portionately to AR. Only 13% of the calculated AR is estimated to be contributed by the 50% of homes below the median concentration of about 25 Bqm^-3 (0.7 pCiL^-1) and about 30% by homes below the mean of about 46 Bqm^-3 (1.25 pCiL^-1). Homes above 148 Bqm^-3 (4 pCiL^-1), the current action level established by the Environmental Protection Agency, contribute about 30% percent of the AR. This contribution to the total AR is indicative of the potential magnitude of avoidable deaths with a risk management program based on the current action guideline. While 10–15 percent of all lung-cancers are estimated to be attributable to indoor radon, eliminating exposures in excess of 148 Bqm^-3 (4 pCiL^-1) would prevent about 3 to 4 percent of all lung-cancers, or, about one-third of the radon-attributable lung-cancers.

The ARs were re-estimated with assumption of thresholds, levels below which cancer risk is not increased, at 37, 74, or 148 Bqm^-3 (1, 2, or 4 pCiL^-1). Even though the committee assumed that risk was most likely linear with exposure at lower levels, this analysis was conducted to illustrate the impact of assuming a threshold on risk-management decisions. Assuming an action level of 148 Bqm^-3 (4 pCiL^-1) for mitigation, postulating a threshold reduces the total number of lung-cancer deaths that are attributable to indoor radon and also the number of lung-cancer deaths that can be prevented by reducing levels in homes to zero. For assumed thresholds below 148 Bqm^-3 (4 pCiL^-1), there is little impact on the