Scientists are working to better understand the broad range of environmental exposures linked to Parkinson’s disease (PD). While the exact cause of Parkinson’s disease is unknown, most experts agree that the condition is caused by a combination of genetic and environmental factors (chemicals from occupational exposure or rural living, head trauma, etc.).
The interactions between genes and environment can be quite complex. Some environmental exposures may lower the risk of PD, while others may increase it. Similarly, some people have inherited a genetic makeup that makes them either more or less vulnerable to the effects of toxic substances than others.
All of this means that the particular combination of factors leading to PD is likely to be unique for each person. These combinations, in different ways, may trigger a common series of biological changes that ultimately lead to this disease.
Scientists are beginning to tease apart the non-genetic factors that influence the risk of developing PD. In particular, epidemiologists are working to identify differences in the experiences of people who develop PD, compared to those who do not. But identifying these risk factors can be difficult. And when we do identify them, they are clues but they do not provide a direct explanation for the cause of Parkinson’s, so scientists must continue to do laboratory experiments in addition to population studies. It is important to note that it is too early to make recommendations from this research, but these results may help us understand the causes of PD and provide direction for future research and therapy development.
Environmental Risk Factors
While age is the biggest risk factor for PD, many chemicals used in various professions — such as welding, farming, military activities and industrial processes — have been associated with the disease. While we know that exposure to these environmental factors increases some people’s risk of developing PD, exposure to any one of these factors does not necessarily mean a person will actually develop the disease. Some specific risk factors include:
- Age: About one percent of people over age 60 have PD.
- Gender: PD is more common in men than in women.
- 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 differences in genetic risk factors. Alternatively, they could be traced to differences in the methods that are used to count people with PD.
- Occupation: Certain occupational categories or job titles have been associated with a higher incidence of PD, but results have been inconsistent. By contrast, lower rates of PD are associated with shift work and jobs involving vigorous physical work.
- Pesticide Exposure: Of all the chemical exposures that have been linked to PD, pesticides (including permethrin and beta-hexachlorocyclohexane) have been reported the most consistently. Recent research has shown higher rates of PD among people exposed to pesticides over a long period of time as part of their work. Investigating pesticide exposure in home use is more challenging.
- Herbicide Exposure/Agent Orange: The herbicide 2,4-dichlorophenoxyacetic acid is one of the chemicals making up Agent Orange, used as a defoliant during the Vietnam war era. Although it has not been proven to cause PD, the U.S. Department of Veterans Affairs has ruled that veterans with PD who served in Vietnam between January 9, 1962 and May 7, 1975 are eligible to receive disability compensation from the Veterans Administration. Research has also been conducted to examine the relationship between paraquat and PD, as well as the fungicide maneb.
- 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. Occupational exposure to TCE was found to be associated with PD among workers whose factory jobs resulted in long-term (eight to 33 years) 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.
- Genetic Predisposition: Often, a person’s genetic makeup will help to determine the effect of an environmental exposure. For example, agricultural workers exposed to pesticides were at an increased risk of PD only if they also had inherited a reduced ability to metabolize toxicants. In another study, head injury was associated with a higher risk of PD only in people with one form of a particular gene. In people without this gene variant, head injury was not associated with a higher risk of PD.
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 the form of coffee or tea may lower risk of development of PD.
- Uric acid or urate: This chemical occurs naturally in blood. High levels, associated with diets high in certain foods, such as 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 Parkinson’s.
- 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 that are used to lower cholesterol levels — is associated with reduced PD risk.
- Vitamin D: There is suggestion 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 Disease later in life.
Page reviewed by Dr. Bhavana Patel, Movement Disorders Fellow at the University of Florida, a Parkinson’s Foundation Center of Excellence.