Prolonged exposure to industrial noise: Cochlear pathology does not correlate with the degree of permanent threshold shift, but is related to duration of exposure

Abstract

It has been shown that the damaging effects of noise on the mammalian inner ear strongly depend upon parameters of exposure, such as the intensity of noise, the duration of exposure, etc., but the relationship between permanent hearing loss and cochlear damage still remains unclear. In this study, we were interested in the damaging effects of exposure to typical steady-state, wide-band industrial noise acting for as long as 12 weeks on guinea pigs. The aim of the study was to evaluate the relationships among the duration of exposure to industrial noise, the level of permanent hearing loss and the degree of cochlear injury and to assess the possible mechanisms of industrial noise-induced hearing loss. The results of the study indicate that 1) prolonged exposure to industrial noise causes in guinea pigs a relatively fast increase in permanent hearing threshold shift (PTS) up to 30 dB, which reaches the asymptotic level after 4 weeks of exposure; 2) the first 30 dB of PTS is caused almost exclusively by the damage to outer hair cells; 3) the degree of cochlear damage is not related to the level of permanent threshold shift; 4) the progression in cochlear pathology depends upon the duration of exposure; 5) both micromechanical and metabolic mechanisms seem to be involved in the development of industrial noise-induced hearing loss.