title: Air Pollution and Autism Spectrum Disorders: Causal or Confounded?
abstract: In the last decade, several studies have examined the association between perinatal exposure to ambient air pollution and risk of autism spectrum disorder (ASD). These studies have largely been consistent, with associations seen with different aspects of air pollution, including hazardous air toxics, ozone, particulate, and traffic-related pollution. Confounding by socioeconomic status (SES) and place of residence are of particular concern, as these can be related to ASD case ascertainment and other potential causal risk factors for ASD. While all studies take steps to address this concern, residual confounding is difficult to rule out. Two recent studies of air pollution and ASD, however, present findings that strongly argue against residual confounding, especially for factors that do not vary over relatively short time intervals. These two studies, conducted in communities around the USA, found a specific association with air pollution exposure during the 3rd, but not the 1st, trimester, when both trimesters were modeled simultaneously. In this review, we discuss confounding possibilities and then explain-with the aid of directed acyclic graphs (DAGs)-why an association that is specific to a particular time window, when multiple exposure windows are simultaneously assessed, argues against residual confounding by (even unmeasured) non-time-varying factors. In addition, we discuss why examining ambient air pollution concentration as a proxy for personal exposure helps avoid confounding by personal behavior differences, and the implications of measurement error in using ambient concentrations as a proxy for personal exposures. Given the general consistency of findings across studies and the exposure-window-specific associations recently reported, the overall evidence for a causal association between air pollution and ASD is increasingly compelling.
authors.affiliation: Department of Environmental Health, Harvard T. H. Chan School of Public Health, Landmark Center, 401 Park Dr, PO Box 15677, Boston, MA, 02215, USA. mweissko@hsph.harvard.edu.|Department of Epidemiology, Harvard T. H. Chan School of Public Health, Boston, MA, USA. mweissko@hsph.harvard.edu.
authors.forename: Marc G
authors.initials: MG
authors.lastname: Weisskopf
authors.affiliation: Department of Environmental Health, Harvard T. H. Chan School of Public Health, Landmark Center, 401 Park Dr, PO Box 15677, Boston, MA, 02215, USA.
authors.forename: Marianthi-Anna
authors.initials: MA
authors.lastname: Kioumourtzoglou
authors.affiliation: Department of Social and Behavioral Sciences, Harvard T. H. Chan School of Public Health, Boston, MA, USA.
authors.forename: Andrea L
authors.initials: AL
authors.lastname: Roberts
mesh_terms: D000397:Air Pollution / Q000009:adverse effects*; D000067877:Autism Spectrum Disorder / Q000209:etiology*; D004781:Environmental Exposure / Q000009:adverse effects; D004784:Environmental Monitoring / Q000379:methods; D005260:Female; D006801:Humans; D052638:Particulate Matter / Q000009:adverse effects; D011247:Pregnancy; D011297:Prenatal Exposure Delayed Effects; D012307:Risk Factors
chemical_list: D052638:Particulate Matter
keywords: Air pollution; Autism spectrum disorders; Causality; Confounding; Epidemiology; Particles