radiochemical data taken at the time of the fallout, estimates of duration of fallout, exposure time, etc. (Sondhaus et al.). Spectrographic data of the fallout showed gamma energy peaks at roughly 100, 700 and 1500 kev. The varied energy distributions, the greater depth dose due to the near 47 geometry of exposure make the fallout. dosimetry significantly different from that of radiation therapy or experimental radiobiology. Table 1 shows the calculated gamma doses (dose in air) for the different island groups. 12.5.2 Skin Dose. Skin exposure was large ly from beta radiation. The dose to the first few millimeters of the body surface were considerably greater than the midline gamma dose. Measurement of beta doses from the skin from Fallout was an exceedingly difficult problem due to the complicated spectrum of different energy beta emitters present, the non-uniform distribution on the skin, and the fact that practical dose meters were not available to adequately discriminate between beta radiation and contaminating gamma radiation. The penetration of beta particles into the skin depended on the beta energies of the component isotopes. Each isotope has its own characteristic spectrum of beta energies. Figure 4 shows roughly the tissue depth necessary to produce 50% attenuation of the beta particles from several isotopes. In animal studies the dose required to produce recognizable skin lesions by a particular beta emitter has been demonstrated to be dependent on the energy of the beta particles. (Henshaw et al., Davis and Alpen, Snider and Raper, Lushbaugh and Hale, Moritz and Henriques). For example it was found that it takes 20,000 to 30,000 rep 35s (avg. energy 0.05 mev) to produce a reaction while it takes only 1,000 to 2,000 rep of 905 5 or 90y (ave. energy 5061302 0.3-0.7 mev) to produce the same reaction