David
E. Baker
University of Minnesota Extension Service
Farm
workers experience one of the highest rates of hearing loss
among all occupations. This is caused in part by the many
potential sources of loud noise on the farm: tractors, combines,
grinders, choppers, shotguns, conveyors, grain dryers, chain
saws, etc. Prolonged exposure to excessive noise can cause
permanent hearing losses unless noise control measures are
taken.
The human ear is composed of three major sections: the external,
middle and inner ear (see Figure 1). Each section has a specific
function in the hearing process.
The
middle ear consists of three tiny bones, or ossicles, that
are suspended in an air-filled space. They connect the eardrum
to the inner ear, which is embedded in the skull. The ossicles
function as a mechanical lever system that delivers sound
from the ear canal to the inner ear. Noise does not affect
the middle ear unless there is an impact sound or pressure
so great that it dislodges or fractures the ossicles.
The
inner ear, or cochlea, is susceptible to damage from continued
exposure to high-level noise. The cochlea (Latin term meaning
snail shell) is a fluid-filled hydraulic system driven by
the piston action of the last ossicle. The vibrating motion
of the ossicle produces a wave motion in a membrane that runs
the full length of the cochlea. If the vibrations are fast
(high-frequency sound), the membrane has its greatest motion
at the base of the cochlea near the vibrating ossicle. If
the vibrations are slow (low-frequency sound), the maximum
membrane motion occurs at the tip, or apex, of the cochlea.
Situated
on top of the moving membrane are thousands of small hair-like
structures with nerves connected to each hair cell. When a
hair cell is bent because of membrane motion, the nerve fires
and the message is transmitted to the brain. Hair cells near
the base transmit information about high-frequency or high-pitch
sound, while those at the apex provide information about low-pitch
sound.
If the
hair cells in a particular region of the cochlea are destroyed,
the nerves will not fire and the brain will not receive any
information. If part of the hair cells are destroyed, the
brain may receive a distorted message that it cannot interpret.
The
whole cochlea is smaller than a dime and is embedded in the
hardest bone in the body. When the microscopic structures
are damaged, there is no way to repair them to restore reasonable
hearing.
Typically,
hair cells are damaged or destroyed when their supporting
structures are overworked. With continued exposure to high-level
noise, the membrane motion is great and the cells that support
the hair cells swell. Eventually, they rupture and the hair
is destroyed or damaged. Only a few hair cells may be lost
at a time, but with repeated exposure over days, months and
years, the cumulative effect can be substantial.
Even
though we cannot see sound, it is a force with real dimensions
and three definite properties: intensity, frequency and duration.
Intensity
is the loudness of a sound, or the pressure it exerts through
the ear. It is measured in units called decibels (dB).
Table
1. Sound intensity levels. |
Decibel
Level (dB) |
Source |
140 |
threshold
of pain: gunshot, siren at 100 feet |
135 |
jet
take off, amplified music |
120 |
chain
saw, jack hammer, snowmobile |
100 |
tractor,
farm equipment, power saw |
90 |
OSHA
limit - hearing damage if excessive exposure to noise
levels above 90 dB |
85 |
inside
acoustically insulated tractor cab |
75 |
average
radio, vacuum cleaner |
60 |
normal
conversation |
45 |
rustling
leaves, soft music |
30 |
whisper |
15 |
threshold
of hearing |
0 |
acute
threshold of hearing - weakest sound |
The ear
is a remarkable organ, responding to sounds ranging from dripping
water to amplified music (Table 1). The normal range of hearing
begins at approximately 0 decibels, a level at which a person
with excellent hearing is able to detect a sound. Typically,
a person begins to identify sounds when a level of 10 to 15
dBs is reached; this is the threshold of hearing.
The
other end of the scale is known as the threshold of pain (140
dB), or the point at which the average person experiences
pain.
In assessing
noise, a special measure called "dBA" indicates damage to
hearing. The dBA rating is provided for many pieces of agricultural
equipment. The higher the dBA number, the greater the risk
of damage to hearing.
Frequency
is the number of sound waves (high and low pressure areas)
produced by a noise source passing a given point per second.
Frequency is measured in cycles per second (cps), also called
hertz (Hz). The higher the number, the higher the frequency.
The
human voice has a range of about 200 to 4,000 Hz. A noise-induced
hearing loss first causes the loss of the ability to hear
sounds at 4,000 Hz. Then hearing loss proceeds until the ear
cannot hear frequencies between 500 and 3,000 Hz, a range
crucial to understanding conversation.
One
of the first signs of loss is the inability to understand
people (especially in a crowd) or other sources of voice communication
such as the television or radio. You become "hard of hearing,"
and sounds seem muffled.
The
most dangerous sounds are high in intensity (dB level) and
have a high frequency. This is because a large number of sound
waves are transmitted to the ears with a force greater than
your ears can tolerate. Noise-induced hearing loss cannot
be reversed, and a hearing aid does little good. Therefore,
prevention is by far the best treatment.
