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 the 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. To provide some perspective as to the magnitude of the ‘“roent- gen” table I is included. 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 that gives rise to a large numberof unstable radioisotopes. Inthe fusion process hydrogen nuclei are joined together. Induced radioactive products result when inert materials capture neutrons that are released during either the fission or fusion process. Generally, these induced radioactive materials are . relatively short-lived and contribute only in a minor way to radiation exposures to man. The principal exception is carbon 14 described in section I F (page 16). Someof these radioactive materials escape as gases and are dispersed and diluted in the atmosphere. 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 particles, together with the associated radioactive nuclides, are swept high into the air by the heat and force of the nuclear explosion. The larger particles and those in the lower levels of the cloud fall nearby. Smaller particles in the upper levels are carried away to be spread worldwide. The worldwide distribution of these radio- active particles follows the same pattern as would occur with any other small particles injected into the same regions of the atmosphere —-radioactivity has essentially no effect on the pattern of distribution. Roughly, a nuclear detonation of one-half million tons or less, fired at a low altitude—but high enough so the fireball does not intersect the ground—results in most of the fission products remaining in the lower atmosphere, the troposphere. They are deposited on the earth’s surface at a rate such that one-half of the amount remaining inthe atmosphere at any one time falls in 2-4 weeks (called tropospheric residence half- time). As the energy yields of the nuclear detonations increase, more and more of the fission products are swept higher and higher into the stratosphere—the layer above the tropo- sphere (fig. 2). The residence half-time here is more like onehalf a year for injection into the lower stratosphere in the polar 2 oc cn 3" CFT 2