Science News - August 11, 2004

Salmon flame retardant research raises new questions

Farmed salmon contain significantly higher levels of the polybrominated diphenyl ether (PBDE) compounds used as flame retardants than do wild salmon, according to research published this week on ES&T’s Research ASAP website (es049548m). The research shows that the PBDEs follow the same trends as other contaminants that salmon take up, such as PCBs and dioxins, but it also raises some important new questions about the nature of these persistent organic pollutants.

The study’s authors, Ronald Hites of Indiana University and colleagues, analyzed the same set of 700 farmed and wild salmon collected from around the world that was also the basis for highly publicized research published earlier this year (Science 2004, 303, 226–229 ). As was the case with the 14 contaminants described in the earlier report—which included pesticides such as toxaphene and dieldrin—the researchers found the highest levels of PBDEs, on average, in farm-raised salmon from Europe (the median level was 3.095 nanograms of total PBDEs per gram of fish tissue (ng/g, wet weight), and the geometric mean was 3.219 ng/g).

In turn, the levels of PBDEs in the farmed North American salmon (median 2.937 ng/g, mean 2.483 ng/g) were significantly higher than the levels in farmed salmon from Chile (median 0.803 ng/g, mean 0.972 ng/g), which were higher than the average levels in wild salmon. In both the farmed and wild salmon, approximately 50% of the total PBDEs that Hites and his colleagues found were in the form of one compound, or congener: brominated diphenyl ether (BDE) 47. This congener is associated with the Penta formulation used in polyurethane foam in furniture, which, together with a second formulation known as Octa, has been banned in Europe and is being discontinued in the United States.

The PBDE uptake patterns in salmon do not correlate at all with the levels found in people; samples of fat and blood from North Americans contain levels 10 times higher, on average, than those of Europeans. Exactly how people are taking up PBDEs remains unclear, but some researchers thought that fish could be the primary source of exposure for PBDEs, says Tom McDonald of the Office of Environmental Health Hazard Assessment of California, the first state to ban PBDEs associated with the Penta formulation. Hites’ data raise significant questions about this theory, McDonald says.

“I’m getting more and more convinced that the high U.S. levels [in people] are due to more of an inhalation route of exposure,” says Åke Bergman of Stockholm University’s department of environmental chemistry, who was one of the first scientists to present evidence that PBDEs were bioaccumulating in humans. “I don’t see why there should be such a difference between North America and Europe otherwise,” he says.

The data, together with other new research investigating PBDEs in foods, raise questions about exposure pathways, agrees Linda Birnbaum, director of the experimental toxicology division of the U.S. EPA’s National Health and Environmental Effects Laboratory.

Hites’ study also begs the question of how the farmed European salmon are being exposed to the PBDEs they’re taking up, Birnbaum says. “Why should the levels, especially of BDE-47, be higher in the European salmon than in the North American salmon, when there’s very little Penta in Europe?” she asks. Birnbaum speculates that the heavier brominated compounds associated with the Deca brominated flame-retardant formulation used more extensively in Europe may be breaking down, or debrominating, to produce lighter compounds such as BDE-47. There is already evidence that some fish can break down the Deca compounds in this way (Environ. Sci. Technol. 2004, 38, 8A–9A), as well as evidence that this can happen environmentally. There are no plans to discontinue the Deca product’s use in North America or Europe

Hites theorizes the food fed to the farmed European and North American salmon may come from the same, or similar, sources in the North Atlantic, which are relatively contaminated. However, he acknowledges that this does not explain why the PBDE levels in farmed European salmon would be higher. Although Hites and his colleagues did not evaluate levels of the contaminant in wild European salmon, Bergman says that he has found that the PBDE levels in these wild salmon are on a par with those Hites reported for the farmed European salmon.

The contamination of wild salmon with PBDEs is “a really strong suggestion of environmental exposure as well as dietary exposure,” says Miriam Jacobs, a nutritionist and toxicologist at the University of Surrey in the U.K., who has also studied contaminants in salmon. Hites and the researchers interviewed for this article agree that the PBDEs are probably reaching the open ocean and getting into the marine food web through atmospheric deposition.

What the researchers find more difficult to explain is why the research shows that wild chinook salmon from British Columbia had the highest levels of contamination of any of the salmon Hites tested. They posit that it has to do with the chinook’s tendency to feed higher in the food web throughout their adult life, eating mainly fish, unlike other salmon species that tend to consume more invertebrates and plankton. Because wild Alaskan chinook tested in the study contained significantly lower PBDE levels, the relative contamination of the waters that the wild chinook inhabit must also play a role, the scientists interviewed for this article agree.

Although Hites and his colleagues did not recommend limiting consumption of farmed salmon based on regulatory guidelines, as they did in their Science article, they suggest that it is “prudent” to consume wild salmon instead, to avoid the higher levels of contaminants found in farmed salmon. The researchers point out that PBDEs are endocrine disrupters that have been shown to have reproductive toxicity, and there is some reason to suspect that they may place a role in cancer formation. “It’s an extra risk we don’t need,” Jacobs concludes.

Compared to PBDE levels in other fish, the levels in farmed and wild salmon as found in the Hites study are low, according to Salmon of the Americas, an industry group.  The total intake of PBDEs from farmed salmon is only a fraction of what it is from other foods, the organization claims. —KELLYN BETTS