How is Projected Hearing Loss Calculated?
Presbycusis can be defined as hearing loss due to aging. Two researchers, Sutton and Spoor, have pooled a variety of research study data to try and predict the level of presbycusis based on current age, gender, and hearing test results. Sutton's equations were derived by combining a number of studies and are adopted for the values in ISO-1999 Annex A (International Standards Organization, Annex A estimates hearing levels for an industrial population that was screened for exposure to gun and intense industrial noise). Spoor's equations combined data from 8 different studies and controlled for noise in the derivations of his equations.
These equations are to help provide guidance and not prove causation. Take the following scenario... If an employee begins work with a preexisting hearing loss, but leaves the job at some point in the future with a more severe hearing loss... is the increase in hearing loss due to the employment or to the effects of aging and his environment? Keep in mind that these equations help evaluate legal causation, what is more likely than not to be true, and not medical causation which requires proof that the individual's conduct was a necessary condition of the hearing loss.
For example... there are MANY reasons why hearing loss may have occurred beyond stated employment and age: listening to loud music, mowing the yard without hearing protection, hunting, etc. Medical causation requires evaluating all these different causes of hearing loss and establishing a direct link between cause and the hearing loss effect.
Indeed... NIOSH does NOT recommend accounting for presbycusis when looking for medical causation of a progressive hearing loss. As per DHHS (NIOSH) Publication No. 98-126 Section 5.1: "NIOSH does not recommend that age correction be applied to an individual's audiogram for significant threshold shift calculations. Although many people experience some decrease in hearing sensitivity with age, some do not. It is not possible to know who will and who will not have an age-related hearing loss. Thus, applying age corrections to a person's hearing thresholds for calculation of significant threshold shift will overestimate the expected hearing loss for some and underestimate it for others, because the median hearing loss attributable to presbycusis for a given age group will not be generalized to that experienced by an individual in that age group."
If you are more interested in the average hearing loss for a given age, check out this webpage.
Robinson, DW and Sutton, GJ. Age effects and hearing. A comparative analysis of published threshold data. Audiology 18:320-334,1979. As also described in Annex A of ISO 1999-01-15.
ISO 1999 Acoustics. Determinaton of occupational noise exposure and estimation of noise-induced hearing impairment. International Standards Organization, Geneva, 1990.
Spoor A. Presbycusis values in relation to noise induced hearing loss. Int. Audiology. 6:48-57,1967.