Radon entry into homes primarily originates from which source?

• A. Water supply
• B. Exterior surfaces
• C. Building materials
• D. Bedrock or soil

Answer: (D)

Radon is produced in the ground from the decay of uranium in soil and rock, and it becomes part of the soil gas that surrounds a building. This gas can move through pore spaces and seep into the home through cracks and openings in the foundation, especially into basements and lower levels. Because this entry pathway comes directly from the underlying soil and bedrock, they’re the primary source of indoor radon. Other routes, like radon in water or emissions from building materials, can contribute some amount, but they typically play a smaller role in overall indoor radon levels. Exterior surfaces don’t deliver radon into the living space in a meaningful way, since the gas originates from beneath the structure and enters the home through the foundation.