Vol. 79 Wednesday, 

No. 242 December 17, 2014 

Part II 

Environmental Protection Agency 

40 CFR Parts 50, 51, 52, et al. 

National Ambient Air Quality Standards for Ozone; Proposed Rule 


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75234 Federal Register / Vol. 79, No. 242 / Wednesday, December 17, 2014 / Proposed Rules 

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 Environmental 

Protection Agency (EPA) proposes to 

make revisions to the primary and 

secondary NAAQS for O3 to provide 

requisite protection of public health and 

welfare, respectively. The EPA is 

proposing to revise the primary 

standard to a level within the range of 

0.065 to 0.070 parts per million (ppm), 

and to revise the secondary standard to 

within the range of 0.065 to 0.070 ppm, 

which air quality analyses indicate 

would provide air quality, in terms of 3- 

year average W126 index values, at or 

below a range of 13–17 ppm-hours. The 

EPA proposes to make corresponding 

revisions in data handling conventions 

for O3 and conforming changes to the 

Air Quality Index (AQI); to revise 

regulations for the prevention of 

significant deterioration (PSD) program 

to add a transition provision for certain 

applications; and to propose schedules 

and convey information related to 

implementing any revised standards. 

The EPA is proposing changes to the O3

monitoring seasons, the Federal 

Reference Method (FRM) for monitoring 

O3 in the ambient air, Federal 

Equivalent Method (FEM) procedures 

for testing, and the Photochemical 

Assessment Monitoring Stations 

(PAMS) network. 

Along with proposing exceptional 

event schedules related to implementing 

any revised O3 standards, the EPA is 

proposing to apply this same schedule 

approach to other future revised 

NAAQS and to remove obsolete 

regulatory language for expired 

exceptional event deadlines. The EPA is 

proposing to make minor changes to the 

procedures and time periods for 

evaluating potential FRMs and 

equivalent methods (including making 

the requirements for nitrogen dioxide 

consistent with the requirements for O3) 

and to remove an obsolete requirement 

for the annual submission of 

documentation by manufacturers of 

certain particulate matter monitors. For 

additional information, see the 

Executive Summary, section I.A. 

DATES: Written comments on this 

proposed rule must be received by 

March 17, 2015. 

Public Hearings: The EPA intends to 

hold three public hearings on this 

proposed rule in January 2015. These 

will be announced in a separate Federal 

Register notice that provides details, 

including specific dates, times, 

addresses, and contact information for 

these hearings. 

ADDRESSES: Submit your comments, 

identified by Docket ID No. EPA–HQ– 

OAR–2008–0699, to the EPA by one of 

the following methods: 

• Federal eRulemaking Portal: http:// 

www.regulations.gov. Follow the online 

instructions for submitting comments. 

• Email: A-and-R-Docket@epa.gov. 

Include docket ID No. EPA–HQ–OAR– 

2008–0699 in the subject line of the 

message. 

• Fax: (202) 566–9744. 

• Mail: Environmental Protection 

Agency, EPA Docket Center (EPA/DC), 

Mailcode 28221T, Attention Docket ID 

No. OAR–2008–0699, 1200 

Pennsylvania Ave. NW., Washington, 

DC 20460. Please include a total of two 

copies. 

• Hand/Courier Delivery: EPA Docket 

Center, Room 3334, EPA WJC West 

Building, 1301 Constitution Ave. NW., 

Washington, DC. Such deliveries are 

only accepted during the Docket’s 

normal hours of operation, and special 

arrangements should be made for 

deliveries of boxed information. 

Instructions: Direct your comments to 

Docket ID No. EPA–HQ–OAR–2008– 

0699. The EPA’s policy is that all 

comments received will be included in 

the public docket without change and 

may be made available online at 

www.regulations.gov, including any 

personal information provided, unless 

the comment includes information 

claimed to be Confidential Business 

Information (CBI) or other information 

whose disclosure is restricted by statute. 

Do not submit information that you 

consider to be CBI or otherwise 

protected through www.regulations.gov 

or email. The www.regulations.gov Web 

site 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 

Web site. This includes documents in 

the rulemaking docket (Docket ID No. 

EPA–HQ–OAR–2008–0699) and a 

separate docket, established for the 

Integrated Science Assessment (ISA) 

(Docket No. EPA–HQ–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: 


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General Information 

What should I consider as I prepare my 

comments for EPA? 

1. Submitting CBI. Do not submit this 

information to the EPA through 

www.regulations.gov or email. Clearly 

mark the part or all of the information 

that you claim to be CBI. For CBI 

information in a disk or CD ROM that 

you mail to EPA, mark the outside of the 

disk or CD ROM as CBI and then 

identify electronically within the disk or 

CD ROM the specific information that is 

claimed as CBI. In addition to one 

complete version of the comment that 

includes information claimed as CBI, a 

copy of the comment that does not 

contain the information claimed as CBI 

must be submitted for inclusion in the 

public docket. Information so marked 

will not be disclosed except in 

accordance with procedures set forth in 

40 CFR part 2. 

2. Tips for Preparing Your Comments. 

When submitting comments, remember 

to: 

• Identify the rulemaking by docket 

number and other identifying 

information (subject heading, Federal 

Register date and page number). 

• Follow directions—The agency may 

ask you to respond to specific questions 

or organize comments by referencing a 

Code of Federal Regulations (CFR) part 

or section number. 

• Explain why you agree or disagree, 

suggest alternatives, and substitute 

language for your requested changes. 

• Describe any assumptions and 

provide any technical information and/ 

or data that you used. 

• Provide specific examples to 

illustrate your concerns, and suggest 

alternatives. 

• Explain your views as clearly as 

possible, avoiding the use of profanity 

or personal threats. 

• Make sure to submit your 

comments by the comment period 

deadline identified. 

Availability of Related Information 

A number of documents relevant to 

this rulemaking are available on EPA 

Web sites. The ISA for Ozone and 

Related Photochemical Oxidants is 

available on the EPA’s National Center 

for Environmental Assessment (NCEA) 

Web site. To obtain this document, go 

to http://www.epa.gov/ncea, and click 

on Ozone in the Quick Finder section. 

This will open a page with a link to the 

February 2013 ISA. The 2014 Policy 

Assessment (PA), Health and Welfare 

Risk and Exposure Assessments (HREA 

and WREA, respectively), and other 

related technical documents are 

available on EPA’s Office of Air Quality 

Planning and Standards (OAQPS) 

Technology Transfer Network (TTN) 

Web site. The final 2014 PA is available 

at: http://www.epa.gov/ttn/naaqs/

standards/ozone/s_o3_2008_pa.html, 

and the final 2014 Health and Welfare 

Risk and Exposure Assessments and 

other related technical documents are 

available at: http://www.epa.gov/ttn/

naaqs/standards/ozone/s_o3_2008_

rea.html. These and other related 

documents are also available for 

inspection and copying in the EPA 

docket identified above. 

Environmental Justice 

Analyses evaluating the potential 

implications of a revised O3 NAAQS for 

environmental justice populations are 

discussed in appendix 9A of the 

Regulatory Impact Analysis (RIA) that 

accompanies this notice of proposed 

rulemaking. The RIA is available on the 

Web, through the EPA’s Technology 

Transfer Network Web site 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 

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 

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 


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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 

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 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 CAA, were peerreviewed by the Clean Air Scientific 

Advisory Committee (CASAC), an 

independent scientific advisory 

committee established by the CAA and 

charged with providing advice to the 

Administrator. The final documents 

reflect the EPA staff’s consideration of 

the comments and recommendations 

made by CASAC and the public on draft 

versions of these documents. 

2. Summary of Major Provisions 

The EPA is proposing that the current 

primary O3 standard set at a level of 

0.075 ppm is not requisite to protect 

public health with an adequate margin 

of safety, and that it should be revised 

to provide increased public health 

protection. Specifically, the EPA is 

proposing to retain the indicator 

(ozone), averaging time (8-hour) and 

form (annual fourth-highest daily 

maximum, averaged over 3 years) of the 

existing primary O3 standard and is 

proposing to revise the level of that 

standard to within the range of 0.065 

ppm to 0.070 ppm. The EPA is 

proposing this revision to increase 

public health protection, including for 

‘‘at-risk’’ populations such as children, 

older adults, and people with asthma or 

other lung diseases, against an array of 

O3-related adverse health effects. For 

short-term O3 exposures, these effects 

include decreased lung function, 

increased respiratory symptoms and 

pulmonary inflammation, effects that 

result in serious indicators of 

respiratory morbidity such as 

emergency department visits and 

hospital admissions, and all-cause (total 

nonaccidental) mortality. For long-term 

O3 exposures, these health effects 

include a variety of respiratory 

morbidity effects and respiratory 

mortality. Recognizing that the CASAC 

recommended a range of levels from 

0.060 ppm to 0.070 ppm, and that levels 

as low as 0.060 ppm could potentially 

be supported, the Administrator solicits 

comment on alternative standard levels 

below 0.065 ppm, and as low as 0.060 

ppm. However, the Administrator notes 

that setting a standard below 0.065 ppm, 

down to 0.060 ppm, would 

inappropriately place very little weight 

on the uncertainties in the health effects 

evidence and exposure/risk information. 

Given alternative views of the currently 

available evidence and information 

expressed by some commenters, the 

EPA is taking comment on both the 

Administrator’s proposed decision to 

revise the current primary O3 standard 

and the option of retaining that 

standard. 

In addition to proposing changes to 

the level of the standard, the EPA is 


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1The legislative history of section 109 indicates 

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

maximum permissible ambient air level . . . which 

will protect the health of any [sensitive] group of 

the population,’’ and that, for this purpose, 

‘‘reference should be made to a representative 

sample of persons comprising the sensitive group 

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

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

2Welfare 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, manmade materials, animals, wildlife, weather, 

visibility and climate, damage to and deterioration 

of property, and hazards to transportation, as well 

as effects on economic values and on personal 

comfort and well-being.’’ 

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 a.m. to 8:00 p.m., 

within the range from 13 ppm-hrs to 17 

ppm-hrs would provide the requisite 

protection against known or anticipated 

adverse effects to the public welfare. 

The EPA solicits comment on this 

proposed conclusion. In considering 

how to achieve that level of air quality, 

the Administrator recognizes that air 

quality data analyses suggest that air 

quality in terms of three-year average 

W126 index values of a range at or 

below 13 to 17 ppm-hrs would be 

provided by a secondary standard level 

within the range of 0.065 to 0.070 ppm, 

and that to the extent areas need to take 

action to attain a standard in the range 

of 0.065 to 0.070 ppm, those actions 

would also improve air quality as 

measured by the W126 metric. Thus, the 

Administrator proposes to revise the 

level of the current secondary standard 

to within the range of 0.065 to 0.070 

ppm. The EPA solicits comments on 

this proposed revision of the secondary 

standard. 

The EPA also solicits comments on 

the alternative approach of revising the 

secondary standard to a W126-based 

form, averaged over three years, with a 

level within the range of 13 ppm-hrs to 

17 ppm-hrs. The EPA additionally 

solicits comments on such a distinct 

secondary standard with a level within 

the range extending below 13 ppm-hrs 

down to 7 ppm-hrs. Further, the EPA 

solicits comments on retaining the 

current secondary standard without 

revision, along with the alternative 

views of the evidence that would 

support retaining the current standard. 

3. Provisions Related to Implementation 

As directed by the CAA, reducing 

pollution to meet national air quality 

standards always has been a shared task, 

one involving the federal government, 

states, tribes and local air agencies. This 

partnership has proved effective since 

the EPA first issued O3 standards more 

than three decades ago, and is 

evidenced by significantly lower O3

levels throughout the country. To 

provide a foundation that helps air 

agencies build successful strategies for 

attaining new O3 standards, the EPA 

will continue to move forward with 

federal regulatory programs, such as the 

proposed Clean Power Plan and the 

final Tier 3 motor vehicle emissions 

standards. To facilitate the development 

of CAA-compliant implementation 

plans and strategies to attain new 

standards, the EPA intends to issue 

timely and appropriate implementation 

guidance and, where appropriate and 

consistent with the law, new 

rulemakings to streamline regulatory 

burdens and provide flexibility in 

implementation. In addition, given the 

regional nature of O3 air pollution, the 

EPA will continue to work with states 

to address interstate transport of O3 and 

O3 precursors. 

This notice contains several proposed 

provisions related to implementation of 

the proposed standards. In addition to 

revisions to the primary and secondary 

NAAQS, the EPA is 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. 7408) 

directs the Administrator to identify and 

list certain air pollutants and then to 

issue air quality criteria for those 

pollutants. The Administrator is to list 

those air pollutants that in her 

‘‘judgment, cause or contribute to air 

pollution which may reasonably be 

anticipated to endanger public health or 

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

ambient air results from numerous or 

diverse mobile or stationary sources;’’ 

and ‘‘for which . . . [the Administrator] 

plans to issue air quality criteria . . . .’’ 

Air quality criteria are intended to 

‘‘accurately reflect the latest scientific 

knowledge useful in indicating the kind 

and extent of all identifiable effects on 

public health or welfare which may be 

expected from the presence of [a] 

pollutant in the ambient air . . . .’’ 42 

U.S.C. 7408(b). Section 109 (42 U.S.C. 

7409) directs the Administrator to 

propose and promulgate ‘‘primary’’ and 

‘‘secondary’’ NAAQS for pollutants for 

which air quality criteria are issued. 

Section 109(b)(1) defines a primary 

standard as one ‘‘the attainment and 

maintenance of which in the judgment 

of the Administrator, based on such 

criteria and allowing an adequate 

margin of safety, are requisite to protect 

the public health.’’ 1 A secondary 

standard, as defined in section 

109(b)(2), must ‘‘specify a level of air 

quality the attainment and maintenance 

of which, in the judgment of the 

Administrator, based on such criteria, is 

requisite to protect the public welfare 

from any known or anticipated adverse 

effects associated with the presence of 

[the] pollutant in the ambient air.’’ 2

The requirement that primary 

standards provide an adequate margin 

of safety was intended to address 

uncertainties associated with 

inconclusive scientific and technical 

information available at the time of 

standard setting. It was also intended to 

provide a reasonable degree of 

protection against hazards that research 

has not yet identified. See State of 

Mississippi v. EPA, 744 F. 3d 1334, 1353 

(D.C. Cir. 2013) (‘‘By requiring an 

‘adequate margin of safety’, Congress 

was directing EPA to build a buffer to 

protect against uncertain and unknown 

dangers to human health’’); see also 

Lead Industries Association v. EPA, 647 

F.2d 1130, 1154 (D.C. Cir 1980); 

American Petroleum Institute v. Costle, 


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3As 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. 

4Lists of CASAC members and of members of the 

CASAC Ozone Review Panel are available at: 

http://yosemite.epa.gov/sab/sabpeople.nsf/Web

CommitteesSubCommittees/Ozone%20Review

%20Panel. 

5The 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 DC 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). 

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 population(s) 3 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. 

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 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 CAA (sections 202 

to 250) which involves controls for 

emissions from mobile sources and 

controls for the fuels used by these 

sources, and new source performance 

standards for stationary sources under 

section 111 of the CAA. For some 

stationary sources, the national 

emissions standards for hazardous air 

pollutants under section 112 of the CAA 

may provide ancillary reductions in O3

precursors. 

After the EPA establishes a new or 

revised NAAQS, the CAA directs the 

EPA and the states to take steps to 

ensure that the new or revised NAAQS 

is met. One of the first steps, known as 

the initial area designations, involves 

identifying areas of the country that 

either are attaining or not attaining the 

new or revised NAAQS along with the 

nearby areas that contribute to the 

violations. Upon designation of 

nonattainment areas, certain states 

would then be required to develop SIPs 

to attain the standards. In developing 

their attainment plans, states would first 

take into account projected emission 

reductions from federal and state rules 

that have been already adopted at the 

time of plan submittal. A number of 

significant emission reduction programs 

that will lead to reductions of O3

precursors are in place today or are 

expected to be in place by the time any 

new SIPs will be due. Examples of such 

rules include the Nitrogen Oxides (NOX) 

SIP Call, Clean Air Interstate Rule 

(CAIR), and Cross-State Air Pollution 

Rule (CSAPR),5 regulations 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

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 


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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 NAAQS to 

fit each region or locale, pointing out 

that Congress was aware of the difficulty 

in meeting standards in some locations 

and had addressed this difficulty 

through various compliance related 

provisions in the CAA. See API v. 

Costle, 665 F.2d 1176, 1184–6 (D.C. Cir. 

1981). 

In 1982, the EPA announced plans to 

revise the 1978 Air Quality Criteria 

document (47 FR 11561), and in 1983, 

the EPA initiated the second periodic 

review of the O3 NAAQS (48 FR 38009). 

The EPA subsequently published the 

1986 Air Quality Criteria for Ozone and 

Other Photochemical Oxidants (U.S. 

EPA, 1986) and the 1989 Staff Paper 

(U.S. EPA, 1989). Following publication 

of the 1986 Air Quality Criteria 

Document (AQCD), a number of 

scientific abstracts and articles were 

published that appeared to be of 

sufficient importance concerning 

potential health and welfare effects of 

O3 to warrant preparation of a 

Supplement (U.S. EPA, 1992). On 

August 10, 1992, under the terms of a 

court order, the EPA published a 

proposed decision to retain the existing 

primary and secondary standards (57 FR 

35542). The notice explained that the 

proposed decision would complete the 

EPA’s review of information on health 

and welfare effects of O3 assembled over 

a 7-year period and contained in the 

1986 AQCD and its 1992 Supplement. 

The proposal also announced the EPA’s 

intention to proceed as rapidly as 

possible with the next review of the air 

quality criteria and standards for O3 in 

light of emerging evidence of health 

effects related to 6- to 8-hour O3

exposures. On March 9, 1993, the EPA 

concluded the review by affirming its 

proposed decision to retain the existing 

primary and secondary standards (58 FR 

13008). 

In August 1992, the EPA announced 

plans to initiate the third periodic 

review of the air quality criteria and O3

NAAQS (57 FR 35542). In December 

1996, the EPA proposed to replace 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. v. EPA, 175 F.3d 1027, 1034– 

1040 (D.C. Cir. 1999) (‘‘ATA I’’). In 

addition, the court directed that, in 

responding to the remand, the EPA 

should consider the potential beneficial 

health effects of O3 pollution in 

shielding the public from the effects of 

solar ultraviolet (UV) radiation, as well 

as adverse health effects. Id. at 1051–53. 

In 1999, the EPA petitioned for 

rehearing en banc on several issues 

related to that decision. The court 

granted the request for rehearing in part 

and denied it in part, but declined to 

review its ruling with regard to the 

potential beneficial effects of O3

pollution. 195 F.3d 4, 10 (D.C. Cir., 

1999) (‘‘ATA II’’). On January 27, 2000, 

the EPA petitioned the U.S. Supreme 

Court for certiorari on the constitutional 

issue (and two other issues), but did not 

request review of the ruling regarding 

the potential beneficial health effects of 

O3. On February 27, 2001, the U.S. 

Supreme Court unanimously reversed 

the judgment of the D.C. Circuit on the 

constitutional issue. Whitman v. 

American Trucking Assoc., 531 U.S. 

457, 472–74 (2001) (holding that section 

109 of the CAA does not delegate 

legislative power to the EPA in 

contravention of the Constitution). The 

Court remanded the case to the D.C. 

Circuit to consider challenges to the O3

NAAQS that had not been addressed by 

that court’s earlier decisions. On March 

26, 2002, the D.C. Circuit issued its final 

decision on remand, finding the 1997 O3

NAAQS to be ‘‘neither arbitrary nor 

capricious,’’ and so denying the 

remaining petitions for review. 

American Trucking Associations, Inc. v. 

EPA, 283 F.3d 355, 379 (D.C. Cir., 2002) 

(‘‘ATA III’’). 

Specifically, in ATA III, the D.C. 

Circuit upheld the EPA’s decision on 

the 1997 O3 standard as the product of 

reasoned decision-making. With regard 

to the primary standard, the court made 

clear that the most important support 

for EPA’s decision to revise the standard 

was the health evidence of insufficient 

protection afforded by the then-existing 

standard (‘‘the record is replete with 

references to studies demonstrating the 

inadequacies of the old one-hour 

standard’’), as well as extensive 

information supporting the change to an 

8-hour averaging time. 283 F.3d at 378. 

The court further upheld the EPA’s 

decision not to select a more stringent 

level for the primary standard noting 

‘‘the absence of any human clinical 

studies at ozone concentrations below 

0.08 [ppm]’’ which supported EPA’s 

conclusion that ‘‘the most serious health 

effects of ozone are ‘less certain’ at low 

concentrations, providing an eminently 

rational reason to set the primary 

standard at a somewhat higher level, at 

least until additional studies become 

available.’’ Id. (internal citations 

omitted). The Court also pointed to the 

significant weight that the EPA properly 

placed on the advice it received from 

CASAC. Id. at 379. In addition, the court 

noted that ‘‘although relative proximity 

to peak background O3 concentrations 

did not, in itself, necessitate a level of 

0.08 [ppm], EPA could consider that 

factor when choosing among the three 

alternative levels.’’ Id. 

Independently of the litigation, the 

EPA responded to the court’s remand to 


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6The 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). 

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 

range of 0.075 to 0.070 ppm (72 FR 

37818). Documents supporting this 

proposed decision included the Air 

Quality Criteria for Ozone and Other 

Photochemical Oxidants (U.S. EPA, 

2006a) and the Staff Paper (U.S. EPA, 

2007) and related technical support 

documents. The EPA also proposed two 

options for revising the secondary 

standard: (1) Replace the current 

standard with a cumulative, seasonal 

standard, expressed as an index of the 

annual sum of weighted hourly 

concentrations cumulated over 12 

daylight hours during the consecutive 3- 

month period within the O3 season with 

the maximum index value, set at a level 

within the range of 7 to 21 ppm-hrs, or 

(2) set the secondary standard identical 

to the proposed primary standard. The 

EPA completed the review with 

publication of a final decision on March 

27, 2008 (73 FR 16436). In that final 

rule, the EPA revised the NAAQS by 

lowering the level of the 8-hour primary 

O3 standard from 0.08 ppm to 0.075 

ppm, not otherwise revising the primary 

standard, and adopting a secondary 

standard identical to the revised 

primary standard. In May 2008, state, 

public health, environmental, and 

industry petitioners filed suit 

challenging the EPA’s final decision on 

the 2008 O3 standards. On September 

16, 2009, the EPA announced its 

intention to reconsider the 2008 O3

standards, and initiated a rulemaking to 

do so. At the EPA’s request, the Court 

held the consolidated cases in abeyance 

pending the EPA’s reconsideration of 

the 2008 decision. 

On January 19, 2010 (75 FR 2938), the 

EPA issued a notice of proposed 

rulemaking to reconsider the 2008 final 

decision. In that notice, the EPA 

proposed that further revisions of the 

primary and secondary standards were 

necessary to provide a requisite level of 

protection to public health and welfare. 

The EPA proposed to decrease the level 

of the 2008 8-hour primary standard 

from 0.075 ppm to a level within the 

range of 0.060 to 0.070 ppm, and to 

change the secondary standard to a new 

cumulative, seasonal standard 

expressed as an annual index of the sum 

of weighted hourly concentrations, 

cumulated over 12 hours per day (8 a.m. 

to 8 p.m.), during the consecutive 3- 

month period within the O3 season with 

a maximum index value, set at a level 

within the range of 7 to 15 ppm-hours. 

The Agency also solicited CASAC 

review of the proposed rule on January 

25, 2010 and solicited additional 

CASAC advice on January 26, 2011. 

After considering comments from 

CASAC and the public, the EPA 

prepared a draft final rule, which was 

submitted for interagency review 

pursuant to Executive Order 12866. On 

September 2, 2011, consistent with the 

direction of the President, the 

Administrator of the Office of 

Information and Regulatory Affairs 

(OIRA), Office of Management and 

Budget (OMB), returned the draft final 

rule to the EPA for further 

consideration. In view of this return and 

the fact that the Agency’s next periodic 

review of the O3 NAAQS required under 

CAA section 109 had already begun (as 

announced on September 29, 2008), the 

EPA deferred the decisions involved in 

the reconsideration until it completed 

its statutorily required periodic review. 

In light of EPA’s decision to 

consolidate the reconsideration with the 

current review, the D.C. Circuit 

proceeded with the litigation on the 

2008 final decision. On July 23, 2013, 

the Court upheld the EPA’s 2008 

primary O3 standard, but remanded the 

2008 secondary standard to the EPA. 

State of Mississippi v. EPA, 744 F.3d 

1334. With respect to the primary 

standard, the court first held that the 

EPA reasonably determined that the 

existing standard was not requisite to 

protect public health with an adequate 

margin of safety, and consequently 

required revision. Specifically, the court 

noted that there were ‘‘numerous 

epidemiologic studies linking health 

effects to exposure to ozone levels 

below 0.08 ppm and clinical human 

exposure studies finding a causal 

relationship between health effects and 

exposure to ozone levels at and below 

0.08 ppm.’’ 744 F.3d at 1345. The court 

also specifically endorsed the weight of 

evidence approach utilized by the EPA 

in its deliberations. Id. at 1344. 

The court went on to reject arguments 

that the EPA should have adopted a 

more stringent primary standard. 

Dismissing arguments that a clinical 

study (as properly interpreted by the 

EPA) showing effects at 0.06 ppm 

necessitated a standard level lower than 

that selected, the court noted that this 

was a single, limited study. Id. at 1350. 

With respect to the epidemiologic 

evidence, the court accepted the EPA’s 

argument that there could be legitimate 

uncertainty that a causal relationship 

between O3 and 8-hour exposures less 

than 0.075 ppm exists, so that 

associations at lower levels reported in 

epidemiologic studies did not 

necessitate a more stringent standard. 

Id. at 1351–52.6

The court also rejected arguments that 

an 8-hour primary standard of 0.075 

ppm failed to provide an adequate 

margin of safety, noting that margin of 

safety considerations involved policy 

judgments by the agency, and that by 

setting a standard ‘‘appreciably below’’ 

the level of the current standard (0.08 

ppm), the agency had made a reasonable 

policy choice. Id. Finally, the court 

rejected arguments that the EPA’s 

decision was inconsistent with 

CASAC’s scientific recommendations 

because CASAC had been insufficiently 

clear in its recommendations whether it 

was providing scientific or policy 

recommendations, and the EPA had 

reasonably addressed CASAC’s policy 

recommendations. Id. at 1357–58. 

With respect to the secondary 

standard, the court held that because the 

EPA had failed to identify a level of air 

quality requisite to protect public 

welfare, the EPA’s comparison between 


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7See http://yosemite.epa.gov/sab/sabproduct.nsf/ 

WebProjectsbyTopicCASAC!OpenView for more 

information on CASAC activities related to the 

current O3 NAAQS review. 

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

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

data/2011_04_OzoneIRP.pdf. 

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

Available: http://www.epa.gov/ttn/naaqs/

standards/ozone/s_o3_2008_pd.html. 

10The 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. 

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 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 policyrelevant 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 ISA, the EPA’s NCEA hosted a 

workshop to review and discuss 

preliminary drafts of key sections of the 

ISA on August 6, 2010 (75 FR 42085, 

July 20, 2010). The CASAC and the 

public reviewed the first external review 

draft ISA (U.S. EPA, 2011a; 76 FR 

10893, February 28, 2011) at a meeting 

held in May 19–20, 2011 (76 FR 23809; 

April 28, 2011). Based on CASAC and 

public comments, NCEA prepared a 

second draft ISA (U.S. EPA, 2011b; 76 

FR 60820, September 30, 2011). CASAC 

and the public reviewed this draft at a 

January 9–10, 2012 (76 FR 236, 

December 8, 2011) meeting. Based on 

CASAC and public comments, NCEA 

prepared a third draft ISA (U.S. EPA 

2012a; 77 FR 36534; June 19, 2012), 

which was reviewed at a CASAC 

meeting in September 2012. The EPA 

released the final ISA (EPA/600/R–10/

076F) in February 2013. 

The EPA presented its plans for 

conducting the Risk and Exposure 

Assessments (REAs) that build on the 

scientific evidence presented in the ISA, 

in two planning documents titled Ozone 

National Ambient Air Quality 

Standards: Scope and Methods Plan for 

Health Risk and Exposure Assessment 

and Ozone National Ambient Air 

Quality Standards: Scope and Methods 

Plan for Welfare Risk and Exposure 

Assessment (henceforth, Scope and 

Methods Plans).9 These planning 

documents outlined the scope and 

approaches that staff planned to use in 

conducting quantitative assessments, as 

well as key issues that would be 

addressed as part of the assessments. 

The EPA released these documents for 

public comment in April 2011, and 

consulted with CASAC on May 19–20, 

2011 (76 FR 23809; April 28, 2011). In 

designing and conducting the initial 

health risk and welfare risk assessments, 

the EPA considered CASAC comments 

(Samet, 2011) on the Scope and 

Methods Plans and also considered 

public comments. In May 2012, the EPA 

issued a memo titled Updates to 

Information Presented in the Scope and 

Methods Plans for the Ozone NAAQS 

Health and Welfare Risk and Exposure 

Assessments that described changes to 

elements of the scope and methods 

plans and provided a brief explanation 

of each change and the reason for it. 

In July 2012, the EPA made the first 

drafts of the Health and Welfare REAs 

available for CASAC review and public 

comment (77 FR 42495, July 19, 2012). 

The first draft PA 10 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 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 


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11A design value is a statistic that describes the 

air quality status of a given location relative to the 

level of the NAAQS. 

subsequently submit the data to the 

EPA. At present, there are 

approximately 1,400 monitors across the 

U.S. reporting hourly O3 averages 

during the times of the year when local 

O3 pollution can be important. Much of 

this monitoring is focused on O3

measurements in urban areas where 

precursor emissions tend to be largest, 

as well as locations directly downwind 

of these areas, but there are also over 

100 sites in rural areas where high 

levels of O3 can periodically exist due 

to transport from upwind sources. Based 

on data from this national network, the 

EPA estimates that approximately 133 

million Americans live in counties 

where O3 concentrations were above the 

level of the existing health-based 

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, 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 a.m. and 8 p.m. As 

O3 precursor emissions have decreased 

across the U.S., O3 design values 11 have 

concurrently shown a modest 

downward trend. Ozone design values 

decreased by approximately 9% on 

average between 2000 and 2012. Air 

quality model simulations estimate that 

peak O3 levels will continue to improve 

over the next decade as additional 

reductions in O3 precursors from power 

plants, motor vehicles, and other 

sources are realized. 

In addition to being affected by 

changing emissions, future O3

concentrations will also be affected by 

climate change. Modeling studies in 

EPA’s Interim Assessment (U.S. EPA, 

2009b) and cited in support of the 2009 

Endangerment Finding (74 FR 66,496; 

Dec. 15, 2009) show that, while the 

impact is not uniform, climate change 

has the potential to cause increases in 

summertime O3 concentrations over 

substantial regions of the country, with 

increases tending to occur during higher 

peak pollution episodes in the summer, 

if offsetting emissions reductions are not 

made. Increases in temperature are 

expected to be the principal factor in 

driving any ozone increases, although 

increases in stagnation frequency may 

also contribute (Jacob and Winner, 

2009). These increases in O3 pollution 

over broad areas of the U.S., including 

in the largest metropolitan areas with 

the worst O3 problems, increase the risk 

of morbidity and mortality. Children, 

people with asthma or other lung 

diseases, older adults, and people who 

are active outdoors, including outdoor 

workers, are among the most vulnerable 

to these O3-related health effects. If 

unchecked, climate change has the 

potential to offset some of the 

improvements in O3 air quality, and 

therefore some of the improvements in 

public health, that are expected from 

reductions in emissions of O3

precursors. 

Another challenging aspect of the O3

issue is the involvement of sources of O3

and O3 precursors beyond those from 

domestic, anthropogenic sources. 

Modeling analyses have suggested that 

nationally the majority of O3

exceedances are predominantly caused 

by anthropogenic emissions from within 

the U.S. However, observational and 

modeling analyses have concluded that 

O3 concentrations in some locations in 

the U.S. can be substantially influenced 

by sources that may not be suited to 

domestic control measures. In 

particular, certain high-elevation sites in 

the western U.S. are impacted by a 

combination of non-local sources like 

international transport, stratospheric O3, 

and O3 originating from wildfire 

emissions. Ambient O3 from these 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 can be episodic 

events with substantial background 

contributions where O3 concentrations 

approach or exceed the level of the 

current NAAQS (i.e., 75 ppb). These 

events are relatively infrequent and the 

EPA has policies that allow for the 

exclusion of air quality monitoring data 

from design value calculations when 

they are substantially affected by certain 

background influences. Wildfires pose a 

direct threat to air quality and public 

safety—threats that can be mitigated 

through management of wildland 

vegetation. The use of wildland 

prescribed fire can influence the 

occurrence of catastrophic wildfires 

which may help manage the 

contribution of wildfires to background 

O3 levels and periodic peak O3 events. 

Prescribed fire mimics a natural process 

necessary to manage and maintain fireadapted ecosystems and climate change 

adaptation, while reducing risk of 

uncontrolled emissions from 

catastrophic wildfires. Wildfire 

emissions may make it more challenging 

to meet the NAAQS. However, the CAA 

requires the EPA to set the NAAQS at 

levels requisite to protect public health 

and welfare without regard to the source 

of the pollutant. API, 665 F. 2d at 1185– 

86. The EPA may consider proximity to 

background levels as a factor in the 

decision whether and how to revise the 

NAAQS when considering levels within 

the range of reasonable values 


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12 Due to rounding convention, the 1997 standard 

level of 0.08 ppm corresponded to 0.084 ppm (84 

ppb). 

13The 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. 

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). 

supported by the air quality criteria and 

judgments of the Administrator. ATA 

III, 283 F. 3d at 379. It is in the 

implementation process that states and 

the EPA can address how to develop 

effective public policy in locations in 

which background sources contribute 

substantially to high O3. Section VII.F 

provides more detail about how 

background O3 can be addressed via 

CAA implementation provisions. 

II. Rationale for Proposed Decision on 

the Primary Standard 

This section presents the 

Administrator’s rationale for her 

proposed decision to revise the existing 

primary O3 standard by lowering the 

level of the standard to within the range 

of 0.065 to 0.070 ppm. As discussed 

more fully below, this rationale draws 

from the thorough review in the ISA of 

the available scientific evidence, 

published through July 2011, on human 

health effects associated with the 

presence of O3 in the ambient air. This 

rationale also takes into account: (1) 

Analyses of O3 air quality, human 

exposures to O3, and O3-associated 

health risks, as presented and assessed 

in the HREA; (2) the EPA staff 

assessment of the most policy-relevant 

scientific evidence and exposure/risk 

information in the PA; (3) CASAC 

advice and recommendations, as 

reflected in discussions of drafts of the 

ISA, REA, and PA at public meetings, in 

separate written 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 ppm 12 to 0.075 

ppm (75 parts per billion (ppb) 13). This 

decision was based on his consideration 

of the available scientific evidence and 

exposure/risk information, the advice 

and recommendations of CASAC, and 

comments from the public. The 

Administrator placed primary emphasis 

on the body of available scientific 

evidence, while viewing the results of 

exposure and risk assessments as 

providing supporting information. 

Specifically, he judged that a standard 

set at 75 ppb would be appreciably 

below the concentration at which 

adverse effects had been demonstrated 

in the controlled human exposure 

studies available at that time (i.e., 80 

ppb), and would provide a significant 

increase in protection compared to the 

then-current standard. The 

Administrator further concluded that 

the body of evidence did not support 

setting a lower standard level, given the 

increasing uncertainty in the evidence 

at lower O3 concentrations (U.S. EPA, 

2014c, Chapter 1). 

In the current review, the EPA’s 

approach to informing decisions on the 

primary O3 standard builds upon the 

general approach used in the last review 

and reflects the broader body of 

scientific evidence, updated exposure/

risk information, and advances in O3 air 

quality modeling now available. This 

approach, described in detail in the PA 

(U.S. EPA, 2014c, section 1.3.1), is 

based most fundamentally on using the 

EPA’s assessment of the available 

scientific evidence and associated 

quantitative analyses to inform the 

Administrator’s judgments regarding a 

primary standard for O3 that is 

‘‘requisite’’ (i.e., neither more nor less 

stringent than necessary) to protect 

public health with an adequate margin 

of safety. Specifically, it is based on 

consideration of the available body of 

scientific evidence assessed in the ISA 

(U.S. EPA, 2013a), exposure and risk 

analyses presented in the HREA (U.S. 

EPA, 2014a), advice and 

recommendations from CASAC (Frey, 

2014a, c), and public comments. Based 

on the application of this approach, the 

PA assesses and integrates the evidence 

and information, and reaches 

conclusions for the Administrator’s 

consideration about the range of policy 

options that could be supported. The 

remainder of this section describes the 

PA’s approach to reviewing the primary 

O3 standard, and to informing the 

Administrator’s proposed decisions on 

the current and alternative standards. 

As an initial matter, the PA recognizes 

that the final decision to retain or revise 

the current primary O3 standard is a 

public health policy judgment to be 

made by the Administrator and will 

draw upon the available scientific 

evidence for O3-attributable health 

effects and on analyses of population 

exposures and health risks, including 

judgments about the appropriate weight 

to assign the range of uncertainties 

inherent in the evidence and analyses. 

The PA’s general approach to informing 

these public health policy judgments 

recognizes that the available health 

effects evidence reflects a continuum 

from relatively higher O3

concentrations, at which scientists 

generally agree that health effects are 

likely to occur, through lower 

concentrations, at which the likelihood 

and magnitude of a response become 

increasingly uncertain. Therefore, the 

conclusions in the PA reflect an 

interpretation of the available scientific 

evidence and exposure/risk information 

that, in the views of the EPA staff, 

neither overstates nor understates the 

strengths and limitations of that 

evidence and information.14 This 

approach is consistent with the 

requirements of sections 108 and 109 of 

the CAA, as well as with how the EPA 

and the courts have historically 

interpreted the CAA. 

The PA draws upon an integrative 

synthesis of the entire body of available 

scientific evidence for O3-related health 

effects, including the evidence newly 

available in the current review and the 

evidence from previous reviews, as 

presented in the ISA (U.S. EPA, 2013a). 

Consideration of the scientific evidence 

is based fundamentally on information 

from controlled human exposure and 

epidemiologic studies, supplemented by 

information from animal toxicology 

studies. In the PA, such evidence 

informs the consideration of the health 


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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). 

16For 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). 

endpoints and at-risk populations 15 on 

which to focus the current review, and 

the consideration of the O3

concentrations at which various health 

effects can occur. 

Since the 2008 review of the O3

NAAQS, the EPA has developed formal 

frameworks for characterizing the 

strength of the scientific evidence with 

regard to health effects associated with 

exposures to O3 in ambient air and 

factors that may increase risk in some 

populations or lifestages. These 

frameworks provide the basis for robust, 

consistent, and transparent processes for 

evaluating the scientific evidence, 

including uncertainties in the evidence, 

and for drawing weight-of-evidence 

conclusions on air pollution-related 

health effects and at-risk populations. 

These frameworks for characterizing the 

strength of the scientific evidence are 

discussed in detail in the ISA (U.S. EPA, 

2013a, Preamble; Chapter 8). 

With regard to characterization of 

health effects, the ISA uses a five-level 

hierarchy to classify the overall weight 

of evidence into one of the following 

categories: causal relationship, likely to 

be a causal relationship, suggestive of a 

causal relationship, inadequate to infer 

a causal relationship, and not likely to 

be a causal relationship (U.S. EPA, 

2013a, Preamble Table II). In using the 

weight-of-evidence approach to inform 

judgments about the degree of 

confidence that various health effects 

are likely to be caused by exposure to 

O3, confidence increases as the number 

of studies consistently reporting a 

particular health endpoint grows and as 

other factors, such as biological 

plausibility and the strength, 

consistency, and coherence of evidence, 

increase. Conclusions about biological 

plausibility and about the consistency 

and coherence of O3-related health 

effects are drawn from the integration of 

epidemiologic studies with mechanistic 

information from controlled human 

exposure and animal toxicological 

studies, as discussed in the ISA (U.S. 

EPA, 2013a, EPA Framework for Causal 

Determination, p. 1viii). The PA places 

the greatest weight on the health effects 

for which the evidence has been judged 

in the ISA to support a ‘‘causal’’ or a 

‘‘likely to be causal’’ relationship with 

O3 exposures. 

The PA further considers the evidence 

base assessed in the ISA with regard to 

the types and levels of exposure at 

which health effects are indicated. This 

consideration of the evidence, which 

directly informs conclusions regarding 

the adequacy of current or alternative 

standards, differs from consideration of 

the evidence in the ISA with regard to 

overarching determinations of causality. 

Therefore, studies that inform 

determinations of causality may or may 

not be concluded to be informative with 

regard to the adequacy of the current or 

alternative standards.16

As with health endpoints, the ISA’s 

characterization of the weight of 

evidence for potential at-risk 

populations is based on the evaluation 

and synthesis of evidence from across 

scientific disciplines. The ISA uses the 

collective evidence to examine the 

coherence of effects across disciplines 

and to determine the biological 

plausibility of reported effects. Based on 

this approach, the ISA characterizes the 

evidence for a number of ‘‘factors’’ that 

have the potential to place populations 

at increased risk for O3-related effects. 

The categories considered in evaluating 

the evidence for these potential at-risk 

factors are ‘‘adequate evidence,’’ 

‘‘suggestive evidence,’’ ‘‘inadequate 

evidence,’’ and ‘‘evidence of no effect.’’ 

For the ‘‘adequate evidence’’ category, 

the ISA concludes that this category is 

appropriate when multiple high-quality 

studies show ‘‘there is substantial, 

consistent evidence within a discipline 

to conclude that a factor results in a 

population or lifestage being at 

increased risk of air pollutant-related 

health effect(s) relative to some 

reference population or lifestage’’ (U.S. 

EPA, 2013a, p. 8–2). In addition, where 

applicable, the ‘‘adequate evidence’’ 

category reflects a conclusion that there 

is coherence in the evidence across 

disciplines. The other categories reflect 

greater uncertainty in the evidence. In 

this review, the PA focuses on those 

factors for which the ISA judges there is 

adequate evidence of increased risk 

(U.S. EPA, 2013a, Table 8–5). At-risk 

populations are discussed in more detail 

in section 3.1.5 of the PA (U.S. EPA, 

2014c) and these categories are 

discussed in more detail in the ISA 

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

Using the available scientific evidence 

to inform conclusions on the current 

and alternative standards is complicated 

by the recognition that a populationlevel 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 

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 cooccurring pollutants) (U.S. EPA, 2013a, 

Chapter 6). Together with animal 

toxicological studies, which can provide 


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17Though the PA recognizes that a broader body 

of studies, including international studies, informs 

the causal determinations in the ISA. 

18For example, see 75 FR 2945–2946 (January 19, 

2010) and 73 FR 16441–16442 (March 27, 2008) 

discussing ‘‘exposures of concern.’’ 

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 

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. 

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 realworld exposures to the range of O3

concentrations in ambient air, as well as 

provide evidence of associations 

between ambient O3 levels and more 

serious acute and chronic health effects 

that cannot be assessed in controlled 

human exposure studies. For these 

studies, the degree of uncertainty 

introduced by confounding variables 

(e.g., other pollutants, temperature) and 

other factors (e.g., effects modifiers such 

as averting behavior) affects the level of 

confidence that the health effects being 

investigated are attributable to O3

exposures, alone and in combination 

with copollutants. 

Available epidemiologic studies have 

generally not indicated a discernible 

population threshold below which O3 is 

no longer associated with health effects 

(U.S. EPA, 2013a, section 2.5.4.4). 

However, the currently available 

epidemiologic evidence indicates 

decreased confidence in reported 

concentration-response relationships for 

O3 concentrations at the lower ends of 

ambient distributions due to the low 

density of data in this range (U.S. EPA, 

2013a, section 2.5.4.4). As discussed 

more fully in Chapter 1 of the PA (U.S. 

EPA, 2014c), the general approach to 

considering the results of epidemiologic 

studies within the context of the current 

and alternative standards focuses on 

characterizing the range of ambient O3

concentrations over which studies 

indicate the most confidence in O3- 

associated health effects, and the 

concentrations below which confidence 

in such health effect associations 

becomes appreciably lower. 

In placing emphasis on specific 

epidemiologic studies, as in past 

reviews, the discussion in the PA 

focuses on the epidemiologic studies 

conducted in the U.S. and Canada. Such 

studies reflect air quality and exposure 

patterns that are likely more typical of 

the U.S. population, since studies 

conducted outside the U.S. and Canada 

may well reflect different demographic 

and air 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 concentrationresponse relationships drawn from 

epidemiologic studies to adjusted air 

quality. Each of these types of 

assessments is discussed briefly below. 

As in the 2008 review, the HREA 

estimates exposures at or above 

benchmark concentrations of 60, 70, and 

80 ppb, reflecting exposure 

concentrations of concern based on the 

available health evidence.18 Estimates of 

exposures of concern, defined as 

personal exposures while at moderate or 

greater exertion to 8-hour average 

ambient O3 levels, at or above these 

discrete benchmark concentrations 

provide perspective on the public health 

risks of O3-related health effects that 

have been demonstrated in controlled 

human exposure and toxicological 

studies. However, because of a lack of 

exposure-response information across a 

range of exposure 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 exposureresponse 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 


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19For 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). 

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). 

22As noted below, for the 70 ppb exposure 

concentration, Schelegle et al. (2009) reported that 

the actual mean exposure concentration was 72 

ppb. 

23The 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. 

concentrations provide some 

perspective on the public health 

impacts of O3-related health effects, 

such as pulmonary inflammation, that 

are plausibly linked to the more serious 

effects seen in epidemiologic studies but 

cannot be evaluated in quantitative risk 

assessments. They also help elucidate 

the extent to which such impacts may 

be reduced by meeting the current and 

alternative standards. Estimates of the 

number of people likely to experience 

exposures of concern cannot be directly 

translated into quantitative estimates of 

the number of people likely to 

experience specific health effects due to 

individual variability in responsiveness. 

Only a subset of individuals can be 

expected to experience such adverse 

health effects, and at-risk populations or 

lifestages, such as people with asthma 

or children, are expected to be affected 

more by such exposures than healthy 

adults. 

The HREA also generates quantitative 

estimates of O3 health risks for air 

quality adjusted to just meet the 

current 19 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 schoolage children, school-age children with 

asthma, adults with asthma, and older 

adults. The available exposure-response 

information does not support this 

approach for other endpoints evaluated 

in controlled human exposure studies 

(U.S. EPA, 2014a, section 2.3). 

The other approach used in this 

review to estimate O3-associated health 

risks is to apply concentration-response 

relationships derived from short- and/or 

long-term epidemiologic studies to air 

quality adjusted to just meet current and 

alternative standards. The 

concentration-response relationships 

drawn from epidemiologic studies are 

based on population exposure 

surrogates, such as 8-hour 

concentrations averaged across monitors 

and over more than one day (U.S. EPA, 

2013a, Chapter 6). The HREA presents 

epidemiologic-based risk estimates for 

O3-associated mortality, hospital 

admissions, emergency department 

visits, and respiratory symptoms (U.S. 

EPA, 2014a, section 2.3). These 

estimates are derived from the full 

distributions of ambient O3

concentrations estimated for the study 

locations.20 In addition, the HREA 

estimates mortality risks associated with 

various portions of distributions of 

short-term O3 concentrations (U.S. EPA, 

2014a). The PA considers risk estimates 

based on the full distributions of 

ambient O3 concentrations and, when 

available, estimates of the risk 

associated with various portions of 

those ambient distributions.21 In doing 

so, the PA takes note of the ISA 

conclusions regarding confidence in 

linear concentration-response 

relationships over distributions of 

ambient concentrations (see above), and 

of the extent to which health effect 

associations at various ambient O3

concentrations are supported by the 

evidence from experimental studies for 

effects following specific O3 exposures. 

B. Health Effects Information 

This section outlines key information 

contained in the ISA (U.S. EPA, 2013a, 

Chapters 4 to 8) and in the PA (U.S. 

EPA, 2014c, Chapters 3 and 4) on the 

known or potential effects on public 

health which may be expected from the 

presence of O3 in the ambient air. The 

information highlighted here 

summarizes: (1) New information 

available on potential mechanisms for 

health effects associated with exposure 

to O3 (II.B.1); (2) the nature of effects 

that have been associated directly with 

both short- and long-term exposure to 

O3 and indirectly with the presence of 

O3 in ambient air (II.B.2); (3) 

considerations related to the adversity 

of O3-attributable health effects (II.B.3); 

and (4) considerations in characterizing 

the public health impact of O3,

including the identification of ‘‘at risk’’ 

populations (II.B.4). 

The decision in the 2008 rulemaking 

emphasized the large number of 

epidemiologic studies published since 

the 1997 review that continued to report 

associations with respiratory hospital 

admissions and emergency department 

visits, as well as additional health 

endpoints, including the effects of acute 

(short-term and prolonged) and chronic 

exposures to O3 on lung function 

decrements and enhanced respiratory 

symptoms in asthmatic individuals, 

school absences, and premature 

mortality. It also emphasized controlled 

human exposure studies showing 

respiratory effects with prolonged O3

exposures at levels below 80 ppb, 

changes in lung host defenses, and 

increased airway responsiveness, and 

animal toxicology studies that provided 

information about mechanisms of 

action. 

The ISA (U.S. EPA, 2013a) prepared 

for this review emphasizes a large 

number of new epidemiologic studies 

published since the last review on 

effects associated with both short- and 

long-term exposures, including new 

epidemiologic studies about risk factors. 

It also emphasizes important new 

information from controlled human 

exposure, dosimetry and toxicology 

studies. Highlights of the new evidence 

included: 

(1) Two controlled human exposure 

studies new since the 2008 review are now 

available that examine respiratory effects 

associated with prolonged, 6.6-hour, O3

exposures to levels of 72 ppb 22 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 

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 


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24 In determining that a causal relationship exists 

for O3 with specific health effects, the EPA has 

concluded that ‘‘[e]vidence is sufficient to conclude 

that there is a causal relationship with relevant 

pollutant exposures’’ (U.S. EPA, 2013a, p. lxiv). 

25 In determining a ‘‘likely to be a causal’’ 

relationship exists for O3 with specific health 

effects, the EPA has concluded that ‘‘[e]vidence is 

sufficient to conclude that a causal relationship is 

likely to exist with relevant pollutant exposures, 

but important uncertainties remain’’ (U.S. EPA, 

2013a, p. lxiv). 

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 newonset asthma in children and increased 

respiratory symptom effects in individuals 

with asthma; additional studies of respiratory 

morbidity and mortality support the 

association between long-term exposure to O3

and a range of respiratory health effects. 

(5) New evidence of risk factors (i.e., 

people with certain genetic variants related 

to antioxidant status or inflammation, and 

people with reduced intake of antioxidant 

nutrients) strengthens our understanding of 

the potential modes of action from O3- 

induced effects. 

1. Overview of Mechanisms 

The purpose of this section is to 

describe the ISA’s characterization of 

the key events and pathways that 

contribute to health effects resulting 

from both short-term and long-term 

exposures to O3. The information in this 

section draws from section 5.3 of the 

ISA (U.S. EPA, 2013a). Mode of action 

refers to a sequence of key events and 

processes that result in a given toxic 

effect. Elucidation of mechanisms 

provides a more detailed understanding 

of these key events and processes. 

Experimental evidence elucidating 

modes of action and/or mechanisms 

contributes to our understanding of the 

biological plausibility of adverse O3- 

related health effects, including 

respiratory effects and effects outside 

the respiratory system (U.S. EPA, 2013a, 

Chapters 6 and 7). 

Figure 3.1 in the PA (U.S. EPA, 2014c) 

shows the current understanding of key 

events in the toxicity pathway of O3, 

based on the available evidence. These 

key events are described briefly here 

and in more detail in section 3.1.1 of the 

PA. The initial key event is the 

formation of secondary oxidation 

products in the respiratory tract (U.S. 

EPA, 2013a, section 5.3). This mainly 

involves direct reactions with 

components of the extracellular lining 

fluid (ELF). Although the ELF has 

inherent capacity to quench (based on 

individual antioxidant capacity), this 

capacity can be overwhelmed, 

especially with exposure to elevated 

concentrations of O3. The resulting 

secondary oxidation products transmit 

signals to the epithelium, pain receptive 

nerve fibers and, if present, immune 

cells (i.e., eosinophils, dendritic cells 

and mast cells) involved in allergic 

responses. Thus, the available evidence 

indicates that the effects of O3 are 

mediated by components of ELF and by 

the multiple cell types found in the 

respiratory tract. Further, oxidative 

stress is an implicit part of this initial 

key event. 

It is well understood that secondary 

oxidation products initiate numerous 

responses at the cellular, tissue, and 

whole organ level of the respiratory 

system. These responses include the 

activation of neural reflexes leading to 

lung function decrements, airway 

obstruction, and extrapulmonary effects 

such as slow resting heart rate; initiation 

of inflammation; alteration of barrier 

epithelial function; sensitization of 

bronchial smooth muscle; modification 

of lung host defenses; and airways 

remodeling (U.S. EPA, 2013a, section 

5.3.10, Figure 5–8). Each of these effects 

is discussed in more detail in section 

3.1.1 of the PA (U.S. EPA, 2014c). 

Persistent inflammation and injury, 

which are observed in animal models of 

chronic and intermittent exposure to O3, 

are associated with airways remodeling 

(see Section 7.2.3 of the ISA, U.S. EPA 

2013). Chronic intermittent exposure to 

O3 has also been shown to result in 

effects on the developing lung and 

immune system. Systemic inflammation 

and vascular oxidative/nitrosative stress 

are also key events in the toxicity 

pathway of O3. Extrapulmonary effects 

of O3 occur in numerous organ systems, 

including the cardiovascular, central 

nervous, reproductive, and hepatic 

systems (U.S. EPA, 2013a, sections 6.3 

to 6.5 and sections 7.3 to 7.5). 

Responses to O3 exposure are variable 

within the population. Studies have 

shown a large range of pulmonary 

function (i.e., spirometric) responses to 

O3 among healthy young adults, while 

responses within an individual are 

relatively consistent over time. Other 

responses to O3 have also been 

characterized by a large degree of 

interindividual variability. For example, 

a 3- to 20-fold difference among subjects 

in their studies in airways inflammation 

(i.e., neutrophilia influx) following O3

exposure has been reported (Schelegle 

et al., 1991 and Devlin et al., 1991, 

respectively). Reproducibility of an 

individual’s inflammatory response to 

O3 exposure in humans, measured as 

sputum neutrophilia, was demonstrated 

by Holz et al (1999). Since individual 

inflammatory responses were relatively 

consistent across time, it was thought 

that inflammatory responsiveness 

reflected an intrinsic characteristic of 

the subject (Mudway and Kelly, 2000). 

While the basis for the observed 

interindividual variability in 

responsiveness to O3 is not clear, 

section 5.4.2 of the ISA discusses 

mechanisms that may underlie the 

variability in responses seen among 

individuals. Certain functional genetic 

polymorphisms, pre-existing conditions 

or diseases, nutritional status, lifestages, 

and co-exposures contribute to altered 

risk of O3-induced effects. Experimental 

evidence for such O3-induced changes 

contributes to our understanding of the 

biological plausibility of adverse O3- 

related health effects, including a range 

of respiratory effects as well as effects 

outside the respiratory system (e.g., 

cardiovascular effects) (U.S. EPA, 2013a, 

Chapters 6 and 7). 

2. Nature of Effects 

The health effects of O3 are described 

in detail and assessed in the ISA (U.S. 

EPA, 2013a). Based on this assessment, 

the ISA determined that a ‘‘causal’’ 

relationship exists between short-term 

exposure to O3 in ambient air 24 and 

effects on the respiratory system and 

that a ‘‘likely to be causal’’ 

relationship 25 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 shortterm 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– 


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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. 

27CASAC 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). 

28The 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. 

8). In the ISA, the EPA additionally 

determined that the currently available 

evidence for additional endpoints is 

‘‘suggestive’’ of causal relationships 

between short-term (central nervous 

system effects) and long-term exposure 

(cardiovascular effects, reproductive 

and developmental effects, central 

nervous system effects and total 

mortality) to ambient O3.

Consistent with emphasis in past 

reviews on O3 health effects for which 

the evidence is strongest, in this review 

the EPA places the greatest emphasis on 

studies of health effects that have been 

judged in the ISA to be caused by, or 

likely to be caused by, O3 exposures 

(U.S. EPA, 2013a, section 2.5.2). This 

section discusses the evidence for 

health effects attributable to O3

exposures, with a focus on respiratory 

morbidity and mortality effects 

attributable to short- and long-term 

exposures, and cardiovascular system 

effects (including mortality) and total 

mortality attributable to short-term 

exposures. This section focuses 

particularly on considering the extent to 

which the scientific evidence available 

in the current review has been 

strengthened since the last review, and 

the extent to which important 

uncertainties and limitations in the 

evidence from the last review have been 

addressed. 

a. Respiratory Effects—Short-Term 

The 2006 O3 AQCD concluded that 

there was clear, consistent evidence of 

a causal relationship between short-term 

O3 exposure and respiratory effects (U.S. 

EPA, 2006a). This conclusion was 

substantiated by evidence from 

controlled human exposure and 

toxicological studies indicating a range 

of respiratory effects in response to 

short-term O3 exposures, including 

pulmonary function decrements and 

increases in respiratory symptoms, lung 

inflammation, lung 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 shortterm increases in ambient O3

concentration were consistently 

associated with decreases in lung 

function in populations with increased 

outdoor exposures, especially children 

with asthma and healthy children; 

increases in respiratory symptoms and 

asthma medication use in children with 

asthma; and increases in respiratoryrelated hospital admissions and asthmarelated 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 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; 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 


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29The 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). 

30Adams (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. 

31Adams (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 (V ̇ 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 V ̇ E of 23 L/min 

per m2 BSA relative to other studies having the 

target V ̇ E of 20 L/min per m2 BSA. The facemask 

exposure is not expected to affect the FEV1

responses relative to a chamber exposure. 

32For the 60 ppb target exposure concentration, 

Schelegle et al. (2009) reported that the actual mean 

exposure concentration was 63 ppb. 

33Adams (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. 

34As noted above, for the 70 ppb exposure group, 

Schelegle et al. (2009) reported that the actual mean 

exposure concentration was 72 ppb. 

assessed in detail in the ISA (U.S. EPA, 

section 6.2.1) and is summarized below. 

Controlled exposures to O3

concentrations that can be found in the 

ambient air can result in a number of 

lung function effects, including 

decreased inspiratory capacity, mild 

bronchoconstriction, and rapid, shallow 

breathing patterns during exercise. 

Reflex inhibition of inspiration results 

in a decrease in forced vital capacity 

(FVC) and total lung capacity (TLC) and, 

in combination with mild 

bronchoconstriction, contributes to a 

decrease in FEV1 (U.S. EPA, 2013a, 

section 6.2.1.1). Accumulating evidence 

indicates that such effects are mediated 

by activation of sensory nerves, 

resulting in the involuntary truncation 

of inspiration and a mild increase in 

airway obstruction due to 

bronchoconstriction (U.S. EPA, 2013a, 

section 5.3.10). 

Data from controlled human exposure 

studies show that increasing the 

duration of O3 exposures and increasing 

ventilation rates decreases the O3

exposure concentrations required to 

impair lung function. Ozone exposure 

concentrations well above those 

typically found in ambient air are 

required to impair lung function in 

healthy resting adults, while exposure 

to O3 concentrations at or below those 

in the ambient air have been reported to 

impair lung function in healthy adults 

exposed for longer durations while 

undergoing intermittent, moderate 

exertion (U.S. EPA, 2013a, section 

6.2.1.1). With repeated O3 exposures 

over several days, FEV1 responses 

become attenuated in both healthy 

adults and adults with mild asthma, 

though this attenuation of response is 

lost after about a week without exposure 

(U.S. EPA, 2013a, section 6.2.1.1; p. 6– 

27). 

When considering controlled human 

exposure studies of O3-induced lung 

function decrements, the ISA and PA 

evaluate both group mean changes in 

lung function and the interindividual 

variability in the magnitude of 

responses. An advantage of O3

controlled human exposure studies (i.e., 

compared to the epidemiologic panel 

studies discussed below) is that 

reported effects necessarily result from 

exposures to O3 itself.29 To the extent 

studies report statistically significant 

decrements in mean lung function 

following O3 exposures after controlling 

for other factors, these studies provide 

greater confidence that measured 

decrements are due to the O3 exposure 

itself, rather than to chance alone. As 

discussed below, group mean changes 

in lung function are often small, 

especially following exposures to 

relatively low O3 concentrations (e.g., 60 

ppb). However, even when group mean 

decrements in lung function are small, 

some individuals could experience 

decrements that are ‘‘clinically 

meaningful’’ (Pellegrino et al., 2005; 

ATS, 1991) with respect to criteria for 

spirometric testing, and/or that could be 

considered adverse with respect to 

public health policy decisions (see 

section II.B.3, below). 

At the time of the last review, a 

number of controlled human exposure 

studies had reported lung function 

decrements in young, healthy adults 

following prolonged (6.6-hour) 

exposures while at moderate exertion to 

O3 concentrations at and above 80 ppb. 

In addition, there were two controlled 

human exposure studies by Adams 

(2002, 2006) that examined lung 

function effects following exposures to 

60 ppb O3. The EPA’s analysis of the 

data from the Adams (2006) study 

reported a small but statistically 

significant O3-induced decrement in 

group mean FEV1 following exposures 

of young, healthy adults to 60 ppb O3

while at moderate exertion, when 

compared with filtered air controls 

(Brown et al., 2008).30 Further 

examination of the post-exposure FEV1

data, and mean data for other time 

points and other concentrations, 

indicated that the temporal pattern of 

the response to 60 ppb O3 was generally 

consistent with the temporal patterns of 

responses to higher O3 concentrations in 

this and other studies (75 FR 2950, 

January 19, 2010). This suggested a 

pattern of response following exposures 

to 60 ppb O3 that was consistent with 

a dose-response relationship, rather 

than random variability. See also State 

of Mississippi v. EPA, F. 3d at 1347 

(upholding EPA’s interpretation of the 

Adams studies). 

Figure 6–1 in the ISA summarizes the 

currently available evidence from 

multiple controlled human exposure 

studies evaluating group mean changes 

in FEV1 following prolonged O3

exposures (i.e., 6.6 hours) in young, 

healthy adults engaged in moderate 

levels of physical activity (U.S. EPA, 

2013a, section 6.2.1.1). With regard to 

the group mean changes reported in 

these studies, the ISA specifically notes 

the following (U.S. EPA, 2013a, section 

6.2.1.1, Figure 6–1): 

1. Prolonged exposure to 40 ppb O3

results in a small decrease in group 

mean FEV1 that is not statistically 

different from responses following 

exposure to filtered air (Adams, 2002; 

Adams, 2006). 

2. Prolonged exposure to an average 

O3 concentration of 60 ppb results in 

group mean FEV1 decrements ranging 

from 1.8% to 3.6% (Adams 2002; 

Adams, 2006; 31 Schelegle et al., 2009; 32

Kim et al., 2011). Based on data from 

multiple studies, the weighted average 

group mean decrement was 2.7%. In 

some analyses, these group mean 

decrements in lung function were 

statistically significant (Brown et al., 

2008; Kim et al., 2011), while in other 

analyses they were not (Adams, 2006; 

Schelegle et al., 2009).33

3. Prolonged exposure to an average 

O3 concentration of 72 ppb results in a 

statistically significant group mean 

decrement in FEV1 of about 6% 

(Schelegle et al., 2009).34

4. Prolonged square-wave exposure to 

average O3 concentrations of 80 ppb, 

100 ppb, or 120 ppb O3 results in 

statistically significant group mean 

decrements in FEV1 ranging from 6 to 

8%, 8 to 14%, and 13 to 16%, 

respectively (Folinsbee et al., 1988; 

Horstman et al., 1990; McDonnell et al., 

1991; Adams, 2002; Adams, 2003; 

Adams, 2006). 

As illustrated in Figure 6–1 of the 

ISA, there is a smooth dose-response 


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35Such 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). 

36The 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. 

37The 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 postexposure), 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). 

38Based on the data available at 60 ppb, 1% of 

subjects experienced decrements >20% (also 

uncorrected for filtered air exposures). 

39Responses to O3 in these studies were adjusted 

for responses observed following exposure to 

filtered air. 

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 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 preexisting 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 extent to which individuals were 

reported to experience FEV1 decrements 

of 10% or greater.36

New studies (Schelegle et al., 2009; 

Kim et al., 2011) add to the previously 

available evidence for interindividual 

variability in the responses of healthy 

adults following exposures to O3. 

Following prolonged exposures to 80 

ppb O3 while at moderate exertion, the 

proportion of healthy adults 

experiencing FEV1 decrements greater 

than 10% was 17% by Adams (2006), 

26% by McDonnell (1996), and 29% by 

Schelegle et al. (2009). Following 

exposures to 60 ppb O3, that proportion 

was 20% by Adams (2002), 3% by 

Adams (2006), 16% by Schelegle et al. 

(2009), and 5% by Kim et al. (2011). 

Across these studies, the weighted 

average proportion (i.e., based on 

numbers of subjects in each study) of 

young, healthy adults with >10% FEV1

decrements is 25% following exposure 

to 80 ppb O3 and 10% following 

exposure to 60 ppb O3, for 6.6 hours at 

moderate exertion (U.S. EPA, 2013a, 

page 6–18 and 6–19).37 38 The ISA notes 

that responses within an individual 

tend to be reproducible over a period of 

several months, indicating that 

interindividual differences reflect 

differences in intrinsic responsiveness. 

Given this, the ISA concludes that 

‘‘[t]hough group mean decrements are 

biologically small and generally do not 

attain statistical significance, a 

considerable fraction of exposed 

individuals experience clinically 

meaningful decrements in lung 

function’’ when exposed for 6.6 hours to 

60 ppb O3 during quasi continuous, 

moderate exertion (U.S. EPA, 2013a, 

section 6.2.1.1, p. 6–20). 

This review has marked an advance in 

the ability to make reliable quantitative 

predictions of the potential lung 

function response to ozone exposure, 

and thus to reasonably predict the 

degree of interindividual response of 

lung function to that exposure. 

McDonnell et al. (2012) and Schelegle et 

al. (2012) developed models using data 

on O3 exposure concentrations, 

ventilation rates, duration of exposures, 

and lung function responses from a 

number of controlled human exposure 

studies. See section 6.2.1.1 of the ISA 

(U.S. EPA 2013a, p. 6–15). The 

McDonnell et al. (2012) and Schelegle et 

al. (2012) studies analyzed large datasets 

to fit compartmental models that 

included the concept of a dose of onset 

in lung function response or a response 

threshold based upon the inhaled O3

dose. The McDonnell et al. (2012) model 

was fit to a dataset consisting of the 

FEV1 responses of 741 young, healthy 

adults (18–35 years of age) from 23 

individual controlled exposure studies. 

Concentrations across individual 

studies ranged from 40 ppb to 400 

ppb,39 activity level ranged from rest to 

heavy exercise, duration of exposure 

was from 2 to 7.6 hours. The extension 

of the McDonnell et al. (2012) model to 

children and older adults is discussed 

in section 6.2.4 of the HREA (U.S. EPA, 

2014a). Schelegle et al. (2012) also 

analyzed a large dataset with substantial 

overlap to that used by McDonnell et al. 

(2012). The Schelegle et al. (2012) 

model was fit to the FEV1 responses of 

220 young healthy adults (taken from a 

dataset of 704 individuals) from 21 

individual controlled exposure studies. 

The resulting 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 


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40Also 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. 

41Unless otherwise specified, the epidemiologic 

studies discussed in the PA evaluate only adults. 

42Reversible loss of lung function in combination 

with the presence of symptoms meets the ATS 

definition of adversity (ATS, 2000). 43Panel studies include repeated measurements 

of health outcomes, such as respiratory symptoms, 

at the individual level (U.S. EPA, 2013a, p. 1x). 

44Evidence 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). 

6.6 hour exposure to 60 ppb O3, and that 

22% would experience such decrements 

following exposure to 80 ppb O3 (U.S. 

EPA, 2013a, p. 6–18 and Figure 6–3).40

Schelegle et al. (2012) estimated that, for 

a prolonged (6.6 hours) O3 exposure 

with moderate, quasi-continuous 

exercise, the average dose of onset for 

FEV1 decrement would be reached 

following 4 to 5 hours of exposure to 60 

ppb, and following 3 to 4 hours of 

exposure to 80 ppb. However, 14% of 

the individuals were estimated to have 

a dose of onset that was less than 40% 

of the average. Those individuals were 

estimated to reach their dose of onset 

following 1 to 2 hours of exposure to 50 

to 80 ppb O3 (U.S. EPA, 2013a, p. 6–16), 

which is consistent with the threshold 

FEV1 responses reported by McDonnell 

et al. (2012). 

CASAC agreed that these models 

mark a significant technical advance 

over the exposure-response modeling 

approach used in the last review (Frey, 

2014a), stating that ‘‘the comparison of 

the MSS [McDonnell-Stewart-Smith] 

model results to those obtained with the 

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 dose threshold below which changes 

in lung function do not occur. 

Epidemiologic studies 41 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 decrements accompanied 

by respiratory symptoms 42 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 studies 43

reported statistically significant 

associations with lung function 

decrements 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). 

Epidemiologic panel studies 

investigating the effects of short-term 

exposure to O3 provided information on 

potential confounding by copollutants 

such as particulate matter with a 

median aerodynamic diameter less than 

or equal to 2.5 microns (PM2.5), 

particulate matter with a median 

aerodynamic diameter less than or equal 

to 10 microns (PM10), nitrogen dioxide 

(NO2), or sulfur dioxide (SO2). These 

studies varied in how they evaluated 

confounding. Some studies of subjects 

exercising outdoors indicated that 

ambient concentrations of copollutants 

such as NO2, SO2, or acid aerosol were 

low, and thus not likely to confound 

associations observed for O3 (Hoppe et 

al., 2003; Brunekreef et al., 1994; Hoek 

et al., 1993). In other studies of children 

with increased outdoor exposures, O3

was consistently associated with 

decreases in lung function, whereas 

other pollutants such as PM2.5, sulfate, 

and acid aerosol individually showed 

variable associations across studies 

(Thurston et al., 1997; Castillejos et al., 

1995; Berry et al., 1991; Avol et al., 

1990; Spektor et al., 1988a). Studies that 

conducted copollutant modeling 

generally found O3-associated lung 

function decrements to be robust (i.e., 

most copollutant-adjusted effect 

estimates fell within the 95% 

confidence interval (CI) of the singlepollutant 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 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 


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45CASAC 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). 

46Referred 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. 

47When 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) 

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 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 

function of other pulmonary tissue, 

leading to diseases such as asthma; (4) 

inflammation can alter the body’s host 

defense response to inhaled 

microorganisms, particularly in 

potentially at-risk populations or 

lifestages such as the very young and 

old; and (5) inflammation can alter the 

lung’s response to other agents such as 

allergens or toxins (U.S. EPA, 2013a, 

section 6.2.3). Thus, lung injury and the 

resulting inflammation provide a 

mechanism by which O3 may cause 

other more serious morbidity effects 

(e.g., asthma exacerbations).45

In the last review, controlled human 

exposure studies reported O3-induced 

airway inflammation following 

exposures at or above 80 ppb and 

animal toxicological studies provided 

evidence for increases in inflammation 

and permeability in rabbits at levels as 

low as 100 ppb O3. In the current 

review, the link between O3 exposures 

and airway inflammation and injury has 

been evaluated in additional controlled 

human exposure studies, as well as in 

recent epidemiologic studies. Controlled 

human exposure studies have generally 

been conducted in young, healthy 

adults or in adults with asthma using 

lavage (proximal airway and 

bronchoalveolar), bronchial biopsy, and 

more recently, induced sputum. These 

studies have evaluated one or more 

indicators of inflammation, including 

neutrophil 46 (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 shortterm exposures to O3 can cause 

pulmonary inflammation. A single acute 

exposure (1–4 hours) of humans to 

moderate concentrations of O3 (200–600 

ppb) while exercising at moderate to 

heavy intensities resulted in a number 

of cellular and biochemical changes in 

the lung, including inflammation 

characterized by increased numbers of 

PMNs, increased permeability of the 

epithelial lining of the respiratory tract, 

cell damage, and production of 

proinflammatory molecules (i.e., 

cytokines and prostaglandins, U.S. EPA, 

2006a). A meta-analysis of 21 controlled 

human exposure studies (Mudway and 

Kelly, 2004) using varied experimental 

protocols (80–600 ppb O3 exposures; 1– 

6.6 hours exposure duration; light to 

heavy exercise; bronchoscopy at 0–24 

hours post-O3 exposure) reported that 

PMN influx in healthy subjects is 

linearly associated with total O3 dose. 

Several studies, including one 

published since the last review (Alexis 

et al., 2010), have reported O3-induced 

increases in PMN influx and 

permeability following exposures at or 

above 80 ppb (Alexis et al., 2010; Peden 

et al., 1997; Devlin et al., 1991), and 

eosinophilic inflammation following 

exposures at or above 160 ppb (Scannell 

et al., 1996; Peden et al., 1997; 

Hiltermann et al., 1999; Vagaggini et al., 

2002). In addition, one recent controlled 

human exposure study has reported O3- 

induced PMN influx following 

exposures of healthy adults to 60 ppb O3

(Kim et al., 2011), the lowest 

concentration at which inflammatory 

responses have been evaluated in 

human studies. 

As with FEV1 responses to O3, 

inflammatory responses to O3 are 

generally reproducible within 

individuals, with some individuals 

experiencing more severe O3-induced 

airway inflammation than indicated by 

group averages (Holz et al., 2005; Holz 

et al., 1999). Unlike O3-induced 

decrements in lung function, which are 

attenuated following repeated exposures 

over several days (U.S. EPA, 2013a, 

section 6.2.1.1), some markers of O3- 

induced inflammation and tissue 

damage remain elevated during repeated 

exposures, indicating ongoing damage 

to the respiratory system (U.S. EPA, 

2013a, section 6.2.3.1). 

