Historically, the term smog referred to a mixture of smoke and fog, hence the name smog. This type of smog was common in Britain due to the burning of coal. This produced a smoke composed of oxides of sulfur. In 1952 this type of smog persisted over London for 4 days and was responsible for over 4000 deaths. The term smog today more often refers to photochemical smog. Photochemical smog results from chemical reactions between various pollutants present in high concentrations. This second type of smog is common in Los Angeles, California and other major metropolitan areas. One way in which the production of photochemical smog is initiated is through the photochemical reaction of nitrogen dioxide (NO2) to form ozone. Vehicle engines, which are extremely numerous in LA, do not completely burn the petroleum they use as fuel. This produces nitrogen dioxide which is released through the vehicle exhaust along with a high concentration of hydrocarbons. The absorption of solar radiation by the nitrogen dioxide results in the formation of ozone (O3). Ozone reacts with many different hydrocarbons to produce a brownish-yellow gaseous cloud which may contain numerous chemical compounds, the combination of which, we call photochemical smog.
Both types of smog can greatly reduce visibility. Even more importantly, they pose a serious threat to our health. They form as a result of extremely high concentrations of pollutants that are trapped near the surface by a temperature inversion. Many of the components which make up these smogs are not only respiratory irritants, but are also known carcinogens.
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