They are exposed to constant low level radiation, they thrive just as well as we would. The problem is the chance of a genetical mutation in the early stages of development of a baby can have terrible results.
I am currently reading my schoolbooks about the effects of long term exposure to low level radiation so i am no expert on it yet, will take till September for that, but i can say that EVERYONE OF YOU has a cancer at this moment. People get it millions of times in there life. The thing is the body is prepared for it and kills the cancer cells before they become a problem in 99,9999% of the cases, sometimes you get it but it is absolutely no threat ( that small red spots on your skin, thick nose on older people, a small piece of skin growing upwards,...) All can be seen as cancer. But sometimes they turn bad and then you have a problem (that red spot turns into a melanoom, aka skincancer) Now, when you put radiation an a body cel (low level, not the instand death level) NOTHING happens to it, thats right, radiation does no damage to your bodycells when there are in working order. The problem is that cells have to split once and a while, in order for someone to grow or heal (that is why radiation on a younger person is a lot more dangerous then on an older person, the younger you are, the more cells split) and when that happens the body cell is vulnerable to a copy error if it gets hit by gamma radiation.
Now, this also explains some of the symptoms af radiation poisoning, the part if the body that has to renew itself the most often is your intestens, thyroïd and eyelens. This is the reason you feel sick and start throwing up when poisoned, your guts are just very badly damaged, thyroïd you feel less but it is the reason you have to take iodine tablets and the eye exposure will make you blind, nothing more. The brain can take something like 25 times the dose of radiation the intestence can so they are no worry, your already a walking corps when you hit those doses of radiation.
will stop here, going off topic a bit lol, just saying radiation isn't that bad, sure, less is better, but we evolved true mutations, who says those mutations that made life possible in the first place weren't caused by radiation?