People exposed to the highest levels of air pollution are about twice as likely to develop Parkinson’s disease dementia or Lewy body disease as those exposed to the lowest levels. Researchers cannot prove that pollution is the cause, but the findings suggest that one of the few risk factors that societies can actually change may influence brain health.
Every inhale carries hitchhikers from your environment into the inner sanctum of your body. The most insidious of these uninvited guests are less than one thirtieth the size of a grain of sand – including microscopic flecks of metal, remnants of burned fossil fuels and wildfire smoke. Scientists now suspect that these particles could be associated with the development of certain kinds of dementia.
By cross-referencing models of air quality with patient data from Denmark’s national patient registry, researchers at Aarhus University found that people who live in neighbourhoods with higher levels of air pollution were more likely to be diagnosed with Parkinson’s disease–related dementia and Lewy body disease – two conditions that continue to increase in prevalence while several other major types of dementia have levelled off.
Their findings have been published in JAMA Network Open.
“This is one of the strongest associations we have seen between air pollution and dementia,” says co-author Jakob Christensen, a consultant neurologist at Aarhus University Hospital and epidemiology researcher at Aarhus University in Denmark. “The correlation is so clear.”
Although researchers emphasise they cannot definitively say air pollution is causing these cases of dementia, these concerning results suggest that government actions could lower the risk of dementia for broad swathes of the population. “Air pollution is one of the things we can do something about as a society,” Christensen notes.
Why are Parkinson’s-related dementias still increasing?
Although many people have heard of the plaques and tangles linked to Alzheimer’s, fewer have heard of alpha-synuclein, a protein that misfolds and clumps inside the neurons of people with Parkinson’s related dementia and Lewy body disease.
“Usually, the cell will clear these misfolded proteins, but for some reason in Parkinson’s disease the cells stop clearing the proteins and they accumulate inside the cell. They are actually toxic,” Christensen explains. “This toxicity can spread to other cells, and eventually a large part of the brain is affected.”
In Parkinson’s, this toxic build-up damages neurons that produce dopamine, a chemical vital to movement. The result can be stiffness, changes in gait and a telltale tremor. Lewy body disease can cause similar physical symptoms and psychosis – specifically visual hallucinations that can be “very disturbing,” Christensen says.
Although the incidence of certain types of dementia, including Alzheimer’s and vascular dementia, seem to have plateaued, Parkinson’s and Lewy body disease continue to become more common worldwide. Researchers increasingly suspect that the modern environment may be part of the explanation.
“We do not know why the incidence of these diseases is still increasing, whereas it is not for other types of dementia,” he adds.
A new suspect enters the frame
Scientists are working to untangle what might trigger these conditions, called alpha-synucleinopathies. Part of the answer may be floating in the air around us – not as a rare toxin, but as a daily exposure shared by millions of people.
“It has been speculated for many years that Parkinson’s in particular is linked to farming and the use of pesticides,” Christensen says.
But in recent years, studies in animal models have suggested that exposure to airborne pollutants can cause changes in the brain that resemble alpha-synucleinopathy. That has given researchers a possible biological explanation for why pollution might be linked to these diseases among people.
How could the air we breathe make changes to our neurons? The most harmful category of airborne pollutant is particulate matter with an aerodynamic diameter smaller than 2.5 µm: PM2.5 (for reference, a grain of sand is about 90 µm in diameter). PM2.5 particles are small enough to evade the body’s defences and infiltrate the tiniest air sacs in the lungs, the muscles of the heart and even the brain.
Researchers suspect that PM2.5 pollutants trigger inflammation, which erodes the blood–brain barrier – the firewall that keeps germs and cells from other parts of the body from straying into the brain – enough for other substances to sneak through. Scientists also suspect that chronic inflammation itself may contribute to the disease process.
“It may not directly be the pollutant itself,” Christensen explains. “It could be proteins from the blood, or it could be the thousands of molecules we are exposed to in daily life. This is quite hard to disentangle.”
Testing the idea in an entire country
To determine whether this idea might hold up outside the laboratory, the researchers examined two streams of data – Denmark’s national patient registry and air quality models that estimate the pollution load for each square kilometre tract. The goal was to test whether the patterns seen in experimental studies also appeared in real populations.
Because every resident of Denmark has a unique identification number, researchers can link health records and residential histories with unusual precision.
First, the researchers identified all Danish citizens older than 65 years who had been diagnosed with Parkinson’s disease dementia or Lewy body disease between 2001 and 2021 – a total of about 6,800 people. They then selected 10 matched controls for each person.
Then, they used their registered address to estimate the airborne pollutants each had been exposed to over the previous 10 years, focusing on PM2.5 and NO2, a gaseous pollutant released by gas stoves, car tailpipes and power plants. PM2.5 includes some of the smallest particles produced by combustion, motor vehicles and other human activities.
The researchers found that exposure to airborne pollutants was associated with a significantly higher risk of developing these types of dementia, suggesting that a factor woven into everyday life may influence diseases of the ageing brain. The findings align with what researchers have seen in experimental studies of pollution exposure.
“Comparing the people with the highest pollution exposure to the people with the lowest, the risk of these types of dementia is more than doubled,” says co-author Henriette Thisted Horsdal, an Aarhus University epidemiologist who conducts register-based research.
People breathing dirtier air faced higher risks
The association was much more pronounced than what has been documented for several other brain-related conditions such as schizophrenia, Christensen emphasises.
One finding surprised the researchers. Lewy body disease appeared much more strongly linked to air pollution than Parkinson’s dementia. In fact, the association with Lewy body disease was consistently stronger throughout the analysis.
“I would assume that it would be more similar based on the shared underlying pathology,” Thisted Horsdal says. Both diseases are driven by the same misfolded alpha-synuclein protein, making the difference unexpected.
Notably, the maximum pollutant exposure in Denmark may be much lower than that experienced by people living in many other densely populated regions of the world.
Strong evidence – but not yet proof
Despite these compelling results, it is too early to say that airborne pollutants cause these types of dementia, the researchers emphasise.
“Even though we found these strong associations, we cannot say anything about causality,” Thisted Horsdal says. She warns against “basing decision-making on this study” without further evidence.
Researchers are still looking for a cellular “smoking gun” that would tie airborne pollutants to these types of dementia. For now, the evidence comes from several directions rather than a single definitive mechanism.
Nevertheless, even Denmark’s unusually detailed data miss some important details. “We have measurements of pollution where people live, but they may work in a factory or be exposed somewhere else,” Christensen says.
Christensen says that the findings fit with a broader picture emerging across health research in which air pollution is increasingly viewed not simply as a respiratory hazard but as a whole-body exposure that may influence how the body ages – including the brain. Evidence from epidemiology, experimental biology and environmental health is increasingly pointing in the same direction.
“It is hard to think of something where air pollution would benefit you. Try to avoid this.”
