Radon exposure and health risks for cave guides and speleologists

Radon is a radioactive trace gas that is originated from uranium decay and released into the atmosphere from rocks and soils. It tends to accumulate in confined environments, representing the largest source of exposure to ionizing radiation for humans. Inhalation of radon and its progeny can cause DNA damage in the lungs, as the emitted alpha particles interact with biological tissues, increasing the risk of cancer with prolonged indoor exposure. Like in many other countries, radon concentration is also regulated in Italian indoor workplaces, underground and ground indoor spaces, with a maximum national reference level of 300 Bq/m3 (200 Bq/m3 for dwellings built after 2024) (Law 101/2020). Although carbonate rocks do not release radon, caves are considered as indoor environments with significant concentrations of 222Rn, especially in locations with poor ventilation. (Cigna, 2005; Ambrosino et al.,2019). Because karst rocks are permeable geological features, this geogenic gas can migrate to the surface via fractures and cavities. According to a recent study, approximately 25,000 people are employed directly as managers and guides in the global business of show caves, which attracts more than 70 million people every year (Chiarini et al., 2022). However, this number does not include researchers studying subterranean ecosystems, who spend an unquanti_ed amount of time underground. Tourist caves, as occupational exposure sites, require appropriate radiological protective measures for cave workers, as well as monitoring programs for the evaluation of the subsurface uctuation of 222Rn levels. This approach was adopted at the Su Mannau Cave (Sardinia), a large karst system (more than 8 km long) formed in Cambrian carbonate rocks, with a 300 m long tourism stretch that attracts over 15,000 tourists each year (the cave is closed to the public from November to March for micrometeorological recovery). 222Rn measurements were carried out using passive nuclear track detectors (CR-39) to determine the radon concentration in the short area of the cave accessible to the public. Six dosimeters were placed along the cave paths, in selected areas where guides spend most of their time during explanations. Radon monitoring was carried out during an observation period between February 2019 and May 2020, with four exposure intervals selected according to the expected seasonal patterns. The results show that radon concentration in Su Mannau Cave ranges from 300 Bq/m3 to 3,500 Bq/m3, with an average of about 1,350 Bq/m3. Higher 222Rn levels were recorded in the deepest part of the cave. Additionally, seasonal changes in Rn activity have been observed with higher concentrations in the summer and lower concentrations in the winter. Although a previous regional survey classified the karst area where Su Mannau Cave is located as having a low probability of exceeding the national reference level (RAS, 2022), this research highlights a discrepancy between generalized studies and the actual Rn exposure risk for cave workers, emphasizing the importance to implement radon monitoring programs in show caves.