As a consequence of coal burning during the Industrial Revolution, black smoke filled the air of big cities like London (and chimney sweeping became a major industry). The smoke would become particularly thick when a weather inversion occurred: this meant that a layer of warmer air a thousand meters or so above the ground trapped a layer of cooler air below, and the smoke could no dissipated. In London, this smoke would mix with fog, creating what a doctor, H.A. des Voex, named smog in 1905. Because of smog, Victorian artists commonly shrouded their cityscapes in an amber wash. Perhaps a thousand deaths per year in London were attributed to smog-induced heart and respiratory failure. Finally, in 1956, following a particularly deadly smog, Britain passed pollution-control laws.

The smog that frequently develops now in cities like Los Angeles looks like the coal-smoke smog of earlier years, but has a different composition. Los Angeles suffers from the development of photochemical smog, so name because it forms when automobile exhaust (carbon monoxide, unburned hydrocarbons, and nitrous oxide) reacts with air in the presence of sunlight to form ozone, nitrogen dioxide, and other hydrocarbons. Weather inversions trap this mixture, along with other pollutants like soot, sulfur oxide, and lead, in the Los Angeles basin, a low area surrounded by high mountains. During some years, there may be over two hundred days when the public health department declares a pollution advisory because breathing can be harmful.

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