If you don't have one already, there may be one waiting for you under the Christmas tree. We're talking about portable music players, like the iPod or Microsoft Zune.

But before you turn them on, or give one to your kids — you should know the risk.

"We have actually clocked an iPod listening to Led Zeppelin's 'Rock and Roll' at 115 decibels on average," says Andy Vermiglio, senior researcher at the House Ear Institute.

How loud is 115 decibels or dba?

The sound of a jackhammer is around 120 decibels.

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Consider this: it is illegal to expose employees to 115 decibels in the workplace for more than 28 seconds without ear protection. And yet, studies show personal stereos routinely pump out that level of sound and people listen to it — a lot.

"My mom tells me all the time don't listen to my iPod, but I do it anyways, even though I know it damages my ear. But I still do it," says Sandra Iryami, a Los Angeles high school student.

She's not alone. Studies have found plenty of kids are beginning to show the side effects of incessant exposure to excessive sound and nearly 30 million Americans have some degree of hearing loss. A third of that is noise-induced.

A recent study of college students in North Carolina found one-third showed evidence of high frequency hearing loss. In Australia, researchers found 25 percent of students played their music at damaging levels.

In the U.S., 50 percent of high school students report symptoms of hearing loss. A study last year at Harvard Medical School concluded listening to personal stereos for more than three minutes puts users at risk for hearing loss.

"The scary part of being exposed to devices like this is that the type of hearing loss can become permanent," says Vermiglio. "And it can become a type of hearing loss that effects not just their ability to understand music, but to understand communication in the world."

It's not that earlier generations didn't listen to loud music. But there are differences today.

For instance, loud music doesn't distort digital players, a cue that used to tell us to turn the music down.

Hearing damage is caused not only by volume but also duration. And most users today don't give their ears a rest. They take their music with them, from the car to the house to the gym, even while studying or reading a book.

The takeaway for users? Consider going into the "settings" feature of your personal stereo and lower how high you can turn up the volume.

That way even when you play it to the max, you're not exceeding around 100 decibels.

William La Jeunesse joined FOX News Channel (FNC) in March 1998 and currently serves as a Los Angeles-based correspondent.