The “London Fog” Of Victorian Novels Was Not Weather. It Was Sulfuric Acid (The Great Smog Of 1952)

The thick yellow “London fog” of Dickens and Sherlock Holmes was smog: natural river fog mixed with the smoke and sulfur dioxide from millions of soft-coal fires. Inside each cold fog droplet, the sulfur dioxide reacted with nitrogen dioxide, another coal-smoke gas, and turned into sulfuric acid, so the city was breathing an acid mist rather than plain water. The worst episode, the Great Smog of 5 to 9 December 1952, made hundreds of tonnes of acid a day under a windless lid of warm air, killed about 4,000 people by the official count (about 12,000 by a later study), and led to the Clean Air Act of 1956, after which the famous fogs stopped.

Open Bleak House and the first thing Dickens hands you is weather. “Fog everywhere,” he writes. Fog up the river, fog down the river, fog in the throats of old men wheezing by the fire. Sherlock Holmes walks out into the same murk, and so does Jack the Ripper. Every Victorian London you have ever pictured comes with that yellow haze.

The odd part is that the fog stopped.

The white mist that drifts off the Thames on a cold morning is still around. What went was the famous stuff, the thick yellow “pea-souper” that could hide your own feet. It vanished within a decade of the 1950s and never came back. Weather does not retire, so something else must have been going on.

That something was a chemical process, and it ran over one of the biggest cities on Earth for about a hundred years. To see it, you first need to know what fog is, and what turns fog into smog.

What Is Smog, And How Is It Different From Fog?

Breathe out on a cold morning, and the little cloud you make is fog in miniature. Warm, damp air meets cold air, and the water in it condenses into droplets too small to fall. But water needs a surface to condense on. In clean country air that surface is a speck of dust or salt, and the fog that forms is thin and white.

London’s air offered a far better surface: soot. Coal smoke is tiny carbon particles, and each one is a ready-made seed for a droplet. The UK Met Office wrote a history of London’s fogs. It says the particles acted as “catalysts for fog,” because water clings to them to make polluted fog, or smog. A city that burns coal makes fog more easily, holds it longer, and colors it.

That color is the tell. The Met Office calls London smog “dirty-yellow or brown,” nothing like the clean white fog of the countryside. Britannica says the fogs were known as “pea-soupers” for their dense, yellow appearance. The word smog itself, smoke plus fog, was coined by a Londoner in 1905. Londoners needed a word for it, since they had been living in it for centuries.

So fog is water droplets, while smog is water droplets grown on soot in air full of coal gases. Those gases came from the particular coal London chose to burn.

Why Did London Burn The Smokiest Coal It Could Find?

Not all coal is the same coal. The kind that matters here is soft, or bituminous, coal. The US Environmental Protection Agency keeps a handbook on burning coal. Bituminous coals, it says, have “higher volatile matter than anthracite.” Volatile matter is the part that boils off as gas and tar before it burns, and more of it means more smoke. Anthracite, the hard coal, has less volatile matter, so it burns hot and clean.

Coal also carries sulfur, which leaves as sulfur dioxide gas when the coal burns. The EPA is blunt about it: sulfur oxide emissions “are directly proportional to the sulfur content of fuel.” More sulfur in the grate means more sulfur dioxide up the chimney. (Our article on why coal piles catch fire by themselves covers what else lurks in a lump of coal.)

London chose the smoky kind, and chose it early. From the 1200s the forests shrank, and Londoners switched to cheap “sea-coal” shipped down from the northeast coast. The EPA’s history of the pea-soupers notes that with sea-coal, “A lot of its energy was spent making smoke, not heat.” King Edward I banned it in 1272. The first person caught burning it was executed. Londoners kept burning it anyway. Anthracite, the EPA notes, “was much cleaner but too expensive.”

Seven centuries later the trade-off still held. In 1952 the good hard coal was going for export, and a cheap soft grade called “nutty slack” went into London’s grates. Now picture a million of those grates lit at once, in a city with no central heating and a cold snap on.

How Bad Was The Smog In London In The 1800s?

Bad enough to kill, and people knew it. The Met Office records a smog “smelling of coal tar” over London in December 1813. People could not see across the street. A fog in December 1873 pushed the death rate across London 40 percent above normal. More deadly fogs followed in 1880, 1882, 1891 and 1892.

