Vaping may be worse for health than conventional smoking, according to new research.
Flavourings in e-cigarettes harm the lungs by triggering inflammation, scientists have warned.
The battery-powered devices are promoted as a tool to help smokers quit the habit.
But even short term use causes as much or more damage as the real thing, said the Greek team.
In experiments on mice they found the additives, including flavourings, caused lung inflammation similar or worse than that seen in traditional cigarette use.
Corresponding author Dr Constaninos Glynos said: “The observed detrimental effects in the lung upon e-cigarette vapour exposure in animal models highlight the need for further investigation of safety and toxicity of these rapidly expanding devices worldwide.”
They simulate smoking a traditional cigarette by dispensing a vapour derived from liquid chemicals in a refillable cartridge that typically contain propylene glycol, nicotine and often flavourings.
Propylene glycol – a colourless and odourless additive – is found in numerous processed food and beverages. It is also used as a solvent in a number pharmaceuticals.
The findings, published in the American Journal of Physiology-Lung Cellular, show e-cigs and refills are not well regulated – and their long-term effects on health are unknown.
So the researchers compared several groups of mice that received whole-body exposure to varying chemical combinations four times each day, with every session separated by 30-minute smoke-free intervals.
Dr Glynos said: “Electronic cigarettes are advertised as a less harmful nicotine delivery system or as a new smoking cessation tool.
“Our findings suggest that exposure to e-cig vapour can trigger inflammatory responses and adversely affect respiratory system mechanics.
“In many cases, the added flavor in e-cigs exacerbated the detrimental effects of e-cig vapour.”
One group received cigarette smoke and three others e-cig vapours containing either propylene glycol, both this and nicotine or these two ingredient and a tobacco flavouring. A fifth batch just got normal, healthy air and acted as a control.
Some animals in each party underwent the regime for three days (short term) and others four weeks (long term).
There was an increase in markers of inflammation, mucus production and altered lung function in all three e-cig groups – after only three days.
But those getting propylene glycol alone showed fewer negative effects with long-term exposure.
Dr Glynos, of the University of Athens, said this suggests the additive elicits only a temporary irritation that eventually subsides with continued use.
In addition, two inflammation-producing proteins became elevated in the flavouring group.
This means some of the many flavouring components on the market may not be safe for even short-term use, said Dr Glynos.
The condition of the e-cig groups alarmed the researchers. The level of oxidative stress – damage at a cellular level – in those exposed to flavourings was equal to or higher than that of the cigarette group.
However, respiratory mechanics were adversely affected only in mice exposed to cigarette smoke – and not to e-cig vapour after prolonged treatment.
Dr Glynos added: “We conclude that both e-cig vaping and conventional cigarette smoking negatively impact lung biology.”
His findings follow a British study earlier this year that also suggested vaping is more harmful than previously thought.
It found some of the effects are similar to those seen in smokers and people with chronic lung disease – boosting the production of inflammatory chemicals in the body.
The University of Birmingham scientists extracted cells from lung samples provided by healthy non-smokers and exposed them to e-cig fluid, condensed vapour or nothing for 24 hours.
Exposure to the vapour increased cell death and the production of inflammatory chemicals.
And the ability of cells to engulf bacteria was significantly impaired in those exposed to vapour.
Public Health England says e-cigs are 95 per cent safer than traditional cigarettes. And they suggest smokers should consider switching to vaping in a bid to help them quit.
But critics warn vaping may cause lung disease, keep people hooked on nicotine, or act as route in to smoking for kids.
Some 7.4million Brits smoke and 2.8million use e-cigs, Office for National Statistics figures show.