The London-based unit is to reveal new findings from research into the controversial use by police of CS sprays and gases and into its delayed effects, particularly on the skin.
Despite claims that CS use in Britain has resulted in few casualties, the service's medical toxicology unit based at Guy's Hospital has been consulted more than 2,000 times by doctors, hospitals and health professionals about patients affected by sprays or gas.
The research, some of which is due to be presented for the first time at a national conference next month, was triggered by a number of reported cases where the onset of effects of CS spray was delayed.
Dr Virginia Murray, consultant medical toxicologist, said: "I and my team are concerned that there seem to be unexpected effects [of CS sprays or gas] which have been occurring. Most of these riot or crowd control agents are designed as short-term irritants which are minimally toxic. What we have found and are still finding is there are some delayed effects ... These effects were not expected and nor was the number of inquiries we have been getting."
Research is believed to have shown that in some cases people did not start to develop skin side-effects, including blisters, until up to three days after exposure to the chemicals. Prolonged exposure through saturated clothing can also cause skin problems.
The report by the unit comes amid growing concern about the use of CS spray, particularly on the mentally ill where the interaction with anti- psychotic medication is not known. Two mentally ill people are taking legal action against the police over the use of the sprays and the mental health charity Mind has called for a ban on them.
In a report on the safety of CS sprays, Dr Robert Jones, a biochemist, said that they should first have been approved by the Medicines Control Agency. "The fact that the complex network of legislation and detailed mandatory testing built up since the thalidomide debacle to safeguard the public has been wholly sidestepped by the Police Scientific Development Branch and the Home Office is unacceptable," he said.