A review was presented to describe the major observations obtained from laboratory studies of experimentally induced hearing losses, both temporary and permanent, resulting from exposure to noise in animal subjects. Studies that measured hearing directly were given the greatest weight in the review. The data evaluated supported several general conclusions. The chinchilla was the most widely used and most appropriate animal model for studies of noise induced hearing loss. With continuous exposures to moderate level noise, thresholds reached asymptotic levels within 18 to 24 hours. Permanent threshold shifts, however, depended upon the level, frequency, and the duration of exposure. Below a critical level of about 115 decibels, permanent threshold shift and cell loss were generally related to the total energy in continuous exposures. Periodic rest periods inserted in an exposure schedule were predictive and resulted in less hearing loss and cochlear damage than equal energy continuous exposures. Under some schedules of periodic exposure, threshold shifts increased over the first few days of exposure, then recovered as much as 30 decibels as the exposure continued.
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