E-cigarettes are not a safe alternative and can harm the lungs, new research has found.
The electronic cigarettes, used by many to quit smoking, generate toxic chemicals similar to tobacco.
Tests on mice showed "vaping" fumes caused damage to lungs and caused the animals to become far more susceptible to respiratory infections.
And their immune responses to both viruses and bacteria were weakened by such an extent that some animals died.
Professor Shyam Biswai, from Johns Hopkins University in the US, who led the research, said: "Our findings suggest that e-cigarettes are not neutral in terms of the effects on the lungs.
"This warrants further study in susceptible individuals, such as COPD (chronic obstructive pulmonary disorder) patients who have switched from cigarettes to e-cigarettes, or to new users of e-cigarettes who may have never used cigarettes."
Researchers said e-cigarettes generated just 1% of the amount of "free radical" toxins in tobacco smoke but this still posed a potential health risk.
Study co-author Dr Thomas Sussan added: "Granted, it's 100 times lower than cigarette smoke, but it's still a high number of free radicals that can potentially damage cells."