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 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; to make corresponding 

revisions in data handling conventions for O3; to make conforming changes to the Air 

Quality Index (AQI); to revise regulations for the prevention of significant deterioration 

(PSD) permitting program to add a grandfathering provision for certain pending permit 

applications from certain requirements with respect to the proposed revisions to the O3

NAAQS; and to propose schedules and convey information related to implementing any 

revised standards. In addition, 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 (FEM) procedures for testing, and the Photochemical 

Assessment Monitoring Stations (PAMS) network.
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The EPA proposes to revise the primary standard for O3 by setting the level of 

that standard to within the range of 0.065 to 0.070 parts per million (ppm) in order to 

provide increased 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. The EPA is proposing conforming changes to the AQI 

for O3, proposing to set an AQI value of 100 equal to the level of the 8-hour primary 

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. The Administrator 

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

within the range from 13 ppm-hours (ppm-hrs) to 17 ppm-hrs would provide the requisite 

protection against known or anticipated adverse effects to the public welfare. To achieve 

that level of air quality, the Administrator proposes to revise the level of the current 

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 three-year average W126 index values, of 

a range at or below 13-17 ppm-hrs. 

In conjunction with proposing exceptional event schedules related to 

implementing any revised O3 standards, the EPA is also proposing to apply this same 

schedule approach to other future revised NAAQS and to remove obsolete regulatory 

language associated with already 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 
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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. 

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-OAR2008-0699, 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 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 
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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 email comment directly to the EPA without going through www.regulations.gov your email 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 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 
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includes documents in the rulemaking docket (Docket ID No. EPA-HQ-OAR-2008-0699) 

and a separate docket, established for the Integrated Science Assessment (Docket No.

EPA-HQ-ORD-2011-0050) 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: 

General Information
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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.
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 Explain your views as clearly as possible, avoiding the use of profanity or 

personal threats.

 Make sure to submit your comments by the comment period deadline identified.

Availability of Related Information

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

sites. The Integrated Science Assessment for Ozone and Related Photochemical Oxidants 

(ISA) 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, Health and Welfare Risk and Exposure Assessments, 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

Policy Assessment 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 
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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

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 
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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

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
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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

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
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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.

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 
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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 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 Clean Air Act, 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 
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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 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 
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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 

Formatted: Highlight
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level of the current 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. The EPA is also taking comment on the Administrator’s proposed 

decision to revise the current secondary standard and the option of retaining that 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.
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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 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 
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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 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 

 

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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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 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 
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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 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 

 

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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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 vehicle and motor 

vehicle fuel control program under title II of the Act (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 

 

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%20Re

view%20Panel.
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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 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 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

 

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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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 feasibility are not relevant considerations in the 

promulgation of the NAAQS. The court also noted that the EPA need not tailor the 
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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). 
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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 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 fourth-highest 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
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(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 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 
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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” which 

supported EPA’s conclusion that “the most serious health effects of ozone are ‘less 

certain’ at low concentrations, providing an eminently rational reason to set the primary 

standard at a somewhat higher level, at least until additional studies become available.” 

Id. (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, EPA could consider that factor when choosing 

among the three alternative levels.” Id.

Finally, the court upheld EPA’s revisions to the secondary standard. Rejecting 

arguments that EPA had improperly evaluated adverse effects of ozone on crops by not 

considering that factors other than ozone also adversely affect crop yield, the court stated 

that EPA had no such obligation: “[t]he Clean Air Act directs EPA to protect public 

welfare from adverse effects of ozone and other pollutants; the Agency cannot escape 

that directive simply because ozone wreaks less havoc than temperature, rainfall, and 

pests.” Id. at 380. The court likewise rejected arguments that EPA had acted based on 

assertedly “crude” estimates of crop-related risks of ozone, noting CASAC’s advice that 
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there was sufficient information to warrant revising the secondary standard, and further 

holding that “nothing in the Clean Air Act requires EPA to wait until it has perfect 

information before adopting a protective secondary NAAQS.” 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 the primary standard within a 
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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 
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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 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 
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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

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 

 

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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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 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 
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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 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 Integrated Science Assessment (ISA), 

the EPA’s National Center for Environmental Assessment (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, 

 

7 See 

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

more information on CASAC activities related to the current O3 NAAQS review.

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

http://www.epa.gov/ttn/naaqs/standards/ozone/data/2011_04_OzoneIRP.pdf
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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/R10/076F) 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 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 

 

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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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 Policy Assessment10 (PA) 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 (U.S. EPA, 2014c) were made available by the EPA in August, 2014. 

These documents reflect staff’s consideration of the comments and recommendations 

 

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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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 
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where O3 concentrations were above the level of the existing health-based 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) high-elevation 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, 
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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 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 

 

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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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 metropolitan areas with 

the worst O3 problems, increase the risk of morbidity and mortality. Children, older 

adults, and people with asthma or other lung diseases 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 nonlocal 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 
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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. 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 
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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 

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 

 

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

0.084 ppm (84 ppb).
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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. 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 thencurrent 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 

 

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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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. 

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 
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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 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. 

 

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). 

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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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 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 
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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 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, based on multiple high-

 

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). 

