Some scientists suggest that there’s a link between exposure to certain environmental toxins, such as pesticides, heavy metals and other substances, and an elevated risk of developing Parkinson’s disease.

Take migrant farm workers or farmers in general: “There’s a substantially increased risk of developing Parkinson’s disease if you are employed in that occupation,” says Ted Dawson, M.D., Ph.D., director of the Institute for Cell Engineering at Johns Hopkins.

Still, he adds, “Genetics probably also play a role because if [toxins] were a major risk factor, we’d have an enormous outbreak of Parkinson’s in farm and migrant workers, and we don’t.”

What he means: Even long-term exposure to any particular toxin, on its own, will never cause Parkinson’s disease. People may be exposed to toxins and never develop the condition. But the link remains as one piece in the puzzle of possible causes.

Environmental Factors in Parkinson’s Disease

Here are environmental factors that may play a role in the development of Parkinson’s disease:

• Pesticides/herbicides: Studies have shown a link between exposure to chemicals in pesticides and herbicides, and the incidence of Parkinson’s disease. These substances include the insecticides rotenone and permethrin (which may be found in clothing or nets treated to kill mosquitoes, for example); organochlorines, such as beta-hexachlorocyclohexane; and the herbicides paraquat and 2,4- dichlorophenoxyacetic acid (2,4-D).

• MPTP: This synthetic neurotoxin has been shown to cause parkinsonism, a syndrome like Parkinson’s disease. Neurologists discovered this link when a group of intravenous drug users in California in the 1980s injected a synthetic heroin that had been contaminated with MPTP and developed immediate symptoms of parkinsonism. 

• Agent Orange: This powerful defoliant, which contains the herbicide 2,4-D, was used extensively during the Vietnam War. Although Agent Orange has not been definitively proven to cause Parkinson’s disease, the U.S. Department of Veterans Affairs added Parkinson’s disease to a list of conditions possibly associated with exposure to it.

• Manganese and other metals: There’s a suggestion that exposure to various metals may be related to the development of Parkinson’s disease. High-dose manganese exposure — linked to certain occupations, such as welding — is known to cause a form of parkinsonism called manganism. Exposure to lead may also be associated with a greater risk of Parkinson’s.

• Solvents: Trichloroethylene, a solvent, has been used in many industrial settings, such as metal degreasing and dry cleaning, and in paint thinners and detergents. Some studies have shown a link between long-term exposure to solvents and development of Parkinson’s. 

• Organic pollutants: PCBs, or polychlorinated biphenyls, were used in various industrial processes until they were banned in the 1970s. Researchers have found high concentrations of PCBs in the brains of people who had Parkinson’s. 

Although environmental exposure to these and other toxins is of continued research interest, it’s hard to determine if any one substance is a culprit. Most often, individual cases of Parkinson’s disease result from a complex interplay between genetics and environmental and other factors.