774
MARTIN
of different radionuclides on fallout-contaminated plants. In the absence
of pertinent measurements, it is reasonable to assume an environ-
mental half-life of 14 days or less for }*!I and for other radionuclides
on fallout-contaminated plants in humid regions. Except for 1317 an
environmental half-life of 28 days should be more accurate for arid
regions. In general, the effective half-life of a given radionuclide on
plants, T,, could be estimated, where T, is the environmental half-life
and T, is the radioactive half-life, by
T.
=e
(7)
Our method! of estimating the effective half-life of Sr in rabbit
bone (T, = 20 days) was made necessary by the absence of pertinent
experimental data, and our only confidence in the accuracy of our estimate is based on the results obtained
(Fig. 5) when we usedit to
solve Eq. 5. Our estimate of the effective half-life of ‘I in rabbit
thyroids (Ta = 2.0 to 2.5 days) was well within the range of experi-
mental results reported by French” (T, = 1.5 to 2.5 days). Whenever
possible, the effective half-lives of radionuclides in animal tissues or
organs should be determined by experimental as well as by empirical
methods. One can then judge which of several possible values should
be applied to a given set of circumstances.
Animal retention factors, e.g., f, andf,, are especially difficult to
evaluate; and, as shown in Eqs. 5 and 6, errors in the estimation of
these parameter values would result in proportional errors in the
prediction of tissue burdens. Our estimates of retention factors for
88Sr (f, = 5.75%) and for 11 (f, = 18% if based on Py or f, = 26% if based
on Sj) represent mathematically arbitrary numbers calculated to obtain reasonably good fits between hypothetical and observed average
tissue burdens. Because of the methods used in the estimation of these
values, their physiological significance is doubtful; but these values
may be just as useful as those obtained from feeding experiments. For
example, French’s experimental results indicated f, values ranging
from <10 to >30% for jackrabbits, whereas the averages obtained for
Dutch rabbits under laboratory conditions were only half as high. We
collected samples of sagebrush and shadscale from the Sedan fallout
field and fed them to Dutch rabbits in the laboratory. The results
reported by Turner’! indicated f, values ranging from 2.0 + 0.6% (based
on samples from the Currant Area) to 12.0 + 8.0% (based on samples
from Groom Valley). Because of the difference in animal species
(Dutch rabbits vs. jackrabbits) and because our field data indicate that
jackrabbits in the Sedan fallout field were feeding on plants other than
sagebrush and shadscale, these results are of dubious value in relation
to the food-chain model.