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 it and does not rise, as warmer air relative to that above it would, but remains trapped under the inversion, close to the ground. Temperature inversions are uncommon but occur more frequently on cold winter nights because the ground cools and water vapor precipitates on low-level dust particles, forming a mist. This caused the thick, smoke-polluted air to be trapped under the inversion. After nightfall, the fog thickened and reduced visibility to only a few meters. The following 114 hours in London experienced visibility less than 500 meters with 48 hours below 50 meters visibility. Heathrow Airport had visibility levels below 10 meters for nearly 48 hours following the morning of December 6. The city was brought to a practical standstill with road, rail, and air transport unable to operate because of the impaired visibility.
Temperature inversions are often reversed in the morning when radiation from the sun warms the ground below the mist. However, on the morning of December 6 the concentrations of smoke were still extremely high, and water vapor continued to condense around the black soot and tar particles. The sun’s radiation was unable to break through the dense smog. This caused the static layer of cooler, polluted air to remain trapped in the lower atmosphere. The fog lasted for 5 days, from December 5 through 10, until winds dispersed the dense air mass and transported the pollution through the Thames Estuary and into the North Sea.
During the week of December 5, the fog, dense with soot and tar particles, reacted with the atmospheric sulfur dioxide and formed a solute sulfuric acid. The heavy fog was inescapable – it was not only on the streets, but also entered into homes.
Causes of Death
The smog-related deaths were primarily attributed to pneumonia, bronchitis, tuberculosis, and heart failure. Many with preexisting conditions, including asthma, died of respiratory distress. Many others died of cardiac distress and asphyxiation. Non-fatal...
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