"We've known for a long time that air pollution is bad for our lungs, and especially for children. We're now beginning to understand how air pollution may affect the brain."
Researchers examined 500 children, mostly boys, who lived in Los Angeles, San Francisco and Sacramento. Half the children had autism. They studied mothers' addresses from birth certificates and residential history then examined data collected by the Environmental Protection Agency from those areas, looking at levels of nitrogen dioxide, as well as fine (PM2.5) and course (PM10) particulate matter. They found that the finest particulate matter, even far from freeways, enters the lungs and blood, and eventually finds its way to the brain.
"Our study found that local estimates of traffic-related air pollution and regional measures of PM2.5, PM10, or nitrogen dioxide at residences were higher in children with autism," researchers wrote.
The new study builds on previous work released in 2010 that found an association between the risk of autism and living within 1,000 feet of a freeway, though Volk would not specify which freeways were studied.
But Volk and other researchers cautioned that their work is not a definitive answer to why more children are being diagnosed with autism.
In 2006, 1 in 110 children in the United States was diagnosed with autism. Now the latest figures from the CDC indicate the rate has increased to 1 in 88.
Genetics, nutrition, and other environmental factors also must be considered. The federal Centers for Disease Control and Prevention estimates that 1 in 88 children are born with an autism-spectrum disorder.
Autism occurs in all racial, ethnic and socioeconomic groups, but is more common among boys. Researchers have looked into various causes, from the age of a child's parents to vaccines. Volk said her team will next look closer at the data to determine how strong the relationship is between environment and autism.
"I think this research is important because we've all talked about how autism is increasing and we don't know why," said Dr. Leslie Richard, pediatrician at the Boone Fetter Clinic at Children's Hospital Los Angeles.
"This study shows there are associations between autism and the environment."
But she added that while research continues, prenatal care should be a mother's priority, no matter where she lives.
Volk and others acknowledged that air quality has improved in Southern California since the 1970s. They also point out that pollutants remain concentrated close to traffic corridors.
Previous studies have linked near-roadway pollution to asthma, lung disease, bronchitis, emphysema and even a hardening of the arteries.
But the quality of those pollutants, how poisonous they are and the concentration remain a mystery because of a lack of consistent and detailed data, said Diane Bailey, a senior scientist with the environmental and health program for the National Resources Defense Council. Earlier this year, the council filed a lawsuit against the South Coast Air Quality Management District, to make the agency install air quality sensors next to busy freeways. The lawsuit is pending.
"So many people live near or next to freeways and it's sort of an unspoken, major hazard that affects the air," Bailey said.
The new research was supported in part by Autism Speaks, an advocacy group that funds research into causes, prevention and treatment.
Alycia Halladay, a senior director for environmental and clinical sciences for Autism Speaks, said the study helps researchers understand environmental risk factors that contribute to the risk of autism, but that the results need to be replicated.
"Researchers now know that there is not one single environmental factor that causes autism," she said. "In fact, there are most likely many different factors working together with genetics to cause a diagnosis. Air pollution may be one of these factors."