~5— that thorium-x applied to the skin results in some percutaneous absorption and entry into the hair shafts and glands. Beta dose to the skin from fallout on the ground will be lergely confined to the lower parts of the body, partioularly the feet and legs, since the beta particles are completely stopped in approximately two meters of air. _..-5 ESTIMATION OF SKIN DOSE Measurement of beta doses to the akin from fallout is an exceedingly difficult problem due to the complicated apectrum of different energy beta emitters esent, the nomniform distribution on the sin, and the fact that practical dose meters have not yet been perfected which will adequately discriminate between the beta radiation and the contaminating gamma component. The penetration of beta particles into the skin depends, of course, on the beta energies of the component isotopes. Each radioisotope has its own characteristic spectrum of beta energies up to a maximum energy. Relatively few particles are of the maximum energy, however, and the average energy (roughl one-third of the maximum energy) and the 50 per cent attenuation thickness of tissue are more meaningful in estimating skin effects. Thus an isotope emit- ting low energy radiation, confined largely to the dead, horny layer of skin, would be relatively ineffective; more energetic radiation, penetrating through the epidermis could result in transepidermal necrosis; and deeper penetration into the dermis could result in more severe ulcerating lesions. Fig. 2 shows roughly the tissue depth necessary to produce 50 per cent attermation of the beta particles from several isotopes. In Table 1 data from animal studies from several investigators show the evergy dependence of beta particles from various isotopes in producing recor~ Co mh os C4 nizable skin reactions. Note that the surface dosea for threshold reaction (erythema, epidermal atrophy) are fairly dependent on the energy of the beta