Addendum

Octamethylcyclotetrasiloxane (D4, CAS No. 556-67-2) and Decamethylcyclopentasiloxane (D5, CAS No. 541-02-6)

SEHSC appreciates Environment's Canada's careful consideration of the available data for the cyclic siloxanes. Environment Canada indicates that the approach taken in this ecological screening assessment was to examine various supporting information and develop conclusions based on a weight-of-evidence approach and using precaution as required under subsection 76.1 of CEPA 1999. However, the conclusions presented in the Final Assessments for D4 and D5 are not consistent with the principles elucidated in the "Cabinet Directive on Regulation" and ignore the doctrine laid out the framework for the "Application of Precaution in Science-based Decision Making About Risk." Similarly, the conclusions found in the Final Assessment don't appear to apply the WoE approach required by section 76.1 of CEPA. All three of these directives provide clear guidance with regard to the use of precaution in science-based regulatory decisions and state that balance is required. In their final assessments, the Minister of the Environment appears to have used the principle in a manner that would not be supported by its original intent -- to act as a bridge in the absence of scientific certainty where there is sufficient evidence to allow a conclusion to be "reasonable" as outlined in the "Application of Precaution in Science-based Decision Making About Risk." Instead, we believe the assessments conclusions are based on environmental exposure scenarios that fail to recognize that D4 and D5 behave in the environment differently than what can be identified by the standard default models and laboratory screening studies.

The ecological risk predicted by Environment Canada is predicated on inaccurately modeled estimates. SEHSC requests that the modeling approach used by Environment Canada for predicting environmental concentrations of D4 and D5 be reassessed, as this will impact the risk quotients (RQs) upon which the need for RM measures is based. In addition, it is important that actual measured environmental monitoring data be used to calibrate the modeling approach for understanding true environmental concentrations.

Environment Canada's assessments of D4 and D5 do not identify a measurable existing environmental hazard posed by these materials. In addition, the modeling approach used to determine environmental exposure inappropriately overestimates environmental concentrations.

While the final screening assessments did consider the scientific data provided by SEHSC, it does not specifically identify any environmental hazard that would be reduced or eliminated by risk management options. In the assessments, the approach taken was to perform a quantitative risk quotient evaluation of exposure and of ecological effects. Environment Canada examined field measurements of the D4 and D5 in water, but then used predicted environmental concentrations (PECs) of aquatic exposure for approximately 1000 municipal discharge sites for comparison to the predicted no effect concentrations (PNEC) to calculate risk quotients values for each compound. Environment Canada calculated predicted environmental concentrations (PECs) for the water compartment based on the use quantities identified from the section 71 survey submissions and estimates of releases from individual industrial sites and from consumers. The assessment conclusions are based on environmental exposure scenarios that fail to recognize that D4 and D5 behave in the environment differently than what can be identified by the standard default models and derived parameters and are overestimated for the aquatic environment. SEHSC recently (March 27, 2009) consulted with Don MacKay (Director Emeritus of the Canadian Centre for Environmental Modelling and Chemistry and Professor Emeritus Trent University) to assess the appropriate approach for modeling cVMS. The guidance provided was that given the chemical-physical properties of these materials a Type-1 or Type-3 chemical assignment was more appropriate than a Type-2 assignment. SEHSC therefore requests that the modeling approach used by Environment Canada for predicting environmental concentrations of D4 and D5 be reassessed.

SEHSC also requests that Environment Canada consider the actual exposure of the aquatic environment to D4 and D5, utilizing the monitoring data already available as well as the data that will become available prior to the end of 2009. The results of these environmental monitoring programs, reflecting actual measures rather than predicted values can be used to calibrate the environmental modeling approach used to predict behavior of these materials. A risk assessment using this approach is provided below.

Air releases of cVMS substances were also mentioned in the Risk Assessments. SEHSC agrees with Environment Canada that these types of releases do not significantly contribute to the environmental exposure of cVMS substances as greater than 98 percent of the cVMS evaporate very quickly. Once in the atmosphere, the cVMS do not create or destroy ozone. They bind to particles, and may be transported long distances, but are degraded by hydroxyl radicals in the air and do not back-deposit. Monitoring data and published environmental studies, some of which were cited in the assessments, indicate that air concentrations at locations remote from point sources are very low to non-detectable.

Assessment documents provided by Environment Canada provide tabular summaries of results from the EQC (ver 2.02) Level III fugacity modeling for D4 (Table 4, page 11) and D5 (Table 4, page 13). The documents state that the fugacity modeling was based on the input parameters listed in Appendix 5, but does not specify if the materials were modeled as Type-1, -2, or -3 chemicals. Our understanding is that siloxane materials should be modeled as Type-1 chemicals (i.e., chemicals that partition into all environmental media) or as Type-3 chemicals (i.e., chemicals that partition into air, biota, and solid phases such as soil and sediment, but are essentially insoluble in water) if the Type-3 partition coefficients are available. However, we are only able to reproduce the values in the results tables when modelling D4 and D5 as Type-2 chemicals using the Type-2 partition coefficients listed in Appendix 5.