Commented [A1]: Appears to be something wrong with this 

sentence. Do you meant: The ISA concludes this category is 

appropriate when multiple high-quality studies show “there is ....
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quality studies, “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 pollutantrelated 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 scientists generally 

agree that adverse health effects are likely to occur, through lower concentrations, at 
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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 longterm 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 lowest-observed-effects levels in those studies. Second, the PA uses these
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studies to help evaluate the extent to which there is confidence in health effect 

associations reported in epidemiologic studies 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. 
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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 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 
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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 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 co-occurring 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. 

 

17 Though the PA recognizes that a broader body of studies, including international 

studies, informs the causal determinations in the ISA. 
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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 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 

 

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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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 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 exposureresponse 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 

 

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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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 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

 

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).

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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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 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 
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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 (nonaccidental) mortality, expanding upon 

 

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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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

 

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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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 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.
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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 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 
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(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 conditions or diseases, nutritional status, lifestages, and coexposures 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 

 

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).
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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. 

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 shortterm (central nervous system effects) and long-term exposure (cardiovascular effects, 

reproductive and developmental effects, central nervous system effects and total 

mortality) to ambient O3.

 

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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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 shortterm O3 exposures, including pulmonary function decrements and increases in respiratory 

symptoms, lung inflammation, lung 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 
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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

concentrations at the low end of the distribution of daily concentrations. Recent 

epidemiologic studies report that associations with respiratory morbidity and mortality 

 

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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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; respiratory symptoms and medication use; lung host defense; 
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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. 

 

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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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 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 
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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 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 

 

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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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 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, 

 

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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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 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 

 

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 V̇

E 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. 

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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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% 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 
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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 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 

 

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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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 et al. (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 

 

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 from all of the studies conducted at 60 ppb, 1% of subjects 

experienced decrements > 20% (also uncorrected for filtered air exposures).
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responsiveness. Given 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. 

 

39 Responses to O3 in these studies were adjusted for responses observed following 

exposure to filtered air.
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(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 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).

 

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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CASAC agreed that these models mark a significant technical advance, stating 

that “the comparison of the MSS [McDonnell-Stewart-Smith] model results to those 

obtained with the exposure-response (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 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 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 pre-existing respiratory diseases such as asthmatic children (U.S. EPA, 

2013a, section 6.2.1.2). Some of these studies reported O3-associated lung function 

 

41 Unless otherwise specified, the epidemiologic studies discussed in the PA evaluate 

only adults.
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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 that associations with lung 

function decrements persisted at relatively low ambient O3 concentrations. For outdoor 

recreation or 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).

 

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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Epidemiologic panel studies investigating the effects of short-term exposure to O3

provided information on potential confounding by copollutants such as PM2.5, PM10, 

NO2, or 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) (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 
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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 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 

 

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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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 summer) can resolve entirely; (2) continued acute inflammation can evolve 

into a chronic inflammatory state; (3) continued inflammation can alter the structure and 
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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, 

 

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).
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including neutrophil46 (PMN) influx, markers 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 

 

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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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 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). 
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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 pro-inflammatory 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 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., 

 

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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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). 

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 
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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-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 
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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; BarrazaVillarreal 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 

 

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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additional evidence that inhaled O3 is likely to be an important source of reactive oxygen 

species in airways and/or may increase pulmonary inflammation via oxidative stressmediated 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 
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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 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, 
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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 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). 
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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 

late-phase 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. 

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). 
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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 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 
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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, and increased 

airway responsiveness, 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 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 

 

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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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 current review, additional 

controlled human exposure studies have evaluated respiratory symptoms following 

exposures to O3 concentrations below 80 ppb and recent epidemiologic studies have 
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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 
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provides consistent evidence for associations between short-term increases in ambient O3

concentrations and increased respiratory 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 

 

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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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 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 
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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

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 

 

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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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 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.
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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.

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 
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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 

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 nonhuman 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., 

 

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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1996; Holz et al., 2002) observed increased airway responsiveness to O3 exposure with 

bronchial allergen challenge in subjects with preexisting allergic airway disease. Ozoneinduced 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 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 
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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 study-durations, were conducted in locations not 

represented by the larger studies, or examined population-specific characteristics that 

 

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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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, populationweighted 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
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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 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 
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asthma hospital admissions (U.S. EPA, 2013a, page 6-157; Silverman and Ito, 2010). In 

addition, an examination of the concentration-response 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 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 short-term 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 
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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. 

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 
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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 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 
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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 physiciandiagnosed 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 new-onset asthma by indicating that the lower risks associated with specific 

genetic variants are found 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 
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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 antiinflammatory 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 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)
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metrics. The short-term 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 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 
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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 
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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 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 
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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 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).
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iv. Respiratory mortality 

A limited number of epidemiologic studies have assessed the relationship between 

long-term 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 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). 
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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 shortterm 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 cardiovascularrelated 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 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
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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 shortterm 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 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) 
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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., ICD-9 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 exposure and cardiovascular mortality in multicity studies and in a multicontinent study. When examining mortality due to CVD, epidemiologic studies 
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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 allcause (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 
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all-year data, effect estimates 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 causespecific mortality. Multi-continent and multicity studies have consistently reported 

positive and statistically significant associations between short-term O3 concentrations 
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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 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 

 

56 Respiratory mortality is discussed in more detail above. 
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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 

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 

 

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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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 

concentration-response 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 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 
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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 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 
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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 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 
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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 

new-onset 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 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 
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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 for adverse health outcomes in active 

healthy adults” (Frey, 2014c, p. 3). However, for children and adults with lung disease, 
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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 
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of ≥10% is a scientifically relevant surrogate for adverse health outcomes for people with 

asthma and lung disease” (Frey, 2014c, p. 3).

