vided at least as much protection as implied by the broad estimate for Army field clothing to the effect that "... the average field issue clothing will stop approximately one-half of the incident beta radia- tion from fall-out material". It has long been a moot question whether within an intense fall-out field the beta burn problem could be the factor limiting the period of exposure of personnel. CASTLE provided an unexcelled oppor- tunity to observe such a situation. Personnel receiving whole body gemma doses of about 200 roentgens suffered no burns under the clothing except where the radioactive material was carried by perspiration or body motion under a collar or cuff. Over unclothed areas of skin, how- ever, burns were sufficiently severe to reduce the efficiency of personnel more than would be predicted from the exposure to the concomi- tent gemma component. Thus, from CASTLE data has come the fairly de- fensible statement that for well clothed personnel in a fall-out field, gamma measurements alone will suffice to determine the radiological effects to be expected and the permissible time of entry into, or period of exposure within, such an area. The Beta Hazard. The term “beta burn” as used here is damage to the skin by ionizing radiation. The radiations primarily effective for this phenomenon are beta particles and low energy gamma rays. Thus, the term "beta burn", as generally used is a misnomer to the extent of the contribution by the low energy gamma or x-rays. Skin burns occur when the quantity of radiation absorbed by the living layers of skin exceeds a certain critical level. The flux of radiation necessary to cause a given degree of skin damage depends upon the energy and kind of radiation arriving in the critical area. The thickness of the outer protective layer of dead tissues, plus the attenuating effect of clothing, determines the radiation dose delivered to the living tissues. . a The energy of gamma radiation is frequently referred to in terms of half-value thicknesses while that of the beta radiation is in 84