Determinants of Occupational Hearing Loss

Abstract

ISEE-220 Objective: It is essential to study predictive factors of occupational hearing loss. The transient threshold shift (TTS) at high frequencies (4 kHz) after exposure to low frequencies (400 Hz) was hypothesized to predict a higher sensibility of the inner ear. Material and Methods: Starting in 1982, all apprentices at an Austrian steel mill underwent an examination including the determination of the TTS at 4 kHz after a 20-minute exposure to 400 Hz/105 dB. TTS was recorded every 30 seconds from 1.5 till 10 minutes after the exposure. Also, before the experimental noise exposure a Bekesy audiogram was recorded. Among others the difference between “no longer heard” and “heard again” at 4 kHz (diff-dB) was noted. In total, 125 of the apprentices remained in the same enterprise and were exposed to noise at work. In regular follow-up examinations audiometry was performed in this group. Results: Hearing loss (PTS) was significantly influenced by the following parameters: noise-level at the workplace, years of noise exposure (or age, because both were highly correlated with each other), TTS at first examination, diff-dB, and the reported frequency of wearing hearing protection. TTS values at every 30 seconds after exposure were highly correlated. So, only TTS after 2 minutes (that was strongest related to PTS) was kept in the final model. Also diff-dB before and at different intervals after exposure were highly correlated. As the best predictor for PTS, the difference between diff-dB before exposure and 2.5 minutes after exposure (d2.5diff-dB) was chosen for the final model. In this final (multiple regression) model the noise level at the workplace was no longer significant. This is likely due to the high correlation between (adverse) noise level and (protective) frequency of personal protective measures. Conclusions: The importance of personal protective measures was again highlighted in this study. But, in addition, individual (preexisting) factors of susceptibility influence the impact of noise exposure.