Neuropharmacological and cochleotoxic effects of styrene can worsen the noise impact

It is well-known that occupational noise exposure can damage workers' hearing. It is also well accepted that a combined exposure to noise with cochleotoxic substances (antibiotics, cisplatin…) or ototoxic agents such as aromatic solvents (toluene, styrene, ethylbenzene) can exacerbate the noise/ototoxic effects. Although solvent-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. And the MER can be precious for preserving the hearing performances of workers.
The aim of the study was to evaluate the effects of a noise (both continuous and impulsive), and a low concentration of styrene [300ppm < (threshold limit value x 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 cochleas. 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 an explanation of theses surprising results. Actually the effects of styrene on the CNS 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.