142. Occupational exposure to chemicals and hearing impairment.

Research conducted over the last two decades has brought attention to the ototoxicity of chemicals in the workplace and their interaction with noise. Chemicals that have been specifically studied for their ototoxicity include solvents, metals, asphyxiants, PCBs and pesticides. Noise exposure is particularly damaging to the cochlea, a part of the peripheral auditory system, whereas chemicals tend to affect both the cochlear structures and the central auditory system. Reduced blood flow and free radical formation are important ototoxic mechanisms shared by noise and chemical exposures. Solvents and asphyxiants may also disrupt intrinsic anti-oxidant defences and make the ear more vulnerable to the effects of e.g. noise exposure. The chemicals reviewed in the present document have all been associated with auditory effects in animals. Some of the solvents and the asphyxiants interact synergistically with noise or potentiate noise effects on the auditory system. Combinations of chemical exposure with noise and other stressors such as physical activity during exposure may lower the concentration of the chemical exposure necessary for induction of an auditory effect. Auditory effects have also been indicated in humans for all agents covered in this document for which there are data. Noise is often present in the occupational arena, which makes prediction of the outcome challenging. As combined exposure (e.g. chemical and noise) is currently not taken care of in the regular occupational exposure limit (OEL) setting procedure, a noise notation can be used to indicate an increased risk of hearing loss after exposure to the chemical at a level close to the OEL with concurrent noise exposure. The strength of evidence for ototoxicity differs between the agents but falls basically into three categories, i.e. agents for which: 1) human data indicate auditory effects under or near existing OELs and robust animal data support an effect on hearing from exposure (styrene, toluene, carbon disulphide, lead, mercury and carbon monoxide), 2) human data are lacking whereas animal data indicate auditory effects under or near existing OELs (p-xylene, ethylbenzene and hydrogen cyanide), 3) human data are poor or lacking and animal data indicate an auditory effect well above the existing OELs (chlorobenzene, trichloroethylene, n-hexane, n-heptane, some solvent mixtures, trimethyltin, acrylonitrile, 3,3'-iminodipropionitrile, pesticides and PCBs)