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UCRL- 3644
thyroid tissue. Following nuclear detonations of the last two years, the thyroid
concentrations of radioactive iodine in pastured cattle reached as high as 0.001
to 0.003 mpC/g (depending upon the quantity of fallout); and the average radiation
exposure, as measured over 3 years, was about 1 r/yr to the thyroid tissue.
This would be of genuine concern to man at similar human burdens of 131,
because it is now known that thyroid tissue is especially sensitive to radiation
induction of tumors. However, cattle fed principally in feed-lots have only
1/100 (or less) as much ti3las range-fed cattle.
Further careful measure-
ment of fresh human thyroid material has been routinely made during the
last two years by techniques that are sensitive and reliable for estimation of
131
content.
Direct measurement shows that human thyroid, at any time of
high uptake of ji3l by bovine thyroid, has less than 1/5000 of the bovine 131
content. It is possible that human thyroids had less than 0.0006 mpC/g during
the latter part of 1956, when range cattle had 1 to 2 mpC/g. It is certain that
the human thyroid exposure during the 1956 period did not exceed 0.001 r/yr,
and the probable value is 0.00016 r/yr or even less. (Interestingly, one human
thyroid showed an activity comparable with bovine thyroid content of 1131, the
case, when traced to its source, proved to be from a man who had previously
been given a small tracer dose of 1/3! in the Donner Laboratory. The observed quantity of 1131 was accounted for by the magnitude of the dose, the
estimated excretion, and the radioactive decay. )
Up to this time, radioiodine from world-wide fallout is not a problem
of concern to humans; and it is not expected that it will become a problem in
the future.
Summary
1.
This paper reports a broad examination of the levels of radiation
exposure incurred from fallout.
The discussion is limited to sr99
Cs137,
and I]
, the only radioactive isotopes reported to become associated with
human environment in detectable quantities.
2.
The world-wide effect of radiation from fallout is now far less
than that of naturally occurring radiation from cosmic rays and from radio-
active elements normally contained in earth, buildings, and body tissue. The
inescapable minimum of natural radiation exposure, for all people, is about
O.l1r/yr. The average person at sea level in the United States is probably
receiving about 0.16 r/yr.
3.
During 1954-55 the Sr?9 concentration in human bones (both in
adults and in stillborn infants) produced an average exposure to the bones
themselves of 0,002 r/yr. jOnly the bones--not the soft tissues--are exposed
to measurable levels of Sr? irradiation. ) At current fallout trends, the irradiation of human bone by Sr?9 will increase to 0.016 r/yr, perhaps even to
0.038 r/yr (Libby). The maximum value projected in this discussion is
0.2 rfyr. (These are average predictions for the northern hemisphere and fo
the major population densities of the earth. )
ee ren
Ark
4. Radioiodine (1i3!) activity has been measured in humans during
periods of likely fallout exposures. Radiation exposure from fallout I 31 is
essentially nil for humans.