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Electronic cigarettes blast out chemicals which are proven to cause lethal cancers, new research has revealed.

A fresh investigation into the health risks of inhaling liquid tobacco vapour has found it is full of the same "free radicals" found in cigarette smoke.

Scientists were "surprised" to see the cancer-causing chemicals in the clouds emitted by "vape" devices, which are at the centre of a growing health scare.

Even though vaping is considered safer than smoking, experiment after experiment has highlighted the many dangers of liquid nicotine.

Professor Shyam Biswal, a lung disease expert at Johns Hopkins University, led the research.

In guarded academic language, he said his research destroys the claim that electronic cigarettes are harmless.

"Our findings suggest that e-cigarettes are not neutral in terms of the effects on the lungs," he said.

"We have observed that they increase the susceptibility to respiratory infections."

(Image: Reuters)

He spent two weeks bombarding mice with e-cigarette vapour before exposing them to pneumonia or flu viruses.

Some rodents which were blasted with the vapour became unable to fight off these lung infections.

"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," said co-researcher Dr Thomas Sussan.

He had one piece of good news for vapers: e-cigarettes produced just 1% of the amount of free radicals in tobacco smoke.

"We were surprised by how high that number was, considering that e-cigarettes do not produce combustion products," added Dr Sussan.

"It's still a high number of free radicals that can potentially damage cells," he continued.

Free radicals are destructive molecules known to inflict serious damage on cells and DNA. They are proven to contribute to the development of cancer.

After this research was published, the Electronic Cigarette Industry Trade Association issued a statement claiming the experiment was not reliable.

The trade body claimed the unlucky mice were hit with more vapour than a human would be exposed to.