Research has demonstrated that commercially available smartphones can theoretically interfere with pacemakers or implanted defibrillators.
A study by the U.S. Food and Drug Administration demonstrated that the Apple Watch 6, marketed as a "lifesaver", is theoretically powerful enough to deactivate a pacemaker or an implanted defibrillator. The reason for this is its built-in, very strong magnet. The iPhone 12 also features a similarly strong magnet; the magnetic fields exceed 10 Gauss in the immediate vicinity.
It is recommended to keep the devices at least 15 cm away from implants. In practice, however, no fatal incident has fortunately occurred.
Based on current knowledge, the magnets in the Apple Watch Series 6 and iPhone 12 pose no health risk to individuals without relevant implants. The magnetic strength is far below the level that would be required to cause physical harm or significantly affect the brain, for example.
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Sources
1 cf. Magazine Welt der Wunder in Article: "Können Smartphones töten?", Edition 11/21, S. 89 (German)
2 cf. National Libary of Medicine in online Article: "Static magnetic field measurements of smart phones and watches and applicability to triggering magnet modes in implantable pacemakers and implantable cardioverter-defibrillators" as of Oktober 18. 2021
pubmed.ncbi.nlm.nih.gov/34600610/