Primarily air pollutants can be caused by primary sources or secondary sources. The pollutants that are a direct result of the process can be called primary pollutants. A classic example of a primary pollutant would be the sulfur-dioxide emitted from factories - See more at: http://www.conserve-energy-future.com/causes-effects-solutions-of-air-pollution.php#sthash.OEe4amQv.dpuf