Abstract Body (Do not enter title and authors here): Background:
Short-term exposure to ambient air pollution, especially particulate matter smaller than 2.5 microns in diameter (PM_2.5), is associated with an increased risk of acute coronary syndrome and is identified as the leading modifiable cause of cardiovascular mortality in the long term. In this study, we set out to examine the effect of acute exposure to high levels of PM_2.5 (≥12 μg/m³) on long-term mortality risk of ST-elevation myocardial infarction (STEMI) patients.

Methods:
From June 2010 to October 2021, 1,553 patients at Liverpool Hospital in Sydney met the 4th Universal Definition of MI criteria for STEMI. The average daily maximum PM_2.5 was measured using publicly available land-based air quality monitors in the catchment area. Mortality risk between the two groups was compared using a Kaplan-Meier plot and further assessed using the Cox regression model.

Results
915 STEMI patients presented on days with maximum PM_2.5 ≥12 μg/m³. These patients had a significantly increased risk of late all-cause mortality with a hazard ratio of 3.08 (CI= 2.01-4.71, p<0.001) compared with those with lower PM_2.5 level exposures. Each 10-unit increase in PM_2.5 exposure was associated with a 10% (p=0.002) higher risk of late mortality.

Conclusions
Patients presenting with STEMI on polluted days are at increased risk of late all-cause mortality, which may be due to increased inflammatory response and myocardial injury at the time of cardiac insult. This finding warrants a more diligent acute care and long-term follow-up strategy for this group of vulnerable patients.