Volcanic ash
According to a 2014 study by KOBEC of Kobe University, in a worst-case scenario if there is a VEI-7 eruption of the Aso Caldera and if the volcanic ash is carried by westerly winds, then pyroclastic flows would cover the 7 million population near the Aso Caldera within two hours. The pyroclastic flows could reach much of Kyushu. Beyond the pyroclastic area is volcanic ash that falls from the sky. If the volcanic ash continuously flows northward, then the ash fall would make it impossible to live normally in large parts of the main islands of Japan, due to paralysis of traffic and lifelines for a limited period (a few days to 2 weeks) until the eruption subsides. In this scenario, the exception would be eastern and northern Hokkaido (the Ryukyu Islands and southern Nanpo Islands would also be excepted). Professor Yoshiyuki Tatsumi, head of KOBEC told the Mainichi Shimbun "the probability of a gigantic caldera eruption hitting the Japanese archipelago is 1 percent in the next 100 years" with a death toll of many 10s of millions of people and wildlife. The potential exists for tens of millions of humans and other living beings to die during a VEI-7 volcanic eruption with significant short-term effects on the global climate. Most casualties would occur in Kyushu by the pyroclastic flows. The potential damage of the volcanic ash depends on the Wind direction. If in another scenario, the wind blows in a western or southern direction then the Volcanic ash could affect the East Asian continent or South-East Asia. If the ash flows eastward then it will spread over the Pacific Ocean. Since the Kikai Caldera is submerged, it is unclear how much damage the hot ash clouds would cause if large quantities of volcanic ash stays beneath the ocean surface. The underwater ash would be swept away by ocean currents.