Polonium-210 Difficult to Detect

This photo released by the family of Alexander Litvinenko shows former Russian security agent Alexander Litvinenko in his hospital bed, at the University College Hospital in central London in this Monday Nov. 20, 2006 file photo. Poisoned Russian spy Alexander Litvinenko died on Thursday, Nov. 23, 2006 in an intensive care ward, London's University College Hospital said. Litvinenko, a fierce critic of the Russian government, suffered a rapid deterioration in his health, but doctors had been unable to determine the cause of his death, a spokesman said in a statement. (AP Photo/Family Hand Out) (AP)

Polonium is so rare that only about 100 grams is believed to be produced each year, said Dr. Mike Keir, a radiation protection adviser at Royal Victoria Infirmary.

"Only a very, very small amount of this would need to be ingested to kill," Keir said. "Unless you can remove the material, there's very little you can do except treat the symptoms."

Given Litvinenko's symptoms _ including hair loss, organ failure and immune system breakdown _ experts said it was understandable why doctors didn't initially recognize polonium-210 as the cause.

"Trying to identify the exact agent that was making him sick was like looking for a needle in a haystack," said Dr. Alistair Hay, a professor of environmental toxicology at Leeds University. Numerous toxins are capable of causing such serious damage without being immediately identified in the body, he said.

The alpha rays emitted by polonium are extremely hard to detect, and a fatal dose of the element may have rapidly penetrated his bone marrow without raising immediate suspicion. Earlier this week, doctors said Litvinenko was in need of a bone marrow transplant.

"As a result of alpha ray radiation, there are very clear genetic changes in the body," Keir said. "But to know for certain that it was polonium radiation, you need to actually find polonium particles."

Polonium was discovered in 1898 by Nobel laureates Marie and Pierre Curie as they were searching for the cause of radiation decay in uranium. They named it polonium in honor of her country of origin, Poland.

The Health Protection Agency said the use of polonium as a deliberate poisoning would be "an unprecedented event." Several experts also said they were unaware of any other known poisonings from the element.

"I've been in radiation sciences for 30-odd years and I'm not aware of any such incident," said Roger Cox, director of the agency's center for radiation, chemicals and environmental hazards.

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Associated Press writer Jill Lawless contributed to this report.