Women who live near California farm fields sprayed with organochlorine pesticides may be more likely to give birth to children with autism, according to a study by state health officials to be published today.
The rate of autism among the children of 29 women who lived near the fields was extremely high, suggesting that exposure to the insecticides in the womb might have played a role. The study is the first to report a link between pesticides and the neurological disorder, which affects one in every 150 children.
But the state scientists cautioned that their finding is highly preliminary because of the small number of women and children involved and lack of evidence from other studies.
"We want to emphasize that this is exploratory research," said Dr. Mark Horton, director of the California Department of Public Health. "We have found very preliminary data that there may be an association. We are in no way concluding that there is a causal relationship between pesticide exposure of pregnant women and autism."
The two pesticides implicated are older-generation compounds developed in the 1950s and used to kill mites, primarily on cotton as well as some vegetables and other crops. Their volumes have declined substantially in recent years.
Examining three years of birth records and pesticide data, scientists from the Public Health Department determined that the Central Valley women lived within 500 meters, or 547 yards, of fields sprayed with organochlorine pesticides during their first trimester of pregnancy. Eight of them, or 28%, had children with autism. Their rate of autism was six times greater than for mothers who did not live near the fields, the study said.
Susan Kegley, senior scientist of Pesticide Action Network North America, a San Francisco-based advocacy group, said the report adds to an existing body of evidence that endosulfan and dicofol, already banned in some countries, are harmful.
"This is one of the first papers that links use of pesticide to incidence of a disease, and autism in particular," she said. "The findings are very strong. This is a sixfold risk factor in comparison to someone who is not exposed. There aren't too many studies that come out like that."
Even though small numbers of children were involved, "it is still one of those things that make you sit up and pay attention," she said.
The findings suggest that 7% of autism cases in the Central Valley during the years studied -- 1996 through 1998 -- might have been connected to exposure to the insecticides drifting off fields into residential areas. Births during those years were analyzed because children born later might not yet be diagnosed with autism.
Children with autism spectrum disorders have impaired social and communication skills. The causes are unknown, but because diagnoses have been increasing, scientists have been exploring various environmental factors, including children's vaccines and chemical pollutants.
"The good news is we've used a new research technology to generate hypotheses and possible associations, so we are making progress in the battle to get more information" about the cause of autism, Horton said.
The goal of the study was to "systematically explore the general hypothesis that residential proximity to agricultural pesticide applications during pregnancy could be associated with autism spectrum disorders in offspring," the authors wrote in their study, published online today in the scientific journal Environmental Health Perspectives.
The scientists collected records of nearly 300,000 children born in the 19 counties of the Sacramento and San Joaquin river valleys. Of those children, 465 had autism. The scientists then compared the addresses during pregnancy to state records that detailed the location of fields sprayed with several hundred pesticides.
For most pesticides, no unusual numbers of autism cases were found, but the exception was a class of compounds called organochlorines. Most, including DDT, were banned in the United States several decades ago because they were building up in the environment. Only dicofol and endosulfan remain.
The autism rate was highest for children of those mothers who lived the closest to the fields and it declined as the distance from the fields increased.
There is no other human or animal evidence that the two chemicals can cause autism. But both affect nerves and the brain -- and cause reproductive effects and alter hormones in animal tests. In addition, dicofol is a possible human carcinogen.
The scientists concluded that "the possibility of a connection between gestational exposure to organochlorine pesticides and autism spectrum disorders requires further study."
A July report by the state Department of Pesticide Regulation said endosulfan can spread far from fields via the air and expose the public, based on air monitoring in Fresno, Monterey and Tulare counties. The agency is likely to designate endosulfan as a toxic air contaminant soon, and dicofol could follow. That designation triggers a review by the agency to see whether steps should be taken to minimize the chemicals drifting off fields into nearby communities.
Glenn Brank, spokesman for the pesticide agency, said officials there are "very interested" in the new autism data but say that "more work" on the potential link is needed before it can carry much weight in assessments of the chemicals' risks.
The two insecticides are now used much less often than in the years in which the possible connection to autism was found. As a result, there is less likelihood that pregnant women are exposed today. Nearly 774,000 pounds were applied in 1996, compared with 277,000 pounds in 2005, down nearly 64%, according to state records.
"In the past couple years, the bottom has dropped out of these two," Brank said.
Insects have built up resistance and cotton farmers have switched to new compounds.
The two chemicals are not found in household or yard pesticides. Traces are found in food, but the study looked only at possible exposure from the air. The chemicals are used most extensively in Fresno, Kings, Imperial and Tulare counties. Dicofol is mostly used on cotton, oranges, beans and walnuts. Endosulfan is used primarily in tomato processing and on lettuce, alfalfa and cotton crops.