Exposure (NCRP 1987). For a variety of reasons, Purpose: Radon risks derive from exposure of bronchio-epithelial however, direct epidemiological assessment of the cells to high-linear energy transfer (LET) a-particles. a-particle risks from domestic radon exposure is diYcult, exposure can result in bystander eVects, where irradiated cells resulting in risk estimates with wide conédence inter- emit signals resulting in damage to nearby unirradiated bystander vals (Lubin et al. 1995b). Consequently, domestic cells. This can result in non-linear dose-response relations, and inverse dose-rate eVects. Domestic radon risk estimates are radon risk estimates are currently based on extrapola- currently extrapolated from miner data, which are at both higher tion of data from miner studies, largely at consider- doses and higher dose-rates, so bystander eVects on unhit cells ably higher radon exposures and exposure rates. At could play a large role in the extrapolation of risks from mines present, a linear extrapolation of the risks from high to homes. Therefore, we extend an earlier quantitative mechan- to low radon exposures is generally considered to istic model of bystander eVects to include protracted exposure, with the aim of quantifying the signiécance of the bystander have the strongest biophysical rationale (NRC 1999). eVect for very prolonged exposures. At an average home radon concentration, few Materials and methods: A model of high-LET bystander eVects, potential target cells in the bronchial epithelium of a originally developed to analyse oncogenic transformation in vitro, home resident will be struck or traversed by an a- is extended to low dose-rates. The model considers radiation particle in, say, 1 year (NRC 1999)—and this obser- response as a superposition of bystander and linear direct eVects. It attributes bystander eVects to a small subpopulation of hyper- vation remains true even at high domestic radon sensitive cells, with the bystander contribution dominating the levels (égure 1). direct contribution at very low acute doses but saturating as This inhomogeneous energy deposition by a-par- the dose increases. Inverse dose-rate eVects are attributed to the ticles is of potential relevance to the radon problem replenishment of the hypersensitive subpopulation during because there is convincing evidence, at least in vitro, prolonged irradiation. Results: The model was étted to dose- and dose-rate-dependent that irradiated cells can send out signals that can radon-exposed miner data, suggesting that one directly hit target result in damage to nearby unirradiated 'bystander' bronchio-epithelial cell can send bystander signals to about 50 cells. The evidence is particularly strong for high- neighbouring target cells. The model suggests that a naõ