Londoners are used to a regular dose of fog, drizzle, and cloud in the winter. But in early December 1952, the city’s fog took a much deadlier turn.

By the time "the big smoke" had lifted just five days later, it had killed 4,000 people and left 150,000 others seriously ill in hospital with respiratory tract infections. In the long term, it’s estimated at least 12,000 people died from the killer fog, along with thousands of animals.

The strange and terrifying event is surprisingly unknown about, even to scientists. But a new study, published in the Proceedings of the National Academies of Sciences, set out to discover the unanswered questions of how the fog came to be so deadly and why it was so dangerously acidic.

An international team of scientists from China, the US, and the UK mimicked the London fog in the lab and compared it to recordings of the atmospheric conditions from two heavily polluted Chinese cities, Beijing and Xi’an.

Sulfates were a key component of the London fog. This gave the fog its definitively thick, smelly, and toxic properties. It’s always been correctly assumed that the London fog was caused by sulfur dioxide released by the burning of low-quality soft coal from chimneys, industry, and power plants. However, why this sulfur dioxide turned into sulfuric acid remained unknown.

A news cast of the London fog in December, 1952.

“Our results showed that this process was facilitated by nitrogen dioxide, another co-product of coal burning, and occurred initially on natural fog," lead author Renyi Zhang of Texas A&M University explained 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.”

A similar chemistry is happening right now in the air of rapidly industrializing Asian cities, many of which are in China. However, a few differences in the nature of the fog stops it from creating sulfuric acid. High levels of ammonia from China’s extensive fertilizer use and road traffic neutralizes the particles. This makes it less acidic, but a still utterly unsavory cocktail of sulfur dioxide and nitrogen dioxide.

“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,” said Zhang. “Reduction in emissions of nitrogen oxides and ammonia is likely effective in disrupting this sulfate formation process... The government has pledged to do all it can to reduce emissions going forward, but it will take time.”