The BBC reports that used vapes gathered at Baxter College in Kidderminster were tested in a laboratory where it was found that children using them could be inhaling more than twice the daily safe amount of lead, and nine times the safe amount of nickel.
Lab co-founder David Lawson said: "In 15 years of testing, I have never seen lead in a device.
"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."
The lab tests also found compound called carbonyls that were present 10 times the level in legal vapes. Some even had more than cigarettes.
High levels of lead exposure in children can affect the central nervous system and brain development, according to the World Health Organization.
University of Nottingham epidemiology professor John Britton, who sits on the Royal College of Physicians Tobacco Group, said inhaling metals could be dangerous.
"Lead is a neurotoxin and impairs brain development, chrome and nickel are allergens and metal particles in general in the bloodstream can trigger blood clotting and can exacerbate cardiovascular disease," he told the BBC.
"The carbonyls are mildly carcinogenic and so with sustained use will increase the risk of cancer - but in legal products, the levels of all of these things is extremely low so the lifetime risk to the individual is extremely small."
He added there had been a much greater rise in illegal products recently and "some of these are hard to distinguish between the ones which are potentially legal".
A YouGov survey in March and April for Action on Smoking and Health suggests a rise in experimental vaping among 11- to 17-year-olds, from 7.7 per cent, last year, to 11.6 per cent, despite it being illegal to sell vapes to under-18s.
The government has allocated £3m to tackle the sale of illegal vapes in England.
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