Before the tearing, the choking and the pouring mucus, tear gas burns. It causes searing pain in the eyes, skin, lungs and mouth—or anywhere it touches. “It can be overwhelming and incapacitating. You can be forced to shut your eyes and cannot open them,” says Sven-Eric Jordt, an anesthesiologist at Duke University. And then comes the coughing and the nausea and the vomiting. What causes these chemicals to have such devastating effects on the human body?

Jordt has studied tear gas for over a decade, but he doesn’t think tear gas is the best term for the weapon. First, he says as a technical point, they’re not gases; they’re powders that billow into the air as a fine mist. “I think of tear gas as a pain gas,” he says. “Because it directly activates pain-sensing receptors.” Weapons like sarin gas cause muscle paralysis that can lead to suffocation. These are designed to kill, while tear gas’ purpose is to repel crowds through maximum misery. Specifically, all tear gas agents activate one of two pain receptors, TRPA1 or TRPV1, and can be classified into two broad categories based on which of those receptors they activate.

The first category, TRPA1-activating agents, includes the chemical called 2-chlorobenzalmalonitrile or CS gas. This is one of the agents used by U.S. law enforcement and, according to a U.S. Customs and Border Protection spokesperson, is what CBP officers fired into crowds of men, women and children at the U.S. southern border on Sunday. “A lot of children fainted. My daughter also got hit. There were pregnant women there and a lot of older men, too,” a witness told the Washington Post on camera on Monday.

These agents are chlorine-containing compounds that blow into the air as a fine particulate. “They are actually dispersed by burning and deposit on the skin or clothing and can persist for a while,” Jordt says. “They chemically react with biomolecules and proteins on the human body,” which can cause a severe burning sensation.

These agents are rarely lethal, Jordt says, but deaths have occurred in cases where they have been used improperly—like a canister fired directly into a crowd and causing head or body injuries, or into confined spaces where people cannot escape. Children are particularly at a high risk for injuries from these agents, Jordt says, because they are so small. “They are shorter, and there are increased concentrations are near the ground. They also have a smaller body surface and lungs so the potential for injury is higher,” he says.

CS gas is the most common of these TRPA1-activating agents but, recently, law enforcement has begun using a newer compound, says Rohini Haar, a doctor with Physicians for Human Rights and a public health scientist at the University of California, Berkeley. “More and more, there’s a higher-level version called CS2 or sometimes CX,” she says. “They [contain silicon] so that they can last longer in the environment and don’t disintegrate as quickly.” The result is a more harmful tear gas that can continue to affect an area for several days.

There are two other TRPA1-activating agents used for riot control: CR gas (dibenzoxazepine) and CN gas (chloroacetophenone, also used in bear spray). Both are more potent than CS gas, Jordt says. During the Arab Spring protests, CN and CR gas were reportedly used alongside CS gas, sometimes to devastating effect. “In the Arab Spring protests, a lot of miscarriages were reported in pregnant women who were exposed,” Jordt says. “Probably due to shock and stress and the chemical exposure.”

The second category of tear gas agents are pepper sprays and activate the TRPV1 pain receptor. These are mostly derived from capsaicin, the spice compound in chili peppers. There are two compounds in common use in this category: OC gas, a concentrated solution of natural capsaicin, and PAVA, a mix of synthetic capsaicin also used by U.S. Customs and Border Protection. “This has fewer chemical or allergic reactions, but it’s also an oil so it’s much harder to get off and can last longer,” Haar says. “It can also cause corneal abrasions if you’re shooting it directly into someone’s eyes.”

Long-term effects from any kind of tear gas are not well known, Haar says, particularly if they are related to infrequent and brief exposure. However, Haar studied residents of the Aida and Dheisheh camps in the West Bank, Palestine, where people reported chronic breathing difficulty, rashes and pain, which the residents attributed to weekly tear gas exposure. “And there’s evidence that [tear gas] can cause emotional trauma, which would have long-term impacts,” Haar says.

Given the risk of injury or damage, Haar says that there are almost no scenarios where the use of tear gas makes sense for controlling crowds. “I can see very few discrete situations where there might be a direct need for tear gas to protect public safety,” she says. “One thing we see more and more now is tear gas causing panic, disorder and chaos. There’s mass deaths from stampedes because tear gas was used.” In 2015, police fired tear gas in a soccer stadium in Egypt in an effort to quell rowdy soccer fans. Twenty-five people died from suffocation or trampling.

Similar stories of exacerbation made Jordt feel skeptical that tear gas is ever useful. “I’ve seen in Germany in the 1980s and 1990s—the use of tear gas escalated the situation,” he says. In May of 1986, protestors responded to tear gas grenades with Molotov cocktails during a May Day demonstration in Berlin’s Kreuzberg neighborhood. “I think [tear gases] cause grief, confusion and increase aggression,” Jordt says. “And they are obviously not as safe as we think.”