Jane Onyanga-Omara

USA TODAY

LONDON — The decades-old mystery of what caused a killer fog that claimed the lives of thousands of people here appears to have been solved by a team of international scientists.

The Great Smog, which blanketed the British capital for five days in December 1952, is estimated by some experts to have killed more than 12,000 people and hospitalized 150,000. Thousands of animals also died.

During a cold snap on Dec. 5 that year, sulphur particles mixed with fumes from burning coal and made the yellow fog smell like rotten eggs. Some Londoners reported being unable to see their feet, and transportation was canceled with the exception of the London Underground. Birds flew into buildings, and robberies increased as thieves were able to make an easy getaway.

The smog eventually lifted on Dec. 9, after cold winds swept the fumes out to the North Sea.

The incident eventually led to the Clean Air Act of 1956, restricting the burning of coal in urban areas in the United Kingdom.

A team of scientists now believe they have solved the mystery of the exact cause and nature of the fog, through lab experiments and measuring the atmosphere in China, which is home to 16 of the 20 most polluted cities in the world.

Their work was published in Proceedings of the National Academy of Sciences.

“People have known that sulfate was a big contributor to the fog, and sulfuric acid particles were formed from sulfur dioxide released by coal burning for residential use and power plants, and other means,” lead author Renyi Zhang, a scientist at Texas A&M University, said in a statement.

“But how sulfur dioxide was turned into sulfuric acid was unclear. Our results showed that this process was facilitated by nitrogen dioxide, another co-product of coal burning, and occurred initially on natural fog.”

The study said that similar conditions often occur in China.

“The difference in China is that the haze starts from much smaller nanoparticles, and the sulfate formation process is only possible with ammonia to neutralize the particles,” Zhang said.

He said sulfur dioxide is mainly emitted by power plants in China, while nitrogen dioxide comes from power plants and vehicles, and ammonia comes from fertilizer use and vehicles.

“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,” he added.