The health risks of using e-cigarettes remain murky due to a lack of conclusive research, but scientists who conducted a recent study on the devices’ possible heart effects are warning they may be “far more dangerous” than previously thought. In fact, vaping may pose the same harms to the heart as smoking tobacco cigarettes, The Telegraph reported.

The research, presented in August at the European Society for Cardiology congress in Rome, found the habit adversely impacts the main artery of the heart.

Researchers analyzed a group of 24 adult smokers whose hearts were monitored while they vaped and while they smoked regular cigarettes. They found that a 30-minute session of vaping had a similar impact on stiffness of the aorta as five minutes spent smoking one cigarette.

“We measured aortic stiffness,” said professor Charalambos Vlachopoulous, lead researcher from the University of Athens Medical School, according to The Telegraph. “If the aorta is stiff, you multiply your risk of dying either from heart disease or from other causes.”

Vlachopoulous further cautioned that long-term risks of vaping are unknown, and that he would not recommend its use as a smoking alternative.

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“The aorta is like a balloon next to the heart,” he said, according to The Telegraph. “The more stiff the balloon is, the more difficult for the heart to pump. It’s the most powerful biomarker we have for estimating cardiovascular risk.”

Other experts reportedly called the findings important and said it was an indicator that more research into long-term effects of e-cigarettes were needed.