Any substance present in the environment in harmful concentration, which adversely alters the environment by damaging the growth rate of a species and by interfering with the food chains, is toxic and affects the health, comfort and property etc. is considered as a pollutant.
Smoke from industries and automobiles, domestic and commercial sewage, radioactive substances from nuclear plants and discarded household articles (tins, bottles, broken crockery etc.) come under the category of pollutants.
Types Of Pollutant:
(a) Primary Pollutants:
These are those which are emitted directly from the source and persist in the form in which they were added to the environment. Typical examples of pollutants included under this category are ash, smoke, fumes, dust, nitric oxide, sulphur dioxide, hydrocarbons etc.
(b) Secondary Pollutants:
These are those which are formed from the primary pollutants by chemical interaction with some constituent present in the atmosphere. Examples are: Sulphur trioxide, nitrogen dioxide, aldehydes, ketones, ozone etc.
Nitrogen oxides and hydrocarbons are two primary pollutants released from automobiles but in the presence of sunlight, they react to form peroxyacyl nitrate (PAN) and ozone, two secondary pollutants which are far more toxic than the primary pollutants from which they are derived. This phenomenon of increased toxicity by chemical interaction among the pollutants is known as Synergism.