Radon in the dry carbon dioxide spa of Mátraderecske, Hungary

The final product of a post-volcanic activity is the exhalation of low-temperature (<100°C) gases containing mostly carbon dioxide. The phenomenon is called mofettes, which are often used for therapeutic treatments in the form of dry CO₂ spas. Along its pathway to the surface, the deep origin gas also intakes different radon isotopes from the rocks and soils; therefore, the risks associated with radon exposures should also be a concern. In this work, we have found that the ²²²Rn activity concentration in the mofette gas of Mátraderecske is particularly high; it is in the order of 200 kBq·m⁻³. However, owing to the carefully designed flow pattern of mofette gas and fresh air, the radon level is about 1 kBq·m⁻³ at the breath level of the staff, accompanying the treatment, which is the radon reference level for workers in Hungary. We have also found that in this dry spa, radon is a good tracer of CO₂; therefore, it can be used to monitor the CO₂ distribution in the treatment pools.