A study was conducted to determine the annual average radon concentrations in and near California residences, to determine the approximate fraction of the California population regularly exposed to residential radon concentrations exceeding 4 pCi/l and 8 pCi/l, and to the extent possible, to identify factors (such as geologic region or building type) which may be useful in predicting high risk areas or groups in California. Annual average indoor radon concentrations were measured with passive (alpha track) samplers sent by mail and deployed by home occupants, who also completed questionnaires on building and occupant characteristics. One quarter of the households also deployed an outdoor sampler. The geographic areas sampled included the Salinas/Santa Cruz area (pretest area - 38 residences), the entire state (310 residences), and portions of the southern Sierra Nevada foothills (37 residences). A subset of the residences was studied in detail to determine factors affecting indoor radon concentrations.
Results of pretest indoor measurements ranged from 0.2 pCi/l to 5.9 pCi/l, with geometric means of 0.85 pCi/l in bedrooms, and 0.93 pCi/l in living rooms. For the statewide main survey, indoor radon concentrations ranged from 0.1 pCi/l to 16 pCi/l, with a geometric mean of 0.83 pCi/l in bedrooms and 0.85 pCi/l in living rooms. When the whole-house value was calculated by averaging the concentrations in the bedroom and living room, the geometric mean was 0.85 pCi/l and the geometric standard deviation was 1.91. The geometric mean of the concentrations in the 27 basements measured was 2.17 pCi/l, which was significantly higher than that of whole-house concentrations. Whole-house, annual average indoor measurements in the Sierra foothills area ranged from 0.4.pCi/l to 8.8 pCi/l with a geometric mean of 1.28 pCi/l. The geometric mean of annual average outdoor radon concentrations was 0.55 pCi/l for the pretest area, 0.42 pCi/l for the statewide survey, and 0.66 pCi/l for the southern Sierra foothill area. The estimated fractions of California residents exposed to radon concentrations exceeding 4 pCi/l and 8 pCi/l are 0.8% and 0.03% respectively. The estimated number of residents exposed to concentrations exceeding 4 pCi/l is 240,000, and the expected number of residents exposed to concentrations exceeding 8 pCi/l is 8,900. The best single predictor of indoor radon concentrations found in this study was the emanation rate of radon from soil. Other variables found to be associated with indoor concentrations were geographic region, ventilation, type of substructure, type and age of residence. In addition to the Sierra foothills, Ventura County was identified as an area with elevated radon concentrations. Residences with rarely-opened windows and doors, dwellings with a concrete slab, single-family houses, and new structures were found in general to have higher radon concentrations.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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