Effects of industrial solvents on hearing and balance: a review.

Industrial hearing loss has generally been associated with noise exposure, but there is a growing awareness that industrial solvents can have an adverse effect on the auditory and vestibular systems in man. Both animal experiments and human studies point to an ototoxic effect of industrial solvents, as well as some central auditory and vestibular disturbances. This review examines the research from the last four decades in an attempt to get an overview of the available evidence. Research shows that industrial solvents are ototoxic in rats. The majority of the solvents studied cause a loss of auditory sensitivity in the mid-frequencies in rats, affecting outer hair cells in the order OHC 3 > OHC 2 > OHC 1 . Inner hair cells are generally unaffected. Spiral ganglion cells are most vulnerable to trichloroethylene. Simultaneous exposure to solvents and noise results in a synergistic effect; the pattern of trauma mirrors that due to solvent exposure rather than noise, but is more enhanced. There is a critical level when synergy occurs. The effects of solvents on the vestibular system are neurotoxic and influence the vestibulo-oculomotor system in both animals and humans; humans also present with problems in postural sway. There is a strong suggestion from human studies that solvents are ototoxic in man, but findings show that both the peripheral and central auditory pathways can be affected. Hearing losses can be in the high frequency region or can affect a wider range of frequencies. Hearing loss and balance disturbances can occur at levels below permitted levels of exposure. The synergistic effect of combined exposure to solvents and noise has also been noted in humans, resulting in greater hearing losses than would be expected from exposure to noise and solvents alone. The findings from both human and animal studies indicate that exposure to industrial solvents or to industrial solvents and noise can have an adverse effect on hearing and balance. The implications for industry and hearing conservation are far reaching.

Audiological findings in workers exposed to styrene alone or in concert with noise.

Audiological testing, interviews and exposure measurements were used to collect data on the health effects of styrene exposures in 313 workers from fiberglass and metal-product manufacturing plants and a mail terminal. The audiological test battery included pure-tone audiometry, distortion product otoacoustic emissions (DPOAE), psychoacoustic modulation transfer function, interrupted speech, speech recognition in noise and cortical response audiometry (CRA). Workers exposed to noise and styrene had significantly poorer pure-tone thresholds in the high-frequency range (3 to 8 kHz) than the controls, noise-exposed workers and those listed in a Swedish age-specific database. Even though abnormalities were noted on DPOAE and CRA testing, the interrupted speech and speech recognition in noise tests were the more sensitive tests for styrene effects. Further research is needed on the underlying mechanisms to understand the effects of styrene and on audiological test batteries to detect changes in populations exposed to solvents.

Effect of exposure to a mixture of solvents and noise on hearing and balance in aircraft maintenance workers.

Aircraft maintenance workers are exposed to a mixture of solvents in the presence of intermittent noise. For this study these workers exposed to solvent mix and noise, were compared with mill workers exposed to noise alone, printed circuit board operatives exposed to solvents alone and those exposed to none who acted as controls. Tympanometry, acoustic reflex thresholds, transient and distortion product otoacoustic emissions, auditory brainstem potentials, nystagmography and posturography were examined. There was a significant effect on pure tone thresholds for both noise and solvents+noise. The distortion product otoacoustic emissions declined with frequency and exhibited lower DP amplitude with noise compared to solvents and noise group. The transient emissions showed a similar effect. Over 32% of subjects with solvent and noise exposure had abnormalities of the auditory brainstem responses in terms of interwave interval prolongation. The mean acoustic reflex thresholds showed a pattern of differences which differentiate noise from solvent and noise groups. The contralateral pathway appears to be differentially affected by solvent exposure. 32% of subjects in the solvents and noise group had an abnormal posturographic finding. In the solvents and noise group 74% had abnormalities of saccades, 56% of pursuit and 45% of optokinetic nystagmus.