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Effects of High Doses of Toluene on Color Vision
Neurotoxicology and Teratology, Vol. 21, No. 1, pp. 41–45, 1999 © 1999 Elsevier Science Inc. Printed in the USA. All rights reserved 0892-0362/99 $–see front matter
PII S0892-0362(98)00027-0
Effects of High Doses of Toluene on Color Vision AXEL MUTTRAY, VOLKMAR WOLTERS, DETLEV JUNG AND JOHANNES KONIETZKO Institute of Occupational, Social, and Environmental Medicine, University of Mainz, Germany IAS, Institute für Arbeits- und Socialhygiene, Stiftung, Karlsruhe, Germany Received 23 February 1996; Accepted 14 June 1998 MUTTRAY, A., V. WOLTERS, D. JUNG, AND J. KONIETZKO. Effects of high doses of toluene on color vision. NEUROTOXICOL TERATOL 21(1) 41–45, 1999.—High exposure to toluene may cause optic neuropathy and retinopathy, both associated with dyschromatopsia. Another solvent, ethanol, is known to induce acute blue–yellow dyschromatopsia. This study investigated the acute effects of high doses of toluene on color vision. Eight male printshop workers were examined before and after cleaning printing containers with pure toluene. After cleaning, concentrations of toluene in blood were between 3.61 and 7.37 mg/l. Color vision was tested with the Farnsworth panel D-15 test, the Lanthony desaturated panel D-15 test, and the Standard Pseudoisochromatic Plates part 2. For control of possible acute effects, eight workers of a metalworking factory without any neurotoxic exposure were tested according to the same procedure. Acute exposure to toluene did not cause impairment of color vision. However, statistical power is limited due to the small number of exposed subjects. Color vision of the printshop workers tested before cleaning was slightly impaired (statistically not significant) when compared with unexposed subjects. © 1999 Elsevier Science Inc. All rights reserved. Toluene
Printer
Occupational exposure
Color vision
THE neurotoxicity of toluene is well established, but there are only a few reports concerning adverse effects on the visual pathway (1,10,18,19,24,34,38,41). In a neurophysiological experiment small doses of toluene were shown to change the c-wave of the electroretinogram and the standing potential of the eye (38), indicating effects on the outer retinal layer. The c-wave originates in the pigment epithelium/receptor layer (13,17,39,45). The standing potential is mainly generated in the pigment epithelium (22,45). A different organic solvent, ethanol, also changes the oculogram and additionally affects the function of blue-sensitive cones, as observed in color vision tests (36,46). In a previous study we could not find adverse effects on color vision of a subacute exposure to toluene over 5 days (31). Here we studied the acute effects on color vision when rotogravure workers were highly exposed. Additionally, we looked for chronic effects comparing color vision ability before acute exposure with that of unexposed subjects.
METHOD
Design To study acute effects, color vision of eight workers of a rotogravure was tested before and after cleaning containers with pure toluene. For control, eight unexposed workers of a metal-working factory were examined by the same procedure. To investigate chronic effects, this control group was not suitable, as there were differences in age, alcohol consumption, and smoking habits. Therefore, control subjects were taken from our data bank containing healthy employed persons. Subjects Study of acute effects. Eight male workers had been working in rotogravure for 9.8 years (65.4). Toluene was the only solvent being used. The medical examination included occupational and past-medical history, physical examination, screen-
Requests for reprints should be addressed to Dr. Axel Muttray, Institute of Occupational, Social, and Environmental Medicine, University of Mainz, Obere Zahlbacher Str. 67, D-55131 Mainz, Germany. Tel: 161-31-17-32-33.
