Pollution
Air can be polluted by many things.  Examples include poisonous gases, sulphur dioxide, nitrogen dioxide, carbon monoxide and very small particulates.  Smoke and harmful gases released by fires, industries, and thermal power plants cause air pollution.  Using coal and wood as fuels for fire causes a lot of air pollution.  Petroleum produces less pollution per ton, but it causes a lot of pollution since a lot of it is burned globally.  Air pollution may cause health problems such as asthma or other breathing problems.