A viral video from last fall shows a protester in Hong Kong neutralizing a tear gas canister by trapping it in a thermos.

As people in several U.S. cities gathered to protest police violence in the wake of George Floyd's death this weekend, some officers have deployed tear gas to disperse protesters.

Here, a tear gas expert weighs in on the ways to neutralize the chemical agent.

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This weekend, tens of thousands of people have gathered in various cities across the U.S. to protest police violence and racial injustice in the wake of the May 25 death of George Floyd, a Black man who died after a white police officer kneeled on his neck in downtown Minneapolis.

While many of the protests have been peaceful, some have turned violent, including in cities like Minneapolis, New York, Los Angeles, and Chicago. To disperse protesters, police officers have deployed tear gas, as captured in videos like this one:

Police and military officers have long used tear gas for riot control. Last year, for example, Hong Kong police notably deployed tear gas to break up pro-democracy demonstrations, but protesters quickly developed several tactics to neutralize the tear gas.

You might remember the viral video below of a Hong Kong protester plucking a smoking tear gas canister from the ground, plopping it into a thermos, and sealing the lid. Then, he shakes it. What comes out of the thermos is a pile of dark grey sludge.

Some have suggested the protester neutralized the tear gas canister with water, while others say liquid nitrogen did the trick. Then a Twitter user reported that the protester in the video reached out to him and revealed the contents of the container: mud.

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If true, it’s simple, effective, and easy to find.

But we were curious about why tear gas is so often used in riots, and especially how to neutralize it. So we reached out to Sven Eric Jordt, a pain pharmacologist and toxicologist at Duke University, to learn more.

What’s Inside a Tear Gas Canister?

It has a very similar chemical composition to what you might find in fireworks, says Jordt.

When ignited, charcoal inside the canister burns, while chemical compounds such as potassium nitrate and potassium chlorate produce oxygen that fuel the fire. Potassium nitrate particularly helps charcoal burn faster, according to Wired . Potassium chlorate breaks down into plumes of potassium chloride gas.

Silicon is sometimes added to the mixture to help the gas form droplets. Magnesium carbonate keeps the environment within the canister from becoming too acidic, which could destabilize the potassium chlorate.

Another secret ingredient? Sugar. Sucrose burns at low temperatures and helps convert tear gas’s key ingredient to noxious fumes. That key ingredient is a chemical compound called 2-Chlorobenzalmalononitrile, or CS. When heated, it creates the pungent burning agent most associated with the gas. In essence, it does most of the damage.

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All of these ingredients are bound together in the canister by a sticky, flammable compound called Nitrocellulose.

Why Does Tear Gas Hurt So Much?

Jordt says the tear gas agent activates receptors on the peripheral sensory nerves, a sinuous network of nerve fibers found all over our body. “When you’re exposed to smoke, it burns your eyes,” Jordt says.

The effects can be gruesome. Those exposed to tear gas have reported coughing, respiratory inflammation, and even asthma attacks. It can burn the eyes and skin, leaving victims incapacitated for hours. Vomiting and nausea have been reported, too.

Jordt and his colleagues have previously found that the receptor that's activated by tear gas is also activated by natural products such as the chemicals found in mustard and wasabi. But tear gas is much more potent, he says.



Can You Use Mud to Neutralize Tear Gas?

Mud would definitely do the trick, Jordt says. “It would extinguish the burning components, block the oxygen supply of the cartridge, and plug openings where aerosol comes out.”

But Jordt wasn’t convinced there was water in the container in the video. “It looks like a powder came out with the charge,” he says. “If there was water in there, you would see water coming out.”

If there was liquid nitrogen in the container, as one tweet suggested, Jordt says it would “massively cool down the charge and stop the chemical reaction.” But there would likely be a lot of fog coming out of the container.

“It's also possible that the charge gets extinguished just by putting it into the thermos,” Jordt says. “It's hard to say. There are different options here.” Whatever was in there, it was effective at stopping the reaction.