Long-term exposure to fine particle air pollution may cause subtle structural changes in the brain that could precede cognitive impairment and hidden brain damage, according to research in the American Heart Association journal Stroke.
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 showed harmful effects on the brain in this study, even at low levels, particularly with older people and even those who are relatively healthy," said Elissa H. Wilker, Sc.D., study lead author and researcher in the Cardiovascular Epidemiology Research Unit at Beth Israel Deaconess Medical Center and the Harvard T.H. Chan School of Public Health in Boston.
Researchers analyzed 943 adults in the Framingham Offspring Study, who were relatively 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 2 microgram per cubic meter of air (μg/m3) increase in PM2.5, a range commonly observed across a metropolitan region, 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 aging," Wilker said.
Fundamental changes in the structure of the cerebral brain volume and smaller brain size are markers of age-associated brain atrophy.
"We found that people who live in areas where there are higher levels of air pollution had smaller total cerebral brain volume and were more likely to have evidence of covert brain infarcts," said Wilker, who is also an instructor of medicine in the Harvard Medical School.
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 said.
Fine particulate matter affects more people than any other pollutant, with chronic exposure causing the most deaths from serious disease, according to the World Health Organization (WHO). PM2.5 may trigger disease because the particles penetrate into the alveoli of the lungs. Fine particulate matter can also contribute to the narrowing of arteries that supply blood to the brain.
These findings are consistent with prior studies that have shown an association between long-term pollution exposure and living close to major roads and first-time stroke and poorer cognitive function in older adults.