Long-term exposure to air pollution, even at moderate levels, can cause brain damage and impair cognitive function in middle-aged and older adults, according to a recent study.

After examining more than 900 participants of the Framingham Heart Study, researchers from Beth Israel Deaconess Medical Center and Boston University School of Medicine found evidence of smaller brain structure and of covert brain infarcts, a type of "silent" ischemic stroke resulting from a blockage in the blood vessels supplying the brain.

"This is one of the first studies to look at the relationship between ambient air pollution and brain structure," Elissa Wilker, a researcher in the Cardiovascular Epidemiology Research Unit at Beth Israel Deaconess Medical Center, said in a statement. "Our findings suggest that air pollution is associated with insidious effects on structural brain aging, even in dementia- and stroke-free individuals."

The study evaluated how far participants lived from major roadways and used satellite imagery to assess prolonged exposure to ambient fine particulate matter, particles with a diameter of 2.5 millionth of a meter, referred to as PM2.5. These particles come from a variety of sources, including power plants, factories, trucks and automobiles and the burning of wood. They can travel deeply into the lungs and have been associated in other studies with increased numbers of hospital admissions for cardiovascular events such as heart attacks and strokes.

They found that an increase of only 2µg per cubic meter in PM2.5, a range commonly observed across metropolitan regions in New England and New York, was associated with being more likely to have covert brain infarcts and smaller cerebral brain volume, equivalent to approximately one year of brain aging.

"These results are an important step in helping us learn what is going on in the brain," Wilker said. "The mechanisms through which air pollution may affect brain aging remain unclear, but systemic inflammation resulting from the deposit of fine particles in the lungs is likely important."

Wilker added that the findings are concerning since it is well known that silent strokes increase the risk of overt strokes and of developing dementia, walking problems and depression.

"We now plan to look at more the impact of air pollution over a longer period, its effect on more sensitive MRI measures, on brain shrinkage over time, and other risks including of stroke and dementia," she said.

The findings are detailed in the journal Stroke.