MONDAY, June 21 (HealthDay News) -- People using MP3 players are leaving themselves open to temporary changes in hearing, which over time might result in permanent hearing loss, Belgian researchers suggest.

Scientists already know that at work, exposure to harmful noise -- noise that is too loud or loud sounds that go on too long -- can eventually lead to hearing loss by damaging the sensitive hair cells in the inner ear. And the same may be true of loud music pumped directly into the ear through MP3 players, the researchers say.

For the study, a team led by Hannah Keppler from Ghent University exposed 21 people to rock music heard through headphones on an MP3 -- a digital audio player using MP3 files -- during six sessions. The participants used two types of headphones and several volume settings on an MP3 player.

The researchers tested the hearing of each person before and after each one-hour session.

Keppler's group found substantial, albeit temporary, changes in hearing sensitivity, which indicates the noise affects the ear's hair cells.

Permanent hearing loss "cannot be predicted from the initial temporary threshold shift [in hearing capacity], but considering the reduction in hearing sensitivity after listening to a portable media player, these devices are potentially harmful," the researchers concluded.

The findings are important because they document a troubling "temporary threshold shift" in hearing associated with MP3 players, said Robert Frisina, associate director of the International Center for Hearing & Speech Research at the University of Rochester Medical School in New York.

Combined with another study that shows temporary hearing loss can lead to permanent but delayed damage to hearing, the findings "become even more significant," he said.

"If one becomes more susceptible to loud-noise damage with aging or age-related hearing loss, MP3 players could be even more of a hazard to middle-aged and older adults," added Frisina, who wasn't involved in the study.

The report was published in the June issue of the Archives of Otolaryngology, Head & Neck Surgery.

One expert says the technology of MP3 music files may also be a factor in the hearing damage seen in this study.

Paul R. Kileny, director of audiology and electrophysiology at the University of Michigan Health System, said in addition to volume levels, which can cause hearing damage, MP3s use a compression system to make music files smaller -- a technique that may also lead to hearing loss.

"The natural peaks and valleys of the music, which is the saving grace for our ears in terms of exposure, have disappeared," he said. "There are less valleys and more peaks. In any piece of music you have less soft breaks than you used to with CDs or vinyl."

In addition, the effect of putting earphones in your ear canal varies with the size of the canal, Kileny said. The younger you are or the smaller you are, the smaller the ear canal. A small ear canal makes music louder than the same music played at the same volume in a larger ear canal, he said.

To prevent hearing loss, Kileny recommends keeping the volume down and reducing the time you listen.

"Prolonged listening at maximum volume, I believe, over time can result in hearing loss," Kileny said. "It's cumulative -- it could take 20 years to 30 years."

Another expert, Robert Fifer, an associate professor of audiology and speech pathology at the University of Miami Leonard M. Miller School of Medicine, said that "the ear doesn't care what kind of sound it is, it really only cares how loud that sound is and for what time duration."

Fifer advises keeping the volume at a level where you can still hear conversation around you.

"If you can still hear what people are saying around you, you are at a safe level," he said. "If the volume is turned so loudly that you can no longer hear conversation around you, or if someone has to shout at you at a distance of about 2 feet or 3 feet to get your attention, then you are up in the hazardous noise range."

More information

For more on noise-related hearing loss, visit the U.S. National Institute on Deafness and Other Comm...cation Disorders.