Polonium, discovered by Marie Curie and named after her native Poland, is A Rare Material and a Surprising Weapon:
Polonium 210 is highly radioactive and very toxic. By weight, it is about 250 million times as toxic as cyanide, so a particle smaller than a dust mote could be fatal. It would also, presumably, be too small to taste.
[...]
Polonium 210 does its damage by emitting alpha particles, which have enough energy to tear apart the genetic machinery of cells, killing them outright or causing them to mutate into tumor-producing forms. It gives off 5,000 times more alpha particles than does the same amount of radium.Alpha-emitters are not picked up by normal radiation-detection devices, a British expert said, so it would be relatively easy to take the substance across a border.
The particles disperse through the body and first destroy fast-growing cells, like those in bone marrow, blood, hair and the digestive tract. That would be consistent with Mr. Litvinenko’s symptoms, which included hair loss, inability to make blood cells and gastrointestinal distress.
It is also a better match than the symptoms caused by thallium, a heavy metal that was first suspected when Mr. Litvinenko fell ill after eating at a sushi bar on Nov. 1. Polonium is so radioactive that it gives off heat, and tiny amounts have been implanted in satellites to make heat and electricity. It was also used in the Soviet Union’s Lunokhod moon rovers.