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ARTICLES Table of Contents   
Year : 2005  |  Volume : 7  |  Issue : 27  |  Page : 49-64
Combined effects of noise and styrene on hearing : Comparison between active and sedentary rats

1 Institut National de Recherche et de Sécurité‚ Vandoeuvre, France
2 Department of Medicine, Unit of Physiology University of Fribourg, Fribourg, Switzerland

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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.

Keywords: solvent, noise, synergism, ototoxicity, risk assessment.

How to cite this article:
Lataye R, Campo P, Loquet G, Morel G. Combined effects of noise and styrene on hearing : Comparison between active and sedentary rats. Noise Health 2005;7:49-64

How to cite this URL:
Lataye R, Campo P, Loquet G, Morel G. Combined effects of noise and styrene on hearing : Comparison between active and sedentary rats. Noise Health [serial online] 2005 [cited 2023 May 31];7:49-64. Available from: https://www.noiseandhealth.org/text.asp?2005/7/27/49/31633

  Introduction Top

Styrene is one of the industrial chemicals proven to impair auditory structures and functions in the rat (Morata and Campo, 2001). The high susceptibility of outer hair cells compared to inner hair cells can be considered as a characteristic of aromatic solvent-induced hearing losses and therefore of styrene-induced hearing losses (Loquet et al., 1999). Besides, it seems important to mention that styrene-induced hearing loss is species dependent. If the rat is sensitive to styrene and more generally to aromatic solvents, the guinea pigs or chinchillas are not (Campo et al., 1993; Fetcher., 1993; Davis et al., 2002). Noise is clearly the predominant occupational hazard to hearing, but complex exposures are not taken into consideration by the noise exposure standards (Directive 2003/10/EC). Literature on noise and on hearing conservation research lead us to realize that noise is often present in occupational settings, where also chemical exposure occurs (Morata et al., 2002). Moreover, there is growing evidence that a number of chemicals used in manufacturing may cause hearing loss, and exacerbate the effects of occupational noise (Sliwinska-Kowalska et al., 2001; 2003).

In its mission to identify and prevent work­-related disorders, it is not unusual that human risk assessment relies on toxicological endpoints obtained with animal models to establish acceptable levels for human exposure to chemical substances or noise. Two recent investigations studied the effects on hearing after exposure to noise and styrene in rats (Lataye et al., 2000; Makitie et al., 2003). However, these studies did not provide information on lower noises which would be valuable for risk assessors. In the experiment carried out by Lataye et al. (2000), the rats were exposed to an octave band noise centered at 8 kHz and emitted at 97 dB, whereas they were exposed to a 100­105 dB industrial noise in Makitie's study. As a result, both studies used a noise exposure with a Leq8h higher than 85 dB. In most European countries, the noise exposure limit for protecting people against work-related noise-induced hearing losses relies on a Leq8h value of 85 dB.

Consequently, these two investigations were not relevant according to risk assessors and decision makers in charge of the legislation of the damage-risk criterion to evaluate the hearing risks for workers exposed to noise. There was obviously no reason for them to propose a reduction of the threshold limit values (TLV) because of the results obtained with these experiments.

Chemicals to which workers are exposed are far too numerous to be studied individually. Consequently, we have narrowed our investigations to styrene as a representative aromatic solvent because of its wide use in many factories (Miller et al., 1994). Furthermore, workers are commonly exposed to styrene fumes in a working environment where noise pollution is also present (Morata et al., 2002; Morata and Campo, 2001). In France, the TLV authorized for work environments is 50 ppm averaged over an 8-h workday. When establishing damage-risk criteria for human exposures based upon data obtained with animal models, a safety factor of 100 and even more is commonly used (Slikker et al., 1996). The size of the safety factor is justified by interspecies variability, extrapolation from subacute to chronic exposures, and increased sensitivity of particular groups within the population such as the elderly or very young, which is called intraspecies variability. Based on this principle, we decided to use a value of 10 as safety factor. This is a lower value than that chosen by most investigators (Fechter et al., 2000, 2002). As a result, we used in this study the following concentrations 300 [30 X 10 (safety factor)], 400 ppm, 500 ppm and 600 ppm [60 X 10] of styrene. One concentration was therefore above the adjusted French TLV (60 ppm X 10), one was right at the adjusted TLV (50 ppm X 10) and two others were below the TLV (30 and 40 ppm X 10). During the combined exposure "noise and styrene", the rats were exposed to 400-ppm (40 ppm X 10) styrene. Theoretically, if we divide 400 ppm by a safety factor of 10, the concentration tested in this experiment would be lower than the recommended French TLV.

