Soil CO 2 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 CO 2 concentrations are not only measured in areas classified as high degassing areas, but lethal CO 2 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 CO 2 concentration above 1.5 vol%. Hazardous CO 2 concentrations are also commonly measured in buildings located in zones where soil CO 2 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 CO 2 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.

Fluid geochemistry monitoring in the Azores involves the regular sampling and analysis of gas discharges from fumaroles and measurements of CO2 diffuse soil gas emissions. Main degassing areas under monitoring are associated with hydrothermal systems of active central volcanoes in S. Miguel, Terceira and Graciosa islands. Fumarole discharge analysis since 1991 show that apart from steam these gas emissions are CO2 dominated with H2S, H2, CH4 and N2 in minor amounts. Mapping of CO2 diffuse soil emissions in S. Miguel Island lead to the conclusion that some inhabited areas are located within hazard-zones. At Furnas village, inside Furnas volcano caldera, about 62% of the 896 houses are within the CO2 anomaly, 5% being in areas of moderate to high risk. At Ribeira Seca, on the north flank of Fogo volcano, few family houses were evacuated when CO2 concentrations in the air reached 8 mol%. To assess and analyse the CO2 soil flux emissions, continuous monitoring stations were installed in S. Miguel (2), Terceira and Graciosa islands. The statistical analysis of the data showed that some meteorological parameters influence the CO2 flux. The average of CO2 flux in S. Miguel stations ranges from 250 g/m 2 /d at Furnas volcano to 530 g/m 2 /d at Fogo volcano. At Terceira Island it is about 330 g/m 2 /d and at Graciosa 4400 g/m 2 /d.

The Azores archipelago is composed of nine volcanic islands located at the triple junction between the North American, Eurasian, and Nubian plates. Nowadays the volcanic activity in the archipelago is characterized by the presence of secondary manifestations of volcanism, such as hydrothermal fumaroles, thermal and cold CO 2 -rich springs as well as soil diffuse degassing areas, and low magnitude seismicity. Soil CO 2 degassing (concentration and flux) surveys have been performed at Pico, Faial, and São Jorge islands to identify possible diffuse degassing structures. Since the settlement of the Azores in the fifteenth Century these three islands were affected by seven onshore volcanic eruptions and at least six destructive earthquakes. These islands are crossed by numerous active tectonic structures with dominant WNW-ESE direction, and less abundant conjugate NNW-SSE trending faults. A total of 2,855 soil CO 2 concentration measurements have been carried out with values varying from 0 to 20.7 vol.%. Soil CO 2 flux measurements, using the accumulation chamber method, have also been performed at Pico and Faial islands in the summer of 2011 and values varied from absence of CO 2 to 339 g m −2 d −1 . The highest CO 2 emissions were recorded at Faial Island and were associated with the Pedro Miguel graben faults, which seem to control the CO 2 diffuse degassing and were interpreted as the pathways for the CO 2 ascending from deep reservoirs to the surface. At São Jorge Island, four main degassing zones have been identified at the intersection of faults or associated to WNW-ESE tectonic structures. Four diffuse degassing structures were identified at Pico Island essentially where different faults intersect. Pico geomorphology is dominated by a 2,351 m high central volcano that presents several steam emissions at its summit. These emissions are located along a NW-SE fault and the highest measured soil CO 2 concentration reached 7.6 vol.% with a maximum temperature of 77 • C. The diffuse degassing maps show that anomalous CO 2 degassing areas are controlled essentially by the tectonic structures and the lithology of the sites since the youngest volcanic systems are characterized by very low CO 2 emissions.