RADIATION STANDARDS, INCLUDING FALLOUT
factor complicates derivation and interpretation of exposure values.
161
Any spe-
cific set of values does not necessarily represent actual risk to the present population, because the population is composed of all ages and is continuously
changing in relation to the selected time interval.
Concentration of Sr® in the skeletal calcium is also dependent on its concentration in the calcium of the diet. Comar (19) has estimated that the concentration in skeletal calcium laid down in thefirst year of life is about one-half
that in the diet. The average Sr” concentration in bone calcium laid down
after the first year or so of life is only 0.25 of that in the calcium of the diet.
The Sr™ concentration in dietary calcium is dependent on its absorption by
the leaves of plants (and, therefore, on fallout rate) and on root uptake from
the soil (dependent on integral surface deposition level}. The most important
factor in Sr” dose evaluation established since the 1959 hearings is the quantitative dependence of dietary Sr™ concentration on fallout rate and integral surface
deposition level. The concentration of Sr® in pe/g of dietary calcium (Qa)
is given by the expression
.
Q2=0.64+0.15B,
in which A is the annual rate of Sr® deposition in millicuries per square mile
per year, and B is the cumulated deposition in millicuries per square mile.
Introduction of this refinement into exposure calculations is largely responsible
for the fact that the present estimates of population exposure dose from Sr”
are no greater than the estimates given at the 1959 hearings, even though an
additional 25 megatons of fission was added to the environment during the
U.S.S.R. 1961 tests.
Another observation since the 1959 hearings which tends to mitigate average
Sr® population exposure estimates is worthy of mention. It now appears that
the accumulated Sr® soil deposit may be undergoing leaching and/or weathering,
decreasing its availability to plant roots at a rate of about 5 percent per year.
This observation is highly significant in that it means that the B term in the
previous expression for dietary Sr” is decreasing at a rate of 7.5 percent per
year instead of 2.5 percent per year from radioactive decay alone. In other
words, the surface deposition component of Sr” dose is decreasing with a half
time of about 9 years instead of 28. However, because this factor is not
sufficiently established quantitatively, it will not be introduced into the present
exposure dose estimations.
Cesium 137
Cesium 137 has a long radiological half-life (28 years), providing a long integration time in the soil, it emits both beta and gammarays, and creates both a
potential external and an internal hazard, both srmatically and genetically.
Internally deposited cesium.—Cesium is chemically very similar to potassium.
a required constitutent in plants and animals. Cesium 137, therefore, tends to
follow potassium ecologically and metabolically as Sr® follows calcium. In the
body, Cstends to deposit in muscle where it has a retention half time of about
140 days, very short compared to that of Sr®. This short biological turnover
time results in rather rapid establishment of equilibrium between body Cs™
and Cs” in the diet with no significant dependence on age.
Since Cs’ emits
gamma rays and the gonads are surrounded by muscle, it creates a potential
genetic hazard. Penetration of bone by gamma rays and deposition of small
amounts of Cs'** in bone result also in a potential somatic hazard to the bone
and bone marrow.
Sinee the 1959 hearings, data from an extensive and intensive 6-year study of
Cs™ levels in the U.S. diet and in the population have been summarized (7).
Two significant conclusions were drawn from these data: (a) The Cslevels
in the U.S. diet and in the population are predominantly dependent on fallout
rate and consequently on the rate of weapons testing and not on the integral
surface deposition level, and (b) the population average genetic, bone, and bone
marrow exposure doses from weapon tests through 1961 are a factor of 2 to 3
lower than predicted at the 1959 hearings, even though the U.S.S.R. has since
‘Injected an estimated 25 megatons of fission into the environment.
External cesium 137 deposition—RBecause of its gamma ray emission, Cs'”
deposited in the environment produces external exposure of the gonads, bone.
and bone marrow. Such exposure is dependent on the integral surface deposition
level and is calculated from the infinite plane dose at 3 feet above the surface,
mitigated by a variety of ill-defined factors, including surface removal (leaching,
weathering, or runoff), shielding (by terrain, buildings, and portions of the body
PERERAHRRBE NESS. ThyeaSASRaookreese cited