The radiation dose from inhaled daughters of 222Rn constitutes about half of the total effective dose equivalent that the general population receives from natural radiation. A variety of results from the United States suggests that Rn concentrations in residences average about 1 pCi/1 (37 Bq/m ). Estimation of the incidence of lung cancer due to the daughters associated with this much 222Rn yields thousands of cases per year among the U.S. population.