E-cigarettes may be as risky as smoking tobacco at increasing the potential for bacterial infections in the lungs, researchers believe.
There is "little difference" in the effect of tobacco smoke and e-cigarette vapour on bacteria often found in the lungs, experts at Queen's University Belfast found.
They found an increase in the potential of bacteria to cause harm when exposed to both cigarette smoke extract and e-cigarette vapour.
The changes could lead to chronic obstructive pulmonary disease (COPD), an umbrella term which includes emphysema and bronchitis, and asthma, they say.
The study authors are calling for "urgent" further research into the long-term effects of vaping.
The researchers compared the effects of smoke extracted from Marlboro red cigarettes to the vapour from a best-selling device, in moths and samples of human cells that line the airway.
The paper reads: "Exposure of respiratory pathogens to e-cigarette vapour induced changes in phenotype and virulence, which may increase bacterial persistence and inflammatory potential.
"These changes were similar, and in some cases exceeded, those observed following bacterial exposure to cigarette smoke and suggest that there is little difference between the effect of (cigarette smoke and e-cigarette vapour).
"There is therefore an urgent need for further robust clinical studies investigating and clarifying the long-term effect of e-cigarette use on both airway cells and respiratory pathogens, to enable a better-informed judgment to be made regarding their safety."
Dr Deirdre Gilpin, researcher from the School of Pharmacy at Queen's University and lead author, added: "This study shows us that vaping may carry the same risk as cigarette smoke in increasing the susceptibility to bacterial infection."
Experts generally agree that vaping is safer than smoking, but the jury is out on the potential long-term implications and how safe it is.
In November, leading cardiologists published research suggesting vaping could damage the brain, heart, blood vessels and lungs, while the European Respiratory Society more recently said it cannot back vaping as a safe way to quit smoking.
Other researchers say they observed clear early benefits in switching from smoking to vaping on vascular function, in a trial by the University of Dundee.
The authors of the current study say that to fully assess the changes, tests on mammal models need to be undertaken.
They also say they may have underestimated the exposure of the bacteria to e-cigarettes because they used similar delivery methods for the vapour and tobacco smoke.
In practical terms, research has found that e-cigarette users typically take larger and longer puffs compared with traditional smokers.
The authors also used unflavoured e-cigarettes in the study, while they say other research has shown a link between flavourings and additives and changes in bronchial cells.
Professor Jose Bengoechea, director of the Wellcome-Wolfson Institute for Experimental Medicine and co-author of the study said: "Worryingly, e-cigarette vapour as well as cigarette smoke increases the harmful potential of already dangerous infections, in addition to the well-known detrimental effect on lung function.
"At the very least this work should open a frank debate on vaping safety."
Prof Peter Hajek, director of the Tobacco Dependence Research Unit, Queen Mary University of London (QMUL), said: "The study explores an interesting topic, but as with all such experiments using petri dishes, moths, and huge levels of exposure, the question arises as to how this translates to humans.
"Some evidence from humans exists, and it points in the opposite direction.
"Not only do smokers with respiratory illness who switch to vaping seem to experience at least the same improvements as unaided quitters, there are some signs that vaping may in fact protect vapers from respiratory infections."