If you held a vote for the world's best-loved gadget, the results wouldn't even be close: It would be the iPod in a landslide. Who doesn't adore the sleek little music machine? Well, one group. For doctors treating hearing loss—not to mention parents worrying about their kids' ears—the iPod is a real source of worry. As it turns out, however, there's not quite so much cause for concern. According to a study presented at a conference straightforwardly called, "Noise-induced Hearing Loss in Children at Work and Play," there's a perfectly safe level for listening to music on an iPod, you just have to know what it is.

It's no secret that loud music can do serious damage to your ears. A whole generation of rock stars are heading into their senior years suffering from the persistent, noised-induced ringing known as tinnitus. The condition is caused by damage to the fine hair cells in the inner ear that convert vibrations into electrical signals, which the brain in turn converts to sound. It's a familiar enough phenomenon to anyone who's left a rock concert or loud stadium with a ringing in the ears that lingers for hours. Now imagine that for life.

Tiny iPods are not giant amplifiers, but they can pack an auditory wallop nonetheless, particularly since the sound is mainlined directly from headphones to ear canals. Researchers at the University of Colorado and Children's Hospital in Boston—affiliated with Harvard University—wanted to determine how much of this the ears can take. The answer is a lot, but only to a point. Analyzing data they collected at Children's Hospital, they concluded that most people can listen to 4.6 hours of iPod music per day, using the standard earphones that come with the unit, provided they don't exceed 70% of maximum volume. Go up to 80% and you're good for only 90 minutes per day. At full volume, you can't do more than five minutes without risking damage. This limit applies to all five genres of music the researchers studied: rock, R&B, dance, Top 40 and country.