LONDON —  Thallium is frequently referred to as the poison of choice: Only a gram of the colorless, odorless, water-soluble heavy metal can kill. It is as toxic as arsenic, and even more so than lead.

Col. Alexander Litvinenko, a former KGB and Federal Security Service agent, was under armed guard at a hospital Monday, fighting for his life after being given the deadly poison in London.

Litvinenko's white blood cell count — generally used as a gauge of the immune system — is down to nearly zero, said Dr. John Henry, a clinical toxicologist involved in his care.

"It shows his bone marrow has been attacked and that he is susceptible to infection," said Henry. Thallium interferes with the cardiovascular and nervous systems, attacking the heart, lungs, liver, kidneys, stomach and intestines.

"One or two grams of thallium would be more than enough to do serious damage," said Dr. Alistair Hay, professor of environmental toxicology at Leeds University, who is not connected to Litvinenko's case. "Thallium doesn't have a striking taste, so it would be relatively easy to mix it into food."

For poisoning purposes, thallium would be in a powdery or crystallized state. Its effects are not immediately noticeable and frequently take weeks to kick in. The poison works by knocking out the body's supply of potassium, essential for healthy cells.

While Litvinenko fell ill on Nov. 1, his hair didn't start to fall out until 10 days later — a sign that led his doctors to suspect poison.

"Hair loss is one of the big giveaways of thallium poisoning," Henry said. Blood tests have since confirmed the diagnosis.

At this late stage, nearly three weeks after the incident, Henry says little can be done to get the thallium out of Litvinenko's system. "It will be naturally excreted. We will give him the potassium antidote, but it won't have a major effect," he said.

Potassium is usually given to thallium-poisoned patients, since it binds to the same sites as thallium in the body, and can help to push it out.

Thallium was used by Saddam Hussein, who poisoned several of his Iraqi opponents. At least two victims survived, after being treated in the United Kingdom.

Henry estimates Litvinenko's recovery could take at least six months. "His main problem in the next two to four weeks, when the acute illness is gone, will be his nervous system," he said. "He will have a lot of muscle weakness and will need major physiotherapy."

In the past, thallium has been used in rat poison. It continues to be used industrially, to manufacture products including glass lenses, semi-conductors, dyes and pigments.

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