Air pollution is caused by harmful gases such as carbon dioxide, sulphur dioxide, nitrogen dioxide, carbon monoxide, and very small particles of carbon. Most of the pollution is caused by factories, vehicles and livestock. Today air pollution has become dangerous to humans. Heavy air pollution may cause breathing problems such as asthma or other health problems. Increase in human activities has also polluted air. Harmful gases may come from people smoking and cooking indoors. Outdoors, harmful gases may come from vehicles and vessels, power stations and volcanoes.