Smoking isn’t good for you. But e-cigarettes, with their sleek, USB bodies and mango-flavored cartridges, promised a sweeter, safer future. No tar, no combustion, no problem. But that picture is getting more complicated. Last week, the Centers for Disease Control announced that it is opening an investigation into the health effects of smoking e-cigarettes, after nearly 100 teenagers in 14 states reported lung illnesses related to vaping. The cases, which were primarily reported among teenagers and young adults, were so severe that some patients were hospitalized and put on ventilators.

So a study out Tuesday in the journal Radiology comes not a moment too soon. In it, researchers show that inhaling e-cigarette vapor—just the vapor, without any nicotine or flavorings—has an immediate, negative impact on the vascular system.

E-cigarettes first appeared on the market in 2007 and in the years since, vaping among teens has skyrocketed. The CDC estimates that one in five high school students use e-cigarettes. From 2017 to 2018, e-cigarette use among teenagers increased by more than 75 percent, prompting the US Surgeon General to call it an “epidemic.”

Yet not much is known about the harms associated with e-cigarettes. Sure, nicotine is harmful even when it isn’t smoked: vaping nicotine is still highly addictive, can harm the development of adolescent brains, and can even cause seizures. But e-cigarettes contain more than nicotine, and the bulk of research so far has largely overlooked how these other ingredients affect users.

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Researchers at the University of Pennsylvania’s Perelman School of Medicine simplified the question. They removed nicotine and flavorings and just looked at what inhaling that basic vapor does to a person’s blood vessels.

Using an MRI, the researchers examined the veins and arteries of 31 people before and after they took a few puffs of an e-cigarette. Their e-cigarettes contained only vape juice, a mixture consisting primarily of water and either glycerol or propylene glycol, which keep everything dissolved inside the cartridge. The test subjects—who were all between the ages of 18 and 35—were nonsmokers and first-time vapers. But after taking 16 three-second puffs, the participants had worse circulation, stiffer arteries, and less oxygen in their blood. “The results of our study defeat the notion that e-cigarette vaping is harmless,” says Felix Wehrli, the study’s principal investigator.

Although glycerol and propylene glycol are considered safe to eat, they may not be safe to inhale. Wehrli’s study shows that when the chemicals are heated and inhaled, they end up passing through the lungs and into the arteries and veins that make up our vascular system. Once there, they irritate the epithelium, a thin layer of cells that lines blood vessels and helps regulate blood flow, blood clotting, and immune responses. The inflamed epithelium then alters how arteries expand and contract. “We did expect an effect, but we never thought the effect was as big as what we found,” notes Wehrli. “It’s not just a little change we detect—it’s a major effect.”

Healthy blood vessels naturally widen and constrict to regulate how much blood is flowing through the body. When Wehrli and his colleagues examined three arteries in the leg, heart, and brain, they found that vaping constricted each one by more than 30 percent. That meant that blood wasn’t flowing as quickly as it was prior to inhaling the vapor. The researchers also found that vaping reduced the amount of oxygen in the blood by 20 percent, and made the walls of the blood vessels more rigid and stiff—a symptom often associated with cardiovascular diseases like hypertension and stroke. Other studies have found similar results in animals, but this is the first such finding in human subjects. “It’s really stunning,” says Sven Jordt, who studies e-cigarettes at Duke University and who was not involved in this study.