Picocuries per liter of air, the US unit for radon concentration. The EPA action level of 4 pCi/L equals 150 becquerels per cubic meter (Bq/m3), the unit most other countries use.
More detail
A picocurie is a trillionth of a curie, a unit of radioactive decay, so the number describes how much decay is happening in each liter of indoor air. For scale, the EPA puts average outdoor air near 0.4 pCi/L (15 Bq/m3) and the average US home near 1.3 pCi/L (50 Bq/m3), with its "consider fixing" band at 2 to 4 pCi/L (75 to 150 Bq/m3). Ohio test reports give results in pCi/L. To turn a European or WHO figure in Bq/m3 into pCi/L, divide by 37. When a report lists two side-by-side devices, Ohio requires both results and their average to be reported, and the average is the number to act on.