Fridge magnets and decorative jewellery could be a killer if you have a weak heart, experts warn.
Noedymium magnets are very strong
A strong type of magnet used in many new commercial products can interfere with pacemakers and implanted heart devices with deadly consequences.
Close contact - within about 3cm - with a neodymium magnet is enough to destabilise these life-saving heart devices, Heart Rhythm journal reports.
The authors suggest manufacturers include a health warning on products.
Ordinary iron or ferrite magnets, which are a dull grey colour with a low magnetic strength, are of little concern.
Very strong magnets made from neodymium-iron-boron, which are shiny and silver in colour, have only recently become available.
But because of their high magnetic field strength and low production costs, they are being used in computer hard drives, headphones and hi-fi speakers, as well as toys, jewellery and even clothes.
Swiss researchers at the University Hospital of Zurich tested the effect of neodymium magnets in 70 heart patients - 41 with pacemakers and 29 with implantable cardioverter defibrillators.
The small 8g magnets tested interfered with all of the patients' devices, regardless of their make or type, when they were in a maximum range of 3cm.
The researchers said larger neodymium magnets would be likely to cause interference at greater distances than this.
Although the devices worked normally again once the magnet was removed, the authors warned permanent damage might occur with prolonged exposure - if someone were to wear a magnetic name badge, for example.
Lead researcher Thomas Wolber said: "Physicians should caution patients about the risks associated with these magnets.
"We also recommend that the packaging includes information on the potential risks."
Ian Asquith, director of Neodymium Magnets UK, a supplier of neodymium magnets to product manufacturers, said his company was aware of the risks and sends warnings out with every magnet.
But consumers were generally unaware, he said, and manufacturers had a responsibility to warn consumers of any risks.
"There is a real danger. These magnets are everywhere. They are in lots of badges, fridge magnets and mobile phones. If you were on a busy bus and someone you are squashed up against had a magnet in their top pocket you could easily come within 3cm of it.
June Davison, of the British Heart Foundation, said anyone with concerns should contact their pacemaker clinic.
"Pacemakers are manufactured to the highest standards, are rigorously tested, and most have a protective case to shield them from outside interference. Problems are rare."