Explain the difference between pure air and polluted air.

Pure air is a mixture of several gases that are invisible and odorless. It consists about 78% nitrogen, 21% is oxygen and less than 1% of Carbon dioxide and other gases like argon, methane and ozone and varying amount of water vapors. Polluted air on the other hand, is contaminated with the substances like smoke, dust and other harmful substances called pollutants. These pollutants can come from natural sources like smoke and dust arising from forest fires or volcanic eruptions. Pollutants can also be added from the human activities. The sources of air pollution are automobile exhausts, power plants, factories and burning of firewood and dung cakes.