Polyneuropathy induced by carbon disulphide in viscose rayon workers

OBJECTIVES

To understand the prevalence of polyneuropathy and correlations among the clinical manifestations, electrophysiological findings, and degree of exposure to carbon disulphide (CS2) in workers who were exposed to variable concentrations of CS2 in a viscose rayon factory.

METHODS

All the 163 workers received a detailed physical and neurological evaluation. Fixed point air samples were analysed for CS2. Nerve conduction velocity was studied in 26 workers with symptoms similar to neuropathy.

RESULTS

Nine workers (53%) with overt polyneuropathy from the fibre cutting department and 19 workers (13%) with oligosymptoms similar to polyneuropathy from various jobs were noted. The fixed point air concentrations of CS2 were 150-300 ppm in the cutting areas and 15 to 100 ppm in the spinning areas. The estimated eight hour time weighted averages in the fibre cutting areas were 40-67 ppm. The occurrence of polyneuropathy was generally correlated with the degree of exposure to CS2. Nerve conduction velocities (NCVs) were significantly different in the overt polyneuropathy and subclinical polyneuropathy groups from the normal controls. The sensitive indicators for CS2 polyneuropathy were distal latency, motor NCV, and amplitude of sensory nerve action potentials in sensory NCVs.

CONCLUSION

The outbreak of polyneuropathy was attributed to higher concentrations of CS2 in fibre cutting areas. Even in other jobs with relatively lower concentrations of CS2, the hazard of subclinical polyneuropathy cannot be overlooked.

Epidemiological study of eye irritation by hydrogen sulphide and/or carbon disulphide exposure in viscose rayon workers

In the framework of an extensive health survey 123 male viscose rayon workers exposed to hydrogen sulphide (H2S) and/or carbon disulphide (CS2) and 67 referents not exposed to either of these chemicals answered questions on eye irritation complaints in a self-administered questionnaire. Personal exposure varied from 4 to 112 mg m-3 for CS2 and from 0.2 to 8.9 mg m-3 for H2S. A combined exposure measure was calculated using principal component analysis. The exposed subjects had a significantly higher prevalence of eye pain, burning and photophobia. Adjustment for age and smoking in logistic regression revealed significantly more eye complaints for all workers that were highly exposed. In a postal survey one-third of the ex-workers of the same viscose rayon factory reported having left the factory because of eye complaints, indicating that their occurrence may be underestimated in the cross-sectional study.

Carbon disulphide. II. Investigations on the uptake of CS2 and the excretion of its metabolite 2-thiothiazolidine-4-carboxylic acid after occupational exposure

The reported investigations on the uptake of carbon disulphide (CS2) and the excretion of its metabolite 2-thiothiazolidine-4-carboxylic acid (TTCA) were based on results from 403 personal air samples (352 passive and 51 active samples) and 362 TTCA determinations in biological material measured during a field study on the adverse effects due to CS2 exposure. The external exposure ranged from below the detection limit (0.2 ppm) to 66 ppm and the urinary TTCA excretion from below the detection limit (0.16 mg./l) to 33.4 mg/l. The excretion of TTCA in postshift urine related to creatinine and volume showed a linear correlation to the CS2 air concentration. On the basis of these results the influence on the internal exposure of physical work load, dermal exposure and individual parameters (age, Brocaindex, disturbed skin barrier) was evaluated. Correlations between the TTCA values in the postshift urine and the individually measured CS2 concentrations were carried out separately for individual departments and persons with and without indications of a disturbed skin barrier. In order to be able to judge the individual internal exposure related to external exposure, a personal quotient was formed from the TTCA level in the urine and the CS2 air concentration measured on the same day (relative internal exposure RIE index = TTCA mg/g creatinine/CS2 in ppm). On investigating interindividual differences, higher relative internal exposures were found in persons with a heavy physical work load and more intensive skin contact. It could be shown for a large group of persons exposed to CS2 that a pathological skin condition leads to an increase in the dermal penetration rate of hazardous substances.(ABSTRACT TRUNCATED AT 250 WORDS)

[Meta-analysis in neurobehavioral toxicological studies]

Meta-analysis was used to deal with the data collected in neurobehavioral toxicological studies to synthesize their findings quantitatively. Results revealed neurotoxic chemicals could cause changes in cognitive abilities, psychomotor function and emotion of persons exposed. However, each toxicant had its distinct effects on neurobehavior, for example, lead mainly impairs touch sense, memory, emotion and cognitive abilities, mercury does intelligence, concentration and motor stability, and carbon disulfide does eye-hand coordination. It suggested meta-analysis could be directive to selecting the best combination of neurobehavioral tests.

