> If background radiation is everywhere, how can there be no safe dose.Easy: there is no absolutely safe dose.
At normal background levels the chance of it causing you significant trouble before something else has long since killed you off in some other way is practically zero so there effectively a “safe enough” dose. But if you are very very unlucky background levels could cause you an embuggerance that becomes life changing or life ending.
It is more complicated than safe doses of most (but obviously not all) drugs/poisons/etc, because for most of the latter they are purged from your system in fairly short order assuming you survive the initial hit, so the next hit if there is one of equal strength is likely to have much the same effect. Radiation tends to hang around a lot longer so repeated exposure to higher levels builds up so the safe dose has to be stated “over time” rather than being simplified to a fixed dose perhaps related to your mass.
There was a village (unfortunately I can't find the reference in a quick search ATM) where there was/is an unusually high incidence of thyroid cancer which was thought to be a genetic pre-disposition as the population stated from a fairly small gene pool, but is now thought to be because the background radiation in the area is a bit higher than elsewhere due to the makeup of the local ground rocks. The difference is nothing to worry about if passing through or visiting regularly, in fact the difference is small enough not to be a major concern to the locals, but a lifetime of the extra exposure is enough to at least be visible in certain health stats.
