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

4 The Nature, Use, Distribution, Concentrations, and Toxicity of Siloxane D5

• 4.1 Use and Release of Siloxane D5 in the Canadian Environment
• 4.2 Distribution and Persistence of Siloxane D5 in the Environment
  • 4.2.1 Use of Models and Tools for Assessing Environmental Fate and Distribution of Siloxane D5
  • 4.2.2 Air
    • 4.2.2.1 Processes Affecting Fate and Distribution in Air
    • 4.2.2.2 Concentrations in Air
  • 4.2.3 Water
    • 4.2.3.1 Processes Affecting Fate and Distribution in Water
    • 4.2.3.2 Concentrations in Water
  • 4.2.4 Sediments
    • 4.2.4.1 Processes Affecting Fate and Distribution in Sediments
    • 4.2.4.2 Concentrations in Sediments
  • 4.2.5 Soils
    • 4.2.5.1 Processes Affecting Fate and Distribution in Soils
    • 4.2.5.2 Concentrations in Soils
• 4.3 Persistence, Bioaccumulation and Trophic Magnification of Siloxane D5 in the Environment
  • 4.3.1 Persistence
  • 4.3.2 Accumulation
    • 4.3.2.1 Bioaccumulation Factor
    • 4.3.2.2 Bioconcentration Factor
    • 4.3.2.3 Biota-sediment Accumulation Factor
    • 4.3.2.4 Biomagnification Factor
    • 4.3.2.5 Trophic Magnification Factor
    • 4.3.2.6 Depuration Rates
    • 4.3.2.7 Fugacity Analysis
  • 4.3.3 Conclusions on Persistence, Bioaccumulation and Trophic Magnification of Siloxane D5 in the Environment
• 4.4 Toxicity of Siloxane D5 to Receptor Organisms in the Environment
  • 4.4.1 Terrestrial Animals
  • 4.4.2 Terrestrial Plants
  • 4.4.3 Aquatic Organisms
  • 4.4.4 Sediment-dwelling Organisms
  • 4.4.5 Conclusions on Toxicity of Siloxane D5 to Receptor Organisms in the Environment

4.1 Use and Release of Siloxane D5 in the Canadian Environment

104. Siloxane D5 is an odourless, colourless liquid that is used in consumer and industrial applications. It is mainly used in blending and formulating personal-care products and cosmetics, and is an intermediate in the production of polydimethylsiloxane silicone polymers. A few commercial dry cleaners in Canada also use Siloxane D5 as a dry-cleaning fluid. Its use in silicone polymers and in dry-cleaning is not considered to be a significant source of release to the environment (EC & HC 2008, p. 9).

105. In Canada and worldwide, the most important uses of Siloxane D5 are in the preparation of personal-care products, including antiperspirants, and hair- and skin-care products (EC & HC 2008, p. 9). Current use of Siloxane D5 in personal-care products in Canada was estimated to be 3.3 million kg/yr in 2010 (SEHSC/CCTFA 2011, p. 21). In 2010, antiperspirants and/or deodorants accounted for 72.2% of Siloxane D5 use in personal-care products. This was followed by hair-care products at 19.4%, skin-care products at 2.7%, colour cosmetics at 2.6%, sunscreens at 1.1%, and several other uses totalling 1.9% (CCTFA 2011a).

106. Due to its large vapour pressure and volatility, the major route of release of Siloxane D5 from personal-care products is to the atmosphere (CCTFA 2011b). For example, in antiperspirants, Siloxane D5 is mostly lost to the air with less than 1% available for wash-off eight hours after application and less than 0.1% available for wash-off after 24 hours.

107. Similar losses to air were observed in other uses where the product was applied directly to the skin or to hair (after washing). However, in the case of hair-care products such as hair conditioners that are rinsed off after in-shower use, approximately 40% of the Siloxane D5 enters drains and will subsequently be transported to wastewater treatment plants (WWTPs) (CCTFA 2011b).

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4.2 Distribution and Persistence of Siloxane D5 in the Environment

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

109. Once released into the environment, chemicals can undergo transformations as they move from one physical location to another and/or among environmental compartments. These transformations can be due to biological and physical-chemical processes, such as hydrolysis and photolysis, and produce substances that are different from the original, parent chemical. The rate and degree of transformation determines the volume of the chemical which is available to potentially accumulate in the environment.

110. In characterising the distribution of Siloxane D5 in various compartments and its persistence in the environment, the Board kept in mind that it is a unique chemical compound, whose behaviour is different from that of other compounds of similar molecular weight and size (Mackay 2011a).

111. Siloxane D5 consists of carbon, silicon, oxygen, and hydrogen in a symmetrical ring structure. This compound has intrinsic chemical and physical properties that result in unique patterns of distribution in the environment (EC 2011a, Tables 4 and 5, p. 21-22, SEHSC. 2011a). These properties were carefully considered in evaluating the exposure of organisms in the environment by Siloxane D5 and the danger, if any, it poses.

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4.2.1 Use of Models and Tools for Assessing Environmental Fate and Distribution of Siloxane D5

112. When a chemical is being evaluated and where there are limited empirical measurements of concentrations in the environment, models can be used to estimate releases to the environment, as well as its fate and distribution after release. Although Siloxane D5 has been in use for over 30 years, there was limited measured information on concentrations in, or effects on, the environment. Consequently, models featured prominently in the Screening Assessment conducted by government officials.

113. The Board carefully reviewed the models and tools applied in the Screening Assessment (EC & HC 2008) and subsequent modifications made to them and to their input parameters. The Board concluded that these models and tools had several limitations and inaccuracies.

114. The Board is of the view that those shortcomings resulted in inaccurate predictions of environmental fates. Consequently, the interpretations based on these models and tools were of limited utility to this review. Now that empirical monitoring data are available, the Board gave greater weight to these measured values than the initial estimates made by the MegaFlush model (EC 2009) and MassFlow tool (EC 2008a).

115. When conducting the Screening Assessment (EC & HC 2008), the MassFlow tool and the MegaFlush model were used to estimate releases to the environment via wastewater. As an input parameter to the MassFlow tool, government officials had estimated that 12.2% of the Siloxane D5 used by industry and in personal care products was released to sewers. (EC & HC 2008 1632, Table 3, p. 12).

116. In this proceeding, the SEHSC proposed that 9-9.5% was a more reasonable estimate of the rate of release to the environment via wastewater (Cowan-Ellsberry and Mackay 2011b, p. 7) and that this was a rate that was also consistent with that specified by Environment Canada in the MassFlow tool documentation (EC 2008b).

117. This estimated rate was confirmed by apportioning the use of Siloxane D5 in Canada (26.9 mg/capita-day) and an average per capita wastewater treatment plant (“WWTP”) flow of 495 L/capita-day, giving a value of 54 µg/L (Cowan-Ellsberry 2011, p. 6). This amount is in agreement with the 95th centile concentration measured in influents from non-industrial sources to WWTPs (approximately 47 µg/L based on tabular data from Wang et al. 2010).

118. Consequently, after considering all the data submitted during this review process, the Board agreed that 9.5% was a more reasonable, yet conservative, estimate of releases of Siloxane D5 to sewers than the value of 12.2% used in the Screening Assessment (EC & HC 2008 1632, Table 3, p. 12).

