Are those head-bobbing, mall-wandering teenagers listening to MP3 players such as Apple iPods damaging their long-term hearing? They just might be if the volume is too loud, or they spend hours a day jamming to their favorite tunes, says Dr. Anand Devaiah, an otolaryngologist at Boston University School of Medicine "Back when the Walkmans came out, people worried then too about increased acoustic trauma to the ear," Devaiah recalls. Yet, he says, there's more to worry about now. "Especially with the newer earbud headphones that sit inside the ear, you don't have as much diffusion of sound as you do with open-air headphones, such as those originally packaged with the Walkman," he says. Still, it's more than just the headphones. "It's how much power you're driving through those headphones," he says. "With these newer devices, it's easy to really crank up the volume, so to speak, and that's what people are doing." The same thing is happening inside the confines of cars equipped with sophisticated sound systems and powerful speakers, he adds. According to the American Academy of Otolaryngology-Head and Neck Surgery Foundation, about 28 million Americans have lost some or all of their hearing, including 17 in 1,000 children under age 18. Of those 28 million, one-third can attribute their hearing loss, at least in part, to noise. While a normal conversation is measured at about 60 decibels, the sound from an iPod Shuffle has been measured at 115 decibels; experts say 80 decibels can be hazardous to sensitive ears, and some people are more susceptible than others. "As you start pushing above 85 or 90 decibels, that puts you in the range of causing damage to your hearing," Devaiah says. Remember, too, that the louder the sound, the shorter the exposure before damage can occur. Loud or moderate yet extended noise causes damage to the hearing nerve and the sensitive hair cells of the inner ear, according to the National Institute on Deafness and Other Communication Disorders. The top portion of the hair cells, known as hair bundles, initiate the nerve impulses to the brain. Different sounds move the hair bundles in different ways, allowing the brain to distinguish one sound from another. One common symptom of noise-induced hearing loss is tinnitus – a ringing, buzzing or roaring in the ears and the head. Symptoms may gradually increase over a period of continuous exposure. Sounds may become distorted or muffled, and the sufferer may not understand speech, particularly the higher pitch of a woman or child's voice. "Typically, the higher frequencies are the ones that are affected first," Devaiah notes. "The nerves that detect that sound are more delicate and thus more susceptible to damage." Devaiah says ears may recover from what's called a "temporary threshold shift," when a sudden, severe sound causes the ear "to shut down the amount of sound that gets in as a manner of protecting itself." A ringing sound may result, which usually goes away. "But if you have ringing that persists because of a sudden acoustic trauma – a gun going off right next to your ear, for example – you're almost assured of having had some level of damage to your ear. Whether it shows up on a hearing test or not is debatable, but most experts will agree that is a pretty bad sign," he says. Devaiah has noticed in his practice that patients with some hearing loss are getting younger and younger. In fact, he says it's not uncommon to see a six or seven year old with hearing loss "without other reasons to explain their problem." While he admits it's easier said than done, he says, "We need to really stress to young people that they need to start protecting their ears now. That's a really big thing that I think primary care physicians need to be doing as well. … It's really just about good aural health." He recommends the use of earplugs in noisy situations, especially for his patients with preexisting hearing loss. For those who work continuously in noisy environments, he suggests investing in earplugs custom-designed to fit the ears and thus evenly block the amount of sound pressure. In answer to criticisms from the hearing advocacy community, Apple released in March a free software update for the iPod nano and fifth-generation iPod that allows customers to easily set their own personal maximum volume limit. The software update also gives parents the ability to set a maximum volume limit on their child's iPod and lock it with a combination code. And how do iPod listeners know their music is too loud? "If they can feel the music, that's a really bad sign," Delaiah says. "Or if it's completely drowning out everything else around them, that's also a bad sign, because that's quite a bit of sound pressure coming out of those earplugs." Meanwhile, according to the National Institute on Deafness, scientists are studying the internal workings of the ear to help prevent and treat noise-induced hearing loss. Recent findings show that hair bundles are capable of rebuilding their structure in 48 hours, which is the common duration of temporary hearing loss. Thus, researchers wonder if permanent hearing loss results when damage is so severe that it overwhelms the self-repair mechanism. Drug therapies, particularly those that would maintain blood circulation to parts of the ear, are also being studied.