A comprehensive radon survey has been carried out in seven caves located in the western half of Romania's most significant karst regions. Touristic and non-touristic caves were investigated with the aim to provide a reliable distribution of their radon levels and evaluate the occupational exposure and associated effective doses. Radon gas concentrations were measured with long-term diffusion-type detectors during two consecutive seasons (warm and cold). All investigated caves exceed the European Union reference level of radon gas at workplaces (300 Bq/m). The radon concentration in these caves ranges between 53 and 2866 Bq/m, reflecting particular cave topography, season-related cave ventilation, and complex tectonic and geological settings surrounding each location. Relatively homogeneous high radon levels occur in all investigated touristic caves and in Tăuşoare and Vântului along their main galleries. Except for Muierii, in all the other caves radon levels are higher during the warm season, compared to the cold one. This suggests that natural cave ventilation largely controls the underground accumulation of radon. The results reported here reveal that the occupational exposure in Urşilor, Vadu Crişului, Tăuşoare, Vântului, and Muierii caves needs to be carefully monitored. The effective doses to workers vary between an average of 0.25 and 4.39 mSv/year depending on the measuring season. The highest values were recorded in show caves, ranging from 1.15 to 6.15 mSv/year, well above the European recommended limit, thus posing a potential health hazard upon cave guides, cavers, and scientists.

A case-control study was conducted in two Romanian counties (Alba and Cluj) in order to investigate lung cancer risk due to residential radon exposure. Two hundred and forty-one subjects were included, of which 104 cases and 137 controls. Residential radon concentrations were monitored using CR-39 track detectors, exposed over a period of at least 6 months. The odds ratios were estimated by using multivariable logistic regression adjusted for age, sex, smoking status, education and family history of lung cancer. The adjusted odds ratios were 1.64 (95% CI: 0.67 -3.98), 2.40 (95% CI: 0.91 -6.35) and 2.12 (95% CI: 0.80 -5.63) for 50-100, 100-147 and more than 147 Bq/m 3 compared with 0-50 Bq/m 3 . The estimated excess odds ratio (EOR) per 100 Bq/m 3 radon concentration was 0.23 (95% CI: -0.11 -0.57) (p = 0.11). The main limitation of the study is represented by the small number of cases and controls, along with the absence of matching of the two samples for age or sex. The results from this study suggest a weak association (generally statistically insignificant) between indoor radon exposure and lung cancer.