Flame retardants are found in a household items such as upholstered furniture. (Tom Mihalek/Reuters)

Levels of flame retardants linked to learning difficulties in children dropped quickly in pregnant women's blood after the chemicals were banned in California, say researchers pointing to benefits of regulation.

Polybrominated diphenyl ethers or PBDEs were used in foam furniture starting in the 1970s to meet fire safety regulations. Researchers have found an expectant mother's exposure to the chemicals, even at low levels, is associated with poorer concentration, attention and reduced IQ in her children.

In Wednesday's online issue of the journal Environmental Science & Technology, researchers say PBDE levels in blood of pregnant women in San Francisco fell by two-thirds from 2008 to 2012. 

Twenty-five women were tested in 2008-2009 and 36 were tested in 2011-2012 when they were between 15 and 24 weeks pregnant. They came to an outpatient clinic serving an ethnically diverse and mainly lower-income population.

Previous studies suggest that house dust in the main way that people are exposed to PBDEs. Consuming fish, meat, dairy products and breast milk are other routes of exposure.

"We were pleasantly surprised by the extent of the decline," Ami Zota, an assistant professor of environmental and occupational health at the George Washington University School of Public Health and Health Services in Washington, DC, said in a release.

Exposures in North America are an order of magnitude higher than in Europe and Asia. In California, residents historically had the world's highest exposures outside of work to one form of the chemicals because of the state's flammability standard for foam furniture. The state banned two forms of PBDEs in 2003-2004.

Last year, an investigation by CBC-TV's Marketplace found some chemical flame retardants used in home furnishings may not help in a house fire and can pose health hazards.

The researchers said while the PBDE exposures in their study population declined significantly, the levels are still higher than those reported in other groups of pregnant women in the U.S.

Nonetheless, the decline "is encouraging from a public policy perspective and demonstrates the power of using biomonitoring to gauge the efficacy of regulatory or public health interventions," Zota's team concluded.

Replacement fire retardants weren't measured. Another limitation is that repeated biological samples weren't collected. The first and second blood samples were analyzed at different labs using similar methods, the researchers said.

The study was funded by the Passport Science Foundation and the U.S. National Institute of Environmental Health Science.