National Ambient Air Quality Standards for Ozone: 

Summary of Final Rule

Briefing for OMB

U.S. Environmental Protection Agency

September 3, 2015
Key Messages

• Court-ordered deadline for final rule signature is October 1, 2015.

• Strong body of scientific evidence supports revising the current 75 ppb primary standard 

level, especially to protect at-risk groups, including children and asthmatics.

• Strong body of scientific evidence supports strengthening the current secondary 

standard

• Additionally, the U.S. Court of Appeals (D.C. Circuit) remanded the secondary standard back to EPA in 

2013

• On November 25, 2014, we proposed to revise both the primary (health) and secondary 

(welfare) ozone standards: 

• Proposed to strengthen the primary standard to a level between 65 – 70 ppb; solicited 

comment on lower levels, down to 60 ppb, and on retaining current standard.

• Proposed to revise the secondary standard to within the range of the primary standard (65 –

70 ppb); solicited comment on separate secondary standard at alternative levels and alternative form (W126).

• Received > 430,000 public comments on the proposal; held 3 public hearings

• The Administrator has concluded that a primary standard of 70 ppb provides requisite 

protection with an adequate margin of safety and a secondary standard of 70 ppb would 

protect the public welfare from any known or anticipated adverse effects. 

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NAAQS Statutory Requirements (CAA 

§109)

• Primary (health-based) standards “shall be ambient air quality standards . . .which 

in the judgment of the Administrator “ are “requisite” to protect public health, including at-risk groups, with an “adequate margin of safety.” 

• Secondary (welfare-based) standards: “...specify a level of air quality the 

attainment and maintenance of which” in the “judgment of the Administrator” is “requisite to protect the public welfare from any known or anticipated adverse effects.”

• An independent scientific review committee (currently the Clean Air Scientific 

Advisory Committee [CASAC]), shall review the science and standards and “shall recommend to the Administrator any new . . . standards and revisions of existing . . . standards as may be appropriate.”

• In setting standards:

• EPA is required to engage in “reasoned decision making”.• EPA may not consider cost in setting the level of the standard. Rather, cost is considered in 

developing control strategies to meet the standards.

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Science-Based Conclusions on Primary Standard

• Over 1,000 new studies since last review; strongest evidence for respiratory effects (including 

respiratory mortality) from short-term ozone exposures. 

• Controlled human exposure studies show wide range of respiratory effects following exposures 

from 60 to 80 ppb. New controlled human exposure studies in this review:

– At 72 ppb: “Adverse” combination of lung function decrements and respiratory symptoms observed 

(Schelegle et al., 2009)

– At 60 ppb: Lung function decrements and airway inflammation observed (Kim et al., 2011); adversity of 

effects is less clear

• At-risk groups include: People with asthma (especially children with asthma) and other lung 

diseases; children and older adults; all people who are active outdoors (especially outdoor 

workers).

• Epidemiological studies show evidence for associations with mortality/morbidity; some studies 

report associations with air quality likely to have been allowed by the current standard.

• EPA conducted a Risk & Exposure Assessment (REA) to estimate ozone exposures of concern 

≥ 60, 70, 80 ppb; the risk of ozone-induced lung function decrements; and the risk of ozoneassociated mortality/morbidity for air quality meeting the current and potential alternative standards.

• CASAC: Science supports revising the level to within the range of 60 – 70 ppb; policy advice 

was to set level below 70 ppb to provide greater margin of safety.

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Overview of Primary Standard Comments

• Most commenters supported either retaining the current standard of 75 ppb or revising 

to 60 ppb; some state agencies supported the proposed range of 65 - 70 ppb.

• Retain current standard level (industry and some states) 

• Lack of support in evidence, exposure/risk assessments; uncertainties too great • Background ozone concentrations near/at/above proposed standard range 

• Negative economic impacts of lower level will create health risks and more nonattainment areas will 

result in job losses

• Set standard level at 60 ppb (environmental and public health advocates)

• Stronger controlled human exposure evidence in this review supports a level of 60 ppb• Adverse health effects at 60 ppb in 6.6-hour exposure studies of healthy, young adults; effects likely 

greater in at-risk populations, including asthmatic children, and with 8-hour exposures

• Point to CASAC advice that: a level of 70 ppb provides “little margin of safety for the protection of 

public health, particularly for sensitive subpopulations”

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Administrator’s Final Decision on Level of Primary Standard

• A standard with a level of 70 ppb is requisite to protect public health with an adequate margin of safety. 

• 70 ppb is below the level at which “adverse” combination of lung function decrements and respiratory 

symptoms observed in clinical studies (e.g., 72 ppb).

• Further considered the protection 70 ppb offers against exposures to ozone concentrations shown in 

controlled human exposure studies to cause respiratory effects.

