Toluene-induced hearing loss: a mid-frequency location of the cochlear lesions

Exploring Embryo-Ototoxic Effects: Insights into Deodorant-Induced Hair Cell Damage in Zebrafish

Our study investigated the embryo-ototoxic effects of deodorant2 (DA2) on zebrafish embryos, which serve as valuable model organisms due to genetic and physiological similarities to humans. We focused on understanding DA2's impact on zebrafish hair cells, which are vital for sensory perception and balance regulation. DA2, provided by the Ministry of Environment, Republic of Korea, was used at 460 μg/mL in dimethyl sulfoxide (DMSO), with a 0.43% DMSO solvent control group. Three experiments, each using 10 zebrafish specimens from each group, showed an initial 13% hair cell count reduction in the DA2-exposed group. Subsequent experiments demonstrated reductions of 37% and 22%, each with one mortality case. Statistical analysis revealed a significant 24% hair cell count reduction in the DA2-exposed group. We also assessed DA2's impact on zebrafish behavior. Although not statistically significant, differences in distances traveled (0.33-0.39, 95% confidence interval: -0.46-1.1, p = 0.2033) and latencies (-0.016-0.018, 95% confidence interval: -0.052-0.021, p = 0.1917) hinted at negative effects. These results highlight DA2's ototoxic properties affecting zebrafish auditory systems and behavior. Further investigation into DA2's effects on aquatic organisms and potential mitigation strategies are essential. These findings contribute to understanding DA2's safety profile, benefiting aquatic ecosystems and human health assessments.

Recommedation for an occupational exposure limit for toluene

The Lower Olefins and Aromatics (LOA) REACH Consortium, which includes toluene registrants in the EU, established a Working Group (WG) to conduct a review of the occupational exposure limit (OEL) for toluene. The review focussed on CNS and neuro-behavioural toxicity, ototoxicity, effects on colour vision, reproductive and developmental effects, as safety signals for these effects were identified. The WG also examined the need for a skin notation and/or a short-term exposure limit (STEL). The WG critically reviewed and discussed the strengths and weaknesses of the available published information describing the effects of toluene in animals and humans, to assess its adequacy as a potential point of departure for the establishment of an OEL for toluene and to derive an OEL. As a result, the WG recommendation for a toluene OEL is 20 ppm 8 h TWA, with a 15 min STEL of 100 ppm and a skin notation.

Experimental animal models of drug-induced sensorineural hearing loss: a narrative review

Objective

This narrative review describes experimental animal models of sensorineural hearing loss (SNHL) caused by ototoxic agents.

Background

SNHL primarily results from damage to the sensory organ within the inner ear or the vestibulocochlear nerve (cranial nerve VIII). The main etiology of SNHL includes genetic diseases, presbycusis, ototoxic agents, infection, and noise exposure. Animal models with functional and anatomic damage to the sensory organ within the inner ear or the vestibulocochlear nerve mimicking the damage seen in humans are employed to explore the mechanism and potential treatment of SNHL. These animal models of SNHL are commonly established using ototoxic agents.

Methods

A literature search of PubMed, Embase, and Web of Science was performed for research articles on hearing loss and ototoxic agents in animal models of hearing loss.

Conclusions

Common ototoxic medications such as aminoglycoside antibiotics (AABs) and platinum antitumor drugs are extensively used to induce SNHL in experimental animals. The effect of ototoxic agents in vivo is influenced by the chemical mechanisms of the ototoxic agents, the species of animal, routes of administration of the ototoxic agents, and the dosage of ototoxic agents. Animal models of drug-induced SNHL contribute to understanding the hearing mechanism and reveal the function of different parts of the auditory system in humans.

Hydrogen Storage for Mobility: A Review

Numerous reviews on hydrogen storage have previously been published. However, most of these reviews deal either exclusively with storage materials or the global hydrogen economy. This paper presents a review of hydrogen storage systems that are relevant for mobility applications. The ideal storage medium should allow high volumetric and gravimetric energy densities, quick uptake and release of fuel, operation at room temperatures and atmospheric pressure, safe use, and balanced cost-effectiveness. All current hydrogen storage technologies have significant drawbacks, including complex thermal management systems, boil-off, poor efficiency, expensive catalysts, stability issues, slow response rates, high operating pressures, low energy densities, and risks of violent and uncontrolled spontaneous reactions. While not perfect, the current leading industry standard of compressed hydrogen offers a functional solution and demonstrates a storage option for mobility compared to other technologies.

Hearing loss in workers exposed to epoxy adhesives and noise: a cross-sectional study

OBJECTIVES

Epoxy adhesives contain organic solvents and are widely used in industry. The hazardous effects of epoxy adhesives remain unclear. The objective of this study was to investigate the risk of hearing loss among workers exposed to epoxy adhesives and noise.

DESIGN

Cross-sectional study.

METHODS

For this cross-sectional study, we recruited 182 stone workers who were exposed to both epoxy adhesives and noise, 89 stone workers who were exposed to noise only, and 43 workers from the administrative staff who had not been exposed to adhesives or noise. We obtained demographic data, occupational history and medical history through face-to-face interviews and arranged physical examinations and pure-tone audiometric tests. We also conducted walk-through surveys in the stone industry. A total of 40 representative noise assessments were conducted in 15 workplaces. Air sampling was conducted at 40 workplaces, and volatile organic compounds were analysed using the Environmental Protection Agency (EPA) TO-15 method.

