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Background: Unfavorable associations between air pollution and myocardial infarction are broadly investigated in recent studies and some of them revealed considerable associations; however, controversies exists between these investigations with regard to culprit components of air pollution and significance of correlation between myocardial infarction risk and air pollution., Methods: The association between exposure to PM 10 , PM 2.5 , ozone, carbon monoxide, sulfur dioxide, and nitrogen dioxide concentration of background air that residents of Tehran, the capital city of Iran, which is ranked as the most air polluted city of Iran and the relative risk of developing ST-elevation myocardial infarction (STEMI) were investigated by a case-crossover design. Our study included 208 patients admitted with a diagnosis of STEMI and undergone primary percutaneous intervention. Air pollutant concentration was averaged in 24-h windows preceding the time of onset of myocardial infarction for the case period. Besides, the mean level of each element of air pollution of the corresponding time in one week, two weeks and three weeks before onset of myocardial infarction, was averaged separately for each day as one control periods. Thus, 624 control periods were included in our investigation such that. Each patient is matched and compared with him/herself., Results: The mean level of PM 10 in case periods (61.47µg/m 3 ) was significantly higher than its level in control periods (57.86µg/m 3 ) (P-value = 0.019, 95% CI: 1.002-1.018, RR = 1.010). Also, the mean level of PM 2.5 in case periods (95.40µg/m 3 ) was significantly higher than that in control days (90.88µg/m 3 ) (P-value = 0.044, 95% CI: 1.001-1.011, RR = 1.006). The level of other components including NO 2 , SO 2 , CO and O 3 showed no significant differences between case and control periods. A 10µg/m 3 increase in PM 10 and PM 2.5 would result in 10.10% and 10.06% increase in STEMI event, respectively. Furthermore, the results of sub-group analysis showed that older patients (equal or more than 60 year-old), diabetic patients, non-hypertensive ones and patients with more than one diseased vessel may be more vulnerable to the harmful effect of particular matters including PM 10 and PM 2.5 on development of STEMI., Conclusion: Air pollution is a worldwide pandemic with great potential to cause terrible events especially cardiovascular ones. PM 2.5 and PM 10 are amongst ambient air pollutant with a high risk of developing STEMI. Thus, more restrictive legislations should be applied to define a safe level of indoor and outdoor air pollutant production., (Copyright © 2017 Elsevier Inc. All rights reserved.)