This report contains the collective views of an international group of experts and does not necessarily represent the decisions or the stated policy of the United Nations Environment Programme, the International Labour Organization, or the World Health Organization.

First draft prepared by R. Newhook and M.E. Meek, Existing Substances Division, Health Canada, Ottawa, Ontario, Canada; and D. Caldbick, Commercial Chemicals Evaluation Branch, Environment Canada, Hull, Quebec, Canada

Published under the joint sponsorship of the United Nations Environment Programme, the International Labour Organization, and the World Health Organization, and produced within the framework of the Inter-Organization Programme for the Sound Management of Chemicals.

World Health Organization
Geneva, 2002

The International Programme on Chemical Safety (IPCS), established in 1980, is a joint venture of the United Nations Environment Programme (UNEP), the International Labour Organization (ILO), and the World Health Organization (WHO). The overall objectives of the IPCS are to establish the scientific basis for assessment of the risk to human health and the environment from exposure to chemicals, through international peer review processes, as a prerequisite for the promotion of chemical safety, and to provide technical assistance in strengthening national capacities for the sound management of chemicals.

The Inter-Organization Programme for the Sound Management of Chemicals (IOMC) was established in 1995 by UNEP, ILO, the Food and Agriculture Organization of the United Nations, WHO, the United Nations Industrial Development Organization, the United Nations Institute for Training and Research, and the Organisation for Economic Co-operation and Development (Participating Organizations), following recommendations made by the 1992 UN Conference on Environment and Development to strengthen cooperation and increase coordination in the field of chemical safety. The purpose of the IOMC is to promote coordination of the policies and activities pursued by the Participating Organizations, jointly or separately, to achieve the sound management of chemicals in relation to human health and the environment.

WHO Library Cataloguing-in-Publication Data

Carbon disulfide.

(Concise international chemical assessment document ; 46)

1. Carbon disulfide - adverse effects 2. Risk assessment 3. Environmental exposure

4. Occupational exposure I. International Programme on Chemical Safety II.Series

ISBN 92 4 153046 4     (NLM Classification: QV 633)

ISSN 1020-6167

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Concise International Chemical Assessment Documents (CICADs) are the latest in a family of publications from the International Programme on Chemical Safety (IPCS) – a cooperative programme of the World Health Organization (WHO), the International Labour Organization (ILO), and the United Nations Environment Programme (UNEP). CICADs join the Environmental Health Criteria documents (EHCs) as authoritative documents on the risk assessment of chemicals.

International Chemical Safety Cards on the relevant chemical(s) are attached at the end of the CICAD, to provide the reader with concise information on the protection of human health and on emergency action. They are produced in a separate peer-reviewed procedure at IPCS.

CICADs are concise documents that provide summaries of the relevant scientific information concerning the potential effects of chemicals upon human health and/or the environment. They are based on selected national or regional evaluation documents or on existing EHCs. Before acceptance for publication as CICADs by IPCS, these documents undergo extensive peer review by internationally selected experts to ensure their completeness, accuracy in the way in which the original data are represented, and the validity of the conclusions drawn.

The primary objective of CICADs is characterization of hazard and dose-response from exposure to a chemical. CICADs are not a summary of all available data on a particular chemical; rather, they include only that information considered critical for characterization of the risk posed by the chemical. The critical studies are, however, presented in sufficient detail to support the conclusions drawn. For additional information, the reader should consult the identified source documents upon which the CICAD has been based.

Risks to human health and the environment will vary considerably depending upon the type and extent of exposure. Responsible authorities are strongly encouraged to characterize risk on the basis of locally measured or predicted exposure scenarios. To assist the reader, examples of exposure estimation and risk characterization are provided in CICADs, whenever possible. These examples cannot be considered as representing all possible exposure situations, but are provided as guidance only. The reader is referred to EHC 170.¹

While every effort is made to ensure that CICADs represent the current status of knowledge, new information is being developed constantly. Unless otherwise stated, CICADs are based on a search of the scientific literature to the date shown in the executive summary. In the event that a reader becomes aware of new information that would change the conclusions drawn in a CICAD, the reader is requested to contact IPCS to inform it of the new information.

The flow chart shows the procedures followed to produce a CICAD. These procedures are designed to take advantage of the expertise that exists around the world – expertise that is required to produce the high-quality evaluations of toxicological, exposure, and other data that are necessary for assessing risks to human health and/or the environment. The IPCS Risk Assessment Steering Group advises the Coordinator, IPCS, on the selection of chemicals for an IPCS risk assessment based on the following criteria:

• there is the probability of exposure; and/or
• there is significant toxicity/ecotoxicity.

Thus, a priority chemical typically

• is of transboundary concern;
• is of concern to a range of countries (developed, developing, and those with economies in transition) for possible risk management;
• is significantly traded internationally;
• has high production volume;
• has dispersive use.

The Steering Group will also advise IPCS on the appropriate form of the document (i.e., EHC or CICAD) and which institution bears the responsibility of the document production, as well as on the type and extent of the international peer review.

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The CICAD Final Review Board has several important functions:

• to ensure that each CICAD has been subjected to an appropriate and thorough peer review;
• to verify that the peer reviewers' comments have been addressed appropriately;
• to provide guidance to those responsible for the preparation of CICADs on how to resolve any remaining issues if, in the opinion of the Board, the author has not adequately addressed all comments of the reviewers; and
• to approve CICADs as international assessments.

Board members serve in their personal capacity, not as representatives of any organization, government, or industry. They are selected because of their expertise in human and environmental toxicology or because of their experience in the regulation of chemicals. Boards are chosen according to the range of expertise required for a meeting and the need for balanced geographic representation.

Board members, authors, reviewers, consultants, and advisers who participate in the preparation of a CICAD are required to declare any real or potential conflict of interest in relation to the subjects under discussion at any stage of the process. Representatives of nongovernmental organizations may be invited to observe the proceedings of the Final Review Board. Observers may participate in Board discussions only at the invitation of the Chairperson, and they may not participate in the final decision-making process.

This CICAD on carbon disulfide was prepared by the Environmental Health Directorate of Health Canada and the Commercial Chemicals Evaluation Branch of Environment Canada based on documentation prepared concurrently as part of the Priority Substances Program under the Canadian Environmental Protection Act (CEPA). The objective of assessments on priority substances under CEPA is to assess potential effects of indirect exposure in the general environment on human health as well as environmental effects. Data identified as of the end of May 1999 were considered in these reviews.² Information on the nature of the peer review and availability of the source document is presented in Appendix 1. Other source documents for the health and environmental effects include reports prepared by IPCS (1979), the Nofer Institute (Rolecki & Tarkowski, 2000), and the United Kingdom Department of Environment (Crookes et al., 1993). Other reviews that were also consulted include BUA (1993) and ATSDR (1996). Information on the peer review of this CICAD is presented in Appendix 2. This CICAD was approved as an international assessment at a meeting of the Final Review Board, held in Ottawa, Canada, on 29 October - 1 November 2001. Participants at the Final Review Board meeting are listed in Appendix 3. The International Chemical Safety Card (ICSC 0022) for carbon disulfide, produced by the International Programme on Chemical Safety (IPCS, 2000), has also been reproduced in this document.

The worldwide production capacity of carbon disulfide (CAS No. 75-15-0) is approximately 1 million tonnes; most of it is used in the production of viscose fibre and cellophane film. It is also released as a by-product in oil and gas processing. Additional industrial releases result from its use in the chemical industry and tire manufacturing. Virtually all anthropogenic and natural releases are to air. Carbon disulfide is also produced naturally by soil and sediment microorganisms, vegetation, forest and grass fires, and volcanoes. Worldwide, at least 40% and possibly as much as 80% of releases are a result of natural or biogenic activity.

Carbon disulfide is ubiquitous throughout the environment. It has been detected in air, water, sediment, and soil; however, it is present primarily in air. The highest concentrations of carbon disulfide in air in the source country for this CICAD (i.e., Canada) have been measured near industrial sources, in particular near natural gas processing plants and sites with sulfur-containing natural gas flares. Carbon disulfide is removed from the air primarily by reaction with hydroxyl radicals, resulting in a half-life of 1-2 weeks. This half-life in air makes it a candidate for long-range transport; however, it is rapidly diluted to natural background levels. Carbon disulfide is rapidly metabolized by organisms and does not bioconcentrate or biomagnify.

Available data upon which to base estimates of human exposure to carbon disulfide are extremely limited; however, air appears to be the major route of exposure for members of the general population. Airborne exposures are estimated to be elevated for populations in the vicinity of industrial point sources.

Carbon disulfide is extensively absorbed by inhalation, but also via the skin. It is metabolized to several metabolites, one of which (2-thiothiazolidine-4-carboxylic acid) is the basis for biomonitoring of exposure at the workplace.

Available data to serve as a basis for assessment of the potential of carbon disulfide to induce irritancy or sensitization are limited. While it has been reported in secondary accounts of seemingly limited early studies that carbon disulfide is severely irritating to the eyes and skin, it has not been possible to verify these data. While inhalation in viscose rayon plants is irritating to the mucous membranes, including the respiratory system, the role of concomitant exposure to hydrogen sulfide and sulfuric acid in induction of these effects is unknown.

Based on the results of studies of workers exposed to carbon disulfide and supporting data from experiments conducted on animals, the nervous system appears to be the critical target for carbon disulfide-induced toxicity, manifested most often as reduced conduction velocity in the peripheral nerves and impaired performance in psychomotor testing. Other effects for which there is considerable weight of evidence in humans exposed to carbon disulfide include alterations in serum lipids and blood pressure that are associated with increased risk of cardiovascular disease, systemic ophthalmological effects, including those on colour vision and damage to the blood vessels of the retina, and (with higher exposures) increased mortality from heart disease.

No evidence of carcinogenicity has been observed in limited epidemiological studies; long-term studies on carcinogenicity in experimental animals have not been reported. There is no clear evidence of genotoxicity, although there is some evidence of weak and/or equivocal clastogenicity in vitro or in vivo.

There are several reports of decreased libido and/or impotence among males occupationally exposed to high concentrations of carbon disulfide, but there is no consistent evidence based on limited study of other adverse reproductive effects in humans. In experimental animals, carbon disulfide is embryotoxic and fetotoxic at high concentrations and causes terata at exposure levels toxic to the dam.

In the sample risk characterization, the estimated mean airborne exposure to carbon disulfide for the general population and for populations in the vicinity of point sources is considerably less than a tolerable concentration of 100 µg/m³. This tolerable concentration was derived based on the benchmark concentration estimated for a 5% excess risk of an abnormal response (defined based on the 5th percentile of the unexposed workers in the critical study) for the most sensitive response variable – i.e., peroneal motor nerve conduction velocity (MCV)³ in viscose rayon workers – adjusting to continuous exposure (24 h/day, 7 days/week) and applying an overall uncertainty factor of 50.

