What do you mean? 

CS gas, or tear gas, isn't a gas; it is a 5 per cent solution of a white crystalline solid dispersed in the solvent 4-methyl-2-pentanone (methylisobutylketone), and it is used as an aerosol spray. The chemical can also be delivered in hand grenades or shells made of soft plastic. Some versions used in defence sprays include a uv dye that can enable subsequent identification of assailants. 

OK, so what is CS anyway? 

It's a molecule called [(2-chlorophenyl)methyl-ene]propanedinitrile (1). 

Who discovered it? 

A couple of chemistry teachers at a college in the US. They reported the synthesis of a series of compounds with similar structures in 1928. They commented that several of them were harmless when wet but 'to handle the dry powder is disastrous'. Symptoms varied; they found that sneezing initiatated by CS caused the face to smart, especially if damp; and the smarting was increased by washing the face. When the related compound [(3-nitrophenyl)-methylene]propanedintrile (2) is inhaled it causes sneezing which produces a mucous that turns bright yellow on exposure to air. No one took much notice of CS until 1958 when scientists at the British Chemical Defence unit at Porton Down (near Salisbury) learned how to use the chemical. During the 1960s the US Army used CS gas in Vietnam to drive the Vietcong out of underground bunkers (its use indoors by police is not recommended because it can induce panic attacks). The agent was used by the Royal Ulster Constabulary in riot control in the Bogside area of Londonderry in 1969, and by US federal agents in the siege of the Branch Davidian building near Waco, Texas in 1993. More recently, French police used tear gas to disperse rioters during nights of unrest in some Paris suburbs. 

How safe is CS gas? 

Toxicologists regard the chemical as a safe riot control gas. A concentration of ca 0.005 mg m^-3 in air causes discomfort; between 1-5 mg m^-3 in air will incapacitate, while the lethal dose is of the order of 70 000 mg m^-3. This is a much greater safety margin than many drugs, such as paracetamol. The chemical is safe to use near animals but not around small children. 

How does CS gas work? 

The CS molecules attach themselves to enzymes in the mucous membrane of the eye and induce a protective release of tears to dissolve and wash away the irritating molecules. 

What are the associated symptoms? 

Pain, burning eyes, conjunctivitis, chest constriction and lots of tears - enough discomfort to give the sufferers something to take their minds off other things for a quarter of an hour.