The ototoxic interaction of styrene and noise

Styrene targets sensory and neural cochlear function through the crossroad between oxidative stress and inflammation

BACKGROUND

Although styrene is an established ototoxic agent at occupational exposure levels, the mechanisms of styrene toxicity in the auditory system are still unclear.

OBJECTIVES

The aim of this study was to identify the consequences of styrene chronic exposure in cochlear structures, looking for the mechanisms of ototoxicity of this organic compound and focusing on cell targets and oxidative stress/inflammatory processes.

METHODS

Male adult Wistar rats were exposed to styrene (400 mg/kg by gavage for 5 days/week, 3 consecutive weeks). Hearing loss was evaluated by measuring auditory brainstem responses (ABR), morphological analysis were performed to evaluate hair cell and spiral ganglion neuron survival, as well as synaptic damage. Analysis of apoptotic (p53) and inflammatory (NF-κB, TNF-α, IL-1β and IL-10) mediators were performed by immunofluorescence analysis and western blot.

RESULTS

Styrene ototoxic effects induced a hearing loss of about 35-40 dB. Immunofluorescence and western blotting analyses demonstrated that styrene administration induced redox imbalance and activated inflammatory processes, targeting sensory hair cell and neural dysfunction by a cross-talk between oxidative and inflammatory mediators.

DISCUSSION

Major findings connect styrene ototoxicity to an interplay between redox imbalance and inflammation, leading to the intriguing assumption of a mixed sensory and neural styrene-induced ototoxicity. Thus, in a clinical perspective, data reported here have important implications for styrene risk assessment in humans.

A Cumulative Risk Perspective for Occupational Health and Safety (OHS) Professionals

Cumulative risk assessment (CRA) addresses the combined risk associated with chemical and non-chemical exposures. Although CRA approaches are utilized in environmental and ecological contexts, they are rarely applied in workplaces. In this perspectives article, we strive to raise awareness among occupational health and safety (OHS) professionals and foster the greater adoption of a CRA perspective in practice. Specifically, we provide an overview of CRA literature as well as preliminary guidance on when to consider a CRA approach in occupational settings and how to establish reasonable boundaries. Examples of possible workplace co-exposures and voluntary risk management actions are discussed. We also highlight important implications for workplace CRA research and practice. In particular, future needs include simple tools for identifying combinations of chemical and non-chemical exposures, uniform risk management guidelines, and risk communication materials. Further development of practical CRA methods and tools are essential to meet the needs of complex and changing work environments.

Cochlear dysfunction is associated with styrene exposure in humans

AIM

Occupational exposure to styrene has been shown to be associated with an increased probability of developing hearing loss. However, the sites of lesions in the auditory system in humans remain unknown. The aim of this study was to investigate the possible adverse effects of styrene exposure on the cochlea of human subjects.

DESIGN

The hearing function of 98 styrene-exposed male workers from the glass fibre-reinforced plastics industry (mean concentration of 55 mg/m3) was evaluated bilaterally using pure-tone audiometry (1000-16000 Hz), distortion product otoacoustic emissions (DPOAEs), and auditory brainstem response (ABR). The results were compared to a group of 111 male workers exposed to noise (above 85 dBA) and 70 male white-collar workers exposed to neither noise nor solvents. Age and noise exposure levels were accounted for as confounding variables in all statistical models.

RESULTS

Styrene exposure was significantly associated with poorer pure-tone thresholds (1-8 kHz), lower DPOAE amplitudes (5-6 kHz), and shorter wave V latencies in both ears compared to control-group subjects. Similar results were found among noise-exposed subjects. A further analysis with wave V latency showed that styrene-exposed subjects showed significantly shorter latencies than expected according to normative data. These results suggest that occupational exposure to styrene at moderate concentrations is associated with cochlear dysfunction, at least at high frequencies. DPOAEs may be considered a valuable diagnostic tool in hearing conservation programs in workers exposed to styrene.

Distortion product otoacoustic emission sensitivity to different solvents in a population of industrial painters

Objective: To evaluate the ototoxic effect of the exposure to different organic solvents and noise using distortion product otoacoustic emissions (DPOAEs).Design: The exposure to different solvents was evaluated by measuring, before and at the end of the work-shift, the urinary concentrations of solvent metabolites used as dose biomarkers. The urinary concentrations of DNA and RNA oxidation products were also measured as biomarkers of oxidative damage. The simultaneous exposure to noise was also evaluated. DPOAEs and pure tone audiometry (PTA) were used as outcome variables, and were correlated to the exposure variables using mixed effect linear regression models.Study sample: Seventeen industrial painters exposed to a solvent mixture in a naval industry. A sample size of 15 was estimated from previous studies as sufficient for discriminating small hearing level and DPOAE level differences (5 dB and 2 dB, respectively) at a 95% confidence level.Results: Statistically significant associations were found between the DPOAE level and the urinary dose biomarkers and the oxidative damage biomarkers. DPOAE level and the logarithm of the metabolite concentration showed a significant negative correlation.Conclusions: DPOAE are sensitive biomarkers of exposure to ototoxic substances and can be effectively used for the early detection of hearing dysfunction.

