Ototoxicity of organic solvents - from scientific evidence to health policy

Audiological Tests Used in the Evaluation of the Effects of Solvents on the Human Auditory System: A Mixed Methods Review

This study aimed to scope the literature, identify knowledge gaps, appraise results, and synthesize the evidence on the audiological evaluation of workers exposed to solvents. We searched Medline, PubMed, Embase, CINAHL, and NIOSHTIC-2 up to March 22, 2021. Using Covidence, two authors independently assessed study eligibility, risk of bias, and extracted data. National Institute of Health Quality Assessment Tools was used in the quality evaluation of included studies; the Downs and Black checklist was used to assess the risk of bias. Of 454 located references, 37 were included. Twenty-five tests were studied: two tests to measure hearing thresholds, one test to measure word recognition in quiet, six electroacoustic procedures, four electrophysiological tests, and twelve behavioral tests to assess auditory processing skills. Two studies used the Amsterdam Inventory for Auditory Disability and Handicap. The quality of individual studies was mostly considered moderate, but the overall quality of evidence was considered low. The discrepancies between studies and differences in the methodologies/outcomes prevent recommending a specific test battery to assess the auditory effects of occupational solvents. Decisions on audiological tests for patients with a history of solvent exposures require the integration of the most current research evidence with clinical expertise and stakeholder perspectives.

Occupational health risk assessment of workplace solvents and noise in the electronics industry using three comprehensive risk assessment models

Background

Occupational hazards such as solvents and noise in the electronics industry are serious. Although various occupational health risk assessment models have been applied in the electronics industry, they have only been used to assess the risks of individual job positions. Few existing studies have focused on the total risk level of critical risk factors in enterprises.

Methods

Ten electronics enterprises were selected for this study. Information, air samples and physical factor measurements were collected from the selected enterprises through on-site investigation, and then the data were collated and samples were tested according to the requirements of Chinese standards. The Occupational Health Risk Classification and Assessment Model (referred to as the Classification Model), the Occupational Health Risk Grading and Assessment Model (referred to as the Grading Model), and the Occupational Disease Hazard Evaluation Model were used to assess the risks of the enterprises. The correlations and differences between the three models were analyzed, and the results of the models were validated by the average risk level of all of the hazard factors.

Results

Hazards with concentrations exceeding the Chinese occupational exposure limits (OELs) were methylene chloride, 1,2-dichloroethane, and noise. The exposure time of workers ranged from 1 to 11 h per day and the frequency of exposure ranged from 5 to 6 times per week. The risk ratios (RRs) of the Classification Model, the Grading Model and the Occupational Disease Hazard Evaluation Model were 0.70 ± 0.10, 0.34 ± 0.13, and 0.65 ± 0.21, respectively. The RRs for the three risk assessment models were statistically different (P < 0.001), and there were no correlations between them (P > 0.05). The average risk level of all of the hazard factors was 0.38 ± 0.18, which did not differ from the RRs of the Grading Model (P > 0.05).

Conclusions

The hazards of organic solvents and noise in the electronics industry are not negligible. The Grading Model offers a good reflection of the actual risk level of the electronics industry and has strong practicability.

Temporary and Permanent Auditory Effects Associated with Occupational Coexposure to Low Levels of Noise and Solvents

This study aimed to assess temporary and permanent auditory effects associated with occupational coexposure to low levels of noise and solvents. Cross-sectional study with 25 printing industry workers simultaneously exposed to low noise (<80 dBA TWA) and low levels of solvents. The control group consisted of 29 industry workers without the selected exposures. Participants answered a questionnaire and underwent auditory tests. Auditory fatigue was measured by comparing the acoustic reflex threshold before and after the workday. Workers coexposed to solvents and noise showed significantly worse results in auditory tests in comparison with the participants in the control group. Auditory brainstem response results showed differences in III−V interpeak intervals (p = 0.046 in right ear; p = 0.039 in left ear). Mean dichotic digits scores (exposed = 89.5 ± 13.33; controls = 96.40 ± 4.46) were only different in the left ear (p = 0.054). The comparison of pre and postacoustic reflex testing indicated mean differences (p = 0.032) between the exposed (4.58 ± 6.8) and controls (0 ± 4.62) groups. This study provides evidence of a possible temporary effect (hearing fatigue) at the level of the acoustic reflex of the stapedius muscle. The permanent effects were identified mainly at the level of the high brainstem and in the auditory ability of binaural integration.

