Where does the indoor radon come from ?

There are many sources of indoor radon. The natural radioactive materials in the soil and rock of building foundation give off radon which may get indoor through cracks and joints of the building. Stone materials used in buildings and home renovations, such as rock, granite, brick and cement, also release radon. Radon in the air outdoor, though usually not at a high activity level, may go indoor through ventilation. The combustion of coal and liquid petroleum gas also releases radon.