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UCRL-3644
calcium in chemical behavior. The levels of Sr? directly measured in young
human bones during the period up to October 1956 are in the vicinity of 0.0038 r/yr
to the bone. Strontium-90 is deposited preferentially in the bone by a factor
of more than 100 over the soft tissues, so that only the bones need be considered
with regard to this isotope.
The Libby report estimates, on the basis of a balance between accumulated fallout of Sr?9 into OF soil and uptake by cattle and man, that in
America the human ratio of Sr’~
to calcium may eventually become 10% to
30% of that observed in the topsoil. The report estimates that sr?9 now held
by the stratosphere, in descending to the earth over the next four years, will
produce a human Sr? concentration of from 0.016 to 0.038 r/yr (0.004 to
0.010 MPC"), assuming that no further Sr?9 is added.
The range of this ex-
pected gain of radiation exposure is equivalent to the extra cosmic radiation
exposure experienced by individuals dwelling at altitudes of 5000 feet (e.g.,
Denver,
Colorado) compared with individuals at sea level.
The estimation
assumes that there is a selection factor! favoring calcium over strontium in
*
.
.
.
MPC = maximum permissible concentration.
Harrison et al. have evidence that elemental strontium-to-calcium ratios,
compared in food, blood plasma, and bone, are strikingly different; for man
they are:
g Sr/g Ca
Food
Plasma
Bone
17
4
x 19-4
x 10-4
2.5 x 10-4
Proportional Units
7
2
1
This is confirmed by Comar in observations using radiostrontium and radio-
calcium simultaneously added to the diet.
In Comar's observations for milk,
the discrimination achieved against strontium in the deposition ratio of Sr/Ca
may be less than that for other food sources, in which strontium and calcium
may have different chemical binding.
The problem of a protective discrimination for humans against the uptake of
the maximum Sr?9/Ca ratio is presented in the Libby report. At this stage of
understanding, this apparent reduction of Sr/Ca in bones of humans compared
with soil, plants, or animals seems to reside partly in the large calcium pool
of the adult cow's body, which constantly dilutes incoming strontium and calcium
so that milk, at present, is always intermediate in Sr/Ca ratio between the
cow's bones and the forage. Similarly, the human calcium pool dilutes incoming Sr/Ca (largely from milk products) so that human bones at this time always
have a lower Sr?9/Ca ratio than cow's milk or cow or calf bones. The content
of children's bones is much higher than in adult or stillbirth material. There
is some evidence for atomic discrimination between strontium and calcium,
but the problem needs further study to determine how much of Sr99 uptake by
bone is lessened at fallout equilibrium. If only dilution operates, withlittle
or no discrimination, humans will develop a higher Sr?9 level than is now expected.