A look at chlorine gas as a weapon

Recent use of chlorine gas as a weapon in Iraq:

Chlorine gas attacks the eyes and lungs within seconds, causing difficulty in breathing and skin irritation in low-level exposure. Inhaled at extremely high levels, it dissolves in the lungs to form hydrochloric acid that burns lung tissue, essentially drowning a person as liquid floods the lungs.

The chemical has been used a number of times recently in insurgent attacks in Iraq.

On Friday, a suspected al-Qaida in Iraq suicide bomber smashed a truck loaded with TNT and toxic chlorine gas into a police checkpoint in Ramadi, killing at least 27 people. It was the ninth such attack since the group's first known use of a chemical weapon in January.

On Jan. 28, a truck carrying explosives and a chlorine tank blew up in Anbar province. More than a dozen people were reported killed.

On Feb. 20, a bomb planted on a chlorine tanker left more than 150 villagers stricken north of Baghdad. The following day, a pickup truck carrying chlorine gas cylinders was blown apart in Baghdad, killing at least five people and sending more than 55 to hospitals gasping for breath and rubbing stinging eyes.

Despite fears over the new tactic, some experts note the insurgents are not yet expert at it, meaning they may be causing widespread fear but not mass casualties.

Chlorine gas causes death by inhalation but experts say the heat from an explosion can render the gas nontoxic.

Anti-terror experts have long believed that terrorists do not need sophisticated knowledge to use toxic industrial chemicals.

Chlorine is easily accessible. It is used for water purification plants, bleaches and disinfectants and obtaining large quantities may not be difficult.

Chlorine gas was first used as a weapon when the German military unleashed it at Ypres, Belgium in 1915.

Chemical poisons killed tens of thousands of soldiers in World War I. Their use caused so much international revulsion that the issue sparked the Geneva Protocol of 1925, which banned the use of chemical weapons.

In general, chlorine is inefficient as a weapon _ it produces a visible greenish cloud and a strong odor, making it easy to detect. Because it is water-soluble, simply covering the mouth and nose with a damp cloth can reduce the effect of the gas.

Chlorine has also been the source of environmental accidents.