Comprehensive Assessment of Radon (222Rn), Thoron (220Rn), and Their Progeny in a Natural Hot Spring Environment: A Case Study of Hin Dad, Kanchanaburi, Thailand

Hin Dad hot spring is a prominent natural geothermal facility in Thailand, widely frequented by domestic and international tourists seeking hydrotherapy. According to UNSCEAR, radon (222Rn), thoron (220Rn), and their short-lived decay products represent the primary sources of natural ionizing radiation exposure to humans. Geothermal waters inherently carry elevated levels of these radionuclides, potentially exposing visitors and facility staff to radiological hazards. This study evaluated 222Rn concentrations in water samples alongside ambient air concentrations of 222Rn, 220Rn, and their respective progeny within the Hin Dad hot spring area. The average 222Rn levels in water across six sampling sites ranged from 3 ± 1 to 24 ± 1 Bq L−1, remaining well below the World Health Organization (WHO) screening level for 222Rn in water (100 Bq L−1). Ambient air monitoring was conducted using both active and passive techniques. Active measurements via an AlphaGuard detector at three designated points yielded average 222Rn and 220Rn concentrations ranging from 12 to 41 Bq m−3 and 23 to 38 Bq m−3, respectively. As expected from the partitioning behavior of radon between water and air, atmospheric concentrations were markedly lower than the corresponding aqueous 222Rn concentrations. Furthermore, long-term passive monitoring over 103 days using RADUET detectors revealed maximum time-integrated average concentrations of 66 ± 1 Bq m−3 for 222Rn and 47 ± 4 Bq m−3 for 220Rn. All atmospheric concentrations complied with the reference levels established by both the WHO and the International Commission on Radiological Protection (ICRP) (100–300 Bq m−3); as this range is defined for indoor environments, it is applied here only as a conservative benchmark for the open and semi-outdoor monitoring stations. Regarding progeny exposure, the maximum Equilibrium Equivalent 222Rn Concentration (EERC) and Equilibrium Equivalent 220Rn Concentration (EETC) were determined to be 21.0 ± 2.5 Bq m−3 and 2.7 ± 0.3 Bq m−3, respectively. The measured concentrations and estimated doses indicate a low radiological concern under the conditions investigated, though the assessment is based on two seasonal sampling rounds and does not include personal dosimetry or wet-season monitoring.