The radon hazard is presumed to be lung cancer induced by alpha irradiation of basal cells in the bronchial epithelium. The effective dose is calculated by a relatively complete analysis which includes (1) the distribution of atmospheric activity by size and daughter, (2) the prediction of alpha decay in the bronchial regions from a model of mucus flow superimposed on calculated regional deposition, (3) measurements of epithelial thickness to determine accessibility of basal cells, and (4) classification of dose by inhaled daughter, size, alpha particle, region, and tissue depth. The cancer-related dose, taken as the largest dose to exceptionally shallow basal cells, associated with one working level of radon daughters is estimated to be 20 rads/yr for nose breathing at 15 l./min, and it may be higher. Comparison with animal experiments suggests that the working level may not be safe and that 30 pc/l. of daughters (0.1 WL) may not be too conservative for the MPC.