There is no satisfactory explanation for the darker dusky-gray color that appeared in some of the skin lesions as healing progressed. The color changes may have been due to alter- ations in local pigment production, vascular changes, or a thinning of the epidermis, rendering it more translucent with resultant darker appearance of the pigment layer. The return to near normal in this pigmentation by 6 months showed the transient nature of this change. The continued absence of pigmentation at the site of the deeper foot lesions at six months and one year later suggests that the pigment-producing elements in these areas may have been permanent ly damaged. The unique features of the lesions such as the marked pigmentation changes, the absence of obvious multiphasic response, the long latent period, and the severe, spotty epidermal injury with minimal dermal injury, may have been partly related to biological factors and partly to physical radiation factors. Among the physical parameters the particulate nature and uneven distribution of the fall- out material was no doubt responsible for the spotty nature of the lesions. Severity of lesions were due to factors related to the skin dose, determined by the time after detonation that con— tamination occurred, and the length of time before decontamination was accomplished, and radiochemical composition of the fallout material particularly in regard to the beta energy spectrum which determined the skin depth dose. Biological factors such as varying degrees of skin pigmentation are known to affect the radiation response. It is generally recognized that blond individuals are more sensitive than brunettes (17). Many of the characteristics noted in these lesions, particularly the marked pigmentation changes, may have been related to the fact that the exposed population was largely a dark-skinned race.* The part of the body involved was important. As was pointed out the flexor surfaces with thinner epithelium are considered to be more sensitive than the extensor surfaces with thicker epithelium. Other inherent differences in sensitivity of different areas may ex1st. In table 3 are listed the approximate surface skin doses required to produce recognizable epidermal injury from beta radiation in animals. It is apparent from the table that beta ray energy is of considerable importance in determining the degree of injury. According to Moritz and Henriques, the difference in dose between that required to produce threshold skin damage and that for permanent damage in pigs is 500 to 1,000 rep (26). One is not justified in com paring animal lesions fron known doses wth lesions in the exposed individuals in this study in order to estimate the skin dose, since species differences in response may exist, and certain radiation factors are not well established, such as accurate knowledge of the beta spectrum of the fallout material and dose rate. Comparison with human data suffers from the drawback of wide differences of radiation doses reported to produce stated effects. The fact that there was so little secondary infection of these lesions may be partly attributed to the daily cleansing and care given them. However, the low incidence of infection, even during the acute stages, also indicate that the humoral and cellular defenses were not sufficiently altered under the circumstances to predispose to infections. This conclusion is emphasized by the fact that an epidemic of upper respiratory infections, as well as other diseases encountered, occurred concomitantly with the skin lesions without untoward results even though significant leukopenia was evident. It is conceivable, however, that with a slightly larger dose of whole-body radiation, the defenses against infection might have been sufficiently impaired to have resulted in serious complications from skin lesions of the sev— erity encountered. Reported clinical experience with radiation skin lesions is based predominantly on the response of white-skinned people, whereas the lesions described herein were observed primarily in Marshallese (Micronesian) people. - 2 any 436 cH *