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Environmental Factors Associated with Autism Spectrum Disorders: A Scoping Review
Autism Spectrum Disorders (ASD) is a major public health concern with the etiology of ASD as it relates to the role of environment remains largely unknown. Over the past few decades, a large body of empirical literature is accruing regarding the role of the environment and ASD. Given the vast literature on this topic, a scoping review was conducted to assess the state of knowledge and identify research gaps relating to the role of the environment and the development of ASD.
Objectives:
The specific objective of the scoping review is to identify the environmental exposures of the preconception, prenatal, perinatal, postnatal, neonatal, and early-life periods that may be associated to an increased or reduced risk of autism spectrum disorders.
Methods:
The scoping review on ASD-related environmental exposures followed the Arksey and O’Malley’s scoping framework. Electronic indexes of MedLine, PsychInfo, and ERIC were used to perform using an a priori search strategy. Searches were limited to articles published between January 1, 2003 and July 12, 2013. Articles eligible for inclusion dealt with environmental factors associated with ASD, Pervasive Developmental Disorders (PDD) or its sub-classifications of autism and Asperger’s syndrome. Environmental factors include biological (e.g., infections, parental conditions), chemical (e.g., metals, air pollution, pesticides), nutritional (e.g., vitamins, folic acid), and social (e.g., SES, stress). Screened articles were excluded if epidemiological associations referred to co-morbidities, the result of living with an ASD, biochemical pathways not directly related to etiology, animal models or strictly genetic studies.
Results:
Between January 2013 and July 2013 there were over 50,000 publications on autism related disorders. Applying environmental factors criteria resulted in 4,844 publications (MedLine: n= 2,542, PsychInfo: n=2,008, ERIC: n=294). After removing duplicates and ineligible studies (e.g., mechanism/pathways), 620 studies were available for review. Of the studies and reviews published, most were based on environmental chemicals (e.g., air pollution, metals) closely followed by biological (e.g., vaccines, pregnancy complications, parental characteristics, immune abnormalies), and nutritional factors (e.g., vitamins). While the causal role of most environmental risks factors remains unclear, prenatal exposure to air pollutants, particularly from nitrogen oxide, particulate matter, and other air pollutants has been consistently associated with increased risk of ASD.
Conclusions:
The large body of literature reflects the interest and potential important role of the environment on the incidence of autism. Results of this scoping review were helpful in assessing the extent, range and nature of research activity to date and will be useful in planning of future directed studies.