The element which raises the greatest concern is MIBK, which is used in paint strippers.

Kevin Blow, of the National Campaign Against CS Spray, says: "Union Carbide, who manufacture it, recommend a safety limit of one part in a million, which is equivalent to a strong smell.

"But when you are getting sprayed in the face with the stuff, you get a lot more than a strong smell."

'Tip of the iceberg'

But Mr Blow claims that could be the tip of the iceberg.

He said a professor at Harvard University in the US, Howard Hu, conducted experiments which suggested the use of CS spray could cause a chronic respiratory disease called RADS (reactive airways dysfunction syndrome).

Mr Blow said there was also research - conducted on rabbits by Union Carbide - that MIBK could cause damage to the cornea of the eye.

CS gas can also come in grenade form, such as these from South Korea

"There is also evidence that it can cause foetal abnormalities," he said.

He said there was also reason to believe it could bring on a cardiac arrest in someone suffering from heart disease.

'No significant threat'

In September 1998 then Home Office Minister Alun Michael declared: "CS spray has been scientifically tested to a level similar to that which would be required for a new pharmaceutical drug, and there is no evidence it poses any significant threat to human health."

But Mr Blow said: "How can they give such blind reassurances when the research just is not there.

"They are not going to spray it in the face of someone with a history of heart problems, or a pregnant woman, to prove it is safe are they?"

He added: "Part of the problem is the way CS spray is used.

"The guidelines specify it is not to be used in confined spaces or at a distance of less than five feet.

"But the PCA report says these guidelines are being regularly breached."

Cancer concerns

There are also concerns about MIBK being carcinogenic.

Earlier this year Police Review magazine published a report by experts from the government's Porton Down chemical and biological weapon laboratories.

It described MIBK as being too toxic to be safe and said it could cause cancer.

Government officials said the tests were theoretical exercises to test the properties of the chemicals involved and took no account of the quantities used in the spray or whether it was used indoors or outdoors.

A Home Office spokesman said: "MIBK does not promote cancer and other countries have been using it for years. The French gendarmerie have been using it since 1984."

The Association of Chief Police Officers (ACPO) said CS spray should be used as a "weapon of last resort" if an officer had serious reason to believe his or someone else's life was in danger.

But Mr Blow said it was too often being used as a crowd control agent.

'Lazy policing'

"An element of laziness is creeping into policing," he said.

The PCA is currently investigating a complaint about an incident in May 1998 in Scunthorpe, Lincolnshire.

Relatives of Mark Bell, who was found asphyxiated in the loft of his home, claimed police officers squirted CS gas into the loft in an attempt to subdue him shortly before he died.

Mr Michael, speaking last September, said about 15,000 officers a year are assaulted in the course of their duty and he said they deserved the best protection on offer.

He said: "That is why I believe CS spray should be available to the police service as an effective self-defence tool."

But Mr Blow said: "Of course the police need to be protected but an easy solution like CS spray is the wrong way of going about it.

"People say 'At least the police don't have guns' but that is a false argument because CS spray can be extremely dangerous."