The study explored the theoretical basis of radon entry into buildings and processes affecting the seasonal variation in indoor radon concentration. The radon source of the model comprises two components, diffusive and convective entry rate. The force driving the convective flow is the pressure differential caused by the indoor-outdoor temperature difference. The physics of diffusive radon entry into buildings, of airflow from soil, driven by the pressure differences, and of air infiltration explain to a considerable extent the observed seasonal variation in indoor radon concentration. The seasonal variation in houses with natural ventilation has been estimated for both typical Northern European climate conditions and for warmer conditions in Central and Southern Europe. In houses with soil gas radon as the dominant radon source a summer minimum is typically 50% of the winter maximum indoor radon concentration. Ventilation through open windows in summertime may result in significantly lower concentration relative to theoretical calculations. The cold climate in North Europe increases radon concentrations by 50% compared to countries in South Europe. The annual average to winter radon concentration ratio was typically 0.7-0.8. This is in agreement with the measured seasonal variations.
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