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In focus-group discussions with students at two high schools in the Netherlands, researchers found that the teens were generally aware that blasting an MP3 player could harm their hearing. Yet most said they usually played their own device at maximum volume and had no plans to change that.

Manufacturers, according to the researchers, could equip MP3 players with an indicator that displays the volume level in terms of decibels, along with a signal such as a flashing light that goes off when decibel levels reach dangerous levels.

Like many teenagers, the students often denied their own personal risk. Most knew the general hazards of loud music, but believed they had a "low personal vulnerability" to hearing loss, the researchers report in the Journal of Pediatrics.

Given this, lead researcher Ineke Vogel said in an email interview, "we strongly

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recommend parents to inform their children and to discuss with their children the use of MP3 players and the potential long-term, irreversible consequences for hearing capacity."

Parents can also look for signs of a problem, like when a child complains of ringing in the ears or sounds being "muffled," according to Vogel and co-researcher Dr. Hein Raat, both based at the University Medical Centre Rotterdam.

Based on the focus-group discussions, though, many parents may be unaware of the hearing risks posed by MP3 players, the researchers note. Of the 73 students in the study, few said their parents had warned them that playing the devices too loud could harm their hearing.

Many students in the study said they did not know how to tell when their MP3 players were too loud. Volumes at or above 90 dB are believed to be hazardous, Vogel's team notes, but noise levels need to reach 120 dB to 140 dB to become uncomfortable or painful.

Vogel and her colleagues recommend that, as a general "rule of thumb," MP3 users set the volume no higher than 60 percent of its full capacity when using "ear bud" style headphones, like those that come with iPods.

With over-the-ear headphones, they recommend 70 percent as the maximum.

Just as there are safety standards for occupational noise exposure, Vogel and her colleagues suggest that more long-range studies are needed to develop safety guidelines for "leisure-time" noise exposure.