E-cigarettes are so dangerous and dependent governments should consider banning evaporation, cardiologists warn.
As evaporation rates around the world increase, new research suggests that habits can damage the brain, heart, blood vessels, and lungs.
Researchers say the whole generation risks being stuck with nicotine because e-cigarettes are so addictive.
Their warning comes a day after British teenager Evan Fisher urged people not to threaten after he had almost died of "catastrophic" breathing failure
Battery-powered e-cigarettes work by heating fluid – usually containing nicotine, propylene glycol, glycerin, and aromas – into an aerosol that the user inhales and exhales.
Professor Thomas Munzel, a lead researcher at the Department of Cardiology at the University Medical Center in Mainz, Germany, said that e-cigarettes are so dangerous and dependent countries should consider banning them.
Public Health England (PHE) insists e-cigarettes are 95% less harmful than smoking.
But Professor Munzel and colleagues have said there is only "little evidence" to support claims that e-cigarettes are a "healthy" alternative to smoking or that they help people quit.
A new study, published in the European Heart Journal, looked at the effect of e-cigarettes on blood flow vapor and stiffness in the brachial artery in the upper arm of 20 smokers before they disappeared 15 minutes later.
They also exposed 151 mice to e-cigarette steam over one, three or five days, for 20 minutes six times daily.
Their results found that one episode of vaporization increased the heart rate and caused the arteries to harden, and the inner lining of the arteries – the endothelium – stopped working properly in smokers.
In mice, blood vessels, including the lungs and brain, appeared to be damaged.
Professor Munzel said: "Our data may indicate that e-cigarettes are not a healthy alternative to traditional cigarettes and their perceived 'safety' is not justified.
"In addition, we do not yet have experience of the health side effects of long-term use of e-cigarettes.
"The e-cigarette epidemic in the US and Europe, especially among our young people, is causing a huge generation of nicotine addicts who are being threatened to switch from traditional e-cigarettes to e-cigarettes.
"Research as ours should serve as a warning of their dangers and aggressive steps should be taken to protect our children from the health risks posed by e-cigarettes.
"We cannot allow the whole generation to become addicted to nicotine."
Studies in the US indicate that more than 3.6 million children use e-cigarettes, while in the UK, a report commissioned by PHE shows that the number of children and young people who smoke is increasing.
In 2018, 11.7% of 11- to 18-year-olds have tried e-cigarettes once or twice at some point – almost twice as much as 6.5% in 2014.
And 3.4% of those surveyed in 2018 said they use e-cigarettes right now – more than double that of 1.6% in 2014.
Commenting on the new study, Deborah Arnoch, chief executive of the charity Smoking and Health, said: "The short-term effects of e-cigarettes on arterial stiffness are very similar to drinking coffee, exercising or watching horror films.
"While evaporation is not risky, it is much less risky than smoking, which kills over 250 people a day in the UK.
"Vapers should not be afraid of smoking – it will be a real public health tragedy."
But Martin Mackey, a professor of European public health at the London School of Hygiene and Tropical Medicine, said the study was "a great piece of detective work".
He said it showed that the concentration of acrolein was nine times higher in e-cigarette vapor than in the liquid itself – and that is the substance that damages arterial function.
"This should be a wake-up call for those who, despite growing evidence to the contrary, continue to promote these products," he said.