Noise Induced Hearing Loss

What is noise-induced hearing loss?

Every day we experience sound in our environment such as the sounds from television and radio household appliances and traffic. Normally these sounds are at safe levels that don’t damage our hearing. However when we’re exposed to harmful noise—sounds that are too loud or loud sounds that last a long time—sensitive structures in our inner ear can be damaged causing noise-induced hearing loss (NIHL).

What causes NIHL?

NIHL can be caused by a one-time exposure to an intense “impulse” sound such as an explosion or by continuous exposure to loud sounds over an extended period of time such as noise generated in a woodworking shop.

Sound is measured in units called decibels. Sounds of less than 75 decibels even after long exposure are unlikely to cause hearing loss. However long or repeated exposure to sounds at or above 85 decibels can cause hearing loss. The louder the sound the shorter the amount of time it takes for NIHL to happen.

Here are the average decibel ratings of some familiar things:

  • The humming of a refrigerator45 decibels

  • Normal conversation60 decibels

  • Noise from heavy city traffic85 decibels

  • Motorcyles95 decibels

  • An MP3 player at maximum volume105 decibels

  • Sirens120 decibels

  • Firecrackers and firearms150 decibels

Although being aware of decibel levels is important to help us protect our hearing how far away we are from the source and how long we are exposed to the sound are equally important. A good rule of thumb is to avoid noises that are too loud too close or last too long.

Who is affected by NIHL?

People of all ages including children teens young adults and older people can develop NIHL. Approximately 15 percent of Americans between the ages of 20 and 69—or 26 million Americans—have hearing loss that may have been caused by exposure to loud sounds or noise at work or in leisure activities. Recreational activities that can put someone at risk for NIHL include target shooting and hunting snowmobile riding listening to MP3 players at high volume through earbuds or headphones playing in a band and attending loud concerts. Harmful noises at home may come from lawnmowers leaf blowers and shop or woodworking tools.

How can noise damage our hearing? 

To understand how loud noises can damage our hearing we have to understand how we hear. Hearing depends on a series of events that change sound waves in the air into electrical signals. Our auditory nerve then carries these signals to the brain through a complex series of steps.

  1. Sound waves enter the outer ear and travel through a narrow passageway called the ear canal which leads to the eardrum.

  2. The eardrum vibrates from the incoming sound waves and sends these vibrations to three tiny bones in the middle ear. These bones are called the malleus incus and stapes.

  3. The bones in the middle ear couple the sound vibrations from the air to fluid vibrations in the cochlea of the inner ear which is shaped like a snail and filled with fluid.  An elastic partition runs from the beginning to the end of the cochlea splitting it into an upper and lower part. This partition is called the basilar membrane because it serves as the base or ground floor on which key hearing structures sit.

  4. Once the vibrations cause the fluid inside the cochlea to ripple a traveling wave forms along the basilar membrane. Hair cells—sensory cells sitting on top of the basilar membrane—ride the wave.

  5. As the hair cells move up and down microscopic hair-like projections (known as stereocilia) that perch on top of the hair cells bump against an overlying structure and bend. Bending causes pore-like channels which are at the tips of the stereocilia to open up. When that happens chemicals rush into the cell creating an electrical signal.

  6. The auditory nerve carries this electrical signal to the brain which translates it into a “sound” that we recognize and understand.

Most NIHL is caused by the damage and eventual death of these hair cells. Unlike bird and amphibian hair cells human hair cells don’t grow back. They are gone for good.

What are the effects and symptoms of NIHL?

When we’re exposed to loud noise over a long period of time we gradually start to lose our hearing. Over time the sounds we hear may become distorted or muffled and it may be difficult to understand other people when they talk. If you have NIHL you might not even be aware of it but it can be detected with a hearing test.

NIHL can also be caused by extremely loud bursts of sound such as gunshots or explosions which can rupture the eardrum or damage the bones in the middle ear. This kind of NIHL could result in immediate hearing loss that may be permanent.

Loud noise exposure can also cause tinnitus—a ringing buzzing or roaring in the ears or head. Tinnitus may subside over time but can sometimes be permanent continuing constantly or occasionally throughout a person’s life.  Hearing loss and tinnitus can occur in one or both ears.

Sometimes exposure to impulse and continuous loud noise causes a temporary hearing loss which disappears 16 to 48 hours later. Recent research suggests however that although the loss of hearing seems to disappear there may be residual long-term damage to your hearing.

Can NIHL be prevented?

NIHL is the only type of hearing loss that is completely preventable. If you understand the hazards of noise and how to practice good hearing health you can protect your hearing for life. Here’s how:

  • Know which noises can cause damage (those at or above 85 decibels).

  • Wear earplugs or other protective devices when involved in a loud activity (activity-specific earplugs and earmuffs are available at hardware and sporting goods stores).

  • Be alert to hazardous noises in the environment.

  • Protect the ears of children who are too young to protect their own.

  • Make family friends and colleagues aware of the hazards of noise.

  • Have your hearing tested if you think you might have hearing loss.  

What research is being done about NIHL?

The National Institute on Deafness and Other Communication Disorders (NIDCD) supports research on the causes diagnosis treatment and prevention of hearing loss. NIDCD-supported researchers have helped to identify some of the many genes important for hair cell development and function and are using this knowledge to explore new treatments for hearing loss.

NIDCD-supported researchers also are investigating potential ways to prevent NIHL after noise exposure. Noise exposure triggers the formation of destructive molecules called free radicals which can cause hair cell death. Researchers are now testing the ability of nutrients such as vitamins and minerals to prevent the damage caused by free radicals and to determine if there is a window of opportunity in which it is possible to rescue hearing from noise trauma or prevent the damage. Researchers are also looking at the protective properties of supporting cells in the inner ear which appear to be capable of lessening the damage to sensory hair cells upon exposure to noise.

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