There is a connection between exposure to air pollution in utero and autism. Research is ongoing into the subject, but currently, air pollution is viewed as a potential risk factor for autism. 

Air Pollution as a Risk Factor for Autism

The question of the strength of the connection between autism and air pollution is a heavily discussed one, especially when it comes to the correlation between the risk of autism spectrum disorder and perinatal exposure to ambient air pollution.

Researchers writing in the Current Environmental Health Reports journal note that a decade’s worth of studies on the subject have found consistent associations between different factors of air pollution, and place of residence and socioeconomic status, being related to autism spectrum disorder, to the point where pollution is broadly grouped with “other potential causal risk factors for ASD.”

Two studies in particular stand out. One, published in the Environment International journal, followed the progress of a large group of children in Shanghai (124 children with autism and 1,240 neurotypical children) from birth to 3 years old. Researchers discovered that exposure to fine particles that emanated from vehicular exhaust, industrial emissions, and other sources of outdoor air pollution increased the risk of developing autism spectrum disorder by as much as 78%. 

Commenting on the study, one of the authors acknowledged that while the causes of autism remain complicated and not yet understood, it has been largely accepted that environmental factors have a role to play in the likelihood of ASD developing in utero. 

The theory behind this is that the developing brains of young children, in gestational states or infancy, are more vulnerable to the exposure of pollutants in their environments and atmospheres. A significant body of research has indicated that the presence of toxic exposures can impact the immune system and the function of the brain. 

The implications of the connection are huge. According to the World Health Organization, up to 4.2 million people die every year as a result of breathing polluted air. Such pollutants are factors in the high rates of disease and premature deaths in countries that have poor environmental standards and densely populated urban areas, like India and China. 

Small Exposures & Delayed Learning

Even in countries that have more environmental regulations, like Australia, air pollution from industrial outputs and consuming fossil fuels has been blamed for 3,000 premature deaths a year. 

An associate professor at Monash University’s School of Public Health and Preventative Medicine was quoted by Science Alert as saying that no amount of air pollution is safe, and that “even exposure to very small amounts of fine particulate matter” might play a role in the development of delayed learning. 

The study in Shanghai found that the smaller the airborne particles, the more likely that they would be able to enter the bloodstream through the lungs.

A Real Link Between Autism & Air Pollution

Another study, this one published in JAMA Pediatrics, noted that an increase in autism rates might have a partial connection to air pollution from vehicular traffic. Researchers examined 132,256 births in Vancouver, Canada, and found a link between exposure to highway pollution in utero and a childhood diagnosis of autism. 

A third study, this one in Environmental Epidemiology, assessed 15,000 infants born between 1989 and 2013 in Denmark. The study discovered a link between exposure to toxic pollutants during the first months of life and a later diagnosis of autism spectrum disorder. 

In all cases, researchers clarified that more work needed to be done to determine the full nature of the connection between autism and air pollution. However, an associate professor and environmental epidemiologist at the University of Wisconsin, Milwaukee, who was not a part of either study, told STAT that the sheer accumulation of research — all of which is in significant agreement — “suggests that the link is real.”

It is important not to cause undue alarm, she noted, but it is equally important to let young and expecting families know that there is a danger that pollution from cars and trucks can pose a risk for the development of autism.

Micropollutants & Fetal Development

While high levels of exposures to pollution have been implicated in early fetal death, where pregnancies do not survive, low-level exposures might have a more subtle effect on brain development. This might be one of the triggers for the development of autism, especially if other factors, such as genetics, are present. 

However, Vancouver and Denmark are both known for their relative clean air quality, even as rates for autism diagnoses have risen. A professor at Simon Fraser University in British Columbia, Vancouver, suggested that the composition of air quality across the world  has changed so much that even in more favorable conditions, there are enough micropollutants in the air to affect fetal development.

