In recent years, smartwatches have evolved from simple step counters to sophisticated health monitors. One of the most talked-about features is the ability to measure blood oxygen saturation, often abbreviated as SpO2. But how does a tiny device on your wrist actually detect the amount of oxygen in your blood? The answer lies in a clever application of physics and sensor technology called photoplethysmography, or PPG.
At its core, a smartwatch that measures blood oxygen uses a combination of light-emitting diodes (LEDs) and photodiodes. Typically, you will find green, red, and infrared LEDs on the back of the watch. For SpO2 readings, the red and infrared lights are the most critical. The principle is based on the fact that oxygenated hemoglobin (the protein in red blood cells that carries oxygen) and deoxygenated hemoglobin absorb light differently. Oxygenated blood absorbs more infrared light and allows more red light to pass through. Conversely, deoxygenated blood absorbs more red light and allows more infrared light to pass.
When you initiate a blood oxygen measurement, usually by opening a health app on the watch, the LEDs flash rapidly. The red and infrared light beams penetrate your skin, tissue, and blood vessels. The photodiodes on the sensor then measure the amount of light that is reflected back or transmitted through. Since blood is pulsing with each heartbeat, the amount of light absorption changes in a rhythmic pattern. The sensor captures these pulsatile changes while filtering out the constant absorption from other tissues like muscle and bone. This dynamic signal is the key to isolating the blood flow component.
The raw data collected by the photodiodes is a complex waveform. The smartwatch’s processor runs an algorithm that analyzes the ratio of red light absorption to infrared light absorption at the peaks and troughs of your pulse. This ratio is then compared to a standard calibration curve that has been established through medical research. Based on this comparison, the device calculates a SpO2 percentage ranging from 0 to 100 percent. In healthy individuals, a normal reading is typically between 95% and 100%. A reading below 90% may be considered low and could indicate a medical issue such as hypoxemia.
However, it is important to recognize that smartwatches are not certified medical devices. Several factors can affect the accuracy of wrist-based SpO2 readings. For instance, movement of the arm or wrist during measurement can introduce noise into the signal. Tattoos, dark skin pigmentation, or thick hair on the wrist can block or scatter the light, leading to unreliable results. Additionally, poor contact between the sensor and the skin, perhaps because the watch band is too loose, will allow ambient light to interfere. For the best results, manufacturers recommend keeping your wrist at heart level, staying still, and ensuring the watch fits snugly but comfortably.
Most modern smartwatches, including the Apple Watch Series 6 and later, various Samsung Galaxy Watches, and Fitbit models, use these PPG sensors. Some advanced devices even combine PPG with additional algorithms to alert you if your SpO2 drops during sleep, which can be a sign of sleep apnea. While these consumer devices are excellent tools for tracking trends over time, they should not replace professional medical equipment like the finger clip oximeter used in hospitals.
In summary, your smartwatch measures blood oxygen by shining red and infrared light through your skin, detecting how much light is absorbed by the blood, and calculating the ratio through a mathematical model. This non-invasive, real-time monitoring has empowered millions of people to take a more active role in understanding their respiratory and overall health. As sensor technology continues to advance, these wrist-worn devices will only become more precise and insightful, bridging the gap between everyday convenience and medical awareness.