One of the early guided summiters, Richard Bass (of Seven Summits fame) responded in an interview about Everest climbers and what it took to survive there, Climbers should have high altitude experience before they attempt the really big mountains. People don't realise the difference between a 20,000-foot mountain and 29,000 feet. It's not just arithmetic. The reduction of oxygen in the air is proportionate to the altitude alright, but the effect on the human body is disproportionate—an exponential curve. People climb Denali [20,320 feet] or Aconcagua [22,834 feet] and think, 'Heck, I feel great up here, I'm going to try Everest.' But it's not like that.