Work out in polluted air and you may miss out on some of the brain benefits of exercise, according to two, large-scale, new studies of exercise, air quality and brain health. The studies, which involved tens of thousands of British men and women, found that, most of the time, people who ran and rode vigorously had larger brain volumes and lower risks for dementia than their less active peers. But if people exercised in areas with even moderate levels of air pollution, the expected brain improvements from exercise almost disappeared.

A large body of evidence demonstrates that, on the whole, exercise bulks up our brains. In studies, active people generally sport more grey matter in many parts of their brains than sedentary people. Grey matter is made up of the brain’s essential, working neurons. Fit people also tend to have healthier white matter, meaning the cells that support and connect neurons. White matter often frays with age, shrinking and developing Swiss-cheese-like lesions even in healthy adults. But fit people’s white matter shows fewer and smaller lesions.

But air pollution has the opposite effects on brains.

Few studies, though, had explored how exercise and air pollution might interact inside our skulls and whether working out in smoggy air would protect our brains from noxious fumes or undermine the good we otherwise gain from working out.

So, for the first of the new studies — published in January in Neurology — researchers at the University of Arizona and University of Southern California [both in the US] pulled records for 8,600 middle-aged adults enrolled in the UK Biobank. A huge trove of health and lifestyle records, the Biobank holds information on about more than 5,00,000 British adults, such as their ages, home locations, socioeconomic status, genomes and extensive health data. Some of the participants also completed brain scans and wore activity monitors for a week to track their exercise habits.

Using established air quality models, they then estimated air pollution levels where the people lived and, finally, compared everyone’s brain scans.

Disappearing benefits

As expected, vigorous exercise was linked, in general, to sturdy brain health. But any beneficial associations almost disappeared when exercisers lived in areas with even moderate air pollution. (Levels in this study were mostly within the bounds considered acceptable for health by European and American air quality standards.) Their grey matter volume was smaller and white matter lesions more numerous than among people living and exercising away from pollution, even if their workouts were similar.

The same scientists repeated aspects of this experiment with another 35,562 older UK Biobank participants, comparing people’s exercise habits, local pollution levels and diagnosis of dementia, if any. The data showed the more people exercised, the less likely they were to develop dementia over time — provided their local air was clear. When it was moderately polluted, though, they had an increased long-term risk of dementia, whether they exercised or not.

‘Alarming’ finding

“These data are of significant importance in terms of our understanding of modifiable risk factors for brain aging,” said Pamela Lein, a professor of neurotoxicity at the University of California, US.

The studies have limitations. They are observational and show links between exercise, pollution and brain health, but cannot prove that bad air directly counteracts the brain benefits of exercise, or how this might occur. They also did not look into where people worked out, only that some lived in places with iffy air.

Boosting brain health

In practice, a number of measures may help to bolster the brain benefits of exercise, experts say.

➡ “Stay away from busy highways, if at all possible,” said David Raichlen, a professor of biological sciences at U.S.C. and co-author of the new studies. Automobile exhausts are among the worst pollutants for human health.

➡ Working out indoors may be no better. “The available evidence suggests pollution levels indoors are about the same as those outside,” Raichlen said, unless a building, such as a gym, has installed extensive air filtration systems. Pollutants can readily enter buildings through open doors or windows or cracks in the structure, and the government doesn’t routinely monitor indoor air quality.

➡ Masking might help. Both surgical and N95 masks filter some unhealthy particulates, such as soot and other matter, said Melissa Furlong, an environmental epidemiologist at the University of Arizona and co-author of the two studies. “If you don’t mind wearing a mask while exercising,” she said, “this would likely result in a reduction of exposure to particulates.”

➡ Most importantly, keep exercising. Exercise has multiple benefits for cardiovascular health, and “we do not want to discourage people from being physically active,” Raichlen said, even if air conditions are not ideal. In the new studies, the brains of people who exercised in polluted air looked no better, he pointed out — but their brains were also no worse than those of people who did not exercise at all.