Mortality of Dutch workers exposed to carbon disulfide

Epidemiological studies carried out in Great Britain, Scandinavia and the United States indicate that workers exposed to carbon disulfide are at an increased risk for cardiovascular disease (CVD) mortality, and in particular for ischemic heart disease (IHD) mortality. In the epidemiological study reported here a retrospective cohort design was used. The total study population consisted of 3322 workers from a Dutch viscose textile plant who had all been employed for at least half a year between 1 January 1947 and 1 January 1980. Only production and maintenance workers were selected for the study. A group of 1434 workers who had been exposed to carbon disulfide was identified from the files available at the plant. The remaining 1888 workers who had not been exposed to carbon disulfide were used as a reference group. The total study population was followed for mortality until 1 January 1988. For the 762 workers who had died before that date, the causes of death were checked at the Central Bureau of Statistics. The results show a slightly but significantly increased risk for CVD mortality (CVD-specific SMR of 115), despite the observation of the so-called healthy worker effect and the statistical uncertainties inherent to this type of study. The results are in accordance with those reported by other investigators. The results indicate that exposure to relatively low levels of carbon disulfide increases the risk of CVD mortality.

Epidemiological study of the effects of carbon disulfide on male sexuality and reproduction

One hundred sixteen male viscose rayon workers exposed to carbon disulfide and 79 other workers, who were not exposed to any toxic agent in the working environment, were asked about their sexual behavior and reproductive record. Forty-three exposed and 35 nonexposed men provided a semen sample. The relationship with occupational exposure was analyzed with univariate and multivariate methods. The results indicate a significant effect of carbon disulfide exposure on libido and potency, but no effects were noted on fertility nor on semen quality.

Carbon disulphide. I. External and internal exposure to carbon disulphide of workers in the viscose industry

Carbon disulphide (CS2), a reagent of rayon production, is known for its high toxic potential and has therefore been the subject of many clinical studies. The aims of the study presented here were to determine the effective exposure to CS2 and to compare the validity of the different exposure indicators. Internal and external exposure to CS2 was investigated in 362 male workers of a viscose factory. The CS2 air concentrations measured through stationary air monitoring in five different work areas (n = 39) were compared with the values of active personal air monitoring (n = 51) and passive personal air monitoring (n = 352). In addition, in all investigations biological monitoring was carried out by measuring the 2-thio-1,3- thiazolidine-carboxylic acid (TTCA) excretion in urine. A significant correlation was found between the data for active and passive personal air sampling (y = 0.99, x + 0.31, r = 0.968). The results of the stationary measurements led, however, to values which were systematically too low. The validity of TTCA excretion in urine as a parameter of biological monitoring could for the first time be verified in a large sample of subjects. There was a linear correlation between TTCA values in the urine related to both volume and creatinine [y (TTCAmg/g creatinine) = 0.315 x (ml CS2/m3 air) + 0.59] and the CS2 values in the air. Nevertheless, TTCA excretion proved to be dependent upon creatinine so that the excreted amounts related to creatinine described the individual exposure more exactly.

Carbon disulfide and the central nervous system: a 15-year neurobehavioral surveillance of an exposed population

Carbon disulfide-induced neurobehavioral effects are well known and do not need further evidence. Carbon disulfide vasculopathy and the syndromic complex resulting in depression, loss of memory and concentration, and behavior disturbances have been widely demonstrated. Less known is the evolution of the symptomatology when the environmental conditions are consistently improved, that is, the reversibility or the progression of the dysfunctions observed. This paper reports on a neurobehavioral follow-up in a viscose rayon factory carried out, in intervals, from 1974 to 1990. Several successive improvements were implemented in the plant through the years, until finally, the most radical changes were made at the end of the Seventies and these resulted in exposure levels far below the current Threshold Limit Values. A total of 493 subjects were examined and some of them were reexamined up to six times. The last examination was completed in September, 1990. In this paper, studies by our group over the 15 years of monitoring are discussed. The results show that the general mental state, as measured by neurobehavioral methods, reflects past and current exposure. This point was explored by dividing the subjects into six groups on the basis of their length of exposure and year of examination and by comparing their performances. The results show that even exposure to levels of carbon disulfide not exceeding 8 mg/m3 may induce absentmindedness and difficulties in perceptive abilities.

