Steve & Judy's Altitude & Acclimatization Page

Last update 6. May 2015 (NHS link)

Mountain section

This page is offered as a discussion of acclimatization in mountaineering, and has some links to relevant sites. The information has been collected from
other sites, conversations with professional mountaineers, common sense and personal experience. Much of the information was gained during research for our ascent of Cotopaxi (5,897 m, 19,347 ft) in January 1998.

Disclaimer : please note that we are not medically qualified, and any information or advice offered here, or via links from our site, is no substitute for professional consultation.

Further links and references will be added from time to time: comments and corrections will be much appreciated. Those already incorporated are identified [thus].

How is this page organised? A general discussion of the problems of high altitude is followed by a section on the process of acclimatization. Then there is a list of topics, reference/conversion tables, and finally links to other sites. As you can see, some navigation aids have been provided, as this page is quite long.


Altitude and Acclimatization

Everyone knows that the air becomes thinner the higher you go. This is why jet aircraft are pressurised, and why Alpine mountaineers become breathless as they climb.

Altitude in the context of high mountains refers to heights above sea level of between about 2,500 metres (a low Alpine peak) and 8,848 metres (the height of Mount Everest). It should be remembered that the effects of climate and weather can make a significant difference to the apparent height, as for example on the Equator the air pressure is higher than at the Poles, and so the thinning of the air at altitude is slightly less apparent. In contrast, a high sub-Arctic mountain such as Denali (Mount McKinley) presents a lower than expected pressure at its summit, as well as extreme cold, and for these reasons alone it is potentially a more dangerous summit than some much higher ones that are nearer the Equator.[1,2]

For comparison, commercial aircraft are pressurised to about 750 mb, equivalent to 2,500 metres above sea level. This is unlikely to cause any adverse symptoms in a normal, healthy person.

Now is a good time to point out that almost all the heights and pressures in this article will use European (metric) units and spellings; a conversion table has been provided for our American or older (!) readers. For now, 1,000 metres (1,000m) is the same as 3,281 feet, and 10,000 feet make 3,048m.

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Acclimatization in this context means the deliberate, temporary modification of the response of the human body to increased altitude. There is a lot more to it than just breathing, but that remains the major problem. There are practical limits to altitude acclimatization, and it is simply not possible to become permanently acclimatized to much above 5,000m, as the body inevitably deteriorates. Short term acclimatization to about 6,500m is the best that a fit climber can hope for.

The scientific relationship between altitude and air pressure and temperature is given in a table below. In brief, the available oxygen falls to about two thirds that at sea level by 4,000m, to half at 6,000m, and to about one third at 8,000m. In other words, for each lungful you inspire at sea level, you need two at 6,000m, and three at 8,000m to get the same mass of air.

Breathing at high altitude

Our metabolism requires an average of about 15 litres of pure oxygen each hour, equivalent to some 75 litres of air at normal pressure (since air contains about 21% oxygen). However, because most of the oxygen we breathe in is simply breathed straight out again, the volume of air we need to inhale and exhale is very much greater than this, 1,000 litres per hour or more [7]. Oxygen is carried in the blood, temporarily bound to a special molecule called haemoglobin, present in the red blood cells, to supply the oxidant for "burning" glucose fuel to power the brain, muscles and other organs. Without oxygen, the "fire" goes out, and we die. The brain, heart and most other organs are utterly dependent on continuous oxygenation, and quite a modest reduction in availability is immediately serious.

The rate of demand for metabolic oxygen varies greatly. A typical resting rate might be 250ml (0.25 litres) per minute, but this increases to between 3 and 4 litres per minute with exercise. Extreme effort, such as climbing a steep slope with a heavy pack, will increase the demand still further.

The lungs have a very convoluted lining, which consists of about 300 million tiny sacs, called alveoli. Each sac has a very thin membrane, through which some of the oxygen in the air is absorbed into the bloodstream by selective diffusion, and carbon dioxide is expired from the blood to the air in the lungs, before being exhaled. Clearly, the less air pressure there is, the less oxygen there will be in each breath, and the less will pass through the membranes into the blood.

The answer is, of course, to breathe more deeply and more frequently, but there are limits to this. It is very tiring to breathe so hard, and since the thin air is almost always very cold, it can be painful. There is a serious danger of freezing the throat and lungs; a cough is inevitable at altitude, but it can rapidly become so bad that muscles in the chest wall may be torn.

