The Great Smog or Big Smoke was a severe air pollution event that affected London in December 1952. A period of cold weather combined with an anticyclone and windless conditions, collected airborne pollutants mostly from the use of coal to form a thick layer of smog over the city. It lasted from Friday 5th to Tuesday the 9th December 1952,and then quickly dispersed after a change in the weather.
Although it caused major disruption due to the effect on visibility, and even penetrated indoor areas, it was not thought to be a significant event at the time. In the following weeks however, medical reports estimated that 4,000 had died prematurely and 100,000 more were made ill due to the smog's effects on the human respiratory tract. More recent research suggests that the number of fatalities was considerably higher at around 12,000.
Source: India Syndicate