The Great Smog of 1952: How a Windless Week Turned London’s Coal Smoke Into Acid

On the morning of Friday, December 5, 1952, Londoners woke to fog. That was nothing new. The city had been famous for its "pea-soupers" for more than a century, and a grey, smoky haze over the rooftops barely rated a mention on the radio. But this fog did not lift. For five days it sat on the capital, thick and yellowish-brown, until people in parts of the East End could not see their own feet.

By the time a west wind finally swept it away on Tuesday, December 9, the Great Smog had killed thousands of people, and the true number would take half a century to pin down. It remains the worst air pollution event in British history. It also left behind a scientific puzzle: why was this fog so much deadlier than the hundreds of London fogs before it? The full answer only arrived in 2016, from a lab studying the haze over Beijing.

Five days in the dark

The first day was unremarkable. According to the Met Office, the fog on December 5 was not especially dense and had a dry, smoky feel. It thickened after dark, and visibility fell to a few meters. On Saturday the low winter sun was too weak to burn it off, and each night after that it closed in again. On the Isle of Dogs, people could not see their own feet (Met Office). By Sunday, December 7, visibility in places was down to about a foot.

Buses stopped running, drivers abandoned cars where they stood, and trains, flights and Thames shipping were disrupted. Only the Underground kept going normally. Road accidents piled up and ambulance services were stretched to the limit. Thieves used the darkness as cover to break into houses and shops and to rob people walking home.

The smog did not stay outside. It slid under doors and through window frames, and on the evening of Monday, December 8, a performance of La Traviata at the Sadler’s Wells Theatre was abandoned after the first act because the audience could no longer see the stage. At the Smithfield Show, the prize livestock fair at Earls Court, one Aberdeen Angus died, twelve cattle had to be put down, and more than a hundred needed a vet (HistoryExtra). The cattle were the first casualties anyone reported. The human toll was noticed only later, by coroners, pathologists and the registrars who recorded deaths.

Why the smoke had nowhere to go

London’s air had been filthy for centuries. The diarist John Evelyn complained about "the aer and smoak of London" in 1661, in what is often called the first book about air pollution. A fog in December 1873 pushed the city’s death rate 40% above normal, and others followed in 1880, 1891 and 1948. What made 1952 different was a rare stack of weather and fuel problems, all arriving in the same week.

A lid of warm air

An anticyclone, a zone of high pressure, settled over southern England on December 4. High pressure pushes air downward, and sinking air warms as it is squeezed. The result was a temperature inversion: a layer of warmer air sitting on top of colder air near the ground. Normally, warm chimney smoke rises and drifts away. Under an inversion it rises only until it meets the warm layer above, then stops. London was, in effect, under a lid.

Fog from the ground up

The night of December 4 was clear and calm, and the ground lost heat quickly to the open sky. Moist air touching the cold ground cooled to its dew point, and water condensed on the countless soot particles already floating over the city. A very light breeze stirred that saturated air into a layer of fog 100 to 200 meters deep, trapped under the inversion (Met Office). With almost no wind, nothing moved it sideways either.

Coal fires in every home

November had been bitterly cold, with heavy snow, and London heated itself with coal. Most homes had open fireplaces, and the coal they burned was poor stuff. After the Second World War, Britain exported much of its better, cleaner coal to pay its debts, and households were left with a cheap, crumbly, sulfur-rich grade sold as "nutty slack." Around the city, coal-fired power stations at Battersea, Bankside, Fulham, Greenwich and elsewhere added their own smoke. So did steam trains, and the diesel buses that had just replaced London’s last electric trams in July 1952.

The Met Office later estimated what all of that put into the trapped air on each day of the smog:

Pollutant	Released each day	What it did
Smoke particles	1,000 tonnes	Blackened the fog and coated lungs; gave the smog its yellow-brown color
Carbon dioxide	2,000 tonnes	Not the main hazard, but a measure of how much coal was burning
Hydrochloric acid	140 tonnes	Irritated eyes, throats and airways
Fluorine compounds	14 tonnes	Added to the toxic, acidic mix
Sulfur dioxide	370 tonnes	Converted into an estimated 800 tonnes of sulfuric acid

Fog was the trap, not the poison.

The chemistry mystery that took 64 years

The sulfur dioxide line in that table is the key to why the Great Smog was so lethal, and it puzzled scientists for decades. Sulfur dioxide was known to turn into sulfuric acid in the air, but the usual reactions are slow. Nobody could explain how so much acid formed so fast, inside a cold fog with very little sunlight to drive the chemistry.

The answer came, unexpectedly, from China. In 2016 a team led by Gehui Wang and Renyi Zhang, the latter an atmospheric scientist at Texas A&M University, was studying why winter haze around Beijing carried far more sulfate than models predicted. In the lab, they found that sulfur dioxide reacts quickly with nitrogen dioxide, another product of burning coal, when both are dissolved in water (PNAS).

In Chinese haze, the particles are tiny, and the reaction would normally stop itself as the droplets turned too acidic. Ammonia from farming neutralizes that acid, so the reaction keeps going and produces sulfate particles. London had no such ammonia. What it had instead was natural fog, made of water droplets that are enormous by comparison. Those big droplets diluted the acid enough that the reaction could keep running inside them overnight.

