Parisian riot police had been bracing themselves for May Day, an annual rally that takes place every May 1 to mark International Workers’ Day. Social media warned that this year’s protests might be particularly violent, as far-left anarchist groups called Black Blocks were calling for a “Revolutionary Day.” The scene certainly seemed revolutionary on Tuesday as 1,200 black-clad protesters were forcibly dispersed by canisters of acrid tear gas.

The chaos in the photos taken by Reuters on Tuesday shows how effectively the chemical weapon used pain to break up the crowds. Technically known as lachrymator agents — colloquially as mace — tear gases have been burning human eyes since 1914, when they were deployed in grenade form by the French army during World War I. The potential for a chemical that could maim but not kill crowds was apparent from the get-go, and in 1919 the United States Chemical Warfare Service developed its own tear gas grenades to quell riots. The rest is history. But even though they now come in many forms, all lachrymators hack the same part of human biology, specifically targeting the nerves that process pain.

Several different compounds have this pain-causing ability. There’s the capsaicin found in chili pepper oil (the basis of the common lachrymator, “pepper spray’); there’s 2-chlorobenzaldene malononitrile (CS), which activates pain receptors; and ω-chloroacetophenone (CN), which is actually so potent that it’s considered toxic (and is now illegal). To be used as tear gases, these solid or liquid agents are aerosolized, which allows them to easily reach the most toxin-sensitive parts of our body: the moist mucus membranes, like the mouth, the nostrils, and yes, the eyes.

In the early 2000s, scientists from Yale University School of Medicine identified how tear gas actually causes its telltale wheezing, choking, crying, and general incapacitation. The active molecules in tear gas specifically bind to certain pain receptors, activating them and triggering these physical responses, all of which are the body’s way of trying to defend itself: The tears elicited by the aerosol are an attempt to wash the chemical out of the eyes.

Tear gas is certainly not as barbaric as other forms of police crowd control, but it’s hardly safe, either. Painful effects aside, it was implicated in a rash of respiratory ailments and miscarriages following its routine use by Bahrain police officers in 2012 and elsewhere. Though the Geneva Convention bans its use in warfare, tear gas is still, controversially, fair game for use on civilians according to international law.

On May Day in 2017, Parisian protestors came armed with Molotov cocktails, which seriously injured at least one police officer; then, too, police responded with cans of tear gas. This year, rioters and police officers continued the cycle, and international policymakers remained undecided on the chemical weapon’s use.