FACT SHEET
FINAL DECISION
OZONE NATIONAL AMBIENT AIR QUALITY STANDARDS

SUMMARY OF ACTION

•	On December 23, 2020, the U.S. Environmental Protection Agency (EPA) acted to retain,
without revision the primary and secondary ozone National Ambient Air Quality Standards
(NAAQS). The standards, established in 2015 by the Obama-Biden Administration, are set at
70 parts per billion (ppb), in terms of a 3-year average of the annual fourth-highest daily
maximum 8-hour average ozone concentrations.

•	With this action, EPA is following the principles established in the earliest days of the Trump
Administration to streamline the NAAQS review process and to fulfill the statutory
responsibility to complete the NAAQS review within five-years.

•	The decision to retain the existing ozone standards comes after careful review and
consideration of the most recent available scientific evidence and technical information,
consultation with the agency's independent science advisors, and consideration of more than
50,000 public comments on the proposal.

•	The Clean Air Act directs EPA to set primary standards to provide public health protection
including for at-risk groups with an adequate margin of safety and the secondary standards to
provide protection against adverse effects to the public welfare, including the environment
and property.

•	The existing primary standard (health-based) provides particular protection for children and
others with asthma. Ozone in outdoor air increases their risk of asthma attacks while playing,
exercising or engaging in strenuous work activities outdoors.

•	The current scientific information supports the conclusion that the primary standard
established in 2015 protects public health with an adequate margin of safety, including the
health of at-risk populations.

o The health effects evidence currently available continues to support the conclusion
that ozone can cause difficulty breathing and other respiratory system effects. For
people with asthma, these effects can lead to emergency room visits and hospital
admissions. Exposure over the long term may lead to the development of asthma.
People most at risk from breathing air containing ozone include people with asthma,
children, the elderly, and outdoor workers.

o Information newly available in this review includes evidence of metabolic effects that
comes primarily from animal toxicological studies. However, the currently available
evidence continues to show respiratory effects associated with the lowest exposures.
The decision includes the conclusion that by focusing on protecting populations at
risk of respiratory effects, the existing standard also provides protection from
metabolic effects.

•	The existing secondary (welfare-based) standard provides protection against an array of
effects, including effects of ozone exposure on vegetation.


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o The current welfare effects evidence continues to demonstrate that repeated exposure
to ozone damages leaves, reduces growth of vegetation and has other harmful effects
on plants and trees that have the potential to impact ecosystems and the benefits they
provide. Ozone distributed globally also has effects on climate.

o Information newly available in this review supports and expands our understanding
on these and other effects on vegetation, ecosystems, and climate.

•	EPA's independent science advisors, the Clean Air Scientific Advisory Committee (CASAC)
advised the Administrator on several aspects of this review, including the standards.

o With regard to the primary standard, the CASAC agreed that the currently available
evidence was generally similar to that available in the last review when the standard
was set. Based on this conclusion, part of CASAC concluded that the primary
standard should be retained. Another part of CASAC expressed support for a lower
standard.

o With regard to the secondary standard, the CASAC agreed that the current evidence
supported retaining the current standard without revision.

•	In reaching its decision on the secondary standard, the EPA has also considered issues raised
by the U.S. Court of Appeals for the D.C. Circuit Court in its remand of the 2015 secondary
standard.

•	The decisions on both standards have also taken into consideration public comment

•	This action is effective upon publication in the Federal Register.

•	Since the beginning of the Trump Administration, EPA has re-designated to attainment eight
nonattainment areas for the 2008 8-hour ozone standards. In this same timeframe, U.S.
nitrogen oxide emissions have dropped ten percent and volatile organic compound emissions
have dropped three percent. National average ozone concentrations have gone down four
percent. Since 1990, national average ozone concentrations have dropped 25 percent.

BACKGROUND

•	The Clean Air Act requires EPA to set national ambient air quality standards for "criteria
pollutants." Currently, ozone and related photochemical oxidants, and five other major
pollutants are listed as criteria pollutants. The others are carbon monoxide, lead, nitrogen
oxides, particulate matter and sulfur oxides. The law also requires EPA to periodically
review, at least every five years, the relevant scientific information and the standards and
revise them, if appropriate, to ensure that the standards provide the requisite protection
for public health and welfare.

FOR MORE INFORMATION

To download a copy of the final decision, go to EPA's Web site at:

•	https://www.epa.gov/ground-level-ozone-pollution/setting-and-reviewing-standards-
control-ozone-pollution

•	Today's decision and other background information are also available either
electronically at http://www.regulations.gov. EPA's electronic public docket and
comment system.


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• For further technical information about the rule, please contact Deirdre Murphy, with
EPA's Office of Air Quality Planning and Standards, at (919) 541-0729 or
murphy.deirdre@epa.gov.


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