these nuclides may become associated with particulate matter from other bomb debris, from dust in the atmosphere or material raised by the explosion, or from moisture condensed in the atmosphere to form rain, and thus will be brought to the earth's surface and become a potential radtOlogical hazard. On the other hand, the nuclides formed in the fusion process (except for the mburned tritium) are not radioactive and do not, of themselves, increase the potential radiological hazard. The radioactive fragments formed in the fission process are chemical elements ranging from zine to europium, the lowest and highest atomic weight elements formed, respectively. A relatively small quanti- ty of radioactive atoms which are not fission fragments are found following atomic detonations. This activity results from the action of neutrons on the nuclei of certain stable atoms. U.S. nuclear weapon war and test experience consists of 65 shots to date. Detonations have been carried out at high, intermediate, and low altitudes, on towers and on and under both land and water surfaces, over a range of yields from less than a kiloton to about 15 megatons TNT equivalent. In addition, the United Kingdom and Soviet Russia have conducted nuclear weapon tests, and much data from the tests by the United Kingdom are available to us. Ground surface and underground bursts result in the incorporation of radioactive fission fragments and the small amount of induced radio-e active materials which are formed, on or into earth particles which provide a vehicle for bringing the contaminant promptly from the atomic cloud to the surface of the earth. An air burst, on the other hand, provides no ready means for bringing the contaminant down quickly, with the result that there is very little local fall-out from this type of burst. Tne height to which an atomic cloud rises above the burst, point depends primarily upon the yield of the detonation. After cloud stabilization, the speed and direction of the winds at all altitudes through which the particles must fall determine the direction of expected 2