evidence: This means that the level of oxygen on the summits of Everest and K2 in winter is as low as it would be at an altitude of around 9,100m during the climbing season. [...] The extremely thin air on the summit of Everest and K2 in winter means that any potential winter climber needs even better acclimatization than usual to deal with the greater risk of acute altitude sickness and the slower pace during the summit push. [...] To sum up, in winter, the combination of extremely low barometric pressure (which signiﬁcantly slows climbing), low air temperatures, and high wind speeds put climbers on Everest or K2 at a remarkably high risk of frostbite, hypothermia, acute mountain sickness, and metabolic deterioration.