Personal noise exposure assessment of Kuwaiti printing industry workers

Recent technological improvements in the printing industry may have altered noise exposures in printeries. Of the eight printery noise assessments published since 2000, none assessed the exposure of workers using computer-to-plate machines and only two used personal noise dosimetry. This method measures noise levels as the worker moves about and permits examination of the impulsiveness of noise levels. In this study, 104 workers wore personal noise dosimeters for one full shift. Computer-to-plate operators experienced noise exposures of 75 dB(A) on average and were rarely exposed to noise levels greater than 85 dB(A). Noise exposure in excess of 85 dB(A) was still common among offset printer operators. In fact, all workers operating web-fed offset machines spent more than half the shift experiencing noise levels greater than 85 dB(A). We found that the 5-min rolling SD of noise levels accurately reflected the impulsivity observed in the noise level profile.

Effects of concomitant exposure to styrene and intense noise on rats' whole lung tissues. Biochemical and histopathological studies

Concurrent exposure to styrene (ST) and noise is common especially in industrial environments. The present study aims to determine the related oxidant-induced changes as the result of combined exposure to ST and noise. For this purpose, 24 male Wistar rats were used in four experimental groups (n = 6/groups): (1) control group, (2) the group exposed to an octave band of noise centered at 8 kHz (100 dB SPL) (6 h/day), (3) the group inhalationally exposed to ST (750 ppm) (6 h/day), (4) the group exposed to noise and ST simultaneously. The DNA damage was measured by assessing the concentration of 8-hydroxyl-2-deoxyguanosine (8-OHdG) using ELISA kit. Levels of lipid peroxidation (MDA), GSH and antioxidative activity of SOD and CAT were also determined in whole lung tissues. The results relatively indicated that sub-acute exposure to both noise and ST can lead to pathological damage in rat lung tissues. Furthermore, enhanced levels of 8-OHdG and MDA production were observed in lung tissues. In contrast, GSH, CAT and SOD were markedly reduced in co-exposed group. The results of the study verified additive interaction between noise and ST on accumulation of DNA oxidation products, progressive morphological damages as well as undermining the antioxidative defense system in the rat lung tissues.

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.

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.

Assessing ototoxicity due to chronic lead and cadmium intake with and without noise exposure in the mature mouse

Exposure to heavy metals may lead to hearing impairment. However, experimental studies have not explored this issue with and without noise exposure in mature animals with environmentally relevant doses. The aim of this study was to investigate ototoxicity produced by lead (Pb) and cadmium (Cd) and noise, singly and in combination, in the adult CBA/CaJ mouse. Metals were delivered via drinking water (0.03 mM, 1 mM, and 3 mM Pb; or 30, 100, and 300 μM Cd) for 12 weeks, resulting in environmentally- and occupationally relevant mean (± standard deviations) blood levels of Pb (2.89 ± 0.44, 38.5 ± 4.9, and 60.1 ± 6.6 μg/dl, respectively) and Cd (1.3 ± 0.23, 6.37 ± 0.87, 27.2 ± 4.1 μg/L, respectively). Metal treatment was also combined with a noise exposure consisting of a 105 dB broadband (2-20 kHz) stimulus for 2 hr or a sham exposure. Auditory performance was determined by comparing auditory brainstem responses (ABR) and distortion product otoacoustic emissions (DPOAE) at baseline and after 11 weeks of metal treatment. Metal-exposed animals did not develop significant auditory deficits and did not exhibit morphological damage to cochlear hair cells. In contrast, noise-exposed animals, including those exposed to combinations of metals and noise, demonstrated significant hair cell loss, reduced DPOAE amplitudes, and ABR threshold shifts of 42.2 ± 13 dB at 32 kHz (105 dB noise alone). No significant potentiation or synergistic effects were found in groups exposed to multiple agents. This study establishes a highly reproducible adult mouse model that may be used to evaluate a variety of environmental exposure mixtures.

Hearing loss, lead (Pb) exposure, and noise: a sound approach to ototoxicity exploration

To determine the state of the research on ototoxic properties of Pb, evaluate possible synergistic effects with concurrent noise exposure, and identify opportunities to improve future research, we performed a review of the peer-reviewed literature to identify studies examining auditory damage due to Pb over the past 50 years. Thirty-eight studies (14 animal and 24 human) were reviewed. Of these, 24 suggested potential ototoxicity due to Pb exposure, while 14 found no evidence of ototoxicity. More animal studies are needed, especially those investigating Pb exposure levels that are occupationally and environmentally relevant to humans. Further investigations into potential interactions of Pb in the auditory system with other hazards and compounds that elicit ototoxicity are also needed in animal models. To better assess the effects of Pb exposure on the human auditory system and the possibility of a synergism with noise, future epidemiological studies need to carefully consider and address four main areas of uncertainty: (1) hearing examination and quantification of hearing loss, (2) Pb exposure evaluation, (3) noise exposure evaluation, and (4) the personal characteristics of those exposed. Two potentially confounding factors, protective factors and mixtures of ototoxicants, also warrant further exploration.

