I have been thinking about hot particles, these were the irksome little specs of radioactive stuff which were released by theChernobylaccident. When one of my readers (My brother) asked me if the ruthenium would enter the metabolism of humans it made me think.
I have come to the conclusion that some of the particles may be hard to digest, as a result it may well be hard to liberate the ruthenium from the particles. Now if you are getting very happy and complacent thinking that if the particles can not be digested then the radioactivity does not pose a threat to the public then think again.
With non radioactive toxic metals if they are in insoluble forms then they tend to be less toxic, this is because the dust will not dissolve in your lungs and thus poison you. Instead you hope that the dust will be slowly lost out of the lungs by the mini-oars which sweep dust out of your lungs.
The dust then passes up into your mouth and then is swallowed together with the mucus; it then passes into the digestive system. If the dust is insoluble in the gastric juices (I assume that if it survives the gastric juices then it will survive the more alkaline environment in the gut) then it will then pass out the other end of you in the poo.
A more soluble metal containing dust such as cadmium oxide or aerosol such as droplets of chromate will either dissolve in the lungs or the digestive system thus giving you a dose of metals.
It is important to note that cigarette smoke contains cadmium oxide particles, if I recall correctly metal foundry workers who are exposed to cadmium fumes tend to suffer from more lung cancer than the general population. This cadmium in cigarette smoke is yet another reason to either not smoke in the first place or to give up the vile habit.
But I guess you did not come here to be reminded of the evils of smoking so back to the topic of interest.
In the case of radioactive particles (and asbestos) the dust is able to exert a malign influence on your lungs without needing to dissolve. The radioactive hot particles emit radiation which damages the lung tissue as long as the particles are in the lungs, if the particles are soluble then they tend to dissolve. The soluble radioactive metals then enter the rest of the body; with some luck the body will then excrete the metals in either urine or poo. So the body is more able to dispose of the metals.
In the case of the insoluble dusts if they are of the wrong particle size then they can be very hard to clear out of the lungs, as a result they can stay in the lungs for years still exerting their malign influence. The worst offender would be an insoluble actinide oxide such as plutonium dioxide; this could sit in the lungs and carry on delivering alpha particles to your cells for many years.
Uranium is likely not to be quite so troublesome, when a DU bullet is used to shoot a tank it tends to ignite on impact which forms uranium oxides. I have been told that one of the signs to look for when trying to tell if a tank was taken out by a DU bullet is a yellow stain around the bullet hole. Many uranium(VI) compounds are yellow or orange in colour.
If uranium metal is burnt in air, I think it is likely to form U3O8 this can be regarded as a mixed U(IV)/U(VI) oxide. A crude way of thinking of it is as UO3.UO3.UO2, it so happens that uranium(VI) oxide dissolves very well in water which contains carbonates. As it so happens that the human body generates carbon dioxide (which forms carbonates when it dissolves in water) then it is likely that uranium(VI) will be soluble in many body fluids as the anionic carbonate complex [UO2(CO3)3]4-, as a result I suspect that small uranium oxide particles will slowly dissolve if they enter a human. While the plutonium dioxide particles will be likely to never dissolve.
It is also important to bear in mind that the surface of uranium dioxide reacts with the oxygen of the air to form uranium trioxide; this process should help speed up the dissolving of uranium oxide if it is left out in the rain.
If you want to read more about radioactive particles then see F.J. Sandalls, M.G. Segal and N. Victorova, Journal of Environmental Radioactivity, 1993, volume 18, pages 5-22.