smog, community-wide polluted air. Its composition is variable. The term is derived from the words smoke and fog, but it is commonly used to describe the pall of automotive or industrial origin that lies over many cities. The term was probably first used in 1905 by H.A. Des Voeux to describe atmospheric conditions over many British towns. It was popularized in 1911 by Des Voeux’s report to the Manchester Conference of the Smoke Abatement League of Great Britain on the more than 1,000 “smoke-fog” deaths that occurred in Glasgow and Edinburgh during the autumn of 1909.
At least two distinct types of smog are recognized: sulfurous smog and photochemical smog. Sulfurous smog, which is also called “London smog,” results from a high concentration of sulfur oxides in the air and is caused by the use of sulfur-bearing fossil fuels, particularly coal. This type of smog is aggravated by dampness and a high concentration of suspended particulate matter in the air. Smoglike air pollution trapped higher in the atmosphere can persist as atmospheric brown clouds, which can cause climatic and health effects. See alsoAsian brown cloud.
Photochemical smog, which is also known as “Los Angeles smog,” occurs most prominently in urban areas that have large numbers of automobiles. It requires neither smoke nor fog. This type of smog has its origin in the nitrogenoxides and hydrocarbon vapours emitted by automobiles and other sources, which then undergo photochemical reactions in the lower atmosphere. The highly toxic gas ozone arises from the reaction of nitrogen oxides with hydrocarbon vapours in the presence of sunlight, and some nitrogen dioxide is produced from the reaction of nitrogen oxide with sunlight. The resulting smog causes a light brownish coloration of the atmosphere, reduced visibility, plant damage, irritation of the eyes, and respiratory distress. Surface-level ozone concentrations are considered unhealthy if they exceed 70 parts per billion for eight hours or longer; such conditions are fairly common in urban areas prone to photochemical smog.