SpO2 — blood oxygen saturation — is the percentage of haemoglobin in your blood that is carrying oxygen. A pulse oximeter measures this non-invasively by shining light through your fingertip. A healthy SpO2 for most adults is 95–100%. Readings below 90% are considered hypoxaemia and require prompt medical attention.
How pulse oximetry works
A pulse oximeter clips to your finger and uses two wavelengths of light — red and infrared — to detect the proportion of oxygenated haemoglobin in your blood. Oxygenated and deoxygenated haemoglobin absorb light differently, and the device calculates SpO2 from the difference.
Pulse oximetry is non-invasive, painless and gives a result within seconds. It became a household tool during the COVID-19 pandemic and is now widely used for home monitoring of respiratory conditions including asthma, COPD and heart failure.
What the numbers mean
| SpO2 reading | Interpretation | Action |
|---|---|---|
| 95–100% | Normal | No action needed |
| 91–94% | Mild hypoxaemia | Speak to your doctor promptly |
| 86–90% | Moderate hypoxaemia | Seek same-day medical review |
| ≤ 85% | Severe hypoxaemia | Call emergency services immediately |
Factors that affect accuracy
Pulse oximeters are highly accurate in most situations but can give misleading readings in certain circumstances. Cold hands reduce peripheral circulation and lower apparent SpO2. Nail varnish — particularly dark or blue shades — blocks the light beam. Poor perfusion (low blood pressure, shock) makes readings unreliable. Motion artefact during exercise or shivering causes fluctuations.
Skin pigmentation has also been shown to affect accuracy. Studies published since 2020 have found that some consumer pulse oximeters overestimate SpO2 in darker-skinned individuals by up to 3–4 percentage points — a meaningful difference when clinical decisions are based on the reading.
- Warm cold hands before measuring — run them under warm water for 30 seconds
- Remove nail varnish or use an alternative finger
- Stay still during the measurement
- Compare to how you feel — if you are breathless with a normal SpO2, the reading may be inaccurate
SpO2 in specific conditions
For people with COPD, acceptable SpO2 targets may be lower (typically 88–92%) because some retain carbon dioxide and high oxygen can suppress their respiratory drive. Always use your own clinician's targets, not generic guidelines, if you have a chronic lung condition.
Altitude reduces atmospheric oxygen pressure, so SpO2 naturally falls at high altitude — even in healthy people. A reading of 90% at 4,500 metres above sea level may be entirely normal.
When to call a doctor
Call your doctor if your SpO2 is consistently below 95% at rest and you don't have a known lung condition explaining it. Seek emergency care if readings fall below 90%, you are breathless at rest, or you feel confused and lightheaded — as these suggest the brain is not receiving adequate oxygen.