What is the average outdoor radon concentration in picocuries per liter (pci/l)?

The average outdoor radon concentration is approximately 0.4 picocuries per liter (pci/l). This value reflects typical levels found in the environment, acknowledging that radon is a naturally occurring radioactive gas produced during the decay of uranium in soil and rock. Outdoors, radon tends to be diluted by the atmosphere, leading to lower concentrations compared to indoor environments, where radon can accumulate in enclosed spaces. This average level is commonly cited in radon studies and reports, making it crucial for awareness and understanding in radon risk assessments and mitigation strategies.