Key facts
- A healthy blood oxygen saturation level is around 95 to 100%, according to Asthma + Lung UK. 1
- If your oxygen level is below 92%, Asthma + Lung UK says a healthcare professional should refer you for an assessment, usually with a blood gas test. 1
- Pulse oximeters overestimate true oxygen levels in people with darker skin tones, and more so at low saturation. 2
- Asthma + Lung UK does not recommend buying a home pulse oximeter unless a healthcare professional has asked you to use one. 1
- The NHS says to call 999 or go to A&E immediately for severe difficulty breathing, such as gasping, choking or being unable to get words out. 7
What is a normal blood oxygen level?
Asthma + Lung UK describes pulse oximetry as a test that measures the oxygen level in your blood, called oxygen saturation or SpO2, using a sensor on a fingertip or earlobe.1 It says a healthy blood oxygen saturation level will be around 95 to 100%.1
The same page says that if your oxygen level is below 92%, your healthcare professional should refer you for an assessment, usually with a blood gas test.1 A change in your usual level may also mean that lung condition symptoms have got worse or that a flare-up is happening.1
This guide gives no percentage at which to call 999. None of the NHS pages read gives one: the NHS describes symptoms, such as severe difficulty breathing, rather than a number.7 If you are struggling to breathe, act on how you are, not on a reading.
Normal and low ranges, with their sources
| Figure | What the source says | Who is speaking | Source |
|---|---|---|---|
| 95 to 100% | A healthy saturation level | Asthma + Lung UK | 1 |
| Below 92% | Your healthcare professional should refer you for assessment, usually with a blood gas test | Asthma + Lung UK | 1 |
| 94 to 98% | A target for oxygen treatment in acutely ill patients not at risk of hypercapnic respiratory failure | British Thoracic Society (for clinicians) | 4 |
| 88 to 92% | A suggested target with COPD or another risk of hypercapnic respiratory failure, pending blood gas results | British Thoracic Society (for clinicians) | 4 |
| 92 to 95% | Not described in the sources read | Not stated | Not stated |
| Level for calling 999 | No percentage given; the NHS describes symptoms | NHS | 7 |
Read on 2026-10-16. The British Thoracic Society figures are targets that clinicians use when giving oxygen. They are not normal values for home use, and the BTS page notes a newer recommendation on target saturations that was not read.
Why not give a single cut-off for “too low”? The independent review of equity in medical devices recommends NICE guidance that emphasises variable accuracy in darker skin tones, and monitoring trends rather than setting absolute thresholds for action.2
How a pulse oximeter works, and what makes it wrong
A pulse oximeter shines two types of light, red and infrared, through the fingertip or earlobe and measures how much light the blood absorbs.1 That is why anything that blocks or alters the light can change the result.
- Nail varnish or false nails can block the light and affect the result. The test asks you to remove them on one finger.1
- Anaemia and Raynaud’s syndrome can also affect results.1
- Black or brown skin. Results may be less accurate because they can show a higher oxygen reading than the true one.1
- The quality of the device. Asthma + Lung UK says home-use pulse oximeters can sometimes give results that are not accurate and may be misleading, and are of lower quality than hospital ones.1
Skin tone: what the review found
The 2024 independent review of equity in medical devices found that pulse oximeters overestimate true oxygen levels in people with darker skin tones, and that the overestimate is greater at low saturation.2 It found evidence of harm in the US but no evidence from NHS studies that this differential performance had affected care, while noting that the potential for harm is clear.2
The Medicines and Healthcare products Regulatory Agency (MHRA) responded on 11 March 2024, listing a scientific review of pulse oximeter inaccuracy on different skin tones and revised guidance among its actions.3 For anyone with a home device, the review recommends looking at changes in readings rather than a single reading.2
Fingertip, wrist and ring: what the evidence says
Wrist and ring devices are not the same as a fingertip oximeter, and the evidence read for this guide is limited and specific to particular models.
| Sensor | Where | Known limits in the sources read | Source |
|---|---|---|---|
| Fingertip oximeter | Fingertip or earlobe | Nail varnish and false nails, anaemia, Raynaud’s syndrome, black or brown skin; some home models give misleading results | 1 |
| Apple Watch Series 7, in a laboratory study | Wrist | Overestimated oxygen saturation, as did a Masimo fingertip oximeter, with accuracy declining at low saturation. The error measure (Arms) below 88% saturation was 5.82% for the watch and 3.52% for the fingertip device, a gap of 2.30 percentage points. Small sample, limited skin tone diversity | 8 |
| Samsung watch and phone, in a laboratory study | Wrist; phone sensor | Root mean square deviation of 1.8% (watch) and 2.6% (phone) against arterial blood in 24 healthy people; the authors say more testing in darker skin tones is needed | 9 |
| Ring | Finger | No validation study of a ring was read for this guide | Not stated |
The two studies tested particular models under controlled conditions. They say nothing about how any other wrist band or ring performs.
The take-away is cautious. Even the fingertip device in the Apple Watch study overestimated at low saturation.8 A consumer wrist reading is a rough indication at best, and not a basis for deciding whether you are ill.
Asthma + Lung UK adds that buying a home oximeter is not recommended unless your healthcare professional has asked you to use one.1 This guide does not recommend buying one either.
