Air Pollution : What is it?

Air pollution is a broad term that covers many areas; as such air pollution cannot be given an exact definition. Air pollution could be loosely defined as "unnaturally high levels of airborne toxins or substances, often emitted by the combustion of fuels for domestic and industrial use, which may occur in a variety of forms including smog, smoke and odours and relate to such deleterious processes as the greenhouse effect and acid rain."

Air pollution in the past has traditionally been related to high levels of smoke and sulphuric oxides, in particular sulphur dioxide (SO2), from fossil fuels burnt for both industrial and domestic purposes. In most cases this situation is improving, however, air pollution from traffic is a major area for concern and, on a world wide scale, is increasing all the time. The chief pollutants released by petrol and diesel combustion engines include carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOX), volatile organic compounds (VOC) and particulates (PM10).

Sulphur dioxide is a particularly harmful compound, having detrimental effects to both the environment and human health. Sulphur compounds are often associated with offensive and unpleasant odours, SO2 is an acidic compound that may combine with water vapour in the atmosphere, cooling and condensing to form cloud, before being precipitated as acid rain. Here it harms the natural environment, acidifying aquatic systems and destroying large areas of forest, as well as damaging man made structures and buildings via acidic corrosion. Sulphur compounds may also have health implications causing breathing difficulties and coughing in asthmatics.

Carbon monoxide is highly toxic, being both odourless and colourless makes it even more dangerous as it is difficult to detect, and can effect the blood systems and breathing of both asthmatic and healthy individuals. Carbon dioxide is less toxic but has implications upon global warming and is a recognised greenhouse gas, it contributes to the layer of gases in the earths atmosphere that prevent reflected solar energy from escaping back into space and lead to atmospheric heating. This in turn may increase the rate at which the polar icecaps are melting, causing sea levels to rise and flood much of the world's present landmass.

Oxides of nitrogen are most commonly associated with the burning of fossil fuels in petrol and diesel engines, as such they are often found in the highest concentration in urban areas or areas where there is a high traffic flow. However, due to their release from fossil fuels, power stations and many industrial processes may release significant levels of these gases too. Oxides of nitrogen, in particular Nitrogen Dioxide (NO2), may lead to respiratory illnesses and suppression of the immune system leading to infections and secondary illness.

Volatile organic compounds (VOC's) are often the result of unburned fuels released from the exhaust which then evaporate into the atmosphere or products of combustion, including aromatic hydrocarbons such as benzene. Particulate matter is often the product of combustion and may take the form of soot released from the exhaust of diesel engines in particular. Finer particulate matter is of particular concern as it stays suspended in the air for long periods and may enter the lungs where it may cause direct damage or act as a carrier for other harmful molecules e.g. VOC's. Particulate matter can often be related to respiratory difficulties and, like VOC's, may have carcinogenic effects or even cause disorders of the nervous system, Report: Poor air quality in Bristol linked to deaths in over-65s >

Other pollutants and areas for concern include heavy metals e.g. lead, which may harm the nervous system or lead to birth defects, and ozone, which is not directly released but forms from NO compounds and may contribute to global warming.

See air pollution archive at Environmental Health Journal >

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