Altitude sickness

Altitude sickness is a reaction to the lower amounts of oxygen available at high altitudes (due to the lower air pressure). Your body will respond in various ways to this: some are normal, some are illnesses. The illnesses are a serious health hazard and can result in death if ignored or left untreated. Altitude sickness is very dangerous for four reasons: it can come on suddenly and progress quickly, it can be fatal, sufferers are often some distance from medical help and are difficult to evacuate swiftly, and in many cases sufferers are reliant on their health because they're doing a lot of physical activity in dangerous environments.

Altitude sickness is a great danger for high-altitude mountaineering (above 4000 or 5000 m), a moderate danger for mountain sports (such as skiing at 3000–4000 m, notably in Colorado), and a moderate danger when flying in to a high-altitude city around 3500 m, notably Tibet (Lhasa), Peru (Cusco, especially for the Inca Trail), and Bolivia (La Paz). For moderate altitudes (such as 3500 m), the main solution is to acclimatize for a night or two at a lower altitude (near 2500 m) and take it easy for the first few days, rather than flying in and immediately going skiing or hiking. Acetazolamide (ACZ) is the most commonly used drug for prevention, and is particularly useful for flying into a high-altitude city. For higher altitudes much more care, preparation, and gradual ascent is necessary, and potent treatments are available. Particularly dangerous are tall, easy mountains, notably Kilimanjaro (5895 m) and Aconcagua (6961 m), where it's easy to get dangerously high quickly. Acclimation requires time, and rushing causes altitude sickness.

There are other risks at altitude that are covered in other articles. It may get very cold; see Cold weather. You may get dehydrated because water evaporates faster at lower air pressure; see Dehydration. There may be danger from strong sun because there is less atmosphere above to protect you; see Sunburn and sun protection. Finally, the terrain may pose dangers such as avalanches or just falling off a mountain; see Mountaineering.

A problem, especially for inexperienced people, are ignorant or malicious tour operators offering hikes to high elevations. If you're not an experienced mountaineer, don't book tours with operators that are not knowledgeable and frank about risks involved, regardless of whether they are well-meaning but ignorant people just trying to make a living, or unscrupulous people in for quick profit. Only book with tour operators that have a detailed plan for how to deal with people suffering from acute mountain sickness (AMS).