Some fogs barely ended, and the EPA describes an 1879 fog that lasted from November to March. In the winter of 1901-02 a fog monitor climbed St Paul’s to check, and average visibility from the top was half a mile (about 800 meters).

None of this stopped the coal. Coal was the Industrial Revolution, and to be against coal was to be against progress. The worst fog of all was still to come.

What Caused The Killer Fog Of 1952?

Three things lined up in the first week of December 1952. The Met Office lays them out.

First, cold. November and early December had been bitter, with heavy snow. Londoners were burning far more coal than usual, and every chimney in the city was working.

Second, an anticyclone. That is a big, slow area of high pressure, and it parked over the region. High pressure pushes air downward, and sinking air warms. So a layer of warm air ended up sitting on top of colder air at street level. That upside-down arrangement is a temperature inversion, and it works like a lid on a pot. On a normal day, warm smoke rises into cooler air and keeps climbing. Under an inversion it rises a short way, meets air warmer than itself, and stops.

Third, fog. Early on Friday 5 December the sky was clear, the wind light, the air near the ground damp. Overnight the ground lost its heat to the clear sky. The damp air touching it cooled to its dew point, and fog formed. The Met Office puts the fog layer at 100 to 200 meters deep (330 to 660 feet), sealed under the inversion. A light easterly wind added pollution from factories on the continent.

Put the three together. You have a city pouring out smoke and sulfur dioxide, a lid stopping it from leaving, and a fog to dissolve it in. Nothing about that weather was unusual for London. The Greater London Authority looked back on the smog after 50 years. Its review calls the weather typical of what still gives London bad air. The only unusual thing was how long the whole arrangement lasted.

How Did Fog Turn Sulfur Dioxide Into Sulfuric Acid?

Most history pages skip the next step, even though it is the reason the smog killed rather than just annoyed.

Sulfur dioxide on its own is a gas, and in dry air it turns into sulfate only slowly. A 2016 study in the Proceedings of the National Academy of Sciences looked at this. It was led by Gehui Wang and Renyi Zhang, with Nobel laureate Mario Molina as senior author. They put the lifetime of that dry route at about a week, and in a week the wind has carried it away.

Fog changes the game, because sulfur dioxide dissolves in water. Inside a droplet it meets other dissolved gases and reacts far faster than it could in open air. Worldwide, droplet reactions make about 80 percent of the sulfate in the lower air. The dry route makes about 20 percent. In other words, the droplet itself is the reaction vessel.

Wang and colleagues showed that the other gas was nitrogen dioxide, which pours out of the same coal fires. In their lab, water exposed to sulfur dioxide alone made no sulfate. Add nitrogen dioxide and the sulfate appeared. The overall reaction they established is:

SO₂ + 2 NO₂ + 2 H₂O → 2 H⁺ + SO₄²⁻ + 2 HONO

In words: one sulfur dioxide, two nitrogen dioxides and two waters make one sulfuric acid. They also make two molecules of nitrous acid gas. (Dissolved sulfuric acid is 2 H⁺ plus SO₄²⁻.) The nitrous acid mostly bubbles back out, while the acid stays in the droplet.

The study then applied this to 1952 and concluded the London Fog formed the same way. The acid, they write, formed faster thanks to “high RH, low temperature, and the presence of large fog droplets.” RH is relative humidity, how close the air is to saturated. Cold, wet, dark, still, and full of both gases: December 1952 was the ideal setup. In chemistry terms, a cold, sealed box full of two reactive gases is a reactor, and that week London was one.

Things got worse when the air warmed and the droplets shrank, because the water left and the acid did not. Drying droplets, the study notes, left behind “concentrated sulfate acid particles.” A dilute acid mist became a fine acid dust, small enough to breathe deep.

How Much Sulfuric Acid Did London Make Each Day?

The Met Office gives the daily bill. On each day of the fog, London’s chimneys released 1,000 tonnes of smoke and 2,000 tonnes of carbon dioxide. Add 140 tonnes of hydrochloric acid and 14 tonnes of fluorine compounds. Then comes the line that matters. Each day, 370 tonnes of sulfur dioxide (about 410 US tons) became 800 tonnes of sulfuric acid (about 880 US tons).

