National Ambient Air Quality Standards

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Counties in the United States where one or more National Ambient Air Quality Standards are not met, as of June 2007.

The National Ambient Air Quality Standards (NAAQS) are standards established by the United States Environmental Protection Agency that apply for outdoor air throughout the country. Primary standards are designed to protect human health, with an adequate margin of safety, including sensitive populations such as children, the elderly, and individuals suffering from respiratory disease. Secondary standards are designed to protect public welfare from any known or anticipated adverse effects of a pollutant (e.g. building facades, visibility, crops, and domestic animals).

NAAQS requires the EPA to set standards on six criteria air contaminants:

Ozone (O₃)
Particulate Matter
- PM₁₀, coarse particles: 2.5 micrometers (μm) to 10 μm in size (although current implementation includes all particles 10 μg or less in the standard)
- PM_2.5, fine particles: 2.5 μm in size or less
Carbon monoxide (CO)
Sulfur dioxide (SO₂)
Nitrogen oxides (NO_x)
Lead (Pb)

[edit] Standards

The standards are listed in Title 40 of the Code of Federal Regulations Part 50.

Pollutant	Type	Standard	Averaging Time^a	Regulatory Citation
SO₂	Primary	0.14 ppm (365 μg/m³)	24-hour	40 CFR 50.4(b)
SO₂	Primary	0.030 ppm (80 μg/m³)	annual	40 CFR 50.4(a)
SO₂	Secondary	0.5 ppm (1,300 μg/m³)	3-hour	40 CFR 50.5(a)
PM₁₀	Primary and Secondary	150 μg/m³	24-hour	40 CFR 50.6(a)
PM_2.5	Primary and Secondary	35 μg/m³	24-hour	40 CFR 50.7(a)
PM_2.5	Primary and Secondary	15 μg/m³	annual	40 CFR 50.7(a)
CO	Primary	35 ppm (40 mg/m³)	1-hour	40 CFR 50.8(a)(2)
CO	Primary	9 ppm (10 mg/m³)	8-hour	40 CFR 50.8(a)(1)
O₃	Primary and Secondary	0.12 ppm (235 μg/m³)	1-hour^b	40 CFR 50.9(a)
O₃	Primary and Secondary	0.075 ppm (235 μg/m³)	8-hour	40 CFR 50.10(a)
NO₂	Primary and Secondary	0.053 ppm (100 μg/m³)	annual	40 CFR 50.11(a) and (b)
Pb	Primary and Secondary	1.5 μg/m³	quarterly	40 CFR 50.12

Note a: Each standard has its own criteria for how many times it may be exceeded, in some cases using a three year average.

Note b: As of June 15, 2005, the 1-hour ozone standard no longer applies to areas designated with respect to the 8-hour ozone standard (which includes most of the United States, except for portions of 10 states).

Source: USEPA