It goes without saying that the performance of a key feature in wearables — heart rate measurement — should be uniformly accurate across users regardless of their ethnicity. But a report from Stat suggests that some of the most popular wearables, including models sold by Fitbit, Samsung, and others, are inaccurately measuring the heart rates of people of color, an issue attributable to reliance on green lights that are more easily absorbed by darker skin.

Stat’s core claim is that most wearables use green LEDs to optically measure heart rates, a decision that generally has merit — and can work properly across a range of skin colors if the LEDs are strong enough, and/or complemented by another type of heart sensor. But in some cases, the green LEDs are both underpowered and unassisted, leading to fluctuations and inaccuracies for users with darker skin.

The potential for skin-related heart rate inaccuracies in some wearables was scientifically established in a 2017 Journal of Personalized Medicine study that found greater and statistically significant error rates in the Fitbit Surge, Samsung Gear, and second-generation Basis Peak with darker skin-toned users. By comparison, the study found that the then-current Apple Watch had the lowest overall error in heart rate measurement, likely due to its use of both bright green and infrared heart sensors.

While the report points a finger in the general direction of wearables made by Fitbit, Samsung, Garmin, and Basis, it suggests that the issues are model-specific, and in some cases may have been addressed in more recent devices. For instance, the report leads with a description of how heart rate measurements from Fitbit’s Charge HR appeared to be inaccurate for one user of color and his friends but notes that Fitbit currently says it’s using “green light at sufficient strength” to enable “the optimum, most consistent performance for users of all skin tones.” The latest Apple Watches — and some new competitors — are adding ECG measurement abilities, which will improve accuracy.

Users with darker skin can reduce the risk of inaccurate heart rate measurement by selecting newer wearables from reputable companies and avoiding those that rely exclusively on cheap, commodity green sensors. Short of ECGs, some companies use amber, red, or infrared lights as backups to their green lights, which are generally considered best for measurement while the user is in motion. In any case, users shouldn’t consider the current state of wearable technology to be truly clinical grade and should check in with their doctors if they notice unusual heart rate fluctuations.

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