinical study (as properly interpreted by the 

EPA) showing effects at 0.06 ppm necessitated a standard level lower than that selected, the 

court noted that this was a single, limited study. Id. at 1350. With respect to the epidemiologic 

evidence, the court accepted the EPA’s argument that there could be legitimate uncertainty that a 

causal relationship between O3 and 8-hour exposures less than 0.075 ppm exists, so that 

associations at lower levels reported in epidemiologic studies did not necessitate a more stringent 

standard. Id. at 1351-52.

6

The court also rejected arguments that an 8-hour primary standard of 0.075 ppm failed to 

provide an adequate margin of safety, noting that margin of safety considerations involved policy 

judgments by the agency, and that by setting a standard “appreciably below” the level of the 

current standard (0.08 ppm), the agency had made a reasonable policy choice. Id. Finally, the 

court rejected arguments that the EPA’s decision was inconsistent with CASAC’s scientific 

recommendations because CASAC had been insufficiently clear in its recommendations whether 

it was providing scientific or policy recommendations, and the EPA had reasonably addressed 

CASAC’s policy recommendations. Id. at 1357-58. 

With respect to the secondary standard, the court held that because the EPA had failed to 

identify a level of air quality requisite to protect public welfare, the EPA’s comparison between 

the primary and secondary standards for determining if requisite protection for public welfare 

was afforded by the primary standard did not comply with the CAA. The court thus rejected the 

 

6 The court cautioned, however, that “perhaps more [clinical] studies like the Adams studies will 

yet reveal that the 0.060 ppm level produces significant adverse decrements that simply cannot 

be attributed to normal variation in lung function,” and further cautioned that “agencies may not 

merely recite the terms ‘substantial uncertainty’ as a justification for their actions.” Id. at 1350, 

1357 (internal citations omitted).
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EPA’s explanation for setting the secondary standard identical to the revised 8-hour primary 

standard, and remanded the secondary standard to the EPA. Id. at 1360-62.

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. On September 29, 2008, the EPA

announced the initiation of a new periodic review of the air quality criteria for O3 and related 

photochemical oxidants and issued a call for information in the Federal Register (73 FR 56581, 

Sept. 29, 2008). A wide range of external experts, as well as the EPA staff, representing a variety 

of areas of expertise (e.g., epidemiology, human and animal toxicology, statistics, risk/exposure 

analysis, atmospheric science, ecology, biology, plant science, ecosystem services) participated 

in a workshop. This workshop was held on October 28-29, 2008 in Research Triangle Park, NC. 

The workshop provided an opportunity for a public discussion of the key policy-relevant issues 

around which the EPA would structure this O3 NAAQS review and the most meaningful new 

science that would be available to inform our understanding of these issues. 

Based in part on the workshop discussions, the EPA developed a draft Integrated Review 

Plan (IRP) outlining the schedule, process, and key policy-relevant questions that would guide 

the evaluation of the air quality criteria for O3 and the review of the primary and secondary O3

NAAQS. A draft of the IRP was released for public review and comment in September 2009. 

This IRP was the subject of a consultation with the CASAC on November 13, 2009 (74 FR 

54562; October 22, 2009).7 The EPA considered comments received from that consultation and 

from the public in finalizing the plan and in beginning the review of the air quality criteria. The 

 

7 See http://yosemite.epa.gov/sab/sabproduct.nsf/WebProjectsbyTopicCASAC!OpenView for 

more information on CASAC activities related to the current O3 NAAQS review.
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EPA’s overall plan and schedule for this review is presented in the Integrated Review Plan for 

the Ozone National Ambient Air Quality Standards.

8

As part of the process of preparing the O3 ISA, the EPA’s NCEA hosted a workshop to 

review and discuss preliminary drafts of key sections of the ISA on August 6, 2010 (75 FR 

42085, July 20, 2010). The CASAC and the public reviewed the first external review draft ISA 

(U.S. EPA, 2011a; 76 FR 10893, February 28, 2011) at a meeting held in May 19-20, 2011 (76 

FR 23809; April 28, 2011). Based on CASAC and public comments, NCEA prepared a second 

draft ISA (U.S. EPA, 2011b; 76 FR 60820, September 30, 2011). CASAC and the public 

reviewed this draft at a January 9-10, 2012 (76 FR 236, December 8, 2011) meeting. Based on 

CASAC and public comments, NCEA prepared a third draft ISA (U.S. EPA 2012a; 77 FR 

36534; June 19, 2012), which was reviewed at a CASAC meeting in September 2012. The EPA 

released the final ISA (EPA/600/R-10/076F) in February 2013.

The EPA presented its plans for conducting the Risk and Exposure Assessments (REAs) 

that build on the scientific evidence presented in the ISA, in two planning documents titled 

Ozone National Ambient Air Quality Standards: Scope and Methods Plan for Health Risk and 

Exposure Assessment and Ozone National Ambient Air Quality Standards: Scope and Methods 

Plan for Welfare Risk and Exposure Assessment (henceforth, Scope and Methods Plans).9 These 

planning documents outlined the scope and approaches that staff planned to use in conducting 

quantitative assessments, as well as key issues that would be addressed as part of the 

assessments. The EPA released these documents for public comment in April 2011, and 

 

8 EPA 452/R-11-006; April 2011; Available: 

http://www.epa.gov/ttn/naaqs/standards/ozone/data/2011_04_OzoneIRP.pdf

9 EPA-452/P-11-001 and -002; April 2011; Available: 

http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_2008_pd.html
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consulted with CASAC on May 19-20, 2011 (76 FR 23809; April 28, 2011). In designing and 

conducting the initial health risk and welfare risk assessments, the EPA considered CASAC 

comments (Samet, 2011) on the Scope and Methods Plans and also considered public comments. 

In May 2012, the EPA issued a memo titled Updates to Information Presented in the Scope and 

Methods Plans for the Ozone NAAQS Health and Welfare Risk and Exposure Assessments that 

described changes to elements of the scope and methods plans and provided a brief explanation 

of each change and the reason for it. 

In July 2012, the EPA made the first drafts of the Health and Welfare REAs available for 

CASAC review and public comment (77 FR 42495, July 19, 2012). The first draft PA10 was 

made available for CASAC review and public comment in August 2012. These documents were 

reviewed by the CASAC O3 Panel at a public meeting in September 2012. The second draft 

REAs and PA, made available by the EPA in January 2014 (79 FR 4694, January 29, 2014),

were prepared with consideration of advice from CASAC (Frey and Samet, 2012a, 2012b) and 

comments from the public. These drafts were reviewed by the CASAC O3 Panel at a public 

meeting on March 25-27, 2014. The CASAC issued final reports on the second drafts of the 

HREA on July 1, 2014 (Frey, 2014a), and the WREA on June 18, 2014 (Frey, 2014b), 

respectively. The CASAC issued a final report on the second draft PA on June 26, 2014 (Frey, 

2014c). The final versions of the HREA (U.S. EPA 2014a), WREA (U.S. EPA, 2014b), and PA 

 

10 The PA is prepared by the staff in the EPA’s Office of Air Quality Planning and Standards 

(OAQPS). It presents a staff evaluation of the policy implications of the key scientific and 

technical information in the ISA and REAs for the EPA’s consideration. The PA provides a 

transparent evaluation, and staff conclusions, regarding policy considerations related to reaching 

judgments about the adequacy of the current standards, and if revision is considered, what 

revisions may be appropriate to consider. The PA is intended to help “bridge the gap” between 

the agency’s scientific assessments presented in the ISA and REAs, and the judgments required 

of the EPA Administrator in determining whether it is appropriate to retain or revise the 

NAAQS.
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(U.S. EPA, 2014c) were made available by the EPA in August, 2014. These documents reflect 

staff’s consideration of the comments and recommendations made by CASAC, as well as

comments made by members of the public, in their review of the draft versions of these 

documents.

E. Ozone Air Quality

Ozone is formed near the Earth’s surface due to chemical interactions involving solar 

radiation and precursor pollutants including volatile organic compounds (VOCs), nitrogen oxides 

(NOX), methane (CH4) and carbon monoxide (CO). The precursor emissions leading to O3

formation can result from both man-made sources (e.g., motor vehicles and electric power 

generation) and natural sources (e.g., vegetation and wildfires). Occasionally, O3 that is created 

naturally in the stratosphere can also contribute to O3 levels near the surface. Once formed, O3

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 surface characteristics.

In order to continuously assess O3 air pollution levels, state and local environmental 

agencies operate O3 monitors at various locations and subsequently submit the data to the EPA. 

At present, there are approximately 1,400 monitors across the U.S. reporting hourly O3 averages 

during the times of the year when local O3 pollution can be important. Much of this monitoring is 

focused on O3 measurements in urban areas where precursor emissions tend to be largest, as well 

as locations directly downwind of these areas, but there are also over 100 sites in rural areas 

where high levels of O3 can periodically exist due to transport from upwind sources. Based on 

data from this national network, the EPA estimates that approximately 133 million Americans 

live in counties where O3 concentrations were above the level of the existing health-based 
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NAAQS of 0.075 ppm at least 4 days in 2012. High O3 values can occur almost anywhere within 

the contiguous 48 states, although locations in California, Texas, and the Northeast Corridor are 

especially subject to poor O3 air quality. From a temporal perspective, the highest daily peak O3

concentrations generally tend to occur during the afternoon within the warmer months due to 

higher 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 

areas of regional production can occasionally result in high nighttime levels of O3, 2) highelevation sites periodically influenced by stratospheric intrusions, and 3) certain locations in the 

western U.S. where large quantities of O3 precursors emissions associated with oil and gas 

development can be trapped by strong inversions associated with snow cover during the colder 

months and efficiently converted to O3. 

One of the challenging aspects of developing plans to reduce emissions leading to high 

O3 concentrations is that the response of O3 to precursor reductions is nonlinear. In particular, 

NOX causes both the formation and destruction of O3. The net impact of NOX emissions on O3

concentrations depends on the local quantities of NOX, VOC, and sunlight which interact in a set 

of complex chemical reactions. In some areas, such as urban centers where NOX emissions 

typically are high, NOX leads to the net destruction of O3, making O3 levels lower in the 

immediate vicinity. This phenomenon is particularly pronounced under conditions that lead to 

low O3 concentrations (i.e. during cool, cloudy weather and at night when photochemical activity 

is limited or nonexistent). However, while NOX can initially destroy O3 near the emission 

sources, these same NOX emissions eventually do react to form more O3 downwind. 

Photochemical model simulations suggest that the additional expected reductions in NOX

emissions will slightly increase O3 concentrations on days with lower O3 concentrations in areas 
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in close proximity to NOX sources, while at the same time decreasing the highest O3

concentrations in outlying areas. See generally, U.S.EPA, 2014a (section 2.2.1).

At present, both the primary and secondary NAAQS use the annual fourth-highest daily 

maximum 8-hour concentration, averaged over 3 years, as the form of the standard. An 

additional air quality metric, referred to as W126, is often used to assess cumulative impact of O3

exposure on ecosystems and vegetation. W126 is a seasonal aggregate of weighted hourly O3

values observed between 8 am and 8 pm. As O3 precursor emissions have decreased across the 

U.S., O3 design values11 have concurrently shown a modest downward trend. Ozone design 

values decreased by approximately 9% on average between 2000 and 2012. Air quality model 

simulations estimate that peak O3 levels will continue to improve over the next decade as 

additional reductions in O3 precursors from power plants, motor vehicles, and other sources are 

realized. 

In addition to being affected by changing emissions, future O3 concentrations will also be 

affected by climate change. Modeling studies in EPA’s Interim Assessment (U.S. EPA, 2009b) 

and cited in support of the 2009 Endangerment Finding (74 FR 66,496; Dec. 15, 2009) show 

that, while the impact is not uniform, climate change has the potential to cause increases in 

summertime O3 concentrations over substantial regions of the country, with increases tending to 

occur during higher peak pollution episodes in the summer, if offsetting emissions reductions are 

not made. Increases in temperature are expected to be the principal factor in driving any ozone 

increases, although increases in stagnation frequency may also contribute (Jacob and Winner, 

2009). These increases in O3 pollution over broad areas of the U.S., including in the largest 

 

11 A design value is a statistic that describes the air quality status of a given location relative to 

the level of the NAAQS.
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metropolitan areas with the worst O3 problems, increase the risk of morbidity and mortality. 

Children, people with asthma or other lung diseases, older adults, and people who are active 

outdoors, including outdoor workers, are among the most vulnerable to these O3-related health 

effects. If unchecked, climate change has the potential to offset some of the improvements in O3

air quality, and therefore some of the improvements in public health, that are expected from 

reductions in emissions of O3 precursors. 

Another challenging aspect of the O3 issue is the involvement of sources of O3 and O3

precursors beyond those from domestic, anthropogenic sources. Modeling analyses have 

suggested that nationally the majority of O3 exceedances are predominantly caused by 

anthropogenic emissions from within the U.S. However, observational and modeling analyses 

have concluded that O3 concentrations in some locations in the U.S. can be substantially 

influenced by sources that may not be suited to domestic control measures. In particular, certain 

high-elevation sites in the western U.S. are impacted by a combination of non-local sources like 

international transport, stratospheric O3, and O3 originating from wildfire emissions. Ambient O3

from these non-local sources is collectively referred to as background O3. See generally section 

2.4 of the Policy Assessment (U.S. EPA, 2014c). The analyses suggest that, at these locations, 

there can be episodic events with substantial background contributions where O3 concentrations

approach or exceed the level of the current NAAQS (i.e., 75 ppb). These events are relatively 

infrequent and the EPA has policies that allow for the exclusion of air quality monitoring data 

from design value calculations when they are substantially affected by certain background 

influences. Wildfires pose a direct threat to air quality and public safety – threats that can be 

mitigated through management of wildland vegetation. The use of wildland prescribed fire can 

influence the occurrence of catastrophic wildfires which may help manage the contribution of 
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wildfires to background O3 levels and periodic peak O3 events. Prescribed fire mimics a natural 

process necessary to manage and maintain fire-adapted ecosystems and climate change 

adaptation, while reducing risk of uncontrolled emissions from catastrophic wildfires. Wildfire 

emissions may make it more challenging to meet the NAAQS. However, the CAA requires the 

EPA to set the NAAQS at levels requisite to protect public health and welfare without regard to 

the source of the pollutant. API, 665 F. 2d at 1185-86. The EPA may consider proximity to 

background levels as a factor in the decision whether and how to revise the NAAQS when 

considering levels within the range of reasonable values supported by the air quality criteria and 

judgments of the Administrator. ATA III, 283 F. 3d at 379. It is in the implementation process 

that states and the EPA can address how to develop effective public policy in locations in which 

background sources contribute substantially to high O3. Section VII.F provides more detail about 

how background O3 can be addressed via CAA implementation provisions.

II. Rationale for Proposed Decision on the Primary Standard

This section presents the Administrator’s rationale for her proposed decision to revise the 

existing primary O3 standard by lowering the level of the standard to within the range of 0.065 to 

0.070 ppm. As discussed more fully below, this rationale draws from the thorough review in the 

ISA of the available scientific evidence, published through July 2011, on human health effects 

associated with the presence of O3 in the ambient air. This rationale also takes into account: (1) 

analyses of O3 air quality, human exposures to O3, and O3-associated health risks, as presented 

and assessed in the HREA; (2) the EPA staff assessment of the most policy-relevant scientific 

evidence and exposure/risk information in the PA; (3) CASAC advice and recommendations, as 

reflected in discussions of drafts of the ISA, REA, and PA at public meetings, in separate written 
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comments, and in CASAC’s letters to the Administrator; and (4) public input received during the 

development of these documents, either in connection with CASAC meetings or separately. 

Section II.A below provides an overview of the approaches used to consider the scientific 

evidence and exposure/risk information as it relates to the primary O3 standard. This includes 

summaries of the approach adopted by the Administrator in the 2008 review of the O3 NAAQS 

and of the approach adopted in the PA in the current review. Section II.B summarizes the body 

of evidence for health effects attributable to short- or long-term O3 exposures, with a focus on 

effects for which the ISA judges that there is a “causal” or a “likely to be causal” relationship 

with O3 exposures. Section II.C summarizes the HREA’s quantitative estimates of O3 exposures 

and health risks, including key results and uncertainties. Sections II.D and II.E present the 

Administrator’s proposed conclusions on the adequacy of the current primary O3 standard and 

alternative primary standards, respectively. 

A. Approach

In the 2008 review of the O3 NAAQS, Administrator Stephen L. Johnson revised the 

level of the 8-hour primary O3 standard from 0.08 ppm12 to 0.075 ppm (75 parts per billion 

(ppb)

13). This decision was based on his consideration of the available scientific evidence and 

exposure/risk information, the advice and recommendations of CASAC, and comments from the 

public. The Administrator placed primary emphasis on the body of available scientific evidence, 

while viewing the results of exposure and risk assessments as providing supporting information. 

 

12 Due to rounding convention, the 1997 standard level of 0.08 ppm corresponded to 0.084 ppm 

(84 ppb).

13 The level of the O3 standard is specified as 0.075 ppm rather than 75 ppb. However, in the PA 

we refer to ppb, which is most often used in the scientific literature and in the ISA, in order to 

avoid the confusion that could result from switching units when discussing the evidence in 

relation to the standard level. Similarly, in the preamble to this notice we refer to ppb. 
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Specifically, he judged that a standard set at 75 ppb would be appreciably below the 

concentration at which adverse effects had been demonstrated in the controlled human exposure 

studies available at that time (i.e., 80 ppb), and would provide a significant increase in protection 

compared to the then-current standard. The Administrator further concluded that the body of 

evidence did not support setting a lower standard level, given the increasing uncertainty in the 

evidence at lower O3 concentrations (U.S. EPA, 2014c, Chapter 1). 

In the current review, the EPA’s approach to informing decisions on the primary O3

standard builds upon the general approach used in the last review and reflects the broader body 

of scientific evidence, updated exposure/risk information, and advances in O3 air quality 

modeling now available. This approach, described in detail in the PA (U.S. EPA, 2014c, section 

1.3.1), is based most fundamentally on using the EPA’s assessment of the available scientific 

evidence and associated quantitative analyses to inform the Administrator’s judgments regarding 

a primary standard for O3 that is “requisite” (i.e., neither more nor less stringent than necessary) 

to protect public health with an adequate margin of safety. Specifically, it is based on 

consideration of the available body of scientific evidence assessed in the ISA (U.S. EPA, 2013a),

exposure and risk analyses presented in the HREA (U.S. EPA, 2014a), advice and 

recommendations from CASAC (Frey, 2014a, c), and public comments. Based on the application 

of this approach, the PA assesses and integrates the evidence and information, and reaches 

conclusions for the Administrator’s consideration about the range of policy options that could be 

supported. The remainder of this section describes the PA’s approach to reviewing the primary 

O3 standard, and to informing the Administrator’s proposed decisions on the current and 

alternative standards. 
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As an initial matter, the PA recognizes that the final decision to retain or revise the 

current primary O3 standard is a public health policy judgment to be made by the Administrator

and will draw upon the available scientific evidence for O3-attributable health effects and on 

analyses of population exposures and health risks, including judgments about the appropriate 

weight to assign the range of uncertainties inherent in the evidence and analyses. The PA’s

general approach to informing these public health policy judgments recognizes that the available 

health effects evidence reflects a continuum from relatively higher O3 concentrations, at which 

scientists generally agree that health effects are likely to occur, through lower concentrations, at 

which the likelihood and magnitude of a response become increasingly uncertain. Therefore, the 

conclusions in the PA reflect an interpretation of the available scientific evidence and 

exposure/risk information that, in the views of the EPA staff, neither overstates nor understates 

the strengths and limitations of that evidence and information.14 This approach is consistent with 

the requirements of sections 108 and 109 of the CAA, as well as with how the EPA and the 

courts have historically interpreted the CAA. 

The PA draws upon an integrative synthesis of the entire body of available scientific 

evidence for O3-related health effects, including the evidence newly available in the current 

review and the evidence from previous reviews, as presented in the ISA (U.S. EPA, 2013a). 

Consideration of the scientific evidence is based fundamentally on information from controlled 

human exposure and epidemiologic studies, supplemented by information from animal 

 

14 Draft versions of the PA were subject to review by CASAC and the final PA reflects 

consideration of the advice received from CASAC during the review process. CASAC concluded 

that “Overall, we find the Second Draft PA to be adequate for its intended purpose of providing a 

strong scientific basis for findings regarding the inadequacy of current primary and secondary 

ozone air quality standards” (Frey, 2014c, p. v). 
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toxicology studies. In the PA, such evidence informs the consideration of the health endpoints 

and at-risk populations15 on which to focus the current review, and the consideration of the O3

concentrations at which various health effects can occur. 

Since the 2008 review of the O3 NAAQS, the EPA has developed formal frameworks for 

characterizing the strength of the scientific evidence with regard to health effects associated with 

exposures to O3 in ambient air and factors that may increase risk in some populations or 

lifestages. These frameworks provide the basis for robust, consistent, and transparent processes 

for evaluating the scientific evidence, including uncertainties in the evidence, and for drawing 

weight-of-evidence conclusions on air pollution-related health effects and at-risk populations. 

These frameworks for characterizing the strength of the scientific evidence are discussed in 

detail in the ISA (U.S. EPA, 2013a, Preamble; Chapter 8). 

With regard to characterization of health effects, the ISA uses a five-level hierarchy to 

classify the overall weight of evidence into one of the following categories: causal relationship, 

likely to be a causal relationship, suggestive of a causal relationship, inadequate to infer a causal 

relationship, and not likely to be a causal relationship (U.S. EPA, 2013a, Preamble Table II). In 

using the weight-of-evidence approach to inform judgments about the degree of confidence that 

 

15 In this review, the term “at-risk population” is used to encompass populations or lifestages that 

have a greater likelihood of experiencing health effects related to exposure to an air pollutant due 

to a variety of factors; other terms used in the literature include susceptible, vulnerable, and 

sensitive. These factors may be intrinsic, such as genetic factors, lifestage, or the presence of 

preexisting diseases, or they may be extrinsic, such as socioeconomic status (SES), activity 

pattern and exercise level, or increased pollutant exposures (U.S. EPA 2013, p. lxx, 8-1, 8-2). 

The courts and the CAA’s legislative history refer to these at-risk subpopulations as 

“susceptible” or “sensitive” populations. See, e.g., American Lung Ass’n v. EPA, 134 F. 3d 388, 

389 (D.C. Cir. 1998) (“NAAQS must protect not only average health individuals, but also 

‘sensitive citizens’ – children, for example, or people with asthma, emphysema, or other 

conditions rendering them particularly vulnerable to air pollution” (quoting S. Rep. No. 91-1196 

at 10).
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various health effects are likely to be caused by exposure to O3, confidence increases as the 

number of studies consistently reporting a particular health endpoint grows and as other factors, 

such as biological plausibility and the strength, consistency, and coherence of evidence, increase. 

Conclusions about biological plausibility and about the consistency and coherence of O3-related 

health effects are drawn from the integration of epidemiologic studies with mechanistic 

information from controlled human exposure and animal toxicological studies, as discussed in 

the ISA (U.S. EPA, 2013a, EPA Framework for Causal Determination, p. 1viii). The PA places

the greatest weight on the health effects for which the evidence has been judged in the ISA to 

support a “causal” or a “likely to be causal” relationship with O3 exposures. 

The PA further considers the evidence base assessed in the ISA with regard to the types 

and levels of exposure at which health effects are indicated. This consideration of the evidence, 

which directly informs conclusions regarding the adequacy of current or alternative standards, 

differs from consideration of the evidence in the ISA with regard to overarching determinations 

of causality. Therefore, studies that inform determinations of causality may or may not be 

concluded to be informative with regard to the adequacy of the current or alternative standards.

16

As with health endpoints, the ISA’s characterization of the weight of evidence for 

potential at-risk populations is based on the evaluation and synthesis of evidence from across 

scientific disciplines. The ISA uses the collective evidence to examine the coherence of effects 

across disciplines and to determine the biological plausibility of reported effects. Based on this 

approach, the ISA characterizes the evidence for a number of “factors” that have the potential to 

 

16 For example, the PA judges that health studies evaluating exposure concentrations near or 

below the level of the current standard and epidemiologic studies conducted in locations meeting 

the current standard are particularly informative when considering the adequacy of the public 

health protection provided by the current standard (U.S. EPA, 2014c, Chapters 3 and 4). 
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place populations at increased risk for O3-related effects. The categories considered in evaluating 

the evidence for these potential at-risk factors are “adequate evidence,” “suggestive evidence,” 

“inadequate evidence,” and “evidence of no effect.” For the “adequate evidence” category, the 

ISA concludes that this category is appropriate when multiple high-quality studies show “there is 

substantial, consistent evidence within a discipline to conclude that a factor results in a 

population or lifestage being at increased risk of air pollutant-related health effect(s) relative to 

some reference population or lifestage” (U.S. EPA, 2013a, p. 8-2). In addition, where applicable, 

the “adequate evidence” category reflects a conclusion that there is coherence in the evidence 

across disciplines. The other categories reflect greater uncertainty in the evidence. In this review, 

the PA focuses on those factors for which the ISA judges there is adequate evidence of increased 

risk (U.S. EPA, 2013a, Table 8-5). At-risk populations are discussed in more detail in section 

3.1.5 of the PA (U.S. EPA, 2014c) and these categories are discussed in more detail in the ISA 

(U.S. EPA, 2013a, chapter 8, Table 8-1). 

Using the available scientific evidence to inform conclusions on the current and 

alternative standards is complicated by the recognition that a population-level threshold has not 

been identified below which it can be concluded with confidence that O3-attributable effects do 

not occur (U.S. EPA, 2013a, section 2.5.4.4). In the absence of a discernible threshold, the PA’s 

general approach to considering the available O3 health evidence involves characterizing 

confidence in the extent to which O3-attributable effects occur, and the extent to which such 

effects are adverse, over the ranges of O3 exposure concentrations evaluated in controlled human 

exposure studies and over the distributions of ambient O3 concentrations in locations where 

epidemiologic studies have been conducted. As noted above, the PA recognizes that the available 

health effects evidence reflects a continuum from relatively high O3 concentrations, at which 
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scientists generally agree that adverse health effects are likely to occur, through lower 

concentrations, at which the likelihood and magnitude of a response become increasingly 

uncertain. Aspects of the approach used in this review to evaluate evidence from controlled 

human exposure and epidemiologic studies, respectively, are discussed below. 

Controlled human exposure studies provide direct evidence of relationships between 

pollutant exposures and human health effects (U.S. EPA, 2013a, p.lx). Controlled human 

exposure studies provide data with the highest level of confidence since they provide human 

effects data under closely monitored conditions and can provide exposure response relationships. 

Such studies are particularly useful in defining the specific conditions under which pollutant 

exposures can result in health impacts, including the exposure concentrations, durations, and 

ventilation rates under which effects can occur. As discussed in the ISA, controlled human 

exposure studies provide clear and compelling evidence for an array of human health effects that 

are directly attributable to acute exposures to O3 per se (i.e., as opposed to O3 and other 

photochemical oxidants, for which O3 is an indicator, or other co-occurring pollutants) (U.S. 

EPA, 2013a, Chapter 6). Together with animal toxicological studies, which can provide 

information about more serious health outcomes as well as the effects of long-term exposures 

and mode of action, controlled human exposure studies also help to provide biological 

plausibility for health effects observed in epidemiologic studies. 

The PA considers the evidence from controlled human exposure studies in two ways. 

First, the PA considers the extent to which controlled human exposure studies provide evidence 

for health effects following exposures to different O3 concentrations, down to the lowestobserved-effects levels in those studies. Second, the PA uses these studies to help evaluate the 

extent to which there is confidence in health effect associations reported in epidemiologic studies 
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down through lower ambient O3 concentrations, where the likelihood and magnitude of O3-

attributable effects become increasingly uncertain. 

The PA considers the range of O3 exposure concentrations evaluated in controlled human 

exposure studies, including concentrations near or below the level of the current standard. The 

PA considers both group mean responses, which provide insight into the extent to which 

observed changes are due to O3 exposures rather than to chance alone, and interindividual 

variability in responses, which provides insight into the fraction of the population that might be 

affected by such O3 exposures (U.S. EPA, 2013a, section 6.2.1.1). When considering the relative 

weight to place on various controlled human exposure studies, the discussion in the PA focuses 

on the exposure conditions evaluated (e.g., exercising versus resting, exposure duration); the 

nature, magnitude, and likely adversity of effects over the range of reported O3 exposure 

concentrations; the statistical precision of reported effects; and the consistency of results across 

studies for a given health endpoint and exposure concentration. In addition, because controlled 

human exposure studies typically involve healthy individuals and do not evaluate the most 

sensitive individuals in the population (U.S. EPA, 2013a, Preamble p. lx), when considering the 

implications of these studies for evaluation of the current and alternative standards, the PA also 

considers the extent to which reported effects are likely to reflect the magnitude and/or severity 

of effects in at-risk groups. 

The PA also considers epidemiologic studies of short- and long-term O3 concentrations in 

ambient air. Epidemiologic studies provide information on associations between variability in 

ambient O3 concentrations and variability in various health outcomes, including lung function 

decrements, respiratory symptoms, school absences, hospital admissions, emergency department 

visits, and premature mortality (U.S. EPA, 2013a, Chapters 6 and 7). Epidemiologic studies can 
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inform understanding of the effects in the study population (which may include at-risk groups) of 

real-world exposures to the range of O3 concentrations in ambient air, as well as provide 

evidence of associations between ambient O3 levels and more serious acute and chronic health 

effects that cannot be assessed in controlled human exposure studies. For these studies, the 

degree of uncertainty introduced by confounding variables (e.g., other pollutants, temperature) 

and other factors (e.g., effects modifiers such as averting behavior) affects the level of 

confidence that the health effects being investigated are attributable to O3 exposures, alone and 

in combination with copollutants.

Available epidemiologic studies have generally not indicated a discernible population 

threshold below which O3 is no longer associated with health effects (U.S. EPA, 2013a, section 

2.5.4.4). However, the currently available epidemiologic evidence indicates decreased 

confidence in reported concentration-response relationships for O3 concentrations at the lower 

ends of ambient distributions due to the low density of data in this range (U.S. EPA, 2013a,

section 2.5.4.4). As discussed more fully in Chapter 1 of the PA (U.S. EPA, 2014c), the general 

approach to considering the results of epidemiologic studies within the context of the current and 

alternative standards focuses on characterizing the range of ambient O3 concentrations over 

which studies indicate the most confidence in O3-associated health effects, and the 

concentrations below which confidence in such health effect associations becomes appreciably 

lower. 

In placing emphasis on specific epidemiologic studies, as in past reviews, the discussion 

in the PA focuses on the epidemiologic studies conducted in the U.S. and Canada. Such studies 

reflect air quality and exposure patterns that are likely more typical of the U.S. population, since 

studies conducted outside the U.S. and Canada may well reflect different demographic and air 
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pollution characteristics.

17 The PA also focuses on studies reporting associations with effects 

judged in the ISA (U.S. EPA, 2013a) to be robust to confounding by other factors, including cooccurring air pollutants. 

To put staff conclusions about O3-related health effects into a broader public health 

context, the PA also considers exposure and risk estimates from the HREA, which develops and 

applies models to estimate human exposures to O3 and O3-related health risks in urban study 

areas across the United States (U.S. EPA, 2014a). The HREA estimates exposures of concern, 

based on interpreting quantitative exposure estimates within the context of controlled human 

exposure study results; lung function risks, based on applying exposure-response relationships 

from controlled human exposure studies to quantitative estimates of exposures; and 

epidemiologic-based risk estimates, based on applying concentration-response relationships 

drawn from epidemiologic studies to adjusted air quality. Each of these types of assessments is 

discussed briefly below. 

As in the 2008 review, the HREA estimates exposures at or above benchmark 

concentrations of 60, 70, and 80 ppb, reflecting exposure concentrations of concern based on the 

available health evidence.18 Estimates of exposures of concern, defined as personal exposures 

while at moderate or greater exertion to 8-hour average ambient O3 levels, at or above these 

discrete benchmark concentrations provide perspective on the public health risks of O3-related 

health effects that have been demonstrated in controlled human exposure and toxicological 

studies. However, because of a lack of exposure-response information across a range of exposure 

 

17 Though the PA recognizes that a broader body of studies, including international studies, 

informs the causal determinations in the ISA. 

18 For example, see 75 FR 2945-2946 (January 19, 2010) and 73 FR 16441-16442 (March 27, 

2008) discussing “exposures of concern.”
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concentrations in these studies, these risks cannot be assessed using a quantitative risk 

assessment. Though this analysis is conducted using discrete benchmark concentrations, 

information from the broad body of evidence indicates that health-relevant exposures are more 

appropriately viewed as a continuum with greater confidence and certainty about the existence of 

health effects at higher O3 exposure concentrations and less confidence and certainty at lower 

exposure concentrations. This approach recognizes that there is no sharp breakpoint within the 

exposure-response relationship for exposure concentrations at and above 80 ppb down to 60 ppb. 

Within the context of this continuum, estimates of exposures of concern at these discrete 

benchmark concentrations provide some perspective on the public health impacts of O3-related 

health effects, such as pulmonary inflammation, that are plausibly linked to the more serious

effects seen in epidemiologic studies but cannot be evaluated in quantitative risk assessments. 

They also help elucidate the extent to which such impacts may be reduced by meeting the current 

and alternative standards. Estimates of the number of people likely to experience exposures of 

concern cannot be directly translated into quantitative estimates of the number of people likely to 

experience specific health effects due to individual variability in responsiveness. Only a subset of 

individuals can be expected to experience such adverse health effects, and at-risk populations or 

lifestages, such as people with asthma or children, are expected to be affected more by such 

exposures than healthy adults.

The HREA also generates quantitative estimates of O3 health risks for air quality adjusted 

to just meet the current19 and alternative standards. One approach to estimating O3 health risks is 

to combine modeled exposure estimates with exposure-response relationships derived from 

 

19 For purposes of the exposure and risk estimates with adjusted air quality, the REA considered 

any value < 76 ppb to be “just meeting” the current 75 ppb standard (U.S. EPA, 2014a).
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controlled human exposure studies of O3-induced health effects. The HREA uses this approach 

to estimate the occurrence of O3-induced lung function decrements in at-risk populations, 

including school-age children, school-age children with asthma, adults with asthma, and older 

adults. The available exposure-response information does not support this approach for other 

endpoints evaluated in controlled human exposure studies (U.S. EPA, 2014a, section 2.3). 

The other approach used in this review to estimate O3-associated health risks is to apply 

concentration-response relationships derived from short- and/or long-term epidemiologic studies 

to air quality adjusted to just meet current and alternative standards. The concentration-response 

relationships drawn from epidemiologic studies are based on population exposure surrogates, 

such as 8-hour concentrations averaged across monitors and over more than one day (U.S. EPA, 

2013a, Chapter 6). The HREA presents epidemiologic-based risk estimates for O3-associated 

mortality, hospital admissions, emergency department visits, and respiratory symptoms (U.S. 

EPA, 2014a, section 2.3). These estimates are derived from the full distributions of ambient O3

concentrations estimated for the study locations.20 In addition, the HREA estimates mortality 

risks associated with various portions of distributions of short-term O3 concentrations (U.S. EPA, 

2014a). The PA considers risk estimates based on the full distributions of ambient O3

concentrations and, when available, estimates of the risk associated with various portions of 

 

20 In previous reviews, including the 2008 review and reconsideration, such risks were separately 

estimated for O3 concentrations characterized as above policy-relevant background 

concentrations. Policy-relevant background concentrations were defined as the distribution of O3

concentrations attributable to sources other than anthropogenic emissions of O3 precursor 

emissions (e.g., VOC, CO, NOX) in the U.S., Canada, and Mexico. The decision in this review to 

estimate total risk across the full range of O3 concentrations reflects consideration of advice from 

CASAC (Frey and Samet, 2012b).
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those ambient distributions.21 In doing so, the PA takes note of the ISA conclusions regarding 

confidence in linear concentration-response relationships over distributions of ambient 

concentrations (see above), and of the extent to which health effect associations at various 

ambient O3 concentrations are supported by the evidence from experimental studies for effects 

following specific O3 exposures. 

B. Health Effects Information

This section outlines key information contained in the ISA (U.S. EPA, 2013a, Chapters 4

to 8) and in the PA (U.S. EPA, 2014c, Chapters 3 and 4) on the known or potential effects on 

public health which may be expected from the presence of O3 in the ambient air. The information 

highlighted here summarizes: (1) new information available on potential mechanisms for health 

effects associated with exposure to O3 (II.B.1); (2) the nature of effects that have been associated 

directly with both short- and long-term exposure to O3 and indirectly with the presence of O3 in 

ambient air (II.B.2); (3) considerations related to the adversity of O3-attributable health effects 

(II.B.3); and (4) considerations in characterizing the public health impact of O3, including the 

identification of “at risk” populations (II.B.4).

The decision in the 2008 rulemaking emphasized the large number of epidemiologic

studies published since the 1997 review that continued to report associations with respiratory 

hospital admissions and emergency department visits, as well as additional health endpoints, 

including the effects of acute (short-term and prolonged) and chronic exposures to O3 on lung 

function decrements and enhanced respiratory symptoms in asthmatic individuals, school 

 

21 In a series of sensitivity analyses, the HREA also evaluates a series of threshold models for 

respiratory mortality associated with long-term O3 concentrations. The PA considers these risk 

estimates based on threshold models, in addition to HREA core estimates based on the linear 

model (U.S. EPA, 2014a, sections 3.2.3.2, 4.4.2.3). 
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absences, and premature mortality. It also emphasized controlled human exposure studies 

showing respiratory effects with prolonged O3 exposures at levels below 80 ppb, changes in lung 

host defenses, and increased airway responsiveness, and animal toxicology studies that provided 

information about mechanisms of action. 

The ISA (U.S. EPA, 2013a) prepared for this review emphasizes a large number of new 

epidemiologic studies published since the last review on effects associated with both short- and 

long-term exposures, including new epidemiologic studies about risk factors. It also emphasizes

important new information from controlled human exposure, dosimetry and toxicology studies. 

Highlights of the new evidence included:

(1) Two controlled human exposure studies new since the 2008 review are now available 

that examine respiratory effects associated with prolonged, 6.6-hour, O3 exposures to 

levels of 72 ppb22 and 60 ppb. These studies observed effects in healthy adults, including 

lung function decrements combined with respiratory symptoms at 72 ppb, and lung 

function decrements and pulmonary inflammation at 60 ppb. These studies expand on 

evidence of lung function decrements with O3 exposure at 60 ppb available in the last 

review, and provide new evidence of airway inflammation, a mechanism by which O3

may cause other more serious respiratory effects (e.g., asthma exacerbations). 

(2) Recent multicity and single city epidemiologic studies continue to report associations 

between short-term O3 exposures and respiratory hospital admissions and respiratory 

emergency department visits. Recent multicity studies and a multi-continent study have 

reported consistent positive associations between short-term O3 exposure and total 

 

22 As noted below, for the 70 ppb exposure concentration, Schelegle et al. (2009) reported that 

the actual mean exposure concentration was 72 ppb.
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(nonaccidental) mortality, expanding upon evidence available in the last review. They 

also observed associations between O3 exposure and cardiovascular and respiratory 

mortality.23

(3) Recent controlled human exposure studies reporting systemic inflammation and 

cardiac changes provide support for the expanded body of epidemiologic evidence for 

cardiovascular mortality, although lack of coherence with epidemiologic studies of 

cardiovascular morbidity remains an important uncertainty.

(4) New epidemiologic studies provide expanded evidence for respiratory effects 

associated with long-term or repeated O3 concentrations (e.g., seasonal average of 1-or 8-

hour daily max concentrations). Recent studies report interactions between exercise or 

different genetic variants and both new-onset asthma in children and increased 

respiratory symptom effects in individuals with asthma; additional studies of respiratory 

morbidity and mortality support the association between long-term exposure to O3 and a 

range of respiratory health effects.

(5) New evidence of risk factors (i.e., people with certain genetic variants related to 

antioxidant status or inflammation, and people with reduced intake of antioxidant 

nutrients) strengthens our understanding of the potential modes of action from O3-

induced effects. 

1. Overview of Mechanisms

The purpose of this section is to describe the ISA’s characterization of the key events and 

pathways that contribute to health effects resulting from both short-term and long-term exposures 

 

23 The consideration of ambient O3 concentrations in the locations of these epidemiologic studies 

are discussed in sections II.D.1.b and II.E.4.a below, for the current standard and alternative 

standards, respectively. 
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to O3. The information in this section draws from section 5.3 of the ISA (U.S. EPA, 2013a). 

Mode of action refers to a sequence of key events and processes that result in a given toxic 

effect. Elucidation of mechanisms provides a more detailed understanding of these key events 

and processes. Experimental evidence elucidating modes of action and/or mechanisms 

contributes to our understanding of the biological plausibility of adverse O3-related health 

effects, including respiratory effects and effects outside the respiratory system (U.S. EPA, 2013a, 

Chapters 6 and 7). 

Figure 3.1 in the PA (U.S. EPA, 2014c) shows the current understanding of key events in 

the toxicity pathway of O3, based on the available evidence. These key events are described 

briefly here and in more detail in section 3.1.1 of the PA. The initial key event is the formation of 

secondary oxidation products in the respiratory tract (U.S. EPA, 2013a, section 5.3). This mainly 

involves direct reactions with components of the extracellular lining fluid (ELF). Although the 

ELF has inherent capacity to quench (based on individual antioxidant capacity), this capacity can 

be overwhelmed, especially with exposure to elevated concentrations of O3. The resulting 

secondary oxidation products transmit signals to the epithelium, pain receptive nerve fibers and, 

if present, immune cells (i.e., eosinophils, dendritic cells and mast cells) involved in allergic 

responses. Thus, the available evidence indicates that the effects of O3 are mediated by 

components of ELF and by the multiple cell types found in the respiratory tract. Further, 

oxidative stress is an implicit part of this initial key event.

It is well understood that secondary oxidation products initiate numerous responses at the 

cellular, tissue, and whole organ level of the respiratory system. These responses include the 

activation of neural reflexes leading to lung function decrements, airway obstruction, and 

extrapulmonary effects such as slow resting heart rate; initiation of inflammation; alteration of 
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barrier epithelial function; sensitization of bronchial smooth muscle; modification of lung host 

defenses; and airways remodeling (U.S. EPA, 2013a, section 5.3.10, Figure 5-8). Each of these 

effects is discussed in more detail in section 3.1.1 of the PA (U.S. EPA, 2014c). 

Persistent inflammation and injury, which are observed in animal models of chronic and 

intermittent exposure to O3, are associated with airways remodeling (see Section 7.2.3 of the 

ISA, U.S. EPA 2013). Chronic intermittent exposure to O3 has also been shown to result in 

effects on the developing lung and immune system. Systemic inflammation and vascular 

oxidative/nitrosative stress are also key events in the toxicity pathway of O3. Extrapulmonary 

effects of O3 occur in numerous organ systems, including the cardiovascular, central nervous, 

reproductive, and hepatic systems (U.S. EPA, 2013a, sections 6.3 to 6.5 and sections 7.3 to 7.5). 

Responses to O3 exposure are variable within the population. Studies have shown a large 

range of pulmonary function (i.e., spirometric) responses to O3 among healthy young adults, 

while responses within an individual are relatively consistent over time. Other responses to O3

have also been characterized by a large degree of interindividual variability. For example, a 3- to 

20-fold difference among subjects in their studies in airways inflammation (i.e., neutrophilia

influx) following O3 exposure has been reported (Schelegle et al., 1991 and Devlin et al., 1991, 

respectively). Reproducibility of an individual’s inflammatory response to O3 exposure in 

humans, measured as sputum neutrophilia, was demonstrated by Holz et al (1999). Since 

individual inflammatory responses were relatively consistent across time, it was thought that 

inflammatory responsiveness reflected an intrinsic characteristic of the subject (Mudway and 

Kelly, 2000). While the basis for the observed interindividual variability in responsiveness to O3

is not clear, section 5.4.2 of the ISA discusses mechanisms that may underlie the variability in 

responses seen among individuals. Certain functional genetic polymorphisms, pre-existing 
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conditions or diseases, nutritional status, lifestages, and co-exposures contribute to altered risk of 

O3-induced effects. Experimental evidence for such O3-induced changes contributes to our 

understanding of the biological plausibility of adverse O3-related health effects, including a 

range of respiratory effects as well as effects outside the respiratory system (e.g., cardiovascular 

effects) (U.S. EPA, 2013a, Chapters 6 and 7).

2. Nature of Effects

The health effects of O3 are described in detail and assessed in the ISA (U.S. EPA, 

2013a). Based on this assessment, the ISA determined that a “causal” relationship exists between 

short-term exposure to O3 in ambient air24 and effects on the respiratory system and that a “likely 

to be causal” relationship25 exists between long-term exposure to O3 in ambient air and 

respiratory effects (U.S. EPA 2013a, pp. 1-6 to 1-7). As stated in the ISA, “[c]ollectively, a very 

large amount of evidence spanning several decades supports a relationship between exposure to 

O3 and a broad range of respiratory effects” (US. EPA, 2013a, p. 1-6). The ISA summarizes the 

longstanding body of evidence for O3 respiratory effects as follows (U.S. EPA, 2013a, p. 1-5):

The clearest evidence for health effects associated with exposure to O3 is provided 

by studies of respiratory effects. Collectively, a very large amount of evidence 

spanning several decades supports a relationship between exposure to O3 and a 

broad range of respiratory effects (see Section 6.2.9 and Section 7.2.8). The 

majority of this evidence is derived from studies investigating short-term 

exposures (i.e., hours to weeks) to O3, although animal toxicological studies and 

recent epidemiologic evidence demonstrate that long-term exposure (i.e., months 

to years) may also harm the respiratory system. 

 

24 In determining that a causal relationship exists for O3 with specific health effects, the EPA has 

concluded that “[e]vidence is sufficient to conclude that there is a causal relationship with 

relevant pollutant exposures” (U.S. EPA, 2013a, p. lxiv).

25 In determining a “likely to be a causal” relationship exists for O3 with specific health effects, 

the EPA has concluded that “[e]vidence is sufficient to conclude that a causal relationship is 

likely to exist with relevant pollutant exposures, but important uncertainties remain” (U.S. EPA, 

2013a, p. lxiv).
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Additionally, the ISA determined that the relationships between short-term exposures to O3 in 

ambient air and both total mortality and cardiovascular effects are likely to be causal, based on 

expanded evidence bases in the current review (U.S. EPA, 2013a, pp. 1-7 to 1-8). In the ISA, the 

EPA additionally determined that the currently available evidence for additional endpoints is 

“suggestive” of causal relationships between short-term (central nervous system effects) and 

long-term exposure (cardiovascular effects, reproductive and developmental effects, central 

nervous system effects and total mortality) to ambient O3.

Consistent with emphasis in past reviews on O3 health effects for which the evidence is 

strongest, in this review the EPA places the greatest emphasis on studies of health effects that 

have been judged in the ISA to be caused by, or likely to be caused by, O3 exposures (U.S. EPA, 

2013a, section 2.5.2). This section discusses the evidence for health effects attributable to O3

exposures, with a focus on respiratory morbidity and mortality effects attributable to short- and 

long-term exposures, and cardiovascular system effects (including mortality) and total mortality 

attributable to short-term exposures. This section focuses particularly on considering the extent 

to which the scientific evidence available in the current review has been strengthened since the 

last review, and the extent to which important uncertainties and limitations in the evidence from 

the last review have been addressed. 

a. Respiratory effects – short-term 

The 2006 O3 AQCD concluded that there was clear, consistent evidence of a causal 

relationship between short-term O3 exposure and respiratory effects (U.S. EPA, 2006a). This 

conclusion was substantiated by evidence from controlled human exposure and toxicological 

studies indicating a range of respiratory effects in response to short-term O3 exposures, including 

pulmonary function decrements and increases in respiratory symptoms, lung inflammation, lung 
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permeability, and airway hyperresponsiveness. Toxicological studies provided additional 

evidence for O3-induced impairment of host defenses. Combined, these findings from 

experimental studies provided support for epidemiologic evidence, in which short-term increases 

in ambient O3 concentration were consistently associated with decreases in lung function in 

populations with increased outdoor exposures, especially children with asthma and healthy 

children; increases in respiratory symptoms and asthma medication use in children with asthma; 

and increases in respiratory-related hospital admissions and asthma-related emergency 

department visits (U.S. EPA, 2013a, pp. 6-1 to 6-2). 

As discussed in detail in the ISA (U.S. EPA, 2013a, section 6.2.9), studies evaluated 

since the completion of the 2006 O3 AQCD support and expand upon the strong body of 

evidence that, in the last review, indicated a causal relationship between short-term O3 exposures

and respiratory health effects. Recent controlled human exposure studies conducted in young, 

healthy adults with moderate exertion have reported forced expiratory volume in 1 second 

(FEV1) decrements and pulmonary inflammation following prolonged exposures to O3

concentrations as low as 60 ppb, and respiratory symptoms following exposures to 

concentrations as low as 72 ppb (based on group mean responses).

26 Epidemiologic studies 

provide evidence that increases in ambient O3 exposures are associated with lung function 

decrements, increases in respiratory symptoms, and pulmonary inflammation in children with 

asthma; increases in respiratory-related hospital admissions and emergency department visits; 

and increases in respiratory mortality. Some of these studies report such associations even for O3

 

26Schelegle et al. (2009) reported a statistically significant increase in respiratory symptoms in 

healthy adults at a target O3 exposure concentration of 70 ppb, averaged over the study period. 

For this 70 ppb target exposure concentration, Schelegle et al. (2009) reported that the actual 

mean exposure concentration was 72 ppb.
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concentrations at the low end of the distribution of daily concentrations. Recent epidemiologic 

studies report that associations with respiratory morbidity and mortality are stronger during the 

warm/summer months and remain robust after adjustment for copollutants. Recent toxicological 

studies reporting O3-induced inflammation, airway hyperresponsiveness, and impaired lung host 

defense continue to support the biological plausibility and modes of action for the O3-induced 

respiratory effects observed in the controlled human exposure and epidemiologic studies. Further 

support is provided by recent studies that found O3-associated increases in indicators of airway 

inflammation and oxidative stress in children with asthma (U.S. EPA, 2013a, section 6.2.9). 

Together, epidemiologic and experimental studies support a continuum of respiratory effects 

associated with O3 exposure that can result in respiratory-related emergency department visits, 

hospital admissions, and/or mortality (U.S. EPA, 2013a, section 6.2.9).

Across respiratory endpoints, evidence indicates antioxidant capacity may modify the 

risk of respiratory morbidity associated with O3 exposure (U.S. EPA, 2013a, section 6.2.9, p. 6-

161). The potentially elevated risk of populations with diminished antioxidant capacity and the 

reduced risk of populations with sufficient antioxidant capacity is supported by epidemiologic 

studies and from controlled human exposure studies. Additional evidence characterizes O3-

induced decreases in antioxidant levels as a key event in the mode of action for downstream 

effects. 

Key aspects of this evidence are discussed below with regard to lung function

decrements; pulmonary inflammation, injury, and oxidative stress; airway hyperresponsiveness; 
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respiratory symptoms and medication use; lung host defense; allergic and asthma-related 

responses; hospital admissions and emergency department visits; and respiratory mortality.

27

i. Lung function decrements

In the 2008 review, a large number of controlled human exposure studies28 reported O3-

induced lung function decrements in young, healthy adults engaged in intermittent, moderate 

exertion following 6.6 hour exposures to O3 concentrations at or above 80 ppb. Although two 

studies also reported effects following exposures to lower concentrations, an important 

uncertainty in the last review was the extent to which exposures to O3 concentrations below 80 

ppb result in lung function decrements. In addition, in the last review epidemiologic panel 

studies had reported O3-associated lung function decrements in a variety of different populations 

(e.g., children, outdoor workers) likely to experience increased exposures. In the current review, 

additional controlled human exposure studies are available that have evaluated exposures to O3

concentrations of 60 or 72 ppb. The available evidence from controlled human exposure and 

panel studies is assessed in detail in the ISA (U.S. EPA, section 6.2.1) and is summarized below. 

Controlled exposures to O3 concentrations that can be found in the ambient air can result 

in a number of lung function effects, including decreased inspiratory capacity, mild 

bronchoconstriction, and rapid, shallow breathing patterns during exercise. Reflex inhibition of 

 

27 CASAC concurred that these were “the kinds of identifiable effects on public health that are 

expected from the presence of ozone in the ambient air” (Frey 2014c, p. 3).

28 The controlled human exposure studies emphasized in the PA utilize only healthy adult 

subjects. In the near absence of controlled human exposure data for children, HREA estimates of 

lung function decrements are based on the assumption that children exhibit the same lung 

function responses following O3 exposures as healthy 18 year olds (U.S. EPA, 2014a, section 

6.2.4 and 6.5). This assumption is justified in part by the findings of McDonnell et al. (1985), 

who reported that children (8-11 years old) experienced FEV1 responses similar to those 

observed in adults (18-35 years old). Thus, the conclusions about the occurrence of lung function 

decrements that follow generally apply to children as well as to adults. 
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inspiration results in a decrease in forced vital capacity (FVC) and total lung capacity (TLC) and, 

in combination with mild bronchoconstriction, contributes to a decrease in FEV1 (U.S. EPA, 

2013a, section 6.2.1.1). Accumulating evidence indicates that such effects are mediated by 

activation of sensory nerves, resulting in the involuntary truncation of inspiration and a mild 

increase in airway obstruction due to bronchoconstriction (U.S. EPA, 2013a, section 5.3.10). 

Data from controlled human exposure studies show that increasing the duration of O3

exposures and increasing ventilation rates decreases the O3 exposure concentrations required to 

impair lung function. Ozone exposure concentrations well above those typically found in 

ambient air are required to impair lung function in healthy resting adults, while exposure to O3

concentrations at or below those in the ambient air have been reported to impair lung function in 

healthy adults exposed for longer durations while undergoing intermittent, moderate exertion 

(U.S. EPA, 2013a, section 6.2.1.1). With repeated O3 exposures over several days, FEV1

responses become attenuated in both healthy adults and adults with mild asthma, though this 

attenuation of response is lost after about a week without exposure (U.S. EPA, 2013a, section 

6.2.1.1; p. 6-27). 

When considering controlled human exposure studies of O3-induced lung function 

decrements, the ISA and PA evaluate both group mean changes in lung function and the 

interindividual variability in the magnitude of responses. An advantage of O3 controlled human 

exposure studies (i.e., compared to the epidemiologic panel studies discussed below) is that 

reported effects necessarily result from exposures to O3 itself.29 To the extent studies report 

 

29 The ISA notes that the use of filtered air responses as a control for the assessment of responses 

following O3 exposure in controlled human exposure studies serves to eliminate alternative 

explanations other than O3 itself in causing the measured responses (U.S. EPA, 2013a, section 

6.2.1.1).
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statistically significant decrements in mean lung function following O3 exposures after 

controlling for other factors, these studies provide greater confidence that measured decrements 

are due to the O3 exposure itself, rather than to chance alone. As discussed below, group mean 

changes in lung function are often small, especially following exposures to relatively low O3

concentrations (e.g., 60 ppb). However, even when group mean decrements in lung function are 

small, some individuals could experience decrements that are “clinically meaningful” (Pellegrino 

et al., 2005; ATS, 1991) with respect to criteria for spirometric testing, and/or that could be 

considered adverse with respect to public health policy decisions (see section II.B.3, below). 

At the time of the last review, a number of controlled human exposure studies had 

reported lung function decrements in young, healthy adults following prolonged (6.6-hour) 

exposures while at moderate exertion to O3 concentrations at and above 80 ppb. In addition, 

there were two controlled human exposure studies by Adams (2002, 2006) that examined lung 

function effects following exposures to 60 ppb O3. The EPA’s analysis of the data from the 

Adams (2006) study reported a small but statistically significant O3-induced decrement in group 

mean FEV1 following exposures of young, healthy adults to 60 ppb O3 while at moderate 

exertion, when compared with filtered air controls (Brown et al., 2008).30 Further examination 

of the post-exposure FEV1 data, and mean data for other time points and other concentrations, 

indicated that the temporal pattern of the response to 60 ppb O3 was generally consistent with the 

temporal patterns of responses to higher O3 concentrations in this and other studies (75 FR 2950, 

January 19, 2010). This suggested a pattern of response following exposures to 60 ppb O3 that 

 

30 Adams (2006) did not find effects on FEV1 at 60 ppb to be statistically significant. In an 

analysis of the Adams (2006) data, Brown et al. (2008) showed that even after removal of 

potential outliers, the average effect on FEV1 at 60 ppb was small, but highly statistically 

significant (p < 0.002) using several common statistical tests.
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was consistent with a dose-response relationship, rather than random variability. See also State of 

Mississippi v. EPA, F. 3d at 1347 (upholding EPA’s interpretation of the Adams studies). 

Figure 6-1 in the ISA summarizes the currently available evidence from multiple 

controlled human exposure studies evaluating group mean changes in FEV1 following prolonged 

O3 exposures (i.e., 6.6 hours) in young, healthy adults engaged in moderate levels of physical 

activity (U.S. EPA, 2013a, section 6.2.1.1). With regard to the group mean changes reported in 

these studies, the ISA specifically notes the following (U.S. EPA, 2013a, section 6.2.1.1, Figure 

6-1): 

1. Prolonged exposure to 40 ppb O3 results in a small decrease in group mean FEV1 that is 

not statistically different from responses following exposure to filtered air (Adams, 2002; 

Adams, 2006). 

2. Prolonged exposure to an average O3 concentration of 60 ppb results in group mean FEV1

decrements ranging from 1.8% to 3.6% (Adams 2002; Adams, 2006;31 Schelegle et al., 

2009;32 Kim et al., 2011). Based on data from multiple studies, the weighted average 

group mean decrement was 2.7%. In some analyses, these group mean decrements in 

 

31 Adams (2006); (2002) both provide data for an additional group of 30 healthy subjects that 

were exposed via facemask to 60 ppb (square-wave) O3 for 6.6 hours with moderate exercise (V̇

E

= 23 L/min per m2 BSA). These subjects are described on page 133 of Adams (2006) and pages 

747 and 761 of Adams (2002). The FEV1 decrement may be somewhat increased due to a target 

V̇

E of 23 L/min per m2 BSA relative to other studies having the target V̇E of 20 L/min per m2

BSA. The facemask exposure is not expected to affect the FEV1 responses relative to a chamber 

exposure.

32 For the 60 ppb target exposure concentration, Schelegle et al. (2009) reported that the actual 

mean exposure concentration was 63 ppb. 
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lung function were statistically significant (Brown et al., 2008; Kim et al., 2011), while in 

other analyses they were not (Adams, 2006; Schelegle et al., 2009).33

3. Prolonged exposure to an average O3 concentration of 72 ppb results in a statistically 

significant group mean decrement in FEV1 of about 6% (Schelegle et al., 2009).34

4. Prolonged square-wave exposure to average O3 concentrations of 80 ppb, 100 ppb, or 120 

ppb O3 results in statistically significant group mean decrements in FEV1 ranging from 6 

to 8%, 8 to 14%, and 13 to 16%, respectively (Folinsbee et al., 1988; Horstman et al., 

1990; McDonnell et al., 1991; Adams, 2002; Adams, 2003; Adams, 2006). 

As illustrated in Figure 6-1 of the ISA, there is a smooth dose-response curve without 

evidence of a threshold for exposures between 40 and 120 ppb O3 (U.S. EPA, 2013a, Figure 6-

1). When these data are taken together, the ISA concludes that “mean FEV1 is clearly decreased 

by 6.6-hour exposures to 60 ppb O3 and higher concentrations in [healthy, young adult] subjects 

performing moderate exercise” (U.S. EPA, 2013a, p. 6-9). 

With respect to interindividual variability in lung function, in an individual with 

relatively “normal” lung function, with recognition of the technical and biological variability in 

measurements, within-day changes in FEV1 of ≥ 5% are clinically meaningful (Pellegrino et al., 

2005; ATS, 1991). The ISA (U.S. EPA, 2013a, section 6.1.) focuses on individuals with >10% 

 

33 Adams (2006) did not find effects on FEV1 at 60 ppb to be statistically significant. In an 

analysis of the Adams (2006) data, Brown et al. (2008) addressed the more fundamental question 

of whether there were statistically significant differences in responses before and after the 6.6 

hour exposure period and found the average effect on FEV1 at 60 ppb to be small, but highly 

statistically significant using several common statistical tests, even after removal of potential 

outliers. Schelegle et al. (2009) reported that, compared to filtered air, the largest change in FEV1

for the 60 ppb protocol occurred after the sixth (and final) exercise period.

34 As noted above, for the 70 ppb exposure group, Schelegle et al. (2009) reported that the actual 

mean exposure concentration was 72 ppb.
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decrements in FEV1 for two reasons. A 10% FEV1 decrement is accepted by the American 

Thoracic Society (ATS) as an abnormal response and a reasonable criterion for assessing 

exercise-induced bronchoconstriction (Dryden et al., 2010; ATS, 2000). (U.S. EPA, 2013a, 

section 6.2.1.1). Also, some individuals in the Schelegle et al. (2009) study experienced 5-10% 

FEV1 decrements following exposure to filtered air. 

In previous NAAQS reviews, the EPA has made judgments regarding the potential 

implications for individuals experiencing FEV1 decrements of varying degrees of severity.35 For

people with lung disease, the EPA judged that moderate functional decrements (e.g., FEV1 

decrements > 10% but < 20%, lasting up to 24 hours) would likely interfere with normal activity 

for many individuals, and would likely result in more frequent use of medication (75 FR 2973, 

January 19, 2010). In previous reviews CASAC has endorsed these conclusions. In the context of 

standard setting, in the last review of the O3 NAAQS CASAC indicated that it is appropriate to 

focus on the lower end of the range of moderate functional responses (e.g., FEV1 decrements 

10%) when estimating potentially adverse lung function decrements in people with lung disease, 

especially children with asthma (Henderson, 2006c; transcript of CASAC meeting, day 8/24/06, 

page 149). More specifically, CASAC stated that “[a] 10% decrement in FEV1 can lead to 

respiratory symptoms, especially in individuals with pre-existing pulmonary or cardiac disease. 

For example, people with chronic obstructive pulmonary disease have decreased ventilatory 

reserve (i.e., decreased baseline FEV1) such that a ≥ 10% decrement could lead to moderate to 

severe respiratory symptoms” (Samet, 2011). In this review, CASAC reiterated its support for 

 

35 Such judgments have been made for decrements in FEV1 as well as for increased airway 

responsiveness and symptomatic responses (e.g., cough, chest pain, wheeze). Ranges of 

pulmonary responses and their associated potential impacts are presented in Tables 3-2 and 3-3 

of the 2007 Staff Paper (U.S. EPA, 2007).
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this conclusion, stating that “[a]n FEV1 decrement of ≥10% is a scientifically relevant surrogate 

for adverse health outcomes for people with asthma and lung disease” (Frey, 2014c p. 3). 

Therefore, in considering interindividual variability in O3-induced lung function decrements in 

the current review, the EPA also focuses on the extent to which individuals were reported to 

experience FEV1 decrements of 10% or greater.36

New studies (Schelegle et al., 2009; Kim et al., 2011) add to the previously available 

evidence for interindividual variability in the responses of healthy adults following exposures to 

O3. Following prolonged exposures to 80 ppb O3 while at moderate exertion, the proportion of 

healthy adults experiencing FEV1 decrements greater than 10% was 17% by Adams (2006), 26% 

by McDonnell (1996), and 29% by Schelegle et al. (2009). Following exposures to 60 ppb O3, 

that proportion was 20% by Adams (2002), 3% by Adams (2006), 16% by Schelegle et al. 

(2009), and 5% by Kim et al. (2011). Across these studies, the weighted average proportion (i.e., 

based on numbers of subjects in each study) of young, healthy adults with >10% FEV1

decrements is 25% following exposure to 80 ppb O3 and 10% following exposure to 60 ppb O3,

for 6.6 hours at moderate exertion (U.S. EPA, 2013a, page 6-18 and 6-19).37, 38 The ISA notes 

that responses within an individual tend to be reproducible over a period of several months, 

indicating that interindividual differences reflect differences in intrinsic responsiveness. Given 

 

36 The approach to using results from controlled human exposure studies conducted in healthy 

adults to provide perspective on the potential public health impacts of O3-related respiratory 

health effects is discussed in section II.A above, and in sections II.C.2 and II.C.3 below.

37 The ISA notes that by considering responses uncorrected for filtered air exposures, during 

which lung function typically improves (which would increase the size of the change, pre-and 

post-exposure), 10% is an underestimate of the proportion of healthy individuals that are likely 

to experience clinically meaningful changes in lung function following exposure for 6.6 hours to 

60 ppb O3 during intermittent moderate exertion (U.S. EPA, 2012, section 6.2.1.1). 

38 Based on the data available at 60 ppb, 1% of subjects experienced decrements > 20% (also 

uncorrected for filtered air exposures).
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this, the ISA concludes that “[t]hough group mean decrements are biologically small and 

generally do not attain statistical significance, a considerable fraction of exposed individuals 

experience clinically meaningful decrements in lung function” when exposed for 6.6 hours to 60 

ppb O3 during quasi continuous, moderate exertion (U.S. EPA, 2013a, section 6.2.1.1, p. 6-20).

This review has marked an advance in the ability to make reliable quantitative predictions 

of the potential lung function response to ozone exposure, and thus to reasonably predict the 

degree of interindividual response of lung function to that exposure. McDonnell et al. (2012) and 

Schelegle et al. (2012) developed models using data on O3 exposure concentrations, ventilation 

rates, duration of exposures, and lung function responses from a number of controlled human 

exposure studies. See section 6.2.1.1 of the ISA (U.S. EPA 2013a, p. 6-15). The McDonnell et al. 

(2012) and Schelegle et al. (2012) studies analyzed large datasets to fit compartmental models 

that included the concept of a dose of onset in lung function response or a response threshold 

based upon the inhaled O3 dose. The McDonnell et al. (2012) model was fit to a dataset 

consisting of the FEV1 responses of 741 young, healthy adults (18-35 years of age) from 23 

individual controlled exposure studies. Concentrations across individual studies ranged from 40 

ppb to 400 ppb,39 activity level ranged from rest to heavy exercise, duration of exposure was 

from 2 to 7.6 hours. The extension of the McDonnell et al. (2012) model to children and older 

adults is discussed in section 6.2.4 of the HREA (U.S. EPA, 2014a). Schelegle et al. (2012) also 

analyzed a large dataset with substantial overlap to that used by McDonnell et al. (2012). The 

Schelegle et al. (2012) model was fit to the FEV1 responses of 220 young healthy adults (taken 

from a dataset of 704 individuals) from 21 individual controlled exposure studies. The resulting 

 

39 Responses to O3 in these studies were adjusted for responses observed following exposure to 

filtered air.
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empirical models can estimate the frequency distribution of individual responses for any 

exposure scenario as well as summary measures of the distribution such as the mean or median 

response and the proportions of individuals with FEV1 decrements > 10%, 15%, and 20%. 

The predictions of the McDonnell and Schelegle models are consistent with the observed 

results from the individual studies of O3-induced FEV1 decrements. Specifically, McDonnell et 

al. (2012) estimated that 9% of healthy exercising adults would experience FEV1 decrements 

greater than 10% following 6.6 hour exposure to 60 ppb O3, and that 22% would experience such 

decrements following exposure to 80 ppb O3 (U.S. EPA, 2013a, p. 6-18 and Figure 6-3).40

Schelegle et al. (2012) estimated that, for a prolonged (6.6 hours) O3 exposure with moderate, 

quasi-continuous exercise, the average dose of onset for FEV1 decrement would be reached 

following 4 to 5 hours of exposure to 60 ppb, and following 3 to 4 hours of exposure to 80 ppb. 

However, 14% of the individuals were estimated to have a dose of onset that was less than 40% 

of the average. Those individuals were estimated to reach their dose of onset following 1 to 2 

hours of exposure to 50 to 80 ppb O3 (U.S. EPA, 2013a, p. 6-16), which is consistent with the 

threshold FEV1 responses reported by McDonnell et al. (2012).

CASAC agreed that these models mark a significant technical advance over the exposureresponse modeling approach used in the last review (Frey, 2014a), stating that “the comparison 

of the MSS [McDonnell-Stewart-Smith] model results to those obtained with the exposureresponse (E-R) model is of tremendous importance. Typically, the MSS model gives results 

about a factor of three higher than the E-R model for school-aged children, which is expected 

because the MSS model includes responses for a wider range of exposure protocols (under 

 

40 Also consistent with the data from published studies (see above), this model predicts that 1% 

of people would experience FEV1 decrements > 20% following 6.6 hour exposure to 60 ppb O3. 
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different levels of exertion, lengths of exposure, and patterns of exposure concentrations) than 

the E-R model” (Frey, 2014a, p. 7). CASAC explicitly found “the updated and expanded lung 

finds the MSS model to be scientifically and biologically defensible.” (Frey, 2014a, pp. 2, 8).

As discussed above and in the ISA (U.S EPA, 2013a, Section 5.3.2), secondary oxidation 

products formed following O3 exposures can activate neural reflexes leading to decreased lung 

function. The McDonnell and Schelegle models included mathematical approaches to simulate 

the potential protective effect of antioxidants in the ELF at lower ambient O3 concentrations, and 

include a dose threshold below which changes in lung function do not occur.

Epidemiologic studies41 have consistently linked short-term increases in ambient O3

concentrations with lung function decrements in diverse populations and lifestages, including 

children attending summer camps, adults exercising or working outdoors, and groups with preexisting respiratory diseases such as asthmatic children (U.S. EPA, 2013a, section 6.2.1.2). Some

of these studies reported O3-associated lung function decrements accompanied by respiratory 

symptoms42 in asthmatic children (Just et al., 2002; Mortimer et al., 2002; Ross et al., 2002; 

Gielen et al., 1997; Romieu et al., 1997; Thurston et al., 1997; Romieu et al., 1996). In contrast, 

studies of children in the general population have reported similar O3-associated lung function 

decrements but without accompanying respiratory symptoms (Ward et al., 2002; Gold et al., 

1999; Linn et al., 1996) (U.S. EPA, 2013a, section 6.2.1.2). 

Several epidemiologic panel studies43 reported statistically significant associations with 

lung function decrements at relatively low ambient O3 concentrations. For outdoor recreation or 

 

41 Unless otherwise specified, the epidemiologic studies discussed in the PA evaluate only adults.

42 Reversible loss of lung function in combination with the presence of symptoms meets the ATS 

definition of adversity (ATS, 2000).

43 Panel studies include repeated measurements of health outcomes, such as respiratory 

symptoms, at the individual level (U.S. EPA, 2013a, p. 1x).
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exercise, associations were reported in analyses restricted to 1-hour average O3 concentrations 

less than 80 ppb (Spektor et al., 1988a; Spektor et al., 1988b), 60 ppb (Brunekreef et al., 1994; 

Spektor et al., 1988a), and 50 ppb (Brunekreef et al., 1994). Among outdoor workers, Brauer et 

al. (1996) found a robust association with daily 1-hour max O3 concentrations less than 40 ppb. 

Ulmer et al. (1997) found a robust association in schoolchildren with 30-minute maximum O3

concentrations less than 60 ppb. For 8-hour average O3 concentrations, associations with lung 

function decrements in children with asthma were found to persist at concentrations less than 

80 ppb in a U.S. multicity study (Mortimer et al., 2002) and less than 51 ppb in a study 

conducted in the Netherlands (Gielen et al., 1997).

Epidemiologic panel studies investigating the effects of short-term exposure to O3

provided information on potential confounding by copollutants such as particulate matter with a 

median aerodynamic diameter less than or equal to 2.5 microns (PM2.5), particulate matter with a 

median aerodynamic diameter less than or equal to 10 microns (PM10), nitrogen dioxide (NO2), 

or sulfur dioxide (SO2). These studies varied in how they evaluated confounding. Some studies 

of subjects exercising outdoors indicated that ambient concentrations of copollutants such as 

NO2, SO2, or acid aerosol were low, and thus not likely to confound associations observed for O3

(Hoppe et al., 2003; Brunekreef et al., 1994; Hoek et al., 1993). In other studies of children with 

increased outdoor exposures, O3 was consistently associated with decreases in lung function, 

whereas other pollutants such as PM2.5, sulfate, and acid aerosol individually showed variable 

associations across studies (Thurston et al., 1997; Castillejos et al., 1995; Berry et al., 1991; Avol 

et al., 1990; Spektor et al., 1988a). Studies that conducted copollutant modeling generally found 

O3-associated lung function decrements to be robust (i.e., most copollutant-adjusted effect 

estimates fell within the 95% confidence interval (CI) of the single-pollutant effect estimates) 
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(U.S. EPA, 2013a, Figure 6-10 and Table 6-14). Most O3 effect estimates for lung function were 

robust to adjustment for temperature, humidity, and copollutants such as PM2.5, PM10, NO2, or 

SO2. Although examined in only a few epidemiologic studies, O3 also remained associated with 

decreases in lung function with adjustment for pollen or acid aerosols (U.S. EPA, 2013a, section 

6.2.1.2). 

Several epidemiologic studies demonstrated the protective effects of vitamin E and 

vitamin C supplementation, and increased dietary antioxidant intake, on O3-induced lung 

function decrements (Romieu et al., 2002) (U.S. EPA, 2013a, Figure 6-7 and Table 6-8).44 These 

results provide support for the new, quantitative models (McDonnell et al., 2012; Schelegle et 

al., 2012), discussed above, which make use of the concept of oxidant stress to estimate the 

occurrence of lung function decrements following exposures to relatively low O3 concentrations.

In conclusion, new information from controlled human exposure studies considerably 

strengthens the evidence and reduces the uncertainties, relative to the evidence that was available 

at the time of the 2008 review, regarding the presence and magnitude of lung function 

decrements in healthy adults following prolonged exposures to O3 concentrations below 80 ppb. 

As discussed in Section 6.2.1.1 in the ISA (U.S. EPA, 2013, p. 6-12), there is information 

available from four separate studies that evaluated exposures to 60 ppb O3 (Kim et al., 2011; 

Schelegle et al., 2009; Adams 2002; 2006). Although not consistently statistically significant, 

group mean FEV1 decrements following exposures to 60 ppb O3 are consistent among these 

studies. Moreover, as is illustrated in Figure 6-1 of the ISA (U.S. EPA, 2013a), the group mean 

FEV1 responses at 60 ppb fall on a smooth intake dose-response curve for exposures between 40 

 

44 Evidence from controlled human exposure studies is mixed, suggesting that supplementation 

may be ineffective in the absence of antioxidant deficiency (U.S. EPA, 2013a, p. 5-63).
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and 120 ppb O3. Based on the data in these studies, 10% of young, healthy adults experience 

clinically meaningful decrements in lung function when exposed for 6.6 hours to 60 ppb O3

during intermittent, moderate exertion. One recent study has also reported statistically significant 

decrements following exposures to 72 ppb O3 (Schelegle et al., 2009). Predictions from newly

developed quantitative models are consistent with these experimental results. Additionally, as 

discussed in more detail in section II.B.4 below, epidemiologic studies continue to provide 

evidence of lung function decrements in people who are active outdoors, including people 

engaged in outdoor recreation or exercise, children, and outdoor workers, at low ambient O3

concentrations. While few new epidemiologic studies of O3-associated lung function decrements 

are available in this review, previously available studies have reported associations with 

decrements, including at relatively low ambient O3 concentrations. 

ii. Pulmonary inflammation, injury, and oxidative stress

Ozone exposures result in increased respiratory tract inflammation and epithelial 

permeability. Inflammation is a host response to injury, and the induction of inflammation is 

evidence that injury has occurred. Oxidative stress has been shown to play a key role in initiating 

and sustaining O3-induced inflammation. Secondary oxidation products formed as a result of 

reactions between O3 and components of the ELF can increase the expression of molecules (i.e., 

cytokines, chemokines, and adhesion molecules) that can enhance airway epithelium 

permeability (U.S. EPA, 2013a, sections 5.3.3 and 5.3.4). As discussed in detail in the ISA (U.S. 

EPA, 2013a, section 6.2.3), O3 exposures can initiate an acute inflammatory response throughout 

the respiratory tract that has been reported to persist for at least 18-24 hours after exposure. 

Inflammation induced by exposure of humans to O3 can have several potential outcomes: 

(1) inflammation induced by a single exposure (or several exposures over the course of a 
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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 asthma; (4) inflammation can alter the body’s host 

defense response to inhaled microorganisms, particularly in potentially at-risk populations or 

lifestages such as the very young and old; and (5) inflammation can alter the lung’s response to 

other agents such as allergens or toxins (U.S. EPA, 2013a, section 6.2.3). Thus, lung injury and 

the resulting inflammation provide a mechanism by which O3 may cause other more serious 

morbidity effects (e.g., asthma exacerbations).45

In the last review, controlled human exposure studies reported O3-induced airway 

inflammation following exposures at or above 80 ppb and animal toxicological studies provided 

evidence for increases in inflammation and permeability in rabbits at levels as low as 100 ppb 

O3. In the current review, the link between O3 exposures and airway inflammation and injury has 

been evaluated in additional controlled human exposure studies, as well as in recent 

epidemiologic studies. Controlled human exposure studies have generally been conducted in 

young, healthy adults or in adults with asthma using lavage (proximal airway and 

bronchoalveolar), bronchial biopsy, and more recently, induced sputum. These studies have 

evaluated one or more indicators of inflammation, including neutrophil46 (PMN) influx, markers 

 

45 CASAC also addressed this issue: “The CASAC believes that these modest changes in FEV1

are usually associated with inflammatory changes, such as more neutrophils in the 

bronchoalveolar lavage fluid. Such changes may be linked to the pathogenesis of chronic lung 

disease” (Frey, 2014a p. 2).

46 Referred to as either neutrophils or polymorphonuclear neutrophils (or PMNs), these are the 

most abundant type of white blood cells in mammals. PMNs are recruited to the site of injury 

following trauma and are the hallmark of acute inflammation. The presence of PMNs in the lung 

has long been accepted as a hallmark of inflammation and is an important indicator that O3

causes inflammation in the lungs. Neutrophilic inflammation of tissues indicates activation of the 

innate immune system and requires a complex series of events, that then are normally followed 

by processes that clear the evidence of acute inflammation.
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of eosinophilic inflammation, increased permeability of the respiratory epithelium, and/or 

prevalence of proinflammatory molecules (U.S. EPA, 2013a, section 6.2.3.1). Epidemiologic 

studies have generally evaluated associations between ambient O3 and markers of inflammation 

and/or oxidative stress, which plays a key role in initiating and sustaining inflammation (U.S. 

EPA, 2013a, section 6.2.3.2). 

There is an extensive body of evidence from controlled human exposure studies 

indicating that short-term exposures to O3 can cause pulmonary inflammation. A single acute 

exposure (1-4 hours) of humans to moderate concentrations of O3 (200-600 ppb) while 

exercising at moderate to heavy intensities resulted in a number of cellular and biochemical 

changes in the lung, including inflammation characterized by increased numbers of PMNs, 

increased permeability of the epithelial lining of the respiratory tract, cell damage, and 

production of proinflammatory molecules (i.e., cytokines and prostaglandins, U.S. EPA, 2006a). 

A meta-analysis of 21 controlled human exposure studies (Mudway and Kelly, 2004) using 

varied experimental protocols (80-600 ppb O3 exposures; 1-6.6 hours exposure duration; light to 

heavy exercise; bronchoscopy at 0-24 hours post-O3 exposure) reported that PMN influx in 

healthy subjects is linearly associated with total O3 dose.

Several studies, including one published since the last review (Alexis et al., 2010), have 

reported O3-induced increases in PMN influx and permeability following exposures at or above 

80 ppb (Alexis et al., 2010; Peden et al., 1997; Devlin et al., 1991), and eosinophilic 

inflammation following exposures at or above 160 ppb (Scannell et al., 1996; Peden et al., 1997; 

Hiltermann et al., 1999; Vagaggini et al., 2002). In addition, one recent controlled human 

exposure study has reported O3-induced PMN influx following exposures of healthy adults to 60 
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ppb O3 (Kim et al., 2011), the lowest concentration at which inflammatory responses have been 

evaluated in human studies. 

As with FEV1 responses to O3, inflammatory responses to O3 are generally reproducible 

within individuals, with some individuals experiencing more severe O3-induced airway 

inflammation than indicated by group averages (Holz et al., 2005; Holz et al., 1999). Unlike O3-

induced decrements in lung function, which are attenuated following repeated exposures over 

several days (U.S. EPA, 2013a, section 6.2.1.1), some markers of O3-induced inflammation and 

tissue damage remain elevated during repeated exposures, indicating ongoing damage to the 

respiratory system (U.S. EPA, 2013a, section 6.2.3.1). 

Most controlled human exposure studies have reported that asthmatics experience larger 

O3-induced inflammatory responses than non-asthmatics.47 Specifically, asthmatics exposed to 

200 ppb O3 for 4-6 hours with exercise show significantly more neutrophils in bronchoalveolar 

lavage fluid (BALF) than similarly exposed healthy individuals (Scannell et al., 1996; Basha et 

al., 1994). Bosson et al. (2003) reported significantly greater expression of a variety of proinflammatory cytokines in asthmatics, compared to healthy subjects, following exposure to 

200 ppb O3 for 2 hours. In addition, research available in the last review, combined with a recent 

study newly available in this review, indicates that pretreatment of asthmatics with 

corticosteroids can prevent the O3-induced inflammatory response in induced sputum, though 

pretreatment did not prevent FEV1 decrements (Vagaggini et al., 2001; 2007). In contrast, 

Stenfors et al. (2002) did not detect a difference in the O3-induced increases in neutrophil 

numbers between 15 subjects with mild asthma and 15 healthy subjects by bronchial wash at the 

 

47 When evaluated, these studies have also reported O3-induced respiratory symptoms in 

asthmatics. Specifically, Scannell et al. (1996), Basha et al. (1994), and Vagaggini et al. (2001, 

2007) reported increased symptoms in addition to inflammation. 
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6 hours postexposure time point, although the neutrophil increase in the asthmatic group was on 

top of an elevated baseline. 

In people with allergic airway disease, including people with rhinitis and asthma, 

evidence available in the last review indicated that proinflammatory mediators also cause 

accumulation of eosinophils in the airways (Jorres et al., 1996; Peden et al., 1995 and 1997; 

Frampton et al., 1997; Hiltermann et al., 1999; Holz et al., 2002; Vagaggini et al., 2002). The 

eosinophil, which increases inflammation and allergic responses, is the cell most frequently 

associated with exacerbations of asthma (72 FR 37846, July 11, 2007).

Studies reporting inflammatory responses and markers of lung injury have clearly 

demonstrated that there is important variation in the responses of exposed subjects (72 FR 

37831, July 11, 2007). Some individuals also appear to be intrinsically more susceptible to 

increased inflammatory responses from O3 exposure (Holz et al., 2005). In healthy adults 

exposed to each 80 and 100 ppb O3, Devlin et al. (1991) observed group average increases in 

neutrophilic inflammation of 2.1- and 3.8-fold, respectively. However, there was a 20-fold range 

in inflammatory responses between individuals at both concentrations. Relative to an earlier, 

similar study conducted at 400 ppb (Koren et al., 1989), Devlin et al. (1991) noted that although 

some of the study population showed little or no increase in inflammatory and cellular injury 

indicators analyzed after exposures to lower levels of O3 (i.e., 80 and 100 ppb), others had 

changes that were as large as those seen when subjects were exposed to 400 ppb O3. The study 

authors concluded that, “while the population as a whole may have a small inflammatory 

response to near-ambient levels of ozone, there may be a significant subpopulation that is very 

sensitive to these low levels” (Devlin et al., 1991). 
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A number of studies report that O3 exposures increase epithelial permeability. Increased 

BALF protein, suggesting O3-induced changes in epithelial permeability, has been reported at 

1 hour and 18 hours postexposure (Devlin et al., 1997; Balmes et al., 1996). A meta-analysis of 

results from 21 publications (Mudway and Kelly, 2004) for varied experimental protocols (80-

600 ppb O3; 1-6.6 hours duration; light to heavy exercise; bronchoscopy at 0-24 hours post-O3

exposure; healthy subjects), showed that increased BALF protein is associated with total inhaled 

O3 dose. As noted in the 2009 PM ISA (U.S. EPA, 2009a), it has been postulated that changes in 

permeability associated with acute inflammation may provide increased access of inhaled 

antigens, particles, and other inhaled substances deposited on lung surfaces to the smooth 

muscle, interstitial cells, immune cells underlying the epithelium, and the blood (U.S. EPA, 

2013a, sections 5.3.4, 5.3.5). As has been observed with FEV1 responses, within individual 

changes in permeability are correlated with changes following sequential O3 exposures (Que et 

al., 2011). Changes in permeability and AHR apear to be mediated by different pathways. 

Animal toxicology studies have provided some support for this hypothesis (Adamson and 

Prieditis, 1995; Chen et al., 2006), though these studies did not specifically evaluate O3

exposures (U.S. EPA, 2009a). 

The limited epidemiologic evidence reviewed in the 2006 O3 AQCD (U.S. EPA, 2006a) 

reported associations between short-term increases in ambient O3 concentrations and airways 

inflammation in children (1-hour max O3 of approximately 100 ppb). In the 2006 O3 AQCD 

(U.S. EPA, 2006a), there was limited evidence for increases in nasal lavage levels of 

inflammatory cell counts and molecules released by inflammatory cells (i.e., eosinophilic 

cationic protein, and myeloperoxidases). Since 2006, as a result of the development of less 

invasive methods, there has been a large increase in the number of studies assessing ambient O3-
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associated changes in airway inflammation and oxidative stress, the types of biological samples 

collected, and the types of indicators. Most of these recent studies have evaluated biomarkers of 

inflammation or oxidative stress in exhaled breath, nasal lavage fluid, or induced sputum (U.S. 

EPA, 2013a, section 6.2.3.2). These recent studies form a larger database to establish coherence 

with findings from controlled human exposure and animal studies that have measured the same 

or related biological markers. Additionally, results from these studies provide further biological 

plausibility for the associations observed between ambient O3 concentrations and respiratory 

symptoms and asthma exacerbations. 

A number of epidemiologic studies provide evidence that short-term increases in ambient 

O3 exposure increase pulmonary inflammation and oxidative stress in children, including those 

with asthma (Sienra-Monge et al., 2004; Barraza-Villarreal et al., 2008; Romieu et al., 2008; 

Berhane et al., 2011). Multiple studies examined and found increases in exhaled nitric oxide

(eNO)48 (Berhane et al., 2011; Khatri et al., 2009; Barraza-Villarreal et al., 2008). In some 

studies of subjects with asthma, increases in ambient O3 concentration at the same lag were 

associated with both increases in pulmonary inflammation and respiratory symptoms (Khatri et 

al., 2009; Barraza-Villarreal et al., 2008). Although more limited in number, epidemiologic 

studies also found associations with cytokines such as IL-6 or IL-8 (Barraza-Villarreal et al., 

2008; Sienra-Monge et al., 2004), eosinophils (Khatri et al., 2009), antioxidants (Sienra-Monge 

et al., 2004), and indicators of oxidative stress (Romieu et al., 2008) (U.S. EPA, 2013a, section 

6.2.3.2). Because associations with inflammation were attenuated with higher antioxidant intake 

in the study by Sienra-Monge et al. (2004), this study provides additional evidence that inhaled 

 

48 Exhaled NO has been shown to be a useful biomarker for airway inflammation in large 

population-based studies (Linn et al., 2009) (U.S. EPA, 2013a, section 7.2.4). 
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O3 is likely to be an important source of reactive oxygen species in airways and/or may increase 

pulmonary inflammation via oxidative stress-mediated mechanisms among all age groups. 

Limitations in some recent studies have contributed to inconsistent results in adults (U.S. EPA, 

2013a, section 6.2.3.2).

Exposure to ambient O3 on multiple days can result in larger increases in pulmonary 

inflammation and oxidative stress, as discussed in section 6.2.3.2 of the ISA (U.S. EPA, 2013a). 

In studies that examined multiple O3 lags, multiday averages of 8-hour maximum or 

8-hour average concentrations were associated with larger increases in pulmonary inflammation 

and oxidative stress (Berhane et al., 2011; Delfino et al., 2010; Sienra-Monge et al., 2004), 

consistent with controlled human exposure (U.S. EPA, 2013a, section 6.2.3.1) and animal studies 

(U.S. EPA, 2013a, section 6.2.3.3) reporting that some markers of pulmonary inflammation 

remain elevated with O3 exposures repeated over multiple days. Evidence from animal 

toxicological studies also clearly indicates that O3 exposures result in damage and inflammation 

in the lung (U.S. EPA, 2013a, section 5.3). In the few studies that evaluated the potential for 

confounding, O3 effect estimates were not confounded by temperature or humidity, and were 

robust to adjustment for PM2.5 or PM10 (Barraza-Villarreal et al., 2008; Romieu et al., 2008; 

Sienra-Monge et al., 2004). 

In conclusion, a relatively small number of controlled human exposure studies evaluating 

O3-induced airway inflammation have become available since the last review. For purposes of 

reviewing the current O3 NAAQS, the most important of these recent studies reported a 

statistically significant increase in airway inflammation in healthy adults at moderate exertion 

following exposures to 60 ppb O3, the lowest concentration that has been evaluated for 

inflammation. In addition, a number of recent epidemiologic studies report O3-associated 
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increases in markers of pulmonary inflammation, particularly in children. Thus, recent studies 

continue to support the evidence for airway inflammation and injury that was available in 

previous reviews, with new evidence for such effects following exposures to lower 

concentrations than had been evaluated previously.

iii. Airway hyperresponsiveness

Airway hyperresponsiveness (AHR) refers to a condition in which the conducting 

airways undergo enhanced bronchoconstriction in response to a variety of stimuli. Airway 

hyperresponsiveness is an important consequence of exposure to ambient O3 because its presence 

reflects a change in airway smooth muscle reactivity, and indicates that the airways are 

predisposed to narrowing upon inhalation of a variety of ambient stimuli including specific 

triggers (i.e., allergens) and nonspecific triggers (e.g., SO2, and cold air). People with asthma are 

generally more sensitive to bronchoconstricting agents than those without asthma, and the use of 

an airway challenge to inhaled bronchoconstricting agents is a diagnostic test in asthma (U.S. 

EPA, 2013, section 6.2.2). Standards for airway responsiveness testing have been developed for 

the clinical laboratory (ATS, 2000), although variation in the methodology for administering the 

bronchoconstricting agent may affect the results (Cockcroft et al., 2005). There is a wide range 

of airway responsiveness in people without asthma, and responsiveness is influenced by a 

number of factors, including cigarette smoke, pollutant exposures, respiratory infections, 

occupational exposures, and respiratory irritants. Dietary antioxidants have been reported to 

attenuate O3-induced bronchial hyperresponsiveness in people with asthma (Trenga et al., 2001). 

Evidence for airway hyperresponsiveness (AHR) following O3 exposures is derived 

primarily from controlled human exposure and toxicological studies (U.S. EPA, 2013a, section 

6.2.2). Airway responsiveness is often quantified by measuring changes in pulmonary function 
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following the inhalation of an aerosolized allergen or a nonspecific bronchoconstricting agent 

(e.g., methacholine), or following exposure to a bronchoconstricting stimulus such as cold air. In 

the last review, controlled human exposure studies of mostly adults (≥ 18 years of age) had 

shown that exposures to O3 concentrations at or above 80 ppb increase airway responsiveness, as 

indicated by a reduction in the concentration of specific (e.g., ragweed) and non-specific (e.g., 

methacholine) agents required to produce a given reduction in lung function (e.g., as measured 

by FEV1 or specific airway resistance) (U.S. EPA, 2013a, section 6.2.2.1). This O3-induced AHR 

has been reported to be dose-dependent (Horstman et al., 1990). Animal toxicology studies have 

reported O3-induced AHR in a number of species, with some rat strains exhibiting 

hyperresponsiveness following 4-hour exposures to O3 concentrations as low as 50 ppb (Depuydt 

et al., 1999). Since the last review, there have been relatively few new controlled human 

exposure and animal toxicology studies of O3 and AHR, and no new studies have evaluated 

exposures to O3 concentrations at or below 80 ppb (U.S. EPA, 2013a, section 6.2.2.1). 

Airway hyperresponsiveness is linked with the accumulation and/or activation of 

eosinophils in the airways of asthmatics, which is followed by production of mucus and a latephase asthmatic response (section II.B.4.a.ii). In a study of 16 intermittent asthmatics, 

Hiltermann et al. (1999) found that there was a significant inverse correlation between the O3-

induced change in the percentage of eosinophils in induced sputum and the concentration of 

methacholine causing a 20% decrease in FEV1. Hiltermann et al. (1999) concluded that the 

results point to the role of eosinophils in O3-induced AHR. Increases in O3-induced nonspecific 

airway responsiveness incidence and duration could have important clinical implications for 

children and adults with asthma, such as exacerbations of their disease. 
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Airway hyperresponsiveness after O3 exposure appears to resolve more slowly than 

changes in FEV1 or respiratory symptoms (Folinsbee and Hazucha, 2000). Studies suggest that 

O3-induced AHR usually resolves 18 to 24 hours after exposure, but may persist in some 

individuals for longer periods (Folinsbee and Hazucha, 1989). Furthermore, in studies of 

repeated exposure to O3, changes in AHR tend to be somewhat less susceptible to attenuation 

with consecutive exposures than changes in FEV1 (Gong et al., 1997; Folinsbee et al., 1994; 

Kulle et al., 1982; Dimeo et al., 1981) (U.S. EPA, 2013a, section 6.2.2). In animal studies a 3-

day continuous exposure resulted in attenuation of O3-induced AHR (Johnston et al., 2005) while 

repeated exposures for 2 hours per day over 10 days did not (Chhabra et al., 2010), suggesting 

that attenuation could be lost when repeated exposures are interspersed with periods of rest (U.S. 

EPA, 2013a, section 6.2.2.2). 

As mentioned above, in addition to human subjects a number of species, including 

nonhuman primates, dogs, cats, rabbits, and rodents, have been used to examine the effect of O3

exposure on AHR , (U.S. EPA, 1996, Table 6-14; and U.S. EPA, 2006a, Annex Table AX5-12, 

p. AX5-36). A body of animal toxicology studies, including some recent studies conducted since 

the last review, provides support for the O3-induced AHR reported in humans (U.S. EPA, 2013a, 

section 6.2.2.2). Although most of these studies evaluated O3 concentrations above those 

typically found in ambient air in cities in the United States (i.e., most studies evaluated O3

concentrations of 100 ppb or greater), one study reported that a very low exposure concentration 

(50 ppb for 4 hours) induced AHR in some rat strains (Depuydt et al., 1999). Additional recent 

rodent studies reported O3-induced AHR following exposures to O3 concentrations from 100 to 

500 ppb (Johnston et al., 2005; Chhabra et al., 2010; Larsen et al., 2010). In characterizing the 

relevance of these exposure concentrations, the ISA noted that a study using radiolabeled O3
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suggests that even very high O3 exposure concentrations in rodents could be equivalent to much 

lower exposure concentrations in humans. Specifically, a 2000 ppb (2 ppm) O3 exposure 

concentration in resting rats was reported to be roughly equivalent to a 400 ppb exposure 

concentration in exercising humans (Hatch et al., 1994). Given this relationship, the ISA noted 

that animal data obtained in resting conditions could underestimate the risk of effects for humans 

(U.S. EPA, 2013a, section 2.4, p. 2-14). 

The 2006 AQCD (U.S. EPA, 2006a, p. 6-34) concluded that spirometric responses to O3

are independent of inflammatory responses and markers of epithelial injury (Balmes et al., 1996; 

Blomberg et al., 1999; Torres et al., 1997). Significant inflammatory responses to O3 exposures 

that did not elicit significant spirometric responses have been reported (Holz et al., 2005). A 

recent study (Que et al., 2011) indicates that AHR also appears to be mediated by a differing 

physiologic pathway. These results from controlled human exposure studies indicate that O3-

induced lung function decrements, inflammatory responses and pulmonary injury (leading to 

increased epithelial permeability), and AHR, are mediated by apparently different physiologic 

pathways. Except for lung function decrments, we do not have concentration or exposure 

response information about the other, potentially more sensitive,49 clinical endpoints (i.e., 

inflammation, increased epithelial permeability, AHR) that would allow us to quantitatively 

estimate the size of the population affetcted and the magnitude of their responses.

In summary, a strong body of controlled human exposure and animal toxicological 

studies, most of which were available in the last review of the O3 NAAQS, report O3-induced 

AHR after either acute or repeated exposures (U.S. EPA, 2013a, section 6.2.2.2). People with 

 

49 CASAC noted that “while measures of FEV1 are quantitative and readily obtainable in 

humans, they are not the only measures — and perhaps not the most sensitive measures — of the 

adverse health effects induced by ozone exposure.” (Henderson, 2006).
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asthma often exhibit increased airway responsiveness at baseline relative to healthy controls, and 

they can experience further increases in responsiveness following exposures to O3. Studies 

reporting increased airway responsiveness after O3 exposure contribute to a plausible link 

between ambient O3 exposures and increased respiratory symptoms in asthmatics, and increased 

hospital admissions and emergency department visits for asthma (U.S. EPA, 2013a, section 

6.2.2.2). 

iv. Respiratory symptoms and medication use

Respiratory symptoms are associated with adverse outcomes such as limitations in 

activity, and are the primary reason for people with asthma to use quick relief medication and 

seek medical care. Studies evaluating the link between O3 exposures and such symptoms allow a 

direct characterization of the clinical and public health significance of ambient O3 exposure. 

Controlled human exposure and toxicological studies have described modes of action through 

which short-term O3 exposures may increase respiratory symptoms by demonstrating O3-induced 

AHR (U.S. EPA, 2013a, section 6.2.2) and pulmonary inflammation (U.S. EPA, 2013a, section

6.2.3).

The link between subjective respiratory symptoms and O3 exposures has been evaluated 

in both controlled human exposure and epidemiologic studies, and the link with medication use 

has been evaluated in epidemiologic studies. In the last review, several controlled human 

exposure studies reported respiratory symptoms following exposures to O3 concentrations at or 

above 80 ppb. In addition, one study reported such symptoms following exposures to 60 ppb O3, 

though the increase was not statistically different from filtered air controls. Epidemiologic 

studies reported associations between ambient O3 and respiratory symptoms and medication use 

in a variety of locations and populations, including asthmatic children living in U.S. cities. In the 
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current review, additional controlled human exposure studies have evaluated respiratory 

symptoms following exposures to O3 concentrations below 80 ppb and recent epidemiologic 

studies have evaluated associations with respiratory symptoms and medication use (U.S. EPA, 

2013a, sections 6.2.1, 6.2.4). 

In controlled human exposure studies available in the last review as well as newly 

available studies, statistically significant increases in respiratory symptoms have been reported in 

healthy adult volunteers engaged in intermittent, moderate exertion following 6.6 hour exposures 

to average O3 concentrations of 80 ppb (Adams, 2003; Adams, 2006; Schelegle et al., 2009) and 

72 ppb (Schelegle et al., 2009). Such symptoms have been reported to increase with increasing 

O3 exposure concentrations, duration of exposure, and activity level (McDonnell et al., 1999). 

Results have been less consistent for lower exposure concentrations. A recent study by 

Schelegle et al. (2009) reported a statistically significant increase in respiratory symptoms in 

healthy adults following 6.6 hour exposures to an average O3 concentration of 72 ppb, but not 60 

ppb. Kim et al. (2011) also did not find statistically significant increases in respiratory symptoms 

following exposures of healthy adults to 60 ppb O3. Adams (2006) reported an increase in 

respiratory symptoms in healthy adults during a 6.6 hour exposure protocol with an average O3

exposure concentration of 60 ppb. This increase was significantly different from initial 

respiratory symptoms, but not from filtered air controls. The findings for O3-induced respiratory 

symptoms in controlled human exposure studies, and the evidence integrated across disciplines 

describing underlying modes of action, provide biological plausibility for epidemiologic 

associations observed between short-term increases in ambient O3 concentration and increases in 

respiratory symptoms (U.S. EPA, 2013a, section 6.2.4).
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In epidemiologic panel studies of respiratory symptoms, data typically are collected by 

having subjects (or their parents) record symptoms and medication use in a diary without direct 

supervision by study staff. Several limitations of symptom reports are well recognized, as 

described in the ISA (U.S. EPA, 2013a, section 6.2.4). Nonetheless, symptom diaries remain a 

convenient tool to collect individual-level data from a large number of subjects and allow 

modeling of associations between daily changes in O3 concentration and daily changes in 

respiratory morbidity over multiple weeks or months. Importantly, many of the limitations in 

these studies are sources of random measurement error that can bias effect estimates to the null 

or increase the uncertainty around effect estimates (U.S. EPA, 2013a, section 6.2.4). Because 

respiratory symptoms are associated with limitations in activity and daily function and are the 

primary reason for using medication and seeking medical care, the evidence is directly coherent 

with the associations consistently observed between increases in ambient O3 concentration and 

increases in asthma emergency department visits, discussed below (U.S. EPA, 2013a, section 

6.2.4).

Most epidemiologic studies of O3 and respiratory symptoms and medication use have 

been conducted in children and/or adults with asthma, with fewer studies, and less consistent 

results, in non-asthmatic populations (U.S. EPA, 2013a, section 6.2.4). The 2006 AQCD (U.S. 

EPA, 2006a, U.S. EPA, 2013a, section 6.2.4) concluded that the collective body of 

epidemiologic evidence indicated that short-term increases in ambient O3 concentrations are 

associated with increases in respiratory symptoms in children with asthma. A large body of 

single-city and single-region studies of asthmatic children provides consistent evidence for 

associations between short-term increases in ambient O3 concentrations and increased respiratory 
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symptoms and asthma medication use in children with asthma (U.S. EPA, 2013a, Figure 6-12, 

Table 6-20, p. 79). 

Methodological differences among studies make comparisons across recent multicity 

studies of respiratory symptoms difficult. Because of fewer person-days of data (Schildcrout et 

al., 2006) or examination of 19-day averages of ambient O3 concentrations (O'Connor et al., 

2008), the ISA did not give greater weight to results from recent multicity studies than results 

from single-city studies (U.S. EPA, 2013a, section 6.2.4.5).50 While evidence from the few 

available U.S. multicity studies is less consistent (O'Connor et al., 2008; Schildcrout et al., 2006; 

Mortimer et al., 2002), the overall body of epidemiologic evidence with respect to the 

association betweeen exposure to O3 and respiratory symptoms in asthmatic children remains 

compelling (U.S. EPA, 2013a, section 6.2.4.1). Findings from a small body of studies indicate 

that O3 is also associated with increased respiratory symptoms in adults with asthma (Khatri et 

al., 2009; Feo Brito et al., 2007; Ross et al., 2002) (U.S. EPA, 2013a, section 6.2.4.2). 

Available evidence indicates that O3-associated increases in respiratory symptoms are not 

confounded by temperature, pollen, or copollutants (primarily PM) (U.S. EPA, 2013a, section 

6.2.4.5; Table 6-25; Romieu et al., 1996; Romieu et al., 1997; Thurston et al., 1997; Gent et al., 

2003). However, identifying the independent effects of O3 in some studies was complicated due 

to the high correlations observed between O3 and PM or different lags and averaging times 

examined for copollutants. Nonetheless, the ISA noted that the robustness of associations in 

some studies of individuals with asthma, combined with findings from controlled human 

exposure studies for the direct effects of O3 exposure, provide substantial evidence supporting 

 

50 Though, as discussed below, for other endpoints (e.g., hospital admissions, emergency 

department visits) the ISA focused primarily on multicity studies. 
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the independent effects of short-term ambient O3 exposure on respiratory symptoms (U.S. EPA, 

2013a, section 6.2.4.5). 

Epidemiologic studies of medication use have reported associations with 

1-hour maximum O3 concentrations and with multiday average O3 concentrations (Romieu et al., 

2006; Just et al., 2002). Some studies reported O3 associations for both respiratory symptoms and 

asthma medication use (Escamilla-Nuñez et al., 2008; Romieu et al., 2006; Schildcrout et al., 

2006; Jalaludin et al., 2004; Romieu et al., 1997; Thurston et al., 1997) while others reported 

associations for either respiratory symptoms or medication use (Romieu et al., 1996; Rabinovitch 

et al., 2004; Just et al., 2002; Ostro et al., 2001). 

In summary, both controlled human exposure and epidemiologic studies have reported 

respiratory symptoms attributable to short-term O3 exposures. In the last review, the majority of 

the evidence from controlled human exposure studies in young, healthy adults was for symptoms 

following exposures to O3 concentrations at or above 80 ppb. Although studies that have become 

available since the last review have not reported increased respiratory symptoms in young, 

healthy adults following exposures with moderate exertion to 60 ppb, one recent study did report 

increased symptoms following exposure to 72 ppb O3. As was concluded in the 2006 O3 AQCD 

(U.S. EPA, 2006a; U.S. EPA, 1996), the collective body of epidemiologic evidence indicates that 

short-term increases in ambient O3 concentration are associated with increases in respiratory 

symptoms in children with asthma (U.S. EPA, 2013a, section 6.2.4). Recent studies of 

respiratory symptoms and medication use, primarily in asthmatic children, add to this evidence. 

In a smaller body of studies, increases in ambient O3 concentration were associated with 

increases in respiratory symptoms in adults with asthma.

v. Lung host defense
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The mammalian respiratory tract has a number of closely integrated defense mechanisms 

that, when functioning normally, provide protection from the potential health effects of 

exposures to a wide variety of inhaled particles and microbes. These defense mechanisms 

include mucociliary clearance, alveolobronchiolar transport mechanism, alveolar macrophages,

51

and adaptive immunity52 (U.S. EPA, 2013a, section 6.2.5). The previous O3 AQCD (U.S. EPA, 

2006a) concluded that animal toxicological studies provided evidence that acute exposure to O3

concentrations as low as 100 to 500 ppb can increase susceptibility to infectious diseases due to 

modulation of these lung host defenses. This conclusion was based, in large part, on animal 

studies of alveolar macrophage function and mucociliary clearance (U.S. EPA, 2013a, section 

6.2.5). 

Integrating animal study results with human exposure evidence, the 2006 Criteria 

Document concluded that available evidence indicates that short-term O3 exposures have the 

potential to impair host defenses in humans, primarily by interfering with alveolar macrophage 

function. Any impairment in alveolar macrophage function may lead to decreased clearance of 

microorganisms or nonviable particles. Compromised alveolar macrophage functions in 

asthmatics may increase their susceptibility to other O3 effects, the effects of particles, and 

respiratory infections (U.S. EPA, 2006a, p. 8–26). These conclusions were based largely on 

studies conducted in animals exposed for several hours up to several weeks to O3 concentrations 

from 100 to 250 ppb (Hurst et al., 1970; Driscoll et al., 1987; Cohen et al., 2002). Consistent 

with the animal evidence, a controlled human exposure study available in the last review had 

 

51 Phagocytic white blood cells within the alveoli of the lungs that ingest inhaled particles.

52 The adaptive immune system, is also known as the acquired immune system. Acquired 

immunity creates immunological memory after an initial response to a specific pathogen, leading 

to an enhanced response to subsequent encounters with that same pathogen. 
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reported decrements in the ability of alveolar macrophages to phagocytize yeast following 

exposures of healthy volunteers to O3 concentrations of 80 and 100 ppb for 6.6 hours during 

moderate exercise (Devlin et al., 1991). 

Alveolobronchiolar transport mechanisms refers to the transport of particles deposited in 

the deep lung (alveoli) which may be removed either up through the respiratory tract (bronchi) 

by alveolobronchiolar transport or through the lymphatic system. The pivotal mechanism of 

alveolobronchiolar transport involves the movement of alveolar macrophages with ingested 

particles to the bottom of the conducting airways. These airways are lined with ciliated epithelial 

cells and cells that produce mucous, which surrounds the macrophages. The ciliated epithelial 

cells move the mucous packets up the resiratory tract, hence the term “mucociliary escalator.” 

Although some studies show reduced tracheobronchial clearance after O3 exposure (U.S. EPA, 

2013a, section 6.2.5.1), alveolar clearance of deposited material is accelerated, presumably due 

to macrophage influx, which in itself can be damaging.

With regard to adaptive immunity, a limited number of epidemiologic studies have 

examined associations between O3 exposure and hospital admissions or emergency department

visits for respiratory infection, pneumonia, or influenza. Results have been mixed, and in some 

cases conflicting (U.S. EPA, 2013a, sections 6.2.7.2 and 6.2.7.3). With the exception of 

influenza, it is difficult to ascertain whether cases of respiratory infection or pneumonia are of 

viral or bacterial etiology. A recent study that examined the association between O3 exposure and 

respiratory hospital admissions in response to an increase in influenza intensity observed an 

increase in respiratory hospital admissions (Wong et al., 2009), but information from 

toxicological studies of O3 and viral infections is ambiguous.
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In summary, relatively few studies conducted since the last review have evaluated the 

effects of O3 exposures on lung host defense. When the available evidence is taken as a whole, 

the ISA concludes that acute O3 exposures impair the host defense capability of animals, 

primarily by depressing alveolar macrophage function and perhaps also by decreasing 

mucociliary clearance of inhaled particles and microorganisms. Coupled with limited evidence 

from controlled human exposure studies, this suggests that humans exposed to O3 could be 

predisposed to bacterial infections in the lower respiratory tract (U.S. EPA, 2013a, section 

6.2.5.5). 

vi. Allergic and asthma-related responses

Effects resulting from combined exposures to O3 and allergens have been studied in a 

variety of animal species, generally as models of experimental asthma. Pulmonary function and 

AHR in animal models of asthma are discussed in detail in Section 6.2.1.3 and Section 6.2.2.2, 

respectively, in the ISA (U.S. EPA, 2013a). Studies of allergic and asthma-related responses are 

discussed in detail in sections 5.3.6 and 6.2.6 of the ISA (U.S. EPA, 2013a). 

Evidence available in the last review indicates that O3 exposure skews immune responses 

toward an allergic phenotype and could also make airborne allergens more allergenic. In humans, 

allergic rhinoconjunctivitis symptoms are associated with increases in ambient O3 concentrations 

(Riediker et al., 2001). Controlled human exposure studies have observed O3-induced changes 

indicating allergic skewing. Airway eosinophils, which are white blood cells that participate in 

allergic disease and inflammation, were observed to increase in volunteers with atopy53 and mild 

asthma (Peden et al., 1997). In a more recent study, expression of IL-5, a cytokine involved in 

 

53 Atopy is a predisposition toward developing certain allergic hypersensitivity reactions. A 

person with atopy typically presents with one or more of the following: eczema (atopic 

dermatitis), allergic rhinitis (hay fever), allergic conjunctivitis, or allergic asthma.
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eosinophil recruitment and activation, was increased in subjects with atopy but not in healthy 

subjects (Hernandez et al., 2010). Epidemiologic studies describe associations between 

eosinophils in both short- (U.S. EPA, 2013a, section 6.2.3.2) and long-term (U.S. EPA, 2013a, 

section 7.2.5) O3 exposure, as do chronic exposure studies in non-human primates. Collectively, 

findings from these studies suggest that O3 can induce or enhance certain components of allergic 

inflammation in individuals with allergy or allergic asthma.

Evidence available in the last review indicates that O3 may also increase AHR to specific 

allergen triggers (75 FR 2970, January 19, 2010). Two studies (Jörres et al., 1996; Holz et al., 

2002) observed increased airway responsiveness to O3 exposure with bronchial allergen 

challenge in subjects with preexisting allergic airway disease. Ozone-induced exacerbation of 

airway responsiveness persists longer and attenuates more slowly than O3-induced lung function 

decrements and respiratory symptom responses and can have important clinical implications for 

asthmatics. Animal toxicology studies indicate that O3 enhances inflammatory and allergic 

responses to allergen challenge in sensitized animals. In addition to exacerbating existing allergic 

responses, toxicology studies indicate that O3 can also act as an adjuvant to produce sensitization 

in the respiratory tract. Along with its pro-allergic effects (inducing or enhancing certain 

components of allergic inflammation in individuals with allergy or allergic asthma), O3 could 

also make airborne allergens more allergenic. When combined with NO2, O3 has been shown to 

enhance nitration of common protein allergens, which may increase their allergenicity (Franze et 

al., 2005). 

vii. Hospital admissions and emergency department visits

The 2006 O3 AQCD evaluated numerous studies of respiratory-related emergency 

department visits and hospital admissions. These were primarily time-series studies conducted in 
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the U.S., Canada, Europe, South America, Australia, and Asia. Based on such studies, the 2006 

O3 AQCD concluded that “the overall evidence supports a causal relationship between acute 

ambient O3 exposures and increased respiratory morbidity resulting in increased emergency 

department visits and [hospital admissions] during the warm season”

54 (U.S. EPA, 2006a). This 

conclusion was “strongly supported by the human clinical, animal toxicologic[al], and 

epidemiologic evidence for [O3-induced] lung function decrements, increased respiratory 

symptoms, airway inflammation, and airway hyperreactivity” (U.S. EPA, 2006a). 

The results of recent studies largely support the conclusions of the 2006 O3 AQCD (U.S. 

EPA, 2013a, section 6.2.7). Since the completion of the 2006 O3 AQCD, relatively fewer studies 

conducted in the U.S., Canada, and Europe have evaluated associations between short-term O3

concentrations and respiratory hospital admissions and emergency department visits, with a 

growing number of studies conducted in Asia. This epidemiologic evidence is discussed in detail 

in the ISA (U.S. EPA, 2013a, section 6.2.7).55

In considering this body of evidence, the ISA focused primarily on multicity studies 

because they examine associations with respiratory-related hospital admissions and emergency 

department visits over large geographic areas using consistent statistical methodologies (U.S. 

EPA, 2013a, section 6.2.7.1). The ISA also focused on single-city studies that encompassed a 

large number of daily hospital admissions or emergency department visits, included long studydurations, were conducted in locations not represented by the larger studies, or examined 

 

54 Epidemiologic associations for O3 are more robust during the warm season than during cooler 

months (e.g., smaller measurement error, less potential confounding by copollutants). Rationale 

for focusing on warm season epidemiologic studies for O3 can be found at 72 FR 37838-37840.

55 The consideration of ambient O3 concentrations in the locations of these epidemiologic studies 

are discussed in sections II.D.1.b and II.E.4.a below, for the current standard and alternative 

standards, respectively. 
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population-specific characteristics that may impact the risk of O3-related health effects but were 

not evaluated in the larger studies (U.S. EPA, 2013a, section 6.2.7.1). When examining the 

association between short-term O3 exposure and respiratory health effects that require medical 

attention, the ISA distinguishes between hospital admissions and emergency department visits 

because it is likely that a small percentage of respiratory emergency department visits will be 

admitted to the hospital; therefore, respiratory emergency department visits may represent 

potentially less serious, but more common outcomes (U.S. EPA, 2013a, section 6.2.7.1). 

Several recent multicity studies (e.g., Cakmak et al., 2006; Dales et al., 2006) and a 

multi-continent study (Katsouyanni et al., 2009) report associations between short-term O3

concentrations and increased respiratory-related hospital admissions and emergency department 

visits. These multicity studies are supported by results from single-city studies also reporting 

consistent positive associations using different exposure assignment approaches (i.e., average of 

multiple monitors, single monitor, population-weighted average) and averaging times 

(i.e., 1-hour max and 8-hour max) (U.S. EPA, 2013a, sections 6.2.7.1 to 6.2.7.5). When 

examining cause-specific respiratory outcomes, recent studies report positive associations with 

hospital admissions and emergency department visits for asthma (Strickland et al., 2010; Stieb et 

al., 2009) and chronic obstructive pulmonary disease (COPD) (Stieb et al., 2009; Medina-Ramon 

et al., 2006), with more limited evidence for pneumonia (Medina-Ramon et al., 2006; Zanobetti 

and Schwartz, 2006). In seasonal analyses (U.S. EPA, 2013a, Figure 6-19, Table 6-28), stronger 

associations were reported in the warm season or summer months, when O3 concentrations are 

higher, compared to the cold season, particularly for asthma (Strickland et al., 2010; Ito et al., 

2007) and COPD (Medina-Ramon et al., 2006). The available evidence indicates that children 
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are at greatest risk for effects leading to O3-associated hospital admissions and emergency 

department visits (Silverman and Ito, 2010; Mar and Koenig, 2009; Villeneuve et al., 2007).

Although the collective evidence across studies indicates a mostly consistent positive 

association between O3 exposure and respiratory-related hospital admissions and emergency 

department visits, the magnitude of these associations may be underestimated to the extent 

members of study populations modify their behavior in response to air quality forecasts, and to 

the extent such behavior modification increases exposure misclassification (U.S. EPA, 2013, 

Section 4.6.6). Studies examining the potential confounding effects of copollutants have reported 

that O3 effect estimates remained relatively robust upon the inclusion of PM and gaseous 

pollutants in two-pollutant models (U.S. EPA, 2013a, Figure 6-20, Table 6-29). Additional 

studies that conducted copollutant analyses, but did not present quantitative results, also support 

these conclusions (Strickland et al., 2010; Tolbert et al., 2007; Medina-Ramon et al., 2006) (U.S. 

EPA, 2013a, section 6.2.7.5).

In the last review, studies had not evaluated the concentration-response relationship 

between short-term O3 exposure and respiratory-related hospital admissions and emergency 

department visits. A preliminary examination of this relationship in studies that have become 

available since the last review found no evidence of a deviation from linearity when examining 

the association between short-term O3 exposure and asthma hospital admissions (U.S. EPA, 

2013a, page 6-157; Silverman and Ito, 2010). In addition, an examination of the concentrationresponse relationship for O3 exposure and pediatric asthma emergency department visits found 

no evidence of a threshold at O3 concentrations as low as 30 ppb (for daily maximum 8-hour

concentrations) (Strickland et al., 2010). However, in both studies there is uncertainty in the 
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shape of the concentration-response curve at the lower end of the distribution of O3

concentrations due to the low density of data in this range (U.S. EPA, 2013a, page 6-157).

viii. Respiratory mortality

The controlled human exposure, epidemiologic, and toxicological studies discussed in 

section 6.2 of the ISA (U.S. EPA, 2013a) provide evidence for respiratory morbidity effects, 

including emergency department visits and hospital admissions, in response to short-term O3

exposures. Moreover, evidence from experimental studies indicates multiple potential pathways 

of respiratory effects from short-term O3 exposures, which support the continuum of respiratory 

effects that could potentially result in respiratory-related mortality in adults (U.S. EPA, 2013a, 

section 6.2.8). The 2006 O3 AQCD found inconsistent evidence for associations between shortterm O3 concentrations and respiratory mortality (U.S. EPA, 2006a). Although some studies 

reported a strong positive association between O3 and respiratory mortality, additional studies 

reported small associations or no associations. New epidemiologic evidence for respiratory 

mortality is discussed in detail in section 6.2.8 of the ISA (U.S. EPA, 2013a). The majority of 

recent multicity studies have reported positive associations between short-term O3 exposures and 

respiratory mortality, particularly during the summer months (U.S. EPA, 2013a, Figure 6-36).

Specifically, recent multicity studies from the U.S. (Zanobetti and Schwartz, 2008b), 

Europe (Samoli et al., 2009), Italy (Stafoggia et al., 2010), and Asia (Wong et al., 2010), as well 

as a multi-continent study (Katsouyanni et al., 2009), reported associations between short-term 

O3 concentrations and respiratory mortality (U.S. EPA, 2013a, Figure 6-37, page 6-259). With 

respect to respiratory mortality, summer-only analyses were consistently positive and most were 

statistically significant. All-year analyses had more mixed results, but most were positive. 
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Of the studies evaluated, only the studies by Katsouyanni et al. (2009) and by Stafoggia 

et al. (2010) analyzed the potential for copollutant confounding of the O3-respiratory mortality 

relationship. Based on the results of these analyses, the ISA concluded that O3 respiratory 

mortality risk estimates appear to be moderately to substantially sensitive (e.g., increased or 

attenuated) to inclusion of PM10. However, in the APHENA study (Katsouyanni et al., 2009), the 

mostly every-6th-day sampling schedule for PM10 in the Canadian and U.S. datasets greatly 

reduced their sample size and limits the interpretation of these results (U.S. EPA, 2013a, section 

6.2.8). 

