RADIATION PROTECTION 217 r/day, respectively. If it is assumed that the fictitious dose rate for man is the same as that for mouse; Curve A corresponds to a dose rate of 12.8 "r"/day and Curves B and C to 2 r/day and 10 r/day respectively. We may now estimate the life shortening in man to be expected from chronic wholebody exposure to roentgen rays of high penetrating power. The simple method worked out for mice will be used and, there fore, only the spontaneous aging Curve A in Figure 2 and the fictitious dose rate applicable to that curve are needed. The curve was extended down to age twenty in order to consider cases of occupational exposure that may begin at that age.* If exposure at an average rate of 0.1 r/day for 40 years, the accumulated dose at age 60 will be 1,460 r. Taking the fictitious dose rate for man as 0.64 "r"/day or 234 "r"/year, the increase in physiologic age will be 1,460 234-6.25 years. Therefore, at age sixty the individual would have the physiologic age of a nonexposed individual 66.25 years old. The life expectancy of United States white males (as of 1950) is sixteen years at age sixty and twelve at age 66.25. Therefore, the life shortening is four years or is Obviously, if the fictitious dose rate for man is 12.8 "r"/day, the life shortening will be approximately 1/20 of a day per r. Therefore, life shortening determined on the basis of a fictitious dose rate of 0.64 "r" /day may be considered to give a reasonable upper limit. There are, of course, some variations in the life shortening per r when the above calculation is made for other ages, because the life expectancy is not a linear function of age, but the maximum is approximately 1.5 days per r. Considering the inherent uncertainties involved, an average value of 1 day per r is a close enough estimate. These calculations have been made for a daily dose of 0.1 r because Curve B of Fig In this country the minimum legal age for work with radiaion is actually 18. ure 2 is for o.1 r/day on the basis of 0.64 "r"/day for Curve A, and the age difference can be read directly from the curves. As pointed out earlier in the case of the mice, so long as the exposure is chronic and the dose rate not too high, the dose accumulated up to the age of interest is all that is needed to determine the approximate life shortening. Thus, in the case of the radiologists in Shields Warren's survey,' if we take 500 r for the average dose accumulated up to age sixty, the age difference is The data of the experiment of Lorenz et al. on chronic irradiation of mice with radium gamma rays have been analyzed in terms of the Gompertz function in such a way that the resulting curves are internally consistent. The straight line for the controls (on a semilog plot) represents the increase in mortality rate with age of the animals. It is customarily assumed that, at least beyond a certain age, the increase is due to the aging process, whatever it may be. The slope of the straight lines for the chronically irradiated mice is greater than for the controls. Therefore, chronic whole body irradiation causes an acceleration of aging in these mice. Irrespective of the mechanism by which radiation causes aging, the effect is additive to the spontaneous one. From the relative positions of the straight lines (Fig. 1) it is then simple to calculate a fictitious dose rate of radiation that would produce the same aging as occurs spontaneously. This turns out to be 12.8 "r" per day. Adding this fictitious 218 RADIATION PROTECTION dose rate to the actual gamma-ray dose rates used in the experiment makes the total accumulated dose at the time when the groups of mice have equal mortality rates the same, irrespective of the dose rate (within the limits of the experiment, 0-8.8 r per day). This is as it should be if the effects of the spontaneous aging process and aging by irradiation are additive. For man the Gompertz curve is known accurately, but varies from country to country and to some extent with the year when the statistical data were compiled. The one given in Figure 2 applies to the white population of the United States. It is not known how this population would respond to chronic irradiation and, therefore, it is impossible to determine directly the fictitious dose rate that would produce aging equivalent to spontaneous aging. However, a value may be obtained by comparing the Gompertz straight lines for the control and chronically irradiated mice (Fig. 1) with the straight line portion of the Gompertz curve for man (Fig. 2 A). It is found that the slope of the straight line for the nonirradiated mice is 20 times greater than the slope of the straight line for man, when the same time scale is used. This means that the mice used