People and all other species have been exposed to some radiation, from cosmic sources, the sun, and from things like radon and uranium on earth. Biological systems are incredibly good at self-repair. So it seems unlikely to me that low level exposure always causes significant or even measurable harm.
We are exposed to elemental toxins such as arsenic, yet even people with slightly elevated levels of arsenic are often okay. As far as anyone can see their health and longevity is not affected. Then again, you cannot run a person's life over from scratch to find out if he might have lived a few years longer with a slightly lower level of arsenic. At the other extreme . . . I do not know whether radiation actually promotes health. I have heard it might, but I have not read the studies, so I cannot judge. But biology is full of surprises, so I would not discount the possibility. - Jed
