How environmental exposures can contribute to autism and ADHD

Each year, biologically based disorders of brain development – autism, attention deficit/hyperactivity disorder (ADHD), mental retardation, dyslexia, and subclinical neurodevelopmental disabilities – affect between 400,000 and 600,000 of the four million babies born in the United States. This means that between 10% and 15% of American children have some kind of learning disability.

Reported rates of autism spectrum disorder (ASD) are sharply increasing.  The CDC reports that the rates of ASD increased by 57% between 2002 and 2006, now affecting 1 of every 110 babies born in the U.S.

With this rise in reported diagnosis, researchers are asking new questions about the causes of autism.  Until recently, most of this research into the causes of ASD has focused on genetic factors.  These investigations have made rapid progress and have identified a series of genetic abnormalities that are linked to autism. Taken together, these identified genetic causes account for about 30 to 40% of cases of autism.

Researchers are learning that environmental exposures also contribute to autism.  Some cases appear to be mainly the result of environmental exposures; others are mainly genetic. And still others are caused by the interplay between genetic susceptibility and environmental exposures.

The case for an environmental contribution

Over the past 50 years, over 80,000 new synthetic chemicals have been developed.  Three thousand are “high production volume” chemicals, meaning they are the most widely used and thus, pose the greatest potential for human exposure.

The brains of infants and children are extremely sensitive to toxic exposures in the environment. New research has identified “critical windows of vulnerability” in fetal life and early childhood, when exposures to toxic chemicals can cause devastating injury to the brain and nervous system. We know that even low levels of toxic exposure during these windows of vulnerability can cause lasting damage to the developing brains of infants and children.

Researchers are now learning that the brain injury caused by prenatal exposures to toxic chemicals can result in learning disabilities and probably also in autism and ADHD. For example, lead, methylmercury, polychlorinated biphenyls (PCBs), manganese, organophosphate pesticides, organochlorine pesticides, phthalates, bisphenol-A (BPA), and brominated flame retardants have all been identified as causes of childhood brain damage.  Lead and phthalates have been linked to ADHD.  And prenatal exposures to certain pesticides as well as to phthalates have been linked to autistic behaviors in children.

Moving forward

Researchers are concerned that this list of chemicals may be only the tip of a much larger problem.  Currently, 200 synthetic chemicals are neurotoxic in adult humans, and another 1,000 chemicals have been identified as neurotoxic in experimental models. Among these 1,200 neurotoxic chemicals, there is a good chance that some have the potential to cause childhood brain injury that result in autism and learning disabilities and that this potential has not yet been recognized.

For this reason, our team at the Mount Sinai Children’s Environmental Health Center has developed a list of the top ten chemicals – all widely distributed in the environment – that we suspect may cause autism and learning disabilities. This list has already been accepted by Environmental Health Perspectives, the most highly ranked journal in environmental health, and will be published later this year.

To learn more about the environmental causes of autism and learning disabilities, read my review “What Causes Autism? Examining the Environmental Contribution.”

Philip J. Landrigan is Director of the Children’s Environmental Health Center, Mount Sinai Medical Center, New York City, NY.

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