Radon concentration in the atmosphere is usually measured in becquerels per cubic meter (Bq/m3), which is an SI derived unit. As a frame of reference, typical domestic exposures are about 100 Bq/m3 indoors and 10–20 Bq/m3 outdoors. In the US, radon concentrations are often measured in picocuries per liter (pCi/l), with 1 pCi/l = 37 Bq/m3.[1]