High 222Rn levels in a show cave (Castañar de Ibor, Spain): Proposal and application of management measures to minimize the effects on guides and visitors

Castañar de Ibor (Caceres, Spain) is a low energy cave showing very high micro-environmental stability throughout the annual cycle and minimum rates of energy exchange with the atmosphere. The radon (222Rn) levels monitored inside Castañar cave reached 50,462 Bq m−3 in April 2005 and had an annual average of 32,246 Bq m−3. Annual variations in Rn concentration seem mainly related to differences in internal and external temperature. The highest values of 222Rn concentration occur during winter and early spring when air-cave temperature surpasses the external air temperature, evidencing very low air exchange rate. These values are the highest recorded in any Spanish cave, either natural or show, and are much higher than the average in most caves around the world. The calculation of the effective dose received by guides during 2004 showed values higher than the maximum effective dose recommended by authorities. Two management measurements were applied to reduce these doses: reduction of the time of visit to a maximum of 60 min, and opening the cave door 1 hour before the entrance of the guides and visitors. These management measures were effective, as they led to a decrease of 10–12% in 222Rn in the cave atmosphere during visits and prevented the guides from being exposed to higher than recommended doses of radiation.