Everything you need to know about Apple Watch blood oxygen (SpO2) monitoring: how the sensor works, real-world accuracy based on clinical studies, the Masimo patent timeline, and tips for getting reliable readings.
Blood oxygen monitoring is one of the most talked-about health features on the Apple Watch. But how does it work, how accurate is it, and what can it actually tell you about your health? This comprehensive guide explains everything you need to know about the Apple Watch Blood Oxygen (SpO2) feature in 2026, including the latest on accuracy research, the Masimo patent saga, and practical tips for getting the most from this sensor.
This article is part of our complete Apple Watch health features guide, which covers every health sensor and feature available on Apple Watch in 2026.
What Is Blood Oxygen Saturation (SpO2)?
Blood oxygen saturation, commonly referred to as SpO2, measures the percentage of hemoglobin molecules in your red blood cells that are carrying oxygen. Hemoglobin is the protein in your blood that binds to oxygen in the lungs and transports it throughout your body.
A healthy blood oxygen level typically falls between 95% and 100%. Levels below 95% may indicate that your body is not getting enough oxygen, a condition known as hypoxemia. Persistent readings below 90% are considered a medical concern and may require immediate attention.
Why Blood Oxygen Matters
Your blood oxygen level is a key indicator of respiratory and cardiovascular health. Low SpO2 can be a sign of:
- Respiratory conditions: Asthma, COPD, pneumonia, or COVID-19 can impair the lungs' ability to absorb oxygen.
- Sleep disorders: Sleep apnea can cause repeated drops in blood oxygen during the night.
- Heart conditions: Some cardiac issues can reduce the efficiency of oxygen circulation.
- High altitude: Reduced atmospheric oxygen at high elevations naturally lowers SpO2.
- Anemia: Low hemoglobin levels reduce the blood's oxygen-carrying capacity.
How the Apple Watch Blood Oxygen Sensor Works
The Apple Watch uses a technology called reflectance pulse oximetry. Traditional pulse oximeters, like the fingertip clip devices used in hospitals, use transmissive pulse oximetry, where light passes through the finger and is measured on the other side. The Apple Watch cannot do this on the wrist, so it uses a different approach.
The Technology
On the back of the Apple Watch, clusters of green, red, and infrared LEDs shine light onto the blood vessels in your wrist. Photodiodes then measure how much light is reflected back. Oxygenated hemoglobin (carrying oxygen) absorbs light differently than deoxygenated hemoglobin (without oxygen). By comparing the absorption ratios at different wavelengths, the watch calculates your blood oxygen percentage. For a deep dive into real-world accuracy numbers, see our 2026 SpO2 accuracy review with clinical data.
This reflectance method is inherently more challenging than transmissive pulse oximetry because the changes in light caused by varying SpO2 levels produce smaller and less stable signals at the wrist. This is why the Apple Watch Blood Oxygen feature is classified as a wellness tool rather than a medical device.
On-Demand vs. Background Readings
The Apple Watch takes blood oxygen measurements in two ways:
- On-demand readings: You open the Blood Oxygen app, hold still for 15 seconds, and receive a single reading. This gives you a snapshot of your SpO2 at that moment.
- Background readings: Throughout the day and night, when you are still, the Apple Watch periodically takes automatic SpO2 measurements. These are stored in the Health app and can reveal trends over time, especially during sleep.
How to Take a Blood Oxygen Reading
- Open the Blood Oxygen app on your Apple Watch.
- Make sure your Apple Watch is snug on your wrist, not too tight and not too loose. The sensors need good skin contact.
- Rest your arm on a flat surface like a table, with the watch face pointing up.
- Keep your wrist and hand completely still.
- Tap Start and remain motionless for 15 seconds.
- View your SpO2 result on the screen.
Tips for Accurate Readings
- Stay completely still. Even small movements can interfere with the optical sensor.
- Ensure a good fit. The watch should be snug against your skin without gaps. Loose bands or tattoos on the wrist can affect readings.
- Avoid cold environments. Cold can constrict blood vessels in your wrist, reducing signal quality.
- Take multiple readings. If a result seems unusual, take two or three readings a minute apart and look at the average.
- Check the band position. The watch should sit about a finger's width above your wrist bone for optimal sensor contact.
How Accurate Is Apple Watch Blood Oxygen?
The accuracy of the Apple Watch SpO2 sensor has been studied extensively. The results are encouraging but come with important caveats.
