Vapes confiscated from UK pupils contain high levels of chemicals including lead, an investigation has found.
Tests on vapes at a school in Kidderminster, near Birmingham, showed children could be inhaling more than twice the safe amount of lead.
The results showed they also have nine times the safe level of nickel and high levels of chromium, according to the study, which was carried out on behalf of the BBC.
Lead is harmful to everyone, but it is particularly dangerous to children and can cause learning and behavioural problems, as well as more serious health issues such as confusion, seizures, coma, and even death.
The vapes were confiscated from secondary school pupils at Baxter College. There were 18 in total and most were illegal, which meant they had not been tested.
David Lawson, the co-founder of the Inter Scientific laboratory, which analysed the vapes, told the BBC he had “never” seen lead in a device in 15 years of testing.
"None of these should be on the market - they break all the rules on permitted levels of metal.
"They are the worst set of results I've ever seen."
Among the vapes, which are designed to look like highlighters, they found 12 micrograms of lead per gram, which is around 2.4 times the safe limit; 9.6 times the safe limit of nickel and 6.6 times the safe level of chromium.
The report said the metals were believed to come from the heating element, but they were also found in the e-liquid.
High levels of a compound called carbonyls, which break down into chemicals including formaldehyde and acetaldehyde as the e-liquid is heated, were found at 10 times the level in legal vapes, higher than some cigarettes.
MHRA head of e-cigarettes Craig Copland said the results will be reviewed to assess whether the vapes posed a health risk.
According to recent figures, there has been a 50 per cent rise in the past year in the proportion of children trying vaping.
The data for Britain shows a rise in experimental vaping among 11 to 17 year olds, from 7.7 per cent last year to 11.6 per cent this year.
Children were asked if they had ever tried vaping once or twice, with the proportion roughly doubling in nine years, from 5.6 per cent in 2014 to 11.6 per cent this year.
Asked what they used, disposable vapes appear to be the e-cigarette of choice among youngsters, while purchases of vapes are mostly made from corner shops.
It is illegal to sell vapes to under-18s, but social media carries posts from teenagers showing vapes and discussing flavours such as pink lemonade, strawberry, banana and mango.