Long-term exposure to air pollution linked to increased risk of Parkinson’s disease

Air pollution is a major public health issue, with long-term exposure to fine particulate matter linked with an increased risk of various diseases, including stroke and dementia. It has also been implicated in neurodegenerative diseases such as Parkinson’s disease, multiple sclerosis and motor neurone disease. However, studies have been underpowered and their findings are often inconsistent or inconclusive.

In a paper published in Environment International, researchers at IMS Epidemiology, University of Cambridge, carried out a systematic review and meta-analysis of existing scientific literature to examine these links further. This approach allowed them to bring together studies that on their own may not provide sufficient evidence, and which sometimes disagree with each other, to provide more robust overarching conclusions.

The analysis included 26 studies on Parkinson’s disease and three each for multiple sclerosis and motor neurone disease.

The team found two air pollutants that were associated with an increased risk of Parkinson’s disease, which affects approximately 6 million people worldwide:

• Particulate matter with a diameter of 2.5 microns or less (PM2.5): a pollutant made up of tiny particles small enough that they can be inhaled deep into the lungs. These particles come from several sources, including exhaust and non-exhaust vehicle emissions, power plants, industrial processes, wood burning stoves and fireplaces, and construction dust. They also form in the atmosphere because of complex chemical reactions involving other pollutants such as sulphur dioxide and nitrogen oxides. The particles can stay in the air for a long time and travel a long way from where they were produced.
• Particulate matter with a diameter of 10 microns or less (PM10): larger particles, but still small enough to be inhaled. These particles come from sources including dust from roads or construction, pollen and mould spores, vehicle emissions (especially from diesel engines), including non-exhaust emissions such brake and tyre wear, and industrial pollution.

Evidence was inconclusive for a link between nitrogen dioxide (NO₂), carbon monoxide (CO), sulphur dioxide (SO₂), ozone and soot, largely due to the low number of research studies and their imprecision or a lack of consistency. Future research should assess these pollutants, and interaction between pollutants, when more evidence becomes available.

Similarly, the researchers found no evidence of a link between air pollution and either multiple sclerosis or motor neurone disease, but this may because there were so few studies in this area.

Dr. Annalan Navaratnam, Clinical Research Fellow, IMS Epidemiology, University of CambridgeThere are still relatively few robust studies that explore the link between air pollution and Parkinson’s disease, but even so, it’s becoming clear that there is a link. We found evidence of a link with two types of pollutants in particular, but even though the evidence was inconclusive for other types, this may be down to how the studies were designed. We urgently need more research, in larger populations, to examine what is a significant public health issue.”

One possible explanation for the link between air pollution and Parkinson’s disease proposed by the researchers is that oxidative stress and neuroinflammation triggered by air pollution – especially PM2.5 – may interact with genetic susceptibility to the disease. In turn, this may lead to the abnormal build-up of the protein α‑synuclein and the loss of dopaminergic neurons, both are which are characteristic of Parkinson’s disease.

First author Alexandra Tien-Smith from IMS Epidemiology, Cambridge, added: “These findings add to a growing and strong body of evidence of the many adverse health impact of air pollution worldwide ranging from a wide variety of diseases across every human organ system to premature mortality.”

The research was partly funded by the European Research Council and the European Union’s Horizon 2020 Programme and Horizon Europe Framework Programme.

To help practitioners and policymakers tackle air pollution in their own regions and cities, Dr. Haneen Khreis and colleagues have previously created an open-access interactive tool mapping the effectiveness of over 1,000 unique policy interventions to reduce traffic-related emissions and air pollution.

The interventions include congestion charging, urban transport planning, housing planning, mass transit development, active travel promotion, electric vehicles and low emissions zones, among others.

Dr. Haneen Khreis, also from IMS Epidemiology, Cambridge, added: “The message is clear: we need to take action now to improve air quality and help reduce the global burden of disease.

“Our open access tool is intended to help researchers, practitioners, policymakers and third sector organizations understand the impact of different interventions on everything from traffic emissions and air pollution to human exposure, health impacts and unexpected co-benefits. We hope it aids and empowers those in a position to make a difference to implement effective and meaningful policies.”

The tool is the first peer-reviewed systematic evidence map to compile international evidence on urban-level policy interventions to reduce traffic emissions the context of human exposure and health effects, whilst also recording other important policy considerations such as enablers, barriers, and co-benefits. The researchers are currently developing this tool further to provide decision-makers with information on the financial investments required, economic benefits and costs, and greenhouse gas emission reductions achievable through the indexed policy interventions.

Source:

University of Cambridge