Combined effects of a simultaneous exposure to noise and toluene on hearing function

MiXie, an Online Tool for Better Health Assessment of Workers Exposed to Multiple Chemicals

There is increasing concern for workers facing multiple chemical exposure. The accumulation of information on occupational conditions indicates the need to incorporate the concept of multiple exposures in the risk assessment process and to develop tools for assessing the potential impacts of multiple exposures on workers’ health. Our objective is to describe the MiXie online decision-making tool that can be used to assess the risk of exposure to multiple chemicals. The description includes the development of MiXie, the structure of its toxicological database according to the target organ or the mode of action, and the algorithm for quantitative analysis of a mixture. Two case studies of its use in evaluating the risks of multiple exposures in real workplace situations are presented. The case study in the printing industry showed increased risk for four toxicological classes (central nervous system damage, ocular damage, skin damage, and ototoxicity) associated with co-exposure to four chemicals during maintenance operations. The MiXie analysis also showed the presence of carcinogenic substances in the mixture and a risk to the development of the foetus. The case study in nail salons showed the presence of carcinogenic and sensitizing chemicals and an increased risk to upper airways. MiXie helps preventers evaluate the possible additive effects of mixtures, providing an easy-to-read diagnosis to identify risks incurred by co-exposed employees. In addition, MiXie identifies risky occupational situations that would go unnoticed without a multiple substance approach.

Inhalation exposure to volatile organic compounds in the printing industry

This study reports on the occupational inhalation exposure to VOCs of workers in the Kuwaiti printing industry. Using the evacuated canister methodology, we targeted 72 VOCs in three printeries and compared the concentrations to previous reports and relevant occupational exposure levels (OELs). We found that recent efforts in the printing industry to reduce VOC usage had been successful, as concentrations of key hazardous VOCs were substantially lower than anticipated. On the other hand, nearly all target VOCs were found. Non-production areas were sampled along with the offset printing areas, another strength of this study, and revealed exposures to hazardous VOCs among administers and digital printer and CTP operators. Exposure to ototoxic VOCs amounted to 1-3% of the OEL, consisting mostly of ethylbenzene, which was likely in use in two of the study printeries. Exposure to carcinogenic or probably carcinogenic VOCs was 15-20% of the OEL at four locations across the three printeries, consisting mostly of vinyl chloride and benzyl chloride. Vinyl chloride VOC was partially sourced from outdoors, but was also likely used inside the study printeries. Interestingly, concentrations of vinyl chloride were similar in most sampling locations to that of CFC-114, a CFC banned by the Montreal Protocol and not commonly used as a refrigerant. This unexpected finding suggests further study is warranted to identify the use of these VOCs in printeries. Exposure to hazardous VOCs up to nearly 50% of the OEL, consisting largely of bromoform and vinyl chloride. Bromoform was found in all the study printeries, sourced partially from outdoor air. The higher concentrations found inside the study printeries likely resulted from the use of the desalinated water for washing. This finding raises of emissions from sources other than blanket washes, and inks, etc. adding to the total VOC load in printery indoor air. Implications: Results from this study indicate that efforts to reduce worker exposure to VOCs particularly dangerous to human health in recent years have been successful, but there is still much to be done to protect workers. Exposures to ototoxic and carcinogenic VOCs were identified, among both production and non-production workers. Unexpected findings included the apparent use in printing activities of the carcinogen vinyl chloride and CFC-114, banned under the Montreal Protocol. Observed lapses in safety procedures included failure to utilize ventilation systems and closing doors between work areas, indicating management and worker education should remain a priority.

The effect of simultaneous exposure to cigarette smoke and noise on distortion product otoacoustic emissions in rats

