UKC

Understanding and Managing the High-Altitude Environment Article

© Jason Sheldrake

Stephen Taylor explores the challenges that living and moving in high-altitude environments present to the human body and how to manage them...


This article seeks to provide an introduction to understanding and managing the high-altitude environment which constitutes the background for mountaineering expeditions. It does so by trying to understand the science behind this context. As a lay person myself (my expertise/research concerns mountain tourism and its impacts) in respect of the underlying science here, it is intended to be a lay person's explanation to the phenomena that underpins and shapes the environment in which we attempt to achieve our high mountain objectives. 

Ama Dablam (6,812m)  © Jason Sheldrake
Ama Dablam (6,812m)
© Jason Sheldrake

After a general exploration of the 'high-altitude' context there are three key themes examined here: (1) Acclimatisation; (2) Eating and drinking; and (3) Sleeping. Arguably, these three items cover the core of the demands of living and succeeding at high-altitude during any expedition. Managing these basics in the correct way will greatly improve your chances of being successful in achieving your planned climbing objectives and your overall enjoyment of the expedition experience! 

Understanding High-Altitude

An essential starting point, before discussing the managing of the high-altitude environment, is to define the term "high-altitude" and to better understand this phenomenon which is central to many expeditions. It is this context which underpins the nature of the environment we encounter and its associated impacts upon us during our expedition. Various sources suggest the following general classification:

Table 1: Typical classification of high-altitude categories

Category

Feet (ft)

Metres (m)

High-altitude

8,000-12,000

2,438-3,658

Very high-altitude

12,000-18,000

3,658-5,487

Extremely high-altitude

18,000 +

5,500+

 

Our focus is really on the second two categories of "very" and "extremely high" altitudes. Some classifications start the first category of "high-altitude" as low as 1,500m as it is from this height that it is possible for people to start to feel its effects (e.g. headaches and altered night vision). For most of us, it is probably around 2,500m that we start to feel the effects of altitude. Interestingly, commercial aircraft are pressurised to an equivalent altitude of 1,500-2,500m. The pressurisation altitude is simply a trade-off, if aircraft were to be pressurised to sea-level, a much heavier and stronger structure would be required to cope with the greater differential pressure at the typical cruising levels of 9,500-11,500m. 

The impact of altitude is a reduction of air (i.e. atmospheric) pressure, through a combination of both decreasing gravity and air density, which results in a lower percentage of oxygen (O2) in a given volume of air, i.e. "thin air". Note, that the overall proportion of O2 in the atmosphere remains constant regardless of altitude – there is simply less pressure and thus a lower density of oxygen in each breath we take, meaning less air is pushed into our lungs and subsequently enters our bloodstream. When we breathe at sea level that intake of air yields a given volume of oxygen in the atmosphere (21% O2). As we increase our altitude, that same breathe contains fewer oxygen molecules, thus the need we experience to take more breaths. 

At sea-level you have the pressure equivalent of 10 metres of water pressing down on you constantly. As you move up a mountain there is less (density of) air above you in the atmosphere, that is, there are fewer molecules present for a given volume of air. Conversely, the smaller the volume into which air is squeezed the stronger the force (this is known as Boyle's Law) – think of a bicycle pump. Furthermore, the air in a bicycle pump gets hotter as the pressure increases and cooler as it decreases (see the section below on altitude and temperature). To get your head around this idea of how pressure impacts the volume of air available in each breathe, take the example of a full bucket of water.

If you were to make a hole at the top of the bucket, water would dribble out quite slowly, but if you then made a hole towards the bottom, the water would escape much more quickly due to the greater weight (i.e. pressure) on that water. So, as we gain altitude there is less pressure to force air into our lungs and ultimately into our bloodstream. The air becomes increasingly thinner as we gain altitude since the 21% O2 found at sea level is spread across a greater volume air – in short, this volume (spreads) increases the higher we go due to the falling pressure.

The single biggest component of the atmosphere is nitrogen at 79%, leaving us our 21% of 02 to make up the remainder. However, because of this decreasing air pressure as we gain altitude, at 1,500m in the same volume (e.g. a breath) of air there is only 17.3% O2, at 5,000m it is 11.2% O2, while at 8,500m it is 7.2% O2 – a mere third of the oxygen available at sea-level. Over 8,000m we are in the so-called "death zone" as here the oxygen concentration is so low that your cells do not receive sufficient oxygen to build new tissue – you are literally, albeit slowly, dying. Table 2 shows effective oxygen levels on some well-known summits.