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 
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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 symptoms of cough and pain on deep inspiration (U.S. EPA, 2013a, section 

6.2.1.1). Some 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, some 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 nonasthmatic 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 
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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 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 nonbiological 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 
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used to encompass these concepts more generally. In the ISA and PA, “at-risk” is the allencompassing 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 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 

 

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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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 atrisk 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, 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 
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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 

short-term 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 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
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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 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
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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. Ozoneinduced increases in neutrophils, IL–8 and protein were found to be significantly higher 

in the BAL fluid from asthmatics compared to healthy subjects, suggesting mechanisms 
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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 

two-phase 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 
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of eosinophils, may indicate an effect on the Th2-related airway response and on airway 

eosinophils. The authors reported 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.
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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 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 
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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 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
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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 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 
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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 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., 
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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 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). 
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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 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).
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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 adults ≥65 years old but not for those adults aged 15 to 64 years
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(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 COPD-related 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. 
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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. 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 (Sienra-Monge 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 
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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 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 
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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 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 
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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 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 
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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 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-

 

59 As noted below (II.C.3.a.ii), asthmatics can experience larger O3-induced respiratory 

effects than non-asthmatic, healthy adults. It is not clear from the available information 

the extent to which the responsiveness of asthmatics to O3 exposures could depend on 

factors such as asthma severity, the effectiveness of asthma control, or the prevalence of 

medication use.

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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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 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 
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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 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 
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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 
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exposures, key 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). 
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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 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 

 

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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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 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 

 

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).
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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 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

 

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).

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 
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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 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 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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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 “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 

 

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). 

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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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). 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 
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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 means), 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 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 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 ozonerelated effects including lung inflammation and airway responsiveness in 

healthy individuals. The 70 ppb-8hr benchmark level reflects the fact that 
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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

 

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 are rounded to nearest thousand unless otherwise indicated. Estimates smaller

than 50 were rounded downward to zero (from U.S. EPA, 2014a, Appendix 5F Table 5F5). 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 0 300 [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

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. 

 

73 As noted below (II.C.3.a.ii), in the case of asthmatics it is not clear from the available 

information the extent to which O3 responsiveness could depend on factors 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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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 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. 
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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 were higher 

than the other adult populations evaluated. The percentages of outdoor workers estimated 

to experience exposures of concern were generally similar to the percentages estimated 

for children (i.e., using the full database of diary profiles) in the 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, 
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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 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. 
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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 long-term 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 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. 

 

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 long-term concentrations. 
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i. Overview and summary of key results

In the current review, the HREA estimates risks of lung function decrements in 

school-aged 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 dose-threshold 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 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 

 

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).

77 Though see below for discussion of uncertainty in lung function responses of children 

and asthmatics. 
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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.
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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

 

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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60 0-0.2 21,000 [2,000] 0.4

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 exposureresponse 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). 
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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., 2000). On this issue, CASAC noted that 

“[a]sthmatic subjects appear to be at least as sensitive, if not more sensitive, than nonasthmatic 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 including the severity of asthma and the prevalence of medication use.

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. 
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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 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 

 

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. Areawide concentrations are discussed in more detail in section 3.1.4 of the PA (U.S. EPA, 

2014c).
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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. 

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 concentration-response 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). 

Commented [A2]: Per our discussion
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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 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, 

 

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 concentrationresponse 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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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, 

primarily because that analysis is based on only one study, though that study is welldesigned, 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 

 

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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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 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 
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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 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 

 

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 
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resulted in larger decreases in mid-range O3 concentrations (25th to 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 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. 

 

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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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 concentrationresponse 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 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-

 

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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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 

concentration-response 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 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 concentrationresponse functions from epidemiologic studies to adjusted air quality. Concentration-
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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.

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; 
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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 longterm 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

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 
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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 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. 
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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 
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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

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 

 

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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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 averaging periods that differ from the 8 hour averaging time of the 

current O3 standard). The PA 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 

 

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. 

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). 
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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 associations in atrisk 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 

 

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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quality in locations where epidemiologic studies have characterized confidence intervals 

around cut point analyses or concentration-response 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 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. 

 

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).
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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 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 

 

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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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 points. In these analyses, effect estimates were based only on the 

subsets of days contributing to averaged O3 concentrations below cut points ranging from 
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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-
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hour O3 concentration generally corresponds to the 98th or 99th percentile of the seasonal 

distribution, depending on the length of the O3 season.

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 pot`ential 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 
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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 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 
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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 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 
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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

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 

 

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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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 

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 
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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 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). 
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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 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. 

 

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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(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 
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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: 

(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: 
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(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) 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. 
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(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%.

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 concentration-response 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 

 

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. 
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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 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 concentrationresponse 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 short-term O3 concentrations, respiratory morbidity associated with short-term O3

 

Area-wide concentrations are discussed in more detail in section 3.1.4 of the PA (U.S. 

EPA, 2014c).
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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. EPA, 2014a, Appendix 7B). Increases were estimated in
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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 ozonerelated 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) 

 

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 concentrationresponse 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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estimates indicating decreased occurrences of O3 exposures of concern and decreased 

health risks upon meeting the current standard, compared to recent air quality. 

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 

 

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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concentration-response 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 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.
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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 

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 
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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 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 
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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 

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 
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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.