Most controlled human exposure 

studies have reported that asthmatics 

experience larger O3-induced 

inflammatory responses than nonasthmatics.47 Specifically, asthmatics 


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reported increased symptoms in addition to 

inflammation. 

48Exhaled 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). 

exposed to 200 ppb O3 for 4–6 hours 

with exercise show significantly more 

neutrophils in bronchoalveolar lavage 

fluid (BALF) than similarly exposed 

healthy individuals (Scannell et al., 

1996; Basha et al., 1994). Bosson et al. 

(2003) reported significantly greater 

expression of a variety of proinflammatory cytokines in asthmatics, 

compared to healthy subjects, following 

exposure to 200 ppb O3 for 2 hours. In 

addition, research available in the last 

review, combined with a recent study 

newly available in this review, indicates 

that pretreatment of asthmatics with 

corticosteroids can prevent the O3- 

induced inflammatory response in 

induced sputum, though pretreatment 

did not prevent FEV1 decrements 

(Vagaggini et al., 2001; 2007). In 

contrast, Stenfors et al. (2002) did not 

detect a difference in the O3-induced 

increases in neutrophil numbers 

between 15 subjects with mild asthma 

and 15 healthy subjects by bronchial 

wash at the 6 hours postexposure time 

point, although the neutrophil increase 

in the asthmatic group was on top of an 

elevated baseline. 

In people with allergic airway disease, 

including people with rhinitis and 

asthma, evidence available in the last 

review indicated that proinflammatory 

mediators also cause accumulation of 

eosinophils in the airways (Jorres et al., 

1996; Peden et al., 1995 and 1997; 

Frampton et al., 1997; Hiltermann et al., 

1999; Holz et al., 2002; Vagaggini et al., 

2002). The eosinophil, which increases 

inflammation and allergic responses, is 

the cell most frequently associated with 

exacerbations of asthma (72 FR 37846, 

July 11, 2007). 

Studies reporting inflammatory 

responses and markers of lung injury 

have clearly demonstrated that there is 

important variation in the responses of 

exposed subjects (72 FR 37831, July 11, 

2007). Some individuals also appear to 

be intrinsically more susceptible to 

increased inflammatory responses from 

O3 exposure (Holz et al., 2005). In 

healthy adults exposed to each 80 and 

100 ppb O3, Devlin et al. (1991) 

observed group average increases in 

neutrophilic inflammation of 2.1- and 

3.8-fold, respectively. However, there 

was a 20-fold range in inflammatory 

responses between individuals at both 

concentrations. Relative to an earlier, 

similar study conducted at 400 ppb 

(Koren et al., 1989), Devlin et al. (1991) 

noted that although some of the study 

population showed little or no increase 

in inflammatory and cellular injury 

indicators analyzed after exposures to 

lower levels of O3 (i.e., 80 and 100 ppb), 

others had changes that were as large as 

those seen when subjects were exposed 

to 400 ppb O3. The study authors 

concluded that, ‘‘while the population 

as a whole may have a small 

inflammatory response to near-ambient 

levels of ozone, there may be a 

significant subpopulation that is very 

sensitive to these low levels’’ (Devlin et 

al., 1991). 

A number of studies report that O3

exposures increase epithelial 

permeability. Increased BALF protein, 

suggesting O3-induced changes in 

epithelial permeability, has been 

reported at 1 hour and 18 hours 

postexposure (Devlin et al., 1997; 

Balmes et al., 1996). A meta-analysis of 

results from 21 publications (Mudway 

and Kelly, 2004) for varied experimental 

protocols (80–600 ppb O3; 1–6.6 hours 

duration; light to heavy exercise; 

bronchoscopy at 0–24 hours post-O3

exposure; healthy subjects), showed that 

increased BALF protein is associated 

with total inhaled O3 dose. As noted in 

the 2009 PM ISA (U.S. EPA, 2009a), it 

has been postulated that changes in 

permeability associated with acute 

inflammation may provide increased 

access of inhaled antigens, particles, 

and other inhaled substances deposited 

on lung surfaces to the smooth muscle, 

interstitial cells, immune cells 

underlying the epithelium, and the 

blood (U.S. EPA, 2013a, sections 5.3.4, 

5.3.5). As has been observed with FEV1

responses, within individual changes in 

permeability are correlated with 

changes following sequential O3

exposures (Que et al., 2011). Changes in 

permeability and AHR apear to be 

mediated by different pathways. Animal 

toxicology studies have provided some 

support for this hypothesis (Adamson 

and Prieditis, 1995; Chen et al., 2006), 

though these studies did not specifically 

evaluate O3 exposures (U.S. EPA, 

2009a). 

The limited epidemiologic evidence 

reviewed in the 2006 O3 AQCD (U.S. 

EPA, 2006a) reported associations 

between short-term increases in ambient 

O3 concentrations and airways 

inflammation in children (1-hour max 

O3 of approximately 100 ppb). In the 

2006 O3 AQCD (U.S. EPA, 2006a), there 

was limited evidence for increases in 

nasal lavage levels of inflammatory cell 

counts and molecules released by 

inflammatory cells (i.e., eosinophilic 

cationic protein, and 

myeloperoxidases). Since 2006, as a 

result of the development of less 

invasive methods, there has been a large 

increase in the number of studies 

assessing ambient O3-associated changes 

in airway inflammation and oxidative 

stress, the types of biological samples 

collected, and the types of indicators. 

Most of these recent studies have 

evaluated biomarkers of inflammation 

or oxidative stress in exhaled breath, 

nasal lavage fluid, or induced sputum 

(U.S. EPA, 2013a, section 6.2.3.2). These 

recent studies form a larger database to 

establish coherence with findings from 

controlled human exposure and animal 

studies that have measured the same or 

related biological markers. Additionally, 

results from these studies provide 

further biological plausibility for the 

associations observed between ambient 

O3 concentrations and respiratory 

symptoms and asthma exacerbations. 

A number of epidemiologic studies 

provide evidence that short-term 

increases in ambient O3 exposure 

increase pulmonary inflammation and 

oxidative stress in children, including 

those with asthma (Sienra-Monge et al., 

2004; Barraza-Villarreal et al., 2008; 

Romieu et al., 2008; Berhane et al., 

2011). Multiple studies examined and 

found increases in exhaled nitric oxide 

(eNO)48 (Berhane et al., 2011; Khatri et 

al., 2009; Barraza-Villarreal et al., 2008). 

In some studies of subjects with asthma, 

increases in ambient O3 concentration at 

the same lag were associated with both 

increases in pulmonary inflammation 

and respiratory symptoms (Khatri et al., 

2009; Barraza-Villarreal et al., 2008). 

Although more limited in number, 

epidemiologic studies also found 

associations with cytokines such as IL– 

6 or IL–8 (Barraza-Villarreal et al., 2008; 

Sienra-Monge et al., 2004), eosinophils 

(Khatri et al., 2009), antioxidants 

(Sienra-Monge et al., 2004), and 

indicators of oxidative stress (Romieu et 

al., 2008) (U.S. EPA, 2013a, section 

6.2.3.2). Because associations with 

inflammation were attenuated with 

higher antioxidant intake in the study 

by Sienra-Monge et al. (2004), this study 

provides additional evidence that 

inhaled 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, 


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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 (BarrazaVillarreal et al., 2008; Romieu et al., 

2008; Sienra-Monge et al., 2004). 

In conclusion, a relatively small 

number of controlled human exposure 

studies evaluating O3-induced airway 

inflammation have become available 

since the last review. For purposes of 

reviewing the current O3 NAAQS, the 

most important of these recent studies 

reported a statistically significant 

increase in airway inflammation in 

healthy adults at moderate exertion 

following exposures to 60 ppb O3, the 

lowest concentration that has been 

evaluated for inflammation. In addition, 

a number of recent epidemiologic 

studies report O3-associated increases in 

markers of pulmonary inflammation, 

particularly in children. Thus, recent 

studies continue to support the 

evidence for airway inflammation and 

injury that was available in previous 

reviews, with new evidence for such 

effects following exposures to lower 

concentrations than had been evaluated 

previously. 

iii. Airway Hyperresponsiveness 

Airway hyperresponsiveness (AHR) 

refers to a condition in which the 

conducting airways undergo enhanced 

bronchoconstriction in response to a 

variety of stimuli. Airway 

hyperresponsiveness is an important 

consequence of exposure to ambient O3

because its presence reflects a change in 

airway smooth muscle reactivity, and 

indicates that the airways are 

predisposed to narrowing upon 

inhalation of a variety of ambient 

stimuli including specific triggers (i.e., 

allergens) and nonspecific triggers (e.g., 

SO2, and cold air). People with asthma 

are generally more sensitive to 

bronchoconstricting agents than those 

without asthma, and the use of an 

airway challenge to inhaled 

bronchoconstricting agents is a 

diagnostic test in asthma (U.S. EPA, 

2013, section 6.2.2). Standards for 

airway responsiveness testing have been 

developed for the clinical laboratory 

(ATS, 2000), although variation in the 

methodology for administering the 

bronchoconstricting agent may affect the 

results (Cockcroft et al., 2005). There is 

a wide range of airway responsiveness 

in people without asthma, and 

responsiveness is influenced by a 

number of factors, including cigarette 

smoke, pollutant exposures, respiratory 

infections, occupational exposures, and 

respiratory irritants. Dietary 

antioxidants have been reported to 

attenuate O3-induced bronchial 

hyperresponsiveness in people with 

asthma (Trenga et al., 2001). 

Evidence for airway 

hyperresponsiveness (AHR) following 

O3 exposures is derived primarily from 

controlled human exposure and 

toxicological studies (U.S. EPA, 2013a, 

section 6.2.2). Airway responsiveness is 

often quantified by measuring changes 

in pulmonary function following the 

inhalation of an aerosolized allergen or 

a nonspecific bronchoconstricting agent 

(e.g., methacholine), or following 

exposure to a bronchoconstricting 

stimulus such as cold air. In the last 

review, controlled human exposure 

studies of mostly adults (≥18 years of 

age) had shown that exposures to O3

concentrations at or above 80 ppb 

increase airway responsiveness, as 

indicated by a reduction in the 

concentration of specific (e.g., ragweed) 

and non-specific (e.g., methacholine) 

agents required to produce a given 

reduction in lung function (e.g., as 

measured by FEV1 or specific airway 

resistance) (U.S. EPA, 2013a, section 

6.2.2.1). This O3-induced AHR has been 

reported to be dose-dependent 

(Horstman et al., 1990). Animal 

toxicology studies have reported O3- 

induced AHR in a number of species, 

with some rat strains exhibiting 

hyperresponsiveness following 4-hour 

exposures to O3 concentrations as low 

as 50 ppb (Depuydt et al., 1999). Since 

the last review, there have been 

relatively few new controlled human 

exposure and animal toxicology studies 

of O3 and AHR, and no new studies 

have evaluated exposures to O3

concentrations at or below 80 ppb (U.S. 

EPA, 2013a, section 6.2.2.1). 

Airway hyperresponsiveness is linked 

with the accumulation and/or activation 

of eosinophils in the airways of 

asthmatics, which is followed by 

production of mucus and a 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). 

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). 


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49CASAC 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). 

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 inflammatory 

responses to O3 exposures that did not 

elicit significant spirometric responses 

have been reported (Holz et al., 2005). 

A recent study (Que et al., 2011) 

indicates that AHR also appears to be 

mediated by a differing physiologic 

pathway. These results from controlled 

human exposure studies indicate that 

O3-induced lung function decrements, 

inflammatory responses and pulmonary 

injury (leading to increased epithelial 

permeability), and AHR, are mediated 

by apparently different physiologic 

pathways. Except for lung function 

decrements, 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 affected 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 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 shortterm 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 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 shortterm increases in ambient O3

concentration and increases in 

respiratory symptoms (U.S. EPA, 2013a, 

section 6.2.4). 

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 individuallevel 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 


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50Though, as discussed below, for other 

endpoints (e.g., hospital admissions, emergency 

department visits) the ISA focused primarily on 

multicity studies. 

51Phagocytic white blood cells within the alveoli 

of the lungs that ingest inhaled particles. 

52The 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. 

and/or adults with asthma, with fewer 

studies, and less consistent results, in 

non-asthmatic populations (U.S. EPA, 

2013a, section 6.2.4). The 2006 AQCD 

(U.S. EPA, 2006a, U.S. EPA, 2013a, 

section 6.2.4) concluded that the 

collective body of epidemiologic 

evidence indicated that short-term 

increases in ambient O3 concentrations 

are associated with increases in 

respiratory symptoms in children with 

asthma. A large body of single-city and 

single-region studies of asthmatic 

children provides consistent evidence 

for associations between short-term 

increases in ambient O3 concentrations 

and increased respiratory symptoms and 

asthma medication use in children with 

asthma (U.S. EPA, 2013a, Figure 6–12, 

Table 6–20, p. 79). 

Methodological differences among 

studies make comparisons across recent 

multicity studies of respiratory 

symptoms difficult. Because of fewer 

person-days of data (Schildcrout et al., 

2006) or examination of 19-day averages 

of ambient O3 concentrations (O’Connor 

et al., 2008), the ISA did not give greater 

weight to results from recent multicity 

studies than results from single-city 

studies (U.S. EPA, 2013a, section 

6.2.4.5).50 While evidence from the few 

available U.S. multicity studies is less 

consistent (O’Connor et al., 2008; 

Schildcrout et al., 2006; Mortimer et al., 

2002), the overall body of epidemiologic 

evidence with respect to the association 

betweeen exposure to O3 and respiratory 

symptoms in asthmatic children 

remains compelling (U.S. EPA, 2013a, 

section 6.2.4.1). Findings from a small 

body of studies indicate that O3 is also 

associated with increased respiratory 

symptoms in adults with asthma (Khatri 

et al., 2009; Feo Brito et al., 2007; Ross 

et al., 2002) (U.S. EPA, 2013a, section 

6.2.4.2). 

Available evidence indicates that O3- 

associated increases in respiratory 

symptoms are not confounded by 

temperature, pollen, or copollutants 

(primarily PM) (U.S. EPA, 2013a, 

section 6.2.4.5; Table 6–25; Romieu et 

al., 1996; Romieu et al., 1997; Thurston 

et al., 1997; Gent et al., 2003). However, 

identifying the independent effects of 

O3 in some studies was complicated due 

to the high correlations observed 

between O3 and PM or different lags and 

averaging times examined for 

copollutants. Nonetheless, the ISA 

noted that the robustness of associations 

in some studies of individuals with 

asthma, combined with findings from 

controlled human exposure studies for 

the direct effects of O3 exposure, 

provide substantial evidence supporting 

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-Nun ̃ ez et al., 2008; Romieu et 

al., 2006; Schildcrout et al., 2006; 

Jalaludin et al., 2004; Romieu et al., 

1997; Thurston et al., 1997) while others 

reported associations for either 

respiratory symptoms or medication use 

(Romieu et al., 1996; Rabinovitch et al., 

2004; Just et al., 2002; Ostro et al., 

2001). 

In summary, both controlled human 

exposure and epidemiologic studies 

have reported respiratory symptoms 

attributable to short-term O3 exposures. 

In the last review, the majority of the 

evidence from controlled human 

exposure studies in young, healthy 

adults was for symptoms following 

exposures to O3 concentrations at or 

above 80 ppb. Although studies that 

have become available since the last 

review have not reported increased 

respiratory symptoms in young, healthy 

adults following exposures with 

moderate exertion to 60 ppb, one recent 

study did report increased symptoms 

following exposure to 72 ppb O3. As 

was concluded in the 2006 O3 AQCD 

(U.S. EPA, 2006a; U.S. EPA, 1996), the 

collective body of epidemiologic 

evidence indicates that short-term 

increases in ambient O3 concentration 

are associated with increases in 

respiratory symptoms in children with 

asthma (U.S. EPA, 2013a, section 6.2.4). 

Recent studies of respiratory symptoms 

and medication use, primarily in 

asthmatic children, add to this 

evidence. In a smaller body of studies, 

increases in ambient O3 concentration 

were associated with increases in 

respiratory symptoms in adults with 

asthma. 

v. Lung Host Defense 

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 immunity 52 (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 shortterm O3 exposures have the potential to 

impair host defenses in humans, 

primarily by interfering with alveolar 

macrophage function. Any impairment 

in alveolar macrophage function may 

lead to decreased clearance of 

microorganisms or nonviable particles. 

Compromised alveolar macrophage 

functions in asthmatics may increase 

their susceptibility to other O3 effects, 

the effects of particles, and respiratory 

infections (U.S. EPA, 2006a, p. 8–26). 

These conclusions were based largely 

on studies conducted in animals 

exposed for several hours up to several 

weeks to O3 concentrations from 100 to 

250 ppb (Hurst et al., 1970; Driscoll et 

al., 1987; Cohen et al., 2002). Consistent 

with the animal evidence, a controlled 

human exposure study available in the 

last review had 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 


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53Atopy 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. 

54Epidemiologic 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. 

55The 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. 

mucous packets up the resiratory tract, 

hence the term ‘‘mucociliary escalator.’’ 

Although some studies show reduced 

tracheobronchial clearance after O3

exposure (U.S. EPA, 2013a, section 

6.2.5.1), alveolar clearance of deposited 

material is accelerated, presumably due 

to macrophage influx, which in itself 

can be damaging. 

With regard to adaptive immunity, a 

limited number of epidemiologic 

studies have examined associations 

between O3 exposure and hospital 

admissions or emergency department 

visits for respiratory infection, 

pneumonia, or influenza. Results have 

been mixed, and in some cases 

conflicting (U.S. EPA, 2013a, sections 

6.2.7.2 and 6.2.7.3). With the exception 

of influenza, it is difficult to ascertain 

whether cases of respiratory infection or 

pneumonia are of viral or bacterial 

etiology. A recent study that examined 

the association between O3 exposure 

and respiratory hospital admissions in 

response to an increase in influenza 

intensity observed an increase in 

respiratory hospital admissions (Wong 

et al., 2009), but information from 

toxicological studies of O3 and viral 

infections is ambiguous. 

In summary, relatively few studies 

conducted since the last review have 

evaluated the effects of O3 exposures on 

lung host defense. When the available 

evidence is taken as a whole, the ISA 

concludes that acute O3 exposures 

impair the host defense capability of 

animals, primarily by depressing 

alveolar macrophage function and 

perhaps also by decreasing mucociliary 

clearance of inhaled particles and 

microorganisms. Coupled with limited 

evidence from controlled human 

exposure studies, this suggests that 

humans exposed to O3 could be 

predisposed to bacterial infections in 

the lower respiratory tract (U.S. EPA, 

2013a, section 6.2.5.5). 

vi. Allergic and Asthma-Related 

Responses 

Effects resulting from combined 

exposures to O3 and allergens have been 

studied in a variety of animal species, 

generally as models of experimental 

asthma. Pulmonary function and AHR 

in animal models of asthma are 

discussed in detail in Section 6.2.1.3 

and Section 6.2.2.2, respectively, in the 

ISA (U.S. EPA, 2013a). Studies of 

allergic and asthma-related responses 

are discussed in detail in sections 5.3.6 

and 6.2.6 of the ISA (U.S. EPA, 2013a). 

Evidence available in the last review 

indicates that O3 exposure skews 

immune responses toward an allergic 

phenotype and could also make 

airborne allergens more allergenic. In 

humans, allergic rhinoconjunctivitis 

symptoms are associated with increases 

in ambient O3 concentrations (Riediker 

et al., 2001). Controlled human 

exposure studies have observed O3- 

induced changes indicating allergic 

skewing. Airway eosinophils, which are 

white blood cells that participate in 

allergic disease and inflammation, were 

observed to increase in volunteers with 

atopy 53 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 non-human primates. 

Collectively, findings from these studies 

suggest that O3 can induce or enhance 

certain components of allergic 

inflammation in individuals with 

allergy or allergic asthma. 

Evidence available in the last review 

indicates that O3 may also increase AHR 

to specific allergen triggers (75 FR 2970, 

January 19, 2010). Two studies (Jo ̈rres et 

al., 1996; Holz et al., 2002) observed 

increased airway responsiveness to O3

exposure with bronchial allergen 

challenge in subjects with preexisting 

allergic airway disease. Ozone-induced 

exacerbation of airway responsiveness 

persists longer and attenuates more 

slowly than O3-induced lung function 

decrements and respiratory symptom 

responses and can have important 

clinical implications for asthmatics. 

Animal toxicology studies indicate that 

O3 enhances inflammatory and allergic 

responses to allergen challenge in 

sensitized animals. In addition to 

exacerbating existing allergic responses, 

toxicology studies indicate that O3 can 

also act as an adjuvant to produce 

sensitization in the respiratory tract. 

Along with its pro-allergic effects 

(inducing or enhancing certain 

components of allergic inflammation in 

individuals with allergy or allergic 

asthma), O3 could also make airborne 

allergens more allergenic. When 

combined with NO2, O3 has been shown 

to enhance nitration of common protein 

allergens, which may increase their 

allergenicity (Franze et al., 2005). 

vii. Hospital Admissions and 

Emergency Department Visits 

The 2006 O3 AQCD evaluated 

numerous studies of respiratory-related 

emergency department visits and 

hospital admissions. These were 

primarily time-series studies conducted 

in the U.S., Canada, Europe, South 

America, Australia, and Asia. Based on 

such studies, the 2006 O3 AQCD 

concluded that ‘‘the overall evidence 

supports a causal relationship between 

acute ambient O3 exposures and 

increased respiratory morbidity 

resulting in increased emergency 

department visits and [hospital 

admissions] during the warm season’’ 54

(U.S. EPA, 2006a). This conclusion was 

‘‘strongly supported by the human 

clinical, animal toxicologic[al], and 

epidemiologic evidence for [O3- 

induced] lung function decrements, 

increased respiratory symptoms, airway 

inflammation, and airway 

hyperreactivity’’ (U.S. EPA, 2006a). 

The results of recent studies largely 

support the conclusions of the 2006 O3

AQCD (U.S. EPA, 2013a, section 6.2.7). 

Since the completion of the 2006 O3

AQCD, relatively fewer studies 

conducted in the U.S., Canada, and 

Europe have evaluated associations 

between short-term O3 concentrations 

and respiratory hospital admissions and 

emergency department visits, with a 

growing number of studies conducted in 

Asia. This epidemiologic evidence is 

discussed in detail in the ISA (U.S. EPA, 

2013a, section 6.2.7).55

In considering this body of evidence, 

the ISA focused primarily on multicity 

studies because they examine 

associations with respiratory-related 

hospital admissions and emergency 

department visits over large geographic 

areas using consistent statistical 

methodologies (U.S. EPA, 2013a, section 

6.2.7.1). The ISA also focused on singlecity 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 populationspecific characteristics that may impact 

the risk of O3-related health effects but 

were not evaluated in the larger studies 

(U.S. EPA, 2013a, section 6.2.7.1). When 


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examining the association between 

short-term O3 exposure and respiratory 

health effects that require medical 

attention, the ISA distinguishes between 

hospital admissions and emergency 

department visits because it is likely 

that a small percentage of respiratory 

emergency department visits will be 

admitted to the hospital; therefore, 

respiratory emergency department visits 

may represent potentially less serious, 

but more common outcomes (U.S. EPA, 

2013a, section 6.2.7.1). 

Several recent multicity studies (e.g., 

Cakmak et al., 2006; Dales et al., 2006) 

and a multi-continent study 

(Katsouyanni et al., 2009) report 

associations between short-term O3

concentrations and increased 

respiratory-related hospital admissions 

and emergency department visits. These 

multicity studies are supported by 

results from single-city studies also 

reporting consistent positive 

associations using different exposure 

assignment approaches (i.e., average of 

multiple monitors, single monitor, 

population-weighted average) and 

averaging times (i.e., 1-hour max and 8- 

hour max) (U.S. EPA, 2013a, sections 

6.2.7.1 to 6.2.7.5). When examining 

cause-specific respiratory outcomes, 

recent studies report positive 

associations with hospital admissions 

and emergency department visits for 

asthma (Strickland et al., 2010; Stieb et 

al., 2009) and chronic obstructive 

pulmonary disease (COPD) (Stieb et al., 

2009; Medina-Ramon et al., 2006), with 

more limited evidence for pneumonia 

(Medina-Ramon et al., 2006; Zanobetti 

and Schwartz, 2006). In seasonal 

analyses (U.S. EPA, 2013a, Figure 6–19, 

Table 6–28), stronger associations were 

reported in the warm season or summer 

months, when O3 concentrations are 

higher, compared to the cold season, 

particularly for asthma (Strickland et al., 

2010; Ito et al., 2007) and COPD 

(Medina-Ramon et al., 2006). The 

available evidence indicates that 

children 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; MedinaRamon et al., 2006) (U.S. EPA, 2013a, 

section 6.2.7.5). 

In the last review, studies had not 

evaluated the concentration-response 

relationship between short-term O3

exposure and respiratory-related 

hospital admissions and emergency 

department visits. A preliminary 

examination of this relationship in 

studies that have become available since 

the last review found no evidence of a 

deviation from linearity when 

examining the association between 

short-term O3 exposure and asthma 

hospital admissions (U.S. EPA, 2013a, 

page 6–157; Silverman and Ito, 2010). In 

addition, an examination of the 

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 respiratoryrelated 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 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 multicontinent 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. Allyear 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 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. 


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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 longterm 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 geneenvironment interactions, mechanisms, 

and biological pathways associated with 

incident asthma. 

In children, the relationship between 

long-term O3 exposure and new-onset 

asthma has been extensively studied in 

the Children’s Health Study (CHS), a 

long-term study that was initiated in the 

early 1990’s which has evaluated effects 

in several cohorts of children. The CHS 

was initially designed to examine 

whether long-term exposure to ambient 

pollution was related to chronic 

respiratory outcomes in children in 12 

communities in southern California. In 

the CHS, new-onset asthma was 

classified as having no prior history of 

asthma at study entry with subsequent 

report of physician-diagnosed asthma at 

follow-up, with the date of onset 

assigned to be the midpoint of the 

interval between the interview date 

when asthma diagnosis was first 

reported and the previous interview 

date. The results of one study 

(McConnell et al., 2002) available in the 

previous review indicated that within 

high O3 communities, asthma risk was 

3.3 times greater for children who 

played three or more outdoor sports as 

compared with children who played no 

sports. 

For this review, as discussed in 

section 7.2.1.1 of the ISA (U.S. EPA, 

2013a), recent studies from the CHS 

provide evidence for gene-environment 

interactions in effects on 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 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 hemeoxygenase (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) metrics. The shortterm O3 studies presented in section 

6.2.7.5 of the ISA (U.S. EPA, 2013a) and 

discussed above in section 3.1.2.1 

continue to indicate that there is 

evidence for increases in both hospital 

admissions and emergency department 

visits in children and adults related to 

all respiratory outcomes, including 

asthma, with stronger associations in 

the warm months. New studies, 

discussed in section 7.2.2 of the ISA 

(U.S. EPA, 2013a) also evaluated longterm 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 longterm 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 concentrationresponse 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 

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 concentrationresponse 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, Ku ̈nzli (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 


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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 

In the 2006 O3 AQCD, few 

epidemiologic studies had investigated 

the effect of chronic O3 exposure on 

pulmonary function. The definitive 8- 

year follow-up analysis of the first 

cohort of the CHS (U.S. EPA, 2013a, 

section 7.2.3.1) provided little evidence 

that long-term exposure to ambient O3

was associated with significant deficits 

in the growth rate of lung function in 

children. The strongest evidence was for 

medium-term effects of extended O3

exposures over several summer months 

on lung function (FEV1) in children, i.e., 

reduced lung function growth being 

associated with higher ambient O3

levels. Short-term O3 exposure studies 

presented in the ISA (U.S. EPA, 2013a, 

section 6.2.1.2) provide a cumulative 

body of epidemiologic evidence that 

strongly supports associations between 

ambient O3 exposure and decrements in 

lung function among children. A later 

CHS study (Islam et al., 2007) included 

in this review (U.S. EPA, 2013a, section 

7.2.3.1) also reported no substantial 

differences in the effect of O3 on lung 

function. However, in a more recent 

CHS study, Breton et al. (2011) 

hypothesized that genetic variation in 

genes on the glutathione metabolic 

pathway may influence the association 

between ambient air pollutant 

exposures and lung function growth in 

children, and found that variation in the 

GSS locus was associated with 

differences in risk of children for lung 

function growth deficits associated 

ambient air pollutants, including O3. A 

recent study (Rojas-Martinez et al., 

2007) of long-term exposure to O3,

described in section 7.2.3.1 of the ISA 

(U.S. EPA, 2013a, p. 7–19), observed a 

relationship with pulmonary function 

declines in school-aged children where 

O3 and other pollutant levels were 

higher (90 ppb at high end of the range) 

than those in the CHS. Two studies of 

adult cohorts provide mixed results 

where long-term exposures were at the 

high end of the range. 

Long-term studies in animals allow 

for greater insight into the potential 

effects of prolonged exposure to O3 that 

may not be easily measured in humans, 

such as structural changes in the 

respiratory tract. Despite uncertainties, 

epidemiologic studies observing 

associations of O3 exposure with 

functional changes in humans can attain 

biological plausibility in conjunction 

with long-term toxicological studies, 

particularly O3-inhalation studies 

performed in non-human primates 

whose respiratory systems most closely 

resemble that of the human. An 

important series of studies, discussed in 

section 7.2.3.2 of the ISA (U.S. EPA, 

2013a), have used nonhuman primates 

to examine the effect of O3 alone, or in 

combination with an inhaled allergen, 

house dust mite antigen (HDMA), on 

morphology and lung function. Animals 

exhibit the hallmarks of allergic asthma 

defined for humans (NHLBI, 2007). 

These studies and others have 

demonstrated changes in pulmonary 

function and airway morphology in 

adult and infant nonhuman primates 

repeatedly exposed to environmentally 

relevant concentrations of O3 (U.S. EPA, 

2013a, section 7.2.3.2). 

The initial observations in adult 

nonhuman primates have been 

expanded in a series of experiments 

using infant rhesus monkeys repeatedly 

exposed to 0.5 ppm O3 starting at 1 

month of age (Plopper et al., 2007; 

Schelegle et al. 2003). The purpose of 

these studies was to determine if a 

cyclic regimen of O3 inhalation would 

amplify the allergic responses and 

structural remodeling associated with 

allergic sensitization and inhalation in 

the infant rhesus monkey; they provide 

evidence of an O3-induced change in 

airway resistance and responsiveness 

provides biological plausibility of longterm 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). 

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 


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evaluated the degree to which models 

incorporating thresholds provided a 

better fit to the data. Based on these 

analyses, Jerrett et al. (2009) reported 

‘‘limited evidence’’ for an effect 

threshold at an O3 concentration of 56 

ppb (p=0.06). 

Additionally, a recent multicity time 

series study (Zanobetti and Schwartz, 

2011), which followed (from 1985 to 

2006) four cohorts of Medicare enrollees 

with chronic conditions that might 

predispose to O3-related effects, 

observed an association between longterm (warm season) exposure to O3 and 

elevated risk of mortality in the cohort 

that had previously experienced an 

emergency hospital admission due to 

COPD. A key limitation of this study is 

the inability to control for PM2.5, 

because data were not available in these 

cities until 1999. 

c. Cardiovascular Effects 

A relatively small number of studies 

have examined the potential effect of 

short-term O3 exposure on the 

cardiovascular system. The 2006 O3

AQCD (U.S. EPA, 2006a, p. 8–77) 

concluded that ‘‘O3 directly and/or 

indirectly contributes to 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 and 

subsequent production of inflammatory 

mediators, which have the potential to 

penetrate the epithelial barrier and to 

initiate toxic effects systemically. In 

addition, animal toxicological studies of 

long-term exposure to O3 provide 

evidence of enhanced atherosclerosis 

and ischemia/reperfusion (I/R) injury, 

corresponding with development of a 

systemic oxidative, proinflammatory 

environment. Recent experimental and 

epidemiologic studies have investigated 

O3-related cardiovascular events and are 

summarized in section 6.3 of the ISA 

(U.S. EPA, 2013a). Overall, the ISA 

summarized the evidence in this review 

as follows (U.S. EPA, 2013a, p. 6–211). 

In conclusion, animal toxicological studies 

demonstrate O3-induced cardiovascular 

effects, and support the strong body of 

epidemiologic evidence indicating O3- 

induced cardiovascular mortality. Animal 

toxicological and controlled human exposure 

studies provide evidence for biologically 

plausible mechanisms underlying these O3- 

induced cardiovascular effects. However, a 

lack of coherence with epidemiologic studies 

of cardiovascular morbidity remains an 

important uncertainty. 

Controlled human exposure studies 

discussed in previous AQCDs have not 

demonstrated any consistent 

extrapulmonary effects. In this review, 

evidence from controlled human 

exposure studies suggests 

cardiovascular effects in response to 

short-term O3 exposure (U.S. EPA, 

2013a, section 6.3.1) and provides some 

coherence with evidence from animal 

toxicology studies. Controlled human 

exposure studies also support the 

animal toxicological studies by 

demonstrating O3-induced effects on 

blood biomarkers of systemic 

inflammation and oxidative stress, as 

well as changes in biomarkers that can 

indicate the potential for increased 

clotting following O3 exposures. 

Increases and decreases in high 

frequency heart rate variability (HRV) 

have been reported following relatively 

low (120 ppb during rest) and high (300 

ppb with exercise) O3 exposures, 

respectively. These changes in cardiac 

function observed in animal and human 

studies provide preliminary evidence 

for O3-induced modulation of the 

autonomic nervous system through the 

activation of neural reflexes in the lung 

(U.S. EPA 2013a, section 5.3.2). 

Overall, the ISA concludes that the 

available body of epidemiologic 

evidence examining the relationship 

between short-term exposures to O3

concentrations and cardiovascular 

morbidity is inconsistent (U.S. EPA, 

2013a, section 6.3.2.9). Across studies, 

different definitions (i.e., ICD–9 

diagnostic codes) were used for both allcause 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 shortterm 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 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 cardiopulmonaryrelated 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 shortterm O3 exposures and mortality found 

evidence that supports the conclusions of the 

2006 AQCD. These new studies reported 

consistent positive associations between 

short-term O3 exposure and all-cause 

(nonaccidental) mortality, with associations 

persisting or increasing in magnitude during 

the warm season, and provide additional 

support for associations between O3 exposure 

and cardiovascular and respiratory mortality. 

The 2006 O3 AQCD reviewed a large 

number of time-series studies of 

associations between short-term O3

exposures and total mortality including 

single- and multicity studies, and metaanalyses. In the large U.S. multicity 


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56Respiratory mortality is discussed in more 

detail above. 

57For 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. 

studies that examined all-year data, 

effect estimates corresponding to singleday 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 cardiopulmonaryrelated mortality’’ (U.S. EPA, 2013a, 

section 6.6.1). 

Recent studies have strengthened the 

body of evidence that supports the 

association between short-term O3

concentrations and mortality in adults. 

This evidence includes a number of 

studies reporting associations with 

nonaccidental as well as cause-specific 

mortality. Multi-continent and multicity 

studies have consistently reported 

positive and statistically significant 

associations between short-term O3

concentrations and all-cause mortality, 

with evidence for larger mortality risk 

estimates during the warm or summer 

months (U.S. EPA, 2013a, Figure 6–27; 

Table 6–42). Similarly, evaluations of 

cause-specific mortality have reported 

consistently positive associations with 

O3, particularly in analyses restricted to 

the warm season (U.S. EPA, 2013a, 

Figure 6–37; Table 6–53).56

In assessing the evidence for O3- 

related mortality, the 2006 AQCD also 

noted that multiple uncertainties 

remained regarding the relationship 

between short-term O3 concentrations 

and mortality, including the extent of 

residual confounding by copollutants; 

characterization of the factors that 

modify the O3-mortality association; the 

appropriate lag structure for identifying 

O3-mortality effects; and the shape of 

the O3-mortality concentration-response 

function and whether a threshold exists. 

Many of the studies, published since the 

last review, have attempted to address 

one or more of these uncertainties. The 

ISA (U.S. EPA, 2013a, section 6.6.2) 

discusses the extent to which recent 

studies have evaluated these 

uncertainties in the relationship 

between O3 and mortality. 

In particular, recent studies have 

evaluated different statistical 

approaches to examine the shape of the 

O3-mortality concentration-response 

relationship and to evaluate whether a 

threshold exists for O3-related mortality. 

In an analysis of the National Morbidity 

and Mortality Air Pollution Study 

(NMMAPS) data, Bell et al. (2006) 

evaluated the potential for a threshold 

in the O3-mortality relationship. The 

authors reported positive and 

statistically significant associations with 

mortality in a variety of restricted 

analyses, including analyses restricted 

to days with 24-hour area-wide average 

O3 concentrations below 60, 55, 50, 45, 

40, 35, and 30 ppb. In these restricted 

analyses O3 effect estimates were of 

similar magnitude, were statistically 

significant, and had similar statistical 

precision. In analyses restricted to days 

with 24-hour average O3 concentrations 

below 25 ppb, the O3 effect estimate was 

similar in magnitude to the effect 

estimates resulting from analyses with 

the higher cutoffs, but had somewhat 

lower 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 concentrationresponse 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 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 cityspecific estimates into an overall 

estimate complicates the interpretation 

of the results. Additional studies in 


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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 

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 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 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, 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 


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58Lifestages, 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. 

medication (U.S. EPA, 2007, p. 3–72; 72 

FR 37849, July 11, 2007). For people 

with lung disease, large functional 

responses (e.g., FEV1 decrements ≥20%, 

lasting longer than 24 hours) and/or 

severe symptomatic responses (e.g., 

persistent uncontrollable cough, severe 

discomfort on exercise or deep breath, 

persistent wheeze accompanied by 

shortness of breath, lasting longer than 

24 hours) would likely interfere with 

normal activity for most individuals and 

would increase the likelihood that these 

individuals would seek medical 

treatment (U.S. EPA, 2007, p. 3–72; 72 

FR 37849, July 11, 2007). In the last O3

NAAQS review, for the purpose of 

estimating potentially adverse lung 

function decrements in people with 

lung disease the CASAC panel indicated 

that a focus on the lower end of the 

range of moderate levels of functional 

responses is most appropriate (e.g., 

FEV1 decrements ≥10%) (Henderson, 

2006; U.S. EPA, 2007, p. 3–76). In 

addition, in their letter advising the 

Administrator on the reconsideration of 

the 2008 final decision, CASAC stated 

that ‘‘[a] 10% decrement in FEV1 can 

lead to respiratory symptoms, especially 

in individuals with pre-existing 

pulmonary or cardiac disease. For 

example, people with chronic 

obstructive pulmonary disease have 

decreased ventilatory reserve (i.e., 

decreased baseline FEV1) such that a 

≥10% decrement could lead to moderate 

to severe respiratory symptoms’’ (Samet, 

2011). In this review, CASAC concurred 

that ‘‘[a]n FEV1 decrement of ≥10% is 

a scientifically relevant surrogate for 

adverse health outcomes for people with 

asthma and lung disease’’ (Frey, 2014c, 

p. 3). 

In judging the extent to which these 

impacts represent effects that should be 

regarded as adverse to the health status 

of individuals, in previous NAAQS 

reviews, the EPA has also considered 

whether effects were experienced 

repeatedly during the course of a year or 

only on a single occasion (U.S. EPA, 

2007). Although some experts would 

judge single occurrences of moderate 

responses to be a nuisance, especially 

for healthy individuals, a more general 

consensus view of the adversity of such 

moderate responses emerges as the 

frequency of occurrence increases. Thus 

it has been judged that repeated 

occurrences of moderate responses, 

even in otherwise healthy individuals, 

may be considered to be adverse since 

they could well set the stage for more 

serious illness (61 FR 65723). The 

CASAC panel in the 1997 NAAQS 

review expressed a consensus view that 

these ‘‘criteria for the determination of 

an adverse physiological response were 

reasonable’’ (Wolff, 1995). In the review 

completed in 2008, estimates of 

repeated occurrences continued to be an 

important public health policy factor in 

judging the adversity of moderate lung 

function decrements in healthy and 

asthmatic people (72 FR 37850, July 11, 

2007). 

Evidence new to this review indicates 

that 6.6-hour exposures to 60 ppb O3

during moderate exertion can result in 

pulmonary inflammation in healthy 

adults (based on study mean). As 

discussed in the ISA, the initiation of 

inflammation can be considered as 

evidence that injury has occurred. 

Inflammation induced by a single O3

exposure can resolve entirely but, as 

noted in the ISA (U.S. EPA, 2013a, p. 6– 

76), ‘‘continued acute inflammation can 

evolve into a chronic inflammatory 

state,’’ which would be adverse. 

Responses measured in controlled 

human exposure studies indicate that 

the range of effects elicited in humans 

exposed to ambient O3 concentrations 

include: Decreased inspiratory capacity; 

mild bronchoconstriction; rapid, 

shallow breathing pattern during 

exercise; and symptoms of cough and 

pain on deep inspiration (U.S. EPA, 

2013a, section 6.2.1.1). Young, healthy 

adults exposed for 6.6 hours to O3

concentrations ≥60 ppb, while engaged 

in intermittent moderate exertion, 

develop reversible, transient decrements 

in lung function. In addition, depending 

on the exposure concentration and the 

duration of exposure, young healthy 

adults have been shown to experience 

symptoms of breathing discomfort and 

inflammation if minute ventilation or 

duration of exposure is increased 

sufficiently (U.S. EPA, 2013a, section 

6.2.1.1). Among healthy subjects there is 

considerable interindividual variability 

in the magnitude of the FEV1 responses, 

but when data were combined across 

studies at 60 ppb (U.S. EPA, 2013a, pp. 

6–17 to 6–18), 10% of healthy subjects 

had >10% FEV1 decrements. Moreover, 

consistent with the findings of the ISA 

(U.S. EPA, 2013a, section 6.2.1.1), 

CASAC concluded that ‘‘[a]sthmatic 

subjects appear to be at least as 

sensitive, if not more sensitive, than 

non-asthmatic subjects in manifesting 

ozone-induced pulmonary function 

decrements’’ (Frey, 2014c, p. 4). The 

combination of lung function 

decrements and respiratory symptoms, 

which has been considered adverse in 

previous reviews, has been 

demonstrated in healthy adults 

following prolonged (6.6 hour) 

exposures, while at intermittent 

moderate exertion, to 72 ppb. For these 

types of effects, information from 

controlled human exposure studies, 

which provides an indication of the 

magnitude and thus adversity of effects 

at different O3 concentrations, combined 

with estimates of occurrences in the 

population from the HREA, provide 

information about their importance from 

a policy perspective. 

4. Ozone-Related Impacts on Public 

Health 

Setting standards to provide 

appropriate public health protection 

requires consideration of the factors that 

put populations at greater risk from O3

exposure. In order to estimate the 

potential for public health impacts, it is 

important to consider not only the 

adversity of the health effects, but also 

the populations at greater risk and 

potential behaviors that may reduce 

exposures. 

a. Identification of At-Risk Populations 

and Lifestages 

The currently available evidence 

expands the understanding of 

populations that were identified to be at 

greater risk of O3-related health effects 

at the time of the last review (i.e., people 

who are active outdoors, people with 

lung disease, children and older adults 

and people with increased 

responsiveness to O3) and supports the 

identification of additional factors that 

may lead to increased risk (U.S. EPA, 

2006, section 3.6.2; U.S. EPA, 2013a, 

Chapter 8). Populations and lifestages 

may be at greater risk for O3-related 

health effects due to factors that 

contribute to their susceptibility and/or 

vulnerability to O3. The definitions of 

susceptibility and vulnerability have 

been found to vary across studies, but in 

most instances ‘‘susceptibility’’ refers to 

biological or intrinsic factors (e.g., 

lifestage, sex, preexisting disease/

conditions) while ‘‘vulnerability’’ refers 

to non-biological or extrinsic factors 

(e.g., socioeconomic status [SES]) (U.S. 

EPA, 2013a, p. 8–1; U.S. EPA, 2010c, 

2009d). In some cases, the terms ‘‘atrisk’’ and ‘‘sensitive’’ have been used to 

encompass these concepts more 

generally. In the ISA and PA, ‘‘at-risk’’ 

is the all-encompassing term used to 

define groups with specific factors that 

increase their risk of O3-related health 

effects. 

There are multiple avenues by which 

groups may experience increased risk 

for O3-induced health effects. A 

population or lifestage 58 may exhibit 

greater effects than other populations or 

lifestages exposed to the same 


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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 preexisting asthma, genetic background, 

and lifestage. A group of people could 

also have extrinsically increased risk, 

which would be through an external, 

non-biological factor, such as 

socioeconomic status (SES) and diet. 

Some groups are at risk of increased 

internal dose at a given exposure 

concentration, for example, because of 

breathing patterns. This category would 

include people who work or exercise 

outdoors. Finally, there are those who 

might be placed at increased risk for 

experiencing greater exposures by being 

exposed to higher O3 concentrations. 

This would include, for example, 

groups of people with greater exposure 

to ambient O3 due to less availability or 

use of home air conditioners such that 

they are more likely to be in locations 

with open windows on high O3 days. 

Some groups may be at increased risk of 

O3-related health effects through a 

combination of factors. For example, 

children tend to spend more time 

outdoors when O3 levels are high, and 

at higher levels of activity than adults, 

which leads to increased exposure and 

dose, and they also have biological, or 

intrinsic, risk factors (e.g., their lungs 

are still developing) (U.S. EPA, 2013a, 

Chapter 8). An at-risk population or 

lifestage is more likely to experience 

adverse health effects related to O3

exposures and/or, develop more severe 

effects from exposure than the general 

population. 

i. People With Specific Genetic Variants 

There is adequate evidence for 

populations with certain genotypes 

being more at-risk than others to the 

effects of O3 exposure on health (U.S. 

EPA, 2013a, section 8.1). Controlled 

human exposure and epidemiologic 

studies have reported evidence of O3- 

related increases in respiratory 

symptoms or decreases in lung function 

with variants including GSTM1, GSTP1, 

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 antiinflammatory genes, which have not 

been investigated in human studies. 

ii. People With Asthma 

Previous O3 AQCDs identified 

individuals with asthma as a population 

at increased risk of O3-related health 

effects. Multiple new epidemiologic 

studies included in the ISA have 

evaluated the potential for increased 

risk of O3-related health effects in 

people with asthma, including: Lung 

function; symptoms; medication use; 

AHR; and airway inflammation (also 

measured as exhaled nitric oxide 

fraction, or FeNO). A study of lifeguards 

in Texas reported decreased lung 

function with short-term O3 exposure 

among both individuals with and 

without asthma; however, the decrease 

was greater among those with asthma 

(Thaller et al., 2008). A Mexican study 

of children ages 6–14 detected an 

association between short-term O3

exposure and wheeze, cough, and 

bronchodilator use among asthmatics 

but not non-asthmatics, although this 

may have been the result of a small nonasthmatic population (Escamilla-Nun ̃ ez 

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 nonasthmatics (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 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 (Herna ́ndez-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 nonusers (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 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 


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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 preexisting allergic disease (Vagaggini et 

al., 2002; Peden et al., 1995). Taken 

together, these results point to several 

mechanistic pathways which could 

account for the enhanced sensitivity to 

O3 in subjects with asthma (U.S. EPA, 

2013a, section 5.4.2.2). 

As noted in the previous review (72 

FR 37846, July 11, 2007) asthmatics 

present a differential response profile 

for cellular, molecular, and biochemical 

parameters (U.S. EPA, 2006a, Figure 8– 

1) that are altered in response to acute 

O3 exposure. Ozone-induced increases 

in neutrophils, IL–8 and protein were 

found to be significantly higher in the 

BAL fluid from asthmatics compared to 

healthy subjects, suggesting 

mechanisms for the increased 

sensitivity of asthmatics (Basha et al., 

1994; McBride et al., 1994; Scannell et 

al., 1996; Hiltermann et al., 1999; Holz 

et al., 1999; Bosson et al., 2003). 

Neutrophils, or PMNs, are the white 

blood cell most associated with 

inflammation. IL–8 is an inflammatory 

cytokine with a number of biological 

effects, primarily on neutrophils. The 

major role of this cytokine is to attract 

and activate neutrophils. Protein in the 

airways is leaked from the circulatory 

system, and is a marker for increased 

cellular permeability. 

Bronchial constriction following 

provocation with O3 and/or allergens 

presents a 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 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. 

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, shortterm exposure to O3 may enhance 

damage in a previously injured lung 

(U.S. EPA, 2013a, section 8.2.2). 

In the 2006 O3 AQCD, the potential 

for individuals with asthma to have 

greater risk of O3-related health effects 

was supported by a number of 

controlled human exposure studies, 

evidence from toxicological studies, and 

a limited number of epidemiologic 

studies. In section 8.2.2, the ISA reports 

that in the recent epidemiologic 

literature some, but not all, studies 

report greater risk of health effects 

among individuals with asthma. Studies 

examining effect measure modification 

of the relationship between short-term 

O3 exposure and altered lung function 

by corticosteroid use provided limited 

evidence of O3-related health effects. 

However, recent studies of behavioral 

responses have found that studies do 

not take into account individual 

behavioral adaptations to forecasted air 

pollution levels (such as avoidance and 

reduced time outdoors), which may 

underestimate the observed associations 

in studies that examined the effect of O3

exposure on respiratory health (Neidell 

and Kinney, 2010). This could explain 

some inconsistency observed among 

recent epidemiologic studies. The 

evidence from controlled human 

exposure studies provides support for 

increased detriments in FEV1 and 

greater inflammatory responses to O3 in 

individuals with asthma than in healthy 

individuals without a history of asthma. 

The collective evidence for increased 

risk of O3-related health effects among 

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 


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growth until about 18–20 years of age 

and are therefore thought to be 

intrinsically more at risk for O3-induced 

damage (U.S. EPA, 2006a). It is 

generally recognized that children 

spend more time outdoors than adults, 

and, therefore, would be expected to 

have higher exposure to O3 than adults. 

Children aged 11 years and older and 

adults have higher absolute ventilation 

rates than younger children aged 1–11 

years. However, younger children have 

higher ventilation rates relative to their 

lung volumes, which tends to increase 

dose normalized to lung surface area. In 

all ages, exercise intensity has a 

substantial effect on ventilation rate, 

high intensity activity results in nearly 

double the ventilation rate for moderate 

activity. For more information on time 

spent outdoors and ventilation rate 

differences by age group, see section 

4.4.1 in the ISA (U.S. EPA, 2013a). 

The 1996 O3 AQCD reported clinical 

evidence that children, adolescents, and 

young adults (<18 years of age) appear, 

on average, to have nearly equivalent 

spirometric responses to O3 exposure, 

but have greater responses than middleaged 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 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, ultrastructural, and morphological levels. 

The studies conducted on infant 

monkeys are most relevant for assessing 

effects in children. Carey et al. (2007) 

conducted a study of O3 exposure in 

infant rhesus macaques, whose 

respiratory tract closely resemble that of 

humans. Monkeys were exposed either 

acutely or in episodes designed to 

mimic human exposure. All monkeys 

acutely exposed to O3 had moderate to 

marked necrotizing rhinitis, with focal 

regions of epithelial exfoliation, 

numerous infiltrating neutrophils, and 

some eosinophils. The distribution, 

character, and severity of lesions in 

episodically exposed infant monkeys 

were similar to that of acutely exposed 

animals. Neither exposure protocol for 

the infant monkeys produced mucous 

cell metaplasia proximal to the lesions, 

an adaptation observed in adult 

monkeys exposed in another study 

(Harkema et al., 1987). Functional and 

cellular changes in conducting airways 

were common manifestations of 

exposure to O3 among both the adult 

and infant monkeys (Plopper et al., 

2007). In addition, the lung growth of 

the distal conducting airways in the 

infant monkeys was significantly 

stunted by O3 and this aberrant 

development was persistent 6 months 

postexposure (Fanucchi et al., 2006). 

Age may also affect the inflammatory 

response to O3 exposure. Toxicological 

studies reported that the difference in 

effects among younger lifestage test 

animals may be due to age-related 

changes in antioxidants levels and 

sensitivity to oxidative stress. Further 

discussion of these studies may be 

found in section 8.3.1.1 of the ISA (U.S. 

EPA, 2013a, p. 8–18). 

The previous and recent human 

clinical and toxicological studies 

reported evidence of increased risk from 

O3 exposure for younger ages, which 

provides coherence and biological 

plausibility for the findings from 

epidemiologic studies. Although there 

was some inconsistency, generally, the 

epidemiologic studies reported positive 

associations among both children and 

adults or just among children. The 

interpretation of these studies is limited 

by the lack of consistency in 

comparison age groups and outcomes 

examined. However, overall, the 

epidemiologic, controlled human 

exposure, and toxicological studies 

provide adequate evidence that children 

are potentially at increased risk of O3- 

related health effects. 

iv. Older adults 

The ISA notes that older adults are at 

greater risk of health effects associated 

with O3 exposure through a variety of 

intrinsic pathways (U.S. EPA, 2013a, 

section 8.3.1.2). In addition, older adults 

may differ in their exposure and 

internal dose. Older adults were 

outdoors for a slightly longer proportion 

of the day than adults aged 18–64 years. 

For more information on time spent 

outdoors by age group, see Section 4.4 

in the ISA (U.S. EPA, 2013a). The 

gradual decline in physiological 

processes that occurs with aging may 

lead to increased risk of O3-related 

health effects (U.S. EPA, 2006a). 

Respiratory symptom responses to O3

exposure appears to increase with age 

until early adulthood and then 

gradually decrease with increasing age 

(U.S. EPA, 1996); lung function 

responses to O3 exposure also decline 

from early adulthood (U.S. EPA, 1996). 

The reductions of these responses with 

age may put older adults at increased 

risk for continued O3 exposure. In 

addition, older adults, in general, have 

a higher prevalence of preexisting 

diseases compared to younger age 

groups and this may also lead to 

increased risk of O3-related health 

effects (U.S. EPA, 2013a, section 

8.3.1.2). With the number of older 

Americans increasing in upcoming 

years (estimated to increase from 12.4% 

of the U.S. population to 19.7% between 

2000 to 2030, which is approximately 

35 million and 71.5 million individuals, 

respectively) this group represents a 

large population potentially at risk of 

O3-related health effects (SSDAN 

CensusScope, 2010a; U.S. Census 

Bureau, 2010). 

The majority of recent studies 

reported greater effects of short-term O3

exposure and mortality among older 

adults, which is consistent with the 

findings of the 2006 O3 AQCD. A study 

(Medina-Ramo ́n and Schwartz, 2008) 


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conducted in 48 cities across the U.S. 

reported larger effects among adults ≥65 

years old compared to those <65 years. 

Further investigation of this study 

population revealed a trend of O3- 

related mortality risk that gets larger 

with increasing age starting at age 51 

(Zanobetti and Schwartz, 2008a). 

Another study conducted in 7 urban 

centers in Chile reported similar results, 

with greater effects in adults ≥65 years 

old (Cakmak et al., 2007). More recently, 

a study conducted in the same area 

reported similar associations between 

O3 exposure and mortality in adults 

aged <64 years old and 65 to 74 years 

old, but the risk was increased among 

the older age group (Cakmak et al., 

2011). A study performed in China 

reported greater effects in populations 

≥45 years old (compared to 5 to 44 year 

olds), with statistically significant 

effects present only among those ≥65 

years old (Kan et al., 2008). An Italian 

study reported higher risk of all-cause 

mortality associated with increased O3

concentrations among individuals ≥85 

year old as compared to those 35 to 84 

years old (Stafoggia et al., 2010). The Air 

Pollution and Health: A European and 

North American Approach (APHENA) 

project examined the association 

between O3 exposure and mortality for 

those <75 and ≥75 years of age. In 

Canada, the associations for all-cause 

and cardiovascular mortality were 

greater among those ≥75 years old. In 

the U.S., the association for all-cause 

mortality was slightly greater for those 

<75 years of age compared to those ≥75 

years old in summer-only analyses. No 

consistent pattern was observed for CVD 

mortality. In Europe, slightly larger 

associations for all-cause mortality were 

observed in those <75 years old in allyear 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 (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. 

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 omega3 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 antioxidantsrich 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 a-tocopherol 

(a form of vitamin E) and ascorbate 

(vitamin C) on spirometric measures of 

lung function after O3 exposure but not 

on the intensity of subjective symptoms 

and inflammatory responses. Dietary 

antioxidants have also afforded partial 

protection to asthmatics by attenuating 

postexposure bronchial 

hyperresponsiveness. Toxicological 

studies discussed in section 8.4.1 of the 

ISA (U.S. EPA, 2013a) provide evidence 

of biological plausibility to the 

epidemiologic and controlled human 

exposure studies. 

Overall, the ISA (U.S. EPA, 2013a) 

concludes adequate evidence is 

available indicating that individuals 

with diets lower in vitamins C and E are 

at risk for O3-related health effects. The 

evidence from epidemiologic studies is 

supported by controlled human 

exposure and toxicological studies. 

vi. Outdoor Workers 

Studies included in the 2006 O3

AQCD reported that individuals who 

participate in outdoor activities or work 

outside to be a population at increased 

risk based on consistently reported 

associations between O3 exposure and 

respiratory health outcomes in these 

groups (U.S. EPA, 2006a). Outdoor 

workers are exposed to ambient O3

concentrations for a greater period of 

time than individuals who spend their 

days indoors. As discussed in section 

4.7 of the ISA (U.S. EPA, 2013a) outdoor 

workers sampled during the work shift 

had a higher ratio of personal exposure 

to fixed-site monitor concentrations 

than health clinic workers who spent 

most of their time indoors. Additionally, 

an increase in dose to the lower airways 

is possible during outdoor exercise due 

to both increases in the amount of air 

breathed (i.e., minute ventilation) and a 

shift from nasal to oronasal breathing. 

The association between FEV1 responses 

to O3 exposure and minute ventilation 

is discussed more fully in section 

6.2.3.1 of the 2006 O3 AQCD (U.S. EPA, 

2006a). 

Previous studies have shown that 

increased exposure to O3 due to outdoor 

work leads to increased risk of O3- 

related health effects, specifically 

decrements in lung function (U.S. EPA, 

2006a). The strong evidence from the 

2006 O3 AQCD, which demonstrated 

increased exposure, dose, and 

ultimately risk of O3-related health 

effects in this population, supports the 


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59As noted below (II.C.3.a.ii), asthmatics can 

experience larger O3-induced respiratory effects 

than non-asthmatic, healthy adults. The 

responsiveness of asthmatics to O3 exposures could 

depend on factors that have not been well-evaluated 

such as asthma severity, the effectiveness of asthma 

control, or the prevalence of medication use. 

60The 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/. 

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 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 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– 

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 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 


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61The 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). 

62The 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. 

63Exposure 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). 

64These 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. 

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 

from a study (Neidell and Kinney, 2010) 

conducted in the 1990’s in Los Angeles, 

CA reports increased asthma hospital 

admissions among children and older 

adults when O3 alert days (1-hour max 

O3 concentration >200 ppb) were 

excluded from the analysis of daily 

hospital admissions and O3

concentrations (presumably thereby 

eliminating averting behavior based on 

high O3 forecasts). If averting behavior 

reduces exposure to ambient O3, then 

epidemiologic studies that do not 

account for averting behavior may 

produce effect estimates that are biased 

toward the null due to exposure 

misclassification (U.S. EPA, 2013, 

section 4.6.6). 

C. Human Exposure and Health Risk 

Assessments 

To put judgments about health effects 

that are adverse for individuals into a 

broader public health context, the EPA 

has developed and applied models to 

estimate human exposures to O3 and O3- 

associated health risks. Exposure and 

risk estimates based on such models are 

presented and assessed in the HREA 

(U.S. EPA, 2014a). In reviewing the draft 

HREA, CASAC expressed the view that 

the document is ‘‘well-written, founded 

based upon comprehensive analyses 

and adequate for its intended purpose’’ 

(Frey, 2014a, p. 1). Analyses in the 

HREA inform consideration of the O3

exposures and health risks that could be 

allowed by the current standard and 

alternative standards, and consideration 

of the kind and degree of uncertainties 

inherent in estimates of O3 exposures 

and health risks. 

The following sections discuss the air 

quality adjustment approach used in the 

HREA for exposure and health risk 

estimates (II.C.1); the approach taken to 

estimate exposures, key 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, pp. 10 and 11). 

The HREA uses a photochemical 

model to estimate sensitivities of O3 to 

changes in precursor emissions in order 

to estimate ambient O3 concentrations 

that would just meet the current and 

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

Chapter 4).61 For the 15 urban study 

areas evaluated in the HREA,62 this 

model-based adjustment approach 

estimates hourly O3 concentrations at 

each monitor location when modeled 

U.S. anthropogenic precursor emissions 

(i.e., NOX, VOC) 63 are reduced. The 

HREA estimates air quality that just 

meets the current and alternative 

standards for the 2006–2008 and 2008– 

2010 periods.64

As discussed in Chapter 4 of the 

HREA (U.S. EPA, 2014a), this approach 

to adjusting air quality models the 

physical and chemical atmospheric 

processes that influence ambient O3

concentrations. Compared to the 

quadratic rollback approach used in 

previous reviews, it provides more 

realistic estimates of the spatial and 

temporal responses of O3 to reductions 

in precursor emissions. Because 

ambient NOX can contribute both to the 

formation and destruction of O3 (U.S. 

EPA, 2014a, Chapter 4), as discussed 

above, the response of ambient O3

concentrations to reductions in NOX

emissions is more variable than 

indicated by the quadratic rollback 

approach. This improved approach to 

adjusting O3 air quality is consistent 

with recommendations from the 

National Research Council of the 

National Academies (NRC, 2008). In 

addition, CASAC strongly supported the 

improved approach, stating that ‘‘the 

quadratic 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), modelbased adjustment to the current and 

alternative standards decreases 


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65Titration is also prominent during time periods 

when photochemistry is limited, and ambient O3

concentrations are relatively low, such as at night 

and on cool, cloudy days (U.S. EPA, 2014a, Chapter 

4). 

66 It is important to note that sensitivity analyses 

in the HREA indicate that the increases in low O3

concentrations are smaller when NOX and VOC 

emissions are reduced than when only NOX

emissions are reduced (U.S. EPA, 2014a, Appendix 

4–D, section 4.7). 

67 In addition, because epidemiology-based risk 

estimates use ‘‘area-wide’’ average O3

concentrations, calculated by averaging 

concentrations across multiple monitors in urban 

study areas (U.S. EPA, 2014c, section 3.2.3.2), risk 

estimates on a given day depend on the daily 

balance between increasing and decreasing O3

concentrations at the individual monitors that are 

averaged together to calculate the ‘‘area-wide’’ 

concentration. 

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. 

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 65), 

model-based adjustment increases 

ambient O3 concentrations compared to 

recent monitored concentrations (U.S. 

EPA, 2014a, section 4.3.3.2; Frey, 2014a, 

p. 10). 

Within urban study areas, the overall 

impacts of model-based air quality 

adjustment are to reduce the O3

concentrations at the upper ends of 

ambient distributions and to increase 

the O3 concentrations at the lower ends 

of those distributions (U.S. EPA, 2014a, 

section 4.3.3.2, Figures 4–9 and 4–10).66

Seasonal means of daily O3

concentrations generally exhibit only 

modest changes upon model 

adjustment, reflecting the seasonal 

balance between daily decreases in 

relatively higher concentrations and 

increases in relatively lower 

concentrations (U.S. EPA, 2014a, 

Figures 4–9 and 4–10). The resulting 

compression in the seasonal 

distributions of ambient O3

concentrations is evident in all of the 

urban study areas evaluated, though the 

degree of compression varies 

considerably across areas (U.S. EPA, 

2014a, Figures 4–9 and 4–10). 

This compression in the distributions 

of ambient O3 concentrations has 

important implications for exposure and 

risk estimates in urban study areas. 

Estimates influenced largely by the 

upper ends of the distribution of 

ambient concentrations (i.e., exposures 

of concern and lung function risk 

estimates, as discussed in sections 3.2.2 

and 3.2.3.1 of the PA (U.S. EPA, 2014c)) 

decrease with adjustment of air quality 

to the current and alternative standards. 

In contrast, seasonal risk estimates 

influenced by the full distribution of 

ambient O3 concentrations (i.e., 

epidemiology-based risk estimates, as 

discussed in section 3.2.3.2 of the PA) 

either decrease or increase in response 

to air quality adjustment, depending on 

the balance between the daily decreases 

in high O3 concentrations and increases 

in low O3 concentrations.67

In their review of the second draft 

HREA, CASAC considered this issue, in 

particular noting that ‘‘reductions in 

nitrogen oxides emissions can lead to 

less scavenging of ozone and free 

radicals, resulting in locally higher 

levels of ozone’’ (Frey, 2014a, p. 10). 

CASAC recommended that ‘‘the EPA 

should identify and discuss whether 

and to what extent health risks in the 

urban core may be affected by NOX

reductions or other possible strategies’’ 

and, in particular, concluded that it 

would ‘‘be of interest to learn if there 

would be any children or outdoor 

workers in the more urban areas who 

would experience significantly higher 

exposures to ozone as a result of 

possible changes in the ozone NAAQS’’ 

(Frey, 2014a, p. 10). Consistent with this 

advice, the exposure and risk 

implications of the spatial and temporal 

patterns of ambient O3 following air 

quality adjustment in urban study areas 

are discussed in the final HREA (U.S. 

EPA, 2014a, Chapter 9) and the final PA 

(U.S. EPA, 2014c, sections 3.2.2, 3.2.3), 

and are summarized below within the 

context of the PA’s consideration of 

exposure estimates (II.D.2.a) and risk 

estimates (II.D.2.b and II.D.2.c). 

2. Exposure Assessment 

This section discusses the HREA 

assessment of human exposures to O3. 

Section II.C.2.a provides an overview of 

the approach used in the HREA to 

assessing exposures and the approach in 

the PA to considering exposure 

estimates, and summarizes key results. 

Section II.C.2.b summarizes the 

important uncertainties in exposure 

estimates. 

a. Overview and Summary of Key 

Results 

The exposure assessment presented in 

the HREA (U.S. EPA, 2014a, Chapter 5) 

provides estimates of the number and 

percent of people exposed to various 

concentrations of ambient O3, while at 

specified exertion levels. The HREA 

estimates exposures in the 15 urban 

study areas for four study groups, all 

school-age children (ages 5 to 18), 

asthmatic school-age children, 

asthmatic adults (ages 19 to 95), and all 

older adults (ages 65 to 95), reflecting 

the evidence indicating that these 

populations are at increased risk for O3- 

attributable effects (U.S. EPA, 2013a, 

Chapter 8). An important purpose of 

these exposure estimates is to provide 

perspective on the extent to which air 

quality adjusted to just meet the current 

O3 NAAQS could be associated with 

exposures to O3 concentrations reported 

to result in respiratory effects.68

Estimates of such ‘‘exposures of 

concern’’ provide perspective on the 

potential public health impacts of O3- 

related effects, including effects that 

cannot currently be evaluated in a 

quantitative risk assessment.69

In the absence of large scale exposure 

studies that encompass the general 

population, as well as at-risk 

populations, modeling is the preferred 

approach to estimating exposures to O3

(U.S. EPA, 2014a, Chapter 5). The use of 

exposure modeling also facilitates the 

estimation of exposures resulting from 

ambient O3 concentrations differing 

from those present during exposure 

studies. In the HREA, population 

exposures to ambient O3 concentrations 

are estimated using the current version 

of the Air Pollutants Exposure (APEX) 

model. The APEX model simulates the 

movement of individuals through time 

and space and estimates their exposures 

to a given pollutant in indoor, outdoor, 

and in-vehicle microenvironments (U.S. 

EPA, 2014a, section 5.1.3). APEX takes 

into account important factors that 

contribute to total human exposure to 

ambient O3, including the temporal and 

spatial distributions of people and O3

concentrations throughout an urban 

area, the variation of O3 concentrations 

within various microenvironments, and 

the effects of exertion on breathing rate 

in exposed individuals (U.S. EPA, 

2014a, section 5.1.3). To the extent 

spatial and/or temporal patterns of 

ambient O3 concentrations are altered 

upon model adjustment, as discussed 

above, exposure estimates reflect 

population exposures to those altered 

patterns. 

The HREA estimates 8-hour exposures 

at or above benchmark concentrations of 


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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 importance, from a 

public health perspective, of the health 

effects that are known or can reasonably 

be inferred to occur as a result of 

exposures at benchmark concentrations 

(U.S. EPA, 2014c, sections 3.1.3, 3.1.5). 

As discussed in more detail above 

(II.B.2), the health evidence that 

supports evaluating exposures of 

concern at or above benchmark 

concentrations of 60, 70, and 80 ppb 

comes from a large body of controlled 

human exposure studies reporting a 

variety of respiratory effects in healthy 

adults. The lowest O3 exposure 

concentration for which controlled 

human exposure studies have reported 

respiratory effects in healthy adults is 

60 ppb (based on changes in group 

mean responses), with more evidence 

supporting this benchmark 

concentration in the current review than 

in the last review. In healthy adults, 6.6 

hour exposures to 60 ppb O3 have been 

reported to decrease lung function and 

to increase airway inflammation. 

Exposures of healthy adults to 72 ppb 

O3 for 6.6 hours have been reported to 

result in larger average lung function 

decrements, compared to 60 ppb, as 

well as in increased respiratory 

symptoms. Exposures of healthy adults 

to 80 ppb O3 for 6.6 hours have been 

reported to result in larger average lung 

function decrements than following 

exposures to 60 or 72 ppb and, 

depending on the study, to increase 

airway inflammation, increase 

respiratory symptoms, increase airways 

responsiveness, and decrease lung host 

defense (based on changes in group 

means) (U.S. EPA, 2014c, section 

3.1.2.1). In commenting on the evidence 

for benchmark concentrations, CASAC 

stated the following (Frey, 2014c, p. 6): 

The 80 ppb-8hr benchmark level represents 

an exposure level for which there is 

substantial clinical evidence demonstrating a 

range of ozone-related effects including lung 

inflammation and airway responsiveness in 

healthy individuals. The 70 ppb-8hr 

benchmark level reflects the fact that in 

healthy subjects, decreases in lung function 

and respiratory symptoms occur at 

concentrations as low as 72 ppb and that 

these effects almost certainly occur in some 

people, including asthmatics and others with 

low lung function who are less tolerant of 

such effects, at levels of 70 ppb and below. 

The 60 ppb-8hr benchmark level represents 

the lowest exposure level at which ozonerelated 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. 

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. 

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 

exposed 70

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 0 100 72 [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 


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70Estimates 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). 71Numbers of children exposed in each urban 

case study area were averaged over the years 2006 

to 2010. These averages were then summed across 

urban study areas. Numbers were rounded to 

nearest thousand unless otherwise indicated. 

Estimates smaller than 50 were rounded downward 

to zero (from U.S. EPA, 2014a, Appendix 5F Table 

5F–5). See below for discussion of uncertainties in 

exposure estimates. 

72As discussed in section 4.3.3 of the HREA, the 

model-based air quality adjustment approach used 

to estimate risks associated with the current and 

alternative standards was unable to estimate the 

distribution of ambient O3 concentrations in New 

York City upon just meeting an alternative standard 

with a level of 60 ppb. Therefore, for the 60 ppb 

standard level the numbers of children and 

asthmatic children reflect all of the urban study 

areas except New York. 

73As noted below (II.C.3.a.ii), in the case of 

asthmatics, responsiveness to O3 could depend on 

factors that have not been well-evaluated, such as 

asthma severity, the effectiveness of asthma control, 

or the prevalence of medication use. 

74The exception to this is lung function 

decrements, as discussed below (and in U.S. EPA, 

2014c, section 3.2.3.1). 

TABLE 1—SUMMARY OF ESTIMATED EXPOSURES OF CONCERN IN ALL SCHOOL-AGE CHILDREN FOR THE CURRENT AND

ALTERNATIVE O3 STANDARDS IN URBAN STUDY AREAS—Continued 

Benchmark 

concentration 

Standard level 

(ppb) 

Average % 

children 

exposed 70

Average number of children exposed 

[average number of asthmatic children] 71

% Children— 

worst year and 

worst area 

60 0–1.2 70,000 [8,000] .............................................................................................. 2.2 

Two or more exposures of concern per season 

≥80 ppb ............. 75 0 600 [100] ...................................................................................................... 0.1 

70 0 0 [0] .............................................................................................................. 0 

65 0 0 [0] .............................................................................................................. 0 

60 0 0 [0] .............................................................................................................. 0 

≥70 ppb ............. 75 0.1–0.6 46,000 [5,000] .............................................................................................. 2.2 

70 0–0.1 5,400 [600] ................................................................................................... 0.4 

65 0 300 [100] ...................................................................................................... 0 

60 0 0 [0] .............................................................................................................. 0 

≥60 ppb ............. 75 3.1–7.6 865,000 [93,000] .......................................................................................... 14.4 

70 0.5–3.5 320,000 [35,000] .......................................................................................... 9.2 

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 healthrelevant exposures as a continuum with 

greater confidence and less uncertainty 

about the existence of adverse health 

effects at higher O3 exposure 

concentrations, and less confidence and 

greater uncertainty as one considers 

lower exposure concentrations. This 

view draws from the overall body of 

available health evidence, which 

indicates that as exposure 

concentrations increase, the incidence, 

magnitude, and severity of effects 

increases. 

Though the PA indicates less 

confidence in the likelihood of adverse 

health effects as O3 exposure 

concentrations decrease, it also notes 

that the controlled human exposure 

studies that provided the basis for 

health benchmark concentrations have 

not evaluated at-risk populations. 

