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A new study is the latest to show changes in cardiovascular function after vaping e-liquids, though in this case, those liquids didn’t even contain nicotine.

The study, published Tuesday in the journal Radiology, concluded that vaping temporarily impacts blood vessel function in healthy people. Using MRI scans, it found, for example, changes in blood flow within the femoral artery in the leg after just one use. The researchers couldn’t determine which chemical might be responsible for the changes they observed.

“After a few minutes, everything normalizes. One could say, big deal, nothing happens,” said study author Felix W. Wehrli, professor of radiologic science and biophysics at the University of Pennsylvania’s Perelman School of Medicine.

“But if someone vapes regularly,” Wehrli continued, there’s a possibility that, over time, things might not go back to normal as readily. The changes his team measured in 31 people, who had never vaped or smoked, reflect “the same processes … known to be initiating steps in the development of cardiovascular disease,” including atherosclerosis. But that takes years to develop, he added.

The study is the most recent addition to a proliferation of research aiming to measure the impact of e-cigarettes on the heart, blood vessels, lungs and brain. But experts say the research remains in its early stages, often taking place in the lab or in animals.

A study in May, for example, found evidence that e-cigarette flavors had toxic effects – including poorer cell survival and signs of increased inflammation – on a type of cardiovascular cell in the lab.

“The use of e-cigarettes is increasing and the data demonstrating potential harm … is also growing,” doctors from the University of Massachusetts Medical School said in a commentary published alongside the May study. “In addition to harm from the nicotine, the additives are a potential source of adverse vascular health and one that is being disproportionately placed on the young.”

While experts have long suspected that vaping poses fewer health risks than smoking cigarettes, the doctors wrote that “little is known about the potential toxicology” of flavorings, particles, heavy metals and other components used in e-cigarettes.

“Nobody knows what it does to the human lung to breathe in and out aerosolized propylene glycol and glycerin over and over. It’s an experiment, frankly,” Dr. Robert Jackler, founder of Stanford Research Into the Impact of Tobacco Advertising, said at a congressional hearing last month.

“We will find out, years from now, the results,” he said.

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Jackler said a number of chemicals used by the flavor industry may be safe when absorbed through the intestine, but we don’t yet know the impact they can have on the lungs over a long period of time.

“There’s no doubt in my mind that vaping is safer than conventional smoking,” Jackler said, “but that doesn’t mean that it’s safe.”