Apple is officially warning inked Apple Watch users that they might have trouble getting reliable heart rate readings.
"Permanent or temporary changes to your skin, such as some tattoos, can also impact heart rate sensor performance," the document now reads. "The ink, pattern, and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings."
During workouts, the Apple Watch's heart rate monitor uses a method called photoplethysmography (or PPG), which shines green light onto the skin and looks for the change in absorption that comes with each pulse. During regular activity, the Watch also measures infrared light reflection, which presumably uses less power than the green LEDs. If a wrist has too much heavy ink, it makes sense that the Watch would have trouble shining a light through the skin.
Keep in mind that not all tattoos will run into problems. Tests of several tattoos by iMore found that solid red and black ink caused the biggest issues, with wildly inaccurate readings and complete failures. Lighter colors produced only slight inaccuracies, and patterned or variegated ink didn't seem to cause any problems.
Why this matters: Fitness tracking is one of the Apple Watch's main features, and a recent test by Consumer Reports found the heart rate monitor to be just as accurate as the group's highest-rated chest-worn monitor. There plenty of other reasons to enjoy the Apple Watch, but users with serious amounts of ink should consider themselves warned.