@crazy-legs – not bad, does make the “percentage of oxygen” mistake though. The percentage of oxygen at the summit of Everest is the same as at sea level, it’s the pressure that’s much less meaning less is drawn into the lungs with each breath and that there isn’t the required pressure to force the gas through the lung walls into the bloodstream.
Someone I know was a guide on Everest in the late 1980s and he described being on the South Col sorting oxygen cylinders in preparation for a summit bid. He was counting and putting the full ones into one pile, the half empty ones into another. Halfway through he’d forget which pile was which so would start again, make the same mistake and start again. It took him half a dozen attempts to finish that simple task. Being in that state and having to tie knots, etc. on which your life depends is fraught with danger.
Back in the 1980s when I was doing most of my altitude climbing I never really fancied Everest. It was “commercial” even back then. K2 was a much more appealing peak. I was on a climbing expedition in the Karakorum in that horrible summer of 1986 when Al Rouse and others died on K2, we walked in at the same time as Rouse’s trip as far as Concordia. I never truly got on with high altitude, I’d randomly black out when up at 7000m or so, I was fine in the Alps at 4000-5000m though.