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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 – carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter and sulfur dioxide. 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

 

Primary Standards

Secondary Standards

Pollutant

Level

Averaging Time

Level

Averaging Time

Carbon Monoxide

9 ppm

(10 mg/m3)

8-hour (1)

None

35 ppm

(40 mg/m3)

1-hour (1)

Lead

0.15 μg/m3(2)

Rolling 3-month Average

Same as Primary

Nitrogen Dioxide

53 ppb(3)

Annual (Arithmetic Average)

Same as Primary

100 ppb

 1-hour (4)

  None

Ozone

0.075 ppm

(2008 std)

8-hour (8)

Same as Primary

0.08 ppm

(1997 std)

8-hour (9)

Same as Primary

0.12 ppm

1-hour (10)

Same as Primary

Particulate Matter (PM10)

150 μg/m3

24-hour (5)

Same as Primary

Particulate Matter (PM2.5)

15.0 μg/m3

Annul  (6)    (Arithmetic Average)

Same as Primary

35 μg/m3

24-hour (7)

Same as Primary

Sulfur Dioxide

0.03 ppm (11)(1971 std)

Annual (Arithmetic Average)

0.5 ppm

3-hour (1)

0.14 ppm (11) (1971 std)

24-hour (1)

 75 ppb (12)  1-hour

None 

 

(1) Not to be exceeded more than once per year.

(2)Final rule signed October 15, 2008. The 1978 lead standard (1.5 µg/m3 as a quarterly average) remains in effect until one year after an area is designated for the 2008 standard, except that in areas designated nonattainment for the 1978 standard, the 1978 standard remains in effect until implementation plans to attain or maintain the 2008 standard are approved.

(3) The official level of the annual NO2 standard is 0.053 ppm, equal to 53 ppb, which is shown here for the purpose of clearer comparison to the 1-hour standard

(4) To attain this standard, the 3-year average of the 98th percentile of the daily maximum 1-hour average at each monitor within an area must not exceed 100 ppb (effective January 22, 2010).

(5) Not to be exceeded more than once per year on average over 3 years.

(6) To attain this standard, the 3-year average of the weighted annual mean PM2.5 concentrations from single or multiple community-oriented monitors must not exceed 15.0 µg/m3.

(7) 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 35 µg/m3 (effective December 17, 2006).

(8) 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 0.075 ppm. (effective May 27, 2008)

(9) (a) 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.
(b) The 1997 standard—and the implementation rules for that standard—will remain in place for implementation purposes as EPA undertakes rulemaking to address the transition from the 1997 ozone standard to the 2008 ozone standard.
(c) EPA is in the process of reconsidering these standards (set in March 2008).

(10) (a) EPA revoked the 1-hour ozone standard in all areas, although some areas have continuing obligations under that standard ("anti-backsliding").
(b) The standard is attained when the expected number of days per calendar year with maximum hourly average concentrations above 0.12 ppm is < 1.

(11) The 1971 sulfur dioxide standards remain in effect until one year after an area is designated for the 2010 standard, except that in areas designated nonattainment for the 1971 standards, the 1971 standards remain in effect until implementation plans to attain or maintain the 2010 standards are approved.

(12) Final rule signed June 2, 2010. To attain this standard, the 3-year average of the 99th percentile of the daily maximum 1-hour average at each monitor within an area must not exceed 75 ppb.

 

 

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