Vernon Ross is a podcasting consultant from St. Louis, Missouri. He has been using a Fitbit Charge HR since he was diagnosed with mild high blood pressure in 2017. As an African American, Ross soon realized his heart rate tracker not only produces fluctuating readings on a regular basis but these readings also do not match the measurements he took via traditional ways.
“Depending on what I would do, my heart rate would jump way off the charts and then fall back down,” he told STAT. “That kind of stuff can cause quite a bit of stress”. Ross later wrote on Fitbit’s website to express his concern as he had heard similar experiences from people around him. They all wonder if the company had tested their products on individuals with different skin tone.
Indeed, Ross is not the only one. There had been some online complaints since the start of the year, questioning whether smart devices would yield inaccurate heart rate readings on individuals with darker skin. The potential inaccuracy may have been a result of an optical technique known as photoplethysmography (PPG) on the heart rate sensors, which is employed to estimate artery volume using light.
The source of inaccuracy
The PPG sensor on a wearable would emit light onto one’s skin. In most incidences, the light that passes through the skin will be absorbed by the body tissues, while some is being reflected. The volume of arteries near the skin surface is one of the factors which determine how much light will be reflected. As arteries tighten and relax in accordance to one’s pulse, the amount of light being reflected will also synchronize that variation, this enables a wearable to estimate its user’s heart rate.
Most wearables, including Fitbits, Samsung smartwatches, and other brands, rely on a green light for their PPG sensors. These green lights are relatively cheaper as compared to infrared lights use in hospital-grade heart rate trackers. However, a downside is the green light has a shorter wavelength so it is more readily absorbed by melanin, a natural skin pigment that is more prevailing on individuals with darker skin tone. Melanin may have hindered the skin’s absorption of green light, thus resulting inaccuracies.
As Tim Collins, Senior Lecturer in Electronic Engineering, Manchester Metropolitan University and Sandra Woolley, Senior Lecturer, Software and Systems Engineering Research, Keele University wrote on The Conversation, blue light has the worst body penetration. Red and infrared light, on the other hand, could easily pass through one’s skin, this means it could easily pick up reflections that are not from the arteries but muscles or tendons. As such, red light PPG sensors may be prone to interference.
Responses from wearable makers
Although green light is inferior to red and infrared light, it is less susceptible to interference and less sensitive to motion errors. Since most wearables are designed with users on the move in mind, perhaps it makes more sense to use green light PPG sensors after all. At the same time, both Collins and Woolley asserted, it remains unclear if darker skin individuals would benefit from a PPG sensor that uses a different light source.
Fitbit representative told STAT that the company uses additional power to boost the green light PPG sensor, to overcome any possible inaccuracy. Meanwhile, Apple’s reply was while their smartwatches use green light for continuous monitoring, there is also the presence of a separate reading taken by infrared light once every five minutes.
This is not a widely researched topic; thus, most scientists are unsure whether the inaccuracies were truly a result of the variations of skin tones or the possibilities of other unknown factors. Nevertheless, the important lessons are: research and improvement of a product should not stop after it has been released to the market. Likewise, there is always a need to take into consideration of a comprehensive group of individuals when developing new technologies.