119. The Board also examined the removal rate parameters used in the MegaFlush model during the conduct of the Screening Assessment. In the Screening Assessment, removal rates for Siloxane D5 were assumed to be 0% for lagoons and 48% for primary treatment (SEHSC. 2011a).

120. Information presented during this review indicated that actual removal rates are greater. For example, data based on the ASTREAT model presented to the Board suggests that the removal rate for lagoons, primary treatment plants, and secondary treatment plants is of the order of 97% (Cowan-Ellsberry 2011, p. 6). This is similar to the actual mean removal rates of 98% recorded in the United Kingdom (Cowan-Ellsberry 2011, p. 7, CCTFA 2011b, p. 432).

121. Concentrations of Siloxane D5 were measured at WWTPs in Canada in summer and winter (Wang et al. 2010, Wang et al. 2011a). These measurements can be used to verify the assumptions used in the MegaFlush model. Although the samples were not matched to the retention time of the WWTP, the concentrations in samples of influent and effluent taken in summer suggested a mean removal rate of Siloxane D5 of 99.2% for lagoons and 96% for WWTPs when using primary treatment. Mean rates of removal of Siloxane D5 by WWTPs using secondary treatment were found to be 97.8% (based on data from Wang et al. 2010).

122. From results of similar samples taken in winter, removal rates of Siloxane D5 from WWTPs using secondary treatment were not significantly influenced by temperature (all were ≥95% between 10 and 25°C) (Wang et al. 2011a, Figure 3). The Board noted, however, that removal rates from lagoons were highly variable at temperatures ≤5°C, ranging from 25% to 99% at the 11 sites tested (Wang et al. 2011a, Figure 4). However, all rates of removal were greater than the 0% worst-case assumption used for lagoons in the Screening Assessment. Similarly, rates of removal from WWTPs using primary treatment were approximately double the assumption of 48% used in the Screening Assessment.

123. As a result, the Board has concluded that less Siloxane D5 is released to receiving waters from WWTPs than was previously estimated from various models and tools, and that the values measured in samples from selected Canadian locations provide reasonable worst-case estimates of exposures in surface waters.

124. The Board agrees with the SEHSC/CCTFA that overall, based on its unique intrinsic properties, its uses, and the types of treatment for wastewater in Canada, a realistic scenario for Siloxane D5 releases into the environment would be 94.5% to air; 0.8% to water; and, 4.7% to soils via biosolids (Powell 2011, p. 2, Cowan-Ellsberry and Mackay 2011b, p. 4).

125. Since the Screening Assessment was completed, the fugacity-based Equilibrium Criterion simulation model ("EQC model”) (CEMC 2003) has been updated. The Board concluded that the revised estimates of environmental fates and the conclusions drawn from the updated model are more accurate than the estimates used in the Screening Assessment. In particular, the revised structure and input parameters of the model provided to the Board (Mackay 2011b, Kim 2011) more accurately reflect the relative distribution of Siloxane D5 between environmental compartments.

126. Furthermore, recent modifications to the EQC model (Mackay 2011b) better estimate the distribution and residence time of Siloxane D5 (time required to degrade to half its initial concentration). These modifications also permitted the model to take into account some of the intrinsic properties of Siloxane D5 and allowed for direct input of the organic carbon-water partition coefficient (“K_OC”).

127. In the previous version of the EQC model used by Environment Canada, the K_OC was calculated from the octanol water partition coefficient (“K_ow”) (Transcript of the Public Hearings, Vol. 4, p. 700), which, in the Board’s opinion, was inappropriate for Siloxane D5 due to its intrinsic properties.

128. The distribution and the residence times in the various compartments of the environment are summarised below (Table 1).

Data from (Kim 2011, Mackay 2011b)

129. Over time, concentrations of chemicals equilibrate between compartments of the environment including air, water, soils, and sediments. The rate of accumulation of a compound such as Siloxane D5 is determined by the relative rates of release and degradation in the environment.

130. With a relatively constant rate of release, which is the case for Siloxane D5, the rate of accumulation is a function of the rates of degradation through various processes. The conditions when the absolute concentrations in the environment and relative concentrations in the various compartments do not change are referred to as steady-state.

131. As noted earlier, Siloxane D5 has been used for more than 30 years in various commercial and industrial applications. Evidence presented to the Board indicated that the concentrations in the environment as a whole and in its relative compartments were not changing, in any material sense, over time (Transcript of the Public Hearings, Vol. 4, pp. 581 & 651). Based on its rate of degradation in the environment, Siloxane D5 has attained a quasi-steady-state. Consequently, concentrations of Siloxane D5 in the environment vary within predictable ranges.

132. As a result, the Board is of the opinion that concentrations of Siloxane D5 in the environment will not increase significantly. Even if uses of Siloxane D5 were to double in the future (a scenario which is not expected to occur), this would not have a measurable impact on the risk posed to the environment (Transcript of the Public Hearings, Vol. 4, p. 656).

133. 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 metabolically in the organs of higher organisms. Siloxane D5 is biotransformed into silanols, which are more soluble and less active than Siloxane D5 (Environment Agency 2010) and, thus, present less risk and danger to the environment.

134. In the following sections, the fate and distribution of Siloxane D5 in various environmental compartments are reviewed. This review focuses on analyses making use of new methods of measuring concentrations. Historically, a major problem with measuring concentrations of all siloxanes in environmental compartments has been the availability of reliable analytical methods. It is only in the last few years that reliable methods have been developed which minimise sample and equipment contamination-related errors (McLachlan et al. 2010, Kierkegaard and McLachlan 2010, EC 2010c, Wang et al. 2011b). Emphasis was placed on data with respect to concentrations measured in locations in Canada.

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4.2.2 Air

4.2.2.1 Processes Affecting Fate and Distribution in Air

135. Siloxane D5 has a larger vapour pressure and is thus more volatile than other molecules of similar molecular weight and size. Consequently, Siloxane D5 tends to partition (i.e., to be released) into air. Once in air, Siloxane D5 can be transported relatively long distances, but deposition to soils or water is predicted to be limited (Table 1) (Transcript of the Public Hearings, Vol. 4, p. 742).

136. The major process of degradation of Siloxane D5 in the atmosphere is indirect photolysis. In this process, hydroxyl radicals (•OH), formed in the atmosphere by UV-B radiation, degrade Siloxane D5 to dimethylsilanediol and ultimately to carbon dioxide, water, and silicon dioxide (sand). Hydroxyl radicals are widely regarded as the cleaning agent of the atmosphere because they convert many atmospheric chemicals, including major air pollutants, into forms that are more water-soluble and therefore more easily removed from the atmosphere in precipitation.

137. The half-life of Siloxane D5 in air has been estimated to be between 0.6 and 9.8 days, depending on temperature, intensity of solar radiation, presence of precursors for •OH, and other parameters (Environment Agency 2010).

