1952 – Great Smog of 1952: A cold fog descends upon London, combining with air pollution and killing at least 12,000 in the weeks and months that follow
. 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, December 5, 1952, to Tuesday, December 9, 1952, and then dispersed quickly after a change of 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, with London having experienced many smog events in the past, so-called "pea soupers." However, government medical reports in the following weeks estimated that up until 8 December 4,000 people had died prematurely and 100,000 more were made ill because of the smog's effects on the human respiratory tract. More recent research suggests that the total number of fatalities was considerably greater, at about 12,000. It is known to be the worst air-pollution event in the history of the United Kingdom, and the most significant in terms of its effect on environmental research, government regulation, and public awareness of the relationship between air quality and health. It led to several changes in practices and regulations, including the Clean Air Act 1956.