4: Summary and Conclusions

• 4.1 Consideration Regarding Related Products

This review of the current science published to 31 March 2008 identified both evidence that decaBDE is bioaccumulating and/or biomagnifying in food chains, and a number of studies suggesting that decaBDE has limited bioaccumulation potential due to low dietary assimilation efficiency and metabolism. Although there are no available measured or experimental evidence to support that decaBDE, as the parent compound, meets the numeric bioaccumulation criteria as identified under the Persistence and Bioaccumulation Regulations of CEPA 1999, several studies nevertheless indicate that the substance is available for uptake and has potential to accumulate rapidly in biota to high levels.

The available studies indicate metabolic pathways in organisms leading to the formation of:

• lower brominated BDEs (down to heptaBDE congeners in mammals; potentially down to pentaBDE congeners in fish). Metabolic transformation has been shown to remove bromine atoms preferentially from the meta and para positons of the PBDE molecule in fish and mammals. Additional removal of ortho position bromine has been suggested in studies with cows;
• hydroxylated BDEs;
• hydroxymethoxylated BDEs; and
• unknown products.

It is also plausible that the lower brominated BDEs are undergoing further metabolism to hydroxylated or hydroxymethoxylated metabolites

With respect to chemical transformation in the environment, this review supports the findings of the Ecological Screening Assessment of PBDEs (Environment Canada 2006a, Environment Canada 2006b) which identified photodegradation and biodegradation as likely mechanisms for transformation in the environment. This review also identifies various new studies which quantify rates of degradation and propose chemical transformation pathways. Together, the new and existing studies provide evidence that transformation of decaBDE is likely occurring 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, based on laboratory studies using activated sludge with and without primers. Based on the available laboratory studies, it is thus reasonable to expect that decaBDE may be transformed in the environment leading to the formation of:

• lower BDEs (including nona- to tetraBDEs). Some laboratory-based biodegradation and abiotic transformation studies propose preferential removal of bromine atoms from meta and para positions, however, removal of bromine from ortho positon has also been observed (with and without additional removal of para bromine) in certain studies;
• brominated dibenzofurans; and
• other unidentified degradation 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 K_ow 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 foodchain) are less than one, mainly due to a low identified dietary assimilation efficiency of decaBDE.

In the absence of metabolism, the BAFs of all potential known metabolites and transformation products of decaBDE were predicted to exceed 5000. When assumptions were made respecting metabolic transformation (a more realistic scenario), approximately 74% of the proposed transformation products still exceeded a BAF of 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, and these exceeded 1 only for a few metabolites. 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.

The importance of environmental transformation of decaBDE as a contributor to the presence of lower BDEs in the environment is still not known. The available monitoring data do not appear to support a conclusion that debromination is a significant transformation mechanism in the environment. However, various studies show that decaBDE will transform to some degree under certain conditions. In the environment, it is possible that transformation could be masked by the prevailing PBDE congener patterns resulting from the use of the commercial pentaBDE and octaBDE formulations; as well, the infrequent analysis of higher brominated PBDEs like octa- and nonaBDEs would make it difficult to detect or confirm transformation. The amounts and potential products of decaBDE transformation will depend upon the rate of the potential degradation processes (e.g., photolysis or biodegradation), the medium in which these processes act, and the medium where decaBDE is likely to be found in the environment, as well as the degradation rates of the transformation products. It is reasonable to expect that under some environmental conditions, decaBDE will remain persistent without evidence of breakdown for at least several years or potentially decades as the study by Sellström et al. (2005) suggests.

Overall, this review confirms that, based on the reviewed materials published up to 31 March 2008, decaBDE is available for uptake in organisms and may accumulate to high and potentially problematic levels in certain species such as birds of prey or mammalian preditors. However, available data do not show that this substance meets Bioaccumulation criteria as defined under the Persistence and Bioaccumulation Regulations under CEPA (1999). Factors such as low assimilation efficiency and metabolic transformation appear to be important determinants of accumulation in organisms. Although uncertainties remain, it is reasonable to conclude that decaBDE may contribute to the formation of bioaccumulative and/or protentially bioaccumulative transformation products such as lower brominated BDEs in organisms and in the environment. As noted in Canada (2006a), levels of decaBDE in sewage sludge and the environment have reached and surpassed mg/kg dw levels, which indicates that these may act as important sources for the formation of bioaccumulatove products over several years or decades.

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 as the commercial decaBDE product. For instance, both are additive flame retardants used in high-impact polystyrene (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 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. 2005) 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 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 the decaBDE and decaBD ethane, the presence of the decaBD ethane in the Canadian wildlife, and the potential for the 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.