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

 

	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
	1.5 µg/m3	Quarterly Average	Same as Primary
Nitrogen Dioxide	53 ppb (3)	Annual (Arithmetic Average)	Same as Primary
	100 ppb	1-hour (4)	None
Particulate Matter (PM10)	150 µg/m3	24-hour (5)	Same as Primary
Particulate Matter (PM2.5)	15.0 µg/m3	Annual (6) (Arithmetic Average)	Same as Primary
	35 µg/m3	24-hour (7)	Same as Primary
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
Sulfur Dioxide	0.03 ppm	Annual (Arithmetic Average)	0.5 ppm	3-hour (1)
	0.14 ppm	24-hour (1)
	75 ppb (11)	1-hour	None

 

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

(2) Final rule signed October 15, 2008.

(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 0.100 ppm (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) (a) 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.

 

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