Noise Induced Hearing Loss

"How much of the hearing loss of aging is due to noise exposure?" Actually, this is a difficult question that often arises in litigation, when an aging patient is seeking compensation for his hearing loss from years of industrial noise exposure. Quite certainly, the two types of loss coexist in many individuals and one aggravates the other. However, characteristic audio-metric differences are noted in the pure forms. The early loss from noise damage shows up as a dip at 4 KHz most often, with recovery at 8 KHz. Sometimes the loss is greater at 3 KHz or 6 KHz but, in any case, a V-shaped "audiometric notch" is present, and this deepens as damage increases (Fig. 7.2).

A typical scenario of acute noise-induced loss might be as follows. An adolescent sits in the front row at a rock concert for 4 hours, with the speakers blaring at 105 dB HL. He comes home with his ears ringing, and this continues for several days. If the hearing is tested in the acute phase, a 40 dB HL drop at 4 KHz might show up on the audiogram. With luck, he completely recovers in a week, with a normal repeat audiogram, although some undetectable hair cell damage may now exist. This

Frequency in Hertz 125 250 500 1K 2K 4K 8K

Frequency in Hertz 125 250 500 1K 2K 4K 8K

Noise Damage Audiogram
Fig. 7.2 Audiogram showing examples of progression of noise-induced hearing loss, right ear.

occurrence is known as a temporary threshold shift (TTS). The outcome can be worse. Even brief, but extremely loud, noises can cause unilateral or bilateral permanent loss. Recently, auto air bags deploying, with tremendous dB levels, have been responsible for a number of cases of permanent loss.

Chronic noise exposure, with its resultant damage, is dose related. However, individuals vary in susceptibility. Much research has been done on the levels of noise and duration of exposure needed to cause damage. Government regulators (i.e., OSHA) have developed exposure criteria requiring ear protection in the workplace. A few concepts are worth explaining. Hearing levels and noise levels are measured in decibels, which actually represent a logarithmic scale with respect to sound energy. An increase of 3 dB in noise level doubles the actual sound intensity. Also, the duration of exposure plays an equally important part. Hence, 96 dB HL of sound for 40 hours is considered twice as damaging as 93 dB HL for the same period. In other words, 5 years of work in a factory averaging 96 dB HL of noise causes the same damage as 10 years at 93 dB HL.

Research has shown that prolonged exposure to 90 dB HL or more may produce noise damage in some individuals, although the percentage of people affected is low. As the sound level and duration of exposure increase, so does the prevalence of damage. Many occupational settings have sustained noise at higher levels than 90 dB HL, and ear protection is mandated by law.

Preventing Noise Damage

The workplace is not, of course, the only source of potential noise-induced loss. Home power tools, chain saws, loud headphones, and firearms are other common menaces to hearing. When these devices are used, ear protection is the answer. (Obviously, the volume to one's headset should be kept down!) In-the-ear plugs and muff-type protectors are available at sports stores. They may be worn together to increase protection, although perspiration may be a problem. Noise levels might be decreased as much as 30 dB HL. For musicians who must hear without distortion, audiologists can fit specialized ear-protection molds that attenuate all the frequencies equally. It is important to advise the patient with chronic noise-induced loss that the damage is permanent and irreversible and emphasize the need to remedy the situation. The negative peak we see in early losses will deepen and widen over time if exposure continues, as shown in Figure 7.2

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