Evaluation of potential health effects associated with occupational and environmental exposure to styrene - an update

The potential chronic health risks of occupational and environmental exposure to styrene were evaluated to update health hazard and exposure information developed since the Harvard Center for Risk Analysis risk assessment for styrene was performed in 2002. The updated hazard assessment of styrene's health effects indicates human cancers and ototoxicity remain potential concerns. However, mechanistic research on mouse lung tumors demonstrates these tumors are mouse-specific and of low relevance to human cancer risk. The updated toxicity database supports toxicity reference levels of 20 ppm (equates to 400 mg urinary metabolites mandelic acid + phenylglyoxylic acid/g creatinine) for worker inhalation exposure and 3.7 ppm and 2.5 mg/kg bw/day, respectively, for general population inhalation and oral exposure. No cancer risk value estimates are proposed given the established lack of relevance of mouse lung tumors and inconsistent epidemiology evidence. The updated exposure assessment supports inhalation and ingestion routes as important. The updated risk assessment found estimated risks within acceptable ranges for all age groups of the general population and workers with occupational exposures in non-fiber-reinforced polymer composites industries and fiber-reinforced polymer composites (FRP) workers using closed-mold operations or open-mold operations with respiratory protection. Only FRP workers using open-mold operations not using respiratory protection have risk exceedances for styrene and should be considered for risk management measures. In addition, given the reported interaction of styrene exposure with noise, noise reduction to sustain levels below 85 dB(A) needs be in place.

Auditory system dysfunction in Brazilian gasoline station workers

Objective: To examine the auditory system of Brazilian gasoline station workers using an extensive audiological test battery. Design: This was a cross-sectional study. The audiological evaluation included a questionnaire, pure-tone audiometry, acoustic immittance tests, transient-evoked otoacoustic emissions (TEOAEs), distortion product otoacoustic emissions (DPOAEs), auditory brainstem response (ABR) and P300 auditory-evoked potentials. Study sample: A total of 77 Brazilian gasoline station workers were evaluated, and their results were compared with those of 36 participants who were not exposed to chemicals or noise at work. The gasoline station employees worked in 18 different gas stations, and the noise area measurements from all gas stations revealed time-weighted averages below 85 dBA. Results: Of the 77 gasoline station workers evaluated, 67.5% had audiometric results within the normal range, but 59.7% reported difficulties in communication in noisy places. Gasoline station workers showed significantly poorer results than non-exposed control participants in one or more conditions of each of the audiological tests used, except P300. Conclusions: The results suggest that the gasoline station workers have both peripheral and central auditory dysfunctions that could be partly explained by their exposure to gasoline.

Hearing loss among older construction workers: Updated analyses

BACKGROUND

A prior study of this construction worker population found significant noise-associated hearing loss. This follow-up study included a much larger study population and consideration of additional risk factors.

METHODS

Data included audiometry, clinical chemistry, personal history, and work history. Qualitative exposure metrics for noise and solvents were developed. Analyses compared construction workers to an internal reference group with lower exposures and an external worker population with low noise exposure.

RESULTS

Among participants (n = 19 127) an overall prevalence of hearing loss of 58% was observed, with significantly increased prevalence across all construction trades. Construction workers had significantly increased risk of hearing loss compared to reference populations, with increasing risk by work duration. Noise exposure, solvent exposure, hypertension, and smoking were significant risk factors in multivariate models.

CONCLUSIONS

Results support a causal relationship between construction trades work and hearing loss. Prevention should focus on reducing exposure to noise, solvents, and cigarette smoke.

