In a damning report it said the electronic devices are not safe – and it’s not clear whether they even help people quit smoking.
Vaping has been touted by producers as a better alternative to cigarette smoking, with the promise they can help people kick the habit.
The WHO said there is not enough evidence to show they help people quit.
But the report added: “There is no doubt they are harmful to health and are not safe.
“It is too early to provide a clear answer on the long-term impact of using them or being exposed to them.”
E-cigs – electronic nicotine delivery systems (ENDS) – pose “significant risks” for pregnant women as they can damage the growing foetus.
And the battery-powered devices – which smokers use to inhale addictive nicotine liquids – also harm the developing minds of young people.
E-cigs are “particularly risky when used by adolescents”, according to the organisation, which also warned teens last year about the dangers.
The report said nicotine is highly addictive and warned that as young people’s brains develop up to their mid-20s, exposure can have long-lasting, damaging effects.
The WHO has called for a ban on advertising aimed at teenagers. At the same time it seems the use of the devices is not keeping teenagers away from conventional cigarettes.
In fact the WHO added teenagers using e-cigs are more likely to move on to them. Those who inhale the second-hand smoke from vaping are also at risk.
The WHO, is the UN’s health agency, also said second-hand exposure to fumes was harmful, pointing out the aerosols “typically contain toxic substances, including glycol which is used to make anti-freeze”.
It added: “ENDS pose risks to users and non-users.”
The WHO warning follows the recent publication of the Health Behaviours in School-aged Children (HBSC) 2018 Study.
The Irish component of the international research was carried out by the Health Promotion Research Centre at NUI Galway.
Of the 15,500 young people surveyed, 22% – or just over 3,400 – reported trying e-cigarettes.
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In September, a top heart doctor at the Galway clinic described e-cigs as the “disaster of the century”.
In a stark warning, Prof Sherif Sultan claimed vaping is “more dangerous than smoking and alcohol combined”.
The world-renowned surgeon said: “We need to ban e-cigarettes immediately. They are the most catastrophic new gadget in the country that have the potential to injure and destroy the well-being of patients.”