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.