Screening Assessment for the Challenge

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Dodecamethylcyclohexasiloxane
(D6)

Chemical Abstracts Service Registry Number
540-97-6

Environment Canada
Health Canada

November 2008

(PDF Version - 849 KB)

Table of Contents

• Synopsis
• Introduction
• Substance Identity
• Physical and Chemical Properties
• Sources
• Uses
• Releases to the Environment
• Environmental Fate
• Persistence and Bioaccumulation Potential
• Potential to Cause Ecological Harm
• Potential to Cause Harm to Human Health
• Conclusion
• References
• Appendix

Synopsis

Pursuant to section 74 of the Canadian Environmental Protection Act, 1999 (CEPA 1999), the Ministers of the Environment and of Health have conducted a screening assessment on dodecamethylcyclohexasiloxane (D6), Chemical Abstracts Service Registry Number 540-97-6. During the categorization process, this substance was identified as a high priority for screening assessment and included in the Ministerial Challenge because it had been initially found to meet the ecological categorization criteria for persistence, bioaccumulation potential and inherent toxicity (PBiT) to non-human organisms and it is known to be in commerce in Canada.

Although the categorization exercise did not determine D6 to be a priority for assessment of potential risks to human health, a human health assessment of D6 was also conducted due to its structure and use pattern similarity to octamethylcyclotetrasiloxane (also known as D4), a high priority for assessment for both human health and ecological risks under CEPA 1999.

D6 is an industrial chemical which was not manufactured in Canada in 2006 in a quantity above the reporting threshold of 100 kg, but which is imported into the country in mixtures with other cyclic siloxanes, as a residual in silicone polymers, and in finished consumer products. From responses to a notice published under section 71 of CEPA 1999, it was determined that between 100 000 and 1 000 000 kg of D6 were imported into Canada in 2006.

D6 may be released to the environment from industrial processes such as blending, formulation and packaging and from its use as an industrial defoamer and degreaser. It is also released from the use and disposal of personal care products. Air, wastewater, and agricultural soil are the principal receiving environmental media for D6 based on its physical-chemical properties and its use patterns.   

In air, D6 is persistent with calculated atmospheric half-lives of more than 2 days. D6 has the potential to be transported over long-distances in the atmosphere. However, it has a low potential to be deposited in water or soil in remote regions. The hydrolysis half-life for D6 is expected to be longer than that of its structurally similar analogues, D4 and D5. A half-life of ~401 days was extrapolated for D6 based on the experimental hydrolysis half-lives of D4 and D5 at neutral pH and ambient temperature. D6 is therefore considered persistent under typical Canadian water conditions. In sediment, D6 is expected to have a half-life longer than 49 to 588 days under realistic Canadian sediment conditions (temperature of 5-25°C), indicating that D6 may be persistent in sediment. D6 is not considered persistent in soil, based on evidence of clay-catalysed degradation, with dimethylsilanediol being the stable hydrolysis product. It is therefore concluded that D6 meets the persistence criterion as set out in the Persistence and Bioaccumulation Regulations.

The empirical bioconcentration factor is less than 2000, consistent with decreased bioavailability of the substance due to its high log Kow value. The read-across laboratory biomagnification factor  values from D4 and D5 indicate that D6 does not biomagnify in the test fish from dietary uptake. Although a modelled bioaccumulation factor over 5000 suggested that the bioaccumulation potential of D6 is high, the predicted bioaccumulation factor alone is not considered to provide sufficient weight to conclude that D6 has a high bioaccumulation potential due to a high degree of uncertainty in this prediction. Thus, while D6 may have some bioaccumulation potential in biota, it is concluded based on empirical bioconcentration factor data and read-across evidence, that D6 does not meet the bioaccumulation criterion as set out in the Persistence and Bioaccumulation Regulations.

Information on the toxicity of D6 to sediment-dwelling organisms is not available; a conservative read-across from D5 was applied and it is concluded that the no-effect concentration for D6 to the benthic community is above 69 mg/kg. The experimental toxicity data showed no adverse effects to pelagic aquatic organisms at concentrations up to 0.0046 mg/L, its approximate water solubility limit. However, reduced bioavailability compared to its close analogues, D4 and D5, suggests that no toxic threshold for adverse effect is expected for D6 at its water solubility limit. Given the low bioavailability and low potential for effects, it is concluded that D6 has low potential to cause ecological harm. 
 
With respect to human health, the liver was identified as a target organ for oral exposures and potentially inhalation exposures of D6. The critical effect level for repeated-dose toxicity was based on increased liver weight, periportal lipidosis and thyroid follicular cell hypertrophy in a 4-week rat study. Comparison of the critical effect level for repeated dose effects via the oral route and the upper-bounding estimates of daily intake of D6 by the general population in Canada results in an adequate margin of exposure. Based on an independent review of a refined exposure assessment for personal care products, an adequate margin of exposure was derived by comparison of the critical effect level for repeated dose effects via the oral route and the conservative upper-bounding estimate of daily intake of D6 via personal care products.

Based on the available information on its potential to cause ecological harm, it is concluded that D6 is not entering the environment in a quantity or concentration or under conditions that have or may have an immediate or long-term harmful effect on the environment or its biological diversity, or that constitute or may constitute a danger to the environment on which life depends.

Based on the available information on its potential to cause harm to human health, it is concluded that D6 is not entering the environment in a quantity or concentration or under conditions that constitute or may constitute a danger in Canada to human life or health.

This substance will be included in the upcoming Domestic Substances List inventory update initiative. In addition and where relevant, research and monitoring will support verification of assumptions used during the screening assessment and, where appropriate, the performance of potential control measures identified during the risk management phase.

Based on the information available, it is concluded that D6 does not meet any of the criteria set out in section 64 of the Canadian Environmental Protection Act, 1999.

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Introduction

The Canadian Environmental Protection Act, 1999 (CEPA 1999) (Canada 1999) requires the Minister of the Environment and the Minister of Health to conduct screening assessments of substances that have met the categorization criteria set out in the Act to determine whether these substances present or may present a risk to the environment or human health. Based on the results of a screening assessment, the Ministers can propose to take no further action with respect to the substance, to add the substance to the Priority Substances List (PSL) for further assessment, or to recommend that the substance be added to the List of Toxic Substances in Schedule 1 of the Act and, where applicable, the implementation of virtual elimination.

Based on the information obtained through the categorization process, the Ministers identified a number of substances as high priorities for action. These include substances that

• met all of the ecological categorization criteria, including persistence (P), bioaccumulation potential (B) and inherent toxicity to aquatic organisms (iT), and were believed to be in commerce in Canada; and/or
• met the categorization criteria for greatest potential for exposure (GPE) or presented an intermediate potential for exposure (IPE), and had been identified as posing a high hazard to human health based on classifications by other national or international agencies for carcinogenicity, genotoxicity, developmental toxicity or reproductive toxicity.

The Ministers therefore published a notice of intent in the Canada Gazette, Part I, on December 9, 2006 (Canada 2006), that challenged industry and other interested stakeholders to submit, within specified timelines, specific information that may be used to inform risk assessment, and to develop and benchmark best practices for the risk management and product stewardship of those substances identified as high priorities.

The substance dodecamethylcyclohexasiloxane, also known as D6, was identified as a high priority for assessment of ecological risk as it was found to be persistent, bioaccumulative and inherently toxic to aquatic organisms and is in commerce in Canada. The Challenge for this substance was published in the Canada Gazette on May 12, 2007 (Canada 2007). A substance profile was released at the same time. The substance profile presented the technical ecological information available prior to December 2005 that formed the basis for categorization of this substance. As a result of the Challenge, more than 100 submissions of information were received for this substance pertaining to its physical and chemical properties, bioaccumulation potential, persistence, ecotoxicology, quantity in commerce and so on.

Although the categorization exercise did not determine D6 to be a priority for assessment with respect to risks to human health, it was recommended that a human health assessment also be conducted due to its structure and use pattern similarity to D4, also known as octamethylcyclotetrasiloxane, a high priority for assessment for both human health and ecological risks under CEPA 1999, and due to the increased use of D6 as an alternative to D4. Therefore, this assessment focuses on information relevant to the evaluation of ecological risks and to the evaluation of risks to human health.

Under CEPA 1999, screening assessments focus on information critical to determining whether a substance meets the criteria for defining a chemical as toxic as set out in section 64 of the Act, where

"64. [...] a substance is toxic if it is entering or may enter the environment in a quantity or concentration or under conditions that

Screening assessments examine scientific information and develop conclusions by incorporating a weight-of-evidence approach and precaution. 

This screening assessment includes consideration of information on chemical properties, hazards, uses and exposure, including the additional information submitted under the Challenge. Data relevant to the screening assessment of this substance were identified in original literature, review and assessment documents, stakeholder research reports and from recent literature searches, up to August 2008 for both human health and ecological sections of the document. Key studies were critically evaluated; modelling results may have been used to reach conclusions. When available and relevant, information presented in hazard assessments from other jurisdictions was considered. The screening assessment does not represent an exhaustive or critical review of all available data. Rather, it presents the most critical studies and lines of evidence pertinent to the conclusion.

Evaluation of risk to human health involves consideration of data relevant to estimation of exposure (non-occupational) of the general population, as well as information on health hazards (based principally on the weight-of-evidence assessments of other agencies that were used for prioritization of the substance). Decisions for human health are based on the nature of the critical effect and/or margins between conservative effect levels and estimates of exposure, taking into account confidence in the completeness of the identified databases on both exposure and effects, within a screening context. The screening assessment does not represent an exhaustive or critical review of all available data. Rather, it presents a summary of the critical information upon which the conclusion is based.

This screening assessment was prepared by staff in the Existing Substances Programs at Health Canada and Environment Canada and incorporates input from other programs within these departments. This assessment has undergone external written peer review/consultation. Comments on the technical portions relevant to human health were received from Toxicology Excellence for Risk Assessment (TERA). While external comments were taken into consideration, the final content and outcome of the screening risk assessment remain the responsibility of Health Canada and Environment Canada. Additionally, the draft of this screening assessment was subject to a 60-day public comment period. The critical information and considerations upon which the assessment is based are summarized below.

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Substance Identity

For the purposes of this document, dodecamethylcyclohexasiloxane will be referred to as D6, an abbreviated name derived from the siloxane notation developed by General Electric (Hurd 1946).

