INTERACTION OF NOISE-INDUCED PERMANENT THRESHOLD SHIFT AND AGE-RELATED THRESHOLD SHIFT

Current medical-legal practices as well as an international standard ( ISO 1999) assume the permanent threshold shifts produced by exposure t o noise add (in dB) to the threshold shifts caused by increased chrono logical age (presbyacusis). This assumption, known as the additivity r ule, was tested in an animal model. Mongolian gerbils, born and raised in a quiet vivarium, were exposed at age 18 months to a 3.5-kHz pure tone for 1 h at 113 dB SPL. At 6-weeks post-exposure, permanent thresh old shifts in the exposed ear were approximately 20 dB in the 4- to 8- kHz region. Thresholds in the nonexposed, control ear were unaffected by the exposure. Animals were then allowed to age in the quiet vivariu m until age 36 months and then were retested. Thus in a given animal, aging-only effects were assessed in one ear (internal control) and noi se-plus-aging effects were assessed in the other (test) ear. A second control was mean age-related threshold shift measured in 48 gerbils wh o were born and raised in the quiet vivarium. This group is referred t o as a non-noise-exposed population (population control). Using the ad ditivity rule, predictions with either the internal or population cont rol significantly overestimated noise-plus-aging effects. Use of the I SO 1999 compression factor reduced the overestimations by 0-5 dB. The intensity rule produced the most accurate predictions. These results s uggest that the interaction of noise-induced permanent threshold shift and age-related threshold shift is not straightforward and that curre nt medical-legal methods using the additivity rule over estimate the c ontribution of ''noise effects.'' (C) 1997 Acoustical Society of Ameri ca.