Climbers summiting Mount Everest have as little oxygen in their bloodstream as residents of coastal areas who are in cardiac arrest—or who are even dead. Four physicians from University College London trekked up Everest and drew their own blood for analysis. They found that because of the altitude, they had about a quarter less oxygen in their blood than is normal for people at sea level. The analysis also confirmed other effects of being at high altitudes, such as the increase in hemoglobin to ferry as much oxygen as possible. Besides helping climbers, the findings, in the January 8 New England Journal of Medicine, could lead to better treatments for oxygen-deprived heart and lung patients on the ground.