Since most carbon disulfide is released to air, for environmental effects, it is terrestrial organisms in the vicinity of industrial sources that are at greatest potential risk. Aquatic organisms close to discharge points might also be potentially affected. However, based on the sample risk characterization, conservative comparisons of estimated exposure with no-effect values indicate that it is unlikely that carbon disulfide causes adverse effects on populations of terrestrial or aquatic organisms.

Carbon disulfide (also known as carbon disulphide, carbon bisulfide, carbon sulfide, and dithiocarbonic anhydride) is a clear, colourless or faintly yellow, mobile liquid at room temperature. Its relative molecular mass is 76.14, its Chemical Abstracts Service (CAS) registry number is 75-15-0, and its Registry of Toxic Effects of Chemical Substances (RTECS) number is FF6650000. Carbon disulfide is highly flammable and highly volatile, with a vapour pressure of 48.210 kPa at 25 ° C and a water solubility of 2100 mg/litre at 20 ° C. The log octanol/water partition coefficient (log K_ow) is 2.14, and the Henry's law constant is 1748 Pa·m³/mol at 25 ° C (Environment Canada & Health Canada, 2000). Information on additional physical/chemical characteristics is presented in the International Chemical Safety Card, reproduced in this document. The conversion factor⁴ for carbon disulfide at 20 °C and 101.3 kPa is 1 ppm = 3.125 mg/m³. The molecular formula of carbon disulfide is:

S = C = S

Analysis of carbon disulfide in air usually involves preconcentration on a sorbent tube followed by thermal or solvent desorption followed by gas chromatography (mass spectrometry, electron capture, photoionization, or flame photometric detector). Detection limits range from 0.6 ng/m³ to 10 µg/m³ (ATSDR, 1996). In samples of water and soil, carbon disulfide is often analysed by purge and trap methods followed by gas chromatography and mass spectrometry. Detection limits are in the low microgram per litre (or kilogram) or milligram per litre (or kilogram) range (ATSDR, 1996). In food, carbon disulfide is usually extracted with acetone and analysed by gas chromatography (electron capture or Sievers chemiluminescence detection). Detection limits are in the nanogram per sample range (ATSDR, 1996). For biological fluids, various purge and trap or solvent extraction methods are applied before analysis by high-performance liquid chromatography or gas chromatography and mass spectrometry. Detection limits are in the nanogram per litre range (ATSDR, 1996).

For the biological monitoring of carbon disulfide, mainly at the workplace, analysis of the metabolite 2-thiothiazolidine-4-carboxylic acid (TTCA) in urine has been extensively applied. The methods most often used are based on high-performance liquid chromatography; the detection limit is <0.1 mg/litre, adequate for assessing 8-h time-weighted average (TWA) exposures below 0.3 mg/m³ (Lowry, 1996).

Wherever possible, data on sources and emissions in a global context are presented. Where such information was not identified, data from the source country of the national assessment on which the CICAD is based (i.e., Canada) are presented as an example.

Carbon disulfide is released into the environment from a wide variety of natural sources. Soils, marshes, and coastal regions tend to be the largest biogenic sources. Production of carbon disulfide from soil and plants occurs naturally from the metabolic action of soil bacteria and plants during the growing season. Increases in soil moisture, temperature, organic content, and light resulted in a direct increase in the rate of production from soil (Staubes et al., 1987). Up to 35 000 tonnes of carbon disulfide may be added to the Canadian environment annually from this natural source alone (Environment Canada, 1980). It is estimated that up to 2280 tonnes of carbon disulfide per year could be released globally as a result of the weathering of sulfide minerals (Stedman et al., 1984). Carbon disulfide is also produced by forest and grass fires and by volcanoes.

Although there is a great deal of uncertainty in the estimates, worldwide, at least 40% and possibly as much as 80% of releases are a result of natural or biogenic activity (Environment Canada & Health Canada, 2000).

Carbon disulfide is used primarily as a solvent in the viscose rayon (about 65%) and cellophane (about 10-12%) industries (BUA, 1993; ATSDR, 1996). In the production of viscose fibres and cellophane film, carbon disulfide is not bound to the end product. Thus, unless appropriate measures are taken, most of the carbon disulfide used in these processes is likely to end up in the atmosphere. If approximately 75% of the world production of about 1 million tonnes is used for these two processes (see section 4.3), some 700 kilotonnes would be released annually.

The 1999 Toxic Release Inventory indicates that approximately 16 kilotonnes of carbon disulfide were released to the environment in the USA from manufacturing and processing facilities in 1999. Virtually all of this was released to the air (TRI, 1999). The estimated annual release of carbon disulfide in the United Kingdom from viscose fibre and cellophane film manufacturing was 15-21 kilotonnes/year (Crookes et al., 1993).

Between 2120 and 2465 tonnes of carbon disulfide were released from Canadian industrial sources in 1996. Nearly all of this was emitted into the atmosphere in the gas sector. Total reported releases from all other industrial sources – including commercial manufacture, distribution, and use of carbon disulfide – were less than 100 tonnes (Environment Canada, 1997b).

Carbon disulfide has been used as a fumigant on stored grain, although its registration for this purpose has been withdrawn in most countries (C. Warfield, personal communication, 1996). It is also released to the environment in cigarette smoke.

Worldwide production of carbon disulfide was estimated to be 1.025 million tonnes in 1984 and 0.9 million tonnes in 1990 (Rolecki & Tarkowski, 2000). In 1985, production in the USA alone was around 143 kilotonnes (ATSDR, 1996). Annual production in the United Kingdom prior to 1992 was estimated to be between 25 and 35 kilotonnes (Crookes et al., 1993).

The major use of carbon disulfide worldwide is the production of viscose (rayon) fibres (65%) and cellophane film (10-15%) (Rolecki & Tarkowski, 2000). Other uses include the manufacture of carbon tetrachloride (apparently only very small amounts at present), sodium sulfite, mineral flotation agents, xanthates, mercaptans, and thioureas. Carbon disulfide is also used as a solvent for fats, lipids, resins, rubbers, sulfur monochloride, and white phosphorus (Crookes et al., 1993).

In 1996, 3.1 kilotonnes of carbon disulfide were manufactured for commercial purposes in Canada (Environment Canada, 1997b). Camford Information Services (1995) reported Canadian domestic production of 10.9 kilotonnes in 1993, down from 25 kilotonnes in 1976. The much lower production figure in recent years reflects the closure in Canada of the rayon and cellulose fibre industry, which had been the major user of carbon disulfide.

In Canada, nearly 1.7 kilotonnes of carbon disulfide were used in 1996 as a precursor in the manufacture of xanthates, which are used as flotation agents in mineral refining processes (Environment Canada, 1997b). Carbon disulfide is also used to produce drilling mud additives to dissolve waxes that interfere with the efficiency and yields of oil and gas wells and in the manufacture of rubber curing accelerators, which are used in the production of rubber tires for vehicles (Camford Information Services, 1995).

In air, carbon disulfide is primarily degraded through photo-oxidation by reactions with hydroxyl radicals and by a secondary route involving triplet oxygen (O(³P)). Based on an assumed concentration of hydroxyl radicals of 5 ´ 10⁵ radicals/cm³, a half-life of about 5.5-15 days is calculated from rate constants between 1.1 ´ 10^-12 and 2.9 ´ 10^-12 cm³/molecule per second (BUA, 1993). Wine et al. (1981) likewise estimated that photo-oxidation in the troposphere results in a half-life in air of 7-14 days. Reaction products include carbonyl sulfide and sulfur dioxide. Carbonyl sulfide has a much longer lifetime (2 years) than carbon disulfide in the atmosphere.

Photolysis of carbon disulfide by radiation at wavelengths above 290 nm occurs in the troposphere. An atmospheric lifetime of 11 days (half-life of 7.7 days) was calculated assuming 12 h of sunlight (Peyton et al., 1976). Wood & Heicklen (1971) demonstrated that direct photolysis of carbon disulfide at 313 nm produces reaction products similar to those of the photo-oxidation reaction – that is, carbon monoxide, carbonyl sulfide, sulfur dioxide, plus an unidentified polymeric material. Wet deposition from the atmosphere is probably a minor removal process, because carbon disulfide interacts only weakly with water (Lovejoy, 1989).

The overall reactivity-based half-life of carbon disulfide in air, as estimated for ChemCAN4 steady-state fugacity modelling (section 5.5), is about 1 week (DMER & AEL, 1996).

Carbon disulfide has little potential to exert effects on the atmospheric environment. Since carbon disulfide is non-halogenated, its ozone-depleting potential is zero, and it will therefore not contribute to the depletion of stratospheric ozone. It is estimated that carbon disulfide has less than 1% of the global warming potential of the reference compound CFC-11, and it is therefore not considered to be involved in climate change. (Carbon disulfide may have an indirect impact on climate change through its main atmospheric transformation product, carbonyl sulfide, but the magnitude of this impact is considered to be small.) Except in proximity to strong point sources, average annual concentrations of carbon disulfide in ambient air are low relative to those for the volatile organic compounds with similar photochemical ozone creation potential that contribute most to the formation of ground-level ozone. Therefore, the contribution of carbon disulfide to ground-level ozone formation is not expected to be significant (Environment Canada & Health Canada, 2000).

Based on a Henry's law constant of 1748 Pa× m³/mol at 25 °C and a vapour pressure of 48.2 kPa at 25 ° C, carbon disulfide released into water is expected to volatilize, with a half-life ranging between 11 min in water (saturated solution) and 2.6 h in a model river (Peyton et al., 1976; Howard, 1989). Carbon disulfide is resistant to hydrolysis in water within the biological pH range (4-10), with a hydrolysis half-life extrapolated to pH 9 of 1.1 years (Peyton et al., 1976). Its predicted rate of biodegradation in water is negligible compared with its rate of volatilization from surface water (ATSDR, 1996). The mean degradation half-life assumed for fugacity modelling by DMER & AEL (1996) (section 5.5) of 5500 h (7.4 months) was based on the estimate of biodegradation half-life by Abrams et al. (1975).

Owing to its low affinity for sorption to organic substances (organic carbon/water partition coefficient [log K_oc] = 1.79), very little carbon disulfide is likely to partition to or remain in sediment. In one study, the soil/sediment microorganism Thiobacillus thioparus (grown aerobically, incubated anaerobically) was able to metabolize carbon disulfide to produce carbonyl sulfide and hydrogen sulfide (Smith & Kelly, 1988). Thus, some biodegradation is expected to occur. The estimated mean reactivity half-life assumed for fugacity modelling (section 5.5) was 5500 h (7.4 months), based on the estimate of biodegradation half-life by Abrams et al. (1975).

No estimates of a half-life for carbon disulfide in soil were identified in the literature. Carbon disulfide has been aerobically degraded by a strain of Thiobacillus thioparus. This particular strain was able to hydrolytically oxidize carbon disulfide sequentially to carbonyl sulfide and hydrogen sulfide; all the carbon was released as carbon dioxide, followed by oxidation of the sulfide to sulfate (Smith & Kelly, 1988). For soil, DMER & AEL (1996) assumed a mean degradation half-life of 5500 h for their fugacity modelling (section 5.5), based on the estimate of biodegradation half-life by Abrams et al. (1975). In the natural environment, carbon disulfide is highly mobile in soil (log K_oc = 1.79) and is subject to rapid volatilization, so it is unlikely to remain in soil long enough to undergo significant biodegradation.