• Protection depends on variations in ozone concentrations across an area, as well as activity patterns and ventilation 

rates.

• A standard level of 70 ppb eliminates almost all exposures to ozone concentrations of 80 and 70 ppb, for which there 

is strongest evidence of adverse effects.

• A standard of 70 ppb also substantially limits exposures to levels as low as 60 ppb.

• While effects have been shown to occur following exposure to 60 ppb, adversity of these effects is less clear. 

However, CASAC did advise limiting exposures to 60 ppb.

• Repeated exposure increases likelihood of more serious effects, leading Administrator to focus on limiting 

multiple (2 or more) exposures.

• A level of 70 ppb protects >98% of children from experiencing repeated exposures to 60 ppb, providing an 

adequate margin of safety.

• A standard of 70 ppb is also estimated to reduce occurrences of ozone-induced lung-function decrements 

≥

10% and 15% and to reduce ambient ozone concentrations associated with mortality and morbidity in 

epidemiologic studies. 

• A level below 70 ppb would be more than “requisite” to protect public health; would place significant emphasis 

on reducing even lower ozone concentrations, despite considerable uncertainty in the occurrence of adverse 

effects at those levels.

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Air Quality Index

• Proposed new breakpoints based on the proposed primary standard and information from the 

health studies examined in the review; EPA proposed:

• Setting the 100 level of the AQI at the same level as the revised primary ozone NAAQS 

• Adjusting AQI breakpoints at the lower end of each range (i.e., AQI values of 50, 150, 200 and 300)

• For the AQI breakpoint between Good and Moderate, we took comment on a range (49-50 or 54-55)

• Almost all commenters supported proposed changes• Final Decision:

• Revising the 100-level AQI value to the level of the standard (70 ppb)

• Revising the “Good/Moderate” breakpoint to be 15 ppb under the standard level (54/55 ppb); this 

proportional adjustment is consistent with past reviews 

• Other category breakpoints (Unhealthy and Very Unhealthy) are revised as proposed

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AQI Category AQI Values Current Breakpoints

ppb, 8-hour average

Final Breakpoints

ppb, 8-hour average

Good 0 - 50 0-59 0-54

Moderate 51 – 100 60-75 55-70

Unhealthy for 

Sensitive Groups

101 - 150 76-95 71-85

Unhealthy 151 – 200 96-115 86-105

Very Unhealthy 201 – 300 116-374 106-200

Hazardous 301 - 500 375 to 

Significant Harm Level

201 to 

Significant Harm Level
Science-Based Conclusions on Secondary Standard

• Ozone has detrimental effects on vegetation, including both crops and trees. 

• Plant response to ozone depends on both the level and cumulative nature of exposures over the growing 

season. Specialized exposure metrics (such as W126 cumulative weighted index) are used to assess potential impacts on vegetation.

• Studies show that effects on sensitive tree species (e.g., reduced growth and productivity) could have 

adverse implications for ecosystems (e.g., reduced ecosystem productivity, alteration of water availability in 

watersheds, reduced carbon sequestration, changes in community composition).

• There is little established quantitative information for informing judgments on adversity of these effects to 

the public welfare. 

• CASAC advised that the secondary standard should be in terms of a cumulative, seasonal 

form (i.e., W126) set at a level within the range of an annual index of 7 to 15 ppm-hrs. 

• EPA proposed to retain the current form and revise the secondary standard level to within 

the range of the primary standard (65 – 70 ppb); solicited comment on separate secondary 

standard with an alternative form (W126).

• Public comments mixed; some supported using same form as primary, some supported 

W126-based form; some supported retention of current standard.

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Administrator’s Final Decision on Secondary Welfare-Based Standard 

• Retain current form of secondary standard and revise level to 70 ppb (same as 

primary standard).

• Such a standard will provide the requisite protection of the public welfare from any 

known or anticipated adverse effects.

• Focus is on ozone effects on tree seedling growth as a proxy (or surrogate) for the full array of 

vegetation-related effects of ozone.

• A cumulative, seasonal exposure metric (the W126 index) is used for assessing growth-related effects 

of ozone and for associated judgments about the desired public welfare protection.

• Consideration is given to CASAC advice on using estimates of relative biomass loss (RBL) for tree 

seedlings and their view that 6% RBL in the median studied species is unacceptably high. 

• The Administrator judges it appropriate for the revised standard to generally limit cumulative seasonal 

exposures to 17 ppm-hrs or lower, in terms of the W126 index, which air quality analyses indicate can 

be achieved by revising the level of the current standard to 70 ppb.

• In accordance with the 2013 judicial remand of the 2008 secondary standard, this 

rulemaking fully explains the Administrator’s conclusions and judgments in support of this decision.

