Vaping just once — even when it doesn't contain nicotine or THC — can damage a person's blood vessels, according to a small study published Tuesday in the journal Radiology.

The new research adds to the growing body of evidence that there may be no harmless form of vaping.

"We've shown that vaping has a sudden, immediate effect on the body's vascular function," study author Felix Wehrli, a professor of radiologic science and biophysics at the University of Pennsylvania, said in a statement.

Previous lab studies have suggested that vaping can have harmful effects. However, studies looking at vaping’s direct effects on humans have been few and far between.

Wehrli and his colleagues recruited 31 healthy people in their 20s and early 30s to participate in the study. None had ever smoked any kind of cigarette — regular or electronic.

For this study, they took 16 puffs of an e-cigarette that contained tobacco flavorings and sweeteners like propylene glycol and glycerol, but no nicotine.

To study how vaping affected blood vessels, the researchers briefly cuffed each person's leg to restrict blood flow to the femoral artery, a large artery in the thigh, and took an MRI image. They did this twice, before and after the participates puffed on the e-cigarettes.

Ordinarily, when the cuff is released, blood should flow much faster, as the body tries to make up for the restricted flow. Indeed, that's what the researchers observed before the participants used the vapes. But after vaping, the scans showed that blood didn't flow as much as before; instead, they observed reduced blood flow and oxygen in the leg — an effect that lasted for about an hour.

Over time, however, "this kind of damage to the body can become cumulative," said Dr. S. Christy Sadreameli, a pediatric pulmonologist at the Johns Hopkins Hospital in Baltimore, and volunteer spokesperson for the American Lung Association. She was not involved in the new research.

That cumulative damage is what could increase the risk of heart problems.

"It may not mean that you are going to have a heart attack soon," Sadreameli said. "But while we all get some damage to our blood vessels with aging, this means it could start happening younger and in a more accelerated fashion."

Wehrli told NBC News that the effect is similar to what's known about conventional tobacco smoking.

"If you smoke a single cigarette or once in a while, you probably won’t get cardiovascular disease or cancer," he said. "It’s the continued regular chronic exposure that leads to damage."

It's unclear what's causing the blood vessel damage. It may be the aerosol, the flavors, or the fact that these molecules change into something more damaging when they're heated.

"The only thing we don’t know is what specifically caused this ... response," Wehrli said. "But we know it’s there. There is strong evidence."

The exact ingredients in e-cigarettes aren't all known, because the Food and Drug Administration does not require vape manufacturers to provide a full list. Though the researchers didn't look at the effects of vaping e-cigarettes with nicotine, it's likely that these effects would also be seen when vaping nicotine, as the other ingredients are also present.

"We do know that they contain ultra-fine particles that are inhaled deep into the lungs," Sadreameli said. Those particles are able to pass through the alveoli, tiny air sacs in the lungs that allow oxygen to get into the blood.

The new findings come as doctors and health officials grapple with a mysterious respiratory illness that is damaging lungs and causing breathing problems nationwide.

Doctors tell NBC News that most patients are otherwise healthy teenagers or young adults who arrive at the hospital with shortness of breath, chest pain, fatigue and coughing.

Their symptoms quickly become worse, and many need to go on ventilators.

The CDC says it's investigating 94 cases in 14 states. The only thing linking the cases is a history of vaping products containing either nicotine or THC, the psychoactive ingredient in marijuana.

Nicotine-free e-cigarettes, like the ones used in this study, account for less than 1 percent of the market share in the U.S., according to another recent study from the CDC and the Truth Initiative, a nonprofit organization that advocates for tobacco cessation.

"However it's still concerning because many people may think of nicotine-free products as harmless," Sadreameli said. "In many cases, these may be the first products a teenager uses."

Sadreameli continued: "The American Lung Association is concerned. I am concerned. Physicians are concerned about the impact of any of these products on the lungs and heart."

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