In summary, recent epidemiologic studies support and reinforce the epidemiologic 

evidence for O3-associated respiratory hospital admissions and emergency department visits 

from the last review. In addition, the evidence for associations with respiratory mortality has 

been strengthened since the last review, with the addition of several large multicity studies. The 

biological plausibility of the associations reported in these studies is supported by the 

experimental evidence for respiratory effects.

b. Respiratory effects – long-term 

Since the last review, the body of evidence indicating the occurrence of respiratory 

effects due to long-term O3 exposure has been strengthened. This evidence is discussed in detail 

in the ISA (U.S. EPA, 2013a, Chapter 7) and summarized below for new-onset asthma and 

asthma prevalence, asthma hospital admissions, pulmonary structure and function, and 

respiratory mortality. 

i. New-onset asthma and asthma prevalence 

Asthma is a heterogeneous disease with a high degree of temporal variability. The on-set, 

progression, and symptoms can vary within an individual’s lifetime, and the course of asthma 
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may vary markedly in young children, older children, adolescents, and adults. In the previous 

review, longitudinal cohort studies that examined associations between long-term O3 exposures 

and the onset of asthma in adults and children indicated a direct effect of long-term O3 exposures 

on asthma risk in adults (McDonnell et al., 1999, 15-year follow-up; Greer et al., 1993, 10-year 

follow-up) and effect modification by O3 in children (McConnell et al., 2002). Since that review, 

additional studies have evaluated associations with new onset asthma, further informing our 

understanding of the potential gene-environment interactions, mechanisms, and biological 

pathways associated with incident asthma. 

In children, the relationship between long-term O3 exposure and new-onset asthma has 

been extensively studied in the Children’s Health Study (CHS), a long-term study that was 

initiated in the early 1990’s which has evaluated effects in several cohorts of children. The CHS 

was initially designed to examine whether long-term exposure to ambient pollution was related 

to chronic respiratory outcomes in children in 12 communities in southern California. In the 

CHS, new-onset asthma was classified as having no prior history of asthma at study entry with 

subsequent report of physician-diagnosed asthma at follow-up, with the date of onset assigned to 

be the midpoint of the interval between the interview date when asthma diagnosis was first 

reported and the previous interview date. The results of one study (McConnell et al., 2002) 

available in the previous review indicated that within high O3 communities, asthma risk was 3.3 

times greater for children who played three or more outdoor sports as compared with children 

who played no sports. 

For this review, as discussed in section 7.2.1.1 of the ISA (U.S. EPA, 2013a), recent 

studies from the CHS provide evidence for gene-environment interactions in effects on newonset asthma by indicating that the lower risks associated with specific genetic variants are found
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in children who live in lower O3 communities. These studies indicate that the risk for new-onset 

asthma is related in part to genetic susceptibility, as well as behavioral factors and environmental 

exposure. The onset of a chronic disease, such as asthma, is partially the result of a sequence of 

biochemical reactions involving exposures to various environmental agents metabolized by 

enzymes related to a number of different genes. Oxidative stress has been proposed to underlie 

the mechanistic hypotheses related to O3 exposure. Genetic variants may impact disease risk 

directly, or modify disease risk by affecting internal dose of pollutants and other environmental 

agents and/or their reaction products, or by altering cellular and molecular modes of action. 

Understanding the relation between genetic polymorphisms and environmental exposure can 

help identify high-risk subgroups in the population and provide better insight into pathway 

mechanisms for these complex diseases.

The CHS analyses (Islam et al., 2008; Islam et al., 2009; Salam et al., 2009) have found 

that asthma risk is related to interactions between O3 and variants in genes for enzymes such as 

heme-oxygenase (HO-1), arginases (ARG1 and 2), and glutathione S transferase P1 (GSTP1). 

Biological plausibility for these findings is provided by evidence that these enzymes have 

antioxidant and/or anti-inflammatory activity and participate in well-recognized modes of action

in asthma pathogenesis. As O3 is a source of oxidants in the airways, oxidative stress serves as 

the link among O3 exposure, enzyme activity, and asthma. Further, several lines of evidence 

demonstrate that secondary oxidation products of O3 initiate the key modes of action that 

mediate downstream health effects (U.S. EPA, 2013a, section 5.3). For example, HO-1 responds 

rapidly to oxidants, has anti-inflammatory and antioxidant effects, relaxes airway smooth 

muscle, and is induced in the airways during asthma. Cross-sectional studies by Akinbami et al. 

(2010) and Hwang et al. (2005) provide further evidence relating O3 exposures with asthma 
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prevalence. Gene-environment interactions are discussed in detail in Section 5.4.2.1 in the ISA 

(U.S. EPA, 2013a). 

ii. Asthma hospital admissions

In the 2006 AQCD, studies on O3-related hospital discharges and emergency department 

visits for asthma and respiratory disease mainly looked at short-term (daily) metrics. The shortterm O3 studies presented in section 6.2.7.5 of the ISA (U.S. EPA, 2013a) and discussed above in 

section 3.1.2.1 continue to indicate that there is evidence for increases in both hospital 

admissions and emergency department visits in children and adults related to all respiratory 

outcomes, including asthma, with stronger associations in the warm months. New studies, 

discussed in section 7.2.2 of the ISA (U.S. EPA, 2013a) also evaluated long-term O3 exposure 

metrics, providing a new line of evidence that suggests a positive exposure-response relationship 

between the first hospital admission for asthma and long-term O3 exposure, although the ISA 

cautions in attributing the associations in that study to long-term exposures since there is 

potential for short-term exposures to contribute to the observed associations.

Evidence associating long-term O3 exposure to first asthma hospital admission in a

positive concentration-response relationship is provided in a retrospective cohort study (Lin et 

al., 2008b). This study investigated the association between chronic exposure to O3 and 

childhood asthma admissions by following a birth cohort of more than 1.2 million babies born in 

New York State (1995-1999) to first asthma admission or until December 31, 2000. Three annual 

indicators (all 8-hour maximum from 10:00 a.m. to 6:00 p.m.) were used to define chronic O3

exposure: (1) mean concentration during the follow-up period (41.06 ppb); (2) mean 

concentration during the O3 season (50.62 ppb); and (3) proportion of follow-up days with O3

levels >70 ppb. The effects of copollutants were controlled, and interaction terms were used to 
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assess potential effect modifications. A positive association between chronic exposure to O3 and 

childhood asthma hospital admissions was observed, indicating that children exposed to high O3

levels over time are more likely to develop asthma severe enough to be admitted to the hospital. 

The various factors were examined and differences were found for younger children (1-2 years), 

poor neighborhoods, Medicaid/self-paid births, geographic region and others. As shown in the 

ISA, Figure 7-3 (U.S. EPA, 2013a, p. 7-16), positive concentration-response relationships were 

observed. Asthma admissions were significantly associated with increased O3 levels for all 

chronic exposure indicators. 

In considering the relationship between long-term pollutant exposures and chronic 

disease health endpoints, where chronic pathologies are found with acute expression of chronic 

disease, Künzli (2012) hypothesizes that if the associations of pollution with events are much 

larger in the long-term studies, it provides some indirect evidence that air pollution increases the 

pool of subjects with chronic disease, and that more acute events are to be expected to be seen 

for higher exposures. The results of Lin et al (2008a) for first asthma hospital admission, 

presented in Figure 7-3 (U.S. EPA, 2013a, p. 7-16), show effects estimates that are larger than 

those reported in a study of childhood asthma hospital admission in New York State (Silverman 

and Ito, 2010), discussed above. The ISA (U.S. EPA, 2013a, p. 7-16) notes that this provides 

some support for the hypothesis that O3 exposure may not only have triggered the events but also 

increased the pool of asthmatic children, but cautions in attributing the associations in the Lin et 

al. (2008) study to long-term exposures since there is potential for short-term exposures to 

contribute to the observed associations. 

iii. Pulmonary structure and function 
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In the 2006 O3 AQCD, few epidemiologic studies had investigated the effect of chronic 

O3 exposure on pulmonary function. The definitive 8-year follow-up analysis of the first cohort 

of the CHS (U.S. EPA, 2013a, section 7.2.3.1) provided little evidence that long-term exposure 

to ambient O3 was associated with significant deficits in the growth rate of lung function in 

children. The strongest evidence was for medium-term effects of extended O3 exposures over 

several summer months on lung function (FEV1) in children, i.e., reduced lung function growth 

being associated with higher ambient O3 levels. Short-term O3 exposure studies presented in the 

ISA (U.S. EPA, 2013a, section 6.2.1.2) provide a cumulative body of epidemiologic evidence 

that strongly supports associations between ambient O3 exposure and decrements in lung 

function among children. A later CHS study (Islam et al., 2007) included in this review (U.S. 

EPA, 2013a, section 7.2.3.1) also reported no substantial differences in the effect of O3 on lung 

function. However, in a more recent CHS study, Breton et al. (2011) hypothesized that genetic 

variation in genes on the glutathione metabolic pathway may influence the association between 

ambient air pollutant exposures and lung function growth in children, and found that variation in 

the GSS locus was associated with differences in risk of children for lung function growth 

deficits associated ambient air pollutants, including O3. A recent study (Rojas-Martinez et al., 

2007) of long-term exposure to O3, described in section 7.2.3.1 of the ISA (U.S. EPA, 2013a, p. 

7-19), observed a relationship with pulmonary function declines in school-aged children where 

O3 and other pollutant levels were higher (90 ppb at high end of the range) than those in the 

CHS. Two studies of adult cohorts provide mixed results where long-term exposures were at the 

high end of the range. 

Long-term studies in animals allow for greater insight into the potential effects of 

prolonged exposure to O3 that may not be easily measured in humans, such as structural changes 
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in the respiratory tract. Despite uncertainties, epidemiologic studies observing associations of O3

exposure with functional changes in humans can attain biological plausibility in conjunction with 

long-term toxicological studies, particularly O3-inhalation studies performed in non-human 

primates whose respiratory systems most closely resemble that of the human. An important 

series of studies, discussed in section 7.2.3.2 of the ISA (U.S. EPA, 2013a), have used nonhuman 

primates to examine the effect of O3 alone, or in combination with an inhaled allergen, house 

dust mite antigen (HDMA), on morphology and lung function. Animals exhibit the hallmarks of 

allergic asthma defined for humans (NHLBI, 2007). These studies and others have demonstrated 

changes in pulmonary function and airway morphology in adult and infant nonhuman primates 

repeatedly exposed to environmentally relevant concentrations of O3 (U.S. EPA, 2013a, section 

7.2.3.2). 

The initial observations in adult nonhuman primates have been expanded in a series of 

experiments using infant rhesus monkeys repeatedly exposed to 0.5 ppm O3 starting at 1 month 

of age (Plopper et al., 2007; Schelegle et al. 2003). The purpose of these studies was to 

determine if a cyclic regimen of O3 inhalation would amplify the allergic responses and 

structural remodeling associated with allergic sensitization and inhalation in the infant rhesus

monkey; they provide evidence of an O3-induced change in airway resistance and responsiveness 

provides biological plausibility of long-term exposure, or repeated short-term exposures, to O3

contributing to the effects of asthma in children.

In addition, significant structural changes in the respiratory tract development, during 

which conducting airways increase in diameter and length, have been observed in infant rhesus 

monkeys after cyclic exposure to O3 (Fanucchi et al., 2006). These effects are noteworthy 

because of their potential contribution to airway obstruction and AHR which are central features 
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of asthma. A number of studies in both non-human primates and rodents demonstrate that O3

exposure can increase collagen synthesis and deposition, including fibrotic-like changes in the 

lung (U.S. EPA, 2013a, section 7.2.3.2).

Collectively, evidence from animal studies strongly suggests that chronic O3 exposure is 

capable of damaging the distal airways and proximal alveoli, resulting in lung tissue remodeling 

and leading to apparent irreversible changes. Potentially, persistent inflammation and interstitial 

remodeling play an important role in the progression and development of chronic lung disease. 

Further discussion of the modes of action that lead to O3-induced morphological changes can be 

found in section 5.3.7 of the ISA (U.S. EPA, 2013a). Discussion of mechanisms involved in 

lifestage susceptibility and developmental effects can be found in section 5.4.2.4 of the ISA 

(U.S. EPA, 2013a). The findings reported in chronic animal studies offer insight into potential 

biological mechanisms for the suggested association between seasonal O3 exposure and reduced 

lung function development in children as observed in epidemiologic studies (U.S. EPA, 2013a, 

section 7.2.3.1).

iv. Respiratory mortality 

A limited number of epidemiologic studies have assessed the relationship between longterm exposure to O3 and mortality in adults. The 2006 O3 AQCD concluded that an insufficient 

amount of evidence existed “to suggest a causal relationship between chronic O3 exposure and 

increased risk for mortality in humans” (U.S. EPA, 2006a). Though total and cardio-pulmonary 

mortality were considered in these studies, respiratory mortality was not specifically considered. 

In the most recent follow-up analysis of the American Cancer Society (ACS) cohort 

(Jerrett et al., 2009), cardiopulmonary deaths were separately subdivided into respiratory and 

cardiovascular deaths, rather than combined as in the Pope et al. (2002) work. Increased O3
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exposure was associated with the risk of death from respiratory causes, and this effect was robust 

to the inclusion of PM2.5. The association between increased O3 concentrations and increased risk 

of death from respiratory causes was insensitive to the use of different models and to adjustment 

for several ecologic variables considered individually. The authors reported that when seasonal 

averages of 1-hour daily maximum O3 concentrations ranged from 33 to 104 ppb, there was no 

statistical deviation from a linear concentration-response relationship between O3 and respiratory 

mortality across 96 U.S. cities (U.S. EPA, 2013a, section 7.7). However, the authors also 

evaluated the degree to which models incorporating thresholds provided a better fit to the data. 

Based on these analyses, Jerrett et al. (2009) reported “limited evidence” for an effect threshold 

at an O3 concentration of 56 ppb (p=0.06). 

Additionally, a recent multicity time series study (Zanobetti and Schwartz, 2011), which 

followed (from 1985 to 2006) four cohorts of Medicare enrollees with chronic conditions that 

might predispose to O3-related effects, observed an association between long-term (warm 

season) exposure to O3 and elevated risk of mortality in the cohort that had previously 

experienced an emergency hospital admission due to COPD. A key limitation of this study is the 

inability to control for PM2.5, because data were not available in these cities until 1999.

c. Cardiovascular effects 

A relatively small number of studies have examined the potential effect of short-term O3

exposure on the cardiovascular system. The 2006 O3 AQCD (U.S. EPA, 2006a, p. 8-77) 

concluded that “O3 directly and/or indirectly contributes to cardiovascular-related morbidity,” 

but added that the body of evidence was limited. This conclusion was based on a controlled 

human exposure study that included hypertensive adult males; a few epidemiologic studies of 
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physiologic effects, heart rate variability, arrhythmias, myocardial infarctions, and hospital 

admissions; and toxicological studies of heart rate, heart rhythm, and blood pressure. 

More recently, the body of scientific evidence available that has examined the effect of 

O3 on the cardiovascular system has expanded. There is an emerging body of animal 

toxicological evidence demonstrating that short-term exposure to O3 can lead to autonomic 

nervous system alterations (in heart rate and/or heart rate variability) and suggesting that 

proinflammatory signals may mediate cardiovascular effects. Interactions of O3 with respiratory 

tract components result in secondary oxidation product formation and subsequent production of 

inflammatory mediators, which have the potential to penetrate the epithelial barrier and to initiate 

toxic effects systemically. In addition, animal toxicological studies of long-term exposure to O3

provide evidence of enhanced atherosclerosis and ischemia/reperfusion (I/R) injury, 

corresponding with development of a systemic oxidative, proinflammatory environment. Recent 

experimental and epidemiologic studies have investigated O3-related cardiovascular events and 

are summarized in section 6.3 of the ISA (U.S. EPA, 2013a). Overall, the ISA summarized the 

evidence in this review as follows (U.S. EPA, 2013a, p. 6-211). 

In conclusion, animal toxicological studies demonstrate O3-induced 

cardiovascular effects, and support the strong body of epidemiologic evidence 

indicating O3-induced cardiovascular mortality. Animal toxicological and 

controlled human exposure studies provide evidence for biologically plausible 

mechanisms underlying these O3-induced cardiovascular effects. However, a lack

of coherence with epidemiologic studies of cardiovascular morbidity remains an 

important uncertainty.

Controlled human exposure studies discussed in previous AQCDs have not demonstrated

any consistent extrapulmonary effects. In this review, evidence from controlled human exposure 

studies suggests cardiovascular effects in response to short-term O3 exposure (U.S. EPA, 2013a, 

section 6.3.1) and provides some coherence with evidence from animal toxicology studies. 

Controlled human exposure studies also support the animal toxicological studies by 
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demonstrating O3-induced effects on blood biomarkers of systemic inflammation and oxidative 

stress, as well as changes in biomarkers that can indicate the potential for increased clotting 

following O3 exposures. Increases and decreases in high frequency heart rate variability (HRV)

have been reported following relatively low (120 ppb during rest) and high (300 ppb with 

exercise) O3 exposures, respectively. These changes in cardiac function observed in animal and 

human studies provide preliminary evidence for O3-induced modulation of the autonomic 

nervous system through the activation of neural reflexes in the lung (U.S. EPA 2013a, 

section 5.3.2). 

Overall, the ISA concludes that the available body of epidemiologic evidence examining 

the relationship between short-term exposures to O3 concentrations and cardiovascular morbidity 

is inconsistent (U.S. EPA, 2013a, section 6.3.2.9). Across studies, different definitions (i.e., ICD9 diagnostic codes) were used for both all-cause and cause-specific cardiovascular morbidity 

(U.S. EPA, 2013a, Tables 6-35 to 6-39), which may contribute to inconsistency in results. 

However, within diagnostic categories, no consistent pattern of association was found with O3. 

Generally, the epidemiologic studies used nearest air monitors to assess O3 concentrations, with 

a few exceptions that used modeling or personal exposure monitors. The inconsistencies in the 

associations observed between short-term O3 and cardiovascular disease (CVD) morbidities are 

unlikely to be explained by the different exposure assignment methods used (U.S. EPA, 2013a, 

section 4.6). The wide variety of biomarkers considered and the lack of consistency among 

definitions used for specific cardiovascular disease endpoints (e.g., arrhythmias, HRV) make 

comparisons across studies difficult. 

Despite the inconsistent evidence for an association between O3 concentration and CVD 

morbidity, mortality studies indicate a consistent positive association between short-term O3
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exposure and cardiovascular mortality in multicity studies and in a multi-continent study. When 

examining mortality due to CVD, epidemiologic studies consistently observe positive 

associations with short-term exposure to O3. Additionally, there is some evidence for an 

association between long-term exposure to O3 and mortality, although the association between 

long-term ambient O3 concentrations and cardiovascular mortality can be confounded by other 

pollutants (U.S. EPA, 2013a). The ISA (U.S. EPA 2013a, section 6.3.4) states that taken 

together, the overall body of evidence across the animal and human studies is sufficient to 

conclude that there is likely to be a causal relationship between relevant short-term exposures to 

O3 and cardiovascular system effects. 

d. Total Mortality

The 2006 O3 AQCD concluded that the overall body of evidence was highly suggestive 

that short-term exposure to O3 directly or indirectly contributes to nonaccidental and 

cardiopulmonary-related mortality in adults, but additional research was needed to more fully 

establish underlying mechanisms by which such effects occur (U.S. EPA, 2013a, p. 2-18). In 

building on the 2006 evidence for mortality, the ISA states the following (U.S. EPA, 2013a, p. 6-

261).

The evaluation of new multicity studies that examined the association between 

short-term O3 exposures and mortality found evidence that supports the 

conclusions of the 2006 AQCD. These new studies reported consistent positive 

associations between short-term O3 exposure and all-cause (nonaccidental) 

mortality, with associations persisting or increasing in magnitude during the warm 

season, and provide additional support for associations between O3 exposure and 

cardiovascular and respiratory mortality. 

The 2006 O3 AQCD reviewed a large number of time-series studies of associations 

between short-term O3 exposures and total mortality including single- and multicity studies, and 

meta-analyses. In the large U.S. multicity studies that examined all-year data, effect estimates 
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corresponding to single-day lags ranged from a 0.5-1% increase in all-cause (nonaccidental) total 

mortality per a 20 ppb (24-hour), 30 ppb (8-hour maximum), or 40 ppb (1-hour maximum) 

increase in ambient O3 (U.S. EPA, 2013a, section 6.6.2). Available studies reported some 

evidence for heterogeneity in O3 mortality risk estimates across cities and across studies. Studies 

that conducted seasonal analyses reported larger O3 mortality risk estimates during the warm or 

summer season. Overall, the 2006 O3 AQCD identified robust associations between various 

measures of daily ambient O3 concentrations and all-cause mortality, which could not be readily 

explained by confounding due to time, weather, or copollutants. With regard to cause-specific 

mortality, consistent positive associations were reported between short-term O3 exposure and 

cardiovascular mortality, with less consistent evidence for associations with respiratory 

mortality. The majority of the evidence for associations between O3 and cause-specific mortality 

were from single-city studies, which had small daily mortality counts and subsequently limited 

statistical power to detect associations. The 2006 O3 AQCD concluded that “the overall body of 

evidence is highly suggestive that O3 directly or indirectly contributes to nonaccidental and 

cardiopulmonary-related mortality” (U.S. EPA, 2013a, section 6.6.1). 

Recent studies have strengthened the body of evidence that supports the association 

between short-term O3 concentrations and mortality in adults. This evidence includes a number 

of studies reporting associations with nonaccidental as well as cause-specific mortality. Multicontinent and multicity studies have consistently reported positive and statistically significant 

associations between short-term O3 concentrations and all-cause mortality, with evidence for 

larger mortality risk estimates during the warm or summer months (U.S. EPA, 2013a, Figure 6-

27; Table 6-42). Similarly, evaluations of cause-specific mortality have reported consistently 
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positive associations with O3, particularly in analyses restricted to the warm season (U.S. EPA, 

2013a, Figure 6-37; Table 6-53).56 

In assessing the evidence for O3-related mortality, the 2006 AQCD also noted that 

multiple uncertainties remained regarding the relationship between short-term O3 concentrations 

and mortality, including the extent of residual confounding by copollutants; characterization of 

the factors that modify the O3-mortality association; the appropriate lag structure for identifying 

O3-mortality effects; and the shape of the O3-mortality concentration-response function and 

whether a threshold exists. Many of the studies, published since the last review, have attempted 

to address one or more of these uncertainties. The ISA (U.S. EPA, 2013a, section 6.6.2) 

discusses the extent to which recent studies have evaluated these uncertainties in the relationship 

between O3 and mortality. 

In particular, recent studies have evaluated different statistical approaches to examine the 

shape of the O3-mortality concentration-response relationship and to evaluate whether a 

threshold exists for O3-related mortality. In an analysis of the National Morbidity and Mortality 

Air Pollution Study (NMMAPS) data, Bell et al. (2006) evaluated the potential for a threshold in 

the O3-mortality relationship. The authors reported positive and statistically significant 

associations with mortality in a variety of restricted analyses, including analyses restricted to 

days with 24-hour area-wide average O3 concentrations below 60, 55, 50, 45, 40, 35, and 30 ppb. 

In these restricted analyses O3 effect estimates were of similar magnitude, were statistically 

significant, and had similar statistical precision. In analyses restricted to days with 24-hour 

average O3 concentrations below 25 ppb, the O3 effect estimate was similar in magnitude to the 

effect estimates resulting from analyses with the higher cutoffs, but had somewhat lower 

 

56 Respiratory mortality is discussed in more detail above. 
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statistical precision, with the estimate approaching statistical significance (i.e., based on 

observation of Figure 2 in Bell et al., 2006). In analyses restricted to days with lower 24-hour 

average O3 concentrations (i.e., below 20 and 15 ppb), effect estimates were similar in magnitude 

to analyses with higher cutoffs, but with notably less statistical precision, and were not 

statistically significant (i.e., confidence intervals included the null, indicating no O3-associated 

mortality, based on observation of Figure 2 in Bell et al., 2006). Ozone was no longer positively 

associated with mortality when the analysis was restricted to days with 24-hour O3

concentrations below 10 ppb. Given the relatively small number of days included in these 

restricted analyses, especially for cut points of 20 ppb and below,57 statistical uncertainty is 

increased. 

Bell et al. (2006) also evaluated the shape of the concentration-response relationship 

between O3 and mortality. Although the results of this analysis suggested the lack of threshold in 

the O3-mortality relationship, the ISA noted that it is difficult to interpret such a curve because: 

(1) there is uncertainty around the shape of the concentration-response curve at 24-hour average 

O3 concentrations generally below 20 ppb; and (2) the concentration-response curve does not 

take into consideration the heterogeneity in O3-mortality risk estimates across cities (U.S. EPA, 

2013a, section 6.6.2.3). 

Several additional studies have used the NMMAPS dataset to evaluate the concentrationresponse relationship between short-term O3 concentrations and mortality. For example, using 

the same data as Bell et al. (2006), Smith et al. (2009) conducted a subset analysis, but instead of 

restricting the analysis to days with O3 concentrations below a cutoff, the authors only included 

 

57 For example, Bell et al. (2006) reported that for analyses restricted to 24-hour O3

concentrations at or below 20 ppb, 73% of days were excluded on average across the 98 

communities. 
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days above a defined cutoff (cutoffs from 15 and 60 ppb). The results of this analysis were 

consistent with those reported by Bell et al. (2006). Specifically, the authors reported consistent 

positive associations for all cutoff concentrations up to concentrations where the total number of 

days available were so limited that the variability around the central estimate was increased (i.e., 

cutoff values at or above about 50 ppb) (U.S. EPA, 2013a, section 6.6.2.3). In addition, using 

NMMAPS data for 1987-1994 for Chicago, Pittsburgh, and El Paso, Xia and Tong (2006)

reported evidence for a threshold around a 24-hour average O3 concentration of 25 ppb, though 

the threshold values estimated in the analysis were sometimes in the range of where data density 

was low (U.S. EPA, 2013a, section 6.6.2.3). Stylianou and Nicolich (2009) examined the 

existence of thresholds following an approach similar to Xia and Tong (2006) using data from 

NMMAPS for nine major U.S. cities (i.e., Baltimore, Chicago, Dallas/Fort Worth, Los Angeles, 

Miami, New York, Philadelphia, Pittsburgh, and Seattle) for the years 1987-2000. The authors 

reported that the estimated O3-mortality risks varied across the nine cities, with the models 

exhibiting apparent thresholds in the 10-45 ppb range for O3 (24-hour average). However, given 

the city-to-city variation in risk estimates, combining the city-specific estimates into an overall 

estimate complicates the interpretation of the results. Additional studies in Europe, Canada, and 

Asia did not report the existence of a threshold (Katsouyanni et al., 2009), with inconsistent 

and/or inconclusive results across cities, or a non-linear relationship in the O3-mortality 

concentration-response curve (Wong et al., 2010). 

3. Adversity of O3 Effects 

In making judgments as to when various O3-related effects become regarded as adverse 

to the health of individuals, in previous NAAQS reviews, the EPA has relied upon the guidelines 

published by the American Thoracic Society (ATS) and the advice of CASAC. In 2000, the ATS 
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published an official statement on “What Constitutes an Adverse Health Effect of Air 

Pollution?” (ATS, 2000), which updated and built upon its earlier guidance (ATS, 1985). The 

earlier guidance defined adverse respiratory health effects as “medically significant physiologic 

changes generally evidenced by one or more of the following: (1) interference with the normal 

activity of the affected person or persons, (2) episodic respiratory illness, (3) incapacitating 

illness, (4) permanent respiratory injury, and/or (5) progressive respiratory dysfunction,” while 

recognizing that perceptions of “medical significance” and “normal activity” may differ among 

physicians, lung physiologists and experimental subjects (ATS, 1985). The 2000 ATS guidance 

builds upon and expands the 1985 definition of adversity in several ways. The guidance 

concludes that transient, reversible loss of lung function in combination with respiratory 

symptoms should be considered adverse. There is also a more specific consideration of 

population risk (ATS, 2000). Exposure to air pollution that increases the risk of an adverse effect 

to the entire population is adverse, even though it may not increase the risk of any individual to 

an unacceptable level. For example, a population of asthmatics could have a distribution of lung 

function such that no individual has a level associated with clinically important impairment.

Exposure to air pollution could shift the distribution to lower levels that still do not bring any 

individual to a level that is associated with clinically relevant effects. However, this would be 

considered to be adverse because individuals within the population would have diminished 

reserve function, and therefore would be at increased risk to further environmental insult (U.S. 

EPA, 2013a, p. lxxi; and 75 FR at 35526/2, June 22, 2010).

The ATS also concluded that elevations of biomarkers such as cell types, cytokines and 

reactive oxygen species may signal risk for ongoing injury and more serious effects or may 

simply represent transient responses, illustrating the lack of clear boundaries that separate 
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adverse from nonadverse events. More subtle health outcomes also may be connected 

mechanistically to health effects that are clearly adverse, so that small changes in physiological 

measures may not appear clearly adverse when considered alone, but may be part of a coherent 

and biologically plausible chain of related health outcomes that include responses that are clearly 

adverse, such as mortality (U.S. EPA, 2014c, section 3.1.2.1). 

In this review, the new evidence provides further support for relationships between O3

exposures and a spectrum of health effects, including effects that meet the ATS criteria for being 

adverse (ATS, 1985 and 2000). The ISA determination that there is a causal relationship between 

short-term O3 exposure and a full range of respiratory effects, including respiratory morbidity 

(e.g., lung function decrements, respiratory symptoms, inflammation, hospital admissions, and 

emergency department visits) and mortality, provides support for concluding that short-term O3

exposure is associated with adverse effects (U.S. EPA, 2013a, section 2.5.2). Overall, including 

new evidence of cardiovascular system effects, the evidence supporting an association between 

short-term O3 exposures and total (nonaccidental, cardiopulmonary) respiratory mortality is 

stronger in this review (U.S. EPA, 2013a, section 2.5.2). And the judgment of likely causal 

associations between long-term measures of O3 exposure and respiratory effects such as newonset asthma, prevalence of asthma, asthma symptoms and control, and asthma hospital 

admissions provides support for concluding that long-term O3 exposure is associated with 

adverse effects ranging from episodic respiratory illness to permanent respiratory injury or 

progressive respiratory decline (U.S. EPA, 2013a, section 7.2.8).

Application of the ATS guidelines to the least serious category of effects related to 

ambient O3 exposures, which are also the most numerous and, therefore, are also potentially 

important from a public health perspective, involves judgments about which medical experts on 
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CASAC panels and public commenters have in the past expressed diverse views. To help frame 

such judgments, in past reviews, the EPA has defined gradations of individual functional 

responses (e.g., decrements in FEV1 and airway responsiveness) and symptomatic responses 

(e.g., cough, chest pain, wheeze), together with judgments as to the potential impact on 

individuals experiencing varying degrees of severity of these responses. These gradations were

used in the 1997 O3 NAAQS review and slightly revised in the 2008 review (U.S. EPA, 1996, p. 

59; 2007, p.3-72; 72 FR 37849, July 11, 2007). These gradations and impacts are summarized in 

Tables 3-2 and 3-3 in the 2007 O3 Staff Paper (U.S. EPA, 2007, pp.3-74 to 3-75).

For active healthy people, including children, moderate levels of functional responses 

(e.g., FEV1 decrements of ≥ 10% but < 20%, lasting 4 to 24 hours) and/or moderate symptomatic 

responses (e.g., frequent spontaneous cough, marked discomfort on exercise or deep breath, 

lasting 4 to 24 hours) would likely interfere with normal activity for relatively few sensitive 

individuals (U.S. EPA, 2007, p.3-72; 72 FR 37849, July 11, 2007); whereas large functional 

responses (e.g., FEV1 decrements ≥ 20%, lasting longer than 24 hours) and/or severe 

symptomatic responses (e.g., persistent uncontrollable cough, severe discomfort on exercise or 

deep breath, lasting longer than 24 hours) would likely interfere with normal activities for many 

sensitive individuals (U.S. EPA, 2007, p.3-72; 72 FR 37849, July 11, 2007) and, therefore,

would be considered adverse under ATS guidelines. For the purpose of estimating potentially 

adverse lung function decrements in active healthy people in the 2008 O3 NAAQS review, the 

CASAC panel for that review indicated that a focus on the mid to upper end of the range of 

moderate levels of functional responses is most appropriate (e.g., FEV1 decrements ≥ 15% but < 

20%) (Henderson, 2006; U.S. EPA, 2007, p. 3-76). In this review, CASAC concurred that the 

“[e]stimation of FEV1 decrements of ≥15% is appropriate as a scientifically relevant surrogate 
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for adverse health outcomes in active healthy adults” (Frey, 2014c, p. 3). However, for children 

and adults with lung disease, even moderate functional (e.g., FEV1 decrements ≥ 10% but < 

20%, lasting up to 24 hours) or symptomatic responses (e.g., frequent spontaneous cough, 

marked discomfort on exercise or with deep breath, wheeze accompanied by shortness of breath, 

lasting up to 24 hours) would likely interfere with normal activity for many individuals, and 

would likely result in additional and more frequent use of medication (U.S. EPA, 2007, p.3-72; 

72 FR 37849, July 11, 2007). For people with lung disease, large functional responses (e.g., 

FEV1 decrements ≥ 20%, lasting longer than 24 hours) and/or severe symptomatic responses 

(e.g., persistent uncontrollable cough, severe discomfort on exercise or deep breath, persistent 

wheeze accompanied by shortness of breath, lasting longer than 24 hours) would likely interfere 

with normal activity for most individuals and would increase the likelihood that these individuals 

would seek medical treatment (U.S. EPA, 2007, p.3-72; 72 FR 37849, July 11, 2007). In the last 

O3 NAAQS review, for the purpose of estimating potentially adverse lung function decrements 

in people with lung disease the CASAC panel indicated that a focus on the lower end of the 

range of moderate levels of functional responses is most appropriate (e.g., FEV1 decrements 

≥10%) (Henderson, 2006; U.S. EPA, 2007, p. 3-76). In addition, in their letter advising the 

Administrator on the reconsideration of the 2008 final decision, CASAC stated that “[a] 10% 

decrement in FEV1 can lead to respiratory symptoms, especially in individuals with pre-existing 

pulmonary or cardiac disease. For example, people with chronic obstructive pulmonary disease 

have decreased ventilatory reserve (i.e., decreased baseline FEV1) such that a ≥10% decrement 

could lead to moderate to severe respiratory symptoms” (Samet, 2011). In this review, CASAC 

concurred that “[a]n FEV1 decrement of ≥10% is a scientifically relevant surrogate for adverse 

health outcomes for people with asthma and lung disease” (Frey, 2014c, p. 3).
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In judging the extent to which these impacts represent effects that should be regarded as 

adverse to the health status of individuals, in previous NAAQS reviews, the EPA has also 

considered whether effects were experienced repeatedly during the course of a year or only on a 

single occasion (U.S. EPA, 2007). Although some experts would judge single occurrences of 

moderate responses to be a nuisance, especially for healthy individuals, a more general

consensus view of the adversity of such moderate responses emerges as the frequency of 

occurrence increases. Thus it has been judged that repeated occurrences of moderate responses,

even in otherwise healthy individuals, may be considered to be adverse since they could well set

the stage for more serious illness (61 FR 65723). The CASAC panel in the 1997 NAAQS review 

expressed a consensus view that these “criteria for the determination of an adverse physiological 

response were reasonable” (Wolff, 1995). In the review completed in 2008, estimates of repeated 

occurrences continued to be an important public health policy factor in judging the adversity of 

moderate lung function decrements in healthy and asthmatic people (72 FR 37850, July 11, 

2007). 

Evidence new to this review indicates that 6.6-hour exposures to 60 ppb O3 during 

moderate exertion can result in pulmonary inflammation in healthy adults (based on study mean). 

As discussed in the ISA, the initiation of inflammation can be considered as evidence that injury 

has occurred. Inflammation induced by a single O3 exposure can resolve entirely but, as noted in 

the ISA (U.S. EPA, 2013a, p. 6-76), “continued acute inflammation can evolve into a chronic 

inflammatory state,” which would be adverse. 

Responses measured in controlled human exposure studies indicate that the range of 

effects elicited in humans exposed to ambient O3 concentrations include: decreased inspiratory 

capacity; mild bronchoconstriction; rapid, shallow breathing pattern during exercise; and 
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symptoms of cough and pain on deep inspiration (U.S. EPA, 2013a, section 6.2.1.1). Young, 

healthy adults exposed for 6.6 hours to O3 concentrations ≥ 60 ppb, while engaged in intermittent 

moderate exertion, develop reversible, transient decrements in lung function. In addition, 

depending on the exposure concentration and the duration of exposure, young healthy adults 

have been shown to experience symptoms of breathing discomfort and inflammation if minute 

ventilation or duration of exposure is increased sufficiently (U.S. EPA, 2013a, section 6.2.1.1). 

Among healthy subjects there is considerable interindividual variability in the magnitude of the 

FEV1 responses, but when data were combined across studies at 60 ppb (U.S. EPA, 2013a, pp. 6-

17 to 6-18), 10% of healthy subjects had >10% FEV1 decrements. Moreover, consistent with the 

findings of the ISA (U.S. EPA, 2013a, section 6.2.1.1), CASAC concluded that “[a]sthmatic 

subjects appear to be at least as sensitive, if not more sensitive, than non-asthmatic subjects in 

manifesting ozone-induced pulmonary function decrements” (Frey, 2014c, p. 4). The combination of 

lung function decrements and respiratory symptoms, which has been considered adverse in 

previous reviews, has been demonstrated in healthy adults following prolonged (6.6 hour)

exposures, while at intermittent moderate exertion, to 72 ppb. For these types of effects, 

information from controlled human exposure studies, which provides an indication of the 

magnitude and thus adversity of effects at different O3 concentrations, combined with estimates 

of occurrences in the population from the HREA, provide information about their importance 

from a policy perspective.

4. Ozone-Related Impacts on Public Health

Setting standards to provide appropriate public health protection requires consideration of 

the factors that put populations at greater risk from O3 exposure. In order to estimate the 

potential for public health impacts, it is important to consider not only the adversity of the health 
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effects, but also the populations at greater risk and potential behaviors that may reduce 

exposures. 

a. Identification of at-risk populations and lifestages

The currently available evidence expands the understanding of populations that were 

identified to be at greater risk of O3-related health effects at the time of the last review (i.e., 

people who are active outdoors, people with lung disease, children and older adults and people 

with increased responsiveness to O3) and supports the identification of additional factors that 

may lead to increased risk (U.S. EPA, 2006, section 3.6.2; U.S. EPA, 2013a, Chapter 8).

Populations and lifestages may be at greater risk for O3-related health effects due to factors that

contribute to their susceptibility and/or vulnerability to O3. The definitions of susceptibility and 

vulnerability have been found to vary across studies, but in most instances “susceptibility” refers 

to biological or intrinsic factors (e.g., lifestage, sex, preexisting disease/conditions) while 

“vulnerability” refers to non-biological or extrinsic factors (e.g., socioeconomic status [SES]) 

(U.S. EPA, 2013a, p. 8-1; U.S. EPA, 2010c, 2009d). In some cases, the terms “at-risk” and 

“sensitive” have been used to encompass these concepts more generally. In the ISA and PA, 

“at-risk” is the all-encompassing term used to define groups with specific factors that increase 

their risk of O3-related health effects.

There are multiple avenues by which groups may experience increased risk for O3-

induced health effects. A population or lifestage58 may exhibit greater effects than other 

populations or lifestages exposed to the same concentration or dose, or they may be at greater 

risk due to increased exposure to an air pollutant (e.g., time spent outdoors). A group with 

 

58 Lifestages, which in this case includes childhood and older adulthood, are experienced by most 

people over the course of a lifetime, unlike other factors associated with at-risk populations. 
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intrinsically increased risk would have some factor(s) that increases risk through a biological 

mechanism and, in general, would have a steeper concentration-risk relationship, compared to 

those not in the group. Factors that are often considered intrinsic include pre-existing asthma, 

genetic background, and lifestage. A group of people could also have extrinsically increased risk, 

which would be through an external, non-biological factor, such as socioeconomic status (SES)

and diet. Some groups are at risk of increased internal dose at a given exposure concentration, for 

example, because of breathing patterns. This category would include people who work or 

exercise outdoors. Finally, there are those who might be placed at increased risk for experiencing 

greater exposures by being exposed to higher O3 concentrations. This would include, for

example, groups of people with greater exposure to ambient O3 due to less availability or use of 

home air conditioners such that they are more likely to be in locations with open windows on 

high O3 days. Some groups may be at increased risk of O3-related health effects through a 

combination of factors. For example, children tend to spend more time outdoors when O3 levels 

are high, and at higher levels of activity than adults, which leads to increased exposure and dose, 

and they also have biological, or intrinsic, risk factors (e.g., their lungs are still developing) (U.S. 

EPA, 2013a, Chapter 8). An at-risk population or lifestage is more likely to experience adverse 

health effects related to O3 exposures and/or, develop more severe effects from exposure than the 

general population.

i. People with Specific Genetic Variants

There is adequate evidence for populations with certain genotypes being more at-risk 

than others to the effects of O3 exposure on health (U.S. EPA, 2013a, section 8.1). Controlled 

human exposure and epidemiologic studies have reported evidence of O3-related increases in 

respiratory symptoms or decreases in lung function with variants including GSTM1, GSTP1, 
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HMOX1, and NQO1. NQO1 deficient mice were found to be resistant to O3-induced AHR and 

inflammation, providing biological plausibility for results of studies in humans. Additionally, 

studies of rodents have identified a number of other genes that may affect O3-related health 

outcomes, including genes related to innate immune signaling and pro- and anti-inflammatory

genes, which have not been investigated in human studies. 

ii. People with Asthma

Previous O3 AQCDs identified individuals with asthma as a population at increased risk 

of O3-related health effects. Multiple new epidemiologic studies included in the ISA have 

evaluated the potential for increased risk of O3-related health effects in people with asthma, 

including: lung function; symptoms; medication use; AHR; and airway inflammation (also 

measured as exhaled nitric oxide fraction, or FeNO). A study of lifeguards in Texas reported 

decreased lung function with short-term O3 exposure among both individuals with and without 

asthma; however, the decrease was greater among those with asthma (Thaller et al., 2008). A 

Mexican study of children ages 6-14 detected an association between short-term O3 exposure and 

wheeze, cough, and bronchodilator use among asthmatics but not non-asthmatics, although this 

may have been the result of a small non-asthmatic population (Escamilla-Nuñez et al., 2008). A 

study of modification by AHR (an obligate condition among asthmatics) reported greater shortterm O3-associated decreases in lung function in elderly individuals with AHR, especially among 

those who were obese (Alexeeff et al., 2007). With respect to airway inflammation, in one study, 

a positive association was reported for airway inflammation among asthmatic children following 

short-term O3 exposure, but the observed association was similar in magnitude to that of 

non-asthmatics (Barraza-Villarreal et al., 2008). Similarly, another study of children in 

California reported an association between O3 concentration and FeNO that persisted both among 
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children with and without asthma as well as those with and without respiratory allergy (Berhane 

et al., 2011). Finally, Khatri et al. (2009) found no association between short-term O3 exposure 

and altered lung function for either asthmatic or non-asthmatic adults, but did note a decrease in 

lung function among individuals with allergies. 

New evidence for difference in effects among asthmatics has been observed in studies 

that examined the association between O3 exposure and altered lung function by asthma 

medication use. A study of children with asthma living in Detroit reported a greater association 

between short-term O3 and lung function (i.e., FEV1) for corticosteroid users compared with 

noncorticosteroid users (Lewis et al., 2005). Conversely, another study found decreased lung 

function among noncorticosteroid users compared to users, although in this study, a large 

proportion of non-users were considered to be persistent asthmatics (Hernández-Cadena et al., 

2009). Lung function was not related to short-term O3 exposure among corticosteroid users and 

non-users in a study taking place during the winter months in Canada (Liu et al., 2009). 

Additionally, a study of airway inflammation reported a counterintuitive inverse association with 

O3 of similar magnitude for all groups of corticosteroid users and non-users (Qian et al., 2009).

Controlled human exposure studies that have examined the effects of O3 on adults with 

asthma and healthy controls are limited. Based on studies reviewed in the 1996 and 2006 O3

AQCDs, subjects with asthma appeared to be more sensitive to acute effects of O3 in terms of 

FEV1 and inflammatory responses than healthy non-asthmatic subjects. For instance, Horstman 

et al. (1995) observed that mild-to-moderate asthmatics, on average, experienced double the 

O3-induced FEV1 decrement of healthy subjects (19% versus 10%, respectively, p = 0.04). 

Moreover, a statistically significant positive correlation between FEV1 responses to O3 exposure 

and baseline lung function was observed in individuals with asthma, i.e., responses increased 
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with severity of disease. Minimal evidence exists suggesting that individuals with asthma have 

smaller O3-induced FEV1 decrements than healthy subjects (3% versus 8%, respectively) 

(Mudway et al., 2001). However, the asthmatics in that study also tended to be older than the 

healthy subjects, which could partially explain their lesser response since FEV1 responses to O3

exposure diminish with age. Individuals with asthma also had significantly more neutrophils in 

the BALF (18 hours postexposure) than similarly exposed healthy individuals (Peden et al., 

1997; Scannell et al., 1996; Basha et al., 1994). Furthermore, a study examining the effects of O3

on individuals with atopic asthma and healthy controls reported that greater numbers of 

neutrophils, higher levels of cytokines and hyaluronan, and greater expression of macrophage 

cell-surface markers were observed in induced sputum of atopic asthmatics compared with 

healthy controls (Hernandez et al., 2010). Differences in O3-induced epithelial cytokine 

expression were noted in bronchial biopsy samples from asthmatics and healthy controls (Bosson 

et al., 2003). Cell-surface marker and cytokine expression results, and the presence of 

hyaluronan, are consistent with O3 having greater effects on innate and adaptive immunity in 

these asthmatic individuals. In addition, studies have demonstrated that O3 exposure leads to 

increased bronchial reactivity to inhaled allergens in mild allergic asthmatics (Kehrl et al., 1999; 

Jorres et al., 1996) and to the influx of eosinophils in individuals with pre-existing allergic 

disease (Vagaggini et al., 2002; Peden et al., 1995). Taken together, these results point to several 

mechanistic pathways which could account for the enhanced sensitivity to O3 in subjects with 

asthma (U.S. EPA, 2013a, section 5.4.2.2).

As noted in the previous review (72 FR 37846, July 11, 2007) asthmatics present a 

differential response profile for cellular, molecular, and biochemical parameters (U.S. EPA, 

2006a, Figure 8–1) that are altered in response to acute O3 exposure. Ozone-induced increases in 
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neutrophils, IL–8 and protein were found to be significantly higher in the BAL fluid from 

asthmatics compared to healthy subjects, suggesting mechanisms for the increased sensitivity of 

asthmatics (Basha et al., 1994; McBride et al., 1994; Scannell et al., 1996; Hiltermann et al., 

1999; Holz et al., 1999; Bosson et al., 2003). Neutrophils, or PMNs, are the white blood cell 

most associated with inflammation. IL–8 is an inflammatory cytokine with a number of 

biological effects, primarily on neutrophils. The major role of this cytokine is to attract and 

activate neutrophils. Protein in the airways is leaked from the circulatory system, and is a marker 

for increased cellular permeability.

Bronchial constriction following provocation with O3 and/or allergens presents a twophase response. The early response is mediated by release of histamine and leukotrienes that 

leads to contraction of smooth muscle cells in the bronchi, narrowing the lumen and decreasing 

the airflow. In people with allergic airway disease, including people with rhinitis and asthma, 

these mediators also cause accumulation of eosinophils in the airways (Bascom et al., 1990; 

Jorres et al., 1996; Peden et al., 1995 and 1997; Frampton et al., 1997a; Michelson et al., 1999; 

Hiltermann et al., 1999; Holz et al., 2002; Vagaggini et al., 2002). In asthma, the eosinophil,

which increases inflammation and allergic responses, is the cell most frequently associated with 

exacerbations of the disease. A study by Bosson et al. (2003) evaluated the difference in O3-

induced bronchial epithelial cytokine expression between healthy and asthmatic subjects. After 

O3 exposure the epithelial expression of IL–5 and GM-CSF increased significantly in asthmatics, 

compared to healthy subjects. Asthma is associated with Th2-related airway response (allergic 

response), and IL–5 is an important Th2-related cytokine. The O3-induced increase in IL–5, and 

also in GM-CSF, which affects the growth, activation and survival of eosinophils, may indicate 

an effect on the Th2-related airway response and on airway eosinophils. The authors reported 
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that the O3-induced Th2-related cytokine responses that were found within the asthmatic group 

may indicate a worsening of their asthmatic airway inflammation and thus suggest a plausible 

link to epidemiological data indicating O3-associated increases in bronchial reactivity and 

hospital admissions.

The accumulation of eosinophils in the airways of asthmatics is followed by production 

of mucus and a late-phase bronchial constriction and reduced airflow. In a study of 16 

intermittent asthmatics, Hiltermann et al. (1999) found that there was a significant inverse 

correlation between the O3-induced change in the percentage of eosinophils in induced sputum 

and the change in PC20, the concentration of methacholine causing a 20% decrease in FEV1. 

Characteristic O3-induced inflammatory airway neutrophilia at one time was considered a 

leading mechanism of airway hyperresponsiveness. However, Hiltermann et al. (1999) 

determined that the O3-induced change in percentage neutrophils in sputum was not significantly 

related to the change in PC20. These results are consistent with the results of Zhang et al. (1995),

which found neutrophilia in a murine model to be only coincidentally associated with airway 

hyperresponsiveness, i.e., there was no cause and effect relationship (U.S. EPA, 2006a, AX 6–

26). Hiltermann et al. (1999) concluded that the results point to the role of eosinophils in O3-

induced airway hyperresponsiveness. Increases in O3-induced nonspecific airway responsiveness 

incidence and duration could have important clinical implications for asthmatics.

Toxicological studies provide additional evidence of the biological basis for the greater 

effects of O3 among those with asthma or AHR (U.S. EPA, 2013a, section 8.2.2). In animal 

toxicological studies, an asthmatic phenotype is modeled by allergic sensitization of the 

respiratory tract. Many of the studies that provide evidence that O3 exposure is an inducer of 

AHR and remodeling utilize these types of animal models. For example, a series of experiments 
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in infant rhesus monkeys have shown these effects, but only in monkeys sensitized to house dust 

mite allergen. Similarly, adverse changes in pulmonary function were demonstrated in mice 

exposed to O3; enhanced inflammatory responses were in rats exposed to O3, but only in animals 

sensitized to allergen. In general, it is the combined effects of O3 and allergic sensitization which 

result in measurable effects on pulmonary function. In a pulmonary fibrosis model, exposure to

O3 for 5 days increased pulmonary inflammation and fibrosis, along with the frequency of 

bronchopneumonia in rats. Thus, short-term exposure to O3 may enhance damage in a previously 

injured lung (U.S. EPA, 2013a, section 8.2.2). 

In the 2006 O3 AQCD, the potential for individuals with asthma to have greater risk of 

O3-related health effects was supported by a number of controlled human exposure studies, 

evidence from toxicological studies, and a limited number of epidemiologic studies. In section 

8.2.2, the ISA reports that in the recent epidemiologic literature some, but not all, studies report 

greater risk of health effects among individuals with asthma. Studies examining effect measure 

modification of the relationship between short-term O3 exposure and altered lung function by 

corticosteroid use provided limited evidence of O3-related health effects. However, recent studies 

of behavioral responses have found that studies do not take into account individual behavioral 

adaptations to forecasted air pollution levels (such as avoidance and reduced time outdoors), 

which may underestimate the observed associations in studies that examined the effect of O3

exposure on respiratory health (Neidell and Kinney, 2010). This could explain some 

inconsistency observed among recent epidemiologic studies. The evidence from controlled 

human exposure studies provides support for increased detriments in FEV1 and greater 

inflammatory responses to O3 in individuals with asthma than in healthy individuals without a 

history of asthma. The collective evidence for increased risk of O3-related health effects among 
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individuals with asthma from controlled human exposure studies is supported by recent 

toxicological studies which provide biological plausibility for heightened risk of asthmatics to 

respiratory effects due to O3 exposure. Overall, the ISA finds there is adequate evidence for 

asthmatics to be an at-risk population. 

iii. Children

Children are considered to be at greater risk from O3 exposure because their respiratory 

systems undergo lung growth until about 18-20 years of age and are therefore thought to be 

intrinsically more at risk for O3-induced damage (U.S. EPA, 2006a). It is generally recognized 

that children spend more time outdoors than adults, and, therefore, would be expected to have 

higher exposure to O3 than adults. Children aged 11 years and older and adults have higher 

absolute ventilation rates than younger children aged 1-11 years. However, younger children 

have higher ventilation rates relative to their lung volumes, which tends to increase dose 

normalized to lung surface area. In all ages, exercise intensity has a substantial effect on 

ventilation rate, high intensity activity results in nearly double the ventilation rate for moderate 

activity. For more information on time spent outdoors and ventilation rate differences by age 

group, see section 4.4.1 in the ISA (U.S. EPA, 2013a). 

The 1996 O3 AQCD reported clinical evidence that children, adolescents, and young 

adults (<18 years of age) appear, on average, to have nearly equivalent spirometric responses to 

O3 exposure, but have greater responses than middle-aged and older adults (U.S. EPA, 1996). 

Symptomatic responses (e.g., cough, shortness of breath, pain on deep inspiration) to O3

exposure, however, appear to increase with age until early adulthood and then gradually decrease 

with increasing age (U.S. EPA, 1996). Complete lung growth and development is not achieved 
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until 18-20 years of age in women and the early 20s for men; pulmonary function is at its 

maximum during this time as well. 

Recent epidemiologic studies have examined different age groups and their risk to 

O3-related respiratory hospital admissions and emergency department visits. Evidence for greater 

risk in children was reported in several studies. A study in Cyprus of short-term O3

concentrations and respiratory hospital admissions detected possible effect measure modification 

by age with a larger association among individuals < 15 years of age compared with those > 

15 years of age; the effect was apparent only with a 2-day lag (Middleton et al., 2008). Similarly, 

a Canadian study of asthma-related emergency department visits reported the strongest 

O3-related associations among 5- to 14-year olds compared to the other age groups (ages 

examined 0-75+) (Villeneuve et al., 2007). Greater O3-associated risk in asthma-related 

emergency department visits were also reported among children (<15 years) as compared to 

adults (15 to 64 years) in a study from Finland (Halonen et al., 2009). A study of New York City 

hospital admissions demonstrated an increase in the association between O3 exposure and 

asthma-related hospital admissions for 6- to 18-year olds compared to those < 6 years old and 

those > 18 years old (Silverman and Ito, 2010). When examining long-term O3 exposure and 

asthma-related hospital admissions among children, associations were determined to be larger 

among children 1 to 2 years old compared to children 2 to 6 years old (Lin et al., 2008). A few 

studies reported positive associations among both children and adults and no modification of the 

effect by age. 

The evidence reported in epidemiologic studies is supported by recent toxicological 

studies which observed O3-induced health effects in immature animals. Early life exposures of 

multiple species of laboratory animals, including infant monkeys, resulted in changes in 
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conducting airways at the cellular, functional, ultra-structural, and morphological levels. The 

studies conducted on infant monkeys are most relevant for assessing effects in children. Carey et 

al. (2007) conducted a study of O3 exposure in infant rhesus macaques, whose respiratory tract 

closely resemble that of humans. Monkeys were exposed either acutely or in episodes designed 

to mimic human exposure. All monkeys acutely exposed to O3 had moderate to marked 

necrotizing rhinitis, with focal regions of epithelial exfoliation, numerous infiltrating neutrophils, 

and some eosinophils. The distribution, character, and severity of lesions in episodically exposed 

infant monkeys were similar to that of acutely exposed animals. Neither exposure protocol for 

the infant monkeys produced mucous cell metaplasia proximal to the lesions, an adaptation 

observed in adult monkeys exposed in another study (Harkema et al., 1987). Functional and 

cellular changes in conducting airways were common manifestations of exposure to O3 among 

both the adult and infant monkeys (Plopper et al., 2007). In addition, the lung growth of the 

distal conducting airways in the infant monkeys was significantly stunted by O3 and this aberrant 

development was persistent 6 months postexposure (Fanucchi et al., 2006). 

Age may also affect the inflammatory response to O3 exposure. Toxicological studies 

reported that the difference in effects among younger lifestage test animals may be due to 

age-related changes in antioxidants levels and sensitivity to oxidative stress. Further discussion 

of these studies may be found in section 8.3.1.1 of the ISA (U.S. EPA, 2013a, p. 8-18). 

The previous and recent human clinical and toxicological studies reported evidence of 

increased risk from O3 exposure for younger ages, which provides coherence and biological 

plausibility for the findings from epidemiologic studies. Although there was some inconsistency, 

generally, the epidemiologic studies reported positive associations among both children and 

adults or just among children. The interpretation of these studies is limited by the lack of 
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consistency in comparison age groups and outcomes examined. However, overall, the 

epidemiologic, controlled human exposure, and toxicological studies provide adequate evidence 

that children are potentially at increased risk of O3-related health effects.

iv. Older adults 

The ISA notes that older adults are at greater risk of health effects associated with O3

exposure through a variety of intrinsic pathways (U.S. EPA, 2013a, section 8.3.1.2). In addition, 

older adults may differ in their exposure and internal dose. Older adults were outdoors for a 

slightly longer proportion of the day than adults aged 18-64 years. For more information on time 

spent outdoors by age group, see Section 4.4 in the ISA (U.S. EPA, 2013a). The gradual decline 

in physiological processes that occurs with aging may lead to increased risk of O3-related health 

effects (U.S. EPA, 2006a). Respiratory symptom responses to O3 exposure appears to increase 

with age until early adulthood and then gradually decrease with increasing age (U.S. EPA, 

1996); lung function responses to O3 exposure also decline from early adulthood (U.S. EPA, 

1996). The reductions of these responses with age may put older adults at increased risk for

continued O3 exposure. In addition, older adults, in general, have a higher prevalence of 

preexisting diseases compared to younger age groups and this may also lead to increased risk of 

O3-related health effects (U.S. EPA, 2013a, section 8.3.1.2). With the number of older 

Americans increasing in upcoming years (estimated to increase from 12.4% of the U.S. 

population to 19.7% between 2000 to 2030, which is approximately 35 million and 71.5 million 

individuals, respectively) this group represents a large population potentially at risk of O3-related 

health effects (SSDAN CensusScope, 2010a; U.S. Census Bureau, 2010). 

The majority of recent studies reported greater effects of short-term O3 exposure and 

mortality among older adults, which is consistent with the findings of the 2006 O3 AQCD. A 
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study (Medina-Ramón and Schwartz, 2008) conducted in 48 cities across the U.S. reported larger 

effects among adults ≥65 years old compared to those < 65 years. Further investigation of this

study population revealed a trend of O3-related mortality risk that gets larger with increasing age 

starting at age 51 (Zanobetti and Schwartz, 2008a). Another study conducted in 7 urban centers 

in Chile reported similar results, with greater effects in adults ≥65 years old (Cakmak et al., 

2007). More recently, a study conducted in the same area reported similar associations between 

O3 exposure and mortality in adults aged < 64 years old and 65 to 74 years old, but the risk was 

increased among the older age group (Cakmak et al., 2011). A study performed in China reported 

greater effects in populations ≥45 years old (compared to 5 to 44 year olds), with statistically 

significant effects present only among those ≥65 years old (Kan et al., 2008). An Italian study 

reported higher risk of all-cause mortality associated with increased O3 concentrations among 

individuals ≥85 year old as compared to those 35 to 84 years old (Stafoggia et al., 2010). The Air 

Pollution and Health: A European and North American Approach (APHENA) project examined 

the association between O3 exposure and mortality for those <75 and ≥ 75 years of age. In 

Canada, the associations for all-cause and cardiovascular mortality were greater among those 

≥75 years old. In the U.S., the association for all-cause mortality was slightly greater for those 

<75 years of age compared to those ≥75 years old in summer-only analyses. No consistent 

pattern was observed for CVD mortality. In Europe, slightly larger associations for all-cause 

mortality were observed in those <75 years old in all-year and summer-only analyses. Larger 

associations were reported among those <75years for CVD mortality in all-year analyses, but the 

reverse was true for summer-only analyses (Katsouyanni et al., 2009). 

With respect to epidemiologic studies of O3 exposure and hospital admissions, a positive 

association was reported between short-term O3 exposure and respiratory hospital admissions for 
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adults ≥65 years old but not for those adults aged 15 to 64 years (Halonen et al., 2009). In the 

same study, no association was observed between O3 concentration and respiratory mortality 

among those ≥65 years old or those 15 to 64 years old. No modification by age (40 to 64 year 

olds versus >64 year olds) was observed in a study from Brazil examining O3 levels and COPDrelated emergency department visits. 

Although some outcomes reported mixed findings regarding an increase in risk for older 

adults, recent epidemiologic studies report consistent positive associations between short-term 

O3 exposure and mortality in older adults. The evidence from mortality studies is consistent with 

the results reported in the 2006 O3 AQCD and is supported by toxicological studies providing 

biological plausibility for increased risk of effects in older adults. Also, older adults may be 

experiencing increased exposure compared to younger adults. Overall, the ISA (U.S. EPA,

2013a) concludes adequate evidence is available indicating that older adults are at increased risk 

of O3-related health effects. 

v. People with diets lower in vitamins C and E

Diet was not examined as a factor potentially affecting risk in previous O3 AQCDs, but 

recent studies have examined modification of the association between O3 and health effects by 

dietary factors. Because O3 mediates some of its toxic effects through oxidative stress, the 

antioxidant status of an individual is an important factor that may contribute to increased risk of 

O3-related health effects. Supplementation with vitamins C and E has been investigated in a 

number of studies as a means of inhibiting O3-mediated damage. 

Two epidemiologic studies have examined effect modification by diet and found 

evidence that certain dietary components are related to the effect O3 has on respiratory outcomes. 

In one recent study, the effects of fruit/vegetable intake and Mediterranean diet were examined. 
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Increases in these food patterns, which have been noted for their high vitamins C and E and 

omega-3 fatty acid content, were positively related to lung function in asthmatic children living 

in Mexico City, and modified by O3 exposure (Romieu et al., 2009). Another study examined 

supplementation of the diets of asthmatic children in Mexico with vitamins C and E (SienraMonge et al., 2004). Associations were detected between short-term O3 exposure and nasal 

airway inflammation among children in the placebo group but not in those receiving the 

supplementation. 

The epidemiologic evidence is supported by controlled human exposure studies, 

discussed in section 8.4.1 of the ISA (U.S. EPA, 2013a), that have shown that the first line of 

defense against oxidative stress is antioxidants-rich extracellular lining fluid (ELF) which 

scavenges free radicals and limit lipid peroxidation. Exposure to O3 depletes antioxidant levels in 

nasal ELF probably due to scrubbing of O3; however, the concentration and the activity of 

antioxidant enzymes either in ELF or plasma do not appear to be related to O3 responsiveness. 

Controlled studies of dietary antioxidant supplementation have demonstrated some protective 

effects of α-tocopherol (a form of vitamin E) and ascorbate (vitamin C) on spirometric measures 

of lung function after O3 exposure but not on the intensity of subjective symptoms and 

inflammatory responses. Dietary antioxidants have also afforded partial protection to asthmatics 

by attenuating postexposure bronchial hyperresponsiveness. Toxicological studies discussed in 

section 8.4.1 of the ISA (U.S. EPA, 2013a) provide evidence of biological plausibility to the 

epidemiologic and controlled human exposure studies. 

Overall, the ISA (U.S. EPA, 2013a) concludes adequate evidence is available indicating 

that individuals with diets lower in vitamins C and E are at risk for O3-related health effects. The 
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evidence from epidemiologic studies is supported by controlled human exposure and 

toxicological studies.

vi. Outdoor workers

Studies included in the 2006 O3 AQCD reported that individuals who participate in 

outdoor activities or work outside to be a population at increased risk based on consistently

reported associations between O3 exposure and respiratory health outcomes in these groups (U.S. 

EPA, 2006a). Outdoor workers are exposed to ambient O3 concentrations for a greater period of 

time than individuals who spend their days indoors. As discussed in section 4.7 of the ISA (U.S.

EPA, 2013a) outdoor workers sampled during the work shift had a higher ratio of personal 

exposure to fixed-site monitor concentrations than health clinic workers who spent most of their 

time indoors. Additionally, an increase in dose to the lower airways is possible during outdoor 

exercise due to both increases in the amount of air breathed (i.e., minute ventilation) and a shift 

from nasal to oronasal breathing. The association between FEV1 responses to O3 exposure and 

minute ventilation is discussed more fully in section 6.2.3.1 of the 2006 O3 AQCD (U.S. EPA, 

2006a). 

Previous studies have shown that increased exposure to O3 due to outdoor work leads to 

increased risk of O3-related health effects, specifically decrements in lung function (U.S. EPA, 

2006a). The strong evidence from the 2006 O3 AQCD, which demonstrated increased exposure, 

dose, and ultimately risk of O3-related health effects in this population, supports the conclusion 

that there is adequate evidence to indicate that increased exposure to O3 through outdoor work 

increases the risk of O3-related health effects.

In some cases, it is difficult to determine a factor that results in increased risk of effects. 

For example, previous assessments have included controlled human exposure studies in which 
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some healthy individuals demonstrate greater O3-related health effects compared to other healthy 

individuals. Interindividual variability has been observed for lung function decrements, 

symptomatic responses, pulmonary inflammation, AHR, and altered epithelial permeability in 

healthy adults exposed to O3, and these results tend to be reproducible within a given individual 

over a period of several months indicating differences in the intrinsic responsiveness. In many 

cases the reasons for the variability is not clear. This may be because one or some of the factors 

described above have not been evaluated in studies, or it may be that additional, unidentified 

factors influence individual responses to O3 (U.S. EPA, 2013a, section 8.5). 

As discussed in chapter 8 of the ISA (U.S. EPA, 2013a), there is a lack of information 

regarding the extent to which some factors may increase risk from O3 exposures. Due to this lack 

of information, the ISA concluded that for some factors, such as sex, SES, and obesity, there is 

only “suggestive” evidence of increased risk, or that for a number of factors the evidence is 

inadequate to draw conclusions about potential increase in risk of effects. Overall, the factors for 

which the ISA concludes there is adequate evidence of increased risk for experiencing O3-related 

effects were related to asthma, lifestage (children and older adults), genetic variability, dietary 

factors, and working outdoors.

b. Size of at-risk populations

One consideration in the assessment of potential public health impacts is the size of 

various population groups for which there is adequate evidence of increased risk for health 

effects associated with O3-related air pollution exposure (U.S. EPA, 2014c, section 3.1.5.2). The 

factors for which the ISA judged the evidence to be “adequate” with respect to contributing to 

increased risk of O3-related effects among various populations and lifestages included: asthma;

childhood and older adulthood; diets lower in vitamins C and E; certain genetic variants; and 
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working outdoors (U.S. EPA, 2013a, section 8.5). No statistics are available to estimate the size 

of an at-risk population based on nutritional status or genetic variability. 

With regard to asthma, Table 3-7 in the PA (U.S. EPA, 2014c, section 3.1.5.2) 

summarizes information on the prevalence of current asthma by age in the U.S. adult population 

in 2010 (Schiller et al. 2012; children - Bloom et al., 2011). Individuals with current asthma 

constitute a fairly large proportion of the population, including more than 25 million people. 

Asthma prevalence tends to be higher in children than adults. Within the U.S., approximately 

8.2% of adults have reported currently having asthma (Schiller et al., 2012) and 9.5% of children 

have reported currently having asthma (Bloom et al., 2011).

59

With regard to lifestages, based on U.S. census data from 2010 (Howden and Meyer, 

2011), about 74 million people, or 24% of the U.S. population, are under 18 years of age and 

more than 40 million people, or about 13% of the U.S. population, are 65 years of age or older. 

Hence, a large proportion of the U.S. population (i.e., more than a third) is included in age 

groups that are considered likely to be at increased risk for health effects from ambient O3

exposure.

With regard to outdoor workers, in 2010, approximately 11.7% of the total number of 

people (143 million people) employed, or about 16.8 million people, worked outdoors one or 

 

59 As noted below (II.C.3.a.ii), asthmatics can experience larger O3-induced respiratory effects 

than non-asthmatic, healthy adults. The responsiveness of asthmatics to O3 exposures could 

depend on factors that have not been well-evaluated such as asthma severity, the effectiveness of 

asthma control, or the prevalence of medication use.
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more days per week (based on worker surveys).60 Of these, approximately 7.4% of the 

workforce, or about 7.8 million people, worked outdoors three or more days per week. 

The health statistics data illustrate what is known as the “pyramid” of effects. At the top 

of the pyramid, there are approximately 2.5 million deaths from all causes per year in the U.S. 

population, with about 250 thousand respiratory-related deaths (CDC-WONDER, 2008). For 

respiratory health diseases, there are nearly 3.3 million hospital discharges per year (HCUP, 

2007), 8.7 million respiratory emergency department visits (HCUP, 2007), 112 million 

ambulatory care visits (Woodwell and Cherry, 2004), and an estimated 700 million restricted 

activity days per year due to respiratory conditions (Adams et al., 1999). Combining small risk 

estimates with relatively large baseline levels of health outcomes can result in quite large public 

health impacts. Thus, even a small percentage reduction in O3 health impacts on 

cardiopulmonary diseases would reflect a large number of avoided cases.

c. Impacts of averting behavior 

The activity pattern of individuals is an important determinant of their exposure (U.S. 

EPA, 2013a, section 4.4.1). Variation in O3 concentrations among various microenvironments 

means that the amount of time spent in each location, as well as the level of activity, will 

influence an individual’s exposure to ambient O3. Activity patterns vary both among and within 

individuals, resulting in corresponding variations in exposure across a population and over time. 

Individuals can reduce their exposure to O3 by altering their behaviors, such as by staying 

 

60 The O*NET program is the nation's primary source of occupational information. Central to the 

project is the O*NET database, containing information on hundreds of standardized and 

occupation-specific descriptors. The database, which is available to the public at no cost, is 

continually updated by surveying a broad range of workers from each occupation. 

http://www.onetcenter.org/overview.html.

http://www.onetonline.org/find/descriptor/browse/Work_Context/4.C.2/.
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indoors, being active outdoors when air quality is better, and by reducing their activity levels or 

reducing the time being active outdoors on high-O3 days (U.S. EPA, 2013a, section 4.4.2). 

The widely reported Air Quality Index (AQI) conveys advice to the public, and 

particularly at-risk populations, on reducing short- or prolonged-exposures on days when 

ambient levels of common, criteria air pollutants (except lead), are elevated (www.airnow.gov). 

Information communicated by the AQI is based on the evidence and exposure/risk information 

assessed in the review of the NAAQS; it is updated and revised as necessary during the review of 

each standard. Proposed changes to the AQI sub-index for O3, based on evidence and 

exposure/risk information assessed in this review, are discussed in section III below. 

The AQI describes the potential for health effects from O3 (and other individual 

pollutants) in six color-coded categories of air-quality, ranging from Good (green), Moderate 

(yellow), Unhealthy for Sensitive Groups (orange), Unhealthy (red), and Very Unhealthy 

(purple), and Hazardous (maroon). Levels in the unhealthy ranges (i.e., Unhealthy for Sensitive 

Groups and above) come with recommendations about reducing exposure. Forecasted and actual 

AQI values for O3 are reported to the public during the O3 season. The AQI advisories explicitly 

state that children, older adults, people with lung disease, and people who are active outdoors, 

may be at greater risk from exposure to O3. People are advised to reduce exposure depending on 

the predicted O3 levels and the likelihood of risk. This advice includes being active outdoors 

when air quality is better, and reducing activity levels or reducing the time being active outdoors 

on high-O3 days. Staying indoors to reduce exposure is not recommended until air quality 

reaches the Very Unhealthy or Hazardous categories. 

Evidence of individual averting behaviors in response to AQI advisories has been found 

in several studies, including activity pattern and epidemiologic studies, especially for the at-risk 
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populations, such as children, older adults, and people with asthma, who are targeted by the 

advisories. Such effects are less pronounced in the general population, possibly due to the 

opportunity cost of behavior modification. Epidemiologic evidence from a study (Neidell and 

Kinney, 2010) conducted in the 1990’s in Los Angeles, CA reports increased asthma hospital 

admissions among children and older adults when O3 alert days (1-hour max O3 concentration

>200 ppb) were excluded from the analysis of daily hospital admissions and O3 concentrations 

(presumably thereby eliminating averting behavior based on high O3 forecasts). If averting 

behavior reduces exposure to ambient O3, then epidemiologic studies that do not account for 

averting behavior may produce effect estimates that are biased toward the null due to exposure 

misclassification (U.S. EPA, 2013, section 4.6.6). 

C. Human Exposure and Health Risk Assessments

To put judgments about health effects that are adverse for individuals into a broader 

public health context, the EPA has developed and applied models to estimate human exposures 

to O3 and O3-associated health risks. Exposure and risk estimates based on such models are 

presented and assessed in the HREA (U.S. EPA, 2014a). In reviewing the draft HREA, CASAC 

expressed the view that the document is “well-written, founded based upon comprehensive 

analyses and adequate for its intended purpose” (Frey, 2014a, p. 1). Analyses in the HREA 

inform consideration of the O3 exposures and health risks that could be allowed by the current 

standard and alternative standards, and consideration of the kind and degree of uncertainties 

inherent in estimates of O3 exposures and health risks. 

The following sections discuss the air quality adjustment approach used in the HREA for

exposure and health risk estimates (II.C.1); the approach taken to estimate exposures, key 
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exposure results, and important uncertainties (II.C.2); and the approaches taken to estimate O3

health risks, key risk results, and important uncertainties (II.C.3). 

1. Air Quality Adjustment

As discussed above (section I.E), O3 is formed near the Earth’s surface due to chemical 

interactions involving solar radiation and precursor pollutants including VOCs, NOX, CH4 and 

CO. The response of O3 to changes in precursor concentrations is nonlinear. In particular, NOX

causes both the formation and destruction of O3. The net impact of NOX emissions on O3

concentrations depends on the local quantities of NOX, VOC, and sunlight, which interact in a set 

of complex chemical reactions. In some areas, such as urban centers where NOX emissions 

typically are high, NOX leads to the net destruction of O3, decreasing O3 concentrations in the 

immediate vicinity. This phenomenon is particularly pronounced under conditions that lead to 

low ambient O3 concentrations (i.e. during cool, cloudy weather and at night when 

photochemical activity is limited or nonexistent). However, while NOX can initially destroy O3

near emission sources, these same NOX emissions eventually react to form O3 downwind of 

those sources. Photochemical model simulations suggest that reductions in NOX emissions will 

slightly increase O3 concentrations near NOX sources on days with lower O3 concentrations, 

while at the same time decreasing the highest O3 concentrations in outlying areas. The 

atmospheric chemistry that influences ambient O3 concentrations is discussed in more detail in 

the ISA (U.S. EPA, 2013a, Chapter 3) and the PA (U.S. EPA, 2014c, Chapter 2) (see also Frey, 

2014a, p. 10 and 11). 

The HREA uses a photochemical model to estimate sensitivities of O3 to changes in 

precursor emissions in order to estimate ambient O3 concentrations that would just meet the 
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current and alternative standards (U.S. EPA, 2014a, Chapter 4).

61 For the 15 urban study areas

evaluated in the HREA,62 this model-based adjustment approach estimates hourly O3

concentrations at each monitor location when modeled U.S. anthropogenic precursor emissions 

(i.e., NOX, VOC)63 are reduced. The HREA estimates air quality that just meets the current and 

alternative standards for the 2006-2008 and 2008-2010 periods.64

As discussed in Chapter 4 of the HREA (U.S. EPA, 2014a), this approach to adjusting air 

quality models the physical and chemical atmospheric processes that influence ambient O3

concentrations. Compared to the quadratic rollback approach used in previous reviews, it 

provides more realistic estimates of the spatial and temporal responses of O3 to reductions in 

precursor emissions. Because ambient NOX can contribute both to the formation and destruction 

of O3 (U.S. EPA, 2014a, Chapter 4), as discussed above, the response of ambient O3

concentrations to reductions in NOX emissions is more variable than indicated by the quadratic 

rollback approach. This improved approach to adjusting O3 air quality is consistent with 

recommendations from the National Research Council of the National Academies (NRC, 2008).

In addition, CASAC strongly supported the improved approach, stating that “the quadratic 

 

61 The HREA uses the Community Multi-scale Air Quality (CMAQ) photochemical model 

instrumented with the higher order direct decoupled method (HDDM) to estimate O3

concentrations that would occur with the achievement of the current and alternative O3 standards 

(U.S. EPA, 2014a, Chapter 4).

62 The urban study areas assessed are Atlanta, Baltimore, Boston, Chicago, Cleveland, Dallas, 

Denver, Detroit, Houston, Los Angeles, New York, Philadelphia, Sacramento, St. Louis, and 

Washington, DC.

63 Exposure and risk analyses for most urban study areas focus on reducing U.S. anthropogenic 

NOX emissions alone. The exceptions are Chicago and Denver. Exposure and risk analyses for 

Chicago and Denver are based on reductions in emissions of both NOX and VOC (U.S. EPA, 

2014a, section 4.3.3.1; Appendix 4D). 

64 These simulations are illustrative and do not reflect any consideration of specific control 

programs designed to achieve the reductions in emissions required to meet the specified 

standards. Further, these simulations do not represent predictions of when, whether, or how areas 

might meet the specified standards.
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rollback approach has been replaced by a scientifically more valid Higher-order Decoupled 

Direct Method (HDDM)” and that “[t]he replacement of the quadratic rollback procedure by the 

HDDM procedure is important and supported by the CASAC” (Frey, 2014a, pp.1 and 3).

Consistent with the O3 chemistry summarized above, in locations and time periods when 

NOX is predominantly contributing to O3 formation (e.g., downwind of important NOX sources, 

where the highest O3 concentrations often occur), model-based adjustment to the current and 

alternative standards decreases estimated ambient O3 concentrations compared to recent 

monitored concentrations (U.S. EPA, 2014a, section 4.3.3.2). In contrast, in locations and time 

periods when NOX is predominantly contributing to O3 titration (e.g., in urban centers with high 

concentrations of NOX emissions, where ambient O3 concentrations are often suppressed and 

thus relatively low65), model-based adjustment increases ambient O3 concentrations compared to 

recent monitored concentrations (U.S. EPA, 2014a, section 4.3.3.2; Frey, 2014a, p. 10). 

Within urban study areas, the overall impacts of model-based air quality adjustment are 

to reduce the O3 concentrations at the upper ends of ambient distributions and to increase the O3

concentrations at the lower ends of those distributions (U.S. EPA, 2014a, section 4.3.3.2, Figures 

4-9 and 4-10).66 Seasonal means of daily O3 concentrations generally exhibit only modest 

changes upon model adjustment, reflecting the seasonal balance between daily decreases in 

relatively higher concentrations and increases in relatively lower concentrations (U.S. EPA, 

2014a, Figures 4-9 and 4-10). The resulting compression in the seasonal distributions of ambient 

 

65 Titration is also prominent during time periods when photochemistry is limited, and ambient 

O3 concentrations are relatively low, such as at night and on cool, cloudy days (U.S. EPA, 2014a,

Chapter 4).

66 It is important to note that sensitivity analyses in the HREA indicate that the increases in low 

O3 concentrations are smaller when NOX and VOC emissions are reduced than when only NOX

emissions are reduced (U.S. EPA, 2014a, Appendix 4-D, section 4.7).
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O3 concentrations is evident in all of the urban study areas evaluated, though the degree of 

compression varies considerably across areas (U.S. EPA, 2014a, Figures 4-9 and 4-10). 

This compression in the distributions of ambient O3 concentrations has important 

implications for exposure and risk estimates in urban study areas. Estimates influenced largely 

by the upper ends of the distribution of ambient concentrations (i.e., exposures of concern and 

lung function risk estimates, as discussed in sections 3.2.2 and 3.2.3.1 of the PA (U.S. EPA, 

2014c)) decrease with adjustment of air quality to the current and alternative standards. In 

contrast, seasonal risk estimates influenced by the full distribution of ambient O3 concentrations 

(i.e., epidemiology-based risk estimates, as discussed in section 3.2.3.2 of the PA) either 

decrease or increase in response to air quality adjustment, depending on the balance between the 

daily decreases in high O3 concentrations and increases in low O3 concentrations.67

In their review of the second draft HREA, CASAC considered this issue, in particular 

noting that “reductions in nitrogen oxides emissions can lead to less scavenging of ozone and 

free radicals, resulting in locally higher levels of ozone” (Frey, 2014a, p. 10). CASAC 

recommended that “the EPA should identify and discuss whether and to what extent health risks 

in the urban core may be affected by NOX reductions or other possible strategies” and, in 

particular, concluded that it would “be of interest to learn if there would be any children or 

outdoor workers in the more urban areas who would experience significantly higher exposures to 

ozone as a result of possible changes in the ozone NAAQS” (Frey, 2014a, p. 10). Consistent with 

this advice, the exposure and risk implications of the spatial and temporal patterns of ambient O3

 

67In addition, because epidemiology-based risk estimates use “area-wide” average O3

concentrations, calculated by averaging concentrations across multiple monitors in urban study 

areas (U.S. EPA, 2014c, section 3.2.3.2), risk estimates on a given day depend on the daily 

balance between increasing and decreasing O3 concentrations at the individual monitors that are 

averaged together to calculate the “area-wide” concentration. 
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following air quality adjustment in urban study areas are discussed in the final HREA (U.S. EPA, 

2014a, Chapter 9) and the final PA (U.S. EPA, 2014c, sections 3.2.2, 3.2.3), and are summarized 

below within the context of the PA’s consideration of exposure estimates (II.D.2.a) and risk 

estimates (II.D.2.b and II.D.2.c). 

2. Exposure Assessment

This section discusses the HREA assessment of human exposures to O3. Section II.C.2.a 

provides an overview of the approach used in the HREA to assessing exposures and the approach 

in the PA to considering exposure estimates, and summarizes key results. Section II.C.2.b 

summarizes the important uncertainties in exposure estimates. 

a. Overview and summary of key results

The exposure assessment presented in the HREA (U.S. EPA, 2014a, Chapter 5) provides 

estimates of the number and percent of people exposed to various concentrations of ambient O3, 

while at specified exertion levels. The HREA estimates exposures in the 15 urban study areas for 

four study groups, all school-age children (ages 5 to 18), asthmatic school-age children, 

asthmatic adults (ages 19 to 95), and all older adults (ages 65 to 95), reflecting the evidence 

indicating that these populations are at increased risk for O3-attributable effects (U.S. EPA, 

2013a, Chapter 8). An important purpose of these exposure estimates is to provide perspective on 

the extent to which air quality adjusted to just meet the current O3 NAAQS could be associated 

with exposures to O3 concentrations reported to result in respiratory effects.68 Estimates of such 

 

68 In addition, the range of modeled personal exposures to ambient O3 provide an essential input 

to the portion of the health risk assessment based on exposure-response functions (for lung 

function decrements) from controlled human exposure studies. The health risk assessment based 

on exposure-response information is discussed below (II.C.3). 
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“exposures of concern” provide perspective on the potential public health impacts of O3-related 

effects, including effects that cannot currently be evaluated in a quantitative risk assessment.

69

In the absence of large scale exposure studies that encompass the general population, as 

well as at-risk populations, modeling is the preferred approach to estimating exposures to O3

(U.S. EPA, 2014a, Chapter 5). The use of exposure modeling also facilitates the estimation of 

exposures resulting from ambient O3 concentrations differing from those present during exposure 

studies. In the HREA, population exposures to ambient O3 concentrations are estimated using the 

current version of the Air Pollutants Exposure (APEX) model. The APEX model simulates the 

movement of individuals through time and space and estimates their exposures to a given 

pollutant in indoor, outdoor, and in-vehicle microenvironments (U.S. EPA, 2014a, section 5.1.3). 

APEX takes into account important factors that contribute to total human exposure to ambient 

O3, including the temporal and spatial distributions of people and O3 concentrations throughout 

an urban area, the variation of O3 concentrations within various microenvironments, and the 

effects of exertion on breathing rate in exposed individuals (U.S. EPA, 2014a, section 5.1.3). To 

the extent spatial and/or temporal patterns of ambient O3 concentrations are altered upon model 

adjustment, as discussed above, exposure estimates reflect population exposures to those altered 

patterns. 

The HREA estimates 8-hour exposures at or above benchmark concentrations of 60, 70, 

and 80 ppb for individuals engaged in moderate or greater exertion (i.e., to approximate 

conditions in the controlled human exposure studies on which benchmarks are based). 

 

69 In this review, the term “exposure of concern” is defined as a personal exposure, while at 

moderate or greater exertion, to 8-hour average ambient O3 concentrations at and above specific 

benchmarks. As discussed below, benchmarks represent exposure concentrations at which O3-

induced health effects are known to occur, or can reasonably be anticipated to occur, in some 

individuals. 
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Benchmarks reflect exposure concentrations at which O3-induced respiratory effects are known 

to occur in some healthy adults engaged in moderate, intermittent exertion, based on evidence 

from controlled human exposure studies (U.S. EPA, 2013a, section 6.2; U.S. EPA, 2014c, 

section 3.1.2.1). The amount of weight to place on the estimates of exposures at or above specific 

benchmark concentrations depends in part on the weight of the scientific evidence concerning 

health effects associated with O3 exposures at those benchmark concentrations. It also depends 

on judgments about the importance, from a public health perspective, of the health effects that 

are known or can reasonably be inferred to occur as a result of exposures at benchmark 

concentrations (U.S. EPA, 2014c, sections 3.1.3, 3.1.5). 

As discussed in more detail above (II.B.2), the health evidence that supports evaluating 

exposures of concern at or above benchmark concentrations of 60, 70, and 80 ppb comes from a 

large body of controlled human exposure studies reporting a variety of respiratory effects in 

healthy adults. The lowest O3 exposure concentration for which controlled human exposure 

studies have reported respiratory effects in healthy adults is 60 ppb (based on changes in group 

mean responses), with more evidence supporting this benchmark concentration in the current 

review than in the last review. In healthy adults, 6.6 hour exposures to 60 ppb O3 have been 

reported to decrease lung function and to increase airway inflammation. Exposures of healthy 

adults to 72 ppb O3 for 6.6 hours have been reported to result in larger average lung function 

decrements, compared to 60 ppb, as well as in increased respiratory symptoms. Exposures of 

healthy adults to 80 ppb O3 for 6.6 hours have been reported to result in larger average lung 

function decrements than following exposures to 60 or 72 ppb and, depending on the study, to 

increase airway inflammation, increase respiratory symptoms, increase airways responsiveness, 

and decrease lung host defense (based on changes in group means) (U.S. EPA, 2014c, section 
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3.1.2.1). In commenting on the evidence for benchmark concentrations, CASAC stated the 

following (Frey, 2014c, p. 6): 

The 80 ppb-8hr benchmark level represents an exposure level for which there is 

substantial clinical evidence demonstrating a range of ozone-related effects 

including lung inflammation and airway responsiveness in healthy individuals. 

The 70 ppb-8hr benchmark level reflects the fact that in healthy subjects, 

decreases in lung function and respiratory symptoms occur at concentrations as 

low as 72 ppb and that these effects almost certainly occur in some people, 

including asthmatics and others with low lung function who are less tolerant of 

such effects, at levels of 70 ppb and below. The 60 ppb-8hr benchmark level 

represents the lowest exposure level at which ozone-related effects have been 

observed in clinical studies of healthy individuals. Based on its scientific 

judgment, the CASAC finds that the 60 ppb-8hr exposure benchmark is relevant 

for consideration with respect to adverse effects on asthmatics.

In considering estimates of O3 exposures of concern at or above benchmarks of 60, 70, 

and 80 ppb, the PA focuses on modeled exposures for school-age children (ages 5-18), including 

asthmatic school-age children, which are key at-risk populations identified in the ISA (U.S. EPA, 

2014c, section 3.1.5). The percentages of children estimated to experience exposures of concern 

are considerably larger than the percentages estimated for adult populations (i.e., approximately 

3-fold larger across urban study areas) (U.S. EPA, 2014a, section 5.3.2 and Figures 5-5 to 5-8).

The larger exposure estimates for children are due primarily to the larger percentage of children 

estimated to spend an extended period of time being physically active outdoors when O3

concentrations are elevated (U.S. EPA, 2014a, sections 5.3.2 and 5.4.1). 

Although exposure estimates differ between children and adults, the patterns of results 

across the urban study areas and years are similar among all of the populations evaluated (U.S. 

EPA, 2014a, Figures 5-5 to 5-8). Therefore, while the PA highlights estimates in children, 

including asthmatic school-age children, it also notes that the patterns of exposures estimated for 

children represent the patterns estimated for adult asthmatics and older adults. 
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Table 1 below summarizes key results from the exposure assessment. Table 1 presents 

estimates of the percentages and numbers of all school-aged children estimated to experience 

exposures of concern when air quality was adjusted to just meet the current and alternative 8-

hour O3 standards. The percentage of all school-age children in the 15 urban study areas

estimated to experience exposures of concern declines when comparing just meeting the current 

standard to just meeting alternative 8-hour O3 standards. Substantial variability is evident across 

years and urban study areas, as indicated by the ranges of averaged estimates and estimates for 

worst-case years and study areas. As discussed below, the interindividual variability in 

responsiveness following exposures of concern means that only a subset of individuals who are 

exposed at and above a given benchmark concentration would actually be expected to experience 

respiratory effects. 
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Table 1. Summary of Estimated Exposures of Concern in All School-age Children for the 

Current and Alternative O3 Standards in Urban Study Areas

Benchmark 

Concentration

Standard 

Level 

(ppb)

Average % 

Children

Exposed70

Average Number of 

Children Exposed 

[Average Number of 

Asthmatic Children]71

% Children -

Worst Year 

and Worst Area

One or more exposures of concern per season

≥ 80 ppb 75 0 - 0.3 27,000 [3,000] 1.1

70 0 - 0.1 3,700 [300] 0.2

65 0 300 [0] 0

60 0 10072 [0] 0

≥ 70 ppb 75 0.6 - 3.3 362,000 [40,000] 8.1

70 0.1 - 1.2 94,000 [10,000] 3.2

65 0 - 0.2 14,000 [2,000] 0.5

60 0 1,400 [200] 0.1

≥ 60 ppb 75 9.5 - 17 2,316,000 [246,000] 25.8

70 3.3 - 10.2 1,176,000 [126,000] 18.9

65 0 - 4.2 392,000 [42,000] 9.5

60 0 - 1.2 70,000 [8,000] 2.2

Two or more exposures of concern per season

≥ 80 ppb 75 0 600 [100] 0.1

70 0 0 [0] 0

65 0 0 [0] 0

60 0 0 [0] 0

≥ 70 ppb 75 0.1 - 0.6 46,000 [5,000] 2.2

70 0 - 0.1 5,400 [600] 0.4

65 0 300 [100] 0

60 0 0 [0] 0

≥ 60 ppb 75 3.1 - 7.6 865,000 [93,000] 14.4

70 0.5 - 3.5 320,000 [35,000] 9.2

 

70 Estimates for each urban case study area were averaged for the years evaluated in the HREA 

(2006 to 2010). Ranges reflect the ranges across urban study areas. Estimates smaller than 0.05% 

were rounded downward to zero (from U.S. EPA, 2014a, Tables 5-11 and 5-12). 

71 Numbers of children exposed in each urban case study area were averaged over the years 2006 

to 2010. These averages were then summed across urban study areas. Numbers were rounded to 

nearest thousand unless otherwise indicated. Estimates smaller than 50 were rounded downward 

to zero (from U.S. EPA, 2014a, Appendix 5F Table 5F-5). See below for discussion of 

uncertainties in exposure estimates. 

72As discussed in section 4.3.3 of the HREA, the model-based air quality adjustment approach 

used to estimate risks associated with the current and alternative standards was unable to 

estimate the distribution of ambient O3 concentrations in New York City upon just meeting an 

alternative standard with a level of 60 ppb. Therefore, for the 60 ppb standard level the numbers 

of children and asthmatic children reflect all of the urban study areas except New York. 
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65 0 - 0.8 67,000 [7,500] 2.8

60 0 - 0.2 5,100 [700] 0.3

b. Key uncertainties

In considering exposure estimates within the context of the current and alternative O3

standards, the PA also notes important uncertainties in these estimates. For example, due to 

variability in responsiveness, only a subset of individuals who experience exposures at or above 

a benchmark concentration can be expected to experience health effects.73 Given the lack of 

sufficient exposure-response information for most of the health effects that informed benchmark 

concentrations, estimates of the number of people likely to experience exposures at or above 

benchmark concentrations generally cannot be translated into quantitative estimates of the 

number of people likely to experience specific health effects.74 The PA views health-relevant 

exposures as a continuum with greater confidence and less uncertainty about the existence of 

adverse health effects at higher O3 exposure concentrations, and less confidence and greater 

uncertainty as one considers lower exposure concentrations. This view draws from the overall

body of available health evidence, which indicates that as exposure concentrations increase, the 

incidence, magnitude, and severity of effects increases. 

Though the PA indicates less confidence in the likelihood of adverse health effects as O3

exposure concentrations decrease, it also notes that the controlled human exposure studies that 

provided the basis for health benchmark concentrations have not evaluated at-risk populations. 

Compared to the healthy individuals included in controlled human exposure studies, members of 

 

73 As noted below (II.C.3.a.ii), in the case of asthmatics, responsiveness to O3 could depend on 

factors that have not been well-evaluated, such as asthma severity, the effectiveness of asthma 

control, or the prevalence of medication use.

74 The exception to this is lung function decrements, as discussed below (and in U.S. EPA, 

2014c, section 3.2.3.1). 
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at-risk populations (e.g., asthmatics, children) could be more likely to experience adverse effects, 

could experience larger and/or more serious effects, and/or could experience effects following 

exposures to lower O3 concentrations. The CASAC expressed similar views in their advice to the 

Administrator (Frey, 2014a, pp. 7 and 14). In considering estimated exposures of concern (U.S. 

EPA, 2014c, section 3.4), the PA notes that concerns about the potential for adverse health 

effects, including effects in at-risk populations must be balanced against the increasing 

uncertainty regarding the likelihood of such effects following exposures to lower O3

concentrations.

Uncertainties associated with the APEX exposure modeling also have the potential to be 

important (U.S. EPA, 2014a, section 5.5.2, Table 5-6). For example, the HREA concludes that 

exposures of concern could be underestimated for some individuals who are frequently and 

routinely active outdoors during the warm season (U.S. EPA, 2014a, section 5.5.2). This could 

include outdoor workers and children who are frequently active outdoors. The HREA

specifically notes that long-term diary profiles (i.e., monthly, annual) do not exist for such 

populations, limiting the extent to which APEX outputs reflect people who follow similar daily 

routines resulting in high exposures, over extended periods of time. 

In order to evaluate one dimension of the potential implications of this uncertainty for 

exposure estimates, the HREA reports the results of limited exposure model sensitivity analyses 

using subsets of activity diaries specifically selected to reflect groups spending a larger 

proportion of time being active outdoors during the O3 season. When diaries were selected to 

mimic activity patterns performed by outdoor workers, the percent of modeled individuals 

estimated to experience exposures of concern was higher than the other adult populations 

evaluated. The percentages of outdoor workers estimated to experience exposures of concern 
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were generally similar to the percentages estimated for children (i.e., using the full database of 

diary profiles) in the worst-case urban study area and year (i.e., urban study area and year with 

the largest percent of children estimated to experience exposures of concern) (U.S. EPA, 2014a, 

section 5.4.3.2, Figure 5-14). In addition, when diaries were restricted to children who did not 

report any time spent inside a school or performing paid work (i.e., to mimic children spending 

large portions of their time outdoors during the summer), the number experiencing exposures of 

concern increased by approximately 30% (U.S. EPA, 2014a, section 5.4.3.1). Though these 

sensitivity analyses are limited to single urban study areas, and though there is uncertainty 

associated with diary selection approaches to mimic highly exposed populations, they suggest 

the possibility that some at-risk groups could experience more frequent exposures of concern 

than indicated by estimates made using the full database of activity diary profiles. 

In further considering activity diaries, the HREA also notes that growing evidence 

indicates that people can change their behavior in response to high O3 concentrations, reducing 

the time spent being active outdoors (U.S. EPA, 2014a, section 5.4.3.3). Commonly termed 

“averting behaviors,” these altered activity patterns could reduce personal exposure 

concentrations. Therefore, the HREA also performed limited sensitivity analyses to evaluate the 

potential implications of averting behavior for estimated exposures of concern. These analyses 

suggest that averting behavior could reduce the percentages of children estimated to experience 

exposures of concern at or above the 60 or 70 ppb benchmark concentrations by approximately 

10 to 30%, with larger reductions possible for the 80 ppb benchmark (U.S. EPA, 2014a, Figure 

5-15). As discussed above for other sensitivity analyses, these analyses are limited to a single 

urban case study area and are subject to uncertainties associated with assumptions about the 

prevalence and duration of averting behaviors. However, the results suggest that exposures of 
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concern could be overestimated, particularly in children (Neidell, 2009; U.S. EPA, 2013, Figures 

4-7 and 4-8), if the possibility for averting behavior is not incorporated into estimates. 

3. Quantitative Health Risk Assessments

For some health endpoints, there is sufficient scientific evidence and information 

available to support the development of quantitative estimates of O3-related health risks. In the 

last review of the O3 NAAQS, the quantitative health risk assessment estimated O3-related lung 

function decrements, respiratory symptoms, respiratory-related hospital admissions, and 

nonaccidental and cardiorespiratory-related mortality (U.S. EPA, 2007). In those analyses, both 

controlled human exposure and epidemiologic studies were used for the quantitative assessment 

of O3-related human health risks. 

In the current review, for short-term O3 concentrations, the HREA estimates lung 

function decrements; respiratory symptoms in asthmatics; hospital admissions and emergency 

department visits for respiratory causes; and all-cause mortality (U.S. EPA, 2014a). For longterm O3 concentrations, the HREA estimates respiratory mortality (U.S. EPA, 2014a).75

Estimates of O3-induced lung function decrements are based on exposure modeling, combined 

with exposure-response relationships from controlled human exposure studies (U.S. EPA, 2014a, 

Chapter 6). Estimates of O3-associated respiratory symptoms, hospital admissions and 

emergency department visits, and mortality are based on concentration-response relationships 

from epidemiologic studies (U.S. EPA, 2014a, Chapter 7). As with the exposure assessment 

discussed above, O3-associated health risks are estimated for recent air quality and for ambient 

concentrations adjusted to just meet the current and alternative O3 standards, based on 2006-2010 

 

75 Estimates of O3-associated respiratory mortality are based on the study by Jerrett et al. (2009). 

This study used seasonal averages of 1-hour daily maximum O3 concentrations to estimate longterm concentrations. 
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air quality and adjusted precursor emissions. The following sections discuss the lung function 

risk assessment (II.C.3.a) and the epidemiology-based morbidity and mortality risk assessments

(II.C.3.b) from the HREA, including important sources of uncertainty in these estimates. 

a. Lung function risk assessment

Section II.C.3.a.i provides an overview of the approach used in the HREA to assessing 

lung function risks, an overview of the approach in the PA to considering lung function risk 

estimates, and a summary of key results. Section II.C.3.a.ii presents a summary of key

uncertainties in lung function risk estimates. 

i. Overview and summary of key results

In the current review, the HREA estimates risks of lung function decrements in schoolaged children (ages 5 to 18), asthmatic school-aged children, and the general adult population for 

the 15 urban study areas. The results presented in the HREA are based on an updated dosethreshold model that estimates FEV1 responses for individuals following short-term exposures to 

O3 (McDonnell et al., 2012), reflecting methodological improvements since the last review 

(II.B.2.a.i, above; U.S. EPA, 2014a, section 6.2.4). The impact of the dose threshold is that O3-

induced FEV1 decrements result primarily from exposures on days with average ambient O3

concentrations above about 40 ppb (U.S. EPA, 2014a, section 6.3.1, Figure 6-9).76

The HREA estimates risks of moderate to large lung function decrements, defined as 

FEV1 decrements > 10%, 15%, or 20%. In evaluating these lung function risk estimates within

 

76 Analysis of this issue in the HREA is based on risk estimates in Los Angeles for 2006 

unadjusted air quality. The HREA shows that more than 90% of daily instances of FEV1

decrements ≥ 10% occur when 8-hr average ambient concentrations are above 40 ppb for this 

modeled scenario. The HREA notes that the distribution of responses will be different for 

different study areas, years, and air quality scenarios (U.S. EPA, 2014c, Chapter 6).
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the context of considering the current and alternative O3 standards, the PA focuses on the percent 

of children estimated to experience one or more and two or more decrements > 10, 15, and 20%, 

noting that the percentage of asthmatic children estimated to experience such decrements is 

virtually indistinguishable from the percentage estimated for all children.77 Compared to 

children, a smaller percentage of adults were estimated to experience O3-induced FEV1

decrements (U.S. EPA, 2014a, section 6.3.1, Table 6-4). As for exposures of concern (see 

above), the patterns of results across urban study areas and over the years evaluated are similar in 

children and adults. Therefore, while the PA highlights estimates in children, it notes that these 

results are also representative of the patterns estimated for adult populations. 

Table 2 below summarizes key results from the lung function risk assessment. Table 2

presents estimates of the percentages of school-aged children estimated to experience O3-induced 

FEV1 decrements > 10, 15, or 20% when air quality was adjusted to just meet the current and 

alternative 8-hour O3 standards. Table 2 also presents the numbers of children, including children 

with asthma, estimated to experience such decrements. As shown in these tables, the percentage 

of school-age children in the 15 urban study areas estimated to experience O3-induced FEV1

decrements declines when comparing just meeting the current standard to just meeting 

alternative 8-hour O3 standards. Substantial variability is evident across years and urban study 

areas, as indicated by the ranges of averaged estimates and estimates for worst-case years and 

locations.

 

77 Though see below for discussion of uncertainty in lung function responses of children and 

asthmatics. 
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Table 2. Summary of Estimated O3-Induced Lung Function Decrements for the Current 

and Potential Alternative O3 Standards in Urban Case Study Areas

Lung 

Function 

Decrement

Alternative 

Standard 

Level

Average 

% 

Children78

Number of Children (5 

to 18 years) [Number of 

Asthmatic Children]79

% Children 

Worst Year and 

Area

One or more decrements per season

≥ 10% 75 14-19 3,007,000 [312,000] 22

70 11-17 2,527,000 [261,000] 20

65 3-15 1,896,000 [191,000] 18

60 5-11 1,404,000 [139,000]80 13

≥ 15% 75 3-5 766,000 [80,000] 7

70 2-4 562,000 [58,000] 5

65 0-3 356,000 [36,000] 4

60 1-2 225,000 [22,000] 3

≥ 20% 75 1-2 285,000 [30,000] 2.8

70 1-2 189,000 [20,000] 2.1

65 0-1 106,000 [11,000] 1.4

60 0-1 57,000 [6,000] 0.9

Two or more decrements per season

≥ 10% 75 7.5-12 1,730,000 [179,000] 14

70 5.5-11 1,414,000 [145,000] 13

65 1.3-8.8 1,023,000 [102,000] 11

60 2.1-6.4 741,000 [73,000] 7.3

≥ 15% 75 1.7-2.9 391,000 [40,000] 3.8

70 0.9-2.4 276,000 [28,000] 3.1

65 0.1-1.8 168,000 [17,000] 2.3

60 0.2-1.0 101,000 [10,000] 1.4

≥ 20% 75 0.5-1.1 128,000 [13,000] 1.5

70 0.3-0.8 81,000 [8,000] 1.1

65 0-0.5 43,000 [4,000] 0.8

60 0-0.2 21,000 [2,000] 0.4

 

78Estimates in each urban case study area were averaged for the years evaluated in the HREA 

(2006 to 2010). Ranges reflect the ranges across urban study areas. 

79Numbers of children estimated to experience decrements in each study urban case study area 

were averaged over 2006 to 2010. These averages were then summed across urban study areas. 

Numbers are rounded to nearest thousand unless otherwise indicated.

80As discussed in section 4.3.3 of the HREA, the model-based air quality adjustment approach 

used to estimate risks associated with the current and alternative standards was unable to 

estimate the distribution of ambient O3 concentrations in New York City upon just meeting an 

alternative standard with a level of 60 ppb. Therefore, for the 60 ppb standard level the numbers 

of children and asthmatic children reflect all of the urban study areas except New York. 
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ii. Key uncertainties

As for exposures of concern discussed above, the PA also considers important 

uncertainties in estimates of lung function risk. In addition to the uncertainties noted for 

exposure estimates, the HREA identifies several key uncertainties associated with estimates of 

O3-induced lung function decrements. An uncertainty with particular potential to impact 

consideration of risk estimates stems from the lack of exposure-response information in children. 

In the near absence of controlled human exposure data for children, risk estimates are based on 

the assumption that children exhibit the same lung function response following O3 exposures as 

healthy 18 year olds (i.e., the youngest age for which controlled human exposure data is 

available) (U.S. EPA, 2014a, section 6.5.3). This assumption is justified in part by the findings of 

McDonnell et al. (1985), who reported that children (8-11 years old) experienced FEV1

responses similar to those observed in adults (18-35 years old). In addition, as discussed in the 

ISA (U.S. EPA, 2013a, section 6.2.1), summer camp studies of school-aged children reported 

O3-induced lung function decrements similar in magnitude to those observed in controlled 

human exposure studies using adults. In extending the risk model to children, the HREA fixes 

the age term in the model at its highest value, the value for age 18. This approach could result in 

either over- or underestimates of O3-induced lung function decrements in children, depending on 

how children compare to the adults used in controlled human exposure studies (U.S. EPA, 

2014a, section 6.5.3). 

A related source of uncertainty is that the risk assessment estimates O3-induced 

decrements in asthmatics using the exposure-response relationship developed from data collected 

from healthy individuals. Although the evidence has been mixed (U.S. EPA, 2013a, section 

6.2.1.1), several studies have reported larger O3-induced lung function decrements in asthmatics 

than in non-asthmatics (Kreit et al., 1989; Horstman et al., 1995; Jorres et al., 1996; Alexis et al., 
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2000). On this issue, CASAC noted that “[a]sthmatic subjects appear to be at least as sensitive, if 

not more sensitive, than non-asthmatic subjects in manifesting ozone-induced pulmonary 

function decrements” (Frey, 2014c, p. 4). To the extent asthmatics experience larger O3-induced 

lung function decrements than the healthy adults used to develop exposure-response 

relationships, the HREA could underestimate the impacts of O3 exposures on lung function in 

asthmatics, including asthmatic children. The implications of this uncertainty for risk estimates 

remain unknown at this time (U.S. EPA, 2014a, section 6.5.4), and could depend on a variety of 

factors that have not been well-evaluated, including the severity of asthma and the prevalence of 

medication use. However, the available evidence shows responses to O3 increase with severity of 

asthma (Horstman et al., 1995) and corticosteroid usage does not prevent O3 effects on lung 

function decrements or respiratory symptoms in people with asthma (Vagaggini et al., 2001, 

2007).

b. Mortality and morbidity risk assessments

As discussed above (II.B.2), epidemiologic studies provide evidence for the most serious 

O3-associated public health outcomes (e.g., mortality, hospital admissions, emergency 

department visits). Section II.C.3.b.i below provides an overview of the approach used in the 

HREA to assessing mortality and morbidity risks based on information from epidemiologic 

studies, discusses the approach in the PA to considering epidemiology-based risk estimates, and 

presents a summary of key results. Section II.C.3.b.ii summarizes key uncertainties in 

epidemiology-base risk estimates. 

i. Overview and summary of key results

Risk estimates based on epidemiologic studies can provide perspective on the most 

serious O3-associated public health outcomes (e.g., mortality, hospital admissions, emergency 
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department visits) in populations that often include at-risk groups. The HREA estimates O3-

associated risks in 12 urban study areas81 using concentration-response relationships drawn from 

epidemiologic studies. These concentration-response relationships are based on “area-wide” 

average O3 concentrations.82 The HREA estimates risks for the years 2007 and 2009 in order to 

provide estimates of risk for a year with generally higher O3 concentrations (2007) and a year 

with generally lower O3 concentrations (2009) (U.S. EPA, 2014a, section 7.1.1).

As in the last review of the O3 NAAQS (U.S. EPA, 2007, pp. 2-48 to 2-54), the PA

recognizes that ambient O3 concentrations, and therefore O3-associated health risks, result from 

precursor emissions from various types of sources. Based on the air quality modeling discussed 

in chapter 2 of the PA (U.S. EPA, 2014c), approximately 30 to 60% of average daytime O3

during the warm season (i.e., daily maximum 8-hour concentrations averaged from April to 

October) is attributable to precursor emissions from U.S. anthropogenic sources (U.S. EPA, 

2014c, section 2.4.4). The remainder is attributable to precursor emissions from international 

anthropogenic sources and natural sources. Because the HREA characterizes health risks from all 

O3, regardless of source, risk estimates reflect emissions from U.S. anthropogenic, international 

anthropogenic, and natural sources. 

 

81 The 12 urban areas evaluated are Atlanta, Baltimore, Boston, Cleveland, Denver, Detroit, 

Houston, Los Angeles, New York, Philadelphia, Sacramento, and St. Louis. 

82 In the epidemiologic studies that provide the health basis for HREA risk assessments, 

concentration-response relationships are based on daytime O3 concentrations, averaged across 

multiple monitors within study areas. These daily averages are used as surrogates for the spatial 

and temporal patterns of exposures in study populations. Consistent with this approach, the 

HREA epidemiologic-based risk estimates also utilize daytime O3 concentrations, averaged 

across monitors, as surrogates for population exposures. In this notice, we refer to these averaged 

concentrations as “area-wide” O3 concentrations. Area-wide concentrations are discussed in 

more detail in section 3.1.4 of the PA (U.S. EPA, 2014c).
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Compared to the weight given to HREA estimates of exposures of concern and lung 

function risks, and the weight given to the evidence (U.S. EPA, 2014c, section 4.4.1), the PA 

places relatively less weight on epidemiologic-based risk estimates. In doing so, the PA notes 

that the overall conclusions from the HREA likewise reflect less confidence in estimates of 

epidemiologic-based risks than in estimates of exposures and lung function risks. The 

determination to attach less weight to the epidemiologic-based estimates reflects the 

uncertainties associated with mortality and morbidity risk estimates, including the heterogeneity 

in effect estimates between epidemiologic study areas, the potential for epidemiologic-based 

exposure measurement error, and uncertainty in the interpretation of the shape of concentrationresponse functions at lower O3 concentrations (discussed below). The PA also notes the HREA 

conclusion that lower confidence should be placed in the results of the assessment of respiratory 

mortality risks associated with long-term O3 exposures, primarily because that analysis is based 

on only one study (even though that study is well-designed) and because of the uncertainty in 

that study about the existence and level of a potential threshold in the concentration-response 

function (U.S. EPA, 2014a, section 9.6). 

In considering the epidemiology-based risk estimates, the PA focuses on mortality risks 

associated with short-term O3 concentrations. In doing so, in addition to noting uncertainty in 

estimates of respiratory mortality associated with long-term O3, the PA notes that the patterns of 

estimated respiratory morbidity risks across urban study areas, over years, and for different 

standards are similar to the patterns of total mortality risk.

The PA considers estimates of total risk (i.e., based on the full distributions of ambient O3

concentrations) and estimates of risk associated with O3 concentrations in the upper portions of 

ambient distributions. A focus on estimates of total risks would place greater weight on the 
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possibility that concentration-response relationships are linear over the entire distribution of 

ambient O3 concentrations, and thus on the potential for morbidity and mortality to be affected 

by changes in relatively low O3 concentrations. A focus on risks associated with O3

concentrations in the upper portions of the ambient distribution would place greater weight on 

the uncertainty associated with the shapes of concentration-response curves for O3

concentrations in the lower portions of the distribution. Given that both types of risk estimates 

could reasonably inform a decision on standard level, depending on the weight placed on 

uncertainties in the occurrence and the estimation of O3-attributable effects at relatively low O3

concentrations, the PA considers both types of estimates. Key results for O3-associated mortality 

risk are summarized in Table 3 below. Table 3 presents estimates of the number of O3-associated 

deaths in urban study areas, for air quality adjusted to just meet the current and alternative 

standards. 
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Table 3. Estimates of O3-Associated Deaths Attributable to the Full Distribution of 8-Hour 

Area-Wide O3 Concentrations and to Concentrations at or above 20, 40, or 60 ppb O3

(Deaths Summed Across Urban Case Study Areas)83

Number of O3-Associated Deaths Summed Across Urban Case Study Areas

Standard 

Level Total O3 20+ ppb 40+ ppb 60+ ppb

2007

75 ppb 7,500 7,500 5,400 500

70 ppb 7,200 7,200 4,900 240

65 ppb 6,500 6,500 2,800 90

60 ppb84 6,400 6,400 2,300 10

2009

75 ppb 7,000 7,000 4,700 270

70 ppb 6,900 6,900 4,300 80

65 ppb 6,400 6,400 2,600 40

60 ppb 6,300 6,300 2,100 10

ii. Key Uncertainties 

Compared to estimates of O3 exposures of concern and estimates of O3-induced lung 

function decrements (discussed above), the HREA conclusions reflect lower confidence in 

epidemiologic-based risk estimates (U.S. EPA, 2014a, section 9.6). In particular, the HREA 

highlights the heterogeneity in effect estimates between locations, the potential for exposure 

measurement errors, and uncertainty in the interpretation of the shape of concentration-response 

 

83 Table 3 is based on the information in Figures 7-2 and 7-3 in the HREA (U.S. EPA, 2014a). 

Estimates of the numbers of O3-associated deaths are based on concentration-response 

relationships for total mortality associated with short-term O3 from the study by Smith et al. 

(2009). Estimates of the numbers O3-associated deaths are rounded to the nearest hundred, unless 

otherwise indicated. 

84As discussed in section 4.3.3 of the HREA, the model-based air quality adjustment approach 

used to estimate risks associated with the current and alternative standards was unable to 

estimate the distribution of ambient O3 concentrations in New York City upon just meeting an 

alternative standard with a level of 60 ppb. Therefore, the total number of deaths indicated for 

the 60 ppb standard level reflect the 60 ppb estimates for all urban study areas except New York 

City. For New York City, the estimated number of O3-associated deaths for the 60 ppb standard 

level was assumed to be equal to the number for the 65 ppb level. 
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functions at lower O3 concentrations (U.S. EPA, 2014a, section 9.6). The HREA also concludes 

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 is based on only one study, 

though that study is well-designed, and because of the uncertainty in that study about the 

existence and identification of a potential threshold in the concentration-response function (U.S. 

EPA, 2014a, section 9.6).85,

86 This section further discusses some of the key uncertainties in 

epidemiologic-based risk estimates, as summarized in the PA (U.S. EPA, 2014c, section 3.2.3.2), 

with a focus on uncertainties that can have particularly important implications for the 

Administrator’s consideration of epidemiology-based risk estimates. 

The PA notes that reducing NOX emissions generally reduces O3-associated mortality and 

morbidity risk estimates in locations and time periods with relatively high ambient O3

concentrations and increases risk estimates in locations and time periods with relatively low 

concentrations (II.C.1, above). When evaluating uncertainties in epidemiologic risk estimates, it 

is important to consider (1) the extent to which the O3 response to reductions in NOX emissions 

appropriately represents the trends observed in ambient O3 following actual reductions in NOX

emissions; (2) the extent to which estimated changes in risks in urban study areas are 

representative of the changes that would be experienced broadly across the U.S. population; and 

(3) the extent to which the O3 response to reductions in precursor emissions could differ with 

 

85 The CASAC also concluded that “[i]n light of the potential nonlinearity of the C-R function 

for long-term exposure reflecting a threshold of the mortality response, the estimated number of 

premature deaths avoidable for long-term exposure reductions for several levels need to be 

viewed with caution” (Frey, 2014a, p. 3).

86 There is also uncertainty about the extent to which mortality estimates based on the long-term 

metric used in the study by Jerrett et al. (2009) (i.e., seasonal average of 1-hour daily maximum 

concentrations) reflects associations with long-term average O3 versus repeated occurrences of 

elevated short-term concentrations.
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emissions reduction strategies that are different from those used in HREA to generate risk 

estimates. 

To evaluate the first issue, the HREA conducted a national analysis evaluating trends in 

monitored ambient O3 concentrations during a time period when the U.S. experienced large-scale 

reductions in NOX emissions (i.e., 2001 to 2010). Analyses of trends in monitored O3 indicate 

that over such a time period, the upper end of the distribution of monitored O3 concentrations 

(i.e., indicated by the 95th percentile) generally decreased in urban and non-urban locations

across the U.S. (U.S. EPA, 2014a, Figure 8-29). During this same time period, median O3

concentrations decreased in suburban and rural locations, and in some urban locations. However, 

median concentrations increased in some large urban centers (U.S. EPA, 2014a, Figure 8-28). As 

discussed in the REA, and above (II.C.1), these increases in median concentrations likely reflect 

the increases in relatively low O3 concentrations that can occur near important sources of NOX

upon reductions in NOX emissions (U.S. EPA, 2014a, section 8.2.3.1). These patterns of 

monitored O3 during a period when the U.S. experienced large reductions in NOX emissions are 

qualitatively consistent with the modeled responses of O3 to reductions in NOX emissions. 

To evaluate the second issue, the HREA conducted national air quality modeling 

analyses. These analyses estimated the proportion of the U.S. population living in locations 

where seasonal averages of daily O3 concentrations are estimated to decrease in response to 

reductions in NOX emissions, and the proportion living in locations where such seasonal 

averages are estimated to increase. Given the close relationship between changes in seasonal 

averages of daily O3 concentrations and changes in seasonal mortality and morbidity risk 

estimates, this analysis informs consideration of the extent to which the risk results in urban 

study areas represent the U.S. population as a whole. This representativeness analysis indicates 
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that the majority of the U.S. population lives in locations where reducing NOX emissions would 

be expected to result in decreases in warm season averages of daily maximum 8-hour ambient O3

concentrations. Because the HREA urban study areas tend to underrepresent the populations 

living in such areas (e.g., suburban, smaller urban, and rural areas), risk estimates for the urban 

study areas are likely to understate the average reductions in O3-associated mortality and 

morbidity risks that would be experienced across the U.S. population as a whole upon reducing 

NOX emissions (U.S. EPA, 2014a, section 8.2.3.2). 

To evaluate the third issue, the HREA assessed the O3 air quality response to reducing 

both NOX and VOC emissions (i.e., in addition to assessing reductions in NOX emissions alone) 

for a subset of seven urban study areas. As discussed in the PA (U.S. EPA, 2014c, section 3.2.1), 

in most of the urban study areas the inclusion of VOC emissions reductions did not alter the NOX

emissions reductions required to meet the current or alternative standards.87 However, the 

addition of VOC reductions generally resulted in larger decreases in mid-range O3 concentrations 

(25thto 75th percentiles) (U.S. EPA, 2014a, Appendix 4D, section 4.7).88 In addition, in all seven 

of the urban study areas evaluated, the increases in low O3 concentrations were smaller for the 

NOX/VOC scenarios than the NOX alone scenarios (U.S. EPA, 2014a, Appendix 4D, section 

4.7). This was most apparent for Denver, Houston, Los Angeles, New York, and Philadelphia. 

Given the impacts on total risk estimates of increases in low O3 concentrations, these results 

 

87 The exceptions are Chicago and Denver, for which the HREA risk estimates are based on 

reductions in both NOX and VOC (U.S. EPA, 2014a, section 4.3.3.1). Emissions of NOX and 

VOC were reduced by equal percentages, a scenario not likely to reflect the optimal combination 

for reducing risks. 

88 This was the case for all of the urban study areas evaluated, with the exception of New York 

(U.S. EPA, 2014a, Appendix 4-D, section 4.7).
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suggest that in some locations optimized emissions reduction strategies could result in larger 

reductions in O3-associated mortality and morbidity than indicated by HREA estimates. 

Section 7.4 of the HREA also highlights some additional uncertainties associated with 

epidemiologic-based risk estimates (U.S. EPA, 2014a). This section of the HREA identifies and 

discusses sources of uncertainty and presents a qualitative evaluation of key parameters that can 

introduce uncertainty into risk estimates (U.S. EPA, 2014a, Table 7-4). For several of these 

parameters, the HREA also presents quantitative sensitivity analyses (U.S. EPA, 2014a, sections 

7.4.2 and 7.5.3). Of the uncertainties discussed in Chapter 7 of the HREA, those related to the 

application of concentration-response functions from epidemiologic studies can have particularly 

important implications for consideration of epidemiology-based risk estimates, as discussed 

below. 

