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

5 Assessment of the Nature and Extent of the Danger to the Environment Posed by Siloxane D5


259. There are several acceptable methods used to assess the potential for adverse effects of chemicals released into the environment. The methodology used to assess chemicals depends upon whether the substances are new or whether they have been used for some time. For a chemical that has been present in the environment for as long a period of time as Siloxane D5, it is possible to conduct a risk assessment based on actual, measured concentrations and any observed effects on the environment.

260. Two basic conditions need to prevail for adverse effects to occur. First, there needs to be exposure. Even for the most hazardous chemicals, if there is no exposure, no harm will be manifested.

261. Second, once exposure has occurred, there needs to be a detrimental or adverse effect. This is referred to as hazard. The magnitude of effect caused by exposure to a defined amount of the chemical is referred to as its potency. The potency of a chemical is described by the dose- or concentration-response relationship, which is derived by exposing organisms to known amounts of the chemical for known periods of time and recording the magnitude of response(s).

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

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

263. When the Screening Assessment was conducted, Siloxane D5 had been in use and entering the environment for more than 30 years. Despite this, there was little information on environmental fate or toxicity available at that time. As a result, the Screening Assessment (EC & HC 2008) focused on a few parameters, such as persistence and potential to bioaccumulate, which, in the Board’s opinion, represents a lower-tiered assessment.

264. Subsequent to 2008, additional information on the basic physical and chemical properties of Siloxane D5 has become available. Furthermore, additional information on the toxicity of Siloxane D5 has also become available.

265. Most importantly, information on concentrations of Siloxane D5 in a number of environmental matrices, including air, water, soils, and sediments is now available. Thus, a more refined assessment of the danger to the environment posed by the use of Siloxane D5 can be made.

266. Risk relates to the probability of adverse outcomes and necessarily relates to the likelihood of exposure and sensitivity to the substance. The lower tiers of ecological risk assessments are based on limited information and, for that reason, are often based on simple ratios of exposure to some threshold for effect (Hazard Quotients or “HQs”). Because of the inherent uncertainty in these approaches, uncertainty factors (“UFs”) are generally applied to reduce the likelihood of a false-negative conclusion that there is little risk when it is, in fact, large.

267. In the lower tiers of assessments, HQs greater than 1.0 suggest that there is the potential for adverse effects while HQs of less than 1.0 suggest little chance of an adverse effect. The HQ is calculated as the ratio of the exposure concentration or dose (multiplied by an UF) to the effect concentration or dose (Equation 1).

Equation for Hazard Quotient

268. These UFs are meant to be conservative and protective rather than being predictive. Assessments move to higher tiers as additional information becomes available and the uncertainty and the need for UFs is reduced.

269. Thus, lower-tiered assessments are intended to identify, and possibly screen out, chemicals that are not of concern. Simply exceeding a HQ of 1.0, or any regulatory threshold, does not necessarily indicate that there will be harm. Rather, it indicates that further, more refined assessments are warranted to better characterise the hazard and/or the risk. This was the case for Siloxane D5.

270. The initial HQs calculated in the Screening Assessment (EC & HC 2008, Figure 1, p. 40) were based, in the Board's opinion, on incorrect assumptions and parameters in models used to predict concentrations at sites receiving effluents from WWTPs (see section 4.2 above). When these errors and uncertainties are taken together, they resulted in an overly conservative estimate of the risk of Siloxane D5.

271. Given the scientific information now available, the Board was able to conduct a more comprehensive or refined risk assessment into the nature and extent of the danger posed by Siloxane D5 to the environment.

272. The Board considered risks in a probabilistic context wherever sufficient data were available. This probabilistic approach is consistent with the views of Environment Canada (Transcript of the Public Hearings, Vol. 9 p. 95) and the SEHSC/CCTFA (Fairbrother et al. 2011). This approach makes use of all of the available data and, while it might not include all possible values for concentrations in Canada, the data were sufficiently representative and robust to use in a probabilistic assessment.

273. Although the Board focused on exposures that were estimated or measured in the Canadian environment, it also took into account relevant data from other jurisdictions.

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5.1 Danger to Terrestrial Animals and Plants

274. To assess risks of Siloxane D5 to terrestrial animals and plants, the Board examined concentrations of Siloxane D5 measured in soils amended with biosolids from WWTPs (Wang et al. 2010). Because samples of soil were taken at different locations within a field on each farm and analysed separately, individual concentrations were treated as independent values that were representative of the spatial variation in concentrations between farms and within fields.