Table 2:  Atmospheric pressure and oxygen levels

Altitude

Approximate Barometric

Pressure (kPa)

Effective Oxygen

Level (%)*

Sea level

(0m)

101.3kPa

21.0% O2

Ben Nevis (1,345m)

84.6kPa

17.6% O2

Mont Blanc (4,808m)

54.0kPa

11.4% O2

Kilimanjaro (5,895m)

47.2kPa

10.1% O2

Aconcagua (6,962m)

41.1kPa

8.7% O2

Everest

 (8848m)

31.5kPa

6.9% O2

*For a fixed volume of air, e.g. a breath. kPa – kilopascals   0m = 101.3 kPa (760mmHg)

 

If you were to suddenly transfer from sea-level to Everest's summit you would, within 30 seconds to a couple of minutes, become unconscious and death would follow not long afterwards. While an extreme (and hypothetical!) example, this illustrates that the human body can only cope with the demands of extreme high-altitude (without supplemental oxygen) if an appropriate acclimatisation strategy (as Reinhold Messner and 207 others, as of 2019, have demonstrated on Everest) is followed to enable our bodies to make the necessary physiological adjustments to cope with the impacts of lower air pressure, albeit for a limited period. We will discuss this under acclimatisation strategies, but also see the recent excellent series here on UKC by Calum Muskett and Jamie Macdonald.

However, it not just at the extremes of Everest that the negative effects of rapid pressure loss would be experienced by a human body suddenly going from sea-level to our extremely high-altitude category. For example, without prior acclimatisation, at 6,000m humans would experience a loss of consciousness within 5-20 minutes. When above 5,500m, we humans are in a very dangerous place, and we need to be very observant as to how we and our expedition partners are coping. Any serious case of altitude sickness, if not appropriately addressed, at best will result in curtailing of the planned expedition, at worst, it can be quickly become a matter of life or death. Potential strategies for acclimatisation are discussed below.

To make everything more complex however, physical altitude as the measurement in metres of a given summit above sea level is not the only factor that determines the actual altitude you will experience. This experienced altitude is known as pressure altitude, and it is a combination of the physical altitude with the additional variables of latitude, temperature, and weather.

Climbing virgin summits in Kyrgyzstan.  © Stephen Taylor
Climbing virgin summits in Kyrgyzstan.
© Stephen Taylor

The effects of latitude on altitude

Another twist here is that air pressure (and air volume) does not vary uniformly around the planet with increasing altitudes. Changes in latitude, as we move away from the equator, sees temperature decrease, which accounts for the climate and humidity variations on the planet's surface, and this creates local variations in air pressure. To illustrate the significance of this, if for example, Mount Everest were to be relocated from its current latitude of 28 degrees north, to say Alaska and placed beside Denali (at a latitude of 63 degrees north), at this higher latitude the summit would feel 914m (3,000ft) higher compared to how it feels at its actual location/latitude. This is because there is a variation in air pressure (it gets less!) caused by lower temperatures – this is the most significant factor associated with latitude – which is due to the reduced amount of sunlight received. A further major contributing factor is that the atmosphere is also thinner – it reduces by 50% - the more we move towards the poles from the equator, i.e. less density = less weight = less pressure. The reduction in sunlight is a function of the greater distance from the equator and corresponding higher angle of solar incidence (the angle at which sunlight hits the planet). This also means, it would also be considerably colder. 

Those who have summited Denali (6,190m) sitting at its northern latitude of 63 degrees and with a lower barometric pressure than most of the World's other high mountains, and have also climbed 6,000m peaks at lower latitudes (e.g. Central Asia/Himalayas), are likely to report that it is a cold mountain that feels higher than its given altitude! Indeed, research suggests that an ascent in very early May (the start of the typical season on Denali) when it is colder, can feel like climbing to an altitude of 1,300 feet higher (nearly 400m!) than an ascent in the warmer temperatures of late July on the same mountain. Certainly, the average change in pressure on Denali from May 1 to July 1 is the equivalent of descending 743 feet according to this research published in 2006. Typically, we focus on the trade-off of the colder temperatures/greater snow cover early season versus the warmer temperatures/less snow cover late season, but here we can see temperature also impacts on the air pressure and thus the experienced/effective level of altitude.  This also highlights why winter ascents of 8,000m peaks are so challenging.   

The above underlines the importance of the distinction between physical altitude versus pressure altitude. That is, the actual altitude – physical height gain above sea level – versus the experienced/perceived altitude that reflects the air pressure. For us, as mountaineers, it is the latter which we are needing to deal with physiologically. So, while physical height is clearly the obvious primary determinant of altitude (or, rather air pressure!), the temperature, the latitude, plus the weather, all ultimately contribute to the actual air pressure experienced. Arguably then, high-altitude mountaineering is more accurately low-pressure mountaineering! This brings us on to the issue of temperature which is a major determinant of pressure altitude.       

Altitude and temperature

The Earth's temperature is, as we all know, dependent upon the heat of the sun which is absorbed by the planet's surface. As the surface warms, this heat spreads into the lower layer of the atmosphere, expanding and becoming less dense as it rises (becomes lighter). The decreasing pressure (lower density) causes the air molecules to expand, causing lower temperatures. This rate at which temperature drops (the adiabatic lapse rate) is 6.5 degrees Celsius per 1,000 metres (see Table 3 for some examples of the effect of the adiabatic lapse rate). Increasing altitudes sees the molecules in the atmosphere moving further apart (lowering the air pressure) and as heat is generated by these molecules colliding, this greater distance between molecules means fewer collisions and thus less heat generated.

