Effects of low-dose styrene exposure on linear and nonlinear otoacoustic emission components

This study investigates the potential effects of low-level styrene exposure on active cochlear mechanisms. To this purpose, transient-evoked otoacoustic emissions (TEOAEs) were recorded and compared in a group of exposed workers and controls. Otoacoustic emissions are low intensity sounds connected to the active mechanisms characterizing the healthy cochlea and can be used to monitor changes in cochlear functionality due to exposure to external agents such as noise, electromagnetic fields, and ototoxic substances. TEOAEs from healthy ears are composed of the contribution of two generation mechanisms, the so-called nonlinear distortion and linear reflection. To detect possible sub-clinical changes in the two generation mechanisms, we applied two advanced and ad hoc analysis techniques: the wavelet transform (WT) and the bispectrum. The WT was used to study linear mechanisms by decomposing TEOAEs into time-frequency components; the bispectrum was applied to detect and extract nonlinear TEOAE contributions as quadratic frequency couplings (QFCs). The exposed group consisted of two cohorts of subjects exposed exclusively to styrene and subjects exposed to styrene and noise. Subjects in the control group were perfectly matched by age and sex to their peers in the two cohorts of the exposed group. In exposed subjects, linear TEOAE components showed significantly lowered SNR at mid to high frequencies (above 1600 Hz) compared to the control group; non-linear contributions, as measured by the QFC, showed in the exposed group a shift towards lower frequencies compared to the control group. No differences were observed in the latency of linear components. We found that TEOAEs characteristics of styrene-exposed workers didn't change before and after the working shift, and that styrene exposure affected the physiological effect of age on cochlear mechanisms. Overall, these results reveal that low level styrene exposure may have induced a sub-clinical modification of cochlear functionality.