The use of Bottled Oxygen

For most of its climbing history, K2 was not usually climbed with bottled oxygen, and small, relatively lightweight teams were the norm. However the 2004 season saw a great increase in the use of oxygen: 28 of 47 summiteers used oxygen in that year.

Acclimatization is essential when climbing without oxygen to avoid some degree of altitude sickness. K2's summit is well above the altitude at which high altitude pulmonary edema (HAPE), or high altitude cerebral edema (HACE) can occur.