The substances which contaminate the air are called air pollutants. Sometimes, such substances may come from natural sources like smoke and dust arising from forest fires or volcanic eruptions.
Pollutants are also added to the atmosphere by certain human activities.
The sources of air pollutants are factories, power plants, automobile exhausts and burning of firewood and dung cakes.
Vehicles produce high levels of pollutants like carbon monoxide, carbon dioxide, nitrogen oxides and smoke . Carbon monoxide is produced from incomplete burning of fuels such as petrol and diesel. It is a poisonous gas. It reduces the oxygen-carrying capacity of the blood.