In winter, of course, there is less heating of the Earth's surface and thus lower winter temperatures on high mountains. In practice this means that outside the summer season, ascents are going to be tougher not just because of the greater cold experienced, but also by the relative increase in altitude experienced (due to the cold decreasing the air pressure). Paradoxically, global warming is increasing air pressure and is potentially making climbing at high-altitude easier as discussed here. This of course, is the flipside of the argument made about the impacts of colder weather and higher latitudes made above with reference to Denali. More generally, global warming is likely to change the traditional timings of when we can climb in the greater ranges (likewise, it is impacting on the summer climbing season in the European Alps). Will winter alpine climbing eventually become the new normal?

Goûter Ridge on Mont Blanc.  © Jason Sheldrake
Goûter Ridge on Mont Blanc.
© Jason Sheldrake

In addition, weather is a further consideration in terms of its impacts upon effective altitude levels. This is because bad weather, associated with low pressure systems means the experienced altitude at a given altitude will feel higher than with a high-pressure weather system. So, if you were sleeping at an altitude of say 6,000m and an intense low-pressure system (e.g. a bad storm) was to suddenly arrive, you would feel as if you had been transported to a higher altitude as the atmospheric pressure has decreased. As I can testify from personal experience, this can have a dramatic effect on how you feel!        

While we are here… with reference to weather systems, in the case of a high-pressure system, here we have a higher ('domes of') density whereby air tends to sink ('subsidence') into the lower atmosphere where warmer temperatures are found and more water vapour can be held. This typically creates a clearer, drier environment as any water droplets that could form clouds tend to evaporate. Conversely, with a low-pressure system ('atmospheric valleys'), the air rises higher in the atmosphere where it is colder. Here there is a lower capacity to hold water vapour which condenses into clouds (billions of water droplets, or ice crystals at higher levels) and then produces precipitation.    

However, as the air cools, the molecules will become closer together BUT this greater proximity results in collisions with less force (and thus less heat generated!) and thus lower air pressure. Confusing? As indicated above, altitude is affected by latitude because at increasing latitude air density decreases AND temperatures decrease. Less pressure means molecules move further apart… less collisions means lower temperatures… lower temperatures mean molecules move closer together, but any collisions have less energy due to the greater proximity of molecules.

 

Table 3: Example of how temperature changes by altitude*

Altitude

Temperature (degrees Celsius)

Sea level (0m)

25

Ben Nevis (1,345m)

16.26

Mont Blanc (4,808m)

-6.25

Kilimanjaro (5,895m)

-13.32

Aconcagua (6,962m)

-20.25

Everest (8848m)

-32.51

*Calculated using an adiabatic lapse rate of 6.5 degrees Celsius per 1,000m and assuming a sea level temperature of 25 degrees Celsius.

Acclimatisation

This is the process of your body adjusting to the lower volume of oxygen due to the reduction in air pressure caused by the increased altitude (plus temperature, latitude, and weather effects!). The body will in the short term make a series of accommodations and, if properly managed in terms of acclimatisation strategy, in the longer term there are temporary adjustments that enable the body to cope more effectively with higher altitudes. See Brendan Scott's short piece here for a quick overview: How does altitude affect the body and why does it affect people differently?

In simple terms, we need oxygen to make the cells in our bodies function. At higher altitudes there is going to be a shortfall of the amount of oxygen in the blood and thus a reduction in the normal functioning of cells. The most immediate and obvious response is that we start breathing more quickly (this can create a fluid build-up in the lungs) in an effort to compensate for the lower number of air molecules, due to low air pressure, in each breath (i.e. an increase in ventilation). In the longer term, the body starts to produce more red blood cells (this leads to a thickening of the blood) which carry O2 around the body, to our muscles for example. For a more detailed, academic explanation of what happens to the human body at high altitudes see John West's paper. Our cardiac output (the volume of blood pumped by our hearts) also increases as an immediate response to the lower air pressure, but this will return to normal after a few days as the number of red blood cells increase and oxygen delivery stabilises.

Increased ventilation/breathing hard means you breath off more carbon dioxide, decreasing the amount of carbon dioxide (acidic) in the blood making your blood more alkaline (rise in PH). Your body reacts to this 'excessive' breathing leading to sporadic breath, particularly when you sleep (see below). To try and decrease the alkalinity of the blood the kidneys increase the bicarbonates in the urine which will become very alkaline.   

Increased urination is experienced at higher altitudes (high-altitude diuresis) as your body tries to avoid an elevation in the blood PH (respiratory alkalosis) caused through the excretion of bicarbonate by the kidneys. This is a normal and healthy indication that your body is trying to acclimatise. This settles once the body becomes more acclimatised. The body also reacts to colder temperatures through increased urination as this reduces the energy requirement otherwise to heat this fluid.      

