The air on our planet is a mixture of some gases in a definite proportion, present of any external element in the air makes it polluted and these elements could be anything like gases, dust, fumes or odour in harmful amounts, these things are called as pollutants.
There are several reasons of air pollution, one the main reason of made made machines which are ejecting millions of dusts and harmful gases in the atmosphere. This uncontrolled pollution effects body both in the home and earth.
To read more about it :- http://en.wikipedia.org/wiki/Air_pollution