Epidemiological study of endocrinological effects of carbon disulfide

One hundred seventeen viscose rayon workers exposed to carbon disulfide, and 66 workers who were not exposed to any toxic agent in the working environment, underwent an endocrinological examination that included determination of thyroxine, luteinizing hormone, follicle-stimulating hormone, prolactin, and testosterone in serum by radioimmunoassay methods. In the viscose rayon factory, the working conditions had not changed subsequent to 1932. Personal monitoring performed in 17 jobs showed that carbon disulfide exposures varied from 4 to 112 mg/m3. A carbon disulfide cumulative exposure index was calculated for each individual. Univariate analysis of the hormone levels versus exposure showed significantly lower levels of prolactin in the exposed versus nonexposed subjects (p = .01). After adjustment for age, alcohol, smoking, body mass index, and stress level, the carbon disulfide cumulative exposure index was not associated with any of the hormones. This discrepancy from previous studies may result from differences in exposure, in selection of the population, or in confounding biases.

Verification of effects on the nervous system of low level occupational exposure to CS2

Neurotoxic effects associated with long term low level occupational exposure to CS2 were reinvestigated four years after the initial study in the same group of workers. The second study concerned 44 exposed and 31 matched control workers. For both studies a personal cumulative exposure Ec was calculated based on function specific exposure levels and the occupational histories, which were carefully re-established. The exposed workers' average Ec was 192 and 213 ppm-years (first and second study respectively). Where possible the values of both data sets were used in a final combined analysis. Effects were found on the motor nerve conduction velocity of the fast (-0.9 m/s) and slow (-1.0 m/s) fibres of the peroneal nerve, the sensory nerve conduction velocity in the hand and arm segment of the median (-2.1 m/s) and ulnar (-1.3 m/s) nerves, and in the sural nerve (-1.3 m/s). An increased refractory period was found in the sural nerve (+ 0.2 ms, + 11%), but not in the peroneal nerve. For the autonomic nervous system an effect was found on the heart frequency response to isometric muscle contraction (-4.7 beats/min, -26%) and maximal forced respiration (-3.2 beats/min, -16%). This study shows the importance of a detailed evaluation of past exposure data. The reinvestigation enabled a more precise estimation of the effects of CS2, which is particularly desirable at around threshold exposure.

A cross-sectional study on the brainstem auditory evoked potential among workers exposed to carbon disulfide

In order to clarify the chronic effect of carbon disulfide (CS2) on the central nervous system (CNS), the brainstem auditory evoked potential (BAEP) was measured in spinning workers exposed to CS2 in a viscose rayon manufacturing factory and unexposed workers in Japan. Workers exposed to CS2 were divided into three groups according to length of exposure: current spinning workers with an exposure duration of more than 240 months (L group, n = 34), current spinning workers with an exposure duration of 24-84 months (S group, n = 25), and former spinning workers with an exposure duration of more than 120 months (R group, n = 16). Unexposed controls were selected from workers in a nylon filament factory using the same criteria (N group; n = 40). BAEP records were analyzed based on the latencies of three main components (I, III, V) and interpeak latencies (I-III IPL, III-V IPL, I-V IPL). Latencies of component V, III-V IPL, and I-V IPL in the L group were significantly greater (P < 0.01, P < 0.05, P < 0.05, respectively). The significantly, higher III-V IPL in the L group suggested that in humans, chronic exposure to CS2 involves the auditory ascending tract in the brainstem. Recovery from this neurotoxic effect is apparently possible, as the BAEP parameters in the R group were not significantly higher than those of the N group despite the long previous CS2 exposure of the R group.

An occupational hygiene survey in a Chinese viscose rayon factory

The retinopathy (microaneurysm/small dot hemorrhage) is an early and specific biological indicator to quantitatively evaluate the CS2 exposure. The appearance of retinal lesions was observed among Yugoslavian, German and American workers exposed to CS2. However, among Finnish CS2 workers a positive result was not obtained. We suggested a different response to CS2 exposure between two ethnic populations. We had an opportunity to do a cross-sectional medical and occupational hygiene survey in a Chinese rayon staple plant. Cross-sectional medical examinations failed to show any chronic CS2 effects on the Chinese workers.

