Analyzing more than 16,000 myocardial infarctions that were treated in the Salt Lake City area of Utah ( U.S.A ) from 1993 to 2014, researchers found a significant association between days that had poor air quality and the incidence of the most severe type of myocardial infarction, ST-segment elevation myocardial infarction ( STEMI ), but only in patients with previously diagnosed coronary artery disease ( CAD ).

For this latest study, researchers studied 16,314 myocardial infarctions treated in hospitals belonging to Intermountain Healthcare, specifically keying on STEMI heart attacks, non-STEMI heart attacks ( NSTEMI ), and unstable angina.
STEMI infarctions are the most severe type because it involves the total blockage of an artery, which then leads to a large portion of the surrounding heart muscle to thicken and possibly die-off from lack of oxygen.

These myocardial infarctions happened to people who resided in or around the area where they were treated, which allowed the researchers to determine the level of air quality in their residence during the time period surrounding their attack.

There was no association between air quality and myocardial infarction incidence for those without a history of coronary artery disease, but the odds of developing a STEMI infarction was approximately 15% higher for those with coronary artery disease who had been exposed to more than 25 micrograms of fine particulate matter per cubic meter of air ( 25 mcg/m3 ).
There were no significant associations between PM2.5 exposure and the other types of myocardial infarction. ( Xagena )

Source: American Heart Association's Scientific Sessions, 2015