That being said, climbing here or almost anywhere at those elevations in the summer or fall is not particularly risky. In fact, my back-of-the-envelope calculations put my chances of dying on the mountain - whether due to volcanic activity or more mundane things like falls, landslides, or exposure - is about the same as the risk I incur driving to the mountain trailheads. More importantly, the combination of all these risks are dwarfed by the exercise benefits.
In short, the approximately half of a statistical life-day I "lost" due to various risks of this hobby are much smaller than the approximately four statistical life-days I "gained" due to the incredible amount of exercise each of these climbs bring. Even if the exercise didn't have this benefit, I'd be perfectly happy to give up a half day of my life if it meant eleven of them were so pleasurable. So while I appreciate your concern, just keep things in perspective - climbing is about as dangerous as a long Sunday drive, and the exercise will help ensure that I stick around to pester you with blog posts for a long, long time.
Oh, and here is Why I Climb. Need I say more?
Ash crater of Adatara-san (Fukushima)
Skies Under Iwate-San (Iwate)
Hakkoda-san in Green (Aomori)
Crater Lake at Zao (Miyagi/Yamagata)
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Top of Japan: Mt Fuji (Yamanashi/Shizuoka)
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1: Assumes 1 fatality per 150 million mile driven. This is about half the US rate, but since this hobby does not involve much driving at night or any while drunk, my odds are better
2: Volcanoes have killed about one hiker per year in Japan historically. Given that there are something like 75 climbable active volcanoes, and what appears to be a couple hundred people climb them per day throughout their 4-6 month seasons, something like 2-3 million climbs are made per year. My one-in-a-million estimate is probably high
3: This is based on my best efforts to track hiking deaths both in Japan and the US, adjusting for the difficulty of my climbs
4: Based on a study by the National Cancer Institute found here: 2.5 hours of moderate exercise or 1.25 hours of vigorous exercise per week increases your lifespan by 3.4 years, or 4.2 if you double up on the exercise. I assume I am already in the "double up" phase, and thus gain 0.8 years for each 2.5/1.25 hours. I also assume that only two-thirds of the exercise I get while climbing is additional, as I probably would have gotten some exercise (but not 6+ hours!) on those days.