Carbon disulfide is expected to have little or no tendency to bioaccumulate or biomagnify in biota, owing to its relatively low log K_ow value (2.14) and rapid metabolism in most animals (Beauchamp et al., 1983).

Fugacity modelling provides an overview of key reaction, intercompartment, and advection (movement out of a system) pathways for carbon disulfide and its overall distribution in the environment (DMER & AEL, 1996). A steady-state, non-equilibrium EQC model (Level III fugacity model) was run using the methods developed by Mackay (1991) and Mackay & Paterson (1991). Values for physical/chemical properties utilized in the modelling are presented in section 2; those for half-lives in various media are presented in sections 5.1-5.3, above. Modelling was based on an assumed default emission rate of 1000 kg/h into a region of 100 000 km², which includes a 10 000-km² area of surface water (20 m deep). The height of the atmosphere is 1000 m. Sediments and soils have an organic carbon content of 4% and 2% and a depth of 1 cm and 10 cm, respectively. The estimated percent distribution predicted by this model is not affected by the assumed emission rate.

Modelling indicates that carbon disulfide partitions differently depending on the medium to which it is released. For example, if emitted into air, 99.8% of the carbon disulfide is present in air; if emitted into soil, the fraction in air is reduced to 73%, with most of the rest in soil. When carbon disulfide is released to water, it is present primarily in water (85%) and, to a lesser extent, in air (15%) (DMER & AEL, 1996). Thus, while the predicted distributions indicate little intermedia transport when carbon disulfide is discharged to air, release to each of soil and (to a lesser extent) water has the potential for substantial transport of carbon disulfide to air (see also sections 5.1-5.3).

Data on concentrations in the environment from the source country of the national assessment on which the CICAD is based (i.e., Canada) are presented here as a basis for the sample risk characterization. Where identified, data from other countries are also presented.

6.1.1 Ambient air

Based on continuous monitoring in ambient air over 2 years at a remote site and at two sites in the vicinity of a sour gas processing plant (carbon disulfide is a minor component of the waste gases emitted from the processing of sour gas), carbon disulfide was not detected in the majority of samples – e.g., in 85-90% of samples at the remote site – and was detected somewhat more frequently at the sour gas sites. Based on extensive data from conventional gas chromatography, combined with some limited data collected over an 8-min sampling period by a sensitive cryofocusing technique, the mean and maximum levels were higher at the sites near the sour gas plant (0.61 and 88 µg/m³, respectively, at an upwind site, and 1.40 and 156 µg/m³, respectively, at a downwind site) than at the remote site (0.51 and 12.5 µg/m³, respectively) (Legge et al., 1990a, 1990b).

The concentration of carbon disulfide in the air downwind from another gas processing site was predicted by the ISC 3 view plume dispersion model (reported release of 1287 tonnes to the atmosphere in 1995, the largest release reported for Canada that year; NPRI, 1996). The highest calculated concentration in air 1 km downwind (a 1-h average) was about 114 µg/m³. The 24-h average maximum ground-level concentration 10 km downwind was 14.3 µg/m³ (The, 1998).

The results of other modelling studies indicate that levels near smaller sour gas wells are somewhat less than those near larger wells. Based on concentrations of carbon disulfide measured in flare gases from a sour gas facility in central Alberta, in combination with plume dispersion modelling, Strosher (1996) predicted the maximum ground-level concentration at 2.02 µg/m³ for a daily average and 0.16 µg/m³ for an annual average.

Carbon disulfide levels were also elevated on the site of Prospec Chemicals, Fort Saskatchewan, Alberta, which uses the compound on-site as a feedstock for xanthates. In monitoring of ambient air outside of the property line (at the point of impingement predicted by dispersion modelling) during the summer of 1997, monthly average concentrations of carbon disulfide ranged from 3 to 6 µg/m³, and hourly maximum concentrations were between 56 and 100 µg/m³ (L. Fu, personal communication, 1997; E. Weiss, personal communication, 1998).

In a small study, the mean level of carbon disulfide in six outdoor air samples in New York City, USA, was 0.30 µg/m³ (Phillips, 1992). Concentrations up to 1.1 and 1.2 µg/m³ in ambient air were measured in the USA and Europe, respectively (Sandalls & Penkett, 1977; Maroulis & Bandy, 1980; Crookes et al., 1993; ATSDR, 1996).

6.1.2 Indoor air

In a small study, carbon disulfide was detected in all nine indoor air samples from a hospital in New York City, USA; the mean concentration was 0.63 µg/m³, which was not significantly higher than the mean level in six outdoor air samples (0.30 µg/m³) (Phillips, 1992).

Concentrations of carbon disulfide were below the limit of detection in samples of indoor air from several office buildings in the USA (Fuortes, 1990; Oldaker et al., 1995).

6.1.3 Surface water and groundwater

Data on levels of carbon disulfide in surface waters are limited to southern Ontario, Canada. Background levels at remote sites in Ontario, largely due to biogenic production, ranged between about 0.005 and 0.4 µg/litre (Caron & Kramer, 1994). In Lake Ontario, in 1981, a median concentration of 0.4 µg/litre and a maximum of 3.9 µg/litre were measured (Kaiser et al., 1983). The authors considered that the lower levels in the open lake were likely due to biogenic activity, while the elevated levels were due mainly to the influence of nearby urban/industrial areas (B. Scott, personal communication, 1998). The highest measured concentration in surface water, 25.0 µg/litre, was associated with a chemical plant on Thompson Creek in the Niagara region that has since closed (Kaiser & Comba, 1983).

In seawater, Lovelock (1974) reported concentrations in the open Atlantic of 0.52 and 0.78 ng/litre off the coast of Ireland and 5.4 ng/litre in stagnant bay water near Ireland. Leck & Rodhe (1991) measured levels of carbon disulfide between 0.83 and 1.18 ng/litre in the open Baltic and North seas. Kim & Andreae (1987) reported concentrations of carbon disulfide in surface waters in the North Atlantic ranging between 0.01 and 4.6 ng/litre.

Data on levels in groundwater were not identified.

6.1.4 Drinking-water

Very few data on the levels of carbon disulfide in drinking-water supplies were identified. In a 1982-1983 survey of raw and treated water samples from 10 Ontario municipalities, carbon disulfide was frequently detected at low levels in each of spring, summer, and winter samplings. Concentrations over the three seasons ranged from non-detectable (<0.1 µg/litre) to trace levels in most cities, from non-detectable to 0.2 µg/litre in Cornwall, and from non-detectable to 0.3 µg/litre in Hamilton (Otson, 1987; R. Otson, personal communication, 1996).

6.1.5 Soil and sediment

Only limited data on concentrations of carbon disulfide in soils were identified. In a 1985-1986 study of background sites in the general vicinity of petrochemical refinery facilities west of Toronto, Ontario, carbon disulfide was detected at one of five sites in Port Credit at 0.000 11 µg/g, but not at any of six sites from Oakville/Burlington (Golder Associates, 1987). In a 1987 survey of organic compounds in surface soils in background areas in the same municipalities, carbon disulfide was detected at 3 of 30 urban residential and parkland sites in Port Credit, Oakville, and Burlington, at concentrations of 0.10, 0.10, and 0.14 µg/g, respectively (Golder Associates, 1987). However, reported levels were near the method detection limit (0.10 µg/g), and the values were not corrected for the observed contamination of the method blank.

In 1988, carbon disulfide was measured in sedi-ment suspensions taken from Lake Ontario, near Burlington, Ontario, and in Harp Lake, near Huntsville, Ontario. Caron & Kramer (1994), using a sulfur-specific gas chromatographic method, were able to detect 5.9 ng carbon disulfide/litre in Lake Ontario sediments and 9.7 ng carbon disulfide/litre in Harp Lake sediments.

6.1.6 Food

Reliable data to serve as a basis for estimation of exposure to carbon disulfide in food were not identified. Carbon disulfide has been used as a fumigant on stored grain, although its registration for this purpose has been withdrawn in most countries (C. Warfield, personal communication, 1996). Carbon disulfide is produced during the metabolism of certain pesticides, such as dithiocarbamates, in plants and soil. Carbon disulfide is also a metabolite produced by plants from naturally occurring sulfur compounds (section 4.1).

The results of a number of food surveys from the USA in which the levels of carbon disulfide were determined have been published (Heikes & Hopper, 1986; Daft, 1987, 1988, 1989; Heikes, 1987). However, the results of these studies are considered to be of limited relevance because they appear to have been conducted before the use of carbon disulfide as a grain fumigant was cancelled and/or the analytical methodology was relatively insensitive.

6.1.7 Consumer products

Based on analysis of mainstream smoke from seven samples of commercial and experimental cigarettes and a single cigar and marijuana cigarette, each of these products delivered approximately 2 µg of carbon disulfide per cigarette/cigar smoked (Horton & Guerin, 1974).

6.1.8 Human tissues and fluids

Carbon disulfide (at the micrograms per litre or micrograms per cubic metre level) and its metabolite TTCA were detected in virtually all samples of breath, blood, urine, and breast milk of subjects with no known occupational exposure in a number of studies (Pellizzari et al., 1982; Phillips, 1992; Brugnone et al., 1994). At least some of the carbon disulfide and/or TTCA may have arisen from exposure to other chemicals of which they are known to be metabolites, such as disulfiram, captan, or dithiocarbamate fungicides, and TTCA is present naturally in brassica vegetables and may be found in the urine at levels above 10 µmol/litre after consumption of such items (Simon et al., 1994, and references therein; Kivisto, 2000).

Data on levels of carbon disulfide in environmental media to serve as the basis for development of sample estimates of population exposure are limited to a small number of surveys of ambient air conducted at few locations in Canada or the USA and limited Canadian surveys in drinking-water and soil in which carbon disulfide was seldom detected. Meaningful probabilistic assessment is precluded, therefore. In this section, mean deterministic estimates of environmental intake from air, water, and soil by members of the general population of Canada have been derived. These are followed by consideration of mean estimates of potential airborne exposures by populations in the vicinity of point sources in Canada, based on the very limited available data.

Point estimates of total daily intake of carbon disulfide by six age groups of the general population of Canada were developed (Table 1), primarily to determine the relative contributions from various media. These estimates indicate that intake from environmental exposure to carbon disulfide is virtually all from inhalation. That air is the principal route of exposure is supported by the results of the EQC fugacity modelling, which indicate that virtually all of the carbon disulfide released to air (industrial releases in Canada are almost entirely to air) will tend to remain in that compartment. Exposure from ingestion of drinking-water and soil appears to be negligible in comparison with that from air. Based on the absence of registered uses for carbon disulfide on food and the results of the fugacity modelling for southern Alberta, which predicted that very low levels of the compound (<1 × 10^-6 µg/g) will accumulate in biota (Environment Canada & Health Canada, 2000), it was assumed that exposure via food will be negligible. For smokers, it is estimated that cigarette smoking can increase the intake of carbon disulfide severalfold.

