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高山症 （也就是高原反应）是一种人在高海拔地区下对于低氧条件的反应（因高海拔地区的气压较低）。你的身体将会以各种各样的方式出现反应：一些是正常的反应，一些是病态的反应。病态的反应是会严重影响健康的一种危险，如果你忽略它或者不加以处置的话，这种反应能够导致死亡。

高山症十分危险，原因有四：它可以突然发作并且会很快恶化，这可能是致命的；患者往往不会与能够获得医疗救助的地方很近；患者很快地撤离也是困难的；而且在很多情况下患者都要依赖他们的健康，因为他们正在危险环境中做大量的体力运动。

高山症对于高海拔（4000m至5000m）的登山运动是一种巨大的危险，一种温和的山地运动的危险（例如在3000m至4000m的地区滑雪，特别是在科罗拉多州），还是一种温和的危险——当你乘坐飞机飞向高海拔的城市时，尤其是中国的西藏（拉萨），秘鲁（库斯科，特别是对于印加小径（Inca Trail））, 以及玻利维亚（拉巴斯）。对于中等的海拔（例如3500m），主要解决办法是在较低海拔地区（在2500m左右）花上一两个晚上适应一下环境，并在前几天保持较为轻松的心态，而不是飞到那里就立即去远足或滑雪。在海拔更高的地方，你要更加小心，要慢慢的登上去，这是非常必要的。适应需要时间,过于急躁的话，是会引起高山症的。

在高海拔地区，也有其他风险，这里并未包括。一个是天气可能会很冷：见寒冷的天气。另一个就是可能有由于强烈的阳光所造成的危险，因为你头顶的大气比较稀薄，是不足以保护你的：见防晒措施。最后，地形可能会造成危险，例如雪崩或者仅仅是从一座山上落下来：见登山运动。

了解

File:Altitude Sickness Warning Mount Evans Colorado.jpg

科罗拉多州埃文斯山上的高山症警告标志

你移动的海拔越高，离海平面越远，气压也就越低。身体在处于高海拔地区以及随之而来的更低的气压下的时候会出现两种问题：

• 低气压下的空气每一口都比正常情况下有更少的氧气。你的身体通过产生更多的红细胞来更加高效率地运输氧气，从而适应这种环境。不过，这个过程需要几天时间，有时要超过一周，在这期间，你可能会生病。

• 在更低的气压下，水蒸发得要更快。这会导致脱水

在高的地方，你的身体的改变是复杂的，可能会十分难于预测。The changes to your body at altitude are complex and can be quite dramatic. The difficulty your body has maintaining a good oxygen supply and keeping related problems under control is directly related to how high up you are, and also to recent changes in your altitude. These are the two major factors that cause altitude sickness. The altitude where you sleep is also important because most of the extra red cells needed to improve your oxygen management are manufactured while you sleep.

Hence this article talks a lot about ascent and descent. Ascending further away from sea level is the risky activity and the time you must be alert. Conversely, descending towards sea level is the single most important factor in reducing or eliminating all forms of altitude sickness.

The article also talks a lot about acclimatisation, giving your body enough time to adapt to higher altitude. This is critically important for avoiding problems.

The information in this page is in no way a substitute for medical advice. Anyone planning a trip to high altitudes should consult their doctor first, and anyone experiencing symptoms at altitude should consider seeing a local doctor.

有多高才算高呢？

La Paz, the world's highest located national capital

A minority of people, about 20%, have some symptoms of altitude sickness if they ascend to about 2500m (8000 feet) above sea level and sleep there. (This is the level of cabin pressurisation of most commercial aircraft apart from the Airbus A380 and Boeing 787). However, most people will acclimatize to 3000m (10,000 feet) with relative ease, perhaps having symptoms after the first night.

Acclimatizing to heights of 3000–5000m (10,000–16,000 feet) is much more difficult, and it is here that it is absolutely necessary to ascend slowly and return to a lower altitude to sleep if you have been travelling around at a higher altitude during the day. Over 50% of people will become ill if they ascend rapidly from sea level to 3500m (11,000 feet) without acclimatization, and everyone will if they ascend rapidly to 5000m (16,000 feet).

It is thought to be impossible to permanently acclimatize to heights above 5500m (18,000 feet). It is possible to spend several weeks sleeping as high as 6000m (20,000 feet) once acclimatized, but gradual deterioration of physical well-being will still occur.

