Parkinson’s disease (PD) is believed to be caused by a complex interaction of genetic and environmental factors. There is now mounting evidence that air pollution exposure is an emerging risk factor in the development of PD.
As the world's largest environmental risk factor for disease and premature death, air pollution has already been linked with heart disease, stroke, lung cancer, respiratory diseases and diabetes, as well as neurogenerative diseases such as Alzheimer’s and Autism Spectrum Disorder (Costa, Cole, Dao, Chang, & Garrick, 2019). However, it remains unclear whether environmental exposures such as air pollution increases the risk of developing PD or simply accelerates a disease process that is already present.
Outdoor air pollution is made up of tiny particles and liquid droplets, called particulate matter (PM) that contaminate the air we breathe. Primarily composed of gases (such as ozone, carbon monoxide, nitrogen dioxide, and sulfur dioxide), these pollutants can come from power plants, cars, trucks, buses, and burning fuels (such as coal, wood and heating oil) as well as from natural sources like forest and grass fires.
PM can range in size from 0.01 microns to 300 microns (a micron is one-millionth of a meter). Particulates less than 10 microns (referred to as “inhalable particles”) can get deep into our lungs and may enter the bloodstream. Particulates that are even smaller, measuring 2.5 microns or less, pose an even greater risk, as they not only enter the lungs, they also easily enter the bloodstream.
A 2022 comprehensive analysis published in the journal, Movement Disorders, “Air Pollution and the Risk of Parkinson's Disease: A Review” (Murata, Barnhill, & Bronstein, 2022) sought to explore several potential pathways by which air pollution may increase the risk of developing PD.
This type of investigation is complex. The authors outline several challenges related to studying environmental contributors of any neurodegenerative disease — including Parkinson’s disease (PD) — such as:
- The lag time (sometimes decades) between a toxic exposure and the emergence of PD symptoms. For example, previous studies have shown that exposure to pesticides Paraquat and/or maneb increased PD risk decades after initial contact (Costello et al. 2009, Am J, Epidemiol, 169(8): 919-926).
- Studies conducted using different methods, making them difficult to compare.
- Some studies fail to account for factors known to be associated with PD risk, such as age, smoking history, pesticide exposure, and time between exposures and disease onset.
- There are differences in what components of air pollution are measured, as well as the timing and methods used to determine exposure.
Nevertheless, as this field is in its early stages, research is valuable and necessary to further the process of scientific discovery. Thus, the authors thoroughly investigated English-language studies published regarding air pollution and neurodegeneration through June 1, 2021 — including epidemiological (the study of disease impact on a population), basic science studies and reviews.
The authors concluded that air pollution is an emerging risk factor in the development of Parkinson’s disease. Researchers identified four pathways as to how air pollution may increase the risk of PD development:
- Direct neurotoxicity and neuroinflammation: Components of air pollution reach the brain through the bloodstream and/or breathing through the nose. Once in the brain, air pollution can be neurotoxic (poisonous to the nervous system) and cause neuroinflammation, which can increase the accumulation of alpha-synuclein (a protein found in the brain that plays a key role in Parkinson’s) and decrease the number of dopaminergic neurons — both classic signs of PD pathology.
- Air pollution – lung – brain connection: Exposure to air pollution can cause inflammation in the airway and lungs, which in turn can cause brain inflammation. This ultimately can lead to brain cell injury and death.
- Air pollution and gut alpha synuclein: Air pollution causes gut inflammation and the local accumulation (clumping) of alpha synuclein. These abnormal alpha-synuclein can spread from the gastrointestinal tract (the gut) through the vagus nerve into the brain, leading to a loss of dopamine.
- Air pollution – microbiome – brain pathway connection: Air pollution can alter the gut microbiome (trillions of microorganisms, such as bacteria, fungi and viruses that live in our gastrointestinal tract). This altered state may lead to inflammation of the brain. Though studies have found evidence of altered microbiota in people with PD, it remains to be seen whether this can indeed affect the development of PD.
What does this mean?
Air pollution is one of the main global health risks for many diseases. According to recent data, 92 percent of the world’s population lives in areas that exceeded the World Health Organization (WHO) guideline for healthy air.
This study described four overarching pathways as to how air pollution may increase the risk of Parkinson’s. In all cases, air pollution can contribute to the development of PD by directly or indirectly damaging the nervous system (neurotoxicity) and/or by an inflammatory response in the brain (neuroinflammation).
In other words, there is a clear association between air pollution and Parkinson’s disease – which is important as this knowledge could inform and impact environmental policies. However, whether air pollution directly causes Parkinson’s has yet to be determined. Similar to other experts in the field, the study authors suggest that a combination of environment and genetics are the cause of Parkinson’s. While most of us cannot control the air we breathe on a day-to-day basis, any time you can limit your exposure to outdoor air pollution is a good idea.
The Parkinson’s Foundation believes in empowering the Parkinson’s community through education. Learn more about PD, air pollution and environmental toxins by viewing Environmental Factors and Genetics & Environmental Interactions, or by calling our free Helpline at 1-800-4PD-INFO (1-800-473-4636) for answers to your Parkinson’s questions.