A noise delivery system for multi-animal multi-level whole body ototoxicity studies

The Naval Medical Research Unit Dayton (NAMRU-D) at Wright-Patterson Air Force Base, Ohio, in conjunction with the U.S. Air Force, studied ototoxic effects of JP-8 in rats. NAMRU-D used a multi-chamber whole body exposure facility for up to 96 test animals and 32 control animals at different exposure levels. The objective was to design a noise delivery system that could provide a white noise source one octave band wide, centered at 8 kHz frequency, delivered from outside the exposure chambers. Sound pressure levels were required to be within ±2 dB at all exposure points within each chamber and within ±2 dB over a 6-h run. Electrodynamic shakers were used to produce input noise in exposure chambers by inducing vibration in chamber plenums. Distribution of sound pressure levels across exposure points was controlled within a ±1.5dB prediction interval (α = 0.05) or better. Stability at a central reference point was controlled over 6-h runs within a ±1 dB prediction interval (α = 0.05) or better. The final system allowed NAMRU-D to deliver noise and whole-body aerosol exposures to multiple animals at different levels simultaneously and study the effects that ototoxins may have on hearing loss.

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 Korean workers: co-exposure to organic solvents and heavy metals in nationwide industries

Background

Noise exposure is a well-known contributor to work-related hearing loss. Recent biological evidence suggests that exposure to ototoxic chemicals such as organic solvents and heavy metals may be additional contributors to hearing loss. However, in industrial settings, it is difficult to determine the risks of hearing loss due to these chemicals in workplaces accompanied by excessive noise exposure. A few studies suggest that the effect of noise may be enhanced by ototoxic chemicals. Therefore, this study investigated whether co-exposure to organic solvents and/or heavy metals in the workplace modifies the risk of noise exposure on hearing loss in a background of excessive noise.

Methods

We examined 30,072 workers nationwide in a wide range of industries from the Korea National Occupational Health Surveillance 2009. Data on industry-based exposure (e.g., occupational noise, heavy metals, and organic solvents) and subject-specific health outcomes (e.g., audiometric examination) were collected. Noise was measured as the daily 8-h time-weighted average level. Air conduction hearing thresholds were measured from 0.5 to 6 kHz, and pure-tone averages (PTA) (i.e., means of 2, 3, and 4 kHz) were computed.

Results

In the multivariate linear model, PTA increment with occupational noise were 1.64-fold and 2.15-fold higher in individuals exposed to heavy metals and organic solvents than in unexposed individuals, respectively.

Conclusion

This study provides nationwide evidence that co-exposure to heavy metals and/or organic solvents may exacerbate the effect of noise exposure on hearing loss in workplaces. These findings suggest that workers in industries dealing with heavy metals or organic solvents are susceptible to such risks.

Interaction between noise and cigarette smoking for the outcome of hearing loss among women: a population-based study

BACKGROUND

We investigated the interaction between exposure to noise and smoking in relation to prevalence of hearing loss among women.

METHODS

A sample of women aged 20-49 years (n = 1,723) from a population-based cross-sectional study carried out in Brazil in 2006 was examined. Hearing loss was assessed using a yes-no validated question. Biological interaction was analyzed using the additive scale and measured with interaction contrast ratio (ICR) and assessment of dose-response relationship.

RESULTS

The combined effect of exposure to noise and cigarette smoking on hearing loss (adjusted prevalence ratio (PRadj) = 3.94, 95% confidence interval (CI): 2.81, 5.52) was greater than expected based on the additive single effects of smoking (PRadj = 1.39, 95% CI: 1.07, 1.81) and noise (PRadj = 2.66, 95% CI: 1.86, 3.82). ICR estimates were not statistically significant. The prevalence of hearing loss among noise-exposed women increased with duration of smoking (P trend = 0.026), number of cigarettes smoked per day (P trend = 0.034), cumulative tobacco use (P trend = 0.030), and early age at smoking initiation (P trend = 0.047).

CONCLUSIONS

Noise and smoking may have a combined effect on hearing loss but further studies are still needed. A dose-response relation of smoking for the noise effect among women is suggested.

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.