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42 ing for congenital dyschromatopsia with the Ishihara plates (16), testing of visual acuity (Rodenstock Sehtestgerät R7, Rodenstock, Ottobrunn-Riemerling, Germany), examination of the central visual field by an Amsler chart, blood picture, blood sugar, glycated haemoglobin (HbAlc), gammaglutamyl transpeptidase, and alanine aminotransferase. No subject had a disease or took drugs known to impair color vision. The eight control subjects were workers of a metalworking factory without any neurotoxic exposure and were tested concerning the same criteria. The average age of the exposed workers and of the controls was 39.1 years (69.4) and 36.6 years (610.2), respectively. All subjects gave informed consent. Study of chronic effects. To investigate chronic effects, healthy blue-collar workers were taken from our data bank for comparison. They had been tested according to the criteria mentioned above and were matched according to gender, age (65 years), alcohol consumption (610 g/day), and smoking habits (never, previously, currently). The average age of the exposed workers and of the controls was 39.1 years (69.4) and 39.9 years (69.9), respectively. The average daily consumption of alcohol was 12 g (617.8) and 11.1 g (615.7), respectively. Exposure The containers for the printing colors were cleaned by hand with pure toluene. The room was small and had poor ventilation. The door and the window were usually kept closed due to low winter temperatures. Exposure was determined by personal air sampling with adsorption on charcoal tubes, desorption with benzylic alcohol, and headspace gas chromatography. Venous blood samples were taken immediately before and after cleaning, and toluene was determined by gas chromatography. Ethanol in blood samples of exposed workers and of controls, taken before testing, was determined enzymatically with alcohol dehydrogenase (Sigma, Germany). None of the workers or of the controls drank alcohol before or during the experiment. Subjects were watched during the whole procedure. We had intended to perform the cleaning at the beginning of the shift when blood concentrations of toluene were low, but this was not possible in most cases due to the working process.
MUTTRAY ET AL. D-15 test (Luneau, Paris) (11), the Lanthony desaturated panel D-15 test (Luneau, Paris) (20), and the Standard Pseudoisochromatic Plates part 2 (SPP2-test) (14). The panel tests were viewed at a distance of 50 cm and the plates at a distance of 75 cm. There were no time limits for the panel tests. After having finished each panel test, the subjects were told to proof the arrangement of the caps. On each plate of the SPP2 test two figures are to be recognized. The subjects were also asked if they could better recognize the left or the right figure. Each plate was demonstrated for 5 s. Statistical Analyses Quantitative evaluation of the panel tests was done by calculating the color confusion indices (CCI) (6,7). As we did not use illuminant C as Bowman did (6,7), we had to calculate the color differences between the color caps. The tristimulus values of the Biolux lamp are Xn 5 95.7, Yn 5 100, and Zn 5 113.9 (information given by Osram). The color confusion index is 1.0, if the arrangement of the caps is correct. In contrast to the manual of the SPP2 test, the left symbol of plate 3 was not evaluated as this number is misread by almost all observers with normal color vision (15,33). We counted “possible” blue–yellow errors, also (25). A “possible” error means that the left symbol of plate 5, 6, 8, or 9 is recognized worse than the right one. There were only slight differences between the results of the single eyes and the means of both eyes. Therefore, only the means are given. Statistics were performed with an SAS package. RESULTS
Exposure The cleaning lasted between 28 and 41 min. The average toluene concentration in air measured by personal air sampling ranged between 1115 and 1358 mg/m3 (100 ppm is approximately 380 mg/m3). The concentrations in blood increased from 0.87 mg/l (60.6) to 4.92 mg/l (61.14). The values of the single subjects are given in Table 1. Blood alcohol levels of all subjects did not exceed 5 mg/dl (0.05º⁄ºº). None of the subjects had any prenarcotic symptoms (e.g., dizziness, headache). One of the examiners, however, left the cleaning room during two experiments due to a headache related to high concentrations of toluene.
Testing Color Vision Regarding acute exposure, the exposed subjects were familiar with the tests as they had participated in a previous study a few months before (31). The control persons from the metal-working factory performed the same tests 2–3 months before the experiments reported in this article, in order to get a comparable level of training. Color vision testing of exposed workers was performed 5–10 min before and after the cleaning with toluene under standard lighting conditions. Control subjects were tested twice at an interval of 40 min, in the meantime performing routine work. To investigate chronic effects, the first measurements of printshop workers, performed on a Monday prior to work shift some months previous, were used for comparison, as the control subjects from our data bank had performed the tests only once. All tests were performed according to the same procedure in a quiet room beneath the work place. We used a selfconstructed gray box with a daylight lamp (Osram L18W/72 Biolux, color temperature 6500 K, illumination 900 lux). The eyes were examined separately with the Farnsworth panel
Color Vision Acute exposure. After acute exposure to toluene no impairment was observed. Table 2 gives the results of the ex-
TABLE 1 BLOOD LEVELS OF TOLUENE Subject
1 2 3 4 5 6 7 8
Toluene (mg/l)*
Toluene (mg/l)†
1.19 1.55 1.77 0.37 0.21 0.22 0.68 0.95
4.72 7.37 5.63 4.70 4.54 4.33 3.61 4.45
* Before cleaning, † after cleaning.
