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National Ambient Air Quality Standards (NAAQS)

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National
Ambient Air Quality Standards (NAAQS)

The Clean Air
Act, which was last amended in 1990, requires EPA to set
National Ambient Air Quality Standards for pollutants
considered harmful to public health and the environment. The Clean
Air Act established two types of national air quality standards.
Primary standards set limits to protect public health,
including the health of "sensitive" populations such as asthmatics,
children, and the elderly. Secondary standards set
limits to protect public welfare, including protection against
decreased visibility, damage to animals, crops, vegetation, and
buildings.

The EPA Office of Air Quality Planning and Standards (OAQPS) has
set National Ambient Air Quality Standards for six principal
pollutants, which are called "criteria" pollutants. They are listed
below. Units of measure for the standards are parts per million
(ppm) by volume, milligrams per cubic meter of air
(mg/m3), and micrograms per cubic meter of air
(µg/m3).

National Ambient Air Quality Standards

Pollutant

Primary Stds.

Averaging Times

Secondary Stds.

Carbon Monoxide

9 ppm (10 mg/m3)

8-hour1

None

35 ppm
(40 mg/m3)

1-hour1

None

Lead

1.5 µg/m3

Quarterly Average

Same as Primary

Nitrogen Dioxide

0.053 ppm
(100 µg/m3)

Annual (Arithmetic Mean)

Same as Primary

Particulate Matter (PM10)

50 µg/m3

Annual2
(Arith. Mean)

Same as Primary

150 ug/m3

24-hour1

Particulate Matter (PM2.5)

15 µg/m3

Annual3
(Arith. Mean)

Same as Primary

65 ug/m3

24-hour4

Ozone

0.08 ppm

8-hour5

Same as Primary

0.12 ppm

1-hour6

Same as Primary

Sulfur Oxides

0.03 ppm

Annual (Arith. Mean)

-------

0.14 ppm

24-hour1

-------

-------

3-hour1

0.5 ppm
(1300
ug/m3)

1 Not to be exceeded more than once per
year.

2 To attain this standard, the expected
annual arithmetic mean PM10 concentration at each monitor within an
area must not exceed 50 ug/m3.

3 To attain this standard, the 3-year
average of the annual arithmetic mean PM2.5
concentrations from single or multiple community-oriented monitors
must not exceed 15 ug/m3.

4 To attain this standard, the 3-year
average of the 98th percentile of 24-hour concentrations at each
population-oriented monitor within an area must not exceed 65
ug/m3.

5 To attain this standard, the 3-year
average of the fourth-highest daily maximum 8-hour average ozone
concentrations measured at each monitor within an area over each
year must not exceed 0.08 ppm.

6 (a) The standard is attained when the
expected number of days per calendar year with maximum hourly
average concentrations above 0.12 ppm is <= 1, as determined by
appendix H. (b) The 1-hour standard is applicable to all areas
notwithstanding the promulgation of 8-hour ozone standards under
Sec. 50.10. On June 2, 2003, (68 FR 32802) EPA proposed several
options for when the 1-hour standard would no longer apply to an
area.

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