Natural Radioactivity in Soil, Indoor Radon Concentrations, and Radiological Risk Assessment near the Mae Tha Fault Zone, San Pong Sub-District, Chiang Mai Basin, Thailand

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.