By MARIA CHENG
The Associated Press
Friday, November 24, 2006; 4:42 PM

LONDON -- Polonium-210 _ the radioactive substance that killed a former Russian spy in London _ is one of the world's rarest elements, first discovered in the 19th century by scientists Marie and Pierre Curie.

It is highly lethal when ingested and extremely hard to detect, experts said Friday.

For days doctors struggled to identify the poison that led to the rapid deterioration of Alexander Litvinenko's health, and ultimately his death late Thursday.

Britain's Health Protection Agency said Friday that polonium-210 was found in his urine.

The agency's chief executive, Pat Troop, said the high level of polonium-210 indicated Litvinenko "would either have to have eaten it, inhaled it or taken it in through a wound."

Police were investigating, but said they were treating it as an "unexplained death" for now.

"This seems to have been a substance carefully chosen for its ability to be hard to detect," said Dr. Philip Walker, a physics professor at the University of Surrey.

Polonium occurs naturally in very low concentrations in the Earth's crust, and experts said small amounts _ but not enough to kill someone _ are used legitimately in Britain and elsewhere for industrial purposes.

Polonium-210 was a critical component in early nuclear weapons, and the former Soviet Union used polonium in power supply systems for spacecraft in the 1970s. It also is used in industrial devices designed to eliminate static electricity.

Professor Dudley Goodhead, a radiation expert at the Medical Research Council, said that "to poison someone, much larger amounts are required and this would have to be manmade, perhaps from a particle accelerator or a nuclear reactor."

The element can be a byproduct from the chemical processing of uranium, but usually is made artificially in a nuclear reactor or particle accelerator. These nuclear facilities are monitored and tightly regulated under international agreements.

Chris Lloyd, a British radiation protection adviser, said it would be relatively easy to smuggle polonium into a country, because its alpha radiation would not set off radiation detectors.

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.