It’s understood that “over half of the most heavily used industrial chemicals are known to be toxic to the brain and nervous system.” (Shabecoff & Shabecoff, 2010, p. 44) This is concerning because “chemicals that have scant or no effect on the adult brain can produce permanent changes in a child’s.” (ibid, p. 27)

A child’s brain will quadruple in size from birth through the teen years. (Johnson, 2001) It nearly triples between birth and age two, and reaches around 90% of its full volume by age 6. It also goes through several major stages of growth and pruning along its way to maturity. (Jensen, 2006) So not only are kids more sensitive to neurotoxins, but they’ve got a lot more going on to disrupt.

What is a neurotoxin?

A neurotoxin is any chemical that disrupts or has the potential to damage the functioning of neurons. Neurons are cells that transmit electrical signals. They are found throughout the body as part of our nervous system and are the source of the magic that happens in the brain, which is why neurotoxin chemicals have such a profound impact on brain development and cognition.

Chemicals that affect the brain

There are a number of chemicals known to be neurotoxic. Most pesticides were actually designed to be neurotoxic; that’s how they kill the bugs. Naturally occurring elements such as lead and mercury are also neurotoxins, as are a number of other man-made chemicals, including PCBs and many byproducts of industrial processes and air pollution. Here are some examples of how toxic exposures can impact the brain:

  • Air pollution has been found to lower IQ and decrease white matter in the brain, as we’ll discuss extensively in later chapters.
  • Lead is quite toxic for the brain, resulting in loss of IQ and other problems, as outlined in our section on lead.
  • Mercury, which crosses the blood-brain barrier and interferes with the functioning of neurons, can cause extensive brain damage in children. The economic cost of this impairment has been pegged at anywhere from $37 billion to $90 billion a year. (Wolf & Groppe, 2014)


Even chemicals that aren’t classified as neurotoxins can cause problems for the brain. For example, particulate matter from air pollution that is inhaled can make its way to the brain. Autopsies of otherwise healthy grade school children living in Mexico City who were killed in car accidents show they often have pervasive legions throughout the brain as a result of all the air pollution.

How chemicals are impacting IQ & intelligence

These effects are often subtle, which is why they don’t get the same attention as something like cancer. But small changes have a way of adding up. A drop in IQ of 5 points (a rather moderate decrease linked with many neurotoxins) would shift the average national IQ down from 100 to 95. This slide would also nearly double the number of retarded adults while halving the number of people considered gifted. (Schettler et al., 2001, p. 16) Studies often find point drops much higher than this (7 to 10 points) on everything from lead to pesticides to air pollution. Keep in mind: this difference is not just an issue for kids living in toxic waste dumps, it emerges FROM NORMAL, EVERYDAY LEVELS OF EXPOSURE, usually by comparing the 20% of kids with the highest exposures to the 20% with the lowest exposures. Those top 20% of kids who get just a little more exposure than everyone else can suffer these kinds of IQ drops.