~24-
UCRL-3644
Sr99/Ca content of milk in the New York area
puc Sr90/g Ca
Date
June
1.2
1954
January 1955
1.6
June 1955
2.0
January 1956
2.7
5 0
September 1956
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The minimumestimate offutureaveragehuman bur denofSr?9,then, |
that Se /g Ca will be presentin the bone,.. This corresponds to the
atest greport ed value formilkconcentration and to the fact that bone acquisition
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of Sr? /Ca in growing children is very similar to milk Sr99 /Ca.
A difficult current problem is the estimation of future Sr?9/Ca in
milk. The level of Sr?9/Ca in milk is increasing, and, by linear extrapolation,
may be expected to raise the Sr?9 concentration in a year's time (by September
1957) to about 7 ppC Sr99/p Ca. At this date, accumulated fallout of sr 90,
based upon the quantity estimated at the time of the Libby report, may be about
25% to 50%° of the amount initially dispersed in the atmosphere. Since the
Libby report was written,
other nuclear detonations have occurred,
so that
it would be very reasonable to assume that fallout, by some 10 years from now,
should have increased milk levels significantly. For lack of better information, we may assume a factor of, say, 3 to 5 times as much as September 1957
(allowing for residual hold-up in the atmosphere and for decay of Sr?9). Thus,
milk levels and human bone levels by 1967 may be 20 to 35 puC Sr90/g Ca.
An additional factor must be considered, which may require that these
future estimates be even higher.
Cows,
in body content of sr90 Ca, may be
expected to lag several years behind the plant and soil levels. This is because
of the large calcium reservoir in their bones and other tissues, and because
the start of growth to milk-producing stage preceded current time by 4 or more
years; moreover, the food consumed by dairy cows is customarily stored for
many months before itis eaten. It is difficult to estimate that point in fallout
time that corresponds to current milk values; it seems likely, however, that
the Sr?9/Ca content of the bones of pasture-fed calves approximates the Sr99/Ca
level that adult cows would secrete in their milk, were they in more rapid
equilibrium with fallout. The following table, derived from values averaged
from Libby's and Eisenbud's reports, shows that calf bones are approximately
60% higher in Sr?9/Ca content than milk. Thus, future estimations of Sr?9
levels should be at least 60% higher than the 20 to 35 puC Sr90/g Ca estimate,
or, say, 30 to 50 puC/g, in round numbers.
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