Why does air pollution occur?

The burning of fossil fuels associated with industrial processes and transport produces large amounts of oxides of nitrogen and microscopic particles. Large urban areas and places close to busy roads have the highest concentrations of nitrogen dioxide. These levels are not generally sufficient to cause harmful effects to most people. The highest ammonia concentration's are found in the countryside. Ammonia is particularly associated with agriculture. Levels of ammonia may be high in areas of arable farming, where fertilisers are used to improve the growth of crops, and in areas of livestock farming, where ammonia is released in animal wastes.