This article is part of Yahoo's 'On This Day' series.
In 1952, a fog so thick that conductors had to walk in front of buses holding flares descended on London - and it left thousands dead.
The Great Smog of 1952, which began on December 19, reportedly saw a herd of cows choked to death in their fields, and settled on the city for five days.
The intense mist - made of a mixture of smoke from coal fires and fog - was so thick that people in the Isle of Dogs in London were unable to see their own feet.
Street crime boomed, cars were abandoned in the streets and cinema films and concerts - even indoors - were cancelled as audiences could not see stages and screens.
Smog had become a regular occurrence in London, and there was little panic in the initial stages of the Great Smog.
Britain’s Met Office explains, ‘Britain has long been affected by mists and fogs, but these became much more severe after the onset of the Industrial Revolution in the late 1700s.
‘Factories belched gases and huge numbers of particles into the atmosphere, which in themselves could be poisonous. The pollutants in the air, however, could also act as catalysts for fog, as water clings to the tiny particles to create polluted fog, or smog.’
But the Great Smog - as it became known - became a catalyst for change on this day in 1956, when the Clean Air act passed.
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The severity of the Great Smog was only realised after the event, when figures were published which showed that up to 4,000 people had died.
Fogs which settled on the city had become a common event in London, due to pollution of the city streets.
As early as 1880, meteorologist Rollo Russell wrote a pamphlet entitled ‘London Fogs’, writing, ‘We may infer that numerous deaths occur in the course of the year from smoke-fogs, not unusually thick, producing or increasing diseases of the lungs.
Initially, people didn’t panic or react as if the Great Smog was anything unusual, says Kate Winkler Dawson, editor of Death in the Air, a history book which discusses the Great Smog.
Dawson told The Verge, ‘We’re talking about 300 years of smog like this. Because of the weather system, this turned into an extraordinarily long and extraordinarily deadly smog but these weather systems, these anticyclones, had come every year.
‘They were always there. They lasted two or three days, usually two, and then they’d be blown away by a wind and all the pollution would float to the atmosphere.’
But the Great Smog would prove to be a catalyst for lasting change.
In 1956, the Clean Air Act - passed in direct response to the Great Smog - established smoke-free areas in cities and restricted the burning of coal in household fires.
Considered a milestone in environmental legislation, it paved the way for subsequent acts which ensure that city air is far cleaner today.
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