Measurements of indoor radon concentrations were performed in 406 residential houses in five counties (Cluj, Bihor, Alba, Bistrita, and Sibiu) using Makrofol and CR-39 alpha-track detectors. From our measurements, an average indoor radon concentration of 82.5 Bq m⁻³ for the Transylvanian population was calculated, i.e. an annual effective dose of 2.4 mSv for the whole body. The calculated dose is 62% higher than that previously reported but yet below the recommended action level of ICRP (International Commission on Radiological Protection). A log-normal distribution of the radon concentration was obtained for the studied counties, excluding some higher values from the Stei region – a radon-prone area in Transylvania. For the Stei region, the data show a dual log-normal distribution of the radon concentration with the second maximum being related to the houses built using uranium waste as a construction material. Assuming a cancer risk coefficient of 10⁻⁴/100 Bq m⁻³ year⁻¹, one can estimate that about 600 lung cancer per year, for about 7,000,000 inhabitants of the Transylvania region, are due to radon inhalation.

Radon represents the most important contribution of population exposure to natural ionising radiation. This article presents the first indoor radon map in some regions of Romania based on 883 surveyed buildings in the Ştei-BăiŢa radon-prone region and 864 in other regions of Romania. Indoor radon measurements were performed in the last 10 y by using CR-39 nuclear track detectors exposed for 3-12 months on ground floor levels of dwellings. Excluding the Ştei-BăiŢa radon-prone region, an average indoor radon concentration of 126 Bq m(-3) was calculated for Romanian houses. In the Ştei-BăiŢa radon-prone area, the average indoor concentration was 292 Bq m(-3). About 21 % of the investigated dwellings in the Ştei-BăiŢa radon-prone region exceed the threshold of 400 Bq m(-3), while 5 % of the dwellings in other areas of Romania exceed the same threshold. As expected, indoor radon concentration is not uniformly distributed throughout Romania. The map shows a high variability among surveyed regions, mainly due to the differences in geology. The radon emanation rate is substantially influenced by the soil characteristics, such as the soil permeability and soil gas radon concentration. Since higher permeability enables the increased migration of soil gas and radon from the soil into the building, elevated levels of indoor radon can be expected in more permeable soil environments.