From the Soil to Man
There is a bewildering variety of radioactive products deposited in the
fallout. Given certain conditions all of them could be dangerous to man.
Actually, very few are.
An example of a radioactive isotope which is produced in large quantity
by the fission process and about which there is some reason to worry,
but actually is not dangerous to man, is iodine¹³¹. This isotope in the
fallout is not dangerous because it has a rather short half-life: eight
days.
During the first weeks after a nuclear explosion some radioactive iodine
may fall out of the cloud and contaminate grazing land. A cow eats
hundreds of pounds of grass in a few days time. Now iodine is found in
the cow’s body or in the body of any mammal mainly in one spot. This is
the thyroid gland located in man near the Adam’s apple. The thyroid
gland is important because it secretes a chemical which regulates many
of the body functions. In man, these include how we burn up our food and
in what mood we are. About twenty per cent of all the iodine which is
taken up, whether radioactive or natural, is concentrated in this one
rather small gland. Such a concentration is precisely the kind of danger
for which we must watch.
Shortly after nuclear tests, cows that graze on range land have been
found with abnormally large amounts of radioactive iodine, although not
so large as to be harmful. In human beings, however, the measured levels
of radioactive iodine are less than a hundredth of what they are in the
cows because by the time this radioactive isotope has reached man, it
has mostly decayed into a stable, harmless variety of xenon gas.
There are many potentially dangerous isotopes in the radioactive debris
of a nuclear explosion. But most of them decay too soon to affect man.
Isotopes which live an extremely long time compared to the human
life-span are also not dangerous to man. A radioactive particle in the
body is not harmful unless it disintegrates and releases its energy
while the individual is still alive.
Two examples of long-lived radioactive isotopes, which are used as fuel
in the bombs and which may be left over from the explosion in large
quantities, are: uranium²³⁵ and plutonium²³⁹. Uranium²³⁵ has a half-life
of 710 million years, which is much too long to be dangerous. Plutonium
has a half-life of 24,000 years and is somewhat more dangerous. The
danger from plutonium arises because it emits an energetic alpha ray.
The danger from radioactivity depends on the kind of particle
emitted—alpha, beta, or gamma rays—and whether these rays attack the
body from the inside or the outside. From the outside the gamma rays are
the most dangerous and the alpha rays the least dangerous. From the
inside the order is just reversed.
To cause damage from the outside the radiation must be very penetrating.
Gamma rays can go through the whole body. Beta rays are stopped in the
skin tissue. Alpha rays cannot even penetrate the outer layer of
non-living, protective skin.
On the inside, however, in the sensitive organs, the short range of the
alpha rays makes them exceedingly dangerous. Their energy is
concentrated in a small amount of tissue to which damage is severe. The
beta rays cause a slightly less concentrated damage, and the gamma rays
the least concentrated of all.
Radioactivity may enter the body as contamination in the food we eat or
in the air we breathe. To be dangerous, however, it must remain in the
body, either in the intestines or the lungs or in other vital organs,
long enough for disintegrations to occur, which will ionize and injure
the living cells.
Fortunately, plutonium in our food is easily excreted from the body.
Only a few thousandths of a per cent of what is eaten, is actually
absorbed. If inhaled, large particles are stopped in the nasal passages.
Small particles get into the lungs but are quickly exhaled. Only
intermediate sized particles are absorbed. However, the plutonium which
is absorbed generally gets laid down in the bones, where it stays for a
long period of time. Altogether, plutonium in the small amounts we
usually deal with is not one of the greater dangers to human beings.
Perhaps its most disagreeable property is that, being an alpha emitter,
it is not very easy to detect. Since alpha particles do not penetrate
through the surface of most radiation meters, special instruments are
needed to find them.
Two fission products which are readily absorbed upon ingestion are:
strontium⁹⁰ (Sr⁹⁰) and cesium¹³⁷ (Cs¹³⁷). Depending somewhat on their
chemical form, approximately thirty-five per cent of the Sr⁹⁰ is
absorbed, and all of the Cs¹³⁷ is absorbed. Both of these isotopes are
plentifully made in the fission process. Moreover they have very
“dangerous” half-lives—about 30 years—which is long enough so that decay
is negligible between the explosion and contact with man, but short
enough so that decay is probable after contact.
