Ototoxic effects of occupational exposure to styrene and co-exposure to styrene and noise

Organic solvents and hearing loss: The challenge for audiology

Organic solvents have been reported to adversely affect human health, including hearing health. Animal models have demonstrated that solvents may induce auditory damage, especially to the outer hair cells. Research on workers exposed to solvents has suggested that these chemicals may also induce auditory damage through effects on the central auditory pathways. Studies conducted with both animals and humans demonstrate that the hearing frequencies affected by solvent exposure are different to those affected by noise, and that solvents may interact synergistically with noise. The present article aims to review the contemporary literature of solvent-induced hearing loss, and consider the implications of solvent-induced auditory damage for clinical audiologists. Possible audiological tests that may be used when auditory damage due to solvent exposure is suspected are discussed.

Potentiation of Chemical Ototoxicity by Noise

High-intensity and/or prolonged exposure to noise causes temporary or permanent threshold shifts in auditory perception. Occupational exposure to solvents or administration of clinically important drugs, such as aminoglycoside antibiotics and cisplatin, also can induce permanent hearing loss. The mechanisms by which these ototoxic insults cause auditory dysfunction are still being unraveled, yet they share common sequelae, particularly generation of reactive oxygen species, that ultimately lead to hearing loss and deafness. Individuals are frequently exposed to ototoxic chemical contaminants (e.g., fuel) and noise simultaneously in a variety of work and recreational environments. Does simultaneous exposure to chemical ototoxins and noise potentiate auditory dysfunction? Exposure to solvent vapor in noisy environments potentiates the permanent threshold shifts induced by noise alone. Moderate noise levels potentiate both aminoglycoside- and cisplatin-induced ototoxicity in both rate of onset and in severity of auditory dysfunction. Thus, simultaneous exposure to chemical ototoxins and moderate levels of noise can potentiate auditory dysfunction. Preventing the ototoxic synergy of noise and chemical ototoxins requires removing exposure to ototoxins and/or attenuating noise exposure levels when chemical ototoxins are present.

Occupational styrene exposure and hearing loss: a cohort study with repeated measurements

OBJECTIVE

Associations between occupational styrene exposure and impairment of hearing function were investigated, guided by three questions: are there hearing losses concerning high frequency and standard audiometric test? Are there dose-response relationships and measurable thresholds of effects? Are there signs of reversibility of possible effects if the workers are examined during times of improvement from their work?

METHODS

A group of workers from a boat building plant, some of whom were laminators, were examined in subgroups of current low (n = 99, mean mandelic acid MA + phenylglyoxylic acid PGA = 51 mg/g creatinine), medium (n = 118, mean 229 mg/g creat.) and high (n = 31, mean 970 mg/g creat.) exposure to styrene. In addition, subgroups chronically exposed to high-long (n = 17) and low-short (n = 34) styrene levels were analysed. The examinations were carried out during normal work days and during the company holidays. Hearing thresholds and transient evoked otoacoustic emissions (TEOAE) were measured. Statistics included multiple co-variance analyses with repeated measures, linear regressions, and logistic regressions.

RESULTS

The analyses of all participants demonstrated no clear exposure effects. Particularly no sufficient proof of dose-response relationship measured against parameters of current exposure (MA + PGA, styrene/blood) and of chronic exposure (cumulative and average life time exposure resp.) was found. The analyses of groups exposed to high levels show elevated thresholds at frequencies up to 1,500 Hz among the subgroup exposed to high styrene levels (e.g. 40-50 ppm as average) for a longer period of time (e.g. more than 10 years). These participants also demonstrated signs of "improvement" at frequencies above 2,000 Hz during work holidays, when they were not exposed to styrene. A significantly elevated odds ratio for cases of hearing loss (more than 25 dB (A) in one ear, 3,000-6,000 Hz) was found among the group exposed to high levels (above 30 ppm as average) for a longer period of time (more than 10-26 years). The measurements of TEOAE did not exhibit significant results related to exposure.

