The observational study was based in London, England, and published in the online journal BMJ Open. (Related: Why cities make people stupid: Air pollution significantly reduces children's IQ, study finds.)
While air pollution has already been established as a contributing factor to several diseases including heart disease, stroke and respiratory illnesses, scientists are eager to also quantify its exact link to the development of dementia. (Related: The four best plants that clean your indoor air.)
With this goal in mind, the research team focused their study on 131,000 people between the ages of 50 and 79 who had not been diagnosed with dementia and who were registered with one of 75 general practices across the greater London area. Health records for these patients were accessed via the Clinical Practice Research Datalink (CPRD), a database of information collected from GPs across the U.K. since 1987.
The health of these patients was monitored for an average of seven years until they either deregistered from the practice, were diagnosed with dementia or died – whichever came first.
Researchers then estimated the volume of pollution in each postcode and from there determined the patients’ likely annual exposure to the air pollutants nitrogen dioxide (NO2), fine particulate matter (PM2.5) and ozone (O3). They also measured the effects of other factors like road noise and heavy traffic when making their deductions.
The BMJ reported as follows on the findings:
During the monitoring period, 2181 patients (1.7%) were diagnosed with dementia, including Alzheimer’s disease.
These diagnoses were associated with ambient levels of NO2 and PM2.5, estimated at the patients’ homes at the start of the monitoring period in 2004.
Those living in areas in the top fifth of NO2 levels ran a 40 per cent 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 associations were consistent and unexplained by known influential factors, such as smoking and diabetes, although when restricted to specific types of dementia, they remained only for patients diagnosed with Alzheimer’s disease.
Since the study was observational in nature, it could not categorically establish that air pollution was the direct cause of dementia in these cases. Nonetheless, the study’s authors note that the link cannot be explained by any of the other factors known to increase the risk of developing dementia.
“The study outcome suggests a linkage [between air pollution and dementia] but cannot inform on the cause,” Frank Kelly, one of the study’s authors, and a professor of environmental health at King’s College London, told The Guardian. “However, I believe that we now have sufficient knowledge to add air pollution to the list of risk factors for dementia. Our calculations suggest that it elevates risk by 7% [overall], so [that would suggest] approximately 60,000 of the total 850,000 dementia cases in the U.K., in mathematical terms.”
Experts advise that governments pay urgent attention to reducing emissions from cars, vans and lorries in order to try to reduce this risk. Learn more about air quality and how it effects health at Environ.news.