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)

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.