What unit does EPA use to express radon concentration in homes?

• A. pCi/L
• B. mg/L
• C. ppm
• D. Bq/m3

Answer: (A)

Radon concentration in homes is an activity concentration, not a mass or simple ratio, so the EPA reports it as picocuries per liter (pCi/L). This unit directly reflects the radioactive decay activity of radon in the air and aligns with the EPA’s recommended action level, which is often cited as 4 pCi/L for remediation decisions. While SI units exist (bq/m3), the U.S. EPA commonly uses pCi/L in residential guidance, and 1 pCi/L corresponds to about 37 Bq/m3, illustrating how the two systems relate. The other units—mg/L (mass per volume) and ppm (parts per million)—don’t describe radon’s activity concentration in air, so they aren’t used for reporting residential radon levels by the EPA.