Environment Canada considers banning silicones in Canada: cyclic silicones out the door.
We invite you to read the full letter sent by CSC president Pierre Beaumier, MCIC, on July 16, 2008. It is available at www.cheminst.ca/public_position.
This report was submitted to Environment Canada in July of 2008.
In the middle of May 2008, the Canada Gazette published recommendations from Environment Canada that call for the 'virtual removal' of 3 cyclosiloxanes [D.sub.4], [D.sub.5] and [D.sub.6] (D = [Me.sub.2]SiO, thus [D.sub.4] is an 8 membered ring, [([Me.sub.2]SiO).sub.4]) from commerce in Canada. Based on their analysis of the open literature, unpublished documents and computer models, it is Environment Canada's assessment that the three cyclosiloxane molecules [D.sub.4]-[D.sub.6] are "persistent, bioaccumulative, and inherently toxic to non-human organisms." This conclusion is at odds with extensive peer-reviewed literature.
These 3 silicones are used in their own right, particularly in personal care products, either individually (previously [D.sub.4] was most commonly used, but over the last decade has been supplanted by [D.sub.5]) or as mixtures (cyclomethicones). They are lipophilic carriers that have a "dry and silky" rather than a greasy feel. In addition, as a consequence of the manufacturing process for silicone polymers, the 3 compounds are present in small quantities in most silicone polymers including oils, gums, greases, elastomers (rubbers), and organofunctional compounds such as surfactants. The absolute concentration associated with the term 'virtual removal' is not defined in the government documents, but if it is set at a low level, resulting legislation could have the effect of removing silicones from nearly all applications and markets in Canada--medical devices excepted.
Silicones have been available commercially since the 1940's because of their unusual properties, which cannot be attained by traditional polymeric materials. For example, silicones are electrically resistive (spark plug wire coatings), thermally stable (O-rings and surface coatings in automotive air bags), flexible and fluid over a wide range of temperatures (flattening agents in paints), water repellent (shoe polish, potting materials for automotive circuits), surface active (defoamers in foods and beverages, bubble stabilizers in polyurethane foams) and have low surface energy (adhesive label backings): over 1,000 medical devices alone, comprised wholly or partly from silicones, are registered with Health Canada. In their brief, Environment Canada notes there are over 6,000 cosmetic and personal care products that are made from or contain cyclosiloxanes. Given the potentially enormous implications such a ban would have for Canadian quality of life, including economic activity, what is the basis of this proposal?
Unlike many compounds in commerce--polymers in particular--silicones have been extensively investigated for both their impact on the environment and human toxicity. Many of these studies arose because of concerns about the safety of silicone gel breast implants: these devices were not available in the US and Canada from 1992-2006, but were recently re-regulated following analysis of the extensive scientific investigations undertaken during the last two decades. The government proposal excludes medical devices from consideration because the three silicones are considered to be safe and non-toxic to humans at normal exposure levels: at high levels of exposure, [D.sub.4] can be a reproductive toxin to rats. The No-Observed-Adverse-Effect-Level (NOAEL) was found to be 700 ppm for male rats and 300 ppm for females. (1,2)
The burden placed by silicones on the environment is affiliated with the chemistry used to polymerize/depolymerize silicones, which occurs under equilibrating conditions. The equilibrium constant for the polymerizations of this type are very close to 1 (Figure 1), (3) and equilibrium is readily established by acid and base catalysts (including clay minerals). As a consequence, cyclic oligomers remain in the reaction mixture at the end of the polymerization. While most of the low molecular weight materials including cyclics can be removed from higher polymers under vacuum, complete removal is not possible. As a consequence, essentially all silicones and products made from them that are sold in Canada will contain small amounts of [D.sub.4]-[D.sub.6].