Although it is technically simpler to use animals at rest for carrying out dose-effect studies, such conditions underestimate the solvent uptake of subjects. Indeed, active and moving animals have a higher pulmonary ventilation and cardiac rates compared to sedentary animals (Bergert and Nestler, 1991) and are thereby closer to workers exposed in their workplace. Moreover, physical exercise has also been shown to affect the susceptibility to noise (Lindgren and Axelsson, 1988). For these reasons, in the present study rats were made to be active during the exposures. This was achieved by placing them inside a home-made running wheel within the inhalation chambers [Figure - 1]a.

Our concern for studying the interaction between noise and solvent in realistic conditions was justified by the fact that firstly, no guideline exists in Europe to prevent from the auditory effects of combined exposure to chemicals and noise, and secondly, the literature does not report the auditory effects of a noise exposure having a permissible Leq8h associated with a low dose of styrene. With the purpose of being closer to the acceptable industrial conditions, the present investigation was designed to study the auditory effects of a simultaneous exposure to 86.2-dB noise (an octave band noise centered at 8 kHz), chosen to cause hearing losses in the frequency range impaired by aromatic solvents (Lataye et al., 1999) for 6 hours (Leq8h=85dB, permissible value according to the European legislation, Directive 2003: L4238) and to 400 ppm [40ppm X 10 (safety factor)]. The 400-ppm exposure of styrene was determined from the dose-effect experiments with active and sedentary rats also reported in this paper. All the findings obtained with active animals were compared with those obtained with sedentary animals exposed to 500, 650, 850 and 1000 ppm in strictly identical conditions (Loquet et al., 1999).

  Materials and Methods Top


Male Long-Evans rats were purchased from Janvier Laboratories in France. A total of 84 rats was employed as subjects in this investigation, 60 for the dose-effect study and 24 for the combined exposure. The rats were 7 weeks old and their weight ranged between 180 and 200 g when they arrived in the animal facility. As soon as they arrived, the animals were located in animal facility setting with a light period from 19h00 to 7h00. Rats are active primarily during the night and we wanted to make them run from 9h00 to 15h00. It was therefore important to change their active period. The animals were housed in individual cages (350 X 180 X 184 mm ) with steam-cleaned pinewood bedding for 1 month before the start of the experiments. Food and tap water were available ad libitum except during the exposure period. The temperature in animal quarters was 22±1°C and the relative humidity ranged from 50 to 55 %. While conducting the research described in this article, the investigators adhered to the Guide for Care and Use of Laboratory Animals, as promulgated by the French Conseil d'Etat through the decret n°87-848 published in the French Journal Officiel on October, 20th, 1987 and the principles of the declaration of Helsinki.

