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Noise-Induced
Hearing Loss
Every day we experience
sound in our environment such as the television, radio, washing
machine, automobiles, buses and trucks. But when an individual
is exposed to harmful sounds—sounds that are too loud or
loud sounds over a long time—sensitive structures of the
inner ear can be damaged causing Noise-Induced Hearing Loss
(NIHL).
How
do we hear?
Hearing is
a series of events in which sound waves in the air produce electrical
signals and cause nerve impulses to be sent to the brain where
they are interpreted as sound. The ear has three main parts:
the outer, middle and inner ear. Sound waves enter through the
outer ear and reach the middle ear where they cause the ear
drum to vibrate.
Image of the inner ear
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The vibrations are
transmitted through three tiny bones in the middle ear, called
the ossicles. These three bones are named the malleus, incus and
stapes (and are also known as the hammer, anvil and stirrup).
The ear drum and ossicles amplify the vibrations and carry them
to the inner ear. The stirrup transmits the amplified vibrations
through the oval window and into the fluid that fills the inner
ear. The vibrations move through fluid in the snail-shaped hearing
part of the inner ear (cochlea) that contains the hair cells.
The fluid in the cochlea moves the top portion of the hair cells,
called the hair bundle, which initiates the changes that lead
to the production of the nerve impulses. These nerve impulses
are carried to the brain where they are interpreted as sound.
Different sounds move to the population of hair cells in different
ways, thus allowing the brain to distinguish among various sounds,
for example, different vowel and consonant sounds.
What
sounds cause NIHL?
NIHL can be caused by a one-time exposure to loud sound as well
as by repeated exposure to sounds at various loudness levels
over an extended period of time. The loudness of sound is measured
in units called decibels. For example, usual conversation is
approximately 60 decibels, the humming of a refrigerator is
40 decibels and city traffic noise can be 80 decibels. Examples
of sources of loud noises that cause NIHL are motorcycles, firecrackers
and small arms fire, all emitting sounds from 120 decibels to
140 decibels. Sounds of less than 75 decibels, even after long
exposure, are unlikely to cause hearing loss.
Exposure to harmful
sounds causes damage to the sensitive hair cells of the inner
ear and to the nerve of hearing. These structures can be injured
by noise in two different ways: from an intense brief impulse,
such as an explosion, or from continuous exposure to noise,
such as that in a woodworking shop.
What
are the effects of NIHL? 
The effect from impulse sound can be instantaneous and can result
in an immediate hearing loss that may be permanent. The structures
of the inner ear may be severely damaged. This kind of hearing
loss may be accompanied by tinnitus, an experience of sound
like 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
intermittently throughout a lifetime.
The damage that occurs
slowly over years of continuous exposure to loud noise is accompanied
by various changes in the structure of the hair cells. It also
results in hearing loss and tinnitus. Exposure to impulse and
continuous noise may cause only a temporary hearing loss. If
the hearing recovers, the temporary hearing loss is called a
temporary threshold shift. The temporary threshold shift largely
disappears within 16 hours after exposure to loud noise.
Both forms of NIHL
can be prevented by the regular use of hearing protectors such
as ear plugs or ear muffs.
What
are the symptoms of NIHL?
The symptoms of NIHL that occur over a period of continuous
exposure increase gradually. Sounds may become distorted or
muffled, and it may be difficult for the person to understand
speech. The individual may not be aware of the loss, but it
can be detected with a hearing test.
Who
is affected by NIHL?
More than 30 million Americans are exposed to hazardous sound
levels on a regular basis. Individuals of all ages including
children, adolescents, young adults and older people can develop
NIHL. Exposure occurs in the work place, in recreational settings
and at home. There is an increasing awareness of the harmful
noises in recreational activities, for example, target shooting
or hunting, snowmobiles, go-carts, woodworking and other hobby
equipment, power horns, cap guns and model airplanes. Harmful
noises at home may come from vacuum cleaners, garbage disposals,
lawn mowers, leaf blowers and shop tools. People who live in
either urban or rural settings may be exposed to noisy devices
on a daily basis. Of the 28 million Americans who have some
degree of hearing loss, about one-third have been affected,
at least in part, by noise.
Can
NIHL be prevented?
Noise-induced hearing loss is preventable. All individuals should
understand the hazards of noise and how to practice good health
in everyday life.
- Know which noises
can cause damage (those above 75 decibels)
- Wear ear plugs
or other hearing protective devices when involved in a loud
activity (special earplugs and ear muffs are available at
hardware stores and sporting good stores)
- Be alert to hazardous
noise in the environment
- Protect children
who are too young to protect themselves
- Make family, friends
and colleagues aware of the hazards of noise.
- 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 identify and measure hearing loss and to rehabilitate persons
with hearing impairments.
What
research is being done for NIHL?
Scientists focusing their research on the mechanisms causing
NIHL hope to understand more fully the internal workings of
the ear, that will result in better prevention and treatment
strategies. For example, scientists have discovered that damage
to the structure of the hair bundle of the hair cell is related
to temporary and permanent loss of hearing. They have found
that when the hair bundle is exposed to prolonged periods of
damaging sound, the basic structure of the hair bundle is destroyed
and the important connections among hair cells are disrupted
which directly lead to hearing loss.
Other studies are
investigating potential drug therapies that may provide insight
into the mechanisms of NIHL. For example, scientists studying
altered blood flow in the cochlea are seeking the effect on
the hair cells. They have shown reduced cochlear blood flow
following exposure to noise. Further research has shown that
a drug which promotes blood flow used for treatment of peripheral
vascular disease (any abnormal condition in blood vessels outside
the heart), maintains circulation in the cochlea during exposure
to noise. These findings may lead to the development of treatment
strategies to reduce NIHL.
Continuing efforts
will provide opportunities that can aid research on noise-induced
hearing loss as well as other diseases and disorders that cause
hearing loss. Research is the way to develop new, more effective
methods to prevent, diagnose, treat and eventually eliminate
these diseases and disorders and improve the health and quality
of life for all Americans.
Where
can I get additional information?
Updated April 1999
NIH Pub. No. 97-4233 |
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