138. The Regulations classify persistent chemicals as those having half-lives in the atmosphere of more than two days. However, an important aspect of the environmental fate of Siloxane D5 is that it is released mostly into the atmosphere. The atmosphere is the compartment where Siloxane D5 predominantly occurs and where it is most rapidly degraded. Because releases are greatest in urban areas, it is noteworthy that half-lives tend to be shorter near urban areas because of the greater concentrations of •OH in these locations (Environment Agency 2010).

139. The Board also considered the question of the potential effects of Siloxane D5 on stratospheric ozone (Shao-Meng 2010). The half-life of Siloxane D5 in the troposphere is considerably less than the 2-3 months that would be required for significant amounts to reach the equatorial regions from where it would be transported to the stratosphere (Transcript of the Public Hearings, Vol. 2, p. 298, Transcript of the Public Hearings, Vol. 3, p. 446, Transcript of the Public Hearings, Vol. 4, p. 657). In addition, Siloxane D5 does not contain ozone depleting substances, such as halogens (Cowan-Ellsberry and Mackay 2011b, Cowan-Ellsberry and Mackay 2011a, Xu 2011c). Consequently, the Board has concluded that stratospheric ozone depletion is not an issue of concern.

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4.2.2.2 Concentrations in Air

140. Siloxane D5 has been measured in ambient air over Sweden at concentrations between 0.3 and 9 ng/m3 (McLachlan et al. 2010), values that are consistent with those predicted from simulation modelling, but less than measured concentrations reported by earlier studies in Sweden (9-170 ng/m³) (Kaj, 2005) and other Nordic countries (5).

141. Total concentrations of siloxanes (of which approximately 75% were Siloxane D5) in ambient air over Canada were 0.3 to 0.4 ng/m³ (EC 2010a). These concentrations were less than those measured in Europe. The Board does not consider these concentrations in Canada to pose a danger to the environment.

142. Total concentrations of siloxanes (of which approximately 75% were Siloxane D5) in air near a WWTP in Canada were greater than in ambient air, with an average of 4,556 ng/m³ and samples downwind from a landfill site were 4,669 ng/m³ for total siloxanes (approximately 75% Siloxane D5) (EC 2010a). These concentrations are more than 80,000-times less than the No-Observed-Adverse-Effect-Level of 380,000,000 ng/m³ reported in rats (Brooke et al. 2009).

143. As for ambient air, where the margins of safety are even greater, the Board determined that these concentrations of Siloxane D5 near sources of emissions did not pose a danger to the environment.

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4.2.3 Water

4.2.3.1 Processes Affecting Fate and Distribution in Water

144. Siloxane D5 undergoes hydrolysis in water, a process of degradation that also involves the hydroxyl ion, OH^–. The final products of degradation of Siloxane D5 in water are carbon dioxide, silicic acid, and/or silicon dioxide.

145. The half-life of Siloxane D5 in freshwater is approximately 315 days at neutral pH and a temperature of 12°C. Hydrolysis is more rapid at pH values greater or less than neutral and also at greater temperature. For example, the hydrolysis half-life of Siloxane D5 at pH 8 and 9°C in saltwater was 64 days (Brooke et al. 2009).

146. In addition to degradation, siloxanes can partition out of water into other environmental compartments (Table 1), which can result in decreases in concentration in the water phase.

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4.2.3.2 Concentrations in Water

147. Concentrations of Siloxane D5 in surface waters downstream from WWTPs processing industrial effluents in Europe were reported as less than the limit of detection (“LOD”) (0.02 µg/L). Concentrations in effluents from these same WWTPs ranged from 0.22 to 26.7 µg/L (summarised in Brooke et al. 2009).

148. A recent study reported concentrations of Siloxane D5 in surface freshwater in locations close to WWTPs in Canada (Wang et al. 2010). In samples taken at 11 locations, from a few meters to 3.1 km away from the outfall, geometric mean concentrations ranged from less than the limit of quantification (“LOQ”) of 0.004 to 1.48 µg/L. The potential risks to aquatic organisms from these measured concentrations are discussed in section 5.2 below. The geometric mean concentrations of Siloxane D5 in samples of effluent from WWTPs, collected at the same time as samples from adjacent surface water, ranged from less than the LOQ (0.004 µg/L) to 1.56 µg/L.

149. In a follow-up study to characterise the effect of seasonality on concentrations of Siloxane D5 in influents and effluents from 13 WWTPs (Wang et al. 2011a), geometric mean concentrations measured in effluents collected in the summer ranged from less than the LOQ (0.004) to 1.56 µg/L (discussed above from Wang et al. 2010), while those collected in winter ranged from arithmetic mean values of 0.53 to 466 µg/L.

150. The greatest concentrations detected in winter were all associated with effluents from one WWTP, Site 8. This particular WWTP received influents from an industrial operation where Siloxane D5 is used. Measured concentrations of Siloxane D5 in influents at that site ranged from 261 to 4,400 µg/L over 16 days. This variation is consistent with unintentional releases from industrial operations. Concentrations of Siloxane D5 in the influent stream in the summer were less, with a geometric mean of 134 µg/L. Site 8 also had the greatest measured concentration of Siloxane D5 in surface waters at a location 1.26 km from the outfall of the WWTP (Wang et al. 2011a). Given the unique circumstances of Site 8, the Board is of the opinion that it represents an extreme worst-case scenario and is not representative of concentrations released into the environment in other locations in Canada.

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4.2.4 Sediments

151. It is important to express concentrations of constituents in sediments consistently. Normalisation to dry-weight (dw) is commonly used. Because Siloxane D5 partitions into organic matter, normalisation to the amount of organic matter or the amount of organic carbon (OC) in the sediments is appropriate and allows assessment of the proportion of the total solubility that has been achieved. This approach is suitable for assessing partitioning from sediments to organisms (see section 4.3.2.3 below).

152. For the purposes of characterising risks, any method of normalisation is appropriate as long as the methods are consistent between toxicity testing and concentrations in the environment. In the following sections, normalisation to (dw) is used.

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4.2.4.1 Processes Affecting Fate and Distribution in Sediments

153. Siloxane D5 has a strong affinity for OC particles and tends to partition into sediments when released into water (Table 1). The Board agreed with the statements of the parties at the hearing that the greatest potential for danger would be in sediments within 3 km downstream from WWTP outfalls (Transcript of the Public Hearings, Vol. 3, p. 542).

154. In addition, Siloxane D5 is more persistent in sediments than in other environmental compartments. Measured half-lives of radiolabelled (14C) Siloxane D5 in natural sediments from Lake Pepin, Minnesota were reported to range from 1,200 to 3,100 days (Xu 2011b). The aerobic half-life in non-sterilised sediment was the shortest (1,200 days). This is the most appropriate value for characterising the rate of dissipation of Siloxane D5 found in sediments in the environment.

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4.2.4.2 Concentrations in Sediments

155. Concentrations of Siloxane D5 in sediments have been measured in a number of locations. Concentrations measured in sediments from Europe and the United Kingdom ranged from non-detectable (1 µg/kg) to 280 µg/kg (dw) (summarised in Brooke et al. 2009). Concentrations in sediments collected in Sweden ranged from non-detectable (variable limits of detection or “variable LOD”) to 190 µg/kg (dw) (Kaj, 2005) and from non-detectable (variable LOD) to 2,000 µg/kg (dw) in other Nordic countries (Nordisk Ministerråd and Nordisk Råd 2005).

