As Donald Rumsfeld would say, there are the known knowns, and then ... never mind. The thing is, there are a lot of variables. People are differently susceptible, although we don't usually know who is until they get sick, and I think beyond a certain level anyone will be killed -- the heroes who actually grasp [something radioactive?} to stop an explosion die within days, I believe. (Someone did that at Los Alamos -- I forget what it was that he grasped.)
I'm sure the rape-a-scans at airports are not safe. The obvious difference between them and medical uses is that there are good reasons to get X-rays, and people submit to them with informed consent. At airports people are bullied into something unnecessary (unless you consider enriching Michael Chertoff to be necessary). I don't know why there isn't more resistance.