Clinical Study Results
A systematic review published in the journal Sensors analyzed multiple studies comparing the Apple Watch to medical-grade pulse oximeters. Key findings include:
- The average absolute difference between Apple Watch and clinical oximeters was less than 1% SpO2 in controlled conditions, which is comparable to many FDA-cleared fingertip pulse oximeters.
- The 95% limits of agreement ranged from plus or minus 2.7% to plus or minus 5.9% SpO2 across different studies. This is within the typical plus or minus 4% to 6% accuracy range for consumer and many clinical pulse oximeters.
- One validation study found that the Apple Watch successfully detected short-term hypoxemia (low oxygen) events as well as a standard medical-grade device.
Limitations of Accuracy
- Normal range bias: The Apple Watch performs best when SpO2 is in the normal range (95% to 100%). At lower saturation levels below 90%, accuracy decreases and readings become less reliable.
- Skin pigmentation: Studies have shown that reflectance pulse oximeters, including the Apple Watch, may overestimate SpO2 in individuals with darker skin tones, particularly at lower saturation levels. This is a known limitation of optical pulse oximetry technology generally, not just Apple Watch.
- Outlier readings: Research published in npj Digital Medicine found that the Apple Watch can occasionally produce outlier readings that are significantly different from actual SpO2. These outliers are more common during movement or poor sensor contact.
- Not medical grade: Apple explicitly states that the Blood Oxygen feature is not intended for medical use, including self-diagnosis or consultation with a doctor. It is a wellness feature designed to provide general fitness and wellness information.
The Masimo Patent Saga
The Blood Oxygen feature on Apple Watch has been at the center of one of the most significant patent disputes in consumer technology. Understanding this history is important because it has directly affected feature availability for millions of users.
Timeline of Events
- 2020: Apple introduces the Blood Oxygen sensor with Apple Watch Series 6.
- 2021: Masimo Corporation, a leading medical device maker specializing in pulse oximetry, sues Apple for patent infringement, claiming Apple hired away Masimo engineers and copied their technology.
- December 2023: Following an International Trade Commission (ITC) ruling, Apple Watch models with the Blood Oxygen feature are temporarily banned from U.S. sales.
- January 2024: The ban is reinstated. Apple begins selling Apple Watch models in the U.S. with the Blood Oxygen feature disabled.
- August 2025: Apple restores the Blood Oxygen feature in the U.S. through a redesigned system that moves SpO2 data processing from the watch to the paired iPhone, working around the ITC import ban.
- November 2025: A federal jury awards Masimo million in damages, covering 43 million Apple Watch units sold with the infringing feature. Apple announces plans to appeal.
- 2026: The ITC opens a new investigation to determine whether Apple's redesigned workaround still infringes. The Blood Oxygen feature remains available on current Apple Watch models in the U.S. as of February 2026, but future availability may depend on the outcome of ongoing legal proceedings.
What This Means for Users
If you currently own an Apple Watch with the Blood Oxygen feature enabled, it will continue to work. However, future software updates or legal rulings could potentially affect the feature. Apple has demonstrated a commitment to keeping the feature available and has invested in engineering workarounds to maintain it for users.
Blood Oxygen During Sleep
One of the most valuable uses of the Apple Watch Blood Oxygen sensor is monitoring SpO2 trends during sleep. When you wear your Apple Watch to bed, it takes periodic background measurements throughout the night.
What Sleep SpO2 Data Can Reveal
- Sleep apnea indicators: Repeated drops in blood oxygen during sleep can be a sign of obstructive sleep apnea. While the separate Sleep Apnea Detection feature uses the accelerometer rather than the SpO2 sensor, blood oxygen data provides additional context.
- Respiratory health: A gradual decline in overnight SpO2 trends could indicate worsening respiratory conditions.
- Altitude effects: If you travel to high altitude, you may notice your overnight SpO2 is lower than usual, which is a normal physiological response.
How to View Sleep Blood Oxygen Data
- Open the Health app on your iPhone.
- Tap Browse, then tap Respiratory.
- Tap Blood Oxygen.
- View your recent readings and trends. You can filter by time period to see overnight data specifically.
