The Great Smog of London, or Great Smog of 1952, was a severe air pollution event that affected London, England, in December 1952. A period of unusually cold weather, combined with an anticyclone and windless conditions, collected airborne pollutants—mostly arising from the use of coal—to form a thick layer of smog over the city. It lasted from Friday 5 December to Tuesday 9 December 1952, then dispersed quickly when the weather changed.The smog caused major disruption by reducing visibility and even penetrating indoor areas, far more severely than previous smog events, called "pea-soupers". Government medical reports in the weeks following the event estimated that up to 4,000 people had died as a direct result of the smog and 100,000 more were made ill by the smog's effects on the human respiratory tract. More recent research suggests that the total number of fatalities may have been considerably greater, with estimates of between 10,000 and 12,000 deaths.London had suffered since the 13th century from poor air quality. The Great Smog is thought to be the worst air pollution event in the history of the United Kingdom, and the most significant for its effects 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.