pCi/L is picocuries per liter, the US measurement of radon concentration. EPA action level is 4.0 pCi/L; the average indoor level is 1.3 pCi/L. (Europe uses Bq/m³; 1 pCi/L = 37 Bq/m³.)
More detail
A picocurie is 10⁻¹² curies; one curie is 37 billion radioactive disintegrations per second. So 4.0 pCi/L means 4 trillionths of a curie of radioactivity per liter of air, or about 148 disintegrations per second per liter. The conversion to international units: 4.0 pCi/L = 148 Bq/m³. The EPA action level (4.0 pCi/L) was set by the EPA in the 1980s based on cost-benefit analysis of available mitigation technology and is intentionally pragmatic rather than ideal; the World Health Organization recommends an action level of 100 Bq/m³ (about 2.7 pCi/L). Either way, mitigation reduces lung-cancer risk and the marginal cost per pCi/L reduced is highest for the first few pCi above the action level. Cincinnati testing equipment context: most local install teams use Sun Nuclear or RadStar continuous radon monitors that report in pCi/L for US compliance and store hourly readings for 48-hour minimum periods. Charcoal-kit screening tests cost $15-$30 from the Ohio Department of Health and report in pCi/L on the lab return. The numerical conversion to Bq/m³ rarely matters for homeowners but does come up if comparing against European or WHO references.