Hearing and Noise in Industry

This book gives the results of what is probably the most definitive study so far of noise-induced hearing loss from exposure to steady noise. The survey was sponsored in 1962 for the investigation of occupational hearing loss from the standpoint of possible prescription under the Industrial Injuries Act and, within 18 months, a team from the MRC Wernher Research Unit on Deafness was examining workers in the special mobile audiometric laboratory at the first of the 32 factories to be visited. Up to three return visits were made over the next five years for retests of hearing and, during this time, the participating scientists and assistants numbered almost 40, including those from the National Physical Laboratory responsible for the noise measurements and for analysis of the data in conjunction with the corresponding hearing measurements. A final report of the study was referred to the Industrial Injuries Advisory Council in February 1969 and an appraisal of the report by the Council has been presented to Parliament. The book is the final report, with some minor amendments. It consists of a 50-page description of the study and summary of the results preceding 16 appendices which make up nearly another 200 pages. Unforeseen difficulties are described, which were encountered in the course of the survey, not the least of which was the very small numbers of noise-exposed subjects found to be suitable for investigation at even the largest factories. Many subjects were unsuitable for reasons of exposure to considerable amounts of gunfire, to previous occupational noise, pathological conditions, and because their present noise exposure appeared to be too variable for accurate specification. Throughout the early stages of the investigation emphasis was given to finding subjects for a prospective study. As this proved virtually impossible, the main results are derived from retrospective and serial studies on nearly 800 subjects who worked in noise levels of 75 to 120 dB(A), and about 100 controls. The mean hearing level measured on the various control groups is shown to be significantly better than ISO standard. Important findings include confirmation of the equalenergy basis for occupational hearing loss and, contrary to the results of other investigations, the continued increase of age-corrected loss beyond 10 years' exposure. On average, the 4-kHz dip was found in all the noise situations of the survey. The statistical distribution of the loss expected in a population exposed to a given noise is given both graphically and as a formula. Temporary threshold shift measurements were undertaken with a view to estimating susceptibility of the individual to occupational hearing loss, as has been attempted by many previous workers. An uncertainty not mentioned by the authors exists here, however, in that a person's noise susceptibility may well vary from day to day, as does his susceptibility to other adverse environmental effects such as to the pressure changes encountered in deep diving. Low correlations were obtained between indices of susceptibility to temporary and permanent effects and therefore a practical test of noise susceptibility is not proposed as a result of this work. Industrial medical officers will be greatly interested in this detailed account of the investigation but the book serves to emphasize how much more remains to be done. Now that the long-awaited results are available, consideration is being given to the legal and practical problems arising over possible prescription. For the purposes of the survey, only pure tone tests were employed but, for dealing with individual cases where compensation is involved, experience has shown that it is essential to have available more sophisticated tests including the newer objective tests of hearing. Should the investigation result in the acceptance of occupational hearing loss as a prescribed disease, a necessary preliminary must be adequate training courses and improved prospects for the many levels of audiological personnel required to conduct large-scale assessments.