An important uncertainty is the shape of concentration-response functions at low ambient 

O3 concentrations (U.S. EPA, 2014a, Table 7-4).89 Consistent with the ISA conclusion that there 

is no discernible population threshold in O3-associated health effects, the HREA estimates 

epidemiology-based mortality and morbidity risks for entire distributions of ambient O3

concentrations, based on the assumption that concentration-response relationships remain linear 

over those distributions. In addition, in recognition of the ISA conclusion that certainty in the 

shape of O3 concentration-response functions decreases at low ambient concentrations, the 

HREA also estimates total mortality associated with various ambient O3 concentrations. The PA 

considers both types of risk estimates, recognizing greater public health concern for adverse O3-

attributable effects at higher ambient O3 concentrations (which drive higher exposure 

 

89 A related uncertainty is the existence, or not, of a threshold. The HREA addresses this issue 

for long-term O3 by evaluating risks in models that include potential thresholds (II.D.2.c). 
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concentrations, section 3.2.2 of the PA (U.S. EPA, 2014c)), as compared to lower 

concentrations. 

A related uncertainty is that associated with the public health importance of the increases 

in relatively low O3 concentrations following air quality adjustment. This uncertainty relates to 

the assumption that the concentration response function for O3 is linear, such that that total risk 

estimates are equally influenced by decreasing high concentrations and increasing low 

concentrations, when the increases and decreases are of equal magnitude. Even on days with 

increases in relatively low area-wide average concentrations, resulting in increases in estimated 

risks, some portions of the urban study areas could experience decreases in high O3

concentrations. To the extent adverse O3-attributable effects are more strongly supported for 

higher ambient concentrations (which are consistently reduced upon air quality adjustment), the 

impacts on risk estimates of increasing low O3 concentrations reflect an important source of 

uncertainty. 

The HREA also notes important uncertainties associated with using a concentrationresponse relationship developed for a particular population in a particular location to estimate 

health risks in different populations and locations (U.S. EPA, 2014a, Table 7-4). As discussed 

above, concentration-response relationships derived from epidemiologic studies reflect the 

spatial and temporal patterns of population exposures during the study. The HREA applies 

concentration-response relationships from epidemiologic studies to adjusted air quality in study 

areas that are different from, and often larger in spatial extent than, the areas used to generate the 

relationships. This approach ensures the inclusion of the actual nonattainment monitors that often 

determine the magnitude of emissions reductions for the air quality adjustments throughout the 

urban study areas. This approach also allows the HREA to estimate patterns of health risks more 
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broadly across a larger area, including a broader range of air quality concentrations and a larger 

population. The HREA notes that it is not possible to quantify the impacts of this uncertainty on 

risk estimates in most urban case study locations, though the HREA notes that mortality effect 

estimates for different portions of the New York City core based statistical area (CBSA) vary by 

a factor of almost 10 (U.S. EPA, 2014a, section 7.5.3). 

An additional, related uncertainty is that associated with applying concentration-response 

functions from epidemiologic studies to adjusted air quality. Concentration-response functions 

from the O3 epidemiologic studies used in the HREA are based on associations between day to 

day variation in “area-wide” O3 concentrations (i.e., averaged across multiple monitors) and 

variation in health effects. Epidemiologic studies use these area-wide O3 concentrations, which 

reflect the particular spatial and temporal patterns of ambient O3 present in study locations, as 

surrogates for the pattern of O3 exposures experienced by study populations. To the extent 

adjusting O3 concentrations to just meet the current standard results in important alterations in 

the spatial and/or temporal patterns of ambient O3, there is uncertainty in the appropriateness of 

applying concentration-response functions from epidemiologic studies (which necessarily reflect 

a different air quality distribution than the modelled distribution) to estimate health risks 

associated with adjusted O3 air quality. In particular, this uncertainty could be important to the 

extent that (1) factors associated with space modify the effects of O3 on health or (2) spatial 

mobility is a key driver of individual-level exposures. Although the impact of this uncertainty on 

risk estimates cannot be quantified (U.S. EPA, 2014a, Table 7-4), it has the potential to become 

more important as model adjustment results in larger changes in spatial and temporal patterns of 

ambient O3 concentrations across urban study areas.
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The use of a national concentration-response function to estimate respiratory mortality 

associated with long-term O3 is a source of uncertainty. Risk estimates generated in sensitivity 

analyses using region-specific effect estimates differ substantially from the core estimates based 

on a single national-level effect estimate (U.S. EPA, 2014a; Table 7-14). Furthermore, the risk 

estimates generated using the regional effect estimates display considerable variability across 

urban study areas (U.S. EPA, 2014a; Table 7-14), reflecting the substantial variability in the 

underlying effect estimates (see Jerrett et al., 2009, Table 4). While the results of the HREA 

sensitivity analyses evaluating this uncertainty point to the potential for regional heterogeneity in 

the long-term risk estimates, the relatively large confidence intervals associated with regional 

effect estimates resulted in the HREA conclusion that staff does not have confidence in the 

regionally based risk estimates themselves. 

Finally, the HREA does not quantify any reductions in risk that could be associated with 

reductions in the ambient concentrations of pollutants other than O3, resulting from control of 

NOX. For example, as discussed in chapter 2 of the PA (U.S. EPA, 2014c), NOX emissions 

contribute to ambient NO2, and NOX and VOCs can contribute to secondary formation of PM2.5 

constituents, including ammonium sulfate (NH4SO4), ammonium nitrate (NH4NO3), and organic 

carbon (OC). Therefore, at some times and in some locations, control strategies that would 

reduce NOX emissions (i.e., to meet an O3 standard) could reduce ambient concentrations of NO2

and PM2.5, resulting in health benefits beyond those directly associated with reducing ambient O3

concentrations. In issuing its advice, CASAC likewise noted the potential reductions in criteria 

pollutants other than ozone as a result of NOx reductions, and the resulting potential public 

health benefits (Frey, 2014a, pp. 10 and 11).

D. Conclusions on the Adequacy of the Current Primary Standard
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The initial issue to be addressed in the current review of the primary O3 standard is 

whether, in view of the advances in scientific knowledge and additional information, the existing 

standard should be revised. In evaluating whether it is appropriate to retain or revise the current 

standard, the Administrator’s considerations build upon those in the 2008 review, including 

consideration of the broader body of scientific evidence and exposure and health risk information 

now available, as summarized above (II.A to II.C). 

In developing conclusions on the adequacy of the current primary O3 standard, the 

Administrator takes into account both evidence-based and quantitative exposure- and risk-based 

considerations. Evidence-based considerations include the assessment of evidence from 

controlled human exposure, animal toxicological, and epidemiologic studies for a variety of 

health endpoints. The Administrator focuses on health endpoints for which the evidence is strong 

enough to support a "causal" or a “likely to be causal” relationship, based on the ISA's 

integrative synthesis of the entire body of evidence. The Administrator's consideration of 

quantitative exposure and risk information draws from the results of the exposure and risk 

assessments presented in the HREA.

The Administrator's consideration of the evidence and exposure/risk information is 

informed by the considerations and conclusions presented in the PA (U.S. EPA, 2014c). The 

purpose of the PA is to help "bridge the gap" between the scientific and technical information 

assessed in the ISA and HREA, and the policy decisions that are required of the Administrator

(U.S. EPA, 2014c, Chapter 1). The PA's evidence-based and exposure-/risk-based considerations

and conclusions are summarized below in sections II.D.1 to II.D.3. CASAC advice to the 

Administrator and public commenter views are summarized in section II.D.4. Section II.D.5
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presents the Administrator’s proposed conclusions concerning the adequacy of the public health 

protection provided by the current standard, and her proposed decision to revise that standard. 

1. Summary of Evidence-Based Considerations in the PA

In considering the available scientific evidence, the PA evaluates the O3 concentrations in 

health effects studies (U.S. EPA, 2014c, section 3.1.4). Specifically, the PA characterizes the 

extent to which effects have been reported for the O3 exposure concentrations evaluated in 

controlled human exposure studies and over the distributions of ambient O3 concentrations in 

locations where epidemiologic studies have been conducted. These considerations, as they relate 

to the adequacy of the current standard, are presented in detail in section 3.1.4 of the PA (U.S. 

EPA, 2014c) and are summarized briefly below for controlled human exposure and 

epidemiologic panel studies (II.D.1.a), epidemiologic studies of short-term O3 exposures 

(II.D.1.b), and epidemiologic studies of long-term O3 exposures (II.D.1.c). Section II.D.1.d 

summarizes the PA conclusions based on consideration of the scientific evidence. 

a. Concentrations in controlled human exposure and panel studies

The evidence from controlled human exposure studies and panel studies is assessed in 

section 6.2 of the ISA (U.S. EPA, 2013a) and is summarized in section 3.1.2 of the PA (U.S. 

EPA, 2014c). As discussed above (II.B), controlled human exposure studies have generally been 

conducted with young, healthy adults, and have evaluated exposure durations less than 8 hours. 

Panel studies have evaluated a wider range of study populations, including children, and have 

generally evaluated associations with O3 concentrations averaged over several hours (U.S. EPA, 

2013a, section 6.2.1.2).90

 

90 The PA focuses on panel studies that used on-site monitoring, and that are highlighted in the 

ISA for the extent to which monitored ambient O3 concentrations reflect exposure concentrations 

in their study populations (U.S. EPA, 2013a, section 6.2.1.2).
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As summarized above (II.B), a large number of controlled human exposure studies have 

reported lung function decrements, respiratory symptoms, airway inflammation, AHR, and/or 

impaired lung host defense in young, healthy adults engaged in moderate, intermittent exertion,

following 6.6-hour O3 exposures. These studies have consistently reported such effects following 

exposures to O3 concentrations of 80 ppb or greater. In addition to lung function decrements, 

available studies have also evaluated respiratory symptoms or airway inflammation following 

exposures to O3 concentrations below 75 ppb. Table 3-1 in the PA highlights the group mean 

results of individual controlled human exposure studies that have evaluated exposures of healthy 

adults to O3 concentrations below 75 ppb (U.S. EPA, 2014c). The studies included in Table 3-1 

of the PA indicate a combination of lung function decrements and respiratory symptoms 

following 6.6 hour exposures to O3 concentrations as low as 72 ppb, and lung function 

decrements and airway inflammation following 6.6 hour exposures to O3 concentrations as low 

as 60 ppb (based on group means). 

The PA also notes consistent results in some panel studies of O3-associated lung function 

decrements. In particular, the PA notes that epidemiologic panel studies in children and adults 

consistently indicate O3-associated lung function decrements when on-site monitored 

concentrations were below 75 ppb, although the evidence becomes less consistent at lower O3

concentrations (U.S. EPA, 2014c, section 3.1.4.1).91

Thus, controlled human exposure studies and panel studies have reported respiratory 

effects in adults and children following exposures to O3 concentrations below 75 ppb (albeit over 

shorter averaging periods than the 8 hour averaging time of the current O3 standard). The PA 

 

91 As indicated in the PA (U.S. EPA, 2014c, Table 3-2), key O3 panel studies evaluated 

averaging periods ranging from 10 minutes to 12 hours. 
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notes that such impairments in respiratory function have the potential to be adverse, based on 

ATS guidelines for adversity and based on advice from CASAC (Frey, 2014c, pp. 5 and 6) (U.S. 

EPA, 2014c, section 3.1.3). In addition, the PA notes that if they become serious enough, these 

respiratory effects could lead to the types of clearly adverse effects commonly reported in O3

epidemiologic studies (e.g., respiratory emergency department visits, hospital admissions). 

Therefore, the PA concludes that the respiratory effects experienced following exposures to O3

concentrations lower than 75 ppb could be adverse in some individuals, particularly if 

experienced by members of at-risk populations (e.g., people with asthma, children).92

b. Concentrations in epidemiologic studies – short-term

The PA also considers distributions of ambient O3 concentrations in locations where 

epidemiologic studies have evaluated O3-associated hospital admissions, emergency department 

visits, and/or mortality (U.S. EPA, 2014c, section 3.1.4.2). When considering epidemiologic 

studies within the context of the current standard, the PA emphasizes those studies conducted in 

the U.S. and Canada. Such studies reflect air quality and exposure patterns that are likely more 

typical of the U.S. population than the air quality and exposure patterns reflected in studies 

conducted outside the U.S. and Canada (U.S. EPA, 2014c, section 1.3.1.2).

93 The PA also 

emphasizes studies reporting associations with effects judged in the ISA to be robust to 

confounding by other factors, including co-occurring air pollutants. In addition to these factors, 

the PA considers the statistical precision of study results, the extent to which studies report 

 

92 These effects were reported in healthy individuals. Consistent with CASAC advice (Samet, 

2011; Frey, 2014a, p. 14; Frey, 2014c, p. 7), it is a reasonable inference that the effects would be 

greater in magnitude and potential severity for at-risk groups. See National Environmental 

Development Ass’n Clean Air Project v. EPA, 686 F. 3d 803, 811 (D.C. Cir. (2012) (making this 

point). 

93 Nonetheless, the PA recognizes the importance of all studies, including international studies, 

in the ISA’s assessment of the weight of the evidence that informs causality determinations.
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associations in at-risk populations, and the extent to which the biological plausibility of 

associations at various ambient O3 concentrations is supported by controlled human exposure 

and/or animal toxicological studies. These considerations help inform the range of ambient O3

concentrations over which the evidence indicates the most confidence in O3-associated health 

effects, and the range of concentrations over which confidence in such associations is 

appreciably lower.

This section summarizes the PA conclusions regarding the extent to which health effect 

associations have been reported for ambient O3 concentrations likely to have met the current O3

standard. Section II.D.1.b.i summarizes PA analyses and conclusions based on analyses 

evaluating the extent to which epidemiologic studies have reported health effect associations in 

locations that would likely have met the current O3 standard. Section II.D.1.b.ii summarizes PA 

conclusions based on analyses evaluating the O3 air quality in locations where epidemiologic 

studies have characterized confidence intervals around cut point analyses or concentrationresponse functions. Section II.D.1.b.iii summarizes the important uncertainties in these analyses. 

i. Associations in locations likely meeting current standard

The PA considers the extent to which U.S. and Canadian epidemiologic studies have 

reported associations with mortality or morbidity in locations that would likely have met the 

current O3 standard during the study period (U.S. EPA, 2014c, section 3.14.2). Addressing this 

issue can provide important insights into the extent to which O3-health effect associations are 

present for distributions of ambient O3 concentrations that would be allowed by the current 

standard. To the extent associations are reported in study areas that would have met the current 

standard, those associations indicate that the current standard could allow the types of clearly 

adverse O3-associated effects reported in epidemiologic studies (e.g., mortality, hospital 
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admissions, emergency department visits).94 In considering these analyses, the PA also notes that 

the lack of such associations in locations meeting the current standard indicates increased

uncertainty in the extent to which O3-associated health effects would persist upon reducing O3

precursor emissions in order to meet that standard.

The PA identifies U.S. and Canadian studies of respiratory hospital admissions, 

respiratory emergency department visits, and mortality (total, respiratory, cardiovascular) from 

the ISA (U.S. EPA, 2013a, Tables 6-28, 6-42, and 6-53, and section 6.2.8; U.S. EPA, 2014c, 

Appendix 3D). Analysis of study area air quality indicates that the large majority of 

epidemiologic study areas evaluated would have violated the current standard during study 

periods (U.S. EPA, 2014c, Appendix 3D). However, the PA notes that a single-city study 

conducted in Seattle, a location that would have met the current standard over the entire study 

period, reported positive and statistically significant associations with respiratory emergency 

department visits in children and adults (Mar and Koenig, 2009). The PA also notes four 

Canadian multicity studies that reported positive and statistically significant associations with 

respiratory morbidity or mortality, and for which the majority of study cities would have met the 

current standard over the entire study periods (Cakmak et al., 2006; Dales et al., 2006; 

Katsouyanni et al., 2009; Stieb et al., 2009).95

The PA concludes that the single-city study by Mar and Koenig (2009) indicates the 

presence of associations with mortality and morbidity for an ambient distribution of O3 that 

 

94 See ATA III, 283 F.3d at 370 (EPA justified in revising NAAQS when health effect 

associations are observed in epidemiologic studies at levels allowed by the NAAQS); State of 

Mississippi v. EPA, 744 F. 3d at 1345 (same).

95 In addition, a study by Vedal et al. (2003) was included in the 2006 AQCD (U.S. EPA, 2006a). 

This study reported positive and statistically significant associations with mortality in Vancouver 

during a time period when the study area would have met the current standard (U.S. EPA, 2007). 

This study was not assessed in the ISA in the current review (U.S. EPA, 2013a).
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would have met the current standard (U.S. EPA, 2014c, section 3.1.4.2). The PA notes that 

interpretation of the air quality concentrations in the multicity study locations evaluated in this 

review is complicated by uncertainties in the extent to which multicity effect estimates can be 

attributed to ambient O3 in the majority of locations, which would have met the current standard, 

versus O3 in the smaller number of locations that would have violated the standard. While 

acknowledging this uncertainty in interpreting air quality in multicity studies, the PA notes that 

multicity effect estimates in the four studies cited above are largely influenced by locations 

meeting the current standard (i.e., given that most study areas would have met this standard). 

Therefore, the PA concludes that Canadian multicity studies, in addition to the single-city study 

in Seattle, suggest confidence in the presence of associations with mortality and morbidity for 

ambient distributions of O3 that would have met the current standard (U.S. EPA, 2014c, section 

3.1.4.2).

ii. Air quality associated with cut point analyses and concentration-response functions 

The PA also considers the extent to which additional epidemiologic studies of mortality 

or morbidity, specifically those conducted in locations that would have violated the current 

standard, can inform consideration of adequacy of the current standard (U.S. EPA, 2014c, 

section 3.1.4.2). In doing so, the PA notes that health effect associations reported in 

epidemiologic studies are influenced by the full distributions of ambient O3 concentrations, 

including concentrations below the level of the current standard. The PA focuses on studies that 

have explicitly characterized O3 health effect associations, including confidence in those 

associations, for various portions of distributions of ambient O3 concentrations.

The U.S. multicity study by Bell et al. (2006) reported health effect associations for air 

quality subsets restricted to ambient O3 concentrations below one or more predetermined cut 
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points. In these analyses, effect estimates were based only on the subsets of days contributing to 

averaged O3 concentrations below cut points ranging from 5 to 60 ppb (Bell et al., 2006, Figure 

2).96 The PA notes that such “cut point” analyses can provide information on the magnitude and 

statistical precision of effect estimates for defined distributions of ambient concentrations, which 

may in some cases include distributions that would meet the current standard (U.S. EPA, 2014c, 

section 3.1.4.2). The cut points below which confidence intervals become notably wider depend 

in large part on data density and, therefore, cut point analyses provide insight into the ambient 

concentrations below which the available air quality information becomes too sparse to support 

conclusions about the nature of concentration-response relationships with a high degree of 

confidence (U.S. EPA, 2014c, section 3.1.4.2). 

The PA considers the extent to which the cut-point analyses reported by Bell et al. (2006) 

indicate health effect associations for distributions of ambient O3 concentrations that would 

likely have met the current standard. The PA particularly focuses on the lowest cut-point for 

which the association between O3 and mortality was reported to be statistically significant (i.e., 

30 ppb, based on visual inspection of Figure 2 in the published study). Based on the O3 air 

quality concentrations that met the criteria for inclusion in the 30 ppb cut point analysis, 95% of 

study areas had 3-year averages of annual 4th highest daily maximum 8-hour O3 concentration at 

or below 75 ppb over the entire study period (U.S. EPA, 2014c, section 3.1.4.2, Table 3-6). 

Though there are important uncertainties in this analysis, as discussed below, the PA concludes 

that these results suggest that the large majority of air quality distributions that provided the basis 

 

96 In the published study, 2-day rolling averages of 24-hour average O3 concentrations were 

calculated in each study location (based on averaging across monitors in study locations with 

multiple monitors). 
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for the positive and statistically significant association with mortality at the 30 ppb cut point 

would likely have met the current O3 standard. 

The PA also analyzes air quality for studies that have reported confidence intervals 

around concentration-response functions over distributions of ambient O3 concentrations (U.S. 

EPA, 2014c, section 3.1.4.2). Confidence intervals around concentration-response functions can 

provide insights into the range of ambient concentrations over which the study indicates the most 

confidence in the reported health effect associations (i.e., where confidence intervals are 

narrowest), and into the range of ambient concentrations below which the study indicates that 

uncertainty in the nature of such associations becomes notably greater (i.e., where confidence 

intervals become markedly wider). As with cut point analyses, the concentrations below which 

confidence intervals become markedly wider are intrinsically related to data density, and do not 

necessarily indicate the absence of an association. 

The PA focuses on two U.S. single-city studies that have reported confidence intervals 

around concentration-response functions (Silverman and Ito, 2010; Strickland et al., 2010). 

Based on the published analyses, the PA identifies the ranges of ambient O3 concentrations over 

which these studies indicate the highest degree of confidence in the reported linear 

concentration-response functions (U.S. EPA, 2014c, section 3.1.4.2). For the lower ends of these

ranges, air quality analyses in the PA indicate that over 99% of days had maximum 8-hour O3

concentrations (i.e., from highest monitors in study locations) at or below 75 ppb. For 

comparison, the annual 4

th highest daily maximum 8-hour O3 concentration generally 

corresponds to the 98th or 99th percentile of the seasonal distribution, depending on the length of 

the O3 season.
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The PA concludes that these analyses of air quality data from the study locations 

evaluated by Silverman and Ito (2010) and Strickland et al. (2010) indicate a relatively high 

degree of confidence in reported statistical associations with respiratory health outcomes on days 

when virtually all monitored 8-hour O3 concentrations were 75 ppb or below (U.S. EPA, 2014c, 

section 3.1.4.2). Though these analyses do not identify true design values, the presence of O3-

associated respiratory effects on such days provides insight into the types of health effects that 

could occur in locations with maximum ambient O3 concentrations at or below the level of the 

current standard. 

iii. Important Uncertainties

In considering the above evidence within the context of developing overall conclusions 

on the current and potential alternative standards, the PA also takes into account important 

uncertainties in these analyses of air quality in locations of epidemiologic study areas. These 

uncertainties are summarized in this section. The PA’s consideration of the evidence, including 

the associated uncertainties, in reaching conclusions on the current and potential alternative 

standards is summarized in sections II.D.3 (current standard) and II.E.4.b (potential alternative 

standards) below. 

The PA notes that while multicity studies generally have greater statistical power and 

geographic coverage than single-city studies, there is often greater uncertainty in conclusions 

about the extent to which multicity effect estimates reflect associations with air quality meeting 

the current standard (U.S. EPA, 2014c, section 1.3.1.2.1). This is particularly the case for the 

multicity studies evaluated in this review with some study locations meeting the current standard 

and others violating that standard. Specifically for the four Canadian multicity studies discussed 

above, the PA notes that interpretation of air quality information is complicated by uncertainties 
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in the extent to which multicity effect estimates can be attributed to ambient O3 in the majority of 

locations, which would have met the current standard, versus O3 in the smaller number of 

locations that would have violated the standard. 

The PA also notes important uncertainties in multicity studies that evaluate the potential 

for thresholds to exist, as was done in the study by Bell et al. (2006). Specifically, the ISA 

highlights the regional heterogeneity in O3 health effect associations as a factor that could 

obscure the presence of thresholds, should they exist, in multicity studies (U.S. EPA, 2013a, 

sections 2.5.4.4 and 2.5.4.5). The ISA notes that community characteristics (e.g., activity 

patterns, housing type, age distribution, prevalence of air conditioning) could be important 

contributors to reported regional heterogeneity (U.S. EPA, 2013a, section 2.5.4.5). Given this 

heterogeneity, the ISA concludes that “a national or combined analysis may not be appropriate to 

identify whether a threshold exists in the O3-mortality [concentration-response] relationship” 

(U.S. EPA, 2013a, p. 2-33). This represents an important source of uncertainty when 

characterizing confidence in reported concentration-response relationships over distributions of 

ambient O3 concentrations, based on multicity studies. The PA notes that this uncertainty 

becomes increasingly important when interpreting concentration-response relationships at lower 

ambient O3 concentrations, particularly those concentrations corresponding to portions of 

distributions where data density decreases notably (U.S. EPA, 2014c, section 3.1.4.2).

Another important uncertainty, related specifically to the PA analysis of cut points by 

Bell et al. (2006), is that EPA staff was unable to obtain the air quality data used to generate the 

cut-point analyses in the published study (U.S. EPA, 2014c, section 3.1.4.2). Therefore, the 

analyses in the PA identified 2-day averages of 24-hour O3 concentrations in study locations 

using the air quality data available in AQS, combined with the published description of study 
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area definitions. An important uncertainty in this approach is the extent to which the PA

appropriately recreated the cut-point analyses in the published study (U.S. EPA, 2014c, section 

3.1.4.2). 

An uncertainty that applies to epidemiologic studies in general is the extent to which 

reported health effects are caused by exposures to O3 itself, as opposed to other factors such as 

co-occurring pollutants or pollutant mixtures. The PA notes that this uncertainty becomes an 

increasingly important consideration as health effect associations are evaluated at lower ambient 

O3 concentrations. In particular, there is increasing uncertainty as to whether the observed 

associations remain plausibly related to exposures to ambient O3, rather than to the broader mix 

of air pollutants present in the ambient air. In considering the potential importance of this 

uncertainty at the relatively low ambient O3 concentrations that are the focus of the PA analyses, 

the PA notes that Silverman and Ito (2010) and Strickland (2010) reported O3 health effect 

associations in co-pollutant models,97 providing support for associations with O3 itself (U.S. 

EPA, 2014c, section 3.1.4.2). The PA also concludes that air quality analyses indicate coherence 

with the results of experimental studies (i.e., in which the study design dictates that exposures to 

O3 itself are responsible for reported effects), and are consistent with the occurrence of O3-

attributable respiratory hospital admissions and emergency department visits, even when 

virtually all monitored concentrations were below the level of the current standard (U.S. EPA, 

2014c, section 3.1.4.2, Tables 3-4, 3-5). 

c. Concentrations in epidemiologic studies – long-term

 

97 In addition, Bell et al. (2006) reported that, based on a previous study (Bell et al., 2004), 

associations with mortality were robust to the inclusion of PM10 in the model. 
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The PA also considers the extent to which epidemiologic studies employing longer-term 

ambient O3 concentration metrics inform our understanding of the air quality conditions 

associated with O3-attributable health effects, and specifically inform consideration of the extent 

to which such effects could occur under air quality conditions meeting the current standard (U.S. 

EPA, 2014c, section 3.1.4.3). Unlike for the studies of short-term O3 discussed above, the 

available U.S. and Canadian epidemiologic studies evaluating long-term ambient O3

concentration metrics have not been conducted in locations likely to have met the current 8-hour 

O3 standard during the study period, and have not reported concentration-response functions that 

indicate confidence in health effect associations at O3 concentrations meeting the current 

standard (U.S. EPA, 2014c, section 3.1.4.3). Therefore, although these studies contribute to 

understanding of health effects associated with long-term or repeated exposures to ambient O3, 

consideration of study area air quality does not inform consideration of the extent to which those 

health effects may be occurring in locations that meet the current standard. 

d. PA conclusions based on consideration of the evidence

As discussed above (II.D.1.a to II.D.1.c), in considering the available scientific evidence, 

including associated uncertainties, as it relates to the degree of public health protection provided 

by the current primary O3 standard, the PA evaluates the extent to which health effects have been 

reported for the O3 exposure concentrations evaluated in controlled human exposure studies and 

over the distributions of ambient O3 concentrations in locations where epidemiologic studies 

have been conducted. The PA concludes that (1) the evidence from controlled human exposure 

studies provides strong support for the occurrence of adverse respiratory effects following 

exposures to O3 concentrations below the level of the current standard and that (2) epidemiologic 

studies provide support for the occurrence of adverse respiratory effects and mortality under air 
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quality conditions that would likely meet the current standard. In further considering the public 

health protection provided by the current standard, the PA next considers the results of exposure 

and health risk assessments. 

2. Summary of Exposure- and Risk-Based Considerations in the PA

In order to further inform judgments about the potential public health implications of the 

current O3 NAAQS, the PA considers the exposure and risk assessments presented in the HREA 

(U.S. EPA, 2014c, section 3.2). Overviews of these exposure and risk assessments, including 

summaries of key results and uncertainties, are provided in section II.C above. This section 

summarizes key observations from the PA related to the adequacy of the current O3 NAAQS, 

based on consideration of the HREA exposure assessment (II.D.2.a), lung function risk 

assessment (II.D.2.b), and mortality/morbidity risk assessments (II.D.2.c). 

a. Exposure assessment – key observations

As discussed above (II.C.2), the exposure assessment provides estimates of the number 

and percent of people who would experience exposures of concern at or above benchmark 

concentrations of 60, 70, and 80 ppb. Benchmarks reflect exposure concentrations at which O3-

induced respiratory effects are known to occur in some healthy adults engaged in moderate, 

intermittent exertion, based on evidence from controlled human exposure studies (U.S. EPA, 

2014c, section 3.1.2.1; U.S. EPA, 2013a, section 6.2). 

The PA focuses on exposure estimates in children. Compared to recent (i.e., unadjusted) 

air quality, the PA notes that adjusting air quality to just meet the current O3 NAAQS 

consistently reduces the estimated occurrence of exposures of concern in children (U.S. EPA, 

2014a, Appendix 5F). When averaged over the years evaluated in the HREA, reductions of up to 

about 70% were estimated. These reductions in estimated exposures of concern, relative to 
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unadjusted air quality, reflect the consistent reductions in the highest ambient O3 concentrations 

upon model adjustment to just meet the current standard (U.S. EPA, 2014c, section 3.2.1; U.S. 

EPA, 2014a, Chapter 4). Such reductions in estimated exposures of concern are evident 

throughout urban study areas, including in urban cores and in surrounding areas (U.S. EPA, 

2014a, Appendix 9A). 

Based on Figures 3-7 to 3-10 in the PA (U.S. EPA, 2014c), and the associated details 

described in the HREA (U.S. EPA, 2014a, Chapter 5), the PA further highlights key observations 

with regard to exposures of concern in children that are estimated to be allowed by the current 

standard. These key observations are summarized below for exposures of concern > 60, 70, and 

80 ppb. 

For exposures of concern at or above 60 ppb, the PA highlights the following key 

observations for air quality adjusted to just meet the current standard: 

(1) On average over the years 2006 to 2010, the current standard is estimated to allow 

approximately 10 to 18% of children in urban study areas to experience one or more

exposures of concern at or above 60 ppb. Summing across urban study areas, these 

percentages correspond to almost 2.5 million children experiencing approximately 4 

million exposures of concern at or above 60 ppb during a single O3 season. Of these 

children, almost 250,000 are asthmatics.

98

(2) On average over the years 2006 to 2010, the current standard is estimated to allow 

approximately 3 to 8% of children in urban study areas to experience two or more 

exposures of concern to O3 concentrations at or above 60 ppb. Summing across the urban

 

98 As discussed above (II.C.2.b), due to variability in responsiveness, only a subset of individuals 

who experience exposures at or above a benchmark concentration can be expected to experience 

adverse health effects. 
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study areas, these percentages correspond to almost 900,000 children (including almost 

90,000 asthmatic children) estimated to experience at least two O3 exposure 

concentrations at or above 60 ppb during a single O3 season. 

(3) In the worst-case years (i.e., those with the largest exposure estimates), the current 

standard is estimated to allow approximately 10 to 25% of children to experience one or 

more exposures of concern at or above 60 ppb, and approximately 4 to 14% to experience 

two or more exposures of concern at or above 60 ppb. 

For exposures of concern at or above 70 ppb, the PA highlights the following key observations

for air quality adjusted to just meet the current standard: 

(1) On average over the years 2006 to 2010, the current standard is estimated to allow up to 

approximately 3% of children in urban study areas to experience one or more exposures

of concern at or above 70 ppb. Summing across urban study areas, almost 400,000

children (including almost 40,000 asthmatic children) are estimated to experience O3

exposure concentrations at or above 70 ppb during a single O3 season.99

(2) On average over the years 2006 to 2010, the current standard is estimated to allow less 

than 1% of children in urban study areas to experience two or more exposures of concern 

to O3 concentrations at or above 70 ppb. 

(3) In the worst-case years, the current standard is estimated to allow approximately 1 to 8% 

of children to experience one or more exposures of concern at or above 70 ppb, and up to 

approximately 2% to experience two or more exposures of concern, at or above 70 ppb.

 

99 As discussed above (II.C.2.b), due to variability in responsiveness, only a subset of individuals 

who experience exposures at or above a benchmark concentration can be expected to experience 

adverse health effects. 
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For exposures of concern at or above 80 ppb, the PA highlights the observation that the current 

standard is estimated to allow about 1% or fewer children in urban study areas to experience 

exposures of concern at or above 80 ppb, even in years with the highest exposure estimates. 

b. Lung function risk assessment – key observations

As discussed above (II.C.3.a), the HREA estimates risks of moderate to large lung 

function decrements (i.e., FEV1 decrements > 10%, 15%, or 20%) in school-aged children (ages 

5 to 18), asthmatic school-aged children, and the general adult population for 15 urban study 

areas. As for exposures of concern, the PA focuses on lung function risk estimates in children

(including children with asthma). 

Compared to risks associated with recent air quality, risk estimates for air quality just 

meeting the current standard are consistently smaller across urban study areas (U.S. EPA, 2014a, 

Appendix 6B). When averaged over the years evaluated in the HREA, risk reductions of up to 

about 40% were estimated compared to recent air quality. These reductions reflect the consistent 

decreases in relatively high ambient O3 concentrations upon adjustment to just meet the current 

standard (U.S. EPA, 2014a, Chapter 4). Such reductions in estimated lung function risks are 

evident throughout urban study areas, including in urban cores and in surrounding areas (U.S. 

EPA, 2014, Appendix 9A).

Based on Figures 3-11 to 3-14 in the PA (U.S. EPA, 2014c), and the associated details 

described in the HREA (U.S. EPA, 2014a, chapter 6), the PA highlights key observations with 

regard to lung function risks estimated in children for air quality adjusted to just meet the current 

standard. These key observations are presented below for FEV1 decrements > 10, 15, and 20%. 

With regard to decrements > 10%, the PA highlights the following key observations for 

air quality adjusted to just meet the current standard: 
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(1) On average over the years 2006 to 2010, the current standard is estimated to allow 

approximately 14 to 19% of children in urban study areas to experience one or more lung 

function decrements > 10%. Summing across urban study areas, this corresponds to 

approximately 3 million children experiencing 15 million O3-induced lung function 

decrements > 10% during a single O3 season. Of these children, about 300,000 are 

asthmatics. 

(2) On average over the years 2006 to 2010, the current standard is estimated to allow 

approximately 7 to 12% of children in urban study areas to experience two or more O3-

induced lung function decrements > 10%. Summing across the urban study areas, this 

corresponds to almost 2 million children (including almost 200,000 asthmatic children) 

estimated to experience two or more O3-induced lung function decrements greater than 

10% during a single O3 season.

(3) In the worst-case years, the current standard is estimated to allow approximately 17 to 

23% of children in urban study areas to experience one or more lung function decrements 

> 10%, and approximately 10 to 14% to experience two or more O3-induced lung 

function decrements > 10%.

With regard to decrements > 15%, the PA highlights the following key observations for air 

quality adjusted to just meet the current standard: 

(1) On average over the years 2006 to 2010, the current standard is estimated to allow 

approximately 3 to 5% of children in urban study areas to experience one or more lung 

function decrements > 15%. Summing across urban study areas, this corresponds to 

approximately 800,000 children (including approximately 80,000 asthmatic children)
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estimated to experience at least one O3-induced lung function decrement > 15% during a 

single O3 season. 

(2) On average over the years 2006 to 2010, the current standard is estimated to allow 

approximately 2 to 3% of children in urban study areas to experience two or more O3-

induced lung function decrements > 15%. 

(3) In the worst-case years, the current standard is estimated to allow approximately 4 to 6% 

of children in urban study areas to experience one or more lung function decrements >

15%, and approximately 2 to 4% to experience two or more O3-induced lung function 

decrements > 15%.

With regard to decrements > 20%, the PA highlights the following key observations for air 

quality adjusted to just meet the current standard: 

(1) On average over the years 2006 to 2010, the current standard is estimated to allow 

approximately 1 to 2% of children in urban study areas to experience one or more lung 

function decrements > 20%. Summing across urban study areas, this corresponds to 

approximately 300,000 children (including approximately 30,000 asthmatic children)

estimated to experience at least one O3-induced lung function decrement > 20% during a 

single O3 season. 

(2) On average over the years 2006 to 2010, the current standard is estimated to allow less 

than 1% of children in urban study areas to experience two or more O3-induced lung 

function decrements > 20%. 

(3) In the worst-case years, the current standard is estimated to allow approximately 2 to 3% 

of children to experience one or more lung function decrements > 20%, and less than 2%

to experience two or more O3-induced lung function decrements > 20%.
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c. Mortality and morbidity risk assessments – key observations

As discussed above (II.C.3.b), risk estimates based on epidemiologic studies can provide 

perspective on the most serious O3-associated public health outcomes (e.g., mortality, hospital 

admissions, emergency department visits) in populations that often include at-risk groups. The 

HREA estimates such O3-associated risks in 12 urban study areas100 using concentrationresponse relationships drawn from epidemiologic studies. These concentration-response 

relationships are based on “area-wide” average O3 concentrations.101 The HREA estimates risks 

for the years 2007 and 2009 in order to provide estimates of risk for a year with generally higher 

O3 concentrations (2007) and a year with generally lower O3 concentrations (2009) (U.S. EPA, 

2014a, section 7.1.1).

In considering these estimates, the PA notes that HREA conclusions reflect somewhat 

lower confidence in epidemiologic-based risk estimates than in estimates of O3 exposures of 

concern and O3-induced lung function decrements (U.S. EPA, 2014a, section 9.6). In particular, 

the HREA highlights the unexplained heterogeneity in effect estimates between locations, the 

potential for exposure measurement errors, and uncertainty in the interpretation of the shape of 

concentration-response functions at lower O3 concentrations (U.S. EPA, 2014a, section 9.6). The 

 

100 The 12 urban areas evaluated are Atlanta, Baltimore, Boston, Cleveland, Denver, Detroit, 

Houston, Los Angeles, New York, Philadelphia, Sacramento, and St. Louis. Morbidity endpoints 

were evaluated in subsets of these areas, based on availability of appropriate studies (U.S. EPA, 

2014a, Chapter 7). 

101 In the epidemiologic studies that provide the health basis for HREA risk assessments, 

concentration-response relationships are based on daytime O3 concentrations, averaged across 

multiple monitors within study areas. These daily averages are used as surrogates for the spatial 

and temporal patterns of exposures in study populations. Consistent with this approach, the 

HREA epidemiologic-based risk estimates also utilize daytime O3 concentrations, averaged 

across monitors, as surrogates for population exposures. In this notice, these averaged 

concentrations are referred to as “area-wide” O3 concentrations. Area-wide concentrations are 

discussed in more detail in section 3.1.4 of the PA (U.S. EPA, 2014c).
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HREA also concludes that lower confidence should be placed in the results of the assessment of 

respiratory mortality risks associated with long-term O3 exposures, primarily because that 

analysis is based on only one study, though that study is well-designed, and because of the 

uncertainty in that study about the existence and identification of a potential threshold in the 

concentration-response function (U.S. EPA, 2014a, section 9.6). These and other uncertainties 

are considered in the PA in reaching conclusions on the current and alternative standards (U.S. 

EPA, 2014c, sections 3.4, 4.6). 

Key observations from the PA are summarized below for mortality and morbidity risks 

associated with air quality adjusted to simulate just meeting the current O3 NAAQS. These 

include key observations for estimates of total (nonaccidental) mortality associated with shortterm O3 concentrations, respiratory morbidity associated with short-term O3 concentrations, and 

respiratory mortality associated with long-term O3 concentrations (U.S. EPA, 2014c, section 

3.2.3.2).

With regard to total mortality or morbidity associated with short-term O3, the PA notes 

the following for air quality adjusted to just meet the current standard:

(1) When air quality was adjusted to the current standard for the 2007 model year (the year 

with generally “higher” O3-associated risks), 10 of 12 urban study areas exhibited either 

decreases or virtually no change in estimates of the number of O3-associated deaths (U.S. 
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EPA, 2014a, Appendix 7B). Increases were estimated in two of the urban study areas

(Houston, Los Angeles)102 (U.S. EPA, 2014a, Appendix 7B).103

(2) In focusing on total risk, the current standard is estimated to allow thousands of O3-

associated deaths per year in the urban study areas. In focusing on the risks associated 

with the upper portions of distributions of ambient concentrations (area-wide 

concentrations > 40, 60 ppb), the current standard is estimated to allow hundreds to 

thousands of O3-associated deaths per year in the urban study areas. 

(3) The current standard is estimated to allow tens to thousands of O3-associated morbidity 

events per year (i.e., respiratory-related hospital admissions, emergency department 

visits, and asthma exacerbations). 

 

102 As discussed above (II.C.1), in locations and time periods when NOX is predominantly 

contributing to O3 formation (e.g., downwind of important NOX sources, where the highest O3

concentrations often occur), model-based adjustment to the current and alternative standards 

decreases estimated ambient O3 concentrations compared to recent monitored concentrations 

(U.S. EPA, 2014a, section 4.3.3.2). In contrast, in locations and time periods when NOX is 

predominantly contributing to O3 titration (e.g., in urban centers with high concentrations of 

NOX emissions, where ambient O3 concentrations are often suppressed and thus relatively low), 

model-based adjustment increases ambient O3 concentrations compared to recent monitored 

concentrations (U.S. EPA, 2014a, section 4.3.3.2). Changes in epidemiology-based risk estimates 

depend on the balance between the daily decreases in high O3 concentrations and increases in 

low O3 concentrations following the model-based air quality adjustment. Commenting on this 

issue, CASAC noted that “controls designed to reduce the peak levels of ozone (e.g., the 4th

highest annual MDA8) may not be effective at reducing lower levels of ozone on more typical 

days and may actually increase ozone levels on days where ozone concentrations are low” (Frey 

2014a, p. 2). CASAC further noted that risk results “suggest that the ozone-related health risks in 

the urban cores can increase for some of the cities as ozone NAAQS alternatives become more 

stringent. This is because reductions in nitrogen oxides emissions can lead to less scavenging of 

ozone and free radicals, resulting in locally higher levels of ozone” (Frey 2014c, p. 10). 

103 For the 2009 adjusted year (i.e., the year with generally lower O3 concentrations), changes in 

risk were generally smaller than in 2007 (i.e., most changes about 2% or smaller). Increases were 

estimated for Houston, Los Angeles, and New York City. 
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With regard to respiratory mortality associated with long-term O3, the PA notes the 

following for air quality adjusted to just meet the current standard: 

(1) Based on a linear concentration-response function, the current standard is estimated to 

allow thousands of O3-associated respiratory deaths per year in the urban study areas. 

(2) Based on threshold models, HREA sensitivity analyses indicate that the number of 

respiratory deaths associated with long-term O3 concentrations could potentially be 

considerably lower (i.e., by more than 75% if a threshold exists at 40 ppb, and by about 

98% if a threshold exists at 56 ppb) (U.S. EPA, 2014a, Figure 7-9).

104

3. Policy Assessment Conclusions on the Current Standard

As an initial matter, the PA concludes that reducing precursor emissions to achieve O3 

concentrations that meet the current standard will provide important improvements in public 

health protection. This initial conclusion is based on (1) the strong body of scientific evidence 

indicating a wide range of adverse health outcomes attributable to exposures to O3

concentrations commonly found in the ambient air and (2) estimates indicating decreased 

occurrences of O3 exposures of concern and decreased health risks upon meeting the current 

standard, compared to recent air quality. 

 

104 Risk estimates for respiratory mortality associated with long-term O3 exposures are based on 

the study by Jerrett et al. (2009) (U.S. EPA, 2014a, Chapter 7). As discussed above (II.B.2.b.iv) 

and in the PA (U.S. EPA, 2014c, section 3.1.4.3), Jerrett et al. (2009) reported that when 

seasonal averages of 1-hour daily maximum O3 concentrations ranged from 33 to 104 ppb, there 

was no statistical deviation from a linear concentration-response relationship between O3 and 

respiratory mortality across 96 U.S. cities (U.S. EPA, 2013a, section 7.7). However, the authors 

reported “limited evidence” for an effect threshold at an O3 concentration of 56 ppb (p=0.06). In 

communications with EPA staff (Sasser, 2014), the study authors indicated that it is not clear 

whether a threshold model is a better predictor of respiratory mortality than the linear model, and 

that “considerable caution should be exercised in accepting any specific threshold.”
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In particular, the PA concludes that strong support for this initial conclusion is provided 

by controlled human exposure studies of respiratory effects, and by quantitative estimates of 

exposures of concern and lung function decrements based on information in these studies. 

Analyses in the HREA estimate that the percentages of children (i.e., all children and children 

with asthma) in urban study areas experiencing exposures of concern, or experiencing abnormal 

and potentially adverse lung function decrements, are consistently lower for air quality that just 

meets the current O3 standard than for recent air quality. The HREA estimates such reductions 

consistently across the urban study areas evaluated and throughout various portions of individual 

urban study areas, including in urban cores and the portions of urban study areas surrounding 

urban cores. These reductions in exposures of concern and O3-induced lung function decrements 

reflect the consistent decreases in the highest O3 concentrations following reductions in precursor 

emissions to meet the current standard. Thus, populations in both urban and non-urban areas 

would be expected to experience important reductions in O3 exposures and O3-induced lung 

function risks upon meeting the current standard.105

The PA further concludes that support for this initial conclusion is also provided by 

estimates of O3-associated mortality and morbidity based on application of concentrationresponse relationships from epidemiologic studies to air quality adjusted to just meet the current 

standard. These estimates, which are based on the assumption that concentration-response 

relationships are linear over entire distributions of ambient O3 concentrations, are associated with 

uncertainties that complicate their interpretation (II.C.3). However, risk estimates for effects 

associated with short- and long-term O3 exposures, combined with the HREA’s national analysis 

 

105 As discussed above (II.C.1), CASAC recommended that the EPA evaluate how health risks in 

urban centers, as well as outside urban centers, change upon reducing NOX emissions, given the 

varying impacts of NOX emissions reductions on ambient O3 concentrations. 
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of O3 responsiveness to reductions in precursor emissions and the consistent reductions estimated 

for the highest ambient O3 concentrations, suggest that O3-associated mortality and morbidity 

would be expected to decrease nationwide following reductions in precursor emissions to meet 

the current O3 standard.

Reductions in O3 precursor emissions (i.e., NOX) could also increase public health 

protection by reducing the ambient concentrations of pollutants other than O3. For example, in 

their advice on the second draft HREA CASAC acknowledged the potential for ambient NO2

concentrations to be affected by changes in NOX emissions (Frey, 2014a, p. 10). Consistent with 

this, the PA notes that NOX emissions contribute to ambient NO2, and that NOX and VOCs can 

contribute to secondary formation of PM2.5 constituents, including ammonium sulfate (NH4SO4), 

ammonium nitrate (NH4NO3), and organic carbon (OC). Therefore, at some times and in some 

locations, control strategies that would reduce NOX emissions (i.e., to meet an O3 standard) could 

reduce ambient concentrations of NO2 and PM2.5, resulting in health benefits beyond those 

directly associated with reducing ambient O3 concentrations.

After reaching the initial conclusion that meeting the current primary O3 standard will 

provide important improvements in public health protection, and that it is not appropriate to 

consider a standard that is less protective than the current standard, the PA considers the 

adequacy of the public health protection that is provided by the current standard. In considering 

the available scientific evidence, exposure/risk information, advice from CASAC (II.D.4, 

below), and input from the public, the PA reaches the conclusion that the available evidence and 

information clearly call into question the adequacy of public health protection provided by the 

current primary standard. In reaching this conclusion, the PA notes that evidence from controlled 

human exposure studies provides strong support for the occurrence of adverse respiratory effects 
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following exposures to O3 concentrations below the level of the current standard. Epidemiologic 

studies provide support for the occurrence of adverse respiratory effects and mortality under air 

quality conditions that would likely meet the current standard. In addition, based on the analyses 

in the HREA, the PA concludes that the exposures and risks projected to remain upon meeting 

the current standard are indicative of risks that can reasonably be judged to be important from a 

public health perspective. Thus, the PA concludes that the evidence and information provide 

strong support for giving consideration to revising the current primary standard in order to 

provide increased public health protection against an array of adverse health effects that range 

from decreased lung function and respiratory symptoms to more serious indicators of morbidity 

(e.g., including emergency department visits and hospital admissions), and mortality. In 

consideration of all of the above, the PA draws the conclusion that it is appropriate for the 

Administrator to consider revision of the current primary O3 standard to provide increased public 

health protection.

4. CASAC Advice

Following the 2008 decision to revise the primary O3 standard by setting the level at 

0.075 ppm (75 ppb), CASAC strongly questioned whether the standard met the requirements of 

the CAA. In September 2009, the EPA announced its intention to reconsider the 2008 standards, 

issuing a notice of proposed rulemaking in January 2010 (75 FR 2938). Soon after, the EPA 

solicited CASAC review of that proposed rule and in January 2011, solicited additional advice. 

This proposal was based on the scientific and technical record from the 2008 rulemaking, 

including public comments and CASAC advice and recommendations. As further described 

above (I.C), in the fall of 2011, the EPA did not revise the standard as part of the reconsideration 

process but decided to defer decisions on revisions to the O3 standards to the next periodic 
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review, which was already underway. Accordingly, in this section we describe CASAC’s advice 

related to the 2008 final decision and the subsequent reconsideration, as well as its advice on this 

current review of the O3 NAAQS that was initiated in September 2008. 

In April 2008, the members of the CASAC Ozone Review Panel sent a letter to EPA 

stating “[I]n our most-recent letters to you on this subject—dated October 2006 and March 

2007—the CASAC unanimously recommended selection of an 8-hour average Ozone NAAQS 

within the range of 0.060 to 0.070 parts per million [60 to 70 ppb] for the primary (human 

health-based) Ozone NAAQS” (Henderson, 2008). The letter continued: 

The CASAC now wishes to convey, by means of this letter, its additional, 

unsolicited advice with regard to the primary and secondary Ozone NAAQS. In 

doing so, the participating members of the CASAC Ozone Review Panel are 

unanimous in strongly urging you or your successor as EPA Administrator to 

ensure that these recommendations be considered during the next review cycle for 

the Ozone NAAQS that will begin next year ... numerous medical organizations 

and public health groups have also expressed their support of these CASAC 

recommendations’ ... [The CASAC did] not endorse the new primary ozone 

standard as being sufficiently protective of public health. The CASAC—as the 

EPA’s statutorily-established science advisory committee for advising you on the 

national ambient air quality standards—unanimously recommended decreasing

the primary standard to within the range of 0.060–0.070 ppm [60 to 70 ppb]. It is 

the Committee’s consensus scientific opinion that your decision to set the primary 

ozone standard above this range fails to satisfy the explicit stipulations of the 

Clean Air Act that you ensure an adequate margin of safety for all individuals, 

including sensitive populations.

In response to the EPA’s solicitation of advice on the EPA’s proposed rulemaking as part 

of the reconsideration, CASAC conveyed support (Samet, 2010).

CASAC fully supports EPA’s proposed range of 0.060 – 0.070 parts per million 

(ppm) for the 8-hour primary ozone standard. CASAC considers this range to be 

justified by the scientific evidence as presented in the Air Quality Criteria for 

Ozone and Related Photochemical Oxidants (March 2006) and Review of the 

National Ambient Air Quality Standards for Ozone: Policy Assessment of 

Scientific and Technical Information, OAQPS Staff Paper (July 2007). As stated 

in our letters of October 24, 2006, March 26, 2007 and April 7, 2008 to former 

Administrator Stephen L. Johnson, CASAC unanimously recommended selection 

of an 8-hour average ozone NAAQS within the range proposed by EPA (0.060 to 

0.070 ppm). In proposing this range, EPA has recognized the large body of data 
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and risk analyses demonstrating that retention of the current standard would leave 

large numbers of individuals at risk for respiratory effects and/or other significant 

health impacts including asthma exacerbations, emergency room visits, hospital 

admissions and mortality.

In response to EPA’s request for additional advice on the reconsideration in 2011, 

CASAC reaffirmed their conclusion that “the evidence from controlled human and 

epidemiological studies strongly supports the selection of a new primary ozone standard within 

the 60 – 70 ppb range for an 8-hour averaging time” (Samet, 2011, p ii). As requested by the 

EPA, CASAC’s advice and recommendations were based on the scientific and technical record 

from the 2008 rulemaking. In considering the record for the 2008 rulemaking, CASAC stated the 

following to summarize the basis for their conclusions (Samet, 2011, pp. ii to iii). 

(1) The evidence available on dose-response for effects of O3 shows associations 

extending to levels within the range of concentrations currently experienced in 

the United States.

(2) There is scientific certainty that 6.6-hour exposures with exercise of young, 

healthy, non-smoking adult volunteers to concentrations ≥ 80 ppb cause 

clinically relevant decrements of lung function.

(3) Some healthy individuals have been shown to have clinically relevant 

responses, even at 60 ppb.

(4) Since the majority of clinical studies involve young, healthy adult populations, 

less is known about health effects in such potentially ozone sensitive 

populations as the elderly, children and those with cardiopulmonary disease. 

For these susceptible groups, decrements in lung function may be greater than 

in healthy volunteers and are likely to have a greater clinical significance.
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(5) Children and adults with asthma are at increased risk of acute exacerbations 

on or shortly after days when elevated O3 concentrations occur, even when 

exposures do not exceed the NAAQS concentration of 75 ppb.

(6) Large segments of the population fall into what the EPA terms a “sensitive 

population group,’’ i.e., those at increased risk because they are more 

intrinsically susceptible (children, the elderly, and individuals with chronic 

lung disease) and those who are more vulnerable due to increased exposure 

because they work outside or live in areas that are more polluted than the 

mean levels in their communities.

With respect to evidence from epidemiologic studies, CASAC stated “while 

epidemiological studies are inherently more uncertain as exposures and risk estimates 

decrease (due to the greater potential for biases to dominate small effect estimates), 

specific evidence in the literature does not suggest that our confidence on the specific 

attribution of the estimated effects of ozone on health outcomes differs over the proposed 

range of 60-70 ppb” (Samet, 2011, p.10). 

Following its review of the second draft PA in the current review, which 

considers an updated scientific and technical record since the 2008 rulemaking, CASAC 

concluded that “there is clear scientific support for the need to revise the standard” (Frey, 

2014c, p. ii). In particular, CASAC noted the following (Frey, 2014c, p. 5): 

[T]he scientific evidence provides strong support for the occurrence of a range of 

adverse respiratory effects and mortality under air quality conditions that would 

meet the current standard. Therefore, CASAC unanimously recommends that the 
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Administrator revise the current primary ozone standard to protect public 

health.106

In supporting these conclusions, CASAC judged that the strongest evidence 

comes from controlled human exposure studies of respiratory effects. The Committee 

specifically noted that “the combination of decrements in FEV1 together with the 

statistically significant alterations in symptoms in human subjects exposed to 72 ppb 

ozone meets the American Thoracic Society’s definition of an adverse health effect” 

(Frey, 2014c, p. 5). CASAC further judged that “if subjects had been exposed to ozone 

using the 8-hour averaging period used in the standard, adverse effects could have 

occurred at lower concentration” and that “the level at which adverse effects might be 

observed would likely be lower for more sensitive subgroups, such as those with asthma”

(Frey, 2014c, p. 5). 

With regard to lung function risk estimates based on information from controlled 

human exposure studies, CASAC concluded that “estimation of FEV1 decrements of 

≥15% is appropriate as a scientifically relevant surrogate for adverse health outcomes in 

active healthy adults, whereas an FEV1 decrement of ≥10% is a scientifically relevant 

surrogate for adverse health outcomes for people with asthma and lung disease” (Frey, 

2014c, p. 3). The Committee further concluded that “[a]sthmatic subjects appear to be at 

least as sensitive, if not more sensitive, than non-asthmatic subjects in manifesting O3-

induced pulmonary function decrements” (Frey, 2014c, p. 4). In considering estimates of 

the occurrence of these decrements in urban study areas, CASAC specifically noted that 

 

106 CASAC provided similar advice in their letter to the Administrator on the REA, stating that 

“The CASAC finds that the current primary NAAQS for ozone is not protective of human health 

and needs to be revised” (Frey, 2014a, p. 15). 
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the current standard is estimated to allow 11 to 22% of school age children to experience 

at least one day with an FEV1 decrement > 10% (Frey, 2014c, p. 7). 

Although CASAC judged that controlled human exposure studies of respiratory 

effects provide the strongest evidence supporting their conclusion on the current standard, 

the Committee judged that there is also “sufficient scientific evidence based on 

epidemiologic studies for mortality and morbidity associated with short-term exposure to 

ozone at the level of the current standard” (Frey, 2014c, p. 5). In support of the biological 

plausibility of the associations reported in these epidemiologic studies, CASAC noted 

that “[r]ecent animal toxicological studies support identification of modes of action and, 

therefore, the biological plausibility associated with the epidemiological findings” (Frey, 

2014c, p. 5). 

Consistent with the advice of CASAC, several public commenters supported 

revising the primary O3 standard to provide increased public health protection. In 

considering the available evidence as a basis for their views, these commenters generally 

noted that the health evidence is stronger in the current review than in past reviews, with 

new evidence for effects attributable to short- and long-term exposures, and new 

evidence for effects at lower O3 exposure concentrations. 

Other public commenters opposed considering revised standards. These 

commenters discussed a variety of reasons for their views. A number of commenters 

expressed the view that the EPA should not lower the level of the standard because a 

lower level would be closer to background O3 concentrations. In addition, several 

commenters challenged the interpretation of the evidence presented in the ISA. With 

respect to the risk assessment, several commenters expressed the view that the EPA 

should only estimate risks above O3 background concentrations, or above threshold 
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concentrations. Some commenters also expressed the view that, based on the mortality 

and morbidity risk estimates in the HREA, there is little to no difference between the 

risks estimated for the current O3 standard and the risks estimated for revised standards 

with lower levels. These commenters concluded that the HREA and PA have not shown 

that the public health improvements likely to be achieved by a revised O3 standard would 

be greater than the improvements likely to be achieved by the current standard.

5. Administrator’s Proposed Conclusions Concerning the Adequacy of the Current Standard

This section discusses the Administrator’s proposed conclusions related to the adequacy 

of the public health protection provided by the current primary O3 standard, resulting in her 

proposed decision to revise that standard. These proposed conclusions, and her proposed 

decision, are based on the Administrator’s consideration of the available scientific evidence, 

exposure/risk information, the comments and advice of CASAC, and public input received thus 

far, as summarized below. 

As an initial matter, the Administrator concludes that reducing precursor emissions to 

achieve O3 concentrations that meet the current primary O3 standard will provide important 

improvements in public health protection, compared to recent air quality. In reaching this initial 

conclusion, she notes the discussion in section 3.4 of the PA (U.S. EPA, 2014c), summarized 

above (II.D.3). In particular, the Administrator notes that this initial conclusion is supported by 

(1) the strong body of scientific evidence indicating a wide range of adverse health outcomes 

attributable to exposures to O3 at concentrations commonly found in the ambient air and (2) 

estimates indicating decreased occurrences of O3 exposures of concern and decreased O3-

associated health risks upon meeting the current standard, compared to recent air quality. Thus, 
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she concludes that it would not be appropriate in this review to consider a standard that is less 

protective than the current standard.107

After reaching the initial conclusion that meeting the current primary O3 standard will 

provide important improvements in public health protection, and that it is not appropriate to 

consider a standard that is less protective than the current standard, the Administrator next 

considers the adequacy of the public health protection that is provided by the current standard. In 

doing so, the Administrator first notes that studies evaluated since the completion of the 2006 O3

AQCD support and expand upon the strong body of evidence that, in the last review, indicated a 

causal relationship between short-term O3 exposures and respiratory health effects. This is the 

strongest causality finding possible under the ISA’s hierarchical system for classifying weight of 

evidence for causation. Together, experimental and epidemiologic studies support conclusions 

regarding a continuum of O3 respiratory effects ranging from small reversible changes in 

pulmonary function, and pulmonary inflammation, to more serious effects that can result in 

respiratory-related emergency department visits, hospital admissions, and premature mortality.

Recent animal toxicology studies support descriptions of modes of action for these respiratory 

effects and augment support for biological plausibility for the role of O3 in reported effects. With 

regard to mode of action, evidence indicates that antioxidant capacity may modify the risk of 

respiratory morbidity associated with O3 exposure, and that the inherent capacity to quench 

(based on individual antioxidant capacity) can be overwhelmed, especially with exposure to 

elevated concentrations of O3. In addition, based on the consistency of findings across studies 

 

107 While not analyzed quantitatively, consistent with CASAC advice (Frey, 2014a, p. 10), the 

Administrator notes that reductions in O3 precursor emissions (e.g., NOX; VOC) to achieve O3

concentrations that meet the current standard could also increase public health protection by 

reducing the ambient concentrations of pollutants other than O3 (i.e., PM2.5, NO2).
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and evidence for the coherence of results from different scientific disciplines, evidence indicates 

that certain populations are at increased risk of experiencing O3-related effects, including the 

most severe effects. These include populations and lifestages identified in previous reviews (i.e., 

people with asthma, children, older adults, outdoor workers) and populations identified since the 

last review (i.e., people with certain genotypes related to antioxidant and/or anti-inflammatory 

status; people with reduced intake of certain antioxidant nutrients, such as Vitamins C and E). 

The Administrator further notes that evidence for adverse respiratory health effects

attributable to long-term, or repeated short-term, O3 exposures is much stronger than in previous 

reviews, and the ISA concludes that there is “likely to be” a causal relationship between such O3

exposures and adverse respiratory health effects (the second strongest causality finding). 

Uncertainties related to the extrapolation of data generated by rodent toxicology studies to the 

understanding of health effects in humans have been reduced by studies in non-human primates 

and by recent epidemiologic studies. The evidence available in this review includes new

epidemiologic studies using a variety of designs and analysis methods, conducted by different 

research groups in different locations, evaluating the relationships between long-term O3

exposures and measures of respiratory morbidity and mortality. New evidence supports

associations between long-term O3 exposures and the development of asthma in children, with 

several studies reporting interactions between genetic variants and such O3 exposures. Studies

also report associations between long-term O3 exposures and asthma prevalence, asthma severity 

and control, respiratory symptoms among asthmatics, and respiratory mortality.

In considering the O3 exposure concentrations reported to elicit respiratory effects, the 

Administrator agrees with the conclusions of the PA and with the advice of CASAC (Frey, 

2014c) that controlled human exposure studies provide the most certain evidence indicating the 
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occurrence of health effects in humans following exposures to specific O3 concentrations. In 

particular, as discussed further in section II.E.4.d below, she notes that the effects reported in 

controlled human exposure studies are due solely to O3 exposures, and interpretation of study 

results is not complicated by the presence of co-occurring pollutants or pollutant mixtures (as is 

the case in epidemiologic studies). Therefore, she places the most weight on information from 

these controlled human exposure studies. 

In considering the evidence from controlled human exposure studies, the Administrator 

first notes that these studies have reported a variety of respiratory effects in healthy adults 

following exposures to O3 concentrations of 60, 72,108 or 80 ppb, and higher. The largest 

respiratory effects, and the broadest range of effects, have been studied and reported following 

exposures of healthy adults to 80 ppb O3 or higher, with most exposure studies conducted at 

these higher concentrations. She further notes that recent evidence includes controlled human 

exposure studies reporting the combination of lung function decrements and respiratory 

symptoms in healthy adults engaged in intermittent, moderate exertion following 6.6 hour

exposures to concentrations as low as 72 ppb, and lung function decrements and pulmonary 

inflammation following exposures to O3 concentrations as low as 60 ppb. As discussed below, 

compared to the evidence available in the last review, these studies have strengthened support for 

the occurrence of abnormal and adverse respiratory effects attributable to short-term exposures to 

O3 concentrations below the level of the current standard.

109 The Administrator concludes that 

 

108 As noted above, for the 70 ppb target exposure concentration, Schelegle et al. (2009) reported 

that the actual mean exposure concentration was 72 ppb.

109 Cf. State of Misisssippi. 744 F.3d 1350 (“Perhaps more studies like the Adams studies will 

yet reveal that the 0.060 ppm level produces significant adverse decrements that simply cannot 

be attributed to normal variation in lung function.”).
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such exposures to O3 concentrations below the level of the current standard are potentially 

important from a public health perspective, given the following:

(1) The combination of lung function decrements and respiratory symptoms reported to 

occur in healthy adults following exposures to 72 ppb O3 or higher, while at moderate 

exertion, meet ATS criteria for an adverse response. In specifically considering the 72 

ppb exposure concentration, CASAC noted that “the combination of decrements in FEV1

together with the statistically significant alterations in symptoms in human subjects 

exposed to 72 ppb ozone meets the American Thoracic Society’s definition of an adverse 

health effect” (Frey, 2014c, p. 5). 

(2) With regard to 60 ppb O3, CASAC agreed that “a level of 60 ppb corresponds to the 

lowest exposure concentration demonstrated to result in lung function decrements large 

enough to be judged an abnormal response by ATS and that could be adverse in 

individuals with lung disease” (Frey, 2014c, p. 7). CASAC further noted that “a level of 

60 ppb also corresponds to the lowest exposure concentration at which pulmonary 

inflammation has been reported” (Frey, 2014c, p. 7).

(3) The controlled human exposure studies reporting these respiratory effects were conducted 

in healthy adults, while at-risk groups (e.g., children, people with asthma) could 

experience larger and/or more serious effects. In their advice to the Administrator, 

CASAC concurred with this reasoning (Frey, 2014a, p. 14; Frey, 2014c, p. 5). 

(4) These respiratory effects are coherent with the serious health outcomes that have been 

reported in epidemiologic studies evaluating exposure to O3 (e.g., respiratory-related 

hospital admissions, emergency department visits, and mortality). 
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As noted above, the Administrator’s proposed conclusions regarding the adequacy of the 

current primary O3 standard place a large amount of weight on the results of controlled human 

exposure studies. In particular, given the combination of lung function decrements and 

respiratory symptoms following 6.6 hour exposures to O3 concentrations as low as 72 ppb, and 

given CASAC advice regarding effects at 72 ppb along with ATS adversity criteria, she 

concludes that the evidence in this review supports the occurrence of adverse respiratory effects 

following exposures to O3 concentrations lower than the level of the current standard. 110 As 

discussed below, the Administrator further considers information from the broader body of 

controlled human exposure studies within the context of quantitative estimates of exposures of 

concern and O3-induced FEV1 decrements. 

In addition to controlled human exposure studies, the Administrator also considers what 

the available epidemiologic evidence indicates with regard to the adequacy of the public health 

protection provided by the current primary O3 standard.111 She notes that recent epidemiologic 

studies provide support, beyond that available in the last review, for associations between shortterm O3 exposures and a wide range of adverse respiratory outcomes (including respiratoryrelated hospital admissions, emergency department visits, and mortality) and with total mortality. 

Associations with morbidity and mortality are stronger during the warm or summer months, and 

remain robust after adjustment for copollutants.

 

110 The use of evidence from controlled human exposure studies conducted in healthy adults to 

characterize the potential for adverse effects, including in at-risk groups such as children and 

asthmatics, is discussed in sections II.C.2 and II.C.3.a, above. CASAC advice on this issue is 

discussed in sections II.D.4 and II.E.4.c. 

111 As noted above, she places less weight on information from epidemiologic studies than on 

information from controlled human exposure studies. 
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In considering information from epidemiologic studies within the context of her 

conclusions on the adequacy of the current standard, the Administrator considers the extent to 

which available studies support the occurrence of O3 health effect associations with air quality 

likely to be allowed by the current standard. In doing so, she places the most weight on air 

quality analyses in locations of single-city studies of short-term O3, as discussed in more detail in 

section II.E.4.d below.112 In particular, she notes that a U.S. single-city study reported 

associations with respiratory emergency department visits in children and adults in a location 

that would likely have met the current O3 standard over the entire study period (Mar and Koenig, 

2009). In addition, even in some single-city study locations where the current standard was likely 

not met (i.e., Silverman and Ito, 2010; Strickland et al., 2010), the Administrator notes PA 

analyses indicating that reported concentration-response functions and available air quality data 

support the occurrence of O3-health effect associations on subsets of days with ambient O3

concentrations below the level of the current standard (II.D.1). Compared to single-city studies, 

the Administrator notes additional uncertainty in interpreting the relationships between air 

quality in individual study cities and health effects based on multicity analyses (discussed further 

in sections II.D.1 and II.E.4.d). While such uncertainties limit the extent to which the 

Administrator bases her conclusions on air quality in locations of multicity epidemiologic 

 

112 As discussed in section II.E.4.d of this preamble, this judgment applies specifically to 

epidemiologic studies of short-term O3 concentrations where multicity effect estimates are 

presented, based on combining the effect estimates from multiple individual cities, and where 

individual city effect estimates are not presented (as is the case for key multicity studies analyzed 

in the PA). Because these reported multicity effect estimates do not allow health effect 

associations to be disaggregated by individual city, it is not possible to assign the health effect 

association to the air quality in any one study location, or to the air quality in a subset of 

locations. In contrast, for epidemiologic studies of long-term concentrations, where multicity 

effect estimates are based on comparisons across cities, different judgments have been made with 

regard to the utility of multicity studies (see, e.g. 78 FR 3086 at 3103/2) (January 15, 2013) (and 

see discussion below of study by Jerrett et al., (2009)).
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studies, she does note that O3 associations with respiratory morbidity or mortality have been 

reported in several multicity studies when the majority of study locations (though not all study 

locations) would likely have met the current O3 standard. When taken together, the 

Administrator reaches the conclusion that single-city epidemiologic studies and associated air 

quality information support the occurrence of O3-associated hospital admissions and emergency 

department visits for ambient O3 concentrations likely to have met the current standard, and that 

air quality analyses in locations of multicity studies provide some support for this conclusion for 

a broader range of effects (i.e., including mortality). 

Beyond her consideration of the scientific evidence, the Administrator also considers the 

results of the HREA exposure and risk analyses in reaching initial conclusions regarding the 

adequacy of the current primary O3 standard. In doing so, as noted above, she focuses primarily 

on exposure and risk estimates based on information from controlled human exposure studies 

(i.e., exposures of concern and O3-induced lung function decrements). She places relatively less 

weight on epidemiologic-based risk estimates, noting that the overall conclusions from the 

HREA likewise reflect less confidence in estimates of epidemiologic-based risks than in 

estimates of exposures and lung function risks (U.S. EPA, 2014, section 9.6). Consistent with the 

conclusions in the PA, her determination to attach less weight to the epidemiologic-based risk 

estimates reflects her consideration of key uncertainties, including the heterogeneity in effect 

estimates between locations, the potential for exposure measurement errors, and uncertainty in 

the interpretation of the shape of concentration-response functions for O3 concentrations in the 

lower portions of ambient distributions (U.S. EPA, 2014, section 9.6) (II.D.2). In particular, she 

concludes that lower confidence should be placed in the results of the assessment of respiratory 

mortality risks associated with long-term O3 exposures, primarily because that analysis is based 
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on only one study (even though that study is well-designed) and because of the uncertainty in 

that study about the existence and level of a potential threshold in the concentration-response 

function (U.S. EPA, 2014a, section 9.6) (II.D.2).113

With regard to estimates of exposures of concern, the Administrator considers the extent 

to which the current standard provides protection against exposures to O3 concentrations at or 

above 60, 70, and 80 ppb, noting CASAC advice that 60 ppb “is an appropriate exposure of 

concern for asthmatic children” (Frey, 2014c, p. 8). She further notes that while single exposures 

of concern could be adverse for some people, particularly for the higher benchmark 

concentrations (70, 80 ppb) where there is stronger evidence for the occurrence of adverse 

effects (discussed further in II.E.4.d, below), she becomes increasingly concerned about the 

potential for adverse responses as the frequency of occurrences increases.114 In particular, she 

notes that repeated occurrences of the types of effects shown to occur following exposures of 

concern can have potentially adverse outcomes. For example, repeated occurrences of airway 

inflammation could potentially result in the induction of a chronic inflammatory state; altered

pulmonary structure and function, leading to diseases such as asthma; altered lung host defense 

response to inhaled microorganisms; and altered lung response to other agents such as allergens 

or toxins (U.S. EPA, 2013a, section 6.2.3). Thus, the Administrator notes that the types of lung 

injury shown to occur following exposures to O3 concentrations from 60 to 80 ppb, particularly if 

experienced repeatedly, provide a mode of action by which O3 may cause other more serious 

 

113 CASAC also called into question the extent to which it is appropriate to place confidence in 

risk estimates for respiratory mortality (Frey, 2014a, p. 11). 

114 Not all people who experience an exposure of concern will experience an adverse effect (even 

members of at-risk populations). For most of the endpoints evaluated in controlled human 

exposure studies (with the exception of O3-induced FEV1 decrements, as discussed below), the 

number of those experiencing exposures of concern who will experience adverse effects cannot 

be reliably quantified. 
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effects (e.g., asthma exacerbations). Therefore, the Administrator places the most weight on 

estimates of two or more exposures of concern (i.e., as a surrogate for the occurrence of repeated 

exposures), though she also considers estimates of one or more, particularly for the 70 and 80 

ppb benchmarks. 

Consistent with CASAC advice (Frey, 2014c), the Administrator focuses on children in 

these analyses of O3 exposures, noting that estimates for all children and asthmatic children are 

virtually indistinguishable (in terms of the percent estimated to experience exposures of 

concern). Though she focuses on children, she also recognizes that exposures to O3

concentrations at or above 60 or 70 ppb could be of concern for adults. As discussed in the 

HREA and PA (and II.C.2.a, above), the patterns of exposure estimates across urban study areas, 

across years, and across air quality scenarios are similar in adults with asthma, older adults, all

children, and children with asthma, though smaller percentages of adult populations are 

estimated to experience exposures of concern than children and children with asthma. Thus, the 

Administrator recognizes that the exposure patterns for children across years, urban study areas, 

and air quality scenarios are indicative of the exposure patterns in a broader group of at-risk 

populations that also includes asthmatic adults and older adults. 

As illustrated in Table 1 (above), the Administrator notes that if the 15 urban study areas 

evaluated in the HREA were to just meet the current O3 standard, fewer than 1% of children in 

those areas would be estimated to experience two or more exposures of concern at or above 70 

ppb, though approximately 3 to 8% of children, including approximately 3 to 8% of asthmatic 

children, would be estimated to experience two or more exposures of concern to O3
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concentrations at or above 60 ppb115 (based on estimates averaged over the years of analysis). To 

provide some perspective on these percentages, the Administrator notes that they correspond to 

almost 900,000 children in urban study areas, including about 90,000 asthmatic children, 

estimated to experience two or more exposures of concern at or above 60 ppb. Nationally, if the 

current standard were to be just met the number of children experiencing such exposures would 

be larger. In the worst-case year and location (i.e., year and location with the largest exposure 

estimates), the Administrator notes that over 2% of children are estimated to experience two or 

more exposures of concern at or above 70 ppb and over 14% are estimated to experience two or 

more exposures of concern at or above 60 ppb. 

Although, as discussed above and in section II.E.4.d, the Administrator is less concerned 

about single occurrences of exposures of concern, she notes that even single occurrences can 

cause adverse effects in some people, particularly for the 70 and 80 ppb benchmarks. Therefore, 

she also considers estimates of one or more exposures of concern. As illustrated in Table 1 

(above), if the 15 urban study areas evaluated in the HREA were to just meet the current O3

standard, fewer than 1% of children in those areas would be estimated to experience one or more 

exposures of concern at or above 80 ppb (based on estimates averaged over the years of 

analysis). However, approximately 1 to 3% of children, including 1 to 3% of asthmatic children, 

would be estimated to experience one or more exposures of concern to O3 concentrations at or 

above 70 ppb and approximately 10 to 17% would be estimated to experience one or more 

exposures of concern to O3 concentrations at or above 60 ppb. In the worst-case year and 

location, the Administrator notes that over 1% of children are estimated to experience one or 

 

115 Almost no children in those areas would be estimated to experience two or more exposures of 

concern at or above 80 ppb. 
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more exposures of concern at or above 80 ppb, over 8% are estimated to experience one or more 

exposures of concern at or above 70 ppb, and about 26% are estimated to experience one or more 

exposures of concern at or above 60 ppb. 

In addition to estimated exposures of concern, the Administrator also considers HREA 

estimates of the occurrence of O3-induced lung function decrements. In doing so, she particularly 

notes CASAC advice that “estimation of FEV1 decrements of ≥ 15% is appropriate as a 

scientifically relevant surrogate for adverse health outcomes in active healthy adults, whereas an 

FEV1 decrement of ≥ 10% is a scientifically relevant surrogate for adverse health outcomes for 

people with asthma and lung disease” (Frey, 2014c, p. 3). The Administrator notes that while 

single occurrences of O3-induced lung function decrements could be adverse for some people, as 

discussed above (II.B.3), a more general consensus view of the potential adversity of such

decrements emerges as the frequency of occurrences increases. Therefore, the Administrator 

focuses primarily on the estimates of two or more O3-induced lung function decrements.

When averaged over the years evaluated in the HREA, the Administrator notes that the 

current standard is estimated to allow about 1 to 3% of children in the 15 urban study areas 

(corresponding to almost 400,000 children) to experience two or more O3-induced lung function 

decrements > 15%, and to allow about 8 to 12% of children (corresponding to about 180,000 

asthmatic children116) to experience two or more O3-induced lung function decrements > 10%. 

Nationally, larger numbers of children would be expected to experience such O3-induced 

decrements if the current standard were to be just met. The current standard is also estimated to 

 

116 As noted above, CASAC concluded that “an FEV1 decrement of ≥10% is a scientifically 

relevant surrogate for adverse health outcomes for people with asthma and lung disease” (Frey, 

2014c, p. 3) and that such decrements “could be adverse for people with lung disease” (Frey, 

2014c, p. 7). 
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allow about 3 to 5% of children in the urban study areas to experience one or more decrements >

15% and about 14 to 19% of children to experience one or more decrements > 10%. In the worstcase year and location, the current standard is estimated to allow 4% of children in the urban 

study areas to experience two or more decrements > 15% (and 7% to experience one or more 

such decrements) and 14% of children to experience two or more decrements > 10% (and 22% to 

experience one or more such decrements). 

In further considering the HREA results, the Administrator considers the epidemiologybased risk estimates. As discussed above, compared to the weight given to HREA estimates of 

exposures of concern and lung function risks, she places relatively less weight on epidemiologybased risk estimates. In giving some consideration to these risk estimates, the Administrator 

notes estimates of total risks (i.e., based on the full distributions of ambient O3 concentrations) 

and risks associated with O3 concentrations in the upper portions of ambient distributions. The 

Administrator notes that estimates of total risks are based on the assumption that concentrationresponse relationships remain linear over the entire distributions of ambient O3 concentrations. 

With regard to total risks, she notes that the HREA estimates thousands of O3-associated hospital 

admissions, emergency department visits, and deaths per year for air quality conditions 

associated with just meeting the current standard in the 12 urban study areas (II.C.3).

However, the Administrator also notes the increasing uncertainty associated with the 

shapes of concentration-response curves for O3 concentrations in the lower portions of ambient 

distributions. She particularly notes that there is less certainty in the shape of concentrationresponse functions for area-wide O3 concentrations at the lower ends of warm season 

distributions (i.e., below about 20 to 40 ppb depending on the O3 metric, health endpoint, and 

study population) (U.S. EPA, 2013a, section 2.5.4.4). The Administrator further notes the 
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evidence from controlled human exposure studies, which provide the strongest support for O3-

induced effects following exposures to O3 concentrations corresponding to the upper portions of 

typical ambient distributions (i.e., 60 ppb and above). Therefore, the Administrator judges it 

appropriate to focus on risks associated with O3 concentrations in the upper portions of ambient 

distributions. Even when considering only area-wide O3 concentrations from the upper portions 

of seasonal distributions, the Administrator notes that the current standard is estimated to allow 

hundreds to thousands of O3-associated deaths per year in urban study areas (II.C.3).

Although the Administrator notes the HREA conclusions indicating somewhat less 

confidence in estimates of O3-associated mortality and morbidity risks, compared to estimates of 

exposures of concern and risk of lung function decrements, she concludes that the general 

magnitude of mortality and morbidity risk estimates suggests the potential for a substantial 

number of O3-associated deaths and adverse respiratory events to occur nationally, even when 

the current standard is met. She especially notes that this is the case based on the risks associated 

with the upper ends of distributions of ambient O3 concentrations, where she has the greatest 

confidence in O3-attributable effects.

In addition to the evidence and exposure/risk information discussed above, the 

Administrator also takes note of the CASAC advice in the current review and in the 2010 

proposed reconsideration of the 2008 decision establishing the current standard. As discussed in 

more detail above, the current CASAC “finds that the current NAAQS for ozone is not protective 

of human health” and “unanimously recommends that the Administrator revise the current 

primary ozone standard to protect public health” (Frey, 2014c, p. 5). The prior CASAC O3 Panel 

likewise recommended revision of the current standard to one with a lower level. This earlier 

recommendation was based entirely on the evidence and information in the record for the 2008 
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standard decision, which, as discussed above, has been substantially strengthened in the current 

review (Samet, 2011; Samet, 2012). 

In consideration of all of the above, the Administrator proposes that the current primary 

O3 standard is not adequate to protect public health, and that it should be revised to provide 

increased public health protection. This proposed decision is based on the Administrator’s initial 

conclusions that the available evidence and exposure and risk information clearly call into 

question the adequacy of public health protection provided by the current primary standard and, 

therefore, that the current standard is not requisite to protect public health with an adequate 

margin of safety. With regard to the evidence, she specifically notes that (1) controlled human 

exposure studies provide support for the occurrence of adverse respiratory effects following 

exposures to O3 concentrations below the level of the current standard (i.e., as low as 72 ppb), 

and that (2) single-city epidemiologic studies provide support for the occurrence of adverse 

respiratory effects under air quality conditions that would likely meet the current standard, with 

multicity studies providing some support for this conclusion for a broader range of effects (i.e., 

including mortality). Courts have repeatedly held that this type of evidence justifies an 

Administrator’s conclusion that it is “appropriate” (within the meaning of section 109 (d)(1) of 

the CAA) to revise a primary NAAQS to provide further protection of public health.117 In 

addition, based on the analyses in the HREA, the Administrator initially concludes that the 

exposures and risks projected to remain upon meeting the current standard can reasonably be 

judged to be important from a public health perspective. Thus, she reaches the proposed 

conclusion that the evidence and information, together with CASAC advice based on their 

consideration of that evidence and information, provide strong support for revising the current 

 

117 See e.g. State of Mississippi, 744 F. 3d at 1345; American Farm Bureau, 559 F. 3d at 525-26.
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primary standard in order to increase public health protection against an array of adverse effects 

that range from decreased lung function and respiratory symptoms to more serious indicators of 

morbidity (e.g., including emergency department visits and hospital admissions), and mortality. 

The Administrator solicits comment on her proposed decision to revise the current 

primary O3 NAAQS, including on her considerations and proposed conclusions based on the 

scientific evidence, exposure/risk information, and CASAC advice. In doing so, she recognizes 

that some have expressed alternative approaches to viewing the evidence and information, 

including alternative approaches to viewing, evaluating, and weighing important uncertainties. In 

some cases, these alternative approaches have led some public commenters to recommend 

retaining the current standard. Given these alternative views, in addition to proposing to revise 

the current primary O3 standard, the Administrator solicits comment on the option of retaining

that standard. In doing so, she also solicits comment on the potential approaches to viewing the 

scientific evidence and exposure/risk information that could support a conclusion that the current 

standard is requisite to protect public health with an adequate margin of safety.

E. Conclusions on the Elements of the Primary Standard

Having reached the proposed conclusion that the currently available scientific evidence 

and exposure/risk information call into question the adequacy of the current O3 standard, the 

Administrator next considers the range of alternative standards supported by that evidence and 

information. Consistent with her consideration of the adequacy of the current standard, the 

Administrator's proposed conclusions on alternative standards are informed by the available 

scientific evidence assessed in the ISA, exposure/risk information presented and assessed in the 

HREA, the evidence-based and exposure-/risk-based considerations and conclusions in the PA, 

CASAC advice, and input from members of the public. The sections below discuss the evidence 
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and exposure/risk information, CASAC advice and public input, and the Administrator's 

proposed conclusions, for the major elements of the NAAQS: indicator (II.E.1), averaging time 

(II.E.2), form (II.E.3), and level (II.E.4). 

1. Indicator

In the last review, the EPA focused on O3 as the most appropriate indicator for a standard 

meant to provide protection against ambient photochemical oxidants. In this review, while the 

complex atmospheric chemistry in which O3 plays a key role has been highlighted, no 

alternatives to O3 have been advanced as being a more appropriate indicator for ambient 

photochemical oxidants. More specifically, the ISA noted that O3 is the only photochemical 

oxidant (other than NO2) that is routinely monitored and for which a comprehensive database 

exists (U.S. EPA, 2013a, section 3.6). Data for other photochemical oxidants (e.g., PAN, H2O2, 

etc.) typically have been obtained only as part of special field studies. Consequently, no data on 

nationwide patterns of occurrence are available for these other oxidants; nor are extensive data 

available on the relationships of concentrations and patterns of these oxidants to those of O3

(U.S. EPA, 2013a, section 3.6). In its review of the second draft PA, CASAC stated “The 

indicator of ozone is appropriate based on its causal or likely causal associations with multiple 

adverse health outcomes and its representation of a class of pollutants known as photochemical 

oxidants” (Frey, 2014c, p. ii).

In addition, the PA notes that meeting an O3 standard can be expected to provide some 

degree of protection against potential health effects that may be independently associated with 

other photochemical oxidants, even though such effects are not discernible from currently 

available studies indexed by O3 alone (U.S. EPA, 2014c, section 4.1). That is, since the precursor 

emissions that lead to the formation of O3 generally also lead to the formation of other 
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photochemical oxidants, measures leading to reductions in population exposures to O3 can 

generally be expected to lead to reductions in population exposures to other photochemical 

oxidants. In considering this information, and CASAC’s advice, the Administrator reaches the 

proposed conclusion that O3 remains the most appropriate indicator for a standard meant to 

provide protection against photochemical oxidants.118

2. Averaging Time

The EPA established the current 8-hour averaging time119 for the primary O3 NAAQS in 

1997 (62 FR 38856). The decision on averaging time in that review was based on numerous 

controlled human exposure and epidemiologic studies reporting associations between 6 to 8 hour 

O3 concentrations and adverse respiratory effects (62 FR 38861). It was also noted that a 

standard with a max 8-hour averaging time is likely to provide substantial protection against 

respiratory effects associated with 1-hour peak O3 concentrations. Similar conclusions were 

reached in the last O3 NAAQS review and thus, the 8-hour averaging time was retained in 2008. 

In reaching a proposed conclusion on averaging time in the current review, the 

Administrator considers the extent to which the available evidence continues to support the 

appropriateness of a standard with an 8-hour averaging time. Specifically, the Administrator 

considers the extent to which the available information indicates that a standard with the current 

8-hour averaging time provides appropriate protection against short- and long-term O3

exposures. 

a. Short-term

 

118The D.C. Circuit upheld the use of O3 as the indicator for photochemical oxidants based on 

these same considerations. American Petroleum Inst. v. Costle, 665 F. 2d 1176, 1186 (D.C. Cir. 

1981).

119This 8-hour averaging time reflects daily max 8-hour average O3 concentrations. 
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As an initial consideration with respect to the most appropriate averaging time for the O3

NAAQS, the Administrator notes that the strongest evidence for O3-associated health effects is 

for respiratory effects following short-term exposures. More specifically, the Administrator notes 

the ISA conclusion that the evidence is “sufficient to infer a causal relationship” between shortterm O3 exposures and respiratory effects. The ISA also judges that for short-term O3 exposures, 

the evidence indicates “likely to be causal” relationships with both cardiovascular effects and 

mortality (U.S. EPA, 2013a, section 2.5.2). Therefore, as in past reviews, the strength of the 

available scientific evidence provides strong support for a standard that protects the public health 

against short-term exposures to O3. 

In first considering the level of support available for specific short-term averaging times, 

the Administrator notes the evidence available from controlled human exposure studies. As 

discussed in more detail in chapter 3 of the PA, substantial health effects evidence from 

controlled human exposure studies demonstrates that a wide range of respiratory effects (e.g., 

pulmonary function decrements, increases in respiratory symptoms, lung inflammation, lung 

permeability, decreased lung host defense, and AHR) occur in healthy adults following 6.6 hour 

exposures to O3 (U.S. EPA, 2013a, section 6.2.1.1). Compared to studies evaluating shorter 

exposure durations (e.g., 1-hour), studies evaluating 6.6 hour exposures in healthy adults have 

reported respiratory effects at lower O3 exposure concentrations and at more moderate levels of 

exertion. 

The Administrator also notes the strength of evidence from epidemiologic studies that 

have evaluated a wide variety of populations (e.g., including at-risk lifestages and populations, 

such as children and people with asthma, respectively). A number of different averaging times 

are used in O3 epidemiologic studies, with the most common being the max 1-hour concentration 
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within a 24-hour period (1-hour max), the max 8-hour average concentration within a 24-hour 

period (8-hr max), and the 24-hour average. These studies are summarized above and assessed in 

detail in chapter 6 of the ISA (U.S. EPA, 2013a). Limited evidence from time-series and panel 

epidemiologic studies comparing risk estimates across averaging times does not indicate that one 

exposure metric is more consistently or strongly associated with respiratory health effects or 

mortality, though the ISA notes some evidence for “smaller O3 risk estimates when using a 24-

hour average exposure metric” (U.S. EPA, 2013a, section 2.5.4.2; p. 2-31). For single- and 

multi-day average O3 concentrations, lung function decrements were associated with 1-hour 

max, 8-hour max, and 24-hour average ambient O3 concentrations, with no strong difference in 

the consistency or magnitude of association among the averaging times (U.S. EPA, 2013a, p. 6-

71). Similarly, in studies of short-term exposure to O3 and mortality, Smith et al. (2009) and 

Darrow et al. (2011) have reported high correlations between risk estimates calculated using 24-

hour average, 8-hour max, and 1-hour max averaging times (U.S. EPA, 2013a, p. 6-253). Thus, 

the Administrator notes that the epidemiologic evidence alone does not provide a strong basis for 

distinguishing between the appropriateness of 1-hour, 8-hour, and 24-hour averaging times. 

Considering the health information discussed above, the Administrator concludes that an 

8-hour averaging time remains appropriate for addressing health effects associated with shortterm exposures to ambient O3. An 8-hour averaging time is similar to the exposure periods 

evaluated in controlled human exposure studies, including recent studies that provide evidence 

for respiratory effects following exposures to O3 concentrations below the level of the current 

standard. In addition, epidemiologic studies provide evidence for health effect associations with 

8-hour O3 concentrations, as well as with 1-hour and 24-hour concentrations. As in previous 

reviews, the Administrator notes that a standard with an 8-hour averaging time (combined with 
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an appropriate standard form and level) would also be expected to provide substantial protection 

against health effects attributable to 1-hour and 24-hour exposures (e.g., 62 FR 38861, July 18, 

1997). This conclusion is consistent with the advice received from CASAC that “the current 8-

hour averaging time is justified by the combined evidence from epidemiologic and clinical 

studies” (Frey, 2014c, p. 6).

b. Long-term

The ISA concludes that the evidence for long-term O3 exposures indicates that there is 

“likely to be a causal relationship” with respiratory effects (U.S. EPA, 2013a, chapter 7). Thus, 

in this review the Administrator also considers the extent to which currently available evidence 

and exposure/risk information suggests that a standard with an 8-hour averaging time can 

provide protection against respiratory effects associated with longer term exposures to ambient 

O3. 

In considering this issue in the last review of the O3 NAAQS, the Staff Paper noted that 

“because long-term air quality patterns would be improved in areas coming into attainment with 

an 8-hr standard, the potential risk of health effects associated with long-term exposures would 

be reduced in any area meeting an 8-hr standard” (U.S. EPA, 2007, p. 6-57). In the current 

review, the PA further evaluates this issue, with a focus on the long-term O3 metrics reported to 

be associated with mortality or morbidity in recent epidemiologic studies. As discussed in 

section 3.1.3 of the PA (U.S. EPA, 2014c, section 4.2), much of the recent evidence for such 

associations is based on studies that defined long-term O3 in terms of seasonal averages of daily 

maximum 1-hour or 8-hour concentrations. 

As an initial consideration, the Administrator notes the risk results from the HREA for 

respiratory mortality associated with long-term O3 concentrations. These HREA analyses
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indicate that as air quality is adjusted to just meet the current 8-hour standard, most urban study 

areas are estimated to experience reductions in respiratory mortality associated with long-term 

O3 concentrations based on the seasonal averages of 1-hour daily maximum O3 concentrations 

evaluated in the study by Jerrett et al. (2009) (U.S. EPA, 2014a, chapter 7).120 As air quality is 

adjusted to meet lower alternative standard levels, for standards based on 3-year averages of the 

annual fourth-highest daily maximum 8-hour O3 concentrations, respiratory mortality risks are 

estimated to be reduced further in urban study areas. This analysis indicates that an O3 standard 

with an 8-hour averaging time, when coupled with an appropriate form and level, can reduce 

respiratory mortality reported to be associated with long-term O3 concentrations. 

In further considering the study by Jerrett et al. (2009), the Administrator notes the PA 

comparison of long-term O3 concentrations following model adjustment in urban study areas

(i.e., adjusted to meet the current and alternative 8-hour standards) to the concentrations present 

in study cities that provided the basis for the positive and statistically significant association with 

respiratory mortality. As indicated in Table 4-3 of the PA (U.S. EPA, 2014c, section 4.2), this 

comparison suggests that a standard with an 8-hour averaging time can decrease seasonal 

averages of 1-hour daily maximum O3 concentrations, and can maintain those O3 concentrations

below the seasonal average concentration where the study indicates the most confidence in the 

reported concentration-response relationship with respiratory mortality (U.S. EPA, 2014c, 

sections 4.2 and 4.4.1). 

The Administrator also notes that the HREA conducted analyses evaluating the impacts 

of reducing regional NOX emissions on the seasonal averages of daily maximum 8-hour O3

 

120 Though the Administrator also notes important uncertainties associated with these risk 

estimates, as discussed above (II.C.3.b). 
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concentrations. Seasonal averages of 8-hour daily max O3 concentrations reflect long-term 

metrics that have been reported to be associated with respiratory morbidity effects in several 

recent O3 epidemiologic studies (e.g., Islam et al., 2008; Lin et al., 2008; Salam et al., 2009). The 

HREA analyses indicate that the large majority of the U.S. population lives in locations where 

reducing NOX emissions would be expected to result in decreases in seasonal averages of daily 

max 8-hour ambient O3 concentrations (U.S. EPA, 2014a, chapter 8). Thus, consistent with the 

respiratory mortality risk estimates noted above, these analyses suggest that reductions in O3

precursor emissions in order to meet a standard with an 8-hour averaging time would also be 

expected to reduce the long-term O3 concentrations that have been reported in recent 

epidemiologic studies to be associated with respiratory morbidity.

c. Administrator’s proposed conclusion on averaging time

Taken together, the Administrator notes that the analyses summarized above indicate that

a standard with an 8-hour averaging time, coupled with the current 4th high form and an 

appropriate level, would be expected to provide appropriate protection against the short- and 

long-term O3 concentrations that have been reported to be associated with respiratory morbidity 

and mortality. The CASAC agreed with this conclusion, stating that “[t]he current 8-hour 

averaging time is justified by the combined evidence from epidemiologic and clinical studies” 

and that “[t]he 8-hour averaging window also provides protection against the adverse impacts of 

long-term ozone exposures, which were found to be “likely causal” for respiratory effects and 

premature mortality” (Frey, 2014c, p. 6). Therefore, considering the available evidence and 

exposure risk information, and CASAC’s advice, the Administrator proposes to retain the current 

8-hour averaging time, and not to set an additional standard with a different averaging time. 

3. Form
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The “form” of a standard defines the air quality statistic that is to be compared to the 

level of the standard in determining whether an area attains that standard. The foremost 

consideration in selecting a form is the adequacy of the public health protection provided by the 

combination of the form and the other elements of the standard. In this review, the Administrator 

considers the extent to which the available evidence and/or information continue to support the 

appropriateness of a standard with the current form, defined by the 3-year average of annual 4th-

highest 8-hour daily maximum O3 concentrations. 

The EPA established the current form of the primary O3 NAAQS in 1997 (62 FR 38856). 

Prior to that time, the standard had a “1-expected-exceedance” form.121 An advantage of the 

current concentration-based form recognized in the 1997 review is that such a form better 

reflects the continuum of health effects associated with increasing ambient O3 concentrations. 

Unlike an expected exceedance form, a concentration-based form gives proportionally more 

weight to years when 8-hour O3 concentrations are well above the level of the standard than

years when 8-hour O3 concentrations are just above the level of the standard.122 It was judged 

appropriate to give more weight to higher O3 concentrations, given that available health evidence 

indicated a continuum of effects associated with exposures to varying concentrations of O3, and 

given that the extent to which public health is affected by exposure to ambient O3 is related to the 

 

121For a standard with a 1-expected-exceedance form to be met at an air quality monitoring site, 

the fourth-highest air quality value in 3 years, given adjustments for missing data, must be less 

than or equal to the level of the standard.

122 As discussed (61 FR 65731), this is because with an exceedance-based form, days on which 

the ambient O3 concentration is well above the level of the standard are given equal weight to 

those days on which the O3 concentration is just above the standard (i.e., each day is counted as 

one exceedance), even though the public health impact of such days would be very different. 

With a concentration-based form, days on which higher O3 concentrations occur would weigh 

proportionally more than days with lower O3 concentrations since the actual concentrations are 

used directly to calculate whether the standard is met or violated. 
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actual magnitude of the O3 concentration, not just whether the concentration is above a specified 

level. 

During the 1997 review, the EPA considered a range of alternative “concentration-based” 

forms, including the second-, third-, fourth- and fifth-highest daily maximum 8-hour 

concentrations in an O3 season. The fourth-highest daily maximum was selected, 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 38856). 

Consideration was also given to setting a standard with a form that would provide a margin of 

safety against possible but uncertain chronic effects, and would provide greater stability to 

ongoing control programs.123 A more restrictive form was not selected, recognizing that the 

differences in the degree of protection afforded by the alternatives were not well enough 

understood to use any such differences as a basis for choosing the most restrictive forms (62 FR 

38856). 

In the 2008 review, the EPA additionally considered the potential value of a percentilebased form. In doing so, the EPA recognized that such a statistic is useful for comparing datasets 

of varying length because it samples approximately the same place in the distribution of air 

quality values, whether the dataset is several months or several years long. However, the EPA 

concluded that a percentile-based statistic would not be effective in ensuring the same degree of 

public health protection across the country. Specifically, a percentile-based form would allow 

more days with higher air quality values in locations with longer O3 seasons relative to places 

 

123 See American Trucking Assn’s v. EPA, 283 F. 3d at 374-75 (less stable implementation 

programs may be less effective, and therefore the EPA can consider programmatic stability in 

determining the form of a NAAQS).
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with shorter O3 seasons. Thus, in the 2008 review, the EPA concluded that a form based on the 

nth-highest maximum O3 concentration would more effectively ensure that people who live in 

areas with different length O3 seasons receive the same degree of public health protection. 

Based on analyses of forms specified in terms of an nth-highest concentration (n ranged 

from 3 to 5), advice from CASAC, and public comment,124 the Administrator concluded that a 

4th-highest daily maximum should be retained (73 FR 16465, March 27, 2008). In reaching this 

decision, the Administrator recognized that “there is not a clear health-based threshold for 

selecting a particular nth-highest daily maximum form of the standard” and that “the adequacy of 

the public health protection provided by the combination of the level and form is a foremost 

consideration” (73 FR 16475, March 27, 2008). Based on this, the Administrator judged that the 

existing form (4th-highest daily maximum 8-hour average concentration) should be retained, 

recognizing the increase in public health protection provided by combining this form with a 

lower standard level (i.e., 75 ppb). 

The Administrator also recognized that it is important to have a form that provides 

stability with regard to implementation of the standard. In the case of O3, for example, he noted 

the importance of a form insulated from the impacts of the meteorological events that are 

conducive to O3 formation. Such events could have the effect of reducing public health 

protection, to the extent they result in frequent shifts in and out of attainment due to 

meteorological conditions. The Administrator noted that such frequent shifting could disrupt an 

 

124In the 2008 review, one group of commenters expressed the view that the standard was not 

adequate and supported a more health-protective form (e.g., a second- or third-highest daily max 

form). Another group of commenters expressed the view that the standard was adequate and did 

not provide any views on alternative forms that would be appropriate should the Administrator 

consider revisions to the standard. The Administrator considered the protection afforded by the 

combination of level and form in revising the standard in 2008 to 75 ppb, as a 3-year average of 

the annual fourth-highest daily max 8-hour concentrations (73 FR 16475, March 27, 2008).
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area’s ongoing implementation plans and associated control programs (73 FR 16474, March 27, 

2008). In his final decision, the Administrator judged that a 4

th high form “provides a stable 

target for implementing programs to improve air quality” (73 FR 16475, March 27, 2008). 

In the current review, the Administrator considers the extent to which newly available 

information provides support for the current form. In so doing, she takes note of the conclusions 

of prior reviews summarized above. She recognizes the value of an nth-high statistic over that of 

an expected exceedance or percentile-based form in the case of the O3 standard, for the reasons 

summarized above. The Administrator additionally takes note of the importance of stability in 

implementation to achieving the level of protection specified by the NAAQS. Specifically, she

notes that to the extent areas engaged in implementing the O3 NAAQS frequently shift from 

meeting the standard to violating the standard, it is possible that ongoing implementation plans 

and associated control programs could be disrupted, thereby reducing public health protection.

In light of this, while giving foremost consideration to the adequacy of public health 

protection provided by the combination of all elements of the standard, including the form, the 

Administrator considers particularly findings from prior reviews with regard to the use of the 

nth-high metric. As noted above, the 4th-highest daily maximum was selected in recognition of 

the public health protection provided by this form, when coupled with an appropriate averaging 

time and level, and recognizing that such a form can provide stability for implementation 

programs. The Administrator concludes that the currently available evidence and information do

not call into question these conclusions from previous reviews. In reaching this conclusion, the 

Administrator notes that CASAC concurred that the O3 standard should be based on the fourth 

highest, daily maximum 8-hour average value (averaged over 3 years), stating that this form 

“provides health protection while allowing for atypical meteorological conditions that can lead to 
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abnormally high ambient ozone concentrations which, in turn, provides programmatic stability” 

(Frey, 2014c, p. 6). Thus, a standard with the current 4th high form, coupled with a level lower 

than 75 ppb as discussed below, would be expected to increase public health protection relative 

to the current standard while continuing to provide stability for implementation programs. 

Therefore, the Administrator proposes to retain the current 4th-highest daily maximum form for 

an O3 standard with an 8-hour averaging time and a revised level, as discussed below. 

4. Level

The Administrator next considers the extent to which alternative levels below 75 ppb

could provide greater protection than the current primary standard against short- and long- term 

exposures to O3 in ambient air, for a standard based on the 3-year average of the annual 4th

highest daily maximum 8-hour O3 concentration. In doing so, she particularly notes the 

evidence-based and exposure-/risk-based considerations in the PA, which take into account the 

experimental and epidemiologic evidence as assessed in the ISA; quantitative estimates of O3

exposures and health risks in at-risk populations provided by the HREA; uncertainties and 

limitations associated with this evidence and information; CASAC advice; and public input (U.S. 

EPA, 2014c, sections 4.4 and 4.5). Section II.E.4.a below summarizes the PA’s approach to 

considering the scientific evidence and the exposure/risk information related to level of the 

primary standard. Section II.E.4.b presents the PA’s conclusions on alternative primary O3

standard levels. Section II.E.4.c summarizes CASAC advice on the level of the primary standard, 

and public input received thus far. Section II.E.4.d presents the Administrator’s proposed 

conclusions on primary O3 standard levels. 

a. PA approach to considering the evidence and information related to alternative levels of the 

primary standard
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The PA's approach to reaching conclusions on alternative standard levels focuses on the 

evidence from controlled human exposure and epidemiologic studies, as assessed in the ISA 

(U.S. EPA, 2013a), and the exposure and health risk analyses presented in the HREA (U.S. EPA, 

2014a). This approach is discussed in detail in Chapters 1 and 4 of the PA (U.S. EPA, 2014c, 

sections 1.3, 4.6), and is summarized below. 

As an initial matter, the PA notes that controlled human exposure studies provide the 

most certain evidence indicating the occurrence of health effects in humans following exposures 

to specific O3 concentrations. Consistent with this, CASAC concluded that “the scientific 

evidence supporting the finding that the current standard is inadequate to protect public health is 

strongest based on the controlled human exposure studies of respiratory effects” (Frey, 2014c, p. 

5). As discussed above and in section 3.1.2.1 of the PA (U.S. EPA, 2014c), controlled human 

exposure studies have reported a variety of respiratory effects in healthy adults following 

exposures to O3 concentrations of 60, 72,125 or 80 ppb, and higher. 

Given the evidence for respiratory effects from controlled human exposure studies, the 

PA considers the extent to which standards with revised levels would be estimated to protect atrisk populations against exposures of concern to O3 concentrations at or above the health 

benchmark concentrations of 60, 70, and 80 ppb (i.e., based on HREA estimates of one or more 

and two or more exposures of concern). In doing so, the PA notes the CASAC conclusion that 

(Frey, 2014c, p. 6): 

The 80 ppb-8hr benchmark level represents an exposure level for which there is 

substantial clinical evidence demonstrating a range of ozone-related effects 

including lung inflammation and airway responsiveness in healthy individuals. 

The 70 ppb-8hr benchmark level reflects the fact that in healthy subjects, 

decreases in lung function and respiratory symptoms occur at concentrations as 

 

125 As noted above, for the 70 ppb exposure concentration Schelegle et al. (2009) reported that 

the actual 6.6-hour mean exposure concentration was 72 ppb. 
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low as 72 ppb and that these effects almost certainly occur in some people, 

including asthmatics and others with low lung function who are less tolerant of 

such effects, at levels of 70 ppb and below. The 60 ppb-8hr benchmark level 

represents the lowest exposure level at which ozone-related effects have been 

observed in clinical studies of healthy individuals.

The PA also notes that, due to individual variability in responsiveness, only a subset of 

people who experience exposures at or above the three benchmark concentrations can be 

expected to experience associated health effects, and that available data are not sufficient to 

quantify that subset of people for most of the endpoints that have been evaluated in controlled 

human exposure studies (i.e., with the exception of FEV1 decrements). The PA views the health 

effects evidence as a continuum with greater confidence and less uncertainty about the 

occurrence of adverse health effects at higher O3 exposure concentrations, and less confidence 

and greater uncertainty as one considers lower exposure concentrations (U.S. EPA, 2014c, 

section 3.2.2, p. 3-101). 

While there is greater uncertainty regarding the occurrence of adverse health effects at 

lower concentrations, the PA also notes that the controlled human exposure studies that provided 

the basis for benchmark concentrations have not evaluated responses in populations at the 

greatest risk from exposures to O3 (e.g., children, people with asthma). Compared to the healthy 

people included in most controlled human exposure studies, members of at-risk populations and 

lifestages are at greater risk of experiencing adverse effects. Thus, the effects reported in healthy 

adults at each of the benchmark concentrations may underestimate effects in these at-risk groups. 

In considering the health evidence within the context of drawing conclusions on alternative 

standard levels, the PA balances concerns about the potential for adverse health effects, 

especially in at-risk populations, with the increasing uncertainty regarding the likelihood of such 

effects following exposures to lower O3 concentrations.
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With respect to the lung function decrements that have been evaluated in controlled 

human exposure studies, the PA considers the extent to which standards with revised levels 

would be estimated to protect healthy and at-risk populations against O3-induced lung function 

decrements large enough to be adverse in some people (based on quantitative risk estimates in 

the HREA). As discussed in section 3.1.3 of the PA (U.S. EPA, 2014c) and section II.B.3 above, 

although some experts would judge single occurrences of moderate responses to be a nuisance, 

especially for healthy individuals, a more general consensus view of the adversity of moderate 

lung function decrements emerges as the frequency of occurrence increases. Repeated 

occurrences of moderate responses, even in otherwise healthy individuals, may be considered to 

be adverse, since they could well set the stage for more serious illness (73 FR 16448). In 

reaching conclusions on alternative standard levels, the PA considers the extent to which 

standards with revised levels would be estimated to protect healthy and at-risk populations 

against one or more, and two or more, moderate (i.e., FEV1 decrements ≥ 10% and ≥ 15%) and 

large (i.e., FEV1 decrements ≥ 20%) lung function decrements.

In evaluating the epidemiologic evidence within the context of drawing conclusions on 

alternative standard levels, the PA considers the extent to which available studies have reported 

associations with emergency department visits, hospital admissions, and/or mortality in locations 

that would likely have met alternative standards with levels below 75 ppb. In evaluating the 

epidemiologic evidence in this way, the PA considers both multicity and single-city studies, 

recognizing the strengths and limitations of each. In particular, while single-city studies are more 

limited than multicity studies in terms of statistical power and geographic coverage, conclusions 

linking air quality in a specific area with health effect associations in that same area can be made 
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with greater certainty for single-city studies (i.e., compared to multicity studies reporting only 

multicity effect estimates). 

The PA also considers the epidemiologic evidence within the context of epidemiologybased risk estimates. Compared to the weight given to HREA estimates of exposures of concern 

and lung function risks, and the weight given to the evidence, the PA places relatively less 

weight on epidemiologic-based risk estimates. In doing so, the PA notes that the overall 

conclusions from the HREA likewise reflect less confidence in estimates of epidemiologic-based 

risks than in estimates of exposures and lung function risks. The determination to attach less 

weight to the epidemiologic-based estimates reflects the uncertainties associated with mortality 

and morbidity risk estimates, including the heterogeneity in effect estimates between locations, 

the potential for exposure measurement errors, and uncertainty in the interpretation of the shape 

of concentration-response functions at lower O3 concentrations (U.S. EPA, 2014a, section 9.6). 

The HREA also concludes that lower confidence should be placed in the results of the 

assessment of respiratory mortality risks associated with long-term O3 exposures, primarily 

because that analysis is based on only one study (even though that study is well-designed) and 

because of the uncertainty in that study about the existence and level of a potential threshold in 

the concentration-response function (U.S. EPA, 2014a, section 9.6). 

In considering the epidemiology-based risk estimates, the PA focuses on the extent to 

which potential alternative O3 standards with levels below 75 ppb are estimated to reduce the 

risk of O3-associated mortality.126 As discussed for the current standard (II.D.2.c), the PA 

considers estimates of total risk (i.e., based on the full distributions of ambient O3

 

126 Differences in estimated respiratory morbidity risks between alternative standard levels are

similar to the differences estimated for total mortality associated with short-term O3

concentrations. 
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concentrations) and estimates of risk associated with O3 concentrations in the upper portions of 

ambient distributions. 

b. PA conclusions on alternative O3 standard levels

Using the approach discussed above to consider the scientific evidence and exposure/risk 

information, CASAC advice (II.E.4.c, below), and public comments, the PA reaches the 

conclusion that it is appropriate for the Administrator to consider alternative primary O3 standard 

levels from 70 to 60 ppb. The basis for this conclusion is discussed in detail in sections 4.4.1 and 

4.4.2 of the PA (U.S. EPA, 2014c), and is summarized below. 

With regard to controlled human exposure studies, the PA considers the lowest O3

exposure concentrations at which various effects have been evaluated and statistically significant 

effects reported. The PA also considers the potential for reported effects to be adverse, including 

in at-risk populations and lifestages. As discussed in section 3.1.2.1 of the PA (U.S. EPA, 

2014c), controlled human exposure studies provide evidence of respiratory symptoms combined 

with lung function decrements (an adverse response based on ATS criteria) in healthy adults 

following 6.6 hour exposures to O3 concentrations as low as 72 ppb, and evidence of potentially 

adverse lung function decrements and airway inflammation following 6.6 hour exposures to O3

concentrations as low as 60 ppb.

Although some studies show that respiratory symptoms also develop during exposures to

60 ppb O3, the increase in symptoms has not been reported to reach statistical significance by the 

end of the 6.6 hour exposure period (Adams, 2006; Schelegle et al., 2009). Thus, while 

significant increases in respiratory symptoms combined with lung function decrements have not 

been reported following exposures to 60 ppb O3, this combination of effects is likely to occur to 

some degree in healthy adults with 6.6-hour exposures to concentrations below 72 ppb, and also 
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are more likely to occur with longer (i.e., 8-hour) exposures.127 In addition, pulmonary 

inflammation, particularly if experienced repeatedly, provides a mechanism by which O3 may 

cause other more serious respiratory morbidity effects (e.g., asthma exacerbations) and possibly 

extrapulmonary effects. As discussed in section 3.1.2.1 of the PA (U.S. EPA, 2014c), the 

physiological effects reported in controlled human exposure studies down to 60 ppb O3 have 

been linked to aggravation of asthma and increased susceptibility to respiratory infection, 

potentially leading to increased medication use, increased school and work absences, increased 

visits to doctors’ offices and emergency departments, and increased hospital admissions.

With regard to the lowest exposure concentration shown to cause respiratory effects (i.e., 

60 ppb),128 the PA notes that most controlled human exposure studies have not evaluated O3

concentrations below 60 ppb. Therefore, 60 ppb does not necessarily reflect an exposure 

concentration below which effects such as lung function decrements and airway inflammation no 

longer occur. This is particularly the case given that controlled human exposure studies were 

conducted in healthy adults, while people with asthma, including asthmatic children, are likely to 

be more sensitive to O3-induced respiratory effects. 

With regard to other O3-induced effects, the PA notes that AHR and impaired lung host 

defense capabilities have been reported in healthy adults engaged in moderate exertion following 

 

127 In addition, CASAC observed that, “adverse health effects in young healthy adults occur with 

exposures to 72 ppb of ozone for 6.6 hours” and that “[i]t is the judgment of CASAC that if 

subjects had been exposed to ozone using the 8-hour averaging period used in the standard, 

adverse effects could have occurred at [a] lower concentration. Further, in our judgment, the 

level at which adverse effects might be observed would likely be lower for more sensitive 

subgroups, such as those with asthma” (Frey, 2014c, p. 5). 

128 As discussed above (II.B.2), prolonged 6.6 exposure to 40 ppb O3 has been shown to result in 

a small decrease in group mean FEV1 that is not statistically different from responses following 

exposure to filtered air (Adams, 2002; Adams, 2006). 
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exposures to O3 concentrations as low as 80 ppb, the lowest concentration evaluated for these 

effects. As discussed in section 3.1.2.1 of the PA (U.S. EPA, 2014c), these physiological effects 

have been linked to aggravation of asthma and increased susceptibility to respiratory infection, 

potentially leading to increased medication use, increased school and work absences, increased 

visits to doctors’ offices and emergency departments, and increased hospital admissions. These 

are all indicators of adverse O3-related morbidity effects, which are consistent with, and provide 

plausibility for, the adverse morbidity effects and mortality effects observed in epidemiologic 

studies. 

Based on consideration of the above evidence, the PA concludes that available controlled 

human exposure studies support considering alternative O3 standard levels from 70 to 60 ppb in 

the current review. In reaching this conclusion, the PA notes that 70 ppb is just below the O3

exposure concentration reported to result in lung function decrements and respiratory symptoms 

in healthy adults (i.e., 72 ppb), a combination of effects that meet ATS criteria for an adverse 

response. In addition, while 70 ppb is well below the 80 ppb exposure concentration shown to 

cause potentially adverse respiratory effects such as AHR and impaired host-defense capabilities, 

these effects have not been evaluated at exposure concentrations below 80 ppb and there is no 

reason to believe that 80 ppb represents a threshold for such effects. In addition, potentially 

adverse lung function decrements and pulmonary inflammation have been demonstrated to occur 

in healthy adults at 60 ppb. Thus, 60 ppb is a short-term exposure concentration that may be 

reasonably concluded to elicit adverse effects in at-risk groups. 

The PA further notes that the range of alternative levels from 70 to 60 ppb is supported 

by evidence from epidemiologic studies and by exposure and risk estimates from the HREA. 

This evidence and exposure/risk information indicate that a level from anywhere in the range of 
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70 to 60 ppb would be expected to result in important public health improvements over the 

current standard. In particular, compared to the current standard a revised standard with a level 

from 70 to 60 ppb would be expected to (1) more effectively maintain short- and long-term O3

concentrations below those present in the epidemiologic studies that reported significant O3

health effect associations in locations likely to have met the current standard; (2) reduce the 

occurrence of exposures of concern to O3 concentrations that result in respiratory effects in 

healthy adults (at or above 60, 70, and 80 ppb); (3) reduce the occurrence of moderate-to-large 

O3-induced lung function decrements; and (4) reduce the risk of O3-associated mortality and 

morbidity, particularly the risk associated with the upper portions of the distributions of ambient 

O3 concentrations. The PA also notes that the range of levels from 70 to 60 ppb corresponds to

the range of levels recommended for consideration by CASAC, based on the available evidence 

and information (Frey, 2014a; Frey, 2014c).

In reaching a conclusion on whether it is appropriate to consider alternative standard 

levels below 60 ppb, the PA notes the following: 

(1) While controlled human exposure studies provide evidence for O3-induced respiratory 

effects following exposures to O3 concentrations as low as 60 ppb, they do not provide 

evidence for adverse effects following exposures to lower concentrations. On this issue, 

CASAC concurred that 60 ppb O3 is an appropriate and justifiable scientifically based 

lower bound for a revised primary standard, based upon findings of “adverse effects, 

including clinically significant lung function decrements and airway inflammation, after 

exposures to 60 ppb ozone in healthy adults with moderate exertion (Adams, 2006; 

Schelegle et al., 2009; Brown et al., 2008; Kim et al., 2011), with limited evidence of 

adverse effects below 60 ppb” (Frey, 2014c, p. 7). 
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(2) Based on the HREA results, meeting an O3 standard with a level of 60 ppb would be 

expected to almost eliminate exposures of concern to O3 concentrations at or above 60 

ppb. To the extent lower exposure concentrations may result in adverse health effects in 

some people, a standard level of 60 ppb would be expected to also reduce exposures to 

O3 concentrations below 60 ppb. 

(3) U.S. and Canadian epidemiologic studies have not reported O3 health effect associations 

based primarily on study locations likely to have met a standard with a level of 60 ppb. 

(4) In all of the urban study areas evaluated, a standard with a level of 60 ppb would be 

expected to maintain long-term O3 concentrations below those where a key study 

indicates the most confidence in a linear concentration-response relationship with 

respiratory mortality. 

Given all of the above considerations the PA concludes that, compared to standards with 

levels from 70 to 60 ppb, the extent to which standards with levels below 60 ppb could result in 

further public health improvements becomes notably less certain. Therefore, the PA concludes

that it is not appropriate in this review to consider standard levels below 60 ppb. 

The following sections summarize the PA’s consideration of the scientific evidence and 

exposure/risk information specifically related to potential alternative O3 standards with levels 

from the upper (70 ppb) (II.E.4.c.i), middle (65 ppb) (II.E.4.c.ii), and lower (60 ppb) (II.E.4.c.iii) 

portions of the range of 70 to 60 ppb. Key exposure/risk information considered in the PA is 

summarized in Tables 4 and 5, below (from U.S. EPA, 2014c, Tables 4-4 and 4-5). 
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Table 4. Summary of Estimated Exposures of Concern for Potential Alternative O3

Standard Levels of 70, 65, and 60 ppb in Urban Case Study Areas129

Benchmark 

Level

Alternative 

Standard 

Level 

(ppb)

Average 

% 

Children

Exposed130

Number of 

Children (5 to 18 

years)

[Number of 

Asthmatic 

Children]131, 132

Average % 

Reduction 

from Current 

Standard133

% Children -

Worst Year 

and Worst 

Area

One or more exposures of concern per season

≥ 70 ppb

70 0.1-1.2 94,000 [10,000] 73 3.2

65 0-0.2 14,000 [2,000] 95 0.5

60 0

134 1,400 [200]135 100 0.1

≥ 60 ppb

70 3.3-10.2 1,176,000 

[126,000] 46 18.9

65 0-4.2 392,000 [42,000] 80 9.5

60 0-1.2 70,000 [8,000] 96 2.2

Two or more exposures of concern per season

≥ 70 ppb

70 0-0.1 5,400 [600] 95 0.4

65 0 300 [100] 100 0

60 0 0 [0] 100 0

≥ 60 ppb

70 0.5-3.5 320,000 [35,000] 61 9.2

65 0-0.8 67,000 [7,500] 92 2.8

60 0-0.2 5,100 [700] 100 0.3

 

129 All alternative standard levels evaluated in the HREA were effective at limiting exposures of 

concern at or above 80 ppb (U.S. EPA, 2014c, Figures 4-1 to 4-4). Therefore, Table 4 focuses on 

exposures of concern at or above the 70 and 60 ppb benchmark concentrations. 