RESULTS

The mean sound pressure level was 87.7 dBA (SD 9.9). The prevalence of noise-induced hearing loss was considerably increased in the stone workers exposed to epoxy adhesives (42%) compared with the stone workers who were not exposed to epoxy adhesives (21%) and the administrative staff group (9.3%). A multivariate logistic regression analysis revealed that exposure to epoxy adhesives significantly increased the risk of hearing loss between 2 and 6 kHz after adjusting for age. Significant interactions between epoxy adhesives and noise and hearing impairment were observed at 3, 4 and 6 kHz.

CONCLUSIONS

Epoxy adhesives exacerbate hearing impairment in noisy environments, with the main impacts occurring in the middle and high frequencies.

Hearing

The main hazard for hearing in the workplace is noise. Organic solvents and heavy metals may increase the danger of developing occupational hearing loss, particularly in the case of co-exposure with noise. While noise produces damage predominantly to the cochlea, chemicals may be responsible for pathologic changes in both peripheral and central parts of the auditory pathway. Noise-induced hearing loss develops slowly over the years, although its progression is most dynamic during the first 10-15 years of exposure. Pure-tone audiometry indicates a bilateral sensorineural hearing loss, affecting predominantly high frequencies, with typical notch at 3-6 kHz in the early stages of the disease. Where there is co-exposure to noise and chemicals, the noise effect on hearing threshold shifts is dominant; however chemicals seem to increase the vulnerability of the cochlea to the damage by noise, particularly at its low and moderate levels. According to European Directive 2003/10/EC, the employer is obliged to implement hearing prevention programs when the A-weighted equivalent 8-hour level of noise (LAEX8 hr) exceeds 80 dB. Since chemicals may impair intelligibility of speech despite a lack of audiometric hearing threshold shift, implementation of speech audiometry, particularly speech in noise tests, is recommended in prevention programs.

Noise-induced hearing loss in workers exposed to urban stressors

The technological and industrial progress together with the intensification of vehicular traffic and the adoption of new social habits are the cause of an increasing noise pollution with possible negative effects on the auditory system. This study aims to assess the noise exposure levels and the effects on the hearing threshold in outdoor and indoor male workers of a big Italian city. The study was carried out on 357 outdoor male workers, exposed to urban noise and on a control group of 357 unexposed indoor workers. Noise levels were measured in 30 outdoor and indoor areas. The subjects underwent tonal liminal audiometry in order to determine the value of their hearing threshold. During their working activity, outdoor and indoor workers are exposed to different noise levels LEX<80 dB(A). At mid-low frequencies (250-2000 Hz), the results show significant differences in the average values of hearing threshold between the two groups in both ears and for all age classes; there are no significant differences between the two groups at higher frequencies. The outdoor noise levels measured are not usually ototoxic and the hearing loss at mid-low frequencies is not characteristic of the exposure to industrial noise. For these reasons the Authors hypothesize that the results may be due to the combined effect of the exposure to noise and to ototoxic air pollutants. The impairment of speech frequencies is disabling and involves the risk of missed forensic recognition.

Self-reported hearing performance in workers exposed to solvents

OBJECTIVE: To compare hearing performance relating to the peripheral and central auditory system between solvent-exposed and non-exposed workers. METHODS: Forty-eight workers exposed to a mixture of solvents and 48 non-exposed control subjects of matched age, gender and educational level were selected to participate in the study. The evaluation procedures included: pure-tone audiometry (500 - 8,000 Hz), to investigate the peripheral auditory system; the Random Gap Detection test, to assess the central auditory system; and the Amsterdam Inventory for Auditory Disability and Handicap, to investigate subjects' self-reported hearing performance in daily-life activities. A Student t test and analyses of covariance (ANCOVA) were computed to determine possible significant differences between solvent-exposed and non-exposed subjects for the hearing level, Random Gap Detection test and Amsterdam Inventory for Auditory Disability and Handicap. Pearson correlations among the three measures were also calculated. RESULTS: Solvent-exposed subjects exhibited significantly poorer hearing thresholds for the right ear than non-exposed subjects. Also, solvent-exposed subjects exhibited poorer results for the Random Gap Detection test and self-reported poorer listening performance than non-exposed subjects. Results of the Amsterdam Inventory for Auditory Disability and Handicap were significantly correlated with the binaural average of subject pure-tone thresholds and Random Gap Detection test performance. CONCLUSIONS: Solvent exposure is associated with poorer hearing performance in daily life activities that relate to the function of the peripheral and central auditory system.

Auditory dysfunction associated with solvent exposure

Background

A number of studies have demonstrated that solvents may induce auditory dysfunction. However, there is still little knowledge regarding the main signs and symptoms of solvent-induced hearing loss (SIHL). The aim of this research was to investigate the association between solvent exposure and adverse effects on peripheral and central auditory functioning with a comprehensive audiological test battery.

