Other names for this illness include altitude anoxia; more serious forms are called high altitude pulmonary edema and high altitude cerebral edema.

Children also are more likely to feel the effects of high altitude. One way to prevent altitude sickness is to ascend heights gradually. Take several days to move up to 8,000 feet. Beyond that height, don't climb any faster than about 1,000 feet a day. Experts also advise sleeping at a lower altitude than you spend the day at. If you're climbing, for instance, don't make camp at the highest point you've reached for the day, but backtrack down the trail at least several hundred feet.

Altitude sickness occurs because the higher up you go, the less oxygen is in the air for you to breathe. It occurs most commonly at elevations above 8,000 feet. If you live in the mountains, your body adapts. At 14,000 feet and above, however, nearly everyone experiences at least mild symptoms of altitude sickness.

These are life-threatening conditions. They need medical care right away. Milder symptoms include shortness of breath, trouble sleeping, headache, insomnia, nausea, rapid pulse, fatigue, and dizziness. More serious symptoms include coughing, tightness in the chest, trouble walking in a straight line, and mental confusion.

If you notice even mild symptoms, don't go any higher. If you have symptoms of altitude sickness, it's important that you return to a lower elevation. For instance, if you are at an altitude of 8,000 to 9,000 feet, you may need to travel down to an elevation of 6,000 feet or lower for your symptoms to go away. A doctor may prescribe medicines to prevent or treat the symptoms of severe high altitude sickness. If your symptoms go away at a lower altitude, you may try to return to a higher elevation after your body adjusts. This may take 1 to 3 days.

You also should stay away from high elevations if you have severe heart disease or sickle cell anemia. If you have mild heart or lung disease, and you are managing it well, you might be fine at high elevations. Check with your doctor to see if it's safe. Travelers should also consider things such as how easy it is to quickly go to lower altitude and how near you are to medical care.