Abstract
This review is a compilation of 11 investigations by different authors regarding the progression of hearing deterioration during severe long-term exposure to noise in mines, shipyards, forges, weaving mills, other factories and industries and from field artillery and hunting. With one exception, the reports concern conditions at times when ear protection was virtually unknown or only seldom used. The different investigations are described in a broad outline with their essential measurement and background data. For the first eight studies the results of the hearing deterioration with increasing exposure time are illustrated in figures showing, on the one hand, the original data and, on the other, the progression of hearing loss with increasing age, though reduced by the ISO 1999 (1990) values from database A for the normal threshold 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 investigations for, particularly, a noise exposure time of about 5 to 10 years, corresponding to an age of around 30 years, in comparison with an exposure 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 higher ages and hearing loss levels of more than 45 to 50 dB it is not possible to distinguish between the effect of the noise, on the one hand, and that of ageing, on the other; the ad hoe assumption of their additivity is no longer valid and thus the term “age correction” inadequate. A series of further figures illustrates the progression of the hearing loss with exposure time in the different investigations at particularly 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 exposure 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 increase 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 progression of the hearing loss for the pure-tone average of 0.5, 1 and 2 kHz and the average of 1, 2, 4 and 8 kHz. The average course of the mean 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.