Neuropharmacological and Cochleotoxic Effects of Styrene. Consequences on noise exposures

Both noise and styrene are damaging for the organ of Corti. Since the solvent is often present with noise, this co-exposure must be considered as a typical framework for workers risk assessment. If styrene impairs the cochlea by a long process, it has quick pharmacological impacts on the central nervous system. It can modify the threshold of the trigger of the middle-ear reflex (MER). In the present investigation, the consequences of a co-exposure to noise, associated with a low concentration of styrene [300ppm<(TLV x 10) safety factor], have been studied in rat. The aim of the study was to evaluate the impacts of a continuous noise regarding to those of an impulse noise, having the same spectrum, when they are, or not, combined with a 300-ppm exposure to styrene. Hearing was tested using distortion product oto-acoustic emissions, and completed by a histological analysis of the cochlea. Although the LEX,8h of the impulse noise was lower (80 dB SPL) than that of the continuous noise (85 dB SPL), it was more harmful for the peripheral auditory receptors. Because a 300-ppm concentration of styrene decreased the threshold value of the MER trigger, the “6-h continuous noise plus 300-ppm styrene” exposure was less damaging than the 6-h continuous noise alone. On the contrary, the 300-ppm styrene exposure potentiated the impulse noise-induced traumatic effects on the organ of Corti. Surprisingly, the cochleotoxic effects of a moderate-intensity continuous noise might be counterbalanced by the pharmacological effect of the solvent. Basic modes of actions of a co-exposure “noise plus solvent” are an important step when dealing with human risk assessment. A fast test for measuring the threshold of the MER trigger should be included in hearing conservation program to detect the pharmacological impact of solvents on the auditory central nervous system.