ISEE-25 Abstract: An association between air pollution and cardiovascular disease (CVD) mortality has been reported. One of the hypothesized arrhythmogenic mechanisms is the impairment of cardiac autonomic control by pollutants. We examined the short-term associations of ambient pollutants (particulate matter < 10 μm in diameter [PM10], O3, CO, NO2, and SO2) with cardiac arrhythmias using data from the baseline examination (1987–1989) of the population-based Atherosclerosis Risk in Communities study. We calculated the following pollutant exposures one-day prior to each participant’s randomly assigned examination date: PM10, CO, NO2, and SO2 as 24-hour averages, and O3 as an 8-hour average of the hourly measures, from the EPA’s Aerometric Information Retrieval System (AIRS). We assessed the presence, frequency, and complexity of arrhythmias from standardized 2-minutes EKG rhythm strips recorded during the baseline clinical examination. After excluding missing data on the ambient exposure measurements, the effective sample sizes for PM10, SO2, NO2, CO, and O3) were 4,001, 7,404, 7,427, 10,699, and 9,059 respectively. The prevalence of arrhythmias was 11%. Among persons without any arrhythmia, the means (SD) of PM10, SO2, NO2, CO, and O3 measured as one day prior their ECG and clinical examination were 29.5 (12.3) μg/m3, 0.005 (0.004) ppm, 0.018 (0.008) ppm, 1.35 (0.60) ppm, and 0.040 (0.017) ppm, respectively. For persons with arrhythmias, the means (SD) were 30.0 (13.0) μg/m3, 0.005 (0.003) ppm, 0.017 (0.008) ppm, 1.33 (0.59) ppm, and 0.040 (0.017) ppm, respectively. Logistic regression models were used to adjust for CVD risk factors, demographic and socioeconomic variables, and relevant meteorological variables to obtain the odds ratios (OR) and 95% confidence intervals (CI) of the presence of arrhythmias associated with one standard deviation increase of each ambient pollutant: the OR for PM10, SO2, NO2, CO, and O3 were 1.00 (0.99, 1.01), 0.94 (0.86, 1.04), 0.98 (0.90, 1.06), 0.94 (0.84, 1.05), and 1.00 (0.91, 1.11), respectively. The frequency and complexity of arrhythmias were not associated with any of the pollutants analyzed. In conclusion, no statistically significant association between short-term increase in ambient pollutants and the presence of arrhythmias was observed in this in healthy, middle aged population.