Pollutants from these sources may not only prove a problem in the immediate vicinity of these sources but can travel long distances, chemically reacting in the atmosphere to produce secondary pollutants such as acid rain or ozone. Evolutionary Trends in Pollution Problems In both developed and rapidly industrialising countries, the major historic air pollution problem has typically been high levels of smoke and SO 2 arising from the combustion of sulphur-containing fossil fuels such as coal for domestic and industrial purposes. These long-term changes in air pollution climate are also seen, often occurring very rapidly, in many developing countries. In both developed and developing countries, the major threat to clean air is now posed by traffic emissions. In addition, photochemical reactions resulting from the action of sunlight on NO2 and VOCs from vehicles leads to the formation of ozone, a secondary long-range pollutant, which impacts in rural areas often far from the original emission site.
Acid rain is another long-range pollutant influenced by vehicle NOx emissions.
However, traffic pollution problems are worsening world-wide. Below is an introduction to the principal pollutants produced by industrial, domestic and traffic sources. Both wet and dry deposition have been implicated in the damage and destruction of vegetation and in the degradation of soils, building materials and watercourses. SO 2 in ambient air is also associated with asthma and chronic bronchitis. The principal source of this gas is power stations burning fossil fuels which contain sulphur.