Uranium and thorium decay to other radioactive atoms including radium which then decays to radon gas.
Radon is a natural part of the earth's atmosphere.
The amount of uranium and radium in soil varies greatly with geographic location and soil type. Therefore, the amount of radon gas released to the atmosphere also varies across the United States. Movement of Radon Radon gas can get into homes and other buildings through cracks or other openings in the foundation and by passing through the foundation materials. The radon gas and its decay products become part of the air that people breathe both inside and outside buildings. However, the amount of radon in air inside buildings is generally much higher than the amount outside because the buildings trap the radon which moves from the soil.