Early childhood exposure to air pollution combined with low exposure to greenness is associated with increased severity of autism symptomsExisting research indicates that air pollution increases the risk of developing autism spectrum disorder (ASD). Specifically, fine particulate matter (PM<sub>2.5</sub>) is believed to have the most harmful effects on children’s health. Additional evidence also suggests that exposure to PM<sub>2.5 </sub>is linked to more severe ASD symptoms. Conversely, research has shown that exposure to green spaces benefits healthy development during early childhood. Few studies, however, have examined how exposure to both fine particulate matter and greenness during the critical developmental period of the first two years of life might influence ASD symptoms. This study was conducted to investigate the independent and combined effects of PM<sub>2.5 </sub>and greenness exposure on ASD symptoms in early childhood.
Children with ASD were recruited from special education institutions in Changsha city, a megacity in Central China with high levels of air pollution and urbanization. Children were eligible to participate in the study if they had a formal ASD diagnosis, were older than 24 months at the time of the study and had resided in Changsha city since birth. Based on these criteria, 108 children were enrolled in the study, including 24 females and 84 males. The majority of children (75%) were diagnosed with ASD before the age of three. The Autism Behavior Checklist (ABC), completed by children’s caregivers, was used to evaluate ASD symptoms. This instrument assessed symptoms and severity across five subscales: sensory, relating, stereotypic behavior (such as repetitive movements or sounds), language, and social independence. In this study, ABC scores ranging from 31 to 67 were indicative of mild symptoms, and scores of 68 or above indicated moderate to severe symptoms. Measures for exposure to air pollution and greenness during the first two years were based on children’s home address zip codes, which were geocoded at the district level. Air pollution exposures were assessed with daily PM<sub>2.5</sub> concentration data from the China High Air Pollutants database. Exposure to greenness was evaluated using the Normalized Difference Vegetation Index (NDVI), with data obtained from Google Earth. The study examined six exposure periods during the first two years of life: 6 months after date of birth (Stage 1), 7-12 months after date of birth (stage 2), 13-18 months after date of birth (stage 3), 19-24 months after date of birth (stage 4), first year after birth (stage 5), and second year after birth (stage 6). Statistical analyses were conducted to examine the relationships between PM<sub>2.5</sub>, greenness, and ASD symptoms.
No significant relationship was found between PM<sub>2.5</sub> exposure and ASD severity, as measured by total ABC scores or subscale scores. However, the occurrence of stereotypic behavior was linked to higher levels of PM<sub>2.5</sub> exposure. Specifically, exposure to PM<sub>2.5</sub> between 13 and 18 months after birth (Stage 3) was linked to an increased likelihood of more severe stereotypic behavior symptoms. On the other hand, exposure to greenness between 19 and 24 months after birth (Stage 4) was significantly linked to a reduced risk of more severe symptoms (for overall ASD symptoms, relating, and social independence). Importantly, each 0.1-unit increase in greenness during Stage 4 was significantly linked to a reduced incidence of moderate or more severe symptoms related to social independence. Greenness exposure at 7–12 months after birth (Stage 2) and at age 1 (Stage 5) was also related to reduced challenges in social independence. Next, the combined effects of air pollution and greenness were examined. High PM<sub>2.5</sub> levels during 13–18 months after birth (Stage 3) combined with low greenness exposure at 6 months after birth (Stage 1) were linked to more severe ASD symptoms. Additionally, during Stage 3 the combination of high PM<sub>2.5</sub> and low greenness was linked with high ASD symptom severity. High levels of PM<sub>2.5</sub> during Stage 3 and low levels of greenness after Stage 1 exhibited a similar relationship with language scores. Notably, these findings revealed “synergistic effects,” suggesting that the joint effects of high PM<sub>2.5</sub> and low greenness were linked to poorer ASD outcomes than when air pollution and greenness were considered independently. Finally, results did not suggest that greenness exposure significantly influenced the relationship between PM<sub>2.5</sub> and ASD symptoms.
The study, which was the first to examine the independent and combined effects of PM<sub>2.5 </sub> and greenness on ASD symptoms, “lends support to the hypothesis that severe symptoms of ASD are linked to exposure to PM<sub>2.5 </sub>and a scarcity of green spaces.” Key findings indicate that: (1) exposure to PM<sub>2.5 </sub>is linked to a heightened risk of more severe ASD symptoms for stereotypic behaviors, and greenness is linked to reduced severity of overall ASD symptoms, and relating and social independence symptoms; (2) the second year of life may be the most sensitive exposure period; and (3) high PM<sub>2.5 </sub>levels combined with low greenness are linked with more severe ASD symptoms. The study contributes new insights into the early-life benefits of green space. Overall findings suggest that “increasing greenness while diminishing air pollution could potentially alleviate the progression of more severe symptoms in children with ASD.” The researchers caution that findings are correlational and do not establish a cause-and-effect link between air pollution and ASD.
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