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post from wikipedia:

The Great Smog befell London starting on December 4, 1952 and lasted until March of 1953. It was a great disaster that killed thousands and formed an important impetus to the modern environmental movement.

In early December of 1952, a cold fog descended upon London. Because of the cold, Londoners began to burn more coal than usual. At the same time, the final conversion of London's electric trams to diesel buses was completed. The resulting air pollution was trapped by the heavy layer of cold air, and the concentration of pollutants built up dramatically. The smog was so thick that it would sometimes make driving impossible. It entered indoors easily, and concerts and screenings of films were cancelled as the audience could not see the stage or screen.

Since London was known for its fog, there was no great panic at the time. In the weeks that followed the medical services compiled statistics and found that the fog had killed 4,000 people - most of whom were very young, elderly, or had pre-existing respiratory problems. Another 8,000 died in the weeks and months that followed.

These shocking revelations led to a rethinking of air pollution. The disaster demonstrated to people around the world that it was a real and deadly problem. New regulations were put in place, restricting the use of dirty fuels in industry and banning black smoke. These included the Clean Air Acts of 1956 and of 1968, and the City of London (Various Powers) Act of 1954.
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This seems terrifying to me!