Regions above 8000m (26,000 feet) are referred to as the death zone: you will deteriorate noticeably while you remain at such high altitudes, some of your body's major systems will shut down and climbers will only remain there for two or three days. Death rates from altitude sickness above 7000m (23,000 feet) are estimated at 4% of all people who venture that high.

If your home is significantly above sea level, you gain a definite leg up on ascending to higher elevations, but that doesn't make you immune to altitude problems; it just pushes the threshold for their onset higher. Most otherwise healthy people who live at elevations of 1500m (5000 feet) to 2500m (8000 feet), an elevation range containing quite a few major cities, experience little trouble going to 3000m (10,000 feet) or a bit higher, but even they will be at risk of altitude problems at 5000m (16,000 feet).

Risk factors

Altitude sickness tends to affect men more than women, especially men between the ages of 16 and 25. It's not clear if there's some unknown biological reason for this, or if it's just the demographic most likely to attempt too much, too soon. It is important to remember that just because you are young and healthy, and haven't experienced altitude sickness in the past doesn't mean you are immune to it on future climbs. Physical fitness is not necessarily a good indicator, and neither are strength or good health. You may react badly to altitude despite being fit, young and healthy. In fact, the fit, young and healthy have a hidden risk: their general physical capacity leads them to believe that they should handle altitude just fine, which is not always true.

Bad health, on the other hand, is a risk factor: particularly cardiac or respiratory problems. Healthy hearts and lungs have a hard enough time getting oxygen to your tissues at high altitudes. Naturally, if you have physical problems that make exertion difficult for you, you have reason to think carefully about exertion at high altitude, where it is much harder!

Scuba diving increases risk of decompression sickness. If you have recently been diving and have not fully gotten rid of the nitrogen in your blood, you should not ascend to a higher altitude (or travel in a plane). See scuba diving for recommendations on how long to wait.

Locations

Other than mountain climbing and other mountain sports, such as skiing, the main travel destinations at high altitudes are Bolivia, Peru (in the Bolivian Plateau), and the Tibetan Plateau (Qinghai and Tibet in China), all of which have significant destinations around 3,500 m, high enough to pose significant risk of altitude sickness. By contrast, other mountainous destinations such as Mongolia, Nepal, Switzerland (mostly below 2,000 m), and even Bhutan (around 2,500 m), are mostly settled at lower elevation, especially in valleys, and pose at most mild risk of altitude sickness.

For flying into Peru for the Inca Trail, Cusco (3,400 m) is quite high, and immediately hitting the Inca Trail is dangerous, occasionally resulting in death. Safer is to leave Cusco and acclimate for a night or two in the Sacred Valley, before returning to Cusco, and thence the Inca Trail. Machu Picchu itself (2,400 m) is not very high, however.

For flying into Lhasa, acclimating for a few days in Kunming (2,000 m) or Xining (2,300 m) helps. Taking the train into Tibet does not: it is first too low, then too high to help acclimation.

For flying into La Paz, acclimate in the low, southern part of town (such as Calacoto or Obrajes) for a few nights.

In terms of large cities, there are under a dozen large cities (at least 100,000 population) above 3,000 m, of which the most significant are La Paz (Bolivia, 3,650 m), Lhasa (China, 3,650 m), and Cusco (Peru, 3,400 m). The countries with many major cities above 2,000 m are Bolivia, Peru, China (Tibetan Plateau), Ecuador, Colombia, and Mexico, while Eritrea, Ethiopia, Guatemala, Yemen all have 1–3 major cities (capital or second city) in 2,000–3,000 m, and Afghanistan and India have a few minor cities at altitude.

List

Comparative elevations of significant travel destinations.

North America

• Colorado
  • Many high ski resorts, e.g., Breckenridge (~3,000–4,000 m)
  • Aspen (2,400 m)

South America

• Bolivia
  • La Paz (3,650 m) – highest capital city
  • El Alto (4,150 m) – highest major metropolis
• Colombia
  • Bogotá (2,600 m)
• Peru
  • Cusco (3,400 m)
  • Inca Trail: Warmiwañusqa “Dead Woman Pass” (4,200 m – highest point, daytime), Pacaymayo (~3,500 m – highest night)
  • Machu Picchu (2,400 m)

Asia

• Bhutan
  • Thimphu (2,250–2,650 m)
• Tibet, China
  • Lhasa (3,500–3,650 m)

Effects of altitude

Precautions at 5360m (17586ft)

Altitude has some physiological effects on all people who are at high altitudes. These effects are not in and of themselves symptoms of illness, although they are signs of the increased difficulty that the body has getting at altitude.

