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Report of the Board of Review for Decamethylcyclopentasiloxane (Siloxane D5)

2 Executive Summary

Conclusions of the Board of Review

1. The Siloxane D5 Board of Review (the “Board”) inquired into the nature and extent of the danger posed by decamethylcyclopentasiloxane, or Siloxane D5, (CAS # 541-02-6; D5) to the Canadian environment or its biological diversity2.

2. Taking into account the intrinsic properties of Siloxane D5 and all of the available scientific information, the Board concluded that Siloxane D5 does not pose a danger to the environment.

3. The evidence presented to the Board demonstrated that Siloxane D5 exceeded the regulatory threshold for persistence. However, Siloxane D5 did not exceed the thresholds established in the Persistence and Bioaccumulation Regulations (“Regulations”).

4. Siloxane D5 does not biomagnify through the food chain, although it can be accumulated into organisms from environmental matrices or food. That is, concentrations of Siloxane D5 do not increase in predators relative to their prey.

5. There is no evidence to demonstrate that Siloxane D5 is toxic to any organism tested up to the limit of solubility in any environmental matrix. The Board is of the view that Siloxane D5 will not accumulate to sufficiently great concentrations to cause adverse effects in organisms in air, water, soils, or sediments.

6. Furthermore, the Board concluded that, based on the information before it, the projected future uses of Siloxane D5 will not pose a danger to the environment.

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Scope of the Mandate of the Board of Review

7. Pursuant to the authority provided in the Canadian Environmental Protection Act (“CEPA 1999”), the Minister of the Environment requested that the Board inquire into the nature and extent of the danger posed by Siloxane D5. The Board consulted with the parties to these proceedings, Environment Canada, the Silicones Environmental, Health and Safety Council of North America (“SEHSC”), the Canadian Cosmetic, Toiletry and Fragrance Association (“CCTFA”), and a coalition consisting of the Canadian Environmental Law Association, the International Institute of Concern for Public Health, Chemical Sensitivities Manitoba, and the Crooked Creek Conservancy Society of Athabasca (the "Coalition"), on the proposed scope of its mandate.

8. Taking into account the issues of concern identified in the Screening Assessment conducted by government officials in 2008, the directions given by the Minister of the Environment establishing this Board, and the nature of the additional information on Siloxane D5 that has become available, the Board determined that it would focus its review on the nature and extent of the danger posed by Siloxane D5 to the environment.

9. The Board has interpreted its mandate to mean that it was to inquire into the nature and extent of the risk posed, if any, by Siloxane D5 to the environment and to determine whether detrimental effects were caused or might be caused. In other words, the Board has conducted what is, in essence, a de novo risk assessment of Siloxane D5 by taking into account the available scientific information. When conducting the de novo risk assessment, the Board was of the opinion that best scientific practice required that it consider all available information about the intrinsic physical and chemical properties of Siloxane D5, along with its toxicity, uses, exposures, and effects.

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Fate of Siloxane D5 in the Environment

10. When chemicals are released into the environment, they move among compartments including air, soils, water, and sediments. The ultimate distribution of a substance among these compartments and the rate at which it moves among these compartments are functions of its chemical and physical properties, and the characteristics of the environment into which it is released.

11. In addition to moving among compartments and from one location to another, chemicals can undergo transformations once released into the environment. These transformations can be due to physical, biological, and/or chemical processes, such as hydrolysis and photolysis, to form products that are different from the original chemical. The rate of transformation determines the concentrations to which chemicals can accumulate in the environment.

12. The behaviour of Siloxane D5 is different from that of other compounds of similar molecular weight and size. It has a molecular structure which consists only of carbon, silicon, oxygen, and hydrogen in a symmetrical ring structure. These physical and structural properties of Siloxane D5 result in unique patterns of distribution in the environment that need to be considered in evaluating the exposure of organisms and the potential danger posed by Siloxane D5.

