What started as a protest against the demolition of Istanbul’s Gezi Park has boiled over into broad anti-government unrest in cities across Turkey. Almost everywhere, the protestors have been met with heavy resistance from riot police and torrents of tear gas. Here’s a crash course in the chemical weapon of the moment.

What is it?

Tear gas isn’t one specific chemical, and it’s usually not even a gas, despite the name. There are a few different compounds that are used as “lachrymatory agents.” Most of them are solids at room temperature, and get mixed with liquid or gas dispersal agents for use.

The variety favored by the Turkish police is known as CS gas, which is made from a powdery compound called 2-chlorobenzylidene malononitrile and a liquid solvent (often methylene chloride), mixed into an aerosol spray.

Where did it come from?

Tear gases in general are something that militaries have been messing around with since World War I. Both France and Germany developed and deployed lachrymatory irritants in battle, but there was evidently a bit of a learning curve. According to the U.S. Army’s Combat Studies Institute, the Germans fired some 3000 tear gas shells during one day of fighting in 1914, but the British troops on the receiving end “suffered no ill effects and never suspected they were under chemical attack.”

In just a few years, the Germans had gotten a handle on things and were using tear-inducing gases to great effect. In 1916, they fired 2000 shells into a French trench system and 2400 French soldiers—blinded, coughing and crying—quickly found themselves surrounded by German troops in protective goggles.

CS gas would come a few decades later. Its active component, 2-chlorobenzylidene malononitrile, was synthesized by American chemists Ben Corson and Roger Stoughton in 1928 and tear gas using it (CS = Corson + Stoughton) was developed and tested during the 1950s and '60s.

What does it do?

Tear gases irritate the mucous membranes of the eyes, nose, mouth, and lungs, and cause tearing, coughing, burning, and stinging sensations, chest tightness and difficulty breathing. At higher concentrations, exposure can cause stomach irritation leading to vomiting and diarrhea.

According to German toxicologist Uwe Heinrich, dispersing the gas at a concentration of one milligram per cubic meter will cause symptoms of irritation. From there, things go sour pretty quickly. A concentration of 10 mg per cubic meter can force trained soldiers to retreat from an area. Ten to 20 mg/m3 or higher can cause serious injury or, depending on the victim and conditions of exposure, death. In one incident reported in a Swiss medical journal, an otherwise healthy adult man was exposed to a tear gas grenade containing just one gram of CS while inside a building. He quickly developed toxic pulmonary edema, a condition where excess fluid collects in the air sacs of the lungs and causes difficulty breathing, and recovered only after weeks of medical treatment.

Sounds nasty. Shouldn’t stuff like this be illegal?

It is. Kind of. Tear gases were used in warfare through most of the 20th century until 1993, when the Chemical Weapons Convention banned the production, stockpiling and use of chemical weapons in battle. The international treaty doesn’t apply to nations’ domestic law enforcement, though, so police officers from Turkey to Texas are free to spray it at civilians.

Repressive Middle Eastern governments really love the stuff, don’t they?

Boy, do they ever! During a protest just last month, Turkish police used 14 tons of water mixed with CS components, and Prime Minister Recep Tayyip Erdoğan has earned the nicknames “Chemical Tayyip” and “Gasman” thanks to police and security forces’ rampant use of tear gas.

Bahraini police also have a gas problem, to the point where the foreign policy writer Steve Fake says the country has “raised the global bar on the usage of tear gas to unprecedented heights. It has become the Tear Gas Regime.” A report released by the organization Physicians for Human Rights last year says, “Preliminary analysis of data suggests that the majority of Shi’a neighborhoods (comprising 80% of all neighborhoods in Bahrain) have been exposed to toxic chemical agent attacks at least once per week since February 2011.” [Emphasis theirs]

How do you treat tear gas exposure?

If you’re outside, the best antidote to any breathing problems is fresh, untainted air, and time. For high dose exposure or exposure in enclosed spaces, bottled oxygen or certain asthma medications may be administered to ease difficulty breathing. Any exposed skin should be washed with soap and water and eyes should be flushed with sterile water or saline solution. A flyer circulated at the Occupy Wall Street protests also recommended spraying the eyes, mouth and throat with a 50/50 mix of water and Maalox. (Though this specifically mentions capsaicin-based tear gases. Not sure if it works for other types, and I’m  not looking to test it out.)

Is there anything else you wanna know about tear gas? Have other questions about current events or sciencey things? Email askmatt@mentalfloss.com.