Methods

Seventy-two solvent-exposed workers and 72 non-exposed workers were selected to participate in the study. The test battery comprised pure-tone audiometry (PTA), transient evoked otoacoustic emissions (TEOAE), Random Gap Detection (RGD) and Hearing-in-Noise test (HINT).

Results

Solvent-exposed subjects presented with poorer mean test results than non-exposed subjects. A bivariate and multivariate linear regression model analysis was performed. One model for each auditory outcome (PTA, TEOAE, RGD and HINT) was independently constructed. For all of the models solvent exposure was significantly associated with the auditory outcome. Age also appeared significantly associated with some auditory outcomes.

Conclusions

This study provides further evidence of the possible adverse effect of solvents on the peripheral and central auditory functioning. A discussion of these effects and the utility of selected hearing tests to assess SIHL is addressed.

Audition and exhibition to toluene - a contribution for the theme

INTRODUCTION

 With the technological advances and the changes in the productive processes, the workers are displayed the different physical and chemical agents in its labor environment. The toluene is solvent an organic gift in glues, inks, oils, amongst others.

OBJECTIVE

 To compare solvent the literary findings that evidence that diligent displayed simultaneously the noise and they have greater probability to develop an auditory loss of peripheral origin.

METHOD

 Revision of literature regarding the occupational auditory loss in displayed workers the noise and toluene.

RESULTS

 The isolated exposition to the toluene also can unchain an alteration of the auditory thresholds. These audiometric findings, for ototoxicity the exposition to the toluene, present similar audiograms to the one for exposition to the noise, what it becomes difficult to differentiate a audiometric result of agreed exposition - noise and toluene - and exposition only to the noise.

CONCLUSION

 The majority of the studies was projected to generate hypotheses and would have to be considered as preliminary steps of an additional research. Until today the agents in the environment of work and its effect they have been studied in isolated way and the limits of tolerance of these, do not consider the agreed expositions. Considering that the workers are displayed the multiples agent and that the auditory loss is irreversible, the implemented tests must be more complete and all the workers must be part of the program of auditory prevention exactly displayed the low doses of the recommended limit of exposition.

Subchronic JP-8 jet fuel exposure enhances vulnerability to noise-induced hearing loss in rats

Both laboratory and epidemiological studies published over the past two decades have identified the risk of excess hearing loss when specific chemical contaminants are present along with noise. The objective of this study was to evaluate the potency of JP-8 jet fuel to enhance noise-induced hearing loss (NIHL) using inhalation exposure to fuel and simultaneous exposure to either continuous or intermittent noise exposure over a 4-wk exposure period using both male and female Fischer 344 rats. In the initial study, male (n = 5) and female (n = 5) rats received inhalation exposure to JP-8 fuel for 6 h/d, 5 d/wk for 4 wk at concentrations of 200, 750, or 1500 mg/m³. Parallel groups of rats also received nondamaging noise (constant octave band noise at 85 dB(lin)) in combination with the fuel, noise alone (75, 85, or 95 dB), or no exposure to fuel or noise. Significant concentration-related impairment of auditory function measured by distortion product otoacoustic emissions (DPOAE) and compound action potential (CAP) threshold was seen in rats exposed to combined JP-8 plus noise exposure when JP-8 levels of 1500 mg/m³ were presented with trends toward impairment seen with 750 mg/m³ JP-8 + noise. JP-8 alone exerted no significant effect on auditory function. In addition, noise was able to disrupt the DPOAE and increase auditory thresholds only when noise exposure was at 95 dB. In a subsequent study, male (n = 5 per group) and female (n = 5 per group) rats received 1000 mg/m³ JP-8 for 6 h/d, 5 d/wk for 4 wk with and without exposure to 102 dB octave band noise that was present for 15 min out of each hour (total noise duration 90 min). Comparisons were made to rats receiving only noise, and thosereceiving no experimental treatment. Significant impairment of auditory thresholds especially for high-frequency tones was identified in the male rats receiving combined treatment. This study provides a basis for estimating excessive hearing loss under conditions of subchronic JP-8 jet fuel exposure.

A weight of evidence approach for the assessment of the ototoxic potential of industrial chemicals

There is accumulating epidemiological evidence that exposure to some solvents, metals, asphyxiants and other substances in humans is associated with an increased risk of acquiring hearing loss. Furthermore, simultaneous and successive exposure to certain chemicals along with noise can increase the susceptibility to noise-induced hearing loss. There are no regulations that require hearing monitoring of workers who are employed at locations in which occupational exposure to potentially ototoxic chemicals occurs in the absence of noise exposure. This project was undertaken to develop a toxicological database allowing the identification of possible ototoxic substances present in the work environment alone or in combination with noise exposure. Critical toxicological data were compiled for chemical substances included in the Quebec occupational health regulation. The data were evaluated only for noise exposure levels that can be encountered in the workplace and for realistic exposure concentrations up to the short-term exposure limit or ceiling value (CV) or 5 times the 8-h time-weighted average occupational exposure limit (TWA OEL) for human data and up to 100 times the 8-h TWA OEL or CV for animal studies. In total, 224 studies (in 150 articles of which 44 evaluated the combined exposure to noise and a chemical) covering 29 substances were evaluated using a weight of evidence approach. For the majority of cases where potential ototoxicity was previously proposed, there is a paucity of toxicological data in the primary literature. Human and animal studies indicate that lead, styrene, toluene and trichloroethylene are ototoxic and ethyl benzene, n-hexane and p-xylene are possibly ototoxic at concentrations that are relevant to the occupational setting. Carbon monoxide appears to exacerbate noise-induced hearing dysfunction. Toluene interacts with noise to induce more severe hearing losses than the noise alone.

