Preliminary results of the study have also discovered that half the people who have complained about being squirted with CS were in police custody or their care at the time of the confrontation.
The findings are part of an inquiry into the use of CS spray set up the independent Police Complaints Authority, in response to growing concern about the gas and official reports that have described the devices as unsafe.
In a series of articles last summer, The Independent on Sunday revealed the extent to which CS spray was being used to suppress mentally ill people and that its health dangers were more serious than had been admitted by the Home Office or the Association of Chief Police Officers (Acpo).
The national inquiry into CS spray is the largest undertaken by the PCA. The decision to focus on CS further calls into question whether the Home Office and police chiefs rushed the decision to arm officers with the spray.
Peter Moorhouse, the chairman of the PCA, which oversees investigations into complaints about the police, told The Independent on Sunday that he was concerned about the number of complaints in which people said they were either in police care or detention when CS was used against them. He also expressed concern at the use of CS against people secured by handcuffs.
A national survey of all 43 forces in England and Wales also revealed that of the three-quarters to respond so far, all have said they are training and equipping volunteer officers, or Specials, to use CS.
Liberty, the civil rights group, argues that unpaid officers with limited experience should not be armed with such potentially dangerous equipment.
The Authority also wants greater scrutiny over the work of Specials; at present they cannot investigate complaints made against the volunteers.
The PCA is carrying out two surveys into CS; the first is to ask all forces in England and Wales about how they use the new weapon carried by 100,000 officers in all but three forces in England and Wales. The sprays, already used in 10,000 incidents, have led to a marked reduction in injuries to police, but the chief constables of Northamptonshire, Nottinghamshire and Sussex have refused to issue them because of health fears.
Further concerns were raised about the device when it emerged in November that a Home Office-commissioned report found that the spray contains a "hazardous" solvent too toxic for safe use.
The second study is looking at all the complaints about CS in the six months from October.
Analysis of the first 15 complaints in October, which were against 10 police forces, found that about half were alleged to have taken place while the person was in police custody.
One man claims he was handcuffed at the time, and in at least three incidents the CS was fired from less than three feet from the complainant's face.
Mr Moorhouse said: "I'm surprised that quite a few of the incidents where we are supervising relate to the use of CS spray in an enclosed space." He acknowledged that "it's not always inappropriate to use it inside" but "once you have somebody handcuffed you couldn't say you should never use it because they could be kicking or lashing out".
The PCA intends to publish a special report on CS next year. It has received 80 complaints about CS spray since April.
The incapacitant takes immediate effect and causes streaming eyes and nose, eyelid spasm, breathing difficulties and, in some cases, blistering of the skin.Reuse content