The progression of noise-induced hearing loss. A survey of workers in selected industries in Canada.

An in-depth investigation of the progression of noise induced hearing loss is reported for individuals working in three major Canadian industries. The particular approach taken was to select occupational groups within each industry, who had the same noise exposure for a continuous period, and on whom standard hearing tests were undertaken at regular intervals. Two methods of evaluating the audiometric data were used. The first was a cross-sectional design, in which one audiogram was analyzed for each employee. The second focused on the rate of hearing loss over time within individuals. The results confirmed previous reports that the effect of continuous noise exposure was maximal in the region of 2-6 kHz. Significant differences in hearing loss were noted across job types. Over a 10 year period the rate of loss within exposed individuals was on the average 1.5 dB per year for 4 kHz, as compared with 0.5 dB for control subjects who held office jobs. The absolute difference between hearing thresholds measured at the beginning and end of this period ranged widely from a slight improvement in hearing to losses often as great as 55 dB. In general the greatest loss occurred at 4 kHz. The number of frequencies at which there was a risk of exceeding a 25 dB fence increased with the number of years of exposure.