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Objective: To evaluate the association between long-term exposure to ambient air pollution and cognitive decline in older adults residing in an urban area., Methods: Data for this study were obtained from 2 prospective cohorts of residents in the northern Manhattan area of New York City: the Washington Heights-Inwood Community Aging Project (WHICAP) and the Northern Manhattan Study (NOMAS). Participants of both cohorts received in-depth neuropsychological testing at enrollment and during follow-up. In each cohort, we used inverse probability weighted linear mixed models to evaluate the cross-sectional and longitudinal associations between markers of average residential ambient air pollution (nitrogen dioxide [NO 2 ], fine particulate matter [PM 2.5 ], and respirable particulate matter [PM 10 ]) levels in the year prior to enrollment and measures of global and domain-specific cognition, adjusting for sociodemographic factors, temporal trends, and censoring., Results: Among 5,330 participants in WHICAP, an increase in NO 2 was associated with a 0.22 SD lower global cognitive score at enrollment (95% confidence interval [CI], -0.30, -0.14) and 0.06 SD (95% CI, -0.08, -0.04) more rapid decline in cognitive scores between visits. Results were similar for PM 2.5 and PM 10 and across functional cognitive domains. We found no evidence of an association between pollution and cognitive function in NOMAS., Conclusion: WHICAP participants living in areas with higher levels of ambient air pollutants have lower cognitive scores at enrollment and more rapid rates of cognitive decline over time. In NOMAS, a smaller cohort with fewer repeat measurements, we found no statistically significant associations. These results add to the evidence regarding the adverse effect of air pollution on cognitive aging and brain health., (© 2020 American Academy of Neurology.)