We were informed that Environment Canada modeled D4 and D5 as Type-2 chemicals based on the recommendations from Dr. Don Mackay at the Canadian Environmental Modeling Centre at Trent University (CEMC). We have discussed this with Dr. Mackay and he was not aware of any discussions that he or anyone in CEMC may have had with Environment Canada concerning the siloxane materials. Moreover, he indicated that if he were asked for a recommendation that he would not have assigned D4 and D5 as a Type-2 chemicals since Type-2 substances have zero (or very low) vapor pressures such that air-to-water partition coefficients (Kaw; also known as the non-dimensional Henry's Law constant) can not be determined. Clearly, D4 and D5 do not fit the description or category of a Type-2 substance. As reported in Appendix 5 of the Screening Assessment Documents, D4 and D5 have relatively high vapor pressures and measured Kaw values are available for both materials. It is unclear as to why Environment Canada would have assigned and modeled D4 and D5 as Type-2 chemicals.

If D4 and D5 are modeled as Type-1 chemicals using the Type-1 properties in Appendix 5 (vapor pressure adjusted so that Kaw is equal to the measured value reported in Appendix 5), substantially different results are obtained. The results in the tables below show that modeling of D4 and D5 using Type-2 partition coefficients, relative to Type-1 properties, overestimates the amounts that will partition to the water compartment and underestimates the amounts that will partition to the soil and sediment compartments.

If the chemical partitioning properties are in thermodynamic balance with the physical-chemical properties, output from the EQC model should be the same regardless if Type-1, Type-2, or Type-3 chemical properties are used. We are not aware of any measured Type-2 partition coefficients that are available for D4 and D5, so we can only assume that the values identified in Appendix 5 of the screening assessment documents represent estimated values. However, it is not identified in the documents how the Type-2 partition coefficients were obtained. If we use the EQC model to estimate the Type-2 partition coefficients from measured Type-1 properties, the values differ by 2-3 orders of magnitude, as shown in the tables below. This indicates that the Type-2 partition coefficients listed in Appendix 5 of the screening assessment documents are not in thermodynamic balance with the measured Type-1 properties listed in the Appendix. This also leads to the conclusion that the fugacity modeling should have been based on Type-1 properties rather than Type-2 partition coefficients.

In Environment Canada's assessments of D4 and D5, the approach taken was to perform a quantitative risk quotient evaluation of exposure and of ecological effects (or potential hazard). Environment Canada examined field measurements of the D4 and D5 in water, but then used predicted environmental concentrations (PECs) of aquatic exposure for approximately 1000 municipal discharge sites for comparison to the predicted no effect concentrations (PNEC) to calculate risk quotients values for each compound.

Environment Canada calculated a predicted environmental concentration (PEC) for the water compartment based on the use quantities identified from the section 71 survey submissions and estimates of releases from individual industrial sites and from consumers. In addition, Environment Canada used Type-2 chemical partitioning properties to evaluate the environmental fate of D4 and D5 after this release. It is not known why Canada did not use Type-1 or Type-3 chemical partitioning properties for the evaluation, which would be more appropriate for this class of materials. Measured Type-2 chemical partitioning properties for D4 and D5 do not exist. When D4 and D5 were modeled using measured Type-1 chemical partitioning properties, the results are significantly different from the results Environment Canada obtained using estimated Type-2 chemical partitioning properties. Canada appears to overestimate the amounts of D4 and D5 that partition to the water compartment. The net result is that the predicted environmental concentrations (PECs) are overestimated for the aquatic environment.

In the Aquatic Risk Distribution Environment Canada have their dilution factor capped at 10. It is unclear why Environment Canada took this very conservative approach. Using data that was used to develop the model being used there are 585 records which include the population served, flow, and type of treatment. Using Environment Canada's factor of 3 to account for low flow compared to average flow which is what is available, then about 522 of the 585 or about 89% have dilution factors > 10. How was a cap in the dilution factor of 10 determined because the data on actual STP releases and river flows from Canadian Water and Wastewater Association suggest that this is highly conservative?

It is also not clear what removals Environment Canada is using for the various types of STP. We would assume that the stabilization ponds, secondary and tertiary treatments would be similar to the ASTREAT predictions for activated sludge of 44% to sludge, 53% to air and 3% to effluent.

Environment Canada also acknowledges uncertainty particularly concerning the assessment of D5. They indicate that "although a risk quotient analysis was conducted for D5, the empirical ecotoxicity evidence suggests that the threshold at which adverse effects in pelagic biota is expected to occur has not been observed in available toxicity tests. Therefore the risk quotients calculated in the above scenario are essentially "unbounded" and may not represent "real" observable effects expected at the above sites."

