Strontium—90:
decades. Some of these were young women who painted
the small numerals on luminous dials, and tipped the
brushes between their lips for the fine work. Some were
suffering from rheumatism or other diseases, and were given
radium as a therapy by some sincere physicians and by
more quacks, in the 1920s before the grave danger
was recognized. In all of these groups, there have been
seen malignancies of bone and of blood—evident consequences of the radium irritation.
Whatis striking is that the incidence of these conditions,
rare in the total population, was large enough in the small
numberof radium-poisoned patients to be recognized without chance of error.
There is a good chance of developing
a bone tumor, or leukemia, if you fix in the bones a mere
speck of radium; less than one microgram, an amount
1/30,000,000 of an ounce, weighing less than the dot over
an inked i. It seems very probable that, if there is a safe
threshold at all here, it is below a tenth of a microgram, an
amount which is a few hundred or a thousand times greater
than that found in the normal skeleton. The present ICRP
recommendations allow a tenth of a microgram of the stuff
for workers in radiation industries, and such an allowance
does not preclude the possibility of undetected subclinical
changes. If we recall that there may be no threshold for
somatic effects, but a straight-line relationship, and that individual responses are variable, it is reasonable to include
at least a factor of 10 for safety in setting the “permissible”
internal bone burden for the population at large. The
UN places the normal average radium dose to the bone at
a value of about 3 rads over 70 years. Somewherein the
range from ten to a hundred times the background value the
“permissible” valuelies.
Nowthere enters a new factor: the element strontium.
f
\HE element strontium is relatively uncommon; its
most common use is for the rich red of its flame
in fireworks and flares. Like radium, it also resem-
bles calcium and it also becomes stored in bone.
The normal skeleton of man has about two pounds of
calcium, and with it from soil and rock there has always
been deposited about a tenth of a teaspoonful of strontium.
Normal (pre-atomic-bomb age) strontium presents no
hazard, so far as anyone knows; it is not radioactive.
But every explosion of a nuclear bomb spreads into the
high air with its fireball a new radioactive isotope of
strontium, strontium-90. It comes from fission, and the
old-style fission bomb and the new-style fission-fusion-
fission bomb both contribute strontium-90. (The yet undeveloped “clean” bomb will contribute proportionately
less as it makes less use of fission energy; no one has yet
discovered how to eliminate the stuff entirely.)
So far some 200 pounds of strontium-90 have been
garried aloft by the churning hot gases of the mushroom
explosions. It has spread worldwide. It falls out of the
upper atmosphere, in a manner not yet fully understood.
It comes down slowly with rain and snow, on river and
reservoir, plant leaf, and soil, Some of it is taken up chemically through the roots of plants to become part of grass
or seed, and eventually part of the glass of milk or the
rice in the bowl. In the U.S., about 80 per cent of the minute quantity which enters our bodies comes to us in our
milk. In Japan more than two-thirds comes from rice. Un-
polished or whole-wheat, unfiltered rain water, and similar
items of diet, such as vegetables, may also contribute a
significant amount of strontium-90. If there never are any
AVERAGE STRONTIUM-90 IN BONES
Units
more explosions, the fallout from tests already made will
reach a maximum about 1970, rising to two and a half or
Children,
three times its present value, and then will decline slowly
for a generation or two.
IN
SSCHENCE”:
127,
269.
1988
There is not much of it. The load of strontium-90 in
0.2
5
0.1]
0
Adults
:
memes
1954-55
1955-56
1956-57
Notable amounts of strontium are absorbed only by
growing bones. Averages above were calculated on
the basis of data compiled from four U.S. cities
106
MARCH 1959
$
your front yard is likely to be about one-tenth of a microgram. It is spread down into the topsoil by rain and plant
growth and hoe and worm; how much is taken up by
plants depends on a wide variety of conditions—upon the
depths of their roots, the chemistry of the soil, the distri-
bution of rainfall, and so forth. But it is out there, and it
enters the food chain and, from there, the bone.
Amongall the scores of constituents of fallout, strontium90 is especially hazardous because it is released in con-
siderable quantity (it forms a few per cent of all radioactive products of fission), it seeks the bone, and it stays
there a long time. Some of the other radioactive products
decay so quickly that they cannot pass through the long
chain of events to reach the target; some which have a long
life do not accumulate in the body; but strontium-90 lasts
on the average about 40 years and it accumulates in bones.
What wili strontium-90 do in bone? Both general
theory and direct animal experiments (with rather heavy