Carbon dioxide (CO2) is permanently released in volcanic areas not only during eruptive events but also during quiescent periods of activity, being one of the main gases emitted by thesoils in the diffuse degassing areas of the Azores archipelago. From a health point of view, CO2 acts as an asphyxiant and in concentrations above 10 vol.% can be lethal. Since 2010 a new anomalous CO2 degassing area has been developing close to Caldeiras da Ribeira Grande site, an area with 15 buildings located in the north flank of Fogo Volcano (S. Miguel Island). High indoor CO2 concentrations were measured in some of the buildings leading, in the end of 2012, to the installation of an indoor CO2 monitoring and alarm system with 52 CO2 sensors inside 12 of the edifices. Data is saved locally and sent to the CVARG/CIVISA installations through internet/radio telemetry systems. The detectors are installed in different compartments of the buildings both in the ground and first floors, when the edifice has two levels. Indoor CO2 values recorded in the ground floor were significantly higher (reaching values as high as 9.5 vol.%) than values measured in the first floor of the buildings. However, in buildings placed over a thermal anomaly, CO2 values reached also higher concentrations in the 1st level (1.5 vol.%), probably due to the decrease of gas density related with the thermal effect. Existence of thermal anomaly associated with the CO2 degassing also interferes with the seasonal patterns of the times series recorded. Buildings located in areas without thermal anomaly showed higher CO2 concentrations during winter time when compared with the summer period, what is in agreement with data recorded in other degassing areas of the archipelago. This behaviour is explained by the effect of meteorological conditions (e.g. decreases in barometric pressure) on the soil gas emissions. However, some buildings located over a thermal anomaly showed higher CO2 concentrations during the summer period, probably consequence of the increase temperature of the soil below the building and consequent increase in the convection of the gas. When CO2 concentration values reach the permissible exposure limit (0.5 vol.%), an alarm is triggered and a report is sent to the local authorities. The installation of the monitoring systems allowed characterizing the indoor CO2 variations in the buildings and identifying the hazardous situations that need mitigation measures (e.g. installation of ventilation systems) or even the evacuation of the residents.