From such arguments as these one concludes that Sr⁹⁰ and Cs¹³⁷ are the
most important isotopes for the internal hazard from the world-wide
fallout. One can be reasonably sure that there are no others of
importance, because careful and extensive research has not found
significant amounts of any in our bodies. We need not fear that one has
been overlooked, because the beta activity of the fission products is
always easy to detect.
The two main questions which we have to answer are these: In what
precise way will the dangerous elements Sr⁹⁰ and Cs¹³⁷ be distributed in
the body? And after they are distributed, what kind of damage will they
produce?
We know too little about the chemistry of the living body to obtain a
complete answer to the second question. Hence it has to be admitted that
the actual danger cannot be stated in a precise way.
Fortunately, enough is known from direct experience to obtain a good
value for the greatest damage that might be produced. In the present
chapter we shall describe what is known about the uptake of the
dangerous elements into the body. In following chapters we shall turn to
the question of the biological consequences.
We may begin by comparing the danger from Cs¹³⁷ with that from Sr⁹⁰.
Both of these isotopes are made in the fission process in about equal
numbers. (Roughly 2 or 2½ per cent of all the fission products are Sr⁹⁰,
and 3 per cent Cs¹³⁷.) They have approximately the same radioactive
half-lives. But they differ in an important respect: The Cs¹³⁷ is
deposited more or less uniformly throughout the body; the Sr⁹⁰ is
concentrated in the bones.
Cs¹³⁷ emits a large part of its radioactive energy in the form of a
gamma ray, which causes ionization uniformly in the body. Sr⁹⁰, on the
other hand, emits all of its energy in the form of two beta rays, which
have ranges of only a small fraction of an inch in the bone. Thus in the
one case the radioactive disintegration energy is distributed in the
whole body; in the other, the energy is deposited in the bones only.
Since the bones comprise about ten per cent of the total body weight,
they are subjected to ten times the radiation dosage. The bones are
quite sensitive to radiation, and an overdosage can cause bone cancer
and interfere with the production of blood cells that goes on in the
marrow. Thus we are led to the conclusion that Sr⁹⁰ is a far greater
potential hazard than Cs¹³⁷. A further point, which leads to the same
conclusion, is that Cs¹³⁷, after being absorbed, is retained in the body
less than six months and then excreted. Sr⁹⁰ is retained for many years.
On the other hand, Cs¹³⁷ can cause a type of damage which Sr⁹⁰ cannot
cause: namely, damage to the reproductive cells. The effect of Sr⁹⁰ is
indeed limited to the bones and adjacent or nearby bone marrow, and does
not reach the reproductive organs. In a later chapter we shall take up
the question of genetic danger, and then we shall be very interested in
Cs¹³⁷. For the remainder of this chapter, however, we may focus our
attention on Sr⁹⁰.
Since a large fraction of the Sr⁹⁰ which enters the body stays there,
the most important questions which remain are: how it gets there and how
much gets there. The essential fact in this connection is that the Sr⁹⁰
generally occurs in the fallout in a chemical form which is easily
dissolved in water. The water is taken up by plants, by absorption
through the leaves and the roots. Animals graze on the plants. Human
beings eat the plants and drink the milk from the grazing animals, and
thus become exposed to Sr⁹⁰. (See pictures 5 and 6.)
One might worry because Sr⁹⁰ is not a naturally occurring isotope but
has been made for the first time by man in the fission process. Here is
an unfamiliar poison being scattered over the earth. Can we have any
idea how much will be taken up by human beings?
The answer depends on a fact which we have emphasized throughout this
book: that isotopes of the same element are chemically and biologically
indistinguishable. The radioactive variety of strontium will behave
exactly like the stable natural variety. In particular, the ratio of
Sr⁹⁰ to stable strontium in the human body must be the same as this
ratio is in our food. From this premise we can predict how much Sr⁹⁰
will reach the human body.
From the total yield of fission energy released in all nuclear tests to
date, one can calculate exactly how much Sr⁹⁰ has been produced. This
amount turns out to be about 100 pounds.
