Altitude and Acclimatisation for the Layman

As you go to even moderate altitudes the air gets ‘thinner’. At around 5,500 metres there is half the amount of air that you would find at sea level(†). Despite there being less air, what air there is is still made up of around 21% of Oxygen, 75% Nitrogen and 3% Carbon Dioxide (plus a small fraction of other gases). This can actually vary quite considerably depending upon the latitude that you are at. There is generally more air concentrated at the equator – the earth bulges at the equator due to centrifugal force which, in turn, means that there is a greater gravitational effect due to the increased mass which attracts more atmosphere. This can also be affected by the time of year. So if you go to Mt McKinley (6,194m) or Aconcagua (6,960m) you can expect the barometric pressure to be similar to what you would get on a 7,500m peak in the Himalayas.

When you consider that humans are not designed to live at altitude it is amazing that the body does actually have a few mechanisms to compensate for the rarified atmosphere. Some of the mechanisms initially have slightly detrimental side affects but they are soon evened out by other adjustments.

For most people the effect of altitude and the rarified atmosphere will first become evident at around 3,500m (for a few of you it will become noticeable at as little as 2,500m). One of the first things that you will notice is that you will become breathless far more easily than at sea level. If you were to monitor your pulse and breathing rate you would find that both are elevated – this is the body’s way of compensating for there being less oxygen available. By breathing more, and by pumping the blood faster, the body can rectify the apparent shortfall.

If you were to keep ascending, steadily increasing your sleeping altitude every day, then eventually your body would not be able to keep up with the ever decreasing oxygen supply, you would become extremely ill and debilitated and would either have to abort your trip or risk dying from altitude related illnesses (more later).

‘Climb High – Sleep Low’
The trick with altitude is to raise your sleeping altitude in moderate steps and to take the occasional ‘rest day’ where you spend 2 nights at the same elevation. There is not a problem with taking exercise and ascending on a ‘rest day’, so long as you return to the same elevation when you sleep. That way, when your body is recovering from the exertions of the day, it is not trying to do it with even less oxygen.

The recommended steps are to raise your altitude above 3,500m by 300m per night and to rest every 4th day, but this is not always possible as water sources and camping areas are not always available at convenient places. Kilimanjaro (5,895m) is a typical example where charity treks are run in 7 days UK to UK! When you take in to account the 300m rule above 3,500m it should take around 7 to 9 days to ascend, maybe another 1 or 2 to descend and 2 for flying to and fro. Little wonder that amongst the higher mountains it has not only the highest failure rate but also the highest mortality rate.

You can average the figures out by, say, jumping 400m, jumping another 400m, having a rest day, jumping another 400m and having another rest day. Gaining 1,200m of ascent in one day and then having 3 rest days is probably not to recommended though. And at the end of the day it is a guideline and there are other factors to be taken in to account as well.

So your body starts by increasing your respiratory rate and circulatory rate – but unfortunately these two actions alone are not enough for your body to acclimatise. Another action that your body will undertake fairly early on is to increase the usable volume of your lungs. There are areas of your lungs that, no matter how fit you are or may have been in the past, you have never utilised. By opening up these unused areas of the lungs the available surface area for oxygen transfer is greatly increased.

Despite there being less oxygen in the air, if likened to a railway system you now have a faster delivery (breathing rate) of the oxygen to the railway sidings (lungs), increased railway sidings for taking on the goods (oxygen), and a faster train (pulse rate) delivering the oxygen to the end user (the muscles). Great.

However there are a couple of drawbacks:

