Radon is drawn into a building by the internal and external air pressure differences that exist or develop by virtue of its design or use. Buildings tend to be warmer inside than the external ambient temperature (this effect is greater during the winter months) and thus have a heating effect on the air contained within them. Since the warmed air is less dense it rises and will escape to atmosphere via open windows and though the buildings upper fabric (walls and roof.) This creates a need for replacement air to be drawn into the building from the lower levels. If there is no ready ingress route i.e. open ground floor doors or windows, depressurisation occurs. This effect is

greatest at the lowest part of the building, that is on the ground floor or in cellars. The external air now being at a higher pressure relevant to the building structure in contact with the soil, is drawn though the soil and into the property. If there is radon in the soil this is also drawn into the building where it may become concentrated relative to the external ambient air. This effect may be enhanced by atmospheric air movements.

See:

The Effect of Atmospheric Air Movements on Radon ingress into Buildings