Autism

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

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 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

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.

• 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

Nutrition

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.