Tear gas has been controlling crowds for a century. How does it work? Can it cause permanent harm?

Christopher Rapuano, chief of the cornea service at Wills Eye Hospital, got a small dose of tear gas a few years ago as he stood on the fringes of a demonstration in Hong Kong. His eyes started to burn. Then they teared up and his vision blurred. He ran to fresher air.

Chris Cramer, a University of Minnesota chemist, got much bigger doses while in the Army, where he was a chemical-weapons specialist. To underscore the value of gas masks, soldiers in training would don the masks, wait until tear gas had been fired, then take them off.

“It feels as though bees are stinging you in your eyes," Cramer said. The tearing is like “onions taken to the hundredth power.” Your throat involuntarily closes. Mucus streams from your nose. Your skin feels sunburned. “It’s a special kind of misery.”

A new generation is learning about tear gas the hard way as authorities try to control demonstrations after the death of George Floyd in Minneapolis. Police used it in Philadelphia on Monday night and in Washington to clear the way for a presidential photo op.

Tear gas can be made of a variety of substances that irritate the mucous membranes of the eyes and cause tearing. It was first used in combat by the French against the Germans in World War I. The U.S. Chemical Warfare Service began using it to disperse crowds in the 1920s. It is now banned for use in war. The U.S. Centers for Disease Control and Prevention calls tear gases “riot control agents.”

The most common ingredients in tear gas, according to the CDC, are chloroacetophenone (CN) and chlorobenzylidenemalononitrile (CS). CS is the ingredient used most often in crowd dispersal, Cramer said. Others include chloropicrin (PS), bromobenzylcyanide (CA), and dibenzoxazepine (CR).

The compounds are actually liquids or powders, not gases. They are released into the air as droplets or particles. Grenade-like canisters are thrown or launched into a crowd to release white, heated vapors, Cramer said. The particles land on mucous membranes, skin, and clothing.

Tear gas activates receptors at the end of pain-sensing nerves in your eyes, nose, throat, and skin, said Sven-Eric Jordt, an associate professor of anesthesiology, pharmacology, and cancer biology at Duke University. If you come in contact with enough of it, it can burn your corneas or skin.

In addition to causing tearing and blurred vision, the gas can irritate the upper respiratory tract, causing coughing and choking. People exposed to tear gas can have a runny nose, difficulty swallowing, chest tightness, rash, nausea, and vomiting.

The amount of exposure matters. “Dose makes the poison,” Cramer said.

“Most of the time, people run away from it, and they have a very brief exposure,” said Edward Jasper, an emergency physician at Jefferson Health.

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Prolonged exposure, especially in an enclosed area, may cause eye scarring, glaucoma, and cataracts, according to the CDC. In a worst-case scenario, tear gas can cause blindness, respiratory failure, and even death. People can also be seriously hurt if hit by a tear gas canister.

Jordt said there is not good evidence about how often people suffer serious consequences. “Most of the data we have about safety were generated in the ’50s and ’60s using outdated techniques,” he said. Police are now using technology that releases higher amounts of chemicals. “There are really no studies backing up safety of tear gas the way it’s used now.”

Yes. The disease spreads less efficiently outside, but, because tear gas causes people to cough, that could help the virus spread in crowded conditions. “I suspect we will see some case uptick,” Cramer said. Jordt added that a study by the Army found that soldiers exposed to tear gas were 2.4 times more likely to get respiratory viruses in the following week than during a previous week of training. “I’m really stunned it’s currently being used to that extent when COVID-19 is around,” he said.

The CDC says the effects of tear gas should diminish after 15 to 30 minutes in fresh air. Jordt calls that optimistic. “Most people are fine after a day or two,” he said.

“If you’re exposed, you want to get out of the area where this is in the air,” Rapuano said. Move as quickly as you can to fresher air and higher ground. Rinse your eyes out with water and wash your skin. Rapuano said you don’t need a lot of water to rinse your eyes. Remove contact lenses. (Better yet, experts said, don’t wear them if you’re worried about tear gas.) Cramer suggests standing in a breeze to let the wind blow particles off. As soon as possible, remove your clothing, which will be contaminated with the chemicals. Don’t pull shirts over your head. Cut them off, the CDC says. The CDC suggests double-bagging clothes that have been exposed to the chemical and disposing of them. Cramer says you can just air clothes out and wash them. You’ll know they’re fine if you sniff them and nothing happens.

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Jasper said people with asthma and underlying lung disease are at risk for worse reactions to tear gas.

Tear gas has been used for years without serious long-term problems, Cramer said. Alternative ways to disperse crowds could be water hoses or noxious noises. “It’s about as humane as any of those other techniques,” he said.

He said, though, that tear gas typically does not deescalate a situation. “If a crowd shows no propensity for violence," he said, "it seems like an awfully violent way to go about doing things.”