Outdoor workouts in high-pollution environments may hurt brain health

How you exercise matters. But neuroscience researchers at USC and the University of Arizona have found that where you exercise may be just as important, especially for brain health.

A new study co-authored by USC Professor David Raichlen indicates that vigorous exercise in a highly polluted area can actually diminish the positive brain benefits of exercise. The paper, published online on Wednesday in Neurology, the medical journal of the American Academy of Neurology, gives a glimpse into the complex impact of air pollution on the human brain.

The positive impacts of an active lifestyle are not limited to improved athletic performance. Regular exercise is often recommended as a way to strengthen the brain and body’s resilience to age-related disease, and the brain also benefits. Researchers have connected physical activity — specially sustained high-intensity, or vigorous, activity — with reduced risk of developing dementia and Alzheimer’s disease, for instance.

Inversely, brain health suffers significantly for people living in highly polluted areas. Air pollution like car exhaust can lead to a higher risk of dementia, poorer cognition and adverse effects on brain volume. Until now, though, scientists had not investigated how living in a polluted environment could offset the benefits of exercise.

Air pollution impacts outdoor workouts

“We found that, while vigorous physical activity was good for brain health, exposure to air pollution seemed to mute some of these benefits,” said Raichlen, the study’s senior author and a professor of human and evolutionary biology at the USC Dornsife College of Letters, Arts and Sciences. “For example, vigorous physical activity reduced white matter lesions, a key marker of brain health, but these benefits were eliminated in areas with high air pollution.

“We’re not recommending we avoid all exercise in air pollution. But, since increased white matter lesions are a risk for stroke and Alzheimer’s disease, we do think we should put more thought into where we exercise, and, for example, avoid areas that are close to vehicle traffic.”

The study expands upon a growing body of USC research that has linked air pollution to a host of health issues from Alzheimer’s disease to asthma.

Interdisciplinary neuroscience approach links brain health and air pollution

To measure physical activity, researchers utilized data from a massive ongoing study run by the UK Biobank, a large-scale biomedical database of more than 500,000 participants in the United Kingdom. For one week, a subset of adult participants wore devices which measured physical exertion. The device, the Axivity AX3, resembles a wrist-worn fitness tracker but gathers more robust information, including measurements of the participants’ intensity and duration of physical activity.

“Every time you land, your body experiences impact forces. The device in this study picks up a combination of these forces — not just what your arm is doing — to help us figure out the intensity of activity,” explained Raichlen.

Subjects also underwent brain scans — multimodal MRI imaging — to measure the volume of brain tissue, and to detect high-signal regions in the brain’s white matter. These regions, known as white matter lesions, are associated with numerous age-related brain diseases including stroke, Alzheimer’s disease and dementia.

The activity and MRI data of 8,600 participants were then measured against the estimated level of air pollution at participants’ residences. Researchers accounted for specific types of pollution: annualized levels of nitrogen dioxide, PM2.5 — particulate matter smaller than 2.5 microns generated by car and power plant exhaust — and PM2.5 absorbance, or black carbon in air.

Melissa Furlong, lead author of the study and an assistant professor at the University of Arizona, said the levels of air pollution measured in this study exist in cities and urban environments across the world.

“Cities like Los Angeles or New York have air pollution levels within the ranges we tested in this study,” she said. “In fact, the amount of air pollution we looked at in this study is well within the normal ranges of air pollution for even cities most people would consider ‘healthy.’”

The results were sobering. Vigorous physical activity was associated with increased gray matter volume and reduced white matter lesions; however, the benefits for white-matter lesions were eliminated when participants hailed from a highly polluted area.

“Air pollution exposure is associated with a heightened risk of reduced brain health and dementia—all sorts of problems,” Raichlen said. “It made sense that during exercise, as your rate of breathing increases, you’re also increasing your exposure to air pollution.”

Study recommends reducing urban air pollution exposure, not exercise

Raichlen reiterated the findings do not recommend eliminating exercise in polluted areas. The team did not find evidence of physical activity in these areas contributing to poor brain health, only that the presence of air pollution diminished the benefits of physical activity for some aspects of brain health. Raichlen added that it is even more important now for scientists to understand how both the environment and lifestyle behaviors like exercise can affect brain health especially as it relates to Alzheimer’s, dementia and other age-related diseases.

“We are increasingly recognizing the importance of exercise as a modifiable lifestyle factor that can reduce the effects of brain aging and the risk for Alzheimer’s disease,” Raichlen said.

“These results really underscore the need to focus on reducing air pollution exposure in urban environments,” Raichlen added. “Cleaning up air in our cities will allow us all to gain the benefits of physical activity regardless of where we live.”

The study was supported by the National Institute on Aging, the National Institute of Environmental Health Sciences, the Arizona Department of Health Services and the McKnight Brain Research Foundation. UK Biobank is generously supported by its founding funders the Wellcome Trust and UK Medical Research Council, as well as the British Heart Foundation, Cancer Research UK, Department of Health, Northwest Regional Development Agency and Scottish Government.

Co-authors with Raichlen include: Melissa Furlong, assistant professor at the Mel and Enid Zuckerman College of Public Health, University of Arizona, Gene E. Alexander, professor, Department of Psychology, University of Arizona; and Yann C. Klimentidis, associate professor, Department of Epidemiology and Biostatistics, University of Arizona.