Compared to the healthy individuals 

included in controlled human exposure 

studies, members of at-risk populations 

(e.g., asthmatics, children) could be 

more likely to experience adverse 

effects, could experience larger and/or 

more serious effects, and/or could 

experience effects following exposures 

to lower O3 concentrations. The CASAC 

expressed similar views in their advice 

to the Administrator (Frey, 2014a, pp. 7 

and 14). In considering estimated 

exposures of concern (U.S. EPA, 2014c, 

section 3.4), the PA notes that concerns 

about the potential for adverse health 

effects, including effects in at-risk 

populations must be balanced against 

the increasing uncertainty regarding the 

likelihood of such effects following 

exposures to lower O3 concentrations. 

Uncertainties associated with the 

APEX exposure modeling also have the 

potential to be important (U.S. EPA, 

2014a, section 5.5.2, Table 5–6). For 

example, the HREA concludes that 

exposures of concern could be 

underestimated for some individuals 

who are frequently and routinely active 

outdoors during the warm season (U.S. 

EPA, 2014a, section 5.5.2). This could 

include outdoor workers and children 

who are frequently active outdoors. The 

HREA specifically notes that long-term 

diary profiles (i.e., monthly, annual) do 

not exist for such populations, limiting 

the extent to which APEX outputs 

reflect people who follow similar daily 

routines resulting in high exposures, 

over extended periods of time. 

In order to evaluate one dimension of 

the potential implications of this 

uncertainty for exposure estimates, the 

HREA reports the results of limited 

exposure model sensitivity analyses 

using subsets of activity diaries 

specifically selected to reflect groups 

spending a larger proportion of time 

being active outdoors during the O3

season. When diaries were selected to 

mimic activity patterns performed by 

outdoor workers, the percent of 

modeled individuals estimated to 

experience exposures of concern was 

higher than the other adult populations 

evaluated. The percentages of outdoor 

workers estimated to experience 

exposures of concern 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 


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75Estimates 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. 

76Analysis 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). 

77Though see below for discussion of uncertainty 

in lung function responses of children and 

asthmatics. 

addition, when diaries were restricted to 

children who did not report any time 

spent inside a school or performing paid 

work (i.e., to mimic children spending 

large portions of their time outdoors 

during the summer), the number 

experiencing exposures of concern 

increased by approximately 30% (U.S. 

EPA, 2014a, section 5.4.3.1). Though 

these sensitivity analyses are limited to 

single urban study areas, and though 

there is uncertainty associated with 

diary selection approaches to mimic 

highly exposed populations, they 

suggest the possibility that some at-risk 

groups could experience more frequent 

exposures of concern than indicated by 

estimates made using the full database 

of activity diary profiles. 

In further considering activity diaries, 

the HREA also notes that growing 

evidence indicates that people can 

change their behavior in response to 

high O3 concentrations, reducing the 

time spent being active outdoors (U.S. 

EPA, 2014a, section 5.4.3.3). Commonly 

termed ‘‘averting behaviors,’’ these 

altered activity patterns could reduce 

personal exposure concentrations. 

Therefore, the HREA also performed 

limited sensitivity analyses to evaluate 

the potential implications of averting 

behavior for estimated exposures of 

concern. These analyses suggest that 

averting behavior could reduce the 

percentages of children estimated to 

experience exposures of concern at or 

above the 60 or 70 ppb benchmark 

concentrations by approximately 10 to 

30%, with larger reductions possible for 

the 80 ppb benchmark (U.S. EPA, 2014a, 

Figure 5–15). As discussed above for 

other sensitivity analyses, these 

analyses are limited to a single urban 

case study area and are subject to 

uncertainties associated with 

assumptions about the prevalence and 

duration of averting behaviors. 

However, the results suggest that 

exposures of concern could be 

overestimated, particularly in children 

(Neidell, 2009; U.S. EPA, 2013, Figures 

4–7 and 4–8), if the possibility for 

averting behavior is not incorporated 

into estimates. 

3. Quantitative Health Risk Assessments 

For some health endpoints, there is 

sufficient scientific evidence and 

information available to support the 

development of quantitative estimates of 

O3-related health risks. In the last 

review of the O3 NAAQS, the 

quantitative health risk assessment 

estimated O3-related lung function 

decrements, respiratory symptoms, 

respiratory-related hospital admissions, 

and nonaccidental and 

cardiorespiratory-related mortality (U.S. 

EPA, 2007). In those analyses, both 

controlled human exposure and 

epidemiologic studies were used for the 

quantitative assessment of O3-related 

human health risks. 

In the current review, for short-term 

O3 concentrations, the HREA estimates 

lung function decrements; respiratory 

symptoms in asthmatics; hospital 

admissions and emergency department 

visits for respiratory causes; and allcause 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. 

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 

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 


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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. 

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. 

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 % 

children 78

Number of children (5 to 18 years) 

[number of asthmatic children] 79

% Children 

worst year and 

area 

One or more decrements per season 

≥10% ................. 75 14–19 3,007,000 [312,000] ..................................................................................... 22 

70 11–17 2,527,000 [261,000] ..................................................................................... 20 

65 3–15 1,896,000 [191,000] ..................................................................................... 18 

60 5–11 1,404,000 [139,000] 80 .................................................................................. 13 

≥15% ................. 75 3–5 766,000 [80,000] .......................................................................................... 7 

70 2–4 562,000 [58,000] .......................................................................................... 5 

65 0–3 356,000 [36,000] .......................................................................................... 4 

60 1–2 225,000 [22,000] .......................................................................................... 3 

≥20% ................. 75 1–2 285,000 [30,000] .......................................................................................... 2.8 

70 1–2 189,000 [20,000] .......................................................................................... 2.1 

65 0–1 106,000 [11,000] .......................................................................................... 1.4 

60 0–1 57,000 [6,000] .............................................................................................. 0.9 

Two or more decrements per season 

≥10% ................. 75 7.5–12 1,730,000 [179,000] ..................................................................................... 14 

70 5.5–11 1,414,000 [145,000] ..................................................................................... 13 

65 1.3–8.8 1,023,000 [102,000] ..................................................................................... 11 

60 2.1–6.4 741,000 [73,000] .......................................................................................... 7.3 

≥15% ................. 75 1.7–2.9 391,000 [40,000] .......................................................................................... 3.8 

70 0.9–2.4 276,000 [28,000] .......................................................................................... 3.1 

65 0.1–1.8 168,000 [17,000] .......................................................................................... 2.3 

60 0.2–1.0 101,000 [10,000] .......................................................................................... 1.4 

≥20% ................. 75 0.5–1.1 128,000 [13,000] .......................................................................................... 1.5 

70 0.3–0.8 81,000 [8,000] .............................................................................................. 1.1 

65 0–0.5 43,000 [4,000] .............................................................................................. 0.8 

60 0–0.2 21,000 [2,000] .............................................................................................. 0.4 

ii. Key Uncertainties 

As for exposures of concern discussed 

above, the PA also considers important 

uncertainties in estimates of lung 

function risk. In addition to the 

uncertainties noted for exposure 

estimates, the HREA identifies several 

key uncertainties associated with 

estimates of O3-induced lung function 

decrements. An uncertainty with 

particular potential to impact 

consideration of risk estimates stems 

from the lack of exposure-response 

information in children. In the near 

absence of controlled human exposure 

data for children, risk estimates are 

based on the assumption that children 

exhibit the same lung function response 

following O3 exposures as healthy 18 

year olds (i.e., the youngest age for 

which controlled human exposure data 

is available) (U.S. EPA, 2014a, section 

6.5.3). This assumption is justified in 

part by the findings of McDonnell et al. 

(1985), who reported that children (8–11 

years old) experienced FEV1 responses 

similar to those observed in adults (18– 

35 years old). In addition, as discussed 

in the ISA (U.S. EPA, 2013a, section 

6.2.1), summer camp studies of schoolaged children reported O3-induced lung 

function decrements similar in 

magnitude to those observed in 

controlled human exposure studies 

using adults. In extending the risk 

model to children, the HREA fixes the 

age term in the model at its highest 

value, the value for age 18. This 

approach could result in either over- or 

underestimates of O3-induced lung 

function decrements in children, 

depending on how children compare to 

the adults used in controlled human 

exposure studies (U.S. EPA, 2014a, 

section 6.5.3). 

A related source of uncertainty is that 

the risk assessment estimates O3- 

induced decrements in asthmatics using 

the exposure-response relationship 

developed from data collected from 

healthy individuals. Although the 

evidence has been mixed (U.S. EPA, 

2013a, section 6.2.1.1), several studies 

have reported larger O3-induced lung 

function decrements in asthmatics than 

in non-asthmatics (Kreit et al., 1989; 

Horstman et al., 1995; Jorres et al., 1996; 

Alexis et al., 2000). On this issue, 

CASAC noted that ‘‘[a]sthmatic subjects 

appear to be at least as sensitive, if not 

more sensitive, than non-asthmatic 

subjects in manifesting ozone-induced 

pulmonary function decrements’’ (Frey, 

2014c, p. 4). To the extent asthmatics 

experience larger O3-induced lung 

function decrements than the healthy 

adults used to develop exposureresponse 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, 


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81The 12 urban areas evaluated are Atlanta, 

Baltimore, Boston, Cleveland, Denver, Detroit, 

Houston, Los Angeles, New York, Philadelphia, 

Sacramento, and St. Louis. 

82 In the epidemiologic studies that provide the 

health basis for HREA risk assessments, 

concentration-response relationships are based on 

daytime O3 concentrations, averaged across 

multiple monitors within study areas. These daily 

averages are used as surrogates for the spatial and 

temporal patterns of exposures in study 

populations. Consistent with this approach, the 

HREA epidemiologic-based risk estimates also 

utilize daytime O3 concentrations, averaged across 

monitors, as surrogates for population exposures. In 

this notice, we refer to these averaged 

concentrations as ‘‘area-wide’’ O3 concentrations. 

Area-wide concentrations are discussed in more 

detail in section 3.1.4 of the PA (U.S. EPA, 2014c). 

section 6.5.4), and could depend on a 

variety of factors that have not been 

well-evaluated, including the severity of 

asthma and the prevalence of 

medication use. However, the available 

evidence shows responses to O3

increase with severity of asthma 

(Horstman et al., 1995) and 

corticosteroid usage does not prevent O3

effects on lung function decrements or 

respiratory symptoms in people with 

asthma (Vagaggini et al., 2001, 2007). 

b. Mortality and Morbidity Risk 

Assessments 

As discussed above (II.B.2), 

epidemiologic studies provide evidence 

for the most serious O3-associated 

public health outcomes (e.g., mortality, 

hospital admissions, emergency 

department visits). Section II.C.3.b.i 

below provides an overview of the 

approach used in the HREA to assessing 

mortality and morbidity risks based on 

information from epidemiologic studies, 

discusses the approach in the PA to 

considering epidemiology-based risk 

estimates, and presents a summary of 

key results. Section II.C.3.b.ii 

summarizes key uncertainties in 

epidemiology-base risk estimates. 

i. Overview and Summary of Key 

Results 

Risk estimates based on 

epidemiologic studies can provide 

perspective on the most serious O3- 

associated public health outcomes (e.g., 

mortality, hospital admissions, 

emergency department visits) in 

populations that often include at-risk 

groups. The HREA estimates O3- 

associated risks in 12 urban study 

areas 81 using concentration-response 

relationships drawn from epidemiologic 

studies. These concentration-response 

relationships are based on ‘‘area-wide’’ 

average O3 concentrations.82 The HREA 

estimates risks for the years 2007 and 

2009 in order to provide estimates of 

risk for a year with generally higher O3

concentrations (2007) and a year with 

generally lower O3 concentrations 

(2009) (U.S. EPA, 2014a, section 7.1.1). 

As in the last review of the O3

NAAQS (U.S. EPA, 2007, pp. 2–48 to 2– 

54), the PA recognizes that ambient O3

concentrations, and therefore O3- 

associated health risks, result from 

precursor emissions from various types 

of sources. Based on the air quality 

modeling discussed in chapter 2 of the 

PA (U.S. EPA, 2014c), approximately 30 

to 60% of average daytime O3 during the 

warm season (i.e., daily maximum 8- 

hour concentrations averaged from 

April to October) is attributable to 

precursor emissions from U.S. 

anthropogenic sources (U.S. EPA, 

2014c, section 2.4.4). The remainder is 

attributable to precursor emissions from 

international anthropogenic sources and 

natural sources. Because the HREA 

characterizes health risks from all O3, 

regardless of source, risk estimates 

reflect emissions from U.S. 

anthropogenic, international 

anthropogenic, and natural sources. 

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 epidemiologicbased 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 longterm O3 exposures, primarily because 

that analysis is based on only one study 

(even though that study is welldesigned) and because of the 

uncertainty in that study about the 

existence and level of a potential 

threshold in the concentration-response 

function (U.S. EPA, 2014a, section 9.6). 

In considering the epidemiologybased risk estimates, the PA focuses on 

mortality risks associated with shortterm 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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83Table 3 is based on the information in Figures 

7–2 and 7–3 in the HREA (U.S. EPA, 2014a). 

Estimates of the numbers of O3-associated deaths 

are based on concentration-response relationships 

for total mortality associated with short-term O3

from the study by Smith et al. (2009). Estimates of 

the numbers O3-associated deaths are rounded to 

the nearest hundred, unless otherwise indicated. 

84As discussed in section 4.3.3 of the HREA, the 

model-based air quality adjustment approach used 

to estimate risks associated with the current and 

alternative standards was unable to estimate the 

distribution of ambient O3 concentrations in New 

York City upon just meeting an alternative standard 

with a level of 60 ppb. Therefore, the total number 

of deaths indicated for the 60 ppb standard level 

reflect the 60 ppb estimates for all urban study areas 

except New York City. For New York City, the 

estimated number of O3-associated deaths for the 60 

ppb standard level was assumed to be equal to the 

number for the 65 ppb level. 

85The 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). 

86There 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. 

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 ppb 84 .......................................................................................................... 6,400 6,400 2,300 10 

2009 

75 ppb .............................................................................................................. 7,000 7,000 4,700 270 

70 ppb .............................................................................................................. 6,900 6,900 4,300 80 

65 ppb .............................................................................................................. 6,400 6,400 2,600 40 

60 ppb .............................................................................................................. 6,300 6,300 2,100 10 

ii. Key Uncertainties 

Compared to estimates of O3

exposures of concern and estimates of 

O3-induced lung function decrements 

(discussed above), the HREA 

conclusions reflect lower confidence in 

epidemiologic-based risk estimates (U.S. 

EPA, 2014a, section 9.6). In particular, 

the HREA highlights the heterogeneity 

in effect estimates between locations, 

the potential for exposure measurement 

errors, and uncertainty in the 

interpretation of the shape of 

concentration-response 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 concentrationresponse function (U.S. EPA, 2014a, 

section 9.6).85 86 This section further 

discusses some of the key uncertainties 

in epidemiologic-based risk estimates, 

as summarized in the PA (U.S. EPA, 

2014c, section 3.2.3.2), with a focus on 

uncertainties that can have particularly 

important implications for the 

Administrator’s consideration of 

epidemiology-based risk estimates. 

The PA notes that reducing NOX

emissions generally reduces O3- 

associated mortality and morbidity risk 

estimates in locations and time periods 

with relatively high ambient O3

concentrations and increases risk 

estimates in locations and time periods 

with relatively low concentrations 

(II.C.1, above). When evaluating 

uncertainties in epidemiologic risk 

estimates, it is important to consider (1) 

The extent to which the O3 response to 

reductions in NOX emissions 

appropriately represents the trends 

observed in ambient O3 following actual 

reductions in NOX emissions; (2) the 

extent to which estimated changes in 

risks in urban study areas are 

representative of the changes that would 

be experienced broadly across the U.S. 

population; and (3) the extent to which 

the O3 response to reductions in 

precursor emissions could differ with 

emissions reduction strategies that are 

different from those used in HREA to 

generate risk estimates. 

To evaluate the first issue, the HREA 

conducted a national analysis 

evaluating trends in monitored ambient 

O3 concentrations during a time period 

when the U.S. experienced large-scale 

reductions in NOX emissions (i.e., 2001 

to 2010). Analyses of trends in 

monitored O3 indicate that over such a 

time period, the upper end of the 

distribution of monitored O3

concentrations (i.e., indicated by the 

95th percentile) generally decreased in 

urban and non-urban locations across 

the U.S. (U.S. EPA, 2014a, Figure 8–29). 

During this same time period, median 

O3 concentrations decreased in 

suburban and rural locations, and in 

some urban locations. However, median 

concentrations increased in some large 

urban centers (U.S. EPA, 2014a, Figure 

8–28). As discussed in the REA, and 

above (II.C.1), these increases in median 

concentrations likely reflect the 

increases in relatively low O3

concentrations that can occur near 

important sources of NOX upon 

reductions in NOX emissions (U.S. EPA, 

2014a, section 8.2.3.1). These patterns of 

monitored O3 during a period when the 

U.S. experienced large reductions in 

NOX emissions are qualitatively 

consistent with the modeled responses 

of O3 to reductions in NOX emissions. 

To evaluate the second issue, the 

HREA conducted national air quality 

modeling analyses. These analyses 

estimated the proportion of the U.S. 

population living in locations where 

seasonal averages of daily O3

concentrations are estimated to decrease 

in response to reductions in NOX

emissions, and the proportion living in 

locations where such seasonal averages 

are estimated to increase. Given the 

close relationship between changes in 


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87The exceptions are Chicago and Denver, for 

which the HREA risk estimates are based on 

reductions in both NOX and VOC (U.S. EPA, 2014a, 

section 4.3.3.1). Emissions of NOX and VOC were 

reduced by equal percentages, a scenario not likely 

to reflect the optimal combination for reducing 

risks. 

88This 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). 

89A 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). 

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 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. 

An important uncertainty is the shape 

of concentration-response functions at 

low ambient O3 concentrations (U.S. 

EPA, 2014a, Table 7–4).89 Consistent 

with the ISA conclusion that there is no 

discernible population threshold in O3- 

associated health effects, the HREA 

estimates epidemiology-based mortality 

and morbidity risks for entire 

distributions of ambient O3

concentrations, based on the 

assumption that concentration-response 

relationships remain linear over those 

distributions. In addition, in recognition 

of the ISA conclusion that certainty in 

the shape of O3 concentration-response 

functions decreases at low ambient 

concentrations, the HREA also estimates 

total mortality associated with various 

ambient O3 concentrations. The PA 

considers both types of risk estimates, 

recognizing greater public health 

concern for adverse O3-attributable 

effects at higher ambient O3

concentrations (which drive higher 

exposure concentrations, section 3.2.2 

of the PA (U.S. EPA, 2014c)), as 

compared to lower concentrations. 

A related uncertainty is that 

associated with the public health 

importance of the increases in relatively 

low O3 concentrations following air 

quality adjustment. This uncertainty 

relates to the assumption that the 

concentration response function for O3

is linear, such that that total risk 

estimates are equally influenced by 

decreasing high concentrations and 

increasing low concentrations, when the 

increases and decreases are of equal 

magnitude. Even on days with increases 

in relatively low area-wide average 

concentrations, resulting in increases in 

estimated risks, some portions of the 

urban study areas could experience 

decreases in high O3 concentrations. To 

the extent adverse O3-attributable effects 

are more strongly supported for higher 

ambient concentrations (which are 

consistently reduced upon air quality 

adjustment), the impacts on risk 

estimates of increasing low O3

concentrations reflect an important 

source of uncertainty. 

The HREA also notes important 

uncertainties associated with using a 

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, concentrationresponse 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 

concentration-response functions from 

epidemiologic studies to adjusted air 

quality. Concentration-response 

functions from the O3 epidemiologic 

studies used in the HREA are based on 

associations between day to day 

variation in ‘‘area-wide’’ O3

concentrations (i.e., averaged across 

multiple monitors) and variation in 

health effects. Epidemiologic studies 

use these area-wide O3 concentrations, 

which reflect the particular spatial and 

temporal patterns of ambient O3 present 

in study locations, as surrogates for the 

pattern of O3 exposures experienced by 

study populations. To the extent 

adjusting O3 concentrations to just meet 

the current standard results in 

important alterations in the spatial and/ 


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90The PA focuses on panel studies that used onsite 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). 

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 concentrationresponse function to estimate 

respiratory mortality associated with 

long-term O3 is a source of uncertainty. 

Risk estimates generated in sensitivity 

analyses using region-specific effect 

estimates differ substantially from the 

core estimates based on a single 

national-level effect estimate (U.S. EPA, 

2014a; Table 7–14). Furthermore, the 

risk estimates generated using the 

regional effect estimates display 

considerable variability across urban 

study areas (U.S. EPA, 2014a; Table 7– 

14), reflecting the substantial variability 

in the underlying effect estimates (see 

Jerrett et al., 2009, Table 4). While the 

results of the HREA sensitivity analyses 

evaluating this uncertainty point to the 

potential for regional heterogeneity in 

the long-term risk estimates, the 

relatively large confidence intervals 

associated with regional effect estimates 

resulted in the HREA conclusion that 

staff does not have confidence in the 

regionally based risk estimates 

themselves. 

Finally, the HREA does not quantify 

any reductions in risk that could be 

associated with reductions in the 

ambient concentrations of pollutants 

other than O3, resulting from control of 

NOX. For example, as discussed in 

chapter 2 of the PA (U.S. EPA, 2014c), 

NOX emissions contribute to ambient 

NO2, and NOX and VOCs can contribute 

to secondary formation of PM2.5

constituents, including ammonium 

sulfate (NH4SO4), ammonium nitrate 

(NH4NO3), and organic carbon (OC). 

Therefore, at some times and in some 

locations, control strategies that would 

reduce NOX emissions (i.e., to meet an 

O3 standard) could reduce ambient 

concentrations of NO2 and PM2.5, 

resulting in health benefits beyond 

those directly associated with reducing 

ambient O3 concentrations. In issuing its 

advice, CASAC likewise noted the 

potential reductions in criteria 

pollutants other than ozone as a result 

of NOx reductions, and the resulting 

potential public health benefits (Frey, 

2014a, pp. 10 and 11). 

D. Conclusions on the Adequacy of the 

Current Primary Standard 

The initial issue to be addressed in 

the current review of the primary O3

standard is whether, in view of the 

advances in scientific knowledge and 

additional information, the existing 

standard should be revised. In 

evaluating whether it is appropriate to 

retain or revise the current standard, the 

Administrator’s considerations build 

upon those in the 2008 review, 

including consideration of the broader 

body of scientific evidence and 

exposure and health risk information 

now available, as summarized above 

(II.A to II.C). 

In developing conclusions on the 

adequacy of the current primary O3

standard, the Administrator takes into 

account both evidence-based and 

quantitative exposure- and risk-based 

considerations. Evidence-based 

considerations include the assessment 

of evidence from controlled human 

exposure, animal toxicological, and 

epidemiologic studies for a variety of 

health endpoints. The Administrator 

focuses on health endpoints for which 

the evidence is strong enough to support 

a ‘‘causal’’ or a ‘‘likely to be causal’’ 

relationship, based on the ISA’s 

integrative synthesis of the entire body 

of evidence. The Administrator’s 

consideration of quantitative exposure 

and risk information draws from the 

results of the exposure and risk 

assessments presented in the HREA. 

The Administrator’s consideration of 

the evidence and exposure/risk 

information is informed by the 

considerations and conclusions 

presented in the PA (U.S. EPA, 2014c). 

The purpose of the PA is to help ‘‘bridge 

the gap’’ between the scientific and 

technical information assessed in the 

ISA and HREA, and the policy decisions 

that are required of the Administrator 

(U.S. EPA, 2014c, Chapter 1). The PA’s 

evidence-based and exposure-/riskbased 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. 

1. Summary of Evidence-Based 

Considerations in the PA 

In considering the available scientific 

evidence, the PA evaluates the O3

concentrations in health effects studies 

(U.S. EPA, 2014c, section 3.1.4). 

Specifically, the PA characterizes the 

extent to which effects have been 

reported for the O3 exposure 

concentrations evaluated in controlled 

human exposure studies and over the 

distributions of ambient O3

concentrations in locations where 

epidemiologic studies have been 

conducted. These considerations, as 

they relate to the adequacy of the 

current standard, are presented in detail 

in section 3.1.4 of the PA (U.S. EPA, 

2014c) and are summarized briefly 

below for controlled human exposure 

and epidemiologic panel studies 

(II.D.1.a), epidemiologic studies of 

short-term O3 exposures (II.D.1.b), and 

epidemiologic studies of long-term O3

exposures (II.D.1.c). Section II.D.1.d 

summarizes the PA conclusions based 

on consideration of the scientific 

evidence. 

a. Concentrations in Controlled Human 

Exposure and Panel Studies 

The evidence from controlled human 

exposure studies and panel studies is 

assessed in section 6.2 of the ISA (U.S. 

EPA, 2013a) and is summarized in 

section 3.1.2 of the PA (U.S. EPA, 

2014c). As discussed above (II.B), 

controlled human exposure studies have 

generally been conducted with young, 

healthy adults, and have evaluated 

exposure durations less than 8 hours. 

Panel studies have evaluated a wider 

range of study populations, including 

children, and have generally evaluated 

associations with O3 concentrations 

averaged over several hours (U.S. EPA, 

2013a, section 6.2.1.2).90

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 


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91As 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. 

92These 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 atrisk groups. See National Environmental 

Development Ass’n Clean Air Project v. EPA, 686 

F. 3d 803, 811 (D.C. Cir. (2012) (making this point). 

93Nonetheless, 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. 

94See 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). 

inflammation following exposures to O3

concentrations below 75 ppb. Table 3– 

1 in the PA highlights the group mean 

results of individual controlled human 

exposure studies that have evaluated 

exposures of healthy adults to O3

concentrations below 75 ppb (U.S. EPA, 

2014c). The studies included in Table 

3–1 of the PA indicate a combination of 

lung function decrements and 

respiratory symptoms following 6.6 

hour exposures to O3 concentrations as 

low as 72 ppb, and lung function 

decrements and airway inflammation 

following 6.6 hour exposures to O3

concentrations as low as 60 ppb (based 

on group means). 

The PA also notes consistent results 

in some panel studies of O3-associated 

lung function decrements. In particular, 

the PA notes that epidemiologic panel 

studies in children and adults 

consistently indicate O3-associated lung 

function decrements when on-site 

monitored concentrations were below 

75 ppb, although the evidence becomes 

less consistent at lower O3

concentrations (U.S. EPA, 2014c, 

section 3.1.4.1).91

Thus, controlled human exposure 

studies and panel studies have reported 

respiratory effects in adults and 

children following exposures to O3

concentrations below 75 ppb (albeit 

over shorter averaging periods than the 

8 hour averaging time of the current O3

standard). The PA notes that such 

impairments in respiratory function 

have the potential to be adverse, based 

on ATS guidelines for adversity and 

based on advice from CASAC (Frey, 

2014c, pp. 5 and 6) (U.S. EPA, 2014c, 

section 3.1.3). In addition, the PA notes 

that if they become serious enough, 

these respiratory effects could lead to 

the types of clearly adverse effects 

commonly reported in O3 epidemiologic 

studies (e.g., respiratory emergency 

department visits, hospital admissions). 

Therefore, the PA concludes that the 

respiratory effects experienced 

following exposures to O3

concentrations lower than 75 ppb could 

be adverse in some individuals, 

particularly if experienced by members 

of at-risk populations (e.g., people with 

asthma, children).92

b. Concentrations in Epidemiologic 

Studies—Short-Term 

The PA also considers distributions of 

ambient O3 concentrations in locations 

where epidemiologic studies have 

evaluated O3-associated hospital 

admissions, emergency department 

visits, and/or mortality (U.S. EPA, 

2014c, section 3.1.4.2). When 

considering epidemiologic studies 

within the context of the current 

standard, the PA emphasizes those 

studies conducted in the U.S. and 

Canada. Such studies reflect air quality 

and exposure patterns that are likely 

more typical of the U.S. population than 

the air quality and exposure patterns 

reflected in studies conducted outside 

the U.S. and Canada (U.S. EPA, 2014c, 

section 1.3.1.2).93 The PA also 

emphasizes studies reporting 

associations with effects judged in the 

ISA to be robust to confounding by 

other factors, including co-occurring air 

pollutants. In addition to these factors, 

the PA considers the statistical 

precision of study results, the extent to 

which studies report 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 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. 

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 


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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). 

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). 

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 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 ConcentrationResponse 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 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 cutpoint 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 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. singlecity 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 4th 

highest daily maximum 8-hour O3

concentration generally corresponds to 

the 98th or 99th percentile of the 

seasonal distribution, depending on the 

length of the O3 season. 

The PA concludes that these analyses 

of air quality data from the study 

locations evaluated by Silverman and 

Ito (2010) and Strickland et al. (2010) 

indicate a relatively high degree of 

confidence in reported statistical 

associations with respiratory health 

outcomes on days when virtually all 

monitored 8-hour O3 concentrations 

were 75 ppb or below (U.S. EPA, 2014c, 

section 3.1.4.2). Though these analyses 

do not identify true design values, the 

presence of O3-associated respiratory 

effects on such days provides insight 

into the types of health effects that 

could occur in locations with maximum 

ambient O3 concentrations at or below 

the level of the current standard. 

iii. Important Uncertainties 

In considering the above evidence 

within the context of developing overall 

conclusions on the current and potential 

alternative standards, the PA also takes 

into account important uncertainties in 

these analyses of air quality in locations 

of epidemiologic study areas. These 

uncertainties are summarized in this 


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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. 

section. The PA’s consideration of the 

evidence, including the associated 

uncertainties, in reaching conclusions 

on the current and potential alternative 

standards is summarized in sections 

II.D.3 (current standard) and II.E.4.b 

(potential alternative standards) below. 

The PA notes that while multicity 

studies generally have greater statistical 

power and geographic coverage than 

single-city studies, there is often greater 

uncertainty in conclusions about the 

extent to which multicity effect 

estimates reflect associations with air 

quality meeting the current standard 

(U.S. EPA, 2014c, section 1.3.1.2.1). 

This is particularly the case for the 

multicity studies evaluated in this 

review with some study locations 

meeting the current standard and others 

violating that standard. Specifically for 

the four Canadian multicity studies 

discussed above, the PA notes that 

interpretation of air quality information 

is complicated by uncertainties 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 concentrationresponse relationships over 

distributions of ambient O3

concentrations, based on multicity 

studies. The PA notes that this 

uncertainty becomes increasingly 

important when interpreting 

concentration-response relationships at 

lower ambient O3 concentrations, 

particularly those concentrations 

corresponding to portions of 

distributions where data density 

decreases notably (U.S. EPA, 2014c, 

section 3.1.4.2). 

Another important uncertainty, 

related specifically to the PA analysis of 

cut points by Bell et al. (2006), is that 

EPA staff was unable to obtain the air 

quality data used to generate the cutpoint 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 cooccurring pollutants or pollutant 

mixtures. The PA notes that this 

uncertainty becomes an increasingly 

important consideration as health effect 

associations are evaluated at lower 

ambient O3 concentrations. In 

particular, there is increasing 

uncertainty as to whether the observed 

associations remain plausibly related to 

exposures to ambient O3, rather than to 

the broader mix of air pollutants present 

in the ambient air. In considering the 

potential importance of this uncertainty 

at the relatively low ambient O3

concentrations that are the focus of the 

PA analyses, the PA notes that 

Silverman and Ito (2010) and Strickland 

(2010) reported O3 health effect 

associations in co-pollutant models,97

providing support for associations with 

O3 itself (U.S. EPA, 2014c, section 

3.1.4.2). The PA also concludes that air 

quality analyses indicate coherence 

with the results of experimental studies 

(i.e., in which the study design dictates 

that exposures to O3 itself are 

responsible for reported effects), and are 

consistent with the occurrence of O3- 

attributable respiratory hospital 

admissions and emergency department 

visits, even when virtually all 

monitored concentrations were below 

the level of the current standard (U.S. 

EPA, 2014c, section 3.1.4.2, Tables 3–4, 

3–5). 

c. Concentrations in Epidemiologic 

Studies—Long-Term 

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 longterm 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 concentrationresponse functions that indicate 

confidence in health effect associations 

at O3 concentrations meeting the current 

standard (U.S. EPA, 2014c, section 

3.1.4.3). Therefore, although these 

studies contribute to understanding of 

health effects associated with long-term 

or repeated exposures to ambient O3, 

consideration of study area air quality 

does not inform consideration of the 

extent to which those health effects may 

be occurring in locations that meet the 

current standard. 

d. PA Conclusions Based on 

Consideration of the Evidence 

As discussed above (II.D.1.a to 

II.D.1.c), in considering the available 

scientific evidence, including associated 

uncertainties, as it relates to the degree 

of public health protection provided by 

the current primary O3 standard, the PA 

evaluates the extent to which health 

effects have been reported for the O3

exposure concentrations evaluated in 

controlled human exposure studies and 

over the distributions of ambient O3

concentrations in locations where 

epidemiologic studies have been 

conducted. The PA concludes that (1) 

the evidence from controlled human 

exposure studies provides strong 

support for the occurrence of adverse 

respiratory effects following exposures 

to O3 concentrations below the level of 

the current standard and that (2) 

epidemiologic studies provide support 

for the occurrence of adverse respiratory 

effects and mortality under air 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. 


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98As 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. 

99As 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. 

2. Summary of Exposure- and RiskBased Considerations in the PA 

In order to further inform judgments 

about the potential public health 

implications of the current O3 NAAQS, 

the PA considers the exposure and risk 

assessments presented in the HREA 

(U.S. EPA, 2014c, section 3.2). 

Overviews of these exposure and risk 

assessments, including summaries of 

key results and uncertainties, are 

provided in section II.C above. This 

section summarizes key observations 

from the PA related to the adequacy of 

the current O3 NAAQS, based on 

consideration of the HREA exposure 

assessment (II.D.2.a), lung function risk 

assessment (II.D.2.b), and mortality/

morbidity risk assessments (II.D.2.c). 

a. Exposure Assessment—Key 

Observations 

As discussed above (II.C.2), the 

exposure assessment provides estimates 

of the number and percent of people 

who would experience exposures of 

concern at or above benchmark 

concentrations of 60, 70, and 80 ppb. 

Benchmarks reflect exposure 

concentrations at which O3-induced 

respiratory effects are known to occur in 

some healthy adults engaged in 

moderate, intermittent exertion, based 

on evidence from controlled human 

exposure studies (U.S. EPA, 2014c, 

section 3.1.2.1; U.S. EPA, 2013a, section 

6.2). 

The PA focuses on exposure estimates 

in children. Compared to recent (i.e., 

unadjusted) air quality, the PA notes 

that adjusting air quality to just meet the 

current O3 NAAQS consistently reduces 

the estimated occurrence of exposures 

of concern in children (U.S. EPA, 2014a, 

Appendix 5F). When averaged over the 

years evaluated in the HREA, reductions 

of up to about 70% were estimated. 

These reductions in estimated 

exposures of concern, relative to 

unadjusted air quality, reflect the 

consistent reductions in the highest 

ambient O3 concentrations upon model 

adjustment to just meet the current 

standard (U.S. EPA, 2014c, section 

3.2.1; U.S. EPA, 2014a, Chapter 4). Such 

reductions in estimated exposures of 

concern are evident throughout urban 

study areas, including in urban cores 

and in surrounding areas (U.S. EPA, 

2014a, Appendix 9A). 

Based on Figures 3–7 to 3–10 in the 

PA (U.S. EPA, 2014c), and the 

associated details described in the 

HREA (U.S. EPA, 2014a, Chapter 5), the 

PA further highlights key observations 

with regard to exposures of concern in 

children that are estimated to be 

allowed by the current standard. These 

key observations are summarized below 

for exposures of concern ≥60, 70, and 80 

ppb. 

For exposures of concern at or above 

60 ppb, the PA highlights the following 

key observations for air quality adjusted 

to just meet the current standard: 

(1) On average over the years 2006 to 

2010, the current standard is estimated 

to allow approximately 10 to 18% of 

children in urban study areas to 

experience one or more exposures of 

concern at or above 60 ppb. Summing 

across urban study areas, these 

percentages correspond to almost 2.5 

million children experiencing 

approximately 4 million exposures of 

concern at or above 60 ppb during a 

single O3 season. Of these children, 

almost 250,000 are asthmatics.98

(2) On average over the years 2006 to 

2010, the current standard is estimated 

to allow approximately 3 to 8% of 

children in urban study areas to 

experience two or more exposures of 

concern to O3 concentrations at or above 

60 ppb. Summing across the urban 

study areas, these percentages 

correspond to almost 900,000 children 

(including almost 90,000 asthmatic 

children) estimated to experience at 

least two O3 exposure concentrations at 

or above 60 ppb during a single O3

season. 

(3) In the worst-case years (i.e., those 

with the largest exposure estimates), the 

current standard is estimated to allow 

approximately 10 to 25% of children to 

experience one or more exposures of 

concern at or above 60 ppb, and 

approximately 4 to 14% to experience 

two or more exposures of concern at or 

above 60 ppb. 

For exposures of concern at or above 

70 ppb, the PA highlights the following 

key observations for air quality adjusted 

to just meet the current standard: 

(1) On average over the years 2006 to 

2010, the current standard is estimated 

to allow up to approximately 3% of 

children in urban study areas to 

experience one or more exposures of 

concern at or above 70 ppb. Summing 

across urban study areas, almost 

400,000 children (including almost 

40,000 asthmatic children) are estimated 

to experience O3 exposure 

concentrations at or above 70 ppb 

during a single O3 season.99

(2) On average over the years 2006 to 

2010, the current standard is estimated 

to allow less than 1% of children in 

urban study areas to experience two or 

more exposures of concern to O3

concentrations at or above 70 ppb. 

(3) In the worst-case years, the current 

standard is estimated to allow 

approximately 1 to 8% of children to 

experience one or more exposures of 

concern at or above 70 ppb, and up to 

approximately 2% to experience two or 

more exposures of concern, at or above 

70 ppb. 

For exposures of concern at or above 

80 ppb, the PA highlights the 

observation that the current standard is 

estimated to allow about 1% or fewer 

children in urban study areas to 

experience exposures of concern at or 

above 80 ppb, even in years with the 

highest exposure estimates. 

b. Lung Function Risk Assessment—Key 

Observations 

As discussed above (II.C.3.a), the 

HREA estimates risks of moderate to 

large lung function decrements (i.e., 

FEV1 decrements ≥10%, 15%, or 20%) 

in school-aged children (ages 5 to 18), 

asthmatic school-aged children, and the 

general adult population for 15 urban 

study areas. As for exposures of 

concern, the PA focuses on lung 

function risk estimates in children 

(including children with asthma). 

Compared to risks associated with 

recent air quality, risk estimates for air 

quality just meeting the current 

standard are consistently smaller across 

urban study areas (U.S. EPA, 2014a, 

Appendix 6B). When averaged over the 

years evaluated in the HREA, risk 

reductions of up to about 40% were 

estimated compared to recent air 

quality. These reductions reflect the 

consistent decreases in relatively high 

ambient O3 concentrations upon 

adjustment to just meet the current 

standard (U.S. EPA, 2014a, Chapter 4). 

Such reductions in estimated lung 

function risks are evident throughout 

urban study areas, including in urban 

cores and in surrounding areas (U.S. 

EPA, 2014, Appendix 9A). 

Based on Figures 3–11 to 3–14 in the 

PA (U.S. EPA, 2014c), and the 

associated details described in the 

HREA (U.S. EPA, 2014a, chapter 6), the 

PA highlights key observations with 

regard to lung function risks estimated 

in children for air quality adjusted to 

just meet the current standard. These 

key observations are presented below 

for FEV1 decrements ≥10, 15, and 20%. 

With regard to decrements ≥10%, the 

PA highlights the following key 

observations for air quality adjusted to 

just meet the current standard: 


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100The 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. Areawide concentrations are discussed in more detail in 

section 3.1.4 of the PA (U.S. EPA, 2014c). 

(1) On average over the years 2006 to 

2010, the current standard is estimated 

to allow approximately 14 to 19% of 

children in urban study areas to 

experience one or more lung function 

decrements ≥10%. Summing across 

urban study areas, this corresponds to 

approximately 3 million children 

experiencing 15 million O3-induced 

lung function decrements ≥10% during 

a single O3 season. Of these children, 

about 300,000 are asthmatics. 

(2) On average over the years 2006 to 

2010, the current standard is estimated 

to allow approximately 7 to 12% of 

children in urban study areas to 

experience two or more O3-induced 

lung function decrements ≥10%. 

Summing across the urban study areas, 

this corresponds to almost 2 million 

children (including almost 200,000 

asthmatic children) estimated to 

experience two or more O3-induced 

lung function decrements greater than 

10% during a single O3 season. 

(3) In the worst-case years, the current 

standard is estimated to allow 

approximately 17 to 23% of children in 

urban study areas to experience one or 

more lung function decrements ≥10%, 

and approximately 10 to 14% to 

experience two or more O3-induced 

lung function decrements ≥10%. 

With regard to decrements ≥15%, the 

PA highlights the following key 

observations for air quality adjusted to 

just meet the current standard: 

(1) On average over the years 2006 to 

2010, the current standard is estimated 

to allow approximately 3 to 5% of 

children in urban study areas to 

experience one or more lung function 

decrements ≥15%. Summing across 

urban study areas, this corresponds to 

approximately 800,000 children 

(including approximately 80,000 

asthmatic children) estimated to 

experience at least one O3-induced lung 

function decrement ≥15% during a 

single O3 season. 

(2) On average over the years 2006 to 

2010, the current standard is estimated 

to allow approximately 2 to 3% of 

children in urban study areas to 

experience two or more O3-induced 

lung function decrements ≥15%. 

(3) In the worst-case years, the current 

standard is estimated to allow 

approximately 4 to 6% of children in 

urban study areas to experience one or 

more lung function decrements ≥15%, 

and approximately 2 to 4% to 

experience two or more O3-induced 

lung function decrements ≥15%. 

With regard to decrements ≥20%, the 

PA highlights the following key 

observations for air quality adjusted to 

just meet the current standard: 

(1) On average over the years 2006 to 

2010, the current standard is estimated 

to allow approximately 1 to 2% of 

children in urban study areas to 

experience one or more lung function 

decrements ≥20%. Summing across 

urban study areas, this corresponds to 

approximately 300,000 children 

(including approximately 30,000 

asthmatic children) estimated to 

experience at least one O3-induced lung 

function decrement ≥20% during a 

single O3 season. 

(2) On average over the years 2006 to 

2010, the current standard is estimated 

to allow less than 1% of children in 

urban study areas to experience two or 

more O3-induced lung function 

decrements ≥20%. 

(3) In the worst-case years, the current 

standard is estimated to allow 

approximately 2 to 3% of children to 

experience one or more lung function 

decrements ≥20%, and less than 2% to 

experience two or more O3-induced 

lung function decrements ≥20%. 

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 areas 100 using concentrationresponse relationships drawn from 

epidemiologic studies. These 

concentration-response relationships are 

based on ‘‘area-wide’’ average O3

concentrations.101 The HREA estimates 

risks for the years 2007 and 2009 in 

order to provide estimates of risk for a 

year with generally higher O3

concentrations (2007) and a year with 

generally lower O3 concentrations 

(2009) (U.S. EPA, 2014a, section 7.1.1). 

In considering these estimates, the PA 

notes that HREA conclusions reflect 

somewhat lower confidence in 

epidemiologic-based risk estimates than 

in estimates of O3 exposures of concern 

and O3-induced lung function 

decrements (U.S. EPA, 2014a, section 

9.6). In particular, the HREA highlights 

the unexplained heterogeneity in effect 

estimates between locations, the 

potential for exposure measurement 

errors, and uncertainty in the 

interpretation of the shape of 

concentration-response functions at 

lower O3 concentrations (U.S. EPA, 

2014a, section 9.6). The 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 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 two of the urban 


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102As 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), modelbased adjustment increases ambient O3

concentrations compared to recent monitored 

concentrations (U.S. EPA, 2014a, section 4.3.3.2). 

Changes in epidemiology-based risk estimates 

depend on the balance between the daily decreases 

in high O3 concentrations and increases in low O3

concentrations following the model-based air 

quality adjustment. Commenting on this issue, 

CASAC noted that ‘‘controls designed to reduce the 

peak levels of ozone (e.g., the 4th highest annual 

MDA8) may not be effective at reducing lower 

levels of ozone on more typical days and may 

actually increase ozone levels on days where ozone 

concentrations are low’’ (Frey 2014a, p. 2). CASAC 

further noted that risk results ‘‘suggest that the 

ozone-related health risks in the urban cores can 

increase for some of the cities as ozone NAAQS 

alternatives become more stringent. This is because 

reductions in nitrogen oxides emissions can lead to 

less scavenging of ozone and free radicals, resulting 

in locally higher levels of ozone’’ (Frey 2014c, p. 

10). 

103For 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. 

104Risk estimates for respiratory mortality 

associated with long-term O3 exposures are based 

on the study by Jerrett et al. (2009) (U.S. EPA, 

2014a, Chapter 7). As discussed above (II.B.2.b.iv) 

and in the PA (U.S. EPA, 2014c, section 3.1.4.3), 

Jerrett et al. (2009) reported that when seasonal 

averages of 1-hour daily maximum O3

concentrations ranged from 33 to 104 ppb, there 

was no statistical deviation from a linear 

concentration-response relationship between O3

and respiratory mortality across 96 U.S. cities (U.S. 

EPA, 2013a, section 7.7). However, the authors 

reported ‘‘limited evidence’’ for an effect threshold 

at an O3 concentration of 56 ppb (p=0.06). In 

communications with EPA staff (Sasser, 2014), the 

study authors indicated that it is not clear whether 

a threshold model is a better predictor of respiratory 

mortality than the linear model, and that 

‘‘considerable caution should be exercised in 

accepting any specific threshold.’’ 

105As 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. 

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 (areawide 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., respiratoryrelated hospital admissions, emergency 

department visits, and asthma exacerbations). 

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 concentrationresponse 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 longterm O3 concentrations could potentially be 

considerably lower (i.e., by more than 75% 

if a threshold exists at 40 ppb, and by about 

98% if a threshold exists at 56 ppb) (U.S. 

EPA, 2014a, Figure 7–9).104

3. Policy Assessment Conclusions on 

the Current Standard 

As an initial matter, the PA concludes 

that reducing precursor emissions to 

achieve O3 concentrations that meet the 

current standard will provide important 

improvements in public health 

protection. This initial conclusion is 

based on (1) the strong body of scientific 

evidence indicating a wide range of 

adverse health outcomes attributable to 

exposures to O3 concentrations 

commonly found in the ambient air and 

(2) estimates indicating decreased 

occurrences of O3 exposures of concern 

and decreased health risks upon 

meeting the current standard, compared 

to recent air quality. 

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 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. 

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 


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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 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 mostrecent letters to you on this subject— 

dated October 2006 and March 2007— 

the CASAC unanimously recommended 

selection of an 8-hour average Ozone 

NAAQS within the range of 0.060 to 

0.070 parts per million [60 to 70 ppb] 

for the primary (human health-based) 

Ozone NAAQS’’ (Henderson, 2008). The 

letter continued: 

The CASAC now wishes to convey, by means 

of this letter, its additional, unsolicited 

advice with regard to the primary and 

secondary Ozone NAAQS. In doing so, the 

participating members of the CASAC Ozone 

Review Panel are unanimous in strongly 

urging you or your successor as EPA 

Administrator to ensure that these 

recommendations be considered during the 

next review cycle for the Ozone NAAQS that 

will begin next year . . . numerous medical 

organizations and public health groups have 

also expressed their support of these CASAC 

recommendations’ . . . [The CASAC did] not 

endorse the new primary ozone standard as 

being sufficiently protective of public health. 

The CASAC—as the EPA’s statutorilyestablished 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 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 doseresponse 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. 

(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 


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106CASAC 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). 

107While 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). 

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

In supporting these conclusions, 

CASAC judged that the strongest 

evidence comes from controlled human 

exposure studies of respiratory effects. 

The Committee specifically noted that 

‘‘the combination of decrements in FEV1

together with the statistically significant 

alterations in symptoms in human 

subjects exposed to 72 ppb ozone meets 

the American Thoracic Society’s 

definition of an adverse health effect’’ 

(Frey, 2014c, p. 5). CASAC further 

judged that ‘‘if subjects had been 

exposed to ozone using the 8-hour 

averaging period used in the standard, 

adverse effects could have occurred at 

lower concentration’’ and that ‘‘the level 

at which adverse effects might be 

observed would likely be lower for more 

sensitive subgroups, such as those with 

asthma’’ (Frey, 2014c, p. 5). 

With regard to lung function risk 

estimates based on information from 

controlled human exposure studies, 

CASAC concluded that ‘‘estimation of 

FEV1 decrements of ≥15% is appropriate 

as a scientifically relevant surrogate for 

adverse health outcomes in active 

healthy adults, whereas an FEV1 

decrement of ≥10% is a scientifically 

relevant surrogate for adverse health 

outcomes for people with asthma and 

lung disease’’ (Frey, 2014c, p. 3). The 

Committee further concluded that 

‘‘[a]sthmatic subjects appear to be at 

least as sensitive, if not more sensitive, 

than non-asthmatic subjects in 

manifesting O3-induced pulmonary 

function decrements’’ (Frey, 2014c, p. 

4). In considering estimates of the 

occurrence of these decrements in urban 

study areas, CASAC specifically noted 

that the current standard is estimated to 

allow 11 to 22% of school age children 

to experience at least one day with an 

FEV1 decrement ≥10% (Frey, 2014c, p. 

7). 

Although CASAC judged that 

controlled human exposure studies of 

respiratory effects provide the strongest 

evidence supporting their conclusion on 

the current standard, the Committee 

judged that there is also ‘‘sufficient 

scientific evidence based on 

epidemiologic studies for mortality and 

morbidity associated with short-term 

exposure to ozone at the level of the 

current standard’’ (Frey, 2014c, p. 5). In 

support of the biological plausibility of 

the associations reported in these 

epidemiologic studies, CASAC noted 

that ‘‘[r]ecent animal toxicological 

studies support identification of modes 

of action and, therefore, the biological 

plausibility associated with the 

epidemiological findings’’ (Frey, 2014c, 

p. 5). 

Consistent with the advice of CASAC, 

several public commenters supported 

revising the primary O3 standard to 

provide increased public health 

protection. In considering the available 

evidence as a basis for their views, these 

commenters generally noted that the 

health evidence is stronger in the 

current review than in past reviews, 

with new evidence for effects 

attributable to short- and long-term 

exposures, and new evidence for effects 

at lower O3 exposure concentrations. 

Other public commenters opposed 

considering revised standards. These 

commenters discussed a variety of 

reasons for their views. A number of 

commenters expressed the view that the 

EPA should not lower the level of the 

standard because a lower level would be 

closer to background O3 concentrations. 

In addition, several commenters 

challenged the interpretation of the 

evidence presented in the ISA. With 

respect to the risk assessment, several 

commenters expressed the view that the 

EPA should only estimate risks above 

O3 background concentrations, or above 

threshold concentrations. Some 

commenters also expressed the view 

that, based on the mortality and 

morbidity risk estimates in the HREA, 

there is little to no difference between 

the risks estimated for the current O3

standard and the risks estimated for 

revised standards with lower levels. 

These commenters concluded that the 

HREA and PA have not shown that the 

public health improvements likely to be 

achieved by a revised O3 standard 

would be greater than the improvements 

likely to be achieved by the current 

standard. 

5. Administrator’s Proposed 

Conclusions Concerning the Adequacy 

of the Current Standard 

This section discusses the 

Administrator’s proposed conclusions 

related to the adequacy of the public 

health protection provided by the 

current primary O3 standard, resulting 

in her proposed decision to revise that 

standard. These proposed conclusions, 

and her proposed decision, are based on 

the Administrator’s consideration of the 

available scientific evidence, exposure/ 

risk information, the comments and 

advice of CASAC, and public input 

received thus far, as summarized below. 

As an initial matter, the Administrator 

concludes that reducing precursor 

emissions to achieve O3 concentrations 

that meet the current primary O3

standard will provide important 

improvements in public health 

protection, compared to recent air 

quality. In reaching this initial 

conclusion, she notes the discussion in 

section 3.4 of the PA (U.S. EPA, 2014c), 

summarized above (II.D.3). In particular, 

the Administrator notes that this initial 

conclusion is supported by (1) the 

strong body of scientific evidence 

indicating a wide range of adverse 

health outcomes attributable to 

exposures to O3 at concentrations 

commonly found in the ambient air and 

(2) estimates indicating decreased 

occurrences of O3 exposures of concern 

and decreased O3-associated health risks 

upon meeting the current standard, 

compared to recent air quality. Thus, 

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 


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108As noted above, for the 70 ppb target exposure 

concentration, Schelegle et al. (2009) reported that 

the actual mean exposure concentration was 72 

ppb. 

109Cf. 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.’’). 

support and expand upon the strong 

body of evidence that, in the last review, 

indicated a causal relationship between 

short-term O3 exposures and respiratory 

health effects. This is the strongest 

causality finding possible under the 

ISA’s hierarchical system for classifying 

weight of evidence for causation. 

Together, experimental and 

epidemiologic studies support 

conclusions regarding a continuum of 

O3 respiratory effects ranging from small 

reversible changes in pulmonary 

function, and pulmonary inflammation, 

to more serious effects that can result in 

respiratory-related emergency 

department visits, hospital admissions, 

and premature mortality. Recent animal 

toxicology studies support descriptions 

of modes of action for these respiratory 

effects and augment support for 

biological plausibility for the role of O3

in reported effects. With regard to mode 

of action, evidence indicates that 

antioxidant capacity may modify the 

risk of respiratory morbidity associated 

with O3 exposure, and that the inherent 

capacity to quench (based on individual 

antioxidant capacity) can be 

overwhelmed, especially with exposure 

to elevated concentrations of O3. In 

addition, based on the consistency of 

findings across studies and evidence for 

the coherence of results from different 

scientific disciplines, evidence indicates 

that certain populations are at increased 

risk of experiencing O3-related effects, 

including the most severe effects. These 

include populations and lifestages 

identified in previous reviews (i.e., 

people with asthma, children, older 

adults, outdoor workers) and 

populations identified since the last 

review (i.e., people with certain 

genotypes related to antioxidant and/or 

anti-inflammatory status; people with 

reduced intake of certain antioxidant 

nutrients, such as Vitamins C and E). 

The Administrator further notes that 

evidence for adverse respiratory health 

effects attributable to long-term, or 

repeated short-term, O3 exposures is 

much stronger than in previous reviews, 

and the ISA concludes that there is 

‘‘likely to be’’ a causal relationship 

between such O3 exposures and adverse 

respiratory health effects (the second 

strongest causality finding). 

Uncertainties related to the 

extrapolation of data generated by 

rodent toxicology studies to the 

understanding of health effects in 

humans have been reduced by studies 

in non-human primates and by recent 

epidemiologic studies. The evidence 

available in this review includes new 

epidemiologic studies using a variety of 

designs and analysis methods, 

conducted by different research groups 

in different locations, evaluating the 

relationships between long-term O3

exposures and measures of respiratory 

morbidity and mortality. New evidence 

supports associations between long-term 

O3 exposures and the development of 

asthma in children, with several studies 

reporting interactions between genetic 

variants and such O3 exposures. Studies 

also report associations between longterm 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 co-occurring pollutants or 

pollutant mixtures (as is the case in 

epidemiologic studies). Therefore, she 

places the most weight on information 

from these controlled human exposure 

studies. 

In considering the evidence from 

controlled human exposure studies, the 

Administrator first notes that these 

studies have reported a variety of 

respiratory effects in healthy adults 

following exposures to O3

concentrations of 60, 72,108 or 80 ppb, 

and higher. The largest respiratory 

effects, and the broadest range of effects, 

have been studied and reported 

following exposures of healthy adults to 

80 ppb O3 or higher, with most exposure 

studies conducted at these higher 

concentrations. She further notes that 

recent evidence includes controlled 

human exposure studies reporting the 

combination of lung function 

decrements and respiratory symptoms 

in healthy adults engaged in 

intermittent, moderate exertion 

following 6.6 hour exposures to 

concentrations as low as 72 ppb, and 

lung function decrements and 

pulmonary inflammation following 

exposures to O3 concentrations as low 

as 60 ppb. As discussed below, 

compared to the evidence available in 

the last review, these studies have 

strengthened support for the occurrence 

of abnormal and adverse respiratory 

effects attributable to short-term 

exposures to O3 concentrations below 

the level of the current standard.109 The 

Administrator concludes that such 

exposures to O3 concentrations below 

the level of the current standard are 

potentially important from a public 

health perspective, given the following: 

(1) The combination of lung function 

decrements and respiratory symptoms 

reported to occur in healthy adults 

following exposures to 72 ppb O3 or 

higher, while at moderate exertion, meet 

ATS criteria for an adverse response. In 

specifically considering the 72 ppb 

exposure concentration, CASAC noted 

that ‘‘the combination of decrements in 

FEV1 together with the statistically 

significant alterations in symptoms in 

human subjects exposed to 72 ppb 

ozone meets the American Thoracic 

Society’s definition of an adverse health 

effect’’ (Frey, 2014c, p. 5). 

(2) With regard to 60 ppb O3, CASAC 

agreed that ‘‘a level of 60 ppb 

corresponds to the lowest exposure 

concentration demonstrated to result in 

lung function decrements large enough 

to be judged an abnormal response by 

ATS and that could be adverse in 

individuals with lung disease’’ (Frey, 

2014c, p. 7). CASAC further noted that 

‘‘a level of 60 ppb also corresponds to 

the lowest exposure concentration at 

which pulmonary inflammation has 

been reported’’ (Frey, 2014c, p. 7). 

(3) The controlled human exposure 

studies reporting these respiratory 

effects were conducted in healthy 

adults, while at-risk groups (e.g., 

children, people with asthma) could 

experience larger and/or more serious 

effects. In their advice to the 

Administrator, CASAC concurred with 

this reasoning (Frey, 2014a, p. 14; Frey, 

2014c, p. 5). 

(4) These respiratory effects are 

coherent with the serious health 

outcomes that have been reported in 

epidemiologic studies evaluating 

exposure to O3 (e.g., respiratory-related 

hospital admissions, emergency 

department visits, and mortality). 

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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110The 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. 

111As noted above, she places less weight on 

information from epidemiologic studies than on 

information from controlled human exposure 

studies. 

112As 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)). 

113CASAC also called into question the extent to 

which it is appropriate to place confidence in risk 

estimates for respiratory mortality (Frey, 2014a, p. 

11). 

114Not all people who experience an exposure of 

concern will experience an adverse effect (even 

members of at-risk populations). For most of the 

endpoints evaluated in controlled human exposure 

studies (with the exception of O3-induced FEV1

decrements, as discussed below), the number of 

those experiencing exposures of concern who will 

experience adverse effects cannot be reliably 

quantified. 

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. 

In considering information from 

epidemiologic studies within the 

context of her conclusions on the 

adequacy of the current standard, the 

Administrator considers the extent to 

which available studies support the 

occurrence of O3 health effect 

associations with air quality likely to be 

allowed by the current standard. In 

doing so, she places the most weight on 

air quality analyses in locations of 

single-city studies of short-term O3, as 

discussed in more detail in section 

II.E.4.d below.112 In particular, she 

notes that a U.S. single-city study 

reported associations with respiratory 

emergency department visits in children 

and adults in a location that would 

likely have met the current O3 standard 

over the entire study period (Mar and 

Koenig, 2009). In addition, even in some 

single-city study locations where the 

current standard was likely not met (i.e., 

Silverman and Ito, 2010; Strickland et 

al., 2010), the Administrator notes PA 

analyses indicating that reported 

concentration-response functions and 

available air quality data support the 

occurrence of O3-health effect 

associations on subsets of days with 

ambient O3 concentrations below the 

level of the current standard (II.D.1). 

Compared to single-city studies, the 

Administrator notes additional 

uncertainty in interpreting the 

relationships between air quality in 

individual study cities and health 

effects based on multicity analyses 

(discussed further in sections II.D.1 and 

II.E.4.d). While such uncertainties limit 

the extent to which the Administrator 

bases her conclusions on air quality in 

locations of multicity epidemiologic 

studies, she does note that O3

associations with respiratory morbidity 

or mortality have been reported in 

several multicity studies when the 

majority of study locations (though not 

all study locations) would likely have 

met the current O3 standard. When 

taken together, the Administrator 

reaches the conclusion that single-city 

epidemiologic studies and associated air 

quality information support the 

occurrence of O3-associated hospital 

admissions and emergency department 

visits for ambient O3 concentrations 

likely to have met the current standard, 

and that air quality analyses in locations 

of multicity studies provide some 

support for this conclusion for a broader 

range of effects (i.e., including 

mortality). 

Beyond her consideration of the 

scientific evidence, the Administrator 

also considers the results of the HREA 

exposure and risk analyses in reaching 

initial conclusions regarding the 

adequacy of the current primary O3

standard. In doing so, as noted above, 

she focuses primarily on exposure and 

risk estimates based on information 

from controlled human exposure studies 

(i.e., exposures of concern and O3- 

induced lung function decrements). She 

places relatively less weight on 

epidemiologic-based risk estimates, 

noting that the overall conclusions from 

the HREA likewise reflect less 

confidence in estimates of 

epidemiologic-based risks than in 

estimates of exposures and lung 

function risks (U.S. EPA, 2014, section 

9.6). Consistent with the conclusions in 

the PA, her determination to attach less 

weight to the epidemiologic-based risk 

estimates reflects her consideration of 

key uncertainties, including the 

heterogeneity in effect estimates 

between locations, the potential for 

exposure measurement errors, and 

uncertainty in the interpretation of the 

shape of concentration-response 

functions for O3 concentrations in the 

lower portions of ambient distributions 

(U.S. EPA, 2014, section 9.6) (II.D.2). In 

particular, she concludes that lower 

confidence should be placed in the 

results of the assessment of respiratory 

mortality risks associated with longterm O3 exposures, primarily because 

that analysis is based on only one study 

(even though that study is welldesigned) 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, 


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115Almost no children in those areas would be 

estimated to experience two or more exposures of 

concern at or above 80 ppb. 

116As 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). 

she notes that repeated occurrences of 

the types of effects shown to occur 

following exposures of concern can 

have potentially adverse outcomes. For 

example, repeated occurrences of 

airway inflammation could potentially 

result in the induction of a chronic 

inflammatory state; altered pulmonary 

structure and function, leading to 

diseases such as asthma; altered lung 

host defense response to inhaled 

microorganisms; and altered lung 

response to other agents such as 

allergens or toxins (U.S. EPA, 2013a, 

section 6.2.3). Thus, the Administrator 

notes that the types of lung injury 

shown to occur following exposures to 

O3 concentrations from 60 to 80 ppb, 

particularly if experienced repeatedly, 

provide a mode of action by which O3

may cause other more serious effects 

(e.g., asthma exacerbations). Therefore, 

the Administrator places the most 

weight on estimates of two or more 

exposures of concern (i.e., as a surrogate 

for the occurrence of repeated 

exposures), though she also considers 

estimates of one or more, particularly 

for the 70 and 80 ppb benchmarks. 

Consistent with CASAC advice (Frey, 

2014c), the Administrator focuses on 

children in these analyses of O3

exposures, noting that estimates for all 

children and asthmatic children are 

virtually indistinguishable (in terms of 

the percent estimated to experience 

exposures of concern). Though she 

focuses on children, she also recognizes 

that exposures to O3 concentrations at or 

above 60 or 70 ppb could be of concern 

for adults. As discussed in the HREA 

and PA (and II.C.2.a, above), the 

patterns of exposure estimates across 

urban study areas, across years, and 

across air quality scenarios are similar 

in adults with asthma, older adults, all 

children, and children with asthma, 

though smaller percentages of adult 

populations are estimated to experience 

exposures of concern than children and 

children with asthma. Thus, the 

Administrator recognizes that the 

exposure patterns for children across 

years, urban study areas, and air quality 

scenarios are indicative of the exposure 

patterns in a broader group of at-risk 

populations that also includes asthmatic 

adults and older adults. 

As illustrated in Table 1 (above), the 

Administrator notes that if the 15 urban 

study areas evaluated in the HREA were 

to just meet the current O3 standard, 

fewer than 1% of children in those areas 

would be estimated to experience two or 

more exposures of concern at or above 

70 ppb, though approximately 3 to 8% 

of children, including approximately 3 

to 8% of asthmatic children, would be 

estimated to experience two or more 

exposures of concern to O3

concentrations at or above 60 ppb 115

(based on estimates averaged over the 

years of analysis). To provide some 

perspective on these percentages, the 

Administrator notes that they 

correspond to almost 900,000 children 

in urban study areas, including about 

90,000 asthmatic children, estimated to 

experience two or more exposures of 

concern at or above 60 ppb. Nationally, 

if the current standard were to be just 

met the number of children 

experiencing such exposures would be 

larger. In the worst-case year and 

location (i.e., year and location with the 

largest exposure estimates), the 

Administrator notes that over 2% of 

children are estimated to experience 

two or more exposures of concern at or 

above 70 ppb and over 14% are 

estimated to experience two or more 

exposures of concern at or above 60 

ppb. 

Although, as discussed above and in 

section II.E.4.d, the Administrator is less 

concerned about single occurrences of 

exposures of concern, she notes that 

even single occurrences can cause 

adverse effects in some people, 

particularly for the 70 and 80 ppb 

benchmarks. Therefore, she also 

considers estimates of one or more 

exposures of concern. As illustrated in 

Table 1 (above), if the 15 urban study 

areas evaluated in the HREA were to 

just meet the current O3 standard, fewer 

than 1% of children in those areas 

would be estimated to experience one or 

more exposures of concern at or above 

80 ppb (based on estimates averaged 

over the years of analysis). However, 

approximately 1 to 3% of children, 

including 1 to 3% of asthmatic children, 

would be estimated to experience one or 

more exposures of concern to O3

concentrations at or above 70 ppb and 

approximately 10 to 17% would be 

estimated to experience one or more 

exposures of concern to O3

concentrations at or above 60 ppb. In 

the worst-case year and location, the 

Administrator notes that over 1% of 

children are estimated to experience one 

or more exposures of concern at or 

above 80 ppb, over 8% are estimated to 

experience one or more exposures of 

concern at or above 70 ppb, and about 

26% are estimated to experience one or 

more exposures of concern at or above 

60 ppb. 

In addition to estimated exposures of 

concern, the Administrator also 

considers HREA estimates of the 

occurrence of O3-induced lung function 

decrements. In doing so, she 

particularly notes CASAC advice that 

‘‘estimation of FEV1 decrements of 

≥15% is appropriate as a scientifically 

relevant surrogate for adverse health 

outcomes in active healthy adults, 

whereas an FEV1 decrement of ≥10% is 

a scientifically relevant surrogate for 

adverse health outcomes for people with 

asthma and lung disease’’ (Frey, 2014c, 

p. 3). The Administrator notes that 

while single occurrences of O3-induced 

lung function decrements could be 

adverse for some people, as discussed 

above (II.B.3), a more general consensus 

view of the potential adversity of such 

decrements emerges as the frequency of 

occurrences increases. Therefore, the 

Administrator focuses primarily on the 

estimates of two or more O3-induced 

lung function decrements. 

When averaged over the years 

evaluated in the HREA, the 

Administrator notes that the current 

standard is estimated to allow about 1 

to 3% of children in the 15 urban study 

areas (corresponding to almost 400,000 

children) to experience two or more O3- 

induced lung function decrements 

≥15%, and to allow about 8 to 12% of 

children (corresponding to about 

180,000 asthmatic children 116) 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 worstcase year and location, the current 

standard is estimated to allow 4% of 

children in the urban study areas to 

experience two or more decrements 

≥15% (and 7% to experience one or 

more such decrements) and 14% of 

children to experience two or more 

decrements ≥10% (and 22% to 

experience one or more such 

decrements). 

In further considering the HREA 

results, the Administrator considers the 

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 


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117See e.g. State of Mississippi, 744 F. 3d at 1345; 

American Farm Bureau, 559 F. 3d at 525–26. 

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

concentrations corresponding to the 

upper portions of typical ambient 

distributions (i.e., 60 ppb and above). 

Therefore, the Administrator judges it 

appropriate to focus on risks associated 

with O3 concentrations in the upper 

portions of ambient distributions. Even 

when considering only area-wide O3

concentrations from the upper portions 

of seasonal distributions, the 

Administrator notes that the current 

standard is estimated to allow hundreds 

to thousands of O3-associated deaths per 

year in urban study areas (II.C.3). 

Although the Administrator notes the 

HREA conclusions indicating somewhat 

less confidence in estimates of O3- 

associated mortality and morbidity 

risks, compared to estimates of 

exposures of concern and risk of lung 

function decrements, she concludes that 

the general magnitude of mortality and 

morbidity risk estimates suggests the 

potential for a substantial number of O3- 

associated deaths and adverse 

respiratory events to occur nationally, 

even when the current standard is met. 

She especially notes that this is the case 

based on the risks associated with the 

upper ends of distributions of ambient 

O3 concentrations, where she has the 

greatest confidence in O3-attributable 

effects. 

In addition to the evidence and 

exposure/risk information discussed 

above, the Administrator also takes note 

of the CASAC advice in the current 

review and in the 2010 proposed 

reconsideration of the 2008 decision 

establishing the current standard. As 

discussed in more detail above, the 

current CASAC ‘‘finds that the current 

NAAQS for ozone is not protective of 

human health’’ and ‘‘unanimously 

recommends that the Administrator 

revise the current primary ozone 

standard to protect public health’’ (Frey, 

2014c, p. 5). The prior CASAC O3 Panel 

likewise recommended revision of the 

current standard to one with a lower 

level. This earlier recommendation was 

based entirely on the evidence and 

information in the record for the 2008 

standard decision, which, as discussed 

above, has been substantially 

strengthened in the current review 

(Samet, 2011; Samet, 2012). 

In consideration of all of the above, 

the Administrator proposes that the 

current primary O3 standard is not 

adequate to protect public health, and 

that it should be revised to provide 

increased public health protection. This 

proposed decision is based on the 

Administrator’s initial conclusions that 

the available evidence and exposure and 

risk information clearly call into 

question the adequacy of public health 

protection provided by the current 

primary standard and, therefore, that the 

current standard is not requisite to 

protect public health with an adequate 

margin of safety. With regard to the 

evidence, she specifically notes that (1) 

controlled human exposure studies 

provide support for the occurrence of 

adverse respiratory effects following 

exposures to O3 concentrations below 

the level of the current standard (i.e., as 

low as 72 ppb), and that (2) single-city 

epidemiologic studies provide support 

for the occurrence of adverse respiratory 

effects under air quality conditions that 

would likely meet the current standard, 

with multicity studies providing some 

support for this conclusion for a broader 

range of effects (i.e., including 

mortality). Courts have repeatedly held 

that this type of evidence justifies an 

Administrator’s conclusion that it is 

‘‘appropriate’’ (within the meaning of 

section 109 (d)(1) of the CAA) to revise 

a primary NAAQS to provide further 

protection of public health.117 In 

addition, based on the analyses in the 

HREA, the Administrator initially 

concludes that the exposures and risks 

projected to remain upon meeting the 

current standard can reasonably be 

judged to be important from a public 

health perspective. Thus, she reaches 

the proposed conclusion that the 

evidence and information, together with 

CASAC advice based on their 

consideration of that evidence and 

information, provide strong support for 

revising the current primary standard in 

order to increase public health 

protection against an array of adverse 

effects that range from decreased lung 

function and respiratory symptoms to 

more serious indicators of morbidity 

(e.g., including emergency department 

visits and hospital admissions), and 

mortality. 

The Administrator solicits comment 

on her proposed decision to revise the 

current primary O3 NAAQS, including 

on her considerations and proposed 

conclusions based on the scientific 

evidence, exposure/risk information, 

and CASAC advice. In doing so, she 

recognizes that some have expressed 

alternative approaches to viewing the 

evidence and information, including 

alternative approaches to viewing, 

evaluating, and weighing important 

uncertainties. In some cases, these 

alternative approaches have led some 

public commenters to recommend 

retaining the current standard. Given 

these alternative views, in addition to 

proposing to revise the current primary 

O3 standard, the Administrator solicits 

comment on the option of retaining that 

standard. In doing so, she also solicits 

comment on the potential approaches to 

viewing the scientific evidence and 

exposure/risk information that could 

support a conclusion that the current 

standard is requisite to protect public 

health with an adequate margin of 

safety. 

E. Conclusions on the Elements of the 

Primary Standard 

Having reached the proposed 

conclusion that the currently available 

scientific evidence and exposure/risk 

information call into question the 

adequacy of the current O3 standard, the 

Administrator next considers the range 

of alternative standards supported by 

that evidence and information. 

Consistent with her consideration of the 

adequacy of the current standard, the 

Administrator’s proposed conclusions 

on alternative standards are informed by 

the available scientific evidence 

assessed in the ISA, exposure/risk 

information presented and assessed in 

the HREA, the evidence-based and 

exposure-/risk-based considerations and 

conclusions in the PA, CASAC advice, 

and input from members of the public. 

The sections below discuss the evidence 


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118The DC 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. 

and exposure/risk information, CASAC 

advice and public input, and the 

Administrator’s proposed conclusions, 

for the major elements of the NAAQS: 

indicator (II.E.1), averaging time (II.E.2), 

form (II.E.3), and level (II.E.4). 

1. Indicator 

In the last review, the EPA focused on 

O3 as the most appropriate indicator for 

a standard meant to provide protection 

against ambient photochemical 

oxidants. In this review, while the 

complex atmospheric chemistry in 

which O3 plays a key role has been 

highlighted, no alternatives to O3 have 

been advanced as being a more 

appropriate indicator for ambient 

photochemical oxidants. More 

specifically, the ISA noted that O3 is the 

only photochemical oxidant (other than 

NO2) that is routinely monitored and for 

which a comprehensive database exists 

(U.S. EPA, 2013a, section 3.6). Data for 

other photochemical oxidants (e.g., 

PAN, H2O2, etc.) typically have been 

obtained only as part of special field 

studies. Consequently, no data on 

nationwide patterns of occurrence are 

available for these other oxidants; nor 

are extensive data available on the 

relationships of concentrations and 

patterns of these oxidants to those of O3

(U.S. EPA, 2013a, section 3.6). In its 

review of the second draft PA, CASAC 

stated ‘‘The indicator of ozone is 

appropriate based on its causal or likely 

causal associations with multiple 

adverse health outcomes and its 

representation of a class of pollutants 

known as photochemical oxidants’’ 

(Frey, 2014c, p. ii). 

