Radon survey in Montenegro - A base to set national radon reference and "urgent action" level

The first nationwide indoor radon survey in Montenegro started in 2002 and year-long radon measurements with CR-39 track-etch detectors, within the national grid of 5 km × 5 km and local grids in urban areas of 0.5 km × 0.5 km, were performed in homes in half of the country's territory. The survey continued in 2014 and measurements in the rest of the country were completed at the end of 2015. The 953 valid results, obtained in the national radon survey, give an average radon activity concentration in Montenegrin homes of 110 Bq/m³. Assuming a log-normal distribution of the experimental results, geometric mean GM = 58.3 Bq/m³ and geometric standard deviation GSD = 2.91 are calculated. However, normality tests show that the experimental data are not log-normal, and that they become closest to a log-normal distribution after subtracting from them radon concentration in the outdoor air of 7 Bq/m³, which is theoretically calculated. Such a transformed distribution has GM_tr = 46.7 Bq/m³ and GSD_tr = 3.54. The estimations derived from positing a priory that the experimental results conform to a log-normal distribution underestimate the percentage of homes with radon concentrations at the thresholds of 300 Bq/m³ and above, which is better estimated by using GM_tr and GSD_tr. Based on the results of radon survey, a new national radon reference level of 300 Bq/m³ and an "urgent action level" of 1000 Bq/m³ are suggested, with estimated fractions of the national dwelling stock above these levels of 7.4% and 0.8% respectively. Fractions of homes with radon concentrations above the suggested levels are also estimated for each of the 23 municipalities in Montenegro, using appropriate GM_tr and GSD_tr. The six municipalities which have more than 10% of homes with radon concentration above 300 Bq/m³ are recommended as radon priority areas.