The week’s wackiest Apple Watch rumor turns out to be true: Tattoos can interfere with the device’s heart rate sensor.
Apple acknowledged as much in updated language on a Web page that explains how the Apple Watch’s heart rate sensor works. The company notes a variety of factors that can affect readings, including permanent or temporary changes to the skin — such as some tattoos. The ink can impede light from the sensor, making it difficult to get reliable readings.
In these cases, Apple recommends connecting an external heart rate monitor, such as a Bluetooth chest strap, to obtain the information.
For those who are curious, skin pigmentation is not an issue. Melanin, the pigment that gives human skin and hair its color, does not block the infrared LEDs.
This article originally appeared on Recode.net.