Radon is a colorless and odorless cancer-causing radioactive gas. It forms naturally from the decay (breaking down) of radioactive elements, such as uranium, which are found in different amounts in soil and rock throughout the world. Radon gas in the soil and rock can move into the air and into underground water and surface water.
High radon areas in the U.S. tend to be in the northern section of the country due to the types of soils found there. States with high concentrations of radon include Iowa, North Dakota, South Dakota, Maine, Minnesota, Illinois, Kansas, Montana, Idaho, Wyoming, Colorado, Pennsylvania, and Virginia. Most or all of these states fall in zone one, which is one designated for parts of the U.S. with the highest levels of radon — four pico Curies per Liter (pCi/L) or higher.