Geogenic determinants of indoor radon exposure in İzmir (West Türkiye).

Radon, a naturally occurring product of uranium decay, is the second leading cause of lung cancer. İzmir Province in western Türkiye, situated within the Aegean extensional regime, comprises complex fault-bounded basins that favor indoor radon accumulation. This study evaluates the spatial variability and geogenic controls of indoor radon to delineate radon-prone zones with public-health relevance. Indoor radon was measured in 79 dwellings distributed across major lithologies and structural settings; detectors were deployed in basements to capture soil-gas infiltration. Concentrations ranged from 12 to 366.5 Bq/m (mean 118 Bq/m), exceeding the national average of 81 Bq/m; 32 % of sites surpassed the EPA action level of 148 Bq/m. Highest values cluster in Bornova, Buca, and Kemalpaşa, coincident with fault-controlled sedimentary basins and permeable units. Spatial mapping highlights the dominant influence of lithology and fault proximity on radon distribution and underscores the limitations of uniform, national-scale mitigation policies. We advocate targeted, geology-aware health policies and urban-planning measures for monitoring and mitigation in geogenically vulnerable districts. These findings contribute to medical geology by providing region-specific evidence of radon risk in one of Türkiye's most seismically active metropolitan areas. These outputs provide decision-ready evidence for monitoring, mitigation, and building-code updates in seismically active metropolitan settings.