Another useful tool out there is the compilation of pre-mitigation indoor air testing data that the CDC has collected. It can be illustrated by county as the median, mean, or maximum Radon concentrations. It is particularly interesting to look at side by side maps showing the mean concentrations and the maximum concentrations. FOr example, the average for the San Diego area is 0.9 pCi/l, but the maximum measured is 45.2 pCi/L. Even a "low risk" area can be a concern. Only way to know is to test.

https://ephtracking.cdc.gov/DataExplorer/