Environmental processing of silicones
The environment is exposed to volatile [D.sub.4]-[D.sub.6] in air, and silicone polymers containing [D.sub.4]-[D.sub.6] in landfills (particularly elastomers such as caulking materials), and in water, where they will normally make their way to, and be found in higher concentrations at, wastewater treatment facilities: [D.sub.4] dissolves in water at concentrations estimated to be about 30-60 ppb: [D.sub.5] and [D.sub.6] are less soluble. Silicones do not affect the operation of wastewater plants, consistent with other evidence that silicones are non-toxic to bacteria, and that microorganism-mediated hydrolysis/ oxidation is occurring at such sites. (4)
When finished silicone products come into contact with catalysts, the equilibrium (Figure 1) can be shifted back to cyclics (5) and in the presence of water, particularly at the very high concentration of water found in the environment, (6) is further pushed through hydrolysis back to the monomer [Me.sub.2]Si[(OH).sub.2] (dimethylsilanediol). Cyclics are not products of the environmental depolymerization process of silicone polymers. (13) In soil, depolymerization of the silicone to monomer can occur in as little as 1-2 weeks. (7-10) Although water is required for hydrolytic depolymerization, the reaction is much faster in drier soils where direct contact between silicone and clay is not hindered by a layer of water. (11,12) Microbes in composted sewage sludge are able to depolymerize silicones including cyclosiloxanes to dimethylsilanediol: thus, both biotic and abiotic mechanisms for silicone depolymerization have been demonstrated. (14)
The second stage of remediation of silicones by the environment involves oxidation. All naturally occurring silicon-derived compounds on the planet are fully oxidized as a consequence of the much higher bond strength of Si-O bonds (ca. 128 kcal [mol.sup.-1], 536 kJ [mol.sup.-1]) compared to Si-C bonds (ca. 88 kcal [mol.sup.-1], 369 kJ [mol.sup.-1]). (15) The oxidative degradation of silicones in the environment, as shown with studies using 14C labelled [D.sub.4], is mediated both by microorganisms (16,17) (and mammals (18,19)), but primarily by abiotic chemical reactions involving hydroxy radicals.
The complete breakdown of [Me.sub.2]Si[(OH).sup.2] to Si[O.sub.2] and C[O.sub.2] (20,21) occurs reasonably rapidly, typically within a few months to years. (22) The reaction cascade involves photooxidation in the presence of "suitable chromophores" such as nitrogen oxides. (23) The role of the nitrogen oxide is to facilitate formation of hydroxy radicals H[O.sup.*]), which is the environmental oxidant. (22) The "tropospheric lifetimes" (24) ranged from ca. 2.5 days for [Me.sub.3]SiOH, 9 days for MM ([Me.sub.3]SiOSi[Me.sub.3]), 10 days for [D.sub.5] to ca. 30 days for [D.sub.3]. (24) [D.sub.4] is similarly understood to be completely degraded in the atmosphere within 10 to 30 days. The degradation of silicones to silica is not associated with lower atmosphere aerosol formation (smog). (25,26)
[FIGURE 1 OMITTED]
[D.sub.4]-[D.sub.6] efficiently volatilize during use, and from water and soil interfaces after disposal in the environment: the compounds undergo oxidative degradation to silica in air. Depolymerization and oxidation also occur in soil: degradation occurs in sediment as well, although significantly less quickly.
Thus, in the environment there is an efficient, closed loop, "silicon cycle." Silicones, including cyclosiloxanes [D.sub.4]-[D.sub.6], produced by human industrial processes from silica are efficiently restored to silica through hydrolysis and oxidation. Both steps are effectively driven by thermodynamics and by biotic and abiotic processes that facilitate conversion from SiC to SiO bonds. These data should reassure Canadians, including the government, that the environmental burden posed by [D.sub.4]-[D.sub.6] is small in the first place, and readily met by normal environmental regenerative processes.
Toxicity to non-aquatic organisms
At least hundreds of millions of kg of silicones in various forms have been imported into Canada over the last six-seven decades as finished products, or as starting materials for further processing. The record of safety of silicones is extraordinarily good. Silicones, including silicone oils, elastomers and volatile fluids, for example, [D.sub.4]-[D.sub.6] which are used in a wide variety of personal care products, are handled by producers and consumers alike with no special precautions required. The lack of special precautions is based on experience with these materials. There is a long history of human exposure to silicones, including [D.sub.4]-[D.sub.6], in developed economies including Canada, with no epidemiological research showing temporary or systemic effects of contact with silicones at their normally used concentrations. (27)
The Draft Assessment acknowledges (surprisingly--since [D.sub.4]-[D.sub.6] have been of concern to Environment Canada since at least 1999) that there is a paucity of Canadian data for quantities released into the environment, in sewage sludge, in biota or in the general environment. This, in the author's view, is unfortunate as it means decisions are going to be made based on computer models and other data that may not be appropriately validated. Although Environment Canada called for public comment on their proposals, it is difficult to effectively respond as unpublished data and protocols were not made available.
Of the empirical data analyzed, most is exceptionally reassuring: the environmental concentrations required for a toxic effect in most aquatic organisms that have been tested are higher than that of a saturated solution of the most soluble of the three cyclics, [D.sub.4]. [D.sub.4] has been shown to accumulate in fathead minnows, reaching a steady state at about seven days. The organism can 'steadily but slowly' eliminate the silicone. (28) During these studies, there was no evidence of mortality or other effects of contact with [D.sub.4].