  1. Cheyne-Stokes Breathing (also known as ‘Sleep apnoea’).Because you are taking on more oxygen you are also getting rid of more (too much) carbon dioxide. When you are awake and conscious you have control over your breathing, but when you are asleep you will find that there is a bizarre side effect called sleep apnoea, which is associated with cheyne stokes breathing.Your subconscious breathing rate is controlled by the amount of carbon dioxide build up rather than your need for oxygen. What happens here is that the brain will detect that there is not enough CO 2 and will tell the lungs to stop breathing. Gradually the CO 2 will build up to such a level that the brain will tell the lungs to start breathing again and you will probably wake up as you gasp for breath. After panting for a couple of lung fulls your breathing will calm down to normal levels and you’ll doze off until you have breathed off too much CO 2 and the cycle starts all over again.Some people find this very disturbing and feel that they didn’t sleep a wink all night. It can also be very disconcerting for your tent partner who wakes up thinking that you have stopped breathing (which, of course, you have). Just as they are about to give you a nudge to see if everything is all right you come to with a gasp.Another side effect is that you may have very vivid (and possibly quite weird) dreams. This is possible associated with the fact that you are not in ‘deep sleep’ but are in and out of consciousness throughout the night.
  2. DiuresisThe second effect is that, as carbon dioxide is an acid and you are getting rid of too much of it, your blood Ph will become alkali. To redress this, your body will initiate a diuresis and you will start to urinate more, which in turn means you are getting rid of alkali and returning your PH to more acceptable levels (Drinking tea and coffee won’t help as caffeine diuresis gets rid of ‘free’ water rather than alkali in the blood). You will eventually find that you need to go 2,3 or even 4 times in the night as you get to higher altitudes until you become sufficiently acclimatised.One thing to bear in mind is that these mechanisms will kick in to effect at different altitudes for different people. Don’t feel that because everyone else is peeing and having disturbed sleep and that you aren’t that you are not acclimatising – it will start soon enough.

Acute Mountain Sickness (AMS)

At first you will probably feel permanently pooped, wonder how you are going to climb your chosen objective and ponder why you didn’t opt for clipping bolts in Thailand. If you listen to your body, don’t overexert yourself and keep well hydrated you will probably weather the storm.

However, most people will at some stage succumb to a bout of AMS. This is not a life threatening condition, more of a feeling of having a permanent hangover. The typical symptoms are :-

  • Mild headache
  • Loss of appetite
  • General lethargy

If you take plenty of fluids and generally don’t over exert yourself then these symptoms will soon pass. There is not a problem with taking Paracetamol to keep the headache at bay. At this stage, as long as the headache recedes with analgesics, there is probably not a problem with continuing with your itinerary.

If, however, you get to a stage where the headache won’t go away, no matter how strong the painkillers are, then it is time to stop, rest and don’t raise your sleeping altitude. You need to stay at this elevation, or even descend to the last altitude where you felt well, to avoid the AMS developing in to a more life threatening condition such as High Altitude Pulmonary Oedema (HAPE) or High Altitude Cerebral Oedema (HACE). Both are avoidable but, tragically, every year people die of these conditions.


Without going in to the mechanisms of what happens basically HACE is fluid leaking in to areas of the brain where it doesn’t belong and causing the brain to be squashed in the cranial cavity.

The typical signs of HACE onset are some (or all) of the following:-

  • the AMS headache becomes an absolute thumper
  • nausea is replaced with vomiting
  • vision is affected
  • hearing is impaired
  • balance is affected
  • temperament and behaviour are altered (the patient may not be aware of the last couple but they may be obvious to team mates).

These are sure signs that the brain is being insulted in some way and it is definitely time to get this person down. If it is night time DO NOT wait until the morning to deal with this person or you may wake up with one less team member.

The drug of choice is Dexamethasone but see the recommended reading at the end of this page to get advice about dosage, possible side effects (PSE), contraindications etc etc.


Again, without in depth analysis HAPE is when fluid leaks in to the alveoli and compromises the breathing. A sure sign is when a person becomes breathless AT REST and may be accompanied with a productive cough with pink frothy sputum. Whilst not as directly life threatening as HACE this is still a killer and MUST be acted upon straight away. The drug of choice is Nifedipine but see the recommended reading at the end of this to get advice about dosage, PSE, C/I etc etc.

If you are embarking upon a high peak (over, say, 6,500m), particularly if it is remote or inaccessible for rescue services, then it may be worth your while investing in, or renting, a hypobaric chamber. These are sausages that are inflated with the casualty inside. This raises the pressure within the chamber, which is the equivalent of descending in altitude. They can be very effective in dealing with an ill patient throughout the night before embarking upon a descent the next day. If a person is ill enough to be put in one of these then they should be leaving the expedition. It is not a temporary cure allowing the person to continue the next day.

