The research, published Wednesday in the journal Environmental Science and Technology, does not prove these compounds, known as PBDEs, caused the rash of hyperthyroidism in the nation's household cats over the past 30 years.
Rather, it lays out a hypothesis, showing that cats are heavily contaminated by these compounds, which leach from household products and are found everywhere, particularly household dust. Cats, meticulous cleaners, ingest PBDE-contaminated dust daily.
The rise of feline hyperthyroidism -- a rare disorder prior to 1980 that today is a leading cause of death in older cats -- matches the increasing sales of PBDEs, or polybrominated diphenyl ethers.
The study was conducted jointly by researchers at the EPA's National Health and Environmental Effects Laboratory and Indiana University. These cats, they note, are the proverbial canaries for humans, who spend upward of 90 percent of the day indoors, according to some estimates.
Pound for pound, a 2-year-old ingests nearly as much dust as a cat, based on EPA estimates. And while the blood of everyone living in the developed world is thought to be contaminated with PBDEs, Americans have by far the highest levels -- 10 to 100 times higher than residents of Japan and Europe.
The spike in hyperthyroidism in cats suggests long-term, low-dose exposure to these compounds is far more endocrine disrupting than laboratory experiments indicate.
"Cats are in the right place to be exposed. They're in our homes, they're on our carpets, sometimes they're on our appliances and furniture," said Janice Dye, an EPA veterinarian and research biologist who was one of the paper's lead authors.
"We know they're being exposed. We know these (PBDEs) have the potential to be endocrine disrupting compounds. We know they can bind to the thyroid hormone." The epidemic of hyperthyroidism in older cats, Dye added, is likely a result of the thyroid gland responding to chronic disruption over a very long period of time.
John Kyte, a spokesman for the Bromine Science and Environment Forum, an industry group, noted the study was small and that other research attributes the epidemic to increased awareness and longer feline life spans.
"In other words, the increase in the number of cases diagnosed is not at all surprising, and it is speculation to try to link it to flame retardants," he said in a statement. "This is an issue that bears watching, and we will see what additional research indicates, but people should not be making broad conclusions based on this single, very limited study"
Millions of pounds of PBDEs known as Penta and Octa were added annually to foam in furniture until 2004, when California and Europe banned them and the nation's sole manufacturer voluntarily agreed to cease production. A third PBDE known as Deca continues to be mixed with hard plastic and is today found in various household appliances, with the United States making up more than half the global market.
The thyroid gland is the body's regulator, controlling how quickly the body burns energy, makes proteins and responds to other hormones.
Before 1980, hyperthyroidism in cats was unheard of; today it is one of the most prevalent health problems in older cats and a leading cause of death. Hyperthyroidism accelerates the body's metabolism. Typical symptoms in cats are increased appetite, weight loss, irritability, lethargy and diarrhea.
"It's quite astounding," said Michael Sozanski, a veterinarian at VCA Albany Animal Hospital. "For us it's become somewhat of a routine diagnosis and treatment. But to see something that's become an epidemic in that time frame is quite scary."
Sozanski easily recalls the day in vet school in 1979 when he gathered in the ward with classmates and heard a professor say they'd likely never see another cat with hyperthyroidism in their careers.
Today he averages a case a day.
But it's the human effects that have researchers concerned. PBDEs, or polybrominated diphenyl ethers, contaminate virtually every house dust sample ever analyzed. Young children, with their smaller size and tendency to mouth objects, are likely as highly contaminated as cats, the EPA researchers estimated. Though children quickly outgrow such tendencies, they're exposed at a critical developmental window to a far higher load of endocrine-disrupting compounds than adults.
"People are just beginning to look to see if we can find anything in the human population," said Linda Birnbaum, research director of the EPA's Health Effects Laboratory and a paper co-author.
"Nobody's done any long-term studies with these."
The findings also lend a sense of urgency to a bill, working its way through the Legislature, to ban these chemicals from commerce, said Arlene Blum, a Berkeley chemist and one of the bill's backers.
The bill, AB 706, would ban any brominated or chlorinated flame retardants from bedding, mattresses and furniture sold in California.
It's no small irony to Blum that her 15-year-old cat, Midnight, suffers from hyperthyroidism.
She's had her house dust tested for PBDEs. It's off the charts. She learned earlier this year that the foam in her furniture -- a J.C. Penney-brand sofa, easy chair and recliner bought in a thrift store in 1987 -- is saturated with PBDEs.
"Can you believe 5 percent of the foam, by weight, in the chair I bought to nurse my daughter is PBDEs?" she asked.
"I wish someone would look at people. If it's happening to Midnight, the rest of us can't be too far behind."
Contact Douglas Fischer at firstname.lastname@example.org or (510) 208-6425.