Concentration Gradient Diffusion

The highest radon concentrations will be nearest the source. Radon concentrations decrease quickly as the surface of the ground is approached. The atmospheric concentration of radon is very low at ground level while the concentration gradient is largest at ground level.  Diffusion is not usually the primary entry route for radon into buildings.

With high concentrations of radon below grade there will be passive diffusion of radon through the foundation from soil/bedrock to indoors even if no air is moving through the soil into a house.  Radon will move from higher concentration to lower concentration areas of the house via molecular diffusion of gas through a porous solid such as solid concrete wall or slabs.