In December 1952, a dense fog fell over London that lasted roughly four days, dropping visibility and making it hard to breathe. At the time, residents paid little attention to the strange event, writing it off as just another natural fog, but once it lifted, people started dying.

The event – referred to as the Great Smog – led to the death of roughly 12,000 people, and the hospitalisation of up to 150,000. But how could something like this happen?

Well, the general hypothesis back then was that coal emissions had somehow mixed with the fog, which led to people being poisoned by the noxious clouds.

This prompted the British Parliament to pass the Clean Air Act in 1956, and the event earned the title of the worst air pollution event in European history.

Despite the Brits ultimately being correct in their suspicions about the coal emissions, no one was quite sure how chemicals from coal-burning managed to infiltrate the fog.

Nw, over 60 years later, an international team of researchers might have finally figured it out, as part of an investigation into China’s modern air pollution issues.

The answer is actually pretty terrifying – it turns out people were breathing in the fog equivalent of acid rain.

How does that work? According to the team, it’s all about sulphate.

"People have known that sulphate was a big contributor to the fog, and sulfuric acid particles were formed from sulphur dioxide released by coal-burning for residential use and power plants, and other means," said team leader Renyi Zhang from Texas A&M University.

The team performed a series of atmospheric experiments in two Chinese megacities – Xi’an and Beijing.

The tests revealed that sulphate can form thanks to interactions caused by the presence of nitrogen dioxide and sulphur dioxide – two compounds that stem from coal burning – mixing with water droplets in fog.

This toxic sulphate builds up in the naturally foggy environment, forming tiny droplets of sulphuric acid that can be blown around the city and breathed in by its residents.

As Zhang explains:

"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 sulphur dioxide to sulphate is that it produces acidic particles, which subsequently inhibits this process. Natural fog contained larger particles of several tens of micrometres in size, and the acid formed was sufficiently diluted. Evaporation of those fog particles then left smaller acidic haze particles that covered the city."

So the compounds released from burning coal and the compounds found inside natural fog – an aqueous medium made of, you guessed it, water – work together to make droplets of sulphuric acid, the same stuff that makes 'acid rain'.

Except, unlike rain, fog is easy to breathe in, leading to many people becoming poisoned.

Even crazier, this sort of thing is happening all of the time in China, though the chemical process is slightly different, requiring ammonia for sulphate to form because the particles being emitted are much smaller.

"In China, sulphur dioxide is mainly emitted by power plants, nitrogen dioxide is from power plants and automobiles, and ammonia comes from fertiliser use and automobiles," Zhang said.

"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."

This means that by understanding an event that happened more than 60 years ago, the team is helping scientists and governments to understand modern pollution problems as well.

They hope officials will use to make better regulations or technologies that limit the amount of air pollution pumped into the skies every year, especially in China.

"The government has pledged to do all it can to reduce emissions going forward, but it will take time," Zhang adds.

"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. Reduction in emissions for nitrogen oxides and ammonia is likely effective in disrupting this sulphate formation process."

Hopefully, the new research will also help countries around the world as they transition to industrial powerhouses like China has over the last century, allowing them to continue making goods without harming both the environment and the people who live in it.

The team’s work was published in Proceedings of the National Academy of Sciences.