The main concern in the Draft Assessment appears to be a study reported in 1995, the toxicity to four marine organisms exposed to [D.sub.4] was measured: (29) daphnids (Daphnia magna), rainbow trout (Oncorhynchus mykiss), mysids (Mysidopsis bahia), and sheepshead minnow (Cyprinodon variegatus). Environment Canada notes, "[D.sub.4] exhibited significant mortality at 0.015 mg/L during the 21-day chronic toxicity study for the water flea, Daphnia magna, an important species of zooplankton in ecosystems. The chronic NOEC for Daphnia magna is 0.008 mg/L for survival and reproduction, and the lowest-observed-effect concentration (LOEC) for survival is 0.015 mg/L (Sousa et al. 1995)." But the authors of the study came to a different conclusion, "it can be concluded from the present work that, under natural environmental conditions, OMCTS (D4) would not be expected to adversely affect aquatic organisms such as fish and invertebrates exposed in the water column." Reconciling these different opinions requires at least more data, particularly data relevant to the Canadian environment.
Much of the concern about cyclic siloxanes expressed by Environment Canada is associated with their persistence in sediment. Related studies to those above were undertaken with silicone contaminated sediment to address this specific concern: PDMS will contain cyclic silicones. The benthic macroinvertebrates Hyalella azteca (amphipod) and larvae of Chironomus tentans (midge) showed no evidence of toxicity due to contact with PDMS over short and long term exposure. (30)
[D.sub.4] was selected in 1984 as an early test case for regulatory examination of compounds of potential concern in the United States (under the Toxic Substances Control Act). (31,32) As a consequence, there was a flurry of scientific data collection at that time to assess the potential of [D.sub.4] to elicit harm to the environment. The factors considered were strikingly similar to those considered in the Draft Screening Assessment: delivery to air, water, soil, sediment; toxicity to organisms; ability of the environment to process [D.sub.4]. However, the conclusion drawn by the US regulatory agencies was "Based on all available evidence, the risk of OMCTS ([D.sub.4]) to aquatic ecosystems is characterized as very low." (33)
The Canadian government has an obligation to protect the health of its citizens and its environment. Careful assessments of the known and potential risks need to be undertaken. It is appropriate to take a conservative stance and, in the case of new risks from new compounds, extra caution is warranted. Cyclic siloxanes do not fall into such a category. [D.sub.4]-[D.sub.6] have been extensively investigated for their behaviour in humans, other organisms, and in the environment and shown to be essentially benign. There is a complete "silicon cycle": silicones, including [D.sub.4]-[D.sub.6], once in the environment, return to the more stable form of silicon--silica--and do so readily. Half-lives are approximately 1-2 weeks for cyclics entering the atmosphere by air--the most common route. This transformation is mediated by both biotic and abiotic hydrolysis and oxidation: the methyl groups on cyclosiloxanes are oxidized just like other organic methyl groups present in naturally occurring biological systems.
Prudence dictates that potential environmental risks, from silicones or other manufactured compounds, should be monitored. Thus, a detailed study of the presence of [D.sub.4]-[D.sub.6] in various geographic locations, and in different compartments of the Canadian environment should be undertaken. However, a careful and objective analysis of the data presented in the Draft Screening Assessments, in this report, and elsewhere, provide compelling evidence that presently [D.sub.4]-[D.sub.6] pose neither a threat to humans nor to the environment and that no regulatory action is currently required. It is not possible to fully comment and analyze additional data on which Environment Canada's proposal is based--they were not made available. Removing [D.sub.4]-[D.sub.6] from commerce would markedly reduce the quality of life of Canadians, including economic outcomes, with no significant beneficial change to the environment.
(1.) Meeks, R. G.; Stump, D. G.; Siddiqui, W. H.; Holson, J. F.; Plotzke, K. P.; Reynolds, V. L., Reproductive Toxicology, 2007, 23, pp. 192-201.
(2.) Siddiqui, W. H.; Stump, D. G.; Reynolds, V. L.; Plotzke, K. P.; Holson, J. F.; Meeks, R. G., Reproductive Toxicology, 2007, 23, pp. 216-225.
(3.) Chojnowski, J., In Siloxane Polymers; Clarson, S. J., Semlyen, J. A., Eds. (Prentice Hall: Englewood Cliffs, NJ, 1993), p 1-.
(4.) Watts, R. J.; Kong, S.; Haling, C. S.; Gearhart, L.; Frye, C. L.; Vigon, B. W., Water Research, 1995, 9, p. 2405.
(5.) Kantor, S. W.; Grubb, W. T.; Osthoff, R. C., Journal of the American Chemical Society, 1954, 76, p. 5190.