130 Estimates for each urban case study area were averaged for the years evaluated in the HREA 

(2006 to 2010). Ranges reflect the ranges across urban study areas. 

131 Numbers of children exposed in each urban case study area were averaged over the years 

2006 to 2010. These averages were then summed across urban study areas. Numbers are rounded 

to nearest thousand unless otherwise indicated. 

132 As noted in section II.C.3.a.ii, the responsiveness of asthmatics to O3 exposures could depend 

on factors that have not been well-evaluated such as asthma severity, the effectiveness of asthma 

control, or the prevalence of medication use.

133 Percent reductions in each urban study area were calculated and averaged across areas. 

134 Estimates smaller than 0.05% were rounded to zero. 

135As discussed in section 4.3.3 of the HREA (U.S. EPA, 2014a), the model-based air quality 

adjustment approach used to estimate risks associated with the current and alternative standards 

was unable to estimate the distribution of ambient O3 concentrations in New York City upon just 

meeting an alternative standard with a level of 60 ppb. Therefore, for the 60 ppb standard level 

the numbers of children and asthmatic children reflect all of the urban study areas except New 

York.
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Table 5. Summary of Estimated Lung Function Decrements for Potential Alternative O3

Standard Levels of 70, 65, and 60 ppb in Urban Case Study Areas

Lung 

Function 

Decrement

Alternative 

Standard 

Level

Average % 

Children136

Number of Children (5 

to 18 years) [Number 

of Asthmatic 

Children]137, 138

Average % 

Reduction from 

Current 

Standard

% Children 

Worst Year 

and Area

One or more decrements per season

≥ 10%

70 11-17 2,527,000 [261,000] 15 20

65 3-15 1,896,000 [191,000] 31 18

60 5-11 1,404,000 [139,000]139 45 13

≥ 15%

70 2-4 562,000 [58,000] 26 5

65 0-3 356,000 [36,000] 50 4

60 1-2 225,000 [22,000] 67 3

≥ 20%

70 1-2 189,000 [20,000] 32 2.1

65 0-1 106,000 [11,000] 59 1.4

60 0-1 57,000 [6,000] 77 0.7

Two or more decrements per season

≥ 10%

70 5.5-11 1,414,000 [145,000] 17 13

65 1.3-8.8 1,023,000 [102,000] 37 11

60 2.1-6.4 741,000 [73,000] 51 7.3

≥ 15%

70 0.9-2.4 276,000 [28,000] 29 3.1

65 0.1-1.8 168,000 [17,000] 54 2.3

60 0.2-1.0 101,000 [10,000] 71 1.4

≥ 20%

70 0.3-0.8 81,000 [8,000] 34 1.1

65 0-0.5 43,000 [4,000] 66 0.8

60 0-0.2 21,000 [2,000] 83 0.4

 

136 Estimates in each urban case study area were averaged for the years evaluated in the HREA 

(2006 to 2010). Ranges reflect the ranges across urban study areas. 

137 Numbers of children estimated to experience decrements in each urban case study area were 

averaged over 2006 to 2010. These averages were then summed across urban study areas. 

Numbers are rounded to nearest thousand unless otherwise indicated. 

138 As noted in section II.C.3.a.ii, the responsiveness of asthmatics to O3 exposures could depend 

on factors that have not been well-evaluated such as asthma severity, the effectiveness of asthma 

control, or the prevalence of medication use.

139As discussed in section 4.3.3 of the HREA (U.S. EPA, 2014a), the model-based air quality 

adjustment approach used to estimate risks associated with the current and alternative standards 

was unable to estimate the distribution of ambient O3 concentrations in New York City upon just 

meeting an alternative standard with a level of 60 ppb. Therefore, for the 60 ppb standard level 

the numbers of children and asthmatic children reflect all of the urban study areas except New 

York.
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i. PA consideration of an O3 standard level of 70 ppb

The PA notes that a level of 70 ppb is below the lowest O3 exposure concentration that 

has been reported to elicit a range of respiratory effects that includes AHR and decreased lung 

host defense, in addition to lung function decrements, airway inflammation, and respiratory 

symptoms (i.e., 80 ppb). A level of 70 ppb is also below the lowest exposure concentration at 

which the combined occurrence of respiratory symptoms and lung function decrements have 

been reported (i.e., 72 ppb), a combination judged adverse by the ATS (U.S. EPA, 2014c, section 

3.1.3). A level of 70 ppb is above the lowest exposure concentration demonstrated to result in 

lung function decrements large enough to be judged an abnormal response by ATS and above the 

lowest exposure concentration demonstrated to result in pulmonary inflammation (i.e., 60 ppb). 

Compared to the current standard, the HREA estimates that a revised O3 standard with a 

level of 70 ppb would reduce exposures of concern to O3 concentrations of 60, 70, and 80 ppb in 

urban study areas, with such a standard level estimated to be most effective at limiting exposures 

at or above the higher health benchmark concentrations and at limiting multiple occurrences of 

such exposures. On average over the years 2006 to 2010, for a standard with a level of 70 ppb, 

up to about 1% of children (i.e., ages 5 to 18) are estimated to experience exposures of concern 

at or above 70 ppb (73% reduction, compared to current standard), and far less than 1% are 

estimated to experience two or more such exposures (95% reduction, compared to current 

standard). In the worst-case location and year (i.e., location and year with the largest exposure 

estimate), about 3% of children are estimated to experience one or more exposures of concern at 

or above 70 ppb, and less than 1% are estimated to experience two or more. Far less than 1% of 
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children are estimated to experience exposures of concern at or above the 80 ppb benchmark 

concentration, even in the worst-case year (Table 4, above).140

As noted above, CASAC advised the EPA that 60 ppb is an appropriate exposure of 

concern with respect to adverse effects on people with asthma, including children (Frey, 2014c, 

pp. 6 and 8). For an O3 standard with a level of 70 ppb, about 3 to 10% of children, including 

asthmatic children, are estimated to experience one or more exposures of concern at or above 60 

ppb in a single O3 season. Compared to the current standard, this reflects about a 46% reduction, 

on average across the urban study areas. About 1% to 4% of children are estimated to experience 

two or more exposures of concern at or above 60 ppb (approximately 60% reduction, compared 

to current standard). In the worst-case location and year, for a standard set at 70 ppb, about 19% 

of children are estimated to experience one or more exposures of concern at or above 60 ppb, and 

9% are estimated to experience two or more such exposures (Table 4, above). 

Compared to the current standard, the HREA estimates that a revised O3 standard with a 

level of 70 ppb would also reduce O3-induced lung function decrements in children. A level of 

70 ppb is estimated to be most effective at limiting the occurrences of moderate and large lung 

function decrements (i.e., FEV1 decrements ≥ 15% and ≥ 20%, respectively), and at limiting 

multiple occurrences of O3-induced decrements. On average over the years 2006 to 2010, for a 

standard with a level of 70 ppb, about 2 to 4% of children in the urban study areas are estimated 

to experience one or more moderate O3-induced lung function decrements (i.e., FEV1 decrement 

≥ 15%), which would be of concern for healthy people, and about 1 to 2.5% of children are 

 

140 As noted above, due to interindividual variability, children (or adults) exposed at these levels 

will not necessarily experience health effects; the information available for some health effects is 

not sufficient to quantify the numbers of children in the urban study areas who might experience 

these effects. 
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estimated to experience two or more such decrements (approximately 30% reduction, compared 

to the current standard). In the worst-case location and year, up to 5% of children are estimated 

to experience one or more O3-induced lung function decrements ≥ 15%, and up to 3% are 

estimated to experience two or more such decrements. For a standard set at 70 ppb, about 2% or 

fewer children are estimated to experience large O3-induced lung function decrements (i.e., FEV1

decrement ≥ 20%), and about 1% or fewer children are estimated to experience two or more such 

decrements, even in the worst-case years and locations (Table 5, above).

On average over the years 2006 to 2010, for an O3 standard set at 70 ppb, about 11 to 

17% of children in the urban study areas are estimated to experience one or more moderate O3-

induced lung function decrements (i.e., FEV1 decrement ≥ 10%), which could be adverse for 

people with lung disease. This reflects an average reduction of about 15%, compared to the 

current standard. About 6 to 11% of children are estimated to experience two or more such 

decrements (17% reduction, compared to current standard). In the worst-case location and year, 

for a standard set at 70 ppb, about 20% of children in the urban study areas are estimated to 

experience one or more O3-induced lung function decrements ≥ 10%, and 13% are estimated to 

experience two or more such decrements (Table 5, above). 

Compared to the current standard, a revised standard with a level of 70 ppb would also 

more effectively maintain short-term ambient O3 concentrations below those present in the 

epidemiologic studies that reported significant O3 health effect associations in locations likely to 

have met the current standard. In particular, the single-city study by Mar and Koenig (2009) 

reported positive and statistically significant associations with respiratory emergency department 

visits in children and adults in a location that likely would have met the current O3 standard over 

the entire study period but violated a revised standard with a level of 70 ppb or below. None of 
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the single-city studies evaluated in section 4.4.1 of the PA (U.S. EPA, 2014c) provide evidence 

for O3 health effect associations in locations meeting a standard with a level of 70 ppb or below. 

While this analysis does not provide information on the extent to which the reported O3-

associated emergency department visits would persist upon meeting an O3 standard with a level 

of 70 ppb, or on the extent to which standard levels below 70 ppb could further reduce the 

incidence of such emergency department visits,141 it suggests that a revised O3 standard with a 

level at or below 70 ppb would require reductions in the ambient O3 concentrations that provided 

the basis for the health effect associations reported by Mar and Koenig (2009).

As discussed above, compared to single-city studies, there is greater uncertainty in 

linking air quality concentrations from individual study cities to multicity effect estimates. With 

regard to the multicity studies in this review, the PA notes that Dales et al. (2006) reported 

significant associations with respiratory hospital admissions based on air quality in 11 Canadian 

cities, most of which would likely have met the current standard over the entire study period, but 

violated a revised standard with a level of 70 ppb or below over at least part of that period (Table 

4-1). This analysis suggests that although the current standard would allow the ambient O3

concentrations in most of the study locations that provided the basis for the association with 

hospital admissions, a revised O3 standard with a level at or below 70 ppb would require 

reductions in those ambient O3 concentrations. As with the study by Mar and Koenig (2009), this 

analysis does not provide information on the extent to which the reported O3-associated hospital 

admissions would persist upon meeting an O3 standard with a level of 70 ppb, or on the extent to 

 

141 Put another way, one cannot infer from this analysis the extent to which effects would occur 

at O3 concentrations below those observed in the study. 
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which standard levels below 70 ppb could further reduce the incidence of such hospital 

admissions.142

With regard to long-term O3 concentrations, the PA evaluates the long-term O3 metrics 

reported to be associated with mortality or morbidity in recent epidemiologic studies (e.g., 

seasonal averages of 1-hour or 8-hour daily max concentrations). Compared to the current 

standard, a revised standard with a level of 70 ppb would be expected to reduce the risk of 

respiratory mortality associated with long-term O3 concentrations, based on information from the 

study by Jerrett et al. (2009), though the PA notes the HREA conclusion, discussed above, that 

lower confidence should be placed in respiratory mortality risk estimates based on this study 

(U.S. EPA, 2014a, section 9.6). In addition, a standard with a level of 70 ppb would be expected 

to more effectively maintain long-term O3 concentrations below those where the study by Jerrett 

et al. (2009) indicates the most confidence in the reported association with respiratory 

mortality.143 Specifically, air quality analyses indicate this to be the case in 9 out of the 12 urban 

study areas for a level of 70 ppb, compared to 6 out of 12 areas for the current standard. Finally, 

a revised standard with a level of 70 ppb would be expected to reduce long-term O3

concentrations based on the types of metrics that have been reported in recent epidemiologic 

 

142 In addition, for the other multicity studies identified in Table 4-1 of the PA (Cakmak et al., 

2006; Stieb et al., 2009; Katsouyanni et al., 2009), and for the study by Bell et al. (2006) (for the 

30 ppb cut point) (Table 4-2 of the PA), the majority of study locations would likely have met a 

standard with a level of 70 ppb (U.S. EPA, 2014c). 

143 As discussed in section 3.1.4.3 of the PA (U.S. EPA, 2014c), the study by Jerrett et al. (2009) 

suggests notably decreased confidence in the reported linear concentration-response function for 

long-term O3 concentrations in the first quartile (i.e., at or below about 53 ppb), given the 

widening in confidence intervals for lower concentrations; the fact that most study cities 

contributing to the linear function had O3 concentrations in the highest three quartiles, 

accounting for approximately 72% of the respiratory deaths in the cohort (based on Table 2 in 

the published study); and the limited evidence presented in the published study for a threshold at 

or near 56 ppb.
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studies to be associated with respiratory morbidity (i.e., seasonal averages of daily maximum 8-

hour concentrations). 

In further considering the potential implications of epidemiology studies for alternative 

standard levels, the PA notes estimates of total mortality associated with short-term O3

concentrations.144 As discussed above, the PA considers estimates of total risk (i.e., based on the 

full distributions of ambient O3 concentrations) and estimates of risk associated with O3

concentrations in the upper portions of ambient distributions. With regard to total risk the PA 

notes that, when summed across urban study areas, a standard with a level of 70 ppb is estimated 

to reduce the number of deaths associated with short-term O3 concentrations by about 4% (2007) 

and 2% (2009), compared to the current standard.145 Based on a national modeling analysis, the 

majority of the U.S. population would be expected to experience reductions in such risks upon 

reducing precursor emissions. 

Compared to the total risk estimates noted above, an O3 standard with a level of 70 ppb is 

estimated to be more effective at reducing the number of deaths associated with short-term O3

concentrations at the upper ends of ambient distributions. Specifically, for area-wide O3

concentrations at or above 40 ppb, a standard with a level of 70 ppb is estimated to reduce the 

number of deaths associated with short-term O3 concentrations by about 10% compared to the 

current standard. In addition, for area-wide concentrations at or above 60 ppb, a standard with a 

 

144 As discussed above, compared to the weight given to the evidence and to HREA estimates of 

exposures of concern and lung function risks, the PA places relatively less weight on 

epidemiologic-based risk estimates. 

145 A standard with a level of 70 ppb is also estimated to reduce respiratory mortality associated 

with long-term O3 concentrations in urban study areas. However, given uncertainties associated 

with these risk estimates, as discussed above, the PA gives them limited weight.
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level of 70 ppb is estimated to reduce O3-associated deaths by about 50% to 70% (U.S. EPA, 

2014c, Figure 4-13). 

The PA noted that in providing the advice that 70 ppb is an appropriate upper bound for 

consideration, CASAC advised that a level of 70 ppb would provide little margin of safety for 

protection of public health, particularly for sensitive subpopulations (Frey, 2014c, p. 8). In 

particular, CASAC stated that:

At 70 ppb, there is substantial scientific certainty of a variety of adverse effects, 

including decrease in lung function, increase in respiratory symptoms, and 

increase in airway inflammation. Although a level of 70 ppb is more protective of 

public health than the current standard, it may not meet the statutory requirement 

to protect public health with an adequate margin of safety (Frey, 2014c, p.8).146

However, the committee also acknowledged that “the choice of a level within the range 

recommended based on scientific evidence [i.e., 70 to 60 ppb] is a policy judgment under the 

statutory mandate of the Clean Air Act” (Frey, 2014c, pp. ii and 8). 

In summary, compared to the current standard, the PA concludes that a revised O3

standard with a level of 70 ppb would be expected to (1) reduce the occurrence of exposures of 

concern to O3 concentrations that result in respiratory effects in healthy adults (at or above 60

and 70 ppb) by about 45 to 95%, almost eliminating the occurrence of multiple exposures at or 

above 70 ppb; (2) reduce the occurrence of moderate-to-large O3-induced lung function 

decrements (FEV1 decrements > 10, 15, 20%) by about 15 to 35%, most effectively limiting the 

occurrence of multiple decrements and decrements > 15, 20%; (3) more effectively maintain 

short- and long-term O3 concentrations below those present in the epidemiologic studies that 

reported significant O3 health effect associations in locations likely to have met the current 

 

146 Also see Frey (2014c, p. ii).
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standard;

147 and (4) reduce the risk of O3-associated mortality and morbidity, particularly the risk 

associated with the upper portions of the distributions of ambient O3 concentrations. 

ii. PA consideration of an O3 standard level of 65 ppb

The PA also considers a standard with a level of 65 ppb. A level of 65 ppb is well below 

80 ppb, an O3 exposure concentration that has been reported to elicit a range of respiratory 

effects that includes airway hyperresponsiveness and decreased lung host defense, in addition to 

lung function decrements, airway inflammation, and respiratory symptoms. A standard level of 

65 ppb is also below the lowest exposure concentration at which the combined occurrence of 

respiratory symptoms and lung function decrements has been reported (i.e., 72 ppb), a 

combination judged adverse by the ATS (U.S. EPA, 2014c, section 3.1.3). A level of 65 ppb is 

above the lowest exposure concentration demonstrated to result in lung function decrements 

large enough to be judged an abnormal response by ATS, where statistically significant changes 

in group mean responses would be judged to be adverse by ATS, and which the CASAC has 

indicated could be adverse in people with lung disease (i.e., 60 ppb). A level of 65 ppb is also 

above the lowest exposure concentration at which pulmonary inflammation has been reported in 

healthy adults (i.e., 60 ppb). 

Compared to the current standard and a revised standard with a level of 70 ppb, the 

HREA estimates that a standard with a level of 65 ppb would reduce exposures of concern to the 

range of O3 benchmark concentrations analyzed (i.e., 60, 70, and 80 ppb). The HREA estimates 

that meeting a standard with a level of 65 ppb would eliminate exposures of concern at or above 

80 ppb in the urban study areas. Such a standard is estimated to allow far less than 1% of 

 

147 Epidemiologic studies also provide some evidence for O3 health effect associations in 

locations likely to have met a standard with a level of 70 ppb, as discussed below for lower 

standard levels. 
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children in the urban study areas to experience one or more exposures of concern at or above the 

70 ppb benchmark level, even in the worst-case years and locations, and is estimated to eliminate 

the occurrence of two or more exposures at or above 70 ppb (Table 4, above). 

In addition, for a standard with a level of 65 ppb, between 0 and about 4% of children 

(including asthmatic children) in urban study areas are estimated to experience exposures of 

concern at or above 60 ppb, which CASAC has indicated is an appropriate exposure of concern 

for people with asthma, including children. This reflects an 80% reduction (on average across 

areas), relative to the current standard. Less than 1% of children are estimated to experience two 

or more exposures of concern at or above 60 ppb (> 90% reduction, compared to current 

standard). In the worst-case location and year, about 10% of children are estimated to experience 

one or more exposures of concern at or above 60 ppb, with about 3% estimated to experience 

two or more such exposures (Table 4, above). 

Compared to the current standard and a revised standard with a level of 70 ppb, the 

HREA estimates that a standard with a level of 65 ppb would also further reduce the occurrence 

of O3-induced lung function decrements. For a level of 65 ppb, about 4% of children, or less, are 

estimated to experience moderate O3-induced FEV1 decrements ≥ 15% (50% reduction, 

compared to current standard), even considering the worst-case location and year. About 2% of 

children, or less, are estimated to experience two or more such decrements. Only about 1% of

children, or less, are estimated to experience large O3-induced lung function decrements (i.e., 

FEV1 decrement ≥ 20%), even in the worst-case year and location.

In addition, for a standard with a level of 65 ppb, about 3 to 15% of children are 

estimated to experience one or more moderate O3-induced lung function decrements (i.e., FEV1

decrement ≥ 10%), which CASAC has indicated could be adverse for people with lung disease. 
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This reflects an average reduction of about 30%, relative to the current standard. About 1 to 9% 

of children in the urban study areas are estimated to experience two or more such decrements 

(37% reduction, compared to current standard). In the worst-case location and year, for a 

standard set at 65 ppb, up to about 18% of these children are estimated to experience one or more 

moderate O3-induced lung function decrements ≥ 10%, and up to 11% are estimated to 

experience two or more such decrements. 

With regard to O3 epidemiologic studies, the PA notes that a revised standard with a level 

of 65 ppb would be expected to maintain short-term ambient O3 concentrations below those 

present in some of the study locations that provided the basis for reported O3 health effect 

associations and that were likely to have met a revised standard with a level of 70 ppb. In 

particular, Katsouyanni et al. (2009) reported statistically significant associations with mortality 

based on air quality in 12 Canadian cities, most of which would likely have met a standard with a 

level of 70 ppb over the entire study period but violated a revised standard with a level of 65 ppb 

or below over at least part of that period (U.S. EPA, 2014c, Table 4-1). This analysis suggests 

that although the current standard or a standard with a level of 70 ppb would allow the ambient 

O3 concentrations in most of the study locations that provided the basis for the association with 

mortality in this study, a revised O3 standard with a level at or below 65 ppb would require 

reductions in those ambient O3 concentrations. As discussed above for a level of 70 ppb, this 

analysis does not provide information on the extent to which O3-associated mortality would 
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persist upon meeting an O3 standard with a level of 65 ppb, or on the extent to which standard 

levels below 65 ppb could further reduce the incidence of this mortality.148

With regard to long-term O3 concentrations, as for 70 ppb (above) the PA evaluates the 

long-term O3 metrics reported to be associated with mortality or morbidity in recent 

epidemiologic studies (e.g., seasonal averages of 1-hour or 8-hour daily max concentrations). 

Compared to the current standard or a revised O3 standard with a level of 70 ppb, a revised 

standard with a level of 65 ppb would be expected to further reduce the risk of respiratory 

mortality associated with long-term O3 concentrations, based on information from the study by 

Jerrett et al. (2009).149 In addition, a standard with a level of 65 ppb would be expected to more 

effectively maintain long-term O3 concentrations below those where the study by Jerrett et al. 

(2009) indicates the most confidence in the reported association with respiratory mortality. 

Specifically, air quality analyses indicate this to be the case in 10 out of the 12 urban study areas

for a level of 65 ppb, compared to 6 out of 12 areas for the current standard and 9 out of 12 for a 

standard with a level of 70 ppb (U.S. EPA, 2014c, Table 4-3). Finally, a revised standard with a 

level of 65 ppb would be expected to further reduce long-term O3 concentrations based on the 

types of metrics that have been reported in recent epidemiologic studies to be associated with 

respiratory morbidity (i.e., seasonal averages of daily maximum 8-hour concentrations). 

 

148 For the other multicity studies identified in Table 4-1 of the PA (Cakmak et al., 2006; Stieb et 

al., 2009; Katsouyanni et al., 2009 (for hospital admissions)), and for the study by Bell et al. 

(2006) (for the 30 ppb cut point) (Table 4-2 of the PA), the majority of study locations would 

have met a standard with a level of 65 ppb (U.S. EPA, 2014c). 

149 Though as discussed above, the PA notes the lower confidence placed in these risk results 

(U.S. EPA, 2014a, section 9.6).
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In further considering the potential implications of epidemiology studies for alternative 

standard levels, the PA notes estimates of total mortality associated with short-term O3.

150 As 

discussed above, the PA considers estimates of total risk (i.e., based on the full distributions of 

ambient O3 concentrations) and estimates of risk associated with O3 concentrations in the upper 

portions of ambient distributions. With regard to total risk the PA notes that, when summed 

across urban study areas, a standard with a level of 65 ppb is estimated to reduce the number of 

deaths associated with short-term O3 exposures by about 13% (2007) and 9% (2009), compared 

to the current standard.151 For area-wide concentrations at or above 40 ppb, a standard level of 65

ppb is estimated to reduce O3-associated deaths by almost 50% compared to the current standard, 

when summed across urban study areas. For area-wide concentrations at or above 60 ppb, a

standard level of 65 ppb is estimated to reduce O3-associated deaths by more than 80% (U.S. 

EPA, 2014c, Figure 4-13).

In summarizing CASAC’s advice regarding a standard with a level of 65, the PA noted 

CASAC’s conclusion that an alternative standard with a level of 65 ppb would further reduce, 

though not eliminate, the frequency of lung function decrements > 15% and would lead to lower 

frequency of short-term premature mortality (i.e., compared to a standard with a level of 70 ppb) 

(Frey, 2014c, p. 8). 

In summary, compared to a standard with a level of 70 ppb, the PA concludes that a 

revised standard with a level of 65 ppb would be expected to further reduce O3 exposures and 

 

150 As discussed above, compared to the weight given to the evidence and to HREA estimates of 

exposures of concern and lung function risks, the PA places relatively less weight on 

epidemiologic-based risk estimates.

151 A standard with a level of 65 ppb is also estimated to reduce respiratory mortality associated 

with long-term O3 concentrations in urban study areas. However, given uncertainties associated 

with these risk estimates, as discussed above, we give them limited weight. 
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health risks. In particular, a standard with a level of 65 ppb is estimated to (1) reduce the 

occurrence of exposures of concern by about 80 to 100%, compared to the current standard, 

decreasing exposures at or above 60 ppb and almost eliminating exposures at or above 70 and 80 

ppb; (2) reduce the occurrence of FEV1 decrements > 10, 15, and 20% by about 30 to 65%, 

compared to the current standard; (3) more effectively maintain short- and long-term O3

concentrations below those present in the epidemiologic studies that reported significant O3

health effect associations in locations likely to have met the current standard;

152 and (4) further 

reduce the risk of O3-associated mortality and morbidity, particularly the risk associated with the 

upper portion of the distribution of ambient O3 concentrations. 

iii. PA consideration of an O3 standard level of 60 ppb

The PA also considers a standard with a level of 60 ppb. A level of 60 ppb is well below 

the O3 exposure concentration that has been reported to elicit a wide range of potentially adverse 

respiratory effects in healthy adults (i.e., 80 ppb). A level of 60 ppb is also below the lowest 

concentration where the combined occurrence of respiratory symptoms and lung function 

decrements was observed, a combination judged adverse by the ATS (i.e., 72 ppb). A level of 60 

ppb corresponds to the lowest exposure concentration demonstrated to result in lung function 

decrements that are large enough to be judged an abnormal response by ATS, that meet ATS 

criteria for adversity based on a downward shift in the distribution of FEV1, and that the CASAC 

indicated could be adverse in people with lung disease. A level of 60 ppb also corresponds to the 

lowest exposure concentration at which pulmonary inflammation has been reported in a single 

controlled human exposure study. 

 

152 Though epidemiologic studies also provide evidence for O3 health effect associations in 

locations likely to have met a standard with a level of 65 ppb, as discussed below for a level of 

60 ppb. 
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Based on the HREA analyses of O3 exposures of concern, a standard with a level of 60 

ppb is estimated to eliminate exposures of concern at or above the 70 and 80 ppb benchmark 

concentrations and to be more effective than the higher standard levels at limiting exposures of 

concern at or above 60 ppb. On average over the years 2006 to 2010, for a standard with a level 

of 60 ppb, between 0 and about 1% of children, including asthmatic children, in urban study 

areas are estimated to experience exposures of concern at or above 60 ppb, which CASAC 

indicated is an appropriate exposure of concern for asthmatic children. This reflects a 96% 

reduction (on average across areas), compared to the current standard. Virtually no children are 

estimated to experience two or more exposures of concern at or above 60 ppb. In the worst-case 

location and year, about 2% of children are estimated to experience exposures of concern at or 

above 60 ppb, with far less than 1% estimated to experience two or more such exposures (Table 

4, above). 

Based on the HREA analyses of O3-induced lung function decrements, a standard with a 

level of 60 ppb would be expected to be more effective than a level of 65 or 70 ppb at limiting 

the occurrence of O3-induced lung function decrements. For a standard with a level of 60 ppb, 

about 2% of children, or less, in the urban study areas are estimated to experience one or more 

moderate O3-induced FEV1 decrements ≥ 15% (almost 70% reduction, compared to current 

standard), and about 1% or less are estimated to experience two or more such decrements (3% in 

the location and year with the largest estimates). About 1% of children, or less, are estimated to 

experience large O3-induced lung function decrements (i.e., FEV1 decrement ≥ 20%), even in the 

worst-case locations and year (Table 5, above).

In addition, for a standard with a level of 60 ppb, about 5 to 11% of children in the urban 

study areas are estimated to experience one or more moderate O3-induced lung function 
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decrements that CASAC indicated could be adverse for people with lung disease (i.e., FEV1

decrements ≥ 10%). This reflects an average reduction of about 45%, compared to the current 

standard. About 2 to 6% of children in these areas are estimated to experience two or more such 

decrements (51% reduction, compared to current standard). In the worst-case location and year, 

for a standard set at 60 ppb, up to about 13% of children are estimated to experience one or more 

moderate O3-induced FEV1 decrements ≥ 10%, and 7% are estimated to experience two or more 

such decrements (Table 5, above). 

With regard to O3 epidemiologic studies, the PA notes that a revised standard with a level 

of 60 ppb would be expected to maintain short-term ambient O3 concentrations below those 

present in some of the study locations that provided the basis for reported O3 health effect 

associations and that were likely to have met a revised standard with a level of 70 or 65 ppb. 

Specifically, in all of the U.S. and Canadian epidemiologic studies evaluated, the majority of 

study cities had ambient O3 concentrations that would likely have violated a standard with a level 

of 60 ppb. Thus, none of the U.S. and Canadian epidemiologic studies analyzed provide evidence 

for O3 health effect associations when the majority of study locations would likely have met a 

standard with a level of 60 ppb (U.S. EPA, 2014c, Tables 4-1 and 4-2). As discussed above, 

while this analysis does not provide information on the extent to which the O3-associated 

morbidity or mortality would persist upon meeting an O3 standard with a level of 60 ppb, it 

suggests that a revised O3 standard with a level of 60 ppb would require reductions in the 

ambient O3 concentrations that provided the basis for those health effect associations. 

With regard to long-term O3 concentrations, compared to the current standard or a revised 

O3 standard with a level of 65 or 70 ppb, a revised standard with a level of 60 ppb would be 

expected to further reduce the risk of respiratory mortality associated with long-term O3
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concentrations, based on information from the study by Jerrett et al. (2009).153 In addition, a 

standard with a level of 60 ppb would be expected to more effectively maintain long-term O3

concentrations below those where the study by Jerrett et al. (2009) indicates the most confidence 

in the reported association with respiratory mortality. Specifically, air quality analyses indicate 

this to be the case in all of the urban study areas evaluated at a level of 60 ppb, compared to 6 out 

of 12 areas for the current standard, 9 out of 12 for a standard with a level of 70 ppb, and 10 out 

of 12 for a standard with a level of 65 ppb (U.S. EPA, 2014c, Table 4-3). Finally, a revised 

standard with a level of 60 ppb would be expected to further reduce long-term O3 concentrations 

based on the types of metrics that have been reported in recent epidemiologic studies to be 

associated with respiratory morbidity (i.e., seasonal averages of daily maximum 8-hour 

concentrations).

In further considering the potential implications of epidemiology studies for alternative 

standard levels, the PA notes estimates of total mortality associated with short-term O3

concentrations.154 As discussed above, the PA considers estimates of total risk (i.e., based on the 

full distributions of ambient O3 concentrations) and estimates of risk associated with O3

concentrations in the upper portions of ambient distributions. With regard to total risk the PA 

notes that, when summed across urban study areas, a standard with a level of 60 ppb is estimated 

to reduce the number of deaths associated with short-term O3 exposures by about 15% (2007) 

 

153 Though as discussed above, the PA notes the lower confidence we place in these risk results 

(U.S. EPA, 2014a, section 9.6).

154 As discussed above, compared to the weight given to the evidence and to HREA estimates of 

exposures of concern and lung function risks, we place relatively less weight on epidemiologicbased risk estimates. 
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and 11% (2009), compared to the current standard (U.S. EPA, 2014c, Figure 4-13).155 For areawide concentrations at or above 40 ppb, a standard with a level set at 60 ppb is estimated to 

reduce O3-associated deaths by almost 60% compared to the current standard. For area-wide 

concentrations at or above 60 ppb, a standard level of 60 ppb is estimated to reduce O3-

associated deaths by over 95% compared to the current standard. 

In summary, compared to a standard with a level of 65 or 70 ppb, the PA concludes that a 

revised standard with a level of 60 ppb would be expected to further reduce O3 exposures and 

health risks. In particular, a standard with a level of 60 ppb is estimated to (1) reduce the 

occurrence of exposures of concern by about 95 to 100%, compared to the current standard, 

almost eliminating exposures at or above 60 ppb; (2) reduce the occurrence of FEV1 decrements 

> 10, 15, and 20% by about 45 to 85%, compared to the current standard; (3) more effectively 

maintain short- and long-term O3 concentrations below those present in the epidemiologic 

studies that reported significant O3 health effect associations in locations likely to have met the 

current standard;

156 and (4) further reduce the risk of O3-associated mortality and morbidity, 

particularly the risk associated with the upper portion of the distribution of ambient O3

concentrations. 

c. CASAC Advice

The PA recognizes that decisions regarding the weight to place on various types of 

evidence, exposure/risk information, and associated uncertainties reflect public health policy 

 

155 A standard with a level of 60 ppb is also estimated to reduce respiratory mortality associated 

with long-term O3 concentrations in urban study areas. However, given uncertainties associated 

with these risk estimates, as discussed above, the PA gives them limited weight.

156 As discussed above, these studies do not provide information on the extent to which O3 health 

effect associations would persist following reductions in ambient O3 concentrations in order to 

meet a standard with a level of 60 ppb. 
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judgments that are ultimately left to the Administrator. To help inform those judgments with 

regard to the range of alternative primary O3 standard levels appropriate for consideration, 

CASAC has provided advice to the Administrator based on their reviews of draft versions of the 

O3 ISA, HREA, and PA. This section summarizes the advice provided by CASAC regarding 

alternative standard levels, as well as the views expressed at the CASAC meetings by public 

commenters. This section includes CASAC advice from the reconsideration of the 2008 final 

decision on the level of the standard, as well as CASAC advice received during the current 

review as it pertains to alternative standards. 

Consistent with its advice in 2008, CASAC reiterated during the reconsideration its 

support for an 8-hour primary O3 standard with a level ranging from 60 to 70 ppb, combined 

with the current indicator, averaging time, and form. Specifically, in response to the EPA’s 

solicitation of CASAC advice during the reconsideration, the CASAC letter (Samet, 2010) to the 

Administrator stated: 

CASAC fully supports EPA’s proposed range of 0.060 – 0.070 parts per million 

(ppm) for the 8-hour primary ozone standard. CASAC considers this range to be 

justified by the scientific evidence as presented in the Air Quality Criteria for 

Ozone and Related Photochemical Oxidants (March 2006) and Review of the 

National Ambient Air Quality Standards for Ozone: Policy Assessment of 

Scientific and Technical Information, OAQPS Staff Paper (July 2007).

Similarly, in response to the EPA’s request for additional advice on the reconsideration in 

2011, CASAC reaffirmed its conclusion that “the evidence from controlled human and 

epidemiologic studies strongly supports the selection of a new primary ozone standard within the 

60 – 70 ppb range for an 8-hour averaging time” (Samet, 2011). CASAC further concluded that 

this range “would provide little margin of safety at its upper end” (Samet, 2011, p. 2). 

In the current review of the Second Draft PA, CASAC concurred with staff’s conclusions 

that it is appropriate to consider retaining the current indicator (O3), averaging time (8-hour 
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average) and form (3-year average of the 4th highest maximum daily 8-hour average. With 

regard to level, CASAC stated the following (Frey, 2014c, pp. ii to iii): 

The CASAC further concludes that there is adequate scientific evidence to 

recommend a range of levels for a revised primary ozone standard from 70 ppb to 

60 ppb. The CASAC reached this conclusion based on the scientific evidence 

from clinical studies, epidemiologic studies, and animal toxicology studies, as 

summarized in the Integrated Science Assessment (ISA), the findings from the 

exposure and risk assessments as summarized in the HREA, and the interpretation 

of the implications of these sources of information as given in the Second Draft 

PA.

The CASAC acknowledges that the choice of a level within the range 

recommended based on scientific evidence [i.e., 70 to 60 ppb] is a policy 

judgment under the statutory mandate of the Clean Air Act. The CASAC advises 

that, based on the scientific evidence, a level of 70 ppb provides little margin of 

safety for the protection of public health, particularly for sensitive subpopulations.

Thus, 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, and taking into account that lower levels 

will provide incrementally greater margins of safety.

The public commenters who expressed the view that the current primary O3 standard is 

not adequate (II.D.3) also submitted comments that supported revising the level of the primary 

O3 standard. Several of these commenters expressed the view that the level should be revised to 

the lower end of the range of 70 to 60 ppb, or in some cases to a level below 60 ppb. These 

commenters often placed a large amount of emphasis on evidence from controlled human 

exposure studies for respiratory effects following exposures to 60 ppb O3. 

In addition, as discussed above (II.D.3), some public commenters expressed the view that 

revision of the current standard is not necessary. Consistent with their view that it would not be 

appropriate to revise the current standard, these commenters did not provide any provisional 

views on alternative levels below 75 ppb that would be appropriate for consideration.

d. Administrator’s proposed conclusions on level 
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This section discusses the Administrator’s proposed conclusions on the level of the 

primary O3 standard. In conjunction with her proposed decisions to retain the current indicator, 

averaging time, and form (II.E.1 to II.E.3, above), the Administrator proposes to revise the level 

of the primary O3 standard to within the range of 65 to 70 ppb. In doing so, she is mindful that

the selection of a primary O3 standard that is requisite to protect public health with an adequate 

margin of safety requires judgments based on an interpretation of the scientific evidence and 

exposure/risk information that neither overstates nor understates the strengths and limitations of 

that evidence and information, nor the appropriate inferences to be drawn therefrom.

157 The 

rationale supporting the Administrator's proposed conclusions on alternative standard levels is 

discussed below. 

The Administrator’s proposed conclusions on alternative standard levels build upon her 

proposed conclusion that the overall body of scientific evidence and exposure/risk information 

call into question the adequacy of public health protection afforded by the current primary O3

standard, particularly for at-risk populations and lifestages (II.D.5). These proposed conclusions 

are based on consideration of the scientific evidence assessed in the ISA (U.S. EPA, 2013a); the

results of the exposure and risk assessments in the HREA (U.S. EPA, 2014a); the evidence-based

and exposure-/risk-based considerations and conclusions in the PA (U.S. EPA, 2014c); CASAC 

advice and recommendations, as reflected in CASAC’s letters to the Administrator and in public 

 

157 As discussed above (I.B), in addressing the requirement for an adequate margin of safety the 

EPA considers such factors as the nature and severity of the health effects, the size of sensitive 

population(s) at risk, and the kind and degree of the uncertainties that must be addressed. The 

selection of any particular approach for providing an adequate margin of safety is a policy choice 

left specifically to the Administrator’s judgment. See Lead Industries Association v. EPA, 647 

F.2d at 1161-62; State of Mississippi, 744 F. 3d at 1353.
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discussions of drafts of the ISA, HREA, and PA; and public input received during the

development of these documents.

In reaching proposed conclusions on alternative levels for the primary O3 standard, the 

Administrator considers the extent to which various alternatives would be expected to protect the 

public, including at-risk populations, against the wide range of adverse health effects that have 

been linked with short- or long-term O3 exposures. At-risk populations include people with 

asthma; children and older adults; people who are active outdoors, including outdoor workers;

people with certain genetic variants; and people with reduced intake of certain nutrients. 

As was the case for her consideration of the adequacy of the current primary O3 standard

(II.D.5), the Administrator places the greatest weight on the results of controlled human exposure 

studies and on exposure and risk analyses based on information from these studies. In doing so, 

she notes that controlled human exposure studies provide the most certain evidence indicating 

the occurrence of health effects in humans following exposures to specific O3 concentrations. 

The effects reported in these studies are due solely to O3 exposures, and interpretation of study 

results is not complicated by the presence of co-occurring pollutants or pollutant mixtures (as is 

the case in epidemiologic studies). She further notes the CASAC judgment that “the scientific 

evidence supporting the finding that the current standard is inadequate to protect public health is 

strongest based on the controlled human exposure studies of respiratory effects” (Frey, 2014c, p. 

5). Consistent with this emphasis, the HREA conclusions reflect relatively greater confidence in 

the results of the exposure and risk analyses based on information from controlled human 

exposure studies (i.e., exposures of concern and risk of lung function decrements) than the 

results of epidemiology-based risk analyses, given the greater uncertainties in the epidemiologybased risk estimates (U.S. EPA, 2014a, section 9.6). For all of these reasons, the Administrator 
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has the most confidence in using the information from controlled human exposure studies to 

reach proposed conclusions on alternative standard levels. 

In considering the evidence from controlled human exposure studies, the Administrator 

first notes that these studies have reported a variety of respiratory effects in healthy adults 

following exposures to O3 concentrations of 60,158 72,159 or 80 ppb, and higher. The largest 

respiratory effects, and the broadest range of effects, have been studied and reported following 

exposures of healthy adults to 80 ppb O3 or higher, with most exposure studies conducted at 

these higher concentrations. Exposures of healthy adults to O3 concentrations of 80 ppb or higher 

have been reported to decrease lung function, increase airway inflammation, increase respiratory 

symptoms, result in airway hyperresponsiveness, and decrease lung host defenses (II.B.2). 

The Administrator notes that O3 exposure concentrations as low as 72 ppb have been 

shown to both decrease lung function and increase respiratory symptoms (Schelegle et al., 2009), 

a combination that meets the ATS criteria for an adverse response. In considering effects at 72 

ppb, CASAC likewise noted that “the combination of decrements in FEV1 together with the 

statistically significant alterations in symptoms in human subjects exposed to 72 ppb ozone 

meets the American Thoracic Society’s definition of an adverse health effect” (Frey, 2014c, p. 

5).

With regard to lower exposure concentrations, the Administrator notes that the 

combination of statistically significant increases in respiratory symptoms and decrements in lung 

 

158 As discussed above (II.B.2), exposures to 60 ppb O3 have been evaluated in studies by Adams 

(2002, 2006), Schelegle et al. (2009), and Kim et al. (2011). In the study by Schelegle, for the 60 

ppb target exposure concentration, study authors reported that the actual mean exposure 

concentration was 63 ppb. 

159 As noted above, for the 70 ppb target exposure concentration, Schelegle et al. (2009) reported 

that the actual mean exposure concentration was 72 ppb.
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function has not been reported. More specifically, she notes that respiratory symptoms have been 

evaluated following 6.6-hour exposures to average O3 concentrations of 60 ppb (Adams, 2006; 

Kim et al., 2011) and 63 ppb (Schelegle et al., 2009) and that none of these studies reported a 

statistically significant increase in respiratory symptoms, compared to filtered air controls.160

Based on this evidence, the Administrator reaches the initial conclusion that the results of 

controlled human exposure studies strongly support setting the level of a revised O3 standard no 

higher than 70 ppb. In reaching this initial conclusion, the Administrator places a large amount 

of weight on the importance of setting the level of the standard well below 80 ppb, the O3

exposure concentration shown in healthy adults to result in the broadest range of respiratory 

effects, and below 72 ppb, the lowest O3 exposure concentration shown in healthy adults to result 

in the adverse combination of respiratory symptoms and lung function decrements. 

In further considering the potential public health implications of a standard with a level of 

70 ppb, the Administrator also considers the extent to which such a standard would be expected 

to limit population exposures to the broader range of O3 concentrations reported in controlled 

human exposure studies to cause respiratory effects. Given the range of effects reported 

following exposures to 80 ppb O3, and the evidence for the adverse combination of lung function 

decrements and respiratory symptoms in healthy adults following exposures as low as 72 ppb, 

the Administrator concludes that the evidence in this review supports the occurrence of adverse 

respiratory effects for exposures to O3 concentrations at or above 72 ppb. 

The Administrator has decreasing confidence that adverse effects will occur following 

exposures to O3 concentrations below 72 ppb. In particular, compared to O3 exposure 

 

160 However, following exposures to 60 ppb O3, several studies have observed decreases in lung 

function and one study (Kim et al., 2011) observed an increase in airway inflammation (II.B.2).
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concentrations at or above 72 ppb, she has less confidence that adverse effects will occur 

following exposures to O3 concentrations as low as 60 ppb. In reaching this conclusion, she notes 

that, as discussed above, statistically significant increases in respiratory symptoms, combined 

with lung function decrements, have not been reported following exposures to 60 or 63 ppb O3, 

though several studies have evaluated the potential for such effects. 

Although she has decreasing confidence in the occurrence of adverse effects following 

exposures to O3 concentrations below 72 ppb, the Administrator notes the CASAC judgment that 

the adverse combination of lung function decrements and respiratory symptoms “almost 

certainly occur in some people” following exposures to lower concentrations (Frey, 2014c, p. 6). 

In particular, when commenting on the extent to which the study by Schelegle et al. (2009) 

suggests the potential for adverse effects following O3 exposures below 72 ppb, CASAC judged 

that: 

[I]f subjects had been exposed to ozone using the 8-hour averaging period used in 

the standard [i.e., rather than the 6.6 hour exposures evaluated in the study],

adverse effects could have occurred at lower concentration. Further, in our 

judgment, the level at which adverse effects might be observed would likely be 

lower for more sensitive subgroups, such as those with asthma [i.e., compared to 

the healthy adults evaluated in the study] (Frey, 2014c, p. 5).

Though CASAC did not provide advice as to how far below 72 ppb adverse effects would likely 

occur, the Administrator agrees that such effects could occur following exposures at least 

somewhat below 72 ppb. 

Based on the evidence and CASAC advice noted above, when considering the extent to 

which a standard with a level of 70 ppb would be expected to limit population exposures to the 

broader range of O3 concentrations shown to cause respiratory effects, the Administrator 

considers the extent to which such a standard would be expected to limit the occurrence of O3
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exposures of concern at or above 60, 70, and 80 ppb.

161 In doing so, she notes that an O3 standard 

established at a particular level can provide protection against a range of exposure 

concentrations, including concentrations below the standard level. This is because the degree of 

protection provided by any NAAQS is due to the combination of all of the elements of the 

standard (i.e., indicator, averaging time, form, level). In the case of the 4th maximum form of the 

O3 NAAQS, which the Administrator is proposing to retain in the current review (II.E.3), the 

large majority of days in areas that meet the standard will have 8-hour O3 concentrations below 

the level of the standard.

In considering exposures of concern at or above 60, 70, and 80 ppb, the Administrator

judges that the evidence supporting the occurrence of adverse respiratory effects is strongest for 

exposures at or above the 70 and 80 ppb benchmarks. While the Administrator has less 

confidence that adverse effects will occur following exposures to O3 concentrations as low as 60 

ppb, she notes the possibility for adverse effects following such exposures given that (1) CASAC 

has indicated the moderate lung function decrements (i.e., FEV1 decrements ≥ 10%) that occur in 

some healthy adults following exposures to 60 ppb O3, which are large enough to be judged an 

abnormal response by ATS, could be adverse to people with lung disease (II.B.3), and that (2) 

airway inflammation has been reported following exposures as low as 60 ppb O3. She also takes 

 

161 As with her consideration of the current standard (II.D.5), the Administrator focuses on 

estimated exposures of concern in children, including asthmatic children, noting the HREA 

analyses indicating that exposures of concern occur in a larger percentage of children than adults 

(given that a larger percentage of children are estimated to spend an extended period of time 

being physically active outdoors when O3 concentrations are elevated) (II.C.2). To the extent 

alternative standards provide an appropriate degree of protection for children, she judges that 

those standards will also protect adult populations (including at-risk adult populations).
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note of CASAC advice that the occurrence of exposures of concern at or above 60 ppb is an 

appropriate consideration for people (including children) with asthma (Frey, 2014c, p. 6).

Due to interindividual variability in responsiveness, the Administrator further notes that 

not every occurrence of an exposure of concern will result in an adverse effect.162 Repeated 

occurrences of some of the effects demonstrated following exposures of concern could increase 

the likelihood of adversity. For example, as discussed in the ISA (U.S. EPA, 2013a, Section 

6.2.3), repeated occurrences of airway inflammation could lead to the induction of a chronic 

inflammatory state; altered pulmonary structure and function, leading to diseases such as asthma; 

altered lung host defense response to inhaled microorganisms, particularly in potentially at-risk 

populations such as the very young and old; and altered lung response to other agents such as 

allergens or toxins. The Administrator notes that the types of lung injury that can occur following 

exposures of concern, particularly if experienced repeatedly, provide a plausible mode of action 

by which O3 may cause other more serious effects. Therefore, the Administrator is most 

concerned about protecting at-risk populations against repeated occurrences of exposures of 

concern. 

Based on the above considerations, the Administrator focuses on the extent to which a 

revised standard would be expected to protect populations from experiencing two or more O3

exposures of concern (i.e., as a surrogate for repeated exposures). While she emphasizes the 

importance of limiting two or more exposures and reducing their occurrence, compared to the 

current standard, she balances this emphasis by noting that (1) not all exposures of concern will 

 

162 For most of the effects demonstrated in controlled human exposure studies (e.g., airway 

inflammation, AHR, decreased lung host defense, respiratory symptoms) the available data are 

not sufficient to quantify the number of people who would experience adverse effects due to O3

exposures. 
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result in adverse effects; (2) she has less confidence in the occurrence of adverse effects at the 60 

ppb benchmark than at the 70 or 80 ppb benchmarks; and (3) the NAAQS are not meant to be 

zero-risk standards.

163 Therefore, in using estimates of exposures of concern to inform her 

decisions on alternative standard levels, the Administrator judges that it would not be appropriate 

to set a standard intended to eliminate all exposures of concern for all benchmarks, particularly 

the 60 ppb benchmark. Her consideration of specific estimates of exposures of concern is 

discussed below. 

As illustrated in Table 1 (above), the Administrator notes that, in urban study areas, a 

revised standard with a level of 70 ppb would be expected to eliminate the occurrence of two or 

more exposures of concern to O3 concentrations at and above 80 ppb and to virtually eliminate 

the occurrence of two or more exposures of concern to O3 concentrations at and above 70 ppb, 

even in the worst-case urban study area and year. For the 70 ppb benchmark, this reflects about a 

95% reduction in the occurrence of two or more exposures of concern, compared to the current 

standard (Table 4). 

Though the Administrator acknowledges greater uncertainty with regard to the 

occurrence of adverse effects following exposures of concern at or above 60 ppb, she notes that a 

revised standard with a level of 70 ppb would also be expected to protect the large majority of 

children in the urban study areas (i.e., about 96% to more than 99% of children in individual 

urban study areas) from experiencing two or more exposures of concern at or above 60 ppb. 

Compared to the current standard, this represents a reduction of more than 60% in the occurrence 

of two or more exposures of concern (Tables 1 and 4). 

 

163 State of Mississippi, 744 F. 3d at 1343.
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Based on the above information, the Administrator concludes that a revised O3 standard 

with a level of 70 ppb would be expected to virtually eliminate the occurrence of two or more O3

exposures of concern for the 70 and 80 ppb benchmarks, and to substantially reduce the 

occurrence of two or more O3 exposures of concern for the 60 ppb benchmark, compared to the 

current standard. 

Although the Administrator is less concerned about single occurrences of exposures of 

concern, she acknowledges that even single exposures to O3 concentrations at or above 

benchmark concentrations (particularly for the 70 and 80 ppb benchmarks) could potentially 

result in adverse effects. To the extent this may be the case, the Administrator notes that a 

standard with a level of 70 ppb would also be expected to (1) virtually eliminate all occurrences 

of exposures of concern at or above 80 ppb, even in the worst-case year and location and (2) 

achieve important reductions, compared to the current standard, in the occurrence of one or more 

exposures of concern at or above 70 and 60 ppb (i.e., about a 70% reduction for the 70 ppb 

benchmark and almost a 50% reduction for the 60 ppb benchmark) (Tables 1 and 4). 

In further evaluating the potential public health impacts of a standard with a level of 70 

ppb, the Administrator also considers the HREA estimates of O3-induced lung function 

decrements. To inform her consideration of these decrements, the Administrator takes note of 

CASAC advice that “estimation of FEV1 decrements of > 15% is appropriate as a scientifically 

relevant surrogate for adverse health outcomes in active healthy adults, whereas an FEV1

decrement of > 10% is a scientifically relevant surrogate for adverse health outcomes for people 

with asthma and lung disease” (Frey, 2014c, p. 3). Consistent with this advice, she considers 

estimates of the occurrence of O3-induced FEV1 decrements > 10 and 15% as surrogates for the 

occurrence of adverse health outcomes. 
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While these surrogates provide perspective on the potential for the occurrence of adverse 

respiratory effects following O3 exposures, the Administrator agrees with the conclusion in past 

reviews that a more general consensus view of the adversity of moderate responses emerges as 

the frequency of occurrence increases (61 FR 65722-3) (Dec, 13, 1996). Specifically, she 

concludes that not every estimated occurrence of an O3-induced FEV1 decrement will be adverse 

and that repeated occurrences of moderate responses, even in otherwise healthy individuals, may 

be considered to be adverse since they could set the stage for more serious illness. Therefore, the 

Administrator becomes increasingly concerned about the potential for adversity as the frequency 

of occurrences increases and, as a result, she focuses primarily on estimates of two or more O3-

induced FEV1 decrements (i.e., as a surrogate for repeated exposures).

Given the above considerations, the Administrator does not believe it would be 

appropriate to set a standard that is intended to eliminate all O3-induced FEV1 decrements. She 

notes that this is consistent with CASAC advice, which did not include a recommendation to set 

the standard level low enough to eliminate all O3-induced FEV1 decrements > 10 or 15% (Frey, 

2014c). Rather, the Administrator considers the extent to which a standard with a level of 70 ppb 

would be expected to protect the population from experiencing O3-induced FEV1 decrements >

10 and 15%, including the extent to which such a standard would be expected to achieve 

reductions in the occurrence of O3-induced FEV1 decrements, relative to the current standard.164

 

164 The Administrator additionally notes that, unlike exposures of concern, the variability in lung 

function risk estimates across urban study areas is often greater than the differences in risk 

estimates between various standard levels (Table 2, above). Given this, and the resulting 

considerable overlap between the ranges of lung function risk estimates for different standard 

levels, although the Administrator has confidence in the lung function risk estimates themselves, 

she views them as providing a more limited basis than exposures of concern for distinguishing 

between the degree of public health protection provided by alternative standard levels. 
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The Administrator notes that a revised O3 standard with a level of 70 ppb is estimated to 

protect about 98 to 99% of children in urban study areas from experiencing two or more O3-

induced FEV1 decrements > 15%, and about 89 to 94% from experiencing two or more 

decrements > 10%.165 Compared to the current standard, these estimates represent decreases in 

the occurrence of two or more O3-induced decrements of about 29 and 17%, respectively (Tables 

2 and 5). Although the Administrator is less concerned about the public health implications of 

single O3-induced lung function decrements, she also gives some consideration to estimates of 

one or more O3-induced FEV1 decrements. In particular, she notes that a revised standard with a 

level of 70 ppb is estimated to reduce the occurrence of one or more O3-induced decrements, 

compared to the current standard, by about 26% (for decrements > 15%) and 15% (for 

decrements > 10%) (Tables 2 and 5). 

Given all of the above information, the Administrator concludes that a revised standard 

with a level of 70 ppb would be expected to provide substantial protection against O3 exposures 

of concern (for benchmark concentrations of 60, 70, 80 ppb) and O3-induced lung function 

decrements, and would be expected to result in important reductions in the occurrence of such 

exposures and decrements, compared to the current standard. This is particularly the case for 

estimates of two or more occurrences of exposures of concern and lung function decrements. 

In next considering the additional protection that would be expected from standard levels 

below 70 ppb, the Administrator evaluates the extent to which a standard with a level of 65 ppb 

would be expected to further limit O3 exposures of concern and O3-induced lung function 

decrements. 

 

165 In the worst-case year and location, a standard with a level of 70 ppb is estimated to protect 

about 97% of children in urban study areas from experiencing two or more O3-induced FEV1

decrements > 15%, and about 87% from experiencing two or more decrements > 10%. 
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In addition to eliminating almost all exposures of concern to O3 concentrations at or 

above 80 and 70 ppb, even in the worst-case years and locations, the Administrator notes that a 

revised standard with a level of 65 ppb would be expected to protect more than 99% of children 

in urban study areas (and 100% of children in some urban study areas) from experiencing two or 

more exposures of concern at or above 60 ppb. Compared to the current standard, this represents 

about a 95% reduction in the occurrence of two or more exposures of concern for the 60 ppb 

benchmark (Tables 1 and 4). In addition, the Administrator notes that a revised standard with a 

level of 65 ppb is estimated to reduce the occurrence of one or more exposures of concern for the 

60 ppb benchmark by about 80%, compared to the current standard (Tables 1 and 4). 

With regard to O3-induced lung function decrements, the Administrator notes that an O3

standard with a level of 65 ppb is estimated to protect about 98% to more than 99% of children 

from experiencing two or more O3-induced FEV1 decrements > 15%, even considering the 

worst-case year and location, and about 91 to 99% from experiencing two or more decrements >

10% (89% in worst-case year and location). These estimates reflect reductions, compared to the 

current standard, of about 54 and 37%, respectively. A revised standard with a level of 65 ppb is 

also estimated to reduce the occurrence of one or more lung function decrements > 15 and 10%, 

compared to the current standard, by about 50 and 31%, respectively. 

Taken together, the Administrator initially concludes that the evidence from controlled 

human exposure studies, and the information from quantitative analyses that draw upon these 

studies (i.e., exposures of concern, O3-induced FEV1 decrements), provide strong support for 

standard levels from 65 to 70 ppb. In particular, she bases this conclusion on the fact that such 

standard levels would be well below the O3 exposure concentration shown to result in the widest 

range of respiratory effects (i.e., 80 ppb), and below the lowest O3 exposure concentration shown 
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to result in the adverse combination of lung function decrements and respiratory symptoms (i.e., 

72 ppb). A standard with a level from 65 to 70 ppb would also be expected to result in important 

reductions, compared to the current standard, in the occurrence of O3 exposures of concern for 

all of the benchmarks evaluated (i.e., 60, 70, and 80 ppb) and in the risk of O3-induced lung 

function decrements > 10 and 15%. 

In further considering the evidence and exposure/risk information, the Administrator 

considers the extent to which the epidemiologic evidence, and the quantitative risk estimates 

based on information from epidemiologic studies, also provide support for standard levels from 

65 to 70 ppb. In doing so, as in her consideration of the adequacy of the current O3 standard, the 

Administrator focuses on epidemiologic studies of respiratory-related hospital admissions, 

emergency department visits, and mortality. These considerations are discussed below. 

The Administrator first considers the extent to which available epidemiologic studies 

have reported associations between short-term O3 concentrations and emergency department 

visits, hospital admissions, and/or mortality in locations that would likely have met alternative 

standards with levels from 65 to 70 ppb (U.S. EPA, 2014c, section 4.4.1). In evaluating the 

epidemiologic evidence in this way, the Administrator places the most weight on single-city 

studies of short-term O3 concentrations, recognizing that there were no multicity studies for 

which air quality data indicated that all cities included in the analyses would likely have met 

alternative standard levels. In particular, she notes that while single-city studies are more limited 

than multicity studies in terms of statistical power and geographic coverage, conclusions linking 

air quality in a given city with health effect associations in that same city can be made with 

greater certainty for single-city studies of short-term O3, compared to health effect associations 

aggregated across multiple cities in multicity studies. In particular, the Administrator notes 
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considerable uncertainty in linking multicity effect estimates (aggregated across multiple cities) 

for short-term O3 with the air quality for subsets of study locations (rather than all locations) 

likely to have met an alternative standard.166

Given the above, the Administrator notes analyses in the PA (U.S. EPA, 2014c, section 

4.4.1) indicating that a revised standard with a level of 65 or 70 ppb would be expected to 

maintain short-term ambient O3 concentrations below those present in the locations of all of the 

single-city studies analyzed. As discussed in the PA (U.S. EPA, 2014c, section 4.4.1), this 

includes several single-city studies conducted in locations that would have violated the current 

standard, and the single-city study by Mar and Koenig (2009) that reported positive and 

statistically significant associations with respiratory emergency department visits with children 

and adults in a location that likely would have met the current standard over the entire study 

period but that would likely not have met a revised standard with a level of 70 ppb or below. 

Thus, the Administrator notes that, while the current standard would allow the ambient O3

concentrations that provided the basis for the health effect associations reported by Mar and 

Koenig (2009), a revised O3 standard with a level at or below 70 ppb would require reductions in 

 

166 In recognizing that multicity studies are often emphasized over single-city studies for 

purposes of making weight of evidence judgments (U.S. EPA, 2013a), the Administrator’s 

judgment in this case applies specifically to interpreting air quality analyses for epidemiologic 

studies of short-term O3 concentrations where multicity effect estimates are aggregated across 

cities, and where individual city effect estimates are not presented (as is the case for the key O3

studies analyzed in the PA, with the exception of the study by Stieb et al. (2009) where none of 

the city-specific effect estimates for asthma emergency department visits were statistically 

significant). Because reported multicity effect estimates do not allow health effect associations to 

be disaggregated by individual city, it is not possible to assign the multicity health effect 

association to the air quality in any one study location, or to the air quality in a particular subset 

of locations. In contrast, for epidemiologic studies of long-term concentrations, where multicity 

effect estimates are based on comparisons across cities, different judgments have been made by 

EPA with regard to the utility of multicity studies (see, e.g. 78 FR 3086 at 3103/2, January 15, 

2013) (and see discussion below of study by Jerrett et al., 2009). 
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those ambient O3 concentrations. While the Administrator acknowledges uncertainty in the 

extent to which the reported O3-associated emergency department visits could be further reduced 

by standard levels below 65 or 70 ppb, she concludes that this analysis indicates that a revised 

standard with a level at least as low as 70 ppb would result in improvements in public health, 

beyond the protection provided by the current standard, in the locations of the single-city 

epidemiologic studies that reported significant health effect associations. 

As discussed above, the Administrator notes the greater uncertainty in interpreting air 

quality in locations of multicity epidemiologic studies of short-term O3 for the purpose of 

evaluating alternative standard levels (II.D.1 and U.S. EPA, 2014c, section 4.4.1). Therefore, she 

places less weight on these studies than on the single-city studies noted above. Despite this 

uncertainty, she notes that PA analyses suggest that standard levels of 65 or 70 ppb would 

require additional reductions, beyond those required by the current standard, in ambient O3

concentrations in several of the epidemiologic study locations that provided the basis for 

statistically significant O3 health effect associations. For example, she notes that Dales et al. 

(2006) reported significant associations with respiratory hospital admissions based on air quality 

in 11 Canadian cities, most of which would likely have met the current standard over the entire 

study period (i.e., seven cities) but would have violated a standard with a level of 70 ppb or 

below over at least part of that period (U.S. EPA, 2014c, Table 4-1). She further notes that 

Katsouyanni et al. (2009) reported statistically significant associations with mortality based on 

air quality in 12 Canadian cities, most of which would likely have met the current standard (i.e., 

eight study cities) and a standard with a level of 70 ppb (i.e., seven study cities) over the entire 

study period, but would have violated a standard with a level of 65 ppb over at least part of that

period (U.S. EPA, 2014c, Table 4-1). While most of the other multicity epidemiologic studies 
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evaluated also suggest that a level from 65 to 70 ppb would result in public health improvements, 

compared to the current standard, the Administrator acknowledges that several multicity 

epidemiologic studies reported O3 health effect associations when the majority of study cities 

would likely have met standards with levels from 65 to 70 ppb. However, given the important 

uncertainties in interpreting the air quality in these multicity studies, the Administrator places 

limited weight on them overall, relative to the single-city studies noted above (and relative to the 

information based on controlled human exposure studies). 

With regard to long-term O3 concentrations, the Administrator considers the long-term O3

metrics reported to be associated with mortality or morbidity in recent epidemiologic studies 

(e.g., seasonal averages of 1-hour or 8-hour daily max concentrations). Compared to the current 

standard, she notes that analyses in the PA (U.S. EPA, 2014c, section 4.4.1) suggest a revised 

standard with a level of 65 or 70 ppb would more effectively maintain long-term O3

concentrations below those where the multicity study by Jerrett et al. (2009) indicates the most 

confidence in the reported association with respiratory mortality (II.B.2, II.D.1). Based on 

additional information from the study by Jerrett et al. (2009), the Administrator also notes HREA 

analyses indicating that a revised standard with a level of 65 or 70 ppb would be expected to 

reduce the risk of respiratory mortality associated with long-term O3 concentrations (though she 

also notes important uncertainties with these risk estimates, as described below). Finally, she 

notes analyses in the HREA suggesting that a revised standard with a level of 65 or 70 ppb 

would be expected to reduce long-term O3 concentrations, defined in terms of O3 metrics similar 

to the long-term metrics that have been reported in recent epidemiologic studies to be associated 

with respiratory morbidity (i.e., seasonal averages of daily maximum 8-hour concentrations). 

Given the above evidence and information, the Administrator concludes that a revised 8-hour 
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standard with a level from 70 to 65 ppb could increase public health protection, compared to the 

current standard, against effects associated with long-term O3 exposures. 

In further evaluating information from epidemiologic studies, the Administrator also 

considers the HREA’s epidemiology-based risk estimates of morbidity and mortality associated 

with short-term O3 (U.S. EPA, 2014a). Compared to the weight given to the evidence from 

controlled human exposure studies, and to HREA estimates of exposures of concern and lung

function risks, she places relatively less weight on epidemiology-based risk estimates. In doing 

so, she notes that the overall conclusions from the HREA likewise reflect relatively less 

confidence in estimates of epidemiology-based risks than in estimates of exposures of concern 

and lung function risks. As discussed above (II.C.3.b), this is based on the greater uncertainties 

associated with mortality and morbidity risk estimates, including the heterogeneity in effect 

estimates between locations, the potential for exposure measurement errors, and uncertainty in 

the interpretation of the shape of concentration-response functions at lower O3 concentrations. 

The Administrator further notes the HREA conclusion that lower confidence should be placed in 

the results of the assessment of respiratory mortality risks associated with long-term O3

exposures, primarily because that analysis is based on only one study (even though that study is 

well-designed) and because of the uncertainty in that study regarding the existence and 

identification of a potential threshold in the concentration-response function (U.S. EPA, 2014a, 

section 9.6). 

In considering epidemiology-based risk estimates, the Administrator focuses on the 

extent to which potential alternative O3 standards are estimated to reduce the risk of mortality 

associated with short-term exposures to O3, noting the similar patterns of risk across urban study 

areas and air quality scenarios for respiratory morbidity endpoints (II.C.3). Given the 
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uncertainties in epidemiology-based risk estimates, the Administrator focuses on the general 

magnitudes of risk changes estimated for standard levels of 65 and 70 ppb, compared to the 

current standard, rather than placing a large amount of weight on the absolute estimates of O3-

associated deaths. In doing so, she notes the CASAC conclusion that “[a]lthough the estimates 

for short-term exposure impacts are subject to uncertainty, the data supports a conclusion that 

there are meaningful reductions in mean premature mortality associated with ozone levels lower 

than the current standard” (Frey, 2014a, p. 10). She further notes that, as discussed above 

(II.C.3.b), the HREA risk estimates for urban study areas are likely to understate the average 

reductions in O3-associated mortality and morbidity risks that would be experienced across the 

U.S. population as a whole upon meeting standards with lower levels. 

The Administrator’s primary focus is on risks associated with O3 concentrations in the 

upper portions of ambient distributions, given the greater uncertainty associated with the shapes 

of concentration-response curves for O3 concentrations in the lower portions of ambient 

distributions.167 The Administrator further notes that experimental studies provide the strongest 

evidence for O3-induced effects following exposures to O3 concentrations corresponding to the 

upper portions of typical ambient distributions. In particular, as discussed above, she notes 

controlled human exposure studies showing respiratory effects following exposures to O3

concentrations at or above 60 ppb (II.B). 

In considering risks associated with O3 concentrations in the upper portions of ambient 

distributions, the Administrator focuses on area-wide O3 concentrations at or above 40 ppb and 

60 ppb. For area-wide O3 concentrations at or above 40 ppb, the Administrator notes that revised 

 

167 The ISA concludes that there is less certainty in the shape of concentration-response functions 

for area-wide O3 concentrations at the lower ends of warm season distributions (i.e., below about 

20 to 40 ppb) (U.S. EPA, 2013a, section 2.5.4.4). 
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standards with levels of 70 or 65 ppb are estimated to reduce the number of premature deaths 

associated with short-term O3 concentrations by about 10% and almost 50%, respectively, 

compared to the current standard.168 In addition, for area-wide concentrations at or above 60 ppb, 

revised standards are estimated to reduce O3-associated premature deaths by about 50% to 70% 

for a standard level of 70 ppb, and by more than 80% for a standard level of 65 ppb.169 Risk 

reductions are smaller when total risks are considered (II.C.3.b). 

Given all of the above evidence, exposure/risk information, and advice from CASAC, the 

Administrator proposes to revise the level of the current primary O3 standard to within the range 

of 65 to 70 ppb. She concludes that a standard with a level from within this range could 

reasonably be judged to be requisite to protect public health with an adequate margin of safety, 

based on her consideration of the evidence and information discussed above. In reaching this 

conclusion, she particularly notes that a level from anywhere within this range would be below 

the lowest O3 exposure concentration shown to result in the adverse combination of respiratory 

symptoms and lung function decrements (i.e., 72 ppb), would be expected to maintain ambient 

O3 concentrations below those in locations where single-city studies assessed in the ISA have 

reported statistically significant O3 health effect associations, and would be expected to result in 

important reductions in O3 exposures and health risks, compared to the current standard.

 

168 For area-wide O3 concentrations at or above 40 ppb, reductions in estimated premature deaths 

are disproportionately larger with the 65 ppb standard level than with the 70 ppb standard level. 

This results from the larger air quality adjustments required to meet the 65 ppb level. Across 

urban study areas, the additional reductions required to meet 65 ppb result in many fewer days 

with area-wide O3 concentrations at or above 40 ppb and, therefore, many fewer O3-associated 

deaths for area-wide concentrations at or above 40 ppb (U.S. EPA, 2014a, Figures 7-2 and 7-3). 

169 Though only a relatively small number of days in urban study areas had area-wide O3

concentrations at or above 60 ppb. 
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The Administrator notes that the determination of what constitutes an adequate margin of 

safety is expressly left to the judgment of the EPA Administrator. She further notes that in 

evaluating how particular standards address the requirement to provide an adequate margin of 

safety, the Administrator must consider such factors as the nature and severity of the health 

effects, the size of sensitive population(s) at risk, and the kind and degree of the uncertainties 

that must be addressed (I.B, above). Consistent with past practice and long-standing judicial 

precedent, she takes the need for an adequate margin of safety into account as an integral part of 

her decision-making on the appropriate level, averaging time, form, and indicator of the 

standard.170

The Administrator notes that the NAAQS are not designed to be zero-risk or background 

standards, and that the sizeable risk reductions that are estimated in the HREA to be associated 

with standard levels of 65 or 70 ppb represent substantial improvements in public health for 

important segments of the population, including at-risk groups such as children and people with 

asthma. Although any rationale supporting a decision to set a specific level within the range of 

65 to 70 ppb would discuss the full body of evidence and information, the Administrator notes 

that certain aspects of this evidence and information could be particularly important in 

distinguishing between the appropriateness of a level closer to 65 ppb versus a level closer to 70

ppb.171

 

170 See, e.g. NRDC v. EPA, 902 F. 2d 962, 973-74 (D.C. Cir. 1990).

171 Although this discussion refers to supporting rationale for a level of 65 ppb or 70 ppb, the 

Administrator is proposing the entire range between 65 and 70 ppb. The Administrator notes that 

although neither the PA nor CASAC reached conclusions or provided advice on a standard set at 

a specific level between 65 ppb and 70 ppb, there is nothing in either the evidence, exposure/risk 

information, or CASAC advice that would preclude such a standard level. 
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For example, a level at or near 65 ppb could be judged requisite to protect public health 

with an adequate margin of safety to the extent the Administrator places greater weight on the 

importance of: (1) eliminating almost all exposures of concern (even single occurrences) at or 

above 70 and 80 ppb, even in worst-case years and locations; (2) almost eliminating the 

occurrence of two or more exposures of concern at or above 60 ppb; (3) achieving additional 

reductions in O3-induced FEV1 decrements, beyond those achieved with a level of 70 ppb (4) 

maintaining ambient concentrations below those in locations of single-city studies and more 

effectively doing so for multicity studies (i.e., more effectively than 70 ppb); and (5) achieving 

substantial reductions, compared to a standard with a level of 70 ppb, in mortality associated 

with the upper portion of the distribution of ambient O3 concentrations, despite uncertainties in 

risk estimates. 

In contrast, a level at or near 70 ppb could be judged requisite to protect public health 

with an adequate margin of safety to the extent the Administrator places a greater amount of 

weight (i.e., greater than for 65 ppb) on the importance of: (1) almost eliminating the occurrence 

of two or more exposures of concern at or above 70 and 80 ppb, even in the worst-case year and 

location; (2) substantially reducing, but not eliminating, the occurrence of two or more exposures 

of concern at or above 60 ppb, noting conclusions regarding increasing uncertainty in adverse 

effects for the 60 ppb benchmark; (3) reducing, but not eliminating, the occurrence of one or 

more exposures of concern, noting that not all exposures of concern result in adverse effects; (4) 

maintaining ambient O3 concentrations below those in locations of single-city epidemiologic 

studies, and uncertainties in analyses of air quality in multicity study locations; and (5) 

recognizing uncertainties in epidemiology-based risk estimates.
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In considering CASAC advice on the range of standard levels, the Administrator first 

notes CASAC’s conclusion that there is adequate scientific evidence to consider a range of levels 

for a primary standard that includes an upper end at 70 ppb. For the reasons discussed above, she 

agrees with this advice. She also notes that while CASAC concluded that a standard with a level 

of 70 ppb “may not meet the statutory requirement to protect public health with an adequate 

margin of safety” (Frey, 2014c, p. 8), it 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, 2014c, p. ii). While she agrees with CASAC that it is appropriate to 

consider levels below 70 ppb, as reflected in her range of proposed levels from 65 to 70 ppb, for 

the reasons discussed above she also concludes that a standard level as high as 70 ppb, which 

CASAC concluded could be supported by the scientific evidence, could reasonably be judged to 

be requisite to protect public health with an adequate margin of safety. 

The Administrator has also considered the appropriateness of standard levels below 65 

ppb. In doing so, she notes the conclusions of the PA and the advice of CASAC that it would be 

appropriate for her to consider standard levels as low as 60 ppb. In particular, she notes that a 

decision to set the primary O3 standard level at 60 ppb would place a large amount of weight on 

the potential public health importance of virtually eliminating even single occurrences of 

exposures of concern at and above 60 ppb, though controlled human exposure studies have not 

reported the adverse combination of respiratory symptoms and decrements in lung function 

following exposures to 60 ppb O3; on the potential public health importance of further reducing 

the occurrence of O3-induced lung function decrements > 10 and 15%; on analyses of ambient 

O3 concentrations in locations of multicity epidemiologic studies, despite uncertainties in linking 

multicity effect estimates for short-term O3 with air quality in individual study cities; and on 

epidemiology-based risk estimates, despite the important uncertainties in those estimates. 
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However, as discussed more fully above, given the uncertainties associated with the adversity of 

exposures to 60 ppb O3, particularly single occurrence of such exposures; uncertainties 

associated with air quality analyses in locations of multicity epidemiologic studies; and 

uncertainties in epidemiology-based risk estimates, particularly uncertainties in the shape of the 

concentration-response functions at lower O3 concentrations and uncertainties associated with 

the heterogeneity in O3 effect estimates across locations, the Administrator does not agree that it 

is appropriate to place significant weight on these factors or to use them to support the 

appropriateness of standard levels below 65 ppb. Compared to O3 standard levels from 65 to 70

ppb, the Administrator concludes that the extent to which standard levels below 65 ppb could 

result in further public health improvements becomes notably less certain. Therefore, she 

concludes that it is not appropriate to propose standard levels below 65 ppb.

172

The Administrator acknowledges that her proposed range of 65 to 70 ppb does not 

include the lower portion of the range supported by CASAC. In reaching the conclusion that this 

is appropriate, she focuses on CASAC’s rationale for levels as low as 60 ppb. In particular, she 

notes the following CASAC advice (Frey, 2014c, p. 7): 

The CASAC concurs that 60 ppb is an appropriate and justifiable scientifically 

based lower bound for a revised primary standard. This is based upon findings of 

adverse effects, including clinically significant lung function decrements and 

airway inflammation, after exposures to 60 ppb ozone in healthy adults with 

moderate exertion (Adams 2006; Schelegle et al., 2009; Brown et al., 2008; Kim 

et al., 2011), with limited evidence of adverse effects below 60 ppb. 

In considering this advice, the Administrator notes that CASAC focused on the 

importance of limiting exposures to O3 concentrations as low as 60 ppb. As discussed above, the 

Administrator agrees with this advice. In particular, she notes that standards within the proposed 

 

172 Although, as discussed below, she solicits comment on standard levels as low as 60 ppb.
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range of 65 to 70 ppb would be expected to substantially limit the occurrence of exposures of 

concern to O3 concentrations at or above 60 ppb, particularly the occurrence of two or more 

exposures.173 When she further considers that not all exposures of concern lead to adverse 

effects, and that the NAAQS are not meant to be zero-risk or background standards, the 

Administrator judges that alternative standard levels below 65 ppb are not needed to further 

reduce such exposures. Therefore, the Administrator’s initial conclusion is that standard levels 

below 65 ppb would be more than requisite to protect public health with an adequate margin of 

safety. 

In reaching this initial conclusion, the Administrator acknowledges that alternative 

approaches to viewing the available scientific evidence and exposure/risk information, and to 

viewing the uncertainties inherent in that evidence and information, could lead one to reach a 

different conclusion. In particular, as noted above, she recognizes that levels as low as 60 ppb 

could potentially be supported, to the extent substantial weight is placed on the public health 

importance of estimates of one or more occurrences of exposures of concern at or above 60 ppb 

and O3-induced lung function decrements > 10%; analyses of ambient O3 concentrations in 

locations of multicity epidemiologic studies; and epidemiology-based estimates of total risk. This 

approach would also place a large amount of weight on the possibility that at-risk groups would 

experience adverse effects at lower levels than the benchmarks derived from clinical studies 

conducted using healthy adult subjects, despite the fact that these studies have not reported a 

statistically significant increase in respiratory symptoms, combined with lung function 

 

173 In fact, as noted above (Table 4), a standard with a level of 70 ppb would be expected to limit 

multiple occurrences of exposures of concern at or above the 60 ppb benchmark to as low as 

0.5% in urban case study areas (and as low as 0% for a standard with a level of 65 ppb). 
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decrements, following exposures to 60 ppb.

174 Such an approach to viewing the evidence and 

exposure/risk information would place very little weight on the uncertainties in these estimates 

and analyses. In some cases, elements of this approach have been supported by public 

commenters, leading some commenters to recommend setting the level of the primary O3

standard at least as low as 60 ppb. In recognition of such an alternative approach to viewing the 

evidence and information, in addition to proposing to set the level of the O3 standard from 65 to 

70 ppb, the Administrator solicits comment on alternative standard levels below 65 ppb, and as 

low as 60 ppb. In doing so, the Administrator reiterates that the CAA does not require the 

establishment of a primary NAAQS at a zero-risk level or at background concentration levels, 

but rather at a level that reduces risk sufficiently so as to protect public health with an adequate 

margin of safety (I.A). 

F. Proposed Decision on the Primary Standard

For the reasons discussed above, and taking into account information and assessments 

presented in the 2013 ISA, 2014 HREA and integration of this information and assessments into 

staff conclusions in the 2014 PA, the advice and recommendations of CASAC, and public 

comments received during the development of these documents, the Administrator proposes to 

retain the current indicator, averaging time and form of the primary O3 standard, and to set a new 

level for the 8-hour primary O3 standard. Specifically, the Administrator proposes to set the level 

of the 8-hour primary O3 standard to within the range of 65 to 70 ppb. The proposed 8-hour 

primary standard would be met at an ambient air monitoring site when the 3-year average of the 

 

174 More specifically, as discussed above, respiratory symptoms have been evaluated following 

6.6-hour exposures to average O3 concentrations of 60 ppb (Adams, 2006; Kim et al., 2011) and 

63 ppb (Schelegle et al., 2009). None of these studies reported a statistically significant increase 

in respiratory symptoms, compared to filtered air controls. 
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annual fourth-highest daily maximum 8-hour average O3 concentration is less than or equal to 

the level of the revised standard that is promulgated. Thus, the Administrator proposes to set a 

standard with a level within this range. For the reasons discussed above, the Administrator also 

solicits comment on setting the level of the primary O3 standard below 65 ppb, and as low as 60 

ppb.

III. Communication of Public Health Information

Information on the public health implications of ambient concentrations of criteria 

pollutants is currently made available primarily through EPA's Air Quality Index (AQI) program. 

The AQI has been in use since its inception in 1999 (64 FR 42530). It provides accurate, timely, 

and easily understandable information about daily levels of pollution (40 CFR 58.50). It is 

designed to tell individual members of the public how clean or unhealthy their air is, whether 

health effects might be a concern, and, if so, measures individuals can take to reduce their 

exposure to air pollution. The AQI focuses on health effects individuals may experience within a 

few hours or days after breathing unhealthy air. The AQI establishes a nationally uniform system 

of indexing pollution concentrations for O3, carbon monoxide, nitrogen dioxide, particulate 

matter and sulfur dioxide. The AQI converts pollutant concentrations in a community's air to a 

number on a scale from 0 to 500. Reported AQI values enable the public to know whether air 

pollution concentrations in a particular location are characterized as good (0 - 50), moderate (51 -

100), unhealthy for sensitive groups (101 - 150), unhealthy (151 - 200), very unhealthy (201 -

300), or hazardous (301 - 500). The AQI index value of 100 typically corresponds to the level of 

the short-term NAAQS for each pollutant. For the O3 NAAQS, an 8-hour average concentration 

of 75 ppb corresponds to an AQI value of 100. An AQI value greater than 100 means that a 

pollutant is in one of the unhealthy categories (i.e., unhealthy for sensitive groups, unhealthy, 
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very unhealthy, or hazardous) on a given day; an AQI value at or below 100 means that a 

pollutant concentration is in one of the satisfactory categories (i.e., moderate or good). An 

additional consideration in selecting breakpoints is for each category to span at least a 15 ppb 

range to allow for more accurate forecasting. Decisions about the pollutant concentrations at 

which to set the various AQI breakpoints, that delineate the various AQI categories, draw 

directly from the underlying health information that supports the NAAQS review.

The Agency recognizes the importance of revising the AQI in a timely manner to be 

consistent with any revisions to the NAAQS. Therefore EPA is proposing conforming changes to 

the AQI, in connection with the Agency's proposed decision on revisions to the O3 NAAQS if 

revisions to the primary standard are promulgated. These conforming changes would include 

setting the 100 level of the AQI at the same level as the revised primary O3 NAAQS and also 

making adjustments based on health information from this NAAQS review to AQI breakpoints at 

the lower end of each range (i.e., AQI values of 50, 150, 200 and 300). The EPA does not 

propose to change the level at the top of the index (i.e., AQI value of 500) that typically is set 

equal to the Significant Harm Level (40 CFR 51.16), which would apply to state contingency 

plans. 

The EPA is proposing to revise the AQI for O3 by setting an AQI value of 100 equal to 

the level of the revised O3 standard (65-70 ppb). The EPA is also proposing to revise the 

following breakpoints: an AQI value of 50 to within a range from 49-54 ppb; an AQI value of 

150 to 85 ppb; an AQI value of 200 to 105 ppb, and an AQI value of 300 to 200 ppb. All these 

levels are averaged over 8 hours. The EPA is proposing to set an AQI value of 50, the breakpoint 

between the good and moderate categories, at 15 ppb below the value of the proposed standard, 

i.e. to within a range from 49 to 54 ppb. The EPA is taking comment on what level within this 
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range to select, recognizing that there is no health message for either at-risk or healthy 

populations in the good category. Thus, the level selected should be below the lowest 

concentration (i.e., 60 ppb) that has been shown in controlled human exposure studies of healthy 

adults175 to cause moderate lung function decrements (i.e., FEV1 decrements ≥ 10%, which could 

be adverse to people with lung disease), large lung function decrements (i.e., FEV1 decrements ≥ 

20%) in a small proportion of people, and airway inflammation.176 The EPA is proposing to set 

an AQI value of 150, the breakpoint between the unhealthy for sensitive groups and unhealthy 

categories, at 85 ppb. At this level, controlled human exposure studies of healthy adults indicate 

that up to 25% of exposed people are likely to have moderate lung function decrements (i.e., 

25% have FEV1 decrements ≥ 10%; 12% have FEV1 decrements ≥ 15%) and up to 7% are likely 

to have large lung function decrements (i.e., FEV1 decrements ≥ 20%) (McDonnell et al., 2012; 

Figure 7). Large lung function decrements would likely interfere with normal activity for many 

healthy people. For people with lung disease, large lung function decrements would likely 

interfere with normal activity for most people and would increase the likelihood that they would 

seek medical treatment (72 FR 37850, July 11, 2007). The EPA is proposing to set an AQI value 

of 200, the breakpoint between the unhealthy and very unhealthy categories, at 105 ppb. At this 

level, controlled human exposure studies of healthy adults indicate that up to 38% of exposed 

people are likely to have moderate lung function decrements (i.e., 38% have FEV1 decrements ≥ 

10%; 22% have FEV1 decrements ≥ 15%) and up to 13% are likely to have large lung function 

decrements (i.e., FEV1 decrements ≥ 20%). The EPA is proposing to set an AQI value of 300, 

 

175 Effects would likely be greater in people with asthma.

176 Exposures to 50 ppb have not been evaluated experimentally, but are estimated to potentially 

affect only a small proportion of healthy adults and with only a half to a third of the moderate to 

large lung function decrements observed at 60 ppb (McDonnell et al., 2012; Figure 7).
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the breakpoint between the very unhealthy and hazardous categories, at 200 ppb. At this level, 

controlled human exposure studies of healthy adults indicate that up to 25% of exposed

individuals are likely to have large lung function decrements (i.e., FEV1 decrements ≥ 20%), 

which would interfere with daily activities for many of them. Large lung function decrements 

would interfere with daily activities for most people with lung disease, and likely cause them to 

seek medical attention. 

Table 6. Proposed AQI Breakpoints

AQI Category Index Values Existing Breakpoints

(ppb, 8-hour 

average)

Proposed Breakpoints

(ppb, 8-hour average)

Good 0 - 50 0 - 59 0 – (49 to 54)

Moderate 51 - 100 60 - 75 (50 to 55) - (65 to 70)

Unhealthy for 

Sensitive Groups

101 - 150 76 - 95 (66 to 71) - 85

Unhealthy 151 - 200 96 - 115 86 - 105

Very Unhealthy 201 - 300 116 - 374 106 - 200

Hazardous 301 - 400 375 - 201 -

401 - 500

EPA believes that the proposed breakpoints reflect an appropriate balance between 

reflecting the health evidence that is the basis for the proposed primary O3 standard and 

providing category ranges that are large enough to be forecasted accurately, so that the new AQI 

for O3 can be implemented more easily in the public forum for which the AQI ultimately exists.

However, the EPA recognizes that some have expressed alternative approaches to viewing the 

evidence and information and solicits comment on these proposed revisions to the AQI.

With respect to reporting requirements (40 CFR Part 58, §58.50), EPA proposes to revise 

40 CFR Part 58, §58.50 (c) to require the AQI reporting requirements to be based on the latest 

available census figures, rather than the most recent decennial U.S. census. This change is 

consistent with our current practice of using the latest population figures to make monitoring 

requirements more responsive to changes in population.
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IV. Rationale for Proposed Decision on the Secondary Standard

This section presents the rationale for the Administrator’s proposed decisions regarding 

the need to revise the current secondary O3 NAAQS and the appropriate revisions to the 

standard, including her proposed decisions that the current secondary standard is not requisite to 

protect public welfare and should be revised to provide additional public welfare protection. 

Based on her consideration of the full body of welfare effects evidence and related analyses, the 

Administrator proposes to conclude that ambient O3 concentrations in terms of a W126 index 

value, averaged across three consecutive years, within the range from 13 ppm-hrs to 17 ppm-hrs 

would provide the requisite protection against known or anticipated adverse effects to the public 

welfare. In considering policy options for achieving that level of air quality, the Administrator 

has further considered the full body of information, including air quality analyses that relate 

ambient O3 concentrations in terms of a three-year average W-126-based metric and in terms of 

the form and averaging time for the current standard. Based on this consideration, the 

Administrator proposes to revise the level of the current secondary standard to within the range 

of 0.065 to 0.070 ppm to achieve the appropriate air quality. 

As discussed more fully below, this proposal is based on a thorough review, in the ISA, 

of the latest scientific information on O3-induced environmental effects. This proposed decision 

also takes into account: (1) staff assessments in the PA of the most policy-relevant information in 

the ISA and WREA analyses of air quality, exposure, and ecological risks and associated 

ecosystem services; (2) CASAC advice and recommendations; and, (3) public comments 

received during the development of these documents, either in connection with CASAC 

meetings or separately. 
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This proposed decision draws on the ISA’s integrative synthesis of the entire body of 

evidence, published through July 2011, on environmental effects associated with the presence of 

O3 and related photochemical oxidants in the ambient air. As summarized in section IV.B below, 

this body of evidence addresses the range of environmental responses associated with exposure 

to ambient levels of O3 (U.S. EPA, 2013a, ISA chapters 9-10), and includes more than four 

hundred new studies that build on the extensive evidence base from the last review. This 

rationale also draws upon the results of quantitative exposure and risk assessments, summarized

in section IV.C below. Section IV.D presents the Administrator’s proposed decisions on the 

adequacy of the current secondary standard (section IV.D.3) drawing on both evidence-based 

and exposure/risk-based considerations in the PA (section IV.D.1) and advice from CASAC 

(section IV.D.2). Proposed conclusions on alternative standards are summarized in section IV.E. 

A. Approach 

In evaluating whether it is appropriate to retain or revise the current secondary O3

standard, the Administrator adopts an approach in this review that builds upon the general 

approach used in the 2008 review177 and reflects the broader body of scientific evidence now 

available, updated exposure/risk information, advances in O3 air quality modeling, and air 

monitoring information. This review of the standard also considers the July 2013 remand of the 

secondary standard by the U.S. Court of Appeals for the D.C. Circuit, such that the proposed 

decision described herein incorporates the EPA’s response to this remand. 

The Administrator’s decisions in the 2008 review were based on an integration of 

information on welfare effects associated with exposure to O3, judgments on the adversity and 

 

177 The 2008 revision of the O3 secondary standard, the proposed reconsideration of the 2008 

decision, and the 2013 court decision on the 2008 revision of the secondary standard are

summarized in section I.C above.
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public welfare significance of key effects, and judgments as to what standard would be requisite 

to protect public welfare. These considerations were informed by air quality and related analyses, 

quantitative exposure and risk assessments, and qualitative assessment of impacts that could not 

be quantified. As a result of the 2008 review, the Administrator concluded the then-current 

secondary standard did not provide the requisite public welfare protection and it was revised. 

The current secondary standard is 75 ppb based on the annual fourth-highest daily maximum 8-

hour average concentration, averaged over three consecutive years, which is identical to the 

current primary standard. In 2008, the Administrator considered the then-available monitoring 

data with regard to relationships between the revised primary standard and degree of protection 

of public welfare from cumulative seasonal O3 exposures, expressed in terms of the W126

exposure index (described in section IV.B.1 below), and decided to revise the secondary standard 

to be equal to the revised primary standard (73 FR 16499-16500, March 27, 2008). In remanding 

the 2008 decision on the secondary standard back to the EPA (described in section I.C above), 

the U.S. Court of Appeals for the D.C. Circuit determined that EPA did not specify what level of 

air quality was requisite to protect public welfare from adverse public welfare effects or explain 

why any such level would be requisite. Mississippi, 744 F.3d at 272-73. 

In addition to reviewing the most recent scientific information as required by the CAA, 

this rulemaking responds to the remand and fully explains the Administrator’s proposed 

conclusions as to the level of air quality requisite to protect public welfare from known or 

anticipated effects. Our general approach in considering the scientific information available in 

this review involves consideration of the integrative synthesis of the entire body of available 

scientific evidence in the ISA (U.S. EPA, 2013a), including information on biologically relevant 

exposure indices, exposure/risk and air quality modeling analyses presented in the WREA (U.S. 
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EPA, 2014b), staff analyses in the PA; advice and recommendations from CASAC (Frey, 2014b, 

c), and public comments. We note that in drawing conclusions on the secondary standard, the 

final decision to retain or revise the standard is a public welfare policy judgment to be made by 

the Administrator. The Administrator’s final decision will draw upon the available scientific 

evidence for O3-attributable welfare effects and on analyses of exposures and public welfare 

risks based on impacts to vegetation, ecosystems and their associated services, as well as 

judgments about the appropriate weight to place on the range of uncertainties inherent in the 

evidence and analyses. Such judgments in the context of this review include: the weight to place 

on the evidence of specific vegetation-related effects estimated to result across a range of 

cumulative seasonal concentration-weighted O3 exposures; the weight to give associated 

uncertainties, including those related to the variability in occurrence of such effects in areas of 

the U.S., especially areas of particular public welfare significance; and, judgments on the extent 

to which such effects in such areas may be considered adverse to public welfare. 

As provided in the CAA, section 109(b)(2), 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.” Effects on welfare include, but are not 

limited to, “effects on soils, water, crops, vegetation, man-made materials, animals, wildlife, 

weather, visibility, and climate, damage to and deterioration of property, and hazards to 

transportation, as well as effects on economic values and on personal comfort and well-being” 

(CAA section 302(h)). As recognized in the last review, the secondary standard is not meant to 

protect against all known or anticipated O3-related effects, but rather those that are judged to be 

adverse to the public welfare (73 FR 16496, March 27, 2008). Thus, the level of protection from 
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known or anticipated adverse effects to public welfare that is requisite for the secondary standard 

is a public welfare policy judgment to be made by the Administrator. In the current review, the 

Administrator’s judgment is informed by conclusions drawn with regard to adversity of effects to 

public welfare in decisions on secondary O3 standards in past reviews. 

In the 2008 decision, the Administrator concluded that the degree to which O3 effects on 

vegetation should be considered to be adverse to the public welfare depends on the intended use 

of the vegetation and the significance of the vegetation to the public welfare, and also applied 

this concept beyond the species level to the ecosystem level (73 FR 16496, March 27, 2008). In 

so doing, the Administrator took note of “a number of actions taken by Congress to establish 

public lands that are set aside for specific uses that are intended to provide benefits to the public 

welfare, including lands that are to be protected so as to conserve the scenic value and the natural 

vegetation and wildlife within such areas, and to leave them unimpaired for the enjoyment of 

future generations” (73 FR 16496, March 27, 2008). The notice for the 2008 decision further

noted that [s]uch public lands that are protected areas of national interest include national parks 

and forests, wildlife refuges, and wilderness areas” (73 FR 16496, March 27, 2008).178,179 The 

Administrator additionally recognized that “States, Tribes and public interest groups also set 

 

178 For example, the National Park Service Organic Act of 1916 established the National Park 

Service (NPS) and, in describing the role of the NPS with regard to “Federal areas known as 

national parks, monuments, and reservations”, stated that the “fundamental purpose” for these 

Federal areas “is to conserve the scenery and the natural and historic objects and the wild life 

therein and to provide for the enjoyment of the same in such manner and by such means as will 

leave them unimpaired for the enjoyment of future generations.” 16 U.S.C. § 1.

179 As a second example, 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).
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aside areas that are intended to provide similar benefits to the public welfare, for residents on 

State and Tribal lands, as well as for visitors to those areas” (73 FR 16496, March 27, 2008). The 

Administrator took note of the “clear public interest in and value of maintaining these areas in a 

condition that does not impair their intended use and the fact that many of these lands contain 

O3-sensitive species” (73 FR 16496, March 27, 2008). Similarly, in judgments of adversity to 

public welfare in the 2010 proposed reconsideration, the Administrator proposed to place the 

highest priority and significance on vegetation and ecosystem effects to sensitive species that are 

known to or are likely to occur in federally protected areas such as national parks and other Class 

I areas,180 or on lands set aside by states, tribes and public interest groups to provide similar 

benefits to the public welfare (75 FR 3023-24, January 19, 2010). 

In the current review, our consideration of the scientific evidence for effects on 

vegetation is based fundamentally on using information from controlled chamber studies, free air 

methodologies, and field-based observational, survey and gradient studies. Such evidence, 

discussed below, informs consideration of welfare endpoints and at-risk species and ecosystems 

on which to focus the current review, and consideration of the ambient O3 conditions under 

which various welfare effects are known or anticipated to occur. As in past reviews, we 

recognize that the available evidence has not provided identification of a threshold in exposure or 

ambient O3 concentrations below which it can be concluded with confidence that O3-attributable 

effects on vegetation do not occur, when considering the broad range of O3-sensitive plant 

species growing within the U.S and the array of effects. This is due in part to the fact that 

 

180 As defined by section 162 of the CAA, Class I areas 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 six thousand acres in size, provided the park or 

wilderness area was in existence on August 7, 1977, as well as other areas designated as Class I 

consistent with that section of the Act. The current Class I areas are specified at 40 CFR part 81.
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research shows that there is variability in sensitivity between and within species and that 

numerous factors, i.e., chemical, physical, biological, and genetic, can influence the direction and 

magnitude of the studied effect (U.S. EPA, 2013a, section 9.4.8). In the absence of evidence for 

a discernible threshold, the general approach to considering the available O3 welfare effects 

evidence involves characterizing the confidence in conclusions regarding O3-attributable 

vegetation effects over the ranges of cumulative seasonal O3 exposure values evaluated in 

chamber studies and in field studies in areas where O3-sensitive vegetation are known to occur, 

as well as characterizing the extent to which these effects can be considered adverse at the plant 

level and beyond. With this approach, we consider the evidence for O3 affecting other ecosystem 

components (such as soils, water, and wildlife) and their associated goods and services, through 

its effects on vegetation, as well as the associated uncertainties. 

Our general approach further recognizes the complexity of judgments to be made 

regarding the identification of particular vegetation effects as welfare effects and regarding the 

point that known or anticipated vegetation-related effects become adverse to the public welfare. 

For example, in addition to the magnitude of the ambient concentrations, the species present, 

their sensitivity to O3, and their public welfare importance are also essential considerations in 

drawing conclusions regarding the significance of public welfare impact. Taking this into 

account, we recognize the existence of a continuum from relatively higher ambient O3

concentrations and conditions, in areas with sensitive species and public welfare significance, for 

which there might be general agreement that effects on public welfare are likely to occur, 

through lower concentrations at which the degree to which public welfare might be expected to 

be affected becomes increasingly uncertain. 
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The evidence base for this review, summarized in section IV.B below, includes 

quantitative information across a broad array of vegetation effects (e.g., growth impairment 

during seedling, sapling and mature tree growth stages, visible foliar injury, and yield loss in 

annual crops) and across a diverse set of exposure methods from laboratory and field studies. 

While considering the full breadth of information available, we place greater weight on U.S. 

studies due to the often species-, site-, and climate-specific nature of O3-related vegetation 

responses, and particularly emphasize those studies that include O3 exposures that fall within the 

range of those likely to occur in the ambient air. We additionally recognize differences across 

different study types in what information they provide (U.S. EPA, 2013a, section 9.2.6). For 

example, because conditions can be controlled in laboratory studies, responses in such studies 

may be less variable and smaller differences may be easier to detect. However, the controlled

conditions may limit the range of responses or incompletely reflect pollutant bioavailability, so 

they may not reflect responses that would occur in the natural environment. Alternatively, field 

data can provide important information for assessments of multiple stressors or where sitespecific factors significantly influence exposure. They are also often useful for analyses of larger 

geographic scales and higher levels of biological organization. However, depending on the type 

of field study, many field study conditions may not be controlled, which can make variability 

higher and differences harder to detect. In some field studies (e.g., gradient studies), the presence 

of confounding factors can also make it difficult to attribute observed effects to specific 

stressors.

In developing quantitative exposure and risk assessments for this review, summarized in 

section IV.C below, we have placed greatest emphasis on studies that have evaluated plant 

response over multiple exposure levels and developed exposure-response (E-R) relationships that 
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allow the estimation of plant responses over the range of O3 exposures pertinent to judgments on 

the current and potential alternative standards. In considering the information from these 

assessments, we focus particularly on the quantitative risks related to three types of O3 effects on 

vegetation and associated ecosystem services: visible foliar injury, biomass loss in trees, and 

crop yield loss. These risks were assessed in a range of analyses primarily involving nationalscale air quality scenarios developed using model adjustments and interpolation methods. We 

consider particularly the national scale assessments for these scenarios, while recognizing the 

uncertainties with regard to the conditions they represent.

With regard to the appropriate characterization of exposures associated with ambient O3

concentrations, as in the 2008 and 1997 reviews, we continue to recognize the relevance of 

cumulative, seasonal, concentration-weighted exposures for assessing vegetation effects. More 

specifically, in the 2008 review, the EPA concluded and the CASAC agreed that the W126 

cumulative exposure metric was the most appropriate to use to evaluate both the adequacy of the 

current secondary standard and the appropriateness of any potential revisions. As discussed in 

section IV.B.1 below, the information available in this review continues to support the use of 

such a metric and it is used in considering potential public welfare impacts in the sections below. 

B. Welfare Effects Information

1. Nature of Effects and Biologically Relevant Exposure Metric

This section describes the nature of O3-induced welfare effects, including the nature of 

the exposures that drive the biological and ecological responses (U.S. EPA, 2013a, chapter 9).

Ozone’s phytotoxic effects were first identified on grape leaves in a study published in 

1958 (Richards et al., 1958). In the almost fifty years that have followed, extensive research has 

been conducted both in and outside of the U.S. to examine the impacts of O3 on plants and their 
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associated ecosystems, since “of the phytotoxic compounds commonly found in the ambient air, 

O3 is the most prevalent, impairing crop production and injuring native vegetation and 

ecosystems more than any other air pollutant” (U.S. EPA, 1989, 1996a). As was established in 

prior reviews, O3 can interfere with carbon gain (photosynthesis) and allocation of carbon within 

the plant. As a result of decreased carbohydrate availability, fewer carbohydrates are available 

for plant growth, reproduction, and/or yield. For seed-bearing plants, these reproductive effects 

will culminate in reduced seed production or yield (U.S. EPA, 1996a, pp. 5-28 and 5-29). Recent 

studies, assessed in the ISA, together with this longstanding and well-established literature on 

O3-related vegetation effects, further contribute to the coherence and consistency of the 

vegetation effects evidence. As described in the ISA, a variety of factors in natural environments 

can either mitigate or exacerbate predicted O3-plant interactions and are recognized sources of 

uncertainty and variability. These include: 1) multiple genetically influenced determinants of O3

sensitivity; 2) changing sensitivity to O3 across vegetative growth stages; 3) co-occurring 

stressors and/or modifying environmental factors (U.S. EPA, 2013a, section 9.4.8). 

Among the studies of vegetation effects, the ISA recognizes controlled chamber studies 

as the best method for isolating or characterizing the role of O3 in inducing the observed plant 

effects, and in assessing plant response to O3 at the finer scales (U.S. EPA, 2013a, sections 9.2 

and 9.3). Recent controlled studies have focused on a variety of plant responses to O3 including 

the underlying mechanisms governing such responses. These mechanisms include: 1) reduced 

carbon dioxide uptake due to stomatal closure (U.S. EPA, 2013a, section 9.3.2.1); 2) the 

upregulation of genes associated with plant defense, signaling, hormone synthesis and secondary 

metabolism (U.S. EPA, 2013a, section 9.3.3.2); 3) the down regulation of genes related to 

photosynthesis and general metabolism (U.S. EPA, 2013a, section 9.3.3.2); 4) the loss of carbon 
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assimilation capacity due to declines in the quantity and activity of key proteins and enzymes 

(U.S. EPA, 2013a, section 9.3.5.1); and 5) the negative impacts on the efficiency of the 

photosynthetic light reactions (U.S. EPA, 2013a, section 9.3.5.1). As described in the ISA, these 

new studies “have increased knowledge of the molecular, biochemical and cellular mechanisms 

occurring in plants in response to O3”, adding “to the understanding of the basic biology of how 

plants are affected by oxidative stress...” (U.S. EPA, 2013a, p. 9-11). The ISA further concludes

that controlled studies “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 (U.S. EPA, 2013a, p. 1-15). 

Such effects at the plant scale can also be linked to an array of effects at larger spatial 

scales. For example, recent field studies at larger spatial scales, together with previously 

available evidence, support the controlled exposure study results and indicate that “ambient O3

exposures can affect ecosystem productivity, crop yield, water cycling, and ecosystem 

community composition” (U.S. EPA, 2013a, p. 1-15; Chapter 9, section 9.4). The current body 

of O3 welfare effects evidence confirms the conclusions reached in the last review on the nature 

of O3-induced welfare effects and is summarized in the ISA as follows (U.S. EPA, 2013a, p. 1-

8).

The welfare effects of O3 can be observed across spatial scales, starting at the 

subcellular and cellular level, then the whole plant and finally, ecosystem-level 

processes. Ozone effects at small spatial scales, such as the leaf of an individual 

plant, can result in effects along a continuum of larger spatial scales. These effects 

include altered rates of leaf gas exchange, growth, and reproduction at the 

individual plant level, and can result in broad changes in ecosystems, such as 

productivity, carbon storage, water cycling, nutrient cycling, and community 

composition.

Based on its assessment of this extensive body of science, the ISA determines that, with 

respect to vegetation and ecosystems, a causal relationship exists between exposure to O3 in 
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ambient air and visible foliar injury effects on vegetation, reduced vegetation growth, reduced 

productivity in terrestrial ecosystems, reduced yield and quality of agricultural crops and 

alteration of below-ground biogeochemical cycles181 (U.S. EPA, 2013a, Table 1-2). In 

consideration of the evidence of O3 exposure and alterations in stomatal performance, “which 

may affect plant and stand transpiration and therefore possibly affecting hydrological cycling,” 

the ISA concludes that “[a]lthough the direction of the response differed among studies,” the 

evidence is sufficient to conclude a likely causal relationship between O3 exposure and the 

alteration of ecosystem water cycling (U.S. EPA, 2013a, section 2.6.3). The ISA also concludes 

that the evidence is sufficient to conclude a likely causal relationship between O3 exposure and 

the alteration of community composition of some terrestrial ecosystems (U.S. EPA, 2013a, 

section 2.6.5). Related to the effects on vegetation growth, productivity and, to some extent, 

below-ground biogeochemical cycles, the ISA additionally determines that a likely causal 

relationship exists between exposures to O3 in ambient air and reduced carbon sequestration 

(also termed carbon storage)182 in terrestrial ecosystems (U.S. EPA, 2013a, p. 1-10 and section 

2.6.2). Modeling studies available in this review consistently found negative impacts of O3 on 

carbon sequestration, although the severity of impact was influenced by “multiple interactions of 

biological and environmental factors” (U.S. EPA, 2013a, p. 2-39). 

The ISA notes that “[t]he suppression of ecosystem [carbon] sinks results in more 

[carbon dioxide] accumulation in the atmosphere” and that a recent study has suggested that “the 

 

181 Based on studies focused on O3-associated alterations in quality and quantity of carbon input 

to soil, microbial community composition, and carbon and nutrient cycling, the ISA concludes 

that the evidence is sufficient “to infer that there is a causal relationship between O3 exposure 

and the alteration of below-ground biogeochemical cycles” (U.S. EPA, 2013a, pp. 2-41 to 2-42). 

182 The terms sequestration and storage are used somewhat interchangeably in the ISA and other 

documents in this review. 
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indirect radiative forcing caused by O3 exposure through lowering the ecosystem [carbon] sink 

could have an even greater impact on global warming than the direct radiative forcing of O3” 

(U.S. EPA, 2013a, p. 2-39). With regard to direct radiative forcing, however, the ISA makes a 

stronger causality conclusion that the evidence supports a causal relationship between changes in 

tropospheric O3 concentrations and radiative forcing183 (U.S. EPA, 2013a, section 2.7.1). There 

are, however, “large uncertainties in the magnitude of the radiative forcing estimate attributed to 

tropospheric O3, making the impact of tropospheric O3 on climate more uncertain than the effect 

of the longer-lived greenhouse gases” (U.S. EPA, 2013a, p. 2-47). In this regard, the ISA 

observes that “radiative forcing does not take into account the climate feedbacks that could 

amplify or dampen the actual surface temperature response,” that “[q]uantifying the change in 

surface temperature requires a complex climate simulation in which all important feedbacks and 

interactions are accounted for” and that “[t]he modeled surface temperature response to a given 

radiative forcing is highly uncertain and can vary greatly among models and from region to 

region within the same model” (U.S. EPA, 2013a, p. 2-47). Even with these uncertainties, the 

ISA notes that “global climate models indicate that tropospheric O3 has contributed to observed 

changes in global mean and regional surface temperatures” and as a result of such evidence 

presented in climate modeling studies, concludes that there is likely to be a causal relationship 

between changes in tropospheric O3 concentrations and effects on climate (U.S. EPA, 2013a, p. 

2-47). The ISA additionally notes, however, that “[i]mportant uncertainties remain regarding the 

effect of tropospheric O3 on future climate change” (U.S. EPA, 2013a, p. 10-31).

 

183 Radiative forcing by a greenhouse gas or aerosol 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 that substance. For example, a reduction in outgoing infrared radiation has been associated 

with O3 by satellite data (U.S. EPA, 2013a, p. 2-47). 
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Given the strong evidence base, and findings of causal or likely causal relationships with

O3 in ambient air, including the quantitative assessments of relationships between O3 exposure 

and occurrence and magnitude of effects, we give a primary focus to three main areas of effects.

The three main areas, for which the evidence is summarized in more detail below, are: 1) impacts 

on tree growth, productivity and carbon storage (section IV.B.1.b); 2) crop yield loss (section 

IV.B.1.c); and 3) visible foliar injury (section IV.B.1.a). Consideration of these three areas 

includes, as appropriate, consideration of evidence of associated effects at larger scales, 

including ecosystems, and on associated ecosystem services.

With regard to biologically based indices of exposure pertinent to O3 effects on 

vegetation, the ISA states the following (U.S. EPA, 2013a, p. 2-44).

The main conclusions from the 1996 and 2006 O3 AQCDs [Air Quality Criteria 

Documents] regarding indices based on ambient exposure remain valid. These 

key conclusions can be restated as follows: ozone effects in plants are cumulative; 

higher O3 concentrations appear to be more important than lower concentrations 

in eliciting a response; plant sensitivity to O3 varies with time of day and plant 

development stage; [and] quantifying exposure with indices that cumulate hourly 

O3 concentrations and preferentially weight the higher concentrations improves 

the explanatory power of exposure/response models for growth and yield, over 

using indices based on mean and peak exposure values.

The long-standing body of available evidence upon which these conclusions are based 

provides a wealth of information on aspects of O3 exposure that are important in influencing 

plant response. Specifically, a variety of “factors with known or suspected bearing on the 

exposure-response relationship, including concentration, time of day, respite time, frequency of 

peak occurrence, plant phenology, predisposition, etc.,” have been identified (U.S. EPA, 2013a, 

section 9.5.2). In addition, the importance of the duration of the exposure and the relatively 

greater importance of higher concentrations over lower concentrations in determining plant 

response to O3 have been consistently well documented (U.S. EPA, 2013a, section 9.5.3). Much 
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of this evidence was assessed in the 1996 AQCD (U.S. EPA, 1996a), while more recent work 

substantiating this evidence is assessed in the subsequent 2006 AQCD and 2013 ISA. 

Understanding of the biological basis for plant response to O3 exposure led to the 

development of a large number of “mathematical approaches for summarizing ambient air 

quality information in biologically meaningful forms for O3 vegetation effects assessment 

purposes” (U.S. EPA, 2013a, section 9.5.3), including those that cumulate exposures over some 

specified period while weighting higher concentrations more than lower (U.S. EPA, 2013a, 

section 9.5.2). As with any summary statistic, these exposure indices retain information on some, 

but not all, characteristics of the original observations. The 1996 AQCD contained an extensive 

review of the published literature on different types of exposure-response metrics, including 

comparisons between metrics, from which the 1996 Staff Paper built its assessment of forms 

appropriate to consider in the context of the secondary NAAQS review. The result of these 

assessments was a decision by the EPA to focus on cumulative, concentration-weighted indices, 

which were recognized as the most appropriate biologically based metrics to consider in this 

context, with attention given primarily to two cumulative, concentration-weighted index forms: 

SUM06 and W126.184

In both the 1997 and 2008 reviews, the EPA concluded that the risk to vegetation comes 

primarily from cumulative exposures to O3 over a season or seasons185 and focused on metrics 

 

184The SUM06 index is a threshold-based approach described as the sum of all hourly O3

concentrations greater or equal to 0.06 ppm observed during a specified daily and seasonal time 

window (U.S. EPA, 2013a, section 9.5.2). The W126 index is a non-threshold approach 

described as the sigmoidally weighted sum of all hourly O3 concentrations observed during a 

specified daily and seasonal time window, where each hourly O3 concentration is given a weight 

that increases from zero to one with increasing concentration (U.S. EPA, 2013a, section 9.5.2).

185In describing the form as “seasonal”, the EPA is referring generally to the growing season of 

O3-sensitive vegetation, not to the seasons of the year (i.e., spring, summer, fall, winter).
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intended to characterize such exposures: SUM06 (61 FR 65716, December 13, 1996) and W126 

(72 FR 37818, July 11, 2007) in the 1997 and 2008 reviews, respectively. Although in both 

reviews the policy decision was made to set the secondary standard to be identical to a revised 

primary standard (with an 8-hour averaging time), the Administrator, in both cases, also 

concluded, consistent with CASAC advice, that a cumulative, seasonal index was the most 

biologically relevant way to relate exposure to plant growth response (62 FR 38856, July 18, 

1997; 73 FR 16436, March 27, 2008; 75 FR 2938, January 19, 2010). This approach for 

characterizing O3 exposure concentrations that are biologically relevant with regard to potential 

vegetation effects received strong support from CASAC in the last review and again in this 

review, including strong support for use of such a metric as the form for the secondary standard 

(Henderson, 2006, 2008; Samet, 2010; Frey, 2014c). 

An alternative to using ambient exposure durations and concentrations to predict plant 

response has been developed in recent years, primarily in Europe, i.e., flux models. While “some 

researchers have claimed that using flux models can be used {sic} to better predict vegetation 

responses to O3 than exposure-based approaches” because flux models estimate the ambient O3

concentration that actually enters the leaf (i.e., flux or deposition) (U.S. EPA, 2013a, p. 9-114), it 

is important to note that “[f]lux calculations are data intensive and must be carefully 

implemented” (U.S. EPA, 2013a, p. 9-114). Further, the ISA states, “[t]his uptake-based 

approach to quantify the vegetation impact of O3 requires inclusion of those factors that control 

the diurnal and seasonal O3 flux to vegetation (e.g., climate patterns, species and/or vegetationtype factors and site-specific factors)” (U.S. EPA, 2013a, p. 9-114). In addition to these data 

requirements, each species has different amounts of internal detoxification potential that may 

protect species to differing degrees. The lack of detailed species- and site-specific data required 
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for flux modeling in the U.S. and the lack of understanding of detoxification processes have 

continued to make this technique less viable for use in vulnerability and risk assessments at the 

national scale in the U.S. (U.S. EPA, 2013a, section 9.5.4).

Therefore, consistent with the ISA conclusions regarding the appropriateness of 

considering cumulative exposure indices that preferentially weight higher concentrations over 

lower for predicting O3 effects of concern based on the long-established conclusions and longstanding supporting evidence described above, and in light of continued CASAC support, we 

continue to focus on cumulative concentration-weighted indices as the most biologically relevant 

metrics for consideration of O3 exposures eliciting vegetation-related effects. Such a metric has 

an “explanatory power” that is improved “over using indices based on mean and peak exposure 

values” (U.S. EPA, 2013a, section 2.6.6.1, p. 2-44). In this review as in the last review, we use 

the W126 cumulative, seasonal metric (U.S. EPA, 2013a, sections 2.6.6.1 and 9.5.2) for 

consideration of the effects evidence and in the exposure and risk analyses in the WREA. 

The subsections below summarize key aspects of the welfare effects information for O3-

elicited visible foliar injury (section IV.B.1.a), effects on forest tree growth, productivity and 

carbon storage (section IV.B.1.b) and reductions in crop yield (section IV.B.1.c), as well as 

associated effects. 

a. Visible Foliar Injury

Visible foliar injury resulting from exposure to O3 has been well characterized and 

documented over several decades of research on many tree, shrub, herbaceous, and crop species 

(U.S. EPA, 2013a, p. 1-10; U.S. EPA, 2006a, 1996a, 1986, 1978). Additionally, O3-induced 

visible foliar injury symptoms on certain plant species, such as black cherry, yellow-poplar and 

common milkweed, are considered diagnostic of exposure to O3 based on the consistent 
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association established with experimental evidence (U.S. EPA, 2013a, p. 1-10). The significance 

of O3 injury at the leaf and whole plant levels depends on an array of factors, and therefore, it is 

difficult to quantitatively relate visible foliar injury symptoms to vegetation effects such as 

individual tree growth, or effects at population or ecosystem levels (U.S. EPA, 2013a, p. 9-39). 

The ISA notes that visible foliar injury “is not always a reliable indicator of other negative 

effects on vegetation” (U.S. EPA, 2013a, p. 9-39). Factors that influence the significance to the 

leaf and whole plant include the amount of total leaf area affected, age of plant, size, 

developmental stage, and degree of functional redundancy among the existing leaf area (U.S. 

EPA, 2013a, section 9.4.2). Visible foliar injury by itself is an indication of phytotoxicity due to 

O3 exposure, which occurs only when sensitive plants are exposed to elevated O3 concentrations 

in a predisposing environment, a major aspect of which is the lack of drought conditions during 

the year such injury is assessed (U.S. EPA, 2013a, section 9.4.2). 

Recent research is consistent with previous conclusions and that O3-induced visible foliar 

injury symptoms are well characterized and considered diagnostic on certain bioindicator plant 

species. Diagnostic usage for these plants has been verified experimentally in exposure-response 

studies, using exposure methodologies such as continuous stirred tank reactors, open-top 

chambers (OTCs), and free-air carbon dioxide (and ozone) enrichment (FACE). Although there 

remains a lack of robust exposure-response functions that would allow prediction of visible foliar 

injury severity and incidence under varying air quality and environmental conditions, 

“experimental evidence has clearly established a consistent association of the presence of visible 

foliar injury symptoms with O3 exposure, with greater exposure often resulting in greater and 

more prevalent injury” (U.S. EPA, 2013a, section 9.4.2, p. 9-41). The research newly available 

in this review includes: 1) controlled exposure studies conducted to test and verify the O3
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sensitivity and response of potential new bioindicator plant species; 2) multi-year field surveys in 

several National Wildlife Refuges (NWR) documenting the presence of foliar injury in valued 

areas; and 3) ongoing data collection and assessment by the U.S. Forest Service’s Forest Health 

Monitoring Forest Inventory and Analysis (USFS FHM/FIA) program, including multi-year 

trend analysis (U.S. EPA, 2013a, section 9.4.2). These recent studies, in combination with the 

entire body of available evidence, thus form the basis for the ISA determinations of a causal 

relationship between ambient O3 exposure and the occurrence of O3-induced visible foliar injury 

on sensitive vegetation across the U.S. (U.S. EPA, 2013a, p. 9-42).

Recently available evidence confirms the evidence available in previous reviews that 

visible foliar injury can occur when sensitive plants are exposed to elevated O3 concentrations in 

a predisposing environment (i.e., adequate soil moisture) (U.S. EPA, 2013a, section 9.4.2). 

Recent evidence also continues to support previous findings that indicated the occurrence of 

visible foliar injury at cumulative ambient O3 exposures previously examined. 

With regard to evidence from controlled exposure studies, a recent study, using 

continuously stirred tank reactor chambers, evaluated the occurrence of O3 characteristic visible 

foliar injury symptoms on 28 species of plants that were suspected of being O3 sensitive and 

most of which grow naturally throughout the northeast and midwest U.S., including in national 

parks and wilderness areas (U.S. EPA, 2013a, section 9.4.2.1; Kline et al., 2008). Across the 28 

tested species, the study reported O3-induced responses in 12, 20, 28 and 28 species at the 30, 60, 
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90 and 120 ppb exposure concentrations,

186 respectively; the plants were exposed for 7 hours per 

each weekday over 21 to 29 summer days (Kline et al., 2008). 

