More Evidence Links Autism to Air Pollution

Veronica Hackethal, MD

October 22, 2014

New research adds to the growing body of evidence linking traffic-related air pollution to the development of autism.

The study, conducted by investigators at the University of Wisconsin-Milwaukee, is the first to examine associations between autism and air pollution in North Carolina.

Its results support those from past studies in California, even though the two states have different climates and weather patterns. The study also linked exposure to air pollution during the third trimester of pregnancy, in particular, to autism in the offspring.

"The evidence is suggesting that some component of vehicle emissions may be associated with autism spectrum disorders," first author Amy Kalkbrenner, PhD, MPH, told Medscape Medical News.

The study has not definitively proven that some component of air pollutants is actually responsible for the development of autism, Dr Kalkbrenner pointed out, but the results add to the evidence pointing in that direction.

"There is now a wealth of studies showing that systemic inflammation in the body may be responsible for the early brain damage that results in autism," said Dr Kalkbrenner. "We also know that exposure to air pollution can cause this body-wide inflammatory response. This could very well change the way the brain is developing."

The study was published online on October 20 in Epidemiology.

Lack of Neurotoxicity Testing?

Previous research, including the 2010 Childhood Autism Risks from Genetics and the Environment (CHARGE) study, by researchers at the University of California, Los Angeles, has shown associations between air traffic pollution during pregnancy and autism in offspring. Moreover, laboratory studies have linked inhaled pollutants to inflammation.

According to current estimates, more than 80,000 man-made chemicals exist in the environment and can be found in the air, drinking water, food, and house dust, said Dr Kalkbrenner.

"There are no requirements that these chemicals be tested for neurodevelopmental toxicity before they're put on the market," said Dr Kalkbrenner.

"There is a great deal that we don't know. Not to increase fear, but there are sound reasons to be concerned about some of these chemicals."

One reason is that during pregnancy, the fetus' developing organ systems are especially vulnerable to potential toxins. Brain connectivity also develops during this time, and abnormalities in synaptic connections have been suggested as a cause of autism.

In the study, researchers looked at exposure to particulate matter (PM10), which varies by geographical location and season. One of the main sources of it appears to be traffic emissions, said Dr Kalkbrenner.

Researchers combined data from autism surveillance systems with birth records for children born in the mid to late 1990s, covering 88,000 children in North Carolina and 77,550 children in the San Francisco Bay region.

They identified children with autism up to age 8 years using the Autism and Developmental Disabilities Monitoring Network, sponsored by the Centers for Disease Control and Prevention. They compared the children with autism to children randomly sampled from birth records who were born in the same counties and years.

Researchers assessed particulate matter exposure from preconception to the first birthday, basing this on the child's birth certificate address. They also used air pollution regulatory monitors to assess PM10 concentrations, as well as a multimethod approach that enabled them to look at PM10 exposure during certain weeks of pregnancy.

The analysis included almost 1000 children with autism. Exposures to PM10 were highest in children in North Carolina born during summer and in children in California born during fall and winter.

Those exposed to PM10 during the third trimester had the greatest odds of developing autism, compared with the second and first trimesters (OR, 1.36, 0.97, and 0.86, respectively). The results took into account other variables that could affect risk for autism, such as maternal education and age, household income, race/ethnicity, urbanization, season of birth, and seasonal variations in PM10.

Strengths and Limitations

Although the researchers performed careful statistical analyses, one potential problem may be that this study relies on group data, Bennett Leventhal, MD, professor of psychiatry and deputy director of child and adolescent psychiatry at the University of California, San Francisco, told Medscape Medical News.

The results may be subject to "ecological fallacy," said Dr Leventhal, which assumes that a group of people have been at the same place at similar times and had similar exposures. Proving this requires analysis on the individual level.

"Until there is a study with individual-level data and reasonable biomarkers of individual exposures and the timing of those exposures, it is almost impossible to make any sound conclusions about the role of any toxin in the etiology of any neurodevelopmental disorder," added Dr Leventhal, who was not involved in this research. "I wish this were different, but I am afraid that is where we are."

Other factors not accounted for in this study could influence exposure to air pollution, Dr Leventhal added, such as time spent outside or at home, air conditioning, and medical conditions in parents or children.

However, one of the study's strengths is that this group of researchers is "very experienced," commented Robert L. Hendren, DO, professor and director of the Autism and Neurodevelopment Program at the University of California, San Francisco. Although not all possible factors could be analyzed, the researchers "carefully examined" many factors that could explain their findings, Dr Hendren added.

"This association [between traffic-related air pollution and autism] points to a gene-by-environment interaction that may be mediated by an inflammatory process," Dr Hendren explained.

"The findings suggest an inflammatory, immune, oxidative stress influence in the third trimester of pregnancy that could be a prevention target. Additional work is needed," he added.

The authors, Dr Hendren, and Dr Leventhal report no relevant financial relationships.

Epidemiology. Published online October 20, 2014. Abstract

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