156. Some measurements of Siloxane D5 were made in samples of sediments from Lake Ontario in 2007 (Powell and Kozerski 2007). In five core samples, concentrations of Siloxane D5 were less than the LOD (4.7 µg/kg ww). However, one “grab” sample from Toronto Harbour contained concentrations of 358 µg/kg (ww) (equivalent to 790 µg/kg (dw)).

157. Concentrations of Siloxane D5 measured in sediments collected downstream from 11 WWTPs in Canada were reported by Wang et al. (2010). Two or more grab samples were taken at each site and these were treated as separate samples. The geometric mean concentrations of triplicate analyses of Siloxane D5 ranged from 0.021 to 7.6 µg/kg (dw).

158. The two greatest geometric mean concentrations, 4.0 and 7.6 µg/kg (dw), were measured in water downstream from Site 8, a site where Siloxane D5 is used in industrial applications (Wang et al. 2011a). These are considered worst-case and not representative of values generally found in WWTP outfalls in Canada.

159. The potential risks to benthic organisms from these measured concentrations are discussed in section 4.4.4 below.

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4.2.5 Soils

160. Based on the evidence reviewed, the Board concluded that there will be negligible deposition of Siloxane D5 from air to soils via rainfall and/or snow. Consequently, precipitation would not represent a source of Siloxane D5 in either the near-field or the far-field (i.e., Arctic).

161. The major pathway of Siloxane D5 into soils is via application of biosolids from WWTPs. Siloxane D5 binds to biosolids which are applied to agricultural lands as an amendment to soils to improve fertility.

162. Concentrations of Siloxane D5 have been documented in biosolids and sewage sludge (summarised for Europe by Brooke et al. 2009) and ranged from 1,100 to 89,000 µg/kg (dw) in Nordic countries. Concentrations in biosolids sampled from WWTPs in Canada ranged from 28,000 to 328,000 µg/kg (dw) (Wang et al. 2010). Since biosolids are not an environmental compartment, but rather a conduit for materials to be transported to soil, the Board focused on the fate and concentration of Siloxane D5 in soils.

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4.2.5.1 Processes Affecting Fate and Distribution in Soils

163. There are two mechanisms by which Siloxane D5 is removed from soils (Xu and Chandra 1999). The first is partitioning to air via moisture-aided volatilisation. This is the dominant process in wet soils. The second is degradation via hydrolysis that is catalysed by clay particles. This process predominates in dry soils (Xu 2011a). The half-life for degradation of Siloxane D5 in air-dry soil is 1.9 hours. From these data, the Board determined that Siloxane D5 will neither persist nor accumulate in soils to which biosolids have been added.

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4.2.5.2 Concentrations in Soils

164. In Canada, concentrations of Siloxane D5 were measured in agricultural soils from 11 sites where soils had been amended with biosolids from WWTPs (Wang et al. 2010). In addition, samples of soils from two sites without amendment were also analysed.

165. Although Siloxane D5 was detected at all sites, concentrations varied among soils taken from different locations on the same farm. When sub-samples on a site were treated as independent measures, concentrations ranged from less than the LOQ (0.003 mg/kg (dw)) to 0.899 mg/kg (dw).

166. Concentrations of Siloxane D5 measured in two samples of soils amended with biosolids in Europe were less than the LOD (0.0001 mg/kg) and 0.01 mg/kg (dw) (Nordisk Ministerråd and Nordisk Råd 2005).

167. Potential risks of adverse effects on plants and soil-dwelling organisms from these measured concentrations are discussed in section 5.1 below.

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4.3 Persistence, Bioaccumulation and Trophic Magnification of Siloxane D5 in the Environment

4.3.1 Persistence

168. The Board was asked to determine if Siloxane D5 presented a danger to the environment. While persistence might be a contributor to the potential for a substance to be a danger, it is not necessarily an indicator of danger in and of itself. There are many natural and synthetic substances that are persistent but do not cause harm or danger to the environment.

169. The persistence of a chemical is an estimate of how long it will remain in the environment or an organism. There can be both global and local persistence. While some chemicals are not transformed or degraded in the environment, others undergo various processes through which they are transformed to other chemicals or are degraded in a manner that they are ultimately mineralised or decomposed into their constituent elements.

170. Whether chemicals degrade, how rapidly they degrade, and into what components they degrade are all important considerations when conducting a risk assessment. Chemicals can also persist in organisms by binding to proteins or being stored in fat tissue from which they are released slowly, enabling the chemical to remain in the body for extended periods of time. In the case of Siloxane D5, this chemical does not bind to molecules in the organism and is not soluble in fats or lipids.

171. In the Screening Assessment, government officials determined that, pursuant to the Regulations, Siloxane D5 was persistent. Based upon the thresholds in the Regulations (see section 3.4 above), a persistent chemical is one for which its half-life is ≥2 days in air; ≥182 days in water; ≥365 days in sediments; or ≥82 days in soils. Those thresholds were established because, during those periods of time, a chemical could possibly contribute to local or widespread effects.

172. From the information in section 4.2, the half-lives of Siloxane D5 in air, water, and sediments exceeded the threshold values in section 3 of the Regulations. The only compartment of the environment where the half-life did not exceed the threshold was for soils. All parties at the hearings and the Board agreed with this characterisation of persistence.

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4.3.2 Accumulation

173. Although Siloxane D5 meets the thresholds for persistence pursuant to the Regulations, it will only be a danger to the environment if its persistence results in exposures that cause adverse effects in the environment.

174. Consequently, 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. Whether this occurs depends upon other intrinsic properties of the chemical and its environment. In this report, the Board discusses how these properties interact with persistence and whether this results in danger to the environment.

175. To cause an effect, a chemical must enter into an organism or at least interact with its external membranes (i.e., skin, gills, etc.). Siloxane D5 can enter organisms through several pathways, including inhalation, or across the integument such as the skin or gills of fish or benthic invertebrates, or the lining of the gastrointestinal tract. However, the primary route of exposure for all organisms is via the diet and/or water. Following this exposure, the process by which neutral, unreactive molecules, such as Siloxane D5, enter into organisms is by molecular diffusion.

176. Diffusion, by definition, is a first-order process that is dependent on the difference in concentration (gradient) and some rate constant that is a function of the physical and chemical properties of the molecule and the membranes of the organism. There are two rates that need to be considered, the rate of accumulation and the rate of depuration, or loss from the organism across all pathways.

177. Under conditions of constant exposure, which is the case when chemicals are considered to be at a steady-state, the concentration that can be achieved in an organism is determined solely by the depuration rate (see section 4.3.2.6 below).

178. There are three mechanisms by which a chemical can be accumulated into organisms. The first is bioconcentration, where the chemical is accumulated in organisms in concentrations that are greater than those in the surrounding environmental compartment. The bioconcentration factor (“BCF”) is the ratio of the concentration in the organism to that in the matrix surrounding the organism.

