During the current wave of protests against systemic racism, police brutality, and the murder of George Floyd taking place in the United States, thousands of people have had encounters with the burning, stinging, and choking sensation of tear gas. But what exactly is this stuff? Why is it so dangerous? And what should you do if you come into contact with it?

Tear gas was first used by the French Army during World War I (1914-1918) as a means to drive the enemy out of trenches. But as of 1997, it's now illegal to use tear gas in international conflict and war.

Under the global Chemical Weapons Convention, it states: "Each State Party undertakes not to use riot control agents as a method of warfare,” defining a riot control agent as "Any chemical not listed in a Schedule, which can produce rapidly in humans sensory irritation or disabling physical effects, which disappear within a short time following termination of exposure.” However, the convention does not cover domestic use against civilians, so use against protestors as "riot control" is fair game in the eyes of international law.

Tear gas is a loose term for a chemical agent that temporarily makes people unable to function by causing irritation to the eyes, mouth, throat, lungs, and skin. According to the Centers for Disease Control and Prevention (CDC), the most commonly used compounds are chlorobenzylidenemalononitrile (sometimes referred to as CS gas) and chloroacetophenone (CN). Another common form of tear gas is pepper spray, which contains capsaicin, the irritant that gives chili peppers their heat.

CS gas causes discomfort by targeting a specific receptor found on cells called TRPA1, which sparks the perception of pain in the nervous system. Once the agent comes into contact with the body, the immune system identifies the foreign substance and attempts to flood out the irritant with body fluids, like tears and mucus, which can cause further discomfort.

In theory, tear gas causes sore eyes, blurred vision, coughing, and a runny nose for up to 30 minutes, but its effect can be much harsher if there’s prolonged or intense exposure. A 2016 medical review on the effects of tear gas found it can be responsible for skin burns, blistering, choking, vomiting, and bleeding. It's especially nasty if the agent comes into close contact with the eyes, where it can cause tearing, bleeding, cornea burning, and even ocular nerve damage. Deaths are rare, but not unheard of.

The effect of tear gas on the respiratory system has also raised concerns that excessive use of the chemical agent by police on protesters during the current US protests could increase the spread of Covid-19, a disease that has hit a staggeringly high proportion of Black people in the US. In an online petition, infectious disease experts in the US warned that tear gas could “increase risk for Covid-19 by making the respiratory tract more susceptible to infection, exacerbating existing inflammation, and inducing coughing.”

If you’ve been exposed to tear gas, there are a number of things you should do. 

First things first, get far away from the gas cloud if possible. As soon as you're able, thoroughly rinse your eyes with clean water or saline solution, as well as wash exposed skin with lots of soap and water. Some activists suggest using milk or mixing water with antacid indigestion tablets, although there’s little scientific evidence to back this up. It’s also important to remove any clothing that may have the chemical agent on it as this could cause further irritation. Take a long shower as soon as possible, until you can't smell the gas anymore, to ensure it is thoroughly rinsed off. This can take as long as 30 minutes.

If your symptoms don’t pass within 30 minutes, then you should consider seeking medical attention.

If you are planning on protesting, there are also things you can do to minimize risk. Glasses and goggles can provide some physical protection, although it’s highly recommended protestors do not wear contact lenses as this can lead to further irritation to the eyes. Wearing make up creates a surface that allows tear gas to stick to your face. Face masks — which are also a good idea to protect against the spread of Covid-19 — will also provide some protection, as will long-sleeved tops and trousers to minimize risk of skin contact.