Objectives: We evaluated the association between short-term exposure to outdoor air pollution and out-of-hospital cardiac arrest in Japan. Methods: We studied 558 residents of Okayama, Japan, who visited hospital emergency departments between January 2006 and December 2010 for out-of-hospital cardiac arrest using a time-stratified case-crossover design. We calculated city-representative average concentrations of different air pollutants and examined the association between air pollution and cardiac arrest. Results: Exposure to air pollution was associated with an elevated risk of cardiac arrest; for example, odds ratios was 1.17 (95% confidence interval, 1.02 to 1.33) per interquartile-range increase in suspended particulate matter concentrations in the previous 48 to 72 hours. We also observed different susceptibilities to suspended particulate matter and ozone exposures by age category. Conclusions: Short-term exposure to outdoor air pollution was associated with increased risk of cardiac arrest.