People continuously exposed to air pollution are at increased risk of dementia, especially if they also suffer from cardiovascular diseases, warn researchers.
According to the study, published in the journal JAMA Neurology, patients with cardiovascular diseases (CVD) who live in polluted environments may require additional support from care providers to prevent dementia.
"Interestingly, we were able to establish harmful effects on human health at levels below current air pollution standards," said study first author Giulia Grande from Karolinska Institutet in Sweden.
"The findings suggest air pollution does play a role in the development of dementia, and mainly through the intermediate step of cardiovascular disease and especially stroke," Grande added.
According to the study, the number of people living with dementia is projected to triple in the next 30 years globally.
No curative treatment has been identified and the search for modifiable risk and protective factors remains a public health priority.
Recent studies have linked both cardiovascular disease and air pollution to the development of dementia, but findings on the air pollution-link have been scarce and inconsistent.
To reach the conclusion, the researchers examined the link between long-term exposure to air pollution and dementia and what role cardiovascular diseases play in that association.
Almost 3,000 adults with an average age of 74 and living in the Kungsholmen district in central Stockholm were followed for up to 11 years. Of those, 364 people developed dementia.
The annual average level of particulate matter 2.5 microns or less in width (PM2.5) are considered low compared to international standards.
For the last five years of exposure, the risk of dementia increased by more than 50 per cent per interquartile range (IQR) difference in mean PM2.5 levels and by 14 per cent per IQR in nitrogen oxide, the researchers said.
Earlier exposures seemed less important. Heart failure and ischemic heart disease both enhanced the dementia risk and stroke explained almost 50 per cent of air pollution-related dementia cases.
Air pollution is an established risk factor for cardiovascular health and because CVD accelerates cognitive decline.
"We believe exposure to air pollution might negatively affect cognition indirectly," said Grande.
"In the study, virtually all of the association of air pollution with dementia seemed to be through the presence or the development of CVD, adding more reason to reduce emissions and optimize treatment of concurrent CVD and related risk factors, particularly for people living in the most polluted areas of our cities," she added.