The developing brain is an awe-inspiring organ - until something goes awry, such as damage from toxic chemicals. It turns out there are more of those than experts realized. Perhaps many more.
In the uterus, cells are multiplying dizzyingly fast and moving into position. They start sending and receiving messages.
That leaves the young brain susceptible to chemical exposure that might not harm fully developed adult brains.
"Things have to happen at a particular time and in a particular sequence," said Philippe Grandjean, adjunct professor in the Harvard School of Public Health in Boston. "If something goes wrong, that is what you'll have to live with the rest of your life."
In a recent paper in the Lancet, Grandjean and Philip Landrigan, director of the Children's Environmental Health Center at the Mount Sinai School of Medicine in New York, said there was a global "silent pandemic of developmental neurotoxicity."
They call it silent, Grandjean said, because it's so subtle. Affected children may not have any recognizable disease; they're just, say, doing less well in school than they would otherwise. They're functioning at a lower level.
The two researchers, who have been studying industrial chemicals for more than three decades, have called for a paradigm shift in how we assess chemical risks. They want mandatory testing of current and new chemicals, plus the formation of an international evaluation clearinghouse.
"Our very great concern," they wrote, "is that children worldwide are being exposed to unrecognized toxic chemicals that are silently eroding intelligence, disrupting behaviors, truncating future achievements, and damaging societies."
In 2006, Grandjean and Landrigan reviewed the scientific literature and identified five industrial chemicals that were recognized causes of neurodevelopment disorders.
They were the familiar bad guys - lead, mercury, arsenic, the solvent and gasoline additive toluene, and now-banned polychlorinated biphenyls.
Eight years later, with more data, the number has more than doubled.
In the Lancet paper, Grandjean and Landrigan listed six more: manganese, fluoride, the pesticides chlorpyrifos and DDT (banned here, but still used abroad), the solvent tetrachloroethylene (known as PERC), and polybrominated diphenyl ethers, which are flame retardants used in couches, mattresses, and other consumer products.
Based on animal studies and other factors, they argue that far more chemicals may adversely affect the developing brain.
One of the newly listed chemicals sure to spark interest is fluoride, added to many public drinking water supplies to fight dental decay. It has come under increasing scrutiny.
Grandjean warned that though their analysis of 27 studies showed a link between higher fluoride levels and a drop in IQ points, "we were unable to document the exact dose dependence. Therefore, we are unable to make any judgment about safety of drinking water fluoridation."
In a statement, the American Chemical Council called the Lancet paper "highly flawed" and said it made unfounded extrapolations about chemicals, creating "confusion and alarm."
The council pointed out that the dose of a chemical - not its mere presence - was the relevant factor.
Grandjean agreed. "Our central point is that because the human brain is so vulnerable to these substances, damage can occur at doses that are otherwise considered safe," he said. "That's why we need to revisit the risk assessments."
Tara Wenger, a toxicologist and geneticist at Children's Hospital of Philadelphia, praised the Lancet paper. She said many of the neurotoxins we now know about were recognized only after a catastrophe - an acute poisoning - in which effects on brain development became clear.
But chemicals that cause subtle differences in learning ability are much harder to identify.
"It's very possible we have chemicals that are already in the environment or that could be developed that could affect the brain in a way that would impair cognitive development," she said, but we don't recognize them because they don't cause something immediately visible.
Advocates, regulators, and industry all have called for an update to the nation's Toxic Substances Control Act, which regulates chemicals. But they differ strongly on what the changes should be.
When the law was adopted in 1976, it grandfathered in 60,000 chemicals then in commercial use. The number now tops 80,000, but the U.S. Environmental Protection Agency has been able to require testing on only about 200 and has regulated or banned only five.
Legislation introduced in 2013 by Sen. Frank R. Lautenberg (D., N.J.) and a Republican colleague has stalled.
More recently, House Republicans have proposed the Chemicals in Commerce Act. Critics say the word commerce reflects its industry-friendly bent.
"GreenSpace," about the environment and health, appears every other week, alternating with Art Carey's "Well Being" column.