London was unusually cool during November of 1952. The famously cold, damp city depended largely on coal-burning stoves to heat houses and buildings, and as the city grew colder people burned more of it; at the same time, the conversion of the city's electric trams to diesel-burning engines was being finished as autumn ended. What this meant was that London's air, already prone to smog, was dirtier than usual as December began.

Normally, the air at the surface of the Earth is warmer than the air above it, because the ground absorbs sunlight and warms the layer of air closest to the surface. Because the warm air is lower density than the cool air above it, it tends to move upwards, and the resulting convection is one of the things that drives the movement of air in the atmosphere. Occasionally, however, a layer of warm air can move over low-lying cold air, forming an atmospheric inversion. The colder, denser air is held at ground level by the blanket of warm air, leaving the air at the surface still and stagnant. Atmospheric inversions are not uncommon at night, but normally the air begins moving again when the sun comes out.

During the beginning of December of 1952, an atmospheric inversion occurred in London. The morning of December 5th was clear, but as the day went on, a thick, smoky fog fell over the city. The fog prevented the sunlight from dispersing the inversion, which meant a pool of still air formed over London. By the next day, the fog was so dense that all traffic stopped in the city. It was impossible to drive, Heathrow Airport was closed, and the rails were halted. Cars were abandoned at the side of the road when drivers left because they couldn't see. The fog didn't remain outdoors, either. It infiltrated buildings, and grew so dense that cinemas were closed and theatrical performances were canceled. A performance of La Traviata at Sadler's Wells was abandoned at intermission because the theater was so foggy that the audience couldn't see the stage.

London was no stranger to smog, of course. The famous "pea-soupers" in Victorian novels are actually reducing smog, a noxious pollution created by the combination of fog and sulfur dioxide-containing coal smoke. Indeed, such was the city's reputation that the word "smog" itself was coined to describe London's air. But the black, particulate-laced smog that fell on December 5th was heavy enough to halt all activity in the city. Even schools were closed, because it was feared that children would become lost in the oppressive gloom.

For most people the unusually heavy smog was only an inconvenience. But the smog was toxic - various atmospheric processes can oxidize sulfur dioxide into sulfur trioxide, which then combines with the moisture in the fog to produce dilute sulfuric acid. Measurements of sulfur dioxide concentration peaked at 700 parts per billion, seven times normal levels. Particulate concentration during the period was fifty-six times normal levels.

No one recognized at first that the heavy cloud filling London was killing people. In fact, the first sign that the death rate was abnormally high was a sudden shortage of coffins. Florists ran out of flowers. Pneumonia, bronchitis, heart failure, and tuberculosis deaths increased drastically. Overall, the death rate in London was three to four times usual levels during the fog.

The fog cleared on December 9th; official figures released in the wake of the fog claimed that about 4,000 people died because of its effects. These figures, however, tallied up only excessive deaths during the actual fog and the two weeks following - but the death rate in London was abnormally high until the following March, and more recent analyses suggest that around 12,000 people died as a result of the fog. Further, official reports claimed that those who died were already sickly, and would not have lasted much longer under any circumstances. Certainly those with preexisting lung and heart problems were particularly vulnerable - but normally, in the sort of event that causes deaths among the elderly and sickly (as with heat waves), there is a corresponding drop in death rates during the following period. Death rates didn't decline to normal until spring, and there was never a drop that would indicate that the fog only killed people who had been barely clinging to life.

Public outrage following the Great Smog led to the passage of England's 1956 Clean Air Act. City councils were given authorization to set up "smokeless zones" and households were provided grants to convert coal stoves to cleaner methods of heating. A 1968 amendment to the Act required tall chimneys to ensure that smoke wouldn't be emitted just above the city. Major improvement has occurred in London's air, but the city still faces major air quality problems. A much smaller smog in 1991 demonstrated that the city's air is still dirty.

Sources

http://news.bbc.co.uk/2/hi/uk_news/england/2545759.stm
http://news.bbc.co.uk/1/hi/health/2545747.stm
http://www.portfolio.mvm.ed.ac.uk/studentwebs/session4/27/greatsmog52.htm
http://www.ace.mmu.ac.uk/eae/Air_Quality/Older/Great_London_Smog.html
http://www.findarticles.com/p/articles/mi_m0CYP/is_12_110/ai_98539853