When does a chemical used as a weapon become a 'chemical weapon'?

The Israeli government has now added its voice to recent reports from Britain and France suggesting that the Syrian government has used chemical weapons. This would obviously be a major development in the ongoing violence in Syria and could have serious implications for U.S. policy, as President Obama has indicated that chemical weapons use would be a  "red line" that would "change my calculus" in terms of U.S. reluctance to directly intervene in the conflict. 

After FP's Josh Rogin reported last January on a State Department cable concluding that Syria had used chemical weapons, the department responded by saying that the reports of chemical weapons were inconclusive but that Assad's forces "did apparently misuse a riot-control gas" during the attack.

So when does a riot control gas become a chemical weapon? According to the Chemical Weapons Convention, a "chemical weapon" is "specifically designed to cause death or other harm" while a riot control gas is one which "can produce rapidly in humans sensory irritation or disabling physical effects which disappear within a short time following termination of exposure." This line isn't always so precisely defined, of course. Even relatively mild CS gas has been found to have serious or even lethal effects at high enough concentrations.

An article in March by Charles Blair and Mila Johns at the Bulletin of the Atomic Scientists took up the distinction:

Evidence does point toward Syrian use of chemical agents designed to be non-lethal -- those that are not entirely banned under the international law. With regard to the Chemical Weapons Convention (CWC), certain agents for purposes of "[l]aw enforcement including domestic riot-control purposes" are permitted. Riot-control agents, commonly lumped together in the public consciousness as tear gas or mace, cause rapid, short-lived irritation of the eyes, skin, and respiratory system, producing symptoms such as shortness of breath, choking, rashes, and temporary blindness due to swelling of the eyes. Riot-control agents are not banned under the CWC and are used by governments throughout the world, including the United States.

Loopholes in the CWC allow for use of other non-lethal agents, including so-called incapacitants, for domestic law enforcement and, in other interpretations, counterterrorism activities. In contrast to riot-control agents, the effects of incapacitants can last for days after exposure. Moreover, their effects are psychological or mental, designed to leave the victim confused, disabled, or, in general, ineffective. (Although Syria is not generally thought to possess such agents, some have theorized that the Assad regime may use incapacitants to test the waters of international reaction.) Also not addressed by the CWC is white phosphorus, an incendiary chemical agent that has been traditionally utilized by militaries to provide ground cover for operations as well as to illuminate targets. Its use as an offensive weapon lies in a legal gray area vis-a-vis chemical weapons law; it can produce severe chemical burns, irritation of mucus membranes (particularly of the eyes), and even death. From Homs forward to last week's attack in Khan al-Assal, the symptoms displayed by victims of alleged chemical weapons are not of a severity that would evince white phosphorus use, though the mere mention of white phosphorus by a doctor treating victims of the Khan-al-Assad incident adds to the growing scrutiny of that chemical. Also, US responses to reports of Syrian use of white phosphorus are complicated by past American and Israeli military operations in which white phosphorus was used.

Israeli forces are now alleging that Assad used Sarin gas -- best known from the 1995 attack on the Tokyo subway -- against civilians near Aleppo and Damascus on March 19. Unlike CS or white phosphorous, sarin is unequivocally a chemical weapon prohibited under the convention, which Syria is not a party to. 

But if Assad was indeed using less severe agents before, as the U.S. statements suggested, it seems possible he may have been building up to this point. Blair and Johns wrote in March that "If chemicals were used in any of incidents to date, they were agents -- white phosphorus, riot-control agents, and incapacitants -- that the CWC either conditionally allows, addresses in a manner that creates a legal gray zone, or ignores altogether.[...] [R]epeated unresolved claims of chemical weapons use slowly normalizes the concept that chemical weapons can be used, eroding the taboo against chemical warfare and desensitizing the public to its horrors."

If the claims are verified, we'll soon see just how sharply defined this red line is. 

Getty Images