The evaluation of the hazard posed to the skin by very small radioactive sources (diameter <1 mm ) has become popularly known as the 'hot particle' problem. Biological data from man and pig on the deterministic effects following beta radiation exposures have been used by ICRP and NCRP task groups and combined with recent epidemiological analyses of radiation induced skin cancer risks in man to form a basis for suggested improved protection criteria for the skin. Both groups agree that the end point of practical concern following hot particle exposures is skin ulceration and that the risk of cancer mortality is negligible. The ICRP have used primarily UK pig skin data to suggest a threshold dose of 1 Sv over an area of 1 cm2 at a depth of about 100-150 µm which would prevent the occurrence of even superficial transient ulceration. The NCRP have used primarily US pig skin data and have produced a criterion which is aimed to prevent the occurrence of a more severe response of acute deep ulceration. The NCRP suggested exposure limit, which is given in terms of a total beta particle exposure of 1010, is less restrictive than that suggested by the ICRP task group by a factor of about 5. The disparity reflects the different levels of acceptable damage and possibly some differences in dose estimates used in the animal studies on which the threshold criteria are based. Both criteria are significantly less restrictive than the hot particle exposure limits previously employed.