Why are farm residents more likely to get Parkinson’s disease? That question, writes Robin Marantz Henig in the National Resources Defense Council’s magazine On Earth, is one epidemiologists started asking in the 1970s.
People who had been exposed to pesticides sporadically over a lifetime were 1.2 times more likely to develop Parkinson’s than those who had not been. And when the exposure was heavy — the kind of lifetime exposure seen in career pesticide applicators, or a single massive exposure as the result of a spill — that increased risk jumped to 2.3 times.
As with many other scientific efforts to establish disease causation through population studies, there will probably never be a smoking gun that settles things once and for all. Population studies can detect associations between certain suspected agents and diseases such as cancer, but it’s hard to draw conclusions about what causes a disease from studies that can register only correlations.