A string of recently published multi-year field studies provide a complementary line of 

field-based evidence by documenting the incidence of visible foliar injury symptoms on a variety 

of O3-sensitive species over multiple years and across a range of cumulative, seasonal exposure 

values in several eastern and midwestern NWRs (U.S. EPA, 2013a, section 9.4.2.1; Davis and 

Orendovici, 2006; Davis, 2007a, b; Davis, 2009). Some of these studies also included 

information regarding soil moisture stress using the Palmer Drought Severity Index (PDSI). 

While environmental conditions and species varied across the four NWRs, visible foliar injury 

was documented to a varying degree at each site. 

By far the most extensive field-based dataset of visible foliar injury incidence is that 

obtained by the USFS FHM/FIA biomonitoring network program. A trend analysis of data from 

the sites located in the Northeast and North Central U.S. for the 16 year period from 1994

through 2009 (Smith, 2012) describes evidence of visible foliar injury occurrence in the field as 

well as some insight into the influence of changes in air quality and soil moisture on visible 

foliar injury and the difficulty inherent in predicting foliar injury response under different air 

quality/soil moisture scenarios (Smith, 2012; U.S. EPA, 2013a, section 9.2.4.1). Study results 

showed that incidence and severity of foliar injury were dependent on local site conditions for 

soil moisture availability and O3 exposure. Overall, there was a declining trend in the incidence 

of visible foliar injury as peak O3 concentrations declined, although the study additionally 

 

186 Two of the target exposure levels, 30 and 60 ppb, fall below the level of the current standard 

(75 ppb), although the exposures were average concentrations for 7-hour exposures across 

durations shorter than a month. Because the form of the current standard targets peak 

concentrations in a season, an area that just meets the current standard can be expected to have 

mean concentrations well below that level due to variability in ambient O3 concentrations. 
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indicated that moderate O3 exposures continued to cause visible foliar injury at sites throughout 

the region (U.S. EPA, 2013a, p. 9-40). In a similar assessment of the USFS FHM/FIA data in the 

West, six years (2000 to 2005) of biomonitoring data, during a period where a large proportion 

of California sites did not meet the current standard, indicated O3-related visible foliar injury in 

25-37% of biosites in California (Campbell et al., 2007; U.S. EPA, 2013a, section 9.4.2.1).187

These recent studies provide additional evidence of O3-induced visible foliar injury in many 

areas across the U.S. and augment the EPA’s understanding of O3-related visible foliar injury 

and of factors, such as soil moisture, that influence associations between O3 exposures or 

concentrations and visible foliar injury. 

b. Effects on Forest Tree Growth, Productivity and Carbon Storage 

Ozone has been shown to affect a number of important U.S. tree species with respect to 

growth, productivity, and carbon storage. Ambient O3 concentrations have long been known to 

cause decreases in photosynthetic rates and plant growth. As discussed in the ISA, research 

published since the 2006 AQCD substantiates prior conclusions regarding O3-related effects on 

forest tree growth, productivity and carbon storage. The ISA states, “previous O3 AQCDs 

concluded that there is strong evidence that exposure to O3 decreases photosynthesis and growth 

in numerous plant species” and that “[s]tudies published since the 2008 review support those 

conclusions” (U.S. EPA, 2013a, p. 9-42). The available studies come from a variety of different 

study types that cover an array of different species, effects endpoints, levels of biological 

organization and exposure methods and durations. The O3-induced effects at the plant scale may

translate to the ecosystem scale, with changes in productivity and carbon storage. As stated in the 

 

187 See: http://www.epa.gov/airtrends/values.html.
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ISA, “[s]tudies conducted during the past four decades have demonstrated unequivocally that O3

alters biomass allocation and plant reproduction” (U.S. EPA, 2013a, p. 1-10).

The previously available strong evidence for trees includes robust E-R functions for 

seedling relative biomass loss (RBL)188 in 11 species developed under the National Health and 

Environmental Effects Research Laboratory-Western Ecology Division program. This series of 

experiments used OTCs to study seedling growth response for a single growing season under a 

variety of O3 exposures (ranging from near background to well above current ambient 

concentrations) and growing conditions (U.S. EPA, 2013a, section 9.6.2; Lee and Hogsett, 

1996). The evidence from these studies shows that there is a wide range in sensitivity across the 

studied species in the seedling growth stage over the course of a single growing season, with 

some species being extremely sensitive and others being very insensitive over the range of 

cumulative O3 exposures studied (U.S. EPA, 2014c, Figure 5-1). At the other end of the 

organizational spectrum, field-based studies of species growing in natural stands have compared 

observed plant response across a number of different sites and/or years when exposed to varying 

ambient O3 exposure conditions. For example, a study conducted in forest stands in the southern 

Appalachian Mountains during a period when O3 concentrations exceeded the current standard 

found that the cumulative effects of O3 decreased seasonal stem growth (measured as a change in 

circumference) by 30-50 percent for most of the examined tree species (i.e., tulip poplar, black 

cherry, red maple, sugar maple) in a high O3 year in comparison to a low O3 year (U.S. EPA, 

2013a, section 9.4.3.1; McLaughlin et al., 2007a). The study also reported that high ambient O3

 

188 These functions for RBL estimate reduction in a year’s growth as a percentage of that 

expected in the absence of O3 (U.S. EPA, 2013a, section 9.6.2; U.S. EPA, 2014b, section 6.2). 
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concentrations can increase whole-tree water use and in turn reduce late-season streamflow 

(McLaughlin et al., 2007b; U.S. EPA, 2013a, p. 9-43). 

The magnitude of O3 impact on ecosystem productivity and on forest composition can 

vary among plant communities based on several factors including: the type of stand or 

community in which the sensitive species occurs (e.g., single species versus mixed canopy), the 

role or position of the species in the stand (e.g., dominant, sub-dominant, canopy, understory), 

the sensitivity of co-occurring species and environmental factors (e.g., drought and other 

factors). For example, O3 has been found to have little impact on white fir, but to greatly reduce 

growth of ponderosa pine in southern California, and cause decreased net primary production of 

most forest types in the Mid-Atlantic region, although only small impacts on spruce-fir forest

(U.S. EPA, 2013a, section 9.4.3.4).

As noted above, long-standing evidence has demonstrated that O3 alters biomass 

allocation and plant reproduction (U.S. EPA, 2013a, section 9.4.3). Several studies published 

since the 2006 O3 AQCD further demonstrate that O3 can alter reproductive processes in 

herbaceous and woody plant species, such as the timing of flowering and the number of flowers, 

fruits and seeds (U.S. EPA, 2013a, section 9.4.3.3). Further, limited evidence in previous reviews 

reported that vegetation effects from a single year of exposure to elevated O3 could be observed 

in the following year. For example, growth affected by a reduction in carbohydrate storage in 

one year may result in the limitation of growth in the following year. Such “carry-over” effects 

have been documented in the growth of some tree seedlings and in roots (U.S. EPA, 2013a, 

section 9.4.8; Andersen, et al., 1997). In the current review, additional field-based evidence 

expands the EPA’s understanding of the consequences of single and multi-year O3 exposures in 

subsequent years. A number of studies were conducted at a planted forest at the Aspen FACE 
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site in Wisconsin. These studies, which occurred in a field setting (more similar to natural forest 

stands than OTC studies), observed tree growth responses when grown in single or two species 

stands within 30-m diameter rings and exposed over a period of ten years to existing ambient 

conditions and elevated O3 concentrations. Some studies indicate the potential for carry-over 

effects, such as those showing that the effects of O3 on birch seeds (reduced weight, germination, 

and starch levels) could lead to a negative impact on species regeneration in subsequent years, 

and that the effect of reduced aspen bud size might have been related to the observed delay in 

spring leaf development. These effects suggest that elevated O3 exposures have the potential to 

alter carbon metabolism of overwintering buds, which may have subsequent effects in the 

following year (Darbah, et al., 2008, 2007; Riikonen et al., 2008; U.S. EPA, 2013a, section 

9.4.3). Other studies found that, in addition to affecting tree heights, diameters, and main stem 

volumes in the aspen community, elevated O3 over a 7-year study period was reported to 

increase the rate of conversion from a mixed aspen-birch community to a community dominated 

by the more tolerant birch, leading the authors to conclude that elevated O3 may alter intra- and 

inter-species competition within a forest stand (U.S. EPA, 2013a, section 9.4.3; Kubiske et al., 

2006; Kubiske et al., 2007). These studies confirm earlier FACE results of aspen growth 

reductions from a 6-7 year exposure to elevated O3 and of cumulative biomass impacts 

associated with changes in annual production in studied tree communities (U.S. EPA, 2013a, 

section 9.4.3; King et al., 2005). 

In addition to individual studies, recent meta-analyses have quantitatively analyzed the 

effect of O3 on trees across large numbers of studies. In particular, a recent meta-analysis of 55 

peer reviewed studies from the past 40 years indicates a negative relationship between O3

concentrations in the northern hemisphere during that period and stomatal conductance and 
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photosynthesis, which decreases growth (U.S. EPA, 2013a, section 9.4.3.1; Wittig et al., 2007). 

In this analysis, younger trees (less than 4 years) were affected less by O3 than older trees (U.S. 

EPA, 2013a, section 9.4.3.1; Wittig, et al., 2007). A second meta-analysis that quantitatively 

compiled 263 peer-reviewed studies “demonstrates the coherence of O3 effects across numerous 

studies and species that used a variety of experimental techniques, and these results support the 

conclusion of the previous AQCD that exposure to O3 decreases plant growth” (U.S. EPA, 

2013a, p. 9-43). Other meta-analyses have examined the effect of O3 exposure on root growth 

and generally found that O3 exposure reduced carbon allocated to roots (U.S. EPA, 2013a, pp. 9-

45 to 9-46).

As noted above, robust E-R functions have been developed for 11 tree species (black 

cherry, Douglas fir, loblolly pine, ponderosa pine, quaking aspen, red alder, red maple, sugar 

maple, tulip poplar, Virginia pine, white pine) from the extensive evidence base of O3–induced 

growth effects that was also available and relied upon in the previous review. While the species 

for which robust E-R functions have been developed represent only a small fraction (0.8 percent) 

of the total number of native tree species in the contiguous U.S. (1,497), this small 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 (U.S. EPA, 2013a, section 9.6.2; 

U.S. EPA, 2014b, section 6.2, Figure 6-2, Table 6-1). Each of these species were studied in 

OTCs, with most species studied multiple times under a wide range of exposure and/or growing 

conditions, with separate E-R functions developed for each combination of species, exposure 

condition and growing condition scenario. These species-specific composite E-R functions have 

been successfully used to predict the biomass loss response from tree seedling species over a 

range of cumulative exposure conditions (U.S. EPA, 2013a, section 9.6.2). The 11 robust 
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composite E-R functions available in the last review, as well as the E-R for eastern cottonwood

(derived from a field study in which O3 and climate conditions were not controlled), are 

described in the ISA and graphed in the WREA to illustrate the predicted responses of these 

species over a wide range of cumulative exposures (U.S. EPA, 2014b, section 6.2, Table 6-1 and 

Figure 6-2; U.S. EPA, 2013a, section 9.6.2). For some of these species, the E-R function is based 

on a single study (e.g., red maple), while for other species there were as many as 11 studies 

available (ponderosa pine). In total, the E-R functions developed for these 12 species (the 11 

with robust composite E-R functions plus eastern cottonwood) reflect 52 tree seedling studies. A 

stochastic analysis in WREA, summarized in section IV.C below, indicates the potential for 

within species variability to contribute appreciably to estimates for each species. Consideration 

of biomass loss estimates in the PA and in discussions below, however, is based on conventional 

method and focuses on estimates for the 11 species for which the robust datasets from OTC 

experiments are available, in consideration of CASAC advice.189 

c. Crop Yield Loss 

The “detrimental effect of O3 on crop production has been recognized since the 1960s”

(U.S. EPA, 2013a, p. 1-10, section 9.4.4). On the whole, the newly available evidence supports 

previous conclusions that exposure to O3 decreases growth and yield of crops. The ISA describes 

average crop yield loss reported across a number of recently published meta-analyses and

identifies several new exposure studies that support prior findings for a variety of crops of 

decreased yield and biomass with increased O3 exposure (U.S. EPA, 2013a, section 9.4.4.1, 

 

189 The CASAC cautioned EPA against placing too much emphasis on the eastern cottonwood 

data, stating that while the cottonwood data are important results, they are not as strong as those 

from other experiments that developed E-R functions based on controlled O3 exposure in OTCs; 

they are from a single gradient study that did not control for O3 and climatic conditions and they 

show extreme sensitivity to O3 compared to other studies (Frey, 2014c, p. 10).
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Table 9-17). Studies have also “linked increasing O3 concentration to decreased photosynthetic 

rates and accelerated aging in leaves, which are related to yield” and described effects of O3 on 

crop quality, such as nutritive quality of grasses, macro- and micronutrient concentrations in 

fruits and vegetable crops and cotton fiber quality (U.S. EPA, 2013a, p. 1-10, section 9.4.4). The 

findings of the newly available studies do not change the basic understanding of O3-related crop 

yield loss since the last review and little additional information is available in this review on 

factors that influence associations between O3 levels and crop yield loss (U.S. EPA, 2013a, 

section 9.4.4.). 

The new evidence has strengthened support for previously established E-R functions for 

10 crops (barley, field corn, cotton, kidney bean, lettuce, peanut, potato, grain sorghum, soybean 

and winter wheat), reducing two important areas of uncertainty, especially for soybean. The 

established E-R functions were developed from OTC-type experiments (U.S. EPA, 2013a, 

section 9.6.3; U.S. EPA, 2014b, section 6.2; U.S. EPA, 2014c, Figure 5-4). In this review, the 

ISA included an analysis comparing OTC data for soybean from the National Crop Loss 

Assessment Network (NCLAN) with field-based data from SoyFACE (Soybean Free Air 

Concentration Enrichment) studies (U.S. EPA, 2013a, section 9.6.3.1).190 Yield loss in soybean 

from O3 exposure at the SoyFACE field experiment was reliably predicted by soybean E-R 

functions developed from NCLAN data, demonstrating a robustness of the E-R functions 

developed with NCLAN data to predict relative yield loss from O3 exposure. A second area of 

 

190 The NCLAN program, which was undertaken in the early to mid-1980s, assessed multiple 

U.S. crops, locations, and O3 exposure levels, using consistent methods, to provide the largest, 

most uniform database on the effects of O3 on agricultural crop yields (U.S. EPA 1996a; U.S. 

EPA, 2006a; U.S. EPA, 2013a, sections 9.2, 9.4, and 9.6, Frey, 2014c, p. 9). The SoyFACE 

experiment was a chamberless (or free-air) field-based exposure study conducted in Illinois from 

2001 – 2009 (U.S. EPA, 2013a, section 9.2.4). 
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uncertainty that was reduced is that regarding the application of the NCLAN E-R functions, 

developed in the 1980s, to more recent cultivars currently growing in the field. Recent studies, 

especially those focused on soybean, provide little evidence that crops are becoming more 

tolerant of O3 (U.S. EPA, 2006a; U.S. EPA, 2013a). A meta-analysis of 53 studies found 

consistent deleterious effects of O3 exposures on soybean from studies published between 1973 

and 2001 (Morgan et al., 2003). Further, Betzelberger et al. (2010) utilized the SoyFACE facility 

to compare the impact of elevated O3 concentrations across 10 soybean cultivars to investigate 

intraspecific variability of the O3 response, finding that the E-R functions derived for these 10 

current cultivars were similar to the response functions derived from the NCLAN studies 

conducted in the 1980s (Heagle, 1989), “suggesting there has not been any selection for 

increased tolerance to O3 in more recent cultivars” (U.S. EPA, 2013a, p. 9-59). Additionally, the 

ISA comparisons of NCLAN and SoyFACE data referenced above “confirm that the response of 

soybean yield to O3 exposure has not changed in current cultivars” (U.S. EPA, 2013a, p. 9-59; 

section 9.6.3.1). Thus, the evidence available in the current review has reduced uncertainties in 

two areas with regard to the use of E-R functions for soybean crop yield loss.

During past O3 NAAQS reviews, there were very few studies that estimated O3 impacts 

on crop yields at large geographical scales (i.e., regional, national or global). Recent modeling 

studies of the impact of O3 concentrations historically found that increased O3 in the past 

generally reduced crop yield, but the impacts varied across regions and crop species, with the 

largest O3-induced crop yield losses estimated to have occurred in high-production areas that had 

been exposed to elevated O3 concentrations, such as the Midwest and the Mississippi Valley 

regions of the U.S. (U.S. EPA, 2013a, Section 9.4.4.1). Among affected crop species, the 

estimated yield loss for wheat and soybean were higher than rice and maize (i.e., field corn). 
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Additionally, satellite and ground-based O3 measurements have assessed soybean yield loss 

estimated to result from O3 over the continuous area of Illinois, Iowa, and Wisconsin, finding a 

relationship which correlates well with the previous results from FACE- and OTC-type 

experiments (U.S. EPA, 2013a, section 9.4.4.1).

Thus, consistent with the conclusions of the 1996 and 2006 CDs, the current ISA 

concludes that O3 concentrations in ambient air can reduce the yield of major commodity crops 

in the U.S. and focuses on use of crop E-R functions based on OTC experiments to characterize 

the quantitative relationship between ambient O3 concentrations and yield loss (U.S. EPA, 

2013a, section 9.4.4). In the PA, as summarized in sections IV.D and IV.E below, relative yield 

loss (RYL) is estimated for 10 different crops using the individual E-R functions described in the 

WREA191 (U.S. EPA, 2014b, section 6.2; U.S. EPA, 2014c, section 6.3).

2. Potential Impacts on Public Welfare

The magnitude of a public welfare impact or the degree to which it may be considered 

adverse is dependent upon the nature and severity of the specific welfare or ecological effect, the 

use or service (and value) of the affected ecosystem and the relevance and significance of that 

use192 to the public welfare. In the preamble of the 2012 final notice of rulemaking on the 

secondary standards for oxides of nitrogen and sulfur (NOx/SOx), the EPA stated that “[a]n

 

191 These functions for RYL estimate reduction in a year’s growth as a percentage of that 

expected in the absence of O3 (U.S. EPA, 2013a, section 9.6.2; U.S. EPA, 2014b, section 6.2). 

192 Ecosystem services have been defined as “the benefits that people obtain from ecosystems” 

(U.S. EPA, 2013a, Preamble, p. 1xxii; UNEP, 2003) and thus are an aspect of the use of a type of 

vegetation or ecosystem. Similarly, a definition used for the purposes of EPA benefits 

assessments states that ecological goods and services are the ‘‘outputs of ecological functions or 

processes that directly or indirectly contribute to social welfare or have the potential to do so in 

the future” and that “[s]ome outputs may be bought and sold, but most are not marketed’’ (U.S. 

EPA, 2006b).
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evaluation of adversity to the public welfare might consider the likelihood, type, magnitude, and 

spatial scale of the effect, as well as the potential for recovery and any uncertainties relating to 

these conditions” (77 FR 20232, April 3, 2012). The EPA additionally stated that 

“[c]onceptually, changes in ecosystem services may be used to aid in characterizing a known or 

anticipated adverse effect to public welfare” (77 FR 20232, April 3, 2012).

193

 

Potential public welfare impacts associated with ecosystems and associated services have 

a range of dimensions, including spatial, temporal, and social, and these likely will vary 

depending on the type of effect being characterized. For example, ecosystems can cover a range 

of spatial scales, and the services they provide can accrue locally or be distributed more broadly, 

such as when crops are sold and eaten locally and/or also sold in regional, national and world 

markets. Accordingly, impacts can be localized or more widely distributed. Further, ecosystem 

services can be realized over a range of temporal scales from immediate up to longer term. The 

size of the societal unit receiving benefits from ecosystem services can also vary dramatically. 

For example, a national park can provide direct recreational services to the thousands of visitors 

that come each year, but also provide an indirect value to the millions who may not visit but 

receive satisfaction from knowing it exists and is preserved for the future (U.S. EPA, 2014b, 

chapter 5, section 5.5.1). 

As recognized in the last review, judgments regarding adverse effects to the public 

welfare depend on the intended use for and significance of the affected vegetation, ecological 

 

193 Ecosystem services analyses were one of the tools used in that review to inform the decisions 

made with regard to adequacy and as such, were used in conjunction with other considerations in 

the discussion of adversity to public welfare (77 FR 20241).
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receptors, ecosystems and resources to the public welfare (73 FR 16496, March 27, 2008).194 For 

example, a number of different types of locations provide services of special significance to the 

public welfare. As emphasized in previous O3 NAAQS decisions, and summarized in section 

IV.A above, Class I areas and other parks have been afforded special federal protection to 

preserve services that provide for the enjoyment of these resources for current and future 

generations. Surveys have indicated that Americans rank as very important the existence of the 

resource, the option or availability of the resource and the ability to bequest or pass on to future 

generations (USDA, 2010). These and other services provided by Class I areas and other areas 

that have been afforded special protection can flow in part or entirely from the vegetation that 

grows there. Aesthetic value and outdoor recreation depend on the perceived scenic beauty of the 

environment. Many outdoor recreation activities directly depend on the scenic value of the area, 

in particular scenic viewing, wildlife-watching, hiking, and camping (U.S. EPA, 2014b, chapters 

5 and 7). Further, analyses have reported that the American public values – in monetary as well 

as nonmonetary ways – the protection of forests from air pollution damage. In fact, studies that 

have assessed willingness-to-pay for spruce-fir forest protection in the southeastern U.S. from air 

pollution and insect damage have found that values held by the survey respondents for the more 

abstract services (existence, option and bequest) were greater than those for recreation or other 

services (U.S. EPA, 2014b, Table 5-6; Haefele et al., 1991; Holmes and Kramer, 1995).

There are several potential public welfare impacts related to the three main categories of 

O3 effects on vegetation (i.e., effects on tree growth, productivity and carbon storage; crop yield 

 

194 As noted in section IV.A above, in judgments regarding public welfare significance in the last 

review, emphasis was placed on vegetation and ecosystem effects to sensitive species that are 

known to or are likely to occur in federally protected areas such as national parks and other Class 

I areas, or on lands set aside by states, tribes and public interest groups to provide similar 

benefits to the public welfare (73 FR 16496, March 27, 2008; 75 FR 3023-24, January 19, 2010). 
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loss; and, visible foliar injury, as described in section IV.B.1 above) and their associated 

ecosystem services. At the same time, these three categories of effects differ with regard to 

aspects important to judging their public welfare significance. Judgments regarding crop yield 

loss, for example, depend on considerations related to the heavy management of agriculture in 

the U.S., while judgments regarding the other categories of effects generally relate to 

considerations regarding forested areas. For example, while both tree growth-related effects and 

visible foliar injury have the potential to be significant to the public welfare through impacts in 

Class I and other protected areas, they differ in how they might be significant and with regard to 

the clarity of the data which describes the relationship between the effect and the services 

potentially affected. 

With regard to effects on tree growth, reduced growth is associated with effects on an 

array of ecosystem services including reduced productivity, altered forest and forest community 

(plant, insect and microbe) composition, reduced carbon storage and altered water cycling (U.S. 

EPA, 2013a, Figure 9-1, sections 9.4.1.1 and 9.4.1.2; U.S. EPA, 2014b, section 6.1). For 

example, forest or forest community composition can be affected through O3 effects on growth 

and reproductive success of sensitive species in the community, with the extent of compositional 

changes dependent on factors such as competitive interactions (U.S. EPA, 2013a, sections 9.4.3 

and 9.4.3.1). Depending on the type and location of the affected ecosystem, services benefitting 

the public in other ways can be affected as well. For example, other services valued by people 

that can be affected by reduced tree growth, productivity and carbon storage include aesthetic 

value, food, fiber, timber, other forest products, habitat, recreational opportunities, climate and 

water regulation, erosion control, air pollution removal, hydrologic and fire regime stabilization 

(U.S. EPA 2013a, sections 9.4.1.1 and 9.4.1.2; U.S. EPA, 2014b, section 6.1, Figure 6-1, section 
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6.4, Table 6-13). Further, impacts on some of these services (e.g., forest or forest community 

composition) may be considered of greater public welfare significance when occurring in Class I 

or other protected areas. 

Consideration of the magnitude of tree seedling growth effects that might cause or 

contribute to adverse effects for trees, forests, forested ecosystems or the public welfare is 

complicated by aspects of, or limitations in, the available information. For example, the evidence 

on tree seedling growth effects, deriving from the E-R functions for 11 species, provides no clear 

threshold or breakpoint in the response to O3 exposure. Additionally, there are no established 

relationships between magnitude of tree seedling growth reduction and forest ecosystem impacts 

and, as noted in section IV.B.1.b above, other factors can influence the degree to which O3-

induced growth effects in a sensitive species affect forest and forest community composition and 

other ecosystem service flows from forested ecosystems. These include: 1) the type of stand or 

community in which the sensitive species is found (i.e., single species versus mixed canopy); 2) 

the role or position the species has in the stand (i.e., dominant, sub-dominant, canopy, 

understory); 3) the O3 sensitivity of the other co-occurring species (O3 sensitive or tolerant); and 

4) environmental factors, such as soil moisture and others. The lack of such established 

relationships complicates judgments as to the extent to which different amounts of tree seedling 

growth would be significant to the public welfare and thus an important consideration in the 

level of protection for the secondary standard. 

During the 1997 review of the secondary standard, views related to this issue were 

provided by a 1996 workshop of 16 then-leading scientists in the context of discussing their 

views for a secondary O3 standard (Heck and Cowling, 1997). In their consideration of tree 

growth effects as an indicator for forest ecosystems and crop yield reduction as an indicator of 
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agricultural systems, the workshop participants identified annual percentages, of RBL for forest 

tree seedlings and RYL for agricultural crops, considered important to their judgments on the 

standard. With regard to forest ecosystems and seedling growth effects as an indicator, the 

participants selected a range of 1-2% RBL per year “to avoid cumulative effects of yearly 

reductions of 2%.” With regard to crops, they indicated an interest in protecting against crop 

yield reductions of 5% RYL yet noted uncertainties surrounding such a percentage which led 

them to identifying 10% RYL for the crop yield endpoint (Heck and Cowling, 1997). The 

workshop report provides no explicit rationale for the percentages identified (2% RBL and 5% or 

10% RYL); nor does it describe their connection to ecosystem impacts of a specific magnitude or 

type and judgments on significance of the effects for public welfare, e.g., taking into 

consideration the intended use and significance of the affected vegetation (Heck and Cowling, 

1997). In recognition of the complexity of assessing the adversity of tree growth effects and 

effects on crop yield in the broader context of public welfare, the EPA’s consideration of those 

effects in both the 1997 and 2008 reviews extended beyond the consideration of various 

benchmark responses for the studied species, and with regard to crops, additionally took note of

their extensive management (62 FR 38856, July 18, 1997; 73 FR 16436, March 27, 2008).

While, as noted above, public welfare benefits of forested lands can be particular to the 

type of area in which the forest occurs, some of the potential public welfare benefits associated 

with forest ecosystems are not location dependent. A potentially extremely valuable ecosystem 

service provided by forested lands and for which the ISA concludes a likely causal relationship 

with O3 in ambient air is carbon storage, a regulating service that is “of paramount importance 

for human society” (U.S. EPA, 2013a, section 2.6.2.1 and p. 9-37). The service of carbon storage

is potentially important to the public welfare no matter in what location the sensitive trees are 
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growing, or what their intended current or future use. In other words, the benefit exists as long as 

the tree is growing, regardless of what additional functions and services it provides. 

Another example of locations potentially vulnerable to O3-related impacts but not 

necessarily identified for such protection might be forested lands, both public and private, where 

trees are grown for timber production, particularly where they are dominated by a single timber 

species stand that is sensitive to O3, such as ponderosa pine. Further, forests in urbanized areas 

provide a number of services that are important to the public in those areas, including air 

pollution removal, cooling of the heat island effect, and beautification (U.S. EPA, 2014b, section 

6.6.2 and Appendix 6D; Akbari, 2002).195 The presence of O3-sensitive trees in such areas may 

place them at risk from elevated O3 exposures, contributing to potential impacts on important 

services provided by urban forests (U.S. EPA, 2014b, sections 6.6.2 and 6.7). There are many 

other tree species, such as species used in the USFS biomonitoring network, and various 

ornamental and agricultural species (i.e., Christmas trees, fruit and nut trees) that provide 

ecosystem services that may be judged important to the public welfare but whose vulnerability to 

impacts from O3 on tree growth, productivity and carbon storage has not been quantitatively 

characterized (U.S. EPA, 2014b, Chapter 6; Abt Associates, 1995).

As noted above, in addition to tree growth-related effects, O3-induced visible foliar injury 

also has the potential to be significant to the public welfare through impacts in Class I and other 

similarly protected areas. Visible foliar injury is a visible bioindicator of O3 exposure in species 

sensitive to this effect, with the injury affecting the physical appearance of the plant. 

Accordingly visible foliar injury surveys are used by federal land managers as tools in assessing 

potential air quality impacts in Class I areas. These surveys may focus on plant species that have 

 

195 For example, see http://www.fs.fed.us/research/urban/environmental-justice.php
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been identified as potentially sensitive air quality related values (AQRVs) due to their sensitivity 

to O3-induced foliar injury (USFS, NPS, FWS, 2010). An AQRV is defined by the National Park 

Services as a “resource, as identified by the FLM for one or more Federal areas that may be 

adversely affected by a change in air quality” and the resource “may include visibility or a 

specific scenic, cultural, physical, biological, ecological, or recreational resource identified by 

the FLM for a particular area” (USFS, NPS, USFWS, 2010).196 No criteria have been 

established, however, regarding a level or prevalence of visible foliar injury considered to be 

adverse to the affected vegetation, and, as noted in section IV.B.1.a above, there is not a clear 

relationship between visible foliar injury and other effects, such as reduced growth and 

productivity. Thus, key considerations with regard to public welfare significance of this endpoint 

have related to qualitative consideration of the plant’s aesthetic value in protected forested areas. 

Depending on the extent and severity, O3-induced visible foliar injury might be expected to have 

the potential to impact the public welfare in scenic and/or recreational areas during the growing 

season, particularly in areas with special protection, such as Class I areas.

 

196 The identification, monitoring and assessment of AQRVs with regard to an adverse effect is 

an approach used for assessing the potential for air pollution impacts from pending permit 

actions in Class I areas (USFS, NPS, USFWS, 2010). An adverse impact is recognized by the 

National Park Service as one that results in diminishment of the Class I areas’s national 

significance or the impairment of the ecosystem structure or functioning, as well as impairment 

of the quality of the visitor experience (USFS, NPS, USFWS, 2010). Federal land managers 

(FLMs) make such adverse impact determinations on a case-by-case basis, using technical and 

other information which they provide for consideration by permitting authorities. The National 

Park Services has developed a document describing an overview of approaches related to 

assessing projects under the National Environmental Policy Act and other planning initiatives 

affecting the National Park System (http://www.nature.nps.gov/air/Pubs/pdf/AQGuidance_2011-

01-14.pdf).
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The ecosystem services most likely to be affected by O3-induced visible foliar injury

(some of which are also recognized above for tree growth-related effects) are cultural services, 

including aesthetic value and outdoor recreation. In addition, several tribes have indicated that 

many of the species identified as O3-sensitive (including bioindicator species) are culturally 

significant (U.S. EPA, 2014c, Table 5-1). The geographic extent of protected areas that may be 

vulnerable to such public welfare effects of O3 is potentially appreciable. Sixty six species that 

occur on U.S. National Park Service (NPS) and U.S. Fish and Wildlife Service lands197 have 

been identified as sensitive to O3-induced visible foliar injury and some also have particular 

cultural importance to some tribes (U.S. EPA, 2014c, Table 5-1 and Appendix 5-A; U.S. EPA, 

2014b, section 6.4.2). Not all species are equally sensitive to O3, however, and quantitative 

relationships between O3 exposure and other important effects, such as seedling growth 

reduction, are only available for a subset of the 66, as described in section IV.B.1. above. A 

diverse array of ecosystem services has been identified for these twelve species (U.S. EPA, 

2014c, Table 5-1). Two of the species in this group that are relatively more sensitive with regard 

to effects on growth are the ponderosa pine and quaking aspen (U.S. EPA, 2014b, section 6.2), 

the ranges for which overlap with many lands that are protected or preserved for enjoyment of 

current and future generations (consistent with the discussion above on Class I and other 

protected areas), including such lands located in the west and southwest regions of the U.S. 

where ambient O3 concentrations and associated cumulative seasonal exposures can be highest 

(U.S. EPA, 2014c, Appendix 2B).198

 

197 See http://www2.nature.nps.gov/air/Pubs/pdf/flag/NPSozonesensppFLAG06.pdf

198 Basal area for resident species in national forests and parks are available in files accessible at: 

http://www.fs.fed.us/foresthealth/technology/nidrm2012.shtml
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With regard to agriculture-related effects, the EPA has recognized other complexities, 

stating that the degree to which O3 impacts on vegetation that could occur in areas and on species 

that are already heavily managed to obtain a particular output (such as commodity crops or 

commercial timber production) would impair the intended use at a level that might be judged 

adverse to the public welfare has been less clear (73 FR 16497, March 27, 2008; 75 FR 3024;

January 19, 2010). We note that while having sufficient crop yields is of high public welfare 

value, important commodity crops are typically heavily managed to produce optimum yields. In 

light of all of the inputs that go into achieving these yields, such as fertilizer, herbicides, 

pesticides, and irrigation, it is difficult to determine at what point O3-induced yield loss creates 

an adverse impact for the producer in the way of requiring increased inputs in order to maintain 

the desired 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. Given these competing impacts on producers 

and consumers, it is unclear how to consider these effects in terms of potential adversity to the 

public welfare (U.S. EPA, 2014c, sections 5.3.2 and 5.7).

When agricultural impacts or vegetation effects in other areas are contrasted with the 

emphasis on forest ecosystem effects in Class I and similarly protected areas, it can be seen that 

the Administrator has in past reviews judged the significance to the public welfare of O3-induced 

effects on sensitive vegetation growing within the U.S. to differ depending on the nature of the 

effect, the intended use of the sensitive plants or ecosystems, and the types of environments in 

which the sensitive vegetation and ecosystems are located, with greater significance ascribed to 

areas identified for specific uses and benefits to the public welfare, such as Class I areas, than to

areas for which such uses have not been established. In summary, several considerations are 
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recognized as important to judgments on the public welfare significance of the array of effects of 

different O3 exposure conditions on vegetation. While there are complexities associated with the 

consideration of the magnitude of key vegetation effects that might be concluded to be adverse to 

ecosystems and associated services, there are numerous locations where O3-sensitive tree species 

are present that may be vulnerable to impacts from O3 on tree growth, productivity and carbon 

storage and their associated ecosystems and services. It is not possible to generalize across all 

studied species regarding which cumulative exposures are of greatest concern, however, as this 

can vary by situation due to differences in exposed species sensitivity, the importance of the 

observed or predicted O3-induced effect, the role that the species plays in the ecosystem, the 

intended use of the affected species and its associated ecosystem and services, the presence of 

other co-occurring predisposing or mitigating factors, and associated uncertainties and 

limitations. These factors contribute to the complexity of the Administrator’s judgments 

regarding the adversity of known and anticipated effects to the public welfare.

C. Exposure and Risk Assessment Information

The WREA characterized ambient O3 exposure and its relationship to tree biomass loss, 

crop yield loss, and visible foliar injury and the associated ecosystem services199 in national-scale 

and case study analyses. The WREA also qualitatively assessed impacts to some ecosystem 

services, including impacts on the hydrologic cycle, pollination regulation, and fire regulation; 

commercial non-timber forest products and insect damage; and aesthetic and non-use values. In 

the quantitative analyses, the WREA characterized effects associated with exposures to O3 in 

ambient air using the W126 metric.

 

199 In its review of drafts of the WREA and PA, the CASAC conveyed support for analyses and 

considerations of ecosystem services that may be affected by O3 exposures (Frey, 2014b, 2014c).
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The following sections summarize the analyses and adjustment approach used to develop 

the O3 concentrations used as inputs to the vegetation risk analyses for tree biomass and crop 

yield loss, and the analyses, including key results and uncertainties, for tree seedling growth, 

productivity, carbon storage and associated ecosystem services (section IV.C.2); crop yield loss 

(section IV.C.3); and visible foliar injury (section IV.C.4).

1. Air Quality Analyses

The WREA evaluated O3 exposure and risks for several national-scale air quality 

scenarios: recent conditions (2006 to 2008),

200 the current secondary standard, and W126 index 

values of 15 ppm-hrs, 11 ppm-hrs, and 7 ppm-hrs, using three-year averages (U.S. EPA, 2014b, 

chapter 4). For each of these scenarios, three-year average W126 index values were estimated at 

each 12 km by 12 km grid cell in a national-scale spatial surface. Additionally, some analyses 

were based on single-year surfaces.201 The method for creating the five scenarios generally 

involved two steps (summarized in Table 5-4 of the PA). The first is derivation of the average 

W126 index value (across the three years) at each monitor location. This value is based on 

unadjusted O3 concentrations from monitoring data for recent conditions and adjusted 

concentrations for the four other scenarios. Concentrations were adjusted based on model 

predicted relationships between O3 and U.S.-wide emissions reductions in oxides of nitrogen 

(NOx). The adjusted air quality does not represent an optimized control scenario that just meets 

the current standard (or target W126 index values for other scenarios), but rather characterizes

 

200 Certain visible foliar injury analyses assessed recent conditions from 2006 to 2010 on an 

annual basis.

201 An analysis using data from USFS FHM/FIA O3 biomonitoring sampling sites (“biosites”)

and a screening-level assessment in 214 national parks were done using national-scale spatial 

surfaces of unadjusted O3 concentrations (in terms of the W126 index) created for each year 

from 2006 through 2010 using the VNA interpolation technique (U.S. EPA, 2014b, section 4.3.2, 

Appendix 4A).
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one potential distribution of air quality across a region when all monitor locations meet the 

standard (U.S. EPA, 2014b, section 4.3.4.2). The development of adjusted concentrations was 

done for each of nine regions independently (see U.S. EPA, 2014b, section 4.3.4.1). In the 

second step, national-scale spatial surfaces (W126 index values for each 12 km x 12 km grid cell

used in the air quality model) were created using the monitor-location values and the Voronoi 

Neighbor Averaging (VNA) spatial interpolation technique (details on the VNA technique are 

presented in U.S. EPA, 2014b, Appendix 4A). 

In the dataset used to create the recent conditions scenario, the three-year average W126 

index values at the monitor locations (before application of the VNA technique) ranged from 

below 5 ppm-hrs to 48.6 ppm-hrs (U.S. EPA, 2014b, Figure 4-4 and Table 4-3). In the nine 

modeling regions, the maximum three-year average W126 index values at monitor locations 

ranged from 48.6 ppm-hrs in the West region down to 6.6 ppm-hrs in the Northwest region.202

After adjustment of the monitor location concentrations to just meet the current standard in each 

region (using relationships described above), the region-specific maximum three-year average 

W126 values ranged from 18.9 ppm-hrs in the West region to 2.6 ppm-hrs in the Northeast 

region (U.S. EPA, 2014b, Table 4-3). With the next step, creation of the national surface of air 

quality values at grid cell centroids, the highest values were reduced, such that all the three-year 

average W126 index values were below 15 ppm-hrs across the national surface with the 

exception of a very small area of the Southwest region (near Phoenix) where average W126 

index values were just above 15 ppm-hrs. Thus, it can be seen that application of the VNA 

 

202 The regions referenced here and also with regard to monitoring data analyses described in 

section IV.D.4 below are NOAA climate regions, as shown in Figure 2B-1 of the PA.
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interpolation method to estimate W126 index values at the centroid of every 12 x 12 km2grid 

cell rather than only at each monitor location results in a lowering of the highest values.

Because the W126 estimates generated for the different air quality scenarios assessed are 

inputs to the vegetation risk analyses for tree biomass and crop yield loss, and also used in the 

foliar injury analyses, any uncertainties in the air quality analyses are propagated into the those 

analyses (U.S. EPA, 2014b, section 8.5). The WREA identified sources of uncertainty for the 

W126 estimates for each air quality scenario and qualitatively characterized the magnitude of 

uncertainty and potential for directional bias (U.S. EPA, 2014b, Table 4-5). As discussed in 

Chapter 4 and 8 of the WREA, an important large uncertainty in the analyses is the assumed 

response of the W126 concentrations to emissions reductions needed to meet the existing 

standard (U.S. EPA, 2014b, section 8.5.1). Any approach to characterizing O3 air quality over 

broad geographic areas based on concentrations at monitor locations will convey inherent 

uncertainty. The model-based adjustments are based on U.S.-wide emissions reductions in NOx

and characterize only one potential distribution of air quality across a region when all monitor 

locations meet the standard (U.S. EPA, 2014b, section 4.3.4.2).203 Additionally, the surface is 

created from the three-year average at the monitor locations, rather than creating a surface for 

 

203 The WREA analyses used U.S.-wide NOx emissions reductions to simulate air quality that 

independently in each region would just meet the existing standard and the three W126 

scenarios. The NOx emissions reductions were determined such that the highest monitor within 

each region would just meet the target level. In this way, the adjustment results in broad regional 

reductions in O3 and includes reductions in O3 at some monitors that were already meeting or 

below the target level. Thus, the adjustments performed to develop a scenario meeting a target 

level at the highest monitor in each region did result in substantial reduction below the target 

level in some areas of the region. This result at the monitors already well below the target 

indicates an uncertainty with regard to air quality expected from specific control strategies that 

might be implemented to meet a particular target level.
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each year and then averaging across years at each grid cell; the potential impact of this on the 

resultant estimates is considered in the WREA (U.S. EPA, 2014b, Appendix 4A).

An additional uncertainty related to the W126 index value estimates for each air quality 

scenario comes from the creation of a national W126 surface using the VNA technique to 

interpolate recent air quality measurements of O3. In general, spatial interpolation techniques 

perform better in areas where the O3 monitoring network is denser. Therefore, the W126 index 

values estimated in the rural areas in the West, Northwest, Southwest, and West North Central 

with few or no monitors (U.S. EPA, 2014b, Figure 2-1) are more uncertain than those estimated 

for areas with denser monitoring. Further, this interpolation method generally underpredicts 

higher 12-hour W126 exposures. Due to the important influence of higher exposures in 

determining risks to plants, the potential for the VNA interpolation approach to underpredict 

higher W126 exposures could result in an underestimation of risks to vegetation in some areas. 

Underestimation of the highest W126 index values for the current standard scenario is an 

additional impact of the interpolation method that is important to consider. 

2. Tree Seedling Growth, Productivity, Carbon Storage and Associated Ecosystem Services

For the WREA assessments related to tree growth, productivity, carbon storage and 

associated ecosystem services, the sections below provide an overview of the analyses along 

with the key results (section IV.C.2.a) and summarize the key uncertainties (section IV.C.2.b).

a. Overview and Summary of Key Results

The assessments to estimate the exposures and risks for tree seedling growth, 

productivity, and carbon storage reflect a range of spatial scales ranging from the county scale up 

to the national park, urban area, and national scales. For the air quality scenarios described 

above, the WREA applied the species-specific E-R functions to develop estimates of O3-
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associated RBL, productivity, carbon storage and associated ecosystem services (U.S. EPA, 

2014b, Chapter 6). Some analyses also apply the median across species E-R functions. 

The WREA examined multiple approaches for characterizing the median tree response to 

O3 exposure based on the 11 robust E-R functions for tree seedlings from the OTC research and 

the E-R function for eastern cottonwood (U.S. EPA, 2014b, section 6.2.1.2 and Figure 6-5). For 

some species, only one study was available (e.g., red maple), and for other species there were as 

many as 11 studies available (e.g., ponderosa pine). To illustrate the effect of within-species 

variability associated with the E-R data available on estimates for a median response across the 

12 species, the WREA performed a stochastic sampling analysis involving multiple iterations of 

random selection of E-R functions from the studies available for each of the 12 species. This 

analysis produced median values at each cumulative exposure level that were higher than 

medians derived by two conventional, deterministic methods (U.S. EPA, 2014b, section 6.2.1.2 

and Figure 6-5).

204 For example, the median seasonal W126 index value for which a two percent 

biomass loss is estimated in seedlings for the studied species ranges from approximately 7 ppmhrs using the conventional methods up to 14 ppm-hrs when derived by the stochastic method.

Although the stochastic method provides some illustration of the effect of within–species 

variability, we focus on the conventional approach that gives equal weight to each studied 

species, calculating the median response based on the composite E-R functions, consistent with 

CASAC advice (Frey, 2014b).

The WREA estimates indicate substantial heterogeneity in plant responses to O3, both 

within species, between species, and across regions of the U.S. The tree species known to be O3-

 

204 These methods were calculating a median using the composite functions and calculating a 

median using all tree seedling studies available.
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sensitive are different in the eastern and western U.S. and the eastern U.S. has far more such 

species. Ozone exposure and risk is somewhat easier to assess in the eastern U.S. because of the 

availability of more data and the greater number of species to analyze. In addition, there are more 

O3 monitors in the eastern U.S. but fewer national parks (U.S. EPA, 2014b, chapter 8). In 

consideration of CASAC advice, the WREA derived RBL and weighted RBL (wRBL) estimates 

separately with and without the eastern cottonwood. The results summarized here are for the 

analyses that exclude cottonwood.205 The WREA reported RBL estimates relative to a 

benchmark of 2% RBL for tree seedlings, as well as relative to other percent RBL values. The 

2% RBL benchmark was considered based on CASAC advice that stated that “focus on a 2% 

loss level for trees ... is appropriate.” (Frey, 2014b, p. 6). The main WREA analyses for effects 

related to tree growth, productivity and carbon storage are summarized below, with the key 

findings for each.

Relative biomass loss nationally was estimated for each of the 12 studied species from 

the composite E-R functions for each species described above and information on the 

distribution of those species across the U.S. (U.S. EPA, 2014b, section 6.2.1.3 and Appendix 

6A). As one example of a tree species near the median of the studied species, relative biomass 

loss estimates (reduced growth) for ponderosa pine in the current standard air quality scenario 

are below two percent for most areas where this species is found but estimates of RBL for this 

species in some areas of the southwest fall above two percent biomass loss (U.S. EPA, 2014b, 

 

205 The CASAC advised that the eastern cottonwood response data “receive too much emphasis” 

in a draft version of the PA, explaining that these “results are from a gradient study that did not 

control for ozone and climatic conditions and show extreme sensitivity to ozone compared to 

other studies” and that “[a]lthough they are important results, they are not as strong as those from 

other experiments that developed E-R functions based on controlled ozone exposure” (Frey, 

2014b, p. 10). 
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Figure 6-8). Maximum estimates of RBL for all areas where ponderosa pine is found decrease to 

just over three percent and just over two percent for the 15 and 7 ppm-hrs scenarios, respectively 

(U.S. EPA, 2014b, Table 6-6).

To provide an indication of ecosystem-level impacts, weighted estimates of RBL (wRBL) 

were also developed for each grid cell nationwide. This is estimated from the species-specific ER functions and a weighting approach based on information on prevalence of the studied species 

across the U.S. (i.e., the proportion of the total basal area modeled by USFS across all species for 

which data were available). An overall wRBL value for each grid cell is generated by summing 

the wRBL values for each studied tree species found within that grid cell. The wRBL is intended 

to be an indication of the potential magnitude of the ecological effect that could occur in some 

ecosystems. In general, the higher the wRBL is in a given ecosystem, the larger the potential 

ecological effect. (U.S. EPA, 2014b, section 6.8, Table 6-25).

For the national-scale analysis, the WREA presents the percent of total basal area with 

wRBL greater than 2%. The estimates for the weighted biomass loss analysis reflecting the 11 

tree species with robust E-R functions are as follows (U.S. EPA, 2014b, Table 6-25):

 For the current standard scenario, the percent of total basal area that exceeds a two 

percent wRBL is 0.2 percent. 

 For the W126 scenarios of 15, 11 and 7 ppm-hrs, the percent of total basal area that 

exceeds a two percent wRBL is 0.2 percent, 0.1 percent, and less than 0.1 percent 

respectively (U.S. EPA, 2014b, Table 6-25). 

In the wRBL analysis for Class I areas, the number of Class I areas with wRBL greater 

than 2% is estimated for the grid cells located in the 145 of the 156 Class I areas for which data 

were available (U.S.EPA, 2014b, Table 6-26).
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 For the current standard scenario, two of the 145 assessed Class I areas have weighted 

RBL values above two percent (U.S. EPA, 2014b, Table 6-26). 

 For the W126 scenarios of 15, 11 and 7 ppm-hrs, there are two, two and one Class I area

with wRBL above two percent, respectively. 

In the county analysis, the WREA estimated the number of U.S. counties in which any of 

the studied tree species is estimated to experience more than two percent RBL, the number of 

species affected, and the number of counties for which the median of the species-specific 

functions exceeds two percent RBL. In addition to the estimates based on all 12 studied species 

and also the 11 species with the exclusion of eastern cottonwood (in response to CASAC 

advice), additional estimates were developed without black cherry to show contribution of that 

sensitive species to the multi-species estimates (U.S. EPA, 2014b, Table 6-7).

 In the current standard scenarios, 66% of the 3,109 assessed counties are estimated to 

have at least one of the 11 species (excluding cottonwood) with an RBL greater than two 

percent, with three counties having three species exceeding two percent. The median 

RBL (across the species present) is above two percent in 239 counties. The maximum 

number of species in any one county with an RBL greater than two percent is three 

(excluding cottonwood). (U.S. EPA, 2014b, Table 6-7). 

 For the 15, 11 and 7 ppm-hrs scenarios, the proportion of 3,109 counties with one or 

more species with an RBL above two percent decreases to 61 percent, 59 percent, and 58 

percent, respectively. For the 7 ppm-hrs scenario, the median RBL is above two percent 

in six percent of the counties (U.S. EPA, 2014b, Table 6-7).

 The county RBL estimates are appreciably influenced by black cherry, a very sensitive 

species that is widespread in the Eastern U.S. For 1,805 of the 1,929 counties estimated 
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to have at least one species with an RBL greater than two percent when air quality is 

meeting the current standard, only black cherry exceeds this level of RBL. If black cherry 

is excluded, the median RBL for the 10 remaining species decreases. For the median 

RBL values, 203 of the 239 counties estimated to have a median RBL above two percent 

when air quality is meeting the current standard are because of the presence of black 

cherry (U.S. EPA, 2014b, Table 6-7).

Additionally, the WREA estimated relative yield loss in timber production and 

associated changes in consumer and producer/farmer economic surplus using E-R functions for 

tree seedlings to calculate relative yield loss (equivalent to relative biomass loss) across full tree 

lifespans and through modeling of the resulting market-based welfare effects. Because the 

forestry and agriculture sectors are related and trade-offs occur between the sectors, the WREA 

calculated the resulting market-based welfare effects of O3 exposure in the forestry and 

agriculture sectors on consumer and producer surplus.206 Because demand for most forestry and 

agricultural commodities is not highly responsive to changes in price, producer surplus (i.e., 

producer gains) often declines. These declines can be more than offset by changes in consumer

surplus gains from lower prices, but, in some cases, lower prices reduce producer gains more 

than can be offset by consumer surplus (U.S. EPA, 2014b, Appendix 6B, Table and B-9).

 In the current standard scenario, estimates of the relative yield loss for timber production 

are below one percent other than in the Southwest, Southeast, Central, and South regions 

 

206 The WREA used the Forest and Agricultural Sector Optimization Model with Greenhouse 

Gases (FASOMGHG). FASOMGHG is a national-scale model that provides a complete 

representation of the impacts of meeting alternative standards on the U.S. forest and agricultural 

sectors. FASOMGHG simulates the allocation of land over time to competing activities in both 

the forest and agricultural sectors. FASOMGHG results include multi-period, multi-commodity 

results over 60 to 100 years in 5-year time intervals when running the combined forestagriculture version of the model. 
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(U.S. EPA, 2014b, section 6.3, Table 6-9) (see U.S. EPA, 2014b, Table 6-8 for 

clarification on region names). The highest yield loss occurs in upland hardwood forests 

in the South Central and Southeast regions at over three percent per year and in Corn Belt 

hardwoods at just over two percent loss per year (U.S. EPA, 2014b, section 6.3, Table 6-

9). 

 For the 15 and 11 ppm-hrs scenarios, relative yield loss estimates for timber production 

are above one percent in parts of the Southeast, Central, and South regions and above two 

percent in parts of the Southeast and Central U.S.

 For the 7 ppm-hrs scenario, relative yield loss estimates for timber production are above 

one percent in the Southeast and South regions (U.S. EPA, 2014b, section 6.3, Table 6-

9). 

The WREA also estimated impacts on tree growth and two ecosystem services provided 

by urban trees: removal of air pollutants and carbon storage. The estimates of the tons of carbon 

monoxide, nitrogen dioxide, ozone and sulfur dioxide removed are for a 25-year period in five 

urban case study areas: Baltimore, Syracuse, the Chicago region, Atlanta, and the urban areas of 

Tennessee (U.S. EPA, 2014b, section 6.7).207

 Estimates for all five urban case study areas indicate increased pollutant removal of O3, 

nitrogen dioxide, carbon monoxide, and sulfur dioxide in the current standard scenario 

(U.S. EPA, 2014b, sections 6.7). The results for the 15 ppm-hrs scenario were very 

similar to those for meeting the current standard. For the 11 and 7 ppm-hrs scenarios, all 

five case study areas indicate smaller additional increases in air pollutant removal beyond 

 

207 The WREA used the i-Tree model for the urban case studies. i-Tree is a peer-reviewed suite 

of software tools provided by USFS.
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moving from current conditions to the current standard (U.S. EPA, 2014b, sections 6.7).

The WREA estimated carbon storage related to O3-induced biomass loss in forests and 

agricultural crops nationally and also in forests in five urban areas using the FASOMGHG and iTree models noted above (U.S. EPA, 2014b, section 6.6). Ozone effects on tree growth affects 

the climate regulation service provided by ecosystems by reducing carbon sequestration and 

storage (U.S. EPA, 2013a, section 9.4.3.4; U.S. EPA, 2014b, chapter 6, section 6.6). Because O3

exposure affects photosynthesis and CO2 uptake by trees, forests sequester less carbon and thus 

more carbon stays in the atmosphere. In the model used to calculate national-level impacts to 

forests and agriculture from O3-related biomass loss, carbon sequestration reflects carbon in 

standing (live and dead) trees, forest soils, the forest understory vegetation, forest floor including 

litter and large woody debris, and wood products both in use and in landfills (U.S. EPA, 2014b, 

chapter 6, Appendix 6B, section 2.7.1).

 Over 30 years for the national-scale analysis, carbon storage in the forestry sector 

estimated for the current standard scenario is just over 89,000 million metric tons of CO2

equivalents (MMtCO2e); this is 11,840 more MMtCO2e storage associated with the 

reduced O3-related growth impact from meeting the current standard as compared with 

recent conditions.

208 The estimates of carbon storage in the agricultural sector are much 

smaller (i.e., 8,469 MMtCO2e for the current standard scenario which is 606 MMtCO2e

more than the recent conditions scenario) (U.S. EPA, 2014b, section 6.6.1 and Appendix 

6B). The forestry sector carbon storage estimated for each of the three W126 scenarios is

 

208 One MMtCO2e is equivalent to 208,000 passenger vehicles or the electricity to run 138,000 

homes for 1 year as calculated by the EPA Greenhouse Gas Equivalencies Calculator (updated 

September 2013 and available at http://www.epa.gov/cleanenergy/energyresources/calculator.html).
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just slightly greater than that estimated for the current standard. As a percentage of the 

current standard estimate, the three scenario estimates are less than 0.1% (13 MMtCO2e), 

just under 1% (593 MMtCO2e) and under 2% (1,600 MMtCO2e) for the 15, 11 and 7 

ppm-hrs scenarios, respectively (U.S. EPA, 2014b, Tables 6-19 and B-10).

 Estimates of the effects of avoided O3-related biomass loss on carbon sequestration in 

forests in the five urban area case studies indicate the potential for an increase in carbon 

sequestration of somewhat more than one MMtCO2e for the current standard scenario 

compared to the recent conditions estimate (U.S. EPA, 2014b, section 6.6.2 and 

Appendix 6D). The additional increases in O3-related carbon sequestration estimated 

across the five case studies for the three W126 scenarios are relatively small (U.S. EPA, 

2014b, section 6.6.2 and Appendix 6D).

Although not discussed in detail here, the WREA also describes qualitative assessments 

for some ecosystem services that may be affected by O3 effects on tree growth and productivity, 

such as commercial non-timber forest products and recreation (U.S. EPA, 2014b, section 6.4), 

aesthetic and non-use values (U.S. EPA, 2014b, section 6.4), increased susceptibility to insect 

attack and fire damage (U.S. EPA, 2014b, sections 5.3 and 5.4, respectively). Other ecological 

effects that are causally or likely causally associated with O3 exposure, such as effects on 

terrestrial productivity, the water cycle, the biogeochemical cycle, and community composition 

(U.S. EPA, 2013a, Table 9-19), were not quantitatively addressed in the WREA due to a lack of 

sufficient quantitative information. 

b. Key Uncertainties

The WREA identified several key limitations and uncertainties in the biomass loss 

assessments for trees, which may have a large impact on both overall confidence and confidence 
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in individual analyses. Key uncertainties that affect the assessment of impacts on ecosystem 

services at the national and case-study scales, as well as across species, U.S. geographic regions 

and future years, include those associated with the interpolated and adjusted O3 concentrations 

used to estimate W126 exposures in the air quality scenarios, the available seedling E-R 

functions, combining effects across sensitive species, the effects of compounding over time, and 

modeling impacts of biomass loss on timber harvesting and urban air pollutant removal.

With regard to the robust seedling E-R functions, the WREA provided some 

characterization of the variability of individual study results and the impact of that on estimates 

of W126 index values that might elicit different percentages of biomass loss in tree seedlings 

(U.S. EPA, 2014b, section 6.2.1.2). Even though the evidence shows that there are additional 

species affected by O3-related biomass loss, the WREA only has E-R functions available to 

quantify this loss for 12 tree species. This limited information only allows a partial 

characterization of the O3-related biomass loss impacts in trees associated with recent O3 index 

values and with just meeting the existing and potential alternative secondary standards. In 

addition, there are uncertainties inherent in these E-R functions, including the extrapolation of 

relative biomass loss rates from tree seedlings to adult trees and information regarding withinspecies variability. The overall confidence in the E-R function varies by species based on the 

number of studies available for that species. Some species have low within-species variability 

(e.g., many agricultural crops) and high seedling/adult comparability (e.g., aspen), while other 

species do not (e.g., black cherry). The uncertainties in the E-R functions for biomass loss and in 

the air quality analyses are propagated into the analysis of the impact of biomass loss on 

ecosystem services, including provisioning and regulating services (U.S. EPA, 2014b, Table 6-

27). The WREA characterizes the direction of potential influence of E-R function uncertainty as 
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unknown, yet its magnitude as high, concluding that further studies are needed to determine how 

accurately the assessed species reflect the larger suite of O3-sensitive tree species in the U.S. 

(U.S. EPA, 2014b, Table 6-27).

Another uncertainty associated with interpretation of the WREA biomass loss-related 

estimates concerns the potential for underestimation of compounding of growth effects across 

multiple years of varying concentrations. Though tree biomass loss impacts were estimated using 

air quality scenarios of three-year average W126 index values, the WREA also conducted an 

analysis to compare the impact of using a variable compounding rate based on yearly variations 

in W126 exposures to that of using a W126 index value averaged across three years. The WREA 

compared the compounded values for an example species occurring in the eastern U.S. and 

another example species occurring in the western U.S. In both examples, one species (tulip polar 

and ponderosa pine, respectively) and one climate region where that species occurred (Southeast 

and Southwest regions, respectively) were chosen and air quality values associated with just 

meeting the existing standard of 75 ppb were used. Within each region, the WREA analysis used 

both the W126 index value at each monitor in the region for each year and the three-year average 

W126 index value using the method described in Chapter 4 of the WREA. The results show that 

the use of the three-year average W126 index value may underestimate RBL values slightly 

(U.S. EPA, 2014b, section 6.2.1.4 and Figure 6-14). In both regions, the three-year average 

W126 index value is sometimes above and sometimes below the individual year W126 index 

value. 

The WREA recognizes uncertainty regarding the extent to which the subset of studied 

tree species encompass the O3 sensitive species in the U.S. and the extent to which it represents 

U.S. vegetation as a whole (U.S. EPA, 2013a, pp. 9-123 to 9-125; U.S. EPA, 2014b, Table 6-27). 
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There are also uncertainties associated with estimating the national scale ecosystem-level 

impacts using wRBL. For example the wRBL estimates are likely biased low as there may be 

other unstudied O3-sensitive tree species in some areas that are also being affected at those 

levels, although this analysis does not take into account the effects of competition, which could 

further affect forest biomass loss.

Uncertainties are recognized in the national-scale analyses of timber production, 

agricultural harvesting, and carbon sequestration, for which the WREA used the FASOMGHG 

model. These uncertainties include those associated with the functions for carbon sequestration, 

the assumptions made regarding proxy species where there are insufficient data, and the nonW126 E-R functions for three crops. The FASOMGHG model does not include agriculture and 

forestry on public lands, changes in exports due to O3 into international trade projections, or 

forest adaptation. Despite the inherent limitations and uncertainties, the WREA concludes that 

the FASOMGHG model reflects reasonable and appropriate assumptions for a national-scale 

assessment of changes in the agricultural and forestry sectors due to changes in vegetation 

biomass associated with O3 exposure (U.S. EPA, 2014b, sections 6.3, 6.5, 6.6, and 8.5.2, and 

Table 6-27). 

In the case study analyses of five urban areas, the WREA used the i-Tree model, which 

includes an urban tree inventory for each area and species-specific pollution removal and carbon 

sequestration functions. However, i-Tree does not account for the potential additional VOC 

emissions from tree growth, which could contribute to O3 formation. Uncertainties are also 

recognized with regard to the base inventory of city trees, the functions used for air pollutant 

removal and for carbon storage (U.S. EPA, 2014b, sections 6.6.2 and 6.7, and Table 6-27). 

Despite the inherent limitations and uncertainties, the WREA concludes that the i-Tree model 
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reflects reasonable and appropriate assumptions for a case study assessment of pollution removal 

and carbon sequestration for changes in biomass associated with O3 exposure (U.S. EPA, 2014b, 

sections 6.6.2, 6.7, and 8.5.2). 

3. Crop Yield

Section IV.C.3.a below provides an overview of the assessments performed in the WREA 

to estimate the exposures and risks for crop yield, as well as the key results. Section IV.C.3.b 

summarizes the key uncertainties.

a. Overview and Summary of Key Results

The WREA conducted two analyses to estimate O3 impacts related to crop yield, 

including annual yield losses estimated for 10 commodity crops grown in the U.S. with E-R 

functions and how these losses affect producer and consumer economic surpluses (U.S. EPA, 

2014b, sections 6.2, 6.5). Summary estimates for crop yield loss related effects in the WREA are 

presented relative to a 5% yield loss benchmark based on consideration of CASAC’s 

recommendation to consider a benchmark of 5% for median crop yield loss and to consider 5% 

yield loss for individual crop species. In addition, other benchmarks levels are considered in the 

WREA (e.g. 10% and 20%). 

The WREA derived estimates of crop RYL estimates nationally and in a county-specific 

analysis. Crop-specific estimates of O3-related RYL nationally were derived for each of the air 

quality scenarios from the 10 E-R functions for crops described above combined with 

information regarding crop distribution (U.S. EPA, 2014b, section 6.5). The WREA also 

reported crop RYL results at the county-level, as well as the number of crop-producing counties 

with greater than five percent RYL (U.S. EPA, 2014b, section 6.5.1, Appendix 6B).

 The largest reduction in O3-induced crop yield loss and yield changes occurs when 
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moving from the recent conditions scenario to the current standard scenario (U.S. EPA, 

2014b, section 6.5). Among the major commercial crops, winter wheat and soybeans are 

more sensitive to ambient O3 levels than other crops. 

 In the current standard scenario, no counties have RYL estimates at or above 5% (U.S. 

EPA, 2014b, section 6.5).209

The WREA also estimated O3-related crop impacts on producer and consumer surplus.210

These are national-scale estimates of the effects of yield loss on agricultural harvesting, which 

supply provisioning services of food and fiber for each of the air quality scenarios. Overall effect 

on agricultural yields and producer and consumer surplus depends on (1) the ability of 

producers/farmers to substitute other crops that are less O3 sensitive, and (2) the responsiveness, 

or elasticity, of demand and supply (U.S. EPA, 2014b, section 6.5).

 Estimates of consumer surplus, or consumer gains, were generally higher in the current 

standard scenario in the agricultural sector because higher productivity under lower O3

concentrations increased total yields and reduced market prices (U.S. EPA, 2014b, Tables

6-17 and 6-18). Combined gains in producer and consumer surplus for forestry and 

agriculture were essentially unchanged for the 15 ppm-hrs scenario, but annualized gains 

increased by $21 million beyond the current standard scenario for the 11 ppm-hrs 

 

209 In the air quality scenario for the current standard, a monitor that already met the current 

standard but was located within the same region as another monitor that was above the current 

standard would have had its concentration adjusted downward. This is due to the fact that 

concentrations were adjusted independently for each region, applying reductions to all monitors 

within the region, such that all monitors located within a region meet the standard (U.S. EPA, 

2014b, section 4.3.4.2). 

210 Welfare economics focuses on the optimal allocation of resources and goods and how those 

allocations affect total social welfare. Total welfare is also referred to as economic surplus, 

which is the overall benefit a society, composed of consumers and producers, receives when a 

good or service is bought or sold, given a quantity provided and a market price. Economic 

surplus is divided into two parts: consumer and producer surplus (U.S. EPA, 2014b, p. ES-6).
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scenario and $231 million for the 7 ppm-hrs scenario. In some cases, lower prices reduce 

producer gains more than can be offset by higher yields (U.S. EPA, 2014b, Table 6-18).

 Because demand for most agricultural commodities is not highly responsive to changes in 

price, producer surplus or producer gains often declined. For agricultural welfare, 

annualized combined consumer and producer surplus gains were estimated to be $2.6 

trillion in 2010 for the current standard scenario (U.S. EPA, 2014b, Table 6-17). 

b. Key Uncertainties

The WREA discusses multiple areas of uncertainty associated with the crop yield loss 

estimates, including those associated with the model-based adjustment methodology as well as 

those associated with the projection of yield loss using the FASOMGHG model at the estimated 

O3 concentrations (U.S. EPA, 2014b, Table 6-27, section 8.5). Because the W126 estimates 

generated in the air quality analyses are inputs to the vegetation risk analyses for crop yield loss, 

any uncertainties in the air quality analyses are propagated into the those analyses (U.S. EPA, 

2014b, Table 6-27, section 8.5). Therefore, the air quality scenarios in the crop yield analyses 

have the same uncertainties and limitations as in the biomass loss analyses (summarized above), 

including those associated with the model-based adjustment methodology (U.S. EPA, 2014b, 

section 8.5).

4. Visible Foliar Injury

Section IV.C.4.a below provides an overview of the assessment in the WREA of O3-

related visible foliar injury and associated ecosystem services impacts, as well as the key results. 

Section IV.C.4.b summarizes the key uncertainties.

a. Overview and Summary of Key Results
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The WREA presents a number of analyses of O3-related visible foliar injury and 

associated ecosystem services impacts (U.S. EPA, 2014b, Chapter 7). An initial analysis using 

USFS FHM/FIA biosite data included the development of benchmark criteria reflecting different 

prevalences of visible foliar injury at different W126 exposures and soil moisture conditions. 

These criteria were then used in a screening-level characterization of the potential risk of foliar 

injury incidence in 214 national parks and a case study assessment of three national parks, which 

also provides limited characterization of the associated ecosystem services. 

In the biosite data analysis, the WREA used the biomonitoring site data from the USFS 

FHM/FIA Network (USFS, 2011),211 associated soil moisture data during the sample years, and 

national surfaces of ambient air O3 concentrations based on spatial interpolation of monitoring 

data from 2006 to 2010 to calculate the proportion of biosites with any visible foliar injury. The 

proportion of biosites metric is derived by first ordering the data (across biosites and sample 

years) by W126 index value estimated for that biosite and year. Then for each W126 index value, 

the proportion of biosites is calculated with any foliar injury for all observations at or below that 

W126 index value. (U.S. EPA, 2014b, section 7.2). This analysis indicates that the proportion of 

biosites showing the presence of any foliar injury increases rapidly from zero to about 20 percent

at relatively low W126 index values. Specifically: (1) the proportion of biosites exhibiting foliar 

injury rises rapidly with increasing W126 index values below approximately 10 ppm-hrs (W126 

<10.46 ppm-hrs), and (2) there is relatively little change in this proportion with increasing W126 

index values above approximately 10 ppm-hrs (W126 >10.46 ppm-hrs). The data for biosites 

during normal moisture years are very similar to the dataset as a whole, with an overall 

 

211 Data were not available for several western states (Montana, Idaho, Wyoming, Nevada, Utah, 

Colorado, Arizona, New Mexico, Oklahoma, and portions of Texas).
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proportion of close to 18 percent for presence of any foliar injury. Among the biosites with a 

relatively wet season, the proportion of biosites showing injury is much higher and the 

relationship with annual W126 index value is much steeper. Much lower proportions of biosites 

show injury with relatively dry seasons (U.S. EPA, 2014b, section 7.2.3, Figures 7-10), 

consistent with the ISA finding that many studies have shown that dry periods tend to decrease 

the incidence and severity of O3-induced visible foliar injury (U.S. EPA, 2013a, section 9.4.2). 

While these analyses indicate the potential for foliar injury to occur under conditions that meet 

the current standard, the extent of foliar injury that might be expected under such conditions is 

unclear from these analyses.

The national-scale screening-level assessment in 214 parks employed benchmark criteria 

developed from the above analysis.212,213 For example, annual O3 concentrations corresponding 

to a W126 index value of 10.46 ppm-hrs represents the O3 exposure concentration where the 

slope of exposure-response relationship changes for FHM biosites, with the percentage of 

biosites showing injury remaining relatively constant for higher W126 index values. The WREA 

refers to this as the “base scenario” benchmark. The W126 benchmarks across this and the other 

 

212 The parks assessed in the WREA include lands managed by the NPS in the continental U.S., 

which includes National Parks, Monuments, Seashores, Scenic Rivers, Historic Parks, 

Battlefields, Reservations, Recreation Areas, Memorials, Parkways, Military Parks, Preserves, 

and Scenic Trails.

213 This analysis considered the approach in Kohut (2007), which assessed the risk of O3-induced 

visible foliar injury on O3 bioindicators (i.e., O3-sensitive vegetation) in 244 parks managed by 

the NPS. Consistent with advice from CASAC (Frey and Samet, 2012a), however, the WREA 

modified the approach used by Kohut (2007) to apply the W126 metric alone. The WREA 

applied different foliar injury benchmarks in this assessment after further investigation into the 

benchmarks applied in Kohut (2007), which were derived from biomass loss rather than visible 

foliar injury. Kohut cited a threshold of 5.9 ppm-hrs for highly sensitive species from Lefohn 

(1997), which was based on the lowest W126 estimate corresponding to a 10 percent growth loss 

for black cherry. For soil moisture, Kohut (2007) qualitatively assessed whether there appeared 

to be an inverse relationship between soil moisture and high O3 exposure.
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four scenarios range from 3.05 ppm-hrs (foliar injury observed at five percent of biosites, normal 

moisture) up to 24.61 ppm-hrs (foliar injury observed at 10 percent of biosites, dry). For the 

scenario of 10 percent biosites with injury, W126 index values were approximately 4, 6, and 25 

ppm-hrs for wet, normal and dry years, respectively. The national-scale screening-level 

assessment applied these benchmarks to 42 parks with O3 monitors and a total of 214 parks with 

O3 exposure estimated from the interpolated national O3 surfaces for individual years from 2006 

to 2010 (U.S. EPA, 2014b, Appendix 7A and section 7.3).

 Based on NPS lists, 95 percent of the 214 parks in this screening-level assessment 

contain at least one vegetation species sensitive to O3-induced foliar injury (U.S. NPS, 

2003, 2006). 

 In the current standard scenario, none of the 214 parks had O3 concentrations estimated to 

exceed the annual benchmark of a W126 index value above 10.46 ppm-hrs (U.S. EPA, 

2014b, section 7.3.3.3).

The case study analyses focused on Great Smoky Mountains National Park (GRSM), 

Rocky Mountain National Park (ROMO), and Sequoia and Kings Canyon National Parks 

(SEKI). Information on visitation patterns, recreational activities and visitor expenditures was 

considered. For example, visitor spending in 2011 exceeded $800 million, $170 million and $97 

million dollars in GRSM, ROMO and SEKI, respectively. In each park, the percent cover of 

species sensitive to foliar injury was estimated and the overlap between recreation areas within 

the park and elevated W126 concentrations was described. (U.S. EPA, 2014b, section 7.4).

 In the current standard scenario, the three-year average W126 index values were at or 

below 7 ppm-hrs in all areas of two of the three parks (GRSM and SEKI). Three-year 

average W126 index values were below 7 ppm-hrs in a little more than half of the area of 
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the third park (ROMO) and between 7 and 11 ppm-hrs in the remainder of the park (U.S. 

EPA, 2014b, section 7.4).

 For the 15, 11 and 7 ppm-hrs scenarios, all areas of the three specific national parks 

evaluated (GRSM, SEKI, and ROMO) had three-year average W126 index values at or 

below 7 ppm-hrs, well below the 10.46 ppm-hrs benchmark. However, the extent of 

foliar injury that might be expected under these scenarios is unclear from these analyses.

Although not discussed in detail here, the WREA also describes qualitative assessments 

for some of the ecosystem services most likely to be affected by O3-induced foliar injury such as 

cultural services, including aesthetic value and outdoor recreation. Aesthetic value and outdoor 

recreation depend on the perceived scenic beauty of the environment. Many outdoor recreation 

activities directly depend on the scenic value of the area, in particular scenic viewing, wildlifewatching, hiking, and camping. These activities and services are of significant importance to 

public welfare as they are enjoyed by millions of Americans every year and generate millions of 

dollars in economic value (U.S. EPA, 2014b, Chapters 5 and 7). Although data are not available 

to explicitly quantify O3 effects on ecosystem services, the WREA includes several qualitative 

analyses.

b. Key Uncertainties

Uncertainties associated with these analyses are discussed in the WREA, sections 7.5 and 

8.5.3, and in WREA Table 7-24, and also summarized in the PA (e.g., U.S. EPA, 2014c, section 

6.3). As discussed in the WREA (section 8.5.3), evaluating soil moisture is more subjective than 

evaluating O3 exposure because of its high spatial and temporal variability within the O3 season, 

and there is considerable subjectivity in the categorization of relative drought. The WREA 

generally concludes that the spatial and temporal resolution for the soil moisture data is likely to 
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underestimate the potential for foliar injury to occur in some areas. In addition, there is lack of a 

clear threshold for drought below which visible foliar injury would not occur. In general, low 

soil moisture reduces the potential for foliar injury, but injury could still occur, and the degree of 

drought necessary to reduce potential injury is not clear. Studies in the ISA provide additional 

information regarding the role of soil moisture in influencing visible foliar injury response, (U.S. 

EPA, 2013a, section 9.4.2). These studies confirm that adequate soil moisture creates an 

environment conducive to greater visible foliar injury in the presence of O3 than drier conditions. 

As stated in the ISA, “[a] major modifying factor for O3-induced visible foliar injury is the 

amount of soil moisture available to a plant during the year that the visible foliar injury is being 

assessed ... because lack of soil moisture generally decreases stomatal conductance of plants 

and, therefore, limits the amount of O3 entering the leaf that can cause injury” (U.S. EPA, 2013a, 

p. 9-39). As a result, “many studies have shown that dry periods in local areas tend to decrease 

the incidence and severity of O3-induced visible foliar injury; therefore, the incidence of visible 

foliar injury is not always higher in years and areas with higher O3, especially with co-occurring 

drought (Smith, 2012; Smith et al., 2003)” (U.S. EPA, 2013a, p. 9-39). This “...partial 

‘protection’ against the effects of O3 afforded by drought has been observed in field experiments 

(Low et al., 2006) and modeled in computer simulations (Broadmeadow and Jackson, 2000)” 

(U.S. EPA, 2013a, p. 9-87). In considering the extent of any protective role of drought 

conditions, however, the ISA also notes that other studies have shown that “drought may 

exacerbate the effects of O3 on plants (Pollastrini et al., 2010; Grulke et al., 2003)” and that 

“[t]here is also some evidence that O3 can predispose plants to drought stress (Maier-Maercker, 

1998)”. Accordingly, the ISA concludes that “the nature of the response is largely species-
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specific and will depend to some extent upon the sequence in which the stressors occur” (U.S. 

EPA, 2013a, p. 9-87).

Due to the absence of biosite injury data in the Southwest region and limited biosite data 

in the West and West North Central regions, the W126 benchmarks for foliar injury that the 

WREA developed and applied in the national park screening assessment may not be applicable 

to these regions. The WREA applied the benchmarks from the national-scale analysis to a 

screening-level assessment of 214 national parks and case studies of three national parks. 

Therefore, uncertainties in the foliar injury benchmarks are propagated into these analyses. 

Other uncertainties associated with these analyses include uncertainty associated with our 

understanding of the number and sensitivity of O3 sensitive species, uncertainties associated with 

spatial assignment of foliar injury biosite data to 12 km x 12 km grid cells, and uncertainties 

associated with O3 exposure data of vegetation and recreational areas within parks (U.S. EPA, 

2014b, Table 7-22).

There are also important uncertainties in the estimated O3 concentrations for the different 

air quality scenarios evaluated (U.S. EPA, 2014b, section 8.5), as discussed earlier in this 

section. These uncertainties only apply to the national park case studies because these are the 

only foliar injury analyses that rely on the air quality scenarios, but any uncertainties in the air 

quality analyses are propagated into those analyses. The WREA identifies additional 

uncertainties that are associated with the national park case studies. Specifically, there is 

uncertainty inherent in survey estimates of participation rates, visitor spending/economic 

impacts, and willingness-to-pay. These surveys potentially double-count impacts based on the 

allocation of expenditures across activities but also potentially exclude other activities with 

economic value. In general, the national level surveys apply standard approaches, which 
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minimize potential bias. Other sources of uncertainty are associated with the mapping, including 

park boundaries, vegetation species cover, and park amenities, such as scenic overlooks and 

trails. In general, the WREA concludes that there is high confidence in the park mapping (U.S. 

EPA, 2014b, Table 7-24).

D. Conclusions on Adequacy of the Current Secondary Standard

The initial issue to be addressed in the current review of the secondary O3 standard is 

whether, in view of the currently available scientific evidence, exposure and risk information and 

air quality analyses, discussed in the PA, the existing standard should be revised. In drawing 

conclusions on adequacy of the current O3 secondary standard, the Administrator has taken into 

account both evidence-based and quantitative exposure- and risk-based considerations, and 

advice from CASAC. Evidence-based considerations draw upon the EPA’s assessment and 

integrated synthesis of the scientific evidence from experimental and field studies evaluating 

welfare effects related to O3 exposure, with a focus on policy-relevant considerations, as 

discussed in the PA. Air quality analyses inform these considerations with regard to cumulative, 

seasonal exposures occurring in areas of the U.S. that meet the current standard. Exposure- and 

risk-based considerations draw upon EPA assessments of risk of key welfare effects, including 

O3 effects on forest growth, productivity, carbon storage, crop yield and visible foliar injury, 

expected to occur in model-based scenarios for the current standard, with appropriate 

consideration of associated uncertainties.

The following sections describe consideration of the evidence and the exposure/risk 

information in the PA and advice received from CASAC, as well as the comments received from 

various parties, and the Administrator’s proposed conclusions regarding the adequacy of the 

current secondary standard.
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1. Evidence- and Exposure/Risk-Based Considerations in the Policy Assessment

Staff assessments in the PA focus on the policy-relevant aspects of the assessment and 

integrative synthesis of the currently available welfare effects evidence in the ISA, analyses of 

air quality relationships with exposure metrics of interest, the exposure and risk assessments in 

the WREA, comments and advice of CASAC and public comment on drafts of the PA, ISA and 

WREA. The PA describes evidence- and exposure/risk-based considerations and presents staff 

conclusions for the Administrator to consider in reaching her proposed decision on the current 

standard. The focus of the initial PA conclusions is consideration of the question: Does the 

currently available scientific evidence and exposure/risk information, as reflected in the ISA and 

WREA, support or call into question the adequacy and/or appropriateness of the protection 

afforded by the current secondary O3 standard?

The PA’s general approach to informing judgments by the Administrator recognizes that 

the available welfare effects evidence demonstrates a range of O3 sensitivity across studied plant 

species and documents an array of O3-induced effects that extend from lower to higher levels of 

biological organization. These effects range from those affecting cell processes and individual 

plant leaves to effects on the physiology of whole plants, as well as the range from species 

effects and effects on plant communities to effects on related ecosystem processes and services. 

Given this evidence, the PA notes that it is not possible to generalize across all studied species 

regarding which cumulative exposures are of greatest concern, as this can vary by situation due 

to differences in exposed species sensitivity, the importance of the observed or predicted O3-

induced effect, the role that the species plays in the ecosystem, the intended use of the affected 

species and its associated ecosystem and services, the presence of other co-occurring 

predisposing or mitigating factors, and associated uncertainties and limitations. Therefore, in 
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developing conclusions in the PA, staff takes note of the complexity of judgments to be made by 

the Administrator regarding the adversity of known and anticipated effects to the public welfare 

and are mindful that the Administrator’s ultimate judgments on the secondary standard will most 

appropriately reflect an interpretation of the available scientific evidence and exposure/risk 

information that neither overstates nor understates the strengths and limitations of that evidence 

and information (U.S. EPA, 2014c, section 5.7). 

In considering the estimates of exposures and risks for air quality scenarios assessed in 

the WREA, the PA: 1) evaluates the weight of the scientific evidence concerning vegetation 

effects associated with those O3 exposures; 2) considers the importance, from a public welfare 

perspective, of the O3-induced effects on sensitive vegetation and associated ecosystem services 

that are known or anticipated to occur as a result of exposures in the assessed air quality 

scenarios; and, 3) recognizes that predictions of effects associated with any given O3 exposure 

may be mitigated or exacerbated by actual conditions in the environment (i.e., co-occurring 

modifying environmental and genetic factors). When considering WREA analyses that involve 

discrete exposure levels or varying levels of severity of effects, the PA’s approach recognizes 

that the available welfare effects evidence demonstrates a wide range in O3 sensitivities across 

studied plant species. The PA additionally considers the uncertainties associated with this 

information.

As an initial matter, the PA recognizes that the CAA does not require that a secondary 

standard be protective of all effects associated with a pollutant in the ambient air, but rather those 

considered adverse to the public welfare (as described in section IV.B.2 above). In considering 

the extent to which it may be appropriate to consider particular welfare effects adverse, the PA 

applies a paradigm used in past reviews. As discussed in section IV.B.2 above, this paradigm 
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recognizes that the significance to the public welfare of O3-induced effects on sensitive 

vegetation growing within the U.S. can vary depending on the nature of the effect, the intended 

use of the sensitive plants or ecosystems, and the types of environments in which the sensitive 

vegetation and ecosystems are located. Accordingly, any given O3-related effect on vegetation 

and ecosystems (e.g., biomass loss, crop yield loss, visible foliar injury) may be judged to have a 

different degree of impact on the public welfare depending, for example, on whether that effect 

occurs in a Class I area, a city park, or commercial cropland. In the last review, the 

Administrator took note of actions taken by Congress to establish public lands that are set aside 

for specific uses that are intended to provide benefits to the public welfare, including lands that 

are to be protected so as to conserve the scenic value and the natural vegetation and wildlife 

within such areas for the enjoyment of future generations (73 FR 16497, March 27, 2008). Such 

public lands that are protected areas of national interest include national parks and forests, 

wildlife refuges, and wilderness areas (73 FR 16497, March 27, 2008). The PA notes that effects 

occurring in such areas would likely have the highest potential for being classified as adverse to 

the public welfare, given the expectation of preserving these areas to ensure their intended use is 

met (U.S. EPA, 2014c, section 5.1). In considering uses of vegetation and forested lands, the 

paradigm also includes consideration of impacts to ecosystem goods and services. In summary, 

the paradigm considered in the PA, consistent with the discussion in section IV.B.2 above, 

integrates the concepts of: 1) variability in public welfare significance given intended use and 

value of the affected entity, such as individual species; 2) relevance of associated ecosystem 

services to public welfare; and 3) variability in spatial, temporal, and social distribution of 

ecosystem services associated with known and anticipated welfare effects. Further, the PA 
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recognizes that there is no bright-line rule delineating the set of conditions or scales at which 

known or anticipated effects become adverse to public welfare. 

With respect to the scientific evidence, the PA takes note of the longstanding evidence 

base that demonstrates O3-induced effects that occur across a range of biological and ecological 

scales of organization, as described in the ISA and summarized in section IV.B.1 above (U.S. 

EPA, 2013a, p. 1-8). Many of the recent studies evaluated in this review have focused on and 

further increased our understanding of the molecular, biochemical and physiological mechanisms 

that explain how plants are affected by O3 in the absence of other stressors (U.S. EPA, 2013a, 

section 9.3). These recent studies, in combination with the extensive and long-standing evidence, 

have further strengthened the coherence and consistency of the entire body of research since the 

last review. Consistent with conclusions in the 2006 AQCD, the ISA determined that a causal 

relationship exists between O3 exposure and visible foliar injury on sensitive vegetation, reduced 

plant growth, reduced productivity in terrestrial ecosystems, reduced yield and quality of 

agricultural crops and alteration of below-ground biogeochemical cycles (U.S. EPA, 2013a, 

Table 1-2 and section 2.6). The relationship between O3 exposures and reduced carbon 

sequestration in terrestrial ecosystems, alteration of terrestrial ecosystem water cycling and 

alteration of terrestrial community composition was concluded to be likely causal (U.S. EPA, 

2013a, Table 1-2). 

The PA recognizes that consistent with conclusions drawn in the last review, the 

currently available evidence base also strongly supports that effects on vegetation are attributable 

to cumulative seasonal O3 exposures. Moreover, on the basis of the entire body of evidence in 

this regard, the ISA concludes that “quantifying exposure with indices that cumulate hourly O3

concentrations and preferentially weight the higher concentrations improves the explanatory 
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power of exposure/response models for growth and yield, over using indices based on mean and 

peak exposure values” (U.S. EPA, 2013a, p. 2-44). Accordingly, as in other recent reviews, the 

evidence continues to provide a strong basis for concluding that it is appropriate to judge impacts 

of O3 on vegetation, related effects and services, and the level of public welfare protection 

achieved, using a cumulative, seasonal exposure metric, such as the W126-based metric. In this 

review, as in the last review, the CASAC concurs with this conclusion (Frey, 2014c, p. iii). Thus, 

based on the consistent and well-established evidence described above, the PA concludes that the 

most appropriate and biologically relevant way to relate O3 exposure to plant growth, and to 

determine what would be adequate protection for public welfare effects attributable to the 

presence of O3 in the ambient air is to characterize exposures in terms of a cumulative seasonal 

form, and in particular the W126 metric. 

In considering the current standard with regard to protection from the array of O3-related 

effects recognized in this review, the PA first considers effects related to forest tree growth, 

productivity and carbon storage, effects for which the ISA concludes the evidence supports a 

causal or likely causal relationship with exposures to O3 in ambient air (U.S. EPA, 2014c, 

sections 5.2 and 5.7). In so doing, the PA notes that while changes in biomass affect individual 

tree species, the overall effect on forest ecosystem productivity depends on the composition of 

forest stands and the relative sensitivity of trees within those stands. In considering the evidence 

for these effects and the extent to which they might be expected to occur under conditions that 

meet the current secondary standard, the PA focused particularly on RBL estimates for the 11 

species for which robust E-R functions have been developed. The PA recognized that recent 

studies, such as multiple-year exposures of aspen and birch, have provided additional evidence 

on tree biomass or growth effects associated with multiple year exposures in the field, including 
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the potential for cumulative and carry-over effects. For example, findings from these studies 

indicate that effects of O3 on birch seeds (reduced weight, germination, and starch levels) could 

lead to a negative impact on species regeneration in subsequent years and may have the potential 

to alter carbon metabolism of overwintering buds, potentially affecting growth in the following 

year. Other studies have reported that multiple-year exposures reduced tree size parameters in an 

aspen community, and increased the rate of conversion from a mixed aspen-birch community to 

a community dominated by the more tolerant birch, such that elevated O3 may alter intra- and 

inter-species competition within a forest stand (U.S. EPA, 2013a, section 9.4.3; U.S. EPA, 

2014c, section 5.2). In giving particular attention to tree seedling biomass loss estimates, the PA 

notes that 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” (Frey, 2014c, p. 10). 

In evaluating the current evidence and exposure/risk information associated with tree 

growth, productivity and carbon storage, with regard to the adequacy of public welfare 

protection afforded by the current standard, the PA considers the evidence of vegetation and 

welfare impacts in areas of the U.S. likely to have met the current standard. With regard to O3

effects on tree growth, productivity and carbon storage and associated ecosystems and services, 

the PA focuses on relative biomass loss estimates based on the OTC-based E-R functions, noting 

that analyses newly performed in this review have reduced the uncertainty associated with using 

OTC E-R functions to predict tree growth effects in the field (U.S. EPA, 2014c, section 5.2.1; 

U.S. EPA, 2013a, section 9.6.3.2). 

In focusing on RBL estimates, the PA recognized that comparison to an array of 

benchmarks would be informative to considerations of significance to public welfare. Included in 
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this array were RBL values of 2% and 6% given emphasis by CASAC (Frey, 2014c). In 

considering the RBL estimates for different O3 conditions associated with the current standard, 

the PA focused first on the median of the species-specific (composite) E-R functions. In so 

doing, the PA takes note of CASAC’s comments that a 6% median RBL is “unacceptably high”, 

and that the 2% median RBL is an important benchmark to consider (Frey, 2014c).214 Based on 

the summary of RBL estimates in the PA, the PA notes that the median species RBL estimate is 

at or below 2% for W126 exposure index values less than or equal to 7 ppm-hrs (U.S. EPA, 

2014c, Table 6-1 and Appendix 5C). The median species RBL is at or above 6% for W126 index 

values of 19 ppm-hrs and higher. 

In recognition of the significance of welfare effects in Class I areas, the PA gives 

appreciable weight to consideration of the occurrence of O3 concentrations associated with the 

potential for RBL estimates above benchmarks of interest in Class I areas that meet the current 

standard. Based on air quality data for the period from 1998 to 2012, the PA focused 

consideration on 22 Class I areas, in which during one or more three-year periods the air quality 

met the current standard and the three-year average W126 index value was at or above 15 ppmhrs (see Table 7 below, drawn from U.S. EPA, 2014c, Table 5-2). Across these 22 Class I areas,

the highest single-year W126 index values for these three-year periods ranged from 17.4 to 29.0 

ppm-hrs. In 20 of the areas, distributed across eight states (AZ, CA, CO, KY, NM, SD, UT, WY) 

 

214 The CASAC provided several comments related to 2% RBL for tree seedlings both with 

regard to its use in summarizing WREA results and with regard to consideration of the potential 

significance of vegetation effects, as summarized in sections IV.D.2 and IV.E.3. In identifying 

2% as an important benchmark, CASAC referenced the 1996 workshop sponsored by the 

Southern Oxidants Study group at which, as noted in section IV.B.2 above, participants 

identified annual percentages of tree seedling growth reduction and crop yield loss they 

considered important to their judgments on a secondary standard. The workshop report provides 

no explicit rationale for the percentages identified or specification with regard to number or 

proportion of species for which such percentages should be met (Heck and Cowling, 1997). 
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and four regions (West, Southwest, West North Central and Central), this range was 19.1 to 29.0 

ppm-hrs, exposure values for which the corresponding median species RBL estimates equal or 

exceed 6%, which CASAC has termed “unacceptably high”. Recognizing that in any given year, 

other environmental factors can influence the extent to which O3 may have the impact predicted 

by the E-R functions, the PA looked beyond single year occurrences of such magnitudes of 

W126 index values. For example, focusing on the highest three-year periods that include these 

highest annual values for 21 areas, the PA notes that in 10 areas (across five states in the West 

and Southwest regions), the three-year average W126 values (for the highest three-year period 

that includes these annual values) are at or above 19 ppm-hrs, ranging up to 22.5 ppm-hrs (for 

which the median species RBL estimate is above 7%). This indicates that the W126 value above 

19 ppm-hrs is not simply a single year in a period of lower years, but that in these cases there 

were sustained higher values that contributed to a three-year W126 also above 19 ppm-hrs. In 

terms of the highest three-year values observed (regardless of single-year values), the PA 

additionally notes that the highest three-year average W126 index value (during periods meeting 

the current standard) was at or above 19 (ranging up to 22.5 ppm-hrs) in 11 areas, distributed 

among five states in the West and Southwest regions (U.S. EPA, 2014c, Table 5-2, Appendix 

5B). 
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Table 7. O3 Concentrations in Class I Areas During Period From 1998 to 2012 that Met the Current Standard and Where 

Three-year Average W126 Index Value was at or Above 15 ppm-hrs 

Class I Area State / County Design Value

(ppb)*

3-year Average W126

(ppm-hrs)*

(# ≥ 19 ppm-hrs, range)

Annual W126

(ppm-hrs)*

(# ≥ 19 ppm-hrs, range)

Number of 3-year 

Periods

Bandelier Wilderness Area QA, DF, PP NM / Sandoval 70-74 15.8-20.8 (2, 20.0-20.8) 12.1-25.3 (4, 19.2-25.3) 8

Bridger Wilderness Area QA, DF WY / Sublette 69-72 15.1-17.4 9.9-19.2 (1, 19.2) 5

Canyonlands National Park QA, DF, PP, UT / San Juan 69-73 15.0-20.5 (2, 19.8-20.5) 9.9-24.8 (5, 19.3-24.8) 9

Carlsbad Caverns National Park PP NM / Eddy 69 15.0-15.3 8.6-26.7 (1, 26.7) 3

Chiricahua National Monument DF, PP AZ / Cochise 69-73 15.7-18.0 13.2-21.6 (2, 19.3-21.6) 7

Grand Canyon National Park QA, DF, PP AZ / Coconino 68-74 15.3-22.2 (7, 19.2-22.2) 11.3-26.7 (7, 19.8-26.7) 12

John Muir Wilderness Area QA, DF, PP CA / Inyo 71-72 16.5-18.6 10.1-25.8 (2, 23.9-25.8) 3

Lassen Volcanic National Park DF, PP CA / Shasta 75 15.3 13.6-18.7 1

Mammoth Cave National Park BC, C, LP, 

RM, SM, VP, YP KY / Edmonson 74 15.9 12.5-22.5 (1, 22.5) 1

Mesa Verde National Park DF CO / Montezuma 67-73 15.5-21.0 (2, 19.0-21.0) 10.7-23.6 (4, 19.7-23.6) 10

Mokelumne Wilderness Area DF, PP CA / Amador 74 17.6 14.8-22.6 (1, 22.6) 1

Petrified Forest National Park AZ / Navajo 70 15.7 12.9-19.2 (1, 19.2) 1

Pinnacles National Monument CA / San Benito 74 15.1 13.1-17.4 1

Rocky Mountain National Park QA, DF, 

PP

CO / Boulder 73-75 15.1-19.3 (1, 19.3) 9.5-25.1 (5, 20.7-25.1) 6

 CO / Larimer 74 15.0-18.3 8.1-25.8 (3, 19.1-25.8) 3

Saguaro National Park DF, PP AZ / Pima 69-74 15.4-18.9 11.0-23.1 (3, 20.0-23.1) 6

Sierra Ancha Wilderness Area DF, PP AZ / Gila 72-75 17.9-22.4 (3, 20.2-22.4) 14.8-27.5 (4, 20.3-27.5) 4

Superstition Wilderness Area PP AZ / Maricopa 75 22.4 (1, 22.4) 14.5-28.6 (2, 27.4-28.6) 1

AZ / Pinal 73-75 18.7-22.5 (2, 20.8-22.5) 14.8-29.0 (3, 22.6-29.0) 3

Weminuche Wilderness Area QA, DF, PP CO / La Plata 70-74 15.0-19.1 (1, 19.1) 10.9-21.0 (2, 20.8-21.0) 5

West Elk Wilderness Area QA, DF CO / Gunnison 68-73 15.6-20.1 (1, 20.1) 12.9-23.9 (3, 21.1-23.9) 8

Wind Cave National Park QA, PP SD / Custer 70 15.4 12.2-20.6 (1, 20.6) 1

Yosemite National Park QA, DF, PP CA / Tuolumne 73-74 20.7-20.8 (2, 20.7-20.8) 19.7-22.1 (4, 19.7-22.1) 2

Zion National Park QA, DF, PP UT / Washington 70-73 17.8-21.1 (2, 20.3-21.1) 14.9-24.2 (5, 19.3-24.2) 4
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*Based on data from http://www.epa.gov/ttn/airs/airsaqs/detaildata/downloadaqsdata.htm. W126 values are truncated after first decimal place.

Superscript letters refer to species present for which E-R functions have been developed. QA=Quaking Aspen, BC=Black Cherry, C=Cottonwood, DF=Douglas 

Fir, LP=Loblolly Pine, PP=Ponderosa Pine, RM=Red Maple, SM=Sugar Maple, VP=Virginia Pine, YP=Yellow (Tulip) Poplar. Sources include USDA-NRCS 

(2014,http://plants.usda.gov), USDA-FS (2014, http://www.fs.fed.us/foresthealth/technology/nidrm2012.shtml) UM-CFCWI (2014, 

http://www.wilderness.net/printFactSheet.cfm?WID=583) and Phillips and Comus (2000).
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In considering the data analysis for 22 Class I areas described above, the PA additionally 

considers the species-specific RBL estimates for quaking aspen and ponderosa pine, two tree 

species that are found in many of these 22 areas and have a sensitivity to O3 exposure that places 

them near the middle of the group for which E-R functions have been established (U.S. EPA, 

2014c, sections 5.2 and 5.7). In the Class I areas where ponderosa pine is present, the highest 

single year W126 index values ranged from 18.7 to 29.0 ppm-hrs and the highest three-year

average W126 values in which these single year values were represented ranged from 15 to 22.5 

ppm-hrs, with these three-year values above 19 ppm-hrs in eight areas across five states. The 

ponderosa pine RBL estimates for 29 and 22.5 ppm-hrs are approximately 12% and 9%, 

respectively (U.S. EPA, 2014c, Appendix 5C). In Class I areas where quaking aspen is present, 

the highest single year W126 index values ranged from 19.2 to 26.7 ppm-hrs and the highest 

three-year average W126 values in which these single year values were represented ranged from 

15.0 to 22.2 ppm-hrs, with these three-year values above 19 ppm-hrs in eight areas across five 

states. The quaking aspen RBL estimates for 26.7 and 22.2 ppm-hrs are approximately 16% and 

13%, respectively (U.S. EPA, 2014c, Appendix 5C).

The PA describes the above observations, particularly in light of advice from CASAC, 

summarized in section IV.D.2 below, as evidence of the occurrence in Class I areas during 

periods where the current standard is met of cumulative seasonal O3 exposures of a magnitude 

for which the tree growth impacts indicated by the estimated median species RBL might 

reasonably be concluded to be important to public welfare (U.S. EPA, 2014c, sections 5.2.1 and 

5.7). 

In considering the WREA analyses of effects on tree growth and associated ecosystem 

services in the air quality scenario for the current standard, the PA first takes note of the potential 
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for the interpolation method used in creating the national surface of O3 concentrations for the air 

quality scenarios to underestimate the higher W126 values such that W126-based exposures 

would be expected to be somewhat higher than those included in each scenario (U.S. EPA, 

2014b, pp. 5-31 to 5-32). While recognizing this, the PA considers results of the WREA analyses 

for the current standard scenario and the 11 species of trees, for which robust E-R functions are 

available. These results indicate that O3 can impact growth of these species across the U.S., as 

well as an array of associated ecosystem services provided by forests, including timber 

production, carbon storage and air pollution removal (U.S. EPA, 2014b, sections 6.2-6.8; U.S. 

EPA, 2014c, section 5.2). 

With regard to WREA analyses of ecosystem services, the PA notes that the nationalscale analysis of O3 impacts on carbon storage indicates appreciably more storage in the air 

quality scenario for the current standard (approximately 11,000 MMtCO2e, over 30 years) 

compared to the scenario for recent, higher O3 conditions (U.S. EPA, 2014b, Appendix 6B, 

Table B-10). The PA additionally considers the WREA estimates of tree growth and ecosystem 

services provided by urban trees over a 25-year period for five urban areas based on case-study 

scale analyses that quantified the effects of biomass loss on carbon storage and pollution removal 

(U.S. EPA, 2014b, sections 6.6.2 and 6.7; U.S. EPA, 2014c, sections 5.2 and 5.7). The urban 

areas included in this analysis represent diverse geography in the Northeast, Southeast, and 

Central regions, although they do not include an urban area in the western U.S. Estimates of the 

effects of O3-related biomass loss on carbon sequestration indicate the potential for an increase 

of somewhat more than a MMtCO2e for the current standard scenario as compared to the recent 

conditions scenario (U.S. EPA, 2014b, section 6.6.2 and Appendix 6D; U.S. EPA, 2014c, 

sections 5.2 and 5.7). The PA also notes the WREA estimates of increased pollution removal in 
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the current standard scenario as compared to the scenario for recent conditions (U.S. EPA, 

2014b, section 6.6.2; U.S. EPA, 2014c, section 5.2.2).

In considering the significance of these WREA analyses of risks for the associated 

ecosystem services for timber production, air pollution removal, and carbon sequestration, the 

PA takes note of the large uncertainties associated with these analyses (see U.S. EPA, 2014b, 

Table 6-27), and the potential for these findings to underestimate the response at the national 

scale. While noting the potential usefulness of considering predicted and anticipated impacts to 

these services in assessing the extent to which the current information supports or calls into 

question the adequacy of the protection afforded by the current standard, the PA also notes that 

staff places limited weight on the absolute magnitude of the risk results for these ecosystem 

service endpoints due to the identification of significant associated uncertainties (U.S. EPA, 

2014c, sections 5.2 and 5.7). 

In reaching conclusions regarding support for the adequacy of the current secondary 

standard provided by the currently available information on O3-induced effects on trees and 

associated services, the PA takes note of: 1) the robust evidence supporting the causal 

relationship between cumulative O3 exposures and effects on tree growth and productivity, and 

information from model simulations supporting the determination of a likely causal relationships 

for carbon storage in terrestrial ecosystems (U.S. EPA, 2013a, sections 2.6.2.1 and 9.4.3); 2) the 

tree seedling E-R functions evidence, which has been strengthened and demonstrates variability 

in sensitivity to O3 across species; 3) estimates of median species RBL at or above 6% associated 

with W126-based exposure levels in several areas when O3 concentrations were at or below the 

current standard; 4) growth effects estimates associated with exposure concentrations in several 

Class I areas based on O3 concentrations from 1998-2012 that were at or below the current 
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standard; 5) evidence that impacts from single year exposures can carry over to the subsequent 

year and/or cumulate over multiple years with repeated annual exposures; 6) evidence from 

recent mechanistic studies and field based studies that support earlier findings from OTC studies; 

and 7) WREA analyses indicating that O3-induced biomass loss can impact ecosystem services 

provided by forests, including timber production, carbon storage, and air pollution removal, even 

when air quality is adjusted to just meet the current standard. Given the above, and noting 

CASAC views (described in section IV.D.2 below), the PA concludes that the current evidence 

and exposure/risk information call into question the adequacy of public welfare protection 

afforded by the current standard from the known and anticipated adverse effects associated with 

O3-induced impacts on tree growth, productivity and carbon storage, including the associated 

ecosystem services assessed in this review. Therefore, the PA concludes that it is appropriate to 

consider revision of the secondary standard to provide increased protection. 

With respect to crops, the PA takes note of the extensive and long-standing evidence on 

the detrimental effect of O3 on crop production, which continues to be confirmed by newly 

available evidence (U.S. EPA, 2013a, section 9.4.4; U.S. EPA, 2014c, sections 5.3 and 5.7). The 

PA additionally notes that recent studies have highlighted the effects of O3 on crop quality, such 

as through decreases in the nutritive quality of grasses, and in the macro- and micro-nutrient 

concentrations in fruits and vegetable crops (U.S. EPA, 2013a, section 9.4.4; U.S. EPA, 2014c, 

section 5.3). Further, the PA notes that there has been little published evidence that crops are 

becoming more tolerant of O3, taking note particularly of the ISA analyses of data from cultivars 

used in NCLAN studies, and yield data for modern cultivars from SoyFACE which confirm that 

the average response of soybean yield to O3 exposure has not changed in current cultivars (U.S. 

EPA, 2006a; U.S. EPA, 2013a, section 9.6.3; U.S. EPA, 2014c, section 5.3). In consideration of 
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the currently available evidence for O3 effects on crops, the PA concludes that the recently 

available evidence, as assessed in the ISA, continues to support the conclusions of the 1996 and 

2006 CDs that ambient O3 concentrations can reduce the yield of major commodity crops in the 

U.S, and that the currently available evidence continues to support the use of the E-R functions 

developed for 10 crops from OTC experiment data. Further, the PA recognizes that important 

uncertainties have been reduced regarding the exposure-response functions for crop yield loss, 

especially for soybean, the second-most planted field crop in the U.S.,215 with the ISA generally 

reporting consistent results across exposure techniques and across crop varieties (U.S. EPA, 

2013a, section 9.6.3.2). 

With regard to consideration of the quantitative impacts of O3 on crop yield, the PA 

considers RYL estimates for O3 conditions associated with the current standard. As in the case of 

the PA considerations of RBL estimates for tree seedlings, the PA recognized CASAC 

comments, which described greater than 5% RYL for the median crop species as “unacceptably 

high” and 5% RYL for the median crop species as adverse, while noting the opportunities to alter 

management of annual crops (Frey, 2014c, pp. iii and 14). The PA notes that staff analyses of 

recent monitoring data (2009-2011) indicate that O3 concentrations in multiple agricultural areas 

in the U.S. that meet the current standard correspond to W126 index levels above 12 ppm-hrs, a 

value for which soybean RYL estimates are greater than 5%. In particular, the PA notes that 

while the design values for two counties in the Midwest met the current standard in 2009-2011, 

 

215 See http://www.ers.usda.gov/topics/crops/soybeans-oil-crops/background.aspx 
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both had a maximum annual W126 of 19 ppm-hrs (in 2011) for which the soybean annual RYL 

estimate, based on the E-R function, is 9%.216

In considering the evidence and exposure/risk-based information for effects on crops, the 

PA notes the CASAC comments regarding the use of crop yields as a surrogate for consideration 

of public welfare impacts, in which it noted that “[c]rops provide food and fiber services to 

humans” and that “[e]valuation of market-based welfare effects of O3 exposure in forestry and 

agricultural sectors is an appropriate approach to take into account damage that is adverse to 

public welfare” (Frey, 2014c, p. 10; U.S. EPA, 2014c, section 5.7). The PA additionally notes, 

however, as recognized in section IV.B.2 above that the determination of the point at which O3-

induced crop yield loss becomes adverse to the public welfare is still unclear, given that crops 

are heavily managed with additional inputs that have their own associated markets and that 

benefits can be unevenly distributed between producers and consumers. The PA further notes 

that to the extent protection is provided by the current standard with regard to impacts on trees, 

protection may also be provided for commodity crops (U.S. EPA, 2014c, sections 5.3 and 5.7). 

In reaching conclusions regarding support provided for the adequacy of the current 

secondary standard by the currently available information on O3-related crop effects, the PA 

notes: (1) the support for a causal relationship between cumulative O3 exposures and effects on 

crop yields and quality (U.S. EPA, 2013a, section 9.4.4); (2) the evidence supporting E-R 

functions for 10 crops, which has been strengthened in this review and which demonstrates 

 

216 The monitoring data reflect observations in locations that meet the current standard. The 

WREA analysis that assessed crop yield loss used a model-developed air quality scenario to 

reflect air quality associated with the current standard (as described in section IV.C.1 above). In 

so doing, adjustments are made to create air quality that meets the standard and when the highest 

monitor in an area is adjusted downward to meet the standard, concentrations at nearby monitors 

that already meet the standard are also reduced.
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variability in sensitivity to O3 across species; (3) evidence from recent mechanistic studies and 

field based studies supporting earlier findings from OTC studies; (4) evidence that crops, and in 

particular soybean, have not become more tolerant of O3 (U.S. EPA, 2013a, section 9.6.3, 

9.4.4.1); and, (5) WREA analysis results indicating that O3-induced crop yield loss can impact 

producer and consumer surpluses and the interaction between agriculture and timber production. 

With regard to visible foliar injury, the PA recognizes the long-standing evidence that has 

established that O3 causes diagnostic visible injury symptoms on studied bioindicator species and 

that soil moisture is a major confounding effect that can decrease the incidence and severity of 

visible foliar injury under dry conditions and vice versa (U.S. EPA, 2014c, sections 5.4 and 5.7). 

As at the time of the last review, the most extensive dataset regarding visible foliar injury 

incidence across the U.S. is that collected by the U.S. Forest Service (USFS) Forest Health 

Monitoring/Forest Inventory and Analysis (FHM/FIA) Program, which has documented 

incidence of visible foliar injury in both the eastern and western U.S. Evidence available in the 

current review includes studies using controlled exposures as well as multi-year field surveys. In 

addition to supporting prior conclusions, the newly available studies also address some 

uncertainties identified in the last review, such as the influence of soil moisture on visible injury 

development (U.S. EPA, 2013a, section 9.4.2). As stated in the ISA, “many studies have shown 

that dry periods in local areas tend to decrease the incidence and severity of O3-induced visible 

foliar injury; therefore, the incidence of visible foliar injury is not always higher in years and 

areas with higher O3, especially with co-occurring drought” (U.S. EPA, 2013a, p. 9-39). The ISA 

additionally concludes, however, that “the nature of the response is largely species-specific and 

will depend to some extent upon the sequence in which the stressors occur” (U.S. EPA, 2013a, p. 

9-87). As recognized in the PA, this area of uncertainty complicates characterization of the 
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potential for visible foliar injury and its severity or extent of occurrence for any given air quality 

conditions and thus complicates identification of air quality conditions that might be expected to 

provide a specific level of protection from this effect (U.S. EPA, 2014c, sections 5.4 and 5.7). 

Information available in this review indicates the occurrence of visible foliar injury in 

some Class I areas during times when O3 concentrations met or would be expected to meet the 

current standard (U.S. EPA, 2014c, sections 5.4.1 and 5.7). In noting this occurrence in Class I 

areas, the PA notes it has particular public welfare significance in light of direction from 

Congress that these areas merit a high level of protection (U.S. EPA, 2014c, sections 5.1, 5.4.1 

and 5.7). The PA also notes that visible foliar injury surveys are used by the federal land 

managers to assess potential O3 impacts in Class I areas (USFS, NPS, FWS, 2010). Given this 

focus on visible foliar injury, the PA concludes that such O3-induced impacts have the potential 

to impact the public welfare in scenic and/or recreational areas on an annual basis. Visible foliar 

injury is associated with important cultural and recreational ecosystem services to the public, 

such as scenic viewing, wildlife-watching, hiking, and camping, that are of significance to the 

public welfare and enjoyed by millions of Americans every year, generating millions of dollars 

in economic value (U.S. EPA, 2014b, section 7.1). In addition, several tribes have indicated that 

many of the O3-sensitive species (including bioindicator species) are culturally significant (U.S. 

EPA, 2014c, Table 5-1). With respect to agricultural species, such visible effects of O3 exposure 

can affect the market value of certain crops and ornamentals for which leaves are the product, 

such as spinach (U.S. EPA, 2006a, p. AX-9 -189). The PA additionally notes CASAC comments 

that “visible foliar injury can impact public welfare by damaging or impairing the intended use 

or service of a resource”, including through “visible damage to ornamental or leafy crops that 

affects their economic value, yield, or usability; visible damage to plants with special cultural 
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significance; and visible damage to species occurring in natural settings valued for scenic beauty 

or recreational appeal” (Frey, 2014c, p. 10).

With regard to the exposure and risk-based information, the PA takes note of the WREA 

analyses of the nationwide dataset (2006-2010) for USFS/FHM biosites, including the 

observation that the proportion of biosites with injury varies with soil moisture conditions and O3

W126 index values (U.S. EPA, 2014b, Chapter 7, Figure 7-10; U.S. EPA, 2014c, section 5.4.2). 

These analyses indicate that the proportion of biosites showing visible foliar injury incidence 

increases steeply with W126 index values up to approximately 10 ppm-hrs, with little difference 

in incidence across higher W126 index levels. The screening-level assessment of national parks 

indicated that risk of visible foliar injury is likely to be lower in most national parks after 

simulating just meeting the current standard, although visible foliar injury would likely continue 

to occur at lower O3 exposures, including some sensitive species growing in National Parks and 

other Class I areas that may provide important cultural ecosystem services to the public. The PA 

also notes the WREA recognition that many of the outdoor recreational activities which directly 

depend on the scenic value of the area are of significant importance to public welfare as they are 

enjoyed by millions of Americans every year and generate millions of dollars in economic value 

(U.S. EPA, 2014b, Chapter 5, Chapter 7). 

In reaching conclusions regarding support for the adequacy of the current secondary 

standard provided by the currently available information on O3-induced visible foliar injury, the 

PA took note of: (1) the evidence for many species of native plants, including trees, that have 

been observed to have visible foliar injury symptoms in both OTC and field settings, some of 

which have also been identified as bioindicators of O3 exposure by the USFS; (2) the finding that 

visible foliar injury incidence can occur at very low cumulative exposures, but due to 
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confounding by soil moisture and other factors, it is difficult to predictively relate a given O3

exposure to plant response; (3) information indicating the occurrence of visible foliar injury in 

some Class I areas under air quality conditions expected to meet the current standard; and, (4) 

WREA analyses, based on USFS biosite data, indicating a relationship of the proportion of 

biosites showing visible foliar injury incidence with W126 index values below approximately 10 

ppm-hrs (U.S. EPA, 2014c, section 5.7). 

The PA additionally recognizes a lack of guidance for federal land managers regarding 

what spatial scale or degree of severity of visible foliar injury is considered sufficient to trigger

protective action for O3 sensitive AQRVs. Further, there does not appear to be any consensus in 

the literature in this regard, and CASAC, while identifying benchmarks to consider for percent 

biomass loss and yield loss for tree seedlings and commodity crops, respectively, did not provide 

a similar recommendation for this endpoint. Likewise, as in previous reviews, the ISA notes the 

difficulty in relating visible foliar injury symptoms to other vegetation effects such as individual 

plant growth, stand growth, or ecosystem characteristics (U.S. EPA, 2013a, section 9.4.2, p. 9-

39) and further notes that the full body of evidence indicates that there is wide variability in this 

endpoint, such that although evidence shows visible foliar injury can occur under very low 

cumulative O3 concentrations, “...the degree and extent of visible foliar injury development 

varies from year to year and site to site..., even among co-members of a population exposed to 

similar O3 levels, due to the influence of co-occurring environmental and genetic factors” (U.S. 

EPA, 2013a, section 9.4.2, p. 9-38). 

Given the above, and taking note of CASAC views, the PA recognizes visible foliar 

injury as an important O3 effect which, depending on severity and spatial extent, may reasonably 

be concluded to be of public welfare significance, especially when occurring in nationally 
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protected areas. While noting the uncertainties associated with describing the potential for visible 

foliar injury and its severity or extent of occurrence for any given air quality conditions, the PA 

notes the occurrence of O3-induced visible foliar injury in areas, including federally protected 

Class I areas that meet the current standard, and suggests it may be appropriate to consider 

revising the standard to achieve greater protection, while recognizing that the degree to which 

O3-induced visible foliar injury would be judged important and potentially adverse to public 

welfare is uncertain (U.S. EPA, 2014c, section 5.7).

With regard to other welfare effects, for which the ISA determined a causal or likely 

causal relationships with O3 in ambient air, such as alteration of ecosystem water cycling and 

changes in climate, the PA concludes there are limitations in the available information which 

affect our ability to consider potential impacts of air quality conditions associated with the 

current standard.

In reaching conclusions on options for the Administrator’s consideration, the PA 

indicates that the final decision to retain or revise the current secondary O3 standard is a public 

welfare policy judgment to be made by the Administrator, based on her judgment as to what 

level of air quality would be requisite (i.e., neither more nor less stringent than necessary) to 

protect the public welfare from any known or anticipated adverse effects. This final decision will 

draw upon the available scientific evidence for O3-attributable welfare effects and on quantitative 

analyses of vegetation and ecosystem exposures and associated risks to vegetation, ecosystems 

and their associated services, and judgments about the appropriate weight to place on the range 

of uncertainties inherent in the evidence and analyses. In making this decision, the Administrator 

will need to weigh the importance of these effects and their associated ecosystem services in the 

overall context of public welfare protection.
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Based on the considerations described in the PA and summarized here, the PA concludes 

that the currently available evidence and exposure/risk information call into question the 

adequacy of the public welfare protection provided by the current standard and provides support 

for considering potential alternative standards to achieve increased public welfare protection, 

especially for sensitive vegetation and ecosystems in federally protected Class I and similarly 

protected areas. In this conclusion, staff gives particular weight to the evidence indicating the 

occurrence in Class I areas that meet the current standard of cumulative seasonal O3 exposures 

associated with estimates of tree growth impacts of a magnitude that may reasonably be 

considered important to public welfare. 

2. CASAC Advice

Beyond the evidence- and exposure/risk-based considerations in the PA discussed above, 

the EPA’s consideration of the adequacy of the current secondary standard also takes into 

account the advice and recommendations of CASAC.

In its advice offered in the current review, based on the updated scientific and technical 

record since the 2008 rulemaking, the CASAC stated that they “support the conclusion in the 

Second Draft PA that the current secondary standard is not adequate to protect against current 

and anticipated welfare effects of ozone on vegetation” (Frey, 2014c, p. iii) and that the PA 

“clearly demonstrates that ozone-induced injury may occur in areas that meet the current 

standard” (Frey, 2014c, p. 12). The Panel further stated “[w]e support EPA’s continued emphasis 

on Class I and other protected areas” (Frey, 2014c, p. 9). Additionally, the CASAC indicated 

support for the concept of ecosystem services “as part of the scope of characterizing damage that 

is adverse to public welfare” and “concurs that trees are important from a public welfare 

perspective because they provide valued services to humans, including aesthetic value, food, 
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fiber, timber, other forest products, habitat, recreational opportunities, climate regulation, erosion 

control, air pollution removal, and hydrologic and fire regime stabilization” (Frey, 2014c, p. 9). 

Similar to comments from CASAC in the last review, including comments on the proposed 

reconsideration, the current CASAC also endorsed the PA discussions and conclusions on 

biologically relevant exposure metrics and the focus on the W126 index accumulated over a 12-

hour period (8am – 8pm) over the three-month summation period of a year resulting in the 

maximum value (Frey, 2014c, p. iii).

In addition, CASAC stated that “relative biomass loss for tree species, crop yield loss, 

and visible foliar injury are appropriate surrogates for a wide range of damage that is adverse to 

public welfare” (Frey, 2014c, p. 10). With respect to relative biomass loss for tree species, 

CASAC states that it is appropriate to “include levels that aim for not greater than 2% RBL for 

the median tree species” and that a median tree species RBL of 6% is “unacceptably high." With 

respect to crop yield loss, CASAC points to a benchmark of 5%, stating that a crop RYL for 

median species over 5% is “unacceptably high” (Frey, 2014c, p. 13).

3. Administrator’s Proposed Conclusions on Adequacy of the Current Standard

In considering the adequacy of the current secondary O3 standard, the Administrator has 

considered the assessment of the current evidence in the ISA, findings of the WREA, including 

associated limitations and uncertainties, considerations and staff conclusions and associated 

rationales presented in the PA, views expressed by CASAC, and public comments. In taking into 

account the information discussed above with regard to the nature of O3-related effects on 

vegetation, the Administrator has taken particular note of: the PA analysis of the magnitude of 

tree seedling growth effects (biomass loss) estimated for different cumulative, seasonal, 

concentration-weighted exposures in terms of the W126 metric; the monitoring analysis in the 
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PA of W126 exposures occurring in locations where the current standard is met, including those 

locations in Class I areas, and associated estimates of tree seedling growth effects; the analyses 

in the WREA illustrating the geographic distribution of tree species for which E-R functions are 

available and relative differences estimated for O3-related growth impacts across areas of the 

U.S. for the air quality scenarios, taking into account the identified potential for the WREA’s 

scenario for the current standard to underestimate the highest W126-based O3 values that would 

be expected to occur. 

