Expectant parents have a lot on their mind these days. If the nightly news is to be trusted, dangerous pollutants lurk in our food, water and furniture, just waiting to invade a pregnant mom's body and harm the developing fetus. The early stages of brain development are indeed uniquely vulnerable to interference from foreign substances—prenatal exposure to many of these chemicals has been linked with lower IQ, behavior problems and mental disorders in kids. Yet the actual risk to a given individual varies widely and is often much lower than the headlines might lead you to believe. Scientific American Mind examined the research to date, as scientific understanding of the effects of environmental pollutants continues to grow.

Flame retardants are a scary business: they have been tied to lower IQs, slowed cognitive development and undescended testes in young boys. Now research has linked prenatal exposure to a certain type of flame retardant to Rett syndrome, a disorder on the autistic spectrum—but scientists suspect that the chemicals may be most harmful to children who have other factors working against them.

Over the past 25 years, polybrominated diphenyl ethers (PBDEs) have been used as flame retardants in a wide range of consumer goods you probably already have in your home: textiles, mattresses, carpets, furniture and electronics. Once they're in your home, they tend to shed into house dust, which then gets picked up on your hands and clothes and breathed in through your lungs. Although we are still figuring out just what PBDEs do to the human body—and at what doses—there are some things we already know. For example, we know that they disrupt the body's use and regulation of thyroid hormones. These hormones are critical for brain development in the womb and early childhood. PBDEs also have an unfortunate knack for sticking around in the environment, our food supply and in our body—particularly fatty tissue, including the brain, which is 60 percent lipids. For that reason, the Environmental Protection Agency now classifies PBDEs as persistent organic pollutants.

A wide array of research during the past few years has shown that PBDEs and their metabolites—what is left over when they break down in the body—are generally bad news. For example, as scientists in Seattle and Beijing discovered in 2013, PBDE-47 interferes with new neuron growth in adults—a process important for learning and memory. Developmental effects, however, are even more significant. Environmental health scientist Julie Herbstman of Columbia University found that children of mothers with higher concentrations of PBDEs in their umbilical cord blood scored lower on mental development tests in early childhood.

But perhaps the most damning evidence against PBDEs is their possible role in autism. For the past few years Janine LaSalle, a microbiologist at the MIND Institute at the University of California, Davis, has been investigating how persistent organic pollutants, including PBDEs, may influence fetal neurodevelopment at the molecular level. When she and her team looked at brain slices from adults, some of whom had autism, they found that persistent organic pollutants, including PBDEs, were present in brain tissue in every sample. They were especially concentrated in the brains of people who had certain types of autism—types known to have significant genetic factors.

LaSalle also tested the effects of PBDEs on pregnant rats specially bred with the genetic mutation associated with Rett syndrome, which involves a lack of verbal ability, repetitive compulsive movements, and physical deformities such as small hands and head. In rats that received daily doses of PBDEs comparable with common human exposure, the female pups had social and behavioral deficits we usually associate with human autism. The gender outcome makes sense: unlike most autistic spectrum patients, where males outnumber females four to one, the vast majority of children with Rett syndrome are girls.

LaSalle thinks the mechanism involved is DNA methylation. Layered over every strand of DNA in all our cells, chemicals called methyl groups influence how our genes are expressed—for instance, turning on genes that build neurons in the brain and turning off those genes elsewhere in the developing body. LaSalle's evidence shows that the brains of people with autism are significantly undermethylated, as were the brains of the rat pups whose mothers had been exposed to levels of PBDEs similar to average human exposure.

If PBDEs are messing with neuronal DNA methylation, that is very worrying. But here is the good news: LaSalle thinks methylation in fetal development works as a kind of “sink”—only if you have enough factors going wrong will the sink overflow and normal brain development be affected [see box below]. For instance, if a pregnant woman happens to be vulnerable to Rett because she carries a rare genetic mutation, and she has significant exposure to PBDEs, and perhaps she does not have enough folic acid or some other detrimental factor, she might end up with a child who has Rett syndrome.

But most of us do not carry the rare mutation in question. Thus, as LaSalle warned an audience at the 2013 annual conference of the Organization for the Study of Sexual Differences, we should not think of PBDEs as a “smoking gun” for autism. They are one possible environmental influence, played out in a complex relation between genetics and environment at a critical stage of fetal development. Happily, the vast majority of our kids are probably going to be just fine. But to be on the safe side, it is probably worth following these tips to reduce PBDE exposure: dust your shelves more regularly with a wet cloth, clean your floors with a wet mop, and wash your hands before you cook and eat. Keep your body fat in check because persistent organic pollutants tend to accumulate in lipids, and take a walk outside when you can—getting out to exercise does double duty because there is no house dust in the park.

One last thing for expecting mothers: a recent paper from the Centers for Disease Control and Prevention and Duke University indicates that biting your nails and licking your fingers is also correlated with more PBDEs in your blood serum, perhaps because of house dust on your hands. So don't nervously bite your nails, even if you might want to—you know, because of the PBDEs.

Credit: Jason Lee