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Air pollution is emerging as an important risk factor for acute coronary syndrome (ACS). In this study, we investigated the association between short-term air pollution exposure and mechanisms of coronary plaque instability evaluated by optical coherence tomography (OCT) imaging in ACS patients. Patients with ACS undergoing OCT imaging were retrospectively selected. Mechanism of culprit lesion instability was classified as plaque rupture (PR) or intact fibrous cap (IFC) by OCT. Based on each case's home address, the mean daily exposures to several pollutants, including particulate matter 2.5 (PM 2.5), on the same day of ACS and in the immediate days (up to 6 days) prior to the index ACS, were collected. 139 ACS patients were included [69 (49.6%) had PR and 70 (50.4%) IFC]. Patients with PR, compared to those with IFC, had higher PM 2.5 exposure levels on the same day of ACS, without differences in the immediate 6 days before index ACS. At multivariate analysis, PM 2.5 exposure on the same day of ACS was the only independent predictor of PR [OR = 1.912 per SD (8.6 μg/m3), CI95 % (1.087–3.364), p = 0.025]. Patients with PR presented a steady increase in PM 2.5 daily exposure levels in the days preceding the occurrence of ACS, with a peak the day of ACS (p for trend = 0.042) This study demonstrates for the first time that a higher short-term PM2.5 exposure, on the same day of ACS, is associated with an increased risk of PR as a pathobiological mechanism of coronary plaque instability. [Display omitted] • Air pollution is emerging as an important risk factor for acute coronary syndrome (ACS). • Acute PM 2.5 exposure, especially on the same day of ACS, is associated with an increased risk of plaque rupture as mechanism of coronary plaque instability. • Mitigation strategies are crucial to reduce the burden of ischemic heart disease, especially in patients with a higher risk of cardiovascular events. [ABSTRACT FROM AUTHOR]