The spray has run into controversy with the death on Saturday of Ibrahima Sey (29), who was restrained by police in London using CS spray.
CS is regarded as one of the safest ways of incapacitating an assailant, but it can cause harm. Police tests with CS sprays last summer were halted for a time when one Metropolitan Police instructor suffered burns.
Dr Alastair Hay, reader in chemical pathology at Leeds University, specialises in toxicology and is chairman of the Working Party on Chemical and Biological Warfare, which has been monitoring agents like CS for many years. "In theory, CS is safe, although those with asthma could react badly to it." Hay believes the police should keep a log of whenever they use it.
CS and other eye irritants have been used by riot police for more than 50 years, and are dispersed in the form of smoke from canisters, hence the name "tear gas". Most were discovered earlier this century as part of military research into chemical warfare agents. The German army was the first to use a tear gas in the First World War when they fired shells filled with benzyl bromide at both Russian positions and French troops.
During that war more than 20 eye irritants were discovered, and interest in tear gas continued. In 1928, two American chemists, Ben Corson and Roger Stoughton, of Middlebury College, Vermont, made a series of new compounds, each with two cyanide units. While most were innocuous materials, they recorded that one had "disastrous" effects when handled. This was a simple molecule consisting of a benzene ring, to which was connected a chlorine atom and a double bond with the two cyanides. Its chemical name was 2-chlorobenzylidene malononitrile; today we know it as CS.
The military gave it this code as one of a series of "C" agents. Others were CN, which stands for w-acetophenone, and this was used as a tear gas until it was discovered to have carcinogenic properties. The worst of the eye irritants is CR, or dibenz-1:4-oxazepine, but this is considered too severe for general use.
All eye irritants act on the nerve endings of the mucous membrane of the eye by triggering certain key enzymes, which unleash a flood of tears to wash away the offending molecules. Eye irritants work by attaching themselves to sulphur sites within the enzymes, and it is molecules that can react with these sites which cause the protective response.
The enzymes are there to monitor and protect the eyes, and we experience their action when we encounter formaldehyde in smoke and thiopropanal oxide from chopped onions. Both produce the symptoms of enzyme overactivity: a stinging sensation, a closing of the eyelids, a flow of tears and inflammation.
Move away from the source and within a few minutes these symptoms disappear. This is also true of CS, whose effects wear off within about 15 minutes. Just one milligram of CS in a cubic metre of air will incapacitate most people, which is why a tear-gas grenade is highly effective at dispersing a crowd.
The health and safety of CS were debated for many years and the government issued its two-part report in 1969 and 1971. This confirmed that it was a suitable agent for riot control because it met the criteria of being effective but harmless, and had a short recovery time without the need for medical attention. CS can pose a threat to health but only at levels several thousand times stronger than that needed for crowd control or in police sprays. Then it may cause serious conditions such as oedema (flooding of the lungs) and people have died because of it.
Dr John Emsley is science writer in residence at Imperial College, London.