Ecological State of the Science Report on Decabromodiphenyl Ether (decaBDE): chapter 4

4. Summary and Conclusions

The existing evidence for the bioaccumulation of decaBDE does not support a conclusion of "bioaccumulative” as set out in the Persistence and Bioaccumulation Regulations. While most available data show that decaBDE has limited potential to bioaccumulate or biomagnify in the environment, some evidence suggests a higher BAF than previously considered for decaBDE, and some new data suggest possible biomagnification. Some studies also show that levels of decaBDE are steadily rising in some biota, and in some cases, measured concentrations have reached levels that can be subjectively described as high. The modelling undertaken to support this evaluation, however, shows uncertainty associated with metabolism in fish as model-predicted aquatic BAFs range from below the 5000 criterion to well above 5000. Predicted terrestrial carnivore BMF values also range from below 1 to greater than 1 depending on the rate of metabolism assumed. Although less relevant than BAF or BMF, experimental BCF measures are below the 5000 criterion. The substance is shown to be increasing in concentrations in some wildlife species, and some data suggest that decaBDE has reached concentrations in some organisms interpreted to be high. Potential factors limiting the bioaccumulation potential of decaBDE include low assimilation efficiency and/or metabolic transformation.

There is still some uncertainty respecting rates and pathways for metabolic transformation organisms. Debromination has been indicated in both mammalian and fish studies, but the amounts of debrominated PBDE product formed is typically very low (e.g., representing less than 1%, up to a few percent of the total dose of decaBDE depending on the study). Formation of nona-, octa- and heptaBDEs has been noted for mammals. In addition to these congener groups, hexa- and pentaBDEs have been noted as bioformed in fish. However, the interpretation of results from metabolic transformation studies is at times complicated by incomplete characterization of impurities in the dosed material and/or dosage of many PBDEs concurrently. Some rodent studies have made inferences, based on mass balance evaluations, that rates of transformation may be higher, with one study suggesting that approximately 45% of the total dose of decaBDE was unaccounted for and may have been metabolized to other compounds (like hydroxylated and hydroxymethoxylated PBDEs) and/or bound as inextricable residues.

With respect to chemical transformation in the environment, this review supports the findings of the Ecological Screening Assessment of PBDEs (Environment Canada 2006a, 2006b), which identified photodegradation and biodegradation as likely mechanisms for transformation in the environment. This review also identifies various new studies that quantify rates of degradation and propose chemical transformation pathways. Together, the new and existing studies provide evidence which makes it plausible to conclude that decaBDE is transforming in the environment.

This evaluation found that decaBDE that is sorbed to dry minerals and particulates appears to undergo relatively rapid phototransformation in the presence of sunlight. In addition, decaBDE sorbed to solids may be subject to biodegradation; however, it appears that this process is occurring at a much slower rate than photodegradation. While photodegradation on solid phases can occur at significant rates, only a very small fraction of the total decaBDE reservoir in the environment (e.g., that fraction adsorbed to particulates or on solid surfaces) that has contact with sunlight would be susceptible to photodegradation. Based on fugacity modelling, < 3.4% of decaBDE in the environment is expected to be associated with bulk air or bulk water phases with potential exposure to sunlight. DecaBDE sorbed to solid surfaces (both anthropogenic and natural) could also be exposed to sunlight. Within these phases, light attenuation and matrix shielding would affect overall exposure to sunlight and potential for photodegradation. While most decaBDE in the environment would partition to sediment and soils (fugacity modelling predicts > 96%), biodegradation has been shown to be a very slow process, with half-lives on the scale of a few years to several decades. Some studies have also not shown any obvious transformation in sediments or soil even after several decades. Hence, it is apparent that evidence of transformation in the environment could be very subtle and shielded by existing PBDE congener patterns dominated by the commercial PBDE products. As well, the infrequent historic analysis of higher brominated PBDEs like octa- and nonaBDEs could make it difficult to detect or confirm transformation.

Based on the available laboratory studies with conditions relevant to the environment, it is reasonable to expect that decaBDE may be transformed in the environment, leading to the formation of lower brominated PBDEs, PBDFs and other unknown products. A number of studies have now shown that decaBDE may transform by photodegradation or biodegradation to nona -, octa-, hepta- and hexaBDEs. One study also indicates trace formations of penta- and tetraBDEs by photodegradation, while another study also indicates biodegradation to penta- and tetraBDEs under enhanced biodegradation conditions. In addition, photodegradation of decaBDE has also been shown to form tri- to octaBDFs as well as unidentified products.

