As powerful neodymium magnets are finding their way into more and more products, we're often asked about pacemaker safety. How close must a magnet be to a pacemaker to affect its operation? If you're using a particular magnet in your product, do you need to be concerned?
We have always been cautious about making broad statements about how pacemakers might behave in strong magnetic fields. Our FAQ has always deferred to medical professionals:
What is a safe distance to keep magnets away from pacemakers? We are not medical professionals, so we cannot offer guidance on pacemaker safety. Please consult a physician for this information.
While this is a safe statement that keeps our lawyers happy, it does little to help an entrepreneur or engineer designing a product that includes neodymium magnets. We understand how strong our magnets are, but understanding the magnetic susceptibility of every pacemaker and ICD (implantable cardioverter defibrillator) ever implanted seemed too daunting a task. Lately, we've been wondering if there's a rough answer that might help folks using our magnets.
We've found a few sources that cite 10 gauss as the maximum magnetic field in which a pacemaker will operate properly. Sources include this report on pacemakers and music players from Boston Scientific, one of the major manufacturers of pacemakers. They say, "As described in the pacemaker and defibrillator instructions for use, exposure to strong magnetic fields >10 gauss (1 mTesla) may alter implanted device function." When we contacted them, the folks at Boston Scientific confirmed the 10 gauss figure.
Research presented at the American Heart Association's Scientific Sessions 2008, as summarized in this article, also indicates that a, "Field strength of 10 gauss at the site of the pacemaker or defibrillator has the potential to interact with the implantable device."
Another good paper also described a 1 mT (1 milliTesla = 10 gauss) limit: Interference of neodymium magnets with cardiac pacemakers and implantable cardioverter-defibrillators: an in vitro study, from the Institute for Biomedical Engineering University in Switzerland. A Swiss public health site also references that paper, and describes safe distances to some neodymium magnet sizes.
We also contacted Medtronic, another major manufacturer of pacemakers and ICDs. They indicated a 5 gauss limit for DC magnetic fields. Since this topic is a critical medical issue, we suggest deferring to the lowest, most conservative value of 5 gauss.
For a given size magnet, you can use our Magnet Calculator to find the magnetic field strength at a distance from the magnet. By changing the input distance, you can find the distance where the field strength drops below 5 gauss.
Note that these numbers are only true along the magnetization axis of the magnet, and for a magnet in free space. Other magnets or steel objects nearby can alter your results. That said, it's a great guideline. If you're unsure, you can always measure the magnetic field of your magnet in your application with a DC magnetometer.
Interestingly, this chart looks much like our Select-A-Size magnet finder page. It's a great tool to find a particular size magnet.
We love to include video clips of our magnets in action whenever we can. Still, it didn't seem prudent to test deactivating a pacemaker for the sake of a good video. Instead, we've shown a short clip of how close proximity to a magnet can temporarily stop a quartz watch. No watches were harmed during testing.
K&J Magnetics makes no warranty concerning the validity of the 5 gauss threshold described here for deactivating implanted medical devices. It's impossible for us to know, or make blanket statements about every medical device ever implanted. This article is intended as more of a guideline to help designers assess risks associated with neodymium magnets. Please consult your physician if you're going to get a strong magnet anywhere near your pacemaker or other medical device.