The debate over the safety of e-cigarettes rages on – and now new research says they could potentially be as dangerous as regular tobacco cigarettes.

The devices which contain nicotine-based liquid could be as harmful as unfiltered cigarettes when it comes to causing DNA damage, according to University of Connecticut (UConn) scientists.

They also found that the vapour from non-nicotine e-cigs caused as much DNA damage as filtered cigarettes, possibly due to the chemical additives.

Cellular mutations caused by DNA damage can lead to cancer.

As well as whether the e-cig contains a nicotine or non-nicotine liquid, the level of DNA damage e-cigs causes also depends on the amount of vapour the user inhales and how many other additives are present.

E-cigarettes – now used by millions – may not be as harmless as we think, scientists warn

Lead author of the study and postdoctoral researcher at UConn Karteek Kadimisetty, said: 'I was shocked the first time I saw the result, so I ran the controls again. I even diluted the samples.

'But the trend was still there – something in the e-cigarettes was definitely causing damage to the DNA.'

Previously, the rise in e-cigarette popularity has been praised for helping nicotine addicts to quit their smoking habit – more than 9 million adults in the US and around 2.9 million in the UK vape.

E-cigs heat a liquid – usually containing nicotine mixed with the chemicals propylene glycol and glycerin, and often flavourings such as menthol or fruit – into a vapour that users can inhale.

The smokeless devices do not produce tar and carbon monoxide – the nasties in tobacco cigarettes.

Therefore, research seems to have found that they are safer than traditional cigarettes in the short term. But because they are still fairly new, their long-term effects are not fully known.

Study is flawed say critics

VAPING TRIGGERS BLADDER CANCER The chemicals in e-cigarettes are linked to cancer-related bladder tissue damage, recent research found. A team from the Vanderbilt-Ingram Cancer Center in Nashville compared the urine of e-cigarettes users with that of nonsmokers. Some 92 percent of e-cigarette users' urine tested positive for two of the five compounds linked to bladder cancer, they discovered. In a second study, the researchers analysed the effect of nicotine and its chemical compounds - including nitrosamines and formaldehyde - on DNA repair in cells lining the bladder. E-cigarettes were found to trigger cancer-related damage to bladder tissue. The research also revealed that nicotine, nitrosamines and formaldehyde block DNA repair, boosting the cancer risk. A different recent study however suggests that e-cigarettes are less addictive than traditional cigarettes. Penn State College of Medicine said vapers typically wait longer to start using their e-cigs after waking up. Advertisement

A spokesman for the United Kingdom Vaping Industry Association has hit out at UConn's findings, which were published in the journal ACS Sensors.

'The public health research in the UK is very clear. Public Health England have found vaping is likely to be 95 per cent less harmful than smoking, he told Metro.

'The Royal College of Physicians found that vaping did not exceed 5 per cent of the harm from smoking. Earlier this year, Cancer Research UK stated that not only is vaping a safer alternative to tobacco, it's long term harmful effects appear to be minimal.'

He also pointed out that UConn admitted in their report that the technology they used to measure toxicity is new and not as comprehensive as current measurement techniques.

NHS says e-cigs are safer but not risk-free

According to the NHS, vaping is 'not risk free, but based on current evidence they carry a fraction of the risk of cigarettes'.

It notes that as well as nicotine, e-cigarette liquid and vapour can contain 'potentially' harmful chemicals.

Though these are 'either at much lower levels than seen in cigarette smoke or at levels not associated with health risk'.

It says: 'E-cigarettes are still fairly new and we won't have a full picture on their safety until they have been in use for many years.

'Public Health England will continue to monitor the evidence as it develops.'