Electronic cigarettes are harmful both to users and bystanders exposed to the fumes, the World Health Organisation (WHO) says in a report, warning that they can damage growing foetuses and affect teenagers' brains.
So-called electronic nicotine delivery systems, or Ends, have been pushed by producers as well as some governments as a safer alternative to smoking, and as a path to kicking the habit.
But in a strongly worded series of questions and answers about such e-cigarettes in a document published on Tuesday, the WHO said that there was not enough evidence to say they help smokers quit, but that there was clear evidence they are dangerous.
"There is no doubt that they are harmful to health and are not safe," the WHO said in one answer, stressing though that "it is too early to provide a clear answer on the long-term impact of using them or being exposed to them".
The battery-powered devices that enable users to inhale addictive nicotine liquids and exhale vapour "are particularly risky when used by adolescents", the WHO said, strengthening initial warnings it made in 2019.
"Nicotine is highly addictive and young people's brains develop up to their mid-20s," it added, stressing that "exposure to nicotine can have long-lasting, damaging effects".
This is of high concern amid skyrocketing popularity of vaping among young people in many countries.
At the same time, there is little indication that using e-cigarettes is keeping teens away from more harmful products, with the WHO pointing out that those using Ends are more likely to end up smoking conventional cigarettes.
The WHO's report on the harmful effect of vaping and e-cigarettes is of high concern amid skyrocketing popularity of vaping among young people in many countries.
And it said that "for pregnant women, Ends pose significant risks as they can damage the growing foetus".
Amid growing fears over serious health consequences linked to e-cigarette use, the WHO warned that so-called vaping increases the risk of heart disease and lung disorders.
The US especially has seen a major health crisis tied to vaping: an acute lung illness epidemic that has killed more than 50 people and sickened more than 2,500.
The illness was later linked to a substance called vitamin E acetate, which is used as a thickening agent for vaping products containing the main psychoactive substance of cannabis - THC - that are often sold on the black market.
At least five other countries have initiated investigations to identify cases of lung injuries related to e-cigarette use, the WHO said.
The agency also said second-hand exposure to e-cigarette fumes was harmful, pointing out that they "typically contain toxic substances, including glycol which is used to make antifreeze".
"Ends pose risks to users and non-users," it said, demanding tight regulation of the products, including bans on marketing to young people and on use in indoor workplaces and public spaces.
The document drew ire from some experts, with Peter Hajek, the head of the Tobacco Dependence Research Unit at the Queen Mary University of London, accusing it of "anti-vaping activism".
Hajek charged that the document was filled with errors and that the authors "should take responsibility for using blatant misinformation to prevent smokers from switching to a much less risky alternative".
NO SMOKING FOUR WEEKS BEFORE OPERATION CUTS RISKS: WHO
Patients who stop smoking at least four weeks before an operation significantly reduce the risk of having post-surgical complications because their blood flow improves, according to a study published on Monday.
The WHO study argued that minor or non-essential operations on regular smokers could be delayed to give them time to quit and thereby improve outcomes such as wound healing and heart function.
The study, conducted in co-operation with Australia's University of Newcastle and the World Federation of Societies of Anaesthesiologists, found that every additional tobacco-free week beyond the four weeks improved health outcomes by 19 per cent.
"The report provides evidence that there are advantages to postponing minor or non-emergency surgery to give patients the opportunity to quit smoking, resulting in a better health outcome," said Dr Vinayak Prasad, head of the WHO's Tobacco Free Initiative.
The study found that nicotine and carbon monoxide, both present in cigarettes, can decrease oxygen levels and greatly increase risk of heart-related complications.
It said the damage on lungs from tobacco smoke also made it difficult for the proper amount of air to flow through, and found that smoking could delay wound healing because of its distorting effect on the patient's immune system.
"Smoking just one cigarette decreases the body's ability to deliver necessary nutrients for healing after surgery," it said.
The WHO said that all countries should build cessation programmes and educational campaigns into their health systems to spread awareness and help people quit smoking.