Vehicles are mainly responsible for more than 80% of total air pollution. The major pollutants released from automobiles, locomotives, aircraft etc., include carbon monoxide, unburnt hydrocarbons and nitrogen monoxide. In the major metropolitan cities like Delhi, Mumbai and Kolkata, vehicular exhaust accounts for 75% of all CO, 45% of all hydrocarbons, 40% of all oxides and 30% of all suspended particulate matter.
Air pollution is also caused by the gaseous and volatile hydrocarbons such as methane, acetylene and ethylene. Some hydrocarbons like ethylene, which undergo chemical reactions in the presence of sunlight and nitrogen oxide form photochemical oxidants that are harmful. Fuel combustion in automobiles also produces nitrogen dioxide.