Since the early-1990s, scientists have known that farmers and other field workers are more likely to succumb to Parkinson’s disease because of their exposure to pesticides and other agricultural chemicals. But these studies fell short on showing a causal relationship between pesticides and the debilitating neurodegenerative disorder.

So, researchers turned to rodent models to prove the link. In the last decade, researchers found that three bug and weed killers promoted neurodegeneration in mice. And now, an independent team has validated those findings in a large epidemiological survey in humans.

The team led by Beate Ritz, an epidemiologist at the University of California-Los Angeles, estimated the average 25-year pesticide exposure for around 700 Californians, about half of whom developed Parkinson’s. Reporting in the European Journal of Epidemiology, the researchers found that people who lived or worked near farmlands treated with two commonly used agricultural fungicides – ziram and maneb – as well as the herbicide paraquat were three times more likely to develop Parkinson’s disease than those Parkinson’s disease than those were not exposed to these agricultural chemicals.

The results follow a previous study of residents from California’s Central Valley showing that people who lived near fields treated by two of these chemicals were […]