Finally, and in passing, it is worth mentioning the drug Acetazolamide (Diamox is its trade name) which can help speed up the acclimatisation process (essentially, it helps to offset the respiratory alkalosis, i.e. the rise in blood PH). I have no personal experience of using this drug and I know very few people who are regular users when going to altitude. It may work well for some individuals, so it is something you might wish to consider.  For more information on Diamox and all matters medical at high-altitude, an excellent free resource and guide to the impact of altitude on the human body is the booklet published by Medical Expeditions (MEDEX) Travel at High Altitude which is available in various languages. This covers everything you need to know about the taking the human body to high altitudes safely! More generally, Jeremy Windsor's Mountain Medicine Blog ('Surviving the Death Zone') covers the topic more broadly and has a wide range of material that many mountaineers will find very interesting (the main target audience is healthcare professionals). Both are highly recommended.

Hypoxia

The air we breathe, through air pressure, is forced into the lungs and from there transferred to the blood where the protein haemoglobin carries the oxygen to your body's cells. The amount of oxygen being carried in the haemoglobin (i.e. the blood oxygen saturation level) is measured, albeit indirectly, by your peripheral capillary oxygen saturation (= SpO2). This is expressed as a percentage of oxygen in the blood – a ratio of oxygenated versus non-oxygenated haemoglobin. A high percentage 90-100% (95-100% is typically considered normal) is needed to ensure muscle function.  Below 90% is considered a poor level of blood oxygenation and is referred to as a state of hypoxia (hypoxemia). 

You can measure your SpO2 value using a pulse oximeter (a non-invasive method to measure arterial blood oxygen saturation and pulse level). These are relatively cheap and widely available and are highly portable. It can be an interesting diversion at high-altitude camps passing round the pulse oximeter – the readings will surprise you! Particularly, when you bear in mind, that at sea-level an SpO2 level below 90% for any prolonged period would typically see the immediate administration of emergency supplementary oxygen.

The Taylor Traverse on virgin peak in Kyrgyzstan.  © Jason Sheldrake
The Taylor Traverse on virgin peak in Kyrgyzstan.
© Jason Sheldrake

High-Altitude Illness

When the human body fails to make the necessary adjustments (acclimatisation response) to the increased altitude this can lead to high-altitude illnesses (HAI). This covers acute mountain sickness (AMS); high-altitude pulmonary edema (HAPE); and high-altitude cerebral edema (HACE). It is worth noting that you can be suffering from all three forms of HAI at the same time! 

If we take the most common and least serious form of HAI, that of AMS, symptoms here are headache; gastrointestinal symptoms (anorexia, nausea, vomiting), sleep disturbance, dizziness, and fatigue. Another symptom is what is called peripheral edema which can manifest itself in the form of a swollen face. Getting some of these symptoms is not uncommon (particularly the headache) and is normally nothing more than a passing phase.  Usually, a couple of days sees the symptoms pass, but if not, then descent might be useful. Typically, this will deal with things quite quickly. 

What we do not want is for things to get more serious in the form of HACE and/or HAPE. Both result from a fluid build-up in the body, in the brain (cerebral) and lungs (pulmonary) respectively. In the case of HAPE this involves a leakage of clear fluids into the oxygen-exchanging sacks of the lungs with symptoms being a persistent dry cough, often with pinkish sputum, fever, and panting, even while resting. In the case of HACE the fluid leakage results in a persistent headache which painkillers do not relieve and several other symptoms such as: an unsteady gait; clumsiness; increased vomiting; gradual loss of consciousness; numbness; and dizziness. See here for an example of HACE involving the inability to form words.

While both conditions can quickly become life threatening and need to be treated with absolute seriousness, perhaps HACE is the most insidious, as sufferers, due to the fact it involves the brain, may not be fully aware of what is happening – this where the importance of keeping an eye on each other on expeditions really comes into play. In the case of HAPE, sufferers are usually very aware of their symptoms!  While there are some drugs that can help (e.g. both Diamox and Dexamethasone – the latter in particular – have been found to be very effective with HACE on a number occasions) the best strategy is typically "descend, descend, descend!" Again, please see the MEDEX guide for more information.

South Ridge Dent Blanche.  © Stephen Taylor
South Ridge Dent Blanche.
© Stephen Taylor

Acclimatisation strategies

We are probably all familiar with the conventional rule-of-thumb which states 'climb high and sleep low' with no daily increase in sleeping altitude of more than 300m per day. However, as discussed below and outlined in Table 4, that is not going to be very useful in practice on many expeditions. Another example of a common rule of thumb is 11.4 days per 1000m of gain. This is considered reliable up to around 18,000m. Above this altitude the research is less clear. Again, this is frequently exceeded in the typical itineraries for ascending popular high peaks. Another interesting issue is how long do we stay acclimatised? Apparently, the body returns to its original state – 'de-acclimate' i.e. a loss of acclimatisation in about 15 days, certainly if we use the red blood cell count. Well, that is the traditional take. More recent research by the University of Colorado's Altitude Research Centre suggests things are a bit more complicated in terms of the metabolic changes involved and that as red blood cells live for around 120 days then so can the changes! That said, I suspect for most of us, personal experience probably suggests the traditional two weeks window seems closer to the mark? 