In our analysis of ecological risk for D4 and D5, we focused on producing an aquatic risk distribution as well, but our intent was a risk distribution or probability based on measured concentrations of each chemical in water, not PECs, and a subsequent comparison to EC's projected PNEC values to examine risk. Fundamentally, a probabilistic assessment of what is the likelihood of an adverse effect occurring from the concentrations of D4 and D5 that are actually being found in the environment. Based on this assessment there is a low potential for risk to the aquatic environment for D4 and D5.

SEHSC also believes that ongoing environmental research and monitoring will substantiate this position and that is it crucial to wait for that data before making any final recommendations on risk management for D4 and D5.

SEHSC scientists have developed a cooperative relationship with Environment Canada scientists to share technical knowledge of sample collection and analytical methods for cVMS. Workshops, presentations at scientific meetings, shared sample-collection expeditions, and cooperative technical guidance form a background for gaining a better understanding of the unique properties of silicone materials and the real-world behaviors of D4 and D5. This relationship is a part of SEHSC's on-going environmental monitoring in the US and Canada (in Toronto Harbor, Lake Ontario, Lake Pepin, Lake Opeongo, and other sites). The EC scientists who participated in the 1st Annual Workshop on Siloxanes in the Environment, held March 27 & 28, 2008 in Burlington, ON, agreed on, and had encountered, the same difficulties in working with these materials both in laboratory studies and in the collection, storage, and analysis of environmental samples. The 2nd Annual Workshop on Siloxanes in the Environment is currently scheduled for April 20-21, 2009 in Burlington ON. Scientists from around the world will be participating in this workshop and discussing the challenges of monitoring for cVMS.

SEHSC has already committed to a comprehensive research and environmental monitoring program in consultation and collaboration with scientists from Environment Canada and other regulatory agencies around the world. Collaborative monitoring programs also are on-going with Norway and the Polar Research Institute, Sweden, and the UK. Focused scientific workshops and scientific forums at SETAC and SETAC Europe further the information exchange between the silicone industry scientists and the scientists in regulatory agencies in North America and Europe on key aspects of quality control, sampling, storage, and transport issues, and analytical techniques. As significant effort has already been invested into this program, SEHSC proposes a more formal joint effort between Environment Canada and key stakeholders. This joint agreement could be developed through the Consultation process with all key stakeholders. This would allow utilization of already developed analytical methodologies and inputs from experts around the world. This underscores the need to comprehensively assess how D4 and D5 behave in the environment before any RM approaches are determined and any socio-economic impacts occur.

Persistence Potential of D4 and D5

The D4 and D5 Final Screening Assessment assumed a read-across to cooler temperatures from laboratory sediment studies conducted at standard study temperatures. Based on this they concluded that the lack of fast degradation of D4 and D5 under some environmental conditions, especially in colder Canadian environments, will result in a half-life in sediment of t_½ > 365 days. In addition for D5, they suggest D5 is expected to persist for relatively long periods at low temperatures and neutral or slightly acidic water conditions in Canadian waters (pH 6-7, temperature 5-10°C). A recent focus on evaluating persistence of organic compounds in environmental media (air, water, soil, sediment) in terms of their single-medium degradation half-lives was undertaken to provide guidance to reviewers of chemical dossiers for POPs and PBTs proposed for action (SETAC, 2008); some of the workshop findings were presented by Dr. R. Boethling, US EPA, Washington, DC, USA at the 2008 SETAC North America Annual Meeting. Dr. Boethling noted the following:

Persistence is determined by a range of transformation processes acting in concert. At the screening level, pH and temperature adjustment does not seem necessary. Further, the question of pH and temperature correction also needs to be addressed in the light of the original intention leading to the selection of the present set of persistence (half-life) criteria. Two intentions seem to have contributed to the selection of the final values. The first was to avoid widespread use of chemicals with properties closely resembling those of "known" POP-type compounds. This approach is often termed the "reference chemical approach". The second was to ensure that levels of a given chemical would significantly decrease within a reasonable timeframe, e.g. within a year, once its emission was reduced or stopped entirely. This approach is also known as the "management approach". It is reasonable to assume that cut-off criteria defined according to the "reference chemical approach" are related to available half-life data for known POPs. Since these half-lives are likely to have been derived under laboratory conditions, these are the conditions for which the comparison should take place. According to the "reference chemical approach" temperature and pH correction therefore does not seem justified.

If Environment Canada were to apply the "reference chemical approach" at the screening level as it was originally intended then D4 would not be considered persistent in water or sediment and D5 should not be considered persistent in water or sediment (based on read across as applied by Environment Canada in this assessment). It appears that Environment Canada advanced to the "management approach" when D4 and D5 did not meet the original screening criteria according to the "reference chemical approach" which seems overly precautionary compared to recent expert conclusions.

Dr. Boethling's observations are consistent with the field and laboratory findings on D4 and D5, where studies have shown that most cVMS are eliminated by sewage treatment plants (STP), either by evaporation or by binding to particles in the sludge and Nordic monitoring results indicate that little is present in the effluent waters (e.g., 0.06-0.98 ppb D5). Similarly, data from Lake Pepin, MN and Lake Opeongo in Canada also have cold water climates, but do not show elevated cVMS concentrations in water. Lake Ontario has detectable levels close to wastewater outflows in Toronto Harbor, but do not demonstrate significant levels beyond the immediate outflow areas.