It is also known that concentrations of carbon disulfide in ambient air are elevated in the vicinity of some point sources in Canada (section 5.1). Based on the range of mean concentrations measured in the vicinity of natural gas processing (1.40 µg/m³; Legge et al., 1990b) and xanthate production facilities (3-6 µg/m³; L. Fu, personal communication, 1997; E. Weiss, personal communication, 1998) in Canada, average exposures by inhalation near such facilities may be increased between 2- and 10-fold over those for the general population.

The database on occupational exposure to carbon disulfide in rayon fibre production is extensive. Concentrations of carbon disulfide in this industry are known to have declined substantially over the several decades encompassed by the available epidemiological studies (Price et al., 1997). The figures below have been selected from relatively recent studies (confined to the 1990s) in different countries.

In a Finnish viscose rayon fibre factory, the mean concentration of carbon disulfide in the air (8-h TWA) was 9.4 mg/m³ (range 4.7-25 mg/m³); in a viscose sheeting production factory, it was 13 mg/m³ (range 0.6-28 mg/m³) (Riihimaki et al., 1992). In a viscose fibre plant in Yugoslavia, exposure to carbon disulfide (TWA) concentrations measured by personal samplers was 63 mg/m³ in the spinning rooms and 19 mg/m³ in the viscose manufacturing departments (Krstev et al., 1993). In a viscose rayon factory in Taiwan, fixed-point airborne concentrations of carbon disulfide were 470-940 mg/m³ in the cutting areas and 47-310 mg/m³ in the spinning areas. The estimated 8-h TWA concentrations in the fibre cutting areas were 125-210 mg/m³ (Chu et al., 1995). In a synthetic fibres factory in Poland, the concentration of carbon disulfide ranged from 9.4 to 23 mg/m³ (Kuligowski, 1996). Geometric mean concentrations of carbon disulfide in a rayon factory in Singapore ranged from 8.4 to 63 mg/m³ (Yang et al., 1996a). Among workers in Taiwan in viscose manufacturing, cellophane processing and ripening, and filament spinning, concentrations of carbon disulfide were highest in the ripening area (170 mg/m³) and the filament spinning area (61 mg/m³) (Kuo et al., 1997). Levels of carbon disulfide fluctuated between <0.6 and 210 mg/m³ in a German viscose rayon factory (Reinhardt et al., 1997b). In a Belgian rayon factory, exposures ranged from 3.1 mg/m³ (centrifuge operator) to 150 mg/m³ (spinning) (Vanhoorne et al., 1991). In a Bulgarian viscose rayon production facility, levels ranged between 9.4 and 63 mg/m³ (Kotseva & De Bacquer, 2000). In a chemical company in Canada, peak exposures to carbon disulfide were in the order of 310-630 mg/m³ (Guidotti & Hoffman, 1999). The average concentrations of carbon disulfide in the workplace in China have decreased to around 10 mg/m³ in recent decades (Yang et al., 1996b; Sun et al., 1998; Lu & Wang, 1999; Q. Wang et al., 1999; Wang & Shiu, 2000).

Dermal absorption can also contribute substantially to occupational exposure for some tasks, although quantitative data were not identified. In a study of German viscose rayon workers, Drexler et al. (1995) observed that the slope of the regression between levels of carbon disulfide in personal air and of TTCA in urine was significantly greater for spinners than for other workers, particularly if they had skin diseases or skin irritation, which they attributed to the increased physical requirements and potential for dermal exposure associated with this task.

There is also exposure of laboratory workers to carbon disulfide, as a consequence of its common use as an analytical solvent, although quantitative data were not identified.

Carbon disulfide is absorbed primarily via the lungs but also through the skin. At the outset of exposure, pulmonary retention is about 80% and declines steadily, reaching a plateau of about 40% within 2 h. Extensive dermal absorption takes place from liquid and even gaseous carbon disulfide, although the available data are limited. Dutkiewicz & Baranowska (1967) reported that rates of absorption of carbon disulfide, determined from analysis of aqueous solutions into which subjects had immersed their hands for 1 h, ranged from 0.232 to 0.789 mg/m² per hour. The experimental procedure was reported only briefly, and hence it is not clear whether factors (other than absorption through the skin) that would have affected these results, such as evaporation, were taken into account. Gastrointestinal absorption has been observed in experimental animals and in case reports in humans (Environment Canada & Health Canada, 2000).

Carbon disulfide is extensively metabolized, with the main metabolites being 2-mercapto-2-thiazolinone-5, thiocarbamide, and TTCA. TTCA represents 2-6% of the total carbon disulfide absorbed in humans and has been utilized for biomonitoring. The information on metabolism of carbon disulfide is somewhat limited. Few data are available on the biotransformation of carbon disulfide in humans, and the metabolic products of carbon disulfide are not completely known. Although the available data indicate that the metabolism of carbon disulfide is generally similar between humans and animals, oxidation to inorganic sulfate may not occur significantly in humans, in contrast to the results obtained in some animal studies (ATSDR, 1996). Different studies show a constant, close correlation between 8-h TWA airborne exposure to carbon disulfide and urinary TTCA concentrations. The concentration of TTCA in an after-shift urine specimen collected towards the end of the work week following exposure to an 8-h TWA of 31 mg carbon disulfide/m³ was approximately 4 mmol/mol creatinine (Lowry, 1996; Rolecki & Tarkowski, 2000).

Carbon disulfide can be metabolized in the liver by the cytochrome P-450 mono-oxygenase system to an unstable oxygen intermediate that either spontaneously generates atomic sulfur, carbonyl sulfide, and carbon dioxide or hydrolyses to form atomic sulfur and mono-thiocarbonate, yielding carbonyl sulfide and carbon dioxide in breath and inorganic sulfates and organosulfur compounds in urine. Alternatively, dithiocarbamates are formed in humans and animals by reaction with amino acids; conjugation of carbon disulfide or carbonyl sulfide with endogenous glutathione forms TTCA and 2-oxythiazolidine-4-carboxylic acid, respectively, which are excreted in urine (ATSDR, 1996).

Owing to the relatively extensive database in humans for critical neurological and cardiovascular effects, emphasis for the review of toxicological data in this context is on their contribution to assessment of biological plausibility and mode of action and characterization of other end-points for which studies in humans are limited or unavailable.

The LC₅₀ for male mice exposed for 60 min to carbon disulfide by inhalation was approximately 690 mg/m³ (Gibson & Roberts, 1972), whereas no mortality occurred in rats exposed to as much as 2470 mg/m³ for 15 h, although neurological effects were observed (HSE, 1981). The oral LD₅₀ for mice over a 24-h period was 3020 mg carbon disulfide/kg body weight. Single oral doses of up to 1260 mg/kg body weight did not cause any deaths or overt toxicity in rats, and only minimal lesions were noted at autopsy (HSE, 1981; ATSDR, 1996).

Although it has been reported that carbon disulfide is irritating to the eyes and skin in secondary accounts of seemingly limited early studies (CEC, 1988), it has not been possible to verify these data.

8.2.1 Inhalation

The repeated-dose toxicity of carbon disulfide has predominantly been investigated in specialized studies of effects on the nervous system. In numerous studies, medium- or long-term exposure of rats to carbon disulfide levels of between 800 and 2500 mg/m³ has been associated with reductions in the nerve conduction velocity in the peripheral nerves or spinal cord (Environment Canada & Health Canada, 2000). In a number of these studies, this effect was accompanied in later stages by neurological impairment and atrophy of the hind limbs and was only partially reversible upon cessation of exposure. In rats exposed to carbon disulfide, hydrogen sulfide, or both, reductions in peripheral nerve conduction velocity were observed only in those exposed to carbon disulfide, and there was no interaction between the compounds (Gagnaire et al., 1986). These reductions in nerve conduction velocity have also been observed in the central nervous system and in the optic pathway, as indicated by increased latencies and decreased amplitudes of somatosensory-, visual-, or brainstem auditory-evoked potentials in rats exposed to 2500 mg carbon disulfide/m³ for periods of 11-15 weeks (Rebert & Becker, 1986; Hirata et al., 1992). In the latter study, there was also a transient increase in the latency of some components of the brainstem auditory-evoked potential at 630 mg/m³ (Hirata et al., 1992). Bokina et al. (1976, 1979) observed deviations in visual-evoked potentials in rabbits exposed for 6 weeks to 0.2 or 2 mg/m³, but these results cannot be critically evaluated, owing to limitations in their reporting. However, it is noted that this end-point was affected only at much higher levels (i.e., 2500 mg/m³) in the (well reported) study in rats by Rebert & Becker (1986).

The reductions in nerve conduction velocity observed in animal studies are accompanied by charac-teristic histopathological lesions in the axon. In a number of studies, rats exposed to between 800 and 2500 mg carbon disulfide/m³ for between 3 and 15 months developed an axonopathy in the peripheral nerves and/or spinal cord (Environment Canada & Health Canada, 2000). The distal portions of the largest and longest myelinated axons (which are the most rapidly conducting axons) are affected first. Structural changes proceed through the development of large axonal swellings composed of disorganized masses of neurofilaments proximal to the nodes of Ranvier, followed by axonal atrophy and Wallerian-like degeneration proximal and distal to the swellings, respectively. These features are characteristic of giant neurofilament axonopathies induced by other compounds, such as 2,5-hexanedione, the neurotoxic metabolite of hexane (Graham et al., 1995).

Neurobehavioural effects have been observed in a number of studies in rats. Neuromuscular effects, most notably reductions in grip strength and gait alterations, were observed following 2-4 weeks of exposure to 1600 and 2500 mg/m³. Gait was also significantly affected after 13 weeks of exposure to 160 mg/m³, although the test values were usually within the normal range (Moser et al., 1998). Exposure to between 610 mg/m³ and roughly 800 mg/m³ or greater inhibited avoidance behaviour in short-term studies (Goldberg et al., 1964a, 1964b) and affected some measures of locomotor activity in chronic studies (Frantik, 1970; Opacka et al., 1984). In those studies that included a recovery period, these neurobehavioural effects were reversible.

The sequence of neurotoxic effects of carbon disulfide was elucidated in a recent collaborative study at the US National Institute for Environmental Health Sciences. In this study, in which rats were exposed to 160, 1600, or 2500 mg/m³, 6 h/day, 5 days/week, for up to 13 weeks, neurofilament protein cross-linking in the spinal cord was observed as early as 2-4 weeks at all exposure levels (Valentine et al., 1997, 1998). Other early indicators were increased expression of nerve growth factor receptor mRNA in the sciatic nerve (an indicator of alterations in the axon-Schwann cell relationship) (Toews et al., 1998) and gait abnormalities (Moser et al., 1998). By 4 weeks, the neuromotor alterations progressed to reductions in grip strength of the hind limbs and forelimbs (Moser et al., 1998). Axonal swelling and degeneration (Sills et al., 1998) and electrophysiological alterations (Herr et al., 1998) in the peripheral nerves and/or spinal cord occurred only in the later stages of the study and at the two highest dose levels.