D6 belongs to a group of cyclic volatile methyl-siloxanes (cVMS) with relatively low molecular weight (<600 g/mol) and high vapour pressure. These cVMS are volatile, low-viscosity silicone fluids consisting of [-Si(CH₃)₂O-]_x  structure units in a cyclic configuration. D6 consists of six of these [-Si(CH₃)₂O-] structure units (x = 6) as shown in the chemical structure below (Table 1).

It should be noted that D6 is also contained under another Chemical Abstracts Service Registry Number. This registry number, CAS RN 69430-24-6, refers to a mixture of dimethyl-substituted cyclosiloxanes of the general structure [-Si(CH₃)₂O-]_x in a cyclic configuration, where x is generally less than 8, and more commonly x  is 3-7 (SEHSC 2007a). This CAS number is associated with the following names: cyclopolydimethylsiloxane, cyclopolydimethylsiloxane (DX), cyclosiloxanes di-Me, dimethylcyclopolysiloxane, polydimethyl siloxy cyclics, polydimethylcyclosiloxane, cyclomethicone and mixed cyclosiloxane. In this report it will be referred to as cyclomethicone, a term commonly used for the mixture in the cosmetics industry.

For this assessment, data from analogue D4 and D5 have also been used based on structural similarity as shown in the table below:

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Physical and Chemical Properties

Table 2 contains experimental and modelled physical and chemical properties of D6 that are relevant to its environmental fate.

Recently, empirical log K_ow values were received and were critically evaluated. The experimental log K_ow of 4.36 for D6 was determined by using the slow stir method following OECD Guideline 123 (Miller 2006). A radio-labelled ¹⁴C-D6 was used in the test. The measurement of K_ow was carried out in triplicate with one blank control. The temperature was maintained constant at 22.5 ± 0.5°C throughout the study. Equilibrium was achieved in 7 days and the weighted average K_ow was calculated based on the sampling from the last four days. A log K_ow of 4.36 was calculated. The analysis of radio-labelled ¹⁴C-D6 in water and octanol was conducted by using liquid scintillation counting (LSC). LSC analysis does not distinguish between a parent compound and its potential hydrolysis products. The author suggested that potential water-soluble hydrolysis products may have been the cause for overestimation of the D6 concentrations in water. However, the hydrolysis potential for D6 is expected to be low under the test conditions that were used (Xu 2006). D6 is expected to have a higher log K_ow value compared to its close analogues D4 and D5 due to its lower water solubility. Considering that acceptable log K_ow values of 4.45-6.49 and 4.76-8.03 have been reported for D4 and D5 respectively, the above value reported for D6 was considered unreliable and was not considered further in this assessment.

A log K_ow value of 9.06 for D6 was estimated by Xu (SEHSC 2007d), extrapolated from the experimental K_owvalues from D4 (Kozerski and Shawl 2007) and D5 (Kozerski 2007) and is used in this screening assessment.

The log K_ow value of 5.86 was determined by Bruggeman et al. (1984) using a high-performance liquid chromatography (HPLC)-retention time method. The measurement was performed on an octadecylsilyl-bonded silica column with 90:10 methanol:water as the mobile phase. Homologous series of n-alkylbenzenes with known log K_ow values were used as reference compounds to calibrate the method. The experimental details of this study are currently unavailable.

Recent experiments on the air-water partition coefficient for D4 and D5 were conducted by Xu (Xu and Kropscott 2007). The partitioning equilibrium among air, water and an organic phase (octanol) was simultaneously achieved during the experiment. The calculated log K_oa and log K_ow values for D4 and D5 were reasonably consistent with the measured experimental values for D4 and D5 reported by Xu (2006) and Kozerski (2007) and are in good agreement with the equilibrium of log K_ow = log K_oa + log K_aw. The study is therefore considered acceptable for this screening assessment report. The average log K_aw of 2.69 and 3.13 for D4 and D5, respectively, were determined by the total D4 and D5 radioactivity in air and water. The log K_aw of D6, therefore, can be extrapolated as 3.3 in accordance with log K_aw = log K_ow - log K_oa. The extrapolated K_aw gives a Henry's Law constant of 4 950 000 Pa·m³/mol. 

For D6, other modelled physical and chemical properties are in good agreement with its measured experimental data. Except for the data discussed above, the most conservative experimental data, when applicable, are used in various model predictions in this screening assessment report.

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Sources

There are no known natural sources of D6.

D6 is an industrial chemical which was not manufactured by any company in Canada in 2006 in a quantity above the reporting threshold of 100 kg, but which is imported into the country in mixtures with other cyclic siloxanes, as a residual in silicone polymers, and as an ingredient in finished consumer products. From responses to a notice published under section 71 of CEPA 1999, it was determined that between 100 000 and 1 000 000 kg of D6 were imported into Canada, as raw materials for formulation and in finished products, in 2006 (Environment Canada 2007). The quantities of D6 imported into Canada have increased significantly since the DSL nomination (Environment Canada 1988).

D6 is a constituent of CAS RN 69430-24-6, termed cyclomethicone in the cosmetics industry. Although cyclomethicone was not directly surveyed under CEPA section 71 by Environment Canada and Health Canada in 2007, it is evident that in some cases, responses to the notice published under section 71 of CEPA 1999 for the 2006 calendar year contained data on the quantity of D6 used or imported as CAS RN 69430-24-6 (Environment Canada 2007).

The quantity of CAS RN 69430-24-6 reported in commerce in Canada during the 1986 calendar year was 2 220 000 kg (Environment Canada 1988). In 2005, Canada was a net importer of 11 500 000 kg of all types of silicone polymers and siloxanes (Will et al. 2007).

D6 is an intermediate in the production of polydimethylsiloxanes (PDMS) silicone polymers, and all PDMS contain residual amounts of volatile cyclosiloxanes, including D6. The lower molecular weight (and consequently lower viscosity) polymers may contain from < 0.1% to 0.5% volatile cyclosiloxanes, and higher molecular weight (and consequently higher viscosity) polymers may contain 1-3% volatile cyclosiloxanes. The proportion of the volatile cyclosiloxanes that consists of D6 is highly product-specific. Release of D6 from some applications of PDMS is expected to occur once the PDMS product is in use (SEHSC 2007b).

D6 has been identified as a high production volume (HPV) chemical by the Organisation for Economic Co-operation and Development (OECD 2007) and the US Environmental Protection Agency (US EPA 2007).

In the United States, there is a trend toward the increased use of volatile methyl-siloxanes, including D6, because of their exemption from volatile organic compound (VOC) legislation in 1994 (US EPA 1994a). Volatile methyl-siloxanes were used as an alternative to chlorofluorocarbons (CFCs) as a means of reducing the regulated VOC content in products (specifically, precision and electronic cleaning applications). According to information from the US EPA, the import/production of D6 in the United States was in the range of 4.5-225 tonnes in 1986, increasing to 450-4500 tonnes in 1990, 1994 and 1998, and then increasing again to 4500-45 000 tonnes in 2002.

In Europe, D6 has been identified as a low production volume (LPV) chemical, with Dow Corning Limited of the United Kingdom as the only producer/importer of D6 (ECB 2007). Currently, there are no controls on D6 in Europe. The quantity of D6 used in the European Union as a site-limited intermediate during 2003-2004 is confidential information.

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Uses

The most important use, worldwide and in Canada, of D6 is as an ingredient in the formulation of personal care products and as an intermediate in the production of polydimethylsiloxanes (PDMS) silicone polymers. As indicated above, D6 is also a constituent of CAS RN 69430-24-6, termed cyclomethicone in the cosmetics industry. Cyclomethicone is a mixture of low molecular weight volatile cyclic siloxanes, the principal ingredients of which are octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6), in varying proportions. In Canada, the most important uses of the mixtures of low molecular weight volatile cyclic siloxanes, which may contain a certain percentage of D6, are in the preparation of personal care products including hair and skin care products, and antiperspirants (Environment Canada 2007). 

All silicone polymers contain trace residual amounts of volatile cyclosiloxanes, including D6. Silicone polymers that contain trace amounts of D6 can be grouped as fluids, gums and resins. Uses of such polymers are described below.

Important uses of silicone fluids include as a formulation component of personal care products for hair and skin care, antiperspirants and deodorants; pharmaceuticals; processing aids such as defoamers; surfactants and mould release agents; lubricants; polishes and coatings on a range of substrates including textiles, carpeting and paper; sealants; architectural coatings; mechanical, heat transfer and dielectric fluids; and reprography (Will et al. 2007).

While it is anticipated that higher molecular weight polymers are used in most of these applications, D6 was reported for use as a defoamer (Environment Canada 2007). Defoamers are employed often at parts per million levels in a range of processing industries including pulp and paper, food, petrochemical, petroleum, and chemical manufacture as well as water treatment. Silicones are also used as defoamers in household products such as cleaners and detergents (Will et al. 2007).

The use of silicone formulants containing D6 in certain pesticide products is regulated in Canada under the Pest Control Products Act (PMRA 2007). 

Silicone fluid/gel mixtures are used for several types of cosmetic and medical devices. Silicone fluids have been approved as active and non-active ingredients in pharmaceuticals in Canada (DPD 2007), the most common use being in anti-flatulence drugs. Other biomedical uses of silicone fluids in Canada are in blood-handling equipment, as a blood defoaming agent, as protective barriers and lubricants, and for surface treatment of wound dressings.

Silicone gums are used in the production of elastomers that are used as sealants and adhesives, and in moulded silicone rubber, coatings and encapsulation. Silicone elastomers are used in the manufacture of consumer products such as pacifiers. Silicone elastomers are also used in a large number of biomedical applications including short- and long-term implants and prostheses, catheters, contact lenses and dentures (Will et al. 2007).

Silicone resins are primarily used in specialty coatings applications, and in the production of silicone-modified polymers (Will et al. 2007). Consumers may be exposed to D6 through the use of these products and by occupying enclosed spaces where coatings, caulking, sealants and silicone rubber are used as building materials or are present in consumer products.

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Releases to the Environment

D6 is not reported as part of Environment Canada's National Pollutant Release Inventory. This substance belongs to a chemical group used in various industry and consumer applications that are associated with the potential for widespread releases.

