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, we hear 

these sounds at safe levels that do not affect our 

hearing. However, when we are 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). These sensitive structures, called hair cells, 

are small sensory cells that convert sound energy 

into electrical signals that travel to the brain. Once 

damaged, our hair cells cannot grow back.

What sounds cause 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. On the 

decibel scale, an increase of 10 means that a sound 

is 10 times more intense, or powerful. To your ears, it 

sounds twice as loud. The humming of a refrigerator 

is 45 decibels, normal conversation is approximately 

60 decibels, and the noise from heavy city traffic can 

reach 85 decibels. Sources of noise that can cause 

NIHL include motorcycles, firecrackers, and small 

firearms, all emitting sounds from 120 to 150 decibels. 

Long or repeated exposure to sounds at or above 

85 decibels can cause hearing loss. The louder the 

sound, the shorter the time period before NIHL can 

occur. Sounds of less than 75 decibels, even after long 

exposure, are unlikely to cause hearing loss. 

Although being aware of decibel levels is an important 

factor in protecting one’s hearing, distance from the 

source of the sound and duration of exposure to the 

sound are equally important. A good rule of thumb is 

to avoid noises that are “too loud” and “too close” or 

that last “too long.”

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 

NIDCD Fact Sheet

Noise-Induced Hearing Loss 

u.s. department of health & human services ∙ national institutes of health ∙ National Institute on Deafness and Other Communication Disorders hearing balance NIH Medical Arts

The Sound Pathway

The sound pathway
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?

NIHL is 100 percent preventable. All individuals should 

understand the hazards of noise and how to practice 

good hearing health in everyday life. To protect 

your hearing:

• Know which noises can cause damage (those at or 

above 85 decibels). 

• Wear earplugs or other hearing protective devices 

when involved in a loud activity (special earplugs 

and earmuffs are available at hardware and sporting 

goods stores). 

• Be alert to hazardous noise 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.

• If you suspect hearing loss, have a medical 

examination by an otolaryngologist (a physician 

who specializes in diseases of the ears, nose, 

throat, head, and neck) and a hearing test by an 

audiologist (a health professional trained to measure 

and help individuals deal with hearing loss).

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 

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—

NIDCD Fact Sheet

Noise-Induced Hearing Loss

hearingbalance

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.
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 higherpitched sounds, such as a cell phone ringing. Those 

nearer the apex, or centermost point, detect lowerpitched sounds, such as a large dog barking. 

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.

•	 Sound waves enter the outer ear and travel through 

a narrow passageway called the ear canal, which 

leads to the eardrum.

• 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.

• 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 

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.

more

Stereocilia

Hair cells in the inner ear
NIDCD supports and conducts research and research training on the 

normal and disordered processes of hearing, balance, smell, taste, 

voice, speech, and language and provides health information, based 

upon scientific discovery, to the public.

hearing, balancesmell, tastevoice, speech, language

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 http://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 

NIDCD Fact Sheet: Noise-Induced Hearing Loss

Publication No. 08-4233

Updated December 2008

For more information, contact:

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

E-mail: nidcdinfo@nidcd.nih.gov

Internet: http://www.nidcd.nih.gov

The NIDCD Information Clearinghouse is a service of the 

National Institute on Deafness and Other Communication 

Disorders, National Institutes of Health, U.S. Department 

of Health and Human Services.

NIDCD also sponsors It’s a Noisy Planet. Protect Their 

Hearing. This national public education campaign, at 

http://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.

For more information, additional addresses and phone 

numbers, or a printed list of organizations, contact:

NIDCD Information Clearinghouse

1 Communication Avenue 

Bethesda, MD 20892-3456

Toll-free Voice: (800) 241-1044

Toll-free TTY: (800) 241-1055

E-mail: nidcdinfo@nidcd.nih.gov