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Protecting Clean Air 

The Clean Air Act, which was last amended in 1990, requires EPA to set National Ambient Air Quality Standards (NAAQS - pronounced "nacks") for pollutants considered harmful to public health and the environment. The Clean Air Act identifies two types of standards for ambient (outdoor) air:

  • A primary standard, designed to protect public health 
  • A secondary standard, designed to protect public welfare. Welfare includes damage to plants and animals, impairment of visibility, and property damage.


National Ambient Air Quality Standards
PollutantAveragePrimary StandardSecondary Standard
Carbon Monoxide8-hour average9 ppmsame as primary
Carbon Monoxide1-hour35 ppmsame as primary
Sulfur Dioxide1-hour75 ppbsame as primary
Sulfur Dioxide3-hournone0.50 ppm
Nitrogen DioxideAnnual53 ppbsame as primary
Nitrogen Dioxide1-hour100 ppbsame as primary
LeadRolling 3-month average0.15 ug/m3same as primary
Ozone8-hour average0.070 ppmsame as primary
PM1024-hour average150 ug/m3same as primary
PM2.524-hour average35 ug/m3same as primary
PM2.5Annual average12 ug/m315 ug/m3