According to experts, e-cigarettes may be more damaging than smoking.
The battery-powered tubes, usually styled to look like real cigarettes, contain a heating element that turns nicotine-laced liquid in a cartridge into a vapour mist that is inhaled.
This produces a sensation similar to smoking the real thing but, say manufacturers, without the carcinogenic chemicals found in cigarette tar, the Daily Mail reported.
To vaporise the nicotine solution, the chemical propylene glycol is put into the cartridges, and accounts for up to 90 per cent of their content.
This can cause 'acute respiratory system irritation', claims Dr Elisabeth Pott, director of the Federal Centre of Health Education in Cologne, Germany, who has studied e-cigarettes.
In 2009, the US Food and Drug Administration analysed e-cigarette cartridges and found traces of the carcinogen nitrosamine and other potentially harmful substances in products from several manufacturers, in addition to ethanol and glycerin.
However, e-cigarette firms point out that nitrosamine is found in much higher concentration in cigarettes.
Professor John Britton, of the Royal College of Physicians' Tobacco Advisory Group, is calling for regulation of e-cigarettes that would ensure a 'guaranteed standard'.
He added that they should be regulated to a degree to make sure there are reasonable levels of nicotine in them.
E-cigarettes have been banned in Canada, Australia, and some American states. Most recently, their use by civil servants in city offices and vehicles has been banned in Hanover, Germany.