Tear gas, formally known as a lachrymatory agent or lachrymator (from lacrima meaning ‘tear’ in Latin), is a chemical weapon that stimulates the corneal nerves in the eyes to cause tears, pain, vomiting, and even blindness. Common lachrymators include pepper spray (OC gas), CS gas, CR gas, CN gas (phenacyl chloride), nonivamide, bromoacetone, xylyl bromide, syn-propanethial-S-oxide (from onions), and Mace (a branded mixture).
Lachrymatory agents are commonly used for riot control. Their use as chemical warfare agents is prohibited by various international treaties. [My emphasis]
The 1993 Chemical Weapons Convention doesn’t apply to domestic law enforcement.
Because the U.S. was concerned that the C.W. Convention could be interpreted to prohibit lethal injection, we were a vocal supporter of this exemption (my emphasis below):
1. ‘Chemical Weapons’ means the following, together or separately:
(a) Toxic chemicals and their precursors, except where intended for purposes not prohibited under this Convention, as long as the types and quantities are consistent with such purposes;
9. ‘Purposes Not Prohibited Under this Convention’s if means:
(d) Law enforcement including domestic riot control purposes.
The most common lacrimator used for riot control is a compound called 2-chlorobenzalmalononitrile, or CS, which is a powder that when mixed with smoke creates an airborne agent with frightening, although not usually lethal, results. But do notice the foregoing Wikipedia list of common lachrymators also includes pepper spray, which Lt. John Pike used as a chemical weapon against peaceful protestors at U. C. Davis in 2011.
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.
According to Sven-Eric Jordt, a Duke University scientist who researches tear gas, speaking with Vox’s Sarah Kliff,
The way these gases work, and this is what we do research on, is that they activate pain receptors — the pain sensing nerves in our body. The cornea is densely covered with these receptors. When tear gas activates these pain receptors, that leads to body reflexes like profuse tear secretion and a muscle cramp in the eyelid that causes them to close. These are all protective responses that the body has to pain, and with the gas they become extremely exaggerated.
There are situations where this can be very dangerous or lethal. If somebody has asthma, for example, or a hypersensitivity or an airwave disease that can be very dangerous.
Tear gas can also lead to profuse mucus production, and that can lead to the feeling of suffocation. That’s especially true if it’s used in closed environments, like what you saw in Cairo. That’s not the case here in Ferguson.
I’m very concerned that, as use has increased, tear gas has been normalized. The attitude now is like, this is safe and we can use it as much as we want.
Regardless of the lengths to which law enforcement will go to justify it, the fact remains that police in the United States – in many cases with inadequate oversight or control – use a chemical banned for use in war, in a domestic “war” on protesters.