The London smogs - poisonous combines of smoke and fog - reached their zenith during the 1950s, and specifically with the Great Smog of December 1952. Trapped by a layer of air derived from an anticyclone, the smoke from the city's factories and coal fires bonded with the freezing fog to coat London and its population for five days - Friday, December 5th until Tuesday, December 9th.
Over the five days, 1,000 tonnes of smoke particles entered London's air environment. Not only was all public transport halted, bar the Underground, but so was London's ambulance service. When the weather changed, the smog disappeared - but more than 4,000 people had died as a direct result, in what remains the worst air-pollution event of the United Kingdom's history.