They found that the children with the autism disorders were 50% more likely than the non-autistic children to be born in areas with higher estimated levels of three metals and two chlorinated solvents: mercury, cadmium, nickel, trichloroethylene and vinyl chloride. No significant link was found with 14 other solvents and metals, including compounds such as lead, benzene and chromium.
The national autism rate is six children per 1,000, so a 50% increase would elevate that rate to nine per 1,000.
The biggest increase came with heavy metals including mercury, a pollutant from power plants, factories and mines that can disrupt brain development.
The Bay Area was chosen for the study because extensive data are readily available there because of a federally funded program to count and track autistic children. The region's toxic air pollution is considered typical for urban areas.
San Francisco County had the highest estimated levels of metals and solvents, including mercury, and Marin County had the lowest of those studied. But the researchers did not compare autism prevalence by county.
In their report, published online Wednesday in the journal Environmental Health Perspectives, the authors said their research "suggests that living in areas with higher ambient levels of hazardous air pollutants, particularly metals and chlorinated solvents, during pregnancy or early childhood, may be associated with a moderately increased risk of autism. These findings illuminate the need for further scientific investigation, as they are biologically plausible but preliminary and require confirmation."
The study is the first to look for a connection between autism among children and levels of hazardous air pollutants at birth. Last year, scientists who compared volumes of industrial mercury emissions in Texas with autism in schoolchildren reported a similar link.
Autism is believed to start in the womb, early in pregnancy, when the brain develops. Genetic factors determine who is susceptible, but experts theorize that environmental factors contribute.
The new study found that mercury was the "most significant correlation with autism," Pessah said, "but every family may not be affected the same way because of their genetic makeup."
Many parents of autistic children blame vaccines that contained a type of mercury called thimerosal. Expert reviews have found no link between vaccines and autism, but some scientists do not consider them definitive.
