C -4- ( critical groups within the population, provided the critical group is small enough to be homogeneous with respect to age, diet and those aspects of behavior that affect the doses received, Such a group should be representative of those individuals in the population expected to receive the highest dose. ICRP believes that it will be reasonable to apply the appropriate dose limit for members of the public to the mean dose of this group. The inate variability within an apparently homogeneous group means that some members of the critical group will receive doses somewhat higher than the dose limit. At the very low levels of risk implied, the health consequence is likely to be minor whether the dose limit is marginally or substantially exceeded. Limitation of exposure of whole populations is achieved partly by limiting the individual doses and partly by limiting the number of persons exposed. It is of the utmost importance to avoic actions that may prove to be a serious hazard later, when correction may be impossible or costly. The ICRP dose limits for individual members of the public are in Table IV. tion is given. No maximum "somatically significant" dose for a populaUsing the linear dose-effect relationship and assuming no-threshold, the ICRP indicates that an annual exposure of active red marrow, averaged over each individual in the population, of 0.5 rem (corresponding to the annual dose limit for members of the public) might at equilibrium lead to an increased incidence of leukemia, at most, of about ten cases per year per million persons exposed. The genetic dose to the population should be kept to the minimum amount consistent with necessity and should certainly not exceed 5 7 8 re -: =e - SO Er ery