  Hearing testing Top

Audiometric testing

Auditory function was tested by recording the auditory evoked potentials (AEP) from the inferior colliculus. To achieve this purpose, the rats were prepared under a deep anesthesia induced by the i.p. administration of a mixture of ketamine and xylazine at a dosage of 45 and 6 mg/kg respectively. Animals were then placed on a stereotaxic table while a tungsten electrode was implanted into the right inferior colliculus. A second electrode was implanted in the rostral cranium just above the dura mater to serve as the reference electrode. Both electrodes were then fastened to a transistor socket and fixed with dental cement to the skull. One month after surgery, audiometric testing was performed in a soundproof booth on awake rats placed in a restraining device. The generation and the signal treatment were performed with a Tucker-Davis Technologies apparatus. The acoustic stimuli (two cycles for the rise/fall ramp, four cycles for the plateau) were gated sinusoidal stimuli at 2, 3, 4, 5, 6, 8, 10, 12, 16, 20, 24 and 32 kHz, presented at a rate of 20/s, with an analysis window of 30 ms. The stimuli were transduced by a speaker (JBL, 2405) positioned 15 cm from the left pinna.

For each frequency, the acoustic calibration was carried out by measuring the sound pressure level emitted by the speaker, when it was driven by a continuous pure tone. The sound field was calibrated by positioning a microphone at a point normally occupied by the center of the animal's head. The targeted intensity is obtained by adjusting the level of the continuous pure tone. The electrical signal from the implanted electrode was amplified (X 2000) and filtered between 30 and 3000 Hz. Averaged auditory evoked potentials (AEP) were obtained from 260 presentations. An amplitude trough-to-peak (N1­P1) of 15 µV of the response was considered as the threshold value. For each animal, an audiogram was obtained prior to styrene exposure (T1), and four weeks post-exposure (T2). Permanent threshold shifts were defined as PTS = T2 - T1.

Surface preparation

After measuring the last AEPs, the rats were deeply anaesthetized. The temporal bones were removed and immediately immersed in 2.5% glutaraldehyde prepared with a 0.1 M cacodylate buffer. The round window membrane was perforated and the perilymphatic space was perfused with the fixative solution. Then cochleae were refrigerated overnight in glutaraldehyde. The following day, cochleae were postfixed with 1.5% OsO 4 in 0.1 M cacodylate buffer using a perilymphatic perfusion. The bony capsule was thinned using a dental drill equipped with a burr. The remaining bone and the lateral wall were then dissected away to expose the organ of Corti. The three turns of the organ of Corti were mounted in glycerin as a surface preparation for counting hair cells (cochleogram). The frequency-place map established by Muller (1991) was used to superimpose the frequency coordinates on the length coordinates of the organ of Corti. Cells were counted as present if either the stereocilia, the cuticular plate, or the cell nucleus could be visualized. A cochleogram showing the percentage of hair cells loss as a function of distance was plotted for each animal. The results were averaged across each group of animals for comparison between groups.

Styrene exposure

In a previous experiment (Loquet et al., 1999), 40 (5 groups X 8) sedentary rats were exposed to styrene. Each animal was housed in an individual cage within an inhalation chamber [Figure - 1]b designed to sustain dynamic and adjustable airflow (10-20 m 3 .h -1 ). The chambers were maintained at a negative pressure of no more than 3 mm H 2 O. Input air was filtered and conditioned to a temperature of 22-24°C and relative humidity of 50-55%. Styrene was vaporized by bubbling an additional airflow through a flask containing the test compound. The originality of the present experiment was to make 16 (4 groups X 4) rats work during the exposure period. To fulfill this condition, a home-made running wheel was manufactured and placed within the inhalation chambers [Figure - 1]a. As a result, the animals had to slowly run for 2 minutes every 3 minutes during the exposures in the dark from 9h00 to 15h00. The rats were exposed by inhalation 6h/d, 5d/w, 4w from 9h00 to 15h00, which can be considered as a subacute exposure. During the exposure period, the animals were in the dark. They were exposed to increasing concentrations of styrene ranging either from 500 to 1000 ppm (n 0 =n 500 =n 650 =n 850 =n 1000 =8) or from 300 to 600 ppm (n 0 =n 300 =n 400 =n 500 =n 600 =4) depending on the experiment. In the combined experiment for which the animals were exposed to noise (n noise =6) or to styrene (n styr =6) or to both noise and styrene (nn oise + sty r=6), the targeted concentration of styrene was 400 ppm. The control groups (n 0 =6) were active and always ventilated with fresh air.