Which Apple Watch Models Support Blood Oxygen?
| Model | Blood Oxygen Support | Notes |
|---|
| Apple Watch Series 6 | Yes | First model with SpO2 sensor |
| Apple Watch Series 7 | Yes | |
| Apple Watch Series 8 | Yes | |
| Apple Watch Series 9 | Yes | Processing moved to iPhone in U.S. |
| Apple Watch Series 10 | Yes | Processing moved to iPhone in U.S. |
| Apple Watch Series 11 | Yes | Processing moved to iPhone in U.S. |
| Apple Watch Ultra | Yes | |
| Apple Watch Ultra 2 | Yes | Processing moved to iPhone in U.S. |
| Apple Watch Ultra 3 | Yes | Processing moved to iPhone in U.S. |
| Apple Watch SE (any) | No | No SpO2 sensor |
Apple Watch Blood Oxygen vs. Fingertip Pulse Oximeters
| Feature | Apple Watch | Fingertip Pulse Oximeter |
|---|
| Measurement Method | Reflectance (wrist) | Transmissive (finger) |
| Accuracy (typical) | +/- 3-6% SpO2 | +/- 2-3% SpO2 |
| Continuous Monitoring | Yes (background) | No (spot check only) |
| Sleep Tracking | Yes | No |
| Medical Grade | No (wellness feature) | Many are FDA-cleared |
| Price | Included with watch | standalone |
| Convenience | Always on your wrist | Must carry separately |
When to See a Doctor
While the Apple Watch Blood Oxygen feature is a wellness tool, certain patterns in your data should prompt a conversation with your healthcare provider:
- Consistently low readings below 95% when you are at rest at sea level.
- A noticeable downward trend in your average SpO2 over days or weeks.
- Frequent drops below 90% during sleep, which may indicate sleep apnea or other respiratory conditions.
- SpO2 readings that do not recover to normal levels after exercise.
If you experience sudden shortness of breath, chest pain, confusion, or blue discoloration of your lips or fingertips, seek emergency medical care immediately. Do not rely on your Apple Watch for emergency medical decisions.
Frequently Asked Questions
Is the Apple Watch Blood Oxygen feature a medical device?
No. Apple classifies the Blood Oxygen feature as a general wellness feature, not a medical device. It is not intended for medical use, self-diagnosis, or consultation with a doctor. For medical-grade pulse oximetry, use an FDA-cleared fingertip pulse oximeter.
Why does my Apple Watch sometimes fail to get a blood oxygen reading?
Failed readings can occur due to movement during the measurement, a loose watch band, tattoos or skin markings on the wrist, poor blood circulation in the extremities, or very cold conditions. Try adjusting your band, staying still, and warming your wrist before retrying.
Can the Apple Watch detect COVID-19 through blood oxygen?
No. While low blood oxygen can be a symptom of severe COVID-19, the Apple Watch cannot diagnose COVID-19 or any other illness. A low SpO2 reading has many possible causes. If you are concerned about COVID-19, get tested through proper medical channels.
Related: Apple Watch Series 11 health sensors tested and Apple Watch ECG accuracy and doctor insights.
Is the Blood Oxygen feature available worldwide?
The Blood Oxygen feature is available in most countries, though availability can vary due to regional regulations. The most significant disruption has been in the United States due to the Masimo patent dispute. As of February 2026, the feature is available in the U.S. through Apple's redesigned processing approach.
Does skin color affect Apple Watch blood oxygen accuracy?
Studies have shown that reflectance pulse oximeters, including the Apple Watch, may overestimate SpO2 in individuals with darker skin tones, particularly at lower saturation levels. This is a known limitation of optical pulse oximetry technology in general. If you have concerns about accuracy, consider using a medical-grade fingertip pulse oximeter for comparison.
Can I use the Blood Oxygen feature during exercise?
The on-demand Blood Oxygen app requires you to stay still for 15 seconds, so it is not designed for use during active exercise. However, background readings are taken periodically when the watch detects you are still, and workout-related SpO2 data may be captured during rest periods.
What is the difference between Blood Oxygen and the Respiratory Rate feature?
Blood Oxygen (SpO2) measures the percentage of oxygen in your blood. Respiratory rate measures how many breaths you take per minute during sleep. Both are tracked by the Apple Watch and displayed in the Health app, but they measure different aspects of respiratory health. Together, they provide a more complete picture of how well your body is oxygenating during rest.
Read more: Apple Watch Health Features Complete Guide 2026 | Apple Watch Series 11 Health Sensors Tested | Apple Watch ECG: Accuracy and Doctor Insights