In addition, the PA notes that meeting 

an O3 standard can be expected to 

provide some degree of protection 

against potential health effects that may 

be independently associated with other 

photochemical oxidants, even though 

such effects are not discernible from 

currently available studies indexed by 

O3 alone (U.S. EPA, 2014c, section 4.1). 

That is, since the precursor emissions 

that lead to the formation of O3

generally also lead to the formation of 

other 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 time 119 for the primary 

O3 NAAQS in 1997 (62 FR 38856). The 

decision on averaging time in that 

review was based on numerous 

controlled human exposure and 

epidemiologic studies reporting 

associations between 6 to 8 hour O3

concentrations and adverse respiratory 

effects (62 FR 38861). It was also noted 

that a standard with a max 8-hour 

averaging time is likely to provide 

substantial protection against 

respiratory effects associated with 1- 

hour peak O3 concentrations. Similar 

conclusions were reached in the last O3

NAAQS review and thus, the 8-hour 

averaging time was retained in 2008. 

In reaching a proposed conclusion on 

averaging time in the current review, the 

Administrator considers the extent to 

which the available evidence continues 

to support the appropriateness of a 

standard with an 8-hour averaging time. 

Specifically, the Administrator 

considers the extent to which the 

available information indicates that a 

standard with the current 8-hour 

averaging time provides appropriate 

protection against short- and long-term 

O3 exposures. 

a. Short-Term 

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 shortterm exposures. More specifically, the 

Administrator notes the ISA conclusion 

that the evidence is ‘‘sufficient to infer 

a causal relationship’’ between shortterm O3 exposures and respiratory 

effects. The ISA also judges that for 

short-term O3 exposures, the evidence 

indicates ‘‘likely to be causal’’ 

relationships with both cardiovascular 

effects and mortality (U.S. EPA, 2013a, 

section 2.5.2). Therefore, as in past 

reviews, the strength of the available 

scientific evidence provides strong 

support for a standard that protects the 

public health against short-term 

exposures to O3. 

In first considering the level of 

support available for specific short-term 

averaging times, the Administrator notes 

the evidence available from controlled 

human exposure studies. As discussed 

in more detail in chapter 3 of the PA, 

substantial health effects evidence from 

controlled human exposure studies 

demonstrates that a wide range of 

respiratory effects (e.g., pulmonary 

function decrements, increases in 

respiratory symptoms, lung 

inflammation, lung permeability, 

decreased lung host defense, and AHR) 

occur in healthy adults following 6.6 

hour exposures to O3 (U.S. EPA, 2013a, 

section 6.2.1.1). Compared to studies 

evaluating shorter exposure durations 

(e.g., 1-hour), studies evaluating 6.6 

hour exposures in healthy adults have 

reported respiratory effects at lower O3

exposure concentrations and at more 

moderate levels of exertion. 

The Administrator also notes the 

strength of evidence from epidemiologic 

studies that have evaluated a wide 

variety of populations (e.g., including 

at-risk lifestages and populations, such 

as children and people with asthma, 

respectively). A number of different 

averaging times are used in O3

epidemiologic studies, with the most 

common being the max 1-hour 

concentration 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 multiday average O3 concentrations, lung 

function decrements were associated 

with 1-hour max, 8-hour max, and 24- 

hour average ambient O3 concentrations, 

with no strong difference in the 

consistency or magnitude of association 

among the averaging times (U.S. EPA, 

2013a, p. 6–71). Similarly, in studies of 

short-term exposure to O3 and mortality, 

Smith et al. (2009) and Darrow et al. 

(2011) have reported high correlations 

between risk estimates calculated using 

24-hour average, 8-hour max, and 1- 

hour max averaging times (U.S. EPA, 

2013a, p. 6–253). Thus, the 

Administrator notes that the 

epidemiologic evidence alone does not 

provide a strong basis for distinguishing 

between the appropriateness of 1-hour, 

8-hour, and 24-hour averaging times. 

Considering the health information 

discussed above, the Administrator 

concludes that an 8-hour averaging time 

remains appropriate for addressing 

health effects associated with short-term 

exposures to ambient O3. An 8-hour 


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120Though the Administrator also notes 

important uncertainties associated with these risk 

estimates, as discussed above (II.C.3.b). 

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. 

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 

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) (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 reflect 

long-term metrics that have been 

reported to be associated with 

respiratory morbidity effects in several 

recent O3 epidemiologic studies (e.g., 

Islam et al., 2008; Lin et al., 2008; Salam 

et al., 2009). The HREA analyses 

indicate that the large majority of the 

U.S. population lives in locations where 

reducing NOX emissions would be 

expected to result in decreases in 

seasonal averages of daily max 8-hour 

ambient O3 concentrations (U.S. EPA, 

2014a, chapter 8). Thus, consistent with 

the respiratory mortality risk estimates 

noted above, these analyses suggest that 

reductions in O3 precursor emissions in 

order to meet a standard with an 8-hour 

averaging time would also be expected 

to reduce the long-term O3

concentrations that have been reported 

in recent epidemiologic studies to be 

associated with respiratory morbidity. 

c. Administrator’s Proposed Conclusion 

on Averaging Time 

Taken together, the Administrator 

notes that the analyses summarized 

above indicate that a standard with an 

8-hour averaging time, coupled with the 

current 4th high form and an 

appropriate level, would be expected to 

provide appropriate protection against 

the short- and long-term O3

concentrations that have been reported 

to be associated with respiratory 

morbidity and mortality. The CASAC 

agreed with this conclusion, stating that 

‘‘[t]he current 8-hour averaging time is 

justified by the combined evidence from 

epidemiologic and clinical studies’’ and 

that ‘‘[t]he 8-hour averaging window 

also provides protection against the 

adverse impacts of long-term ozone 

exposures, which were found to be 

‘‘likely causal’’ for respiratory effects 

and premature mortality’’ (Frey, 2014c, 

p. 6). Therefore, considering the 

available evidence and exposure risk 

information, and CASAC’s advice, the 

Administrator proposes to retain the 

current 8-hour averaging time, and not 

to set an additional standard with a 

different averaging time. 

3. Form 

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-expectedexceedance’’ form.121 An advantage of 

the current concentration-based form 

recognized in the 1997 review is that 


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122As 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. 

123See 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). 

124 In 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). 

such a form better reflects the 

continuum of health effects associated 

with increasing ambient O3

concentrations. Unlike an expected 

exceedance form, a concentration-based 

form gives proportionally more weight 

to years when 8-hour O3 concentrations 

are well above the level of the standard 

than years when 8-hour O3

concentrations are just above the level 

of the standard.122 It was judged 

appropriate to give more weight to 

higher O3 concentrations, given that 

available health evidence indicated a 

continuum of effects associated with 

exposures to varying concentrations of 

O3, and given that the extent to which 

public health is affected by exposure to 

ambient O3 is related to the 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 fifthhighest daily maximum 8-hour 

concentrations in an O3 season. The 

fourth-highest daily maximum was 

selected, recognizing that a less 

restrictive form (e.g., fifth highest) 

would allow a larger percentage of sites 

to experience O3 peaks above the level 

of the standard, and would allow more 

days on which the level of the standard 

may be exceeded when the site attains 

the standard (62 FR 38856). 

Consideration was also given to setting 

a standard with a form that would 

provide a margin of safety against 

possible but uncertain chronic effects, 

and would provide greater stability to 

ongoing control programs.123 A more 

restrictive form was not selected, 

recognizing that the differences in the 

degree of protection afforded by the 

alternatives were not well enough 

understood to use any such differences 

as a basis for choosing the most 

restrictive forms (62 FR 38856). 

In the 2008 review, the EPA 

additionally considered the potential 

value of a 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 4thhighest daily maximum should be 

retained (73 FR 16465, March 27, 2008). 

In reaching this decision, the 

Administrator recognized that ‘‘there is 

not a clear health-based threshold for 

selecting a particular nth-highest daily 

maximum form of the standard’’ and 

that ‘‘the adequacy of the public health 

protection provided by the combination 

of the level and form is a foremost 

consideration’’ (73 FR 16475, March 27, 

2008). Based on this, the Administrator 

judged that the existing form (4thhighest 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 4th high 

form ‘‘provides a stable target for 

implementing programs to improve air 

quality’’ (73 FR 16475, March 27, 2008). 

In the current review, the 

Administrator considers the extent to 

which newly available information 

provides support for the current form. In 

so doing, she takes note of the 

conclusions of prior reviews 

summarized above. She recognizes the 

value of an nth-high statistic over that 

of an expected exceedance or percentilebased form in the case of the O3

standard, for the reasons summarized 

above. The Administrator additionally 

takes note of the importance of stability 

in implementation to achieving the level 

of protection specified by the NAAQS. 

Specifically, she notes that to the extent 

areas engaged in implementing the O3

NAAQS frequently shift from meeting 

the standard to violating the standard, it 

is possible that ongoing implementation 

plans and associated control programs 

could be disrupted, thereby reducing 

public health protection. 

In light of this, while giving foremost 

consideration to the adequacy of public 

health protection provided by the 

combination of all elements of the 

standard, including the form, the 

Administrator considers particularly 

findings from prior reviews with regard 

to the use of the nth-high metric. As 

noted above, the 4th-highest daily 

maximum was selected in recognition of 

the public health protection provided by 

this form, when coupled with an 

appropriate averaging time and level, 

and recognizing that such a form can 

provide stability for implementation 

programs. The Administrator concludes 

that the currently available evidence 

and information do not call into 

question these conclusions from 

previous reviews. In reaching this 

conclusion, the Administrator notes that 

CASAC concurred that the O3 standard 

should be based on the fourth highest, 

daily maximum 8-hour average value 

(averaged over 3 years), stating that this 

form ‘‘provides health protection while 

allowing for atypical meteorological 

conditions that can lead to 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 


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125As 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. 

as discussed below, would be expected 

to increase public health protection 

relative to the current standard while 

continuing to provide stability for 

implementation programs. Therefore, 

the Administrator proposes to retain the 

current 4th-highest daily maximum 

form for an O3 standard with an 8-hour 

averaging time and a revised level, as 

discussed below. 

4. Level 

The Administrator next considers the 

extent to which alternative levels below 

75 ppb could provide greater protection 

than the current primary standard 

against short- and long- term exposures 

to O3 in ambient air, for a standard 

based on the 3-year average of the 

annual 4th highest daily maximum 8- 

hour O3 concentration. In doing so, she 

particularly notes the evidence-based 

and exposure-/risk-based considerations 

in the PA, which take into account the 

experimental and epidemiologic 

evidence as assessed in the ISA; 

quantitative estimates of O3 exposures 

and health risks in at-risk populations 

provided by the HREA; uncertainties 

and limitations associated with this 

evidence and information; CASAC 

advice; and public input (U.S. EPA, 

2014c, sections 4.4 and 4.5). Section 

II.E.4.a below summarizes the PA’s 

approach to considering the scientific 

evidence and the exposure/risk 

information related to level of the 

primary standard. Section II.E.4.b 

presents the PA’s conclusions on 

alternative primary O3 standard levels. 

Section II.E.4.c summarizes CASAC 

advice on the level of the primary 

standard, and public input received 

thus far. Section II.E.4.d presents the 

Administrator’s proposed conclusions 

on primary O3 standard levels. 

a. PA Approach to Considering the 

Evidence and Information Related to 

Alternative Levels of the Primary 

Standard 

The PA’s approach to reaching 

conclusions on alternative standard 

levels focuses on the evidence from 

controlled human exposure and 

epidemiologic studies, as assessed in 

the ISA (U.S. EPA, 2013a), and the 

exposure and health risk analyses 

presented in the HREA (U.S. EPA, 

2014a). This approach is discussed in 

detail in Chapters 1 and 4 of the PA 

(U.S. EPA, 2014c, sections 1.3, 4.6), and 

is summarized below. 

As an initial matter, the PA notes that 

controlled human exposure studies 

provide the most certain evidence 

indicating the occurrence of health 

effects in humans following exposures 

to specific O3 concentrations. Consistent 

with this, CASAC concluded that ‘‘the 

scientific evidence supporting the 

finding that the current standard is 

inadequate to protect public health is 

strongest based on the controlled human 

exposure studies of respiratory effects’’ 

(Frey, 2014c, p. 5). As discussed above 

and in section 3.1.2.1 of the PA (U.S. 

EPA, 2014c), controlled human 

exposure studies have reported a variety 

of respiratory effects in healthy adults 

following exposures to O3

concentrations of 60, 72,125 or 80 ppb, 

and higher. 

Given the evidence for respiratory 

effects from controlled human exposure 

studies, the PA considers the extent to 

which standards with revised levels 

would be estimated to protect 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 ozone-related effects including lung 

inflammation and airway responsiveness in 

healthy individuals. The 70 ppb-8hr 

benchmark level reflects the fact that in 

healthy subjects, decreases in lung function 

and respiratory symptoms occur at 

concentrations as low as 72 ppb and that 

these effects almost certainly occur in some 

people, including asthmatics and others with 

low lung function who are less tolerant of 

such effects, at levels of 70 ppb and below. 

The 60 ppb-8hr benchmark level represents 

the lowest exposure level at which ozonerelated 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 atrisk populations and lifestages are at 

greater risk of experiencing adverse 

effects. Thus, the effects reported in 

healthy adults at each of the benchmark 

concentrations may underestimate 

effects in these at-risk groups. In 

considering the health evidence within 

the context of drawing conclusions on 

alternative standard levels, the PA 

balances concerns about the potential 

for adverse health effects, especially in 

at-risk populations, with the increasing 

uncertainty regarding the likelihood of 

such effects following exposures to 

lower O3 concentrations. 

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 


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126 Differences in estimated respiratory morbidity 

risks between alternative standard levels are similar 

to the differences estimated for total mortality 

associated with short-term O3 concentrations. 

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). 

128As discussed above (II.B.2), prolonged 6.6 

exposure to 40 ppb O3 has been shown to result in 

a small decrease in group mean FEV1 that is not 

statistically different from responses following 

exposure to filtered air (Adams, 2002; Adams, 

2006). 

department visits, hospital admissions, 

and/or mortality in locations that would 

likely have met alternative standards 

with levels below 75 ppb. In evaluating 

the epidemiologic evidence in this way, 

the PA considers both multicity and 

single-city studies, recognizing the 

strengths and limitations of each. In 

particular, while single-city studies are 

more limited than multicity studies in 

terms of statistical power and 

geographic coverage, conclusions 

linking air quality in a specific area with 

health effect associations in that same 

area can be made 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 epidemiologicbased estimates reflects the 

uncertainties associated with mortality 

and morbidity risk estimates, including 

the heterogeneity in effect estimates 

between locations, the potential for 

exposure measurement errors, and 

uncertainty in the interpretation of the 

shape of concentration-response 

functions at lower O3 concentrations 

(U.S. EPA, 2014a, section 9.6). The 

HREA also concludes that lower 

confidence should be placed in the 

results of the assessment of respiratory 

mortality risks associated with longterm O3 exposures, primarily because 

that analysis is based on only one study 

(even though that study is welldesigned) and because of the 

uncertainty in that study about the 

existence and level of a potential 

threshold in the concentration-response 

function (U.S. EPA, 2014a, section 9.6). 

In considering the epidemiologybased 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

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 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 

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 


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129All 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. 

130Estimates 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. 

131Numbers 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. 

132As noted in section II.C.3.a.ii, the 

responsiveness of asthmatics to O3 exposures could 

depend on factors that have not been well-evaluated 

such as asthma severity, the effectiveness of asthma 

control, or the prevalence of medication use. 

133Percent reductions in each urban study area 

were calculated and averaged across areas. 

134Estimates smaller than 0.05% were rounded to 

zero. 

135As discussed in section 4.3.3 of the HREA 

(U.S. EPA, 2014a), the model-based air quality 

adjustment approach used to estimate risks 

associated with the current and alternative 

standards was unable to estimate the distribution of 

ambient O3 concentrations in New York City upon 

just meeting an alternative standard with a level of 

60 ppb. Therefore, for the 60 ppb standard level the 

numbers of children and asthmatic children reflect 

all of the urban study areas except New York. 

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 hostdefense 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 atrisk 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 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). 

(2) Based on the HREA results, meeting an 

O3 standard with a level of 60 ppb would be 

expected to almost eliminate exposures of 

concern to O3 concentrations at or above 60 

ppb. To the extent lower exposure 

concentrations may result in adverse health 

effects in some people, a standard level of 60 

ppb would be expected to also reduce 

exposures to O3 concentrations below 60 

ppb. 

(3) U.S. and Canadian epidemiologic 

studies have not reported O3 health effect 

associations based primarily on study 

locations likely to have met a standard with 

a level of 60 ppb. 

(4) In all of the urban study areas 

evaluated, a standard with a level of 60 ppb 

would be expected to maintain long-term O3

concentrations below those where a key 

study indicates the most confidence in a 

linear concentration-response relationship 

with respiratory mortality. 

Given all of the above considerations 

the PA concludes that, compared to 

standards with levels from 70 to 60 ppb, 

the extent to which standards with 

levels below 60 ppb could result in 

further public health improvements 

becomes notably less certain. Therefore, 

the PA concludes that it is not 

appropriate in this review to consider 

standard levels below 60 ppb. 

The following sections summarize the 

PA’s consideration of the scientific 

evidence and exposure/risk information 

specifically related to potential 

alternative O3 standards with levels 

from the upper (70 ppb) (II.E.4.c.i), 

middle (65 ppb) (II.E.4.c.ii), and lower 

(60 ppb) (II.E.4.c.iii) portions of the 

range of 70 to 60 ppb. Key exposure/risk 

information considered in the PA is 

summarized in Tables 4 and 5, below 

(from U.S. EPA, 2014c, Tables 4–4 and 

4–5). 

TABLE 4—SUMMARY OF ESTIMATED EXPOSURES OF CONCERN FOR POTENTIAL ALTERNATIVE O3 STANDARD LEVELS OF

70, 65, 60 PPB IN URBAN CASE STUDY AREAS 129

Benchmark level 

Alternative 

standard level 

(ppb) 

Average % 

children 

exposed 130

Number of children (5 to 18 years) 

[number of asthmatic children] 131 132

Average % 

reduction from 

current 

standard 133

% 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 

60 134 0 1,400 [200] 135 .................................................................. 100 0.1 

≥60 ppb ............. 70 3.3–10.2 1,176,000 [126,000] .......................................................... 46 18.9 

65 0–4.2 392,000 [42,000] ............................................................... 80 9.5 


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136Estimates 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. 

137Numbers 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. 

138As noted in section II.C.3.a.ii, the 

responsiveness of asthmatics to O3 exposures could 

depend on factors that have not been well-evaluated 

such as asthma severity, the effectiveness of asthma 

control, or the prevalence of medication use. 

139As discussed in section 4.3.3 of the HREA 

(U.S. EPA, 2014a), the model-based air quality 

adjustment approach used to estimate risks 

associated with the current and alternative 

standards was unable to estimate the distribution of 

ambient O3 concentrations in New York City upon 

just meeting an alternative standard with a level of 

60 ppb. Therefore, for the 60 ppb standard level the 

numbers of children and asthmatic children reflect 

all of the urban study areas except New York. 

TABLE 4—SUMMARY OF ESTIMATED EXPOSURES OF CONCERN FOR POTENTIAL ALTERNATIVE O3 STANDARD LEVELS OF

70, 65, 60 PPB IN URBAN CASE STUDY AREAS 129—Continued 

Benchmark level 

Alternative 

standard level 

(ppb) 

Average % 

children 

exposed 130

Number of children (5 to 18 years) 

[number of asthmatic children] 131 132

Average % 

reduction from 

current 

standard 133

% Children— 

worst year and 

worst area 

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 

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 % 

children 136

Number of children (5 to 18 years) 

[number of asthmatic children] 137 138

Average % 

reduction from 

current 

standard 

% Children 

worst year and 

area 

One or more decrements per season 

≥10% ................. 70 11–17 2,527,000 [261,000] .......................................................... 15 20 

65 3–15 1,896,000 [191,000] .......................................................... 31 18 

60 5–11 1,404,000 [139,000] 139 .................................................... 45 13 

≥15% ................. 70 2–4 562,000 [58,000] ............................................................... 26 5 

65 0–3 356,000 [36,000] ............................................................... 50 4 

60 1–2 225,000 [22,000] ............................................................... 67 3 

≥20% ................. 70 1–2 189,000 [20,000] ............................................................... 32 2.1 

65 0–1 106,000 [11,000] ............................................................... 59 1.4 

60 0–1 57,000 [6,000] ................................................................... 77 0.7 

Two or more decrements per season 

≥10% ................. 70 5.5–11 1,414,000 [145,000] .......................................................... 17 13 

65 1.3–8.8 1,023,000 [102,000] .......................................................... 37 11 

60 2.1–6.4 741,000 [73,000] ............................................................... 51 7.3 

≥15% ................. 70 0.9–2.4 276,000 [28,000] ............................................................... 29 3.1 

65 0.1–1.8 168,000 [17,000] ............................................................... 54 2.3 

60 0.2–1.0 101,000 [10,000] ............................................................... 71 1.4 

≥20% ................. 70 0.3–0.8 81,000 [8,000] ................................................................... 34 1.1 

65 0–0.5 43,000 [4,000] ................................................................... 66 0.8 

60 0–0.2 21,000 [2,000] ................................................................... 83 0.4 

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


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140As 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. 

141Put another way, one cannot infer from this 

analysis the extent to which effects would occur at 

O3 concentrations below those observed in the 

study. 

142 In addition, for the other multicity studies 

identified in Table 4–1 of the PA (Cakmak et al., 

2006; Stieb et al., 2009; Katsouyanni et al., 2009), 

and for the study by Bell et al. (2006) (for the 30 

ppb cut point) (Table 4–2 of the PA), the majority 

of study locations would likely have met a standard 

with a level of 70 ppb (U.S. EPA, 2014c). 

standard with a level of 70 ppb would 

reduce exposures of concern to O3

concentrations of 60, 70, and 80 ppb in 

urban study areas, with such a standard 

level estimated to be most effective at 

limiting exposures at or above the 

higher health benchmark concentrations 

and at limiting multiple occurrences of 

such exposures. On average over the 

years 2006 to 2010, for a standard with 

a level of 70 ppb, up to about 1% of 

children (i.e., ages 5 to 18) are estimated 

to experience exposures of concern at or 

above 70 ppb (73% reduction, compared 

to current standard), and far less than 

1% are estimated to experience two or 

more such exposures (95% reduction, 

compared to current standard). In the 

worst-case location and year (i.e., 

location and year with the largest 

exposure estimate), about 3% of 

children are estimated to experience one 

or more exposures of concern at or 

above 70 ppb, and less than 1% are 

estimated to experience two or more. 

Far less than 1% of children are 

estimated to experience exposures of 

concern at or above the 80 ppb 

benchmark concentration, even in the 

worst-case year (Table 4, above).140

As noted above, CASAC advised the 

EPA that 60 ppb is an appropriate 

exposure of concern with respect to 

adverse effects on people with asthma, 

including children (Frey, 2014c, pp. 6 

and 8). For an O3 standard with a level 

of 70 ppb, about 3 to 10% of children, 

including asthmatic children, are 

estimated to experience one or more 

exposures of concern at or above 60 ppb 

in a single O3 season. Compared to the 

current standard, this reflects about a 

46% reduction, on average across the 

urban study areas. About 1% to 4% of 

children are estimated to experience 

two or more exposures of concern at or 

above 60 ppb (approximately 60% 

reduction, compared to current 

standard). In the worst-case location and 

year, for a standard set at 70 ppb, about 

19% of children are estimated to 

experience one or more exposures of 

concern at or above 60 ppb, and 9% are 

estimated to experience two or more 

such exposures (Table 4, above). 

Compared to the current standard, the 

HREA estimates that a revised O3

standard with a level of 70 ppb would 

also reduce O3-induced lung function 

decrements in children. A level of 70 

ppb is estimated to be most effective at 

limiting the occurrences of moderate 

and large lung function decrements (i.e., 

FEV1 decrements ≥15% and ≥20%, 

respectively), and at limiting multiple 

occurrences of O3-induced decrements. 

On average over the years 2006 to 2010, 

for a standard with a level of 70 ppb, 

about 2 to 4% of children in the urban 

study areas are estimated to experience 

one or more moderate O3-induced lung 

function decrements (i.e., FEV1

decrement ≥15%), which would be of 

concern for healthy people, and about 1 

to 2.5% of children are estimated to 

experience two or more such 

decrements (approximately 30% 

reduction, compared to the current 

standard). In the worst-case location and 

year, up to 5% of children are estimated 

to experience one or more O3-induced 

lung function decrements ≥15%, and up 

to 3% are estimated to experience two 

or more such decrements. For a 

standard set at 70 ppb, about 2% or 

fewer children are estimated to 

experience large O3-induced lung 

function decrements (i.e., FEV1

decrement ≥20%), and about 1% or 

fewer children are estimated to 

experience two or more such 

decrements, even in the worst-case 

years and locations (Table 5, above). 

On average over the years 2006 to 

2010, for an O3 standard set at 70 ppb, 

about 11 to 17% of children in the 

urban study areas are estimated to 

experience one or more moderate O3- 

induced lung function decrements (i.e., 

FEV1 decrement ≥10%), which could be 

adverse for people with lung disease. 

This reflects an average reduction of 

about 15%, compared to the current 

standard. About 6 to 11% of children 

are estimated to experience two or more 

such decrements (17% reduction, 

compared to current standard). In the 

worst-case location and year, for a 

standard set at 70 ppb, about 20% of 

children in the urban study areas are 

estimated to experience one or more O3- 

induced lung function decrements 

≥10%, and 13% are estimated to 

experience two or more such 

decrements (Table 5, above). 

Compared to the current standard, a 

revised standard with a level of 70 ppb 

would also more effectively maintain 

short-term ambient O3 concentrations 

below those present in the 

epidemiologic studies that reported 

significant O3 health effect associations 

in locations likely to have met the 

current standard. In particular, the 

single-city study by Mar and Koenig 

(2009) reported positive and statistically 

significant associations with respiratory 

emergency department visits in children 

and adults in a location that likely 

would have met the current O3 standard 

over the entire study period but violated 

a revised standard with a level of 70 ppb 

or below. None of the single-city studies 

evaluated in section 4.4.1 of the PA 

(U.S. EPA, 2014c) provide evidence for 

O3 health effect associations in locations 

meeting a standard with a level of 70 

ppb or below. While this analysis does 

not provide information on the extent to 

which the reported O3-associated 

emergency department visits would 

persist upon meeting an O3 standard 

with a level of 70 ppb, or on the extent 

to which standard levels below 70 ppb 

could further reduce the incidence of 

such emergency department visits,141 it 

suggests that a revised O3 standard with 

a level at or below 70 ppb would require 

reductions in the ambient O3

concentrations that provided the basis 

for the health effect associations 

reported by Mar and Koenig (2009). 

As discussed above, compared to 

single-city studies, there is greater 

uncertainty in linking air quality 

concentrations from individual study 

cities to multicity effect estimates. With 

regard to the multicity studies in this 

review, the PA notes that Dales et al. 

(2006) reported significant associations 

with respiratory hospital admissions 

based on air quality in 11 Canadian 

cities, most of which would likely have 

met the current standard over the entire 

study period, but violated a revised 

standard with a level of 70 ppb or below 

over at least part of that period (Table 

4–1). This analysis suggests that 

although the current standard would 

allow the ambient O3 concentrations in 

most of the study locations that 

provided the basis for the association 

with hospital admissions, a revised O3

standard with a level at or below 70 ppb 

would require reductions in those 

ambient O3 concentrations. As with the 

study by Mar and Koenig (2009), this 

analysis does not provide information 

on the extent to which the reported O3- 

associated hospital admissions would 

persist upon meeting an O3 standard 

with a level of 70 ppb, or on the extent 

to which standard levels below 70 ppb 

could further reduce the incidence of 

such hospital admissions.142

With regard to long-term O3

concentrations, the PA evaluates the 

long-term O3 metrics reported to be 

associated with mortality or morbidity 

in recent epidemiologic studies (e.g., 


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143As discussed in section 3.1.4.3 of the PA (U.S. 

EPA, 2014c), the study by Jerrett et al. (2009) 

suggests notably decreased confidence in the 

reported linear concentration-response function for 

long-term O3 concentrations in the first quartile 

(i.e., at or below about 53 ppb), given the widening 

in confidence intervals for lower concentrations; 

the fact that most study cities contributing to the 

linear function had O3 concentrations in the highest 

three quartiles, accounting for approximately 72% 

of the respiratory deaths in the cohort (based on 

Table 2 in the published study); and the limited 

evidence presented in the published study for a 

threshold at or near 56 ppb. 

144As 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 epidemiologicbased risk estimates. 

145A 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. 

146Also see Frey (2014c, p. ii). 

147Epidemiologic 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. 

seasonal averages of 1-hour or 8-hour 

daily max concentrations). Compared to 

the current standard, a revised standard 

with a level of 70 ppb would be 

expected to reduce the risk of 

respiratory mortality associated with 

long-term O3 concentrations, based on 

information from the study by Jerrett et 

al. (2009), though the PA notes the 

HREA conclusion, discussed above, that 

lower confidence should be placed in 

respiratory mortality risk estimates 

based on this study (U.S. EPA, 2014a, 

section 9.6). In addition, a standard with 

a level of 70 ppb would be expected to 

more effectively maintain long-term O3

concentrations below those where the 

study by Jerrett et al. (2009) indicates 

the most confidence in the reported 

association with respiratory 

mortality.143 Specifically, air quality 

analyses indicate this to be the case in 

9 out of the 12 urban study areas for a 

level of 70 ppb, compared to 6 out of 12 

areas for the current standard. Finally, 

a revised standard with a level of 70 ppb 

would be expected to reduce long-term 

O3 concentrations based on the types of 

metrics that have been reported in 

recent epidemiologic studies to be 

associated with respiratory morbidity 

(i.e., seasonal averages of daily 

maximum 8-hour concentrations). 

In further considering the potential 

implications of epidemiology studies for 

alternative standard levels, the PA notes 

estimates of total mortality associated 

with short-term O3 concentrations.144

As discussed above, the PA considers 

estimates of total risk (i.e., based on the 

full distributions of ambient O3

concentrations) and estimates of risk 

associated with O3 concentrations in the 

upper portions of ambient distributions. 

With regard to total risk the PA notes 

that, when summed across urban study 

areas, a standard with a level of 70 ppb 

is estimated to reduce the number of 

deaths associated with short-term O3

concentrations by about 4% (2007) and 

2% (2009), compared to the current 

standard.145 Based on a national 

modeling analysis, the majority of the 

U.S. population would be expected to 

experience reductions in such risks 

upon reducing precursor emissions. 

Compared to the total risk estimates 

noted above, an O3 standard with a level 

of 70 ppb is estimated to be more 

effective at reducing the number of 

deaths associated with short-term O3

concentrations at the upper ends of 

ambient distributions. Specifically, for 

area-wide O3 concentrations at or above 

40 ppb, a standard with a level of 70 

ppb is estimated to reduce the number 

of deaths associated with short-term O3

concentrations by about 10% compared 

to the current standard. In addition, for 

area-wide concentrations at or above 60 

ppb, a standard with a level of 70 ppb 

is estimated to reduce O3-associated 

deaths by about 50% to 70% (U.S. EPA, 

2014c, Figure 4–13). 

The PA noted that in providing the 

advice that 70 ppb is an appropriate 

upper bound for consideration, CASAC 

advised that a level of 70 ppb would 

provide little margin of safety for 

protection of public health, particularly 

for sensitive subpopulations (Frey, 

2014c, p. 8). In particular, CASAC stated 

that: 

At 70 ppb, there is substantial scientific 

certainty of a variety of adverse effects, 

including decrease in lung function, increase 

in respiratory symptoms, and increase in 

airway inflammation. Although a level of 70 

ppb is more protective of public health than 

the current standard, it may not meet the 

statutory requirement to protect public health 

with an adequate margin of safety (Frey, 

2014c, p. 8).146

However, the committee also 

acknowledged that ‘‘the choice of a level 

within the range recommended based 

on scientific evidence [i.e., 70 to 60 ppb] 

is a policy judgment under the statutory 

mandate of the Clean Air Act’’ (Frey, 

2014c, pp. ii and 8). 

In summary, compared to the current 

standard, the PA concludes that a 

revised O3 standard with a level of 70 

ppb would be expected to (1) reduce the 

occurrence of exposures of concern to 

O3 concentrations that result in 

respiratory effects in healthy adults (at 

or above 60 and 70 ppb) by about 45 to 

95%, almost eliminating the occurrence 

of multiple exposures at or above 70 

ppb; (2) reduce the occurrence of 

moderate-to-large O3-induced lung 

function decrements (FEV1 decrements 

≥10, 15, 20%) by about 15 to 35%, most 

effectively limiting the occurrence of 

multiple decrements and decrements 

≥15, 20%; (3) more effectively maintain 

short- and long-term O3 concentrations 

below those present in the 

epidemiologic studies that reported 

significant O3 health effect associations 

in locations likely to have met the 

current standard; 147 and (4) reduce the 

risk of O3-associated mortality and 

morbidity, particularly the risk 

associated with the upper portions of 

the distributions of ambient O3

concentrations. 

ii. PA Consideration of an O3 Standard 

Level of 65 ppb 

The PA also considers a standard with 

a level of 65 ppb. A level of 65 ppb is 

well below 80 ppb, an O3 exposure 

concentration that has been reported to 

elicit a range of respiratory effects that 

includes airway hyperresponsiveness 

and decreased lung host defense, in 

addition to lung function decrements, 

airway inflammation, and respiratory 

symptoms. A standard level of 65 ppb 

is also below the lowest exposure 

concentration at which the combined 

occurrence of respiratory symptoms and 

lung function decrements has been 

reported (i.e., 72 ppb), a combination 

judged adverse by the ATS (U.S. EPA, 

2014c, section 3.1.3). A level of 65 ppb 

is above the lowest exposure 

concentration demonstrated to result in 

lung function decrements large enough 

to be judged an abnormal response by 

ATS, where statistically significant 

changes in group mean responses would 

be judged to be adverse by ATS, and 

which the CASAC has indicated could 

be adverse in people with lung disease 

(i.e., 60 ppb). A level of 65 ppb is also 

above the lowest exposure 

concentration at which pulmonary 

inflammation has been reported in 

healthy adults (i.e., 60 ppb). 

Compared to the current standard and 

a revised standard with a level of 70 

ppb, the HREA estimates that a standard 

with a level of 65 ppb would reduce 

exposures of concern to the range of O3

benchmark concentrations analyzed 

(i.e., 60, 70, and 80 ppb). The HREA 

estimates that meeting a standard with 

a level of 65 ppb would eliminate 

exposures of concern at or above 80 ppb 

in the urban study areas. Such a 

standard is estimated to allow far less 

than 1% of children in the urban study 

areas to experience one or more 

exposures of concern at or above the 70 


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148For 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). 

149Though as discussed above, the PA notes the 

lower confidence placed in these risk results (U.S. 

EPA, 2014a, section 9.6). 

150As 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 epidemiologicbased risk estimates. 

151A 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. 

ppb benchmark level, even in the worstcase years and locations, and is 

estimated to eliminate the occurrence of 

two or more exposures at or above 70 

ppb (Table 4, above). 

In addition, for a standard with a level 

of 65 ppb, between 0 and about 4% of 

children (including asthmatic children) 

in urban study areas are estimated to 

experience exposures of concern at or 

above 60 ppb, which CASAC has 

indicated is an appropriate exposure of 

concern for people with asthma, 

including children. This reflects an 80% 

reduction (on average across areas), 

relative to the current standard. Less 

than 1% of children are estimated to 

experience two or more exposures of 

concern at or above 60 ppb (≤ 90% 

reduction, compared to current 

standard). In the worst-case location and 

year, about 10% of children are 

estimated to experience one or more 

exposures of concern at or above 60 

ppb, with about 3% estimated to 

experience two or more such exposures 

(Table 4, above). 

Compared to the current standard and 

a revised standard with a level of 70 

ppb, the HREA estimates that a standard 

with a level of 65 ppb would also 

further reduce the occurrence of O3- 

induced lung function decrements. For 

a level of 65 ppb, about 4% of children, 

or less, are estimated to experience 

moderate O3-induced FEV1 decrements 

≥15% (50% reduction, compared to 

current standard), even considering the 

worst-case location and year. About 2% 

of children, or less, are estimated to 

experience two or more such 

decrements. Only about 1% of children, 

or less, are estimated to experience large 

O3-induced lung function decrements 

(i.e., FEV1 decrement ≥20%), even in the 

worst-case year and location. 

In addition, for a standard with a level 

of 65 ppb, about 3 to 15% of children 

are estimated to experience one or more 

moderate O3-induced lung function 

decrements (i.e., FEV1 decrement 

≥10%), which CASAC has indicated 

could be adverse for people with lung 

disease. This reflects an average 

reduction of about 30%, relative to the 

current standard. About 1 to 9% of 

children in the urban study areas are 

estimated to experience two or more 

such decrements (37% reduction, 

compared to current standard). In the 

worst-case location and year, for a 

standard set at 65 ppb, up to about 18% 

of these children are estimated to 

experience one or more moderate O3- 

induced lung function decrements 

≥10%, and up to 11% are estimated to 

experience two or more such 

decrements. 

With regard to O3 epidemiologic 

studies, the PA notes that a revised 

standard with a level of 65 ppb would 

be expected to maintain short-term 

ambient O3 concentrations below those 

present in some of the study locations 

that provided the basis for reported O3

health effect associations and that were 

likely to have met a revised standard 

with a level of 70 ppb. In particular, 

Katsouyanni et al. (2009) reported 

statistically significant associations with 

mortality based on air quality in 12 

Canadian cities, most of which would 

likely have met a standard with a level 

of 70 ppb over the entire study period 

but violated a revised standard with a 

level of 65 ppb or below over at least 

part of that period (U.S. EPA, 2014c, 

Table 4–1). This analysis suggests that 

although the current standard or a 

standard with a level of 70 ppb would 

allow the ambient O3 concentrations in 

most of the study locations that 

provided the basis for the association 

with mortality in this study, a revised 

O3 standard with a level at or below 65 

ppb would require reductions in those 

ambient O3 concentrations. As 

discussed above for a level of 70 ppb, 

this analysis does not provide 

information on the extent to which O3- 

associated mortality would persist upon 

meeting an O3 standard with a level of 

65 ppb, or on the extent to which 

standard levels below 65 ppb could 

further reduce the incidence of this 

mortality.148

With regard to long-term O3

concentrations, as for 70 ppb (above) the 

PA evaluates the long-term O3 metrics 

reported to be associated with mortality 

or morbidity in recent epidemiologic 

studies (e.g., seasonal averages of 1-hour 

or 8-hour daily max concentrations). 

Compared to the current standard or a 

revised O3 standard with a level of 70 

ppb, a revised standard with a level of 

65 ppb would be expected to further 

reduce the risk of respiratory mortality 

associated with long-term O3

concentrations, based on information 

from the study by Jerrett et al. (2009).149

In addition, a standard with a level of 

65 ppb would be expected to more 

effectively maintain long-term O3

concentrations below those where the 

study by Jerrett et al. (2009) indicates 

the most confidence in the reported 

association with respiratory mortality. 

Specifically, air quality analyses 

indicate this to be the case in 10 out of 

the 12 urban study areas for a level of 

65 ppb, compared to 6 out of 12 areas 

for the current standard and 9 out of 12 

for a standard with a level of 70 ppb 

(U.S. EPA, 2014c, Table 4–3). Finally, a 

revised standard with a level of 65 ppb 

would be expected to further reduce 

long-term O3 concentrations based on 

the types of metrics that have been 

reported in recent epidemiologic studies 

to be associated with respiratory 

morbidity (i.e., seasonal averages of 

daily maximum 8-hour concentrations). 

In further considering the potential 

implications of epidemiology studies for 

alternative standard levels, the PA notes 

estimates of total mortality associated 

with short-term O3.150 As discussed 

above, the PA considers estimates of 

total risk (i.e., based on the full 

distributions of ambient O3

concentrations) and estimates of risk 

associated with O3 concentrations in the 

upper portions of ambient distributions. 

With regard to total risk the PA notes 

that, when summed across urban study 

areas, a standard with a level of 65 ppb 

is estimated to reduce the number of 

deaths associated with short-term O3

exposures by about 13% (2007) and 9% 

(2009), compared to the current 

standard.151 For area-wide 

concentrations at or above 40 ppb, a 

standard level of 65 ppb is estimated to 

reduce O3-associated deaths by almost 

50% compared to the current standard, 

when summed across urban study areas. 

For area-wide concentrations at or above 

60 ppb, a standard level of 65 ppb is 

estimated to reduce O3-associated 

deaths by more than 80% (U.S. EPA, 

2014c, Figure 4–13). 

In summarizing CASAC’s advice 

regarding a standard with a level of 65, 

the PA noted CASAC’s conclusion that 

an alternative standard with a level of 

65 ppb would further reduce, though 

not eliminate, the frequency of lung 

function decrements ≥15% and would 

lead to lower frequency of short-term 

premature mortality (i.e., compared to a 

standard with a level of 70 ppb) (Frey, 

2014c, p. 8). 

In summary, compared to a standard 

with a level of 70 ppb, the PA concludes 

that a revised standard with a level of 


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152Though 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. 

153Though as discussed above, the PA notes the 

lower confidence we place in these risk results 

(U.S. EPA, 2014a, section 9.6). 

154As 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. 

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; 152 and (4) further 

reduce the risk of O3-associated 

mortality and morbidity, particularly 

the risk associated with the upper 

portion of the distribution of ambient O3

concentrations. 

iii. PA Consideration of an O3 Standard 

Level of 60 ppb 

The PA also considers a standard with 

a level of 60 ppb. A level of 60 ppb is 

well below the O3 exposure 

concentration that has been reported to 

elicit a wide range of potentially adverse 

respiratory effects in healthy adults (i.e., 

80 ppb). A level of 60 ppb is also below 

the lowest concentration where the 

combined occurrence of respiratory 

symptoms and lung function 

decrements was observed, a 

combination judged adverse by the ATS 

(i.e., 72 ppb). A level of 60 ppb 

corresponds to the lowest exposure 

concentration demonstrated to result in 

lung function decrements that are large 

enough to be judged an abnormal 

response by ATS, that meet ATS criteria 

for adversity based on a downward shift 

in the distribution of FEV1, and that the 

CASAC indicated could be adverse in 

people with lung disease. A level of 60 

ppb also corresponds to the lowest 

exposure concentration at which 

pulmonary inflammation has been 

reported in a single controlled human 

exposure study. 

Based on the HREA analyses of O3

exposures of concern, a standard with a 

level of 60 ppb is estimated to eliminate 

exposures of concern at or above the 70 

and 80 ppb benchmark concentrations 

and to be more effective than the higher 

standard levels at limiting exposures of 

concern at or above 60 ppb. On average 

over the years 2006 to 2010, for a 

standard with a level of 60 ppb, between 

0 and about 1% of children, including 

asthmatic children, in urban study areas 

are estimated to experience exposures of 

concern at or above 60 ppb, which 

CASAC indicated is an appropriate 

exposure of concern for asthmatic 

children. This reflects a 96% reduction 

(on average across areas), compared to 

the current standard. Virtually no 

children are estimated to experience 

two or more exposures of concern at or 

above 60 ppb. In the worst-case location 

and year, about 2% of children are 

estimated to experience exposures of 

concern at or above 60 ppb, with far less 

than 1% estimated to experience two or 

more such exposures (Table 4, above). 

Based on the HREA analyses of O3- 

induced lung function decrements, a 

standard with a level of 60 ppb would 

be expected to be more effective than a 

level of 65 or 70 ppb at limiting the 

occurrence of O3-induced lung function 

decrements. For a standard with a level 

of 60 ppb, about 2% of children, or less, 

in the urban study areas are estimated 

to experience one or more moderate O3- 

induced FEV1 decrements ≥15% (almost 

70% reduction, compared to current 

standard), and about 1% or less are 

estimated to experience two or more 

such decrements (3% in the location 

and year with the largest estimates). 

About 1% of children, or less, are 

estimated to experience large O3- 

induced lung function decrements (i.e., 

FEV1 decrement ≥20%), even in the 

worst-case locations and year (Table 5, 

above). 

In addition, for a standard with a level 

of 60 ppb, about 5 to 11% of children 

in the urban study areas are estimated 

to experience one or more moderate O3- 

induced lung function decrements that 

CASAC indicated could be adverse for 

people with lung disease (i.e., FEV1

decrements ≥10%). This reflects an 

average reduction of about 45%, 

compared to the current standard. 

About 2 to 6% of children in these areas 

are estimated to experience two or more 

such decrements (51% reduction, 

compared to current standard). In the 

worst-case location and year, for a 

standard set at 60 ppb, up to about 13% 

of children are estimated to experience 

one or more moderate O3-induced FEV1

decrements ≥10%, and 7% are 

estimated to experience two or more 

such decrements (Table 5, above). 

With regard to O3 epidemiologic 

studies, the PA notes that a revised 

standard with a level of 60 ppb would 

be expected to maintain short-term 

ambient O3 concentrations below those 

present in some of the study locations 

that provided the basis for reported O3

health effect associations and that were 

likely to have met a revised standard 

with a level of 70 or 65 ppb. 

Specifically, in all of the U.S. and 

Canadian epidemiologic studies 

evaluated, the majority of study cities 

had ambient O3 concentrations that 

would likely have violated a standard 

with a level of 60 ppb. Thus, none of the 

U.S. and Canadian epidemiologic 

studies analyzed provide evidence for 

O3 health effect associations when the 

majority of study locations would likely 

have met a standard with a level of 60 

ppb (U.S. EPA, 2014c, Tables 4–1 and 

4–2). As discussed above, while this 

analysis does not provide information 

on the extent to which the O3-associated 

morbidity or mortality would persist 

upon meeting an O3 standard with a 

level of 60 ppb, it suggests that a revised 

O3 standard with a level of 60 ppb 

would require reductions in the ambient 

O3 concentrations that provided the 

basis for those health effect associations. 

With regard to long-term O3

concentrations, compared to the current 

standard or a revised O3 standard with 

a level of 65 or 70 ppb, a revised 

standard with a level of 60 ppb would 

be expected to further reduce the risk of 

respiratory mortality associated with 

long-term O3 concentrations, based on 

information from the study by Jerrett et 

al. (2009).153 In addition, a standard 

with a level of 60 ppb would be 

expected to more effectively maintain 

long-term O3 concentrations below 

those where the study by Jerrett et al. 

(2009) indicates the most confidence in 

the reported association with respiratory 

mortality. Specifically, air quality 

analyses indicate this to be the case in 

all of the urban study areas evaluated at 

a level of 60 ppb, compared to 6 out of 

12 areas for the current standard, 9 out 

of 12 for a standard with a level of 70 

ppb, and 10 out of 12 for a standard 

with a level of 65 ppb (U.S. EPA, 2014c, 

Table 4–3). Finally, a revised standard 

with a level of 60 ppb would be 

expected to further reduce long-term O3

concentrations based on the types of 

metrics that have been reported in 

recent epidemiologic studies to be 

associated with respiratory morbidity 

(i.e., seasonal averages of daily 

maximum 8-hour concentrations). 

In further considering the potential 

implications of epidemiology studies for 

alternative standard levels, the PA notes 

estimates of total mortality associated 

with short-term O3 concentrations.154


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155A 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. 

156As 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. 

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).155 For area-wide concentrations at 

or above 40 ppb, a standard with a level 

set at 60 ppb is estimated to reduce O3- 

associated deaths by almost 60% 

compared to the current standard. For 

area-wide concentrations at or above 60 

ppb, a standard level of 60 ppb is 

estimated to reduce O3-associated 

deaths by over 95% compared to the 

current standard. 

In summary, compared to a standard 

with a level of 65 or 70 ppb, the PA 

concludes that a revised standard with 

a level of 60 ppb would be expected to 

further reduce O3 exposures and health 

risks. In particular, a standard with a 

level of 60 ppb is estimated to (1) reduce 

the occurrence of exposures of concern 

by about 95 to 100%, compared to the 

current standard, almost eliminating 

exposures at or above 60 ppb; (2) reduce 

the occurrence of FEV1 decrements ≥10, 

15, and 20% by about 45 to 85%, 

compared to the current standard; (3) 

more effectively maintain short- and 

long-term O3 concentrations below 

those present in the epidemiologic 

studies that reported significant O3

health effect associations in locations 

likely to have met the current 

standard; 156 and (4) further reduce the 

risk of O3-associated mortality and 

morbidity, particularly the risk 

associated with the upper portion of the 

distribution of ambient O3

concentrations. 

c. CASAC Advice 

The PA recognizes that decisions 

regarding the weight to place on various 

types of evidence, exposure/risk 

information, and associated 

uncertainties reflect public health 

policy judgments that are ultimately left 

to the Administrator. To help inform 

those judgments with regard to the 

range of alternative primary O3 standard 

levels appropriate for consideration, 

CASAC has provided advice to the 

Administrator based on their reviews of 

draft versions of the O3 ISA, HREA, and 

PA. This section summarizes the advice 

provided by CASAC regarding 

alternative standard levels, as well as 

the views expressed at the CASAC 

meetings by public commenters. This 

section includes CASAC advice from 

the reconsideration of the 2008 final 

decision on the level of the standard, as 

well as CASAC advice received during 

the current review as it pertains to 

alternative standards. 

Consistent with its advice in 2008, 

CASAC reiterated during the 

reconsideration its support for an 8-hour 

primary O3 standard with a level 

ranging from 60 to 70 ppb, combined 

with the current indicator, averaging 

time, and form. Specifically, in response 

to the EPA’s solicitation of CASAC 

advice during the reconsideration, the 

CASAC letter (Samet, 2010) to the 

Administrator stated: 

CASAC fully supports EPA’s proposed range 

of 0.060–0.070 parts per million (ppm) for 

the 8-hour primary ozone standard. CASAC 

considers this range to be justified by the 

scientific evidence as presented in the Air 

Quality Criteria for Ozone and Related 

Photochemical Oxidants (March 2006) and 

Review of the National Ambient Air Quality 

Standards for Ozone: Policy Assessment of 

Scientific and Technical Information, 

OAQPS Staff Paper (July 2007). 

Similarly, in response to the EPA’s 

request for additional advice on the 

reconsideration in 2011, CASAC 

reaffirmed its conclusion that ‘‘the 

evidence from controlled human and 

epidemiologic studies strongly supports 

the selection of a new primary ozone 

standard within the 60–70 ppb range for 

an 8-hour averaging time’’ (Samet, 

2011). CASAC further concluded that 

this range ‘‘would provide little margin 

of safety at its upper end’’ (Samet, 2011, 

p. 2). 

In the current review of the Second 

Draft PA, CASAC concurred with staff’s 

conclusions that it is appropriate to 

consider retaining the current indicator 

(O3), averaging time (8-hour average) 

and form (3-year average of the 4th 

highest maximum daily 8-hour average. 

With regard to level, CASAC stated the 

following (Frey, 2014c, pp. ii to iii): 

The CASAC further concludes that there is 

adequate scientific evidence to recommend a 

range of levels for a revised primary ozone 

standard from 70 ppb to 60 ppb. The CASAC 

reached this conclusion based on the 

scientific evidence from clinical studies, 

epidemiologic studies, and animal toxicology 

studies, as summarized in the Integrated 

Science Assessment (ISA), the findings from 

the exposure and risk assessments as 

summarized in the HREA, and the 

interpretation of the implications of these 

sources of information as given in the Second 

Draft PA. 

The CASAC acknowledges that the choice 

of a level within the range recommended 

based on scientific evidence [i.e., 70 to 60 

ppb] is a policy judgment under the statutory 

mandate of the Clean Air Act. The CASAC 

advises that, based on the scientific evidence, 

a level of 70 ppb provides little margin of 

safety for the protection of public health, 

particularly for sensitive subpopulations. 

Thus, our policy advice is to set the level 

of the standard lower than 70 ppb within a 

range down to 60 ppb, taking into account 

your judgment regarding the desired margin 

of safety to protect public health, and taking 

into account that lower levels will provide 

incrementally greater margins of safety. 

The public commenters who 

expressed the view that the current 

primary O3 standard is not adequate 

(II.D.3) also submitted comments that 

supported revising the level of the 

primary O3 standard. Several of these 

commenters expressed the view that the 

level should be revised to the lower end 

of the range of 70 to 60 ppb, or in some 

cases to a level below 60 ppb. These 

commenters often placed a large amount 

of emphasis on evidence from 

controlled human exposure studies for 

respiratory effects following exposures 

to 60 ppb O3. 

In addition, as discussed above 

(II.D.3), some public commenters 

expressed the view that revision of the 

current standard is not necessary. 

Consistent with their view that it would 

not be appropriate to revise the current 

standard, these commenters did not 

provide any provisional views on 

alternative levels below 75 ppb that 

would be appropriate for consideration. 

d. Administrator’s Proposed 

Conclusions on Level 

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 


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157As 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. 

158As 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. 

159As noted above, for the 70 ppb target exposure 

concentration, Schelegle et al. (2009) reported that 

the actual mean exposure concentration was 72 

ppb. 

160However, following exposures to 60 ppb O3, 

several studies have observed decreases in lung 

function and one study (Kim et al., 2011) observed 

an increase in airway inflammation (II.B.2). 

inferences to be drawn therefrom.157

The rationale supporting the 

Administrator’s proposed conclusions 

on alternative standard levels is 

discussed below. 

The Administrator’s proposed 

conclusions on alternative standard 

levels build upon her proposed 

conclusion that the overall body of 

scientific evidence and exposure/risk 

information call into question the 

adequacy of public health protection 

afforded by the current primary O3

standard, particularly for at-risk 

populations and lifestages (II.D.5). 

These proposed conclusions are based 

on consideration of the scientific 

evidence assessed in the ISA (U.S. EPA, 

2013a); the results of the exposure and 

risk assessments in the HREA (U.S. 

EPA, 2014a); the evidence-based and 

exposure-/risk-based considerations and 

conclusions in the PA (U.S. EPA, 

2014c); CASAC advice and 

recommendations, as reflected in 

CASAC’s letters to the Administrator 

and in public discussions of drafts of 

the ISA, HREA, and PA; and public 

input received during the development 

of these documents. 

In reaching proposed conclusions on 

alternative levels for the primary O3

standard, the Administrator considers 

the extent to which various alternatives 

would be expected to protect the public, 

including at-risk populations, against 

the wide range of adverse health effects 

that have been linked with short- or 

long-term O3 exposures. At-risk 

populations include people with 

asthma; children and older adults; 

people who are active outdoors, 

including outdoor workers; people with 

certain genetic variants; and people 

with reduced intake of certain nutrients. 

As was the case for her consideration 

of the adequacy of the current primary 

O3 standard (II.D.5), the Administrator 

places the greatest weight on the results 

of controlled human exposure studies 

and on exposure and risk analyses based 

on information from these studies. In 

doing so, she notes that controlled 

human exposure studies provide the 

most certain evidence indicating the 

occurrence of health effects in humans 

following exposures to specific O3

concentrations. The effects reported in 

these studies are due solely to O3

exposures, and interpretation of study 

results is not complicated by the 

presence of co-occurring pollutants or 

pollutant mixtures (as is the case in 

epidemiologic studies). She further 

notes the CASAC judgment that ‘‘the 

scientific evidence supporting the 

finding that the current standard is 

inadequate to protect public health is 

strongest based on the controlled human 

exposure studies of respiratory effects’’ 

(Frey, 2014c, p. 5). Consistent with this 

emphasis, the HREA conclusions reflect 

relatively greater confidence in the 

results of the exposure and risk analyses 

based on information from controlled 

human exposure studies (i.e., exposures 

of concern and risk of lung function 

decrements) than the results of 

epidemiology-based risk analyses, given 

the greater uncertainties in the 

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 60,158 72,159 or 80 

ppb, and higher. The largest respiratory 

effects, and the broadest range of effects, 

have been studied and reported 

following exposures of healthy adults to 

80 ppb O3 or higher, with most exposure 

studies conducted at these higher 

concentrations. Exposures of healthy 

adults to O3 concentrations of 80 ppb or 

higher have been reported to decrease 

lung function, increase airway 

inflammation, increase respiratory 

symptoms, result in airway 

hyperresponsiveness, and decrease lung 

host defenses (II.B.2). 

The Administrator notes that O3

exposure concentrations as low as 72 

ppb have been shown to both decrease 

lung function and increase respiratory 

symptoms (Schelegle et al., 2009), a 

combination that meets the ATS criteria 

for an adverse response. In considering 

effects at 72 ppb, CASAC likewise noted 

that ‘‘the combination of decrements in 

FEV1 together with the statistically 

significant alterations in symptoms in 

human subjects exposed to 72 ppb 

ozone meets the American Thoracic 

Society’s definition of an adverse health 

effect’’ (Frey, 2014c, p. 5). 

With regard to lower exposure 

concentrations, the Administrator notes 

that the combination of statistically 

significant increases in respiratory 

symptoms and decrements in lung 

function has not been reported. More 

specifically, she notes that respiratory 

symptoms have been evaluated 

following 6.6-hour exposures to average 

O3 concentrations of 60 ppb (Adams, 

2006; Kim et al., 2011) and 63 ppb 

(Schelegle et al., 2009) and that none of 

these studies reported a statistically 

significant increase in respiratory 

symptoms, compared to filtered air 

controls.160

Based on this evidence, the 

Administrator reaches the initial 

conclusion that the results of controlled 

human exposure studies strongly 

support setting the level of a revised O3

standard no higher than 70 ppb. In 

reaching this initial conclusion, the 

Administrator places a large amount of 

weight on the importance of setting the 

level of the standard well below 80 ppb, 

the O3 exposure concentration shown in 

healthy adults to result in the broadest 

range of respiratory effects, and below 

72 ppb, the lowest O3 exposure 

concentration shown in healthy adults 

to result in the adverse combination of 

respiratory symptoms and lung function 

decrements. 

In further considering the potential 

public health implications of a standard 

with a level of 70 ppb, the 

Administrator also considers the extent 

to which such a standard would be 

expected to limit population exposures 

to the broader range of O3

concentrations reported in controlled 

human exposure studies to cause 

respiratory effects. Given the range of 

effects reported following exposures to 

80 ppb O3, and the evidence for the 

adverse combination of lung function 

decrements and respiratory symptoms 

in healthy adults following exposures as 

low as 72 ppb, the Administrator 

concludes that the evidence in this 

review supports the occurrence of 

adverse respiratory effects for exposures 

to O3 concentrations at or above 72 ppb. 

The Administrator has decreasing 

confidence that adverse effects will 

occur following exposures to O3

concentrations below 72 ppb. In 

particular, compared to O3 exposure 


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161As 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). 

162For 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. 163State of Mississippi, 744 F. 3d at 1343. 

concentrations at or above 72 ppb, she 

has less confidence that adverse effects 

will occur following exposures to O3

concentrations as low as 60 ppb. In 

reaching this conclusion, she notes that, 

as discussed above, statistically 

significant increases in respiratory 

symptoms, combined with lung 

function decrements, have not been 

reported following exposures to 60 or 63 

ppb O3, though several studies have 

evaluated the potential for such effects. 

Although she has decreasing 

confidence in the occurrence of adverse 

effects following exposures to O3

concentrations below 72 ppb, the 

Administrator notes the CASAC 

judgment that the adverse combination 

of lung function decrements and 

respiratory symptoms ‘‘almost certainly 

occur in some people’’ following 

exposures to lower concentrations (Frey, 

2014c, p. 6). In particular, when 

commenting on the extent to which the 

study by Schelegle et al. (2009) suggests 

the potential for adverse effects 

following O3 exposures below 72 ppb, 

CASAC judged that: 

[I]f subjects had been exposed to ozone using 

the 8-hour averaging period used in the 

standard [i.e., rather than the 6.6 hour 

exposures evaluated in the study], adverse 

effects could have occurred at lower 

concentration. Further, in our judgment, the 

level at which adverse effects might be 

observed would likely be lower for more 

sensitive subgroups, such as those with 

asthma [i.e., compared to the healthy adults 

evaluated in the study] (Frey, 2014c, p. 5). 

Though CASAC did not provide advice 

as to how far below 72 ppb adverse 

effects would likely occur, the 

Administrator agrees that such effects 

could occur following exposures at least 

somewhat below 72 ppb. 

Based on the evidence and CASAC 

advice noted above, when considering 

the extent to which a standard with a 

level of 70 ppb would be expected to 

limit population exposures to the 

broader range of O3 concentrations 

shown to cause respiratory effects, the 

Administrator considers the extent to 

which such a standard would be 

expected to limit the occurrence of O3

exposures of concern at or above 60, 70, 

and 80 ppb.161 In doing so, she notes 

that an O3 standard established at a 

particular level can provide protection 

against a range of exposure 

concentrations, including 

concentrations below the standard level. 

This is because the degree of protection 

provided by any NAAQS is due to the 

combination of all of the elements of the 

standard (i.e., indicator, averaging time, 

form, level). In the case of the 4th 

maximum form of the O3 NAAQS, 

which the Administrator is proposing to 

retain in the current review (II.E.3), the 

large majority of days in areas that meet 

the standard will have 8-hour O3

concentrations below the level of the 

standard. 

In considering exposures of concern 

at or above 60, 70, and 80 ppb, the 

Administrator judges that the evidence 

supporting the occurrence of adverse 

respiratory effects is strongest for 

exposures at or above the 70 and 80 ppb 

benchmarks. While the Administrator 

has less confidence that adverse effects 

will occur following exposures to O3

concentrations as low as 60 ppb, she 

notes the possibility for adverse effects 

following such exposures given that (1) 

CASAC has indicated the moderate lung 

function decrements (i.e., FEV1

decrements ≥10%) that occur in some 

healthy adults following exposures to 60 

ppb O3, which are large enough to be 

judged an abnormal response by ATS, 

could be adverse to people with lung 

disease (II.B.3), and that (2) airway 

inflammation has been reported 

following exposures as low as 60 ppb 

O3. She also takes note of CASAC advice 

that the occurrence of exposures of 

concern at or above 60 ppb is an 

appropriate consideration for people 

(including children) with asthma (Frey, 

2014c, p. 6). 

Due to interindividual variability in 

responsiveness, the Administrator 

further notes that not every occurrence 

of an exposure of concern will result in 

an adverse effect.162 Repeated 

occurrences of some of the effects 

demonstrated following exposures of 

concern could increase the likelihood of 

adversity. For example, as discussed in 

the ISA (U.S. EPA, 2013a, Section 6.2.3), 

repeated occurrences of airway 

inflammation could lead to the 

induction of a chronic inflammatory 

state; altered pulmonary structure and 

function, leading to diseases such as 

asthma; altered lung host defense 

response to inhaled microorganisms, 

particularly in potentially at-risk 

populations such as the very young and 

old; and altered lung response to other 

agents such as allergens or toxins. The 

Administrator notes that the types of 

lung injury that can occur following 

exposures of concern, particularly if 

experienced repeatedly, provide a 

plausible mode of action by which O3

may cause other more serious effects. 

Therefore, the Administrator is most 

concerned about protecting at-risk 

populations against repeated 

occurrences of exposures of concern. 

Based on the above considerations, 

the Administrator focuses on the extent 

to which a revised standard would be 

expected to protect populations from 

experiencing two or more O3 exposures 

of concern (i.e., as a surrogate for 

repeated exposures). While she 

emphasizes the importance of limiting 

two or more exposures and reducing 

their occurrence, compared to the 

current standard, she balances this 

emphasis by noting that (1) not all 

exposures of concern will 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 zerorisk standards.163 Therefore, in using 

estimates of exposures of concern to 

inform her decisions on alternative 

standard levels, the Administrator 

judges that it would not be appropriate 

to set a standard intended to eliminate 

all exposures of concern for all 

benchmarks, particularly the 60 ppb 

benchmark. Her consideration of 

specific estimates of exposures of 

concern is discussed below. 

As illustrated in Table 1 (above), the 

Administrator notes that, in urban study 

areas, a revised standard with a level of 

70 ppb would be expected to eliminate 

the occurrence of two or more exposures 

of concern to O3 concentrations at and 

above 80 ppb and to virtually eliminate 

the occurrence of two or more exposures 

of concern to O3 concentrations at and 

above 70 ppb, even in the worst-case 

urban study area and year. For the 70 

ppb benchmark, this reflects about a 

95% reduction in the occurrence of two 

or more exposures of concern, compared 

to the current standard (Table 4). 

Though the Administrator 

acknowledges greater uncertainty with 

regard to the occurrence of adverse 

effects following exposures of concern 

at or above 60 ppb, she notes that a 

revised standard with a level of 70 ppb 

would also be expected to protect the 

large majority of children in the urban 

study areas (i.e., about 96% to more 


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164The 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. 

165 In the worst-case year and location, a standard 

with a level of 70 ppb is estimated to protect about 

97% of children in urban study areas from 

experiencing two or more O3-induced FEV1

decrements ≥15%, and about 87% from 

experiencing two or more decrements ≥10%. 

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 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 increases and, 

as a result, she focuses primarily on 

estimates of two or more O3-induced 

FEV1 decrements (i.e., as a surrogate for 

repeated exposures). 

Given the above considerations, the 

Administrator does not believe it would 

be appropriate to set a standard that is 

intended to eliminate all O3-induced 

FEV1 decrements. She notes that this is 

consistent with CASAC advice, which 

did not include a recommendation to set 

the standard level low enough to 

eliminate all O3-induced FEV1

decrements ≥10 or 15% (Frey, 2014c). 

Rather, the Administrator considers the 

extent to which a standard with a level 

of 70 ppb would be expected to protect 

the population from experiencing O3- 

induced FEV1 decrements ≥10 and 15%, 

including the extent to which such a 

standard would be expected to achieve 

reductions in the occurrence of O3- 

induced FEV1 decrements, relative to 

the current standard.164

The Administrator notes that a 

revised O3 standard with a level of 70 

ppb is estimated to protect about 98 to 

99% of children in urban study areas 

from experiencing two or more O3- 

induced FEV1 decrements ≥15%, and 

about 89 to 94% from experiencing two 

or more decrements ≥10%.165 Compared 

to the current standard, these estimates 

represent decreases in the occurrence of 

two or more O3-induced decrements of 

about 29 and 17%, respectively (Tables 

2 and 5). Although the Administrator is 

less concerned about the public health 

implications of single O3-induced lung 

function decrements, she also gives 

some consideration to estimates of one 

or more O3-induced FEV1 decrements. 

In particular, she notes that a revised 

standard with a level of 70 ppb is 

estimated to reduce the occurrence of 

one or more O3-induced decrements, 

compared to the current standard, by 

about 26% (for decrements ≥15%) and 

15% (for decrements ≥10%) (Tables 2 

and 5). 

Given all of the above information, 

the Administrator concludes that a 

revised standard with a level of 70 ppb 

would be expected to provide 

substantial protection against O3

exposures of concern (for benchmark 

concentrations of 60, 70, 80 ppb) and 

O3-induced lung function decrements, 

and would be expected to result in 

important reductions in the occurrence 

of such exposures and decrements, 

compared to the current standard. This 

is particularly the case for estimates of 

two or more occurrences of exposures of 

concern and lung function decrements. 

In next considering the additional 

protection that would be expected from 

standard levels below 70 ppb, the 

Administrator evaluates the extent to 

which a standard with a level of 65 ppb 

would be expected to further limit O3

exposures of concern and O3-induced 

lung function decrements. 

In addition to eliminating almost all 

exposures of concern to O3

concentrations at or above 80 and 70 

ppb, even in the worst-case years and 

locations, the Administrator notes that a 

revised standard with a level of 65 ppb 

would be expected to protect more than 

99% of children in urban study areas 

(and 100% of children in some urban 

study areas) from experiencing two or 

more exposures of concern at or above 

60 ppb. Compared to the current 

standard, this represents about a 95% 

reduction in the occurrence of two or 

more exposures of concern for the 60 

ppb benchmark (Tables 1 and 4). In 

addition, the Administrator notes that a 

revised standard with a level of 65 ppb 

is estimated to reduce the occurrence of 

one or more exposures of concern for 

the 60 ppb benchmark by about 80%, 

compared to the current standard 

(Tables 1 and 4). 

With regard to O3-induced lung 

function decrements, the Administrator 

notes that an O3 standard with a level 

of 65 ppb is estimated to protect about 

98% to more than 99% of children from 

experiencing two or more O3-induced 

FEV1 decrements ≥15%, even 

considering the worst-case year and 

location, and about 91 to 99% from 


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166 In recognizing that multicity studies are often 

emphasized over single-city studies for purposes of 

making weight of evidence judgments (U.S. EPA, 

2013a), the Administrator’s judgment in this case 

applies specifically to interpreting air quality 

analyses for epidemiologic studies of short-term O3

concentrations where multicity effect estimates are 

aggregated across cities, and where individual city 

effect estimates are not presented (as is the case for 

the key O3 studies analyzed in the PA, with the 

exception of the study by Stieb et al. (2009) where 

none of the city-specific effect estimates for asthma 

emergency department visits were statistically 

significant). Because reported multicity effect 

estimates do not allow health effect associations to 

be disaggregated by individual city, it is not 

possible to assign the multicity health effect 

association to the air quality in any one study 

location, or to the air quality in a particular subset 

of locations. In contrast, for epidemiologic studies 

of long-term concentrations, where multicity effect 

estimates are based on comparisons across cities, 

different judgments have been made by EPA with 

regard to the utility of multicity studies (see, e.g. 78 

FR 3086 at 3103/2, January 15, 2013) (and see 

discussion below of study by Jerrett et al., 2009). 

experiencing two or more decrements 

≥10% (89% in worst-case year and 

location). These estimates reflect 

reductions, compared to the current 

standard, of about 54 and 37%, 

respectively. A revised standard with a 

level of 65 ppb is also estimated to 

reduce the occurrence of one or more 

lung function decrements ≥15 and 10%, 

compared to the current standard, by 

about 50 and 31%, respectively. 

Taken together, the Administrator 

initially concludes that the evidence 

from controlled human exposure 

studies, and the information from 

quantitative analyses that draw upon 

these studies (i.e., exposures of concern, 

O3-induced FEV1 decrements), provide 

strong support for standard levels from 

65 to 70 ppb. In particular, she bases 

this conclusion on the fact that such 

standard levels would be well below the 

O3 exposure concentration shown to 

result in the widest range of respiratory 

effects (i.e., 80 ppb), and below the 

lowest O3 exposure concentration 

shown to result in the adverse 

combination of lung function 

decrements and respiratory symptoms 

(i.e., 72 ppb). A standard with a level 

from 65 to 70 ppb would also be 

expected to result in important 

reductions, compared to the current 

standard, in the occurrence of O3

exposures of concern for all of the 

benchmarks evaluated (i.e., 60, 70, and 

80 ppb) and in the risk of O3-induced 

lung function decrements ≥10 and 15%. 

In further considering the evidence 

and exposure/risk information, the 

Administrator considers the extent to 

which the epidemiologic evidence, and 

the quantitative risk estimates based on 

information from epidemiologic studies, 

also provide support for standard levels 

from 65 to 70 ppb. In doing so, as in her 

consideration of the adequacy of the 

current O3 standard, the Administrator 

focuses on epidemiologic studies of 

respiratory-related hospital admissions, 

emergency department visits, and 

mortality. These considerations are 

discussed below. 

The Administrator first considers the 

extent to which available epidemiologic 

studies have reported associations 

between short-term O3 concentrations 

and emergency department visits, 

hospital admissions, and/or mortality in 

locations that would likely have met 

alternative standards with levels from 

65 to 70 ppb (U.S. EPA, 2014c, section 

4.4.1). In evaluating the epidemiologic 

evidence in this way, the Administrator 

places the most weight on single-city 

studies of short-term O3 concentrations, 

recognizing that there were no multicity 

studies for which air quality data 

indicated that all cities included in the 

analyses would likely have met 

alternative standard levels. In particular, 

she notes that while single-city studies 

are more limited than multicity studies 

in terms of statistical power and 

geographic coverage, conclusions 

linking air quality in a given city with 

health effect associations in that same 

city can be made with greater certainty 

for single-city studies of short-term O3, 

compared to health effect associations 

aggregated across multiple cities in 

multicity studies. In particular, the 

Administrator notes considerable 

uncertainty in linking multicity effect 

estimates (aggregated across multiple 

cities) for short-term O3 with the air 

quality for subsets of study locations 

(rather than all locations) likely to have 

met an alternative standard.166

Given the above, the Administrator 

notes analyses in the PA (U.S. EPA, 

2014c, section 4.4.1) indicating that a 

revised standard with a level of 65 or 70 

ppb would be expected to maintain 

short-term ambient O3 concentrations 

below those present in the locations of 

all of the single-city studies analyzed. 

As discussed in the PA (U.S. EPA, 

2014c, section 4.4.1), this includes 

several single-city studies conducted in 

locations that would have violated the 

current standard, and the single-city 

study by Mar and Koenig (2009) that 

reported positive and statistically 

significant associations with respiratory 

emergency department visits with 

children and adults in a location that 

likely would have met the current 

standard over the entire study period 

but that would likely not have met a 

revised standard with a level of 70 ppb 

or below. Thus, the Administrator notes 

that, while the current standard would 

allow the ambient O3 concentrations 

that provided the basis for the health 

effect associations reported by Mar and 

Koenig (2009), a revised O3 standard 

with a level at or below 70 ppb would 

require reductions in those ambient O3

concentrations. While the Administrator 

acknowledges uncertainty in the extent 

to which the reported O3-associated 

emergency department visits could be 

further reduced by standard levels 

below 65 or 70 ppb, she concludes that 

this analysis indicates that a revised 

standard with a level at least as low as 

70 ppb would result in improvements in 

public health, beyond the protection 

provided by the current standard, in the 

locations of the single-city 

epidemiologic studies that reported 

significant health effect associations. 

