Any of a group of substances, most often synthetic organic halogen compounds, that irritate the mucous membranes of the eyes, causing a stinging sensation and tears.
They may also irritate the upper respiratory tract, causing coughing, choking, and general debility. Tear gas was first used in warfare in World War I, but since its effects are short-lasting and rarely disabling, it came into use by law-enforcement agencies as a means of dispersing mobs, disabling rioters, and flushing out armed suspects without the use of deadly force.