In this study, an assessment of indoor radon concentration and gamma dose rates were carried out in 16 Portuguese thermal spas. Indoor gamma dose rates measurements were made using a Geiger counter (Gamma Scout -GS3) and radon concentration measurements were carried out using CR-39 detectors. The detectors were exposed for an extended period of time (in average 42 days). The results showed that gamma doses rates are generally low but, in several cases, radon concentration exceeded national as well as international standards, namely the reference level recommended by the Directive 2013/59/EURATOM (300 Bq/m3) and the threshold for protection (400 Bq/m3) from the Portuguese legislation. The annual effective doses deriving from external radiation (gamma dose rates) and indoor radon concentration showed the need to implement measures to optimize the radiation protection of the workers against ionizing radiation.

The concentration of radon was assessed in the indoor air of 14 thermal spas and in the home of a selected worker from each thermal spa. The measurements of the indoor radon concentration were carried out using CR-39 detectors, placed in both the workplaces and workers' homes, exposed for an average period of 45 days, in different seasons of the year. For the thermal spas, the indoor radon concentration ranged between 73 and 4335 Bq m −3 , with a geometric mean of 445 Bq m −3 and an arithmetic mean of 687 Bq m −3 , and within the workers' homes ranged between 68 and 4051 Bq m −3 , with a geometric mean of 410 Bq m −3 and an arithmetic mean of 785 Bq m −3. The annual effective doses due to radon inhalation were calculated considering two different exposure scenarios for the thermal spas. For the worst-case scenario, assuming that workers do not have job rotation, the values varied between 1 and 31 mSv y −1 , while for an exposure scenario considering job rotation, the calculated dose ranged between 0.6 and 16 mSv y −1. For workers' homes, the calculated dose ranged between 2 and 102 mSv y −1. The results showed that the EU reference level of 300 Bq m −3 (Directive 2013/59/EURATOM) was exceeded in several cases. In 36% of the thermal spas the exposure of the workers is likely to exceed the effective dose of 6 mSv y −1 and in this case, according to the Directive, it should be considered as "a planned exposure situation".

Abstract. Radon concentration measurements were performed in indoor air and in natural mineral waters in seventeen

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