In December 1952, there was an unusual episode of heavy smog in London, which lasted till March 1953. The light winds and high moisture content created ideal conditions for smog formation. That winter was also extremely cold which caused an increase in coal combustion and the increase in car travel, which caused a combination of black soot, sticky particles of tar and gaseous sulphur dioxide. This freak occurrence lead to the heaviest winter smog episode recorded.The smog episode killed almost 12,000 people, mainly children, elderly people and people with chronic respiratory diseases. The number of deaths during the smog episode was three to four times more than on a normal day. The deaths were attributed to lung disease, tuberculosis and heart failure. Mortality from bronchitis and pneumonia increased more than sevenfold. Most deaths occurred due to the breathing in of acid aerosols, which irritate and inflame bronchial tubes. The acidity was not measured at the time, but estimates show that pH levels could have fallen to as low as 2 at the peak of the smog episode.The highest death rate recorded was between December 8th and 9th, with an average of 900 deaths a day. In some of the poorer parts of the city, death rates were nine times their normal number.
The heavy pollution and the resulting death toll increased awareness of the seriousness of air pollution. The London smog disaster resulted in the introduction of the first Clean Air Acts in 1956.