179. The second mechanism is bioaccumulation, the process by which chemicals accumulate in organisms from the medium surrounding the organisms, including sources such as food. The bioaccumulation factor (“BAF”) is the ratio of the concentration in the organism to that in the surrounding environmental medium. If sediments are included as a matrix, this ratio is the biota-sediment accumulation factor (“BSAF”).

180. The BSAF is defined as the ratio of the concentration in an animal, such as a benthic invertebrate or a fish, normalised to lipid content of the animal, divided by the concentration of the chemical of interest in the sediment, normalised to the OC content of the sediment.

181. The BSAF is a thermodynamic constant that allows the prediction of concentrations in organisms from that in sediments by correcting for the effects of lipids and OC on the distribution characteristics of a chemical. The BSAF has the units of kg OC divided by kg lipid (kg OC/kg lipid). BSAF values tend to be site-specific due to the many environmental factors that affect it. For this reason, the BSAF is better used in a comparative, rather than a predictive, manner.

182. The BSAF can best be used to interpret the degree to which a chemical will likely be accumulated into organisms from sediments. Since some Siloxane D5 is expected to partition to sediments, the BSAF is a relevant parameter to use in assessing the behaviour of Siloxane D5 in the environment, particularly for organisms associated with sediments, such as benthic invertebrates. Chemicals with BSAF values of about 500 would normally be expected to biomagnify (Gobas et al. 2011 pp. 26-28).

183. The third mechanism is biomagnification, or trophic magnification, in which concentrations in organisms become greater with successively higher trophic levels in the food chain. In this process, the organism at the higher trophic level, such as a predator, accumulates a greater concentration than the organism at the lower trophic level, such as the prey.

184. Biomagnification occurs for persistent compounds such as certain polychlorinated biphenyls (“PCBs”) that are slowly, or not, biotransformed. This causes concentrations of more persistent residues that associate with lipids in the prey to increase which, in turn, results in a concentration gradient that forces the more persistent compound into the organism eating the prey. This concentration gradient and the resulting propensity of the chemical to migrate from the greater to the lesser concentration are referred to as fugacity (see the discussion beginning in paragraph 189 below).

185. At steady-state, the ratio of the concentration of the compound in an organism to that in its food is the biomagnification factor (“BMF”). The relationship between concentrations of a chemical in organisms at various trophic levels in a food chain and their position in the food chain is the trophic magnification factor (“TMF”).

186. Information about BMF and TMF is important for assessing exposures in organisms higher on the food chain. In characterising bioaccumulation and trophic magnification of Siloxane D5, the primary focus of the Board was on aquatic organisms, for which measured data were available.

187. TMFs are derived under field conditions from analyses of organisms at various trophic levels for the chemical in question as well as from the ratio of two isotopes of nitrogen, ¹⁵N/¹⁴N. Organisms higher on the food chain concentrate the ¹⁵N isotope and this is used as a surrogate for trophic level.

188. TMF values less than 1.0 indicate biodilution where organisms biotransform the chemical more rapidly than they accumulate it and the concentrations decrease with increasing trophic level. A TMF greater than 1.0 is indicative of biomagnification up the food chain.

189. Fugacity is a concept that relates to movement of chemicals among compartments of the environment. At steady-state, while individual molecules are still moving between and among compartments, concentrations of a chemical of concern in each compartment do not change.

190. Fugacity can be thought of as a forcing function with the units of pressure (Pa). That is to say, fugacity is the tendency for a substance to move from one compartment into another. The fugacity capacity (Z) of a system allows comparison of concentrations of a chemical in different compartments by expressing them on a common basis.

191. The fugacity ratio is an indicator of the propensity of a chemical to bioconcentrate. More specifically, it is the ratio of the fugacity of a chemical in an organism relative to that in the compartment of the environment to which it is exposed, such as water or sediments. A chemical which biomagnifies has a fugacity ratio that exceeds 1.0.

192. When determining the potential for a chemical to produce adverse effects, it is important to assess whether organisms are likely to be exposed to the chemical. One aspect of this exposure assessment is to estimate the potential for the compound to enter into organisms by examining the BCF, BAF, BSAF, and/or TMF. Several approaches can be taken depending on the information available and the tier of the risk assessment. Accumulation factors can be measured under either controlled laboratory conditions, estimated from field exposures, or predicted by models.

193. If a chemical is being assessed and little information is available, its accumulation or persistence profile could be predicted by the use of simple linear free-energy models (correlations) from first or second principles. These are measures of the physical and chemical properties of the compound of interest and can include measures of properties in its pure form, such as water solubility, and distribution between water and organic solvents, such as the K_ow.

194. In the simplest approach, the values of BAF, BCF, or some surrogate estimator such as K_ow are compared to reference values. Based on historical information for other classes of chemicals, section 4 of the Regulations establishes threshold values of 5,000 for the BAF and BCF. In circumstances where measured values are available, such as in this case for BCFs, it is not necessary to consider the K_ow.

195. If a chemical meets a threshold, it is identified as a chemical of concern because of its potential for accumulating into organisms and in the environment. These thresholds should be used primarily to screen chemicals out from further consideration or to prioritise chemicals for further study. Simply because the BCF or BAF of a chemical exceeds the reference value does not, in and of itself, determine that the chemical will pose a danger. It is instead an indicator of the potential to accumulate.

196. As the Board noted earlier, section 5 of the Regulations requires consideration of the intrinsic properties of a chemical. In the case of Siloxane D5, this would include:

• its unique physical and chemical properties;
• how it enters the environment;
• how it moves among compartments; and,
• the manner by which it is degraded.

197. These additional factors are important in determining the exposure and potential for effects of Siloxane D5 in the environment. Consequently, the Board considered two aspects of accumulation:

• Do the values of BAF or BCF for Siloxane D5 meet the bioaccumulation thresholds according to the Regulations?
• Do the intrinsic properties of Siloxane D5 affect the analysis of its bioaccumulation characteristics?

198. Since the Board has the mandate to assess the nature and extent of the danger (or risk) posed by Siloxane D5 to the environment, the Board considered bioaccumulation, trophic magnification, and the intrinsic properties of Siloxane D5 as they relate to its distribution and fate in the environment. The Board therefore considered how Siloxane D5 is released to the environment, its distribution into the various compartments of the environment, how it is degraded, and the toxicity of the molecule once it reaches a site of action in an organism.

199. In conducting this assessment, the Board considered all the relevant scientific information available, including that information which was available in 2008, when the Screening Assessment was conducted.

200. The Board considered bioaccumulation and trophic magnification in relation to the exposures that organisms in the environment could experience to determine if these would pose a danger to the environment. The Board also recognised that accumulation of a substance from the matrix or food in an assay, or test, for toxicity (acute or chronic) inherently considers BCF or BMF, as well as the relevance of the concentrations that accumulate in an organism, even if these are not measured.

201. If toxicity is not observed in a long-term assay, then the accumulation that occurred in an organism as a result of that exposure did not produce adverse effects. Therefore, the exposure tested in the assay represented a de minimis risk.

202. In essence, accumulation in and of itself is not necessarily harmful. It is only harmful when the accumulation results in a concentration in the organism that exceeds its threshold of toxicity, or that of its predators.