An additional problem faced as the oxygen partial pressure (known as PaO2, the proportional amount of oxygen in the thinner air) falls is that the mechanism for its absorption into the blood is less efficient. The details of the mechanism are beyond the scope of this article, but in essence, if the air pressure is too low, the diffusion through the alveolar membrane doesn't take place properly. There are other unpleasant side effects on the lungs, such as High Altitude Pulmonary Oedema (edema in America, hence HAPE as the abbreviation) to worry about, too.

The body deals with all these problems by increasing the efficiency of oxygen intake from the lungs and transport in the blood. An important part of acclimatization is the production of more red blood cells, which takes place mainly during rest. However, the extra red cells and general dehydration thicken the blood, which brings its own problems [1]. I have read that the body actually resets its target for the "thickness" of the blood, presumably a response which evolution has not had an opportunity to correct. Additional drinking and aspirin may help to counter this effect.

As one is forced to breathe more deeply and more frequently to inspire enough oxygen, an unfortunate side-effect is that the CO2 balance is upset, and one has to excrete large amounts of bicarbonate in the urine to restore it. Diamox (acetazolamide) can help with this process.

Supplementary Oxygen

Breathing supplementary oxygen used to be de rigueur at very high altitude (say above 7,500m), and is still common among guided parties in the Himalaya. However, the weight and inconvenience of carrying oxygen cylinders, and the potential for failure of the regulator valve, has led to many climbers doing without.

A serious problem could arise if the supply was suddenly exhausted or the equipment failed, leaving the climber "out of his depth", so to speak, having relied on the oxygen as a substitute for proper acclimatization. This may have contributed to the 1996 tragedy on Everest (with too many commercial parties on the 1953 route, ironically involving some of the best-known and most experienced guides) [2,5].

There is also an ethical viewpoint that using supplementary oxygen is somehow "cheating", especially if the cylinders are carried up by porters until needed by the clients [2]. However, it seems unlikely that anyone would consider it "cheating" to carry supplementary food, water or clothing.

Even if not used during climbing, a modest supply of additional oxygen might be useful in emergency, and it could certainly help to restore normal sleep and keep the body's core temperature up during the night, when metabolism and respiration are naturally depressed. Its use always has to be a matter of judgement.

General health and medication

As the body is already working beyond its normal range, the additional load placed on it by even minor infections can rapidly become serious. People have died from illnesses that would hardly be noticed at lower levels. Minor cuts and grazes are inevitable, and will normally be tolerable, though slow to heal. However, deeper wounds or a persistent, hacking cough must be treated at lower altitude.

Apart from its use in pain relief [5], Aspirin is sometimes recommended to people going to high altitude for use as a prophylactic, at about 75mg per day. It helps to thin the blood, and reduces the risk of thrombosis (unwanted clotting). On the other hand, it can increase the risk of stomach ulcers, and may prolong bleeding after an injury [2]. There may be complications with Diamox. Like so much advice, it's a balance: our doctor (GP), though not an altitude medicine expert, strongly recommended aspirin even for the long-haul flights, after checking that we had no stomach problems. For most people, moderate amounts of aspirin probably do more good than harm. 

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How to Acclimatize

There is one Golden Rule for safe acclimatization: gain height slowly, and be prepared to lose height quickly at the first sign of real sickness.

The extent to which a formal and managed acclimatization programme is required on any climb depends on the absolute altitude, the overall altitude gain, and especially on the amount of time for which it is planned to stay high [1]. For example, in the European Alps, a quick dash to a relatively easy 4,000 metre summit, such as the Mönch from the Jungfraujoch station, might be feasible for a fit and experienced party without any acclimatization, if it can be guaranteed that they will be descending the same day. They may get headaches and mild nausea, but it should be surviveable, though even one night spent at the Mönchsjochhütte beforehand would be a better idea (and would allow for an earlier start).

On the other hand, rushing by tramway to the Nid d'Aigle and setting straight off up Mont Blanc (4,807m) could be serious, if a night has to be spent unexpectedly at high altitude. On such a climb it is better to spend at least one night during the ascent, preferably two, and then if there is an emergency owing to weather or injury and the party is forced to bivouac, a night might safely be passed even as high as the Vallot shelter (at least from the point of view of AMS).

That is the European scale of things. In the greater ranges such as the Andes or the Himalaya, a more measured acclimatization programme is vital. There must be long periods of build-up at increasing altitude, laying siege to the altitude as much as to the mountain, and having frequent descents to allow the body to get itself used to the physiological changes taking place.

