veéd Various units have been used to express exposure to radiation such as the roentgen, rep, rem, and rad. All are intended to express some relationship between radiation energy absorbed and biological effects. Since it is not critical for the following discussions to understand the technical differences among the units, only the “"roentgen" will be used. esi To provide some perspective as to the magnitude of the "roentgen" of exposure the following table (Table 1) is included. area (Table 1.) unc 2. Sources and Nature of Fallout. The major source of radioactive materials in fallout is the fissioning or splitting of atoms of uranium and plutonium, forming a large number of unstable radionuclides. Other induced radioactive products result from inert materials capturing neutrons that are released during either the fission or fusion process. (Fusion is the process wherein hydrogen nuclei are joined together.) Generally, these induced radio- active materials are relatively short-lived and contribute only ina minor way to radiation exposures to man. The principal exception is carbon-14 described in Section I. F. (Page 20 ). Some of these nuclides escape as gases and are dispersed and diluted in the atmosphere. —— K Most of the fission products, however, become incorporated into or attached onto minute inert particles of dust and debris from the immediate environment of the bomb. The dust par- ticles, together with the associated radioactive nuclides, are swept