Autism,1 also known as autism spectrum disorder (ASD), is a disorder2 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.3
The Autism and the Environment (525KB) fact sheet provides a summary of NIEHS-supported research on autism.
What are some of the symptoms of autism?
Although people with autism have a range 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 autism diagnosed?
In 2013 the American Psychiatric Association updated the criteria for diagnosing ASD in the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5).
The new DSM-5 criteria encourage diagnosis prior to school age. Children with ASD must show symptoms from early childhood, although the symptoms may not be recognized until later. Sometimes symptoms are not evident until children are old enough to be in social situations that challenge their capacity to respond.
What causes autism?
Although recent studies indicate that the rate of ASD is rising, 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
- Maternal lifestyle and environmental risk factors for autism spectrum disorders (International Journal of Epidemiology)
- Disentangling the heterogeneity of autism spectrum disorder through genetic findings (Nature Reviews, Neurology)
What are some of the environmental factors researchers believe may be associated with autism?
The clearest evidence for environmental risk factors in ASD 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
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 near a freeway at birth were twice as likely to develop autism4 A distance of 1,014 feet, or a little less than 3.5 football fields, was considered near a freeway.
- Building on those findings, researchers reported an increased risk of autism5 associated with exposure to traffic-related air pollution and regional air pollutants in 2013.
- 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.6
Researchers funded by NIEHS discovered that problems with the immune system, as well as maternal conditions during pregnancy, are linked with higher autism risk.
- Researchers found that some mothers of autistic children have antibodies that may interfere with fetal brain development in ways that could lead to autism.7
- Maternal diabetes and obesity may have a role in autism. These metabolic conditions, which are associated with inflammation, both have strong links to the likelihood of having a child with autism or another developmental disability.8
- Additional studies have shown that maternal inflammation during pregnancy may be linked to autism. Elevated levels of inflammation, which can come from an infection, were assessed by measuring C-reactive protein in the mother’s blood. This finding may help to identify preventive strategies.9
According to NIEHS-funded research, prenatal vitamins may help lower autism risk.
- Women were less likely to have a child with autism if they took a daily prenatal vitamin during the three months before pregnancy and the first month of pregnancy, compared to women not taking the supplements. This was more evident in genetically susceptible women or children, suggesting that a gene-environment interaction could be responsible.10
- A later study identified folic acid as the source of 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.11
Mercury and other contaminants
There continues to be concern about autism and mercury exposure. NIEHS funds research examining exposure to mercury and 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 .12
- 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 in children without autism.13
- 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.
- No link between autism and vaccines containing thimerosal, a mercury-based compound, has been found.
- Autism and the Environment (525KB)
What is NIEHS Doing?
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. Recently published findings from CHARGE showed that pregnant women who lived near farms using certain pesticides were more likely to have children with ASD or developmental delay than pregnant women who lived more than 1.5 kilometers from farms using these pesticides.
In a separate 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 while in the womb. This finding helps explain why some previous studies that focused exclusively on genetic variation and ASD development have proven inconclusive.
NIEHS involvement with the IACC
NIEHS is one of the federal members of the Interagency Autism Coordinating Committee (IACC). Dr. Linda Birnbaum, NIEHS director, is the primary NIEHS representative to the IACC and Dr. Cindy Lawler, chief of the NIEHS Genes, Environment, and Health branch, is the alternate.
NIEHS works in partnership with other members of the IACC to summarize advances in autism research and participate in strategic planning for research needs. The IACC hears directly from members of the public regarding their concerns about autism and recommendations for research needs.
Stories from the Environmental Factor (NIEHS Newsletter)
- Baby Teeth Link Autism with Metal Uptake, in NIEHS-funded Study (July 2017)
- Environmental and Genetic Factors Jointly Influence Autism-related Genes (January 2017)
- Bioinformatics Sheds Light on the Genetics of Autism (June 2016)
- Certain Fungicides May Play a Role in Autism, Neurodegenerative Diseases (May 2016)
- Asian Pacific Autism Conference Includes Focus on the Environment (December 2015)
- Autism Studies Build on past Investments and Guide Future Research (January 2014)
- New Evidence of Gene-environment Interaction in Autism (January 2014)
- Parents Are Right: Children with Autism Experience More GI Symptoms (January 2014)
- Baby Teeth Link Autism and Heavy Metals, NIH Study Suggests (June 1, 2017)
- Prenatal Inflammation Linked to Autism Risk (Jan. 24, 2013)
- Autism and The Environment Meeting Report (214KB) (September 2010)
To identify novel opportunities and mechanisms to accelerate research on environmental factors and autism, a diverse group of scientists came together on September 8, 2010 at the National Institute of Environmental Health Sciences (NIEHS) Keystone Campus in Durham, NC to share ideas and expertise.
- Healthy People 2020 - Healthy People provides science-based, 10-year national objectives for improving the health of all Americans.
- NIEHS Virtual Forum: Autism and the Environment (April 2014)
NIEHS hosted a virtual forum on autism spectrum disorders and the environment on April 22, 2014.
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