The perception that e-cigarettes are safe should be treated with caution, scientists have said, as new research suggests vapour could harm lung cells.

A study, published in the journal Thorax, found vapour may cause inflammation and impair the activity of the cells that remove potentially damaging dust particles, bacteria and allergens.

Researchers from the University of Birmingham say some of the effects are similar to those seen in cigarette smokers and people with chronic obstructive pulmonary disease (COPD).

"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," says lead author Professor David Thickett.

"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," he says.