CS spray and its `safe' components

molecule of the month John Emsley on the history of the controversial crowd disperser

Police in Britain now carry spray cans of what is misleadingly referred to as CS gas. In fact, CS is a white solid which melts at 96C, and the cans contain a solution of CS dissolved in a solvent. When a jet of this is fired into an attacker's eyes, he or she will immediately be disabled by uncontrollable weeping.

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.

Start your day with The Independent, sign up for daily news emails
ebooks
ebooksAn introduction to the ground rules of British democracy
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Recruitment Genius: Developer

£35000 - £40000 per annum: Recruitment Genius: Are you familiar with the sayin...

Recruitment Genius: Hospitality Assistant

£6 - £8 per hour: Recruitment Genius: This care organisation take pride in del...

Recruitment Genius: Care Worker

£6 - £7 per hour: Recruitment Genius: This care organisation take pride in del...

Recruitment Genius: Marketing Assistant

£18000 - £25000 per annum: Recruitment Genius: A fantastic opportunity has ari...

Day In a Page

John Palmer: 'Goldfinger' of British crime was murdered, say police

Murder of the Brink’s-MAT mastermind

'Goldfinger' of British crime's life ended in a blaze of bullets, say police
Forget little green men - aliens will look like humans, says Cambridge University evolution expert

Forget little green men

Leading evolutionary biologist says aliens will look like humans
The Real Stories of Migrant Britain: An Algerian scientist adjusts to life working in a kebab shop

The Real Stories of Migrant Britain

An Algerian scientist struggles to adjust to her new life working in a Scottish kebab shop
Bodyworlds museum: Dr Gunther von Hagens has battled legal threats, Parkinson's disease, and the threat of bankruptcy

Dying dream of Doctor Death

Dr Gunther von Hagens has battled legal threats, Parkinson's disease, and the threat of bankruptcy
UK heatwave: Temperature reaches 39.8 degrees on Central Line - the sweatiest place in London

39.8 degrees recorded on Tube

There's hot (London) and too damn hot (the Underground). Simon Usborne braved the Central line to discover what its passengers suffer
Kitchens go hi-tech: From robot chefs to recipe-shopping apps, computerised cooking is coming

Computerised cooking is coming

From apps that automatically make shopping lists from your recipe books to smart ovens and robot chefs, Kevin Maney rounds up innovations to make your mouth water
Jessie Cave interview: The Harry Potter star has published a feminist collection of cartoons

Jessie Cave's feminist cartoons

The Harry Potter star tells Alice Jones how a one-night stand changed her life
Football Beyond Borders: Even the most distruptive pupils score at homework club

Education: Football Beyond Borders

Add football to an after-school homework club, and even the naughtiest boys can score
10 best barbecue books

Fire up the barbie: 10 best barbecue books

We've got Bibles to get you grilling and smoking like a true south American pro
Wimbledon 2015: Nick Bollettieri - Junk balls and chop and slice are only way 5ft 1in Kurumi Nara can live with Petra Kvitova’s power

Nick Bollettieri's Wimbledon Files

Junk balls and chop and slice are only way 5ft 1in Kurumi Nara can live with Petra Kvitova’s power
Ron Dennis exclusive: ‘This is one of the best McLaren teams ever – we are going to do it’

‘This is one of the best McLaren teams ever – we are going to do it’

Ron Dennis shrugs off a poor start to the season in an exclusive interview, and says the glory days will come back
Seifeddine Rezgui: What motivated a shy student to kill 38 holidaymakers in Tunisia?

Making of a killer

What motivated a shy student to kill 38 holidaymakers in Tunisia?
UK Heatwave: Temperatures on the tube are going to exceed the legal limit for transporting cattle

Just when you thought your commute couldn't get any worse...

Heatwave will see temperatures on the Tube exceed legal limit for transporting cattle
Exclusive - The Real Stories of Migrant Britain: Swapping Bucharest for London

The Real Stories of Migrant Britain

Meet the man who swapped Romania for the UK in a bid to provide for his family, only to discover that the home he left behind wasn't quite what it seemed
Cheaper energy on the way, but it's not all sunshine and rainbows

Cheaper energy on the way, but it's not all sunshine and rainbows

Solar power will help bring down electricity prices over the next five years, according to a new report. But it’s cheap imports of ‘dirty power’ that will lower them the most