A new study is casting yet another shadow over e-cigarettes. Researchers at the University of Pennsylvania say participants who inhaled only vape juice—no nicotine, no flavorings—suffered temporary changes to their cardiovascular system, Wired reports. The study found that brain, heart, and leg arteries in the 31 participants constricted by over 30%, which reduced oxygen flow in the blood by 20%. Arteries also stiffened, a symptom already linked to cardiovascular ailments like stroke and hypertension. "The results of our study defeat the notion that e-cigarette vaping is harmless," says study leader Felix Wehrli.
But Juul, which controls over 70% of the American vaping market, pointed out a possible limitation—that participants inhaled 16 times for three seconds each. The San Francisco-based company called it "a forced puffing regime that is unrealistically high in volume with very limited time between puffs." And not only vaping causes such responses: Caffeine use and exercise also temporarily affect vascular activity. But after dozens came down with breathing illnesses linked to vaping, and another study raised red flags about vape juice, caution may be the best bet. "There's no doubt in my mind that vaping is safer than conventional smoking," a doctor tells CNN, "but that doesn't mean that it's safe." (Read more vaping stories.)