Hearing impairment in F-111 maintenance workers: the study of health outcomes in aircraft maintenance personnel (SHOAMP) general health and medical study

BACKGROUND

We sought to examine hearing loss in a group from the Royal Australian Air Force who undertook fuel tank maintenance on F-111 aircraft, with exposure to formulations containing ototoxins, relative to two different comparison groups.

METHODS

Using pure-tone audiometry, hearing thresholds were assessed in 614 exposed personnel, 513 technical-trade comparisons (different base, same job), and 403 non-technical comparisons (same base, different job). We calculated percentage loss of hearing (PLH) and used regression models to examine whether there was an association between PLH and F-111 fuel tank maintenance, adjusting for possible confounders. In addition, the difference between the observed hearing thresholds and the expected thresholds based on an otologically normal population (ISO-7029-2003) was determined.

RESULTS

The PLH ranged from nil to 96 (median 1.5, quartiles 0.3, 5.5). A logistic regression model showed no statistically significant difference in PLH among the three exposure groups (exposed vs. non-technical controls 1.1: 95% CI 0.7, 2.0 and exposed vs. technical OR 0.9: 95% CI 0.6, 1.3). The model also highlighted a number of other risk factors for PLH including age, tinnitus, smoking, depression, and use of depression medications. However, at all eight frequencies measured, all populations had lower than expected hearing thresholds based on published ISO-7029 medians.

CONCLUSIONS

Although there was no difference in PLH between the three exposure groups, the study did reveal a high degree of hearing loss between the 3 groups and a normal population.

Ototoxic potential of JP-8 and a Fischer-Tropsch synthetic jet fuel following subacute inhalation exposure in rats

This study was undertaken to identify the ototoxic potential of two jet fuels presented alone and in combination with noise. Rats were exposed via a subacute inhalation paradigm to JP-8 jet fuel, a kerosene-based fuel refined from petroleum, and a synthetic fuel produced by the Fischer-Tropsch (FT) process. Although JP-8 contains small ( approximately 5%) concentrations of aromatic hydrocarbons some of which known to be ototoxic, the synthetic fuel does not. The objectives of this study were to identify a lowest observed adverse effect level and a no observed adverse effect level for each jet fuel and to provide some preliminary, but admittedly, indirect evidence concerning the possible role of the aromatic hydrocarbon component of petroleum-based jet fuel on hearing. Rats (n = 5-19) received inhalation exposure to JP-8 or to FT fuel for 4 h/day on five consecutive days at doses of 500, 1000, and 2000 mg/m(3). Additional groups were exposed to various fuel concentrations followed by 1 h of an octave band of noise, noise alone, or no exposure to fuel or noise. Significant dose-related impairment in the distortion product otoacoustic emissions (DPOAE) was seen in subjects exposed to combined JP-8 plus noise exposure when JP-8 levels of at least 1000 mg/m(3) were presented. No noticeable impairment was observed at JP-8 levels of 500 mg/m(3) + noise. In contrast to the effects of JP-8 on noise-induced hearing loss, FT exposure had no effect by itself or in combination with noise exposure even at the highest exposure level tested. Despite an observed loss in DPOAE amplitude seen only when JP-8 and noise were combined, there was no loss in auditory threshold or increase in hair cell loss in any exposure group.

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.

Ethyl benzene should be considered ototoxic at occupationally relevant exposure concentrations

Organic solvents can produce ototoxic effects in both man and experimental animals. The objective of this study was to review the literature on the effects of low-level exposure to ethyl benzene on the auditory system and consider its relevance for the occupational settings. Both human and animal investigations were evaluated only for realistic exposure concentrations based on the permissible exposure limits. In Quebec, the Time-Weighed Average Exposure Value for 8A h (TWAEV) is 100A ppm (434A mg/m(3)) and the Short-Term Exposure Value for 15A min (STEV) is 125A ppm (543A mg/m(3)). In humans, the upper limit for considering ototoxicity data relevant to the occupational exposure situation was set at STEV. Animal data were evaluated only for exposure concentrations up to 100 times the TWAEV. In workers, there is no evidence of either ethyl benzene-induced hearing losses or ototoxic interaction after combined exposure to ethyl benzene and noise. In rats, ethyl benzene affects the auditory function mainly in the cochlear mid-frequency range and ototoxic interaction was observed after combined exposure to noise and ethyl benzene. Further studies with sufficient data on the ethyl benzene exposure of workers are necessary to make a definitive conclusion. Given the current evidence from animal studies, we recommend considering ethyl benzene as an ototoxic agent.

