Apportionment of noise-induced hearing disability and its prognosis in a medicolegal context: a modelling study

Apportionment of liability for noise-induced hearing loss is required in medicolegal work when two or more separate instances of noise exposure have occurred. In general there are insufficient audiometric records to determine how much hearing loss was caused by each noise exposure, and hence there is insufficient information on which to base apportionment of liability. Apportionment must then be based on a theoretical understanding of the way in which hearing loss is related to patterns of noise exposure and how the increased hearing threshold levels lead to greater disability. A model is developed which combines existing models of the relationship of hearing impairment to noise exposure and age, and the relationship of hearing disability to impairment. The model is evaluated for a wide range of notional patterns of noise exposure, leading to a simple equation which predicts the relative attribution of disability to previous noise exposures sufficiently accurately for the purposes of apportionment. The model also shows that disability due to noise exposure generally increases slightly with increasing age even after noise exposure has ceased. Thus the difference in disability between a person exposed to noise and a noise-free control of the same age and sex can be expected to increase somewhat as the subject and control grow older. However, the magnitude of this effect, often termed prognosis, is shown to be small and can be assumed to be zero in practice.