Not much is known about factors that cause autism, a developmental disorder that impairs social functioning in children. Though genetic links are known, environmental factors like exposure to toxic substances may also play a role. A study from University of Southern California and University of California, Davis explored the link between residential proximity to freeways and major roads and autism; proximity to roads was used as a proxy marker of traffic-related air pollution. The results showed that living near a freeway, especially during the third trimester of pregnancy, was associated with autism in the child.
Autism causes significant impairment of social interaction and communication in children. In recent years, the prevalence of autism and all autism spectrum disorders has increased. Genetics is known to play an important role in autism, but the rising prevalence indicates that environmental factors may play an important role as well. It has been shown that exposure to air pollutants (like nitrogen dioxide, black carbon, etc.) during pregnancy and early life can impair brain function. Researchers have previously demonstrated that the concentration of hazardous air pollutants, including arsenic, diesel exhaust particles and nickel, in early childhood is associated with autism. The authors of this study examined the relationship between traffic-related air pollutants and autism, and also focused on the role of timing for this exposure during pregnancy or early life.
• The authors utilized data from the Childhood Autism Risks from Genetics and the Environment (CHARGE) study, which enrolled preschool children. Three-hundred-and-four autism cases and 259 typically developing children in the 2 to 5 years age group were selected for the analysis.
• Autism was diagnosed if both the autism diagnostic observation schedules (ADOS) and the autism diagnostic interview-revised (ADI-R) were positive.
• Parents were interviewed and details of household exposures, social and medical information, and occupational and residential histories were noted. The residential history noted addresses and the corresponding dates the mother and child lived at each location, beginning three months prior to conception and extending to the most recent place of residence.
• Distance to the nearest freeway and distance to the nearest major road were used as proxies for traffic-related pollutant exposure and information from the residential history was used to estimate the exposure to traffic during the first, second and third trimesters of pregnancy.
• Living within 1,000 feet of a freeway at birth was associated with autism (odds ratio of 1.86, implying a nearly two times greater risk). When compared to individuals living farthest from the freeway (more than 3.2 miles), the risk was even greater.
• The association between living closest to the freeway and autism was observed in all trimesters, but was significant only during the third trimester where the risk factor was more than two times greater. This implied a late pregnancy or an early life effect of traffic-related air pollutants on autism.
• No consistent pattern of association of autism with proximity to a major road was evident across all trimesters of pregnancy.
The number of children evaluated in this study was small. More importantly, traffic is likely to be variable across various roads; residential history or proximity to major roads may not correctly indicate the concentration of traffic-related air pollutants. In individuals who moved during pregnancy, the level and duration of exposure would have been variable. Larger studies, in which the concentration of various air pollutants are measured and correlated to autism, are required to confirm the link that is indicated in this study. Moreover, trimester-specific analysis and early life analysis must be included in such studies, to correctly identify the critical time/period of exposure.
It is known that exposure to pollutants and toxins during pregnancy and early childhood impairs normal brain development. The link between traffic-related air pollutants and autism that has emerged from this research seems plausible. Increase in the prevalence of autism has been attributed to better diagnostic criteria and techniques, but it seems that it may also be linked to environmental factors. This study is a significant work that paves the way for further research into the link between neurodevelopmental disorders and environmental pollutants. Though this is a preliminary study, it is safe to assume that the exposure to air pollutants must be minimized during pregnancy and early life. “Examination of gene-pollution interactions may also help us learn about causal pathways involved in autism, identify potentially susceptible populations and lead to prevention strategies.”