Mountain climbing is becoming a popular sport, but it is also a potentially dangerous one. People can fall. They may also become ill. One of the most common dangers to climbers is altitude sickness, which can affect even very experienced climbers. Altitude sickness usually begins when a climber goes above 8,000 or 9,000 feet. The higher one climbs, the less oxygen there is in the air. When people don't get enough oxygen, they often begin to gasp for air. They may also feel dizzy and light-headed. Besides these symptoms of altitude sickness, others such as headache and fatigue may also occur. At heights of over 18,000 feet, people may be climbing in a constant daze. This state of mind can have an adverse effect on their judgment. A few precautions can help most climbers avoid altitude sickness. The first is not to go too high, too fast. If you climb to 10,000 feet, stay at that height for a day or two. Your body needs to get used to a high altitude before you climb to an even higher one. Or if you do climb higher sooner, come back down to a lower height when you sleep. Also, drink plenty of liquids and avoid tobacco and alcohol. When you reach your top height, do light activities rather than sleep too much. You breathe less when you sleep, so you get less oxygen. The most important warning is this if you have severe symptoms, and they don't go away, go down. Don't risk injury or death because of over-confidence or lack of knowledge.