in the experiment of Lorenz et al. age 20 times faster than man. It is reasonable to assume then that the fictitious dose rate for man is 1/20 of that for these mice. Obviously, if the life span of mouse is shorter than that of man, either the aging agent (whatever it may be) is more powerful in mouse than in man, or the biologic system of man is more stable than that of mouse. Be that as it may, the time rate of aging differs by a factor of 20. If we use this factor to determine the fictitious dose rate for man we find that it is 0.64 "r" per day. On the same basis the aging produced in mice by 1 r per day should be produced in man by 1 r in twenty days; that is, by 0.05 r per day. This makes possible the estimation of the life shortening in man to be expected from chronic exposure at any given dose rate (not in excess of 8.8/20=0.44 r per day, 8.8 r per day be ing the highest dose rate used by Lorenz et al.). A simple method for doing this is described in the text. The most widely quoted estimate of the life shortening to be exepcted in man from exposure to radiation is one made by Hardin B. Jones, which is fifteen days per roentgen. The value derived by us is ap proximately one day per roentgen of aċ cumulated dose for chronic exposure at a dose rate not in excess of 0.5 r per day. The National Committee on Radiation Protec tion has recommended a maximum permissible accumulated dose of 50 r in ten years for individuals occupationally exposed to roentgen or gamma rays. This amounts to 210 r in forty-two years (age eighteen to sixty). According to our method of estimation, the life shortening attributable to this accumulated dose is about two-thirds of a year. On the same basis the life shortening to be expected in radiologists who in the past (when the dangers of exposure to radiation were not well known) may have accumulated whole-body doses of 500 r is 1.5 years. It should be noted that the method of extrapolation from animal to man developed by us has only one new feature-the assignment of a fictitious dose rate to spontaneous aging. This is really a necessary consequence of the application of the Gompertz function to the mortality of chronically irradiated animals. Therefore, if this is justified, the extrapolation according to our method is correct in principle. How ever, owing to the small numbers of animals used in chronic exposure experiments, the derived numerical values for man may have a considerable error, but this would not be so large as to make the life shortering in man equal to anything like fiftee days per roentgen. It is well to point ou: explicitly that the present discussion af plies only to chronic exposure at low dos rates not to the effect of a large dose re ceived in a short time. G. Failla, Sc. D. 630 West 168th Street New York 32, New York REFERENCES 1. BERLIN, N. I., and DIMAGGIO, F. L. A Survey of BOCHE, R. D. In: Zirkle, R. E., Editor. Biological 4. Jones, H. B. Factors in longevity. Kaiser Found. M. Bull., 1956, 4, 329–341. 5. LEWIS, E. B. Leukemia and ionizing radiation. 6. Lorenz, E., and others. In: Zirkle, R. E., Editor. DUBLIN, L. I., and SPIEGELMAN, M. Mortality of 7. WARREN, S. Longevity and causes of death from irradiation in physicians. J.A.M.A., 1956, medical specialists, 1938-1942. J.A.M.A., 1948, 137, 1519-1524. 162, 464. 58454 0-60————15 Representative HOLIFIELD. Our next witness will be Dr. James Crow, professor of genetics and zoology, University of Wisconsin. STATEMENT OF DR. JAMES CROW,1 PROFESSOR OF GENETICS AND ZOOLOGY, UNIVERSITY OF WISCONSIN Dr. CROW. Thank you, sir. Representative HOLIFIELD. It is nice to have you with us again, sir. Dr. CROW. Thank you. I should like, Mr. Chairman, to join those others who have commented favorably on the careful, detailed, unbiased, and conscientious efforts of the Special Subcommittee on Radiation and its chairman and staff. The large amount of material assembled from diverse sources and made available to the public has been and will continue to be invaluable. The work of this committee, the NCRP, the ICRP, the National Academy committees, the United Nations committees, the Federal Radiation Council, and others insure that we are moving into the nuclear age with careful consideration of the health problems. This has not always been true of new technical developments. There has been criticism that too much attention has been given to radiation effects, and that chemical industry has developed with much less public discussion of toxicity levels and health hazards. This may be true, and to the extent that trained manpower that is in short supply is taken from more urgent or more basic problems to work on radiation protection, the attention to the hazard becomes itself a