Background: Acute changes in pollution have been associated with cardiac morbidity and mortality. Air pollution might impact cardiac morbidity and mortality through precipitation of cardiac arrhythmia in susceptible individuals. Despite gradual improvement in air quality over the past decades in North America, the links between days of higher pollution and cardiovascular morbidity and mortality still exist. This suggests that further research, at lower levels of ambient air pollution is warranted. The objective of this study is to examine the influence of acute changes in air pollution in Ontario, Canada on the frequency of discharges from implantable cardioverter defibrillators (ICDs) in response to cardiac arrhythmias.Methods: Using a case-crossover design, we compared ambient air pollution concentrations on the day of an ICD discharge to other days in the same month and year in 10320 patients with ICDs. We adjusted for weather, lagged the data from 0 to 3 days, and stratified the results by several patient-related characteristics.Results: Median (interquartile range) for SO2, NO2, ozone and PM2.5, were 1.0 ppb (0.4, 2.1), 10.0 ppb (6.0, 15.3), 26.0 ppb (19.4, 33.0) and 6.6 µg/m3 (4.3,10.6) respectively. Odds ratios (95%CI) for an ICD discharge associated with an IQR increase in pollutant were 1.0 (0.91, 1.0) for SO2, 1.0 (0.9, 1.1) for NO2, 1.0 (0.9,1.1) for ozone, and 1.0 (0.9, 1.1) for PM2.5.Conclusion: Small day to day changes in air pollution at the relatively low levels seen in our study did not appear to be a risk factor for cardiac arrhythmias in patients with ICDs.