From building material, water and soil.
Radon in building materials:
Radon levels inside because of the radon content in the wall, the walls´ material density, surface density, the density of the wall’s other side of the area, exchange of air inside, the air pressure etc..
Radon stored in soil:
Through leaks in the building’s foundation radon can be drawn into the housing from the ground. Having negative pressure in the ventilation system increases the risk that radon sucked in a house.