Ototoxicity of toluene and styrene: state of current knowledge

Noise is the most common preventable cause of irreversible sensorineural hearing loss. During recent years, the results of experimental and human investigations have raised the level of concern about the potential ototoxicity of chemical agents and their interaction with noise. European Directive 2003/10/EC on the minimum health and safety requirements regarding the exposure of workers to the risks arising from noise specifies that the employer shall give particular attention, when carrying out the risk assessment, to, among others, any effects on workers' health and safety resulting from interactions between noise and work-related ototoxic substances. There is, however, currently very little awareness in the occupational health community of the chemical hazards to hearing. The main objective of this review was to analyze the available scientific literature on the ototoxic effects of styrene and toluene, in order to examine dose-response/effect relationships and the relevance of the prevention strategy for people exposed to these solvents. While both solvents appear clearly ototoxic in rats, human data are less straightforward and the existing evidence does not allow characterization of the dose-response/effect relationships; further research is needed. However, once hearing loss is incurred, it is irreversible, and one should be alert to the possible hearing loss induced by toluene and styrene and to the possible additive, potentiating, or synergistic ototoxic effects in case of combined exposure to several chemicals and in case of combined exposure to noise and chemical substances.