Data examined over a seven-year period revealed 48 million Americans -- about one in five -- aged 12 and older suffer from some form of hearing loss, far exceeding previous estimates of 21 to 29 million people.

The study also found rates of hearing loss nearly doubled with every decade of age, and that women and blacks were significantly less likely to have hearing loss at any age. While researchers are still unsure why that is, they plan to study whether the female hormone estrogen and the melanin pigment in darker skin has a protective effect on the inner ear.

So what causes hearing loss, especially in younger people? Loud music could be to blame.

Dr. Rob Jackler, Stanford University School of Medicine professor and hearing committee chairman of the American Academy of Otolaryngology, said a ringing or hollow sound after listening to a steady stream of loud noise could potentially cause permanent damage.

"The tricky thing about loud noise exposure is that most people won't see the impact for many years later," adds study researcher Dr. Frank R. Lin, an assistant professor of otolaryngology-head and neck surgery and epidemiology at Johns Hopkins School of Medicine. "So consumers aren't aware they are damaging their hearing until it's too late."

In other words, if you still want to be able to hear later in life, keep the volume on those iPods turned down.

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