Tear gas, formally known as a lachrymatory agent or lachrymator (from lacrima meaning "a tear" in Latin), is a chemical compound that stimulates the corneal nerves in the eyes to cause tearing, pain, and even blindness. Common lachrymators include OC, CS, CR, CN, nonivamide, bromoacetone, phenacyl bromide, xylyl bromide and syn-propanethial-S-oxide (from onions). Lacrymators often share the structural element Z=C-C-X, where Z indicates carbon or oxygen, and X indicates bromide or chloride.
Tear gas works by irritating mucous membranes in the eyes, nose, mouth and lungs, and causes crying, sneezing, coughing, hard breathing, pain in the eyes, temporary blindness, etc. Lachrymators are thought to act by attacking sulphydryl 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 1915, xylyl bromide was a popular tearing agent since it was easily brewed.
Lachrymatory agents are commonly used as riot control agents and chemical warfare agents. For example, tear gas and pepper spray are commonly used for riot control. During World War I more toxic lachrymatory agents were used. Certain lachrymatory agents are often used by police to assist in bringing targeted persons under control, most notably tear gas, but also in some countries (Finland, Australia, and the USA) another issued substance is mace, which is used as a personal attack repellent.
Although lachrymatory agents are commonly used as riot control agents, the most common use of some is in laboratory synthesis, where their lachrymatory properties are an undesirable side effect. Common examples include benzyl chloride, thionyl chloride and acetic anhydride.
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