[Effects of occupational noise exposure on asymmetric hearing loss in automobile manufacturing workers]

Objective: To investigate the characteristics of asymmetric hearing loss in automobile manufacturing workers and the effect of occupational noise exposure on asymmetric hearing loss. Methods: A cross-sectional approach was used in this study. From March 2017 to February 2018, Subjects (7066) from four complete vehicle factories were given a pure tone audiometry (Hearing thresholds were measured at frequencies of 0.5, 1, 2, 3, 4 and 6 kHz in each ear) and were required to complete a health-related information questionnaire. According to the inclusion criteria, a total of 6339 workers were selected. The mean hearing thresholds for the left and right ears at overall frequencies were compared using the repeated means analysis of variance (ANOVA) test. The threshold differences at each frequency were compared using paired t tests. Results: The overall mean left minus right threshold difference across all frequencies was determined to be 0.58 dB, which met statistical significance (P<0.01) . Hearing threshold in the left ear was statistically significantly higher compared with the right ear at each frequency. The differences between binaural threshold shifts at each frequency among subjects with a asymmetry in terms of worse left ear and worse right ear were at the range of 6.17-9.87 dB and 6.39-10.92 dB, respectively. Hearing threshold in the left ear was statistically significantly higher compared with the right ear at only 2, 3 kHz of subjects with high-frequency hearing threshold shifts (HFHTs) more than 25 dB. Hearing threshold in the left ear was statistically significantly higher compared with the right ear at only 3 kHz of subjects with high-frequency hearing threshold shifts (HFHTs) more than 30 dB. With the increase of HFHTs, the proportion of subjects with a asymmetry at 2 and 3 kHz of more than 10 dB in terms of worse left ear and worse right ear increased. Conclusion: The average hearing threshold of the left ear across overall frequencies is higher compared with the right ear, the proportion of the cases with a higher left ear hearing threshold is higher that that of the cases with a higher right ear hearing threshold. As hearing loss caused by occupational noise exposure getting worse, the proportion of the cases with a higher left ear hearing threshold and the cases with a higher right ear hearing threshold may tent to be the same.