5.1 Introduction Fottowixe A Nuciear detonationin the spring of 1954, a large group of people were contaminated with fission products. In addition to a sublethal external gamma radiation exposure and beta irradiation of the skin, delectable amounts of radiomiclides were deposited internally. It has been assumedthat in all situations resulting from a contaminating event, the ratio of external to internal dose would be exceedingly high. However, a detailed ‘study of the internal contamination in the exposed human population and in animals was madeto determine the kind and degree of internal deposition. Three general problems were investigated: (1) The determination of the contribution of the internal contamination to the acute radiation syndrome observed; (2) The possibility of long term effects, and (3) The qualitative and quantitative nature of the internal contamination produced by exposure of individuals to mixed fission products. There was no previous situation in which human beings were exposed to an environment contaminated with mixed fission products. Concurrent studies were undertaken by the Japanese, however, on radioactive materials to which a small group of Japanese fishermen, near Rongelap at the time of the detonation, were exposed. The report of the extensive investigations undertaken on the ashes by the Japanese have been published (4). Evaluation of the internal contamination of the human beings was made by a study of the radioelements excreted. As very little information is presently available concerning the ratio of excreted radioelements to the amount deposited in the body, it was necessary to base the evaluation on data obtained from animals which had been contaminated in the same event. Detailed studies of animal tissues and animal excreta then provided data on which estimates of the human body burden were based. 5.2 General Nature of Internal Radiation Toxicity Tue Narre Or the radiation hazard from in- ternally deposited fission products can best be understood in terms of the biophysical behavior of the radionuclides. Fission products entering the body through inhalation or ingestion concentrate in various tissues and act as sources of internal radiation. The ability of a radionuclide to enter the blood stream is determined byits solubility, chemical properties and physical state. The radioelements formedinfission are predominantly oxides which have a limited solubility in body fluids. Onthis basis, only a few of the radioelements can become available to the body. However, the amount which can produce injurious effects when deposited within the body is minute because of the close proximity of the isotope to the tissues it irradiates, and because the isotope continues to irradiate these tissues until it is removed by biological turnoveroris rendered harmless by radioactive decay. The effects of radiation from internally deposited emitters are the same as those from external radiation. The distinguishing feature of in- ternal radiation, however, is its long continuing nature. Radioactive isotopes follow the same metabolic processes in the body as the naturally occurring inactive isotopes of the same element and of chemically similar elements. Thus strontium and barium, which are analogous chemically to calcium, are deposited in the calcifying tissue of the bone. Although nearly two hundred radioisotopes are produced in thefission process, only a few are potential chronic internal radiation hazards. These fission products, which are listed in Table 5.1, constitute a high percentageofthe fission yield, and localize chiefly in bone. The “bone-seekers” have, in 67