Bioaccumulation Potential of D4 and D5

Environmental exposure of aquatic organisms to lipophilic compounds may occur through the water column, food, and sediment. However, uptake of highly hydrophobic or lipophilic chemicals (i.e., log KOW values greater than ~5) from water is considered to be negligible for most fish, compared to uptake via consumption of contaminated foodstuffs (Bruggeman et al., 1984; Muir et al., 1985; Thomann, 1989; Nichols et al., 2004). A compound is said to 'biomagnify' when lipid-normalized concentrations (or fugacity) of accumulated chemical residues in biological organisms increases with increasing trophic position (Fisk et al., 2001; Hu et al., 2006). It has been proposed that bioaccumulative substances be defined as substances which biomagnify in the food web - i.e., lipid-normalized concentrations (or fugacity) increase with increasing trophic position (Gobas et al., 2008). Based on this definition, it was concluded that that the most relevant criterion for assessing chemical bioaccumulation was the Trophic Magnification Factor (TMF), and that the most conclusive evidence to demonstrate that a chemical substance biomagnifies was a TMF > 1 (SETAC, 2008).

TMFs are derived from the slope of the regression of log lipid-normalized concentrations of the chemical in organisms from a food web versus their trophic level. As such, a TMF represents the trophic level increase of a chemical substance that is averaged across the food web rather than a single predator-prey relationship. Consequently, TMFs are preferable to single predator-prey relationships for comparing biomagnification between ecosystems and are particularly useful for comparing bioaccumulation of individual chemicals within a well-defined food web. In addition, TMFs may be broadly applied across systems that differ considerably in their location and characteristics (Houde et al., 2008).

Biomagnification of cyclic volatile methylsiloxane (cVMS) materials, specifically, D4, D5, and D6, was determined in Lake Pepin, Minnesota, a natural flood-plain lake on the Mississippi River located approximately 80 km downstream from the Twin Cities of Minneapolis and Saint Paul, Minnesota USA (Powell and Woodburn, 2009). TMFs for the three cVMS materials were determined for a benthic invertebrate-to-fish food web using the stable isotopes of nitrogen (15N) and carbon (13C) to estimate trophic positions and carbon flow to consumers (Post, 2002; Jardine et al., 2006). Several key points should be noted from the Lake Pepin cVMS field results (Powell and Woodburn, 2009):

• In the Lake Pepin food web, lipid-adjusted concentrations of D4, D5, and D6 all declined with increasing trophic level. This indicates that bioaccumulation of these cVMS compounds is not due to simple water-to-lipid partitioning, i.e., bioconcentration via the gills. If bioconcentration was the dominant process for bioaccumulation, the lipid-normalized cVMSconcentrations should be approximately equal across the various organisms of the food web. The Lake Pepin data show that simple water-to-lipid partitioning was not the key process for accumulation, thereby demonstrating that the bioconcentration process was not highly pertinent to uptake of D4, D5, and D6 into aquatic organisms. Hence, bioaccumulation of D4, D5, and D6 in the Lake Pepin food web occurred predominately via dietary biomagnification processes controlled by food uptake and associated mitigation processes such as metabolism, growth dilution, low uptake and assimilation efficiencies, and reduced bioavailability due to chemical sorption in the water/sediment phase(s). Collectively these mechanisms led to TMFvalues of 0.30, 0.16, and 0.14 for D4, D5, and D6, respectively. The results from Lake Pepin showed that cVMS materials undergo trophic dilution in aquatic food webs, i.e., TMFs < 1 are indicative of materials that do not biomagnify and are considered to be non-bioaccumulative (SETAC, 2008). In conclusion, the Lake Pepin field data demonstrate that laboratory BCF values for the three cVMS materials are not appropriate measures for regulation of the 'B' potential of these chemicals, as dietary uptake predominates. As noted by Environment Canada (EC), the laboratory BMF values for D4 and D5 are <1, and the Lake Pepin TMF field data <1 confirm that the laboratory BMFs are more appropriate and suitable measures of the 'B' trophic dilution processes observed for the cVMS materials.
• The hierarchical ranking of 'B' with (1) TMF, (2) BMF, and then (3) BCF measures for cVMS materials was reinforced by findings from a recent SETAC workshop on bioaccumulation processes (SETAC, 2008); some of the workshop findings were presented by Dr. Frank Gobas of Simon Frazier University at the 2008 SETAC North America Annual Meeting. Dr. Gobas noted the following:
  
  "Despite the lack of a recognized definition for a B substance, we defined a B(ioaccumulative) substance as a substance which biomagnifies in the food-web, i.e. increases in normalized concentration (or fugacity) with increasing trophic position. It was concluded that the most relevant B criterion is the TMF(Trophic Magnification Factor), and that the most conclusive evidence to demonstrate that a chemical substance biomagnifies is a TMF > 1 (emphasis added). In absence of data on the TMF, the BMF (i.e., Biomagnification Factor) is an indicator of the chemical's potential to biomagnify through the food chain. While the BCF is generally used to characterize B substances, the BCF is not a good surrogate for BMF or TMF in terrestrial food-webs. The BCF is an acceptable surrogate for the BMF or TMF in aquatic food-webs if the route of exposure (water vs. diet) does not affect biotransformation rate of the chemical in the organisms and if bioavailability issues are not introducing significant experimental artifacts" (Gobas et al., 2008).

