In Central America, volcanoes and earthquakes pose major environmental threats to populations. In particular, degassing volcanoes may adversely affect the health of local inhabitants. For example the recent gas emission crisis at Masaya, Nicaragua, which began in 1979, exposed thousands of people to hazardous levels of SO2 and sulphate aerosols downwind. During different phases of their activity other volcanoes may emit toxic plumes into populated areas, e.g. San Cristóbal, Nicaragua. Poás, a composite basaltic-andesite cone situated on the Cordillera Central, of Costa Rica, rises 1300mn above its base at 1400m. Historically it has been in a state of nearly continuous mild activity. The active crater is roughly circular with a diameter of about 1100m and a crater lake has existed since 1965. Until recently the heat input from the presumed shallow magma body was balanced by the heat output from the fumaroles and crater lake, with degassing occurring through the central vent beneath the lake. Since 1987, gas emissions from Poás volcano have increased and have been reported to affect vegetation and cause health problems in the populated areas downwind. During this period the hot crater lake has undergone an annual cycle in which its level rises and falls with the wet and dry seasons; in the dry season it becomes strongly acid because of enhanced evaporation and concentration of anions. Therefore, in addition to gas from the crater fumaroles the plume can contain copious amounts of concentrated acid aerosol from the lake which may greatly increase the health hazard from inhaling contaminated air. The changes at Poas probably reflect a dramatic increase in power input consistent with the injection of shallow magma beneath the crater lake and wich may have coincided with an increase in the flux of gases and aerosols since 1986