CONCLUSION

This study found, that chronic and intensive styrene exposure increases the hearing thresholds. At levels of about 30-50 ppm as an average inhaled styrene per work day over a period of about 15 years with higher exposure levels above 50 ppm in the past, an elevated risk for impaired hearing thresholds can be expected. The formerly published results on ototoxic effects below 20 ppm could not be confirmed. With few exceptions (at frequencies of 1,000 and 1,500 Hz) no dose-response relationship between threshold and exposure data was found. Improvements of hearing thresholds during work- and exposure-free period are possible.

Occupational noise, smoking, and a high body mass index are risk factors for age-related hearing impairment and moderate alcohol consumption is protective: a European population-based multicenter study

A multicenter study was set up to elucidate the environmental and medical risk factors contributing to age-related hearing impairment (ARHI). Nine subsamples, collected by nine audiological centers across Europe, added up to a total of 4,083 subjects between 53 and 67 years. Audiometric data (pure-tone average [PTA]) were collected and the participants filled out a questionnaire on environmental risk factors and medical history. People with a history of disease that could affect hearing were excluded. PTAs were adjusted for age and sex and tested for association with exposure to risk factors. Noise exposure was associated with a significant loss of hearing at high sound frequencies (>1 kHz). Smoking significantly increased high-frequency hearing loss, and the effect was dose-dependent. The effect of smoking remained significant when accounting for cardiovascular disease events. Taller people had better hearing on average with a more pronounced effect at low sound frequencies (<2 kHz). A high body mass index (BMI) correlated with hearing loss across the frequency range tested. Moderate alcohol consumption was inversely correlated with hearing loss. Significant associations were found in the high as well as in the low frequencies. The results suggest that a healthy lifestyle can protect against age-related hearing impairment.

Central auditory processing effects induced by solvent exposure

OBJECTIVES

Various studies have demonstrated that organic solvent exposure may induce auditory damage. Studies conducted in workers occupationally exposed to solvents suggest, on the one hand, poorer hearing thresholds than in matched non-exposed workers, and on the other hand, central auditory damage due to solvent exposure. Taking into account the potential auditory damage induced by solvent exposure due to the neurotoxic properties of such substances, the present research aimed at studying the possible auditory processing disorder (APD), and possible hearing difficulties in daily life listening situations that solvent-exposed workers may acquire.

MATERIALS AND METHODS

Fifty workers exposed to a mixture of organic solvents (xylene, toluene, methyl ethyl ketone) and 50 non-exposed workers matched by age, gender and education were assessed. Only subjects with no history of ear infections, high blood pressure, kidney failure, metabolic and neurological diseases, or alcoholism were selected. The subjects had either normal hearing or sensorineural hearing loss, and normal tympanometric results. Hearing-in-noise (HINT), dichotic digit (DD), filtered speech (FS), pitch pattern sequence (PPS), and random gap detection (RGD) tests were carried out in the exposed and non-exposed groups. A self-report inventory of each subject's performance in daily life listening situations, the Amsterdam Inventory for Auditory Disability and Handicap, was also administered.

RESULTS

Significant threshold differences between exposed and non-exposed workers were found at some of the hearing test frequencies, for both ears. However, exposed workers still presented normal hearing thresholds as a group (equal or better than 20 dB HL). Also, for the HINT, DD, PPS, FS and RGD tests, non-exposed workers obtained better results than exposed workers. Finally, solvent-exposed workers reported significantly more hearing complaints in daily life listening situations than non-exposed workers.

CONCLUSIONS

It is concluded that subjects exposed to solvents may acquire an APD and thus the sole use of pure-tone audiometry is insufficient to assess hearing in solvent-exposed populations.

Occupationally-acquired noise-induced hearing loss: a senseless workplace hazard

OBJECTIVES

Occupational noise-induced hearing loss (ONIHL) describes an acquired hearing deficiency directly attributable to excessive workplace noise exposure. Data suggest that excessive noise attributes to approximately 37% of all adult causes of hearing loss and remains a significant contributor to employment-related morbidity internationally. Typically insidiously-acquired, often without frank progressive symptomatology, regional medical agencies continue to struggle with this potentially debilitating condition. The aim of the study was to provide a synopsis of the current understanding of ONIHL, its impact on individual workers and the wider international community, and to identify barriers to more uniform adoption of personal hearing protection.

