Uranium and its daughter products radium and radon, are found in nearly all rocks and soils. Thorium, which is even more common, also produces radium. In most areas of the United States, the soil gas contains between 200 and 2,000 pCi of radon per liter. L. As radon gas moves through underground fissures, it usually decays into solid particles after several feet. But it travels much farther in dry, permeable soils, like gravel or course sand.
Radon is soluble in water and underground streams can carry it long distances.
Sources of indoor radon Radon gas naturally moves into the permeable disturbed soil and the gravel bed surrounding foundations and then, into the buildings through openings and pores in concrete. Radon from soil is by far the main source of indoor radon. Building materials like rocks, bricks, and concrete also give off radon. The higher temperature and lower pressure indoors release the gas. Water in sump pits and floor drains releases vapor and radon.