It is well-established that air pollution is a risk factor for heart disease, stroke and respiratory disease, but the role it plays in neurodegenerative conditions, such as Alzheimer’s disease, remains unclear.
In a new study, U.K. researchers set out to investigate a potential link between poor air quality and dementia. They used carefully calculated estimates of air and noise pollution levels across Greater London. To determine any potential links with new dementia diagnoses, they reviewed data from anonymized patient health records in the Clinical Practice Research Datalink (CPRD). This is a long-term collection of data from participating general practices across the UK since 1987.
The researchers narrowed down the data to focus on just under 131,000 patients ages 50 to 79 in 2004, who had not been diagnosed with dementia, and who were registered at one of 75 general practices located within the London orbital M25 motorway, a major highway that encircles almost all of Greater London.
Based on residential zip codes, the researchers estimated the subjects’ yearly exposure to air pollutants — specifically nitrogen dioxide (NO2), fine particulate matter (PM2.5), and ozone (O3) — as well as proximity to heavy traffic and road noise, using well-established modeling methods, validated with recorded measurements.
The health of these patients was then tracked for an average of 7 years, until a diagnosis of dementia, death, or deregistration from the practice, whichever came first.
During the monitoring period, 2,181 patients (1.7%) were diagnosed with dementia, including Alzheimer’s disease. These diagnoses were associated with immediately nearby levels of NO2 and PM2.5, estimated at the patients’ homes at the start of the monitoring period in 2004.
Patients living in areas in the top fifth of NO2 levels ran a 40 percent heightened risk of being diagnosed with dementia than those living in the bottom fifth. A similar increase in risk was observed for higher PM2.5 levels.
These links were consistent and could not be explained by other risk factors, such as smoking and diabetes. When restricted to specific types of dementia, the associations remained only for patients diagnosed with Alzheimer’s disease.
“Traffic related air pollution has been linked to poorer cognitive development in young children, and continued significant exposure may produce neuroinflammation and altered brain innate immune responses in early adulthood,” the researchers write.
Since the study is observational, it can’t establish cause, and the findings may be applicable only to London. In addition, the researchers were not able to investigate long-term exposure, which may be relevant as Alzheimer’s disease may take many years to develop.
Many factors may play a role in the development of dementia, the exact cause of which is still unknown, the researchers point out. And while there are several potential pathways for air pollutants to reach the brain, it remains unclear how how they might contribute to neurodegeneration.
Still, the researchers say that even if the impact of air pollution were relatively modest, the public health gains would be significant if it emerged that reducing exposure to it might delay the progression of dementia.