Photoplethysmography in Diverse Skin Tones: Evaluating Bias in Smartwatch Health Monitoring

Heart disease remains one of the most pressing global health burdens, and the growth of wearable technology has introduced new possibilities for prevention and monitoring. Smartwatches commonly use photoplethysmography (PPG) to estimate heart rate by detecting changes in light reflected from the skin. However, melanin's absorption of green light raises concerns about measurement accuracy in individuals with darker skin tones. This narrative review, conducted in accordance with the PRISMA extension for narrative reviews, examined studies published between May 2017 and May 2025. Searches were performed in PubMed, Google Scholar, and the American College of Cardiology (ACC) databases, yielding 50 records, of which 23 met the inclusion criteria. Only English-language studies were included, and duplicates were removed through automated and manual screening. Findings demonstrated significant variability across devices. One validation study of 60 participants reported that a major smartwatch brand showed mean heart rate differences of less than 5 bpm across skin tones (95% CI: -3.2 to +4.1). By contrast, other brands underestimated heart rate by 10-15 bpm at rest and by more than 20% during vigorous activity in darker-skinned users (n = 75, p < 0.01). Devices using alternate operating systems displayed inconsistent calibration, with error rates reaching up to 12%. Large-scale studies involving over 400,000 participants reported greater than 95% sensitivity for atrial fibrillation detection, though stratification by skin tone was not conducted. Skin-tone-related bias in smartwatch PPG sensors poses risks that extend beyond fitness tracking to arrhythmia detection, hypertension monitoring, and telehealth reliability. To improve equity, standardized validation protocols stratified by Fitzpatrick skin type and activity level are essential, alongside regulatory mandates for inclusive testing. Future research should include longitudinal studies across diverse cohorts to ensure wearable health technologies become reliable tools for all populations.