Fewer than 300 humans have climbed the seven highest summits on each continent. Each summit poses a different challenge to the physical, mental, and emotional resources of the climber. Due to differing interpretations between political and geological boundaries, there are varying definitions of what composes the seven summits. But most mountaineers abide by Reinhold Messner’s list, which seeks the highest possible climb.

Altitude Sickness
The term altitude sickness covers a group of symptoms—from bothersome to fatal--that may appear once someone is more than 8,000 feet (2,400 meters) above sea level. Not everyone gets altitude sickness and there is no way to predict who is at risk. Most symptoms are thought to be caused by low CO² levels that raise blood alkalinity. This explains why extra oxygen does not always alleviate the symptoms. Altitude sickness can often be prevented by ascending in slow stages, with returns to lower altitudes to sleep. The physical ease of climbing Kilimanjaro lends itself to altitude sickness, as people often ascend it too quickly without giving their bodies a chance to acclimatize.

Milder symptoms that indicate altitude sickness include vomiting, appetite loss, and nausea; persistent headaches; trouble sleeping; dizziness; fatigue; shortness of breath when active; a pins and needles sensation; and swollen hands, feet, and ankles. If altitude sickness advances to the point of cerebral edema (swelling of the brain) or pulmonary edema (fluid in the lungs) it can be fatal.