The cloud of smog that choked London in 1952 killed 4,000 people with a deadly, foggy suspension of sulfuric acid. For decades, scientists weren’t sure where the toxic compound came from. As dramatized in an episode of Netflix’s new show, The Crown, the Queen’s researchers knew, early on, that coal-burning factories spewing sulfur dioxide were to blame. But how the gas was converted into a lung-searing acid remained a mystery — until now.

Drawing parallels to the Great Fog with extreme pollution in Beijing — thought to kill 4,000 people daily — a global team of researchers associated with Texas A&M University uncovered the mechanics of death by smog. In their paper, published in the Proceedings of the National Academy of Scientists this week, they explain that London’s historic pollution disaster was very much a team effort involving several molecules.

London's Great Fog lasted 5 days and hospitalized 150,000 people.

It all started with sulfur dioxide. This chemical is released when fossil fuels like coal are burned, and it reacts with other compounds in the air to create sulfates, tiny particles that create a vision-impairing haze; in London, this haze was so dense it literally blocked out the sun and made the world dark. In the PNAS article, the scientists show that the conversion of this already-dangerous chemical to sulfates also produced sulfuric acid, the active compound in forest-scalding acid rain.

Sulfate production, they write, is the result of the unholy marriage of sulfur dioxide with a chemical known as nitrogen dioxide, which is also released when coal is burned. As pollution and actual fog intermingled in London, the water vapor in the fog presented the perfect environment for sulfates — and sulfuric acid — to be produced. The molecules of sulfuric acid mixed together with particles of water vapor, and as those acidic fog droplets evaporated, they “left smaller acidic haze particles that covered the city,” lead author Renyi Zhang, Ph.D., told Zhang says this throwback is informative for modern science, in that “[reduction] in emissions for nitrogen oxides and ammonia is likely effective in disrupting this sulfate formation process.”

The researchers are hoping that this lesson from the past can inform our scientific future — and inspire us not to repeat our mistakes. In 1956, British Parliament passed the Clean Air Act, an effort to curb pollution from coal-burning plants, in response to the killer fog, setting what many would consider a precedent for governments to respond to human-prompted catastrophes — which may include one in our future.

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