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The lead singer of the Australian band AC/DC says it’s auto racing, not rock music that has brought him to the brink of deafness. But Brian Johnson’s recent withdrawal from a North American tour provides a cautionary tale for anyone who likes their music loud, hearing specialists say.

The singer has been diagnosed with noise-induced hearing loss, a condition the World Health Organization says threatens up to half of the world’s teens. The WHO said last year that hearing can be damaged in as little as 15 minutes when people are exposed to loud sound, and that chronic use of headphones and earbuds put adolescents and young adults at extreme risk of future hearing loss.

For parents, whose own hearing may be compromised from AC/DC concerts back in the 1980s, this presents a challenge: How to regulate their children’s listening habits when there is no clear way to know how loud is too loud. Doctors know that sound above 85 decibels can damage hearing after eight hours of exposure (above 100 decibels, it takes just 15 minutes), but without a sound-level meter, which costs a couple of hundred dollars for an inexpensive one, how can you measure the sound?

“Even the average audiologist can’t tell,” said Dr. Brian Fligor, chief audiology officer at Lantos Technologies in Boston, and the former director of diagnostic audiology at Boston Children’s Hospital.

Some hearing loss is inevitable with age. But there are general guidelines that, when heeded, can help to prevent, as Fligor says, your 15-year-old from becoming a 25-year-old with a 55-year-old’s hearing.

And don’t blame rock music.

“Classical music is just as dangerous as heavy metal,” he said.

How sound can destroy

The human ear, which is fully developed at birth, consists of three parts: outer, middle and inner. It’s the inner ear, which looks like a snail, that does most of the heavy lifting of hearing. Microscopic structures called hair cells send electrical signals through the auditory nerve, which sends them on to the brain, said Dr. Melissa Tribble, a pediatric audiologist at Stanford Children’s Health.

“When loud noises are presented, these hair cells are overworked, and when they’re overworked a lot, they lose their efficiency,” Tribble said.

When the sound is loud enough to do damage, it's game over. The cells die, and they can't be regenerated, nor will we grow more. We're born with all we will ever have.

“Noise-related hearing loss is completely preventable, however it’s also completely irreversible,” said Barbara Kelley, executive director of the Hearing Loss Association of America in Bethesda, Maryland.

And the sound doesn't have to be painfully loud for damage to occur, Kelley said. A baby exposed to loud music at an event like a wedding reception can suffer damage even when asleep, she said.

Of course, most children and teens are aware that they're listening to music at heart-thumping decibels. They like it that way.

"It sounds really good (to them); the louder, the better," Kelley said. "And kids don't think anything bad is ever going to happen to them."

And, Fligor said, “There’s a good bit of science behind why we like music loud. There’s a dose at which endorphins kick in. And while the negative consequences don’t happen too terribly soon, the reward happens immediately."

Electrical engineer and author Barry Blesser says loud music stimulates regions of the brain that control reward and emotion. "Raising the loudness of music, like a double shot of whisky, elevates the intensity of the experience," Blesser wrote. (The stimulating effect of music on the brain is so potent that USA Track & Field, the governing body in long-distance racing, bans portable music devices among people vying for prizes. The group says it gives them an unfair psychological advantage.)

Listening for danger

When trying to decide if music is too loud, parents should keep a blender and a lawn mower in mind.

Generally, sound below 80 decibels — roughly, the equivalent of someone shouting — does not cause damage, Fligor said.

“Most everyone can listen to that all day, and you’re fine. When you get to about 85 decibels, about the level of a blender, you can listen, for the most part, eight hours, and you’re still OK. But above that, the amount of time you can safely listen drops off really fast," Fligor said.

The damage is cumulative. Listening to music at 85 decibels for eight hours (or 100 decibels for 15 minutes) may cause temporary hearing loss or buzzing in the ears that subsides, but it also causes "wear-and-tear" in the inner ear, Fligor said.

"Depending on how much wear-and-tear is already there, further damage could be permanent. It's why it often takes years of noise exposure to end up with a measurable hearing loss, but of course what you've lost is gone for good, and the loss only accumulates further from there."

Sound advice

Trying to listen to music amid other sound — think, a teenager zoning out with headphones in the backseat while the rest of the family converses — can heighten risk, since the listener has to ratchet the volume up to overcome competing sound. Noise-canceling headphones can help.

Live concerts, however, are uniquely dangerous, delivering sound exceeding 110 decibels for hours. "If you leave a concert and your ears feel stuffed or plugged, that's the hair cells in your cochlea dying," the Hearing Loss Association's Kelley said. "If I go to a concert, I wear earbuds, and so does my husband."

The couple also will fashion earbuds out of napkins if they find themselves in a restaurant where the music is too loud.

While Johnson, the AC/DC singer, recently said he believes his hearing was damaged at an auto racetrack, not four decades of throbbing rock music, research has shown that musicians suffer hearing loss in greater proportion than the general public, and classical musicians are not exempt.

Parents whose children play an instrument can purchase earplugs especially designed for musicians, and anyone who is regularly exposed to loud noise, such as landscapers or hunters, should also invest in protective earmuffs or earbuds.

The National Institutes of Health also notes that even farms can be dangerously noisy: Grain dryers and chain saws can be as loud as an AC/DC concert.

The National Institute on Deafness and Other Communication Disorders, a division of the NIH, wants families to come to see earplugs as necessary as bike helmets. It offers resources for parents on its website, including an interactive sound ruler that helps children understand decibel levels of common sounds such as sirens and dishwashers.

It's also a good idea for parents to ask their pediatrician for a hearing test if their schools don't provide one, Kelley said. If your child frequently doesn't respond when you ask her to do something, she said, "don't assume she's not paying attention."

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