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UCRL-3644
uptake from soils into the plant and into the cow and into the human bones,
that 70% to 90% of the soil strontium is rejected in favor of calcium.
so
Both human adults and stillborn babies have similar concentrations
of Sr? (i.e., similar Sr?9/Ca ratios). This is to be expected, since the
developing child draws its calcium from the maternal calcium pool, which is
in partial equilibrium with maternal bone. Both these human sources of measured sr99/Ca have been placed during 1954 and 1955 at approximately 1/6 of
the value for cow's milk; the resultant adult human bone irradiation value for
this period is about 0.0019 r/yr (0.0005 MPC) from the Sr?9 content. Reported values for adults did not exceed 0.004 r/yr in the sample studied, ex-
cept for one individual measured at 0.008 r/yr.
in terms of radiation effect.
This is a very small number
If, in the fallout to be expected, the discrimination against Sr99 in
its course from soil to plant to human bone is by only a factor of 50% instead
of a factor of 70% to 90%, Libby's estimate of the future Sr’~
concentration
would have to be increased to 0.075 r/yr (0.020 MPC), based on the present
stratospheric and soil burdens. This level of Sr?9 would represent an additional radiation exposure to the bone, equivalent to the additional cosmic radiation experienced by those who dwell at 10,000 feetin this latitude.
Libby has estimated, from soil calcium levels, that if the entire sr99
burden reached the soil and humans came into equilibrium with the top 2 inches
of average soil, humans would eventually approach a maximum value of
40 pyc Sr79 /p Ca, or about 0.15 r/yr of bone irradiation. Such a value would
approximately double bone irradiation over natural radiation.
Estimation from Human Bone Assays
of Future Human Bone Concentrations of Strontium-90
The uptake of Sr?9 has been directly measured in human bones as a
function of age, and of location and time of collection (Libby, 5 Kulp et al. ).
The following summary conclusions can be drawn from analysis of this information:
1. Strontium-90 content of the bones in human stillbirths is increasing and, on the average, is estimated from Libby as follows.
U.S.A,
uC Sr99/p Ca
increase
December 1953
0.14
December 1954
0. 30
114
December 1955
0. 66
120
December 1956
(1. 3)
(extrapolated)
(100)