A radiation protection approach to assessing population risk for threshold-type radiobiological effects

The potential harm to a population exposed to radiation from a nuclear accident, such as the one that recently occurred at the Chernobyl plant in the Soviet Union, is of concern to many individuals. The average dose to a population is a useful index of harm (IH) only for linear, nonthreshold-type, quantal (i.e., all-or-none) effects. For such radiobiological effects, the expected harm to the population is linearly related to the average dose. However, for nonstochastic effects, it is not. An IH is proposed for threshold-type nonstochastic effects which is based on a form of the Weibull model where, at low to moderate doses, the individual risk at dose X = D/D50 is given by the approximation Risk = ln(2)XV; where D50 is the absorbed radiation dose that produces the specified effect in one-half the population, D is the absorbed radiation dose, and V is a positive parameter. The dose, X, is in units of the D50. Use of this form of the Weibull model is limited to doses such that X is small in comparison to 1. An IH for the population can be obtained by defining a new variable, P = XV, in dimensionless units, because the individual risk is linearly related to P at low and moderate doses. The average value for P (given by [P]) for an exposed population can be used as an IH for the population when the maximum value for P does not exceed 1. Both P and [P] can be regarded as theoretical doses. The average risk for the population in terms of the average dose [P] is given by ln(2)*[P], and the expected cases of nonstochastic effects among N individuals by N*ln(2)*[P]. As an example of the application of the average dose [P], the expected cases of temporary sterility in males among the approximately 135,000 people evacuated within 30 km of the Chernobyl plant is calculated to be about 200. The cases of sterility would be expected to come from those males exposed to doses to the testes of about 0.35 Gy or higher. No cases of sterility would be expected for individuals exposed to lower doses.