E-cigarettes could damage blood vessels in the brain and countries should consider banning them, scientists have warned.
Tests on humans and mice has suggested vaping stiffens the arteries and speeds up the heart, raising blood pressure and risking brain damage.
It also produces a chemical which forces a naturally-occurring enzyme in the body to trigger internal tissue damage, German researchers say.
The University Medical Center in Mainz research comes in a week which has already seen horror stories about vaping and scientific studies adding fuel to the fire.
Just yesterday, 19-year-old Ewan Fisher, from Nottingham, told how vaping caused his lungs to fail. A study claimed the devices are just as bad for the heart as tobacco. And a man in the US was revealed to be the first to have a double lung transplant because of vaping.
Cardiologists now say countries should think about following the example of India and Singapore and banning e-cigarettes outright.
Ewan Fisher, 19, from Nottingham, had to be hooked up to life support and almost died from serious respiratory failure triggered by his vaping habit when he was a 16-year-old
Experts at the German university measured blood flow and vessel stiffness in arteries in the arms of 20 humans before and 15 minutes after they used an e-cigarette.
And they also exposed 151 mice to e-cigarette vapour for two hours per day over one, three or five days.
Doctors at a Michigan hospital say they have performed the first double lung transplant on a man whose lungs were irreversibly damaged from vaping.
No other details about the transplant were released by the Henry Ford Health System in Detroit on Monday.
The patient has reportedly asked his medical team to share photographs and an update to warn others about the dangers of vaping, which will be presented later today.
It comes amid the slew of vaping-related lung illnesses that have sickened 2,051 Americans since March and have killed at least 40 in 24 states and the nation's capitol.
Most of the victims are male and under the age of 35, with the ages of those who died ranging from 17 to 75, according to the report.
There have been three deaths each confirmed in California, Georgia, Illinois, Indiana, Massachusetts and Minnesota and two deaths each in Kansas, Oregon and Tennessee.
In the human participants – who were all smokers in real life but still considered 'healthy' – they found the vapour made their arteries stiffen up.
It also reduced the function of the cells in the blood vessel linings and increased oxidative stress – a chemical imbalance which can cause tissue damage – in blood vessels in the brain.
In the rodents, the scientists found an enzyme which is found naturally in the body, named NOX-2, was being affected by the chemical acrolein which came from the e-cigarettes.
NOX-2, they said, was responsible for damage to blood vessels including ones in the lungs and brain. Mice without the enzyme were not harmed.
Professor Thomas Münzel said: 'The results identified several mechanisms whereby e-cigarettes can cause damage to the blood vessels, lungs, heart and brain.
'This is a consequence of toxic chemicals that are produced by the vaping process and may also be present at lower concentrations in the liquid itself.
'Importantly, we identified an enzyme, NOX-2, that mediated all the effects of e-cigarettes on the brain and cardiovascular system, and we found that a toxic chemical called acrolein, which is produced when the liquid in e-cigarettes is vaporised, activated the damaging effects of NOX-2.
'Our data may indicate that e-cigarettes are not a healthy alternative to traditional cigarettes, and