0018933 but theyare also important for an appraisal of the effects of nuclear weapons used in warfare, It has been recognized since the earliest weapons tests that a substantial portion of the radionuclides formed in a nuclear detonation are deposited throughout the world, thereby becoming available for general biological assimilation. The total fallout is usually comsidered as being divided into two classes, designated as local and world-wide fallout. Ina general way, local fallout is thought of as consisting of relatively large particles, which reach the earth's surface in a few hours, whereas world wide fallout is composed of finely-divided material which may remain sus- pended in the atmosphere for months or years and be deposited at long distances from the source, A more precise differentiation is needed for specific situations, one of the most important considerations being the location of the detonation site in relation to world centers of population, For explosions at the Pacific Proving Grounds, the boundary between the two classes has been chosen at a particle falling velocity of 3 inches per second; material settling out more slowly than this is likely to be transported beyond the ccean areas and deposited in inhabited regions, SAN BRUNO ERE The ratio of local to world-wide fallout is governed by the size distribution of the particles in the nuclear cloud which act as collectors for the radioactive fission- product atoms. If many large particles with fast falling 12