Soil CO₂ diffuse degassing constitutes a permanent risk in quiescent volcanic–hydrothermal areas, as is the case in the Azores archipelago. Since the early 1990s geochemical studies carried out in São Miguel Island showed that some villages are placed in anomalous high degassing areas, and indoor measurements performed in various dwellings highlight the risk to the population. These high indoor CO₂ concentrations are not only measured in areas classified as high degassing areas, but lethal CO₂ concentrations are also registered in buildings located in areas previously defined as low- and medium-risk zones. These lethal values are measured in non-ventilated environments and basements in areas with soil CO₂ concentration above 1.5 vol%. Hazardous CO₂ concentrations are also commonly measured in buildings located in zones where soil CO₂ is higher than 5 vol%. Considering the dangerous values registered and the fact that indoor gas concentration can increase several orders of magnitude owing to peculiar meteorological conditions, updated values are suggested for the correlation between soil gas concentration and CO₂ exposure. This study highlights that both the use of soil degassing maps by land-use planners and appropriate construction rules for buildings placed in degassing areas are necessary.