Altitude sickness is caused by a lack of oxygen in the air – the higher up you go, the less oxygen there is in the air around and the red blood cells responsible for delivering oxygen to the cells of the body will be unable to provide it with the amount of oxygen it requires. A protracted lack of oxygen is very hard on the body, causing serious damage and in some cases even death.

Humans are not genetically built to live at high altitudes: there are very few large settlements at altitudes above 4,000 meters with the two highest large cities in the world being Lhasa in Tibet (3,656 m/11,955 ft) and La Paz in Bolivia (3,640 m/11,942 ft). Not for nothing – at an altitude of 3,500 m/11,484 ft the concentration of oxygen in the air drops to 64% and at 5,500 m/18,044 ft it reaches half of the amount of oxygen at sea level. Above an altitude of 8,000 m/26,247 ft, humans are unable to survive beyond a short visit. Among mountain climbers, these heights are known as the death zone and they aim to minimize their time there as much as possible.

Table of oxygen concentration in the air by altitude:

Altitude % Oxygen
Sea level 100%
2,500m/8,202 ft 73%
3,000m/9,843 ft 68%
3,500m/11,483 ft 64%
4,000m/13,123 ft 60%
4,500m/14,764 ft 57%
5,000m/16,404 ft 53%
5,500m/18,044 ft 50%
6,000m/19,685 ft 47%
7,000m/22,966 ft 41%
8,000m/26,247 ft 36%
8,848m/29,028 ft – Everest summit 33%


Altitude sickness is typically divided into two degrees of severity.

  1. Acute Mountain Sickness – usually begins to appear at altitudes above 3,000 meters/9,843 feet.

The symptoms include:

  1. headache
  2. lack of appetite, nausea and vomiting
  3. inability to sleep and/or sleep disturbances
  4. exhaustion and dizziness



Here there is a deterioration from the first phase of the sickness – usually 24 hours after the appearance of symptoms of the previous phase and following the failure of the body to acclimatize to the altitude, one of the following dangerous phases will begin to appear:

HACE (High-Altitude Cerebral Edema)

In this state of the illness, the brain begins to fill with liquid which causes edema and the sick person begins to feel instability, fogginess and confusion. This stage is very dangerous and if it is not immediately treated can lead to loss of consciousness and even death.

HAPE (High-Altitude Pulmonary Edema)

High-altitude pulmonary edema is the accumulation of fluids causing edema in the lungs. Its common symptoms include: shortness of breath, coughing, increased heart rate, sleepiness, weakness, and blueness of the lips. If not immediately treated, HAPE can lead to loss of consciousness and even death.

It is sometimes difficult to distinguish between the different degrees of the sickness, and certainly for other travelers accompanying the afflicted person, but there are several rules of thumb that indicate the transition to the more dangerous stage, such as: non-stop headaches, general protracted weakness, or if the traveler loses coordination and cannot walk a straight line even with slow and deliberate steps.

These signs should raise a major red flag and the best way to manage at this stage is quick descent in altitude by at least 1,000 meters/3,281 ft.

It is important to remember that the second stage of the illness is relatively rare at altitudes where hiking happens (generally up to an altitude of 5,500 m/18,044 ft) and

most people suffer from some degree of stage one symptoms. Despite the relative rarity however, it can occur and hikers do die as a result of altitude sickness.

The most important tip to anyone who plans to hike at a high altitude is not to take the matter lightly. Even if the altitude sickness does not develop to its more severe state, there are more than a few cases of hikers who “just” had their trip ruined and were forced to stop their hike before completing it. After all the investment of time and money in getting to a place from afar, it is a shame to ruin the experience for having neglected to follow a few simple rules.

The 3 safest and most effective ways to manage symptoms of altitude sickness are:


A.Slow and measured ascent – it is highly recommended at altitudes of over 3,000 meters (9,842 feet), not to spend the night at an altitude of over 500 meters (1,640 feet) more than you slept at the previous night and that for every stop to sleep at an altitude of 1,000 meters (3,280 feet) more than the previous night, you stay an additional day to rest and acclimatize.

The emphasis here is on spending the night – you can definitely ascend more than 500 meters/1,640 feet throughout a day of hiking – for example at mountain passes – just as long as you follow the rules regarding sleeping.

The break allows the body time to adjust to the altitude and to adapt itself to the shortage of oxygen. What actually happens is that the body begins to produce more red blood cells that help it to compensate for the scarcity of oxygen and to deliver the required amount of oxygen to the cells.


B.It is important to take pills to prevent altitude sickness (such as Diamox or Uramox) before entering the region.

The pills speed up the acclimatization of the body and are very effective in preventing the initial symptoms of altitude sickness (stage 1). They are safe for use and for most people they don’t cause major side effects, although a small portion of the population has allergic reactions to the pill.

Always consult with a doctor before starting to take the medication.

The pills should be taken starting 48 hours before arriving to an altitude of 3,000 meters/9,840 feet, after which 1-2 pills should be taken daily until one day after leaving the high altitude area. Another possibility which is less advisable is just to take the pills along with you and only beginning to take them in the event of altitude sickness symptoms appearing. The pills help tremendously and for some people they make the difference between finishing the trek or not.


C.Descent to lower altitude – In the event that you went up too fast and your body is not managing to adapt and your first stage symptoms don’t seem to be going away, it is recommended to descend and spend another night at a lower altitude (500 m less). If there is concern that you may have entered the second, more severe phase of the illness, you must descend 1,000 meters/3,281 feet as fast as possible and if possible, to an altitude of less than 3,000 meters/9,842 feet.

Additionally, it is important to know that the body’s sensitivity to altitude differs between individuals and is unrelated to physical fitness but rather to one’s specific genetics. There are people who will only experience symptoms of altitude sickness at a greater altitude than others and there is no way to know before entering the mountains. As such, it is recommended that everyone take the preventative measures mentioned above, and when hiking as a group, to avoid explicitly or implicitly putting pressure on anyone feeling unwell to continue to climb on.

WARNING: This article is not a replacement for medical advice. It is intended for hikers who wish to know a little more about the conditions of travel in high mountains, the dangers of reaching significant altitude and its effect on the health of the hiker.


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