MATERIALS AND METHODS

A review of the contemporary literature was performed using defined keyword searches and OVID, PubMed, and Google Scholar as primary electronic search engines.

RESULTS

A number of published works were identified, describing aspects of the relationship between workplace-related noise exposure and subsequent development of employee hearing impairment, which demonstrate an overwhelming gender imbalance, with up to 97% of affected individuals being male. Industry-specific associations (e.g., mining, manufacturing and heavy construction) were well documented, as were links to toxin-specific exposures, in the recognized development of hearing loss. However, evidence of integration of appraisal of the topically-current area of genetic susceptibility was often lacking. Much discordance still exists among international agencies in the prescriptive regulation and enforcement of "safe" exposure limits.

CONCLUSIONS

Despite a high level of public awareness regarding the importance of hearing preservation and increasingly stringent international occupational health, safety and welfare requirements mandating provision of safer work environments, ONIHL continues to be a significant occupational hazard. ONIHL is permanent and may cause significant disability, for which there currently exists no cure, but is largely overtly-preventable. The impact of ONIHL on the global transition toward dominant communication-rich white-collar employment roles is difficult to quantitate, but is likely to be substantive upon the afflicted individual. In the mainstream setting, exposure-avoidance strategies aimed to reduce the incidence of ONIHL remain the focus of public health and occupational medicine approaches.

Ototoxicity of organic solvents - from scientific evidence to health policy

The scientific workshop, organized under the 6th European Framework Programme, the Marie Curie Host Fellowship for the Transfer of Knowledge "NoiseHear" Project, by the Nofer Institute of Occupational Medicine (Łódź, Poland, 15-16 November 2006), gathered world specialists in noise, chemicals, and ototoxicity, including hearing researchers, toxicologists, otolaryngologists, audiologists and occupational health physicians.The workshop examined the evidence and the links between isolated exposure to organic solvents, combined exposure to noise and solvents, and effects on the auditory system. Its main purpose was to review the key scientific evidence to gather the necessary knowledge for developing adequate occupational health policies. This paper summarizes the workshop sessions and subsequent discussions.

Effects of Aromatic Solvents on Acoustic Reflexes Mediated by Central Auditory Pathways

From previous in vivo investigations, it has been shown that toluene can mimic the effects of cholinergic receptor antagonists and may thereby modify the response of protective acoustic reflexes. The current study aimed to define the relative effects of aromatic solvents on the middle ear and inner ear acoustic reflexes. Toward this end, the cochlear microphonic (CMP) elicited with a band noise centered at 4 kHz, and the compound action potential (CAP) elicited with 4-kHz tone pips was measured in rats. Both potentials were recorded before, during, and after triggering the protective reflexes by a 110-dB SPL contralateral octave band noise centered at 12.5 kHz (12.5 kHz-OBN). In several rats, the middle ear muscles were severed to identify the relative effects of toluene on the two reflexes. While the reflex elicitor was capable of decreasing both the CMP and CAP amplitudes, an injection of 116.2 mM toluene cancelled this suppressor effect induced by the contralateral sound. In the rats with nonfunctional middle ear muscles, a solvent injection did not modify the electrophysiological responses of the cochlea. Different solvents were tested to study the relationship of the chemical structure of the solvents on the acoustic reflexes. The present study showed that aromatic solvents can inhibit the action of the middle ear reflex by their anticholinergic effect on the efferent motoneurons. An aromatic nucleus and the presence of one side chain of no more than 3 C seem to be required in the solvent structure to inhibit the efferent motoneurons.