Then came the morning. As the weak sun partly burned off the fog each day, the water evaporated, but the acid did not. What was left, Zhang explained, was a city full of concentrated acid droplets small enough to breathe deep into the lungs.

Now you have a mixture of sulfuric acid and water. All you evaporate is the water. You make very concentrated sulfuric acid droplets. (Renyi Zhang, Texas A&M University, to NPR, November 2016)

That cycle, fog forming at night and partly clearing by day, repeated for five days. Zhang told NPR it was "a combination of meteorology and chemistry," and noted that the same industrial trick is used on purpose to make strong sulfuric acid (NPR).

Counting the dead: 4,000 or 12,000?

At first, almost nobody grasped how many people had died. Fog was part of London life, and the city had come through the Blitz a decade earlier. Weeks later, the Ministry of Health’s figures showed roughly 4,000 more deaths in Greater London during and just after the smog than would normally be expected, most of them among the very old, the very young and people with heart or lung disease. Around 100,000 more were made ill. Most died of bronchitis and pneumonia, often on top of existing lung or heart disease.

Deaths stayed high for months, though. A preliminary government report blamed those later deaths on a flu outbreak. E. T. Wilkins, the government’s leading air pollution scientist, disagreed. He charted London’s weekly deaths that winter against the year before and found roughly 8,000 extra deaths in the months after the smog, on top of the first 4,000.

In 2004, Michelle Bell, Devra Davis and Tony Fletcher re-examined the records in the journal Environmental Health Perspectives. They concluded that influenza could account for only a fraction of the later deaths, and that it would have taken a flu season twice as deadly and four times as widespread as the one a general medical practice actually recorded that winter to explain most of them (Environmental Health Perspectives). Their findings support a total close to Wilkins’s original estimate of about 12,000.

Myths and facts about the Great Smog

Myth: it was just an unusually bad fog. Fog was the trap, not the poison. London had dense fogs every winter. The difference in 1952 was the amount of coal smoke and acid held inside it, and how long the weather kept it there.

Myth: everyone could see it was a disaster. Many Londoners shrugged it off at the time. The death toll emerged in statistics weeks later, and even then the health minister, Iain Macleod, was dismissive. "Really, you know, anyone would think fog had only started in London since I became Minister," he said when questioned about it (History and Policy).

Myth: factories caused it. Industry and power stations contributed, but the government committee that investigated the smog concluded that low-level smoke from ordinary home fireplaces was a central part of the problem, not just in 1952 but every winter.

Myth: the Clean Air Act ended smog overnight. It took years. Another fog in December 1962 killed about 750 Londoners, according to the Met Office, before smokeless fuels and central heating finally won out.

How London cleaned up its air

The government resisted for months, preferring to treat the smog as a natural disaster that no one could have prevented. Public and press pressure built, and in July 1953 it appointed the industrialist Sir Hugh Beaver to lead a Committee on Air Pollution. London had been here before: a century earlier, the Great Stink of 1858 pushed Parliament into rebuilding the city’s sewers. This time, the Beaver Committee’s reports, in November 1953 and November 1954, argued that smoke from domestic chimneys as well as industry had to be outlawed in towns, and estimated that smoke pollution was costing Britain about £250 million a year (History and Policy).

The Conservative MP Gerald Nabarro forced the pace with a private member’s bill, and the government followed with its own. The Clean Air Act received royal assent on July 5, 1956 (legislation.gov.uk). It let local councils declare smoke control areas, where only approved smokeless fuels could be burned, offered grants to help households convert their fireplaces, and banned dark smoke from chimneys. A second Clean Air Act in 1968 pushed reluctant councils further. It is often described as one of the first comprehensive national air pollution laws anywhere; the United States passed its own Clean Air Act in 1970 (National Geographic).

What the Great Smog still tells us

The smog’s effects did not end in 1953. In a 2023 study in the Journal of Health Economics, Stephanie von Hinke and Emil Sørensen compared people who were in the womb or in infancy during the smog with people born slightly later, or at the same time in unaffected areas. Decades on, those exposed had lower fluid intelligence and worse respiratory health, with some evidence of fewer years in school (arXiv).

London’s air today is far cleaner than in 1952, but its main problems have changed rather than disappeared. Traffic fumes and fine particles have replaced coal smoke, and the same overnight inversions that trapped the Great Smog still trap pollution in cities from Delhi to Salt Lake City every winter. The 2016 chemistry also showed something uncomfortable: the reactions that made London’s fog so acidic are still at work wherever coal smoke, traffic exhaust and damp, stagnant air meet. The lesson of the Great Smog was that you cannot change the weather, but you can change what you put into it.

The bottom line

The Great Smog of 1952 was not a freak of nature. It was ordinary London smoke caught by a rare spell of cold, still weather, and turned by fog chemistry into a cloud of acid that killed perhaps 12,000 people. It took the government years to act and scientists more than six decades to fully explain the chemistry. But the response, smoke control areas and cleaner fuels, worked, and it is the reason no London fog since has come close.

Related on aglena: The Summer of the Dry Fog: How Iceland’s Laki Eruption Poisoned a Nation and Darkened Europe, The Night Lake Nyos Exhaled: How a Quiet Crater Lake Suffocated 1,700 People

Further reading