On December 5, 1952 fog slowly descended on the city of London. Over the course of the day, the fog thickened and acquired a yellowish hue and a scent like rotten eggs.
At the time, an unusual cold snap had chimneys and smokestacks working overtime, and a high-pressure system parked itself over the area, containing the noxious fumes, reports Ben Guarino for The Washington Post.
The smelly cloud grew to be 30-miles wide and was so thick that buses couldn’t run, planes were grounded and even boat traffic came to a halt. Over the next five days, the fog coated sidewalks with black ooze and left smudges on the faces of anyone who dared to walk through it.
But even worse, the Great Smoke, as the incident came to be known, was deadly.
By the time the fog lifted on December 9, 150,000 had been hospitalized and at least 4,000 died from the fog, according to a press release. In 2004, that estimate was revised since many died in the months following the incident. The final death tally was up to 12,000 people.
The Great Smoke was tragic, but it also spurred action. “The 1952 smog was a real knock to the psyche,” Christine Corton, author of the book London Fog tells Sabrina Tavernise at The New York Times. “People had been through so much—the war, the Blitz. People said we didn’t go through all those deprivations to die from coal smoke. They were fed up. They wanted a better quality of life.” In 1956, Britain passed the Clean Air Act.
While everyone knew the incident was driven by the sulfur dioxide in coal smoke, which created an acid when it combined with water vapor, no one could figure out exactly how the process worked. In order for a fog to form, the water vapors must be nearly neutral, not acidic. But a new study, published in the journal Proceedings of the National Academies of Sciences, that focuses on pollution in China may have finally cracked the cold, foggy case.
Guarino reports that an international group of researchers looked at the chemistry of the heavy air pollution in the Chinese cities of Xian and Beijing. Like 1950s London, those cities have high levels of sulfur dioxide. But they wondered why the smog in Britain turned deadly while the pollution in China, while harmful in the long run, does not convert into deadly concentrations of acid.
The researchers collected air samples from the cities and also conducted experiments in the lab to figure out the chemistry of the Chinese pollution. What they discovered is that the hazes form by two different mechanisms. Burning coal releases sulfur dioxide and nitrogen dioxide, which interacted in the water droplets common in the London skies. Initially, the water particles in the fog were large enough to dilute the acid, forming a near neutral fog. But as water vapor evaporated, the fog became acidic, eventually concentrating so strongly that breathing it in was enough to damage the lungs.
In China, however, the researchers found that a third compound entered the mix: ammonia, which comes from agriculture and automobiles. These chemicals all combine to create smog, but the ammonia helps neutralize the fog's acidity, allowing the fog to form and preventing it from becoming deadly.
Still, the research, led by Texas A&M researcher Renyi Zhang does show that controlling nitrogen dioxide and ammonia could help combat some of China’s pollution problems. “We think we have helped solve the 1952 London fog mystery and also have given China some ideas of how to improve its air quality,” Zhang says in the press release. “Reduction in emissions for nitrogen oxides and ammonia is likely effective in disrupting this sulfate formation process.”
The research could also be beneficial to other cities under siege by pollution. New Delhi in India, for instance, was forced to close schools for three days last week when air pollution levels reached 1,000 micrograms per cubic meter, 16 times the level deemed safe.
Update November 21, 2016: This article has been updated to clarify that in order for fog to form the vapors must be neutral.