Exposure to lead, mercury, and certain other toxic chemicals that can affect developing brains seems to be on the decline in the U.S., suggests a new study published in the journal Molecular and Cellular Endocrinology.
Children and pregnant women today are exposed to less lead and mercury than they were two decades ago, probably because of years of restrictions on the use of these heavy metals, the researchers say.
Despite the good news, the new study points to serious problems that remain. Lead is a significant risk, for example. And the researchers suggest that flame retardants—chemicals that may be less recognized for their potentially harmful effects on the brain—now represent a greater risk to cognitive development than lead, mercury, and organophosphates (a type of pesticide).
What's more, the study looked at only four chemicals. There are many more that are believed to be toxic to developing brains, the researchers note in the paper, and still others that may yet be undiscovered.
"The overall decline is a good thing," says Leonardo Trasande, M.D., a pediatrician and children's environmental health researcher at New York University and senior author of the new study. But he adds that the research shows "a new suite of environmental chemicals"—flame retardants and organophosphates, for example—that are less well known than heavy metals but may pose serious risks to developing brains.
Understanding the Chemicals
Trasande and colleagues studied four groups of chemicals with known or suspected effects on child brain development. Those included the heavy metals lead and mercury, which occur naturally in the earth and can contaminate food and water. (Lead was removed from gasoline and paint in the 1970s and 1980s, though it can still be found in older houses.)
They also looked at two man-made groups of chemicals, flame retardants called polybrominated diphenyl ethers (PBDEs) and organophosphate pesticides.
Lead and mercury are known to have direct effects on the brain, while PBDEs and organophosphate pesticides may influence the brain indirectly by altering levels of thyroid hormone—a key player in brain development, Trasande says.
The researchers chose to focus on these four groups of chemicals because of "extensive research on their ability to cause neurodevelopmental damage and cross the placental barrier," according to the study. They looked at children and pregnant women because the brains of young children and fetuses are most sensitive to these potentially toxic effects.
Flame retardants are used in a number of products—from furniture and bedding to TVs, computers, and other electronics—to stop or slow the spread of fire. Reducing combustibility of household goods is important, though a large body of research shows that some flame retardants leech out of products and into the household environment, where they can be absorbed into the body. As a result, their use has become controversial.
Chemical manufacturers began voluntarily phasing out PBDE flame retardants in 2004, after these chemicals, which were linked to health problems, started showing up in high levels in people's bodies. Because they persist in the environment, they can still be found in homes and in food, even though they are not common in new products.
Some researchers have expressed concern over some of the chemicals that have been used to replace PBDEs in products, which include certain types of organophosphates. These chemicals also have been linked to health problems and bear a strong chemical resemblance to organophosphate pesticides.
While the EPA banned most residential uses of organophosphate pesticides in 2001, they continue to be widely used in agriculture.
What the Study Found
The researchers used a national dataset to estimate average exposures to these substances among pregnant women (for mercury, PBDEs, and organophosphates) and children at age 5 (for lead) over a roughly 16-year period.
Exposure to these four types of chemicals combined was on the decline, they found.
Then the researchers used previous research to estimate the IQ point loss—a tool used to quantify potential cognitive effects—that could be expected based on those exposures.
From 2001 to 2002, the researchers estimated, exposure to these chemicals contributed to a loss of 27 million IQ points among U.S. kids. From 2015 to 2016, the total IQ loss was still large but had dropped to an estimated loss of 9 million IQ points.
How much is 9 million IQ points? That's roughly equivalent to about 2 points per child born in the U.S. during those years, Trasande says.
The loss of a few IQ points might not be noticeable in an individual child, says Bruce Lanphear, M.D., a children's environmental health scientist at Simon Fraser University in Canada, who was not involved with the study. "But across the entire population, it adds up," he says. At the same time, Lanphear says, that loss in IQ points likely isn't distributed equally across the population because exposures are not the same from person to person.
The greatest estimated loss in IQ points during the 16-year period came from exposure to PBDE flame retardants, the study found. PBDEs contributed about to twice the IQ point loss of lead or organophosphates, while mercury exposures were estimated to contribute to less than 1 percent of total IQ point loss.
What the Study Can't Tell Us
According to the Centers for Disease Control and Prevention, most developmental disabilities are thought to be caused by a complex mix of factors. Those may include genetics, parental behaviors such as smoking or drinking, complications during pregnancy or birth, and exposures of the mother or child to toxic chemicals in the environment.
In most cases, developmental disabilities aren't thought to be caused by any single factor, and this new study can't show that exposures to flame retardants, for example, directly caused IQ point loss or intellectual disability.
"It is not possible to conclude that chemicals—and in particular, flame retardants—are the cause of neurodevelopmental health issues in children," the American Chemistry Council, an industry trade association for U.S. chemical companies, said in emailed comments to Consumer Reports.
This study also extrapolated IQ loss based on other, smaller studies; it did not measure it directly.
The new study was observational, a type of research scientists often use to assess relationships between environmental exposures and health. Such studies "are never going to give us a perfect picture of what's going on in the real world, but they serve as an incredibly important scientific tool in identifying potential areas of concern," says Thomas Zoeller, Ph.D., an epidemiologist at the University of Massachusetts Amherst, who was not involved in the study.
How to Protect Yourself and Your Kids
Kids today may be exposed to less of these chemicals than in the past. Still, there are steps that parents can take to further reduce risk.
Buy organic as much as possible. Organic fruits and veggies may contain lower levels of pesticide residues and other contaminants.
Bust dust. Contaminants, including flame retardants and heavy metals such as lead, can attach to dust particles in the home. Frequently vacuuming (with a HEPA filter vacuum) and wet mopping can help to reduce dust in the home.
Wash hands frequently. Young kids crawl around on the floor, where they may be more exposed to dust. "Frequent hand-washing is a simple way to limit exposure," says Kenneth Spaeth, M.D., an occupational and environmental health specialist at Northwell Health in New Hyde Park, N.Y.
Opt for furniture that doesn't contain flame retardants. Even though PBDEs have largely been phased out of furniture sold after 2015, newer flame retardants may behave similarly in the environment and people's bodies. These are often added to foam cushioning. Polyester- or wool-filled furniture is less likely to contain added flame retardants, according to the Green Science Policy Institute, a nonprofit that's focused on reducing harmful chemicals in products and is based in California.
The institute also recommends avoiding new or secondhand foam-filled products with a TB 117 label, which indicates the presence of flame retardants. If you see a TB-117-2013 label (PDF), make sure the box next to "contains no added flame retardant chemicals" is checked.
Check your home for lead. Lead was removed from house paint in 1978, but old lead-based paint remains the most common cause of lead poisoning in kids. Test for lead in paint and dust if your home was built before 1978. Lead can also be found in some water pipes in homes built before 1986. "Consider having the plumbing inspected for lead if you live in an older home," says Don Huber, director of product safety at Consumer Reports.
Choose low-mercury fish. If you're pregnant or breastfeeding, opt for seafood that is lower in mercury, including cod, haddock, trout, shrimp, and canned light tuna. Avoid higher-mercury species, such as shark, swordfish, and tilefish.
