Five days of fog in London, thousands dead, one new law.
From 5 to 9 December 1952 a windless high-pressure zone sat over London. Coal smoke from millions of stoves stayed trapped, visibility dropped to a few metres, and buses crawled behind walking guides. Early estimates counted 4,000 deaths; later analyses put it at up to 12,000. The Clean Air Act followed in 1956.
Useful: Air-quality limits feel abstract until you know what a city without them looked like.
Source: en.wikipedia.org