Chemical weapons threat in Iraq

The United States has said US-led forces approaching Baghdad have crossed a "red line" that could trigger a chemical attack by Iraqi forces.

What are the most likely weapons?

Mustard gas as well as nerve agents sarin and VX are the most frequently cited.

¤ VX nerve gas causes death by suffocation by disrupting the functioning of nerves that control breathing.

¤ Sarin causes death by suffocation. Iraq is believed to have used sarin against Kurdish Iraqis in the 1980s. A cult released sarin in a Tokyo subway in 1995, killing 12 people.

¤ Mustard gas is a blistering agent that has been called the "king" of chemical weapons. It causes horrific casualties rather than many fatalities. The gas produces painful, long-lasting blisters all over the body, and often leads to blindness. Iraq used mustard gas against Iranian troops during the Iran-Iraq war of the 1980s.

How could the weapons be used?

US defence and intelligence officials say Iraq possesses a hidden stockpile of chemical and biological agents and has numerous ways to deliver them to their target, including artillery shells, rockets and small planes with spray tanks.

US troops refer to such an attack as being "slimed". US military officials say they have a plan to retaliate should US forces come under attack from Iraqi forces using poison gas or any other weapon of mass destruction.

How can US-Led forces protect themselves?

US and British troops are equipped to deal with most threats posed by chemical and biological weapons. The troops have NBC (nuclear, biological and chemical) protection suits and respirators and are provided with medication to inject themselves. These protection measures are supported by a range of detection and warning systems.

The suits and respirators are supposed to allow troops to continue operating but their use in the temperatures of Iraq makes them uncomfortable and raises the risk of heat exhaustion.

NBC suits consist of boots, trousers and a tunic made of three protective layers and a charcoal lining which acts as a filter against any deadly agents. The suit can provide indefinite protection, but is usually worn for a maximum of 72 hours before a new one is issued.

Portable detection units, which are about the size of a small telescope, can alert troops to chemical agents released into the air.

Most modern fighting vehicles have air filtration systems that can keep out chemical agents.

What other chemical weapons may Iraq have?

Independent experts have said Iraq may possess:

¤ Phosgene, a choking agent that causes fluid buildup in the lungs. It can burn skin and eyes.

¤ Chlorine gas, a choking agent, was the first chemical weapon used effectively in large quantities in World War One.

¤ Cyanide, a blood agent that interferes with the use of oxygen in the body, preventing blood from transporting oxygen.

What biological weapons may Iraq have? Independent experts have said Iraq may possess:

¤ Anthrax, a deadly bacterial disease.

¤ Smallpox, a viral disease that killed millions over the centuries but was declared eradicated worldwide as a naturally occurring disease more than two decades ago.

¤ Ricin, among the most toxic naturally occurring substances, is relatively easy to produce.

¤ Botulism, a muscle-paralysing bacterial disease.