Effects on Populations of Exposure to Low Levels of Ionizing Radiation (1972) / Chapter Skim
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Chapter V GENETIC EFFECTS OF IONIZING RADIATION I Introduction and Brief History 42 A
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Chapter V GENETIC EFFECTS OF IONIZING RADIATION I Introduction and Brief History This chapter reviews briefly the information now available on the genetic risk to the human population from low levels of ionizing radiation and gives the Subcommittee,s conclusions and recommendations.
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have for man.
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"The previous recommendation should be reconsidered pepodically with the view to keeping the reproductive cell dose at the lowest practicable level. If it is feasible to reduce medical exposures, industrial exposures, or both, the total should be reduced accordingly." The present subcommittee concurs with this recommendation for periodic review and it is in this spirit that the present study has been undertaken.
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after another is being understood in molecular and chemical terms. Human chromosomes can now be studied with great precision, whereas at the time of the first BEAR report not even the correct number of human chromosomes was known.
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cuous or lethal effect. That this class of mutations exists was realized earlier, but whereas at the time of the 1956 report they were estimated to occur with a frequency two or three times that of lethals it is now estimated that they are at least 10 times as frequent as lethals among spontaneous mutations.
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there is good reason to think from animal experiments and from fragmentary human evidence that mutant genes are often lost from the popu-. lation because of mild dominant effects on viability and fertility when the gene is heterozygous.
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However, they occasionally give rise to unbalanced gametes leading to embryonic death or to congenital anomalies. Radiation does not appear to be a major cause of these.
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species the rate of evolution is hardly ever, if ever, limited by the mutation rate. The difference between rapidly evolving and slowly evolving species is far more easily explained on the basis of such factors as availability of ecological opportunity and stability of the environment than by differences in mutation rate.
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Table 1. Sources of genetically significant radiation.
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VI. Risks versus Benefits It is not a part of the Subcommittee,s assigned task to balance benefits against risks.
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As mentioned earlier, the natural level of radiation averages about 100 mrem per year. This varies considerably from one region to another, depending especially on the kinds of minerals present in the earth and on the altitude.
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and 200 rem. If we consider the mutation rate for dominant visible mutations -- estimated with considerable uncertainty as about 2 x 10-9 per rem -- the doubling dose is 100 rem or more unless the spontaneous rate is less than 2 x 10-7The extensive studies in Hiroshima and Nagasaki permit a direct approach to the doubling dose based entirely on human data (see Explanatory Note 6)
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We can get some support for the numerical values in Table 2 by a different calculation. This again uses mouse data for the induced rate, since there are no suitable human data.
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Table 3. Estimates of cytogenetic effects from 5 rem per generation.
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total incidence is less than 0.5 percent. Serious diseases caused by chromosome aberrations also have an incidence of about one-half percent, but these are not thought to be greatly increased by low level radiation (See the previous section)
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Table 4. Estimated effect of 5 rem per generation on a population of one million.
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might permit greater hazards from other sources to replace the overestimated radiation hazard thus avoided. A more serious error would be to underestimate the effect.
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one case) to show measurable effects on viability or fertility.
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year of the current Radiation Protection Guides. Accordingly, the 170 mrem seems to provide an unnecessarily large cushion.
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for a population group would have to be approximately 1/3 of the maximum amount, or again 170 mrem per year. Based on the published recommendations of the NCRP and ICRP, the population average of 170 mrem was adopted by the Federal Radiation Council in 1960.
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recessive detrimental changes and mutations associated with small dominant effects. Nevertheless, induced hereditary changes leading to skeletal anomalies (37)
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Much more important, we think, is the absence of significant effects on physical measurements or on health and survival.
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means that they must be eliminated from the population through heterozygous effects (55, 52)
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likely to be part of the same ion-cluster than from independent ion tracks. So we expect the linear component to predominate greatly at doses that commonly apply to the human population.
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to the mature oocyte are very resistant to mutation; hardly any mutations are produced. In the mouse the duration of the mature oocyte is about 7 weeks.
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close to zero if the female is relatively insensitive to radiation effects.
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cause per million live births is about 55. A small fraction of this number would survive to produce live births with congenital defects.
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led to two quite different interpretations. Reports from one laboratory (84, 85)
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section VIII-D we estimated that the equilibrium amount of illness from 5 rem of exposure per 30-year generation would be increased between 0.5 and 5 percent. One rem would produce an increase of 0.1 to 1.0 percent.
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(30) Green, E
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(70) Russell, W
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