D6 will be released to the environment during the use of personal care products such as hair and skin care products, antiperspirants and others and these releases will be to air and wastewater. It is estimated that more than 90% of D6 used in personal care products enters the atmosphere (Allen et al.1997). D6 may also be emitted from industrial processes in which it is used in blending, formulation and packaging operations, and as intermediate in producing PDMS polymers. All of these operations take place in Canada (Environment Canada 2007). Industrial releases of D6 may also occur when silicone polymers are used in process industries as foam control agents, as mould release agents, as lubricants, and in other applications (Environment Canada 2007). The releases from industrial processes are expected to be to the atmosphere and wastewater. The release of D6 to sewage treatment plants as wastewater can lead to its association with sludges that may then be sent to landfills, incinerated or be applied to agricultural soils as soil enrichment. Disposal of consumer and industrial products containing D6 can also lead to the transfer of D6 to landfills.

Detection of D6 at sewage treatment plants, landfills and near industrial plants as well as in indoor and ambient air away from industrial activity is evidence that both point sources and disperse sources contribute to the concentration of D6 in the environment (Norden 2005, Kaj et al. 2005). The application of D6-containing pesticides on crops and the disposal of sewage sludge on agricultural lands or in landfills will result in the release of D6 to environmental media. There is some evidence that D6 is a transient degradation product of PDMS in contact with soil, while the principal degradation products are silanols (Xu et al. 1998). Thus, in addition to release of residual D6 from PDMS manufacture, there may be de novo synthesis of D6 occurring in landfills and agricultural lands where sewage sludge containing PDMS is spread, although the overall contribution of PDMS degradation is not considered significant under environmental conditions.

Mass Flow Tool

To estimate the potential release of D6 to the environment at different stages of its life cycle, a Mass Flow Tool was used. Empirical data concerning releases of specific substances to the environment are seldom available. Therefore, for each identified type of use of the substance, the proportion and quantity of release to the different environmental media are estimated, as are the proportions of the substance chemically transformed or sent for waste disposal. Assumptions and input parameters used in making these estimates are based on information obtained from a variety of sources including responses to regulatory surveys, Statistics Canada, manufacturers' websites and technical databases. Of particular relevance are emission factors, which are generally expressed as the fraction of a substance released to the environment, particularly during its manufacture, transformation, and use associated with industrial processes. Sources of such information include emission scenario documents, often developed under the auspices of the Organisation for Economic Co-operation and Development (OECD), and default assumptions used by different international chemical regulatory agencies. It is noted that the level of uncertainty in the mass of substance and quantity released to the environment generally increases further down the life cycle.

Based on the Mass Flow Tool (Table 3), the substance is mainly released to air and wastewater (sewer). Air receives the highest potential of releases (77.7%) as a result of the use of consumer products such as skin creams or sun creams and also volatilization of residues in silicone polymers, especially during the first year of use. Releases to wastewater are estimated to be approximately 12.3% from industrial use during on-site formulation and from consumer use of the personal care products. Another 9.8% of the substance is transferred for waste disposal.

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Environmental Fate

Based on its physical and chemical properties (Table 2) and the results of Level III fugacity modelling (Table 4; model input parameters are listed in Appendix 5 of this screening assessment), D6 may partition in significant quantities to any environmental medium, depending on the compartment of release.

Based on the information available (Table 3), the environmental release of D6 is estimated to be mainly to air (77.7%). A vapour pressure of 4-4.73 Pa and a Henry's Law constant of 4 950 000 Pa·m³/mol, as well as a long half-life in air, indicate that when D6 is released to air it will remain there (> 99%) (Table 4).

When D6 is released to water (12.3%, Table 3), it is expected to adsorb to suspended solids, such as sewage sludge and sediments, based on its high estimated log K_oc values (> 6). Results of the Level III fugacity simulation for release to water show that approximately 83% will reside in the solid phase (suspended sediment and bed sediments) and 16% will reside in the aqueous phase (water column). Although volatilization from water surfaces is also expected based upon the air-water partition coefficient (K_aw), the high sorption of D6 by solid matter significantly reduces D6's volatilization (0.9%, Table 4).

When D6 is released to moist soils through, for example, application of sewage sludge on agricultural lands, it is expected to adsorb strongly to soil (i.e., expected to be immobile), which reduces the potential for volatilization and further partitioning. The results of the Level III fugacity model support this idea, showing that if D6 is released to soil, 68.1% will remain in soil associated with solids, for the same reasons as described for sediment. Approximately 31.8% of the mass fraction is estimated to partition to air. This estimate is consistent with the observation by Xu (1999) that volatilization is the major loss process of cyclic siloxanes from moist soils. However, the percent volatilization of D6 is less than that of D5 (~71%) due to its high absorptivity to solid matter (K_oc > 6). In dry soil, D6 is expected to be quickly hydrolyzed by clay minerals to form dimethylsilanediol as the final breakdown product (Xu 1999, Xu and Chandra 1999).

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Persistence and Bioaccumulation Potential

Environmental Persistence

Atmospheric Degradation

The Level III fugacity model results indicate that D6, when released to air, will mainly remain in air, where it is expected to be slowly oxidized by the gas-phase reaction with photochemically produced hydroxyl (OH) radicals. No experimental data are available, but the atmospheric oxidation half-life for D6 is predicted to be ~6 days based on a model calculation assuming hydroxyl radical concentrations of 1.5 × 10⁶OH/cm³ (Table 5b). The half-lives of 2.6-12.8 days can be estimated by AOPWIN based on a measured daily average hydroxyl concentration of 3.5 × 10⁶OH/cm³ and 0.7 × 10⁶ OH/cm³(daily average concentration = maximum concentration/2), in summer and winter respectively (Ren et al. 2003, 2006). It is therefore concluded that D6 could be degraded more rapidly in urban centers in summer seasons when the atmospheric hydroxyl radicals are highest. However, when a yearly average removal half-life is estimated, it is consistent with the half-life of ~6 days estimated with hydroxyl radical concentration of 1.5 × 10⁶OH/cm³. D6 is not expected to react, or react appreciably, with other photo-oxidative species in the atmosphere, such as O₃, nor is it likely to degrade via direct photolysis (Atkinson 1991). Therefore, it is expected that reactions with hydroxyl radicals will be the most important fate process in the atmosphere for the substance. Therefore, the degradation half-life of 5.69 days is considered critical and will be used for D6 in environmental fate modelling.

Navea et al. (2007) have investigated the effects of ozone, aerosols and solar radiation on the fate of D4 and D5 in a simulated environment chamber. They concluded that mineral aerosols such as kaolinite and hematite can significantly accelerate the removal of D4 and D5 from the gas phase of the atmosphere, especially under daytime conditions. The finding also indicated that ozone can further accelerate these removal processes for D4 and D5. Although obtained data suggested that mineral aerosols, combined with ozone, may have significant effects on the environmental fate of cVMS in the air, it is difficult to quantitatively extrapolate the results of the simulations to realistic environmental conditions. First, it should be noted that the study was conducted under unrealistically high concentrations of cVMS, mineral aerosols and ozone. Second, mineral and carbon black samples used in the study were high-purity (> 99%) analytical samples that provided maximum surface area and thus, maximally available sorption sites, i.e., ideal conditions for D4/D5 absorption. The degree to which these pure minerals are representative of the particulate matter in air is questionable. Third, it is reasonable to believe that minerals such as kaolinite and hematite can be found in atmospheric particular matter (PM); however, they are unlikely to be the most common and abundant components in atmospheric dust. In addition, it should be mentioned that the study was conducted in a simulated environment chamber and involved reacting the mineral aerosols with only one cVMS (D4 or D5) at a time. Under actual environmental conditions, thousands of chemicals compete for aerosols' adsorption sites. Therefore, in such conditions, the "effectiveness" of D4/D5 removal from the ambient air could be significantly lower than that observed in the chamber's mono-component atmosphere. 

Thus, it may be concluded that the degree to which aerosols and ozone accelerate the degradation of cVMS in air under realistic environmental conditions is uncertain.

Thus, the data demonstrate that this substance is persistent in air (half-life > 2 days) in accordance with the Persistence and Bioaccumulation Regulations (Canada 2000).

Degradation in Water and Sediment

Empirical hydrolysis data are not available for D6. Therefore, a read-across approach was applied using data from other similar siloxanes. Experimental hydrolysis studies were available for its structurally similar analogues D4 (Durham and Kozerski 2005) and D5 (Durham 2006) and were critically reviewed by internal experts (Bidleman 2008); the results of these studies and critical reviews are summarized below. The hydrolysis kinetics of D4 and D5 were determined by measuring the disappearance of radio-labelled parent compounds as a function of time based on OECD Guideline 111. Reactions were investigated in flame-sealed borosilicate glass tubes at pH of 4, 7 and 9 and temperatures of 10°C, 25°C and 35°C. Additional tests were performed for D5 at pH of 5.5 and 8, and temperature of 25°C. The hydrolysis rates of D4 and D5 were reported to be pH-dependent and followed pseudo first-order kinetics. Both D4 and D5 were found to undergo hydrolysis, with half-lives (t_½) of 0.04-45 days for D4 and 0.9-733 days for D5 under realistic Canadian environmental conditions (pH 6-9, temperature 5-25°C) (GEMStat 2008, NOAA 2008). The hydrolysis half-lives increased from D4 to D5. Therefore D6, structurally similar to D4 and D5, will be expected to undergo hydrolysis in the Canadian environment (pH 6-9, temperature 5-25°C) with half-lives of > 0.9-733 days. A half-life of ~401 days was extrapolated for D6 at neutral pH and under ambient temperature based on the hydrolysis data of D4 and D5 (Xu 2007).

Recent information received on microbial degradation indicates that D6 is not likely to be biodegraded in water. The 28-day ready-biodegradability test was performed in sealed vessels in accordance with OECD Draft Guideline 310 and data showed limited biodegradation (4.47%) of D6 over 28 days in a ready-biodegradation test (Springborn Smithers Laboratories 2005). This is approximately equal to a half-life of 411 days assuming first-order rate kinetics. These data are further supported by two of the models in Table 5b. These models indicate that the probability of biodegradation of D6 in water is effectively zero. Therefore, D6 is expected to be persistent in water (t_½> 180 days). Also, BIOWIN (2000) reported an overall weighted conclusion of "not readily biodegradable" based on the combined results of the BIOWIN3 and BIOWIN5 models.

