A study, published in journal BMJ and ScienceDaily, found vapour may cause inflammation and impair the activity of alveolar macrophages, cells which remove potentially damaging dust particles, bacteria and allergens.
The researchers said some of the effects are similar to those seen in cigarette smokers and people with chronic obstructive pulmonary disease (COPD).
Professor David Thickett, lead author from the University of Birmingham, 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, then that’s something we need to know about.”
He added: “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.”