Comparative study of audiometrics tests on metallurgical workers exposed to noise only as well as noise associated to the handling of chemical products

Exposure to ototoxic chemical products in the presence or absence of noise can cause irreversible injury to the hearing of workers for a significantly short period of exposure period. Aim: to perform a comparative study, through audiometric tests, in workers exposed to noise only and noise associated with chemical products. Materials and Methods:155 steel workers (18 - 50 years) exposed to noise (group I), and exposed to noise and chemical products (group II) for a period that varies from 3 to 20 years. Results: significant difference in the rate of occupational hearing loss in the right ear between groups I (3.6%) and II (15.5%). A significantly higher rate of occupational hearing loss in group II (18.3%) and I (6%). With respect to the average time of exposure to aggressive agents, group I was exposed by a significantly higher time. Retrospective study. Discussion: The fact that the right ear was more affected in group II is controversial and should be better investigated in the future, since some studies show that the left ear would be more prone to noise-induced hearing loss. Conclusion: group II had proportionally a higher rate of hearing loss when compared with group I, even after having been exposed to aggressive agents for a lower average time.

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.

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 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.

A proposed methodology for setting occupational exposure limits for hydrocarbon solvents

Occupational exposure limit (OEL) development for hydrocarbon solvents is complicated because most of these solvents have complex compositions and only a few representative constituents have been studied in detail. A proposed solution to this problem is to group constituents with similar physical, chemical, and toxicological properties and to assign "guidance values" to each group. A unique OEL can then be calculated for each solvent, using a reciprocal calculation procedure (RCP) based on the liquid composition. This procedure follows the American Conference of Governmental Industrial Hygienists' (ACGIH) generic advice for complex mixtures and is recommended by the U.K. Health and Safety Executive for OEL calculations by hydrocarbon solvent manufacturers. The RCP is justified, as the toxicological properties of the constituents are additive and the differences between the vapor and liquid compositions do not substantially affect the calculated exposure limits. The guidance values are based principally on acute central nervous system depression and eye and respiratory tract irritation, effects that are the most sensitive indicators of hydrocarbon solvent exposure. One benefit of this procedure is that it is a relatively simple but practical procedure that requires limited compositional information. Further, it provides OEL recommendations that are consistent with occupational experience and current regulatory advice. Groupings and guidance values are proposed, and sample calculations are provided.

Exacerbation of noise-induced hearing loss by co-exposure to workplace chemicals

BACKGROUND

: Numerous organic solvents applied in industry, like toluene, styrene, xylene and n-hexane have been demonstrated to impair hearing in animals. However, the effects of exposure to a given organic solvent and the interaction of noise and solvents on hearing in humans are still not fully recognized.

STUDY DESIGN

: The study was aimed to assess the effects of occupational exposure to solvents alone or in combination with noise on hearing in 1117 employees of yacht, ship, plastic, shoe, and paint and lacquer industry. These persons were exposed either to the mixture of organic solvents with xylene as the main component, or predominantly to styrene, or to the mixture of n-hexane and toluene. Detailed questionnaire data and pure-tone audiometry were compared with data of the reference group that included white collar workers, exposed neither to solvents nor noise and metal factory workers exposed to noise only. In all statistics, the confounding factors were recognized as gender, age, exposure to noise, and other variables occurring at a different rate in study and reference group.

RESULTS

: Odds ratio (OR) of hearing loss related with the particular exposure to chemicals was 2.4 (95%CI 1.59-3.74) in case of solvent mixture, 3.9 (95%CI 2.4-6.2) in case of styrene and 5.3 (95%CI 2.6-10.9) in case of n-hexane and toluene exposure. The odds of developing hearing loss substantially increased in the case of combined exposure to organic solvents and noise as compared to isolated exposure to each of these hazards. The highest OR (over 20-fold) was demonstrated in subgroups of subjects exposed simultaneously to noise and two ototoxic solvents (i.e. styrene and toluene or n-hexane and toluene). The mean hearing thresholds were significantly higher in the solvent-exposed groups than in the reference group. The differences in thresholds were observed at high frequencies in the solvent mixture- and n-hexane + toluene-exposed groups and at all frequencies in the styrene-exposed group. A positive linear relationship existed between exposure to solvents and hearing thresholds at high frequencies.

CONCLUSIONS

: The results of the study provide the epidemiological evidence that exposure to organic solvents in humans is associated with an increased risk of hearing loss. The simultaneous exposure to organic solvents and noise seems to enhance the hearing deficit if compared with isolated exposures.

Effects of Concurrent Noise and Jet Fuel Exposure on Hearing Loss

We sought to examine the effects of occupational exposure to jet fuel on hearing in military workers.

METHODS

Noise-exposed subjects, with or without jet fuel exposure, underwent hearing tests. Work histories, recreational exposures, protective equipment, medical histories, alcohol, smoking, and demographics were collected by questionnaire. Jet fuel, solvent, and noise exposure data were collected from records. Fuel exposure estimates were less than 34% of the OSHA Threshold Limit Values.

RESULTS

Subjects with 3 years of jet fuel exposure had a 70% increase in adjusted odds of hearing loss (OR = 1.7; 95% CI = 1.14-2.53) and the odds increased to 2.41 (95% CI = 1.04-5.57) for 12 years of noise and fuel exposure.

