terms of Eax’ These are difficult terms to use when assessing the degree of damage caused by a spectral mixture of beta particles and gemma rays. It is more convenient and useful to talk in terms of the per cent of total beta and gamma radiation that is absorbed in the critical layers of skin. The outer layer of protective dead skin cells varies in thick- ness from 0.1 m, weighing 10 mg/cm®, over the anterior aspect of the erm to a thickness many times this value over the sole of the foot and the palmar surface of the hand. The minimum beta energy necessary to penetrate a thickness greater than 10 ng/em@ is 80 kev. is the minimm beta energy to produce skin damage. Thus, 80 kev It has been calcu- lated that somewhat more than 50% of the beta rays emitted during the first few weeks after fall-out has occurred have less than this cut-off value and thus would be absorbed in the protective cornified layer and produce no biological effect 2Y/ The somewhat less than 50% of the beta particles remaining has an average energy of about 600 kev and can penetrate well into the body with a large portion being absorbed by the critical living layers of skin, and this constitutes the hazerdous fraction of the beta spectrum. The effective energy for gemma radiation to produce skin burns lies between 1 and 100 kev. It is estimated that about 10% of the total gemma radiation from fall-out is within this range. The quantity of beta radiation required to cause injury to the skin, or beta burns, has been found in the laboratory by using pure beta emitters with beta energies known to be about 500 to 700 kev. The investigators found that 2000 roentgen equivalent physical (rep) is the minimm beta dose which will cause visible reddening of the skin. 2h/ Cronkite, E.P., et al., Study of the Response of Human Beings Ac- 25/ Personal Communication, Health Physics Division, Hanford Operations, February 1955. cidentally Exposed to Significant Fall-out Radiation, Final Report Project CASTLE 4.1, WI-923, October 1954, CONFIDENTIAL. 85