As discussed above, the 

Administrator notes the greater 

uncertainty in interpreting air quality in 

locations of multicity epidemiologic 

studies of short-term O3 for the purpose 

of evaluating alternative standard levels 

(II.D.1 and U.S. EPA, 2014c, section 

4.4.1). Therefore, she places less weight 

on these studies than on the single-city 

studies noted above. Despite this 

uncertainty, she notes that PA analyses 

suggest that standard levels of 65 or 70 

ppb would require additional 

reductions, beyond those required by 

the current standard, in ambient O3

concentrations in several of the 

epidemiologic study locations that 

provided the basis for statistically 

significant O3 health effect associations. 

For example, she notes that Dales et al. 

(2006) reported significant associations 

with respiratory hospital admissions 

based on air quality in 11 Canadian 

cities, most of which would likely have 

met the current standard over the entire 

study period (i.e., seven cities) but 

would have violated a standard with a 

level of 70 ppb or below over at least 

part of that period (U.S. EPA, 2014c, 

Table 4–1). She further notes that 

Katsouyanni et al. (2009) reported 

statistically significant associations with 

mortality based on air quality in 12 

Canadian cities, most of which would 

likely have met the current standard 

(i.e., eight study cities) and a standard 

with a level of 70 ppb (i.e., seven study 

cities) over the entire study period, but 

would have violated a standard with a 

level of 65 ppb over at least part of that 

period (U.S. EPA, 2014c, Table 4–1). 

While most of the other multicity 

epidemiologic studies 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 


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167The 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). 

168For 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 areawide O3 concentrations at or above 40 ppb and, 

therefore, many fewer O3-associated deaths for areawide concentrations at or above 40 ppb (U.S. EPA, 

2014a, Figures 7–2 and 7–3). 

169Though only a relatively small number of days 

in urban study areas had area-wide O3

concentrations at or above 60 ppb. 

have met standards with levels from 65 

to 70 ppb. However, given the important 

uncertainties in interpreting the air 

quality in these multicity studies, the 

Administrator places limited weight on 

them overall, relative to the single-city 

studies noted above (and relative to the 

information based on controlled human 

exposure studies). 

With regard to long-term O3

concentrations, the Administrator 

considers the long-term O3 metrics 

reported to be associated with mortality 

or morbidity in recent epidemiologic 

studies (e.g., seasonal averages of 1-hour 

or 8-hour daily max concentrations). 

Compared to the current standard, she 

notes that analyses in the PA (U.S. EPA, 

2014c, section 4.4.1) suggest a revised 

standard with a level of 65 or 70 ppb 

would more effectively maintain longterm O3 concentrations below those 

where the multicity study by Jerrett et 

al. (2009) indicates the most confidence 

in the reported association with 

respiratory mortality (II.B.2, II.D.1). 

Based on additional information from 

the study by Jerrett et al. (2009), the 

Administrator also notes HREA analyses 

indicating that a revised standard with 

a level of 65 or 70 ppb would be 

expected to reduce the risk of 

respiratory mortality associated with 

long-term O3 concentrations (though she 

also notes important uncertainties with 

these risk estimates, as described 

below). Finally, she notes analyses in 

the HREA suggesting that a revised 

standard with a level of 65 or 70 ppb 

would be expected to reduce long-term 

O3 concentrations, defined in terms of 

O3 metrics similar to the long-term 

metrics that have been reported in 

recent epidemiologic studies to be 

associated with respiratory morbidity 

(i.e., seasonal averages of daily 

maximum 8-hour concentrations). Given 

the above evidence and information, the 

Administrator concludes that a revised 

8-hour standard with a level from 70 to 

65 ppb could increase public health 

protection, compared to the current 

standard, against effects associated with 

long-term O3 exposures. 

In further evaluating information from 

epidemiologic studies, the 

Administrator also considers the 

HREA’s epidemiology-based risk 

estimates of morbidity and mortality 

associated with short-term O3 (U.S. 

EPA, 2014a). Compared to the weight 

given to the evidence from controlled 

human exposure studies, and to HREA 

estimates of exposures of concern and 

lung function risks, she places relatively 

less weight on epidemiology-based risk 

estimates. In doing so, she notes that the 

overall conclusions from the HREA 

likewise reflect relatively less 

confidence in estimates of 

epidemiology-based risks than in 

estimates of exposures of concern and 

lung function risks. As discussed above 

(II.C.3.b), this is based on the greater 

uncertainties associated with mortality 

and morbidity risk estimates, including 

the heterogeneity in effect estimates 

between locations, the potential for 

exposure measurement errors, and 

uncertainty in the interpretation of the 

shape of concentration-response 

functions at lower O3 concentrations. 

The Administrator further notes the 

HREA conclusion that lower confidence 

should be placed in the results of the 

assessment of respiratory mortality risks 

associated with long-term O3 exposures, 

primarily because that analysis is based 

on only one study (even though that 

study is well-designed) and because of 

the uncertainty in that study regarding 

the existence and identification of a 

potential threshold in the concentrationresponse 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 estimates for 

urban study areas are likely to 

understate the average reductions in O3- 

associated mortality and morbidity risks 

that would be experienced across the 

U.S. population as a whole upon 

meeting standards with lower levels. 

The Administrator’s primary focus is 

on risks associated with O3

concentrations in the upper portions of 

ambient distributions, given the greater 

uncertainty associated with the shapes 

of concentration-response curves for O3

concentrations in the lower portions of 

ambient distributions.167 The 

Administrator further notes that 

experimental studies provide the 

strongest evidence for O3-induced 

effects following exposures to O3

concentrations corresponding to the 

upper portions of typical ambient 

distributions. In particular, as discussed 

above, she notes controlled human 

exposure studies showing respiratory 

effects following exposures to O3

concentrations at or above 60 ppb (II.B). 

In considering risks associated with 

O3 concentrations in the upper portions 

of ambient distributions, the 

Administrator focuses on area-wide O3

concentrations at or above 40 ppb and 

60 ppb. For area-wide O3 concentrations 

at or above 40 ppb, the Administrator 

notes that revised standards with levels 

of 70 or 65 ppb are estimated to reduce 

the number of premature deaths 

associated with short-term O3

concentrations by about 10% and 

almost 50%, respectively, compared to 

the current standard.168 In addition, for 

area-wide concentrations at or above 60 

ppb, revised standards are estimated to 

reduce O3-associated premature deaths 

by about 50% to 70% for a standard 

level of 70 ppb, and by more than 80% 

for a standard level of 65 ppb.169 Risk 

reductions are smaller when total risks 

are considered (II.C.3.b). 

Given all of the above evidence, 

exposure/risk information, and advice 

from CASAC, the Administrator 

proposes to revise the level of the 

current primary O3 standard to within 

the range of 65 to 70 ppb. She concludes 

that a standard with a level from within 

this range could reasonably be judged to 

be requisite to protect public health 

with an adequate margin of safety, based 

on her consideration of the evidence 

and information discussed above. In 

reaching this conclusion, she 

particularly notes that a level from 

anywhere within this range would be 

below the lowest O3 exposure 

concentration shown to result in the 


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170See, e.g., NRDC v. EPA, 902 F. 2d 962, 973– 

74 (D.C. Cir. 1990). 

171Although 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. 

adverse combination of respiratory 

symptoms and lung function 

decrements (i.e., 72 ppb), would be 

expected to maintain ambient O3

concentrations below those in locations 

where single-city studies assessed in the 

ISA have reported statistically 

significant O3 health effect associations, 

and would be expected to result in 

important reductions in O3 exposures 

and health risks, compared to the 

current standard. 

The Administrator notes that the 

determination of what constitutes an 

adequate margin of safety is expressly 

left to the judgment of the EPA 

Administrator. She further notes that in 

evaluating how particular standards 

address the requirement to provide an 

adequate margin of safety, the 

Administrator must consider such 

factors as the nature and severity of the 

health effects, the size of sensitive 

population(s) at risk, and the kind and 

degree of the uncertainties that must be 

addressed (I.B, above). Consistent with 

past practice and long-standing judicial 

precedent, she takes the need for an 

adequate margin of safety into account 

as an integral part of her decisionmaking on the appropriate level, 

averaging time, form, and indicator of 

the standard.170

The Administrator notes that the 

NAAQS are not designed to be zero-risk 

or background standards, and that the 

sizeable risk reductions that are 

estimated in the HREA to be associated 

with standard levels of 65 or 70 ppb 

represent substantial improvements in 

public health for important segments of 

the population, including at-risk groups 

such as children and people with 

asthma. Although any rationale 

supporting a decision to set a specific 

level within the range of 65 to 70 ppb 

would discuss the full body of evidence 

and information, the Administrator 

notes that certain aspects of this 

evidence and information could be 

particularly important in distinguishing 

between the appropriateness of a level 

closer to 65 ppb versus a level closer to 

70 ppb.171

For example, a level at or near 65 ppb 

could be judged requisite to protect 

public health with an adequate margin 

of safety to the extent the Administrator 

places greater weight on the importance 

of: (1) Eliminating almost all exposures 

of concern (even single occurrences) at 

or above 70 and 80 ppb, even in worstcase years and locations; (2) almost 

eliminating the occurrence of two or 

more exposures of concern at or above 

60 ppb; (3) achieving additional 

reductions in O3-induced FEV1

decrements, beyond those achieved 

with a level of 70 ppb (4) maintaining 

ambient concentrations below those in 

locations of single-city studies and more 

effectively doing so for multicity studies 

(i.e., more effectively than 70 ppb); and 

(5) achieving substantial reductions, 

compared to a standard with a level of 

70 ppb, in mortality associated with the 

upper portion of the distribution of 

ambient O3 concentrations, despite 

uncertainties in risk estimates. 

In contrast, a level at or near 70 ppb 

could be judged requisite to protect 

public health with an adequate margin 

of safety to the extent the Administrator 

places a greater amount of weight (i.e., 

greater than for 65 ppb) on the 

importance of: (1) Almost eliminating 

the occurrence of two or more exposures 

of concern at or above 70 and 80 ppb, 

even in the worst-case year and 

location; (2) substantially reducing, but 

not eliminating, the occurrence of two 

or more exposures of concern at or 

above 60 ppb, noting conclusions 

regarding increasing uncertainty in 

adverse effects for the 60 ppb 

benchmark; (3) reducing, but not 

eliminating, the occurrence of one or 

more exposures of concern, noting that 

not all exposures of concern result in 

adverse effects; (4) maintaining ambient 

O3 concentrations below those in 

locations of single-city epidemiologic 

studies, and uncertainties in analyses of 

air quality in multicity study locations; 

and (5) recognizing uncertainties in 

epidemiology-based risk estimates. 

In considering CASAC advice on the 

range of standard levels, the 

Administrator first notes CASAC’s 

conclusion that there is adequate 

scientific evidence to consider a range 

of levels for a primary standard that 

includes an upper end at 70 ppb. For 

the reasons discussed above, she agrees 

with this advice. She also notes that 

while CASAC concluded that a standard 

with a level of 70 ppb ‘‘may not meet 

the statutory requirement to protect 

public health with an adequate margin 

of safety’’ (Frey, 2014c, p. 8), it further 

acknowledged that ‘‘the choice of a level 

within the range recommended based 

on scientific evidence is a policy 

judgment under the statutory mandate 

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

While she agrees with CASAC that it is 

appropriate to consider levels below 70 

ppb, as reflected in her range of 

proposed levels from 65 to 70 ppb, for 

the reasons discussed above she also 

concludes that a standard level as high 

as 70 ppb, which CASAC concluded 

could be supported by the scientific 

evidence, could reasonably be judged to 

be requisite to protect public health 

with an adequate margin of safety. 

The Administrator has also 

considered the appropriateness of 

standard levels below 65 ppb. In doing 

so, she notes the conclusions of the PA 

and the advice of CASAC that it would 

be appropriate for her to consider 

standard levels as low as 60 ppb. In 

particular, she notes that a decision to 

set the primary O3 standard level at 60 

ppb would place a large amount of 

weight on the potential public health 

importance of virtually eliminating even 

single occurrences of exposures of 

concern at and above 60 ppb, though 

controlled human exposure studies have 

not reported the adverse combination of 

respiratory symptoms and decrements 

in lung function following exposures to 

60 ppb O3; on the potential public 

health importance of further reducing 

the occurrence of O3-induced lung 

function decrements ≥10 and 15%; on 

analyses of ambient O3 concentrations 

in locations of multicity epidemiologic 

studies, despite uncertainties in linking 

multicity effect estimates for short-term 

O3 with air quality in individual study 

cities; and on epidemiology-based risk 

estimates, despite the important 

uncertainties in those estimates. 

However, as discussed more fully above, 

given the uncertainties associated with 

the adversity of exposures to 60 ppb O3,

particularly single occurrence of such 

exposures; uncertainties associated with 

air quality analyses in locations of 

multicity epidemiologic studies; and 

uncertainties in epidemiology-based 

risk estimates, particularly uncertainties 

in the shape of the concentrationresponse functions at lower O3

concentrations and uncertainties 

associated with the heterogeneity in O3

effect estimates across locations, the 

Administrator does not agree that it is 

appropriate to place significant weight 

on these factors or to use them to 

support the appropriateness of standard 

levels below 65 ppb. Compared to O3

standard levels from 65 to 70 ppb, the 

Administrator concludes that the extent 

to which standard levels below 65 ppb 

could result in further public health 

improvements becomes notably less 

certain. Therefore, she concludes that it 


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172Although, as discussed below, she solicits 

comment on standard levels as low as 60 ppb. 

173 In fact, as noted above (Table 4), a standard 

with a level of 70 ppb would be expected to limit 

multiple occurrences of exposures of concern at or 

above the 60 ppb benchmark to as low as 0.5% in 

urban case study areas (and as low as 0% for a 

standard with a level of 65 ppb). 

174More specifically, as discussed above, 

respiratory symptoms have been evaluated 

following 6.6-hour exposures to average O3

concentrations of 60 ppb (Adams, 2006; Kim et al., 

2011) and 63 ppb (Schelegle et al., 2009). None of 

these studies reported a statistically significant 

increase in respiratory symptoms, compared to 

filtered air controls. 

is not appropriate to propose standard 

levels below 65 ppb.172

The Administrator acknowledges that 

her proposed range of 65 to 70 ppb does 

not include the lower portion of the 

range supported by CASAC. In reaching 

the conclusion that this is appropriate, 

she focuses on CASAC’s rationale for 

levels as low as 60 ppb. In particular, 

she notes the following CASAC advice 

(Frey, 2014c, p. 7): 

The CASAC concurs that 60 ppb is an 

appropriate and justifiable scientifically 

based lower bound for a revised primary 

standard. This is based upon findings of 

adverse effects, including clinically 

significant lung function decrements and 

airway inflammation, after exposures to 60 

ppb ozone in healthy adults with moderate 

exertion (Adams 2006; Schelegle et al., 2009; 

Brown et al., 2008; Kim et al., 2011), with 

limited evidence of adverse effects below 60 

ppb. 

In considering this advice, the 

Administrator notes that CASAC 

focused on the importance of limiting 

exposures to O3 concentrations as low 

as 60 ppb. As discussed above, the 

Administrator agrees with this advice. 

In particular, she notes that standards 

within the proposed range of 65 to 70 

ppb would be expected to substantially 

limit the occurrence of exposures of 

concern to O3 concentrations at or above 

60 ppb, particularly the occurrence of 

two or more exposures.173 When she 

further considers that not all exposures 

of concern lead to adverse effects, and 

that the NAAQS are not meant to be 

zero-risk or background standards, the 

Administrator judges that alternative 

standard levels below 65 ppb are not 

needed to further reduce such 

exposures. Therefore, the 

Administrator’s initial conclusion is 

that standard levels below 65 ppb 

would be more than requisite to protect 

public health with an adequate margin 

of safety. 

In reaching this initial conclusion, the 

Administrator acknowledges that 

alternative approaches to viewing the 

available scientific evidence and 

exposure/risk information, and to 

viewing the uncertainties inherent in 

that evidence and information, could 

lead one to reach a different conclusion. 

In particular, as noted above, she 

recognizes that levels as low as 60 ppb 

could potentially be supported, to the 

extent substantial weight is placed on 

the public health importance of 

estimates of one or more occurrences of 

exposures of concern at or above 60 ppb 

and O3-induced lung function 

decrements ≥10%; analyses of ambient 

O3 concentrations in locations of 

multicity epidemiologic studies; and 

epidemiology-based estimates of total 

risk. This approach would also place a 

large amount of weight on the 

possibility that at-risk groups would 

experience adverse effects at lower 

levels than the benchmarks derived 

from clinical studies conducted using 

healthy adult subjects, despite the fact 

that these studies have not reported a 

statistically significant increase in 

respiratory symptoms, combined with 

lung function decrements, following 

exposures to 60 ppb.174 Such an 

approach to viewing the evidence and 

exposure/risk information would place 

very little weight on the uncertainties in 

these estimates and analyses. In some 

cases, elements of this approach have 

been supported by public commenters, 

leading some commenters to 

recommend setting the level of the 

primary O3 standard at least as low as 

60 ppb. In recognition of such an 

alternative approach to viewing the 

evidence and information, in addition to 

proposing to set the level of the O3

standard from 65 to 70 ppb, the 

Administrator solicits comment on 

alternative standard levels below 65 

ppb, and as low as 60 ppb. In doing so, 

the Administrator reiterates that the 

CAA does not require the establishment 

of a primary NAAQS at a zero-risk level 

or at background concentration levels, 

but rather at a level that reduces risk 

sufficiently so as to protect public 

health with an adequate margin of safety 

(I.A). 

F. Proposed Decision on the Primary 

Standard 

For the reasons discussed above, and 

taking into account information and 

assessments presented in the 2013 ISA, 

2014 HREA and integration of this 

information and assessments into staff 

conclusions in the 2014 PA, the advice 

and recommendations of CASAC, and 

public comments received during the 

development of these documents, the 

Administrator proposes to retain the 

current indicator, averaging time and 

form of the primary O3 standard, and to 

set a new level for the 8-hour primary 

O3 standard. Specifically, the 

Administrator proposes to set the level 

of the 8-hour primary O3 standard to 

within the range of 65 to 70 ppb. The 

proposed 8-hour primary standard 

would be met at an ambient air 

monitoring site when the 3-year average 

of the annual fourth-highest daily 

maximum 8-hour average O3

concentration is less than or equal to the 

level of the revised standard that is 

promulgated. Thus, the Administrator 

proposes to set a standard with a level 

within this range. For the reasons 

discussed above, the Administrator also 

solicits comment on setting the level of 

the primary O3 standard below 65 ppb, 

and as low as 60 ppb. 

III. Communication of Public Health 

Information 

Information on the public health 

implications of ambient concentrations 

of criteria pollutants is currently made 

available primarily through EPA’s Air 

Quality Index (AQI) program. The AQI 

has been in use since its inception in 

1999 (64 FR 42530). It provides 

accurate, timely, and easily 

understandable information about daily 

levels of pollution (40 CFR 58.50). It is 

designed to tell individual members of 

the public how clean or unhealthy their 

air is, whether health effects might be a 

concern, and, if so, measures 

individuals can take to reduce their 

exposure to air pollution. The AQI 

focuses on health effects individuals 

may experience within a few hours or 

days after breathing unhealthy air. The 

AQI establishes a nationally uniform 

system of indexing pollution 

concentrations for O3, carbon monoxide, 

nitrogen dioxide, particulate matter and 

sulfur dioxide. The AQI converts 

pollutant concentrations in a 

community’s air to a number on a scale 

from 0 to 500. Reported AQI values 

enable the public to know whether air 

pollution concentrations in a particular 

location are characterized as good (0– 

50), moderate (51–100), unhealthy for 

sensitive groups (101–150), unhealthy 

(151–200), very unhealthy (201–300), or 

hazardous (301–500). The AQI index 

value of 100 typically corresponds to 

the level of the short-term NAAQS for 

each pollutant. For the O3 NAAQS, an 

8-hour average concentration of 75 ppb 

corresponds to an AQI value of 100. An 

AQI value greater than 100 means that 

a pollutant is in one of the unhealthy 

categories (i.e., unhealthy for sensitive 

groups, unhealthy, very unhealthy, or 

hazardous) on a given day; an AQI value 

at or below 100 means that a pollutant 

concentration is in one of the 

satisfactory categories (i.e., moderate or 

good). An additional consideration in 


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175Effects would likely be greater in people with 

asthma. 

176Exposures to 50 ppb have not been evaluated 

experimentally, but are estimated to potentially 

affect only a small proportion of healthy adults and 

with only a half to a third of the moderate to large 

lung function decrements observed at 60 ppb 

(McDonnell et al., 2012; Figure 7). 

selecting breakpoints is for each 

category to span at least a 15 ppb range 

to allow for more accurate forecasting. 

Decisions about the pollutant 

concentrations at which to set the 

various AQI breakpoints, that delineate 

the various AQI categories, draw 

directly from the underlying health 

information that supports the NAAQS 

review. 

The Agency recognizes the 

importance of revising the AQI in a 

timely manner to be consistent with any 

revisions to the NAAQS. Therefore EPA 

is proposing conforming changes to the 

AQI, in connection with the Agency’s 

proposed decision on revisions to the O3

NAAQS if revisions to the primary 

standard are promulgated. These 

conforming changes would include 

setting the 100 level of the AQI at the 

same level as the revised primary O3

NAAQS and also making adjustments 

based on health information from this 

NAAQS review to AQI breakpoints at 

the lower end of each range (i.e., AQI 

values of 50, 150, 200 and 300). The 

EPA does not propose to change the 

level at the top of the index (i.e., AQI 

value of 500) that typically is set equal 

to the Significant Harm Level (40 CFR 

51.16), which would apply to state 

contingency plans. 

The EPA is proposing to revise the 

AQI for O3 by setting an AQI value of 

100 equal to the level of the revised O3

standard (65–70 ppb). The EPA is also 

proposing to revise the following 

breakpoints: An AQI value of 50 to 

within a range from 49–54 ppb; an AQI 

value of 150 to 85 ppb; an AQI value of 

200 to 105 ppb, and an AQI value of 300 

to 200 ppb. All these levels are averaged 

over 8 hours. The EPA is proposing to 

set an AQI value of 50, the breakpoint 

between the good and moderate 

categories, at 15 ppb below the value of 

the proposed standard, i.e. to within a 

range from 49 to 54 ppb. The EPA is 

taking comment on what level within 

this range to select, recognizing that 

there is no health message for either atrisk 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 175 to cause moderate 

lung function decrements (i.e., FEV1

decrements ≥10%, which could be 

adverse to people with lung disease), 

large lung function decrements (i.e., 

FEV1 decrements ≥20%) in a small 

proportion of people, and airway 

inflammation.176 The EPA is proposing 

to set an AQI value of 150, the 

breakpoint between the unhealthy for 

sensitive groups and unhealthy 

categories, at 85 ppb. At this level, 

controlled human exposure studies of 

healthy adults indicate that up to 25% 

of exposed people are likely to have 

moderate lung function decrements (i.e., 

25% have FEV1 decrements ≥10%; 12% 

have FEV1 decrements ≥15%) and up to 

7% are likely to have large lung 

function decrements (i.e., FEV1

decrements ≥20%) (McDonnell et al., 

2012; Figure 7). Large lung function 

decrements would likely interfere with 

normal activity for many healthy 

people. For people with lung disease, 

large lung function decrements would 

likely interfere with normal activity for 

most people and would increase the 

likelihood that they would seek medical 

treatment (72 FR 37850, July 11, 2007). 

The EPA is proposing to set an AQI 

value of 200, the breakpoint between the 

unhealthy and very unhealthy 

categories, at 105 ppb. At this level, 

controlled human exposure studies of 

healthy adults indicate that up to 38% 

of exposed people are likely to have 

moderate lung function decrements (i.e., 

38% have FEV1 decrements ≥10%; 22% 

have FEV1 decrements ≥15%) and up to 

13% are likely to have large lung 

function decrements (i.e., FEV1

decrements ≥20%). The EPA is 

proposing to set an AQI value of 300, 

the breakpoint between the very 

unhealthy and hazardous categories, at 

200 ppb. At this level, controlled human 

exposure studies of healthy adults 

indicate that up to 25% of exposed 

individuals are likely to have large lung 

function decrements (i.e., FEV1

decrements ≥20%), which would 

interfere with daily activities for many 

of them. Large lung function decrements 

would interfere with daily activities for 

most people with lung disease, and 

likely cause them to seek medical 

attention. 

TABLE 6—PROPOSED AQI BREAKPOINTS

AQI category Index values 

Existing 

breakpoints 

(ppb, 8-hour 

average) 

Proposed breakpoints 

(ppb, 8-hour average) 

Good .......................................................................................................... 0–50 0–59 0–(49 to 54). 

Moderate .................................................................................................... 51–100 60–75 (50 to 55)–(65 to 70). 

Unhealthy for Sensitive Groups ................................................................. 101–150 76–95 (66 to 71)–85. 

Unhealthy ................................................................................................... 151–200 96–115 86–105. 

Very Unhealthy .......................................................................................... 201–300 116–374 106–200. 

Hazardous .................................................................................................. 301–400 375– 201–. 

401–500 

EPA believes that the proposed 

breakpoints reflect an appropriate 

balance between reflecting the health 

evidence that is the basis for the 

proposed primary O3 standard and 

providing category ranges that are large 

enough to be forecasted accurately, so 

that the new AQI for O3 can be 

implemented more easily in the public 

forum for which the AQI ultimately 

exists. However, the EPA recognizes 

that some have expressed alternative 

approaches to viewing the evidence and 

information and solicits comment on 

these proposed revisions to the AQI. 

With respect to reporting 

requirements (40 CFR part 58, § 58.50), 

EPA proposes to revise 40 CFR part 58, 

§ 58.50 (c) to require the AQI reporting 

requirements to be based on the latest 

available census figures, rather than the 

most recent decennial U.S. census. This 

change is consistent with our current 

practice of using the latest population 

figures to make monitoring 

requirements more responsive to 

changes in population. 


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177The 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. 

IV. Rationale for Proposed Decision on 

the Secondary Standard 

This section presents the rationale for 

the Administrator’s proposed decisions 

regarding the need to revise the current 

secondary O3 NAAQS and the 

appropriate revisions to the standard, 

including her proposed decisions that 

the current secondary standard is not 

requisite to protect public welfare and 

should be revised to provide additional 

public welfare protection. Based on her 

consideration of the full body of welfare 

effects evidence and related analyses, 

the Administrator proposes to conclude 

that ambient O3 concentrations in terms 

of a W126 index value, averaged across 

three consecutive years, within the 

range from 13 ppm-hrs to 17 ppm-hrs 

would provide the requisite protection 

against known or anticipated adverse 

effects to the public welfare. In 

considering policy options for achieving 

that level of air quality, the 

Administrator has further considered 

the full body of information, including 

air quality analyses that relate ambient 

O3 concentrations in terms of a threeyear average W–126-based metric and in 

terms of the form and averaging time for 

the current standard. Based on this 

consideration, the Administrator 

proposes to revise the level of the 

current secondary standard to within 

the range of 0.065 to 0.070 ppm to 

achieve the appropriate air quality. 

As discussed more fully below, this 

proposal is based on a thorough review, 

in the ISA, of the latest scientific 

information on O3-induced 

environmental effects. This proposed 

decision also takes into account: (1) 

Staff assessments in the PA of the most 

policy-relevant information in the ISA 

and WREA analyses of air quality, 

exposure, and ecological risks and 

associated ecosystem services; (2) 

CASAC advice and recommendations; 

and, (3) public comments received 

during the development of these 

documents, either in connection with 

CASAC meetings or separately. 

This proposed decision draws on the 

ISA’s integrative synthesis of the entire 

body of evidence, published through 

July 2011, on environmental effects 

associated with the presence of O3 and 

related photochemical oxidants in the 

ambient air. As summarized in section 

IV.B below, this body of evidence 

addresses the range of environmental 

responses associated with exposure to 

ambient levels of O3 (U.S. EPA, 2013a, 

ISA chapters 9–10), and includes more 

than four hundred new studies that 

build on the extensive evidence base 

from the last review. This rationale also 

draws upon the results of quantitative 

exposure and risk assessments, 

summarized in section IV.C below. 

Section IV.D presents the 

Administrator’s proposed decisions on 

the adequacy of the current secondary 

standard (section IV.D.3) drawing on 

both evidence-based and exposure/riskbased 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 review 177 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 thencurrent secondary standard did not 

provide the requisite public welfare 

protection and it was revised. The 

current secondary standard is 75 ppb 

based on the annual fourth-highest daily 

maximum 8-hour average concentration, 

averaged over three consecutive years, 

which is identical to the current 

primary standard. In 2008, the 

Administrator considered the thenavailable 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 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 vegetationrelated 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 . . . 


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178For 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. 

179As 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). 

180As 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. 

is requisite to protect the public welfare 

from any known or anticipated adverse 

effects associated with the presence of 

such air pollutant in the ambient air.’’ 

Effects on welfare include, but are not 

limited to, ‘‘effects on soils, water, 

crops, vegetation, man-made materials, 

animals, wildlife, weather, visibility, 

and climate, damage to and 

deterioration of property, and hazards to 

transportation, as well as effects on 

economic values and on personal 

comfort and well-being’’ (CAA section 

302(h)). As recognized in the last 

review, the secondary standard is not 

meant to protect against all known or 

anticipated O3-related effects, but rather 

those that are judged to be adverse to 

the public welfare (73 FR 16496, March 

27, 2008). Thus, the level of protection 

from known or anticipated adverse 

effects to public welfare that is requisite 

for the secondary standard is a public 

welfare policy judgment to be made by 

the Administrator. In the current 

review, the Administrator’s judgment is 

informed by conclusions drawn with 

regard to adversity of effects to public 

welfare in decisions on secondary O3

standards in past reviews. 

In the 2008 decision, the 

Administrator concluded that the degree 

to which O3 effects on vegetation should 

be considered to be adverse to the 

public welfare depends on the intended 

use of the vegetation and the 

significance of the vegetation to the 

public welfare, and also applied this 

concept beyond the species level to the 

ecosystem level (73 FR 16496, March 

27, 2008). In so doing, the Administrator 

took note of ‘‘a number of actions taken 

by Congress to establish public lands 

that are set aside for specific uses that 

are intended to provide benefits to the 

public welfare, including lands that are 

to be protected so as to conserve the 

scenic value and the natural vegetation 

and wildlife within such areas, and to 

leave them unimpaired for the 

enjoyment of future generations’’ (73 FR 

16496, March 27, 2008). The notice for 

the 2008 decision further noted that 

[s]uch public lands that are protected 

areas of national interest include 

national parks and forests, wildlife 

refuges, and wilderness areas’’ (73 FR 

16496, March 27, 2008).178 179 The 

Administrator additionally recognized 

that ‘‘States, Tribes and public interest 

groups also set aside areas that are 

intended to provide similar benefits to 

the public welfare, for residents on State 

and Tribal lands, as well as for visitors 

to those areas’’ (73 FR 16496, March 27, 

2008). The Administrator took note of 

the ‘‘clear public interest in and value 

of maintaining these areas in a 

condition that does not impair their 

intended use and the fact that many of 

these lands contain O3-sensitive 

species’’ (73 FR 16496, March 27, 2008). 

Similarly, in judgments of adversity to 

public welfare in the 2010 proposed 

reconsideration, the Administrator 

proposed to place the highest priority 

and significance on vegetation and 

ecosystem effects to sensitive species 

that are known to or are likely to occur 

in federally protected areas such as 

national parks and other Class I areas,180

or on lands set aside by states, tribes 

and public interest groups to provide 

similar benefits to the public welfare (75 

FR 3023–24, January 19, 2010). 

In the current review, our 

consideration of the scientific evidence 

for effects on vegetation is based 

fundamentally on using information 

from controlled chamber studies, free 

air methodologies, and field-based 

observational, survey and gradient 

studies. Such evidence, discussed 

below, informs consideration of welfare 

endpoints and at-risk species and 

ecosystems on which to focus the 

current review, and consideration of the 

ambient O3 conditions under which 

various welfare effects are known or 

anticipated to occur. As in past reviews, 

we recognize that the available evidence 

has not provided identification of a 

threshold in exposure or ambient O3

concentrations below which it can be 

concluded with confidence that O3- 

attributable effects on vegetation do not 

occur, when considering the broad 

range of O3-sensitive plant species 

growing within the U.S and the array of 

effects. This is due in part to the fact 

that research shows that there is 

variability in sensitivity between and 

within species and that numerous 

factors, i.e., chemical, physical, 

biological, and genetic, can influence 

the direction and magnitude of the 

studied effect (U.S. EPA, 2013a, section 

9.4.8). In the absence of evidence for a 

discernible threshold, the general 

approach to considering the available O3

welfare effects evidence involves 

characterizing the confidence in 

conclusions regarding O3-attributable 

vegetation effects over the ranges of 

cumulative seasonal O3 exposure values 

evaluated in chamber studies and in 

field studies in areas where O3-sensitive 

vegetation are known to occur, as well 

as characterizing the extent to which 

these effects can be considered adverse 

at the plant level and beyond. With this 

approach, we consider the evidence for 

O3 affecting other ecosystem 

components (such as soils, water, and 

wildlife) and their associated goods and 

services, through its effects on 

vegetation, as well as the associated 

uncertainties. 

Our general approach further 

recognizes the complexity of judgments 

to be made regarding the identification 

of particular vegetation effects as 

welfare effects and regarding the point 

that known or anticipated vegetationrelated 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 climateVerDate Sep<11>2014 20:05 Dec 16, 2014 Jkt 235001 PO 00000 Frm 00081 Fmt 4701 Sfmt 4702 E:\FR\FM\17DEP2.SGM 17DEP2

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specific nature of O3-related vegetation 

responses, and particularly emphasize 

those studies that include O3 exposures 

that fall within the range of those likely 

to occur in the ambient air. We 

additionally recognize differences 

across different study types in what 

information they provide (U.S. EPA, 

2013a, section 9.2.6). For example, 

because conditions can be controlled in 

laboratory studies, responses in such 

studies may be less variable and smaller 

differences may be easier to detect. 

However, the controlled conditions may 

limit the range of responses or 

incompletely reflect pollutant 

bioavailability, so they may not reflect 

responses that would occur in the 

natural environment. Alternatively, 

field data can provide important 

information for assessments of multiple 

stressors or where 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 exposure-response (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 

risks were assessed in a range of 

analyses primarily involving nationalscale air quality scenarios developed 

using model adjustments and 

interpolation methods. We consider 

particularly the national scale 

assessments for these scenarios, while 

recognizing the uncertainties with 

regard to the conditions they represent. 

With regard to the appropriate 

characterization of exposures associated 

with ambient O3 concentrations, as in 

the 2008 and 1997 reviews, we continue 

to recognize the relevance of 

cumulative, seasonal, concentrationweighted 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 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) cooccurring stressors and/or modifying 

environmental factors (U.S. EPA, 2013a, 

section 9.4.8). 

Among the studies of vegetation 

effects, the ISA recognizes controlled 

chamber studies as the best method for 

isolating or characterizing the role of O3

in inducing the observed plant effects, 

and in assessing plant response to O3 at 

the finer scales (U.S. EPA, 2013a, 

sections 9.2 and 9.3). Recent controlled 

studies have focused on a variety of 

plant responses to O3 including the 

underlying mechanisms governing such 

responses. These mechanisms include: 

(1) Reduced carbon dioxide uptake due 

to stomatal closure (U.S. EPA, 2013a, 

section 9.3.2.1); (2) the upregulation of 

genes associated with plant defense, 

signaling, hormone synthesis and 

secondary metabolism (U.S. EPA, 2013a, 

section 9.3.3.2); (3) the down regulation 

of genes related to photosynthesis and 

general metabolism (U.S. EPA, 2013a, 

section 9.3.3.2); (4) the loss of carbon 

assimilation capacity due to declines in 

the quantity and activity of key proteins 

and enzymes (U.S. EPA, 2013a, section 

9.3.5.1); and (5) the negative impacts on 

the efficiency of the photosynthetic light 

reactions (U.S. EPA, 2013a, section 

9.3.5.1). As described in the ISA, these 

new studies ‘‘have increased knowledge 

of the molecular, biochemical and 

cellular mechanisms occurring in plants 

in response to O3’’, adding ‘‘to the 

understanding of the basic biology of 

how plants are affected by oxidative 

stress . . .’’ (U.S. EPA, 2013a, p. 9–11). 

The ISA further concludes that 

controlled studies ‘‘have clearly shown 

that exposure to O3 is causally linked to 

visible foliar injury, decreased 

photosynthesis, changes in 

reproduction, and decreased growth’’ in 

many species of vegetation (U.S. EPA, 

2013a, p. 1–15). 

Such effects at the plant scale can also 

be linked to an array of effects at larger 

spatial scales. For example, recent field 

studies at larger spatial scales, together 

with previously available evidence, 

support the controlled exposure study 

results and indicate that ‘‘ambient O3

exposures can affect ecosystem 

productivity, crop yield, water cycling, 

and ecosystem community 

composition’’ (U.S. EPA, 2013a, p. 1–15; 

Chapter 9, section 9.4). The current 

body of O3 welfare effects evidence 

confirms the conclusions reached in the 

last review on the nature of O3-induced 

welfare effects and is summarized in the 

ISA as follows (U.S. EPA, 2013a, p. 1– 

8). 

The welfare effects of O3 can be observed 

across spatial scales, starting at the 

subcellular and cellular level, then the whole 

plant and finally, ecosystem-level processes. 

Ozone effects at small spatial scales, such as 

the leaf of an individual plant, can result in 

effects along a continuum of larger spatial 

scales. These effects include altered rates of 

leaf gas exchange, growth, and reproduction 

at the individual plant level, and can result 


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181Based 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). 

182The terms sequestration and storage are used 

somewhat interchangeably in the ISA and other 

documents in this review. 

183Radiative 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). 

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 cycles 181 (U.S. EPA, 

2013a, Table 1–2). In consideration of 

the evidence of O3 exposure and 

alterations in stomatal performance, 

‘‘which may affect plant and stand 

transpiration and therefore possibly 

affecting hydrological cycling,’’ the ISA 

concludes that ‘‘[a]lthough the direction 

of the response differed among studies,’’ 

the evidence is sufficient to conclude a 

likely causal relationship between O3

exposure and the alteration of 

ecosystem water cycling (U.S. EPA, 

2013a, section 2.6.3). The ISA also 

concludes that the evidence is sufficient 

to conclude a likely causal relationship 

between O3 exposure and the alteration 

of community composition of some 

terrestrial ecosystems (U.S. EPA, 2013a, 

section 2.6.5). Related to the effects on 

vegetation growth, productivity and, to 

some extent, below-ground 

biogeochemical cycles, the ISA 

additionally determines that a likely 

causal relationship exists between 

exposures to O3 in ambient air and 

reduced carbon sequestration (also 

termed carbon storage) 182 in terrestrial 

ecosystems (U.S. EPA, 2013a, p. 1–10 

and section 2.6.2). Modeling studies 

available in this review consistently 

found negative impacts of O3 on carbon 

sequestration, although the severity of 

impact was influenced by ‘‘multiple 

interactions of biological and 

environmental factors’’ (U.S. EPA, 

2013a, p. 2–39). 

The ISA notes that ‘‘[t]he suppression 

of ecosystem [carbon] sinks results in 

more [carbon dioxide] accumulation in 

the atmosphere’’ and that a recent study 

has suggested that ‘‘the 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 forcing 183 (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 longerlived greenhouse gases’’ (U.S. EPA, 

2013a, p. 2–47). In this regard, the ISA 

observes that ‘‘radiative forcing does not 

take into account the climate feedbacks 

that could amplify or dampen the actual 

surface temperature response,’’ that 

‘‘[q]uantifying the change in surface 

temperature requires a complex climate 

simulation in which all important 

feedbacks and interactions are 

accounted for’’ and that ‘‘[t]he modeled 

surface temperature response to a given 

radiative forcing is highly uncertain and 

can vary greatly among models and from 

region to region within the same model’’ 

(U.S. EPA, 2013a, p. 2–47). Even with 

these uncertainties, the ISA notes that 

‘‘global climate models indicate that 

tropospheric O3 has contributed to 

observed changes in global mean and 

regional surface temperatures’’ and as a 

result of such evidence presented in 

climate modeling studies, concludes 

that there is likely to be a causal 

relationship between changes in 

tropospheric O3 concentrations and 

effects on climate (U.S. EPA, 2013a, p. 

2–47). The ISA additionally notes, 

however, that ‘‘[i]mportant uncertainties 

remain regarding the effect of 

tropospheric O3 on future climate 

change’’ (U.S. EPA, 2013a, p. 10–31). 

Given the strong evidence base, and 

findings of causal or likely causal 

relationships with O3 in ambient air, 

including the quantitative assessments 

of relationships between O3 exposure 

and occurrence and magnitude of 

effects, we give a primary focus to three 

main areas of effects. The three main 

areas, for which the evidence is 

summarized in more detail below, are: 

1) impacts on tree growth, productivity 

and carbon storage (section IV.B.1.b); 2) 

crop yield loss (section IV.B.1.c); and 3) 

visible foliar injury (section IV.B.1.a). 

Consideration of these three areas 

includes, as appropriate, consideration 

of evidence of associated effects at larger 

scales, including ecosystems, and on 

associated ecosystem services. 

With regard to biologically based 

indices of exposure pertinent to O3

effects on vegetation, the ISA states the 

following (U.S. EPA, 2013a, p. 2–44). 

The main conclusions from the 1996 and 

2006 O3 AQCDs [Air Quality Criteria 

Documents] regarding indices based on 

ambient exposure remain valid. These key 

conclusions can be restated as follows: ozone 

effects in plants are cumulative; higher O3

concentrations appear to be more important 

than lower concentrations in eliciting a 

response; plant sensitivity to O3 varies with 

time of day and plant development stage; 

[and] quantifying exposure with indices that 

cumulate hourly O3 concentrations and 

preferentially weight the higher 

concentrations improves the explanatory 

power of exposure/response models for 

growth and yield, over using indices based 

on mean and peak exposure values. 

The long-standing body of available 

evidence upon which these conclusions 

are based provides a wealth of 

information on aspects of O3 exposure 

that are important in influencing plant 

response. Specifically, a variety of 

‘‘factors with known or suspected 

bearing on the exposure-response 

relationship, including concentration, 

time of day, respite time, frequency of 

peak occurrence, plant phenology, 

predisposition, etc.,’’ have been 

identified (U.S. EPA, 2013a, section 

9.5.2). In addition, the importance of the 

duration of the exposure and the 

relatively greater importance of higher 

concentrations over lower 

concentrations in determining plant 

response to O3 have been consistently 

well documented (U.S. EPA, 2013a, 

section 9.5.3). Much of this evidence 

was assessed in the 1996 AQCD (U.S. 

EPA, 1996a), while more recent work 

substantiating this evidence is assessed 

in the subsequent 2006 AQCD and 2013 

ISA. 

Understanding of the biological basis 

for plant response to O3 exposure led to 

the development of a large number of 

‘‘mathematical approaches for 

summarizing ambient air quality 

information in biologically meaningful 

forms for O3 vegetation effects 

assessment purposes’’ (U.S. EPA, 2013a, 

section 9.5.3), including those that 

cumulate exposures over some specified 

period while weighting higher 

concentrations more than lower (U.S. 

EPA, 2013a, section 9.5.2). As with any 

summary statistic, these exposure 

indices retain information on some, but 

not all, characteristics of the original 

observations. The 1996 AQCD contained 

an extensive review of the published 


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184The SUM06 index is a threshold-based 

approach described as the sum of all hourly O3

concentrations greater or equal to 0.06 ppm 

observed during a specified daily and seasonal time 

window (U.S. EPA, 2013a, section 9.5.2). The W126 

index is a non-threshold approach described as the 

sigmoidally weighted sum of all hourly O3

concentrations observed during a specified daily 

and seasonal time window, where each hourly O3

concentration is given a weight that increases from 

zero to one with increasing concentration (U.S. 

EPA, 2013a, section 9.5.2). 

185 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). 

literature on different types of exposureresponse 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, concentrationweighted indices, which were 

recognized as the most appropriate 

biologically based metrics to consider in 

this context, with attention given 

primarily to two cumulative, 

concentration-weighted index forms: 

SUM06 and W126.184

In both the 1997 and 2008 reviews, 

the EPA concluded that the risk to 

vegetation comes primarily from 

cumulative exposures to O3 over a 

season or seasons 185 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 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 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 longestablished conclusions and longstanding supporting evidence described 

above, and in light of continued CASAC 

support, we continue to focus on 

cumulative concentration-weighted 

indices as the most biologically relevant 

metrics for consideration of O3

exposures eliciting vegetation-related 

effects. Such a metric has an 

‘‘explanatory power’’ that is improved 

‘‘over using indices based on mean and 

peak exposure values’’ (U.S. EPA, 

2013a, section 2.6.6.1, p. 2–44). In this 

review as in the last review, we use the 

W126 cumulative, seasonal metric (U.S. 

EPA, 2013a, sections 2.6.6.1 and 9.5.2) 

for consideration of the effects evidence 

and in the exposure and risk analyses in 

the WREA. 

The subsections below summarize key 

aspects of the welfare effects 

information for O3-elicited visible foliar 

injury (section IV.B.1.a), effects on forest 

tree growth, productivity and carbon 

storage (section IV.B.1.b) and reductions 

in crop yield (section IV.B.1.c), as well 

as associated effects. 

a. Visible Foliar Injury 

Visible foliar injury resulting from 

exposure to O3 has been well 

characterized and documented over 

several decades of research on many 

tree, shrub, herbaceous, and crop 

species (U.S. EPA, 2013a, p. 1–10; U.S. 

EPA, 2006a, 1996a, 1986, 1978). 

Additionally, O3-induced visible foliar 

injury symptoms on certain plant 

species, such as black cherry, yellowpoplar 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, opentop chambers (OTCs), and free-air 

carbon dioxide (and ozone) enrichment 

(FACE). Although there remains a lack 

of robust exposure-response functions 

that would allow prediction of visible 

foliar injury severity and incidence 

under varying air quality and 

environmental conditions, 

‘‘experimental evidence has clearly 

established a consistent association of 

the presence of visible foliar injury 

symptoms with O3 exposure, with 

greater exposure often resulting in 

greater and more prevalent injury’’ (U.S. 

EPA, 2013a, section 9.4.2, p. 9–41). The 

research newly available in this review 

includes: 1) controlled exposure studies 

conducted to test and verify the O3


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186Two 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. 

187See: http://www.epa.gov/airtrends/

values.html. 

188These 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). 

sensitivity and response of potential 

new bioindicator plant species; 2) multiyear 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 multiyear trend analysis (U.S. EPA, 2013a, 

section 9.4.2). These recent studies, in 

combination with the entire body of 

available evidence, thus form the basis 

for the ISA determinations of a causal 

relationship between ambient O3

exposure and the occurrence of O3- 

induced visible foliar injury on sensitive 

vegetation across the U.S. (U.S. EPA, 

2013a, p. 9–42). 

Recently available evidence confirms 

the evidence available in previous 

reviews that visible foliar injury can 

occur when sensitive plants are exposed 

to elevated O3 concentrations in a 

predisposing environment (i.e., 

adequate soil moisture) (U.S. EPA, 

2013a, section 9.4.2). Recent evidence 

also continues to support previous 

findings that indicated the occurrence of 

visible foliar injury at cumulative 

ambient O3 exposures previously 

examined. 

With regard to evidence from 

controlled exposure studies, a recent 

study, using continuously stirred tank 

reactor chambers, evaluated the 

occurrence of O3 characteristic visible 

foliar injury symptoms on 28 species of 

plants that were suspected of being O3

sensitive and most of which grow 

naturally throughout the northeast and 

midwest U.S., including in national 

parks and wilderness areas (U.S. EPA, 

2013a, section 9.4.2.1; Kline et al., 

2008). Across the 28 tested species, the 

study reported O3-induced responses in 

12, 20, 28 and 28 species at the 30, 60, 

90 and 120 ppb exposure 

concentrations,186 respectively; the 

plants were exposed for 7 hours per 

each weekday over 21 to 29 summer 

days (Kline et al., 2008). 

A string of recently published multiyear field studies provide a 

complementary line of field-based 

evidence by documenting the incidence 

of visible foliar injury symptoms on a 

variety of O3-sensitive species over 

multiple years and across a range of 

cumulative, seasonal exposure values in 

several eastern and midwestern NWRs 

(U.S. EPA, 2013a, section 9.4.2.1; Davis 

and Orendovici, 2006; Davis, 2007a, b; 

Davis, 2009). Some of these studies also 

included information regarding soil 

moisture stress using the Palmer 

Drought Severity Index (PDSI). While 

environmental conditions and species 

varied across the four NWRs, visible 

foliar injury was documented to a 

varying degree at each site. 

By far the most extensive field-based 

dataset of visible foliar injury incidence 

is that obtained by the USFS FHM/FIA 

biomonitoring network program. A 

trend analysis of data from the sites 

located in the Northeast and North 

Central U.S. for the 16 year period from 

1994 through 2009 (Smith, 2012) 

describes evidence of visible foliar 

injury occurrence in the field as well as 

some insight into the influence of 

changes in air quality and soil moisture 

on visible foliar injury and the difficulty 

inherent in predicting foliar injury 

response under different air quality/soil 

moisture scenarios (Smith, 2012; U.S. 

EPA, 2013a, section 9.2.4.1). Study 

results showed that incidence and 

severity of foliar injury were dependent 

on local site conditions for soil moisture 

availability and O3 exposure. Overall, 

there was a declining trend in the 

incidence of visible foliar injury as peak 

O3 concentrations declined, although 

the study additionally indicated that 

moderate O3 exposures continued to 

cause visible foliar injury at sites 

throughout the region (U.S. EPA, 2013a, 

p. 9–40). In a similar assessment of the 

USFS FHM/FIA data in the West, six 

years (2000 to 2005) of biomonitoring 

data, during a period where a large 

proportion of California sites did not 

meet the current standard, indicated O3- 

related visible foliar injury in 25–37% 

of biosites in California (Campbell et al., 

2007; U.S. EPA, 2013a, section 

9.4.2.1).187 These recent studies provide 

additional evidence of O3-induced 

visible foliar injury in many areas across 

the U.S. and augment the EPA’s 

understanding of O3-related visible 

foliar injury and of factors, such as soil 

moisture, that influence associations 

between O3 exposures or concentrations 

and visible foliar injury. 

b. Effects on Forest Tree Growth, 

Productivity and Carbon Storage 

Ozone has been shown to affect a 

number of important U.S. tree species 

with respect to growth, productivity, 

and carbon storage. Ambient O3

concentrations have long been known to 

cause decreases in photosynthetic rates 

and plant growth. As discussed in the 

ISA, research published since the 2006 

AQCD substantiates prior conclusions 

regarding O3-related effects on forest 

tree growth, productivity and carbon 

storage. The ISA states, ‘‘previous O3

AQCDs concluded that there is strong 

evidence that exposure to O3 decreases 

photosynthesis and growth in numerous 

plant species’’ and that ‘‘[s]tudies 

published since the 2008 review 

support those conclusions’’ (U.S. EPA, 

2013a, p. 9–42). The available studies 

come from a variety of different study 

types that cover an array of different 

species, effects endpoints, levels of 

biological organization and exposure 

methods and durations. The O3-induced 

effects at the plant scale may translate 

to the ecosystem scale, with changes in 

productivity and carbon storage. As 

stated in the ISA, ‘‘[s]tudies conducted 

during the past four decades have 

demonstrated unequivocally that O3

alters biomass allocation and plant 

reproduction’’ (U.S. EPA, 2013a, p. 1– 

10). 

The previously available strong 

evidence for trees includes robust E–R 

functions for seedling relative biomass 

loss (RBL) 188 in 11 species developed 

under the National Health and 

Environmental Effects Research 

Laboratory-Western Ecology Division 

program. This series of experiments 

used OTCs to study seedling growth 

response for a single growing season 

under a variety of O3 exposures (ranging 

from near background to well above 

current ambient concentrations) and 

growing conditions (U.S. EPA, 2013a, 

section 9.6.2; Lee and Hogsett, 1996). 

The evidence from these studies shows 

that there is a wide range in sensitivity 

across the studied species in the 

seedling growth stage over the course of 

a single growing season, with some 

species being extremely sensitive and 

others being very insensitive over the 

range of cumulative O3 exposures 

studied (U.S. EPA, 2014c, Figure 5–1). 

At the other end of the organizational 

spectrum, field-based studies of species 

growing in natural stands have 

compared observed plant response 

across a number of different sites and/ 

or years when exposed to varying 

ambient O3 exposure conditions. For 

example, a study conducted in forest 

stands in the southern Appalachian 

Mountains during a period when O3

concentrations exceeded the current 

standard found that the cumulative 


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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 whole-tree water use and in 

turn reduce late-season streamflow 

(McLaughlin et al., 2007b; U.S. EPA, 

2013a, p. 9–43). 

The magnitude of O3 impact on 

ecosystem productivity and on forest 

composition can vary among plant 

communities based on several factors 

including: the type of stand or 

community in which the sensitive 

species occurs (e.g., single species 

versus mixed canopy), the role or 

position of the species in the stand (e.g., 

dominant, sub-dominant, canopy, 

understory), the sensitivity of cooccurring species and environmental 

factors (e.g., drought and other factors). 

For example, O3 has been found to have 

little impact on white fir, but to greatly 

reduce growth of ponderosa pine in 

southern California, and cause 

decreased net primary production of 

most forest types in the Mid-Atlantic 

region, although only small impacts on 

spruce-fir forest (U.S. EPA, 2013a, 

section 9.4.3.4). 

As noted above, long-standing 

evidence has demonstrated that O3

alters biomass allocation and plant 

reproduction (U.S. EPA, 2013a, section 

9.4.3). Several studies published since 

the 2006 O3 AQCD further demonstrate 

that O3 can alter reproductive processes 

in herbaceous and woody plant species, 

such as the timing of flowering and the 

number of flowers, fruits and seeds 

(U.S. EPA, 2013a, section 9.4.3.3). 

Further, limited evidence in previous 

reviews reported that vegetation effects 

from a single year of exposure to 

elevated O3 could be observed in the 

following year. For example, growth 

affected by a reduction in carbohydrate 

storage in one year may result in the 

limitation of growth in the following 

year. Such ‘‘carry-over’’ effects have 

been documented in the growth of some 

tree seedlings and in roots (U.S. EPA, 

2013a, section 9.4.8; Andersen, et al., 

1997). In the current review, additional 

field-based evidence expands the EPA’s 

understanding of the consequences of 

single and multi-year O3 exposures in 

subsequent years. A number of studies 

were conducted at a planted forest at the 

Aspen FACE site in Wisconsin. These 

studies, which occurred in a field 

setting (more similar to natural forest 

stands than OTC studies), observed tree 

growth responses when grown in single 

or two species stands within 30-m 

diameter rings and exposed over a 

period of ten years to existing ambient 

conditions and elevated O3

concentrations. Some studies indicate 

the potential for carry-over effects, such 

as those showing that the effects of O3

on birch seeds (reduced weight, 

germination, and starch levels) could 

lead to a negative impact on species 

regeneration in subsequent years, and 

that the effect of reduced aspen bud size 

might have been related to the observed 

delay in spring leaf development. These 

effects suggest that elevated O3

exposures have the potential to alter 

carbon metabolism of overwintering 

buds, which may have subsequent 

effects in the following year (Darbah, et 

al., 2008, 2007; Riikonen et al., 2008; 

U.S. EPA, 2013a, section 9.4.3). Other 

studies found that, in addition to 

affecting tree heights, diameters, and 

main stem volumes in the aspen 

community, elevated O3 over a 7-year 

study period was reported to increase 

the rate of conversion from a mixed 

aspen-birch community to a community 

dominated by the more tolerant birch, 

leading the authors to conclude that 

elevated O3 may alter intra- and interspecies 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 peerreviewed studies ‘‘demonstrates the 

coherence of O3 effects across numerous 

studies and species that used a variety 

of experimental techniques, and these 

results support the conclusion of the 

previous AQCD that exposure to O3

decreases plant growth’’ (U.S. EPA, 

2013a, p. 9–43). Other meta-analyses 

have examined the effect of O3 exposure 

on root growth and generally found that 

O3 exposure reduced carbon allocated to 

roots (U.S. EPA, 2013a, pp. 9–45 to 9– 

46). 

As noted above, robust E–R functions 

have been developed for 11 tree species 

(black cherry, Douglas fir, loblolly pine, 

ponderosa pine, quaking aspen, red 

alder, red maple, sugar maple, tulip 

poplar, Virginia pine, white pine) from 

the extensive evidence base of O3– 

induced growth effects that was also 

available and relied upon in the 

previous review. While the species for 

which robust E–R functions have been 

developed represent only a small 

fraction (0.8 percent) of the total number 

of native tree species in the contiguous 

U.S. (1,497), this small subset includes 

eastern and western species, deciduous 

and coniferous species, and species that 

grow in a variety of ecosystems and 

represent a range of tolerance to O3 (U.S. 

EPA, 2013a, section 9.6.2; U.S. EPA, 

2014b, section 6.2, Figure 6–2, Table 6– 

1). Each of these species were studied in 

OTCs, with most species studied 

multiple times under a wide range of 

exposure and/or growing conditions, 

with separate E–R functions developed 

for each combination of species, 

exposure condition and growing 

condition scenario. These speciesspecific composite E–R functions have 

been successfully used to predict the 

biomass loss response from tree seedling 

species over a range of cumulative 

exposure conditions (U.S. EPA, 2013a, 

section 9.6.2). The 11 robust composite 

E–R functions available in the last 

review, as well as the E–R for eastern 

cottonwood (derived from a field study 

in which O3 and climate conditions 

were not controlled), are described in 

the ISA and graphed in the WREA to 

illustrate the predicted responses of 

these species over a wide range of 

cumulative exposures (U.S. EPA, 2014b, 

section 6.2, Table 6–1 and Figure 6–2; 

U.S. EPA, 2013a, section 9.6.2). For 

some of these species, the E–R function 

is based on a single study (e.g., red 

maple), while for other species there 

were as many as 11 studies available 

(ponderosa pine). In total, the E–R 

functions developed for these 12 species 

(the 11 with robust composite E–R 

functions plus eastern cottonwood) 

reflect 52 tree seedling studies. A 

stochastic analysis in WREA, 

summarized in section IV.C below, 

indicates the potential for within 

species variability to contribute 

appreciably to estimates for each 

species. Consideration of biomass loss 

estimates in the PA and in discussions 


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189The 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). 

190The 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). 

191These 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). 

192Ecosystem 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). 

below, however, is based on 

conventional method and focuses on 

estimates for the 11 species for which 

the robust datasets from OTC 

experiments are available, in 

consideration of CASAC advice.189

c. Crop Yield Loss 

The ‘‘detrimental effect of O3 on crop 

production has been recognized since 

the 1960s’’ (U.S. EPA, 2013a, p. 1–10, 

section 9.4.4). On the whole, the newly 

available evidence supports previous 

conclusions that exposure to O3

decreases growth and yield of crops. 

The ISA describes average crop yield 

loss reported across a number of 

recently published meta-analyses and 

identifies several new exposure studies 

that support prior findings for a variety 

of crops of decreased yield and biomass 

with increased O3 exposure (U.S. EPA, 

2013a, section 9.4.4.1, Table 9–17). 

Studies have also ‘‘linked increasing O3

concentration to decreased 

photosynthetic rates and accelerated 

aging in leaves, which are related to 

yield’’ and described effects of O3 on 

crop quality, such as nutritive quality of 

grasses, macro- and micronutrient 

concentrations in fruits and vegetable 

crops and cotton fiber quality (U.S. EPA, 

2013a, p. 1–10, section 9.4.4). The 

findings of the newly available studies 

do not change the basic understanding 

of O3-related crop yield loss since the 

last review and little additional 

information is available in this review 

on factors that influence associations 

between O3 levels and crop yield loss 

(U.S. EPA, 2013a, section 9.4.4.). 

The new evidence has strengthened 

support for previously established E–R 

functions for 10 crops (barley, field 

corn, cotton, kidney bean, lettuce, 

peanut, potato, grain sorghum, soybean 

and winter wheat), reducing two 

important areas of uncertainty, 

especially for soybean. The established 

E–R functions were developed from 

OTC-type experiments (U.S. EPA, 

2013a, section 9.6.3; U.S. EPA, 2014b, 

section 6.2; U.S. EPA, 2014c, Figure 5– 

4). In this review, the ISA included an 

analysis comparing OTC data for 

soybean from the National Crop Loss 

Assessment Network (NCLAN) with 

field-based data from SoyFACE 

(Soybean Free Air Concentration 

Enrichment) studies (U.S. EPA, 2013a, 

section 9.6.3.1).190 Yield loss in soybean 

from O3 exposure at the SoyFACE field 

experiment was reliably predicted by 

soybean E–R functions developed from 

NCLAN data, demonstrating a 

robustness of the E–R functions 

developed with NCLAN data to predict 

relative yield loss from O3 exposure. A 

second area of uncertainty that was 

reduced is that regarding the application 

of the NCLAN E–R functions, developed 

in the 1980s, to more recent cultivars 

currently growing in the field. Recent 

studies, especially those focused on 

soybean, provide little evidence that 

crops are becoming more tolerant of O3

(U.S. EPA, 2006a; U.S. EPA, 2013a). A 

meta-analysis of 53 studies found 

consistent deleterious effects of O3

exposures on soybean from studies 

published between 1973 and 2001 

(Morgan et al., 2003). Further, 

Betzelberger et al. (2010) utilized the 

SoyFACE facility to compare the impact 

of elevated O3 concentrations across 10 

soybean cultivars to investigate 

intraspecific variability of the O3

response, finding that the E–R functions 

derived for these 10 current cultivars 

were similar to the response functions 

derived from the NCLAN studies 

conducted in the 1980s (Heagle, 1989), 

‘‘suggesting there has not been any 

selection for increased tolerance to O3 

in more recent cultivars’’ (U.S. EPA, 

2013a, p. 9–59). Additionally, the ISA 

comparisons of NCLAN and SoyFACE 

data referenced above ‘‘confirm that the 

response of soybean yield to O3

exposure has not changed in current 

cultivars’’ (U.S. EPA, 2013a, p. 9–59; 

section 9.6.3.1). Thus, the evidence 

available in the current review has 

reduced uncertainties in two areas with 

regard to the use of E–R functions for 

soybean crop yield loss. 

During past O3 NAAQS reviews, there 

were very few studies that estimated O3

impacts on crop yields at large 

geographical scales (i.e., regional, 

national or global). Recent modeling 

studies of the impact of O3

concentrations historically found that 

increased O3 in the past generally 

reduced crop yield, but the impacts 

varied across regions and crop species, 

with the largest O3-induced crop yield 

losses estimated to have occurred in 

high-production areas that had been 

exposed to elevated O3 concentrations, 

such as the Midwest and the Mississippi 

Valley regions of the U.S. (U.S. EPA, 

2013a, Section 9.4.4.1). Among affected 

crop species, the estimated yield loss for 

wheat and soybean were higher than 

rice and maize (i.e., field corn). 

Additionally, satellite and ground-based 

O3 measurements have assessed soybean 

yield loss estimated to result from O3

over the continuous area of Illinois, 

Iowa, and Wisconsin, finding a 

relationship which correlates well with 

the previous results from FACE- and 

OTC-type experiments (U.S. EPA, 

2013a, section 9.4.4.1). 

Thus, consistent with the conclusions 

of the 1996 and 2006 CDs, the current 

ISA concludes that O3 concentrations in 

ambient air can reduce the yield of 

major commodity crops in the U.S. and 

focuses on use of crop E–R functions 

based on OTC experiments to 

characterize the quantitative 

relationship between ambient O3

concentrations and yield loss (U.S. EPA, 

2013a, section 9.4.4). In the PA, as 

summarized in sections IV.D and IV.E 

below, relative yield loss (RYL) is 

estimated for 10 different crops using 

the individual E–R functions described 

in the WREA 191 (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 use 192 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, 


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193Ecosystem 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). 

194As 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). 

2012). The EPA additionally stated that 

‘‘[c]onceptually, changes in ecosystem 

services may be used to aid in 

characterizing a known or anticipated 

adverse effect to public welfare’’ (77 FR 

20232, April 3, 2012).193

Potential public welfare impacts 

associated with ecosystems and 

associated services have a range of 

dimensions, including spatial, temporal, 

and social, and these likely will vary 

depending on the type of effect being 

characterized. For example, ecosystems 

can cover a range of spatial scales, and 

the services they provide can accrue 

locally or be distributed more broadly, 

such as when crops are sold and eaten 

locally and/or also sold in regional, 

national and world markets. 

Accordingly, impacts can be localized 

or more widely distributed. Further, 

ecosystem services can be realized over 

a range of temporal scales from 

immediate up to longer term. The size 

of the societal unit receiving benefits 

from ecosystem services can also vary 

dramatically. For example, a national 

park can provide direct recreational 

services to the thousands of visitors that 

come each year, but also provide an 

indirect value to the millions who may 

not visit but receive satisfaction from 

knowing it exists and is preserved for 

the future (U.S. EPA, 2014b, chapter 5, 

section 5.5.1). 

As recognized in the last review, 

judgments regarding adverse effects to 

the public welfare depend on the 

intended use for and significance of the 

affected vegetation, ecological receptors, 

ecosystems and resources to the public 

welfare (73 FR 16496, March 27, 

2008).194 For example, a number of 

different types of locations provide 

services of special significance to the 

public welfare. As emphasized in 

previous O3 NAAQS decisions, and 

summarized in section IV.A above, 

Class I areas and other parks have been 

afforded special federal protection to 

preserve services that provide for the 

enjoyment of these resources for current 

and future generations. Surveys have 

indicated that Americans rank as very 

important the existence of the resource, 

the option or availability of the resource 

and the ability to bequest or pass on to 

future generations (Cordell et al., 2008). 

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 significance. Judgments 

regarding crop yield loss, for example, 

depend on considerations related to the 

heavy management of agriculture in the 

U.S., while judgments regarding the 

other categories of effects generally 

relate to considerations regarding 

forested areas. For example, while both 

tree growth-related effects and visible 

foliar injury have the potential to be 

significant to the public welfare through 

impacts in Class I and other protected 

areas, they differ in how they might be 

significant and with regard to the clarity 

of the data which describes the 

relationship between the effect and the 

services potentially affected. 

With regard to effects on tree growth, 

reduced growth is associated with 

effects on an array of ecosystem services 

including reduced productivity, altered 

forest and forest community (plant, 

insect and microbe) composition, 

reduced carbon storage and altered 

water cycling (U.S. EPA, 2013a, Figure 

9–1, sections 9.4.1.1 and 9.4.1.2; U.S. 

EPA, 2014b, section 6.1). For example, 

forest or forest community composition 

can be affected through O3 effects on 

growth and reproductive success of 

sensitive species in the community, 

with the extent of compositional 

changes dependent on factors such as 

competitive interactions (U.S. EPA, 

2013a, sections 9.4.3 and 9.4.3.1). 

Depending on the type and location of 

the affected ecosystem, services 

benefitting the public in other ways can 

be affected as well. For example, other 

services valued by people that can be 

affected by reduced tree growth, 

productivity and carbon storage include 

aesthetic value, food, fiber, timber, other 

forest products, habitat, recreational 

opportunities, climate and water 

regulation, erosion control, air pollution 

removal, hydrologic and fire regime 

stabilization (U.S. EPA 2013a, sections 

9.4.1.1 and 9.4.1.2; U.S. EPA, 2014b, 

section 6.1, Figure 6–1, section 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 


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195For example, see http://www.fs.fed.us/

research/urban/environmental-justice.php. 

196The identification, monitoring and assessment 

of AQRVs with regard to an adverse effect is an 

approach used for assessing the potential for air 

pollution impacts from pending permit actions in 

Class I areas (USFS, NPS, USFWS, 2010). An 

adverse impact is recognized by the National Park 

Service as one that results in diminishment of the 

Class I areas’s national significance or the 

impairment of the ecosystem structure or 

functioning, as well as impairment of the quality of 

the visitor experience (USFS, NPS, USFWS, 2010). 

Federal land managers (FLMs) make such adverse 

impact determinations on a case-by-case basis, 

using technical and other information which they 

provide for consideration by permitting authorities. 

The National Park Services has developed is a 

document describing an overview of approaches 

related to assessing projects under the National 

Environmental Policy Act and other planning 

initiatives affecting the National Park System 

(http://www.nature.nps.gov/air/Pubs/pdf/

AQGuidance_2011–01–14.pdf). 

197See http://www2.nature.nps.gov/air/Pubs/pdf/ 

flag/NPSozonesensppFLAG06.pdf. 

this issue were provided by a 1996 

workshop of 16 then-leading scientists 

in the context of discussing their views 

for a secondary O3 standard (Heck and 

Cowling, 1997). In their consideration of 

tree growth effects as an indicator for 

forest ecosystems and crop yield 

reduction as an indicator of agricultural 

systems, the workshop participants 

identified annual percentages, of RBL 

for forest tree seedlings and RYL for 

agricultural crops, considered important 

to their judgments on the standard. With 

regard to forest ecosystems and seedling 

growth effects as an indicator, the 

participants selected a range of 1–2% 

RBL per year ‘‘to avoid cumulative 

effects of yearly reductions of 2%.’’ 

With regard to crops, they indicated an 

interest in protecting against crop yield 

reductions of 5% RYL yet noted 

uncertainties surrounding such a 

percentage which led them to 

identifying 10% RYL for the crop yield 

endpoint (Heck and Cowling, 1997). The 

workshop report provides no explicit 

rationale for the percentages identified 

(2% RBL and 5% or 10% RYL); nor does 

it describe their connection to 

ecosystem impacts of a specific 

magnitude or type and judgments on 

significance of the effects for public 

welfare, e.g., taking into consideration 

the intended use and significance of the 

affected vegetation (Heck and Cowling, 

1997). In recognition of the complexity 

of assessing the adversity of tree growth 

effects and effects on crop yield in the 

broader context of public welfare, the 

EPA’s consideration of those effects in 

both the 1997 and 2008 reviews 

extended beyond the consideration of 

various benchmark responses for the 

studied species, and with regard to 

crops, additionally took note of their 

extensive management (62 FR 38856, 

July 18, 1997; 73 FR 16436, March 27, 

2008). 

While, as noted above, public welfare 

benefits of forested lands can be 

particular to the type of area in which 

the forest occurs, some of the potential 

public welfare benefits associated with 

forest ecosystems are not location 

dependent. A potentially extremely 

valuable ecosystem service provided by 

forested lands and for which the ISA 

concludes a likely causal relationship 

with O3 in ambient air is carbon storage, 

a regulating service that is ‘‘of 

paramount importance for human 

society’’ (U.S. EPA, 2013a, section 

2.6.2.1 and p. 9–37). The service of 

carbon storage is potentially important 

to the public welfare no matter in what 

location the sensitive trees are growing, 

or what their intended current or future 

use. In other words, the benefit exists as 

long as the tree is growing, regardless of 

what additional functions and services 

it provides. 

Another example of locations 

potentially vulnerable to O3-related 

impacts but not necessarily identified 

for such protection might be forested 

lands, both public and private, where 

trees are grown for timber production, 

particularly where they are dominated 

by a single timber species stand that is 

sensitive to O3, such as ponderosa pine. 

Further, forests in urbanized areas 

provide a number of services that are 

important to the public in those areas, 

including air pollution removal, cooling 

of the heat island effect, and 

beautification (U.S. EPA, 2014b, section 

6.6.2 and Appendix 6D; Akbari, 

2002).195 The presence of O3-sensitive 

trees in such areas may place them at 

risk from elevated O3 exposures, 

contributing to potential impacts on 

important services provided by urban 

forests (U.S. EPA, 2014b, sections 6.6.2 

and 6.7). There are many other tree 

species, such as species used in the 

USFS biomonitoring network, and 

various ornamental and agricultural 

species (i.e., Christmas trees, fruit and 

nut trees) that provide ecosystem 

services that may be judged important to 

the public welfare but whose 

vulnerability to impacts from O3 on tree 

growth, productivity and carbon storage 

has not been quantitatively 

characterized (U.S. EPA, 2014b, Chapter 

6; Abt Associates, 1995). 

As noted above, in addition to tree 

growth-related effects, O3-induced 

visible foliar injury also has the 

potential to be significant to the public 

welfare through impacts in Class I and 

other similarly protected areas. Visible 

foliar injury is a visible bioindicator of 

O3 exposure in species sensitive to this 

effect, with the injury affecting the 

physical appearance of the plant. 

Accordingly visible foliar injury surveys 

are used by federal land managers as 

tools in assessing potential air quality 

impacts in Class I areas. These surveys 

may focus on plant species that have 

been identified as potentially sensitive 

air quality related values (AQRVs) due 

to their sensitivity to O3-induced foliar 

injury (USFS, NPS, FWS, 2010). An 

AQRV is defined by the National Park 

Services as a ‘‘resource, as identified by 

the FLM for one or more Federal areas 

that may be adversely affected by a 

change in air quality’’ and the resource 

‘‘may include visibility or a specific 

scenic, cultural, physical, biological, 

ecological, or recreational resource 

identified by the FLM for a particular 

area’’ (USFS, NPS, USFWS, 2010).196 No 

criteria have been established, however, 

regarding a level or prevalence of visible 

foliar injury considered to be adverse to 

the affected vegetation, and, as noted in 

section IV.B.1.a above, there is not a 

clear relationship between visible foliar 

injury and other effects, such as reduced 

growth and productivity. Thus, key 

considerations with regard to public 

welfare significance of this endpoint 

have related to qualitative consideration 

of the plant’s aesthetic value in 

protected forested areas. Depending on 

the extent and severity, O3-induced 

visible foliar injury might be expected to 

have the potential to impact the public 

welfare in scenic and/or recreational 

areas during the growing season, 

particularly in areas with special 

protection, such as Class I areas. 

