How to Acclimatize to Altitude Safely
Altitude acclimatization is governed almost entirely by one variable: how fast you ascend. Above roughly 3,000 metres, increase your sleeping altitude by no more than 300 to 500 metres per night, and take a full rest day every third day or every 1,000 metres gained. Climb high and sleep low if the terrain allows. Everything else — hydration, food, avoiding alcohol, and in some cases acetazolamide — helps at the margins, but none of it substitutes for going up slowly.
The reason this advice is so specific is that acclimatization is a set of physiological changes that take a fixed amount of time. You cannot will them to happen faster.
What your body is actually adjusting to
The proportion of oxygen in air is the same at 5,000 metres as it is at sea level: a little under 21 percent. What changes is the total pressure of the atmosphere pushing that oxygen into your lungs.
At sea level, atmospheric pressure averages about 1,013 hectopascals. At 2,500 metres it is around 750. At 3,500 metres roughly 660, at 4,500 metres about 580, and at the summit of Everest a little over 330. Because the partial pressure of oxygen scales with the total, the gradient driving oxygen across the lung membrane into your blood falls in proportion.
Your body has a sequence of responses, and they run on different clocks:
Within minutes. Chemoreceptors in the carotid arteries detect the drop and increase your breathing rate and depth. This is the single largest and fastest compensation.
Within hours. Heart rate rises, and blood is redistributed. Breathing off extra carbon dioxide makes the blood more alkaline, which paradoxically limits how much further your breathing can increase.
Within one to four days. The kidneys excrete bicarbonate, correcting that alkalinity and releasing the brake on your breathing. This is the central step in acclimatization, and it is why day three at a new altitude usually feels better than day one.
Within one to three weeks. Erythropoietin drives up red blood cell production, increasing the oxygen-carrying capacity of the blood. Capillary density and muscle metabolism adapt over weeks to months.
The kidney step is the bottleneck for a normal trip, and it cannot be rushed.
The ascent rules that matter
The guidance below is drawn from the wilderness medicine consensus and is worth following even if you have been to altitude before without trouble:
- Above 3,000 metres, gain no more than 300 to 500 metres of sleeping altitude per night. What matters is where you sleep, not the highest point you touched during the day.
- Take a rest day every 1,000 metres, or every third night. A rest day means sleeping at the same altitude two nights running.
- Climb high, sleep low. Day hikes to a higher point followed by a descent to sleep are a genuine acclimatization stimulus and are safer than sleeping high.
- Treat the flight or drive in as part of the ascent. Flying straight into a city at 3,600 metres is a very rapid gain. Plan two easy days before doing anything demanding.
- Never ascend further with symptoms. If you have a headache, nausea or unusual fatigue, stay put until it resolves.
Rule five is the one people break. The symptoms of acute mountain sickness resolve on their own in most cases, but only if you stop going up.
What helps, and what does not
Hydration. You lose more water at altitude through faster breathing and dry air. Drink enough that your urine stays pale. But overdrinking does not prevent altitude sickness, and drinking to excess can cause its own problems.
Carbohydrate. A diet weighted toward carbohydrate marginally improves the respiratory exchange ratio at altitude. The effect is small. Eating enough of anything matters more, and appetite often disappears.
Alcohol and sedatives. Avoid both for the first 48 hours. They suppress breathing during sleep, which is exactly when oxygen levels are already at their lowest.
Acetazolamide. A prescription diuretic that accelerates the kidney step described above, effectively giving you a head start on acclimatization. It has good evidence behind it for prevention when a slow ascent is not possible. It also causes tingling in the fingers and makes carbonated drinks taste odd. This is a decision for a doctor who knows your history, not a self-prescription.
Fitness. Does not protect you. Fit people often ascend faster, which increases risk. Prior altitude performance is a better predictor than any fitness measure.
Pre-acclimatization. Sleeping in a hypoxic tent or spending time at moderate altitude in the weeks before does help, if done for long enough. A weekend at 2,000 metres a month earlier does very little.
If you get migraines
People who are sensitive to barometric change often find altitude a distinct experience, and it is worth understanding why. A storm at sea level might drop the pressure 20 to 30 hectopascals over a day. Driving from the coast to 3,000 metres drops it by around 300, and it stays there.
This means altitude is a sustained low rather than a rate of change. Some people who react strongly to fast-moving weather systems do fine at a stable high altitude; others find the opposite. The distinction is covered more fully in the comparison of acute mountain sickness and migraine, and telling them apart matters because the treatments differ.
If you already log symptoms against the barometer at home — somewhere with a genuinely active one, say Inverness, where Atlantic depressions produce some of the largest natural swings in Britain — an altitude trip gives you a useful contrast at the far end of the scale. Keep logging while you are up there. The pattern of whether symptoms track the ascent itself or the days after arrival is informative, and it is the kind of thing a migraine tracker app with automatic logging will capture more reliably than memory.
Carry your usual acute migraine treatment. Altitude is a recognised trigger, and a migraine at 4,000 metres is still a migraine, though it needs to be distinguished from altitude sickness before you treat it as one.
Red flags
Stop, and descend, for any of these:
- Confusion, difficulty concentrating, or personality change
- Loss of coordination, particularly an unsteady heel-to-toe walk
- Breathlessness at rest, or a cough producing frothy sputum
- A headache that does not respond to painkillers and rest
- Symptoms that worsen rather than improve after a night at the same altitude
These suggest high altitude cerebral or pulmonary oedema. Both are life-threatening and both improve rapidly with descent. Descending 500 to 1,000 metres is usually enough to turn things around, and it should not wait for morning.
FAQ
How long does altitude acclimatization take?
The main kidney-driven adjustment takes one to four days at a given altitude. Fuller adaptation, including increased red cell mass, takes one to three weeks. Most people feel substantially better by day three.
Does acclimatization carry over to the next trip?
Partly, and briefly. Returning to altitude within about a week of coming down is noticeably easier. After a month at sea level, most of the benefit has gone.
Is 2,000 metres high enough to need acclimatization?
Most people have no trouble below about 2,500 metres, which is the usual threshold for acute mountain sickness. Some sensitive individuals notice symptoms lower, particularly if they arrive quickly by car or cable car.
Can I fly straight to a high-altitude city?
You can, but treat the first two days as rest days and avoid alcohol and exertion. Cities above 3,000 metres are the ones that catch people out, because arriving by plane skips every stage of a gradual ascent. The related question of what happens in the cabin on the way is covered in cabin pressure explained.
Does oxygen at the hotel help?
Supplemental oxygen relieves symptoms while you are breathing it, but it does not speed acclimatization and symptoms often return when you stop. It is a comfort measure, not a treatment for the underlying adjustment.