Kilimanjaro altitude sickness: how acclimatisation works and how to lower your risk

Why does Kilimanjaro cause altitude sickness?

The summit stands at 5,895 m. As you climb, the air still holds 21 percent oxygen, but the air pressure falls, so every breath brings in fewer oxygen molecules. Given time, the body adjusts. That adjustment is acclimatisation. Climb faster than it can happen and altitude illness follows.

TravelHealthPro, the UK's travel health service, notes that altitude illness usually starts above 2,500 m, and it classes anything over 5,500 m as extreme altitude. Kilimanjaro's summit is in that band.

Studies on the mountain show how common it is:

Why do longer routes give you a better chance?

Because they come closer to the ascent rate doctors recommend. CDC, the NHS and the Wilderness Medical Society give the same core rule: once above 3,000 m, raise your sleeping altitude by no more than 500 m a night. TravelHealthPro and the NHS add a rest day at the same height every three or four days.

Short itineraries can't do that. In the 2009 Davies study, climbers on the Marangu route slept at huts at 2,700 m, 3,700 m and 4,700 m, which means gains of 1,000 m from one hut to the next before a summit attempt to 5,895 m. Summit rates and AMS were similar on the 4- and 5-day versions, and the authors concluded that trekkers need a more gradual route profile.

A single extra day isn't a fix on its own. The 2010 Jackson study found no clear benefit from one rest day at 3,700 m at those fast ascent rates. What it did find was much less AMS in people who had already acclimatised before the climb.

The 2022 Croughs study of 1,237 hikers found that climbing over more days was one of the factors linked to reaching the summit. The British Mountaineering Council's advice matches: choose 7 to 9 days rather than a rushed 5-day programme. Compare route lengths on our Kilimanjaro page.

What does "climb high, sleep low" mean?

The height you sleep at matters more than the highest point you reach during the day. That's the Wilderness Medical Society's position, and it's the logic behind the old saying. Walking up to a higher point and coming back down to sleep lets your body meet thinner air without spending the night in it.

In the Croughs study, climbing higher during the day was one of the predictors of summit success. Good itineraries build these walks in.

What are the symptoms of AMS, HACE and HAPE?

Acute mountain sickness (AMS) is the common, milder form. Its main sign is a headache, plus one or more of: nausea or loss of appetite, vomiting, dizziness, tiredness or weakness. Poor sleep is common. CDC compares it to a hangover. It usually starts 6 to 10 hours after you arrive at a new height.

High altitude cerebral oedema (HACE) is swelling of the brain. Watch for confusion, stumbling or clumsiness, and someone who is very drowsy or hard to wake.

High altitude pulmonary oedema (HAPE) is fluid on the lungs. It starts with breathlessness on effort and a dry cough, then breathlessness at rest, a wet cough, frothy or blood-stained spit, and chest tightness.

HACE and HAPE are uncommon but can kill. They tend to appear at least 2 days after reaching heights above 3,000 to 4,000 m. TravelHealthPro's rule is blunt: at altitude, symptoms are altitude illness until proven otherwise.

What should you do if you feel ill on the mountain?

1. Tell your guide straight away. Hiding a headache to keep up with the group is how mild AMS becomes serious.
2. Stop going up. With mild symptoms, rest at the same height. Never climb higher to sleep while you have symptoms.
3. Go down if it gets worse, or if it hasn't improved after a day or two. A descent of 300 to 1,000 m usually helps.
4. Signs of HACE or HAPE mean descend now and get urgent medical help.
5. Never leave someone with altitude illness alone.

This matters more than it sounds. In the Croughs study, most hikers kept climbing despite mild or even severe symptoms. The only response to mild symptoms linked to fewer severe ones was not climbing further.

Should I take Diamox (acetazolamide)?

Talk to your doctor. The Wilderness Medical Society says acetazolamide aids acclimatisation and should be strongly considered for travellers at moderate to high risk. TravelHealthPro describes preventive medicine as something added to a gradual ascent, never a replacement for it. It isn't suitable for everyone, which is why the decision and the dose belong to a doctor who knows your health.

How fit do I need to be?

Fit enough to walk for several hours a day, several days in a row, and enjoy it. But be clear about what fitness does. CDC states that training and physical fitness do not affect the risk of altitude illness. Very fit people get altitude sickness too.

Train with long walks on hills, carrying a light pack, on back-to-back days. It makes the trek more comfortable and the steep summit night less of a shock.

Things that do change your risk, according to TravelHealthPro: a past history of altitude illness, living below 900 m, and hard exertion on arrival at altitude. How you've coped at altitude before is probably the best guide. Some climbers acclimatise first on nearby Mount Meru (4,566 m), which the British Mountaineering Council suggests. Skip alcohol on the climb.

Last, check that your travel insurance covers the maximum altitude you'll reach and helicopter evacuation. Ready to choose a route? Start on plan your trip.