STRONTIUM-90 IN MILE
In making comparisons of radiation sources, it must be taken into account
that Sr-90 radiation is localized in the bone andall of the other sources of external radiation can not be compared directly because of différences in penetrating power. However, Libby (74) has pointed out that it is possible to compare
yn ineom-
rage disdiet need
directly Sr-90 and casmie ray radiation at the bone level. The f-particles elimi-
1 lonizing
many in-
nated by Sr-90 and Y-90 decomposition have a tissue penetration of about 2 mm.
and this is localized in the bone wherecell formation is taking place. On the other
hand, cosmie rays penetrate the whole body, and the ionization density along
the tracks of the ~-mesons, which are the principal cosmie ray components to
consider in the lower air, is similar to that of the Sr-90 and Y-90 decomposition.
Hence, it is possible to equate cosmic-ray dosage and Sr-90 dosage directly, and
express cosmie ray radiation in terms of Sunshine Units. Libby (14) notes that
the annual cosmic-ray radiation dosage at Washington, D. C. (or any place at
sea level at this latitude) is about 37 mr per year and at Denver, Colorado
(5,000 ft.), 60 mr per year. This Is equal to about 23 mr per year difference, or
is equivalent to 8 S.U. (1 8.U. is about the same as 3 mr per year). The present
level of Sr-90 in the bones of children is between 0.5 and 1.0 8.U. and, if testing
were to continue at the same rate indefinitely to equilibrium, the level of Sr-90
in human bone would be in the range of 4-21 8.U. (8, 11,14}. This level compared to 8 8.U. derived from cosmic rays at Denver over Washington, suggests
that if the level were strong enough to cause leukemia and bone cancer there
should be a correlation of such diseases with altitude. Examination by Libby of
Public Health records showed that this is not the case (74).
It is apparent that the present levels of Sr-90 in bone are contributing a small
amount of radiation compared to other sources. If testing were to continue for
the next 50 yr. at the same rate, the contribution from Sr-90 in bones would be
about equivalent to moving from sea level to 5,000 ft. altitude.
At present, the maximiim permissible dose of Sr-90 in bone is 100 8.U. for
the average population. The present average level in children is less than 1%
of this and will rise only to 4-20% of this, at the extreme, if testing continues.
One should also not underestimate the abilities of the atomic scientists. Since it
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is the U-235 of the atomic bomb detonator which produces Sr-90, and not the
hydrogen bombitself, the developmentof a ‘‘clean’’ means of starting the hydrogen bomb would lower or eliminate Sr-90 formation. The radiation produced
from a hydrogen bomb is chiefly due to tritium, which is an extremely weak 8
particle emitter. Attempts are now being made to develop such a system.
Genetic effects of radiation. While this review has been coneerned chiefly
with the significance of Sr-90 in milk, it is unwise to neglect the genetic considerations of the fallout problem. This is necessary because there is not a clear
division point between the various phases of the fallout problem. As a source of
radiation to the gonads, the Sr-90 derived from food and present in bone is of
small consequence because of the low level present and the short penetrating
power of the 8 particles. However, the fallout of other isotopes produced in the
fission reaction has increased the background radiation. Although this increase
[13 ]