Poor air quality in urban areas is mainly caused by the combustion of fuel by industries, households and vehicles, as well as forest fires and dust. Pollutants are either emitted directly or are the result of chemical reactions.
Pollutants provoke a wide range of negative health effects such as lung and heart malfunctions, bronchitis or asthma. The risk of cancer also increases as the quality of air quality declines.
Inadequate urban planning, the establishment of satellite cities and the preference of private over public transport result in increasing motor vehicle mileage, which consequently raises the levels of air pollution in urban conglomerates.
Fast growing industries applying out-dated technology, the use of poor quality fuel or coal, and the lack of land-use planning allowing heavily polluting factories to produce in urban dwelling centres also contribute to the deterioration of air quality.
If and when air pollution is addressed, the focus is almost always limited to metropolitan areas.