Noise exposure

The rats were exposed to an octave band noise centered at 8 kHz inside the home-made running wheel within the inhalation chambers. The spectrum of the noise [Figure - 2] was intentionally chosen to cause hearing losses in the most sensitive frequency range and in the frequency range impaired by aromatic solvents. The rats were exposed to an octave band noise centered at 8 kHz emitted at 86.2 dB SPL. For the noise exposure (n noise =6) or for the combined exposure (n noise + styr =6) lasting 6h/d, 5d/w, 4w, the rats must slowly run for 2 minutes every 3 minutes.

Statistical analysis

The effects on hearing of the styrene concentrations or the noise were tested by running a 2-way ANOVA for which frequency was a within-subject and treatment a between­ subject factor. The interaction between noise and styrene was tested using a linear model. The fixed effects studied were the styrene, the noise or the interaction effects. The comparison between the results obtained with active and sedentary animals was tested using linear regressions.

  Results Top

Dose-effect study: electrophysiological data

[Figure - 3]a presents the PTS values obtained with active rats exposed to styrene concentrations ranging from 300 to 600 ppm. In the whole, the audiograms from all groups were parallel within the lower frequency range [2-6 kHz], but only the shifts (8 and 7 dB) at 2 and 3 kHz were significant at 600 ppm. As expected, the auditory threshold shifts increased significantly [F(4,263)=16.60, p<0.001] in the mid-frequency range [10-24 kHz] as a function of the styrene concentration. For instance, a 19-dB shift was obtained at 16-20 kHz with 600 ppm, 14-dB with 500 ppm, and only 5-dB at 20 kHz with 400 ppm. Although, the frequency effect was significant [F(11,263)=5.52, p<0.001], there was no significant interaction "frequency X treatment" .

[Figure - 3]b shows a significant increase of the auditory threshold shifts with sedentary rats exposed to styrene concentrations ranging from 500 to 1000 ppm [F styr (4,42)=48.58, p<0.001].

There were also a significant frequency effect [F freq (11,378)=15.31, p<0.001] and a significant interaction [F styrxfreq(44,378)=5.97, p<0.001].

Comparison between active and sedentary rats

Because the PTSs induced by 600ppm styrene were significant at 2 and 3 kHz and in the mid frequency range from 8 to 24 kHz, the PTS values obtained across all the tested frequencies were averaged in order to obtain a single representative value of the styrene-induced hearing loss (SIHL) for each concentration [300 - 600 ppm] and [500 - 1000 ppm] depending on the activity of the rats [Table - 1].

[Figure - 4] provides a theoretical method for establishing a simple relation between concentration and SIHL from sedentary subjects. A linear continuous model was run for elaborating a predicting model. The equation of the model which is a regression line was "dose = 25.18 x + 559.26".

The theoretical model could be considered as a predicting model for determining a styrene concentration required to cause the same amount of SIHL with sedentary and active rats. For example, [Table - 1] indicates that a 9.7-dB SIHL was obtained at 600 ppm with active rats. Applying this level of SIHL in [Figure - 4], a 9.7-dB SIHL gives a 802-ppm exposure with sedentary animals. As a result, a 202-ppm higher styrene exposure (802-600=202) was required for the sedentary rats to theoretically obtain the same amount of SIHL compared to the active rats.

Using this predicting model, the concentrations of styrene providing equivalent SIHLs for sedentary or active rats are reported in [Table - 2]. Based on this table, an average difference of 213 - ppm was found depending on animal activity.

Dose-effect study: histological data

[Figure - 5]a shows a cochleogram obtained from active rats exposed from 300 to 600 ppm. Each cochleogram (n=4/group) illustrates the hair cell losses along the organ of Corti following styrene exposures to 300, 400, 500 and 600 ppm. In each cochleogram, the most significant loss was located at the third row of outer hair cells (OHC3), the second row (OHC2) being less damaged than the third row, but more than the first row (OHC1).