Experimental and modelled biodegradation data indicate that D6 has little potential to biodegrade in aqueous environments. Therefore, hydrolysis is likely the major degradation process for D6 in water. The read-across data based on D4 and D5 hydrolysis half-lives and the extrapolated half-life for D6 suggest that D6 is expected to persist for long periods under Canadian water conditions. It is therefore concluded that D6 meets the criterion of persistence in water (t_½ > 182 days) under the Persistence and Bioaccumulation Regulations (Canada 2000).

No sediment degradation information was available for D6. However, a preliminary degradation study for D4 in a water/sediment system has been received recently (Xu and Miller 2008). A modified OECD 308 guideline was followed. The study was conducted at ambient temperature (22-25°C) with natural sediment (sandy silt, high OC content, ~70% water content and 11% organic matter, pH ~7) and water collected from deep under an uncontaminated lake. Radio-labelled D4 in diethylene glycol methyl ether was added via syringe at 10-15 locations on the surface sediment in each flask after the overlying water was carefully removed. Overlying water was again added onto the spiked sediment with minimum disturbance of sediment. Spiking of sediment instead of water ensured the substance's distribution in sediment. This properly addressed the substance's specific physical and chemical properties (high volatility and potential hydrolysis) and improved the reproducibility of the study. The concentrations of D4 measured from day 6 to day 22 (test termination) indicated that a steady state had been reached between water and sediment, with more than 95% D4 and radioactivity being detected in sediment. As demonstrated in the hydrolysis study of D4, the degradation products in sediment/water were oligomer diol intermediates, HO(Me2SiO)_nH (n=2-4), while dimethylsilanediol (DMSD) was the final degradation product. The calculated half-life for D4 degradation in sediment was 49 days at 22-25°C. The same degradation products observed in the study and in the hydrolysis study of D4 suggested that hydrolysis was the major degradation process in the sediment/water system. The major uncertainty in the study is the lack of test replicates. Since no data were available for degradation half-lives at lower temperatures, a read-across approach using the D4 hydrolysis data was applied. The hydrolysis half-lives of D4 in water were ~ 6-12 times longer when water temperatures were decreased to 5-10°C from 25°C. Assuming a similar trend of decreases in sediment, the estimated half-lives for D4 in sediment are 294 and 588 days, at 10°C and 5°C, respectively.

The sediment degradation half-lives of D4 are considered as read-across for D6 based on the similarity of the two substances. It is therefore concluded that D6 under some (colder) Canadian environmental conditions will have a half-life in water of > 182 days and in sediment of > 365 days. D6 is thus considered persistent in water and sediment in accordance with criteria defined under the Persistence and Bioaccumulation Regulations (Canada 2000). The extrapolation from D4 sediment degradation at lower temperatures based on hydrolysis, however, is not without uncertainty. 

Degradation in Soil

Although no empirical data on biodegradation in soils are available, biodegradation of D6 in water is negligible as noted above, based on the ready-biodegradation test (Springborn Smithers Laboratories 2005) and predictions of five of the six biodegradation models (Table 5b).

Xu (Xu 1999, Xu and Chandra 1999) has extensively investigated potential degradation pathways of cyclosiloxanes, including D4, D5 and D6, in Wahiawa soil from Hawaii at room temperature and 32% relative humidity. He concluded that the ring-opening polymerization reaction to form polydimethylsiloxane (PDMS) and the demethylation for cyclosiloxanes were insignificant in soils at concentrations < 200 mg/kg dry weight. Clay-catalyzed hydrolysis of D6 was observed in highly weathered Wahiawa soils under dry soil conditions. The degradation half-life of D6 on Wahiawa soil (high clay content of 55% with water content of 2.1%) was 1.38 days under dry soil conditions. It was suggested that the dryness of soil severely limits biological activity but promotes abiotic reactions such as surface-acid-catalyzed hydrolysis of PDMS, a polymer with the same dimethylsiloxane backbone as cyclosiloxanes (Xu 1999). The degradation rates of cyclosiloxanes were determined by soil moisture, clay type and clay content, as well as the size of the siloxane molecules that determine the rate of diffusion to the surface catalytic sites. The degradation rate of D6 was much slower than that of D4 and D5 under the same soil conditions (~1 and ~2 hours half-life, respectively for D4 and D5) and supported the statement that degradation rates among the three cyclosiloxanes were expected to decrease as the molecular weight increases: D4 > D5 >> D6. The degradation half-lives of D4 on Londo soil were 3.54-5.25 days at relative humidity of less than 93%. A slower degradation half-life would be expected for D6 in the temperate Londo soil. The weight of evidence suggests that D6 may undergo clay-catalyzed hydrolysis and may not be persistent under dry soil conditions. The lack of hydrolysis degradation of D4 in water-saturated Londo soil also suggests that degradation of D6 is negligible under the same conditions. Volatilization becomes the major loss mechanism for D6 under such soil conditions in an open system, and at a slower rate than D4 and D5.

Based on the available empirical studies that show the potential for rapid clay-catalyzed hydrolysis in surface soils, D6 is not considered persistent in soil, since its degradation half-life is less than the criterion of t_½ > 182 days stated in the Persistence and Bioaccumulation Regulations (Canada 2000).

The available empirical and modelled data indicate that D6 meets the persistence criteria for air (half-life ≥ 2 days), water (half-life ≥ 182 days) and sediments (half-life ≥ 365 days) as set out in the Persistence and Bioaccumulation Regulations (Canada 2000), but that it does not meet the half-life criterion for soil (i.e., its half-life is > 182 days).

Long-range Transport Potential

The Transport and Persistence Level III model (TaPL3 2000), a regional model, was used to estimate the characteristic travel distance (CTD) of D6. CTD is defined as the maximum distance travelled in air by 63% of the substance. Beyer et al.(2000) have proposed that CTDs of > 2000 km represent high long-range atmospheric transport potential (LRATP), those of 700-2000 km represent moderate LRATP, and those of < 700 km represent low LRATP. Based on the CTD estimate of 2966 km, the long-range atmospheric transport potential of D6 is judged to be high. This means that D6 is subject to atmospheric transport to remote regions such as the Arctic.

The OECD POPs Screening Model can be used to help identify chemicals with high persistence and long-range transport potential (Scheringer et al. 2006). The OECD model is a global model which compartmentalizes the earth into air, water and soil. This model is "transport-oriented" rather than "target-oriented," as it simply identifies the CTD without indicating specifically where a substance may be transported (Fenner et al. 2005). Klasmeier et al. (2006) have suggested that a threshold of 5098 km, based on the model's CTD estimate for 2,2',3',4,4',5,5'-Heptachlorobiphenyl (PCB-180), can be used to identify substances with high long-range transport potential. PCB-180 has been found in remote regions. The CTD calculated for D6 using the OECD model is 2963 km, indicating that D6 still has a significant potential for transport in air, but is below the boundary suggested for global pollutants by Klasmeier et al. (2006).

The OECD POPs Screening Model also calculates the transfer efficiency (TE), which is the percentage of emission flux to air that is deposited to the surface (water and soil) in a remote region (TE = D/E × 100, where E is the emission flux to air and D is the deposition flux to surface media in a target region). The TE for D6 was calculated to be 1.6E-06%, which is well below the boundary of 4.65E-04% (2,4,4'-trichlorobiphenyl, or PCB-28) established for the model's reference substances that are empirically known to be deposited from air to soil or water. The low TE means that D6 has the potential for long-range travel in the atmosphere without being deposited to Earth's surface in any particular remote region. In addition, the log K_oa and log K_aw of D6 suggest that it will also have a low Arctic contamination potential (ACP) when examined using chemical partitioning space plots as described by Wania (2003, 2006).

A preliminary monitoring study of a remote ecosystem was conducted in Lake Opeongo, the largest lake in Algonquin Provincial Park, Ontario, Canada. The lake is relatively remote from potential sources of cVMS from sewage and runoff. Therefore, it was assumed that the only significant source of cVMS to the lake would be from atmosphere deposition (Powell 2008). Preliminary analysis of sediments and zooplankton samples for cVMS found no D6, suggesting that atmospheric deposition is not a significant source of D6 to Lake Opeongo. Limits of detection were 19.0 ng (background corrected mass) for both sediments and zooplankton.

It is therefore concluded that D6 has the potential to be transported over long distances in the atmosphere. However, the modelled TE for D6 is low, which suggests that it lacks the potential to be deposited in water or soil in remote regions. The monitoring results of Lake Opeongo also supported the low atmospheric deposition potential for D6. It is expected that airborne D6 will be eventually degraded by hydroxyl radicals in air.

Potential for Bioaccumulation

The empirical and modelled log K_ow values for D6 (Table 2) suggest that this substance has the potential to bioaccumulate in biota.

In the Aquatic Compartment

New experimental data were received on bioconcentration studies in aquatic organisms. A bioconcentration study of ¹⁴C-D6 (radiochemical purity 99.57%) in fathead minnows (Pimephales promelas) was carried out in a flow-through system at concentrations of 4.4 µg/L and 0.41 µg/L for 49 days, followed by 98 days of depuration (Drottar 2005). The mean measured steady-state bioconcentration factor (BCF_ss) was calculated to be 1160 L/kg wet weight based on concentrations measured from day 35 to day 49. The kinetic bioconcentration factor (BCF_k) was calculated to be 1660 L/kg based on the uptake and depuration rates (k₁/k₂) for the 0.41 µg/L treatment groups. Fish tissue analysis also indicated that the depuration half-life for radio-labelled D6 was 30 days. The low BCF values for D6 (compared to D4 BCF_ss at 12 600 and BCF_k at 13 400; and D5 BCF_ss at 7060 L/kg and BCF_k at 13 300) could contribute to the low bioavailability of D6 (higher molecular weight and log k_ow ~9 compared to its structurally similar analogues, D4 and D5) and may also be caused by D6 adsorbing onto suspended particles (unconsumed food and feces).

Experimental data have been received on dietary bioaccumulation studies of D4 and D5 (Drottar 2006, 2007; Domoradzki 2008a, 2008b; SEHSC 2008b). The resultant lipid-normalized BMF values for D4 and D5 are in the range of 0.6-0.9. It is therefore considered that D6, a close analogue of D4 and D5, may have similar low biomagnification potential as suggested by the laboratory fish dietary studies of D4 and D5.

Another bioconcentration study was carried out for invertebrates (Dow Corning Corporation 1985). Daphnia magna were exposed to a concentration of 4 µg/L of [¹⁴C] D6 for 32 days. The steady state was achieved after 21 days of exposure and the BCF_ss was calculated as 2400 L/kg wet weight. The study was not well documented. The sources and acclimation of the test organisms, test conditions, and the test system design were not detailed in the study. A critical review of the study indicated the reliability of the study to be low.