In practice, acclimatisation seems to resemble something akin to a 'black art' than anything matching the scientific based advice. For example, if we take our 300m daily gain and 11.4 days per 1000m of gain, which is based on science, and look at the actual itineraries adopted on many popular high-altitude objectives, there is typically a significant gap in this theory and the actual practice. I illustrate this 'gap' in Figure 1 with reference to one such objective, the 7,134 metres Lenin Peak on the Kyrgyzstan/Tajikistan border and outline a potential acclimatisation strategy that, within the typically allocated commercial timescale, has been experienced to greatly improve acclimatisation (and successful ascents) in practice.

If we take the location of camps on established high-altitude routes on well-known peaks, these tend to reflect a combination of suitability of the terrain and the acceptable gain in altitude from the preceding camp. Thus, these tend to embody some generalised understanding of what constitutes a reasonable gain in altitude. More fundamentally, the positioning of these camps will typically shape the acclimatisation strategies that might be adopted for that specific mountain.

Table 4: Examples of camp locations and altitude gains on popular high mountains

Camps with high gains shown

Everest

South (Nepal)

Denali

West Buttress

Lenin Peak

Razedelnaya Route

Base Camp

5,350m

2,195m

3,600m

Camp 1

5,900m (+550m)

2,377m (+182m)

4,300m (+700m)*

Camp 2

6,500m (+600m)

3,414m (+914m)

5,300m (+1000m)

Camp 3

7,200m (+700m)

4,328m (+915m)

6,100m (+800m)

Camp 4

8,000m (+800m)

5,243m (+915m)

N/A**

Summit

8,848m (+848m)

6,190m (+947m)

7,134m (+1,034m)

 *Officially ABC, subsequent camps being 1; 2 & 3. **Camp '4' 6,400m (+300m) rarely used.

 

A classic example of the influence of the territory is present in all three examples above in respect to the transition from the respective base camps to the first high camps. All three involve covering not insignificant distances and/or difficult ground. In the case of Everest this involves crossing the dangerous Khumbu icefall, in the case of Denali it is 5.5 miles along the Kahiltna Glacier. Thereafter the range of height between camps (excluding highest camp to summit) varies from 600m-1,000m, with an average gain between camps of 830.5m. This underlines the reality of trying to follow conventional advice regarding daily height gains. Additionally, we are individuals and acclimatise at our own rates – for most of us this is within a similar envelope of time, but other people just acclimatise a lot more slowly than the 'average'. If that is you, or someone in your team, then that too needs to be factored in to the timescale for your expedition.     

With reference to Figure 1 below, we can see three recommended strategies for acclimatisation and ascent of Lenin Peak (see my earlier article on this mountain here on UKC). This excludes rest days or delays due to bad weather but highlights the general approach to be taken. Here each strategy takes two rotations up to the high camp (Camp 2, 6,100m) with the summit bid on the second rotation. What is notable is the descent all the way to basecamp (3,600m) in both strategies 1 and 2 (days 12 and 11 respectively) after sleeping at the highest camp. This is known as the "Russian rest" it was widely practised by Soviet-era climbers and introduced to Western climbers by the legendary Anatoli Boukreev and his peers. This involves sleeping at within around 1,000m of the target summit and then descending as low as one can (i.e. basecamp) for a few days' rest. Indeed, it is not uncommon to see "local" climbers spend three or so days at Lenin basecamp after sleeping at Camp 2 before moving back up the mountain for the summit bid. The belief is that this time at this lower altitude greatly improves the acclimatisation process. It is not clear if this has been scientifically verified, but it does appear to have become established practice by many throughout the former Soviet Union. The other traditional practice of using vodka drinking to help with acclimatisation, however, seems to have been abandoned!         

Lenin Peak.  © Jason Sheldrake
Lenin Peak.
© Jason Sheldrake

Figure 1: Three acclimatisation strategies for Lenin Peak

             
 

Strategy 1

 

Strategy 2

 

Strategy 3

 

Day

Sleep

(m)

Daily high point

Sleep

(m)

Daily high point

Sleep

(m)

Daily high point

1

3,600

3,600

3,600

3,600

3,600

3,600

2

3,600

4,300

3,600

4,300

3,600

4,300

3

4,300

4,300

4,300

4,300

4,300

4,300

4

5,300

5,300

5,300

5,300

5,300

5,300

5

5,300

5,600

5,300

5,600

5,300

5,600

6

3,600

3,600

4,300

4,300

4,300

4,300

7

4,300

4,300

5,300

5,300

5,300

5,300

8

5,300

5,300

6,100

6,100

6,100

6,100

9

6,100

6,100

6,100

6,200

6,100

6,200

10

6,100

6,200

4,300

4,300

4,300

4,300

11

4,300

4,300

3,600*

3,600

4,300

4,300

12

3,600*

3,600

4,300

4,300

5,300

5,300

13

4,300

4,300

5,300

5,300

6,100

6,100

14

5,300

5,300

6,100

6,100

6,100

7,134

15

6,100

6,100

6,100

7,134

4,300

4,300

16

6,100

7,134

4,300

4,300

3,600

3,600

17

4,300

4,300

3,600

3,600

   
             

*Russian rest.

