Notes: The National Residential Radon Survey (U.S. EPA, 1992a) estimated radon levels in the U.S. housing stock in the early 1990s. The number of homes at or above 4 pCi/L in subsequent years was estimated by applying the results of the Radon Survey (U.S. EPA, 1992a) to U.S. Census data on the number of households nationwide. The 1992 National Residential Radon Survey was based on the housing stock that would be covered by EPA’s radon testing policy (i.e., homes that should test). This included only homes intended for regular (year-round) use and covers single-family homes, mobile homes on permanent foundations, and multi-unit and group quarters (U.S. EPA, 1992b). The 1992 residential survey estimated that about one in 15 homes in EPA’s “should test” category would have a radon level of 4 pCi/L or more.
  