CHAPTER XIFrom the Soil to Man

All of the exposed persons were evacuated to Kwajalein as soon as the Task Force facilities would permit. But it was not until a week or so after the explosion that arrangements could be made for men with radiation measuring instruments to tour the atolls and determine what the levels of exposure had been.

On the southern tip of Rongerik they measured the activity and calculated that the American servicemen had received approximately 78 roentgens. This was good news because a dosage of 50 to 100 roentgens is not lethal and only in rare cases leads to any sickness. In any event full recovery could be expected within a few days.

As they prowled around Rongerik atoll, the measuring crew found places where the radiation levels had been much higher. At the northern end a person would have received more than 200 roentgens.

On Ailinginae the measured values were comparable to those on Rongerik. The estimated dosage to the Ailinginae people was 69 roentgens.

On Rongelap the situation was much worse. Measurements in the southern part of the atoll showed that the Rongelap people had gotten a dose of about 175 roentgens. Such a dose would not be fatal, but at least some of the people would probably be sick.

The crew then went on to explore the rest of the atoll. As they moved north, the dose levels rose higher and higher. In the middle of the atoll, only ten or fifteen miles from the inhabited part, a person would have received 400 roentgens of radiation. At this level he would have a fifty-fifty chance of surviving.

On the northern tip of the atoll, about thirty miles away, the dose would have been over a thousand roentgens. Such a dose means certain death in less than a month.

The following table contains a summary of what happened:

On Kwajalein the Marshallese were cared for and underwent medical observation. As soon as possible their skin and hair were scrubbed with soap and water. The coconut oil in their hair made decontamination difficult.

During all this time the presence of the Japanese fishing boat in the area was not even suspected. Not until two weeks after the explosion, when the little boat returned to Yaizu harbor, did the world find out. By this time the 23 fishermen were pretty sick. We do not know precisely what dose the fishermen received, but the best guess is about 200 roentgens. Unhappily, one of the fishermen died, presumably from complications associated with the exposure to radiation.[10]The other 22, however, are in good health and back at work.

Our medical information on the Marshallese islanders is complete. After staying three months on Kwajalein they were removed to Majuro atoll, where homes were built for them and where they have been cared for and under continuous surveillance since the incident. Frequent and thorough medical examinations have been conducted, handicapped somewhat by the problem of communicating through an interpreter.

In the first twenty-four hours some of the victims complained of nausea, fever, and stomach-ache. But these symptoms abated promptly in every case without treatment. There was also some complaint of skin itching and a burning sensation, but these symptoms also lasted only a couple of days.Then followed a week or so of comfort and no complaint. After that skin lesions and loss of hair began to occur.

Fifty to eighty per cent of the beta rays during the exposure period had an average energy of 0.3 million electron-volts. Much of this energy was stopped in the outer layer of skin, which is two thousandths of an inch thick. The remainder of the beta rays had an average energy of 0.6 million electron-volts; these beta rays could easily penetrate into the deeper layer of live skin. The most important fact, however, was that clothing of any kind, even a thin cotton fabric, provided protection against all the beta rays. Lesions developed only on the exposed parts of the body and in a few other places such as the armpits and the creases of the neck where material tends to accumulate. Bare feet were especially bad. During the acute period some of the people walked on their heels.

At the end of six months lost hair had grown out again unaltered in texture and color, and the skin lesions had healed. Everyone appeared healthy and normal with no apparent after effects.

There had been four pregnancies amongst Rongelap women at the time of the exposure. One baby was born dead, but the other three were quite normal. There was no evidence that the stillbirth had been due to radiation effects. In fact the percentage of stillbirths amongst the Rongelapese is normally high. Statistically, one in four is not an unusual ratio.

Today, more than three years since the accident, all of the Marshallese and American victims seem to be fully recovered. No malignancies or leukemias have shown up, but these long-term effects are still being carefully watched for by an AEC medical group.

All in all some serious but limited harm has been done. It was a close shave. To see how close, one only needs to glance at the map below, which shows the roentgen dosage for 48 hours of exposure. At the southern tip of Rongelap, wherethe inhabitants lived, the dosage was 175 roentgens. But at the northern tip, less than thirty miles away, the dosage was more than a thousand roentgens. If the wind had veered just a little bit farther to the south, probably all of the people on Ailinginae, Rongelap, and Rongerik would have been killed.

