If you're unlucky, all it takes is one loud concert to spark a lifetime of ear problems -- a constellation of symptoms that include not just hearing loss but also ringing in the ears, sound sensitivity, a feeling of aural fullness and even chronic ear pain.
Scientists are just now starting to understand the more nuanced workings of the inner ear, or cochlea, a tiny, snail shaped organ buried deep inside a skull bone -- and about how noise exposure can gum up the complicated system in multiple ways.
Many people are familiar with muffled hearing and ringing ears -- called tinnitus -- after a concert or loud sporting event. Even if these symptoms go away within days, they can portend permanent ear damage, even years later.
The effect of noise is cumulative, insidious and, researchers say, irreversible.
"Over the course of one's lifetime, the damage builds up," said Paul Fuchs, a professor of otolaryngology at the Johns Hopkins School of Medicine.
According to the National Institutes of Health, about 50 percent of Americans age 75 and older have a disabling hearing loss, where their trouble understanding speech becomes apparent to them and to others.
Among teens, many of whom are wedded to ear buds and loud music, nearly 20 percent report some hearing loss. Tinnitus, a usually relentless ringing that can be much more distressing than hearing loss, plagues 10 to 15 percent of adults, according to various studies.
Chris Munson, 55, an engineer and former home-audio enthusiast from suburban Dallas, loved his music loud in his younger days.
He also had tinnitus that came and went. In hindsight, he said, "it was absolutely a warning sign, but if you don't know how to read those warning signs, you ignore them."
One day eight years ago -- having listened to excerpts from films including "The Matrix" with his elaborate home-theater setup the previous night -- Munson awoke with "my head in a ball of sound," he said. The ringing worsened over time, spreading from one ear to both and expanding from one steady tone to several fluctuating ones. This time, the ringing didn't go away. Instead, it worsened over time.
Soon afterward, Munson also developed mild hyperacusis -- a sensitivity that renders everyday noises uncomfortably loud or even painful. He describes his tinnitus as a screaming, constant multi-tone with no real-world correlation. Now he gets an ache in the ear canal from the hum of the refrigerator and the snap of a pop top. To avoid clinking dishware, which causes him ear pain, his family eats from paper plates.
The home theater gathers dust.
"Your ears have a budget," he said. "Spend it too quickly and you're broke."
Why noise can hurt
How does loud or unending noise damage hearing?
Basically, a sound wave vibrates the eardrum and then passes to the cochlea, which contains rows of microscopic hair cells bathed in fluid. These hair cells move with the sound and send signals through the auditory nerve to the brain, which interprets the sound.
Noise that's too loud or long-lasting destroys the hair cells, causing hearing loss or partial deafness. But that's not all. Recent studies show that noise also severs connections between nerves and brain, a likely cause of such abnormalities as the inability to separate background from foreground sound. People typically notice the problem when conversing in a crowded restaurant.
The mechanisms of tinnitus remain a mystery. One study used electrodes to measure the brain activity of a 50-year-old man with tinnitus and hearing loss. The patient had been a recreational firearm user in his younger days.
The effects of the tinnitus permeated many parts of his brain, while a matching tone activated only the part of the brain that processes hearing. In other words, the "noise" of tinnitus affects the brain far differently than a similar real noise does. That may explain why tinnitus is so distressing, said Phillip Gander, a postdoctoral research scholar at the University of Iowa, who is the study's co-author.
Noise can also activate pain fibers in the inner ear. These fibers, which have been identified in mice though not yet in humans, probably explain the pain that occurs with an intensely loud noise as well as pain that can linger after the noise stops, said Jaime Garcia-Anoveros, an associate professor at Northwestern University and the senior author of a new study on "auditory nociception" or, in lay terms, "noise-induced pain."
Another unexplained symptom -- the feeling of aural fullness or pressure in the ear canal, not unlike the pressure felt during an airplane descent -- may be caused by these same pain fibers, Garcia-Anoveros said.
"What they're detecting is not necessarily sound. They could be detecting spilled contents of damaged cells -- a sensation from your ear that is not a hearing sensation."
Noise doesn't even have to be all that loud to be damaging. A long exposure to less-intense noise, such as a job in a noisy restaurant, can be especially pernicious. Bryan Pollard, president of the nonprofit Hyperacusis Research, says people report ear problems caused by all sorts of commonplace hazards, from lawn mowers to smoke alarms to power tools.
Once noise-caused sensitivity has set in, hyperacusis patients report crippling ear pain, sometimes from things as simple as a shopping trip filled with "noises they were not aware could be dangerous or surprise noise they did not anticipate," Pollard said.
Even if people are aware that exposure to excessive noise can be bad, "I don't think they have a sense of what it means should they acquire a hearing impairment," said Gregory Flamme, an associate professor of audiology at Western Michigan University. "I don't think they know how or when to protect themselves."
People face more day-to-day noise than they realize, he said, from vacuum cleaners, blenders, hair dryers, movies and other things. Flamme uses noise dosimeters, which people wear to measure the total noise dosage during a day or other time period. One rousing basketball game could give a person what would normally be a month's worth of exposure.
The ensuing damage depends in large part on individual susceptibility. In mouse studies, a gene governing susceptibility to noise-induced hearing loss has just been discovered, but it remains impossible to predict whose ears will prove to be "tough" or "tender," as researchers put it.
Christine Reiners, 49, never thought twice about the loud tunes she listened to as a teenager. A few years ago, she started taking exercise classes. They ran just twice a week for an hour, but the music blared.
Last year, she woke one night with "the sound of an alarm" in her head. It never stopped. "It's horrible -- a high-pitched screech," she said, sometimes joined by a chirp. She has trouble sleeping and concentrating.
Reiners has visited many doctors, receiving such misdiagnoses as an ear infection, intracranial hypertension and sinusitis. She even had sinus surgery, which didn't help.
"It's hard knowing that it's not going to get any better," said Reiners, a mother of two from North East, Maryland. "I'm praying it doesn't get worse."
According to hearing specialists, limiting the volume and duration of noise exposure goes a long way toward safety, as does the proper use of hearing protection such as earplugs or protective earmuffs.
A rule of thumb: Earplugs are needed when the noise is so loud that people sitting next to each other must raise their voices to be heard.
Because there are no ways to fix noise-induced hearing problems, "the only solution is prevention," said Larry E. Roberts, a professor emeritus and auditory neuroscientist at McMaster University in Canada. He views loud noise as a public-health hazard akin to smoking, and he would like to see aggressive public-awareness campaigns.
Research is starting on ways to reverse hearing impairments, but "the challenge of restoring functional hearing with molecular engineering is great and is likely decades away," Roberts said. "Think of smoking. We can do lung transplants, but this is not the solution for lung cancer."