TOLUENE EFFECTS ON COLOR VISION
43 TABLE 2
COLOR CONFUSION INDICES OF WORKERS ACUTELY EXPOSED TO TOLUENE AND OF CONTROL SUBJECTS Farnsworth Panel D-15 Test Exposed Subjects
1 2 3 4 5 6 7 8 Control Subjects
1 2 3 4 5 6 7 8
Lanthony Desaturated Panel D-15 Test
Before Exposure
After Exposure
Difference (a.-b.)
1.0 1.0 1.0 1.0 1.0 1.09 1.0 1.30
1.0 1.0 1.0 1.0 1.0 1.07 1.0 1.23
0 0 0 0 0 20.02 0 20.07
1st Test
2nd Test
Difference (a.-b.)
1.0 1.0 1.0 1.0 1.26 1.17 1.0 1.0
1.0 1.0 1.0 1.0 1.40 1.0 1.0 1.0
0 0 0 0 0.14 20.17 0 0
posed workers and of the control subjects on the Farnsworth panel D-15 test and on the Lanthony desaturated panel D-15 test. We calculated the differences of the CCIs before and after cleaning and the control situation, respectively. The medians of the differences were not statistically different (20.08 and 20.02, respectively, Wilcoxon–Mann–Whitney test, onesided tested). The results of the panel tests were confirmed by those of the SPP2 test. No subject made any real error. The median difference of the number of the possible blue–yellow errors after and before cleaning and the work of the controls, respectively, was 0. Chronic exposure. All eight control subjects performed well on the Farnsworth Panel D-15 test, but the color confusion indices of three printshop workers were slightly elevated (Table 3). Regarding the Lanthony desaturated panel D-15 test, which is more difficult, the performance of the exposed workers was worse (medians 1.18 and 1.05, respectively, p 5 0.06, Wilcoxon–Mann–Whitney test, one-sided tested), but the difference did not yield statistical significance. The individual results are given in Table 3. The errors made by the exposed workers were of blue–yellow type. All subjects recognized all figures on the SPP2 test correctly. The number of the possible blue–yellow errors was not elevated in the exposed group, when compared with the control group (medians 0.25 and 0.5, respectively). DISCUSSION
The question if an acute exposure to toluene can impair color vision is interesting from a practical as well as from a toxicological point of view concerning the common use of toluene and the high sensitivity of color vision to toxic effects of several solvents and mixtures (5,9,21,27–30,32,35,36,40,46). In this study acute exposure to toluene resulting in a mean concentration of 4.92 mg/l in blood was not associated with an impairment of color vision. However, a limitation of the study is due to the small number of exposed subjects that could be tested.
Before Exposure
After Exposure
1.46 1.21 1.0 1.05 1.0 1.65 1.37 1.53
Difference (a.-b.)
20.23 0.02 0 20.05 0 20.35 20.11 20.23
1.23 1.23 1.0 1.0 1.0 1.30 1.26 1.30
1st Test
2nd Test
1.0 1.0 1.09 1.12 1.85 1.43 1.0 1.06
Difference (a.-b.)
1.08 1.0 1.26 1.07 1.82 1.35 1.0 1.0
0.08 0 0.17 20.05 20.03 20.08 0 20.06
It was intended to perform the cleaning at the beginning of the shift, yielding minimal baseline concentrations of toluene in blood, but this was not possible. The highest concentration before cleaning was 1.77 mg/l. In a previous study (31) workers with toluene concentrations in this magnitude had no impairment of color vision. Therefore, we do not think that the initially elevated concentrations could have masked a possible impairment of color vision. Furthermore, the subjects with low concentrations before cleaning did not deteriorate and the increase of toluene concentration in blood was rather high, with a mean of 4.05 mg/l. Blue-sensitive cones generally are more susceptible to toxic substances than red- and green-sensitive cones (46). This also refers to the chronic effects of organic solvents (26,32) and the acute effects of ethanol (36,46). Therefore, the color vision tests selected had to be sensitive to blue–yellow dyschromatopsia. As the study was performed during the working process, tests could not be time consuming. The Lanthony desaturated panel D-15 test meets both criteria. Its sensitivity
TABLE 3 COLOR CONFUSION INDICES OF WORKERS EXPOSED TO TOLUENE (1ST EXAMINATION) AND OF CONTROL SUBJECTS TAKEN FROM DATABANK Farnsworth Panel D-15 Test
Lanthony Desaturated Panel D-15 Test
Exposed Subjects
Control Subjects
Exposed Subjects
Control Subjects
1.02 1.0 1.0 1.0 1.0 1.05 1.08 1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1.19 1.21 1.0 1.05 1.09 1.21 1.52 1.16
1.0 1.14 1.09 1.0 1.0 1.03 1.27 1.06
44
MUTTRAY ET AL.
