The gas gives Japan's onsen resorts an unmistakable sulphurous smell, but it can be dangerous if inhaled in high enough concentrations. Authorities said there is a possibility the three inhaled the gas, the Asahi Shimbun reported. At safe levels, the gas is said to be good for high blood pressure, joint pain and other ailments – health benefits that have long attracted visitors to the green-white waters of the 400-year-old resort. However, it can also be hazardous outdoors in poorly ventilated spaces and in high concentrations. The victims – the manager of Kagetsu Highland Hotel, a member of staff and the managing director of the company that runs the hotel – had been conducting routine maintenance checks at the hot spring's source when they were believed to have got into difficulty, Japanese media reported. Authorities were alerted on Monday evening when the men in their 50s and 60s failed to return to the hotel. A doctor at the scene confirmed they had died, the Asahi Shimbun newspaper said, but authorities have yet to determine the official cause of death.