If the material is very insoluble, the little that remains in the lung dissolves very slowly. And the lung eventually clears virtually all the radioactivity in it. Some people are skeptical about the degree of confidence scientists can have in their understanding of depleted uranium's effects because little research has been done on it. Harley says scientists can be so sure because there's been a great deal of research on natural uranium. This is the same substance that's in natural uranium.
Natural uranium has three isotopes in different ratios.
Uranium 238 is by far the majority. Then you have a little uranium 235 and a little uranium 234. These are the fissionable isotopes, and U235 is what you use to make fuel for nuclear reactors and weapons. They take out this very small mass and you're left with U238, which is depleted uranium. But otherwise you're actually talking about the exact same material.