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[Narrator] In World War I,

toxic gas was a weapon of choice.

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Today, law enforcement uses a less potent recipe

to pacify a room or control a crowd.

But what's inside a can of tear gas?

It starts with charcoal.

That's wood burned until almost nothing is left but carbon,

just like in your backyard barbecue.

Charcoal keeps things smoking.

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But instead of lighter fluid, we add a dusting

of ancient chemistry, potassium nitrate.

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In the 13th Century, it was known as Chinese snow,

a key ingredient of gunpowder and fireworks.

KNO3 makes charcoal easy to ignite

and as it burns it releases oxygen fueling the fire.

At 2,500 degrees, tiny grains of silicon

melt into super hot pellets of glass

that spatter and ignite other flammables in the midst

like potassium chlorate which makes smoke.

But it's also dangerously explosive

if conditions get too acidic.

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That's where magnesium carbonate comes in.

The same stuff you find in fire extinguishers.

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It neutralizes acid, keeping the whole thing

from turning into a bomb.

So instead of blowing up, all that sound and fury

activates o-chlorobenzalmalononitrile,

the lachrymator, or tear producer.

It's a king of irritants attacking the mucus membranes

of your ears, nose, and lungs, making you cough,

choke, and go temporarily blind.

It would be deadly at concentrations

more than 6,000 times greater.

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