As an initial matter, the Administrator recognizes the appropriateness and usefulness of 

the W126 metric, as described in sections IV.B.1 and IV.D.1 above, in evaluating O3 exposures 

of potential concern for vegetation effects. In so doing, the Administrator additionally notes 

support conveyed by CASAC for such a use for this metric.

With regard to considering the adequacy of public welfare protection provided by the 

current secondary standard, the Administrator focuses first on welfare effects related to reduced 

native plant growth and productivity in terrestrial ecosystems, taking note of the ISA conclusion 

of a causal relationship between O3 in the ambient air and these effects. In considering the 

assessment of the information available in this review with regard to O3 effects on vegetation 

growth and productivity, the Administrator takes note of the evidence from OTC studies of the 

effects of O3 exposure on tree seedling growth that support robust E-R functions for 11 tree 

seedling species, and the characterization of growth effects across these species for different 

cumulative seasonal concentration-weighted exposures using the W126 metric. Reductions in 

growth of sensitive species, as recognized in section IV.B above, have the potential to result in 

effects on ecosystem productivity, as well as, on forest and forest community composition. The 

Administrator takes particular note of the evidence, described in section IV.D.1 above, of the 
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occurrence in Class I areas during periods where the current standard is met of cumulative 

seasonal O3 exposures for which median species RBL estimates are of a magnitude that CASAC 

has termed “unacceptably high.” In so doing, the Administrator also takes note of a number of 

actions taken by Congress to establish public lands that are 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 for the enjoyment of 

future generations. Such public lands that are protected areas of national interest include national 

parks and forests, wildlife refuges, and wilderness areas (many of which have been designated 

Class I areas).217 

While recognizing the variability in the various environmental factors that can influence 

the occurrence and severity of the effect of ambient O3 concentrations on vegetation in different 

locations, the Administrator concludes that the information referenced above including the 

currently available, extensive evidence base and also factors affecting the significance of impacts 

to public welfare, as well as WREA estimates regarding the potential for occurrence of impacts 

important to public welfare, provides an appropriate basis to inform a conclusion as to whether 

the current standards provide adequate protection against O3-related vegetation effects on public 

welfare. With regard to the results of the monitoring analysis, the Administrator takes note of the 

PA conclusions that the impacts on tree growth (and the potential for associated ecosystem 

effects) estimated for W126 values found to occur in Class I areas when meeting the current 

standard are reasonably concluded to be important from a public welfare standpoint in terms of 

 

217 As noted in section IV.A above, Congress has established areas such as national parks and 

wilderness areas with specific purposes including the preservation of the areas for future 

generations, and has identified many of those areas as Class I areas.
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both the magnitude of the vegetation effects and the significance to public welfare of such effects 

in such areas, calling into question the adequacy of the current secondary standard. 

The Administrator also recognizes the causal relationships between O3 in the ambient air 

and visible foliar injury, reduced yield and quality of agricultural crops and alteration of belowground biogeochemical cycles associated with effects on growth and productivity. As to visible 

foliar injury, the Administrator takes note of the complexities and limitations in the evidence 

base regarding characterizing air quality conditions with respect to the magnitude and extent of 

risk for visible foliar injury. She additionally recognizes the challenges of associated judgments 

with regard to adversity of such effects to public welfare. In taking note of the conclusions with 

regard to crops, she recognizes the complexity of considering adverse O3 impacts to public 

welfare due to the heavy management common for achieving optimum yields and market factors 

that influence associated services and additionally takes note of the PA conclusions that placing 

emphasis on the protection afforded to trees inherently also recognizes a level of protection 

afforded for crops.

Based on her consideration of the conclusions in the PA, and with particular weight given 

to PA findings pertaining to tree growth-related effects, as well as with consideration of 

CASAC’s conclusion that the current standard is not adequate, the Administrator proposes to 

conclude that the current standard is not requisite to protect public welfare from known or 

anticipated effects and that revision is needed to provide increased public welfare protection, 

especially for sensitive vegetation and ecosystems in federally protected Class I areas and in 

other areas providing similar public welfare benefits. The Administrator further concludes that 

the scientific evidence and quantitative analyses on tree growth-related effects provide strong 

support for consideration of alternative standards that would provide increased public welfare 
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protection beyond that afforded by the current O3 secondary standard. She further notes that a 

revised standard would provide increased protection for other growth-related effects, including 

for carbon storage and for areas for which it is more difficult to determine public welfare 

significance, as recognized in section IV.B.2 above, as well other welfare effects of O3, 

including visible foliar injury and crop yield loss.

In giving particular focus to tree growth-related effects of O3 on public welfare, the 

Administrator additionally recognizes that there are alternative approaches to viewing the 

evidence and information, including alternative approaches to viewing, evaluating, and weighing 

important uncertainties. In some cases, these alternative approaches have been expressed by 

public commenters, leading some public commenters to recommend retaining the current 

standard. Given these alternative views, in addition to proposing to revise the current secondary 

standard, the Administrator also solicits comment on the option of retaining the standard without 

revision.

E. Consideration of Alternative Secondary Standards

Given her proposed conclusion that the current secondary standard is inadequate, the 

Administrator has then considered what revisions to the standard may be appropriate, focusing 

on revisions to the key standard elements of indicator, form, averaging time, and level. On the 

basis of the strength and coherence of the vegetation effects evidence indicating a cumulative, 

seasonal, concentration-weighted metric as the most appropriate approach for judging potential 

impacts of and protection from O3 in ambient air, the Administrator judges that it is appropriate 

to consider revisions to the secondary standard that reflect this understanding and to use such a 

metric in identifying an appropriate level of protection and considering the protection afforded 

by potential alternative standards. The Administrator also judges that the current averaging time 
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and form may also provide protection to vegetation when set at an appropriate level. Therefore, 

the Administrator considered whether revision to the level of the current secondary standard 

might provide sufficient protection to also achieve the level of air quality that is determined 

requisite to protect the public welfare. 

The sections below address the indicator for the secondary standard (section IV.E.1), 

consideration of a cumulative, seasonal exposure-based standard in the PA (section IV.E.2), 

CASAC advice and public input (section IV.E.3), analyses of air quality in the PA and 

subsequent to the PA (section IV.E.4) and the Administrator’s proposed conclusions regarding 

an alternative secondary standard (section IV.E.5) 

1. Indicator

In the last review of the air quality for O3 and other photochemical oxidants and of the O3

standard, as in other prior reviews, the EPA focused on a standard for O3 as the most appropriate 

surrogate for ambient photochemical oxidants. Ozone is a long-established surrogate for ambient 

photochemical oxidants, among which it is by far the most widely studied with regard to effects 

on welfare and specifically on vegetation. The information available in this review adds to the

understanding of the atmospheric chemistry for photochemical oxidants and O3 in particular (as 

described in the ISA, sections 3.2 and 3.6, and summarized in section 2.2 in the PA). The 1996 

Staff Paper noted that the database on vegetation effects is generally considered to raise concern 

at levels found in the ambient air for O3 and, therefore, control of ambient O3 levels has 

previously been concluded to provide the best means of controlling other photochemical oxidants 

of potential welfare concern (U.S. EPA, 1996b, p. 277). In the current review, while the complex 

atmospheric chemistry in which O3 plays a key role has been highlighted, no alternatives to O3

have been advanced as being a more appropriate surrogate for ambient photochemical oxidants.
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Ozone continues to be the only photochemical oxidant (other than nitrogen dioxide) that is 

routinely monitored and for which a comprehensive database exists (U.S. EPA, 2013a, section 

3.6).

Thus, the Administrator concludes that ozone is the appropriate indicator and proposes to 

continue to use O3 as indicator for a secondary standard that is intended to address effects 

associated with exposure to O3, alone and in combination with related photochemical oxidants. 

In so doing, the Administrator recognizes that measures leading to reductions in ecosystem 

exposures to O3 will also reduce exposures to other photochemical oxidants.

2. Consideration of a Cumulative, Seasonal Exposure-based Standard in the Policy Assessment

In recognition of the extensive evidence supporting a cumulative, seasonal exposure 

index as a biologically relevant metric for assessing potential for O3 effects on vegetation, 

discussed in sections IV.B.1 above, as well as advice from CASAC in the current and last O3

NAAQS reviews, summarized in sections IV.D.3 above and IV.E.3 below, the PA focused its 

consideration of alternative standards on a revised secondary standard based on a cumulative, 

seasonal, concentration-weighted form. The PA considered the currently available information 

that has been critically analyzed and characterized in the ISA, the risk and exposure information 

presented in the WREA, and CASAC advice and public comment with regard to support for 

consideration of options for alternative standards that might be expected to provide increased 

protection from ambient O3 exposures over the current standard. 

a. Form and Averaging Time

In considering potential forms for a revised secondary standard, the PA considers the 

characterization of the evidence in the ISA, summarized in section IV.B.1 above, including the 

ISA conclusion that exposure indices that cumulate and differentially weight the higher hourly 
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average concentrations over a season and also include the mid-level values, such as the W126 

index, offer the most scientifically defensible approach for characterizing vegetation response to 

ambient O3 and comparing study findings, as well as for defining indices for vegetation 

protection (U.S. EPA, 2013a, section 2.6.6.1). The PA additionally considers CASAC advice in 

the current review, as well as that from the last review, all of which provided support for such a 

form. Thus, in considering alternative forms of a revised standard, the PA concludes that it is 

reasonable and appropriate to consider a cumulative, concentration-weighted form to provide 

protection against cumulative, seasonal exposures to O3 that are known or anticipated to harm 

sensitive vegetation or ecosystems. The PA recognizes that such a metric is specifically designed 

to focus on the kind of O3 exposures that have been shown to cause harm to vegetation and states 

that it would have a distinct advantage over the form of the current standard in characterizing air 

quality conditions potentially of concern for vegetation and in more directly demonstrating that 

the desired degree of protection against those conditions was being achieved (U.S. EPA, 2014c, 

sections 6.2 and 6.6). 

With regard to the appropriate index for a cumulative seasonal form, the PA considers the 

evidence and background for a number of different cumulative concentration weighted indices 

that have been developed and evaluated in the scientific literature and in past NAAQS reviews in 

terms of their ability to predict vegetation response and their usefulness in the NAAQS context 

(U.S. EPA, 2006a, pp. 9-11 to 9-15 and pp. AX9-159 to AX9-187; U.S. EPA, 2007, pp. 7-15 to 

7-16). While these various forms have different strengths and limitations, the PA notes the ISA 

conclusion that the W126 index, described in section IV.B.1 above, has some important 

advantages over other non-sigmoidally weighted cumulative indices, including its lack of a cutoff in its weighting scheme which allows for cumulation of lower O3 concentrations (U.S. EPA, 
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2013a, section 9.5; U.S. EPA, 2014c, sections 6.2 and 6.6). Additionally, the W126 metric adds 

increasing weight to hourly concentrations from about 40 ppb to about 100 ppb, which is an 

important feature because “as hourly concentrations become higher, they become increasingly 

likely to overwhelm plant defenses and are known to be more detrimental to vegetation” (U.S. 

EPA, 2013a, p. 9-104). The PA additionally takes note of CASAC advice in the current and last 

review that concurred with a focus on the W126 form (Frey, 2014c, p. iii; Henderson, 2006; 

Samet, 2010). Based on the considerations summarized here, the PA concludes that the W126 

index is the most appropriate cumulative seasonal form to consider in the context of the 

secondary O3 NAAQS review. 

The PA next considers the exposure periods – diurnal and seasonal – over which the 

W126 index would be summed in any given year. The currently available information continues 

to provide support for a definition of the diurnal period of interest as the 12-hour period from 

8:00 am to 8:00 pm, which the EPA identified in past reviews as appropriately capturing the 

diurnal window with most relevance to the photosynthetic process (U.S. EPA, 2013a, section 

9.5.3; 72 FR 37900, July 11, 2007). The CASAC has generally supported this 12-hour daylight 

period as well (Frey, 2014c; Henderson, 2006, 2007). Based on these considerations, the PA

concludes that the 12-hour daylight window (8:00 am to 8:00 pm) represents the portion of the 

diurnal exposure period that is most relevant to predicting or inducing plant effects related to 

photosynthesis and growth and thus is an appropriate diurnal period to use in conjunction with a 

W126 cumulative metric (U.S. EPA, 2014c, sections 6.2 and 6.6). With regard to a seasonal 

period of interest, the current evidence base continues to provide support for a seasonal period 

with a minimum duration of three months, as described more fully in the ISA and considered in 

the PA (U.S. EPA, 2013a, section 9.5.3; U.S. EPA, 2014c, sections 6.2 and 6.6). The CASAC 
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has also indicated support for such a three month period (Frey, 2014c; Samet, 2010; Henderson, 

2006). The PA thus concludes that it is appropriate to identify the seasonal W126 index value as 

that derived from the consecutive 3-month period within the O3 season with the highest W126 

index value. 

The PA additionally considers the period of time over which a cumulative seasonal 

W126-based standard should be evaluated, considering the support for both a single year form 

and a form averaged over three years (U.S. EPA, 2014c, pp. 6-29 through 6-33). The PA 

considers the evidence of effects associated with single year and multiple year exposures as well 

as their potential public welfare significance. The PA also considers comments from CASAC, 

including their comment in the current review that “[t]he CASAC does not recommend the use 

of a three-year averaging period” and that they “favor a single-year period for determining the 

highest three-month summation which will provide more protection for annual crops and for the 

anticipated cumulative effects on perennial species” (Frey, 2014c, p. iii). 

The PA considered O3-induced effects that can occur with a single year’s exposure, 

including visible foliar injury, growth reduction in annual and perennial species and yield loss in 

annual crops (U.S. EPA, 2014c, section 6.3). While recognizing that there are a number of O3-

induced effects that have the potential for public welfare significance within the annual 

timeframe, the PA also notes the uncertainties associated with these effects that complicate 

consideration of the level of appropriate protection on an annual basis for such effects in order to 

protect the public welfare from known or anticipated adverse effects, and thus recognizes the 

possibility that a multiple-year form could be considered to provide a more consistent target level 

of protection for certain effects (U.S. EPA, 2014c, pp. 6-29 to 6-31). With regard to visible foliar 

injury, the ISA notes that “the degree and extent of visible foliar injury development varies from 
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year to year and site to site ... even among co-members of a population exposed to similar O3

levels, due to the influence of co-occurring environmental and genetic factors” (U.S. EPA, 

2013a, p. 9-38; U.S. EPA, 2014c, p. 6-30). Additionally, the PA takes note of the difficulty and 

complexity shown by the WREA analyses with regard to identifying W126 index values that 

would provide consistent protection on an annual basis given likely fluctuations in annual O3 and 

soil moisture conditions (U.S. EPA, 2014c, p. 6-30).

The PA additionally notes evidence of some O3 effects on perennial species that may

result from a single season’s elevated O3 exposures, such as reduced bud size or starch content, 

which may have the potential for some “carry over” of effects on plant growth or reproduction in 

the subsequent season. Another effect where such potential for “carry over” has been noted with 

elevated O3 exposure is reduction in below-ground carbohydrate reserves which can impair 

growth in subsequent seasons (U.S. EPA, 2014c, pp. 6-30 to 6-31; U.S. EPA, 2013a, pp. 9-43 to 

9-44 and p. 9-86). The PA notes that the occurrence of such annual effects of elevated O3

exposures over multiple years may contribute to a potential to be compounded, increasing the 

potential for effects at larger scales (e.g., population, ecosystem). In the PA, staff notes that 

multiple consecutive years of critical O3 exposures might be expected to result in larger impacts 

on forested areas than intermittent occurrences of such exposures due to the potential for 

compounding or carry-over effects on tree growth (U.S. EPA, 2014c, pp. 6-29 to 6-31).

In light of the above summarized considerations for potential compounding of carry-over 

effects, the PA concludes that the public welfare significance of the effects that can occur as a 

result of three-year O3 exposures are potentially greater than those associated with a single year 

of such exposure. Thus, to the extent that the focus for public welfare protection to be afforded 

by the secondary O3 standard is on long-term effects that occur in sensitive tree species in natural 
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forested ecosystems, including federally protected areas such as Class I areas or on lands set 

aside by States, Tribes and public interest groups to provide similar benefits to the public 

welfare, the PA concludes that a standard with a form that evaluates the cumulative seasonal 

index across multiple years might be considered to provide a more appropriate match to the 

nature of O3-related effects on vegetation upon which the secondary O3 standard is focused. In 

considering such forms, the PA focuses on one that averages the W126 index values across three 

years (U.S. EPA, 2014c, section 6.2).

With regard to single-year and three-year forms, the PA considers a WREA analysis that 

examined the extent to which cumulative RBL across a three-year period might be 

underestimated when each year’s RBL is derived from the three-year average W126 index value 

versus each single-year W126 index value for each of three years (in which no other influence on 

plant growth is presumed to change). This analysis indicates that use of the three-year average 

may lead to an underestimation, although of relatively small magnitude (U.S. EPA, 2014b, 

section 6.2.1.4). The PA notes that this limited analysis does not account for moisture levels and 

other environmental factors that could affect plant growth and that vary from year to year. When 

considering an appropriate level for a form that averages W126 index values across three years, 

the PA also recognizes the importance of considering the extent to which the cumulative effect 

of different average W126 exposures across the three-year period would be judged adverse (U.S. 

EPA, 2014c, p. 6-31).

Although single-year W126 index values were not separately analyzed in the PA analysis 

of recent monitoring data, the data indicate appreciable variation in cumulative, seasonal O3

concentrations among monitor locations meeting different levels of a standard of the current 

form (U.S. EPA 2014c, section 6.Appendix 2B).Therefore, a standard with an annual form 
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would have the cumulative seasonal index values be at or lower than the level of the standard in 

all years and, noting the inter-annual observed variability in seasonal W126 index values, could 

be appreciably below the standard level in some years. For a standard with a form that averages 

the cumulative seasonal index values across three consecutive years, the annual seasonal index 

value could be above the level in some years, but would have to be below it in others within the 

same three-year period, thus restricting the air quality for a given area to have no more than two 

years out of three with a W126 index value above the standard level, and depending on 

magnitude of each year’s index, potentially having no more than one.

In its consideration of one year as compared to three year forms, the PA also considers 

implications with regard to stability of air quality programs that implement the NAAQS (U.S. 

EPA, 2014c, pp. 6-31 to 6-32). The PA notes that a standard based on a single year W126 index 

would be expected to have a less stability relative to a standard based on a form that averages 

seasonal indices across three consecutive years, given the potential for large year-to-year 

variability in annual W126 index values in areas across the country. Thus, a three-year 

evaluation period can contribute to greater public welfare protection by limiting year-to-year 

disruptions in ongoing control programs that would occur if an area was frequently shifting in 

and out of attainment due to extreme year-to-year variations in meteorological conditions. This 

greater stability in air quality management programs would thus facilitate achievement of the 

protection intended by a standard. Such considerations of stability often receive particular weight 

in NAAQS reviews, such as those resulting in selection of the form for the current O3 primary 

and secondary standards (62 FR 38856, July 18, 1997), as well as the primary standards for 

nitrogen dioxide (75 FR 6474, February 9, 2010) and sulfur dioxide (75 FR 35520, June 22, 
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2010). See also ATA III, 283 F. 3d at 374-75 (recognizing programmatic stability as a legitimate 

consideration in the NAAQS standard-setting process).

Thus, to the extent that emphasis continues to be placed on protecting against effects 

associated with multi-year exposures and maintaining more year-to-year stability of public 

welfare protection, the PA concludes that it is appropriate to consider a secondary standard form 

that is an average of the seasonal W126 index values across three consecutive years. The PA 

concludes that such a form might be appropriate for a standard intended to achieve the desired 

level of protection from longer-term effects, including those associated with potential 

compounding, and that such a form might be concluded to contribute to greater stability in air 

quality management programs, and thus, greater effectiveness in achieving the desired level of 

public welfare protection, than that might result from a single year form (U.S. EPA, 2014c, 

section 6.6).

The PA additionally recognized that to the extent the Administrator finds it useful to 

consider the public welfare protection that might be afforded by a revised primary standard, this 

is appropriately judged by evaluating the impact of attainment of such a revised primary standard 

on O3 exposures in terms of the cumulative seasonal W126-based exposure index. 

b. Level

In considering an appropriate range of levels to consider for a W126-based standard, the 

PA notes that, due to the variability in the importance of the associated ecosystem services 

provided by different species at different exposures and in different locations, as well as 

differences in associated uncertainties and limitations, both the species present and their public 

welfare significance, in addition to the magnitude of the ambient concentrations, are essential 

considerations in drawing conclusions regarding the significance or magnitude of public welfare 
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impact. Therefore, in development of the PA conclusions, staff took note of the complexity of 

judgments to be made by the Administrator regarding the adversity of known and anticipated 

effects to the public welfare and recognized that the Administrator’s ultimate judgments on the 

secondary standard will most appropriately reflect an interpretation of the available scientific 

evidence and exposure/risk information that neither overstates nor understates the strengths and 

limitations of that evidence and information. 

As described in section IV.D.1 above, the PA employed a paradigm, which has evolved 

over the course of the O3 and other secondary NAAQS reviews, to assist in putting the available 

science and exposure/risk information into the public welfare context (U.S. EPA, 2014c, section 

5.1). This paradigm recognizes that the significance to the public welfare of O3-induced effects 

on sensitive vegetation growing within the U.S. can vary depending on the nature of the effect, 

the intended use of the sensitive plants or ecosystems, and the types of environments in which 

the sensitive vegetation and ecosystems are located. Accordingly, any given O3-related effect on 

vegetation and ecosystems (e.g., biomass loss, crop yield loss, visible foliar injury) may be 

judged to have a different degree of impact on or significance to the public welfare depending, 

for example, on whether that effect occurs in a Class I area, a city park, or commercial cropland. 

This approach also includes consideration of impacts to ecosystem goods and services, which are 

an important category of public welfare effects with an obvious relationship to consideration of 

intended use (73 FR 16492, March 27, 2008). 

In considering potential levels for an alternative standard based on the W126 metric, the 

PA focused primarily on impacts on tree growth, crop yield loss, and visible foliar injury, as well 

as impacts on the associated ecosystem services, while taking note of the uncertainties and 

limitations associated with several key aspects of this information. In addition to uncertainties 
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related to the WREA air quality scenarios and assessments summarized in section IV.C above, 

the PA also recognized uncertainties associated with the evidence underlying the tree seedling 

and crop E-R functions (U.S. EPA, 2014c, section 6.3). These include uncertainties regarding 

intra-species variability due to the different numbers of studies that exist for different species so 

that the weight of evidence is not the same for each species. Those species with more than one 

study show variability in response and E-R functions. The potential variability in less wellstudied species is, however, unknown (U.S. EPA, 2013a, pp. 9-123 to 9-125; U.S. EPA, 2014b, 

section 6.2.1.2, and Table 6-27). The PA also recognizes uncertainty regarding the extent to 

which tree seedling E-R functions can be used to represent mature trees since seedling sensitivity 

has been shown in some cases to not reflect mature tree O3 sensitivity in the same species and 

uncertainty in the relationship of O3 effects on tree seedlings (e.g., relative biomass loss) in one 

or a few growing seasons to effects that might be expected to accrue over the life of the trees 

extending into adulthood (U.S. EPA, 2013a, section 9.6 and pp. 9-52 to 9-53; U.S. EPA, 2014b, 

sections 6.2.1.1, 6.2.1.4 and Tables 6-5 and 6-27). 

With respect to tree growth, the PA gave primary consideration to relative biomass loss 

estimates derived from the E-R functions, described in section IV.B.1.b above and in the PA, 

while also considering WREA risk/exposure estimates related to this effect (U.S. EPA, 2014c, 

section 6.4). The PA takes note of the different index value estimates presented in Table 6-1 of 

PA (Table 8 below) with regard to the number of studied species below different response 

benchmarks, as well as with regard to the median response. The PA additionally considers the 

WREA estimates regarding: (1) percent of assessed geographic area exceeding 2% weighted 

relative biomass (U.S. EPA, 2014c, Table 6-2); (2) number of assessed Class I areas with tree 

seedling weighted relative biomass loss estimates below 2% (U.S. EPA, 2014c, Table 6-3); and 
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(3) the percent median biomass loss across counties for different air quality scenarios (U.S. EPA, 

2014c, Table 5-5). The PA further notes other WREA estimates for effects on ecosystem services 

related to public welfare, such as carbon sequestration and air pollutant removal. With respect to 

crop yield loss, the PA notes the summary of RYL estimates for individual crop species and for 

the median across species (Table 8), and the WREA risk/exposure estimates (U.S. EPA, 2014b, 

Section 6.3). The PA also notes information available on visible foliar damage to species 

occurring in areas preserved for their natural character, such as federal Class I areas, and the 

analyses in the WREA evaluating biosite data and several benchmarks of injury (U.S. EPA, 

2014b, section 5.4.2). 

Table 8. Tree Seedling Biomass Loss and Crop Yield Loss Estimated for O3 Exposure over 

a Season

W126 value

for exposure 

period

Tree seedling RBLA Crop RYLC

Median Value Individual Species Median Value Individual Species

21 ppm-hrs Median species w. 

6.8% loss B

< 2% loss: 3/11 species 

< 5% loss: 5/11 species

<10% loss: 7/11 species

<15% loss: 10/11 species

>40% loss: 1/11 species

Median species 

w. 7.7 % loss D

< 5% loss: 4/10 species

>5,<10% loss: 3/10 

species

>10,<20% loss: 3/10 

species

19 ppm-hrs Median species w. 

6.0% loss B

< 2% loss: 3/11 species

<5% loss: 5/11 species

<10% loss: 7/11 species

<15% loss: 10/11 species

>30% loss: 1/11 species

Median species 

w. 6.4 % loss D

< 5% loss: 5/10 species

>5, <10% loss: 3/10 

species

>10,<20% loss: 2/10 

species

17 ppm-hrs Median species w. 

5.3% loss B

< 2% loss: 5/11 species

<5% loss: 5/11 species

<10% loss: 9/11 species

<15% loss: 10/11 species

>30% loss: 1/11 species

Median species 

w. 5.1 % loss D

< 5% loss: 5/10 species

>5, <10% loss: 3/10 

species

>10,<20% loss: 2/10 

species 

15 ppm-hrs Median species w. 

4.5% loss B

< 2% loss: 5/11 species

<5% loss: 6/11 species

<10% loss: 10/11 species

>30% loss: 1/11 species

Median species 

w.<5% loss D

< 5% loss: 6/10 species

>5, <10% loss: 4/10 

species

13 ppm-hrs Median species w. 

3.8% loss B

< 2% loss: 5/11 species

<5% loss: 7/11 species

<10% loss: 10/11 species

>20% loss: 1/11 species

Median species 

w.<5% loss D

< 5% loss: 6/10 species

>5, <10% loss: 4/10 

species
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11 ppm-hrs Median species w. 

3.1% loss B

< 2% loss: 5/11 species

<5% loss: 8/11 species

<10% loss: 10/11 species

>20% loss: 1/11 species

Median species 

w. <5% loss D

< 5% loss: 9/10 species

>5, <10% loss: 1/10 

species

9 ppm-hrs Median species w. 

2.4% loss B

< 2% loss: 5/11 species

<5% loss: 10/11 species

>20% loss: 1/11 species

Median species 

w. <5% loss D

< 5% loss: all species

7 ppm-hrs Median species w. 

<2% loss B

< 2% loss: 7/11 species

<5% loss: 10/11 species

>15% loss: 1/11 species

Median species 

w. <5% loss D

< 5% loss: all species

A Estimates are based on the 11 E-R functions for tree seedlings described in WREA, Appendix 6F and discussed in 

the PA, section 5.2.1, with the exclusion of cottonwood in consideration of CASAC comments on differences of that 

study from the other controlled E-R studies (Frey, 2014b, 2014c).

B This is the median of the composite E-R functions for 11 tree species from the WREA, Appendix 6F (discussed in 

the PA, section 5.2.1). 

C Estimates here are based on the 10 E-R functions for crops (from the PA, Appendix 6F and section 5.3.1).

D This median value is the median of the composite E-R functions for 10 crops from WREA, Appendix 6F (also 

discussed in the PA, section 5.3.1). 

Given the wide variation in sensitivity of studied tree species to O3-induced relative 

biomass loss, the PA focused consideration on both median species values and individual species 

responses and RBL estimates for a given range of W126 index values. In this consideration, the 

PA took note of CASAC’s advice regarding RBL levels, specifically their emphasis on a 

benchmark of median relative tree biomass loss at or below 2% and their view that a 6% median 

relative biomass loss is “unacceptably high.” The median tree species RBL estimate is at or 

below 2% only at the lowest W126 level assessed, 7 ppm-hrs. At incrementally higher W126 

index levels, the median RBL is also incrementally higher, so that at W126 index values of 9, 11, 

13, 15, 17 and 19 ppm-hrs, the median RBL increases to 2.4%, 3.1%, 3.8%, 4.5%, 5.3% and 

6.0%, respectively. Thus, the median species biomass loss is below 6%, the level characterized 

by the CASAC as unacceptably high, across the W126 range of 7 to 17 ppm-hrs, for which it 

varies from approximately 2% to approximately 5%. Given this finding, the PA discussion of a 

range of levels appropriate to consider focuses on this range. In focusing on this range, the PA 

considers the full array of CASAC advice with regard to interpretation of the evidence and 
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exposure/risk information on vegetation-related effects of O3, as well as the role of the 

Administrator’s judgments in identifying the level of air quality that is requisite to protect public 

welfare from adverse effects, as noted in section IV.A above.218

The PA recognizes that public welfare judgments may reasonably be informed by a range 

of biomass loss benchmarks, in contexts of considering both median RBL estimates and RBL 

estimates for individual species. Accordingly, in considering individual tree species estimates, 

the PA notes the value of additionally characterizing the RBL estimates in comparison to higher 

loss levels such as 10% or 15%. 

For every W126 value over the full range from 7 to 17 ppm-hrs, the RBLs for each of 

five species is less than 2% (Table 8), which is the lower benchmark that CASAC identified for 

tree species. Accordingly, the PA focused attention on the remaining six more sensitive studied 

species (i.e., eastern white pine, aspen, tulip poplar, ponderosa pine, red alder, and black cherry) 

to evaluate the protection against tree seedling biomass loss at different W126 levels within the 

range from 17 to 7 ppm-hrs. At a W126 index value of 17 ppm-hrs, one of these six species (red 

alder) has a RBL estimate below 6%, while at the W126 index value of 7 ppm-hrs, five of these 

six species have RBLs below 6% (eastern white pine, aspen, tulip poplar, ponderosa pine, red 

alder). Taken together with the more tolerant species, the proportion of the studied tree species 

with RBLs below 6% are 6 of 11, 7 of 11, 8 of 11, and 10 of 11 at W126 index values of 17, 15, 

13, and 11 ppm-hrs, respectively. 

 

218 In the context of the O3 standard, such judgments include: the weight to give the evidence of 

specific vegetation-related effects estimated to result across a range of cumulative seasonal 

concentration-weighted O3 exposures; the weight to give associated uncertainties, including 

those related to the variability in occurrence of such effects in specific areas of U.S., such as 

those of particular public welfare significance; and, judgments on the extent to which such 

effects in such areas may be considered adverse to public welfare.
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With regard to other, higher, RBL benchmark levels and estimates for all 11 species, the 

PA notes that 9 of 11 studied tree species have a predicted RBL below 10% at the W126 level of 

17 ppm-hrs, while 10 of 11 species have a predicted RBL below 10% for W126 levels of 15 to 7 

ppm-hrs. In addition, 10 of 11 studied tree species have a predicted RBL below 15% for W126 

levels of 17 to 7 ppm-hrs. The PA notes that the RBL estimates for black cherry, the most 

sensitive of the 11 species, remain above 15% for W126 index values across the range from 17 to 

7 ppm-hrs, making unclear the extent to which black cherry estimates might inform 

consideration of different W126 exposures within this range (U.S EPA, 2014c, section 6.6 and 

Table 6-1; U.S. EPA, 2014b, section 6.2 and Appendix 6A).

While recognizing the limitations and uncertainties associated with the WREA air quality 

scenarios with regard to their representation of conditions just meeting different three-year 

average W126 index values (as summarized in section IV.C.1 above), including the potential 

underestimation of the highest O3 concentrations, the PA additionally considers several WREA 

RBL analyses (U.S. EPA, 2014c, section 6.3). In the WREA characterization of the number of 

counties where the median RBLs were greater than 2%, 7% of the counties have median RBLs 

greater than 2% in the 15 and 11 ppm-hrs W126 scenarios, as compared to 8% for the current 

standard (U.S. EPA, 2014c, Table 5-5; U.S. EPA, 2014b, Table 6-7). The percentage is 6% in the 

7 ppm-hrs W126 scenario. Of the 221 counties (7% of counties) estimated to have a median RBL 

above 2% for the 15 ppm-hrs scenario, 203 of those counties have a RBL greater than 2% 

because of the presence of black cherry (U.S. EPA, 2014c, section 6.3).

In considering the potential magnitude of the ecosystem impact of O3-related biomass 

effects on tree growth, the PA additionally focused on the WREA estimates of weighted RBL for 

the W126 air quality scenarios (U.S. EPA, 2014b, section 6.8). For the current standard and the 
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three W126 scenarios, the percent of total assessed area having weighted RBL greater than 2% 

was 0.2%, 0.2%, 0.1% and <0.1%, respectively (U.S. EPA, 2014c, Table 6-2; U.S. EPA 2014b, 

Table 6-25). In giving particular attention to estimates for Class I areas, the PA notes that for all

four scenarios, the WREA estimates indicate weighted RBL greater than 2% in one or two of the 

145 assessed nationally protected Class I areas (U.S. EPA, 2014c, sections 6.3 and 6.6). 

In considering potential impacts on ecosystem services related to reductions in O3 effects 

on tree growth, the PA particularly recognizes that impacts on climate regulation can reasonably 

be concluded to be potentially significant from a public welfare perspective. In additionally 

recognizing that carbon sequestration has been identified as a potentially important tool for 

managing anthropogenic impacts on climate, the PA considers the WREA estimates of potential 

increases in forestry carbon storage for ambient O3 reductions in the three W126 air quality 

scenarios (U.S. EPA, 2014c, sections 6.3 and 6.6; U.S. EPA, 2014b, section 6.6.1). The WREA 

estimates additional forestry carbon storage potential of 13, 593 and 1,600 MMtCO2e (over 30 

years) for the W126 scenarios of 15, 11 and 7 ppm-hrs, respectively, as compared to the current 

standard (U.S. EPA, 2014b, Table 6-18). Compared to the absolute estimate for the current 

standard scenario (approximately 89,000 MMtCO2e, over 30 years), these amounts represent 

additional storage of less than 0.1 %, just under 1% and under 2% for the 15, 11 and 7 ppm-hrs 

scenarios, respectively (U.S. EPA, 2014b, section 6.6.1 and Appendix 6B).

The PA additionally considers the WREA estimates for five urban areas of how reduced 

growth of O3-sensitive trees in urban forests may affect air pollutant removal (U.S. EPA, 2014b,

sections 6.6.2 and 6.7 and Appendix 6D). As with the national estimates, estimates for all five 

case study areas indicate generally small differences between the current standard and the three 

W126 scenarios (U.S. EPA, 2014c, Table 6-5). The PA additionally notes significant 
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uncertainties and limitations associated with WREA estimates related to carbon sequestration 

and air pollution removal (U.S. EPA, 2014b, Table 6-27; U.S. EPA, 2014c, sections 6.3 and 6.6), 

some of which are summarized in section IV.C.2.b above. The PA recognizes that, as with 

consideration of other pertinent evidence and exposure/risk information, the Administrator’s 

consideration of WREA estimates for these ecosystem services will involve judgments regarding 

the appropriate weight to place on such uncertainties as well as the potential impacts to the 

public welfare of the estimates. 

The PA additionally considers the biomass effects of O3 on crops estimated for different 

W126 index values across the range identified above. For this consideration, the PA focuses on 

the 10 crops for which robust E-R functions have been established, as described in section 

IV.B.1 above: barley, lettuce, field corn, grain sorghum, peanut, winter wheat, field cotton, 

soybean, potato and kidney bean (U.S. EPA, 2013a; U.S. EPA, 2014b, section 6.5 and Figure 6-

3). In evaluating this information, the PA takes note of CASAC’s comment regarding 

significance of 5% for median crop relative yield loss (RYL). The PA finds that the median crop 

RYL is at or below 5% for all W126 index values from 7 to 17 ppm-hrs and observes that this 

finding makes it unclear to what extent this information informs consideration of levels within 

this range. The RBL estimates for half of the ten individual species are below 5% RYL at 17 

ppm-hrs. The number of the ten individual crops with RYL below 5% is six for W126 values of 

15 and 13 ppm-hrs, nine for a W126 value of 11 ppm-hrs and ten for W126 levels of 9 and 7 

ppm-hrs. Recognizing that different crops are likely to have different values or importance to 

public welfare, the PA also considers the RYL estimates across the W126 range for individual 

species. 
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In considering these RYL estimates, the PA recognizes that they do not reflect the 

influence of the heavy management of agricultural crops that is common in the U.S. and so 

cannot be easily interpreted with regard to potential public welfare significance. In light of the 

median RYL estimates of approximately 5% or lower for W126 index values at and below 17 

ppm-hrs, the PA gives less emphasis to consideration of crop RYL, while noting that this 

information indicates that a secondary standard revised to provide additional protection for 

vegetation with attention to tree growth, would be expected to also provide additional protection 

to crops over that provided by the current standard (U.S. EPA, 2014c, section 6.6).

The PA also considers the evidence and exposure/risk information with regard to visible 

foliar injury and the extent to which that might inform consideration of potential alternative 

secondary standards appropriate for the Administrator to consider. Specifically, the PA notes the 

findings of the WREA analyses of the nationwide USFS/FHM biosite dataset (2006-2010) that 

while soil moisture conditions influence the proportion of biosites with O3-related visible foliar 

injury, as described in section IV.B.1.a above, the proportion of such sites increases appreciably 

with increasing W126 index values up to approximately 10 ppm-hrs, while relatively little or no 

change in incidence of injury is seen with O3 exposures at higher W126 index values (U.S. EPA 

2014b, Chapter 7, Figure 7-10). The PA additionally notes that visible foliar injury has been 

identified by the federal land managers as a diagnostic tool for informing conclusions regarding 

potential O3 impacts on potentially sensitive AQRVs (USFS, NPS, FWS, 2010), which the PA 

concludes indicates that such O3-induced impacts might be considered to have the potential to 

impact the public welfare in scenic and/or recreational areas during years they occur. 

The PA was unable, however, to identify any guidance for federal land managers 

regarding at what spatial scale or what degree of severity visible foliar injury might be sufficient 
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to trigger protective action based on this potential impact on AQRVs. The PA states that there 

does not appear to be consensus in the literature regarding severity of visible foliar injury and 

risks to plant functions or services, additionally noting that CASAC, while identifying percent 

biomass loss and yield loss benchmarks for tree seedlings and commodity crops, respectively, 

did not provide any benchmark or criteria for consideration of O3 impacts related to this 

endpoint. Further, as in previous reviews, the ISA concludes visible foliar injury is not always a 

reliable indicator of other negative effects on vegetation, making it difficult to relate visible 

foliar injury symptoms to other vegetation effects such as individual plant growth, stand growth, 

or ecosystem characteristics (U.S. EPA, 2013a, section 9.4.2, p. 9-39). Additionally, although 

evidence shows visible foliar injury can occur under very low cumulative O3 exposures, “...the 

degree and extent of visible foliar injury development varies from year to year and site to site..., 

even among co-members of a population exposed to similar O3 levels, due to the influence of cooccurring environmental and genetic factors” (U.S. EPA 2013a, section 9.4.2, p. 9-38). Thus, 

while the PA recognizes visible foliar injury as an important O3 effect which, depending on 

severity and spatial extent may reasonably be concluded to be of public welfare significance, 

most particularly in nationally protected areas such as Class I areas, it additionally recognizes the 

appreciable variability in this endpoint, which poses challenges to giving it primary emphasis in 

identifying potential alternative standard levels. 

On the basis of all the considerations described above, including the evidence and 

exposure/risk analyses, and advice from CASAC, the PA concludes that a range of W126 index 

values appropriate for the Administrator to consider in identifying a secondary standard that 

might be expected to provide the requisite protection to the public welfare from any known or 

anticipated adverse effects, extends from 7 to 17 ppm-hrs. The PA notes, however, the role of 
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judgments by the Administrator in such decisions, as recognized above. In selecting this range, 

the PA primarily considers the evidence- and exposure/risk-based information for cumulative 

seasonal O3 exposures represented by W126 index values (including those represented by the 

WREA average W126 scenarios) associated with biomass loss in studied tree species, both in 

and outside areas that have been afforded special protections. The PA recognizes that tree 

biomass loss can be an indicator of more significant ecosystem-wide effects which might 

reasonably be concluded to be significant to public welfare. For example, when biomass loss 

occurs over multiple years at a sufficient magnitude, it is linked to some level of effects on an 

array of ecosystem-level processes, such as nutrient and water cycles, changes in above and 

below ground communities, carbon storage and air pollution removal, that benefit the public 

welfare (U.S. EPA, 2014c, Figure 5-1). In focusing on tree biomass effects, the PA gave 

emphasis to CASAC’s judgment that a 6% median RBL is unacceptably high, and that the 2% 

median RBL is an important benchmark to consider. The PA notes that for the lower W126 value 

of 7 ppm-hrs that the median tree species biomass loss is at or below 2% and that for the upper 

value of 17 ppm-hrs that the median tree biomass loss is below 6%.

219

In considering the stability and potential for associated greater public welfare protection 

offered by a three-year form, as well as based on the recognition that in any given year in the 

environment, other environmental factors can influence the extent to which O3 may have the 

impact predicted by the E-R functions on which much of the range discussion above focuses, the

PA gave careful consideration to the support for consideration of potential alternative W126 

 

219 We note that a W126 index value of 19 ppm-hrs is estimated to result in a median RBL value 

of 6%, as shown in Table 2 above.
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based standards with levels in the range identified above (17 ppm-hrs to 7 ppm-hrs) with a threeyear average form.

Thus, the PA concludes that in staff’s view, the evidence- and exposure/risk-based 

information relevant to tree biomass loss and the associated ecosystem services important to the 

public welfare support consideration of a W126-based secondary standard with index values 

within the range of 7 to 17 ppm-hrs, and a form averaged over three years. In reaching this 

conclusion, the PA gave particular consideration to the importance of considering the lasting or 

carry-over effects that can derive from single year exposures of perennial plants, recognizing the 

importance of considering the available evidence and exposure/risk based information related to 

such effects, as well as associated uncertainties. The PA additionally recognized that there is 

limited information to discern differences in the level of protection afforded for cumulative 

growth-related effects by potential alternative W126-based standards of a single year form as 

compared to a three-year average form. Lastly, the PA recognizes the role of policy judgments 

required of the Administrator with regard to the public welfare significance of identified effects, 

the appropriate weight to assign the range of uncertainties inherent in the evidence and analyses, 

and, ultimately, in identifying the requisite protection for the secondary O3 standard. Examples 

of areas where the Administrator’s judgments would be expected include those stemming from 

consideration of the effects associated with longer-term conditions and the role that year-to-year 

exposure variability may play in associated public welfare impacts, as well as the objectives for 

consideration of tree species biomass loss estimates in relationship to identified benchmarks 

(e.g., 2% or greater). 

The PA also concludes that, to the extent the Administrator finds it useful to consider the 

public welfare protection that might be afforded by a revised primary standard, this is 
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appropriately judged through the use of a cumulative seasonal W126-based exposure metric, a 

metric considered appropriate for evaluating impacts on vegetation. For example, comparison of 

the air quality conditions (expressed in terms of W126 exposures) expected to result from a 

revised primary standard to the W126-based exposures concluded to provide requisite public 

welfare protection would thus inform a judgment of whether a secondary standard set identical to 

a revised primary standard would be expected to achieve the appropriate level of air quality. The 

PA notes that such a comparison would be in terms of a metric considered appropriate for 

evaluating impacts on vegetation which inform conclusions on public welfare impacts. The PA

further concludes that the drawing of conclusions with regard to the public welfare protection 

afforded by such a standard should entail consideration of the air quality conditions likely to be 

achieved in terms of the cumulative seasonal W126-based metric described above. 

Accordingly, the PA describes several analyses of air quality data that might inform such 

consideration (U.S. EPA, 2014c, section 6.4), and notes the importance of taking into account 

associated uncertainties, including those associated with the limited monitor coverage in many 

rural areas, such as those in the West and Southwest regions and at high elevation sites. 

Additional such analyses, based on more recent O3 monitoring data, have been developed since 

the completion of the PA. All of these analyses are summarized in section IV.E.4 below. In 

reaching conclusions on appropriate policy options for a revised secondary standard the 

Administrator has considered the findings of these analyses, as described in section IV.E.5 

below.

3. CASAC Advice 

Beyond the evidence- and exposure/risk-based considerations in the PA discussed above, 

the EPA’s consideration of a revised secondary standard also takes into account the advice and 
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recommendations of CASAC. The EPA also considered public comments received to date, some 

of which urged the consideration of a secondary standard with a cumulative seasonal form using 

the W126 metric and a level within the range of 7 to 15 ppm-hrs or in the low end of this 

range,220 while others have urged retaining the existing form and averaging time due to their 

view of a lack of new information to support a distinct secondary standard.

In advice offered on a revised secondary standard in the current review, similar to advice 

in the last review, including advice offered on the 2010 proposed reconsideration, the CASAC 

recommended “retaining the current indicator (ozone) but establishing a revised form of the 

secondary standard to be the biologically relevant W126 index accumulated over a 12-hour 

period (8 a.m. – 8 p.m.) over the 3-month summation period of a single year resulting in the 

maximum value of W126” (Frey, 2014c, p. iii). With regard to the level, the CASAC 

recommended that “that the level associated with this form be within the range of 7 ppm-hrs to 

15 ppm-hrs to protect against current and anticipated welfare effects of ozone” and that “CASAC 

does not support a level higher than 15 ppm-hrs” (Frey, 2014c, p. iii). The CASAC additionally 

stated that “[i]n 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,” while 

also acknowledging “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, 2014c, p. 

iii). 

 

220 Public comment received thus far in this review are in the docket EPA–HQ–OAR-2008-0699, 

accessible at www.regulations.gov 
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In providing advice on a range for the secondary standard, the CASAC noted a W126 

index value for which the median tree species RBL estimate was 6 percent, and the median crop 

species RBL estimate was over 5 percent, stating that “[t]hese levels are unacceptably high” 

(Frey, 2014c, p. iii).

221 In addition, regarding consideration of relative biomass loss benchmarks 

for tree seedlings, the CASAC stated that “[a] 2% biomass loss is an appropriate scientifically 

based value to consider as a benchmark of adverse impact for long-lived perennial species such 

as trees, because effects are cumulative over multiple years” (Frey, 2014c, p. 14). In so stating, 

the CASAC referenced findings for biomass loss in aspen exposed to elevated O3 over seven 

years, citing Wittig et al., 2009.222 The CASAC additionally pointed to the report of the 1996 

workshop sponsored by the Southern Oxidants Study group (Heck and Cowling, 1997, noted in 

section IV.B.2 above) which described a general consideration of 1-2% per year growth 

reduction in making judgments the group identified as appropriate for the endpoint of growth 

effects in trees, without providing an explicit rationale for the identified percentages (Frey, 

2014c, p. 14). The CASAC also commented that “it is appropriate to identify a range of levels of 

alternative W126-based standards that includes levels that aim for not greater than 2% RBL for 

the median tree species” (Frey, 2014c, p. 14). The CASAC noted that the “level of 7 ppm-hrs is 

the only level analyzed for which the relative biomass loss for the median tree species is less 

 

221 The CASAC made this comment while focusing on Table 6-1 in the second draft PA and the 

entry for 17 ppm-hrs. That table was revised for inclusion in the final PA in consideration of 

CASAC comments on the E-R function for eastern cottonwood, such that the RBL estimates for 

17 ppm-hrs in the final table (see Table 2 above) are below the values CASAC viewed as 

“unacceptably high”.

222 The way in which the statement pointing to the aspen seven-year biomass loss value from 

Wittig et al (2009) relates to CASAC’s view with regard to 2%, however, is unclear as the 

original source for this finding (cited in Wittig et al., 2009) indicates yearly relative biomass loss 

values during this seven year exposure that are each well above 2%, and, in fact, are all above 

20% (King, et al., 2005).
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than or equal to 2 percent” indicating that 7 ppm was appropriate lower bound for the 

recommended range (Frey, 2014c, p. 14).

With regard to consideration of effects on crops, the CASAC, as noted above, described 

median species RYL over 5% yield loss as “unacceptably high.” The CASAC further noted that 

“[c]rop loss appears to be less sensitive than these other indicators, largely because of the 

CASAC judgment that a 5% yield loss represents an adverse impact, and in part due to more 

opportunities to alter management of annual crops” (Frey, 2014c, p. 14).

The CASAC 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”, while further providing its own policy recommendations, including the following (Frey, 

2014c, p. iii).

[T]he CASAC advises that a level of 15 ppm-hrs for the highest 3-month sum in a 

single year is requisite to protect crop yield loss, but that lower levels provide 

additional protection against crop yield loss. Furthermore, there are specific 

economically significant crops, such as soybeans, that may not be protected at 15 

ppm-hrs but would be protected at lower levels. A level below 10 ppm-hrs is 

required to reduce foliar injury. A level of 7 ppm-hrs is protective of relative 

biomass loss for trees and offers additional protection against crop yield loss and 

foliar injury. Therefore, 7 ppm-hrs is protective of ecosystem services. Thus, 

lower levels within the recommended range offer a greater degree of protection of 

more endpoints than do higher levels within the range. 

Additionally, in regard to consideration of form, the CASAC noted that “[i]f, as a policy 

matter, the Administrator prefers to base the secondary standard on a three-year averaging period 

for the purpose of program stability, then the level of the standard should be revised downward 

such that the level for the highest three-month summation in any given year of the three-year 

period would not exceed the scientifically recommended range of 7 ppm-hrs to 15 ppm-hrs” 

(Frey, 2014c, pp. iii and iv). In related manner, the CASAC noted that a three-year average 
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W126 level of 13 ppm-hrs may be appropriate depending on consideration of year-to-year 

variability and such policy considerations (Frey, 2014c, p. iv).

Lastly, in comments recognizing uncertainties associated with the evidence and exposure 

and risk analyses, the CASAC stated that “there is sufficient scientific evidence, and sufficient 

confidence in the available research results, to support the advice we have given above for this 

review cycle of the primary and secondary standards” (Frey, 2014c, p. iv).

4. Air Quality Analyses 

As described in section II.D. above, the PA concludes with regard to the primary standard 

that it is appropriate for the Administrator to consider revision of the level to within the range of 

60 to 70 ppb. In consideration of this conclusion for the primary standard, although the PA also 

concludes it is appropriate to consider a revised secondary standard with a cumulative, seasonal, 

concentration-weighted form, the PA recognized that, it may be practical to consider the extent 

to which a revised secondary standard in the form of the current secondary standard might be 

expected to also reduce and provide protection from cumulative seasonal exposures of concern, 

noting that, for example, if a clear and robust relationship was found to exist between 8-hour 

daily peak O3 concentrations and cumulative, seasonal exposures, the averaging time and form of 

the current standard might be concluded to have the potential to be effective as a surrogate (U.S. 

EPA, 2014c, section 6.4). 

Therefore, the PA evaluated what the available information indicated with regard to 

control of cumulative O3 exposures that might be afforded by alternative secondary standards 

with the averaging time and form of the current standard (a three-year average of 4th highest 8-

hour average concentrations). The available information addressing this point includes a “focus 

study” in the ISA, and several air quality analyses described in the PA, chapters 2, 5 and 6 and 
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Appendix 2b.223 Additionally, a similar air quality analysis performed with more recent 

monitoring data is now available and is also described here.

The focus study described in the ISA examined the diel variability in O3 concentrations in 

six rural areas between 2007 and 2009 (U.S. EPA, 2013a, pp. 3-131 to 3-133). The ISA reported 

that “[t]here was considerable variability in the diel patterns observed in the six rural focus 

areas” with the three mountainous eastern sites exhibiting a “generally flat profile with little

hourly variability in the median concentration and the upper percentiles,” while the three western 

rural areas demonstrated a “clear diel pattern to the hourly O3 data with a peak in concentration 

in the afternoon similar to those seen in the urban areas,” which was especially obvious at the 

San Bernardino National Forest site, 90 km east of Los Angeles at an elevation of 1,384 meters 

(U.S. EPA, 2013a, p. 3-132). Thus, while the western sites that are influenced by upwind urban 

plumes may have increased cumulative seasonal values coincident with increased daily 8-hour 

peak O3 concentrations, this analysis indicates that, in sites without such an urban influence (the 

eastern sites in this analysis), such a relationship does not occur (U.S. EPA, 2013a, section 

3.6.3.2). Thus, the lack of such a relationship indicates that in some locations, O3 air quality 

patterns can lead to elevated cumulative, seasonal O3 exposures without the occurrence of 

elevated daily maximum 8-hour average O3 concentrations (U.S. EPA, 2013a, section 3.6.3.2).

Further, staff notes that the prevalence and geographic extent of such locations is unclear, since 

as in the last review, there continue to be relatively fewer monitors in the western U.S., including 

in high elevation remote sites. In considering the findings of this analysis, the PA additionally 

recognized, however, that the cumulative seasonal values for the eastern rural sites, where 

 

223 This information and analyses were included in the second draft PA (U.S. EPA, 2014j), 

reviewed by CASAC in early 2014, and drafts of the ISA, reviewed by CASAC earlier in the 

review.
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cumulative seasonal O3 concentrations appear to be relatively less related to daily maximum 8-

hour concentrations, are lower in general than those of the western, urban-influenced sites.

In addition to the focus study described in the ISA (U.S. EPA, 2013a, section 3.6.3.2), the 

PA considers additional analyses of air quality monitoring data. For example, Chapter 2 of the 

PA characterized recent monitoring data of O3 air quality in rural areas. While approximately 80

percent of the O3 monitoring network is urban focused, about 120 rural monitors are divided 

among CASTNET, NCore, and portable O3 monitors (POMs) sites (U.S. EPA, 2014c, Chapter 2, 

pp. 2-2 to 2-3, Figure 2.1). Specifically, as stated in Chapter 2 of the PA, “[a]lthough rural 

monitoring sites tend to be less directly affected by anthropogenic pollution sources than urban 

sites, rural sites can be affected by transport of O3 or O3 precursors from upwind urban areas and 

by local anthropogenic sources such as motor vehicles, power generation, biomass combustion, 

or oil and gas operations” (U.S. EPA, 2013a, section 3.6.2.2). In addition, O3 tends to persist 

longer in rural than in urban areas due to lower rates of chemical scavenging in non-urban 

environments. At higher elevations, increased O3 concentrations can also result from 

stratospheric intrusions (U.S. EPA, 2013a, sections 3.4, 3.6.2.2). As a result, O3 concentrations 

measured in some rural sites can be higher than those measured in nearby urban areas (U.S. 

EPA, 2013a, section 3.6.2.2) and the ISA concludes that “cumulative exposures for humans and 

vegetation in rural areas can be substantial, and often higher than cumulative exposures in urban 

areas” (U.S. EPA, 2013a, p. 3-120). These known differences between urban and rural sites 

suggest that there is the potential for an inconsistent relationship between 8-hour daily peak O3

concentrations and cumulative, seasonal exposures in those areas. However, the PA also notes 

that reductions in NOx emissions that occur in urban areas to attain primary standards would also 
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have the effect of reducing downwind, rural concentrations over the season (U.S. EPA, 2014c, 

section 6.4). 

In addition, as was done in both the 1997 and 2008 reviews, the PA analyzed 

relationships between O3 levels in terms of the current averaging time and form and a W126 

cumulative form, based on recent air quality data. One analysis in the PA describes the W126 

index values and current standard design values at each monitor for two periods: 2001-2003 and 

2009-2011 (e.g., U.S. EPA, 2014c, Appendix 2B, Figures 2B-2 and 2B-3). This shows that 

between the two periods, during which broad scale O3 precursor emission reductions occurred, 

O3 concentrations in terms of both metrics were reduced. There is a fairly strong, positive degree 

of correlation between the two metrics (U.S. EPA, 2014c, Appendix 2B).224 Focusing only on the 

latter dataset (2009-2011), it can be seen that at monitors just meeting the current standard 

(three-year average fourth-highest daily maximum 8-hour average concentration equal to 0.075 

ppm), W126 index values (in this case three-year averages) varied from less than 3 ppm-hrs to 

approximately 20 ppm-hrs (U.S. EPA, 2014c, Appendix 2B, Figure 2B-3b). At sites with a threeyear average fourth-highest daily maximum 8-hour average concentration at or below a potential 

alternative primary standard level of 70 ppb, three-year W126 index values were above 17 ppmhrs at no monitors, above 15 ppm-hrs at one monitor, and above 13 ppm-hrs at 8 monitors in the 

West and Southwest NOAA climate regions. At sites with a three-year average fourth-highest 

daily maximum 8-hour average concentration at or below a potential alternative primary standard 

level of 65 ppb, three-year W126 index values were above 11 ppm-hrs at no monitors, above 9

ppm-hrs at three monitors, and above 7 ppm-hrs at 9 monitors (distributed across five regions). 

 

224 Appendix 2B in the PA additionally observes that the program implemented for reducing 

precursor emissions, especially NOx, appears to have been an effective strategy for lowering 

both design values and W126 index values.
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The majority of these nine monitoring sites are located in the West and Southwest regions and 

include the states of Arizona, California, Colorado, Nevada, New Mexico, and Utah. At sites 

with a three-year average fourth-highest daily maximum 8-hour average concentration at or 

below a potential alternative primary standard level of 60 ppb, three-year W126 index values 

were at or below 7 ppm-hrs at all monitors (U.S. EPA, 2014c, Figure 2B-3b).

Another analysis in Chapter 2 of the PA presents the data for sets of recent three-year 

periods back to 2006-2008 and indicates that among the counties with O3 concentrations that met 

the current standard, the number of counties with three-year W126 index values above 15 ppmhrs ranges from fewer than 10 to 24 (U.S. EPA, 2014c, Appendix 2B, Figure 2B-9). In general 

during this longer period, W126 index values above 15 ppm-hrs and meeting the current standard 

were predominantly in Southwest region. As the first analysis in Appendix 2B of the PA (for the 

2001-2003 and 2009-2011 periods) indicates, monitors in the West and Southwest tend to have 

higher W126 index values relative to their design values than do monitors in other regions. This 

pattern is noteworthy because the Southwest region has a less dense monitoring network than 

regions in the eastern U.S. (see U.S. EPA, 2014c, Figure 2-1), so that the extent to which this 

pattern occurs throughout these regions is uncertain. 

An additional air quality analysis was performed for this review that is documented in a 

technical memorandum (Wells, 2014). This analysis examines the relationships between O3

levels in terms of the form and averaging time for the current standard (the “4th max” metric) and 

a three-year average, W126-based metric. The first part of the analyses focus on the air quality 

values for the most recent three-year period, 2011-2013. Based on this information, it can be 

seen that at monitors just meeting the current standard (three-year average fourth-highest daily 

maximum 8-hour average concentration equal to 0.075 ppm), W126 index values (in this case 
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three-year averages) varied from less than 3 ppm-hrs to up to 23 ppm-hrs (Figure 5a). At sites 

with a three-year average fourth-highest daily maximum 8-hour average concentration at or 

below a level of 70 ppb (566 monitors distributed across all regions of the U.S.), three-year

W126 index values were above 17 ppm-hrs at no monitors, above 15 ppm-hrs at 4 monitors, and 

above 13 ppm-hrs at 16 monitors (1% of the monitors in full dataset and less than 3% in this 

group). These 16 monitors are located in the Southwest (15 monitors) and West North Central 

NOAA climate regions and include the states of Arizona, Colorado, New Mexico, Utah and 

Wyoming. At sites with a three-year average fourth-highest daily maximum 8-hour average 

concentration at or below a level of 65 ppb (220 monitors distributed across all regions of the 

U.S.225), three-year W126 index values were above 11 ppm-hrs at no monitors, above 7 ppm-hrs

at 15 monitors. These 15 monitoring sites are predominantly located in the West North Central 

and Southwest regions. At all sites with a three-year average fourth-highest daily maximum 8-

hour average concentration at or below a level of 60 ppb, three-year W126 index values were at 

or below 7 ppm-hrs (Wells, 2014, Figure 5b).

Further analysis in the technical memorandum focused on a comparison of monitors with 

a three-year average fourth-highest daily maximum 8-hour average concentration at or below a 

level of 70 ppb and a three-year W126 index values above 13 ppm-hrs for sets of three-year 

periods between 2001-2003 and 2011-2013 (Wells, 2014, Figure 8). This analysis found that the 

number of sites meeting 70 ppb while exceeding 13 ppm-hrs has remained relatively constant 

over the past decade, with these sites consistently being limited to a small number in the West 

and Southwest. In addition, the number of sites meeting both 70 ppb and 13 ppm-hrs has 

 

225 This memo utilizes the same regional specifications as are used in the PA and WREA (e.g., 

U.S. EPA, 2014c, Appendix 2B, Figure 2B-1).
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increased over time, while the number of sites exceeding both 70 ppb and 13 ppm-hrs has 

decreased by a similar amount.

The second part of the analysis in the technical memorandum focused on trends in the 

relationships between O3 levels in terms of the 4th high metric and a three-year average W126 

metric, starting with the 2001-2003 period and ending with the 2011-2013 period. Based on 

analysis of 729 monitors, trends in both the 4

th high metric and the three-year average, W126 

metric showed decreasing values between 2001-2003 and 2011-2013. In addition, the amount of 

year-to-year variability in the two metrics tended to decrease over time with decreasing O3

concentrations, especially for the W126 metric. Most sites in the eastern U.S. and California saw 

large, widespread decreases in both the 4th high metric and the three-year average W126 metrics

over the past decade as a result of regional NOx control programs. In the inter-mountain west, 

where control programs have been more localized, the decreases observed in the 4th high metric 

and three-year average W126 metrics were typically much smaller in magnitude, with a small 

number of sites showing significant increases.

As part of this analysis, regional comparisons were included on the relative changes in 

the relationships between O3 levels in terms of the 4th high metric and a three-year average W126 

metric between the periods of 2001-2003 and 2011-2013. Figure 12 in the technical 

memorandum shows that a positive, linear relationship persists within each region between the 

changes in 4th high and three-year average W126 metrics. Nationally, the three-year average 

W126 metric decreased by approximately 0.7 ppm-hrs per unit ppb decrease in the 4th high 

metric. In addition, the Southwest and West regions, which have the greatest potential for sites to 

measure elevated cumulative, seasonal O3 exposures without the occurrence of elevated daily 
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maximum 8-hour average O3 concentrations, exhibited the greatest response in W126 value 

change per unit change in 4

th high metric (Wells, 2014, Table 6).

The technical memorandum concludes that the 4

th high metric and a three-year average 

W126 metric are highly correlated, as are the relative changes in these two metrics over the past 

decade. In this way, the technical memorandum concludes that that future control programs 

designed to help meet a revised primary O3 standard based on the three-year average of the 4th

highest daily maximum 8-hour concentration are expected to also result in decreases in the 

values of a three-year average W126 metric. 

The above information suggests that depending on the level for a standard of the current 

averaging time and form, the current form and averaging time of the secondary standard can be 

expected to achieve control of cumulative seasonal O3 exposures, providing air quality that may 

meet specific three-year average W126 index values. As discussed above, we recognize 

limitations in the dataset and associated analyses, including those related to monitor coverage, 

which may contribute uncertainties to conclusions related to the relationships described. With 

respect to monitor coverage, the current O3 monitoring network is urban focused, with fewer 

monitors in some parts of the country, particular rural areas of the southwestern and western U.S. 

Because of this, there are potential uncertainties in the extent to which the monitoring 

information discussed above represents air quality patterns and relationships that would occur in 

areas without monitors. There is some information suggesting that there is a potential for 

inconsistencies in the relationship between W126 measures of seasonal O3 concentrations and 

the fourth highest peak O3 concentrations assessed by the current standard averaging time and 

form, but the available data suggest that air quality in areas meeting a standard of the current 

form and averaging time with a level in the range of 65 to 70 ppb would also meet a three-year 
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W126 index value falling in the range of 13 to 17 ppm-hrs, and that to the extent areas need to 

take action to attain a primary standard in the range of 65 to 70 ppb, those actions would also 

improve air quality as measured by the W126 metric.226 To the extent to which the monitoring 

data can be expected to describe future relationships in air quality, we acknowledge potential 

uncertainties in specifying future air quality but note that these uncertainties are limited by the 

fact that the data analysis includes over a decade of O3 measurements, with similar patterns and 

trends observed in air quality over this period of time.

5. Administrator’s Proposed Conclusions 

In considering what revisions to the secondary standard are appropriate, the 

Administrator has drawn on the ISA conclusions regarding the weight of the evidence for a range 

of welfare effects associated with O3 in ambient air, and associated areas of uncertainty; 

quantitative risk and exposure analyses in the WREA for different adjusted air quality scenarios 

and associated limitations and uncertainties; staff evaluations of the evidence, exposure/risk 

information and air quality information in the PA; additional air quality analyses of relationships 

between air quality metrics based on form and averaging time of the current standards and a 

cumulative seasonal exposure index; and CASAC advice; and, public comments received thus 

far in the review. 

As described in section IV.E.1 above, the Administrator concludes it is appropriate to 

continue to use O3 as the indicator for a secondary standard intended to address adverse effects to 

public welfare associated with exposure to O3 alone and in combination with related

 

226 EPA notes that areas can be expected to have air quality at least as good as that specified by 

the primary standard, so to the extent there are inconsistencies between fourth highest peak 

concentrations and W126 values such that some areas meeting a standard of 0.065 to 0.070 ppm 

might be well below the range of 13 to17 ppm-hours, those inconsistencies are less relevant to 

consideration of the appropriate form and level for the secondary standard. 
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photochemical oxidants. In this review, no alternatives to O3 have been advanced as being a 

more appropriate surrogate for ambient photochemical oxidants. Thus, as is the case for the 

primary standard (discussed above in section II.E.1), the Administrator proposes to continue to 

use O3 as the indicator for a standard that is intended to address effects associated with exposure 

to O3 alone and in combination with related photochemical oxidants. In so doing, the 

Administrator recognizes that measures leading to reductions in ecosystem exposures to O3

would also be expected to reduce exposures to other photochemical oxidants.

The Administrator has next considered the array of information with regard to identifying 

policy options for a revised secondary standard for O3 that in her judgment would provide 

appropriate protection for public welfare effects associated with O3 in ambient air. This 

information includes ISA conclusions, WREA analysis findings, staff considerations and 

conclusions in the PA and CASAC advice, as well as the Administrator’s conclusions in the last 

review, with regard to a biologically relevant exposure metric for O3 vegetation-related effects. 

The information also includes PA conclusions and CASAC advice with regard to key aspects of

the definition of such a metric, as summarized in section IV.E.2 and IV.E.3 above. Additionally, 

the Administrator has considered findings of staff evaluations in the PA with regard to potential 

impacts on vegetation and forested ecosystems associated with a range of values for such a 

metric and identified uncertainties and limitations of such information, as summarized in section 

IV.E.2 above. Additionally important to her deliberations here are findings of air quality analyses 

of relationships between the W126-based exposure metric and levels of a standard of the same 

form and averaging time as the current standards, as described in section IV.E.4 above. Based on 

consideration of this array of information, as described below, the Administrator has drawn 

conclusions with regard to policy options for a revised secondary standard. In drawing 
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conclusions on such options, she recognizes that the Act does not require that NAAQS be set at 

zero-risk or background levels, but rather at levels that reduce risk sufficiently to protect public 

welfare from adverse effects.

As an initial matter, the Administrator recognizes the longstanding evidence, described in 

the ISA, of O3 effects on vegetation and associated terrestrial ecosystems. Further, in reaching a 

proposed conclusion on the appropriate form and averaging time for a revised secondary 

standard that would provide increased protection against vegetation-related effects on public 

welfare, the Administrator takes note of the conclusions drawn in the ISA, the PA and by 

CASAC in this review that the scientific evidence continues to demonstrate the cumulative 

nature of O3-induced plant effects and the need to give greater weight to higher concentrations, 

as summarized in sections IV.B.1, IV.D.1, IV.D.2, IV.E.2.a and IV.E.3 above. Based on these 

considerations, the Administrator concurs with the CASAC that a cumulative, seasonal, 

concentration-weighted exposure-based form and averaging time provides the most direct link 

between O3 in ambient air and O3-related effects on vegetation. The Administrator further 

concludes that in judging the extent of public welfare protection that might be afforded by a 

revised standard, it is appropriate to use a cumulative, seasonal concentration-weighted metric.

In identifying a cumulative, seasonal, concentration-weighted metric for use in judging 

public welfare protection, the Administrator gives weight to the PA conclusions regarding 

consideration of a revised secondary standard in terms of the cumulative, seasonal, 

concentration-weighted form, the W126 index. As described in section IV.B.1 above, the ISA 

has recognized the strength of the W126 index in its weighting of potentially damaging O3

concentrations that contributes to the advantages it offers over other weighted cumulative 

indices. The Administrator notes the PA conclusions regarding the W126 metric, specifically use 
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of the three consecutive month period within the O3 season with the maximum index value as the 

seasonal period over which to cumulate hourly O3 exposures and the cumulation of daily 

exposures for the 12-hour period from 8:00 am to 8:00 pm. The Administrator additionally takes 

note of CASAC support for consideration of the W126 index defined in this way and concludes

it is appropriate to use the cumulative seasonal W126-based metric derived in this way. 

In further considering the PA conclusions regarding a revised secondary standard in 

terms of the W126 index, the Administrator takes note of considerations in the PA of a threeyear or single-year evaluation period. Such considerations include the variability in ambient air 

O3 concentrations from year to year, as well as variability and uncertainties related to 

environmental factors that influence the occurrence and magnitude of O3-related effects. The 

Administrator additionally notes the PA observation of greater significance for effects associated 

with multiple-year exposures. Based on these and related considerations described in the PA (and 

summarized in section IV.E.2 above), the Administrator, in identifying a metric for use in 

judging public welfare protection afforded, agrees with the PA conclusion that it is appropriate to 

consider a form that averages W126 index values across three consecutive years, and to do so in 

conjunction with identification of levels for such a form that might be judged to provide the 

appropriate degree of public welfare protection from O3 effects across multiple years. In so 

doing, the Administrator takes note of the ISA conclusions regarding the role of environmental 

factors in variability associated with effects of ambient air O3 and the year-to-year variability 

commonly observed in such environmental factors. Further, the Administrator also recognizes 

uncertainties associated with determining the degree of vegetation impacts for annual effects that 

would be adverse to public welfare. Even in the case of annual crops, the assessment of public 

welfare significance is unclear for the reasons discussed below related to agricultural practices. 
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The considerations identified here lead the Administrator to conclude it is appropriate to use an 

index averaged across three years.

In reaching this conclusion regarding a three-year average metric, the Administrator has 

considered CASAC comments that it favors a W126-based secondary standard with a single year 

form and that its recommended range of levels relates to such a form. The Administrator concurs 

with CASAC that it is important to consider impacts associated with a single year that may be of 

a magnitude concluded to represent an adverse effect on public welfare. The Administrator 

further concludes that such an occurrence can be addressed through use of a three-year average 

metric, chosen with consideration of the relevant factors. As noted above, the Administrator 

gives consideration to the variabilities, as well as the uncertainties, associated with single year 

and multiple year impacts. Based on all of these considerations, the Administrator recognizes 

greater confidence in judgments related to public welfare impacts based on a three-year average 

metric.

Thus, based on all of the above, the Administrator proposes, for purposes of judging the 

extent of public welfare protection that might be afforded by a revised standard and whether it 

meets the appropriate level of protection, to use the average W126 index value across three 

years, with each year’s value identified as that for the three-month period yielding the highest 

seasonal value and with daily O3 exposures within a three-month period cumulated for the 12-

hour period from 8:00 am to 8:00 pm. 

In reaching a conclusion on the appropriate range of W126 index values that describe the 

O3 conditions expected to provide the requisite protection of public welfare, the Administrator 

has given careful consideration to the following: (1) the nature and degree of effects of O3 to the 

public welfare, including what constitutes an adverse effect; (2) the strengths and limitations of 
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the evidence that is available regarding known or anticipated adverse effects from cumulative, 

seasonal exposures, and its usefulness in informing selection of a proposed range; and (3) 

CASAC’s views regarding a range of W126 levels appropriate to consider, as well as on the 

strength of the evidence and its adequacy to inform a range of levels. In this consideration, the 

Administrator recognizes that the choice of a range of W126 index values (and the form of the 

W126 index) that might be expected to provide protection of the public welfare from any known 

or anticipated adverse effects requires judgments about the interpretation of the evidence and 

other information, such as the quantitative analyses of air quality monitoring, exposure and risk, 

that neither overstates nor understates the strengths and limitations of the evidence and 

information nor the appropriate inferences to be drawn as to risks to public welfare. The CAA 

does not require that a secondary standard be protective of all effects associated with a pollutant 

in the ambient air but rather those considered adverse to the public welfare (as described in 

section IV.B.2 above). The Administrator additionally recognizes that there is not a bright line 

clearly directing the choice of a range of W126 index values and that the choice of what is 

appropriate is a public welfare policy judgment entrusted to the Administrator. 

In determining the range of three-year average W126 index values that might be expected 

to provide the appropriate level of public welfare protection, the Administrator first considers the 

nature and degree of effects of O3 on the public welfare. The Administrator recognizes that the 

significance to the public welfare of O3-induced effects on sensitive vegetation growing within 

the U.S. can vary, depending on the nature of the effect, the intended use of the sensitive plants 

or ecosystems, and the types of environments in which the sensitive vegetation and ecosystems 

are located. Any given O3-related effect on vegetation and ecosystems (e.g., biomass loss, visible 

foliar injury), therefore, may be judged to have a different degree of impact on the public 
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depending, for example, on whether that effect occurs in a Class I area, or a residential or 

commercial setting. The Administrator notes that such a distinction is supported by CASAC 

advice in this review. In her judgment, like those of the Administrator in the last review, it is 

appropriate that this variation in the significance of O3-related vegetation effects should be taken 

into consideration in making judgments with regard to the level of ambient O3 concentrations 

that is requisite to protect the public welfare from any known or anticipated adverse effects. As a 

result, the Administrator concludes that of those known and anticipated O3-related vegetation 

and ecosystem effects identified and discussed in this notice, particular significance should be 

ascribed to those that occur on sensitive species that are known to or are likely to occur in 

federally protected areas such as Class I areas227 or on lands set aside by States, Tribes and 

public interest groups to provide similar benefits to the public welfare, for residents on those 

lands, as well as visitors to those areas. 

Likewise, the Administrator also notes that the same known or anticipated O3-induced 

effects occurring in other areas may call for less protection. For example, the maintenance of 

adequate agricultural crop yields is extremely important to the public welfare and is currently 

achieved through the application of intensive management practices. With respect to commercial 

production of commodities, the Administrator notes that judgments about the extent to which O3-

related effects on commercially managed vegetation are adverse from a public welfare 

perspective are particularly difficult to reach, given that the extensive management of such 

 

227 For example, 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).
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vegetation (which, as CASAC noted, may reduce yield variability) may also to some degree 

mitigate potential O3-related effects. The management practices used on these lands are highly 

variable and are designed to achieve optimal yields, taking into consideration various 

environmental conditions. In addition, changes in yield of commercial crops and timber may 

affect producers and consumers differently, further complicating the question of assessing 

overall public welfare impacts. Thus, the Administrator concludes that agricultural crops do not 

have same need for additional protection from the NAAQS as forested ecosystems and, while 

research on agricultural crop species remains useful in illuminating mechanisms of action and 

physiological processes, information from this sector on O3-induced effects is considered less 

useful in informing judgments on what level(s) would be sufficient but not more than necessary 

to protect the public welfare. The CASAC identified a crop RYL benchmark of 5% for the 

median species and indicated they found higher percentages unacceptably high. Although the 

Administrator has not drawn a conclusion with regard to this specific benchmark, the 

Administrator finds the public welfare impacts associated with crop yield loss to be a less 

important consideration in this review for the reasons discussed here, including the extensive 

management of crop yields and the dynamics of agricultural markets, and thus is not focusing on 

crop yield loss in selecting a revised standard. She notes, however, the PA finding that median 

species crop RYL estimates for W126 index values in the PA identified range (17 to 7 ppm-hrs) 

fall below the 5% benchmark emphasized by CASAC for this endpoint. The Administrator also

notes that a standard revised to increase protection for forested ecosystems would also be 

expected to provide some increased protection for agricultural crops. 

The Administrator also recognizes that O3-related effects on sensitive vegetation can 

occur in other areas that have not been afforded special federal protections, ranging from effects 
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on vegetation growing in managed city parks and residential or commercial settings, such as 

ornamentals used in urban/suburban landscaping or vegetation grown in land use categories that 

are heavily managed for commercial production of commodities such as timber. For vegetation 

used for residential or commercial ornamental purposes, the Administrator believes that there is 

not adequate information at this time to establish a secondary standard based specifically on 

impairment of these categories of vegetation, but notes that a secondary standard revised to 

provide protection for sensitive natural vegetation and ecosystems would likely also provide 

some degree of protection for such vegetation.

Based on the above, the Administrator finds that the type of information most useful in 

informing the selection of an appropriate range of protective levels is appropriately focused on 

information regarding exposures and responses of sensitive trees and other native species known 

or anticipated to occur in protected areas such as Class I areas or on lands set aside by States, 

Tribes and public interest groups to provide similar benefits to the public welfare, for residents 

on those lands, as well as visitors to those areas. 

With regard to the available evidence, the Administrator finds the coherence and strength 

of the weight of evidence from the large body of available literature compelling. This evidence 

addresses a broad array of O3-induced effects on a variety of tree species across a range of 

growth stages (i.e., seedlings, saplings and mature trees) using diverse field-based (e.g., free air, 

gradient and ambient) and OTC exposure methods. The Administrator gives particular attention 

to the effects related to native tree growth and productivity, recognizing their relationship to a 

range of ecosystem services, including forest and forest community composition. 

With regard to selection of the values for use with the W126 index for the purpose of 

identifying a range of O3 conditions expected to provide the appropriate level of protection from 
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vegetation effects of particular concern, the Administrator, as an initial matter, takes note of the 

PA conclusion that, with regard to a target level of protection for a revised standard, it is 

appropriate to give consideration to a range of levels from 17 ppm-hrs to 7 ppm-hrs, expressed in 

terms of the W126 index averaged across three consecutive years. As summarized in section 

IV.E.2.b above, this PA conclusion draws heavily on considerations related to estimates of tree 

seedling growth impacts (in terms of relative biomass loss) associated with a range of W126-

based index values developed from the robust E-R functions for 11 tree species. This conclusion 

also gives weight to CASAC comments as to an unacceptably high magnitude of relative 

biomass loss (6%) for the median species and a magnitude of median relative biomass loss on 

which to focus considerations (2%). The Administrator takes particular note of the CASAC view 

of a median species RBL of 6% as unacceptably high.

In considering the basis for the range of W126 index levels identified by the PA, for 

which 17 ppm-hrs is the upper end, the Administrator considers the CASAC advice, including 

their view that a 6% median tree seedling species RBL is unacceptably high, their consideration 

of Table 6-1 in the second draft PA which indicated such a RBL estimate for a W126 index value 

of 17 ppm-hrs, and their consequent lack of support for levels higher than 15 ppm-hrs (Frey, 

2014c, p. iii; U.S. EPA 2014j, Table 6-1). As noted in section IV.E.3 above, revisions to this 

table in the final PA, made in consideration of CASAC comments have resulted in changes to the 

median species RBL estimates such that the median species RBL estimate for a W126 index 

value of 17 ppm-hrs in this table in the final PA (5.3%) is nearly identical to the median species 

estimate for 15 ppm-hrs (the value corresponding to the upper end of the CASAC-identified 

range) in the second draft PA (5.2%) (U.S. EPA, 2014c, Table 6-1; U.S. EPA, 2014j, Table 6-1).
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The Administrator additionally takes note of the PA observations that the number and 

proportion of individual species with RBL estimates at or below 2%, a benchmark given 

emphasis by CASAC, do not vary across W126 index values from 17 ppm-hrs down to 9 ppmhrs (as seen in Table 8 above), providing little distinction with regard to the significance of 

growth impacts for exposures across this large portion of the PA range. The Administrator also 

notes the CASAC recommendation regarding a lowering of the level with consideration of a 

three-year average index; however, the Administrator’s judgments on a three-year average index, 

as described above, focus on confidence in conclusions that might be drawn with regard to single 

as compared to multiple year impacts. For example, the Administrator, while recognizing the 

strength of the evidence with regard to quantitative characterization of O3 effects on growth of 

tree seedlings and crops, in addition to noting the additional difficulties for assessing welfare 

impacts of crops, takes note of the uncertainty associated with drawing conclusions with regard 

to the extent to which small percent reductions in annual growth contribute to adverse effects on 

public welfare and the role of annual variability in environmental factors that affect plant 

responses to O3. Moreover, as explained above, the Administrator concludes that concerns 

related to the possibility of a singly unusually damaging year can be addressed through use of a 

three-year average metric, chosen with consideration of the relevant factors. Accordingly, she 

judges it appropriate to include 17 ppm-hrs, without adjustment, in the range of three-year 

average W126 index values appropriate to consider in determining what secondary standard will 

provide air quality associated with the appropriate level of public welfare protection. She thus 

judges it appropriate to focus on a range for three-year average W126 levels with 17 ppm-hrs at 

the upper end. In so doing, she additionally notes CASAC’s recognition that, within a 
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scientifically appropriate range, the choice of levels is a public policy judgment by the 

Administrator. 

In turning to consideration of the low end for the W126 index range, the Administrator 

considers the full range of W126 levels identified in the PA with regard to the evidence and 

exposure/risk-based information, and associated uncertainties, identified in the PA, as well as 

CASAC advice. The Administrator notes the CASAC policy view regarding protection provided 

for trees and associated ecosystem services from a W126 index value of 7 ppm-hrs, which is 

based on the W126 index value for which the median species estimate falls below 2% RBL. The 

Administrator recognizes, however, as noted above, the greater uncertainty associated with the 

extent to which estimates of benefits in terms of ecosystem services and reduced effects on 

vegetation at lower O3 exposures might be judged significant to the public welfare. 

The Administrator additionally notes the results of the EPA’s quantitative exposure and 

risk assessments for the air quality scenarios for W126 levels at and below 11 ppm-hrs, including 

the relatively small additional benefits and increased uncertainty with the ecosystem services 

estimates in these lower W126 scenarios. With regard to the PA evaluation of RBL estimates, the 

Administrator, while noting the PA observations of similarity in the number of species with less 

than 2% RBL across the W126 range from 17 to 9 ppm-hrs, as stated above, additionally notes 

PA observations of a similar number of studied species with RBL estimates below 5% for W126 

index values of 13 and 11 ppm-hrs. Thus, to the extent that weight is given to the importance of 

5% RBL for individual species, both W126 index values are observed to provide RBL estimates 

below this benchmark. 

With regard to considerations of O3 effects beyond biomass loss in tree seedlings, the 

Administrator takes note of the lack of new quantitative E-R relationships for larger trees 
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growing in the field that would help inform consideration of a standard level within the lower 

part of PA range. Thus, the Administrator recognizes that important uncertainties remain in 

interpreting the quantitative O3-related growth effects for tree seedlings assessed in OTC studies 

for the purpose of characterizing long-term growth effects, and other more subtle but important 

effects on sensitive tree species, natural forests, and forested ecosystems in the broader context 

of protection of public welfare. Additionally, while the Administrator notes that there is evidence 

that O3-related visible foliar injury can occur at such lower levels (below a W126 index value of 

13 ppm-hrs), she recognizes, as summarized in sections IV.C.3.c and IV.D.1 above, the 

significant challenges in judging the extent to which such effects should be considered adverse to 

public welfare, in light of the variability and the lack of clear quantitative relationship with other 

effects on vegetation, as well as the lack of established criteria or objectives that might inform 

consideration of potential public welfare impacts related to this vegetation effect. 

Thus, in the Administrator’s judgment, focus on a three-year average W126 index value 

below 13 ppm-hrs would not give sufficient attention to the important uncertainties and 

limitations inherent in the currently available scientific evidence and in the quantitative 

assessments conducted for the current review. Taking into account the uncertainties that remain 

in interpreting the evidence, the Administrator observes that the likelihood of obtaining benefits 

to public welfare decreases with a standard set below a level of 13 ppm-hrs, while the likelihood 

of requiring reductions in ambient concentrations that go beyond those that are needed to reduce 

adverse impacts to public welfare increases.

Based on the above considerations and based on the entire body of evidence and 

information currently available, the Administrator identifies the range of three-year average 

W126 index values extending from 13 to 17 ppm-hrs as appropriate to consider in identifying the 
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ambient O3 concentrations that would provide the appropriate level of public welfare protection. 

In so doing, the Administrator notes CASAC recognition that a three-year average W126 level of 

13 ppm-hrs may be appropriate depending on consideration of year-to-year variability and policy 

considerations. Thus, based on the discussion above, and with consideration of CASAC advice 

on these issues, the Administrator proposes that ambient O3 concentrations resulting in 

cumulative seasonal O3 exposures of a level within the range from 13 ppm-hrs to 17 ppm-hrs, in 

terms of a W126 index averaged across three consecutive years, would provide the requisite 

protection against known or anticipated adverse effects to the public welfare. The EPA solicits 

comments on levels within this range. 

The Administrator next turns to consideration of policy options for a revised secondary 

standard that would provide this level of protection. The Administrator takes note of staff 

conclusions that it is appropriate to consider a revised secondary standard in terms of the 

cumulative, seasonal, concentration-weighted form, the W126 index. Further, she gives 

extensive consideration to CASAC advice to set such a secondary standard. Such a standard, as 

mentioned above, would be directly linked to O3 exposures to which vegetation are most 

responsive and thus might be expected to provide some confidence that such exposures of 

concern would be controlled.

In considering different policy options for a revised secondary standard, the 

Administrator finds it useful to consider what can be concluded from the available information 

with regard to relationships between air quality characterized in terms of the current form and 

averaging time and also in terms of the W126 metric. She has considered particularly what such 

analyses and relationships indicate with regard to the extent to which W126-based O3

concentrations may be controlled by a revised secondary standard set identical to a revised 
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primary standard (in the range identified in section II.F above). In so doing, she considers the air 

quality analyses in the PA and also the analyses of more recent air quality data summarized in 

the EPA technical memo (described in section IV.E.4 above), focusing particularly on analyses 

examining the W126-based O3 exposure achieved in locations found to meet potential alternative 

standards within the range of primary standards proposed in section II.F above. 

Findings from these analyses of recent O3 measurements and trends in the relationship 

between the current standard and the W126 metrics were substantially similar for the various 

time periods examined over the past decade. There is some information suggesting that there is a 

potential for inconsistencies in the relationship between W126 measures of seasonal O3

concentrations and the fourth highest peak O3 concentrations assessed by the current standard

averaging time and form, but the available data suggest that air quality in areas meeting a 

primary standard in the range of 65 to 70 ppb would also meet a three-year W126 index value 

falling in the range of 13 to 17 ppm-hrs, and that to the extent areas need to take action to attain a 

standard in the range of 0.065 to 0.070 ppm, those actions would also improve air quality as 

measured by the W126 metric. The Administrator also recognizes that the relatively lesser 

density of monitors in rural areas, including in areas of the West and Southwest NOAA climatic 

regions currently meeting the current standard where O3 W126 index values are generally higher, 

makes uncertain the degree to which a revised level for the current standard would provide the 

appropriate degree of protection for vegetation-related effects on public welfare in these areas. 

The Administrator takes note of the PA finding, referenced in section IV.D.3 above, that 

reductions in NOx emissions that occur in urban areas to attain primary standards would also 

have the effect of reducing downwind, rural concentrations over the season. Thus, while the 
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potential for underprotection may exist, depending on the specific levels chosen, the extent of 

such areas and of such a risk is not clear. 

Based on the most recent period of monitoring data, the Administrator notes that in all 

areas in which the O3 concentrations would have met a primary standard with a revised level of 

70 ppb (which includes over 500 monitors distributed across all regions of U.S), the three-year 

average W126 index values are at or below 17 ppm-hrs. In the same areas, only 16 monitors (or 

less than 3% of all monitors in this group, all but one of which is located in the Southwest 

region) had three-year average W126 index values above 13 ppm-hrs. She further notes that in 

all areas in which the O3 concentrations would have met a primary standard with a revised level 

of 65 ppb (which includes 220 monitors distributed across all regions of U.S), the three-year 

average W126 index values are at or below 13 ppm-hrs.

In considering these findings regarding cumulative seasonal O3 exposures in areas that 

would have met a primary standard with a revised level within the proposed range, the 

Administrator also takes note of the high correlation observed between the design value for the 

current secondary (and primary) standard and values for the three-year average, W126 metric, as 

well as the high correlation in the relative changes in these two metrics based on air quality 

analyses of O3 measurements from over the past decade. This finding supports a conclusion that 

the air quality analyses indicate that future control programs designed to reduce O3

concentrations to help meet a revised primary O3 standard that retains the current form and 

averaging time (three-year average of the 4th highest daily maximum 8-hour concentration) 

would also be expected to result in reductions in three-year average, W126 index values. Further, 

she notes the conclusion from the air quality analysis that the Southwest and West regions, which 

have the greatest potential for sites to measure elevated cumulative, seasonal O3 exposures 
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without the occurrence of elevated daily maximum 8-hour average O3 concentrations, exhibited 

the greatest response in W126 index value change per unit change in metric based on the current 

standard form and averaging time. While recognizing the limitations of such analyses in 

projections of future air quality patterns, the Administrator also notes that the time period over 

which the analyses focused involved emissions control programs to achieve O3 reductions such 

that their findings would be expected to be informative of further similar control activities, such 

as those to meet a revised standard with a lower level, in the future. 

Based on the findings from these analyses, the Administrator finds it appropriate to 

consider the policy option of retaining the form and averaging time of the current secondary 

standard and revising the level to within the range of 65 to 70 ppb. In such consideration, the 

Administrator first notes her proposed conclusion that the requisite protection from known or 

anticipated adverse effects to public welfare may be achieved by cumulative, seasonal, 

concentration-weighted O3 concentrations characterized in terms of a W126 index value that falls 

within the range from 13 to 17 ppm-hrs. Her final decision on the W126 index value in this range 

that affords the requisite protection will be based on a series of judgments, as described above. 

Given the focus on tree seedling growth effects in identifying this range, such judgments will 

include the weight to give the evidence of specific vegetation-related effects estimated to result 

from W126 index values within this range, including the objectives for consideration of tree 

species biomass loss estimates in relationship to identified benchmarks (e.g., median species 

RBL of 2% and greater), the weight to give associated uncertainties, including those related to 

the variability in occurrence of such effects in forested areas, the associated ecosystem services 

including those of particular public welfare significance, and judgments on the extent to which 

such effects in forested areas may be considered adverse to public welfare. This final decision 
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will also take into account judgments with regard to the weight to give the evidence and 

quantitative analyses, and associated uncertainties, related to other effects of O3 (summarized in 

sections IV.C, IV.D.1 and IV.E.2 above), particularly including those for which the ISA 

concludes causal or likely causal relationships with O3 exposures. As noted above, a standard 

that provides the appropriate level of protection for growth effects would also be expected to 

provide additional protection for other effects including visible foliar injury, crops and carbon 

storage.

The Administrator notes that based on the above analyses, the proposed range of levels 

for a revised primary standard provide air quality, in terms of three-year average W126 index 

values, of a range at or below the range which the Administrator has identified for consideration 

with regard to the requisite public welfare protection. Thus, depending on final judgments on 

revisions to the primary standard and the requisite protection for the secondary standard, a 

revised secondary standard identical to the revised primary standard may provide sufficient 

protection for public welfare. Therefore, the Administrator proposes to retain the current 

averaging time and form of the secondary standard and revise the level to within the range of 65 

to 70 ppb.

In reaching such a conclusion, the Administrator recognizes that such a strengthening of 

the secondary standard would be expected to provide significant additional protection for public 

welfare, including effects related to vegetation and associated ecosystem services (and others 

discussed above), over that afforded by the current secondary standard. 

Thus, based on her consideration of the full range of information as described above, the 

Administrator judges that ambient O3 concentrations in terms of a three-year average W126 

index value within the range extending from 13 ppm-hrs to 17 ppm-hrs would provide requisite 
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public welfare protection. She further judges that it would be appropriate to achieve that level of 

air quality by retaining the existing averaging time and form, and revising the level to within the 

range of 65 to 70 ppb. In recognition of CASAC’s recommendation and the PA conclusion with 

regard to a distinct secondary standard, the Administrator additionally solicits comment on the 

policy option of revising the form and averaging time for the secondary standard to a W126 

index value, averaged across three years, with each year’s value identified as that for the threemonth period yielding the highest seasonal value and with daily O3 exposures within a threemonth period cumulated for the 12-hour period from 8:00 am to 8:00 pm, and a level within the 

range from 13 ppm-hrs to 17 ppm-hrs. 

F. Proposed Decision on the Secondary Standard

The Administrator proposes to revise the level of the current secondary standard within 

the range of 0.065 to 0.070 ppm. The EPA solicits comments on this proposed revision of the 

secondary standard. Further, the EPA solicits comments on the proposed conclusion that air

quality in terms of a W126 index value, averaged across three consecutive years, within the 

range of 13 ppm-hrs to 17 ppm-hrs would provide requisite protection against known or 

anticipated adverse effects to the public welfare. Additionally, the EPA solicits comments on 

alternative values for a three-year average W126 index for such a purpose within the range 

extending below 13 ppm-hrs down to 7 ppm-hrs. 

The EPA also solicits comments on the alternative approach of revising the secondary 

standard to a cumulative, seasonal, concentration-weighted form, the W126 index based on the 

three consecutive month period within the O3 season with the maximum index value, with daily 

exposures cumulated for the 12-hour period from 8:00am to 8:00pm and with a form that 

averages seasonal W126 values across three consecutive years and a level within the range of 13 
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to 17 ppm-hrs. The EPA additionally solicits comments on such a distinct secondary standard 

with a level within the range extending below 13 ppm-hrs down to 7 ppm-hrs. Further, the EPA 

solicits comments on retaining the current secondary standard without revision, along with the 

alternative views of the evidence that would support retaining the current standard. 

V. Appendix U: Interpretation of the Primary and Secondary NAAQS for O3

A. Background

The EPA is proposing to create Appendix U to 40 CFR part 50 to reflect the proposed 

revisions to the primary and secondary NAAQS for O3 discussed in previous sections of this 

preamble. The proposed Appendix U explains the computations necessary for determining when 

the proposed primary and secondary O3 NAAQS are met at an ambient air quality monitoring 

site, similar to Appendix P to 40 CFR part 50 which deals with interpretation of the O3 NAAQS 

promulgated in 2008. Specifically, the proposed Appendix U addresses data selection 

requirements (section V.B), data reporting and data handling requirements (section V.C), and 

data completeness requirements. The EPA is proposing to maintain the data completeness 

requirements from the previous O3 NAAQS.

Given that the EPA is soliciting public comment on a distinct secondary standard based 

on the W126 metric, section V.D of this preamble contains a discussion of additional data 

handling requirements that would be adopted in Appendix U in the event that the Administrator 

decides to set a distinct secondary standard based on public comments received.

The proposed Appendix U also provides specific requirements for the handling of data 

affected by exceptional events in accordance with 40 CFR 50.14. Section V.E of this preamble 

addresses O3-specific deadlines related to the flagging and submission of demonstrations for 

exceptional event data for the proposed O3 NAAQS.
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B. Data Selection Requirements

The EPA is proposing to clarify which data are to be used in comparisons with the 

NAAQS. First, the EPA proposes to maintain the existing regulatory requirements that only O3

data collected by a federal reference method specified in Appendix D to 40 CFR part 50, or an 

equivalent method designated in accordance with 40 CFR part 53, and meeting all applicable 

monitoring requirements listed in 40 CFR part 58, are eligible for comparison to the proposed O3

NAAQS.

Second, the EPA is proposing in Appendix U that O3 design values are to be calculated 

on a site-level basis. Past practice has been to calculate a design value for each individual O3

monitor. However, this practice could be viewed as inconsistent with the stated purpose of the 

previous O3 data handling appendix, which is to determine “whether the national 8-hour primary

and secondary ambient air quality standards for ozone (O3) specified in §50.15 are met at an 

ambient O3 air quality monitoring site.” (40 CFR part 50, Appendix P, section 1 (emphasis 

added)). Given the level of consistency in the measurement data obtained across the various

federal reference and equivalent O3 monitoring instruments currently in operation (U.S. EPA, 

2013a, section 3.5.2.1), the EPA believes that it would be appropriate to combine data across O3

monitors operating at the same site. Therefore, the EPA is proposing an analytic approach for 

combining data collected from multiple O3 monitors at a site in order to obtain a single set of 

hourly O3 concentration data for each site.

The proposed approach allows the monitoring agencies to designate one monitor as the 

“primary monitor” for each site. In the absence of a primary monitor designation, the primary 

monitor would default to the monitor with the most complete hourly dataset in each year. Once a 

primary monitor has been determined for the site, missing hourly O3 concentrations for the 
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primary monitor would be substituted from any other monitors at the site. In the event of three or 

more monitors operating at the same site, missing hourly O3 concentrations for the primary 

monitor would be substituted with hourly values averaged across the other monitors. The EPA 

notes that at the time of this proposal, there were approximately 20 sites operating two monitors 

simultaneously, and no O3 sites operating three or more monitors simultaneously. This proposed 

approach for combining data across monitors at a site is consistent with the existing approach 

described in Appendix N to Part 50 for the PM2.5 NAAQS. The EPA invites public comment on 

the scientific validity of combining data across O3 monitors, and the merits of the proposed 

approach for combining data across multiple O3 monitors at a site.

Third, the EPA proposes to maintain the existing practice of combining data from nearby

monitoring sites in order to determine a valid design value, known as a “site combination”. Site 

combinations typically involve situations where sites have been replaced or relocated a short 

distance away, and the monitoring agency wishes to combine the data from the two sites in order 

to maintain a continuous data record. The EPA regional offices have approved over 100 site 

combinations for O3 since the promulgation of the 1997 O3 NAAQS. The EPA has maintained 

records of approved site combinations, but these records are not easily accessible by the public.

The EPA proposes to replace the current procedure for approving O3 site combinations 

with a more formal procedure in Appendix U, which would allow states to submit site 

combination requests to the appropriate Regional Administrator. Site combinations may be 

approved by the Regional Administrator, after he or she has determined that the measured air 

quality concentrations do not differ substantially between the two sites. In order to make this 

determination, the Regional Administrator may request additional information from the states, 
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including detailed information on the locations and distance between the two sites, levels of 

ambient concentrations measured at the two sites, and local emissions or meteorology data.

In order to improve transparency, the EPA will make records of all approved site 

combinations available in their Air Quality System (AQS) database, and will update design value 

calculations in AQS so that approved site combinations are implemented. The EPA invites public 

comment on the merits of the proposed process for approving site combinations in order to 

obtain valid design values for the O3 NAAQS.

C. Data Reporting and Data Handling Requirements

The EPA is proposing to maintain the requirement that hourly O3 concentration data be 

reported in parts per million (ppm) to three decimal places. Any decimal digits reported beyond 

three decimal digits will be truncated, consistent with past practice (40 CFR part 50, Appendix P,

section 2.1) and the typical measurement uncertainty associated with most O3 monitoring 

instruments. The proposed Appendix U clarifies that hourly O3 concentrations are to be reported 

in Local Standard Time (LST), consistent with how the values are currently stored in AQS.

The EPA is proposing to maintain the existing procedures for calculating moving 8-hour 

averages from the hourly O3 data (40 CFR part 50, Appendix P, section 2.1), with one minor 

exception. In instances where fewer than six hourly O3 concentrations are available during an 8-

hour period (i.e. less than 75% completeness), the EPA is proposing to substitute zero (i.e. 0.000 

ppm) instead of one half of the O3 monitoring instrument’s minimum detectable limit (MDL) for 

the missing concentration values to determine if the resulting 8-hour average is greater than the 

level of the NAAQS. The purpose of this “data substitution test” is to identify any 8-hour periods 

that do not meet the requirements for a valid 8-hour average, but have reported concentrations 

that are so high that the NAAQS is exceeded even when substituting low values for the missing 
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concentrations. The EPA believes that a constant substitution value of zero is preferable to 1/2 

MDL, which may vary across O3 monitoring instruments. The MDL value for most O3

monitoring instruments is 0.005 ppm, and the 1/2 MDL value is 0.002 ppm (with truncation);

thus, in practice, the difference is slight. The EPA notes that a value of zero micrograms per 

cubic meter (ug/m3) is used in data substitution tests for 24-hour average PM2.5 concentrations,

as specified in Appendix N to 40 CFR part 50. The EPA invites public comment on the merits of 

using zero instead of 1/2 MDL for the 8-hour average data substitution test.

The EPA is proposing new procedures for determining daily maximum 8-hour average 

O3 concentrations. Past practice allows for daily maximum 8-hour average O3 concentrations 

from two consecutive days to have some hours in common (40 CFR part 50, Appendix P, section 

2.1). One implication of this is that an O3 site may be counted as having exceeded the NAAQS 

on two distinct days based on two 8-hour periods having up to 7 hours in common. Theoretically, 

this could result in an annual fourth-highest value greater than the NAAQS based on high 

overnight O3 concentrations occurring only twice during the year.

The EPA performed an analysis based on ambient O3 concentration data from 2004 to 

2013 (Wells, 2014b), which showed that at least one instance of overlapping daily maximum 8-

hour averages occurred at 99.5% of O3 sites during that time period. Overlapping daily 

maximum 8-hour averages were infrequent at most sites, but in some cases, these values 

occurred quite regularly (up to 60 times per year). Overlapping daily maximum 8-hour averages 

contributed to additional exceedances of the proposed O3 NAAQS at 14% of sites for a level of 

0.070 ppm, and at 23% of sites for a level of 0.065 ppm. In addition, 8% of sites had overlapping 

daily maximum 8-hour averages which contributed to a higher annual fourth-highest daily 

maximum value in one or more years. Finally, the analysis showed that O3 sites located in non-
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urban areas affected by long-range transport, especially those sites at higher elevations, were 

most likely to have additional exceedances of the proposed O3 NAAQS due to the occurrence of 

overlapping daily maximum 8-hour averages.

Based on this analysis, the EPA initially concludes that overlapping daily maximum 8-

hour averages are more likely to contribute to additional exceedances of the O3 NAAQS as the 

level of the standard is lowered. Therefore, the EPA is proposing a new procedure for 

determining daily maximum 8-hour average O3 concentrations for the proposed NAAQS that is 

based on 17 consecutive 8-hour periods in each day, beginning with the 8-hour period from 7:00 

AM to 3:00 PM, and ending with the 8-hour period from 11:00 PM to 7:00 AM. Given that 8-

hour averages are stored in the beginning hour of each period, this corresponds to the 8-hour 

averages from 7:00 AM to 11:00 PM.

The rationale for the proposed approach is twofold. First, it avoids any possibility of 

“double counting” exceedances of the NAAQS based on 8-hour periods with one or more hours 

in common, while continuing to make use of all of the hourly concentration data, and keeping the 

calculations simple and straightforward. Second, it is more consistent with the physical processes 

involved in the formation and transport of ground-level O3. Specifically, the chemical reactions 

involved in the formation of new ground-level O3 require sunlight. Therefore, it is appropriate to 

begin the “O3 day” at sunrise, which for simplicity is assumed to be 7:00 AM LST. Similarly, 

any daily maximum 8-hour averages occurring after sunset are assumed to be caused by transport 

of O3 molecules which originated before sunset. Therefore, it is appropriate to end the “O3 day” 

with the 8-hour period beginning at 11:00 PM and ending at 7:00 AM.

In order to accommodate the above proposed approach to the hours considered in an “O3

day”, the EPA is also proposing to modify the requirement for determining whether a daily 
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maximum 8-hour average O3 concentration is valid for assessing compliance with the NAAQS

(40 CFR part 50, Appendix P, section 2.1). The proposed Appendix U requires valid 8-hour 

averages for 13 of the 17 8-hour periods in a day in order to determine a valid daily maximum 

value. The requirement of 13 valid 8-hour averages was chosen because 13/17 is the smallest 

ratio greater than 75%, which is consistent with the long standing requirement of 75% data 

completeness for daily and annual NAAQS-related statistics. In addition, the EPA is proposing 

to maintain the existing provision allowing daily maximum 8-hour averages greater than the 

level of the NAAQS to be considered valid (40 CFR part 50, Appendix P, section 2.1). The EPA 

invites public comment on the merits of the proposed procedure for determining daily maximum 

8-hour average O3 concentrations, and the merits of the proposed daily validity criteria.

Finally, the EPA has included additional language in the proposed Appendix U codifying 

existing data handling procedures for the previous O3 NAAQS. First, the proposed Appendix U 

maintains the provision that hourly O3 concentrations approved under 40 CFR 50.14 as having 

been affected by exceptional events are to be counted as missing or unavailable when calculating 

8-hour averages, and that these concentrations are to be included when determining whether the 

daily validity criteria have been met for a given day. Effectively, this means that it is possible for 

an 8-hour period affected by exceptional events to lack sufficient data to determine an 8-hour 

average, yet the 8-hour period may still be counted toward meeting the daily validity criteria. 

Second, the proposed Appendix U maintains the existing practice of including monitored days 

outside of the O3 monitoring season when determining the annual fourth-highest daily maximum 

value. Finally, the proposed Appendix U maintains the existing practice of using only daily 

maximum 8-hour average values for days where the daily validity criteria have been met when 

determining the annual fourth-highest daily maximum value.
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D. Considerations for the Possibility of a Distinct Secondary Standard

Given that the EPA is soliciting public comment on setting a distinct secondary O3

NAAQS based on the W126 index, the EPA is including a discussion on the data handling 

requirements for a distinct secondary standard. In the event that the Administrator decides to set 

a distinct secondary O3 standard based on the W126 index, the EPA will adopt data handling 

requirements for the secondary standard similar to those proposed during the reconsideration of 

the 2008 O3 NAAQS in 2010 (see 75 FR 3049-3052, January 19, 2010).

Two changes would need to be made to the data handling provisions for the secondary 

standard proposed in 2010 in order to provide consistency with what the EPA is proposing for 

the primary standard in Appendix U. First, the secondary standard design value (i.e. the 3-year 

average of the annual W126 index) would be truncated after the decimal point, instead of being 

rounded to the nearest whole number. Second, paragraph 4(c)(ii) would be modified to read:

“If one or more months during the ozone monitoring seasons of three consecutive years 

has less than 75% data completeness, the three years shall nevertheless be used in the 

computation of a valid design value for the site, if, after adjusting the monthly W126 index 

values for the months with less than 75% data completeness by a factor of 4/3, the resulting 

design value is greater than the level of the standard.”

E. Exceptional Events Information Submission Schedule

States228 are responsible for identifying air quality data that they believe warrant special 

consideration, including data affected by exceptional events. States identify such data by 

flagging (making a notation in a designated field in the electronic data record) specific values in 

 

228 References to “state” are meant to include state, local and tribal agencies responsible for 

preparing and submitting exceptional event documentation as identified in the Exceptional 

Events Rule (72 FR 13560, March 22, 2007).
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the AQS database. States flag the data and submit supporting documentation showing that the 

data have been affected by exceptional events if they wish the EPA to consider excluding the 

data in regulatory decisions, including determining whether or not an area is attaining the 

proposed revised O3 NAAQS, if a different standard is finalized.

All states and areas of Indian country that include areas that could exceed or contribute to 

an exceedance of any revised O3 NAAQS in a nearby area and could therefore be designated as 

nonattainment have the potential to be affected by this rulemaking. Therefore, this action applies 

to all states; to local air quality agencies to which a state has delegated relevant responsibilities 

for air quality management including air quality monitoring and data analysis; and to tribal air 

quality agencies, where appropriate. 

The “Treatment of Data Influenced by Exceptional Events; Final Rule” (72 FR 13560, 

March 22, 2007), known as the Exceptional Events Rule and codified at 40 CFR 50.1, 50.14 and 

51.930, contains generic deadlines for a state to submit to the EPA specified information about 

exceptional events and associated air pollutant concentration data. Under this generic flagging 

schedule in 40 CFR 50.14(c)(2)(iii), a state must initially notify the EPA that data have been 

affected by an event by July 1 of the calendar year following the year in which the event 

occurred. This is done by flagging the data in AQS and providing an initial event description. 

According to the generic demonstration schedule in 40 CFR 50.14(c)(3)(i), the state must also, 

after notice and opportunity for public comment, submit a demonstration to justify any claim 

within 3 years after the quarter in which the data were collected. This section of the regulation 

also states that if the EPA must make a regulatory decision based on the data, the state must 

submit all information to the EPA no later than 1 year before the decision is to be made.
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These generic deadlines in the Exceptional Events Rule apply to data influencing 

redesignation efforts or other regulatory decisions made by the EPA after the EPA promulgates 

initial area designations for a new or revised NAAQS. However, these same generic deadlines in 

the Exceptional Events Rule may not work well with the timing of the initial area designation 

process and schedule under a new or revised NAAQS. Until the EPA promulgates the level and 

form of the NAAQS, a state does not know whether the criteria for excluding data (which are 

tied to the level and form of the NAAQS) were met for a given event. In some cases, the generic 

deadlines, especially the deadlines for flagging some relevant data, may have already passed by 

the time the EPA promulgates the new or revised NAAQS. This scheduling constraint could 

result in the EPA’s being unable to consider whether an exceptional event has affected the data 

relied on for initial area designations and further result in an area being designated nonattainment 

based on data that might have been excluded as having been influenced by an exceptional event 

if the EPA had been able to consider it during the designation process. For this reason, the EPA 

has historically undertaken rulemaking as part of the NAAQS promulgation process to adjust the 

generic deadlines in sections 50.14(c)(2)(iii) and 50.14(c)(3)(i) of the Exceptional Events Rule to 

accommodate the initial area designation process and schedule under a new or revised NAAQS.

The Exceptional Events Rule at section 50.14(c)(2)(vi) indicates “when EPA sets a 

NAAQS for a new pollutant or revises the NAAQS for an existing pollutant, it may revise or set 

a new schedule for flagging exceptional event data, providing initial data descriptions and 

providing detailed data documentation in AQS for the initial designations of areas for those 

NAAQS.” The EPA intends to issue its final action promulgating a revised O3 NAAQS or 

determine that it is not necessary to do so in October 2015. 
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The CAA provides requirements regarding the schedule for initial area designations. 

Section 107(d)(1) of the CAA states that, “By such date as the Administrator may reasonably 

require, but not later than 1 year after promulgation of a new or revised national ambient air 

quality standard for any pollutant under section 109, the Governor of each state shall ... submit 

to the Administrator a list of all areas (or portions thereof) in the State, designating...” those 

areas as nonattainment, attainment, or unclassifiable.229 No later than 120 days prior to 

promulgating designations, the EPA is required to notify states of any intended modifications to 

their designation recommendations as the EPA may deem necessary. Section 107(d)(1)(B)(i) 

further provides, “Upon promulgation or revision of a NAAQS, the Administrator shall 

promulgate the designations of all areas (or portions thereof) ... as expeditiously as practicable, 

but in no case later than 2 years from the date of promulgation. Such period may be extended for 

up to one year in the event the Administrator has insufficient information to promulgate the 

designations.” As described in more detail in section VII.C of this proposal, the EPA intends to 

complete designations for any revised O3 NAAQS promulgated in 2015 following the standard 

2-year process. The EPA is required by Court Order to take final action for this O3 NAAQS 

review no later than October 1, 2015. The EPA does not intend to establish a date earlier than the 

1 year submission period provided in CAA section 107(d)(4); thus, state Governors (and tribes, if 

they choose) would be required to submit their initial designation recommendations for any 

revised NAAQS no later than 1 year after promulgation (i.e., by October 1, 2016, if the EPA 

promulgates a revised NAAQS on October 1, 2015). State Governors (and tribes, if they choose) 

would likely use air quality data from the years 2013 to 2015 as the basis for their 

 

229 While the CAA says “designating” with respect to the Governor’s letter, in the full context of 

the CAA section it is clear that the Governor actually makes a recommendation.
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recommendations. The EPA would notify states and tribes of intended modifications to their 

recommendations no later than June 2017 and the EPA would promulgate initial designations for 

any revised NAAQS in October 2017. We anticipate that the EPA’s notification of intended 

modifications and the final designations would be based on air quality data from the years 2014

to 2016, because air quality data from 2016 is required to be certified by the state no later than 

May 1, 2017, and thus would be available for consideration for purposes of initial area 

designations by October 2017.

As indicated above, and as explained in additional detail in section VII.C of this

preamble, section 107(d)(1)(B) of the CAA allows the Administrator to extend the designations 

schedule for up to 1 year in the event the Administrator has insufficient information to 

promulgate the designations for a newly promulgated NAAQS. If the EPA were to determine 

that it is necessary to extend the schedule for designating areas for a revised O3 NAAQS 

(promulgation in October 2015) from 2017 to 2018, then it is possible that air quality data from 

2017 could be considered for designations. This could raise concerns about whether influences 

from exceptional events in 2017 could be investigated and submitted by the state and reviewed 

by the EPA in sufficient time for consideration during the designation process. 

For purposes of initial designations, where the EPA considers the most recent air quality 

monitoring data in a relatively quick timeframe, the EPA is proposing revisions to the flagging 

and data submission schedule in 40 CFR 50.14 applicable to the initial area designations process. 

The proposed exceptional events schedule is based on following a standard 2 year designation 

process. However, because the CAA also provides for a 3-year process in the event the 

Administrator has insufficient information to promulgate the designations for a newly 

promulgated NAAQS within 2 years and provides for the promulgation of designations as 
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“expeditiously as practicable,” which could include accelerating the designations schedule ahead 

of the 2-year schedule, the proposed exceptional event schedule also includes provisions for both 

an accelerated designations process and a 3-year process. If the EPA were to pursue a 

designations schedule other than a 2- or 3-year process, the EPA would notify the state 

Governors of the intended date for final designations through notification letters, guidance and/or 

Federal Register notices. 

These proposed revised exceptional event scheduling provisions would, if promulgated, 

apply to submission of information supporting claimed exceptional events affecting pollutant 

data for initial area designations under any new or revised NAAQS, including any revised O3

NAAQS promulgated in October 2015. The general data flagging deadlines in the Exceptional 

Events Rule at 40 CFR 50.14(c)(2)(iii) and the general schedule for submission of 

demonstrations at 40 CFR 50.14(c)(3)(i) would continue to apply to regulatory decisions other 

than those related to the initial area designations process under a new or revised NAAQS. The 

EPA believes these proposed revisions to the exceptional events scheduling provisions will 

provide adequate time for states to determine whether data have been influenced by an 

exceptional event, to notify the EPA by flagging the relevant data and providing an initial 

description in AQS, and to submit documentation to support claims for exceptional events. 

Therefore, using the authority provided in CAA section 319(b)(2), the EPA proposes to 

modify the schedule for data flagging and submission of demonstrations for exceptional events 

data considered for initial area designations by replacing the deadlines and information in Table 

1 in 40 CFR 50.14 with the deadlines and information presented in Table 9. The EPA is also 

providing Table 10 to illustrate how the proposed schedule might apply to the designations 

process for any revised O3 NAAQS promulgated in October 2015 or to designations processes 
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for future new or revised NAAQS. The EPA invites comment on these proposed changes, shown 

in Table 9, to the exceptional event data flagging and documentation submission deadlines for 

future new or revised NAAQS, including any revised O3 NAAQS promulgated in 2015. 

Table 9. Proposed Schedule for Exceptional Event Flagging and Documentation 

Submission for Data to be Used in Initial Area Designations 

Exceptional Event / Regulatory 

Action Exceptional Event Deadline Scheduled

Exceptional event data flagging and 

initial description deadline for data 

years 1, 2 and 3.a

If state and tribal initial designation 

recommendations for the new/revised NAAQS 

are due August through January, then the 

flagging and initial description deadline will be 

the July 1 prior to the recommendation 

deadline. If state and tribal recommendations 

for the new/revised NAAQS are due February 

through July, then the flagging and initial 

description deadline will be the January 1 prior 

to the recommendation deadline. 

Exceptional event demonstration 

submittal deadline for data years 1, 2 

and 3.a

No later than the date that state and tribal 

recommendations are due to EPA.

Exceptional event data flagging, 

initial description, and exceptional 

event demonstration submittal 

deadline for data year 4band potential 

data year 5.c

By the last day of the month that is 1 year and 7

months after promulgation of a new or revised 

NAAQS, unless either option a or b applies.

a. If the EPA follows a 3-year designation 

schedule, the deadline is 2 years and 7 

months after promulgation of a new or 

revised NAAQS. 

b. If the EPA notifies the state/tribe via Federal 

Register notice, letter or guidance that it 

intends to complete the initial area 

designations process according to a schedule 

other than a 2-year or 3-year timeline, the 

deadline is 5 months prior to the date 

specified for final designations decisions in 

such EPA notification.

aWhere data years 1, 2, and 3 are those years expected to be considered in state and tribal 

recommendations.

bWhere data year 4 is the additional year of data that the EPA may consider when it makes final 

area designations for the new/revised NAAQS under the standard designations schedule. 

cWhere data year 5 is the additional year of data that the EPA may consider when it makes 

final area designations for the new/revised NAAQS under an extended designations schedule.
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dThe date by which air agencies must certify their ambient air quality monitoring data in AQS 

is annually on May 1 of the year following the year of data collection. The EPA cannot require 

air agencies to certify data prior to this date. In some cases, however, air agencies may choose 

to certify a prior year's data in advance of May 1 of the following year, particularly if the EPA 

has indicated its intent to promulgate final designations in the months of May, June, July or 

August. Exceptional event flagging, initial description, and demonstration deadlines for “early 

certified” data will follow the deadlines for “year 4” and “year 5” data. 
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Table 10. Examples by Month of How the Proposed Revised Schedule for Exceptional Event Flagging and Documentation 

Submission for Data to be Used in Initial Area Designations Would Apply 

Month of NAAQS Promulgation, State and Tribal Recommendation, and Final Designations

Exceptional Event 

/ Regulatory 

Action

Exceptional Event Deadline 

Schedulec

Oct Nov Dec Jan Feb Mar Apr Mayd Jund Juld Augd Sep Oct

Oct 

2015

Nov 

2015

Dec 

2015

Jan 

2016

Feb 

2016

Mar 

2016

Apr 

2016

May 

2016

Jun 

2016

Jul 

2016

Aug 

2016

Sep 

2016

Oct 

2016

Exceptional event 

data flagging and 

initial description 

deadline for data 

years 1, 2, and 3.a

If state and tribal recommendations for 

the new/revised NAAQS are due 

August through January, then the 

flagging and initial description deadline 

will be the July 1 prior to the 

recommendation deadline. If state and 

tribal recommendations for the 

new/revised NAAQS are due Feb

through July, then the flagging and 

initial description deadline will be the 

January 1 prior to the recommendation 

deadline. 

