Electronic cigarettes, more commonly known as e-cigarettes or e-cigs, have become a controversial topic since their arrival on the market.
What are e-cigarettes?
E-cigarettes, also known as personal vaporisers or an electronic nicotine delivery system, supply the user with nicotine without the burning of tobacco leaves, although not all use nicotine.
When the user sucks on the e-cigarette, liquid nicotine is vaporised and absorbed through the mouth. When they breathe out, a plume of what appears to be smoke is emitted but it is actually largely water vapour.
A battery-powered heating coil heats the liquid to form the vapour, with some of the designs involving a pressure sensor that is activated by the user taking a puff, while others have a button to heat them automatically.
How popular are they?
Inventor Hon Lik was the first to have his idea patented in his native China in 2003, and it has since become an industry worth around £2 billion. Anti-smoking group Ash estimates there are now 2.6 million vapers in the UK.
What are the health risks?
The Royal College of Physicians have said that e-cigarettes should be widely promoted as a substitute to smoking as they are much less harmful.
There is a possibility that the devices may result in some long-term harm because of the inhalation of the ingredients other than nicotine - but the harm that could be caused is substantially smaller than that caused by smoking, the report authors said.
While e-cigarettes are far less dangerous than conventional cigarettes, health experts are not encouraging people to take up the habit for the sake of it.
The emergence of e-cigarettes has given way to fears that they will act as a gateway to smoking conventional cigarettes among those who have never smoked - particularly children - but there is no evidence to support this.
Although many youngsters report having tried vaping, as Professor Peter Hajek, director of the Tobacco Dependence Research Unit at Queen Mary, University of London, and a co-author of the PHE report, said: "People who are attracted to e-cigarettes are the same people who are attracted to smoking. People who drink white wine are more likely to try red wine than people who do not drink alcohol."
In addition to the risks posed by inhaling the vapour, a nationwide safety alert was recently issued after a . It's fair to say, however, that they still only cause a fraction of the fires discarded cigarettes do.
Are there any other concerns?
The European Society of Cardiology recently warned that while e-cigarettes may be effective in helping smokers quit, they needed the same restrictions as cigarettes to avoid uptake by young people and non-smokers.