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Dangerous emissions from flavourings used in e-cigarettes damage the lungs, according to a new study.

Although safer than smoking real cigarettes, e-cigarettes are still toxic - with some flavourings causing more lung stress than others.

"Our research affirms that e-cigarettes may pose significant health risks and should be investigated further.

"It seems that every day a new e-cigarette product is launched without knowing the harmful health effects of these products," said Irfan Rahman, a professor at the University of Rochester, where the research was carried out.

The research team looked at the impact of vaping on human lung cells and mice. In both cases the lung cells became inflamed.

The harmful vapours seem to be created by when the element inside the e-cigarette is activated to heat the "juice" into an aerosol that can be inhaled like regular cigarette smoke.

It's these vapours that contain nano-particles of heavy metals and other carcinogens that aggravate the lung tissue.

Cinnamon-flavoured e-liquid was one of the worst offenders, while sweet flavours were generally more damaging than straight nicotine.

One worrying trend is a technique known as "dripping" where smokers drip e-liquid directly onto the heating element to get a stronger nicotine hit and allow for an easy way to switch between flavours. This releases more lung-damaging chemicals.

The study follows two leading cancer organisations calling for more restrictions on e-cigarettes at a time where there's not enough evidence of the value of vaping in quitting smoking.