Learning and Developmental Disabilities
According to the U.S. Centers for Disease Control and Prevention (CDC), the number of U.S. children with developmental disabilities has been climbing over the past decade, reaching nearly one in six by 2008. The rising incidences of autism and attention deficit hyperactivity disorder account for most of this increase.Coleen A. Boyle, et al. “Trends in the Prevalence of Developmental Disabilities in US Children, 1997–2008,” Pediatrics, 127 (6) (June 2011): 1034-1042.
The National Academy of Sciences estimates that environmental factors, including toxic chemicals, cause or contribute to at least a quarter of learning and developmental disabilities in American children. Committee on Developmental Toxicology, Board on Environmental Studies and Toxicology, National Research Council. Scientific Frontiers in Developmental Toxicology and Risk
Intellectual disability (ID, formerly referred to as mental retardation) impacts 2%, or approximately 1.4 million, children. Attention deficit hyperactivity disorder (ADHD) is conservatively estimated to occur in 3–6%, or approximately 2 million, children. Almost 1% of 8-year-old children are diagnosed with autism spectrum disorder, a 10-fold increase over just a 15-year period.Steven G. Gilbert, “The Scientific Consensus Statement on Environmental Agents Affiliated with Neurodevelopmental Disorders,” (Bolinas, CA: Collaborative on Health and the Environment, 2008), abstracted in Neurotoxicology and Teratology, 31, no 4, (July–August 2009): 241–2., 31, no 4, (July–August 2009): 241–2.)National Institute of Mental Health, “NIMH’s Response to New Autism Prevalence Estimate,” http://www.nimh.nih.gov/about/director/updates/2009/nimhsresponse-to-new-autism-prevalenceestimate.shtml (November 4, 2009).Catherine Rice, “Prevalence of autism spectrum disorders — Autism and Developmental Disabilities Monitoring Network, United States, 2006, National Center on Birth Defects and Developmental Disabilities,” MMWR Surveillance Summaries, 58, no. SS10 (December 18, 2009): 1-20. http://www.cdc.gov/mmwr/preview/mmwrhtml/ss5810a1.htm.) About 30% of this dramatic rise in autism cannot be explained by changes in the age of diagnosis and the inclusion of milder cases.Irva Hertz-Picciotto and Lora Delwiche, “The Rise in Autism and the Role of Age in Diagnosis,” Epidemiology, 20, no. 1 (2009): 84–90.
These conditions impose tremendous psychological and economic costs on the affected children, their families, and communities. Just the cost of providing special education services to students with disabilities amounted to $77.3 billion in 1999–2000, an average of $12,474 per student. Steven G. Gilbert, “The Scientific Consensus Statement on Environmental Agents Affiliated with Neurodevelopmental Disorders,” (Bolinas, California: Collaborative on Health and the Environment, 2008), abstracted in Neurotoxicology and Teratology, 31, no 4 (July–August 2009): 241–2. According to the U.S. CDC individuals with an autism spectrum disorder have average medical expenditures that exceed those without the disorder by $4,110–$6,200 per year.Centers for Disease Control and Prevention, “CDC Statement on Autism Data,” http://www.cdc.gov/ncbddd/autism/data.html (accessed October 10, 2009).Tom T. Shimabukuro, Scott D. Grosse SD, and Catherine Rice, “Medical Expenditures for Children with an Autism Spectrum Disorder in a Privately Insured Population,” Journal of Autism and Developmental Disorders, 38, no. 3 (March 2008): 546–52. A 2006 study reported that the economic costs associated with autism in the U.S. are approximately $35 billion dollars per year.M.L. Ganz, “The Costs of Autism,” Understanding Autism: From Basic Neuroscience to Treatment, Firtst Edition, Steven O. Molden, ed, and John Rubenstein, ed, (Boca Raton, Florida: CRC Press, 2006): 475–502.
The human brain: more susceptible during development
As with cancer, much of what we know about chemicals that can cause neurological problems comes from studies of occupational exposures. Research on occupational exposures and epidemics of industrial chemical poisoning have led to the identification of lead, methyl-mercury, polychlorinated biphenyls (PCBs), arsenic, and toluene as known causes of neurodevelopmental disorders. Industrial chemicals have been identified in the peer reviewed scientific literature as causing neurological effects in adults, mostly through occupational exposures (see Table 2). Many of these chemicals are in common use and are produced in high volumes.
However, in recent decades, many scientists have begun to focus on the effects of chemicals on the brain during fetal development and childhood. In the years since the Toxic Substances Control Act (TSCA) became law, evidence has been accumulating that lead, mercury and other neurotoxic chemicals have a profound effect on the developing brain at levels that were once thought to be safe. Scientists have learned that the developing human brain is much more susceptible to toxic substances than the adult brain. Windows of unique vulnerability occur as the brain begins to develop in utero and continue through adolescence, along a precise and delicate step-by-step sequence involving various neurobiological processes. A chemical exposure at three months gestation may result in a different effect than exposure to the same chemical at six months gestation or at two years of age. If chemicals inhibit, interfere with, or halt a developmental process, the damage may be permanent.