has been demonstrated in epidemiological studies on effects of solvents (26,32). Although the exposed subjects were familiar with the tests and the Farnsworth panel D-15 test, which is much easier to perform than the Lanthony desaturated panel D-15 test, was administered first, the median color confusion index of the desaturated panel was slightly lower after exposure than before. This effect is probably due to training and was observed in previous studies (4,31). The comparison with the control subjects indicates that training has probably not masked a possible effect of toluene. Subjects exposed to 380 mg toluene/m3 had a slight impairment of color discrimination compared to controls without exposure (2), but the effect could not be reproduced (3). Furthermore, the significance of the test applied is not known. The concentrations of toluene in venous blood were not given. To make a comparison with our results possible, we estimated the probable concentrations of Baelum’s subjects using the results of kinetic studies conducted in exposure chambers with comparable exposure (8,23,37,42,43). We assume that the concentrations were about 1 mg/l blood. Considering our subjects having blood concentrations ranging between 3.61 and 7.37 mg/l, this concentration is relatively low. Printers with a subacute exposure and concentrations of toluene in blood above 1 mg/l did not have an impairment of color vision (31). In monkeys, even small doses of toluene cause alterations of the c-wave and the standing potential (38), which are due to changes of the potentials of the receptor–pigment epithelium complex (13,17,22,39,45). The retinal pigment epithelium has three major functions: vitamin A transport, daily phagocytosis of the upper tenth of the photoreceptor outer segment, and potassium buffering, in addition to transport and metabolism of different substances (45). Color vision reacts sensitively to changes of these parameters. Chronic changes of pigment epithelium’s function may cause effects other than dyschromatopsia; impairment of phagocytosis can result in pigmentation, impairment of vitamin A metabolism in nightblindness, and impaired transport in macular edema (45). To our knowledge, these symptoms have not yet been described in humans, neither in patients inhaling toluene nor in exposed workers. Patients suffering from chronic abuse of toluene and developing visual disturbances had optic neuropathies and
changes in the electroretinograms, which were different from those observed in the monkeys (38,41). The authors concluded that the chronic inhalation of toluene could impair any part of the visual pathway, including the distal parts of the retina and the pigment epithelium (41). Even if the electrophysiological findings are different, an acute as well as a chronic exposure to toluene can impair the outer retinal layer and easily could result in an imbalance of signals from the spectrally different cone mechanisms resulting in color vision disturbances. However, our investigation, which took place under a high occupational exposure to toluene, failed to demonstrate an acute effect on color vision. With regard to possible chronic effects of toluene, the exposed workers performed the Lanthony desaturated panel D-15 test slightly worse than the controls, indicating that there could be an adverse effect on color vision. Recently, an association between occupational exposure to toluene and color vision impairment has been reported (44). The study design does not permit to differentiate between subacute and chronic effects. Subjects were tested on Wednesday prior to their shift. Blood levels of toluene were below 0.095 mg/l, indicating that acute effects are improbable. Color vision of 20 workers chronically exposed to styrene was impaired before and after holidays (12), indicating a chronic, but not an acute, effect on blue–yellow vision. These and our results indicate that occupational exposure to solvents may be associated with chronic rather than acute impairment of color vision. However, oral ingestion of ethanol, reaching intoxication levels, caused acute blue–yellow dyschromatopsia (36,46), explained by an adverse effect on the blue-sensitive cones (46). Different reasons for the divergent results are possible. The exposure to toluene and to styrene might not have been sufficiently high to cause acute dyschromatopsia; those solvents only cause chronic impairment of color vision, and their toxicity may be different from that of ethanol. Further studies on possible acute and on the reversibility of chronic effects are necessary before definite conclusions can be drawn. ACKNOWLEDGEMENTS
The authors thank Prof. E. Zrenner, Tübingen, for valuable advice. Data of this paper are part of the thesis of V. Wolters (in preparation).
