of principal concern, because of their greater penetrating power. The most energetic beta particles travel only a few yards in air and are of concern only when the fallout materials remain in contact with or ino very close proximity to the skin, or when the emitting materials find their way into the body. The amount of alpha emitting isotopes associated with fallout material is considered to be of relatively minor consequence. EXTERNAL GAMMA EXPOSURE The gamma radiation dose that one may actually receive and the biological effects are dependent upon a number of factors, as follows: 1. Radiological decay. The decrease in radioactivity of fallout material roughly follows the relationship of (time)~1-2, This means that, for every sevenfold lapse of time after a nuclear explosion, there will be a tenfold reduction in dose rate. For example, if fallout occurs one hour after a detonation, such as might occur for twenty or thirty miles around ground zero of a high yield weapon, the dose rate will be onetenth of its initial value by the seventh hour. An additional tenfold reduction would require seven times seven hours or approximately two additional days of waiting. The theoretical* dose accumulated from the first to seventh hour after detonation would be approximately the * Calculations of theoretical doses are based on (a) the radio- activity decreasing according to (time)-1-2, (b) there is no loss” of activity by weathering effects, and (c) the person is out-ofdoors for the time considered. ‘ -6-