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What is noise-induced hearing loss?

Illustration showing the sound pathway.
The sound pathway



What sounds cause NIHL?





What are the effects of NIHL?

Exposure to harmful sounds causes damage to the hair cells as well as the auditory, or hearing, nerve (see figure). Impulse sound can result in immediate hearing loss that may be permanent. This kind of hearing loss may be accompanied by tinnitus—a ringing, buzzing, or roaring in the ears or head—which may subside over time. Hearing loss and tinnitus may be experienced in one or both ears, and tinnitus may continue constantly or occasionally throughout a lifetime.

Continuous exposure to loud noise also can damage the structure of hair cells, resulting in hearing loss and tinnitus, although the process occurs more gradually than for impulse noise.

Exposure to impulse and continuous noise may cause only a temporary hearing loss. If a person regains hearing, the temporary hearing loss is called a temporary threshold shift. The temporary threshold shift largely disappears 16 to 48 hours after exposure to loud noise. You can prevent NIHL from both impulse and continuous noise by regularly using hearing protectors such as earplugs or earmuffs.

Scientists believe that, depending on the type of noise, the pure force of vibrations from the noise can cause hearing loss. Recent studies also show that exposure to harmful noise levels triggers the formation of molecules inside the ear that damage hair cells and result in NIHL. These destructive molecules play an important role in hearing loss in children and adults who listen to loud noise for too long.


What are the symptoms of NIHL?

When a person is exposed to loud noise over a long period of time, symptoms of NIHL will increase gradually. Over time, the sounds a person hears may become distorted or muffled, and it may be difficult for the person to understand speech. Someone with NIHL may not even be aware of the loss, but it can be detected with a hearing test.


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 high frequency 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, woodworking and other hobbies, playing in a band, and attending rock concerts. Harmful noises at home may come from lawnmowers, leaf blowers, and shop tools.


Can NIHL be prevented?


  • Be alert to hazardous noise in the environment.


How we hear

High resolution hair cells of the inner ear
Hair cells in the inner ear

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.

  • The bones in the middle ear amplify, or increase, the sound vibrations and send them to the inner ear—also called the cochlea—which is shaped like a snail and is filled with fluid. An elastic membrane runs from the beginning to the end of the cochlea, splitting it into an upper and lower part. This membrane is called the “basilar” membrane because it serves as the base, or ground floor, on which key hearing structures sit.
  • The sound vibrations cause the fluid inside the cochlea to ripple, and a traveling wave forms along the basilar membrane. Hair cells—sensory cells sitting on top of the membrane—“ride the wave.”
  • As the hair cells move up and down, their bristly structures bump up against an overlying membrane and tilt to one side. This tilting action causes pore-like channels, which are on the surface of the bristles, to open up. When that happens, certain chemicals rush in, creating an electrical signal.
  • The auditory nerve carries this electrical signal to the brain, which translates it into a “sound” that we recognize and understand.
  • Hair cells near the base of the cochlea detect higher-pitched sounds, such as a cell phone ringing. Those nearer the apex, or centermost point, detect lower-pitched sounds, such as a large dog barking.


What research about NIHL is being conducted?

The National Institute on Deafness and Other Communication Disorders (NIDCD) researches the causes, diagnosis, treatment, and prevention of hearing loss. Most hearing loss is caused by damaged hair cells, which do not grow back in humans and other mammals. NIDCD-supported researchers have helped to identify some of the many genes important for ear development and hearing; they have also been studying the possibility of using gene therapy to regrow hair cells in mammals.

NIDCD researchers also are investigating a potential way to prevent NIHL after noise exposure. Noise exposure triggers the formation of destructive molecules, called free radicals, which cause hair cell death. Researchers initially had thought that antioxidants—chemicals that protect against cell damage from free radicals—might prevent NIHL only if the antioxidants were given before noise exposure. In a recent study, however, the antioxidants in salicylate (aspirin) and Trolox (vitamin E) were given to guinea pigs as long as three days after noise exposure and still significantly reduced hearing loss. These results suggest that there is a window of opportunity in which it is possible to rescue hearing from noise trauma. NIDCD-funded researchers are now testing the ability of nutrients, such as vitamins and minerals, to prevent NIHL in military personnel and college students.


Where can I get more information?

NIDCD maintains a directory of organizations that can answer questions and provide printed or electronic information on NIHL. Please see the list of organizations at www.nidcd.nih.gov/directory.

Use the following keywords to help you search for organizations that are relevant to NIHL:

  • Noise-induced hearing loss
  • Hard-of-hearing

For more information, additional addresses and phone numbers, or a printed list of organizations, contact the NIDCD Information Clearinghouse.

NIDCD sponsors It’s a Noisy Planet. Protect Their Hearing. This national public education campaign, at www.noisyplanet.nidcd.nih.gov, is designed to increase awareness among parents of children ages 8 to 12 about the causes and prevention of NIHL.

NIDCD Information Clearinghouse
1 Communication Avenue
Bethesda, MD 20892-3456
Toll-free Voice:  (800) 241-1044
Toll-free TTY:  (800) 241-1055
Fax:  (301) 770-8977


NIH Pub. No. 97-4233

Updated: October 2008

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