Tear gas is banned for use in warfare by the Geneva Convention. It's allowed, however, to be used by police as "domestic riot control", as seen in protests in Turkey, Bahrain, and now Ferguson, Missouri. Here's what you need to know about tear gas: What it is, and what it does to the human body.
What's in it?
The term "tear gas" is a bit of a misnomer: The canisters in a police department's arsenal contain a solid powered chemical, rather than a gas. When activated, the chemical is aerosolised, creating a dense plume of crystallised chemicals that hang in the air.
Almost universally, those clouds contain one of two chemicals. Oleum capsicum (OC) tear gas is made of chilli pepper oil, the same ingredient used in pepper spray. CS gas uses a chemical called 2-chlorobenzalmalononitrile as its active ingredient, with a couple of variations that range in potency.
Various forms of tear gas have been in use since World War I, some of which are now banned due to toxicity. The ones that remain are said to be nontoxic and — when used appropriately — nonlethal. But it wasn't until just a few years ago that we fully understood how tear gas works.
What does it do?
Exposure to tear gas causes a trademark set of symptoms: severe burning of the eyes, mouth, throat, and skin. Victims cough, choke, and retch, with tears and mucous streaming uncontrollably from their eyes, noses, and throats. In response to the pain, victims' eyelids snap shut. Some report temporary blindness.
Depending on the type of tear gas used, symptoms come on nearly instantly. CS gas kicks in within 30 to 60 seconds, causing irritation to the skin, eyes, and breathing passages. Pepper-based OC gas sets in even more swiftly, and can even incapacitate its victims.
Tear gas is so viciously effective because, again, it's not a gas; it's a substance that binds and reacts to moisture. The aerosolised crystals glom on to the wet surfaces of a person's eyes, mouth, skin, and breathing passages. The body's defence mechanisms go into overdrive, trying to flush the chemicals away with tears and mucus. The pain and panic, combined with the body's swift reaction, can incapacitate or disorient a victim.
If a victim can escape the tear gas and get to open air, the symptoms usually wear off within one hour, though burning eyes and skin, dizziness, and disorientation can last much longer. While researchers are studying ways to block the chemicals' effects, currently there's no cure for victims other than getting away into open air.
Ever since tear gas saw its first use in WWI, scientists assumed it acted as a simple irritant. But recent research shows that the chemicals used today aren't just aerosolized itching powder: They're actually nerve agents.
Sven-Eric Jordt is a professor of pharmacology at the Yale University School of Medicine. Dr. Jordt's research team focuses on the nerve pathways of pain. In 2006 and 2009, the Jordt lab published papers explaining for the first time the neurological basis of how tear gas works. As Dr. Jordt explained to National Geographic in 2013:
Tear gases are nerve gases that specifically activate pain-sensing nerves. Spelled out like that, people can better compare them to other nerve agents out there. That's the major discovery we made, that they are not benign or just irritants. The receptors are designed to warn a human or animal about exposure to a noxious chemical, so the animal removes itself from the exposure. They increase survival.
In other words, they're not just there to incapacitate. They act directly to cause pain.
When it goes wrong
Tear gas is meant to work long enough to subdue a crowd, then dissipate with no permanent repercussions. A 2003 study states that there is "no evidence that a healthy individual will experience long-term health effects from open-air exposures to CS". But that's not always how it goes.
Reports from demonstrations in Egypt in 2013 state that 37 people died of asphyxiation after a tear gas canister was fired into the vehicle carrying them. Others have been blinded, maimed or killed by tear gas canisters shot from close range. The War Resisters League reports on deaths caused by exposure to tear gas in closed spaces, miscarriages and stillbirths after exposure to the chemicals. People with respiratory diseases like asthma are especially susceptible to tear gas, sometimes requiring long-term hospitalisation. Overexposure can cause burns to the skin or eyes, the latter frequently causing blindness.
Tear gas is meant to be a non-lethal crowd dispersing agent. But to its victims, and the people who study it, that distinction isn't always so clear. As Dr Jordt told National Geographic, the legacy of tear gas doesn't always live up to that non-lethal claim:
There are enough examples where people suffered severe injury and burns, especially in enclosed environments or city streets with several-story buildings. Residents who live near Tahir Square in Cairo that have gotten a lot of tear gas have had long-term exposure, leading to respiratory problems. Long-term exposure is very problematic.
People with asthma or other conditions can have very severe reactions. Tear gases are very serious chemical threats. I think it is very problematic to use them.
When asked by National Geographic if tear gas should be used on civilians, Dr Jordt's response was telling:
Tear gas under the Geneva Convention is characterised as a chemical warfare agent, and so it is precluded for use in warfare, but it is used very frequently against civilians. That's very illogical.
It's unknown how many canisters of tear gas have been fired in Ferguson, Missouri. Media reports have been limited, but by all accounts the Ferguson police are relying on it extensively to control and subdue protestors. Given the mixed history of this weapon, and its potential for injury or death, it makes an already unstable situation even more troubling.
Picture: Police advance through smoke, Wednesday, Aug. 13, 2014, in Ferguson, Mo. (AP Photo/Jeff Roberson)