The authors used the state cancer registry to recruit 173 white and Latino seniors in Tulare, Fresno and Kern counties who had been diagnosed with prostate cancer between August 2005 and July 2006. They compared them with 162 men without prostate cancer, found through Medicare and tax records.
Researchers then traced where the men lived and worked from 1974 to 1999 and compared those locations with state records of pesticide application. Those who lived within 500 meters of places where methyl bromide, captan and eight other organochlorine pesticides had been applied, they found, were more likely to have developed prostate cancer.
"This is some evidence that we're doing a very bad job of controlling how you apply pesticides," said Myles Cockburn, an associate professor of preventive medicine at USC's Keck School of Medicine who was among the authors of the study published this spring in the American Journal of Epidemiology.
The researchers chose to examine prostate cancer in part because, unlike other forms of cancer, the risk factors are relatively few, Cockburn said.
They chose to focus on residential rather than occupational pesticide exposure because of the Central Valley's demographics and because they suspected that previous studies were skewed toward those who handle pesticides, a population also more likely to have worn protective gear, he said.
"California's Central Valley has by far the largest use of pesticides and the largest population potentially exposed to them in the United States," he said.
The study was funded by the National Cancer Institute, National Institute of Environmental Health Sciences and Department of Defense Prostate Cancer Research Program.
Critics questioned the study's findings and definition of pesticide exposure.
"Pesticide use doesn't equal pesticide exposure of bystanders," said Robert Krieger, a toxicologist at UC Riverside, who said "contact with potential for absorption" would be more accurate.
"Just because you lived in the vicinity of an application doesn't guarantee you were exposed," Krieger said. He questioned whether the men involved in the study could accurately report details that might skew the results, such as whether they used household pesticides.
"The attempts to reconstruct exposure in retrospect is extremely uncertain," he said, adding that he and other researchers have focused on workplace pesticide exposures that they can more easily quantify and trace.
Krieger said state air monitoring has confirmed that so-called pesticide drift occurs from fields into neighborhoods but that people working with pesticides face much greater exposure than bystanders.
Officials at the California Department of Pesticide Regulation had not reviewed the USC study Tuesday, but a spokeswoman defended efforts to guard against pesticide drift.
"California's drift regulations are the toughest in the nation," said Lea Brooks, a department spokeswoman. "They include buffer zones to address urban encroachment of agricultural lands and labor-intensive crops."
She said the department works with county agricultural commissioners to track pesticide drift, immediately investigate reports and issue civil penalties.