(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.
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(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 Administrator revise the current 

primary ozone standard to protect public health.106

 

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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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 
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that 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 shortand 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. 
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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 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 
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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, 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 

 

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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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 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 antiinflammatory 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 
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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 longterm 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 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 cooccurring 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 
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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 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 

 

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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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). 

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 
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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 short-term O3 exposures and a wide range of 

adverse respiratory outcomes (including respiratory-related 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 

 

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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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 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 
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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 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 

 

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 
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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 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 

 

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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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 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 

 

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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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 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 
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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 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 worst-case 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 

 

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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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 

epidemiology-based 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 epidemiology-based 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 concentration-response 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 

concentration-response 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 evidence from controlled human exposure studies, which 

provide the strongest support for O3-induced effects following exposures to O3
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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 
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evidence and information in the record for the 2008 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 strong 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 

 

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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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 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 also 

solicits comment on the option of retaining that standard. 

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
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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-/riskbased considerations and conclusions in the PA, CASAC advice, and input from 

members of the public. The sections below discuss the evidence 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 
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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 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 

 

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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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

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 short-term 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 
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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 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, 
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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 short-term 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 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
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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 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)
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(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 concentrations. Seasonal averages of 8-hour daily max O3 concentrations 

 

120 Though the Administrator also notes important uncertainties associated with these risk 

estimates, as discussed above (II.C.3.b). 
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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 
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current 8-hour averaging time, and not to set an additional standard with a different 

averaging time. 

3. Form

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 

 

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.
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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 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 

 

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. 

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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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 

percentile-based 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 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 

 

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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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 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 nthhigh 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
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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 

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-
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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

The PA's approach to reaching conclusions on alternative standard levels focuses 

on the evidence from controlled human exposure and epidemiologic studies, as assessed 
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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 at-risk 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 ozonerelated effects including lung inflammation and airway responsiveness in 

healthy individuals. The 70 ppb-8hr benchmark level reflects the fact that 

 

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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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.

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, 
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especially in at-risk populations, with the increasing uncertainty regarding the likelihood 

of such effects following exposures to lower O3 concentrations.

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 
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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 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 

epidemiology-based 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 concentrationresponse 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). 
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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 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

 

126 Differences in estimated respiratory morbidity risks between alternative standard 

levels are similar to the differences estimated for total mortality associated with shortterm O3 concentrations. 
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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 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 

 

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). 
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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 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 

 

128 As discussed above (II.B.2), prolonged 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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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 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 
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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 
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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

Average % 

Reduction 

from Current 

Standard132

% 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

133 1,400 [200]134 100 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 Percent reductions in each urban study area were calculated and averaged across areas. 

133 Estimates smaller than 0.05% were rounded to zero. 

134As 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.

Commented [A3]: Both tables 4 and 5 should have a the std 

footnote re: the numbers of children/severity of asthma/medication 

management.
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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 % 

Children135

Number of Children (5 

to 18 years) [Number 

of Asthmatic 

Children]136

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]137 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

 

135 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. 

136 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. 

137As 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 
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ppb, and less than 1% are estimated to experience two or more. Far less than 1% of 

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).138

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 

 

138 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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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 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). 
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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 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,139 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 

 

139 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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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 which standard levels below 70 ppb could further reduce the incidence of such 

hospital admissions.140

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 

 

140 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). 
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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.141

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 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.142 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 

 

141 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 concentrationresponse 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.

142 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. 
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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.143 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 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 

 

143 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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not meet the statutory requirement to protect public health with an 

adequate margin of safety (Frey, 2014c, p.8).144

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 longterm 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;

145 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

 

144 Also see Frey (2014c, p. ii).

145 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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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 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-
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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 worstcase year and location.
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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. 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, compared to a 

standard with a level of 70 ppb, a revised standard with a level of 65 ppb would more 

effectively maintain short-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. 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 
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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 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.146

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).147 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 

 

146 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). 

147 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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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.

148 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.149 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 

 

148 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.

149 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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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 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;

150 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 

 

150 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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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. 

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). 
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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 worstcase 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 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, compared to a 

standard with a level of 65 or 70 ppb, a revised standard with a level of 60 ppb would 
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more effectively maintain short-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. 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 longterm O3 concentrations, based on information from the study by Jerrett et al. (2009).151 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. 

 

151 Though as discussed above, the PA notes the lower confidence we place in these risk 

results (U.S. EPA, 2014a, section 9.6).
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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.152 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) and 11% (2009), compared to the 

current standard (U.S. EPA, 2014c, Figure 4-13).153 For area-wide concentrations at or 

above 40 ppb, a standard with a level set at 60 ppb is estimated to reduce O3-associated 

 

152 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 epidemiologic-based risk estimates. 

153 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.
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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;

154 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 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 

 

154 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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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). 
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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 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. 
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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 

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.

155 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 

 

155 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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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-/riskbased 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 

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. Atrisk 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 
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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 epidemiology-based risk estimates (U.S. EPA, 2014a, section 9.6). 

For all of these reasons, the Administrator 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 60,156 72,157 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 

 

156 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. 

157 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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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). 

Most of these effects have also been shown in healthy adults following exposures 

to O3 concentrations lower than 80 ppb, with generally smaller effects following 

exposures to lower concentrations. In particular, 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, although the combination of statistically significant 

increases in respiratory symptoms and lung function decrements has not been reported,158

the Administrator notes that exposures to 60 ppb O3 have been shown to decrease lung 

function in several studies and, in one study (Kim et al., 2011), to also increase airway 

inflammation (II.B.2). 