Based on the foregoing, the best advice would appear to be to go gradually increasingly high enough to stimulate the required physiological responses and allow enough time for your body to start to adapt to the altitude. Perfect acclimatisation is something we might aspire to, but it will probably be about getting it 'good enough' so that you can achieve your set objective safely.   

Eating and drinking at altitude

Effective rehydration and maintaining energy intake are key factors in the success of any expedition. For most people, appetite declines with altitude gain. This is quite natural as the digestive system slows down at altitude. Energy use however increases. It is this combination that results in weight loss on expeditions. At higher altitudes hormones such as adrenaline, noradrenaline and cortisol become elevated, and these increase the use of carbohydrates as fuel. Overall, in terms of macronutrients, the body's preferred source of fuel at altitude is carbohydrates and the recommendation is that we aim to intake at least 60% in this form in terms of solids and liquids. Carbohydrates can replace depleted muscle glycogen, prevents muscle being used for energy and uses around 8-10% less oxygen than fat and protein for metabolism. There is less advice regarding recommended protein and fat intakes, but around 20-30% daily intake of protein to support muscle recovery is widely suggested. Less is typically mentioned about fat intakes at higher altitudes but, given its high calorie load, this would seem to be useful in preventing too much weight loss. However, as indicated above, fat does need more oxygen than carbohydrates for metabolism.

Climbing a virgin peak in Kyrgyzstan.  © Jason Sheldrake
Climbing a virgin peak in Kyrgyzstan.
© Jason Sheldrake

There is some research to suggest that with regards to micronutrients, taking a vitamin E supplement can help with countering some of the effects of altitude upon blood flow and physical performance. Maintaining blood flow/blood volume appears to be a common theme with regards performance at high altitudes. This points towards the importance of iron which has a key role in helping red blood cells deliver oxygen and in energy metabolism. However, this would suggest that magnesium, zinc, folate, and vitamin B12 are also going to be important too as these all, along with iron, contribute to the production of red blood cells. As such, this suggests some form of dietary supplement to ensure you are getting these micronutrients is worth consideration as the type of foods being consumed at high-altitude are unlikely to be particularly good sources of these, as here the primary emphasis is more upon consuming the macronutrients to get the required calories/energy.       

As noted above, increased dehydration is to be expected. This is a combination of increased respiration in cold dry air, sweating and increased urination, which combine to reduce overall blood volume. This reduces the amount of blood and oxygen going to muscles etc. which leads to a decrease in exercise performance. Some research suggests a 3% decline for every 300 metres gained above 1500 metres. If that is accurate, even at 4,500 metres that would be a 30% decline in performance!  The general advice is that we should drink an extra 1 to 1.5 litres per of liquid per day at high-altitude, a total of 3-5 litres per day.  As always, the colour of your urine is the best guide here to your water intake – if it's light you've got it right, if it's dark – knock it back!  

The importance of rehydration suggests some understanding of the physical effects of altitude on boiling water is going to be useful – the boiling point lowers with increasing altitude due to decreasing pressure (see Table 4 for examples with reference to well-known summits). More technically, as atmospheric pressure decreases with altitude; vapour pressure (the tendency of molecules to escape a liquid's surface into the gas phase) increases with water temperature. Any liquid will boil when its vapour pressure is equal to the atmospheric pressure. So, with higher altitudes, due to the lower atmospheric pressure, the point as which vapour pressure is reached in water will require a lower temperature. In simple terms: water will boil at increasingly lower temperatures as altitude increases (and thus air pressure decreases). This will increase cooking times! Conversely, we can raise the boiling point above 100 degrees Celsius if we increase the pressure – this is the principle of pressure cookers – they allow us to use create higher pressure (circa 2 bar = 120 degrees Celsius) achieving a higher temperature which shortens the cooking time. To calculate the boiling point for a specific altitude see here.  

Table 4: Boiling points at various altitudes

Altitude

Boiling point (Degrees Celsius)

Sea level (0m)

100

Ben Nevis (1,345m)

95.6

Mont Blanc (4,808m)

83.5

Kilimanjaro (5,895m)

79.5

Aconcagua (6,962m)

75.5

Everest (8848m)

68.0

 

Which foods?

Given the logistics of high-altitude ascents and the need to minimise weight combined with the physical impacts of high-altitude, the focus here is upon the most effective and efficient route to ensuring rehydration and the consumption of sufficient daily calories. On this basis, most of these daily calories are going to be consumed in the form of freeze-dried or dehydrated foods. This also means that the capacity to melt snow and boil water is central to the whole endeavour. Initially, we will look at the issue of food and then we will briefly examine the important issue of stove selection for high-altitude.