Shipping & Pesticides

Traffic is often cited as the main contributing pollutant to autism, but it is not the only source. Researchers conducting the Denmark study noted that sulfur dioxide, potentially coming from the shipping industry, was also linked to autism. The American Journal of Psychiatry also published the reports of a study in 2018 that connected a mother’s exposure to specific pesticides and autism.

Scientists encourage governments to locate schools and other childcare facilities away from busy highways and traffic areas, to reduce the risk that long-term contact between pregnant women and young children will create a greater likelihood for autism spectrum disorder to develop. 

None of this research will offer any kind of proof that “pollution causes autism.” This is primarily because there can never be any research that deliberately exposes children or pregnant mothers to pollution, but also because autism’s many causes and risk factors are still not fully known. One researcher commented that while there is no definitive evidence that autism causes air pollution, “collectively, we have enough [evidence].”

How Wildfire Smoke Could Affect the Development of Autism

While the pollution from vehicles and factories is most often cited as a potential risk factor in the development of autism spectrum disorder, more natural sources of air pollution, such as the smoke from wildfires, can also play a part. At the University of Washington, researchers found that the air quality in Seattle during summer wildfires was similar to that found in locations that have poor air quality indexes due to industrial and vehicular emissions.

These researchers conducted animal experiments and observed that exposure to diesel exhaust during development could cause minute changes in the structure of the cerebral cortex in the brain. These changes are also seen in the brains of people who have autism spectrum disorder.

The study found that infants who had pre-birth exposure to nitric oxide (one of the pollutants) had a small increase in rates of autism diagnoses.

Writing in the Brain, Behavior and Immunity journal, the researchers felt that their work added to the concerns that air pollution is a “possible etiological (contributing to the development of a medical condition) factor for developmental and neurodegenerative disorders.”

The studies found that exposure to pollutants increases neuroinflammation and decreases the production of a protein that activates a signaling pathway that is needed for the proper positioning of neurons in the brain. Both phenomena are typical with autism spectrum disorders.

Exposure During Pregnancy

Researchers out of the Harvard School of Public Health found that children whose mothers were exposed to high amounts of fine particulate pollution late in their pregnancies were twice as likely to develop autism spectrum disorder, compared to children of mothers who experienced no such exposure and breathed cleaner air.

The risk increased with the degree of exposure to the fine particulates that are emitted by industrial smokestacks, vehicles, and fires.

Past research has also supported the theory that there is a link to a pregnant woman’s environmental and atmospheric health and the risk of autism. Specifically, if an expectant mother was living near a freeway during her third trimester, the risk of autism in her child would double.

On the other hand, there was no connection between autism and exposure to fine particulate pollution before pregnancy, during early pregnancy, or after the child was born. The window of risk appeared to be exclusively during the third trimester.

Even as the rate of autism diagnoses has skyrocketed (from 1 in 150 children in 2000, to 1 in 68 children in 2010, according to the American Psychological Association, debate remains whether this is due to greater incidence of autism or simply greater awareness of the disability.

Even with this uncertainty, the dramatic increase in incidence has encouraged researchers to look at environmental causes. While autism has a genetic basis, the sheer rise in the numbers cannot be explained by genetic factors alone.

Nonetheless, epidemiologists concur that an expectant mother’s exposure to air pollution does increase the possibility that her child might develop autism.

How to Assess the Air Quality in Your Area

If you are concerned that the quality of air in your area is polluted enough to pose health risks for your family, you can look at the World Air Quality Index Project, a nonprofit group with the stated goal of providing information about the air quality of major cities around the world. Many local news organizations and weather stations also report on current air quality levels of their respective areas. There are also a number of smartphone apps that can give you a measurement of the air quality index specific to your location.

You can also talk with your doctor or pediatrician about environmental concerns. Mitigating the effects of pollution from roadways could entail wearing a mask or moving to a different area if possible. However, the Environmental Health journal points out that the ability (or lack thereof) to move or commute to get away from freeways and industrial areas is directly linked to socioeconomic levels. Women and families at lower income levels do not often have this option.