The EPA recommends considering a fix in that range, since no level is risk-free. Many Central Ohio households run a long-term test of more than 90 days across the heating season to learn the year-round average before deciding.
More detail
The EPA's lifetime risk table puts the gap in plain numbers. Among 1,000 people who never smoked, about 4 could get lung cancer from a lifetime at 2 pCi/L and about 7 at 4 pCi/L; among smokers, about 32 and 62. A summer short-term result in the 2 to 4 band is the weakest basis for a decision, because a closed winter house usually pulls harder on the soil. A long-term device set out in November and collected in February answers the question for most Columbus homes. Weigh who uses the lowest level, too: a basement bedroom or a home office down there changes the math. Our decision guide for borderline results walks through each band.