Table
2. Permissible noise exposure scale1 |
Duration
- hours per day |
Sound
level (dBA) |
8 |
90 |
4 |
95 |
2 |
100 |
1 |
105 |
1/2 |
110 |
1/4
or less |
115 |
1
- Based on OSHA Noise Standard. |
Duration
is the amount of time you are exposed to a sound level. In Table
2, the right-hand column lists various high sound levels and
the left-hand column indicates the length of exposure that is
safe for the corresponding noise level during a day. These figures
have been determined after years of research on noise-induced
hearing loss and are accepted as the standard for allowable
noise level exposures.
The
average person can be exposed to a sound source producing
90 dBA for a maximum of eight hours. If the sound level is
at 100 dBA, then the maximum exposure is two hours. An unprotected
ear can be exposed to 115 dBA for a maximum of only 15 minutes
a day. Your ears should not be directly exposed for any length
of time to sounds greater than 115 dBA.
For
every 5 dB increase above 90 dBA, the permissible exposure
time is reduced by half. For example, if you purchased a tractor
with a 95 dBA rating, you would be risking a hearing loss
after four hours of exposure. If the tractor had a 90 dBA
rating, you could use the tractor for eight hours before reaching
the same risk level.
The
effects of noise can be broken into three areas:
- Physiological
effects: noise-induced hearing loss or aural pain, nausea
and reduced muscular control.
- Psychological
effects: noise can startle, annoy and disrupt concentration
of sleep.
- Interference
with communications.
The
ears provide two warning signs for overexposure to noise:
temporary threshold shift (TTS) and ringing in the ears (tinnitus).
After leaving a noisy area or piece of equipment, many people
commonly experience both of these symptoms. The temporary
hearing loss is difficult to detect unless a hearing test
is performed.
This
temporary hearing loss was taken into consideration in the
exposure limits listed in Table 2. For example, should you
be exposed to a noise level of 100 dBA for two hours, the
remaining 22 hours of that day's exposure should be at a noise
level below 90 dBA. This allows the ear to recover from the
temporary hearing loss. This recovery period varies, depending
upon the individual, the severity and length of exposure.
Hearing
usually returns almost completely in 12 to 14 hours if there
is no more noise exposure. Any amount of hearing that does
not return becomes a permanent threshold shift (PTS) or permanent
hearing loss. With repeated exposure, the effects are cumulative.
Tinnitus
is a general symptom of the auditory system not functioning
properly. If you have persistent tinnitus, consult a physician.
If you experience tinnitus after exposure to noise, it is
a sign of overexposure. Since some people are more susceptible
to noise damage than others, one person may experience more
tinnitus or damage than another with exposure to the same
noise.
People
who have damaged their ears permanently from overexposure
to noise often have constant ringing in their ears. Some just
learn to live with the ringing. Others cannot stand it and
seek professional help. For the most part, physicians and
audiologists can do very little to relieve tinnitus.
The
permanent damage that occurs from overexposure to noise results
in a hearing loss that is most annoying and deceptive. In
general, most noises damage the hair cells near the base of
the cochlea, where high-frequency information is processed.
High-frequency hearing loss creates several problems:
- Under
ideal listening conditions, speech may still sound mumbly
(especially women's and children's voices). The listener
is aware that someone is speaking but cannot understand
the message. Unfortunately, normal-hearing people expect
that if speech can be detected, it can be understood. When
this doesn't occur, they lose patience with the hearing-impaired
listener.
- The
ability to identify the source of sound is greatly reduced
and the listener seems inattentive.
- When
there is background noise, the listener with a high-frequency
hearing loss cannot separate one voice from another. Since
all the voices are jumbled together, he or she understands
no one. Social functions and group meetings become a chore,
and the individual begins to avoid them.
- With
noise-induced hearing loss, there is often a reduced tolerance
for loud sounds. Thus the level of a sound must be high
before it is understood, but if it is slightly higher, it
may be intolerably loud.
- With
noise-induced hearing loss, hearing aids can help, but they
do not totally restore the quality of hearing that was present
before the loss.
The
psychological effects of noise are more difficult to describe.
Psychological effects such as depression and nervousness are
a result of the ear's inability to adjust to sound. The eye
has a very effective means of adjusting to light, but people
never get "used" to noise. Instead, they usually adjust their
mental attitude rather than hearing compensation. Subconscious
frustrations can result when noise is endured, but the body
system cannot adjust to it.
The
effects of noise on communications are quite obvious. Did
you ever try to communicate with someone while a tractor was
running or around a grain dryer without yelling? This can
be a major problem in an emergency situation.
Noise-induced
hearing loss is a major problem because people are unaware
of its warning signs and effects until it is too late. Since
there is strong social pressure to have normal hearing, an
individual rarely admits to having a hearing problem until
the effects are very substantial. Early awareness and corrective
action are essential to eliminating noise as a hearing hazard.