Cigarette smoking is a possible risk factor for hearing loss. However, the impact of simultaneous exposure to noise and smoke on hearing has remained controversial. This study investigated the combined effect of exposure to cigarette smoking and noise on hearing loss. Three groups of male Wistar rats (275 ± 25 g) were subjected to white noise (102 ± 0.5 dB), cigarette smoking (20 cigarettes), and both cigarette smoking and noise for 8 h and 10 days inside the exposure chamber. The control group was exposed to neither noise nor smoke. Distortion product otoacoustic emissions (DPOAE) were measured before any intervention, and it was repeated 1, 7, and 21 days after the last exposure. One-day postexposure to noise, cigarette smoking, and both cigarette smoking and noise, the mean of DPOAE amplitudes decreased significantly ( p < 0.05) between, respectively, 5.7-30.7, 1.5-7.5, and 5.2-32.6 dB within the frequency range of 4620-9960. Temporal DPOAE change in rats exposed to noise or both cigarette smoking and noise was not significantly different ( p > 0.05). DPOAE amplitudes returned to the baseline values in the group subjected to smoking 21 days postexposure. The most permanent change was observed in rats exposed to both cigarette smoking and noise. Accordingly, simultaneous subacute exposure to noise and cigarette smoking increases the effect of noise on permanent hearing loss. Therefore, smoking workers exposed to noise might be at a greater risk of developing hearing loss, and it is recommended that authorities in charge take note of this evidence.

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.

Chemical exposure in occupational settings and related health risks: a neglected area of research in Pakistan

In Pakistan a huge number of workers is routinely exposed to various types of chemical contaminants but there is a dearth of information as to the impact of these agents, due to a lack of a routine surveillance system and proper reporting. Prolonged and sometimes acute occupational exposures to varied organic chemicals may result in numerous health related problems. Studies from all over the world have shown adverse health outcomes of chemicals that are commonly used in various occupations. Such chemical exposures are not just confined to the workplace, but the residents surrounding industrial sites also face significant health risks due to indirect chemical exposure. Occupational exposure is a multidimensional risk factor that varies from one occupation to another, and is associated with health decline in workers. Common determinants of workplace hazards include improper, or lack of use of self-protective equipment, active and passive exposure to cigarette smoke as well as the socio-demographic and economic background of workers. There may be more than one cause of occupational stress and psychophysical disturbance among workers such as workload, lower salaries, and lack of social and medical facilities; indeed, their general health is poor. Therefore, in Pakistan, it is particularly important to focus on these issues and set rules and regulations to create occupational hazard awareness among workers, which will promote health safety at work places. If priorities are given to the correct use of self-protective equipment, adopting proper hygiene at the workplace and to avoid smoking, occupational exposures and consequent health risks may be minimized significantly.

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.

Effect of combined occupational exposure to noise and organic solvents on hearing

Noise exposure has been commonly regarded as the main hazard of occupational hearing loss. Recent studies indicate that several chemicals, including organic solvents have ototoxic effects. This study aimed at evaluating the hearing of workers exposed to both noise and a mixture of organic solvents at concentrations anticipated as safe. The study comprised three groups. The first one included 70 workers exposed to noise only, the second group consisted of 93 workers exposed to organic solvents and noise, and the control group included 59 individuals exposed to neither noise nor organic solvents. The three groups were matched for age, socioeconomic status, and smoking habit. The results of this study revealed that there was no statistically significant difference between the two exposed groups as regards the duration of exposure. There was a highly statistically significant difference between the two exposed groups as regards the different types of hearing loss (conductive deafness, sensory neural hearing loss, and mixed type) compared with the control one. Our study reported that sensory neural hearing loss occurred earlier in subjects with combined exposure to noise and solvents at a mean duration of exposure (16.38 ± 9.44 years) compared to (24.53 ± 9.59 years) the subjects with sole exposure to noise. The difference between the two groups was statistically significant regarding this type of hearing impairment ( p < 0.05). There was a positive significant correlation between hearing impairment and duration of exposure in the two exposed groups. As regards the results of the environmental monitoring, both noise exposure levels (dB) and levels of different organic solvents measured (mg/m3) in different work departments were less than the levels recommended by Egyptian Environmental Law No. 4 for 1994. It is recommended that in the case of combined exposure, noise and solvent levels should be lowered than the permissible limits recommended for either alone.

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.

Mechanisms Involved in Ototoxicity

The modern era of evidence-based ototoxicity emerged in the 1940s following the discovery of aminoglycosides and their ototoxic side effects. New classes of ototoxins have been identified in subsequent decades, notably loop diuretics, antineoplastic drugs, and metal chelators. Ototoxic drugs are frequently nephrotoxic, as both organs regulate fluid and ion composition. The mechanisms of ototoxicity are as diverse as the pharmacological properties of each ototoxin, though the generation of toxic levels of reactive oxygen species appears to be a common denominator. As mechanisms of cytotoxicity for each ototoxin continue to be elucidated, a new frontier in ototoxicity is emerging: How do ototoxins cross the blood-labyrinth barrier that tightly regulates the composition of the inner ear fluids? Increased knowledge of the mechanisms by which systemic ototoxins are trafficked across the blood-labyrinth barrier into the inner ear is critical to developing new pharmacotherapeutic agents that target the blood-labyrinth barrier to prevent trafficking of ototoxic drugs and their cytotoxic sequelae.

