Does Car Pollution Relate to Autism?

Air pollution and autism rates possibly linked in a recent study

/ Author:  / Reviewed by: Robert Carlson, M.D

(RxWiki News) With the increase in cases of autism, researchers are constantly looking for possible causes of the condition. A new study looks at whether air pollution from traffic might play a role.

The limited study compared the rates of autism among children in California to their exposure to traffic-related air pollution. They also looked at mothers' exposure during pregnancy.

The researchers found an association between air pollution exposure and autism. However, the study is very small and contains many weaknesses. More research is necessary, and the association may not have anything to do with air pollution contributing to autism.

"Ask your pediatrician about air pollution. "

The study was led by Heather E. Volk, PhD, MPH, of the Department of Preventive Medicine in the Keck School of Medicine and Children's Hospital Los Angeles at the University of Southern California.

Autism is thought by many in the research community to result from a combination of genetic and environmental factors. Dr. Volk and her colleagues sought to find out whether traffic-related air pollution might be one of these environmental factors. The researchers included 279 children with autism and 245 children with typical development in the study. The children with autism were diagnosed using multiple standard psychiatric assessments.

Using questionnaires and birth certificates, the researchers gathered all the residential addresses where the mothers of the children had lived from pregnancy through their child's first birthday. They used a mathematical model to estimate how much traffic-related air pollution probably existed at each residence.

This data was only an estimate based on where the mother and her child lived, and it did not take into account how much time or where mothers or children spent their time away from home

The results found that children living in the areas with the highest amount of estimated pollution based on the math models were three times more likely to have autism than children living in areas with the lowest estimated pollution. Likewise, women living in the area with the highest estimated amount of air pollution were twice as likely to have a child with autism.

The researchers did not find any associations with the second or third highest areas for air pollution, based on their mathematical estimates. They also did not find any difference in autism rates between children living in urban areas versus rural areas.

The researchers adjusted their findings to account for the children's gender and ethnicity, their parents' education, their mother's age and whether the mother smoked during pregnancy.

However, the adjusted numbers and the unadjusted numbers were nearly the same in all the measures the researchers reported.

They also did not adjust for father's age, which previous studies have shown may be linked to autism. They also did not adjust for mothers who smoked after their child was born.

In the graph of results shown in the study, the shape of the autism rates line did not resemble the shape of graph lines which usually show that an increasing exposure of something is related to an increasing number of disease cases.

Limitations of the study include its small size and the use of estimated approximations of air pollution for different addresses. Another limitation is that other factors could be explored that might explain the small association the researchers found.

For example, because the higher rate of autism cases were found in certain areas with the highest amount of air pollution – but not in other areas of higher pollution – there could be other factors about those neighborhoods or the women who live there that could relate to autism.

In addition, if air pollution was a strong factor relating to autism, it would be expected that rates of autism would be highest in cities and in countries with very poor air quality. However, other countries with high pollution rates have not reported epidemics of autism that exceed the cases in the US.

Glen Elliott, PhD, MD, a clinical professor at Stanford University's Department of Psychiatry and Behavioral Sciences, pointed out that a similar question may be raised regarding pollution rates in the US.

“The apparent marked upsurge in autism prevalence has understandably caused both deep consternation and a strong desire to identify possible causes. The results of this study certainly are of potential interest," Dr. Elliott said.

"However, if true, the study may raise more questions than it answers since, overall, there has been a decline in air pollution over the past several decades, not an increase," Dr. Elliott said. "Unless the authors can show a specific change in the type of air pollution, their results lead ineluctably to the question of why autism rates were not soaring in Los Angeles and other areas when smog was at its worst.”

According to an Environmental Protection Agency report looking at air pollution from 1900 to 1998, most chemicals related to car exhaust peaked in the 1970s and have been decreasing since. As the authors stated, more research will need to be done to determine if there is an actual link between traffic-related air pollution and autism.

The study was published November 26 in the Archives of General Psychiatry. The study lists its funding as the Environmental Protection Agency, grants from the National Institute of Environmental Health Sciences and matching funds from MIND Institute.

Dr. Volk also received funding from Autism Speaks to present the findings of this study at a conference. A psychiatrist who works with Autism Speaks, Geraldine Dawson, PhD, wrote an editorial accompanying this study which promotes its value. However, Dr. Dawson did not disclose her organization's funding related to the study in her editorial. Two other authors are employed by Sonoma Technology, Inc. Another author has received money from an air quality violations settlement agreement between the South Coast Air Quality Management District and British Petroleum.

Reviewed by: 
Review Date: 
November 24, 2012
Last Updated:
April 11, 2013