1. Exposure to small particle air pollution during pregnancy was associated with a dose-dependent increased risk of childhood autism.
2. The association was greatest for exposure during the third trimester.
Evidence Rating Level: 3 (Average)
Study Rundown: The diagnosis of autism spectrum disorder (ASD) has increased in prevalence in recent years from a rate of one in 2000 children (0.05%) in the 1970s to an estimated one in 150 children (0.67%) today. The cause of this increase has been a topic of debate, with some attributing the increase to a rise in awareness, others arguing there has been an objective increase in cases and most experts fall somewhere in between. Regardless, rates of autism in the United States country are high. While the cause of autism remains unclear, it is generally believed that both genetics and environmental exposures play a role. Air pollution contains a number of toxins that have been associated with fetal neurotoxicity and several studies have identified associations with living closer to a freeway, being exposed to diesel particulate matter and certain metals and an increased odds of having a child with autism. In this study, the largest to date, researchers assessed rates of autism and air pollution at specific time periods before, during and after pregnancy.
Women exposed to the highest levels of fine particulate air pollution during pregnancy were two times more likely to have a child with autism than those exposed to the lowest amount. This association was greatest during the third trimester. Strengths of this study included a large, well-defined cohort and exposure ascertainment at multiple time periods both during and outside of pregnancy. Results were limited by recall bias, as all data were self-reported. Additionally, all participants were nurses, a group with above-average medical literacy such that findings may not be generalizable to the greater public. Prospective investigation including objective measures of environmental exposure (e.g. urine samples) is merited.
Relevant Reading: Residential proximity to freeways and Autism in the CHARGE study
In-Depth [case-control study]: Researchers randomly selected children with autism spectrum disorder (n=245) and controls without autism (n=1522) from participants in the Nurses’ Health Study II, a prospective cohort of US female nurses followed by biennial mailed questionnaires. Autism diagnosis was based on maternal report and validated against the Autism Diagnostic Interview-Revised in a randomly selected subset of participants. Monthly averages of exposure to two sizes of environmental particulate matter (PM2.5=diameter≤2.5μm; PM2.5-10, diameter 2.5-10μm) before, during, and after pregnancy were predicted by residential addresses.
Women in the highest quartile of PM2.5 exposure during pregnancy were more likely to have a child diagnosed with autism than those in the lowest quartile (OR=2.06, 95% CI 1.17-3.63). Exposure to PM2.5 before and after pregnancy had a weaker association with autism than exposure during pregnancy. PM2.5 exposure was associated with autism in all three trimesters, with the greatest association seen in the third trimester (OR=1.49 per quartile of PM2.5 exposure, 95% CI=1.20-1.85). There was no significant association between PM2.5-10 exposure and autism.
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