thumb|upright|Tear gas in use in France 2007
thumb|An exploded tear gas canister airborne in Greece
thumb|right|A tear gas canister in Minneapolis during [[Operation Metro Surge]]
Tear gas, also known as a lachrymatory agent or lachrymator (), sometimes colloquially known as "mace" after the early commercial self-defense spray, is a chemical weapon that stimulates the nerves of the lacrimal gland in the eye to produce tears. In addition, it can cause severe eye and respiratory pain, skin irritation, bleeding, and blindness. Common lachrymators both currently and formerly used as tear gas include pepper spray (OC gas), PAVA spray (nonivamide), CS gas, CR gas, CN gas (phenacyl chloride), bromoacetone, xylyl bromide, chloropicrin (PS gas) and Mace (a branded mixture).
While lachrymatory agents are commonly deployed for riot control by law enforcement and military personnel, their use in warfare is prohibited by various international treaties. During World War I, increasingly toxic and deadly lachrymatory agents were used.
The short and long-term effects of tear gas are not well studied. The published peer-reviewed literature consists of lower quality evidence that do not establish causality. Tear gas works by irritating mucous membranes in the eyes, nose, mouth and lungs. It causes crying, coughing, difficulty breathing, pain in the eyes, and temporary blindness.
With CS gas, symptoms of irritation typically appear after 20 to 60 seconds of exposure and commonly resolve within 30 minutes of leaving the area.
Risks
thumb|The aftermath of exposure to tear gas during protests in [[Oakland, California]]
As with all non-lethal or less-lethal weapons, there is a risk of serious permanent injury or death when tear gas is used. A case of serious vascular injury from tear gas shells occurred in Iran, with high rates of associated nerve injury (44%) and amputation (17%), and instances of head injuries in young people. Novel findings suggest that menstrual changes are one of the most commonly reported health issues in women.
Skin exposure to CS gas may cause chemical burns 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.
In the 2019–20 Chilean protests, people have had complete and permanent loss of vision in one or both eyes as result of the impact of tear gas grenades.
The majority (2,116; 93.8%) of protestors exposed to tear gas during the 2020 protests in Portland, Oregon, reported physical (2,114; 93.7%) or psychological (1,635; 72.4%) health issues experienced immediately after (2,105; 93.3%) or days following (1,944; 86.1%) the exposure. The majority (1,233; 54.6%) of respondents exposed to tear gas received or planned to seek medical or mental healthcare for their tear gas-related health issues.
Uses
Warfare
During World War I, forms of tear gas were used in combat and tear gas was the most common form of chemical weapon used. None of the belligerents believed that the use of irritant gases violated the Hague Convention of 1899 which prohibited the use of "poison or poisoned weapons" in warfare. Grenades with the lacrimatory agent ethyl bromoacetate were the first chemical weapon used in World War I. It first was used in August 1914 by the French, whose police had previously used it domestically starting in 1912; followed by the Germans deploying the lung irritant dianisidine chlorosulphonate they called "Nerst Ni-Shrapnel" or "ni-shells" (derived from the word for sneezing powder) against British and Indian troops in October, and the lacrimatory irritant xylyl bromide against the Russians in January of 1915. Alone, these riot control agents' use was small scale and ineffective and over shadowed by the stronger chemical weapons that were introduced later in 1915. Tear gas continued to be used until the end of the war.
Although the French were the first to deploy tear gas in war, an American schoolteacher first proposed using artillery shells filled with chlorine gas in 1862.
Use of tear gas in interstate warfare, as with all other chemical weapons, was prohibited by the Geneva Protocol of 1925: it prohibited the use of "asphyxiating gas, or any other kind of gas, liquids, substances or similar materials", a treaty that most states have signed. Police and civilian self-defense use is not banned in the same manner. The United States adopted the protocol under the stipulation that it did not apply to tear gas use in warfare as the agents were nonlethal and used domestically, and did not ratify the protocol until 1975 following controversy over its use in the Vietnam War.
Tear gas was used in combat by Italy in the Second Italo-Ethiopian War, by Japan in the Second Sino-Japanese War, by Iraq in the Iran–Iraq War, by Spain in the Rif War, by the United States in the Vietnam War, and by Israel in the Israel–Palestine conflict.
Tear gas exposure is an element of military training programs, typically as a means of improving trainees' tolerance to tear gas and encouraging confidence in the ability of their issued protective equipment to prevent chemical weapons exposure.
Riot control
Certain lachrymatory agents, most notably tear gas, are often used by police to force compliance. Xylyl bromide, CN and CS are the oldest of these agents. CS gas is the most widely used. CN has the most recorded toxicity. 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.
Counter-measures
thumb|right|People using umbrellas to defend from tear gas canisters and less-than-lethal munitions
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, and covering as much skin as possible.
Activists in United States, the Czech Republic, Venezuela and Turkey have used antacid solutions such as Maalox diluted with water to repel effects of tear gas attacks, with Venezuelan chemist Mónica Kräuter recommending the usage of diluted antacids as well as baking soda. There have been reports of these antacids being helpful for tear gas,
thumb|right|250x250px|thumbtime=00:10|Protestors in Hong Kong using traffic cones to put out tear gas canisters
During the 2019 Hong Kong protests, frontline protesters became adept at extinguishing tear gas: they formed special teams that sprang into action as soon as it was fired. These individuals generally wore protective clothing, including heat-proof gloves, or covered their arms and legs with cling film to prevent the painful skin irritation. Canisters were sometimes picked up and lobbed back at police or extinguished straight away with water, or neutralized using objects such as traffic cones. Similarly, Chilean protesters of Primera Línea had specialized individuals collecting and extinguishing the tear gas grenades. Others acted as tear gas medics, and another group, the "shield-bearers," protected the protesters from the direct physical impact of the grenades.
Treatment
upright=1.1591|thumb|A [[paramedic tending to an opposition protester during the 2014 Venezuelan protests]]
250px|thumb|[[Fabiola Campillai, a Chilean woman left blind in both eyes by a direct hit of a tear-gas grenade in her face]]
There is no specific antidote to common tear gases. At the first sign of exposure or potential exposure, masks are applied when available. People are removed from the affected area when possible. Immediate removal of contact lenses has also been recommended, as they can retain particles.
Blowing the nose to get rid of the chemicals is recommended, as is avoiding rubbing of the eyes. Some evidence suggests that Diphoterine, a hypertonic amphoteric salt 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 adhere to the skin. Clothes, shoes and accessories that come 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. It is unclear how effective these remedies are. In particular, vinegar can burn the eyes and prolonged inhalation can irritate the airways.
Vegetable oil and vinegar have been reported as helping relieve burning caused by pepper spray.