The Arnot-Gobas model (Arnot and Gobas 2003) can be used to predict the bioaccumulation factor (BAF) of this substance, while taking into account any potential metabolism using a metabolic rate constant (k_M). The available BCF in vivo test data were used to derive an in vivo-based metabolic rate constant according to the method of Arnot et al. (2008a). In this method, k_M is derived according to the following equation:

where

The method of Arnot et al. (2008a) provides for the estimation of confidence factors (CF) for the k_M to account for error associated with the in vivo data (i.e., measurement variability, parameter estimation uncertainty and model error). A mean CF of ± 2.7 was calculated for the available BCF data.

Because metabolic potential can be related to body weight and temperature (e.g., Hu and Layton 2001, Nichols et al.2007), the k_M was further normalized to 15°C and then corrected for the body weight of the middle trophic level fish in the Arnot-Gobas model (184 g) (Arnot et al.2008b). The middle trophic level fish was used to represent overall model output as suggested by the model developer (Arnot, personal communication with Bonnell M. of Environment Canada 2008; unreferenced) and is most representative of fish weight likely to be consumed by an avian or terrestrial piscivore. After normalization routines, the k_M ranges from ~0.002 to 0.01 with a median value of ~0.005.

The calculated k_M values based on in vivoexperiments suggest that the rate of metabolism of D6 is quite low (≤ 0.01 day-1 at best). The experimental BCF study on fathead minnows (Drottar 2005) demonstrated certain metabolism, even though limited, of D6. The calculated BCF of 302 using the lower percentile rate constant (~0.002) is considerably lower than the BCF values reported by Fackler et al. (1995); this rate constant is therefore considered a best case. A corresponding BAF of 323 594 was calculated for D6 for fish in Canadian waters (Table 7) using this metabolic rate constant.

In the Sediment Compartment

No bioaccumulation information for D6 was available for the sediment compartment. However, biota-sediment accumulation factor (BSAF) studies on benthic organisms are available for its structurally close analogues, D5 (Springborn Smithers Laboratories 2003) and D4 (Kent et al. 1994). The read-across data were used in the weight-of-evidence approach to determine the sediment accumulation potential of D6, even though D6 is expected to be even less bioavailable (greater sorption to organic matter) in sediments than D5 and D4, and consequently may exhibit lower accumulation potential.

The biota-sediment accumulation factors (BSAFs) for D4 were calculated using data from a Chirononmus tentans (midge) sub-chronic toxicity study (Kent et al. 1994). Midgewere exposed to D4 in sediment of low (LOC), medium (MOC), and high (HOC) organic carbon content ranging from 0.27% to 4.1%. The average BSAFs were calculated (BSAF = C_{organism tissue}mg/kg / C_sediment mg/kg) to be 0.7, 1.3 and 2.2, respectively, for the HOC, MOC and LOC phases. Biota-sediment accumulation factors were calculated for D5 using data from a midge (Chironomus riparius) acute/chronic toxicity study (Springborn Smithers Laboratories 2003). The midge larvae were exposed to D5 concentrations ranging from 13 to 180 mg/kg. Tests were conducted with sediments of 2.0% organic carbon content. The midge BSAFs were calculated to be 1.2, 1.1, 0.83 and 0.46 at treatment levels of 13, 30, 73 and 180 mg/kg dry weight, respectively.

The empirical results indicate that both D4 and D5 may have some potential, though limited (BSAF < 2), to bioaccumulate from sediment exposures. The BSAF data of D5 may be used as read-across values for D6 as a worst-case conservative scenario. D6 is therefore considered to have some potential to bioaccumulate through exposure to sediment, although it is likely that D6 would exhibit a lower BSAF due to its reduced bioavailability. However, neither sediment accumulation study specified whether the gut contents of test organisms had been purged before calculation of BSAF values. The BSAF value may thus be over-estimated due to the presence of test substances on sediments within the gut of the invertebrates.

In the Soil Compartment

No bioaccumulation information for D6 was available for the soil compartment.

In the Terrestrial Compartment

The Gobas mass-balance bioaccumulation model for terrestrial organisms (Gobas et al. 2003) uses a chemical's octanol-air and octanol-water partition coefficient (K_oaand K_ow) to estimate the chemical's biomagnification (BMF) potential in terrestrial food chains. It was estimated that chemicals with a log K_oa > 5 can biomagnify in terrestrial food chains if log K_ow > 2 and the rate of chemical transformation or metabolism is low. A log K_oa of 5.76 indicates that D6 may have the potential to biomagnify in terrestrial food chains. However, the metabolism of cVMS demonstrated in laboratory mammals could reduce the biomagnification potential in the terrestrial food web.

Summary of the Bioaccumulation Potential of D6

Overall, there is conflicting evidence for the bioaccumulation potential of D6. Empirical bioconcentration testing has shown that the BCF is < 5000, likely a result of the lower aquatic bioavailability of D6 compared with D4 and D5 (as suggested by the physical and chemical properties of cVMS). The bioconcentration potential in other aquatic and benthic organisms at other trophic levels is also expected to be low based on the low reported BCF for fish and expected low bioavailability.

The predicted BCF, corrected for metabolism, is also relatively low compared with D4 and D5. The predicted BAF, also corrected for metabolism, is quite high and exceeds 5000. The mass-balance kinetic model used is based on "first principles," meaning that the most important domain of the model is that a chemical obeys the principal mechanism of the model, in this case passive diffusion. D6 meets this domain and is within the model's log K_owand molecular weight boundaries as well. Therefore, the predictions for bioaccumulation are considered to be applicable to D6. However, there is less agreement between metabolism-corrected BCF values and empirical BCF values, suggesting that BCF and BAF model results for D6 have a high degree of uncertainty due to the relative absence of empirical BCF and BAF values at D6's range of log K_owused in the model (i.e., ≥ 9). Also, the possibility that super-hydrophobic chemicals do not reach steady state in the environment (which the model assumes), adds to the uncertainty (Arnot and Gobas 2006).

Sediment BSAF values for D4 and D5 would suggest a relatively low level of accumulation in sediment macroinvertebrates for D6 as well. As this is the only sediment bioaccumulation test available for cVMS and there are no predictive models for sediment organisms, testing or field evidence at more realistic environmental loadings would help verify these values. No fish BMF studies have been conducted for D6. Using D4 and D5 BMF testing for read-across purposes, it is likely that the biomagnification factor in fish for D6 would also be low, given its lower bioavailability. However, there is currently no evidence to suggest that this may be the case for other trophic levels. Field mesocosm studies are currently underway to examine trophic transfer of cVMS in aquatic food webs, but these data are not yet available for full evaluation and have not currently been considered for this assessment. 

Currently, there is only a modelled BAF > 5000 to suggest that the bioaccumulation potential of D6 is high. This value used alone as the only measure of high bioaccumulation potential presents some degree of uncertainty when other evidence suggesting low bioaccumulation potential is considered. Given the conflicting evidence, the predicted BAF alone is not considered to provide sufficient reliable weight to conclude that D6 has a high bioaccumulation potential. It is, however, reasonable to conclude that D6 has some bioaccumulation potential in biota, but this potential is likely to be lower than that of D4 and D5. It is therefore concluded that based on the lack of high bioconcentration potential as well as read-across evidence for low bioaccumulation via dietary exposures in fish, D6 does not meet the bioaccumulation criterion (BCF or BAF ≥ 5000) as set out in the Persistence and Bioaccumulation Regulations (Canada 2000).

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Potential to Cause Ecological Harm

Ecological Effects Assessment

A - In the Aquatic Compartment

New empirical ecotoxicity data have been received for D6 and are discussed in detail below (Table 8a). Both studies indicate this substance does not exhibit adverse effects on fish and Daphnia exposed at concentrations up to its water solubility limit (~0.005 mg/L).

Juveniles of Daphnia magna were exposed to D6 concentrations of 0.27, 0.57, 1.2, 2.5 and 4.6 µg/L (measured) for 21 days under static-renewal conditions, and the test solutions were renewed every 24 hours (Springborn Smithers Laboratories 2006). Acetone was used as a solubilizer at a concentration of 0.1 mg/L. The pH was maintained between 7.4 and 8.4, temperature oscillated between 18 and 21°C, and dissolved oxygen remained at > 60% of saturation. The test water had a hardness of 120-190 mg CaCO₃/L. No mortality, reduced reproduction or other biologically significant effects were observed at the highest test concentration of 4.6 µg/L (no-observed-effect concentration ≥ 4.6 µg/L). The temperature control and water hardness deviated from OECD Guideline 211, but were not considered to significantly limit the test. However, feeding during the test was 0.3 mg C/daphnid/day–higher than the OECD guideline of 0.1-0.2 mg C/daphnid/day–which is likely to favour the health of the test organisms and increase their resistance to toxic effects. A higher carbon load to the test system may also act to reduce bioavailability of D6 by providing a sorption substrate. Nonetheless, these experimental data were considered acceptable for the purposes of this screening assessment, noting possible test limitations as mentioned.

The aquatic toxicity of D6 to fathead minnow (Pimephales promelas) was examined during a BCF study as described in the "Potential for Bioaccumulation" section (Drottar 2005). Juveniles of fathead minnow were exposed to concentrations of 0.41 and 4.4 µg/L (measured) for 49 days under flow-through conditions. No biologically significant effects were observed at the highest test concentration of 4.4 µg/L (NOEC ≥ 4.4 µg/L) during the 49-day fish uptake phase. Although the fish toxicity was determined as part of a BCF study; the test design is not equivalent to a long-term fish early-life stage toxicity test. No experimental data on the toxicity of D6 to early-life stages of fish is available for D6. However, a 93-day fish early life-stage toxicity study with D4 was reported by Sousa et al. (1995). Rainbow trout embryos were exposed to D4 at concentration levels of 0.00025 - 0.0044 mg/L. No adverse effects were observed to embryo viability, hatching success, larval survival, and growth at all treatment levels. Therefore, the chronic NOEC for D4 93-day fish early life-stage is 0.0044 mg/L, the highest concentration tested. It is reasonable to conclude that due to its reduced bioavailability, D6 would also not exhibit adverse effects at the concentration of 0.0044 mg/L.