While many of us probably think that 'dehydrated' and 'freeze-dried' foods are essentially the same thing, they are not! Dehydration removes 90-95% of moisture content while freeze drying removes in the region of 98-99%. This lower moisture means longer shelf life, greater retention of the nutritional value of the fresh product, that the food is more palatable, and it lowers the weight of the food, plus it is faster to prepare. With dehydrated foods cooking is required and with the lower boiling points at higher-altitudes this means longer cooking times (over 6,000m this can add 15 minutes or more). By some estimates it is an extra one minute for every 300m. With freeze-dried it involves just the adding of water (hot or cold). Either way, it means less fuel. Dehydrated foods typically will cost less but this saving needs to be balanced with the other 'costs' in respect of the advantages of freeze-dried foods.            

Whichever of these two types of food you select, as discussed above, the focus is upon ensuring most of the calories are from carbohydrates. These are the most effective fuel source for your body at high-altitude. As it is, carbohydrates are usually the main macronutrient in these pre-packaged foods and typically these meals have around 600 calories per serving, but larger portions providing up to around a 1,000 calories are available. So, for planning purposes, three portions of freeze-dried food per person per day (e.g. breakfast porridge; evening meal and dessert) will generate circa. 1,800 calories and this can be supplemented during the day when you are on the go with energy bars (circa. 200 calories per portion) and protein bars (circa. 250 calories per portion) to support recovery. However, in practice, the actual amounts of food available will reflect individual preferences and practicalities like carrying to high-altitude and an individual's capacity/desire to consume food, which as indicated above, does tend to diminish. Regardless of calories intake (a deficit here is normal in practice), the importance of liquid intake cannot be stressed too much… getting this wrong will have a much more immediate effect than any calorie shortages. Underpinning the intake of both food and liquids is the capacity to melt snow… this takes us to stoves.

Camp high in Caucasus near Elbrus.   © Jason Sheldrake
Camp high in Caucasus near Elbrus.
© Jason Sheldrake

Which stove?

As you have probably guessed, this too is not quite as straightforward as it might first appear. That said, for some it probably is… i.e.  that the best choice is a mixed fuel stove such as the well proven MSR XGK… in terms of performance at high-altitude (and they are correct!) … but performance is arguably not the only consideration. In terms of convenience and ease of use then a gas stove (with a butane-propane mix) such as the MSR Reactor is recognised as an excellent and proven system for melting snow at altitudes to at least 6,500m.  Other branded stoves are available of course, but these two stoves are good examples of the mixed fuel and gas systems, respectively. So, how to decide?

In terms of outright performance, the mixed fuel stove is widely acknowledged as the best choice. However, it is not as easy to use. It needs to be primed and pressurised, plus it needs to be carefully maintained. Taking a maintenance kit with you would be strongly advised. Moreover, you really need to be experienced in their use and confident you can get it operating whatever comes your way. But again, in cold, low pressure environments, mixed fuel is the best performer and depending on the location of your trip, might be the only option if gas cannisters cannot be sourced locally.

In terms of convenience, ease of use (lightweight with a combined heat exchanger/pot system), and assuming you can source the fuel cannisters locally, gas stoves are a more attractive proposition to many people. However, if you are going to use one at high-altitude then ensure it has a pressure regulator – most gas stoves do not! In addition, placing the gas cannister in a container of warm water helps with performance too. While the use of any stove inside an enclosed space is not advised, high-altitude expeditions typically necessitate this very activity (sufficient ventilation is essential!) and here the relative ease of use of the gas stove (e.g. the use of a hanging kit) seems very attractive to me. Finally, use of a tight lid regardless of stove type is essential!

Cooking at an ABC in Kyrgyzstan.  © Jason Sheldrake
Cooking at an ABC in Kyrgyzstan.
© Jason Sheldrake

Sleeping at altitude

This can be more of an issue than it might first appear since a normal phenomenon at altitude is periodic breathing which is most obvious during sleep.  This sees periods of hyperpnea (very deep or rapid breaths) followed by apnea (no breathing – this is typically of 3-10 seconds duration but can be as long as 15 seconds). It can be very disruptive of sleep and can induce feelings of panic when waking up suddenly after a bout of apnea. Some slow deep breathing can help here. The other thing that might disturb your sleep is the frequent need to urinate. Here, particularly at high camps, having a well organised system for using a pee bottle is essential (this is considerably more challenging if you are not male). A wide mouthed litre Nalgene bottle works well, although making sure it can be identified by touch (use of tape/elastic bands) is strongly advised to avoid unpleasant confusion. Overall, if you are not acclimatising well, nights can be long and unpleasant. If this persists, a period of descent is probably advisable. Lastly, due to the potential for noise from snoring and high winds, a pair of ear plugs are worth taking.

