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UCRL-3644
Strontium-90 contents of various materials (in pyC/g Ca)
Mid- 1953
Mid-1955
Mid-1956
Milk
1.1
2.1
3.6
Calf bones
1.4
3.5
5.7
Alfalfa (Wisconsin)
6.7
Soil (Wisconsin)
2 to6in. depth
18
-
0 to 2 in. depth
-
9
=
35
-
It appears that the Sr?9 /Ca of cow's milk is a close index of the concentration of strontium in newly acquired humanbone. Gurrent milk.levels
—
suggest that children's bones in the next decade will approach an average concentration of approximately 50 ppc $199 /g Ca. This is in close agreement
with the estimation by Libby of a minimum average concentration of Sr99 of
10 to 40 puC Sr?9/g Ca. These estimates do not consider local variance in
the United States, nor, with respect to future concentrations, the special
problem of high-rainfall or low-calcium areas.
The upper value of approximately 40 pyC S199 /g Ca has been set by
Libby upon the consideration that this is the projected specific concentration
ratio when all the fallout is complete and mixed with the average calcium content of 2 inches of topsoil. There does not seem to be a way of independently
confirming the upper average limit of radiostrontium concentration from observation of milk or bone. The biological concentrations are increasing rapidly
with respect to time, approximately following the level of total accumulated
fallout, and 40 uyC Sr90/g Ca may not truly be a limit.
Whatever the speculation concerning future levels of Sr99 in humans,
we can be certain that current values (1956) represent a low level.
If we
translate a small dose such as 0.0038 r/yr (0.001 MPC) into numbers predicting an increase in leukemia mortality (an estimate may be based upon
tentative data that leukemia tendency may be doubled by 50 r whole-body ex-
posure™), an increment of
0.0038 r/yr x 50 years mean life span _ 0.004
50 r/tumor doubling
Ot
,
or 0.4% increase in leukemias, is estimated. Since there are only approximately 8,000 cases of leukemia deaths reported in the population of the United
ec
-
o.
This number may be high, since it is based upon whole-body radiation ex-
posure, while induction of leukemia by Sr?
exposure is the result of direct
irradiation of bone and marrow, the specific tissues involved in the leukemia
change.
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,