Why can radon accumulate to risky levels inside houses?

Radon gas accumulates to risky levels inside houses primarily due to its natural origin and the way it enters and remains within buildings. Radon is a radioactive gas produced by the decay of uranium present in soil and rocks, particularly in areas with granite or other uranium-rich geological formations.1

Because radon is a gas, it can diffuse through soil and rock, entering buildings through foundations, basements, or water supplies derived from wells. Once inside, the gas can accumulate to dangerous concentrations, especially in homes that are poorly ventilated. Efforts to conserve energy by lowering air-exchange rates in buildings have further exacerbated this problem by preventing the dilution of indoor contaminants.12

Exposure to high levels of radon and its radioactive decay products is a serious health concern, as it is a leading cause of lung cancer. Reducing these risks requires appropriate building construction and maintenance, as well as ensuring adequate ventilation to prevent the buildup of the gas.132