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Can dirty air be bad for the brain? A new study suggests that smog and other air pollutants may contribute to memory loss and dementia.

The findings, part of a large and ongoing study of nurses in their 70s, found that long-term exposure to air pollution may speed up cognitive decline in older adults. Women who lived in areas with the worst quality air scored lower on tests of memory and thinking than those who lived in cleaner areas.

Exposure to polluted air contributed to the equivalent of about a two-year decline in brain function, which might lead to an earlier onset of Alzheimer’s and other forms of dementia. That translates to about two million cases of Alzheimer’s over a 40-year period, the researchers estimate.

The report, from the Archives of Internal Medicine, is one of the few to study the effects of air pollution on brain health.

It is difficult to establish a direct link between environmental toxins and a disease like Alzheimer’s, because so many factors are involved, and correlation does not equal causation. But this study examined thinking skills over a four-year period and involved a large sample size of more than 19,000 women living in different parts of the United States.

The levels of pollution in the study linked to memory and thinking problems were typical of exposure levels found in many areas of the United States.

Other studies have linked air pollution to an increased risk of heart attacks and strokes. Cardiovascular problems have likewise been linked to dementia and poor brain health.

“Unlike other factors that may be involved in dementia, such as diet and physical activity, air pollution is something we can intervene on as a society at large through policy, regulation and technology,” said Jennifer Weuve, assistant professor at the Rush Medical College in Chicago and the principal investigator of the study. “If our findings are confirmed in other research, air pollution reduction is a potential means for reducing the future population burden of age-related cognitive decline, and eventually, dementia.”

Air pollutants contain chemicals and metals that become suspended in the air. Fine particles may be particularly damaging, reaching deep into the lungs and possibly even penetrating into the brain. Sources of such emissions include cars, diesel-powered equipment and various industrial processes.

In earlier studies, levels of beta-amyloid, a toxic protein that builds up in the brains of those with Alzheimer’s disease, were higher among residents of polluted cities. Another report, from Germany, found that older women who lived near busy roads did worse on memory and thinking tests than similar women who lived in rural areas. Exposure to so-called black carbon, a product of traffic, over the previous one to 11 years has been linked to worse cognitive function in a study from China.

Animal studies have recorded higher levels of brain inflammation in animals exposed to polluted air. Increasingly, scientists believe that inflammation may play a role in Alzheimer’s and heart disease.

By ALZinfo.org, The Alzheimer’s Information Site. Reviewed by William J. Netzer, Ph.D., Fisher Center for Alzheimer’s Research Foundation at The Rockefeller University.

Source: Jennifer Weuve, Robin C. Puett, Joel Schwartz, et al: “Exposure to Particulate Air Pollution and Cognitive Decline in Older Women.” Archives of Internal Medicine, Vol. 172 (No. 3), pages 219-227, 2012.