A new research study has found that nicotine e-cigarettes and vaping products could pose greater risks to heart health than cigarettes or other traditional tobacco products.

Researchers from the Smidt Heart Institute at Cedars-Sinai Medical Center in Los Angeles presented these findings at this week’s annual American Heart Association Scientific Sessions. In the study, doctors measured the blood flow in the hearts of 10 non-smokers, 10 tobacco smokers, and 10 e-cigarette smokers. These measurements were taken before and after subjects performed a hand-grip exercise that simulates physiological stress. Nicotine users’ blood flow was also tested before and after smoking.

The study found that after exercising, blood flow increased in the hearts of non-smokers. In cigarette smokers, blood flow increased slightly after smoking, but decreased during the hand-grip exercise. But for e-cigarette users, blood flow decreased after vaping and after the exercise. “This suggests e-cigarettes cause an abnormality that impedes blood flow regulation in the heart,” said study co-author Dr. Florian Rader, a heart specialist at Cedars-Sinai, to Time.

It’s common knowledge that smoking tobacco can increase the risk of lung cancer, but smoking actually poses a greater risk to heart health. Around 210,000 Americans die from smoking-induced cardiovascular disease each year, compared to 140,000 annual lung cancer deaths, according to the Office of the Surgeon General. The Smidt heart study is one of the first to explore whether or not vaping nicotine can also damage cardiovascular health.

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“We have known for decades that smoking increases your risk for heart attack and dying from heart disease,” said Christine Albert, MD, MPH, founding chair of the newly established Department of Cardiology at the Smidt Heart Institute. “Now, with this study, we have compelling evidence suggesting that newer methods of electronic nicotine delivery may be equally, or potentially more, harmful to your heart as traditional cigarettes.”

“Our results suggest that e-cigarette use is associated with coronary vascular dysfunction at rest, even in the absence of physiologic stress,” said Susan Cheng, MD, MPH, MMsc, director of Public Health Research at the Smidt Heart Institute and director of Cardiovascular Population Sciences at the Barbra Streisand Women’s Heart Center, in a statement. “These findings indicate the opposite of what e-cigarette and vaping marketing is saying about their safety profile.”

The results of this study are worrying, especially considering the fact that the number of high school students using e-cigarettes increased from 20.8 percent last year to 27.5 percent this year. The small sample size of this study makes it difficult to draw definite conclusions, though. Rader told Time that the blood flow “averages were very different for the e-cigarette smokers compared to the tobacco smokers. For e-cigarette smokers, I would say this provides another cautionary note, and it’s also justification for larger research studies.”

The researchers did not test whether or not cannabis vaping products posed any risks to heart health. Participants were not even asked if they smoked or vaped cannabis, which may further confound the results of the research.

The study does not establish any links to the recent outbreak of EVALI (e-cigarette, or vaping, product use associated lung injury), which has killed dozens of Americans and sickened thousands more since this summer. But the troubling idea that vaping could damage both heart and lungs might signal that it is time to re-evaluate whether vaping is actually a safer alternative to traditional methods of tobacco combustion.