The effect of carbon disulfide on lipid metabolism has been extensively studied. In several studies, exposure of rats to between 230 and 1700 mg/m³ for periods of between 6 and 15 months resulted in significant increases in serum levels of cholesterol (and often phospholipids and triglycerides). The content of total cholesterol and cholesterol esters in the aorta was sig-nificantly increased in rats and rabbits as a result of medium- or long-term exposure to 1000 mg carbon disulfide/m³. Exposure to 1000 mg carbon disulfide/m³ exacerbated the effect of an atherogenic diet on the levels of lipids in the serum, heart, or walls of the coronary blood vessels (Environment Canada & Health Canada, 2000).

There is only limited evidence of other effects induced by inhalation of carbon disulfide. In the collaborative National Institute for Environmental Health Sciences study, medium-term exposure to 160-2500 mg/m³ did not cause histopathological lesions in a range of organs (brain, heart, aorta, lung, and female reproductive tract), with the exception of the peripheral nervous system and spinal cord (Sills et al., 1998). However, there are a number of reports in which elevated exposure to levels of several hundred milligrams per cubic metre or greater affected visual function and optic nerve/retinal cellular structure in monkeys and rats, renal histopathology in mice and rabbits, and hepatic metabolism in rats and mice (BUA, 1993; ATSDR, 1996).

8.2.2 Oral

While it has often been assumed that the cardiovascular effects of carbon disulfide are secondary to its arteriosclerotic effects, the results of several studies in rats suggest that these may be the result of a direct effect on the heart. Short-term exposure of restrained and anaesthetized rats to between 126 and 253 mg/kg body weight per day had a cardiodepressive effect on electrophysiological and mechanical parameters and decreased left ventricular contractility, increased blood pressure, and caused electrocardiograph alterations indicative of myocardial ischaemia following administration of epinephrine or norepinephrine (Hoffmann & Klapperstück, 1990; Hoffmann & Müller, 1990; Klapperstück et al., 1991). However, in conscious unrestrained normotensive rats, the highest dose did not alter mean arterial blood pressure or heart rate, although it significantly reduced body weight (Hoffmann & Klapperstück, 1990).

Short-term administration of 300 mg carbon disulfide/kg body weight per day to mice was not hepatotoxic but reduced the hepatic microsomal cytochrome P-450 content and the activities of several associated mono-oxygenases (Masuda et al., 1986).

While short-term exposure of mice to between 138 and 1102 mg/kg body weight per day altered thymus weight, it was not immunotoxic, as indicated by white blood cell differentials, spleen weight, and natural killer cell activity (Keil et al., 1996).

Available data are inadequate as a basis for assessment of the carcinogenicity of carbon disulfide. Identified data are confined to a single screening study of lung tumour induction in strain A mice (Adkins et al., 1986).

There is no clear evidence from in vitro studies that carbon disulfide is genotoxic (Environment Canada & Health Canada, 2000). In several studies in bacteria, carbon disulfide did not induce point mutations in Salmonella typhimurium or in Escherichia coli, both with and without metabolic activation. In studies of mammalian cells exposed to carbon disulfide in the presence of metabolic activation, there were small and/or equivocal increases in chromatid gaps in human lymphocytes, in unscheduled DNA synthesis in diploid WI-38 cells derived from human embryonic lung tissue, and in sister chromatid exchanges in human lymphocytes. In one study (Le & Fu, 1996), in human sperm exposed to carbon disulfide in vitro, there was a significant increase in the frequency of chromosomal aberrations and in the frequency of chromosomal breaks.

Available in vivo data on the genotoxicity of carbon disulfide are limited. In male and female rats inhaling 63 or 125 mg carbon disulfide/m³, 7 h/day for 1 or 5 days, there was no significant increase in the frequency of chromosomal aberrations in bone marrow cells (Belisles et al., 1980). In contrast, Vasil'eva (1982) reported that oral exposure to carbon disulfide induced chromosomal aberrations and polyploid cells in the bone marrow of female rats and in rat embryos exposed on days 10-13 of gestation. It is difficult to assess the validity of these findings, as the reporting was brief (e.g., the statistical significance was often not indicated) and the effective dose was not reported, except to indicate that it was one-tenth of the LD₅₀.

When male rats were exposed to 63-125 mg carbon disulfide/m³, 7 h/day for 5 days, there was no significant increase in dominant lethal mutations, nor was there a dose-related increase in sperm abnormalities in rats or mice exposed according to the same protocol (Belisles et al., 1980), although lack of an effect on sperm abnormalities in positive control rats undermines somewhat the significance of these observations.

In a small number of studies, exposure of male rats to 1875 mg carbon disulfide/m³ (but not to 1090 mg/m³), 5 h/day, 5 days/week, for several weeks, affected copulatory behaviour, reducing times to mount and ejaculate. There were no clear effects on sperm counts, circulating levels of reproductive hormones, or testicular histology (Tepe & Zenick, 1984; Zenick et al., 1984).

In an early study, prolonged estrous cycle in rats was reported after exposure to 10 mg/m³ or more for 4 months (Acadzhanova, 1978). However, in a more recent study (WIL Research Laboratories, Inc., 1992), there were no effects on estrous cycling, mating index, or fertility index in rats exposed to up to 1560 mg carbon disulfide/m³, 6 h/day, before and during mating and throughout gestation. This dose adversely affected maternal weight and weight gain and increased pup mortality, decreased pup viability, and decreased live litter size, but development of the pups was otherwise unaffected. There were no effects at 780 mg/m³, except for a small increase in the length of gestation (also at 1560 mg/m³), which was within the range of historical controls.

There was a significant reduction in the number of viable fetuses in rats and mice after exposure to 2000 mg carbon disulfide/m³ for 2 h/day during pregnancy; no significant gross toxicological effects in the dams were reported at this high level of exposure (Yaroslavskii, 1969). In another early series of investigations in rats, inhalation of 100 or 200 mg/m³ for several hours daily during gestation was reported to be fetotoxic and cause malformations, most often club foot and hydrocephalus (Tabacova et al., 1978, 1983), while 10 mg/m³ reduced postnatal survival, delayed the development of postnatal milestones, and impaired motor coordination (Tabacova et al., 1981).

Behavioural changes in the offspring, most often reduced exploratory activity in open field tests, were also reported at levels between 0.03 and 200 mg/m³ (Hinkova & Tabacova, 1978; Tabacova et al., 1978, 1981, 1983). Exposure over two generations appeared to result in greatly increased sensitivity to the teratogenic effects of carbon disulfide, causing malformations at as little as 0.03 mg/m³ in the second generation, compared with 100 mg/m³ in the first (Tabacova et al., 1983). In general, no significant maternal toxicity was reported at exposure levels of 100 mg/m³ and below. However, it is difficult to evaluate the validity of these findings. The studies are generally only briefly reported, and important information (e.g., concerning maternal toxicity in each case) is often not provided. There is also some inconsistency in the findings; for example, Tabacova et al. (1981) reported that in utero exposure to as little as 0.03 and 10 mg/m³ increased motor activity in open field tests, whereas motor activity was impaired at the same doses in their other studies. Moreover, the results of subsequent studies, most of which are better reported, have generally failed to confirm the teratogenic findings reported by Tabacova and colleagues, although it should be noted that some of the studies conducted by these investigators differed somewhat in their design (e.g., exposure over two generations). In utero exposure of rats to levels of 1250 or 2500 mg/m³ did not induce a significant increase in the incidence of club foot, although it did cause maternal and fetal toxicity and minor skeletal anomalies (Saillenfait et al., 1989). (There was no effect at 625 mg/m³.) In another study (Belisles et al., 1980; Hardin et al., 1981), there was no evidence of embryo/fetotoxicity or of teratogenicity in rats exposed to levels of 63 or 125 mg/m³. However, there is some weak support for the behavioural effects reported in the early studies from a small study by Lehotzky et al. (1985), in which the latency of a conditioned avoidance response was significantly lengthened by in utero exposure to concentrations of 10-2000 mg/m³, although there was no clear dose-response.

In rabbits, inhalation of 1875 or 3750 mg carbon disulfide/m³ during organogenesis decreased fetal body weight and increased post-implantation losses. Significant increases in visceral and skeletal malformations were also observed at the higher, maternally toxic, dose level (PAI, 1991). In another study, there was no evidence of embryo/fetotoxicity or teratogenicity in rabbits exposed to much lower concentrations (63 or 125 mg/m³) prior to and during gestation (Belisles et al., 1980; Hardin et al., 1981). However, it is difficult to assess the validity of these results, owing to mortality among the dams from causes that were apparently unrelated to the chemical exposure.

There was no compound-related increase in malformations, and no clear evidence of embryo- or fetotoxicity, in rats exposed orally to maternally toxic doses of carbon disulfide (between 100 and 600 mg/kg body weight per day) during the period of organogenesis. Fetal body weights were decreased in rats exposed to 200 mg/kg body weight per day and more (Jones-Price et al., 1984a). In contrast, in rabbits gavaged with 25, 75, or 150 mg/kg body weight per day in a similar study, carbon disulfide was embryo- and fetotoxic at all dose levels (increases in percent resorptions, non-live fetuses, and affected [non-live and malformed] fetuses), although this was accompanied by maternal toxicity at the two highest doses. The highest dose also induced significant increases in the frequency of malformed fetuses (Jones-Price et al., 1984b).

As reviewed by Graham et al. (1995), it has been postulated that the axonal degeneration that underlies the central-peripheral neuropathy caused by carbon disulfide is the result of the reaction of carbon disulfide and carbonyl sulfide with protein amino groups to yield initial adducts (dithiocarbamate derivatives). The adducts decompose to an electrophile (isothiocyanate for carbon disulfide and isocyanate for carbonyl sulfide), which in turn reacts with protein nucleophiles on the neurofilaments to cause protein cross-linking. (Such cross-links have been demonstrated in vitro and in erythrocyte spectrin and neurofilaments in rats exposed to carbon disulfide [Valentine et al., 1993, 1995, 1997].) Progressive cross-linking of the neurofilament occurs during its transport along the axon, and covalently cross-linked masses of neurofilaments may occlude axonal transport at the nodes of Ranvier, ultimately resulting in axonal swelling and degeneration.

In a number of early reports of poisoning following pulmonary exposure to 1560-3125 mg carbon disulfide/m³, a range of psychiatric disturbances was reported, while concentrations of approximately 15 625 mg/m³ resulted in central nervous system depression, coma, respiratory paralysis, and death. In several case reports, ingestion of approximately 18 g caused neurological signs, cyanosis, peripheral vascular collapse, and hypothermia, followed by death due to central nervous system depression and respiratory paralysis within a few hours (HSE, 1981).

