"A substance or condition that contaminates air, water, or soil. Pollutants can be artificial substances, such as pesticides and PCBs, or naturally occurring substances, such as oil or carbon dioxide, that occur in harmful concentrations in a given environment."pollutant. (n.d.). The American Heritage® Science Dictionary. Retrieved February 15, 2007, from Dictionary.com website: http://dictionary.reference.com/browse/pollutant
"physical, chemical, biological, [or] radioactive . . . substance or matter which is emitted into or otherwise enters the ambient air." (CAA, sec. 302(g))"
The Clean Air Act in the U.S.