For long-term indoor radon measurements (91 or more days), home occupants are allowed by the USEPA recommendations to leave their house windows open whenever weather allows. Open windows diminish the partial vacuum normally found in homes (caused by warm air rising through the house), and consequently reduces the inward movement of radon into the house from surrounding soil. However, even under these open-house conditions, indoor radon in our study area increases during rainfall, and long-term radon measurements are higher if taken during a season of above average precipitation. This seasonal effect can result in higher than normal short-term (2-7 day) indoor radon measurements, often taken by home inspectors during the sale of a house.
Mose, Douglas G.; Mushrush, George W.; and Siaway, George
"Indoor Radon: Short-Term And Long-Term Influence Of Prolonged Precipitation,"
Proceedings of the Annual International Conference on Soils, Sediments, Water and Energy:
Vol. 12, Article 20.
Available at: http://scholarworks.umass.edu/soilsproceedings/vol12/iss1/20