Approximately one half of this amount has been deposited in and near the
test sites in the close-in fallout. (Most of the radioactivity comes
from the big bombs, and most of these have been burst on the ground or
over shallow water.) A small portion of the 100 pounds has disintegrated
in the cloud. The remainder, roughly 50 pounds, is partially still in
the stratosphere and partially has been disseminated around the world in
the tropospheric and stratospheric fallout. At the present time
measurements show that 25 or 30 pounds have actually been returned to
the surface of the earth. Local values vary from about one third to more
than twice the average world-wide value.
In the northern part of the United States, in the regions of frequent
rainfall, the measured values are about twice the world-wide average. In
the latitudes between 10°S and 50°N the average value is about 50 per
cent greater than the world-wide average. For the rest of the world one
finds, with some variations, about one third the world-wide average.
Most of the Sr⁹⁰ fallout is caught in the top two or three inches of the
soil. It exists there in a water-soluble form that is readily
assimilated by plants. Also in the soil, chemically inseparable from the
Sr⁹⁰, is stable natural strontium. Plants, animals, and human beings
have no way of distinguishing between the two.
It is not easy to determine how much natural strontium is in a form
which is available to the plants. Some of the natural strontium is
insoluble; and some is below the root depth. Our best estimate is that
there are about 60 pounds per acre actually available for uptake by the
plants. This is, of course, an average.
The amount of natural strontium in the human body is a quantity we know
rather well. It has been carefully measured and is about 0.7 gram in the
average adult, with proportionately less in children. Now since we know
how greatly Sr⁹⁰ has been diluted in the soil and how much natural
strontium there is in our bodies, we can calculate the expected quantity
of Sr⁹⁰ in our bones. Considering the many uncertainties in the
calculation one should not expect too good an agreement. The remarkable
fact is that the quantity of Sr⁹⁰ measured in small children does agree
with the calculated amount. For adults the measured value is quite a bit
less than the calculated amount because adult bones have been made for
the most part before there was any Sr⁹⁰ in the environment.
The fact that we can calculate how much Sr⁹⁰ is at present in the body
is most important because it gives us confidence that we understand what
is happening. It is especially important for us to understand what is
happening so that we can predict how nuclear tests which are carried out
today will affect future levels of Sr⁹⁰ in the body.
From arguments such as we have given, plus a record of the Sr⁹⁰ content
of bones over the last several years, it seems unlikely that the level
of Sr⁹⁰ will increase by more than a factor of two or so due to tests
already conducted. Actually this factor may be even smaller both because
of the mixing of the strontium with the deeper layers of the soil, and
because the radioactive strontium which stays in the ground for a long
time tends to become chemically less soluble and mixed more thoroughly
with that part of the natural strontium which is chemically unavailable.
This latter process is called “chemical aging.”
To follow radioactive strontium and normal strontium from the soil into
the food and the bones is not an easy matter. We must worry about the
question of the strontium depth in the soil and the chemical form of the
strontium. The complete identity of Sr⁹⁰ and normal strontium holds only
if both are near the same place and in the same chemical form. A further
difficulty is that until recently little was known about the behavior of
normal strontium and knowledge is accumulating slowly.
Much more is known about calcium. Now calcium and strontium do not
behave in an identical way, but they do behave similarly. In passing
from soil to man the ratio of calcium to strontium does not remain the
same but at least it changes in a more or less definite manner. Actually
most work on Sr⁹⁰ uptake has been done by comparing Sr⁹⁰ with calcium.
In order to use the data on calcium one has to find out how the calcium
to strontium ratio is changed when the material is taken up into the
human body. In the soil there is, on the average, about 1 part of
strontium to 100 parts of calcium. In the human body the ratio is about
1 to 1400.
Thus the strontium is discriminated against relative to calcium in going
from the soil to man by a factor of about 14. This is a factor of
protection.
It is good to double-check this conclusion and to find out how the
calcium to strontium ratio changes step by step in going from the soil
to man. One finds a factor of 1.4 in going from the soil to the plant, a
factor of 7 in going from the plant to the milk, and a factor of about 2
in going from the milk to man. Actually, if we put all these factors
together we should expect that on the way from the soil to man the
calcium to strontium ratio increases by a factor 20. This is in
reasonable but not in excellent agreement with the ratio 14 given above.