Epidemiological study of the cardiovascular effects of carbon disulphide

In an extensive health survey 115 male viscose rayon workers exposed to carbon disulphide (CS2) and 76 referents not exposed to this chemical were interviewed using the WHO cardiovascular questionnaire and responded to a self-administered questionnaire. A 12-lead ECG was taken and coded using the Minnesota code. Blood pressure (BP) was measured electronically and blood was examined for lipoproteins. In the viscose rayon factory the working conditions had not changed since 1932. Personal monitoring performed in 17 jobs, showed exposures varying from 4 to 112 mg/m3. For each individual a CS2 cumulative exposure index (CS2 index) was calculated. There were no significant differences between exposed and referents concerning the prevalence of angina, history of myocardial infarction, intermittent claudication and ECG signs of ischaemia. Blood pressure, low density lipoprotein (LDL)-cholesterol and the apolipoproteins A1 and B rose while high density lipoprotein (HDL)-cholesterol, the HDL-cholesterol/apolipoprotein A1 and the LDL-cholesterol/apolipoprotein B ratios decreased significantly with increasing exposure. Adjustment for age, body mass index, smoking, alcohol consumption, stress and tension at work, shift work, noise exposure and educational level in multiple linear regression analysis showed significant effects of the CS2 index on systolic BP, diastolic BP, cholesterol, HDL-cholesterol, LDL-cholesterol, apolipoprotein A1, apolipoprotein B, the LDL cholesterol/apolipoprotein B and HDL cholesterol/apolipoprotein A1 ratios; there were no significant effects on the triglycerides.

Detection of carbon disulfide in breath and air: a possible new risk factor for coronary artery disease

Carbon disulfide (CS2) is toxic to the heart and arteries; chronic exposure can result in accelerated atherosclerosis and coronary artery disease in humans and animals. Exposure to CS2 was investigated in normal volunteers working in New York City, using a new and highly sensitive assay. Volatile organic compounds in breath and air were captured in adsorbent traps containing graphitized carbon and molecular sieve, then thermally desorbed, concentrated by two-stage cryofocusing, and assayed for CS2 by gas chromatography/mass spectroscopy. Breath CS2 assays were performed in 42 normal volunteers, as well as in room air and in outdoor air collected at sites in and around New York City. The assay was linear, reproducible, and sensitive to picomolar (10(-12) mol/l) quantities. CS2 was detected in all samples of breath and indoor and outdoor air (mean concentrations were 5.25 pmol/l, SD = 3.89 in breath, 8.26 pmol/l in indoor air, and 3.92 pmol/l in outdoor air) (NS). The alveolar CS2 gradient (alveolar-inspired CS2) ranged from -20.0 to 8.0 nmol/l, separating subjects into either "excreters" or "retainers" of CS2. In view of the known toxicity of CS2, atmospheric pollution with CS2 merits attention as a possible new risk factor for the development of accelerated atherosclerosis and coronary artery disease.

The effects of long-term occupational exposure to carbon disulphide on serum lipids

No definite conclusion can be made from many epidemiological and clinical studies of lipid metabolism in workers exposed to carbon disulphide (CS2). The aim of our investigation was to study the serum lipid levels in workers exposed to different CS2 levels to establish possible exposure-response relationships. The first group, spinners, were exposed to high CS2 levels; the second group, workers in viscose production, to low CS2 levels; and the third group included non-exposed workers. As a part of a large examination, total and free serum cholesterol, total lipid and triglyceride levels were determined. Mean total lipid levels were similar in high and low CS2 exposed workers, but significantly lower in non-exposed workers. The total serum cholesterol mean levels were also similar in both groups, but significantly higher in the non-exposed group. Only serum triglyceride values were related to the CS2 levels: the most elevated were in the high exposed workers, somewhat lower in the low exposed, and the lowest in the non-exposed group. The finding is especially important as triglycerides are the most specific for developing atherosclerosis, which is often reported in workers with long-term exposure to carbon disulphide.

