The operation of a pacemaker can be affected by the proximity of the magnets, as they cause the pacemaker to operate in a mode that does not respond to the user's own heart rhythm. The way pacemakers respond to magnetic fields varies by manufacturer, so people with pacemakers should not hold strong magnets close to their chests.
Will strong magnets damage my smartphone or tablet?
Small to medium size strong magnets will not have any adverse effect on your smartphone or tablet. These smart devices likely already contain small magnets that enable them to perform certain functions. However, using large, powerful magnets should be kept away from any electronic equipment, as strong magnetic fields can damage mechanical parts.
Can a strong magnet damage a watch?
If the tiny parts of a mechanical watch are made of ferrous materials, they may become magnetized when they are near a strong magnetic field. If the mechanical ferrous metal parts are magnetized, they will attract each other and into the inside of the case, causing the watch to run fast or slow or stop working altogether.
A lot of watches are now made of "non-magnetic" materials, so they can resist fairly weak magnetic fields. For safety, you should always keep your mechanical watch away from strong magnetic fields. If your watch does become magnetized, a watch repair shop should be able to demagnetize it and restore it to normal operating condition.