Assessment of population exposure to terrestrial gamma radiation in South Korea.

The objective of this study is to quantitatively estimate the annual population exposure to terrestrial gamma radiation in South Korea by conducting a high-resolution car-borne survey across the entire country. From 2013 to 2017, a total of 723 052 gamma-ray measurements were collected using a vehicle-mounted NaI(Tl) scintillation detector. The measured count rates were converted into the activity concentrations of radionuclides (²²⁶Ra,²³²Th, and⁴⁰K), and subsequently into absorbed dose rates in air (nGy h^-1) using Beck's formula. The spatial distribution of dose rates was visualized using both inverse distance weighting and kriging interpolation methods. Based on the 2020 population census data, population-weighted averages were calculated at both administrative and national levels. Annual effective doses were subsequently estimated by applying dose conversion coefficient and occupancy factors. The national population-weighted outdoor absorbed dose rate in air was 108 ± 19.5 nGy h^-1, which is approximately 1.8 times higher than the global average of 59 nGy h^-1reported by UNSCEAR, and about 16% higher than the previous national survey result (93 nGy h^-1). The corresponding annual effective dose was estimated to be 0.87 mSv, with approximately 85% attributable to indoor exposure. Among the radionuclides,²³²Th showed the highest contribution to the total dose, and a strong positive correlation was observed between²³²Th and⁴⁰K (r= 0.6530). In contrast, a weak but significant positive correlation was found between outdoor terrestrial gamma dose rates and indoor radon concentrations (r= 0.2063), suggesting that indoor radon levels are influenced by various environmental factors beyond gamma radiation, which should be considered when evaluating or predicting indoor radon exposure. This study presents the most comprehensive and representative estimates of terrestrial gamma radiation exposure in South Korea, based on the largest nationwide dataset of terrestrial gamma dose rates to date. These results can serve as a scientific basis for developing and optimizing radiation protection strategies under existing exposure situations.