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PII S0892-0362(98)00027-0
Effects of High Doses of Toluene on Color Vision AXEL MUTTRAY, VOLKMAR WOLTERS, DETLEV JUNG AND JOHANNES KONIETZKO Institute of Occupational, Social, and Environmental Medicine, University of Mainz, Germany IAS, Institute für Arbeits- und Socialhygiene, Stiftung, Karlsruhe, Germany Received 23 February 1996; Accepted 14 June 1998 MUTTRAY, A., V. WOLTERS, D. JUNG, AND J. KONIETZKO. Effects of high doses of toluene on color vision. NEUROTOXICOL TERATOL 21(1) 41–45, 1999.—High exposure to toluene may cause optic neuropathy and retinopathy, both associated with dyschromatopsia. Another solvent, ethanol, is known to induce acute blue–yellow dyschromatopsia. This study investigated the acute effects of high doses of toluene on color vision. Eight male printshop workers were examined before and after cleaning printing containers with pure toluene. After cleaning, concentrations of toluene in blood were between 3.61 and 7.37 mg/l. Color vision was tested with the Farnsworth panel D-15 test, the Lanthony desaturated panel D-15 test, and the Standard Pseudoisochromatic Plates part 2. For control of possible acute effects, eight workers of a metalworking factory without any neurotoxic exposure were tested according to the same procedure. Acute exposure to toluene did not cause impairment of color vision. However, statistical power is limited due to the small number of exposed subjects. Color vision of the printshop workers tested before cleaning was slightly impaired (statistically not significant) when compared with unexposed subjects. © 1999 Elsevier Science Inc. All rights reserved. Toluene
Printer
Occupational exposure
Color vision
THE neurotoxicity of toluene is well established, but there are only a few reports concerning adverse effects on the visual pathway (1,10,18,19,24,34,38,41). In a neurophysiological experiment small doses of toluene were shown to change the c-wave of the electroretinogram and the standing potential of the eye (38), indicating effects on the outer retinal layer. The c-wave originates in the pigment epithelium/receptor layer (13,17,39,45). The standing potential is mainly generated in the pigment epithelium (22,45). A different organic solvent, ethanol, also changes the oculogram and additionally affects the function of blue-sensitive cones, as observed in color vision tests (36,46). In a previous study we could not find adverse effects on color vision of a subacute exposure to toluene over 5 days (31). Here we studied the acute effects on color vision when rotogravure workers were highly exposed. Additionally, we looked for chronic effects comparing color vision ability before acute exposure with that of unexposed subjects.
METHOD
Design To study acute effects, color vision of eight workers of a rotogravure was tested before and after cleaning containers with pure toluene. For control, eight unexposed workers of a metal-working factory were examined by the same procedure. To investigate chronic effects, this control group was not suitable, as there were differences in age, alcohol consumption, and smoking habits. Therefore, control subjects were taken from our data bank containing healthy employed persons. Subjects Study of acute effects. Eight male workers had been working in rotogravure for 9.8 years (65.4). Toluene was the only solvent being used. The medical examination included occupational and past-medical history, physical examination, screen-
Requests for reprints should be addressed to Dr. Axel Muttray, Institute of Occupational, Social, and Environmental Medicine, University of Mainz, Obere Zahlbacher Str. 67, D-55131 Mainz, Germany. Tel: 161-31-17-32-33.
