A large amount of SO2 gas was discharged from Miyakejima volcano since the end of September, 2000. The averaged SO2 flux was about 50,000 ton/day in 2000. About 4,000 people could not returned to the island, because SO2 gas has been discharged, and more than 5 ppm of SO2 gas concentrations has been observed frequently around the island. In generally, volcanic gas contains mainly water vapor. The dry gas is largely composed of HF, HCl, SO2 , H2S and CO2 with small amount of N2, H2, He, Ar, CH4 and CO. HF, HCl, SO2 and H2S gases have high toxicity. The volcanic gas disaster occupied about 2.5% of volcanic disaster based on the dead since 1900. In Japan, disasters by volcanic gas occurred 28 times around eleven active volcanoes and 49 people were died during the recent 50 years. Occurrence of volcanic gas disaster is mainly dominated by the configuration of ground around fumarolic area and meterological conditions such as windless, cloudy, foggy weather and formation of inversion layer. SO2 has been released at Oyama volcano in Miyakejima island for more than two years, and the inhabitants had to leave the island. Volcanic gas disaster is influenced by meteorological conditions and local geology. In case of big eruption, volcanic gas ascends to the stratospheric, and aerosol causes low temperature in the world. To reduce the disaster of volcanic gas, it is needed to monitor with a selective gas sensor and to alarm people the forecasted dangers.
