In the days since protesters were driven out of Lafayette Square coughing and limping, their eyes burning amid clouds of smoke, the Trump administration has insisted federal authorities did not use tear gas on the crowd.

“No tear gas was used, and no rubber bullets were used,” White House press secretary Kayleigh McEnany told reporters Wednesday, echoing similar claims from the U.S. Park Police and the president’s allies.

McEnany was correct, but only to the extent that police did not use products labeled “tear gas” and “rubber bullets.”

The Park Police acknowledged firing “pepper balls,” a projectile munition that lofts irritant powder into the air, and “smoke canisters” to scatter the crowd Monday. The agency has not provided more details about the contents of the smoke it deployed, and a spokesman for the Park Police did not respond to requests seeking clarification Wednesday.

At least one spent canister recovered from the streets outside the park Monday by reporters was clearly labeled “Skat Shell OC.”

The OC stands for oleoresin capsicum, an oily substance derived from chile peppers that is often used in topical ointments and “heat” creams for arthritis relief and muscle pain. When it gets into the eyes, noses and lungs, however, it triggers searing, debilitating pain, coughs, sneezes and mucus secretion.

As police across the country use such chemical agents to quell protests and rioting, they also are triggering those major vectors for the spread of coronavirus.

Like the pepper-ball projectiles, the OC gas detected Monday is very much a lachrymatory product designed to produce tears. It clears crowds by making the air sting and burn — one reason the Park Police and other federal forces wore gas masks as they cleared a path for Trump to walk across the street and pose for photos outside a vandalized church.

Isabelle Riley, a volunteer physician who was at the square Monday, said she treated several protesters who had “symptoms consistent with a chemical irritant” — not the kind of thing that comes out of a fog machine or a campfire.

“Acute exposure to smoke would not cause the severity of what I was seeing,” she said. “We’ve all been sitting near a bonfire, and you’re not walking away crying in agony because you can’t open your eyes.”

Riley and other medical responders used milk and antacids to treat the protesters, she said, which “neutralizes the resin in the pepper.”

OC gas is considered less harsh than other airborne chemical agents that also fall under the catchall phrase “tear gas” and are widely deployed as “riot control agents” by military and police forces around the world.

Chuck Wexler, executive director of the Police Executive Research Forum, which studies and advocates best practices among law enforcement agencies, said the use of chemical agents by local law enforcement is rare and “only a last resort when everything else doesn’t work.”

“If a crowd is peaceful, you would never use it on them,” Wexler said. “And if you did, you would want to announce it. Think about the impact of tear gas on those with some underlying medical or health issues.”

According to the Centers for Disease Control and Prevention, riot-control agents are “chemical compounds that temporarily make people unable to function by causing irritation to the eyes, mouth, throat, lungs, and skin.”

Several compounds fall under this category, according to the CDC.

Among others, they include chloroacetophenone (CN), more commonly referred to as “mace,” or pepper sprays. Such compounds are all typically referred to as “tear gas” because their most prominent effect is to irritate mucus membranes, including the eyes, which secrete tears as a protective response.

Riot-control agents are designed to cause irritation within seconds of exposure, making people want to flee. And, indeed, toxicologists advise that getting away from the gas is the best and first thing to do to mediate the impact.

The most common symptoms of exposure, according to the CDC, can include excessive watering and burning of the eyes, a runny nose, a burning mouth, chest tightness, coughing, skin burns, nausea and vomiting.

Health experts have watched with growing alarm in recent days at the widespread, increased use of smoke and irritating chemicals on American protesters in the middle of the pandemic. They fear the tear gas is accelerating the coronavirus’s spread and making people more susceptible to it.

An open letter started at the University of Washington has now been signed by 1,288 infectious-disease experts and others working in public health detailing those concerns.

“When you get sprayed with tear gas or pepper spray, you cough, shout and scream. That projects the virus so much further. It gives the virus droplets super spreading power,” said Peter Chin-Hong, an infectious-disease specialist at the University of California at San Francisco who helped lead that effort. “For those who don’t have the virus, the first thing you’re going to do is touch your eyes, nose, mouth. Even if you were wearing a mask, that’s now soaked with irritant — you’re not going to keep it on anymore. From a transmission perspective, it’s a disaster.”

Because of how such chemicals are designed to irritate the body’s airways, they could also make protesters more susceptible to infection. Tear gas is actually not gas but an aerosol version of irritant liquid or powder that inflames the pain-sensing nerves in eyes, nose and throat, said Sven-Eric Jordt, an associate professor of anesthesiology, pharmacology and cancer biology at Duke University.

The chemicals can weaken and damage tissue in the airways — including the fine cilia hairs that help protect the airways from foreign intruders, such as a virus.

Research shows that people with weakened airways — from smoking, pollution or chronic respiratory conditions — are much more susceptible to flu, pneumonia and other respiratory diseases. The question is whether tear gas, which is usually a shorter-term exposure, could do the same.

“If we’re talking about low-dose exposure for a minute, I don’t know what the effect is,” Chin-Hong said. “But with higher doses, more prolonged exposure and underlying conditions, that’s worrisome.”

At least three research papers published in 2014 documented worrying signs of lung injury and increased incidences of respiratory illnesses caused by CS gas — a stronger form of tear gas — among military recruits and troops. As a result, Jordt noted, the U.S. military deliberately lowered the amount of tear gas that recruits were exposed to in training.

“The amount and ease that our country is using tear gas right now is something we should think about,” Chin-Hong said. “And if we’re going to use it, then we need to be weighing the effects of that.”