  Dr. Gobas's observations are consistent with the field and laboratory findings on D4 and D5, where laboratory-measured BMF values <1 were predictive of field behavior (i.e., Lake Pepin TMF values <1 for all three materials). This finding correlates with research from the scientific literature, which indicates that uptake of highly hydrophobic chemicals (log KOW >5) from water (i.e., bioconcentration) is considered to be negligible for most aquatic species, compared to uptake via consumption of contaminated foodstuffs (Bruggeman et al., 1984; Muir et al., 1985; Thomann, 1989; Nichols et al., 2004).

• An in-depth modeling analysis of the TMF data from Lake Pepin, MN (USA) has been performed by HydroQual (2009), on behalf of the Silicone Health Environment and Safety Council; this report is attached. The field data were modeled using the Thomann-Farley bioaccumulation/foodchain model (Thomann et al., 1992), available through Manhattan College; this model is functionally similar to other foodchain models (Arnot and Gobas, 2004). A necessary modification made to the model was to exclude the standard relationship used to estimate chemical assimilation efficiency based on log Kow, as this equation significantly overestimated the uptake efficiency for the cVMS materials; the modeled efficiency used to successfully fit the Lake Pepin field data for D4, D5, and D6 was 10%. In addition, the modeled gill uptake efficiency varied between 50% for D4, 15% for D5, and 1.5% for D6. A composite cumulative loss (metabolism + elimination) rate constant (kloss) was set at approximately 0.01 (day-1) for D4 and D5, and zero for D6. In brief, the model was able to successfully describe the field data, illustrating that the primary route of exposure was through the diet (>50% for D4 and >90% for D5 and D6), and not bioconcentration via the water column. In addition, the model predicts cVMS concentrations in fish decline over time, due to growth and elimination rates that are faster than the rates of accumulation from diet and water exposures; this effect is largely due to poor dietary assimilation. The paper concludes by examining the unique physico-chemical properties of cVMS compounds that may play a part in their limited uptake rate across membrane interfaces and suggests that a combination of high hydrogen bonding basicity (i.e., high Abraham hydrogen bond B values) and high log Kow values (>5) is consistent with limited gut uptake rates (Lipinski et al., 1997).

Fish Metabolism rates for D5

There appears to be some confusion regarding the fish metabolism of D5 and, given the complexity of the information submitted to EC, this is understandable. In the work of Springer et al. (2007) and Domoradzki et al. (2007), ¹⁴C-D5 was administered to adult rainbow trout (~1 kg) in a bolus dose via oral gavage at ~12 µg/g bw and followed via blood and tissue monitoring for 96 hours. Of the recovered ¹⁴C activity, 75% was eliminated via the feces and only 25% was absorbed by the fish; of the absorbed ¹⁴C activity, 14% was attributed to D5 metabolites collected in the fish (blood, gastrointestinal tract, bile, urine, liver tissue, gonads, and fat). A subsequent examination of the data by Woodburn and Domoradzki (2008) used pharmacokinetic compartmental modeling of the ¹⁴C-D5 and metabolite residues to determine explicit elimination and metabolism rate constants (i.e., k₂ and k_m values, respectively) for D5 in trout. This model used the measured ¹⁴C measured concentrations of parent D5 and total metabolites in fish blood collected at regular intervals during the 96-hour study. Research has shown previously in work with hydrophobic organic compounds in fish that blood concentrations of such compounds are routinely used to reflect both the magnitude and kinetics of chemical concentrations expressed on a whole-body basis (Barron et al., 1991; Barron et al., 1993; Nichols et al., 2004). In summary, Woodburn and Domoradzki (2008) calculated a trout metabolism rate constant (k_m) of ~0.007 hr^-1 (0.17 day^-1) for ¹⁴C-labeled D5 by using a simple compartmental model and available data on D5 and metabolites in trout blood; this fish metabolism rate constant is consistent with the collected trout blood residue data over 96 hours.

