Radon is a naturally-occurring radioactive gas.  Radon is colorless, odorless and tasteless.  Found everywhere, radon results from the decay of radium, a radioactive element found in most soils.  The radium remains in the ground, but radon gas seeps through the soil and enters buildings.  Radon frequently enters from the soil underneath the basement or slab, and radon levels are usually highest in the lowest level of the building. Radon levels vary considerably from location to location and homes next door to each other can have very different radon concentrations.  The US EPA has created maps showing the potential for high radon concentrations.  Elevated radon gas concentrations have been found in every county in the United States.  Radon can only be detected by testing.  

Radon is denoted by the chemical symbol Rn, has an atomic number of 86 and is a noble gas.  Radon concentration in soil and rock is maintained by the decay of its longer-lived progenitors: radium, uranium, and thorium.