Engineering
controls, when practical and economically feasible, are the
most effective ways to reduce noise exposure because they reduce
sound level at the source. Some examples are:
- Replacing
worn, loose or unbalanced machine parts to cut down on the
amount of vibration generated.
- Making
sure that machine parts are well-lubricated to cut down
on noise exposure created by friction.
- Installing
a good, high-quality muffler on all engine-powered equipment
to reduce vibration produced by airflow.
- Isolating
yourself from the noise source with an acoustically designed
cab. In recent years, farm machinery manufacturers have
designed cabs that reduce noise level exposure to safe limits.
Many of the new tractor cabs may reduce an operator's noise
exposure by at least 10 to 15 dBs.
Before
purchasing a new tractor, consider the use of an acoustical
roll-over protective cab. Just as tractors are different,
so are cabs in their ability to reduce noise levels. Sound
data is available on most new tractors as part of the Nebraska
Tractor Test Report.
To get
information on the sound levels of various tractor models,
write to: Nebraska Tractor Test Data, Department of Agricultural
Engineering, University of Nebraska-Lincoln, College of Agriculture,
NE 68583. Ask for publication MP37. Tractor dealers should
also have this information for any tractor tested in Nebraska.
Reduction
of noise exposure on new tractors is only a part of the total
noise problem in agriculture. Additional noise engineering
must be done on grain dryers, combines, pickers, elevators,
chain saws, shellers, grinders, mixers, pulverizers, snow
blowers, conveyors and grain roller mills, to name just a
few. Noise not eliminated by engineering must be controlled
by altered work schedules.
Altered
work schedules are a second alternative to prevent noise-related
problems by reducing the amount of exposure to high sound
levels on farms. When practical, arrange work schedules so
that farm workers do not exceed the allowable exposure limit
to a high noise source.
For
example, for a tractor that produces a noise level of 95 dBA,
the safe exposure is four hours per day per person. Try to
arrange work schedules to let farm workers exchange work activities
so that no one person is exposed to the noise for more than
four hours.
Personal
protection equipment is the final alternative for farm workers
who wish to cut down on noise exposure. The two basic types
of hearing protection are ear muffs and ear plugs.
Ear
muffs are the most effective. The attenuation (noise reduction)
provided by ear muffs varies widely due to differences in
size, shape, seal material, shell mass and type of suspension.
Some may attenuate sound by as much as 40 dBs.
To get
good quality muffs, deal with a reputable firm. Examine them
for comfort, construction, seal and attenuation. Manufacturers
supply attenuation data for their product, so you can evaluate
their effectiveness.
Ear
plugs are available as pre-formed inserts made of rubber,
plastic or foam and handformed inserts of disposable materials
such as wax or Swedish Wool. For agricultural use, wax or
Swedish Wool have little value from a sanitation standpoint
(they must be changed daily and must be shaped by hand before
inserting) and because of their lower attenuation level.
Pre-formed
ear plugs may be cheaper, but due to the difference in the
shape of a person's ear canal, trained personnel should fit
each individual for plugs. The wearer must also know how to
properly insert the ear plug. When purchasing ear plugs, follow
the directions closely so that a snug, tight fit is obtained
in the ear canal when the plug is inserted.
Warning:
Cotton should never be used for the purpose of reducing noise
exposure. Cotton cannot block out high-frequency sound and
will provide no protection from high sound levels.
Ear
protective devices will not block out all sounds. They will
block out only those sounds that are dangerous to hearing.
Machinery sounds different when you are wearing ear protection,
but with continuous use, you can learn the new sounds and
still be able to determine whether the machinery is operating
properly.
Operators
who have suffered previous hearing losses may find that they
are unable to detect certain sounds necessary to assure proper
machine operation when they are using ear protection. In these
cases, it may be necessary for the operator to remove the
ear protection to check for these sounds. But remember to
properly replace the ear protection to protect against further
hearing loss.
If
you are continually exposed to high sound levels, you should
have a hearing test periodically. This test, called an audiogram,
will reveal signs of hearing loss as a result of high sound
level exposure. If a hearing loss is noted, take the necessary
steps to reduce exposure and eliminate further damage to your
hearing.
If
you allow your ears to be exposed to the invisible dangers of
agricultural noise or other high sound level sources, the result
could be a permanent loss of hearing. You can make the difference
between a life with all the joys of sound or a life of silence
by following the safe work procedures outlined in this guide.
Disclaimer
and Reproduction Information: Information in NASD does not
represent NIOSH policy. Information included in NASD appears
by permission of the author and/or copyright holder. More
NASD Review: 04/2002
This document
is
GO1962
,
a series of the University Extension, the University of Missouri-Columbia,
Columbia, MO 65211. Publication date: October 1993.
David
E. Baker, Department of Agricultural Engineering, University
of Missouri-Columbia, Columbia MO 65211.
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