(i) It is mainly caused by industries and automobiles. Automobiles are the greatest pollutior of the atmosphere and are responsible for `75%` of noise pollution and about `60-80%` of air pollution of big cities.

(ii) The combustion of fossil fuels (coal, oil, gas etc.) releases `CO`, `CO_2`, nitrogen oxides, fluorides, hydrocarbons etc. into atmosphere usually in the form of smoke causing air pollution.

(iii) Particulate matters released by mills, factories and mines also cause air pollution.

(iv) Some of the natural air pollutants are pollen grains, spores, volcanoes etc.