Why Does Altitude Sickness
Happen Just from Going Higher?
Climb a high mountain and you may get a headache or feel sick. This is altitude sickness. You are breathing, yet you feel as if you were in a stuffy tunnel. The cause is thought to lie not in the "amount" of air, but in how "thick" the air is.
A hiker is heading up a high mountain on a day off. They take a bus or car straight to a trailhead at high altitude and set off briskly. The weather is good, and they feel in perfect shape.
But once they pass about 3,000 metres, their head starts to feel heavy, and they run out of breath after just a few steps. Resting doesn't help, and before long they feel nauseous.
This is altitude sickness. It is said to have little to do with how fit you are.
Altitude sickness has just two causes
The make-up of the air doesn't change, but as air pressure falls, the amount of oxygen you can take in with each breath falls too. This is a physical fact that can't be avoided.
The body can adapt bit by bit to low-oxygen surroundings, but this "acclimatisation" takes days. The faster you climb, the more your body falls behind.
Let's take them one at a time.
Cause 1: Air becomes like "watered-down juice"
The share of oxygen in air is about 21%, whether you are at ground level or on a summit. What changes is the air pressure, that is, how "thick" the air itself is.
It is a bit like diluting juice with water. The "proportion" of sugar in the juice stays the same, but the more water you add, the less sugar there is in each glass. Going higher is like adding more and more water to the juice that is air. The amount of oxygen that reaches your lungs in a single breath falls as you climb.
People often assume altitude sickness only affects Everest-class peaks, but in many people symptoms are said to start at around 2,500–3,000 m. Japan's Mt. Fuji (3,776 m) falls squarely in that range. Because a bus can take you straight to the fifth station, it is also pointed out that climbers can easily set off without giving the body time to adjust.
Cause 2: The body needs time to adjust
The body can adjust bit by bit to thin air. It breathes faster and makes more red blood cells, which carry oxygen, so as to secure oxygen even in thin air. This is called "altitude acclimatisation".
The problem is that this adjustment can't be finished in a few hours or even a day. If you gain height faster than your body can adapt, oxygen shortage carries on, and it shows up as symptoms such as headache and nausea. The idea that "I'm young and healthy, so I'll be fine" rarely holds, because this is not a matter of fitness but a matter of time to adapt.
Symptoms start with headache, nausea and tiredness, which are hard to tell apart from a cold or plain fatigue. It is said to be important not to push on here, but to rest or at least stop climbing higher. Unsteadiness, dizziness so bad you can't stay standing, and vomiting that won't stop are signs that things are getting worse.
So what should you do?
- Climb slowly when you gain heightAbove 2,500 m in particular, it is advised to limit how much height you gain in a day. This gives the body time to adjust.
- If symptoms appear, don't go any higherIf you feel a headache or nausea, stay where you are and rest. Pressing on because "we came all this way" is the most dangerous thing you can do.
- If it gets worse, go down without hesitatingLosing height is said to be the surest remedy. In many cases, symptoms ease once you descend.
Symptoms such as being unsteady and unable to walk, feeling confused, or being breathless even when keeping still may mean the illness has moved on to a serious, life-threatening stage. Do not go any higher. Move to lower ground as soon as you can.
Hesitating to descend because "the summit is so close" or "I don't want to trouble everyone" is said to be one reason harm gets worse. Not delaying the decision to descend matters more than anything.
There are also medicines for prevention and treatment, but whether you can take them depends on your constitution and any existing conditions, so it is advised to talk to a doctor beforehand. Please don't decide to use them on your own.
Summary
Altitude sickness happens just from going higher because two things work at once: a physical reason, ① falling air pressure makes the air itself thinner, and a bodily reason, ② the body takes time to get used to thin oxygen. The faster you climb, the wider the gap between the two becomes.
It isn't that there's too little oxygen.
You have simply gone too high before your body could adjust.
The same change in air pressure also acts on your ears, on aeroplanes and mountain roads. See also "Why do your ears hurt before a plane lands?"
- Take a sealed bag of crisps or similar, just as it is, up a mountain or to the observation deck of a tall building
- Watch the bag puff up more and more as you gain height
The air inside the bag hasn't changed, but as the air pressure outside falls, the pressure inside becomes relatively higher and the bag looks swollen. It lets you feel for yourself the point made above, that air pressure falls as you go up.
For those who want to know more ― terms, formulas and links to textbooksEach part is labelled with its level, from junior-high science to university specialist courses
- Junior highCovered in junior-high science
- High schoolCovered in high-school "Basic Physics" or "Basic Earth Science"
- High school+High-school Maths III, advanced science, or column-level content
- UniversityUniversity specialist subjects (physiology, mountain medicine) not taught in high school
- ResearchTopics not yet taught as settled fact even at university, still being studied by researchers
Junior highTerms: words around altitude sickness
- Air pressure: the pressure caused by the weight of air. It falls as you go higher, because there is less air above you.
