Acclimatisation is a process that results in the body becoming accustomed to lower levels of oxygen at altitude. It is only achieved by spending extended periods of time at various levels of altitude before progressing higher.
Climbers typically refer to three main levels of altitude. The first level, aptly called ‘high altitude’, describes the altitude zone between 2,500m and 3,500m. Altitudes between 3,500m and 5,500m are referred to as ‘very high altitude’. The third level is called ‘extreme altitude’ and is characterised by zones above 5,500m. There are other zones above extreme altitude, but these are limited to the highest mountains in the Himalayas – like the ‘death zone’ which is above 8,000m!
When it comes to trekking at altitude, most people can ascend from sea level (say your hometown) to 2,400m without experiencing altitude illness symptoms. Once you get beyond this height though, changes in oxygen and pressure levels begin influencing the body’s physiology.
To illustrate these changes consider the following. At sea level oxygen air saturation is about 21 per cent and barometric pressure is approximately 760mmHg (millimetres of mercury). As one ascends oxygen saturation stays about the same but air density decreases, which means the percentage of oxygen per breath reduces. You can imagine this as oxygen molecules moving further and further away from each other at higher altitudes, hence the term ‘thin air’.
The good news is that as oxygen per breath decreases, the body very quickly and cleverly starts to adapt to the new environment. Four main changes occur. Firstly, breathing becomes a lot faster and deeper (even at rest). Secondly, blood concentration changes – red blood cells, the part of your blood responsible for carrying oxygen, increases. The third change occurs in your lungs and is a result of increased pressure in your pulmonary capillaries. This forces blood into areas of the lungs that are not used when breathing at sea level. The final change involves the secretion of an enzyme that promotes more effective transfer of oxygen from your haemoglobin to your blood tissue.
With these changes and, assuming you have given yourself enough time to rest at a ‘reasonable’ starting altitude, the body will acclimatise. Progressing to higher altitudes on a slow and incremental scale thus becomes possible.
To demonstrate acclimatisation in action, trekkers and mountaineers have come up with a term called the acclimatisation line – an arbitrary altitude line that describes the point at which someone’s altitude sickness symptoms appear.
For instance, let’s assume that your altitude line is around 3,000m. On the first day of your trek you ascend to 3,000m and remain asymptomatic as you are on or just below your acclimatisation line. Resting at this altitude for a night or two should ensure that you are fully acclimatised to this altitude and your line might move to around 3,800m. Trekking to 3,700m will ensure you remain asymptomatic, but if you continued further to say, 4,000m, it is likely that you will start experiencing altitude illness symptoms. Descent back down to below your acclimatisation line (in this example, 3,800m) to rest for a few days should resolve any symptoms. Continued ascent would only result in the deterioration of your condition.
So what is the learning? If you want to acclimatise properly make sure you build in rest days as you ascend, make sure to take note of your altitude if symptoms present themselves. Near your acclimatisation line altitude sickness symptoms typically resolve after a day or two, but continued ascent before you have properly acclimatised will almost certainly lead to the worsening of your symptoms and further acclimatisation will not occur. The only sure way to improve is to get below the altitude line where your symptoms first occurred.