Increase in cochlear microphonic potential after toluene administration

Human and animal studies have shown that toluene can cause hearing loss. In the rat, the outer hair cells are first disrupted by the ototoxicant. Because of their particular sensitivity to toluene, the cochlear microphonic potential (CMP) was used for monitoring the cochlea activity of anesthetized rats exposed to both noise (band noise centered at 4 kHz) and toluene. In the present experiment, the conditions were specifically designed to study the toluene effects on CMP and not those of its metabolites. To this end, 100-microL injections of a vehicle containing different concentrations of solvent were made into the carotid artery connected to the tested cochlea. Interestingly, an injection of 116.2-mM toluene dramatically increased in the CMP amplitude (approximately 4 dB) in response to an 85-dB SPL noise. Moreover, the rise in CMP magnitude was intensity dependent at this concentration suggesting that toluene could inhibit the auditory efferent system involved in the inner-ear or/and middle-ear acoustic reflexes. Because acetylcholine is the neurotransmitter mediated by the auditory efferent bundles, injections of antagonists of cholinergic receptors (AchRs) such as atropine, 4-diphenylacetoxy-N-methylpiperidine-methiodide (mAchR antagonist) and dihydro-beta-erythroidine (nAchR antagonist) were also tested in this investigation. They all provoked rises in CMP having amplitudes as large as those obtained with toluene. The results showed for the first time in an in vivo study that toluene mimics the effects of AchR antagonists. It is likely that toluene might modify the response of protective acoustic reflexes.

Assessment of central auditory processing in a group of workers exposed to solvents

CONCLUSION

Despite having normal hearing thresholds and speech recognition thresholds, results for central auditory tests were abnormal in a group of workers exposed to solvents. Workers exposed to solvents may have difficulties in everyday listening situations that are not related to a decrement in hearing thresholds. A central auditory processing disorder may underlie these difficulties.

OBJECTIVE

To study central auditory processing abilities in a group of workers occupationally exposed to a mix of organic solvents.

MATERIALS AND METHODS

Ten workers exposed to a mix of organic solvents and 10 matched non-exposed workers were studied. The test battery comprised pure-tone audiometry, tympanometry, acoustic reflex measurement, acoustic reflex decay, dichotic digit, pitch pattern sequence, masking level difference, filtered speech, random gap detection and hearing-in-noise tests.

RESULTS

All the workers presented normal hearing thresholds and no signs of middle ear abnormalities. Workers exposed to solvents had lower results in comparison with the control group and previously reported normative data, in the majority of the tests.

Ototoxicity of the three xylene isomers in the rat

Numerous experiments have shown that the aromatic solvents can affect the auditory system in the rat, the cochlea being targeted first. Solvents differ in cochleotoxic potency: for example, styrene is more ototoxic than toluene or xylenes. The goal of this study was to determine the relative ototoxicity of the three isomers of xylene (o-, m- or p-xylene). Moreover, by dosing with the two urinary metabolites of xylene, methylhippuric (MHAs) and mercapturic acids (MBAs), this study points toward a causal relationship between the cochleotoxic effects and potential reactive intermediates arising from the biotransformation of the parent molecules. Separate groups of rats were exposed by inhalation to one isomer following this schedule: 1800 ppm, 6 h/d, 5 d/wk for 3 wk. Auditory thresholds were determined with brainstem-auditory evoked potentials. Morphological analysis of the organ of Corti was performed by counting both sensory and spiral ganglion cells. Among the three isomers, only p-xylene was cochleotoxic. A 39-dB permanent threshold shift was obtained over the tested frequencies range from 8 to 20 kHz. Whereas outer hair cells were largely injured, no significant morphological change was observed within spiral ganglia. The concentrations of urinary p-, o- or m-MHA were greater (p-MHA: 33.2 g/g; o-MHA: 7.8 g/g; m-MHA: 20.4 g/g) than those obtained for MBAs (p-MBA: 0.04 g/g; o-MBA: 6.2 g/g; m-MBA: 0.03 g/g). Besides, there is a large difference between o-MBA (6.2 g/g) and p-MBA (0.04 g/g). As a result, since the cysteine conjugates are not determinant in the ototoxic process of xylenes, the location of the methyl groups around the benzene nucleus could play a key role.