203. Before turning to the Board’s analysis of bioaccumulation in relation to Siloxane D5, it is important to note that section 4 of the Regulations, which guided the Board, establishes a hierarchical order for considering the appropriate bioaccumulation metric.

204. The first metric prescribed in the Regulations is the BAF. If that factor cannot be determined, then evaluators shall assess the chemical’s BCF. If neither the BAF nor the BCF can be determined, evaluators shall then assess the K_ow. As noted earlier, because the BCF was measured, it was not necessary to consider the K_ow.

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4.3.2.1 Bioaccumulation Factor

205. As established in the Regulations, the BAF is the first factor to be considered in assessing bioaccumulation. The Screening Assessment cited BAFs which were calculated from models. This is because no empirical BAFs were available (EC & HC 2008). However, as noted in the Screening Assessment, government officials were unable to conclude that Siloxane D5 met the regulatory threshold for bioaccumulation (BCF or BAF ≥5,000) due to conflicting evidence. (EC & HC 2008, p. iii, EC 2011a, p. 3). Environment Canada reiterated this conclusion in its January 20, 2011 State of the Science Report where it stated that it “considers the BAF modelling for D5 to be equivocal” (EC 2011a, p. 67-68).

206. Even though no additional BAF data were provided to the Board during this review, Environment Canada, in its closing submissions, took the position that “D5 is bioaccumulative chemical and exceeds the Canadian criterion of BAF and/or BCF ≥5,000 set out in the Regulations. This determination is based on a weight of evidence that considered 15 bioaccumulation metrics” (EC 2011b, p. 7).

207. This statement is at odds with the position taken in the Screening Assessment and in the more recent State of the Science Report. Nevertheless, having considered the submissions of the parties and the relevant scientific information, the Board has concluded that the values for BAF are equivocal and do not support a conclusion that the regulatory threshold for BAF has been met.

208. Due to the absence of measured BAF values, the Board also considered alternative methodologies of expressing bioaccumulation. These included the multimedia BAF (“mmBAF”) and the relative BAF (McLachlan 2011). However, neither method is currently in use in a regulatory or risk assessment context (Transcript of the Public Hearings, Vol. 3, p. 476 and Transcript of the Public Hearings, Vol. 4, p. 613). Furthermore, the techniques are new and have not been validated against other approaches to estimate bioaccumulation. Consequently, the Board was of the opinion that it would not be appropriate to rely on either mmBAF or relative BAF in the context of this inquiry.

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4.3.2.2 Bioconcentration Factor

209. According to the Regulations, the BCF is the second factor to consider in assessing bioaccumulation. Estimates of potential to accumulate into organisms can be used in initial assessments or the lower tiers of a tiered risk assessment process by comparing empirical values to thresholds. Paragraph 4(b) of the Regulations establishes a value of 5,000 as the screening level threshold for the BCF.

210. At the time that the Screening Assessment was conducted, there were three values available for the BCF (EC & HC 2008). These values ranged from 1,950 to 13,300 L/kg wet weight (ww) (Annelin and Frye 1989, Drottar 2005, Opperhuizen et al. 1987).

211. To supplement those empirical data, the Arnot-Gobas model (2003) was used in the Screening Assessment to estimate the BCF for middle trophic-level fish. All values calculated using that model were less than 5,000 (EC & HC 2008, Table 9b).

212. The only new information about the BCF that was provided to the Board during this review was the results of an early-life-stage chronic toxicity test conducted on fathead minnows. The authors of the study reported values that ranged from 4,000 to 5,000 L/kg (ww) (EC 2010d).

213. The Screening Assessment concluded that, “while D5 has the potential to accumulate in biota, it is not possible to conclude at this time that D5 meets the criteria for bioaccumulation as set out in the Regulations based on considerations of the conflicting evidence from laboratory studies and predictive models” (EC & HC 2008, pp. ii-iii). Furthermore, the January 10, 2011 State of the Science Report (EC 2011a, p. 55) found the data on BCF for Siloxane D5 to be “equivocal”, even considering the early-life-stage fathead minnows chronic toxicity test study discussed above.

214. However, in its closing submissions, Environment Canada stated that Siloxane D5 has a BAF and/or BCF ≥5,000 (EC 2011b, p. 7). Here again, the Board noted that Environment Canada had changed its position with respect to BCF from that in the Screening Assessment and in the more recent State of the Science Report. Nevertheless, having considered all parties' submissions and the relevant scientific information, the Board has concluded that the values for BCF are equivocal and do not support a conclusion that the regulatory threshold for bioconcentration has been met.

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4.3.2.3 Biota-sediment Accumulation Factor

215. The BSAF was not considered in the Screening Assessment (EC & HC 2008). However, during the hearings, this parameter was raised by Environment Canada and the SEHSC/CCTFA. BSAFs of Siloxane D5 have been measured in a number of studies on sediment-dwelling organisms under laboratory and field conditions (Wildlife International 2008, Norwood et al. 2010, Dow Corning Corporation 2009). All values were less than 10 kg OC/kg lipid. These values are less than 500 kg OC/kg lipid, which is the normal threshold of concern for persistent, neutral, organic molecules.

216. In the case of Siloxane D5, the BSAF would need to exceed approximately 725 kg OC/kg lipid before it would be expected to biomagnify to a concentration that would exceed the threshold for an adverse effect (Gobas et al. 2011, p. 28). Siloxane D5 has a greater threshold of total concentration because it tends to partition more to OC in sediments than other neutral, organic molecules of similar molecular size (see discussion of K_oc in section 4.2.1 above).

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4.3.2.4 Biomagnification Factor

217. Of the two studies which measured BMFs for Siloxane D5, the Environment Agency in the United Kingdom considered only one, that of Drottar (2007) to be valid (Brooke et al. 2009). Drottar studied rainbow trout and reported a BMF of 0.22 (ww) and 0.63 when normalised to lipid (Drottar 2007).

218. Biomagnification does not occur when values of BMFs are less than 1.0. This is discussed in detail with respect to depuration rates in section 4.3.2.6 below. Having reviewed the available scientific information respecting the BMF, the Board is of the opinion that Siloxane D5 does not biomagnify.

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4.3.2.5 Trophic Magnification Factor

219. No information on the TMF was available when the Screening Assessment was conducted. Information has now become available that allowed TMF values to be calculated.

220. TMF values for Siloxane D5 measured using nitrogen isotopes in Lake Pepin, in the Mississippi River, south of Minneapolis-St Paul, Minnesota, were less than 1.0, which indicates trophic dilution. Based on all species analysed, the TMF was 0.18. When aggregated into feeding habits (trophic guilds), the TMF for all species was 0.11 (Dow Corning Corporation 2009).

221. The use of nitrogen isotopes to characterise trophic levels in terrestrial and freshwater environments might be confounded by inputs of nitrogen from anthropogenic sources or by difficulties in measuring concentrations of lipids in some organisms (Powell and Seston 2011), and this could have influenced the measurements of TMF for Siloxane D5. However, a similar characterisation of the food web in Lake Pepin was undertaken with a number of PCBs, which are known to biomagnify in the food chain. All TMFs for the PCBs were greater than 1.0 (from 1.5 to 5.1, depending on the congener of PCB) (Powell and Seston 2011) and were consistent with the behaviour of these well-studied compounds in other systems.