Dosage in First 48 Hours After Fallout Began

This shot proved what had been argued for many years: that radioactivity is not just an incidental part of an atomic explosion. The people on Rongelap were far outside the area of danger from blast and thermal effects. But they received a sizeable dose of radiation. In fact, a person could have stood unprotected at a distance of thirty miles from the explosion and been perfectly safe from the blast and thermal radiation. But at that same distance in a downwind direction he would have accumulated a lethal dose of radiation within a matter of minutes after the fallout began.

Because of the radioactive fallout, the test sites must be located in remote parts of the world. It would be desirable if sites could be found which are so remote from populous areas that the tests could be conducted without regard to the direction of the winds. Unfortunately the bombs are too big and the planet is too small.

As a result the winds must be watched before every test; and the tests must be delayed until the winds are favorable. What happened to the Marshallese was an accident which might have been avoided if the winds had been blowing more directly toward the north at shot time. Since this accident the wind requirements for the tests have become far more stringent, our knowledge of the danger has increased, and the rules of safety have in all respects improved. Many large yield weapons have been tested since March 1, 1954, but no other accidents have occurred. We can be confident that accidents of this kind are now very improbable.

At the U. S. test site in Nevada there has been no instance of a major fallout on a populated area. Probably the most worrisome situation which has occurred there was in the spring of 1953 during the Upshot-Knothole test series. After the ninth shot of the series the cloud drifted eastward over St. George, Utah, a town of about 5000 people. Some fallout occurred shortly before nine o’clock in the morning. About nine-thirty AEC officials issued a warning advising the residents to stay indoors. By noon the warning was withdrawn and people were allowed to continue with their normal affairs. The incident left everyone a little bit scared, but no one had received a radiation dose greater than two or three roentgens.

We have been talking about the local fallout which occurs within a few hundred miles of the test site. Not all the radioactivity which is made in the explosion goes into this fallout. Some of it travels for really long distances, not hundreds but actually thousands of miles from ground zero. This part of the radioactivity is disseminated world-wide and completely escapes the control of man. To be sure, by the time this radioactivity is distributed over a large fraction of the earth’s surface, the dosage levels of radiation are very tiny, less than a ten thousandth of a roentgen for a megaton explosion. There is no danger whatever that a person would die or even becomemildly sick from this amount of radiation. There is, however, the possibility of long-range effects such as bone cancer, leukemia, and genetic mutation.

The world-wide danger is, of course, primarily due to the big bombs. The little ones, such as are tested in Nevada, release about ten kilotons (TNT equivalent) of fission energy. Some of the big ones in the Pacific release a few megatons of fission energy. Since the amount of radioactivity is proportional to the fission energy released, one big bomb is equivalent to several hundred or possibly a thousand little ones. Altogether in Nevada, to date, there have been only sixty or seventy shots. It may be desirable to minimize the world-wide fallout from the big shots in the Pacific. But for the little shots in Nevada, it is probably more important to minimize the local fallout. How much radioactivity goes into the local fallout, how much into the world-wide, and how these relative amounts can be controlled, are the main topics for the remainder of this chapter.

Not all the radioactivity which is made in the explosion contributes to the fallout, either local or world-wide. Some of the radioactive fission fragments (gamma emitters) have such short half-lives[11]that they actually disintegrate before the bomb has disassembled. A great many others disintegrate in the first few minutes while the atomic cloud is rising. The energetic beta and gamma rays released in these early, rapid disintegrations are stopped in short distances and merely add to the havoc at the scene of the explosion.

USAEC—Joint Office of Test Information1. A shallow underground explosion. The radioactivity and the ground dirt are thoroughly mixed.

USAEC—Lookout Mountain Laboratory, USAF2. An atomic test tower—five hundred feet high.

USAEC3. A tower shot. Ground dirt rises along the stem, but very little actually mixes with the fireball.

Elton P. Lord—USAEC4. An air shot—3,500 feet above ground. No dirt.

