Acclimatisation and Human Responses to High Altitude

Wg Cdr Jon Naylor MBChB FRCP DAvMed  DMountainMed MRAeS RAF

1.            What is high altitude?

Most humans live at or near sea-level altitude and it therefore follows that human physiology is best suited to the level of oxygen found at sea level. Conversely, human performance rapidly drops off at higher altitudes as a result of the reduction in oxygen available.

Definitions of high altitude vary but it is generally accepted that altitudes of greater than 3000m above sea level constitute “High Altitude”, while those above 6000m are termed “Extreme Altitude”.

2.            What are the normal human responses on acute exposure to high altitude (low barometric pressures)?

The partial pressure of atmospheric oxygen falls progressively as barometric pressure decreases with increasing altitude. Correspondingly, the ability to perform work (eg walking or climbing) reduces as the oxygen available to body tissues falls.

Acute exposure to extreme altitude, as would be experienced in a sudden decompression event in an aircraft incident, is a huge physiological challenge and may prove fatal. The human body is unable to adapt to the rapid reduction in available oxygen and a period of cognitive impairment may be followed by coma and death.

However, most altitude exposure is more gradual and thus the body is able to adapt and acclimatise. With a slow rate of ascent it is possible for humans to operate at high altitude without supplemental oxygen and, as has been proven on a number of occasions, humans can venture into extreme altitudes and function in a limited capacity. It is a remarkable coincidence that the highest altitude which humans can tolerate, albeit only for a limited time, is about the same as the highest point on the planet on which we live: 8848m, the summit of Mt Everest.

3.            What is acclimatisation?

High altitude acclimatisation refers to the physiological changes which occur in humans when they go to high altitudes (above 1500m) from their usual lowland habitats. These adaptations are generally believed to occur up to altitudes of 5000m but become less effective once extreme altitudes are reached. It is well recognised that acclimatisation to altitudes above 6000m cannot occur and that a steady physical decline occurs which makes prolonged survival at these altitudes impossible.

Assuming a steady exposure to increasing altitude above 2500m, within recommended ascent rates (no more than 300m increase in sleeping altitude per day with a rest day after ever 3rd day), most people will acclimatise well with few problems.  More rapid ascent profiles risk significant altitude illness – Acute Mountain Sickness (AMS), High Altitude Pulmonary Oedema (HAPE) or High Altitude Cerebral Oedema (HACE).

Although responses to altitude and acclimatisation rates vary among individual, exposure to the reduced barometric pressure of altitude produces a predictable series of physiological responses which together lead to acclimatisation. These responses can be broken down into short-term, medium term and long-term; however, all are trying to achieve the same thing – increased oxygen supply to the body’s tissues.


1.            Increased respiratory rate leading to increased oxygen in the alveoli of the lungs

2.            Increased heart rate helping to increase delivery of oxygen to tissues


1.            Increased urine output leading to reduced plasma volume and concentration of the haemoglobin in the blood

2.            Increased production of erythropoietin and therefore increased haemoglobin in the blood


1.            Increased density of capillaries in the tissues

2.            Changes in hormones and enzymes in cells


4.            What human responses could be expected at the following altitudes?

I.             Barometric pressure less than 60% of sea-level pressure

ii.            Air temperature 0 to -20 Celsius

iii.           Arterial oxygen saturations 80-85%

Base Camp (BC) for the Makalu SE Ridge 2014 Expedition will be at 4800m, about the same height as the summit of Mt Blanc, the highest peak in the Alps.

On arrival, the team members will be experiencing most of the effects of altitude exposure, including a rapid heart rate and rapid breathing. Significant exercise will be difficult and sleep will also be disturbed. Many people at this altitude will have unusual breathing patterns at night (alternating periods of rapid and shallow breathing with occasional pauses, called periodic breathing). Those who are acclimatising less well will have mild AMS symptoms such as headache, nausea, poor appetite, tiredness and difficulty sleeping.

Most people will have acclimatised within a few days of arrival at Base Camp, and although heart rate and breathing rate remain higher than at sea level, AMS symptoms should have worn off and most will be able to exercise to a reasonable degree.

b.            6000m

i.             Barometric pressure <50% of sea-level pressure

ii.            Temperature -5 to -25 Celsius

iii.           Arterial oxygen saturations approx. 75%

Advanced Base Camp (ABC) on the SE Ridge will be at about 5800m.

Above this level further acclimatisation is impossible and gradual physical decline is inevitable. It will be vitally important for the climbers to be well acclimatised and in good physical shape at ABC. Physical exertion above 6000m becomes increasingly difficult and the plan for progress up the ridge will include periods of time for climbers to recuperate at lower altitudes (ABC or BC). Relentless loss of weight and other evidence of progressive deterioration will occur with any length of time spent above this altitude

c.             7000m

i.             Barometric pressure <45% of sea-level pressure

ii.            Air temperature -10 to -30 Celsius

iii.           Arterial oxygen saturations approx. 70%

Beyond Camp 2 (6800m) on the SE Ridge the effects of altitude will be compounded by extreme environmental conditions with high winds and very low temperatures. Climbers will notice progressive difficulty carrying out any physical activity. Sleep at this altitude is very difficult and likely to be severely disturbed. Despite heavy workloads and high energy requirements, most climbers at this altitude have poor appetites and struggle to take in enough food and fluid to compensate for losses (partly for practical reasons as melting snow, boiling water and preparing food is very time consuming).

d.            8000m

i.             Barometric pressure <40% of sea-level pressure

ii.            Temperature -15 to -40 Celsius

iii.           Arterial oxygen saturations 60-70%

Beyond the Black Gendarme (7500m) and on towards the summit (8463m) is the so-called “Death Zone”. Even at rest the work of breathing is very high and any exertion produces extreme hyperventilation. Exercise capacity is severely limited – a few steps at a time with long periods of rest between. Many climbers find that they are lightheaded and rather detached from reality at these extreme altitudes. Upward progress is very slow and the risk of altitude illness, and from the effects of the extreme cold, is magnified many times. Physical and mental preparation for this phase of the climb is crucial – a successful and safe summit bid may depend on the tiniest detail of the plan.