Every year millions of people go to the mountains for backpacking, skiing, mountain climbing and other activities. If you're planning a trip to altitudes over 8,000 feet, before you go, talk with your doctor about high-altitude illness, or mountain sickness as it is often called.

What causes high-altitude illness?

The low amount of oxygen in the air at high altitudes causes high-altitude illness. The amount of oxygen in the air goes down as you climb higher above sea level and becomes very low at altitudes above 8,000 feet. If you travel to a high altitude, you may feel ill because the air has less oxygen in it. If you stay at a high altitude for a long time, your body gets used to the low oxygen level, and you don't get sick from it.

There are three main types of high-altitude illness: (1) acute mountain sickness (also called AMS), (2) high-altitude pulmonary edema (also called HAPE), which affects the lungs, and (3) high-altitude cerebral edema (also called HACE), which affects the brain. These illnesses can be serious, but they can also be prevented.

How can I prevent high-altitude illness?

You can do two important things to prevent high-altitude illness: (1) take your time traveling to higher altitudes and (2) sleep at an altitude that is lower than the altitude you were in during the day. For example, if you ski at an elevation of 10,000 feet during the day, sleep the night before and the night after at an elevation of 8,500 feet.

When you travel to a high altitude, your body will begin adjusting right away to the amount of oxygen in the air, but it takes several days for your body to adjust completely. If you're healthy, you can probably go safely from sea level to an altitude of 8,000 feet in a few days. But when you reach an altitude above 8,000 feet, don't go up faster than 1,000 feet per day. The closer you live to sea level, the more time your body will need to get used to a high altitude. Plan your trip so your body has time to get used to the high altitude before you start your physical activity.

How do I know if I am getting a high-altitude illness?

Some of the first signs of high-altitude illness are headache, lightheadedness, weakness, trouble sleeping and an upset stomach. If you have these symptoms, stop going up to a higher altitude or go back down to a lower altitude until your symptoms go away. More severe symptoms include difficulty breathing while you're resting, coughing, confusion and inability to walk in a straight line. If you get these symptoms, go to a lower altitude right away and get help from a doctor.

What should I do if I get high-altitude illness?

The best treatment for any of the three high-altitude illnesses is to go down to a lower altitude right away. But if you have only mild symptoms, you may be able to stay at that altitude and let your body adjust. If you do this, don't exercise at all, just rest until you feel better.

If you have severe symptoms, go down 1,500 to 2,000 feet right away to see if your symptoms get better. Keep going down until your symptoms go away completely.

A medicine called acetazolamide (brand name: Diamox) can be used to treat high-altitude illness. When you talk with your doctor about your upcoming trip, ask about getting a prescription for this medicine to take with you in case you get mountain sickness. If you start to have symptoms of high-altitude illness, you can take 125 to 250 mg of the medicine by mouth twice a day.

Don't ignore signs of high-altitude illnesss. People can die of high-altitude illness if they don't recognize the signs or if they don't believe their illness is caused by the high altitude. When you have signs of high-altitude illness, don't go to a higher altitude until you feel better and your symptoms have completely gone away.

Is it safe to go to a high altitude if I have a chronic illness like heart disease or lung disease?

It might be safe. The answer depends on the type of chronic illness you have and how severe it is. Most people with chronic illnesses such as heart or lung disease can safely spend time at a high altitude if their disease is under good control. People with coronary artery disease, mild emphysema or high blood pressure aren't at greater risk of high-altitude illness than people without these diseases. They also don't risk making their disease worse by traveling to a high altitude. Being overweight does not increase the risk of getting high-altitude illness.

But some diseases make going to a high altitude very dangerous. People with sickle cell anemia shouldn't go to a high altitude. A high altitude is also dangerous in people with severe lung disease, such as chronic obstructive pulmonary disease (COPD) and severe emphysema, and in people with severe heart disease. If you have a chronic disease, ask your doctor if it's safe for you to travel to a high altitude.

Is going to a high altitude dangerous during pregnancy?

There isn't much information about the risk of high-altitude illness during pregnancy, so it's hard to say if going to a high altitude is safe for pregnant women. Some experts recommend that pregnant women not travel to an altitude above 8,000 feet. If you're pregnant, ask your doctor for advice before you travel to a high altitude.

What about children and high altitudes?

It's usually safe for children to go to high altitudes, but they're more likely to get high-altitude illness because their bodies have a hard time adjusting to the low oxygen in the air. A child may not be able to recognize the symptoms of high-altitude illness, so parents and other adults must carefully watch for any signs of high-altitude illness in children.