CS gas (commonly called tear gas), or chlorobenzylidene malonitrile, is a non-lethal riot control agent. Tear gas is a chemical compound (often generated by a burning process) which, in humans, causes immediate tearing of the eyes, and irritation of the skin and mucous membranes. Tear gas is available in a number of different chemical formulations with effects ranging from mild tearing of the eyes to immediate vomiting and prostration.
Tear gas is usually deployed as a burning canister or grenade.
Members of the armed forces of the United States of America and other countries are often exposed to CS during initial training to show the importance of proper wearing of a gas mask. As the agent's presence quickly reveals improper fit or seal of the mask's rubber gaskets against the face, it is sometimes used during training refresher courses or equipment maintenance exercises as well.