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Saturday, April 28, 2012

Brain Drain: Could Environmental Chemicals Cause Autism?

From the Natural Resources Defense Council (NRDC) Blog Smarter Living: Family Health

"We've created a situation where we are exposing our children and grandchildren every day to new chemicals that didn't exist [until recently]," says Landrigan. "We've never tested them, and we don't have a clue what these chemicals do to early development."

There are few new parents today who haven't considered the possibility that autism might affect their child. And many parents are wondering if chemicals in our air, food, and water are to blame for what has been called the "autism epidemic."

The number of children with autism spectrum disorders (ASDs) has been rising steadily, and the Centers for Disease Control and Prevention (CDC) now estimate that ASD affects about 1 in 110 children in the U.S. 40 percent of these children have intellectual disabilities, and all suffer from impairments in social interactions. Some also have problems with communication and behavior.

The cause of this rise in autism, which many studies have shown cannot be explained by changes in diagnosis methods, is still unknown. Although a few percent of ASD cases can be traced to inherited genes, our genes don't change dramatically over the span of just a few decades.

One thing that has changed dramatically over the last several decades is human exposure to toxic chemicals and metals in the environment. Many of these chemicals and metals are known to affect the developing fetal brain. These include lead, methylmercury, polychlorinated biphenyls (PCBs), arsenic, cadmium, and manganese.

The developing brain is highly sensitive during the first three months of growth. In children who are susceptible, exposure to a neurotoxic metal or chemical during this window could confer a lasting change in brain structure and function.

The chemicals known to cause harm to the developing fetal and infant brain are part of a larger family of 200 chemicals known from workplace studies to cause neurological harm in humans, according to a review article by Dr. Philip J. Landrigan of the Children's Environmental Health Center at the Mount Sinai School of Medicine published in the January, 2010 issue of Current Opinion in Pediatrics. Children are exposed to roughly 3,000 chemicals in personal care products, building materials, cleaning products and motor vehicle fuels, yet fewer than 20 percent of these chemicals have been tested thoroughly to see if they harm the developing brain. "We've created a situation where we are exposing our children and grandchildren every day to new chemicals that didn't exist [until recently]," says Landrigan. "We've never tested them, and we don't have a clue what these chemicals do to early development."

When it comes to environmental exposures, much of the public focus has been on finding links between vaccines and autism. Landrigan argues, however, that it is time to move on. "There have been a dozen well-conducted epidemiological studies that have failed to detect a connection between vaccines and autism," Landrigan says, "so I think as a matter of research priority it is time to look at other hazards."

Chemicals suspected of harming the developing brain include phthalates (found in personal care products), bisphenol A (BPA), found in the linings of food cans), brominated flame retardants (found in old computers, television sets and foam padding), chlorinated solvents used in industry, the now-banned organochlorine pesticide DDT, and organophosphate pesticides. Although these chemicals have not been directly linked to ASD, the fact that they can cause learning and behavioral problems supports the idea that chemicals in the environment could cause ASD.

A further line of evidence for a link between environmental chemicals and ASD comes from a handful of known examples where chemicals taken during pregnancy appear to have caused the disorder. These chemicals include the drug thalidomide, the anti-seizure medication valproic acid, and the drug misoprostol, says Landrigan. A link has also been found between prenatal exposure to the organophosphate pesticide chlorpyrifos and increased risk of pervasive developmental disorder, a form of autism. Rubella infection during pregnancy also appears to increase the risk of ASD. "That series of observations raises the question in my mind, could there be other environmental causes, other chemicals or other environmental factors that cause autism?" says Landrigan.

How might environmental chemicals contribute to the risk of developing autism? One possibility is that these chemicals trigger ASD in children who inherited genes that make them susceptible to autism. These genes could be "turned on" or activated by an exposure in the womb, during childbirth, in early life, or during the toddler years.

Another possibility is that chemicals in the environment cause spontaneous gene alterations, called "de novo" mutations because they arise anew rather than being inherited. Mutations are fairly common but normally our DNA-repair mechanisms keep them from causing disease. When DNA-repair mechanisms fail, these mutations can lead to diseases such as cancer. Several de novo mutations have been detected in children with ASD. Some of these mutations are in genes related to brain development.

Environmental chemicals also could cause de novo mutations in the one or both of the parents. If these mutations occurred in egg or sperm cells, they could be passed on to the next generation. This could help explain why older fathers and mothers are more likely to give birth to a child with ASD.

It may be that multiple factors contribute to the range of conditions we call ASD. Many researchers now view ASD as an array of related disorders with similar symptoms but potentially with different causes.

