The Science Behind Tear Gas

Tear gas has become a go-to way for police departments worldwide to try and break up a crowd. But there’s a lot of confusion out there about these chemicals—what they are, what they do, and whether they can cause long-term harm. So we’re going to talk about the science of tear gas. And look, we’re not here to tell you how you should feel about these chemicals or their use in any context. But we can explain what is known about them—and what isn’t. Tear gases belong to a category of compounds called lacrimators. 

They’re chemicals that irritate skin and mucous membranes and cause symptoms like pain, coughing, an uncontrollable shutting of the eyelids, and, well, tears. Hence the name. Though, tear gases aren’t actually gases. Instead, they’re liquids or solids that are turned into a spray or cloud with solvents or smoking reactions. Their effects can incapacitate people or just be so overwhelmingly awful debilitating that people run away. Though they don’t tend to last long, which is why these chemicals are used as riot control agents. Now, humans have used chemical weapons, in some form or another, for a long, long time. We’ve found archaeological evidence of this dating back to the Roman empire. But the modern concept of tear gas and its use by law enforcement seems to have gotten started around 1912 when French police forces began using chemicals to incapacitate criminals. 

Then, shortly after that came World War I, which involved the widespread use of chemical weapons, including noxious gases. And now, in 2020, most of those chemicals are banned from being developed, stockpiled, or used, per an international agreement called the Chemical Weapons Convention. But tear gases are an exception of sorts. Most countries have agreed not to use them in war, but the CWC doesn’t actually ban the use of tear gases domestically. So these compounds have continued to be manufactured and sold to law enforcement agencies around the globe. Today, there are a number of compounds that might fall under the header of tear gases. 

Some of the most common ones are as follows: CS gas; CN, which is actually the active ingredient in Mace; CR gas; OC; and finally, PAVA. So, basically, a lot of letters, because no one wants to say their chemical names over and over again. Of these, CS and OC are the most commonly used. But they all cause similar symptoms. And they break down into two major categories based on how they do that. CS, CN, and CR are synthetic compounds known as electrophilic agents because they bind to and accept electron pairs from other compounds. There are electrophilic agents that don’t cause pain. But what makes these tear gases special is that they fit into and take electrons from a specific protein in our bodies called TRPA1. This is a receptor found on cells throughout the body, including nerve cells. And in nerve cells, in particular, one of its functions seems to be detecting potentially harmful chemicals. 

When one of these electrophilic agents reaches this receptor, it binds to electron-rich groups inside it and ultimately induces nerve signals that are registered as heat or pain. Non-tear-gas compounds, like the ones found in wasabi or cinnamon, also bind to this receptor. But CS, CN, and CR are extremely powerful and selective. A 2008 study called them the most potent activators of TRPA1 known to date. The other big category of tear gases is the capsaicinoids, which are compounds derived from chili peppers or their mimics. For instance, OC — which stands for oleoresin capsicum — is an oily resin made from ripe chili peppers in the genus Capsicum. Because OC is derived from plants, it can actually contain over 100 compounds, but the most notable is —the major spicy compound in chili peppers. PAVA is a similar, though slightly different,spicy molecule that also occurs naturally in some chili peppers. 

Though, the stuff in riot control agents was likely synthesized in a lab. And, yes, you could call these “pepper sprays” or “pepper balls” instead of “tear gases”, but since they’re used in similar ways, researchers tend to group them together. Plus, these pepper sprays are a lot stronger than you might assume. In fact, capsaicinoids are regulated as pesticides in the US, since they’re used to discourage mammals from eating crops and in bear deterrent sprays—at about a third to a tenth the concentration of police pepper sprays. Chemically, capsaicinoids aren’t electrophiles like CS, and they don’t mess with TRPA1. Instead, they activate a related receptor on cells called TRPV1. Capsaicinoids fit into a pocket about halfway through this gate-like receptor, and when they bind there, the gate opens. This affects the electrical charge inside the cell and ultimately leads to the sensation of heat or pain. 

So, even though they hit different targets, both kinds of tear gas have similar downstream effects: they hurt. The idea behind using these kinds of chemicals on people is that, though profoundly unpleasant, they’re relatively safe and are considered to have a wide safety margin. In other words, the dose it takes to incapacitate someone is much smaller than the dose that’s fatal. Like, five or six orders of magnitude smaller. That said, from a research standpoint, there may be some potential blind spots here. The studies to determine that incapacitating dose are generally conducted on small numbers of healthy, adult men, for starters. Like, for CS, most were military recruits tested in the 1960s. So, it’s not clear from that research how people with conditions like asthma, or women or kids would fare. And case studies have suggested, though not for certain, that those distinctions can matter. Women, children, the elderly, and people with chronic conditions all seem to be more vulnerable to tear gases for various reasons. Dose studies also may miss long-term or chronic effects, as they only follow subjects for a short while. For instance, the EPA’s exposure guidelines for CS note just one human experimental study that reported on symptoms more than a few hours after exposure. And as of 2014, there was no human data on repeated exposures. 