Look hard at that last pair, because it tests what you just learned. Acid should outweigh gas, since each sulfur dioxide molecule gains an oxygen and a water on its way to sulfuric acid. So the product is heavier than the ingredient, and how much heavier is fixed by the molecules. Using masses from the NIST Chemistry WebBook:

1. Sulfur dioxide weighs 64.06 grams per mole. Sulfuric acid weighs 98.08.
2. One molecule of gas becomes one molecule of acid, so the mass ratio is 98.08 ÷ 64.06 = 1.53.
3. Turn all 370 tonnes of gas into acid: 370 × 1.53 = about 570 tonnes of pure sulfuric acid (about 620 US tons).

The Met Office says 800. The periodic table allows only 570. So 800 tonnes cannot be pure acid. It must include water held in the acid droplets, or come from a looser estimate the page does not explain. Either way, the scale stands: hundreds of tonnes of acid, every day, over one city.

The air measurements agree. The London County Council kept a monitor at County Hall in Lambeth. On 4 December it read 0.49 milligrams of smoke per cubic meter of air, and by noon on the 5th it was 2.46. On the 7th and 8th it was 4.46, nine times the starting value. Sulfur dioxide went from 0.41 to 3.83 over the same days, also nine times. The GLA review notes that the peak was the highest recorded at County Hall since measurements began in 1932. In 2001, Michelle Bell and Devra Davis went back over the smog in Environmental Health Perspectives. They put those levels at “5-19 times above current regulatory standards and guidelines.”

How Long Did The Great Smog Of 1952 Last, And What Stopped It?

Five days, from Friday 5 December to Tuesday 9 December. The GLA review walks through them. Sunday the 7th was probably the worst, with almost all of Greater London under dense smog. On Monday the 8th a light wind cleared Westminster but not the East End, and the smog came back that night. On Tuesday the 9th a southwesterly wind arrived and cleared it everywhere. So the smog ended when the wind came back, and nobody had switched anything off.

In between, the city stopped working. On the Isle of Dogs, the Met Office notes, people could not see their feet. Britannica records that transport was cut to the Underground, and cars were abandoned in the road. Plays were canceled because audiences could not see the stage. At Sadler’s Wells, the GLA notes, La Traviata was stopped after the first act. The theater had filled with smog.

The first reported victims were prize cattle at the Smithfield Show, which began to choke. The GLA records that one died and twelve had to be slaughtered. Sixty more needed serious veterinary treatment. The human toll only became clear afterward.

How Many People Died In The Great Smog Of London?

The number depends on how far out you count, and both figures are real.

The count at the time was about 4,000. The Ministry of Health’s own committee put it at 3,500 to 4,000 more deaths than expected, according to the GLA review. The Met Office and Britannica both give about 4,000.

The higher figure comes from Bell and Davis, who reassessed the episode in 2001. Deaths stayed high for months. The official view had blamed the extra deaths in January and February on a flu outbreak. Bell and Davis checked insurance claims, hospital admissions and flu reports against the year before. They rejected the flu explanation. Their conclusion: “about 12,000 excess deaths occurred from December 1952 through February 1953.” Britannica now gives 12,000 as the present-day estimate.

So: about 4,000 in the week itself, and roughly 12,000 once the following winter is counted. The two figures do not contradict each other, because they count deaths over different stretches of time.

What Did Winston Churchill Do About The Smog?

Almost nothing, and the record is clear. The Churchill Project at Hillsdale College searched its 80-million-word archive. It found no comment from Churchill on the Great Smog at all. He was in London that week, and back at his desk on the Budget Estimates by Monday 8 December. Hansard, the record of everything said in Parliament, has nothing on the fog until February 1953. Parliament that week did find time to debate whether the phrase is “boo to a goose” or “bo to a goose.”

The Crown shows a prime minister who ignores the fog until it forces his hand, and the Churchill Project calls that artistic license. The first parliamentary question came on 12 February 1953. One MP blamed the cheap “nutty slack” burned at home. Geoffrey Lloyd, the Minister of Fuel and Power, replied. “Even good coal produces smoke…but it also produces warmth, which is very much required at the present time.”

