Hazardous indoor CO₂ concentrations in volcanic environments

Carbon dioxide is one of the main soil gases released silently and permanently in diffuse degassing areas, both in volcanic and non-volcanic zones. In the volcanic islands of the Azores (Portugal) several villages are located over diffuse degassing areas. Lethal indoor CO₂ concentrations (higher than 10 vol %) were measured in a shelter located at Furnas village, inside the caldera of the quiescent Furnas Volcano (S. Miguel Island). Hazardous CO₂ concentrations were detected not only underground, but also at the ground floor level. Multivariate regression analysis was applied to the CO₂ and environmental time series recorded between April 2008 and March 2010 at Furnas village. The results show that about 30% of the indoor CO₂ variation is explained by environmental variables, namely barometric pressure, soil water content and wind speed. The highest indoor CO₂ concentrations were recorded during bad weather conditions, characterized by low barometric pressure together with rainfall periods and high wind speed. In addition to the spike-like changes observed on the CO₂ time series, long-term oscillations were also identified and appeared to represent seasonal variations. In fact, indoor CO₂ concentrations were higher during winter period when compared to the dry summer months. Considering the permanent emission of CO₂ in various volcanic regions of the world, CO₂ hazard maps are crucial and need to be accounted by the land-use planners and authorities. - Highlights: • CO₂ may be a permanent and silent hazard in active and quiescent volcanic areas. • Soil CO₂ is influenced by meteorological conditions in degassing environments. • Hazardous indoor CO₂ concentrations are caused by stormy weather conditions. • Seasonal cycles are detected with higher CO₂ concentrations recorded during winter. - Release of CO₂ from soils in volcanic regions is dependent on the meteorological conditions that explain up to 30% of the indoor CO₂ variations, with consequent impact for human health.