Newly restored photos have shown London's deadly sulfuric acid fog of 1952.

In early December of 1952, an unusual fog descended on London. Over the next five days, 4,000 people ended up choking to death in the smog, with a further 100,000 people suffering from respiratory illness as a result of it. More recent estimates argue that unusually high excess deaths in the months following the pea-souper - the name given to thick yellow, black or greenish fog -  with estimates going as high as 12,000.

Photos of the fog - which were restored by Stuart Humphryes aka Babel Colour - show just how little visibility there was after the smog descended.

So what caused this smog, and turned it so very deadly? The answer is our old friend and eventual planet-killer: fossil fuels.

“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,” Texas A&M researcher Renyi Zhang said in a statement, having examined the cause of the fog in 2016.

“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. 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.”

In November and December, the weather had been unusually cold, and people began to burn more coal to keep themselves warm. Normally, smoke from the coal would rise into the atmosphere where it would disperse. However, an anticyclone was above the city at the time, pushing air - and any pollutants from homes and factories alike - downwards towards the ground, trapping polluted particles from dispersing at higher altitudes.

"On each day during the foggy period, the following pollutants were emitted," the UK's Met Office explained. "1,000 tonnes of smoke particles, 2,000 tonnes of carbon dioxide, 140 tonnes of hydrochloric acid and 14 tonnes of fluorine compounds. In addition, and perhaps most dangerously, 370 tonnes of sulphur dioxide were converted into 800 tonnes of sulphuric acid."

The killer fog spurned on the government to create clean air laws in 1956 and 1968, banning black smoke emissions and forcing residents and factory owners to switch to smokeless fuels. Deaths of this scale, attributed to polluted fog, have never happened again.