 

158 More specifically, the Administrator 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). None of these studies 

reported a statistically significant increase in respiratory symptoms, compared to filtered 

air controls, though Adams (2006) reported a significant increase in symptoms, compared 

to initial symptoms. 

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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 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 
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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 in 

some people, the Administrator considers the extent to which such a standard would be 

expected to limit the occurrence of O3 exposures of concern at or above 60, 70, and 80 
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ppb.

159 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 

 

159 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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inflammation has been reported following exposures as low as 60 ppb O3. She also takes 

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.160 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. 

 

160 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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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 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.

161 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). 

 

161 State of Mississippi, 744 F. 3d at 1343.
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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). 

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 
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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. 

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 
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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.162

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%.163 Compared to the current standard, these 

 

162 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. 

163 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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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. 

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 
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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 
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below the lowest O3 exposure concentration shown 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 
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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 singlecity studies of short-term O3, compared to health effect associations aggregated across 

multiple cities in multicity studies. In particular, the Administrator notes 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.164

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 

 

164 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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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 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 
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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 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 longterm 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 
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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 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 epidemiologybased 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 
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(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 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 
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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.165 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 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.166 In 

 

165 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). 

166 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, 
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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.167 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, 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 

 

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). 

167 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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part of her decision-making on the appropriate level, averaging time, form, and indicator 

of the standard.168

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.169

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) setting the level of the standard well below the lowest 

O3 exposure concentration shown to result in the adverse combination of respiratory 

symptoms and lung function decrements (This judgment would emphasize that 8-hour 

exposures could result in effects at lower concentrations than indicated in 6.6-hour

controlled human exposure studies and that members of at risk populations could 

 

168 See, e.g. NRDC v. EPA, 902 F. 2d 962, 973-74 (D.C. Cir. 1990).

169 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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experience more serious effects than the healthy subjects evaluated in these studies); (2) 

eliminating almost all exposures of concern (even single exposures) at or above 70 and 

80 ppb, even in worst-case years and locations; (3) almost eliminating the occurrence of 

two or more exposures of concern at or above 60 ppb; (4) achieving additional reductions 

in O3-induced FEV1 decrements, beyond those achieved with a level of 70 ppb (5) 

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 

(6) 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) setting the level 

of the standard just below the lowest O3 exposure concentration shown to result in the 

adverse combination of respiratory symptoms and lung function decrements (i.e., 72 ppb) 

(This judgment would emphasize the increasing uncertainty in the extent to which 

adverse effects occur following lower exposure concentrations); (2) 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; (3) 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; (4) 

reducing, but not eliminating, the occurrence of one or more exposures of concern, noting 
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that not all exposures of concern result in adverse effects; (5) 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 (6) recognizing 

uncertainties in epidemiology-based risk estimates.

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 
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eliminating even single occurrences of exposures of concern at and above 60 ppb; on the 

potential public health importance of providing substantially greater protection (i.e., than 

that provided by a level of 65 ppb) against 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. As discussed more 

fully above, given the uncertainties associated with such estimates, analyses, and 

assumptions, the Administrator does not agree that it is appropriate to place significant 

weight on these factors. 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.

170

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; 

 

170 Although, as discussed below, she solicits comment on standard levels as low as 60 

ppb.
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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 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.171 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 

 

171 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 > 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 effects at lower levels than the benchmarks derived from clinical studies 

conducted using healthy adult subjects. 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 
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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 a 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 annual fourthhighest 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 
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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, 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). 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. An additional consideration in selecting breakpoints is for each category 

to span at least a 15 ppb range to allow for more accurate forecasting.

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 
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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 level 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 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 adults 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.172 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 

 

172 Exposures to 50 ppb have not been evaluated experimentally, but are estimated to 

potentially affect only a small proportion of individuals 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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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 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). 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, 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 people are likely to have large lung function decrements (i.e., FEV1 decrements 

≥ 20%), which would interfere with daily activities for most healthy people and likely 

cause many of them to seek medical attention. Effects would likely be greater in people 

with asthma.

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
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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.

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.

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 
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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 requisite 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. 

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 
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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 review173 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 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 

 

173 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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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. 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 
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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 
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the public welfare (73 FR 16496, March 27, 2008). Thus, the level of protection from 

known or anticipated adverse effects to public welfare that is requisite for the secondary 

standard is a public welfare policy judgment to be made by the Administrator. In 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, 
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wildlife refuges, and wilderness areas” (73 FR 16496, March 27, 2008).174,175 The 

Administrator additionally recognized that “States, Tribes and public interest groups also 

set aside areas that are intended to provide similar benefits to the public welfare, for 

residents on 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,176 or on 

 

174 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.

175 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).

176 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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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 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 
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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. 

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 climatespecific 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 
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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 site-specific 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 exposureresponse (E-R) relationships that 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 
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risks were assessed in a range of analyses primarily involving national-scale 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 associated ecosystems, since “of the phytotoxic 
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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 
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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 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, 
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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 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 cycles177

(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 

 

177 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). 
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relationship exists between exposures to O3 in ambient air and reduced carbon 

sequestration (also termed carbon storage)178 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 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 forcing179 (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 

 

178 The terms sequestration and storage are used somewhat interchangeably in the ISA 

and other documents in this review. 

179 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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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).

