I worked at a nuclear plant during a college internship and can vouch for the strange attitude toward radiation exposure. There were guys who would work outages every year, receive their annual rad dose in a few months, and make enough money to take the rest of the year off. Then back next year to do it again. The max dose of 5 rem is about 20x the cosmic background - so 20 yrs of radiation in a year! I chose a different career soon after figuring that out.
Radiation can widely vary and very little is known about low dose radiation. The nuclear industry follows a rule called ALARA As Low As Reasonaobly Achievable which really just allows for these strict minimums. There is little proof that 5 rems will actually cause a higher probability of cancer.
