The combined influence of fault proximity, soil radioactivity, and house characteristics on residential exposure has rarely been examined in tropical Southeast Asia. This study provides an integrated baseline of soil radioactivity and indoor radon concentrations in the San Pong sub-district, located near the Mae Tha Fault Zone, northern Thailand. Fifty soil samples were analyzed using high-purity germanium gamma spectrometry, and indoor radon concentrations were measured in 50 houses using RADUET detectors over a three-month period. The mean activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K (73.99 ± 7.73 Bq kg⁻¹, 70.53 ± 6.34 Bq kg⁻¹, and 610.54 ± 65.74 Bq kg⁻¹, respectively) exceeded the UNSCEAR (2024) worldwide and Thai national averages; consequently, the outdoor absorbed dose rate in air (102.24 ± 8.96 nGy h⁻¹), annual effective dose (0.13 ± 0.01 mSv y⁻¹), and excess lifetime cancer risk (0.52 × 10⁻³) were approximately twice the worldwide values. In contrast, indoor radon concentration was low (21.27 to 47.02 Bq m⁻³; mean 33.38 ± 5.38 Bq m⁻³), and inhalation dose (0.84 mSv y⁻¹) remained low. Neither soil radionuclides nor indoor radon increased toward the fault. Natural ventilation of the tropical houses may keep indoor radon low, whereas outdoor gamma exposure warrants continued monitoring.

Background: Understanding the existence of natural radioactivity in different soil origins is essential for evaluating associated radiological risks. Objectives: The purpose of this study was to determine the natural radioactivity concentrations in soil samples from Mae Chaem district using a HPGe detector. Materials and methods: Thirty-five soil samples were randomly collected for laboratory analysis from four geological regions, ten from Triassic granite (Trgr), Silurian Devonian-Carboniferous (SDCtp), and Ordovician (O) locations and five from the Quaternary sediments, namely the Terrace deposits (Qt) area. The radioactivity of 226Ra, 232Th, and 40K was measured using a gamma spectrometry with a high-purity germanium detector. The three radiation hazard indices for soil samples, including absorbed dose rate in outdoor air (Dair), the annual effective dose equivalent (AEDout), and the excess lifetime cancer risk outdoor (ELCRout) were determined. Results: The concentration of natural radioactivity for 226Ra of Trgr, SDCtp, O, and Qt were found in the range of 50 to 172, 46 to 179, 59 to 285, and 46 to 77 Bq kg-1, respectively. The activity concentration of 232Th varied from 69 to 148, 53 to 239, 60 to 273, and 51 to 120 Bq kg-1, whereas the activity concentration of 40K ranged from 351 to 1100, 299 to 1356, 190 to 1364, and 779 to 1188 Bq kg-1, respectively. The Dair varied from 88 to 207, 68 to 275, 94 to 345, and 85 to 146 nGy h-1, with average values 139, 131, 170, and 110 nGy h-1 for Trgr, SDCtp, O and Qt samples, respectively. All sample values were higher than the worldwide population-weighted value of 59 nGy h-1 reported by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR, 2000). The AEDout and the ELCRout values also were higher than global average values by UNSCEAR. Conclusion: In soil samples collected from four specific regions in the Mae Chaem district, the average natural activity concentrations of 226Ra, 232Th, and 40K were found to be above the worldwide population-weighted value. The Dair, AEDout in external terrestrial radiation from outdoor sources and ELCRout values were higher than reported by UNSCEAR. These results indicate a high radiological health risk associated with prolonged exposure to natural radionuclides in soils from the Mae Chaem district.