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

Summary

In July 2006, the final decision on the screening assessment of substances - polybrominated diphenyl ethers (PBDEs) - was published by the Minister of the Environment and the Minister of Health in the Canada Gazette, Part I.  It was concluded that PBDEs--i.e., tetrabromodiphenyl ether (tetraBDE), pentabromodiphenyl ether (pentaBDE), hexabromodiphenyl ether (hexaBDE), heptabromodiphenyl ether (heptaBDE), octabromodiphenyl ether (octaBDE), nonabromodiphenyl ether (nonaBDE) and decabromodiphenyl ether (decaBDE)--which are found in commercial PentaBDE, octaBDE and decaBDE technical formulations, are entering the environment in a quantity or concentration or under conditions that have or may have an immediate or long-term harmful effect on the environment or its biological diversity, and thus meet the criteria set out in paragraph 64(a) of the Canadian Environmental Protection Act, 1999 (CEPA 1999). In addition, it was concluded that all seven PBDEs homologues met criteria for persistence, but only tetra- to hexaBDE met the criteria for bioaccumulation as defined in the Persistence and Bioaccumulation Regulations. The analysis also noted that the higher brominated diphenyl ethers, and decaBDE in particular, could accumulate to some degree in biota and debrominate to bioaccumulative and persistent transformation products.

Since the completion of the Ecological Screening Assessment, a large amount of new information has been published regarding the accumulation of decaBDE in biota and the potential transformation of decaBDE to persistent and bioaccumulative products. The purpose of the present report is to provide an updated analysis of bioaccumulation and transformation of decaBDE, by summarizing evidence considered in the original Screening Assessment, and then examining the related new science published up to August 25, 2009. 

Overall, available data do not show that the decaBDE itself meets the numeric criteria for bioaccumulation as defined in the Persistence and Bioaccumulation Regulations. With regard to bioaccumulation, potential factors such as low assimilation efficiency and/or metabolic transformation appear to be important determinants of accumulation in organisms. Nevertheless, some recent studies have shown concentrations of decaBDE to be increasing steadily in some wildlife species and there are a few equivocal reports of biomagnification factors (BMFs) exceeding 1. In some cases, such as in the tissues of kestrel, sparrowhawk, peregrine falcon, glaucous gull, red fox, shark, harbour porpoise and whitebeaked dolphin, measured concentrations of decaBDE in tissues are interpreted as high. While trophic magnification or bioaccumulation is a potential explanation for these high concentrations, it is also very possible that some biota are exposed to very high exposure concentrations of decaBDE by consuming contaminated refuse and/or inhabiting decaBDE hotspots close to industrialized areas.

This report also considers it reasonable to conclude that decaBDE may contribute to the formation of lesser-brominated PBDEs and other metabolic products in organisms--potentially those that are bioaccumulative. Although there is some uncertainty, the evaluation found evidence that fish and mammals appear to have some capacity to metabolically break down decaBDE. In fish, decaBDE appears to form hepta- to nonaBDEs, and potentially penta- and hexaBDEs. In mammals, debromination of decaBDE down to heptaBDEs has been observed. In both fish and mammals, formation of lower brominated PBDEs appears to be very low and only a fraction (typically on the order of a few percent) of the total amount of decaBDE administered to the organism. However, 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 (such as hydroxylated and hydroxymethoxylated PBDEs) and/or bound as inextricable residues.

The evaluation of transformation in the environment identified numerous laboratory studies that provide evidence that decaBDE may break down in the environment, particularly as a result of photodegradation and biodegradation. Studies of photodegradation of decaBDE sorbed to solids in aqueous and dry systems have demonstrated transformation of decaBDE to tri- to nonaBDEs, tri- to octabrominated dibenzofurans (octaBDFs) and unidentified products. While relevant to the environment, the actual fraction of decaBDE exposed to sunlight adsorbed to atmospheric and aquatic particulates, or solids (anthropogenic or natural), would be a small fraction of the total amount of decaBDE in the environment. Biodegradation studies have also shown potential breakdown of decaBDE mainly to nona-, octa- and heptaBDEs, while transformation to triBDEs has also been shown under enhanced laboratory conditions. Overall, biodegradation appears to occur at a much slower rate than that of phototransformation, with half-lives in the range of several years to several decades. The photolytic half-life of decaBDE adsorbed to house dust and exposed to sunlight has been reported to range from approximately 1-2 months (assuming 8 hours of sunlight per day).

Modelling of bioaccumulation factors (BAFs) and BMFs was conducted to estimate whether transformation products of decaBDE resulting from processes in organisms and in the general environment could be bioaccumulative. The evaluation found that many of the identified transformation products could be bioaccumulative (i.e., have BAFs in excess of 5000) and some could biomagnify in food chains. The analysis also indicated potential transformation of decaBDE to products (i.e., tetra- to hexaBDEs) that have been established as bioaccumulative based on empirical evidence.

While laboratory studies on the transformation of decaBDE support a conclusion that transformation to lower BDEs and BDFs should be occurring in the environment, the phenomenon has not been conclusively shown through monitoring studies to occur in the environment. This suggests that the process of environmental transformation may be very slow and subtle, and possibly may have relevance to a small fraction of the total decaBDE reservoir in the environment. Evidence of transformation may be shielded by existing patterns of PBDEs in the environment which are dominated by congeners found in the commercial products. The fact that fewer studies have historically measured octa- and nonaBDE congeners in environmental samples makes the elucidation of debromination patterns of decaBDE in the environment challenging.

While this report has focused on decaBDE, its analyses and conclusions provide useful inferences on alternative flame retardants with similar chemical structures and use patterns, such as decabromodiphenyl ethane (decaBD ethane). DecaBDE and decaBD ethane have only minor structural differences relating to the bond between their aromatic rings and, thus, these substances may have similarities in physical and chemical properties, persistence, transformation patterns, and accumulation in organisms. Based on the similarity in properties between decaBDE and decaBD ethane, the presence of decaBD ethane in Canadian wildlife, and the potential for decaBD ethane to be used as a large-scale replacement for decaBDE, there is also a need to further understand the potential risks from decaBD ethane in the environment and its capacity to accumulate in wildlife and transform to potentially bioaccumulative products. Understanding the risk from alternatives generally will help to ensure that substitutions of flame retardants are made on an informed basis.

 

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