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PBT assessment

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PBT assessment: overall result

dodecamethylcyclohexasiloxane D6 / 540-97-6 / 208-762-8
Type of composition:
boundary composition of the substance
State / form:
Reference substance:
dodecamethylcyclohexasiloxane D6 / 540-97-6 / 208-762-8
PBT status:
the substance is not PBT / vPvB

The substance does not meet the definitive criteria for persistence in the soil compartments; it is not P or vP. The criteria for persistence (P/vP) in the aquatic compartment are met. The criteria for persistence (P/vP) in the sediment compartment are met.

The substance does not meet the definitive criteria for bioaccumulation based on weight of evidence determination using expert judgement; it is not B or vB.

The substance does not meet the definitive criteria for toxicity; it is not T.


The purpose of the following sections is to summarise the Registrants’ approach to weight of evidence concerning persistence and bioaccumulation, and to acknowledge that scientific understanding of persistency and bioaccumulation is growing and changing. REACH guidance on the use of weight-of-evidence for PBT/vPvB assessment is limited; this is an area of science still changing, and areas of development and uncertainty are still being discussed amongst technical leaders in the field.



Registrants’ opinion concerning weight of evidence on persistence


Persistence is not intrinsic to a substance; it will depend instead on the experimental conditions at the time and place of testing. Due to the physical properties of D6, standard laboratory tests for persistence, especially sealed systems, are not appropriate for extrapolation to a complex environment.

Based on hydrolysis rate, D6 would be considered to be persistent in water based on a standard laboratory study, however due to the very low water solubility, it will volatilise from the water or bind (reversibly) to organic carbon rather than being dissolved in the water and these properties limit the presence or persistence in water in the environment. Environmental presence is driven by the integration of all partitioning properties and degradation rates for all environmental compartments. A holistic multimedia fate and transport assessment to characterize chemical exposure (Webster et al., 2004) is the most important first step to ensure the most accurate assessment of the role of partitioning and degradation on the fate of a substance in the environment. For example, even though these substances bind to organic carbon, which allows them to enter the sediment compartment, this is not irreversible. The environment exists in an equilibrium and the high air/water and low organic carbon/air-water ratio dictates that even though D6 partitions to sediment it will prefer air even relative to organic carbon.

For substances such as D6, it is Poverall(Pov)or P in the final sink (air) that matters in the global context.Consequently, chemical persistence should be assessed with respect to a compound’s Povin an evaluative multimedia regional or global environment.The overall persistence (POV) is more important for D6 than for other classes of chemicals, such as the classical PBTs/POPs. Since D6 partitions readily to atmosphere (major compartment) where it is degraded more rapidly than in other matrices, D6’s presence in the global environment is much shorter (months) than the classical POPs where global lifetimes are much longer (several years). The predicted Povfor D6 is 126 days indicating different persistence characteristics compared to classical POPs which have half lives in years.

There are no guidelines for using Povto classify chemicals as P and vP; however, expert judgment of the results of global modeling shows that global half-lives in air are short and, if use were to cease, partitioning into and then rapid degradation in the atmosphere would result in complete dissipation in a few years. As these processes are ongoing, this means that concentrations in the global environment are in a quasi-steady state at this time and will not increase with continued use and release (Bridges and Solomon, 2016).

D6 tends to be distributed mainly in air (> 95%). In air, it is well established that D6 readily degrades by reaction with OH radicals (Atkinson, 1991; Latimeret al, 1998, Sommerladeet al1993). D6 released to air is mainly released from the urban centers where the OH radical concentrations are much higher than the global average OH radical concentration used to estimate their current half-lives (Suzuki et al., 1984; Nunnermacker et al. 1998; Dillon et al., 2002; Ren et al. 2002; Hjorth 1984; Schade et al. 2002). Recent work using actual air monitoring data demonstrates the real life degradation of D6 (and other VMS) in air may be much faster than the value that is currently estimated (Xu et al., 2019) and involve other mechanisms beyond OH radical degradation. This work suggests a calculated real-life half-life (empirical t-value) of 3 days in the air for D6.

In summary, the Registrants acknowledge that D6 meets the individual laboratory criteria for persistent in water, and would be expected to meet the laboratory criteria for sediment (based on read-across). However, the available lines of evidence show consistently that various forms of degradation of D6 are possible and that it is necessary to take into account the evidence concerning rates of reaction in assessing if D6 would be truly persistent in the environment. In addition, the potential for transfer between compartments also needs to be considered. The combination of degradation and potential for transfer between compartments is reflected in the overall persistence (POV) assessment and POV is most appropriate for assessing persistence in the global context. 