Modelling of BAFs and BMFs played an important supplemental role in this review and was used to suggest whether decaBDE and its transformation products may be bioaccumulative or biomagnify in food chains.

The metabolism-corrected model predicted aquatic BAFs for decaBDE ranged from below the 5000 criterion of the Persistence and Bioaccumulation Regulations to well above 5000. The predictions demonstrate the uncertainty associated with the metabolism potential of decaBDE in fish and log Kow determinations for this substance. Given the exceptionally low water solubility limit of decaBDE, it is not expected that this substance will be appreciably taken up from the water phase by aquatic organisms. Although less relevant than BAF or BMF, experimental BCF measures are below the 5000 criterion for decaBDE. With consideration given to decaBDE metabolism, terrestrial BMF predictions (based on a wolf food chain) indicate lack of or a low level of biomagnification of decaBDE.

In the absence of metabolism, the BAFs of all potential metabolites and transformation products of decaBDE were predicted to exceed 5000. When assumptions were made respecting metabolic transformation (a more realistic scenario), almost all proposed transformation products still yielded BAFs exceeding 5000. In the absence of metabolism, the BMFs of potential transformation products are also predicted to be very high; however, when assumptions were made respecting metabolic transformation, the predicted BMFs are significantly lower, but are still all greater than one. This analysis suggests that many possible decaBDE metabolites/transformation products could be highly bioaccumulative and some metabolites may have the capacity to biomagnify in food chains.

Overall, this review confirms that, based on the reviewed materials published up to August 25, 2009, decaBDE is not shown to meet bioaccumulation criteria as defined under the Persistence and Bioaccumulation Regulations under CEPA 1999. However, some studies show that levels of decaBDE are steadily rising in some biota, and in some cases, measured concentrations are considered high. In addition, some equivocal evidence suggests potential biomagnification in food chains. Although uncertainties remain, it is reasonable to conclude that decaBDE may also contribute to the formation of bioaccumulative and/or potentially bioaccumulative transformation products, such as lower brominated BDEs, in organisms and the environment.

4.1 Consideration Regarding Related Products

While this review has focused on decaBDE, its analyses and conclusions are relevant to alternative flame retardants with similar chemical structures and use patterns. For instance, decabromodiphenyl ethane (or decaBD ethane)--(1,2-bis(pentabromodiphenyl)ethane; 1,1"-(ethane-, 1,2-diyl) bis[pentabromobenzene])--is a replacement for the decaBDE commercial product, having the same or similar applications. Both are additive flame retardants used in HIPS and in textiles used in the manufacture of television cabinets, cable insulation and adhesives (Kierkegaard 2007). In Japan , Watanabe and Sakai (2003) have shown that there has been a clear shift in consumption away from the decaBDE commercial product to decaBD ethane.

The only structural difference between decaBD ethane and decaBDE is the carbon bond between the aromatic rings of decaBD ethane (for decaBDE, the aromatic rings are linked with an oxygen atom) (see Appendix F). Based on structural similarities, the two substances likely have similar physical-chemical properties, characteristics of persistence, transformation, and accumulation in organisms (Kierkegaard 2007).

DecaBD ethane has been identified in sewage sludge, both in Canada (Konstantinov et al. 2006) and in Spain (Eljarrat et al. 2005), has been measured in walleye and burbot in Lake Winnipeg (Law et al. 2006), and detected in herring gull eggs from the Great Lakes area of Canada (Letcher et al. 2007). The United Kingdom Environment Agency recently published a detailed risk assessment for this substance ( United Kingdom 2007b). While direct risks resulting from toxic effects of this substance were considered low, concerns were identified over this substance’s potential to accumulate in wildlife and transformation to other chemical products. The Agency also identified a need for further work on decaBD ethane to confirm the findings of their assessment, particularly to provide more reliable measures of this substance’s potential to bioaccumulate and degrade in the environment.

Based on concerns expressed for decaBDE in this State of Science report, the similarity in properties between decaBDE and decaBD ethane, the presence of the decaBD ethane in Canadian wildlife, and the potential for decaBD ethane to be used as a large-scale replacement for decaBDE, there is a need to further understand the potential risks from decaBD ethane in the environment and its capacity to accumulate in wildlife and transform to bioaccumulative products. Understanding the risk from alternatives will help to ensure that substitutions of flame retardants are made on an informed basis.

 

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