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.
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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 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 
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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 W126.180

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 seasons181 and 

focused on metrics 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 

 

180The 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 nonthreshold 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).

181 In 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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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 vegetation-type 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 for flux 
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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 long-standing 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). 
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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 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-
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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 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. 
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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, 90 and 120 ppb exposure concentrations,

182

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. 

 

182 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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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 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).183 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 

 

183 See: http://www.epa.gov/airtrends/values.html.
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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 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)184 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, 

 

184 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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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 concentrations can increase wholetree 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 
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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 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 
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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 intraand 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 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
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“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 ER 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 composite E-R functions available in the last review, as well 
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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.185 

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 

 

185 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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crops. The ISA describes average crop yield loss reported across a number of recently 

published meta-analyses and 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, 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, 
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2013a, section 9.6.3.1).186 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 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) 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 

 

186 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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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 

occured 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). Additionally, satellite and 

ground-based O3 measurements have assessed soybean yield loss that 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 
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individual E-R functions described in the WREA187 (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 use188 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 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).

189

 

 

187 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). 

188 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).

189 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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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 receptors, ecosystems and resources to the public welfare (73 FR 16496, 

March 27, 2008).190 For example, a number of different types of locations provide 

services of special significance to the public welfare. As emphasized in previous O3

 

190 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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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 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 
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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, 
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section 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 
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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 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 
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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 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).191 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 

 

191 For example, see http://www.fs.fed.us/research/urban/environmental-justice.php.
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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 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).192 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 

 

192 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. 
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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.

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 lands193 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 

 

193 See http://www2.nature.nps.gov/air/Pubs/pdf/flag/NPSozonesensppFLAG06.pdf
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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).194

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 

 

194 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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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 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 
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O3-induced effect, the role that the species plays in the ecosystem, the intended use of the 

affected species and its associated ecosystem and services, the presence of other cooccurring predisposing or mitigating factors, and associated uncertainties and limitations. 

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 

services195 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.

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).

 

195 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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1. Air Quality Analyses

The WREA evaluated O3 exposure and risks for several national-scale air quality 

scenarios: recent conditions (2006 to 2008),

196 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.197 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 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). 

 

196 Certain visible foliar injury analyses assessed recent conditions from 2006 to 2010 on 

an annual basis.

197 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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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.198 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 

interpolation method to estimate W126 index values at the centroid of every 12 x 12 km2

 

198 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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grid 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 
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when all monitor locations meet the standard (U.S. EPA, 2014b, section 4.3.4.2).199

Additionally, the surface is created from the three-year average at the monitor locations, 

rather than creating a surface for 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. 

 

199 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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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-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 
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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).

200 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 ppm-hrs 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-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 

 

200 These methods were calculating a median using the composite functions and 

calculating a median using all tree seedling studies available.
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the eastern cottonwood. The results summarized here are for the analyses that exclude 

cottonwood.201 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, 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 

 

201 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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(wRBL) were also developed for each grid cell nationwide. This is estimated from the 

species-specific E-R 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).

 For the current standard scenario, two of the 145 assessed Class I areas have 
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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 
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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 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 
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surplus.202 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 (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). 

 

202 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 forest-agriculture version of the model. 
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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).203

 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 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 i-Tree 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 

 

203 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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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.

204 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 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 

 

204 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/energy-resources/calculator.html).
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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 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 
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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 within-species 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 lossrelated 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. 
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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). 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 non-W126 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 
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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 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%). 
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The WREA derived estimates of crop RYL estimates nationally and in a countyspecific 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 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).205

The WREA also estimated O3-related crop impacts on producer and consumer 

surplus.206 These are national-scale estimates of the effects of yield loss on agricultural 

 

205 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). 

206 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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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 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 
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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

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. 
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In the biosite data analysis, the WREA used the biomonitoring site data from the 

USFS FHM/FIA Network (USFS, 2011),207 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 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 

 

207 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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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.208,209 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 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 

 

208 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.

209 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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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 
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than half of the area of 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, wildlife-watching, 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 
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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 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 
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(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-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 
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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 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 
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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.

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?
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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 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 
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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 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 
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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 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. 
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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 
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concentrations improves the explanatory 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 
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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 

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 
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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 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).210 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 

 

210 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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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 ppm-hrs (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) 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 threeyear 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 
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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
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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

*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). 
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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 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 

national-scale 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 
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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 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, 
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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 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 
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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 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.,211 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 

 

211 See http://www.ers.usda.gov/topics/crops/soybeans-oil-crops/background.aspx.
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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, 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%.212

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 

 

212 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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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 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 
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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 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 
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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 

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 
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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 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 
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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 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 
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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.

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 
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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, 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 
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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 PA of W126 

exposures occurring in locations where the current standard is met, including those 
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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 ER 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 concentrationweighted 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
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takes particular note of the evidence, described in section IV.D.1 above, of the 

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).213 

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 

 

213 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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current standard are reasonably concluded to be important from a public welfare 

standpoint in terms of 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 below-ground 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 
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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 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 
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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 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 longestablished 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 
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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. 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 
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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 average concentrations over a season and also include the midlevel 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 
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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 cut-off in its 

weighting scheme which allows for cumulation of lower O3 concentrations (U.S. EPA, 

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 
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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 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 
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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 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 
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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 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 
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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 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 
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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-toyear 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, 2010). See also ATA III, 283 F. 3d at 374-75 (recognizing programmatic stability as a 

legitimate consideration in the NAAQS standard-setting process).
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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 impact. Therefore, in development of the PA 
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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 
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of the uncertainties and limitations associated with several key aspects of this 

information. In addition to uncertainties 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 well-studied 

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 
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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 (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
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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

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. 
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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 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.214

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 

 

214 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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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. 

