E-cigarettes are being pitched as potentially safer alternatives to traditional tobacco cigarettes, but it's starting to look like they may share some of the same deleterious effects. A new study out of Johns Hopkins University looks at e-cigarettes' effects on mice, and it finds that exposure to their vapor limited of the mice's immune systems and impaired their lungs' ability to fight off viral and bacterial infections. They also found that the vapor contained harmful molecules known as free radicals. The findings were published this week in the journal PLOS One.

"They had more of the virus in their lungs."

The research team exposed the mice to e-cigarette vapor, from the commercial brand Njoy, for two weeks. They attempted to simulate human conditions by measuring the level of nicotine in the mice's blood and matching it with the levels that occur in human smokers. After two weeks of vapor exposure, some mice were exposed to pneumonia, other mice were exposed to the flu, and others were left in fresh air. In both cases with disease exposure, the mice had more trouble fighting back: with pneumonia, the bacteria had an easier time multiplying; with the flu, the mice lost more weight — some even died.

"We saw that they got sicker" after disease exposure, Thomas Sussan, the paper's lead author and an assistant scientist at the Bloomberg School of Public Health, tells The Verge. "They had more of the virus in their lungs, they had more inflammation in their lungs, [and] they were more susceptible to death."

The results suggest something in e-cigarette vapor is inhibiting the immune system within the mice's lungs, the researchers say. Though it's not clear what the cause is, one candidate is the free radicals the researchers detected in the vapor. These molecules are capable of damaging cells and DNA, and they're present in high levels in tobacco smoke. The researchers didn't expect to find them in e-cig vapor because there's no combustion involved, but it appears that just heating up e-cig liquid may be enough. "We were actually surprised that we found quite a bit," Sussan says, "but the levels were considerably lower than what you'd see in cigarette smoke." Far lower — more than 100 times beneath cigarette levels.

But what's impeding the mice's immune systems may be something else entirely. The researchers suggest that the culprit may be related to an additive in e-cig liquid called "propylene glycol," which is also found in fog machines and seems to cause similar lung problems in people who work with them. This study, however, didn't investigate that link. Njoy declined to comment.

"They did work with one particular brand, one particular flavor, one particular level of nicotine," David Peyton, a chemist at Portland State University who was not involved with the study, tells The Verge. "Critically, I would like to see this group follow up with no-nicotine e-cigarette fluid, I would like to see no flavoring at all, I would like to see ... the glycerine alone, and see what effects those have." Though Peyton sees no problems with the study's protocols, its limited scope means that it's a starting point for a lot more research. "As a scientist there are a whole host of question that this study opens up."

The study doesn't say if this is better or worse than cigarette smoke

That type of uncertainty speaks to the general ambiguity in the current debate around e-cigarette safety and regulation. Though it's widely expected that they'll be less harmful than traditional cigarettes, it isn't clear what the risks will be — just because the technology is so new. That's led to much debate over how strictly e-cigarettes should be regulated: loose regulations could mean e-cigs being picked up by non-smokers, exposing them to harm, while strict restrictions could prevent smokers from switching to a less-harmful alternative.

Sussan's team doesn't determine how the effects of e-cigarette vapor compare to the effects of traditional cigarette smoke, but their work does show that similar effects are present. "I can't say that it's better or worse," Sussan says, "but we see that same type of [immune] dysfunction." In particular, the researchers say this may be cause for concern should a smoker who's developed a lung disease attempt to switch to e-cigarettes for safety, as that safety may not actually be there.

Sussan notes that vaping activists often — rightly or otherwise — find fault with research into the effects of e-cigs. He believes that his research group has found a way around this by developing a more-realistic animal model. In fact, Sussan says, their model appears to be the first of its kind, with an animal being exposed to vapor in a way that actually simulates smoking.

"You can't really train mice to vape."

"This type of exposure certainly has precedent in the tobacco research area, so it's got that kind of legitimacy around it," Peyton says. That said, "You can't really train mice to vape," so there are always going to be some differences between this model and a human study. And Peyton thinks that this study may well find itself facing criticism — "and perhaps unfairly so" — simply because it doesn't look at how the effects of e-cigarette vapor compare to the effects of cigarette smoke. "People will push back on it and say, 'you're saying that this is not supporting e-cigarettes as a cessation device,'" he says. "What they've done is compared e-cigarette vaping to non-vaping."

Chances are, we're going to see more use of animal models in the future. The debate around e-cigarettes' health effects isn't going to subside any time soon, and any model that gets results people view as realistic is likely to see some traction. "Clearly this is the first study," Peyton says. "There are a lot more [e-cig] systems that this group can do exactly the same study on but with a whole different series of devices to work on.

Update February 6th, 1:04PM ET: this article has been updated to note that Njoy declined a request to comment.