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Tear gas in use in France in 2007.
Exploded teargas canister on the fly
Tear gas shells used in Istanbul in 2013
Tear gas, formally known as a lachrymatory agent or lachrymator (from lacrima meaning "tear" in Latin), is a possibly lethal  chemical weapon that stimulates the corneal nerves in the eyes to cause tears, pain, and even blindness. Common lachrymators include OC, CS, CR, CN (phenacyl chloride), nonivamide, bromoacetone, xylyl bromide and syn-propanethial-S-oxide (from onions).
Effects and use 
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, temporary blindness, etc. 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 and mouth. First used in 1914, xylyl bromide was a popular tearing agent since it was easily brewed.
Lachrymatory agents are commonly used as riot control and chemical warfare agents. During World War I, more toxic lachrymatory agents were used. 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, which is the same as CN and is used as a self-defense weapon, in small spray cans. Xylyl bromide, CN and CS are the oldest of these agents, and CS is the most widely used.
Use of tear gas in warfare (as all other chemical weapons) is prohibited by various international treaties that most states have signed. Police and private self-defense use is not banned in the same manner. Armed forces can use tear gas for drills (practicing with gas masks) and for riot control.
See also 
Further reading