Exposure to air pollution has been known to affect respiratory diseases, lung function and cardiac health, but a new study led by researchers at the Keck School of Medicine of USC shows that it may also have a negative impact on how the brain’s white matter ages.

The research indicates that older women who lived in geographic locations with higher levels of fine particulate matter in ambient air had significantly smaller white matter volumes across a wide range of brain areas.

Fine particulate matter is smaller than 2.5 micrometers and is known as PM2.5, a form of pollution that easily enters the lungs and possibly the bloodstream. The California Air Resources Board maintains a database of locations with high PM2.5 rates, and inland areas of Southern California often are among the state’s worst locations. White matter connects brain regions and determines how information is processed in the brain.

“Investigating the impact of air pollution on the human brain is a new area of environmental neurosciences,” said Jiu-Chiuan Chen, MD, MPH, ScD, associate professor of preventive medicine at the Keck School of Medicine and lead author of the research. “Our study provides convincing evidence that several parts of the aging brain, especially the white matter, are an important target of neurotoxic effects induced by long-term exposure to fine particles in the air.”

The study found that women ages 71-89 who had lived in places with greater PM2.5 exposures had significantly smaller volumes of white matter, and that this could not be explained by the geographic region in which they lived or by their race, ethnic background, socioeconomic status, lifestyle or medical conditions.

The researchers examined magnetic resonance imaging (MRI) scans of the brains of 1,403 women who are part of the Women’s Health Initiative Memory Study (WHIMS), a nationwide group based out of Wake Forest Baptist Medical Center in Winston-Salem, N.C. The researchers also used residential histories and air monitoring data to estimate the participants’ exposure to air pollution in the previous six to seven years.

This is the first study to differentiate between white and gray matter while examining the neurotoxic effects of PM2.5 on brain volumes of older people. White matter contains nerve fibers and connects brain regions with each other by traveling deep within the brain and passing nerve signals throughout it. Gray matter consists primarily of neuronal cell bodies, dendrites, glial cells and capillaries. The study did not find impacts from exposure to air pollution in participants’ gray matter.

The WHIMS study began in 1996 at Wake Forest Baptist Medical Center for the purpose of studying how postmenopausal hormone treatment affects cognitive impairment and brain aging.

The research appears in the June 15 issue of the Annals of Neurology. Co-authors include Xinhui Wang and Helena Chui of the Keck School of Medicine of USC; John McArdle of the USC Dornsife College of Letters, Arts and Sciences; Gregory Wellenius of Brown University; Mark Serre of the University of North Carolina; Ira Driscoll pf the University of Wisconsin; Ramon Casanova and Mark Espeland of Wake Forest Baptist Medical Center; and JoAnn Manson of Harvard Medical School.

The collaborative study was funded in part by the National Institutes of Health, as well as by the Rosenblith Award from the Health Effects Institute, an organization jointly funded by the U.S. Environmental Protection Agency and certain auto and engine manufacturers. The work was also supported by the Southern California Environmental Health Sciences Center. Other support came from the National Heart, Lung and Blood Institute, the National Institute on Aging, the U.S. Department of Health and Human Services and Wyeth Pharmaceuticals, Inc. Wake Forest Baptist Medical Center funds the Women’s Health Initiative Program and its memory study.

— Leslie Ridgeway