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These results were part of a cascade of findings pointing to a connection between environmental toxins, especially pesticides, and Parkinson's disease. As long ago as the 1970s, epidemiologists noticed that Parkinson's was more likely to occur in people who had grown up in rural areas, especially those who had lived on farms. But they were not sure which aspect of a rural background was relevant. Living near livestock? Drinking well water? Being exposed to pesticides? "It's been very difficult to pin down an explanation," Kamel says.
Pesticides seemed the most likely culprit. "Animal models have shown that specific pesticides can cause parkinsonian changes," Kamel says, "and we have mechanistic data also" -- that is, evidence of biological processes at the level of the interaction between brain cells and the chemicals in common pesticides -- that can explain how a cause-and-effect relationship might work. "To the degree we understand the neurological mechanisms that may be related to Parkinson's disease," Kamel says, "it seems that certain specific pesticides might play a role."
"Despite remaining uncertainties and data gaps," wrote the authors of the 2008 report by the Science and Environmental Health Network -- Jill Stein, Ted Schettler, Ben Rohrer, and Maria Valenti -- "the body of evidence linking pesticide exposure to Parkinson's disease fulfills generally accepted criteria for establishing causation." When combined with "extensive laboratory animal data" specifying the underlying biology of this relationship, they wrote, "collectively, this evidence supports the conclusion that pesticide exposures can cause Parkinson's disease in some people."
Like most other population studies, this one has no way of proving that, for any one individual, X definitely led to Y -- that Jackie Christensen's early-onset Parkinson's disease, for instance, was caused by her exposure to pesticides as a teenager. To Christensen, however, the causal connection is clear. Growing up in rural Minnesota, she spent summers working on local farms. In her early teens, this meant engaging in a practice known as "walking beans." A pickup truck would drop off a bunch of youngsters, including Christensen, at one end of a field, and they would walk the rows of soybeans, weeding as they went. Later, Christensen and her friends rode a "bean buggy," a rig attached to the front of a tractor from which they would spray the herbicide Roundup, sometimes dyed purple so they could see where it was landing, carefully aiming for the weeds and trying to avoid the beans. Often she was dressed in nothing more than a bathing suit and a baseball cap. "I had a great tan those summers," she wrote in the introduction to her book, The First Year: Parkinson's Disease; An Essential Guide for the Newly Diagnosed, "and I had no idea nor gave any thought whatsoever to what I might be exposing myself to, or what the effects might be. After the first day or two of spraying, I could no longer smell the odor of the herbicide. I do remember that when I would come home, my mother would immediately tell me to take a shower because I smelled like chemicals."
As a young adult, Christensen had a single massive chemical exposure, during a political demonstration that involved wading into the Mississippi River in St. Louis. Wastewater treatment runoff made the water as neon green as Mountain Dew. She says it's "anybody's guess" what was in the water, but since many of the industries in St. Louis at the time discharged their wastes into the river, she says the brew probably included organophosphate pesticides, dry cleaning solvents, and other compounds. "After that action, within an hour I had a headache," she says, "and I was nauseated and felt fatigued and lousy for a week. I know now that those are common symptoms of acute pesticide poisoning. At the time I didn't think about what was causing it. I was 25 and thought I was bulletproof."
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