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According to some scholars, toxic exposures contributed to the collapse of the Roman Empire. Roman rulers were poisoned by water cisterns and food containers fashioned from lead. These extreme exposures could have gradually made the Patrician families deranged and infertile – leaving no competent leaders to run the Empire. A similar fate seems to have befallen the Samurai regime in Japan. When bones from a castle burial were analyzed, lead concentrations in the children suggested severe lead poisoning. With damaged mental abilities, the Samurai descendants would likely have been incapable of dealing with political crisis, possibly contributing to the downfall of the Shogunate.

Today, our brains are being put to another extreme test, this time from a combination of toxic chemicals that includes mercury, arsenic, pesticides and persistent industrial compounds. The hidden threat that we now face is what I call chemical brain drain. It is insidious and silent, as it is usually not linked to any medical diagnosis, and it is serious, as the combined deficits are affecting the brains of a whole generation of children, upon whom our future relies.

Only recently has it become clear that the brain is extremely vulnerable, especially during its development in the womb and during infancy. In my early career, I encountered some surprises that medical school had not prepared me for, surprises that were fundamental to our understanding of the frailty of the human brain. According to the medical tradition, the fetus is well-protected inside the pregnant mother’s womb. But contrary to this comforting notion, the placenta allows many toxic chemicals to seep through, and some of these compounds can damage the brain’s sensitive developmental processes. The mother may escape unscathed, but for her child, such damage can be catastrophic.

Our highly sophisticated brains start out as a tiny strip of cells. A couple of weeks after conception, these cells are ready to multiply. At its peak, about 12,000 cells are generated every minute – 200 per second. Most do not remain in the same place, but move to specific locations within the developing brain. Overall, our brains develop by multiplication, migration, maturation and messaging – complex steps, each of which has to happen in a specific fashion, in the correct order and at the right time.