246 |
S. H. COHN
excretion, it can be seen that the body burden does appear .o be approaching
an equilibrium level at a value below the 17 pc/g calcium estimated, This
value also agrees with the equilibrium value of 23 nc (23 pe/g Ca) calculated
from tne urinary excretion data [9].
Caesium-137
The Cs!°7 urinary excretion level for the six months immediately
following exposure can be expressed as a single exponential function with
a biological half-life of 110 d (Fig. 5). This value is in agreement with the
mean value obtained in a study of four adult males reported by RICHMOND
{13].
one
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o
L
a
z
P=
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a
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ne
—
~
w
5
2
100
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ao
0
a
30
a
60
lw,
90
3
120
t
180
1
180
J
210
TIME (4)
Fig.6
Cs137 excretion in urine of exposed Marshallese people for 6 months following exposure
The vertical bars indicate estimated standard deviation.
The mean body burden of Cs}*! calculated in 1957 from urine data
(before the return of the Marshallese to their home island) was 7 nc, or
comparable to the mean level measured in the population of the United States
at that time. Four Rongelap inhabitants measured in 1957 in the whole-body
counter at Argonne National Laboratory also had approximately the same
mean Cs137 body burden, 11 ne [14]. This value for Cs}*" at this time
indicates tnat the residual activity from the original acute exposure in 1954
was very low three years later. Actually the original Cs}37 level in 1954
(88 nc) decreased to essentially zero by virtue of the effective half-life of
Cs13T being 110 d. Thus the body burden of 7-11 ne in 1957 indicates continued exposure to world-wide fall-out of Cs!37 during the 3-yr absence
of the Marshallese from Rongelap.
The mean body burden of Cs}increased rapidly after the return of
the Marshallese to Rongelap, reaching an equilibrium value varying between