I’m posting this in the event that this information will be helpful to someone. I’m not qualified to ascertain the validity of the concern.
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But rather one warning too much, than too little!
The pacemakers with which I am familiar (my own, for example) do not behave this way. Instead, the magnet causes the pacemaker to pace at a specific rate and is one way cardiologists estimate the remaining lifetime of the battery. That being said, pacemaker patients are routinely cautioned to avoid strong magnetic fields such as welding equipment, MRI installations, etc (although pacemakers slightly newer than mine are “MR compatible”).
The FDA has since sort of debunked the dramatic aspect of that study and confirmed Apple’s recommendation (if you have a pacemaker, you should stay away from magnets anyway, generally speaking).
The upshot is that anything with a strong magnet should be kept at a distance from implanted pacemakers and defibrillators. As @jec0047 has noted, for the most part placing a magnet over a pacemaker will cause it to change to an “asynchronous” mode in which it will pace but will not sense intrinsic heart electrical activity. While the risk of this is low, it is not zero and so this should not be done outside of the appropriate medical circumstances. It is also correct that pacemakers will, in response to a magnet application, pace at a rate that gives information about battery longevity. Also, removing the magnet will cause the pacemaker to revert to its normally programmed operation.
The situation is different for implanted defibrillators. For most, magnet application will result in deactivation of the defibrillator function. If the patient should sustain a cardiac arrest rhythm at that time, the ICD would not function. Some defibrillators can have this “magnet mode” programmed off. Again, removing the magnet causes the defibrillator to revert to its programmed settings. However, for some defibrillators, they can be set such that application of a magnet for a certain length of time (eg 30 seconds) will switch the defibrillator from an active to a disabled mode. Reapplication of the magnet will reverse this switch. As a result, if the magnet is applied for a certain length of time the defibrillator could be deactivated and left off when the magnet is removed. Again, a magnet should not be applied to a defibrillator outside of an appreciate medical situation / with the advice of a qualified practitioner.
It is also true that some pacemakers and defibrillators are now labelled as “MRI-contingent” which means that patients with these devices can under MRI scanning. There are specific requirements for the scanner (eg field strength), programming of the pacemaker/defibrillator before and after the scan, etc that must be followed for this to be done with a proper safety margin. Note that the MRI poses risk both because of the static magnetic field and because of the high frequency RF field.
The risk applies to iPhones with suitably strong magnets, but is generally a low risk. However, cell phones should never be placed in close proximity to these devices, such as in a shirt pocket over the implantation site. Similar issues can arise with other magnets, such as the magnets in speakers, for example.
Welding is another issue. Arc welding is a problem again because of the RF field created by the arc. Acetyline torch welding is not an issue.
Good to know, thanks @nlippman.