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The poison that had Londoners so worried last week is a natural toxin that occurs in the husks of the castor bean, which can be grown all over the world. Around 1 million tons of the beans are processed each year to produce oil, mainly as an industrial lubricant. Between 5% and 10% of the residue is ricin. So it is easy for terrorists to obtain the toxin. Though making a pure and therefore more potent form of the substance is difficult, impure versions can be made using equipment available in most kitchens. The bean husks are ground into a powder and dissolved in a solution, then dried into crystalline form.

No antidote currently exists for ricin, tiny amounts of which can cause death in 36 to 48 hours from respiratory and circulatory failure. But according to Philippe Pouletty, head of France Biotech, a national industry association, a potential treatment could be developed that would be similar to the type of therapeutic antibodies used to treat serpent bites. The cost of developing this for ricin, according to Pouletty, would be around 550 million to 5100 million, roughly the price tag of a jet fighter. "Governments have to decide which will save more lives," Pouletty says. The main factor holding back the development is lack of government funding.

While experts agree that developing an antidote is crucial, there is less agreement about whether a vaccine is necessary, since ricin isn't contagious and is unlikely to be used as a weapon of mass destruction. Still, an antidote would be useful because the substance is readily available and a small amount — one milligram, the size of a grain of salt — is enough to kill an adult.

Weaponizing the poison is more problematic. Ricin can be administered by inhalation, injection or ingestion. Bioterrorism experts believe that the poisoning of food or drink would be the terrorists' obvious choice, because as an aerosol ricin is less effective than other deadly gases. Injections, though invariably lethal, would only make sense as an assassination tool.

Another possibility might be for terrorists to use ricin in an aerosol with some other toxin, since its inhalation damages the lungs, making it easier for other pathogens to infect the body. Ricin could be mixed with a gas — cyanide, VX, GA (tabun) or GB (sarin) — and dispersed over a civilian population or troops as a deadly one-two punch. Until a vaccine or antidote becomes available, the threat remains.

• KATE NOBLE/London
• Terror poison is a natural born killer