Abstract: A comprehensive radon survey has been carried out in B?i?a radon-prone area, Transylvania, Romania, in 4 localities (B?i?a, Nucet, Fîna?e, and Cîmpani) situated in the vicinity of former Romanian uranium mines. Indoor radon concentrations have been measured in 1128 ground floor rooms and cellars of 303 family houses by using CR-39 diffusion type radon detectors. The annual average of indoor radon concentration for B?i?a area was found to be 241 ± 178 Bq m? 3, which is about two times higher than the average value of 126 Bq m? 3, computed for Romania. About 28% of investigated houses exceed the reference level of radon gas in dwellings of 300 Bq m? 3. The indoor radon measurements on each house have been carried out in several rooms simultaneously with the aim of obtaining a more detailed picture on the exposure to radon in the studied area. An analysis on the variability of radon levels among floors (floor-to-floor variation) and rooms (room-to-room variation) and also the influence of factors like the presence of cellar or the age of the building is presented. The coefficient of variation (CV) within ground floor rooms of the same house (room-to-room variation) ranged between 0.9 and 120.8%, with an arithmetic mean of 46.2%, a large variability among rooms within surveyed dwellings being clearly identified. The mean radon concentration in bedrooms without cellar was higher than in bedrooms above the cellar, the difference being statistically significant (t test, one tail, p < 0.001, n = 82). For houses built during 1960?1970 an increasing trend for radon levels was observed, but overall there was no significant difference in indoor radon concentrations by age of dwelling (one-way ANOVA test, p > 0.05).

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