TUESDAY, Nov. 13 (HealthDay News) -- People who have had a head injury at some point in their lives and also have lived or worked near areas where the herbicide paraquat was used may have a threefold increased risk of developing Parkinson's disease, according to a new study.
Paraquat, which can be deadly to humans and animals, is commonly used on crops to control weeds.
The study participants all lived in an agricultural area of central California. The group included about 350 people with Parkinson's disease and about 750 people without the disorder. They reported whether they had ever had a head injury that resulted in a loss of consciousness for more than five minutes.
The researchers also used state data and other sources of information to assess paraquat use within about one-third of a mile of the participants' homes and workplaces.
The investigators found that Parkinson's disease patients were nearly twice as likely as those without the disease (12 percent versus 7 percent) to have suffered a head injury that resulted in loss of consciousness for more than five minutes.
In addition, Parkinson's disease patients were 36 percent more likely than those without the disease to have had exposure to paraquat, according to the study, which appears in the Nov. 13 issue of the journal Neurology.
"While each of these two factors is associated with an increased risk of Parkinson's on their own, the combination is associated with greater risk than just adding the two factors together," study author Dr. Beate Ritz, of the Fielding School of Public Health at the University of California, Los Angeles, said in an American Academy of Neurology news release.
"This study suggests that the physiological process that is triggered by a head injury may increase brain cells' vulnerability to attacks from pesticides that can be toxic to the brain, or the other way around," she explained. "For example, chronic low-dose exposure to pesticides may increase the risk of Parkinson's after a head injury."
Although the study found an association between development of Parkinson's and previous head injury and paraquat exposure, it did not prove a cause-and-effect relationship.
The U.S. National Institute of Neurological Disorders and Stroke has more about Parkinson's disease.