Police and military forces around the world use tear gas for a reason: It disperses crowds effectively through the application of temporary physiological discomfort. A crowd of rioters may intend to storm a barricade one moment, but priorities have a way of changing when physical pain and irritation enter the picture. Gassed people quickly become "incapable of effective concerted action," to quote the 1969 "Police Chemical Agents Manual."
The term "tear gas" may refer to any of several riot control agents, including chloroacetophenone (CN), an ingredient in the chemical spray Mace. Collectively, we refer to these chemicals as tear producers, or lacrimators. Yet modern tear gas almost always boils down to a particular chemical agent: orthochlorobenzalmalononitrile (CS) or C10H5ClN2, a crystalline powder with a peppery odor.
Chemists first synthesized CS in the late 1950s as a crowd suppressant. Within a few years, it quickly replaced the less-powerful CN as the go-to tear gas. It remains in use around the world to this day.
Don't let the tears fool you. Lacrimators are irritants, not mood-altering chemicals. CS gas causes a severe burning sensation upon contact with skin. Your sensory nerves sound the alarm to your brain, sending the memo, "Hey, please remove this awful chemical from your skin before it physically harms you."
And indeed, prolonged exposure to the CS can cause rashes and chemical burns. When the irritant encounters the human eye, the stakes are much higher. The sensory nerves send a signal to your brain stem, which in turn sends hormones to tear glands in the eyelids. These glands pump out a salty wash of protein, water, mucus and oil to help rid your sensitive peepers of the irritant as quickly as possible.
This is how tear gas produces tears, but the effects of exposure don't necessarily stop with the ocular immune system. Inhale CS gas and the burning sensation will kick-start similar defenses in the nose and the respiratory system: flowing mucus and hacking coughs, all in an attempt to rid the body of its irritant. Nausea and vomiting also may occur.
The good news is that in most cases, these symptoms vanish within an hour of exposure. An affected person generally flees from the source of the exposure and all that crying, coughing and vomiting helps rid the body of the chemical in no time. Then the irritation subsides.
Except when it doesn't.