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ESTIMATION OF THE INTERNAL
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RADIATION HAZARD
The potential radiation effects that maybe pro-
duced by specific quantities of internallv-deposited
radioisotopes can be only roughly predicted from
clinically observed effects of known amounts of internally-deposited radium. These effects do not
appear unul a period of years (10 to 15) has
elapsed. Thus, although it is possible to estimate
the porential hazard in terms of the concentration
of internal emitters, clinical observations made
within a few years after contamination yield no
data on the degree of damage that may ultimately
be produced.
In evaluating the long-term effects produced by
an acute interna! exposure and exposureto residual contamination, Sr®® is clearly the critical element. Particular effort was therefore made to
determine its levels in the urine of the Marshallese, and thus to estimate the body burdens.
Of the gamma-emitting fission products, Cs'*” is
of the greatest interest, even thoughit is of minor
significance as an internal radiation hazard. Like
Sr°°, Cs*** has a gaseous precursor with a half-life
sufficiently long to avoid early condensation in the
fireball. Cs'*” thus follows Sr®° into the stratosphere. Since the fission yields and the half-lives
of the two radioelements are nearly equal, they
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are present in the fallout in like quantities. While
they have different ecological cycles because of
their different chemical properties, Cs'*” nevertheless provides a useful tracer for studying the movement of Sr®® through the biosphere, since its gamma-emitting properties make it readily detectable.
There is also some interest in the neutron-induced radioelement Zn*®, even thoughit, too, does
not appear in levels hazardous to human beings.
The interest centers chiefly around thefact thatit
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is definitely transmitted through marinelife, and
thus provides a clear example of the transmission
of a radioelement throughthe food chain to man.
Other fission products and neutron-induced
activities (Fe?55%, Co97-38-89) Mans) Celtt-Pritt,
Zr°5-Nb*, and Ru'*-Rh?°*) also appear in small
amounts in the soil and the food chain, and thus
may appear ultimately in man, but the levels are
11859935
The body burden of fAssion products can be
determined in three ways. The method of choice
is the direct in vivo measurement bv whole-body
spectrometry. [he limitations of this method are
that few whole-bodv counters are in existence,
they require enormouslybulky shielding and thus
are not easily transported to various sites, and
their absolute calibration is difficult. Further. this
methodis restricted to analysis of gamma-emitting
isotopes, since, to date, a whole-body beta counter
has not been developed.
Asecone nethod for calculating bodv burden.
particularly for counting beta emitters such as
Sr°°. is the estimation of the internal deposition
from data obtained by radiochemical analysis of
the urine.
Finally, itis possible to make a completely indirect estimate of the human body burden of
radioisotopes by what may becalled the environ-
mental approach. In this method, the estimate of
the body burden ts based on the concentrations of
the fission products present in the environment,
chiefly the soil and the important components of
the diet. In order to make this estimate, data must
be obtained on the transfer of the fission products
between successive elements of the ecological
chain leading from soil to bone. For example, al-
though Sr and Ca are chemicaily similar and thus
appeartogether in the various componentsofthe
ecological chain, Ca is taken up preferentially by
plants and animals so thatit is necessary to determune the discrimination factor for each step. When
these factors are known, it is possible to estimate
the concentration of a radionuclide in man from
its concentration in any step of the ecological
chain.
All three of these approaches to the estimation
of the body burdens in the Marshallese people will
be considered in this report.
EXPERIMENTAL PROCEDURES
Whole-Body Counting
The gamma-ray activity from the internallydeposited fission products and the neutron-induced activities in 227 of the Marshallese people
were measured with a whole-body gamma scintillation spectrometer. The technique of in vie
gamma-ray measurement of human beings was
a
creases, though manvfold, have remained far below maximum permissible levels.
so low thatlittle effort was made to quantify these
elements in the Marshallese until whole-body
counting techniques madeit feasible.
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been reflected in ageimcrease in body burdens of
some radionuclides..As will be shown, the in-