What is Acclimatisation? Definition and Meaning

For the past 16 years at Indiahikes, we have noticed many mountain terms being tossed around in conversations, sometimes correctly, but more times incorrectly. 

We are making The Trek Glossary series, breaking down terms relevant to trekking. These are quick reads with accurate information that will help you understand mountain terminology. 

Today, I will dedicate my piece to explaining what is acclimatisation to high altitude. 

Every year, thousands of trekkers step into the Himalayas touching altitudes as high as 20,000 feet and a concerning percentage of them get affected by altitude sickness. There’s a need for awareness. 

Understanding of this term is not just limited to the trekking world. It is relevant for everyone travelling to high-altitude regions. 

What is Acclimatisation to High Altitude?

In simple terms, acclimatisation to high altitude is the process of adaptation to the lesser atmospheric pressure (barometric pressure).

There’s a science behind acclimatisation. When individuals ascend to high altitudes, the air pressure decreases, resulting in oxygen molecules being spaced farther apart. So you’re getting lesser oxygen with every breath. While the percentage of oxygen in the atmosphere remains constant at 20.8%, the body needs to adapt to the reduced air pressure. 

Here is what occurs physiologically:

1. Immediate Response:

a. Increased heart rate (HR) and respiratory rate (RR) to improve oxygen intake.

b. The body expels excess bicarbonate (urine) as a response to imbalances caused by the decreased oxygen levels.

2. 4-7 Days at High Altitude:

a. Increased production of red blood cells (RBCs) to carry more oxygen.

b. A rise in 2,3-DPG (2,3-diphospoglycerate), a compound aiding oxygen release to tissues.

3. Beyond 7 Days:

a. Growth of additional blood capillaries to enhance oxygen transport.

At What Height Does Acclimatisation Become Necessary? 

Acclimatisation is a critical process for trekkers/travellers travelling to high altitudes. From what we have noticed, for most people, the effects of high altitude start becoming noticeable at elevations above 8,000 feet (approximately 2,400 meters). However, for individuals travelling from sea-level locations, acclimatisation may be necessary at altitudes as low as 5,000–6,000 feet (~1,500–1,800 meters). 

For instance, someone travelling from cities like Chennai or Mumbai to Mussoorie, which is at an elevation of 6,000 feet, could experience mild symptoms if they ascend too quickly, as the body has had insufficient time to adapt to lesser oxygen pressure.

In a conversation about acclimatisation, Diptarka Gupta, medical trainer at Indiahikes, categorically mentioned that the speed of ascent (altitude gain) plays a significant role in the need for acclimatisation. 

I will elaborate it through an example: A person travelling by air from Chennai or Mumbai (sea level locations) to Dehradun and then driving to Mussoorie within a few hours might be at risk of altitude-related issues due to the rapid altitude gain. Similarly, trekkers arriving in Manali directly from a sea-level city and heading to treks the same day are also vulnerable to altitude sickness. The ascent trajectory and origin location very much influence acclimatisation requirements.

If I go further in detail, you can compare two different treks. One instance is the Bhrigu Lake trek, where the altitude climbs rapidly—within just two days, trekkers reach Rola Kholi, which is at an altitude of approximately 12,500 feet. As a result, there more cases of Acute Mountain Sickness (AMS) can be seen on this route. In contrast, the Hampta Pass trek also ascends to 14,000 feet (approx), yet the occurrence of AMS is much lower. This clearly shows that the pace of ascent plays a crucial role in acclimatisation.

Similarly, our previous itinerary for Kuari Pass trek involved trekkers arriving at Joshimath, then driving to Auli the next day, followed by a trek to Padiyar. This route required an ascent from 6,000 to 9,000 feet (approx)—a steep 3,000-feet climb (approx) —which led to a higher number of AMS cases. One of the key reasons we switched to an alternate route was to enhance safety and improve acclimatisation. 

Taking the time and necessary steps to acclimatise to high altitude lowers the risk of altitude sickness, but it does not eliminate the risk completely. 

How Do We Define Altitude?

Below I am sharing a diagram that gives a better understanding of how we define altitude

What are the Natural Methods for Acclimatisation?

Following these five pointers can help your body acclimatise well to high altitude.

