cause this serves as the basis for estimating the number of radiation-induced chromosome mutations in the population as a whole. Byreferring to the tables, the average dose may be taken to be in the order of 10 millirads for the 4-year period covered bythis report. This is small when compared with the gammaradiation re- ceived from natural sources by populations throughout the world. Libby (6) estimates that the average external dose from natural sources, both terrestrial and cosmic, is of the order of 75 millirads per year, or 300 millirads in four years. On this basis, the gammadose delivered from fallout is about 3 percent of the average gamma dose from natural sources. Thus, even the maximum theoretical dose from fallout to date is a small fraction added to the gammadose received from natural radioactivity, and this slight incrementis considerably smaller in magnitude than tium-90 in debris which falls out relatively close to the site of detonation and a corresponding enrichment from debris Rural populations are less shielded by buildings and do not have the advantage of large paved areas to encourage runoff during storms. Nevertheless, the true situ- which falls out at greater distances. This may explain the low ratios of measured- ation is never the infinite smooth plane on whichthese calculations are based, and are relatively close to the Nevada site. and other factors. enriched in strontium-90 by as much as a Significance of Findings higher than the predicted values. Based In interpreting the significance of these data, one needs to consider only the esti- factor of 2, which explains why the measured values at the other stations were on this soil study, one might be justified in doubling the strontium-90 values given, but this factor was not used in preparing the tables. The estimates of gammadose, like the estimates of strontium-90 were derived by calculations from estimates of the fall- out of mixed fission products. It should ties in the terrain, the plowing of fields, 00 L of T Similarly, fallout analyzed at great distances from detonations is known to be some reduction is afforded by irregulari- oO to-predicted values for Albuquerque, Salt Lake City, and Grand Junction, which e7 mates of gamma dose anddistribution of strontium-90. The total accumulations of mixed fission products are important only insofar as they serve as the basisforesti- mating the gammadose delivered and the amount of strontium-90 present. Geneticists are concerned with the average gamma dose to populations be- °4 0/4 a tium-90 in any given particle of dust is variable—there is a depletion of stron- e/ oO fireball. The relative abundanceof stron- 0/6 Ze ri is formed relatively late in the life of the 24 ™~ of 33 seconds, some of the strontium-90 cal properties. In the upper { foot of the earth’s crust, radium is present in amounts approximating 1000 millicuries per square mile. This radium,like other trace elements, is absorbedintoall living things. Adult North Americans contain about PREDICTED~Sp an? OS SS os wT & T T TT Hunter and Ballou curvesof relative iso- topic abundance in conjunction with the daily measurement of mixedfission products. Because strontium-90 is derived following fission from its precursor krypton-90, an inert gas which has a half-life be noted that the assumptions underlying the calculation of gamma dose tend to | produce values which are much higher than the doses to which populations are actually exposed. The reported values do not allow for weathering or shielding. The effects of weathering and shielding cannot be treated quantitatively. For populationsin cities, the true dose would be very much reduced by the fact that fallout to the surface is soon washed into gutters and storm sewers. For these reasons, it is likely that the actual dose to urban populations does not exceed 10 percent of the values reported here. n of measured to predicted strontium-90. These findings are consistent with present knowledge of the way strontium-90 is formed and distributed. The estimates of strontium-90 were.obtained by using the the tables can be understood in relationship to the occurrence of natural radium, with which strontium shares many chemi- AH Fig. 1. Cumulative deposition of mixed fission products in millicuries per square mile at the stations shown. These data are for fallout from October 1952 to September {955 and are extrapolated to 1 January 1956. the normal variations in natural dose which occur from place to place throughout the world. The significance of the deposition of strontium-90 in the quantities shown in 4 6 8 # Zz MEASURED Sr? "me? 4A & Fig. 2. Plot of measured versus predicted strontium-90 for soils from 17 locations in the United States: 1, Albuquerque; 2, Atlanta; 3, Binghamton; 4, Boise; 5, Des Moines; 6, Detroit; 7, Grand Junction; 8, Jacksonville ; 9, Los Angeles; 10, Memphis; 11, New Orleans; 12, New York: 13, Philadelphia; /¢4, Rapid City; /5, Rochester; 16, Salt Lake City; /7, Seattle.