A new study suggest that e-cigarettes may not be as healthy as previously thought.
The study, conducted at the University of Birmingham, found that in some cases the vapour from e-cigarettes disabled important immune system cells within the lungs.
Those responsible for the study said that their results urge "caution against the widely held opinion that e-cigarettes are safe".
That said, they are still better for you than smoking. Public Health England has previously advised that e-cigarettes are much less harmful than cigarettes and advised that they should be used to help aid quitting smoking habits.
The small University of Birmingham study was led by Professor David Thickett and looked at the effects of vaping on the lung tissue of eight non-smokers.
They set up a machine that replicated the effect of vaping and studied what happened to the samples of tissue.
They discovered that in some of the cases the vapour causes the tissue to become inflamed. It also impaired the function of certain cells that remove bad things like dust, bacteria and allergens called alveolar macrophages.
However, on top of the fact that as a method of quitting, it's obviously better for you than continuing to smoke, they also said that the results were only discovered in a laboratory over a short space of time.
They said that we need to do further research before it can be claimed to be as accurate as possible.
Earlier this year, an independent review concluded that e-cigarettes should be available on prescription due to the fact that there is "overwhelming evidence" that they are better for you than smoking.
Professor Thickett warned that, despite them being safer than fags, more research is definitely needed.
He said: "In terms of cancer causing molecules in cigarette smoke, as opposed to cigarette vapour, there are certainly reduced numbers of carcinogens.
"They are safer in terms of cancer risk - but if you vape for 20 or 30 years and this can cause COPD [chronic obstructive pulmonary disease], then that's something we need to know about.
"I don't believe e-cigarettes are more harmful than ordinary cigarettes - but we should have a cautious scepticism that they are as safe as we are being led to believe."
Martin Dockrell, who is the lead for tobacco control at Public Health England, said: "E-cigarettes are not 100% risk-free but they are clearly much less harmful than smoking.
"Any smoker considering e-cigarettes should switch completely without delay."