Neuropharmacological and cochleotoxic effects of styrene : Consequences on noise exposures

Abstract
Introduction
Occupational noise exposure can damage workers' hearing, particularly when combined with exposure to chemicals such as styrene. Although styrene-induced cochlear impairments become apparent after a long incubation period, the pharmacological impact of styrene on the central nervous system (CNS) can be rapidly measured by determining the threshold of the middle-ear acoustic reflex (MER) trigger. The aim of the study was to evaluate the effects of a combined exposure to noise and styrene on hearing.
Method
Two different noises (continuous vs. impulse) were tested. If the LEX,8h of the impulse noise was lower (80 dB SPL) than that of the continuous noise (85 dB SPL), the spectrum was identical. That was an octave band noise centered at 8 kHz. Two concentrations of styrene were generated: 300 ppm and 600 ppm. The exposure lasted 6 hours per day, 5 days a week for 4 weeks.
The impact on hearing was assessed using distortion product oto-acoustic emissions, and histological analysis of cochleae (cochleograms).
Results
Although the LEX,8h of the impulse noise was lower than that of the continuous noise, it was more detrimental to the peripheral auditory receptors. Surprisingly, a co-exposure to 300-ppm styrene and continuous noise was less damaging than exposure to continuous noise alone. In contrast, a co-exposure to 600-ppm styrene and continuous noise was more damaging than exposure to continuous noise alone. Regardless of the styrene concentration, the traumatic effects of impulse noise on the cochlea were enhanced by co-exposure to the solvent.
Discussion
The pharmacological effect of styrene on the CNS is determinant to explain these results. Styrene can change the threshold of the MER trigger. Therefore, the CNS effects of styrene play a crucial role in a combined exposure. Based on these results, the temporal structure of the noise should be reintroduced as a key parameter in hearing conservation regulations.