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 lands 197

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 


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198Basal area for resident species in national 

forests and parks are available in files accessible at: 

http://www.fs.fed.us/foresthealth/technology/

nidrm2012.shtml. 

199 In its review of drafts of the WREA and PA, 

the CASAC conveyed support for analyses and 

considerations of ecosystem services that may be 

affected by O3 exposures (Frey, 2014b, 2014c). 

200Certain visible foliar injury analyses assessed 

recent conditions from 2006 to 2010 on an annual 

basis. 

201An 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). 

only available for a subset of the 66, as 

described in section IV.B.1. above. A 

diverse array of ecosystem services has 

been identified for these twelve species 

(U.S. EPA, 2014c, Table 5–1). Two of 

the species in this group that are 

relatively more sensitive with regard to 

effects on growth are the ponderosa pine 

and quaking aspen (U.S. EPA, 2014b, 

section 6.2), the ranges for which 

overlap with many lands that are 

protected or preserved for enjoyment of 

current and future generations 

(consistent with the discussion above on 

Class I and other protected areas), 

including such lands located in the west 

and southwest regions of the U.S. where 

ambient O3 concentrations and 

associated cumulative seasonal 

exposures can be highest (U.S. EPA, 

2014c, Appendix 2B).198

With regard to agriculture-related 

effects, the EPA has recognized other 

complexities, stating that the degree to 

which O3 impacts on vegetation that 

could occur in areas and on species that 

are already heavily managed to obtain a 

particular output (such as commodity 

crops or commercial timber production) 

would impair the intended use at a level 

that might be judged adverse to the 

public welfare has been less clear (73 FR 

16497, March 27, 2008; 75 FR 3024; 

January 19, 2010). We note that while 

having sufficient crop yields is of high 

public welfare value, important 

commodity crops are typically heavily 

managed to produce optimum yields. In 

light of all of the inputs that go into 

achieving these yields, such as fertilizer, 

herbicides, pesticides, and irrigation, it 

is difficult to determine at what point 

O3-induced yield loss creates an adverse 

impact for the producer in the way of 

requiring increased inputs in order to 

maintain the desired yields. Moreover, 

based on the economic theory of supply 

and demand, increases in crop yields 

would be expected to result in lower 

prices for affected crops and their 

associated goods, which would 

primarily benefit consumers. Given 

these competing impacts on producers 

and consumers, it is unclear how to 

consider these effects in terms of 

potential adversity to the public welfare 

(U.S. EPA, 2014c, sections 5.3.2 and 

5.7). 

When agricultural impacts or 

vegetation effects in other areas are 

contrasted with the emphasis on forest 

ecosystem effects in Class I and 

similarly protected areas, it can be seen 

that the Administrator has in past 

reviews judged the significance to the 

public welfare of O3-induced effects on 

sensitive vegetation growing within the 

U.S. to differ depending on the nature 

of the effect, the intended use of the 

sensitive plants or ecosystems, and the 

types of environments in which the 

sensitive vegetation and ecosystems are 

located, with greater significance 

ascribed to areas identified for specific 

uses and benefits to the public welfare, 

such as Class I areas, than to areas for 

which such uses have not been 

established. In summary, several 

considerations are recognized as 

important to judgments on the public 

welfare significance of the array of 

effects of different O3 exposure 

conditions on vegetation. While there 

are complexities associated with the 

consideration of the magnitude of key 

vegetation effects that might be 

concluded to be adverse to ecosystems 

and associated services, there are 

numerous locations where O3-sensitive 

tree species are present that may be 

vulnerable to impacts from O3 on tree 

growth, productivity and carbon storage 

and their associated ecosystems and 

services. It is not possible to generalize 

across all studied species regarding 

which cumulative exposures are of 

greatest concern, however, as this can 

vary by situation due to differences in 

exposed species sensitivity, the 

importance of the observed or predicted 

O3-induced effect, the role that the 

species plays in the ecosystem, the 

intended use of the affected species and 

its associated ecosystem and services, 

the presence of other co-occurring 

predisposing or mitigating factors, and 

associated uncertainties and limitations. 

These factors contribute to the 

complexity of the Administrator’s 

judgments regarding the adversity of 

known and anticipated effects to the 

public welfare. 

C. Exposure and Risk Assessment 

Information 

The WREA characterized ambient O3

exposure and its relationship to tree 

biomass loss, crop yield loss, and visible 

foliar injury and the associated 

ecosystem services 199 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). 

1. Air Quality Analyses 

The WREA evaluated O3 exposure 

and risks for several national-scale air 

quality scenarios: recent conditions 

(2006 to 2008),200 the current secondary 

standard, and W126 index values of 15 

ppm-hrs, 11 ppm-hrs, and 7 ppm-hrs, 

using three-year averages (U.S. EPA, 

2014b, chapter 4). For each of these 

scenarios, three-year average W126 

index values were estimated at each 12 

km by 12 km grid cell in a national-scale 

spatial surface. Additionally, some 

analyses were based on single-year 

surfaces.201 The method for creating the 

five scenarios generally involved two 

steps (summarized in Table 5–4 of the 

PA). The first is derivation of the 

average W126 index value (across the 

three years) at each monitor location. 

This value is based on unadjusted O3

concentrations from monitoring data for 

recent conditions and adjusted 

concentrations for the four other 

scenarios. Concentrations were adjusted 

based on model predicted relationships 

between O3 and U.S.-wide emissions 

reductions in oxides of nitrogen (NOx). 

The adjusted air quality does not 

represent an optimized control scenario 

that just meets the current standard (or 

target W126 index values for other 

scenarios), but rather characterizes one 

potential distribution of air quality 

across a region when all monitor 

locations meet the standard (U.S. EPA, 

2014b, section 4.3.4.2). The 

development of adjusted concentrations 

was done for each of nine regions 

independently (see U.S. EPA, 2014b, 

section 4.3.4.1). In the second step, 

national-scale spatial surfaces (W126 

index values for each 12 km x 12 km 


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202The 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. 

203The 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. 

204These methods were calculating a median 

using the composite functions and calculating a 

median using all tree seedling studies available. 

grid cell used in the air quality model) 

were created using the monitor-location 

values and the Voronoi Neighbor 

Averaging (VNA) spatial interpolation 

technique (details on the VNA 

technique are presented in U.S. EPA, 

2014b, Appendix 4A). 

In the dataset used to create the recent 

conditions scenario, the three-year 

average W126 index values at the 

monitor locations (before application of 

the VNA technique) ranged from below 

5 ppm-hrs to 48.6 ppm-hrs (U.S. EPA, 

2014b, Figure 4–4 and Table 4–3). In the 

nine modeling regions, the maximum 

three-year average W126 index values at 

monitor locations ranged from 48.6 

ppm-hrs in the West region down to 6.6 

ppm-hrs in the Northwest region.202

After adjustment of the monitor location 

concentrations to just meet the current 

standard in each region (using 

relationships described above), the 

region-specific maximum three-year 

average W126 values ranged from 18.9 

ppm-hrs in the West region to 2.6 ppmhrs 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 km 2 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 

when all monitor locations meet the 

standard (U.S. EPA, 2014b, section 

4.3.4.2).203 Additionally, the surface is 

created from the three-year average at 

the monitor locations, rather than 

creating a surface for each year and then 

averaging across years at each grid cell; 

the potential impact of this on the 

resultant estimates is considered in the 

WREA (U.S. EPA, 2014b, Appendix 4A). 

An additional uncertainty related to 

the W126 index value estimates for each 

air quality scenario comes from the 

creation of a national W126 surface 

using the VNA technique to interpolate 

recent air quality measurements of O3. 

In general, spatial interpolation 

techniques perform better in areas 

where the O3 monitoring network is 

denser. Therefore, the W126 index 

values estimated in the rural areas in the 

West, Northwest, Southwest, and West 

North Central with few or no monitors 

(U.S. EPA, 2014b, Figure 2–1) are more 

uncertain than those estimated for areas 

with denser monitoring. Further, this 

interpolation method generally 

underpredicts higher 12-hour W126 

exposures. Due to the important 

influence of higher exposures in 

determining risks to plants, the 

potential for the VNA interpolation 

approach to underpredict higher W126 

exposures could result in an 

underestimation of risks to vegetation in 

some areas. Underestimation of the 

highest W126 index values for the 

current standard scenario is an 

additional impact of the interpolation 

method that is important to consider. 

2. Tree Seedling Growth, Productivity, 

Carbon Storage and Associated 

Ecosystem Services 

For the WREA assessments related to 

tree growth, productivity, carbon storage 

and associated ecosystem services, the 

sections below provide an overview of 

the analyses along with the key results 

(section IV.C.2.a) and summarize the 

key uncertainties (section IV.C.2.b). 

a. Overview and Summary of Key 

Results 

The assessments to estimate the 

exposures and risks for tree seedling 

growth, productivity, and carbon storage 

reflect a range of spatial scales ranging 

from the county scale up to the national 

park, urban area, and national scales. 

For the air quality scenarios described 

above, the WREA applied the speciesspecific 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 on estimates for a median 

response across the 12 species, the 

WREA performed a stochastic sampling 

analysis involving multiple iterations of 

random selection of E–R functions from 

the studies available for each of the 12 

species. This analysis produced median 

values at each cumulative exposure 

level that were higher than medians 

derived by two conventional, 

deterministic methods (U.S. EPA, 

2014b, section 6.2.1.2 and Figure 6– 

5).204 For example, the median seasonal 

W126 index value for which a two 

percent biomass loss is estimated in 

seedlings for the studied species ranges 

from approximately 7 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, 


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205The 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). 

206The 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 

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 the eastern cottonwood. The 

results summarized here are for the 

analyses that exclude cottonwood.205

The WREA reported RBL estimates 

relative to a benchmark of 2% RBL for 

tree seedlings, as well as relative to 

other percent RBL values. The 2% RBL 

benchmark was considered based on 

CASAC advice that stated that ‘‘focus on 

a 2% loss level for trees . . . is 

appropriate.’’ (Frey, 2014b, p. 6). The 

main WREA analyses for effects related 

to tree growth, productivity and carbon 

storage are summarized below, with the 

key findings for each. 

Relative biomass loss nationally was 

estimated for each of the 12 studied 

species from the composite E–R 

functions for each species described 

above and information on the 

distribution of those species across the 

U.S. (U.S. EPA, 2014b, section 6.2.1.3 

and Appendix 6A). As one example of 

a tree species near the median of the 

studied species, relative biomass loss 

estimates (reduced growth) for 

ponderosa pine in the current standard 

air quality scenario are below two 

percent for most areas where this 

species is found but estimates of RBL for 

this species in some areas of the 

southwest fall above two percent 

biomass loss (U.S. EPA, 2014b, Figure 

6–8). Maximum estimates of RBL for all 

areas where ponderosa pine is found 

decrease to just over three percent and 

just over two percent for the 15 and 7 

ppm-hrs scenarios, respectively (U.S. 

EPA, 2014b, Table 6–6). 

To provide an indication of 

ecosystem-level impacts, weighted 

estimates of RBL (wRBL) were also 

developed for each grid cell nationwide. 

This is estimated from the speciesspecific 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 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 speciesspecific functions exceeds two percent 

RBL. In addition to the estimates based 

on all 12 studied species and also the 11 

species with the exclusion of eastern 

cottonwood (in response to CASAC 

advice), additional estimates were 

developed without black cherry to show 

contribution of that sensitive species to 

the multi-species estimates (U.S. EPA, 

2014b, Table 6–7). 

• In the current standard scenarios, 

66% of the 3,109 assessed counties are 

estimated to have at least one of the 11 

species (excluding cottonwood) with an 

RBL greater than two percent, with three 

counties having three species exceeding 

two percent. The median RBL (across 

the species present) is above two 

percent in 239 counties. The maximum 

number of species in any one county 

with an RBL greater than two percent is 

three (excluding cottonwood). (U.S. 

EPA, 2014b, Table 6–7). 

• For the 15, 11 and 7 ppm-hrs 

scenarios, the proportion of 3,109 

counties with one or more species with 

an RBL above two percent decreases to 

61 percent, 59 percent, and 58 percent, 

respectively. For the 7 ppm-hrs 

scenario, the median RBL is above two 

percent in six percent of the counties 

(U.S. EPA, 2014b, Table 6–7). 

• The county RBL estimates are 

appreciably influenced by black cherry, 

a very sensitive species that is 

widespread in the Eastern U.S. For 

1,805 of the 1,929 counties estimated to 

have at least one species with an RBL 

greater than two percent when air 

quality is meeting the current standard, 

only black cherry exceeds this level of 

RBL. If black cherry is excluded, the 

median RBL for the 10 remaining 

species decreases. For the median RBL 

values, 203 of the 239 counties 

estimated to have a median RBL above 

two percent when air quality is meeting 

the current standard are because of the 

presence of black cherry (U.S. EPA, 

2014b, Table 6–7). 

Additionally, the WREA estimated 

relative yield loss in timber production 

and associated changes in consumer and 

producer/farmer economic surplus 

using E–R functions for tree seedlings to 

calculate relative yield loss (equivalent 

to relative biomass loss) across full tree 

lifespans and through modeling of the 

resulting market-based welfare effects. 

Because the forestry and agriculture 

sectors are related and trade-offs occur 

between the sectors, the WREA 

calculated the resulting market-based 

welfare effects of O3 exposure in the 

forestry and agriculture sectors on 

consumer and producer surplus.206


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Federal Register / Vol. 79, No. 242 / Wednesday, December 17, 2014 / Proposed Rules 75325 

100 years in 5-year time intervals when running the 

combined forest-agriculture version of the model. 

207The WREA used the i-Tree model for the 

urban case studies. i-Tree is a peer-reviewed suite 

of software tools provided by USFS. 

208One 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). 

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). 

The WREA also estimated impacts on 

tree growth and two ecosystem services 

provided by urban trees: removal of air 

pollutants and carbon storage. The 

estimates of the tons of carbon 

monoxide, nitrogen dioxide, ozone and 

sulfur dioxide removed are for a 25-year 

period in five urban case study areas: 

Baltimore, Syracuse, the Chicago region, 

Atlanta, and the urban areas of 

Tennessee (U.S. EPA, 2014b, section 

6.7).207

• Estimates for all five urban case 

study areas indicate increased pollutant 

removal of O3, nitrogen dioxide, carbon 

monoxide, and sulfur dioxide in the 

current standard scenario (U.S. EPA, 

2014b, sections 6.7). The results for the 

15 ppm-hrs scenario were very similar 

to those for meeting the current 

standard. For the 11 and 7 ppm-hrs 

scenarios, all five case study areas 

indicate smaller additional increases in 

air pollutant removal beyond 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 nationallevel impacts to forests and agriculture 

from O3-related biomass loss, carbon 

sequestration reflects carbon in standing 

(live and dead) trees, forest soils, the 

forest understory vegetation, forest floor 

including litter and large woody debris, 

and wood products both in use and in 

landfills (U.S. EPA, 2014b, chapter 6, 

Appendix 6B, section 2.7.1). 

• Over 30 years for the national-scale 

analysis, carbon storage in the forestry 

sector estimated for the current standard 

scenario is just over 89,000 million 

metric tons of CO2 equivalents 

(MMtCO2e); this is 11,840 more 

MMtCO2e storage associated with the 

reduced O3-related growth impact from 

meeting the current standard as 

compared with recent conditions.208

The estimates of carbon storage in the 

agricultural sector are much smaller 

(i.e., 8,469 MMtCO2e for the current 

standard scenario which is 606 

MMtCO2e more than the recent 

conditions scenario) (U.S. EPA, 2014b, 

section 6.6.1 and Appendix 6B). The 

forestry sector carbon storage estimated 

for each of the three W126 scenarios is 

just slightly greater than that estimated 

for the current standard. As a percentage 

of the current standard estimate, the 

three scenario estimates are less than 

0.1% (13 MMtCO2e), just under 1% (593 

MMtCO2e) and under 2% (1,600 

MMtCO2e) for the 15, 11 and 7 ppm-hrs 

scenarios, respectively (U.S. EPA, 

2014b, Tables 6–19 and B–10). 

• Estimates of the effects of avoided 

O3-related biomass loss on carbon 

sequestration in forests in the five urban 

area case studies indicate the potential 

for an increase in carbon sequestration 

of somewhat more than one MMtCO2e 

for the current standard scenario 

compared to the recent conditions 

estimate (U.S. EPA, 2014b, section 6.6.2 

and Appendix 6D). The additional 

increases in O3-related carbon 

sequestration estimated across the five 

case studies for the three W126 

scenarios are relatively small (U.S. EPA, 

2014b, section 6.6.2 and Appendix 6D). 

Although not discussed in detail here, 

the WREA also describes qualitative 

assessments for some ecosystem 

services that may be affected by O3

effects on tree growth and productivity, 

such as commercial non-timber forest 

products and recreation (U.S. EPA, 

2014b, section 6.4), aesthetic and nonuse 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 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 


tkelley on DSK3SPTVN1PROD with PROPOSALS2
75326 Federal Register / Vol. 79, No. 242 / Wednesday, December 17, 2014 / Proposed Rules 

209 In the air quality scenario for the current 

standard, a monitor that already met the current 

standard but was located within the same region as 

another monitor that was above the current 

standard would have had its concentration adjusted 

downward. This is due to the fact that 

concentrations were adjusted independently for 

each region, applying reductions to all monitors 

within the region, such that all monitors located 

within a region meet the standard (U.S. EPA, 2014b, 

section 4.3.4.2). 

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 unknown, yet its 

magnitude as high, concluding that 

further studies are needed to determine 

how accurately the assessed species 

reflect the larger suite of O3-sensitive 

tree species in the U.S. (U.S. EPA, 

2014b, Table 6–27). 

Another uncertainty associated with 

interpretation of the WREA biomass 

loss-related estimates concerns the 

potential for underestimation of 

compounding of growth effects across 

multiple years of varying 

concentrations. Though tree biomass 

loss impacts were estimated using air 

quality scenarios of three-year average 

W126 index values, the WREA also 

conducted an analysis to compare the 

impact of using a variable compounding 

rate based on yearly variations in W126 

exposures to that of using a W126 index 

value averaged across three years. The 

WREA compared the compounded 

values for an example species occurring 

in the eastern U.S. and another example 

species occurring in the western U.S. In 

both examples, one species (tulip polar 

and ponderosa pine, respectively) and 

one climate region where that species 

occurred (Southeast and Southwest 

regions, respectively) were chosen and 

air quality values associated with just 

meeting the existing standard of 75 ppb 

were used. Within each region, the 

WREA analysis used both the W126 

index value at each monitor in the 

region for each year and the three-year 

average W126 index value using the 

method described in Chapter 4 of the 

WREA. The results show that the use of 

the three-year average W126 index value 

may underestimate RBL values slightly 

(U.S. EPA, 2014b, section 6.2.1.4 and 

Figure 6–14). In both regions, the threeyear average W126 index value is 

sometimes above and sometimes below 

the individual year W126 index value. 

The WREA recognizes uncertainty 

regarding the extent to which the subset 

of studied tree species encompass the 

O3 sensitive species in the U.S. and the 

extent to which it represents U.S. 

vegetation as a whole (U.S. EPA, 2013a, 

pp. 9–123 to 9–125; U.S. EPA, 2014b, 

Table 6–27). 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 speciesspecific pollution removal and carbon 

sequestration functions. However, i-Tree 

does not account for the potential 

additional VOC emissions from tree 

growth, which could contribute to O3

formation. Uncertainties are also 

recognized with regard to the base 

inventory of city trees, the functions 

used for air pollutant removal and for 

carbon storage (U.S. EPA, 2014b, 

sections 6.6.2 and 6.7, and Table 6–27). 

Despite the inherent limitations and 

uncertainties, the WREA concludes that 

the i-Tree model reflects reasonable and 

appropriate assumptions for a case 

study assessment of pollution removal 

and carbon sequestration for changes in 

biomass associated with O3 exposure 

(U.S. EPA, 2014b, sections 6.6.2, 6.7, 

and 8.5.2). 

3. Crop Yield 

Section IV.C.3.a below provides an 

overview of the assessments performed 

in the WREA to estimate the exposures 

and risks for crop yield, as well as the 

key results. Section IV.C.3.b summarizes 

the key uncertainties. 

a. Overview and Summary of Key 

Results 

The WREA conducted two analyses to 

estimate O3 impacts related to crop 

yield, including annual yield losses 

estimated for 10 commodity crops 

grown in the U.S. with E–R functions 

and how these losses affect producer 

and consumer economic surpluses (U.S. 

EPA, 2014b, sections 6.2, 6.5). Summary 

estimates for crop yield loss related 

effects in the WREA are presented 

relative to a 5% yield loss benchmark 

based on consideration of CASAC’s 

recommendation to consider a 

benchmark of 5% for median crop yield 

loss and to consider 5% yield loss for 

individual crop species. In addition, 

other benchmarks levels are considered 

in the WREA (e.g. 10% and 20%). 

The WREA derived estimates of crop 

RYL estimates nationally and in a 

county-specific analysis. Crop-specific 

estimates of O3-related RYL nationally 

were derived for each of the air quality 

scenarios from the 10 E–R functions for 

crops described above combined with 

information regarding crop distribution 

(U.S. EPA, 2014b, section 6.5). The 

WREA also reported crop RYL results at 

the county-level, as well as the number 

of crop-producing counties with greater 

than five percent RYL (U.S. EPA, 2014b, 

section 6.5.1, Appendix 6B). 

• The largest reduction in O3-induced 

crop yield loss and yield changes occurs 

when moving from the recent 

conditions scenario to the current 

standard scenario (U.S. EPA, 2014b, 

section 6.5). Among the major 

commercial crops, winter wheat and 

soybeans are more sensitive to ambient 

O3 levels than other crops. 

• In the current standard scenario, no 

counties have RYL estimates at or above 

5% (U.S. EPA, 2014b, section 6.5).209


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Federal Register / Vol. 79, No. 242 / Wednesday, December 17, 2014 / Proposed Rules 75327 

210Welfare 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). 

211 Data were not available for several western 

states (Montana, Idaho, Wyoming, Nevada, Utah, 

Colorado, Arizona, New Mexico, Oklahoma, and 

portions of Texas). 

212The 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. 

213This 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. 

The WREA also estimated O3-related 

crop impacts on producer and consumer 

surplus.210 These are national-scale 

estimates of the effects of yield loss on 

agricultural harvesting, which supply 

provisioning services of food and fiber 

for each of the air quality scenarios. 

Overall effect on agricultural yields and 

producer and consumer surplus 

depends on (1) the ability of producers/ 

farmers to substitute other crops that are 

less O3 sensitive, and (2) the 

responsiveness, or elasticity, of demand 

and supply (U.S. EPA, 2014b, section 

6.5). 

• Estimates of consumer surplus, or 

consumer gains, were generally higher 

in the current standard scenario in the 

agricultural sector because higher 

productivity under lower O3

concentrations increased total yields 

and reduced market prices (U.S. EPA, 

2014b, Tables 6–17 and 6–18). 

Combined gains in producer and 

consumer surplus for forestry and 

agriculture were essentially unchanged 

for the 15 ppm-hrs scenario, but 

annualized gains increased by $21 

million beyond the current standard 

scenario for the 11 ppm-hrs scenario 

and $231 million for the 7 ppm-hrs 

scenario. In some cases, lower prices 

reduce producer gains more than can be 

offset by higher yields (U.S. EPA, 2014b, 

Table 6–18). 

• Because demand for most 

agricultural commodities is not highly 

responsive to changes in price, producer 

surplus or producer gains often 

declined. For agricultural welfare, 

annualized combined consumer and 

producer surplus gains were estimated 

to be $2.6 trillion in 2010 for the current 

standard scenario (U.S. EPA, 2014b, 

Table 6–17). 

b. Key Uncertainties 

The WREA discusses multiple areas 

of uncertainty associated with the crop 

yield loss estimates, including those 

associated with the model-based 

adjustment methodology as well as 

those associated with the projection of 

yield loss using the FASOMGHG model 

at the estimated O3 concentrations (U.S. 

EPA, 2014b, Table 6–27, section 8.5). 

Because the W126 estimates generated 

in the air quality analyses are inputs to 

the vegetation risk analyses for crop 

yield loss, any uncertainties in the air 

quality analyses are propagated into the 

those analyses (U.S. EPA, 2014b, Table 

6–27, section 8.5). Therefore, the air 

quality scenarios in the crop yield 

analyses have the same uncertainties 

and limitations as in the biomass loss 

analyses (summarized above), including 

those associated with the model-based 

adjustment methodology (U.S. EPA, 

2014b, section 8.5). 

4. Visible Foliar Injury 

Section IV.C.4.a below provides an 

overview of the assessment in the 

WREA of O3-related visible foliar injury 

and associated ecosystem services 

impacts, as well as the key results. 

Section IV.C.4.b summarizes the key 

uncertainties. 

a. Overview and Summary of Key 

Results 

The WREA presents a number of 

analyses of O3-related visible foliar 

injury and associated ecosystem 

services impacts (U.S. EPA, 2014b, 

Chapter 7). An initial analysis using 

USFS FHM/FIA biosite data included 

the development of benchmark criteria 

reflecting different prevalences of 

visible foliar injury at different W126 

exposures and soil moisture conditions. 

These criteria were then used in a 

screening-level characterization of the 

potential risk of foliar injury incidence 

in 214 national parks and a case study 

assessment of three national parks, 

which also provides limited 

characterization of the associated 

ecosystem services. 

In the biosite data analysis, the WREA 

used the biomonitoring site data from 

the USFS FHM/FIA Network (USFS, 

2011),211 associated soil moisture data 

during the sample years, and national 

surfaces of ambient air O3

concentrations based on spatial 

interpolation of monitoring data from 

2006 to 2010 to calculate the proportion 

of biosites with any visible foliar injury. 

The proportion of biosites metric is 

derived by first ordering the data (across 

biosites and sample years) by W126 

index value estimated for that biosite 

and year. Then for each W126 index 

value, the proportion of biosites is 

calculated with any foliar injury for all 

observations at or below that W126 

index value. (U.S. EPA, 2014b, section 

7.2). This analysis indicates that the 

proportion of biosites showing the 

presence of any foliar injury increases 

rapidly from zero to about 20 percent at 

relatively low W126 index values. 

Specifically: (1) the proportion of 

biosites exhibiting foliar injury rises 

rapidly with increasing W126 index 

values below approximately 10 ppm-hrs 

(W126 <10.46 ppm-hrs), and (2) there is 

relatively little change in this 

proportion with increasing W126 index 

values above approximately 10 ppm-hrs 

(W126 >10.46 ppm-hrs). The data for 

biosites during normal moisture years 

are very similar to the dataset as a 

whole, with an overall proportion of 

close to 18 percent for presence of any 

foliar injury. Among the biosites with a 

relatively wet season, the proportion of 

biosites showing injury is much higher 

and the relationship with annual W126 

index value is much steeper. Much 

lower proportions of biosites show 

injury with relatively dry seasons (U.S. 

EPA, 2014b, section 7.2.3, Figures 7– 

10), consistent with the ISA finding that 

many studies have shown that dry 

periods tend to decrease the incidence 

and severity of O3-induced visible foliar 

injury (U.S. EPA, 2013a, section 9.4.2). 

While these analyses indicate the 

potential for foliar injury to occur under 

conditions that meet the current 

standard, the extent of foliar injury that 

might be expected under such 

conditions is unclear from these 

analyses. 

The national-scale screening-level 

assessment in 214 parks employed 

benchmark criteria developed from the 

above analysis.212 213 For example, 

annual O3 concentrations corresponding 

to a W126 index value of 10.46 ppm-hrs 

represents the O3 exposure 

concentration where the slope of 

exposure-response relationship changes 

for FHM biosites, with the percentage of 

biosites showing injury remaining 

relatively constant for higher W126 

index values. The WREA refers to this 

as the ‘‘base scenario’’ benchmark. The 


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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 biosites with 

injury, W126 index values were 

approximately 4, 6, and 25 ppm-hrs for 

wet, normal and dry years, respectively. 

The national-scale screening-level 

assessment applied these benchmarks to 

42 parks with O3 monitors and a total 

of 214 parks with O3 exposure estimated 

from the interpolated national O3

surfaces for individual years from 2006 

to 2010 (U.S. EPA, 2014b, Appendix 7A 

and section 7.3). 

• Based on NPS lists, 95 percent of 

the 214 parks in this screening-level 

assessment contain at least one 

vegetation species sensitive to O3- 

induced foliar injury (U.S. NPS, 2003, 

2006). 

• In the current standard scenario, 

none of the 214 parks had O3

concentrations estimated to exceed the 

annual benchmark of a W126 index 

value above 10.46 ppm-hrs (U.S. EPA, 

2014b, section 7.3.3.3). 

The case study analyses focused on 

Great Smoky Mountains National Park 

(GRSM), Rocky Mountain National Park 

(ROMO), and Sequoia and Kings 

Canyon National Parks (SEKI). 

Information on visitation patterns, 

recreational activities and visitor 

expenditures was considered. For 

example, visitor spending in 2011 

exceeded $800 million, $170 million 

and $97 million dollars in GRSM, 

ROMO and SEKI, respectively. In each 

park, the percent cover of species 

sensitive to foliar injury was estimated 

and the overlap between recreation 

areas within the park and elevated 

W126 concentrations was described. 

(U.S. EPA, 2014b, section 7.4). 

• In the current standard scenario, the 

three-year average W126 index values 

were at or below 7 ppm-hrs in all areas 

of two of the three parks (GRSM and 

SEKI). Three-year average W126 index 

values were below 7 ppm-hrs in a little 

more than half of the area of 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 ppmhrs, 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 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 cooccurring drought (Smith, 2012; Smith 

et al., 2003)’’ (U.S. EPA, 2013a, p. 9–39). 

This ‘‘. . . partial ‘protection’ against 

the effects of O3 afforded by drought has 

been observed in field experiments 

(Low et al., 2006) and modeled in 

computer simulations (Broadmeadow 

and Jackson, 2000)’’ (U.S. EPA, 2013a, 

p. 9–87). In considering the extent of 

any protective role of drought 

conditions, however, the ISA also notes 

that other studies have shown that 

‘‘drought may exacerbate the effects of 

O3 on plants (Pollastrini et al., 2010; 

Grulke et al., 2003)’’ and that ‘‘[t]here is 

also some evidence that O3 can 

predispose plants to drought stress 

(Maier-Maercker, 1998)’’. Accordingly, 

the ISA concludes that ‘‘the nature of 

the response is largely species-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 screeninglevel 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 × 12 

km grid cells, and uncertainties 

associated with O3 exposure data of 

vegetation and recreational areas within 

parks (U.S. EPA, 2014b, Table 7–22). 

There are also important uncertainties 

in the estimated O3 concentrations for 

the different air quality scenarios 

evaluated (U.S. EPA, 2014b, section 

8.5), as discussed earlier in this section. 

These uncertainties only apply to the 

national park case studies because these 

are the only foliar injury analyses that 

rely on the air quality scenarios, but any 

uncertainties in the air quality analyses 

are propagated into those analyses. The 

WREA identifies additional 

uncertainties that are associated with 


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the national park case studies. 

Specifically, there is uncertainty 

inherent in survey estimates of 

participation rates, visitor spending/

economic impacts, and willingness-topay. These surveys potentially doublecount 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 policyrelevant considerations, as discussed in 

the PA. Air quality analyses inform 

these considerations with regard to 

cumulative, seasonal exposures 

occurring in areas of the U.S. that meet 

the current standard. Exposure- and 

risk-based considerations draw upon 

EPA assessments of risk of key welfare 

effects, including O3 effects on forest 

growth, productivity, carbon storage, 

crop yield and visible foliar injury, 

expected to occur in model-based 

scenarios for the current standard, with 

appropriate consideration of associated 

uncertainties. 

The following sections describe 

consideration of the evidence and the 

exposure/risk information in the PA and 

advice received from CASAC, as well as 

the comments received from various 

parties, and the Administrator’s 

proposed conclusions regarding the 

adequacy of the current secondary 

standard. 

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 evidenceand exposure/risk-based considerations 

and presents staff conclusions for the 

Administrator to consider in reaching 

her proposed decision on the current 

standard. The focus of the initial PA 

conclusions is consideration of the 

question: Does the currently available 

scientific evidence and exposure/risk 

information, as reflected in the ISA and 

WREA, support or call into question the 

adequacy and/or appropriateness of the 

protection afforded by the current 

secondary O3 standard? 

The PA’s general approach to 

informing judgments by the 

Administrator recognizes that the 

available welfare effects evidence 

demonstrates a range of O3 sensitivity 

across studied plant species and 

documents an array of O3-induced 

effects that extend from lower to higher 

levels of biological organization. These 

effects range from those affecting cell 

processes and individual plant leaves to 

effects on the physiology of whole 

plants, as well as the range from species 

effects and effects on plant communities 

to effects on related ecosystem processes 

and services. Given this evidence, the 

PA notes that it is not possible to 

generalize across all studied species 

regarding which cumulative exposures 

are of greatest concern, as this can vary 

by situation due to differences in 

exposed species sensitivity, the 

importance of the observed or predicted 

O3-induced effect, the role that the 

species plays in the ecosystem, the 

intended use of the affected species and 

its associated ecosystem and services, 

the presence of other co-occurring 

predisposing or mitigating factors, and 

associated uncertainties and limitations. 

Therefore, in developing conclusions in 

the PA, staff takes note of the 

complexity of judgments to be made by 

the Administrator regarding the 

adversity of known and anticipated 

effects to the public welfare and are 

mindful that the Administrator’s 

ultimate judgments on the secondary 

standard will most appropriately reflect 

an interpretation of the available 

scientific evidence and exposure/risk 

information that neither overstates nor 

understates the strengths and 

limitations of that evidence and 

information (U.S. EPA, 2014c, section 

5.7). 

In considering the estimates of 

exposures and risks for air quality 

scenarios assessed in the WREA, the PA: 

(1) Evaluates the weight of the scientific 

evidence concerning vegetation effects 

associated with those O3 exposures; (2) 

considers the importance, from a public 

welfare perspective, of the O3-induced 

effects on sensitive vegetation and 

associated ecosystem services that are 

known or anticipated to occur as a 

result of exposures in the assessed air 

quality scenarios; and, (3) recognizes 

that predictions of effects associated 

with any given O3 exposure may be 

mitigated or exacerbated by actual 

conditions in the environment (i.e., cooccurring 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 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 


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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. 

With respect to the scientific 

evidence, the PA takes note of the 

longstanding evidence base that 

demonstrates O3-induced effects that 

occur across a range of biological and 

ecological scales of organization, as 

described in the ISA and summarized in 

section IV.B.1 above (U.S. EPA, 2013a, 

p. 1–8). Many of the recent studies 

evaluated in this review have focused 

on and further increased our 

understanding of the molecular, 

biochemical and physiological 

mechanisms that explain how plants are 

affected by O3 in the absence of other 

stressors (U.S. EPA, 2013a, section 9.3). 

These recent studies, in combination 

with the extensive and long-standing 

evidence, have further strengthened the 

coherence and consistency of the entire 

body of research since the last review. 

Consistent with conclusions in the 2006 

AQCD, the ISA determined that a causal 

relationship exists between O3 exposure 

and visible foliar injury on sensitive 

vegetation, reduced plant growth, 

reduced productivity in terrestrial 

ecosystems, reduced yield and quality 

of agricultural crops and alteration of 

below-ground biogeochemical cycles 

(U.S. EPA, 2013a, Table 1–2 and section 

2.6). The relationship between O3

exposures and reduced carbon 

sequestration in terrestrial ecosystems, 

alteration of terrestrial ecosystem water 

cycling and alteration of terrestrial 

community composition was concluded 

to be likely causal (U.S. EPA, 2013a, 

Table 1–2). 

The PA recognizes that consistent 

with conclusions drawn in the last 

review, the currently available evidence 

base also strongly supports that effects 

on vegetation are attributable to 

cumulative seasonal O3 exposures. 

Moreover, on the basis of the entire 

body of evidence in this regard, the ISA 

concludes that ‘‘quantifying exposure 

with indices that cumulate hourly O3

concentrations and preferentially weight 

the higher concentrations improves the 

explanatory power of exposure/response 

models for growth and yield, over using 

indices based on mean and peak 

exposure values’’ (U.S. EPA, 2013a, p. 

2–44). Accordingly, as in other recent 

reviews, the evidence continues to 

provide a strong basis for concluding 

that it is appropriate to judge impacts of 

O3 on vegetation, related effects and 

services, and the level of public welfare 

protection achieved, using a cumulative, 

seasonal exposure metric, such as the 

W126-based metric. In this review, as in 

the last review, the CASAC concurs 

with this conclusion (Frey, 2014c, p. 

iii). Thus, based on the consistent and 

well-established evidence described 

above, the PA concludes that the most 

appropriate and biologically relevant 

way to relate O3 exposure to plant 

growth, and to determine what would 

be adequate protection for public 

welfare effects attributable to the 

presence of O3 in the ambient air is to 

characterize exposures in terms of a 

cumulative seasonal form, and in 

particular the W126 metric. 

In considering the current standard 

with regard to protection from the array 

of O3-related effects recognized in this 

review, the PA first considers effects 

related to forest tree growth, 

productivity and carbon storage, effects 

for which the ISA concludes the 

evidence supports a causal or likely 

causal relationship with exposures to O3

in ambient air (U.S. EPA, 2014c, 

sections 5.2 and 5.7). In so doing, the 

PA notes that while changes in biomass 

affect individual tree species, the overall 

effect on forest ecosystem productivity 

depends on the composition of forest 

stands and the relative sensitivity of 

trees within those stands. In considering 

the evidence for these effects and the 

extent to which they might be expected 

to occur under conditions that meet the 

current secondary standard, the PA 

focused particularly on RBL estimates 

for the 11 species for which robust E– 

R functions have been developed. The 

PA recognized that recent studies, such 

as multiple-year exposures of aspen and 

birch, have provided additional 

evidence on tree biomass or growth 

effects associated with multiple year 

exposures in the field, including 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 interspecies competition within a forest 

stand (U.S. EPA, 2013a, section 9.4.3; 

U.S. EPA, 2014c, section 5.2). In giving 

particular attention to tree seedling 

biomass loss estimates, the PA notes 

that CASAC ‘‘concurs that biomass loss 

in trees is a relevant surrogate for 

damage to tree growth that affects 

ecosystem services such as habitat 

provision for wildlife, carbon storage, 

provision of food and fiber, and 

pollution removal’’ (Frey, 2014c, p. 10). 

In evaluating the current evidence 

and exposure/risk information 

associated with tree growth, 

productivity and carbon storage, with 

regard to the adequacy of public welfare 

protection afforded by the current 

standard, the PA considers the evidence 

of vegetation and welfare impacts in 

areas of the U.S. likely to have met the 

current standard. With regard to O3

effects on tree growth, productivity and 

carbon storage and associated 

ecosystems and services, the PA focuses 

on relative biomass loss estimates based 

on the OTC-based E–R functions, noting 

that analyses newly performed in this 

review have reduced the uncertainty 

associated with using OTC E–R 

functions to predict tree growth effects 

in the field (U.S. EPA, 2014c, section 

5.2.1; U.S. EPA, 2013a, section 9.6.3.2). 

In focusing on RBL estimates, the PA 

recognized that comparison to an array 

of benchmarks would be informative to 

considerations of significance to public 

welfare. Included in 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, 


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214The 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). 

the PA takes note of CASAC’s comments 

that a 6% median RBL is ‘‘unacceptably 

high’’, and that the 2% median RBL is 

an important benchmark to consider 

(Frey, 2014c).214 Based on the summary 

of RBL estimates in the PA, the PA notes 

that the median species RBL estimate is 

at or below 2% for W126 exposure 

index values less than or equal to 7 

ppm-hrs (U.S. EPA, 2014c, Table 6–1 

and Appendix 5C). The median species 

RBL is at or above 6% for W126 index 

values of 19 ppm-hrs and higher. 

In recognition of the significance of 

welfare effects in Class I areas, the PA 

gives appreciable weight to 

consideration of the occurrence of O3

concentrations associated with the 

potential for RBL estimates above 

benchmarks of interest in Class I areas 

that meet the current standard. Based on 

air quality data for the period from 1998 

to 2012, the PA focused consideration 

on 22 Class I areas, in which during one 

or more three-year periods the air 

quality met the current standard and the 

three-year average W126 index value 

was at or above 15 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 three-year 

periods that include these highest 

annual values for 21 areas, the PA notes 

that in 10 areas (across five states in the 

West and Southwest regions), the threeyear average W126 values (for the 

highest three-year period that includes 

these annual values) are at or above 19 

ppm-hrs, ranging up to 22.5 ppm-hrs 

(for which the median species RBL 

estimate is above 7%). This indicates 

that the W126 value above 19 ppm-hrs 

is not simply a single year in a period 

of lower years, but that in these cases 

there were sustained higher values that 

contributed to a three-year W126 also 

above 19 ppm-hrs. In terms of the 

highest three-year values observed 

(regardless of single-year values), the PA 

additionally notes that the highest threeyear 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). 

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 


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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—Continued 

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 

Superstition Wilderness 

Area PP.

AZ/Maricopa ..... 75 22.4 (1, 22.4) ....................... 14.5–28.6 (2, 27.4–28.6) ..... 1 

AZ/Pinal ............ 73–75 18.7–22.5 (2, 20.8–22.5) ..... 14.8–29.0 (3, 22.6–29.0) ..... 3 

Weminuche Wilderness 

Area QA, DF, PP.

CO/La Plata ...... 70–74 15.0–19.1 (1, 19.1) .............. 10.9–21.0 (2, 20.8–21.0) ..... 5 

West Elk Wilderness 

Area QA, DF.

CO/Gunnison .... 68–73 15.6–20.1 (1, 20.1) .............. 12.9–23.9 (3, 21.1–23.9) ..... 8 

Wind Cave National Park QA,

PP.

SD/Custer ......... 70 15.4 ...................................... 12.2–20.6 (1, 20.6) .............. 1 

Yosemite National Park QA,

DF, PP.

CA/Tuolumne ... 73–74 20.7–20.8 (2, 20.7–20.8) ..... 19.7–22.1 (4, 19.7–22.1) ..... 2 

Zion National Park QA, DF, PP UT/Washington 70–73 17.8–21.1 (2, 20.3–21.1) ..... 14.9–24.2 (5, 19.3–24.2) ..... 4 

* 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). 

In considering the data analysis for 22 

Class I areas described above, the PA 

additionally considers the speciesspecific 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 ppmhrs in eight areas across five states. The 

ponderosa pine RBL estimates for 29 

and 22.5 ppm-hrs are approximately 

12% and 9%, respectively (U.S. EPA, 

2014c, Appendix 5C). In Class I areas 

where quaking aspen is present, the 

highest single year W126 index values 

ranged from 19.2 to 26.7 ppm-hrs and 

the highest three-year average W126 

values in which these single year values 

were represented ranged from 15.0 to 

22.2 ppm-hrs, with these three-year 

values above 19 ppm-hrs in eight areas 

across five states. The quaking aspen 

RBL estimates for 26.7 and 22.2 ppm-hrs 

are approximately 16% and 13%, 

respectively (U.S. EPA, 2014c, 

Appendix 5C). 

The PA describes the above 

observations, particularly in light of 

advice from CASAC, summarized in 

section IV.D.2 below, as evidence of the 

occurrence in Class I areas during 

periods where the current standard is 

met of cumulative seasonal O3

exposures of a magnitude for which the 

tree growth impacts indicated by the 

estimated median species RBL might 

reasonably be concluded to be 

important to public welfare (U.S. EPA, 

2014c, sections 5.2.1 and 5.7). 

In considering the WREA analyses of 

effects on tree growth and associated 

ecosystem services in the air quality 

scenario for the current standard, the PA 

first takes note of the potential 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 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 


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215See http://www.ers.usda.gov/topics/crops/

soybeans-oil-crops/background.aspx 

216The monitoring data reflect observations in 

locations that meet the current standard. The WREA 

analysis that assessed crop yield loss used a modeldeveloped 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. 

weight on the absolute magnitude of the 

risk results for these ecosystem service 

endpoints due to the identification of 

significant associated uncertainties (U.S. 

EPA, 2014c, sections 5.2 and 5.7). 

In reaching conclusions regarding 

support for the adequacy of the current 

secondary standard provided by the 

currently available information on O3- 

induced effects on trees and associated 

services, the PA takes note of: (1) the 

robust evidence supporting the causal 

relationship between cumulative O3

exposures and effects on tree growth 

and productivity, and information from 

model simulations supporting the 

determination of a likely causal 

relationships for carbon storage in 

terrestrial ecosystems (U.S. EPA, 2013a, 

sections 2.6.2.1 and 9.4.3); (2) the tree 

seedling E–R functions evidence, which 

has been strengthened and demonstrates 

variability in sensitivity to O3 across 

species; (3) estimates of median species 

RBL at or above 6% associated with 

W126-based exposure levels in several 

areas when O3 concentrations were at or 

below the current standard; (4) growth 

effects estimates associated with 

exposure concentrations in several Class 

I areas based on O3 concentrations from 

1998–2012 that were at or below the 

current standard; (5) evidence that 

impacts from single year exposures can 

carry over to the subsequent year and/ 

or cumulate over multiple years with 

repeated annual exposures; (6) evidence 

from recent mechanistic studies and 

field based studies that support earlier 

findings from OTC studies; and (7) 

WREA analyses indicating that O3- 

induced biomass loss can impact 

ecosystem services provided by forests, 

including timber production, carbon 

storage, and air pollution removal, even 

when air quality is adjusted to just meet 

the current standard. Given the above, 

and noting CASAC views (described in 

section IV.D.2 below), the PA concludes 

that the current evidence and exposure/ 

risk information call into question the 

adequacy of public welfare protection 

afforded by the current standard from 

the known and anticipated adverse 

effects associated with O3-induced 

impacts on tree growth, productivity 

and carbon storage, including the 

associated ecosystem services assessed 

in this review. Therefore, the PA 

concludes that it is appropriate to 

consider revision of the secondary 

standard to provide increased 

protection. 

With respect to crops, the PA takes 

note of the extensive and long-standing 

evidence on the detrimental effect of O3

on crop production, which continues to 

be confirmed by newly available 

evidence (U.S. EPA, 2013a, section 

9.4.4; U.S. EPA, 2014c, sections 5.3 and 

5.7). The PA additionally notes that 

recent studies have highlighted the 

effects of O3 on crop quality, such as 

through decreases in the nutritive 

quality of grasses, and in the macro- and 

micro-nutrient concentrations in fruits 

and vegetable crops (U.S. EPA, 2013a, 

section 9.4.4; U.S. EPA, 2014c, section 

5.3). Further, the PA notes that there has 

been little published evidence that 

crops are becoming more tolerant of O3,

taking note particularly of the ISA 

analyses of data from cultivars used in 

NCLAN studies, and yield data for 

modern cultivars from SoyFACE which 

confirm that the average response of 

soybean yield to O3 exposure has not 

changed in current cultivars (U.S. EPA, 

2006a; U.S. EPA, 2013a, section 9.6.3; 

U.S. EPA, 2014c, section 5.3). In 

consideration of the currently available 

evidence for O3 effects on crops, the PA 

concludes that the recently available 

evidence, as assessed in the ISA, 

continues to support the conclusions of 

the 1996 and 2006 CDs that ambient O3

concentrations can reduce the yield of 

major commodity crops in the U.S, and 

that the currently available evidence 

continues to support the use of the E– 

R functions developed for 10 crops from 

OTC experiment data. Further, the PA 

recognizes that important uncertainties 

have been reduced regarding the 

exposure-response functions for crop 

yield loss, especially for soybean, the 

second-most planted field crop in the 

U.S.,215 with the ISA generally reporting 

consistent results across exposure 

techniques and across crop varieties 

(U.S. EPA, 2013a, section 9.6.3.2). 

With regard to consideration of the 

quantitative impacts of O3 on crop yield, 

the PA considers RYL estimates for O3

conditions associated with the current 

standard. As in the case of the PA 

considerations of RBL estimates for tree 

seedlings, the PA recognized CASAC 

comments, which described greater than 

5% RYL for the median crop species as 

‘‘unacceptably high’’ and 5% RYL for 

the median crop species as adverse, 

while noting the opportunities to alter 

management of annual crops (Frey, 

2014c, pp. iii and 14). The PA notes that 

staff analyses of recent monitoring data 

(2009–2011) indicate that O3

concentrations in multiple agricultural 

areas in the U.S. that meet the current 

standard correspond to W126 index 

levels above 12 ppm-hrs, a value for 

which soybean RYL estimates are 

greater than 5%. In particular, the PA 

notes that while the design values for 

two counties in the Midwest met the 

current standard in 2009–2011, both 

had a maximum annual W126 of 19 

ppm-hrs (in 2011) for which the 

soybean annual RYL estimate, based on 

the E–R function, is 9%.216

In considering the evidence and 

exposure/risk-based information for 

effects on crops, the PA notes the 

CASAC comments regarding the use of 

crop yields as a surrogate for 

consideration of public welfare impacts, 

in which it noted that ‘‘[c]rops provide 

food and fiber services to humans’’ and 

that ‘‘[e]valuation of market-based 

welfare effects of O3 exposure in forestry 

and agricultural sectors is an 

appropriate approach to take into 

account damage that is adverse to public 

welfare’’ (Frey, 2014c, p. 10; U.S. EPA, 

2014c, section 5.7). The PA additionally 

notes, however, as recognized in section 

IV.B.2 above that the determination of 

the point at which O3-induced crop 

yield loss becomes adverse to the public 

welfare is still unclear, given that crops 

are heavily managed with additional 

inputs that have their own associated 

markets and that benefits can be 

unevenly distributed between producers 

and consumers. The PA further notes 

that to the extent protection is provided 

by the current standard with regard to 

impacts on trees, protection may also be 

provided for commodity crops (U.S. 

EPA, 2014c, sections 5.3 and 5.7). 

In reaching conclusions regarding 

support provided for the adequacy of 

the current secondary standard by the 

currently available information on O3- 

related crop effects, the PA notes: (1) the 

support for a causal relationship 

between cumulative O3 exposures and 

effects on crop yields and quality (U.S. 

EPA, 2013a, section 9.4.4); (2) the 

evidence supporting E–R functions for 

10 crops, which has been strengthened 

in this review and which demonstrates 

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 


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the interaction between agriculture and 

timber production. 

With regard to visible foliar injury, 

the PA recognizes the long-standing 

evidence that has established that O3

causes diagnostic visible injury 

symptoms on studied bioindicator 

species and that soil moisture is a major 

confounding effect that can decrease the 

incidence and severity of visible foliar 

injury under dry conditions and vice 

versa (U.S. EPA, 2014c, sections 5.4 and 

5.7). As at the time of the last review, 

the most extensive dataset regarding 

visible foliar injury incidence across the 

U.S. is that collected by the U.S. Forest 

Service (USFS) Forest Health 

Monitoring/Forest Inventory and 

Analysis (FHM/FIA) Program, which 

has documented incidence of visible 

foliar injury in both the eastern and 

western U.S. Evidence available in the 

current review includes studies using 

controlled exposures as well as multiyear 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 

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 riskbased information, the PA takes note of 

the WREA analyses of the nationwide 

dataset (2006–2010) for USFS/FHM 

biosites, including the observation that 

the proportion of biosites with injury 

varies with soil moisture conditions and 

O3 W126 index values (U.S. EPA, 2014b, 

Chapter 7, Figure 7–10; U.S. EPA, 

2014c, section 5.4.2). These analyses 

indicate that the proportion of biosites 

showing visible foliar injury incidence 

increases steeply with W126 index 

values up to approximately 10 ppm-hrs, 

with little difference in incidence across 

higher W126 index levels. The 

screening-level assessment of national 

parks indicated that risk of visible foliar 

injury is likely to be lower in most 

national parks after simulating just 

meeting the current standard, although 

visible foliar injury would likely 

continue to occur at lower O3 exposures, 

including some sensitive species 

growing in National Parks and other 

Class I areas that may provide important 

cultural ecosystem services to the 

public. The PA also notes the WREA 

recognition that many of the outdoor 

recreational activities which directly 

depend on the scenic value of the area 

are of significant importance to public 

welfare as they are enjoyed by millions 

of Americans every year and generate 

millions of dollars in economic value 

(U.S. EPA, 2014b, Chapter 5, Chapter 7). 

In reaching conclusions regarding 

support for the adequacy of the current 

secondary standard provided by the 

currently available information on O3- 

induced visible foliar injury, the PA 

took note of: (1) The evidence for many 

species of native plants, including trees, 

that have been observed to have visible 

foliar injury symptoms in both OTC and 

field settings, some of which have also 

been identified as bioindicators of O3

exposure by the USFS; (2) the finding 

that visible foliar injury incidence can 

occur at very low cumulative exposures, 

but due to confounding by soil moisture 

and other factors, it is difficult to 

predictively relate a given O3 exposure 

to plant response; (3) information 

indicating the occurrence of visible 

foliar injury in some Class I areas under 

air quality conditions expected to meet 

the current standard; and, (4) WREA 

analyses, based on USFS biosite data, 

indicating a relationship of the 

proportion of biosites showing visible 

foliar injury incidence with W126 index 

values below approximately 10 ppm-hrs 

(U.S. EPA, 2014c, section 5.7). 

The PA additionally recognizes a lack 

of guidance for federal land managers 

regarding what spatial scale or degree of 

severity of visible foliar injury is 

considered sufficient to trigger 

protective action for O3 sensitive 

AQRVs. Further, there does not appear 

to be any consensus in the literature in 

this regard, and CASAC, while 

identifying benchmarks to consider for 

percent biomass loss and yield loss for 

tree seedlings and commodity crops, 

respectively, did not provide a similar 

recommendation for this endpoint. 

Likewise, as in previous reviews, the 

ISA notes the difficulty in relating 

visible foliar injury symptoms to other 

vegetation effects such as individual 

plant growth, stand growth, or 

ecosystem characteristics (U.S. EPA, 

2013a, section 9.4.2, p. 9–39) and 

further notes that the full body of 

evidence indicates that there is wide 

variability in this endpoint, such that 

although evidence shows visible foliar 

injury can occur under very low 

cumulative O3 concentrations, ‘‘. . .the 

degree and extent of visible foliar injury 

development varies from year to year 

and site to site . . ., even among comembers of a population exposed to 

similar O3 levels, due to the influence 


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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 

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 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 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 locations in Class I 

areas, and associated estimates of tree 

seedling growth effects; the analyses in 

the WREA illustrating the geographic 

distribution of tree species for which E– 

R functions are available and relative 

differences estimated for O3-related 

growth impacts across areas of the U.S. 

for the air quality scenarios, taking into 

account the identified potential for the 

WREA’s scenario for the current 

standard to underestimate the highest 

W126-based O3 values that would be 

expected to occur. 

As an initial matter, the Administrator 

recognizes the appropriateness and 

usefulness of the W126 metric, as 

described in sections IV.B.1 and IV.D.1 

above, in evaluating O3 exposures of 

potential concern for vegetation effects. 

In so doing, the Administrator 

additionally notes support conveyed by 

CASAC for such a use for this metric. 

With regard to considering the 

adequacy of public welfare protection 

provided by the current secondary 

standard, the Administrator focuses first 

on welfare effects related to reduced 


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217As 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. 

native plant growth and productivity in 

terrestrial ecosystems, taking note of the 

ISA conclusion of a causal relationship 

between O3 in the ambient air and these 

effects. In considering the assessment of 

the information available in this review 

with regard to O3 effects on vegetation 

growth and productivity, the 

Administrator takes note of the evidence 

from OTC studies of the effects of O3

exposure on tree seedling growth that 

support robust E–R functions for 11 tree 

seedling species, and the 

characterization of growth effects across 

these species for different cumulative 

seasonal concentration-weighted 

exposures using the W126 metric. 

Reductions in growth of sensitive 

species, as recognized in section IV.B 

above, have the potential to result in 

effects on ecosystem productivity, as 

well as, on forest and forest community 

composition. The Administrator takes 

particular note of the evidence, 

described in section IV.D.1 above, of the 

occurrence in Class I areas during 

periods where the current standard is 

met of cumulative seasonal O3

exposures for which median species 

RBL estimates are of a magnitude that 

CASAC has termed ‘‘unacceptably 

high.’’ In so doing, the Administrator 

also takes note of a number of actions 

taken by Congress to establish public 

lands that are set aside for specific uses 

intended to provide benefits to the 

public welfare, including lands that are 

to be protected so as to conserve the 

scenic value and the natural vegetation 

and wildlife within such areas for the 

enjoyment of future generations. Such 

public lands that are protected areas of 

national interest include national parks 

and forests, wildlife refuges, and 

wilderness areas (many of which have 

been designated Class I areas).217

While recognizing the variability in 

the various environmental factors that 

can influence the occurrence and 

severity of the effect of ambient O3

concentrations on vegetation in different 

locations, the Administrator concludes 

that the information referenced above 

including the currently available, 

extensive evidence base and also factors 

affecting the significance of impacts to 

public welfare, as well as WREA 

estimates regarding the potential for 

occurrence of impacts important to 

public welfare, provides an appropriate 

basis to inform a conclusion as to 

whether the current standards provide 

adequate protection against O3-related 

vegetation effects on public welfare. 

With regard to the results of the 

monitoring analysis, the Administrator 

takes note of the PA conclusions that 

the impacts on tree growth (and the 

potential for associated ecosystem 

effects) estimated for W126 values 

found to occur in Class I areas when 

meeting the current standard are 

reasonably concluded to be important 

from a public welfare standpoint in 

terms of 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 

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, concentrationweighted metric as the most appropriate 

approach for judging potential impacts 

of and protection from O3 in ambient 

air, the Administrator judges that it is 

appropriate to consider revisions to the 

secondary standard that reflect this 

understanding and to use such a metric 

in identifying an appropriate level of 

protection and considering the 

protection afforded by potential 

alternative standards. The 

Administrator also judges that the 

current averaging time 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 


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standard in the PA (section IV.E.2), 

CASAC advice and public input (section 

IV.E.3), analyses of air quality in the PA 

and subsequent to the PA (section 

IV.E.4) and the Administrator’s 

proposed conclusions regarding an 

alternative secondary standard (section 

IV.E.5) 

1. Indicator 

In the last review of the air quality for 

O3 and other photochemical oxidants 

and of the O3 standard, as in other prior 

reviews, the EPA focused on a standard 

for O3 as the most appropriate surrogate 

for ambient photochemical oxidants. 

Ozone is a long-established surrogate for 

ambient photochemical oxidants, among 

which it is by far the most widely 

studied with regard to effects on welfare 

and specifically on vegetation. The 

information available in this review 

adds to the understanding of the 

atmospheric chemistry for 

photochemical oxidants and O3 in 

particular (as described in the ISA, 

sections 3.2 and 3.6, and summarized in 

section 2.2 in the PA). The 1996 Staff 

Paper noted that the database on 

vegetation effects is generally 

considered to raise concern at levels 

found in the ambient air for O3 and, 

therefore, control of ambient O3 levels 

has previously been concluded to 

provide the best means of controlling 

other photochemical oxidants of 

potential welfare concern (U.S. EPA, 

1996b, p. 277). In the current review, 

while the complex atmospheric 

chemistry in which O3 plays a key role 

has been highlighted, no alternatives to 

O3 have been advanced as being a more 

appropriate surrogate for ambient 

photochemical oxidants. Ozone 

continues to be the only photochemical 

oxidant (other than nitrogen dioxide) 

that is routinely monitored and for 

which a comprehensive database exists 

(U.S. EPA, 2013a, section 3.6). 

Thus, the Administrator concludes 

that ozone is the appropriate indicator 

and proposes to continue to use O3 as 

indicator for a secondary standard that 

is intended to address effects associated 

with exposure to O3, alone and in 

combination with related 

photochemical oxidants. In so doing, 

the Administrator recognizes that 

measures leading to reductions in 

ecosystem exposures to O3 will also 

reduce exposures to other 

photochemical oxidants. 

2. Consideration of a Cumulative, 

Seasonal Exposure-based Standard in 

the Policy Assessment 

In recognition of the extensive 

evidence supporting a cumulative, 

seasonal exposure index as a 

biologically relevant metric for assessing 

potential for O3 effects on vegetation, 

discussed in sections IV.B.1 above, as 

well as advice from CASAC in the 

current and last O3 NAAQS reviews, 

summarized in sections IV.D.3 above 

and IV.E.3 below, the PA focused its 

consideration of alternative standards 

on a revised secondary standard based 

on a cumulative, seasonal, 

concentration-weighted form. The PA 

considered the currently available 

information that has been critically 

analyzed and characterized in the ISA, 

the risk and exposure information 

presented in the WREA, and CASAC 

advice and public comment with regard 

to support for consideration of options 

for alternative standards that might be 

expected to provide increased 

protection from ambient O3 exposures 

over the current standard. 

a. Form and Averaging Time 

In considering potential forms for a 

revised secondary standard, the PA 

considers the characterization of the 

evidence in the ISA, summarized in 

section IV.B.1 above, including the ISA 

conclusion that exposure indices that 

cumulate and differentially weight the 

higher hourly 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, concentrationweighted form to provide protection 

against cumulative, seasonal exposures 

to O3 that are known or anticipated to 

harm sensitive vegetation or ecosystems. 

The PA recognizes that such a metric is 

specifically designed to focus on the 

kind of O3 exposures that have been 

shown to cause harm to vegetation and 

states that it would have a distinct 

advantage over the form of the current 

standard in characterizing air quality 

conditions potentially of concern for 

vegetation and in more directly 

demonstrating that the desired degree of 

protection against those conditions was 

being achieved (U.S. EPA, 2014c, 

sections 6.2 and 6.6). 

With regard to the appropriate index 

for a cumulative seasonal form, the PA 

considers the evidence and background 

for a number of different cumulative 

concentration weighted indices that 

have been developed and evaluated in 

the scientific literature and in past 

NAAQS reviews in terms of their ability 

to predict vegetation response and their 

usefulness in the NAAQS context (U.S. 

EPA, 2006a, pp. 9–11 to 9–15 and pp. 

AX9–159 to AX9–187; U.S. EPA, 2007, 

pp. 7–15 to 7–16). While these various 

forms have different strengths and 

limitations, the PA notes the ISA 

conclusion that the W126 index, 

described in section IV.B.1 above, has 

some important advantages over other 

non-sigmoidally weighted cumulative 

indices, including its lack of a 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 

continues to provide support for a 

definition of the diurnal period of 

interest as the 12-hour period from 8:00 

a.m. to 8:00 p.m., 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 a.m. to 8:00 p.m.) 

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 


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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 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 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 longterm 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 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 threeyear forms, the PA considers a WREA 

analysis that examined the extent to 

which cumulative RBL across a threeyear 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 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 


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three-year evaluation period can 

contribute to greater public welfare 

protection by limiting year-to-year 

disruptions in ongoing control programs 

that would occur if an area was 

frequently shifting in and out of 

attainment due to extreme year-to-year 

variations in meteorological conditions. 

This greater stability in air quality 

management programs would thus 

facilitate achievement of the protection 

intended by a standard. Such 

considerations of stability often receive 

particular weight in NAAQS reviews, 

such as those resulting in selection of 

the form for the current O3 primary and 

secondary standards (62 FR 38856, July 

18, 1997), as well as the primary 

standards for nitrogen dioxide (75 FR 

6474, February 9, 2010) and sulfur 

dioxide (75 FR 35520, June 22, 2010). 

See also ATA III, 283 F. 3d at 374–75 

(recognizing programmatic stability as a 

legitimate consideration in the NAAQS 

standard-setting process). 

Thus, to the extent that emphasis 

continues to be placed on protecting 

against effects associated with multiyear 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 longerterm 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 conclusions, 

staff took note of the complexity of 

judgments to be made by the 

Administrator regarding the adversity of 

known and anticipated effects to the 

public welfare and recognized that the 

Administrator’s ultimate judgments on 

the secondary standard will most 

appropriately reflect an interpretation of 

the available scientific evidence and 

exposure/risk information that neither 

overstates nor understates the strengths 

and limitations of that evidence and 

information. 

As described in section IV.D.1 above, 

the PA employed a paradigm, which has 

evolved over the course of the O3 and 

other secondary NAAQS reviews, to 

assist in putting the available science 

and exposure/risk information into the 

public welfare context (U.S. EPA, 2014c, 

section 5.1). This paradigm recognizes 

that the significance to the public 

welfare of O3-induced effects on 

sensitive vegetation growing within the 

U.S. can vary depending on the nature 

of the effect, the intended use of the 

sensitive plants or ecosystems, and the 

types of environments in which the 

sensitive vegetation and ecosystems are 

located. Accordingly, any given O3- 

related effect on vegetation and 

ecosystems (e.g., biomass loss, crop 

yield loss, visible foliar injury) may be 

judged to have a different degree of 

impact on or significance to the public 

welfare depending, for example, on 

whether that effect occurs in a Class I 

area, a city park, or commercial 

cropland. This approach also includes 

consideration of impacts to ecosystem 

goods and services, which are an 

important category of public welfare 

effects with an obvious relationship to 

consideration of intended use (73 FR 

16492, March 27, 2008). 

In considering potential levels for an 

alternative standard based on the W126 

metric, the PA focused primarily on 

impacts on tree growth, crop yield loss, 

and visible foliar injury, as well as 

impacts on the associated ecosystem 

services, while taking note of the 

uncertainties and limitations associated 

with several key aspects of this 

information. In addition to uncertainties 

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 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, 


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218 In the context of the O3 standard, such 

judgments include: The weight to give the evidence 

of specific vegetation-related effects estimated to 

result across a range of cumulative seasonal 

concentration-weighted O3 exposures; the weight to 

give associated uncertainties, including those 

related to the variability in occurrence of such 

effects in specific areas of U.S., such as those of 

particular public welfare significance; and, 

judgments on the extent to which such effects in 

such areas may be considered adverse to public 

welfare. 

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 RBL a Crop RYL c

Median value Individual species Median value Individual species 

21 ppm-hrs ........ Median species w. 6.8% 

loss b.

≤ 2% loss: 3/11 species ........

≤5% loss: 5/11 species .........

≤10% loss: 7/11 species .......

≤15% loss: 10/11 species .....

>40% loss: 1/11 species .......

Median species w. 7.7% 

loss d.

≤5% loss: 4/10 species. 

>5, <10% loss: 3/10 species. 

>10, <20% loss: 3/10 species. 

19 ppm-hrs ........ Median species w. 6.0% 

loss b.

≤2% loss: 3/11 species .........

<5% loss: 5/11 species .........

≤10% loss: 7/11 species .......

≤15% loss: 10/11 species .....

>30% loss: 1/11 species .......

Median species w. 6.4% 

loss d.

≤5% loss: 5/10 species. 

>5, <10% loss: 3/10 species. 

>10, <20% loss: 2/10 species. 

17 ppm-hrs ........ Median species w. 5.3% 

loss b.

≤2% loss: 5/11 species .........

<5% loss: 5/11 species .........

≤10% loss: 9/11 species .......

15% loss: 10/11 species .......

>30% loss: 1/11 species .......

Median species w. 5.1% 

loss d.

≤5% loss: 5/10 species. 

>5, <10% loss: 3/10 species. 

>10, <20% loss: 2/10 species. 

15 ppm-hrs ........ Median species w. 4.5% 

loss b.

≤2% loss: 5/11 species .........

≤5% loss: 6/11 species .........

≤10% loss: 10/11 species .....

>30% loss: 1/11 species .......

Median species w. ≤5% 

loss d.

≤5% loss: 6/10 species. 

>5, <10% loss: 4/10 species. 

13 ppm-hrs ........ Median species w. 3.8% 

loss b.

≤2% loss: 5/11 species .........

<5% loss: 7/11 species .........

<10% loss: 10/11 species .....

>20% loss: 1/11 species .......

Median species w. ≤5% 

loss d.

≤5% loss: 6/10 species. 

>5, <10% loss: 4/10 species. 

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. 

aEstimates 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). bThis 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). cEstimates here are based on the 10 E–R functions for crops (from the PA, Appendix 6F and section 5.3.1). 

dThis median value is the median of the composite E–R functions for 10 crops from WREA, Appendix 6F (also discussed in the PA, section 

5.3.1). 

Given the wide variation in sensitivity 

of studied tree species to O3-induced 

relative biomass loss, the PA focused 

consideration on both median species 

values and individual species responses 

and RBL estimates for a given range of 

W126 index values. In this 

consideration, the PA took note of 

CASAC’s advice regarding RBL levels, 

specifically their emphasis on a 

benchmark of median relative tree 

biomass loss at or below 2% and their 

view that a 6% median relative biomass 

loss is ‘‘unacceptably high.’’ The 

median tree species RBL estimate is at 

or below 2% only at the lowest W126 

level assessed, 7 ppm-hrs. At 

incrementally higher W126 index levels, 

the median RBL is also incrementally 

higher, so that at W126 index values of 

9, 11, 13, 15, 17 and 19 ppm-hrs, the 

median RBL increases to 2.4%, 3.1%, 

3.8%, 4.5%, 5.3% and 6.0%, 

respectively. Thus, the median species 

biomass loss is below 6%, the level 

characterized by the CASAC as 

unacceptably high, across the W126 

range of 7 to 17 ppm-hrs, for which it 

varies from approximately 2% to 

approximately 5%. Given this finding, 

the PA discussion of a range of levels 

appropriate to consider focuses on this 

range. In focusing on this range, the PA 

considers the full array of CASAC 

advice with regard to interpretation of 

the evidence and exposure/risk 

information on vegetation-related effects 

of O3, as well as the role of the 

Administrator’s judgments in 

identifying the level of air quality that 

is requisite to protect public welfare 

from adverse effects, as noted in section 

IV.A above.218

The PA recognizes that public welfare 

judgments may reasonably be informed 

by a range of biomass loss benchmarks, 

in contexts of considering both median 

RBL estimates and RBL estimates for 

individual species. Accordingly, in 

considering individual tree species 

estimates, the PA notes the value of 

additionally characterizing the RBL 


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estimates in comparison to higher loss 

levels such as 10% or 15%. 

For every W126 value over the full 

range from 7 to 17 ppm-hrs, the RBLs 

for each of five species is less than 2% 

(Table 8), which is the lower benchmark 

that CASAC identified for tree species. 

Accordingly, the PA focused attention 

on the remaining six more sensitive 

studied species (i.e., eastern white pine, 

aspen, tulip poplar, ponderosa pine, red 

alder, and black cherry) to evaluate the 

protection against tree seedling biomass 

loss at different W126 levels within the 

range from 17 to 7 ppm-hrs. At a W126 

index value of 17 ppm-hrs, one of these 

six species (red alder) has a RBL 

estimate below 6%, while at the W126 

index value of 7 ppm-hrs, five of these 

six species have RBLs below 6% 

(eastern white pine, aspen, tulip poplar, 

ponderosa pine, red alder). Taken 

together with the more tolerant species, 

the proportion of the studied tree 

species with RBLs below 6% are 6 of 11, 

7 of 11, 8 of 11, and 10 of 11 at W126 

index values of 17, 15, 13, and 11 ppmhrs, 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 

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 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 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 ppmhrs. 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 


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219We note that a W126 index value of 19 ppmhrs is estimated to result in a median RBL value of 

6%, as shown in Table 2 above. 

additional protection for vegetation with 

attention to tree growth, would be 

expected to also provide additional 

protection to crops over that provided 

by the current standard (U.S. EPA, 

2014c, section 6.6). 

The PA also considers the evidence 

and exposure/risk information with 

regard to visible foliar injury and the 

extent to which that might inform 

consideration of potential alternative 

secondary standards appropriate for the 

Administrator to consider. Specifically, 

the PA notes the findings of the WREA 

analyses of the nationwide USFS/FHM 

biosite dataset (2006–2010) that while 

soil moisture conditions influence the 

proportion of biosites with O3-related 

visible foliar injury, as described in 

section IV.B.1.a above, the proportion of 

such sites increases appreciably with 

increasing W126 index values up to 

approximately 10 ppm-hrs, while 

relatively little or no change in 

incidence of injury is seen with O3

exposures at higher W126 index values 

(U.S. EPA 2014b, Chapter 7, Figure 7– 

10). The PA additionally notes that 

visible foliar injury has been identified 

by the federal land managers as a 

diagnostic tool for informing 

conclusions regarding potential O3

impacts on potentially sensitive AQRVs 

(USFS, NPS, FWS, 2010), which the PA 

concludes indicates that such O3- 

induced impacts might be considered to 

have the potential to impact the public 

welfare in scenic and/or recreational 

areas during years they occur. 

The PA was unable, however, to 

identify any guidance for federal land 

managers regarding at what spatial scale 

or what degree of severity visible foliar 

injury might be sufficient to trigger 

protective action based on this potential 

impact on AQRVs. The PA states that 

there does not appear to be consensus 

in the literature regarding severity of 

visible foliar injury and risks to plant 

functions or services, additionally 

noting that CASAC, while identifying 

percent biomass loss and yield loss 

benchmarks for tree seedlings and 

commodity crops, respectively, did not 

provide any benchmark or criteria for 

consideration of O3 impacts related to 

this endpoint. Further, as in previous 

reviews, the ISA concludes visible foliar 

injury is not always a reliable indicator 

of other negative effects on vegetation, 

making it difficult to relate visible foliar 

injury symptoms to other vegetation 

effects such as individual plant growth, 

stand growth, or ecosystem 

characteristics (U.S. EPA, 2013a, section 

9.4.2, p. 9–39). Additionally, although 

evidence shows visible foliar injury can 

occur under very low cumulative O3

exposures, ‘‘. . . the degree and extent 

of visible foliar injury development 

varies from year to year and site to site 

. . ., even among co-members of a 

population exposed to similar O3 levels, 

due to the influence of 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) 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 ecosystemwide 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 ppmhrs that the median tree biomass loss is 

below 6%.219

In considering the stability and 

potential for associated greater public 

welfare protection offered by a threeyear 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. 