5. Leg bone of a three-month-old rabbit killed ten minutes after injection of Sr⁸⁹. The darkened areas show where the strontium has been deposited. Sr⁹⁰ and normal Sr⁸⁸ would be deposited in the same places. It is an important fact that the deposition is fairly uniform in the calcified portions of the bone.From a chapter by Vaughan, Tutt, and Kidman in the bookBiological Hazards of Atomic Energy,edited by Haddow, published by Oxford University Press, 1952

From a chapter by Vaughan, Tutt, and Kidman in the bookBiological Hazards of Atomic Energy,edited by Haddow, published by Oxford University Press, 1952

6. Leg bone of a woman who died of radium poisoning. The bright regions show where the radium has been deposited. Hot spots are clearly visible.From an article, “The Late Effects of Internally Deposited Radioactive Materials in Man,” by Aub et al., inMedicine—a professional journal, Vol. 31, No. 3, September, 1952

6. Leg bone of a woman who died of radium poisoning. The bright regions show where the radium has been deposited. Hot spots are clearly visible.

From an article, “The Late Effects of Internally Deposited Radioactive Materials in Man,” by Aub et al., inMedicine—a professional journal, Vol. 31, No. 3, September, 1952

USAEC—Knolls Atomic Power Laboratory7. Capsules of cobalt⁶⁰, shielded in a water tank. One hundred and thirty million dollars’ worth of radium, twice the world’s present supply, would be needed to equal the rays from this powerful gamma source.

USAEC8. Cobalt irradiation.

1. The metallic element cobalt is machined into wafers slightly larger than a dime.

2. The wafers are placed edge to edge in aluminum containers, then inserted into an atomic furnace, or reactor.

3. Under bombardment of neutrons, the nuclei of the cobalt atoms become excited and emit radiation, or rays.

4. After “cooking” in the reactor a certain time, the cobalt is removed and placed in shielded containers for shipment.

5. The now radioactive cobalt goes from the Savannah River Plant to Oak Ridge for re-shipment to medical centers all over the country.

6. At medical centers, it is placed in tele-therapy machines. Its powerful rays aid medical specialists in the fight against cancer.

NTO—Lookout Mountain Laboratory Photo9. The smoke-ring cloud from the air-defense atomic weapon.

Wide World Photo10.

University of California Radiation Laboratory11. The streaks are condensation trails produced by charged particles in a Wilson Cloud Chamber. They appear bright because the chamber is illuminated and the condensation trails reflect light just as an ordinary cloud does.

University of California Radiation Laboratory12. Another picture in the Wilson Cloud Chamber. A large number of closely-spaced tracks form a cloud. (The tracks are curved because of the presence of a magnetic field.)

USAEC—Argonne National Laboratory13. Cutaway section of a nuclear reactor. The heart of the reactor is a small region at the center where the fission energy is generated. Most of the weight and volume are needed for cooling apparatus and shielding material to keep in nuclear radiation.

For the radioactivity to affect areas at a large distance from the point of the explosion, considerable time must elapse while the atomic cloud rises and drifts in the horizontal winds. During this time more disintegrations occur, due mainly to the short-lived nuclei. The rate at which they occur keeps diminishing as the short-lived nuclei disappear. Roughly speaking, the rate diminishes simply in proportion to the time. More precisely, the rate drops somewhat faster, decreasing by a factor of ten when the time increases by a factor of seven. A minute after the explosion the activity is less than one per cent of what it is at a second. After an hour it is less than one per cent of its value at a minute. This law for the decrease in activity of fission products is, of course, quite different from the simple law of radioactive decay. The latter law applies to a single radioactive species. The fission products consist at any instant of many different radioactive species. Each one obeys the simple law of radioactive decay, but the totality follows a different law.

It should be kept in mind that the product nucleus of a radioactive disintegration may itself be radioactive with a different half-life. For example, there is strontium⁹⁰. Only a small amount of this isotope is made directly in the fission process. The fission process yields large quantities of krypton⁹⁰, which decays with a half-life of one-half minute into rubidium⁹⁰. The latter has a half-life of three minutes and decays into strontium⁹⁰. This is how practically all of the strontium⁹⁰ is made in the explosion. Thus both the intensity and the nature of the radioactivity keep changing with time.

These facts are important because they determine the magnitude and the character of the danger when the radioactivity finally falls out of the cloud and is deposited on the surface of the earth. Those radioactive particles which disintegrate while still in the cloud need not worry us since this radiation can have no effect on living organisms that may be underneath. Provided that the cloud is more than a few hundred feet above the ground, the beta and gamma rays released in these disintegrations merely dissipate their energy in ionizing the air.

