How Clean Is Your Outdoor Air?
The Clean Air Act, last amended in 1990, is the comprehensive federal law that regulates air emissions from stationary and mobile sources. These pollution limits are called National Ambient Air Quality Standards (NAAQS). The Clean Air Act requires the EPA to set NAAQS for pollutants considered harmful to public health and the environment. The standards track six common "criteria" pollutants: particulate matter (PM), ground-level ozone (O3), carbon monoxide (CO), sulfur dioxide (SO2), nitrogen oxides (NO2) and lead (Pb).
When an area's monitored air pollution exceeds the NAAQS a certain number of times, EPA designates it as a "nonattainment" area. This designation is determined by a formula that uses the number of times the standard is exceeded each year. The formula includes three years of data to ensure that an area isn't designated as nonattainment because of one unusual year and the law also includes a periodic review process.