Relation between hearing threshold level and its component parts.

A model is presented showing how different components of threshold shift combine. It has long been evident that simple arithmetical addition cannot apply, but it is only recently that experimental data have been shown to validate a simple interaction between age- and noise-related components. The combination formula extends to sensory loss from other causes. Used in conjunction with growth functions for the age and noise components, it facilitates both the prediction of future hearing loss and the 'retrodiction' of past hearing threshold levels at any point in the life of the average person, given the present age and the noise exposure history. By reconstructing the hearing thresholds for a notional 'life' in which an episode of noise exposure is eliminated, the formula enables the specific effect of that episode to be evaluated at later times. As with any such system, uncertainties arise in the application to individuals. Possible ways forward are discussed.