Having a head injury can be risky enough, but a new study finds people who suffered a traumatic brain injury and lived in area with exposure to the pesticide called paraquat may be three times more likely to develop Parkinson's disease.
"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, a professor of epidemiology and environmental health sciences at UCLA's Fielding School of Public Health, said in a press release.
Parkinson's disease is an incurable neurological condition that causes tremors, slowness of movement, stiffness or rigidity of the arms, legs or trunk and trouble with balance (called postural instability), according to the National Parkinson Foundation. It is caused by the loss of brain cells that produce dopamine, a natural chemical that helps regulate the body's movement.
About 50,000 to 60,000 new cases are diagnosed in the U.S. each year, adding to the 1 million already diagnosed. Parkinson's is the 14th leading cause of death in the United States.
For the study, published Nov. 12 in Neurology, researchers compared a group of 357 people with Parkinson's to 754 people without the disease. All participants lived in an agricultural area in central California, and they were asked to report any head injuries they had ever received that caused loss of consciousness longer than five minutes.
Some previous research suggested a traumatic brain injury could increase risk for Parkinson's disease, but the effect has not been seen across old studies, suggesting another factor might be at play. Using records of pesticide applications on California crops that date back to 1974, the researchers mapped out exposure to paraquats, a toxic herbicide used primarily for weed and grass control.
Ingesting the chemical can lead to serious health problems like organ failure, coma, or respiratory failure that leads to death according to the Centers for Disease Control and Prevention. Based on previous animal studies, the chemical could also affect dopamine-producing areas of the brain.
Their study found 42 of the 357 people with Parkinson's -- or 12 percent -- reported having had such a head injury, compared to 50 of the 754 people without the disease (7 percent). That means people with Parkinson's were twice as likely to have had a head injury.
But, those with Parkinson's were also 36 percent more likely to report exposure to paraquat than those without the disease. The greatest risk was seen in people who reported both head injury and paraquat exposure.
"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," Ritz said. "For example, chronic low dose exposure to pesticides may increase the risk of Parkinson's after a head injury."
An expert not involved in the research said the study supports the idea that Parkinson's could be caused by several different factors.
"Parkinson's disease is likely multifactorial in origin, with many genetic and environmental factors interacting to increase or decrease an individual's risk of developing the disease," Dr. Andrew Feigen, a Parkinson's researcher at The Feinstein Institute for Medical Research in Manhasset, N.Y. Feigen, who was not involved in the study, told CBSNews.com in an email, "It is important to note, however, that the paper is addressing risk or probability of getting PD, not cause and effect. Most people with these exposures will not develop PD, and conversely, most people with PD have not had these exposures."