Air pollution can never be given a precise and clear cut definition. It would be appropriate to define air pollutant than air pollution. Air pollutants are generally defined as aerial substances that have some adverse effects on plants, animals (including man) or materials.
When the rate of pollution increases or the self-purifying capacity beyond certain limits decreases, accumulation of pollutants occur causing serious health problems.
Air borne pollutants can be classified into two main groups –
1. Gaseous pollutants
2. Panic late pollutants.
(1) Gaseous Pollutants:
Gaseous pollutants include substances that are gaseous in nature and are of two types:
(a) Primary pollutants, and
(b) Secondary pollutants.
(a) Primary pollutants are admitted directly from different sources and mixing up with the atmosphere, i.e. carbon dioxide, carbon monoxide, sulphur dioxide, hydrogen sulphite, hydrogen fluoride and oxides of nitrogen.
The burning of fossil fuels like coal, natural gas and oil releases considerable amount of carbon dioxide, carbon monoxide and super dioxides and hydrogen sulphides to the atmosphere. Carbon monoxide is produced by incomplete oxidation of carbonaceous fuels. Other sources are photoxidation of organic matter and biological oxidation of marine organizations.
Sulphur dioxide and hydrogen sulphide are mainly released from the combustion of sculpture containing fossil fuels. But the main sources are from industries like copper, lead, zinc where sulphur containing ores are roasted. Oil refineries, sulphuric acid manufacturing plants and fertiliser, paper and pulp industries, Hydrogen fluoride is liberated mainly from aluminum smelting industries and oxides of nitrogen are mainly released from automobile exhausts.
(b) Secondary pollutants are formed in the atmosphere either from the primary pollutants or from compounds by interaction in the atmosphere. Exhaust gases of automobiles have particular importance in the formation of secondary pollutants such as, ozone, aldehydes and ketenes.
(2) Particulate pollutants:
Air-borne paniculate materials consist of both solid and liquid particles mainly dust, fume, mist, spray and smoke. The main sources are from industry and factory chimneys and automobile exhausts and from mining areas.