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Of all of London’s pea-soup fogs, the Great Smog was the deadliest.

When a late November chill persisted into early December 1952, Londoners heaped extra coal into their hearths to stay warm. The smoke from the chimneys added to the usual factory exhaust. Because much of Britain’s finer-quality coal had been sold to pay the country’s postwar debts, what was burned instead was cheap, sulfurous stuff. At the same time, an anticyclone — a column of sinking cool air around a high-pressure zone — settled over the capital city, trapping the smoke below.

The Big Smoke was born. Particles of sulfuric acid circulated within the haze — in unusually concentrated amounts, and in places lethally so. A team of scientists, writing recently in the Proceedings of the National Academy of Sciences, said they have figured out the chemical process needed to create the dangerous acid fog.

Over London the wet, clear sky shaped into fog so thick that visibility was as low as three feet. Traffic came to a standstill. It was, as the Guardian reported 60 years ago, a “busy time for thieves.”

It was also a busy time for florists and undertakers. The city reportedly ran out of coffins. By the time the fog lifted five days later, roughly 4,000 people had died from inhaling toxins in the smoke. Some 150,000 were hospitalized. A 2004 study calculated the fog claimed a total of 12,000 lives in the months that followed, as the residents succumbed to complications from breathing the killer fog.

(One of the better episodes of Netflix’s “The Crown” revolves around the event.)

The blame was pinned, accurately, on the sulfur dioxide in the coal smoke, which mixed with the fog’s water vapor and became an acid. In the fog of December 1952, as Britain’s Met Office noted, 370 metric tons of sulfur dioxide produced 800 metric tons of sulfuric acid particles.

Chinese citizens documented the dark haze that enveloped northern Chinese cities in December 2015 as air pollution levels jumped far above the level considered safe by the World Health Organization. (Jenny Starrs and Jason Aldag/The Washington Post)

“People have known that sulfate was a big contributor to the fog, and sulfuric acid particles were formed from sulfur dioxide released by coal burning for residential use and power plants, and other means,”  said Texas A&M researcher Renyi Zhang in a news release.

But how that acid formed — and in turn gave the pea soup its deadly bite — was not fully understood until now, according to Zhang and a group of atmospheric chemistry experts from Texas A&M University and universities in Beijing and Xian, China.

The researchers measured air pollution in the megacities of Xian and Beijing, two Chinese metropolises that have recently struggled with poor air quality and haze. Combined with laboratory tests, the study filtered out what made London so immediately toxic and the haze in Chinese cities not quite so lethal. (That is not to say it is not harmful, as China’s higher rates of asthma and pollution-related deaths demonstrate.) 

In London, the anticyclone trapped wet air. Within the fog, the sulfur dioxide released by coal mixed with another compound, nitrogen dioxide — that, too, a byproduct of coal burning — to form the sulfate particles. As water in the foggy cloud evaporated, the left-behind acid particles were concentrated into toxic levels. “Evaporation of those fog particles,” Zhang said, “then left smaller acidic haze particles that covered the city.” When inhaled, the particles wreaked havoc on the lungs, leaving Londoners susceptible to fatal respiratory tract infections.

Sulfates also existed in the Chinese haze, Zhang noted, though the air pollution in Xian and Beijing had close to a neutral pH, not an acidic one like London’s. The particulate matter in Chinese air pollution also tended to be smaller. Where clouds helped produce sulfate in the Great Smog, the scientists said that ammonia fertilizer played a similar role in China. Zhang and his colleagues concluded that a focus on ammonia and nitrogen oxide emissions would help prevent another killer fog.

Although the Big Smoke spurred Britain to pass its 1956 Clean Air Act, air pollution remains a problem around the world. In India, as the New York Times reported earlier in November, a polluted haze forced New Delhi schools to close for three days.

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