The majority of the available epidemiological studies are of workers in the viscose rayon production industry, in which there is exposure to airborne carbon disulfide, along with lesser quantities of hydrogen sulfide,⁵ at several stages during the process. Concentrations of carbon disulfide in this industry are known to have declined substantially over the several decades encompassed by the available epidemiological studies (Price et al., 1997), and the results of epidemiological studies suggest that some of the effects induced by carbon disulfide (e.g., reductions in peripheral nerve conduction velocity) are not completely reversible. In addition, it is clear that exposures are markedly higher for certain tasks (e.g., operators of spinning machinery) than for the workplace as a whole. Consequently, when possible, those studies in which exposures and/or processes were reported to have remained the same for many years, and those in which personal monitoring data were collected, have been highlighted in the following sections.

There are numerous early clinical reports of pronounced psychological and central nervous system damage following single or long-term exposure to high, but poorly characterized, levels of carbon disulfide in the rubber and viscose rayon industries. Extended exposure under such conditions led to the recognition of chronic carbon disulfide intoxication, characterized by psychoses, polyneuropathy of the lower extremities, gastrointestinal disturbances, myopathy of the calf muscles, nerasthenic syndrome, optic neuritis, and atherosclerotic vasculencephalopathy (IPCS, 1979; O'Donoghue, 1985). Exposures in the workplace were reduced in order to prevent such overt toxicity; more recent studies in which effects of these lower levels of exposure were examined are the focus of the discussion that follows.

9.2.1 Effects on the nervous system

Neurophysiological effects on both the peripheral and central nervous systems, as well as behavioural and neuropathological effects, have been reported in a number of cross-sectional studies of workers exposed to carbon disulfide in the viscose rayon industry. The most common observations are of effects on the peripheral nervous system, most often characterized by reduced conduction velocity in the motor and, in some instances, sensory nerves, and generally most pronounced in the more distal portions of the nervous system (e.g., in the lower limbs).

In an early neurophysiological study, male Finnish viscose rayon workers with long-term exposure to carbon disulfide and hydrogen sulfide at 31-94 mg/m³ (with higher peak and historical levels) were compared with unexposed paper mill workers of similar age distribution (Seppäläinen & Tolonen, 1974). In exposed workers as a whole, there were significant reductions in MCV of the deep peroneal, posterior tibial, and ulnar nerves and in slow motor fibre conduction velocities in the deep peroneal and ulnar nerves. Results were comparable in workers who were currently exposed and in those removed from exposure for a number of years.

Effects on peripheral nervous system conduction were also associated with lower exposures to carbon disulfide in a cross-sectional study of 156 white male workers employed for at least 1 year (actual mean of 12.2 years) in a US viscose rayon plant (Johnson et al., 1983). After excluding data from workers with other possible neurotoxic exposures/conditions (diabetes, excessive alcohol consumption, or elevated blood lead) and adjusting for age, exposed workers had significantly reduced MCV and amplitude ratio of muscle action potentials following peroneal nerve stimulation and reduced MCV and increased discrete amplitude of the nerve action potential in the sural nerve, compared with 233 unexposed workers at two synthetic textile plants at the same site. These differences were observed primarily in the workers who were most highly exposed at the time of the study, with median 8-h personal airborne levels of 24 mg/m³, although conduction velocities in both nerves were slightly lower in workers with moderate (median 13 mg/m³) and low (median 3 mg/m³) exposures (the mean peroneal MCV in the high, medium, and low exposed workers were 43.7, 43.4, and 41.8 m/s, respectively, compared with 45.3 m/s in the unexposed workers). Personal air sampling was conducted at the same time as the investigation of a wide range of health effects; 81.8% of exposed employees had worked in their current job assignment for the duration of their employment with the company, and, based on area samples, exposures were stable over more than 20 years prior to the study. In contrast to the findings for nerves in the legs, none of the neurophysiological variables in the ulnar nerve was associated with carbon disulfide exposure. In behavioural testing of this population, there were no remarkable findings in psychological, psychomotor, cognitive-perceptual, or vision testing, although exposed workers reported symptoms of neurobehavioural ailments signifiantly more frequently (Putz-Anderson et al., 1983).

In another study in which exposures were well characterized, there were significant reductions in MCV of the peroneal nerve, after adjustment for potential confounders (age, weight, height, glucose tolerance, and cigarette and alcohol consumption), in workers exposed to carbon disulfide (median 13 mg/m³ in personal air). There were also reductions in sensory nerve conduction velocity (SCV) of the sural nerve in workers from those departments with high exposure compared with workers from departments with low exposure (Reinhardt et al., 1997a). The authors questioned the significance of these results, based primarily on the lack of effects on other neurophysiological parameters and the lack of significant dose-response among exposed workers. However, it is considered that the changes observed by Reinhardt et al. (1997a) represent a compound-related effect.

The results of several other studies confirm that exposure to carbon disulfide at mean concentrations of 15-<30 mg/m³ is associated with reductions in MCV and SCV in the peripheral nerves, most often in the lower limbs, although exposures were not well characterized in most of these studies (Vasilescu & Florescu, 1980; Sandrini et al., 1983; Hirata et al., 1996; Takebayashi et al., 1998).

In contrast, there was little indication of effects on the peripheral nervous system in a small study of Italian viscose rayon workers who had been exposed to slightly lower carbon disulfide levels – i.e., mostly less than 10 mg/m³ (Cirla & Graziano, 1981). In this study, MCV of the peroneal nerve was non-significantly slower in exposed workers than in well matched controls. Based on needle electromyography and neurological examinations, 5 out of 50 exposed subjects had peripheral nerve impairment, compared with only 2 out of 50 controls. There were no significant differences in the results of neuropsychological testing of intelligence, performance, and memory conducted on half of the subjects.

In several studies in which exposures were substantially higher, effects on the peripheral nervous system were more pronounced, as indicated by reductions in the MCV and SCV of a wider range of nerves (including those in the upper limbs) and/or alterations in other peripheral neurophysiological variables (Gilioli et al., 1978; Ruijten et al., 1993; Chu et al., 1995; Vanhoorne et al., 1995). In the subset of these studies in which subgroup analyses were conducted, there was an exposure-response relationship, with reductions in peroneal MCV among exposed workers being related to the exposure concentrations (Gilioli et al., 1978; Vanhoorne et al., 1995) or most pronounced in workers engaged in tasks that would most likely have entailed the heaviest exposure to carbon disulfide (Chu et al., 1995).

Chu et al. (1996) reported histopathological findings in a male viscose rayon worker exposed to a TWA concentration of 125-209 mg/m³, with clinical and neurophysiological signs of peripheral neuropathy. The results of sural nerve biopsy revealed ultrastructural changes similar to those in the peripheral nervous system of animals exposed to carbon disulfide (axonal degeneration with disorganized neurofilaments) (section 8.2.1).

In four studies, workers with long-term exposure to approximately 30-90 mg carbon disulfide/m³ (often with higher historical exposures) performed significantly more poorly than unexposed workers on a variety of neurobehavioural tests, most often on psychomotor tests of motor speed or dexterity (Hänninen, 1971; Cassitto et al., 1978; Hänninen et al., 1978; De Fruyt et al., 1998). The evidence that such effects are associated with lower exposures is conflicting, although there were no remark-able differences in the results of extensive neurobehavioural testing in workers exposed to similar or slightly higher levels of carbon disulfide in several well described studies (Cirla & Graziano, 1981; Putz-Anderson et al., 1983; Reinhardt et al., 1997b; Takebayashi et al., 1998); however, there were significant increases in the frequency of reported central nervous system symptoms in some of these studies (Cirla & Graziano, 1981; Putz-Anderson et al., 1983; Takebayashi et al., 1998).

There was no clear evidence of effects on the results of electroencephalography conducted on workers exposed to carbon disulfide (Environment Canada & Health Canada, 2000), although this end-point has not been extensively investigated.

In epidemiological studies of more specific effects on the nervous system, exposure to mean levels of carbon disulfide in the range of 15-30 mg/m³ was associated with vestibular alterations, changes in the wave pattern of brainstem auditory-evoked potentials, and effects on the dopaminergic system. However, in all of these studies, the group sizes were fairly small, and there was often historical exposure to higher levels (Environment Canada & Health Canada, 2000).

9.2.2 Cardiovascular disease

In an early study, Hernberg et al. (1970, 1971, 1973; Tolonen et al., 1975) reported a significant excess of deaths from coronary heart disease over the first 5 years in 343 workers exposed to carbon disulfide in a Finnish viscose rayon plant, compared with a well matched group of workers from a paper mill (14 exposed, 3 control deaths, relative risk [RR] 4.8, P < 0.007). There were also significant increases in indicators of cardiovascular morbidity (non-fatal myocardial infarction, chest pain) and of risk factors for coronary heart disease (increased blood pressure). The workers had been exposed to airborne concentrations of carbon disulfide of 31-94 mg/m³ during the period when the study was initiated, although short-term and historical exposures were much higher. After these results were reported, exposures were reduced to less than 31 mg/m³, and the majority of the cohort was removed from exposure. In a subsequent (13-year) follow-up (Tolonen et al., 1979; Hernberg & Tolonen, 1981; Nurminen et al., 1982), there was still a significant excess of deaths from coronary heart disease, but this was entirely due to the almost 5-fold excess in the initial 5 years.

Cardiovascular mortality was significantly greater in the most highly exposed workers in a cohort of 2939 male workers at a viscose rayon factory in the United Kingdom (Sweetnam et al., 1987). Among spinners with at least 10 years of employment in the industry, who were considered to have the highest continuous exposures, mortality was significantly in excess for all causes, ischaemic heart disease (73 deaths, standardized mortality ratio [SMR] 172, P < 0.001), and other circulatory diseases combined (33 deaths, SMR 165, P < 0.01), compared with the general population. There was also a significant excess of mortality from ischaemic heart disease in non-process fitters (nine deaths, SMR 290, P < 0.01). A significant trend between mortality from ischaemic heart disease among long-term older workers and cumulative exposure score or exposure score over the last 2 years was observed. These patterns were not evident in workers who had left employment or those with less than 10 years of exposure. Based on a report of an earlier follow-up, levels in the spinning department frequently exceeded 63 mg/m³ (Tiller et al., 1968). While there was concomitant exposure to hydrogen sulfide, the excess of mortality from ischaemic heart disease was similar among workers with high-level exposure to carbon disulfide alone or to both compounds.

Findings were similar in a larger cohort of 10 418 male workers employed for at least 1 year at one of four US viscose rayon plants (MacMahon & Monson, 1988). In workers with the greatest exposure (based on their job titles – principally spinners and cutters), there was a significant excess of mortality from arteriosclerotic heart disease compared with the general population (242 deaths, SMR 124, P < 0.01); this occurred principally in workers with 15 or more years of exposure. No data were presented on exposures to carbon disulfide or other chemicals, nor on other known risk factors for heart disease.