Lastly, by the very nature of D6 (volatile, strongly bound to organic carbon, subject to degradation), the presence of D6 in sediment will be constrained by these properties and the maximum absorptive capacity of the sediment (Bridges and Solomon 2016). Therefore, sediment does not possess an unlimited potential for uptake of D6 into biota, and the presence (or persistence) of D6 in sediment does not lead to an increased uncertainty in the estimation of risk to this compartment. A quantitative risk assessment can be conducted near emission points to ensure there is no risk from D6 to this compartment. 


Registrants’ opinion concerning weight of evidence on bioaccumulation

The Registrants consider that D6 is not bioaccumulative in the sense that D6 will not biomagnify to increasing and unpredictable concentrations in the food web. Although water may be an exposure route for lower trophic level organisms based on the BCF, a concern for bioaccumulation would require presence in water, low potential for elimination from biota at the lower trophic levels, little metabolism or excretion by higher species, and the potential for toxicity expression in these organisms. This is not the case for D6 because D6 is volatile, poorly soluble, and D6’s presence in surface water is therefore low to non-existent. In addition, in terrestrial and other air breathing organisms, both metabolism and elimination of D6 via exhalation prevent the accumulation of D6. D6 will not therefore biomagnify to increasing unpredictable concentrations in the food web and could never accumulate and pose a threat to top predators and humans through biomagnification. This is supported by field data assessing trophic magnification.

Factors that control the accumulation of D6 in biota, laboratory and field    

Mackay et al., 2015 examines the role of physical-chemical properties important in evaluating the potential for accumulation from “superhydrophobic” compounds such as D6. The term superhydrophobic is applicable to compounds with log Kowgreater than about 7, which includes D6 (log Kow8.87 at 23.6°C). The authors compiled a series of conventional uptake equations and a simple accumulation model for aquatic organisms. The model was applied to D6 simulating conditions in standard toxicity tests. The authors then discussed the reasons for the apparent lack of accumulation to concentrations of toxicity with these substances, and the potential to accumulate to toxic levels under field conditions in which diet is likely the principal source of lipophilic chemical loading to aquatic organisms (notably fish) that occupy intermediate and higher trophic level positions in aquatic food webs. Among the important factors that were examined were the roles of hydrophobicity and metabolism.

Hydrophobic substances are predicted to substantially accumulate in aquatic organisms in the absence of metabolism or growth dilution. Hydrophobicity serves to constrain the ability of a compound to accumulate by limiting the aqueous solubility. Metabolism plays a key role in the accumulation and toxicity of hydrophobic substances. As metabolic rates increase, greater amounts of a compound must be accumulated in order to cause toxicity. From the model calculations by Mackayet al. (2015), even modest rates of metabolism can substantially affect the accumulation and toxicity of D6. The authors concluded that the extent of possible biomagnification of a persistent substance is controlled by the chemical’s hydrophobicity, dietary assimilation efficiency, elimination rates, and the nature of the diet. If there is even slow biotransformation such that the biotransformation rate (kM) approaches or exceeds k2,biomagnification is unlikely to be significant. They further indicate that the preferred approach for elucidating the possibility of biomagnification is to obtain monitoring data for representative food webs and convert the concentrations to fugacities or chemical activities to reveal their relative equilibrium status. Additional research is underway with independent experts assessing the biotransformation capacities of D4 and D5 in fish and benthic organisms. A series of dietary in vivo exposures of D5 to juvenile rainbow trout were conducted to determine dietary uptake efficiencies and biotransformation rates in fish intestinal and somatic tissues (Kim et al., 2019; Gobas, personal communication). In this research, following dietary uptake of D5, the intestinal content was separated from the rest of the fish, and both tissue types (i.e. digestive tract and remainder of the fish body) were measured by GC-MS for parent D5. From this work, it was illustrated that while both somatic and intestinal biotransformation was observed for D5, the intestinal biotransformation of D5 was ~ 150 times greater than the biotransformation in the rest of the fish body. Since exposure of hydrophobic substances like D5 occurs primarily through dietary assimilation, solely using D5 data from waterborne BCF studies will therefore overestimate the bioaccumulation potential and not accurately predict the propensity for biomagnification in the food web.