CONCLUSIONS

These findings suggest that jet fuel has a toxic affect on the auditory system.

Relative ototoxicity of 21 aromatic solvents

Some aromatic solvents (e.g. toluene, p-xylene, styrene, and ethylbenzene) show, in the rat, striking ototoxicity characterized by an irreversible hearing loss, as measured by behavioural or electrophysiological methods, associated with damage to outer hair cells in the cochlea of the exposed animals. To broaden the range of aromatic solvents studied concerning their potential ototoxicity and to compare their ototoxicity quantitatively, 21 aromatic solvents were administered orally by gastric intubation to Sprague-Dawley rats for 5 days/week for a 2-week period. The dose used was 8.47 mmol kg(-1) body weight day(-1). The possible ototoxicity of the aromatic solvents was evaluated by morphological investigation of the cochlea. Whole-mount surface preparations of the organ of Corti were made to quantify the number of missing hair cells (cytocochleogram). Among the 21 solvents studied, eight (toluene, p-xylene, ethylbenzene, n-propylbenzene, styrene, alpha-methylstyrene, trans-beta-methylstyrene, and allylbenzene) caused histological lesions of the organ of Corti. They differed widely in their potency. The least ototoxic solvents caused outer hair cell (OHC) loss in the middle turn of the organ of Corti. The OHC loss was slight in the first row, and greater in the second and third rows. The most ototoxic solvents caused high losses in the three rows of the outer hair cells along the entire length of the basilar membrane. There were also occasional inner hair cell (ICH) losses in the most affected animals. Although no measurements were made of the chemical concentrations reached in the blood or the brain, tentative ranking of an increasing ototoxicity of the eight aromatic solvents could be proposed on the basis of the histological losses observed-alpha-methylstyrene<trans-beta-methylstyrene=toluene< or =p-xylene<n-propylbenzene<styrene=ethylbenzene<allylbenzene. There was no relationship between the degree of ototoxicity and the lipophilic properties of the ototoxic agents as expressed by the octanol/water partition coefficients. However, it seemed that some structural constraint was essential to induce ototoxicity. It seems there must be a single side-chain on the aromatic ring for ototoxicity, except with p-xylene. The other aromatic solvents with two side-chains were not ototoxic. When the saturated side-chain was branched (isopropylbenzene, isobutylbenzene, sec-butylbenzene, tert-butylbenzene), no ototoxicity was observed. The ototoxic potency increased when the length of the saturated side-chain extended from one carbon atom to two carbon atoms. Beyond that point, the ototoxic effect decreased with n-propylbenzene and disappeared with n-butylbenzene. Moreover, unsaturation of the side-chain of allylbenzene increased the ototoxicity of n-propylbenzene substantially. Branching of the unsaturated chain (alpha-methylstyrene and trans-beta-methylstyrene) decreased the ototoxicity of styrene.

Cochlear function in young and adult Fischer 344 rats

Fischer 344 (F344) rats are often used as an animal model for investigation of the mechanisms underlying age-related hearing loss. The aim of this study was to assess cochlear function in young (1-month-old) and adult (6-month-old) F344 rats using recording of otoacoustic emissions and auditory brainstem responses (ABRs). The results were compared with control groups of Long Evans (LE) rats of the same ages. The results demonstrate a significant increase in the hearing threshold in F344 rats in comparison with LE rats, expressed mainly at low frequencies (1-2 kHz). In F344 rats, transient evoked otoacoustic emissions were not measurable and distortion product otoacoustic emissions could be detected within a frequency range of 2.4-6.3 kHz. Tympanometric measurements did not reveal any differences in middle ear parameters between F344 and LE rats. The amplitudes of click-evoked ABRs were significantly lower in 6-month-old F344 rats than in LE rats, but other parameters of the ABRs were almost identical in both rat strains. The results demonstrate a significant deficit in low-frequency hearing and altered otoacoustic emissions in both young and adult F344 rats, suggesting a defect of the inner ear sensory epithelium at the apical part of the cochlea.

Hexachlorobenzene, a dioxin-like compound, disrupts auditory function in rat

Hexachlorobenzene (HCB) is a dioxin-like compound widely distributed in the environment. In this study, we investigated the effects of HCB on the cochlea. Conscious free-moving rats were given HCB per os daily for 4 weeks at doses of 0.16, 4 or 16 mg/kg in olive oil, whereas the control group received olive oil only. The effects of HCB were evaluated at various time intervals, by measuring auditory nerve acoustic thresholds and plasma thyroid hormone concentration by radioimmunoassay. Histological evaluation involved surface preparation and scanning electron microscopy observations of cochlear hair cells. At a dose of 0.16 mg/kg, HCB induced no loss of acoustic sensitivity, whereas at 4 mg/kg, it induced cochlear sensitivity deficits at the mid-frequencies (2-16 kHz) with complete recovery once treatment was stopped. At a dose of 16 mg/kg, permanent threshold shifts were observed at all frequencies tested (from 1 to 32 kHz). Morphological studies showed no cochlear hair cell loss or alteration of stereocilia. HCB treatment reduced circulating thyroxine concentrations. Thyroidectomy had no effect on cochlear sensitivity in control animals. Thus, HCB is a potent oto-toxicant, and its ototoxicity may be independent of its thyroidal effects.

