Scientists are working to better understand the broad range of environmental exposures linked to Parkinson's disease (PD).
Most experts agree that PD is caused by a combination of genetic and environmental factors (chemicals, toxins, head trauma). The interactions between genes and the environment can be quite complex. Some environmental exposures may lower the risk of PD, while others may increase it.
Similarly, some people may have a genetic makeup that makes them more vulnerable to the effects of toxic substances than others. Researchers believe that a combination of factors may trigger biological changes that ultimately lead to Parkinson's.
Environmental Risk Factors
- Head Injury: Traumatic brain injury — injury that results in alteration in level of consciousness — has been associated with an increased risk of developing PD years after the injury; however, the mechanisms underlying this are unclear.
- Area of Residence: There are differences in the geographic distribution of PD. These could be due to differences in environmental factors and genetic risk factors.
- Occupation: Certain occupational categories or job titles have been associated with a higher incidence of PD, but results have been inconsistent.
- Exposure to Metals: Occupational exposures to various metals have been suggested to be related to the development of PD. But long-term exposure to metals is not easily measured and the results of studies measuring PD risk and specific metals have been inconsistent.
- Solvents and Polychlorinated Biphenyls (PCBs): Trichloroethylene (TCE) is a solvent used in many industries and is the most common organic contaminant in groundwater. Exposure to TCE was found to be associated with PD among workers whose factory jobs resulted in long-term exposure. PCBs have been found in relatively high concentrations in the brains of people who had PD. Occupational exposure to PCBs has been associated with greater risk of Parkinson's in women, but not in men.
Pesticide and Herbicide Exposure
A strong link has been shown between PD and exposure to pesticides and herbicides. We need more Parkinson’s-specific research to better understand what causes PD and to work to prevent it and help eliminate the risk of getting the disease, when it comes to all environmental risk factors ― and whether genetics can cause an increased risk in developing Parkinson’s.
One herbicide that has been linked to Parkinson’s is paraquat, a widely used commercial herbicide in the U.S. that is banned in 32 countries, including the European Union and China. The Parkinson’s Foundation, along with the Unified Parkinson’s Advocacy Council, signed two letters to the U.S. Environmental Protection Agency (EPA) encouraging them to cancel the registration of paraquat based on strong scientific research linking the herbicide to Parkinson’s disease. In October 2020, the EPA re-approved paraquat for use in the U.S. Without additional action, paraquat will remain legal for sale and use in the U.S. for the next 15 years.
Other Risk Factors
- Age: Largest risk factor for developing PD. About one percent of people over age 60 have PD.
- Gender: PD is more common in men than in women.
Potential Protective Factors
Scientists have also found certain factors that may reduce the risk of developing PD. As with risk factors, not enough is known about these and they should not be tried without the counsel of a doctor.
- Caffeine: Consumption of caffeine in coffee or tea may lower risk of developing PD.
- Uric acid or urate: This chemical occurs naturally in blood. High levels, associated with diets high in certain foods, like meats, can cause gout and kidney stones. However, researchers have found that men with uric acid levels in the high end of the normal range have a lower incidence of PD, though a similar effect was not observed in women.
- Anti-inflammatory Drugs: Several studies have shown that people who regularly take anti-inflammatory drugs such as ibuprofen have a lower risk of PD.
- Smoking: Many studies have associated cigarette smoking with a decreased risk of PD perhaps due to the protective factor of nicotine.
- Cholesterol Levels: Some studies have suggested that the use of statins — drugs used to lower cholesterol levels — is associated with reduced PD risk.
- Vitamin D: It has been suggested that those with higher vitamin D levels were at lower risk of developing PD, however additional studies are needed to support this.
- Exercise: Increase physical activity early in life has been associated with a lower risk of developing Parkinson's later in life.