National Ambient Air Quality Standard (NAAQS)

The Clean Air Act requires EPA to set National Ambient Air Quality Standards for pollutants considered harmful to public health and the environment. The EPA has set National Ambient Air Quality Standards for six criteria pollutants: sulfur dioxide (S0₂), particulates (PM_2.5/PM₁₀), nitrogen oxides (NO_x), carbon monoxide (CO), ozone (O₃), and lead (Pb).  Periodically, the standards are reviewed and may be revised.