Like a number of other wearables, the Apple Watch comes with a heart rate sensor. It can measure heart rate at a specific time (like during a workout) and also at a regular interval and later present that information through the Apple Health app. Reports began to surface earlier this week that tattoos could interfere with the sensor, resulting in incorrect readings. Apple has now confirmed that this is indeed the case.
Permanent or temporary changes to your skin, such as some tattoos, can also impact heart rate sensor performance. The ink, pattern, and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings.
The Apple Watch heart rate sensor uses a technology called photoplethysmography. The sensor flashes its green LED lights hundreds of times per second. As blood absorbs green light, the more blood is flowing through your wrist at any one time, the less of the light will be reflected back to light-sensitive photodiodes. Essentially, the less light is bouncing back, the higher your heart rate is. Dark tattoo inks can block light from reaching blood vessels and capillaries, affecting the sensor’s performance.
Apple suggests that you connect your Apple Watch to an external heart monitor such as a Bluetooth chest strap if you find that your tattoos are preventing the heart rate sensor from giving you reliable data.
It should also be pointed out that this issue is not limited to the Apple Watch. For example, the Fitbit Charge HR uses the same technology and users will be susceptible to the same problem.