New data suggest PBDE byproducts are ubiquitous in U.S. waters
Kellyn Betts
Environmental Science and Technology
05/27/2009
Research published in ES&T (DOI 10.1021/es9003679) shows for the first time that compounds produced when PBDE flame retardants are exposed to wastewater treatment can generate dioxins. Kris McNeill, Bill Arnold, and their University of Minnesota colleagues say they believe that the photochemically created brominated and mixed halogenated dibenzo-p-dioxins they discovered are likely to be “ubiquitous in aquatic environments.” Experts agree that both the dioxins and the compounds that produce them, hydroxylated PBDEs (OH-PBDEs), could be impacting aquatic wildlife, and humans as well.
Buttressing the finding’s significance is a report by the U.S. National Atmospheric and Oceanic Administration (NOAA) released on April 1, which documents “that PBDEs are clearly ubiquitous” in U.S. coastal waters. When the report was issued, John H. Dunnigan, assistant administrator of NOAA’s National Ocean Service, said: “Scientific evidence strongly documents that these contaminants impact the food web and action is needed to reduce the threats posed to aquatic resources and human health.” The report points out that laboratory toxicity studies have connected PBDEs to liver, thyroid, and neurobehavioral development impairments. It also says PBDEs “show the potential for adverse human health effects.”
The NOAA report is the most comprehensive assessment of the presence of PBDEs in the U.S. coastal environment, says Susan Shaw, director of the Marine Environmental Research Institute, a nonprofit organization. Shaw is the lead author of a 2009 paper documenting the biomagnification of PBDEs in northwest Atlantic marine food chains, including predatory fish and seals.
Although the NOAA scientists measured neither OH-PBDEs nor the dioxins that they can spawn, McNeill and Arnold predict that scientists are likely to find these byproducts wherever PBDEs are present in the water environment. “It’s logical to assume if you have PBDEs [in wastewater], you’ll have OH-PBDEs. . .and if the OH-PBDEs are exposed to sunlight, you’re going to get brominated dioxins formed in the water,” Arnold says.
When considered together, the new research and report suggest that additional research is needed to investigate whether the mechanism discussed in the paper could produce more toxic dioxins from other PBDE compounds, says Linda Birnbaum, director of the National Institute of Environmental Health Sciences and one of the world’s foremost authorities on dioxin toxicity. She says that the dioxins produced in the new experiment seem unlikely to be very toxic, however.
Birnbaum says she is also concerned about the potential health effects from OH-PBDEs. “They appear to be relatively persistent and do appear to have endocrine properties,” she says. According to the new paper, a growing body of research suggests that they, too, are ubiquitous in the water environment.
Fascinated by mechanisms
Arnold is a chemical engineer and McNeill a chemist. They credit their ongoing fascination with molecular mechanisms, together with what Arnold calls “chemical intuition”, with their discovery of this new route by which dioxins can be generated in the environment. They say they were inspired by the realization that OH-PBDEs are virtually identical, structurally speaking, to triclosan, an antibacterial agent that also reaches U.S. waters via wastewater treatment. In 2003, the team was the first to discover that triclosan can produce a chlorinated dioxin under similar conditions.
In their new work, Arnold and McNeill’s team documented that four OH-PBDEs—including two hydroxylated polybrominated/chlorinated diphenyl ether (OH-PBCDE) compounds—produced dioxins in the presence of sunlight. They used water from nearby sources, such as the Mississippi River, and calculated yields of 0.7−3.6%.
“While yields are low. . .the results are important because of the ubiquity of PBDEs and OH-PBDEs in the environment,” says Derek Muir, senior research scientist at Environment Canada. In addition, because the dioxins “may bioaccumulate and biomagnify. . .it would be useful to have more environmental measurements with which to assess their relative importance.”
The OH-PBDE compounds McNeill and Arnold tested in the new experiment are associated with BDE-47, one of the most bioaccumulative PBDE compounds, or congeners. BDE-47 is one of the congeners included in the penta-BDE flame retardant formulation, which is one of two PBDE formulations banned in the EU and discontinued in the U.S. in 2004. A third formulation, deca-BDE, is still used in electronics products in many parts of the world. However, the NOAA report points out that “even with a global reduction of PBDE use, the continued release into the environment of PBDEs is inevitable for years to come due to their persistent nature.”
Dioxin’s toxicity revisited
Dioxin is a general term that describes a group of hundreds of structurally and chemically related compounds, including polychlorinated dibenzo-p-dioxins (known as PCDDs) and polychlorinated dibenzofurans (known as PCDFs). Of the 419 chlorinated dioxins and related compounds that have been identified—including some dioxin-like PCBs—only about 30 are considered to have significant toxicity, according to a World Health Organization website. The most toxic is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a known carcinogen.
Chlorinated dioxins are known to bioaccumulate in aquatic and terrestrial food chains. Animal studies suggest that “exposure to low levels of dioxins over long periods (or high level exposures at sensitive times) might result in reproductive or developmental effects,” according to a website of the U.S. Food and Drug Administration’s Center for Food Safety and Applied Nutrition.
