ved B, Whole Body Exposures Background Information -. Fallout particles consisting of inert materials together with the associated radioactive materials, settle to the earth's surface where, of course, most of them will remain and thus will never get inside our bodies. These external, man-made radionuclides, however, will irradiate eel: the whole body by their penetrating gamma radiations while their shorter jrea range beta particles will contribute a much lesser biologically signifi- cant exposure to the skin. Of the radionuclides that contribute to external radiation, the unc most important single one is cesium-137. approximately 27 years. Its radioactive half-life is Thus, it is possible for cesium-137 to remain in our environment for long periods of time without losing much of its activity, although there can be loss or reduction in availability of the material through normal weathering processes. Yet cesium-137 does have a short enough life so that most of the radiations are released within the lifetime of a man. All radioactive materials in fallout, except cesium-137, which remain outside the body may be conveniently lumped together to estimate their contribution to external exposures. These usually are called "short-lived" even though some do have half-lives of upwards of one year. In spite of the fact that nearly all of the radiation exposure received from these short-lived radionuclides is completed within a year after the radionuclides are created the total amount of exposure may be greater than that received from cesium-137 within 30 years.