Environmental Services Program

National Ambient Air Quality Standards

The air quality of outdoor air breathed by the public is determined by carefully comparing the monitored level of air pollutants with air quality standards established by the EPA. The National Ambient Air Quality Standards set the limits for six air pollutants that have documented effects on public health.

The National Ambient Air Quality Standards contain primary and secondary standards for each of these six air pollutants.

If then...
A primary standard is exceeded health risks exist
A secondary standard is exceeded crops, trees and buildings may be damaged

Most of the air quality standards are based on a particular time length for averaging, and the average concentration during that time. For a violation to be recorded the average concentration, usually rounded to the same number of significant digits as the standard, must be greater than the standard during the specified averaging period.



1. Carbon Monoxide or CO

Carbon Monoxide Standards
Averaging Time Primary Standard Secondary Standard
One-hour 35 ppm None
Eight-hour 9 ppm None

Neither standard is allowed to be exceeded more than once per year.

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2. Lead or Pb

Lead Standards
Averaging Time Primary Standard Secondary Standard
Three-month (Rolling average) 0.15 ug/m3 Same as Primary
Quarterly Average 1.5 ug/m3 Same as Primary

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3. Nitrogen Dioxide or NO2

Nitrogen Dioxide Standards
Averaging Time Primary Standard Secondary Standard
One-year 53 ppb Same as Primary
One-hour 100 ppb None

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.

The one-hour standard is a 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).

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4. Ozone or O3

Ozone Standards
Averaging Time Primary Standard Secondary Standard
Eight-hour (standards) 0.075 ppm 0.075 ppm

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5. Sulfur Dioxide or SO2

Sulfur Dioxide Standards
Averaging Time Primary Standard Secondary Standard
24-hour 0.14 ppm None
One-year 0.03 ppm None
One-hour/Three-year 75 ppb None

To attain the one-hour/three-year 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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6. Particulate Matter

Particulate matter (PM) is an airborne mixture of liquid droplets and solid particles made up of organic chemicals, metals, acids or dust particles. There are two groups of PM that matter most since they can easily be inhaled: inhalable particulates or PM10 and fine iInhalable particulates or PM2.5.

Inhalable Particulates or PM10

Inhalable Particulates (PM10) Standards
Averaging Time Primary Standard Secondary Standard
24-hour (standards) 150 ug/m3 150 ug/m3

Fine Inhalable Particulates or PM2.5

Fine Inhalable Particulates (PM2.5) Standards
Averaging Time Primary Standard Secondary Standard
One-year (standards) 15.0 ug/m3 15.0 ug/m3
24-hour 35 ug/m3 35 ug/m3

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For more information about the pollutants and their standards visit EPA National Ambient Air Quality Standards.

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