Blood Oxygen Level Guide
Look up what any SpO2 reading means from a pulse oximeter.
Explains normal (95-100%), low (90-94%), and critically low levels with symptoms and guidance.
Blood oxygen level (SpO2) measures the percentage of hemoglobin molecules in your blood that are carrying oxygen. Pulse oximeters estimate this non-invasively using light absorption at two wavelengths (red ~660 nm and infrared ~940 nm).
The underlying formula (Beer-Lambert Law basis):
SpO2 = (Ratio of oxygenated Hb / Total Hb) × 100
In practice, pulse oximeters use a calibration curve derived from this ratio (R):
R = (AC660 / DC660) / (AC940 / DC940)
Where AC = pulsatile (arterial) signal and DC = baseline signal at each wavelength. SpO2 is then looked up from a stored R-curve. You don’t calculate this manually — your oximeter does it automatically.
What the numbers mean:
| SpO2 Range | Interpretation |
|---|---|
| 95–100% | Normal — healthy oxygenation |
| 91–94% | Mild hypoxemia — monitor closely |
| 86–90% | Moderate hypoxemia — seek medical attention |
| Below 86% | Severe hypoxemia — emergency |
Normal variation factors:
- At sea level, healthy adults average 97–99%
- At high altitude (8,000+ ft), 92–95% is normal
- During deep sleep, SpO2 may briefly dip to 90–92% even in healthy people
- Smokers and people with COPD often have lower baseline readings
Accuracy notes:
- Nail polish (especially dark colors) and acrylic nails can cause falsely low readings: use a fingertip without polish or try the earlobe
- Poor circulation (cold hands) reduces accuracy significantly: warm up before measuring
- Carbon monoxide poisoning produces falsely normal SpO2 readings: oximeters cannot distinguish COHb from O2Hb
- For clinical accuracy, arterial blood gas (ABG) testing is the gold standard
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