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Monitoring Aspects of the Rule –

Finalized as Proposed with Minor Updates

• Updates to the Photochemical Assessment Monitoring Stations (PAMS) 

Network (reduce redundancy, improve geographic distribution, add flexible component)

• Extension of the ozone monitoring season in 32 states and D.C. (majority 

of changes were by 1 month)

• Approved Federal Reference Method (FRM) for ambient measurements 

of ozone while retaining existing FRM and Federal Equivalent Methods (FEMS)

• New Appendix U to Part 50 (data handling provisions for determining 

compliance with the revised NAAQS)

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PAMS Network Design

• We are replacing the existing 20 year-old multi-site, enhanced ozone network design 

with an updated 2-part network design

• Simplify existing network design by requiring PAMS measurements to be collocated with 

existing NCore sites in areas with population of 1 million or more irrespective of O3 

NAAQS attainment status

• Require states with moderate or above ozone non-attainment areas and states in the 

Ozone Transport Region to develop and implement an Enhanced Monitoring Plan 

(EMP) to support flexible approaches for collecting data to understand ozone issues in new and existing high ozone areas

• PAMS monitoring at NCore sites will become effective by June 1, 2019

• EMPs submitted within two years of designations or by July 1, 2020, whichever is later

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Ozone Monitoring Seasons

• EPA requires ozone monitoring only during the “ozone season” – the time of year 

when weather conditions are most favorable for ozone formation. This season varies by state: in some states with warmer climates, monitoring is required yearround; however, in states where the climate is colder, ozone monitoring is required for as little as four months during the summertime. 

• Final rule extends the ozone monitoring season for 32 states and D.C.

• One month for 23 of the 32 states and the District of Columbia• Additional extensions of two months to seven months including states where ozone can be 

elevated during the winter 

• Year-round seasons for all NCore multi-pollutant sites

• Regional Administrators will still be allowed to approve changes to states’ ozone 

monitoring seasons without rulemaking

• Expanded monitoring season requirements will become effective January 1, 2017

• Impact mitigated by the high proportion of monitors already being voluntarily 

operated on year-round basis 

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PSD Permit Grandfathering Provision

• Finalizing, as proposed, a grandfathering provision that will allow certain permits to be 

issued based on applicant’s demonstrated compliance with 2008 NAAQS rather than 

2015 NAAQS. A permit application will qualify for grandfathering if either:

• The application is determined to be complete prior to the signature date of final NAAQS rule, or 

• The draft permit has been proposed for public comment prior to the effective date of NAAQS.

• Note: This is the same set of criteria finalized for the 2012 PM2.5 NAAQS. Not all reviewing 

authorities make formal completeness determinations, but for those that do path #1 generally provides a larger window for grandfathering than path #2.

• Comments and Considerations:

• Industry groups and some states support some type of grandfathering.• Opposing comments by coalition of 8 environmental groups include several legal arguments 

concluding the Act does not allow grandfathering and EPA’s interpretation of authority is flawed.

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Schedule for Submitting Demonstrations of 

Exceptional Events

• To ensure EPA has adequate time to assess any state-submitted exceptional events 

(EE) demonstrations for 2014 – 2016 that would substantively affect designations (e.g., nonattainment vs. attainment), we are establishing EE demonstration submission deadlines that split the available submission and review time between EPA and states 

as follows:

• October 1, 2016 ... for events affecting 2014 and 2015 ozone data• May 31, 2017 ... for events affecting 2016 ozone data (1 month prior to 120-day letters)• May 31, 2018 ... for 2017 ozone data (would apply only under the optional 3-yr designations schedule)

• [Note: The schedule is actually a ‘delta schedule’ that calculates submission deadlines based on the 

promulgation date of the NAAQS. Applying this delta schedule to an October 1, 2015 NAAQS promulgation results in the identified dates.]

• Comments and Considerations:

• Schedule is consistent with previous designations efforts and supported by many commenters.

• Several states requested more time to develop and submit demos for 2016 events.

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Ozone Designation and Implementation Schedules

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Designation Schedule

Schedule Tentative Date

State and Tribe Recommendations

Within 1 year after NAAQS promulgation

October 2016(Air quality data years: 2013 – 2015)

Final Designation With 2 years after

NAAQS promulgation (Administrator has discretion to use additional year to collect more information.)

October 2017Effective date may vary(Air quality data years: 2014 –2016)

Implementation ScheduleInfrastructure SIP Within 3 years after

NAAQS promulgation

October 2018

Nonattainment SIP Within 36 - 48 months 

after designations depending on classification

October 2020

Attainment Schedule by Classification

Schedule

Marginal 3 years to attainModerate 6 years to attainSerious 9 years to attainSevere 15 to 17 years to attainExtreme 20 years to attain

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