Hearing and Hearing Loss Prevention

Exposure to loud noise destroys the hair cells in the inner ear. Once destroyed the hair cells cannot be replaced. Destroyed hair cells equate to loss of hearing. The loss of inner ear hair cells can also result in a loss of lateral inhibition whereby surviving adjacent hair cells and nerve endings are no longer "controlled" by the missing hair cells and fire spontaneously. This is one explanation for an accompanying condition to a noise-induced hearing loss called TINNITUS, A CONSTANT RINGING IN THE EARS WHICH CAN BE MORE DEBILITATING AND ANNOYING THAN THE HEARING LOSS ITSELF.

For more information on tinnitus visit the sites below:

HEARUSA
American Tinnitus Association
USACHPPM Tinnitus Fact Sheet

How noise induced hearing loss affects speech

Hearing loss due to noise exposure is Progressive. It occurs over a long period of time and is called noise induced hearing loss. Loss of hearing due to noise exposure is Painless. By the time hearing loss is noticed, the damage has been done. Hearing loss due to noise exposure is Permanent. Hearing that is lost cannot be regained. Unprotected exposure to noise over time may also lead to other health problems such as high blood pressure, and an increased anxiety level.

Hearing loss due to noise exposure can also occur as a result of a one time very loud, very short duration noise such as weapon discharge. Hearing loss of this nature is called acoustic trauma. Acoustic trauma may permanently damage the inner, middle and or outer ear instantaneously.

Sometimes the sound pressure level that creates a noise can be so intense that it can cause damage to soft tissue organs in the body organs such as the lungs or liver. An explosion is a good example of the kind of noise that can damage body organs. The damage to body organs is called a non-auditory effect of noise exposure. The intense sound pressure that creates the noise is referred to as blast over pressure in the scientific world. For more information about blast over pressure, contact the USACHPPM Ergonomics Program.

The graph below is of an audiogram. Audiograms are used to graph an individual’s ability to hear. The numbers across the top that go from 125 - 8000 Hertz (Hz) represent frequency or pitch. The frequencies progress from low (125 Hz) to high (8000 Hz). The numbers along the left side of the graph, represent intensity or loudness. Higher numbers represent greater sound intensity. For example, 30 dB is louder than 10 dB. 0 dB does not mean the absence of sound, it represents the softest level at which the normal human ears can detect sound is present. When a hearing test is given, tones are presented to each ear separately for each frequency. The softest level at which an individual responds to the tones is called threshold and is recorded on the graph for each ear. Usually circles are used to responses from the right ear an “X” is used to record responses from the left ear. The greater the amount of intensity needed to hear a tone, the greater the hearing loss. 0-25dB is a normal hearing range for adults.

Hearing loss due to noise exposure usually occurs in the high frequencies. Because speech sounds that give meaning to words (ch, th, sh, f, p) are high frequency sounds, hearing loss due to noise exposure causes difficulty understanding words.

When is sound harmful?

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When steady state sounds (sounds lasting .5sec or longer) reach a level of 85 dB(A) and the length of exposure is 8 hours or longer, hearing loss can occur if the ear is not adequately protected. The chart to the right shows the relationship between sound level and exposure time limit. Notice that as the sounds get louder the exposure time gets shorter.

Most people do not walk around with a device to measure noise levels so it might not be obvious when sound is hazardous. A good rule of thumb to use is the "three foot rule," which says, "if you must shout above the noise to be heard by someone standing three feet away then the noise could be damaging your hearing."

Also, if after leaving a potentially noise hazardous area, you experience tinnitus (ringing in the ears) or people talking to you sound as though they are mumbling, you could be experiencing temporary hearing loss due to noise exposure. Over time, temporary hearing losses may lead to permanent hearing loss.

Impulse or impact noise such as the discharge of a weapon or explosion can damage hearing instantly When impulse noise reaches a level of 140 dB (P) hearing loss can occur if the ear is not adequately protected.

Sound Level	Exposure Limit
85 dB (A)	8 hours
88 dB (A)	4 hours
91 dB (A)	2 hours
94 dB (A)	1 hour
97 dB (A)	30 minutes
100 dB (A)	15 minutes
103 dB (A)	7.5 minutes
106 dB (A)	3.75 minutes

Preventing hearing loss due to hazardous noise exposure

The best way to prevent hearing loss due to noise exposure is to eliminate or reduce the noise level so that it is not hazardous. When noise cannot be eliminated, individuals must be protected from the noise by wearing hearing protection. Other ways to prevent hearing loss due to noise exposure is to limit the amount of time around noise, or increase your distance from the source of the noise.

Noise hazards are not only found on the job they can also be found at home and when participating in recreational activities. Ensure the same hearing loss prevention techniques are practiced at home and on the job.

Commanders and Supervisors must provide a hearing conservation program to protect the hearing of workers exposed to hazardous noise levels on the job. Employees must wear hearing protection when working around noise hazards. Military personnel exposed to noise hazards are required to wear hearing protection when working in industrial settings and during training. It is the responsibility of the employee/soldier to practice hearing loss prevention. It is also important to remember to practice hearing loss prevention off the job as well.

Personnel exposed to noise must also receive an annual hearing test to monitor their hearing.