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). 


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220Public comment received thus far in this 

review are in the docket EPA–HQ–OAR–2008– 

0699, accessible at www.regulations.gov 

221The 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’’. 

222The 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). 

The PA also concludes that, to the 

extent the Administrator finds it useful 

to consider the public welfare 

protection that might be afforded by a 

revised primary standard, this is 

appropriately judged through the use of 

a cumulative seasonal W126-based 

exposure metric, a metric considered 

appropriate for evaluating impacts on 

vegetation. For example, comparison of 

the air quality conditions (expressed in 

terms of W126 exposures) expected to 

result from a revised primary standard 

to the W126-based exposures concluded 

to provide requisite public welfare 

protection would thus inform a 

judgment of whether a secondary 

standard set identical to a revised 

primary standard would be expected to 

achieve the appropriate level of air 

quality. The PA notes that such a 

comparison would be in terms of a 

metric considered appropriate for 

evaluating impacts on vegetation which 

inform conclusions on public welfare 

impacts. The PA further concludes that 

the drawing of conclusions with regard 

to the public welfare protection afforded 

by such a standard should entail 

consideration of the air quality 

conditions likely to be achieved in 

terms of the cumulative seasonal W126- 

based metric described above. 

Accordingly, the PA describes several 

analyses of air quality data that might 

inform such consideration (U.S. EPA, 

2014c, section 6.4), and notes the 

importance of taking into account 

associated uncertainties, including 

those associated with the limited 

monitor coverage in many rural areas, 

such as those in the West and 

Southwest regions and at high elevation 

sites. Additional such analyses, based 

on more recent O3 monitoring data, have 

been developed since the completion of 

the PA. All of these analyses are 

summarized in section IV.E.4 below. In 

reaching conclusions on appropriate 

policy options for a revised secondary 

standard the Administrator has 

considered the findings of these 

analyses, as described in section IV.E.5 

below. 

3. CASAC Advice 

Beyond the evidence- and exposure/

risk-based considerations in the PA 

discussed above, the EPA’s 

consideration of a revised secondary 

standard also takes into account the 

advice and recommendations of 

CASAC. The EPA also considered 

public comments received to date, some 

of which urged the consideration of a 

secondary standard with a cumulative 

seasonal form using the W126 metric 

and a level within the range of 7 to 15 

ppm-hrs or in the low end of this 

range,220 while others have urged 

retaining the existing form and 

averaging time due to their view of a 

lack of new information to support a 

distinct secondary standard. 

In advice offered on a revised 

secondary standard in the current 

review, similar to advice in the last 

review, including advice offered on the 

2010 proposed reconsideration, the 

CASAC recommended ‘‘retaining the 

current indicator (ozone) but 

establishing a revised form of the 

secondary standard to be the 

biologically relevant W126 index 

accumulated over a 12-hour period (8 

a.m.–8 p.m.) over the 3-month 

summation period of a single year 

resulting in the maximum value of 

W126’’ (Frey, 2014c, p. iii). With regard 

to the level, the CASAC recommended 

that ‘‘that the level associated with this 

form be within the range of 7 ppm-hrs 

to 15 ppm-hrs to protect against current 

and anticipated welfare effects of 

ozone’’ and that ‘‘CASAC does not 

support a level higher than 15 ppm-hrs’’ 

(Frey, 2014c, p. iii). The CASAC 

additionally stated that ‘‘[i]n reaching 

its scientific judgment regarding the 

indicator, form, summation time, and 

range of levels for a revised secondary 

standard, the CASAC has focused on the 

scientific evidence for the identification 

of the kind and extent of adverse effects 

on public welfare,’’ while also 

acknowledging ‘‘that the choice of a 

level within the range recommended 

based on scientific evidence is a policy 

judgment under the statutory mandate 

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

iii). 

In providing advice on a range for the 

secondary standard, the CASAC noted a 

W126 index value for which the median 

tree species RBL estimate was 6 percent, 

and the median crop species RBL 

estimate was over 5 percent, stating that 

‘‘[t]hese levels are unacceptably high’’ 

(Frey, 2014c, p. iii).221 In addition, 

regarding consideration of relative 

biomass loss benchmarks for tree 

seedlings, the CASAC stated that ‘‘[a] 

2% biomass loss is an appropriate 

scientifically based value to consider as 

a benchmark of adverse impact for longlived perennial species such as trees, 

because effects are cumulative over 

multiple years’’ (Frey, 2014c, p. 14). In 

so stating, the CASAC referenced 

findings for biomass loss in aspen 

exposed to elevated O3 over seven years, 

citing Wittig et al., 2009.222 The CASAC 

additionally pointed to the report of the 

1996 workshop sponsored by the 

Southern Oxidants Study group (Heck 

and Cowling, 1997, noted in section 

IV.B.2 above) which described a general 

consideration of 1–2% per year growth 

reduction in making judgments the 

group identified as appropriate for the 

endpoint of growth effects in trees, 

without providing an explicit rationale 

for the identified percentages (Frey, 

2014c, p. 14). The CASAC also 

commented that ‘‘it is appropriate to 

identify a range of levels of alternative 

W126-based standards that includes 

levels that aim for not greater than 2% 

RBL for the median tree species’’ (Frey, 

2014c, p. 14). The CASAC noted that the 

‘‘level of 7 ppm-hrs is the only level 

analyzed for which the relative biomass 

loss for the median tree species is less 

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 


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223This 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. 

protection against crop yield loss and foliar 

injury. Therefore, 7 ppm-hrs is protective of 

ecosystem services. Thus, lower levels within 

the recommended range offer a greater degree 

of protection of more endpoints than do 

higher levels within the range. 

Additionally, in regard to 

consideration of form, the CASAC noted 

that ‘‘[i]f, as a policy matter, the 

Administrator prefers to base the 

secondary standard on a three-year 

averaging period for the purpose of 

program stability, then the level of the 

standard should be revised downward 

such that the level for the highest threemonth 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 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.223 Additionally, a 

similar air quality analysis performed 

with more recent monitoring data is 

now available and is also described 

here. 

The focus study described in the ISA 

examined the diel variability in O3

concentrations in six rural areas 

between 2007 and 2009 (U.S. EPA, 

2013a, pp. 3–131 to 3–133). The ISA 

reported that ‘‘[t]here was considerable 

variability in the diel patterns observed 

in the six rural focus areas’’ with the 

three mountainous eastern sites 

exhibiting a ‘‘generally flat profile with 

little hourly variability in the median 

concentration and the upper 

percentiles,’’ while the three western 

rural areas demonstrated a ‘‘clear diel 

pattern to the hourly O3 data with a 

peak in concentration in the afternoon 

similar to those seen in the urban 

areas,’’ which was especially obvious at 

the San Bernardino National Forest site, 

90 km east of Los Angeles at an 

elevation of 1,384 meters (U.S. EPA, 

2013a, p. 3–132). Thus, while the 

western sites that are influenced by 

upwind urban plumes may have 

increased cumulative seasonal values 

coincident with increased daily 8-hour 

peak O3 concentrations, this analysis 

indicates that, in sites without such an 

urban influence (the eastern sites in this 

analysis), such a relationship does not 

occur (U.S. EPA, 2013a, section 3.6.3.2). 

Thus, the lack of such a relationship 

indicates that in some locations, O3 air 

quality patterns can lead to elevated 

cumulative, seasonal O3 exposures 

without the occurrence of elevated daily 

maximum 8-hour average O3

concentrations (U.S. EPA, 2013a, 

section 3.6.3.2). Further, staff notes that 

the prevalence and geographic extent of 

such locations is unclear, since as in the 

last review, there continue to be 

relatively fewer monitors in the western 

U.S., including in high elevation remote 

sites. In considering the findings of this 

analysis, the PA additionally 

recognized, however, that the 

cumulative seasonal values for the 

eastern rural sites, where cumulative 

seasonal O3 concentrations appear to be 

relatively less related to daily maximum 

8-hour concentrations, are lower in 

general than those of the western, 

urban-influenced sites. 

In addition to the focus study 

described in the ISA (U.S. EPA, 2013a, 

section 3.6.3.2), the PA considers 

additional analyses of air quality 

monitoring data. For example, Chapter 2 

of the PA characterized recent 

monitoring data of O3 air quality in rural 

areas. While approximately 80 percent 

of the O3 monitoring network is urban 

focused, about 120 rural monitors are 

divided among CASTNET, NCore, and 

portable O3 monitors (POMs) sites (U.S. 

EPA, 2014c, Chapter 2, pp. 2–2 to 2–3, 

Figure 2.1). Specifically, as stated in 

Chapter 2 of the PA, ‘‘[a]lthough rural 

monitoring sites tend to be less directly 

affected by anthropogenic pollution 

sources than urban sites, rural sites can 

be affected by transport of O3 or O3

precursors from upwind urban areas 

and by local anthropogenic sources such 

as motor vehicles, power generation, 

biomass combustion, or oil and gas 

operations’’ (U.S. EPA, 2013a, section 

3.6.2.2). In addition, O3 tends to persist 

longer in rural than in urban areas due 

to lower rates of chemical scavenging in 

non-urban environments. At higher 

elevations, increased O3 concentrations 

can also result from stratospheric 

intrusions (U.S. EPA, 2013a, sections 

3.4, 3.6.2.2). As a result, O3

concentrations measured in some rural 

sites can be higher than those measured 

in nearby urban areas (U.S. EPA, 2013a, 

section 3.6.2.2) and the ISA concludes 

that ‘‘cumulative exposures for humans 

and vegetation in rural areas can be 

substantial, and often higher than 

cumulative exposures in urban areas’’ 

(U.S. EPA, 2013a, p. 3–120). These 

known differences between urban and 

rural sites suggest that there is the 

potential for an inconsistent 

relationship between 8-hour daily peak 

O3 concentrations and cumulative, 

seasonal exposures in those areas. 

However, the PA also notes that 

reductions in NOx emissions that occur 

in urban areas to attain primary 

standards would also 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


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224Appendix 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. 

225This 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). 

precursor emission reductions occurred, 

O3 concentrations in terms of both 

metrics were reduced. There is a fairly 

strong, positive degree of correlation 

between the two metrics (U.S. EPA, 

2014c, Appendix 2B).224 Focusing only 

on the latter dataset (2009–2011), it can 

be seen that at monitors just meeting the 

current standard (three-year average 

fourth-highest daily maximum 8-hour 

average concentration equal to 0.075 

ppm), W126 index values (in this case 

three-year averages) varied from less 

than 3 ppm-hrs to approximately 20 

ppm-hrs (U.S. EPA, 2014c, Appendix 

2B, Figure 2B–3b). At sites with a threeyear average fourth-highest daily 

maximum 8-hour average concentration 

at or below a potential alternative 

primary standard level of 70 ppb, threeyear W126 index values were above 17 

ppm-hrs at no monitors, above 15 ppmhrs at one monitor, and above 13 ppmhrs at 8 monitors in the West and 

Southwest NOAA climate regions. At 

sites with a three-year average fourthhighest daily maximum 8-hour average 

concentration at or below a potential 

alternative primary standard level of 65 

ppb, three-year W126 index values were 

above 11 ppm-hrs at no monitors, above 

9 ppm-hrs at three monitors, and above 

7 ppm-hrs at 9 monitors (distributed 

across five regions). The majority of 

these nine monitoring sites are located 

in the West and Southwest regions and 

include the states of Arizona, California, 

Colorado, Nevada, New Mexico, and 

Utah. At sites with a three-year average 

fourth-highest daily maximum 8-hour 

average concentration at or below a 

potential alternative primary standard 

level of 60 ppb, three-year W126 index 

values were at or below 7 ppm-hrs at all 

monitors (U.S. EPA, 2014c, Figure 2B– 

3b). 

Another analysis in Chapter 2 of the 

PA presents the data for sets of recent 

three-year periods back to 2006–2008 

and indicates that among the counties 

with O3 concentrations that met the 

current standard, the number of 

counties with three-year W126 index 

values above 15 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 fourthhighest daily maximum 8-hour average 

concentration equal to 0.075 ppm), 

W126 index values (in this case threeyear averages) varied from less than 3 

ppm-hrs to up to 23 ppm-hrs (Figure 

5a). At sites with a three-year average 

fourth-highest daily maximum 8-hour 

average concentration at or below a 

level of 70 ppb (566 monitors 

distributed across all regions of the 

U.S.), three-year W126 index values 

were above 17 ppm-hrs at no monitors, 

above 15 ppm-hrs at 4 monitors, and 

above 13 ppm-hrs at 16 monitors (1% of 

the monitors in full dataset and less 

than 3% in this group). These 16 

monitors are located in the Southwest 

(15 monitors) and West North Central 

NOAA climate regions and include the 

states of Arizona, Colorado, New 

Mexico, Utah and Wyoming. At sites 

with a three-year average fourth-highest 

daily maximum 8-hour average 

concentration at or below a level of 65 

ppb (220 monitors distributed across all 

regions of the U.S.225), three-year W126 

index values were above 11 ppm-hrs at 

no monitors, above 7 ppm-hrs at 15 

monitors. These 15 monitoring sites are 

predominantly located in the West 

North Central and Southwest regions. At 

all sites with a three-year average 

fourth-highest daily maximum 8-hour 

average concentration at or below a 

level of 60 ppb, three-year W126 index 

values were at or below 7 ppm-hrs 

(Wells, 2014, Figure 5b). 

Further analysis in the technical 

memorandum focused on a comparison 

of monitors with a three-year average 

fourth-highest daily maximum 8-hour 

average concentration at or below a 

level of 70 ppb and a three-year W126 

index values above 13 ppm-hrs for sets 

of three-year periods between 2001– 

2003 and 2011–2013 (Wells, 2014, 

Figure 8). This analysis found that the 

number of sites meeting 70 ppb while 

exceeding 13 ppm-hrs has remained 

relatively constant over the past decade, 

with these sites consistently being 

limited to a small number in the West 

and Southwest. In addition, the number 

of sites meeting both 70 ppb and 13 

ppm-hrs has increased over time, while 

the number of sites exceeding both 70 

ppb and 13 ppm-hrs has decreased by 

a similar amount. 

The second part of the analysis in the 

technical memorandum focused on 

trends in the relationships between O3

levels in terms of the 4th high metric 

and a three-year average W126 metric, 

starting with the 2001–2003 period and 

ending with the 2011–2013 period. 

Based on analysis of 729 monitors, 

trends in both the 4th high metric and 

the three-year average, W126 metric 

showed decreasing values between 

2001–2003 and 2011–2013. In addition, 

the amount of year-to-year variability in 

the two metrics tended to decrease over 

time with decreasing O3 concentrations, 

especially for the W126 metric. Most 

sites in the eastern U.S. and California 

saw large, widespread decreases in both 

the 4th high metric and the three-year 

average W126 metrics over the past 

decade as a result of regional NOX

control programs. In the inter-mountain 

west, where control programs have been 

more localized, the decreases observed 

in the 4th high metric and three-year 

average W126 metrics were typically 

much smaller in magnitude, with a 

small number of sites showing 

significant increases. 

As part of this analysis, regional 

comparisons were included on the 

relative changes in the relationships 

between O3 levels in terms of the 4th 

high metric and a three-year average 

W126 metric between the periods of 

2001–2003 and 2011–2013. Figure 12 in 

the technical memorandum shows that 

a positive, linear relationship persists 

within each region between the changes 

in 4th high and three-year average W126 

metrics. Nationally, the three-year 

average W126 metric decreased by 

approximately 0.7 ppm-hrs per unit ppb 

decrease in the 4th high metric. In 

addition, the Southwest and West 

regions, which have the greatest 

potential for sites to measure elevated 

cumulative, seasonal O3 exposures 

without the occurrence of elevated daily 

maximum 8-hour average O3

concentrations, exhibited the greatest 


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226EPA 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 to 17 ppmhours, those inconsistencies are less relevant to 

consideration of the appropriate form and level for 

the secondary standard. 

response in W126 value change per unit 

change in 4th high metric (Wells, 2014, 

Table 6). 

The technical memorandum 

concludes that the 4th high metric and 

a three-year average W126 metric are 

highly correlated, as are the relative 

changes in these two metrics over the 

past decade. In this way, the technical 

memorandum concludes that that future 

control programs designed to help meet 

a revised primary O3 standard based on 

the three-year average of the 4th highest 

daily maximum 8-hour concentration 

are expected to also result in decreases 

in the values of a three-year average 

W126 metric. 

The above information suggests that 

depending on the level for a standard of 

the current averaging time and form, the 

current form and averaging time of the 

secondary standard can be expected to 

achieve control of cumulative seasonal 

O3 exposures, providing air quality that 

may meet specific three-year average 

W126 index values. As discussed above, 

we recognize limitations in the dataset 

and associated analyses, including those 

related to monitor coverage, which may 

contribute uncertainties to conclusions 

related to the relationships described. 

With respect to monitor coverage, the 

current O3 monitoring network is urban 

focused, with fewer monitors in some 

parts of the country, particular rural 

areas of the southwestern and western 

U.S. Because of this, there are potential 

uncertainties in the extent to which the 

monitoring information discussed above 

represents air quality patterns and 

relationships that would occur in areas 

without monitors. There is some 

information suggesting that there is a 

potential for inconsistencies in the 

relationship between W126 measures of 

seasonal O3 concentrations and the 

fourth highest peak O3 concentrations 

assessed by the current standard 

averaging time and form, but the 

available data suggest that air quality in 

areas meeting a standard of the current 

form and averaging time with a level in 

the range of 65 to 70 ppb would also 

meet a three-year W126 index value 

falling in the range of 13 to 17 ppm-hrs, 

and that to the extent areas need to take 

action to attain a primary standard in 

the range of 65 to 70 ppb, those actions 

would also improve air quality as 

measured by the W126 metric.226 To the 

extent to which the monitoring data can 

be expected to describe future 

relationships in air quality, we 

acknowledge potential uncertainties in 

specifying future air quality but note 

that these uncertainties are limited by 

the fact that the data analysis includes 

over a decade of O3 measurements, with 

similar patterns and trends observed in 

air quality over this period of time. 

5. Administrator’s Proposed 

Conclusions 

In considering what revisions to the 

secondary standard are appropriate, the 

Administrator has drawn on the ISA 

conclusions regarding the weight of the 

evidence for a range of welfare effects 

associated with O3 in ambient air, and 

associated areas of uncertainty; 

quantitative risk and exposure analyses 

in the WREA for different adjusted air 

quality scenarios and associated 

limitations and uncertainties; staff 

evaluations of the evidence, exposure/

risk information and air quality 

information in the PA; additional air 

quality analyses of relationships 

between air quality metrics based on 

form and averaging time of the current 

standards and a cumulative seasonal 

exposure index; and CASAC advice; 

and, public comments received thus far 

in the review. 

As described in section IV.E.1 above, 

the Administrator concludes it is 

appropriate to continue to use O3 as the 

indicator for a secondary standard 

intended to address adverse effects to 

public welfare associated with exposure 

to O3 alone and in combination with 

related 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 vegetationrelated effects. The information also 

includes PA conclusions and CASAC 

advice with regard to key aspects of the 

definition of such a metric, as 

summarized in section IV.E.2 and IV.E.3 

above. Additionally, the Administrator 

has considered findings of staff 

evaluations in the PA with regard to 

potential impacts on vegetation and 

forested ecosystems associated with a 

range of values for such a metric and 

identified uncertainties and limitations 

of such information, as summarized in 

section IV.E.2 above. Additionally 

important to her deliberations here are 

findings of air quality analyses of 

relationships between the W126-based 

exposure metric and levels of a standard 

of the same form and averaging time as 

the current standards, as described in 

section IV.E.4 above. Based on 

consideration of this array of 

information, as described below, the 

Administrator has drawn conclusions 

with regard to policy options for a 

revised secondary standard. In drawing 

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 vegetationrelated 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 by a 

revised standard, it is appropriate to use 


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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, concentrationweighted 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 a.m. to 8:00 p.m. 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 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 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 threeyear 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 threemonth 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 a.m. to 8:00 p.m. 

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 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 known and anticipated O3- 

related vegetation and ecosystem effects 


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227For 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). 

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 areas 227 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 

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 ppmhrs) 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, 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 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 ppmhrs (Frey, 2014c, p. iii; U.S. EPA 2014j, 

Table 6–1). As noted in section IV.E.3 

above, revisions to this table in the final 

PA, made in consideration of CASAC 

comments have resulted in changes to 

the median species RBL estimates such 

that the median species RBL estimate 

for a W126 index value of 17 ppm-hrs 

in this table in the final PA (5.3%) is 

nearly identical to the median species 

estimate for 15 ppm-hrs (the value 

corresponding to the upper end of the 

CASAC-identified range) in the second 

draft PA (5.2%) (U.S. EPA, 2014c, Table 

6–1; U.S. EPA, 2014j, Table 6–1). 


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The Administrator additionally takes 

note of the PA observations that the 

number and proportion of individual 

species with RBL estimates at or below 

2%, a benchmark given emphasis by 

CASAC, do not vary across W126 index 

values from 17 ppm-hrs down to 9 ppmhrs (as seen in Table 8 above), providing 

little distinction with regard to the 

significance of growth impacts for 

exposures across this large portion of 

the PA range. The Administrator also 

notes the CASAC recommendation 

regarding a lowering of the level with 

consideration of a three-year average 

index; however, the Administrator’s 

judgments on a three-year average 

index, as described above, focus on 

confidence in conclusions that might be 

drawn with regard to single as 

compared to multiple year impacts. For 

example, the Administrator, while 

recognizing the strength of the evidence 

with regard to quantitative 

characterization of O3 effects on growth 

of tree seedlings and crops, in addition 

to noting the additional difficulties for 

assessing welfare impacts of crops, takes 

note of the uncertainty associated with 

drawing conclusions with regard to the 

extent to which small percent 

reductions in annual growth contribute 

to adverse effects on public welfare and 

the role of annual variability in 

environmental factors that affect plant 

responses to O3. Moreover, as explained 

above, the Administrator concludes that 

concerns related to the possibility of a 

singly unusually damaging year can be 

addressed through use of a three-year 

average metric, chosen with 

consideration of the relevant factors. 

Accordingly, she judges it appropriate 

to include 17 ppm-hrs, without 

adjustment, in the range of three-year 

average W126 index values appropriate 

to consider in determining what 

secondary standard will provide air 

quality associated with the appropriate 

level of public welfare protection. She 

thus judges it appropriate to focus on a 

range for three-year average W126 levels 

with 17 ppm-hrs at the upper end. In so 

doing, she additionally notes CASAC’s 

recognition that, within a scientifically 

appropriate range, the choice of levels is 

a public policy judgment by the 

Administrator. 

In turning to consideration of the low 

end for the W126 index range, the 

Administrator considers the full range 

of W126 levels identified in the PA with 

regard to the evidence and exposure/

risk-based information, and associated 

uncertainties, identified in the PA, as 

well as CASAC advice. The 

Administrator notes the CASAC policy 

view regarding protection provided for 

trees and associated ecosystem services 

from a W126 index value of 7 ppm-hrs, 

which is based on the W126 index value 

for which the median species estimate 

falls below 2% RBL. The Administrator 

recognizes, however, as noted above, the 

greater uncertainty associated with the 

extent to which estimates of benefits in 

terms of ecosystem services and reduced 

effects on vegetation at lower O3

exposures might be judged significant to 

the public welfare. 

The Administrator additionally notes 

the results of the EPA’s quantitative 

exposure and risk assessments for the 

air quality scenarios for W126 levels at 

and below 11 ppm-hrs, including the 

relatively small additional benefits and 

increased uncertainty with the 

ecosystem services estimates in these 

lower W126 scenarios. With regard to 

the PA evaluation of RBL estimates, the 

Administrator, while noting the PA 

observations of similarity in the number 

of species with less than 2% RBL across 

the W126 range from 17 to 9 ppm-hrs, 

as stated above, additionally notes PA 

observations of a similar number of 

studied species with RBL estimates 

below 5% for W126 index values of 13 

and 11 ppm-hrs. Thus, to the extent that 

weight is given to the importance of 5% 

RBL for individual species, both W126 

index values are observed to provide 

RBL estimates below this benchmark. 

With regard to considerations of O3

effects beyond biomass loss in tree 

seedlings, the Administrator takes note 

of the lack of new quantitative E–R 

relationships for larger trees 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 ppmhrs, while the likelihood of requiring 

reductions in ambient concentrations 

that go beyond those that are needed to 

reduce adverse impacts to public 

welfare increases. 

Based on the above considerations 

and based on the entire body of 

evidence and information currently 

available, the Administrator identifies 

the range of three-year average W126 

index values extending from 13 to 17 

ppm-hrs as appropriate to consider in 

identifying the 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-toyear 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 


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confidence that such exposures of 

concern would be controlled. 

In considering different policy 

options for a revised secondary 

standard, the Administrator finds it 

useful to consider what can be 

concluded from the available 

information with regard to relationships 

between air quality characterized in 

terms of the current form and averaging 

time and also in terms of the W126 

metric. She has considered particularly 

what such analyses and relationships 

indicate with regard to the extent to 

which W126-based O3 concentrations 

may be controlled by a revised 

secondary standard set identical to a 

revised primary standard (in the range 

identified in section II.F above). In so 

doing, she considers the air quality 

analyses in the PA and also the analyses 

of more recent air quality data 

summarized in the EPA technical memo 

(described in section IV.E.4 above), 

focusing particularly on analyses 

examining the W126-based O3 exposure 

achieved in locations found to meet 

potential alternative standards within 

the range of primary standards proposed 

in section II.F above. 

Findings from these analyses of recent 

O3 measurements and trends in the 

relationship between the current 

standard and the W126 metrics were 

substantially similar for the various time 

periods examined over the past decade. 

There is some information suggesting 

that there is a potential for 

inconsistencies in the relationship 

between W126 measures of seasonal O3

concentrations and the fourth highest 

peak O3 concentrations assessed by the 

current standard averaging time and 

form, but the available data suggest that 

air quality in areas meeting a primary 

standard in the range of 65 to 70 ppb 

would also meet a three-year W126 

index value falling in the range of 13 to 

17 ppm-hrs, and that to the extent areas 

need to take action to attain a standard 

in the range of 0.065 to 0.070 ppm, 

those actions would also improve air 

quality as measured by the W126 

metric. The Administrator also 

recognizes that the relatively lesser 

density of monitors in rural areas, 

including in areas of the West and 

Southwest NOAA climatic regions 

currently meeting the current standard 

where O3 W126 index values are 

generally higher, makes uncertain the 

degree to which a revised level for the 

current standard would provide the 

appropriate degree of protection for 

vegetation-related effects on public 

welfare in these areas. The 

Administrator takes note of the PA 

finding, referenced in section IV.D.3 

above, that reductions in NOX emissions 

that occur in urban areas to attain 

primary standards would also have the 

effect of reducing downwind, rural 

concentrations over the season. Thus, 

while the 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 ppmhrs. In the same areas, only 16 monitors 

(or less than 3% of all monitors in this 

group, all but one of which is located in 

the Southwest region) had three-year 

average W126 index values above 13 

ppm-hrs. She further notes that in all 

areas in which the O3 concentrations 

would have met a primary standard 

with a revised level of 65 ppb (which 

includes 220 monitors distributed 

across all regions of U.S), the three-year 

average W126 index values are at or 

below 13 ppm-hrs. 

In considering these findings 

regarding cumulative seasonal O3

exposures in areas that would have met 

a primary standard with a revised level 

within the proposed range, the 

Administrator also takes note of the 

high correlation observed between the 

design value for the current secondary 

(and primary) standard and values for 

the three-year average, W126 metric, as 

well as the high correlation in the 

relative changes in these two metrics 

based on air quality analyses of O3

measurements from over the past 

decade. This finding supports a 

conclusion that the air quality analyses 

indicate that future control programs 

designed to reduce O3 concentrations to 

help meet a revised primary O3 standard 

that retains the current form and 

averaging time (three-year average of the 

4th highest daily maximum 8-hour 

concentration) would also be expected 

to result in reductions in three-year 

average, W126 index values. Further, 

she notes the conclusion from the air 

quality analysis that the Southwest and 

West regions, which have the greatest 

potential for sites to measure elevated 

cumulative, seasonal O3 exposures 

without the occurrence of elevated daily 

maximum 8-hour average O3

concentrations, exhibited the greatest 

response in W126 index value change 

per unit change in metric based on the 

current standard form and averaging 

time. While recognizing the limitations 

of such analyses in projections of future 

air quality patterns, the Administrator 

also notes that the time period over 

which the analyses focused involved 

emissions control programs to achieve 

O3 reductions such that their findings 

would be expected to be informative of 

further similar control activities, such as 

those to meet a revised standard with a 

lower level, in the future. 

Based on the findings from these 

analyses, the Administrator finds it 

appropriate to consider the policy 

option of retaining the form and 

averaging time of the current secondary 

standard and revising the level to within 

the range of 65 to 70 ppb. In such 

consideration, the Administrator first 

notes her proposed conclusion that the 

requisite protection from known or 

anticipated adverse effects to public 

welfare may be achieved by cumulative, 

seasonal, concentration-weighted O3

concentrations characterized in terms of 

a W126 index value that falls within the 

range from 13 to 17 ppm-hrs. Her final 

decision on the W126 index value in 

this range that affords the requisite 

protection will be based on a series of 

judgments, as described above. Given 

the focus on tree seedling growth effects 

in identifying this range, such 

judgments will include the weight to 

give the evidence of specific vegetationrelated 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 threeyear average W126 index values, of a 


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range at or below the range which the 

Administrator has identified for 

consideration with regard to the 

requisite public welfare protection. 

Thus, depending on final judgments on 

revisions to the primary standard and 

the requisite protection for the 

secondary standard, a revised secondary 

standard identical to the revised 

primary standard may provide sufficient 

protection for public welfare. Therefore, 

the Administrator proposes to retain the 

current averaging time and form of the 

secondary standard and revise the level 

to within the range of 65 to 70 ppb. 

In reaching such a conclusion, the 

Administrator recognizes that such a 

strengthening of the secondary standard 

would be expected to provide 

significant additional protection for 

public welfare, including effects related 

to vegetation and associated ecosystem 

services (and others discussed above), 

over that afforded by the current 

secondary standard. 

Thus, based on her consideration of 

the full range of information as 

described above, the Administrator 

judges that ambient O3 concentrations 

in terms of a three-year average W126 

index value within the range extending 

from 13 ppm-hrs to 17 ppm-hrs would 

provide requisite 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 

period cumulated for the 12-hour period 

from 8:00 a.m. to 8:00 p.m., and a level 

within the range from 13 ppm-hrs to 17 

ppm-hrs. 

F. Proposed Decision on the Secondary 

Standard 

The Administrator proposes to revise 

the level of the current secondary 

standard within the range of 0.065 to 

0.070 ppm. The EPA solicits comments 

on this proposed revision of the 

secondary standard. Further, the EPA 

solicits comments on the proposed 

conclusion that air quality in terms of a 

W126 index value, averaged across three 

consecutive years, within the range of 

13 ppm-hrs to 17 ppm-hrs would 

provide requisite protection against 

known or anticipated adverse effects to 

the public welfare. Additionally, the 

EPA solicits comments on alternative 

values for a three-year average W126 

index for such a purpose within the 

range extending below 13 ppm-hrs 

down to 7 ppm-hrs. 

The EPA also solicits comments on 

the alternative approach of revising the 

secondary standard to a cumulative, 

seasonal, concentration-weighted form, 

the W126 index based on the three 

consecutive month period within the O3

season with the maximum index value, 

with daily exposures cumulated for the 

12-hour period from 8:00am to 8:00pm 

and with a form that averages seasonal 

W126 values across three consecutive 

years and a level within the range of 13 

to 17 ppm-hrs. The EPA additionally 

solicits comments on such a distinct 

secondary standard with a level within 

the range extending below 13 ppm-hrs 

down to 7 ppm-hrs. Further, the EPA 

solicits comments on retaining the 

current secondary standard without 

revision, along with the alternative 

views of the evidence that would 

support retaining the current standard. 

V. Appendix U: Interpretation of the 

Primary and Secondary NAAQS for O3

A. Background 

The EPA is proposing to create 

Appendix U to 40 CFR part 50 to reflect 

the proposed revisions to the primary 

and secondary NAAQS for O3 discussed 

in previous sections of this preamble. 

The proposed Appendix U explains the 

computations necessary for determining 

when the proposed primary and 

secondary O3 NAAQS are met at an 

ambient air quality monitoring site, 

similar to Appendix P to 40 CFR part 50 

which deals with interpretation of the 

O3 NAAQS promulgated in 2008. 

Specifically, the proposed Appendix U 

addresses data selection requirements 

(section V.B), data reporting and data 

handling requirements (section V.C), 

and data completeness requirements. 

The EPA is proposing to maintain the 

data completeness requirements from 

the previous O3 NAAQS. 

Given that the EPA is soliciting public 

comment on a distinct secondary 

standard based on the W126 metric, 

section V.D of this preamble contains a 

discussion of additional data handling 

requirements that would be adopted in 

Appendix U in the event that the 

Administrator decides to set a distinct 

secondary standard based on public 

comments received. 

The proposed Appendix U also 

provides specific requirements for the 

handling of data affected by exceptional 

events in accordance with 40 CFR 50.14. 

Section V.E of this preamble addresses 

O3-specific deadlines related to the 

flagging and submission of 

demonstrations for exceptional event 

data for the proposed O3 NAAQS. 

B. Data Selection Requirements 

The EPA is proposing to clarify which 

data are to be used in comparisons with 

the NAAQS. First, the EPA proposes to 

maintain the existing regulatory 

requirements that only O3 data collected 

by a federal reference method specified 

in Appendix D to 40 CFR part 50, or an 

equivalent method designated in 

accordance with 40 CFR part 53, and 

meeting all applicable monitoring 

requirements listed in 40 CFR part 58, 

are eligible for comparison to the 

proposed O3 NAAQS. 

Second, the EPA is proposing in 

Appendix U that O3 design values are to 

be calculated on a site-level basis. Past 

practice has been to calculate a design 

value for each individual O3 monitor. 

However, this practice could be viewed 

as inconsistent with the stated purpose 

of the previous O3 data handling 

appendix, which is to determine 

‘‘whether the national 8-hour primary 

and secondary ambient air quality 

standards for ozone (O3) specified in 

§ 50.15 are met at an ambient O3 air 

quality monitoring site.’’ (40 CFR part 

50, Appendix P, section 1 (emphasis 

added)). Given the level of consistency 

in the measurement data obtained 

across the various federal reference and 

equivalent O3 monitoring instruments 

currently in operation (U.S. EPA, 2013a, 

section 3.5.2.1), the EPA believes that it 

would be appropriate to combine data 

across O3 monitors operating at the 

same site. Therefore, the EPA is 

proposing an analytic approach for 

combining data collected from multiple 

O3 monitors at a site in order to obtain 

a single set of hourly O3 concentration 

data for each site. 

The proposed approach allows the 

monitoring agencies to designate one 

monitor as the ‘‘primary monitor’’ for 

each site. In the absence of a primary 

monitor designation, the primary 

monitor would default to the monitor 

with the most complete hourly dataset 

in each year. Once a primary monitor 

has been determined for the site, 

missing hourly O3 concentrations for the 

primary monitor would be substituted 

from any other monitors at the site. In 

the event of three or more monitors 

operating at the same site, missing 

hourly O3 concentrations for the 

primary monitor would be substituted 

with hourly values averaged across the 

other monitors. The EPA notes that at 

the time of this proposal, there were 

approximately 20 sites operating two 


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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 

locations and distance between the two 

sites, levels of ambient concentrations 

measured at the two sites, and local 

emissions or meteorology data. 

In order to improve transparency, the 

EPA will make records of all approved 

site combinations available in their Air 

Quality System (AQS) database, and 

will update design value calculations in 

AQS so that approved site combinations 

are implemented. The EPA invites 

public comment on the merits of the 

proposed process for approving site 

combinations in order to obtain valid 

design values for the O3 NAAQS. 

C. Data Reporting and Data Handling 

Requirements 

The EPA is proposing to maintain the 

requirement that hourly O3

concentration data be reported in parts 

per million (ppm) to three decimal 

places. Any decimal digits reported 

beyond three decimal digits will be 

truncated, consistent with past practice 

(40 CFR part 50, Appendix P, section 

2.1) and the typical measurement 

uncertainty associated with most O3

monitoring instruments. The proposed 

Appendix U clarifies that hourly O3

concentrations are to be reported in 

Local Standard Time (LST), consistent 

with how the values are currently stored 

in AQS. 

The EPA is proposing to maintain the 

existing procedures for calculating 

moving 8-hour averages from the hourly 

O3 data (40 CFR part 50, Appendix P, 

section 2.1), with one minor exception. 

In instances where fewer than six 

hourly O3 concentrations are available 

during an 8-hour period (i.e. less than 

75% completeness), the EPA is 

proposing to substitute zero (i.e. 0.000 

ppm) instead of one half of the O3

monitoring instrument’s minimum 

detectable limit (MDL) for the missing 

concentration values to determine if the 

resulting 8-hour average is greater than 

the level of the NAAQS. The purpose of 

this ‘‘data substitution test’’ is to 

identify any 8-hour periods that do not 

meet the requirements for a valid 8-hour 

average, but have reported 

concentrations that are so high that the 

NAAQS is exceeded even when 

substituting low values for the missing 

concentrations. The EPA believes that a 

constant substitution value of zero is 

preferable to 1/2 MDL, which may vary 

across O3 monitoring instruments. The 

MDL value for most O3 monitoring 

instruments is 0.005 ppm, and the 1/2 

MDL value is 0.002 ppm (with 

truncation); thus, in practice, the 

difference is slight. The EPA notes that 

a value of zero micrograms per cubic 

meter (ug/m3) is used in data 

substitution tests for 24-hour average 

PM2.5 concentrations, as specified in 

Appendix N to 40 CFR part 50. The EPA 

invites public comment on the merits of 

using zero instead of 1/2 MDL for the 8- 

hour average data substitution test. 

The EPA is proposing new procedures 

for determining daily maximum 8-hour 

average O3 concentrations. Past practice 

allows for daily maximum 8-hour 

average O3 concentrations from two 

consecutive days to have some hours in 

common (40 CFR part 50, Appendix P, 

section 2.1). One implication of this is 

that an O3 site may be counted as having 

exceeded the NAAQS on two distinct 

days based on two 8-hour periods 

having up to 7 hours in common. 

Theoretically, this could result in an 

annual fourth-highest value greater than 

the NAAQS based on high overnight O3

concentrations occurring only twice 

during the year. 

The EPA performed an analysis based 

on ambient O3 concentration data from 

2004 to 2013 (Wells, 2014b), which 

showed that at least one instance of 

overlapping daily maximum 8-hour 

averages occurred at 99.5% of O3 sites 

during that time period. Overlapping 

daily maximum 8-hour averages were 

infrequent at most sites, but in some 

cases, these values occurred quite 

regularly (up to 60 times per year). 

Overlapping daily maximum 8-hour 

averages contributed to additional 

exceedances of the proposed O3 NAAQS 

at 14% of sites for a level of 0.070 ppm, 

and at 23% of sites for a level of 0.065 

ppm. In addition, 8% of sites had 

overlapping daily maximum 8-hour 

averages which contributed to a higher 

annual fourth-highest daily maximum 

value in one or more years. Finally, the 

analysis showed that O3 sites located in 

non-urban areas affected by long-range 

transport, especially those sites at 

higher elevations, were most likely to 

have additional exceedances of the 

proposed O3 NAAQS due to the 

occurrence of overlapping daily 

maximum 8-hour averages. 

Based on this analysis, the EPA 

initially concludes that overlapping 

daily maximum 8-hour averages are 

more likely to contribute to additional 

exceedances of the O3 NAAQS as the 

level of the standard is lowered. 

Therefore, the EPA is proposing a new 

procedure for determining daily 

maximum 8-hour average O3

concentrations for the proposed NAAQS 

that is based on 17 consecutive 8-hour 

periods in each day, beginning with the 

8-hour period from 7:00 a.m. to 3:00 

p.m., and ending with the 8-hour period 

from 11:00 p.m. to 7:00 a.m. Given that 

8-hour averages are stored in the 

beginning hour of each period, this 

corresponds to the 8-hour averages from 

7:00 a.m. to 11:00 p.m. 

The rationale for the proposed 

approach is twofold. First, it avoids any 

possibility of ‘‘double counting’’ 

exceedances of the NAAQS based on 8- 

hour periods with one or more hours in 

common, while continuing to make use 

of all of the hourly concentration data, 

and keeping the calculations simple and 

straightforward. Second, it is more 

consistent with the physical processes 

involved in the formation and transport 

of ground-level O3. Specifically, the 

chemical reactions involved in the 

formation of new ground-level O3

require sunlight. Therefore, it is 

appropriate to begin the ‘‘O3 day’’ at 

sunrise, which for simplicity is assumed 

to be 7:00 a.m. LST. Similarly, any daily 

maximum 8-hour averages occurring 

after sunset are assumed to be caused by 

transport of O3 molecules which 


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228References 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). 

originated before sunset. Therefore, it is 

appropriate to end the ‘‘O3 day’’ with 

the 8-hour period beginning at 11:00 

p.m. and ending at 7:00 a.m. 

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 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 fourthhighest 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: 

‘‘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 

States 228 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 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 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 


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229While 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. 

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 

unclassifiable.229 No later than 120 days 

prior to promulgating designations, the 

EPA is required to notify states of any 

intended modifications to their 

designation recommendations as the 

EPA may deem necessary. Section 

107(d)(1)(B)(i) further provides, ‘‘Upon 

promulgation or revision of a NAAQS, 

the Administrator shall promulgate the 

designations of all areas (or portions 

thereof) . . . as expeditiously as 

practicable, but in no case later than 2 

years from the date of promulgation. 

Such period may be extended for up to 

one year in the event the Administrator 

has insufficient information to 

promulgate the designations.’’ As 

described in more detail in section VII.C 

of this proposal, the EPA intends to 

complete designations for any revised 

O3 NAAQS promulgated in 2015 

following the standard 2-year process. 

The EPA is required by Court Order to 

take final action for this O3 NAAQS 

review no later than October 1, 2015. 

The EPA does not intend to establish a 

date earlier than the 1 year submission 

period provided in CAA section 

107(d)(4); thus, state Governors (and 

tribes, if they choose) would be required 

to submit their initial designation 

recommendations for any revised 

NAAQS no later than 1 year after 

promulgation (i.e., by October 1, 2016, 

if the EPA promulgates a revised 

NAAQS on October 1, 2015). State 

Governors (and tribes, if they choose) 

would likely use air quality data from 

the years 2013 to 2015 as the basis for 

their recommendations. The EPA would 

notify states and tribes of intended 

modifications to their recommendations 

no later than June 2017 and the EPA 

would promulgate initial designations 

for any revised NAAQS in October 

2017. We anticipate that the EPA’s 

notification of intended modifications 

and the final designations would be 

based on air quality data from the years 

2014 to 2016, because air quality data 

from 2016 is required to be certified by 

the state no later than May 1, 2017, and 

thus would be available for 

consideration for purposes of initial area 

designations by October 2017. 

As indicated above, and as explained 

in additional detail in section VII.C of 

this preamble, section 107(d)(1)(B) of 

the CAA allows the Administrator to 

extend the designations schedule for up 

to 1 year in the event the Administrator 

has insufficient information to 

promulgate the designations for a newly 

promulgated NAAQS. If the EPA were 

to determine that it is necessary to 

extend the schedule for designating 

areas for a revised O3 NAAQS 

(promulgation in October 2015) from 

2017 to 2018, then it is possible that air 

quality data from 2017 could be 

considered for designations. This could 

raise concerns about whether influences 

from exceptional events in 2017 could 

be investigated and submitted by the 

state and reviewed by the EPA in 

sufficient time for consideration during 

the designation process. 

For purposes of initial designations, 

where the EPA considers the most 

recent air quality monitoring data in a 

relatively quick timeframe, the EPA is 

proposing revisions to the flagging and 

data submission schedule in 40 CFR 

50.14 applicable to the initial area 

designations process. The proposed 

exceptional events schedule is based on 

following a standard 2 year designation 

process. However, because the CAA also 

provides for a 3-year process in the 

event the Administrator has insufficient 

information to promulgate the 

designations for a newly promulgated 

NAAQS within 2 years and provides for 

the promulgation of designations as 

‘‘expeditiously as practicable,’’ which 

could include accelerating the 

designations schedule ahead of the 2- 

year schedule, the proposed exceptional 

event schedule also includes provisions 

for both an accelerated designations 

process and a 3-year process. If the EPA 

were to pursue a designations schedule 

other than a 2- or 3-year process, the 

EPA would notify the state Governors of 

the intended date for final designations 

through notification letters, guidance 

and/or Federal Register notices. 

These proposed revised exceptional 

event scheduling provisions would, if 

promulgated, apply to submission of 

information supporting claimed 

exceptional events affecting pollutant 

data for initial area designations under 

any new or revised NAAQS, including 

any revised O3 NAAQS promulgated in 

October 2015. The general data flagging 

deadlines in the Exceptional Events 

Rule at 40 CFR 50.14(c)(2)(iii) and the 

general schedule for submission of 

demonstrations at 40 CFR 50.14(c)(3)(i) 

would continue to apply to regulatory 

decisions other than those related to the 

initial area designations process under a 

new or revised NAAQS. The EPA 

believes these proposed revisions to the 

exceptional events scheduling 

provisions will provide adequate time 

for states to determine whether data 

have been influenced by an exceptional 

event, to notify the EPA by flagging the 

relevant data and providing an initial 

description in AQS, and to submit 

documentation to support claims for 

exceptional events. 

Therefore, using the authority 

provided in CAA section 319(b)(2), the 

EPA proposes to modify the schedule 

for data flagging and submission of 

demonstrations for exceptional events 

data considered for initial area 

designations by replacing the deadlines 

and information in Table 1 in 40 CFR 

50.14 with the deadlines and 

information presented in Table 9. The 

EPA is also providing Table 10 to 

illustrate how the proposed schedule 

might apply to the designations process 

for any revised O3 NAAQS promulgated 

in October 2015 or to designations 

processes for future new or revised 

NAAQS. The EPA invites comment on 

these proposed changes, shown in Table 

9, to the exceptional event data flagging 

and documentation submission 

deadlines for future new or revised 


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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 schedule d

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 4 b and 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. 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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EP17DE14.000</GPH>tkelley on DSK3SPTVN1PROD with PROPOSALS2

Table 10. Examples by Month of How the Proposed Revised Schedule for Exceptional Event Flagging a

Submission for Data to be Used in Initial Area Would Apply 

Exceptional Event 

I Regulatory 

Action 

Exceptional event 

data flagging and 

initial description 

deadline for data 

years I, 2, and 3." 

Exceptional event 

demonstration 

submittal deadline 

for data years I, 2, 

and 3." 

AQS Q&Aand 

data certi !ication 

Exceptional Event Deadline 

Schedule' 

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 l prior to the 

recommendation deadline. If state and 

tribal recommendations for the 

new/revi,ed NAAQS are due Feb 

through July, then the flagging and 

initial description deadline will be the 

January I prior to the recommendation 

deadline. 

'-To later than the date that state and 

tribal recommendations are due to EPA. 

Annually on May I of the year 

following the vear of data collection 

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 orb applies. 

a. Tfthe EPA lollows a 3 year 

designation schedule, the deadline is 2 years and 7 months after 

promulgation of a new or revised 

Exceptional event NAAQS. 

data flagging, b. If the EPA notifies the state/trihe via 

initial description, Federal Register nmice, letter or 

and exceptional guidance that it imends to complete 

event the initial area designations process 

demonstration according to a schedule other than a 

submittal deadline 2-year or 3-year time line, the 

for data year 4" and deadline is 5 months prior to the date 

potential data year specified for final designations 

5.c decisions in such EPA notification. 

State & Tribal Recommendations to EPA 

Oct Oct 

2015 

July I, 

2016 

(data 

years 

2013. 

2014. 

2015) 

by Oct 

2016 

(data 

years 

2013, 

2014, 

2015) 

May 1 

by May 

31, 

2017 

(data 

year 

2016) 

Oct 

2016 

Nov 

Nov 

2015 

July I, 

2016 

(data 

years 

2013, 2014, 

2015) 

by Nov 

2016 

(data 

years 

2013, 2014, 

2015) 

May I 

hy June 

30, 

2017 

(data 

year 

2016) 

Nov 

2016 

Month ofNAAQS Promulgation, State and Tribal Recommendation, and Fina

Dec 

Dec 

2015 

July I, 

2016 

(data 

years 

2013, 2014, 

2015) 

by Dec 

2016 

(data 

years 

2013, 2014, 

2015) 

May 1 

by July 

31, 

2017 

(data 

year 

2016) 

Dec 

2016 

Jan Jan 

2016 

July I, 

2016 

(data 

years 

2013, 2014, 

2015) 

by Jan 

2017 

(data 

years 

20!3, 2014, 

2015) 

May 1 

by Aug 

31, 

2017 

(data 

year 

2016 

and 

potentia 

lly 

2017) 

.Jan 

2017 

.Feb 

Feb 

2016 

Jan I, 

2017 

(data 

years 

2013, 2014, 

2015) 

by Feb 

2017 

(data 

years 

2013, 2014, 

2015) 

May 1 

by Sep 

30, 

2017 

(data 

year 

2016 

and 

potentia 

lly 

2017) 

Feb 

2017 

Mar 

Mar 

2016 

Jan I. 

2017 

(data 

years 

2013, 

2014, 

2015) 

by Mar 

2017 

(data 

years 

2013, 

2014. 

2015) 

May 1 

by Oct 

31, 

2017 

(data 

year 

2016 

and 

potentia 

lly 

2017) 

Mar 

2017 

Apr Apr 

2016 

Jan 1, 

2017 

(data 

years 

2013, 2014, 

2015) 

by Apr 

2017 

(data 

years 

2013, 2014, 

2015) 

May 1 

by Nov 

30, 

2017 

(data 

year 

2016 

and 

potentia 

lly 

2017) 

Apr 

2017 

May" 

May 

2016 

Jan I, 

2017 

(data 

years 

2013, 

2014, 

2015) 

by May 

2017 

(data 

years 

2013, 

2014, 

2015) 

May 1 

by Dec 

31, 

2017 

(data 

year 

2016 

and 

potentia 

lly 

2017) 

May 

2017 

Jund 

Jun 

2016 

Jan I, 

2017 

(data 

years 

2014, 2015, 

2016) 

by June 

2017 

(data 

years 

2014, 2015, 

2016) 

May I 

by Jan 

31, 

2018 

(data 

year 

2017) 

.June 

2017 

Ju

J

20

Ja

2

(

y

22

2

by 

2

(

y

2

2

2

M

by

28

2

(

2

,Ju

20
Additionally, in conjunction with 

proposing exceptional event schedules 

related to implementing any revised O3

standards, the EPA is also proposing to associated with exceptional event 

remove obsolete regulatory language 


EP17DE14.001</GPH>tkelley on DSK3SPTVN1PROD with PROPOSALS2

EPA notifies States/Tribes of intended modifications to June July Aug Sep Oct Nov Dec Jan Feb M

recommendations (RAs send 120-day letters) 2017 2017 2017 2017 2017 2017 2017 2018 2018 20

Oct Nov Dec Jan Feb Mar Apr May June Ju

Administrator Promulgates Final Designations 2017 2017 2017 2018 2018 2018 2018 2018 2018 20

a - Where data years 1, 2. and 3 arc those years expected to be considered in state and tribal recommendations. 

h- 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 designat

d-The date by which air agencies must their ambient air quality monitoring data in AQS is annually on May I of the year following the year uf data collediun. The EP

certifY data prim· to this date. In some cases, however, air agencies may choose to certiJy a prior year's data in advance of May 1 of the following year. particularly if the EPA 

final designations in the months of \1ay, June, July or August. Exceptional event flagging, initial description, and demonstration deadlines for "early certi!led" data will follo

5" data. 
75358 Federal Register / Vol. 79, No. 242 / Wednesday, December 17, 2014 / Proposed Rules 

230See 40 CFR part 58 Appendix D, section 4.1, 

Table D–3 for a table of required O3 seasons. 

231Certain states, such as California and Arizona, 

have approved shorter seasons for a subset of O3

sites, based on Regional Administrator review and 

approval (see 40 CFR part 58, Appendix D, section 

4.1(i) for the waiver authority). 

232Approximately 800 O3 monitors are currently 

operated year-round, representing greater than 50% 

of the total O3 monitoring network of about 1500 

monitors. They include monitors that are mandated 

to operate year-round due to the required O3 season 

and other monitors that are voluntarily operated 

year-round by states and other organizations 

including EPA-operated monitors at Clean Air 

Status and Trends Network (CASTNET) sites. 

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 reevaluate 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 oxidechemiluminescence (NO–CL) 

methodology. Because of the similarity 

of this new chemiluminescence 

technique to the existing ethylenechemiluminescence (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 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., seasonallydependent factors such as ambient 

temperature, strength of solar insolation, 

and length of day) differ by location.230

In some locations, conditions conducive 

to O3 formation are limited to the 

summer months of the year. For 

example, in states with colder climates 

such as Montana and South Dakota, the 

currently required O3 monitoring season 

is four months long. However, in other 

states with warmer climates such as 

California, Nevada, and Arizona, the 

currently required O3 monitoring season 

is year-round.231

Based on the O3 NAAQS revision that 

was finalized in 2008, as well as the 

proposed NAAQS revisions discussed 

in this rulemaking, the EPA has 

determined that lengthening the O3

monitoring seasons may be appropriate. 

Ambient O3 concentrations could 

approach or exceed the level of the 2008 

NAAQS, as well as the proposed 

NAAQS, more frequently and during 

more months of the year. The EPA has 

done an analysis to address the issue of 

whether extensions of currently 

required monitoring seasons are 

appropriate (Rice, 2014). In this 

analysis, we determined the number of 

days where one or more monitors had 

a daily maximum 8-hour O3 average 

equal to or above 0.060 ppm in the 

months outside the currently-required 

state O3 monitoring season using data 

from monitors that collected O3 data 

year-round in 2010–2013.232 We find 

that this level, taking into consideration 

reasonable uncertainty, serves as an 

appropriate indicator of ambient 

conditions that may be conducive to the 

formation of O3 concentrations that 

approach or exceed the 2008 NAAQS or 

the proposed 8-hour average range of 

0.065 to 0.070 ppm. Although we refer 

to these days as ‘‘exceedance days’’ in 

the analysis, this 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 yearto-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 yearround monitor was identified as ‘‘yearround’’ 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). 


tkelley on DSK3SPTVN1PROD with PROPOSALS2
Federal Register / Vol. 79, No. 242 / Wednesday, December 17, 2014 / Proposed Rules 75359 

233Public reporting requirements are detailed in 

40 CFR part 58 Appendix G, Uniform Air Quality 

Index (AQI) and Daily Reporting. 

234See http://airnow.gov/. 

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, Colorado, Wyoming, and Utah) 

experiencing between 31 and 230 outof-season ‘‘exceedance days’’ from 2010 

to 2013 at monitors operating yearround. 

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 manner 233 and to support 

comparisons of an area’s air pollution 

levels against the NAAQS. The EPA 

believes that frequency of ‘‘exceedance 

days’’ in which daily maximum of 8- 

hour O3 levels are observed to be greater 

than or equal to a threshold level of 

0.060 ppm in months outside the 

currently required O3 monitoring season 

supports the proposed lengthening of 

the O3 monitoring season requirements 

for certain states. 

The operation of O3 monitors during 

periods of time when ambient levels 

approach or exceed the level of the 

proposed NAAQS ensures that persons 

unusually sensitive to O3 are alerted to 

potential levels of health concern 

allowing them to take precautionary 

measures. The majority of O3 monitors 

in the U.S. report to AIRNOW,234 as 

well as to state-operated Web sites and 

automated phone reporting systems. 

These programs support many 

objectives including real-time air quality 

reporting to the public, O3 forecasting 

programs, and the verification of realtime 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, 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, 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 Administratorgranted 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-ofseason 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 multipollutant 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 longterm, 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 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 yearround due to existing requirements, as 


tkelley on DSK3SPTVN1PROD with PROPOSALS2
75360 Federal Register / Vol. 79, No. 242 / Wednesday, December 17, 2014 / Proposed Rules 

235Additional information on the O3 designation 

process can be obtained at EPA’s O3 designations 

Web page at http://www.epa.gov/groundlevelozone/ 

designations/. 236One Type 2 site and either a Type 1 or a Type 

3 site are currently required. 237Refer to 40 CFR part 51.905 

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 proposed O3

season. If we determine, based on any 

such comments that implementation 

could occur earlier in such cases, we 

could proceed to final action requiring 

earlier implementation. 

C. Revisions to the Photochemical 

Assessment Monitoring Stations (PAMS) 

Section 182 (c)(1) of the CAA required 

the EPA to promulgate rules for 

enhanced monitoring of O3, oxides of 

nitrogen, and VOCs for nonattainment 

areas classified as serious (or above) to 

obtain more comprehensive and 

representative data on O3 air pollution. 

In addition, Section 185B of the CAA 

required the EPA to work with the 

National Academy of Sciences (NAS) to 

conduct a study on the role of O3

precursors in tropospheric O3 formation 

and control. In 1992, the NAS issued the 

report entitled, ‘‘Rethinking the Ozone 

Problem in Urban and Regional Air 

Pollution’’, (NAS, 1991). 

In response to the CAA requirements 

and the recommendations of the NAS 

report, on February 12, 1993 (58 FR 

8452), the EPA revised the ambient air 

quality surveillance regulations to 

require PAMS in each O3 nonattainment 

area classified as serious, severe, or 

extreme (‘‘PAMS areas’’).235 As noted in 

EPA’s Technical Assistance Document 

(TAD) for Sampling and Analysis of 

Ozone Precursors (U.S. EPA, 1998), the 

objectives of the PAMS program are to: 

(1) Provide a speciated ambient air 

database which is both representative 

and useful in evaluating control 

strategies and understanding the 

mechanisms of pollutant transport by 

ascertaining ambient profiles and 

distinguishing among various 

individual VOCs; (2) provide local, 

current meteorological and ambient data 

to serve as initial and boundary 

condition information for 

photochemical grid models; (3) provide 

a representative, speciated ambient air 

database which is characteristic of 

source emission impacts to be used in 

analyzing emissions inventory issues 

and corroborating progress toward 

attainment; (4) provide ambient data 

measurements which would allow later 

preparation of unadjusted and adjusted 

pollutant trends reports; (5) provide 

additional measurements of selected 

criteria pollutants for attainment/

nonattainment decisions and to 

construct NAAQS maintenance plans; 

and (6) provide additional 

measurements of selected criteria and 

non-criteria pollutants to be used for 

evaluating population exposure to air 

toxics as well as criteria pollutants. 

The original PAMS requirements 

called for two to five sites per area 

depending on the area’s population. 

Four types of PAMS sites were 

identified including upwind (Type 1), 

maximum precursor emission rate (Type 

2), maximum O3 (Type 3), and extreme 

downwind (Type 4) sites. Each PAMS 

site was required to measure O3, NO, 

NO2, speciated VOCs, selected carbonyl 

compounds, and selected 

meteorological parameters. In addition, 

upper air meteorological monitoring 

was required at one site in each PAMS 

area. 

In the October 17, 2006 monitoring 

rule (71 FR 61267), the EPA revised the 

PAMS requirements to only require two 

PAMS sites per PAMS area.236 The 

intent of the revision was to ‘‘allow 

PAMS monitoring to be more 

customized to local data needs rather 

than meeting so many specific 

requirements common to all subject O3

nonattainment areas; the PAMS changes 

would also give states the flexibility to 

reduce the overall size of their PAMS 

programs—within limits—and to use 

the associated resources for other types 

of monitoring they consider more 

useful.’’ In addition to reducing the 

number of required sites per PAMS area, 

the 2006 revisions also limited the 

requirement for carbonyl measurements 

(specifically formaldehyde, 

acetaldehyde, and acetone) to areas 

classified as serious or above for the 8- 

hour O3 standard. This change was 

made in recognition of carbonyl 

sampling issues which were believed to 

cause significant uncertainty in the 

measured concentrations. 

Twenty-two areas were classified as 

serious or above O3 nonattainment at 

the time the PAMS requirements were 

promulgated in 1993. On July 18, 1997 

(62 FR 38856), the EPA revised the 

averaging time of the O3 NAAQS from 

a 1-hour averaging period to an 8-hour 

averaging period. On June 15, 2005 (70 

FR 44470), the EPA revoked the 1-hour 

standard in most areas of the country; 

however, PAMS requirements were 

identified as requirements that had to be 

retained in the anti-backsliding 

provisions237 included in that action. 

Therefore, PAMS requirements continue 

to be applicable to areas that were 

classified as serious or above 

nonattainment for the 1-hour O3

standard as of June 15, 2004. Currently, 

25 areas are subject to the PAMS 

requirements with a total of 75 sites. As 

will be discussed in detail later, the 

current PAMS sites are concentrated in 

the North East and California with 

relatively limited coverage in the rest of 

the country (Cavender, 2014). 

As discussed above, the first PAMS 

sites began operation in 1994, and have 

been in operation for over 20 years. 

Many changes have occurred during 

that time that have changed the O3

problem in the U.S. as well as our 

understanding of it. The O3 standard has 

been revised multiple times since the 

PAMS program was first implemented. 

On July 18, 1997, the EPA revised the 

O3 NAAQS to a level of 0.08 ppm, with 

a form based on the 3-year average of 

the annual fourth-highest daily 

maximum 8-hour average O3

concentration. On March 28, 2008 (73 

FR 16436), the EPA revised the O3

standard to a level of 0.075 ppm, with 

a form based on the 3-year average of 

the annual fourth-highest daily 

maximum 8-hour average O3

concentration. These changes in the 

level and form of the O3 NAAQS, along 

with notable decreases in O3 levels 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 


tkelley on DSK3SPTVN1PROD with PROPOSALS2
Federal Register / Vol. 79, No. 242 / Wednesday, December 17, 2014 / Proposed Rules 75361 

238PAMS 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. 

(June 15, 2005), only five areas were 

classified as serious or above for the 8- 

hour standard as compared to 22 areas 

that were classified as serious or above 

for the 1-hour standard.238 While the 

number of serious and above areas 

decreased, the number of nonattainment 

areas remained nearly the same. In 

addition, much of the equipment used 

at PAMS sites is old and in need of 

replacement. New technologies have 

been developed since the inception of 

the PAMS program that should be 

considered for use in the network. For 

these reasons, the EPA determined that 

it would be appropriate to re-evaluate 

the PAMS program and associated 

requirements in light of current O3

issues. 

In 2011 (U.S. EPA, 2011c), the EPA 

initiated an effort to re-evaluate the 

PAMS requirements in light of changes 

in the needs of PAMS data users and the 

improvements in monitoring 

technology. The EPA consulted with 

CASAC’s, Air Monitoring and Methods 

Subcommittee (AMMS) to seek advice 

on potential revisions to the technical 

and regulatory aspects of the PAMS 

program; including changes to required 

measurements and associated network 

design requirements. The EPA also 

requested advice on appropriate 

technology, sampling frequency, and 

overall program objectives in the 

context of the most recently revised O3

NAAQS and changes to atmospheric 

chemistry that have occurred over the 

past 10–15 years in the significantly 

impacted areas. The CASAC AMMS met 

on May 16 and May 17, 2011, and 

provided a report with their advice on 

the PAMS program on September 28, 

2011 (U.S. EPA, 2011c). In addition, the 

EPA met multiple times with the 

National Association of Clean Air 

Agencies (NACAA) Monitoring Steering 

Committee (MSC) to seek advice on the 

PAMS 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 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. 

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 


tkelley on DSK3SPTVN1PROD with PROPOSALS2
75362 Federal Register / Vol. 79, No. 242 / Wednesday, December 17, 2014 / Proposed Rules 

239Although 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. 

240While 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. 

241Monitoring 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. 

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 each state. Accordingly, the EPA is 

proposing to require PAMS 

measurements at any existing NCore site 

in an O3 nonattainment area (either 

based on the 2008 O3 NAAQS or the 

2015 O3 NAAQS if finalized) in lieu of 

the current PAMS network design 

requirements.239 The NCore network is 

a multi-pollutant monitoring network 

consisting of 80 sites (63 urban, 17 

rural) and is intended to support 

multiple air quality objectives including 

the development and model evaluation 

of photochemical models (including 

both PM2.5 and O3 models), and the 

tracking of regional precursor trends. 

NCore sites are sited in typical 

neighborhood scale locations which are 

more suitable than source impacted 

locations for evaluation of grid models 

typical of current photochemical models 

and tracking of trends in pre-cursor 

concentrations. The EPA believes NCore 

sites are well suited for O3 model 

development and evaluation. 

The proposal to require PAMS 

measurements at existing NCore sites in 

O3 nonattainment areas would replace 

the existing PAMS network design.240

This change would keep roughly the 

same number of required PAMS sites 

while improving spatial coverage 

(Cavender, 2014). Based on the range of 

the O3 NAAQS being proposed today 

and current O3 design value estimates 

(based on 2011–2013 air quality data), 

the number of required sites is 

estimated to be between 48 and 65, 

which compares to 50 currently 

required sites, and 75 currently 

operating sites. Potential redundancy in 

the existing network would be reduced 

while important network coverage in 

the Southeast and Midwest would be 

added. The improved spatial coverage 

will also improve the 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 proposed 

requirements are promulgated.241 As 

discussed later in this preamble, the 

EPA is proposing a staggered 

compliance schedule for the proposed 

PAMS requirements in recognition of 

the need for capital investment and staff 

training at these sites. 

The EPA recognizes that in limited 

situations, existing NCore sites may not 

be the most appropriate locations for 

making PAMS measurements. For 

example, an existing PAMS site in an O3

nonattainment area may be sited at a 

different location than the existing 

NCore site. In this case, it may be 

appropriate to continue monitoring at 

the existing PAMS site to support 

ongoing research and to maintain trends 

information. To account for these 

situations, the EPA is also proposing to 

provide the EPA Regional Administrator 

the authority to approve an alternative 

location for a required PAMS sites 

where appropriate. 

The EPA seeks comment on the 

network design revision proposed 

above, the requirement for PAMS 

measurements at NCore sites in O3

nonattainment areas, and the removal of 

current multi-site PAMS network design 

requirements. The EPA also solicits 

comment on whether, instead of 

requiring PAMS measurements at all 

NCore sites in nonattainment areas, we 

should instead adopt one of the other 

options discussed above, for example, 

using both attainment status and 

population thresholds, that may result 

in a fixed PAMS monitoring network 

that is either smaller or larger than what 

will result from the proposed 

requirement. 

The second part of the proposed 

PAMS network design includes 

monitoring agency directed enhanced 

O3 monitoring activities intended to 

provide data needed to understand an 

area’s specific O3 issues. To implement 

this part of the PAMS network design, 

the EPA is proposing to add a 

requirement for states with O3

nonattainment areas to develop an 

‘‘Enhanced 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 


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242 Data of longer than a 1-hour average are often 

not used in model evaluations due to the 

complexity of trying to accommodate non-hourly 

averaged data. 

243Carbonyls 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. 

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 nonmethane 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 

measure and report hourly speciated 

VOCs using an auto GC. More complete 

and consistent speciated VOC data 

nationally would better help meet 

certain objectives of the PAMS program 

described above (e.g., a speciated 

ambient air database useful in 

evaluating control strategies, analyzing 

emissions inventory issues, 

corroborating progress toward 

attainment, and evaluating population 

exposure to air toxics). Furthermore, as 

noted by the CASAC AMMS, hourly 

VOC data are ‘‘particularly useful in 

evaluating air quality models and 

performing diagnostic emission 

attribution studies. These data can be 

provided on a near real-time basis and 

presented along with other precursor 

species (e.g., oxides of nitrogen and 

carbon monoxide) collected over similar 

averaging times.’’ Longer time-averaged 

data are of significantly lower value for 

model evaluation.242 In addition, 

creating consistent monitoring 

requirements across the network will 

provide better data for analyzing 

regional trends and spatial patterns. 

At the time the original PAMS 

requirements were promulgated, the 

canister options were included because 

the EPA recognized that the 

technologies necessary to measure 

hourly average speciated VOCs 

concentrations were relatively new and 

may not have been suitable for broad 

network use. At that time, gas 

chromatographs designed for laboratory 

use were equipped with auto-samplers 

designed to ‘‘trap’’ the VOC compounds 

from a gas sample, and then ‘‘purge’’ the 

compounds onto the GC column. The 

EPA did not believe that auto-GCs were 

universally appropriate due to the 

technical skill and effort necessary at 

that time to properly operate an autoGC. 

While the basic principles of auto-GC 

technology have not changed, the 

hardware and software of modern autoGCs are greatly improved over that 

available at the time of the original 

PAMS requirements. Based on advice 

from the CASAC AMMS, the EPA has 

initiated an evaluation of current autoGCs 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. 

3. Carbonyl Sampling 

Carbonyls include a number of 

compounds important to O3 formation 

that cannot currently be measured using 

the auto-GCs or canisters used at PAMS 

sites to measure speciated VOCs.243

The current method for measuring 

carbonyls in the PAMS program is 

Compendium Method TO–11A (U.S. 

EPA, 1999). In this method, carbonyl 

compounds are adsorbed and converted 

into stable hydrazones using 

dinitrophenylhydrazine (DNPH) 

cartridges. These cartridges are then 

analyzed for the individual carbonyl 

compounds using liquid 

chromatography (LC) techniques. Three 

carbonyls (formaldehyde, acetaldehyde, 

and acetone) are currently required to be 

measured in the PAMS program. 

In 2006, the EPA revised the PAMS 

requirements such that carbonyl 

sampling was only required in areas 

classified as serious or above 

nonattainment for O3 under the 8-hour 

O3 standard which effectively reduced 

the applicability of carbonyl sampling to 

a few areas in California. This change 

was made in recognition that there were 

a number of issues with Method TO– 

11A that raised concerns with the 

uncertainty in the carbonyl data being 

collected. These issues include 

interferences (humidity and O3) and 

breakthrough (i.e., overloading of the 

DNPH cartridge) at high concentrations. 

While solutions for these issues have 

been investigated, these improvements 

have not been incorporated into Method 

TO–11A. 

A recent evaluation of the importance 

of VOCs and carbonyls to O3 formation 

determined that carbonyls, especially 

formaldehyde, are very important to O3

formation (Cavender, 2013). CASAC 

AMMS (U.S. EPA, 2011c) also noted the 

importance of carbonyls 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 


tkelley on DSK3SPTVN1PROD with PROPOSALS2
75364 Federal Register / Vol. 79, No. 242 / Wednesday, December 17, 2014 / Proposed Rules 

244Nitrogen 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. 

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. 

In conventional NOX analyzers, NO2

is determined as the difference between 

the measured NO and NOX

concentrations. However, due to the 

non-selective reduction of oxidized 

nitrogen compounds by the 

molybedenum converter used in 

conventional NOX monitors, the NO2

measurement made by conventional 

NOX monitors can be biased high due to 

the varying presence of NOz compounds 

that may be reported as NO2.244 The 

unknown bias from the NOz compounds 

is undesirable when attempting to 

understand O3 chemistry. 

Improvements in reactive nitrogen 

measurements have been made since the 

original PAMS requirements were 

promulgated that allow for improved 

NO2 measurements. Selective photolytic 

converters have been developed that are 

not significantly biased by NOz

compounds (Ryerson et al., 2000). 

Monitors using photolytic converters are 

commercially available and have been 

approved as FEMs for the measurement 

of NO2. In addition, methods that 

directly read NO2 have been developed 

that allow for very accurate readings of 

NO2 without some of the issues inherent 

to the ‘‘difference method’’ used in 

converter based NOX analyzers. 

However, these direct reading NO2

analyzers generally do not provide an 

NO estimate, and would need to be 

paired with a converter-based NOX

monitor or NOy monitor in order to also 

measure NO. 

As discussed above, the EPA is 

proposing to change the PAMS network 

design such that PAMS measurements 

would be required at existing NCore 

sites in O3 nonattainment areas. NCore 

sites currently are required to measure 

NO and NOy. NCore sites are not 

currently required to measure NO2. Due 

to the importance of accurate NO2 data 

to the understanding of O3 formation, 

the EPA is proposing to require NO2

measurements at required PAMS sites. 

Since existing NCore sites currently 

measure NOy, either a direct reading 

NO2 analyzer or a photolytic-converter 

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. 

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 


tkelley on DSK3SPTVN1PROD with PROPOSALS2
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245The 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. 

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 importance of mixing height 

measurements for O3 modeling, the EPA 

is proposing to require monitoring 

agencies to measure mixing height at 

PAMS sites. The EPA is aware of a large 

network of ceilometers operated by the 

National Oceanic and Atmospheric 

Administration (NOAA) as part of the 

Automated Surface Observing System 

(ASOS). The EPA has been in 

discussions with NOAA regarding the 

potential for these systems to provide 

the needed mixing height data, 

however, the ASOS ceilometers are not 

currently equipped to provide mixing 

height data. Nonetheless, the EPA will 

continue to work with NOAA to 

determine if the ASOS ceilometers can 

be upgraded to meet the need for mixing 

height data, and is including proposed 

regulatory language that will allow 

states a waiver to use nearby mixing 

height data from ASOS or other sources 

to meet the requirement to collect 

mixing height data at required PAMS 

sites. 

