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Warnings over e-cigarettes are alarmist - and increasing their use could save many lives, researchers have said.

For every million smokers who switch to e-cigarettes, more than 6,000 lives a year could be saved, according to the University College London team.

Meanwhile another group of London-based experts has attacked criticism of e-cigarettes as "misleading".

Last week the World Health Organization called for e-cigarette use to be banned in public places and workplaces.

The WHO said this was because they could increase the levels of some toxins and nicotine in the air.

Its report also warned about the risk of e-cigarettes acting as a gateway by which non-smokers might start smoking real cigarettes.

But the UCL team said the numbers of non-smokers using e-cigarettes amounted to less than 1% of the population, according to the Smoking Toolkit study, a monthly survey of smokers in England.

Prof Robert West added that even though some toxins were present in vapour from e-cigarettes the concentrations were very low.

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"You have to be a bit crazy to carry on smoking conventional cigarettes when there are e-cigarettes available," he said.

"The vapour contains nothing like the concentrations of carcinogens and toxins as cigarette smoke.

"In fact, concentrations are almost all well below a twentieth of cigarettes."

Using these estimates it would mean 6,000 lives a year being saved for every million smokers who exchanged real cigarettes for e-cigarettes, he said.

If all nine million UK smokers used them that would equate to 54,000 lives saved out of the current 60,000 premature deaths, Prof West said.

His concerns were echoed by researchers at the National Addiction Centre based at King's College London and the Tobacco Dependence Unit at Queen Mary University.

They carried out an analysis - published in the journal Addiction - of the WHO research which contributed to last week's report.

They concluded that some of the assumptions WHO had made were "misleading".

'Little hard data'

Lead researcher Prof Peter Hajek said: "I think any responsible regulator proposing restricting regulation has to balance reducing risks with reducing potential benefits.

"In this case the risks are unlikely, some already proven not to exist, while the benefits are potentially enormous. It really could be a revolutionary intervention in public health if smokers switched from cigarettes to electronic cigarettes.

"So killing benefits, which are huge, for risks which are small is like asking people to stop using mobile phones and tablets, or restrict their use and further development, because of a one in 10 million chance that the battery might overheat in your device."

The WHO has yet to respond to the criticisms of its work.

Prof John Ashton, president of the Faculty of Public Health, which has been one of the bodies that has expressed concerns, said he did not want to see a ban.

But he added: "We do want to be sure that any benefits they may have don't undo all the hard work that's been done over decades to save lives by reducing smoking. We are particularly concerned that 'vaping' may lead to young people starting to smoke cigarettes."

And he added: "At the moment, there is very little hard data about e-cigarettes: until we get some solid facts on their impact on people's health, we need proper regulation."

Shirley Cramer, chief executive officer of the Royal Society of Public Health, said the argument was not clear cut.

"Emerging evidence from the States suggests significant numbers of non smokers are using e-cigarettes, with the potential for them to get hooked on nicotine.

"We need to curb the appeal of 'e-cigarettes' to non-smokers - it would help if we stop marketing what is essentially a medicinal product as a cool or trendy fashion accessory."