Neuronal circuits involved in the middle-ear acoustic reflex

Human and animal studies have shown that certain aromatic solvents such as toluene can cause hearing loss and can exacerbate the effects of noise. The latter effects might be due to a modification of responses of motoneurons controlling the middle-ear acoustic reflex. In the present investigation, the audition of Long-Evans rats was evaluated by measuring cubic (2f1 - f2) distortion otoacoustic emissions (f1 = 8000 Hz; f2 = 9600 Hz; f1/f2 = 1.2) prior to, during, and after activation of the middle-ear acoustic reflex. A noise suppressor was used to modify the amplitude of the 2f1 - f2 distortion otoacoustic emissions. It was delivered either contralaterally (band noise centered at 4 kHz), or ipsilaterally (3.5 kHz sine wave) to test the role played by the central auditory nuclei. This audiometric approach was used to study the physiological efficiency of the middle-ear acoustic reflex during an injection of a bolus of Intralipid (as a vehicle) containing 58.4, 87.4, or 116.2mM toluene via the carotid artery. The results showed that toluene could either increase or decrease middle-ear acoustic reflex efficiency, depending on the toluene concentration and the ear receiving noise suppressor. A new neuronal circuit of the middle-ear acoustic reflex has been proposed to explain findings obtained in this investigation. Finally, the depressing action of toluene on the central auditory nuclei driving the middle-ear acoustic reflex might explain the synergistic effects of a co-exposure to noise and aromatic solvents.

The association between low levels of lead in blood and occupational noise-induced hearing loss in steel workers

As the use of leaded gasoline has ceased in the last decade, background lead exposure has generally been reduced. The aim of this study was to examine the effect of low-level lead exposure on human hearing loss. This study was conducted in a steel plant and 412 workers were recruited from all over the plant. Personal information such as demographics and work history was obtained through a questionnaire. All subjects took part in an audiometric examination of hearing thresholds, for both ears, with air-conducted pure tones at frequencies of 500, 1000, 2000, 3000, 4000, 6000 and 8000 Hz. Subjects' blood samples were collected and analyzed for levels of manganese, copper, zinc, arsenic, cadmium and lead with inductive couple plasma-mass spectrometry. Meanwhile, noise levels in different working zones were determined using a sound level meter with A-weighting network. Only subjects with hearing loss difference of no more than 15 dB between both ears and had no congenital abnormalities were included in further data analysis. Lead was the only metal in blood found significantly correlated with hearing loss for most tested sound frequencies (p<0.05 to p<0.0001). After adjustment for age and noise level, the logistic regression model analysis indicated that elevated blood lead over 7 microg/dL was significantly associated with hearing loss at the sound frequencies of 3000 through 8000 Hz with odds ratios raging from 3.06 to 6.26 (p<0.05-p<0.005). We concluded that elevated blood lead at level below 10 microg/dL might enhance the noise-induced hearing loss. Future research needs to further explore the detailed mechanism.

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.

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.

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.

Evaluation of auditory fatigue in combined noise, heat and workload exposure

This study was performed in a climatic chamber to evaluate the combined effects of noise intensity, heat stress, workload, and exposure duration on both noise-induced temporary threshold shift (TTS) and the recovery time by adopting Taguch's method. Fourteen subjects without previous significant noise exposure and smoking history were recruited to participate in this study. All hearing threshold levels at eight different frequencies (250 to 8,000 Hz) of better ear were measured in an audiometric booth by using the ascending method in 2 dB steps before each exposure condition. The test was also carried out after exposure to evaluate TTS at various times. The TTS recovery time was assessed using an audiometric test on all subjects at post-exposure times of 2, 20, 40, 60, 80 and 120 min, respectively. It was found that TTS depended mainly on the exposed noise dose and was enhanced by workload and heat stress. The TTS recovery time is dependent upon the magnitude of the initial hearing loss. In conclusion, TTS driven by noise exposure is enhanced by heat and workload. Further studies are required to evaluate the effects of workload with extreme temperature in a workplace environment.

