Cardiac patients whose blood type is A, B or AB are at increased risk for myocardial infarction (MI) in highly polluted environments, according to research out of two Utah medical centers.

The study, which was presented at the American Heart Association’s Scientific Sessions 2017 conference Tuesday, expanded on previous research that detailed the risks heart patients face during stretches of significant air pollution, according to a release from Intermountain Medical Center. Prior studies have found that ingestion of fine particulate matter like carbon monoxide, sulfur dioxide and ozone raises the risk of cardiovascular disease just as much as risk factors like hypertension and diabetes.

Lead investigator Benjamin Horne, PhD, and colleagues went a step further with that knowledge.

“We wondered, if someone has a specific variation in this ABO gene, are they more or less likely to experience a heart attack in times of higher pollution?” he said in the Intermountain release. “The primary mutation we studied differentiates between O blood types and non-O, which includes positive and negative A, B and AB blood types. The one that’s been found in genetic studies to be lower risk is O. The other three were higher risk.”

The ABO gene, found in individuals with A, B or AB blood types, is the only gene that’s been proven to predict heart attacks in patients with coronary disease. Other studies have validated different genes as predictors of coronary artery disease in healthy patients.

Horne said his own previous research yielded a risk threshold for non-O blood type patients: 25 micrograms per cubic meter of pollution. The increase in PM2.5 is linear, he said, and below that level there is “little, if any, difference in risk.” Pollution levels can—rarely—reach 100 micrograms per cubic meter, but between 50 and 60 is more common.

Horne and his team wrote that O-type heart patients are still at risk for heart attacks in densely polluted areas, but the risk is much smaller than for other blood types. Either way, Horne said, people suffering from coronary disease don’t develop the condition and experience a heart attack the next day. He said staying indoors, exercising indoors and staying compliant with taking heart medication all work to reduce potential harm.

“You have to have other characteristics for coronary disease to progress to a heart attack,” he said. “The association between heart attacks and pollution in patients with non-O blood isn’t something to panic over, but it is something to be aware of.”