Radiation Doses at Which Clear Evidence of Cancer Risks Is Shown

 For x- or γ-rays, good evidence of an increase in risk for cancer is shown at acute doses >50 mSv, and reasonable evidence for an increase in some cancer risks at doses above ≈5 mSv. As expected from basic radiobiology , the doses above which statistically significant risks are seen are somewhat higher for protracted exposures than for acute exposures; specifically, good evidence of an increase in some cancer risks is shown for protracted doses >100 mSv, and reasonable evidence for an increase in cancer risk at protracted doses above ≈50 mSv.

It seems unlikely that we will be able to directly estimate risks at significantly lower doses than these because of the practical limits of epidemiology discussed above. Of course, the fact that risks cannot be directly estimated at doses below, say, 5 mSv, does not imply any conclusion as to whether risks actually exist at these lower doses. As we discuss below, at lower doses inferences with regard to risk need to be based on understanding underlying mechanisms.