It is important, but sometimes extremely difficult, to distinguish the more serious symptoms of AMS from the headache and general feeling of malaise that commonly arise at high altitude, since even a safe ascent may result in quite unpleasant symptoms [5]. The notorious soroche headache in the Andes can be manageable, provided that it is not confused with HACE. However, if in doubt, go down. A headache that gets easier purely as a result of descent is almost certainly caused by AMS, in which case no re-ascent should be attempted until it has completely gone.

The general routine on longer ascents (say, above 6,000m) is to gain only a few hundred metres each day, and to try to sleep lower than the high point. Certainly, a break from climbing should be taken every couple of days, and a descent of a few hundred to a thousand metres made to recuperate. The safe height gain each day depends on the absolute height and the individual, and sheer fitness is not necessarily a guide (though it undoubtedly helps).

It is difficult, and could be misleading, to quote exact figures. Safe rates of ascent of as little as 600 m per day are quoted in the literature, but that is clearly impractical on many routes. Typical European summit days may exceed 1,500 m in height gain, although the expectation is often that the climber will descend to the starting point, or lower, in the evening. However, the presence of extremely high refuges such as that on the Signalkuppe (Monte Rosa group) at over 4,500m suggests that some risks can be taken on a regular basis. On long climbs with a good infrastructure, such as commercial expeditions to the 8,000m peaks, there will (should!) be a series of controlled ascents and descent, working gradually up the mountain like a ratchet.

To summarize: lay siege to the altitude, as much as to the mountain. Most importantly, no further height gain should be attempted until the temporary symptoms of AMS have disappeared. If they persist, go down, and stay down until they do disappear. If this turns out to be too repetitive a process, and lasting progress towards acclimatization is not being made, abandon the ascent.


Topics

  High Altitude Cerebral Oedema
    Cheyne-Stokes (Periodic) Breathing
      Cold & Dehydration
        Dexamethasone & Diamox
          Diet & Nutrition
            Menstruation and Contraception
              High Altitude Pulmonary Oedema
                Sleep

Cerebral Oedema (HACE)

This is a potentially lethal condition, in which the brain expands within the skull, owing to the accumulation of excessive fluid. The typical symptom of HACE is a general loss of brain functions, such as inability to think logically or walk straight (ataxia). Symptoms can also include extreme headache, but a headache alone does not necessarily indicate HACE.

Lives have been lost through careless or irrational behaviour induced by HACE. Many cases have been recorded of climbers dropping their gloves, forgetting to tie in, or not fastening their harness properly. Maurice Herzog famously dropped his gloves while descending Annapurna, but completely forgot that he had taken with him a spare pair of socks for just such an emergency. He subsequently lost his fingers to frostbite. But for the bravery and self-sacrifice of their companions, he and Lachenal would never have got down alive.

HACE can be tested for by the traditional test for drunkenness, getting the patient to walk heel-to-toe along a straight line. Do this as soon as the condition is suspected, as evacuation is much easier before the ability to walk safely is lost.

The only proper treatment for HACE is immediate evacuation to a lower altitude. Even a reduction of 500m may be enough, but time is critical. If a Gamow bag (a proprietary form of portable pressure vessel) is available, this may be a temporary alternative to evacuation, and may also aid evacuation by getting a person mobile [2], but is is very important to use it correctly.

The symptoms and treatment have some similarity to those of Pulmonary Oedema. The powerful drug Dexamethasone is probably best reserved for emergencies in which such an evacuation is impossible, through other incapacitation or bad weather, and in any event should only be administered under expert medical supervision.

Cheyne-Stokes Breathing

This is an unnatural breathing rhythm in which the sufferer alternates between panting and temporary cessation of breathing. It often occurs during sleep, and can be quite alarming for a companion if the pause is more than about 30 seconds (it may also cause the sleeper to wake up). The cause is related to the two mechanisms by which the body regulates involuntary breathing.

