Workers whose occupations involve contact with pesticides appear to have an increased risk of having Parkinson's disease, says a report published in the medical journal Archives of Neurology.

The development of Parkinson's disease related to chemical exposure was identified in the late 20th century, background information in the report says. Since then, occupations such as farming, teaching, and welding have been suspected of increasing the risk of Parkinson's disease. Associations have been inconsistent, however, and few previous studies have evaluated the direct relationship between occupational chemical exposure and disease risk.

Dr. Caroline M. Tanner, of the Parkinson's Institute, in Sunnyvale, Calif., and colleagues studied 519 people with Parkinson's disease and 511 "control" subjects who were the same age and sex and lived in the same location. Participants were surveyed about their occupational history and exposure to toxins, including solvents and pesticides.

Working in agriculture, education, health care, or welding wasn't associated with Parkinson's disease, nor was any other specific occupation studied after adjustment for other factors.

Among the patients with Parkinson's disease, 44—or 8.5 percent—reported pesticide exposure compared with 27, or 5.3 percent, of the control-group participants, and occupational pesticide exposure was associated with an increased risk of the disease.

"Growing evidence suggests a causal association between pesticide use and parkinsonism. However, the term 'pesticide' is broad and includes chemicals with varied mechanisms," the authors say. "Because few investigations have identified specific pesticides, we studied eight pesticides with high neurotoxic plausibility based on laboratory findings. Use of these pesticides was associated with higher risk of parkinsonism, more than double that in those not exposed."

Three individual compounds—an organochloride, an herbicide, and an insecticide—were associated with a more than threefold increased risk of Parkinson's. All three have been shown to have effects on neurons affected by Parkinson's disease.

"This convergence of epidemiologic and laboratory data from experimental models of Parkinson's disease lends credence to a causative role of certain pesticides in the neurodegenerative process," the authors say. "Other pesticide exposures such as hobby gardening, residential exposure, wearing treated garments, or dietary intake were not assessed. Because these exposures may affect many more subjects, future attention is warranted."