41
42 ing for congenital dyschromatopsia with the Ishihara plates (16), testing of visual acuity (Rodenstock Sehtestgerät R7, Rodenstock, Ottobrunn-Riemerling, Germany), examination of the central visual field by an Amsler chart, blood picture, blood sugar, glycated haemoglobin (HbAlc), gammaglutamyl transpeptidase, and alanine aminotransferase. No subject had a disease or took drugs known to impair color vision. The eight control subjects were workers of a metalworking factory without any neurotoxic exposure and were tested concerning the same criteria. The average age of the exposed workers and of the controls was 39.1 years (69.4) and 36.6 years (610.2), respectively. All subjects gave informed consent. Study of chronic effects. To investigate chronic effects, healthy blue-collar workers were taken from our data bank for comparison. They had been tested according to the criteria mentioned above and were matched according to gender, age (65 years), alcohol consumption (610 g/day), and smoking habits (never, previously, currently). The average age of the exposed workers and of the controls was 39.1 years (69.4) and 39.9 years (69.9), respectively. The average daily consumption of alcohol was 12 g (617.8) and 11.1 g (615.7), respectively. Exposure The containers for the printing colors were cleaned by hand with pure toluene. The room was small and had poor ventilation. The door and the window were usually kept closed due to low winter temperatures. Exposure was determined by personal air sampling with adsorption on charcoal tubes, desorption with benzylic alcohol, and headspace gas chromatography. Venous blood samples were taken immediately before and after cleaning, and toluene was determined by gas chromatography. Ethanol in blood samples of exposed workers and of controls, taken before testing, was determined enzymatically with alcohol dehydrogenase (Sigma, Germany). None of the workers or of the controls drank alcohol before or during the experiment. Subjects were watched during the whole procedure. We had intended to perform the cleaning at the beginning of the shift when blood concentrations of toluene were low, but this was not possible in most cases due to the working process.
MUTTRAY ET AL. D-15 test (Luneau, Paris) (11), the Lanthony desaturated panel D-15 test (Luneau, Paris) (20), and the Standard Pseudoisochromatic Plates part 2 (SPP2-test) (14). The panel tests were viewed at a distance of 50 cm and the plates at a distance of 75 cm. There were no time limits for the panel tests. After having finished each panel test, the subjects were told to proof the arrangement of the caps. On each plate of the SPP2 test two figures are to be recognized. The subjects were also asked if they could better recognize the left or the right figure. Each plate was demonstrated for 5 s. Statistical Analyses Quantitative evaluation of the panel tests was done by calculating the color confusion indices (CCI) (6,7). As we did not use illuminant C as Bowman did (6,7), we had to calculate the color differences between the color caps. The tristimulus values of the Biolux lamp are Xn 5 95.7, Yn 5 100, and Zn 5 113.9 (information given by Osram). The color confusion index is 1.0, if the arrangement of the caps is correct. In contrast to the manual of the SPP2 test, the left symbol of plate 3 was not evaluated as this number is misread by almost all observers with normal color vision (15,33). We counted “possible” blue–yellow errors, also (25). A “possible” error means that the left symbol of plate 5, 6, 8, or 9 is recognized worse than the right one. There were only slight differences between the results of the single eyes and the means of both eyes. Therefore, only the means are given. Statistics were performed with an SAS package. RESULTS
Exposure The cleaning lasted between 28 and 41 min. The average toluene concentration in air measured by personal air sampling ranged between 1115 and 1358 mg/m3 (100 ppm is approximately 380 mg/m3). The concentrations in blood increased from 0.87 mg/l (60.6) to 4.92 mg/l (61.14). The values of the single subjects are given in Table 1. Blood alcohol levels of all subjects did not exceed 5 mg/dl (0.05º⁄ºº). None of the subjects had any prenarcotic symptoms (e.g., dizziness, headache). One of the examiners, however, left the cleaning room during two experiments due to a headache related to high concentrations of toluene.
Testing Color Vision Regarding acute exposure, the exposed subjects were familiar with the tests as they had participated in a previous study a few months before (31). The control persons from the metal-working factory performed the same tests 2–3 months before the experiments reported in this article, in order to get a comparable level of training. Color vision testing of exposed workers was performed 5–10 min before and after the cleaning with toluene under standard lighting conditions. Control subjects were tested twice at an interval of 40 min, in the meantime performing routine work. To investigate chronic effects, the first measurements of printshop workers, performed on a Monday prior to work shift some months previous, were used for comparison, as the control subjects from our data bank had performed the tests only once. All tests were performed according to the same procedure in a quiet room beneath the work place. We used a selfconstructed gray box with a daylight lamp (Osram L18W/72 Biolux, color temperature 6500 K, illumination 900 lux). The eyes were examined separately with the Farnsworth panel
Color Vision Acute exposure. After acute exposure to toluene no impairment was observed. Table 2 gives the results of the ex-
TABLE 1 BLOOD LEVELS OF TOLUENE Subject
1 2 3 4 5 6 7 8
Toluene (mg/l)*
Toluene (mg/l)†
1.19 1.55 1.77 0.37 0.21 0.22 0.68 0.95
4.72 7.37 5.63 4.70 4.54 4.33 3.61 4.45
* Before cleaning, † after cleaning.
