London smog disaster

London smogLondon smogLondon, England was the site of a dense smog caused by heavy coal combustion from December 5 to December 9, 1952, that killed about 12,000 people.

Causes of the Smog

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 μg/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 μg/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 health effects from the smog included short-term chest pains, lung inflammation and diminished breathing ability, damaged respiratory cells, permanent lung damage, and increased incidence of asthma attacks. It is also thought that the smog could have increased the population’s risk of cancer.

The implications of the fog were not immediately clear. It was not until the deaths peaked on the 8th and 9th of December at 900 per day that the people knew something was wrong. During the smog and for two weeks following, approximately 4,000 people were killed. Some reports indicate that death rates remained above-normal for the entire winter and it is now thought that approximately 12,000 deaths can be tied to the great smog in the winter of 1952. The death toll could be thousands higher if it were known how many died from complications of smog-related illnesses in the following months and years.

At the time, officials reported that the smog had caused the deaths of mainly the old and those already suffering from chronic cardiovascular and respiratory illness. It was later determined that only two-thirds of the original 4,000 dead were over 65 years of age. Deaths in the middle-age range of 45 to 64 years experienced death rates three times greater than normal during the event. Infants were also highly-susceptible to the pollution-laden smog and infant mortality doubled during the week of December 5, 1952.

Fifty years later, scientists examined well-preserved archival autopsy tissues of people known to have died from the smog exposure. That research supports the hypothesis the deaths were due to the effects ultrafine carbonaceous and metal particulate matter.

Aftermath

The smog-related deaths spurred the British government to take action and clean up the nation’s air. Society was becoming aware of the connection between fuel combustion, atmospheric pollution, and damages to public health. The 1956 Clean Air Act gave local governments the authority to provide funds to households to convert their coal-fired heaters for use of cleaner sources of energy such as gas, oil, smokeless coal, or electricity. The 1968 Clean Air Act was aimed at industry and introduced the use of taller chimneys which allowed the pollution from coal combustion to be released higher into the atmosphere.

As residents and businesses were necessarily given time to convert, however, fogs continued to be smoky for some time after the Act of 1956 was passed. In 1962, for example, 750 Londoners died as a result of a fog, but nothing on the scale of the 1952 Great Smog has ever occurred again.

These policies may have alleviated the immediate pollution impacts of coal combustion, we are now aware that taller chimney stacks have led to long-range transport of sulfur dioxide, or transboundary pollution. Transboundary pollution has been discovered as the cause of acid rain in regions without significant local emissions of sulfur dioxide.

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