Hyperventilation

You will naturally breathe faster at higher altitudes to compensate for the lower air pressure. It's possible you won't notice this: a similar effect happens during air travel.

Altitude diuresis

Increased urine output is a response to hypoxia : increased output of bicarbonate makes it possible to increase breathing. This will make you urinate a lot at altitude. If you aren't urinating much more than you usually would, then you might actually be dehydrated.

Periodic breathing

Because of the disruption to oxygen and carbon dioxide levels in your blood due to the body chemistry changes and hyperventilation that occurs at altitude, your body's "when to breathe" chemical signals become confused. While you're awake you will remember to breathe, but when you sleep it is common to have interrupted breathing: holding your breath for up to fifteen seconds and then breathing very rapidly when you start breathing again.

This can be very alarming when you wake up knowing that you weren't breathing or were short of breath; or when you notice someone else has stopped breathing. But it is a normal response to altitude, and happens to almost everyone. Acclimatisation only improves it a little.

Illnesses at altitude

Climbers at the top of Mount Kilimanjaro — a mountain where you easily and quickly can trek up to dangerous heights

As well as the less dangerous physiological effects, altitude makes you susceptible to actual illnesses, several of them very dangerous. While not all the effects of altitude can be avoided, you should take sensible steps to avoid actual illness, and take it very seriously if it does occur.

Dehydration

You need to increase your fluid intake at high altitudes. The loss of appetite, a precursor to nausea, can lead you in to a dehydration headache. Unfortunately, it is easy to mistake dehydration headaches for acute mountain sickness (AMS) headaches (below) and vice versa. If a headache does not improve after drinking a litre of fluids it should be considered as an AMS effect.

Dehydration headache can also be recognised by comparing pulse rates: if your pulse rate goes up more than 20% when standing up after lying down for five minutes, you need more fluids.

Acute mountain sickness

Acute mountain sickness (AMS) is the most common unhealthy response to altitude: it's a collection of signs that your body is becoming ill and has not adapted successfully to a higher altitude.

For your own safety, assume any illness at altitude is AMS. The most common reasons that people fail to descend as soon as they should are bad assumptions. They assume that having AMS is a sign of weakness; that their level of fitness means they can't have AMS; or mistake their symptoms for the flu or another illness. Assume AMS first: it happens to healthy strong people, and if it turns out you are indeed sick with something else, descending to a lower altitude will make it easier for your body to heal anyway.

In particular, if you've recently ascended, and you have a headache and any other symptom, you have AMS. The other signs of AMS vary for different people, but include:

• fatigue
• dizziness
• loss of appetite
• nausea or vomiting
• confusion
• difficulty walking (called gait ataxia)
• rattling breath
• feeling generally extremely ill

The last three signs in particular are signs that you are becoming quite ill, but you should not wait for the onset of these symptoms before acknowledging you have AMS: they're fairly reliable indicators of the onset of high altitude cerebral edema (HACE) or high altitude pulmonary edema (HAPE).

You and your party should keep an eye on each other for signs of AMS, and if you have AMS, for signs of it worsening. Very sick people can become confused and not realise how ill they are. Loss of appetite is a particularly good sign: anyone who has been walking or climbing at altitude for a day should be hungry for a good meal in the evening.

If you have symptoms of AMS, do not ascend further. Consider descending.

If you have signs of HACE or HAPE, descend immediately. Your life may depend on it.

High altitude cerebral edema

Climbing Mount Everest

High altitude cerebral edema (HACE) is the end-stage of AMS (conversely AMS can be thought of as the mild form of HACE). When you have HACE, your brain swells and stops working properly.

HACE symptoms include a number of signs of mental functions failing: confusion, fatigue and weird behaviour. But the most reliable one is gait ataxia, and you can test it by walking heel to toe along a straight line on the ground. Healthy people can pass this test easily, anyone who has difficulty balancing while they do it is showing signs of HACE.

HACE is extremely serious, and you may only have a few hours to help someone with HACE. The main treatment is descent, but a person experiencing these symptoms will need significant help. Dexamethasone is one drug that can be used to relieve symptoms, but it is just a temporary bridge to give more time for descent.

A 2008 medical study Why Climbers Die On Mount Everest shows HACE as the leading cause of death.

