FACT SHEET

PROPOSED DECISION
REVIEW OF THE PRIMARY NATIONAL AMBIENT AIR QUALITY STANDARDS

FOR OXIDES OF NITROGEN

ACTION

•	On July 14, 2017, the U.S. Environmental Protection Agency (EPA) proposed to retain,
without revision, the health based or primary National Ambient Air Quality Standards
(NAAQS) for Oxides of Nitrogen.

•	EPA proposed that the current standards protect public health with an adequate margin of
safety, and that the available evidence and information does not support the consideration
of potential alternative standards that would provide a different degree of public health
protection.

•	This decision is based on a review of the full body of scientific evidence and of the
information available from analyses of potential nitrogen dioxide (NO2) exposures. EPA's
independent scientific advisors, the Clean Air Scientific Advisory Committee (CASAC), also
concluded that the available information supports retaining the current 1-hour and annual
primary NO2 standards without revision.

•	NO2 is the component of oxides of nitrogen of greatest concern for health and is the
indicator for the primary NAAQS. There are currently two primary NO2 standards:

1.	a 1-hour standard, established in 2010, at a level of 100 parts per billion (ppb). It is
based on the 98th percentile of the annual distribution of daily maximum 1-hour NO2
concentrations, averaged over 3 years; and

2.	an annual standard, originally set in 1971, at a level of 53 ppb. It is based on annual
average NO2 concentrations.

NO2 AND PUBLIC HEALTH

•	There is strong evidence that both short- and long-term exposure to NO2 are associated
with asthma-related health effects. Exacerbation of asthma symptoms, in some cases
resulting in hospitalization, has been shown to occur following short-term exposures (i.e.,
typically hours to days). The development of asthma in children has been shown to be
associated with long-term exposures (typically years).

•	Consistent with the prior review, the evidence indicates that people with asthma, children
(under the age of 18), and older adults (over the age of 65) are at increased risk for NO2-
related health effects.

•	Exposures to elevated ambient concentrations of NO2 can occur near important emissions
sources, such as major roadways.

PROGRESS IN REDUCING EMISSIONS AND EXPOSURE

•	Currently, there are no monitors in the United States measuring air quality values that
exceed either the 1-hour or annual standard.

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•	Nationwide estimates indicate a 61 percent decrease in total NOx emissions from 1980 to
2016 as a result of federal, state, local and tribal efforts to improve air quality. NOx is a term
commonly used to describe the combination of NO and NO2.

•	As NOx emissions have declined, ambient concentrations of NO2 have also declined broadly
across the U.S. Since 1980, the median annual NO2 level compared to the NAAQS, known as
the design value, has decreased by about 65 percent and the median 1-hour design value
has decreased by about 50 percent.

BACKGROUND

•	The Clean Air Act requires EPA to set national ambient air quality standards for criteria
pollutants. Currently, oxides of nitrogen and five other major pollutants are listed as criteria
pollutants. The others are ozone, lead, carbon monoxide, sulfur oxides, and particulate
matter. The law also requires EPA to periodically review the relevant scientific information
and the standards, and to revise the standards as appropriate to ensure that they provide
the requisite protection for public health, allowing an adequate margin of safety, and the
public welfare.

•	In the prior review, which was completed in 2010, EPA increased the protection provided
against NO2 exposures by adding the current 1-hour standard. This decision was based on
scientific evidence, together with analyses of NO2 exposures and health risks, showing that
the annual standard alone would not be sufficiently protective against the respiratory
effects that can occur following short-term NO2 exposures, particularly in people with
asthma.

•	While NOx is emitted from a wide variety of source types, the top three categories of
sources of NOx emissions are from highway vehicles, off-highway vehicles, and stationary
fuel combustion sources. NOx emissions also come from electric utility sources, both coal
and gas-fired.

HOW TO COMMENT

•	EPA will accept comment on the proposal for 60 days after publication in the Federal
Register. Comments, identified by Docket ID No. EPA-HQ-OAR-2013-0146, may be
submitted 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-2013-0146 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 EPA-HQ-OAR-2013-0146,1200 Pennsylvania Avenue, NW.,
Washington, DC 20460.

¦	Hand/Courier Delivery: EPA Docket Center, Room 3334, EPA WJC West Building, 1301
Constitution Avenue, NW., Washington, DC 20004. Such deliveries are only accepted

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during the Docket's normal hours of operation, and special arrangements should be
made for deliveries of boxed information.

•	If one is requested, EPA will hold a public hearing. In that case, EPA will announce the place
and time.

FOR MORE INFORMATION

•	To download a copy of the proposal, go to EPA's Web site at: https://www.epa.gov/no2-
pollution.

•	Today's proposed decision and other background information are also available either
electronically at http://www.regulations.gov. EPA's electronic public docket and comment
system, or in hardcopy at the EPA Docket Center's Public Reading Room.

¦	The Public Reading Room is located in the EPA Headquarters, Room Number 3334 in the
EPA West Building, located at 1301 Constitution Avenue, NW, Washington, DC. Hours of
operation arpe 8:30 a.m. to 4:30 p.m. eastern standard time, Monday through Friday,
excluding Federal holidays.

¦	Visitors are required to show photographic identification, pass through a metal
detector, and sign the EPA visitor log. All visitor materials will be processed through an
X-ray machine as well. Visitors will be provided a badge that must be visible at all times.

¦	Materials for this proposed action can be accessed using Docket ID No. EPA-HQ-OAR-
2013-0146.

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