Tear gas, formally known as a lachrymatory agent or lachrymator (from the Latin lacrima, meaning "tear"), is a chemical weapon that causes severe eye, respiratory, and skin irritation, pain, vomiting, 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).
Lachrymatory agents are commonly used for riot control. Their use in warfare is prohibited by various international treaties. During World War I, increasingly toxic lachrymatory agents were used.