| Summary: | 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.
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