The hope many smokers have of being able to enjoy their habit without restrictions might go up in smoke as the World Health Organisation (WHO) calls for a ban on e-cigarettes indoors. The organisation believes this alternative to conventional cigarettes might not be safe for secondary smokers, as e-cigarettes could increase the levels of some toxins and nicotine in the air.

E-cigarettes work by heating liquid nicotine - the substance in tobacco which makes it addictive - and turning it into vapour, which is breathed in by the smoker. Since there's no smoke, puffing on e-cigarettes is being called 'vaping'.

According to estimates, the number of e-cigarette users in the UK alone has tripled over two years to more than 2 million, in a market worth more than £90m a year.

Many think e-cigarettes let smokers avoid legislation put in place about seven years ago to protect staff in the workplace. Vaping bars have sprung up in London, but their future remains uncertain. A growing concern about public health has led many pubs, train companies and offices to ban the use of these devices.

The British Medical Association is joining the WHO in asking for restrictions on e-cigarettes. They fear marketing them as fashion accessories or medical aids might undo their campaign against smoking. Ram Mooray from the organisation, said: "Our biggest concern is that smoking becomes glamorous again."

So, do the disadvantages of e-cigarettes outweigh the benefits? The debate is likely to heat up. Research by University College London says that for every million smokers who switch to e-cigarettes, more than 6,000 lives a year could be saved. Some smokers are using them to reduce their craving for cigarettes.

Professor Peter Hayek, Director of the Tobacco Dependence Research Unit at Queen Mary University of London, warns against restricting e-cigarettes. He says it's "like asking people to stop using mobile phones and tablets because of a one-in-10-million chance that the battery might overheat in your device."