222. When considering these results, the Board concluded that these measurements add confidence to the TMF values for Siloxane D5 and that it does not undergo trophic magnification. The Board also agreed with the suggestion that a benchmark chemical that is not biotransformed, such as PCB 153 or PCB 180, could be used a reference to allow the estimation of a relative TMF (Powell and Seston 2011). This method compensates for potential sources of error and allows for better characterisation of trophic magnification of chemicals such as Siloxane D5 in freshwater food-chains.

223. In Lake Pepin, concentrations of Siloxane D5 were greater in the lowest trophic level (detritivores) than they were in sediments. However, concentrations of Siloxane D5 were inversely proportional to the ratio of nitrogen isotopes in all the other trophic levels. Trophic magnification did not occur. Trophic dilution of Siloxane D5 was also observed in Oslo Fjord (Powell 2009), the Humber Estuary, and the Baltic Sea (Gobas 2011, slides 11&12).

224. After reviewing all of the scientific information with respect to TMFs and taking into account the uncertainties noted by the parties, the Board concluded that Siloxane D5 does not biomagnify.

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4.3.2.6 Depuration Rates

225. The rate at which Siloxane D5 is biotransformed by higher organisms determines, in part, the rate of accumulation and depuration (loss from the organism along all pathways including diffusion, active transport, and degradation).

226. As food is consumed, some of the OC is degraded and accumulated into the organism where it is converted to energy. Thus, the OC content, and in particular the lipid content, of the food decreases as it is digested and biotransformed. Under conditions of constant exposure, which is the case at steady-state, the concentration that can be achieved in an organism is determined only by the depuration rate.

227. Information on the rate of depuration can be used in two ways. First, it can be used to interpret whether a compound, such as Siloxane D5 is biotransformed. Siloxane D5 is biotransformed, to varying degrees, in mammals (Varaprath et al. 2003) and in fish (Springer 2007). The rate constants for depuration that have been reported for two species of fish and one species of worm and range from 0.029 to 0.25/day (Gobas 2011, slide 4). These measured rates of depuration are greater than those predicted from models in which it was assumed that Siloxane D5 is not biotransformed. This confirms that Siloxane D5 is biotransformed by organisms.

228. Second, the depuration rate can also be compared to the uptake rate. If the rate of depuration (expressed in units of mass of chemical per mass of animal per unit time) exceeds the rate of uptake (expressed in the same units), then the chemical cannot be biomagnified.

229. In the case of Siloxane D5, if it is assumed that an animal eats 4% of its body mass in food per day (a high rate) and that the assimilation efficiency is 50% (which is a typical value for neutral organic chemicals in fish), a maximum uptake rate can be calculated (Gobas et al. 2011, p. 3 ). When this was done, a rate constant of 0.02 /day was calculated (Gobas 2011, slide 4). The fact that the measured depuration rates all exceed this value also indicates that Siloxane D5 does not biomagnify.

230. Based on the available information on the BMF, measured concentrations in the environment, and the pathways and processes of degradation, the Board concluded that Siloxane D5 does not biomagnify. From this conclusion, it follows that organisms at higher trophic levels will not be exposed to greater concentrations of Siloxane D5 than organisms below them in the food web.

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4.3.2.7 Fugacity Analysis

231. As noted above, fugacity is the tendency of chemicals to move from one compartment into another in the environment and/or organisms. When the fugacity ratios for Siloxane D5 were calculated, they were all less than 1.0 (Gobas et al. 2011, Figure 3, p. 59). For bioaccumulative organic compounds, such as the non-biotransformed PCBs, the fugacity ratios are always greater than 1.0. The evidence indicated that Siloxane D5 is biotransformed as its fugacity ratios were less than 1.0.

232. Analyses of fugacities can also be used to determine whether Siloxane D5 would cause adverse effects. Based on calculations made for some average environmental conditions, the maximum fugacity that Siloxane D5 could achieve at its solubility limits would be approximately 33 Pa (Gobas et al. 2011, p. 32-39, Figures 2 and 3, p. 58-59).

233. When fugacities of Siloxane D5 were calculated for a range of environmental compartments and organisms, the fugacities were always less than 33 Pa and decreased with increasing trophic level. The fugacity values ranged from a maximum of 10 Pa in effluent from WWTPs to 0.0000001 Pa in marine mammals (Gobas et al. 2011, Figure 3, p. 59). The no-observed-effect-concentration (“NOEC”) values for all organisms tested were at fugacity magnitudes that exceeded 33 Pa. These fugacity magnitudes are thermodynamically impossible to attain in the environment (Gobas et al. 2011, Figure 3, p. 59).

234. The results of this analysis are consistent with the conclusion of the Board that Siloxane D5 does not bioconcentrate and, based on its solubility limits, Siloxane D5 cannot exceed concentrations that could give rise to adverse effects in organisms.

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4.3.3 Conclusions on Persistence, Bioaccumulation and Trophic Magnification of Siloxane D5 in the Environment

235. From the scientific information before it, the Board has concluded that Siloxane D5 meets the criteria to be classified as a persistent chemical under the Regulations. However, it will only be a danger to the environment if Siloxane D5 persists in such a way as to result in 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. The Board's conclusions with respect to adverse effects are described in the following sections.

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

237. The Screening Assessment did not consider all of the intrinsic properties of Siloxane D5 and their effect on fate and transport in the environment and subsequent exposure to organisms. The Screening Assessment concluded, in part, that Siloxane D5 should be classified as bioaccumulative simply by comparing the values of the BCF and/or BAF to the regulatory threshold of 5,000, even though the data were equivocal. Such an approach might be appropriate in less robust, lower-tiered screening assessments and in the absence of additional information about the intrinsic properties of a substance. However, given the availability of additional information, the Board conducted a refined assessment of the potential danger posed to the environment from Siloxane D5 and concluded that it does not biomagnify through the food chain.

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4.4 Toxicity of Siloxane D5 to Receptor Organisms in the Environment

238. Toxicity is the potential of a chemical to produce adverse, or detrimental, effects in organisms. The magnitude of the adverse effect is determined by the concentration and duration of exposure to the chemical and how it interacts with the organism.

239. The interaction of the chemical with the organism includes toxicokinetic and toxicodynamic factors. Toxicokinetic factors include the rates of absorption, distribution, biotransformation, and, finally, excretion. Toxicodynamic factors also include the rate at which damage is caused and the rate of recovery, if any, from the damage.

240. There are many processes in organisms that affect the amount of the chemical reaching target sites in tissues. Indeed, organisms can be exposed to some concentration of a chemical for a very long period without any adverse effect. This is referred to as the incipient effective concentration. Organisms, including humans, are continuously exposed to toxic chemicals, but it is only when the critical incipient effect concentration is exceeded for a sufficient period of time for damage to accumulate, that adverse effects will occur.

