Neurotoxins and the Developing Brain

Photo: Thester11 via CC BY 3.0

We have long known that there is a critical period during a child’s development during which the brain is considerably more malleable than an adult’s brain. In other words, outside influences — both positive and negative — have a much greater impact on neuronal formation and function from gestation to the teenage years. Indeed, by the time you are two years old, almost all of the billions of brain cells that you will ever have exist in your head. With this in mind, recent research regarding the neurotoxic effects of environmental chemicals on children is especially alarming.

One study, published in The Lancet by Drs. Phillipe Grandjean and Philip J. Landrigan, analyze the detrimental impact of eleven common chemicals on the developing brain — something they have deemed a “silent pandemic.” These include:

  • lead
  • methylmercury
  • polychlorinated biphenyls
  • arsenic
  • toluene
  • manganese
  • fluoride
  • chlorpyrifos
  • dichlorodiphenyltrichloroethane
  • tetrachloroethylene
  • polybrominated diphenyl ethers

These substances, found in both the environment and everyday items like furniture and clothing, are believed to cause not just lower IQs, but may also have a role in attention deficit hyperactivity disorder and autism spectrum disorder. You might be wondering what role genetics play in the development of neurobehavioral conditions. Experts believe that about 30 to 40 percent can be accounted for by genes alone, meaning the environment must make up the other part.

Many of the chemicals they listed as neurotoxic to children fall under the category of heavy metals. The mechanism by which heavy metals cause brain damage is multifaceted, as this working paper from the National Scientific Council on the Developing Child details. They include the “disruption of neural cell migration from one part of the brain to another, as well as the formation of synapses (i.e., connections among cells)” and can also “interfere with neurotransmitters, which are the natural body chemicals that carry signals from one cell to another.”

Evidence also suggests that air pollution can be detrimental to young brains. One study measured the impact of prenatal exposure to airborne polcyclic aromatic hydrocarbons (PAHs). These are defined by the Agency for Toxic Substances and Disease Registry as “a group of over 100 different chemicals that are formed during the incomplete burning of coal, oil and gas, garbage, or other organic substances like tobacco or charbroiled meat.” The results found exposures in late pregnancy to high levels of PAHs predicted lower cognitive development at 3 years of age and lower IQ at 5 years.

The evidence shows that small things can make a big difference in two very important ways, the first of which is that the oft-repeated adage “the dose makes the poison” is not necessarily relevant when it comes to children. Dr. David Bellinger, a professor of neurology at Harvard Medical School, explains to The Atlantic, “Are the blood lead levels of contemporary humans generally below the threshold of toxicity? Let us hope so, but the conclusion that they are is based more on faith than on evidence.”

Indeed, even relatively small changes in the amounts of chemicals can have an observable effect. For instance, Dr. Bruce Lanphear and colleagues report that “a flurry of recent studies shows that as the level of PBDEs, a type of flame retardant, increases in pregnant women’s blood from 10 to 100 ppb, the IQ scores of their children drop by about 5 points.”

This reveals the second important way in which seemingly “little things” in the environment have major consequences for brain development. As Lanphear and colleagues state, “Widespread exposure to a neurotoxin, like lead, that causes a 5-point drop in IQ across a population would result in a 57 percent increase in the number of U.S. children who are challenged, from 6 million to 9.4 million … When you add the impact of another brain-damaging toxin, like flame retardants, the number of challenged kids would rise to over 11 million.”

Arguably the most effective way to protect developing brains from potential damage is to enact more stringent testing and regulation of chemicals. When it comes to our children, it seems wise to shape our actions according to the maxim: “Better safe than sorry.”

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