I spent the first 25 years of my life at sea level and then lived in Santa Fe New Mexico for a couple months.
Now a normal oxygen reading for me on a pulse oximeter is 98% saturated. The first thing the manager did when Ishowed up to my job in Santa Fe was check my "sats".. they were 92 %.
I also remember the first time I ran when I was there.. everything started to get black

I got used to it after a week or so but when I went up to the mountains to ski, I'd still feel the effects.