In a historical cohort study of 3322 Dutch male viscose rayon workers, mortality from circulatory diseases was significantly increased among the 1434 workers exposed to carbon disulfide compared with the general population (Swaen et al., 1994). Among workers from the bleaching and spinning departments, who had continuous exposure to carbon disulfide, there was a significant excess of mortality from cardiovascular diseases (103 deaths, SMR 126, 95% confidence interval [CI] 103-154) and a non-significant excess from ischaemic heart disease (65 deaths, SMR 125, CI 96-162). Among these workers, mortality from cardiovascular diseases and ischaemic heart disease was inversely related to cumulative exposure, although this was estimated from personal air samples collected late in the study period, and historical exposures were most likely higher. The risk for cardiovascular disease was reported to be most pronounced 20-30 years after the first exposure. In contrast to the results of other studies (Hernberg & Tolonen, 1981; Sweetnam et al., 1987), the risk for cardiovascular mortality did not decrease after termination of exposure. No information was available on other risk factors for heart disease, but there was no excess of cardiovascular diseases in unexposed workers, who were considered to be similar to the exposed workers in terms of lifestyle.

Mancuso (1981) conducted a historical cohort study of more than 9000 males and females employed at a US viscose rayon plant. In the 26-year follow-up, there was significant excess mortality from coronary heart disease among males (453 deaths, SMR 111, CI 101-122). There were no quantitative exposure data, but the SMRs for coronary heart disease increased with increasing duration of exposure and were significantly increased in male workers employed for more than 10 years in those tasks for which exposure was considered high (spinning and twisting, maintenance, and mechanics). In females, findings were similar but less pronounced and generally not statistically significant.

Among a historical cohort of 2291 Polish viscose rayon workers who had been diagnosed with chronic carbon disulfide poisoning, there were significant excesses of deaths from diseases of the circulatory system (SMR 139, CI 125-154), including ischaemic heart disease (SMR 137, CI 114-164) and cerebrovascular disease (SMR 188, CI 143-242), and a non-significant excess of mortality from arteriosclerosis among males (SMR 120, CI 94-151) (Peplonska et al., 1996). Results were similar among women but were based on few cases and were often not statistically significant. Exposures to carbon disulfide, although apparently heavy, were poorly characterized.

There is also evidence from cross-sectional studies of overt toxicity to the cardiovascular system, most often reported as an increased frequency of angina or non-fatal myocardial infarction or of abnormal electrocardiograph. However, the increases were often non-significant and were based on small numbers of cases, and there is no clear dose-response across studies (although exposures were poorly characterized in most of these investigations) (Environment Canada & Health Canada, 2000).

As discussed in Drexler et al. (1995), shift work, as is common in viscose production, tends to be associated with a series of risk factors for cardiovascular disease and hence is a potential confounding factor in epidemiological studies. However, in those studies in which shift work was taken into account (Vanhoorne et al., 1992; Drexler et al., 1995), the results were similar to those of other studies (Environment Canada & Health Canada, 2000).

9.2.3 Risk factors for cardiovascular disease

Egeland et al. (1992) observed a significant association between increases in serum levels of low-density lipoprotein cholesterol (LDL-C) and diastolic blood pressure and increasing exposure to carbon disulfide among male workers exposed to median levels of 3-24 mg/m³ at a US viscose rayon plant, compared with unexposed workers at three synthetic textile plants, and after adjustment for potential confounders. There was no association between exposure and high-density lipoprotein cholesterol (HDL-C), triglyceride, blood glucose, or systolic blood pressure. The levels of LDL-C, total cholesterol, and diastolic blood pressure were significantly greater in the high-exposure group than in the low-exposure workers. The results of area sampling indicated that exposures were stable for more than 20 years prior to the study.

In a study of 237 Polish women exposed to levels of carbon disulfide in the same range (i.e., 16-22 mg/m³) in viscose fibre production (Stanosz et al., 1994), exposed women had significantly increased levels of total cholesterol and LDL-C and significantly reduced levels of HDL-C compared with a control group of female textile workers of similar age. Effects on these blood lipids were confined to women aged 40-55 and to those with greater than 10 years of exposure. No subgroup analyses by exposure level were conducted.

These findings are supported by two studies of viscose rayon workers (Wronksa-Nofer & Laurman, 1987; Vanhoorne et al., 1992) in which exposure to carbon disulfide at levels generally in excess of 31 mg/m³ was associated with significant increases in serum cholesterol and LDL-C and decreases in HDL-C and, in the latter study (which adjusted for several potential confounders), with increases in blood pressure.

In contrast to the above studies, findings were negative in two investigations in which exposure levels were slightly less than those for the US workers studied by Egeland et al. (1992). In a study of German male viscose rayon workers exposed to a median personal airborne concentration of 13 mg/m³ (Drexler et al., 1995), there was no association between various measures of exposure (exposure category, levels in personal air, or TTCA levels in urine) and blood pressure or blood levels of cholesterol, LDL-C, HDL-C, triglycerides, apolipoproteins, electrolytes, or glucose. HDL-C and apolipoprotein levels were associated with duration of employment in jobs with exposure, but this was also observed in the controls, and the authors suggested that this was the result of long-term shift work. Similarly, Cirla & Graziano (1981) reported no significant differences in blood pressure or serum levels of blood lipid and lipoproteins between workers exposed to mean carbon disulfide levels of 5.0-20 mg/m³ and controls who were well matched for age and a series of lifestyle factors.

There are a number of early clinical reports of decreased glucose tolerance and increased prevalence of diabetes among patients with severe carbon disulfide poisoning (reviewed in Candura et al., 1979; HSE, 1981). In some cross-sectional studies, viscose rayon workers exposed to unknown concentrations of carbon disulfide had a significantly increased prevalence of diabetes mellitus (Goto & Hotta, 1967) or decreased glucose tolerance (Goto et al., 1971; Candura et al., 1979). However, in a number of similar studies in which exposure levels were characterized, there was no association between exposure to carbon disulfide or glucose tolerance in workers exposed to mean or median concentrations of approximately 3-90 mg/m³, often with higher historical exposures (Hernberg et al., 1971; Cirla & Graziano, 1981; Franco et al., 1981, 1982; Egeland et al., 1992; Chrostek Maj & Czeczotko, 1995; Drexler et al., 1995; Takebayashi et al., 1998).

9.2.4 Effects on the eye

Exposure to carbon disulfide at levels greater than 31 mg/m³ was associated with damage to the retinal capillaries, in the form of microaneurysms or haemorrhages, in a number of cross-sectional studies. However, there appears to be considerable variation in the susceptibility to this effect among populations, and there is no clear evidence that exposure to lower levels of carbon disulfide is associated with retinopathy. In addition, such effects are of uncertain clinical significance (Environment Canada & Health Canada, 2000).

The association of exposure to carbon disulfide with other effects on the eye has not been extensively investigated. There are two reports of effects on colour vision in viscose rayon workers with current or historical exposures to carbon disulfide at levels greater than 31 mg/m³ (Raitta et al., 1981; Vanhoorne et al., 1996), while colour vision was not affected in workers exposed to lower levels (Albright et al., 1984; Ruijten et al., 1990). In these populations, there were no other effects on measures of vision, including visual acuity, visual field, eye motility, depth perception, and pupillary reaction.

9.2.5 Carcinogenicity

In those epidemiological studies in which mortality from non-cardiovascular causes was presented, there was no consistent excess of mortality from all cancers combined or from cancers at any specific site. However, the number of cancer deaths at any given site was small or modest in all of these studies (Environment Canada & Health Canada, 2000).

9.2.6 Effects on reproduction and development

With the exception of several reports of reduced libido and/or impotence in male workers exposed to (mostly) high concentrations of carbon disulfide in the viscose rayon industry (Cirla et al., 1978; Cirla & Graziano, 1981; Wägar et al., 1981; Vanhoorne et al., 1994), there is no clear evidence of effects on human reproduction and development.

Semen quality, fertility, and pregnancy outcomes were not associated with exposure of male viscose rayon workers to carbon disulfide in the better documented of the available studies (Meyer, 1981; Selevan et al., 1983; Vanhoorne et al., 1994). The potential effects of carbon disulfide on female reproduction have not been adequately investigated, and there are conflicting reports of the frequency of abnormal menstrual duration or pain/bleeding in populations of female Chinese viscose rayon workers (Cai & Bao, 1981; Zhou et al., 1988; He et al., 1996; Q. Wang et al., 1999; Zhang et al., 1999). The findings of two early overlapping, small exploratory Finnish studies (Hemminki et al., 1980; Hemminki & Niemi, 1982) of an increased frequency of spontaneous abortions associated with parental employment in the viscose rayon industry were not confirmed in several subsequent studies, some of which were of inherently stronger design, although in all cases the number of abortions was small (Cai & Bao, 1981; Selevan et al., 1983; Zhou et al., 1988; Bao et al., 1991; Lindbohm et al., 1991; He et al., 1996; Q. Wang et al., 1999; Zhang et al., 1999).⁶

In a study reported as an abstract, the incidence of all birth defects was increased (RR 2.0, CI 1.1-3.6) among Chinese women who had worked at least 6 months in a viscose factory; the difference remained after adjusting for several possible confounding factors, but was not related to estimated exposure (>10 versus <10 mg/m³) (Bao et al., 1991). No effect on terata was observed in three other small Chinese studies, where the reporting was similarly limited (He et al., 1996; Q. Wang et al., 1999; Zhang et al., 1999).

9.2.7 Other effects

There are a number of epidemiological investigations of the association between exposure to carbon disulfide and a variety of other effects, most often alterations in circulating levels of thyroid hormones, gonadotropins, or adrenal and/or testicular hormones and increases in the prevalence of diabetes or decreased glucose tolerance (Environment Canada & Health Canada, 2000). However, the findings in the available studies of these effects were inconsistent and, in some instances, contradictory and have often not been confirmed in those studies in which the study design was stronger and/or the reporting was more detailed.

The mode of action for the toxicity of carbon disulfide varies from species to species. In microorganisms, carbon disulfide may interfere with the general metabolism of a nitrifier species or with the primary oxidative reactions. In higher life forms, carbon disulfide can form dithiocarbamates, which are metal chelating, or it can form elemental sulfur during oxidative desulfurization in the liver (Beauchamp et al., 1983). Acute toxicity is confined mainly to neurotoxic effects.

Mammals appear to have relatively high tolerance to single or short-term exposure to carbon disulfide (Crookes et al., 1993). While investigations of effects on wild mammals have not been reported, effects on laboratory mammals have been extensively studied. In a flow-through inhalation study using mice, an approximate 1-h LC₅₀ for vapour exposure of 690 mg/m³ was estimated by Gibson & Roberts (1972). This was the lowest acute toxicity value identified from the literature (see section 8.1).

