The Great Smog of December 1952 - which started 64 years ago today - was a deadly event in London’s history. A combination of dense fog, cold weather and the smoke from London’s coal fires led to the ‘pea souper’ smog enveloping the city for a four day period – which claimed the lives of as many as 12,000 people.

Although coal fires were always known to be the cause – indeed the event led to the passing of the Clean Air Act in 1956 – new research has revealed the process which made the smog so deadly.

An international research team has discovered that the same chemical processes can be seen at work in modern day Chinese cities and other polluted areas.

In a study published in the journal Proceedings of the National Academies of Sciences, researchers have pinpointed sulphate as the cause of the deadly fog.

“People have known that sulphate was a big contributor to the fog, and sulphuric acid particles were formed from sulphur dioxide released by coal burning for residential use and power plants, and other means,” explained Renyi Zhang from Texas A&M university in a statement.

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“But how sulphur dioxide was turned into sulphuric 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.

“Another key aspect in the conversion of sulphur dioxide to sulphate is that it produces acidic particles, which subsequently inhibits this process.

[In pictures: The great smogs of the 1950s]

“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.”

The study also revealed that similar chemical processes are at work in China – home to 16 of the 20 most polluted cities in the world.

The Great Smog started on December 5 1952 when the dense cloud enveloped London – already in the grip of a particularly cold winter which saw coal fires burning across the city. At first it appeared little different from a natural fog, but conditions soon deteriorated. Visibility was as low as three feet in many parts of the city. Public transport was shut down and pedestrians were forced to link arms in human chains to help them make their way home.