LC C: SECRETRES7sRLCED Deh cTeA 0018933 but they are 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 totz] fallout is usually considered as being divided into two classes, designzted as loc2l and world-wide fallout. Ina general way, local fallout is thought of 2s consisting of relatively large particles, which reach the earth's surface in a few hours, where2s world wide fallout is composed of finely-divided materi2l which may remain suspended in the atmosphere for months or years and be deposited at long distances from the sources A more precise differentiztion 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; m2teri@l settling out more slowly than this is likely to be transported beyond the ocean arezs and deposited in inhebited regions, if it attains an altitude of 190,000 ft. . #_so The ratio of local to world-wide fallout is/governed by the heiynt at tained by the nuclear cloud and the size distribution / of the particles in the nuclear cloud which act as collectors for the radioactive fission-product ztoms, If many large particles with fast falling ]