Small increases of fine particulate matter can lead to silent strokes that in turn increase the risk of overt strokes and of developing dementia, walking problems and depression among the old REUTERS

Long-term exposure to air pollution can cause damage to brain structures and impair cognitive function in middle-aged and older adults, and put them at a 46% higher risk of silent strokes.

The study led by scientists from Beth Israel Deaconess Medical Center and Boston University School of Medicine, conducted on 900 participants above the age of 60, showed that even a small increase of two micrograms per cubic metre in the tiny particulate matter PM2.5 was associated with covert brain infarcts (silent strokes) and smaller cerebral brain volume, equivalent to approximately one year of brain aging.

This range of air pollution is commonly observed across metropolitan regions in New England and New York.

"This is concerning since we know 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," says Sudha Seshadri, MD, a Professor of Neurology at Boston University School of Medicine and Senior Investigator, the Framingham Study.

The researchers evaluated how far participants lived from major roadways and used satellite imagery to assess prolonged exposure to PM 2.5, particles with a diameter of 2.5 millionth of a metre.

These particles come from a variety of sources, but largely originate from fossil fuel burning in power plants, factories and vehicle exhaustion, besides in burning of wood. Being fine, they can travel deeply into the lungs and have been associated with respiratory issues like chronic bronchitis and lung cancer, as well as cardiovascular events such as heart attacks and strokes as they can enter the bloodstream.

Participants were free of dementia and stroke. The evaluation included total cerebral brain volume, a marker of age-associated brain atrophy; hippocampal volume, which reflect changes in the area of the brain that controls memory; white matter hyperintensity volume, which can be used as a measure of pathology and aging; and covert brain infarcts.

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

Air pollution is a rising issue in many developing economies like India and China. Construction waste, vehicular exhaust and industrial activities here contribute most to the air pollution. Beijing has recently imposed several measures to control vehicular movement and polluting factories.

A University of Chicago study had estimated that 99.5% of India's 1.2 billion people are breathing in air polluted above safe levels determined by WHO, cutting 660 million lives short by about three years each. The estimates could be conservative as they were based on 2012 satellite data that underestimated PM2.5 levels.