(6.) Spivack, J.; Dorn, S. B., Environmental Science and Technology, 1994, 28, p. 2345.
(7.) Buch, R. R.; Ingebrigstson, D. N., Environmental Science and Technology, 1979, 13, p. 676.
(8.) Lehmann, R. G.; Varaprath, S.; Annelin, R. B.; A. Arndt, J. L., Environmental Toxicology and Chemistry, 1995, 14, p. 1299.
(9.) Lehmann, R. G.; Frye, C. L.; Tolle, D. A.; Zwick, T. C., Water, Air & Soil Pollution, 1995, 83, p. 1.
(10.) Carpenter, J. C.; Celia, J. A.; Dorn, S. B., Environmental Science and Technology, 1995, 29, p. 864.
(11.) Lehmann, R. G.; Varaprath, S.; Frye, C. L., Environmental Toxicology and Chemistry, 1994, 13, p. 1061.
(12.) Lehmann, R. G.; Miller, R. L.; Xu, S.; Singh, U. B.; Reece, C. F., Environmental Science and Technology, 1998, 32, p. 1260.
(13.) Stevens, C., Journal of Inorganic Biochemistry, 1998, 69, p. 203-207.
(14.) Grumping, R.; Michalke, K.; Hirner, A. V.; Hensel, R., Applied Environmental Microbiology, 1999, 65, pp. 2276-2278.
(15.) Brook, M. A., In Silicon in Organic, Organometallic and Polymer Chemistry; (Wiley: New York, 2000), p. 27-38.
(16.) Lehmann, R. G.; Varaprath, S.; Frye, C.L., Environmental Toxicology and Chemistry, 1994, 13, p.1753.
(17.) Lehmann, R. G.; Miller, J. R.; Collins, H. P., Water, Air & Soil Pollution, 1998, 106, pp. 111-122.
(18.) McKim, J. M., Jr.; Kolesar, G. B.; Jean, P. A.; Meeker, L. S.; Wilga, P. C.; Schoonhoven, R.; Swenberg, J. A.; Goodman, B. L.; Gallavan, R. H.; Meeks, R. G., Toxicoloigcal and Applied Pharmacology, 2001, 172, pp. 83-92.
(19.) Sarangapani, R.; Teeguarden, B.; Plotzke, K. P.; McKim, J. M.; Andersen, M. E., Toxicological Sciences 2002, 67, 159-172.
(20.) Lehmann, R. G.; Miller, J. R., Environmental Science and Technology, 1996, 15, pp. 1455-1460.
(21.) Atkinson, R., Environmental Science and Technology, 1996, 25, p. 863.
(22.) Lentz, C. W., Industrial Research and Development, 1980, p. 139.
(23.) Buch, R. R.; Lane, T. H.; Annelin, R. B.; Frye, C. L., Environmental Science and Technology, 1984, 3, p. 215.
(24.) Sommerlade, R.; Parlar, H.; Wrobel, D.; Kochs, P., Environmental Science and Technology, 1993, 27, p. 2435.
(25.) Carter, W. P. L., Air Waste, 1994, 44, p. 881.
(26.) Carter, W. P. L.; Atkinson, R., Environmental Science and Technology, 1989, 23, p.864.
(27.) Safety of Silicone Breast Implants; (Institute of Medicine, National Academy Press: Washington, 2000).
(28.) Fackler, P. H.; Dionne, E.; Hartley, D. A.; Hamelink, J. L., Environmental Toxicology and Chemistry, 1995, 14, pp. 1649-1656.
(29.) Sousa, J. V.; McNamara, P. C.; Putt, A. E.; Machao, M. W; Surprenant, D. C.; Mahelink, J. L.; Kent, D. J.; Silberhom, E. M.; Hobson, J. F., Environmental Toxicology and Chemistry, 1995, 14, pp. 1639-1647.
(30.) Henry, K. S.; Wieland, W H.; Powell, D. E.; Giesy, J. P., Environmental Toxicology and Chemistry, 2001, 20, pp. 2611-2616.
(31.) Hobson, J. F., Environmental Toxicology and Chemistry, 1995, 14, pp. 1635-1638.
(32.) Walker, J. D.; Smock, W. H., Environmental Toxicology and Chemistry, 1995, 14, pp. 1631-1634.
(33.) Hobson, J. F.; Silberhorn, E. M., Environmental Toxicology and Chemistry, 1995, 14, pp. 1667-1673.
Michael A. Brook, MCIC, is a professor of chemistry at McMaster University. His research focuses on polymer and materials synthesis, mostly using silicon chemistry.
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|Author:||Brook, Michael A.|
|Publication:||Canadian Chemical News|
|Date:||Nov 1, 2008|
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