Assessment of natural background radiation in the tourist area of Biserna Cave based on in situ measurements

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.