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Par Robert Lamb | 15 November 2011.

Trailing white fumes, a silver canister arches over the no-man’s-land between the armored riot police and the bulk of your fellow protesters.

You duck, but the canister falls short and lands amid the people in front of you. The white fumes begin to billow up like fake smoke at a magic show, but there’s nothing fake about the cries you hear. People shriek. They wail. They flee backward through the crowd, tears and mucus flowing freely from reddened faces. They cough and rub their eyes maddeningly. Some of them double over and retch.

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.

You turn to flee, too, but that’s when the cloud of white powder hits you full in the face. You pinch your eyes shut at the sudden sting. You gasp, and the burning sensation seers its way into your sinuses and down your throat. It’s as if you’ve swallowed fire.

Forcing your eyes open, you find the world has blurred. The screaming crowd scatters, and all is chaos. You feel as if you’ll never breathe or see again.

Yes, you’ve been hit with tear gas, a substance used around the world to disperse crowds and control riot situations. While you’ll probably recover fully in an hour’s time, it’s not an experience you’ll forget. Just what is this dreadful stuff, and why does it feel like you’ve been licked full in the face by the devil himself?

In this article, we’ll pop open the silver canister and discuss what tear gas is, why it’s legal and what you can do to make the burning stop.

When Tear Gas Fails

Not every exposure to CS gas goes according to plan. For starters, some people exhibit varying degrees of genetic or acquired immunity to the irritant — a reality most frequently encountered in boot camp.

Members of the U.S. Army undergo routine tear gas exposure as part of their basic training. After all, as a short-term irritant, CS gas injects a healthy dose of realism into a test aimed to boost soldiers’ confidence in the gear and procedures that protect them from nuclear, biological and chemical threats.

The soldiers strap on their gas masks and enter an enclosed mask confidence chamber filled with CS gas. Then their drill sergeant orders them to remove their masks and answer questions such as, “What is your name, rank and serial number?” This helps to ensure that the solider breathes the gas-tainted surrounding air.

For most soldiers, what follows is intense burning. A small minority, however, experiences far milder symptoms — perhaps just a mere sneeze or two, thanks to a natural immunity. Drill sergeants, on the other hand, who regularly encounter CS gas in these tests, may benefit from acquired immunity. Over time, they’ve simply built up a degree of tolerance in the same way you might build up a tolerance to spicy food or even poison. These veterans also know what to expect from the gas and don’t panic when the symptoms pop up.

Other folks, however, exhibit heightened sensitivity to CS gas. Children, the elderly and individuals with pre-existing conditions (such as asthma or a compromised immune system) are likely to experience prolonged symptoms, the severity of which depends entirely on the method, duration and intensity of the exposure.

Tear gas exposure inside a closed space is far more intense than exposure in the open air — and it also introduces additional dangers. As we’ll discuss on the following page, heat often plays a role in the dispersal of CS gas from a canister or grenade. Studies have found that, in an enclosed space, such thermal dispersion can generate additional semivolatile organic air contaminants [source: Lenhart]. For this reason, training programs usually use specially designed CS capsules rather than tear gas canisters.

In addition, while police forces often employ tear gas to force an adversary out of an enclosed space, the gas can induce panicked behavior if an exit isn’t readily accessible [source: PBS].

Long-term or heavy exposure to CS gas can result in severe symptoms or even permanent damage in the form of glaucoma or blindness. In rare cases, exposure can end in death due to serious internal chemical burns or respiratory failure. In 2011, a 36-year-old Palestinian woman named Jawaher Abu Rahmeh died from respiratory failure and then cardiac arrest following exposure at a West Bank protest [source: The Guardian].

The occasional ingestion of CS produces a certain amount of abdominal turmoil but no permanent damage.

So how does tear gas find its way to a target? Find out on the next page.