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With many injured, some with mutilated legs or arms, questions were raised whether these wounds were inflicted by tear gas or other weapons.

Tear gas comprises chemicals such as chloroacetophenone (CN), chlorobenzalmalononitrile (CS), dibenzoxazepine (CR), oleoresin capsicum (OC or 'pepper spray') and pelargonic acid vallinylamide (PAV). It causes irritation to the skin, eyes, respiratory system, gastrointestinal tract and lung membranes.

The effects can last from 10 to 30 minutes, 24 hours or even up to three days after exposure, Dr Noppawat Samankhathiwat said. In highly concentrated doses or when used in a badly ventilated place, it could cause more harm.

The gas stimulates the corneal nerves in the eyes to cause tears, pain and even temporary blindness, while also leading to a runny nose, burning sensation in the nose and throat areas, coughing, restricted breathing or vomiting.

Exposure to this gas in a high dose could also lead to pulmonary edema in 12 to 24 hours. When it comes into contact with the skin, it causes a burning sensation and swelling, while prolonged exposure could lead to severe burns, Noppawat said.

As regards treatment, victims must be removed from the area to a better ventilated place so that most symptoms can go away without special treatment. The exposed person's clothes must be removed and the skin cleaned with a lot of water and soap. In case of severe breathing difficulty, the person might need a medicine to expand the respiratory tract.

Although tear gas is generally nonlethal, it could be fatal if asthma or emphysema patients are exposed to it. There have been reports of death from exposure to tear gas due to bleeding in the lungs, pneumonia and suffocation when the exposure took place in badly ventilated places. A lab study also found that the tear gas might cause gene mutation.

In 1969, 80 countries voted that tear gas was a chemical weapon, prohibited for use in a war according to the Geneva Protocol, but it is still used for riot control in many countries.

Meanwhile, Thai Police Quartermaster and Ordnance Division chief Pol MajGeneral Puwadol Wutthakanok and experts yesterday held a demonstration of firing a 38mm teargas bullet at a leg made of clay from 13 metres. The result showed that the clay leg was not torn out.

"Tear gas cannot cause a mutilated leg," he said. He said teargas launchers, which police used in the morning, were also fragile. In fact, they could cause harm to the shooter and the teargas bullet has no shrapnel but only gas.

Preventivemedicine expert Dr Chaiyos Khunanuson said from what he saw in television footage, the police use of tear gas to disperse protesters in front of Parliament was aimed at the people, not at the area near the protesters as previously.

He said tear gas by itself could not cause a mutilated leg, but if people were exposed to the gas in very high doses it could lead to severe lung complications and even death.