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The flavourings used in some e-cigarettes contain potentially dangerous chemicals, researchers have claimed.

American experts studied a small sample of 30 products and found a "significant" number contained aldehydes - compounds which are thought to damage the lungs.

'Vapers' have almost 8,000 flavour to choose from including menthol, cherry, coffee and bubble gum, the study found.

If vapers inhale 5 millilitres of e-liquid a day, researchers claimed they would be exposed to twice the recommended occupational exposure limits of benzaldehyde and vanillin.

The study has prompted warnings that smokers are being put off swapping to vaping - which is known to be a healthier alternative.

Researchers at Portland State University in Oregon admitted their sample was small and said: "Nevertheless, the results obtained are likely to be similar to what a broad survey would have revealed, and in any case strongly suggest that very high levels of some flavour chemicals are undoubtedly present in a great number of the thousands of products currently available.

"Regulatory actions that should be considered include requiring ingredient identification, limiting levels of some individual flavour chemicals, and limiting total levels of flavour chemicals."

A separate study published in the BMJ found 5.8% of 10 to 11-year-olds had tried e-cigs - far more than had tried tobacco (1.6%).

Among children aged 11 to 16, some 12.3% said they had tried e-cigarettes.

But just 1.5% - including 0.3% who did not smoke tobacco - reported regular e-cigarette use.

(Image: PA)

The study said there is evidence that e-cigarettes are being used by growing numbers of young non-smokers, prompting "significant concerns that, if unregulated, and marketed to young people, e-cigarettes could seriously undermine the success of recent tobacco control strategies".

"The carcinogens and other toxins within e-cigarettes are one concern; harm reduction arguments do not hold where e-cigarettes are used by young people who would not otherwise have been using tobacco," the authors added.

"Furthermore, it has been suggested that e-cigarette use could act as a new gateway into nicotine addiction and tobacco use for young people."

A debate is currently raging over the safety of e-cigs, with contradictory evidence offered by scientists on both sides.

Some experts fear e-cigs are a health timebomb, whilst many others highlight claims they are an order of magnitude safer than old-school smoking.

E-cig advocates hit back at the studies.

Clive Bates, a former director of the anti-smoking charity ASH, said: "We should stay focussed on the huge benefits gained when people switch to smoking, and do what is reasonable to reduce any unnecessary risks with vaping.

"The danger of being alarming about flavours is that it may put people off switching from vaping to smoking - and that's where the real harm would be caused."

Tom Pruen, chief scientific officer at the Electronic Cigarette Industry Trade Association, added: "It is likely that as more information becomes available the levels of some flavourings may have to be reduced to further increase the safety of the products.

"It is, however, important to bear in mind that even now this is comparing potential respiratory irritants in e-cigs with known, potent, carcinogens in tobacco smoke. It does little to contradict the idea that e-cigs are vastly safer than smoking."