Breathing in common air pollutants over many years may substantially raise a person’s risk of developing certain neurodegenerative diseases, pointing to an environmental driver for cognitive decline. A new study published in JAMA Network Open found that prolonged exposure to fine particulate matter and nitrogen dioxide is linked to higher rates of Lewy body dementia and Parkinson’s disease dementia. These results suggest that improving air quality could serve as a preventative measure to protect brain health in aging populations.
Lewy body dementia and Parkinson’s disease dementia are related neurological conditions that severely impact memory, thinking, and behavior. Both disorders involve the abnormal buildup of a specific protein called alpha-synuclein in the brain. Proteins are the microscopic structures that carry out essential functions inside our cells. When these proteins misfold and clump together, they disrupt normal cellular activity and eventually cause brain cells to die.
In Lewy body dementia, these protein clumps typically cause early cognitive problems, visual hallucinations, and unpredictable shifts in alertness. In Parkinson’s disease, the damage initially affects movement, causing tremors and stiffness, but many patients eventually develop dementia as the disease spreads through the brain. Researchers want to identify environmental triggers that might cause this destructive protein buildup. If external factors contribute to these diseases, modifying our environment might help prevent the onset of symptoms.
Dimitry S. Davydow, a psychiatrist and researcher at the University of Florida College of Medicine, led the investigation into these environmental factors. He collaborated with Gregory M. Pontone, a psychiatrist at the University of Florida, along with a team of environmental scientists and epidemiologists from Aarhus University in Denmark. They aimed to track pollution exposure over a long period to see how it affected older adults. The research team focused specifically on two ubiquitous pollutants found in nearly all modern cities.
The first pollutant, fine particulate matter, consists of tiny airborne particles that are much thinner than a single human hair. Because they are so small, these particles can be inhaled deep into the lungs and easily pass into the bloodstream. The second pollutant is nitrogen dioxide, a toxic, reddish-brown gas. Both of these substances are primarily generated by combustion processes, such as the burning of fossil fuels in car engines and power plants.
The brain is usually protected from harmful substances in the blood by a strict biological filter known as the blood-brain barrier. Some incredibly small particles and gases can bypass this defense system and enter brain tissue directly. Once inside, these pollutants might trigger an aggressive immune response from the brain’s defense cells. Chronic inflammation resulting from this immune response can damage neurons and potentially encourage proteins to misfold.
Another possible entry point for these pollutants is the human nose. The olfactory system, which handles the sense of smell, provides a direct neural path from the outside environment to the brain. People who develop Lewy body dementia or Parkinson’s disease often lose their sense of smell early in the disease process. High levels of air pollution are also associated with a weakened ability to smell, suggesting the nasal cavity could be a gateway for toxic particles.
To investigate these patterns, the research team analyzed national health and population records from Denmark. They gathered de-identified data covering more than two million Danish citizens aged 65 to 95 between the years 2001 and 2021. Denmark maintains comprehensive health registries that track medical diagnoses and residential addresses for its entire population. This detailed record-keeping allowed the researchers to look far back into the patients’ lives with high accuracy.
From this massive dataset, the researchers identified just over 3,000 people diagnosed with Lewy body dementia. They also found about 3,800 individuals diagnosed with Parkinson’s disease dementia. The investigators matched each of these patients with ten control subjects who did not have either condition. The control subjects were chosen to be the exact same sex and born within fourteen days of the patients they were matched with.
Next, the team had to calculate how much pollution each person had breathed in over time. They used a high-resolution mapping system that models air pollution levels across Denmark on a very localized scale. By combining this environmental data with the historical residential addresses of the subjects, they calculated a ten-year average exposure for each person. This average covered the entire decade right before a dementia diagnosis was recorded.
The researchers adjusted their statistical models to account for a wide variety of background factors that might influence brain health. They included the socioeconomic status of the individuals, such as their highest level of education, employment status, and income bracket. They also factored in the general economic conditions of the subjects’ immediate neighborhoods. Finally, they included detailed medical histories, taking into account other physical illnesses and prior psychiatric conditions.
The data revealed a clear connection between higher pollution levels and increased dementia risk. For every small incremental increase in the concentration of fine particulate matter, the risk of developing Lewy body dementia nearly quadrupled. The same incremental increase in this particulate matter was associated with more than double the risk of developing Parkinson’s disease dementia.
Nitrogen dioxide exposure showed a similar, though slightly less dramatic, pattern in the analysis. A fixed increase in the average concentration of this gas almost doubled a person’s chances of developing Lewy body dementia. For Parkinson’s disease dementia, the elevated gas exposure corresponded to a fourteen percent higher risk. In both cases, the connection to pollution was stronger for Lewy body dementia than for the dementia associated with Parkinson’s disease.
The researchers also grouped the subjects based on their total pollution exposure to look for a consistent dose-response relationship. They compared the people who breathed the dirtiest air to those who enjoyed the cleanest air. The group with the highest exposure to fine particulate matter had more than twice the risk for both types of dementia compared to the lowest exposure group.
“These are pollutants most people are exposed to every day,” said Dimitry S. Davydow, M.D., M.P.H., the Lauren and Lee Fixel Professor at the Norman Fixel Institute for Neurological Diseases at UF Health. “They come from things like traffic, shipping and other forms of combustion.”
“While this research does not establish causation, it does show a clear association between air pollution exposure and increased risk of these dementias,” said Gregory Pontone, M.D., M.H.S., the Louis and Roberta Fixel Endowed Chair. “It’s an important step in understanding how environmental factors may contribute to disease development.”
The study relied heavily on diagnoses made in hospitals or specialty clinics, which presents a minor limitation. This means the researchers might have missed milder cases of dementia or patients who never sought specialized medical care. If milder cases were missing from the registries, the exact risk calculations might be slightly underestimated by the final analysis.
The team also lacked access to certain personal details that consistently affect health outcomes in older adults. The national databases do not record lifestyle habits like diet, alcohol consumption, or daily exercise routines. The registries also omit details about specific occupational hazards, meaning the team could not account for people who work in heavily polluted industrial settings.
Additionally, fine particulate matter and nitrogen dioxide are often emitted from the exact same sources, such as highway traffic. Because the two pollutants frequently exist together in the air, it is incredibly difficult to separate their individual effects on the human body. The researchers noted that these elements might work together simultaneously to cause neurological harm.
Future research could explore exactly how these invisible particles initiate the neurodegenerative process on a cellular level. Scientists hope to investigate whether blocking the brain’s inflammatory response might slow or stop the damage caused by inhaled pollutants. Further studies could also look at how agricultural chemicals, like pesticides, might combine with air pollution to impact brain health over a lifetime.
The study, “Exposure to Air Pollutants and Lewy Body and Parkinson Disease–Related Dementias,” was authored by Dimitry S. Davydow, Gregory M. Pontone, Michael S. Okun, Melissa J. Armstrong, Theresa Wimberley Böttger, Camila Geels, Lise Marie Frohn, Jørgen Brandt, Julie Werenberg Dreier, Jakob Christensen, Carsten Bøcker Pedersen, and Henriette Thisted Horsdal.
