In the summer of 2019, an outbreak hit America. More than 2,800 people were hospitalized and 68 people died, US health authorities reported. The most likely common cause – a chemical found in some vapes that damages lung tissue, leaving people gasping for air and coughing.

That lung injury outburst spurred on researchers investigating the health harms of vaping. Five years later, the evidence from large, lengthy studies is steadily mounting that using e-cigarettes is linked to higher risks of respiratory disease, heart attacks, and stroke.

Now, a new study presented at last week's annual scientific meeting of the American College of Cardiology adds to those findings. Researchers found that people aged 18 years and over who have used e-cigarettes at any point in their lives are 19 percent more likely to experience heart failure compared to people who have never used them.

Unlike a sudden heart attack, heart failure is the slow weakening or stiffening of the heart, so much so it struggles to pump blood around the body.

"More and more studies are linking e-cigarettes to harmful effects and finding that it might not be as safe as previously thought," Yakubu Bene-Alhasan, a physician and researcher at MedStar Health in Baltimore, said in a statement after presenting the results. "The difference we saw was substantial."

The prospective study, the largest of its kind exploring the link between e-cigarettes and heart failure, has not yet been peer-reviewed.

Bene-Alhasan and colleagues analyzed health records of 175,667 US adults involved in the US National Institutes of Health's All of US Research program. Of this sample, some 28,660 people reported ever having used e-cigarettes, and 3,242 experienced heart failure in the nearly four years of median follow-up data collected.

People who already had one recorded heart failure at the start of the study were excluded from the analysis, and the researchers adjusted for age, sex, and other factors linked to heart failure, such as diabetes, high blood pressure, high cholesterol, and smoking status.

Roughly 60 percent of the participants were women, most were white, and the average age was 52 years old. Before this, large surveys typically focused on younger adults, who have a relatively low occurrence of heart attacks and strokes.

Compared to people who had never vaped, former and current e-cigarette users had a 19 percent increased risk of heart failure, specifically the type where the heart muscle stiffens and doesn't properly fill with blood. The researchers didn't ask people how often they used e-cigarettes, or what ingredients the vapes they used contained, such as nicotine or flavorings.

"It does make me concerned that what we've suspected about e-cigarettes may be true, that they have some harm to themselves related to the nicotine consumption," Yu-Ming Ni, a cardiologist at the MemorialCare Heart and Vascular Institute in California, told Healthline.

We know nicotine is highly addictive, and that young people who vape are three times more likely to take up cigarette smoking. There is also little evidence that nicotine e-cigarettes help people quit smoking.

What's more, vapes labeled 'nicotine-free' often contain the substance. Research shows that nicotine-free vapes can harm lung tissue and blood vessels too. The next stop after the lungs and bloodstream is the heart, but more research is needed to pinpoint which compounds are causing harm.

Among the 28,660 e-cigarette users, those who also said they smoked cigarettes saw their risk of heart failure jump to 59 percent compared to people who vaped but didn't smoke.

However, Ni notes that this latest observational study can only point to associations between e-cigarette use and heart failure: "All we can say is that there is an association between those two things."

Other researchers not involved in the work want to see future studies investigate how often people use e-cigarettes and the relative risk of heart failure with varying exposure levels.

"I think this research is long overdue, especially considering how much e-cigarettes have gained traction," Bene-Alhasan said.

"With more research, we will get to uncover a lot more about the potential health consequences and improve the information out to the public."