PA's criteria air pollutant emissions inventory indicates that releases of all criteria air pollutants except nitrogen oxides have been in decline since the passage of the 1970 Clean Air Act. Overall air quality across the country has improved significantly since the 1980s. These improvements, however, have not eliminated air quality problems, and major efforts to control pollution sources are still required to ensure everyone breathes air that meets Clean Air Act standards.
Air pollution comes from a wide variety of sources that are usually classified into three groups: mobile, area, and point sources. Mobile sources (like cars and trucks) are responsible for about 75% of carbon monoxide pollution, and for more nitrogen oxide and volatile organic compound emissions than area or point sources. Area sources (like small businesses) are responsible for over 50% of particulate matter emissions and for nearly half of all volatile organic compound emissions. Point sources (like power plants and industrial factories) account for nearly 90% of sulfur dioxide emissions. See Scorecard's national summary of criteria air pollution emissions.