This new understanding of the developing brain’s unique vulnerability suggests that there may be a thousand or more other chemicals that can impact the developing brain, although no authoritative estimate of the true number of neurotoxicants is available.
Chemicals of Concern
In recent years, peer-reviewed scientific studies continue to identify chemicals and categories of chemicals for which there is sufficient evidence to warrant serious concern over the effects of those chemicals on brain development. The toxic chemicals listed below are just three examples of such chemicals.
Certain Brominated Flame Retardants: Used in upholstery, electronics, carpet, building materials, bedding, mattresses, and many other products. In laboratory studies, low doses caused deficits in learning, memory and hearing, changes in behavior and delays in sensory-motor development in mice and rats.
A study of 329 mothers who delivered in hospitals in lower Manhattan following the attacks of 9/11/2001 found an association between levels of brominated flame retardants in their babies’ cord blood and delays in mental and physical development measured at 1,2,3,4 and 6 years of age.Herbstman, JB, et al. “Prenatal Exposure to PBDEs and Neurodevelopment,” Environmental Health Perspectives. 2010 May; 118(5):712-9.
Perchlorate: Used in rocket fuel and widely found in U.S. drinking water.
In 2010, scientists with the Office of Environmental Health Hazard Assessment examined records of blood samples drawn from the heels of 497,458 newborns in 1998 in California. The researchers found that babies born in areas of California where tap water was contaminated with perchlorate had a 50% chance of having a poorly performing thyroid gland.Steinmaus C, Miller MD, Smith AH, “Perchlorate in drinking water during pregnancy and neonatal thyroid hormone levels in California,” J Occup Environ Med. 2010 Dec; 52(12):1217-524.
The thyroid is a gland in the throat that produces hormones essential for healthy nerve and brain development. Perchlorate, brominated flame retardants and Bisphenol A (see below) are all under investigation for their potential to interfere with thyroid hormones and function.
Bisphenol A (BPA): A plasticizer that mimics estrogen in the body, BPA is found in hard plastics, food and soda can linings, and cash register receipts among other uses.
In August 2007, a group of 38 leading scientists expressed unanimous concern that recent trends in human disease may be related to BPA exposures, based on their assessment of hundreds of peer-reviewed research studies. Experiments with animals have established links between BPA and prostate and breast cancers, genital abnormalities in male babies and decline in sperm quality, early onset of puberty in girls and neurobehavioral problems such as attention deficit hyperactivity disorder.vom Saal, FS, SM Belcher, LJ Guillette, R Hauser, JP Myers, GS Prins, WV Welshons, JJ Heindel et al. 2007. Chapel Hill Bisphenol A Expert Panel Consensus Statement: “Integration of mechanisms, effects in animals and potential impact to human health at current exposure levels,” Reproductive Toxicology 24:131-138.
A 2009 study of 249 women and their babies found that the daughters of the women who had higher exposure levels to BPA while pregnant were more likely to show aggressive and hyperactive behavior as two-year olds. Joe M. Braun, et al., “Prenatal Bisphenol A Exposure and Early Childhood Behavior,” Environmental Health Perspectives, 117(12) December 2009.
The chemicals highlighted above are widespread in products, the environment and people. The CDC finds that more than 90 percent of Americans have detectable levels of the brominated flame retardants, perchlorate and Bisphenol A in their bodies. As described, all of these chemicals are linked to problems with brain development.
Chemicals, learning and developmental disabilities, and TSCA
There is an emerging scientific understanding that disparate chemicals used for different purposes in multiple products can result in similar effects on a particular system in the body. The National Academy of Sciences – the nation’s most esteemed body of scientists – recommends that review of chemicals for safety should include an assessment of cumulative chemical exposures – meaning exposures from multiple chemicals that relate to the same or similar adverse effects. This approach to assessing chemicals is critical to protecting the developing brain from harm. Under a revised TSCA, EPA is directed to assess cumulative exposures of toxic chemicals wherever possible.
There is very solid and mounting scientific evidence on a limited number of chemicals, including those described above, to show that these chemicals are harmful to brain development. Where the weight of the evidence warrants concern, TSCA should require swift action to replace known toxins with safer alternatives.
However, for most of the thousands of chemicals on the market, we have very little data on their effects on the developing nervous system. Of the 3,000 chemicals produced in highest volume (over one million pounds per year), only 12 have been adequately tested for neurotoxicity. To ensure healthy brain development for future generations, TSCA must be updated to require that all existing and new chemicals are shown to be safe for pregnant women, children, workers, and other vulnerable populations.This entire section is informed by the article “Developmental Neurotoxicity of Industrial Chemicals,” by P. Grandjean and P.J. Landrigan, published in the journal Lancet, 368, no. 9553 (December 16, 2006): 2167–78.
