Agents that bring about an undesirable change of our environment are called pollutants.
The United States Environment Protection Agency (EPA) has defined pollutant as any substance introduced into the environment that adversely affects the usefulness of a resource.
According to Environmental (Protection) Act, (1986) of India, pollutant means any solid, liquid or gaseous substance present in such concentration as may be or tend to be injurious to the environment.
There are two basic types of pollutants on the basis of their degradation:
1. Non-biodegradable pollutants:
Materials like waste plastic bottles, polyethenelene bags, used soft drink cans, waste glasses, poisons like mercuric and cadmium salts, chlorinated hydrocarbon pesticides like DDT, BHC etc. either do not degrade or degrade very slowly in the natural environmental conditions. Such pollutants are not acted upon by microbs. These pollutants accumulate in the environment, enter into food chain and are biologically magnified.
2. Biodegradable pollutants:
Materials like plant and animal remaining domestic sewage, market garbage, livestock wastes etc. are rapidly decomposed by the natural processes i.e. by action of microorganisms such as bacteria and fungi and are called Biodegradable pollutants. Some of them are decomposed through primary, secondary or tertiary treatments by human engineered sewage treatment plants. However, serious problems arise when the input of biodegradable pollutants exceeds the decomposition or dispersal capacity of the environment.
Primary and Secondary Pollutants:
Primary pollutants are released directly from their source, e.g. oxides of carbon and sulphur from burning of fossil fuels, unburt hydrocarbons from automobiles, smoke, fly-ash from factories or dusts of cement, asbestos, mills, and factories. Their toxicity is comparatively less.
Secondary pollutants are formed by combination of two or more primary pollutants in environment by photochemical reactions. Example: Photochemical smog formed by unburnt hydrocarbons of automobiles, oxides of nitrogen and ultra-violet radiation of sunlight. They are not only toxic but also mutagenic and/or carcinogenic.