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Noise is hazardous to health mainly because it can damage the ear, but it may also influence other bodily functions. A temporary or permanent decrease in hearing acuity, such as that from noise exposure (noise induced hearing loss, NIHL), may impair speech communication. Noise can also mask speech and warning signals and thus poses a risk to safety and to the general health of workers.
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The most apparent and best-known health risk from noise is damage to hearing, so this will be addressed first. The other effects of noise are dealt with later in the chapter.
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EFFECT OF NOISE ON HEARING
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In this chapter, we use the word noise to describe sound that may be damaging to hearing, because this word has traditionally had negative connotations and thus will be identified more readily with health hazards. The potential of noise to damage hearing, however, is entirely related to its physical properties. The amount of NIHL that is acquired is related to the intensity and durations of the noise exposure and the character of the noise (spectrum and time pattern). The character of the noise—whether it is continuous or transient and its spectrum—also plays a role and different types of noise pose different degrees of risk to hearing, even though the overall intensity of the noises is the same; impulsive sounds such as that from gunshots generally pose a greater risk than continuous noise.
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Low-frequency sounds are considered to be less damaging than high-frequency sounds of the same physical intensity. Therefore, when noise intensity is measured with a sound-level meter for predicting its effect on hearing, a frequency weighting is used. The commonly used weighting (A-weighting) gives energy at low frequencies less weight than energy at high frequencies. The importance of the temporal pattern of noise is more difficult to represent in standard measurement of noise level.
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Since it is the physical characteristics of the sound that determines its potential for causing hearing loss, the origin of the sound has no influence upon the degree of risk it presents for hearing damage, and sounds to which people are exposed during recreational activities pose as great a risk to hearing as noise that is associated with work activities such as in industry. Activities where people are exposed to gunshot noise in particular pose a high risk of inducing NIHL.
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There is great variation in an individual person's susceptibility to noise-induced hearing loss (NIHL) and, therefore, only the average probability for acquiring a hearing loss can be predicted on the basis of knowledge about the physical characteristics of noise and the duration of exposure to noise.
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Temporary Threshold Shift and Permanent Threshold Shift
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The first effect noticed when an ear is exposed to sounds above a certain intensity and for a certain time is a reduction ...