How Altitude Hits Differently Above 4,000m — and What Helps

Above 2,500 metres, the air has noticeably less oxygen. By 4,000 metres you're breathing roughly 60% of the oxygen available at sea level. By 5,000 metres it's closer to 50%. That's the simple physics behind altitude sickness.

What it actually feels like. Mild acute mountain sickness (AMS) feels like a hangover: headache, nausea, no appetite, broken sleep. About a third of hikers above 4,000 m get some version of this. It's uncomfortable but manageable.

The dangerous version. High-altitude pulmonary edema (HAPE) and cerebral edema (HACE) are rare but real. The big warning signs are: a wet cough, breathlessness at rest, severe headache, confusion, loss of coordination. If any of these appear, the answer is always the same: descend, immediately.

What helps before the trip. Cardio fitness doesn't prevent AMS, but it does mean you arrive less wrecked. If you can sleep at altitude in advance (e.g. a night in Naivasha or Nyahururu before flying to the trailhead), you'll gain a small edge.

What helps on the trail. Walk slowly. Truly slowly. We tell new guides: if your client isn't a little annoyed by your pace, you're going too fast. Drink 3-4 litres a day. Eat even when you don't feel like it. Take the acclimatisation day seriously.

What about Diamox? Acetazolamide (Diamox) is well-studied and effective. We don't prescribe — talk to your doctor — but a majority of our Kili clients take it prophylactically. Side effects (tingling fingers, frequent urination, fizzy taste in carbonated drinks) are minor.

When to turn around. No summit is worth your life. Our guides will turn the group around if anyone shows symptoms of HACE or HAPE. We don't apologise for this. We're proud of our 0% serious incident record across nearly a decade.