Decades of exposure to air pollution not only damages the brain but shrinks it, as well.

Long known to be a deadly hazard to our lungs and respiratory system, air pollution has now been shown to have equally devastating effects on our brain that manifest themselves as we grow older, said a new study by the Boston University School of Medicine and Beth Israel Deaconess Medical Center.

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This is the first study to be conducted on adult brains that researches the effects of air pollution, observed in terms of brain size and prevalence of strokes.

More significantly, the study linked prolonged exposure to air pollution to damage suffered by the brain. That damage also includes shrinking the size of the brain; increasing the risk for strokes and probably dementia, a class of diseases that include Alzheimer's disease.

The study concluded the effect of pollution on brain structure affects cognitive function, most especially in middle-aged and old people.

It reported a decrease in brain volume in people with prolonged exposure to polluted air. More precisely, the study linked a reduction of brain size by 0.32 percent with an increase of two micrograms of fine-particle polluted matter per cubic meter.

That change in brain volume "is equivalent to about one year of brain aging," said study author Elissa Wilker, a researcher in the cardiovascular epidemiology research unit at Beth Israel Deaconess Medical Center in Boston.

The two microgram threshold has also been linked to a 46 percent increase in a patient's odds of getting a "silent stroke". These are very small strokes that are asymptomatic, but can be identified on brain scans. Silent strokes are a risk factor in the onset of deadlier strokes.

An increase of 46 percent in the chance of getting this condition "is concerning since we know that silent strokes increase the risk of overt strokes and of developing dementia, walking problems and depression", said Dr Sudha Seshadri, Professor of Neurology at the Boston University School of Medicine.

The study said on average, participants that live in more polluted areas had the brain volume of someone a year older than those that lived in less polluted areas. The exact mechanism by which air pollution modifies the brain structure has not been fully understood but will be the aim of future studies.

What doctors know is that inflammation may cause the reduction of brain volume and that polluted air may cause inflammation.

The study was published April 23 in the American Heart Association journal Stroke.