Ototoxicity in rats exposed to ethylbenzene and to two technical xylene vapours for 13 weeks

Male Sprague-Dawley rats were exposed to ethylbenzene (200, 400, 600 and 800 ppm) and to two mixed xylenes (250, 500, 1,000 and 2,000 ppm total compounds) by inhalation, 6 h/day, 6 days/week for 13 weeks and sacrificed for morphological investigation 8 weeks after the end of exposure. Brainstem auditory-evoked responses were used to determine auditory thresholds at different frequencies. Ethylbenzene produced moderate to severe ototoxicity in rats exposed to the four concentrations studied. Increased thresholds were observed at 2, 4, 8 and 16 kHz in rats exposed to 400, 600 and 800 ppm ethylbenzene. Moderate to severe losses of outer hair cells of the organ of Corti occurred in animals exposed to the four concentrations studied. Exposure to both mixed xylenes produced ototoxicity characterized by increased auditory thresholds and losses of outer hair cells. Ototoxicity potentiation caused by ethylbenzene was observed. Depending on the mixed xylene studied and the area of the concentration-response curves taken into account, the concentrations of ethylbenzene in mixed xylenes necessary to cause a given ototoxicity were 1.7-2.8 times less than those of pure ethylbenzene. Given the high ototoxicity of ethylbenzene, the safety margin of less or equal to two (LOAEL/TWA) might be too small to protect workers from the potential risk of ototoxicity. Moreover, the enhanced ototoxicity of ethylbenzene and para-xylene observed in mixed xylenes should encourage the production of mixed xylenes with the lowest possible concentrations of ethylbenzene and para-xylene.

Field study to explore possible effects of styrene on auditory function in exposed workers

OBJECTIVES

We conducted this study to examine, whether occupational styrene exposures are associated with reduced hearing ability.

METHODS

The auditory function was investigated by pure tone audiometry and registration of transitory evoked otoacoustic emissions (TEOAE) in 32 workers of a fibre-reinforced plastic boat building factory. Sixteen subjects were laminators (mean age: 41 yr (SD: 8)) and therefore regularly exposed to styrene with mean duration of exposure to styrene of 7.5 yr (SD 5.0). The tests were applied to a reference group of 16 workers (mean age: 39 yr (SD: 8)) who were not directly exposed to styrene but had a similar noise exposure.

RESULTS

A few and isolated correlations between the parameters of hearing acuity and exposure indices, such as current internal styrene exposures (sum of MA and PGA) and duration of styrene exposure, were statistically significant, but no consistent association was found.

CONCLUSION

The results of this study do not support the assumption of an ototoxic effect of chronic styrene exposure in workers.

The ototoxicity of styrene: a review of occupational investigations

OBJECTIVES

The objective of this study was to review critically a number of occupational investigations of the exposure and effect relation between inhaled styrene vapour and hearing loss. There is concern that workers' hearing may be impaired by exposure to styrene, as used in industries making plastics and fibreglass-reinforced products.

METHODS

Seven occupational studies, each dealing with the ototoxicity of styrene, were examined. Factors assessed included the experimental design and number of subjects within exposure groups, measurement of the styrene-in-air concentration, confirmation of the styrene exposure by blood or urine analysis, determination of the hearing threshold levels for the exposure and control groups, and measurement of any occupational noise in the subjects' workplaces. Consideration was also given to statistical relations between high-frequency hearing loss and lifetime exposure indices for styrene and noise.

RESULTS

The results are equivocal. Four investigations failed to find any effect of styrene on hearing thresholds. In contrast, other investigations claimed to have demonstrated styrene-induced hearing loss in industrial populations, with synergism between styrene and noise. However, these reports exhibited shortcomings of experimental design and data analysis.

CONCLUSIONS

Considering the body of evidence as a whole, hearing deficits due to occupational exposure to styrene at low concentrations have not been demonstrated by scientifically reliable argument. There is some suggestion of an association between styrene exposure, occupational noise, and hearing dysfunction. Further studies in humans are necessary to clarify this question.

