The concentrations of indoor radon in the basements of homes located in southern Maryland average 1.3 times the first-floor radon concentrations. Particular geological units tend to be associated with higher indoor radon. In the study area, homes underlain by phyllite are generally above 4 pCi / L (the U.S. Environmental Protection Agency “action level”). Comparative studies between indoor radon and total-gamma aeroradioactivity show that aeroradioactivity can be accurately used to estimate community radon hazards. When combined, geology and aeroradioactivity can be used to identify problem homes.