Once the factor of protection is established we can get a value of the
expected strontium uptake from the way in which the radioactive material
is diluted by calcium rather than by normal strontium. This is a less
straightforward but, for the time being, a more practical method than
the direct Sr⁹⁰—normal strontium comparison. It is particularly
important when one compares soils of rather different calcium content.
Plants and animals require calcium. When they do not get it, they
develop a calcium-hunger. Since strontium is chemically similar to
calcium, a lack of calcium in the soil is readily substituted by
available strontium. One would expect that plants grown on calcium-poor
soil and animals raised on such land would exhibit abnormally high
natural strontium content and also a proportionately high Sr⁹⁰ content.
The high Sr⁹⁰ content has in fact been verified. Some sheep in Wales,
for example, appear to have about ten times the average amount of Sr⁹⁰
in their bodies.
Fortunately most people derive their food from many areas widely
separated from each other. Soil that is deficient in calcium is not
likely to supply more than a small part of an individual’s sustenance.
However, the possibility of a large fluctuation cannot be ignored. In
this event corrective measures would be needed. One simple measure would
be to fertilize deficient soil with additional calcium.
That soil can be successfully treated in this way is illustrated by the
present situation in Wales. The sheep with the abnormally high Sr⁹⁰
content all come from the steep, poor pastures which are not limed. The
sheep from the lower pastures, which are limed (not because of the
fallout but for economic reasons), show an activity of only one third
the value mentioned above.
The point we have tried to make in this chapter is that the present
human levels of Sr⁹⁰ can be satisfactorily accounted for by simple
arguments based on the chemical similarity of elements and the identity
of isotopes. These arguments give us confidence that we correctly
understand how Sr⁹⁰ and how much Sr⁹⁰ is getting from the soil to the
human body.
At the same time we have seen how many factors influence the eventual
uptake into the human body: geographical latitude, frequency of
rainfall, the chemical form in which strontium is found, the calcium
content of the soil, the method of agriculture. Even though the United
States has pushed this investigation vigorously since 1952 the bulk of
the work is still ahead of us.
For instance, in the United States, dairy products provide most of the
calcium and strontium in our diets. In Japan, however, the situation is
somewhat different. There the main source of calcium and strontium is
rice. As a result, the ratio of strontium to calcium may be passing
differently from the soil to man. Also the fallout strontium might be
washed deeper into the soil and the soluble to non-soluble ratio might
be different.
Considering the complex nature of the Sr⁹⁰ uptake into man, it is
important to keep close track of the actual Sr⁹⁰ levels in the soil, in
our food, and in our own bodies. The following graphs show how these
levels have risen in the last several years due to the bomb tests:
[Illustration: Sr⁹⁰ in the soil—measured in thousandths of a gram
per square mile.]
[Illustration: Average Sr⁹⁰ in U.S. milk—measured in trillionths of
a gram per quart.]
[Illustration: Average radiation doses from Sr⁹⁰ in bones of young
children (U.S.)—measured in roentgens per year.]
The actual amounts of Sr⁹⁰ in the soil, in the milk, and in the bones of
young children are only approximately known. But the main point that we
are trying to illustrate, is that since 1954 the buildup of Sr⁹⁰ has
gone on at a rather steady rate. How far will this buildup continue?
More radioactivity was released in tests in the year 1954 than in all
other years put together. Probably more than one-half of that activity
has already been deposited. Since that time the fission energy produced
in U.S. tests has steadily decreased. Furthermore, we have learned how
to minimize the world-wide fallout by employing ground bursts which
deposit most of their activity in the close-in fallout near the test
site. It is also possible to place chemical additives near the bomb in
order to convert the strontium into a more insoluble form or else into a
form which will more readily fall out in the immediate neighborhood of
the explosion. And what is most important—we are developing clean
nuclear weapons, which produce blast and heat but greatly reduced
radioactivity. In the future these clean weapons may eliminate the
additional radioactivity altogether.
It is hard to make predictions about the plans of all nations. If we
find—and others also find—that clean weapons are the most desirable, the
total strontium contamination is not likely to become more than perhaps
two to four times the present value. We believe that all reasons—respect
for human life, military considerations and simple sanity—lead to one
conclusion. In the development of nuclear explosives we must endeavor to
make them clean. But the real reason for this does not lie in the small
contamination due to tests. The real reason is that war could turn
contamination into a danger to countless people.