Researchers have found that metals are leaking from the components of e-cigarettes into the vapour - which could potentially be toxic when breathed in by the user.
A study discovered that five metals - including lead, which is linked to brain damage - were found in the liquid of the device, which is then heated up and inhaled.
In the study, researchers at Johns Hopkins University in Baltimore, US, examined five brands' cigarette-like models of first generation vaping devices, which more closely resemble traditional cigarettes.
In 'cig-a-like' devices, liquid is stored in a cartridge along with the coil, which increases the liquid's exposure to the coil, even in the absence of heating.
The five brands tested are sold across the United States in big-box retail stores, convenience stores, and petrol stations, as well as online.
Five well known brands across the US were tested for the metals (Image: iStockphoto)
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Three of the five brands constituted 71% of total market share in 2015.
To test the liquid for metal levels, the researchers extracted samples of the liquid that had not been heated by the coil prior to extraction. The liquid is a mixture of propylene glycol, glycerin, nicotine, and flavorings.
Because liquid volume varied considerably from brand to brand, the research team tested for concentrations of metals in micrograms per litre, and if a brand came in more than one flavour, the researchers chose one flavour for the sake of consistency.
Scientists aren't sure if the presence of metals could be dangerous (Image: Getty)
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The metals - cadmium, chromium, lead, manganese, and nickel - were all found in the tested liquids.
Ana Maria Rule, an assistant scientist in the Department of Environmental Health and Engineering, who led to the study, said: "We do not know if these levels are dangerous but their presence is troubling and could mean that the metals end up in the aerosol that e-cigarette users inhale.
"One of the things that is troubling is that the metals in e-cigarette coils, which heat the liquid that creates the aerosol, are toxic when inhaled, so perhaps regulators might want to look into an alternative material for e-cigarette heating coils."
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She added: "It was striking, the varying degrees to which the metals were present in the liquid.
"This suggests that the FDA should consider regulating the quality control of e-cigarette devices along with the ingredients found in e-cigarette liquids."