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Transportation Air Quality Selected Facts and Figures

Air Quality Standards

What Are the National Ambient Air Quality Standards (NAAQS)?

The NAAQS are set by EPA to protect public health and welfare. Primary standards are designed to protect against adverse health effects, while secondary standards protect against welfare effects, such as damage to crops, vegetation, buildings, and visibility.

Primary Air Quality Standards for Transportation-Related Pollutants (2015)


Type of Average



8-hour 1-hour

9 ppm

35 ppm


1-hour Annual

100 ppb

53 ppb



0.070 ppm


Annual 24-hour

12 μg/m3

35 μg/m3



150 μg/m3

ppm = parts per million. ppb = parts per billion.

μg/m3 = micrograms per cubic meter

Source: U.S. EPA,

An area is in violation of a standard if it exceeds the concentration level for its evaluation time frames. For example, for an area to attain the 8-hour ozone standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone concentrations measured at each monitor within the year must not exceed 0.070 ppm.

What Are Nonattainment Areas?

Areas that do not meet the NAAQS are designated as nonattainment areas. The CAA requires States to develop plans, known as State Implementation Plans (SIPs), to attain the standards for each area designated nonattainment for a NAAQS. If the areas do not meet these and other requirements, they face CAA-required sanctions and other penalties, including loss of highway funds. Metropolitan planning organizations and the U.S. Department of Transportation (U.S. DOT) must ensure that transportation plans, programs, and projects conform to these SIPs.

What Are Maintenance Areas?

A maintenance area is any geographic region of the United States that EPA previously designated as a nonattainment area for one or more pollutants pursuant to the Clean Air Act Amendments of 1990 (CAAA), and subsequently redesignated as an attainment area subject to the requirement to develop a maintenance plan under Section 175A of the Clean Air Act, as amended. The maintenance plan normally contains an emissions or modeling demonstration that shows how the area will stay in compliance through the 20-year maintenance period.

Updated: 5/3/2016
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