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Air Pollution 
Definition of air pollution According to the World Health Organization (WHO), air pollution is defined as "limited to situations in which the outdoor ambient atmosphere contains materials in concentration which are injurious to humans and their surrounding environment." Air pollutants in cities is mostly a mixture of many different pollutants, some visible such as dust and soot, but many invisible, such as very small particles and gases. The more visible components are evident, the more small particles and gases can be expected. Pollution is classified either as primary or secondary. Primary pollutants are substances directly produced by a process, such as ash from a volcanic eruption, or the carbon monoxide gas from a motor vehicle exhaust. Secondary pollutants are not emitted. Rather, they are formed in the air when the primary pollutants react or interact. Ozone is an example of a secondary pollutant. Brief introduction to the principal air pollutants Carbon Monoxide. It is an odourless, colourless gas produced by incomplete oxidation (burning) of carbon. Carbon monoxide is produced naturally by oxidation of methane, in the oceans and air, produced from organic decomposition. In cities, the motor vehicle is by far the largest source, although any combustion process may produce it. Sulphur Dioxide. Sulphur dioxide in the atmosphere arises from both natural and human activities. Natural processes like volcanic eruptions release sulphur compounds. The main human activities producing sulphur dioxide are the smelting of mineral ores containing sulphur, and the combustion of fossil fuels. Sulphur dioxide dissolves in water, forming sulphuric acid which is highly corrosive in nature. It is very damaging to materials, plants and animal tissues. Particulate Matter. Particulate Matter is a collective term used to denote a broad array of finely divided solid or liquid dispersed in air. Particulate matter may include living organisms like bacteria, viruses, moulds, algae, pollen grains etc. The non-viable particulates in the atmosphere are mists, smokes, fumes, dusts etc. Particulate matter of health-threatening size may be emitted from a number of sources like volcanoes and dust storms, power plants, industrial processes, and incinerators. The size of such particulate matter ranges from less than 0.1 micrometres (µm) to hundreds of micrometres. Particles that have an aerodynamic diameter of less 10 µm are referred to as PM10. As these particles are small enough to enter the human respiratory system, they are also called 'respirable particulate matter'. Particles that are greater than 10 microns are normally retained by the cilia in the nose, and do not enter the respiratory tract. Therefore, particles larger than 10 microns do not cause much harm except irritation of the nose and the eyes. Lead. The major source of lead in the air is leaded fuel used in motor vehicles. The introduction of unleaded fuel in 1985 has resulted in a substantial decrease in the concentration of lead in the air. Lead is a heavy metal, and when present in the body, it can impair brain function, especially in children. Oxides of Nitrogen. The major human activity which generates oxides of nitrogen is fuel combustion, especially in motor vehicles. Oxides of nitrogen form in the air when the fuel is burnt at high temperatures. This is mostly in the form of nitric oxide, with less than 10 percent as nitrogen dioxide. Once emitted, nitric oxide combines with oxygen ('oxidises') to form nitrogen dioxide. These oxides of nitrogen may remain in the atmosphere for several days, and during this time, chemical processes may generate nitric acid and particles like nitrates and nitrites. These oxides of nitrogen play a major role in the chemical reactions that generate photochemical smog. Ozone. Ozone is a colourless gas formed by the chemical reactions between reactive organic gases and oxides of nitrogen in the presence of sunlight. Ozone is one of the secondary irritant pollutants present in urban-generated smog. There is also a stratospheric layer of ozone at 12-50 km altitude. Contrary to the ozone near the surface, this layer is of paramount importance to prevent the deadly UV-radiation from reaching the ground. Last update: 16 April 2013
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