The National Ambient Air Quality Standards

PROPOSED UPDATES TO THE AIR QUALITY STANDARDS FOR GROUND-LEVEL OZONE:
INFORMATION ABOUT PROPOSED PRIMARY AND SECONDARY STANDARDS, AND
UPDATES TO THE AIR QUALITY INDEX (AQI)

On Nov. 25, 2014, the U.S. Environmental Protection Agency (EPA) proposed to update the nation's
air quality standards for ground-level ozone to improve public health and environmental protection.
The proposed updates would particularly help children, older adults, and people of all ages with lung
diseases, such as asthma. The proposal will expand the ozone monitoring season in many states, and
it will update the Air Quality Index to ensure people are notified when air quality is unhealthy. And it
will improve the health of trees, plants and ecosystems.

EPA will take comment on the proposal for 90 days after it is published in the Federal Register and will
hold three public hearings. The Agency will issue a final rule by Oct. 1, 2015.

States would have time to develop and implement plans to meet revised standards, and existing and
proposed federal rules will help by making significant strides toward reducing ozone-forming
pollution. EPA projections show the vast majority of U.S. counties would meet the proposed
standards by 2025 just with the rules and programs now in place or under way.

AIR QUALITY STANDARDS

•	The Clean Air Act requires EPA to set two types of outdoor air quality standards for ozone: a
primary standard, to protect public health, and a secondary standard, to protect the public
welfare. The law requires EPA to review the
standards every five years.

Proposed Primary Standard to Protect Public Health:

•	EPA's current primary (health) standard is an 8-
hour standard of 75 parts per billion (ppb), set in
2008. An area meets the primary standard if the
fourth-highest maximum daily 8-hour ozone
concentration per year, averaged over three years,
is equal to or less than 75 ppb.

•	The Clean Air Act requires that primary standards
be "requisite to protect public health with an
adequate margin of safety," including the health of
groups of people considered more at risk. EPA is
proposing that the current ozone standard is not
adequate to protect public health as the law
requires and that the standard should be revised to
improve public health protection for millions of
Americans.

Protecting Public Health with an
Adequate Margin of Safety

The Clean Air Act requires EPA to set
primary air quality standards to reduce
risk sufficiently to protect public health
with an "adequate margin of safety,"
including the health of at-risk groups. In
making this judgment, EPA considers
factors such as the nature and severity of
health effects, the size of the at-risk
groups affected, and the degree of
certainty and uncertainty in the science.
EPA's task is to set standards that are
"requisite" - neither more nor less
stringent than necessary - to accomplish
this. The law does not require EPA to set
primary standards at a zero-risk level.

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•	EPA is proposing to strengthen the health standard from a level of 75 ppb to a level within a
range from 65 to 70 ppb. The agency is proposing to retain the form and averaging time for the
standard. An area would meet the primary standard if the fourth-highest maximum daily 8-hour
ozone concentration per year, averaged over three years, is equal to or less than the level of the
standard.

•	EPA also is seeking comment on levels for the health standard as low as 60 ppb. The agency will
accept comments on all aspects of the proposal, including on retaining the existing standard.

•	A significantly expanded body of scientific evidence shows that ozone can cause a number of
harmful effects on the respiratory system, including difficulty breathing and inflammation of the
airways. For people with lung diseases such as asthma and COPD (chronic obstructive pulmonary
disease), these effects can lead to emergency room visits and hospital admissions. Ozone
exposure also is likely to cause premature death from lung or heart diseases. In addition, evidence
indicates that long-term ozone exposure is likely to result in harmful respiratory effects, including
the development of asthma.

•	People most at risk from ozone exposure include: children; people with asthma and other
respiratory diseases; older adults; people who are active outdoors, especially outdoor workers;
people with certain genetic characteristics; and people with reduced intake of certain nutrients,
such as vitamins C and E.

•	An estimated 25.9 million people have asthma in the U.S., including almost 7.1 million children.
Asthma disproportionately affects children, families with lower incomes, and minorities, including
Puerto Ricans, Native Americans/Alaska Natives and African-Americans.

