A California Cluster

Pesticides may be added to the growing list of possible autism triggers. A group of scientists in California examined the statistics on autism and proximity to agricultural fields. They obtained data from the state on pesticide application and autism records from the California Department of Developmental Services. What they found was a 19-county area, known as the Central California Valley, where the risk of autism disorders increased with the nearness to pesticide-sprayed fields.

They studied women living in this area, who could have been exposed to chemicals drifting on the wind. The authors found an autism rate of 14.3 cases per 1,000 births for mothers who lived within 500 meters of the fields, compared with the baseline risk of 6.5/1,000.

“We looked for patterns in space and time,” Eric M. Roberts, lead author of the study, says. “First trimester organochlorines popped out.”

Records from women pregnant between 1996 and 1998 were compared with pesticide records on a number of different factors: time of application, type of chemical and proximity to fields. Only one combination showed a statistically significant pattern: higher rates of autism for mothers who were exposed to a group of chemicals called organochlorines during their first trimester of pregnancy.

Organochlorines are a group of chemicals used as an insecticide on fruits, vegetables, beans, nuts and cotton. DDT is the most famous organochlorine, and all others have been banned except endosulfan and dicofol, which is chemically similar to DDT.

“Organochlorine usage is not going up,” Roberts was careful to caution. “There are several classes of pesticides that may be connected to autism. We’re looking at a million possibilities.”

Little is known about what is happening at the molecular and cellular level in patients with autism. Scientists are still unsure whether the key differences in brain development are linked to a specific hormone or neurotransmitter. So tracking down which chemical could be causing the disorder is like trying to find the proverbial needle in a haystack—while blindfolded.

“It’s not something someone’s offering as an answer,” Roberts adds. “It’s just something to keep in mind.”

Sonya Lunder, a senior analyst with the Environmental Working Group, is interested in pesticides as a potential culprit but is cautious because of autism’s tangled web of cause-and-effect.

But, Lunder points out that pesticides cannot possibly be the explanation behind the autism epidemic. Pesticides have been used widely for decades, and autism rates have only recently been on the rise.

“Everyone has been trying to sift out exposures and outcomes,” says Lunder, who named air pollution as another possible environmental stressor. “The idea that it would be one chemical trigger is just totally blown away by the complexity of the disorder.”

The Environmental Protection Agency (EPA) and the chemical industry test rigorously for side effects from synthetic chemicals, including pesticides. Tests range from short-term exposure effects on eyes and skin to birth defects to nervous system damage.

In regards to children and infants, the EPA built in a tenfold safety factor when assessing the risk of pesticide exposure. But the agency has yet to develop a sound method to study cumulative effects of pesticides, or how different pesticides with similar chemical properties could interact in the body. Their website attests that the agency is “developing a methodology for this type of assessment.”

Pesticides are products that have shifted over the years, replaced when better technology is developed and better molecules are discovered. DDT was phased out more than 30 years ago. Still, Lunder wants to keep research focused on environmental triggers, including pesticides. “It’s not just a genetic disorder,” she says of autism.

Roberts wants to look further into the potential pesticide link, too, and is applying for money from the Centers for Disease Control and Prevention and the National Institute of Health.

“We have a really impressive amount of health records and pesticide surveillance,” he says. He would like to expand the study over a whole decade, unlike the three years covered in the original paper, and also look at other categories of pesticides. Geneticists and biologists may be able to use this data to study similar developmental problems.

“That way the entire scientific community moves forward,” he says.