CS is an abbreviation for O-chlorobenzylidene malonontrite. The properties of this compound were first discovered by American chemists in 1928, and the potential chemical warfare uses were suggested by a Dutch writer in 1934. During World War Two scientists in various countries studied the effects of the compound but it wasn't seriously developed as a weapon until the mid 1950's. The first widespread use of it was during the Vietnam War and since then it has remained a weapon in the arsenal of armies and police forces the world over. The Effects of CS Gas The canisters that are carried by the police in the UK contain three ingredients: the active chemical (CS), a liquid solvent to dissolve and carry it, and an inert gas to act as a propellant for the spray. The active ingredient, CS, is one of a group of chemical compounds called lachrymators. These chemicals are tear producing agents, hence the euphemism 'tear gas'. Exposure to them causes severe eye irritation, a profuse flow of tears, skin irritation (especially on moist areas of the body) and irritation of the upper respiratory tract, causing sneezing, coughing and difficulty in breathing.