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RADIATION DOSAGES Summary table
rads/litetime (USA)
EXTERNAL
doses) suggest that the longer-ranging rays of strontium-90
(and its radioactive decay product yttrium-90) are in fact
only one-fifth or one-tenth as damaging as radium rays,
energy for energy. On the other hand, the concentration of
strontium-90 may show “hot spots,” in the same way as
does radium. And localized damage will tend to reflect not
the average but the maximum insult done to living cells.
Prudence dictates that we increase our estimate of damage
by a factor of five or ten to take into account the possible
effect of the concentrating tendency; that is the practice of
both the ICRP and the UN.
It appears that 1 S.U.* contributes about 1/5 of a rad in
a seventy-year residence in bone. Various experiments
suggest that children whose bones are being formed from
current milk supplies are retaining strontium-90 at a level
between 14 and 14 of the $.U.’s in their milk. On the basis
of these facts we can set a rough maximum “permissible”
burden of strontium-90. If we follow the argument already
made for radium, which seems the one closest to the real
(approximate)
Maximum “permissible” dosage
for the population at large
30
Natural: cosmic rays, soil, rock
4 to 10
Man-made, at present: diagnostic X rays
§ to 10
fallout
up to 4%
other sources, Including TV tubes
up to 4%
Approximate total
10 to 20
Balance to reach “permissible” level
20 to 10
INTERNAL
Natural: carbon-14, potassium-40, radium
2 to 6
Man-made strontium-90 (bone dose):
Maximum “permissible” dosage calculated from the ICRP recommendation
of 100 S.U.
up to 20°
Assuming cessation of bomb tests—
For young children, 5 to 10
years hence
1% toa I*
For aduits (bones mostly formed
in pre-atomic bomb era)
up to 1/10
Assuming continuation of tests at past rate—
For young children, 50 to 60
years hence
5*
situation, we could allow a burdenof nearly 150 5.U. before
*Not taking inte account natural decay or deceleration of bone
growth with age
somewhat arbitrarily-selected factors at several stages in
OTHER FALLOUT PRODUCTS
maximum “permissible” dose might lie somewhere in the
large range from as little as a fifth of the 150 5.U. to as
much as ten times that.
The isotope carbon-14 is a normal component of the air
and of our food; it contributes a little to the normal in.
ternal burden, rather uniformly over the body. But it also
is made in appreciable quantity by all nuclear weapons,
fission or fusion, dirty or clean. These already have contributed a good deal of the stuff to the air, enough to
raise the amount presentin living things by about one-third
of 1 per cent. This isotope is very long-lived, averaging
about 8000 years. In the present generation, the effects of
carbon-14 will be small, compared to the genetic and
possible leukemia-inducing effects of the other components of fallout. But since it lasts such a long time,
it can be shown that its cumulative genetic effect is some
10 or 20 times greater than that of the other fallout components, stretched out as congenital defects of posterity.
Another isotope, cesium-137, is copious in fallout. It
can be detected from within the living body because, like
potassium, it emits penetrating radiation. It bears much
the samerelation to the essential potassium that strontium
bears to calcium. But it is a rather short-time resident of
the body, staying there a matter of months, rather than
reaching the 30 rads which weset as a rough “permissible”
burden for the bones of an average individual. The use of
such calculations may be viewed as suggesting that a
It does not seem safe, however, to apply to the poorly
known and highly variable bone burden, affecting the
whole world’s people, any smaller safety factor than that
used for external radiation. In this case one would again set
the limit at something near 30 rads, several times the
natural internal burden. This gives us our 150 S.U. again,
which agrees well enough with the ICRP limit of 1955,
put at 100 S.U.
But it must be emphasized that any “permissible” level
obviously is predicated on the assumption that there is a
threshold dose for strontium-90 below which any effects
are insignificant. The absence of a threshold would mean
that bone damage would be proportional to the dosage in
any amount, and the concept of a tolerance dose would then
imply accepting as socially tolerable an unknown number
of radiation-diseased individuals.
The summarytable on this page presents average ranges
for radiation dosages, based on authoritative estimates. It
should be emphasized that these figures are approximate.
Even if the population has an average dose rate within the
“permissible” limit, there are many individuals, of course,
who are receiving much more.
*Abbreviation for Strontium Unit. Expresses the number of micromicrocuries of strontium-90 per gram of calcium.
the decades typical for strontium. The overall effect of
cesium is to increase the dose received from radiations
like that of potassium by about 15 percent.
The possibility of damage by other fission-producedisotopes (such as plutonium-239) not yet studied extensively
should not be forgotten; the whole fallout problem raises
new tasks for those who contro] the wholesomeness of
modern foods, water, and air.
For results of CU’s study, see the following pages
CONSUMER REPORTS
107