Law enforcement officers in Ferguson, Mo., have used tear gas extensively in the wake of the police-shooting death of Michael Brown, an 18-year-old unarmed African-American. The conflict in Ferguson led one PolitiFact reader to ask us to check the accuracy of a social-media meme now circulating that addresses the legality of tear gas.

The meme -- posted by the group OurTime.org, an advocacy group for young Americans -- said, "Tear gas has been classified as a chemical weapon and banned in international conflict since 1993. Why is its use allowed by U.S. police forces?" The post had garnered 143,000 likes, 42,000 shares, and 35,000 comments by late August. We wanted to know if it was accurate.

First, some background on tear gas. It is a broad term for chemical compounds that temporarily make people unable to function by causing irritation to the eyes, mouth, throat, lungs, and skin, according to the Centers for Disease Control and Prevention . Pepper spray and CS gas are among the most commonly used.

The CDC says that "prolonged exposure, especially in an enclosed area, may lead to long-term effects such as eye problems including scarring, glaucoma, and cataracts, and may possibly cause breathing problems such as asthma." However, the agency adds that "if symptoms go away soon after a person is removed from exposure to riot control agents, long-term health effects are unlikely to occur."

Has tear gas been 'banned in international conflict since 1993'?

This is close to being accurate. The Chemical Weapons Convention bans the development, production, acquisition, stockpiling, retention, transfer or use of chemical weapons. And tear gas qualifies under the convention as a chemical weapon.

Specifically, Article I (5) of the convention says , "Each State Party undertakes not to use riot control agents as a method of warfare," while Article II (7) defines "riot control agent" as: "Any chemical not listed in a Schedule, which can produce rapidly in humans sensory irritation or disabling physical effects which disappear within a short time following termination of exposure."

The meme is incorrect, however, when it comes to the year the convention came into force. It was finalized in 1993 but took effect on April 29, 1997 -- 180 days after the 65th country, Hungary, ratified the treaty, as the convention set forth. The United States is covered by this provision, the Senate having ratified the convention five days before it went into force.

In addition, there may be a gray area for use by the military. When President Gerald Ford signed an earlier, decades-old agreement covering chemical weapons, the Geneva Gas Protocol of 1925, the United States reserved the right to use tear gas in a limited number of contexts, such as for controlling a riot at a prisoner-of-war detention area -- just not against troops engaged in battle. Anthony Clark Arend, a Georgetown University professor of government and foreign service, said he believes that this limited exception would also hold water under the Chemical Weapons Convention.

How relevant is that to police?

This part is more murky, because the same convention being touted by the meme’s authors specifically states that law enforcement use within a country is permitted. "Law enforcement including domestic riot control purposes" is "not prohibited under this convention," says Article II (9) (d).

This bifurcation between a permitted domestic use and a banned international use is unusual in such accords, said Brian Finlay, managing director of the Stimson Center, a think tank that focuses on global security issues.

As for how it happened, there’s a backstory.

According to the Organization for the Prohibition of Chemical Weapons , the international group that helps enforce the Chemical Weapons Convention, riot control agents were "the topic of long and heated debates" during negotiations of the convention. In the end, a compromise was reached, the group says, allowing the use of tear gas for riot control but prohibiting it for warfare.

Indiana University law professor David P. Fidler, who has studied the issue, said that while there’s been some controversy over what this article of the convention permits in some contexts, "law enforcement use of tear gas has not been one of them."

When we contacted OurTime.org, co-founder Jarrett Moreno told us that "the focus of our post was raising an ethical and moral question: If we can't use tear gas on our enemies, why is it acceptable to use on our own citizens? After Ferguson where we saw children, disabled people, and members of the press being hit with tear gas while exercising their First Amendment rights, why are they being contained with something that we don't even use on the battlefield?"

Experts acknowledged that the treatment of tear gas under the convention is somewhat paradoxical, and it stems in part from horse-trading by convention negotiators. But they added that there are other, more substantive reasons as well.

• The use of any type of gas on a battlefield is problematic. "Part of the thinking is that soldiers in the field don't have the ability to readily distinguish in the heat of battle if a gas being used is tear gas or something more lethal," said Richard Price, a political scientist at the University of British Columbia who has studied the issues. The negotiators, he said, thought that, "as a practical matter, it was best to ban them all" on the battlefield.

• There are few immediate alternatives to tear gas for riot control. There are strategies to prevent riots, including better community relations, a less militaristic appearance, and improved training, all of which have been raised in relation to Ferguson. But once rioting is under way, police need tools to control it -- and "even though tear gas is far from perfect," said David A. Koplow, a Georgetown University law professor, "it continues to be used in that role because there’s nothing else better."

Israel has deployed an organic riot-control agent dubbed "skunk water," described by a BBC journalist as the "worst, most foul thing you have ever smelled. An overpowering mix of rotting meat, old socks that haven't been washed for weeks -- topped off with the pungent waft of an open sewer."

However, it is sprayed from water cannons, which in the popular imagination are associated with their use against civil rights protesters in the 1960s, making them highly unsuitable for use in a scenario like the one in Ferguson. Projectiles such as rubber bullets can be effective, but they can cause serious injury or, if poorly aimed, death.

An emerging technology called the Active Denial System -- or more colloquially, the "pain ray" -- creates millimeter-wave frequencies that impart a searing sensation of heat on the skin, but amid concerns, the United States military has been slow to adopt it. Meanwhile, new forms of "incapacitating chemical agents" that use anesthetic chemicals have been proposed, but they have drawn the concern of such groups as the International Committee of the Red Cross .

The problem with new-generation chemical techniques, said Fidler of Indiana University, "is that their physiological effects are stronger than tear gas, which creates the potential for more injuries and other health harms. Tear gas is well understood as a riot control agent, and it seems to function well enough for law enforcement purposes.

• A lack of desire for change. There is little sign that the convention’s signatories have any desire to reconcile the tear gas paradox.

"There is certainly an argument to be made that the fundamental segregation of these two issues is inherently unreasonable," said Finlay of the Stimson Center. "That said, this is very unlikely to change, as it is both a matter of treaty law as well as customary international law today. There is not much appetite among states to alter the status quo."

Our ruling

The meme said police in the United States use tear gas even though it "has been classified as a chemical weapon and banned in international conflict since 1993."

The Chemical Weapons Convention did outlaw the use of tear gas in warfare, though that went into effect in 1997, not 1993. However, the meme glosses over some context. It tries to leverage the Chemical Weapons Convention’s decision to ban tear gas as evidence of why the technique should be illegal for policing, yet that very same convention explicitly allows its use for domestic law enforcement purposes.

The claim is accurate but needs clarification, so we rate it Mostly True.