TOLUENE EFFECTS ON COLOR VISION
43 TABLE 2
COLOR CONFUSION INDICES OF WORKERS ACUTELY EXPOSED TO TOLUENE AND OF CONTROL SUBJECTS Farnsworth Panel D-15 Test Exposed Subjects
1 2 3 4 5 6 7 8 Control Subjects
1 2 3 4 5 6 7 8
Lanthony Desaturated Panel D-15 Test
Before Exposure
After Exposure
Difference (a.-b.)
1.0 1.0 1.0 1.0 1.0 1.09 1.0 1.30
1.0 1.0 1.0 1.0 1.0 1.07 1.0 1.23
0 0 0 0 0 20.02 0 20.07
1st Test
2nd Test
Difference (a.-b.)
1.0 1.0 1.0 1.0 1.26 1.17 1.0 1.0
1.0 1.0 1.0 1.0 1.40 1.0 1.0 1.0
0 0 0 0 0.14 20.17 0 0
posed workers and of the control subjects on the Farnsworth panel D-15 test and on the Lanthony desaturated panel D-15 test. We calculated the differences of the CCIs before and after cleaning and the control situation, respectively. The medians of the differences were not statistically different (20.08 and 20.02, respectively, Wilcoxon–Mann–Whitney test, onesided tested). The results of the panel tests were confirmed by those of the SPP2 test. No subject made any real error. The median difference of the number of the possible blue–yellow errors after and before cleaning and the work of the controls, respectively, was 0. Chronic exposure. All eight control subjects performed well on the Farnsworth Panel D-15 test, but the color confusion indices of three printshop workers were slightly elevated (Table 3). Regarding the Lanthony desaturated panel D-15 test, which is more difficult, the performance of the exposed workers was worse (medians 1.18 and 1.05, respectively, p 5 0.06, Wilcoxon–Mann–Whitney test, one-sided tested), but the difference did not yield statistical significance. The individual results are given in Table 3. The errors made by the exposed workers were of blue–yellow type. All subjects recognized all figures on the SPP2 test correctly. The number of the possible blue–yellow errors was not elevated in the exposed group, when compared with the control group (medians 0.25 and 0.5, respectively). DISCUSSION
The question if an acute exposure to toluene can impair color vision is interesting from a practical as well as from a toxicological point of view concerning the common use of toluene and the high sensitivity of color vision to toxic effects of several solvents and mixtures (5,9,21,27–30,32,35,36,40,46). In this study acute exposure to toluene resulting in a mean concentration of 4.92 mg/l in blood was not associated with an impairment of color vision. However, a limitation of the study is due to the small number of exposed subjects that could be tested.
Before Exposure
After Exposure
1.46 1.21 1.0 1.05 1.0 1.65 1.37 1.53
Difference (a.-b.)
20.23 0.02 0 20.05 0 20.35 20.11 20.23
1.23 1.23 1.0 1.0 1.0 1.30 1.26 1.30
1st Test
2nd Test
1.0 1.0 1.09 1.12 1.85 1.43 1.0 1.06
Difference (a.-b.)
1.08 1.0 1.26 1.07 1.82 1.35 1.0 1.0
0.08 0 0.17 20.05 20.03 20.08 0 20.06
It was intended to perform the cleaning at the beginning of the shift, yielding minimal baseline concentrations of toluene in blood, but this was not possible. The highest concentration before cleaning was 1.77 mg/l. In a previous study (31) workers with toluene concentrations in this magnitude had no impairment of color vision. Therefore, we do not think that the initially elevated concentrations could have masked a possible impairment of color vision. Furthermore, the subjects with low concentrations before cleaning did not deteriorate and the increase of toluene concentration in blood was rather high, with a mean of 4.05 mg/l. Blue-sensitive cones generally are more susceptible to toxic substances than red- and green-sensitive cones (46). This also refers to the chronic effects of organic solvents (26,32) and the acute effects of ethanol (36,46). Therefore, the color vision tests selected had to be sensitive to blue–yellow dyschromatopsia. As the study was performed during the working process, tests could not be time consuming. The Lanthony desaturated panel D-15 test meets both criteria. Its sensitivity
TABLE 3 COLOR CONFUSION INDICES OF WORKERS EXPOSED TO TOLUENE (1ST EXAMINATION) AND OF CONTROL SUBJECTS TAKEN FROM DATABANK Farnsworth Panel D-15 Test
Lanthony Desaturated Panel D-15 Test
Exposed Subjects
Control Subjects
Exposed Subjects
Control Subjects
1.02 1.0 1.0 1.0 1.0 1.05 1.08 1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1.19 1.21 1.0 1.05 1.09 1.21 1.52 1.16
1.0 1.14 1.09 1.0 1.0 1.03 1.27 1.06
44
MUTTRAY ET AL.
