Air pollution

Air pollution is a general term that covers a broad range of contaminants in the atmosphere. Pollution can occur from natural causes or from human activities. Discussions about the effects of air pollution have focused mainly on human health but attention is being directed to environmental quality and amenity as well. Air pollutants are found as gases or particles, and on a restricted scale they can be trapped inside buildings as indoor air pollutants. Urban air pollution has long been an important concern for civic administrators, but increasingly, air pollution has become an international problem.
The most characteristic sources of air pollution have always been combustion processes. Here the most obvious pollutant is smoke. However, the widespread use of fossil fuels have made sulfur and nitrogen oxides pollutants of great concern. With increasing use of petroleum-based fuels, a range of organic compounds have become widespread in the atmosphere.
In urban areas, air pollution has been a matter of concern since historical times. The use of coal throughout the centuries has caused cities to be very smoky places. Along with smoke, large concentrations of sulfur dioxide were produced. It was this mixture of smoke and sulfur dioxide that typified the foggy streets of Victorian London, paced by such figures as Sherlock Holmes and Jack the Ripper, whose images remain linked with smoke and fog (smog). Such situations are far less common in the cities of North America and Europe today. However, until recently, they have been evident in other cities, such as Ankara, Turkey, and Shanghai, China, that rely heavily on coal.
Coal is still burnt in large quantities to produce electricity or to refine metals, but these processes are frequently undertaken outside cities. Within urban areas, fuel use has shifted towards liquid and gaseous hydrocarbons (petrol and natural gas). These fuels typically have a lower concentration of sulfur, so the presence of sulfur dioxide has declined in many urban areas. However, the widespread use of liquid fuels in automobiles has meant increased production of carbon monoxide, nitrogen oxides, and volatile organic compounds (VOCs).
Primary pollutants such as sulfur dioxide or smoke are the direct emission products of the combustion process. Today, many of the key pollutants in the urban atmospheres are secondary pollutants, produced by processes initiated through photochemical reactions. The photochemical smog is now characteristic of urban atmospheres dominated by secondary pollutants.
Although the automobile is the main source of air pollution in contemporary cities, there are other equally significant sources. Stationary sources are still important and the oil-burning furnaces that have replaced the older coal-burning ones are still responsible for a range of gaseous emissions and fly ash. Incineration is also an important source of complex combustion products, especially where this incineration burns a wide range of refuse. These emissions can include chlorinated hydrocarbons such as dioxin. When plastics, which often contain chlorine, are incinerated, hydrochloric acid results in the waste gas stream. Metals, especially where they are volatile at high temperatures, can migrate to smaller, respirable particles. The accumulation of toxic metals, such as cadmium, on fly ash gives rise to concern over harmful effects from incinerator emissions.