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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) |
15.0 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 15.0 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 NAAQS
will no longer apply to an area one year after the effective date of the designation
of that area for the 8-hour ozone NAAQS. The effective designation date for most
areas is June 15, 2004. (40 CFR 50.9; see Federal Register of April 30, 2004
(69 FR 23996).)
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