Noise-Induced Hearing Loss

Exposure to sufficiently intense noise for a long enough duration results in damage to the inner ear and thus decreases one's ability to hear. In addition to a general decrease in the ability to detect sounds, the quality and clarity of auditory perception can be affected, as well. While these effects are often temporary, it is not uncommon for some residual permanent damage to persist for the remainder of the affected person's life. (Miller, 1979a)

Whether temporary or permanent, hearing loss due to noise exposure primarily affects the inner ear, especially when the noise is presented over a significant period of time. Specifically, it is the organ of corti that is most commonly affected (Bugliarello, Alexandre, Barnes, & Wakstein, 1976; Miller, 1979a). (View discussion of the anatomy and physiology of the ear). Sataloff (1965) describes the effect, "sound induced motion of the fluid in the cochlea induces shearing and bending movements of the hair cells in the Organ of Corti, which, in turn, result in electrical stimuli transmitted by the auditory nerve. Prolonged and excessive noise eventually produces deterioration and, finally destruction of hair cells, and thus disrupts the sound transmission mechanism" (p. 225). Liberman (1990) continues, "the most vulnerable elements in the ear are the sensory cells themselves, in particular their stereocilia and the rootlets which anchor them to the cuticular plate" (p. 17).

In general, the effects of noise exposure on hearing can be divided into three categories: acoustic trauma, noise-induced temporary threshold shift (NITTS), and noise-induced permanent threshold shift (NIPTS). Acoustic trauma, which results from a single or relatively few exposures, is defined as "immediate organic damage to the ear from excessive sound energy" (Melnick, 1979, p. 9-1). If the noise is intense enough, other structures outside the inner ear may also be affected, such as the eardrum, which may become ruptured. However, Miller (1979a) notes that such damage is rare, and occurs only in instances involving "extremely intense noise and blasts" (p. 114). Additionally, acoustic trauma often causes some degree of permanent damage to the auditory system (Melnick).

The second and third types of effects on the hearing system that are produced by noise, NITTS and NIPTS, involve an increase in a person's auditory threshold. That is, a person's auditory sensitivity decreases. The difference between these two effects is that in NITTS the threshold eventually returns to its original level, whereas in NIPTS the threshold shift is permanent. Thus, a person with NIPTS has "no possibility of further recovery" (Melnick, 1979, p. 9-1). Though NIPTS can sometimes result from acoustic trauma, as described above, it more often results from long periods of repeated exposure to noise. This exposure is often the result of one's occupation (Hallberg, 1996), and hearing damage has been shown to increase with higher average occupational noise levels (van Dijk, Souman, and de Vries, 1987).

On to Non-auditory physiological effects