Unnatural history of a simple but lethal by-product of castor oil plants

The mysterious assassination of Georgi Markov, a Bulgarian dissident, on Waterloo Bridge in 1978 is lodged in the collective memory. It was the day an ordinary domestic umbrella was turned into a lethal weapon by secret agents of a foreign power who committed murder on London's streets.

Markov, an author and broadcaster who had defected to Britain nine years earlier, was stabbed in the thigh by unknown assailants, believed to be from the Bulgarian secret service. He died an agonising death three days later as doctors struggled to discover what was wrong with him. At the post-mortem examination a tiny pellet the size of a pinhead was discovered and ricin poisoning was identified as the cause of death.

Ricin is a plant toxin, which is easily extracted from the beans of the castor oil plant. The fatal dose by injection, the most lethal way of administering it, is thought to be around one microgram per kilogram of body weight. For a 10-stone adult that would be equivalent to the material in a pinch of salt.

The toxin is easy to extract, store and transport and is therefore potentially widely available. Globally, one million tons of castor beans are processed annually to make castor oil. Its main use today is as a laxative, but it was formerly used as a lubricant in aircraft.

Ricin is contained in the waste mash from castor oil production. It can be produced in liquid or crystalline form or as a dry powder. Although it is not particularly volatile it could be sprayed as an aerosol, but its use as a weapon of mass destruction is limited by the difficulty of distributing it in sufficient dose.

Pat Troop, the deputy chief medical officer, said yesterday that an aerosol would have to be sprayed directly into a victim's face to be an effective weapon. It would be unlikely to be lethal if pumped into the London Underground or an office air-conditioning system.

People exposed to sub-lethal accidental releases of ricin aerosol in the 1940s suffered symptoms four to eight hours later including fever, chest tightness, coughing, breathlessness and nausea. Later they began to sweat profusely, which was the sign of the end of the symptoms and recovery.

If large numbers of patients in one location began to suffer these symptoms, it could suggest poisoning with aerosolised ricin. The rapid progression of symptoms and deterioration of individuals' medical condition would tend to rule out an infection as the cause.

Ricin is also lethal if ingested, and accidental poisonings have occurred following the swallowing of castor oil beans. Symptoms include vomiting, abdominal pain and bloody diarrhoea. Although the toxin is poorly absorbed through the gut, one to three beans chewed by a child and as few as eight chewed by an adult may be fatal, according to a Department of Health briefing posted on the Public Health Laboratory Service website.

There is no vaccine or antidote to the toxin. Victims would be offered nursing care only to deal with their symptoms and make them as comfortable as possible.

Yesterday there were reports that large stockpiles of ricin had been found in several countries in the Middle East, and that Iraq had included the toxin in its chemical and biological weapons programme. But Britain was experimenting with the poison long before these. In the Second World War, efforts to create a ricin bomb were made at the Porton Down Chemical Defence Establishment. Ricin was known as "compound W" in the US, and the weapon was known as the W bomb. According to Elizabeth Sigmund, author of Rage Against the Dying, published in 1980, which covered the history of chemical weapons, the aim was to produce an aerosol that people would inhale.

A government adviser on chemical weapons said yesterday: "A lot of work was done on ricin at that time as a means of turning it into a weapon. But it was abandoned because it was decided it would never be a battlefield weapon with the potential for mass destruction. The nerve gases were thought to be more effective."

In the 1960s research continued on the structure of ricin at Exeter University, but this was not specifically weapons-related. Pharmaceutical companies became interested in the compound as a potential "magic bullet" for the treatment of cancer.

Ricin is a double chain protein. Its unique structure means it can use one side to penetrate the cell wall while the other side poisons the cell. Scientists believed its structure could be harnessed to target and destroy tumour cells. But that research also came to nothing.

Today it is banned under the International Chemical Weapons Convention, along with saxitoxin, a poison derived from shellfish. As a naturally occurring compound it is also classed as a biological weapon.

Daniel Feakey, research fellow at the Sussex Harvard Programme on Chemical and Biological Weapons at Sussex University, said: "They are banned for all but a very limited set of purposes linked with pharmaceutical and medical research. They are not supposed to be held except in very limited quantities."

In the United States, four members of the Patriots Council, an extremist and anti-government group, were arrested in 1991 for plotting to kill a US marshal with ricin. They planned to mix the agent with a solvent and then smear it on the door handles of the victim's vehicles.

There was another ricin-related case in 1995, when customs officials stopped a man entering Canada from Alaska who had in his possession several guns and a container of white powder. The powder was identified as ricin.

Two years later, US investigators found ricin in a makeshift basement laboratory belonging to a man who had shot his stepson.

The real threat of an attack involving ricin is the terror it could provoke. The government adviser on chemical weapons said yesterday : "A terror group could create a ricin aerosol. I suspect it would kill a few people but not many. But it would cause panic. That is the worry."