Long-term exposure to air pollution is associated with lower scores on tests of mental acuity, researchers have found. And one reason may be that air pollution causes changes in brain structure that resemble those of Alzheimer’s disease.

The scientists studied 998 women ages 73 to 87 and free of dementia, periodically giving them tests of learning and memory. They used magnetic resonance imaging to detect brain atrophy, or wasting, and then scored the deterioration on its degree of similarity to the brain atrophy characteristic of Alzheimer’s disease. They matched Environmental Protection Agency data on air pollution to the women’s residential addresses.

Over 11 years of follow-up, they found that the greater the women’s exposure to PM 2.5, the tiny particulate matter that easily penetrates the lungs and bloodstream, the lower their scores on the cognitive tests.

After excluding cases of dementia and stroke, they also found a possible reason for the declining scores: The M.R.I. results showed that increased exposure to PM 2.5 was associated with increased brain atrophy, even before clinical symptoms of dementia had appeared. The study is in the journal Brain.