July 1, 

2016

(data 

years 

2013, 

2014, 

2015)

July 1, 

2016

(data 

years 

2013, 

2014, 

2015)

July 1, 

2016

(data 

years 

2013, 

2014, 

2015)

July 1, 

2016

(data 

years 

2013, 

2014, 

2015)

Jan 1, 

2017

(data 

years 

2013, 

2014, 

2015)

Jan 1, 

2017

(data 

years 

2013, 

2014, 

2015)

Jan 1, 

2017

(data 

years 

2013, 

2014, 

2015)

Jan 1, 

2017

(data 

years 

2013, 

2014, 

2015)

Jan 1, 

2017

(data 

years 

2014, 

2015, 

2016)

Jan 1, 

2017

(data 

years 

2014, 

2015, 

2016)

July 1, 

2017

(data 

years 

2014, 

2015, 

2016)

July 1, 

2017

(data 

years 

2014, 

2015, 

2016)

July 1, 

2017

(data 

years 

2014, 

2015, 

2016)

Exceptional event 

demonstration 

submittal deadline 

for data years 1, 2, 

and 3.a

No later than the date that state and 

tribal recommendations are due to EPA.

by Oct 

2016

(data 

years 

2013, 

2014, 

2015)

by Nov 

2016

(data 

years 

2013, 

2014, 

2015)

by Dec 

2016

(data 

years 

2013, 

2014, 

2015)

by Jan 

2017

(data 

years 

2013, 

2014, 

2015)

by Feb 

2017

(data 

years 

2013, 

2014, 

2015)

by Mar 

2017

(data 

years 

2013, 

2014, 

2015)

by Apr 

2017

(data 

years 

2013, 

2014, 

2015)

by May 

2017

(data 

years 

2013, 

2014, 

2015)

by June 

2017

(data 

years 

2014, 

2015, 

2016)

by July 

2017

(data 

years 

2014, 

2015, 

2016)

by Aug 

2017

(data 

years 

2014, 

2015, 

2016)

by Sep 

2017

(data 

years 

2014, 

2015, 

2016)

by Oct 

2017

(data 

years 

2014, 

2015, 

2016)

AQS Q&A and 

data certification

Annually on May 1 of the year 

following the year of data collection May 1 May 1 May 1 May 1 May 1 May 1 May 1 May 1 May 1 May 1 May 1 May 1 May 1

Exceptional event 

data flagging, 

initial description, 

and exceptional 

event 

demonstration 

submittal deadline 

for data year 4band 

potential data year 

5.c

By the last day of the month that is 1 

year and 7 months after promulgation 

of a new or revised NAAQS, unless 

either option a or b applies.

a. If the EPA follows a 3 year 

designation schedule, the deadline is 

2 years and 7 months after 

promulgation of a new or revised 

NAAQS.

b. If the EPA notifies the state/tribe via 

Federal Register notice, letter or 

guidance that it intends to complete 

the initial area designations process 

according to a schedule other than a 

2-year or 3-year timeline, the 

deadline is 5 months prior to the date 

specified for final designations 

decisions in such EPA notification.

by May 

31, 

2017

(data 

year 

2016)

by June 

30, 

2017

(data 

year 

2016)

by July 

31, 

2017

(data 

year 

2016)

by Aug 

31, 

2017

(data 

year 

2016 

and 

potentia

lly 

2017)

by Sep 

30, 

2017

(data 

year 

2016 

and 

potentia

lly 

2017)

by Oct 

31, 

2017

(data 

year 

2016 

and 

potentia

lly 

2017)

by Nov 

30, 

2017

(data 

year 

2016 

and 

potentia

lly 

2017)

by Dec 

31, 

2017

(data 

year 

2016 

and 

potentia

lly 

2017)

by Jan 

31, 

2018

(data 

year 

2017)

by Feb 

28/29, 

2018

(data 

year 

2017)

by Mar 

31, 

2018

(data 

year 

2017)

by Apr 

30, 

2018

(data 

year 

2017)

by May 

31, 

2018

(data 

year 

2017)

State & Tribal Recommendations to EPA 

Oct 

2016

Nov 

2016

Dec 

2016

Jan 

2017

Feb 

2017

Mar 

2017

Apr 

2017

May 

2017

June 

2017

July 

2017

Aug 

2017

Sep 

2017

Oct 

2017
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EPA notifies States/Tribes of intended modifications to 

recommendations (RAs send 120-day letters)

June 

2017

July 

2017

Aug 

2017

Sep 

2017

Oct 

2017

Nov 

2017

Dec 

2017

Jan 

2018

Feb 

2018

Mar 

2018

Apr 

2018

May 

2018

June 

2018

Administrator Promulgates Final Designations

Oct 

2017

Nov 

2017

Dec 

2017

Jan 

2018

Feb 

2018

Mar 

2018

Apr 

2018

May 

2018

June 

2018

July 

2018

Aug 

2018

Sep 

2018

Oct 

2018

a - Where data years 1, 2, and 3 are those years expected to be considered in state and tribal recommendations.

b - Where data year 4 is the additional year of data that the EPA may consider when it makes final area designations for the new/revised NAAQS. 

c - Where data year 5 is the additional year of data that the EPA may consider when it makes final area designations for the new/revised NAAQS under an extended designations schedule.

d -The date by which air agencies must certify their ambient air quality monitoring data in AQS is annually on May 1 of the year following the year of data collection. The EPA cannot require air agencies to 

certify data prior to this date. In some cases, however, air agencies may choose to certify a prior year's data in advance of May 1 of the following year, particularly if the EPA has indicated its intent to promulgate 

final designations in the months of May, June, July or August. Exceptional event flagging, initial description, and demonstration deadlines for "early certified" data will follow the deadlines for "year 4" and “year 

5” data. 
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Additionally, in conjunction with proposing exceptional event schedules related to 

implementing any revised O3 standards, the EPA is also proposing to remove obsolete regulatory 

language associated with exceptional event schedules for historical standards. The EPA expects 

to propose additional revisions to the Exceptional Events Rule in a future notice and comment 

rulemaking effort and will solicit public comment on other, non-schedule related, aspects of the 

Exceptional Events Rule at that time. 

VI. Ambient Monitoring Related to Proposed O3 Standards

A. Background

The EPA is proposing to: revise the state-by-state O3 monitoring seasons; revise the 

PAMS monitoring requirements; revise the FRM for measuring O3; and revise the FEM testing 

requirements. The EPA is also proposing to make additional minor changes to the FEM testing 

requirements for NO2 and particulate matter in part 53 as discussed below.

The EPA is proposing to extend the length of the required O3 monitoring season in some 

states to be appropriate for the O3 NAAQS revision finalized in 2008, as well as a final revised 

O3 standard, if a revision is finalized in 2015. 

The EPA is proposing to make changes to the PAMS monitoring requirements in 40 CFR 

part 58, Appendix D section 5. Section VI.C of this preamble provides background on the current 

PAMS monitoring requirements, recent efforts to re-evaluate the current PAMS requirements, 

and a summary of the proposed PAMS requirement revisions.

The EPA is proposing to revise the FRM to establish a new, additional technique for 

measuring O3 in the ambient air. This new technique is based on nitric oxide-chemiluminescence 

(NO-CL) methodology. Because of the similarity of this new chemiluminescence technique to 

the existing ethylene-chemiluminescence (ET-CL) methodology, the EPA proposes that it be 
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incorporated into the existing O3 FRM, using the same calibration procedure. Appendix D of 40 

CFR part 50 would be revised to include both the original ET-CL as well as the new NO-CL 

methodology. A minor change is proposed to the existing O3 FRM calibration procedure, which 

would be applicable to both of the chemiluminescence FRM methodologies. The proposed 

change in section 4.5.2.3 of the calibration procedure in appendix D provides for more flexibility 

in the range of the linearity test. 

The only substantial changes proposed to the requirements of 40 CFR part 53 are in 

Tables B-1 and B-3 of subpart B. Table B-1 has been updated in recent years with regard to 

FRM and FEM methods for SO2 (74 FR 64877, December 8, 2009) and CO (76 FR 54294, 

August 31, 2011) to be more consistent with current analyzer performance capabilities. Similar 

changes to Table B-1 are proposed here for methods for O3. Modest changes to Table B-3 would 

add new interferent test concentrations specifically for NO-CL analyzers, adding a test for NO2. 

Also, the table would clarify that the existing test concentrations apply to ET-CL O3 analyzers.

In addition, the EPA is making minor additional changes to Part 53 including: 

conforming changes to the FEM testing requirements in Table B-1 and Figure B-5 for NO2; 

extending the period of time for the Administrator to take action on a request for modification of 

a FRM or FEM from 30 days to 90 days; and removing an obsolete provision for manufacturers 

to submit Product Manufacturing Checklists for certain PM monitors.

B. Revisions to the Length of the Required O3 Monitoring Seasons

Unlike the ambient monitoring requirements for other criteria pollutants that mandate 

year-round monitoring, O3 monitoring is only required during the seasons of the year that are 

conducive to O3 formation. These seasons vary in length from place to place as the conditions 

conducive to the formation of O3 (i.e., seasonally-dependent factors such as ambient temperature, 
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strength of solar insolation, and length of day) differ by location.230 In some locations, conditions 

conducive to O3 formation are limited to the summer months of the year. For example, in states 

with colder climates such as Montana and South Dakota, the currently required O3 monitoring 

season is four months long. However, in other states with warmer climates such as California, 

Nevada, and Arizona, the currently required O3 monitoring season is year-round.

231

Based on the O3 NAAQS revision that was finalized in 2008, as well as the proposed 

NAAQS revisions discussed in this rulemaking, the EPA has determined that lengthening the O3

monitoring seasons may be appropriate. Ambient O3 concentrations could approach or exceed 

the level of the 2008 NAAQS, as well as the proposed NAAQS, more frequently and during 

more months of the year. The EPA has done an analysis to address the issue of whether 

extensions of currently required monitoring seasons are appropriate (Rice, 2014). In this

analysis, we determined the number of days where one or more monitors had a daily maximum 

8-hour O3 average equal to or above 0.060 ppm in the months outside the currently-required state 

O3 monitoring season using data from monitors that collected O3 data year-round in 2010-

2013.

232 We find that this level, taking into consideration reasonable uncertainty, serves as an 

appropriate indicator of ambient conditions that may be conducive to the formation of O3

concentrations that approach or exceed the 2008 NAAQS or the proposed 8-hour average range 

of 0.065 to 0.070 ppm. Although we refer to these days as “exceedance days” in the analysis, this

 

230 See 40 CFR part 58 Appendix D, section 4.1, Table D-3 for a table of required O3 seasons.

231 Certain states, such as California and Arizona, have approved shorter seasons for a subset of 

O3 sites, based on Regional Administrator review and approval (see 40 CFR part 58, Appendix 

D, section 4.1(i) for the waiver authority).

232Approximately 800 O3 monitors are currently operated year-round, representing greater than

50% of the total O3 monitoring network of about 1500 monitors. They include monitors that are 

mandated to operate year-round due to the required O3 season and other monitors that are 

voluntarily operated year-round by states and other organizations including EPA-operated 

monitors at Clean Air Status and Trends Network (CASTNET) sites.
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0.060 ppm threshold is simply a conservative benchmark that is below the levels proposed for 

the revised NAAQS. Proposals for revising each state’s required monitoring season are based on 

the observed “exceedance days” where the 8-hour average daily maximum was ≥ 0.060 ppm in 

and surrounding the state. The EPA considered a number of factors including out-of-season 

“exceedance days” either before or after the current O3 monitoring season, the pattern of 

“exceedance days” in the out-of-season months, and regional consistency. We note that seasonal 

O3 patterns vary year-to-year due primarily to highly variable meteorological conditions 

conducive to the formation of early or late season elevated O3 concentrations in some years and 

not others. The EPA believes it is important that O3 monitors operate during all periods when 

there is a reasonable possibility of ambient levels approaching the level of the proposed NAAQS.

The EPA reviewed the year-round, O3 data for 2010 through 2013. A year-round monitor 

was identified as “year-round” if it had at least 20 daily observations in all 12 months, for at least 

1 year of the 4 year period. During the 2010-2013 data period, all states operated a portion of 

their monitoring network outside of their required O3 monitoring season and reported the data to 

the EPA Air Quality System (AQS). The EPA’s analysis found the frequency of observed

“exceedance days” of daily maximum 8-hour average O3 readings of ≥ 0.060 ppm to be quite 

high in several states across the country in months outside of the currently required monitoring 

season. A total of 43 states experienced at least one “exceedance day” outside of their current O3

season; 21 states had “exceedance days” only before the required monitoring season; 4 states had 

“exceedance days” only after the required monitoring season; and 18 states had “exceedance 

days” both before and after the required monitoring season. In some cases, the frequency of 

“exceedance days” before the current O3 season was high, with four states (South Dakota, 
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Colorado, Wyoming, and Utah) experiencing between 31 and 230 out-of-season “exceedance 

days” from 2010 to 2013 at monitors operating year-round.

Basing O3 monitoring season requirements on the goal of ensuring monitoring when 

ambient O3 levels approach or exceed the level of the proposed NAAQS supports established 

monitoring network objectives described in Appendix D of Part 58, including the requirement to 

provide air pollution data to the general public in a timely manner233 and to support comparisons 

of an area's air pollution levels against the NAAQS. The EPA believes that frequency of 

“exceedance days” in which daily maximum of 8-hour O3 levels are observed to be greater than 

or equal to a threshold level of 0.060 ppm in months outside the currently required O3

monitoring season supports the proposed lengthening of the O3 monitoring season requirements 

for certain states.

The operation of O3 monitors during periods of time when ambient levels approach or 

exceed the level of the proposed NAAQS ensures that persons unusually sensitive to O3 are 

alerted to potential levels of health concern allowing them to take precautionary measures. The 

majority of O3 monitors in the U.S. report to AIRNOW,234 as well as to state-operated websites 

and automated phone reporting systems. These programs support many objectives including realtime air quality reporting to the public, O3 forecasting programs, and the verification of real-time 

air quality forecast models.

The specific proposed changes to the required state O3 monitoring seasons are detailed in 

the proposed changes to Table D-3 of 40 CFR part 58, Appendix D (O3 Monitoring Season by 

State). Although 43 states had at least one exceedance day outside the current monitoring season, 

 

233 Public reporting requirements are detailed in 40 CFR part 58 Appendix G, Uniform Air 

Quality Index (AQI) and Daily Reporting.

234 See http://airnow.gov/
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changes are proposed for only 33 of those states. These proposed changes would entail an

increase of 1 month for 23 states (Connecticut, Delaware, District of Columbia, Idaho, Illinois, 

Iowa, Kansas, Maryland, Massachusetts, Minnesota, Missouri, Nebraska, New Hampshire, New 

Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, South Carolina, Northern 

Texas, Virginia, and West Virginia), an increase of one and one half months for Wisconsin, an 

increase of two months for four states (Indiana, Michigan, Montana, and North Dakota), an 

increase of four months for Florida and South Dakota, an increase of five months for Colorado, 

and an increase of seven months for Utah. For Wyoming, we are proposing to add three months 

at the beginning of the season and remove one month at the end of the season, resulting in a net 

increase of two months. Ozone season requirements are currently split by Air Quality Control 

Region (AQCR) in Louisiana and Texas. Included in the state-by-state accounting is the proposal 

to lengthen the required season in the northern part of Texas (AQCR 022, 210, 211, 212, 215, 

217, and 218) by one month. Southern Texas O3 monitors in AQCRs 106, 153, 213, 214, and 

216 would remain on a year-round schedule. In some states with limited available data and few 

exceedance days outside the current season, proposed changes were made by considering 

regional consistency and using supporting information from the surrounding states; these 

changes were all minor, involving the proposed addition of 1 month to the current required 

season in Iowa, Missouri, and West Virginia. 

The EPA solicits comment on the proposed changes to the required O3 monitoring 

seasons. We note that EPA Regional Administrators have previously approved certain deviations 

from the required O3 monitoring seasons through rulemakings (64 FR 3028, January 20, 1999; 

67 FR 57332, September 10, 2002; and 69 FR 52836, August 30, 2004). The current ambient 

monitoring rule, in paragraph 4.1(i) of 40 CFR part 58 Appendix D (71 FR 61319, October 17, 
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2006), allows the EPA Regional Administrators to approve changes to the O3 monitoring season 

without rulemaking. The EPA is retaining the rule language allowing such deviations from the 

required O3 monitoring seasons in the proposed revision to paragraph 4.1(i) of 40 CFR part 58, 

Appendix D. The proposed changes to O3 monitoring season requirements, if finalized, will 

revoke previous Regional Administrator-granted waiver approvals. As appropriate, monitoring 

agencies could seek new waivers. Post-final rule requests submitted along with relevant 

supporting information by states for monitoring season waivers from the revised requirements 

will be reviewed by Regional Administrators using, at a minimum, occurrences of the moderate 

AQI level, the frequency of out-of-season O3 NAAQS exceedances, and regional consistency.

Any deviations based on the Regional Administrator’s waiver of requirements must be described 

in the state’s annual monitoring network plan and updated in the AQS.

Current regulations permit O3 monitors located at NCore multi-pollutant stations to be 

counted toward meeting minimum network monitoring requirements. The NCore network 

requirements were promulgated in the October 17, 2006 (71 FR 61317) revisions to ambient 

monitoring regulations in order to build a long-term, nationwide network that supports multiple 

objectives including air quality trends analyses, model evaluation, ecosystem studies, and 

assessment of transport between urban and rural areas. In the 2006 rulemaking, the EPA did not 

propose a different O3 monitoring season for NCore stations.

NCore stations are required to operate a full suite of gaseous and particulate matter 

monitors as well as basic meteorology to support the objectives. Given the potential value of 

NCore data to support year-round scientific studies, the EPA believes that it is appropriate to 

require O3 monitors at NCore stations to be operated year-round. Accordingly, the EPA proposes 

that the required monitoring season for NCore stations be January through December regardless 
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of the length of the required O3 monitoring season for the remainder of the SLAMS (State and 

Local Air Monitoring Stations) monitors within a state.

The EPA has estimated the cost of the proposed changes to the O3 seasons. The results 

are detailed in the EPA ICR #2313.03 and summarized in Section VIII.B., “Paperwork 

Reduction Act”. The estimated cost is $1,668,433 which is about 7% of the total average annual 

cost of $24,115,182 for the national O3 monitoring network. This estimate is based on the current 

requirements in 40 CFR part 58 and the proposed requirements in this rule. We note however, 

that greater than 50% of the monitors are currently operated year-round due to existing 

requirements, as well as other monitors that are voluntarily operated year-round by the states. 

Taking into consideration the number of year-round O3 monitors that are operated due to existing 

requirements, as well as on a discretionary basis by states, the incremental cost of these proposed 

changes is reduced from $1,668,433 to approximately $230,000, which is less than 1% of the 

total average annual cost of the national O3 monitoring network. 

Considering the timing of this proposal and the final rulemaking (court ordered deadline 

of October 1, 2015) and associated burden on state/local monitoring agencies, we propose that 

implementation of the revised O3 seasons become effective at SLAMS (including NCore sites) 

on January 1, 2017. The EPA is proposing to add paragraph 58.13 (g) of 40 CFR part 58 to 

require that monitors operating under the requirements of section 4.1 of 40 CFR part 58, 

Appendix D operate on the applicable required O3 monitoring seasons effective January 1, 2017 

as listed in Table D-3 of appendix D to this part. We solicit comment on whether the revised 

seasons could be implemented beginning January 1, 2016 for all monitors or for a subset of 

monitors, such as those currently operating year-round or on a schedule that corresponds to the 
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proposed O3 season. If we determine, based on any such comments that implementation could 

occur earlier in such cases, we could proceed to final action requiring earlier implementation. 

C. Revisions to the Photochemical Assessment Monitoring Stations (PAMS) 

Section 182 (c)(1) of the CAA required the EPA to promulgate rules for enhanced 

monitoring of O3, oxides of nitrogen, and VOCs for nonattainment areas classified as serious (or 

above) to obtain more comprehensive and representative data on O3 air pollution. In addition, 

Section 185B of the CAA required the EPA to work with the National Academy of Sciences 

(NAS) to conduct a study on the role of O3 precursors in tropospheric O3 formation and control.

In 1992, the NAS issued the report entitled, “Rethinking the Ozone Problem in Urban and 

Regional Air Pollution”, (NAS, 1991).

In response to the CAA requirements and the recommendations of the NAS report, on 

February 12, 1993 (58 FR 8452), the EPA revised the ambient air quality surveillance 

regulations to require PAMS in each O3 nonattainment area classified as serious, severe, or 

extreme (“PAMS areas”). 235 As noted in EPA’s Technical Assistance Document (TAD) for 

Sampling and Analysis of Ozone Precursors (U.S. EPA, 1998), the objectives of the PAMS 

program are to: 1) provide a speciated ambient air database which is both representative and 

useful in evaluating control strategies and understanding the mechanisms of pollutant transport

by ascertaining ambient profiles and distinguishing among various individual VOCs; 2) provide 

local, current meteorological and ambient data to serve as initial and boundary condition 

information for photochemical grid models; 3) provide a representative, speciated ambient air 

database which is characteristic of source emission impacts to be used in analyzing emissions 

 

235 Additional information on the O3 designation process can be obtained at EPA’s O3

designations webpage at http://www.epa.gov/groundlevelozone/designations/
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inventory issues and corroborating progress toward attainment; 4) provide ambient data 

measurements which would allow later preparation of unadjusted and adjusted pollutant trends 

reports; 5) provide additional measurements of selected criteria pollutants for 

attainment/nonattainment decisions and to construct NAAQS maintenance plans; and 6) provide 

additional measurements of selected criteria and non-criteria pollutants to be used for evaluating 

population exposure to air toxics as well as criteria pollutants.

The original PAMS requirements called for two to five sites per area depending on the 

area’s population. Four types of PAMS sites were identified including upwind (Type 1), 

maximum precursor emission rate (Type 2), maximum O3 (Type 3), and extreme downwind 

(Type 4) sites. Each PAMS site was required to measure O3, NO, NO2, speciated VOCs, selected 

carbonyl compounds, and selected meteorological parameters. In addition, upper air 

meteorological monitoring was required at one site in each PAMS area.

In the October 17, 2006 monitoring rule (71 FR 61267), the EPA revised the PAMS 

requirements to only require two PAMS sites per PAMS area.

236 The intent of the revision was to 

“allow PAMS monitoring to be more customized to local data needs rather than meeting so many 

specific requirements common to all subject O3 nonattainment areas; the PAMS changes would 

also give states the flexibility to reduce the overall size of their PAMS programs—within 

limits—and to use the associated resources for other types of monitoring they consider more 

useful.” In addition to reducing the number of required sites per PAMS area, the 2006 revisions 

also limited the requirement for carbonyl measurements (specifically formaldehyde, 

acetaldehyde, and acetone) to areas classified as serious or above for the 8-hour O3 standard. 

 

236 One Type 2 site and either a Type 1 or a Type 3 site are currently required.
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This change was made in recognition of carbonyl sampling issues which were believed to cause 

significant uncertainty in the measured concentrations.

Twenty-two areas were classified as serious or above O3 nonattainment at the time the 

PAMS requirements were promulgated in 1993. On July 18, 1997 (62 FR 38856), the EPA 

revised the averaging time of the O3 NAAQS from a 1-hour averaging period to an 8-hour 

averaging period. On June 15, 2005 (70 FR 44470), the EPA revoked the 1-hour standard in 

most areas of the country; however, PAMS requirements were identified as requirements that 

had to be retained in the anti-backsliding provisions237 included in that action. Therefore, PAMS 

requirements continue to be applicable to areas that were classified as serious or above 

nonattainment for the 1-hour O3 standard as of June 15, 2004. Currently, 25 areas are subject to 

the PAMS requirements with a total of 75 sites. As will be discussed in detail later, the current 

PAMS sites are concentrated in the North East and California with relatively limited coverage in 

the rest of the country (Cavender, 2014). 

As discussed above, the first PAMS sites began operation in 1994, and have been in 

operation for over 20 years. Many changes have occurred during that time that have changed the 

O3 problem in the U.S. as well as our understanding of it. The O3 standard has been revised 

multiple times since the PAMS program was first implemented. On July 18, 1997, the EPA 

revised the O3 NAAQS to a level of 0.08 ppm, with a form based on the 3-year average of the 

annual fourth-highest daily maximum 8-hour average O3 concentration. On March 28, 2008 (73 

FR 16436), the EPA revised the O3 standard to a level of 0.075 ppm, with a form based on the 3-

year average of the annual fourth-highest daily maximum 8-hour average O3 concentration.

These changes in the level and form of the O3 NAAQS, along with notable decreases in O3 levels 

 

237 Refer to 40 CFR part 51.905
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in most parts of the U.S., have changed the landscape of the O3 problem in the U.S. At the time 

of the first round of designations for the 8-hour standard (June 15, 2005), only five areas were 

classified as serious or above for the 8-hour standard as compared to 22 areas that were classified 

as serious or above for the 1-hour standard.238 While the number of serious and above areas 

decreased, the number of nonattainment areas remained nearly the same. In addition, much of the 

equipment used at PAMS sites is old and in need of replacement. New technologies have been 

developed since the inception of the PAMS program that should be considered for use in the 

network. For these reasons, the EPA determined that it would be appropriate to re-evaluate the 

PAMS program and associated requirements in light of current O3 issues.

In 2011 (U.S. EPA, 2011c), the EPA initiated an effort to re-evaluate the PAMS 

requirements in light of changes in the needs of PAMS data users and the improvements in 

monitoring technology. The EPA consulted with CASAC’s, Air Monitoring and Methods 

Subcommittee (AMMS) to seek advice on potential revisions to the technical and regulatory 

aspects of the PAMS program; including changes to required measurements and associated 

network design requirements. The EPA also requested advice on appropriate technology, 

sampling frequency, and overall program objectives in the context of the most recently revised 

O3 NAAQS and changes to atmospheric chemistry that have occurred over the past 10-15 years 

in the significantly impacted areas. The CASAC AMMS met on May 16 and May 17, 2011, and 

provided a report with their advice on the PAMS program on September 28, 2011 (U.S. EPA, 

2011c). In addition, the EPA met multiple times with the National Association of Clean Air 

Agencies (NACAA) Monitoring Steering Committee (MSC) to seek advice on the PAMS 

 

238 PAMS requirements continue to apply to all areas classified as serious or above as of June 15, 

2005 due to anti-backsliding provisions of 40 CFR 51.905.
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program. The MSC includes monitoring experts from various state and local agencies actively 

engaged in ambient air monitoring and many members of the MSC have direct experience with 

running PAMS sites. As discussed in more detail in the appropriate sections below, the EPA took 

into consideration advice from the CASAC AMMS and the MSC in proposing changes to the 

PAMS requirements. 

Based on the findings of the PAMS evaluation and the consultations with the CASAC 

AMMS and NACAA MSC, the EPA is proposing to revise several aspects of the PAMS 

monitoring requirements including changes in 1) network design, 2) VOC sampling, 3) carbonyl 

sampling, 4) nitrogen oxides sampling, and 5) upper air meteorology measurements. The 

following paragraphs describe the changes being proposed including the rationale for the 

proposed changes. Timing and other implementation issues associated with these proposed 

changes are discussed at the end of this section. 

1. Network Design

As discussed above, the current PAMS network design calls for two sites (a Type 2, and a 

Type 1 or Type 3) per PAMS area. In their report (U.S EPA, 2011c), the CASAC AMMS found

“that the existing uniform national network design model for PAMS is outdated and too resource 

intensive,” and recommended “that greater flexibility for network design and implementation of 

the PAMS program be transferred to state and local monitoring agencies to allow monitoring, 

research and data analysis to be better tailored to the specific needs of each O3 problem area.” 

While stating that the current PAMS objectives were appropriate, the AMMS report also stated 

that “objectives may need to be revised to include both a national and regional focus because 

national objectives may be different from regional objectives.” The NACAA MSC also advised 
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the EPA that the existing PAMS requirements were too prescriptive and may hinder state efforts 

to collect other types of data that were more useful in understanding their local O3 problems.

The EPA agrees with CASAC that the PAMS objectives include both local and national 

objectives, and believes that the current PAMS network design is no longer suited for meeting 

either sets of objectives. As part of the PAMS evaluation, it was determined that at the national 

level the primary use of the PAMS data has been to evaluate photochemical model performance.

Due to the locations of the current PAMS areas and the current network design, existing PAMS 

sites are clustered along the northeast and west coasts leading to significant redundancy in these 

areas and very limited coverage throughout the remainder of the country (Cavender, 2014). The 

resulting uneven spatial coverage greatly limits the value of the PAMS data for evaluation of 

model performance. CASAC (U.S. EPA, 2011c) noted the spatial coverage issue and advised 

that EPA should consider requiring PAMS measurements in areas in addition to “areas classified 

as serious and above for the O3 NAAQS to improve spatial coverage.” The EPA also agrees with 

CASAC and the NACAA that the PAMS requirements should be revised to provide monitoring 

agencies greater flexibility in meeting local objectives. 

The EPA is proposing changes to the network design requirements that we believe will 

better serve both national and local objectives. The EPA is proposing a two part network design. 

The first part of the design includes a network of fixed sites (required PAMS sites) intended to 

support O3 model development and the tracking of trends of important O3 precursor 

concentrations. The second part of the network design includes monitoring agency directed 

Enhanced Monitoring Plans which allow monitoring agencies the needed flexibility to 

implement additional monitoring capabilities to suit the needs of their area. 
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The EPA considered a number of options to revise the fixed site portion of the network 

design (Cavender, 2014). An initial option considered was to require all NCore sites to make 

PAMS measurements regardless of O3 attainment status. This option would take advantage of the 

existing NCore infrastructure and would result in a relatively wide geographic distribution of 

sites. However, it was noted that this option would place some PAMS measurements in areas 

with relatively low O3 levels and would also result in a network of approximately 80 required 

sites, which would strain existing resources with a somewhat larger network than the current 

situation, and could make it difficult to also implement the desired state-directed Enhanced 

Monitoring Plans. The second option considered was to require only NCore sites in O3

nonattainment areas to collect PAMS measurements. This option would provide the benefits 

discussed above for collecting PAMS measurements at existing NCore sites. This option would 

also reduce the total number of sites required and focus efforts in areas with higher, nonattaining, levels of O3. The final option considered would add a population limit in addition to 

the consideration of O3 attainment status at NCore sites. An illustration of this example would be 

a PAMS requirement that applied only to NCore sites in O3 nonattainment areas with a 

population greater than a given threshold, for example, Core Based Statistical Areas with 

1,000,000 people or more. This approach would continue the current practice of focusing PAMS 

resources in areas of elevated O3 readings with an additional consideration that measurements in 

these larger population areas would be sufficient to characterize O3 formation on a national basis.

After considering the above options as well as the comments of CASAC and NACAA, 

the EPA believes that an approach focused primarily on the use of the existing NCore sites in O3

nonattainment areas provides an appropriate balance to the consideration of O3 levels as well as 

population, noting that a majority of NCore sites are already located in the larger urban areas of 
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each state. Accordingly, the EPA is proposing to require PAMS measurements at any existing 

NCore site in an O3 nonattainment area (either based on the 2008 O3 NAAQS or the 2015 O3

NAAQS if finalized) in lieu of the current PAMS network design requirements.239 The NCore 

network is a multi-pollutant monitoring network consisting of 80 sites (63 urban, 17 rural) and is 

intended to support multiple air quality objectives including the development and model 

evaluation of photochemical models (including both PM2.5 and O3 models), and the tracking of 

regional precursor trends. NCore sites are sited in typical neighborhood scale locations which are 

more suitable than source impacted locations for evaluation of grid models typical of current 

photochemical models and tracking of trends in pre-cursor concentrations. The EPA believes 

NCore sites are well suited for O3 model development and evaluation. 

The proposal to require PAMS measurements at existing NCore sites in O3 nonattainment 

areas would replace the existing PAMS network design.240 This change would keep roughly the 

same number of required PAMS sites while improving spatial coverage (Cavender, 2014). Based 

on the range of the O3 NAAQS being proposed today and current O3 design value estimates

(based on 2011-2013 air quality data), the number of required sites is estimated to be between 48

and 65, which compares to 50 currently required sites, and 75 currently operating sites. Potential

redundancy in the existing network would be reduced while important network coverage in the 

Southeast and Midwest would be added. The improved spatial coverage will also improve the 

 

239 Although enhanced monitoring for O3, oxides of nitrogen, and VOCs is specifically required 

for areas classified at least serious for the O3 NAAQS by section 182(c)(1) of the CAA, the EPA 

has concluded that requiring enhanced monitoring for all O3 nonattainment areas is appropriate 

for the purposes of monitoring ambient air quality and better understanding O3 pollution.

240 While the EPA is proposing to replace the multi-site design, monitoring agencies would be 

encouraged to identify the type of PAMS site the NCore site represents. In most cases, NCore 

sites would likely be classified as either a Type 2 or Type 3 site. In limited situations, rural 

NCore sites might be subject to these proposed requirements, in which case, these sites would 

likely be either Type 1 or Type 4 sites. 
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EPA’s ability to track trends in precursor concentrations regionally. The EPA notes that in 

limited situations, an O3 nonattainment area may not have an NCore site and in those cases, the 

area would only be subject to the requirement for an Enhanced Monitoring Plan as discussed in 

more detail below. The EPA believes that the network coverage provided by existing NCore sites 

in O3 nonattainment areas would be adequate for the national PAMS objectives discussed above, 

and that requiring PAMS sites, in addition to Enhanced Monitoring Plans, in those O3

nonattainment areas without NCore sites would not substantially improve the network coverage.

The EPA notes that the proposed network design change would provide significant cost 

efficiencies. By adding PAMS measurements to existing NCore sites, the PAMS network would 

be taking advantage of existing infrastructure and measurements currently being collected at 

NCore sites. NCore sites already have the larger, climate-controlled shelters that are necessary to 

operate the automated gas chromatographs (“auto-GCs”) used to collect speciated VOCs. In 

addition, existing NCore sites currently collect data on many of the required PAMS 

measurements including O3, CO, total reactive nitrogen (NOy), and meteorological 

measurements including wind speed and direction, temperature, and relative humidity.

While the EPA believes these proposed changes will result in fixed network cost savings 

for the overall network on a national basis, individual monitoring agencies may see either an 

increase or a decrease in burden as a result of these proposed changes. Monitoring agencies in O3

nonattainment areas who are not currently affected by the existing PAMS requirements would be 

required to add PAMS measurements to their existing NCore sites, while several monitoring 

agencies with existing PAMS sites would not be required to continue PAMS monitoring if these 
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proposed requirements are promulgated.241 As discussed later in this preamble, the EPA is 

proposing a staggered compliance schedule for the proposed PAMS requirements in recognition 

of the need for capital investment and staff training at these sites. 

The EPA recognizes that in limited situations, existing NCore sites may not be the most 

appropriate locations for making PAMS measurements. For example, an existing PAMS site in 

an O3 nonattainment area may be sited at a different location than the existing NCore site. In this 

case, it may be appropriate to continue monitoring at the existing PAMS site to support ongoing 

research and to maintain trends information. To account for these situations, the EPA is also 

proposing to provide the EPA Regional Administrator the authority to approve an alternative 

location for a required PAMS sites where appropriate.

The EPA seeks comment on the network design revision proposed above, the 

requirement for PAMS measurements at NCore sites in O3 nonattainment areas, and the removal 

of current multi-site PAMS network design requirements. The EPA also solicits comment on 

whether, instead of requiring PAMS measurements at all NCore sites in nonattainment areas, we 

should instead adopt one of the other options discussed above, for example, using both 

attainment status and population thresholds, that may result in a fixed PAMS monitoring network 

that is either smaller or larger than what will result from the proposed requirement.

The second part of the proposed PAMS network design includes monitoring agency 

directed enhanced O3 monitoring activities intended to provide data needed to understand an 

area’s specific O3 issues. To implement this part of the PAMS network design, the EPA is 

proposing to add a requirement for states with O3 nonattainment areas to develop an “Enhanced 

 

241 Monitoring agencies would be able to seek approval to shut down non-required PAMS sites at 

their discretion pursuant to the requirements detailed in 40 CFR 58.14.
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Monitoring Plan.” These Enhanced Monitoring Plans, which are to be submitted as part of their 

required Annual Monitoring Network Plan (40 CFR 58.10), would be reviewed and approved by 

the EPA Regional Administrator as part of the annual plan review process. The purpose of the 

Enhanced Monitoring Plan is to improve monitoring for ambient concentrations of O3, NOx/NOy, 

VOCs, and meteorology. The goal of the Enhanced Monitoring Plan is to allow monitoring 

agencies flexibility in determining and collecting the data they need to understand their O3

problems, consistent with this purpose and the advice obtained from the CASAC AMMS and the 

NACAA MSC. Types of activities that might be included in the Enhanced Monitoring Plan

include (but are not limited to) additional PAMS sites (e.g., upwind or downwind sites), 

additional O3 and NOx monitoring, ozonesondes or other aloft measurements, rural 

measurements, mobile PAMS sites, additional meteorological measurements, and episodic or 

intensive studies. The savings from a smaller less costly fixed network of required PAMS sites 

would be available for re-investment in the development and implementation of the proposed 

Enhanced Monitoring Plans.

2. Speciated VOC Measurements

Measurement of speciated VOCs important to O3 formation is a key aspect of the PAMS 

program. Currently, the existing PAMS requirements allow for a number of options in measuring 

speciated VOCs at PAMS sites which include 1) hourly measurements using an auto-GC, 2) 

eight 3-hour samples daily using canisters, or 3) one morning and one afternoon sample with a 3-

hour or less averaging time daily using canisters plus continuous total non-methane hydrocarbon 

(TNMHC) measurements.

The EPA believes that the options provided for VOC measurements limit the comparative 

value of the data being collected, and is proposing to require instead that all required PAMS sites 
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measure and report hourly speciated VOCs using an auto GC. More complete and consistent 

speciated VOC data nationally would better help meet certain objectives of the PAMS program 

described above (e.g., a speciated ambient air database useful in evaluating control strategies, 

analyzing emissions inventory issues, corroborating progress toward attainment, and evaluating 

population exposure to air toxics). Furthermore, as noted by the CASAC AMMS, hourly VOC 

data are “particularly useful in evaluating air quality models and performing diagnostic emission 

attribution studies. These data can be provided on a near real-time basis and presented along with 

other precursor species (e.g., oxides of nitrogen and carbon monoxide) collected over similar 

averaging times.” Longer time-averaged data are of significantly lower value for model 

evaluation.242 In addition, creating consistent monitoring requirements across the network will 

provide better data for analyzing regional trends and spatial patterns. 

At the time the original PAMS requirements were promulgated, the canister options were 

included because the EPA recognized that the technologies necessary to measure hourly average 

speciated VOCs concentrations were relatively new and may not have been suitable for broad 

network use. At that time, gas chromatographs designed for laboratory use were equipped with 

auto-samplers designed to “trap” the VOC compounds from a gas sample, and then “purge” the 

compounds onto the GC column. The EPA did not believe that auto-GCs were universally 

appropriate due to the technical skill and effort necessary at that time to properly operate an autoGC.

While the basic principles of auto-GC technology have not changed, the hardware and 

software of modern auto-GCs are greatly improved over that available at the time of the original 

 

242 Data of longer than a 1-hour average are often not used in model evaluations due to the 

complexity of trying to accommodate non-hourly averaged data.
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PAMS requirements. Based on advice from the CASAC AMMS, the EPA has initiated an 

evaluation of current auto-GCs potentially suitable for use in the PAMS network. Based on the 

preliminary results, the EPA believes that typical NCore site operators, with appropriate training, 

will have the skill necessary to operate a modern auto-GC successfully. Considering the 

advances in auto-GC technology, the added value obtained from hourly data, and the proposed 

move of PAMS measurements to NCore sites in O3 nonattainment areas, the EPA is proposing to 

require hourly speciated VOC sampling at all PAMS sites. The EPA notes that this proposed 

requirement would effectively prevent the use of canisters to collect speciated VOCs at the 

required PAMS sites. However, canister sampling may continue to be an appropriate method for 

collecting speciated VOCs at other locations as part of the proposed Enhanced Monitoring Plans. 

While the EPA believes that the proposed transition to hourly speciated VOC sampling is 

the appropriate strategy to take advantage of improved technology and to broaden the utility of 

collected data, we are also mindful of the additional rigidity that the proposed mandatory use of 

auto-GCs may have for monitoring agencies, especially those that have experience with and have 

established effective and reliable canister sampling programs. Therefore, the EPA is requesting 

comment on the proposed requirement for hourly VOC sampling as well as the range of 

alternatives that might be appropriate in lieu of a strict requirement. Such alternatives could 

range from a more formal process where monitoring agencies could request a Regional 

Administrator-granted waiver from the hourly VOC requirements through the Annual 

Monitoring Network Plan process to collect canister-based speciated VOC data, to a more 

flexible set of alternatives where canister sampling could be retained based on each monitoring 

agency’s evaluation of programmatic needs as well as their own logistical and technical 

capabilities.
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3. Carbonyl Sampling

Carbonyls include a number of compounds important to O3 formation that cannot 

currently be measured using the auto-GCs or canisters used at PAMS sites to measure speciated 

VOCs.243 The current method for measuring carbonyls in the PAMS program is Compendium 

Method TO-11A (U.S. EPA, 1999). In this method, carbonyl compounds are adsorbed and 

converted into stable hydrazones using dinitrophenylhydrazine (DNPH) cartridges. These 

cartridges are then analyzed for the individual carbonyl compounds using liquid chromatography 

(LC) techniques. Three carbonyls (formaldehyde, acetaldehyde, and acetone) are currently 

required to be measured in the PAMS program.

In 2006, the EPA revised the PAMS requirements such that carbonyl sampling was only 

required in areas classified as serious or above nonattainment for O3 under the 8-hour O3

standard which effectively reduced the applicability of carbonyl sampling to a few areas in 

California. This change was made in recognition that there were a number of issues with Method 

TO-11A that raised concerns with the uncertainty in the carbonyl data being collected. These 

issues include interferences (humidity and O3) and breakthrough (i.e., overloading of the DNPH 

cartridge) at high concentrations. While solutions for these issues have been investigated, these 

improvements have not been incorporated into Method TO-11A.

A recent evaluation of the importance of VOCs and carbonyls to O3 formation 

determined that carbonyls, especially formaldehyde, are very important to O3 formation

(Cavender, 2013). CASAC AMMS (U.S. EPA, 2011c) also noted the importance of carbonyls 

 

243 Carbonyls compounds including formaldehyde and acetaldehyde are difficult to analyze by 

GC with Flame Ionization Detectors (FID). Both of these compounds in their free state, do not 

respond well to FID detectors. GC analysis is difficult due to the chemical composition of these 

compounds, increased polarity and their inherently low boiling points.
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stating that “There are many compelling scientific reasons to measure carbonyls. They are a very 

important part of O3 chemistry almost everywhere.” Due to the importance of carbonyls to 

understanding O3 chemistry, the EPA believes the need for carbonyl data outweighs the concerns 

over the uncertainty in the data. Therefore, the EPA is proposing to require all required PAMS 

sites to measure formaldehyde, acetaldehyde, and acetone. In addition, EPA is investigating 

alternatives to further reduce uncertainties in carbonyl data as described below.

To improve the carbonyl data that would be collected at required PAMS sites (and 

National Air Toxics Trends Station, or NATTS sites which are also currently measuring 

carbonyls), the EPA has undertaken an effort to improve carbonyl sampling and analysis 

methods to reduce the uncertainty in carbonyl data. This effort will lead to improvements to the 

current Method TO-11A by incorporating solutions to sampling and analysis issues that have 

been identified since Method TO-11A was finalized in 1999, such as the inclusion of an O3

scrubber in the sampling system to reduce the interference from oxidants such as O3. Also as part 

of this effort, the EPA is investigating alternative cartridge materials that have been identified in 

the literature as a replacement for DNPH that may have better collection efficiency with fewer 

interferences.

4. Nitrogen Oxides Sampling

It is well known that NO and NO2 play important roles in O3 formation (U.S. EPA, 

2011a, Section 3.2.2). Under the current network design, Type 2 PAMS sites are required to 

measure NOx (which by definition is the sum of NO and NO2), and Types 1, 3, and 4 sites are 

required to measure NOy which by definition includes NO, NO2, and other oxidized nitrogen 

compounds (NOz). NCore sites are also currently required to measure NOy but are not required 

to measure NO2. 
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In conventional NOx analyzers, NO2 is determined as the difference between the 

measured NO and NOx concentrations. However, due to the non-selective reduction of oxidized 

nitrogen compounds by the molybedenum converter used in conventional NOx monitors, the 

NO2 measurement made by conventional NOx monitors can be biased high due to the varying 

presence of NOz compounds that may be reported as NO2.

244 The unknown bias from the NOz

compounds is undesirable when attempting to understand O3 chemistry.

Improvements in reactive nitrogen measurements have been made since the original 

PAMS requirements were promulgated that allow for improved NO2 measurements. Selective 

photolytic converters have been developed that are not significantly biased by NOz compounds

(Ryerson et al., 2000). Monitors using photolytic converters are commercially available and have 

been approved as FEMs for the measurement of NO2. In addition, methods that directly read 

NO2 have been developed that allow for very accurate readings of NO2 without some of the 

issues inherent to the “difference method” used in converter based NOx analyzers. However, 

these direct reading NO2 analyzers generally do not provide an NO estimate, and would need to 

be paired with a converter-based NOx monitor or NOy monitor in order to also measure NO.

As discussed above, the EPA is proposing to change the PAMS network design such that 

PAMS measurements would be required at existing NCore sites in O3 nonattainment areas. 

NCore sites currently are required to measure NO and NOy. NCore sites are not currently 

required to measure NO2. Due to the importance of accurate NO2 data to the understanding of O3

formation, the EPA is proposing to require NO2 measurements at required PAMS sites. Since 

 

244 Nitrogen compounds that would likely be reported (along with NO2) as NO2 with a 

conventional NOx monitor include peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), 

peroxymethacryloyl nitrate (MPAN), and nitric acid (HNO3), and as well as other nitrogen 

compounds not listed here.
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existing NCore sites currently measure NOy, either a direct reading NO2 analyzer or a photolyticconverter NOx analyzer should be used to meet the proposed requirement. The EPA believes 

conventional NOx analyzers would not be appropriate for making PAMS measurements due to 

the uncertainty caused by interferences caused by NOz compounds.

5. Meteorology Measurements

Monitoring agencies are currently required to collect surface meteorology at all PAMS 

sites. As noted in EPA’s TAD (U.S. EPA, 1998) for the PAMS program, the PAMS 

requirements do not provide specific surface meteorological parameters to be monitored. As part 

of the implementation efforts for the original PAMS program, a list of recommended parameters

was developed and incorporated into the TAD which includes wind direction, wind speed, 

temperature, humidity, atmospheric pressure, precipitation, solar radiation, and ultraviolet (UV) 

radiation. Currently, NCore sites are required to measure the above parameters with the 

exceptions of atmospheric pressure, precipitation, solar radiation, and UV radiation. In 

recognition of the importance of these additional measurements for O3, the EPA is proposing to 

specify that required PAMS sites are required to collect wind direction, wind speed, temperature, 

humidity, atmospheric pressure, precipitation, solar radiation, and UV radiation. This proposed 

revision will provide clarity and consistency to the collection of surface meteorological 

parameters important to the understanding of O3 formation. If PAMS measurements are moved 

to NCore sites in O3 nonattainment areas, as is being proposed, the net impact of this proposed 

revision to the surface meteorological requirements for PAMS sites is to add the requirement for 

the monitoring of atmospheric pressure, precipitation, solar radiation, and UV radiation at 

affected NCore sites. 
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The existing PAMS requirements also require the collection of upper air meteorological 

measurements at one site in each PAMS area. The term “upper air meteorological” is not well 

defined in the existing PAMS requirements. As part of the implementation efforts for the original 

PAMS program “mixing height” was added to the PAMS TAD as a recommended 

meteorological parameter to be monitored.

Most monitoring agencies installed radar profilers to meet the requirement to collect 

upper air meteorology. Radar profilers provide data on wind and speed at multiple heights in the 

atmosphere. Radio acoustic sounding system (RASS) profilers are often included with radar 

profilers to obtain atmospheric temperature at multiple heights in the atmosphere and to estimate 

mixing height. The EPA recognizes that the upper air data on wind speed and wind direction 

from radar profilers can be very useful in O3 modeling. However, many of the current PAMS 

radar profilers are old and in need of replacement or expensive maintenance. In addition, the cost 

to install and operate radar profilers at all NCore sites would be prohibitive. Therefore, the EPA 

is not proposing to require upper air wind speed and direction as required meteorological 

parameters to be monitored at PAMS sites. Where monitoring agencies find the radar profiler 

data valuable, continued operation of existing radar profilers or the installation of new radar 

profilers would be appropriate to consider as part of the state’s Enhanced Monitoring Plan.

As discussed above, mixing height is one upper air meteorological measurement that has 

historically been measured at PAMS sites. A number of methods can be used to measure mixing 

height in addition to radar profiler technology discussed above. Recent developments in 

ceilometer technology allow for the measurement of mixing height by changes in particulate 

concentrations at the top of the boundary layer (Eresmaa et al., 2006). Ceilometers provide the 

potential for continuous mixing height data at a fraction of the cost of radar profilers. Due to the 
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importance of mixing height measurements for O3 modeling, the EPA is proposing to require 

monitoring agencies to measure mixing height at PAMS sites. The EPA is aware of a large 

network of ceilometers operated by the National Oceanic and Atmospheric 

Administration (NOAA) as part of the Automated Surface Observing System (ASOS). The EPA 

has been in discussions with NOAA regarding the potential for these systems to provide the 

needed mixing height data, however, the ASOS ceilometers are not currently equipped to 

provide mixing height data. Nonetheless, the EPA will continue to work with NOAA to 

determine if the ASOS ceilometers can be upgraded to meet the need for mixing height data, and 

is including proposed regulatory language that will allow states a waiver to use nearby mixing 

height data from ASOS or other sources to meet the requirement to collect mixing height data at 

required PAMS sites.

6. PAMS Season

Currently, PAMS measurements are required to be taken during the months of June, July, 

and August. This 3-month period is referred to as the “PAMS Season”. As part of the PAMS reevaluation, the EPA considered changes to the PAMS season. The 3-month PAMS season was 

originally selected to represent the most active period for O3 formation. However, the EPA notes 

that in many areas the highest O3 concentrations are observed outside of the PAMS season.245 As 

an example, the highest O3 concentrations in the Mountain-West often occur during the winter 

months. Data collected during the current PAMS season would have limited value in 

understanding winter O3 episodes. 

 

245 The current O3 monitoring season by state in 40 CFR part 58, appendix D, requires 

monitoring seasons from 4 to 12 months. As noted in section VI.B. of this preamble, the EPA 

proposes to lengthen the seasons further for 33 states. 
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The CASAC AMMS (U.S. EPA, 2011c) noted in their report to the EPA that “it would 

be desirable to extend the PAMS monitoring season beyond the current June, July, August 

sampling period,” but that “the monitoring season should not be mandated and rigid; it should be 

flexible and adopted and coordinated on a regional airshed basis (i.e., within the same O3

region).” The EPA agrees with CASAC on the need for flexibility in determining when PAMS 

measurements should be taken to meet local monitoring needs but also agrees with CASAC that 

the flexibility “should not conflict with national goals for the PAMS program.” A significant 

benefit of the standard PAMS season is that it ensures data availability from all PAMS sites for 

national- or regional-scale modeling efforts.

While the EPA agrees with the potential benefit of extending the availability of PAMS 

measurements outside of the current season, we also considered the burden of requiring 

monitoring agencies to operate additional PAMS measurements (e.g., hourly speciated VOC) for 

periods that in some cases, might be much longer than the current 3-month season, for example, 

if the PAMS season was extended to match each state’s required O3 monitoring season. Being 

mindful of the potential burden associated with a lengthening of the PAMS season as well as the 

potential benefits of the additional data, the EPA is proposing to maintain the current 3-month 

PAMS monitoring season for required PAMS sites rather than extending the PAMS season to 

other periods where elevated O3 may be expected. The EPA believes that the 3-month PAMS 

season will provide a consistent data set of O3 and O3 precursor measurements for addressing the 

national PAMS objectives. Monitoring agencies are encouraged to consider collecting PAMS 

measurements in additional periods beyond the required PAMS season as part of the proposed 

Enhanced Monitoring Plan. The monitoring agencies should consider factors such as the periods 
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of expected O3 exceedances and regional consistency when determining potential expansion of 

the specific monitoring periods beyond the required PAMS season.

7. Timing and Other Implementation Issues

The EPA recognizes that the proposed changes to the PAMS requirements will require 

resources and a reasonable implementation schedule if they are promulgated. The proposed 

network design changes would require monitoring agencies to start collection of PAMS 

measurements at many NCore sites that are not currently collecting PAMS measurements. These 

affected monitoring agencies would need to make capital investments (primarily for the 

installation of auto-GCs, NO2 monitors, and ceilometers). Monitoring agencies will also need 

time to develop the expertise, by training existing staff or otherwise, to successfully collect 

PAMS measurements. The EPA believes that the current national funding level of the PAMS 

program is sufficient to support these proposed changes, especially in light of the staggered 

deployment schedule described below. The current grant guidance includes the maintenance of a 

PAMS capital equipment reserve that could be used to assist monitoring agencies with the 

purchase of needed equipment. We also recognize that the proposed revisions would result in a 

potential shifting of PAMS resources, and we would work with the regional offices, affected 

states, and monitoring organizations such as the NACAA and the Association of Air Pollution 

Control Agencies (AAPCA) to facilitate any shifts in funding during the implementation phase 

of the program.

For these reasons, the EPA is proposing a staggered deployment schedule for the 

proposed changes to the PAMS requirements (including both the monitoring at required PAMS 

sites and the Enhanced Monitoring Plans). For areas currently designated as nonattainment for 

O3 based on the 2008 NAAQS, the EPA is proposing to require monitoring agencies to 
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incorporate the proposed PAMS requirements into their next annual monitoring network plan 

following promulgation of these proposed changes (due July 1, 2016, based on current 

schedules) and to comply with these proposed PAMS requirements by the following PAMS 

season (June 1, 2017, based on current schedules). For new areas designated as O3 nonattainment 

based on the initial round of designations following the promulgation of a revised O3 standard, 

the EPA is proposing to require monitoring agencies to incorporate the proposed PAMS 

requirements into their next annual monitoring network plan following designations (due July 1, 

2018, based on current schedules) and to comply with new PAMS requirements by the following

PAMS season (June 1, 2019, based on current schedules). Finally, the EPA is proposing that 

areas designated as O3 nonattainment following the initial round of designations be allowed 2 

years after designation to comply with the proposed PAMS requirements. The EPA believes that 

the proposed compliance schedule will allow monitoring agencies adequate time to implement 

the proposed PAMS requirements. The EPA solicits comments on whether the proposed 

implementation schedule is practicable, or whether additional time would be warranted for 

installation of new PAMS sites, the development of Enhanced Monitoring Plans, or other 

specific new PAMS requirements. 

D. Addition of a New Federal Reference Method (FRM) for O3

To be used in a determination of compliance with the O3 NAAQS, O3 monitoring data 

must be obtained using either a FRM or a FEM, as defined in 40 CFR parts 50 and 53. Nearly all 

the monitoring methods for O3 currently used by state and local monitoring agencies are FEM 

continuous analyzers utilizing a measurement principle based on quantitative measurement of the 

absorption of UV light by O3. This type of O3 analyzer was introduced into the monitoring 

networks in the 1980s and has since become the most predominant type of method used because 
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of its all-optoelectronic design and ease of installation and use. The existing O3 FRM utilizes a 

measurement principle based on quantitative measurement of the chemiluminescence from the 

reaction of O3 with ethylene. Ozone analyzers based on this FRM principle are no longer used 

for routine O3 field monitoring and are no longer commercially available. The current list of all 

approved FRMs and FEMs capable of providing ambient O3 data for use in NAAQS attainment 

decisions may be found on the EPA’s website and in the docket for this action (U.S. EPA, 

2014g).

The EPA proposes to revise the FRM to establish a new technique for measuring O3 in 

the ambient air. This new technique would be a new type of analyzer based on Nitric Oxidechemiluminescence (NO-CL) methodology. Because of the similarity of this new 

chemiluminescence technique to the existing ethylene-chemiluminescence (ET-CL) 

methodology, the EPA proposes that it be incorporated into the existing O3 FRM, using the same 

calibration procedure. Appendix D of 40 CFR part 50 would be revised to include both the 

original ET-CL as well as the new NO-CL methodology. A minor change is proposed to the 

existing O3 FRM calibration procedure, which would be applicable to both chemiluminescence 

FRM methodologies. The proposed change in section 4.5.2.3 of the calibration procedure in Part 

50 provides for more flexibility in the range of the linearity test. 

FRMs, as set forth in several appendices to 40 CFR part 50, serve two primary purposes. 

The first is to provide a specified, definitive methodology for routinely measuring concentrations 

of various ambient air pollutants for comparison to the NAAQS in Part 50, for quality assurance 

assessment of monitoring data, and for other air monitoring objectives. The second is to provide 

a standard of comparison for determining equivalence to the specified reference method of 
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alternative and perhaps more practical pollutant measurement methods (equivalent methods, or 

FEMs) that can be used in lieu of the FRM for routine monitoring.

Some of the FRMs contained in appendices to Part 50 (such as the original SO2 FRM and 

the lead FRM) are manual methods that are completely specified in a step-by-step manner. 

Others (such as the O3 FRM) are in the form of a measurement principle along with an 

associated calibration procedure that must be implemented in a commercially-produced FRM 

analyzer model. Such FRM-type analyzers must be tested and shown to meet explicit 

performance and other qualification requirements that are set forth in 40 CFR part 53 (Ambient 

Air Monitoring Reference and Equivalent Methods). Each analyzer model is then considered to 

be an FRM only upon specific designation as an FRM by the EPA under the provisions of 40 

CFR 53.2 (General requirements for a reference method determination). 

As pollutant measurement technology advances and changes, the reference methods in 

part 50 are assessed by the EPA to determine if improved or more suitable measurement 

technology is available to better meet current FRM needs as well as potential future FRM 

requirements. New technology can either be presented to the EPA for evaluation by an FEM 

applicant under 40 CFR 53.16 (Supersession of reference methods), or (as in this case) the EPA 

can originate the process as provided in 40 CFR 53.7 (Testing of methods at the initiative of the 

Administrator). 

The current FRM for measuring O3 in the ambient air was promulgated on April 30, 1971 

(36 FR 8186), in conjunction with the EPA’s establishment (originally as 42 CFR part 410) of 

the first national ambient air quality standards for six criteria pollutants (including O3), as now 

set forth in 40 CFR part 50. On February 8, 1979 (44 FR 8224), the original O3 FRM calibration 

procedure was changed from a wet-chemical standard to a UV photometric calibration 
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procedure. Minor updates to technical references were made on July 18, 1997 (62 FR 38895). 

This FRM is specified as a measurement principle and calibration procedure in Appendix D of 

Part 50. The measurement principle of the FRM is based on the quantitative measurement of 

chemiluminescent light intensity emitted by the chemical reaction of O3 in an air sample with 

ethylene gas mixed in a measurement cell. This ET-CL measurement is calibrated by the 

specified calibration procedure, which is based on photometric assay of O3 calibration 

concentrations in a dynamic flowing system, using measurement of the absorption of UV light 

by the O3 calibration concentrations at a nominal wavelength of 254 nm.

At the time of the FRM’s original promulgation, analyzers based on the ET-CL FRM 

were widely used for field monitoring of O3. Laboratory testing prior to, during, and following 

analyzer development indicated that interferences to which the method was susceptible were few 

and relatively minor in magnitude. Further, subsequent field experience with the FRM analyzers 

showed them to be stable, accurate, and reliable. Operation of these FRM analyzers requires a 

supply of ethylene gas, provided by an attendant high-pressure compressed gas cylinder. 

Installation of this high-pressure cylinder of flammable and potentially explosive gas proved 

problematic at many field-monitoring sites due to fire codes or other safety restrictions. Further, 

the ethylene gas cylinder required periodic replacement – a considerable cost and operational 

inconvenience.

Following the development of FEM O3 analyzers based on UV absorption, use of these 

newer UV FEM analyzers eventually supplanted the ET-CL FRM analyzers because the UV 

analyzers required no gas supply or other reagents and were much easier to install and operate. 

Currently, nearly all compliance monitoring in the U.S. is carried out with UV absorption type 

FEM analyzers (Long, 2014). This transition from ET-CL FRM analyzers to UV absorption 
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analyzers in U.S. (as well as world-wide) monitoring networks has become so extensive that 

analyzer manufacturers no longer manufacture the ET-CL FRM analyzers. The last new O3 FRM 

analyzer was designated by EPA in 1979. As a result, no FRM O3 analyzers are commercially 

available to serve as reference standards for testing and designation of new O3 FEM analyzers, 

for O3 compliance monitoring, and for quality assurance of field monitors. FRM units 

manufactured years ago are becoming increasingly difficult to maintain in operational condition 

due to aging of components and lack of replacement parts (several of the original FRM analyzer 

manufacturers no longer exist).

Until the last few years, relatively few measurement techniques have been successfully 

implemented in a continuous ambient O3 analyzer model that has achieved designation by the 

EPA as either an FRM or FEM (U.S. EPA, 2014g). These include the ET-CL technique, the UV 

absorption technique and differential optical absorption spectroscopy (DOAS, an open-path 

method represented by two FEM instrument models from different manufacturers). A relatively 

new technology is nitric oxide (NO)-O3 chemiluminescence, which is represented by two FEM 

instrument models from a single manufacturer. An even newer technology is a “scrubberless” 

UV absorption technique that is represented by a single analyzer model for which FEM 

designation was recently achieved.

As noted above, the ET-CL technique is technically advantageous as an FRM, but its 

ethylene supply requirement and the lack of commercially available analyzers severely limit its 

ability to fulfill the needs for an O3 FRM. DOAS analyzers are not suitable for some FRM 

purposes because of their open-path nature.

Commercial availability of conventional UV-absorption O3 analyzers is excellent, and 

their widespread use makes the measurement technique desirable for consideration as an FRM. 
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However, the technique is susceptible to potential measurement interference from mercury, some 

volatile aromatic hydrocarbons, water, and other compounds that sometimes occur in ambient air 

(Spicer et al., 2010). These interferences are substantially reduced by the use of scrubbers (as 

discussed below) in UV FEM analyzers, such that the technique can be used extensively for 

compliance monitoring. Although the interferences are substantially reduced by the use of 

scrubbers, the potential for interferences prevents the technique from consideration as an FRM.

It is important to make a distinction between use of the UV-absorption measurement 

technique for assay of O3 concentrations, as described in the FRM calibration procedure of Part 

50, Appendix D, and use of the UV absorption technique for measurement of O3 in ambient air. 

For assay of calibration concentrations, the technique is used in a system with clean, zero air (air 

that must be free of contaminants which would cause a detectable response from the O3 analyzer) 

such that potential ambient-air-borne interferences are not an issue. Under these clean-air 

conditions, the UV assay technique is very accurate and highly reproducible, so much so that the 

National Institute of Standards and Technology (NIST) utilizes it for its O3 Standard Reference 

Photometer.

In contrast, use of the UV-absorption technique to measure O3 in ambient air is much 

more difficult because of the need to deal with UV-absorbing (and hence potential interfering) 

species present in ambient air. Ambient UV O3 monitors typically suppress interferences by 

using an “O3 scrubber” that attempts to remove O3 from ambient air without removing 

potentially interfering species, to create a zero- O3 reference air that still contains any potentially 

interfering species. In a differential measurement process that compares the UV absorption 

measurement of O3 in the ambient air sample with that in this zero-O3 reference air, the net effect 

of interferences is minimized by cancellation. FEM analyzers using such O3 scrubbers are able to 
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meet the FEM interference test requirements of 40 CFR part 53 and provide adequate O3

monitoring data at most typical O3 monitoring sites. 

On October 7, 2011, the EPA designated two NO-CL O3 analyzers as FEMs (76 FR 

62402). These analyzers use a variation of the current FRM measurement principle, based on 

measurement of the chemiluminescence produced by the chemical reaction of O3 with NO rather 

than with ethylene. As explained below, the EPA believes that this variation has performance 

suitable for an O3 FRM and offers a substantial implementation advantage over the existing 

FRM.

The NO-CL measurement technique for O3 is quite similar to the existing ET-CL FRM 

technique, in that both are based on the measurement of the intensity of the chemiluminescence 

resulting from a chemical reaction of a reactant with the O3 in the ambient air sample. The 

principle difference is that the reactant is NO rather than ethylene. As a potential variation of the 

FRM measurement principle, the measurement would be calibrated with the same calibration 

procedure specified in the FRM.

The performance of NO-CL analyzers has been shown to be very similar to the 

performance of ET-CL FRM analyzers, providing stable, accurate, highly reproducible 

measurements of ambient O3 with minimal potential interferences (U.S. EPA, 2014h). As with 

ET-CL, some minor interference from variable humidity in ambient air can be minimized with a 

sample air dryer. The analyzers require a supply of NO gas, typically from a high-pressure 

compressed gas cylinder. However, unlike ethylene, NO is neither flammable nor explosive, so 

use of the method in field applications is eased considerably relative to use of ET-CL analyzers. 

Nitric oxide gas is toxic, but it is possible to use a cylinder of much less toxic, non-combustible 

nitrous oxide (N2O) gas with a photolytic N2O-to-NO converter to supply NO gas for the 
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instrument as needed. There will be no requirement for states to switch to NO-CL analyzers; 

therefore, UV-absorption FEM analyzers can still be used for routine O3 monitoring. As noted 

previously, the EPA has designated two NO-CL FEM analyzers (from the same manufacturer), 

both of which would qualify for re-designation as FRMs if the NO-CL technique is finalized as 

an FRM. NO-CL analyzers would then be available for those applications where an FRM 

analyzer is needed. 

Because of the similarity of the NO-CL technique to the existing ET-CL technique, the 

EPA is proposing to amend the ET-CL FRM by adding the NO-CL technique as a variation to 

the existing FRM measurement principle specified in Appendix D of Part 50. The specified 

calibration procedure would be applicable to both FRM ET-CL and NO-CL measurement 

techniques. Since the existing ET-CL FRM measurement principle remains a technically 

adequate FRM, and the proposed new NO-CL FRM is technically adequate, it is prudent to 

retain the existing FRM measurement principle. The designation of all currently designated O3

FEMs is based on comparison to the ET-CL FRM, so retention of the ET-CL FRM allows those 

FEM designations to be retained. 

Adding the proposed NO-CL measurement technique to the current O3 FRM would allow 

at least two commercially available FRM analyzer models (currently FEMs) to be re-designated 

as FRMs to fulfill FRM analyzer needs. Some older FRM analyzers based on the existing ET-CL 

measurement principle may still be in operable condition, and there is no technical reason to 

cancel their designation by withdrawing the original ET-CL FRM technique. Additionally, 

retaining the existing ET-CL FRM technique allows for the possibility of an instrument 

manufacturer offering an ET-CL FRM analyzer in the future.
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The second of the newly introduced O3 measurement techniques is known as the 

scrubberless UV absorption (UV-SL) technique. It utilizes the UV-absorption measurement 

technique that is widely used in O3 monitoring networks. The new UV-SL technique specifies 

removal of O3 from the sample air for the zero reference by a gas-phase reaction with NO rather 

than via a conventional chemical scrubber. The NO reacts with the O3 much faster than with 

other potential interfering compounds and is very effective at removing the O3 without affecting 

other compounds that may be present in the ambient air sample. The differential UV 

measurement can effectively eliminate interferences to an insignificant level. Other potential 

interference arising from changes in water vapor concentration can be minimized with a sample 

air dryer.

The UV-SL technique appears to have characteristics that are advantageous for meeting 

the requirements of a new O3 FRM. Analyzers implementing this technique require a supply of 

NO (such as a high-pressure gas cylinder). As noted previously in connection with the NO-CL 

technique, NO is neither flammable nor explosive, so use of the method in field applications is 

eased considerably relative to use of ET-CL analyzers. Use of N2O gas, also supplied in 

compressed gas cylinders but less toxic than NO, is also possible with a photolytic N2O to NO 

converter. One commercially available UV-SL analyzer was approved as an FEM on June 18, 

2014 (79 FR 34734). The performance of the analyzer, as reported by the manufacturer246 and 

some initial field and laboratory studies performed by the EPA (U.S. EPA, 2014h), suggests that 

the analyzer may meet existing, as well as the proposed, requirements for an O3 FRM. 

The CASAC AMMS provided a peer review of the proposed FRM and changes to the 

Part 53 requirements on April 3, 2014. The CASAC AMMS recommended that the EPA

 

246 2B FEM test data via http://www.twobtech.com/model_211.htm
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consider the UV-SL as a FRM. The EPA is independently conducting further laboratory and 

field tests of the UV-SL analyzer to verify its performance. Although this new UV-SL 

methodology shows substantial promise for future consideration as a new O3 FRM, there is 

currently insufficient documented test and performance information available on the method to 

propose it as a new FRM at this time. The EPA is continuing to study the method and assess its 

potential suitability as a new O3 FRM, and the EPA solicits comment on its potential and 

suitability as an FRM.

The EPA is not proposing to supersede (replace) the existing O3 FRM measurement 

principle under the provisions of 40 CFR 53.16. Rather, for the reasons in the preamble and 

having conducted the necessary tests, the EPA is proposing, consistent with 40 CFR 53.7 , to 

revise the existing O3 FRM to widen the scope of its ET-CL measurement principle to include 

the NO-CL measurement technique as well. 

Following promulgation of the proposed revised O3 FRM measurement principle, any 

new candidate O3 FRM analyzers would be required to use either the ET-CL or NO-CL 

measurement principle, and would also be subject to the O3 FRM performance requirements 

proposed in 40 CFR part 53. The FRM calibration procedure specified in Appendix D would 

apply to both O3 FRM measurement techniques.

A substantial number of laboratory tests have confirmed the excellent performance of the 

NO-CL analyzers as well as very close agreement with both ET-CL and UV analyzers in 

collocated field tests. Therefore, the EPA believes the proposed FRM measurement principle that 

incorporates the NO-CL methodology is the best approach to improve the availability of FRM 

analyzers for O3. No other currently known approach or alternative methodology appears to be 

more appropriate for a new FRM. Adding the NO-CL technology to the existing O3 FRM is also 
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endorsed by the EPA’s CASAC AMMS. The EPA solicits comment on the proposal to retain the 

existing O3 FRM measurement principle and amend it to include the NO-CL variation as well. 

Comments are also solicited on the nature and adequacy of the proposed revised FRM.

The generic description of the FRM measurement principle for the existing ET-CL FRM 

in Appendix D would be amended to include the NO-CL variation (see the proposed rule text for 

Appendix D). As noted previously, the new NO-CL technique would also use the same 

calibration procedure in Appendix D and would be similarly coupled with the explicit O3 FRM 

analyzer performance requirements specified in subpart B of 40 CFR part 53. In addition to the 

incorporation of the NO-CL methodology, numerous minor clarifications, wording changes, 

additional details, and a more refined numbering system are being proposed for Appendix D. 

Accordingly, the entire Appendix D is proposed to be revised as identified in the proposed 

regulatory text.

Because the new NO-CL technique is proposed to be added to the existing FRM 

measurement principle, while the existing ET-CL FRM principle would be retained and remain 

in effect, all existing designated FEM analyzer models will continue their designated status. 

Thus, this action would cause no negative consequences on monitoring agencies, and no 

disruption of, or required change to, their O3 monitoring programs. Comparative testing has been 

carried out at several field monitoring sites under a variety of ambient conditions, and the results 

confirm that the proposed new NO-CL FRM measurement technique provides ambient O3

measurements that compare and correlate excellently with measurements using the existing ETCL measurement principle, with no significant bias, offset, or discrepancy (U.S. EPA, 2014h). 

E. Revisions to the Procedures for Testing Performance Characteristics and Determining 

Comparability Between Candidate Methods and Reference Methods
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The only substantial changes proposed to the requirements of Part 53 are in Tables B-1 

and B-3 of Subpart B. Table B-1 has been updated in recent years with regard to FRM and FEM 

methods for SO2 (74 FR 64877, December 8, 2009) and CO (76 FR 54294, August 31, 2011).

Similar update changes to Table B-1 are proposed here for O3. Modest changes proposed for

Table B-3 would add new interferent test concentrations specifically for NO-CL analyzers, 

adding a test for NO2. The table would also clarify that the existing test concentrations apply to 

ET-CL O3 analyzers. Figure B-5 is revised to correct a minor inconsistency in the “Calculations” 

column for the two “Precision” rows to change “% URL” to “% Standard Deviation.”

Several changes to the performance requirements given in Table B-1 are proposed for O3.

The performance requirements for “standard range” instruments would be updated to be more 

consistent with current O3 analyzer performance capabilities. The noise requirement limit would 

be reduced from 0.005 to 0.001 ppm for O3 analyzers, the lower detectable limit would be 

reduced from 0.010 to 0.003 ppm, and the maximum interference equivalent limits would be 

reduced from 0.02 to 0.005 ppm for each potential interfering agent (interferent). The 

performance limit requirement for the total of all interferents is proposed to be withdrawn for O3

methods. This withdrawal is appropriate because O3 analyzer test performance, as reported in 

recent FEM applications, has shown that the limits established for individual interferents are 

sufficiently effective to define adequate analyzer performance, and the separate limit for the total 

of all interferences is unnecessary. 

Maximum zero drift for O3 analyzers would be reduced from 0.02 to 0.004 ppm. The 

existing limit for span drift at 20% of the upper range limit (URL) is proposed to be withdrawn.

Analyzer performance test results have clearly shown that the existing 80% URL limits are fully 

adequate and better specify span drift performance and that the 20% URL span drift limits are 
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ineffective and unnecessary. The span drift limit applicable to O3 analyzers is proposed to be 

reduced from ±5.0% to ±3.0%. Lag time limits would be reduced from 20 to 2 minutes, and rise 

and fall time limits would be similarly reduced from 15 to 2 minutes.

For precision, the EPA proposes to change the form of the precision limit specifications 

(at both 20% and 80% of URL) for O3 analyzers from ppm to percent (of the URL). This change 

would make the limits responsive to higher and lower measurement ranges, as appropriate, and is 

consistent with the same change previously made in the corresponding precision requirements 

for SO2 and CO analyzers. Both limits would be set at 2% for O3 analyzers, which is equivalent 

to, and, therefore effectively unchanged, from the existing limits of 0.01 ppm (for a URL of 0.5 

ppm). Although the changes to Part 53 proposed here are generally restricted to methods for O3, 

this change in form for the precision limits is proposed to be extended to methods for NO2 as 

well, to simplify Table B-1 and make it consistent for all pollutants covered by the Table. The 

precision limits that would be applicable to methods for NO2 are proposed to be changed to 4% 

and 6% of the URL (for 20% and 80% of the URL, respectively). These values are exactly 

equivalent to the existing limits of 0.020 ppm and 0.030 ppm, respectively, for the specified 

URL of 0.5 ppm. Therefore, these precision limits for NO2 remain effectively unchanged, but 

specified as a percent rather than an absolute concentration. A new footnote is proposed for 

Table B-1 to clarify that these revised precision limits are given as “standard deviation expressed 

as percent of the URL.” Therefore, Figure B-5 will be revised to correct a minor inconsistency in 

the “Calculation” column for the two “Precision” rows to change the “% URL” to “% Standard 

Deviation.”

The EPA has reviewed the documented performance of currently designated FRM and 

FEM methods for O3 (that are still in commercial production or in service in monitoring 
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networks) and has verified that all would meet the proposed new performance requirements for 

O3 methods (Long, 2014). Therefore, adoption of the proposed new performance requirements in 

Table B-1 would not require the withdrawal or cancelation of the FRM or FEM designation of 

any such O3 analyzers.

Finally, to meet a need for analyzers with more sensitive measurement ranges for 

monitoring in relatively clean areas, new, “lower range” performance limit requirements are 

proposed for O3 analyzers. These lower range limits are set forth in a new “lower range” column 

in Table B-1 and would be optional. But where a lower measurement range is included in the 

FRM or FEM designation, these proposed new requirements would provide more stringent 

performance for analyzers commensurate with greater accuracy for low-level measurements in 

lower-level concentration ranges.

The EPA believes that these proposed changes in the performance requirements of Tables 

B-1 and B-3 are appropriate, based on analyzer performance data available from analyzer 

manuals and recent FRM and FEM applications. The EPA solicits comment as to whether the 

proposed changes are reasonable, appropriate, beneficial, and achievable without undue burden.

The EPA is proposing minor changes to the general provisions in subpart A of Part 53 to 

ease the administrative burden associated with processing and reviewing modification requests to 

existing FRMs and FEMs. This change in 40 CFR 53.14(c) will extend the length of time for the 

Administrator to take action on a request for modification of a reference or equivalent method 

from 30 days to 90 days. Section 53.14(c) would read: "Within 90 calendar days after receiving a 

report under paragraph (a) of this section, the Administrator will take one or more of the 

following actions:" The EPA is also proposing to remove the obsolete provision that

manufacturers who offered PM2.5 or PM10-2.5 samplers or analyzers for sale as part of a FRM or 
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FEM may continue to do so only so long as updates of the Product Manufacturing Checklist are 

submitted annually. This change is accomplished through the removal of section (i) from 40 CFR 

53.9 and Figure E-2 from subpart E of Part 53.

VII. Implementation of Proposed O3 Standards

The proposed revisions to the primary and secondary O3 NAAQS discussed in sections 

II.E and IV.G of this preamble, if finalized, would trigger a process under which states247 make 

recommendations to the Administrator regarding area designations, and the EPA promulgates the 

final area designations. States would also be required to review, modify, and supplement their 

existing SIPs. The proposed O3 NAAQS revisions would also affect the transportation 

conformity and general conformity processes. The revised O3 NAAQS and the subsequent

designations process could affect which preconstruction permitting requirements apply to O3 in 

some areas and the nature of those requirements in others.

The EPA has regulations in place addressing the requirements for SIPs and several 

provisions in these existing rules cover O3 (40 CFR part 51). States likewise have provisions in 

their SIPs to address air quality for O3 and to implement the existing O3 NAAQS. The EPA has 

also provided general guidance on the development of SIPs for all pollutants and administration 

of construction permitting programs, as well as specific guidance on implementing the O3

NAAQS in some contexts under the CAA and the EPA regulations. 

When the EPA proposes to revise a NAAQS for a particular criteria pollutant, it 

considers the extent to which existing EPA regulations and guidance are sufficient to implement 

the standard and whether any revisions or updates to those regulation and guidance would be 

 

247 This and all subsequent references to “state” are meant to include state, local, and tribal 

agencies responsible for the implementation of an O3 control program.
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helpful or appropriate in facilitating the implementation of the revised standard by states. The 

CAA does not require that the EPA promulgate new implementing regulations every time that a 

NAAQS is revised. Likewise, the CAA does not require the issuance of additional implementing 

regulations or guidance by the EPA before a revised NAAQS becomes effective. Existing EPA 

regulations may be sufficient in many cases to enable the EPA and the states to begin the process 

of implementing a revised NAAQS. However, where the nature of revisions to a NAAQS 

indicate that additional EPA regulations or guidance (or revisions to existing regulations or 

guidance) may be helpful to implement unique aspects of the revised standard, the EPA 

endeavors to provide those regulations and guidance in a timely way to facilitate preparation of 

SIPs plans. It is important to note, however, that the existing EPA regulations in 40 CFR part 51 

applicable to SIPs generally and to particular pollutants continue to apply even without such 

updates. Accordingly, the discussion below provides the EPA’s current thoughts about the extent 

to which revisions to existing regulations and additional guidance might be helpful or 

appropriate to aid in the implementation of a revised O3 NAAQS, should one be finalized 

through this rulemaking. 

This section provides background information for understanding the possible 

implications of the proposed NAAQS changes in some areas, and describes the EPA’s plans for 

providing revised rules or additional guidance on some subjects in a timely manner to assist 

states with their implementation efforts under the requirements of the CAA. This section also 

describes existing EPA interpretations of CAA requirements and other EPA guidance relevant to 

implementation of revised O3 NAAQS. Relevant CAA provisions that provide potential 

flexibility with regard to meeting implementation timelines are also discussed.
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This section contains a discussion of how existing requirements to reduce the impact on 

O3 concentrations from the stationary source construction in permit programs under the CAA 

may be affected by the proposed revisions of the O3 NAAQS. These are the PSD and 

Nonattainment New Source Review (NNSR) programs. To facilitate the timely implementation 

of the PSD requirements, the EPA proposes as part of this rulemaking to add a grandfathering 

provision to its regulations that would apply to certain PSD permit applications that are pending 

on the effective date of the revised O3 NAAQS. If the proposed NAAQS revisions are finalized, 

this grandfathering provision could be finalized at the same time as the revised NAAQS (see 

section VII.D of this preamble).

The EPA intends to propose additional regulations and issue additional guidance, as 

necessary, related to the implementation requirements for any revised O3 NAAQS resulting from 

this proposal. The EPA intends to take these actions on a schedule that provides timely assistance 

to air agencies. Accordingly, in this section, the EPA solicits comment on several issues that the 

agency anticipates addressing in future guidance or regulatory actions to assist with 

implementation of the revised O3 NAAQS. Because these issues are not relevant to the 

establishment of the NAAQS, and the CAA does not require that the EPA provide 

implementation rules or guidance for each revised NAAQS, the EPA does not expect to respond, 

nor is the agency required to respond, to these comments in the final action on this proposal. 

However the EPA expects these comments will be helpful as future guidance and regulations are 

developed. 

A. NAAQS Implementation Plans

1. Background
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As directed by theCAA, reducing pollution to meet national air quality standards always 

has been a shared task, one involving the federal government, states, tribes and local air quality 

management agencies. The EPA develops regulations and strategies to reduce pollution on a 

broad scale, while states and tribes are responsible for implementation planning and any 

additional emission reduction measures necessary to bring areas into attainment. The agency 

supports implementation planning with technical resources and guidance, while states and local 

agencies use their knowledge of local needs and opportunities in designing emission reduction 

strategies that will work best for their industries and communities.

This partnership has proved effective since the EPA first issued O3 standards more than 

three decades ago. For example, 101 areas were designated as nonattainment for the 1-hour O3 

standards issued in 1979. As of the end of 2013, air quality in 98 of those areas meets the 1-hour 

standards. The EPA strengthened the O3 standards in 1997, shifting to an 8-hour standard to 

improve public health protection, particularly for children, the elderly, and other sensitive 

individuals, against effects such as reduced lung function and respiratory symptoms, hospital and 

emergency room visits for asthma, and possible irreversible damage to the lungs. The 1997 

standards drew significant public attention when they were proposed, with numerous parties 

voicing concerns about states’ ability to comply. However, after close collaboration between the 

EPA, states, tribes and local governments to reduce O3-forming pollutants, significant progress 

has been made. Air quality in 90% of the original 113 areas designated as nonattainment for the 

1997 O3 NAAQS now meets the 1997 standards. The EPA designated 46 areas as nonattainment 

for the 2008 O3 NAAQS in 2012. We expect these areas to make similar progress in achieving 

clean air.
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The majority of man-made NOX and VOC emissions that contribute to O3 formation in 

the U.S. come from the following sectors: on-road and nonroad mobile sources, industrial 

processes (including solvents), consumer and commercial products, and the electric power 

industry. In 2011, the most recent year for which the National Emissions Inventory (NEI) is 

available, onroad and nonroad mobile sources accounted for about 60% of annual NOX

emissions; and the electric power industry accounted for about nearly 15%. With respect to 

VOC, industrial processes (including solvents) accounted for about 57% of manmade VOC 

emissions; and mobile sources accounted for about 39%. Emissions from natural sources, such as 

trees, also comprise around 70% of total VOC emissions nationally, with a higher proportion 

during the O3 season and in areas with more vegetative cover. See section VII.F of this preamble

for more detail on background O3.

Since 2000, the EPA has issued numerous emissions and fuels standards for on-road and 

nonroad mobile sources, as well as emissions standards for many types of stationary sources. 

Benefits from new engine standards increase each year as older, more-polluting vehicles and 

engines are replaced with newer, cleaner models. Benefits from fuel programs generally begin as 

soon as a new fuel is available. The ongoing emission reductions from federal programs such as 

these will provide for substantial emissions reductions well into the future, and will complement 

state and local efforts to attain any revised O3 NAAQS.

Over the past 15 years, the EPA has established new emissions standards under title II of 

the CAA, 42 U.S.C. 7521–7574, for numerous classes of automobile, truck, bus, motorcycle, 

earth mover, aircraft, and locomotive engines, and for the fuels used to power these engines. The 

EPA also established new standards for the smaller engines used in small watercraft, and lawn 

and garden equipment. In March 2008, the EPA promulgated new standards for locomotive and 
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for marine diesel engines and in April 2010 the EPA promulgated new standards for Category 3 

(C3) engines installed on U.S. ocean-going vessels and to marine diesel fuels produced and 

distributed in the U.S. In September 2011, the EPA and the National Highway Transportation 

Safety Administration established greenhouse gas and fuel efficiency standards for new 2014 –

2018 model year medium and heavy-duty engines and vehicles. In addition to improving fuel 

efficiency and reducing greenhouse gas emissions this rule reduces emissions of NOX from the 

subject vehicles. In March 2014, the EPA promulgated Tier 3 standards for tailpipe and 

evaporative emissions from passenger cars, light-duty trucks, medium-duty passenger vehicles, 

and some heavy-duty vehicles. The associated gasoline sulfur standard will enable more 

stringent vehicle emissions standards and will make existing emissions control systems more 

effective. Compared to current standards, the VOC and NOX tailpipe standards for light-duty 

vehicles represent approximately an 80% reduction from today’s fleet average, and the heavyduty tailpipe standards represent about a 60% reduction in VOC and NOX.

The emission reductions from all of these mobile source programs are significant and will 

continue to be realized throughout the implementation period for any revised O3 NAAQS. The 

EPA projects that between 2011 and 2025, onroad and nonroad mobile NOX will decline by 

more than 60% and onroad and nonroad mobile VOC will decline by more than 50%.

248

The reduction of VOC emissions from industrial processes has been achieved either 

directly or indirectly through implementation of control technology standards, including 

maximum achievable control technology (MACT), reasonably available control technology 

(RACT), and best available control technology (BACT) standards; or is anticipated due to 

 

248 “Regulatory Impact Analysis for the Proposed Revisions to the National Ambient Air Quality 

Standards for Ground-Level Ozone,” (December 2014) at http://www.epa.gov/ttnecas1/ria.html. 
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proposed or upcoming proposals based on generally available control technology or best 

available controls under provisions related to consumer and commercial products. These 

standards have resulted in VOC emission reductions of almost a million tons per year 

accumulated starting in 1997 from a variety of sources including combustion sources, coating 

categories, and chemical manufacturing. The EPA also finalized emission standards and fuel 

requirements for new stationary engines. In the area of consumer and commercial products, the 

EPA finalized new national VOC emission standards for aerosol coatings in 2008 and will 

review and revise, as necessary, existing rules for household and institutional consumer products, 

architectural and industrial maintenance coatings, and automobile refinish coatings. Additionally, 

in O3 nonattainment areas, we anticipate reductions of an additional 10,000 tons per year as states 

adopt rules implementing control techniques recommendations issued in 2008 for four additional 

categories of consumer and commercial products, such as surface coatings and adhesives used in 

industrial manufacturing operations. These emission reductions primarily result from solvent 

controls and typically occur where and when the solvent is used, such as during manufacturing 

processes.

As noted above, the power industry is responsible for a nearly 15 % of NOX emissions 

across the U.S. Power industry emission sources include large electric generating units (EGU) 

and some large industrial boilers and turbines. The EPA’s Clean Air Interstate Rule (CAIR), 

issued on March 10, 2005 (70 FR 25612; May 12, 2005), was designed to permanently reduce 

power industry emissions of NOX in the eastern U.S. The first phase of the cap was to begin in 

2009, and a lower second phase cap was to begin in 2015. The EPA had projected that by 2015, 

the CAIR and other programs would reduce NOX emissions during the O3 season by about 50%

and annual NOX emissions by about 60% from 2003 levels in the Eastern U.S. However, on July 
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11, 2008, and December 23, 2008, the U.S. Court of Appeals for the D.C. Circuit (D.C. Circuit) 

issued decisions on petitions for review of the CAIR. In its July 11 opinion, the court found 

CAIR unlawful and decided to vacate CAIR and its associated Federal implementation plans 

(FIPs) in their entirety. State of North Carolina v. EPA, 531 F. 3d 896. On December 23, 2008, 

however, the court granted EPA’s petition for rehearing to the extent that it remanded without 

vacatur for EPA to conduct further proceedings consistent with the Court’s prior opinion. Under 

this decision, CAIR will remain in place only until replaced by EPA with a rule that is consistent 

with the Court’s July 11 opinion. 

The EPA issued the final CSAPR on July 6, 2011 (76 FR 48208; August 8, 2011), to 

replace CAIR. CSAPR requires states to significantly improve air quality by reducing power 

plant emissions that contribute to O3 and/or fine particle pollution in other states. CSAPR 

requires a total of 28 states to reduce annual SO2 emissions, annual NOX emissions and/or O3 

season NOX emissions to assist downwind states in attaining the 1997 O3 and fine particle and 

2006 PM2.5 NAAQS. On December 30, 2011, the D.C. Circuit issued an order staying CSAPR 

and ordering the EPA to continue implementing CAIR. Subsequently, on August 21, 2012, the 

D.C. Circuit issued an opinion vacating CSAPR. EME Homer City Generation LP v. EPA, 696 

F. 3d 7. In its decision the Court again instructed the EPA to continue administering CAIR. The 

U.S. and other parties appealed the D.C. Circuit decision to the U.S. Supreme Court and on April 

29, 2014, the U.S. Supreme Court issued an opinion reversing the judgment of the D.C. Circuit,

upholding the EPA’s interpretation of the CAA “good neighbor” provision (CAA section 110 

(a)(2)(d)(ii)), and remanding the case back to the D.C. Circuit for further proceedings consistent 

with the Supreme Court opinion. EME Homer City Generation LP v. EPA, 134 S. Ct. 1584. On 

June 26, 2014, the U.S. Government filed a motion with the D.C. Circuit to lift the stay of the 
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CSAPR. The D.C. Circuit has since lifted the stay of the rule. Order, Document #1518738, EME 

Homer City Generation, L.P. v. EPA, Case #11-1302 (D.C. Cir. Oct. 23, 2014).

The EPA proposed the Clean Power Plan for existing power plants on June 2, 2014 (79 

FR 34830; June 18, 2014). In this action the EPA proposed state-specific rate-based goals for 

CO2 emissions from the power sector, as well as guidelines for states to follow in developing 

plans to achieve the state-specific goals. This rule, as proposed, would continue progress already 

underway to reduce CO2 emissions from existing fossil fuel-fired power plants in the U.S. 

Actions taken to comply with the proposed guidelines would reduce emissions of CO2 and other 

air pollutants, including SO2, NOx and directly emitted PM2.5, from the electric power industry. 

The EPA estimates that the Clean Power Plan, as proposed, would reduce precursors for both O3

and particulate matter leading to decreases in the concentrations of those pollutants of 

approximately 25% in 2030.

It should also be noted, in general, that new EGUs are subject to NOX limits under New 

Source Performance Standards (NSPS) under CAA section 111, as well as either PSD or NNSR 

requirements. The EPA’s regulations for commercial, industrial and solid waste incinerators set 

standards for NOX and several air toxics for all commercial incinerators, as required under 

Section 129 of the Act. Air toxics rules for industrial boilers will yield co-benefit NOX

reductions as a result of tune-ups and energy efficiency measures, especially from boilers that 

burn coal. And several new source performance standards and air toxics standards are expected 

to make further cuts to NOX and VOC emissions from new and existing sources of pollution. 

These include upcoming review and revisions for gas turbines and municipal waste combustors, 

along with proposed requirements for the petroleum refining industry. The NSPS and air toxics 

standards that have recently taken effect for stationary engines will also make cuts to NOX and 
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VOC emissions. The EPA also anticipates reductions in O3 precursors to result from 

implementation of the Mercury and Air Toxics Standard rule, as well as from measures to 

address Regional Haze best available retrofit technology (BART) determinations.

While the EPA uses its regulatory opportunities to reduce NOX and VOCs, the agency 

also is pursuing non-regulatory efforts as we strive toward cleaner air. Energy Star, a joint 

program of the EPA and the U.S. Department of Energy, protects the environment and saves 

money through energy efficient products and practices. Improving energy efficiency in homes, 

buildings and industry helps reduce all emissions from the power sector – including NOX – while 

reducing compliance costs for electricity providers. As part of its new Advance Program, the 

EPA is working collaboratively with state, local, and tribal governments that want to take steps 

to reduce air pollution in O3 and particulate matter attainment areas. Although these areas are not 

currently subject to nonattainment planning requirements, Advance Program participants are 

interested in undertaking their own planning efforts with the goal of keeping their air healthy and 

creating an improved buffer against future air quality violations. Participating areas are 

implementing a mix of voluntary and mandatory measures relating to mobile, area, and point 

sources as well as energy efficiency measures, and they are also pursuing education and 

awareness programs to improve their communities’ understanding of air quality issues.

The EPA recognizes that a number of areas of the country have been working to reduce

O3 precursors for many years and now may need to turn to newer, more innovative approaches 

for reducing emissions as they develop their implementation plans. These approaches, such as 

smart growth policies and renewable energy portfolios, hold great promise for improved air 

quality and health, and the EPA is working with air quality agencies and stakeholders to identify 

ways to include these types of programs in implementation plans. For example, the EPA 
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developed a roadmap for giving SIP credit to energy efficiency/renewable energy projects.249

Recognition of innovative programs will allow states and tribes to pursue effective strategies that 

address some of the more challenging issues affecting air quality, such as land use planning, ever 

increasing motor vehicle use, and planning for long-term energy needs.

With respect to agricultural sources, the U.S. Department of Agriculture (USDA) has 

approved conservation systems and activities that reduce agricultural emissions of NOX and 

VOC. The EPA recognizes that USDA has been working with the agricultural community to 

develop site-specific conservation systems and activities to control emissions of O3 precursors. 

The EPA will continue to work with USDA on these activities with efforts to identify and/or 

improve the control efficiencies, prioritize the adoption of these conservation systems and 

activities, and ensure that appropriate criteria are used for identifying the most effective 

application of conservation systems and activities.

The EPA will work together with USDA and with states to identify appropriate measures 

to meet the primary and secondary standards, including site-specific conservation systems and 

activities. Based on prior experience identifying conservation measures and practices to meet the 

PM NAAQS requirements, the EPA will use a similar process to identify measures that could 

meet the O3 requirements. The EPA anticipates that certain USDA approved conservation 

systems and activities that reduce agricultural emissions of NOX and VOC may be able to satisfy 

the requirements for applicable sources to implement reasonably available control measures for 

purposes of attaining the primary and secondary O3 NAAQS.

 

249 “The Roadmap for Incorporating Energy Efficiency/Renewable Energy Policies and Programs 

into State and Tribal Implementation Plans,” (July 2012) at http://epa.gov/airquality/eere/.
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The agency also is active in work to reduce the international transport of O3 and other 

pollutants that can contribute to “background” O3 levels in the U.S. Under the Convention on 

Long-Range Transboundary Air Pollution (LRTAP) of the United Nations Economic 

Commission for Europe, the U.S. has been a party to the Protocol to Abate Acidification, 

Eutrophication, and Ground-level Ozone (known as the Gothenburg Protocol) since 2005. The 

U.S. is also active in the LRTAP Task Force for Hemispheric Transport of Air Pollution, which 

in 2010 produced a comprehensive assessment of the intercontinental transport of air pollution 

(including O3) in the northern hemisphere. 

The U.S. has worked bilaterally with Canada under the US-Canada Air Quality 

Agreement to adopt an Ozone Annex to address transboundary O3 impacts. The EPA also 

continues to work with rapidly growing countries such as China on air quality management 

activities and the development of analytical tools to help these countries address significant air 

quality problems, including the emissions of O3-forming pollutants. This work includes 

supporting China’s efforts to rapidly deploy power plant pollution controls that can achieve NOX

reductions of at least 80 to 90%.

We know that developing the implementation plans that outline the steps a nonattainment 

area will take to meet an air quality standard requires a significant amount of work on the part of 

state, tribal or local air agencies. The EPA routinely looks for ways to reduce this workload, 

including assisting with air quality modeling by providing inputs such as emissions, 

meteorological and boundary conditions; and sharing national-scale model results that states can 

leverage in their development of their attainment demonstrations. At the same time, we work 

with air agencies to provide implementation flexibility to the extent allowed by law. 

2. Timing of Rules and Guidance
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In public comment periods associated with several recent rulemakings, the EPA received 

comments from a variety of states and organizations asking for rules and guidance associated 

with a revised NAAQS to be issued in a timely manner. Although issuance of such rules and 

guidance is not a part of the NAAQS review process, National Ass’n of Manufacturers v. EPA, 

750 F. 3d 921, 926-27 (D.C. Cir. 2014), toward that end the EPA intends to produce appropriate 

revisions to necessary implementation rules and provided additional guidance in time frames that 

would be more useful to states when developing their implementation plans than has been the 

case with some previous rules and guidance. 

Certain requirements under the PSD preconstruction permit review program apply 

immediately to a revised NAAQS upon the effective date of that NAAQS, unless the EPA has 

established a grandfathering provision through rulemaking. To ensure a smooth transition to a

revised O3 NAAQS, the EPA is proposing a grandfathering provision similar to the one finalized 

in the 2012 PM2.5 NAAQS Rule. See section VII.D of this preamble for more details on the PSD 

program.

Promulgation of the NAAQS starts a clock for the EPA to designate areas as either 

attainment or nonattainment. State recommendations for area designations are due to the EPA 

within 12 months of promulgation of the NAAQS. In an effort to allow states to make more 

informed recommendations, the EPA intends to issue guidance concerning the designations 

process within 4 months of promulgation of the NAAQS, or approximately 8 months before state 

recommendations are due. The EPA has issued designation guidance for several NAAQS in 

recent years. While generally the EPA considers information related to the same factors in 

making designation decisions, the guidance is tailored to the particular NAAQS. The EPA 

anticipates that the guidance for a revised NAAQS resulting from this proposal would be similar 
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to the designation guidance for the 2008 O3 NAAQS. The EPA generally completes area 

designations 2 years after promulgation of a NAAQS. See section VII.C of this preamble for 

additional details on designations.

Clean Air Act section 110 requires SIPs to be submitted within 3 years of promulgation 

of a revised NAAQS. These SIPs are referred to as “infrastructure SIPs.” The EPA issued 

general guidance on submitting infrastructure SIPs on September 13, 2013.250 It should be noted 

that this guidance did not address certain state planning and emissions control requirements 

related to interstate pollution transport. Should this guidance need to be modified for this 

prospective O3 NAAQS, the EPA intends to issue that updated guidance no later than 1 year after 

promulgation of a revised O3 NAAQS. See section VII.B.3 of this preamble for additional 

information on infrastructure SIPs. 

The EPA intends to propose any appropriate rules for assisting with implementing any 

revised O3 NAAQS resulting from this proposal within 1 year after a revised NAAQS is 

established. The rules that EPA is considering, as with implementation of previous NAAQS, 

would address nonattainment area classification methodologies, SIP due dates, attainment dates, 

and required implementation programs such as NNSR and conformity. At that same time the 

EPA intends to address any modifications needed as a result of this revised NAAQS to guidance 

pertaining to developing nonattainment area emissions inventories and attainment 

demonstrations, and demonstrating conformity. The EPA anticipates finalizing these items by the 

time areas are designated nonattainment. Finalizing rules and guidance by this time would 

provide air agencies with the information to develop any CAA-required SIPs associated with 

 

250 See memorandum from Stephen D. Page to Regional Air Directors, “Guidance on 

Infrastructure State Implementation Plan (SIP) Elements under Clean Air Act Sections 110(a)(1) 

and 110(a)(2)” September 13, 2013.
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nonattainment designations. In an area designated as nonattainment, new major sources and 

major modifications at existing sources are required to comply with NNSR requirements 

including the application of “lowest achievable emission rate” (LAER) and emissions offsets at 

ratios prescribed by the CAA. See section VII.B.4 of this preamble for additional information on 

nonattainment SIPs.

3. Section 110 State Implementation Plans

The CAA section 110 specifies the general requirements for SIPs. Within 3 years after 

the promulgation of revised NAAQS (or such shorter period as the Administrator may 

prescribe251) each state must adopt and submit ‘‘infrastructure’’ SIPs to the EPA to address the 

requirements of section 110(a)(1) and (2), as applicable. These ‘‘infrastructure SIPs’’ establish 

the basic state programs to implement, maintain, and enforce revised NAAQS and provide 

assurances of state resources and authorities. States are to develop and maintain an air quality 

management infrastructure that includes enforceable emission limitations, a permitting program, 

an ambient monitoring program, an enforcement program, air quality modeling capabilities, and 

adequate personnel, resources, and legal authority. Section 110(b) of the CAA provides that the 

EPA may extend the deadline for the ‘‘infrastructure’’ SIP submission for a revised secondary 

standard by up to 18 months beyond the initial 3 years. If both the primary NAAQS and a 

distinct secondary NAAQS are finalized, the EPA currently believes it would be more efficient 

for states and the EPA if each affected state submits a single section 110 infrastructure SIP that 

addresses both standards at the same time (i.e., within 3 years of promulgation of the O3 

NAAQS), because the EPA does not at present discern any need for there to be any significant 

 

251 While the CAA allows the EPA to set a shorter time for submission of these SIPs, the EPA 

does not currently intend to do so.
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substantive difference in the infrastructure SIPs for the two standards. However, the EPA also 

recognizes that states may prefer the flexibility to submit the secondary NAAQS infrastructure 

SIP at a later date. The EPA solicits comment on these infrastructure SIP submittal timing 

considerations, and specifically on challenges that would justify needing 18 additional months to 

complete the submission of an infrastructure SIP for the secondary standard. 

It is the responsibility of each state to review its air quality management program’s 

infrastructure SIP provisions in light of each revised NAAQS. Most states have revised and 

updated their infrastructure SIPs in recent years to address requirements associated with recently 

revised NAAQS. It may be the case that for a number of infrastructure elements, the state may 

believe it has adequate state regulations already adopted and approved into the SIP to address a 

particular requirement with respect to the revised O3 NAAQS. For such portions of the state’s 

infrastructure SIP submittal, the state may provide a ‘‘certification’’ specifying that certain 

existing provisions in the SIP are adequate. Although the term ‘‘certification’’ does not appear in 

the CAA as a type of infrastructure SIP submittal, the EPA sometimes uses the term in the 

context of infrastructure SIPs, by policy and convention, to refer to a state’s SIP submission. If a 

state determines that its existing EPA-approved SIP provisions are adequate in light of the 

revised O3 NAAQS with respect to a given infrastructure SIP element (or sub-element), then the 

state may make a ”certification” that the existing SIP contains provisions that address those 

requirements of the specific CAA section 110(a)(2) infrastructure elements. In the case of a 

certification, the submittal does not have to include another copy of the relevant provision (e.g., 

rule or statute) itself. Rather, the submittal may provide citations to the already SIP-approved 

state statutes, regulations, or non-regulatory measures, as appropriate, which meet the relevant 

CAA requirement. Like any other SIP submittal, such certification can be made only after the 
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state has provided reasonable notice and opportunity for public hearing. This ‘‘reasonable notice 

and opportunity for public hearing’’ requirement for infrastructure SIP submittals appears at 

section 110(a), and it comports with the more general SIP requirement at section 110(l) of the 

CAA. Under the EPA’s regulations at 40 CFR part 51, if a public hearing is held, an 

infrastructure SIP submittal must include documentation by the state that the public hearing was 

held in accordance with the EPA’s procedural requirements for public hearings. See 40 CFR part 

51, Appendix V, paragraph 2.1(g), and 40 CFR 51.102.

4. Nonattainment Area Requirements 

Part D of the CAA describes the various program requirements that apply to states with 

nonattainment areas for different NAAQS. Section 182 (found in subpart 2 of Part D) includes 

the SIP requirements that govern the O3 program, and supplements the more general 

nonattainment area requirements in sections 172 and 173. Under CAA section 182, states 

generally are required to submit attainment demonstration SIPs within 3 or 4 years of the 

effective date of area designations by the EPA, depending on the classification of the area. These 

plans need to show how the nonattainment area will attain the primary O3 standard ‘‘as 

expeditiously as practicable,’’ but no later than within the relevant time frame from the effective 

date of designations associated with the classification of the area. 

Section 181(a)(1) of the CAA establishes classification categories for areas designated 

nonattainment for the primary O3 NAAQS. These categories range from “Marginal,” the lowest 

O3 classification with the fewest requirements associated with it, to “Extreme,” the highest 

classification with the most required programs. Areas with worse O3 problems are given more 

time to attain the NAAQS and more associated emission control requirements. Pursuant to 

previous O3 NAAQS reviews, the EPA set the secondary NAAQS equal to the primary NAAQS. 
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Thus, previous implementation programs for O3 standards did not include a separate 

classification threshold methodology for the secondary NAAQS. For this NAAQS review, which 

may result in a secondary standard different in form and level compared to the primary standard, 

the EPA is considering methodologies for establishing the air-quality based thresholds for 

assigning the section 181 classifications to areas out of attainment with a secondary O3 NAAQS.

Any such methods would be proposed for comment if the EPA finalizes a distinct secondary 

NAAQS.

There are two main EPA rulemakings relating to implementation of the 2008 O3 NAAQS. 

In May 2012, the EPA issued the final Classifications Rule (77 FR 30160; May 21, 2012). The 

Classifications Rule detailed the classifications approach, established attainment deadlines and 

revoked the 1997 O3 NAAQS for purposes of transportation conformity. In June 2013, the EPA 

proposed a SIP Requirements Rule (78 FR 34178; June 6, 2013) to provide rules and guidance to 

states regarding development of SIPs to attain the 2008 O3 NAAQS. The EPA believes that the 

overall framework and policy approach of the proposed SIP Requirements Rule for the 2008 O3 

NAAQS provides an effective and appropriate template for the general approach states would 

follow in planning for attainment of a revised primary O3 standard. The EPA intends to develop 

and propose a new SIP Requirements Rule that will address, to the extent necessary, any new 

implementation requirements that would result from any revised O3 NAAQS. The EPA intends 

to propose this implementation rule within 1 year after the revised O3 NAAQS are promulgated, 

and finalize the implementation rule by no later than the time the area designations process is 

finalized (approximately 2 years after promulgation of the O3 NAAQS).

In general, when developing an attainment plan, the state begins with the evaluation of 

the air quality improvements the nonattainment area can expect in the future due to ‘‘on the 
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books’’ existing federal, state, and local emission reduction measures. The state then must 

conduct a further assessment of relevant NOX and VOC emission sources in the nonattainment 

area, and the additional reasonably available control measures (RACM) and reasonably available 

control technology (RACT) that can be implemented by these sources, in determining how soon 

the area can attain the standard. Under section 172(c)(1) of the CAA as interpreted by the EPA, 

attainment demonstrations must include a RACM analysis showing that no additional reasonably 

available measures could be adopted and implemented such that the SIP could specify an 

attainment date that is 1 or more years earlier.

The evaluation of these potential emissions reductions and associated air quality 

improvement is commonly performed with sophisticated air quality modeling tools. Given that 

O3 concentrations are affected both by regionally-transported O3 and O3 precursor emissions and 

emissions of precursors from local sources in the nonattainment area (e.g., industrial sources, 

EGUs, and on-road mobile sources), the EPA recommends the use of regional grid-based models 

(such as CMAQ and CAMx) to develop O3 attainment strategies. Although, as described above,

the EPA projects significant improvements in O3 concentrations regionally resulting from a 

number of ongoing emission reduction programs already in place (e.g., mobile source engine and 

fuel standards and regulations for power plants) and from a number of recently promulgated 

rules such as the Cross State Air Pollution Rule (76 FR 48208; August 8, 2011), the Mercury and 

Air Toxics Standards rule (77 FR 9304; February 16, 2012) and the Tier 3 rule (79 FR 23414; 

April 28, 2014) that will result in VOC and NOX reductions from many geographically dispersed 

sources, local reductions of direct O3 precursors can also result in important health benefits. 

States must also ensure that a nonattainment area will make ‘‘reasonable further 

progress’’ (RFP) in accordance with subpart 2 of the CAA from the time of the nonattainment 
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designation to its attainment date. The amount of RFP required is based on the classification of 

the nonattainment area. Under the approach outlined in the proposed SIP Requirements Rule for 

the 2008 O3 NAAQS, areas designated nonattainment and classified as Moderate would 

generally be required to reduce emissions by 15% over the first six years after the effective date 

of designations. Areas classified higher than Moderate would be required to produce additional 

emission reductions after this 6-year period for an area that average 3% reductions per year. All 

RFP and attainment plans must also include contingency measures which would apply without 

significant delay in the event the area fails to attain by its attainment date or meet RFP 

milestones.

The EPA expects that the same general approach for determining attainment of the 

previous 1997 and 2008 8-hour O3 primary standards by the attainment deadline would be 

followed for determining attainment with any revised primary O3 standard. Attainment would be 

evaluated based on the 3 most recent years of certified, complete, and quality-assured air quality 

data in the nonattainment area. Areas are able to obtain up to two 1-year attainment date 

extensions provided under CAA section 181 under certain circumstances. Under previous 8-hour 

O3 NAAQS rules, an area whose design value based on the most recent 3 years of data exceeds 

the standard could receive a 1-year attainment date extension if the air quality concentration for 

the third year alone does not exceed the level of the standard. Similarly, an area that has received 

a 1-year extension could receive a second 1-year extension if the average of the area’s air quality 

concentration in the ‘‘extension year’’ and the previous year does not exceed the level of the 

standard. 

B. Implementing a Distinct Secondary O3 NAAQS, if One is Established 
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In each of the previous O3 NAAQS reviews the secondary standard was set equal to the 

primary standard. As discussed in section IV of this preamble, the EPA is proposing to retain the 

current averaging time and form of the secondary standard and to revise the level. The EPA is 

also soliciting comment on the alternative approach of revising the secondary standard to a 

cumulative, seasonal, concentration-weighted form based on the W126 index.

If the EPA were to establish a distinct secondary standard, there would be unique 

implementation issues to consider. These could include issues related to, but not limited to, PSD 

implementation, nonattainment area classification thresholds, attainment planning, and 

conformity demonstrations. These issues would be addressed in future implementation rules and 

guidance, as necessary. The EPA solicits comments on the specific kinds of implementationrelated issues (with examples, where possible) that air agencies and affected sources would face 

if a separate and distinct secondary standard is established.

C. Designation of Areas

After the EPA establishes or revises a NAAQS, the CAA directs the EPA and the states 

to take steps to ensure that the new or revised NAAQS is met. One of the first steps, known as 

the initial area designations, involves identifying areas of the country that either do not meet the 

new or revised NAAQS along with the nearby areas that contribute to the violations. 

Section 107(d)(1) of the CAA provides that, "By such date as the Administrator may 

reasonably require, but not later than 1 year after promulgation of a new or revised national 

ambient air quality standard for any pollutant under section 109, the Governor of each state 

shall... submit to the Administrator a list of all areas (or portions thereof) in the state" that 

designates those areas as nonattainment, attainment, or unclassifiable. The EPA must then 

promulgate the area designations according to a specified process, including procedures to be 
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followed if the EPA intends to modify a recommendation. The CAA defines an area as 

nonattainment if it is violating the NAAQS or if it is contributing to a violation in a nearby area. 

Section 107(d)(1)(B)(i) further provides, "Upon promulgation or revision of a national 

ambient air quality standard, the Administrator shall promulgate the designations of all areas (or 

portions thereof)... as expeditiously as practicable, but in no case later than 2 years from the date 

of promulgation of the new or revised national ambient air quality standard. Such period may be 

extended for up to one year in the event the Administrator has insufficient information to 

promulgate the designations.” In certain contexts, with respect to the NAAQS, the term 

“promulgation” has been interpreted by the courts to be signature and widespread dissemination 

of a final NAAQS rule.252 By no later than 120 days prior to promulgating area designations, the 

EPA is required to notify states of any intended modifications to their recommendations that the 

EPA may deem necessary. States then have an opportunity to demonstrate why any proposed 

modification is inappropriate. Whether or not a state provides a recommendation, the EPA must 

timely promulgate the designation that the agency deems appropriate. 

While section 107 of the CAA specifically addresses states, the EPA intends to follow the 

same process for tribes to the extent practicable, pursuant to CAA section 301(d) regarding tribal 

authority and the Tribal Authority Rule (63 FR 7254, February 12, 1998). To provide clarity and 

consistency in doing so, the EPA issued a 2011 guidance memorandum on working with tribes 

during the designation process.253

 

252 American Petroleum Institute v. Costle, 609 F.2d 20 (D.C. Cir. 1979).

253 Page, S. (2011). Guidance to Regions for Working with Tribes during the National Ambient 

Air Quality Standards (NAAQS) Designations Process, Memorandum from Stephen D. Page, 

Director, EPA Office of Air Quality Planning and Standards to Regional Air Directors, Regions 

I-X, December 20, 2011. Available:

http://www.epa.gov/ttn/oarpg/t1/memoranda/20120117naaqsguidance.pdf.

xx 
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As discussed in sections II and IV of this preamble, the EPA is proposing to revise both 

the primary and secondary O3 NAAQS, which currently are identical 8-hour standards that were 

set at 0.075 ppm in the 2008 NAAQS rule (73 FR 16436; March 27, 2008). If the EPA revises 

the primary and secondary O3 NAAQS based on this proposal, the EPA intends to complete 

designations for both NAAQS following the standard 2-year process discussed above. The EPA 

is required to sign the final rule for this O3 NAAQS review no later than October 1, 2015, under a 

court-ordered deadline. In accordance with section 107(d)(4) of the CAA, state Governors (and 

tribes, if they choose) should submit their initial designation recommendations for a revised 

primary and secondary NAAQS no later than 1 year after promulgation of any revised O3 

NAAQS (for example, by October 1, 2016, if the EPA promulgates such NAAQS on October 1, 

2015.) If the EPA intends to modify any state recommendation, the EPA would notify the 

appropriate state Governor (or tribal leader) no later than 120 days prior to making final 

designation decisions. A state or tribe that believes the modification is inappropriate would then 

have the opportunity to demonstrate to EPA why it believes its original recommendation (or a 

revised recommendation) is more appropriate. The EPA would take any additional input into 

account in making the final designation decisions. 

Consistent with previous designations, the EPA intends to use area-specific multi-factor 

analyses to support area boundary decisions for any revised primary or secondary O3 standards. 

Historically, the EPA has evaluated information related to the following factors for designations: 

air quality data, emissions-related data, meteorology, geography/topography, and jurisdictional 

boundaries. The EPA solicits comment related to establishing area designation boundaries for the 

proposed revised primary and secondary NAAQS, including any relevant technical information 

that should be considered by the EPA and the extent to which different considerations may be 
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relevant to establishing boundaries for a distinct secondary NAAQS. As noted earlier, the EPA 

intends to issue designation guidance to the states shortly after the promulgation of any revised

O3 NAAQS to provide information on the designation process and to assist states in developing 

their recommendations. The EPA invites preliminary comment on all aspects of the designation 

process at this time, which the EPA will consider in developing that guidance.

D. Prevention of Significant Deterioration and Nonattainment New Source Review

Programs for the Proposed Revised Primary and Secondary O3 NAAQS 

The CAA, at parts C and D of title I, contains NSR requirements that constitute 

preconstruction review and permitting programs applicable to new major stationary sources and 

major modifications of existing major sources. The preconstruction review of each new major 

source and major modification generally applies on a pollutant-specific basis, and the 

requirements that apply for each pollutant generally depend on whether the area is designated as 

attainment (or unclassifiable) or nonattainment for that pollutant. For the O3 NAAQS, in areas 

designated attainment and unclassifiable, the PSD requirements under part C apply. In 

nonattainment areas for O3, the NNSR requirements under part D apply. Collectively, those two 

sets of permit requirements are commonly referred to as the “major NSR programs.”

Until areas are designated for the proposed revised O3 NAAQS, the NSR provisions 

applicable under an area’s designation for the 2008 NAAQS (including any applicable antibacksliding requirements) would continue to apply. See 40 CFR 51.166(i)(2) and 52.21(i)(2). 

When the new designations for any revised O3 NAAQS are effective, they generally will serve to 

determine whether the PSD or nonattainment NSR program applies. 

1. Prevention of Significant Deterioration (PSD)
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The statutory requirements for a PSD permit program set forth under part C (sections 160 

through 169 of the CAA) are addressed by the EPA’s PSD regulations found at 40 CFR 51.166 

(minimum requirements for an approvable SIP) and 40 CFR 52.21 (federal PSD permit program 

for areas lacking an EPA-approved PSD program in the applicable SIP and for lands owned by 

the federal government and tribal lands). Both sets of regulations already apply to O3. See 40 

CFR 51.166(b)(23), (49); 40 CFR 52.21(b)(23), (50).. Among other things, in attainment and 

unclassifiable areas, the PSD program requires a new major stationary source or a major 

modification to an existing major source to apply BACT for each applicable pollutant and to 

conduct an air quality impact analysis to demonstrate that the proposed source or project will not 

cause or contribute to a violation of any NAAQS or PSD increment (see CAA section 165(a)(3)-

(4), 40 CFR 51.166(j)-(k), 40 CFR 52.21(j)-(k)). PSD requirements may also include, in 

appropriate cases, an analysis of potential adverse impacts on Class I areas (see CAA sections 

162 and 165). 254 These existing requirements of the PSD program will remain applicable to O3 

and the demonstration required under 40 CFR 51.166(k) and 52.21(k) will apply to any revised 

O3 NAAQS when such NAAQS become effective, except to the extent that a pending permit 

application is subject to a grandfathering provision that the EPA establishes through rulemaking. 

To address ambient O3 impacts of VOC and NOX precursor emissions from individual 

stationary sources, Appendix W to 40 CFR part 51 currently directs states to consult with the 

applicable EPA Regional Office to determine the appropriate techniques on a case-by-case basis, 

which may or may not involve the use of air quality models, for evaluating whether a PSD 

 

254 Congress established certain Class I areas in section 162(a) of the CAA, including 

international parks, national wilderness areas, and national parks that meet certain criteria. Such 

Class I areas, known as mandatory federal Class I areas, are afforded special protection under the 

CAA. In addition, states and tribal governments may establish Class I areas within their own 

political jurisdictions to provide similar special air quality protection.
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source causes or contributes to a violation of the O3 NAAQS (40 CFR part 51, Appendix W, 

section 5.2.1.c). At present, the EPA is evaluating the models and techniques available to address 

atmospheric chemistry of O3 formation in assessing such single source impacts, and as part of 

that evaluation has conducted discussions of such tools with the regulatory modeling community. 

Consistent with its commitment to engage in a rulemaking process to determine whether updates 

to Appendix W in 40 CFR part 51 are warranted,255 the EPA is planning to propose a rulemaking 

in the spring of 2015 to consider whether to update Appendix W. If the EPA concludes that it is 

technically and scientifically appropriate, it will propose appropriate regulatory updates to 

Appendix W as part of that rulemaking and may also make related updates to technical guidance, 

as appropriate. In the meantime, in order to demonstrate that a proposed source or modification 

does not cause or contribute to a violation of the applicable O3 NAAQS, PSD permit applicants 

would follow the current provisions in Appendix W until any revisions to them are in effect.

For PSD, a “major stationary source” is one with the potential to emit 250 tons per year 

(TPY) or more of any regulated NSR pollutant, unless the new or modified source is classified 

under a list of 28 source categories contained in the statutory definition of “major emitting

facility” in section 169(1) of the CAA. For those 28 source categories, a “major stationary 

source” is one with the potential to emit 100 TPY or more of any regulated NSR pollutant. A 

“major modification” is a physical change or a change in the method of operation of an existing 

major stationary source that results first, in a significant emissions increase of a regulated NSR 

 

255 See Letter from Gina McCarthy, Assistant Administrator, to Robert Ukeiley, at 1 (Jan. 4, 

2012), available at

http://www.epa.gov/scram001/10thmodconf/review_material/Sierra_Club_Petition_OAR-11-

002-1093.pdf.
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pollutant at a project, and second, in a significant net emissions increase of that pollutant at the 

source.256 See 40 CFR 51.166(b)(2)(i), 40 CFR 52.21(b)(2)(i). 

The EPA’s regulations define the term “regulated NSR pollutant” to include “[a]ny 

pollutant for which a [NAAQS] has been promulgated and any pollutant identified [in EPA 

regulations] as a constituent or precursor to such pollutant” (40 CFR 51.166(b)(49); 40 CFR 

52.21(b)(50)). These regulations identify VOC and NOX as precursors to O3 in all attainment and 

unclassifiable areas (40 CFR 51.166(b)(49)(i)(a); 40 CFR 52.21(b)(50)(i)(a)). Thus, for O3, the 

PSD program currently requires the review and control of emissions of VOC and NOX, as 

applicable, as precursors of O3. 

