Even at low levels, long term exposure to polluted air can make your brain a year older, leading to thinking and memory problems and other hidden damage.
Fine particle air pollution - smaller than 2.5 micrometers in diameter (PM2.5) - may be the most common and hazardous type of air pollution. It comes from burning wood or coal, car exhaust and other sources.
"Long-term exposure to air pollution at low levels showed harmful effects on the brain in this study, particularly with older people and even those who are relatively healthy," said lead study author Elissa Wilker, instructor of medicine at the Harvard Medical School.
Researchers analysed 943 adults who were healthy and free of dementia and stroke.
The participants lived in the greater Boston area and throughout New England and New York -- regions where air pollution levels are low compared to other parts of the nation and the world.
During 1995-2005, researchers used magnetic resonance imaging (MRI) to determine the effect of long-term exposure to air pollution on markers of brain structure.
They found a two microgram per cubic metre of air increase in particulate matter (PM) 2.5 was associated with a 0.32 percent smaller total cerebral brain volume and a 46 percent higher risk of covert brain infarcts, a type of silent stroke.
"The magnitude of association that we observed for brain volume was similar to approximately one year of brain ageing," Wilker said.
Fundamental changes in the structure of the cerebral brain volume and smaller brain size are markers of age-associated brain atrophy.
The small infarcts, typically located in deep regions of the brain, have been associated with neurological abnormalities, poorer cognitive function, dementia, and are thought to reflect small vessel disease, she concluded.
The findings appeared in the journal Stroke.