Styrene enhances the noise induced oxidative stress in the cochlea and affects differently mechanosensory and supporting cells

Experimental and human investigations have raised the level of concern about the potential ototoxicity of organic solvents and their interaction with noise. The main objective of this study was to characterize the effects of the combined noise and styrene exposure on hearing focusing on the mechanism of damage on the sensorineural cells and supporting cells of the organ of Corti and neurons of the ganglion of Corti. The impact of single and combined exposures on hearing was evaluated by auditory functional testing and histological analyses of cochlear specimens. The mechanism of damage was studied by analyzing superoxide anion and lipid peroxidation expression and by computational analyses of immunofluorescence data to evaluate and compare the oxidative stress pattern in outer hair cells versus the supporting epithelial cells of the organ of Corti. The oxidative stress hypothesis was further analyzed by evaluating the protective effect of a Coenzyme Q₁₀ analogue, the water soluble Q_ter, molecule known to have protective antioxidant properties against noise induced hearing loss and by the analysis of the expression of the endogenous defense enzymes. This study provides evidence of a reciprocal noise-styrene synergism based on a redox imbalance mechanism affecting, although with a different intensity of damage, the outer hair cell (OHC) sensory epithelium. Moreover, these two damaging agents address preferentially different cochlear targets: noise mainly the sensory epithelium, styrene the supporting epithelial cells. Namely, the increase pattern of lipid peroxidation in the organ of Corti matched the cell damage distribution, involving predominantly OHC layer in noise exposed cochleae and both OHC and Deiters' cell layers in the styrene or combined exposed cochleae. The antioxidant treatment reduced the lipid peroxidation increase, potentiated the endogenous antioxidant defense system at OHC level in both exposures but it failed to ameliorate the oxidative imbalance and cell death of Deiters' cells in the styrene and combined exposures. Current antioxidant therapeutic approaches to preventing sensory loss focus on hair cells alone. It remains to be seen whether targeting supporting cells, in addition to hair cells, might be an effective approach to protecting exposed subjects.

Oxidative stress biomarkers and otoacoustic emissions in humans exposed to styrene and noise

OBJECTIVE

Evaluating the correlation between otoacoustic emission levels, styrene exposure, and oxidative stress biomarkers concentration in styrene-exposed subjects, to investigate the role of oxidative stress in outer hair cell damage.

DESIGN

Distortion product otoacoustic emissions were measured in the exposed workers and in a control group. Separation between the distortion and reflection otoacoustic components was performed by time-frequency-domain filtering. The urinary concentration of the DNA and RNA oxidation products, namely 8-oxo-7,8-dihydroguanine (oxoGua), 8-oxo-7,8-dihydro-2'-deoxyguanosine (oxodGuo), and 8-oxo-7,8-dihydroguanosine (oxoGuo), were evaluated.

STUDY SAMPLE

Nine subjects exposed to styrene in a fiberglass factory, eight control subjects. The two groups were statistically equivalent in mean age.

RESULTS

Statistically significant differences were found in the distortion component levels between the exposed and the control group. High levels of the oxidative damage biomarkers were found in the workers exposed to high levels of styrene. Significant negative correlation was found between the otoacoustic emission distortion component levels and the concentration of the oxoGuo biomarker.

CONCLUSIONS

Exposure-induced damage of the cochlear amplifier is shown in the mid-frequency range, confirming animal experiments, in which hair cells in the cochlear middle turn were damaged. Hearing damage is consistent with the outer hair cell apoptosis pathway associated with oxidative stress.

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.

Otoacoustic emission sensitivity to exposure to styrene and noise

The ototoxic effect of the exposure to styrene is evaluated, also in the presence of simultaneous exposure to noise, using otoacoustic emissions as biomarkers of mild cochlear damage. Transient-evoked and distortion product otoacoustic emissions were recorded and analyzed in a sample of workers (15 subjects) exposed to styrene and noise in a fiberglass manufacturing facility and in a control group of 13 non-exposed subjects. Individual exposure monitoring of the airborne styrene concentrations was performed, as well as biological monitoring, based on the urinary concentration of two styrene metabolites, the Mandelic and Phenylglyoxylic acids. Noise exposure was evaluated using wearable phonometers, and hearing loss with pure tone audiometry. Due to their different job tasks, one group of workers was exposed to high noise and low styrene levels, another group to higher styrene levels, close to the limit of 20 ppm, and to low noise levels. A significant negative correlation was found between the otoacoustic emission levels and the concentration of the styrene urinary metabolites. Otoacoustic emissions, and particularly distortion products, were able to discriminate the exposed workers from the controls, providing also a rough estimate of the slope of the dose-response relation between otoacoustic levels and styrene 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.

