Geogenic Radon Risk in Italian Show Caves: A Review

The natural release of radon from rocks is one of the main environmental risk of ionizing radiation exposure affecting human health. This radioactive element derives from radium-226 decay into the crystalline matrix. Being a gas, radon diffuses into underground spaces, leading to dangerous concentration in poorly ventilated environments, such as caves. Radon level in karst systems is controlled by different factors, such as the underground geometry, the amplitude of the cave opening, the sediment filling composition, the external meteorology, etc., and it can vary spatially and temporally from a site to another. For this reason, it is very difficult to accurately predicting radon exposure in caves and its monitoring is important to perform radiation assessment of cave workers primarily to reduce the health risks. Currently, more than 1,200 cavities worldwide are commercially exploited as touristic georesources (the so-called show caves) and in many countries they represent a very engaging and potentially competitive economic sector, favouring a rural development in remote area or extending the tourist period to the all seasons. In Italy, more than 60 natural caves are currently equipped for visits and show caves represent a valuable natural and cultural heritage, attracting hundreds of thousands of tourists every year. In these peculiar underground work spaces, the radon concentration is regulated by the Italian Law 101/2020, which establishes a reference level of 300 Bq/m3, expressed as the annual average value of radon activity concentration in the air. However, the geogenic nature of radon and the environmental conditions of caves, particularly the low ventilation and the variable microclimates, can lead to significant radon concentrations. This review critically analyses and synthesizes the existing literature on radon concentration in Italian show caves with the aim to provide an up-to-date overview of the current knowledge on occupational health in tourist caves, to highlight the current problems in estimating the exact radiation dose for cave workers and to explore the future challenges for the implementation of the adopted regulatory frameworks of radiation protection with a specific protocol for cave environment mitigation.