275. Cumulative frequency distributions were used to compare measured concentrations of Siloxane D5 in the environment to toxicity values. The distributions were constructed as probability plots by plotting the concentrations in a log scale on the X-axis of a graph and then plotting the ranks of the values on the Y-axis using a % probability scale. The plotting position on the Y-axis was calculated using the Weibull formula.9 A probability plot of concentrations measured in soils (Figure 1) showed that greatest measured concentration was 100-fold less than the IC50 (Table 3) for the most sensitive terrestrial organism (barley). The probability of exceeding this concentration was less than 1% and the risk was determined by the Board to be de minimis for soils. This is because soils amended with biosolids represent a maximum or worst-case scenario for concentrations of Siloxane D5 in this matrix. Concentrations of Siloxane D5 in unamended soils would all be less than those where biosolids have been added.

Figure 1. Concentrations of Siloxane D5 in Canadian soils amended with biosolids compared to toxicity values for the most sensitive terrestrial plants and animals. The 95th centile is indicated by the horizontal dashed line.

Figure 1 is a graph which describes the concentrations of siloxane D5 in amended soils compared to toxicity values for the most sensitive terrestrial species

276. The Board concluded that concentrations of Siloxane D5 would not increase in soils over time and that this route of exposure did not present a danger to terrestrial animals and plants. This is conclusion is based upon:

  • the volatility of Siloxane D5;
  • its short half-life in dry soils (0.1 to 13 days, Environment Agency 2010) and combined measured data for all soils (2.7 to 83 days, Brooke et al. 2009); and,
  • the amount and frequency of application of biosolids to farmlands authorised throughout Canada (CCME 2010).

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5.2 Danger to Aquatic Organisms

277. Concentrations of Siloxane D5 measured in surface waters near discharges from WWTPs were available for selected sites in Canada (Wang et al. 2010). These samples were taken at sites ranging from 5 m to 3.1 km from the point of discharge and are representative of sites close to the release. Concentrations of Siloxane D5 at five of these locations were less than the LOQ, and the greatest geometric mean concentration in all of the sites measured was 1.48 µg/L.

278. In reviewing data from other countries, the Board noted that concentrations in surface waters ranged from less than the LOD to 0.4 µg/L (EC 2011c). These data suggest that concentrations are generally small and all are less than the maximum solubility in water of 17 µg/L.

279. In addition, none of the concentrations of Siloxane D5 measured in effluent from WWTPs in Canada (Wang et al. 2010) exceeded its maximum solubility in water. This is noteworthy because some of the WWTPs in this study were selected from a much larger dataset to represent worst-case scenarios of release in Canada (Wang et al. 2010).

280. Taking into account the evidence before it, the Board prepared a probability plot of the concentrations measured in surface waters (Figure 2). The greatest measured concentration was 10-fold less than the maximum solubility of Siloxane D5 in water, which is also the NOEC for aquatic organisms. This is consistent with observations reported from other jurisdictions.

Figure 2. Concentrations of Siloxane D5 in Canadian surface waters within 3.1 km of discharges from waste-water treatment plants compared to the maximum solubility in water. The 95th centile is indicated by the horizontal dashed line.

Figure 2 concentrations of siloxane D5 in Canadian surface waters within 3.1 km of discharges from waste-water treatment plants compared to the maximum solubility in water.

281. Adverse effects of Siloxane D5 are not expected in aquatic organisms exposed to concentrations equal to or less than its maximum solubility in water. The Board has therefore concluded that concentrations of Siloxane D5 in surface waters present minimal risk and are not a danger to aquatic organisms, even in surface waters in proximity to discharges from WWTPs.

282. Based on an analysis of all the information presented to it, the Board concluded that there is no risk posed by Siloxane D5 to aquatic organisms. Indeed, no adverse effects were observed in aquatic organisms at concentrations equal to the solubility of Siloxane D5 in any matrix, which is the maximum concentration that can occur in the environment.

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5.3 Danger to Sediment-dwelling Organisms

283. Because of the strength of binding of Siloxane D5 to sediments and its relatively long half-life in this compartment of the environment (Xu 2011b), sediment-dwelling organisms are more likely to be exposed to Siloxane D5 than other organisms.

284. To assess risks to these organisms, the Board examined data on concentrations of Siloxane D5 in sediments sampled in close proximity (5 m to 3.1 km) to discharges from WWTPs in Canada (Wang et al. 2010). These samples of sediment were taken at the same time as the samples of water discussed in section 5.2 above and, from the tabulated values (Wang et al. 2010, Table 3), two or three separate samples of sediment were taken at each site. Each sample was analysed in duplicate or triplicate.