The acute and chronic toxicity of D6 was predicted using ECOSAR (2004) models. Predicted results are given in Table 8b.

The modelled results for D6 using ECOSAR suggests that there is a possibility of a high level of chronic adverse effects (i.e., chronic NOEC < 0.1 mg/L) in aquatic organisms at the limit of solubility. However, the reliability of the predicted toxicity values is considered to be low. The log K_ow for D6 (9.07) is higher than the recommended ECOSAR log K_ow cut off (8.0) for prediction of chronic toxicity using the neutral organic structure-activity relationships and the lack of empirical siloxane data in the model's training set limit their consideration in this assessment. The model predicted aquatic toxicity data were therefore not considered further.

The empirical evidence suggests that D6 does not exhibit adverse effects to pelagic biotaat concentrations below its solubility limit (0.005 mg/L). Nonetheless, the NOEC of 0.0046 mg/L from the 21-day Daphnia toxicity test was selected for comparison to predicted environmental concentrations in water.

B - In Other Environmental Compartments

In the Sediment Compartment

No toxicity studies for sediment organisms were found for D6. However, chronic toxicity studies on benthic organisms are available for its structurally close analogue, D5 (Springborn Smithers Laboratories 2003, Krueger et al. 2007, 2008). These experimental data were used in the weight-of-evidence approach to determine the sediment toxicity potential of D6. However, D6 is expected to be even less bioavailable (greater sorption to organic matter) in sediments than D5 and consequently may exhibit lower toxicity potential.

Chronic toxicity studies of D5 (Springborn Smithers Laboratories 2003, Krueger et al. 2008) were carried out on midge (Chironomus riparius). The no-effect concentration for male development rate in these 28-day studies was 69-70 mg/kg dry weight. The lowest-effect concentration for growth and male development rate observed at 160-180 mg/kg dry weight.

The chronic toxicity of D5 was also tested on blackworms (Lumbriculus variegates) in a 28-day study (Krueger et al. 2007). The no-effect concentration for survival and reproduction for D5 was 1272 mg/kg dry weight.

Based on the chronic data on D5, Chironomus ripariuspopulations areconsidered to be more sensitive than blackworm populations to cyclosiloxanes. Although it is likely that D6 would exhibit a lower toxicity towards benthic organisms due to reduced bioavailability and hydro-availability, the chronic D5 toxicity values may be used as read-across data for D6 sediment toxicity as a reasonable worst-case exposure and bioavailability potential. A conservative NOEC (no-observed-effect concentration) of 69 mg/kg dry weight and a conservative LOEC (lowest-observed-effect concentration) of 160 mg/kg dry weight can thus be derived for D6.

In the Soil Compartment

No effects studies for soil organisms were found for D6 or its analogues.

In the Terrestrial Compartment

No ecological studies were identified for terrestrial wildlife. Laboratory studies on mammals are discussed under the "Potential to Cause Harm to Human Health" section in this screening assessment.

Ecological Exposure Assessment

In Air

In Canada, preliminary environmental measurements of volatile methyl-siloxanes, including D6, were conducted in the Great Lakes region during February and March of 2006 (Backus 2007). Eighteen outdoor air samples were collected from rural and urban areas in Ontario and D6 was present in almost all the samples at concentration levels of < 1 µg/m³, with the exception of one relatively high D6 concentration detected at ~16 µg/m³ in the Toronto urban area. This result is in agreement with what has been reported in other jurisdictions (Table 9a).

It is, however, possible that the detection of D6 in ambient air is in part a result of sample contamination. Volatile cyclosiloxanes are present in a wide variety of commercial products, and both Canadian and Nordic monitoring programs have reported problems of high levels of cyclosiloxanes in sample blanks. The methodology for measuring and analyzing air concentrations at ng/m³ to the low µg/m³level is still under development. Very few duplicate measurements are available for outdoor air monitoring and the few that are available exhibit poor reproducibility (Backus 2007).

In Water

In Canada, water from a total of nine sewage treatment plants (STPs), including conventional secondary and tertiary water treatment plants and lagoons, in large urban centres in southwestern Ontario was sampled in October 2005 and winter 2005 (Backus 2007). D6 was detected at concentrations of 0.49-27.33 µg/L and 0.97-2.71 µg/L, respectively, in the influents and effluents. Seasonal differences in D6 concentrations in influents at STPs were only present in the three lagoon influents, while seasonal differences of D6 in effluents were more significant (from 0.97-1.00 in fall to 2.32-2.71 in winter).

Similar monitoring results have been reported in other jurisdictions (Table 9b). In the United States, D6 was detected in drinking water concentrates from two cities–New Orleans, Louisiana and Cincinnati, Ohio (Lucas 1984, as cited in US EPA 1992).

In Sediments

In Canada, surface sediments and sediment cores were collected from Lake Ontario in July 2006 and analyzed for D4, D5 and D6 (Powell and Kozerski 2007). Surface sediments consisting of the upper 5 cm of sediment were collected from Toronto Harbour and the Kingston Basin. Sediment cores, which were sectioned into strata 5 mm thick, were collected from the Rochester, Mississauga and Niagara basins. The surface sediments from Toronto Harbour and Kingston Basin contain moderate total organic carbon (TOC = 2.1-2.4% dw), while sediment cores contain high TOC (4-5% dw). Loss-on-ignition analysis of sediments also demonstrated lower water contents in surface sediments (55-70% ww) than in sediment cores (80-89%). Sediments in Toronto Harbour and the four sedimentary basins are known to be contaminated with a variety of organic compounds that enter the lake through direct discharges of treated wastewater, flow from the upper Great Lakes (Erie, Huron and Michigan) and the Niagara River, and atmospheric deposition. Surface sediments from Toronto Harbour contained the highest concentration of D6, at 0.2 µg/g dry weight. In contrast, concentrations of the cyclic siloxane materials in the surface sediments and sediment cores from the four sedimentary basins were all less than the analytical method detection limit, which was 0.01 µg/g for D6. Similar monitoring results have been reported in other jurisdictions, where D6 was detected in surface sediments from urban areas and point sources (Table 9c).

A preliminary monitoring study of a remote ecosystem was conducted in Lake Opeongo, the largest lake in Algonquin Provincial Park, Ontario, Canada. The lake is relatively remote from potential sources of cVMS from sewage and runoff (Powell 2008). Preliminary analysis of surface sediment and sediment core samples found no D6, with a limit of detection of 19.0 ng (background corrected mass).

The sediment monitoring results from Lake Opeongo and the Lake Ontario area suggest that D6 contamination is more likely to be found near urban centres and point sources.

In Soil

D6 may enter soil from land application of sewage sludge. No monitoring data for D6 in sewage sludge are available for Canada. In Europe, D6 is present in sewage sludge at levels ranging from the low mg/kg dry weight level up to 14.0 mg/kg dry weight (Norden 2005, Kaj et al. 2005, NILU 2007).

No monitoring data are available for D6 in Canadian soil. D6 concentrations in two soil samples from the Faroe Islands were below detection limit (< 4 ng/g dw) (Norden 2005).

In Biota

A preliminary monitoring study of a remote ecosystem was conducted in Lake Opeongo, the largest lake in Algonquin Provincial Park, Ontario, Canada. The lake is relatively remote from potential sources of cVMS from sewage and runoff (Powell 2008). Preliminary analysis of zooplankton samples found no D6. Bulk zooplankton samples were pooled into a single sample for each of the two locations from the lake without being sorted into species. The limit of detection was 19.0 ng (background corrected mass).

In Europe, D6 was detected in livers of marine fish but not freshwater fish, and mainly from areas representing urban or diffuse sources. Concentrations generally ranged from < 5 to 10 ng/g wet weight (ww), except for one sample from Norway of cod liver with extremely high siloxane concentrations (74 ng/g ww D6). The follow-up environmental monitoring program conducted by the Norwegian government confirmed the originally reported levels of D6 (NILU 2007; see Table 9d of this assessment). D6 was also found in common mussels, flounder livers and fillets, and in cod stomach contents from Norway in the same monitoring program. The concentrations varied with species, gender and age. Among marine mammals monitored, D6 was detected in seal blubber in Denmark at the level of 7.9 ng/g ww (Norden 2005). D6 was not detected in fish muscle samples in Sweden (Kaj et al. 2005).

The presence of D6 in European biota indicates that despite the low detected concentrations or even non-detection of the substance in or near fish habitats, D6 is available in the environment for biota to take up and accumulate.

Characterization of Ecological Risk

The approach taken in this ecological screening assessment was to examine various supporting information and develop conclusions based on a weight-of-evidence approach and using precaution as required under subsection 76.1 of CEPA 1999. Particular consideration was given to risk quotient analysis, persistence, bioaccumulation potential, toxicity, sources and fate in the environment.

Based on the information available, D6 is highly persistent in water; it also has some bioaccumulation potential in aquatic organisms as suggested by the experimental BCF and its presence in certain European biota. A quantitative risk quotient evaluation of exposure and of ecological effects was therefore conducted as part of the weight-of-evidence evaluation of D6's potential to cause harm.

In the aquatic compartment, two experimental chronic toxicity studies–one with Daphnia magna and the other with fathead minnow–were critically reviewed and accepted. D6 exhibited no adverse effects at 0.0044 to 0.0046 mg/L, a concentration that is very close to the water solubility limit of the substance (~ 0.005 mg/L). No application factor was applied to the chronic NOEC of 0.0046 mg/L.  The PNEC is therefore 0.0046 mg/L.

A risk quotient (RQ) analysis, integrating level of exposure with a toxicity threshold, was performed for D6. In order to address the potential risk of D6 on a national scale in Canada, a distribution profiling risk quotients in water at multiple release sites where the substance can be released by industry or by consumers (i.e., municipal sewage treatment plants) was determined. This type of analysis provides a line of evidence for the risk assessment of a substance when the full range of geographic locations of the industrial and consumer releases of the substance cannot be fully established.

Specifically, when a substance is used by a number of industry sectors but the actual facilities involved cannot be identified, the aquatic exposure can be estimated for all sites where facilities related to these sectors are located. In addition to this, information on potential releases from consumer use can be integrated into the calculations. A predicted environmental concentration (PEC) for the water compartment is determined based on the use quantities identified from the section 71 survey submissions and estimates of releases from individual industrial sites and from consumers. The receiving water is either a watercourse or a lake, and a dilution factor based on the size of the receiving water–up to a maximum of 10–is used to estimate the PEC. The risk quotient at each site is then determined for the water column. The distribution indicates not only the proportion or number of threshold-exceeding sites, but also the magnitude of the exceedence at each of these sites. Further details on the approach are provided in Environment Canada (2008b).

