People of color may be at risk of getting inaccurate readings from their smartwatches or fitness trackers after complaints were received about the devices’ effectiveness when worn by people with darker skin.

Most of these devices rely on green lights, which are simpler and cheaper than infrared lights used in hospital grade heart rate trackers. The short wavelengths of green lights are more readily absorbed by melanin, the natural pigments on human skin. Darker skin tends to contain more melanin, which may result in blocking green light and thus affecting the accuracy of the device.

One complaint came from an African American man who claimed the heart rate monitor on his device would not go above 140 for a sustained period while running. Others said having tattoos or body hair around the wrists also gave the device trouble reading their heart rates. If proved correct, the phenomenon could have widespread implications as an estimated 40 million people in the US have smartwatches or fitness trackers. Researchers and scientists claim more research on the issue was needed to establish the effect of melanin on green light absorption.