Researchers have found that levels of some of the most troublesome contaminants for human health are lower than in previous years but persist in most Americans’ blood, even decades after being banned. The new data provide the most recent and comprehensive picture to date of exposure to persistent organic pollutants (POPs) in the U.S. Blood samples were collected in 2003−2004 from approximately 1800 Americans aged 12 years and older as part of the National Health and Nutrition Examination Survey (NHANES), which is administered by the National Center for Health Statistics at the Centers for Disease Control and Prevention (CDC). Previous CDC studies reported levels in 1999−2000 and 2001−2002.

Researchers at the CDC, led by Donald Patterson (now heading EnviroSolutions Consulting, Inc.), measured levels of 21 types of POPs in blood serum. The survey focused on chemicals that have been banned, including PCBs and some agricultural chemicals, or that are being considered for bans, such as polybrominated flame retardants and perfluorinated compounds, under two international treaties, the Stockholm Convention on POPs and the POPs Protocol to the 1979 Geneva Convention on Long-Range Transboundary Air Pollution. By definition, all POPs are persistent in the environment, bioaccumulate up the food chain, and have toxic effects in humans and the environment.

Levels of all of the POPs tested in Americans’ blood were lower than in the previous NHANES surveys. “This is good and it’s important to point out,” Patterson notes, “but whether that’s a trend that will continue in the same direction, we cannot say.” Some of the analytical methods changed during the years of the survey, which makes data from different years difficult to compare statistically, he notes. Patterson says that more statistical tests may be possible after the next set of data, collected in 2005−2006, is analyzed and released in late 2009 or early 2010.

Despite the statistical shortcomings, researchers are able to make some cautious comparisons over time. For example, levels of dioxins and furans, both of which are carcinogenic, have declined in the overall U.S. population, says risk-assessment specialist Judy LaKind, president of the consulting firm LaKind Associates and associate professor at the University of Maryland School of Medicine. Her analysis of NHANES data is published in the Journal of Exposure Science and Environmental Epidemiology (DOI 10.1038/jes.2008.63). “Levels [of dioxins] have decreased over time in the younger part of the population, but have held steady in older people,” LaKind says. “As pointed out by CDC, this is consistent with today’s younger people being exposed to lower levels in the environment.”

Many persistent compounds can still be detected in Americans’ blood decades after U.S. bans. For example, DDT was banned by the U.S. EPA in 1973, but its breakdown product DDE was detected in 99.7% of people in the 2003−2004 samples. Likewise, the fungicide hexachlorobenzene was detected in 99.9% of people tested despite a 1984 ban. This continuing occurrence in blood may result at least in part from consumption of foods that were imported from countries that still use these chemicals, the authors say.

The new study provides valuable information to evaluate the effectiveness of international bans, says Heidelore Fiedler of the UN Environment Programme’s Chemicals Branch, which helps to manage Stockholm Convention programs. However, she notes that NHANES uses different protocols than the Stockholm Convention and therefore not all of the U.S. results can be compared with other countries’.

Further studies by epidemiologists, toxicologists, and risk assessors are needed to interpret how POPs levels in the new study relate to human health. With the new data, researchers can better understand how and why POPs levels vary in certain populations, says epidemiologist Russ Hauser of Harvard University. For example, the study found significantly lower total toxicity levels among Mexican Americans compared with other ethnic groups, but across all groups, levels were generally higher in women than in men.

“It’s that sort of finding that demands additional research to figure out what’s going on,” says epidemiologist David Garabrant, who leads a study of dioxin exposure at the University of Michigan. “These are clues to important factors that determine your levels of these chemicals.”