National Academy of Sciences reports exposure to toxic chemicals contribute to learning and developmental disabilities in American children

The number of American children with learning and developmental disabilities has been climbing over the past decade, reaching nearly one in six by 2008.^[1] The increasing prevalence of autism and attention deficit hyperactivity disorder accounts for most of this change.^[2]The National Academy of Sciences estimates that combinations of environmental factors, including exposure to toxic chemicals, along with genetic susceptibility, cause or contribute to at least 25% of learning and developmental disabilities in American children.^[3]

Intellectual disability (formerly referred to as mental retardation) affects 2%, or approximately 1.4 million, children in the United States.^[4] As of 2009, 9% of children—roughly 50 million kids—were diagnosed with attention deficit hyperactivity disorder (ADHD).^[5][6] According to the U.S. Centers for Disease Control and Prevention (CDC), an estimated 1 in 88 children in the United States have an autism spectrum disorder.^[7] Between 1997 and 2008, the prevalence of autism increased nearly 300% nationally.^[8] In a seminal study of California’s dramatic rise in autism rates, researchers found that about 30% of the rise could not be explained by changes in the age of diagnosis or the inclusion of milder cases.^[9]

These conditions impose tremendous psychological and economic costs on the affected children, their families, and communities. On average, it costs twice as much to educate a child who has a learning or developmental disability as to educate a child who does not.^[10] According to the CDC, individuals with an autism spectrum disorder have average medical expenditures that exceed those without the disorder by $4,110–$6,200 per year.^[11] A 2006 study reported that the economic costs associated with autism in the U.S. are approximately $35 billion dollars per year.^[12]

The human brain: more susceptible during development

Much of what we know about chemicals that can cause neurological problems comes from studies of adults—often in the workplace—and from animal studies. For example, lead, mercury, and various organic solvents have been identified in the peer-reviewed, scientific literature as causing neurological effects in adults, mostly through occupational exposures (see Table 2).^[13] Many of these chemicals are in common use and are produced in high volumes,^[14] but for many, we have very little knowledge about their neurologic impact in children. A large number of chemicals have never been evaluated for their neurological impacts in children or adults.

In the last few decades, extensive evidence has accumulated showing that neurotoxic chemicals can have a profound effect on the developing brain at levels that were once thought to be safe, and that may have little or no discernible impacts in adults.^[15] Beginning in utero and continuing through adolescence, exposures to certain chemicals during particular time windows of vulnerability can disrupt normal developmental processes with profound and often life-long consequences.^[16][17]

Lead, mercury, arsenic, PCBs, certain flame-retardants (PBDEs), and pesticides are among the chemicals for which the special vulnerability of the developing brain has been extensively demonstrated.^[18][19] Our understanding of the developing brain’s unique vulnerability suggests that there may be hundreds or even thousands of additional chemicals that can have an impact. We have no authoritative estimate of the actual number, primarily because relatively few chemicals have been examined for effects in the developing brain of laboratory animals or children.

Ten chemicals suspected of causing developmental neurotoxicity

In spring 2012, scientists from the National Institute of Environmental Health Sciences and the Mount Sinai School of Medicine listed “10 chemicals and mixtures widely distributed in the environment that are already suspected of causing developmental neurotoxicity.” These are:

1. Lead: a heavy metal banned from gasoline in the 1970s, found in old paint, lead pipes and sinkers, toys, jewelry, and other items made of vinyl plastic.
2. Methylmercury: released into the air from coal-burning power plants; also found in some medical equipment, switches, personal care products, and fluorescent bulbs.
3. Polychlorinated biphenyls (PCBs): used in electrical transformers; banned in the late 1970s but still widely found in lakes, rivers, soil, fish, and people.
4. Organophosphate pesticides: pesticides containing phosphorous that work by disrupting the nervous system; used to kill insects on crops and lawns, and in buildings.
5. Organochlorine pesticides: pesticides containing chlorine that work by disrupting the nervous system; used to kill insects on crops and lawns, and in buildings; many but not all have been banned in the United States.
6. Endocrine disruptors: chemicals that disrupt the hormone system, including phthalates and bisphenol A (both widely used in plastics), PCBs, brominated flame retardants, perfluorinated compounds, dioxins, organochlorine pesticides, among others.
7. Automotive exhaust
8. Polycyclic aromatic hydrocarbons: air pollutants from fuel combustion in vehicles, coal-fired power plants, heating, and cooking; also found in tobacco smoke.
9. Brominated flame retardants: flame retardant chemicals added to furniture, electronics, building materials, bedding, and a wide range of other products.
10. Perfluorinated compounds: used in stain-resistant and nonstick products.

Lead, methylmercury, PCBs, some endocrine disruptors, brominated flame retardants, and perfluorinated compounds are among chemicals subject to regulation by the Toxic Substances Control Act.^[39]

Toxic flame retardants example Scientists continue to identify chemicals with enough evidence to raise serious concern over their effects on brain development. A category of flame-retardant chemicals called polybrominated diphenyl ethers (PBDEs) provides a clear illustration of how current chemical laws fail to protect public health.

