Though it is well known that air pollution raises the risk of stroke, now new research suggests that long-term exposure can even cause damage to brain structures and impair cognitive function in middle-aged and older adults.

In studying more than 900 participants who were part of the Framingham Heart Study, researchers found evidence of smaller brain structure and covert brain infarcts, a type of "silent" ischemic stroke resulting from a blockage in the blood vessels supplying the brain.

Traffic is a major source of air pollution, so a team of scientists from Beth Israel Deaconess Medical Center and Boston University School of Medicine evaluated how far participants lived from major roadways. They then 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 previous research has linked this form of air pollution with increased numbers of hospital admissions for cardiovascular events, such as heart attacks and strokes.

"This is one of the first studies to look at the relationship between ambient air pollution and brain structure," Elissa Wilker, one of the researchers, 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 found that an increase of only 2µg per cubic meter in PM2.5 was associated with being more likely to have covert brain infarcts and smaller cerebral brain volume - that's the equivalent of roughly one year of brain aging.

What's concerning is that this much air pollution is commonly seen across major US cities, in regions like New England and New York. Also, participants living in more polluted areas had a 46 percent higher risk of silent strokes on MRI - which are risk factors for developing dementia, walking problems and depression.

"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."

The researchers plan to continue studying air pollution and how it affects our health on a long-term basis, focusing on brain shrinkage over time, and other risks including stroke and dementia.

The results were published in the journal Stroke.

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