Radon emanates naturally from the ground and from some building materials all over the world, wherever there are traces of uranium or thorium, and particularly in regions with soils containing granite or shale, which have a higher concentration of uranium. In every 1 square mile of surface soil, the first 6 inches (150 mm) (of depth) contains about 0.035 oz of radium (0.4 g per km2) which releases radon in small amounts to the atmosphere.[1] Sand used in making concrete is the major source of radon in buildings.[11]