Scientists from John Hopkins University in Maryland, US have found that exposure to e-cigarette vapour can compromise the immune system in the lungs, making them more susceptible to infections such as pneumonia and flu.
Not only that, but the same free radical chemicals previously thought only to be found in tobacco cigarettes and air pollutants, have been found in e-cigarette vapours too.
"We were surprised by how high that number was, considering that e-cigarettes do not produce combustion products," said Thomas Sussan, PhD, lead author and an assistant scientist in the Department of Environmental Health Sciences at the Bloomberg School.
"Granted, it's 100 times lower than cigarette smoke, but it's still a high number of free radicals that can potentially damage cells."
Effect of e-cigarette vapour on mice
The results of the study, entitled Exposure to electronic cigarettes impairs pulmonary anti-bacterial and anti-viral defenses in a mouse model, are published in the journal PLOS ONE.
The researchers exposed one group of mice to the same amount of e-cigarette vapour that an actual human would inhale for two weeks, while the other group was just exposed to air.
Then, both groups were divided into three sub-groups each.
One sub-group of each original group received nasal drops containing Influenza A, while a second sub-group on each side received nasal drops containing Streptococcus pneumoniae, a bacteria responsible for pneumonia and sinusitis, among other illnesses. The final two sub-groups received nothing at all.
The mice exposed to e-cigarette vapour were significantly more likely to develop compromised immune responses to both the virus and the bacteria, which in some cases killed the mice.
"E-cigarette vapour alone produced mild effects on the lungs, including inflammation and protein damage," said Sussan.
"However, when this exposure was followed by a bacterial or viral infection, the harmful effects of e-cigarette exposure became even more pronounced.
"The e-cigarette exposure inhibited the ability of mice to clear the bacteria from their lungs, and the viral infection led to increased weight loss and death indicative of an impaired immune response."
What exactly are e-cigarettes?
E-cigarettes, also known as electronic cigarettes, are slim hand-held battery-powered devices that provide inhaled doses of vaporised nicotine.
An atomiser heats a solution made of nicotine and either propylene glycol or glycerine, which produces vapour that the user can inhale.
As such, due to the fact that tobacco is not being burned, e-cigarettes have been touted as being healthier, as they are free from almost all the toxic chemicals that accompany nicotine in regular cigarette smoke.
However, recently more and more evidence is coming to light that e-cigarettes may still be harmful. In August 2014, the World Health Organisation called for increased legislation to ban the use of e-cigarettes indoors and to prevent them from being sold to minors.
In November 2014, research commissioned by the Japanese Ministry of Health found a number of e-cigarette products to have 10 times the level of cancer-causing carcinogens that normal cigarettes have.