Meanwhile, the estimates of lethality come from studies on animals, not people. That’s for obvious reasons. Still, as we've mentioned a lot here on SciShow, studies in animals like mice don’t necessarily translate perfectly to humans. There are plenty of chemicals that lab animals tolerate way better than people—and vice versa. All this means that we just don’t have a lot of experimental data on how these chemicals affect a real-world population of people. Instead, scientists often have to rely on after-the-fact case studies written up by doctors, retrospective studies, or reviews of these. And while these can be helpful at letting us know the possible consequences of exposure, they’re not perfect, either. Like, they’re usually not set up to tell us how outcomes change with dose, since they’re done after exposure. Plus, they can have gaps—like, if patients or their doctors don’t report everything accurately. And ultimately, that means they’re not so good at telling us how common more serious reactions to tear gases are. But we do know they exist. 

Even the initial studies for CS noted things like kidney damage in animals. And case studies contain a number of other worrisome effects. In the respiratory system, for example, teargas exposure can cause potentially life-threatening conditions. These include pulmonary edema—which is when the lungs fill with fluid—and respiratory arrest, which is when the person stops breathing. Tear gases can also cause chronic inflammation of the lung tissue and coughing that can last for weeks. One Army study even suggests CS exposure might make subjects more susceptible to respiratory infections. So people exposed to tear gases may be at higher risk for catching the flu... or coronaviruses. Cases have suggested tear gases can also trigger heart attacks, as well as cause digestive tract symptoms, first and second-degree burns, and miscarriages. There’s even some evidence to suggest that they may be carcinogens, meaning they could speed the development of certain cancers. That might seem like a wide swath of issues. And it is. That may be because TRPA1 and TRPV1 are found in many types of tissue and might do quite a few different things in the body. They’ve been connected to widespread immune activation, for example. TRPA1 was even called a “gatekeeper” of inflammation by one study. And because they cause pain, they activate the body’s fight or flight network. 

That means they can affect all sorts of body parts and functions — like heart rates, which may lead to erratic, potentially dangerous changes in heart rhythms. In the end, the safety of tear gas is really controversial. And despite being supposedly “non-lethal” or “sub-lethal”, there have been deaths linked to different types. Given all of this information, it’s only natural to wonder whether there are any good treatments or antidotes to tear gases. And, sadly, no. There isn’t a universal cure. Most doctors say preventing or limiting exposure is key. Experts have noted that gas masks can filter out the particles in the air. And goggles, long sleeves, and cloth covering the nose and mouth can prevent chemicals from reaching the skin and mucous membranes. So those can all reduce the amount of teargas that gets on and into a person’s body. Experts have also recommended removing makeup or contacts, as particles may stick to them. But if someone is exposed to tear gas, the most important thing is for them to get away from the source, and, maybe, get somewhere high up, as the chemical clouds are heavier than air and tend to sink. Now, some people swear by certain treatments. Like, you may have seen videos or pictures of people rinsing their eyes with milk. 

That kind of seems like it would make sense since people chug milk to counteract the capsaicinoids in spicy foods. And research has found that milk quenches the mouth-burn of spicy meals. Unfortunately, the best way to manage the symptoms of tear gas exposure is still debated by the researchers studying this, and the number of studies of different options is limited. Like, we’re-lucky-if-it’s-been-studied-at-all-in-some-cases,type of limited. Milk is actually a good example of this. In the few studies that have looked at it, it didn’t seem to be much if at all better than plain old water. But also, those studies usually had participants rinse off prior to applying washcloths soaked with milk to their skin. Which is not really what would happen in a riot-control situation. It may also depend on what tear gas was used. Which gets tricky, since it can be really hard to tell them apart at the moment. Not only is there confusion from the situation—many of these chemicals are hard to distinguish by look or smell. Like, CS and OC both smell pretty peppery. 

Still, overall, experts largely agree that rinsing the eyes with clean, uncontaminated water or saline and washing the skin with soap and water is one of the key steps to treating tear gas exposure. One study has suggested that an alkaline solution, like a mix of baking soda and water, could speed up the breakdown of CS, and the antacid Maalox might be a bit more effective on skin irritation from OC in particular. So, maybe, if the type of tear gas is known, those could be used. But again, the research on them is very limited, so it might be safer to stick to water and soap. It’s also important to know that tear gas chemicals linger in the environment and on clothing. That means they can transfer to healthcare workers trying to help out. And contaminated clothes should be washed multiple times separately from the rest of a person’s laundry. Also, surfaces may need to be scrubbed and exposed to food tossed. In extreme cases, specific, professional cleaners who can deal with noxious chemicals may be needed. So, there you have it. That’s the rundown on tear gas. 

That term can refer to any of a number of compounds, all of which get their tear-producing effects by triggering pain receptors of some kind. And though they’re often called non-lethal, they can affect many parts of the body, with serious, sometimes fatal consequences. And there are still major gaps in our knowledge on their effects and how best to treat them. That’s where things stand. I hope you found this information helpful! And as always, like this information then feel free to share this blog with your friends and family and keep supporting :))