[Figure - 5]b depicts the cochleogram obtained with sedentary animals exposed from 500 to 1000 ppm. The pattern of the losses is quite similar to that shown with active rats [Figure - 5]a: the losses obtained at OHC3 were superior to those obtained at OHC2, and the losses of OHC2 were superior to those obtained at OHC1. Regardless of the activity of the rats, the inner hair cells were well preserved. For both cochleogram, a significant outer hair cell loss around 4-5 kHz at the level of the second and first row is noticeable.

Comparison between active and sedentary rats

Both [Figure - 5] a and b show that the hair cell losses are spread out along the cochlea except at the basal part of the cochlea. This means that it was possible to establish a predicting model [Figure - 6]a,b,c which would indicate the styrene concentration required to obtain same amount of OHC loss with active and sedentary animals. This model could thus give a unique value representative of the averaged losses at the level of each row. The averaged values are reported in [Table - 3].

To make the procedure clear, one example is shown in [Figure - 6] for each row of outer hair cells. [Table - 3] indicates that 32% of OHC3 was lost at 400 ppm with active rats. As previously, the predicting model [Figure - 6]a : regression line having the equation y = 8.02 x + 459.34] indicates that a 716-ppm concentration of styrene would have been needed with sedentary rats to obtain the same amount of OHC loss. In the same manner, 15% of OHC2 at 500 ppm with active rats leads to 693 ppm with sedentary rats [Figure - 6]b : y=10.85 x + 530.37) and 10% of OHC1 at 600 ppm to 723 ppm [Figure - 6]c: y = 17.88 x + 544.35).

To have an idea of the OHC loss induced by the styrene exposures at the level of the organ of Corti, the values obtained with the predicting models were averaged across the rows to obtain only one unique value. These values were reported in [Table - 4], which allows the comparison between the OHC losses obtained at the three rows of OHCs in the sedentary and active rats. Based on this table, an average difference of 218 ppm was found depending on the activity of the animals.

Combined exposure to noise and styrene: electrophysiological data

All the animals was active during this experiment. [Figure - 7] shows the PTS obtained after a low-intensity noise (OBN: 8kHz at 86.2 dB SPL), after a low-dose of styrene (400 ppm) and after the combined exposure of noise and styrene for 6 h/d, 5 d/w, 4 w. While the treatment [F(3,216)=7.08, p<0.001] is significant, post hoc tests show that there is no difference between the "noise" and the "noise x styrene" groups. So, the low increase of PTS values in the high-frequency range for the "noise x styrene" group is not statistically significant at 95%.

Combined exposure to noise and styrene: histological data

The rats were exposed to an octave band noise centered at 8 kHz emitted at 86.2 dB SPL. The trauma induced by noise alone was too subtle to be quantified with a cochleogram. On the contrary, the combined exposure caused a clear increase in OHC loss, particularly at the level of OHC2,3 at the vicinity of 4 and 20 kHz as illustrated in [Figure - 8].

