Polluted air over a community. The term, a combination of “smoke” and “fog,” was popularized in the early 20th century and now commonly refers to the pall of automotive or industrial origin that lies over many cities. Sulfurous smog results from the use of sulfur-bearing fossil fuels, particularly coal, and is aggravated by dampness. Photochemical smog requires neither smoke nor fog. Nitrogen oxides and hydrocarbon vapours emitted from automobiles and other sources undergo reactions in the presence of sunlight that produce a light brownish coloration of the atmosphere, reduced visibility, plant damage, irritation of the eyes, and respiratory distress.

This entry comes from Encyclopædia Britannica Concise.
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