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.

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.

Hearing loss in workers exposed to toluene and noise

In this study we investigated the risk of hearing loss among workers exposed to both toluene and noise. We recruited 58 workers at an adhesive materials manufacturing plant who were exposured to both toluene and noise [78.6-87.1 A-weighted decibels; dB(A)], 58 workers exposed to noise only [83.5-90.1 dB(A)], and 58 administrative clerks [67.9-72.6 dB(A)] at the same company. We interviewed participants to obtain sociodemographic and employment information and performed physical examinations, including pure-tone audiometry tests between 0.5 and 6 kHz. A contracted laboratory certified by the Council of Labor in Taiwan conducted on-site toluene and noise exposure measurements. The prevalence of hearing loss of >or=25 dB in the toluene plus noise group (86.2%) was much greater than that in the noise-only group (44.8%) and the administrative clerks (5.0%) (p<0.001). The prevalence rates were 67.2, 32.8, and 8.3% (p<0.001), respectively, when 0.5 kHz was excluded from the estimation. Multivariate logistic regression analysis showed that the toluene plus noise group had an estimated risk for hearing loss>or=25 dB, 10.9 times higher than that of the noise-only group. The risk ratio dropped to 5.8 when 0.5 kHz was excluded from the risk estimation. Hearing impairment was greater for the pure-tone frequency of 1 kHz than for that of 2 kHz. However, the mean hearing threshold was the poorest for 6 kHz, and the least effect was observed for 2 kHz. Our results suggest that toluene exacerbates hearing loss in a noisy environment, with the main impact on the lower frequencies.

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.

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.

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.

Chemical exposure as a risk factor for hearing loss

In 2002, the National Institute for Occupational Safety and Health and the National Hearing Conservation Association cosponsored the "Best Practices Workshop: Combined Effects of Chemicals and Noise on Hearing." This article summarizes the main results of the Workshop. Its goals were to review the knowledge of chemical ototoxicity and to stimulate participant discussion on how to address this risk. Speakers provided an overview of the effects of chemicals on the auditory system (http://www.cdc.gov/niosh/noise/noiseandchem/noiseandchem.html). Research priorities were discussed in concurrent working group sessions. The Workshop concluded with a panel of the groups' facilitators reporting on these sessions. The following key issues were identified: rationale and proposal of a list of priority chemicals; valid procedures for exposure (animal studies), exposure assessment, and audiological testing; need for mechanistic research and a Response Level; recommendations for preventive actions; and information dissemination.

Solvent ototoxicity in the rat and guinea pig

There is clear evidence that aromatic solvents can disrupt the auditory system in humans and animals. As far as animal models are concerned, solvent-induced hearing loss seems to be species-dependent. Indeed, most published data have been obtained with the rat, which shows mid-frequency cochlear deficits, whereas the guinea pig does not show any permanent hearing loss after solvent exposure. In the current investigation, the effects of two solvents, toluene (600 ppm) and styrene (1000 ppm), were studied in both Long-Evans rats and pigmented guinea pigs exposed 6 h/day for 5 consecutive days. Cochlear function was tested by using distortion product otoacoustic emissions (DPOAE) measured prior to the solvent exposure, 20 min after the end of the exposure and successively at 2 and 4 weeks post-exposure. In addition to cochlear testing, solvent concentrations in blood and urinary metabolites were measured. A cochlear histological analysis was performed at the end of the experiment. No decrease in DPOAE amplitude was observed in the guinea pig, even immediately following the end of exposure. The rat model showed severe disruption of auditory function and cochlear pathology, whereas the guinea pig had no disruption of DPOAE or cochlear pathological alterations. Therefore, the vulnerability of the cochlear function was strictly dependent on the species. As expected, an important difference in the styrene concentration in blood was observed: the solvent concentrations were fourfold higher in the rat than in the guinea pig. Therefore, it is clear that a pharmacokinetic or an uptake difference might explain the difference in susceptibility observed between the two species. Moreover, the metabolism pathways of the solvents were different depending on the species. Attempts to explain differences of vulnerability between the rat and guinea pig are addressed in the present paper.

