trapolation to very low radiation levels establishes the maximum value that may reasonably be expected, but it does not preclude the possibility that there may be no effect at all insofar as somatic effects are concerned. 4.2 Genetic Damage Genetic damage from Sr*° is generally accepted as negligible because this element does not concentrate in the reproductive organs. There may be, however, some minor effect from its presence in the circulating blood or even from incorporation into the chromosomes themselves. Until more is known about such possibilities, calculations about genetic damage must continue to be based on the increase of background radiation due to long-lived gamma-rayemitting isotopes in the fallout. It is estimated in the NAS report that in the United States the accumulated 30 year dose will be about 0.1 r, if weapons testing continues at the average rate of the previous five years. If the dose corresponding to the spontaneous mutation rate (doubling dose) in man is 50 r in 30 years (30 to 80 r is mentioned as the probable range in the NAS report), this means that the mutation rate will be increased by 0.2 per cent. Even if the doubling dose were aslittle as 10 r, which is probably the reasonable minimum,the increase would only amount to 1.0 per cent. In the NAS report it is stated that 2 per cent of the total live births in the United States have tangible defects of genetic origin that appear prior to sexual maturity. According to genetic principles, it may be expected that the number of such defective individuals will ultimately be increased by 0.2 to 1.0 per cent of the present frequency by the predicated increase in background radiation resulting from gammaray fallout. There are approximately 4,000,000 children born alive per year in the United States. The ultimate increase in geneti- cally defective children will therefore be 0.2 to 1.0 per cent of 4,000,000 (160 to 800 per year as compared to 80,000 per year resulting from the spontaneous mutation rate). One may get larger absolute numbers by extending the calculation to the world’s population of 2.7 billion. Assuming the same birth rate and the same estimated gonad dose of 0.1 r in 30 years as for the United States, the ultimate world-wide increase of defective children is 2500 to 13,000 per year. It should be noted that this is the ultimate increase; that is, the increase that would occur if the additional 0.1 r in 30 years persisted for a great many generations. In the first + Sitti generation, the increase might amount to 10 per cent of the ultimate value. If the radiation from fallout were not to continue after the first generation, the manifestation of mutations would continue to increase for some generations and then would gradually return to the initial level. It should be noted also that the world-wide average increase in background radiation ascribable to fallout, is considerably lower than in the United States. Therefore, the above world-wide estimates are definitely too high, by a considerable factor. The NAS report also stresses the conclusion that in any evaluation of genetic damage the total damage must include the effects of many mutations that do not produce tangible defects, at least when inherited from only one parent. The total damage, in fact, is more nearly equal to the frequency of mutations induced. The NAS report gives the estimates of six geneticists on the Committee that the induced mutation rate is probably about 0.5 per cent per roentgen per individual. If this figure is used as the basis of calculation, the total number of detrimental mutations induced by a gammaray fallout dose of 0.1 r would be 2000 in the United States and 32,000 for the entire world, in contrast to a spontaneous frequency 100 to 800 times higher. 4.3. Leukemia It is well known from animal experiments and from observations on humansthat exposure to radiation in sufficient amounts induces leukemia in susceptible individuals. That some sort of susceptibility of unknown nature (perhaps genetic) is involved in the process, is evident from the study by W. M. Court Brown and R. Doll (Medical Research Council report) of the incidence of leukemia in ankylosing spondylitis patients treated with x-rays. Of the patients treated with maximum bone marrow doses of 2750 r or more, 0.176 per cent developed leukemia. Neither the above mentioned study nor any other made so far provides the necessary in- formation to decide whether there is a threshold dose that must be exceeded before leukemia is induced in man, but some animal experiments indicate the existence of a threshold. Also, the absence of a threshold is considered doubtful by most authorities in the field of hematology. 272 ;