Air exchange in the building
The exchange between indoor air and outdoor air has a significant influence on the level of radon concentration indoors. Leaky windows and doors lead to higher air exchange rates. If the air exchange rate is reduced by installing tightly sealed windows and doors, the indoor air concentration of radon can increase considerably.
Building condition
The tighter the building envelope is against the ground, the better the protection against radon. The permeability of a building to the soil air is crucial. This applies to the foundation area as well as masonry in contact with the ground. Penetration possibilities exist via gaps and cracks as well as along cable and pipe penetrations. If cellars or other areas of the building in contact with the ground are open to the living area, radon can spread particularly easily into the living area.
Inhabited buildings act like suction cups, especially in the cold season. The reason for this is a low negative pressure that draws radon-containing soil air into the building. The effect is called the "chimney effect" and is caused by temperature differences between indoor and outdoor air or by wind pressure.
Condition of the substrate
The composition of soil and rock (uranium and radium content) and the grain size of the rock play an important role. The grain size provides information on how much radon is released into the soil air. The permeability of the subsoil determines whether the radon-containing soil air is transported further.
Particular caution should be exercised with regard to increased radon concentrations indoors in cones of rubble and slopes, weathered granite, karst and gravel soils. This contrasts with very compact soils, for example loamy soils.