Portable music players have sparked recent concern that they could contribute to noise-induced hearing loss, since they are used frequently and with earphones that deliver the sound directly to the ear. As a result, they were investigated thoroughly by Cory Portnuff, a graduate student at the University of Colorado, and Dr. Brian Fligor, ScD, CCC-A, Director of Diagnostic Audiology at Children's Hospital Boston and Harvard Medical School in Boston.

Portnuff and Fligor measured specific sound levels that come out of five portable music players: the Apple iPod, the Apple iPod Nano, and the Apple iPod Mini; the Creative Zen Micro; and the SanDisk Sansa. For each player, they measured sound levels in music transmitted several different types of earphones, ranging from the "stock" earphones that come with the products to "isolator" earphones that block outside noise, to "supra-aural" earphones that fit over the ears.

Interestingly, they found that all five music players output very similar sound levels, especially at the highest volume levels. They also found no significant differences between sound levels in five different genres of popular music, especially when played at the highest volume.

But those highest volumes, the researchers point out, can be dangerous - so dangerous that the researchers only recommend listening at those levels a few minutes a day, if at all. To come up with their recommended listening times, the researchers compared the players' volume levels to 85 dBA, the minimum sound level determined to pose the risk of hearing damage according to National Institute of Occupational Safety and Health (NIOSH) guidelines. The unit "dBA" stands for "A-weighted" decibels, a scale that takes into account the fact that the human ear has different sensitivities to different frequency levels and reduces the weight for frequencies below 1,000 hertz and above 7,000 hertz, to which the ears are less sensitive.

The typical individual can tolerate about two hours of 91 dBA per day before risking hearing loss, Dr. Fligor says. Loud sounds stress and potentially damage the delicate hair cells in the inner ear, which convert mechanical vibrations (sound) to electrical signal that the brain interprets as sound. Over time, the hair cells can become permanently damaged and no longer work, producing hearing loss.

With this in mind, Portnuff and Dr. Fligor say that typical individuals can listen to their iPods for a total of 1.2 hours a day with the supplied earphones if the volume is at 80 percent of maximum levels. Listening to iPods at full volume is not recommended for more than 5 minutes per day if using the supplied earphones or other earbuds, three minutes per day if using "isolator" earphones that block out background noise, or 18 minutes per day if using "supra-aural" earphones that are placed over the ears rather than inside them. These guidelines take into account, for example, that earbud-style earphones are inserted closer to the ear and they typically deliver sounds at levels 5.5 dBA higher than the supra-aural ones. The maximum recommended listening times represent the amount of time that a typical person could listen to their portable music player every day without greatly increasing their risk of hearing loss.

The Sight & Hearing Association was represented at the first-ever conference, "NIHL in Children at Work and at Play," held in Cincinnati, Ohio, in October.