At the summit of Everest the air is still about 21% oxygen, the same as at the beach. What drops at high altitude is air pressure, and that one change explains nearly everything a trekker feels up there.
Why there is less oxygen at altitude
At sea level the atmosphere presses down at roughly 1,013 millibars. At 5,500 meters it’s about half that. The proportion of oxygen is unchanged, but each breath holds half as many molecules.
That pressure drop controls how much oxygen crosses from your lungs into your blood, and how much reaches your muscles and brain. It’s why “about half the oxygen at 5,500 meters” is a fair shorthand even though the air’s composition is the same.
It also starts lower than you might expect. At 3,050 meters, the oxygen pressure in each breath is already down to 69% of its sea level value.
| Band | Range | What to expect | Examples |
|---|---|---|---|
| High altitude | 1,500 to 3,500 m | Harder effort and worse sleep. Altitude illness becomes possible once you sleep above about 2,500 m | Cusco (3,400 m), Namche Bazaar (3,440 m) |
| Very high altitude | 3,500 to 5,500 m | The high days of the big treks, where altitude illness is a real consideration | Dead Woman’s Pass (4,215 m), Everest Base Camp (5,364 m) |
| Extreme altitude | Above 5,500 m | Short visits only. Nobody lives permanently this high | Kilimanjaro’s summit (5,895 m) |
How your body acclimatizes, and how long it takes
Acclimatization is a sequence of adaptations that run on very different clocks. Knowing which is which explains why itinerary length matters so much, and why training can’t stand in for it.
| When | What changes | On a two week trek? |
|---|---|---|
| Minutes | Breathing gets faster and deeper | Yes, straight away |
| First 3 to 5 days | Kidneys adjust blood chemistry so breathing can rise further. Heart rate rises | Yes, and it’s most of what you feel |
| Weeks | More red blood cells | Very little |
Within minutes you breathe more, deeper and faster. This is the hypoxic ventilatory response. It’s the first and largest compensation you have, and the reason you find yourself panting on gentle ground.
Over the first few days the extra breathing blows off carbon dioxide, which makes your blood more alkaline and holds your breathing back. Your kidneys correct this by excreting bicarbonate, and breathing can then rise further. This acute phase takes roughly 3 to 5 days after each ascent, and it’s most of what an acclimatization day buys you.
In those same days your heart rate rises to move blood faster, and plasma volume drops, which concentrates the red cells you already have. Production of erythropoietin, the hormone that drives red cell production, goes up within hours of arrival.
Over weeks new red blood cells appear in meaningful numbers. This is the adaptation people associate with altitude, and on a two week trek you get very little of it. The improvement you feel by day eight is almost entirely breathing and kidney chemistry.
That timeline is why gym work can’t replace days on the mountain. The process only starts when the pressure drops.
How high altitude affects sleep, appetite and thinking
These five effects make a trek uncomfortable well before oxygen delivery becomes dangerous.
Sleep breaks up. Above about 2,700 meters, some degree of periodic breathing at night is close to universal: breathing deepens, pauses, then restarts, often waking you. It feels alarming and is usually normal. The cost is broken sleep on the days you most need recovery.
You dry out. Breathing faster through cold, dry air loses more water than you notice. Altitude also makes you urinate more in the first days. Dehydration doesn’t cause altitude sickness, but its headache and fatigue look the same, which makes it harder to tell what’s going on.
Appetite drops. It falls just as your energy needs rise. It’s common to eat much less than you need above 4,000 meters and then run short of energy several days in.
Thinking gets worse. Reaction time, memory and judgment all decline with low oxygen, usually before you notice. This is a safety issue, because decisions get harder at the point where you’re less equipped to make them.
Recovery slows. Catching your breath after a steep section takes much longer than at home, so short pauses on climbs are normal.
Altitude sickness: AMS, HAPE and HACE
There are three conditions. They sit on a rough spectrum, though they don’t always progress in order.
| AMS | HAPE | HACE | |
|---|---|---|---|
| What it is | Acute mountain sickness | Fluid in the lungs | Swelling of the brain |
| How common | Common. About 25% of visitors sleeping above 2,450 m in Colorado | Uncommon. Up to 1 in 100 travelers above 4,300 m | Rare, especially below 4,300 m |
| Typical onset | 2 to 12 hours after arriving at a new altitude | Usually 2 to 4 days after arriving | Usually follows worsening AMS |
| Key sign | Headache plus nausea, fatigue or dizziness | Breathless at rest | Loss of coordination, confusion |
| What to do | Stop ascending. Go down if it gets worse | Descend now | Descend now |
Acute mountain sickness (AMS)
This is the common one. It usually appears 2 to 12 hours after arriving at a new altitude, and anyone sleeping above about 2,500 meters is at risk.
