G-Web

Air pollution is defined as the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or damages the natural environment into the atmosphere. Pollutants can be in the form of solid particles, liquid droplets, or gases. Pollutants may be classified as either primary or secondary pollutants. A primary pollutant is one entering the atmosphere directly, factory and transport emmissions, or volcanic ash or other natural sources. Secondary pollutants form in the air when primary pollutants react or interact. An important example of a secondary pollutant is ground level ozone — one of the many secondary pollutants that make up photochemical smog. Some pollutants may be both primary and secondary, i.e. they originate as a primary, then undergo chemical reaction to become secondary.