  Discussion Top

Both electrophysiological and histological results [Figure - 3],[Figure - 5] were consistent with previous reports, in which the auditory function of the rat was impaired by a styrene exposure (Loquet et al., 1999; Morata and Campo, 2001). One of the scopes of the present study was to evaluate the influence of the styrene concentration on the permanent auditory threshold shift (PTS) when the rats were active. No significant PTS was obtained in the sedentary rats exposed to 650-ppm styrene exposure [Figure - 3]b compared to controls, however 15­dB and 20-dB threshold shifts were measured at 16 kHz in the active animals [Figure - 3]a which were exposed to lower concentrations of styrene: 500 and 600 ppm. These results show that the ototoxic potency of the solvent exposure depends on the physical activity of the rats or, more generally, of the subjects. Such findings were expected since several authors have already shown a strong relationship between the uptake of a gaseous solvent via the lungs, and physical activity (Bergert and Nestler, 1991). In fact, both elevated pulmonary ventilation and cardiac output are parameters that condition the uptake rate in subjects (Astrand, 1975). These physiological parameters are difficult, or impossible, to control. Furthermore it is technically simpler to use sedentary rather than active animals [Figure - 1]a,b for carrying out dose-effect studies. However, based on the results in this study we strongly recommend the use of active animals in studies aimed for the derivation of acceptable human exposures. The goal in this study was to estimate the influence of the rate of uptake on the resulting internal concentration and therefore on the toxic effect of the substance. By using sedentary animals, we clearly underestimate the ototoxic effect of styrene. The error in the estimate of the ototoxic potency of the solvent is not negligible. By establishing a straight relationship between styrene-induced hearing loss and concentration of styrene [Table - 2], [Table - 4], a mean error of 213 ppm was calculated for the electrophysiological end­point. In our experimental context, the organ of Corti is considered as the critical target organ. Interestingly, the relationship between the cochlear data and the concentration of styrene also led to a mean error of 218 ppm, which is in the same magnitude to that obtained with the electrophysiological data. Thus, it is reasonable to claim that a styrene exposure by inhalation in inactive subjects can be decreased by approximately 200 ppm to estimate the ototoxic effects that would be obtained with an active population.

The most commonly used approach for risk assessment of ototoxicants, like styrene, is the introduction of a safety factor (Slikker et al., 1996; Fechter et al., 2000; 2002). This common approach proposes that the acceptable daily intake or exposure of contaminants is derived from the no observed adverse effect level in an animal divided by a factor 100 (Ecetoc, 1995). This calculated level is often used as an aid to guidelines establishment. The safety factor accounts for interspecies and intraspecies (ageing effect) differences in sensitivity or for extrapolation from subacute to chronic exposures (Barnes and Douson, 1988). In our calculations of the styrene exposure level used for the combined exposure experiment (400 ppm), we used a safety factor of 10 to compare the exposure of the rats to humans. The use of a safety factor of 10 cannot be considered exaggerated since risk assessors often use factors up to 100 (Ecetoc, 1995; Slikker et al., 1996).

An exposure defined by a concentration of 400 ppm [40 ppm X 10 (safety factor)] styrene corresponds to an exposure which is lower than the French recommended values for styrene exposures (50 ppm). Although no significant auditory threshold shift was shown in the elec­trophysiological data, an OHC loss of approximately 50% was measured at the third row of OHC [Figure 5a], verifying a toxic effect induced by this concentration. The absence of electrophysiological effect can be explained by the fact that at such a concentration, only the outer hair cells of the third row were affected and partially lost, and they do not play a crucial elec­trophysiological role (Loquet et al., 1999).

As far as the combined effects are concerned, the results and especially the histological data [Figure - 8] show a risk of potentiation of the styrene-induced hearing loss by the noise, even at a concentration of 400 ppm styrene. The potentiation between the solvent and the noise is mainly located around 4 kHz [Figure - 8]. At the 4 kHz frequency, the performances of the audiometric technique is not excellent (Lataye et al., 1999). Thus, the lack of electrophysiological effect despite the cochlear insults is not surprising. In fact, the outer hair cell losses were not sufficient to cause a significant increase of the PTS amplitudes as measured with our elec­trophysiological technique. Therefore, there is not a disagreement between the electrophysio­logical and histological data, our results illustrate mainly a difference of sensitivity of the techniques used in the present investigation.

How might such exposure conditions relate to human exposure and human health?

Earlier reports have indicated that occupational exposures to solvents are sufficiently high to be associated with hearing deficits (Morata et al., 1993, 1997, 2002; Morioka et al, 2000; Sliwinska-Kowalska et al., 2003, 2004), even though the exposure level necessary to cause hearing loss in animal studies is much higher. Until now, this difference in the lowest level necessary to cause an effect in humans and rats has not been explained satisfactorily. In the present study, the modification of the styrene effects on the auditory system by simultaneous exposure to noise and/or physical exercise was examined to determine if such concurrent exposures could explain this difference. Our results suggest that the auditory effects of solvents may have been observed at lower concentrations in humans because humans are generally exposed to solvents in combination with a multitude of other factors (several exposures, physical demands, etc) whereas animal experiments typically involve isolated solvent exposures.

