Radon is a colorless, odorless, radioactive gas produced by the decay of uranium, which is present in virtually all rocks and soils, typically at concentrations of 1 to 4 parts per million (ppm). Radon can accumulate inside homes and other buildings at concentrations that are commonly tens of times greater than in outdoor air and, in some cases, may be hundreds or even thousands of times greater. Most indoor radon is derived from uranium in underlying soil and rock and gradually seeps into buildings through cracks or other openings in the ground floor. Houses with unusually high concentrations of indoor radon are typically built on rock and soil that contain unusually high uranium concentrations.