If you wear a smartwatch or fitness ring to track workouts, sleep or recovery, the numbers you see each morning come from a sensor reading light reflected through your skin. That's exactly where the trouble starts for millions of people. Roughly 107 million Americans now own a wearable, and research reviewed in 2025 shows that devices perform inconsistently across skin tones — some watches underestimated heart rate by 10 to 15 beats per minute in darker-skinned users during moderate to vigorous exercise, even as one major brand stayed within five bpm across the board.
The tech behind it is called photoplethysmography, or PPG. A device shines green light into the skin and measures how much bounces back as blood pulses through your vessels. The problem: melanin absorbs and scatters that same light. More melanin means more signal loss, and darker skin tones are more vulnerable to bad data. And this isn't just about workout stats. Heart rate readings can flag irregular rhythms that point toward clots, stroke or heart failure, so an inaccurate sensor has real stakes.
It's not purely about skin tone, either. Cardiologist Michael Rosenberg of the University of Colorado Anschutz told Outside that "noise" — tattoos, body hair, sweat, sunscreen residue, wrist size, even freckles — all interfere with readings.
The good news is that the big players are actively fixing this. WHOOP built an algorithm that compensates for melanin's light absorption and boosts LED brightness when the return signal weakens, accepting shorter battery life as the trade-off. Apple adjusts light transmission and signal amplification and says every Watch generation is designed so users with the darkest skin get equivalent performance, even using a spectrophotometer to measure skin colour objectively in testing. Oura points to its ring's advantage: skin on the palm side of a finger is more evenly pigmented and sits over larger arteries, producing a stronger signal, and its "smart sensing" continuously picks the strongest light path. Google says it recruits diverse testers globally for Fitbit and Pixel Watch development.
One workaront stands out for anyone who needs reliable numbers: ECG. Chest straps like Polar's H10 measure the heart's electrical signals directly through the skin, so pigmentation doesn't matter, and Polar recommends ECG wherever precision is the priority. Several Apple Watch, Polar and WHOOP models offer on-demand ECG too.
Practical steps within your control: wear the device snug and not on a bone, keep the sensor clean of sweat and lotion, position it away from tattoos, and install updates promptly. And remember these are commercial products, not medical devices — they're good at spotting overall trends, but the odd inaccurate reading is normal.
Whether you're training for a trek, tracking recovery on the road, or just relying on your watch's heart data, it's worth knowing how your device behaves on your skin — and choosing one whose maker has tested for it. Credit: Outside Online