Research Report 193 describes a novel study by Jiu-Chiuan Chen and colleagues examining possible associations between long-term exposure to ambient particulate matter (PM) and changes in the brains of older women in the United States. The study focused on brain volumes and neurocognitive outcomes, specifically mild cognitive impairment and dementia. Dr. Chen used neuroimaging and cognitive outcome data from women enrolled in the Women’s Health Initiative Memory Study and estimated their exposure to ambient PM2.5 and to diesel PM.