Living in places with exposure to very fine air pollutant particles doubled dementia risk in older women, poor performance on memory tests was found in 19,000 retired nurses for every additional 10µg/m3 of polluted air they breathed, MRI scans reveal more brain atrophy in people that live close to the highway in a dose-dependent manner in more than one study. These associations held up even after adjusting for factors such as education, smoking, diabetes, and cardiovascular disease.

Although many observational studies are now linking air pollution to dementia it is difficult to establish causation. However, a growing body of animal studies are establishing causation between breathing in particulate matter and brain inflammation and atrophy.