has been demonstrated in epidemiological studies on effects of solvents (26,32). Although the exposed subjects were familiar with the tests and the Farnsworth panel D-15 test, which is much easier to perform than the Lanthony desaturated panel D-15 test, was administered first, the median color confusion index of the desaturated panel was slightly lower after exposure than before. This effect is probably due to training and was observed in previous studies (4,31). The comparison with the control subjects indicates that training has probably not masked a possible effect of toluene. Subjects exposed to 380 mg toluene/m3 had a slight impairment of color discrimination compared to controls without exposure (2), but the effect could not be reproduced (3). Furthermore, the significance of the test applied is not known. The concentrations of toluene in venous blood were not given. To make a comparison with our results possible, we estimated the probable concentrations of Baelum’s subjects using the results of kinetic studies conducted in exposure chambers with comparable exposure (8,23,37,42,43). We assume that the concentrations were about 1 mg/l blood. Considering our subjects having blood concentrations ranging between 3.61 and 7.37 mg/l, this concentration is relatively low. Printers with a subacute exposure and concentrations of toluene in blood above 1 mg/l did not have an impairment of color vision (31). In monkeys, even small doses of toluene cause alterations of the c-wave and the standing potential (38), which are due to changes of the potentials of the receptor–pigment epithelium complex (13,17,22,39,45). The retinal pigment epithelium has three major functions: vitamin A transport, daily phagocytosis of the upper tenth of the photoreceptor outer segment, and potassium buffering, in addition to transport and metabolism of different substances (45). Color vision reacts sensitively to changes of these parameters. Chronic changes of pigment epithelium’s function may cause effects other than dyschromatopsia; impairment of phagocytosis can result in pigmentation, impairment of vitamin A metabolism in nightblindness, and impaired transport in macular edema (45). To our knowledge, these symptoms have not yet been described in humans, neither in patients inhaling toluene nor in exposed workers. Patients suffering from chronic abuse of toluene and developing visual disturbances had optic neuropathies and
changes in the electroretinograms, which were different from those observed in the monkeys (38,41). The authors concluded that the chronic inhalation of toluene could impair any part of the visual pathway, including the distal parts of the retina and the pigment epithelium (41). Even if the electrophysiological findings are different, an acute as well as a chronic exposure to toluene can impair the outer retinal layer and easily could result in an imbalance of signals from the spectrally different cone mechanisms resulting in color vision disturbances. However, our investigation, which took place under a high occupational exposure to toluene, failed to demonstrate an acute effect on color vision. With regard to possible chronic effects of toluene, the exposed workers performed the Lanthony desaturated panel D-15 test slightly worse than the controls, indicating that there could be an adverse effect on color vision. Recently, an association between occupational exposure to toluene and color vision impairment has been reported (44). The study design does not permit to differentiate between subacute and chronic effects. Subjects were tested on Wednesday prior to their shift. Blood levels of toluene were below 0.095 mg/l, indicating that acute effects are improbable. Color vision of 20 workers chronically exposed to styrene was impaired before and after holidays (12), indicating a chronic, but not an acute, effect on blue–yellow vision. These and our results indicate that occupational exposure to solvents may be associated with chronic rather than acute impairment of color vision. However, oral ingestion of ethanol, reaching intoxication levels, caused acute blue–yellow dyschromatopsia (36,46), explained by an adverse effect on the blue-sensitive cones (46). Different reasons for the divergent results are possible. The exposure to toluene and to styrene might not have been sufficiently high to cause acute dyschromatopsia; those solvents only cause chronic impairment of color vision, and their toxicity may be different from that of ethanol. Further studies on possible acute and on the reversibility of chronic effects are necessary before definite conclusions can be drawn. ACKNOWLEDGEMENTS
The authors thank Prof. E. Zrenner, Tübingen, for valuable advice. Data of this paper are part of the thesis of V. Wolters (in preparation).
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