Promotion of noise-induced hearing loss by chemical contaminants

Recent studies have underscored the ability of a wide range of chemical agents to potentate noise-induced hearing loss. Given the ubiquitous nature of noise exposure particularly in many work settings, the high rate of noise-induced hearing loss, the limited degree to which auditory function can recover following damage to the inner ear, and the disparate chemical structures that appear capable of impairing hearing, this issue appears to have great public health significance. A compendium of chemicals known to potentiate noise induced hearing loss is presented along with a hypothesis that might explain at least one basis for potentiation of noise-induced hearing loss by certain chemical toxicants. The use of benchmark dose analysis to undertake a risk assessment for promotion of noise-induced hearing loss by both carbon monoxide and hydrogen cyanide is described.

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.

Acrylonitrile potentiates noise-induced hearing loss in rat

Acrylonitrile, one of the 50 most commonly produced industrial chemicals, has recently been identified as a promoter of noise-induced hearing loss (NIHL). This agent has the potential to produce oxidative stress through multiple pathways. We hypothesize that acrylonitrile potentiates NIHL as a consequence of oxidative stress. The objectives of this study were to characterize acrylonitrile exposure conditions that promote permanent NIHL in rats and determine the ability of this nitrile to produce auditory dysfunction by itself. Additionally, we sought to determine whether a spin-trap agent that can form adducts with ROS would protect against the effects of acrylonitrile. Acrylonitrile administration produced significant elevation in NIHL detected as a loss in compound action potential sensitivity. The effect was particularly robust for high-frequency tones and particularly when acrylonitrile and noise were given on repeated occasions. Acrylonitrile by itself did not disrupt threshold sensitivity. Administration of the spin-trap agent phenyl- N- tert-butylnitrone (PBN), given to rats prior to acrylonitrile and noise, did block the elevation of NIHL by acrylonitrile. However, PBN at the dose and time interval given was ineffective in protecting auditory function in subjects exposed to noise alone. The results suggest that oxidative stress may play a role in the promotion of NIHL by acrylonitrile.

Occupational exposure to chemicals and sensory organs: a neglected research field

The effect of industrial chemicals on the sensory perception of exposed workers has received scant attention from the medical community to date, and the scientific literature is mainly limited to some case-reports or isolated studies. Possible explanations for this include the complexity of sensory perception, and the lack of agreement among researchers on methods for testing large groups of subjects. Nevertheless, some published studies showed that vision, hearing and olfactory function can be affected by various industrial metals and solvents, and some data exist also for touch and taste. This review discusses the main industrial chemicals involved. The pathogenesis of the toxicity of chemicals to sensory perception may be related to an action on receptors, nerve fibers, and/or the brain; probably, different pathogenetic mechanisms are involved. One of the main problems in this research field is that most of the studies to date evaluated the effect of a single industrial chemical on a single sense: as an example, we know that styrene exposure can impair smell and also hearing and vision but we have little idea whether different senses are impaired in the same worker, or whether each impairment is independent. In addition, workers are frequently exposed to different chemicals: co-exposure may have no effect, or result in both an increase or a decrease of the effect, as was observed for hearing loss, but studies on this aspect are largely insufficient. Research shows that both occupational and environmental exposure to industrial chemicals can affect sense organs, and suggests that the decline of perception with age may be, at least partly, related to this exposure. Nevertheless, available evidence is incomplete, and is largely inadequate for an estimation of a "safe" threshold of exposure. Good quality further research in this field is needed. This is certainly complex and demands adequate resources, but is justified by the ultimate result: the possibility to prevent an avoidable part of the decline in sensory function with age.

Neural adaptation to pulsatile acoustical stimulation in the cochlear nucleus of the rat

This study, carried out in adult Long-Evans rats, was designed to investigate the adaptive properties of the cochlear nucleus to pulsatile acoustical stimuli. To achieve this purpose, near-field evoked potentials were picked up from the ventral cochlear nucleus in awake animals. Individual auditory thresholds were measured and responses to 250 ms trains of repetitive clicks with pulse rates ranging from 100 to 2000 pulses per second were collected. The amplitude of the first negative (N(1)) component of the evoked potentials to consecutive individual pulses in the train was measured by using a subtraction method. As expected, a rapid amplitude decrement of the responses in the train was obtained and a three phase adaptation was described. The decrease of individual N(1) component amplitude was fitted for each rate of stimulation with exponential decrease equations and time constants were calculated. Such an analysis allowed us to characterize three distinct adaptive processes which were discussed. The results were comparable to those obtained in previous studies in the auditory nerve and suggest that the adaptation recorded in the ventral cochlear nucleus by using near-field evoked potentials reflects the adaptive properties of auditory nerve fibers.

