1. Pollution may be defined as contamination of air, water or soil by undesirable amounts of materials or heat and is caused by the concentration of substances which have harmful effects.
2. Toxic and otherwise harmful substances which have an undesirable impact on different components of the environment and life forms are known as pollutants.
3. Air pollution means degradation of air quality due to concentration of harmful contaminants which affect human, plant and animal lives. 

Sulphur dioxide, hydrogen sulphide, carbon monoxide and hydrocarbons.

Compounds of sulphur like sulphur dioxide, sulphur trioxide and hydrogen sulphide are pollutants.

Harmful effects of oxides of sulphur:

(a) It causes headache, vomiting and even death due to respiratory failure.

(b) It destroys vegetation and weakens building materials/constructions.

(c) It mixes with smoke and fog to form smog, which is very harmful.

It is oxidised by atmospheric oxygen into sulphur trioxide (SO₃) which combines with water to form sulphuric acid.

2SO_2(g) + O_2(g) →2SO_3(g)

SO_3(g) + H₂O_(l) → H₂SO_4(l)

1. Volcanoes, decaying vegetation, forest fires and dust storms
2. Automobiles, factories, industrial processes and decay of crop residue in rural areas