Noise-induced hearing loss and associated factors among vector control workers in a Malaysian state

This study aims to determine the prevalence and associated factors of noise-induced hearing loss (NIHL) among vector control workers in the state of Negeri Sembilan, Malaysia. This was an analytical cross-sectional study conducted on 181 vector control workers who were working in district health offices in a state in Malaysia. Data were collected using a self-administered questionnaire and audiometry. Prevalence of NIHL was 26% among this group of workers. NIHL was significantly associated with the age-group of 40 years and older, length of service of 10 or more years, current occupational noise exposure, listening to loud music, history of firearms use, and history of mumps/measles infection. Following logistic regression, age of more than 40 years and noise exposure in current occupation were associated with NIHL with an odds ratio of 3.45 (95% confidence interval = 1.68-7.07) and 6.87 (95% confidence interval = 1.54-30.69), respectively, among this group of vector control workers.

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.

Ototoxicity of trichloroethylene in concentrations relevant for the working environment

Organic solvents can cause hearing loss themselves or promote noise-induced hearing loss. The objective of this study was to review the literature on the effects of low-level exposure to trichloroethylene 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 Quebec permissible exposure limits: 50 ppm 8-h time-weighed average exposure value (TWAEV) and 200 ppm short-term exposure value (STEV). In humans, the upper limit for considering ototoxicity data relevant to the occupational exposure situation was set at the STEV. Animal data were evaluated only for exposure concentrations up to 100 times the TWAEV. There is no convincing evidence of trichloroethylene-induced hearing losses in workers. In rats, trichloroethylene affects the auditory function mainly in the cochlear mid- to high-frequency range with a lowest observed adverse effect level (LOAEL) of 2000 ppm. No studies on ototoxic interaction after combined exposure to noise and trichloroethylene were identified in humans. In rats, supra-additive interaction was reported. Further studies with sufficient data on the trichloroethylene exposure of workers are necessary to make a definitive conclusion. In the interim, we recommend considering trichloroethylene as an ototoxic agent.

Occupational ototoxicity of n-hexane

The ability of chemicals to produce hearing loss themselves or to promote noise-induced hearing loss has been reported for some organic solvents. The objective of this study was to review the literature on the effects of low-level exposure to n-hexane on the auditory system and consider its relevance for occupational settings. Both human and animal investigations were evaluated only for realistic exposure concentrations based on the permissible inhalation exposure limits. In Quebec, the time-weighted average exposure value (TWAEV) for 8 h is 50 ppm. In humans, the upper limit for considering ototoxicity data relevant to the occupational exposure situation was set at five times the TWAEV. Animal data were evaluated only for exposure concentrations up to 100 times the TWAEV. There is no convincing evidence of n-hexane-induced hearing loss in workers. In rats, n-hexane seems to affect auditory function; however, the site of these alterations cannot be determined from the present data. Further studies with sufficient data on the exposure of workers to n-hexane are necessary to make a definitive conclusion. In the interim, we recommend considering n-hexane as a possibly ototoxic agent.

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.

Selective inhibition of human heteromeric alpha9alpha10 nicotinic acetylcholine receptors at a low agonist concentration by low concentrations of ototoxic organic solvents

Ethylbenzene and para-xylene (p-xylene), but not the chemically closely related organic solvents ortho-xylene (o-xylene) and meta-xylene (m-xylene), are known to cause ototoxicity and irreversible hearing loss, though the underlying mechanisms are still unknown. In this study, effects of ethylbenzene and of p-, o-, and m-xylene on human heteromeric alpha9alpha10 nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus oocytes were investigated using the two-electrode voltage clamp technique. ACh dose-dependently evoked an alpha9alpha10 nAChR-mediated ion current with an EC(50) of 137 microM. When ACh is applied at a low concentration (10 microM), the nAChR-mediated ion current is inhibited by a low concentration (10 microM) of ethylbenzene and p-xylene, but not by the same concentration of the non-ototoxic solvents. At a high solvent concentration (300 microM), all solvents cause inhibition of the ion currents evoked by 10 microM ACh. Ion currents evoked by a near maximum-effective concentration ACh (1mM) are inhibited by the selected organic solvents only at 300 microM. These results demonstrate that low concentrations of the known ototoxic solvents ethylbenzene and p-xylene inhibit alpha9alpha10 nAChR-mediated ion currents, whereas the structurally related, non-ototoxic solvents m-xylene and o-xylene do not, indicating that the alpha9alpha10 nAChR is a potential target for solvent-induced ototoxicity.