What Is Blood Oxygen?

Blood oxygen, or SpO2, refers to the percentage of oxygen in your blood. A healthy level is generally 90% or higher, with 95% to 100% being typical for most people. Levels below 88% are concerning and require immediate medical attention.

It’s normal for blood oxygen to dip slightly during exercise due to increased oxygen demand. You may have seen athletes using oxygen masks between sessions to boost recovery when levels drop. Smartwatches with SpO2 sensors can help monitor these changes, providing insights into your health.

Measuring Blood Oxygen with Pulse Oximeters

Most smartwatches use SpO2 sensors, also known as pulse oximeters, to measure blood oxygen levels. These sensors, located on the back of the watch, align with your wrist to monitor oxygen saturation in the blood, a critical indicator of overall health.

Pulse oximeters, commonly used by medical professionals, measure the oxygen carried by haemoglobin, the protein in your blood responsible for oxygen transport. This is especially useful for those with respiratory conditions like asthma. In smartwatches, these sensors are miniaturized, using green, red, and infrared LEDs to shine light through the blood in your wrist. Photodiodes then measure the reflected light, and algorithms analyze the data to determine blood colour—darker blood indicates lower oxygen levels—displaying your SpO2 on the watch or its app.

Measuring Blood Oxygen with Reflective Pulse Oximetry

Some smartwatches use reflective pulse oximetry, which measures reflected light rather than light passing through the body. This allows blood oxygen levels to be assessed from areas like the wrist or chest. Light emitted by the watch’s sensor is reflected by subsurface tissue or bone back to the sensor, which calculates the difference between emitted and reflected light to determine SpO2 levels.

Why Use Blood Oxygen Sensors on Your Smartwatch?

Understanding how smartwatches measure blood oxygen is one thing, but what are the practical benefits? Here’s how SpO2 sensors can enhance your wellness:

• Overall Wellbeing: Monitor your general health and fitness, especially during exercise when SpO2 levels may drop.
• Sleep Apnoea: Detect signs of sleep apnoea, a condition causing shallow breathing or pauses during sleep, which can lead to daytime fatigue.
• Respiratory Conditions: Help manage conditions like asthma by indicating when an inhaler may be needed.
• High-Altitude Activities: Track how your body adapts to high altitudes during activities like hiking or mountaineering.

Limitations of Blood Oxygen Measurements

While SpO2 sensors on smartwatches are useful, they have limitations. Wrist-based measurements are generally less accurate than fingertip pulse oximeters, which shine light through the entire finger. Smartwatch sensors rely on light reflection, a less precise method.

These measurements are intended for general wellness, not medical diagnosis. Don’t rely solely on your smartwatch’s readings for health decisions. They provide a baseline, but if your SpO2 drops significantly below normal, consult a doctor and consider verifying with a fingertip pulse oximeter.

How to Measure Blood Oxygen on Different Smartwatches

Not all smartwatches measure blood oxygen the same way. Below are step-by-step guides for popular brands:

Fitbit: Installing an SpO2 Watch Face

Fitbit users can display SpO2 levels directly on the watch face with these steps:

1. Tap the Today tab in the Fitbit app.
2. Go to your profile.
3. Tap the device image for the watch you want to update.
4. Tap Gallery, then the Clocks tab.
5. Find the SpO2 watch face (tap See All for the full list).
6. Select the SpO2 watch face and tap Install.

Garmin: Viewing Sleep Pulse Ox Stats

To check blood oxygen levels during sleep on a Garmin smartwatch, follow these steps:

1. Open the Garmin Connect app.
2. On iOS, tap More; on Android, tap the three-bar icon.
3. Tap Health Stats, then Sleep.
4. Select Pulse Ox from the Stages, Pulse Ox, and Respiration options to view sleep data.

Frequently Asked Questions

How Does a Smartwatch Measure Blood Oxygen?

Smartwatches typically use a pulse oximeter on the back, with colored LEDs that shine through your wrist’s blood vessels. Photodiodes measure reflected light, and algorithms calculate your SpO2 level based on blood color.

How Does a Smartwatch Measure Oxygen Saturation?

Using SpO2 sensors, smartwatches emit light through your wrist and measure reflected light with photodiodes. Algorithms then estimate oxygen saturation by analyzing light absorption.

How Reliable Are Smartwatch Blood Oxygen Measurements?

Smartwatch SpO2 measurements are reasonably accurate for general wellness but less precise than fingertip pulse oximeters. Use them to monitor trends, not for medical diagnoses.

Conclusion

Smartwatches measure blood oxygen using pulse oximeters or reflective pulse oximetry, with LEDs and photodiodes analyzing light reflected from your wrist’s blood vessels to calculate SpO2 levels. While useful for tracking general health, sleep apnoea, respiratory conditions, or high-altitude activities, these measurements are not medical-grade. Always consult a doctor for significant health concerns.