Air pollution exposure is a critical health risk factor for many chronic illnesses and has been linked to dementia. However, there are a number of gaps in our knowledge of how air pollution contributes to dementia, and these are the focus of our proposal. First, most air pollution exposure studies do not differentiate between causes of dementia. Second, they tend to rely on cross-sectional analyses. Third, they do not consider dementia biomarkers. To address those issues, we chose to focus on longitudinal studies of aging and brain health where we can distinguish among two of the most common causes of dementia, i.e., Alzheimer's disease (AD) and vascular cognitive impairment (VCI), which may also co-exist. Specifically, we will leverage data from four diverse longitudinal studies of brain aging including more than 23,000 participants from metro-Atlanta and Georgia spanning a wide range of age, socioeconomic status, and cognitive status. These longitudinal studies focus on unaffected or minimally affected individuals since studying those individuals affords us the best chance of determining the relative contribution of AD and VCI to overall cognition. Each study performs detailed neurocognitive testing, standard cerebrospinal fluid (CSF) biomarkers of AD and brain volumetric biomarkers of AD and VCI and we will assign high-resolution fine particle ambient air pollution concentrations (PM2.5 and its components) to all study participants based on their current and past residential addresses. Leveraging these rich data allows to test for association between air pollution and different facets of AD- or VCI. To understand how air pollution contributes to AD, VCI, or both, we will 1) investigate associations between air pollution and indicators of AD characterized by subjective memory complaints, short-term memory or language impairment based on neurocognitive testing, positive AD CSF biomarkers (i.e., Aβ42, total-Tau, and phospho-Tau) and smaller hippocampal volumes; 2) investigate associations between air pollution and indicators of VCI characterized by impairment in the executive function (measured by neurocognitive testing) and cerebral microvascular ischemia changes measured by brain MRI; and 3) investigate associations between air pollution and accelerated epigenetic aging and dementia-related DNAm patterns. Critically, we will also evaluate how sex, race/ethnicity, socioeconomic factors, genetic risk, depression, cardiovascular disease and diabetes modify the associations between air pollution and indicators of AD and VCI. This study will provide biological insights of how air pollution affects early manifestations of the two most common causes of dementia (AD and VCI) and how modifiers (e.g., sex and race/ethnicity) affect vulnerability. Therefore, we expect this work to have an important and sustained impact on the field.