Tear gas, also known as a lachrymatory agent or lachrymator (from Latin lacrima 'tear'), 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. Exposure to tear gas agents may produce numerous short-term and long-term health effects, including development of respiratory illnesses, severe eye injuries and diseases (such as traumatic optic neuropathy, keratitis, glaucoma, and cataracts), dermatitis, damage of cardiovascular and gastrointestinal systems, and death, especially in cases with exposure to high concentrations of tear gas or application of the tear gases in enclosed spaces.
Tear gas generally consists of aerosolized solid or liquid compounds (bromoacetone or xylyl bromide), not gas. 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.
As with all non-lethal or less-lethal weapons, there is a risk of serious permanent injury or death when tear gas is used. Risks include from being hit by tear gas cartridges that may cause severe bruising, loss of eyesight, or skull fracture, resulting in immediate death. 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.
While the medical consequences of the gases are typically limited to minor skin inflammation, delayed complications are also possible. People with pre-existing respiratory conditions such as asthma are particularly at risk. They are likely to need medical attention and may sometimes require hospitalization or even ventilation support.
Skin exposure to CS gas 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.
Venezuelan chemist Mónica Kräuter studied thousands of tear gas canisters fired by Venezuelan authorities since 2014. She concluded that the majority of canisters used the main component CS gas, and that 72% of the tear gas used was expired. She noted that expired tear gas "breaks down into cyanide oxide, phosgenes and nitrogens that are extremely dangerous".
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. Health issues associated with exposure to tear gas often require medical attention.
TRPA1 ion channels expressed on nociceptors have been implicated as the site of action for CS gas, CR gas, CN gas (phenacyl chloride), chloropicrin and bromoacetone in rodent models.
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. The US Chemical Warfare Service developed tear gas grenades for use in riot control in 1919.
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.
Certain lachrymatory agents, most notably tear gas, are often used by police to force compliance. In some countries (e.g., Finland, Australia, and United States), another common substance is mace. The self-defense weapon form of mace is based on pepper spray which comes in small spray cans. Versions including CS are manufactured for police use. Xylyl bromide, CN and CS are the oldest of these agents. CS gas is the most widely used. CN has the most recorded toxicity.