Hearing: the effects of chemicals

Recent studies of human beings exposed to environmental chemicals, as well as experimental animal studies, have identified a number of chemical agents that are commercial products, chemical intermediaries, waste products, or contaminants that are potentially ototoxic. The classes of compounds discussed in this review include organic solvents, asphyxiant gases, and heavy metals that are present in the environment as industrial pollutants or byproducts. Both human and animal investigations are summarized in discussing the actions of these ototoxic compounds. The suggested gaps in our knowledge are highlighted to help direct future research.

Epidemiological study of gastrointestinal and liver effects of carbon disulfide

In the framework of an extensive health survey, 119 viscose rayon workers exposed to carbon disulfide (CS2) and 79 workers from other plants not exposed to any toxic agent in the working environment underwent a gastrointestinal examination including a self-administered questionnaire, abdominal palpation, percussion and a number of liver function tests. In the viscose rayon factory the working conditions have not changed since 1932. Personal monitoring performed in 17 jobs showed CS2 exposures varying from 4 to 112 mg.m-3. For each individual a cumulative CS2 exposure index (CS2 index) was calculated. Univariate analysis of the questionnaire findings showed significantly higher prevalences of anorexia (37.0% vs 11.4%, P less than 0.0001), vomiting (12.6% vs 3.8%, P = 0.04), and recent weight loss (10.9% vs 1.3%, P = 0.009) in the exposed than in the non-exposed workers. The exposed workers had a larger total number of gastrointestinal complaints and a larger proportion reported at least one complaint at least once a month than the non-exposed ones. Abdominal tenderness was very rare in both groups. In multiple logistic regression analysis, adjusting for alcohol intake, smoking, body mass index (BMI), coffee consumption, commuting time, stress at work, shift work, educational level, family history of ulcer disease and intake of medication, significant associations with exposure were found for anorexia (P = 0.0001), nausea (P = 0.009), vomiting (P = 0.002) and flatulence (P = 0.03). Stress at work was a determinant for quite a few digestive complaints, but shift work was not significantly associated with any of them.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of pharmaceuticals in industrial workers--possible implications for epidemiological studies

A total of 129 workers exposed to carbon disulphide (CS2) and 81 non-exposed controls were asked about their current use of pharmaceuticals, using a self-administered questionnaire. In all, 31% of the exposed and 19.8% of the non-exposed used some medicine (P = 0.08). The average number of pharmaceuticals per subject amounted to 0.71 in the exposed vs. 0.36 in the non-exposed (P = 0.049). Predominant types of medicines used were analgesics (12.4% in the exposed vs. 8.6% in the non-exposed, P = 0.50) and sedatives/hypnotics (10.1% in the exposed vs. 4.9% in the non-exposed, P = 0.21). The pharmaceuticals consumed can cause numerous (side) effects that are similar to the toxic effects of CS2. To take into account these possibly confounding agents, a classification system for possible (side) effects of pharmaceuticals was developed, taking the dose into account. According to this method, many (side) effects of pharmaceuticals that could occur were recorded with higher frequency and intensity in the exposed subjects. Potential (side) effects that occurred significantly more frequently in the exposed than in the non-exposed were: tiredness, sedation, dizziness (20.9% vs. 4.9%, P = 0.001), excitation, anxiety (10.9% vs. 2.5%, P = 0.03), vision disturbances (7.0% vs. 0%, P = 0.01), and erection decrease (5.4% vs. 0%, P = 0.045). The implications of these findings for epidemiological studies are discussed.

[Changes in the activity of mixed-function oxidases in workers in an artificial silk plant]

The carbon sulfide effect on the oxidase mixed function activity (OMF) is studied in 172 workers from plant producing artificial silk, distributed in 5 groups, according to index of exposure, defining the strength and duration of the toxic effect. The OMF activity is evaluated by giving aminopyrine and determining its metabolites in urine. The exposure to carbon sulfide effects the OMF activity. It is expressed by inhibition of OMF, manifested most frequently in quality controllers and transporters (31%) and heads of shift and section foremen (25%). In the remaining groups the OMF inhibition, estimated after the first aminopyrine metabolite, varies from 16% to 21%. Even more significant are the changes in the excretion of the second metabolite, where the deviation of the referent values reaches 56% for IInd group, 55% in IIIrd group and 45% in Ist group.