6. PAMS Season 

Currently, PAMS measurements are 

required to be taken during the months 

of June, July, and August. This 3-month 

period is referred to as the ‘‘PAMS 

Season’’. As part of the PAMS reevaluation, the EPA considered changes 

to the PAMS season. The 3-month 

PAMS season was originally selected to 

represent the most active period for O3

formation. However, the EPA notes that 

in many areas the highest O3

concentrations are observed outside of 

the PAMS season.245 As an example, the 

highest O3 concentrations in the 

Mountain-West often occur during the 

winter months. Data collected during 

the current PAMS season would have 

limited value in understanding winter 

O3 episodes. 

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 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 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 


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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 of its alloptoelectronic 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 Web site 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 ethylenechemiluminescence (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 NOCL 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 

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 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 highpressure 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 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 


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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. 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 ambientair-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 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 ETCL 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 N2Oto-NO converter to supply NO gas for 

the instrument as needed. There will be 

no requirement for states to switch to 

NO-CL analyzers; therefore, UVabsorption 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 NOCL 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 redesignated 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. 

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 


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246 2B FEM test data via http://

www.twobtech.com/model_211.htm. 

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 

manufacturer 246 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 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 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 ET–CL 

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 

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 


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247This and all subsequent references to ‘‘state’’ 

are meant to include state, local, and tribal agencies 

responsible for the implementation of an O3 control 

program. 

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 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 

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 

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 states 247 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 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 


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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. 

This section contains a discussion of 

how existing requirements to reduce the 

impact on O3 concentrations from the 

stationary source construction in permit 

programs under the CAA may be 

affected by the proposed revisions of the 

O3 NAAQS. These are the PSD and 

Nonattainment New Source Review 

(NNSR) programs. To facilitate the 

timely implementation of the PSD 

requirements, the EPA proposes as part 

of this rulemaking to add a 

grandfathering provision to its 

regulations that would apply to certain 

PSD permit applications that are 

pending on the effective date of the 

revised O3 NAAQS. If the proposed 

NAAQS revisions are finalized, this 

grandfathering provision could be 

finalized at the same time as the revised 

NAAQS (see section VII.D of this 

preamble). 

The EPA intends to propose 

additional regulations and issue 

additional guidance, as necessary, 

related to the implementation 

requirements for any revised O3 NAAQS 

resulting from this proposal. The EPA 

intends to take these actions on a 

schedule that provides timely assistance 

to air agencies. Accordingly, in this 

section, the EPA solicits comment on 

several issues that the agency 

anticipates addressing in future 

guidance or regulatory actions to assist 

with implementation of the revised O3

NAAQS. Because these issues are not 

relevant to the establishment of the 

NAAQS, and the CAA does not require 

that the EPA provide implementation 

rules or guidance for each revised 

NAAQS, the EPA does not expect to 

respond, nor is the agency required to 

respond, to these comments in the final 

action on this proposal. However the 

EPA expects these comments will be 

helpful as future guidance and 

regulations are developed. 

A. NAAQS Implementation Plans 

1. Background 

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 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. 

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 for marine diesel 

engines and in April 2010 the EPA 

promulgated new standards for Category 

3 (C3) engines installed on U.S. oceangoing 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 


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248 ‘‘Regulatory Impact Analysis for the Proposed 

Revisions to the National Ambient Air Quality 

Standards for Ground-Level Ozone,’’ (December 

2014) at http://www.epa.gov/ttnecas1/ria.html. 

standards for light-duty vehicles 

represent approximately an 80% 

reduction from today’s fleet average, 

and the heavy-duty tailpipe standards 

represent about a 60% reduction in VOC 

and NOX. 

The emission reductions from all of 

these mobile source programs are 

significant and will continue to be 

realized throughout the implementation 

period for any revised O3 NAAQS. The 

EPA projects that between 2011 and 

2025, onroad and nonroad mobile NOX

will decline by more than 60% and 

onroad and nonroad mobile VOC will 

decline by more than 50%.248

The reduction of VOC emissions from 

industrial processes has been achieved 

either directly or indirectly through 

implementation of control technology 

standards, including maximum 

achievable control technology (MACT), 

reasonably available control technology 

(RACT), and best available control 

technology (BACT) standards; or is 

anticipated due to 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 11, 2008, 

and December 23, 2008, the U.S. Court 

of Appeals for the DC Circuit (DC 

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 

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 statespecific rate-based goals for CO2

emissions from the power sector, as well 

as guidelines for states to follow in 

developing plans to achieve the statespecific 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 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 


tkelley on DSK3SPTVN1PROD with PROPOSALS2
75372 Federal Register / Vol. 79, No. 242 / Wednesday, December 17, 2014 / Proposed Rules 

249 ‘‘The Roadmap for Incorporating Energy 

Efficiency/Renewable Energy Policies and Programs 

into State and Tribal Implementation Plans,’’ (July 

2012) at http://epa.gov/airquality/eere/. 

available retrofit technology (BART) 

determinations. 

While the EPA uses its regulatory 

opportunities to reduce NOX and VOCs, 

the agency also is pursuing nonregulatory 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 developed a roadmap for giving SIP 

credit to energy efficiency/renewable 

energy projects.249 Recognition of 

innovative programs will allow states 

and tribes to pursue effective strategies 

that address some of the more 

challenging issues affecting air quality, 

such as land use planning, ever 

increasing motor vehicle use, and 

planning for long-term energy needs. 

With respect to agricultural sources, 

the U.S. Department of Agriculture 

(USDA) has approved conservation 

systems and activities that reduce 

agricultural emissions of NOX and VOC. 

The EPA recognizes that USDA has been 

working with the agricultural 

community to develop site-specific 

conservation systems and activities to 

control emissions of O3 precursors. The 

EPA will continue to work with USDA 

on these activities with efforts to 

identify and/or improve the control 

efficiencies, prioritize the adoption of 

these conservation systems and 

activities, and ensure that appropriate 

criteria are used for identifying the most 

effective application of conservation 

systems and activities. 

The EPA will work together with 

USDA and with states to identify 

appropriate measures to meet the 

primary and secondary standards, 

including site-specific conservation 

systems and activities. Based on prior 

experience identifying conservation 

measures and practices to meet the PM 

NAAQS requirements, the EPA will use 

a similar process to identify measures 

that could meet the O3 requirements. 

The EPA anticipates that certain USDA 

approved conservation systems and 

activities that reduce agricultural 

emissions of NOX and VOC may be able 

to satisfy the requirements for 

applicable sources to implement 

reasonably available control measures 

for purposes of attaining the primary 

and secondary O3 NAAQS. 

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 

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 


tkelley on DSK3SPTVN1PROD with PROPOSALS2
Federal Register / Vol. 79, No. 242 / Wednesday, December 17, 2014 / Proposed Rules 75373 

250See 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. 

251While the CAA allows the EPA to set a shorter 

time for submission of these SIPs, the EPA does not 

currently intend to do so. 

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 to the designation guidance for 

the 2008 O3 NAAQS. The EPA generally 

completes area designations 2 years 

after promulgation of a NAAQS. See 

section VII.C of this preamble for 

additional details on designations. 

Clean Air Act section 110 requires 

SIPs to be submitted within 3 years of 

promulgation of a revised NAAQS. 

These SIPs are referred to as 

‘‘infrastructure SIPs.’’ The EPA issued 

general guidance on submitting 

infrastructure SIPs on September 13, 

2013.250 It should be noted that this 

guidance did not address certain state 

planning and emissions control 

requirements related to interstate 

pollution transport. Should this 

guidance need to be modified for this 

prospective O3 NAAQS, the EPA 

intends to issue that updated guidance 

no later than 1 year after promulgation 

of a revised O3 NAAQS. See section 

VII.B.3 of this preamble for additional 

information on infrastructure SIPs. 

The EPA intends to propose any 

appropriate rules for assisting with 

implementing any revised O3 NAAQS 

resulting from this proposal within 1 

year after a revised NAAQS is 

established. The rules that EPA is 

considering, as with implementation of 

previous NAAQS, would address 

nonattainment area classification 

methodologies, SIP due dates, 

attainment dates, and required 

implementation programs such as NNSR 

and conformity. At that same time the 

EPA intends to address any 

modifications needed as a result of this 

revised NAAQS to guidance pertaining 

to developing nonattainment area 

emissions inventories and attainment 

demonstrations, and demonstrating 

conformity. The EPA anticipates 

finalizing these items by the time areas 

are designated nonattainment. 

Finalizing rules and guidance by this 

time would provide air agencies with 

the information to develop any CAArequired SIPs associated with 

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 prescribe 251) 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 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 EPAapproved 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 nonregulatory measures, as appropriate, 

which meet the relevant CAA 

requirement. Like any other SIP 

submittal, such certification can be 

made only after the 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 


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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. 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 

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 

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 

In each of the previous O3 NAAQS 

reviews the secondary standard was set 

equal to the primary standard. As 

discussed in section IV of this preamble, 

the EPA is proposing to retain the 

current averaging time and form of the 

secondary standard and to revise the 

level. The EPA is also soliciting 

comment on the alternative approach of 

revising the secondary standard to a 

cumulative, seasonal, concentrationweighted form based on the W126 

index. 

If the EPA were to establish a distinct 

secondary standard, there would be 


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252American Petroleum Institute v. Costle, 609 

F.2d 20 (D.C. Cir. 1979). 

253Page, S. (2011). Guidance to Regions for 

Working with Tribes during the National Ambient 

Air Quality Standards (NAAQS) Designations 

Process, Memorandum from Stephen D. Page, 

Director, EPA Office of Air Quality Planning and 

Standards to Regional Air Directors, Regions I–X, 

December 20, 2011. Available: http://www.epa.gov/ 

ttn/oarpg/t1/memoranda/

20120117naaqsguidance.pdf. 

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 implementation-related issues 

(with examples, where possible) that air 

agencies and affected sources would 

face if a separate and distinct secondary 

standard is established. 

C. Designation of Areas 

After the EPA establishes or revises a 

NAAQS, the CAA directs the EPA and 

the states to take steps to ensure that the 

new or revised NAAQS is met. One of 

the first steps, known as the initial area 

designations, involves identifying areas 

of the country that either do not meet 

the new or revised NAAQS along with 

the nearby areas that contribute to the 

violations. 

Section 107(d)(1) of the CAA provides 

that, ‘‘By such date as the Administrator 

may reasonably require, but not later 

than 1 year after promulgation of a new 

or revised national ambient air quality 

standard for any pollutant under section 

109, the Governor of each state 

shall . . . submit to the Administrator 

a list of all areas (or portions thereof) in 

the state’’ that designates those areas as 

nonattainment, attainment, or 

unclassifiable. The EPA must then 

promulgate the area designations 

according to a specified process, 

including procedures to be followed if 

the EPA intends to modify a 

recommendation. The CAA defines an 

area as nonattainment if it is violating 

the NAAQS or if it is contributing to a 

violation in a nearby area. 

Section 107(d)(1)(B)(i) further 

provides, ‘‘Upon promulgation or 

revision of a national ambient air 

quality standard, the Administrator 

shall promulgate the designations of all 

areas (or portions thereof) . . . as 

expeditiously as practicable, but in no 

case later than 2 years from the date of 

promulgation of the new or revised 

national ambient air quality standard. 

Such period may be extended for up to 

one year in the event the Administrator 

has insufficient information to 

promulgate the designations.’’ In certain 

contexts, with respect to the NAAQS, 

the term ‘‘promulgation’’ has been 

interpreted by the courts to be signature 

and widespread dissemination of a final 

NAAQS rule.252 By no later than 120 

days prior to promulgating area 

designations, the EPA is required to 

notify states of any intended 

modifications to their recommendations 

that the EPA may deem necessary. 

States then have an opportunity to 

demonstrate why any proposed 

modification is inappropriate. Whether 

or not a state provides a 

recommendation, the EPA must timely 

promulgate the designation that the 

agency deems appropriate. 

While section 107 of the CAA 

specifically addresses states, the EPA 

intends to follow the same process for 

tribes to the extent practicable, pursuant 

to CAA section 301(d) regarding tribal 

authority and the Tribal Authority Rule 

(63 FR 7254, February 12, 1998). To 

provide clarity and consistency in doing 

so, the EPA issued a 2011 guidance 

memorandum on working with tribes 

during the designation process.253

As discussed in sections II and IV of 

this preamble, the EPA is proposing to 

revise both the primary and secondary 

O3 NAAQS, which currently are 

identical 8-hour standards that were set 

at 0.075 ppm in the 2008 NAAQS rule 

(73 FR 16436; March 27, 2008). If the 

EPA revises the primary and secondary 

O3 NAAQS based on this proposal, the 

EPA intends to complete designations 

for both NAAQS following the standard 

2-year process discussed above. The 

EPA is required to sign the final rule for 

this O3 NAAQS review no later than 

October 1, 2015, under a court-ordered 

deadline. In accordance with section 

107(d)(4) of the CAA, state Governors 

(and tribes, if they choose) should 

submit their initial designation 

recommendations for a revised primary 

and secondary NAAQS no later than 1 

year after promulgation of any revised 

O3 NAAQS (for example, by October 1, 

2016, if the EPA promulgates such 

NAAQS on October 1, 2015.) If the EPA 

intends to modify any state 

recommendation, the EPA would notify 

the appropriate state Governor (or tribal 

leader) no later than 120 days prior to 

making final designation decisions. A 

state or tribe that believes the 

modification is inappropriate would 

then have the opportunity to 

demonstrate to EPA why it believes its 

original recommendation (or a revised 

recommendation) is more appropriate. 

The EPA would take any additional 

input into account in making the final 

designation decisions. 

Consistent with previous 

designations, the EPA intends to use 

area-specific multi-factor analyses to 

support area boundary decisions for any 

revised primary or secondary O3

standards. Historically, the EPA has 

evaluated information related to the 

following factors for designations: air 

quality data, emissions-related data, 

meteorology, geography/topography, 

and jurisdictional boundaries. The EPA 

solicits comment related to establishing 

area designation boundaries for the 

proposed revised primary and 

secondary NAAQS, including any 

relevant technical information that 

should be considered by the EPA and 

the extent to which different 

considerations may be 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 


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254Congress 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. 

255See 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. 

256As 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. 

257 In the past the EPA has asserted the discretion 

to take such grandfathering action, under 

appropriate circumstances, either by rulemaking or 

through a case-by-case determination for a specific 

permit application. The United States Court of 

Appeals for the Ninth Circuit recently vacated a 

decision by the EPA to issue an individual PSD 

permit grandfathering a permit applicant from 

certain requirements. See Sierra Club v. EPA, 762 

F.3d 971 (9th Cir. 2014). In light of that decision, 

the EPA is no longer asserting authority to 

grandfather permit applications on a case-by-case 

basis. However, in the same opinion the court also 

stated that it did ‘‘not doubt, or express any opinion 

on, the EPA’s traditional authority to employ formal 

rulemaking to implement grandfathering’’ and 

distinguished that authority from the permitspecific 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. 

new designations for any revised O3

NAAQS are effective, they generally 

will serve to determine whether the PSD 

or nonattainment NSR program applies. 

1. Prevention of Significant 

Deterioration (PSD) 

The statutory requirements for a PSD 

permit program set forth under part C 

(sections 160 through 169 of the CAA) 

are addressed by the EPA’s PSD 

regulations found at 40 CFR 51.166 

(minimum requirements for an 

approvable SIP) and 40 CFR 52.21 

(federal PSD permit program for areas 

lacking an EPA-approved PSD program 

in the applicable SIP and for lands 

owned by the federal government and 

tribal lands). Both sets of regulations 

already apply to O3. See 40 CFR 

51.166(b)(23), (49); 40 CFR 52.21(b)(23), 

(50).. Among other things, in attainment 

and unclassifiable areas, the PSD 

program requires a new major stationary 

source or a major modification to an 

existing major source to apply BACT for 

each applicable pollutant and to 

conduct an air quality impact analysis 

to demonstrate that the proposed source 

or project will not cause or contribute to 

a violation of any NAAQS or PSD 

increment (see CAA section 165(a)(3)– 

(4), 40 CFR 51.166(j)–(k), 40 CFR 

52.21(j)–(k)). PSD requirements may 

also include, in appropriate cases, an 

analysis of potential adverse impacts on 

Class I areas (see CAA sections 162 and 

165).254 These existing requirements of 

the PSD program will remain applicable 

to O3 and the demonstration required 

under 40 CFR 51.166(k) and 52.21(k) 

will apply to any revised O3 NAAQS 

when such NAAQS become effective, 

except to the extent that a pending 

permit application is subject to a 

grandfathering provision that the EPA 

establishes through rulemaking. 

To address ambient O3 impacts of 

VOC and NOX precursor emissions from 

individual stationary sources, Appendix 

W to 40 CFR part 51 currently directs 

states to consult with the applicable 

EPA Regional Office to determine the 

appropriate techniques on a case-bycase basis, 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,255

the EPA is planning to propose a 

rulemaking in the spring of 2015 to 

consider whether to update Appendix 

W. If the EPA concludes that it is 

technically and scientifically 

appropriate, it will propose appropriate 

regulatory updates to Appendix W as 

part of that rulemaking and may also 

make related updates to technical 

guidance, as appropriate. In the 

meantime, in order to demonstrate that 

a proposed source or modification does 

not cause or contribute to a violation of 

the applicable O3 NAAQS, PSD permit 

applicants would follow the current 

provisions in Appendix W until any 

revisions to them are in effect. 

For PSD, a ‘‘major stationary source’’ 

is one with the potential to emit 250 

tons per year (TPY) or more of any 

regulated NSR pollutant, unless the new 

or modified source is classified under a 

list of 28 source categories contained in 

the statutory definition of ‘‘major 

emitting facility’’ in section 169(1) of 

the CAA. For those 28 source categories, 

a ‘‘major stationary source’’ is one with 

the potential to emit 100 TPY or more 

of any regulated NSR pollutant. A 

‘‘major modification’’ is a physical 

change or a change in the method of 

operation of an existing major stationary 

source that results first, in a significant 

emissions increase of a regulated NSR 

pollutant at a project, and second, in a 

significant net emissions increase of that 

pollutant at the source.256 See 40 CFR 

51.166(b)(2)(i), 40 CFR 52.21(b)(2)(i). 

The EPA’s regulations define the term 

‘‘regulated NSR pollutant’’ to include 

‘‘[a]ny pollutant for which a [NAAQS] 

has been promulgated and any pollutant 

identified [in EPA regulations] as a 

constituent or precursor to such 

pollutant’’ (40 CFR 51.166(b)(49); 40 

CFR 52.21(b)(50)). These regulations 

identify VOC and NOX as precursors to 

O3 in all attainment and unclassifiable 

areas (40 CFR 51.166(b)(49)(i)(a); 40 CFR 

52.21(b)(50)(i)(a)). Thus, for O3, the PSD 

program currently requires the review 

and control of emissions of VOC and 

NOX, as applicable, as precursors of O3. 

As noted above, section 165(a)(3) of 

the CAA and the implementing PSD 

regulations require the owner or 

operator of a proposed facility to, among 

other things, demonstrate that 

‘‘emissions from construction or 

operation of such facility will not cause, 

or contribute to, air pollution in excess 

of any . . . national ambient air quality 

standard in any air control region.’’ See 

also 40 CFR 51.166(k), 40 CFR 52.21(k). 

The EPA has interpreted this 

requirement to include any NAAQS that 

is in effect as of the date a permit is 

issued, unless it has grandfathered 

permit applications from the 

requirement to demonstrate that the 

proposed facility does not cause or 

contribute to a violation of the new or 

revised NAAQS.257 See, e.g., 73 FR 

28321, 28324, 28340 (May 16, 2008); 78 

FR 3253 (Jan. 15, 2013); Memorandum 

from Stephen D. Page, Director, Office of 

Air Quality Planning & Standards, 

‘‘Applicability of the Federal Prevention 

of Significant Deterioration Permit 

Requirements to New and Revised 

National Ambient Air Quality 

Standards’’ (April 1, 2010). Consistent 

with this interpretation, any revised O3

NAAQS finalized through this 

rulemaking will need to be addressed by 

PSD permit applicants and permitting 

authorities, in permits issued on or after 

the date when the revised NAAQS 

become effective, unless the permit 

application has been grandfathered 

through rulemaking, as described below 

in this proposal. 

Because the complex chemistry of O3

formation poses significant challenges 


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258Letter 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. 

259See, e.g., id. 

260See id. at 1 and 3. 

261The 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. 

for the assessing the impacts of 

individual stationary sources on O3

formation, the EPA’s judgment has been 

that it was not technically sound to 

designate a specific air quality model 

that must be used in the PSD permitting 

process to make this demonstration for 

O3.258 The EPA has explained that 

sources must make the demonstration 

required under CAA section 165(a)(3) 

and the implementing regulations, that 

this demonstration necessarily involves 

an analysis, and has established a 

process to determine on a case-by-case 

basis, in consultation with the 

appropriate EPA Regional Office, what 

analytical techniques should be used to 

assess the impact of an individual 

source 259 (40 CFR part 51, Appendix W, 

Section 5.2.1.c). The EPA has, however, 

granted a petition from Sierra Club 

requesting, among other things, that it 

initiate rulemaking to designate air 

quality models for O3, and consistent 

with that petition grant, has been going 

through a process to evaluate potential 

updates to Appendix W.260 While that 

process is underway, individual sources 

should continue to follow the existing 

procedures to determine what method is 

appropriate to use to evaluate their 

impacts on O3 formation. 

The PSD rules in 40 CFR 51.166(i)(2) 

and 52.21(i)(2) contain an exemption for 

particular pollutants from the PSD 

requirements if the owner or operator of 

the source demonstrates that the area in 

which the facility is located is 

designated as nonattainment for that 

pollutant under CAA section 107. Thus, 

new major sources and modifications 

will generally be subject to the PSD 

program requirements for O3 if they are 

locating in an area that does not have a 

current nonattainment designation 

under CAA section 107 for O3. As 

explained in the recent proposal for the 

implementation rule for the 2008 O3

NAAQS, references to historical 

nonattainment designations for a 

revoked standard should not be viewed 

as current ‘‘nonattainment 

designation[s] under CAA section 107’’ 

within the meaning of 40 CFR 

51.166(i)(2) and 52.21(i)(2) and, 

therefore, do not trigger the exemption 

from PSD requirements that would 

otherwise result from those provisions 

(78 FR 34216, June 6, 2013). 

a. PSD Grandfathering Provision 

Recognizing that some PSD 

applications may have already been 

submitted and could be in the review 

process when a revised O3 NAAQS 

becomes effective, the EPA is proposing 

a transition plan that would enable 

certain PSD applications to make the 

demonstration that the proposed project 

will not cause or contribute to a 

violation of any NAAQS with respect to 

the O3 NAAQS that were in effect on the 

date the reviewing authority determines 

the permit application complete or the 

date the public notice on the draft 

permit or preliminary determination is 

first published (depending on which 

grandfathering provision applies), rather 

than the revised O3 NAAQS.261

The EPA is proposing and taking 

comment on adding a grandfathering 

provision to EPA’s regulations at 40 CFR 

51.166 and 52.21 that would apply 

specifically to two categories of PSD 

permit applications that are pending 

when the EPA issues the revised O3

NAAQS: (1) Applications for which the 

reviewing authority has formally 

determined that the application is 

complete on or before the signature date 

of the revised NAAQS; and (2) 

applications for which the reviewing 

authority has first published a public 

notice of a draft permit or preliminary 

determination before the effective date 

of the revised NAAQS. These two 

categories are proposed because some 

states do not do completeness 

determinations as part of their permit 

process. 

As explained above, the EPA 

interprets the CAA and implementing 

PSD regulations at 40 CFR 52.21(k)(1) 

and 51.166(k)(1) to require that PSD 

permit applications must include a 

demonstration that new major sources 

and major modifications will not cause 

or contribute to a violation of any 

NAAQS that is in effect as of the date 

the PSD permit is issued. Thus, if the 

EPA revises the O3 NAAQS, any 

proposed new source or modification 

with a PSD permit application pending 

at the time the revised O3 NAAQS takes 

effect would be expected to conduct an 

analysis to demonstrate that it does not 

cause or contribute to a violation of that 

NAAQS, absent some type of transition 

provision exempting the application 

from that requirement. This 

demonstration, as noted above, should 

be completed in consultation with the 

applicable EPA Regional Office. 

Nevertheless, the agency has 

previously recognized that the CAA 

provides discretion for the EPA to 

grandfather PSD permit applications 

from requirements that become 

applicable while the applications are 

pending (45 FR 52683, August 7, 1980; 

52 FR 24672, July 1, 1987; 78 FR 3086, 

January 15, 2013). As discussed in more 

detail in these referenced actions, 

section 165(a)(3) of the CAA requires 

that a permit applicant demonstrate that 

its proposed project will not cause or 

contribute to a violation of any NAAQS. 

At the same time, section 165(c) of the 

CAA requires that a PSD permit be 

granted or denied within 1 year after the 

permitting authority determines the 

application for such permit to be 

complete. In addition, section 301 of the 

CAA authorizes the Administrator ‘‘to 

prescribe such regulations as are 

necessary to carry out his functions 

under this chapter.’’ When read in 

combination, these three provisions of 

the CAA provide the EPA with the 

discretion to issue regulations to 

grandfather pending permit applications 

from having to address a revised 

NAAQS where necessary to achieve 

both CAA objectives to protect the 

NAAQS and to avoid delays in 

processing PSD permit applications. 

Moreover, in a recent opinion the U.S. 

Court of Appeals for the Ninth Circuit 

recognized the EPA’s traditional 

exercise of grandfathering authority 

through rulemaking and indicated that 

this approach was consistent with 

statutory requirement to ‘‘enforce 

whatever regulations are in effect at the 

time the agency makes a final decision’’ 

because it involved identifying ‘‘an 

operative date, incident to setting the 

new substantive standard, and the 

grandfathering of pending permit 

applications was explicitly built into the 

new regulations.’’ Sierra Club v. EPA, 

762 F.3d 971, 983 (9th Cir. 2014). 

In the EPA’s most recent adoption of 

a grandfathering provision for PSD, it 

adopted a provision for PM2.5 that 

provides a reasonable transition for 

implementing certain new PSD 

requirements related to the 2012 PM2.5

NAAQS for pending permit applications 

that have met certain criteria. As 

finalized, the PM2.5 grandfathering 

provision included the same two 

categories of permit applications that 


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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 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 

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 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 

The EPA has historically allowed the 

use of screening tools to help facilitate 

the implementation of the NSR program 


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262This language is contained in a footnote in the 

PSD regulations at 40 CFR 51.166(i)(5)(i) and 

52.21(i)(5)(i), and it has not been revisited by the 

EPA since the issuance of the 8-hour O3 NAAQS. 

These values do not reflect a categorical conclusion 

by the EPA that sources emitting less than 100 tpy 

of VOCs or NOX will not cause or contribute to a 

violation of the current (or any revised) O3 NAAQS, 

nor does it reflect a conclusion that such sources 

should be categorically excluded from the 

requirement for an ambient impact analysis. 

Instead, the EPA recommends consultation with the 

appropriate EPA Regional Office in accordance with 

section 5.2.1.c of Appendix W when a review of an 

application for a new source or modification 

involves emissions less than 100 tpy of either O3

precursor. See Letter from Gina McCarthy, Assistant 

Administrator, to Robert Ukeiley, at 4 (Jan. 4, 2012), 

available at http://www.epa.gov/scram001/

10thmodconf/review_material/Sierra_Club_

Petition_OAR-11-002-1093.pdf. 

263Any 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. 

264See, 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). 

265See, 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). 

266 78 FR 3261 (January 15, 2013); Stephen D. 

Page, Director, Office of Air Quality Planning and 

Standards, to Regional Air Division Directors, 

‘‘Guidance Concerning Implementation of the 1- 

hour SO2 NAAQS for the Prevention of Significant 

Deterioration Program,’’ (August 23, 2010). 

267The 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. 

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.’’ 262 The EPA intends to consider 

whether it is appropriate to make any 

revisions to the PSD regulations related 

to the screening tools for O3 in a 

separate rulemaking that will 

specifically address various 

implementation issues for O3. However, 

there are no such revisions being 

proposed in today’s rulemaking. Until 

any rulemaking to amend existing 

regulations is completed, permitting 

decisions should continue to be based 

on the existing 40 TPY SER for O3

precursors (NOX and VOC) in existing 

regulations. Further decisions regarding 

the need for an analysis to assess the 

impact of an individual source on the 

O3 NAAQS and the method of analysis 

depend on the nature of the source and 

its emissions, and, as noted above, 

should be determined in consultation 

with the EPA Regional Office on a caseby-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.263 This raises the question 

as to how a source can be issued a PSD 

permit in light of known existing 

ambient violations of the revised 

NAAQS. 

Section 165(a)(3)(B) of the CAA 

requires that a proposed source may not 

construct unless it demonstrates that it 

will not cause or contribute to a 

violation of any NAAQS. This statutory 

requirement is implemented through a 

provision contained in the PSD 

regulations at 40 CFR 51.166(k) and 

52.21(k). If a source cannot make this 

demonstration or if its initial air quality 

impact analysis shows that the source’s 

impact does cause or contribute to a 

violation, a PSD permit may not be 

issued until that adverse impact is 

mitigated.264 The PSD regulations, 

however, do not explicitly specify 

remedial actions that a prospective 

source can take to address such a 

situation. Nevertheless, the EPA has 

historically recognized in regulations 

and through other actions that sources 

applying for PSD permits may utilize 

offsets as part of the required PSD 

demonstration under the CAA section 

165(a)(3)(B), even though the PSD 

provisions of the Act do not expressly 

reference offsets, in contrast to the 

NNSR provisions of the Act.265

The EPA has looked to the procedures 

contained in a separate set of 

regulations at 40 CFR 51.165(b) to guide 

the process by which a source that is 

located in an area designated as 

attainment or unclassifiable for a 

NAAQS, but that is determined to cause 

or contribute to a violation of that 

NAAQS in any area, can use offsets to 

mitigate its adverse impact on the 

NAAQS and ultimately meet the PSD 

demonstration requirement under CAA 

section 165(a)(3)(B) and the 

implementing regulations.266

Section 51.165(b) states that plans 

shall include a preconstruction review 

permit program (or its equivalent) to 

satisfy the requirements of CAA section 

110(a)(2)(D)(i) for major sources and 

major modifications, and that the 

program shall apply to any major 

stationary source or major modification 

locating in an area designated 

attainment or unclassifiable for any 

NAAQS, when that source would cause 

or contribute to a NAAQS violation.267

Paragraph (b)(3) of that regulation 

provides that the required permit 

program may include a provision 

allowing a proposed major source or 

major modification to reduce the impact 

of its emissions on air quality by 

obtaining sufficient emissions 

reductions to, at a minimum, 

compensate for its adverse ambient 

impact where the source or modification 

would otherwise cause or contribute to 

a violation of any NAAQS. Although 

section 51.165(b) refers explicitly to 

CAA section 110(a)(2)(D)(i), which now 

addresses transport issues, but not CAA 

section 165(a)(3)(B), the EPA has 

previously explained that 51.165(b) may 

also be interpreted to apply to the 

section 165(a)(3)(B) demonstration 


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268Briefly, in 1980, the EPA had determined that 

the statutory requirements under CAA section 

165(a)(3)(B), taken together with the requirements 

of CAA section 110(a)(2)(D) required all major 

sources locating outside a nonattainment area, but 

causing or contributing to a NAAQS violation to 

reduce the impact on air quality so as to assure 

attainment and maintenance of the NAAQS. 45 FR 

31310 (May 13, 1980). In a footnote, the EPA further 

indicated that this offset requirement must apply to 

sources causing or contributing to a newly 

discovered NAAQS violation until the area is 

designated nonattainment. See 45 FR 31310 (May 

13, 1980). In the 1980 rule, the EPA adopted section 

51.18(k), which was later renumbered section 

51.165(b). The EPA revised 51.165(b) to expressly 

authorize an offset program to meet the 

requirements of CAA section 110(a)(2)(D)(i), but 

this provision may also be interpreted to apply to 

section 165(a)(3)(B), consistent with the EPA’s 

reading of section 51.18(k) in 1980. It is also worth 

noting that at the time of these rules, before the 

1990 CAA amendments, section 110(a)(2)(D) 

required each state to have ‘‘a permit or equivalent 

program for any major emitting facility . . . to 

assure (i) that national air quality standards are 

achieved and maintained . . ..’’ 

269See Appendix S, Part I; 40 CFR 52.24(k). 

270As 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 CFR 52.24(k). 

based on the regulatory history (78 FR 

3262, n. 256).268

Generally, the offset needed to 

compensate for a proposed source’s 

adverse impact would be determined by 

the ability of any particular emissions 

reduction to mitigate the source’s 

adverse impact at the location of the 

violation. As long as the emissions 

reduction or offset can be shown to 

compensate for the source’s adverse 

impact, there is no implied requirement 

that the amount of the emissions 

reduction be equal to or greater than the 

proposed emissions increase. See 44 FR 

3278 (January 16, 1979). (‘‘Although full 

emissions offsets are not required, such 

a source must obtain emissions offsets 

sufficient to compensate for it air 

quality impact where the violation 

occurs.’’) 

In previous discussions of the use of 

emissions offsets to help make the 

demonstration required under CAA 

section 165(a)(3)(B), the EPA has 

explained that any emissions used for 

PSD purposes must meet applicability 

criteria that are at least as stringent as 

the offset criteria set forth in the NNSR 

requirements for offsets under 40 CFR 

51.165(a)(3). See 78 FR 3262. The EPA 

continues to believe that these criteria 

provide the most appropriate criteria for 

determining the creditability of PSD 

offsets. 

d. PSD for a Distinct Secondary 

Standard, if One Is Established 

As noted above, the CAA requires that 

proposed new major stationary sources 

and major modifications demonstrate 

that their emissions increases will not 

cause or contribute to a violation of any 

NAAQS, which includes the primary 

and secondary NAAQS. For O3, the 

existing primary and secondary NAAQS 

are defined in the same form and at the 

same level. As described earlier in this 

preamble, the Administrator is 

proposing to retain the current 

averaging time and form and revise the 

level of the current secondary standard 

to within the range of 70 to 65 ppb. In 

addition, among other things, the 

agency is seeking comment on the 

alternative approach of revising the 

secondary standard to establish a 

distinct O3 secondary standard. If the 

agency were to finalize a secondary 

standard that differs from the primary 

standard, PSD permit applicants would 

be required to provide an analysis that 

specifically addresses the revised 

secondary standard and make the 

necessary showing of compliance with 

that standard, as well as any revised 

primary standard. Moreover, if such a 

secondary standard is expressed in a 

distinctly different form than the 

primary standard, the required analysis 

for making the compliance 

demonstration would need to be 

consistent with that form. 

Should the Administrator decide to 

establish a distinct secondary NAAQS 

for O3, the EPA would consider whether 

the approaches put forth in any 

regulatory updates to Appendix W and 

associated guidance, as noted in this 

preamble above, are sufficient for 

making the necessary compliance 

demonstration for that standard for 

purposes of PSD. If appropriate, the EPA 

may consider establishing a surrogacy 

policy that would allow a source to 

make the PSD-required demonstration 

of compliance with a distinct secondary 

O3 NAAQS solely through a 

demonstration of compliance with the 

primary NAAQS. Therefore, the EPA 

expects that projects subject to the 

revised O3 NAAQS could generally 

move forward consistent with the PSD 

program requirements and NNSR 

program requirements as subject to the 

revised primary and secondary O3

NAAQS. The EPA seeks comment on 

this potential approach as well as any 

other options that should be considered 

for showing compliance with any 

revised primary and secondary O3

NAAQS. 

2. Nonattainment New Source Review 

Part D of title I of the CAA includes 

preconstruction review and permitting 

requirements for new major stationary 

sources and major modifications when 

they locate in areas designated 

nonattainment for a particular pollutant. 

As explained in section VII.D.1 of this 

preamble, the relevant part D 

requirements are typically referred to as 

the NNSR program. The EPA’s 

regulations for the NNSR programs are 

contained in 40 CFR 51.165, 52.24 and 

Part 51, Appendix S. For example, the 

EPA has developed minimum program 

requirements for an NNSR program that 

is approvable in a SIP, and those 

requirements, which include 

requirements for O3, are contained in 40 

CFR 51.165. In addition, 40 CFR part 51, 

Appendix S contains requirements 

constituting an interim NNSR program. 

This program governs NNSR permitting 

in nonattainment areas that lack a SIPapproved NNSR permitting program, 

and applies during the time between the 

date of the relevant designation and the 

date that the EPA approves into the SIP 

a NNSR program.269 This program is 

commonly known as the Emissions 

Offset Interpretative Rule, and is 

applicable to O3 as well.270

As with PSD, the NNSR requirements 

apply on a pollutant-specific basis. 

However, in nonattainment areas, NNSR 

applies only to nonattainment 

pollutants, that is, pollutants for which 

an area is designated nonattainment on 

the date when the permit is issued. As 

explained in section VII.D.1 of this 

preamble, prior to the designation of 

areas for any revised O3 NAAQS, 

applicability of either PSD or NNSR for 

O3 to a proposed major new or modified 

source will depend on an area’s current 

designations with regard to the O3

NAAQS. Accordingly, a major 

stationary source or major modification 

proposing to locate in an area currently 

designated nonattainment for the 2008 

O3 NAAQS must satisfy the NNSR 

permit requirements for O3. The EPA is 

not proposing any new or revised NNSR 

requirements in this proposal. As 

explained in section VII.A.2 of this 

preamble, the CAA requires that area 

designations for new or revised NAAQS 

be addressed subsequent to the effective 

date of such new or revised NAAQS. 

The EPA anticipates that the area 

designation process for any revised O3

NAAQS will be completed within 2 

years after the revised NAAQS become 

effective. Accordingly, any revisions to 

the existing NNSR requirements for O3

will be proposed at a later date 

consistent with the designation process 

for any revised O3 NAAQS. The EPA 

will also at the same time propose any 

necessary revisions to the NNSR 

requirements under Appendix S so that 

states will be able to issue NNSR 

permits for the revised O3 NAAQS on 

and after the effective date of 


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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 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? 

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 Web site 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 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 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 


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271Note that the relief mechanisms discussed 

here do not include the CAA’s interstate transport 

provisions found in sections 110(a)(2)(D) and 126. 

The interstate transport provisions are intended to 

address the cross-state transport of O3 and O3

precursor emissions from man-made sources within 

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 

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 

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 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: 271


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the continental U.S. rather than background O3 as 

it is defined in this section. 

272A 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.’’ 

273Federal Register (2007). Treatment of Data 

Influenced by Exceptional Events; Final Rule. 40 

CFR 50 and 51; Federal Register 72:13560. 

274The 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. 

275U.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. 

276U.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. 

• 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 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, non-routine 

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 

event-influenced exceedances or 

violations of a NAAQS, including the 

proposed O3 NAAQS, provided the 

event meets the statutory requirements 

in section 319 of the CAA: 

• The event ‘‘affects air quality.’’ 

• The event ‘‘is not reasonably 

controllable or preventable.’’ 

• The event is ‘‘caused by human 

activity that is unlikely to recur at a 

particular location or [is] a natural 

event.’’ 272

The EPA’s implementing regulations, 

the 2007 Exceptional Events Rule, 

further specify that states must provide 

evidence that: 273

• ‘‘There is a clear causal relationship 

between the measurement under 

consideration and the event that is 

claimed to have affected the air quality 

in the area;’’ 

• ‘‘The event is associated with a 

measured concentration in excess of 

normal historical fluctuations, including 

background;’’ and 

• ‘‘There would have been no 

exceedance or violation but for the 

event.’’ 

The ISA contains discussions of 

natural events that may contribute to O3

or O3 precursors. These include 

stratospheric O3 intrusion and wildfire 

events.274 As indicated above, to satisfy 

the exceptional event requirements and 

to qualify for data exclusion under the 

Exceptional Events Rule, a state must 

develop and submit evidence 

addressing each of the identified 

criteria. The extent to which a 

stratospheric O3 intrusion event or a 

wildfire event contribute to O3 levels 

can be uncertain, and in most cases 

requires detailed investigation and 

analysis to adequately determine. 

Strong stratospheric O3 intrusion 

events, most prevalent at high elevation 

sites during winter or spring, can be 

identified based on measurements of 

low relative humidity, evidence of deep 

atmospheric mixing, and a low ratio of 

CO to O3 based on ambient 

measurements. Accurately determining 

the extent of weaker intrusion events 

remains challenging (U.S. EPA 2013a, p. 

3–34). Although states have submitted 

only a few exceptional event 

demonstrations for stratospheric O3

intrusion, the EPA recently approved a 

demonstration from Wyoming for a June 

2012 stratospheric O3 event.275

While stratospheric O3 intrusions can 

increase monitored ground-level 

ambient O3 concentrations, wildfire 

plumes can either suppress or enhance 

O3 depending upon a variety of factors 

including fuel type, combustion stage, 

plume chemistry, aerosol effects, 

meteorological conditions and distance 

from the fire (Jaffe and Wigder, 2012). 

As such, determining the impact of 

wildfire emissions on specific O3

observations is challenging. The EPA 

recently approved an exceptional event 

demonstration for wildfires affecting 1- 

hour O3 levels in Sacramento, California 

in 2008 that successfully used a variety 

of analytical tools (e.g., regression 

modeling, back trajectories, satellite 

imagery, etc.) to support the exclusion 

of O3 data affected by large fires.276

Because of previously expressed 

stakeholder feedback regarding 

implementation of the Exceptional 

Events Rule and specific stakeholder 

concerns regarding the analyses that can 

be used to support O3-related 

exceptional event demonstrations, the 

EPA intends to propose revisions to the 

Exceptional Events Rule in a future 

notice and comment rulemaking effort 

and will solicit public comment at that 

time. 

Additionally, the EPA intends to 

develop guidance to address 

implementing the Exceptional Events 

Rule criteria for wildfires that could 

affect ambient O3 concentrations. 

Wildfire emissions are a component of 

background O3 (Jaffe and Wigder, 2012) 

and can significantly contribute to 

periodic high O3 levels (Emery, 2012). 

Besides their effect on air quality, 

wildfires pose a direct threat to public 

safety—a threat that can be mitigated 

through management of wildland 

vegetation. Attempts to suppress 

wildfires have resulted in unintended 

consequences, including increased risks 

to both humans and ecosystems. Indeed, 

‘‘Fire policy that focuses on [wildfire] 

suppression only, delays the inevitable, 

promising more dangerous and 

destructive future . . . fires’’ (Stephens, 

S. et al., 2013). The use of wildland 


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277Note 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. 

278For 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. 279U.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. 

280U.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. 

prescribed fire can influence the 

occurrence of catastrophic wildfires 

which may help manage the 

contribution of wildfires to background 

O3 levels and periodic peak O3 events. 

Additionally prescribed fires can have 

benefits to those plant and animal 

species that depend upon natural fires 

for propagation, habitat restoration, and 

reproduction, as well as myriad 

ecosystem functions (e.g., carbon 

sequestration). As previously indicated, 

the CAA and the EPA’s implementing 

regulations allow for the exclusion of air 

quality monitoring data from design 

value calculations when they are 

substantially affected by certain 

background influences. Additionally, 

the CAA requires the EPA to set the 

NAAQS at levels requisite to protect 

public health and welfare without 

regard to the source of the pollutant. 

However, EPA understands the 

importance of prescribed fire which 

mimics a natural process necessary to 

manage and maintain fire-adapted 

ecosystems and climate change 

adaptation, while reducing risk of 

uncontrolled emissions from 

catastrophic wildfires. The EPA is 

committed to working with federal land 

managers, tribes, and states to 

effectively manage prescribed fire use to 

reduce the impact of wildland-fire 

related emissions on ozone through 

policies and regulations implementing 

these standards. 

3. Rural Transport Areas 

Clean Air Act section 182(h) 

authorizes the EPA Administrator to 

determine that an area designated 

nonattainment can be treated as a rural 

transport area. In accordance with the 

statute, a nonattainment area may 

qualify for this determination if it meets 

the following criteria: 

• The area does not contain emissions 

sources that make a significant 

contribution to monitored O3

concentrations in the area, or in other 

areas; and 

• The area does not include and is 

not adjacent to a Metropolitan Statistical 

Area.277

Historically, the EPA has recognized 

few nonattainment areas under this 

statutory provision.278 The EPA has not 

issued separate written guidance to 

further elaborate on the interpretation of 

these CAA qualification criteria. 

However, the EPA developed draft 

guidance in 2005 that explains the kinds 

of technical analyses that states could 

use to establish that transport of O3 and/ 

or O3 precursors into the area is so 

overwhelming that the contribution of 

local emissions to an observed 8-hour 

O3 concentration above the level of the 

NAAQS is relatively minor and 

determine that emissions within the 

area do not make a significant 

contribution to the O3 concentrations 

measured in the area or in other 

areas.279 While this guidance was not 

prepared specifically for rural transport 

areas, it could be useful to states for 

developing technical information to 

support a request that the EPA treat a 

specific O3 nonattainment area as a rural 

transport area. 

An area that qualifies for treatment as 

a rural transport area is deemed to have 

fulfilled all O3-related planning and 

control requirements if it meets the 

CAA’s requirements for areas classified 

Marginal, which is the lowest 

classification specified in the CAA. 

Therefore, a state would not need to 

develop an attainment plan or an 

attainment demonstration for such an 

area or adopt the various mandatory 

measures required in nonattainment 

areas classified as Moderate or above. 

The only requirements that would 

apply, regardless of the level of O3 air 

quality, would be NNSR (at the 

Marginal major source threshold and 

offset ratio), conformity requirements 

associated with a nonattainment 

designation, and the emission inventory 

and source emission statement 

requirements. 

4. International Transport 

Clean Air Act section 179B recognizes 

the possibility that certain 

nonattainment areas may be heavily 

impacted by O3 or O3 precursor 

emissions from international sources 

beyond the regulatory jurisdiction of the 

state. The EPA’s science review suggests 

that the influence of international 

sources on U.S. O3 levels will be largest 

in locations that are in the immediate 

vicinity of an international border with 

Canada or Mexico, but other locations 

can also potentially be affected when 

conditions are favorable for long-range 

transport (U.S. EPA 2013a, p.3–140). 

Section 179B allows states to consider 

in their attainment demonstrations 

whether an area might have met the O3

NAAQS by the attainment date ‘‘but 

for’’ emissions contributing to the area 

originating outside the U.S. If a state is 

unable to demonstrate attainment in 

such an area after adopting all 

reasonably available control measures 

(RACM, including RACT, as required by 

CAA section 182(b)), the EPA can 

nonetheless approve the CAA-required 

state attainment plan and demonstration 

using the authority in section 179B. 

When the EPA approves this type of 

attainment plan, states avoid potential 

sanctions and FIPs, and there would be 

no adverse consequence for a finding 

that the area failed to attain the NAAQS 

by the relevant attainment date. For 

example, the area would not be 

reclassified to the next highest 

classification or required to implement 

a section 185 penalty fee program. 

Section 179B authority does not allow 

the EPA to avoid designating an area as 

nonattainment or for the area to be 

classified with a lower classification 

than is indicated by actual air quality. 

Generally, monitoring data influenced 

by international transport may not be 

excluded from regulatory 

determinations, unless the data are 

influenced by an excludable exceptional 

event. Section 179B also does not 

provide for any relaxation of mandatory 

emissions control measures (including 

contingency measures) or the prescribed 

emissions reductions necessary to 

achieve RFP. 

The EPA’s guidance on ‘but for’ 

demonstrations involving international 

emissions indicates that states may want 

to consider conducting air quality 

modeling using O3 episodes that do not 

involve international transport of 

emissions (U.S. EPA 1991)280, running 

the model with boundary conditions 

that reflect general U.S. background 

concentrations, and analyzing 

monitoring data if a dense network has 

been established. Additional 

information that may be helpful at 

nonattainment areas abutting 

international borders could include 

evaluating changes in O3 with changes 

in wind direction at monitors near the 

border, and comparing emissions on 

both sides of the border. States are 

encouraged to consult with their EPA 

Regional Office to establish appropriate 


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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 Jua ́rez, 

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 Jua ́rez were not available. 

VIII. Statutory and Executive Order 

Reviews 

Additional information about these 

statutes and Executive Orders can be 

found at http://www2.epa.gov/lawsregulations/laws-and-executive-orders. 

A. Executive Order 12866: Regulatory 

Planning and Review and Executive 

Order 13563: Improving Regulation and 

Regulatory Review 

This action is an economically 

significant regulatory action that was 

submitted to the Office of Management 

and Budget (OMB) for review. Any 

changes made in response to OMB 

recommendations have been 

documented in the docket. The EPA 

prepared an analysis of the potential 

costs and benefits associated with this 

action. This analysis is contained in the 

document, Regulatory Impact Analysis 

for the O3 NAAQS, November, 2014. A 

copy of the analysis is available in the 

RIA docket (EPA–HQ–OAR–2013–0169) 

and the analysis is briefly summarized 

here. The RIA estimates the costs and 

monetized human health and welfare 

benefits of attaining four alternative O3

NAAQS nationwide. Specifically, the 

RIA examines the alternatives of 60 ppb, 

65 ppb, 70 ppb, and 75 ppb. The RIA 

contains illustrative analyses that 

consider a limited number of emissions 

control scenarios that states and 

Regional Planning Organizations might 

implement to achieve these alternative 

O3 NAAQS. However, the CAA and 

judicial decisions make clear that the 

economic and technical feasibility of 

attaining ambient standards are not to 

be considered in setting or revising 

NAAQS, although such factors may be 

considered in the development of state 

plans to implement the standards. 

Accordingly, although an RIA has been 

prepared, the results of the RIA have not 

been considered in issuing this 

proposed rule. 

B. Paperwork Reduction Act 

The information collection 

requirements in this proposed rule have 

been submitted for approval to the 

Office of Management and Budget 

(OMB) under the Paperwork Reduction 

Act (PRA). The Information Collection 

Request (ICR) document prepared by 

EPA has been assigned EPA ICR 

#2313.03. You can find a copy of the 

ICR in the docket for this rule, and it is 

briefly summarized here. 

The information collected and 

reported under 40 CFR part 58 is needed 

to determine compliance with the 

NAAQS, to characterize air quality and 

associated health and ecosystems 

impacts, to develop emission control 

strategies, and to measure progress for 

the air pollution program. We are 

proposing to extend the length of the 

required O3 monitoring season in 33 

states and propose that the revised O3

monitoring seasons become effective on 

January 1, 2017. We are also proposing 

revisions to the PAMS monitoring 

requirements that reduce the number of 

required PAMS sites while improving 

spatial coverage, and proposing to 

require states with O3 non-attainment 

areas to develop an enhanced 

monitoring plan as part of the PAMS 

requirements. For areas currently 

designated as nonattainment for O3

based on the 2008 NAAQS, we propose 

that these areas comply with the PAMS 

requirements by June 1, 2017. For new 

areas designated based on a revised 

NAAQS, if finalized, we propose that 

those areas comply with the PAMS 

requirements by January 1, 2019. In 

addition, we are proposing to revise the 

O3 FRM to establish a new, additional 

technique for measuring O3 in the 

ambient air. We propose that it be 

incorporated into the existing O3 FRM, 

using the same calibration procedure in 

Appendix D of 40 CFR part 50. We also 

propose to make changes to the 

procedures for testing performance 

characteristics and determining 

comparability between candidate FEMs 

and reference methods. 

For the purposes of ICR #2313.03, the 

burden figures represent the burden 

estimate based on the requirements 

contained in the proposed rule. The 

burden estimates are for the 3-year 

period from 2015 through 2017. The 

implementation of the PAMS changes, if 

finalized, will occur beyond the time 

frame of this ICR with likely 

implementation dates between 2017 and 

2019. The cost estimates for the PAMS 

network (including proposed revisions) 

will be captured in future routine 

updates to the Ambient Air Quality 

Surveillance ICR that are required every 

3 years by OMB. The proposal for a new 

FRM in 40 CFR part 50 and revisions to 

the O3 FEM procedures for testing 

performance characteristics in 40 CFR 

part 53 does not add any additional 

information collection requirements. 

The ICR burden estimates are 

associated with the proposed changes to 

the O3 seasons. This information 

collection is estimated to involve 158 

respondents for a total cost of 

approximately $24,115,182 (total 

capital, labor, and operation and 

maintenance) plus a total burden of 

339,930 hours for the support of all 

operational aspects of the entire O3

monitoring network. The labor costs 

associated with these hours are 

$19,813,692. Also included in the total 

are other costs of operations and 

maintenance of $2,210,132 and 

equipment and contract costs of 

$2,091,358. The actual labor cost 

increase to expand the O3 monitoring 

seasons is $1,668,433. In addition to the 

costs at the state, local, and tribal air 

quality management agencies, there is a 

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 EPA–HQ–OAR–2008–0699. 

Submit any comments related to the ICR 

to EPA and OMB. Send comments to the 

EPA at the Air and Radiation Docket 

and Information Center Docket in the 

EPA Docket Center (EPA/DC), EPA 

West, Room 3334, 1301 Constitution 

Ave. NW., Washington, DC. The Docket 

Center Public Reading Room is open 

from 8:30 a.m. to 4:30 p.m., Monday 

through Friday, excluding legal 

holidays. The telephone number for the 

Reading Room is (202) 566–1744, and 

the telephone number for the Air and 

Radiation Docket and Information 

Center Docket is (202) 566–1742. An 

electronic version of the public docket 

is available at www.regulations.gov. 

Send comments to OMB at the Office of 


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75386 Federal Register / Vol. 79, No. 242 / Wednesday, December 17, 2014 / Proposed Rules 

281As 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. 

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 December 17, 2014, a 

comment to OMB is best assured of 

having its full effect if OMB receives it 

by December 17, 2014. The final rule 

will respond to any OMB or public 

comments on the information collection 

requirements contained in this proposal. 

C. Regulatory Flexibility Act 

I certify that this action will not have 

a significant economic impact on a 

substantial number of small entities 

under the Regulatory Flexibility Act 

(RFA). The reason is that this proposed 

rule will not impose any requirements 

on small entities. Rather, this rule 

establishes national standards for 

allowable concentrations of O3 in 

ambient air as required by section 109 

of the CAA. See also American Trucking 

Associations v. EPA, 175 F. 3d at 1044– 

45 (NAAQS do not have significant 

impacts upon small entities because 

NAAQS themselves impose no 

regulations upon small entities). 

Similarly, the proposed revisions to 40 

CFR part 58 address the requirements 

for states to collect information and 

report compliance with the NAAQS and 

will not impose any requirements on 

small entities. Similarly, the addition of 

a new FRM in 40 CFR part 50 and 

revisions to the FEM procedures for 

testing in 40 CFR part 53 will not 

impose any requirements on small 

entities. 

D. Unfunded Mandates Reform Act 

This action does not contain any 

unfunded mandate as described in the 

Unfunded Mandates Reform Act 

(UMRA), 2 U.S.C. 1531—1538, and does 

not significantly or uniquely affect small 

governments. Furthermore, as indicated 

previously, in setting a NAAQS the EPA 

cannot consider the economic or 

technological feasibility of attaining 

ambient air quality standards, although 

such factors may be considered to a 

degree in the development of state plans 

to implement the standards. See also 

American Trucking Associations v. 

EPA, 175 F. 3d at 1043 (noting that 

because the EPA is precluded from 

considering costs of implementation in 

establishing NAAQS, preparation of a 

Regulatory Impact Analysis (RIA) 

pursuant to the Unfunded Mandates 

Reform Act would not furnish any 

information which the court could 

consider in reviewing the NAAQS). 

E. Executive Order 13132: Federalism 

This action does not have federalism 

implications. It will not have substantial 

direct effects on the states, on the 

relationship between the national 

government and the states, or on the 

distribution of power and 

responsibilities among the various 

levels of government. 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

This action does not have tribal 

implications, as specified in Executive 

Order 13175. It does not have a 

substantial direct effect on one or more 

Indian Tribes as tribes are not obligated 

to adopt or implement any NAAQS. In 

addition, tribes are not obligated to 

conduct ambient monitoring for O3 or to 

adopt the ambient monitoring 

requirements of 40 CFR part 58. Thus, 

Executive Order 13175 does not apply 

to this rule. 

The EPA specifically solicits 

comment on this rule from tribal 

officials. Prior to finalization of this 

proposal, the EPA intends to conduct 

outreach consistent with the EPA Policy 

on Consultation and Coordination with 

Indian Tribes. Outreach to tribal 

environmental professionals will be 

conducted through participation in the 

Tribal Air call, which is sponsored by 

the National Tribal Air Association. In 

addition, the EPA intends to offer 

formal consultation to the tribes during 

the public comment period. If 

consultation is requested, a summary of 

the result of that consultation will be 

presented in the notice of final 

rulemaking and will be available in the 

docket. 

G. Executive Order 13045: Protection of 

Children From Environmental Health & 

Safety Risks 

This action is subject to Executive 

Order 13045 because it is an 

economically significant regulatory 

action as defined by Executive Order 

12866, and the EPA believes that the 

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 populations281 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 at-risk lifestage, we have 

carefully evaluated the environmental 

health effects of exposure to O3

pollution among children. Discussions 

of the results of the evaluation of the 

scientific evidence, policy 

considerations, and the exposure and 

risk assessments pertaining to children 

are contained in sections II.B and II.C of 

this preamble. 

H. Executive Order 13211: Actions That 

Significantly Affect Energy Supply, 

Distribution, or Use 

This action is not a ‘‘significant 

energy action’’ because it is not likely to 

have a significant adverse effect on the 

supply, distribution, or use of energy. 

The purpose of this rule is to establish 

revised NAAQS for O3, establish an 

additional FRM, revise FEM procedures 

for testing, and revises air quality 

surveillance requirements. The rule 

does not prescribe specific pollution 

control strategies by which these 

ambient standards and monitoring 

revisions will be met. Such strategies 

will be developed by states on a caseby-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 


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allow for the approval of an FEM for O3

that is similar to the FRM. Any method 

that meets the performance criteria for 

a candidate equivalent method may be 

approved for use as an FEM. This 

approach is consistent with EPA’s 

PBMS. The PBMS approach is intended 

to be more flexible and cost-effective for 

the regulated community; it is also 

intended to encourage innovation in 

analytical technology and improved 

data quality. The EPA is not precluding 

the use of any method, whether it 

constitutes a voluntary consensus 

standard or not, as long as it meets the 

specified performance criteria. 

J. Executive Order 12898: Federal 

Actions To Address Environmental 

Justice in Minority Populations and 

Low-Income Populations 

The EPA believes the human health or 

environmental risk addressed by this 

action will not have potential 

disproportionately high and adverse 

human health or environmental effects 

on minority, low-income or indigenous 

populations because it does not affect 

the level of protection provided to 

human health or the environment. This 

action proposes the strengthening of the 

O3 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 Web site at http://

www.epa.gov/ttn/naaqs/standards/

ozone/s_o3_index.html. 

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List of Subjects 

40 CFR Part 50 

Environmental protection, Air 

pollution control, Carbon monoxide, 

Lead, Nitrogen dioxide, Ozone, 

Particulate matter, Sulfur oxides. 


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75396 Federal Register / Vol. 79, No. 242 / Wednesday, December 17, 2014 / Proposed Rules 

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 

Environmental protection, 

Administrative practice and procedure, 

Air pollution control, Reporting and 

recordkeeping requirements. 

40 CFR Part 58 

Environmental protection, 

Administrative practice and procedure, 

Air pollution control, Intergovernmental 

relations, Reporting and recordkeeping 

requirements. 

Dated: November 25, 2014. 

Gina McCarthy, 

Administrator. 

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 paragraph (c)(2)(iii); 

■ b. Removing and reserving paragraphs 

(c)(2)(iv) and (v); 

■ c. Revising paragraph (c)(2)(vi) 

introductory text and Table 1; 

■ d. Revising paragraph (c)(3)(i); and 

■ e. Removing and reserving paragraphs 

(c)(3)(ii) and (iii); 

■ 3. The revisions read 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. 

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 schedule d

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 4 b

and 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 a 2-year 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. 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. 

(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


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concentration is less than or equal to 

(0.065–0.070) ppm, as determined in 

accordance with appendix U to this 

part. 

(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 fourthhighest 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 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. 

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 

(a) 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 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 Beer-Lambert absorption law, 

Where: 

a = 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, 


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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. 

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 signalprocessing 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 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 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. 

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 re-assay 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 

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 flowrates 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. 

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. 


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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. 

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. 

Where: 

[O3]OUT = O3 concentration, ppm 

a = 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 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. 

4.5.5.6 Plot the O3 analyzer responses 

(vertical or Y-axis) versus the corresponding 

O3 standard concentrations (horizontal or Xaxis). 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: 

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 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). 

8. Ollison, W.M.; Crow, W.; Spicer, C.W. 

‘‘Field testing of new-technology ambient air 


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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, medium-response 

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.] 

BILLING CODE 6560–50–P 


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0 > FLOW 

---.-i CONTROLLER 

ZERO 

AlA 

I 

I 

L 

FLOW 

CONTROLLER 

UV PHOTOMETER 

DETECTOR 

SIGNAL 

PROCESSING 

ELECTRONICS 

o, 

GENERATOR 

VENT 

OUTPUT 

MANIFOLD 

~---7 VENT 

1 EXTRA OUTLETS CAPPED 

WHEN NOT IN USE 

TO INLET OF ANAL VZER 

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1 

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ABSORPTION CELL 0 

FLOWMETER FLOW 

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L__ _ ___J 

---7 EXHAUST I 

Figure 2. Schematic diagram of a typical UV photometric calibration system. 
75402 Federal Register / Vol. 79, No. 242 / Wednesday, December 17, 2014 / Proposed Rules 

BILLING CODE 6560–50–C 

■ 5. 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. 

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. 

(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 


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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 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 

a.m. to 3:00 p.m. and ending with the 8-hour 

period from 11:00 p.m. to 7:00 a.m. (i.e., the 

8-hour averages for 7:00 a.m. to 11:00 p.m.). 

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 a.m. to 11:00 p.m. In 

addition, in the event that fewer than 13 

valid 8-hour averages are available, a daily 

maximum 8-hour average O3 concentration 

shall also be considered valid if it is greater 

than the level of the NAAQS. Hourly O3

concentrations that have been approved 

under § 50.14 as having been affected by 

exceptional events shall be included when 

determining whether these criteria have been 

met. 

(e) The primary and secondary O3 design 

value statistic is the annual fourth-highest 

daily maximum 8-hour O3 concentration, 

averaged over three years, expressed in parts 

per million. The fourth-highest daily 

maximum 8-hour O3 concentration for each 

year shall be determined based only on days 

meeting the validity criteria in 3(d). The 3- 

year average shall be computed using the 

three most recent, consecutive years of 

ambient O3 monitoring data. Design values 

shall be reported to three decimal places, 

with additional digits to the right of the third 

decimal place truncated. 

4. Comparisons With the Primary and 

Secondary Ozone NAAQS 

(a) The primary and secondary national 

ambient air quality standards for O3 are met 

at an ambient air quality monitoring site 

when the 3-year average of the annual fourthhighest 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 days 

within O3 monitoring season 

1st highest daily 

max 8-hour O3

(ppm) 

2nd highest daily 

max 8-hour O3

(ppm) 

3rd highest daily 

max 8-hour O3

(ppm) 

4th highest daily 

max 8-hour O3

(ppm) 

5th highest daily 

max 8-hour O3

(ppm) 

2014 ............ 100 0.082 0.080 0.075 0.069 0.068 

2015 ............ 96 0.074 0.073 0.065 0.062 0.060 

2016 ............ 98 0.070 0.069 0.067 0.066 0.060 

Average ....... 98 ............................... ............................... ............................... 0.065 ...............................

As shown in Example 1, this site meets the 

primary and secondary O3 NAAQS because 

the 3-year average of the annual fourthhighest 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 ...............................


tkelley on DSK3SPTVN1PROD with PROPOSALS2
75404 Federal Register / Vol. 79, No. 242 / Wednesday, December 17, 2014 / Proposed Rules 

282NO2 precision in Table B–1 is also changed to 

percent to agree with the calculation specified in 

53.23(e)(10)(vi). 

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 

(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 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. 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: 

§ 53.23 Test procedures. 

* * * * * 

(e) * * * 

(1) * * * 

(vi) Precision: Variation about the 

mean of repeated measurements of the 

same pollutant concentration, denoted 

as the standard deviation expressed as 

a percentage of the upper range 

limits.

282

* * * * * 

■ 14. Revise Table B–1 to Subpart B of 

Part 53 to read as follows: 

TABLE B–1 TO SUBPART B OF PART 53—PERFORMANCE LIMIT SPECIFICATIONS FOR AUTOMATED METHODS

Performance parameter Units 1

SO2 O3 CO NO2

(Std. 

range) 

Definitions and test 

Std. procedures 

range 3

Lower 

range 2,3

Std. 

range 3

Lower 

range 2,3

Std. 

range 3

Lower 

range 2,3

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 .................................. ppm ............ ±0.005 4±0.005 ±0.005 ±0.005 ±1.0 ±0.5 ±0.02 Sec. 53.23(d). 

Total, all interferents ........................... ppm ............ ................ ................ ................ ................ ................ ................ 0.04 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 


tkelley on DSK3SPTVN1PROD with PROPOSALS2
Federal Register / Vol. 79, No. 242 / Wednesday, December 17, 2014 / Proposed Rules 75405 

TABLE B–1 TO SUBPART B OF PART 53—PERFORMANCE LIMIT SPECIFICATIONS FOR AUTOMATED METHODS—Continued 

Performance parameter Units 1

SO2 O3 CO NO2

(Std. 

range) 

Definitions and test 

Std. procedures 

range 3

Lower 

range 2,3

Std. 

range 3

Lower 

range 2,3

Std. 

range 3

Lower 

range 2,3

20% of upper range limit .................... Percent ....... ................ ................ ................ ................ ................ ................ ±20.0 Sec. 53.23(e). 

80% of upper range limit .................... Percent ....... ±3.0 ±3.0 ±3.0 ±3.0 ±2.0 ±2.0 ±5.0 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 .................... .................... ................ ................ ................ ................ ................ ................ ................ Sec. 53.23(e). 

Percent 5 ..... 2 2 2 2 1.0 1.0 4 Sec. 53.23(e). 

.................... ................ ................ ................ ................ ................ ................ ................ Sec. 53.23(e). 

80% of upper range limit .................... Percent 5 ..... 2 2 2 2 1.0 1.0 6 Sec. 53.23(e). 

1To 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. 2Tests 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. 3For 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 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. 4For nitric oxide interference for the SO2 UVF method, interference equivalent is ±0.0003 ppm for the lower range. 5Standard deviation expressed as percent of the URL. 

BILLING CODE 6560–50–P 


tkelley on DSK3SPTVN1PROD with PROPOSALS2

EP17DE14.010</GPH>tkelley on DSK3SPTVN1PROD with PROPOSALS2

Table B-3 to Subpart B of Part 53-Interferent Test Concentration,1 Parts per M

(.) 

....... ..... 

Q 

..... 0 0 C\l '"0 <l) §< 

;.... 

- ....... 

..!:1 

..... <l) :>< g 

<l) ;> 

C\l N (.) 0 bJ)<l) <l) <l) (!) 0 <l) <l) 

1-<'"0 bO'"O (!) <l) 

- 0 

....., ;>--. 0 8 8-o (.) o-o ..... ;::l 

- ..... :::i ....... 0 ...... ..... ,.0 ....... 

- <l) ,

- ro 0 -o-o -oc;::: c. :>< ..... :>< .b ..... :>< >... 0 - - ;>--. :>--.- - 0 0 C\l 0 ...c.:: N I ro r

0 :::i ....... ..... -' 

Po; ::r:: g r;l \/1 '"0 z:.s z u:.S u.1 0 8 u 

so2 Ultraviolet 50.1 4 0.14 0.5 0.5 0.5 0.2 20,000 

fluorescence 

so2 Flame photometric 0.01 4 0.14 750 3 20,000 50 so2 Gas 0.1 4 0.14 750 3 20,00 50 

chromatography 0 

so2 Spectrophotometri 0.2 0.1 0.1 4 0.14 0.5 750 0.5 

c-wet chemical 

(pararosanaline) 

so2 Electrochemical 0.2 0.1 0.1 4 0.14 0.5 0.5 0.2 0.5 3 20,00 

0 

so2 Conductivity 0.2 0.1 4 0.14 0.5 750 so2 Spectrophotometri 4 0.14 0.5 0.5 0.2 

c-gas phase, 

including DOAS 

03 Ethylene 3 0.1 750 3 20,00 

chemiluminescene 0.08 0 

03 NO- 3 0.1 0.5 750 0.08 20,000 

chemiluminescene 

OJ Electrochemical 3 0.1 0.5 0.5 ,0.08 

.. 

EP17DE14.011</GPH>tkelley on DSK3SPTVN1PROD with PROPOSALS2

03 Spectrophotometri 3 0.1 0.5 0.5 0.5 0.08 

c-wet chemical 

(potassium iodide) 

03 Spectrophotometri 0.5 0.5 0.5 0.08 0.02 20,000 

c-gas phase, 

including 

ultraviolet 

absorption and 

DOAS 

co Non-dispersive 750 20,000 410 

Infrared 

co Gas 20,000 410 

chromatography 

with flame 

ionization detector 

co Electrochemical 0.5 0.2 20,000 410 co Catalytic 0.1 750 0.2 20,000 410 

combustionthermal detection 

co IR fluorescence 750 20,000 4 10 co Mercury 0.2 410 

replacement-UV 

photometric 

N02 Chemiluminescent 3 0.1 0.5 4 0.1 0.5 20,000 

N02 Spectrophotometri 0.5 4 0.1 0.5 750 0.5 

c-wet chemical 

( azo-dyereaction) 

EP17DE14.012</GPH>tkelley on DSK3SPTVN1PROD with PROPOSALS2

NOz Electrochemical 0.2 3 0.1 0.5 4 0.1 0.5 750 0.5 20,000 50 NOz Spectrophotometri 3 0.1 0.5 4 0.1 0.5 0.5 20,000 50 

c-gas phase 

1. Concentrations of interferents listed must be prepared and controlled to ±1 0 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 t

5. If candidate method utilizes an elevated-temperature scrubber for removal of aromatic hydrocarbons, perfor

6. If naphthalene test concentration cannot be accurately quantified, remove the scrubber, use a test concentrati

scale response, reattach the scrubber, and evaluate response for interference. 

EP17DE14.013</GPH>tkelley on DSK3SPTVN1PROD with PROPOSALS2

TEST 

PARAMETERS 

ZERO 

DRIFT 

SPAN 

DRIFT 

PREClSI ON 

12 

HOUR 

24 

HOUR 

24 

HOUR 

20% 

URL 

0 

URL 

(Pso) 

CALCULATION OF ZERO DRIFT, SPAN DRIFT, AND PRECISION 

Applicant _________________________ _ 

CALCULATIONS 

12ZD = Cmax - Cmin 

Z = (L1 + L2 )/2 

24ZD = Zn - Zn-1 24ZD = -

12 

Sn 

i=7 

s -s SDn = n'"' n-l X 100% 

n-1 

S -Sf 

SD = n n-1 X 1000r. 

n sf 70 

n-1 

P2o =%STANDARD 

DEVIATION OF (P1 ... P5) P80 =%STANDARD DEVIATION OF (P7 ••• Pu) 

TEST DAY (n) 

1 2 3 4 5 6 7 I 8 I 9 10 I 11 I 12 I 13 

Figure B-5. Form for calculating zero drift, span drift, and precision(§ 53.23(e)). 
75410 Federal Register / Vol. 79, No. 242 / Wednesday, December 17, 2014 / Proposed Rules 

BILLING CODE 6560–50–C 

* * * * * 

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. 

* * * * * 

Figure E–2 to Subpart E of Part 53 

[Removed] 

■ 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. 

(11) The annual monitoring network 

plan shall include the Enhanced 

Monitoring Plan (EMP) for areas 

designated as O3 nonattainment, as 

required under 40 CFR part 58 

Appendix D, section 5(f) beginning with 

the annual monitoring plans due on July 

1, 2016. 

* * * * * 

■ 21. Amend § 58.13 by adding 

paragraphs (g) and (h) to read as follows: 

§ 58.13 Monitoring network completion. 

* * * * * 

(g) The O3 monitors required under 

appendix D, section 4.1 of this part must 

operate on the first day of the applicable 

required O3 monitoring season in effect 

January 1, 2017. 

(h) The Photochemical Assessment 

Monitoring sites required under 40 CFR 

part 58 Appendix D, section 5(a) must 

be physically established and operating 

under all of the requirements of this 

part, including the requirements of 

appendix A, C, D, and E of this part, no 

later than June 1, 2017, or two years 

following designation as O3

nonattainment. 

Subpart F—Air Quality Index Reporting 

■ 22. Amend § 58.50 by revising 

paragraph (c) to read as follows: 

§ 58.50 Index reporting. 

* * * * * 

(c) The population of a metropolitan 

statistical area for purposes of index 

reporting is the latest available U.S. 

census population. 

Subpart G—Federal Monitoring 

■ 23. Amend Appendix D to Part 58, 

under section 4, by revising section 

4.1(i) and Table D–3 to Appendix D of 

part 58 and by revising section 5 to read 

as follows: 

Appendix D to Part 58—Network Design 

Criteria for Ambient Air Quality Monitoring 

* * * * * 

4. Pollutant-Specific Design Criteria for 

SLAMS Sites 

4.1 Ozone (O3) Design Criteria. * * * 

(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 out-of-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. 

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. 

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. 

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. 


tkelley on DSK3SPTVN1PROD with PROPOSALS2
Federal Register / Vol. 79, No. 242 / Wednesday, December 17, 2014 / Proposed Rules 75411 

TABLE D–3 1 TO APPENDIX D OF PART

58—OZONE MONITORING SEASON

BY STATE—Continued 

State Begin month End month 

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, 

(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, 

(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. 

[FR Doc. 2014–28674 Filed 12–16–14; 8:45 am] 

BILLING CODE 6560–50–P 


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