Neuropharmacological and cochleotoxic effects of styrene. Consequences on noise exposures

• Styrene exposure has raised concerns for hearing.
• Despite similar energy, impulse and continuous noises do not have the same traumatic impact on cochlea.
• Styrene potentiates the effects of impulse noise.
• Moderate exposure to styrene reduced the auditory damage caused by continuous noise.
• Previous studies showed that styrene lowers the trigger threshold for the middle-ear reflex.

Occupational noise exposure can damage workers' hearing, particularly when combined with exposure to cochleotoxic chemicals such as styrene. Although styrene-induced cochlear impairments only 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 noise (both continuous and impulse), and a low concentration of styrene [300 ppm < (threshold limit value × 10) safety factor] on the peripheral auditory receptor, and on the CNS in rats. The impact of the different conditions on hearing loss was assessed using distortion product oto-acoustic emissions, and histological analysis of cochleae. Although the LEX,8h (8-hour time-weighted average exposure) of the impulse noise was lower (80 dB SPL sound pressure level) than that of the continuous noise (85 dB SPL), it appeared more detrimental to the peripheral auditory receptors.A co-exposure to styrene and continuous noise was less damaging than exposure to continuous noise alone. In contrast, the traumatic effects of impulse noise on the organ of Corti were enhanced by co-exposure to styrene. The pharmacological effects of the solvent on the CNS were discussed to put forward a plausible explanation of these surprising results. We hypothesize that CNS effects of styrene may account for this apparent paradox. Based on the present results, the temporal structure of the noise should be reintroduced as a key parameter in hearing conservation regulations.