Ecological Effects Assessment

Request for reconsideration of D5 Algal Growth Study:
Acute and chronic studies with D5 on rainbow trout and daphnia were reviewed by EC and deemed acceptable for use in the screening assessment. All of the studies reviewed had no-effect concentrations (NOECs) at the limit of D5 functional water solubility (~15 µg/L or ppb) or at the highest concentration that could be maintained in the test. Sediment acute and chronic studies with chironomids (midges) and Lumbriculus sp. were also reviewed and considered valid for use in the screening assessment. The only study for D5 classified as 'invalid' by EC was a 96-hr algal growth study with the freshwater green algae, Pseudokirchneriella subcapitata (Springborn, 2001). It is unclear, however, why that is the case. The study was performed to satisfy both US EPA and OECD guidelines, by measuring both cell density and growth rate (OECD, 2006). Performance and acceptance criteria for the control organisms were met, as required per guidelines. Media concentrations of D5 were measured and reported. At test initiation, the nominal D5 concentration was 20 µg/L and the measured was 12 µg/L. It is not uncommon for a difficult-to-test, highly volatile compound such as D5 to have an initial measured concentration in water lower than the nominal dose, and the dose level will often decline in a first-order manner over the period of static exposure. In order to maintain concentrations throughout the test, the study was run in a closed, minimal headspace system. This is particularly unique for an algal study as the organisms need a source of carbon to continue to grow and reproduce adequately. As a result, sodium bicarbonate (NaHCO₃) was added to all treatment and control vessels in order to ensure a healthy supply of carbon for the test system, as recommended by the OECD Guidance Document 23 on difficult-to-test substances (OECD, 2000). The study was run as a limit test, starting above the limit of D5 water solubility (~15 µg/L). As stated, growth in the control vessels met the acceptance criteria (OECD, 2006). The spiked, measured D5 concentration was 12 µg/L on Day 0, 9.2 µg/L on Day 1, and 2.1 µg/L on Day 4; the geometric mean concentration over the 4-day period was 6.2 µg/L. Even though the concentrations by the end of the study were lower than at initiation, the results of the study show that when you put D5 in water, in a worst-case closed system, there are no long-term impacts to algae. In addition, the dissipation of D5 in the algal media appears to strictly follow first-order kinetics (i.e., C = Co * e^-k*time), as shown in Text Figure 1, with a measured D5 dissipation half-life of 37 hours; this dissipation of D5 is likely predominately due to volatility losses, which would be consistent with first-order kinetics. In conclusion, we submit that the algal growth study on D5 (geometric mean concentration of 6.2 µg/L over four days) should be considered 'acceptable' or 'acceptable with limitations' and that the overall conclusion of considering D5 inherently toxic is not supported by the available empirical data for D5 with fish, invertebrates, and algae.

Text Figure 1: D5 dissipation curve from algal growth study

Critical Body Burdens of D4, D5, and D6
The accumulated tissue residue or "burden" of a chemical in aquatic biota has been proposed to be a more appropriate indicator of adverse effects in aquatic organisms than water concentrations, as the overall tissue residue is considered to represent a more toxicologically relevant "dose" (McCarty and Mackay, 1993; Barron et al., 2002); this approach is commonly referred to as the "critical body residue" or CBR method. A CBR of 0.2 to 0.8 mmol/kg has been calculated for fathead minnows for observation of sublethal effects due to a narcosis mode of action by nonpolar chemicals (McCarty and Mackay, 1993). Examination of accumulated body residue levels for aquatic vertebrates collected with the Lake Pepin, MN field study produces the following table (Text Table 1) of body residue levels for D4, D5, and D6:

A body burden 'safety factor' may be calculated for D4, D5, and D6 by comparing the mean accumulated residues to the lower level sublethal effects (narcosis, nonpolar organic chemicals) CBR of 0.2 mmol/kg (McCarty and Mackay, 1993). For D4, D5, and D6, the body burden safety factors for the accumulated residues from Lake Pepin, MN aquatic vertebrates, when compared to a CBR for sublethal effects due to narcosis effects from nonpolar chemicals, are approximately 8500, 1000, and 24000, respectively. In conclusion, these data indicate that if Lake Pepin accumulated residues are taken as representative of urban environmental systems, that safety factors of at least 1000-fold exist between accumulated cVMS residues and sublethal effect levels based on narcosis effects from nonpolar chemicals.

Silicone Industry's Risk Assessment on D4, D5, and D6:
Octamethylcyclotetrasiloxane (D4; CAS No. 556-67-2)
Decamethylcyclopentasiloxane (D5; CAS No. 541-02-6)
Dodecamethylcyclohexasiloxane (D6; CAS No. 540-97-6)

(A) Ecological Risk Assessment of D4, D5, D6 - Water:
In their screening assessments of D4, D5, and D6, EC examined both field measurements of the cVMS materials in water, but also used predicted environmental concentrations (PECs) of aquatic exposure for approximately 1000 municipal discharge sites for comparison to calculate risk quotients from PNEC values for each compound. The use of a PEC database of approximately 1000 sites produced an aquatic risk distribution for each chemical, based on this method. In our analysis of ecological risk for each cVMS material, we focused on producing an aquatic risk distribution as well, but our intent was a risk distribution or probability based on measured concentrations of each chemical in water, not PECs, and a subsequent comparison to EC's projected PNEC values to examine risk probability. Fundamentally, a probabilistic assessment frames risk in the proper context, i.e., what is the likelihood of an adverse effect occurring (Woodburn, 2000)?