1. Rest Well: Adequate rest upon reaching high altitude is essential. For instance, travellers/trekkers flying directly to Leh (elevation: ~11,000 feet) must rest on arrival before engaging in any activities. Immediate exertion can heighten the risk of altitude-related problems.
2. Ascend Slowly: Gradual altitude gain is the safest approach. Trekkers/climbers are advised not to increase their sleeping altitude by more than 1,000 feet (~300 meters) per day. This practice significantly helps in acclimatisation and reduces the likelihood of altitude sickness.
3. Climb High, Sleep Low: It is a natural acclimatisation technique. Our most experienced trek leaders advise this. This strategy involves ascending to a higher elevation during the day and descending to a lower altitude to sleep. For example, mountaineers on expeditions often transport supplies to higher camps and then descend to lower camps for the night. This is again the reason why trek leaders take trekkers through the acclimatisation walk. When you hike (with the purpose of exploring) to a slightly higher altitude and then descend, your body is getting a chance to adjusting to the altitude. It is a simple yet effective way of acclimatisation.
4. Good Hydration: Staying well-hydrated is crucial. Your body loses more water because of the increased rate of respiration. Water supports the efficiency of red blood cells to collect oxygen in the lungs.
5. A Carbohydrate-Rich Diet: A carbohydrate-rich diet is beneficial as carbohydrates are quickly metabolised and provide energy efficiently. We would never recommend High-protein and fatty foods, like meat, as the body takes longer to digest and requires more energy to break the high-protein diet.

How to Acclimatise with Diamox?

Over the years, we have noticed trekkers taking diamox (tablets) for better acclimatisation.

Acetazolamide (Diamox) helps speed up acclimatisation. Our experts have shared a note on how Diamox works as a preventive (helping acclimatisation) and curative measure.

What Happens When Your Body is Not Acclimatised? Or What Causes Altitude Sickness?

Essentially, failing to acclimatize means your body is unable to adapt to the altitude, leading to AMS symptoms

Anyone who ascends to high altitude becomes hypoxic. This is a condition which signals insufficient oxygen in the blood. 

Atmospheric pressure (barometric pressure) falls as the elevation increases. Normally, oxygen diffuses from the lung into the blood along a pressure differential. It diffuses from higher gas pressure in the lung to lower gas pressure in the blood. At high altitudes, the reduced barometric pressure lowers the oxygen pressure in the lungs, which in turn decreases the amount of oxygen that can diffuse into the bloodstream. 

To explain the point further, I will take the help of an example. Think about an average healthy individual: at sea level, the blood is about 95% saturated with oxygen. However, at 18,000 feet (5,400 meters), due to the drop in atmospheric pressure, their blood's oxygen saturation falls to just 71%. This means the blood carries 24% less oxygen. 

The reduced amount of oxygen in the blood is the main reason for altitude-related sickness.

The reason why some people are affected with altitude sickness and others don’t is still not fully known.

Indiahikes Trek Leaders and Medical Experts Recommend

Along with the above-mentioned ways of natural acclimatisation, our most experienced trek leaders recommend these two additional pointers.

• Climb at Your Own Pace: Avoid overexertion. Trekkers and travellers must listen to their bodies and not yield to peer pressure or strict schedules.
• DO NOT Ignore Symptoms: Early signs of altitude sickness include headache, nausea, fatigue, and dizziness. Descend immediately if severe symptoms occur. I would recommend you read these two pieces on AMS. What is Acute Mountain Sickness?  Following to it, read How To Treat Acute Mountain Sickness (AMS), High Altitude Pulmonary Edema (HAPE), High Altitude Cerebral Edema (HACE)
• A key recommendation from Indiahikes trek leaders and medical experts. Take Diamox if you’re trekking to altitudes above 14,000 feet.

When asked about Sherpas, Diptarka shares: ”Studies indicate that the Sherpas of Nepal and communities in the Andes, have evolved unique genetic adaptations, enabling them to thrive in high-altitude environments. These populations have a higher density of capillaries and more efficient oxygen utilisation, which minimises the effects of low oxygen availability.”

In Conclusion:

That is all you need to know about acclimatisation to higher altitude. If you have any questions you’d like to ask, drop them in the comments section.

Please let me know in the comments section if you want me to cover any specific trek term. I will cover it in my upcoming posts.