13. The pathway of release of Siloxane D5 into the environment is important in determining its distribution and persistence. The unique properties of Siloxane D5 combined with its primary uses and types of releases into the environment are important when considering its potential to cause harm. Due to its relatively great vapour pressure and volatility, Siloxane D5 tends to partition primarily into air. In other words, irrespective of where Siloxane D5 is initially deposited in the environment, it will migrate mainly into the air. While it can be transported relatively long distances in the air, deposition from the air to soils or water will be very limited.

14. Once in the air, Siloxane D5 degrades relatively rapidly through a process of indirect photolysis, in which naturally occurring hydroxyl radicals, formed in the atmosphere by sunlight, degrade it into smaller molecules called silanols and ultimately to carbon dioxide, water, and silicon dioxide (the main constituent of sand). These products do not present a danger to the environment. Thus, an important aspect of the environmental fate of Siloxane D5 is that the compartment into which it is mostly released and most likely to occur is also the compartment where it also undergoes the most rapid rate of transformation.

15. Although, in the Board's view, Siloxane D5 meets the criteria to be classified as a persistent chemical under the Persistence and Bioaccumulation Regulations (the “Regulations”), it will only be a danger to the environment if this intrinsic property results in exposures that cause adverse effects in the environment. Thus, persistence must be accompanied by accumulation in one or more compartment(s) of the environment (or organisms) to the point that these exposures exceed the dose or concentration that causes an adverse effect. This was the principal focus of the Board's review.

16. The rate of accumulation of Siloxane D5 in air, water, soils, and sediments, as well as in organisms, is determined by its rate of release to the environment and, once there, its rates of movement to other compartments of the environment (dissipation) and transformation. Because Siloxane D5 has been used in commercial and industrial applications for a relatively long time – more than 30 years – and, given its rates of dissipation and transformation in the environment, current concentrations of Siloxane D5 are at a quasi-steady-state.

17. Concentrations of a chemical are considered to be in a quasi-steady-state in environmental compartments when concentrations remain approximately constant in the long-term, although they could fluctuate in the short-term due to sporadic releases or seasonal changes in the environment. This means that concentrations in each compartment vary within a predictable range and that concentrations of Siloxane D5 in the environment are not changing significantly over time.

18. In addition to the physical processes of degradation, there are biologically-mediated processes of transformation, referred to as biotransformation. Biotransformation can occur due to the actions of bacteria and fungi, or in the bodies of higher organisms as part of normal metabolic functions or as adaptive responses. Siloxane D5 is biotransformed into silanols, which are more soluble than Siloxane D5 and present less risk to the environment. The rate at which Siloxane D5 is biotransformed by higher animals, determines, in part, the rate of loss from the animal involving all pathways including diffusion and active transport. The rate of biotransformation in organisms also determines the concentrations that can be accumulated in plants and animals.

19. The importance of these understandings is that, at current rates of use, concentrations of Siloxane D5 in the environment will not increase significantly. If, in the future, the use and subsequent release of Siloxane D5 to the environment were to increase, the change in concentration in the various compartments of the environment would be directly proportional to the increase in release and thus be predictable based upon current information.

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Exposure of Organisms in the Environment

20. Exposure measures the concentration of a chemical available to enter into an organism where it might produce adverse effects. When predicting concentrations of chemicals to which organisms might be exposed, it is also important to consider bioavailability. Bioavailability is the fraction of the total concentration of a chemical that is available to be accumulated into organisms. Siloxane D5 has a particularly strong affinity for organic particles in sediments and wet soils, and is not readily available to be accumulated into organisms.

21. When predicting concentrations of Siloxane D5 that would occur in various compartments of the environment, it is also important to consider the limits of solubility in that “matrix” and the fraction that is biologically available to organisms. It is theoretically impossible for Siloxane D5 to exceed its solubility limits in water or the organic matter in sediments or soils. Consequently, the Board advises readers of this report to be cautious about drawing conclusions with respect to bioaccumulation, persistence, and toxicity that are based upon concentrations that exceed the theoretical solubility limit.

