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Background: Air pollution, particularly particulate matter less than 2.5 micrometers in diameter (PM2.5), is a significant risk factor for cardiovascular morbidity. Current studies have been primarily based on the general, typically healthy, population and there is limited information for individuals with pre-existing disease. We used the EPA CARES resource to examine the association between short-term PM2.5 exposure and heart rate (HR) in heart failure (HF) patients. Additionally, we examine potential effect modification by beta-blockers, a common medication class that modifies HR. Methods: We analyzed 3,048,856 heart rate (HR) measurements on 26,634 HF patients between January 2014 and December 2016, compiled using electronic health records from University of North Carolina affiliated hospitals. Satellite data, land use, and ground based monitoring were used to estimate daily average concentrations of PM2.5 at 1km resolution, and immediate (lag 0), delayed (lag 1 to 4), and 5 day moving average (5dMA) exposures at each primary address were computed. We used generalized additive mixed models to associate PM2.5 with HR while adjusting for age, sex, race, season, time-trend, daily temperature, and relative humidity, with a random intercept for individual. Results: PM2.5 exposure was associated with HR for lag 2 and 3 (beta = 0.006, CI = 0.002, 0.009; beta = 0.005, CI = 0.002, 0.010). Associations were stronger in individuals not taking beta-blocker medications at any time prior to HR measurement, with associations seen at all lags and strongest for 5dMA (beta = 0.086, CI = 0.073, 0.099). Conclusions: Elevated PM2.5 is associated with increased HR in HF patients. Associations are at best weak for the entire population, but strong and consistent across lags for measurements prior to beginning beta-blockers, suggesting that beta-blocker medication regimes may substantially attenuate effects of PM2.5 on HR. This abstract does not necessarily reflect the policies of the U.S. EPA.