The new magnetic circular array introduced in iPhone 12 smartphones last year to support the MagSafe charging technology can disrupt implantable cardioverter-defibrillator (ICD) medical devices.
The warning comes from three cardiac electrophysiology doctors from the Henry Ford Heart and Vascular Institute at the Henry Ford Hospital in Detroit, Michigan.
In a letter published in a medical journal [PDF] last month, doctors warned that the new iPhone magnets could potentially "inhibit lifesaving therapy in a patient, particularly when the phone is carried in an upper chest pocket."
These magnets, arranged in a circle, play a role in aligning the iPhone with a MagSafe charger for wireless charging operations.
Research published in 2009 has previously shown that any type of magnet, radio, or electronic equipment that generates a magnetic field stronger than 10 gauss can trigger internal systems inside ICD devices and stop their operations.
The Henry Ford Hospital doctors said they carried out tests with the new iPhone 12, released last year, and found that the new magnets are strong enough to trigger these switches.
"Once the iPhone was brought close to the ICD over the left chest area, immediate suspension of ICD therapies was noted and persisted for the duration of the test (Figure 1). This result was reproduced multiple times with different positions of the phone over the pocket," the doctors said.
"Contemporary studies [1, 2] have shown minimal risk of electromagnetic interference from ICDs and older-generation smartphones not having a magnetic array."
The new warning comes to supersede an Apple support page published last year on the same topic.
In that page, Apple estimated that even if iPhone 12 models contained more magnets, the new models were "not expected to pose a greater risk of magnetic interference to medical devices than prior iPhone models."
The tech giant did advise users of implanted pacemakers and defibrillators that in order to "avoid any potential interactions," they should keep their iPhones and MagSafe chargers at a safe distance from their implants of more than 12 inches (30 cm).
Furthermore, Apple said that if users suspected that their iPhone or any MagSafe accessories are interfering with their medical devices, they should stop using their iPhone or MagSafe accessories right away.