(CNN) -- A new study suggests that vaping e-liquids, specifically propylene glycol and glycerin, may lead to some inflammation in the lungs -- but more research is needed to determine just how much inflammation may occur over a prolonged period of time.
The study, published in the journal Cancer Prevention Research on Wednesday, found that in people who had never smoked, after using e-cigarettes just twice daily for a month, levels of propylene glycol in their system was linked with changes in inflammatory cell counts in their lungs, although the magnitude of changes was small.
The study is the "first experimental demonstration" of the impact that e-cigarette use may have on inflammation in human lungs, the researchers wrote.
The study findings came as no surprise to Dr. Peter Shields, a medical oncologist with a focus on lung cancer at the Ohio State University Comprehensive Cancer Center in Columbus, who was the study's lead author.
"Obviously inhaling stuff is going to have some impact on the lungs," Shields said.
"This is another piece of information," he said about the study. "I would not make the conclusion that this shows that e-cigs are harmful to your health, but one piece of evidence for that. I would say that we have choices in the world and it makes sense that if you're not a smoker, you shouldn't start using e-cigs."
The study has published at the same time an outbreak of lung injury associated with e-cigarette use is sweeping the United States and currently under federal investigation -- but the study has no direct connection with the outbreak.
Among those lung injury cases, "most patients report a history of using tetrahydrocannabinol (THC)-containing products," according to the US Centers for Disease Control and Prevention. "Exclusive use of nicotine containing products has been reported by some patients with lung injury cases, and many patients with lung injury report combined use of THC- and nicotine-containing products."
The new study examined specific constituents of e-liquids used in e-cigarettes -- propylene glycol and vegetable glycerine -- which are carriers for nicotine and flavors in e-liquids.
"But it is still unknown what role, if any, nicotine containing e-cigs are involved in the patients with severe respiratory illness," Shields said. "In that context, a lot of us are really looking hard for what might be unique to e-cigs that we wouldn't see in traditional smokers."
The study included 30 young adults, ages 21 to 30, who did not have a history of smoking and had not used e-cigarettes.
The adults, who were recruited for the study between 2015 and 2017, were randomly assigned into two groups: one was a control group and the other was instructed to use e-cigarettes at least twice daily, taking 20 puffs during one hour each time, for one month.
The adults in the e-cigarette group were given refillable tank devices with LED screens that had a puff counter to measure their daily e-cigarette use. They vaped using e-liquids that contained 50% propylene glycol and 50% vegetable glycerine, but no nicotine and no flavors.
At the start of the study and then five weeks later, all of the adults underwent bronchoscopy -- a procedure that looks inside the lung airways -- so that the researchers could analyze their lung tissue and lung health.
Between the control group and the e-cigarette group, the researchers found no significant differences in the levels of inflammatory cells and proteins in the adults' lungs.
However, when looking specifically at the lungs of the adults in the e-cigarette group, the researchers found a correlation between the amount of propylene glycol they inhaled and the concentration of inflammatory cells in their lungs.
The researchers knew how much propylene glycol each person in the e-cigarette group consumed by measuring propylene glycol levels in their urine samples.
"So people with higher changes in levels actually inhaled more, and when you look at that, you see an increase in inflammation," Shields said.
"But then the question is so what? How much is there?" he said about the inflammation. "Given that these people were no different than the control group, what it says is that we measured an increase but they were still within the normal range."
The study had some limitations, including that a small sample size of adults were involved and the study went on for only about a month. Also, the e-cigarettes in the study did not have nicotine or flavors.
If the study continued for more than one month and if the e-cigarettes were used more than twice a day or contained flavors and nicotine, "it's quite possible that more use would give more inflammation," Shields said, "and now we have direct evidence for inflammation from an important part of e-liquids."
The study is the most recent addition to a proliferation of research aimed at measuring the impact of e-cigarette use on the heart, blood vessels, lungs and brain -- but experts caution much of that research remains in its early stages, often taking place in the lab or in animals.
"This is a nice start to hopefully more longitudinal studies with more participants, if able in the future," said Dr. Maria Rahmandar, pediatrician and medical director of the Substance Use & Prevention Program at Ann & Robert H. Lurie Children's Hospital of Chicago, who was not involved in the new study.
"Really all we currently know is that the FDA calls propylene glycol and vegetable glycerin safe for ingestion or putting on skin. These have not been deemed safe to inhale," Rahmandar said.
"We're still learning so much about e-cigarettes. Even these components that seem like they should be safe, we have no idea what they do once they're heated up, aerosolized and broken down into their byproducts, and what effect those have, especially when they're broken down into teeny tiny ultrafine particles that can go into deep parts of the lung," she said. "This, I think, is a great beginning to what we need to look at more deeply."
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