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 
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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 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 
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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 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 
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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. 

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).
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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 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, 
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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 co-occurring 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 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) 
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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%.

215

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 based standards with levels in the range 

identified above (17 ppm-hrs to 7 ppm-hrs) with a three-year average form.

 

215 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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Thus, the PA concludes that in staff’s view, the evidence- and exposure/riskbased 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 
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standard, this is 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 
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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 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,216 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 

 

216 Public comment received thus far in this review are in the docket EPA–HQ–OAR2008-0699, accessible at www.regulations.gov.
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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). 

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).

217 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., 

2009.218 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, 

 

217 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”.

218 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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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 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 
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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 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 
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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 Appendix 2b.219 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 

 

219 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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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 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-
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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 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 
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(U.S. EPA, 2014c, Appendix 2B).220 Focusing only on the latter dataset (2009-2011), it 

can be seen that at monitors just meeting the current standard (three-year average fourthhighest 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 three-year 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 ppm-hrs 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, threeyear 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). 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, threeyear 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 threeyear periods back to 2006-2008 and indicates that among the counties with O3

 

220 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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concentrations that met the current standard, the number of counties with three-year

W126 index values above 15 ppm-hrs 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 three-year averages) 

varied from less than 3 ppm-hrs to up to 23 ppm-hrs (Figure 5a). At sites with a threeyear 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 
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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.221), 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 increased over time, while the 

number of sites exceeding both 70 ppb and 13 ppm-hrs has decreased by a similar 

amount.

 

221 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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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 

4

th 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 threeyear 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 maximum 8-hour average O3
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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
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of the current form and averaging time with a level 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 primary standard in the range of 65

to 70 ppb, those actions would also improve air quality as measured by the W126 

metric.222 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 

 

222 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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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 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 
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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 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, concentrationweighted 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 
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by a revised standard, it is appropriate to use a cumulative, seasonal concentrationweighted 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 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 

three-year 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 
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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. 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 
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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 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 
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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 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 
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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 areas223 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 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 

 

223 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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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 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, 
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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 vegetation effects of particular concern, the Administrator, as an initial 
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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 ppm-hrs (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, 
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she additionally notes CASAC’s recognition that, within a 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. 
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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 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 
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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 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.
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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 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 
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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 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 
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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 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 
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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 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 
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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 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 three-month period 

yielding the highest seasonal value and with daily O3 exposures within a three-month 
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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 proposed for the primary standard 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 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
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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.

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 
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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 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 
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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, including detailed information on the 
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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 
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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 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 
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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-urban areas affected by longrange 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 groundlevel O3. Specifically, the chemical reactions involved in the formation of new groundlevel O3 require sunlight. Therefore, it is appropriate to begin the “O3 day” at sunrise, 
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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 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 
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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.

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:
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“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

E. Exceptional Events Information Submission Schedule

States224 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 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 

 

224 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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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.

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 
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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. 

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 
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unclassifiable.225 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 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 

 

225 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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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 “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 
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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 for future new or revised NAAQS. The EPA 
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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 MaydJundJuld AugdSep 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.226 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.

227

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.

228 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

 

226 See 40 CFR part 58 Appendix D, section 4.1, Table D-3 for a table of required O3 seasons.

227 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).

228Approximately 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 manner229 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,230 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, 

 

229 Public reporting requirements are detailed in 40 CFR part 58 Appendix G, Uniform Air 

Quality Index (AQI) and Daily Reporting.

230 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”). 231 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 

 

231 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.

232 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. 

 

232 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 provisions233 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 

 

233 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.234 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 

 

234 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.235 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.236 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 

 

235 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.

236 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.237 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 

 

237 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.238 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 

 

238 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.239 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 

 

239 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.

240 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 

 

240 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.241 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. 

 

241 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 manufacturer242 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

 

242 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 states243 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 

 

243 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

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%.

244

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 

 

244 “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.245

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.

 

245 “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, 2013.246 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 

 

246 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 

prescribe247) 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 

 

247 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 

revise the level of the current secondary standard to a level within the range proposed for 

the primary standard. 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 
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promulgate the area designations according to a specified process, including procedures to be 

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.248 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.249

 

248 American Petroleum Institute v. Costle, 609 F.2d 20 (D.C. Cir. 1979).

249 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 
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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 

 

I-X, December 20, 2011. Available:

http://www.epa.gov/ttn/oarpg/t1/memoranda/20120117naaqsguidance.pdf.

xx 
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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 

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. 
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1. Prevention of Significant Deterioration (PSD)

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). 250 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, 

 

250 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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which may or may not involve the use of air quality models, for evaluating whether a PSD 

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,251 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 

 

251 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.252 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 

contribute to a violation of the new or revised NAAQS.253 See, e.g., 73 FR 28321, 28324, 28340 

 

252 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. 

253In 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 
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(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.

254 The EPA has explained that 

sources must make the demonstration required under CAA section 165(a)(3) and the 

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 

 

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.

254 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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individual source255 (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.256 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

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 

 

255 See, e.g., id.

256 See id. at 1 and 3.
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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.257

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.