Proposed Secondary Standard to Protect the Public Welfare

•	The Clean Air Act also requires EPA to set secondary standards that specify the level of air quality
that is "requisite to protect the public welfare from any known or anticipated adverse effects."
These effects include effects on soils, water, crops, vegetation, man-made materials, weather,
visibility and climate, among others.

•	The current secondary standard for ozone is identical to the primary standard, an 8-hour standard
of 75 ppb.

•	New studies since the last review of the standards add to evidence showing that repeated
exposure to ozone reduces growth and has other harmful effects on plants and trees. These types
of effects have the potential to impact ecosystems and the benefits they provide.

•	EPA is proposing that the existing secondary standard is not adequate to protect the public
welfare, particularly against harm to trees, plants and ecosystems. The agency is proposing a
two-step approach to provide appropriate protection: defining a target level of protection and
revising the standard to achieve that level of protection.

•	The impact of ozone exposure on trees, plants and ecosystems often is assessed using a seasonal
index known as a "W126 index." A W126 index, named after portions of the equation used to

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calculate it, is a weighted index designed to reflect the cumulative exposures that can damage
plants and trees during the consecutive three months in the growing season when daytime ozone
concentrations are the highest and plant growth is most affected.

•	The agency is proposing that air quality
meeting a W126 index value between 13 and
17 parts per million-hours (ppm-hours),

averaged over three years, would provide the
"requisite protection" that the law requires.

•	To achieve that level of protection, EPA is
proposing to revise the level of the secondary
standard to a level within the same range
proposed for the primary standard (an 8-hour
standard in the range of 65 to 70 ppm).

Analyses show that a standard in this range
would provide protection equivalent to a W126
index of 13 to 17 ppm-hours. EPA analyzed data
from air quality monitors and found that setting
a standard in a W126form would not provide
additional protection beyond an 8-hour
standard.

•	The agency is proposing to retain the form and
averaging time of the current secondary
standard. An area would meet the secondary
standard if the annual fourth highest maximum
daily 8-hour average, averaged over three years,
is equal to or less than the level of the standard.

•	EPA is seeking comment on setting the standard based on the W126 metric within a range of 13
to 17 ppm-hours, averaged over three years. The Agency also is seeking comment on defining a
target protection level in terms of a W126 index value as low as 7 to 13 ppm-hours. The agency
will accept comments on all aspects of the proposal, including on retaining the existing standard.

PROPOSED REVISIONS TO THE AIR QUALITY INDEX

•	EPA is proposing updates to the Air Quality Index (AQI) for ozone. The AQI is EPA's color-coded
tool for telling the public how clean or polluted the air is, and recommending steps people can
take to reduce their daily exposure to pollution.

•	The AQI converts ozone concentrations to a number on a scale from 0 to 500. This scale is used by
cities and states across the country to report current and daily ozone concentrations and for daily
ozone air quality forecasting.

Calculating the W126 Index:

A W126 index is a seasonal index that scientists

often use designed to assess the impact of

ozone on plants and trees. Here's how it's

calculated for EPA's proposal:

•	Measure hourly ozone concentrations for
each hour from 8 a.m. to 8 p.m. - the time
when ozone has the greatest potential to
cause harm;

•	Weight each hourly measurement, with
more weight given to higher ozone
concentrations (higher concentrations are
more damaging to plants);

•	Add the 12 weighted hourly values to get a
daily value;

•	Add the daily values for each month to get a
monthly value;

•	Add the monthly values in each consecutive
three-month period during the ozone
season. The highest of these three-month
sums is the seasonal index value.

•	Average the seasonal index values over the
most recent three years.

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• EPA is proposing to change the breakpoints for each AQI category, based on the level of the
proposed primary standard and information from the health studies examined as part of the
review of the standards.

•	EPA is proposing to set the 100 value of the index at the level of the primary 8-hour ozone
standard (in a range from 65 to 70 ppb). An AQI of 100 is the upper end of the "Moderate" or
"Code Yellow" range, and marks the level above which EPA begins cautioning at-risk groups. The
"Unhealthy for Sensitive Groups" or "Code Orange" range (AQI of 101-150) would begin 1 ppb
above the level of the proposed standard (66 to 71 ppb) and would extend to 85 ppb.

