Altitude & Physiology
Your Blood Oxygen on Kilimanjaro
What the pulse oximeter actually measures — and why the number on your finger matters more than you think.
At 4,000 metres your blood oxygen saturation drops to 80-85%. At base camp, most trekkers have no idea what that number means or whether they should be alarmed. Some ignore it entirely. Others panic and descend when they could have continued safely. Both responses are dangerous in different ways.
This post explains what SpO2 is, what numbers are normal on Kilimanjaro, which readings signal genuine trouble, and how Bobby Tours guides use pulse oximetry as one input — never the only input — in their summit-day decisions.
What SpO2 Actually Measures
A pulse oximeter clips onto your finger and shines infrared light through the tissue. Haemoglobin that is carrying oxygen absorbs light differently from haemoglobin that is not. The device calculates the ratio and expresses it as SpO2 — saturation of peripheral oxygen. At sea level a healthy reading is 95-100%.
The Acute Mountain Sickness risk zone begins when SpO2 falls below 90% at rest, or when it drops more than 4 percentage points from your personal baseline within a 24-hour period. Most Kilimanjaro climbers cross that threshold by their second day on the mountain — and that is entirely normal.
One phenomenon that surprises many climbers: SpO2 drops further during sleep above 4,000 metres. This is sleep desaturation — breathing becomes irregular and oxygen levels can fall another 5-8% below daytime readings. It is why many climbers wake feeling worse than when they went to sleep, even before they start moving.
Fitness does not protect against hypoxia.
Athletes with high VO2 max do not have higher SpO2 readings at altitude. Fitness determines how efficiently your body uses whatever oxygen is available — it does not change how much your blood actually carries. A well-trained runner and a casual walker will show identical SpO2 readings at 4,700 metres.
Real Kili Numbers — What Guides See
On the Machame route, our guides see a consistent SpO2 trajectory across a 7-day climb:
Day 1 — Moshi (900m)
Baseline reading before ascending.
Day 2 — Shira Camp (3,840m)
First significant drop. Most climbers notice mild headache.
Day 4-5 — Kibo Base Camp (4,700m)
Normal for this altitude. The body is actively acclimatising.
Summit night (5,895m)
Expected during the ascent. Cold, exercise, and sleep deprivation compound the drop.
On summit night, SpO2 drops most severely. Cold temperatures cause peripheral vasoconstriction — blood is shunted away from the extremities to preserve core temperature — which can give falsely low readings on a finger pulse oximeter. Nail polish causes the same problem. Motion artifact from a shaky hand on the climb also produces false lows. Our guides are trained to recognise these false readings and cross-check with pulse palpation and visual observation before drawing conclusions.
At each camp, our guides perform a standard check: SpO2 reading plus heart rate plus Lake Louise Score plus direct observation. No single number triggers a summit-no-go decision.
When Low SpO2 Means Turn Back
The most common — and most dangerous — mistake climbers make on Kilimanjaro is comparing their SpO2 reading to their partner's. Baseline physiology varies enormously between individuals. One climber at 5,000 metres with SpO2 of 72% may be adapting well and heading for the summit; another at the same altitude with SpO2 of 78% may be deteriorating fast. The number only has meaning in the context of that individual's baseline, symptoms, and trend over time.
Bobby Tours guides apply a three-factor protocol before recommending descent:
- SpO2 reading: Under 75% at rest, or below 65% at any point.
- Lake Louise Score: A structured symptom checklist scoring headache, gastrointestinal upset, fatigue, dizziness, and difficulty sleeping. A score of 6 or more with at least a headache indicates AMS.
- Guide observation: Is the climber communicating clearly? Are they eating and drinking? Can they walk at pace without stopping to breathe?
The descent decision.
SpO2 under 75% at rest, or any SpO2 reading below 65% combined with a headache, equals immediate descent. No exceptions. This is not a grey area — it is the point at which altitude illness risks becoming life-threatening HACE or HAPE. The descent itself typically reverses symptoms within hours.
A real example from our operations: a climber at 5,000 metres with SpO2 of 72% and no headache, normal Lake Louise Score, eating lunch and talking coherently — our guide kept her on the mountain and she summited successfully the following morning. Another climber with SpO2 of 68% and a moderate headache at the same camp was descended immediately to Shira Camp, where his oxygen levels recovered to 84% within three hours and he felt well enough to walk down to Mti Mkubwa the next day. Both were correct decisions.
The difference was not the number — it was the combination of factors our guides assessed alongside it.
How to Track Your Own SpO2 on Kili
Before departure, buy a portable pulse oximeter — a reliable consumer model costs around $30-50. Look for one with a clear OLED display and a perfusion index readout, which tells you how reliable the reading is at a given moment. On the mountain:
Morning at camp
Measure before you start walking. Sit for two minutes first — exertion raises heart rate and lowers the reading artificially.
Evening at camp
Measure before dinner. Use this as your baseline for the following morning.
Any symptom onset
Measure immediately if you develop a headache, nausea, dizziness, or unusual fatigue. Write down the number, your heart rate, and your Lake Louise Score.
Record your readings in a small notebook: SpO2 percentage, heart rate, Lake Louise Score, and any symptoms. This log gives your guide a trend line — and a trend is far more informative than any single reading.
Bobby Tours provides SpO2 monitoring at every camp as part of our standard guide protocol. All of our guides carry pulse oximeters and are trained in Lake Louise scoring. You do not need to track SpO2 alone — but having your own device and understanding the numbers gives you a more active role in your own safety.
Download the Altitude Health Checklist
SpO2 tracking log, Lake Louise Score sheet, symptom diary, and descent decision guide — everything you need to monitor your own altitude health on the mountain.
Download Free ChecklistCommon Questions
What is a normal SpO2 reading on Kilimanjaro?
Normal sea-level SpO2 is 95-100%. On Kilimanjaro it drops progressively: 95-97% at Moshi (900m), 88-92% at Shira Plateau (3,840m), 80-85% at Kibo Base Camp (4,700m), and 70-78% during the summit attempt (5,895m). These are normal physiological responses to altitude, not indicators of illness on their own.
When does low SpO2 mean I should descend on Kilimanjaro?
SpO2 under 75% at rest, or any SpO2 drop below 65% combined with a headache, warrants immediate descent. The critical principle is context: a single number never dictates a decision. Bobby Tours guides combine SpO2 readings with Lake Louise Score and direct observation before recommending descent. Comparing your SpO2 to your climbing partner's is meaningless — baseline physiology varies significantly between individuals.
Does fitness affect SpO2 on Kilimanjaro?
No. Athletes with high VO2 max do not have higher SpO2 readings at altitude. Fitness improves your cardiovascular system's ability to use whatever oxygen is available, but it does not change how much oxygen your blood actually carries. A marathon runner and a casual walker will show similar SpO2 readings at 4,700m. Acclimatisation — not fitness — is what determines your SpO2 trajectory on the mountain.