Evaluation of the utility of a standard history questionnaire in assessing the neurological effects of solvents

Using a standard battery of medical surveillance questions, a study was undertaken to determine if an increase in reported neurologic symptoms was resulting from solvent exposure at a pharmaceutical research, development, and manufacturing site. The prevalence of positive responses to 13 interval history questions pertaining to neurological symptoms was compared between those enrolled in exposed surveillance programs (n = 840) and those enrolled in other, non-solvent exposed surveillance programs (n = 1,042). The ratio of positive responders between the exposed and unexposed groups was used to generate a relative prevalence ratio (RPR). No significantly elevated RPRs were seen when the analysis was adjusted for the confounding factors of age, sex, smoking, alcohol use, noise exposure, and number of interval histories. These results suggest that workplace solvent exposures in the employees studied did not appear to result in obvious neurologic symptoms. However, low-level neurotoxic exposures can cause asymptomatic or sub-clinical disorders. Therefore, more sensitive neurotoxic surveillance systems need to be developed.

Assessment of exposure to carbon disulfide in viscose production workers from urinary 2-thiothiazolidine-4-carboxylic acid determinations

The follow-up of environmental carbon disulfide (CS2) exposure and urinary excretion of 2-thiothiazolidine-4-carboxylic acid (TTCA) among 20 operatives over a 4-day working week in two viscose producing factories confirmed earlier observations that TTCA is a sensitive and reliable indicator of exposure to CS2. Exposure to as low as 0.5-1.0 ppm (1.6-3.2 mg/m3) of CS2 (8-hour time-weighted average [TWA]) was associated with detectable amounts of TTCA in end-of-shift urine. Moreover, the excretion of TTCA, relative to estimated CS2 uptake, appeared surprisingly constant in the studied work force. Approximately 3% (range 2-6.5%) of absorbed CS2 was detected in urine as TTCA. The proportional TTCA excretion did not show dose dependency in the estimated CS2 dose range which varied by about 20-fold. TTCA elimination exhibited both a fast (T 1/2 6 hour) and a slow (T 1/2 68 hour) phase. The slow elimination is compatible with a high lipid solubility and reversible protein binding of CS2. Consequently, urinary excretion of TTCA, relative to CS2 exposure, increased by about a third during the workweek. Urinary TTCA concentration of 4.5 mmol/mol creatinine in a postshift sample corresponded to a TWA exposure to 10 ppm CS2 towards the end of the working week.

Effects of paternal occupational exposure on spontaneous abortions

BACKGROUND

Paternal exposure to mutagenic agents has been suggested to affect pregnancy outcome adversely.

METHODS

A nationwide data base of medically diagnosed spontaneous abortions and other pregnancies and national census data was used to evaluate the effects of men's occupational exposures on risk of spontaneous abortion in 99,186 pregnancies in Finland. Census data from the years 1975 and 1980 provided information about the occupation, industry, and socioeconomic status. A job-exposure classification was developed to classify women and their husbands according to possible occupational exposures on the basis of their occupational title and industry.

RESULTS

In 10% of the pregnancies, the husband was exposed to one or more of the mutagens, and the rate of spontaneous abortion was unaffected (OR = 1.0). Of the 25 specific mutagenic exposures evaluated, paternal exposure to four (ethylene oxide, rubber chemicals, solvents used in refineries, and solvents used in the manufacturing of rubber products) was associated with an increased relative risk of spontaneous abortion. In addition, the risk of spontaneous abortion was higher among wives of rubber products workers than among unexposed men.

CONCLUSIONS

Although there is some biological rationale for the findings of this study, these findings need to be confirmed by studies in which individual exposures can be measured directly.

Finger tremor after carbon disulfide-based pesticide exposures

Index finger tremor accompanying voluntary movement was studied in 19 age-matched control subjects and in 19 grain industry employees chronically exposed to carbon disulfide-based fumigants. Visual judgments of tremor amplitude made by neurologists during clinical examinations equaled the sensitivity of computerized tremor amplitude measurements. Tremor frequency variations detectable only with computerized measurement were present in grain workers with and without increased tremor amplitudes. Frequency differences discriminated between normal subjects and 74% of the grain workers. The distribution of tremor frequency power in the grain workers was often sequestered at 5 to 7 Hz, reminiscent of tremor in idiopathic Parkinson's disease. These findings suggest that the measurement of subtle tremor frequency changes may provide an early indication of chronic carbon disulfide poisoning.