Background: Previous studies have shown that air pollution particulate matter (PM) is associated with an increased risk for myocardial infarction. The effects of air pollution on the risk of ST-elevation myocardial infarction (STEMI), in particular the role of gaseous air pollutants such as NO2 and O-3 and the susceptibility of specific populations, are still under debate. Methods: All patients entered in the Belgian prospective STEMI registry between 2009 and 2013 were included. Based on a validated spatial interpolation model from the Belgian Environment Agency, a national index was used to address the background level of air pollution exposure of Belgian population. A time-stratified and temperature-matched case-crossover analysis of the risk of STEMI was performed. Results: A total of 11,428 STEMI patients were included in the study. Each 10 mu g/m(3) increase in PM10, PM2.5 and NO2 was associated with an increased odds ratio (ORs) of STEMI of 1.026 (CI 95%: 1.005-1.048), 1.028 (CI 95%: 1.003-1.054) and 1.051 (CI95%: 1.018-1.084), respectively. No effect of O3 was found. STEMI was associated with PM10 exposure in patients >= 75 y.o. (OR: 1.046, CI95%: 1.002-1.092) and with NO2 in patients