The effect of vibration on hearing may be due to the transfer of vibration to the inner ear, which could directly affect the hearing ability or may increase subject's susceptibility to NIHL, but we did not find a which study measured the exact amount of WBV really reaching the inner ear.
The limitation of our study was unavailability of pre-employment audiograms in some cases, hence, we considered the first available audiogram in the medical records as the baseline audiogram for the comparison. While most experimental studies evaluated the effects of sinusoidal vibration, we assessed the combined interaction of WBV and noise on human body in the real field wherein the complex vibration pattern is more common than the sinusoidal pattern. Furthermore, this study was designed as historical cohort, while previous researches had a cross-sectional design.
In our study, despite the unauthorized exposure of train drivers to noise and WBV, the STS and hearing threshold shift at higher frequencies were not more prevalent compared with shunters who were exposed only to impermissible noise levels; hence, no interaction was found between noise and vibration in this study. Further prospective research in the real workplaces using advanced audiometric methods such as otoacoustic emission and auditory brainstem response is recommended.