GOING VAPE? More than a million adults use e-cigs to stop smoking - but is vaping safe?
But, many experts have cautioned that the health effects of vaping remain largely unknown.
Rule and her colleagues, including lead author Pablo Olmedo, a postdoctoral researcher at the Johns Hopkins University's Bloomberg School at the time of the study, recruited 56 daily e-cigarette users from vaping conventions and e-cigarette shops around Baltimore in autumn 2015.
Vaping is thought to be as much as 95 percent safer than cigarette smoking, but mounting evidence suggests that the devices could still pose serious risks.
Only a few weeks ago, United Kingdom health bodies suggested electronic cigarettes should be in hospital shops to encourage smokers to wean themselves off their habit. How arsenic got into these e-liquids is yet another mystery.
Part of the purported safety advantage of e-cigarettes over combustible tobacco is that they use liquid vapor instead of smoke from burning plant, paper and chemical products.
But their heating mechanisms - battery-powered atomizers, or heating coils - have been the subjects of little research.
The difference indicated that the metals nearly certainly had come from the coils. The scientists tested and found the presence of 15 metals in the e-liquids in the coil-containing e-cigarette tanks and in the generated aerosols.
In e-cigarettes, electric current passes through a metal coil to heat nicotine-containing "e-liquids", creating an aerosol-a mix including vaporized e-liquid and tiny liquid droplets.
Since these devices don't use traditional smoke, people are under the assumption that they are safe for you. This can hinder brain development in teens.
Although the Food and Drug Administration has the authority to regulate e-cigarettes, it is considering its options. Precisely how metals get from the coil into the surrounding e-liquid is another mystery.
But once the liquid reached the tank where it was exposed to the heating coil, levels spiked significantly.
But most worrisome were the types and quantities of metals found in the vapor that the e-cigarette-users were liberally puffing on every day.
The aerosols contained potentially unsafe levels of lead, chromium, manganese and/or nickel, the scientists reported. Nearly 50 percent of aerosol samples had lead concentrations higher than health-based limits defined by the Environmental Protection Agency. "The actual levels of these metals varied greatly from sample to sample, and often were much higher than safe limits".
It can stay dormant in teeth and bones, and be reactivated during pregnancy, seeping into the blood where it can reach and poison a developing fetus with the potential to cause significant brain damage.
"We don't know yet whether metals are chemically leaching from the coil or vaporizing when it's heated", Rule said.
"It's important for the FDA, the e-cigarette companies and vapers themselves to know that these heating coils, as now made, seem to be leaking toxic metals, which then get into the aerosols that vapers inhale", says senior researcher Ana María Rule, assistant scientist in environmental health and engineering in Johns Hopkins University's Bloomberg School of Public Health.
They also found high levels of arsenic - which can cause many illnesses, including cancer - in vapor from 10 of the e-cigarettes, though it is unclear how it found its way into the vapor.