Taylor & Selvidge (1984) investigated the effects of gaseous carbon disulfide on bush beans (Phaseolus vulgaris) in a closed system, with three replicate exposures, and reported no effect on transpiration or photosynthesis at any of the measured concentrations (0.42 ´ 10⁶ to 5.6 ´ 10⁶ µg/m³ for 6 h) or visible injury at the single measured concentration at which it was assessed (1.0 ´ 10⁷ µg/m³). In another study, of all the reduced sulfur gases examined, carbon disulfide had the lowest internal flux rate from leaf surface to leaf interior in all three plant species (bush bean Phaseolus vulgaris, soybean Glycine max, and tomato Lycopersicon esculentum) tested (Taylor et al., 1983). This may account in part for its relatively low toxicity compared with that of other sulfur gases, since flux to the interior of a leaf is the major determinant of the ability of a compound to cause leaf injury.

Few other studies on plants were identified in the literature; however, the effects on seeds from the use of carbon disulfide as a fumigant were examined by two separate investigators (Kamel et al., 1975; Verma, 1991). The most sensitive species was seed of the wheat plant, Giza 135 variety. The reduction in germination was 55% when grains with a 15% moisture content were exposed to a concentration of 5.05 × 10⁸ µg carbon disulfide/m³ (Kamel et al., 1975). In general, seeds with higher moisture content were more sensitive. Overall, a concentration of 2.53 × 10⁸ µg carbon disulfide/m³ for a 24-h period can be considered to be without adverse effect for wheat seed when the moisture content does not exceed 15%.

Carbon disulfide fumigation affects all life stages of invertebrates with varying degrees of toxicity (Crookes et al., 1993). The 7-day LC₅₀ value for the most sensitive identified species, the mite Lepidoglyphus destructor, was 1.1 × 10⁶ µg/m³ (Barker, 1982).

In a 5-day study of the effects of carbon disulfide on nitrification in soils using sealed containers, Bremner & Bundy (1974) reported nearly complete inhibition at nominal concentrations as low as 0.5 µg/g. The ecological significance of this result is uncertain, however, because concentrations in test soils were not measured, and the effect nearly disappeared when the test duration was increased to 14 days.

Van Leeuwen et al. (1985) investigated the effects of carbon disulfide on several aquatic species, from algae to the guppy (Poecilia reticulata). Under controlled conditions in a sealed container to prevent evaporative loss, the most sensitive species was Daphnia magna, with a 48-h LC₅₀ of 2.1 mg/litre. At higher concentrations, 3 mg/litre and above, reduced hatching and developmental effects, particularly notochord deformities, were observed in the frog Microhyla ornata (Ghate, 1985). The most sensitive fish species studied was the guppy, with a 96-h LC₅₀ of 4 mg/litre (van Leeuwen et al., 1985). The 96-h EC₅₀ for the green alga Chlorella pyrenoidosa was 21 mg/litre, based on inhibition of growth (van Leeuwen et al., 1985).

11.1.1 Hazard identification

The most relevant data for hazard characterization are those from epidemiological studies of populations exposed to carbon disulfide in the workplace. In this section, the available data for those effects that are potentially critical (i.e., effects on the nervous and cardiovascular systems) are evaluated in the context of the traditional criteria for causality for epidemiological studies. (While the epidemiological data regarding the association between exposure to carbon disulfide and damage to the retinal capillaries [section 9.2.4] would satisfy some of the criteria for causality, such effects are considered to be of uncertain clinical significance. The weight of evidence for the remaining categories of effects discussed in sections 8 and 9, including carcinogenicity, genotoxicity, reproductive, developmental and other systemic or organ system effects, is considered inadequate.)

Effects on the nervous system, including neurophysiological, behavioural, and pathological effects, were reported in a large number of cross-sectional studies of viscose rayon workers (section 9.2.1). The most common, consistent findings were reduced conduction velocity in the motor and sensory nerves, generally most pronounced in the more distal portions of the nervous system (e.g., in the lower limbs). There are also a small number of reports of impaired performance on neuropsychological testing, most often on psychomotor tests of motor speed or dexterity, in workers exposed to relatively high levels of carbon disulfide.

In most instances when subgroups of the study populations were analysed separately, the reductions in nerve conduction velocities were most pronounced in those workers with exposure to the highest concentrations, those employed in tasks that were considered to entail the highest exposures, or those with the greatest cumulative exposures. Considering the most reliable studies as a whole, there was an apparent gradient in response across studies, with reductions in the nerve conduction velocities of a wider range of nerves, including those in the upper limbs, observed in the most highly exposed populations, compared with effects only on the lower limbs in populations with exposure to moderate or low concentrations, and no significant effects on conduction velocities in the least exposed populations. In addition, effects on peripheral nerve conduction velocity were generally observed at lower levels than were other effects on the nervous system, particularly the psychomotor effects, both within and between studies. Reductions in nerve conduction velocity in workers who were removed from exposure for a number of years were less pronounced than in workers who were currently exposed in some studies, but not in others.

The effects reported on the peripheral nervous system are supported by the results of studies of animals exposed over the medium or long term by inhalation, in which nerve conduction velocity in the peripheral nerves or spinal cord was consistently reduced, accompanied by histopathological lesions and biochemical changes similar to those induced by certain other compounds that cause axonopathy (e.g., 2,5-hexanedione, the neurotoxic metabolite of hexane) (section 8.2.1). In several studies in rats, exposure to carbon disulfide also affected performance in neurobehavioural testing or altered levels of catecholamines in the brain or adrenals (section 8.2.1).

While the populations of viscose rayon workers in which effects on the nervous system were observed had concomitant exposure to carbon disulfide and hydrogen sulfide, the available evidence indicates that the effects on peripheral nerve conduction velocity were due to carbon disulfide alone. In these studies, concentrations of hydrogen sulfide were typically much less than concentrations of carbon disulfide. In one study, motor tail nerve conduction velocity in rats was reduced by exposure to carbon disulfide but was not affected by hydrogen sulfide alone, nor did hydrogen sulfide modify the effect of carbon disulfide in combined exposures. Medium-term exposure of Sprague-Dawley rats to up to 114 mg hydrogen sulfide/m³ (a concentration that reduced body weights) did not cause neuropathology (CIIT, 1983).

Excess mortality from coronary heart disease has been observed in a number of cohorts of viscose rayon workers exposed to carbon disulfide. While there was inadequate account taken of factors known to affect heart disease (e.g., smoking) in most of these studies, there were consistent excesses in all of the more powerful investigations. The strength of the association was moderate to high, with relative risks ranging from 1.1 to 4.8. There was evidence of a dose-response relationship in most of the studies in which it was examined. The excesses were generally much less pronounced following elimination or reduction of exposure.

In a number of cross-sectional studies in which potential confounding factors were accounted for, occupational exposure to carbon disulfide was associated with clinical changes that increase the risk of heart disease, including increases in blood pressure and in serum levels of total cholesterol and LDL-C and decreases in serum levels of HDL-C. In those studies where internal comparisons were made, these effects were related to the extent of exposure, although there are a number of inconsistencies in this context across studies. There have been some reports of increases in overt manifestations of coronary heart disease, such as angina and coronary electrocardiograph, in workers exposed to carbon disulfide; in the available studies, however, there was often no precise information on the extent of exposure, and the increases were often non-significant and/or based on small numbers of cases.

The biological plausibility of the associations between exposure to carbon disulfide and clinical changes or adverse outcomes for heart disease is supported by the results of studies in animals, in which long-term exposure of rats to high levels of airborne carbon disulfide consistently altered lipid metabolism, resulting in increased serum levels of cholesterol and other blood lipids and exacerbating the atherogenic effects of a lipid-rich diet. Hence, the traditional criteria for causality for associations observed in epidemiological studies are fulfilled, at least in part, for cardiovascular effects associated with exposure to carbon disulfide.

There are several reports of decreased libido and/or impotence among males occupationally exposed to high concentrations of carbon disulfide, but there is no consistent evidence based on limited study of other adverse reproductive effects in humans. In experimental animals, carbon disulfide is embryo- and fetotoxic at high concentrations and causes terata at exposure levels toxic to the dam.

11.1.2 Exposure-response analyses and criteria for setting tolerable intakes/concentrations or guidance values

It is not possible at present to identify a quantitative relationship between a given decrease in nerve conduction velocity and an expected degree of loss of function. However, it is noted that nerve conduction velocity is a relatively crude indicator of effects of carbon disulfide on the nerves, because function is not impaired until axonal degeneration has actually occurred, in contrast to agents that produce demyelination or have a direct effect on conduction. An additional concern is that, although the effect is measured on the peripheral nervous system, because carbon disulfide produces a central-peripheral distal axonopathy, it is likely that the long axons of the central nervous system are also affected. Further, there is only limited capability for regeneration of the peripheral nervous system, and even less in the central nervous system. In short, while reduced nerve conduction velocity (subclinical in the key available studies) by itself may not produce an adverse health outcome, it is indicative of, and a precursor of, other changes that clearly are adverse; given the limited reversibility of this effect, a precautionary approach is warranted. Consequently, the critical effect for the characterization of exposure-response is defined as a statistically significant, compound-related decrease in peripheral nerve conduction velocity.

The lowest levels associated with the reductions in peripheral nerve conduction velocity in exposed humans are very similar among the key studies (i.e., those in which exposure and/or processes were reported to be the same for many years and for which personal monitoring data were collected), ranging from 13 to <31 mg/m³. Those levels without significant effect in the key studies are also very similar, ranging from <10 to 13 mg/m³; in both studies, however, there were reductions in nerve conduction velocity, albeit not statistically significant, in the peroneal and/or sural nerves even at these concentrations.

However, in only one of the available epidemiological studies in which there was an association between exposure to carbon disulfide and reductions in peripheral nerve conduction velocity was the exposure of the study population adequately characterized to permit quantitative exposure-response analyses – i.e., in the study reported by Johnson et al. (1983). In addition, the design of the study by Johnson et al. (1983) was among the strongest of the available studies – it was conducted on a sizable population, for which a range of exposure levels were well characterized using personal sampling and for which exposures had been stable for more than 20 years; the exclusionary criteria and analyses would have taken account of a number of potential confounders; and the study also included examination of other manifestations of effects on the nervous system, including peripheral nervous system symptoms and neurobehavioural testing.

Based on the results of the study by Johnson et al. (1983), a benchmark concentration (BMC) for the association between exposure to carbon disulfide and effects on peripheral nerve conduction has been calculated as a measure of exposure-response (Appendix 4).

To convert continuous end-points to quantal ones for which meaningful benchmark doses can be developed, it is necessary to define an "abnormal" response. An abnormal response in this case was defined as that for the 5th percentile of the unexposed population (i.e., measures more extreme than this were considered to be abnormal). The benchmark is then defined as the concentration at which the excess risk of an abnormal response is 5%. On this basis, the BMCL₀₅s (the lower 95% confidence limits for the BMC₀₅s) were 20 mg/m³ (6.3 ppm) for peroneal MCV and 31 mg/m³ (9.9 ppm) for sural SCV (Table A-1; see Appendix 4 for details of the calculations).⁷ While serum LDL-C was also significantly associated with exposure to carbon disulfide, it is noted that the weight of evidence for cardiovascular effects is not as great as for effects on the nervous system, and the BMC calculated for this end-point was greater