The Great Smog befell London starting on December 4, 1952. It was a great disaster and formed one of the most important impetuses 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. Also at the same time the final conversion of London's electric trams to diesel buses was completed. This air pollution was trapped by the heavy layer of cold air and the concentration of pollutants built up dramatically.
The fog was so thick that it would sometimes make driving impossible. It entered indoors easily as well and concerts and movies were cancelled as the audience could not see the end of the theatre.
Since London was known for its fog, there was no great panic from the weather at the time. It was only in the weeks that followed when the medical services compiled statistics and found that the fog had killed 4000 people - most of whom were either very young, elderly or had pre-existing respiratory problems. Another 8000 died in the weeks and months that followed.
These revelation caused a great shock that lead to a rethinking of air pollution. The disaster demonstrated to people around the world that it was a real, and a deadly problem. New regulations were put in place restricting the use of dirty fuels in industry and banning black smoke.
