eye know the regs. wondering the theory behind the reg.
NCRP 1960, page 485 states:
"we recommend that the population permissible dose for man-made radiation be based on the average natural background level. Although it is not our responsibility to determine the exact level, we believe that the population permissible somatic dose for man-made radiations, excluding medical and dental sources, should not be larger than that due to natural background radiation, without a careful examination of the reasons for, and the expected benefits to society from a larger dose”
In calculations performed in this report they regularly used an average background dose of 100 mrem per year. So while the report never explicitly stated an annual limit of 100 mrem. It was deduced from the usage of that value that it was their intent.
So there's no real health justification for a limit of 100 mrem per year, just the idea that the public shall receive zero dose per year from industrial non-medical sources, leaving natural background as the only remaining source. And if natural background causes 100 mrem/year then that should be the limit.