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Bioaccumulation in Organisms

22. Siloxane D5 cannot produce toxicity by interacting with molecules on the outside surface of an organism. To cause effects, compounds like Siloxane D5 must enter into organisms. Siloxane D5 can enter organisms through several pathways, including inhalation from the air or across external surfaces such as the skin and gills of fish or benthic invertebrates, the roots of plants, and the lining of the gastro-intestinal system. However, the primary route of exposure for all organisms would be through the diet and/or from water.

23. There are three concepts related to the accumulation of chemicals into organisms. The first is bioaccumulation, which is the process of the chemical entering into an organism. The second is bioconcentration, where concentrations of the chemical are accumulated to values that are greater than, but proportional to, those in the surrounding medium. Finally, there is the concept of biomagnification or trophic magnification in which predators accumulate greater concentrations of the chemical than those in their prey. When expressed appropriately, concentrations of Siloxane D5 in organisms can be greater than those in the surrounding medium, i.e., it can bioconcentrate to some degree. While Siloxane D5 can accumulate in organisms, it does not biomagnify through the food-chain.

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Toxicity of Siloxane D5

24. Toxicity is the potential of a chemical to produce adverse effects in organisms. The severity of the effect caused is determined by the duration and magnitude of exposure to the chemical and its potency. The potency of a chemical is described by the dose-response relationship, which is derived by exposing organisms to known quantities of a chemical for known periods of time and recording the magnitude of response.

25. Once an organism has been exposed, the damage produced is related to the rates of biotransformation, depuration (excretion), and repair of any damage caused. Thus, organisms can be exposed to some concentration of a chemical for a very long period without exerting any apparent effects. In addition, animals and plants have the ability to adapt to exposures to some chemicals so that the normal functioning of the organism is not adversely affected. Such adaptive responses are not considered adverse responses.

26. In assessing the potential adverse effects and, thus, the hazard posed by a chemical, it is useful to know the mechanism of toxic action of that chemical. That is to say, how the chemical causes toxicity. There are multiple known mechanisms of toxic action. Chemicals can have a “specific” mechanism of action due to the interaction of the molecule with a particular receptor. For instance, the physical shape of some molecules is such that it fits into structures on biomolecules such as proteins. Chemicals can also elicit effects by mimicking biological molecules or blocking active receptor sites.

27. In addition to these specific mechanisms of action, all molecules have what is termed a minimal or basal toxicity. This is referred to as “narcosis” and occurs when the molecule dissolves in membranes in the body and produces changes in their structural and/or chemical properties. This process is reversible and does not necessarily result in permanent damage. For neutral (uncharged) molecules such as Siloxane D5, there is no known specific mechanism of toxicity. Toxic effects are therefore caused by narcosis.

28. In the scientific literature, there are few reports of the toxicity of Siloxane D5. However, due to its non-specific mechanism of action, it is possible to predict with acceptable certainty, the toxicity to different species and the critical body burden, or concentration required to produce an adverse effect in an organism. This is because the physiologies and membranes of animals are similar and there is little variation in sensitivity among organisms. Consequently, it is very unlikely that there will be uniquely sensitive species. For this reason, a smaller set of data on toxicity, such as that which exists for plants, fish, and mammals, is sufficiently robust to make accurate conclusions about the potential for effects on organisms.

29. Siloxane D5 has not been found to cause toxicity in any organisms tested up to its limit of solubility in any environmental compartments, or matrices. This, coupled with the fact that it is theoretically impossible for Siloxane D5 to exceed its solubility in any given matrix, led the Board to conclude that it is virtually impossible for Siloxane D5 to accumulate to sufficient concentrations to produce adverse effects to organisms in air, water, soils, or sediments.

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Risk Assessment

30. Risk assessments can be conducted in tiers of increasing complexity, depending on the amount of information available. Assessments of new chemicals are restricted to the lower tiers and are based on:

  • the physical and chemical properties of the compounds;
  • the results of simple simulations that predict environmental fates; and,
  • a few tests or models to determine toxicity.

