FRIDAY, Oct. 20 (HealthDay News) -- Maxing out the volume on a typical MP3 player -- such as the popular Apple "iPod" -- for anything more than five minutes a day can permanently damage a listener's hearing, new research says.

And listening to an MP3 in a noisy environment appears to encourage higher-than-safe volume use, which should be avoided altogether or offset by using noise-reduction-style earphones that allow for listening at lower volumes, the researchers added.

Both cautionary notes were struck Thursday by researchers presenting studies at a conference titled Noise-Induced Hearing Loss in Children at Work and Play, in Covington, Ky. Organizers described it as the first conference ever convened in the United States to specifically address hearing loss among children.

The study authors said their research applies equally to a five music players tested, including the iPod, iPod Nano, and iPod Mini, as well as Sandisk Sansa and Creative Zen Micro players.

"What people should think about is that all personal music players are capable of producing levels that are potentially dangerous to their hearing, but all of them can be used in a safe manner as well," said Brian J. Fligor, a co-author of two conference studies and director of the diagnostic audiology program at Children's Hospital Boston.

In one study, Fligor teamed with audiology doctoral student Cory Portnuff from the University of Colorado, Boulder, to measure sound levels produced by MP3 players through the stock in-ear "bud" headphones that come with the players. The researchers also tested optional "isolator" earphones that block background noise and "supra-aural" earphones that sit over the ear.

After determining that all the players operated at similar volume levels, Fligor and Portnuff sought to define uniform safe listening recommendations. The researchers' listening advisory was set with an eye toward established U.S. government guidelines that indicate hearing loss can begin at volumes of 85 decibels.

With the caveat that not all people are "typical" -- with varying levels of "tenderness" and "toughness" when it comes to hearing tolerance -- the researchers concluded that most people can listen to an MP3 player for 4.6 hours a day at 70 percent of full volume.

When set to 80 percent of full volume, 1.2 hours of daily listening is the maximum, they suggested, while full volume listening should never exceed five minutes a day when using a bud earphone, three minutes with a noise-reduction earphone, or 18 minutes with an over-the-ear set.

The authors pointed out that the differences reflect the fact that ear-bud style headphones deliver higher levels of sound to the ear than over-the-ear varieties.

A second study looked at how people -- in this case doctoral students -- actually deal with volume control in light of two variables: background noise and earphone types. The study was done by Fligor and Terri E. Ives, an assistant professor with the PCO School of Audiology in Elkins Park, Pa.

In general, men listened to music at higher volumes than women. But, overall, only about 6 percent of the study participants chose to listen at "risky" levels -- above 85 decibels -- while in quiet conditions. However, in noisy conditions, those students with background-noise-reducing earphones set their volumes lower than those with regular earphones.

About 80 percent of those study participants with non-noise reducing earphones turned their volumes up to "risky" levels while in noisy conditions. That figure fell to just 20 percent among those using noise reduction earphones.

"It's very clear that the amount of background noise is the number one thing that dictated whether or not people listened too loud," said Fligor. "Not the kind of headphone. Except when we provided people with an isolating earphone to isolate background noise. That did reduce the potential risk for hearing loss, because it did cause people to modify their hearing use to lower levels.

"So, I suggest, it's really worth the investment to get the noise reduction earphones," he added.

That advice was seconded by Dr. Anil K. Lalwani, chairman of the department of otolaryngology at New York University School of Medicine and Medical Center in New York City.

"We have to be cognizant of the fact that, if we're trying to listen to our portable devices in a noisy environment, we are putting our ears at risk," he said. "And I would agree that those headphones that reduce background noise are going to reduce your risk, because you're not going to turn the volume up as high."

More information

To learn more about preventing hearing loss, visit the Better Hearing Institute.