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.