A pollutant defined as any solid, liquid or gaseous substance present in such a concentration which may be or tend to be injurious to the environment. For example carbon dioxide and carbon monoxide are present in small amount in the atmosphere however when their concentration become so excess so as to affect human health it is considered as a pollutant. Similarly small amount of organic matter become excessive due to fertilizer run off, domestic effluents, detergents etc they are considered as pollutant.
Few other examples for pollutant are fluorides, metal ions, chloro fluro carbons (CFC), radioactive wastes, noise, heat etc. Since pollutant are present in trace amount. Their concentration are usually expressed as parts per million(ppm or parts per billion or parts per trillion.
Pollutants are also classified a biodegradable or non biodegradable. Metal ions, mercury salts, DDT detergents, etc are non degradable while organic compounds are biodegradable.