[t0mas88]

Were there other issues involved in the fatalities around 12-14k ft? Freezing etc?

In aviation rules you can have passengers at 12k ft without oxygen for an unlimited amount of time. The crew needs to use oxygen if you're between 10k and 13k for more than 30 minutes. Above 13k both crew and passengers must use oxygen immediately (EASA rules, FAA is different).

So they seem to consider 12k to not be dangerous to passengers.

[simplyluke]

Speaking from experience in the mountains: 12k at rest and 12k under subsequent days of exercise produce very different responses. What might be a mild headache sitting in the back of a plane could be a pretty distinct AMS case lugging a pack up and down mountains.

[antonvs]

The main issue is that exercise exacerbates hypoxia. You're climbing a mountain, your muscles need oxygen, but you're not getting enough.

But the temperature does make it worse. Your body is trying to generate heat, which increases oxygen demand. Your blood vessels are constricting, so circulation is less efficient, limiting oxygen distribution. All together, this creates all sorts of health risks.

Sitting in a comfy plane seat browsing the internet is not the same thing. Besides, even healthy adults just sitting can experience mild hypoxia effects above about 10k ft - fatigue, mild cognitive effects, headaches. But if you're just sitting, it's generally tolerable. Of course, you don't want pilots working in those conditions for any length of time.

[happosai]

There are like a half dozen cities with 100.000+ population at over 12k feet altitude. And towns and villages a lot higher. So right, 12k is no's dangerous per se.

The problems come if you haven't acclimatized.