does that mean the damage from radiation occurs when it decays so a lengthy shelf life might not even hurt someone who inhales a bit of radioactive dust if the nasty stuff already decayed?
This is one of those questions that is coming from a wide mark, so the only answer is a start from scratch explanation.
Radioactive material, by definition, is material that contains atoms that have an unstable nucleus and are therefore subject to spontaneously decay. Such an unstable nucleus, at any particular moment, might decay, or it might not. It
might never decay, or it might decay instantly. But, since probability does work, if you have a lump of this material containing millions of such atoms you can say with certainty that over some specified period of time half of them will decay and half of them won't...that's half-life.
So, if you have one lone atom of radioactive material in you it might decay, or it might not. If you carry it around for the determined period that is the half life for that particular isotope there is a coin flip's chance that it will have decayed or it wouldn't have.
Now, damage. Carrying that one atom around isn't, in itself, going to do any damage (unless it does some sort of
chemical interaction that's bad for you, but as far as the radioactivity goes). For the fact that it is radioactive to matter it has to have had that moment when it happens to decay. But if it does, then it has the potential to damage you in a bunch of ways.
For one, it could decay by emitting a very high energy 'particle,' that can also be considered as a 'wave' the same way light can. This 'high energy photon' or 'x-ray' or whatever you want to call it can do things to you as it passes through.
For another, it could also emit an electron at a very high velocity. Chemical bonds are held together by electrons, and a high energy electron racing by can disrupt those bonds...maybe breaking a water molecule that your body is using into a hydroxyl ion that you don't really want there and a free hydrogen atom that you also don't want there, or causing some other chemical change that's even more obviously bad like disrupting a DNA molecule.
And worst of all, this decay process can change what an atom
is. For a convenient example, even though it has a short enough half life to make it unlikely to be a real problem, consider O-15. O-15 is oxygen, and chemically it works just like any other oxygen. You breath oxygen, and you need oxygen. Your red blood cells use a chemical process to pick oxygen out of the air in your lungs and deliver it to all your cells, where it is used in chemical reactions. So, if a cell is using this oxygen atom for whatever it is using it for, and all of a sudden this oxygen atom
isn't an oxygen atom any more that's a problem. How bad of a problem depends on what the cell was trying to do at the time, but whatever it was doing it can't do it with a nitrogen atom, which is what it will suddenly be stuck with.
Which brings us back to your question. If you breathe in this one O-15 atom, and breath it back out, and it didn't happen to decay somewhere along the line, it may as well have been a typical stable oxygen atom, because you would never detect any difference. But if you breath in
a million O-15 atoms then you are going to have to deal with the fact that half a million of them will be nitrogen within the next couple minutes, and another quarter million will be nitrogen within the next couple minutes after that, etc.
Got it?
