A number of volcano observatories have implemented their own SO2 guidelines. At Mt. Aso crater, Japan, for example, visitors are evacuated when SO2 levels exceed 0.2 ppm continuously for 1 minute or instantaneous levels exceed 5.0 ppm. These levels were reduced from >5 ppm for 5 minutes following gas related fatalities in the 1990's (Ng'Walali et al., 1999). In 2000, Hawaii Volcanoes National Park in collaboration with the USGS Hawaiian Volcano Observatory introduced a set of SO2 advisories to protect staff and visitors to the park (below).