Researchers investigated the effects of MP3 player headphones, most of which contain the magnetic substance neodymium, on the operation of implanted cardiac devices (abstract P662).

It should be common sense that electronic devices can cause interference with other electronic devices (remember the old pacemaker / microwave problem?). I wrote earlier about RFID tags interfering with medical devices, a study released on Sunday shows that headphones might throw off pacemakers and implantable cardioverter defibrillators (ICDs) when placed within an inch of the medical devices.