Sooooo, you're taking an early autumn hike with your friends on a Japanese mountain when it suddenly starts to erupt! What a crazy situation. Had flashbacks to 9/11, seeing that ash cloud envelope the hikers.
I'm not sure what the best course of action would be for the folks high up on the slope. You can't outrun the ash cloud. If there's a building, I suppose sheltering inside would be best. If out in the open, perhaps fashioning a dust mask would be the first priority. That ash apparently turns into almost like a concrete in your lungs if you breathe it in. Reports say 7 hikers lost consciousness in eruption.
It doesn't seem like the ash cloud was hot enough to kill anyone outright, at least according to the reports I've seen so far. So, I guess it's a simple ashfall rather than a flesh-cooking pyroclastic flow, but who knows, perhaps it's simply too early for the fatalities to be found. Here's
a first person POV of the eruption by someone on the mountain. Here's
a helicopter view of the eruption.