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Researchers from Johns Hopkins University in the US found that significant numbers of the devices generated aerosols with potentially unsafe levels of lead, chromium, manganese and/or nickel.
Chronic inhalation of these metals has been linked to lung, liver, immune, cardiovascular and brain damage, and even cancers.
"It's important for the e-cigarette companies and vapers themselves to know that these heating coils, as currently made, seem to be leaking toxic metals - which then get into the aerosols that vapers inhale," said Ana Maria Rule, assistant scientist at Johns Hopkins University.
E-cigarettes typically use a battery-supplied electric current that passes through a metal coil to heat nicotine-containing "e-liquids," creating an aerosol - a mix including vaporised e-liquid and tiny liquid droplets.
Vaping, the practice of inhaling this aerosol as if it were cigarette smoke, is now popular especially among teens, young adults and former smokers.
A 2017 survey of 8th-, 10th- and 12th-grade students in public and private schools, sponsored by the National Institute on Drug Abuse, found that about one in six had used e-cigarettes in the previous 30 days.
Evidence that vaping is not entirely safe continues to accumulate, however. Recent studies have found that e-cigarette liquids contain flavourings and other chemicals that harm cells in standard toxicology tests.
Working with participants' devices, the scientists tested for the presence of 15 metals in the e-liquids in the vapers' refilling dispensers, the e-liquids in their coil-containing e-cigarette tanks and in the generated aerosols.
They found minimal amounts of metals in the e-liquids within refilling dispensers, but much larger amounts of some metals in the e-liquids that had been exposed to the heating coils within e-cigarette tanks.
The difference indicated that the metals almost certainly had come from the coils. Scientists showed that the metal contamination carried over to the aerosols produced by heating the e-liquids.
Of the metals significantly present in the aerosols, lead, chromium, nickel and manganese were the ones of most concern, as all are toxic when inhaled.
E-cigarette heating coils typically are made of nickel, chromium and a few other elements, making them the most obvious sources of metal contamination, although the source of the lead remains a mystery.
"We don't know yet whether metals are chemically leaching from the coil or vaporising when it's heated," Rule said.