The cold. Brrrr!
Anecdotally the #1 thing which prevents climbers succeeding without Os is the cold. Summit temperatures are commonly in the -40F (-40C) range, although one climber I spoke with made it in -54F (-48C). Here is a shot of Mt Everest’s current summit conditions. Just an average week, and a low of -87F (-66C) wind chill.
The two risks to the climber are hypothermia and frostbite.
To keep you alive your body conserves heat in your core, where all the really important things are. It first closes the blood vessels to your extremities, which is why your hands feel icy cold on a chilly day. In normal conditions your body reopens these vessels when they cool below 50F (10C) to flush them with warm blood and stop them freezing. However, if your core is already cold this won’t happen, and without that blood flow – and no matter what you are wearing – the tissue will eventually freeze.
Within your core, without sufficient body heat your temperature will fall and you will get hypothermia. It is a dangerous condition, especially on the mountain. It causes physical conditions such as a weakness and unconsciousness, but also dizziness, a lack of coordination, poor judgement and confusion.
In severe hypothermia cases they have found climbers lying frozen to death next to a pile of neatly folded clothes. In the later stages the little muscles which close the blood vessels fail, dumping warm core blood into the extremities and causing a burning sensation on the skin. It’s that feeling I’m sure you have experienced when you get indoors with really cold hands and they hurt as they warm up. This hot sensation causes the delirious victim to remove their clothes. Delirious and confused, it is believed the clothes are folded to bring some ‘order to the chaos’ of their situation.
High altitude raises the risks of hypothermia as the amount of heat your body can generate is lower, for the same reason you climb slower as you go higher. Although covered head-to-toe in a down suit and other technical layers, all these do is efficiently retain the heat your body generates. It helps a lot, but there is a limit to how much you can wear. Lower heat generation and longer exposure makes hypothermia more likely.
Additionally, hypoxic blood conditions further constrict your vessels, reducing your ability to maintain blood flow to the extremities. That results in a higher risk of frostbite.
Indeed, one of the key reasons to climb using bottled oxygen is to reduce these risks and to push the envelope further – hopefully all the way to the summit. Climbing without Os just means that window of opportunity is much smaller.
The advanced gear you wear on Everest is a big help in lowering these risks. Here is a picture of the boots I will be wearing. $1,000 well spent. Yep that is a LOT of money for boots, but at $100 a toe they are cheaper than buying toeless shoes for the rest of my life. Plus they are Italian made!