The principal mechanism is a response to carbon dioxide (CO2) in the lungs: when a certain concentration, is reached, there is a reflex to exhale*. The lungs then refill with fresh, oxygenated air from outside. This may be why some climbers report that smoking helps them to breathe better at altitude, as the additional CO2 from burning tobacco may stimulate the exhalation reflex. Contrary to popular supposition, there is no such reflex for a low concentration of oxygen in the lungs, which is why it can be easier to avoid breathing (such as when one is close to drowning) when the lungs are empty than when they are full. The second regulatory mechanism concerns the proportion of oxygen in the blood, which is measured in the brain stem, presumably a very ancient evolutionary development. Too low a value causes an involuntary inhalation (as in a yawn). It is apparently the alternation of these two mechanisms that causes Cheyne-Stokes breathing.

Cheyne-Stokes breathing, solely as a direct result of increased altitude, is common, and is generally not a cause for concern, unless it continues after acclimatization would have been expected, or is accompanied by unexpected symptoms.

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Cold

At high altitudes, the air is colder than at sea level, roughly by 6.5 degrees Celsius (11.7 degrees Fahrenheit) for every 1000 m of height (refer to the conversion table below). Bad weather, especially wind, can obviously make matters much worse, and since these are relative changes, the temperatures at the summits of the more polar mountains are dangerously low, even in good weather.

Apart from requiring the climber to wear suitable clothing, the cold has a significant effect on the metabolism, in a number of ways. Perhaps most importantly, the additional energy required to keep the body warm can only come from "burning" food, and this requires oxygen, which is of course in short supply. The cold air makes breathing painful, and the efficiency of the chemical processes to do with the metabolism of oxygen is impaired. The dreadful combination of extreme altitude, cold, and resulting fatigue is a common factor in deaths in high mountains.

The dangers of frostbite and hypothermia are serious, but are not yet covered by this article.

Dehydration

Adequate hydration is essential, to allow the body to regulate its chemical balance in response to the change in altitude. However, the air at high altitude is always very dry; above 5,000 m there is virtually no water vapour present. This has the effect of stripping water from the lungs, and making sweat evaporate very rapidly. These factors, combined with the effort of climbing, lead to extreme dehydration unless a special effort is made to drink more fluid than usual.

Especially during prolonged exposure to high altitude, the importance of fluid throughput cannot be overstated. To quote, "unless you're pissing gin-clear and lots of it, you're not adequately hydrated" [3]. The rate of fluid output (micturation) is a good guide, as unless you are having to get up several times a night to eliminate it, you are definitely not drinking enough. This can be a great inconvenience, but there is really no alternative.

Unfortunately, fetching enough ice or snow, melting it and making even cold drinks, never mind hot ones, take a lot of fuel, effort, and especially time. A cold drink may be almost useless anyway, as it will quickly freeze solid during the day's climb. In Europe, generally climbing from refuges, we have found that an energy drink such as Isostar, made up with very hot water in a plastic juice bottle, keeps warm for several hours in an insulating sleeve of the sort generally used to keep a wine bottle cool. A vacuum flask is a heavy luxury, but the taste of real coffee on top of Mont Blanc was worth every gramme...

Hot, weak black tea is an excellent drink, and a good way of getting sugar into the system [3], but some feel that coffee should be avoided (although it is invariably offered for breakfast in Alpine refuges) as it is a diuretic (but then, so is tea). Remember that Diamox (acetazolamide) taken to assist with acclimatization increases the need for drinking. At least 3 litres (nearly 7 pints) per day of water or watery drinks should be consumed every 24 hrs, and 4 litres is a better target, even if this is rarely achieved outside the lower camps.

A serious effect of dehydration is that the blood becomes thickened, with additional risk of strain on the heart and circulatory system, clotting and general impairment of performance. Aspirin can be taken to offset this.

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Dexamethasone

This is a prescription (ethical) steroid drug (also known as Decadron) which can be used to reduce the pressure in the brain that is associated with High Altitude Cerebral Oedema (HACE, see reference). The typical symptom of HACE is a general loss of brain functions, such as inability to think logically or walk straight (ataxia). An intense and persistent headache may also occur. Dexamethasone can also decrease Pulmonary Oedema.

Diamox

Diamox is a trade name for the prescription (ethical) drug Acetazolamide, a carbonic anhydrase inhibitor. Its principal medical use is in the treatment of glaucoma (excessive pressure in the front of the eye, between the lens and cornea: acetazolamide inhibits the formation of the aqueous humour), but it has been used successfully by mountaineers to counter moderate altitude AMS. It works in this context by increasing the excretion of bicarbonate through the kidneys, and so increasing the acidity of the blood, which helps it to carry more oxygen. It is apparently much less effective at very high altitudes.

