Why Isn’t Tear Gas Illegal?

It’s a chemical weapon, isn’t it?

Tear gas was used on protesters in Tehran

Iranian security forces fired tear gas into crowds of anti-government protesters Thursday. Tear gas was also used in Sri Lanka this week and in Venezuela and Haiti last month. The State Department stockpiles it at embassies around the world. Aren’t chemical weapons like tear gas illegal?

Yes, but only in war. The 1993 Chemical Weapons Convention doesn’t apply to domestic law enforcement. (The United States was a major proponent of the exemption, fearing that the convention might be interpreted to prohibit lethal injection.) The most common lacrimator used for riot control is a compound called 2-chlorobenzalmalononitrile , or CS. (It’s not actually a gas so much as a powder that’s usually mixed with smoke to create an airborne agent.) CS has no long-term effects when used properly, but the treaty still bans its use in battle since it’s difficult to distinguish from more dangerous agents in the fog of war. (You don’t want the other side to think they’re being attacked with, say, sarin, and respond in kind.) It’s also considered inhumane to deploy CS as a means of weakening enemy soldiers so they might be killed by conventional means—although that didn’t stop the United States from using tear gas to flush the Viet Cong out of tunnels and then bombarding them as they fled.

Signatories to the Chemical Weapons Convention are free to manufacture riot-control agents— defined as chemicals that cause short-lived irritation or disabling effects—as long as they report their holdings to an international body and don’t make so much that their stockpile is inconsistent with domestic use. When deployed in open spaces, the effects of tear gas are indeed temporary. Victims may experience crying, uncontrollable blinking, burning in the throat, sneezing, coughing, retching, and sometimes temporary blindness—but all that should subside within hours.

In enclosed spaces, however, the chemical agent can have much more serious effects. When police plan to use tear gas grenades to flush suspects out of a house, they start by comparing the dose of CS with the volume of the building and calculating a “lethal concentration time.” That’s the number of minutes it will take before most people inside would die from exposure. If the lethal concentration time is nearing, and the suspects haven’t yet emerged, the police start breaking windows for ventilation.

It’s not entirely clear how many people have been killed by CS. Amnesty International said 50 Palestinians died from inhalation in the late 1980s—prompting a brief suspension of tear gas sales to Israel—but those conclusions are disputed. The FBI used CS in its raid on the Branch Davidian compound (PDF) in Waco, but the ensuing fire left it unclear how, exactly, the cult members were killed. Such incidents have prompted a search for less toxic crowd-dispersing chemicals such as malodorants, but none has proven as effective as tear gas. Russia appears to be moving in the other direction, using the powerful opiate fentanyl to incapacitate rebels during a 2002 hostage crisis. That approach ended up killing more than 100 innocent people.

The United States is so enthusiastic about riot-control agents that it has a standing Executive Order reserving the right to use them on the battlefield, in spite of the Chemical Weapons Convention’s prohibition, to protect convoys or prevent the use of civilian shields. While the U.S. hasn’t invoked the order since ratifying the Convention in 1997, Donald Rumsfeld made news in 2003 when he raised the possibility.

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Explainer thanks Dennis Cole, co-author of the Chemical Agents Instructor Handbook, Nishal Mohan of the Federation of American Scientists, and Jonathan B. Tucker of the James Martin Center for Nonproliferation Studies. Become a fan of Slate  or the Explainer  on Facebook. Follow us on Twitter.