Radon exposure of the skin: I. Biological effects

Abstract

Radon progeny can plate out on skin and give rise to exposure ofthe superficial epidermis from alpha emitters Po-218 (7.7 MeV, range∼66 µm) and Po-214(6 MeV, range ∼44 µm). Dose rates from beta/gamma emitters Pb-214 and Bi-214 are low and only predominateat depths in excess of the alpha range. This paper reviews the evidence for a causal linkbetween exposure from radon and its progeny, and deterministic and stochastic biologicaleffects in human skin.Radiation induced skin effects such as ulceration and dermal atrophy, which requireirradiation of the dermis, are ruled out for alpha irradiation from radon progeny becausethe target cells are considerably deeper than the range of alpha particles. They have notbeen observed in man or animals. Effects such as erythema and acute epidermal necrosishave been observed in a few cases of very high dose alpha particle exposures in man andafter acute high dose exposure in animals from low energy beta radiations with similardepth doses to radon progeny. The required skin surface absorbed doses are in excess of100 Gy. Such effects would require extremely high levels of radon progeny. They wouldinvolve quite exceptional circumstances, way outside the normal range of radon exposuresin man.There is no definitive identification of the target cells for skin cancer induction in animals orman. The stem cells in the basal layer which maintain the epidermis are the most plausiblecontenders for target cells. The majority of these cells are near the end of the range of radonprogeny alpha particles, even on the thinnest body sites. The nominal depth of these cells,as recommended by the International Commission on Radiological Protection (ICRP), is70 µm. There is evidence however that some irradiation of the hair follicles and/or the deeperdermis, as well as the inter-follicular epidermis, is also necessary for skin cancer induction.Alpha irradiation of rodent skin that is restricted to the epidermis does not produce skincancer. Accelerator generated high energy helium and heavy ions can produce skin cancerin rodents at high doses, but only if they penetrate deep into the dermis. The riskfigures for radiation induced skin cancer in man recommended by the ICRP in1990 are based largely on x and beta irradiated cohorts, but few data exist belowabsorbed doses of about 1 Gy. The only plausible finding of alpha-radiation inducedskin cancer in man is restricted to one study in Czech uranium miners. Thereis no evidence in other uranium miners and the Czech study has a number ofshortcomings.This review concludes that the overall balance of evidence is against causality of radonprogeny exposure and skin cancer induction. Of particular relevance is the finding inanimal studies that radiation exposure of cells which are deeper than the inter-follicularepidermis is necessary to elicit skin cancer. In spite of this conclusion, a follow-on paperevaluates the attributable risk of radon to skin cancer in the UK on the basis that targetcells for skin cancer induction are the cells in the basal layer of the inter-follicularepidermis—since this is the conservative assumption made by international bodies such asthe International Commission on Radiological Protection (ICRP) for general radiologicalprotection purposes.