Susceptibility to the ototoxic properties of toluene is species specific

Toluene is the most widely used industrial solvent. It has been shown to be ototoxic in mice and rats, and to increase permanent threshold shift in conjunction with exposure to noise. Chinchillas are widely used for studying noise effects on the cochlea. The present study was initiated to study toluene and noise interaction in chinchillas. Thirty-three chinchillas were exposed to a 95 dBA 500 Hz octave band noise plus 2000 ppm toluene, 8 or 12 h per day for 10 days. Auditory function was estimated using the auditory brainstem response (ABR) to tones between 500 Hz and 16 kHz. There was no effect on the ABR of toluene alone. Noise alone produced a threshold shift. There was no interaction of noise and toluene on the ear. The present study suggests that chinchillas are markedly less susceptible to the ototoxic effect of toluene than mice and rats. A working hypothesis as to the species differences was that chinchilla liver was able to detoxify the toluene. Hepatic microsomes from chinchillas, rats and humans were tested for their ability to convert toluene to the more water-soluble compound - benzyl alcohol. Chinchilla livers were found to contain more of the P450 enzymes CYP2E1 and CYP2B than rats or humans. In addition, the data show that the P450 enzymes are more active in chinchillas than in rats and humans. In conclusion, the results suggest that rats and mice are a more appropriate model for human toluene ototoxicity. However, chinchillas may provide a valuable model for investigating how ototoxic agents can be detoxified to less damaging compounds.

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.

Simultaneous exposure to ethyl benzene and noise: synergistic effects on outer hair cells

The effects on hearing of simultaneous exposure to the ototoxic organic solvent ethyl benzene and broad-band noise were evaluated in rats. The effects of three ethyl benzene concentrations (0, 300 or 400 ppm) and three noise levels (95 or 105 dB(lin) SPL or background noise at 65 dB(lin) SPL) and all their combinations were investigated for a 5 day exposure at 8 h/day. Distortion product otoacoustic emissions and compound action potentials were affected after 105 dB noise alone, and after 105 dB noise in combination with ethyl benzene (300 and 400 ppm). However, the amount of loss for these combinations did not exceed the loss for 105 dB noise alone. Outer hair cell (OHC) loss after exposure to 300 ppm ethyl benzene was located in the third row of OHCs. At 400 ppm, the loss spread out to the second and first row of OHCs. Noise alone hardly affected the OHC counts except for a minor loss in the first row of OHCs after 105 dB SPL. Noise at 105 dB in combination with ethyl benzene at 300 and 400 ppm, however, showed OHC loss greater than the sum of the losses induced by noise and ethyl benzene alone.

Potentiation of noise-induced hearing loss by amikacin in guinea pigs

Noise and aminoglycosides initially attack cochlear outer hair cells (OHCs). Distortion product otoacoustic emissions (DPOAEs) are used for the early diagnosis of damage to OHCs. The effects of sub-damaging doses of amikacin, an aminoglycoside antibiotic agent, on noise-induced hearing loss (NIHL) were examined in guinea pigs. Animals were grouped by gender and exposed to broadband noise at 105 dB SPL for 12 h and/or injected i.m. with either amikacin (100 mg/kg/day) or saline for 10 days. Auditory brainstem response (ABR) thresholds, along with DPOAE amplitudes, were measured serially before and after noise exposure. DPOAE amplitudes decreased and ABR thresholds elevated immediately after noise exposure and then gradually recovered. At all frequencies, the emission amplitudes recovered completely to pre-exposure baseline values by 4 days after noise exposure. There was no effect of amikacin on either the ABR threshold or DPOAE amplitudes, in animals treated with amikacin only. However, amikacin significantly prolonged the effect of noise exposure on DPOAE amplitude but not on the noise-induced temporary threshold shift (TTS) of the ABR. In animals treated with a combination of noise and amikacin, significant changes in DPOAE amplitudes were still observed at 4 weeks after cessation of noise exposure. No gender difference in the responses to noise and/or amikacin could be demonstrated. The present findings indicate that even sub-damaging dosages of amikacin might impair recovery from NIHL in guinea pigs.

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.

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.

