SSSR PRUEPST SREB 2 TY vo
RADIATION STANDARDS, INCLUDING FALLOUT
91
This meansthat the rate-dependent process will be dominant. The
level of Cs**7 in the diet has not fallen as rapidly as expected from the
decrease in fallout rate, probably because of a holdup in the root mat
of pastures,
For future predictions it is probably best to base estimates of the
cumulative component on comparison with Sr®° assuming that ab-
sorption of Cs'*? from soil is less by a factor of 10 to 25, but that
efficiency of secretion into milk is greater by a factor of 10.
The next chart shows the same type of approach for cesium 137,
wherethe factors are 1.4 for the rate-dependent component and 0.05 for
the cumulative-dependent component.
I must say, however, that this relationship is on nowhere near a
firm a basis as the one for strontium. This 1s about the best we can
do with the data at hand. It does bring out the point, though, at
steady state one must not ignore the cumulative component because
in the years to come the amount of cesium we see in the diet may come
almostequally from the soil as comparedto rate.
I would like to say a word about radioactivity in the total diet
simply to give you a comparison of relative daily intakes of various
radionuclides both naturally and man made.
The next chart, please. One notices that the daily intake expressed
in terms of picocuries of natural potassium 40 is about 4,000 picocuries
per day; cesium 137, about 50 in 1961; strontium 90, about 10, cerium
144, about 4; lead 210, which is a natural radionuclide, about 4; radium 226, which is a natural radionuclide, about 2; and plutonium
239, estimated at one-tenth.
Representative Price. Which of this group, aside from those you
have gone into already in detail, do you consider of some importance?
Dr. Comar. The Prediction Panel later on will present some dose
predictions which will answer your question quantitatively.
There is no question that strontium 90 and, from the short-term
standpoint, iodine 131 are the most important. Cesium 187 is of much
lesser importance. The others are of even lesser importance.
_It should be noted, of course, that possible effects on the population.
Chairman Horirrenp. Let me ask you a question. I am not quite
sure.
Why do you rate potassium, K-40, along with these other radioactive materials? Is it radioactive, also?
Dr. Comar. Potassium 40 is radioactive.
Chairman Horirteip. It comes in the food naturally.
Dr. Comar. It isin the food naturally.
Chairman Houtrrerp. By afactor of 4,000 units or picocuries as
against 40 from cesium 137.
Mr. Ramey, That would be natural background ?
Dr. Comar. Yes.
Representative Price. Whatis the half-life?
Dr. Comar. About 10 to the ninth year, I believe.
Representative Hosmer. It is excreted by the body ?
Dr. Comar. It is turned over by the body with a half time of
something like 60 to 80 days.
Representative Hosmer. The difference, then, is that these other
elements tend to remain in the body a longer period of time?
86853 O—62—pt. 1——7
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