Study: How Many Kids Have Hearing Loss?

July 2 -- Scientists are finding what many parents will say they knew all along — loud music hurts kids' hearing.

But what moms and dads may not know is that more than 12 percent of children between the ages of six and 19 suffer from noise-induced hearing loss — about 5.2 million kids.

So says a Centers for Disease Control study appearing in the July issue of Pediatrics.

How Many Kids Are Suffering?

And though hearing experts have suspected for a long time that long-term hearing loss begins in childhood, the study is the first to show how common hearing impairment is among kids.

"Historically, people have been looking only at adult hearing loss and assuming that this is not a problem among children," said Amanda Niskar, a nurse at the CDC and lead author of the study. "What we have found here for the first time is that this is not true. [Hearing loss] is a progression, and it starts when you're very young."

Some hearing experts say the problem of hearing loss in kids will likely worsen, considering rising levels of environmental noise.

Niskar's study, which looked at more than 5,000 kids, measured a preliminary type of hearing loss known as a noise-induced-hearing threshold shift, or NITS. In short, kids with NITS have difficulty hearing certain pitches or levels of sound.

Chronic Exposure Leads to Damage

"The first signs that people notice when they have NITS is that they'll hear an entire conversation, but they'll have difficulty discerning certain words," Niskar said. "And even if a child has temporary NITS, they might have problems understanding what teachers in a classroom are saying."

Regular exposure to loud noises worsens NITS by damaging nerve cells in the ear called hair cells. As the name suggests, these cells have tiny hairs that detect sound vibrations and turn them into signals sent to the brain. But while soft noises only cause the hairs to vibrate, loud noises can break them.

Brief instances of exposure to loud noise may only temporarily damage these hairs. Niskar said two hours of loud music on headphones or seven minutes next to the speakers at a rock concert result in NITS that may last for only a few days. However, chronic exposures can damage the hair cells — and hearing — permanently.