On Dec. 5, 1952, the first hints of what would eventually become the United Kingdom's worst environmental disaster began to appear.

"You had this swirling," funeral director Stan Cribb told NPR in 2002 of the grey wisps that began to drift down streets. "[It was] like somebody had set a load of car tires on fire."

By Dec. 8, the 30-mile-wide fog (referred to by Londoners as a "pea-souper") enveloped the entire city, reducing visibility to less than three feet and making outdoor excursions of any kind unbearable. Black ooze covered the sidewalks. Scores of animals died at local markets. And birds, unable to see in the blinding mist, began crashing into buildings and littering the sidewalks.

"The air was not only dark, tinged with yellow, but stank of rotten eggs," the BBC recounted. "Those who ventured out into the soot-choked air recall returning home with their faces and clothes — even petticoats — blackened. Some were bought to their knees, coughing uncontrollably."

You can see an old MovieTone news reel on the event, showing just how miserable the conditions were:

By the time the fog cleared on Dec. 9, an estimated 12,000 men, women and children had died and more than 150,000 had been hospitalized.

Learning from one disaster to prevent another

While the burning of millions of residential coal fires has always been correctly blamed as the chief culprit of the disaster, scientists had been puzzled about why this particular event was so toxic. Thanks to an international team of scientists from China, the U.S., and the U.K., we now know the answer.

In a paper published in the Proceedings of the National Academies of Sciences, the team describes how sulfate emitted from the burning of coal interacted with the fog to form a toxic concentration of sulfuric acid.

"Our results showed that this process was facilitated by nitrogen dioxide, another co-product of coal burning, and occurred initially on natural fog," Texas A&M researcher Renyi Zhang said in a statement. "Another key aspect in the conversion of sulfur dioxide to sulfate is that it produces acidic particles, which subsequently inhibits this process. Natural fog contained larger particles of several tens of micrometers in size, and the acid formed was sufficiently diluted. Evaporation of those fog particles then left smaller acidic haze particles that covered the city."

The team was motivated to solve the chemistry behind the 1952 disaster less from curiosity and more to better understand the dramatic smog events currently plaguing China's major cities.

"In China, sulfur dioxide is mainly emitted by power plants, nitrogen dioxide is from power plants and automobiles, and ammonia comes from fertilizer use and automobiles," said Zhang. "Again, the right chemical processes have to interplay for the deadly haze to occur in China. Interestingly, while the London fog was highly acidic, contemporary Chinese haze is basically neutral."

The researchers hope that by analyzing how smog events form, the information will better help Chinese officials curb crippling air pollution events in the future.

“A better understanding of the air chemistry holds the key for development of effective regulatory actions in China,” Zhang added.