Indoor radon levels and their dynamics in relation with the geological characteristics of La Garrotxa, Spain

Indoor radon concentration has been measured in workplaces and dwellings of the volcanic region of La Garrotxa and in the vicinity of the recently active Amer fault, in northeastern Spain. The radon levels obtained do not constitute a relevant risk for workers. Higher indoor radon levels have been found on volcanic materials (mean 133 Bq m−3, range 15-1464 Bq m−3) than on non-volcanic materials (mean 67 Bq m−3, range 16-327 Bq m−3). A summertime elevation of radon levels that does not happen in buildings placed on non-volcanic materials has been observed as consequence of convective airflow from the soil to indoor environments through a network of subterranean channels present in volcanic materials. This effect is very important if the subterranean channel reaches the indoor environment through natural holes called blowholes. A possible influence of the Amer fault has been seen in La Vall d'en Bas, one of the five villages studied, which has a more homogeneous underlying geology.