Air is never clean. Natural sources have polluted our breathable air for centuries. Sources such as volcanic ash, salt released from oceanic waves crashing, smoke from forest fires and windblown dirt particles all contribute to natural air pollution. Humans and their ability to manipulate these natural pollutants have accentuated their natural effect. For example, farmers often use fire to burn off vegetation on their field and to re-oxygenate their fields. However, human production and consumption …showed first 75 words of 4123 total…

…showed last 75 words of 4123 total…Microsoft Corporation. 3) "Carbon Monoxide": Microsoft Encarta Online Encyclopedia, 2001, http://encarta.msn.com (Sept. 30, 2001) 4) Http://w3.iac.net/~ard/ 5) http://www.epa.gov/otaq/03-co.html 6) J.G. Calvert et al., "Achieving Acceptable Air Quality: Some Reflections on Controlling Vehicle Emissions," Science, 261 (July 2, 1993) 38-39. 7) Innes, Robert Regulating Automobile Pollution under Certainty, Competition, and Imperfect Information, Department of Agricultural and Resource Economics, 31, 219-239 (1996). 8) Lutgens, Frederick and Tarbuck, Edward. The Atmosphere: New Jersey, Prentice Hall, 6th ed. 1995: 323-331.