The notion that some substances in the environment can damage the nervous system has an ancient history. The neurotoxicity of lead was recognized more than 2,000 years ago by the Greek physician Dioscerides, who wrote, “Lead makes the mind give way.” In the intervening millennia many other substances have been added to the list of known or suspected neurotoxicants. Despite this accumulation of knowledge, there is still much that isn’t understood about how neurotoxicants affect the developing brain, especially the effects of low-dose exposures. Today researchers are taking a hard look at low-dose exposures in utero and during childhood to unravel some of the mysteries of impaired neurodevelopment.
About 17% of school-age children in the United States suffer from a disability that affects their behavior, memory, or ability to learn, according to a study published in the March 1994 issue of Pediatrics by a team from the Centers for Disease Control and Prevention (CDC). The list of maladies includes attention deficit/hyperactivity disorder (ADHD), autistic spectrum disorders, epilepsy, Tourette syndrome, and less specific conditions such as mental retardation and cerebral palsy. All are believed to be the outcome of some abnormal process that unfolded as the brain was developing in utero or in the young child.
These disorders have an enormous impact on families and society. According to the 1996 book Learning Disabilities: Lifelong Issues, children with these disorders have higher rates of mental illness and suicide, and are more likely to engage in substance abuse and to commit crimes as adults. The overall economic cost of neurodevelopmental disorders in the United States is estimated to be $81.5-167 billion per year, according to a report published in the December 2001 issue of EHP Supplements.
Potentially even more disturbing is that a number of epidemiologic studies suggest that the incidence of certain disorders is on the rise. In the United States, the diagnosis of autistic spectrum disorders increased from 4-5 per 10,000 children in the 1980s to 30-60 per 10,000 children in the 1990s, according to a report in the August 2003 Journal of Autism and Developmental Disorders. Similarly, notes a report in the February 2002 issue of CNS Drugs, the diagnosis of ADHD grew 250% between 1990 and 1998. The number of children in special education programs classified with learning disabilities increased 191% between 1977 and 1994, according to an article in Advances in Learning and Behavioral Disabilities, Volume 12, published in 1998.
So what is going on? The short answer is that no one really knows. There’s not even consensus on what the soaring rates actually mean. Heightened public awareness could account for the surge in the numbers, or it may be that physicians are getting better at diagnosing the conditions. Some autism researchers believe the rise in that condition’s prevalence simply reflects changes in diagnostic criteria over the last 25 years. On the other hand, some scientists believe that the rates of neurodevelopmental disease are truly increasing, and that the growing burden of chemicals in the environment may play a role.
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