As noted above, section 165(a)(3) of the CAA and the implementing PSD regulations 

require the owner or operator of a proposed facility to, among other things, demonstrate that 

“emissions from construction or operation of such facility will not cause, or contribute to, air 

pollution in excess of any ... national ambient air quality standard in any air control region.” See 

also 40 CFR 51.166(k), 40 CFR 52.21(k). The EPA has interpreted this requirement to include 

any NAAQS that is in effect as of the date a permit is issued, unless it has grandfathered permit 

applications from the requirement to demonstrate that the proposed facility does not cause or 

 

256 As explained in 40 CFR 52.21(a)(2)(iv)(a) and 51.166(a)(7)(iv)(a), “[t]he project is not a 

major modification if it does not cause a significant emissions increase. If the project causes a 

significant emissions increase, then the project is a major modification only if it also results in a 

significant net emissions increase.” The PSD regulations at 40 CFR 51.166(a)(7) and 52.21(a)(2) 

also explain in more detail the two-pronged test for determining whether a proposed project at a 

facility is a major modification. 
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contribute to a violation of the new or revised NAAQS.257 See, e.g., 73 FR 28321, 28324, 28340 

(May 16, 2008); 78 FR 3253 (Jan. 15, 2013); Memorandum from Stephen D. Page, Director, 

Office of Air Quality Planning & Standards, “Applicability of the Federal Prevention of 

Significant Deterioration Permit Requirements to New and Revised National Ambient Air 

Quality Standards” (April 1, 2010). Consistent with this interpretation, any revised O3 NAAQS 

finalized through this rulemaking will need to be addressed by PSD permit applicants and 

permitting authorities, in permits issued on or after the date when the revised NAAQS become 

effective, unless the permit application has been grandfathered through rulemaking, as described 

below in this proposal.

Because the complex chemistry of O3 formation poses significant challenges for the 

assessing the impacts of individual stationary sources on O3 formation, the EPA’s judgment has 

been that it was not technically sound to designate a specific air quality model that must be used 

in the PSD permitting process to make this demonstration for O3.

258 The EPA has explained that 

sources must make the demonstration required under CAA section 165(a)(3) and the 

 

257In the past the EPA has asserted the discretion to take such grandfathering action, under 

appropriate circumstances, either by rulemaking or through a case-by-case determination for a 

specific permit application. The United States Court of Appeals for the Ninth Circuit recently 

vacated a decision by the EPA to issue an individual PSD permit grandfathering a permit 

applicant from certain requirements. See Sierra Club v. EPA, 762 F.3d 971 (9th Cir. 2014). In 

light of that decision, the EPA is no longer asserting authority to grandfather permit applications 

on a case-by-case basis. However, in the same opinion the court also stated that it did “not doubt, 

or express any opinion on, the EPA’s traditional authority to employ formal rulemaking to 

implement grandfathering” and distinguished that authority from the permit-specific 

grandfathering at issue in the case before it. Id., at 982, n.7 & 982-983. Thus, the EPA does not 

interpret this opinion to limit its authority to grandfather through rulemaking, but rather believes 

that the decision offers support for such authority.

258 Letter from Gina McCarthy, Assistant Administrator, to Robert Ukeiley, at 2 (Jan. 4, 2012), 

available at http://www.epa.gov/scram001/10thmodconf/review_material/Sierra 

_Club_Petition_OAR-11-002-1093.pdf. 
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implementing regulations, that this demonstration necessarily involves an analysis, and has 

established a process to determine on a case-by-case basis, in consultation with the appropriate

EPA Regional Office, what analytical techniques should be used to assess the impact of an 

individual source259 (40 CFR Part 51, Appendix W, Section 5.2.1.c). The EPA has, however, 

granted a petition from Sierra Club requesting, among other things, that it initiate rulemaking to 

designate air quality models for O3, and consistent with that petition grant, has been going 

through a process to evaluate potential updates to Appendix W.260 While that process is 

underway, individual sources should continue to follow the existing procedures to determine 

what method is appropriate to use to evaluate their impacts on O3 formation.

The PSD rules in 40 CFR 51.166(i)(2) and 52.21(i)(2) contain an exemption for 

particular pollutants from the PSD requirements if the owner or operator of the source 

demonstrates that the area in which the facility is located is designated as nonattainment for that 

pollutant under CAA section 107. Thus, new major sources and modifications will generally be 

subject to the PSD program requirements for O3 if they are locating in an area that does not have 

a current nonattainment designation under CAA section 107 for O3. As explained in the recent 

proposal for the implementation rule for the 2008 O3 NAAQS, references to historical 

nonattainment designations for a revoked standard should not be viewed as current 

“nonattainment designation[s] under CAA section 107” within the meaning of 40 CFR 

51.166(i)(2) and 52.21(i)(2) and, therefore, do not trigger the exemption from PSD requirements 

that would otherwise result from those provisions (78 FR 34216, June 6, 2013). 

a. PSD Grandfathering Provision

 

259 See, e.g., id.

260 See id. at 1 and 3.
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Recognizing that some PSD applications may have already been submitted and could be 

in the review process when a revised O3 NAAQS becomes effective, the EPA is proposing a 

transition plan that would enable certain PSD applications to make the demonstration that the 

proposed project will not cause or contribute to a violation of any NAAQS with respect to the O3

NAAQS that were in effect on the date the reviewing authority determines the permit application 

complete or the date the public notice on the draft permit or preliminary determination is first 

published (depending on which grandfathering provision applies), rather than the revised O3

NAAQS.261

The EPA is proposing and taking comment on adding a grandfathering provision to 

EPA’s regulations at 40 CFR 51.166 and 52.21 that would apply specifically to two categories of 

PSD permit applications that are pending when the EPA issues the revised O3 NAAQS: (1) 

applications for which the reviewing authority has formally determined that the application is 

complete on or before the signature date of the revised NAAQS; and (2) applications for which 

the reviewing authority has first published a public notice of a draft permit or preliminary 

determination before the effective date of the revised NAAQS. These two categories are 

proposed because some states do not do completeness determinations as part of their permit 

process.

 

261 The proposed grandfathering provision is intended to apply to pending PSD permit 

applications that meet one or both of the specified criteria and that are for sources locating in 

areas where PSD continues to apply with respect to O3 at the time the permit is issued. The 

proposed grandfathering provision is not intended to apply to sources locating in areas where 

NNSR applies at the time of permit issuance (for example, if the area had been designated as 

attainment for O3 when the permit application was submitted but was subsequently designated as 

nonattainment for O3 and that nonattainment designation would be in effect when the permit

would be issued). For such sources, the permit application must be resubmitted in accordance 

with the applicable NNSR requirements.
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As explained above, the EPA interprets the CAA and implementing PSD regulations at 

40 CFR 52.21(k)(1) and 51.166(k)(1) to require that PSD permit applications must include a 

demonstration that new major sources and major modifications will not cause or contribute to a 

violation of any NAAQS that is in effect as of the date the PSD permit is issued. Thus, if the 

EPA revises the O3 NAAQS, any proposed new source or modification with a PSD permit 

application pending at the time the revised O3 NAAQS takes effect would be expected to conduct 

an analysis to demonstrate that it does not cause or contribute to a violation of that NAAQS, 

absent some type of transition provision exempting the application from that requirement. This 

demonstration, as noted above, should be completed in consultation with the applicable EPA 

Regional Office.

Nevertheless, the agency has previously recognized that the CAA provides discretion for 

the EPA to grandfather PSD permit applications from requirements that become applicable while 

the applications are pending (45 FR 52683, August 7, 1980; 52 FR 24672, July 1, 1987; 78 FR 

3086, January 15, 2013). As discussed in more detail in these referenced actions, section 

165(a)(3) of the CAA requires that a permit applicant demonstrate that its proposed project will 

not cause or contribute to a violation of any NAAQS. At the same time, section 165(c) of the 

CAA requires that a PSD permit be granted or denied within 1 year after the permitting authority 

determines the application for such permit to be complete. In addition, section 301 of the CAA 

authorizes the Administrator “to prescribe such regulations as are necessary to carry out his 

functions under this chapter.” When read in combination, these three provisions of the CAA 

provide the EPA with the discretion to issue regulations to grandfather pending permit 

applications from having to address a revised NAAQS where necessary to achieve both CAA 

objectives to protect the NAAQS and to avoid delays in processing PSD permit applications. 
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Moreover, in a recent opinion the U.S. Court of Appeals for the Ninth Circuit recognized the 

EPA’s traditional exercise of grandfathering authority through rulemaking and indicated that this 

approach was consistent with statutory requirement to “enforce whatever regulations are in effect 

at the time the agency makes a final decision” because it involved identifying “an operative date, 

incident to setting the new substantive standard, and the grandfathering of pending permit 

applications was explicitly built into the new regulations.” Sierra Club v. EPA, 762 F.3d 971, 

983 (9th Cir. 2014).

In the EPA’s most recent adoption of a grandfathering provision for PSD, it adopted a 

provision for PM2.5 that provides a reasonable transition for implementing certain new PSD 

requirements related to the 2012 PM2.5 NAAQS for pending permit applications that have met 

certain criteria. As finalized, the PM2.5 grandfathering provision included the same two 

categories of permit applications that today are being proposed for O3. See 40 CFR 51.166(i)(10) 

and 52.21(i)(11). In the rulemaking, adding the grandfathering provision for the PM2.5 NAAQS, 

the EPA also provided a detailed rationale and legal basis for including the grandfathering 

provision in the PSD program. See 78 FR 3087 at 3253-59 (January 15, 2013); see also 77 FR 

39023-24 (June 29, 2012). 

When the PM2.5 NAAQS grandfathering provision was originally proposed, the EPA 

provided for only one category of pending PSD applications—applications for which the 

reviewing authority has published a public notice on the draft permit prior to the effective date of 

the revised PM2.5 NAAQS. A majority of the commenters supported the adoption of a 

grandfathering provision but some responded that a grandfathering milestone based on the 

submittal of a complete application would be more appropriate in order to avoid significant 

burdens associated with having to withdraw an application. These commenters pointed out the 
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significant level of effort, resources and time involved in preparing all of the information 

necessary for a complete permit application. They claimed that it would be unfair to establish 

grandfathering milestones beyond the complete application date because the processes and 

timeframe involved in generating the draft permit or preliminary determination materials and 

publishing the public notice are largely out of the control of the permit applicant and vary from 

agency to agency.

Based on this and other pertinent information provided by the commenters, the EPA 

concluded in that rulemaking that it should add an additional grandfathering milestone to avoid 

substantial additional burden and delay for permit applications that have reached a stage in the 

review process by which significant resources have been expended to complete PSD analyses 

and demonstrations that would have to be redone to address the revised NAAQS. Accordingly, 

the EPA adopted a grandfathering provision for the PM2.5 NAAQS in the final rule that included 

two milestones for establishing grandfathering eligibility. The EPA believes that these 

considerations and this rationale also apply to pending PSD permit applications that would be 

affected by a revised O3 NAAQS. Accordingly, the EPA is proposing to apply these same two 

milestones in this proposed rulemaking for the revised O3 NAAQS. 

The proposed grandfathering provision does not apply to any applicable PSD 

requirements related to O3 other than the requirement to demonstrate that the proposed source 

does not cause or contribute to a violation of any revised O3 NAAQS. Sources with projects 

qualifying under the grandfathering provision will be required to apply BACT to all applicable 

pollutants, demonstrate that the project emissions will not cause or contribute to a violation of 

the existing O3 NAAQS, and address any Class I area and additional O3-related impacts in 

accordance with the PSD regulatory requirements. 
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For the reasons provided both herein and in the prior EPA actions referenced above, the 

EPA proposes to amend the federal PSD permitting regulations at 40 CFR 52.21 to add the 

described grandfathering provision for the proposed O3 NAAQS revision. Specifically, the 

proposed provision provides that qualifying new sources and modifications seeking PSD permits 

under 40 CFR 52.21 shall not be required to demonstrate that their proposed emissions will not 

cause or contribute to a violation of the revised O3 NAAQS, but instead must demonstrate that 

their proposed emissions will not cause or contribute to a violation of the O3 NAAQS in effect on 

the date the reviewing authority determines the permit application complete or the date the public 

notice on the draft permit or preliminary determination is first published, depending on which 

prong of the grandfathering provision is applicable for that source. See proposed 40 CFR 

52.21(i)(12).

For sources subject to the PSD program under section 52.21, it should be noted that the 

EPA intends for a source that satisfies either milestone in the proposed revisions to section 

52.21(i) to be grandfathered from this requirement if those revisions are finalized. Accordingly, 

if a particular source does not qualify under the first milestone based on a complete application, 

it may qualify under the second milestone based on the issuance of a public notice. Conversely, a 

source may qualify for grandfathering under the first milestone, even if it does not satisfy the 

second. As explained below, states with EPA-approved PSD programs in their SIPs would have 

additional flexibility for implementing the proposed grandfathering provision to the extent that 

any alternative approach is at least as stringent as the federal provision.

The EPA also proposes that states that issue PSD permits under a SIP-approved PSD 

permit program should have discretion to “grandfather” pending PSD permits in the same 

manner under these same circumstances. Therefore, the EPA is proposing to revise its rules at 40 
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CFR 51.166 to provide a comparable exemption applicable to SIP-approved PSD programs, 

although such states are under no obligation to grandfather. See proposed 40 CFR 51.166(i)(11). 

The EPA recognizes that such states interested in grandfathering PSD sources for O3 will not 

have time to revise their rules and submit them to the EPA for approval into the SIP, since the 

need to grandfather sources will occur immediately upon the effective date of the revised O3

NAAQS. As explained in an earlier rulemaking, the EPA believes that states implementing a 

SIP-approved PSD program have the discretion to allow grandfathering consistent with the 

grandfathering provision contained in the federal rule provisions, even in the absence of an 

express grandfathering provision in their state rules, if the particular state’s laws and regulations 

may be interpreted to provide such discretion. See 78 FR 3086 at 3258.

Because state SIPs cannot be less stringent than federal requirements, the states’ 

discretion must be limited to applying grandfathering consistent with the federal rule provisions 

for O3. However, we believe that such consistent application affords states with ample flexibility 

for implementing the provision. Accordingly, a state may elect to apply both milestones or it 

may elect to rely solely upon one of the milestones for grandfathering PSD permits for O3. For 

example, in states that do not issue a formal completeness determination, the complete 

application milestone would not serve any practical purpose for grandfathering a PSD source, so 

the state may choose not to use this milestone. These states may elect to rely solely upon the 

public notice milestone, regardless of whether it issues formal completeness determinations. 

However, the EPA anticipates that once a decision is made concerning either the use of both 

milestones or only one, states will apply the provision consistently to all PSD permit applications 

that would qualify under the elected milestone(s).
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The EPA seeks comments on all aspects of the proposed grandfathering provisions under 

either 40 CFR 52.21 or 51.166 as they would apply to exempt certain pending PSD permit 

applications from having to address the revised O3 NAAQS. 

b. PSD Screening Tools

The EPA has historically allowed the use of screening tools to help facilitate the 

implementation of the NSR program by reducing the source’s burden and streamlining the 

permitting process for circumstances where pollutant emissions or ambient impacts could be 

considered de minimis. For example, the EPA has established significant emission rates or SERs 

that are used to determine when the NSR requirements should be applied to a particular new or 

modified source with regard to each regulated NSR pollutant. See 40 CFR 51.166(b)(23) and 

52.21(b)(23). For O3, the EPA established a separate SER in these regulations of 40 tpy for 

emissions of each O3 precursor--VOC and NOX. For PSD, these SER values for VOC and NOX

are used to determine when the proposed major source or major modification must complete 

PSD review for that precursor, including complying with BACT for that precursor and 

completing the appropriate air quality analyses associated with the proposed emissions increase 

of that precursor. 

Another key screening tool commonly used for PSD is the significant impact level (SIL). 

This particular tool is used to determine the extent to which an ambient impact analysis must be 

completed for the applicable pollutant. The EPA has not established a SIL for O3. The PSD 

regulations currently state that “[n]o de minimis air quality level is provided for ozone. However, 

any net emissions increase of 100 tons per year or more of [VOC] or [NOX] subject to PSD 

would be required to perform an ambient impact analysis, including the gathering of ambient air 
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quality data.” 262 The EPA intends to consider whether it is appropriate to make any revisions to 

the PSD regulations related to the screening tools for O3 in a separate rulemaking that will 

specifically address various implementation issues for O3. However, there are no such revisions 

being proposed in today’s rulemaking. Until any rulemaking to amend existing regulations is 

completed, permitting decisions should continue to be based on the existing 40 TPY SER for O3 

precursors (NOX and VOC) in existing regulations. Further decisions regarding the need for an 

analysis to assess the impact of an individual source on the O3 NAAQS and the method of 

analysis depend on the nature of the source and its emissions, and, as noted above, should be 

determined in consultation with the EPA Regional Office on a case-by-case basis in accordance 

with section 5.2.1.c. of Appendix W to 40 CFR part 51. 

c. Other PSD Transition Issues

As explained earlier in this section, the EPA anticipates that the existing O3 air quality in 

some areas will no longer be in attainment of the primary O3 standard when it is revised, and that 

these areas will be designated as “nonattainment” at a later date consistent with the designation 

process set forth for O3 under the CAA. However, until such nonattainment designation occurs, 

proposed new major sources or major modifications located in any area designated attainment or 

 

262 This language is contained in a footnote in the PSD regulations at 40 CFR 51.166(i)(5)(i) and 

52.21(i)(5)(i), and it has not been revisited by the EPA since the issuance of the 8-hour O3

NAAQS. These values do not reflect a categorical conclusion by the EPA that sources emitting 

less than 100 tpy of VOCs or NOX will not cause or contribute to a violation of the current (or any 

revised) O3 NAAQS, nor does it reflect a conclusion that such sources should be categorically 

excluded from the requirement for an ambient impact analysis. Instead, the EPA recommends 

consultation with the appropriate EPA Regional Office in accordance with section 5.2.1.c of 

Appendix W when a review of an application for a new source or modification involves 

emissions less than 100 tpy of either O3 precursor. See Letter from Gina McCarthy, Assistant 

Administrator, to Robert Ukeiley, at 4 (Jan. 4, 2012), available at 

http://www.epa.gov/scram001/10thmodconf/review_material/Sierra _Club_Petition_OAR-11-

002-1093.pdf.
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unclassifiable for the 2008 O3 NAAQS will continue to be required to obtain a PSD permit.263

This raises the question as to how a source can be issued a PSD permit in light of known existing 

ambient violations of the revised NAAQS. 

Section 165(a)(3)(B) of the CAA requires that a proposed source may not construct 

unless it demonstrates that it will not cause or contribute to a violation of any NAAQS. This 

statutory requirement is implemented through a provision contained in the PSD regulations at 40 

CFR 51.166(k) and 52.21(k). If a source cannot make this demonstration or if its initial air 

quality impact analysis shows that the source’s impact does cause or contribute to a violation, a 

PSD permit may not be issued until that adverse impact is mitigated.264 The PSD regulations, 

however, do not explicitly specify remedial actions that a prospective source can take to address 

such a situation. Nevertheless, the EPA has historically recognized in regulations and through 

other actions that sources applying for PSD permits may utilize offsets as part of the required 

PSD demonstration under the CAA section 165(a)(3)(B), even though the PSD provisions of the 

Act do not expressly reference offsets, in contrast to the NNSR provisions of the Act.265

The EPA has looked to the procedures contained in a separate set of regulations at 40 

CFR 51.165(b) to guide the process by which a source that is located in an area designated as 

 

263 Any proposed major stationary source or major modification for O3 that does not receive its 

PSD permit by the effective date of a new nonattainment designation for the area where the 

source would locate would then be required to satisfy all of the applicable NNSR preconstruction 

permit requirements for O3. 

264 See, e.g., Stephen D. Page, Director, Office of Air Quality Planning and Standards, to 

Regional Air Division Directors, “Guidance Concerning Implementation of the 1-hour SO2

NAAQS for the Prevention of Significant Deterioration Program,” (August 23, 2010); 44 FR 

3278 (January 16, 1979).

265 See, e.g., In re Interpower of New York, Inc., 5 E.A.D. 130, 141 (EAB 1994) (describing an 

EPA Region 2 PSD permit that relied in part on offsets to demonstrate the source would not 

cause or contribute to a violation of the NAAQS). 52 FR 24698 (July 1, 1987); 78 FR 3261-62 

(Jan. 15, 2013).
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attainment or unclassifiable for a NAAQS, but that is determined to cause or contribute to a 

violation of that NAAQS in any area, can use offsets to mitigate its adverse impact on the 

NAAQS and ultimately meet the PSD demonstration requirement under CAA section 

165(a)(3)(B) and the implementing regulations.266

Section 51.165(b) states that plans shall include a preconstruction review permit program 

(or its equivalent) to satisfy the requirements of CAA section 110(a)(2)(D)(i) for major sources 

and major modifications, and that the program shall apply to any major stationary source or 

major modification locating in an area designated attainment or unclassifiable for any NAAQS, 

when that source would cause or contribute to a NAAQS violation.267 Paragraph (b)(3) of that 

regulation provides that the required permit program may include a provision allowing a 

proposed major source or major modification to reduce the impact of its emissions on air quality 

by obtaining sufficient emissions reductions to, at a minimum, compensate for its adverse 

ambient impact where the source or modification would otherwise cause or contribute to a 

violation of any NAAQS. Although section 51.165(b) refers explicitly to CAA section 

110(a)(2)(D)(i), which now addresses transport issues, but not CAA section 165(a)(3)(B), the 

 

266 78 FR 3261 (January 15, 2013); Stephen D. Page, Director, Office of Air Quality Planning 

and Standards, to Regional Air Division Directors, “Guidance Concerning Implementation of the 

1-hour SO2 NAAQS for the Prevention of Significant Deterioration Program,” (August 23, 

2010).

267 The definition of “major stationary source” and “major modification” in this regulation is 

based on the respective definitions in the NNSR regulations at 40 CFR 51.165(a), which are 

more inclusive than the respective PSD definitions, but clearly include major sources covered by 

the PSD requirements.
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EPA has previously explained that 51.165(b) may also be interpreted to apply to the section 

165(a)(3)(B) demonstration based on the regulatory history (78 FR 3262, n. 256).

268

Generally, the offset needed to compensate for a proposed source’s adverse impact would 

be determined by the ability of any particular emissions reduction to mitigate the source’s 

adverse impact at the location of the violation. As long as the emissions reduction or offset can 

be shown to compensate for the source’s adverse impact, there is no implied requirement that the 

amount of the emissions reduction be equal to or greater than the proposed emissions increase. 

See 44 FR 3278 (January 16, 1979). (“Although full emissions offsets are not required, such a 

source must obtain emissions offsets sufficient to compensate for it air quality impact where the 

violation occurs.”) 

In previous discussions of the use of emissions offsets to help make the demonstration 

required under CAA section 165(a)(3)(B), the EPA has explained that any emissions used for 

PSD purposes must meet applicability criteria that are at least as stringent as the offset criteria set 

forth in the NNSR requirements for offsets under 40 CFR 51.165(a)(3). See 78 FR 3262. The 

 

268 Briefly, in 1980, the EPA had determined that the statutory requirements under CAA section 

165(a)(3)(B), taken together with the requirements of CAA section 110(a)(2)(D) required all 

major sources locating outside a nonattainment area, but causing or contributing to a NAAQS 

violation to reduce the impact on air quality so as to assure attainment and maintenance of the 

NAAQS. 45 FR 31310 (May 13, 1980). In a footnote, the EPA further indicated that this offset 

requirement must apply to sources causing or contributing to a newly discovered NAAQS 

violation until the area is designated nonattainment. See 45 FR 31310 (May 13, 1980). In the 

1980 rule, the EPA adopted section 51.18(k), which was later renumbered section 51.165(b). The 

EPA revised 51.165(b) to expressly authorize an offset program to meet the requirements of 

CAA section 110(a)(2)(D)(i), but this provision may also be interpreted to apply to section 

165(a)(3)(B), consistent with the EPA’s reading of section 51.18(k) in 1980. It is also worth 

noting that at the time of these rules, before the 1990 CAA amendments, section 110(a)(2)(D) 

required each state to have “a permit or equivalent program for any major emitting facility...to 

assure (i) that national air quality standards are achieved and maintained....”
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EPA continues to believe that these criteria provide the most appropriate criteria for determining 

the creditability of PSD offsets.

d. PSD for a Distinct Secondary Standard, if One is Established

As noted above, the CAA requires that proposed new major stationary sources and major 

modifications demonstrate that their emissions increases will not cause or contribute to a 

violation of any NAAQS, which includes the primary and secondary NAAQS. For O3, the 

existing primary and secondary NAAQS are defined in the same form and at the same level. As 

described earlier in this preamble, the Administrator is proposing to retain the current averaging 

time and form and revise the level of the current secondary standard to within the range of 70 to 

65 ppb. In addition, among other things, the agency is seeking comment on the alternative 

approach of revising the secondary standard to establish a distinct O3 secondary standard. If the 

agency were to finalize a secondary standard that differs from the primary standard, PSD permit 

applicants would be required to provide an analysis that specifically addresses the revised 

secondary standard and make the necessary showing of compliance with that standard, as well as 

any revised primary standard. Moreover, if such a secondary standard is expressed in a distinctly 

different form than the primary standard, the required analysis for making the compliance 

demonstration would need to be consistent with that form. 

Should the Administrator decide to establish a distinct secondary NAAQS for O3, the 

EPA would consider whether the approaches put forth in any regulatory updates to Appendix W 

and associated guidance, as noted in this preamble above, are sufficient for making the necessary 

compliance demonstration for that standard for purposes of PSD. If appropriate, the EPA may 

consider establishing a surrogacy policy that would allow a source to make the PSD-required 

demonstration of compliance with a distinct secondary O3 NAAQS solely through a 
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demonstration of compliance with the primary NAAQS. Therefore, the EPA expects that 

projects subject to the revised O3 NAAQS could generally move forward consistent with the 

PSD program requirements and NNSR program requirements as subject to the revised primary 

and secondary O3 NAAQS. The EPA seeks comment on this potential approach as well as any 

other options that should be considered for showing compliance with any revised primary and 

secondary O3 NAAQS. 

2. Nonattainment New Source Review

Part D of title I of the CAA includes preconstruction review and permitting requirements

for new major stationary sources and major modifications when they locate in areas designated 

nonattainment for a particular pollutant. As explained in section VII.D.1 of this preamble, the 

relevant part D requirements are typically referred to as the NNSR program. The EPA’s

regulations for the NNSR programs are contained in 40 CFR 51.165, 52.24 and Part 51, 

Appendix S. For example, the EPA has developed minimum program requirements for an NNSR 

program that is approvable in a SIP, and those requirements, which include requirements for O3, 

are contained in 40 CFR 51.165. In addition, 40 CFR part 51, Appendix S contains requirements 

constituting an interim NNSR program. This program governs NNSR permitting in 

nonattainment areas that lack a SIP-approved NNSR permitting program, and applies during the 

time between the date of the relevant designation and the date that the EPA approves into the SIP 

a NNSR program.269 This program is commonly known as the Emissions Offset Interpretative 

Rule, and is applicable to O3 as well. 270

 

269 See Appendix S, Part I; 40 CFR 52.24(k). 

270 As appropriate, certain NNSR requirements under 40 CFR 51.165 or Appendix S can also 

apply to sources and modifications located in areas that are designated attainment or 

unclassifiable in the Ozone Transport Region. See, e.g., CAA 184(b)(2), 40 C.F.R. 52.24(k).
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As with PSD, the NNSR requirements apply on a pollutant-specific basis. However, in 

nonattainment areas, NNSR applies only to nonattainment pollutants, that is, pollutants for which 

an area is designated nonattainment on the date when the permit is issued. As explained in 

section VII.D.1 of this preamble, prior to the designation of areas for any revised O3 NAAQS, 

applicability of either PSD or NNSR for O3 to a proposed major new or modified source will 

depend on an area’s current designations with regard to the O3 NAAQS. Accordingly, a major 

stationary source or major modification proposing to locate in an area currently designated 

nonattainment for the 2008 O3 NAAQS must satisfy the NNSR permit requirements for O3. The 

EPA is not proposing any new or revised NNSR requirements in this proposal. As explained in 

section VII.A.2 of this preamble, the CAA requires that area designations for new or revised 

NAAQS be addressed subsequent to the effective date of such new or revised NAAQS. The EPA 

anticipates that the area designation process for any revised O3 NAAQS will be completed within 

2 years after the revised NAAQS become effective. Accordingly, any revisions to the existing 

NNSR requirements for O3 will be proposed at a later date consistent with the designation 

process for any revised O3 NAAQS. The EPA will also at the same time propose any necessary 

revisions to the NNSR requirements under Appendix S so that states will be able to issue NNSR 

permits for the revised O3 NAAQS on and after the effective date of designations of new 

nonattainment areas for O3 until such time that their own NNSR program is approved as part of 

their SIP, where the state does not already have an approved NNSR program applicable to O3.

This section provides an explanation of some of the key requirements of the NNSR 

program as it currently applies to O3. For NNSR, “major stationary source” is generally defined 

as a source with the potential to emit at least 100 tpy of the pollutant for which the area is 

designated nonattainment. In some cases, however, the CAA and the NNSR regulations define 
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“major stationary source” for NNSR in terms of a lower rate dependent on the pollutant. For O3, 

in addition to the general threshold level of 100 tpy, lower major source thresholds have been 

defined for O3 nonattainment areas based on the stringency of the area’s classification. The 

NNSR program requires the review and control of emissions of both VOC and NOx as 

precursors of O3, and both are reviewed separately in accordance with the applicable major 

source threshold. For example, the threshold for O3 nonattainment areas classified as Serious is 

50 tpy for both VOC and NOx. See 40 CFR 51.165(a)(1)(iv)(A)(1)(i) and (a)(1)(iv)(A)(2)(iv), 

respectively.

As explained earlier in section VII.D.1 of this preamble, a major modification is a 

physical change or change in the method of operation of an existing major stationary source that 

results in both a significant emissions increase, and a significant net emissions increase. To 

determine whether an emissions increase is significant, the NNSR rules define significant 

emissions rates or SERs for each applicable pollutant. The SER for VOC is 40 tpy, as is the SER 

for NOx. See 40 CFR 51.165(a)(1)(x)(A). It should be noted that there are additional more 

stringent criteria that must be considered in determining a major modification in nonattainment 

areas classified as Serious, Severe or Extreme for O3. See 40 CFR 51.165(a)(1)(x)(B), (C) and 

(E).

New major stationary sources and major modifications for O3 must comply with the 

LAER as defined in the CAA and NNSR rules, as well as performing other analyses as required

under section 173 of the CAA. In addition, appropriate emissions reductions, known as 

emissions offsets, must be secured to offset the proposed emissions increase of the precursors 

that trigger NNSR for O3. The appropriate emissions offset needed for a particular source will 

depend upon the classification for the O3 nonattainment area in which the source or modification 
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will locate. Generally, the ratio of the total emissions reduction to the emissions increase is at 

least 1:1; however, more stringent ratios apply to O3 nonattainment areas according to the area 

classification. See, e.g., 40 CFR 51.165(a)(9) and 40 CFR part 51, Appendix S, IV.G.2. 

E. Transportation and General Conformity Programs

1. What are Transportation and General Conformity?

Conformity is required under CAA section 176(c) to ensure that federal actions are 

consistent with (“conform to”) the purpose of the SIP. Conformity to the purpose of the SIP 

means that federal activities will not cause new air quality violations, worsen existing violations, 

or delay timely attainment of the relevant NAAQS or interim reductions and milestones. 

Conformity applies to areas that are designated nonattainment, and those nonattainment areas 

redesignated to attainment with a CAA section 175A maintenance plan after 1990 (“maintenance 

areas”). 

The EPA’s Transportation Conformity Rule (40 CFR 51.390 and Part 93, subpart A) 

establishes the criteria and procedures for determining whether transportation activities conform 

to the SIP. These activities include adopting, funding or approving transportation plans, 

transportation improvement programs (TIPs) and federally supported highway and transit 

projects. For further information on conformity rulemakings, policy guidance and outreach 

materials, see the EPA’s Website at http://www.epa.gov/otaq/stateresources/transconf/index.htm. 

The EPA may issue future transportation conformity guidance as needed to implement a revised 

O3 NAAQS.

With regard to general conformity, the EPA first promulgated general conformity 

regulations in November 1993. (40 CFR part 51, subpart W, 40 CFR part 93, subpart B) 

Subsequently the EPA finalized revisions to the general conformity regulations on April 5, 2010. 
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(75 FR 17254-17279). Besides ensuring that federal actions not covered by the transportation 

conformity rule will not interfere with the SIP, the general conformity program also fosters 

communications between federal agencies and state/local air quality agencies, provides for 

public notification of and access to federal agency conformity determinations and allows for air 

quality review of individual federal actions. More information on the general conformity 

program is available at http://www.epa.gov/air/genconform/.

2. Why Is the EPA Discussing Transportation and General Conformity in this Proposed 

Rulemaking?

The EPA is discussing transportation and general conformity in this proposed O3 NAAQS 

rulemaking in order to provide affected parties with information on when and how conformity 

must be implemented after nonattainment areas are designated for a revised O3 NAAQS. The 

information presented here is consistent with existing conformity regulations and statutory 

provisions that are not addressed by this O3 NAAQS rulemaking. Affected parties would include 

state and local transportation and air quality agencies, metropolitan planning organizations 

(MPOs), and federal agencies including the U.S. Department of Transportation (DOT), the U.S. 

Department of Defense, the U.S. Department of Interior, and the U.S. Department of Agriculture. 

3. When Would Transportation and General Conformity Apply to Areas Designated 

Nonattainment for a Revised O3 NAAQS, if One is Established?

Transportation and general conformity apply one year after the effective date of 

nonattainment designations for a revised O3 NAAQS. This is because CAA section 176(c)(6) 

provides a 1-year grace period from the effective date of initial designations for any revised 

NAAQS before transportation and general conformity apply in areas newly designated 

nonattainment for a specific pollutant and NAAQS. 
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4. Will Transportation and General Conformity Apply to a Distinct Secondary O3 NAAQS, if 

One is Established? 

Section 176(c)(1)(A) of the CAA states that conformity to a SIP means “conformity to an 

implementation plan’s purpose of eliminating or reducing the severity and number of violations 

of the national ambient air quality standards and achieving expeditious attainment of such 

standards...” In other words, because the CAA refers to the NAAQS without distinguishing 

between them, conformity applies to both the primary and secondary NAAQS for all criteria 

pollutants. Therefore, if a distinct secondary O3 NAAQS is established, both transportation and 

general conformity will apply in any areas designated nonattainment for such a NAAQS.

Current transportation and general conformity regulations already apply to such a 

secondary NAAQS, and nothing in this proposal affects those transportation and general 

conformity requirements. The EPA will consider the need to issue additional guidance 

concerning the implementation of transportation and general conformity in areas designated 

nonattainment for a distinct secondary O3 NAAQS, if one is established. 

5. What Impact Would the Implementation of a Revised O3 NAAQS Have on a State’s 

Transportation and/or General Conformity SIP? 

If the EPA revises the O3 NAAQS, but does not make specific changes to its 

transportation or general conformity regulations, then states should not need to revise their 

transportation and/or general conformity SIPs. The EPA is not proposing any changes to its 

transportation or general conformity regulations. While we are not proposing any revisions to the 

general conformity regulations at this time, we recommend, when areas develop SIPs for a 

revised O3 NAAQS, that state and local air quality agencies work with federal agencies with 

large facilities that are subject to the general conformity regulations to establish an emissions 
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budget for those facilities in order to facilitate future conformity determinations under the 

conformity regulations. Such a budget could be used by federal agencies in determining 

conformity or identifying mitigation measures if the budget level is included and identified in the 

SIP. However, because some federal agencies may not have an established facility-wide 

emissions budget in the SIP for the purpose of meeting general conformity requirements, state, 

local and tribal agencies are encouraged to maintain ozone SIP emissions inventories on an 

annual basis, at a minimum, to facilitate compliance of federal agencies with CAA section

176(c). Finally, states with new nonattainment areas may also need to revise conformity SIPs in 

order to ensure the state regulations apply in any newly designated areas if the existing SIP does 

include current conformity provisions. 

If this is the first time that transportation conformity will apply in a state, such a state is 

required by the statute and EPA regulations to submit a SIP revision that addresses three specific 

transportation conformity requirements that address consultation procedures and written 

commitments to control or mitigation measures associated with conformity determinations for 

transportation plans, TIPs or projects. (40 CFR 51.390) Additional information and guidance can 

be found in the EPA’s “Guidance for Developing Transportation Conformity State 

Implementation Plans” 

(http://www.epa.gov/otaq/stateresources/transconf/policy/420b09001.pdf).

F. How Background O3 Is Addressed in CAA Implementation Provisions

1. Introduction

The EPA and state, local and tribal air agencies, need to determine how to most 

effectively and efficiently use the CAA’s various provisions to provide required public health 

and welfare protection from the harmful effects of O3. In most cases, reducing man-made 
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emissions of NOX and VOCs will reduce O3 formation and provide additional health and welfare 

protection. The EPA recognizes, however, that “background” O3 levels, which can be significant 

in some areas on some days, may present a challenge to air agencies in preparing clean air plans.

That is, O3 and O3-forming pollution from natural and international sources could prevent 

ambient levels from reaching attainment levels in locations where the impacts of such sources 

are large relative to the impact of controllable man-made sources of NOX and VOC emissions 

within the U.S., especially in locations with few remaining untapped opportunities for local 

emission reductions.

Climate change may also influence future O3 concentrations. Modeling studies in EPA’s 

Interim Assessment (U.S. EPA, 2009b) and cited in support of the 2009 Endangerment Finding 

(74 FR 66,496; Dec. 15, 2009) show that, while the impact is not uniform, simulated climate 

change causes increases in summertime O3 concentrations over substantial regions of the 

country, with increases tending to occur during higher peak pollution episodes in the summer. 

Increases in temperature are expected to be the principal factor in driving any O3 increases, 

although increases in stagnation frequency may also contribute (Jacob and Winner, 2009). These 

temperature increases could lead to more prevalent wildfires, the impacts of which may lessened 

by various mitigation measures including taking steps to minimize fuel loading in areas 

vulnerable to fire.

The term “background” O3 is often used to refer to O3 that originates from natural sources 

of O3 (e.g., wildfires and stratospheric O3 intrusions) and O3 precursors, as well as from 

manmade international emissions of O3 precursors. Using the term generically, however, can 

lead to confusion as to what sources of O3 are being considered. The PA provides three specific 

definitions of background O3: natural background, North American background, and United 
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States background. Natural background (NB) is defined as the O3 that would exist in the absence 

of any manmade O3 precursor emissions. North American background (NAB) is defined as that 

O3 that would exist in the absence of any manmade O3 precursor emissions from North America. 

U.S. background (USB) is defined as that O3 that would exist in the absence of any manmade 

emissions inside the U.S. Because background O3 is difficult to measure, air quality modeling is 

conducted to estimate NA, NAB, and USB.

The PA identifies several key findings related to background O3. First, background O3

can comprise a considerable fraction of total seasonal mean O3 across the U.S. Studies have 

estimated that seasonal mean USB 8-hour O3 values across U.S. locations varied between 25 to

50 ppb in 2007 (U.S. EPA, 2014c, Figure 211). The largest seasonal average values of 

background are modeled to occur at locations in the intermountain western U.S. and the highest 

daily USB levels are highest in the spring and early summer seasons. Second, the modeling 

indicates that U.S. anthropogenic emission sources are the dominant contributor to the majority 

of modeled O3 exceedances of the NAAQS across the U.S. This conclusion is based on results 

that indicate background contributions are generally similar on high O3 days as on all other O3

days. As a result, the proportional influence of background sources tends to be lower on high O3

days. Third, while the majority of modeled O3 exceedances have local and regional emissions as 

their primary cause, there can be events where O3 levels approach or exceed the concentration 

levels being proposed in this notice (i.e., 60-70 ppb) in large part due to background sources. 

These cases of high USB levels on high O3 days typically result from stratospheric intrusions of 

O3, wildfire O3 plumes, or long-range transport of O3 from sources outside the U.S. In most 

locations in the U.S., these events are relatively infrequent and the CAA contains provisions that 
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can be used to help deal with certain events, including providing varying degrees of regulatory 

relief for air agencies and potential regulated entities.

Regulatory relief associated with U.S. background O3 may include: 271

 Relief from designation as a nonattainment area (through exclusion of data affected by 

exceptional events)

 Relief from the more stringent requirements of higher nonattainment area classifications 

(through treatment as a rural transport area; through exclusion of data affected by 

exceptional events; or through international transport provisions)

 Relief from adopting more than reasonable controls to demonstrate attainment (through 

international transport provisions)

None of these relief mechanisms are completely burden-free, meaning they all require 

some level of assessment or demonstration by a state and/or EPA to legally invoke. In no case 

does the CAA authorize a blanket exclusion from the basic application of an air quality 

management regime because an area is significantly impacted by background O3. While any

prediction of the exact nature of future implementation challenges associated with alternative 

prospective standards is inherently uncertain, there is no question that, as the levels of alternative 

prospective standards are lowered, background will represent increasingly larger fractions of 

total O3 levels and may subsequently complicate efforts to attain these standards. For a 

prospective standard of 70 ppb, the EPA does not believe that background O3 would create 

significant implementation-related challenges at locations throughout the U.S. and prevent 

 

271 Note that the relief mechanisms discussed here do not include the CAA’s interstate transport 

provisions found in sections 110(a)(2)(D) and 126. The interstate transport provisions are 

intended to address the cross-state transport of O3 and O3 precursor emissions from man-made 

sources within the continental U.S. rather than background O3 as it is defined in this section.
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attainment of the NAAQS. However, as the levels of prospective standards are lowered, the areas 

that would most likely need to use the relief mechanisms discussed in this section as part of 

attaining the lower prospective levels are rural locations in the western U.S., consistent with the 

previously mentioned locations where we have estimated the largest seasonal average values of 

background occur. The remainder of this section discusses these relief mechanisms and the 

methods associated with legally invoking them. These relief mechanisms depend on 

distinguishing background O3 by the following types of drivers: routine natural emissions, nonroutine natural events and international emissions. The EPA welcomes comment on any of these 

issues related to O3 background and implementation.

2. Exceptional Events Exclusions

A state can request and the EPA can agree to exclude data associated with eventinfluenced exceedances or violations of a NAAQS, including the proposed O3 NAAQS, provided 

the event meets the statutory requirements in section 319 of the CAA: 

 The event “affects air quality.”

 The event “is not reasonably controllable or preventable.”

 The event is “caused by human activity that is unlikely to recur at a particular location or 

[is] a natural event.”272

The EPA’s implementing regulations, the 2007 Exceptional Events Rule, further specify 

that states must provide evidence that: 273

 

272 A natural event is further described in 40 CFR 50.1(k) as “an event in which human activity 

plays little or no direct causal role.”

273 Federal Register (2007). Treatment of Data Influenced by Exceptional Events; Final Rule. 40 

CFR 50 and 51; Federal Register 72:13560.
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 “There is a clear causal relationship between the measurement under consideration and 

the event that is claimed to have affected the air quality in the area;”

 “The event is associated with a measured concentration in excess of normal historical 

fluctuations, including background;” and

 “There would have been no exceedance or violation but for the event.”

The ISA contains discussions of natural events that may contribute to O3 or O3 precursors. 

These include stratospheric O3 intrusion and wildfire events.

274 As indicated above, to satisfy the 

exceptional event requirements and to qualify for data exclusion under the Exceptional Events 

Rule, a state must develop and submit evidence addressing each of the identified criteria. The 

extent to which a stratospheric O3 intrusion event or a wildfire event contribute to O3 levels can 

be uncertain, and in most cases requires detailed investigation and analysis to adequately 

determine.

Strong stratospheric O3 intrusion events, most prevalent at high elevation sites during 

winter or spring, can be identified based on measurements of low relative humidity, evidence of 

deep atmospheric mixing, and a low ratio of CO to O3 based on ambient measurements. 

Accurately determining the extent of weaker intrusion events remains challenging (U.S. EPA 

2013a, p.3-34). Although states have submitted only a few exceptional event demonstrations for 

 

274 The preamble to the Exceptional Events Rule (72 FR 13560, March 22, 2007) identifies both 

stratospheric O3 intrusions and wildfires as natural events that could also qualify as exceptional 

events under the CAA and Exceptional Event Rule criteria. Note that O3 resulting from routine 

natural emissions from vegetation, microbes, animals and lightning are not exceptional events 

authorized for exclusion under the section 319 of the CAA.
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stratospheric O3 intrusion, the EPA recently approved a demonstration from Wyoming for a June 

2012 stratospheric O3 event.275

While stratospheric O3 intrusions can increase monitored ground-level ambient O3

concentrations, wildfire plumes can either suppress or enhance O3 depending upon a variety of 

factors including fuel type, combustion stage, plume chemistry, aerosol effects, meteorological 

conditions and distance from the fire (Jaffe and Wigder, 2012). As such, determining the impact 

of wildfire emissions on specific O3 observations is challenging. The EPA recently approved an 

exceptional event demonstration for wildfires affecting 1-hour O3 levels in Sacramento, 

California in 2008 that successfully used a variety of analytical tools (e.g., regression modeling, 

back trajectories, satellite imagery, etc.) to support the exclusion of O3 data affected by large 

fires.

276

Because of previously expressed stakeholder feedback regarding implementation of the 

Exceptional Events Rule and specific stakeholder concerns regarding the analyses that can be 

used to support O3-related exceptional event demonstrations, the EPA intends to propose 

revisions to the Exceptional Events Rule in a future notice and comment rulemaking effort and 

will solicit public comment at that time. 

Additionally, the EPA intends to develop guidance to address implementing the 

Exceptional Events Rule criteria for wildfires that could affect ambient O3 concentrations. 

Wildfire emissions are a component of background O3 (Jaffe and Wigder, 2012) and can 

 

275 U.S. EPA (2014) Treatment of Data Influenced by Exceptional Events: Examples of 

Reviewed Exceptional Event Submissions. U.S. Environmental Protection Agency, Research 

Triangle Park, NC. Available at http://www.epa.gov/ttn/analysis/exevents.htm.

276 U.S. EPA (2014) Treatment of Data Influenced by Exceptional Events: Examples of 

Reviewed Exceptional Event Submissions. U.S. Environmental Protection Agency, Research 

Triangle Park, NC. Examples of O3-related exceptional event submissions available at 

http://www.epa.gov/ttn/analysis/exevents.htm. 
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significantly contribute to periodic high O3 levels (Emery, 2012). Besides their effect on air 

quality, wildfires pose a direct threat to public safety – a threat that can be mitigated through 

management of wildland vegetation. Attempts to suppress wildfires have resulted in unintended 

consequences, including increased risks to both humans and ecosystems. Indeed, “Fire policy 

that focuses on [wildfire] suppression only, delays the inevitable, promising more dangerous and 

destructive future...fires” (Stephens, S. et al., 2013). The use of wildland prescribed fire can 

influence the occurrence of catastrophic wildfires which may help manage the contribution of 

wildfires to background O3 levels and periodic peak O3 events. Additionally prescribed fires can 

have benefits to those plant and animal species that depend upon natural fires for propagation, 

habitat restoration, and reproduction, as well as myriad ecosystem functions (e.g., carbon 

sequestration). As previously indicated, the CAA and the EPA’s implementing regulations allow 

for the exclusion of air quality monitoring data from design value calculations when they are 

substantially affected by certain background influences. Additionally, the CAA requires the EPA 

to set the NAAQS at levels requisite to protect public health and welfare without regard to the 

source of the pollutant. However, EPA understands the importance of prescribed fire which 

mimics a natural process necessary to manage and maintain fire-adapted ecosystems and climate 

change adaptation, while reducing risk of uncontrolled emissions from catastrophic wildfires. 

The EPA is committed to working with federal land managers, tribes, and states to effectively 

manage prescribed fire use to reduce the impact of wildland-fire related emissions on ozone 

through policies and regulations implementing these standards.

3. Rural Transport Areas
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Clean Air Act section 182(h) authorizes the EPA Administrator to determine that an area 

designated nonattainment can be treated as a rural transport area. In accordance with the statute, 

a nonattainment area may qualify for this determination if it meets the following criteria:

 The area does not contain emissions sources that make a significant contribution to 

monitored O3 concentrations in the area, or in other areas; and

 The area does not include and is not adjacent to a Metropolitan Statistical Area.

277

Historically, the EPA has recognized few nonattainment areas under this statutory 

provision.278 The EPA has not issued separate written guidance to further elaborate on the 

interpretation of these CAA qualification criteria. However, the EPA developed draft guidance in 

2005 that explains the kinds of technical analyses that states could use to establish that transport 

of O3 and/or O3 precursors into the area is so overwhelming that the contribution of local 

emissions to an observed 8-hour O3 concentration above the level of the NAAQS is relatively 

minor and determine that emissions within the area do not make a significant contribution to the 

O3 concentrations measured in the area or in other areas.

279 While this guidance was not prepared 

specifically for rural transport areas, it could be useful to states for developing technical 

information to support a request that the EPA treat a specific O3 nonattainment area as a rural 

transport area.

 

277 Note that the EPA interprets the rural transport provisions of section 182(h) would not apply 

to an O3 monitor that is located in a relatively rural location, but is heavily influenced by shortrange upwind contributions from a nearby urbanized area. The EPA will work closely with states 

to determine whether a particular monitor violating the NAAQS is considered to be affiliated 

with a nearby urban area, or is an isolated rural area monitor.

278 For the 1979 1-hour O3 standard, Essex County, New York, and Smyth County, Virginia 

(White Top Mountain) were recognized by the EPA as rural transport areas.

279 U.S. Environmental Protection Agency (2005). Criteria For Assessing Whether an Ozone 

Nonattainment Area is Affected by Overwhelming Transport [Draft EPA Guidance]. U.S. 

Environmental Protection Agency, Research Triangle Park, NC. June 2005. Available at 

http://www.epa.gov/scram001/guidance/guide/owt_guidance_07-13-05.pdf.
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An area that qualifies for treatment as a rural transport area is deemed to have fulfilled all 

O3-related planning and control requirements if it meets the CAA’s requirements for areas 

classified Marginal, which is the lowest classification specified in the CAA. Therefore, a state 

would not need to develop an attainment plan or an attainment demonstration for such an area or 

adopt the various mandatory measures required in nonattainment areas classified as Moderate or 

above. The only requirements that would apply, regardless of the level of O3 air quality, would 

be NNSR (at the Marginal major source threshold and offset ratio), conformity requirements 

associated with a nonattainment designation, and the emission inventory and source emission 

statement requirements. 

4. International Transport

Clean Air Act section 179B recognizes the possibility that certain nonattainment areas 

may be heavily impacted by O3 or O3 precursor emissions from international sources beyond the 

regulatory jurisdiction of the state. The EPA’s science review suggests that the influence of 

international sources on U.S. O3 levels will be largest in locations that are in the immediate 

vicinity of an international border with Canada or Mexico, but other locations can also 

potentially be affected when conditions are favorable for long-range transport (U.S. EPA 2013a, 

p.3-140). Section 179B allows states to consider in their attainment demonstrations whether an 

area might have met the O3 NAAQS by the attainment date “but for” emissions contributing to 

the area originating outside the U.S. If a state is unable to demonstrate attainment in such an area 

after adopting all reasonably available control measures (RACM, including RACT, as required 

by CAA section 182(b)), the EPA can nonetheless approve the CAA-required state attainment 

plan and demonstration using the authority in section 179B. 
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When the EPA approves this type of attainment plan, states avoid potential sanctions and 

FIPs, and there would be no adverse consequence for a finding that the area failed to attain the 

NAAQS by the relevant attainment date. For example, the area would not be reclassified to the 

next highest classification or required to implement a section 185 penalty fee program. 

Section 179B authority does not allow the EPA to avoid designating an area as 

nonattainment or for the area to be classified with a lower classification than is indicated by 

actual air quality. Generally, monitoring data influenced by international transport may not be 

excluded from regulatory determinations, unless the data are influenced by an excludable 

exceptional event. Section 179B also does not provide for any relaxation of mandatory emissions 

control measures (including contingency measures) or the prescribed emissions reductions 

necessary to achieve RFP.

The EPA’s guidance on ‘but for’ demonstrations involving international emissions 

indicates that states may want to consider conducting air quality modeling using O3 episodes that 

do not involve international transport of emissions (U.S. EPA 1991)280), running the model with 

boundary conditions that reflect general U.S. background concentrations, and analyzing 

monitoring data if a dense network has been established. Additional information that may be 

helpful at nonattainment areas abutting international borders could include evaluating changes in 

O3 with changes in wind direction at monitors near the border, and comparing emissions on both 

sides of the border. States are encouraged to consult with their EPA Regional Office to establish 

appropriate technical requirements for these analyses.

 

280 U.S. Environmental Protection Agency (1991). Criteria for Assessing the Role of Transported 

Ozone/Precursors in Ozone Nonattainment Areas. EPA-450/4-91-015. U.S. Environmental 

Protection Agency, Office of Air Quality Planning and Standards, Research Triangle Park, NC. 

May 1991.
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The EPA has used section 179B authority previously to approve attainment plans for 

Mexican border areas in El Paso, TX (O3, PM10, and CO plans); Nogales, AZ (PM10 plan); and 

Imperial Valley, CA (PM10 plan). The 1-hour O3 attainment plan for El Paso, TX was approved 

by EPA as sufficient to demonstrate attainment of the NAAQS by the Moderate classification 

deadline of November 15, 1996, taking into account “but for” international emissions sources in 

Ciudad Juárez, Mexico (69 FR 32450, June 10, 2004). The state’s demonstration included 

airshed modeling using only the U.S. emissions data because emissions data from Ciudad Juárez 

were not available.

VIII. Statutory and Executive Order Reviews

Additional information about these statutes and Executive Orders can be found at 

http://www2.epa.gov/laws-regulations/laws-and-executive-orders.

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: 

Improving Regulation and Regulatory Review

This action is an economically significant regulatory action that was submitted to the 

Office of Management and Budget (OMB) for review. Any changes made in response to OMB 

recommendations have been documented in the docket. The EPA prepared an analysis of the 

potential costs and benefits associated with this action. This analysis is contained in the 

document, Regulatory Impact Analysis for the O3 NAAQS, November, 2014. A copy of the 

analysis is available in the RIA docket (EPA-HQ-OAR-2013-0169) and the analysis is briefly 

summarized here. The RIA estimates the costs and monetized human health and welfare benefits 

of attaining four alternative O3 NAAQS nationwide. Specifically, the RIA examines the 

alternatives of 60 ppb, 65 ppb, 70 ppb, and 75 ppb. The RIA contains illustrative analyses that 
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consider a limited number of emissions control scenarios that states and Regional Planning 

Organizations might implement to achieve these alternative O3 NAAQS. However, the CAA and 

judicial decisions make clear that the economic and technical feasibility of attaining ambient 

standards are not to be considered in setting or revising NAAQS, although such factors may be 

considered in the development of state plans to implement the standards. Accordingly, although 

an RIA has been prepared, the results of the RIA have not been considered in issuing this 

proposed rule.

B. Paperwork Reduction Act

The information collection requirements in this proposed rule have been submitted for 

approval to the Office of Management and Budget (OMB) under the Paperwork Reduction Act 

(PRA). The Information Collection Request (ICR) document prepared by EPA has been assigned 

EPA ICR #2313.03. You can find a copy of the ICR in the docket for this rule, and it is briefly 

summarized here. 

The information collected and reported under 40 CFR part 58 is needed to determine 

compliance with the NAAQS, to characterize air quality and associated health and ecosystems 

impacts, to develop emission control strategies, and to measure progress for the air pollution 

program. We are proposing to extend the length of the required O3 monitoring season in 33 states

and propose that the revised O3 monitoring seasons become effective on January 1, 2017. We are 

also proposing revisions to the PAMS monitoring requirements that reduce the number of 

required PAMS sites while improving spatial coverage, and proposing to require states with O3

non-attainment areas to develop an enhanced monitoring plan as part of the PAMS requirements. 

For areas currently designated as nonattainment for O3 based on the 2008 NAAQS, we propose 

that these areas comply with the PAMS requirements by June 1, 2017. For new areas designated 
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based on a revised NAAQS, if finalized, we propose that those areas comply with the PAMS 

requirements by January 1, 2019. In addition, we are proposing to revise the O3 FRM to establish 

a new, additional technique for measuring O3 in the ambient air. We propose that it be 

incorporated into the existing O3 FRM, using the same calibration procedure in Appendix D of 

40 CFR part 50. We also propose to make changes to the procedures for testing performance 

characteristics and determining comparability between candidate FEMs and reference methods.

For the purposes of ICR #2313.03, the burden figures represent the burden estimate based 

on the requirements contained in the proposed rule. The burden estimates are for the 3-year 

period from 2015 through 2017. The implementation of the PAMS changes, if finalized, will 

occur beyond the time frame of this ICR with likely implementation dates between 2017 and 

2019. The cost estimates for the PAMS network (including proposed revisions) will be captured 

in future routine updates to the Ambient Air Quality Surveillance ICR that are required every 3 

years by OMB. The proposal for a new FRM in 40 CFR part 50 and revisions to the O3 FEM 

procedures for testing performance characteristics in 40 CFR part 53 does not add any additional 

information collection requirements.

The ICR burden estimates are associated with the proposed changes to the O3 seasons. 

This information collection is estimated to involve 158 respondents for a total cost of 

approximately $24,115,182 (total capital, labor, and operation and maintenance) plus a total 

burden of 339,930 hours for the support of all operational aspects of the entire O3 monitoring 

network. The labor costs associated with these hours are $19,813,692. Also included in the total 

are other costs of operations and maintenance of $2,210,132 and equipment and contract costs of 

$2,091,358. The actual labor cost increase to expand the O3 monitoring seasons is $1,668,433. In 

addition to the costs at the state, local, and tribal air quality management agencies, there is a 
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burden to EPA of 41,418 hours and $2,617,591. Burden is defined at 5 CFR 1320.3(b). State, 

local, and tribal entities are eligible for state assistance grants provided by the Federal 

government under the CAA which can be used for related activities. An agency may not conduct 

or sponsor, and a person is not required to respond to, a collection of information unless it 

displays a currently valid OMB control number. The OMB control numbers for EPA’s 

regulations in 40 CFR are listed in 40 CFR part 9. 

To comment on the Agency’s need for this information, the accuracy of the provided 

burden estimates, and any suggested methods for minimizing respondent burden, EPA has 

established a public docket for this rule, which includes this ICR, under Docket ID number EPAHQ-OAR-2008-0699. Submit any comments related to the ICR to EPA and OMB. Send 

comments to the EPA at the air and Radiation Docket and Information Center Docket in the EPA 

Docket Center (EPA/DC), EPA West, Room 3334, 1301 Constitution Ave. NW., Washington 

DC. The Docket Center Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday 

through Friday, excluding legal holidays. The telephone number for the Reading Room is (202) 

566-1744, and the telephone number for the Air and Radiation Docket and Information Center 

Docket is (202) 56-1742. An electronic version of the public docket is available at 

www.regulations.gov. Send comments to OMB at the Office of Information and Regulatory 

Affairs, Office of Management and Budget, 725 17th Street, NW, Washington, DC 20503, 

Attention: Desk Office for EPA. Since OMB is required to make a decision concerning the ICR 

between 30 and 60 days after [INSERT DATE OF PUBLICATION IN THE FEDERAL 

REGISTER], a comment to OMB is best assured of having its full effect if OMB receives it by 

[INSERT DATE 30 DAYS AFTER PUBLICATION IN THE FEDERAL REGISTER]. The
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final rule will respond to any OMB or public comments on the information collection 

requirements contained in this proposal.

C. Regulatory Flexibility Act

I certify that this action will not have a significant economic impact on a substantial 

number of small entities under the Regulatory Flexibility Act (RFA). The reason is that this 

proposed rule will not impose any requirements on small entities. Rather, this rule establishes 

national standards for allowable concentrations of O3 in ambient air as required by section 109 of 

the CAA. See also American Trucking Associations v. EPA, 175 F. 3d at 1044-45 (NAAQS do 

not have significant impacts upon small entities because NAAQS themselves impose no 

regulations upon small entities). Similarly, the proposed revisions to 40 CFR part 58 address the 

requirements for states to collect information and report compliance with the NAAQS and will 

not impose any requirements on small entities. Similarly, the addition of a new FRM in 40 CFR 

part 50 and revisions to the FEM procedures for testing in 40 CFR part 53 will not impose any 

requirements on small entities.

D. Unfunded Mandates Reform Act

This action does not contain any unfunded mandate as described in the Unfunded 

Mandates Reform Act (UMRA), 2 U. S. C. 1531 – 1538, and does not significantly or uniquely 

affect small governments. Furthermore, as indicated previously, in setting a NAAQS the EPA 

cannot consider the economic or technological feasibility of attaining ambient air quality 

standards, although such factors may be considered to a degree in the development of state plans 

to implement the standards. See also American Trucking Associations v. EPA, 175 F. 3d at 1043 

(noting that because the EPA is precluded from considering costs of implementation in 

establishing NAAQS, preparation of a Regulatory Impact Analysis (RIA) pursuant to the 
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Unfunded Mandates Reform Act would not furnish any information which the court could 

consider in reviewing the NAAQS). 

E. 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. 

F. 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 as tribes are not obligated 

to adopt or implement any NAAQS. In addition, tribes are not obligated to conduct ambient 

monitoring for O3 or to adopt the ambient monitoring requirements of 40 CFR part 58. Thus, 

Executive Order 13175 does not apply to this rule.

The EPA specifically solicits comment on this rule from tribal officials. Prior to 

finalization of this proposal, the EPA intends to conduct outreach consistent with the EPA Policy 

on Consultation and Coordination with Indian Tribes. Outreach to tribal environmental 

professionals will be conducted through participation in the Tribal Air call, which is sponsored 

by the National Tribal Air Association. In addition, the EPA intends to offer formal consultation 

to the tribes during the public comment period. If consultation is requested, a summary of the 

result of that consultation will be presented in the notice of final rulemaking and will be available 

in the docket.

G. Executive Order 13045: Protection of Children from Environmental Health & Safety Risks

This action is subject to Executive Order 13045 because it is an economically significant 

regulatory action as defined by Executive Order 12866, and the EPA believes that the 
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environmental health risk addressed by this action may have a disproportionate effect on 

children. The rule will establish uniform national ambient air quality standards for O3; these 

standards are designed to protect public health with an adequate margin of safety, as required by 

CAA section 109. However, the protection offered by these standards may be especially 

important for children because children, especially children with asthma, along with other at-risk

populations

281 such as all people with lung disease and people active outdoors, are potentially 

susceptible to health effects resulting from O3 exposure. Because children are considered an atrisk lifestage, we have carefully evaluated the environmental health effects of exposure to O3

pollution among children. Discussions of the results of the evaluation of the scientific evidence, 

policy considerations, and the exposure and risk assessments pertaining to children are contained 

in sections II.B and II.C of this preamble. 

H. Executive Order 13211: Actions that Significantly Affect Energy Supply, Distribution, or Use

This action is not a “significant energy action” because it is not likely to have a 

significant adverse effect on the supply, distribution, or use of energy. The purpose of this rule is 

to establish revised NAAQS for O3, establish an additional FRM, revise FEM procedures for 

testing, and revises air quality surveillance requirements. The rule does not prescribe specific 

pollution control strategies by which these ambient standards and monitoring revisions will be 

met. Such strategies will be developed by states on a case-by-case basis, and the EPA cannot 

predict whether the control options selected by states will include regulations on energy 

suppliers, distributors, or users. Thus, the EPA concludes that this rule is not likely to have any 

 

281 As used here and similarly throughout this document, the term population refers to people

having a quality or characteristic in common, including a specific pre-existing illness or a 

specific age or life stage.
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adverse energy effects and does not constitute a significant energy action as defined in Executive 

Order 13211.

I. National Technology Transfer and Advancement Act

This rulemaking involves environmental monitoring and measurement. Consistent with 

the Agency’s Performance Based Measurement System (PBMS), the EPA proposes not to 

require the use of specific, prescribed analytical methods. Rather, the Agency plans to allow the 

use of any method that meets the prescribed performance criteria. Ambient air concentrations of 

ozone are currently measured by the Federal reference method (FRM) in 40 CFR part 50, 

Appendix D (Measurement Principle and Calibration Procedure for the Measurement of Ozone 

in the Atmosphere) or by Federal equivalent methods (FEM) that meet the requirements of 40 

CFR part 53. Procedures are available in part 53 that allow for the approval of an FEM for O3

that is similar to the FRM. Any method that meets the performance criteria for a candidate 

equivalent method may be approved for use as an FEM. This approach is consistent with EPA’s 

PBMS. The PBMS approach is intended to be more flexible and cost-effective for the regulated 

community; it is also intended to encourage innovation in analytical technology and improved 

data quality. The EPA is not precluding the use of any method, whether it constitutes a voluntary 

consensus standard or not, as long as it meets the specified performance criteria.

J. Executive Order 12898: Federal Actions to Address Environmental Justice in Minority 

Populations and Low-Income Populations

The EPA believes the human health or environmental risk addressed by this action will 

not have potential disproportionately high and adverse human health or environmental effects on 

minority, low-income or indigenous populations because it does not affect the level of protection 

provided to human health or the environment. This action proposes the strengthening of the O3
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NAAQS. If the proposed revisions are finalized, the revised O3 NAAQS will increase public 

health protection. Analyses evaluating the potential implications of a revised O3 NAAQS for 

environmental justice populations are discussed in appendix 9A of the Regulatory Impact 

Analysis (RIA) that accompanies this notice of proposed rulemaking. The RIA is available on 

the Web, through the EPA’s Technology Transfer Network website at 

http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_index.html. 

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Page 581 of 625

Page 581 of 625 – O3 NAAQS Proposed Rule

List of Subjects 

40 CFR Part 50

Environmental protection, Air pollution control, Carbon monoxide, Lead, Nitrogen dioxide, 

Ozone, Particulate matter, Sulfur oxides.

40 CFR Part 51

Environmental protection, Administrative practices and procedures, Air pollution control, 

Intergovernmental relations.

40 CFR Part 52

Environmental Protection, Administrative practices and procedures, Air pollution control, 

Incorporation by reference, Intergovernmental relations. 

40 CFR Part 53

[to be filled in]

40 CFR Part 58

Environmental protection, Administrative practice and procedure, Air pollution control, 

Intergovernmental relations, Reporting and recordkeeping requirements

________________________________

Dated:

_________________________________

Gina McCarthy,

Administrator.
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For the reasons set forth in the preamble, chapter I of title 40 of the Code of Federal Regulations 

is proposed to be amended as follows:

PART 50 – NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY 

STANDARDS

1. The authority citation for part 50 continues to read as follows:

Authority: 42 U.S.C. 7401 et seq.

2. Amend §50.14 by:

a. Revising paragraphs (c)(2)(iii) and (c)(2)(vi);

b. Removing and reserving paragraphs (c)(2)(iv) and (c)(2)(v);

c. Revising paragraph (c)(2)(vi), Table 1 heading and its entries;

d. Revising paragraph (c)(3)(i); and

e. Removing and reserving paragraphs (c)(3)(ii) and (c)(3)(iii);

3. The revised text reads as follows: 

§ 50.14 Treatment of air quality monitoring data influenced by exceptional events.

* * * * *

(c) ***

(2) ***

(iii) Flags placed on data as being due to an exceptional event together with an initial description 

of the event shall be submitted to EPA not later than July 1st of the calendar year following the 

year in which the flagged measurement occurred, except as allowed under paragraph (c)(2)(vi) of 

this section.

* * * * *

(vi) Table 1 identifies the data submission process for new or revised NAAQS, beginning with 

the 2015 Ozone NAAQS. This process shall apply to those data that will or may influence the 

initial designation of areas for any new or revised NAAQS. 
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Table 1. Schedule for Exceptional Event Flagging and Documentation Submission for Data 

to be Used in Initial Area Designations 

Exceptional Event / Regulatory 

Action Exceptional Event Deadline Scheduled

Exceptional event data flagging and 

initial description deadline for data years 

1, 2 and 3.a

If state and tribal recommendations for the 

new/revised NAAQS are due August through 

January, then the flagging and initial 

description deadline will be the July 1 prior 

to the recommendation deadline. If state and 

tribal recommendations for the new/revised 

NAAQS are due February through July, then 

the flagging and initial description deadline 

will be the January 1 prior to the 

recommendation deadline. 

Exceptional event demonstration 

submittal deadline for data years 1, 2 

and 3.a

No later than the date that state and tribal 

recommendations are due to EPA.

Exceptional event data flagging, initial 

description, and exceptional event 

demonstration submittal deadline for 

data year 4band potential data year 5.c

By the last day of the month that is 1 year 

and 7 months after promulgation of a new or 

revised NAAQS, unless either option a or b 

applies.

a. If the EPA follows a 3-year 

designation schedule, the deadline is 

2 years and 7 months after 

promulgation of a new or revised 

NAAQS.

b. If the EPA notifies the state/tribe via 

Federal Register notice, letter or 

guidance that it intends to complete 

the initial area designations process 

according to a schedule other than a 

2-year or 3-year timeline, the 

deadline is 5 months prior to the date

specified for final designations 

decisions in such EPA notification.

a Where data years 1, 2, and 3 are those years expected to be considered in state and tribal 

recommendations.

b Where data year 4 is the additional year of data that the EPA may consider when it makes final 

area designations for the new/revised NAAQS under a 2-year designations schedule. 

c Where data year 5 is the additional year of data that the EPA may consider when it makes final 

area designations for the new/revised NAAQS under an extended designations schedule.
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d The date by which air agencies must certify their ambient air quality monitoring data in AQS 

is annually on May 1 of the year following the year of data collection. The EPA cannot require 

air agencies to certify data prior to this date. In some cases, however, air agencies may choose 

to certify a prior year's data in advance of May 1 of the following year, particularly if the EPA 

has indicated its intent to promulgate final designations in the months of May, June, July or 

August. Exceptional event flagging, initial description, and demonstration deadlines for "early 

certified" data will follow the deadlines for "year 4" and “year 5” data.

(3) Submission of demonstrations. (i) Except as allowed under paragraph (c)(2)(vi) of this 

section, a State that has flagged data as being due to an exceptional event and is 

requesting exclusion of the affected measurement data shall, after notice and opportunity 

for public comment, submit a demonstration to justify data exclusion to EPA not later 

than the lesser of, 3 years following the end of the calendar quarter in which the flagged 

concentration was recorded or, 12 months prior to the date that a regulatory decision must 

be made by EPA. A State must submit the public comments it received along with its 

demonstration to EPA.

*****

3. Section 50.19 is added to read as follows:

§ 50.19 National primary and secondary ambient air quality standards for ozone.

(a) The level of the national 8-hour primary ambient air quality standard for ozone (O3) is 

(0.065-0.070) parts per million (ppm), daily maximum 8-hour average, measured by a reference 

method based on appendix D to this part and designated in accordance with part 53 of this 

chapter or an equivalent method designated in accordance with part 53 of this chapter.

(b) The 8-hour primary O3 ambient air quality standard is met at an ambient air quality 

monitoring site when the 3-year average of the annual fourth-highest daily maximum 8-hour 

average O3 concentration is less than or equal to (0.065-0.070) ppm, as determined in accordance 

with appendix U to this part.
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Page 585 of 625

(c) The level of the national secondary ambient air quality standard for O3 is (0.065-0.070) ppm, 

daily maximum 8-hour average, measured by a reference method based on appendix D to this 

part and designated in accordance with part 53 of this chapter or an equivalent method 

designated in accordance with part 53 of this chapter.

(d) The 8-hour secondary O3 ambient air quality standard is met at an ambient air quality 

monitoring site when the 3-year average of the annual fourth-highest daily maximum 8-hour 

average O3 concentration is less than or equal to (0.065-0.070) ppm, as determined in accordance 

with appendix U to this part.

4. Revise Appendix D to part 50 under Subchapter C to read as follows:

APPENDIX D TO PART 50 — REFERENCE MEASUREMENT PRINCIPLE AND CALIBRATION 

PROCEDURE FOR THE MEASUREMENT OF OZONE IN THE ATMOSPHERE (CHEMILUMINESCENCE 

METHOD)

1.0 Applicability.

1.1 This chemiluminescence method provides reference measurements of the

concentration of ozone (O3) in ambient air for determining compliance with the national primary 

and secondary ambient air quality standards for O3 as specified in 40 CFR part 50. This 

automated method is applicable to the measurement of ambient O3 concentrations using 

continuous (real-time) sampling and analysis. Additional quality assurance procedures and 

guidance are provided in 40 CFR part 58, appendix A, and in Reference 14.

2.0 Measurement Principle.

2.1 This reference method is based on continuous automated measurement of the 

intensity of the characteristic chemiluminescence released by the gas phase reaction of O3 in 

sampled air with either ethylene (C2H4) or nitric oxide (NO) gas. An ambient air sample stream 
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and a specific flowing concentration of either C2H4 (ET-CL method) or NO (NO-CL method) are 

mixed in a measurement cell, where the resulting chemiluminescence is quantitatively measured 

by a sensitive photo-detector. References 8-11 describe the chemiluminescence measurement 

principle.

2.2 The measurement system is calibrated by referencing the instrumental 

chemiluminescence measurements to certified O3 standard concentrations generated in a 

dynamic flow system and assayed by photometry to be traceable to a National Institute of 

Standards and Technology (NIST) standard reference photometer for O3 (see Section 4, 

Calibration Procedure, below).

2.3 An analyzer implementing this measurement principle is shown schematically in 

Figure 1. Designs implementing this measurement principle must include: an appropriately 

designed mixing and measurement cell; a suitable quantitative photometric measurement system 

with adequate sensitivity and wave length specificity for O3; a pump, flow control, and sample 

conditioning system for sampling and drying the ambient air and moving it into and through the 

measurement cell; a means to supply, meter, and mix a constant, flowing stream of either C2H4

or NO gas of fixed concentration with the sample air flow in the measurement cell; suitable 

electronic control and measurement processing capability; and other associated apparatus as may 

be necessary. The analyzer must be designed and constructed to provide accurate, repeatable, and 

continuous measurements of O3 concentrations in ambient air, with measurement performance 

that meets the requirements specified in subpart B of part 53 of this chapter.

2.4 An analyzer implementing this measurement principle and calibration procedure will 

be considered a federal reference method (FRM) only if it has been designated as a reference

method in accordance with part 53 of this chapter.
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2.5 Sampling considerations. The use of a particle filter on the sample inlet line of a 

chemiluminescence O3 FRM analyzer is required to prevent buildup of particulate matter in the 

measurement cell and inlet components. This filter must be changed weekly (or at least often as 

specified in the manufacturer’s operation/instruction manual), and the sample inlet system used 

with the analyzer must be kept clean, to avoid loss of O3 in the O3 sample air prior to the 

concentration measurement.

3.0 Interferences.

3.1 Except as described in 3.2 below, the chemiluminescence measurement system is 

inherently free of significant interferences from other pollutant substances that may be present in

ambient air.

3.2 A small sensitivity to variations in the humidity of the sample air is minimized by a 

sample air dryer. Potential loss of O3 in the inlet air filter and in the air sample handling 

components of the analyzer and associated exterior air sampling components due to buildup of 

airborne particulate matter is minimized by filter replacement and cleaning of the other inlet 

components.

4.0 Calibration Procedure.

4.1 Principle. The calibration procedure is based on the photometric assay of O3

concentrations in a dynamic flow system. The concentration of O3 in an absorption cell is 

determined from a measurement of the amount of 254 nm light absorbed by the sample. This 

determination requires knowledge of (1) the absorption coefficient (α) of O3 at 254 nm, (2) the 

optical path length (l) through the sample, (3) the transmittance of the sample at a nominal 

wavelength of 254 nm, and (4) the temperature (T) and pressure (P) of the sample. The 

transmittance is defined as the ratio I/I0, where I is the intensity of light which passes through the 
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cell and is sensed by the detector when the cell contains an O3 sample, and I0 is the intensity of 

light which passes through the cell and is sensed by the detector when the cell contains zero air.

It is assumed that all conditions of the system, except for the contents of the absorption cell, are 

identical during measurement of I and I0. The quantities defined above are related by the BeerLambert absorption law,

Transmitance =

I

I0

= e

−αcl

 (1)

where:

α = absorption coefficient of O3 at 254 nm = 308 ±4 atm−1 cm−1 at 0 °C and 760 

torr,1, 2, 3, 4, 5, 6, 7

c = O3 concentration in atmospheres, and

l = optical path length in cm.

A stable O3 generator is used to produce O3 concentrations over the required calibration 

concentration range. Each O3 concentration is determined from the measurement of the 

transmittance (I/I0) of the sample at 254 nm with a photometer of path length l and calculated 

from the equation,

c(atm) = −

1

αl

(ln I

I0

) (2a)

or

c(ppm) = −

106

αl

(ln I

I0

). (2b)

The calculated O3 concentrations must be corrected for O3 losses, which may occur in the 

photometer, and for the temperature and pressure of the sample.
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4.2 Applicability. This procedure is applicable to the calibration of ambient air 

O3 analyzers, either directly or by means of a transfer standard certified by this procedure. 

Transfer standards must meet the requirements and specifications set forth in Reference 12. 

4.3 Apparatus. A complete UV calibration system consists of an O3 generator, an output 

port or manifold, a photometer, an appropriate source of zero air, and other components as 

necessary. The configuration must provide a stable O3 concentration at the system output and 

allow the photometer to accurately assay the output concentration to the precision specified for 

the photometer (4.3.1). Figure 2 shows a commonly used configuration and serves to illustrate 

the calibration procedure, which follows. Other configurations may require appropriate 

variations in the procedural steps. All connections between components in the calibration system 

downstream of the O3 generator must be of glass, Teflon, or other relatively inert materials.

Additional information regarding the assembly of a UV photometric calibration apparatus is 

given in Reference 13. For certification of transfer standards which provide their own source of 

O3, the transfer standard may replace the O3 generator and possibly other components shown in 

Figure 2; see Reference 12 for guidance.

4.3.1 UV photometer. The photometer consists of a low-pressure mercury discharge 

lamp, (optional) collimation optics, an absorption cell, a detector, and signal-processing 

electronics, as illustrated in Figure 2. It must be capable of measuring the transmittance, I/I0, at a 

wavelength of 254 nm with sufficient precision such that the standard deviation of the 

concentration measurements does not exceed the greater of 0.005 ppm or 3% of the 

concentration. Because the low-pressure mercury lamp radiates at several wavelengths, the 

photometer must incorporate suitable means to assure that no O3 is generated in the cell by the 

lamp, and that at least 99.5% of the radiation sensed by the detector is 254 nm radiation. (This 
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can be readily achieved by prudent selection of optical filter and detector response 

characteristics.) The length of the light path through the absorption cell must be known with an 

accuracy of at least 99.5%. In addition, the cell and associated plumbing must be designed to 

minimize loss of O3 from contact with cell walls and gas handling components. See Reference 13 

for additional information.

4.3.2 Air flow controllers. Air flow controllers are devices capable of regulating air flows 

as necessary to meet the output stability and photometer precision requirements.

4.3.3 Ozone generator. The ozone generator used must be capable of generating stable 

levels of O3 over the required concentration range.

4.3.4 Output manifold. The output manifold must be constructed of glass, Teflon, or other 

relatively inert material, and should be of sufficient diameter to insure a negligible pressure drop 

at the photometer connection and other output ports. The system must have a vent designed to 

insure atmospheric pressure in the manifold and to prevent ambient air from entering the 

manifold.

4.3.5 Two-way valve. A manual or automatic two-way valve, or other means is used to 

switch the photometer flow between zero air and the O3 concentration.

4.3.6 Temperature indicator. A device to indicate temperature must be used that is 

accurate to ±1 °C.

4.3.7 Barometer or pressure indicator. A device to indicate barometric pressure must be 

used that is accurate to ±2 torr.

4.4 Reagents.

4.4.1 Zero air. The zero air must be free of contaminants which would cause a detectable 

response from the O3 analyzer, and it must be free of NO, C2H4, and other species which react 
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with O3. A procedure for generating suitable zero air is given in Reference 13. As shown in 

Figure 2, the zero air supplied to the photometer cell for the I0 reference measurement must be 

derived from the same source as the zero air used for generation of the O3 concentration to be 

assayed (I measurement). When using the photometer to certify a transfer standard having its 

own source of O3, see Reference 12 for guidance on meeting this requirement.

4.5 Procedure.

4.5.1 General operation. The calibration photometer must be dedicated exclusively to use 

as a calibration standard. It must always be used with clean, filtered calibration gases, and never 

used for ambient air sampling. A number of advantages are realized by locating the calibration 

photometer in a clean laboratory where it can be stationary, protected from the physical shock of 

transportation, operated by a responsible analyst, and used as a common standard for all field 

calibrations via transfer standards.

4.5.2 Preparation. Proper operation of the photometer is of critical importance to the 

accuracy of this procedure. Upon initial operation of the photometer, the following steps must be 

carried out with all quantitative results or indications recorded in a chronological record, either in 

tabular form or plotted on a graphical chart. As the performance and stability record of the 

photometer is established, the frequency of these steps may be reduced to be consistent with the 

documented stability of the photometer and the guidance provided in Reference 12.

4.5.2.1 Instruction manual. Carry out all set up and adjustment procedures or checks as 

described in the operation or instruction manual associated with the photometer.

4.5.2.2 System check. Check the photometer system for integrity, leaks, cleanliness, 

proper flow rates, etc. Service or replace filters and zero air scrubbers or other consumable 

materials, as necessary.
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4.5.2.3 Linearity. Verify that the photometer manufacturer has adequately established that 

the linearity error of the photometer is less than 3%, or test the linearity by dilution as follows:

Generate and assay an O3 concentration near the upper range limit of the system or appropriate 

calibration scale for the instrument, then accurately dilute that concentration with zero air and reassay it. Repeat at several different dilution ratios. Compare the assay of the original 

concentration with the assay of the diluted concentration divided by the dilution ratio, as follows

E =

A1− A2/R

A1

× 100% (3)

where:

E = linearity error, percent

A1 = assay of the original concentration

A2 = assay of the diluted concentration

R = dilution ratio = flow of original concentration divided by the total flow

The linearity error must be less than 5%. Since the accuracy of the measured flow-rates 

will affect the linearity error as measured this way, the test is not necessarily conclusive.

Additional information on verifying linearity is contained in Reference 13.

4.5.2.4 Inter-comparison. The photometer must be inter-compared annually, either 

directly or via transfer standards, with a NIST standard reference photometer (SRP) or 

calibration photometers used by other agencies or laboratories.

4.5.2.5 Ozone losses. Some portion of the O3 may be lost upon contact with the 

photometer cell walls and gas handling components. The magnitude of this loss must be 

determined and used to correct the calculated O3 concentration. This loss must not exceed 5%.

Some guidelines for quantitatively determining this loss are discussed in Reference 13.
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4.5.3 Assay of O3 concentrations. The operator must carry out the following steps to 

properly assay O3 concentrations.

4.5.3.1 Allow the photometer system to warm up and stabilize.

4.5.3.2 Verify that the flow rate through the photometer absorption cell, F, allows the cell 

to be flushed in a reasonably short period of time (2 liter/min is a typical flow). The precision of 

the measurements is inversely related to the time required for flushing, since the photometer drift 

error increases with time.

4.5.3.3 Ensure that the flow rate into the output manifold is at least 1 liter/min greater 

than the total flow rate required by the photometer and any other flow demand connected to the 

manifold.

4.5.3.4 Ensure that the flow rate of zero air, Fz, is at least 1 liter/min greater than the flow 

rate required by the photometer.

4.5.3.5 With zero air flowing in the output manifold, actuate the two-way valve to allow 

the photometer to sample first the manifold zero air, then Fz. The two photometer readings must 

be equal (I = I0).

NOTE: In some commercially available photometers, the operation of the two-way valve 

and various other operations in section 4.5.3 may be carried out automatically by the photometer.

4.5.3.6 Adjust the O3 generator to produce an O3 concentration as needed.

4.5.3.7 Actuate the two-way valve to allow the photometer to sample zero air until the 

absorption cell is thoroughly flushed and record the stable measured value of Io.

4.5.3.8 Actuate the two-way valve to allow the photometer to sample the O3

concentration until the absorption cell is thoroughly flushed and record the stable measured value 

of I.
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4.5.3.9 Record the temperature and pressure of the sample in the photometer absorption 

cell. (See Reference 13 for guidance.)

4.5.3.10 Calculate the O3 concentration from equation 4. An average of several 

determinations will provide better precision.

[O3]OUT = (

−1

αl

ln I

I0

) (

T

273) (

760

P

) ×

106

L

(4)

where:

[O3]OUT = O3 concentration, ppm

α = absorption coefficient of O3 at 254 nm = 308 atm−1 cm−1 at 0° C and 760 torr

l = optical path length, cm

T = sample temperature, K

P = sample pressure, torr

L = correction factor for O3 losses from 4.5.2.5 = (1−fraction of O3 lost).

NOTE: Some commercial photometers may automatically evaluate all or part of equation 

4. It is the operator's responsibility to verify that all of the information required for equation 4 is 

obtained, either automatically by the photometer or manually. For “automatic” photometers 

which evaluate the first term of equation 4 based on a linear approximation, a manual correction 

may be required, particularly at higher O3 levels. See the photometer instruction manual and 

Reference 13 for guidance.

4.5.3.11 Obtain additional O3 concentration standards as necessary by repeating steps 

4.5.3.6 to 4.5.3.10 or by Option 1.

4.5.4 Certification of transfer standards. A transfer standard is certified by relating the 

output of the transfer standard to one or more O3 calibration standards as determined according 
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to section 4.5.3. The exact procedure varies depending on the nature and design of the transfer 

standard. Consult Reference 12 for guidance.

4.5.5 Calibration of ozone analyzers. Ozone analyzers must be calibrated as follows, 

using O3 standards obtained directly according to section 4.5.3 or by means of a certified transfer 

standard.

4.5.5.1 Allow sufficient time for the O3 analyzer and the photometer or transfer standard 

to warm-up and stabilize.

4.5.5.2 Allow the O3 analyzer to sample zero air until a stable response is obtained and 

then adjust the O3 analyzer's zero control. Offsetting the analyzer's zero adjustment to +5% of 

scale is recommended to facilitate observing negative zero drift (if any). Record the stable zero 

air response as “Z”.

4.5.5.3 Generate an O3 concentration standard of approximately 80% of the desired upper 

range limit (URL) of the O3 analyzer. Allow the O3 analyzer to sample this O3 concentration 

standard until a stable response is obtained.

4.5.5.4 Adjust the O3 analyzer's span control to obtain the desired response equivalent to 

the calculated standard concentration. Record the O3 concentration and the corresponding 

analyzer response. If substantial adjustment of the span control is necessary, recheck the zero and 

span adjustments by repeating steps 4.5.5.2 to 4.5.5.4.

4.5.5.5 Generate additional O3 concentration standards (a minimum of 5 are 

recommended) over the calibration scale of the O3 analyzer by adjusting the O3 source or by 

Option 1. For each O3 concentration standard, record the O3 concentration and the corresponding 

analyzer response.
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4.5.5.6 Plot the O3 analyzer responses (vertical or Y-axis) versus the corresponding 

O3 standard concentrations (horizontal or X-axis). Compute the linear regression slope and 

intercept and plot the regression line to verify that no point deviates from this line by more than 2 

percent of the maximum concentration tested.

4.5.5.7 Option 1: The various O3 concentrations required in steps 4.5.3.11 and 4.5.5.5 

may be obtained by dilution of the O3 concentration generated in steps 4.5.3.6 and 4.5.5.3. With 

this option, accurate flow measurements are required. The dynamic calibration system may be 

modified as shown in Figure 3 to allow for dilution air to be metered in downstream of the 

O3 generator. A mixing chamber between the O3 generator and the output manifold is also 

required. The flow rate through the O3 generator (Fo) and the dilution air flow rate (FD) are 

measured with a flow or volume standard that is traceable to a NIST flow or volume calibration 

standard. Each O3 concentration generated by dilution is calculated from:

[O3]′OUT = [O3]OUT (

FO

FO+FD

) (5)

where:

[O3]′OUT = diluted O3 concentration, ppm

FO = flow rate through the O3 generator, liter/min

FD = diluent air flow rate, liter/min

Note: Additional information on calibration and pollutant standards is provided in 

Section 12 of Reference 14.

5.0 Frequency of Calibration.

5.1 The frequency of calibration, as well as the number of points necessary to establish 

the calibration curve, and the frequency of other performance checking will vary by analyzer; 

however, the minimum frequency, acceptance criteria, and subsequent actions are specified in 
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Appendix D of Reference 14: Measurement Quality Objectives and Validation Templates. The 

user's quality control program shall provide guidelines for initial establishment of these variables 

and for subsequent alteration as operational experience is accumulated. Manufacturers of 

analyzers should include in their instruction/operation manuals information and guidance as to 

these variables and on other matters of operation, calibration, routine maintenance, and quality 

control.

6.0 References.

1. E.C.Y. Inn and Y. Tanaka, “Absorption coefficient of Ozone in the Ultraviolet and 

Visible Regions”, J. Opt. Soc. Am., 43, 870 (1953).

2. A. G. Hearn, “Absorption of Ozone in the Ultraviolet and Visible Regions of the 

Spectrum”, Proc. Phys. Soc. (London), 78, 932 (1961).

3. W. B. DeMore and O. Raper, “Hartley Band Extinction Coefficients of Ozone in the 

Gas Phase and in Liquid Nitrogen, Carbon Monoxide, and Argon”, J. Phys. Chem., 68, 412 

(1964).

4. M. Griggs, “Absorption Coefficients of Ozone in the Ultraviolet and Visible 

Regions”, J. Chem. Phys., 49, 857 (1968).

5. K. H. Becker, U. Schurath, and H. Seitz, “Ozone Olefin Reactions in the Gas Phase. 1. 

Rate Constants and Activation Energies”, Int'l Jour. of Chem. Kinetics, VI, 725 (1974).

6. M. A. A. Clyne and J. A. Coxom, “Kinetic Studies of Oxy-halogen Radical 

Systems”, Proc. Roy. Soc., A303, 207 (1968).

7. J. W. Simons, R. J. Paur, H. A. Webster, and E. J. Bair, “Ozone Ultraviolet Photolysis. 

VI. The Ultraviolet Spectrum”, J. Chem. Phys., 59, 1203 (1973).
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8. Ollison, W.M.; Crow, W.; Spicer, C.W. “Field testing of new-technology ambient air 

ozone monitors.” J. Air Waste Manage. Assoc., 63 (7), 855-863 (2013).

9. Parrish, D.D.; Fehsenfeld, F.C. “Methods for gas-phase measurements of ozone, ozone 

precursors and aerosol precursors.” Atmos. Environ., 34 (12-14), 1921-1957(2000).

10. Ridley, B.A.; Grahek, F.E.; Walega, J.G. “A small, high-sensitivity, mediumresponse ozone detector suitable for measurements from light aircraft.” J. Atmos. Oceanic 

Technol., 9 (2), 142-148(1992).

11. Boylan, P., Helmig, D., and Park, J.H. “Characterization and mitigation of water 

vapor effects in the measurement of ozone by chemiluminescence with nitric oxide.” Atmos. 

Meas. Tech. 7, 1231-1244 (2014).

12. Transfer Standards for Calibration of Ambient Air Monitoring Analyzers for Ozone, 

EPA publication number EPA-454/B-13-004, October 2013. EPA, Office of Air Quality 

Planning and Standards, Research Triangle Park, NC 27711. [Available at 

www.epa.gov/ttnamti1/files/ambient/qaqc/OzoneTransferStandardGuidance.pdf.] 

13. Technical Assistance Document for the Calibration of Ambient Ozone Monitors, 

EPA publication number EPA-600/4-79-057, September, 1979. [Available at 

www.epa.gov/ttnamti1/files/ambient/criteria/4-79-057.pdf.]

14. QA Handbook for Air Pollution Measurement Systems—Volume II. Ambient Air 

Quality Monitoring Program. EPA-454/B-13-003, May 2013. [Available at 

http://www.epa.gov/ttnamti1/files/ambient/pm25/qa/QA-Handbook-Vol-II.pdf.]
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Figure 1. Gas-phase chemiluminescence analyzer schematic diagram, where PMT means 

photomultiplier tube.
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Figure 2. Schematic diagram of a typical UV photometric calibration system.
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Figure 3. Schematic diagram of a typical UV photometric calibration system (Option 1).

Appendix U to Part 50 is added to read as follows:

Appendix U to Part 50 -- Interpretation of the Primary and Secondary National Ambient 

Air Quality Standards for Ozone

1. General

(a) This appendix explains the data handling conventions and computations necessary for 

determining whether the primary and secondary national ambient air quality standards (NAAQS) 

for ozone (O3) specified in §50.19 are met at an ambient O3 air quality monitoring site. Data 

reporting, data handling, and computation procedures to be used in making comparisons between 
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reported O3 concentrations and the levels of the O3 NAAQS are specified in the following 

sections.

(b) Whether to exclude or retain the data affected by exceptional events is determined by the 

requirements under §§50.1, 50.14 and 51.930.

(c) The terms used in this appendix are defined as follows:

8-hour average refers to the moving average of eight consecutive hourly O3 concentrations 

measured at a site, as explained in section 3 of this appendix.

Annual fourth-highest daily maximum refers to the fourth highest value measured at a site during 

a particular year.

Collocated monitors refers to the instance of two or more O3 monitors operating at the same site.

Daily maximum 8-hour average O3 concentration refers to the maximum calculated 8-hour 

average value measured at a site on a particular day, as explained in section 3 of this appendix.

Design value refers to the metric (i.e., statistic) that is used to compare ambient O3 concentration 

data measured at a site to the NAAQS in order to determine compliance, as explained in section 

4 of this appendix.

Minimum data completeness requirements refer to the amount of data that a site is required to 

collect in order to make a valid determination that the site is meeting the NAAQS.

Monitor refers to a physical instrument used to measure ambient O3 concentrations.

O3 monitoring season refers to the span of time within a year when individual states are required 

to measure ambient O3 concentrations, as listed in Appendix D to part 58 of this chapter.

Site refers to an ambient O3 air quality monitoring site.

Site data record refers to the set of hourly O3 concentration data collected at a site for use in 

comparisons with the NAAQS.
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Year refers to calendar year.

2. Selection of Data for use in Comparisons with the Primary and Secondary Ozone NAAQS

(a) All valid hourly O3 concentration data collected using a federal reference method specified in 

Appendix D to this part, or an equivalent method designated in accordance with part 53 of this 

chapter, meeting all applicable requirements in part 58 of this chapter, and submitted to EPA’s 

Air Quality System (AQS) database, or otherwise available to EPA, shall be used in design value 

calculations. Data not meeting these requirements shall not be used in design value calculations.

(b) All design value calculations shall be implemented on a site-level basis. If data are reported 

to EPA from collocated monitors, those data shall be combined into a single site data record as 

follows:

(i) The monitoring agency may designate one monitor as the primary monitor for the site. 

If a primary monitor has not been designated by the monitoring agency, the monitor with 

the largest number of hourly O3 concentrations reported for the year shall be designated 

as the primary monitor.

(ii) Hourly O3 concentration data from a collocated monitor shall be substituted into the 

site data record whenever a valid hourly O3 concentration is not obtained from the 

primary monitor. In the event that hourly O3 concentration data are available for two or 

more collocated monitors, the hourly concentration data for those monitors shall be 

averaged and substituted into the site data record.

(c) In certain circumstances, including but not limited to site closures or relocations, data from 

two nearby sites may be combined into a single site data record for the purpose of calculating a 

valid design value. The appropriate Regional Administrator may approve such combinations 

after taking into consideration factors such as distance between sites, spatial and temporal 
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patterns in air quality, local emissions and meteorology, jurisdictional boundaries, and terrain 

features.

3. Data Reporting and Data Handling Conventions

(a) Hourly average O3 concentrations shall be reported in parts per million (ppm) to the third 

decimal place, with additional digits to the right of the third decimal place truncated. Each hour 

shall be identified using local standard time (LST).

(b) Moving 8-hour averages shall be computed from the hourly O3 concentration data for each 

hour of the year and shall be stored in the first, or start, hour of the 8-hour period. An 8-hour 

average shall be considered valid if at least 6 of the hourly concentrations for the 8-hour period 

are available. In the event that only 6 or 7 hourly concentrations are available, the 8-hour average 

shall be computed on the basis of the hours available, using 6 or 7 as the divisor. In addition, in 

the event that 5 or fewer hourly concentrations are available, the 8-hour average shall be 

considered valid if, after substituting zero for the missing hourly concentrations, the resulting 8-

hour average is greater than the level of the NAAQS. The 8-hour averages shall be reported to 

three decimal places, with additional digits to the right of the third decimal place truncated. 

Hourly O3 concentrations that have been approved under §50.14 as having been affected by 

exceptional events shall be counted as missing or unavailable in the calculation of 8-hour 

averages.

(c) The daily maximum 8-hour average O3 concentration for a given day is the highest of the 17 

consecutive 8-hour averages beginning with the 8-hour period from 7:00 AM to 3:00 PM and 

ending with the 8-hour period from 11:00 PM to 7:00 AM (i.e., the 8-hour averages for 7:00 AM 

to 11:00 PM). Daily maximum 8-hour average O3 concentrations shall be determined for each 
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day with ambient O3 monitoring data, including days outside the O3 monitoring season if those 

data are available.

(d) A daily maximum 8-hour average O3 concentration shall be considered valid if valid 8-hour 

averages are available for at least 13 of the 17 consecutive 8-hour periods starting from 7:00 AM 

to 11:00 PM. In addition, in the event that fewer than 13 valid 8-hour averages are available, a 

daily maximum 8-hour average O3 concentration shall also be considered valid if it is greater 

than the level of the NAAQS. Hourly O3 concentrations that have been approved under §50.14 as 

having been affected by exceptional events shall be included when determining whether these 

criteria have been met.

(e) The primary and secondary O3 design value statistic is the annual fourth-highest daily 

maximum 8-hour O3 concentration, averaged over three years, expressed in parts per million. 

The fourth-highest daily maximum 8-hour O3 concentration for each year shall be determined 

based only on days meeting the validity criteria in 3(d). The 3-year average shall be computed 

using the three most recent, consecutive years of ambient O3 monitoring data. Design values 

shall be reported to three decimal places, with additional digits to the right of the third decimal 

place truncated.

4. Comparisons with the Primary and Secondary Ozone NAAQS

(a) The primary and secondary national ambient air quality standards for O3 are met at an 

ambient air quality monitoring site when the 3-year average of the annual fourth-highest daily 

maximum 8-hour average O3 concentration (i.e., the design value) is less than or equal to (0.065 

– 0.070) ppm.

(b) A design value greater than the level of the NAAQS is always considered to be valid. A 

design value less than or equal to the level of the NAAQS must meet minimum data 
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completeness requirements in order to be considered valid. These requirements are met for a 3-

year period at a site if valid daily maximum 8-hour average O3 concentrations are available for at 

least 90% of the days within the O3 monitoring season, on average, for the 3-year period, with a 

minimum of at least 75% of the days within the O3 monitoring season in any one year.

(c) When computing whether the minimum data completeness requirements have been met, 

meteorological or ambient data may be sufficient to demonstrate that meteorological conditions 

on missing days were not conducive to concentrations above the level of the NAAQS. Missing 

days assumed less than the level of the NAAQS are counted for the purpose of meeting the 

minimum data completeness requirements, subject to the approval of the appropriate Regional 

Administrator.

(d) Comparisons with the primary and secondary O3 NAAQS are demonstrated by examples 1 

and 2 as follows:

Example 1: Site Meeting the Primary and Secondary O3 NAAQS

Year

Percent valid 

days within O3

monitoring 

season

1st highest 

daily max 8-

hour O3 (ppm)

2nd highest 

daily max 8-

hour O3

(ppm)

3rd highest 

daily max 8-

hour O3

(ppm)

4th highest 

daily max 8-

hour O3

(ppm)

5th highest 

daily max 8-

hour O3

(ppm)

2014 100 0.082 0.080 0.075 0.069 0.068

2015 96 0.074 0.073 0.065 0.062 0.060

2016 98 0.070 0.069 0.067 0.066 0.060

Average 98 0.065

As shown in Example 1, this site meets the primary and secondary O3 NAAQS because the 3-

year average of the annual fourth-highest daily maximum 8-hour average O3 concentrations (i.e., 
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0.065666 ppm, truncated to 0.065 ppm) is less than or equal to (0.065 – 0.070) ppm. The 

minimum data completeness requirements are also met because the average percent of days 

within the O3 monitoring season with valid ambient monitoring data is greater than 90%, and no 

single year has less than 75% data completeness.

Example 2: Site Failing to Meet the Primary and Secondary O3 O3 NAAQS

Year

Percent valid 

days within O3

monitoring 

season

1st highest 

daily max 8-

hour O3 (ppm)

2nd highest 

daily max 8-

hour O3 (ppm)

3rd highest 

daily max 8-

hour O3 (ppm)

4th highest 

daily max 8-

hour O3 (ppm)

5th highest 

daily max 8-

hour O3 (ppm)

2014 96 0.085 0.080 0.079 0.074 0.072

2015 74 0.084 0.083 0.072 0.071 0.068

2016 98 0.083 0.081 0.081 0.075 0.074

Average 89 0.073

As shown in Example 2, this site fails to meet the primary and secondary O3 NAAQS because 

the 3-year average of the annual fourth-highest daily maximum 8-hour average O3 concentrations 

(i.e., 0.073333 ppm, truncated to 0.073 ppm) is greater than (0.065 – 0.070) ppm, even though 

the annual data completeness is less than 75% in one year and the 3-year average data 

completeness is less than 90%.

PART 51—REQUIREMENTS FOR PREPARATION, ADOPTION, AND SUBMITTAL 

OF IMPLEMENATION PLANS

6. The authority citation for part 51 continues to read as follows:

Authority: 23 U.S.C. 101; 42 U.S.C. 7401-7671q.

Subpart I–—Review of New Sources and Modifications
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7 Amend § 51.166 by adding paragraph (i)(11) to read as follows: 

§ 51.166 Prevention of significant deterioration of air quality.

* * * * * 

(i) * * *

(11) The plan may provide that the requirements of paragraph (k)(1) of this section shall 

not apply to a stationary source or modification with respect to the national ambient air quality 

standards for ozone in effect on [EFFECTIVE DATE OF FINAL RULE] if:

(i) The reviewing authority has determined a permit application subject to this section to 

be complete on or before [SIGNATURE DATE OF FINAL RULE]. Instead, the requirements in 

paragraph (k)(1) of this section shall apply with respect to the national ambient air quality 

standards for ozone in effect at the time the reviewing authority determined the permit 

application to be complete; or

(ii) The reviewing authority has first published before [EFFECTIVE DATE OF FINAL 

RULE] a public notice of a preliminary determination or draft permit for the permit application 

subject to this section. Instead, the requirements in paragraph (k)(1) of this section shall apply 

with respect to the national ambient air quality standards for ozone in effect at the time of first 

publication of a public notice of the preliminary determination or draft permit.

* * * * * 

PART 52—APPROVAL AND PROMULGATION OF IMPLEMENTATION PLANS

8. The authority citation for part 52 continues to read as follows:

Authority: 42 U.S.C. 7401 et seq.

9. Amend § 52.21 by adding paragraph (i)(12) to read as follows:

§ 52.21 Prevention of significant deterioration of air quality.
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* * * * *

(i) * * *

(12) The requirements of paragraph (k)(1) of this section shall not apply to a stationary 

source or modification with respect to the national ambient air quality standards for ozone in 

effect on [EFFECTIVE DATE OF FINAL RULE] if:

(i) The Administrator has determined a permit application subject to this section to be 

complete on or before [SIGNATURE DATE OF FINAL RULE]. Instead, the requirements in 

paragraph (k)(1) of this section shall apply with respect to the national ambient air quality 

standards for ozone in effect at the time the Administrator determined the permit application to 

be complete; or

(ii) The Administrator has first published before [EFFECTIVE DATE OF FINAL RULE] 

a public notice of a preliminary determination or draft permit subject to this section. Instead, the 

requirements in paragraph (k)(1) of this section shall apply with respect to the national ambient 

air quality standards for ozone in effect on the date the Administrator first published a public 

notice of a preliminary determination or draft permit.

* * * * *

PART 53—AMBIENT AIR MONITORING REFERENCE AND EQUIVALENT 

METHODS

10. The authority citation for part 53 continues to read as follows:

Authority: Sec. 301(a) of the Clean Air Act (42 U.S.C. sec. 1857g(a)), as amended by 

sec. 15(c)(2) of Pub. L. 91-604, 84 Stat. 1713, unless otherwise noted.

Subpart A—General Provisions§ 53.9 [Amended]

11. Amend § 53.9 by removing paragraph (i).
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12. Amend § 53.14 by revising paragraph (c) introductory text to read as follows: 

§ 53.14 Modification of a reference or equivalent method.

* * * * *

(c) Within 90 calendar days after receiving a report under paragraph (a) of this section, 

the Administrator will take one or more of the following actions:

* * * * *

Subpart B—Procedures for Testing Performance Characteristics of Automated 

Methods for SO2, CO, O3, and NO2 

13. Amend § 53.23 by revising paragraph (e)(1)(vi) to read as follows:

* * * * *

 (e) * * *

(1) * * * 
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 (vi) Precision: Variation about the mean of repeated measurements of the same 

pollutant concentration, denoted as the standard deviation expressed as a percentage of the upper 

range limits.

282

* * * * *

14. Revise Table B-1 to Subpart B of Part 53 to read as follows:

 

282 NO2 precision in Table B-1 is also changed to percent to agree with the calculation specified 

in 53.23(e)(10)(vi).
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Table B-1 to Subpart B of Part 53—Performance Limit Specifications for Automated Methods

Performance parameter Units1

SO2 O3 CO NO2 

(Std.

range)

Definitions and 

Std. range test procedures 3 Lower 

range2,3

Std.

range3

Lower 

range2,3

Std.

range3

Lower 

range2,3

1. Range ppm 0-0.5 <0.5 0-0.5 <0.5 0-50 <50 0-0.5 Sec. 53.23(a).

2. Noise ppm 0.001 0.0005 0.001 0.0005 0.2 0.1 0.005 Sec. 53.23(b).

3. Lower detectable limit ppm 0.002 0.001 0.003 0.001 0.4 0.2 0.010 Sec. 53.23(c).

4. Interference equivalent 

Each interferent

Total, all interferents

ppm

ppm

±0.005

---

4±0.005

---

±0.005

---

±0.005

---

±1.0

---

±0.5

---

±0.02

0.04

Sec. 53.23(d).

Sec. 53.23(d).

5. Zero drift, 12 and 24 hour ppm ±0.004 ±0.002 ±0.004 ±0.002 ±0.5 ±0.3 ±0.02 Sec. 53.23(e).

6. Span drift, 24 hour 

20% of upper range limit

80% of upper range limit

Percent

Percent

---

±3.0

---

±3.0

 

---

±3.0

---

±3.0

---

±2.0

---

±2.0

±20.0

±5.0

Sec. 53.23(e)

Sec. 53.23(e).

7. Lag time Minutes 2 2 2 2 2.0 2.0 20 Sec. 53.23(e).

8. Rise time Minutes 2 2 2 2 2.0 2.0 15 Sec. 53.23(e).

9. Fall time Minutes 2 2 2 2 2.0 2.0 15 Sec. 53.23(e).

10. Precision

20 % of upper range limit

80 % of upper range limit

 

Percent5

Percent5

---

2

---

2

---

2

---

2

---

2

---

2

---

2

---

2

--

1.0

---

1.0

---

1.0

---

1.0

4

6

Sec. 53.23(e).

Sec. 53.23(e).

Sec. 53.23(e).

Sec. 53.23(e).

1. To convert from parts per million (ppm) to μg/m3

at 25 °C and 760 mm Hg, multiply by M/0.02447, where M is the molecular 

weight of the gas. Percent means percent of the upper measurement range limit.

2. Tests for interference equivalent and lag time do not need to be repeated for any lower range provided the test for the standard range 

shows that the lower range specification (if applicable) is met for each of these test parameters.

3. For candidate analyzers having automatic or adaptive time constants or smoothing filters, describe their functional nature, and 

describe and conduct suitable tests to demonstrate their function aspects and verify that performances for calibration, noise, lag, rise, 

fall times, and precision are within specifications under all applicable conditions. For candidate analyzers with operator-selectable 
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time constants or smoothing filters, conduct calibration, noise, lag, rise, fall times, and precision tests at the highest and lowest settings 

that are to be included in the FRM or FEM designation.

4. For nitric oxide interference for the SO2 UVF method, interference equivalent is ±0.0003 ppm for the lower range.

5. Standard deviation expressed as percent of the URL.
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15. Revise Table B-3 to Subpart B of Part 53 to read as follows:

Table B-3 to Subpart B of Part 53—Interferent Test Concentration,1 Parts per Million

PollutantAnalyzer typeHydrochloric acidAmmoniaHydrogensulfideSulfur dioxideNitrogen dioxideNitric oxideCarbon dioxideEthyleneOzonem-XyleneWater vaporCarbonmonoxideMethaneEthaneNaphthalene

SO2 Ultraviolet 

fluorescence

5

0.1 40.14 0.5 0.5 0.5 0.2 20,000 6

0.05

SO2 Flame photometric 0.01 4 0.14 750 320,000 50

SO2 Gas 

chromatography

0.1 40.14 750 320,000 50

SO2 Spectrophotometric

-wet chemical 

(pararosanaline)

0.2 0.1 0.1 40.14 0.5 750 0.5

SO2 Electrochemical 0.2 0.1 0.1 40.14 0.5 0.5 0.2 0.5 320,000

SO2 Conductivity 0.2 0.1 40.14 0.5 750

SO2 Spectrophotometric

-gas phase, 

including DOAS

4

0.14 0.5 0.5 0.2

O3 Ethylene 

chemiluminescene

3

0.1 750 4

0.08

3

20,000

O3 NOchemiluminescene

3

0.1 0.5 750 0.08 20,000

O3 Electrochemical 30.1 0.5 0.5 0.08
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O3 Spectrophotometric

-wet chemical 

(potassium iodide)

3

0.1 0.5 0.5 0.5 0.08

O3 Spectrophotometric

-gas phase, 

including 

ultraviolet 

absorption and 

DOAS

0.5 0.5 0.5 0.08 0.02 20,000

CO Non-dispersive 

Infrared

750 20,000 410

CO Gas 

chromatography 

with flame 

ionization detector

20,000 410 0.5

CO Electrochemical 0.5 0.2 20,000 410

CO Catalytic 

combustion-thermal 

detection

0.1 750 0.2 20,000 410 5.0 0.5

CO IR fluorescence 750 20,000 410 0.5

CO Mercury 

replacement-UV 

photometric

0.2 410 0.5

NO2 Chemiluminescent 30.1 0.5 40.1 0.5 20,000

NO2 Spectrophotometric

-wet chemical 

(azo-dyereaction)

0.5 40.1 0.5 750 0.5
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1. Concentrations of interferents listed must be prepared and controlled to ±10 percent of the stated value.

2. Analyzer types not listed will be considered by the Administrator as special cases.

3. Do not mix with the pollutant.

4. Concentration of pollutant used for test. These pollutant concentrations must be prepared to ±10 percent of the stated value.

5. If candidate method utilizes an elevated-temperature scrubber for removal of aromatic hydrocarbons, perform this interference test.

6. If naphthalene test concentration cannot be accurately quantified, remove the scrubber, use a test concentration that causes a full 

scale response, reattach the scrubber, and evaluate response for interference.

16. Amend Appendix A to Subpart B of Part 53 by revising “Figure B-5” to read as follows:

Appendix A to Subpart B of Part 53—Optional Forms for Reporting Test Results

* * * * *

CALCULATION OF ZERO DRIFT, SPAN DRIFT, AND PRECISION

Applicant____________________________________________________________ Date_________________________

Analyzer____________________________________________________________ Pollutant_____________________

TEST

PARAMETERS CALCULATIONS TEST DAY (n)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

ZERO 

DRIFT

12 

HOUR 12ZD = Cmax − Cmin

24 Z = (L1 + L2)/2

NO2 Electrochemical 0.2 30.1 0.5 40.1 0.5 750 0.5 20,000 50

NO2 Spectrophotometric

-gas phase

3

0.1 0.5 40.1 0.5 0.5 20,000 50
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HOUR 24ZD = Zn − Zn−1

24ZD = Zn

′ − Zn−1

′

SPAN 

DRIFT

24 

HOUR

Sn =

1

6

∑Pi

12

i=7

SDn =

Sn − Sn−1

Sn−1

× 100%

SDn =

Sn − Sn−1

′

Sn−1

′ × 100%

PRECISION

20% 

URL 

(P20)

P20 = % STANDARD 

DEVIATION OF (P1...P6)

80% 

URL 

(P80)

P80 = % STANDARD 

DEVIATION OF (P7...P12)

Figure B-5. Form for calculating zero drift, span drift, and precision (§ 53.23(e)).

* * * * *
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Subpart C—Procedures for Determining Comparability Between Candidate 

Methods and Reference Methods

17. Amend §53.32 by revising paragraph (g)(1)(iii) to read as follows:

§53.32 Test procedures for methods for SO2, CO, O3, and NO2.

* * * * *

 (g) * * *

(1) * * * 

 (iii) The measurements shall be made in the sequence specified in table C-2 of this 

subpart.

* * * * *

Subpart E [Amend]

18. Amend Subpart E by removing “Figure E-2 to Subpart E of Part 53”.

PART 58—AMBIENT AIR QUALITY SURVEILLANCE

19. The authority citation for part 58 continues to read as follows:

AUTHORITY: 42 U.S.C. 7403, 7405, 7410, 7414, 7601, 7611, 7614, and 7619.

Subpart B—Monitoring Network

20. Amend §58.10 by adding paragraphs (a)(10) and (11) to read as follows:

§58.10 Annual monitoring network plan and periodic network assessment

(a) * * *

(10) The annual monitoring network plan shall provide for the required O3 sites to be 

operating on the first day of the applicable required O3 monitoring season in effect on January 1, 

2017 as listed in Table D-3 of appendix D of this part.
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(11) The annual monitoring network plan shall include the Enhanced Monitoring Plan 

(EMP) for areas designated as O3 nonattainment, as required under 40 CFR part 58 Appendix D, 

section 5(f) beginning with the annual monitoring plans due on July 1, 2016.

* * * * *

21. Amend §58.13 by adding paragraphs (g) and (h) to read as follows:

§58.13 Monitoring network completion.

* * * * *

(g) The O3 monitors required under appendix D, section 4.1 of this part must operate on 

the first day of the applicable required O3 monitoring season in effect January 1, 2017. 

(h) The Photochemical Assessment Monitoring sites required under 40 CFR part 58 

Appendix D, section 5(a) must be physically established and operating under all of the 

requirements of this part, including the requirements of appendix A, C, D, and E of this part, no 

later than June 1, 2017, or two years following designation as O3 nonattainment.

Subpart F—Air Quality Index Reporting

Amend § 58.50 by revising paragraph (c) to read as follows: 

§ 58.50 Index reporting.

* * * * *

(c) The population of a metropolitan statistical area for purposes of index reporting is the latest 

available U.S. census population. 

* * * * *

Subpart G—Federal Monitoring

22. Amend Appendix D to part 58 by:
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a. Under section 4—by revising section 4.1(i) and “Table D-3 to Appendix D of part

58; and

b. Revising section 5 and its heading.

The revised text reads as follows:

Appendix D to part 58—Network Design Criteria for Ambient Air Quality 

Monitoring

* * * * *

4. POLLUTANT-SPECIFIC DESIGN CRITERIA FOR SLAMS SITES

* * * * *

4.1 * * * 

(i) Since O3 levels decrease significantly in the colder parts of the year in many areas, O3

is required to be monitored at SLAMS monitoring sites only during the “ozone season” as 

described below in Table D–3 of this appendix. These ozone seasons are also identified

in the AQS files on a state-by-state basis. Deviations from the O3 monitoring season must 

be approved by the EPA Regional Administrator. These requests will be reviewed by 

Regional Administrators taking into consideration, at a minimum, the frequency of outof-season O3 NAAQS exceedances, as well as occurrences of the Moderate air quality 

index level and regional consistency. Any deviations based on the Regional 

Administrator’s waiver of requirements must be described in the annual monitoring 

network plan and updated in AQS. Changes to the O3 monitoring season requirements in 

Table D-3 revoke any previously approved Regional Administrator waivers for affected 

states. Requests for monitoring season waivers must be accompanied by relevant 

supporting information. Information on how to analyze O3 data to support a change to the 
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O3 season in support of the 8-hour standard for a specific state can be found in reference 

8 to this appendix. O3 monitors at NCore stations are required to be operated year-round

(January to December). 

TABLE D-3

1

TO APPENDIX D OF PART 58. OZONE MONITORING SEASON BY STATE.

State Begin Month End Month

Alabama March October

Alaska April October

Arizona January December

Arkansas March November

California January December

Colorado January December

Connecticut March September

Delaware March October

District of Columbia March October

Florida January December

Georgia March October

Hawaii January December

Idaho April September

Illinois March October

Indiana March October

Iowa March October

Kansas March October

Kentucky March October
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Louisiana (Northern) AQCR 

019,022

March October

Louisiana (Southern) AQCR 106 January December

Maine April September

Maryland March October

Massachusetts March September

Michigan March October

Minnesota March October

Mississippi March October

Missouri March October

Montana April September

Nebraska March October

Nevada January December

New Hampshire March September

New Jersey March October

New Mexico January December

New York March October

North Carolina March October

North Dakota March September

Ohio March October

Oklahoma March November

Oregon May September

Pennsylvania March October

Puerto Rico January December
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Rhode Island March September

South Carolina March October

South Dakota March October

Tennessee March October

Texas (Northern) AQCR

022, 210, 211, 212, 215, 217, 218

March November

Texas (Southern) AQCR 

106, 153, 213, 214, 216

January December

Utah January December

Vermont April September

Virginia March October

Washington May September

West Virginia March October

Wisconsin March October 15

Wyoming January September

American Samoa January December

Guam January December

Virgin Islands January December

1The required O3 monitoring season for NCore stations is January through December.

* * * * *

5. NETWORK DESIGN FOR PHOTOCHEMICAL ASSESSMENT MONITORING STATIONS (PAMS)

AND ENHANCED OZONE MONITORING
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(a) State and local monitoring agencies are required to collect and report the following 

PAMS measurements at each NCore site required under paragraph 3(a) of this appendix located 

in an area designated as nonattainment for O3. 

(b) PAMS measurements include:

(1) Hourly averaged speciated volatile organic compounds (VOCs),

(2) 8 3-hour averaged carbonyls daily,

(3) Hourly averaged O3,

(4) Hourly averaged nitrogen oxide (NO), nitrogen dioxide (NO2), and total 

reactive nitrogen (NOy), 

 (5) Hourly averaged 3 meter ambient temperature,

(6) Hourly vector-averaged 10 meter wind direction,

(7) Hourly averaged 10 meter wind speed,

(8) Hourly average atmospheric pressure,

(9) Hourly averaged relative humidity, and

(10) Hourly averaged mixing-height.

(c) The EPA Regional Administrator may grant a waiver to allow the collection of 

required PAMS measurements at an alternative location where the monitoring agency can 

demonstrate that the alternative location will provide representative data useful for regional or 

national scale modeling and the tracking of trends in O3 precursors. 

(d) The EPA Regional Administrator may also grant a waiver to allow representative 

meteorological data from nearby monitoring stations to be used to meet the requirements to 

collect temperature, wind direction, wind speed, atmospheric pressure, relative humidity, or 
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hourly averaged mixing height where the monitoring agency can demonstrate the data is 

collected in a manner consistent with EPA quality requirements for these measurements.

(e) At a minimum, the monitoring agency shall collect the required PAMS measurements 

during the months of June, July, and August.

(f) States with O3 nonattainment areas are required to develop and implement an 

Enhanced Monitoring Plan (EMP) detailing enhanced O3 and O3 precursor monitoring activities 

to be performed which is subject to review and approval by the EPA Regional Administrator. 

The EMP will include monitoring activities deemed important to understanding the O3 problems 

in the state. Such activities may include, but are not limited to, the following:

(1) Additional O3 monitors beyond the minimally required under paragraph 4.1 of this 

appendix, 

(2) Additional NOX or NOy monitors beyond those required under 4.3 of this appendix,

(3) Additional speciated VOC measurements including data gathered during different 

periods other than required under paragraph 5(e) of this appendix, or locations other than those 

required under paragraph 5(a) of this appendix, and

(4) Enhanced upper air measurements of meteorology or pollution concentrations.