The chemicals produced by e-cigarettes combine inside people's lungs to make entirely new combinations that are toxic to living cells, scientists have found.
Chemicals that produce flavours such as vanilla, berry and cinnamon can mix up with other solvents in the gadgets and become a danger to health.
Whether using e-cigarettes - known as vaping - is safe has been a topic of debate for years as their use has increased rapidly, particularly among young people.
Scientists generally agree that they are safer than smoking tobacco, which is proven to cause more than a dozen types of cancer and contribute to other diseases.
But evidence is increasingly showing that vaping isn't good for you either.
The new research shows that the chemicals inside the devices can become toxic to cells living in the lungs or the blood vessels and heart.
Chemicals used in e-cigarettes (stock image) combine to create not-seen-before mixtures which are damaging to cells in the lungs and the linings of blood vessels, researchers said
'We consistently observed that the new chemicals formed from the flavours and e-liquid solvents were more toxic than either of their parent compounds,' said Professor Sven-Eric Jordt, a pharmacologist at Duke University in North Carolina.
He and colleagues at Yale University isolated chemicals used in e-cigarettes and put them onto human lung cells in a lab.
The cells were those that occur in the lining of the bronchi, which are the main airways that connect the windpipe to the insides of the lungs.
Vaping doesn't work as an aid for quitting tobacco and actually has the opposite effect, suggests new research.
Researchers said there was 'no evidence that e-cigarettes were helpful in the quit attempt' in a large-scale study in the US.
Scientists from the University of California in San Diego carried out an analysis of data on 45,971 Americans included in the government-commissioned Population Assessment of Tobacco and Health (PATH) Study.
Participants in the study were interviewed in 2013 and 2014 and are interviewed every year.
A quarter of smokers who tried to quit in the first years of their enrolment used e-cigarettes to help with their attempt. And at their follow-up interview one year later, 9.6 per cent had managed to stay away from tobacco over the previous 12 months.
However, even though some had stopped smoking, researchers say the number who quit was hardly different at all from smokers who didn't use e-cigs.
One of the studies' authors, Dr John P. Pierce, said: 'Among this representative sample of US smokers trying to quit, we found no evidence that e-cigarettes were helpful in the quit attempt.
'This lack of effectiveness was also apparent in the sub-sample who used e-cigarettes on a daily basis for this quit attempt.'
The study was published in the journal PLOS One.
Chemicals they looked at included the flavourings