A new Canadian study has found that exposure to certain air pollutants during pregnancy and early childhood may increase the risk of autism spectrum disorder (ASD) in children.
The research, published in JAMA Network Open, was led by a multi-institutional team that included scientists from Health Canada.
It examined nearly 2.2 million children born in Ontario over a 20-year period and identified more than 19,000 autism diagnoses.
Scientists have long known that breathing in fine particulate matter — tiny particles in the air smaller than 2.5 micrometers (PM2.5) — can harm health. These particles come from sources such as car exhaust, industrial emissions, and burning wood or fossil fuels.
Earlier studies have suggested links between PM2.5 exposure and developmental conditions, including autism, but most research treated PM2.5 as a single category rather than looking at its different chemical ingredients.
In this new study, researchers explored whether specific components of fine particulate matter — such as sulfate, ammonium, nitrate, and black carbon — might be more harmful than others.
They also examined the timing of exposure during pregnancy to find out which stages were most sensitive. In addition, they considered how early-life exposure to ozone, a gas formed when sunlight reacts with pollution, might contribute to autism risk.
To estimate exposure, the team used a combination of satellite data, chemical transport models, ground-based air quality monitors, and land-use maps.
They calculated weekly air pollution levels for each mother based on her postal code, from conception through 36 weeks of pregnancy, and for the child’s first year of life. This allowed them to identify which pollutants and time periods were most strongly linked to later autism diagnoses.
The results showed that prenatal exposure to certain components of PM2.5 — particularly sulfate and ammonium — was linked to a higher risk of autism.
The risk was most pronounced in the second and third trimesters, roughly between weeks 21 and 36 of pregnancy. Children whose mothers were exposed to higher levels of these substances had about a 10 to 15 percent greater chance of developing autism.
Ozone exposure during the first year of life also appeared to increase autism risk by around 9 percent. By contrast, other pollutants like black carbon, sea salt, and organic matter did not show a significant link after researchers accounted for other factors such as socioeconomic conditions and parental health.
Interestingly, the associations were stronger in urban areas compared to rural ones, likely due to higher air pollution levels. The link was also more noticeable among boys than girls — consistent with the fact that autism is diagnosed more often in males.
In lower- and middle-income neighborhoods, and in areas with higher proportions of racialized or newcomer populations, the estimated risks were more pronounced, highlighting environmental inequality in exposure.
According to the authors, these findings suggest that certain pollutants — rather than overall air quality — may play a specific role in brain development during pregnancy.
They note that sulfate and ammonium are common byproducts of industrial pollution and vehicle emissions, which can accumulate in fine airborne particles small enough to enter the bloodstream.
The study supports the idea that the developing brain is particularly vulnerable to environmental exposures during critical stages of pregnancy. It also underscores the importance of clean air policies and pollution control, especially in urban centers and for expectant mothers.
While the research cannot prove cause and effect, it adds to growing evidence that prenatal and early-life air pollution can have long-term effects on children’s neurological development. More studies are needed to understand the biological mechanisms involved and to identify strategies for reducing risk.
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The study is published in JAMA Network Open.
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