The BCF is in reality a laboratory screening metric; substances of high hydrophobicity inevitably have a high BCF, however fast they metabolise, and so for such substances BCF should not be used as a definitive criterion.


Activity or Fugacity Analysis to Assess Biomagnification Potential of D6

Dimensionless chemical activities are related to chemical fugacities (units of Pa), which are often described as the escaping tendency of a chemical from one matrix to another. Chemical activities allow the expression of all data being examined to range from 0 to 1. Chemical activities are easy to calculate and allow the comparison of concentration data that are of differing units.

Chemical activity of aqueous concentrations are calculated as the ratio of the concentration with the solubility (for D6, the solubility is ca. 5 µg/l at 23°C) in water. Activities of sediment and soil concentrations are the ratio of organic carbon (OC) based concentrations with the OC-based ‘maximum sorptive capacity’. The OC-based maximum sorptive capacity of D6 is 5465 mg/kg-OC. Activities of concentrations in biota are the ratio of the lipid-based concentration and the apparent solubility of the chemical in lipid, which is approximated by the compound’s Kowand aqueous solubility values (see Section 4.3.3 for further details).


Relationship to determination of TMF

The discussion of activities allows an overview of many measurements. It provides a line of evidence regarding field measurements and helps discussion of weight of evidence as concluded by Bridges and Solomon (2016); they conducted a weight of evidence (WOE) analysis for D6. The WOE analysis included studies that examined data on concentrations in various environmental matrices, persistence, bioaccumulation, and toxicity. TMF studies reported in the literature could be considered as particular subsets of measurements.

Several field studies from locations in Europe, North America, and Asia have shown that D6 biodilutes as you move up the food chain. Trophic food web biodilution behavior of D6 has been documented in systems varying from freshwater to marine and pelagic to benthicpelagic in food web structure, and reliable reported D6 TMF values are less than one. 


The Registrants acknowledge that there is a lack of full agreement of the TMFs across various study areas and this may reflect differences in TMF study design. TMFs may also be biased because of environmental conditions and sample collection. There is often a lack of common sampling areas for the species considered in the TMF calculation and the presence of point sources such as wastewater treatment plants can cause concentration gradients in the sampling area. These gradients can have a significant impact on study outcomes, as shown by Gobaset al., 2015, Kimet al., 2016 and Mackayet al., 2016. Therefore, the Industry continues to conduct additional work to add more certainty to these assessments.

Furthermore, as demonstrated by the Multibox-AQUAWEB model, TMF values above 1 can occur if sampling has been carried out with high spatial variation. Work is currently underway with external experts to update a Multibox-AQUAWEB model that takes into consideration these variables and confounding factors. As noted above,research is underway assessing the biotransformation capacities of D4 and D5. The additional understanding of biotransformation rates and uptake efficiencies in aquatic organisms will be used to further calibrate the Multibox-AQUAWEB model for assessing field TMFs. 

Additional field studies supporting a lack of biomagnification/trophic magnification have also been published (Powellet al., 2017; Powellet al., 2018).


Overall weight of evidence on bioaccumulation

The various documents mentioned above indicate and advocate an approach that identifies specific lines of evidence for specific areas and then brings these lines of evidence together to apply a weighting to each line in terms of its relative importance on the conclusion. The approach applied is qualitative only in that there was no agreed ‘scoring’ methodology in this context. In many cases the weighting is based on expert judgment and argument, rather than a quantitative weighting or scoring system. As indicated in these submissions a robust quantitative and transparent weight of evidence PBT assessment on D6 by academic experts (Bridges and Solomon, 2016), has been addressed.

The results of this robust quantitative weight-of-evidence (WoE) (Bridges and Solomon, 2016) as well as additional lines of evidence on biomagnification potential of D6 is presented.

