A 2014 study by the US military has also found that exposure to tear gas and pepper spray make people vulnerable to respiratory infections in the week following exposure compared with the week before exposure. Army recruits exposed to CS tear gas just once during basic training had a much higher likelihood of developing respiratory illnesses such as viral illnesses like influenza, pneumonia, or bronchitis. That study and others led the American Thoracic Society in 2020 to call for a moratorium on the use of tear gas and other chemical agents during racial justice protests. One worry was that their use could increase the chances of getting sick with Covid-19.

“The science used to justify the use of tear gas is outdated,” says Sven-Eric Jordt, an associate professor of anesthesiology, pharmacology, and cancer biology at Duke University School of Medicine who helped author the American Thoracic Society recommendations.

Much of the research into the health effects of exposure to tear gas and other chemical agents is based on military research that was conducted in the 1950s, ’60s, and ’70s on young, healthy men. He says those studies do not address the potential health effects on a broader civilian population. They also don't take into account more advanced launching technologies developed in recent years that allow for the deployment of much higher amounts of tear gas over longer distances.

In some cases, the effects of these agents are not due to the chemicals themselves. “People can be injured when in contact with the canisters used to disperse the gas, and this can include blunt trauma or burn injuries,” Calello says. In Los Angeles, federal agents reportedly fired hard plastic pepper balls, also known as pepper spray projectiles, at protesters.

Injuries can also occur from blunt-tipped rubber bullets. Their name is a bit misleadi...