Little, M. P. and Wakeford, R. The Bystander Effect in C3H 10T(1/2) Cells and Radon-Induced Lung Cancer. Radiat. Res. 156, 695-699 (2001).Bystander effects, whereby cells that are not directly exposed to radiation exhibit adverse biological effects, have been observed in a number of experimental systems, including C3H 10T(1/2) cells exposed to alpha-particle radiation. The bystander effect implies that risks from exposure to low doses of radiation obtained by linear extrapolation from data for high-dose exposures might be substantial underestimates. The best estimate of the ratio of the lung cancer risk among persons exposed to low (residential) doses of radon daughters to that among persons (underground miners) exposed to high doses of radon daughters is in the range of 2.4-4.0, with an upper 95% confidence limit of about 14. Assuming that the bystander effect observed in the in vitro C3H 10T(1/2) cell system applies to human lung cells in vivo, these epidemiological data imply that the central estimate of the number of neighboring cells that can contribute to the bystander effect is between 0 and 1, with an upper 95% confidence limit of about 7. As a consequence, the bystander effect observed in the experimental C3H 10T(1/2) cell system probably does not play a large part in the process of radon-induced lung carcinogenesis in humans.