Smog may be a heart disease patient’s worst enemy, according to new research presented Sunday at the annual American Heart Association's Scientific Sessions.

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

"Our research indicated that during poor air quality days, namely those with high levels of PM 2.5, patients with heart disease are at a higher risk of suffering from a STEMI heart attack," said lead author Dr. Kent Meredith in a statement . PM2.5 is defined as air pollution that contains fine particles smaller than 2.5 millimeters in diameter, such as soot and car exhaust, and its exposure to humans has been tied to a wide array of chronic health problems as well as premature death .

For this latest study, the researchers studied 16,314 heart attacks treated in hospitals belonging to Intermountain Healthcare, specifically keying on STEMI heart attacks, non-STEMI heart attacks (NSTEMI), and unstable anginas. STEMI heart attacks 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 heart attacks 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 heart attack incidence for those without a history of CAD, but the odds of developing a STEMI heart attack was approximately 15 percent higher for those with CAD who had been exposed to more than 25 micrograms of fine particulate matter per cubic meter of air (25 μg/m 3 ). There were no significant associations between PM2.5 exposure and the other types of heart attacks.

Though this study cannot definitively conclude a link between air pollution and heart attack risk, Meredith and his team believe it points to possible interventions for people with CAD.

"The study suggests that during many yellow air quality days, and all red quality air days, people with known coronary artery disease may be safer if they limit their exposure to particulate matter in the air by exercising indoors, limiting their time outdoors, avoiding stressful activities, and remaining compliant with medications," said Dr. Meredith. "These activities can reduce inflammation in the arteries, and therefore make patients less sensitive to the fine particulate matter present on poor air quality days."

The Air Quality Index , run by the Environmental Protection Agency, is comprised of six color-based categories, ranging from good to hazardous air quality. The red (4th) level is considered “unhealthy”, denoting a level of local air pollution that has the potential to cause adverse health effects among the entire local population.

Source: Meredith K, Pope C, Muhlstein J, et al. Short-term Exposure to Fine Particulate Matter Air Pollution Preferentially Increases the Risk of ST-Elevation Acute Coronary Events. American Heart Association's Scientific Sessions. 2015.