Urban air quality

The nature of urban air pollution has clearly varied through both time and space. In most developed countries, pollutants which are largely invisible are now the major source of concern, while in developing countries the traditional visible pollutants, such as smoke, and continue to be a major problem. Urban areas, with their high population densities and high levels of energy consumption, are associated with a range of environmental problems of which air pollution is only one. Health effects are the primary area of concern related to urban air quality, resulting from exposure to pollutants both indoors and outdoors. The environmental stresses of rapid urbanization are exacerbated by higher population densities in the cities of the developing world. With few exceptions, urbanization has been accompanied by industrial development, often with housing and industry being mixed together. As widespread industrialization relied on coal, urbanization was accompanied by smoking chimneys. Smoke and SO₂ were, therefore, the first major urban pollutants.