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others 1999), and Maine (Hess and others 1982). In the Nevada study, Price and others (1994) used the same method as Hopper and others (1991) and measured 50 sites across the state during a 30-d period in the summer of 1992. Sites were chosen to reflect different rock types and represent the principal population centers in Nevada. About half the sites were in residential areas, and the rest were in remote areas near rock outcrops. Results indicate that radon in soil gas corresponds well with the geology, outdoor radon concentration and indoor radon. Measurements across the state ranged from 2.6 to 52 Bq m-3; the geometric mean was 13.1 Bq m-3. The range and values of concentrations were generally very similar to what Hopper and others found for the United States.
As part of a statewide radon study, indoor radon was measured at 300 sites throughout California (Liu and others 1991). At 68 of those sites, outdoor radon was also measured by using alpha-track detectors in cups suspended 1–2 m above the ground and exposed for a year starting in April 1988. Indoor radon was found to correlate well with broad geologically defined areas of the state. The geometric mean outdoor radon concentration was 15.54 Bq m-3, and the range was 0.3 to 55.5 Bq m-3.
Steck and others (1999) measured annual average atmospheric radon concentrations at 111 locations across Iowa beginning in 1993 and ending in the spring of 1997. They also measured ambient radon at 64 selected sites in western and northern Minnesota during 1995–1996. Comparisons were made with indoor radon; at some sites seasonal variations and variation with height were tested. Large-volume alpha-track detectors were enclosed in protective housings and placed 1.5–2 m above the ground for a year at each site. In Minnesota, concurrent annual average indoor radon measurements were also made with the same type of device. In Iowa, the researchers found that elevated outdoor radon concentrations twice the annual average reported by Hopper and others persisted over long periods and covered wide areas of the state. In both states, some outdoor concentrations were the same as or higher than the national average indoor radon concentration (46 Bq m-3). In general, outdoor radon concentrations were distributed in a geographically similar pattern to indoor measurements.
Etched-track detectors were used to measure outdoor radon and multi-room indoor radon at 100 sites in Maine from October 1980 to May 1981 to determine integrated average radon concentrations during the heating season (Hess and others 1982). The outdoor cups were in open sheltered areas on porches, garages, and sides of homes approximately one meter above the ground. Over half the houses were in geologic regions where high concentrations of radon in water were previously found. The remaining measurements were made in regions of low or intermediate concentrations of radon in water. Outdoor radon corresponded well with geology and in some instances was comparable with indoor radon concentrations. The average ambient radon concentration of 26 Bq m-3 reported by Hess and others (1982) is nearly twice the US average reported by Hopper and others (1991).