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The weather in Greater London had been unusually cold for several weeks leading up to the event. Because of the cold weather, households were burning more coal than usual to keep warm. The smoke from approximately one million coal-fired stoves, in addition to the emissions from local industry, was released into the atmosphere. Increases in smoke and sulfur emissions from the combustion of coal had been occurring since the Industrial Revolution and the British were familiar with these types of smog events. At times, the smoke and emissions were so heavy that residents referred to the events as ‘pea soupers’ because the fog was as dense as pea soup. However, while the area had experienced heavy smog in the past, no event had caused such problems as the weather event in December, 1952.

Formation of the Deadly Smog

Thousands of tons of black soot, tar particles, and sulfur dioxide had accumulated in the air from the heavy coal combustion. Estimates of PM10 concentrations during December, 1952, range between 3,000 and 14,000 ?g/m³ with the high range being approximately 50 times higher than normal levels at the time. PM10 is particulate matter less than 10 micrometers in diameter. Conditions for Londoners today are much better with PM 10 concentrations around 30 ?g/m³. Estimates also suggest that sulfur dioxide levels during December of 1952 were 7 times greater than normal at 700 parts per billion (ppb).

A light fog had lingered in the city throughout the day of December 5, although it was nothing unusual. However, as night came, light winds, cool air, and high humidity at ground-level were ideal conditions for the formation of thick, smoky fog, or smog. The smoke and fumes from the heavy coal combustion settled close to the ground and due to a temperature inversion, remained motionless and created dense smog.

A temperature inversion occurs when the air closer to the ground is cooler than the air above it. This cool air is denser than the warmer air above