Use of DPOAEs for assessing hearing loss caused by styrene in the rat

The study was carried out to test whether or not cubic distortion otoacoustic emissions were more sensitive than auditory-evoked potentials for assessing styrene-induced hearing losses in the Long-Evans rat. For the purposes of comparison, changes in cubic distortion product otoacoustic emissions (DeltaDPOAE), evoked potential permanent threshold shifts (PTS) and outer hair cell losses were measured in a population of styrene-treated rats. Each rat was exposed to either 650 or 750 ppm of styrene for 4 weeks, 5 days per week, 6 h per day. Only the 750 ppm exposure caused significant hearing losses. For this concentration, DPOAEs appeared as sensitive to styrene as the audiometry performed with evoked potentials, but not more. A high coefficient of correlation [0.84< or =r< or =0.91] between DeltaDPOAE and PTS was obtained across the styrene-induced effects for frequencies ranging from 5 to 12 kHz. This experiment demonstrates that DPOAEs can be used to monitor the ototoxicity induced by styrene even though they cannot be considered as a more sensitive index of cochlear pathology than the evoked potentials, at least under our experimental conditions. Likewise evoked potentials, normal DPOAEs may not guarantee a normal cochlear status and therefore results of DPOAE measurements should be interpreted cautiously. The use of both techniques and the determination of the ratio DeltaDPOAE/PTS may be useful in determining the cause of hearing loss: mechanical or chemical process. Moreover, because of its non-invasive and objective characteristics, the use of DPOAEs could play a greater role in a prevention policy.

Styrene-induced hearing loss: a membrane insult

Styrene is an aromatic solvent widely used as a precursor for polystyrene plastics in many factories which produce glass-reinforced plastic. This solvent has been shown to disrupt the auditory system in both humans and animals. In order to study the sequence of events which could explain the cochlear impairments, a time course experiment was carried out with 6-month-old rats. Male Long Evans rats were exposed to 1000 ppm styrene for 6 h/day, 5 days/week, for either 1, 2, 3, or 4 consecutive weeks. Auditory function was tested by recording the near field evoked potentials from the inferior colliculus, and histological analyses of the cochleae were performed with light and transmission electron microscopy. The electrophysiological results support a toxic mid-frequency process which keeps worsening even after the end of the exposure. The histological findings demonstrate that supporting cells are the first targets of the solvent. Then, the outer hair cells of the third row (OHC3) are disrupted, followed successively by OHC2 and OHC1 from the basal (20 kHz) to the upper turn (4 kHz) of the cochlea. Basically, the disorganization of the membranous structures could be the starting point for the cochlear injury induced by styrene. This paper presents a hypothesis that the accumulation of K+ in the spaces of Nuel underlies the toxic effects of styrene.

Cochlear pathology induced by styrene

Hair cells, spiral fibers and spiral ganglion cells (SGCs) coming from cochleae of styrene-treated Long-Evans rats were counted in order to assess the extent and location of the cochlear injury after the solvent inhalation. If the hair cells, and more specifically the outer hair cells (OHCs), were undoubtedly the first targets of inhaled styrene, the histological results of the present study would seem to indicate that neurons of the spiral ganglion were also injured with increasing styrene doses. The degenerative process of SGCs and spiral fibers within the osseous lamina was predominant in the middle and mid-basal turn. The electrophysiological data, obtained by recording near-field potentials from the inferior colliculus, reflected the damages of the SGCs and fibers but were not consistent with the histopathological data of the organ of Corti. Because of the weak correlation between the styrene-induced injury at the level of the organ of Corti and that induced at the level of the spiral ganglion, it is likely that two different intoxication routes exist within the cochlea. Such an assumption is discussed in the present paper.

Low-level toluene disrupts auditory function in guinea pigs

Toluene appears to have adverse effects on the human auditory system, but it is difficult to estimate its potency since it is commonly present in the workplace in combination with noise exposure; workplace noise exposures are often highly variable. Studies designed to assess toluene ototoxicity specifically have been limited to high-dose studies in a single laboratory animal model, the rat. Here permanent hearing loss has been observed at concentrations of 1000 ppm toluene and greater after inhalation exposure for 5 days, 6 h/day. The OSHA threshold limit value for toluene is only 100 ppm. The current study focuses on the onset of toluene ototoxicity acutely in the guinea pig and in adducing a mechanism of effect. In this study, evidence is presented for the impairment of auditory function by toluene in the guinea pig, at a concentration substantially lower than that used for studying permanent impairment in the rat. The impaired function was correlated with reduced energy metabolism in outer hair cells. Assessment of auditory function was made using distortion product otoacoustic emissions (DPOAE) with subsequent measurement of succinate dehydrogenase (SDH) staining density in hair cells using surface preparations. Temporary disruption of auditory function in guinea pigs is seen in subjects exposed to 250, 500, and 1000 ppm toluene for 8 h/day, 5 day/week for 1 and 4 weeks. Concentrations as low as 250 ppm toluene were able to disrupt auditory function acutely in the guinea pig, and 500 and 1000 ppm toluene produced greater acute dysfunction. SDH staining suggests that reduced enzyme activity in the midfrequency region of the cochlea occurs acutely following toluene exposure. Although the auditory dysfunction progressed between 1 and 4 weeks of exposure, a permanent loss did not develop for these subjects and hair cell death was not seen. The current study identifies early evidence of auditory system impairment in the guinea pig at low toluene concentration and evidence for impairment of energy production in hair cells. While even a transient auditory impairment has implications for workplace safety, additional study on the transition from such acute effects to permanent impairment is essential.