The consumer releases used a database of approximately 1000 municipal discharge sites accounting for about 2/3 of the Canadian population. The industrial release analysis was done for 27 sites relating to 87 industrial facilities identified by NAICS code as possible users of D6. Under these scenarios, a total of 31 (~3.2%) of all evaluated municipal discharge sites across Canada showed a risk to aquatic organisms, with RQs exceeding 1 (Figure 1). The equation and inputs used to calculate the PEC in the receiving watercourses are described in Environment Canada (2008c). 

Figure 1. Aquatic risk distribution for D6 (Environment Canada 2008b, 2008c)

Although a risk quotient analysis was conducted for D6, the empirical ecotoxicity evidence suggests that the threshold at which adverse effects in pelagic biota is expected to occur has not been observed in available toxicity tests. Therefore, the RQs calculated in the above scenario are essentially "unbounded" and do not represent "real" observable effects expected at the above sites.

The very high log K_ow for D6 would suggest that it is well into the class of "super-hydrophobic" chemicals which are often associated with low bioavailability to pelagic and benthic biota. Lack of appreciable bioconcentration in test organisms and relatively low tissue residue values from available field monitoring data also suggest that the potential for body burdens reaching critical internal levels from water and sediment exposures is mitigated by low bioavailability.

D6 is imported into Canada in significant quantities. Based on the information available, the quantities of D6 imported and used in Canada have increased significantly since the DSL nomination (Environment Canada 1988). Given the trend of increasing use, the environmental release of and potential for environmental exposure to D6 will increase. D6 has also been determined to be persistent in the environment, and has a relatively low potential to bioaccumulate in biota. Therefore, because the physical-chemical property, bioconcentration and ecotoxicity profiles for D6 provide a consensus basis for a weight of evidence, it can be reasonably concluded that D6 has a low potential to cause ecological harm in the Canadian environment.

Uncertainties in Evaluation of Ecological Risk

D6 is imported into Canada in significant quantities. Based on the information available, the quantities of D6 imported and used in Canada have increased significantly since the DSL nomination (Environment Canada 1988). Given the trend of increasing use, its environmental release and potential for environmental exposure will increase. D6 is also one of the major components in CAS RN 69430-24-6. It is also present in PDMS at up to 3%. The Challenge to industry and other stakeholders issued by the Government of Canada (Canada 2007) did not survey CAS RN 69430-24-6 (cyclomethicone, the mixture) or PDMS. Even though there is evidence that some companies did report individual cVMS in the mixture under the survey, the quantities of these substances imported into Canada and their uses in 2006 are not completely known, and their releases into the Canadian environment are not considered fully in this assessment report.

For the risk distribution analysis, the PNEC was derived from a concentration that is very close to its water solubility limit (~ 0.005 mg/L), and no application factor was applied to extrapolate laboratory data into the field due to the lack of observed adverse effects at this concentration. Since no release information from industrial operations was available for the risk distribution analysis, it is assumed that releases to wastewater were uniformly distributed among 27 industrial sites evaluated. In reality, certain industrial sites may use higher quantities of D6 than others, resulting in higher releases to the municipal discharging sites associated with these industrial sites and therefore a higher risk than predicted. The distribution concentrations in the analysis applied instantaneous dilution of the effluent from sewage treatment plants (STP) into the receiving water. However, under realistic environmental conditions, instantaneous dilution may not be achieved over a certain distance from the discharge point, and the area near the discharge point of an STP may present a higher risk than predicted.

Sediment is an important media of concern for D6. The D4 sediment degradation studies were used as read-across values for D6 and those studies are not without uncertainties. In addition, extrapolation of half-lives at low temperature in sediment based on hydrolysis data may also contribute to the overall uncertainty in sediment persistence.

The available bioconcentration study and read-across biomagnification factor and sediment accumulation values from the D6 analogues showed a low potential for D6 to bioaccumulate in laboratory tests. There are a lack of field data on bioaccumulation potential via the food web and in non-aquatic organisms.

Model predictions were also included in this screening assessment as a line of evidence. Higher uncertainties associated with predicted values, as few siloxanes or chemicals with high log K_ow values (> 8), as for D6, have been studied and included in the models.

Environmental monitoring data in Canada and elsewhere are limited. Sample contamination is a potential problem in environmental monitoring due to D6's widespread uses. Data on environmental concentrations of D6 in biota and surface water in Canada are lacking and few environmental concentrations have been reported outside of urban areas in Canada. Consequently, monitoring data from European countries have been presented in this report. However, monitoring has been identified as a key component in the Chemicals Management Plan in Canada and D6 is being considered for environmental monitoring under the Plan. Environmental monitoring will contribute to a better understanding of the environmental presence and "true" environmental accumulation potential of the substance in the environment.

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Potential to Cause Harm to Human Health

Exposure Assessment

The data on levels of D6 found in environmental media including ambient air near and away from point sources, surface waters, sediments, sewage sludge and biota are described in this report in the section entitled "Ecological Exposure Assessment." Unpublished data from Canada include measurements taken of air near and away from point sources (Backus 2007, unpublished results); air, influent and effluent water at wastewater treatment plants (Backus 2007, unpublished results); and Great Lakes sediment (Powell and Kozerski 2007). Many analyses of volatile siloxanes have been confounded by sample contamination during collection and analysis, resulting in siloxanes being detected in blanks at levels comparable in some cases to those in samples taken near point sources. Results of extensive sampling and measurement of siloxanes in environmental media in Scandinavia have been published by the Nordic Council of Ministers and the Swedish Environmental Research Institute (Norden 2005, Kaj et al. 2005). Data from these reports were considered reliable and were used to produce the upper-bounding estimates of exposure to siloxanes in air, water and soil by the general population in Canada.

The upper-bounding estimates of daily intake of D6 for six age groups in the Canadian population are shown in Appendix 1. The estimates of intake from environmental media and diet range from 28.7 µg/kg body weight/day (µg/kg-bw/day) for adults aged 60 years and older to 87 µg/kg-bw/day for children aged 6 months to 4 years. The most significant contribution to daily intake from environmental media is inhalation of indoor air, based on a study of 400 homes in Sweden in which D6 was detected in 142 samples, approximately twice as many as for D4. The mean concentration of D6 in indoor air measured in children's bedrooms in this study was 7.9 µg/m³ and the maximum was 164 µg/m³ (Kaj et al. 2005).

Confidence in the upper-bounding estimate of exposure to D6 through environmental media and diet is moderate. No Canadian data were used, but data from studies in Scandinavia were available for ambient and indoor air, water and soil. The maximum concentration of D6 measured in indoor air in the Swedish residential study is higher than the maxima measured for D4 and D5 in Swedish homes (Kaj et al. 2005)–a result not readily explained, given that D6 has a lower vapour pressure compared to its lower molecular weight analogues and given the expected relative prevalence of use of the three cyclosiloxanes in 2004. The use of a regulatory limit for DMPS (CAS RN 9006-65-9, dimethylpolysiloxane) in one quarter of dairy products and one half of processed food may overestimate the dietary contribution to total exposure, but the estimated contribution from all food to exposure of the general population from environmental media and diet is less than five percent of the contribution from air.

Using ConsExpo 4.1, software developed to estimate exposure to consumer products, the potential systemic dose of D6 through the use of personal care products was estimated for women that use skin care products, hair care products and antiperspirants (RIVM 2006). Similar calculations for D4 showed that the estimated adult female dose was much higher than that for adult males, chiefly because of leave-on skin care products. Therefore, an estimated systemic dose for adult women only is presented.

Manufacturers of personal care products are required to notify Health Canada of the concentration, within broad ranges, of siloxanes, including D6 and polydimethylcyclosiloxanes, termed cyclomethicone in personal care products^[1]. Health Canada has been notified of approximately 530 cosmetic products that contain D6 as the sole siloxane or in mixtures with other specified siloxanes, and that department has also been notified of approximately 6000 cosmetic products that contain cyclomethicone or cyclomethicone mixtures which may contain D6 (CNS 2007). The data on the concentration of D6 in personal care products were obtained principally from the information provided by Canadian industry (Environment Canada 2007, CNS 2007) and were supplemented by information from other sources noted in Appendix 2. Market share data were not used to determine the concentration of D6 in the dominant products in each use category. In cases where the concentration for a product category reported in response to a notice published under section 71 of CEPA 1999 was higher than concentrations reported in the CNS database, a lower concentration of D6 for that product category, consistent with the range reported to the CNS, was used in estimating the systemic dose (Environment Canada 2007, CNS 2007).

It was assumed that 20% of the D6 in a product left on the skin evaporated and the remaining 80% was available for dermal absorption. This approach maintains the conservative bias of screening assessments while considering the relative vapour pressure of D6 compared to D4 for which evaporation data are available (Zareba et al. 2002). Experimental data to provide guidance on partitioning of personal care products arising from evaporation on skin were not located.

Studies of oral dosing of F344 rats showed that approximately 12% of ¹⁴C-D6 was absorbed systemically (Dow Corning 2004). Of ¹⁴C-D5 applied topically, 0.17% was absorbed across rat skin (Jovanovic et al. 2008). In a 6-hr inhalation study in rats, 1-2% of ¹⁴C-D5 was retained in the body (Tobin et al. 2008). It was proposed to use figures of 12% absorption by ingestion, 0.17% absorption via the dermal route and 2% absorption by inhalation for the purpose of estimating dose from personal care products. The dermal and inhalation absorption figures used for D6 are those determined for D5, as shown above. This approach was deemed to be consistent with a conservative bias in screening assessments while avoiding an overestimation of absorption factors. A distinction was made between products that are washed off and those that are left on the body. Other assumptions are noted in Appendix 2. 

The results of a sample calculation for the application of hair styling lotion are shown in Appendix 2, and summaries of the estimated systemic dose arising from the use of personal care products by women are shown in Table 1. For adult women, the upper-bounding estimate of daily systemic dose from the modelled personal care products, aggregated over inhalation, dermal and oral exposure, is 0.10 mg/kg-bw/day. In the screening assessment of octamethylcyclotetrasiloxane (D4) it was shown that the systemic dose received by women from the use of personal care products was higher than the dose received by men and this is expected to be the case for D6.

