Interaction of noise-induced hearing loss and presbyacusis.

A medical-legal and scientific topic of longstanding interest is the interaction between presbyacusis and noise-induced permanent threshold shift (NIPTS). Current medical-legal practices as well as international standard, ISO 1999 International Organization for Standards: Acoustics: Determination of Occupational Noise Exposure and Estimation of Noise-Induced Hearing Impairment. ISO 1999. Geneva, International for Standards, 1990 assume that NIPTS and hearing loss caused by the aging process add in dB. Results of laboratory studies with animals are inconsistent in their support of the "additivity assumption". When intense, short-duration exposures are used, the predictions of the combined effects of noise and age are too large. The additivity model appears to be supported with long-duration exposures, but we question the accuracy of such predictions. The animal studies reviewed here suggest that the allocation of hearing loss in an older individual into a noise component and an aging component is much more complex than "additivity in dB".