31. However, for existing chemicals, there is more information available that can be considered in assessing the potential for harm. If chemicals have been released into the environment, as is the case with Siloxane D5, “real-world” measurements can be used to assess properties and to validate models and predictions of both exposure and effects.

32. There are two basic conditions that need to prevail for harm to occur. First, there needs to be exposure. Even for the most hazardous chemicals, no harm will occur if there is no exposure. Second, once exposure has occurred, there needs to be a detrimental or harmful effect. This is referred to as hazard. The magnitude of effect caused by exposure to a defined amount of a chemical is referred to as its potency.

33. In the 2008 Screening Assessment conducted by Health Canada and Environment Canada, government officials assessed the potential for Siloxane D5 to produce adverse effects in the environment. At that time, there was less information on environmental fate and toxicity than is currently available. Consequently, the Screening Assessment was limited to the lowest tiers of assessment, which make use of the least information and thus have the greatest uncertainty. The Screening Assessment was limited to a few basic comparisons of parameters, such as persistence and potential to bioaccumulate. There was insufficient information upon which a more detailed assessment could be made. In fact, in most cases, surrogate information based on the basic physical and chemical properties of Siloxane D5 or analogous chemicals had to be used.

34. As has already been discussed, since Siloxane D5 has unique properties for a molecule of its size, the basis for such extrapolations was uncertain. Since 2008, additional information on the basic physical and chemical properties of Siloxane D5 has become available so that more refined simulations of environmental fate can now be made.

35. Furthermore, additional information on the hazard of Siloxane D5 is now available. Most importantly, better methods for quantifying Siloxane D5 in various matrices have become available and these methods of analysis have allowed additional information to be collected on concentrations of Siloxane D5 in environmental matrices, including air, water, soils, and sediments, as well as in organisms. Thus, a more refined assessment of the danger posed by use of Siloxane D5 can be made at this time. The Board has relied on this most recent information in making its assessment of the risks posed by Siloxane D5 to the environment.

36. Risk relates to the probability of adverse outcomes and is always related to probabilities of exposure and effects. Lower-tiered risk assessments are based on limited information and for that reason are often based on simple ratios of exposure to some threshold for effect. Because of the inherent uncertainty in these approaches, uncertainty factors are generally applied as safety factors. These factors are meant to be conservative and protective rather than being predictive. As additional information becomes available and assessments move to higher tiers, the uncertainty becomes less and the need for uncertainty factors is reduced.

37. Lower-tiered assessments are designed to screen out, rather than to screen in, chemicals of concern. They are meant to be conservative so as to minimise the likelihood of improperly classifying chemicals that might produce adverse effects. Exceeding one of the individual indicators of persistence or bioaccumulation does not imply that there will harmful exposures. Rather, it indicates that further, more refined assessments are warranted.

38. In the Screening Assessment of Siloxane D5, government officials followed a conservative, or precautionary, approach since there was insufficient information on toxicity and no information on concentrations in the environment. In contrast, the Board had access to monitoring information to substantiate the results of more refined estimates of both exposure and effects and was able to make use of this scientific information to conduct a more robust evaluation at higher tiers of assessment.

39. The Board considered the unique physical and chemical properties and the mechanism and potency of toxicity of Siloxane D5, and concluded that it is virtually impossible for Siloxane D5 to occur in any environmental matrix at concentrations sufficient to cause damage. Consequently, the Board has determined that current uses of Siloxane D5 do not pose a risk of danger to the environment.

40. The Board has further concluded that there is no potential for future uses of Siloxane D5 to result in danger to the environment since current concentrations of Siloxane D5 in the environment are at a quasi-steady-state. Concentrations of Siloxane D5 are unlikely to change in the future, except in direct proportion to the growth of the population.

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2 Hereinafter, the term “environment” refers to the phrase “environment or its biological diversity”.

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