This article is a travel topic
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.
Another risk at altitude is that it may get very cold. This article does not cover that; see Travelling in cold weather.
The further you move away from sea level up into higher altitudes, the lower the air pressure is. The body has two main problems with high altitude and the corresponding lower air pressure:
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.
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.
Altitude sickness is very dangerous for four reasons: its rapid progression; its deadliness; the fact sufferers are usually some distance from medical help and are difficult to evacuate swiftly; and the fact that in many cases sufferers are reliant on their health because they're doing a lot of physical activity in dangerous environments.
 How high is high?
A minority of people, about 20%, have some symptoms of altitude sickness if they ascend to about 2500 meters (8000 feet) above sea level and sleep there. However, most people will acclimatize to 3000 meters (10,000 feet) with relative ease, perhaps having symptoms after the first night.
Acclimatizing to heights of 3000–5000 meters (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 3500 meters (11,000 feet) without acclimatization, and everyone will if they ascend rapidly to 5000 meters (16,000 feet).
It is thought to be impossible to permanently acclimatize to heights above 5500 meters (18,000 feet). It is possible to spend several weeks sleeping as high as 6000 meters (20,000 feet) once acclimatized, but gradual deterioration of physical well-being will still occur.
Regions above 7500 meters (25,000 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 7000 meters (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 1500 meters (5000 feet) to 2500 meters (8000 feet), an elevation range containing quite a few major cities, experience little trouble going to 3000 meters (10,000 feet) or a bit higher, but even they will be at risk of altitude problems at 5000 meters (16,000 feet).
 Risk factors
Altitude sickness tends to affect men more than women, especially men between the ages of 16 and 25, for unknown reasons. 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.
 Effects of altitude
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.
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
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.
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 AMS (below) headaches and vice versa. If a headache does not improve after drinking a liter of water 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 (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:
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 HACE or 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 (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 for this 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 (HAPE) is another severe altitude illness. It sometimes occurs in conjunction with AMS or HACE, but sometimes doesn't — 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 10,000 feet (3000 meters), 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 (DCS, also known as the bends or caisson disease) is a severe illness in which bubbles of nitrogen forms 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.
 Keep hydrated
Remember to drink adequately - around one liter 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.
 Acclimatize to altitude gradually
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:
You can, of course, ascend more gradually than these rates. Many people ascending from sea level choose to spend several nights at 2500 meters (8000 feet) to 3000 meters (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 3700 meters (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 5000 meters (16,000 feet) and some mountains over 8000 meters (26,000 feet), so do not set out until you are thoroughly acclimatised in Lhasa.
Where possible, avoid ascents more rapid than recommended above, particularly any sudden ascent to 3000 meters (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 2000 meters (7000 feet) to 5000 meters (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 3000 meters (10,000 feet) at least spend a few days at some intermediate destination en route. If flying to a more moderate altitude above 2500 meters (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.
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 nighttime, 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.
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 has a mild to moderate diuretic effect , so adequate amounts of fluids 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 Lhasa); or taking the landroute there from Kathmandu , 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.