The widespread and prolonged use of Diamox in the treatment of glaucoma means that there is a great repository of information about its safety and side effects. These include numbness or tingling in the fingers and toes; strange tastes in the mouth; and chemical changes in the urine. A suitably experienced doctor should be consulted before using Diamox, as it is a Sulphonamide drug and there are several reasons why some patients should not take it. There may be problems if it is used in combination with aspirin. A trial course, to evaluate side effects before going to the mountains, is a good idea.

There may be some advantage in taking half the regular 250 mg dose twice a day, morning and night (that is, 125 mg, half a tablet, each time) [5].

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Diet and Nutrition

It's very difficult to maintain appetite at high altitude. The senses of smell and taste are greatly inhibited, and the general feeling of lethargy and nausea that can accompany mild altitude sickness compound to put one off eating. It's also a chore to prepare meals, as to save weight, high altitude rations are generally dehydrated, and fetching enough ice or snow, melting it and heating the food take a lot of fuel, effort, and especially time. It is vitally important to motivate oneself (and one's companions) to eat a high calorie diet, whether one is hungry or not.

Specific food cravings occur, and this can be turned to advantage by careful advance selection of the high-altitude rations. Sir John (now the late Lord) Hunt tells a wonderful tale in his "Ascent of Everest" of his delight at finding a tin of tuna fish left on the South Col by the Swiss expedition. Snack foods not needing cooking are great, too: biscuits, crackers, sweets, chocolate, etc... [2]. We find chewy cereal bars are an especially good pick-me-up, if you can carry enough. 

Since the high-altitude experience is generally brief, there is no need to worry too much about a balanced diet. Go for readily digested, starchy carbohydrates such as pasta, mashed potato and thick soups. Fatty foods are difficult to digest, and should be avoided. There may be some advantage in taking vitamin and mineral supplements, particularly potassium and iron, to maintain the nerves and blood, and of course salt is vital as so much is lost through sweat evaporation in the very dry air.

Constipation and diarrhoea tend to alternate, and this can be dangerous as well as distressing. Some medicinal assistance may help to regulate the system. On many routes, water pollution is a very serious issue, and great care should be taken to avoid faecal contamination of drinking water. Sterilization by boiling is impossible, as water boils at too low a temperature, so chemical treatment with iodine may be required. The objectionable taste of iodine can be neutralised by dissolving Vitamin C (ascorbic acid) in the water after purification. We find that effervescent tablets work best.

Menstruation and Contraception

Menstruation (periods) may be upset during the acclimatization process, and while this seems unlikely to cause harm, it could be inconvenient. Lady climbers may wish to take additional "requisites" or make medicinal arrangements beforehand. Oral contraceptives may be less effective at high altitude, and there may also be an increased risk of thrombosis, as with any oestrogen-based medicine. These matters should be discussed with one's doctor.

Pulmonary Oedema (HAPE)

This is another potentially lethal condition in which the lungs (actually the alveoli) fill with fluid, probably as a result of osmosis through the very thin membranes that line them. This obviously reduces the area available for oxygen intake, rapidly leading to a vicious circle of reduced oxygenation and further increased HAPE, so the condition may worsen very quickly. The patient may cough up pink froth, and breathing will be noisy.

Other symptoms and treatment have some similarity to those of Cerebral Oedema. There is no cure except immediate descent, although Nifedipine (Adalat) may relieve some of the symptoms of HAPE (by reducing blood pressure).

Sleep and sleeping pills

Sleep can be elusive above 4,000m. Quite apart from the altitude (which makes it difficult to develop a relaxed involuntary breathing pattern) the cold, noise and general discomfort are impediments to a good night's rest. There is also likely to be worry about one's situation, and the likelihood of an early start. However, taking a sleeping pill is extremely dangerous, as the body is already lethargic, and it is easy to slip into a coma.

Diamox may help to establish a relaxed sleeping pattern, by reducing the likelihood of Cheyne-Stokes breathing.

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Tables

Altitude, Pressure and Temperature

The relationship between altitude and air pressure and temperature is complex and depends on a number of secondary factors such as the latitude and the humidity. However, for most purposes the following tables (taken from Kaye & Laby, 1959) will be accurate enough.



