These effects can vary dramatically depending on the exact timing of exposures. Here is a sampling of the toxins that can misdirect the development of a child's brain. Young monkeys exposed to lead show symptoms including heightened distractability and inappropriate responses to stimuli. One million American children currently live with blood lead levels above the threshold recognized by EPA as affecting behavior and cognition. Millions more would be added to this list if EPA's threshold were updated to take account of the most current science on the effects of lead in children.
Mercury enters the environment through waste incinerators and coal-burning power plants.
It bioaccumulates in fish in its most toxic form, methylmercury. Not surprisingly, such pesticides can disrupt the development and functioning of the human nervous system by the same mechanisms. Animal studies show that neurotoxic pesticides can produce permanent changes in brain structure and functioning when exposures occur on a single critical day of development. For example, some effects occurred in newborn mice if exposures occurred on day 10 of development, but not if exposures occurred on day 3 or 19. The two communities were very similar in ethnic composition and culture, but one community practiced chemical-intensive agriculture while the other used few farm chemicals.