- Partial pressure of oxygen: the share of the total air pressure that oxygen accounts for. It is a guide to how much oxygen the body can actually take in.
- Altitude acclimatisation: the process by which the body gradually adapts to low-oxygen surroundings.
High schoolChecking with a formula: how much does air pressure fall as you go up?
The change in air pressure with height can be estimated with an approximate formula like this.
Pressure ≒ ground pressure × exp(−altitude ÷ 8000)
| Pressure | Estimated pressure at that altitude [hPa] |
| Ground pressure | Estimated pressure at 0 m altitude, about 1013 hPa |
| Altitude | Height above the ground [m] |
| exp( ), 8000 | The symbol for a power of the base of natural logarithms (e), and a rough guide to the height at which pressure falls to 1/e [m] |
The higher the altitude, the more negative the number inside exp becomes, and the smaller the pressure value gets.
| Factor at the summit of Mt. Fuji (3776 m) | About 0.62 |
| Factor at 5000 m | About 0.535 |
| Factor at the summit of Everest (8848 m) | About 0.331 |
* The figure of 8000 m is a rough guide for approximation. Actual air pressure also varies with temperature and weather.
| Pressure at the summit of Mt. Fuji | 1013 × 0.62 ≒ 628 hPa |
| Pressure at 5000 m | 1013 × 0.535 ≒ 542 hPa |
| Pressure at the summit of Everest | 1013 × 0.331 ≒ 335 hPa |
| What % of ground pressure is the summit of Mt. Fuji? | (628 ÷ 1013) × 100 ≒ 62% |
| What % of ground pressure is the summit of Everest? | (335 ÷ 1013) × 100 ≒ 33% |
On the summit of Mt. Fuji, you are breathing air about 62% as thick as at ground level. On the summit of Everest, it is about one third of ground level. This is why oxygen cylinders are considered essential on Everest and other high peaks.
High school+Why does "climbing slowly" work?
When the body is put in a low-oxygen environment, it first tries to cope by breathing more often. Over several days, it increases the number of red blood cells that carry oxygen and changes the properties of the blood, gradually becoming a body that can stay active on thin oxygen. This series of changes is said to take from several days to one or two weeks. If you gain height faster than this adjustment, the body can't keep up and oxygen shortage carries on. "Climbing slowly" works because it gives the body time to adjust.
UniversityHow does the body react to oxygen shortage?
When the partial pressure of oxygen falls, the breathing centre is stimulated first, and breathing rate and ventilation increase. This is called the "hypoxic ventilatory response". Over-breathing temporarily tips the blood towards alkaline, so the kidneys start adjusting to correct this at the same time. Over several days, the kidneys are also prompted to release a hormone, and production of oxygen-carrying red blood cells increases. In severe cases, fluid can build up in the lungs or brain. This is more specialist material, covered in university physiology and mountain medicine.
ResearchWhat is still unclear
- It is still hard to predict accurately in advance who will get altitude sickness. It is said to have little to do with age or fitness, and research continues into why individual differences are so large, including genetic factors.
- Research is still under way on how well preventive drugs work and how best to use them. Which drugs can be used depends on constitution and existing conditions, and the aim is to establish safer, more effective ways of prevention.
- Researchers disagree on how far short-term high-altitude training can actually speed up acclimatisation. Studies that measure the effects of altitude training precisely are also continuing in sports science.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science: air pressure, properties of air | Basics of how altitude, air pressure and oxygen are linked |
| High school | Basic Earth Science: structure of the atmosphere | Air pressure falling with height |
| High school+ | Maths III: exponential functions | The whole calculation in ① to ④ above |
| University | Physiology, mountain medicine | Hypoxic ventilatory response, increased red blood cell production |
| Research | Mountain medicine, sports science (ongoing research) | Predicting individual differences, research on preventive drugs, effects of altitude training |
| ― | Disaster prevention, outdoor safety education | Climb slowly, don't climb once symptoms appear, deciding to descend |
- Explanatory materials on the symptoms, prevention and treatment of altitude sickness from mountain-medicine societies and specialist bodies.
- General descriptions of how air pressure varies with height in earth science and meteorology textbooks (including approximate values from the International Standard Atmosphere).
- General descriptions of the ventilatory response and red blood cell production in low-oxygen environments in physiology textbooks.
- Advisory materials on altitude sickness on Mt. Fuji from mountaineering organisations and local governments.
* Figures for air pressure, altitude and so on are approximations to help you understand how it works. Actual values vary with temperature, weather and region.
* This article is a general science explainer. For decisions about your health, and for real mountaineering safety decisions, follow the instructions of doctors and other specialists and the information from mountaineering organisations. The figures given are approximations to help you understand how it works.