Combined effects of noise and mixed solvents exposure on the hearing function among workers in the aviation industry

This study aims to evaluate the effect of occupational exposure to noise and organic solvents on hearing loss in the aviation industry. The study population comprised 542 male workers, who worked in avionics jobs in Kimhae, Korea, who kept records of work environment evaluations and medical examinations. The Cumulative Exposure Index (CEI) was constructed to assess the lifetime cumulative exposure of the workers, and pure tone audiometry (PTA) data of the workers from their biannual medical surveillance was used to assess hearing loss. The prevalence of hearing loss found in the group exposed to noise and mixed solvents simultaneously (54.9%) was higher than those in the other groups (6.0% in the unexposed, 17.1% in the noise-only, and 27.8 % in the exposed to only a solvents mixture). The relative risks, adjusted for age, were estimated to be 4.3 (95 % CI 1.7-10.8) for the noise-only group, 8.1 (95% CI 2.0-32.5) for the noise and solvents group, and 2.6 (95 % CI 0.6-10.3) for the solvents-mixture group. These suggest that chronic exposure to mixed solvents had a toxic effect on the auditory system. This raises the issue of whether hearing conservation regulations should be applied to all workers exposed to solvents.

Prospective noise induced changes to hearing among construction industry apprentices

AIMS

To characterise the development of noise induced damage to hearing.

METHODS

Hearing and noise exposure were prospectively monitored among a cohort of newly enrolled construction industry apprentices and a comparison group of graduate students, using standard pure tone audiometry and distortion product otoacoustic emissions (DPOAEs). A total of 328 subjects (632 ears) were monitored annually an average of 3.4 times. In parallel to these measures, noise exposure and hearing protection device (HPD) use were extensively monitored during construction work tasks. Recreational/non-occupational exposures also were queried and monitored in subgroups of subjects. Trade specific mean exposure L(eq) levels, with and without accounting for the variable use of hearing protection in each trade, were calculated and used to group subjects by trade specific exposure level. Mixed effects models were used to estimate the change in hearing outcomes over time for each exposure group.

RESULTS

Small but significant exposure related changes in DPOAEs over time were observed, especially at 4 kHz with stimulus levels (L1) between 50 and 75 dB, with less clear but similar patterns observed at 3 kHz. After controlling for covariates, the high exposure group had annual changes in 4 kHz emissions of about 0.5 dB per year. Pure tone audiometric thresholds displayed only slight trends towards increased threshold levels with increasing exposure groups. Some unexpected results were observed, including an apparent increase in DPOAEs among controls over time, and improvement in behavioural thresholds among controls at 6 kHz only.

CONCLUSIONS

Results indicate that construction apprentices in their first three years of work, with average noise exposures under 90 dBA, have measurable losses of hearing function. Despite numerous challenges in using DPOAEs for hearing surveillance in an industrial setting, they appear somewhat more sensitive to these early changes than is evident with standard pure tone audiometry.

Prevalence of Hearing Impairment in a Rural Midwestern Cohort: Estimates from the Keokuk County Rural Health Study, 1994 to 1998

OBJECTIVE

The current paper estimates the prevalence of hearing problems in a rural population, analyzes the prevalence of hearing problems across age groups, and compares the prevalence of hearing problems in this population with results obtained in other populations.

DESIGN

Data were obtained from a random sample of the residents of a rural county, stratified by farm, rural non-farm, and town residence. Hearing test results were obtained from participants between the ages of 8 and 92 years (N = 1972; 47% male). Pure-tone thresholds were classified as normal or impaired using a number of metrics, including speech intelligibility index values. Selected comparisons of crude rates were made with previous population-based studies of hearing loss prevalence.

RESULTS

Nearly all (99%) of the participants in this study had significant hearing impairment. Atypical hearing impairment is most prevalent at 6 kHz. In males, this excess impairment shifts to lower frequencies with age but monotonically decreases in females. Notched configurations were most common among those between 30 and 59 years old. In females, the prevalence of hearing impairments sufficient to interfere with speech understanding begins to rapidly increase in the 6th decade (4th decade in males). Comparisons with prior population-based studies in the United States and Great Britain identified few significant differences.

CONCLUSIONS

Significant hearing impairment is common in rural populations. The high prevalence in this population is similar to that found in other population-based studies. Future studies are needed to examine (1) the risk factors for hearing impairment, (2) the natural course of hearing problems across the life span, and (3) the effect of programs for the prevention of hearing impairment and rehabilitation for persons with existing hearing impairments.

