The perils of polonium
News Analysis Polonium-210, the highly toxic radioactive isotope found in the body of poisoned former Russian spy Alexander Litvinenko is a very rare, exotic material that is difficult to obtain, scientists say.
The UK's Health Protection Agency says Litvinenko, who died last week in a London hospital, had a significant amount of the radioactive isotope in his body.
But how it got there and where it came from is a mystery.
Radiation and chemistry experts say large-scale equipment, such as a nuclear reactor, would be needed to produce sufficient amounts to kill.
"It is not as simple as the idea that somebody might have broken into a radioactivity cabinet at some local hospital and walked off with some polonium," says Dr Andrea Sella, a lecturer in chemistry at University College London.
Although scientists would not speculate on the source of the polonium, Sella says Litvinenko's death is not a random killing.
"This is not a tool chosen by a group of amateurs. These people had some serious resources behind them," he says.
Polonium is a by-product of uranium that was discovered by Polish chemist Marie Curie in the late 19th century.
It's a rare but naturally occurring metalloid found in the soil and atmosphere, and even in the human body.
But at high doses it is highly toxic if ingested or inhaled; it emits alpha particles that can damage the body's tissues and organs.
"[Alpha particles] are unable to penetrate a sheet of paper and so it is not a hazard unless ingested," says Professor William Gelletly of the University of Surrey.
Long-term exposure to radiation can cause mutations and cancer. But exposure to a short, intense burst of radiation causes major damage to key control centres in cells.
"An alpha particle strikes a strand of DNA. It snips it in two, which is bad news, or glues two strands together. Either way normal cell repair mechanisms may be unable to sort that out," says Sella.
"The result is that essentially the cellular command and control network [in the body] falls apart. That is what radiation sickness is all about."
Professor David Ray, of the University of Nottingham, says even if a high dose of radiation could not be detected externally after Litvinenko was admitted to hospital a fatal dose could have concentrated in deep tissues such as bone marrow.
"The limited information that has been released about Mr Litvinenko's condition and the timing of his death is consistent with either radiation poisoning or chemicals that stop cell division," he says.
Polonium-210 also has a very short half-life of 138 days, at which point it loses half of its radioactivity.
"That is long enough so you can handle it and deliver it to your target and it will pack a punch," Sella says.
Polonium-210 is used in research and medicine, as well as a heating source for space components.
But in those forms it is not conducive to easy poisoning, scientists say.