45
4
been reflected in’an increase in body burdens of
some radionuclides. As will be shown, the increases, though manyfold, have remained far be-
low maximum permissible levels.
ESTIMATION OF THE INTERNAL
RADIATION HAZARD
The potential radiation effects that may be pro-
duced byspecific quantities of internally-deposited
radioisotopes can be only roughly predicted from
clinically observed effects of known amounts ofinternally-deposited radium. These effects do not
appear until a period of years (10 to 15) has
elapsed. Thus, althoughit is possible to estimate
the potential hazard in termsof the concentration
of internal emitters, clinical observations made
within a few years after contaminationyield no
data on the degree of damagethat 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 thusto estimate the body burdens.
Of the gamma-emittingfission products, Cs'*? is
of the greatest interest, even thoughit is of minor
significance as an internal radiation hazard. Like
Sr*°, Cs*3" has a gaseous precursor with a half-life
sufficiently long to avoid early condensation in the
fireball. Cs'*’ thus follows Srinto the stratosphere. Since the fission yields and the half-lives
of the two radioelements are nearly equal, they
are present in thefallout in like quantities. While
they have different ecological cycles because of
their different chemical properties, Cs'** neverthe-
less provides a useful tracer for studying the movement of Sr** through the biosphere,since its gamma-emitting properties makeit readily detectable.
There is also some interest in the neutron-induced radioelement Zn®, even thoughit, too, does
not appearin levels hazardous to human beings.
The interest centers chiefly around the fact thatit
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®*-*?, Co°?-58-8°) Mn**, Ce?#-Pr',
Zr*-Nb®*, and Ru’°*-Rh’®*) also appear in small
amounts in the soil and the food chain, and thus
mayappear ultimately in man, but the levels are
so lowthat litdle effort was made to quanufy these
elements in the Marshallese until whole-body
counting techniques madeit feasible.
The body burden of fission products can be
determined in three ways. The method ofchoice
is the direct in vivo measurement by whole-body
spectrometry. Thelimitations of this method are
that few whole-body counters are in existence,
they require enormously bulky shielding and thus
are not easily transported to varioussites, 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.
DOEARCHIVES
A second methodfor calculating
urden,
particularly for counting beta emitters such as
Sr®°, is the estimation of the interna] deposition
from data obtained by radiochemical analysis of
the urine.
Finally, it is possible to make a completely indirect estimate of the human body burden of
radioisotopes by what maybe called the environmental approach. In this method, the estimate of
the body burdenis 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 onthetransfer 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
appeartogetherin the various components ofthe
ecological chain, Ca is taken up preferentially by
plants and animals so thatit is necessaryto 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 anystep 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-rayacuvity from the internally-
deposited fission products and the neutron-induced activities in 227 of the Marshallese people
were measured with a whole-body gamma scin-
tillation spectrometer. The technique of in vwo
gamma-ray measurement of human beings was
Sl