High altitude pulmonary edema

High altitude pulmonary edema (HAPE) is another severe altitude illness. It sometimes occurs in conjunction with AMS or HACE, but sometimes on its own — it's thought to have different causes. When you have HAPE, your lungs fill with fluid. Signs include extreme fatigue; breathlessness (when not due to interrupted breathing — give yourself 30 seconds to recover upon waking); a cough, especially if it is wet and has blood in it; rattling or gurgling breath; chest congestion; very fast heart rate; very fast breathing; and blue extremities. A fever is sometimes present. It most commonly sets in at night.

HAPE is another extremely serious illness, and like HACE should be treated as a critical emergency. Nifedipine is the drug of choice for the treatment of HAPE, but it can only provide temporary relief and rapid descent is very important.

Cheyne Stokes breathing

Above 3000m (10,000 feet), some people experience a periodic breathing during sleep known as Cheyne-Stokes respirations. The pattern begins with a few shallow breaths and increases to deep sighing respirations then falls off rapidly. Respirations may cease entirely for a few seconds and then the shallow breaths begin again. During the period when breathing stops the person often becomes restless and may wake with a sudden feeling of suffocation. This can disturb sleeping patterns, exhausting the climber.

Acetazolamide is helpful in relieving the periodic breathing. This type of breathing is not considered abnormal at high altitudes. However, if it occurs first during an illness (other than altitude illnesses) or after an injury (particularly a head injury) it may be a sign of a serious disorder.

Decompression sickness

Decompression sickness (DCS, also known as the bends or caisson disease) is a severe illness in which bubbles of nitrogen form in your blood, blocking blood supply to parts of your body. Symptoms include persistent tingling or joint pain, fatigue, itching, rashes, confusion and collapse. Decompression sickness is brought on by extremely sudden changes in air pressure (effectively an increase in altitude), such as loss of cabin pressure in a plane you are flying in. Even a fast ascent to most altitudes (such as by plane) would not normally cause decompression sickness. The exception is for anyone who has recently been scuba diving, who should avoid ascents above the altitude that their dive took place at for between 12 and 24 hours depending on dive activity. See the Scuba diving article for more information.

Prevention

Keep hydrated

Remember to drink adequately - around one litre extra of fluids per day. Pushing large volumes of water does not protect against AMS, and can give the same symptoms (headaches, nausea, vomiting and more) as severe AMS from electrolyte imbalance.

Check blood oxygen content

You can use a pulse oximeter to measure your blood oxygen content, which can help you detect problems before you get symptoms. These are inexpensive and easily available, with good accuracy. Medical staff at altitude typically carry these, but it is prudent to purchase one (or two, as a backup) yourself.

Acclimatize to altitude gradually

The Base Camp, Mount Everest

Acclimatization is the process of getting your body to adapt to the lower oxygen levels by ascending slowly into higher altitudes, spending some time at each one to adapt. The most important factor is to increase your sleeping elevation (the altitude where you're spending the night) slowly. If you're on a hiking or climbing holiday, a typical strategy is to spend a day (or initially part of a day) at a higher altitude and return to a lower altitude to sleep. This also works for people doing winter sports at high altitudes: ski at the top of the resort and sleep at the bottom.

Here are the recommended maximum increases in your sleeping elevation which will stop most people from proceeding to AMS:

• Go no higher than 2400m (8000 feet) the first night.
• Increase your sleeping elevation by 300m (1000 feet) per night after 3000m (10,000 feet).
• Every 1000m (3000 feet), you should spend a second night at the same altitude. This will be every fourth night if you have been ascending at the maximum pace recommended above.

You can, of course, ascend more gradually than these rates. Many people ascending from sea level choose to spend several nights at 2500m (8000 feet) to 3000m (10,000 feet) before beginning acclimatization to higher altitude.

During acclimatisation, drink a lot of non-alcoholic drinks. Some people find vegetarian food slightly accelerates the acclimatisation. Local recipes like mate de coca in Peru (tea from coca leaves) are fine, but their effect is doubtful.

