In addition, the results may be skewed because children in urban areas have more access to doctors and clinics where they are more likely to be diagnosed, said Irva Hertz-Picciotto, an environmental epidemiologist at the University of California, Davis, who studies autism.
The new study used information from 325 mothers, all nurses from around the country, who gave birth after 1987 to a child later diagnosed with autism. The researchers divided these children into five groups based on their mothers’ estimated air pollution exposure during pregnancy and compared their autism rates to 22,000 non-autistic children born from 1987 to 2002. The U.S. Environmental Protection Agency's pollution estimates were broken down by census tract. The income and education level of the families were factored in, since they also can be linked to air pollution.
For mercury and diesel, the mothers in the highest exposure group were twice as likely to have an autistic child. Lead, manganese, nickel, methylene chloride and overall metal exposure also were associated with higher incidences of autism. Twenty-six of 180 pollutants had a significant association between exposure and autism rates.
“Since so many [pollutants] were linked to higher autism rates, we can’t tell from the study which ones might be the causes,” Roberts said.
Autism disorders are development disabilities – with a wide range in severity -- that are marked by social impairment, difficulty with communication and repetitive behaviors. Rates have been increasing, but many experts think this cannot be entirely explained by increased diagnoses by doctors.
Boys are about four times more likely than girls to be identified as having the disorder, according to the CDC. In Roberts’ study, the researchers saw associations between exposures and higher autism rates when looking at the group as a whole and at just boys. However, when they looked only at girls, there were no statistically significant links between pollution and autism.
It’s not clear why, Roberts said. Boys are more likely to have autism, so it may be easier to “push them over the threshold into autism” through mothers’ pollutant exposure, Roberts said. But both she and Kalkbrenner said the sample size of girls – 14 percent of the children with autism – is too small for the gender differences to be significant.
Previously, several industrial pollutants – methylene chloride, quinolone and styrene – were linked to higher autism rates in children born in North Carolina and West Virginia, according to a 2010 study. Pregnant mothers’ exposure to metals and chlorinated solvents was linked to increased risk of having an autistic child, according to a 2006 study of San Francisco area children.