The composition of pure air contains 78% nitrogen, 21% oxygen, 0.03% carbon dioxide and 0.97% other gases. Other gases include methane, argon, ozone and water vapours etc. Pure air is also considered to be free of dust, aerosols and reactive gaseous contaminants of anthropogenic origin.
The air is said to be polluted when the composition of air is altered by the addition of harmful substances or gases such as sulphur dioxide, carbon monoxide, nitrogen dioxide and particulate matter. The presence of such substances in the atmosphere are harmful to the health of humans and other living beings and can also cause damage to the climate.