Several recent studies have suggested that electronic cigarettes may not be as safe as we may think, and a new study now adds that these popular devices leak harmful metals—some of them highly toxic.

Ever since their release on the global market, electronic cigarettes (e-cigarettes) have been hailed, generally, as a safer alternative to the regular ones.

These devices work by heating up a flavored liquid that sometimes—though not always—contains nicotine.

Instead of smoke, e-cigarettes release aerosols, or "vapors," which is why users of e-cigarettes are often referred to as "vapers," and the act of "smoking" using this device is dubbed "vaping."

Despite the popular belief that e-cigarettes are better for our health than traditional ones, recent research has indicated that these devices could bring about fresh, and little understood, risks.

It has been suggested that people who use e-cigarettes could have a higher risk of cardiovascular problems and cancer, and a study published earlier this month found that a few certain e-cigarette flavors are particularly toxic.

Now, a team of scientists from the Johns Hopkins Bloomberg School of Public Health in Baltimore, MD, concludes that e-cigarette vapors could also be harmful. Senior study author Ana María Rule and team surmise that the danger may arise from the toxic content of e-cigarette heating coils.

"It's important for the FDA [Food and Drug Administration], 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."

Ana María Rule

The researchers' findings are now published in the journal Environmental Health Perspectives.

Concentration 'higher than safe limits'

In a previous study, Rule and colleagues identified a series of toxic metals—cadmium, chromium, lead, manganese, and nickel—in e-cigarette liquids.

The new research took these findings and went further, testing the e-cigarettes of actual users in order to try to understand how exposed people were to these toxic substances, and under what circumstances.

Rule and team worked with 56 participants who used e-cigarettes on a daily basis. The researchers tested the participants' e-cigarettes, verifying the presence of 15 metals in the refilling dispensers, the vaping liquids "loaded" into the e-cigarettes, and the vapors that resulted from the liquids' heating.

The researchers were able to confirm that toxic metals were present in the e-liquids from refilling dispensers, though in fairly small concentrations. But a completely different story unfolded in the case of tank-filling solutions that had already been heated by the inbuilt coils.

These e-liquids had much higher concentrations of toxic metals, which—the researchers say—suggests that the solutions themselves are not the source of these substances. Instead, Rule and colleagues infer that the toxic metals may come from the heating coils.

Due to the contamination of the e-liquid, traces of toxic metals were also found in the aerosols released by the e-cigarettes.

The metals that the study's authors express most concern about are lead, chromium, nickel, and manganese, all of which have been linked with important health risks, including cancer, brain damage, and disorders of the respiratory system.

In aerosols, the median concentration for lead alone was around 15 micrograms per kilogram, and 48% of the vapor samples that the team tested had lead concentrations that exceeded the limit recommended by the United States Environmental Protection Agency.

"These were median levels only," says Rule. "The actual levels of these metals [including those of nickel, chromium, and manganese] varied greatly from sample to sample, and often were much higher than safe limits."

Mysterious sources of toxic metals

As previous research has confirmed, the heating coils contained in e-cigarettes are often made of a combination of nickel and chromium, plus other substances. This supports the thesis that many of the toxic metals whose traces were found in e-liquids and aerosols must have leaked from those coils.

However, it remains unclear where the lead is coming from, or how all of these metals contaminate the e-liquid.

"We don't know yet whether metals are chemically leaching from the coil or vaporizing when it's heated," says Rule.

Study co-author Angela Aherrera also led a separate investigation using data from the same 56 participants. Her additional inquiry revealed that the e-cigarette users had increased levels of nickel and chromium in their urine and saliva.

These were consistent with the concentration of these metals identified in e-cigarette vapors.

Interestingly, however, toxic metal traces were found in higher concentrations in vapors produced by e-cigarettes whose heating coils were frequently replaced.

This suggests that the leaks likely occur from new heating coils, and perhaps less so in the case of older elements.

Another worrying result indicated that arsenic—a highly toxic substance—was found in refill e-liquid, tank liquid, and vapors in no fewer than 10 out of the 56 sets of samples provided by the study participants. Why arsenic was present remains to be clarified.

"We've established with this study that there are exposures to these metals, which is the first step," says Rule, adding that there is a "need also to determine the actual health effects."