It’s worth making sure that they have been to the loo beforehand, that they are insulated on a sleeping mat and in a sleeping bag, that they have some water, a head torch and an altimeter watch in there with them. There is also usually a window so make sure that you are keeping in touch with them for reassurance. Once they are inside you must keep pumping to maintain the pressure (there is a safety valve so you can’t overdo it), the patient needs to be aware that they may need to equalise the pressure in their ears (holding their nose and gently blowing with a closed mouth).

If they need to come out at any time DO NOT just unzip and open up or you could make them very ill again.

The downsides of a hyperbaric chamber are that a) the patient immediately becomes immobile and b) when they do come out there are immediately back to the elevation that they were unwell at. It can be a great way to look after someone through the night … but so is descent which can’t be achieved if they are in a chamber. They are also quite labour intensive in that someone needs to keep pumping which is actually a lot more effort than you would have thought.

For stabilising a casualty prior to being collected by helicopter the next day they are great. For physically getting someone to a lower elevation they are not.


The problem with Hypobaric chambers is that you are not going to be moving for the period that you have got someone in there. Yes they are great and get the patient down to a lower elevation – but only for as long as they are in there. As soon as they re-emerge from the chamber then they are back at the same altitude and they still have to get down to a lower altitude.

Another problem is that for as long as you have someone in the chamber someone has to pump, it’s hard work so that person will need relieving every so often, and someone has to monitor the patient. Then you’ll probably be needing a brew or two so you’ll have a cook up and about and what if the patient needs to go to the toilet, or vomit, or needs medical attention?

To that end oxygen is an excellent alternative. And remember that oxygen is a drug and is therefore medication. Whilst someone is on oxygen they are also potentially mobile (or can be carried) and can actually descend, rather than hypothetically descend, which is what they need to do to get in to a better environment.

If you are putting someone on oxygen then there is a balance between flow rate and length of time the supply will last. So cranking the flow to 4 litres a minute might be an initial consideration but you will only get 3 or 4 hours before needing to change to another cylinder. This is fine if you have a plentiful supply but if you have a limited immediate availability of oxygen then turning it down 2 (or even in some cases) 1 litre a minute may be sufficient for the patient and will give between 8 and 16 hours.

It is worth bearing in mind that if oxygen is being administered it is often worth considering using this in conjunction with other medication to treat the symptoms – not to just rely on oxygen alone.


It’s amazing how much fluid your body requires at altitude. You’ll be aware that you are sweating in to your shoes, you’ll be sweating with the daily exertions of walking and climbing at altitude and you may be carrying a heavy sack as well. But what you don’t realise is just how much moisture you breathe out.
For you to be able to effectively assimilate the oxygen from the air the air needs to be moist. So with every breathe that you take in moisture is ‘added’ – but this is then lost when you breathe out again.

You can usually reckon on needing 4 to 5 litres of water per day to account for the loss of fluid at altitude. This requirement may be more if you are on a particularly hard trip.

If you become dehydrated you will be more prone to dry cracked lips (very debilitating), you will be more prone to AMS, you’re more likely to become constipated and will not be able to carry on performing at the same level.
There are times when you just won’t be able to maintain the fluid intake – especially if you have a few BIG days and no water supply. Melting ice and snow takes time and you need to get on. In which case make sure that you take ‘little and often’ and also make sure that you factor in a couple of rest days soon.

If you just gulp down a litre then your body will be slightly overwhelmed with the sudden intake, and your kidneys will start processing and expelling the fluid – pretty soon you are going to pee it out. If, however, you sip occasionally you’ll be able to just keep topping up, preventing those lips becoming parched, and will be less likely to pee the water out. But remember – if you aren’t taking on 4 to 5 litres then it’s not enough and you WILL become dehydrated sooner rather than later.

A good indicator is that your urine output should be clear and copious.

For further information, particularly with reference to recommended medication and dosages, please have a look at the following sites:

The High Altitude Medicine Guide – just about everything you need to know about acclimatisation, AMS, HACE etc
The Himalayan Rescue Association
Outdoor Action Guide to High Altitude: Acclimatization and Illnesses
High Altitude Medicine pages from the ‘American Family Physician’
Altitude / AMS / HACE & HAPE
Everest Base Camp Clinic