More research is needed to explore the environmental aspects that contribute to the risk of developing ASD. Over a lifetime, the cost of care is estimated to reach $3.2 million per individual. Autism's costs are felt not just by the parents of children with ASD but also by society as a whole.

Parents who already have a child with autism should consider genetic consultation, recommends Landrigan. "If they have one child with autism, it is wise to check to see if they have one of the known genetic risk factors for autism," he says.

What You Can Do

To reduce the risk of giving birth to a child with learning or behavioral problems, says Landrigan, adults who plan to become parents should try to live as healthily as possible. This includes eating a healthy diet, getting adequate sleep and exercise, and limiting your exposure to known and suspected neurotoxic chemicals. Here are some tips:
  • Have your home or apartment tested for lead paint before beginning any sanding or other renovations (lead exposures have been declining over the years so lead is unlikely to be a cause of autism, but it is linked to lasting harm to the brain and can result in lower performance on tests and behavioral problems)
  • Avoid fish high in PCBs and methylmercury (see NRDC's Consumer Guide to Mercury in Fish.)
  • Minimize exposure to pesticides by eating organic foods and not using pesticides around the home and yard
  • Even though the evidence is not definitive, people should reduce their exposure to phthalates in personal care products and avoid canned foods because the can linings contain BPA.
Learn More

For a good overview see, CDC's Autism Spectrum Disorder page (with stats and definition of autism)

For 1 in 110 children developing autism, see "Prevalence of Autism Spectrum Disorders --- Autism and Developmental Disabilities Monitoring Network, United States, 2006." The authors note that, "Although improved ascertainment accounts for some of the prevalence increases documented in the ADDM sites, a true increase in the risk for children to develop ASD symptoms cannot be ruled out."

For Landrigan's literature survey on environmental chemicals and metal related to learning disorders and the possible links between environmental contaminants and autism, see Landrigan, P.J. "What causes autism? Exploring the environmental contribution." Curr Opin Pediatr. 2010 Jan 16.

For flame retardants as a potential risk factor, see: Messer A. "Mini-review: Polybrominated diphenyl ether (PBDE) flame retardants as potential autism risk factors." Physiol Behav. 2010 Jan 25. [Epub ahead of print]

For solvents such as trichloroethylene, and vinyl chloride, see: Windham GC, Zhang L, Gunier R, Croen LA, Grether JK. "Autism spectrum disorders in relation to distribution of hazardous air pollutants in the San Francisco Bay area." Environ Health Perspect2006;114(9):1438–44.

For organophosphates, see the following three studies:

Eskenazi B, Rosas LG, Marks AR, et al. "Pesticide toxicity and the developing brain."Basic Clin Pharmacol Toxicol 2008; 102:228–236.

Roberts EM, English PB, Grether JK, Windham GC, Somburg L, Wolff C. "Maternal residence near agricultural pesticide applications and autism spectrum disorders among children in the California Central Valley." Environ Health Perspect 2007;115(10):1482–9.

Rauh VA, Garfinkel R, Perera FP, et al. "Impact of prenatal chlorpyrifos exposure on neurodevelopment in the first 3 years of life among inner-city children." Pediatrics 2006; 118:e1845–e1859.

The Childhood Autism Risks from Genetics and Environment study has a goal of addressing a wide spectrum of chemical and biologic exposures, susceptibility factors and their interactions with regard to ASDs. This older paper provides a good overview of the potential genetic and environmental causes and the interaction of the two:

Hertz-Picciotto et al. "The CHARGE Study: An Epidemiologic Investigation of Genetic and Environmental Factors Contributing to Autism"

For a discussion of "de novo" mutations caused by environmental mutagens see: "Environmental risk factors for autism: Do they help cause de novo genetic mutations that contribute to the disorder?" Dennis K. Kinney et al. Medical Hypotheses 74 (2010) 102–106

For parental age as risk factor, see: Shelton JF, Tancredi DJ, Hertz-Picciotto I. "Independent and dependent contributions of advanced maternal and paternal ages to autism risk." Autism Res. 2010 Feb;3(1):30-9.

Copy number variations are mutations resulting in extra or decreased numbers of genes. For an association between copy number variations and autism, see the following two studies:
Sebat J. et al. "Strong Association of De Novo Copy Number Mutations with Autism."Science 20 April 2007: Vol. 316. no. 5823, pp. 445 - 449

Glessner JT et al. "Autism genome-wide copy number variation reveals ubiquitin and neuronal genes." Nature. 2009 May 28;459(7246):569-73. Epub 2009 Apr 28.

1 comment:

  1. Fantastic article. Well written ... leaves the reader wanting to check things out further. Have BO.LTed this for you.