Combined effects of ototoxic solvents and noise on hearing in automobile plant workers in Iran

Exposure of workers to mixtures of organic solvents and to occupational noise is frequent in a number of industries. Recent studies suggest that exposure to both can cause a more severe hearing loss than exposure to noise alone. Our cross-sectional study included 411 workers of a large automobile plant divided in three groups. The first group included assembly workers exposed to noise alone; the second included workers in a new paint shop, who were exposed to a mixture of organic solvents at a permissible level; and the third group included paint shop workers exposed to both noise and higher than permissible levels of organic solvents in an old paint shop. These groups were compared in terms of low-frequency hearing loss (model 1; average hearing threshold >25 dB at 0.5 kHz, 1 kHz, and 2 kHz) and high-frequency hearing loss (model 2; average hearing threshold >25 dB at 3 kHz, 4 kHz, 6 kHz, and 8 kHz). High-frequency hearing loss was more common in workers exposed to a combination of noise and mixed organic solvents even at permissible levels than in workers exposed to noise alone even after correction for confounding variables. This study shows that combined exposure to mixed organic solvents and occupational noise can exacerbate hearing loss in workers. Therefore, an appropriate hearing protection programme is recommended, that would include short-interval audiometric examinations and efficient hearing protectors.

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.

Relation between outer hair cell loss and hearing loss in rats exposed to styrene

The relationship between outer hair cell (OHC) loss and cochlear sensitivity is still unclear, because in many animal models there exist surviving but dysfunctional OHCs and also injured/dead inner hair cells (IHC). Styrene is an ototoxic agent, which targets and destroys OHCs starting from the third row to the second and first rows depending on the exposure level. The remaining cells may be less affected. In this experiment, rats were exposed to styrene by gavage at different doses (200-800 mg/kg/day) for varying periods (5 days/week for 3-12 weeks). An interesting finding was that the cochlear sensitivity was not affected in a few rats with all OHCs in the third row being destroyed by styrene. A further loss of OHCs was usually accompanied with a linear input/output (I/O) function of cochlear compound action potentials (CAP), indicating the loss of cochlear amplification. However, normal CAP amplitudes at the highest stimulation level of 90 dB SPL were often observed when all OHCs were destroyed, indicating normal function of the remaining IHCs. The OHC-loss/hearing-loss relation appeared to be a sigmoid-type function. Initially, styrene-induced OHC losses (<33%) did not result in a significant threshold shift. Then CAP threshold shift increased dramatically with OHC loss from 33% to 66%. Then, CAP threshold changed less with OHC loss. The data suggest a tri-modal relationship between OHC loss and cochlear amplification. That is, under the condition that all surviving OHCs are ideally functioning, the cochlear amplifier is not affected until 33% of OHCs are absent, then the gain of the amplifier decreases proportionally with the OHC loss, and at last the amplifier may fail completely when more than 67% of OHCs are lost.

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.

Hearing levels of firefighters: risk of occupational noise-induced hearing loss assessed by cross-sectional and longitudinal data

OBJECTIVE

It is well known that firefighters face serious risk to their health and safety in the performance of their duties. To determine whether individuals who work as firefighters sustain occupational noise-induced hearing losses, we evaluated results of hearing tests completed by firefighters and compared them with age-matched, non-occupationally exposed groups of individuals.

DESIGN

The results of annual audiometric testing and a related questionnaire collected as part of a company-wide hearing conservation program were obtained from two large urban fire departments. The records from 12,609 tests conducted over an 11-year period were evaluated. Hearing levels obtained from the tests were compared with age-matched control populations from an American national standard, and a longitudinal study comparing the rate of hearing loss to that expected due to presbycusis alone was conducted.

RESULTS

Comparisons of the data to the national standard indicated that firefighters do not exhibit excessive loss of hearing compared with age-matched, non-occupationally exposed control subjects. The longitudinal study that examined the regression of firefighters' hearing with age compared with the expected presbycasic regression function indicated that the hearing of firefighters in the study declined over the 7-year period at a rate that was less than that expected due to age alone.

CONCLUSIONS

The results of this large-scale, cross-sectional, and longitudinal study indicate that firefighters are not at risk for occupational noise-induced hearing loss, even though they work nonstandard shifts and are occasionally exposed to high levels of noise. The results are consistent with the findings but not the conclusions of several other studies of firefighters' hearing.

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

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

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.