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?