High radon levels in subterranean environments: monitoring and technical criteria to ensure human safety (case of Castañar cave, Spain)

• Castañar cave contains the highest radon concentration in Spain (average: 31.9 kBq m−3).
• The Rn concentration decreases by advection processes during the dry periods (Text > Tcave).
• The most connected areas inside the cave accumulate more Rn in winter and ventilate more in summer.
• Public visitors received 12.1% of the total effective dose permitted per visit based on legal constrains.
• Passive Rn detectors introduce a significant error in the effective dose calculations.

Castañar cave contains the highest radon gas (222Rn) concentration in Spain with an annual average of 31.9 kBq m−3. Seasonal variations with summer minimums and maximum values in fall were recorded. The reduction of air-filled porosity of soil and rock by condensation or rainfalls hides the radon exchange by gas diffusion, determining this seasonal stair-step pattern of the radon activity concentration in underground air. The effective total dose and the maximum hours permitted have been evaluated for the guides and public safety with a highly detailed radon measurement along 2011 and 2012. A network of 12 passive detectors (kodalphas) has been installed, as well as, two radon continuous monitoring in the most interesting geological sites of the subterranean environment.A follow up of the recommended time (max. 50 min) inside the underground environment has been analysed since the reopen to public visitors for not surpassing the legal maximum effective dose for tourists and guides. Results shown that public visitors would receive in fall a 12.1% of the total effective dose permitted per visit, whereas in summer it is reduced to 8.6%, while the cave guide received a total effective dose of 6.41 mSv in four months.The spatial radon maps allow defining the most suitable touristic paths according to the radon concentration distribution and therefore, appropriate fall and summer touristic paths are recommended.