Headache is the defining symptom, usually with at least one of: nausea or loss of appetite, unusual fatigue, dizziness, or occasionally vomiting.
It feels a lot like a hangover, and it’s often mistaken for dehydration, poor sleep or being unfit.
What to do: stop ascending and tell your guide. Rest, drink, eat. AMS generally settles within 12 to 48 hours if you don’t go higher. If it gets worse despite rest, go down.
The rule that matters most: never go up to sleep at a higher altitude while you have symptoms, however minor. HACE usually develops from AMS that someone kept climbing with.
High altitude pulmonary edema (HAPE)
Fluid accumulating in the lungs. It’s uncommon, usually appears two to four days after arriving at altitude, and can develop in someone who never had AMS.
Signs: at first a cough and more breathlessness on exertion than the climb explains. Then breathlessness that persists at rest, a cough that turns wet, chest tightness and unusual fatigue on easy ground.
The distinguishing feature is breathlessness when you aren’t doing anything. Being winded on a climb is normal. Being winded sitting down is a warning.
High altitude cerebral edema (HACE)
Swelling of the brain. It’s rare, and usually a progression from severe AMS.
Signs: confusion, unusual behavior, drowsiness and ataxia, a loss of coordination that looks like being drunk. Guides often check for it by asking someone to walk a straight line heel to toe. Failing that test at altitude is treated as an emergency. Without treatment or descent, HACE can go from ataxia to coma within 12 to 24 hours.
What to do
For HAPE or HACE, descend immediately. Descent is the definitive treatment. Guidelines call for losing 300 to 1,000 meters, and improvement is often quick.
For AMS, stop and rest. Descend if it gets worse or doesn’t settle.
Supplemental oxygen and portable hyperbaric bags help, and good operators on Kilimanjaro and in the Khumbu carry them. A hyperbaric bag simulates a descent of roughly 1,500 to 1,800 meters. Both buy time, and losing altitude is still the goal.
- Never leave someone with suspected HACE or HAPE alone, and never send them down unaccompanied.
- Descend at night if you have to. With HACE, hours matter.
Who gets altitude sickness
No test predicts it reliably.
Fitness doesn’t change your risk. The CDC says it directly: training and physical fitness do not affect risk. Fit people get AMS like everyone else. What fitness changes is how much each day costs you, and that matters for everything else on the trek. The one trap is pace, because a strong aerobic base makes it easy to climb faster than you’re adapting.
Age doesn’t work the way people expect. Children are as susceptible as adults, and people over 50 have slightly lower risk.
Your own history is the best guide. How you responded at a similar altitude and ascent rate is the most reliable predictor, and even that isn’t infallible. Someone who was fine at 5,000 meters last year can struggle this year.
Pulse oximeters get used a lot, and guides on Kilimanjaro often check saturation daily. They’re useful for tracking a trend across days. A single low reading isn’t a diagnosis, and a reassuring one doesn’t rule anything out.
Readings vary widely between healthy people at the same altitude, and cold fingers alone can produce a bad number. Treat it as one input among several.
How to lower your risk of altitude sickness
Training isn’t on this list.
- Ascent rate, measured by where you sleep. Above 3,000 meters, the guideline is to raise your sleeping altitude by no more than 500 meters a night and add an extra night for every 1,000 meters gained.
- A longer itinerary. On Kilimanjaro, the number of days on your route is the biggest lever you have, and it’s a booking decision made months out.
- Acclimatization days used actively. Walk up a few hundred meters, then come back down to sleep.
- Arriving early where the trip allows it, such as extra days in Cusco or Namche Bazaar.
- No alcohol for the first 48 hours at altitude.
- Drinking and eating steadily, which won’t prevent AMS but keeps you from stacking dehydration and depletion on top of it.
- Telling your guide the truth. Underreporting symptoms to protect a summit attempt is how mild AMS turns into an evacuation.
- Being willing to turn around.
Medications for altitude exist and are widely used. They involve trade-offs, they depend on your health history, and they’re a conversation with a doctor experienced in travel medicine, weeks before you fly. No medication replaces a sensible ascent profile.
This article is general awareness and does not replace medical advice. If you have any pre-existing condition, particularly cardiovascular or respiratory, discuss your trek with a doctor before booking. For the clinical version, the CDC’s altitude illness chapter is the reference.
Fitness won’t acclimatize you. It does decide how much you have left each day while altitude takes its share, and that affects how well you eat, sleep and recover across a two week trek.
Our Trekking Fitness Plan builds that fitness with strength work, stair climber sessions and aerobic conditioning.