241. Knowledge of the mechanism of action of a chemical is useful in assessing its hazard to organisms. A chemical can have a specific mode of action due to its interaction with a particular receptor. For instance, the shape of some molecules is such that they fit into structures on biomolecules such as enzymes or receptors on the surface of a cell. Chemicals can elicit effects by mimicking biological molecules or blocking key receptors. Chemicals can interact with other chemicals enabling inactive chemicals to become active. It also should be emphasised that, in the environment, chemicals do not exist as sole entities but are part of a mixture.

242. In addition to these specific mechanisms of action, all chemicals have what is termed the minimal or basal toxicity. This is referred to as narcosis and is caused when the molecule partitions, or dissolves, into the membranes of the cells of the organism and produces changes in their structural or chemical properties. This process is generally reversible and does not cause permanent damage unless exposures are of sufficient duration and magnitude. For neutral (uncharged) molecules such as Siloxane D5, there is no known specific mechanism of action, so adverse effects can only be induced by narcosis (Transcript of the Public Hearings, Vol. 3, p. 516, Transcript of the Public Hearings, Vol. 7, p. 992).

243. Concentrations of Siloxane D5 required to produce adverse effects would be expected to vary little among species because the physiologies and membranes of most organisms are quite similar. Consequently, it is unlikely that there will be uniquely sensitive species. For this reason, a smaller set of data on toxicity is sufficiently robust to make meaningful conclusions about the potential for adverse effects on organisms.

244. The evidence available indicates that Siloxane D5 is not toxic to mammals (Environment Agency 2010). Adverse effects on other species are only observed at very large concentrations which, based on the evidence before this Board, cannot be attained in the environment as a result of normal releases.

245. In some studies on benthic organisms (summarised in EC 2011c, Table 2, p. 19), adverse effects were noted. However, in these studies, concentrations exceeded the theoretical solubility limits of Siloxane D5 in the OC component of the sediments. It is the opinion of the Board that these studies do not accurately represent concentrations of Siloxane D5 that are likely to be found in the environment.

246. There is no toxicity information available for some classes of animals such as amphibians or birds (EC & HC 2008, Brooke et al. 2009). There were no data available as to whether Siloxane D5 interacted with other chemicals in the environment as part of a mixture. However, the Board is of the opinion that this lack of information would not change its conclusion, nor does it introduce an unacceptable level of uncertainty about hazard and risk.

247. Results of studies of the toxicity of Siloxane D5 to terrestrial and aquatic organisms that were conducted after the Screening Assessment was released have added to the understanding of the sensitivities of several groups of organisms. The toxicity data contained within these new studies are summarised and discussed in the following sections. These data represent the values associated with the most sensitive response observed in each study.

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4.4.1 Terrestrial Animals

248. Toxicity concentrations from studies conducted after 2008 were available for earthworms and springtails and are summarised in Table 2. The most sensitive organism tested was the springtail with an IC50 based on production of young at 767 mg/kg (dw).

^a IC₅₀ is the concentration that causes 50% inhibition of the response.
^b Toxicity concentrations are based on measured concentrations at the initiation of the study.
Data from (EC 2010b).

249. During preliminary tests, after 14 days, concentrations of Siloxane D5 decreased to 50% of the initial, nominal concentrations. These losses were due to degradation and volatility. However, the values in Table 2 above are based on concentrations of Siloxane D5 measured in soils at the beginning of the study, the only time when concentrations were measured (EC 2010b). Despite some of the uncertainties associated with the study, the Board concluded that the results of these tests were appropriate for assessment of risks.

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4.4.2 Terrestrial Plants

250. Information on toxicity was available for four species of plants (Table 3). All these data were generated after 2008. As for terrestrial animals (above), IC50 values for effects of Siloxane D5 on terrestrial plants are based on the concentrations of Siloxane D5 measured in soils at the beginning of the study. The Board determined that this did not invalidate the results of the tests since this dissipation would occur in the field after the addition of biosolids containing Siloxane D5 to the soil.

^a IC50 is the concentration that causes 50% inhibition of the response.
^b IC50s are based on measured values at the initiation of the study.
^c IC50s based on lack of observed response in the range-finding study.
Data from (EC 2010b).

251. The most sensitive plant was barley with an IC50 value based on dry mass of roots of 209 mg/kg (dw). This value was used in the risk assessment discussed below (section 5.1 below).

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4.4.3 Aquatic Organisms

252. Although some information on toxicity was available when the Screening Assessment was conducted (EC & HC 2008), additional data have been presented to the Board and are included in this review. The concentration values for all of the studies are summarised (Table 4) and include two studies on early life-stage.

NOECs are based on measured concentrations except for^a, which is based on nominal concentrations in a flow-through assay.
Data from (EC 2011c).

253. No adverse effects were observed in any of the studies. In all cases, the NOECs were the greatest concentration tested or measured in the studies. The concentrations were all greater than or equal to the maximum solubility of Siloxane D5 in water (17 µg/L) (EC 2011a, Table 5, p. 22).

254. In several of these studies, it was not possible to maintain concentrations of Siloxane D5 that were close to or greater than its maximum solubility. For example, in the study on the fathead minnow (EC 2010d), the greatest nominal concentration tested (17 µg/L) was measured to be 8.7 µg/L. This is consistent with the vapour pressure and the air-water partition coefficient, K_aw, of Siloxane D5 and its tendency to dissipate from water to air (section 4.2 above).

255. Because the mechanism of action of Siloxane D5 is narcosis (Transcript of the Public Hearings, Vol. 3, p. 546), the Board determined that the toxicity values available for Siloxane D5 in aquatic organisms were representative of other aquatic species. The Board further concluded that no adverse effects would be expected at concentrations less than or equal to its maximum solubility in water (17 µg/L).

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4.4.4 Sediment-dwelling Organisms

256. Toxicity data for sediment-dwelling organisms were available for the mud-worm (Lumbriculus variegatus), and larvae of the midge (Chironmus riparius) in 2008. Additional data for these species and the scud (Hyalella azteca) were made available subsequently. All of the data are summarised in Table 5 and are presented relative to dry-weight of sediments and relative to the amount of OC in the sediments.

^a This is the only value where the concentration of Siloxane D5 is less than saturation in the OC component of the sediment.
Data from (Fairbrother et al. 2011).

257. The Board noted that all but one of the NOECs in Table 5 were at concentrations greater than the maximum solubility limit in the OC fraction of the sediment used in the test. Based on the measured K_oc for Siloxane D5, the concentration at saturation would be 2,516 µg/g OC (Mackay 2011a). Unless an unintentional industrial release occurred, concentrations greater than this value could not result from normal diffusion and partitioning in the environment.

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4.4.5 Conclusions on Toxicity of Siloxane D5 to Receptor Organisms in the Environment

258. From the scientific information presented to the Board, Siloxane D5 is not toxic to any organisms tested up to and greater than the limit of solubility in the environmental matrix through which they were exposed. As noted above in section 3.3.7, it is theoretically impossible for Siloxane D5 to partition into any matrix to such an extent that its concentration is greater than its maximum solubility in that medium. Consequently, the Board is of the opinion that Siloxane D5 will not accumulate to sufficiently great concentrations to produce adverse effects in organisms in air, water, soils, or sediments. Moreover, Siloxane D5 does not appear to interact with other chemicals in the environmental mixture to cause harm to the environment or organisms.