Avoid rapid ascents

Rapid ascents are the opposite of acclimatization; you make a rapid ascent when you're gaining altitude faster than recommended. This may mean climbing and camping higher than recommended, but you can also make an even more rapid ascent by driving to a high altitude location, and flying from low altitude to high altitude is an even more rapid ascent. For example, flying from sea level to Lhasa, Tibet, which is 3700m (12,000 feet) high, is distinctly unwise. Consider spending a week or so at an intermediate altitude; see Overland to Tibet for some possibilities. If you are going to travel around Tibet — where some inhabited areas are over 5000m (16,000 feet) and some mountains over 8000m (26,000 feet), so do not set out until you are thoroughly acclimatised in Lhasa. The same is true if you travel to the Andes; destinations such as Cuzco, La Paz or the Inca Trail are located well over 3000m above the sea level.

Where possible, avoid ascents more rapid than recommended above, particularly any sudden ascent to 3000m (10,000 feet) or higher. Even if you are taking Acetazolamide (below) a rapid ascent makes it more likely you'll get AMS and makes AMS progress to serious illness faster, so you will have less time to respond and descend.

Be particularly wary with oxygen equipment: some tourists have died at altitude when their equipment failed and they were utterly unacclimatized.

Consider road or rail travel rather than flying directly to somewhere with a very high altitude — but remember that the surface option often involves a lot higher altitudes: the Manali-Leh road for example will take you from below 2000m (7000 feet) to 5000m (16,000 feet) in less than a day. Or fly in stages, stopping somewhere at moderate altitude in between. If you must fly to any destination about 3000m (10,000 feet) at least spend a few days at some intermediate destination en route. If flying to a more moderate altitude above 2500m (8000 feet), you will still want to spend several nights at that altitude before setting off into higher country.

Refrain from smoking and alcohol when you arrive by plane in a high altitude area from lower altitudes.

Treatment

Mountaineer cemetery, Aconcagua

As soon as the symptoms of AMS appear, your first priority is recovering. You must not ascend any further until the symptoms have disappeared. This may take up to 48 hours, if it takes longer, descend. You could also descend on the onset of symptoms, this will make them disappear much faster, probably within hours.

If you are getting sicker or showing signs of HACE or HAPE, you must descend to a lower altitude as quickly as possible. If it is night time, do not wait for morning if you have a choice at all. You should descend at least as far as you were the last night you had no AMS symptoms. You may need to seek hospital care.

People with HACE and HAPE are frequently confused or exhausted, and are likely to need help with the descent. Help them down!

There is some equipment available to treat people with HACE or HAPE at high altitudes, including hyperbaric bags in which the sufferer can lie in a higher pressure atmosphere. Likewise, because the main cause of these illnesses is a lack of oxygen, breathing oxygen from a tank will slow their onset and may provide some temporary relief of symptoms. Either treatment buys some time if it is too dangerous to descend, but they are not a substitute for descent.

Sufferers of DCS needed to be hospitalised and treated in a recompression chamber: descent to sea level is not sufficient to alleviate DCS symptoms. As with HACE and HAPE, breathing oxygen may provide some temporary relief of symptoms allowing for rescue. Scuba diving organisations can advise further.

Acetazolamide

This drug (sold as Diamox) stimulates your breathing. The drug was originally designed as a treatment for glaucoma, but a side effect of increased breathing rates and depth have proven useful to climbers. It has the effect of increasing acclimatisation rates; improving periodic breathing; and helping people recover from AMS more quickly. There are some side effects. The drug acts as a diuretic and can cause easy dehydration, so drinking plenty of water is important. This drug can be useful for people who have had AMS in the past; people on a forced ascent (for example, flying into Tibet); and anyone who has AMS, particularly if they are choosing not to descend. Many climbers also take it as a prophylaxis.

Acetazolamide is not an absolute preventative measure, particularly in the case of forced ascents. A prescription is necessary, and a doctor should be consulted about proper dosages.

Other drugs

Salmeterol (Serevent), temazepam (Temaze), nifedipine and dexamethasone. Some of these drugs are found in capsules sold in China e.g. Gao Yuan Kang (高原康), which contains dexamethasone. Some herbal preparations are also purported to prevent/treat high altitude illness, such as gingko biloba and a combination capsule called Gao Yuan Ning (高原宁), sold in China. The effectiveness of these preparations remain scientifically unproven, although Gao Yuan Ning (高原宁) is used by Chinese military personnel in cases of rapid ascent.

It is extremely important to note that all these drugs can have significant side effects, especially dexamethasone, a potent steroid medication. Tourists are advised to consult their doctor prior to obtaining these medications. Foreign tourists should procure any necessary medications in their home countries and note the ingredients contained in the medications.