The unit pCi/L is used to measure which quantity in the context of radon?

• A. Dose rate
• B. Energy of radiation
• C. Radioactivity per liter of air
• D. Time-integrated activity

Answer: (C)

The main idea is that pCi/L expresses radioactivity concentration in air. It tells you how many radioactive decays occur per second in each liter of air, i.e., the activity per unit volume of air. That’s radon’s radioactivity in the room, not how much energy is deposited in tissue (dose rate) and not the energy per decay or a time-integrated total. In practice, reporting radon levels in pCi/L communicates ambient radon concentration; for reference, 1 pCi/L equals 37 Bq/m^3, and common action levels are discussed around several pCi/L.