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 

 

257 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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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. 

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, 
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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 

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 
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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. 

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 
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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 

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 
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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).

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
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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 

quality data.” 258 The EPA intends to consider whether it is appropriate to make any revisions to 

 

258 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 
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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 

unclassifiable for the 2008 O3 NAAQS will continue to be required to obtain a PSD permit.259

 

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.

259 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. 
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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.260 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.261

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 

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 

 

260 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).

261 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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NAAQS and ultimately meet the PSD demonstration requirement under CAA section 

165(a)(3)(B) and the implementing regulations.262

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.263 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 

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).

264

 

262 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).

263 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.

264 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 
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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 

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 

 

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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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 revise the level of the 

current secondary standard within the range proposed for the primary standard 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 

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. 
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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.265 This program is commonly known as the Emissions Offset Interpretative 

Rule, and is applicable to O3 as well. 266

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 

 

265 See Appendix S, Part I; 40 CFR 52.24(k). 

266 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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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 

“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 
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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 

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?
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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. 

(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 
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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. 

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 
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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 

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 
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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 

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 
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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 

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 
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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 

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: 267

 

267 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 
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 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 

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 

 

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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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.”268

The EPA’s implementing regulations, the 2007 Exceptional Events Rule, further specify 

that states must provide evidence that: 269

 “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.”

 

268 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.”

269 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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The ISA contains discussions of natural events that may contribute to O3 or O3 precursors. 

These include stratospheric O3 intrusion and wildfire events.

270 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 

stratospheric O3 intrusion, the EPA recently approved a demonstration from Wyoming for a June 

2012 stratospheric O3 event.271

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 Widger, 2012). As such, determining the impact 

 

270 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.

271 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.
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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.

272

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 

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 inevitable, promising more dangerous and 

destructive future...fires” (Stephens, S. et al., 2013). The use of prescribed wildland fire can 

influence the occurrence of catastrophic wildfires which may help manage the contribution of 

 

272 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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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. 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. The EPA is committed to working with Federal land managers, Tribes 

and the States to address the impact of fire-related emissions on O3 through appropriate 

implementation policies.

3. Rural Transport Areas

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.

273

Historically, the EPA has recognized few nonattainment areas under this statutory 

provision.274 The EPA has not issued separate written guidance to further elaborate on the 

 

273 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.
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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.

275 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.

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

 

274 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.

275 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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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. 

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.
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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)276), 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.

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

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

Improving Regulation and Regulatory Review

 

276 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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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, Month, 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 

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 (PRA)

The information collection requirements in this proposed rule have been submitted for 

approval to the Office of Management and Budget (OMB) under the 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 

Field Code Changed
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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 

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.
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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 

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 
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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

final rule will respond to any OMB or public comments on the information collection 

requirements contained in this proposal.

C. Regulatory Flexibility Act (RFA)

I certify that this action will not have a significant economic impact on a substantial 

number of small entities under the RFA. 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 (UMRA)

This action does not contain any unfunded mandate as described in UMRA, 2 U. S. C. 

1531 – 1538, and does not significantly or uniquely affect small governments. Furthermore, as 
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Page 604 of 679

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 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 
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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 

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

277 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 

 

277 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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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 

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
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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

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 X of Y – 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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(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).
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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 

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.
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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.

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.
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(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 

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 
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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 

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 
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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 

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 1st highest 2nd highest 3rd highest 4th highest 5th highest 
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days within O3

monitoring 

season

daily max 8-

hour O3 (ppm)

daily max 8-

hour O3

(ppm)

daily max 8-

hour O3

(ppm)

daily max 8-

hour O3

(ppm)

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., 

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
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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

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
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(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.

* * * * *

(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 
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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).

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.

278

* * * * *

14. Revise Table B-1 to Subpart B of Part 53 to read as follows:

 

278 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. range3 test procedures 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

50.1 4 0.14 0.5 0.5 0.5 0.2 20,000 6

0.05

SO2 Flame photometric 0.01 4 0.14 750 3 20,000 50

SO2 Gas 

chromatography

0.1 4 0.14 750 3 20,000 50

SO2 Spectrophotometric

-wet chemical 

(pararosanaline)

0.2 0.1 0.1 4 0.14 0.5 750 0.5

SO2 Electrochemical 0.2 0.1 0.1 4 0.14 0.5 0.5 0.2 0.5 3 20,000

SO2 Conductivity 0.2 0.1 4 0.14 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 3 0.1 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 4 10

CO Gas 

chromatography 

with flame 

ionization detector

20,000 4 10 0.5

CO Electrochemical 0.5 0.2 20,000 4 10

CO Catalytic 

combustion-thermal 

detection

0.1 750 0.2 20,000 4 10 5.0 0.5

CO IR fluorescence 750 20,000 4 10 0.5

CO Mercury 

replacement-UV 

photometric

0.2 4 10 0.5

NO2 Chemiluminescent 3 0.1 0.5 4 0.1 0.5 20,000

NO2 Spectrophotometric

-wet chemical 

(azo-dyereaction)

0.5 4 0.1 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 3 0.1 0.5 4 0.1 0.5 750 0.5 20,000 50

NO2 Spectrophotometric

-gas phase

3 0.1 0.5 4 0.1 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 G—Federal Monitoring

22. Amend Appendix D to part 58 by:

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
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* * * * *

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 

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
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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

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
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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

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
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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

(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,
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(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 

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, 
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(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.