Radon is a radio active, colorless, tasteless and odorless gas. Every home has some radon. The outside air has .4 pCi/L.

Radon comes from the natural radioactive decay of radium and uranium found in the soil beneath the house. The amount of radon in the soil depends on complex soil chemistry, that varies from one house to the next. One home may have a radon level of 2 pCi/L and the home next door could have a reading of 17 pCi/L. Radon levels in the soil range from a few hundred to several thousands of pCi/L. The amount of radon that escapes from the soil to enter the house depends on the weather, soil porosity, soil moisture, and the suction within the house. Radon is often found in areas which have granite uranium, shale, and phosphate.

Additional Information:

http://www.radon.utah.gov/

http://www.epa.gov/radon/

The EPA has printed a very helpful pamphlet called The Citizen's Guide to Radon. It can be found at:
http://www.epa.gov/radon/pubs/citguide.html#myths