PROGRESSION OF HEARING-LOSS CAUSED BY OCCUPATIONAL NOISE

This review is a compilation of II investigations by different authors regarding the progression of hearing deterioration during severe long -term exposure to noise in mines, shipyards, forges, weaving mills, ot her factories and industries and from field artillery and hunting. Wit h one exception, the reports concern conditions at times when ear prot ection was virtually unknown or only seldom used. The different invest igations are described in a broad outline with their essential measure ment and background data. For the first eight studies the results of t he hearing deterioration with increasing exposure time are illustrated in figures showing, on the one hand, the original data and, on the ot her, the progression of hearing loss with increasing age, though reduc ed by the ISO 1999 (1990) values from database A for the normal thresh old of hearing as a function of age. This is followed by a discussion of the results and a compilation of the data from the 11 investigation s for, particularly, a noise exposure time of about 5 to 10 years, cor responding to an age of around 30 years, in comparison with an exposur e time of 30 to 40 years, i.e. at an age range of about 50 to 60 years . Despite the great diversity in the character and level of the noise, the compilation shows for the higher ages in the range of 3 to 8 kHz a similar median hearing loss from nearly all investigations; however, at 1 kHz and, particularly, at 2 kHz the differences in the character of the noise are apparent in a wide spread of the median hearing loss between the different studies. In addition, it was found that at high er ages and hearing loss levels of more than 45 to 50 dB it is not pos sible to distinguish between the effect of the noise, on the one hand, and that of ageing, on the other; the ad hoc assumption of their addi tivity is no longer valid and thus the term ''age correction'' inadequ ate. A series of further figures illustrates the progression of the he aring loss with exposure time in the different investigations at parti cularly 1 kHz, 2 kHz and 4 kHz. These groups of curves differ greatly regarding the average increase in hearing loss during the first 10 to 12 years of noise exposure, from about 5 to 9 dB per decade of exposur e at 1 kHz to about 20 dB at 2 kHz and 35 to 50 dB at 4 kHz; however, during the following 20 to 30 years of work in the noise the further i ncrease is fairly similar, at 1 kHz on an average 5 to 6 dB per decade and at 2 and 4 kHz 7 to 10 dB per decade. Also presented is the progr ession of the hearing loss for the pure-tone average of 0.5, 1 and 2 k Hz and the average of 1, 2, 4 and 8 kHz. The average course of the mea n of the 0.5, 1 and 2 kHz values was found very similar to that of the 1 kHz curves and of the mean of the 1, 2, 4 and 8 kHz values similar to that of the 2 kHz curves.