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Recently, Science News included a report that certain cat foods, especially fish-flavored canned entrees, deliver substantial quantities of brominated flame retardants to the pets' diets (SN: 8/25/07, p. 125). The finding helps explain why blood concentrations of these ubiquitous chemicals—known as polybrominated diphenyl ethers (PBDEs)—tend to be much higher in cats than in people in the United States.

However, that's not to say people are only mildly exposed. Several studies show that both children and adults can accumulate substantial amounts of these hormone-perturbing agents. Certain foods can deliver significant quantities, especially fish, chicken livers, and certain sausages. Moreover, recent studies have found that mothers typically pass PBDEs along to their children—both in the womb and during breastfeeding.

In general, manufacturers add PBDEs to plastics and foam products to reduce the likelihood they will catch fire. However, treated products can shed the chemicals into air, water, and dust. Through exposure routes that remain somewhat sketchy, PBDEs have been entering the U.S. food supply.

What's emerging as an apparently far greater source of human exposure is ingestion of PBDE-laced house dust. That puts toddlers at greatest risk of accumulating the chemicals because of the time they spend crawling on carpets and mouthing toys and other items that are indoor-dust magnets.

Indeed, authors of the new cat study argue in the Sept. 15 issue of Environmental Science & Technology that house cats "may serve as sentinels to better assess human exposure and adverse health outcomes related to low-level but chronic PBDE exposure."

What are those risks? Studies have shown that at least in young animals, brief, early PBDE exposures can mildly impair learning and reduce thyroid-hormone concentrations in blood. Exposures later in life can delay puberty in male rodents, and adult exposures can block the activity of cellular receptors for male sex hormones. Then there's the new cat study. It tentatively links PBDE exposures to the development of a potentially lethal thyroid disease.

No one knows whether these flame retardants pose similar risks to people—and if so, what concentrations might be harmful. The data just aren't in. Until they are, the animal data suggest that cautious parents might want to vacuum frequently and limit their small children's intake of certain foods.

Youngsters get the most

Not quite a year ago, Douglas Fischer of the Oakland (Calif.) Tribune teamed up with a number of international scientists. The team then twice, several months apart, descended on a California family. The goal was to evaluate the extent to which family members, who ostensibly share a common lifestyle and environment, might differ in their accumulation of PBDEs.

In the October 2006 Environmental Health Perspectives, the team reported that at its first visit, in September, Dad and Mom had blood concentrations of 32 and 50 parts per billion (ppb) of PBDEs in blood, respectively. By contrast, a 5-year-old daughter had 137 ppb PBDEs in her blood and an 18-month-old, partially weaned toddler son had 245 ppb of PBDEs.

Three months later, PBDE values had dropped in all the family members. However, the earlier trend remained: Dad and Mom's blood had about 4 ppb of PBDEs—roughly one-tenth as much as their toddler's blood contained and a quarter to a third as much as their 5-year-old's did.

The scientists can't explain the big seasonal difference in contamination levels, but they suspect that family members might have encountered a large temporary source of the flame retardants in September. The team ruled out the likelihood that the differences stem from measuring errors because of validation tests it ran at each sampling.

House dust accounted for the major difference in PBDE contaminations levels between adults and children, the authors speculate. They note that a 2005 study found evidence that house dust may account for 80 percent of total daily PBDE exposures for toddlers—almost six times its share of adult exposures. However, diet can also bump up a youngster's burden of the chemicals. On the basis of previous studies of PBDE values in breast milk, the authors say they suspect that breastfeeding contributed "significantly to the higher levels of [some particular PBDEs] observed in the toddler."

The researchers conclude that children appear to "have higher PBDE concentrations than adults, concentrations that may be high enough to cause harm."

Just last month, researchers in Spain homed in on PBDE exposures attributable to breast milk. They measured concentrations of PBDEs in 92 children at birth and in the blood of another 244 preschoolers at about age 4. Although babies were born with about 3 ppb of PBDEs in their blood, values were much higher in the preschoolers—about 13 ppb if they were formula-fed as babies and close to 70 ppb if they had been breastfed.

Indeed, the scientists report in the July 15 Environmental Science & Technology that despite a relatively short breastfeeding period for these youngsters—typically 4.5 months—"breastfeeding was the determining factor for the body burden of these compounds at 4 years of age."

Where do PBDEs come from?

Last year, a British team of scientists reported PBDE concentrations in air that had been sampled in homes, office buildings, a post office, a coffee shop, a restaurant, and car interiors.

Overall, the researchers found, cars "displayed the highest average concentrations of PBDEs" among the indoor environments. However, airborne concentrations of the flame retardants tended to diminish with vehicle age, suggesting that flame-retardants gradually escape from foam and plastic auto-interior components, the researchers reported in the Dec. 15 Environmental Science & Technology.

Although some mid-century homes could sport airborne PBDE values of 100 picograms per cubic meter, most homes—especially those constructed in the past 17 years—registered in the very low pg/m³ range. These trillionths of a gram values are quite small, and no one knows if they're biologically active in people.

What about another source, foods? Several studies of commonly eaten foods have pointed to high-fat dairy goods and meat as especially important sources of PBDEs in the human diet. Among foods that stood out last fall in a study of goods from a Dallas supermarket:

• Fish. Median and maximum PBDE concentrations for uncooked fish were 616 and 3,700 parts per trillion (ppt), respectively.
• Chicken livers. PBDE concentrations topped 2,800 ppt.
• Sausages and lunch meats. PBDE values for these meats fairly consistently ran around 1,000 to 1,400 ppt.
• Poultry. Average and maximum PBDE values were about 500 and nearly 1,300 ppt, respectively.
• Cheeses. PBDE concentrations ranged from about 100 ppt to 700 ppt.

The authors noted in the October 2006 Environmental Health Perspectives that PBDE contamination values can vary considerably within product categories. For instance, among meats, bacon typically had PBDE values of no more than 39 ppt, whereas one pork sausage contained 1,426 ppt of the flame retardants.

For perspective, the researchers point out that PBDE values in sampled breast milk typically ranges from 1,000 to 21,000 ppt.

The role of dust

Both the new study of house cats and last October's survey of PBDE contamination in supermarket foods present a conundrum. Linda Birnbaum, a toxicologist with the Environmental Protection Agency and coauthor on both studies, explains that the two sets of data indicate that food doesn't account for much of the amount of flame-retardants found in either people or pets in the United States.

Indeed, her team has calculated a likely daily PBDE intake from the foods and found it unimpressive. The total intake would probably lead to blood-PBDE values of less than 10 ppb, the researchers note. Yet the average blood value exceeds 30 ppb in U.S. residents, and 5 percent of people have blood-PBDE values 10 to 100 times that amount.

Such numbers "suggest that house dust is an important source [of PBDEs]," Birnbaum told Science News. Supporting that suspicion are data from a study published in the March 1 Environmental Science & Technology. Thomas F. Webster of Boston University and his colleagues showed that among new mothers in the Boston area, PBDE concentrations "in breast milk and house dust were strongly and positively correlated."

Overall, breast milk from women whose homes contained house dust rich in flame-retardant residues had PBDE concentrations 2.6 times those of milk from women whose homes had low PBDE concentrations in dust.

Birnbaum and her collaborators say that because house cats may occupy the same environmental niche around the home as young children—both crawling around and getting dirt in their mouths— a better understanding of how cats become exposed to flame retardants and the hormonal impacts of those exposures "may have public health ramifications for both veterinary and human patients alike."

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