So, what actually happens, down at the molecular and cellular level, when a person is exposed to alpha radiation? If it’s coming from outside the body, not all that much. The outer layer of dead skin cells is enough to soak up most of the damage, and it’s not like alpha particles can make it that far through the air, anyway. This is good news for Londoners worried about exposure (I note that reports have at least three sites there showing traces of radioactivity). I strongly discourage anyone from standing around next to an alpha source, but there are a lot worse things that you can stand next to - a gamma or fast neutron source, for example, either of which will penetrate your tan and keep on going.
But inside the body, that’s a different story. Alexander Litvinenko was given polonium in his food or drink, and from there the stuff distributes fairly widely across many tissues. At lower radioactive doses, that pattern is probably a good thing. When you have a radionuclide that concentrates in a particular tissue, like iodine in the thyroid, a dose that would be bearable across the entire body can cause a lot of local damage when it piles up. At higher doses, though, the situation can flip around. People can survive with damaged thyroid glands, or after total bone marrow transplants or the like. But general tissue damage is much harder to deal with.
Polonium ends up concentrating in the kidneys, to the extent that it concentrates anywhere, and attempts have been made to minimize radiation damage there. But by then an awful lot of destruction has occurred elsewhere – the blood-forming tissues, the linings of the gastrointestinal tract and the blood vessels themselves, and others. Note that these are all fast-dividing cell populations.
Zooming in, the mechanisms for all that mayhem are complex, and they’re still not completely understood. The first thing you can imagine is the alpha particle smacking into something, which to a first approximation is exactly what happens. They don’t get far – less than 100 micrometers. But along the way they can bash into quite a few things, losing some energy each time, which shows up as flung-off electrons, various strengths of photons, and doubtless some good old kinetic bouncing around. Eventually, when the particle slows down enough, it drags off a couple of electrons in passing and settles down as a peaceful atom of helium. That leaves some positive charges to account for, though, since those electrons were otherwise employed before being press-ganged, and this ionization (along with that caused by those stray electrons along the way) is one of the major sources of cellular damage.
All this can take place either in the nucleus or out in the cytoplasm, with different effects. This sort of thing can damage the cell's outer membrane, for one thing, which can lead to trouble. In the nucleus, one of the more dramatic events is sudden double-strand DNA breakage. That's never a good thing, since the strands don't always get put back together correctly. A couple of years ago, a group from the Netherlands was able to come up with dramatic images of chromosome breakage along the tracks made by alpha particles in living cells.
Then there’s also the complication of the “bystander effect”. Untouched cells in the vicinity of one that has taken an ionizing radiation hit also show changes, which seem to be at least partly related to an inflammation response. This seems to happen mostly after damage to the nucleus.
All this focused destruction has long since drawn the attention of people who actually want to kill off cells, namely oncology researchers. Alpha sources conjugated to antibodies are a very big deal in cancer treatment, and a huge amount of work is going on in the area. The antibodies can, in theory, deliver the radiation source specifically to certain cell types, which soak up most of the exposure.
So there's a use for everything. But one of those uses, this time, was assassination. Alexander Litvinenko's killers knew exactly what they were doing, and exactly what would happen to him. I hope that they're eventually found and dealt with proportionately.