The time which the radioactive debris spends in the cloud depends most critically on one factor: the proximity of theexplosion to the ground surface. The nature of the surface, whether it is soil or water, also plays a role. If the explosion has taken place right on the ground, on a soil surface, a lot of big, heavy dirt particles become incorporated into the fireball and begin to fall under the action of gravity even before the cloud stops rising. This fallout continues for a period of several hours to perhaps a half day. At the same time some of the radioactive fission products which have adhered to these dirt particles also fall out. This is the origin of the so-called close-in or local fallout, which extends for a distance downwind of the explosion of a few miles to a few hundred miles, according to the energy of the bomb and the strength of the winds. Approximately eighty per cent or so of all the fission products are accounted for by this close-in fallout in the case of a surface explosion. The shot on March 1, 1954 was of this variety.

There are several possibilities for influencing the amount of close-in fallout. One is to explode the bomb over deep water. In this case the close-in fallout amounts to between thirty and fifty per cent. This is because many of the water drops to which radioactive particles have adhered evaporate before they hit the ground. Over shallow water, however, if the fireball actually touches the bottom, the close-in fallout resembles the case of a land explosion and is again about eighty per cent or so. The close-in fallout for underground or underwater explosions will be even higher than for the surface explosions. In fact a really deep underground or underwater explosion would be completely contained and no activity would be spread around.

Another possibility for reducing the close-in fallout is to detonate the bomb on a tower so tall that the fireball cannot touch the surface. In this case the amount of close-in fallout is reduced from eighty per cent to approximately five per cent. Of course, it is not feasible to build towers for really big bombs whose fireballs may be a mile or so in diameter. Inthis case the bomb might be dropped from an airplane to produce the same effect. The Hiroshima explosion was an example of an air burst of a small bomb. The close-in fallout in that case was very small. Such radiation sickness as occurred there was due to the direct gamma rays and neutrons released in the explosion itself.

In the case of a near-surface explosion, where the fireball almost touches the ground, the close-in fallout is also only about five per cent. This is a somewhat surprising fact since in this case photographs show large quantities of surface material being sucked up into the cloud, just as they are in a true surface explosion.

This material certainly consists of large, heavy dirt particles which subsequently fall out of the cloud. Yet most of them somehow fail to come in contact with the radioactive fission products.

This peculiar phenomenon can be understood by looking at the details of how the fireball rises. At first the central part of the fireball is much hotter than the outer part and thus rises more rapidly. As it rises, however, it cools and falls back around the outer part, creating in this way a doughnut-shaped structure. The whole process is analogous to the formation of an ordinary smoke ring. In most of the photographs one sees, the doughnut is obscured by the cloud of water that forms, but sometimes when the weather is particularly dry, it becomes perfectly visible. During the rather orderly circulation of air through the hole, the bomb debris and the dirt that has been sucked up remain separated. (Seepictures 1-4.)

The close-in fallout accounts for only a portion of the radioactivity, ranging from less than a per cent for a high altitude shot to almost complete deposition for some ground shots. For the world-wide fallout we are interested in what happens to the remainder. This depends on how the atomic cloud is carried by the upper winds for long distances. In this connection it is important to distinguish between a big bomband a little bomb. It is also important to distinguish between the lower and higher portions of the atmosphere called, respectively, the troposphere and the stratosphere.

The atmosphere is heated by the sun in an indirect way. The sun’s rays pass through air without warming it. They heat up instead the bottom of the atmosphere, that is, the solid ground. The atmosphere is heated in the same manner in which a boiling pot is heated on the kitchen range. The heat is delivered from below and is carried in rising currents to the top.

Only in the case of the atmosphere there is no sharp upper limit. The currents rise to an altitude of thirty to fifty thousand feet, then turn and descend. This boiling part of the atmosphere is called the troposphere or region of heat. Above it there is less vertical motion. The upper region is called the stratosphere or stratified region.

For a little bomb the atomic cloud stops rising before it reaches the stratosphere. For a big bomb, above about a megaton of energy (a million tons of TNT equivalent), the cloud pokes right into the stratosphere and keeps going to a height of a hundred thousand feet or so.

The most important fact about the stratosphere is this: It has very little weather. Most of the weather phenomena such as clouds, rain, snow, fog, mist, etc., are confined to the lower portion of the atmosphere, the troposphere. The stratosphere, however, contains practically no water.