Pressure from the press, doctors and MPs told in the end. Harold Macmillan was the minister responsible. A University of London Press history records that he proposed a committee in a memo dated 18 November 1953. That was almost a year after the event. It became the Committee on Air Pollution under Sir Hugh Beaver. Its reports led straight to the law that ended the fogs.

How Did London Get Rid Of Smog? The Clean Air Act 1956

The Beaver Committee found that the biggest source of London’s smoke was home fires rather than factories. Its final report came in 1954. The GLA review notes it recommended smokeless zones. It also proposed smoke control areas, where burning soft coal at home would be restricted, and grants to convert home fires to smokeless fuel. The City of London went first and banned smoke in the Square Mile in 1954, according to the London Museum.

Then came the law: the Clean Air Act 1956 received Royal Assent on 5 July 1956. Its full title is one line: “An Act to make provision for abating the pollution of the air.” Eleven words, nearly 700 years after Edward I tried the same thing. Section 1 says “dark smoke shall not be emitted from a chimney of any building.” Section 11 let any local authority declare a smoke control area. Inside one, burning ordinary coal at home became an offense. Britannica adds that homeowners got grants to switch to oil, gas or electricity.

It did not work overnight, since people were given time to convert. The Met Office records that a fog in 1962 still killed 750 Londoners, and a second Clean Air Act followed in 1968. But the trend was set: coal fires gave way to gas and central heating, and power stations moved out of the city. The Met Office’s verdict is that “nothing on the scale of the 1952 Great Smog has ever occurred again.” The pea-souper became a museum piece. That makes it a rare environmental story: a law that fixed the problem it was written for.

Has Anything Like The Great Smog Happened Again?

Yes, because the recipe was never unique to Britain. Take soft coal or a smelter, a valley or a city, and a temperature inversion, and the same thing can happen anywhere.

The American version came first. Donora, Pennsylvania, is a mill town about 40 kilometers (25 miles) southeast of Pittsburgh. In the last week of October 1948 an inversion sealed it under the smoke from a steel plant and a zinc works. A 2018 review in the American Journal of Public Health records that 20 people died. Another 5,900, 43 percent of the town, fell ill. The inversion sat at about 46 meters (150 feet), below the height at which many residents lived. It held for five days until rain broke it. A sign in Donora’s museum reads “Clean Air Started Here.” The review says that is not hyperbole, because Donora fed straight into America’s own Clean Air Act. Eighteen years earlier, a fog in Belgium’s Meuse Valley had killed more than 60 people in a day. Doctors warned it would happen again, and it did.

The chemistry has not gone away either. The 2016 PNAS study that solved London’s sulfur puzzle was written to explain the winter haze in Chinese cities. That haze runs on the same sulfur dioxide plus nitrogen dioxide reaction, on damp fine particles instead of fog droplets. Spread the same coal-sulfur-to-acid path over a continent and you get the acid rain crisis of the 1980s. The same reaction sits behind all three disasters.

Smog is still a problem in London, though not this kind. Today’s pollution comes mainly from road vehicles, the London Museum notes, and you cannot see it. London fights it with tools like the Ultra Low Emission Zone, introduced in 2019. The air is cleaner, but not clean. The museum cites a 2019 study that linked roughly 6,000 excess deaths in the capital that year to air pollution. (See how much vehicles contribute to air pollution for the modern version.)

So, Was The London Fog Really Sulfuric Acid?

Partly, and the “partly” is the point. London sits in a river valley, and it makes real fog on its own; nobody manufactured the mist Dickens wrote about. What the city added was everything that made the mist famous: the color, the smell, the way it hung around for days, and the sting.

Soft coal, burned in a million grates, sent up soot, sulfur dioxide and nitrogen dioxide. The soot seeded the fog and made it thick and yellow. The fog dissolved the two gases and let them react into sulfuric acid, hundreds of tonnes a day. An inversion kept the whole brew over the city until the wind came back. In December 1952 that chain ran long enough to kill thousands.

Then the fuel changed, and the chain broke at its first link, which is why the fog stopped. The gas lamps, the hansom cabs and the detective are still in the books. The yellow haze around them was mostly a heating problem, and it was solved.

So the next time a Sherlock Holmes adaptation rolls the fog in, you will know what you are looking at. You are looking at the memory of a chemical reaction. It ran in the open over one of the great cities of the world for about a hundred years.