Most expeditions use oxygen masks and tanks above . Everest can be climbed without supplementary oxygen, but only by the most accomplished mountaineers and at increased risk. Humans' ability to think clearly is hindered with low oxygen, and the combination of extreme weather, low temperatures, and steep slopes often requires quick, accurate decisions. While about 95 percent of climbers who reach the summit use bottled oxygen in order to reach the top, about five percent of climbers have summited Everest without supplemental oxygen. The death rate is double for those who attempt to reach the summit without supplemental oxygen. Travelling above 8,000 feet altitude is a factor in cerebral hypoxia. This decrease of oxygen to the brain can cause dementia and brain damage, as well as other symptoms. One study found that Mount Everest may be the highest an acclimatised human could go, but also found that climbers may suffer permanent neurological damage despite returning to lower altitudes.