Vaping makes users more likely to catch pneumonia – just like smoking tobacco or breathing in traffic fumes, experts suggested last night.
The vapour from e-cigarettes helps bacteria that cause the condition to stick to the cells that line the airways, they said.
The effect occurs with traditional cigarette smoke and those who are exposed to air pollution high in particulates from vehicle exhausts.
Researcher Jonathan Grigg, professor of paediatric respiratory and environmental Medicine at Queen Mary University of London, said the study shows that vaping, especially in the long term, could raise the risk of bacterial lung infection, reports Daily Mail.
He said "Pneumococcal bacteria can exist in our airways without causing illness. In some cases they can invade the lining cells causing pneumonia or septicaemia.
"We know that exposure to cigarette smoke helps these bacteria stick to airway lining cells, increasing the risk of infection. We wanted to see whether or not e-cigarettes might have the same effect."
Professor Grigg and his team began by looking at human nose lining cells in the lab. They exposed some cells to e-cigarette vapours both with and without nicotine, while other cells were not exposed.
Cells exposed to either nicotine-containing or nicotine-free vapour produced levels of the molecule PAFR – which pneumonia sticks to – that were three times higher than those not exposed to the vapour.
When pneumococcal bacteria was introduced to these cells, researchers found that exposure to either nicotine or nicotine-free vapour doubled the amount of bacteria that stuck to airway cells. Further tests were carried out on 11 e-cigarette users and six non-smokers.
The vapers were asked to take at least ten puffs on their e-cigarettes over five minutes before bacteria levels were compared between both groups. The vapers' levels of PAFR molecules were three times higher.
Professor Grigg said: "Together, these results suggest vaping makes the airways more vulnerable to bacteria sticking to lining cells. If this occurs when a vaper gets exposed to pneumococcal bacterium, this could increase the risk of infection.
"Some people may be vaping because they think it is safe or in an attempt to quit smoking, but this study adds to growing evidence that inhaling vapour has the potential to cause adverse health effects. Other aids to quitting such as patches or gum do not result in airway cells being exposed to concentrations of potentially toxic compounds."
But Professor Peter Openshaw, professor of experimental medicine at Imperial College London, said: "The evidence that vaping might increase the risk of lung infection is only indirect. Although vaping might increase susceptibility to pneumonia, the effect is likely to be lower than from smoking itself.
"This study should not be used as a reason to continue to smoke rather than vape. The evidence is that e-cigarettes are far less harmful."
It comes days after health officials suggested pregnant women who smoke should be encouraged to use e-cigarettes. Public Health England said patients should also be allowed to vape in hospital.