D4:
For D4, the water residues examined in a probabilistic approach were taken from Norden (2005) and NILU (2007), as these data represent the most recent, publicly-available information on this chemical. Sewage treatment plant (STP) influents and landfill runoff water samples were excluded from analysis, as such water does not represent aquatic ecosystems supporting organisms and populations; STP effluent samples were included in the analysis, and zero dilution was assumed, representing a worst-case situation for aquatic populations. Figure 1 presents the D4 aquatic field data as a cumulative probability distribution, with a comparison to the D4 PNEC value of 0.2 µg/L; non-detectable residues of D4 were presumed to be present at 50% of the detection limit. In an available dataset of N=37 samples, no exceedence of the PNEC value was observed, indicating a less than 3% probability (1/(N+1)) of D4 water concentrations exceeding this residue level in field situations, including STP effluent discharges. In conclusion, the available field measurement data on D4 in aquatic systems indicate that the probability of real world, aquatic ecosystem concentrations of this material exceeding the proposed PNEC value of 0.2 µg/L is quite small, less than 3%.

D5:
For D5, the water residues examined in a probabilistic approach were taken from Norden (2005), NILU (2007), Boehmer and Gerhards (2003), and Sparham (2008), as these data represent the most recent, publicly-available information on this chemical. STP influents and landfill runoff water samples were again excluded from analysis, as such waters do not represent aquatic ecosystems supporting organisms and populations; silicone manufacturing STP effluent samples were included in the analysis (Boehmer and Gerhards, 2003), and zero dilution was assumed, representing a worst-case situation for aquatic populations. Figure 2 presents the D5 aquatic field data in a cumulative probability distribution, with a comparison to the D5 PNEC value of 15 µg/L; non-detectable residues of D5 were presumed to be present at 50% of the detection limit. In an available dataset of N=94 samples, only two samples in exceedence of the PNEC value were observed, and both were present in STP effluent from a silicone-producing facility in Germany (Boehmer and Gerhards, 2003). These data indicate there is a less than 3% probability of D5 water concentrations exceeding this residue level in field situations, including STP effluent discharges (2/(N+1)). In conclusion, the available field measurement data on D5 in aquatic systems indicate that the probability of real world, aquatic ecosystem concentrations of this material exceeding the proposed PNEC value of 15 µg/L is quite small, less than 3%, and that such exceedences would most likely occur in effluent from silicone-producing STPs.

D6:
For D6, the water residues examined in a probabilistic approach were taken from Norden (2005) and NILU (2007), as these data represent the most recent, publicly-available information on this chemical. STP influents and landfill runoff samples were excluded from analysis, as such waters do not represent aquatic ecosystems supporting organisms and populations; silicone manufacturing STP effluent samples were included in the analysis (Boehmer and Gerhards, 2003), and zero dilution was assumed, representing a worst-case situation for aquatic populations. Figure 3 presents the D6 aquatic field data in a cumulative probability distribution, with a comparison to the D6 PNEC value of 4.6 µg/L; non-detectable residues of D6 were presumed to be present at 50% of the detection limit. In an available dataset of N=37 samples, no exceedence of the PNEC value was observed, indicating a less than 3% probability (1/(N+1)) of D6 water concentrations exceeding this residue level in field situations, including STP effluent discharges. In conclusion, the available field measurement data on D6 in aquatic systems indicate that the probability of real world, aquatic ecosystem concentrations of this material exceeding the proposed PNEC value of 4.6 µg/L is quite small, less than 3%.

(B) Ecological Risk Assessment of D4, D5, D6 - Sediment:
The lipophilic nature of the cVMS materials suggests that sediment adsorption is a likely removal mechanism of these compounds from water and empirical evidence supports this concept (Norden, 2005; NILU 2007). Risk quotients for sediment were not explicitly examined in the EC screening assessments of D4, D5, and D6, but probabilistic comparisons of measured cVMS sediment residues to sediment NOEC values may still be made for each material.

D4:
For D4, the sediment residues examined in a probabilistic approach were taken from Norden (2005), NILU (2007), and Powell and Kozerski (2007), as these data represent the most recent, publicly-available information on this chemical. Figure 4 presents the D4 sediment field data in a cumulative probability distribution, with a comparison to the D4 NOEC value of 44 µg/g dw (measured) from a prolonged (28 d) midge toxicity study (Krueger et al., 2008a); non-detectable residues of D4 were presumed to be present at 50% of the detection limit. In the available dataset of N=33 samples, none of the sediment residues were in exceedence of the NOEC value and there is an approximate ≥500-fold safety factor between the D4/midge chronic NOEC and the 95^th centile sediment concentration, i.e., the concentration that encompasses 95% of the available sediment field data on D4 or 95% of available sediment concentrations are at or less than 0.08 µg-D4/g dw. Collectively, these data indicate that there is less than a 1% probability of D4 sediment levels achieving or exceeding the sediment NOEC for this material and a minimum 700-fold safety factor exists between the NOEC and ≥95% of the available sediment field data on this chemical. In conclusion, the available sediment field data on D4 indicate that the likelihood of sediment concentrations of this material achieving levels within a factor of 700 of the chronic sediment NOEC value is quite small, less than 5%, and the probability of achieving or exceeding the NOEC is less than 1%, indicating little risk to benthic species.

