Abstract
This study presents the first detailed in situ assessment of natural background radiation in the tourist-accessible portion of Biserna Cave, Bulgaria. Using portable gamma-ray dosimeter and radon detectors, we measured ambient gamma-ray dose rates and radon concentrations along the cave route. Radon was measured at eleven depth-profile points, and a time-weighted average inhalation dose was calculated using a 40% – 20% – 40% time-distribution model (entry – final – travel). The resulting 1-hour inhalation dose (radon) is ~ (3.81 – 10.695) µSv, including a mean gamma-ray component (0.177 µSv/h), the max total is ≈ 10.872 µSv. For a seasonal visitor population of 2,000 – 2,500, the max collective dose is 21.7 – 27.2 mSv. Tour guides, with ~ 115 h exposure annually, receive ~ (0.42 – 1.22 mSv) ± (0.126 – 0.450) mSv. These values lie well below regulatory limits for public exposure, but the elevated radon concentration levels in deeper zones merit attentive occupational monitoring. The study underscores the importance of depth-resolved profiling and site-specific dose modelling in cave ecotourism settings.