According to recent tests by the Mayo Clinic in Rochester, Minn., cell phones operating in the direct vicinity of medical devices in hospitals do not have an impact on them. The researchers were looking for any sign of noticeable interference, CNN/Reuters reports. This flies in the face of the policy at many hospitals which bans the use of cell phones.

But not all electronic devices seem so benign in association with medical devices. For instance, the Mayo Clinic also published a report that found that portable CD players can provoke abnormal readings in electrocardiograms when used in close proximity to the device.

More alarming, Mayo clinic researchers found that anti-theft systems used in many retail stores can have harmful effects on pacemakers and defibrillators. The researchers reported one case in which a patient with a pacemaker collapsed in a store near a security system; another case involved a man with an implanted defibrillator that shocked him when he walked near a security device.

There are over 1 million electronic article surveillance systems in use around the world, according to the researchers, CNN/Reuters reports.

These findings were first published in the March issue of Mayo Clinic Proceedings.