Exposure to two pesticides commonly used in agriculture since 1974—paraquat and maneb—can cause a three-fold increase in developing Parkinson’s disease in those whose genes may be susceptible. Parkinson’s disease can include tremors, muscle stiffness, and lack of balance. The degenerative nerve disease is caused by how pesticides affect dopamine levels in the brain. A study of people who developed Parkinson’s were compared to those that were healthy. All were asked about their pesticide exposure at home, and from drift from neighboring farms. The key to the study is that it found that a person with a susceptible gene did not develop Parkinson’s if they were not also exposed to the pesticide.