Time to Noise-Induced Hearing Loss among Different Type of Shift Work among Steel Workers: A Survival Study

BACKGROUND: There have been few reports on relationships between Shift Work (SW) and time to Noise-Induced Hearing Loss (NIHL). This research explores the relation between SW and time to NIHL among male workers. METHODS: Between 1990 and 2015, this historical cohort study was conducted. Two different definitions events (Low Frequency Hearing (LFH) and High Frequency Hearing (HFH)) of NIHL An average pure-tone hearing threshold was thresholds at 1, 1.5 and 2 kHz for LFH(un-normal>20) and 4, 6 and 8 kHz for HFH (un-normal>20) for Air (AC) and Bone (BC) conduction, in both ears, respectively. In this study Kaplan–Meier and Multilevel Cox Model in addition R version 3.2.1 and MLWiN software used to analysis data. P-values less than 0.05 are considered to be statistically significant RESULTS: There were 6632 male workers in the research survey (mean age=35(±5)). Among these subjects, 2678 (40%),278 (4%) and 3676 (56%) were Day Workers (DW), Weekly Rotating Shift workers (WRS) and Routinely Rotating Shift workers (RRS), respectively. The result of this study showed more hazard risk of ear problem in LFH (Hazard=1.55 with 95% CI= (1.06–1.39)) and HFH (Hazard=1.08 with 95% CI= (1.04–1.13)) in RRS rather than day workers. In WRS group ear problem has been seen just in LFH (Hazard=1.55 with 95% CI= (1.2–2.0)). CONCLUSION: The findings of the 15-year historical cohort study generally support a relationship between SW and time to NIHL. Therefore, more actions in the field of noise control, greater use of safety devices as well as increased staff training on the use of safety devices recommended for shift worker.