285. The geometric means of the replicated analyses were used to characterise concentrations of Siloxane D5 in the sediments. The multiple samples from each site were treated as independent so that the distribution of values obtained represented the spatial variance between and within sites. Because all samples of sediments were taken at about the same time, it was not possible to characterise temporal variance, such as seasonality.

286. Using the data in Table 5, the Board characterized the probability of concentrations of Siloxane D5 in sediments to exceed thresholds for toxicity to sediment-dwelling invertebrates. These are displayed in Figure 3. As there were multiple values for all of the species tested, the geometric mean was used to represent a single NOEC value for each species.

Figure 3. Concentrations of Siloxane D5 in sediments from Canadian surface waters sampled within 3.1 km of discharges from waste-water treatment plants compared to NOECs for sediment-dwelling organisms. The 5th and 95th centiles are indicated by the horizontal dashed lines.

Figure 3 is a graph which describes the concentrations of siloxane D5 in sediments from Canadian surface waters sampled within 3.1 km of discharges from waste-water treatment plants compared to the no-observed-effect-concentrations for sediment-dwelling organisms. The 5th and 95th centiles are indicated by the horizontal dashed lines.

287. It was not possible to calculate a geometric mean for the mud-worm, L. variegatus ( symbol on graph), as both of the NOEC values were greater than the maximum concentration tested. However, these data for the mud-worm were included in the ranking but not included in the regression of the toxicity data. The Board used the Hazen equation to calculate the plotting positions.10

288. Regardless of the units of expression of exposure concentrations, the Board determined that risks were de minimis. For concentrations expressed in µg/g (dw), there is a 1% probability that the 5th centile of the distribution of NOECs will be exceeded (Figure 3A, Table 6). For concentrations expressed in µg/g OC, there is a 1.1% probability that the 5th centile of the distribution of NOECs will be exceeded (Figure 3B, Table 6).

Table 6. Regression coefficients and intercepts for the distribution of NOECs for chronic exposures of sediment-dwelling organisms to Siloxane D5 and exposure concentrations in sediments from surface waters receiving effluents from WWTPs in Canada.
Parameter (units)y = ax + bCentile intercepts (μg/g (dw) or OC)Probability of exceeding the 5th centile of the distribution of NOECs
nr2ab5%95%(dw)(OC)
NOECs (μg/g (dw))31.002.22-5.0534   
NOECs (μg/g OC)31.002.98-11.211627   
Conc. in sediments (μg/g (dw))220.961.130.60 8.41.0% 
Conc. in sediments (μg/g OC)220.960.89-0.58 311 1.1%

In the above regression formula, “a” is the slope of the function and “b” is the y-intercept of the function.
Data from (Wang et al. 2010).

289. These conclusions are based only on concentrations in sediments at locations in Canada. However, similar results were obtained when data from locations in other jurisdictions were included in the characterisation of exposures by use of a similar methodology to that discussed above (Fairbrother et al. 2011). Again, because a probabilistic approach was used, it is theoretically impossible to say there was no overlap between the distributions of measured concentrations in sediments and the toxicity values. Since no adverse effects were observed in any of the toxicity tests conducted up to the limit of solubility of Siloxane D5 in sediments, the Board concluded, based on the evidence before it, that there is no risk posed by D5 on sediment-dwelling benthic invertebrates.

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5.4 Conclusions on the Nature and Extent of the Danger of Siloxane D5 to the Environment

290. Taking into account the nature of Siloxane D5, its intrinsic properties, and all of the available scientific information, the Board concluded that Siloxane D5 does not pose a danger to the environment. The Board also concluded that current concentrations of Siloxane D5 in the environment are at a quasi-steady-state. Consequently, concentrations of Siloxane D5 are expected to remain approximately constant over the long-term.

291. Due to its unique chemical and physical properties, mechanism of action, and lack of toxicity at concentrations less than its limit of solubility, it is virtually impossible for Siloxane D5 to occur in any environmental matrix at concentrations sufficient to produce harm to the environment. Thus, the Board concluded that Siloxane D5 does not currently pose a danger to the environment and that, based on the best available information, future uses of Siloxane D5 will not pose a danger to the environment.

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9 The Weibull formula (p = i/n+1 x 100), where p is the plotting position, i is the rank of the data point and n is the total number of data points in the set) is used to calculate plotting positions for the purposes of constructing a cumulative frequency distribution. The Weibull formula is normally used with larger data sets (n > 10).

10 The Hazen equation (p = (i – 0.5)/n x 100) is used for smaller data sets (n < 10).

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