National Ambient Air Quality Standards

The Environmental Protection Agency sets limits on safe concentrations in the air of six emissions:

• Particulate matter (less than 10 micrometers in diameter)
• Sulfur dioxide
• Ozone
• Nitrogen dioxide
• Carbon monoxide
• Lead.

The standards are for average concentrations over various lengths of time, from three hours to one year, depending on the emission. The standards are designed to protect human health (primary), to prevent other types of damage (secondary), or to do both. The current standards are as follows:

National Ambient Air Quality Standards

 

Pollutant	Primary Stds.	Averaging Times	Secondary Stds.
Carbon Monoxide	9 ppm (10 mg/m3)	8-hour(1)	None
	35 ppm (40 mg/m3)	1-hour(1)	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)	Revoked(2)	Annual(2) (Arith. Mean)	Revoked(2)
	150 µg/m3	24-hour(3)	Same as Primary
Particulate Matter (PM2.5)	15.0 µg/m3	Annual(4) (Arith. Mean)	Same as Primary
	35 µg/m3	24-hour(5)	Same as Primary
Ozone	0.08 ppm	8-hour(6)	Same as Primary
	0.12 ppm	1-hour(7) (Applies only in limited areas)	Same as Primary
Sulfur Dioxide	0.03 ppm	Annual (Arith. Mean)	[see below]
	0.14 ppm	24-hour(1)	[see below]
	[see above]	3-hour(1)	0.5 ppm (1300 µg/m3)

 

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

(2) Due to a lack of evidence linking health problems to long-term exposure to coarse particle pollution, the agency revoked the annual PM10 standard in 2006 (effective December 17, 2006).

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

(4) 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.

(5) 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).

(6) 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.

(7) (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) As of June 15, 2005 EPA revoked the 1-hour ozone standard in all areas except the fourteen 8-hour ozone nonattainment Early Action Compact (EAC) Areas.

 

Source http://www.epa.gov/air/criteria.html