D5:
For D5, the sediment residues examined in a probabilistic approach were taken from Norden (2005), NILU (2007), Boehmer and Gerhards (2003), and Powell and Kozerski (2007), as these data represent the most recent, publicly-available information on this chemical. Figure 5 presents the D5 sediment field data in a cumulative probability distribution, with a comparison to the lowest available D5 NOEC values of 69-70 µg/g dw from a full life-cycle midge toxicity study (Springborn Smithers, 2003; Krueger et al., 2008b); non-detectable residues of D5 were presumed to be present at 50% of the detection limit. In the available dataset of N=53 samples, none of the sediment residues were in exceedence of the NOEC value and there is an approximate ≥100-fold safety factor between the D5/midge chronic NOEC and the 95^th centile sediment concentration, i.e., 95% of available field sediment concentrations are at or less than 0.6 µg-D5/g dw. Collectively, these data indicate that there is less than a 1% probability of D5 sediment levels achieving or exceeding the sediment NOEC for this material and a minimum 100-fold safety factor exists between the full life-cycle D5 sediment NOEC and ≥95% of the available sediment field data on this chemical. In conclusion, the available sediment field data on D5 indicate that the likelihood of real sediment concentrations of this material achieving levels within a factor of ~100 of the full-life cycle sediment NOEC value is quite small, less than 5%, and the probability of achieving or exceeding the NOEC is less than 1%, indicating little risk to benthic species, and any such exceedences would most likely occur in areas adjacent to effluent from silicone-producing STPs (Boehmer and Gerhards, 2003).

D6:
For D6, the sediment residues examined in a probabilistic approach were taken from Norden (2005), NILU (2007), and Powell and Kozerski (2007), as these data represent the most recent, publicly-available information on this chemical. Figure 6 presents the D6 sediment field data in a cumulative probability distribution, with a comparison to the lowest available NOEC value of 69-70 µg/g dw from a full life-cycle midge toxicity study on D5 (Springborn Smithers, 2003; Krueger et al., 2008b); non-detectable residues of D6 were presumed to be present at 50% of the detection limit. In the available dataset of N=33 samples, none of the sediment residues were in exceedence of the NOEC value and there is an approximate ≥250-fold safety factor between the D6/midge chronic NOEC and the 95th centile sediment concentration, i.e., 95% of available field sediment concentrations are at or less than 0.3 µg-D6/g dw. Collectively, these data indicate that there is less than a 1% probability of D6 sediment levels achieving or exceeding the sediment NOEC for this material and a minimum 250-fold safety factor exists between the full life-cycle D6 sediment NOEC and ≥95% of the available sediment field data on this chemical. In conclusion, the available sediment field data on D6 indicate that the likelihood of real sediment concentrations of this material achieving levels within a factor of ~250 of the full-life cycle sediment NOEC value is quite small, less than 5%, and the probability of achieving or exceeding the NOEC is less than 1%, indicating little risk to benthic species from this compound.

In conclusion, risk assessment calculations on D4, D5, and D6 in water and sediment were conducted using probabilistic techniques. These methods are useful in framing risk in the proper context, i.e., what is the likelihood of an adverse effect occurring? Results of the risk calculations clearly indicate that both in water and sediment environments, the likelihood of any adverse ecological result occurring as a result of chemical exposure to a cVMS material is quite small, less than 3% at worst and generally less than 1%. The most likely environment for an ecologically significant exposure appears to be at the outfalls of industrial STP systems, where exposure residues are, of course, maximized. Given the lipophilic nature of cVMS materials, benthic organisms in sediments systems would appear to have a greater exposure opportunity than pelagic organisms. However, given the low toxicity of cVMS materials to benthic species, comparison of NOEC levels to measured field exposure concentrations show that the 95% concentrations are 100- to 500-fold lower than the long-term NOEC levels for tested benthic species, such as worms and midges.

Figure 1. D4 aquatic field data, expressed as a cumulative probability distribution, compared to Environment Canada PNEC of 0.2 µg/L

Figure 2. D5 aquatic field data, expressed as a cumulative probability distribution, compared to Environment Canada PNEC of 15 µg/L

Figure 3. D6 aquatic field data, expressed as a cumulative probability distribution, compared to Environment Canada PNEC of 4.6 µg/L

Figure 4. D4 sediment field data, expressed as a cumulative probability distribution, compared to sediment NOEC of 44 µg/g dw

Figure 5. D5 sediment field data, expressed as a cumulative probability distribution, compared to sediment NOEC of 69 µg/g dw

Figure 6. D6 sediment field data, expressed as a cumulative probability distribution, compared to sediment NOEC of 69 µg/g dw

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