hazard. But I believe that the thought, the discussion, and the detailed work done on this question are far better than the haphazard way in which many other health risks have grown. In some respects knowledge of radiation effects is very deep. Measuring instruments have a sensitivity such that extremely minute amounts can be detected. The physical knowledge of different kinds of radiation is solid. The qualitative effects of radiation on plants and animals are known in great detail, and extrapolation to man is fairly secure. It is when we come to the quantitative effects of low doses that the great uncertainty appears. As has been brought out repeatedly before this committee and elsewhere, the genetic effects of radiation seem to have no threshold and the best evidence is that, at the low doses to which the public is likely to be exposed in peacetime, the genetic risk is, to a first approximation at least, simply proportional to the total dose prior to reproduction. Russell's work on mice, confirmed for Drosophilla in Muller's labora tory, shows that in the early germ cell stages chronic irradiation is less effective than acute. This and other complications make the quantitative assessment of genetic risks seem even more uncertain than they did a few years ago. But none of these complications are such as to necessitate a revision of the previous conclusion that there is no threshold below which the risk is zero. In the past few years there has been an increasing tendency to use the same concept in setting dose limits for somatic exposure. This is reflected in the report of the ad hoc committee of the NCRP and by the Federal Radiation Council. This view is adopted not because it is proven, but because it is not unlikely that at least some components of somatic effects follow such a rule and this seems like a prudent, conservative principle to follow. At the 1959 hearings I suggested, along with Dr. Schubert and others, that the natural background level of radiation be used as a yardstick for setting general population standards. This is a dose to which the population has been exposed throughout its history and, though hardly beneficial, it has not been disastrous. This was not a new idea at the time, for it was then a part of the thinking of the ad hoc committee and had come up earlier in NCRP and ICRP discussions. I am pleased that the Federal Radiation Council has adopted this view. In its background statement it says, and I quote: We believe that the current population exposure resulting from background radiation is a most important starting point in the establishment of radiation protection guides for the general population. I am departing slightly from my prepared statement in the interest, I think, of greater clarity. For the genetic dose the National Academy of Sciences committee recommended that the population average of manmade radiation from all sources be kept below 10 roentgens in a 30-year period. This is about three times the background level but includes medical and dental radiation. The NCRP recommendation is similar. The ICRP has recommended 5 rems for 30 years, plus the lowest possible contribution from medical exposure. The Federal Radiation Council has adopted this same value for its radiation protection guide. With respect to the somatic dose, the ICRP has suggested that for planning purposes the average concentration of such isotopes or mixtures thereof in air or water applicable to the population at large should not exceed one-thirtieth of the occupational values. For whole body radiation this is 1.70 millirems per year, or about 1.7 times the background level. The ad hoc committee recommended that the maximum permissible dose of manmade radiation, excluding medical and dental sources, should not be substantially higher than the background without a careful examination of the reasons for higher values. They suggested that the background level be arbitrarily taken to be one-tenth rem, or 100 millirem per year. The Federal Radiation Council recommends 170 millirem per year for whole body average for population groups. Thus there is a considerable agreement among all these groups. The various recommendations are of course not completely independent, for each group had previous reports of the other groups and several persons served in more than one group. But that all these groups agree that the natural background level is an appropriate yardstick and that the population average level should be in the range of 1 to 2 times background is significant. According to the Federal Radiation Council these values for the general population and the NCRP recommendations for occupationally exposed groups do not appear to restrict unduly the beneficial use of |