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Photo of a child with autism



Autism, also known as ASD, is a spectrum of disorders that causes impairment in social interaction, as well as the presence of repetitive, restricted behaviors and interests. It is usually first diagnosed in early childhood.

The term spectrum refers to the wide range of symptoms, skills, and levels of impairment that those with ASD can have. Some are mildly impaired by their symptoms, while others are severely disabled.

According to the Centers for Disease Control and Prevention, ASD affects roughly 1 in 68 children.

What are some of the symptoms of ASD?

Although people with autism have a variety of symptoms that vary in severity, they all have difficulties communicating and interacting with others, and show restricted and repetitive patterns of behavior and interests. Most symptoms are noticeable by the time a child is 2-3 years old, but many children are not diagnosed until later. Early intensive behavioral intervention can improve communication, learning, and social skills in children with autism. 

Autism affects people for their entire lives, and often comes with other conditions, such as epilepsy, sleep disturbances, and gastrointestinal problems. Currently, no drugs have proven effective for treating core autism symptoms.

How is ASD diagnosed?

Historically, those with ASD were placed into one of four related, yet distinctly separate, diagnostic subtypes:

  • Autistic disorder
  • Childhood disintegrative disorder
  • Pervasive developmental disorder – not otherwise specified (PDD-NOS)
  • Asperger syndrome

In 2013, however, the American Psychiatric Association released the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), which replaced these subtypes with the current spectrum system.

What causes ASD?

Although recent studies indicate that the rate of ASD appears on the rise, the causes of these disorders are not well-understood. Over time, scientists have found that rare gene changes, or mutations, as well as small common genetic variations, are associated with ASD, thus implying a genetic component. However, a growing area of research indicates that ASD may be caused by an interaction of genetic and environmental factors.

For example, one hypothesis states that ASD may be triggered by a mother’s exposure to environmental agents while pregnant. These exposures, in turn, could cause or contribute to the child’s development of ASD.

Related studies on ASD, genes, and the environment

What are some of the environmental factors researchers believe may be associated with ASD?

The clearest evidence for environmental risk factors in autism involves events before and during birth. They may include:

  • Advanced parental age at time of conception
  • Prenatal exposure to air pollution
  • Maternal obesity or diabetes
  • Extreme prematurity and very low birth weight
  • Any birth difficulty leading to periods of prenatal oxygen deprivation to the baby’s brain
  • Prenatal exposure to certain pesticides

Again, however, these factors alone are unlikely to cause ASD. Rather, they appear to increase a child’s chances for developing ASD, when combined with the aforementioned genetic factors.

Environmental factors play a role in autism

Air pollution

Work supported by NIEHS indicates that early-life exposure to air pollution is a risk factor for autism.

  • A 2011 study reported that children living within 1,014 feet, or a little less than 3.5 football fields, of a freeway, at birth, were twice as likely to develop autism.
  • Building on those findings, in 2013, researchers reported an association between exposure to traffic-related air pollution, as well as components of regional air pollution, and an increased risk of autism.
  • A 2014 study pointed to a likely gene-environment interaction. Children whose genetic makeup causes them to be more susceptible to the health effects of high levels of air pollution showed the highest risk for autism.
Prenatal conditions

Researchers funded by NIEHS discovered that problems with the immune system, as well as maternal conditions during pregnancy, are linked with higher autism risk.

  • Research showed that some children are born to mothers with antibodies that interfere with fetal brain development in ways that could lead to autism.
  • Maternal diabetes and obesity, which are associated with inflammation, both have strong links to the likelihood of having a child with autism or another developmental disability.
  • During pregnancy, elevated levels of inflammation, which can come from an infection, were linked with an increased risk of having a child with autism. This finding may help to identify preventive strategies.

According to NIEHS-funded research, prenatal vitamins may help lower autism risk.

  • Women who took a daily prenatal vitamin during the three months before and during the first month of pregnancy, were less likely to have a child with autism than women not taking the supplements. This was more evident in genetically susceptible women or children, suggesting that a gene-environment interaction could be responsible.
  • A later study identified folic acid as the source of the protective effects of prenatal vitamins. Women who consumed the daily recommended dosage during the first month of pregnancy had a reduced risk of having a child with autism.
Mercury and other contaminants

There continues to be concern about autism and mercury exposure. NIEHS funds research examining this and exposures to other contaminants.

  • Eating fish is the primary way that we are exposed to organic mercury. A 2013 study examined people in the Republic of Seychelles, where fish consumption is high. The study found no association between prenatal organic mercury exposure and autism behaviors.
  • Scientists can test for recent exposure to organic mercury with blood tests. Researchers found that after adjusting for dietary and other mercury sources, children with autism had blood mercury levels that were similar to those found in children without autism.
  • Researchers are also studying other contaminants, such as bisphenol A (BPA), phthalates, heavy metals, flame retardants, polychlorinated biphenyls (PCBs), and pesticides, to see if they affect early brain development and play a role in autism.

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What NIEHS is Doing on Autism 

NIEHS has steadily increased funding of autism research over the last decade, and this investment is producing important new discoveries that may help prevent autism. For example, NIEHS-funded researchers have shown that taking folic acid and avoiding infections during pregnancy can help lower autism risk. Researchers have also shown that problems with the immune system are involved in autism, and that early-life exposure to high levels of air pollution may increase risk, especially for children whose genetic makeup causes them to be more susceptible.

The NIEHS Autism Research Program has attracted talented scientists from toxicology, epidemiology, and other areas. These researchers are using new ways to measure prenatal exposures, screen for contaminants that affect brain development, and understand how environmental factors interact with genes to lead to autism.

Notable NIEHS studies on ASD

A team of NIEHS-funded scientists at the University of California (UC), Davis are searching to address the environmental contributors to ASD through their continued work on the Childhood Autism Risk from Genetics and the Environment (CHARGE) Study.

CHARGE is an ongoing exploration of more than 1,800 children, to clarify the roles of genetics and environmental exposures in ASD development.

Through an array of interdisciplinary approaches, UC Davis researchers hope to:

  • Identify specific environmental agents that affect risk of ASD
  • Learn how those exposures affect basic mechanisms of brain development
  • Examine the role of immune system dysfunction in ASD

To learn more about the CHARGE Study, please visit the following resources:

In a second study led by NIEHS-funded scientists from the University of Southern California, researchers found that children possessing a specific genetic risk factor appear more likely to develop ASD when exposed to high levels of air pollution during gestation. This finding helps explain why some previous studies that focused exclusively on genetic variation and ASD development have proven inconclusive.

The study was published in the January 2014 issue of the journal Epidemiology, and is available online, along with a companion article in the NIEHS newsletter, the Environmental Factor.

Other ASD stories from the Environmental Factor:

NIEHS press releases on ASD

General Information 

Related Topics 

Research Links 

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