Radon-222 in karst caves of Southwestern China: Sources, seasonal dynamics, and human exposure risks

Karst cave systems represent significant reservoirs of Radon-222 (²²²Rn), which is a natural radioactive and class-I carcinogenic pollutant whose risk is amplified by increasing cave tourism. However, the dominant sources, driving mechanisms, and resulting exposure levels in these subterranean environments remain poorly understood. Therefore, we conducted a comprehensive monitoring campaign across 24 karst caves in Southwest China while integrating field measurements with controlled simulations and dose assessment. Results indicate that both the carbonate bedrock and overlying soil layer act as primary sources of cave ²²²Rn, with soil contributing substantially more due to secondary radionuclide enrichment during weathering. Seasonal ventilation, driven by temperature-induced airflow (chimney effect), was identified as the dominant factor controlling ²²²Rn variability, resulting in three distinct seasonal patterns: summer-high/winter-low, winter-high/summer-low, and annual-average. Further, we further elucidated the source-transport-sink dynamics governing ²²²Rn behavior in cave systems and highlighted the critical role of cave structure in modulating airflow and ²²²Rn accumulation. Although the short-term exposure risk for tourists is negligible, the annual effective radiation dose experienced by occupational groups in nine tourist caves exceeds regulatory limits, underscoring the necessity of the development of targeted protective measures. This study elucidates the source-transport-exposure pathway of ²²²Rn pollution in karst cave environments and provides a scientific basis for stratified risk management. Our findings emphasize the importance of integrating environmental process research into public health protection frameworks for high-risk micro-environments.