Influence of age on noise- and styrene-induced hearing loss in the Long-Evans rat

This paper reviews different investigations carried out with Long-Evans rats on the influence of age on the ototoxicity induced by styrene and on the vulnerability to noise. The first part of this article is focused on the differences in auditory susceptibility to noise (92 or 97dB octave band noise centered at 8kHz, 6 h/day, 5 days/week, 4 weeks) and styrene (700ppm, 6h/d, 5 d/w, 4 w) between young (three and half months) and old (24 months) Long-Evans rats. Auditory evoked potential measures revealed that the old rats tend to be more sensitive than young rats to higher noise levels (97dB), but equally vulnerable to moderate levels (92dB). By contrast, the aged rats were virtually insensitive to 700ppm styrene compared to the young animals. Two additional studies were performed controlling and examining the influence of body weight and post-natal age on the sensitivity to styrene. Rats of the same age (21 weeks) and but having different body weight (∼310g versus ∼410g) did not show any difference of sensitivity to 700ppm styrene, whereas 14-week-old rats with the same body weight as 21-week-old rats (∼350g) revealed increased sensitivity to styrene. These results show that weight does not play a key role in the sensitivity to styrene, and suggest a long period of increased sensitivity to styrene during the first months of life.

Combined effects of noise and styrene on hearing : Comparison between active and sedentary rats

In this study, two investigations were carried out with adult Long-Evans rats exposed to increasing concentrations of styrene. In the first experiment, the hearing of rats, which were forced to walk in a special wheel during the exposure, was compared to that of rats which were sleepy in their cage. The active rats were exposed to styrene concentrations ranging from 300 to 600 ppm, whereas the sedentary rats were exposed from 500 to 1000 ppm for 4 weeks, 5 days per week, 6 hours per day. In the second experiment, designed to evaluate the hearing risks at threshold limit values, active rats were exposed either to a noise having a Leq8h of 85 dB (equivalent level of a continuous noise for a typical 8-h workday), or to 400-ppm styrene or to a simultaneous exposure to noise and styrene. In both experiments, auditory function was tested by auditory-evoked potentials from the inferior colliculus and completed by morphological analyses of the organ of Corti. The results of the first experiment showed that the same amount of styrene-induced hearing loss can be obtained by using concentrations approximately 200 ppm lower in active rats than in sedentary rats. The second investigation showed that, in spite of the low-intensity noise and the low-concentration of styrene, there is a clear risk of potentiation of styrene-induced hearing loss by noise. These findings and exposure conditions were discussed and extrapolated with regard to the risk assessment for human beings. The authors propose to decrease the French threshold limit value of styrene for ensuring a high level of protection for human hearing.

Predictors of hearing threshold levels and distortion product otoacoustic emissions among noise exposed young adults

AIM

To examine the relations between noise exposure and other risk factors with hearing function as measured by audiometric thresholds and distortion product otoacoustic emissions.

METHODS

A total of 456 subjects were studied (393 apprentices in construction trades and 63 graduate students). Hearing and peripheral auditory function were quantified using standard, automated threshold audiometry, tympanometry, and distortion product otoacoustic emissions (DPOAEs). The analysis addressed relations of noise exposure history and other risk factors with hearing threshold levels (HTLs) and DPOAEs at the baseline test for the cohort.

RESULTS

The cohort had a mean age of 27 (7) years. The construction apprentices reported more noise exposure than students in both their occupational and non-occupational exposure histories. A strong effect of age and years of work in construction was observed at 4, 6, and 8 kHz for both HTLs and DPOAEs. Each year of construction work reported prior to baseline was associated with a 0.7 dB increase in HTL or 0.2 dB decrease DPOAE amplitude. Overall, there was a very similar pattern of effects between the HTLs and DPOAEs.

