Most people spend the large majority of their lives indoors, breathing indoor air.
It is often dirtier than the air outside, and almost nobody checks it.
Sources
US EPA — A Citizen's Guide to Radon: the 4 pCi/L action level, the recommendation to consider action between 2 and 4, and that there is no known safe level of exposure
US EPA / CPSC — The Inside Story: A Guide to Indoor Air Quality: particles from incomplete combustion lodging in the lung, pollutants including radon attaching to small particles, and the advice to use exhaust fans over gas ranges and keep burners adjusted