PBDEs are used in many products, including upholstery, electronics, carpet, building materials, bedding, and mattresses.^[20][21] Despite some regulatory restrictions on PBDEs, these chemicals remain a serious problem because they break down into more toxic forms and are used in many recycled products like carpet padding. In addition, they persist in the environment for a long time, build up in people’s bodies and in breast milk, and are often found at higher levels in children than adults.^[22][23]

An extensive body of scientific literature documents the neurodevelopmental impacts of PBDEs in laboratory animals. In laboratory studies, low doses of some PBDEs cause deficits in learning, memory and hearing, changes in behavior, and delays in sensory-motor development in mice and rats.^{[24][25][26][27][28][29][30]}

A landmark study published in 2010 provided the first evidence of the adverse effects of these chemicals on human brain development. The study tracked 329 women who gave birth in lower Manhattan hospitals following the terrorist attacks of September 11, 2001. The researchers found an association between levels of PBDE flame retardants in the babies’ cord blood and delays in mental and physical development measured at 1, 2, 3, 4, and 6 years of age.^[31] In 2004 and 2009, the U.S. government reached a voluntary agreement with chemical manufacturers to begin phasing out three commercial mixtures of PBDEs.^[32][33] Since 2002, at least twelve states have banned one or more of these flame retardants due to the mounting evidence of harm to human health.^[34] Some manufacturers have responded by replacing the three PBDE mixtures with different chemicals that they claimed were safer, with no publicly available information to support their claims.

These replacement flame retardants are now showing up in the environment, including in the atmosphere, sediments, and seagull eggs around the Great Lakes, raising concerns that we have simply moved from one dangerous set of chemicals to another, without adequate safety testing.^[35][36] And, to the extent that these new chemicals are also persistent and bioaccumulative, we will be living with their toxic consequences for years to come.

How chemical policy reform can help

There is solid and mounting scientific evidence on a limited number of chemicals, including those described above, to show that they are harmful to brain development. Where the weight of the evidence warrants concern, TSCA reform should include swift action to replace known toxic chemicals with safer alternatives.

However, for most of the thousands of chemicals on the market, we have virtually no information about their effects on the developing nervous system. Of the 3,000 chemicals produced in highest volume (over one million pounds per year), few have been adequately tested for toxicity to the developing brain.^[37] To ensure healthy brain development for future generations, TSCA must be updated to require that all existing and new chemicals are evaluated for their safety for pregnant women, children, workers, and other vulnerable populations.

Table 2: Some Chemicals Known to be Neurotoxic to Humans[38]*
Metals and inorganic compounds	Organic solvents	Other organic substances
Aluminum compounds Arsenic and arsenic compounds Azide compounds Barium compounds Bismuth compounds Carbon monoxide Cyanide compounds Decaborane Diborane Ethylmercury Fluoride compounds Hydrogen sulphide Lead and lead compounds Lithium compounds Manganese and manganese compounds Mercury and mercuric compounds Methylmercury Nickel carbonyl Pentaborane Phosphine Phosphorus Selenium compounds Tellurium compounds Thallium compounds Tin compounds	Acetone Benzene Benzyl alcohol Carbon disulphide Chloroform Chloroprene Cumene Cyclohexane Cyclohexanol Cyclohexanone Dibromochloropropane Dichloroacetic acid 1,3-Dichloropropene Diethylene glycol N,N-Dimethylformamide 2-Ethoxyethyl acetate Ethyl acetate Ethylene dibromide Ethylene glycol n-Hexane Isobutyronitrile Isophorone Isopropyl alcohol Isopropyl acetone Methanol Methyl butyl ketone Methyl cellosolve Methyl ethyl ketone Methylcyclopentane Methylene chloride Nitrobenzene 2-Nitropropane 1-Pentanol Propyl bromide Pyridine Styrene Tetrachloroethane Tetrachloroethylene Toluene 1,1,1-Trichloroethane Trichloroethylene Vinyl chloride Xylene	Acetone cyanohydrin Acrylamide Acrylonitrile Allyl chloride Aniline 1,2-Benzenedicarbonitrile Benzonitrile Butylatedtriphenyl phosphate Caprolactam Cyclonite Dibutyl phthalate 3-(Dimethylamino)-propanenitrile Diethylene glycol diacrylate Dimethyl sulphate Dimethylhydrazine Dinitrobenzene Dinitrotoluene Ethylbis(2-chloroethyl)amine Ethylene Ethylene oxide Fluoroacetamide Fluoroacetic acid Hexachlorophene Hydrazine Hydroquinone Methyl chloride Methyl formate Methyl iodide Methyl methacrylate p-Nitroaniline Phenol p-Phenylenediamine Phenylhydrazine Polybrominated biphenyls Polybrominated diphenyl ethers Polychlorinated biphenyls Propylene oxide TCDD Tributyl phosphate 2,2,2-Trichlorotriethylamine Trimethyl phosphate Tri-o-tolyl phosphate Triphenyl phosphate
*It is important to note that the listed chemicals are industrial chemicals. Table 2 does not include neurotoxic pesticides, which are regulated under a different federal statue.

Citations:

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