O-29B2-4 Background/Aims: We systematically examined the time-course and acute effects of fine particles (PM2.5) on following 5 major domains of cardiac electrophysiologic parameters: (1) heart rate variability (HRV) as measures of cardiac autonomic modulation (CAM), (2) heart-rate corrected QT intervals as measures of ventricular repolarization, (3) ST-segment height as measures of ventricular ischemia, (4) P-wave and PR durations as measures of atrial fibrillation vulnerability, and (5) frequency of ectopy. Methods: We obtained 24-hour beat-to-beat ECGs using a 1000 Hz Holter ECG system. After visual identification and removal of artifacts and arrhythmic beats, we calculated the 5 major domains on each of the 30-minute segments. We used a personal PM2.5 monitor to measure 24-hour individual-level real-time PM2.5 exposures and calculated 30-minute time-specific average PM2.5 exposures. Distributed lag models were used to assess the autocorrelation corrected relationship between 30-minute PM2.5 and each of the 5 ECG domains. In these models, lag 0 indicates the spontaneous relationship between PM2.5 and the ECG outcomes, and lag 1 indicates 30 minutes between the PM2.5 and ECG outcomes, and so on. Results: The mean (SD) of personal PM2.5 exposure was 14 (22) μg/m3. We indentified 2 patterns of significant adverse effects of PM2.5 concentration on ECG parameters: (a) acute effects on decreasing CAM up to lag 7 and prolonging ventricular repolarization up to lag 8; and on increasing ST-height and atrial fibrillation vulnerability up to lag 4; and (b) ultra acute effects on increasing ectopy up to lag 1. Conclusion: Higher PM2.5 is adversely associated with all 5 major ECG parameters we analyzed, with the adverse effects occurring acutely (within 2–4 hours of elevated PM2.5) or ultra acutely (within 0.5 hr of elevated PM2.5). These adverse effects on ECG parameters may trigger the onset of acute cardiac events and cumulatively over time, may result in increased risk of cardiac disease.