"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."
Source: Polonium-210 one of world's rarest elements - CNN.com