Lung conditions and different target ranges
COPD is the NHS name for a group of lung conditions that cause breathing difficulties, including emphysema and chronic bronchitis.5 The British Thoracic Society suggests a lower target saturation range of 88 to 92% for people with COPD or other risk of hypercapnic respiratory failure, pending blood gas results.4
That is a target for clinicians giving oxygen in an emergency setting. It is not a figure for you to aim at, and you should not adjust any oxygen yourself. The NHS warns that buying oxygen online and using it without a prescription can be dangerous.6
If you have a lung condition, ask your healthcare team what applies to you. Asthma + Lung UK says a change in your oxygen level may mean your symptoms have got worse or a flare-up is happening.1
Reading a home oximeter sensibly, if you have been asked to use one
Only the points below come from the sources read. The pages gave no step-by-step timing for a reading, so this guide does not invent one.
- Remove nail varnish or false nails from the finger you use.1
- Keep using the device if your healthcare professional told you to, and watch for changes.1
- Look at changes in readings rather than a single reading.2
- Watch for other warning signs that the review lists: shortness of breath, cold hands and feet, chest pain and fast heart rate.2
- On brown or black skin, bluish change may be easier to see on the palms of the hands.7
For heart rate checks you can do by hand, see how to measure your resting heart rate.
When to speak to a GP, call NHS 111 or call 999
This page cannot judge your breathing. Do not wait for a reading if you are struggling to breathe. The NHS advises the following.
- Call 999 or go to A&E immediately if you have severe difficulty breathing, such as gasping, choking or being unable to get words out.7
- Call 999 if your chest feels tight or heavy, you have pain that spreads to your arms, back, neck or jaw, you feel suddenly confused, or your lips or skin turn very pale, blue or grey.7 On brown or black skin, the change may be easier to see on the palms of the hands.7
- Contact NHS 111 if you have difficulty breathing or are more short of breath than usual.7
- Make a GP appointment if your shortness of breath gets worse with normal activities, gets worse when lying down, or comes with swollen ankles.7
- If your oxygen level is below 92%, a healthcare professional should refer you for assessment.1
In a medical emergency in the United Kingdom, call 999. For urgent advice that is not an emergency, call NHS 111.
Frequently asked questions
What is a normal blood oxygen level?
Asthma + Lung UK says a healthy blood oxygen saturation level is around 95 to 100%.
Is 94 a bad oxygen level?
The sources read do not describe a reading of 94 as bad or good. Asthma + Lung UK gives 95 to 100% as healthy and says below 92% should prompt a referral for assessment. Pulse oximeters can be inaccurate, so one reading should not be over-read. Look at changes, and at how you feel.
What oxygen level is dangerous?
None of the NHS pages read gives a percentage at which to call 999. The NHS describes symptoms: severe difficulty breathing, a tight or heavy chest, or lips or skin turning very pale, blue or grey. Call 999 for those whatever a reading says.
Do smartwatches measure blood oxygen accurately?
Not reliably, on the evidence read. In a laboratory study an Apple Watch overestimated saturation, as did a Masimo fingertip device, with accuracy declining at low saturation. Another study of a Samsung watch found a deviation of 1.8% in 24 healthy people and called for testing in darker skin tones.
Can skin tone affect oximeter readings?
Yes. The 2024 independent review of equity in medical devices found pulse oximeters overestimate true oxygen levels in people with darker skin tones, more so at low saturation. Asthma + Lung UK says results on black or brown skin may be less accurate.
Should I buy a pulse oximeter for home?
Asthma + Lung UK says buying one is not recommended unless your healthcare professional has asked you to use one, as home devices can give results that are not accurate and may be misleading.
When should I call 999 about my breathing?
The NHS says to call 999 or go to A&E immediately for severe difficulty breathing, such as gasping, choking or being unable to get words out, or a tight or heavy chest, pain spreading to the arms, back, neck or jaw, sudden confusion, or lips or skin turning very pale, blue or grey. Use NHS 111 for difficulty breathing or being more short of breath than usual.
Does a low reading with COPD mean something different?
People with COPD may have a different target range, which clinicians set. The British Thoracic Society suggests 88 to 92% for clinicians treating those at risk of hypercapnic respiratory failure. Ask your healthcare team what applies to you.
Sources
- Asthma + Lung UK, Pulse oximetry (last reviewed 2 March 2026), Asthma + Lung UK. Retrieved 16 October 2026
- Department of Health and Social Care, Equity in medical devices: independent review, summary report (11 March 2024), GOV.UK. Retrieved 16 October 2026
- MHRA, Response to the Equity in medical devices independent review (11 March 2024), GOV.UK. Retrieved 16 October 2026
- British Thoracic Society, Emergency oxygen: December 2019 update to the BTS guideline, British Thoracic Society. Retrieved 16 October 2026
- NHS, Chronic obstructive pulmonary disease (COPD) (last reviewed 11 April 2023), NHS. Retrieved 16 October 2026
- NHS, Home oxygen therapy (last reviewed 2 August 2023), NHS. Retrieved 16 October 2026
- NHS, Shortness of breath (last reviewed 30 January 2024), NHS. Retrieved 16 October 2026
- Jiang Y et al., Performance of wearable pulse oximetry during controlled hypoxia induction, JMIR Form Res 2026 (abstract), PubMed. Retrieved 16 October 2026
- Browne SH et al., Evaluation of pulse oximetry accuracy in a commercial smartphone and smartwatch during human hypoxia laboratory testing, Sensors 2025 (abstract), PubMed. Retrieved 16 October 2026