The alternative smoking trend that’s taking over international markets might be more dangerous than tobacco companies are letting on—heat-not-burn tobacco’s negative impact on blood vessel function matches the damage done by smoking regular cigarettes, the American Heart Association reported at its Scientific Sessions 2017.

Heat-not-burn products aim to avoid smoke exposure, according to an AHA release, by raising the temperature of tobacco to a level where nicotine-containing vapor is released, but not cranking up the heat enough to burn the tobacco. Though these products have been marketed overseas as a healthier alternative to smoking, the health effects of using heat-not-burn tobacco are still largely unknown.

A preliminary animal study presented at this week’s Scientific Sessions tested heat-not-burn products in rats, whose reactions to the vapor were recorded. Researchers monitored the rats’ blood vessel response to the vapor released by heat-not-burn tobacco and recorded the vessels’ ability to widen in response to increased blood flow.

The scientists found that not only was the amount of nicotine in the rats’ blood after exposure to heat-not-burn products higher than it would have been had the rats smoked one traditional cigarette, but after ten 15-second exposures over five minutes to heat-not-burn vapor, the rats’ blood vessel function decreased by 58 percent.

After ten 5-second exposures to the vapor, the rats saw a 60 percent decrease in blood vessel function. The researchers said these results were comparable to minimized vessel function in regular cigarette smokers.

With the decreasing popularity of e-cigarettes, analysts have predicted heat-not-burn tobacco products could find a successful home in U.S. markets. Philip Morris International, which distributes the iQOS (“I Quit Ordinary Smoking”) used in the rat trials, has already entered the FDA approval process to start marketing and sales in America.