Metres    Feet  Pressure  Relative  Air Temp  *     Typical
                   mb     Pressure  Deg C   Deg F   Environment
---------------------------------------------------------------
     0       0   1,013    100.0%     15.0    59.0   Sea level
 1,000   3,281     899     88.7%      8.5    47.3
 2,000   6,562     795     78.5%      2.0    35.6   Foothills
 3,000   9,843     701     69.2%     -4.5    23.9
 4,000  13,123     616     60.8%    -11.0    12.2   European Alps
 5,000  16,404     540     53.3%    -17.5     0.5
 6,000  19,685     472     46.6%    -24.0   -11.2   Andes
 7,000  22,966     411     40.5%    -30.5   -22.9
 8,000  26,247     356     35.1%    -37.0   -34.6   Himalaya
 9,000  29,528     307     30.3%    -43.5   -46.3
10,000  32,808     264     26.1%    -50.0   -58.0   Aircraft (outside)

Examples

 4,808  15,774     554     54.7%                    Mont Blanc
 5,897  19,347     479     47.2%                    Cotopaxi
 8,848  29,029     314     31.0%                    Everest

* Note: these are theoretical still air temperatures, at temperate
  latitudes, and take no account of humidity or wind, which can
  dramatically lower the apparent temperature.  Nearer the poles,
  absolute temperatures are of course very much lower.


Conversion Table, Metres and Feet


metres    feet    feet   metres
-------------------------------
   100     328     250       76
   250     820     500      152
   500   1,640   1,000      305
 1,000   3,281   2,500      762
 1,500   4,921   5,000    1,524
 2,000   6,562   7,500    2,286
 2,500   8,202  10,000    3,048
 3,000   9,843  12,500    3,810
 4,000  13,123  15,000    4,572
 5,000  16,404  17,500    5,334
 6,000  19,685  20,000    6,096
 7,000  22,966  22,500    6,858
 8,000  26,247  25,000    7,620
 9,000  29,528  27,500    8,382
10,000  32,808  30,000    9,144

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Links

We cannot be responsible for the content of other sites' material, but we have prepared rough and ready pages of information sources thrown up by search engines, not all tested, on our own site, HACE links and HAPE links.

Off-site links are gathered here (partial update May 2015).

The (UK) NHS website now carries some information on Altitude Sickness with some supplementary pages.

The British Mountaineering Council website has excellent information pages on Altitude, including acute mountain sickness (AMS) and related topics. Dr Charles Clarke is a world authority on high-altitude medicine.

We've also been advised of a new booklet available for free download from Medex (scroll down the page).

Here are some further links, turned up by search engines or kindly sent by readers, all with some relevance to altitude, but offered with no guarantee as to the validity of their content :

Apex "A High Altitude Resource" which also maintains the HAPE.Org.UK "worldwide database of people who have previously suffered from HAPE" (High Altitude Pulmonary (O)Edema).

High Altitude Medicine Guide Concise advice on AMS etc and lots of Himalayan links
Lung volumes and capacities from Loyola University, if you are interested in pulmonary physiology (moved)
National Outdoor Leadership School (USA) (moved - home page only)
Supplemental Oxygen for the General Aviation Pilot by Dr Carlson, a Professor of Medicine at Stanford University Medical School, as well as an instrument rated private pilot

A link passed to us by David Pardo [sic] on 13/4/1999 refers to the use of hypobaric chambers for altitude conditioning - Colorado Altitude Training

"Altitude Illness", P W Barry, A J Pollard, an article found on British Medical Journal website and posted by Anthony Cox on uk.rec.climbing, which itself has many links and references (registration required).

May 2005 : The Altitude Centre, UK, supply and use simulated altitude equipment. Note : this is a commercial site, but may be of interest to mountaineers or athletes.

Books

The following books have been recommended by readers of this page, but I cannot offer any personal endorsement :

"Going Higher: The Story of Man & Altitude" by Charles Houston M.D. [3,4]
"High Altitude Medicine" by Herb Hultgren, M.D. (commercial page at http://www.highaltitudemedicine.com/)[4]

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Acknowledgements:

[1] Dr Henry Lickorish
[2] Peter G. Green {home page broken)
[3] David Van Baak
[4] Hal Murray
[5] Mike Trueman(home page broken)
[6] David Pardo (links)
[7] Ian Oliver (in the context of SCUBA diving)

We are most grateful to these named contributors, and to several others, for their valued peer review, helpful suggestions and contributions to this page.

Disclaimer : please note that we are not medically qualified, and any information or advice offered here, or via links from our site, is no substitute for professional consultation.


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Cheshire,   England
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