Polonium, the substance discovered in the urine of the former Russian spy Alexander Litvinenko before his death last night, is highly radioactive and extremely toxic.

Discovered in 1898 by Marie Curie, the Polish-French physicist who became the first two time Nobel laureate, it was named after her homeland, Poland.

When ingested or inhaled, polonium starts attacking organs and can cause irreversible damage to the kidneys, liver and spleen. As was the case with Litvinenko, it would also make its victims hair fall out.

A by-product of uranium, it has the chemical symbol Po and lies in the group below oxygen in the periodic table. Chemically similar to tellurium and bismuth, it has a metallic appearance and dissolves in dilute acids, so could easily have been slipped into Litvnenko's food or a drink.

Britain’s independent Health Protection Agency said that polonium-210 - the polonium isotope identified in the case - can represent a radiation hazard if it is ingested, through breathing, eating or in a wound. However, it is not a danger so long as it remains outside the body and as such, would have not caused any harm to those using it as a poison.

Professor Dudley Goodhead, of the Medical Research Council & Genome Stability Unit, said that polonium has a half life of 138 days which makes it relatively easy to identify. This also means there is easily enough time for someone to use it as a poison, compared with other radioactive materials, some of which have a half life only as long as three minutes.

"There are very small amounts of polonium-210 in the natural environment from decay of uranium in the earth and in fact everyone has a small amount of polonium 210 in their body," Professor Goodhear said.

"But to poison someone, much larger amounts are required and this would have to be man-made, perhaps from a particle accelerator or a nuclear reactor."

Small amounts of polonium-210 are made every year in specialist laboratories and it has been studied for the possible use of heating spacecraft. A great deal of energy is released by its decay and a single gramme generates 140 watts of heat energy. It has also been used in devices that eliminate static in textile mills and on brushes to remove accumulated dust from photographic films.

When Maria Curie chose to name her discovery after Poland, it was in the hope it would bring notoriety to her homeland. At the time, Poland was under Russian, Prussian and Austrian domination.

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