Relationship between styrene exposure and hearing loss: review of human studies

Styrene is an aromatic solvent belonging to the alkylbenzene family. Occupational exposure to styrene occurs mainly in the manufacturing of fiberglass-reinforced polyester products, e.g. reinforced plastics and composites. Since 1988, nine studies have been published on the relationship between occupational exposure to styrene and hearing loss. All studies were the cross-sectional epidemiological studies or clinical studies from occupational health clinics. A total of more than 1000 workers exposed to styrene, both with and without concurrent noise exposure, were examined using different outcome measures for hearing loss. Exposure assessment was usually based on styrene measurements in the breathing zone during several hours of one working day. Some of the studies employed also the biological monitoring of styrene exposure based on determination of its urinary metabolites. The current exposures to styrene varied between 2 and 35 ppm. In some studies, lifetime exposure was calculated using company records and questionnaire data. The current exposure to noise was estimated by noise dosimetry or standard noise measurements. Lifetime noise exposure was assessed using questionnaire data and occupational noise estimates. In many studies, noise-exposed groups were used as controls together with the unexposed workers. Of the nine studies, seven show some effects on the auditory system that were associated with styrene-alone exposure. These effects are examined using different outcome measures such as pure tone audiometry, high frequency hearing loss, and central hearing tests. In some studies, an increased risk for hearing loss was associated with exposure estimates.