Risk estimates for radiation-induced cancer - the epidemiological evidence

The risk of low-dose radiation exposures has for a variety of reasons - bee n highly politicised. This has led to a frequently exaggerated perception o f the potential health effects, and to lasting public controversies. A bala nced view requires a critical reassessment of the epidemiological basis of current assumptions. There is reliable quantitative information available o n the increase of cancer rates due to moderate and high doses. This provide s a firm basis for the derivation of probabilities of causation, e.g. after high radiation exposures. For small doses or dose rates, the situation is entirely different: potential increases of cancer rates remain hidden below the statistical fluctuations of normal rates, and the molecular mechanisms of cancerogenesis are not sufficiently well known to allow numerical predi ctions. Risk coefficients for radiation protection must, therefore, be base d on the uncertain extrapolation of observations obtained at moderate or hi gh doses. While extrapolation is arbitrary, it is, nevertheless, used and m ostly with the conservative assumption of a linear dose dependence with no threshold (LNT model). All risk estimates are based on this hypothesis. The y are, thus, virtual guidelines, rather than firm numbers. The observations on the A-bomb survivors are still the major source of information on the h ealth effects of comparatively small radiation doses. A fairly direct inspe ction of the data shows that the solid cancer mortality data of the A-bomb survivors are equally consistent with linearity in dose and with reduced ef fectiveness at low doses. In the leukemia data a reduction is strongly indi cated. With one notable exception - leukemia after prenatal exposure - thes e observations are in line with a multitude of observations in groups of pe rsons exposed for medical reasons. The low-dose effects of densely ionizing radiations - such as alpha-particles from radon decay products or high-ene rgy neutrons - are a separate important issue. For neutrons, there is littl e epidemiological information. This has facilitated exaggerated claims of h igh neutron effects with reference to alleged dangers from transports of re actor fuel. However, in spite of limited information, it can be shown that the data from Hiroshima exclude the stated claims. New dosimetric informati on on neutrons may turn out to be highly informative with regard to an uppe r limit for the potential effects of neutrons and equally with regard to a reassessment - and a possible reduction - of risk estimates for gamma-rays.