Since 1964, paraquat has been used in the United States to kill broadleaf weeds and grasses before the planting or emergence of various crops, to control weeds in orchards, and to desiccate plants before harvest. Scientific research indicates that paraquat can interfere with dopaminergic neurons in the brain by creating oxidative stress that results in the degeneration and death of dopaminergic neurons (and many other types of animal cells). Many epidemiological studies (studies of the patterns and causes of disease in defined populations) have found an association between paraquat exposure and Parkinson's disease, including multiple studies finding a two- to five-fold or greater increase in the risk of Parkinson's in populations with occupational exposure to paraquat compared to populations without such exposure.