Scientists have documented for the first time that banned flame retardants have declined in people in the United States, where levels of the chemicals had been growing exponentially.

The small study, published today, reported that levels in pregnant California women were 65 percent lower than in a similar group of women tested three years earlier.

The two flame retardants have been banned in the United States since 2004. But many experts have been concerned about their persistence because they break down slowly in human tissues and were widely used in products, such as sofas, that people keep for many years.

“Our study is the first to look at the potential impacts of the ban on chemical body burden in the blood of U.S. residents," said Ami Zota, a reproductive health scientist at George Washington University and lead author of the study published in the journal Environmental Science & Technology.

“I think the takeaway is that chemical regulations matter. They can have an impact on individual lives,” Zota said.

Linda Birnbaum, director of the National Institute of Environmental Health Sciences, cautioned that while the results are suggestive, they are not definitive. “They are comparing two very small and selective groups of women,” she said.

The compounds, polybrominated diphenyl ethers, or PBDEs – were banned after evidence emerged that they were doubling in women’s breast milk every five years.

Studies suggest that PBDEs can disrupt thyroid hormones in pregnant women and newborns. Thyroid hormones are important for brain development. Prenatal exposure to PBDEs has been linked to poorer concentration, attention and reduced IQs.

The researchers tested the blood of 36 women in the San Francisco Bay Area who were in the second trimester of their pregnancies in 2011 and 2012. They were predominantly black or Latina and low income; all were receiving abortions at a San Francisco clinic.

Their levels of total PBDEs were 65 percent lower than levels found in women tested in 2008 and 2009, who had some of the highest levels ever seen worldwide.

In addition, breakdown products of some PBDEs declined six-fold between the two groups. When some chemicals are metabolized, byproducts are formed. For flame retardants, scientists don’t know a lot about these compounds and what they may do, although some research suggests they may disrupt thyroid hormones.

The researchers could not compare the same women over time. The groups were demographically similar, but there is no way to confirm that the differences represent a real decline.

“If we truly are seeing a decline, it is good news and in line with what we might expect,” Birnbaum said.

These compounds have been declining in the environment, including house dust, over the past decade. But the decline in human bodies was expected to take longer, especially since people may be continuously exposed through their old furniture.

Experts expect the decline in PBDEs to mirror that of other persistent compounds, called PCBs, banned in 1978 – a rapid drop followed by a leveling-off period in which they persist in people at low levels.

Dr. Arnold Schecter, a professor of environmental and occupational health at the University of Texas School of Public Health, said the study “shows a trend in the direction we would like to see to protect public health and individual health.”

Yet he said it remains unclear what amount of PBDEs in the body, if any, is a safe amount. “I don’t think anyone can say how concerned we can be or whether there is a safe level,” he said.

The levels found in the California women are still higher than those reported in other contemporary groups of U.S. pregnant women, Zota said. Low-income and minority communities seem to have higher exposures. In addition, Californians are believed to have the highest levels in the world, largely due to the state’s flammability standard for furniture.

While levels of PBDEs may be decreasing, newer flame retardants, such as Firemaster 550, another brominated compound, have been going up, according to earlier research. The authors of the new study did not test the women for newer flame retardants.

“We don’t know as much about the health effects of replacement flame retardants, but few studies have shown that these new chemicals may also disrupt hormone function,” said Heather Stapleton, an environmental chemist at Duke University.

Also, another PBDE, known as deca, still is found in many electronic products. Its use has been banned in the United States but “deca is very stable in the environment and its use far exceeds that of banned PBDEs,” Birnbaum said.

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