The discovery of large gas emissions from the ground inside the caldera of Furnas, and especially in the town, has drawn attention to the possible hazard posed by unusual or unexpected gas emanation inside dwellings or spaces below ground. Two common gases are found at high concentrations in the area where people live: carbon dioxide and radon, associated with asphyxia and lung cancer respectively. In order to evaluate the risk of this soil degassing, a map at a scale of 1/2000 was drawn, covering the inhabited area of the caldera, showing the degassing zones. The map indicates correlations between soil degassing and air pollution inside dwellings. This study does not take into account any sudden gas burst triggered for instance by a local or regional earthquake. The conditions of temperature and pressure of the upper hydrothermal reservoir (Oskarsson et al. 1994) suggest that such an event could occur inside the caldera. A huge amount of gas and steam can be released in a very short time, creating a flow of gas, the effects of which will depend on the site of the explosion and the topography of the environment.