Furnas Volcano is one of the three active central volcanoes of S. Miguel Island, in the Azores Archipelago. Fumarole grounds, thermal springs, CO₂ rich mineral cold waters and several soil diffuse degassing areas are the main secondary volcanic phenomena that can be found on Furnas Volcano surface. Several surveys of ²²²Rn soil diffuse degassing have been performed in the soil CO₂ diffuse degassing area previously identified for seismovolcanic monitoring and public health purposes. The ²²²Rn concentration map obtained allowed to realise that several dwellings at Furnas Village are placed on ²²²Rn concentration anomalous zones.

When dispersed in atmospheric air ²²²Rn does not seem to represent a hazard for the health, however, when it accumulates in confined spaces, as in dwellings, it can represent a serious threat. Being a radioactive gas it can be easily introduced into the human body through inhalation, where the alpha particles can damage the lounge tissue conducting to the development of lounge cancer. 

A continuous monitoring test to measure indoor ²²²Rn and CO₂ concentration was performed during November and December 2005 in the basement of a dwelling located at Furnas village. ²²²Rn concentration values oscillated between 0 and 13272.91 Bq/m³ and CO₂ concentration values varied from 0 to 6 %vol. The highest values were obtained when the ventilation efficiency was reduced, or null, being the ²²²Rn values, on those periods, significantly higher than the security level (400 Bq/m³) recommended by the portuguese law. In these conditions, also the concentration of CO₂ reached values above the recommended ones by the International Organizations for indoor air quality. In this study, the influences of several factors such as changes on the meteorological conditions, indoor ventilation efficiency, soil CO₂ flux and soil ²²²Rn concentration variations were taken into account. Indoor ²²²Rn and CO₂ concentration seems to depend essentially, on ventilation, which in this case is controlled by the wind speed and residents’ habits.