LONDON

Puffing on slim metal tubes loaded with pale yellow liquid, two London businessmen say they have between their lips a cure for what the U.N. calls "one of the biggest public health threats the world has ever faced".

Electronic cigarettes are the future, they argue. Cheaper, cleaner and cooler than smoking, "vaping" - using a vaporiser to inhale nicotine infused with exotic flavours ranging from pina colada to bubblegum - will spell the end of tobacco.

"After I first tried this, I left half a cigarette in the ashtray and never went back," says Zoltan Kore, who co-runs the newly opened London e-cigarette shop "Smoke No Smoke".

"I'm not a smoker now, I'm a vaper," says business partner Gabor Kovacs. "The awful morning coughing fits have gone, and the waking up in the night struggling to breathe has gone, too."

Such stories - and hopes of persuading the rest of the world's billion smokers to stub out their tar and toxin-loaded cigarettes, cutting a catalogue of chronic disease risks as they do - are tantalising for public health experts.

And since "vaping" doesn't entail kicking the addiction - either to the stimulant nicotine or to the behavioural habits of smoking - some say it can help smokers quit much more effectively than nicotine gum or patches.

COOL ALTERNATIVE, OR DANGEROUS GATEWAY?

All the top tobacco companies are now placing bets on e-smokes, which some analysts predict may outsell conventional cigarettes in 10 years, raising the counter-intuitive prospect that Big Tobacco could actually help people quit smoking.

Celebrities like Bruno Mars and Courtney Love are also endorsing them, a further inducement to makers of iconic cigarette brands like Marlboro and Camel to invest.

Yet e-cigarettes are far from universally accepted as a public health tool; regulators are agonising over whether to restrict them as "gateway" products to nicotine addiction and tobacco smoking, or embrace them as treatments for would-be quitters.

A big issue is the lack of long-term scientific evidence to support the safety and effectiveness of e-cigarettes, prompting critics like the British Medical Association (BMA) to warn of the dangers of their unregulated use.

"These devices may also undermine efforts to prevent or stop smoking by making cigarette use seem normal in public and at work," argues the BMA, which has called for vaping to be banned in public places in Britain, just as smoking is.

The World Health Organisation (WHO) is equally wary, saying that until e-cigarettes have been endorsed as safe and effective by national regulators, "consumers should be strongly advised not to use any of these products".

Supporters of e-cigarettes scoff at suggestions they are a hazard or could be a slippery slope for previously addiction-free young people to get hooked on nicotine.

There is, they argue, no evidence of any harm from nicotine consumption and it would be crazy to impose tougher restrictions on e-smokes than on toxic "death sticks" that are freely available to buy on almost every street corner worldwide.

As Adrian Everett, chief executive of Britain's leading e-cigarette company E-Lites put it in a comment to Reuters: "Comparing electronic cigarettes to tobacco is like comparing playing football to juggling live hand grenades."

BIG KILLER

While the debate rumbles, smoking is killing half of all those who do it. Tobacco has an annual death toll of 6 million people, and that could exceed 8 million by 2030 unless something urgent is done to stop people smoking, according to the WHO.

As well as causing lung cancer and other chronic respiratory diseases, smoking is also a major contributor to cardiovascular diseases, the world's number one killer.

"This could be the most effective method of smoking reduction that we have ever had," says Konstantinos Farsalinos of the Onassis Cardiac Surgery Center in Greece, who has conducted several studies exploring the risks of vaping.

His work, some of which has had some funding from makers of e-cigarettes, has found no adverse effects on heart function, nor any notable cancer risks to cells in the lungs.

Other research, however, suggests "vaping" may reduce lung capacity, and the German Cancer Research Center said last month it was concerned e-cigarette liquids contained ingredients that can irritate the airways, while poor quality products could contain carcinogens.

Against this background, a growing number of regulators see a need to control standards in a largely unregulated sector.

Britain became the latest to take the plunge this week by opting to regulate e-cigarettes as non-prescription medicines, after finding widely varying nicotine levels and contaminants in some existing products. This means manufacturers will need a licence from 2016, though they will still be sold in general stores.

A few countries have banned them outright - such as Brazil, Norway and Singapore - while others are opting for varying degrees of regulation, in some cases including limits on advertising and curbs on their use in public places.

France said last month it would impose the same restrictions on e-cigarettes as on conventional ones.

The European Union is proposing to limit the amount of nicotine they can hold before regulation kicks in, while the U.S. Food and Drug Administration has so far adopted a light touch, saying it plans to regulate e-cigarettes as it does tobacco.

Greater regulation, in one form or another, may sink smaller players that cannot afford to navigate through various licensing systems. But the rest will benefit from a halo of legitimacy.

In particular, that could play into the deep pockets of Big Tobacco - a prospect that makes some campaigners uneasy.

"Tobacco companies seem to be playing both sides of the game by selling cigarettes that cause thousands of deaths a year but also selling products designed to reduce the harm," says Martin Dockrell of British campaign group Action on Smoking and Health.

"There are some real risks here that need to be managed."

NO-BRAINER FOR TOBACCO FIRMS

For tobacco companies seeking to offset the decline in traditional smoking, judicious investment in e-cigarettes has become a no-brainer.

Altria, owner of Philip Morris, maker of the world's top-selling cigarette brand Marlboro, became the last of the major tobacco companies to jump on the bandwagon this week.

It is following in the steps of U.S. rivals Reynolds American, maker of Camel, and Lorillard, which paid $135 million to acquire Blu Ecigs in 2012. British American Tobacco and Imperial Tobacco are investing too.

Market consultant Euromonitor estimates the world market for e-cigarettes was more than $2 billion last year, with the United States accounting for a quarter of that. And the market is growing at breakneck speed.

Bonnie Herzog, an analyst at Wells Fargo bank in New York, believes the U.S. market alone will top $1 billion this year. By 2023, she predicts, there could be more vapers than smokers.

That poses a challenge to makers of nicotine-replacement products such as gum and patches, sold by drugmakers including Pfizer and GlaxoSmithKline, but it is a huge opportunity for the newcomers.

They include hundreds of small firms selling vaping devices made in China, where e-cigarettes were first invented, as well as a few larger pure-play e-smoke groups like U.S.-based NJOY.

NJOY this month raised $75 million from entrepreneurs including Sean Parker, co-founder of the music-sharing site Napster - another feather in its cap after recently luring former U.S. Surgeon General Richard Carmona onto its board.

At Kore and Kovacs' green, sharped-lined shop in Camden in trendy North London, the watermelon and cherry flavoured vaping liquids and shiny offers of Swarovski-crystal-encrusted e-cigarettes are a far cry from the smoky reds and dusty browns one would expect in an old-style tobacconist's.

The pair say they are pursuing a deliberately clean image, designed to show how vaping can detoxify nicotine addiction.

These are exciting but uncertain times for e-smoke pioneers facing a wave of new rules in a hitherto unregulated market.

For now, Kore welcomes plans by Britain to regulate e-cigarettes as medicines. It will ensure safe standards and keep what he calls the "dodgy companies who mix nicotine liquids in the basement" out of the market.

But he's clear that excessively stringent rules would be dangerously counterproductive, banishing e-cigarettes to an inaccessible niche market and crushing an opportunity to improve health and save lives.

"To make people go back to smoking poisonous, stinking cigarettes is something that just must not happen," he said.