Scientists have finally determined what caused a dense fog that killed thousands of people in London in 1952. CBS News reports that a team of researchers from the U.S., China and the U.K. studied the incident, which is believed to have killed more than 12,000 people and sent more than 150,000 to the hospital.

“Researchers have for a long time connected emissions from burning coal with the killer fog, but the specific chemical processes that led to the deadly mix of pollution and fog were not fully understood,” the report notes.

“Study lead author Renyi Zhang, an atmospheric scientist at Texas A&M University, said that sulfate was a big contributor to the deadly London fog,” the story reports. “Sulfuric acid particles, which formed from the sulfur dioxide that was released from the burning of coal, were also a component of the fog. The question was, How did sulfur dioxide get turned into sulfuric acid?”

In a statement, Zhang said: “Our results showed that this process was facilitated by nitrogen dioxide, another co-product of coal burning, and occurred initially on natural fog. Another key aspect in the conversion of sulfur dioxide to sulfate is that it produces acidic particles, which subsequently inhibits this process.”

The report adds: “The natural fog contained larger particles, Zhang explained, with the smaller acidic particles evenly distributed throughout. When those fog particles evaporated, an acidic-haze was left covering the city.”