December 1952 brought an episode of heavy smog to London, which lasted until March 1953. Light winds and a high moisture content created ideal conditions for smog formation. The unusual cold in London in the winter of 1952-1953 caused additional coal combustion and many people travelled only by car, which caused the occurrence of a combination of black soot, sticky particles of tar and gaseous sulphur dioxide. This resulted in the heaviest winter smog episode known to men.

Measurements suggested that the concentration of particulate matter in the air had reached 56 times its normal level. Sulphur dioxide concentrations increased to seven times its peak level. The smoke particles trapped in the fog gave it a yellow-black colour. Sulphur dioxide reacted with substances in foggy droplets to form sulphuric acid, adding an intense form of acid rain to the process.

By night of December 5 the smog was so dense that visibility dropped to only a few meters. Smog easily entered buildings, causing cinemas, theatres, and stores to be closed. Transport became largely impossible. Motor vehicles were abandoned, trains were disrupted, and airports were also closed.

The smog episode killed approximately 12.000 people, mainly children, elderly people and people suffering from chronic respiratory or cardiac disease. The number of deaths during the smog disaster was three or four times that on a normal day. They could be attributed to lung disease, tuberculosis and heart failure. Mortality from bronchitis and pneumonia increased more than sevenfold.

Peaks of smoke and sulphur dioxide were in line with peaks in deaths. However, most deaths occurred because of breathing in acid aerosols, which irritates or inflames the bronchial tubes. Acidity was not measured, but estimates show that the pH probably fell to 2 during the peaks in the smog episode.

The highest death rate during the smog episode occurred on December 8 and December 9, at 900 deaths per day. In some parts of the city death rates even increase to nine times the normal number. Until spring the death rate remained high at almost a thousand more deaths per week than expected in a normal winter.

This heavy pollution and its resulting death toll made people aware of the seriousness of air pollution. The London smog disaster resulted in the introduction of the first Clean Air Acts in 1956.