Costello, S, M Cockburn, J Bronstein, X Zhang and B Ritz. 2009.
Parkinson's disease and residential exposure to maneb and paraquat from agricultural applications in the Central Valley of California.
American Journal of  Epidemiology 169: 919-926.

Synopsis by Jonathan Chevrier, Ph.D.

The risk of Parkinson's disease increases in people who live near farm fields sprayed with a combination of pesticides.

A recent study conducted in California's Central Valley found that people who lived near fields sprayed with a combination of pesticides used on crops such as potatoes, dry beans and tomatoes had an increased risk of Parkinson's disease.

This is the first study to evaluate associations between exposure to a combination of pesticides and the risk of Parkinson's disease.

These results add to the growing literature suggesting that exposure to multiple chemicals may be more harmful than exposure to individual chemicals and contribute to the debate of evaluating chemical safety one at a time rather than in combination.

The cause of Parkinson's disease is still a mystery to scientists but reports of higher risks of this ailment in farmers and in rural populations have lead some to hypothesize that exposure to pesticide mixtures may be a contributor.  

The scientists found that people who live within 500 meters of a field sprayed with the pesticides maneb and paraquat in combination, but not individually, had a 75 percent higher risk of Parkinson's disease relative to controls. Being exposed to the mixture at a younger age resulted in an even higher risk. Individuals potentially exposed to these pesticides when they were 60 years old or younger were 5 times more likely to be diagnosed with Parkinson's disease.

These results are predicted by studies which showed that exposing rodents to maneb and paraquat together resulted in reduced motor activity, nerve cell loss and decreased levels the neurotransmitter dopamine in certain areas of the brain as observed in Parkinson's patients.  Animal studies also predicted Costello's finding that effects of these pesticides would be more important when exposure occurred at a younger age.

Researchers obtained these results after comparing potential exposure to pesticides in 368 people with Parkinson's diseases and 341 people without living in an agricultural area. Exposure was estimated using land-use maps and data from the California Pesticide Use Report, a program which requires that the precise date, chemical and location of spraying be reported to the State.

However, biological markers, such as pesticide concentrations in urine and blood, were not measured. Other factors associated with living close to certain fields may explain the reported association.

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