smog

Smog was coined by Henry Antoine Des Voeux in 1905 at a Public Health Congress in London. He combined 'smoke' and 'fog' to describe the lethal combination of coal smoke and natural fog that hung over London and other industrial cities. The word was needed because 'fog' alone was insufficient — London's fog was not natural. It was manufactured. The city's million coal fires produced a toxic mixture that the natural fog trapped at ground level.

The Great London Smog of December 1952 killed an estimated 12,000 people in five days. Visibility dropped to zero. The smog entered buildings. The death toll exceeded that of the cholera epidemics that had prompted London's sewer system a century earlier. The disaster led directly to the Clean Air Act of 1956, the first major air-quality legislation in British history. The word smog became the argument for the law.

Los Angeles developed its own smog in the 1940s, but it was chemically different — photochemical smog, produced by the interaction of automobile exhaust and sunlight rather than coal smoke and fog. Arie Haagen-Smit, a chemist at Caltech, identified the mechanism in 1950. The same word was applied to a different phenomenon because the visible result — a brown, acrid haze — looked similar enough. Smog became a generic term for urban air pollution regardless of its chemistry.

Beijing, Delhi, and other megacities now experience smog events that combine both types — industrial emissions, vehicle exhaust, and meteorological inversions. The Air Quality Index, developed in the 1970s, quantified what Des Voeux's portmanteau had named: the measurable danger of breathing in a city. The word that began as a clever coinage became a global health category.