Brominated and mixed halogenated dioxins have been the subject of far less study than their chlorinated cousins. “There is a big knowledge gap in this field,” says Lillemor Asplund of Stockholm University’s Department of Applied Environmental Science. Asplund is a coauthor of a 2007 ES&T paper which raises the possibility that polybrominated dibenzo-p-dioxins similar to the ones McNeill and Arnold have reported could be an important new class of marine toxins.
The most comprehensive assessment of brominated dioxins’ toxicity was published in 2003 by a team led by Birnbaum. The researchers found that the “available literature suggests that brominated compounds have similar toxicity profiles to their chlorinated homologs.” They also predicted that “it is likely that human, as well as wildlife, exposure to brominated dioxins and furans will increase.”
Birnbaum says that the dioxins of highest concern—such as TCDD—have four or more chlorine and/or bromine atoms in specific positions. This configuration is known to activate the high degree of aryl hydrocarbon receptor (AHR) binding that scientists generally agree is responsible for TCDD’s toxicity. Of the dioxins that McNeill and Arnold identified in the new study, none have halogens in all these positions.
Birnbaum predicts that, as is the case with chlorinated dioxins and dioxin-like compounds, less than 10% of the brominated and mixed halogenated dioxins will prove to be toxic.
Arnold and McNeill say that they are continuing to explore the identities of additional dioxins that can be produced from other OH-PBDE compounds. Scientists do not yet have a firm handle on how many different OH-PBDEs are present in fresh- and saltwater as the result of oxidation of the 39 PBDE congeners known to have been included in commercial flame-retardant formulations. McNeill and Arnold roughly estimate that there are dozens of OH-PBDEs, which they believe are capable of producing a similar number of dioxins in aquatic environments.
The Baltic situation
In a 2005 ES&T paper, Asplund and colleagues published one of the first papers to report brominated dibenzo-p-dioxins in wildlife. They found the dioxins in blue mussels from the Baltic Sea, where OH-PBDEs are believed to be produced naturally via red algae. The concentrations of two brominated dioxins containing three halogens in the mussels “are as high as. . .0.30−220 nanograms per gram lipid weight,” Asplund says. These levels are significantly higher than the concentrations of PCB-153, one of the most bioaccumulative PCBs, and chlorinated dioxins in the same samples, she says.
In a 2007 paper, Asplund and her coauthors discussed their investigations into dioxin-like potency of these compounds, including their AHR binding affinity. Their work suggested that the concentrations found in some Baltic species came close to or exceeded the European Commission’s maximum residue limits for TCDD dioxin equivalents. Asplund says that her coauthor Peter Haglund of Umeå University (Sweden) is initiating further investigations into the compounds’ toxicity. Her team believes that the more heavily brominated dioxins are more likely to be of concern.
Asplund says that brominated dioxins similar to the ones she and her colleagues found have also been documented in food chains outside the Baltic, including in the U.K. She considers it possible that the reaction discussed in McNeill and Arnold’s new paper is responsible for at least some of the brominated dioxins in the marine environment.
OH-PBDE questions
The researchers interviewed for this article agree that more data is needed on almost every aspect of OH-PBDEs. The toxicological data available to date shows they “are more persistent in the body than we would ever have expected,” Birnbaum says. One of ES&T’s top papers of 2008 shows that an OH-PBDE associated with BDE-47 disrupts oxidative phosphorylation, a metabolic pathway found in all living things that converts carbohydrates to energy. Scientists are now investigating whether this effect, which can also be triggered by other environmental contaminants such as perfluorinated compounds, could explain why emaciated seals are being found in the Baltic Sea.
Initial work with cell assays also suggests that some OH-PBDEs may be more potent antiandrogens than the PBDEs themselves—“as potent as some of the classic pharmacological agents,” says Tom Webster, associate chairman of environmental health at Boston University’s School of Public Health.
Earlier this year, scientists involved with a study being carried out by the Harvard School of Public Health published research linking men living in homes with higher concentrations of PBDE flame retardants in their dust with proportionally lower levels of testosterone. Researchers do not yet know if OH-PBDEs could be playing a role in these findings, Webster says.
Because toxicologists have documented that OH-PBDEs can be produced metabolically from PBDEs in rats, mice, and perhaps humans, it is unclear if the OH-PBDEs in animals and people are produced by their bodies or if they result from direct environmental exposures—or both, Webster says. The compounds have been detected in seals, birds, and humans, as well as Baltic fish and shellfish.
There are no known natural sources of OH-PBDEs in freshwater. Scientists believe that the OH-PBDEs detected in samples collected near wastewater treatment plants are produced by oxidation of the PBDEs in wastewater during sewage treatment. Last year, an international team of researchers also documented OH-PBDEs in both rain and snow. They posit that the reaction of airborne PBDEs with the OH radical generates OH-PBDEs in the atmosphere.
Implications of NOAA’s report
The NOAA report on PBDEs is based on samples from the more than 300 sites monitored via the agency’s Mussel Watch program, which has tracked contaminants for more than 23 years. By comparing samples collected in 1996 with ones collected from 2004 to 2007, the NOAA researchers found that the incidence and concentrations of PBDEs had increased significantly.
