How the Apple Watch measures heart rate, and why it (hopefully) won't give you a rash

By Jonah Comstock
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watch-heart-rate-monitor-heroJust a few days before the first units start shipping, Apple has posted some support documents that pull back the curtain on some of the technical details of its heart rate monitor, and also give some insight into steps Apple has taken to head off potential skin irritation issues. Apple blog 9to5Mac first spotted the two new pages.

According to the new heart rate page, Apple Watch monitors the user's heart rate every 10 minutes and stores the data in the Health app. This data is stored as it's own metric, but also used in calculating workout intensity and calories burned. The page also explains the specific method the Watch uses to track heart rate, based on green LEDs.

"The heart rate sensor in Apple Watch uses what is known as photoplethysmography," the page explains. "This technology, while difficult to pronounce, is based on a very simple fact: Blood is red because it reflects red light and absorbs green light. Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. When your heart beats, the blood flow in your wrist — and the green light absorption — is greater. Between beats, it’s less. By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute — your heart rate."

An infrared sensor in the Watch can also act as a back-up to the LEDs, and is used for the routine every-10-minute heart rate check. The page also warns that a number of factors can hamper the iPhone's heart rate readings: wearing the band too loosely, exceptionally cold weather, and irregular movements like tennis or boxing can all create inaccurate readings. In that case, the Apple Watch can be connected to a Bluetooth-enabled chest strap heart monitor.

The other page, "Wearing Apple Watch", preemptively addresses concerns about the watch itself. On this page, Apple details the materials in the watch that could create allergy issues -- nickel and methacrylates -- and stresses that steps were taken to make sure there would be no skin irritations with the device.

"Every material that touches your skin has gone through extensive evaluation in accordance with our specification," the page says, "This includes: thousands of material composition tests, more than a thousand prototypes worn for trial studies, hundreds of toxicological assessments, and consultations with board-certified dermatologists."

Though there's no way to say for sure if Apple was inspired by Fitbit's problems with the Fitbit Force, which prompted a voluntary recall for causing rashes in a number of users, it seems likely the company was watching the activity tracker space for potential pitfalls during the development process. Apple has even gone as far as to publish a separate page detailing it's internal specifications for wearables and listing out seven pages of restricted chemicals the company won't use.

Finally, a third page details the accessibility features of the watch, including voiceover and large print options for the visually impaired and taptic feedback for those with impaired hearing.