It should be noted that application of a weight-of-evidence approach that considers all available information relating to the substances does not lead to the conclusion that D6 is bioaccumulative or very bioaccumulative (Bridges and Solomon, 2016). The evaluation concluded:

A transparent quantitative weight-of-evidence (WoE) evaluation of each study was needed to characterize their properties. Measurements of concentrations of cVMSs in the environment are challenging but, at this time, concentrations measured in robust studies are generally small and all less than thresholds of toxicity. The cVMSs are slightly persistent in air with half-lives ≤11 d. They are rapidly degraded in dry soils and partition from wet soils into the atmosphere. They are not classifiable as persistent in soils. Persistence in water and sediment is variable but greatest concentrations in the environment are observed in sediments. Based on their overall persistence in the environment, cVMSs should not be classified as persistent. The overall weight of evidence for the studies in food-webs supports a conclusion that the cVMSs clearly do not meet the criteria for biomagnification in the environment, a conclusion that is consistent with results of toxicokinetic studies. Toxicity was not observed at the solubility limit in water or the maximum sorption capacity in soils and sediments. Combining all of these lines of evidence shows that the traditional measures of persistence and biomagnification used to classify the legacy POPs are not suitable for the cVMSs. Refined approaches are needed and, when applied, these show that these materials are not classifiable as persistent, bioaccumulative or toxic in the environment.



Weight of evidence approach - summary

When using the weight of evidence assessment (Bridges and Solomon, 2016), the Registrants are convinced that D6 is neither vPvB nor PBT.

The R11 guidance states:

PBT substances are substances that are persistent, bioaccumulative and toxic, while vPvB substances are characterised by a particular high persistency in combination with a high tendency to bio-accumulate, which may, based on experience from the past with such substances, lead to toxic effects and have an impact in a manner which is difficult to predict and prove by testing, regardless of whether there are specific effects already known or not. These properties are defined by the criteria laid down in Annex XIII of the Regulation…

Experience with PBT/vPvB substances has shown that they can give rise to specific concerns that may arise due to their potential to accumulate in parts of the environment and: that the effects of such accumulation are unpredictable in the long-term; such accumulation is practically difficult to reverse as cessation of emission will not necessarily result in a reduction in chemical concentration.

Furthermore, PBT or vPvB substances may have the potential to contaminate remote areas that should be protected from further contamination by hazardous substances resulting from human activity because the intrinsic value of pristine environments should be protected.


Some key conclusions can be reached:

  • There is a high probability that B/vB is not met in the environment itself. Definitive methodology for assessment of trophic magnification factor (TMF) is still needed. Therefore, B/vB cannot be considered as being met with certainty, and therefore PBT/vPvB may not apply under consideration of weight-of-evidence. Fugacity ratio calculations support that biomagnification is not occurring.
  • Any ecotoxicological or mammalian toxicological effects are understood and are not irreversible or feasible under realistic exposure conditions
  • D6 is not found in remote regions except when associated with human activity, and the concentrations in the environment are not increasing such as to suggest a high level of overall persistence.
  • The absolute concentrations of D6 measured in the environment are consistent with the local release pattern and do not indicate unexpectedly high persistence, i.e. there is no indication that half-lives in the environment are longer than the laboratory data suggest. In addition, analysis of available air monitoring data suggests the half-lives are shorter than the laboratory data suggest.
  • D6 has uses in the personal care sector which result in a predictable level of the substance in environmental compartments (see section 10). The fraction of D6 that is released that can ultimately be found in compartments such as fresh water and marine sediments is very small, and the tonnage released directly to the environment is low. The majority is released to, and will degrade in, air. Modelling data using standard and advanced methods shows that the scope for movement of the substance by long-range mechanisms is limited by degradation in air and a very low potential for deposition to soil or water from air. This is strongly supported by the monitoring data obtained to date.
  • PEC values are low in absolute terms. The environments in which D6 is found are protected by use of PEC/PNEC methodology. The exposures and risk characterisation ratios are discussed in detail in Sections 9 and 10. Monitoring data available also supports that the environments in which D6 is found are protected.


Industry recognizes that there are differences in global scientific opinions on how to use weight of evidence to evaluate PBT and vPvB potential.

The silicones industry continues to work proactively to address regulators’ concerns. Our monitoring activities in particular, both to water/sediment compartment and air aim at achieving a common understanding of the fate of D6 in the environment, if and when the substance is emitted.

Registrants continue to work with the most knowledgeable scientific partners from academia to:

·      address remaining uncertainties e.g. on the presence and behaviour of D6 in air;

·      review data and studies pertaining to PBT/vPvB assessment such as new biomagnification information and biotransformation/elimination/uptake efficiency of D6 in aquatic organisms;

·      improve modelling and apply probabilistic risk assessment methods to D6 to re-confirm the lack of risk in aquatic systems.





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Volumes 622–623, 1 May 2018, Pages 127-139.