Succinate dehydrogenase (SDH) activity in hair cells: a correlate for permanent threshold elevations

Hair cell loss is often used as a histological correlate of hearing loss. However, the histological and the physiological data are not always well correlated. This paper investigates the use of succinate dehydrogenase (SDH) activity in the hair cells as a marker of cellular dysfunction and so the loss of auditory sensitivity. In our previous studies, potentiation of noise-induced auditory threshold elevation by carbon monoxide (CO) was observed [Chen and Fechter, 1999; Chen et al., 1999]. However, its histological basis is still unclear. In this study, rats were exposed to 100-dB octave-band noise (center frequency=13.6 kHz, 2 h) or to the combination of the noise and CO (1200 ppm). Threshold elevation of compound action potential (CAP) and cochlear histological changes were assessed 4 weeks after exposure. The noise alone caused CAP threshold elevations with little if any or without hair cell loss. However, the SDH activity in the hair cells decreased after the exposure. The SDH reduction, especially in the inner hair cells, was well related to the loss of auditory sensitivity. The combined exposure to noise and CO caused more severe CAP threshold elevation and SDH activity reduction than did the noise alone and it also caused significant outer hair cell loss. However, across all the test frequencies, neither the hair cell loss nor the SDH reduction alone had good correlation to the reduction of the auditory sensitivity. Under this situation, CAP threshold elevation seemed to follow OHC loss at high frequencies and to follow SDH reductions in the IHCs at low frequencies, where no hair cell loss occurred.

Combined effects of noise and styrene exposure on hearing function in the rat

Combined exposure to both noise and aromatic solvents such as styrene is common in many industries. In order to study the combined effects of simultaneous exposure to both noise and styrene on hearing, male adult Long-Evans rats were exposed either to 750 ppm styrene alone, to a 97 dB SPL octave band of noise centered at 8 kHz, or to a combination of noise and styrene. The exposure duration was 6 h/day, 5 days/week, for 4 consecutive weeks. Auditory function was tested over a frequency range from 2 to 32 kHz by recording near field potentials from the inferior colliculus, whereas histopathological analyses of the cochleae were performed with conventional morphometric approaches. Whereas both noise and styrene each caused permanent threshold shifts, the mechanisms of cochlear damage were different. Noise-induced hearing loss was mainly related to injuries of the stereocilia, whereas styrene-induced hearing loss was related to outer hair cell losses. Following the combined exposure, the threshold elevations as well as the cell losses exceeded the summed loss caused by noise and by styrene alone in the range of 8-16 kHz. Therefore, these results suggest that the two ototoxicants can cause a permanent synergistic loss of auditory sensitivity.

Comparison of toluene-induced and styrene-induced hearing losses

Toluene and styrene are industrial solvents that can severely damage the auditory function in adult rats. In the present study, toluene (1000 to 2000 ppm) and styrene doses (500 to 1500 ppm) were investigated according to the same schedule: 6 hours per day, 5 days per week, for 4 consecutive weeks. The auditory function of the animals was tested by recording evoked potentials from the inferior colliculus over a frequency range from 2 to 32 kHz, whereas pathological data were evaluated by conventional histologic techniques. The permanent threshold shifts (PTS) were obtained with a styrene dose 2.4 times lower than that of the toluene. The slope of the regression line (PTS/doses) was 2.1 steeper with styrene than that obtained with toluene in the same experimental conditions. The sequence of histopathological events along the organ of Corti, especially the orderliness and the location of the traumas, was similar for paired concentrations of styrene and toluene, which were respectively 650 ppm, 1500 ppm for the first match, and 850 ppm, 1750 ppm for the second one. Both electrophysiological and histological findings point out the higher ototoxic potency of the styrene compared to that of the toluene. Assumptions concerning the ototoxic mechanism are addressed in the present paper.

Toluene ototoxicity in rats: assessment of the frequency of hearing deficit by electrocochleography

To identify the frequency range most sensitive to toluene-induced auditory damage, the auditory function of adult Long-Evans rats exposed to 1750 ppm of toluene (6 h/day, 5 days/week, 4 weeks), was tested by recording auditory-evoked potentials directly from the round window of the cochlea. The present electrocochleographic findings do not support a specific mid- to high-frequency loss of auditory sensitivity. On the contrary, the electrophysiologic data, obtained for audiometric frequencies ranging from 2 to 32 kHz, showed a hearing deficit not only in the mid-frequency region (12-16 kHz), but also in the mid-low-frequency region (3-4 kHz). Actually, the effect of toluene was independent of the frequency in our experimental conditions. Histological analysis was consistent with electrophysiologic data because a broad loss of outer hair cells occurred in both mid- and mid-apical coil of the organ of Corti.