A new study from University of Southern California researchers finds that children exposed to more air pollution had higher rates of autism. Though there is no conclusive answer about whether pollution can cause autism, the lead author says “it may be a risk factor for autism. Autism is a complex disorder and it’s likely there are many factors contributing.”
Studying 500 children California cities, the researchers found those likely exposed to the most pollution — estimated based on traffic, vehicle emissions, wind patterns, and regional data — are two to three times more likely to be diagnosed with the disorder. Some children may be more susceptible because of genetics.
TIME describes the growing body of research that links autism to pollution:
Even so, the latest study findings suggest that air pollution may be one of the best characterized environmental risk factors for autism. In an earlier study published in 2010, Volk and colleagues showed that kids with autism were much more likely than kids without the disorder to have been born to mothers living within 1,000 feet of a freeway. Other researchers have shown that kids with autism are also unusually likely to have exposure to high levels of diesel exhaust particles and metals (mercury, cadmium, and nickel) and to other air-pollutant chemicals, such as those used to make rubber, plastics, and dyes.
These associations continued to remain strong even after researchers adjusted for other characteristics, like poverty, that may also be connected to pollution. Unlike asthma, for example, autism rates are not consistently higher among lower income populations. In Volk’s study, the links between air pollution and autism risk were virtually unchanged after accounting for parents’ race and ethnicity, educational attainment, and smoking status, as well as for the area’s population density.
Some questions do remain, such as why autism diagnoses have increased since 2006 to 1 in 88 children without any major changes in pollution. Although scientists need to further examine that link, outdoor pollutants are already a known trigger in asthma, which has also become more common in recent years.