The Relationship between Occupational Noise Exposure and Noise Induced Hearing Loss (NIHL) in Small-Scale Industries: A Case Study in the City of Damavand, Iran

Exposure to the excessive levels of occupational noise is one of the principal harmful agents affecting the workers’ health. This study aimed to investigate the relationship between the occupational noise exposure and the hearing loss caused by working in small-scale service industries in the city of Damavand, close to the metropolitan capital city of Tehran, Iran. This descriptive cross-sectional study investigated the occupational noise levels within the 90 small-scale industries (mainly service industries and workshops) working under the supervision of Damavand healthcare network governed by the Iranian ministry of health and medical education. A sound level meter (Bruel and Kjær 2250) was employed to determine the noise exposure levels based on the dB A, and according to the standard ISO 9612: 2009. The audiometric exam tests were performed by an audiometer (model MEVOX SA-900). The obtained data were then analysed by SPSS 16, using linear regression and t-test. The highest measured 8-hour equivalent continuous sound pressure levels (Leqs) were associated with auto body mechanics (89.2 dB A), foundry and casting workers (88.8 dB A), aluminium products fabrication workers (86.32 dB A), blacksmiths and forging (85.8 dB A) carpenters (84.93 dB A), and cabinet manufacturers, respectively (84 dB A). Results from the hearing threshold shifts of the workers from the studied occupational groups revealed that the highest work-related hearing loss associated with the right ear occurred among the auto body mechanics, aluminium products fabrication workers and carpenters. However, the most significant work-related hearing loss associated with the left ear was noticed among carpenters, aluminium products fabrication workers, and auto body mechanics, respectively. Pearson correlation coefficient was tested between Leqs, work experience and hearing loss, and the results implied that the progress of workers’ hearing loss was correlated with the increase in work history and experience. The 8-hour Leqs and work experience were, respectively, the most important factors affecting the rate of hearing loss among the participants of this study.