These are solid, liquid or gaseous substance present in the atmosphere in such concentrations, as may be or tend to be injurious to human beings or other living creatures, plants and property.
Any substance present in environment in such a concentration which adversely alters the environment by damaging the growth rate of a species and by interfering with the food chain is toxic and affects health, comfort and property.
eg: smoke from domestic and commercial areas and industries, radioactive substances from nuclear power plants, discarded and waste household articles.
The air pollutants are classified on the basis of origin and physical state.
(a) On the basic of origin: pollutants are of two types:
(i) Primary pollutants:
Those pollutants that are emitted directly from the sources and are present in the atmosphere in the same form in which they were emitted area called primary pollutants eg. CO, NO2, SO2 and hydrocarbon, emitted by burning of coal, fuels, vehicular exhaust and NH3 emitted from agricultural processes. Radioactive pollutants produced by nuclear explosions and process (radon gas) pollen grains, micro organisms, bacteria, spoils.
(ii) Secondary pollutants:
Those pollutants that are formed in the atmosphere by chemical interactions between primary pollutants and atmospheric constituents are called secondary pollutants. These are usually formed by oxidation, hydrolysis and photochemical reactions e.g. SO3, O3, peroxyacylnitrate, (PAN), aldehydes and ketones etc.
Hydro carbons + (NO)x + sunlight PAN, peroxides, formaldehyde + aldehyde + acrolin
(b) On the basic of state of matter: pollutants are of two types:
(i) Gaseous pollutants:
The pollutants which occur in gaseous state at normal temperature and pressure e.g CO, CO2, SO, NO2, hydrocarbon and photochemical oxidants.
(ii) Particulate pollutants:
All atmospheric substances that are not gases but may be suspended droplets, solid particles or mixture of the two are generally referred as particulates.