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Have you ever heard of The Great Smog of London in 1952 that killed 12,000 people? Did you know that death by smog was even possible? Join Hank for a new episode of SciShow where we unpack this deadly pollution event and discuss how smog can actually affect life expectancy.
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London is famously foggy. Sometimes, that can mean a wistful stroll or another excuse for a cup of tea, but when fog mixes with smoke and chemicals produced by industry it becomes something new: smog.

And, for a couple of centuries, London's smog could kill. Really bad smogs could kill a thousand people in a few days, but no one did much about it until 1952 when a 5-day smog in London killed an estimated 12,000 people. It was called the Great Smog of London and it helped wake up the country and the world to the dangers of unrestricted pollution.

Fog is just a cloud that forms down here on the ground, which, by itself, isn't that bad. You might not be able to see well when you're driving or you might not be able to land your plane, but it's nice.

But clouds can act like sponges, forming around and trapping whatever's already in the air. This wasn't a problem until the 1200s when a lot of London switched from wood to coal for heating their homes. Burning coal creates soot and smoke, which can irritate your lungs, and also creates poisons like sulfur dioxide and carbon monoxide.

Sulfur dioxide, which is a sulfur atom bonded to two oxygen atoms, reacts with water to form sulfuric acid, which can harm your internal organs, as you might imagine. And carbon monoxide, which is one carbon and one oxygen, binds with the hemoglobin in your blood to stop oxygen from getting around your body.

So, when London started burning coal, all the smoke and chemicals mixed with the natural fog and it became thicker and darker as years passed.

It wasn't a health crisis at first, but people did complain that the smoke smelled terrible. And this was back in the 1200s when everything smelled terrible already. But it was easier to keep burning coal than to switch back to wood, so, for centuries, they just accepted the occasional thick, dark, smelly cloud hanging over the city, as you do.

Things really got dangerous when the Industrial Revolution happened in the 1700s. Because now, coal wasn't just heating homes. It powered huge factories throughout the city, and all that extra smoke and soot made the air in London's fogs much darker. It became so common that a physician named Harold Antoine des Voeux invented the term smog to describe it.

Really smoggy days completely blacked out the center of the city so that you couldn't see more than a few meters ahead of you even in the middle of the day. The soot also irritated people's lungs, causing illnesses like bronchitis to become more common. Some people even suffocated from breathing so much smoke or the poisons in the air.

Individual smogs in 1873 and 1892 each killed over a thousand humans and livestock. And we don't even know how many people died early from collecting soot in their lungs over the course of their lives. But coal kept London flourishing, so nobody did anything to stop it.

Then came the Great Smog. On December 5th, 1952, a thick fog rolled in and mixed with London's dirty air, just like it did most winters. But this time, high-pressure weather systems surrounded London and kept the cloud from moving on. So an especially dense, black smog stopped on London for miserable days. The smog was so thick that flights were grounded, most public transportation was cancelled, trains collided and theaters and movies stopped because people couldn't see what they were watching. This is difficult to imagine. This was 1952, not that long ago.

An estimated 4,000 people died in those five awful days before the smog dissipated. A lot of them suffocated because their lungs were inflamed from breathing in so much soot. And, with sulfur dioxide from the burning coal reacting with water vapor in the smog, Londoners also spent those five days breathing air full of sulfuric acid. That, and the smoke, contributed to respiratory and other health problems which killed around another 8,000 people in the following months.

Ultimately, roughly one in a 1,000 Londoners died because of the Great Smog. Some people argued afterward that the spike in deaths was due to a flu epidemic, but scientists have investigated that in all sorts of ways and it's really unlikely that the flu could have been anywhere near as devastating as the smog itself.

Four years later, Parliament finally passed a Clean Air Act that dictated what kinds of fuels could be burned within the city. It, and other laws, have helped rein in the smog problem in London. But even today, London's air pollution lowers the life expectancy of a lot of people, and is indirectly linked to tens of thousands of early deaths every year throughout the United Kingdom.

Despite the Great Smog's devastation, it took a while for other industrial powerhouses to take the hint. New York City had a series of smogs in the 1960s that affected more than 16 million people, and black, soot-filled rain coated Boston around the same time.

But, eventually, lawmakers around the world stepped in. Starting in the 1970s, laws got serious about limiting pollution, forcing car companies to make more efficient engines, and factories to produce fewer emissions. Because, it turns out, turning air into poison: not a great idea.

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