45
clinically observed effects of known amountsof in-
ternally-deposited radium. These effects do not
appear until a period of years (10 to 15) has
elapsed. Thus, althoughit is possible to estimate
the potenual hazardin termsof the concentration
of internal emitters, clinical observations made
within a few years after contamination yield no
data on the degree of damage that mayultimately
be produced.
In evaluating the long-term effects produced by
an acute internal 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 internalradiation hazard. Like
Sr*°, Cs'37 has a gaseous precursor with a half-life
sufficiently long to avoid early condensation in the
fireball. Cs’*’ thus follows Sr? into the strato-
sphere. Since the fission yields and the half-lives
of the two radioelements are nearly equal, they
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 move-
ment of Sr®° through the biosphere, since its gamma-emitting properties make it readily detectable.
The body burden of fission products can be
determined in three ways. The method of choice
is the direct im vive measurement by whole-body
spectrometry. The limitations of this method are
that few whole-body counters are in existence,
thev require enormously bulky shielding and thus
are not easily transported to various sites, and
their absolute calibration is difficult. Further, this
method is restricted to analysis of gamma-emitting
isotopes, since, to date, a whole-body beta counter
has not been developed.
A second methodfor calculating body burden,
particularly for counting beta emitters such as
Sr?”, is the estimation of the internal deposition
from data obtained by radiochemicalanalysis of
the ur >.
Fin..uy, it 1s possible to make a completely in-
direct estimate of the human body burden of _
radioisotopes by what may be called the environmental approach. In this method, the estimate of
the body burden is 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 chemically similar and thus
appear together in the various componentsof the
ecological chain, Ga is taken up preferentially by
plants and animals so thatit is necessary to determine 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
There is also some interest in the neutron-induced radioelement Zn*, even thoughit, too, does
of the body burdens in the Marshallese people will
be considered in this report.
The interest centers chiefly around the fact thatit
EXPERIMENTAL PROCEDURES
not appearin levels hazardous to humanbeings.
is definitely transmitted through marine life, 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®*99, Co%?-35-6°, Mn3* Cel#s-Prit,
Zr°°-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
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 vivo
gamma-ray measurement of human beings was
Ww.
The potential radiation effects that maybe produced byspecific quantities of internally-deposited
radioisotopes can be onlv roughlypredicted from
counting techniques madeit feasible.
“ou
ESTIMATION OF THE INTERNAL
RADIATION HAZARD
elements in the Marshallese until whole-body
=
«
so low thatlittle effort was made to quantily these
Ny Te
been reflected in amiacrease in body burdens of
some radionuclides..As will be shown, the increases, though manyfold, have remained far below maximum permissible levels.