Polonium-210 one of world's rarest elements

LONDON, England (AP) -- Polonium-210 -- the radioactive substance used to poison 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.

 
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.

 
On Friday, British police said trace amounts of polonium had been found in Litvinenko's urine, and that it had been deliberately used to kill him.

 
"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.

 
The former Soviet Union reportedly used polonium in its space program in the 1970s, and it is used also in devices designed to eliminate static electricity.

 
Polonium is so exceedingly 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 his symptoms -- including hair loss, organ failure and immune system breakdown -- experts said that earlier hypotheses, such as thallium poisoning, were reasonable.

 
"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. There are numerous toxins capable of causing such serious damage without being immediately identified in the body, he said.