Where are indoor radon concentrations commonly highest?

Indoor radon concentrations are commonly highest near the source, which includes areas like ground-contacted rooms such as basements and crawl spaces. Radon gas is a naturally occurring radioactive gas that is produced from the decay of uranium in the soil. Since it originates from the ground, it tends to infiltrate homes through cracks in floors and walls, gaps around service pipes, and other openings in the foundation. Therefore, areas that are in direct contact with the ground—like basements—are particularly susceptible to elevated radon levels.

While poor ventilation can contribute to higher radon levels, the concentration is primarily a function of the proximity to the source. Ventilation improvements can help reduce radon concentrations throughout a home, but they are particularly relevant in rooms that are already affected by radon ingress. Rooms such as attics, meanwhile, are not typically associated with high indoor radon concentrations, as they are not in direct connection with the ground source where radon would enter the home.

Hence, the highest indoor radon concentrations are indeed found near the source, making ground-contacted rooms the critical areas of concern for radon exposure and mitigation efforts.