RICIN is a deadly poison, easily made and lethal in very small doses.

Its sudden appearance in the heart of the United States Capitol building has revived memories of the deadly anthrax attacks of October 2001, when anthrax-laced envelopes were sent to Capitol Hill offices, including that of Tom Daschle, then the Senate leader, and to media offices. Five people died and 17 others suffered illness.

Ricin. which is made from the seeds of the castor bean, is deadliest when injected below the skin. As little as 70 micrograms, about the weight of a grain of salt, is enough to kill an average-sized adult.

Although it was unclear last night who was responsible for sending the ricin in the post to the office of Bill Frist, the Republican Senate Leader, many on Capitol Hill are aware that al-Qaeda has a well-established interest in ricin.

The first solid evidence that the terror group had experimented with ricin before September 11 was found by Anthony Loyd, of The Times, in Kabul in the days after the Taleban fled the Afghan capital in November 2001. In the cellar of an abandoned al-Qaeda safe house once used as a terrorist training centre, he discovered detailed instructions for preparing the poison, including the observation: “A dose equal to seven seeds will kill a child.”

Inhaling ricin, the most probable route in the Senate case yesterday, is slightly less dangerous, requiring 30 times as much to create a lethal dose.

Ricin can also be delivered in the form of a powder, a mist or a pellet, or dissolved in water or weak acid.

Ricin operates by killing cells and the initial symptoms are not unlike flu. When inhaled, symptoms include difficulty breathing, fever, cough, nausea, sweating and tightness in the chest.

Death follows, usually within a few days, when blood pressure falls and breathing stops. There is no specific antidote, but ricin poisoning can be treated with respirators, by flushing the stomach, with fluids and measures to sustain blood pressure. Unlike anthrax or smallpox, ricin is a poison not a germ, so it cannot spread from person-to-person.

Ricin is highly stable and easy to transport. The most plausible use of the poison by terrorists would be to contaminate food or water. Simply leaving it lying around, as in this case, is not an efficient means of distribution. It could also be injected, as in the infamous murder in 1978 of the Bulgarian writer Georgi Markov, at a bus stop near Bush House, the BBC World Service’s offices in London.

In that case a tiny pellet containing ricin was injected into his thigh by a Bulgarian agent, using the tip of an umbrella as a syringe.

Markov fell ill within hours and died three days later of multiple organ failure. The pellet was discovered only after his death.

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