A study publicized by U.S. researchers at an American Heart Association meeting in New Orleans claims that the headphones of portable audio players (iPods, other generic MP3 players) may interfere with the functionality of cardiac equipment. The devices in question are pacemakers and implantable defibrillators, which can malfunction if in close proximity to headphones.

Previously it had been thought that the players themselves were the problem, but a study done earlier this year by the Food and Drug Administration (FDA) showed conclusively that they posed no harm.

The headphones plugged into them, however, are another story. The small speakers are based on equally small but powerful magnets. When you place them within 1.2 inches of the cardiac equipment, a quarter of patients will get interference, the study shows.

The team at the Medical Device Safety Institute at Beth Israel Medical Center in Boston was lead by Dr. William Maisel, and tested eight different models of headphones and ear buds, which were connected to Apple iPods. The study was carried out on 60 patients; of those, fourteen patients experienced problems, and it was found that defibrillators are twice as likely to be affected as pacemakers.

''For patients with pacemakers, exposure to the headphones can force the device to deliver signals to the heart, causing it to beat without regard to the patients' underlying heart rhythm,'' as Dr. Maisel has said in a press statement. ''Exposure of a defibrillator to the headphones can temporarily deactivate the defibrillator.''

The results come as no great surprise since users of this kind of cardiac equipment are specifically prohibited from being in areas with powerful magnetic fields, due to the devices’ sensitivity.

Maisel said that, while certain brands have been found to have stronger magnetic fields than others, people should not focus on brands, but rather keep their headphones at least 1.2 inches (3 cm) away from the device. That means don’t keep the headphones in your breast pocket or leave it dangling on your chest.

Otherwise, using MP3 players and headphones is perfectly safe, according to Dr. Maisel: ''The main message here is: it's fine for patients to use their headphones normally, meaning they can listen to music and keep the headphones in their ears. But what they should not do is put the headphones near their device.''

MP3 headphones are not the only devices to come under scientific scrutiny for fear of them posing a danger to people with heart problems. For instance, other studies that have been conducted by the American Heart Foundation in regards to popular electronics have found no interference to be caused by Bluetooth earpieces, the MP3 players themselves, electric blankets, or metal detectors used at airports and hip-hop parties.