It has been considered true for many years, probably because of commentary in US-EPA publications for the general public, that winter concentrations of indoor radon are greater than summer concentrations. The higher amount in the winter is attributed to the observation that people normally keep their windows closed during the winter, allowing indoor radon concentrations to rise. The lower radon concentrations in the summer might occur because often open their windows, allowing low-radon outside air to enter the home. It now appears that seasonal rainfall can cause unexpected indoor radon concentrations. In our study of over 1000 homes, where indoor radon was measured seasonally over an entire year, a summer season that had above normal rainfall had higher indoor radon than the prior winter, which had below normal precipitation.
Mose, Douglas G.; Mushrush, George W.; and Saiway, George
"Summer Indoor Radon Exceeds Winter Indoor Radon,"
Proceedings of the Annual International Conference on Soils, Sediments, Water and Energy: Vol. 11
, Article 13.
Available at: https://scholarworks.umass.edu/soilsproceedings/vol11/iss1/13