The "death zone" starts at 26,000 feet. The peak of the mountain is about 1,000 feet under the death zone so supplemental oxygen isn't a requirement. That bein

The "death zone" starts at 26,000 feet. The peak of the mountain is about 1,000 feet under the death zone so supplemental oxygen isn't a requirement. That being said the effect of lower oxygen is you go slower. It took them a week to ascend 10k feet, and if you'll allow me to assume a spherical cow then a hypothetically similar climb at sea level could probably be done in a couple of days going by how fast top climbers climb El Capitan.