Air Quality

The U.S. Environmental Protection Agency set national air quality standards for six common pollutants (also referred to as "criteria" pollutants). These standards are known as National Ambient Air Quality Standards.
 
Pollutant
[links to historical tables of NAAQS reviews]
Primary/
Secondary
Averaging Time Level Form
Carbon Monoxide (CO) primary 8 hours 9 ppm Not to be exceeded more than once per year
1 hour 35 ppm
Lead (Pb) primary and
secondary
Rolling 3 month average 0.15 μg/m3 (1) Not to be exceeded
Nitrogen Dioxide (NO2) primary 1 hour 100 ppb 98th percentile of 1-hour daily maximum concentrations, averaged over 3 years
primary and
secondary
1 year 53 ppb (2) Annual Mean
Ozone (O3) primary and
secondary
8 hours 0.070 ppm (3) Annual fourth-highest daily maximum 8-hour concentration, averaged over 3 years
Particle Pollution (PM2.5) primary

secondary
1 year 12.0 μg/m3
15.0 μg/m3
annual mean, averaged over 3 years
primary and
secondary
24 hours 35 μg/m3 98th percentile, averaged over 3 years
Particle Pollution (PM10) primary and
secondary
24 hours 150 μg/m3 Not to be exceeded more than once per year on average over 3 years
Sulfur Dioxide (SO2) primary 1 hour 75 ppb (4) 99th percentile of 1-hour daily maximum concentrations, averaged over 3 years
secondary 3 hours 0.5 ppm Not to be exceeded more than once per year

(1) In areas designated nonattainment for the Pb standards prior to the promulgation of the current (2008) standards, and for which implementation plans to attain or maintain the current (2008) standards have not been submitted and approved, the previous standards (1.5 µg/m3 as a calendar quarter average) also remain in effect.

(2) The level of the annual NO2 standard is 0.053 ppm. It is shown here in terms of ppb for the purposes of clearer comparison to the 1-hour standard level.

(3) Final rule signed October 1, 2015, and effective December 28, 2015. The previous (2008) O3 standards additionally remain in effect in some areas. Revocation of the previous (2008) O3 standards and transitioning to the current (2015) standards will be addressed in the implementation rule for the current standards.

(4) The previous SO2 standards (0.14 ppm 24-hour and 0.03 ppm annual) will additionally remain in effect in certain areas: (1) any area for which it is not yet 1 year since the effective date of designation under the current (2010) standards, and (2)any area for which implementation plans providing for attainment of the current (2010) standard have not been submitted and approved and which is designated nonattainment under the previous SO2 standards or is not meeting the requirements of a SIP call under the previous SO2 standards (40 CFR 50.4(3)), A SIP call is an EPA action requiring a state to resubmit all or part of its State Implementation Plan to demonstrate attainment of the require NAAQS.