CONCLUSIONS

This analysis shows a relatively good correspondence between the associations of noise exposures and other risk factors with DPOAEs and the associations observed with pure-tone audiometric thresholds in a young adult working population. The results provide further evidence that DPOAEs can be used to assess damage to hearing from a variety of exposures including noise. Clarifying advantages of DPOAEs or HTLs in terms of sensitivity to early manifestations of noise insults, or their utility in predicting future loss in hearing will require longitudinal follow up.

Consequences of noise- or styrene-induced cochlear damages on glutamate decarboxylase levels in the rat inferior colliculus

Both noise and styrene can injure the cochlea, resulting in a reduction of incoming inputs from the cochlea to the central nervous system. In addition, styrene is known to have neurotoxic properties at high doses. The loss of inputs caused by noise has been shown to be compensated by a new equilibrium between excitatory and inhibitory influences within the inferior colliculus (IC). The main goal of this study was to determine whether styrene-induced hearing loss could also be counterbalanced by a GABAergic adjustment in the IC. For this purpose, rats were exposed to noise (97 dB SPL octave band noise centered at 8 kHz), or to a non-neurotoxic dose of styrene for 4 weeks (700 ppm, 6 h/day, 5 days/week). Auditory sensitivity was tested by evoked potentials, and cochlear damage was assessed by hair cell counts. Glutamate decarboxylase (GAD) was dosed in the IC by indirect competitive enzyme-linked immunosorbent assay. Both noise and styrene caused PTSs that reached 27.0 and 14.6 dB respectively. Outer hair cell (OHC) loss caused by noise did not exceed 9% in the first row, on the other hand OHC loss induced by styrene reached 63% in the third row. Only the noise caused a decrease of GAD of 37% compared to that measured in the controls. No significant modification of GAD concentration has been shown after styrene exposure. Thus, central compensation for cochlear damage may depend on the nature of the ototoxic agent. Unless styrene directly affects IC function, it is reasonable to assume that noise causes a modification of inhibitory neurotransmission within the structure because of impairment of afferent supply to the auditory brainstem. The present findings suggest that central compensation for cochlear damage can preferably occur when afferent fibers are altered.

Effect of an initial noise induced hearing loss on subsequent noise induced hearing loss

The effect of previous noise induced hearing loss (NIHL) on subsequent NIHL was studied in rats. Three groups of animals were initially exposed to different durations of 113 dB SPL broad band noise (21 days, 3 days or 0 days--unexposed). Their permanent threshold shifts (PTS) from this exposure (PTS1) were evaluated using auditory nerve-brainstem evoked responses (ABR). All the animals were then noise-exposed for an additional 12 days, and the incremental PTS following this exposure (PTS2) was also assessed. The 21 day group showed the greater PTS1 [mean +/- SD: 27.03 +/- 6.78 dB, compared with 11.67 +/- 10.47 dB (3 day group)] and the lowest PTS2 [9.84 +/- 8.19 dB, compared with 13.33 +/- 14.60 dB (3 day group) and 24.04 +/- 12.4 dB (0 day group)]. This group also showed the highest total PTS and lowest SD following the two noise exposures [36.88 +/- 6.29 dB, compared with 25.00 +/- 12.68 dB (3 day group) and 26.35 +/- 11.93 dB (0 day group)]. The results may be explained by the lower effective intensity of the second noise exposure for the animals with a large PTS1 compared to those with little or no NIHL from the first noise exposure.

Effects of Coexposure to Noise and Mixture of Organic Solvents on Hearing in Dockyard Workers

Questionnaire and audiometric data of 701 dockyard workers (517 noise and organic solvent mixture-exposed and 184 noise-only-exposed) were referred to 205 control subjects not exposed to either noise or solvents. The odds ratio (OR) of hearing loss was significantly increased by approximately 3 times in the noise-only group and by almost 5 times in the noise and solvent group. A moderate effect of solvent ototoxicity, in addition to noise, was observed on hearing threshold at a frequency 8 kHz. ORs for hearing loss were 1.12 for each increment of 1 year of age, 1.07 for every decibel of lifetime noise exposure (dB-A), and 1.004 for each increment of the index of lifetime exposure to solvents. The results suggest an additive damaging effect of coexposure to noise and organic solvents to the auditory organ.