More obviously, getting the correct sleeping bag and mat combination is important. While selecting the warmest sleeping bag you can get sounds simple enough, the issue of weight and bulk (plus cost!) enters the equation here. It is worth bearing in mind that you will have a range of clothing with you (e.g. down jacket, perhaps down trousers) which can be added to your sleep system in addition to being used during the day. This can easily boost a minus -15 bag, such as the ME Fireflash or the Rab Mythic 600 to cope with all but the coldest conditions. Given that the newest sleeping bags in this category are coming in at around the 1kg mark, this is an attractive and feasible option for many high-altitude objectives. Just check you can comfortably fit in the bag with the extra layers! To this we can add the benefits of insulation and comfort from the latest generation of inflatable mats.  The down filled ones having a particularly high level of insulation and comfort. There is a wide range of options here (e.g. Exped Downmat 9M  or the ME Aerostat Down 7.0) with varying combinations of weight and insulation (see UKC review of this category here). In terms of flexibility and added protection (plus in extreme cases… a backup!) pairing the inflatable mat with a closed-cell mat such as the Exped Flexmat or Therm-a-Rest Z Lite is highly recommended.

Final words

The high-altitude environment is the arena in which mountaineering expeditions take place and is the backdrop against the specific technical and physical challenges of any given ascent. Having a basic understanding of this environment and its effects can enable us to plan better and perform closer to our normal physical levels and achieve our chosen goals. To quote Thomas Jefferson "…that knowledge is power, that knowledge is safety, and that knowledge is happiness".

Acknowledgements: Many thanks to Jason Sheldrake for permission to use his excellent images.

About the author: Dr J. Stephen Taylor lectures and researches in Natural Area Tourism at Edinburgh Napier University. His main research interests concern the governance of natural areas and the topic of mountaineering tourism more generally. The current focus for this research is Lenin Peak and the impacts of mountain tourism in Kyrgyzstan more generally. As well as climbing regularly in the UK and the Alps, he has had great fun participating on ISM's Virgin Peaks Expedition to Kyrgyzstan over the last ten years.

UKC Articles and Gear Reviews by JSTaylor



Support UKC

As climbers we strive to make UKClimbing.com the kind of website we would love to visit, with the most up-to-date news, diverse and interesting articles, comprehensive gear reviews, breathtaking photographs and a vast and useful logbook system. As a result, an incredible community has formed around the site - we’ve provided the framework but it’s you who make the website what it is today. If you appreciate the content we offer then you can help us by becoming an official UKC Supporter. This can be a one-off single annual payment or a more substantial payment paid monthly or yearly which includes full access to Rockfax Digital and discounts on Rockfax print publications.

If you appreciate UKClimbing.com then please help us by becoming a UKC Supporter.

UKC Supporter

  • Support the website we all know and love
  • Access to a year's subscription to Rockfax Digital.
  • Plus 30% off Rockfax guidebooks
  • Plus Show your support UKC Supporter badge on your profile and forum posts
UKC/UKH/Rockfax logo

A lot of interesting info there. I've tried to make headway with the "there's just as much oxygen, but in percentage terms there isn't", but I'm struggling. If oxygen takes up 21% of a given volume of air at sea level but is around half of that at 5000m, does this mean there is a higher percentage of nitrogen accounting for the rest?

24 Sep

I'm not a scientist, but I believe it's to do with density, i.e. the air composition doesn't change but just becomes less dense as you go higher, meaning less availability of o2. Someone will correct me no doubt if that's not right.

But if the percentage of oxygen per volume of air decreases, this suggests that the air composition does change?

24 Sep

As the mountaineer ascends to higher altitudes, so air pressure decreases, and this has the effect of spreading out the essential oxygen molecules we depend on to survive and perform. To counter this decrease in oxygen intake, the body adapts by gradually increasing the red blood cells over a number of days. Other mechanisms come into play. We breathe harder and more frequently, and air-sacs in the extremities of the lungs begin to function. However, if the body is gradually worked hard over a couple of weeks such as 6-10 hours of strenuous activity daily, then the body will naturally increase the red blood cell count. It does not need to be at high altitudes for this to occur. Consequently, climbers and mountaineers had learned to ‘acclimatize’ to this ‘thin air’ as they ascend. The higher they ascend, the longer this period of acclimatization. Like all things however, it is a law of diminishing returns, and above a certain height no matter how long the climber acclimatizes, there is little or no improvement. It could be argued that the reverse is true. There comes a point when the body starts to deteriorate.

I should perhaps add to all this the importance of hydration, as this goes hand in hand with oxygen intake. The water molecules are also more spread out. So with the loss of pressure there is also less water vapour in the air we breathe, so more intake of H2O is required to avoid such dehydration.

Another interesting issue is the boiling point of water, which decreases with the higher altitude gain. So there comes a point (around 60,000ft,) when the water content in our bodies starts to boil!

Hope this helps.

24 Sep

An interesting article, with a lot of good and useful information, but the early use of % of oxygen is very confusing.... to be devil's advocate, what completes the 100% as oxygen decreases?

More Comments
Loading Notifications...
Facebook Twitter Copy Email LinkedIn Pinterest