Oxygen saturation (SpO₂) is the most direct compass for understanding how your body is responding to altitude. A pulse oximeter tells you, in real time, what percentage of your blood's haemoglobin is carrying oxygen. But normal values at altitude are different from those at sea level — confusing the two leads to poor decisions.
Check the oxygen percentage available at your target altitude with the Oxymeter calculator.
What is Oxygen Saturation (SpO₂)?
SpO₂ measures the percentage of haemoglobin in peripheral capillary blood that is carrying oxygen. A value of 98% means that 98% of haemoglobin molecules are loaded with O₂. At sea level, a healthy adult at rest has an SpO₂ between 95% and 100%.
A pulse oximeter calculates this value using photoplethysmography: two LEDs — one red (660 nm) and one infrared (940 nm) — pass through the tissue of the finger, and a sensor measures the absorbed light. Because oxygenated and deoxygenated haemoglobin absorb the two wavelengths differently, the ratio allows SpO₂ to be calculated with a standard margin of error of ±2%.
Why SpO₂ Drops With Altitude
As you gain altitude, atmospheric pressure decreases. The percentage of oxygen in the air remains constant (20.9%), but the partial pressure of oxygen (pO₂) decreases proportionally. The lungs receive less "push" to transfer oxygen into the blood, and SpO₂ falls.
This is precisely the parameter the Oxymeter calculator measures in real time: the availability of oxygen as a percentage relative to sea level, calculated using the ICAO Standard Atmosphere formula.
SpO₂ Normal Values by Altitude: Reference Table
The values below are the expected averages for a healthy adult who is not acclimatized on first exposure to altitude, and for an acclimatized individual after several days:
| Altitude | Baro. pressure | O₂ available | SpO₂ unacclimatized | SpO₂ acclimatized |
|---|---|---|---|---|
| 0 m (sea level) | 760 mmHg | 100% | 97–100% | 97–100% |
| 1,000 m | 674 mmHg | 89% | 96–99% | 96–99% |
| 1,500 m | 634 mmHg | 84% | 95–98% | 96–99% |
| 2,000 m | 596 mmHg | 79% | 93–97% | 95–98% |
| 2,500 m | 560 mmHg | 74% | 91–95% | 94–97% |
| 3,000 m | 526 mmHg | 70% | 88–93% | 92–96% |
| 3,500 m | 493 mmHg | 65% | 85–91% | 90–95% |
| 4,000 m | 462 mmHg | 61% | 82–88% | 88–93% |
| 4,500 m | 432 mmHg | 57% | 79–85% | 85–91% |
| 5,000 m | 405 mmHg | 54% | 75–82% | 82–89% |
| 5,500 m | 379 mmHg | 50% | 70–79% | 78–86% |
| 6,000 m | 354 mmHg | 47% | 65–75% | 73–82% |
| 7,000 m | 309 mmHg | 41% | 57–68% | 65–77% |
| 8,000 m | 267 mmHg | 35% | 50–62% | 58–70% |
| 8,849 m (Everest) | 253 mmHg | 33% | 42–58% | 55–70% |
Note: these are average ranges. Individual variability is significant. An SpO₂ of 85% at 4,000 m in an acclimatized, asymptomatic individual is not pathological. Context is everything.
How to Interpret SpO₂ Values in the Mountains
SpO₂ alone is not sufficient: it must always be integrated with clinical status (are symptoms present or absent?) and with the trend (is it dropping or stable?).
Practical interpretation guide
| SpO₂ at rest | Interpretation | Recommended action |
|---|---|---|
| > 90% | Good acclimatization for the altitude | Continue — monitor daily |
| 85–90% | Marginal acclimatization | Reduce physical activity, hydrate, do not ascend further |
| 80–85% | Insufficient acclimatization | Consider descent, consult the group, consider supplemental oxygen |
| < 80% | Potential emergency | Descend immediately — the altitude is too high for this body |
The three SpO₂ warning patterns
Regardless of the absolute value, these three patterns require immediate attention:
- Drop > 5% compared to the previous day at the same altitude — indicates that the body is not acclimatizing but deteriorating
- SpO₂ that does not recover to > 90% within 10 minutes of rest after physical exertion
- Variability > 5% between two consecutive resting measurements — a signal of physiological instability
How to Use a Pulse Oximeter in the Mountains
When to measure
- On waking, after 5 minutes of rest: this is the most representative measurement of baseline status
- After 15 minutes of moderate walking: assesses the cardiovascular response to exertion
- Before sleeping: identifies deterioration that occurred during the day
Factors that make readings unreliable
- Cold hands (peripheral hypothermia): vasoconstriction reduces capillary blood flow to the finger
- Nail polish (especially dark colours or glitter): absorbs the LED wavelengths
- Movement during measurement: motion artefacts generate falsely low readings
- Direct sunlight on the sensor: interferes with photoplethysmography
The solution: always measure in shade, after warming the hands, with the finger still for at least 30 seconds before reading the value.
Fingertip oximeter vs smartwatch
Dedicated fingertip devices (Nonin, Masimo, Beurer) have a ±2% margin of error and are the standard in mountain medicine. Smartwatches with a wrist SpO₂ sensor (Apple Watch, Garmin) offer the advantage of continuous overnight monitoring, but with lower precision (±3–5%) and more motion artefacts.
For demanding expeditions, the recommendation is to use both: the smartwatch for overnight trend monitoring, the fingertip device for reference measurements on which decisions are based.
SpO₂ and the Oxymeter Calculator: Using Them Together
The Oxymeter calculator tells you how much oxygen is available in the environment (% O₂ relative to sea level). A pulse oximeter tells you how your body is responding to that environment. They are complementary pieces of information:
- Low ambient O₂ + high SpO₂ = good acclimatization
- Moderate ambient O₂ + low SpO₂ = insufficient acclimatization — slow the ascent
Using both together, recording both values daily in an expedition diary, is the most effective way to monitor physiological progression.
SpO₂ on Specific Peaks
For a complete profile — altitude, expected SpO₂ range, difficulty, and oxygen comparison bar — visit each peak page:
- Mont Blanc (4,810 m) — Expected SpO₂: 80–91%
- Matterhorn (4,478 m) — Expected SpO₂: 83–93%
- Kilimanjaro (5,895 m) — Expected SpO₂: 72–83%
- Aconcagua (6,962 m) — Expected SpO₂: 65–77%
Read more: All guides on altitude health
Frequently Asked Questions
What are normal oxygen saturation values at altitude?
At 3,000 m an SpO₂ between 88 and 93% is within the normal range for a non-acclimatized individual; 92–96% for an acclimatized one. At 5,000 m, 75–82% is expected without acclimatization and 82–89% with acclimatization. The absolute value must always be contextualized with symptoms present and the trend over the previous 24 hours.
Below what SpO₂ value should I descend?
A stable SpO₂ below 80% at rest, or one that does not recover above 75–80% after prolonged rest, is a signal requiring descent. UIAA guidelines state: do not ascend further with resting SpO₂ below 80% at any altitude. Descent of even 500–1,000 m almost always produces rapid recovery.
Is a wrist pulse oximeter reliable in the mountains?
Dedicated fingertip devices have ±2% error and are the mountain medicine standard. Smartwatches offer continuous overnight monitoring with lower precision (±3–5%). For demanding expeditions, use both.
What is the difference between available oxygen and SpO₂?
Available oxygen (from the Oxymeter calculator) measures the environmental oxygen supply. SpO₂ measures how your body is utilizing it. A mismatch — low supply with high SpO₂, or adequate supply with low SpO₂ — tells you about your acclimatization status. See the complete altitude oxygen table for more detail.