An exposure assessment for use of D6, including personal care product uses, was submitted to the Government of Canada under the Challenge Program (SEHSC 2008a). The methodology is different from that shown in Appendix 2, Table 1, as a Monte Carlo probabilistic analysis was conducted and aggregate general population exposures from all sources (including personal care products) and routes (inhalation, dermal, ingestion) were derived. The contribution of the use of personal care products to total exposure via separate exposure routes (inhalation, dermal, oral) was characterized and then summed to arrive at an aggregated exposure estimate. Due to the fact that D4 and D6 were used as analogues to determine oral toxicity of D5, D5 is used here as a surrogate for validation of the D6 probabilistic exposure assessment. Thus, a probabilistic exposure assessment for use of D5 was also submitted to the Government of Canada under the Challenge Program (SEHSC 2008b). Independent review of the submitted probabilistic assessment showed that this assessment evaluated exposure to both user and non-user groups (see Appendix 4). The data were re-analyzed based on just user groups to allow comparison with the deterministic exposure assessment in this screening assessment. Based on user groups only, the probabilistic exposure values for adult females (most highly exposed adult group) were 10-16 times lower than the deterministic values shown for D5. Therefore, based on D5 used as an analogue to determine probabilistic exposure values for D6, it is expected that probabilistic exposure values for adult females (most highly exposed adult group) will be 10-16 times lower than the deterministic values shown in Appendix 2, Table 1. Note that due to the requirement for detailed analysis and validation of probabilistic exposure assessments, such assessments are normally outside the scope for conducting the exposure component of a screening assessment.

Based on user groups only in the D5 probabilistic exposure assessment, the exposure values for children aged 0-6 months (most highly exposed children's group) were in the range of 0.016-0.032 mg/kg-bw/day (see Appendix 4). A comparison with a deterministic exposure assessment for children was not possible due to the lack of sufficient product use data required for modelling children's exposure in a deterministic exposure assessment. When the children's probabilistic exposure values are compared with the adult female deterministic exposure values, they are 5-10 times lower for D5. Therefore, based on D5 used as an analogue to determine probabilistic exposure values for D6, it is expected that if the children's probabilistic exposure values for D6 were compared to the adult female deterministic exposure values, they would be 5-10 times lower for D6.

Other types of consumer products such as surface coatings, caulking, polishes and cleaners were deemed to contribute significantly less to daily exposure through daily use and were not further considered in the modelling of daily dose through consumer exposure scenarios. Both personal care products and other consumer products contribute to the concentration of D6 in indoor and ambient air and thus exposure by inhalation. Their contribution to total exposure of non-occupationally exposed individuals is estimated via indoor air in the multi-media environmental exposure model discussed in the preceding text of this section.

Confidence in the estimate of systemic dose of D6 through the use of personal care products is low. All estimates were made by the use of models, and the use pattern data were not from Canadian studies. The extent of use of D6 in personal care products and the concentration of D6 in products on the market currently may be lower than used in estimating the systemic dose reported above and in Appendix 2 (Environment Canada 2007, SEHSC 2007a). Consequently, these values are expected to be overestimates of exposure to D6 from use of personal care products.

Health Effects Assessment

Appendix 3 contains a summary of the available health effects information for dodecamethylcyclohexasiloxane (D6).

No international agency has classified D6 for carcinogenicity, genotoxicity or reproductive/developmental toxicity. Only one national review on the health effects of cyclosiloxanes was identified to date, that of the Danish Environmental Protection Agency (EPA). They reviewed health effects for D4 and D5 but not D6 (Lassen et al. 2005).

No carcinogenicity data for D6 were identified. Results of mutagenicity assays using Salmonella typhimurium or Escherichia coli (NOTOX 1990c) were negative. No other genotoxicity studies were identified.

With regard to reproductive toxicity, a lowest-observed-effect level (LOEL) of 1000 mg/kg-bw/day was determined based on an increased number (non-statistical) of sperm-positive, non-gravid females in a repeated-dose reproductive and developmental toxicity study. No developmental effects were observed at any dose. In this study, male and female Sprague-Dawley (SD) rats were administered 0, 100, 330 or 1000 mg/kg-bw/day of D6 in corn oil by oral gavage daily, seven days/week for up to 45 consecutive days (Dow Corning 2006).

In the repeated-dose portion of this study, a LOEL of 100 mg/kg-bw/day was determined based on increased liver weights and periportal lipidosis in the liver of females and follicular cell hypertrophy of the thyroid in both sexes of rats (dose-related in females) when they were administered the same daily oral doses of D6 for 28 days (Dow Corning 2006). The value of 100 mg/kg-bw/day is considered to be the critical level for repeated-dose toxicity in this assessment based on both the liver and thyroid effects. Note that for D4 (octamethylcyclotetrasiloxane) and D5 (decamethylcyclopentasiloxane), the liver was also identified as a target organ for oral exposures. The Danish EPA identified the liver as a target organ for D4 exposures, and liver effects were observed in both oral and inhalation studies using D5 (Lassen et al. 2005).

Only one other repeated-dose study was identified. In that study no systemic toxicity was observed in SD rats treated by oral gavage to 0 or 1500 mg/kg-bw/day of D6 for four weeks (Dow Corning 1990).

No published studies on the toxicokinetics of D6 were identified.

The confidence in the toxicity database is low. There was limited information on acute, short-term, developmental, reproductive and genotoxicity data based mainly on oral exposure, but there was a lack of subchronic and chronic toxicity/carcinogenicity data. In addition, no data were identified based on inhalation or dermal (except for one acute study) exposures.

Characterization of Risk to Human Health

The critical effect level for repeated-dose toxicity of D6 is considered to be 100 mg/kg-bw/day via the oral route. This is based on increased periportal lipidosis and dose-related increases in liver weights and thyroid follicular cell hypertrophy in a 4-week rat study. Although repeated-dose inhalation data were not identified, it is also expected that the liver would be a target organ for inhalation exposures to D6 because liver effects were also observed upon inhalation exposures to the structurally similar compounds, D4 and D5 (see Health Effects Assessment section).

Comparison of the critical effect level for repeated dosing via the oral route (100 mg/kg-bw/day) and the upper-bounding estimate of daily intake of D6 by the general population in Canada results in a margin of exposure of approximately 40 000. This is based on adjusting the inhalation contribution to daily intake by an inhalation absorption value of 2.0%, resulting in a systemic exposure of 2.5 µg/kg-bw/day. The margins of exposure for repeated-dose effects and exposure via environmental media to the general population are considered to be adequate to account for uncertainties in the databases on exposure and effects.

The apparent intake dose of 0.09 mg/kg-bw/day from personal care products which incorporates absorption factors for dermal, inhalation and oral exposure (Table 1, Appendix 2) was corrected by applying the reciprocal of the oral absorption factor to calculate the equivalent systemic dose of 0.19 mg/kg-bw/day. Using this calculated upper-bounding estimate of 0.19 mg/kg-bw/day, a comparison with the critical effect dose level for repeated dosing via the oral route (100 mg/kg-bw/day) resulted in a margin of exposure of approximately 500 from use of personal care products. However, it is considered that the exposure estimates presented above are overestimates of actual exposure based on a submitted probabilistic assessment and information indicating that the percentage of personal care products containing D6 on the Canadian market may be lower than assumed in deriving exposure estimates. Based on values derived from an independent review of the probabilistic exposure assessment for D5, which is considered to be a surrogate for the D6 probabilistic exposure assessment, it appears that the margin of exposure from use of personal care products would be at least 10 times higher for adults and at least 5 times higher for children from that shown above (i.e. > 2000). On the basis of the above considerations, including consideration for the extent of its database, D6 is considered not to meet the criteria under paragraph 64(c) of CEPA 1999.

Uncertainties in Evaluation of Risk to Human Health

The scope of this screening assessment does not take into consideration a full analysis of the mechanism of action of dodecamethylcyclohexasiloxane and it does not take into account possible differences between humans and experimental species in sensitivity to effects induced by this substance.

There is uncertainty regarding the estimation of exposure and systemic dose because of the use of modelling and a lack of Canadian data. There is uncertainty associated with the use of models and the choice of variables related to the use of consumer products including quantity and frequency of use, absorbed fraction and environmental parameters.

The cumulative exposures of the other cyclosiloxanes in polydimethylsiloxanes (PDMS) are not considered in this assessment. However, D4 and D5 are considered in separate assessments.

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Conclusion

Based on the information presented in this screening assessment, it is concluded that dodecamethylcyclohexasiloxane (D6) is not entering the environment in a quantity or concentration or under conditions that have or may have an immediate or long-term harmful effect on the environment or its biological diversity or that constitute or may constitute a danger to the environment on which life depends. 

Based on the available information on its potential to cause harm to human health, it is concluded that dodecamethylcyclohexasiloxane is not entering the environment in a quantity or concentration or under conditions that constitute or may constitute a danger in Canada to human life or health.

It is therefore concluded that dodecamethylcyclohexasiloxane does not meet the definition of toxic as set out in section 64 of CEPA 1999. Additionally, dodecamethylcyclohexasiloxane does not meet the criteria for persistence and bioaccumulation potential as set out in the Persistence and Bioaccumulation Regulations (Canada 2000).

The conclusion in this screening assessment is based on the available information at this time and acknowledges that there are uncertainties associated with this assessment. Research on cVMS is currently being conducted to help address these uncertainties, but some of this research has not been completed at this time. In the context of the Challenge program, any new information provided after the final screening assessment may be considered during the risk management phase.

Monitoring has also been identified as a key component in the Chemicals Management Plan in Canada and D6 is being considered for environmental monitoring under the Plan. Field level data will contribute to a better understanding of the distribution of D6 in the environment and its bioaccumulation potential in relevant food webs.

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Appendix

Appendix of the Screening Assessment

• Appendix 1: Upper-bounding Estimates of Daily Intake of D6 by the General Population in Canada (µg/kg-bw per Day by Various Age Groups)
• Appendix 2: Consumer Exposure Modelling
• Appendix 3: Summary of Health Effects Information for Dodecamethylcyclohexasiloxane (D6)
• Appendix 4: Review of D5 Probabilistic Exposure Assessment
• Appendix 5: Multimedia Modelling Input Parameters for D6 in the Ecological Screening Assessment

Footnote

^[1] Cosmetic Regulations, C.R.C., c. 869.