Often, in its mission to identify and prevent work-related disorders, occupational health relies on findings obtained from toxicological studies using animals. But, extrapolation from an animal model to conclusions about human effects requires precautions. Even if the best model of a human is a human, the use of animal models as surrogates for humans to investigate toxic effects is sometimes unavoidable. Also the choice of the specific animal model is determinant. In the current study, the rat was chosen because of its solvent metabolism, which is not far from that of human beings (Ramsey and Andersen, 1984; Begninus et al., 1998; Pryor, 1991; Nakajima et al., 1993; 1994). It is essential to verify that the test species is a relevant model system for man particularly in terms of similar metabolic processing of the substance of interest. Guinea-pigs or chinchillas would not have been good animal models since their solvent metabolism differs from humans in several ways (Davis et al., 2002). Another reason for choosing the rat as a model is that most of the data concerning chemically-induced hearing loss derive from rats, which facilitates the comparison of the results with other studies. In addition, our exposure conditions were quite realistic compared with those reported in the literature, specially for the combined exposure for which they were compatible with the standard values. By the way, the authors would want to draw the attention of the readers on the fact that 400 ppm cannot be rigorously considered as a no observed adverse effect level but rather as a low effect level since the third row of OHC was largely injured. Consequently, 400 ppm cannot be considered as a demarcation between non hazardous and possibly hazardous exposures.

Due to the precautions taken in the present investigation, we think that the results gained from the active animals can reasonably be extrapolated to humans.

  Conclusions Top

When using animal data for risk assessment for humans, it is important to consider the activity of the animals during inhalation exposure. If this is not done the risk of solvent-induced hearing loss might be underestimated. A safe estimation of the ototoxic risks encountered by people in their working environment would be to subtract approximately 200 ppm from the concentration tested by investigators using sedentary rats. Another adjustment to take into consideration is that, given the risks encountered by subjects exposed to solvents and noise, it is reasonable to wonder whether the solvent and the noise TLV as well, are really pertinent in mixed occupational exposures.

  Recommendation Top

In the experimental conditions in this study, the noise exposure was harmless and the concentration of the solvent [40 ppm X 10] lower than the TLV recommended by the French legislation. Due to the findings obtained in this study, a safety effort should be made to decrease the styrene TLV rather than the Leq value. We firmly recommend that workers should not be exposed at or above 40 ppm of styrene for safety purposes. In France, the TLV of styrene authorized in working environments is 50 ppm averaged over an 8-h workday. In our opinion, a TLV below 30 ppm should ensure a higher level of protection for human health. As an example, TLV as low as 20 ppm has already be adopted by several European countries, such as Sweden, Finland, Germany and Austria or 25 ppm in Denmark and Norway.

  Acknowledgments Top

This study was supported by European grant QLK4-2000-00293 and by Institut National de Recherche et de Securite. The authors would like to thank Dr A.C. Johnson and Dr T. Morata for their precious comments on the manuscript and C. Barthelemy for his technical help[31].

  References Top

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31.Sliwinska-Kowalska M, Zamyslowska-Szmytke E, Szymczak W, Kotylo P, Fiszer M, Wesolowski W, Pawlaczyk-Luszczynska M, Bak M, Gajda-Szadkowska A. (2004). Effects of coexposure to noise and mixture of organic solvents on hearing in dockyard workers. JOEM. 46(1) , 30-8.  Back to cited text no. 31    

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DOI: 10.4103/1463-1741.31633

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  [Table - 1], [Table - 2], [Table - 3], [Table - 4]

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