Now suppose a little bomb whose cloud will remain in the troposphere has been exploded at one of the United States test sites. The Nevada test site is at a latitude of 37°N and the Pacific test site at 12°N. In these middle latitudes, in the troposphere, the winds blow mainly from west to east with an average speed of approximately 20 miles an hour. There will be a slight southerly or northerly motion on top of this. But by and large the radioactive cloud will stay in a pretty narrowband around the latitude at which the explosion took place.

After the first few hours, when the close-in fallout has dwindled, the radioactive particles remaining in the cloud are too light and too fine to fall any more under the action of gravity. At this point the weather becomes important. Rain, fog, or mist captures the radioactive particles, and returns them to the ground in the rainfall. This results in the so-called tropospheric fallout.[12]The average time for this fallout to occur is approximately two weeks to a month. During this time, while staying more or less in the latitude of the explosion, the radioactive particles may actually have encircled the earth.

The clouds of the big bombs rise high into the stratosphere. The winds in the stratosphere do not blow so predominantly in a latitudinal direction. What is more important, they stay in the stratosphere for years, in which time the radioactivity is distributed to all areas of the globe. The fallout from the big bombs is thus really world-wide.

The tropospheric fallout takes about a month. The stratospheric fallout takes 5 to 10 years. The reason for this difference is the weather, or rather the lack of it. In the stratosphere there is no rain or fog to catch the radioactive particles and hence no effective mechanism for producing the fallout. In fact, since the radioactive particles are too fine to fall by gravity, they must simply wait until some turbulent motions impel them downward back into the troposphere. This process requires a long time.

That rainfall is the most important mechanism for producing the world-wide fallout has been shown by examining the fallout in certain dry regions of southern California and South America. In every case the fallout was found to be considerablysub-normal. In one place in Chile, where there is never any rain, the fallout was found to be only one per cent of what might be expected on the basis of the average fallout at the same latitude.

In regions having at least a few inches of rain per year, the fallout tends to be proportional to the rainfall on the average. However, the proportionality to rainfall depends on the nature of the weather so that, say, twenty inches of rain in one part of the world may not give as much fallout as the same amount of rain in other weather zones. We are rapidly learning about this.

Having said what the age is of the various kinds of fallout, we are in a position to say which radioactive species are still present when the radioactivity is deposited on the ground. The close-in fallout, being only a few hours old, still includes many short-lived isotopes, which disintegrate before there is a possibility of ingestion or inhalation into the body. Consequently the danger from the close-in fallout results from external exposure, mainly to gamma radiation on the whole body, and to a lesser extent to energetic beta rays on the skin. Clothes and ordinary housing provide relatively little shielding against gamma rays. Special protective shelters are needed. During a war if the enemy were to bomb our cities with super-megaton weapons surface-burst, the close-in fallout would be a far greater agent of destruction against an unsheltered populace than either blast or thermal radiation.

In the stratospheric world-wide fallout, however, all of the short-lived radioactivity has disappeared, since a period of many years has elapsed since the explosion. After a year or so the only gamma emitter which is left in appreciable quantity is cesium¹³⁷, with a half-life of 30 years. Its gamma ray, however, is not very penetrating. In spite of this fact cesium¹³⁷ is considered to be the second most important hazard for the long term fallout. The first is strontium⁹⁰, which is a beta emitter with a half-life of 28 years. This is long enough sothat most of these nuclei will still be present even after spending a long time in the stratosphere. Since strontium is chemically similar to calcium, it contaminates our foodstuffs and is easily incorporated into our bodies. Once inside it stays for long periods of time, deposited in our bones. We shall see in a later chapter how serious this danger may be.

The tropospheric fallout, and to a lesser extent, the stratospheric, includes some other radioactive species besides cesium¹³⁷ and strontium⁹⁰, and we shall discuss these in the next chapter. But by and large they are of little consequence (with the possible exception of iodine¹³¹) either because they are not easily absorbed in the body or else because their radiation is not very energetic. The world-wide hazard is thus narrowed down to just two isotopes, an internal beta emitter and a weak gamma emitter.

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 radioactiveiodine, 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 towhich 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 extensiveresearch 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 radiationdosage. 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. (Seepictures 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 strontiumin 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 acreactually 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 strontiumdepth 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 importantwhen 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 strontiumis 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:

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