Tear gas, formally known as a lachrymator agent or lachrymator (from the Latin lacrima, meaning "tear"), sometimes colloquially known as mace,[NB 1] is a chemical weapon that causes severe eye, respiratory, and skin irritation, pain, bleeding, and even blindness. In the eye, it stimulates the nerves of the lacrimal gland to produce tears. Common lachrymators include pepper spray (OC gas), CS gas, CR gas, CN gas (phenacyl chloride), nonivamide, bromoacetone, xylyl bromide, syn-propanethial-S-oxide (from onions), and Mace (a branded mixture).
Tear "gas" consists of aerosolized solid compounds, not gas. Tear gas works by irritating mucous membranes in the eyes, nose, mouth and lungs, and causes crying, sneezing, coughing, difficulty breathing, pain in the eyes, and temporary blindness. With CS gas, symptoms of irritation typically appear after 20–60 seconds of exposure and commonly resolve within 30 minutes of leaving (or being removed from) the area. With pepper spray (also called "oleoresin capsicum", capsaicinoid or OC gas), the onset of symptoms, including loss of motor control, is almost immediate. There can be considerable variation in tolerance and response, according to the National Research Council (US) Committee on Toxicology.
The California Poison Control System analyzed 3,671 reports of pepper spray injuries between 2002 and 2011. Severe symptoms requiring medical evaluation were found in 6.8% of people, with the most severe injuries to the eyes (54%), respiratory system (32%) and skin (18%). The most severe injuries occurred in law enforcement training, intentionally incapacitating people, and law enforcement (whether of individuals or crowd control).
Lachrymators are thought to act by attacking sulfhydryl functional groups in enzymes. One of the most probable protein targets is the TRPA1 ion channel that is expressed in sensory nerves (trigeminal nerve) of the eyes, nose, mouth and lungs.
As with all non-lethal, or less-lethal weapons, there is some risk of serious permanent injury or death when tear gas is used. This includes risks from being hit by tear gas cartridges, which include severe bruising, loss of eyesight, skull fracture, and even death. A case of serious vascular injury from tear gas shells has also been reported from Iran, with high rates of associated nerve injury (44%) and amputation (17%), as well as instances of head injuries in young people.
While the medical consequences of the gases themselves are typically limited to minor skin inflammation, delayed complications are also possible: people with pre-existing respiratory conditions such as asthma, who are particularly at risk, are likely to need medical attention and may sometimes require hospitalization or even ventilation support. Skin exposure to CS may cause chemical burns or induce allergic contact dermatitis. When people are hit at close range or are severely exposed, eye injuries involving scarring of the cornea can lead to a permanent loss in visual acuity. Frequent or high levels of exposure carry increased risks of respiratory illness.
Reports of expired tear gas canisters picked up by protesters in Egypt led to theories that it could be more toxic, but Steve Wright of Leeds Metropolitan University said if enough time has elapsed that the chemicals have broken down inside the can, then it makes the canister less effective.  However, a study carried out by Mónica Krauter, a Venezuelan professor of Simón Bolívar University, collected thousands of tear gas canisters fired by Venezuelan authorities in 2014, showed that 72% of the tear gas used was expired and noted that expired tear gas "breaks down into cyanide oxide, phosgenes and nitrogens that are extremely dangerous".
Use of tear gas in warfare (as with all other chemical weapons) is prohibited by various international treaties[NB 2] that most states have signed. Police and private self-defense use is not banned in the same manner. Armed forces can legally use tear gas for drills (practicing with gas masks) and for riot control. First used in 1914, xylyl bromide was a popular tearing agent since it was easily prepared.
Certain lachrymatory agents are often used by police to force compliance, most notably tear gas. In some countries (e.g., Finland, Australia, and the United States), another common substance is mace. The self-defense weapon form of mace is based on pepper spray, and comes in small spray cans, and versions including CS are manufactured for police use. Xylyl bromide, CN and CS are the oldest of these agents, and CS is the most widely used. CN has the most recorded toxicity. Tear gas exposure is a standard in Australia for military, police and prison officer training programs.
Typical manufacturer warnings on tear gas cartridges state "Danger: Do not fire directly at person(s). Severe injury or death may result." Such warnings are not necessarily respected, and in some countries, disrespecting these warnings is routine. In the 2013 protests in Turkey, there were hundreds of injuries among protesters targeted with tear gas projectiles. In the Israeli-occupied territories, Israeli soldiers have been routinely documented by Israeli human rights group in firing direct tear gas canisters at activists, some of which resulted in fatalities. Amnesty International criticized the usage of tear gas by Venezuelan authorities noting canisters being fired directly at individuals while also being shot into residential buildings.
However, tear gas guns do not have a manual setting to adjust the range of fire. The only way to adjust the projectile's range is to aim towards the ground at the correct angle. Incorrect aim will send the capsules away from the targets, causing risk for non-targets instead. For example, this occurred during the 2013 protests in Brazil, 2014 Hong Kong Protests and 2014–17 Venezuelan protests.
Tear gas being used against opposition protesters during the 2014 Venezuelan protests
A variety of protective equipment may be used, including gas masks and respirators. In riot control situations, protesters sometimes use equipment (aside from simple rags or clothing over the mouth) such as swimming goggles and adapted water bottles.
Activists in the United States, the Czech Republic, Venezuela and Turkey have reported using antacid solutions such as Maalox diluted with water to repel effects of tear gas attacks with Venezuelan chemist Mónica Krauter recommending the usage of diluted antacids as well as baking soda. There have also been reports of these antacids being helpful for tear gas, and for capsaicin-induced skin pain.
There is no specific antidote to common tear gases. Getting clear of gas and into fresh air is the first line of action. Removing contaminated clothing and avoiding shared use of contaminated towels could help reduce skin reactions. Immediate removal of contact lenses has also been recommended, as they can retain particles.
Once a person has been exposed, there are a variety of methods to remove as much chemical possible and relieve symptoms. The standard first aid for burning solutions in the eye is irrigation (spraying or flushing out) with water. There are reports that water may increase pain from CS gas, but the balance of limited evidence currently suggests water or saline are the best options. Some evidence suggests that Diphoterine solution, a first aid product for chemical splashes, may help with ocular burns or chemicals in the eye.
Bathing and washing the body vigorously with soap and water can remove particles that adhered to the skin while clothes, shoes and accessories that have came into contact with vapors must be washed well since all untreated particles can remain active for up to a week. Some advocate using fans or hair dryers to evaporate the spray, but this has not been shown to be better than washing out the eyes and it may spread contamination.
Oral analgesics may help relieve eye pain.
Vinegar, petroleum jelly, milk and lemon juice solutions have also been used by activists. It is unclear how effective these remedies are. In particular, vinegar itself can burn the eyes and prolonged inhalation can also irritate the airways. Though vegetable oil and vinegar have also been reported as helping relieve burning caused by pepper spray, Krauter does not suggest the usage of vinegar, toothpaste or menthol creams, stating that "they trap the particles emanating from the gas near the airways and make it more feasible to inhale". A small trial of baby shampoo for washing out the eyes did not show any benefit.
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