Seven years ago the Kremlin critic and ex-spy Alexander Litvinenko met two Russians in a London hotel. What happened next was one of the most brazen assassinations of modern times. According to British prosecutors, Litvinenko's companions, Andrei Lugovoi and Dmitry Kovtun, slipped a colourless, odourless substance into his tea. Litvinenko drank. Not much, but enough for him to die in agony three weeks later in University College hospital.

The substance was polonium-210, a rare and highly radioactive isotope that a Swiss team has discovered in Yasser Arafat's exhumed corpse. It is extremely hard to detect. Scientists only identified it in Litvinenko hours before his death. A former FSB officer, and teetotaller, Litvinenko was a fitness fanatic. Doctors say it was only because he was in such good shape that he lasted so long. If he had died sooner, the cause of death would probably never have been uncovered.

Polonium-210 occurs at very low levels naturally, but is manufactured for use by industrial plants to prevent the buildup of static electricity.

It is an effective and convenient poison. It emits pure alpha particles, which outside the body can be stopped by a sheet of tissue paper. But if ingested, it causes widespread damage as it passes into organs. The radiation releases energy that creates reactive particles called free radicals. These in turn form toxic compounds that are deadly to surrounding cells.

Because polonium emits only alpha particles, it can be safely carried in glass vials and will not set off radiation detectors at airports. Once ingested, it is hard to detect, because all the radiation remains in the body. A lethal dose could be as little as a few milligrams, which could be administered as a powder or dissolved in liquid.