Of the 1238 dwellings in the CNHS, 471 dwellings (38.0%) were in areas below 300 persons per square mile. Comparison to the USEPA estimates based on population density (USEPA, 1974; Schomer et al., 2011) shows that Table I extends the results to lower population densities and gives values consistent with the available measurements (Schomer et al., 2011).