Who, What, Why: How dangerous is tear gas?

Protesters in Cairo have reported severe symptoms after inhaling tear gas used by police. Just how dangerous is tear gas?

Doctors and activists in and around Tahrir Square have claimed a new type of tear gas is being used by police, due to the extreme reactions being reported.

One medical volunteer at a field hospital on the edge of the square said he saw people suffering problems with their nervous system and epileptic fits. Others said people were coughing up blood and collapsing.

This has led some to suggest the Egyptian security forces have been using stronger forms of the gas.

The chemical compound used in most tear gas canisters is known as CS, and is often used as a means of crowd control by military and police units to force people to disperse quickly.

There are other, stronger chemical structures of the gas. One is called CN, which was a precursor of CS. And then there is CR, which is known to be a particularly potent version that is six times more powerful than CS. CR gas is rarely used and it's banned in the US because it can cause cancer.

But so far there is no evidence that anything other than CS gas has been used, says the BBC's Jon Leyne in Cairo.

"None of the cans we saw had evidence they were the more poisonous CR or CN gas. Indeed, of the many journalists and human rights activists investigating the issue, none has come up with a used canister of CR or CN gas."

So how dangerous is CS gas and what could be the reason why symptoms appear to be so severe?

Experts agree that typical effects of common forms of tear gas are a burning, watery sensation in the eyes, difficulty breathing, chest pain, excessive saliva and skin irritation. Heavy exposure can also induce vomiting.

Symptoms begin 20 to 30 seconds after exposure but ease about 10 minutes later once the person has escaped to fresh air, says Neil Gibson, an analyst with IHS Jane's, an intelligence and security publication.

Different types of tear gas with different compounds have their own toxicological effects and levels, says Mr Gibson. "The effects differ mostly in high dosage, but in lower concentrations they are similar."

Authorities use tear gas, namely the CS version, because "it means avoiding the use of something more physical or live rounds [of ammunition]," says Alastair Hay, professor of environmental toxicology at the University of Leeds.

Death is rare but not impossible, he says. It is often the result of a combination of tear gas and other factors such as restraint in police custody where breathing can be further restricted.

"Generally, deaths from exposure to riot control agents are minimal - that's why most countries have them as something to resort to."

But repeated use of tear gas in narrow spaces, such as Mohammed Mahmoud Street near Tahrir Square where many protests have taken place, means people are exposed to the gas in a smaller space for longer. That has led to more severe problems.

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Also, the strain caused by physical exertion such as a running makes someone coming in contact with tear gas more susceptible to more severe symptoms, says Mr Hay. Breathing becomes more restricted, which could lead to violent coughing that produces blood.

Repeated or prolonged exposure is dangerous, says Steve Wright of Leeds Metropolitan University, who has researched technologies related to policing.

Not only is the gas harmful, but people could also be hurt by being hit with a canister, says Mr Wright. Tear gas canisters work much like hand grenades in that a pin is pulled, which triggers an ignition that sends the chemical into the air. Tear gas can also be fired from a gun.

Reports of expired tear gas canisters picked up by protesters in Egypt led to theories that it could be more toxic.

But Mr Wright says if enough time has elapsed that the chemicals have broken down inside the can, then it makes the canister less effective.