A Breath of Not-So-Fresh Air

A paper published last week in Environmental Health Perspectives had the autism community all atwitter last week, quite literally. It was clear something was up when my Twitter feed suddenly filled up with tweets about a national study, led by Andrea Roberts and colleagues at Harvard University, “confirming the link between autism and air pollution.”

Utilizing Environmental Protection Agency records of hazardous air pollutant concentrations, the researchers examined levels of air pollution in the year and location in which women lived when they gave birth to their children. These mothers were enrolled in the Nurses Health Study II, a large sample of female nurses that has been followed since 1989. To focus their investigation, the researchers studied air pollutants previously found to be associated with autism, such as metals and diesel particulate matter. Overall, they found positive associations between perinatal exposure to several air pollutants and autism, meaning the greater the concentration of these pollutants, the greater chance of a child having autism. They also showed that some of these associations were stronger for boys than for girls.

However, this highly publicized study on air pollution is not exactly a breath of fresh air. Several things reveal weaknesses in the design and interpretation of this study:

1. On Twitter, Forbes writer Emily Willingham, made an interesting point:

Screen Shot 2013-06-26 at 8.20.42 PMAir pollution may be related to autism, just as countless environmental factors are, but it clearly isn’t a major player in its etiology, or causation (as much as I hate to use that word). For more of Emily’s thoughts on the autism/air pollution relationship, see this insightful post about a previous study.

2. While the Nurses Health Study provided a large sample, there is a significant confound with only studying children born to nurses. In an Autism Research paper, Gayle Windham and colleagues demonstrate a relationship between maternal occupational exposure to chemicals, as well as other potential toxins, and autism. Particularly, maternal exposure to disinfectants, as can be seen in nurses and other medical professionals, seemed to be related to autism.  While this finding doesn’t negate the air pollution finding, it certainly complicates the picture.

3. The children in the autism group in this study were noted as having autism, Asperger’s and PDD-NOS, or, as was explained more vaguely, “may have been on the autism spectrum.” Including multiple subtypes of autism in a study sample is common, but, again, it provides another confound, especially considering the subtype-specific findings related to smoking during pregnancy, which I discuss in this post.

4. Regarding the researchers’ interpretation of these findings, there was much to be desired. Yes, an association between air pollutants and autism was demonstrated. But why or how? In other words, what potential genetic or developmental underlying mechanisms are set into motion by air pollution exposure? For now this is just a correlational, not causational, finding. Simply stating the supposed existence of an association doesn’t provide many answers.

While an interesting concept, the autism and air pollution “link” needs further exploration, and, hopefully, a more reserved reception as future studies are published.

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Tuesday Thoughts: Autism, Hurricanes, and Mud

A big theory in autism research involves what is called prenatal stress, or maternal stress.  The idea is that a stressful event, such as a hurricane, experienced during a woman’s pregnancy may lead to an increased risk of her child having autism. Several studies supporting this idea have been published.  For example, one of my advisor’s papers, published in 2005, demonstrated that the experience of stressors during a specific time period during pregnancy (weeks 21 to 32) may be related to an increased risk of autism.  Stressors were assessed via surveys and included events like the death of a spouse or being fired from a job.

Confusingly, however, these results conflict with those of very recent studies.   It seems this idea may not be as simple as originally thought. This past June, a paper was published that did not find an increased risk of autism associated with prenatal stress.  This study examined many of the same stressors experienced by pregnant women as those of the above paper, yet the authors did not find the same results.

Results from another study, presented at the 2012 International Meeting for Autism Research, further complicate this picture.  The study examined the incidence of autism in children of mothers who experienced psychosocial stressors, such as physical abuse, during pregnancy.  The twist is that physical abuse experienced during pregnancy was not associated with an increased risk of autism.  Instead, children of women who experienced fear of their partner or physical or emotional abuse in the years just before giving birth had a higher risk of autism.

Why did two very similar studies produce opposite results? Why does it seem that physical abuse during pregnancy is not associated with autism when abuse beforehand is? Many reasons may lie beneath these conflicting and confusing results.  Potentially, gene variants associated with autism are playing a role alongside prenatal stress in causing this disorder. Or different techniques used to carry out the above studies may have contributed to the differing findings. Otherwise, the reasons are about as clear as mud.  For now, the best we can do is put on our finest analytical goggles, jump into the data, and start sorting through the mud.

Photo source: http://earthobservatory.nasa.gov/IOTD/view.php?id=79008