Exposure to air pollution during pregnancy is linked to an increased risk of autism spectrum disorder, according to the results of a new study reported by CNN.
Pregnant women in Vancouver who were exposed to the highest level of environmental nitric oxide, an airborne, traffic-related pollutant, were more likely to give birth to children later diagnosed with autism, the researchers say.
Lief Pagalan, lead author of the study and a member of the faculty of health sciences at Simon Fraser University, cautions that the study, published Monday in the medical journal JAMA Pediatrics, showed only an association between prenatal exposure to nitric oxide and autism rates. It did not prove that air pollution caused autism.
Experts emphasize that the exact causes of autism remain unknown, and some say the researchers in this study did not analyze every potential risk factor.
The research, which analyzed the records of 129,436 children born in Vancouver between 2004 and 2009, "adds to the growing concern that there may be no safe levels of exposure to air pollution," Pagalan said in an email.
"Not only did we have access to rich data, enabling us to develop one of the largest studies to date, but we were also able to conduct this study in a city with relatively lower levels of air pollution," he said.
How was the research conducted?
"We analyzed air pollution data in Vancouver over the same period to assess air pollution exposures in the pregnant woman," he said. "Their children were followed up for at least 5 years to see if they were diagnosed with autism spectrum disorder."
Overall, about 1% of the kids were diagnosed with autism by age 5, the researchers found. They compared autism rates among the children of women who had been exposed to the least amount of air pollution during pregnancy against rates among the children of women exposed to the most.
All three measures of air pollution (particulate matter, nitrogen dioxide and nitric oxide) showed a similar association with autism.
What were the results?
The odds of developing autism among children prenatally exposed to higher levels of PM2.5 (particulate matter less than 2.5 micrometers in diameter) were 1.04%, the odds of autism in children exposed to higher levels of nitrogen dioxide were 1.06%, and the odds of autism in children exposed to higher levels of nitric oxide were 1.07%. This final increased risk proved to be "statistically significant," which means it crossed the line from random chance and shows a true relationship.
Though this study builds on previous research showing a link between air pollution and autism, other studies have yielded different results:
Studies in the United States, including one in Los Angeles County, have shown that living close to a highway where air quality is poor may be a possible trigger of autism, yet three European studies -- including one that looked at Sweden, the Netherlands, Spain and Italy -- have not proved an association, the authors of the new study noted.