Electronic cigarettes, or e-cigarettes, are devices for delivering nicotine vapour instead of tobacco smoke. There are two types of devices: 'tank models' and 'cigalikes' which, as the name suggests, are made to resemble cigarettes.
Their use has grown rapidly, with an estimated 2.6 million British adults using them.
Scientific evidence that e-cigarettes can help people quit smoking has only recently started to appear.
Many people are using them to help stop smoking, other people use them as a way to carry on smoking while avoiding cigarette smoke.
In May 2016, the EU’s revised Tobacco Products Directive came into force so that electronic cigarettes below a certain nicotine threshold can be sold with health warnings. Above this nicotine threshold, e-cigarettes will have to be authorised as medicinal products.
The limit is 20mg/ml, which the health group ASH says will not have much of an impact on most people's vaping, as most products are already below this level.
The legislation also requires safety standards and child resistant refills.
E-cigarettes are easily bought in the UK, in shops and online.
The sale of e-cigarettes is illegal to under-18s. The same legislation makes it illegal for adults to buy electronic cigarettes for children.
An e-cigarette contains three components: a battery, an atomiser and a cartridge containing nicotine suspended in propylene glycol and water. When the user sucks on the e-cigarette, the liquid in the cartridge is heated so that some of it evaporates. This vapour delivers nicotine into the user's lungs.
There is no smoke, but some nicotine vapour is released into the surrounding air as the user exhales.
Are e-cigarettes regulated?
E-cigarettes are not covered by general legislation banning smoking in public places.
It is legal to 'vape' an e-cigarette in a public place because there is no burning and no smoke is emitted - only odourless vapour.
However, they may still be subject to bans in some places, including aircraft, trains, shopping centres and hospitals.
The medicines regulator MHRA says the UK wants to ensure that electronic cigarettes meet appropriate standards of safety, quality and efficacy to help reduce the harms from smoking.
The regulator is encouraging companies to voluntarily submit medicines licence applications for electronic cigarettes as medicines.
E-cigarettes will be subject to the same advertising restrictions as tobacco products.
In January 2013, the Advertising Standards Authority banned a claim that the vapour inhaled and exhaled when using a type of e-cigarette is "a completely harmless vapour". The ASA considered the claims that the product was not harmful had not been substantiated due to an absence of adequate evidence.
Is 'passive vaping' harmful? Are e-cigarettes safe?
Evidence about the safety of e-cigarettes has grown rapidly.
There have been claims that e-cigarettes are harmful and these have centred on media headlines about high levels of formaldehyde.
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