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CHAPTER 16 



will not be evident when added to the 

 number of genetic deaths resulting from 

 spontaneous mutation. It' the one rad gon- 

 adal exposure were repeated every genera- 

 tion, an equilibrium would eventually be 

 established in which, for every generation, 

 100 to 4.000 people per million would show 

 the effects of radiation-induced mutants in 

 the form of genetie death. However, since 

 the kinds of phenotypic effects produced by 

 the radiation-induced mutants would be the 

 same as those from mutants which occur 

 normally, we would not be able to recognize 

 specifically those people hurt by the radia- 

 tion. 



What part of our normal load of mutants 

 comes from naturally-occurring penetrating 

 radiation? Since human beings receive 

 about five rads in the course of a reproduc- 

 tive generation — that is, in 30 years — it is 

 possible that as little as % , or y 6 , of our 

 mutations normally are radiation-induced. 



How much additional radiation are we 

 exposed to during medical treatment? If 

 medical use of radiation were to continue at 

 its present level, it has been estimated that 

 each person in the United States would re- 

 ceive a total dose to the sex cells of about 

 three r per generation. Of course, some 

 people do not receive this amount of radia- 

 tion, while others get considerably more. 

 But this average radiation dose to the germ 

 cells from medical uses alone is 60% of the 

 amount received spontaneously and is rais- 

 ing the mutation rate about 10% above the 

 spontaneous rate. In the years to come, 

 with increased use of radiation for diagnosis 

 and therapy, the average dose from medical 

 radiation might increase greatly. Already 

 radioactive materials have been used in one 

 million medical treatments in a single year. 

 Many governments as well as private dental 

 and medical groups are investigating such 

 radiation exposure, and many ways of re- 

 ducing radiation exposure without hinder- 

 ing its usefulness are being instituted, 



It is difficult to determine the number of 

 germ-line mutations resulting from the radia- 

 tion associated with fallout following atomic 

 explosions, because some radiation reach- 

 ing the gonads could come from fallout on 

 the ground, breathed in, or ingested with 

 food. In the latter case, the distribution of 

 particular radioactive substances in the body 

 makes a large difference in the amount of 

 radiation reaching the sex cells. With re- 

 spect to sex cells, the three most important 

 radioactive substances in fallout are cesium- 

 137, strontium-90, and carbon- 14. Because 

 cesium is distributed through the tissues — 

 including the gonads — more or less evenly 

 whereas strontium is preferentially localized 

 in bone, we expect cesium- 137 to produce 

 more gonadal radiation damage from in- 

 gested fallout than strontium-90 produces. 



The period of time over which radioac- 

 tive substances produce new mutations also 

 varies. The induction of mutations by rela- 

 tively short-lived radioactive substances, like 

 strontium-90 and cesium- 137, is restricted 

 almost entirely to a few generations. On 

 the other hand, carbon- 14 — C-14 — is long- 

 lived, having a half-life of 6,000 years. So, 

 if the exposure to C-14 in the environment 

 does not change, there will be about half as 

 many new mutations induced after 200 gen- 

 erations as there are in the first generation. 

 Because of its abundance and long half-life, 

 carbon- 14's potential for delivering radia- 

 tion to the gonads has been calculated as 

 being 4 to 17 times more than radioactive 

 cesium's and strontium's combined and, 

 therefore, carbon- 14 is capable of producing 

 proportionally more point mutations. 



In the United States National Academy of 

 Sciences — National Research Council report 

 of 1956 (see References), the gonadal dose 

 expected from fallout — if weapons of the 

 same type continued to be tested at the same 

 rate — was given as about 0.1 rad in the next 

 thirty years. On the basis of the United 

 Nations report, we could expect approxi- 



