Michigan’s bald eagles are among the most contaminated birds on the planet when it comes to phased-out flame retardant chemicals in their livers, according to new research.

The study, published last month in the Journal of Great Lakes Research, found that the top predators in the Great Lakes are highly exposed to banned flame retardants, still widespread in the environment.

Michigan’s population of bald eagles is stable, but the compounds have been linked in other birds to impaired reproduction, weird behavior and development, and hormone disruption.

“While the sensitivity of eagles to PBDEs has yet to be determined, there is a possibility that the exposures reported here may be associated with sub-clinical effects,” Nil Basu, an associate professor at McGill University who led study while at the University of Michigan, said in an email.

More than four decades ago, companies started putting polybrominated diphenyl ethers, PBDEs, into furniture cushions, electronics and clothing in an effort to slow the spread of flames if they catch fire.

The chemicals quickly built up in people and the environment. Starting in the early 2000s, phase-outs began. PBDEs have been found in air, dirt and people in virtually every corner of the globe, including the Great Lakes region.

The compounds can leach out of products they were used in and stay in the environment for a long time. It’s most likely the eagles are exposed through eating contaminated fish. But the chemicals also can enter landfills, latch onto dust and be inhaled, or be licked off the feathers, Basu said.

The chemicals "are everywhere,” Basu said. “They build up in the food chains so that top predators – such as bald eagles – accumulate high levels.”

Flame-retardants have been found in birds all over the world – from the United States to China.

The researchers tested the liver tissues of 33 dead bald eagles collected from 2009 to 2011 by the Michigan Department of Natural Resources. They tested for four common types of PBDEs; all but two of the birds had all four compounds in their liver.

PBDE concentrations were "among the highest found in liver tissues of any wildlife,” the authors wrote, with one eagle measuring 1,538 parts per billion PBDEs in its liver. Americans have some of the highest levels of PBDEs in their bodies worldwide, with studies of U.S. breast milk finding median PBDE concentrations of about 30 ppb, though the types of PBDEs vary.

Tom Cooley, a wildlife pathologist with Michigan’s Department of Natural Resources, is in charge of figuring out why animals die in Michigan. In bald eagles it’s often things such as electrocutions, getting hit by cars, or lead poisoning, he said. The state does not usually test wildlife for flame-retardants.

The four major PBDE compounds found in the bald eagles were from the flame retardant sold commercially as penta-PBDE mixture, used in plastics, wire insulation, cars and some textiles.

In 2009, penta, along with another common mixture octa, were added to the United Nations Stockholm Convention as persistent pollutants to be phased out by countries around the world.

But, just as its chemical predecessors, PBDEs are “very persistent” once they get into the environment, said Marta Venier, an assistant scientist with Indiana University.

“We can expect to see PBDEs in wildlife for a long time,” she said. In the current study, the researchers also found PBDEs in the liver tissue of all 35 dead river otters that were collected throughout Wisconsin from 2009 to 2010.

Bald eagles nest statewide in Michigan, Cooley said. The state’s birds – just like the rest of the nation – were on the brink of extinction in the 1950s and early 1960s, mostly due to chemicals such as PCBs and DDT in pesticides.

In the late 1960s researchers recognized the problem and chemical bans and species protection spurred a comeback.

Michigan’s population is strong and has been going up, Cooley said. “We have about 750 active nests throughout the state,” he said.

Research on PBDEs suggests the compounds might harm highly exposed birds.

There is “great concern” about the exposure because eagles are not only exposed to flame retardants but “hundreds of other potentially toxic chemicals,” Basu said. “We know very little of how contaminants interact to affect health,” he said.

Much of the research on what PBDE exposure might mean to birds has been done on captive kestrels. Canadian researchers have found that exposure to the compounds as an embryo caused development and reproduction problems in male birds.

They’ve also linked exposure to lower nest temperatures, smaller eggs with thinner shells, delayed egg laying, and odd behavior such as fewer mating calls and eating less.

Chemicals do not impact all bird species in the same way so it is unclear how, if at all, the compounds impact Michigan’s bald eagles.

“If there were health problems, it would be kind of under the radar and we wouldn’t necessarily notice in the [dead] birds we got in here,” Cooley said.

The bald eagles might have more flame-retardants in them than recent research indicates. “They focused on a small range of PBDEs,” Venier said, adding that there are many other mixtures outside of the four the group studied.

Basu said that banned PBDEs should eventually decrease in the eagles but, he cautioned, “it can take years or decades for this to occur.”

In addition, there are now replacement flame-retardants, too, with unknown but potentially worrisome environmental health risks.

“The market of flame retardants is a bit like a whack-a-mole game,” Venier said. “One gets phased out and another one is introduced keeping properties similar to the one it replaced.

“It goes on and on.”

Read more about the health of the world’s birds in our series Winged Warnings.

This article originally ran at Environmental Health News, a news source published by Environmental Health Sciences, a nonprofit media company.