Data from survivors of the Hiroshima and Nagasaki bombs, as well as from nuclear accidents, suggest that a dose of more than 15 sieverts kills within days, as the radiation destroys gut tissue. People exposed to less than 5 Sv usually live for longer than three weeks and may even survive the initial poisoning. A dose within the range of 5-15 Sv is equivalent to the amount of radiation received by someone standing within 800 metres of the Hiroshima bomb.

If ingested, perhaps as little as one ten-millionth of a gram of polonium-210 could deliver this dose. Estimates on the lethal dose vary widely.

...Police are still investigating, but it is possible that Litvinenko spread tiny amounts of polonium-210 after being poisoned. The substance could have come out in his sweat or tears, for example.

Theories about the planes that have been grounded after finding hints of radiation on board are even more speculative. Authorities have not said what level of radiation has been detected or even whether the source is polonium-210.

If polonium-210 is found to be the source, further questions need to be answered. The substance decays by emitting alpha particles, which can be stopped by something as flimsy as a sheet of paper. If the polonium-210 was brought from Russia, as many observers have speculated, contamination could easily have been prevented by simply keeping it in a tightly sealed bottle.