I. Background

A. Executive Summary

This section summarizes information about the purpose of this regulatory action in 

section 1, the major provisions of this proposal in section 2, and provisions related to 

implementation in section 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.) 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 did not 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 by the U.S. Court of Appeals (D.C. 

Circuit) of the Agency’s 2008 decision on the secondary standard. Between 2008 and 2014, the 

EPA prepared draft and final versions of the Integrated Science Assessment, the Health and 

Welfare Risk and Exposure Assessments, and the Policy Assessment. Multiple drafts of these 

documents were available for public review and comment, and as required by the Clean Air Act, 

were peer-reviewed by the Clean Air Scientific Advisory Committee (CASAC), an independent 

scientific advisory committee established by the CAA and charged with providing advice to the 

Administrator. The final documents 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 of 0.075 ppm is not adequate 

to protect public health, 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 place very little weight on the uncertainties in the health effects 

evidence and exposure/risk information, which she proposes would be inappropriate at this time.

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

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

current primary O3 standard and the option of retaining that standard. In addition to proposing 

changes to the level of the standard, the EPA is proposing conforming changes to the Air Quality 

Index (AQI) by proposing to set an AQI value of 100 equal to the level of the 8-hour primary O3 

standard, and proposing proportional changes to the AQI values of 50, 150, 200 and 300.
The EPA also proposes to revise the secondary standard to provide increased protection 

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

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

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

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

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

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

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

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

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

within the range proposed for the primary standard 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 same range proposed 

for the primary O3 standard of 0.065 to 0.070 ppm. The EPA solicits comments on this proposed 

revision of the secondary standard. 

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

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

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

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

is also taking comment on the Administrator’s proposed decision to revise the current secondary 

standard and the option of retaining that standard.

3. Provisions Related to Implementation
As directed by the CAA, reducing pollution to meet national air quality standards always 

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

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

three decades ago, and is evidenced by the 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. These proposed provisions are intended to ensure future implementation 

actions are not overly burdensome and are clearly essential for achieving and maintaining clean 

air. 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. With respect to monitoring, 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.

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.