Exercising in a polluted area can be bad for the brain.
Exercise
Activities such as running, tennis and football can help to reverse some of the brain ageing linked with conditions such as dementia but people who are active in polluted areas do not get the same benefit.
Experts looked at more than 8,000 people whose physical activity was measured with a fitness tracker for a week.
The most active had reduced levels of white matter lesions - damage to the inner layer of the brain - but only if they lived in a low-pollution area.
Tiny pollution particles can potentially cause inflammation in the body and damage to blood vessels which may have an effect on the brain.
Dr. Melissa Furlong, who led the study from the University of Arizona, said: "Vigorous exercise may increase exposure to air pollution, and prior studies have shown adverse effects of air pollution on the brain."