•	EPA is not proposing to change the level at the top of the index (an AQI value of 500). This level is
typically set equal to the Significant Harm Level, a level that represents imminent danger. The
Significant Harm Level for ozone is 600 ppb, averaged over two hours. Some states are required
to have contingency plans in place to avoid reaching this level.

•	The proposed AQI breakpoints are outlined in the table below.

AQI Category

Index Values

Current Breakpoints

(2008 AQI)
(ppb, 8-hour average)

Proposed Breakpoints
(ppb, 8-hour average)

Note: parentheses indicate a range

Good

0-50

0-59

0 to (49-54)

Moderate

51 -100

60-75

(50-55) to (65-70)

Unhealthy for
Sensitive Groups

101-150

76-95

(66-71) to 85

Unhealthy

151-200

96-115

86-105

Very Unhealthy

201-300

116-374

106-200

Hazardous

301-500

375 to

the Significant Harm Level*

201 to

the Significant Harm Level*

*The Significant Harm Level for ozone is 600 ppb, two-hour average

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BACKGROUND:

•	The Clean Air Act requires EPA to review the ozone standards every five years to determine
whether they should be revised in light of the latest science. Reviewing the NAAQS is a lengthy
undertaking and includes the following major steps before EPA issues a proposed rule: planning; a
comprehensive review, synthesis and evaluation of the science on ozone (referred to as the
Integrated Science Assessment); risk and exposure assessments for public health and the public
welfare; and a staff policy assessment.

•	Scientific review during the development of each of these documents is thorough and extensive.
Drafts of all documents are reviewed by EPA's independent science advisers (the Clean Air
Scientific Advisory Committee, or CASAC), and the public has an opportunity to comment on
them.

•	The EPA Administrator evaluates all of this information, along with advice from the CASAC, in
determining whether to propose revisions to a standard. Proposed rules are made available for
public comment, and the agency generally holds public hearings. EPA carefully considers
comments received on the proposal before issuing a final rule.

•	EPA issued the first national air quality standards for ozone in 1971. The agency has revised the
standards three times - in 1979,1997 and 2008 - to ensure they continue to protect public health
and welfare. (In 1993, the agency reviewed the standards but determined that revisions were not
warranted; in 2010, the agency proposed, but did not finalize, revisions as part of a
reconsideration of the 2008 standards.) A table of historical ozone standards is available at:
http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_history.html

•	EPA last updated the ozone standards in March 2008 and began the regularly scheduled review of
that standard six months later. The proposed revisions are a result of that regularly scheduled
review.

•	EPA proposed to revise the standards Nov. 26, 2014.

o On January 21, 2014, the Sierra Club, American Lung Association, Environmental Defense
Fund and Natural Resources Defense Council sued EPA for not completing the review of
the standards within five years - by March 2013. The groups asked the U.S. District Court
for the Northern District of California to order EPA to complete the five-year review of the
2008 standards. The court ordered the agency to sign a proposed rule by Dec. 1, 2014 and
a final rule by Oct. 1, 2015.

•	In July 2013, the U.S. Court of Appeals for the D.C. Circuit upheld the 2008 primary ozone
standard but remanded the secondary standard to EPA, on the grounds that the agency had not
specified the level of air quality that was requisite to protect public welfare as required by the
Clean Air Act, and had not clearly shown how the secondary standard provided this protection.

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The proposed revisions to the ozone standards respond to this remand.

•	On Oct. 6, 2014, the U.S. Supreme Court declined to hear an appeal of the D.C. Circuit Court's July
2013 decision.

FOR MORE INFORMATION:

•	To read the proposal and additional fact sheets, visit http://www.epa.gov/glo/actions.html

•	To comment on the proposal

•	For technical documents related to this review of the standards, see:
http://www.epa.gOv/ttn/naaqs/standards/ozone/s o3 index.html

•	For your local air quality forecasts and information on current air quality, visit www.airnow.gov

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