Long-term exposure to tiny particles of air pollution may be linked to subtle changes in the brain that could lead to thinking and memory problems, a new study suggests.

These fine particles, smaller than 2.5 micrometers in diameter (much smaller than a pinhead), are released by burning wood or coal, car exhaust and other sources. Long-term exposure may shrink the brain and increase the risk of silent strokes, the researchers suggested.

With a silent stroke, a people might not even be aware they have experienced one.

"Our findings suggest that air pollution is associated with insidious effects on brain aging, even in relatively healthy older people," said lead researcher Elissa Wilker, an instructor in medicine at Harvard Medical School in Boston.

The toll of long-term exposure to this type of pollution is about equal to an extra year of brain aging, she said.

"The association with silent strokes is of concern, because these are associated with increased risk of serious strokes, walking problems and depression," Wilker said.

However, the study did not prove that air pollution actually ages the brain or causes silent strokes.

The report was published online April 23 in the journal Stroke.

For the study, Wilker's team collected data on 943 adults who took part in the Framingham Offspring Study, a long-term study that included people from Framingham, Mass. All were free of dementia and had not had a stroke at the start of the study.

Wilker pointed out that the participants lived in and around the New England region and New York, areas where levels of air pollution are low compared with other places in the world. The only way to limit exposure to air pollution is to live in areas where it is low, she said.

From 1995 to 2005, the researchers used MRIs to determine the effect of long-term exposure to air pollution on the brain.

They found that even the relatively low amount of tiny particle pollution participants were exposed to resulted in a 0.3 percent smaller brain size and a 46 percent higher risk of silent strokes.

These small strokes, which usually strike in areas deep in the brain, have been associated with dementia and loss of mental functioning. They may signal damage to the small vessels in the brain, Wilker added.

Dr. Sam Gandy, director of the Center for Cognitive Health at Mount Sinai Hospital in New York City, said, "There have been several studies like this one that link air pollutants to chronic inflammatory diseases, such as diabetes and lung disease as well as to brain disease."

He pointed out that "in Mexico City, where the air pollution is at record levels, brain inflammation, including brain damage linked to Alzheimer's, has been found even in teenagers."

It is possible that air pollutants may aggravate many of the chronic diseases that people suffer, Gandy said.

"This study brings stroke into focus as a potential effect of pollutants, such that stroke joins lung inflammation, brain inflammation and insulin resistance as major medical problems that are exacerbated by exposure to air pollution," he said.

According to the World Health Organization, fine air pollution causes more deaths from chronic airway diseases than any other type of air pollution because these tiny particles can get into the tiny sacs in the lungs -- the alveoli. In addition, fine-particle air pollution can also contribute to the narrowing of arteries that supply blood to the brain.

Wilker said the findings are consistent with studies that have reported a link between long-term pollution exposure and the risk of stroke and decreased mental function among older adults living alongside major roads.