Volcanic activity can inject large quantities of gases and aerosols into the atmosphere both during and between eruptions, creating a health risk for the local population. The paper describes how the volcanic gas concentration in the air can be computed by a flow model simulating the wind field over a digital terrain model of the volcano coupled with a Lagrangian particle model that uses the known (measured) gas emission rates to simulate gas dispersion. The coupling provides hazard maps for a number of meteorological conditions, introduced as boundary and initial conditions to the wind flow model, and permits the estimation of the risk both for actual and increased emission rates. An application for Vulcano Island (Sicily, Italy) is presented. According to the results, the risk at Vulcano is mainly related to: i) SO2 from the fumaroles of La Fossa cone; ii) the diffuse soil emission of CO2 from the cone flanks and from the plane around the cone. Although the conclusion is that there is no major hazard for passive degassing at present flux rates, the situation could change in the future. If gaseous emission rates would increase by one order of magnitude above the peak values of a few years ago, the gas concentrations could reach dangerous thresholds for the people of Vulcano Porto village, 1 km NW to the cone. ¿ 1999 American Geophysical Union