The liquid inside e-cigarettes contains high levels of toxic metals, a new study led by researchers at the Johns Hopkins Bloomberg School of Public Health has found.

In an examination of five e-cigarette brands' first-generation devices, researchers found varying levels of cadmium, chromium, lead, manganese, and nickel in the liquid component that, when heated, delivers ingredients such as nicotine and flavors to the user. Researchers believe the main source of the metals—which can be toxic or carcinogenic when inhaled—is the coil that heats the liquid to create the aerosol, which is commonly (but erroneously) referred to as vapor.

"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," says Ana María Rule, an assistant scientist in the Department of Environmental Health and Engineering who led study, which appears in the January issue of Environmental Research. "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."

For their study, the researchers examined "cig-a-like" models of first generation devices, which resemble traditional cigarettes. Many newer models are boxier and bulkier, resembling a cassette player with a mouthpiece attachment.

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.

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.

The five brands tested are sold across the United States in big-box retail stores, convenience stores, and gas stations, as well as online. Three of the five brands constituted 71 percent of total market share in 2015. Because liquid volume varied considerably from brand to brand, the research team tested for concentrations of metals in micrograms per liter, and if a brand came in more than one flavor, the researchers chose one flavor for the sake of consistency.

The metals were present in all five brands, but concentrations of the metals varied. For example, the brand with the highest concentration of all five metals contained 400 times the concentration of nickel of one brand, and 240 times the concentration of manganese of another brand.

"It was striking, the varying degrees to which the metals were present in the liquid," Rule says. "This suggests that the FDA should consider regulating the quality control of e-cigarette devices along with the ingredients found in e-cigarette liquids."

The Food and Drug Administration began regulating e-cigarettes last year, but has not yet issued warnings. For now, regulations require e-cigarette makers to submit ingredient lists and information about potentially harmful ingredients. In addition to the coil, the researchers believe some of the metals may come from the components of the e-cigarette device or the manufacturing process.

The researchers did not examine the presence of metals in the aerosol "vapor."