Introduction We aimed to prospectively assess the combined effect of smoking and occupational noise exposure on hearing loss.

Methods A prospective study was conducted using health check-up data collected between 2008 and 2015 in the Japan Epidemiology Collaboration on Occupational Health Study. The participants were 31 444 employees, who were aged 30–59 years, free of hearing loss at baseline. Smoking (never, former, or current) and occupational noise exposure (yes or no) were identified by a self-administered questionnaire at baseline. Major outcomes were high (hearing thresholds at 4 kHz >40 dB in at least one ear) and low (hearing thresholds at 1 kHz >30 dB in at least one ear) frequency hearing loss. Cox proportional hazards model was used to estimate the combined effect of smoking and occupational noise exposure on hearing loss, adjusting for covariates.

Results During follow-up (a median of 6 years), 2101 individuals developed high-frequency hearing loss, and 979 developed low-frequency hearing loss. Compared with never smokers who reported no exposure to occupational noise, the hazard ratio (95% confidence interval) for high-frequency hearing loss was 1.17 (1.01,1.35) for past smokers without occupational noise exposure, 1.61 (1.43, 1.82) for current smoker without occupational noise exposure, 1.58 (1.32, 1.89) for never smokers with occupational noise exposure, 1.71 (1.39, 2.11) for past smokers with occupational noise exposure, 2.04 (1.75, 2.38) for current smoker with occupational noise exposure. For low-frequency hearing loss, the corresponding hazard ratios were 0.85 (0.68, 1.05), 1.08 (0.91, 1.29), 1.17 (0.92, 1.49), 1.06 (0.75, 1.48), 1.42 (1.13, 1.78). Dose response relationship between smoking intensity and hearing loss was observed in both people with and without noise exposure.

Conclusion Smoking is a risk factor for hearing loss, independent of occupational noise exposure, and its combined effect on hearing loss with occupational noise exposure is additive.