Which statement best describes seasonal and regional differences affecting indoor radon exposure?

• A. Radon production varies mainly with soil type and uranium content
• B. Radon levels are unaffected by temperature
• C. The rate of radon exhalation from soil is constant everywhere
• D. Seasonal and regional differences in indoor-outdoor temperature differences affecting the suction of soil gas into buildings

Answer: (D)

Seasonal and regional differences in indoor radon exposure come from how indoor–outdoor temperature differences create pressure differences that suck soil gas into buildings. When it’s colder outside and warmer inside, the stack effect and related air movement lower pressure at the foundation and pull more radon-laden soil gas through cracks into the living space, causing higher indoor radon levels in winter and varying by region with different climates and building styles. Radon production in soil does depend on uranium content, but that explains variation in soil radon supply, not the seasonal indoor fluctuations caused by building ventilation and pressure changes. The rate of radon exhalation from soil isn’t constant everywhere, and temperature does matter for indoor levels, so the statement tying seasonal and regional temperature-driven suction of soil gas into buildings best describes the pattern.