The higher you go above sea level, the less oxygen there is to breathe. When you reach elevations above 2,000 metres (6,500 feet), your body doesn't always adjust quickly enough to the decrease in oxygen. This can lead to altitude sickness, which is actually a group of potentially life-threatening ailments. Acute mountain sickness (AMS) is the most common type. Other forms of altitude sickness attack the lungs and brain.
Anyone can develop altitude sickness. Not everyone gets it; the effects of altitude vary from one person to the next. Some adjust to the decrease in oxygen more easily than others. But the higher and faster a person climbs, the more likely altitude sickness will develop. About 20% of people who ascend above 2,500 metres (8,000 feet) in a day and about 40% of people who ascend above 10,000 feet (3,000 m) in a day will experience symptoms. Children are the most susceptible. Surprisingly, the risk is twice as high for people under the age of 60 years.